Bruno Yun
email: yun@lirmm.fr

Pierre Bisquert
email: pierre.bisquert@inra.fr

Patrice Buche
email: patrice.buche@inra.fr

Madalina Croitoru
email: croitoru@lirmm.fr

Jean Guillard
email: guillard@univ-montp2.fr

Valérie V Guillard

Rallou Thomopoulos
email: rallou.thomopoulos@inra.fr

Choice of environment-friendly food packagings through argumentation systems and preferences

Keywords: . Food Packaging, Logic-Based Argumentation, Argumentation Tool, Preference Management, Decision Support System

Introduction

We propose a Multi-Criteria Decision Support system (MCDSS) which permits to take into account the points of view of several stakeholders of a food chain about a question under debate. In this paper, we want to be able to choose a packaging solution in a given list of possible alternatives, for a given food to pack. The case study chosen in this paper is fresh strawberries. Stakeholders' opinions (consumers, scientists, manufacturers, etc.) in favor or against specific options are expressed on different criteria (for instance the environmental impact of the packagings). The MCDSS, which implements an argumentation process, must be able to help the manager in charge of the decision (for instance, a strawberry producer) to determine a ranked list of the alternative solutions taking into account food chain stakeholders' opinions and preferences expressed on the associated criteria.

For instance, a strawberry producer expresses the need for a new packaging to pack strawberries. The design of this new packaging needs to take into consideration the packaging industry constraints (ability to scale-up the production process, the availability of the raw material, etc.), the waste management administration rules about packaging end of life (biodegradability, recyclability, incineration, burying, etc.) and consumer preferences (trans-parent packaging, environment-friendly packaging, no extra-cost due to packaging, etc.).

In order to gather consumers' viewpoints, multiple methods can be used: text mining, gathering reviews, etc. We chose to focus on online polls so as to easily gather arguments from a variety of consumers.

Stakeholders' opinions are expressed as text arguments. As illustrated in Figure 1, these arguments are the input of the argumentation system which distinguishes for each option (wood packaging, open plastic packaging, etc.) the reasons leading to its acceptance or its rejection. Then, the argumentation system detects the conflicts among the arguments and computes the sets of coherent arguments which defend themselves against contradicting arguments. After that, it ranks the packaging solutions under debate using a given prioritization of the requirements. Thus, packagings have to be selected according to several aspects or criteria (food conservation, shock protection, packaging end of life management, etc.) highlighted by arguments expressed by the stakeholders involved in the project. The problem at hand does not simply consist in addressing a multi-criteria optimization problem [START_REF] Bouyssou | Decision-making process -Concepts and Methods[END_REF], since we want the MCDSS to be able to justify why certain packagings are chosen. To this aim, we use argumentation theory [START_REF] Dung | On the acceptability of arguments and its fundamental role in nonmonotonic reasoning, logic programming and n-persons games[END_REF]; [START_REF] Besnard | Elements of Argumentation[END_REF]; [START_REF] Rahwan | Argumentation in Artificial Intelligence[END_REF], in which some approaches combine argumentation and multi-criteria decision making such as Amgoud and Prade (2009) or recently [START_REF] Delhomme | Designing an argumentative decision-aiding tool for urban planning[END_REF]. This paper details how arguments are modeled within a structured argumentation system and how the delivered justified conclusions can be used in the packaging ranking process. It extends the first stage presented in [START_REF] Yun | Arguing About End-of-Life of Packagings: Preferences to the Rescue[END_REF] with several new contributions: (i) beside textual arguments, survey results are now integrated as a possible knowledge source; (ii) this raises a scaling-up issue, since high data volumes now have to be managed and automatically analyzed; (iii) the reasoning process, which was based on the computation of several coherent viewpoints, is now able to rank them using a prioritization of criteria.

The main contributions of the work are the following:

1. A MCDSS based on an argumentation system (AS). Arguments may be either manually entered or automatically generated from a set of responses to a given web survey.

2. A MCDSS designed to allow the ranking of packaging alternative solutions using the consensual sets of arguments (called extensions) computed by the argumentation system and a prioritization of requirements.

3. An evaluation of the MCDSS tool, based on the strawberry case study, in the framework of the Pack4Fresh project with an interdisciplinary collaboration between experts of packaging research, consumer behavior research, and computer science research.

The paper is structured as follows: in Section 2, we present the MCDSS global workflow which implements the desired functionalities expressed by the partners of the Pack4Fresh INRA-CIRAD project which financed this work. In Section 3, we briefly recall Dung's argumentation framework, used to compute extensions (maximal consistent sets of arguments) and we present the structured argumentation model we use and the way we automatically generate arguments from a set of answers to a given web survey. In Section 4, we present the model proposed to rank extensions according to a prioritization on requirements. Section 5 presents the case study and its results.

Section 6 is dedicated to the implementation of the approach and Section 7 to related works. Finally, Section 8 recalls our contributions and introduces some perspectives. • Task 1: Argument structuring: in this task, a textual opinion is encoded into a logic-based structured argument thanks to a dedicated graphical user-friendly interface (GUI).

MCDSS workflow overview

• Task 2: Automatic argument generation: this task automatically transforms some poll's answers into formal arguments made of concepts and rules using the framework described in Section 3.

• Task 3: Logical arguments derivation: Using the framework described in Section 3.2, this task builds all possible arguments by a derivation process.

• Task 4: Attacks detection: According to the definition of attacks defined in Section 3.2, this task computes an argumentation graph made of arguments (nodes) and attacks (edges).

• Task 5: Extensions computation: This task computes the set of extensions, i.e. the subsets of non-conflicting (consistent) arguments which defend themselves from attacking arguments (cf. Section 3.1). To scale up and manage high volumes of arguments from web survey results, connection with the Aspartix platform [START_REF] Dvorak | Making use of advances in answer-set programming for abstract argumentation systems[END_REF] is performed.

• Task 6: Extension rankings: the computation of extensions delivers one or several extensions. In the case of several extensions, the system uses the prioritization on criteria using the framework described in Section 4.1 in order to rank the extensions and to select the top-ranked. Finally, the selected extension is then used to extract preferences associated with its arguments.

Next section introduces the model we propose for argument formalization and the way arguments may be automatically generated from a poll.

Logic argumentation model and poll-based arguments generation

In this section, we recall Dung's argumentation principles and present an instantiation of this framework thanks to a logical language, then we show how arguments are automatically generated from a set of answers to a given web survey.

Dung argumentation principles

A Dung's argumentation framework (AF ) [START_REF] Dung | On the acceptability of arguments and its fundamental role in nonmonotonic reasoning, logic programming and n-persons games[END_REF]

Logic argumentation model

A knowledge base contains the concepts of the considered domain expressed using a logical language L (such as propositional logic in this paper), the alternative choices in debate and two reserved concepts ACC, REJ referring to the decisions (respectively the accepted and rejected denominations) with ¬ACC = REJ and conversely. An argumentation system AS = (L, ¬, R s , R d ) is composed of the logical language L, a negation function, a set of strict rules R s and a set of defeasible rules R d . A strict subsumption, denoted , expresses natural inclusion in the domain, as "Plastic trays are packagings". A defeasible subsumption, denoted , expresses an inclusion which is not always true, as "Plastic packagings can be reusable".

A knowledge base in an AS = (L, ¬, R s , R d ) is a tuple (K, Cr) such that K ∪ Cr ⊆ L, where K contains the alternative choices in debate and Cr contains the reasons/criteria that may underlie an argument.

An argument A is of the form ∅ c 0 1 c 1 2 c 21 , where i ∈ { , }, We say that an argument A attacks an argument B iff at least one of the two following conditions is satisfied:

c 0 ∈ K, c 1 ∈ Cr, c 2 ∈ {ACC, REJ} and for all i ∈ {1, 2}, there exists a strict (resp. defeasible) rule in R s (resp. R d ) of the form c i-1 c i if i = (resp. c i-1 c i if i = ).
• Choice(A) = Choice(B), Den(A) = Den(B) and B is of the form ∅ c 0 1 c 1 c 2 . • Choice(A) = Choice(B), Den(A) = Den(B) = ACC and B is of the form ∅ c 0 1 c 1 c 2 .
Example 2. We consider the following arguments expressed about biodegradability of packaging materials considered here as one possible alternative of end of life management:

• Life Cycle Analysis (LCA) results are not in favor of biodegradable materials, regarding their high environmental impact,

• Consumers are in favor of biodegradable materials since they help to protect the environment.

We model these arguments by using the proposed logical language as follows:

• BP is a concept referring to biodegradable packaging materials,

• P EV , HIP are concepts referring to packagings which respectively protect the environment and have a high environmental impact (according to LCA),

• ACC, REJ are concepts referring to the decisions (accepted, rejected).

The set of rules R = R s ∪ R d is:

• R s = {BP HIP, ¬HIP ¬BP, HIP REJ, ¬REJ ¬HIP } • R d = {BP P EV, P EV ACC}
Please notice that strict rules are used to model reliable knowledge based on measured parameters by using well-defined and stated procedures, or expressed with linguistic terms such as "must", "shall", "mandatory", "important", etc..

Instead, defeasible rules model knowledge based on empirical observations or expressed with linguistic terms such as "may", "can", "optional", etc. Here, the rules involve HIP are considered as strict and those involving P EV are defeasible.

The following structured arguments can be built on the knowledge base

Poll-based argument generation

Let us now describe the process used to generate poll-based arguments.

It is composed of several steps:

•

Step 1: Creation of the poll: as defined in Section 3.2, elements of K represent the alternatives that are in discussion. They may be different packagings, products, etc. We propose to design a set Q of general questions that can be answered by "Yes", "No" or "Neutral" about concepts, i.e. elements of Cr, which will be used as criteria to rank the alternatives under discussion. An example of a question can be q 1 ="Do you think that x ∈ K protects the environment?" or q 2 ="Do you think that x ∈ K is harmful for strawberries?" The set of questions Q = {q 1 , q 2 , . . . , q m } is asked for every alternative of K.

Please note that we denote by Con(q 1 ) = P rotect_environment (resp.

Con(q 2 ) = Harmf ul), the underlying concept of question q 1 (resp. q 2 ).

We also define a function σ : Cr → {ACC, REJ}, given by domain experts, that tells us if a concept is an element in favor (ACC) or against (REJ) a given alternative. For instance, σ(Con(q 1 )) = ACC (resp.

σ(Con(q 2 )) = REJ).

•

Step 2: Getting the answers: The poll is proposed to an audience composed of n persons. The result of the poll can be represented with three functions:

positive : Q × K → N that takes as input a question and an alternative and returns the number of persons that answered "Yes", It is obvious that for every k i ∈ K and every question q j ∈ Q, positive(q j , k i ) + negative(q j , k i ) + neutral(q j , k i ) = n.

-negative : Q × K → N that
• Step 3: Processing the answers: Once the answers received, we process them using an aggregation function agg for filtering purposes.

agg(q j , k i ) =            0 if neutral(q j , k i ) > positive(q j , k i ) + negative(q j , k i ) -1 else if positive(q j , k i ) < negative(q j , k i ) 1 otherwise
We do not use answers to questions with agg(q j , k i ) = 0 because the answers are not pertinent enough w.r.t. the metric used.

• Step 4: Creating the arguments: In this step, we first select a "certainty" threshold α ∈ {0, 1, . . . , n} and create the following arguments:

∀k i ∈ K, ∀q j ∈ Q : -if agg(q j , k i ) = 1 and |positive(q j , k i ) -negative(q j , k i )| > α then ∅ k i Con(q j ) σ(Con(q j )), -if agg(q j , k i ) = 1 and |positive(q j , k i ) -negative(q j , k i )| ≤ α then ∅ k i Con(q j ) σ(Con(q j )), -if agg(q j , k i ) = -1 and |positive(q j , k i ) -negative(q j , k i )| > α then ∅ k i ¬Con(q j ) ¬σ(Con(q j )), -if agg(q j , k i ) = -1 and |positive(q j , k i ) -negative(q j , k i )| ≤ α then ∅ k i ¬Con(q j ) ¬σ(Con(q j )).
Example 3. Suppose that there is a question q = "Do you think that x protects strawberries from shocks?" and that P lastic_not_closed is an alternative in K corresponding to a plastic packaging that is not closed. We ask the question q to the consumers and we get that 394 persons answered "No", 179 persons answered "I do not know" and 272 persons answered "Yes".

Since we have that neutral(q, P lastic_not_closed) ≤ positive(q, P lastic_not_closed)

+ negative(q, P lastic_not_closed) and positive(q, P lastic_not_closed) < negative(q, P lastic_not_closed), we compute that agg(q , P lastic_not_closed) = -1. Now, if we define α = 200, the only argument produced, meaning that "not closed plastic packagings are rejected because they do not protect strawberries from shocks", is:

∅ P lastic_not_closed ¬Shocks_protection REJ

Ranking extensions

We suppose in this section that arguments generated from polls as described in Section 3.3 or manually entered by experts are available in the knowledge base. Extensions are computed using the semantics recalled in Section 3.1. We explain in this section the proposed method to rank extensions according to preferences expressed on requirements.

We first define the necessary notions used in this section. Let E be an extension. We define the accepted requirements and the rejected requirements of an extension E as:

AReq(E) = A=∅ c 0 1 c 1 2 ACC∈E {c 1 } RReq(E) = A=∅ c 0 1 c 1 2 REJ∈E {c 1 }
Considering the definition of attacks provided in Section 3.2, it must be noticed that for a given extension E, AReq(E), if not empty, gathers positive arguments in favor of a given alternative in debate and RReq(E) gathers negative arguments against all the other alternatives in debate.

Refining extensions using semantics

In this section, we introduce our method for ranking a set of extensions E using the locally, Pareto and globally optimal semantics inspired by [START_REF] Croitoru | Introducing Preference-Based Argumentation to Inconsistent Ontological Knowledge Bases[END_REF]. These semantics return subsets of the original set of extensions. We introduce here the three notions which are based on the notion of domination (preference) between concepts of the accepted requirements.

An extension E is said not to be locally optimal if we can find another extension E such that the concepts of E are either included in E or dominated by elements of E (there is at most one concept dominated).

Definition 1. We say that an extension E ∈ E is locally optimal if and only if x ∈ AReq(E) and a concept y such that there exists E ∈ E\{E}, ((AReq(E)\{x}) ∪ {y}) ⊆ AReq(E ) and x < y.

An extension E is said not to be Pareto optimal if we can find another extension E such that the concepts of E are either included in E or dominated by elements of E (they are dominated by a single concept).

Definition 2. We say that an extension E ∈ E is Pareto optimal if and only if X ⊆ AReq(E) and a concept y and X = ∅ such that there exists E ∈ E\{E}, ((AReq(E)\X) ∪ {y}) ⊆ AReq(E ) and for every x ∈ X, x < y.

An extension E is said not to be globally optimal if we can find another extension E such that the concepts of E are either included in E or dominated by elements of E (no restrictions).

Definition 3. We say that an extension E ∈ E is globally optimal if and only if X ⊆ AReq(E) and a set of concepts Y and X = ∅ such that there exists E ∈ E\{E}, ((AReq(E)\X) ∪ Y ) ⊆ AReq(E ) and for every x ∈ X, there exists y ∈ Y such that x < y.

Note that while those semantics allow to refine the set of considered extensions, they may be unable to output only one extension. This is of course dependent of the preferences the user has expressed: the more preferences are used, the more refinements are going to happen. Note also that it is possible to use the preferences differently, namely in a more "quantitative" fashion based on argument count. We study this new approach in the next section.

Ranking methods using scores

This new approach using scores is interesting in many ways. First, it is obviously easier and faster to compute than the approach introduced in the previous section (and based on [START_REF] Croitoru | Introducing Preference-Based Argumentation to Inconsistent Ontological Knowledge Bases[END_REF]). Furthermore, an extension can be accurately scored (using the preferences) even if we do not have the entire set of extensions. This can be useful in the event that we do not have enough time to compute all the extensions. In this section, we introduce two scores for ranking extensions. A research issue is to find a way to combine the two scores in order to produce a more efficient ranking. This can be achieved by using multicriteria methods. We provide a naive way to combine the two scores, namely

Score 3 (E) = Score 1 (E) + Score 2 (E).

Use-case

The use case is coming from the INRA Glofoods Pack4Fresh project which, as explained in the introduction, aims at designing innovative packaging solutions for fresh food products. For best packaging selection support, one aspect to take into account is the consumers' expectations in terms of packaging characteristics. In the project it has been tested for strawberries.

Four packaging options have been considered (see Figure 4):

• an opened plastic basket (without lid or film)

• a wood packaging (without lid)

• a plastic basket with rigid lid

• a plastic basket with plastic film

Automated generation of arguments from the poll

A survey upon a sample of 840 people has provided the following 38 arguments using the poll-based argument automatic generation process pre-

Extensions computation

Using the argumentation model presented in Section 3.2, 1519 attacks have been generated upon the 46 arguments. Thanks to these arguments and attacks, five preferred extensions have been calculated using Aspartix.

Please note that the preferred semantics is used because it is simple and allows to preserve every existing point of view (cf. Section 3.1). One can observe that the first four extensions are composed of:

• the set of positive arguments in favor of a given alternative,

• the set of negative arguments against the other alternatives in debate.

For instance, extension E 4 is associated with the alternative Wood Packaging. Arguments a1, a2, a3, a4, a5, a6, a7, a8 are positive arguments in favor of Wood Packaging and arguments a11,a12,a13,a14,a15,a16,a18,a23,a30,a33,a37,a40,a41,a42,a44,a46 are negative arguments against the three other alternatives. , a10, a17, a19, a20, a23, a30, a33, a37, a39, a42, a45, a46} E 2 {a9, a11, a12, a13, a14, a15, a16, a18, a23, a31, a32, a34, a35, a36, a38, a39, a42, a44} E 3 {a9, a11, a12, a13, a14, a15, a16, a18, a21, a22, a24, a25, a26, a27, a28, a29, a30, a33, a37, a39, a43, a44, a48} E 4 {a1, a2, a3, a4, a5, a6, a7, a8, a11, a12, a13, a14, a15, a16, a18, a23, a30, a33, a37, a40, a41, a42, a44, a46} E 5 {a9, a11, a12, a13, a14, a15, a16, a18, a23, a30, a33, a37, a39, a42, a44, a46} Table 4: Preferred extensions of the use-case.

E 1 {a9
The remaining extension E 5 contains all the negative arguments associated with all the alternatives. In this use case, this last extension will be considered as useless since negative arguments are already available in the other extensions.

Scenario analysis

We will consider the following three scenarios:

• Scenario SECURE: "not nefast effect" (i.e. not harmful) concept is preferred to all the other concepts.

• Scenario GREEN: "Protect_environment", "recyclable" and "reusable"

are preferred to all the other concepts.

• Scenario PLEASURE: "can see", "can smell", "protect flavor" and "incite to eat" are preferred to all the other concepts.

In the following, we only detail the results obtained for scenario SECURE and we present globally the results obtained for the three scenarios. The in-Locally optimal {P lastic_with_plastic_f ilm, P lastic_rigid_lid, W ood_packaging} Pareto optimal {P lastic_with_plastic_f ilm, P lastic_rigid_lid} Globally optimal {P lastic_with_plastic_f ilm, P lastic_rigid_lid} Table 5: Results obtained for scenario SECURE refining extensions using the locally, Pareto and globally optimal semantics. terested reader will find the detailed results for the other scenarios in Section AppendixA.

Preferences associated with concepts for scenario SECURE are the following:

• P rotect_f lavor < ¬N ef ast_ef f ect • Can_see < ¬N ef ast_ef f ect

• P rotect_environment < ¬N ef ast_ef f ect • ¬P rotect_environment < ¬N ef ast_ef f ect • Shocks_protection < ¬N ef ast_ef f ect • ¬Shocks_protection < ¬N ef ast_ef f ect • Reusable < ¬N ef ast_ef f ect • Recyclable < ¬N ef ast_ef f ect • ¬Reusable < ¬N ef ast_ef f ect • ¬Recyclable < ¬N ef ast_ef f ect • Incite_to_eat < ¬N ef ast_ef f ect
• Can_smell < ¬N ef ast_ef f ect • ¬Can_smell < ¬N ef ast_ef f ect • N ef ast_ef f ect < ¬N ef ast_ef f ect • ¬Good_f ridge_conservation < ¬N ef ast_ef f ect • ¬Good_ambiant_conservation < ¬N ef ast_ef f ect • Good_ambiant_conservation < ¬N ef ast_ef f ect • Good_f ridge_conservation < ¬N ef ast_ef f ect • Concentrate_smell < ¬N ef ast_ef f ect • Condensation < ¬N ef ast_ef f ect • ¬Concentrate_smell < ¬N ef ast_ef f ect
We can see in and Score 1 . Indeed, in scenarios SECURE and PLEASURE, Score 3 provides an advantage to alternatives with less negative arguments which are P lastic_rigid_lid and W ood_packaging.

The same scenarios have been presented to a food packaging expert in order to assess the MCDSS results. Concerning scenario SECURE, the expert agrees with results obtained with Globally_optimal and Score 1 indicators and disagrees with result obtained with Score 3 . Indeed, the expert prefers P lastic_with_plastic_f ilm to P lastic_rigid_lid as the first one permits to control in a better way modified atmosphere which extends shelf life (expressed in Argument a45) and avoids moisture and microorganism growth.

It may be noticed that this last argument was not present in the MCDSS knowledge base as an expert argument but it exits as a consumer argument (a20). The addition of this new argument will not change the ranking for all the indicators as the MCDSS does not take into account the fact that the same argument may be expressed by different stakeholders. An option could be to introduce a weight which will provide more power to arguments which are supported by several stakeholders; such an approach could benefit from the notion of ranking semantics such as Amgoud and Ben-Naim ( 2013 Concerning scenario GREEN, the expert has defined three individual rankings for each of the criteria Protect environment, Reusable and Recyclable:

• Protect environment: P lastic_with_plastic_f ilm > W ood_packaging > P lastic_rigid_lid = P lastic_not_closed considering that P lastic_with_plastic_f ilm (with modified atmosphere) permits to reduce waste and W ood_packaging has less impact on environment than P lastic_rigid_lid and P lastic_not_closed in terms of biodegradability duration.

• Reusable: W ood_packaging = P lastic_not_closed > P lastic_rigid_lid > P lastic_with_plastic_f ilm considering the practical point of view of reuse of the packaging material for another usage.

• Recyclable: W ood_packaging > P lastic_not_closed = P lastic_rigid_lid = P lastic_with_plastic_f ilm considering that none of the three plastic materials are recyclable at the state of the art and that wood packaging is the only recyclable one.

Considering that W ood_packaging is the only one appearing in first position for "Reusable" and "Recyclable" and in second position for "Protect environment", we can state that the expert agrees with the result proposed by the MCDSS for the three indicators Globally_optimal, Score 1 and Score 3 .

Concerning scenario PLEASURE, the expert did not want to assess the criterion Incite_to_eat as it is a question of consumer's perception. However, the expert considers that all packagings are ex-aequo for the three remaining criteria (can_see, can_smell and protect_f lavor). This corresponds to the result expressed by the MCDSS for the indicators Globally_ optimal and Score 1 , except for the case of P lastic_with_plastic_f ilm which is ranked behind the other packagings by the MCDSS. This is due to the fact that consumers consider the P lastic_with_plastic_f ilm packaging not to allow smelling the strawberries (Argument a18), whereas the expert considers this is compensated by its ability to concentrate smell. The latter compensation effect, however, is not coded in the MCDSS.

We may note that in several of the above evaluation cases, discordances between MCDSS and expert rankings are not due to the ranking method itself but to missing information to be included into the MCDSS, or pieces of information included in the MCDSS but not taken into account by the expert (by example negative arguments). This highlights the interest of an iterative process for argument elicitation in order to obtain complete information in the MCDSS, as recommended in [START_REF] Thomopoulos | An iterative approach to build relevant ontology-aware data-driven models[END_REF]; [START_REF] Johnson | Making ontology-based knowledge and decision trees interact: an approach to enrich knowledge and increase expert confidence in data-driven models[END_REF]; [START_REF] Thomopoulos | Artificial intelligence-based decision support system to manage quality of durum wheat products[END_REF]. On the contrary, similar information lead to similar rankings, which constitutes a positive expert validation feedback on the MCDSS reasoning engine. Another significant finding was that in complex cases, as in the GREEN scenario for instance, providing a unique global ranking was a difficult task for the expert. Thus we can conclude that (i) there is a recognized added value of providing MCDSS results and (ii)

expert evaluation has to be achieved firstly on simple cases, which can be intuitively apprehended by human reasoning. Interestingly, these remarks are in line with a well-known distinction between different approaches to decision support [START_REF] Tsoukiàs | On the concept of decision aiding process: an operational perspective[END_REF]. The normative approach, more common in the Anglo-Saxon school of decision support, derives decision models from rationality norms established a priori. Expert decision deviating from these norms is interpreted as a mistake which highlights the need for MCDSS aid in order to decide in a rational way [START_REF] Fishburn | Utility Theory for Decision Making[END_REF]. On the contrary, in the descriptive approach, more common in the European school of decision support, decision models are derived from observing how expert make decisions, in order to reproduce their way of reasoning in the MCDSS Bell et al. (1988). In our system, normative decision support is expected from the MCDSS in complex cases, whereas the descriptive approach is used for MCDSS evaluation in simple cases.

Implementation of the approach

The MCDSS has been implemented as a Java GXT/GWT web application (although the access is restricted). This MCDSS takes as input a collection of textual arguments in favor or against a set of alternatives under debate. It implements the entire process described in Section 2 from argument elicitation to extension ranking and it also provides several GUIs for visualisation purposes. The main interface of the system is illustrated in Figure 5; it displays the graphical representation of the formalized concepts and arguments. We integrated a simple and intuitive interface in the web application for inputting preferences which enables users to clearly visualize the preferences implied (see Figure 6c). The preferences are saved in a database and are specific to a particular argumentation case. We also implemented all the preferences methods discussed in this paper. The processing of the argumentation framework is hidden to the user and only the different extensions produced are displayed (see Figure 6a). The user can then add preferences and use the refining methods introduced in Section 4.1 (see Figure 6b).

Related work

This work presents a novel application of preference based logical argumentation systems for food science. As illustrated in Figure 2, our approach follows the following work-flow: generation of arguments (from text or polls), attack computation and generation of argumentation framework and, last, use of preferences for extension ranking.

Regarding the first step of the work-flow, i.e. the argument generation, we used the structured argument definition of Prakken ( 2010) but changed to our particular application needs (our arguments are always in favour or not of an option). In the second work-flow step, the attack used in this paper, albeit satisfying the rationality postulates of [START_REF] Caminada | On the evaluation of argumentation formalisms[END_REF], also follows the intuition of [START_REF] Prakken | An abstract framework for argumentation with structured arguments[END_REF]. Last, the preferences are applied to the extensions of the argumentation framework built upon the first two steps. Ordinal preference handling follows the work of [START_REF] Croitoru | Introducing Preference-Based Argumentation to Inconsistent Ontological Knowledge Bases[END_REF].

This work differs from classical argumentation approaches (for an overview please check [START_REF] Modgil | A general account of argumentation with preferences[END_REF]) in the fact that the attack relation is not modified (i.e. changed, deleted) but the preferences are used directly on the outputted extensions. The numerical preference handling takes this work further in a cardinal setting. A discussion on the rationales of different kinds of attacks can be found in [START_REF] Yun | Defining argumentation attacks in practice: An experiment in food packaging consumer expectations[END_REF].

This work uses the software interface described in [START_REF] Tamani | An argumentation system for eco-efficient packaging material selection[END_REF] for logical argument elicitation from text. This software, similarly to other argumentation software such as Araucaria [START_REF] Reed | Araucaria: Software for argument analysis, diagramming and representation[END_REF], Argunet "Quaestion-it.com" [START_REF] Evripidou | Quaestio-it.com: a social intelligent debating platform[END_REF] which is a social intelligence debating platform that demonstrates a question-and-answer web application providing support for user-posed questions; Carneades [START_REF] Gordon | Introducing the carneades web application[END_REF], which provides software tools based on a common computational model of argument graphs useful for policy deliberations, etc.

Conclusion

In this paper we proposed a complete methodology, from texts and online polls, until final decision support, in order to (i) model possibly conflicting arguments from various actors involved and regarding several criteria, (ii)

2 www.debategraph.org structure an argumentation system, (iii) deliver justified conclusions based on extension computation, (iv) use criteria prioritization to rank the solutions. Using this methodology, a case study concerning the choice of the most suitable eco-packaging for fresh food products is presented and its expert evaluation discussed.

This system is a significant breakthrough in two different fields. On the one hand, it extends explanatory approaches of multi-criteria and multi-actor decision by allowing for scaling up to high data volumes, which have to be managed and automatically analyzed, due to the use of online polls as a data source. On the other hand, it opens the way to sustainable choices to reduce the post-harvest environmental impact of fresh foods, since food packaging plays a crucial part in it. Moreover, in opposition to classical "black box" approaches, users can access and assess the reasons behind the provided decision, which allows the iterative process of adding new arguments if some pieces of information are missing. This guarantees the fact that decision biases can be corrected by knowledge enrichment.

The aim of this paper was to present, assess and show the relevance of the MCDSS workflow. An interesting future methodological study would be to fine-tune the current MCDSS workflow parameterization, notably with regards to the aggregation function used to compute arguments from the polls, the "certainty" threshold used to distinguish between strict and defeasible arguments, and the semantics used to compute extensions.

Moreover, as a future work, this methodology is promising to support

Figure 1 :

 1 Figure 1: Global insight of the MCDSS.

Figure 2

 2 Figure 2 presents the main tasks of the MCDSS workflow.

Figure 2 :

 2 Figure 2: MCDSS workflow overview.

Figure 3 :

 3 Figure 3: Examples of different Dung semantics.

  We denote by Choice(A) = c 0 of logical symbols used in arguments concerned by the argument A, Reason(A) = c 1 the reason associated with the argument A and Den(A) = c 2 the decision associated with the argument A.

(

  K, Cr) with K = {BP } and Cr = {HIP, P EV }: • A : ∅ BP HIP REJ • B : ∅ BP P EV ACC Argument A attacks argument B since Choice(A) = Choice(B), Den(A) = REJ, Den(B) = ACC and B : ∅ BP P EV ACC.

  takes as input a question and an alternative and returns the number of persons that answered "No" and neutral : Q×K → N that takes as input a question and an alternative and returns the number of persons that answered "Neutral".

4. 2

 2 .1. First scoring: Higher score based on positive arguments means less dominated The first method only considers positive arguments in favor of one of the alternatives in debate. It gives the highest score to the extension that is the least dominated. Namely, the score of an extension E is: Definition 4. Score 1 (E) = a∈AReq(E) |{c | c is a concept and c < a}| With this score, the best extension is the one with the highest score. 4.2.2. Second scoring: Higher score based on negative arguments means less dominated The second method only considers negative arguments against the alternatives in debate. It gives the highest points to the extension whose negative arguments are the most dominated. Namely, the score of an extension E is: Definition 5. Score 2 (E) = a∈RReq(E) |{c | c is a concept and a < c}| Again, with this score, the best extension is the one with the highest score.

Figure 4 :

 4 Figure 4: Four considered packaging options: Plastic not closed, Wood packaging, Plastic rigid lid, Plastic with plastic film.

  );Amgoud et al. (2016);[START_REF] Bonzon | A comparative study of ranking-based semantics for abstract argumentation[END_REF];Baroni et al. (2018) where arguments' strength is computed based on the attacks in the framework. So, Score 3 seems to bring an additional piece of information which is not taken into account by the expert.

Figure 5 :

 5 Figure 5: Main interface of the argumentation system showing a global overview on the alternatives under debate.

Figure 6 :

 6 Figure 6: Interfaces for preference management and extension ranking.

  [START_REF] Schneider | Argunet-a software tool for collaborative argumentation analysis and research[END_REF] and DebateGraph 2 , allows the expression of arguments as texts to manually formalize them as hypothesis and conclusions but also to compute the extensions and the preference induced ranking. In this respect, our interface is the only software allowing to compute all steps of the workflow described in Figure2.While this work presents a significant and original application of argumentation theory in food science, let us also highlight other numerous argumentation applications developed recently in various fields: ArgTrust[START_REF] Parsons | Argtrust: Decision making with information from sources of varying trustworthiness[END_REF], in which the authors considered argumentation frameworks for decision making; CISpaces framework[START_REF] Toniolo | Argumentation-based collaborative intelligence analysis in cispaces[END_REF], which supports col-

  is a tuple (A, C), where C ⊆ A × A is a binary attack relation on the set of arguments A. For each argument X ∈ A, X is acceptable w.r.t. a set of arguments E ⊆ A if and only if any argument attacking X is attacked by an argument of E. A set of arguments E ⊆ A is conflict free if and only if ∀X, Y ∈ E, (X, Y ) /

	extension if and only if it is the set inclusion minimal complete extension;
	E is a stable extension if and only if it is preferred and ∀Y / ∈ E, ∃X ∈ E
	such that (X, Y ) ∈ C. For a given semantics, the set of extensions of an
	argumentation framework is denoted by E.
	Example 1. Figure 3 illustrates some examples of argumentation graphs,
	upon which extensions under the Dung's semantics (admissible, complete,
	preferred, grounded and stable) are computed (nodes in green color). Note
	that sub-graphs (b) and (c) illustrate the two preferred extensions in the ar-
	gumentation graph.
	∈ C.
	E is an admissible extension if and only if it is conflict-free and ∀X ∈ E, X is
	acceptable w.r.t. E; E is a complete extension if and only if E is admissible and
	X ∈ E whenever X is acceptable w.r.t. E; E is a preferred extension if and
	only if it is a set inclusion maximal complete extension; E is the only grounded

Table 6 :

 6 Results obtained for scenario SECURE ranking extensions using scoring functions.

	Packaging	Score 1 Score 2 Score 3
	W ood_packaging	0	14	14
	P lastic_with_plastic_f ilm	21	8	29
	P lastic_not_closed	0	12	12
	P lastic_rigid_lid	21	14	35

Table 7 :

 7 Table7that the results obtained using the two indicators Globally_optimal and Score 1 are the same for the alternatives in first position. Score 3 indicator is more discriminant than Globally_optimal Summary of the results obtained for the three scenarios.

	Scenario	Globally optimal	Score 1	Score 3
	SECURE	{P lastic	P lastic	P lastic_rigid_lid
		_with_plastic_f ilm, _with_plastic_f ilm	> P lastic
		P lastic_rigid_lid}	∼ P lastic_rigid_lid	_with_plastic_f ilm
			> W ood_packaging	> W ood_packaging
			∼ P lastic_not_closed > P lastic_not_closed
	GREEN	W ood_packaging	W ood_packaging	W ood_packaging
			> P lastic_rigid_lid	> P lastic_rigid_lid
			∼ P lastic_not_closed > P lastic_not_closed
			> P lastic	> P lastic
			_with_plastic_f ilm	_with_plastic_f ilm
	PLEASURE {P lastic_not_closed,	W ood_packaging	W ood_packaging
		P lastic_rigid_lid,	∼ P lastic_rigid_lid	∼ P lastic_rigid_lid
		W ood_packaging}	∼ P lastic_not_closed > P lastic_not_closed
			> P lastic	> P lastic
			_with_plastic_f ilm	_with_plastic_f ilm

The notation ∅ c 0 indicates that the alternative c 0 is given and does not necessitate any justification in general.

innovation by guiding the design of new-generation, biosourced, "intelligent", eco-efficient food packagings. Research is active in this area but mainly focused on technical aspects such as the properties of the materials in an extremely small size scale. However, to be acceptable and used, these newgeneration solutions have to take into account all the considerations and expectations raising from end-users all along the supply chain, from production to consumption and after use, with a life-cycle sight.

Appendix AppendixA: Detailed results for the use case Locally optimal {P lastic_rigid_lid, W ood_packaging} Pareto optimal W ood_packaging Globally optimal W ood_packaging Locally optimal {P lastic_not_closed, P lastic_rigid_lid, W ood_pack} Pareto optimal {P lastic_not_closed, P lastic_rigid_lid, W ood_pack} Globally optimal {P lastic_not_closed, P lastic_rigid_lid, W ood_pack} Baroni, P., Rago, A., and Toni, F. (2018). How many properties do we need for gradual argumentation? In Proceedings of the Thirty-Second


Jeong-Won Yoon

Hyun-Suk Chun

Ja-Myeong Koo

Seung-Boo Jung

AU-SN FLIP-CHIP SOLDER BUMP FOR MICROELECTRONIC AND OPTOELECTRONIC APPLICATIONS

As an alternative to the time-consuming solder pre-forms and pastes currently used, a co-electroplating method of eutectic Au-Sn alloy was used in this study. Using a coelectroplating process, it was possible to plate the Au-Sn solder directly onto a wafer at or near the eutectic composition from a single solution. Two distinct phases, Au 5 Sn and AuSn, were deposited at a composition of 30at.%Sn. The Au-Sn flip-chip joints were formed at 300 and 400℃ without using any flux. In the case where the samples were reflowed at 300℃, only an (Au,Ni) 3 Sn 2 IMC layer formed at the interface between the Au-Sn solder and Ni UBM. On the other hand, two IMC layers, (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn, were found at the interfaces of the samples reflowed at 400℃. As the reflow time increased, the thickness of the (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn IMC layers formed at the interface increased and the eutectic lamellae in the bulk solder coarsened.

INTRODUCTION

With the rapid advancement of information technologies which has been seen in recent years, the use of optoelectronic packages, such as planar light-wave circuit modules, is increasing rapidly. In these packages, solder alloys are commonly employed for the purpose of mounting active devices, such as laser diodes, on the substrate of the package [START_REF] Tew | Au/Sn solder for facedown bonding of AlGaAs/GaAs ridge waveguide laser diodes[END_REF]. The solder interconnection of the module not only provides conventional functions, such as heat dissipation, electrical connection and selfaligning effects, but also helps to maintain the precise alignment between the laser diode and the waveguide during the use of the module. The solders used for the application of these modules, therefore, must be resistant to the creep deformation induced by stresses, such as thermal stress, in the modules. Solders for bonding applications in microelectronic/optoelectronic packages are classified as either soft solder or hard solder depending on their melting temperature [START_REF] Tsai | Controlling the microstructures from the gold-tin reaction[END_REF]. Soft solders, such as Sn and In alloys, have low melting temperatures, but exhibit lower yield strengths, which lead to lower creep resistance. Solder creep reduces the reliability of optoelectronic packages, because the alignment of the devices cannot be maintained over time. On the other hand, hard solders, including Au-rich Au-Sn, Au-Si, and Au-Ge alloys, have higher melting temperatures and higher yield strengths. The advantages of solders with a higher melting temperature include superior thermal stability and long term reliability. Among these hard solders, eutectic Au-30at.% Sn is the preferred alloy, because of its relatively low melting point, low elastic modulus, high thermal conductivity and high strength, as compared with those of the other solders [START_REF] Doesburg | Microstructure and preferred orientation of Au-Sn alloy plated deposits[END_REF][START_REF] Djurfors | Pulsed electrodeposition of the eutectic Au/Sn solder for optoelectronic packaging[END_REF][START_REF] Djurfors | Microstructural characterization of pulsed electrodeposited Au/Sn alloy thin films[END_REF]. Au-Sn eutectic solder has a lower melting temperature (278°C) compared to other hard solders, such as Au-Si (363°C) and Au-Ge (356°C). This property makes it useful for bonding devices that are sensitive to high processing temperatures, but need good creep resistance, such as GaAs or large Si dies on alumina. In addition, the high thermal conductivity of Au-Sn (57 W/m℃) makes it particularly useful for bonding higher power devices that demand good heat dissipation. Au-Sn solder also offers many other advantages when making solder joints, such as the ability to solder without using flux, the formation of a hermetic seal, good mechanical and electrical properties and low intermetallic growth rates when used over Ni, Pd or Pt. The drawback of this solder is that maintaining the desired eutectic composition requires extreme accuracy and precise control. Au-30at.% Sn solder has traditionally been applied using solder pre-forms, paste, or electron-beam evaporation [START_REF] Doesburg | Microstructure and preferred orientation of Au-Sn alloy plated deposits[END_REF]. Solder pre-forms are problematic due to the alignment difficulties that are encountered and the oxidation of the solder prior to bonding. Solder paste also suffers from oxidation prior to bonding, in addition to the possibility of solder contamination during bonding originating from the organic binder in the paste. Electron-beam evaporation is advantageous for Au-Sn solder deposition in that the amount of oxide formed prior to bonding can be reduced and the thickness and position of the solder can be precisely controlled. The sequential evaporation of Au and Sn layers to produce a deposit of the desired ©TIMA Editions/DTIP 2006 ISBN: 2-916187-03-0 Authors Title. Cut at approx. 70 characters if it is too long.... composition has been successfully employed, along with co-evaporation techniques. The electroplating of eutectic Au-Sn solder is also an attractive alternative, because it offers the advantages of low cost and high speed as compared to evaporation techniques, while providing a similar level of control to that of pre-forms and paste. Au-Sn solder has been deposited by electroplating Au and Sn layers sequentially from separate Au and Sn solutions. Recently, however, Au-Sn solder has been co-deposited by electroplating [START_REF] Djurfors | Microstructural characterization of pulsed electrodeposited Au/Sn alloy thin films[END_REF]. The alloy electroplating of Au-Sn promises better composition control, lower mechanical stress, and finer dimensional capability, along with lower processing complexity, higher throughput, and a lower capital cost. Co-deposition is also advantageous in that the level of Sn oxidation is kept to a minimum during the electroplating process, since the wafer does not need to be removed from the solution until plating is complete. Amongst the major trends observed in the development of semiconductor devices is the small volume of the products that was achieved using integrated circuits (ICs) and the larger size and functionality per unit area of the modules used in the products. To facilitate these trends, flip chip (FC) interconnection technology was developed. An FC interconnection is the connection of an IC chip to a carrier or substrate with the active face of the chip facing toward the substrate. FC technology is generally considered as the ultimate first level connection, because it allows for the highest density and shortest path length to be achieved, so that the optimal electrical characteristics can be obtained. Flip-chip solder connections have to be fluxless, because flux residue seriously affects both the performance and reliability of flip-chip assemblies [START_REF] Kim | Fluxless flip-chip Sn-Au solder interconnect on thin Si wafers and Cu laminated polyimide films[END_REF]. Fluxless solder reflowing is also an environmentally benign technology. The most commonly used Pb-free solder material for fluxless bonding is an alloy of Au-20(wt.%)Sn [START_REF] Elger | Development of an assembly process and reliability investigations for flip-chip LEDs using AuSn soldering[END_REF]. During reflowing, the solder alloy melts and then reacts with the substrate or the metallization of the chip to form intermetallic compounds (IMCs) at the joining interface. The brittle nature of these IMCs, as well as extensive intermetallic growth, can reduce the reliability of solder joints. In other words, interfacial phenomena may be directly related to the reliability of the solder joint in electronic packages. Therefore, knowledge of the formation of IMCs during soldering in microelectronic packaging is essential. In this paper, we report the fluxless Au-Sn flip-chip bumping technique using coelectroplated Au-Sn alloy. In addition, we examined the interfacial reactions between the Au-Sn solder and the Ni UBM for various reflow conditions.

EXPERIMENTAL PROCEDURES

Silicon wafers with a diameter of 4 inches were metallized with 0.2 ㎛-thick Ti and 0.8 ㎛-thick Cu. The Ti and Cu layers are used as an adhesion layer and interconnection layer, respectively. The metallized wafers had area array pad arrangements at a pitch of 300 ㎛ with a rectangular pad opening having a width of 50 ㎛. The bumping for the flip chip devices was performed using electrolytic Ni with a thickness of 10 ㎛. The temperature of the solution was controlled by means of a heater situated underneath the electroplating tank, while a thermometer placed inside the electroplating solution continuously read the temperature. A 50 ㎛ layer of Au-Sn was electroplated in a bath at a current density of 0.5A/dm 2 for 120 minutes. The plating bath temperature and pH value were 35℃ and 4.3, respectively. After electroplating, the microstructural features of all of the electroplated samples were examined using a scanning electron microscopy (SEM, Philips XL 40 FEG and/or HITACHI S-3000H), equipped with an energy dispersive x-ray (EDX) spectroscope.

Authors

Title. Cut at approx. 70 characters if it is too long.... In addition, X-ray diffraction (XRD) phase analysis was carried out using a Rigaku (Japan) diffractometer. A Cu coupon was electroplated under the same conditions for the XRD analysis. The filament voltage and current were set to 30kV and 100mA, respectively. The sample was scanned between 10˚ and 90˚ at a rate of 5˚/min. Then, the electroplated Au-Sn samples were reflowed in a reflow machine (RF-430-N2, Japan Pulse Laboratory Co. Ltd., Japan) with a maximum temperature of 300℃ for 60 sec. Reflows were conducted consecutively for between one and five times. In addition, we performed reflowing to isothermally observe the morphological changes in the joint interface at 400℃ for periods ranging from 2min to 20min. Subsequently, the samples were mounted in epoxy and metallographically polished for microstructural characterization. Their microstructures and chemical compositions were observed with a SEM equipped with an EDX system. Also, the compositions of the phases formed at the interface were determined using a JEOL JXA-8900R (Tokyo, Japan) electron probe micro analyzer (EPMA) equipped with a wavelength-dispersive X-ray (WDX) analyzer. For each compositional analysis, at least five measurements were performed and the average value was reported. The total area of the interfacial IMC layer was measured using image analysis software. The layer areas were divided by the interface length shown in the cross-section to yield the average layer thickness.

RESULTS AND DISCUSSION

Au-Sn alloy electroplating was directly performed on the Ni UBM of the Si wafer. Figure 2 shows the SEM plan view image of the electroplated Au-Sn alloy and the corresponding EDX analysis result. The surface of the plating layer had a gray and rough grain structure.

According to the EDX analysis result, the composition of the electroplated Au-Sn alloy was approximately 70at.% Au and 30at.% Sn. Unlike in conventional alternate electroplating, the method of co-electroplating of Au-Sn solder used in this study allowed for the direct deposition of the desired Au-Sn alloy.

Figure 3 shows the diffraction pattern obtained from the Au-Sn alloy electroplated on the Cu coupon. As expected, the alloy is a mixture of the two phases, AuSn and Au 5 Sn. The Au-Sn eutectic alloy consists of the Au 5 Sn (ζ-phase) and AuSn (δ-phase) phases, as shown in Fig. 4. Using a co-electroplating process, it was possible to plate the Au-Sn solder alloy directly onto the wafer at or near the eutectic composition from a single solution. From Figs. 2 and3, it was verified that the appropriate combination of these two phases (Au 5 Sn and AuSn) resulted in the eutectic composition of the Au-Sn alloy. According to the Au-Sn phase diagram (Fig. 4), the eutectic temperature of the composition is 280℃. In

Authors

Title. Cut at approx. 70 characters if it is too long.... practice, however, heating above 300℃ is necessary for the complete melting of the alloy, because the gradient of the solid-liquid line at the eutectic composition of Au-20Sn (in wt.%) is very steep. The Au-Sn flip-chip joints are formed at 300℃ without using any flux. Figure 5 shows the SEM images of the Au-Sn solder bumps reflowed at 300℃ for 60sec. After reflowing, the average diameter of the solder bump was approximately 150 ㎛. The resulting Au-Sn solder bumps were smooth and exhibited a metallic silver color. Figure 7 shows the cross-sectional SEM images of the interfaces between the Au-Sn solder and Ni UBM after multiple reflows. The high magnification BSE (Back Scattered Electron) image mode of SEM was used to clarify the morphology of the phase formed at the interface. Only one irregular-shaped reaction product was found between the Au-Sn solder and the Ni UBM after one reflow (Fig. 7(a)). The composition of the reaction product was 25.7at.%Au-33.5at.%Ni-40.8at.%Sn. The ratio of the atomic percentage of (Au + Ni) to that of Sn was (25.7 + 33.5):(40.8), which is close to 3:2. Therefore, it is suggested that the reaction product was the (Au,Ni) 3 Sn 2 IMC. It is known that some binary phases such as AuSn, Ni 3 Sn 4 and Ni 3 Sn 2 in the Au-Ni-Sn system have a very high solubility of the third element, due to the similarity in the chemical and physical properties of Au and Ni [START_REF] Tsai | Controlling the microstructures from the gold-tin reaction[END_REF]. It seems that Au enters into the Ni 3 Sn 2 lattice and substitutes for the Ni atoms. A ternary IMC often has a lower Gibbs free energy than a binary compound of the same structure from the entropy argument [START_REF] Tsai | Controlling the microstructures from the gold-tin reaction[END_REF]. Therefore, Ni 3 Sn 2 has a natural tendency to absorb Au to reach its saturated composition. A similar Ni-seeking mechanism has been proposed and widely accepted in the literature for the formation of the (Au,Ni)Sn phase and/or the resettlement of the (Au,Ni)Sn 4 phase [START_REF] Tsai | Controlling the microstructures from the gold-tin reaction[END_REF][START_REF] Lee | Growth and ripening of (Au,Ni)Sn 4 phase in Pb-free and Pb-containing solders on Ni/Au metallization[END_REF]. The average thickness of the (Au,Ni) 3 Sn 2 IMC layer formed at the interface was approximately 0.6 ㎛. Besides the thin (Au,Ni) 3 Sn 2 layer, rod-shape (Au,Ni) 3 Sn 2 phases were found as well. The rod-shape (Au,Ni) 3 Sn 2 phase had a higher Au content, as compared to the interfacial thin (Au,Ni) 3 Sn 2 layer (see Fig. 7(a)). This phase was analyzed and found to be composed of 32.4at.% Au, 26.8at.% Ni and 40.8at.% Sn. Since Ni originates from the UBM (or substrate) and Au from the solder, it is reasonable for the Ni content of the interfacial (Au,Ni) 3 Sn 2 layer to be higher, since it is closer to the Ni UBM. A peculiar phenomenon was observed in the reflowed solder matrix. Figure 9 shows the cross-sectional SEM images of the solder matrix in the Au-Sn solder/Ni UBM joint reflowed at 400 for 2 min ℃ . As shown in Fig. 9, the spalled (Au,Ni) 3 Sn 2 phase was embedded in the AuSn phase. The dark core region is (Au,Ni) 3 Sn 2 phase, and the light exterior layer surrounding the (Au,Ni) 3 Sn 2 phase is AuSn (δ-phase). The measured average compositions of the (Au,Ni) 3 Sn 2 and AuSn phases are shown in Table 2. Similar phenomena are also observed in the interfacial SEM images of the samples reflowed at 300 and 400℃ (see Figs.

  Figure 1 shows the resulting Ni UBM (Under Bump Metallization) arrays employed in this study. The Ni UBM serves as both an adhesion layer and a diffusion barrier layer between the Cu and solder. The lateral overlap of the Ni UBM on the chip passivation layer was approximately 10 ㎛ . Coelectroplating of Au-Sn alloy was performed on the Ni UBM. A commercially available single plating solution was used for the deposition of the Au-Sn alloy. The cathode was an Si wafer electroplated with Ni, and Pt was used as the anode. The electroplating cell was set up with the cathode facing the anode and the two spaced 90 mm apart. A mechanical stirrer with a controllable speed was used to supply the agitation in the electroplating solution.

Fig. 1

 1 Fig.1 SEM images of Ni UBMs on a wafer.

Fig. 2 (

 2 Fig.2 (a) SEM plan view image of electroplated Au-Sn alloy and (b) corresponding EDX analysis result.

Fig. 3

 3 Fig.3 XRD spectra from electroplated Au-Sn sample.

Fig. 4

 4 Fig.4 Au-rich portion of the Au-Sn phase diagram.

Fig. 5

 5 Fig.5 SEM images of the Au-Sn solder bumps reflowed on a wafer. The typical cross-sectional microstructure of the Au-Sn solder is shown in Fig. 6. The microstructure of the solder matrix was composed of AuSn (δ-phase) and Au 5 Sn (ζphase). The bright constituent in the eutectic microstructure is the ζ-phase (nominally Au 5 Sn) while the darker constituent is the δ-phase (AuSn).

Fig. 6

 6 Fig.6 Cross-sectional SEM image of the Au-Sn solder reflowed at 300 for 60sec. ℃ The lighter phase is Au 5 Sn and the darker phase is AuSn.

Fig. 7

 7 Fig.7 Cross-sectional SEM images of the Au-Sn solder/Ni UBM interfaces reflowed at 300 for 60sec ℃ ; (a) 1 reflow and (b) 5 reflows.

Figure 7

 7 Figure 7(b) shows the cross-sectional SEM image of the interface between the Au-Sn solder and Ni UBM after 5 reflows. Although the thickness of the interfacial (Au,Ni) 3 Sn 2 IMC layer increased with increasing number of reflows, the interfacial structure was very similar to that of the single-reflow sample. In addition, many IMC particles were found at the interface. However, these IMC particles were actually cross-sections of the rod-shape IMCs grown randomly from different locations on the interface. Figure 8 shows the cross-sectional SEM images of the Au-Sn solder/Ni UBM interfaces reflowed at 400 for ℃ different reaction times. After reflowing at 400 for ℃ 2 min, two IMC layers, (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn, were formed at the interface, as shown in Fig. 8(b). The (Au,Ni) 3 Sn 2 phase is initially formed when the liquid Au-Sn solder reacts with the Ni UBM, and then the (Au,Ni) 3 Sn phase is formed when the (Au,Ni) 3 Sn 2 phase reacts with the Ni UBM. The thickness of the (Au,Ni) 3 Sn layer formed on the Ni UBM was very thin. In addition, (Au,Ni) 3 Sn 2 IMC particles were widely dispersed in the matrix of the solder alloy (see Fig. 8(a)). As a whole, the thickness of the (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn IMC layers increased with increasing reaction time, as shown in Fig. 8. After reflowing at 400 for ℃ 20 min, the thickness of the (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn IMC layers were about 2.1 and 0.4 ㎛, respectively. The measured average compositions of the (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn layers in Fig. 8(f) are shown in Table 1. The composition varied

  7 and 8). As the temperature and time of the reflow increased, the δ(AuSn)-phase adjacent to the interfacial layer was gradually replaced by the ζ(Au 5 Sn)phase, as shown in Figs. 7(a), 7(b) and 8(e). Eventually, the ζ-phase covered mainly the interfacial layer, as shown in Fig. 8(e). In other words, the phase distribution at the interface changed by the interfacial reaction proceeded, due to reflowing.

Fig. 9

 9 Fig.9 SEM images of the solder matrix in the Au-Sn solder/Ni UBM joint reflowed at 400 for 2 min. ℃

Table 1

 1 Chemical compositions (in at.%) of the IMC phases formed at the interface of Fig. 8.(f)

	Phases	Au	Ni	Sn
	(Au,Ni) 3 Sn 2	15.9	43.8	40.3
	(Au,Ni) 3 Sn	4.9	72.5	22.6

ACKNOWLEDGEMENT

This work was supported by grant No. RTI04-03-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy(MOCIE).

Authors

Title. Cut at approx. 70 characters if it is too long....

Authors

Title. Cut at approx. 70 characters if it is too long....

CONCLUSION

In this study, we fabricated eutectic or near eutectic Au-Sn flip-chip bumps from a single plating bath using the principle of alloy co-electroplating. This process is well suited to the deposition of Au-Sn alloy with a composition of Au-30at.%Sn. Fluxless soldering can be performed with the plated Au-Sn solder bump. After reflowing, the average diameter of the solder bump was approximately 150 ㎛. The microstructure of the solder matrix was composed of the eutectic phases of the Au-Sn alloy, AuSn (δ-phase) and Au 5 Sn (ζ-phase). We also studied the initial microstructure and microstructural evolution of the eutectic Au-Sn solder bumps on the Ni UBM during the reflow reaction. In the case of the samples reflowed at 300 , only ℃ an (Au,Ni) 3 Sn 2 IMC layer formed at the interface between the Au-Sn solder and Ni UBM. On the other hand, two IMC layers, (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn, were found at the interfaces of the samples reflowed at 400 .

℃ The (Au,Ni) 3 Sn 2 phase is initially formed when the liquid Au-Sn solder reacts with the Ni UBM, and then the (Au,Ni) 3 Sn phase is formed when the (Au,Ni) 3 Sn 2 phase reacts with the Ni substrate. As the reflow time increased, the thickness of the interfacial (Au,Ni) 3 Sn 2 and (Au,Ni) 3 Sn IMC layers increased.


S W Youn

M Takahashi

H Goto

T Kobayashi

R Maeda

AFM, SEM AND NANO/MICRO-INDENTATION STUDIES OF THE FIB-MILLED GLASSY CARBON SURFACE HEAT-TREATED AT DIFFERENT CONDITIONS

This study investigates the changes in the mechanical and morphological properties (hardness, elastic modulus, surface roughness and dimensions) of the FIB-milled GC sample heat-treated at 1400 o C for different times (0, 10, 20 and 30 min) using nano/micro-indentation, optical microscope, optical interferometer, scanning electron microscope and atomic force microscope. The FIB milled structures had white surfaces, and they were surrounded by a faint circular area. As the temperature hold-time increased, their surfaces became dim as a result of both the diffusion of gallium into the surface and the evaporation of exhaustible gallium source. Additionally, FIB-milled depth was 25-30 nm increased after heattreatment regardless of increasing the temperature hold time. The surface roughness for both milled and unmilled surfaces increased to less than R a = 5 nm with the increase of the temperature hold time. Both the uniform distributions and the highest values of the hardness and elastic modulus for the GC mold were obtained after heattreatment at 1400 o C for 10-20 min.

INTRODUCTION

The needs for high-throughout, high-resolution and costeffective forming techniques of glasses have increased in the application fields of micro/nano devices such as optical-lenses, filters, bio-chips, and others [START_REF] Hirai | Fine pattern fabrication on glass surface by imprint lithography[END_REF][START_REF] Takahashi | Nanoimprint of glass materials with glassy carbon molds fabricated by focused-ion-beam etching[END_REF][START_REF] Iliescu | Characterization of masking layers for deep wet etching of glass in an improved HF/HCl solution[END_REF]. Recently, glass imprint processes, where the nano/micro patterns on the mold are physically imprinted into glass material at elevated temperature and pressure, have been actively studied for this purpose [START_REF] Hirai | Fine pattern fabrication on glass surface by imprint lithography[END_REF][START_REF] Takahashi | Nanoimprint of glass materials with glassy carbon molds fabricated by focused-ion-beam etching[END_REF].

Because conventional mold materials such as silicon (or silicon oxide) and nickel are not feasible for glass forming (due to their poor heat resistance and difficulty of release after imprinting), there is also a great demand for a new mold material with properties (such as hardness and thermal expansion coefficient) more suitable for the glass imprint application. A glassy carbon (GC) with a wide range of applications, such as electrodes, sliding parts and biomaterials [START_REF] Kuhnke | MicroFabrication of flow field channels in glassy carbon by a combined laser and reactive ion etching process[END_REF][START_REF] Nakao | Microindentation measurements of glassy carbon implanted with high-energy titanium ions[END_REF][START_REF] Iwashita | Elasto-plastic deformation of glass-like carbons heat-treated at different temperatures[END_REF], is one of promising candidates because it has high operating temperature (2000 o C), chemical stability, high hardness, wear resistance and gas impermeability. Moreover, replicated glass structure can be easily removed from GC mold after imprinting since the cohesion between GC and glasses is generally poor. The thermal expansion coefficient is especially important in glass imprinting, where a temperature over 1300°C is typically required in the imprinting step for quartz. A thermal mismatch between the mold and the glass substrate could result in pattern distortions or a stress build-up during the cooling cycle, which would affect the pattern fidelity. In this regard, GC mold and glass substrates make a very good pair for the imprint process.

The issue of creating low-cost nano/micro mold has been greatly alleviated due to the advancement of mechanical molding process, such as nanoimprint [START_REF] Guo | Recent progress in nanoimprint technology and its applications[END_REF]. FIB milling technique is one of the most promising techniques for this purpose because it offers various advantages such as the high flexibility in the working shapes, the dimensions, and the material selectivity [START_REF] Reyntjens | A review of focused ion beam applications in microsystem technology[END_REF]. Even though FIB milling has been considered too slow for mass production, this technique has some distinct advantages when used in combination with pattern transfer techniques, such as imprint. Recently, promising works concerning focused ion beam (FIB) milling for the fabrication of nano/micro mold have been published [START_REF] Takahashi | Nanoimprint of glass materials with glassy carbon molds fabricated by focused-ion-beam etching[END_REF][START_REF] Sun | Optimization and experimentation of nanoimprint lithography based on FIB fabricated stamp[END_REF].

However, it should be considered that the beam of Ga+ ion with an accelerated voltage of 30-50 keV can induce the damage of sample surface, and then produce artifacts including implantation, amorphization, and mixing [START_REF] Reyntjens | A review of focused ion beam applications in microsystem technology[END_REF]. Such phenomena result in the changes in both mechanical properties and dimensions of the FIB milled mold. Furthermore, the implanted gallium in the FIB milled material can desorb during heating cycles and redeposit on and diffuse into the material surface [START_REF] Weiland | Inline failure analysis on productive wafers with dual beam SEM/FIB systems[END_REF]. For example, a GC mold surface can be contaminated by the precipitation of gallium ions during heating in the imprint process because the implanted gallium ions precipitate during annealing above 250 o C [START_REF] Takahashi | Nanoimprint of glass materials with glassy carbon molds fabricated by focused-ion-beam etching[END_REF].

It was also reported that this kind of defect can be improved by pre-heating in vacuum at a higher temperature than imprint temperature [START_REF] Takahashi | Nanoimprint of glass materials with glassy carbon molds fabricated by focused-ion-beam etching[END_REF]. However, further works are still needed because the reliability and lifetime of mold is one of the most important issues for the acceptance of imprint-based technology in industrial applications.

In this study, the effects of heat-treatment conditions (1400 o C -0, 10, 20 and 30 min) on the mechanical and morphological properties (hardness, elastic modulus, surface roughness and dimensions) of a FIB milled GC sample were investigated. Changes in morphological properties such as dimensions, surface appearance and roughness, were investigated using optical microscope (OM), scanning electron microscope (SEM), atomic force microscope (AFM) and optical interferometer. Furthermore, indentation tests were performed to evaluate the elastic-plastic properties (hardness and elastic modulus) of FIB-milled GC at the submicro-scale. Understanding of the deformation in FIB-milled GC will aid in improving the reliability and lifetime of GC mold.

EXPERIMENTAL PROCEDURE

FIB milling, heat-treatment and glass

Commercially available glassy carbon (GS-20, Tokai Carbon, Japan) with porosities of 2-3 vol.% was used as a mold material. A computer-controlled FIB system (model EIP-5400, Elionix, Japan) was used for the FIB milling of GC. The FIB instrument is equipped with a Ga liquidmetal ion source and is operated at the accelerating voltage of 40 kV. The beam currents of 2 pA and 100 nA correspond to the spot sizes of 30 nm and 8 ㎛ , respectively. The desired structure was first defined in the WEACAD software before FIB milling. In order to keep FIB milled surfaces flat and clean, a multi-step milling method was used. In this method, FIB milling with a constant dwell-time (t d1 ) was repeated to control milling depth. Twenty five squares (size of each square: 10×10 µm2) were milled on the GC surface with the milling number of 10 times (t d1 =10 µs) under the same FIB milling condition (emission voltage: 6.1 kV, emission current: 2.66 µA, and probe current: 1 nA).

Both heat-treatment and glass imprint tests were performed in vacuum using a hot-embossing equipment (max. load: 10 kN, max. heating temp.: 1400 o C and vacuum pressure: 0.07 Pa), with an imprint velocity at 0.3 mm/min. The heat-treatment procedure was as follows. A FIB milled GC sample was heated to 1400 o C and the temperature was maintained for different times (0, 10, 20 and 30 min). Subsequently, the GC was cooled to below 200 o C without any coolant. The imprint procedure for quartz was as follows. A quartz sample was heated to an elevated temperature (1300 o C) above the glass transition temperature, and the pressure and the temperature were maintained before completion of the imprinting and cooling. After cooling to a temperature (about 200 o C) below the T g of the glasses, the GC mold was removed from the glass substrate. Prior to imprint process, a FIB milled GC mold was heat-treated in vacuum at 1400 o C for 10 min.

Mechanical and morphological characterizations

Indentation tests were performed using a commercial nanoindentation system (ENT-2100, Elionix, Japan) with a Berkovich diamond tip. The detailed description of the theory behind the nanoindentation test has been provided in numerous references [START_REF] Oliver | An improved technique for determine hardness and elastic modulus using load and displacement sensing indentation experiments[END_REF][START_REF] Li | A review of nanoindentation continuous stiffness measurement technique and its applications[END_REF]. Nanoindentation hardness is defined as the indentation load divided by the projected contact area of the indentation. It is the mean pressure that a material will support under load. From the load-displacement curve, hardness can be obtained at the peak load as

A P E max = ( 1
)
where A is the projected contact area. For an indenter with a known geometry such as the Berkovich tip, the projected contact area is the function of contact depth, which is measured by the nanoindenter system in situ during indentation. Therefore, the projected area A can be measured and calculated directly from the indentation displacement.

The elastic modulus was calculated using the Oliver-Pharr data analysis procedure [START_REF] Oliver | An improved technique for determine hardness and elastic modulus using load and displacement sensing indentation experiments[END_REF] beginning by fitting the unloading curve to a power-law relation. The unloading stiffness can be obtained from the slope of the initial portion of the unloading curve, S = dP/dH. Based on relationships developed by Sneddon for the indentation of an elastic half space by any punch that can be described as a solid of revolution of a smooth function, a geometry independent relation involving contact stiffness, contact area, and elastic modulus can be derived as follows [START_REF] Oliver | An improved technique for determine hardness and elastic modulus using load and displacement sensing indentation experiments[END_REF]:

r E A S π β 2 = ( 2
)
where β is a constant which depends on the geometry of the indenter (β = 1.034 for a Berkovich indenter), and E r is the reduced elastic modulus which accounts for the fact that elastic deformation occurs in both the sample and the indenter. E r is given by

i i r E E E 2 2 1 1 1 ν ν - + - = (3)
where E and ν i are the elastic modulus and Poisson's ratio for the sample, and E i and ν i are the same quantities for the indenter. For diamond, E i = 1141 GPa and ν i = 0.07.

Especially, at low indentation depths, the tip radius and the surface roughness on the GC surface do not allow a correct interpretation of the data due to the change of the contact area in a variety of ways [START_REF] Oliver | An improved technique for determine hardness and elastic modulus using load and displacement sensing indentation experiments[END_REF][START_REF] Li | A review of nanoindentation continuous stiffness measurement technique and its applications[END_REF]. According to the pre-test results, indentation tests were carried out under the maximum load of 100 mN (which corresponded to the depth of about 1 µm).

Optical microscope (OM), atomic force microscope (AFM), scanning electron microscope (SEM) and optical interferometer were used to investigate the morphologies of the processed samples. Although FIB image could be used to measure the dimension in situ, this imaging mode damages the finished pattern. Additionally, a small amount of Ga+ ions is implanted in the sample during imaging.

RESULTS AND DISCUSSION

Fig. 1 shows the optical images of the FIB-milled glassy carbon surface heat-treated at 1400 o C for different times (t H = 0, 10, 20 and 30 min). The heating temperature was set at 100 o C above forming temperature for quartz (about 1300 o C). As shown in Fig. 1 (a), the FIB milled structures had white surfaces, and they were surrounded by a faint circular area. As the t H increased, their surfaces became dim as a result of both the diffusion of gallium into the surface and the evaporation of exhaustible gallium source. After heat-treatment at 1400 o C for 20 min, there was no difference in color between the FIB milled and un-milled surfaces, as shown in Fig. 1 (c).

Surface roughness of FIB-milled and un-milled glassy carbon surfaces heat-treated at 1400 o C for different holdtime (0, 10, 20 and 30 min) was measured using the contact-mode AFM (C-AFM). In Fig. 2, the C-AFM images of FIB-milled surfaces are shown as examples. Variations of surface roughness for both regions are shown in Fig. 3. As a whole, surface roughness increased to less than R a = 5 nm with the hold-time at the maximum temperature (1400 o C) mainly resulting from the increase of the micro-pores volume.

One problem of AFM measurement system is that the AFM tip cannot be perpendicular to the sample surface. This limit leads to the measured profile of the pattern cross section being asymmetric. Therefore, the morphological change of the milled surfaces was measured using an optical interferometer, as shown in Fig. 4. The variation of the FIB-milled depth is shown in Fig. 5. One distinctive aspect is that the milled depth was 25-30 nm increased after heat-treatment at 1400 o C. However, the effect of the temperature hold-time on the milled depth was negligible. Indentation tests were carried out at each region A, B, C of GC sample (as marked in Fig. 6) heat-treated at 1400 o C for different times. In each region, indentation S. W. Youn, M. Takahashi, H. Goto, T. Kobayashi and R. Maeda AFM, SEM and Nano/micro-indentation Studies of the FIB-milled Glassy Carbon….... test was repeated from 5 to 10 times at different positions (which are randomly chosen) on the surface of the sample before and after heat-treatment. As an example, the optical image of indents made on FIB-milled surfaces (Region A) heat-treated at 1400 o C for 30 min are shown in Fig. 7. Fig. 8 shows the variation of load-displacement curves for each region A, B, C of the sample surface with different heat-treatment conditions. Before heat-treatment, the penetration of the indenter for the FIB-milled surface (region A) was shallower than that for the un-milled surfaces (region B and C). With increasing the temperature hold time (t H ), the difference in the shape of load-displacement curve among the regions A, B, C decreased due to the diffusion of Ga, and that was minimized at t H = 20 min. In addition, the increase in surface roughness by heat-treatment (previously mentioned in Fig. 3) had almost no influence at an initial stage of the loading curve, as shown in Fig. 8. Fig. 9 shows the relationship between heat-treatment condition and mechanical properties including indentation hardness and elastic modulus. Both hardness and elastic modulus of FIB-milled surfaces were improved more than those of un-milled surfaces. Higher hardness and elastic modulus of FIB-milled surface than un-milled surface may be explained by the formation of a dense amorphous surface layer. It is well established that ion irradiation of glassy carbon with energetic ions leads to the formation of a dense amorphous surface layer [START_REF] Mcculloch | Molecular dynamics and experimental studies of preferred orientation induced by compressive stress[END_REF]. Such amorphization causes the wear resistance to improve. It should be noted that the actual hardness and modulus of the FIB-milled buried layer may be higher than the measured apparent values, because the amorphous layer is so thin that the indentation load could be influenced from the soft substrate. In all regions A, B and C, the highest values of hardness and elastic modulus were obtained by the 10 min-1400 o C heat-treatment. This may be explained by the Ga+ concentration in region C dropped due to both further diffusion of Ga+ ion. A relaxation of the residual stress by the heat-treatment was considered as another possible reason. Fig. 10 shows an array of sample subwavelength splitring resonators (SRRs) structures milled by FIB before and after heat-treatment at 1400 o C for 10 min. The width of the smallest line structure was about 1 µm. As expected, the difference in color between the FIB-milled surface and un-milled surface decreased after heat-treatment. In Fig. 11 and Fig. 12, a GC mold for lens pattern were prepared by multi-step FIB milling (emission voltage: 5.89 kV, emission current: 3.19 µA, and probe current: 1 nA) and subsequently heat-treated at 1400 o C for 10 min. The maximum depth of master structures before and after heat-treatment measured using optical interferometer was 158 nm and 181 nm, respectively. Fig. 13 shows the lens pattern replicated on quartz surface. Lens pattern was fabricated with good fluidity Fig. 13 3D images of lens pattern replicated on quartz surface measured using optical interferometer

CONCLUSIONS

In order to obtain the proper heat-treatment condition for GC mold fabrication, FIB-milled GC sample was maintained at 1400 o C for different times (t H = 0, 10, 20 and 30 min) and its mechanical properties, surface roughness and dimensional changes were investigated. Obtained results can be summarized as follows (1) SEM and OM observation result showed that the FIB-milled structures had white surfaces, and they were surrounded by a faint circular area. As the t H increased, their surfaces became dim as a result of both the diffusion of gallium into the surface and the evaporation of exhaustible gallium source. After heat-treatment at 1400 o C for 20 min, there was no difference in color between the FIB-milled and un-milled surfaces.

(2) Morphological data measured using an optical interferometer showed that the FIB-milled depth was 25-30 nm increased after heat-treatment regardless of increasing the t H . Such dimensional change should be considered when a mold is designed.

(3) C-AFM analysis results showed that the surface roughness for both milled and un-milled surfaces increased to less than R a = 5 nm with increasing the t H .

(4) In both hardness and elastic modulus, the FIBmilled surface indicated larger values than the un-milled surfaces due to the formation of a dense amorphous surface. Both the uniform distributions and the highest values of mechanical properties for the GC mold were obtained after heat-treatment at 1400 o C for 10-20 min.

Fig. 1

 1 Fig. 1 Optical images of FIB-milled glassy carbon surfaces heat-treated at 1400 o C for (a) 0 (raw), (b) 10, (c) 20 and (d) 30 min

Fig. 3

 3 Fig. 3 Surface roughness of FIB-milled and un-milled GC surfaces heat-treated at 1400 o C for different times.

Fig. 4

 4 Fig. 4 Morphological change of FIB-milled glassy carbon surfaces heat-treated at 1400 o C for (a) 0 (raw), (b) 10, (c) 20 and (d) 30 min measured using optical interferometer.

Fig. 5

 5 Fig. 5 Variation of FIB-milled depths at different heattreatment conditions.

Fig. 11 (

 11 Fig. 11 (a) Optical and (b) SEM images of FIB-milled GC mold for optical lens.

Fig. 8

 8 Fig. 8 Load-displacement curves for the indentation on glassy carbons heat-treated at 1400 o C for (a) 0 (raw), (b) 10, (c) 20 and (d) 30 min (the maximum indentation load: 100 mN).

Fig. 6 Fig. 9 (Fig. 10

 6910 Fig. 6 Optical image of FIB-milled GC surface, where A, B, C is region for indentation tests

©TIMA Editions/DTIP 2006ISBN: 2-916187-03-0


X C Shan

Q D Zhang

Y F Sun

R Maeda

A MICRO TURBINE DEVICE WITH ENHANCED MICRO AIR BEARINGS

As part of progress in developing a micro gas turbine engine, this paper presents the design, fabrication and testing of a silicon-based micro turbine device, which is driven by compressed air. To improve its rotational speed and stability, the turbine device has enhanced journal air bearing and thrust air bearings. The thrust air bearings are utilized for supporting the rotor from both its top-and bottom-sides. The top thrust air bearing employs pump-in type spiral grooves, and the bottom bearing uses pump-out type grooves. The journal air bearing is a plain journal with a critical short journal length (L) and a small radial clearance (C). Aspect ratio (L:C) above 20:1 is realized using optimized DRIE process. The micro turbine device has been fabricated and tested. The turbine device can sustain stable operations, and a rotation speed of 15,000 rpm has been reported during test.

INTRODUCTION

Micro gas turbine engine [START_REF] Epstein | Millimeter-scale MEMS gas turbine engines[END_REF][START_REF] Isomura | Development of micromachined gas turbine engine for portable power generation[END_REF] is one of the promising solutions to provide high-density power source for microsystems. We are developing a silicon-based micro gas turbine engine [START_REF] Shan | Studies on a micro combustor for gas turbine engines[END_REF], which consists of a combustor, a radial inflow turbine and a centrifugal compressor. A micro combustor composed of seven layers of silicon structures has been developed, which can sustain a stable combustion with an exit temperature as high as 1600 K [START_REF] Shan | Studies on a micro combustor for gas turbine engines[END_REF]. The micro turbine is another critical component in micro gas turbine engine, which will generate power output and drive the compressor.

The critical concerns for a turbine device are friction and wear during operation. Micro air bearing is preferred in microsystems due to its miniaturized dimension and capability to achieve high speed and low wear operations. Some researchers have employed micro air bearings into rotational microsystems [START_REF] Tanaka | Turbo test rig with hydro-inertia air bearings for a palmtop gas turbine[END_REF][START_REF] Hara | Infrared polarization modulator using a MEMS-based air turbine[END_REF][START_REF] Frechette | Demonstration of a microfabricated high-speed turbine supported on gas bearings[END_REF].

A conventional rotor has a dimension of a few millimeters along its axial direction. However, the rotor in a silicon-based micro turbine has an axial dimension (L) of less than one millimeter and a diameter (D) of several millimeters. The commonly accepted assumptions and design rules for conventional air bearings need to be revised due to the consequences of scaling down as well as the current limitations in micro fabrication techniques. Piekos [START_REF] Piekos | Numerical simulation of gas-lubricated journal bearings for micro-fabricated machines[END_REF] has investigated the performance of a short journal bearing in micro turbine system. A categorization of non-dimensional variables was suggested including loading and fluid.

This paper reports on design, fabrication and testing of a silicon-based micro turbine device. The turbine device consists of dynamic journal bearing and thrust air bearings, which are designed based on numerical simulation and fabricated through DRIE process. The turbine is driven by compressed air instead of exhausted gas from a micro combustor during test. Experimental investigations demonstrate that stable operations can be sustained, and a rotation speed of 15,000 rpm has been achieved.

DESIGN OF MICRO TURBINE STRUCTURE

The turbine device to be developed consists of three layers of silicon dies and two acrylic plates for tubing and clamping. Fig. 1 shows the schematic design of the turbine device and Fig. 2 illustrates the exploded view with the acrylic plates omitted. There are three flow paths, which are for rotor driving, static journal air bearing and thrust air bearing, respectively. The 3 rd wafer provides a platen for bottom-side dynamic thrust air bearing and the flow paths for driving air as well as for static air bearings. The blades profile for both the rotor and stator are fabricated from the 2 nd wafer. The 1 st wafer provides the air outlet and a platen for topside dynamic thrust air bearing.

Silicon wafers of 0.4 mm thick are used for 1 st and 3 rd layers of wafers. A wafer of 0.8 mm thick is used for the 2 nd wafer, from which both the rotor and stator are

DESIGN OF DYNAMIC AIR BEARINGS

Governing equations

The investigations on micro air bearings are performed based on numerical simulation. The governing equation for dynamic air bearings is Reynolds equation [START_REF] Hamrock | Fundamentals of fluid lubrication[END_REF].

t ph x ph R z p ph z x p ph x ∂ ∂ + ∂ ∂ = ∂ ∂ ∂ ∂ + ∂ ∂ ∂ ∂ ) ( 12 ) ( 6 ) ( ) ( 3 3 µ µω (1)
where h is fluid film height; µ is the viscosity of fluid; and p is the pressure. where p is pressure; h, fluid film height; R, radius; µ, viscosity; ω, angular speed; C, bearing clearance; ε, eccentricity ratio; h g , groove depth, p a , ambient pressure, l, journal bearing length; r i , r e , inner and outer radius of thrust bearing, respectively.

With the above boundary conditions, Eq. ( 1) can be solved to obtain the pressure distribution and other dynamic parameters. A commercial software "ARMD 5.4" plus some codes developed on Matlab were used to execute the simulation.

Design of micro journal bearing

Due to the limitation of manufacturing process, the length of journal bearing (L) is set to one half of wafer thickness, i.e. 0.4 mm. Considering the effective diameter of rotor (D) is 8.28 mm, the aspect ratio of L/D (L/D =0.0483) is extremely low. The load capacity of a plain journal bearing is investigated by employing the above-expressed solutions [START_REF] Zhang | Performance Analysis of Air Bearing in a Micro System[END_REF]. Fig. 3 Where p a = 1.0135x10 5 Pa, ε in Fig. 3(a) stands for the ratio of bearing eccentricity e versus bearing clearance C (ε=e/C). The load capacity of the journal bearing will increase with the increase of radius of the journal bearing. At the same rotor radius, higher load capacity can also be obtained by reducing the radial clearance C of the journal bearing, as shown in Fig. 3(b). The radial clearance of a journal bearing is defined by the width of a trench between the rotor and stator. A trench of 20 µm wide and 400 µm deep with an aspect ratio of 20 was set in our design.

Design of micro thrust bearings

The dynamic characteristics of self-acting thrust air bearings in double-side configuration are investigated. Based on the actual turbine structure, a pump-in configuration with spiral grooves is designed for top thrust air bearing, and a pump-out configuration is for bottom bearing, as shown in Figs. 4 (a)-(c). Due to the miniaturized dimension and small weight of thrust plate, the axial load is mainly caused by the pressure difference between the top and bottom thrust air bearings.

X. C. Shan, Q. D. Zhang, Y. F. Sun and R. Maeda A Micro Turbine Device with Enhanced Micro Air-Bearings

Some parameters, such as outer radius R o , inner radius R i and groove radius R m , of a spiral grooves are determined by the turbine dimension and structure. Other parameters that affect the load capacity are investigated. These parameters include groove numbers N g , groove angle α, axial clearance A c , groove depth g d , and groove width ratio a g (a g = groove width/(groove pitch)). The axial clearance A c is the film height on the ridge region of thrust bearing, as shown in Fig. 4(c), and groove depth ratio G d (G d =g d /A c ) is adopted in the investigation.

The load carrying capacity and stiffness are investigated based on an assumed rotating speed of 100,000 rpm. If rotation speed increases, the load capacity and stiffness will also increase almost linearly with the increase of the speed.

Fig. 5 shows that load capacity versus groove numbers (N g ) of thrust bearing. The non-dimensional load capacity W is defined as Where ω stands for rotation speed, and µ for dynamic viscosity of air with µ = 18.1x10 -6 Pa s at 20 °C, A c for axial clearance with A c = 10 µm in the simulation. It can be seen from Fig. 5 that the load capacity increases with the increase of groove numbers. The effect of groove angle on the load capacity is shown in Fig. 6. It is observed that with the increase of the groove numbers, the peak values of the load capacity gradually move to higher values of groove angles. The investigation on the effect of groove width ratio illustrates that the maximum load capacity can be obtained when the groove width ratio (a g ) ranges from 0.5 to 0.7, provided that the groove numbers are between 6 and 50.

Table 1 illustrates the parameters of dynamic thrust bearings fabricated for the turbine device.

FABRICATION

The DRIE (Deep Reactive Ion Etch) process based on inductively coupled plasma is used for fabricating the turbine device. Key process parameters including plasma source power, bias power and process pressure have been optimized to realize high aspect ratio etching with straight sidewalls.

In the 1 st wafer, the top thrust bearing using pump-in spiral grooves is first fabricated from the bottom-side surface, followed by through-etching from topside. Since the rotor will be supported by top-and bottom-thrust air bearings, the total axial clearance A c will be shared by clearance A ct (clearance between rotor and top thrust bearing) and A cb (clearance between rotor and bottom bearing). The design value of total clearance A c is 15 µm with A ct and A cb being 5 µm and 10 µm, respectively. Hence, the etching depths of grooves for the top-and bottom-bearings are 15 µm and 36 µm, respectively.

In the 2 nd wafer, DRIE process is executed from the top to generate the blade profiles of turbine, followed by DRIE etching from bottom to define the high aspect ratio trench between the rotor and stator. The trench functions as the clearance of journal bearing. Fig. 7 shows the topand bottom-views of the 2 nd wafer. A clearance of 20 µm with an aspect ratio of 20 is achieved. The surface roughness of the journal bearing is evaluated by measuring the inner sidewall of stator and outer sidewall of rotor using a Zygo® interferometer. The results show that surface roughness is less than 0.2 µm.

In the 3 rd wafer, the bottom thrust bearing using pump-out spiral grooves is fabricated from the top of wafer, followed by DRIE through-etching from the other side. Figs. 8 and9 show the fabricated thrust bearings. The rotor of the turbine rotates in clockwise direction relative to top bearing while in anti-clockwise direction relative to the bottom bearing.

INTEGRATION AND TESTING

The performance of micro air bearings has been evaluated by measuring the rotation speed of the fabricated micro turbine device. The micro turbine is mounted in an acrylic rig, and then connected to an air distribution system, which consists of the compressed air source, pressure transducers, valve regulators and mass flow meters. Fig. 10 shows the experimental setup. An optical fiber sensor is used to monitor the rotation of the micro turbine.

Fig. 11 demonstrates the real time output signal of the optical fiber sensor, which shows that the rotational speed is 3516 rpm when the flow rate of compressed air is 7.2 slpm (standard liter per minute). The signal from the fiber sensor can also be used for Fast Fourier Transform (FFT) to calculate the rotational speed and stability. Fig. 12 demonstrates one example. The spectrum analysis based on FFT indicates that the rotation speed is relatively uniform during operation while the flow rate is being kept constant. Stable operations of 15,000 rpm have been achieved during testing.

CONCLUSION

We have developed a silicon-based micro turbine device, which consists of both journal bearing and thrust air bearings with parameters recommended by simulation. The characteristics and performances of micro air bearings are studied through numerical simulation. It is found that the load capacity of a micro journal bearing with low aspect ratio (L/D=0.0483) will increase with rotor diameter, and almost linearly increase with the bearing number (rotation speed). Higher load capacity can be achieved by increasing the value of L/D or decreasing the radial clearance of a journal bearing.

The investigations on the dynamic thrust bearings demonstrate that the load capacity of thrust bearing increases with groove numbers. With the increase of the groove numbers, the peak values of the load capacity gradually move to higher values of the groove angle. A silicon-based micro turbine device has been designed and fabricated. A micro journal bearing with aspect ratio above 20:1 is realized using optimized DRIE process. Experimental tests demonstrate that the turbine device can sustain stable operations, and a rotation speed of 15,000 rpm has been achieved.

Fig. 1 Fig. 2

 12 Fig.1 The schematic diagram of the micro turbine device. Three flow paths are for driving air, static journal bearing and static thrust bearing. The dynamic thrust bearings are designed on the 1 st and 3 rd wafer dies. 1st 11111111 1 st wafer

  (a) shows the simulation results of non-dimensional load capacity W versus rotor radii at the aspect ratio of L/D = 0.0483. In the results,

  Load capacity versus bearing radius (D/2) Load capacity versus radial clearance Fig. 3 Load capacity of a journal bearing

  Fig.4 Schematic views of thrust air bearings consisting of spiral grooves. A c stands for the axial clearance, g d for groove depth.

Fig. 5

 5 Fig.5 Dynamic load capacity versus groove numbers of a thrust bearing with pump-in configuration

Fig. 8

 8 Fig.7 The 2 nd wafer fabricated using DRIE process

  X. C. Shan, Q. D. Zhang, Y. F. Sun and R. Maeda A Micro Turbine Device with Enhanced Micro Air-Bearings

Fig. 9

 9 Fig. 9 Pump-out spirals for bottom bearing. Air is pumped out while rotor relatively rotates in anti-clockwise direction

ACKNOWLEDGEMENTS

The authors would like to thank SIMTech and the Agency for Science, Technology and Research (A*Star), Singapore for the project funding.