Abdooulaye Guissé

François Lévy

Adeline Nazarenko

From regulatory texts to BRMS: How to guide the acquisition of business rules?

This paper tackles the problem of rule acquisition, which is critical for the development of BRMS. The proposed approach assumes that regulations written in natural language (NL) are an important source of knowledge but that turning them into formal statements is a complex task that cannot be fully automated. The present paper focuses on the first phase of this acquisition process, the normalization phase that aims at transforming NL statements into controlled language (CL), rather than on their formalization into an operational rule base. We show that turning a NL text into a set of self-sufficient and independent CL rules is itself a complex task that involves some lexical and syntactic normalizations but also the restoration of contextual information and of implicit semantic entities to get a set of self-sufficient and unambiguous rule statements. We also present the SemEx tool that supports the proposed acquisition methodology based on the selection of the relevant text fragments and their progressive and interactive transformation into CL rule statements.

0 This work was realized as part of the FP7 231875 ONTORULE project (http://ontorule-project.eu). We thank our partners for the fruitful discussions, especially

Introduction

Checking the conformance of a process with respect to regulations is a growing domain of application for business rule management systems (BRMS). For instance, in order to export cars in various countries, car manufacturers have to satisfy safety and quality tests described in UNO regulations (e.g. 50 pages without annexes for the sole safety belts) and others constraints from the European and national authorities. Moreover, these regulations evolve over time (the UNO text has been modified 10 times between 2005 and 2009). Even if efficient rule systems are now able to exploit and maintain large rule bases, rule acquisition remains a bottleneck, and text-based rule acquisition is an important challenge.

We propose a method for the acquisition of rules and a tool, SemEx, which supports that method and guides the acquisition process. This approach relies on two strong assumptions. First of all, we believe that acquiring conformance rules from regulations cannot be fully automated. This is due to the complexity of human natural language (NL). The acquisition strategy that we propose relies on the cooperation of a domain expert and local automated processes. The expert controls the transformation of the regulation but automatic processes ease up the expert work. Second, we consider that it is difficult if not impossible to translate directly NL regulations into an operational rule bases expressed in formal language. We rather propose to decompose the formalization work into two main phases and to use controlled languages (CL) such as SBVR structured English (SBVR-SE 1 ) as an intermediate language. A domain expert designs a set of CL rules from the source NL regulations and this set of candidate rules is then passed on to a specialist of information technologies (IT) that formalizes the candidate rule base taking the characteristics of the final application and the constraints of the rule engine into account. We focus here on the first phase, the normalization of the source regulation into a set of candidate rules written in CL, rather than on the second phase, the transformation of the CL statements into rules, which has been more studied. We show how this normalization process can be divided into intermediary steps, which allows to decompose the expert work and to guide each step with specific helping tools.

This rule acquisition method and the associated SemEx tool have been developed as part of the ONTORULE project, which aim was to define an integrated platform for acquisition, maintenance and execution of business-oriented knowledge bases combining ontologies and rules. The work has been tested on two industrial use cases. The AAdvantage use case aimed at developing a classification application to determine the benefits that an airline customer retention program member has earned over a given period. The business rule model had been designed from the documentation downloaded from the American Airlines (AA) web site, in particular the Terms and Conditions (5,744 words), which describes the membership statuses and the associated benefits. In the Audi use case, a rule application has been being defined to certify the conformance of Audi procedures with vehicle safety international regulation. The present experiment is based on the aforementioned UNO regulation. In each use case, the normalization process has been guided by a domain ontology that had been built beforehand.

Section 2 presents the state of the art. The normalization methodology and the architecture of the SemEx tool are described in Section 3. Section 4 details the framework that we propose for the progressive translation of texts into CL, showing the complexity of the involved linguistic and semantic transformations. Section 5 presents the results of that normalization processes in our use cases.

Related works

BRMS are useful for propagating automatically the changes in the business of organizations into their information systems [START_REF] Bajec | Issues and challenges in business rule-based information systems development[END_REF]. According to [START_REF] Halle | Business Rule Revolution: Running Business the Right Way[END_REF], the different forms of business rules can be seen as a continuous flow of models: the rules evolve from an initial state (the rules are included in documents specifying the system), to a final state (they are formalized and integrated in information systems). However, some problems remain unsolved regarding 1) the acquisition of rules from specification documents, 2) their modeling in a formal language that enable their automation, 3) their integration in BRMS (storage, exploitation and maintenance).

The present work focuses on the first two points. This question, already raised by [START_REF] Brg | Defining business rules what are they really? The Business Rules Group : formerly, known as the GUIDE Business Rules Project -Final Report revision 1[END_REF][START_REF] Halle | Business Rule Revolution: Running Business the Right Way[END_REF][START_REF] Dubauskaite | An open issues in business rules based information system development[END_REF], concerns the transformation from informal to formal knowledge and the translation of text fragments written in NL into formal rules. This translation is difficult to automate, due to the complexity of NL and reduced expressivity of formal languages. Even the translation into SPARQL of LN queries, which are much simpler than texts, is acknowledged as a complex problem. To the best of our knowledge, only [START_REF] Dinesh | Reasoning about conditions and exceptions to laws in regulatory conformance checking[END_REF] considers a direct translation of legal texts but, after a parsing step, the abstract syntax trees are translated by hand into CTL2 .

Controlled languages have been proposed as intermediate languages in this translation process [START_REF] Wagner | First-version controlled english rule language[END_REF]. They allow to reformulate rules in a way that is still readable for the user and is easier to formalize than NL. In the Business Rules domain, CLs are used in Oracle Policy Modeling Suite3 , in IBM SPARCLE policy workbench [START_REF] Brodie | An empirical study of natural language parsing of privacy policy rules using the sparcle policy workbench[END_REF]. Other in use controlled languages have been described by RuleSpeak [START_REF] Ross | Principles of the Business Rule Approach[END_REF], by Atempto Controlled English (http://attempto.ifi.uzh.ch/site/docs/). SBVR (Semantics of Business Vocabulary and Rules) can be seen as a synthesis of several efforts and a standard independent of English or any natural language. It has been accepted by the OMG (Object Management Group) 4 . We refer to the English version of SBVR CL, namely SBVR Structured English (SBVR-SE). SBVR relies on formulas (Figure 1) combining linguistic basic templates with logical, modal or quantification operators.

The NL to CL translation is a complex task, the automation of which has been rarely considered. Recently, [START_REF] Bajwa | Sbvr business rules generation from natural language specification[END_REF] has proposed NL2SBVR 5 , a tool to automatically translate NL into SBVR-SE. According to the reported experiments, the complexity of the translation depends on the number of clauses that compose a sentence. Only so-called simple rules, composed of at most two clauses, are translated with a 80% success rate. This is the reason why, in SemEx, translation into CL and simplification go along for complex and long NL rules.

NL simplification has been studied to ease translation [START_REF] Siddharthan | Syntactic simplification and text cohesion[END_REF], human understanding (esp. in case of understanding disorders [START_REF] Max | Simplification interactive pour la production de textes adaptés aux personnes souffrant de troubles de la compréhension[END_REF]), text summarization [START_REF] Gasperin | Learning when to simplify sentences for natural text simplification[END_REF][START_REF] Chandrasekar | Automatic induction of rules for text simplification[END_REF], foreign language learning (see [START_REF] Chandrasekar | Motivations and methods for text simplification[END_REF] for a general presentation). Methods give a significant role to the lexical part, trying to stick to a privileged vocabulary, and to the brevity of sentences. Our transformation process (see Section 4) relies on the same methods but differs in its goal. The above works aims at preserving the discursive structure of texts while sometimes simplifying the information content, whereas, in business rule acquisition, the simplification of text should give a set of independent rule statements but preserve their meaning.

3 Normalization process

Overall approach

The normalization process takes a regulatory text as input and outputs a new "text" composed of a list of independent, self-sufficient rules. The rules are written in a language that is as controlled as possible and the set of rule forms a draft of a business rule model (the basis of a formal rule model). The proposed approach relies on the selection of the relevant text fragments (sentences or sequences of sentences) that convey rule information, and on their normalization, i.e. on their translation into CL. Both steps are difficult to handle automatically. The selection step calls for browsing facilities, since missing some important passages leads to a partial BR model. Only simple natural language statements can be automatically and reliability translated in CL. An interactive approach is therefore adopted, which consists in transforming step-by-step a regulatory text fragment into one or several independent rules written in controlled language.

This approach is illustrated on Figure 2. The underlying methodology is supported by the SemEx tool6 , which offers two main acquisition functionalities for the selection and normalization of rules. Those functionalities and the corresponding perspectives of SemEx are presented in sections 3.3 and 3.4 7 .

For traceability, diagnosis and revision purposes, the whole set of transformation results needs to be stored and mined. SemEx therefore relies on a rich annotation scheme to encode the rule base under construction and the source text from which it derives. Once it is built, the annotation structure can be explored through dedicated search functionalities. This annotation scheme is described in Section 3.2.

SemEx is built on W3C standards and technologies, which enables the reuse of resources (OWL ontologies) and components (SPARQL search engine).

Annotation scheme

The annotation scheme describes the data structure in which the source document and the business rules that derive from it are encoded. The basic elements of this scheme are the textual units that are either tokens or sequences of tokens (elementary character strings resulting from a segmentation process). The source document is actually represented as a corpus which is composed of one or several documents. A document is a list of sentences, which are themselves represented as sequences of tokens. The rule base is composed of a set of candidate rules which are also sequences of tokens. Two types of annotation relations are defined. The high-level ones relate sentences and candidate rules. The rule base is similar to a document but the candidate rules that compose it are partially ordered by an annotation relation. A sentence as a whole can be annotated by a candidate rule which results from its selection and a candidate rule as such can be annotated by one or several candidate rules that derive from it. Low-level annotation relations link the textual units that compose the sentences and the candidate rules to elements of the conceptual (or lexical) and logical (or grammatical) vocabularies: the ontological elements (concepts, properties or individuals) that compose the domain ontology chosen to interpret the source document, and the keywords that serve as grammatical words in the controlled language.

In technical terms, this data structure is encoded as a RDF graph. Annotation links are encoded in RDFa: the RDF annotations are anchored in textual units of XML documents and refer to resources that are OWL entities or candidate rules.

For visualization, low-level annotations are usually represented in a SBVR-style, where the annotated units are colored in blue, red, green and orange according to the type of element (concept, instance, property or known key word) that is referred to. High-level annotations are encoded as explicit references in the source sentence or candidate rule that point to the target candidate rules.

Selection of regulatory fragments

The first challenge for the knowledge engineer who develops a business rule model is to identify the relevant parts of the document and to select the fragments that convey regulatory information. He/she mainly has to read through the source text but the relevant information is often scattered in large and complex documents. At this step, precision must be favored over recall, as it is harder to recover missed fragments than to drop irrelevant ones.

SemEx proposes several devices to help that selection task. The first one is the low level of annotations. When browsing the annotated text in which all the recognized textual units are colored, the expert can focus on passages in which many textual units have been identified. The keywords are especially useful: sentences with a lot of annotated keywords are likely to be relevant.

The second device is a small information extraction engine that allows to design extraction patterns in an interactive mode and look for matching fragments. For instance, a sentence that follows the structure "if. . . [then]. . . must. . . " is likely to convey relevant information. Only a rudimentary version of the extraction engine is integrated in SemEx for now but it could be extended. The goal is to store the patterns so that the most generic and reliable ones can be reused from one application to another.

The third device is a semantic search engine that returns a list of sentences or candidate rules in answer to a semantic query. For instance, if one looks for all the sentences that mention the concept participant, one gets all the sentences that contain the word "participant" but also in which the participant is designated as "a participating company" or even as a "member of the program". This ensures a high recall level.

Normalization

Once the relevant fragment are identified, they must be normalized into CL. The goal is to get rid of ambiguities, to homogenize the lexical and syntactic turns and to make explicit all useful information. This is a complex process that cannot be fully automated. SemEx methodology supports an interactive and progressive process that transforms the initial sentence extracted from the source document into a standardized one, which is written in CL or as close as possible.

In technical terms, the expert can derive a new candidate rule from an existing one. The derivation is encoded as an annotation link and any intermediary step can be restored at any moment. The next section details the transformations that can be applied during the normalization of a rule base. Figure 4 on page 12 gives an example of a derivation tree. This section presents the types of elementary transformations that are needed to translate a NL rule into controlled language. Each of them is explained and illustrated on our use cases. Transformations aim at clarifying the text provided to IT specialists in charge of implementing the rules. This involves reformulating ambiguous or tricky sentences, while preserving the meaning of the underlying rules. Each rule must also be formulated in a self sufficient way, so that its operative meaning can be determined without referring to the source text or to other rules.

Our target language is close to SBVR-SE. The main difference is that we exploit a lexicalized ontology [START_REF] Omrane | Lexicalized ontology for a business rules management platform: An automotive use case[END_REF] to represent the domain and its conceptual vocabulary. This vocabulary has a narrower scope than that of SBVR, which often includes specific and general-purpose dictionaries 8 whereas general purpose vocabulary remain mostly out of the scope of our transformations.

We identified four types of transformation that are presented below. The lexical normalizations replaces the terms of a candidate rule so as to stick to the domain vocabulary. The decontextualization makes explicit the contextual elements of meaning, so that the resulting rules be understandable independently of the source text and other rules. The syntactic normalization simplifies the syntactic structure of the sentence so that it is unambiguous and easy to understand. The semantic normalization operates at the semantic level, where discourse entities not explicitly referred in the text must often be introduced.

Lexical normalization

The operation of lexical normalization aims at checking the business vocabulary of a candidate rule and at replacing all the mentioned terms by their preferred forms. This transformation process takes as input a candidate rule and a lexicalized ontology, which specifies not only the relevant concepts and properties for the field of application but also the preferred and alternative terms to refer to them [START_REF] Omrane | Lexicalized ontology for a business rules management platform: An automotive use case[END_REF]. The goal is that the candidate rule conform to that vocabulary. The rule terms must be disambiguated and made as specific as possible with respect to the terminology associated to the domain ontology.

The lexical normalization is based on the recognition of the terms of a candidate rule. This operation relies on the annotation of the candidate rule with respect to the ontology: a semantic annotator is integrated in SemEx [START_REF] Lévy | An environment for the joint management of written policies and business rules[END_REF]. Terms recognized as preferred terms in the ontology are left as is but alternate terms are replaced by their associated preferred ones. These annotation and replacement processes is based on a lemmatized version of the text as output by a part-of-speech tagger 9 to ensure the linguistic correctness of the resulting rule. The normalization process also requires that the ambiguity of terms which can get several annotations be solved. The expert has to identify the relevant term meaning in the context and select the proper unambiguous preferred terms.

For instance in Two belts or restraint systems are required for the buckle inspection and the low-temperature buckle test.

The concepts BuckleInspection and LowTemperatureBuckleTest are represented by their preferred terms "buckle inspection" and "low-temperature buckle test", but "belts" and "restraint systems", which stand for the concepts SeatBelt and ChildRestraintSystem, are replaced by the preferred forms of these concepts ("seat belt" and "child restraint system"). This lead to the following transformed candidate rule:

Two seat belts or child restraint systems are required for the buckle inspection and the low-temperature buckle test.

Lexical normalization also involves nominalizations when a domain concept or entity is mentioned through a verbal phrase. For instance, "be tested for strength" should be replaced by "undergo a strength test" in the following rule:

All the adjustment devices shall be tested for strength as prescribed in paragraph 7.5.1.

Decontextualization

Decontextualization extends lexical normalization in that it tracks references to business concepts which are not made by the specific business vocabulary stored in the lexicalized ontology, but by a word or phrase (the referent) which co-refer to a pre-mentioned word or phrase (the antecedent) and whose meaning depends on the antecedent's one. The co-reference link must be broken and the actual meaning of the referent must be made explicit so that the rule can be understood independently of its context. Various types of referent can be found.

Grammatical words Pronouns and possessive adjectives often embed a reference to a business entity. In the following rule, "They" should be replaced by "The adjustment devices".

All the adjustment devices shall undergo a strength test as prescribed in paragraph 7.5.1. They must not break or become detached under the tension set up by the prescribed load.

This type of coreference can often be solved automatically using an anaphora solver, which identifies the referring items and their antecedent. We plan to integrate such an anaphora solver in SemEx and to test the benefit of this additional helping tool but we do not have such an experimental feedback yet.

Generic business terms referring to high-level concepts are often used to refer to more specific ones. This is a stylistic way to avoid repetitions when the context is clear enough, but those references must be made explicit in contextindependent candidate rules. In the UNO regulation, "Test" is often used for the specific test under description. In the following example, "test" means "microslip test" and the generic term must be replaced by the specific one.

The samples to be submitted to the micro-slip test shall be kept for a minimum of 24 hours in an atmosphere having a temperature of 20 ±5 • C and a relative humidity of 65 ±5%. The test shall be carried out at a temperature between 15 and 30 • C Individual constraints are often left implicit to skip straightforward details. For instance, in Mileage credit will be credited only to the account of the AAdvantage member who flies, etc.

"Mileage credit" does not refer to the plain general concept MileageCredit, but to a specific instance earned by the AAdvantage member who took the mentioned flight ("who flies"). Decontextualization yields to Mileage credit awarded for a flight will be credited only to the account of the AAdvantage member who takes that flight, etc.

Searching for implicit individual constraints is difficult. We plan to compare the concepts in the rule to configuration of roles in the ontology, as the triangle which links the mileage credit earned for a ticket, an AAdvantage member who buys a ticket and a flight for which the ticket is delivered.

Reference keys are symbols or numbers which refer to a distant piece of text. We observed that in regulation texts, the accompanying text can take various forms but that reference keys are often used to introduce exceptions. Clarification is therefore both important and cumbersome.

In the following example, two load determination procedures must be defined depending on the fact that the buckle is part of the attachment or not. A complex reorganization of the candidate rule is therefore necessary. (7.5.1) The buckle shall be connected to the tensile-testing apparatus and the load shall then build up to 980 daN . . . If the buckle is part of the attachment, the buckle shall be tested with the attachment, in conformity with paragraph 7.5.2. below, (7.5.2) The attachments shall be tested in the manner indicated in paragraph 7.5.1., but the load shall be 1,470 daN.

Such a text reorganization cannot be made automatically, but navigation facilities can be proposed so that the expert can easily identify reference keys and get a quick access to the referred parts of text.

Syntactic normalization

Syntactic normalization aims at giving a more standard phrasing of the simple candidate rules or at splitting them into several simpler ones that are easier to understand. This normalization is close to the text simplification operations that have been proposed for English [START_REF] Siddharthan | Syntactic simplification and text cohesion[END_REF][START_REF] Candido | Supporting the adaptation of texts for poor literacy readers: a text simplification editor for brazilian portuguese[END_REF], but some of these simplifications are not adequate in our business rules context and we propose more specific structures and transformations for our specialized texts.

Sentence reordering reorganizes the sentence to stick to the order of a logical rule pattern. This often lead to exchange the main and subordinate clauses in a candidate rule. In the following example, the elliptic "upgrade" is understood as a coreference and the clauses are reordered:

Upgrades are void if sold for cash or other consideration. If upgrades are sold for cash or other consideration, these upgrades are void.

Splitting enumerations Enumerations are a well-known factor of sentence complexity and splitting enumerations leads to decompose candidate rules into simpler ones. Enumerations have various linguistic forms : pairs of connectors such as either...or, neither...nor, not only...but also, whether...or, etc., coordinating conjunctions (and, or ) or plain juxtaposition. The enumerated list can be the subject, the object or even the verb of the clause. For instance, the following candidate rule should be split into three independent sentences: Neither accrued mileage, nor award tickets, nor upgrades are transferable by the member upon death. Accrued mileage is not transferable by the member upon death. Award tickets are not transferable by the member upon death. Upgrades are not transferable by the member upon death.

Enumerations are difficult to handle automatically. Coordination markers are easy to detect but the scopes of the enumerations are not. Their interpretation is sometimes difficult: splitting is correct only if the enumeration clusters independent conditions; otherwise, it may lead to errors, as in:

Mileage credit may not be combined among AAdvantage members, their estates, successors and assigns.

Splitting rules Independently of enumerations, complex candidate rules often need to be split. Solving a coreference is a frequent cause that should be often handled automatically when an anaphora solver will be integrated into Semex. In the following example, the decontextualization of the pronouns "which" and "yours" leads to split the candidate rule into three independent ones:

The membership year, which is the period in which your elite benefits are available, runs from March 1 through the last day of February of the following year.

The membership year is a period. Member's elite benefits are available in the membership year. The membership year runs from March 1 through the last day of February of the following year.

Semantic restoration

Semantic restoration is a fourth kind of transformation that is often implied by decontextualization or syntactic normalization : discourse entities, which are implicit in the source documents, often have to be restored during normalization.

Restoring an entity to solve a reference In some cases of decontextualization, there is no unambiguous designation available for solving a coreference and a new entity must be introduced.

In the following example, expliciting "which" by "two perpendicular axes" misses the coreference, using "these perpendicular axes" does not solve it but introducing a reference to a SensitivityTestAxes concept enables the rule split.

When retractors are being tested for sensitivity to vehicle deceleration they shall be tested at the above extraction along two perpendicular axes, which are horizontal if the retractor is installed in a vehicle as specified by the safety-belt manufacturer.

When retractors are being tested for sensitivity to vehicle deceleration they shall be tested at the above extraction along the sensitivity test axes. Sensitivity test axes are perpendicular. Sensitivity test axes are horizontal if the retractor is installed in a vehicle as specified by the safety-belt. manufacturer.

Restoring an interval to express constraints It often happens that constraints between entities are only expressible with the help of an interval that is not mentioned as such in the text.

In the following example, the transformation depends on the concepts available to refer to time entities and the proposed solution assumes there is none and defines them all:

The breaking load shall be determined within 5 minutes after the strap is removed from the conditioning atmosphere or from the receptacle. The determination time is the time when the breaking load is determined. The removing time is the time when the strap is removed from the conditioning atmosphere or from the receptacle. The delay between the removing time and the determination time will be less than 5 minutes These cases frequently occur for temporal and spatial constraints and keyword search (e.g. after, until, since) should help to detect the problematic rules.

Experiments and Results

This normalization method has been tested on the two ONTORULE industrial use cases for which two real BR models and rule applications had to be defined, exploiting available sources of information. In each case, a rule base has been designed by extracting a set of candidate rules from the source regulatory texts and by normalizing them, using as many transformation steps as necessary. Transformations which are not yet proposed by a tool incorporated in SemEx have been handled manually by the expert, so as to test the completeness and the correctness of the description. Figure 4 shows the example of Rule R5, on which a decontextualization transformation (R125) and then a syntactic normalization (R124) are applied before it is decomposed into two elementary and independent sub-rules (R126 and R127). We tried to automatically translate the initial and the final candidate rules into SBVR, using the NL2SBVR tool [START_REF] Bajwa | Sbvr business rules generation from natural language specification[END_REF].The result (Figure 5) shows that the meaning of the initial sentence is lost in the translation (statement 1), but that the translation works for the transformed final candidate rules giving valid SBVR statements (2 and 3).

The following tables present the types and size of the rule bases that have been designed out of the source regulatory texts for each use case. Four types of transformations are considered here: the normalization of the vocabulary; the restoration of contextual information, the syntactic transformation and the decomposition of rules. Since the syntactic and semantic transformations are often performed together, the semantic transformations are not isolated here.

Table 1 shows the results of the selection phase. It gives the size of the initial rule bases with respect to the size of the source texts. In the use cases, 1/4 at least of the sentences have been selected as relevant regulatory information. This high rates are due to the fact that the source text are short but dense. This table also shows the initial structure of the rule base. More than 2/3 of the extracted rule are structural rules, some of them are operative rules and few of them are derivation rules. Table 2 details the types of transformations that have been made for each use case. Four types of transformation are considered: the first three ones respectively affect the vocabulary, the context and the syntax; the last one is the decomposition of one candidate rule into several ones. In terms of number of transformations, the syntactic normalization is the most expensive one: it requires twice as much transformation steps than other types of transformations. All the initial rules have undergone a syntactic normalization (100%). The vocabulary transformation and decontextualization also affect more than half of the initial candidate rules, whereas decomposition is required in less numerous cases (resp. 35% and 20% for the AAdvantage and Audi use cases). 2. Distribution of the different types of transformation (α = rate of a given type of transformation when considering all the transformations that have been made; β = rate of initial candidate rules that have undergone a given type of transformation)

The last table [START_REF] Brg | Defining business rules what are they really? The Business Rules Group : formerly, known as the GUIDE Business Rules Project -Final Report revision 1[END_REF] presents the structure of the resulting rule base, the number and types of the final candidate rules. As expected, there are more final rules than initial ones since some of them have undergone a decomposition. At the end of the normalization process, the regulatory information written in the AAdvantage text (245 sentences) has been reduced to a set of 104 candidate rules which are autonomous SBVR statements. The reduction is even higher for the Audi use case (225 sentences, 65 candidate rules), whose source text is more detailed.

Conclusion

This paper tackles the rule acquisition problem, assuming that regulations written in NL are a rich source of knowledge but that turning NL into formal statements is a complex task than cannot be fully automated. We propose to decompose the acquisition process into two main phases: the translation of NL statements into CL and their formalization into an operational rule base.

The present paper focuses on the first "normalization" phase. It shows that transforming NL statements into CL is itself a complex task that involves some lexical and syntactic normalizations but also the restoration of contextual information and of implicit semantic entities to get a set of self-sufficient, unambiguous and easy to understand rule statement. We also present the SemEx tool that supports the proposed acquisition methodology based on the selection of the relevant text fragments and their normalization into a SBVR-like CL.

SemEx has been designed as an interactive rule acquisition tool. It guides the domain expert through a sequence of steps that produces elementary candidate rules, helps the detection of relevant keywords and controls the results. Some helping tools have already been plugged in SemEx: e.g. a semantic annotator that takes a lexicalized ontology as input and annotates a text with respect to that ontology and a keyword search that helps locating the most relevant text fragments or identifying the problematic features (e.g. anaphoric pronouns) in the selected fragments.

In the next future, we plan to exploit more intensively NL processing tools to guide and help the acquisition task. Part of the morphological and syntactic calculus could be automated using a parser. Some anaphora could be solved and syntactic transformation patterns could be exploited. We are currently testing these technologies to enrich SemEx with additional helping tools. We are also planning to integrate a SBVR parser to check the syntactic conformity of the final candidate rules with SBVR-SE. Successful checking would indicate that the transformation phase is achieved and failures would give some indication on how to complete it. The last and most challenging tool would be a semantic parser able to check the conformity of the final candidate rule to the underlying ontology. That would help to identify the semantic shortcuts that need to be made explicit in the candidate rules.

Fig. 1 .

 1 Fig. 1. SVBR SE business rule example (www.brcommunity.com)

Fig. 2 .Fig. 3 .

 23 Fig. 2. Rule acquisition overall approach. Two final candidate rules (4 and 6) have been derived from the same initial candidate rule (1) extracted from the source text.

Fig. 4 .

 4 Fig. 4. Example of a derivation tree

Fig. 5 .

 5 Fig. 5. SBVR translation

Table 1 .

 1 Results of the selection phase on the AAdvantage and Audi use cases (CR = candidate rule; SR = structural rule; OR = operative rule; DR = derivation rule)

	Use Case # of sentences # of initial CR Selection rate # of SR # of OR # of DR
	AAdvantage	245	74	30%	54	14	6
	Audi	221	54	25%	45	9	0

  Normalization types AAdvantage α AAdvantage β Audi α Audi β

	Vocabulary	19%	65%	20%	61%
	Context	18%	60%	19%	57%
	Syntax	43%	100%	47% 100%
	Decomposition	21%	35%	14%	20%
	Table

Table 3 .

 3 Structure of the final rule base

	Use Case SR OR DR Final CR
	AAdvantage 71 27 6	104
	Audi	54 11 0	65

CTL is a modal temporal logic

www.oracle.com/technology/products/applications/policy-automation

http://www.omg.org

http://www.cs.bham.ac.uk/ isb855/nl2ocl/projects.html

http://www-lipn.univ-paris13.fr/ guisse/index.php?n=Semex.Semex

Additional functionalities are accessible via SemEx for text annotation and for mining the text and the resulting business model, but they are not described in detail here. See[START_REF] Lévy | An environment for the joint management of written policies and business rules[END_REF]. for an overall description of the tool.


Wissam Sabbah

Raphaël Riva

Stanislas Hascoët

Cyril Buttay

Stephane Azzopardi

Eric Woirgard

Dominique Planson

Bruno Allard

Régis Meuret

Evaluation of silver-sintering die attach

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The development of power components based on silicon carbide (SiC) allows for the design of power converter operating at high temperature (above 200 or 300°C). SiC is a semiconductor material with a large band gap that is not only able to operate at temperatures exceeding 300°C but also offers fast switching speed, high voltage blocking capability and high thermal conductivity compared to silicon technology components. The classical die attach technology uses high temperature solder alloys which melt at around 300°C. However, even a die attach with such high melting point can only operate up to a much lower temperature. A 0.8 ratio between the melting and operating temperatures (both expressed in Kelvin) is commonly accepted [START_REF] Knoerr | Reliability Assessment of Sintered Nano-Silver Die Attachment for Power Semiconductors[END_REF]. This means that the solder which melts at 300°C (573 K) can only operate at temperatures below 185°C (573 x 0.8 = 458 K). Alternative die attach solutions have recently been proposed: Transient Liquid Phase Bonding [START_REF] Guth | New assembly and interconnects beyond sintering methods Proceedings of PCIM[END_REF], soldering with higher melting point alloys such as ZnSn [START_REF] Manikam | Die Attach Materials for High Temperature Applications: A Review Components, Packaging and Manufacturing Technology[END_REF], or silver sintering [START_REF] Göbl | Low temperature sinter technology Die attachment for automotive power electronic applications Automotive Power Electronics[END_REF][START_REF] Lei | Low-Temperature Sintering of Nanoscale Silver Paste for Attaching Large-Area (>100mm²) Chips[END_REF].

In this study, we will focus on assessing the electrical performance of this latter technique which is becoming mature. Indeed, silver sintering is a very interesting technology, as silver offers very good thermal conductivity (429W/m.K, better than copper), relatively inexpensive (compared to alternative solutions which often use gold), and has a very high melting point (961°C).

In particular, two aspects of the silver sintering technology will be investigated: the thermal conductivity of the die attach, and the possible formation of conductive silver filaments by migration. Of course, this constitutes only a partial evaluation of the technology, additional tests are needed, such as thermal cycling, to achieve a complete characterization.

Assessment of thermal resistivity

Description of the vehicle test assembly

In order to assess the influence of the silver joint on the overall thermal resistivity (Rth), four preliminary test vehicles (TV) are assembled with different sintering process:

• Two using a paste based on silver micro particles (Heraeus, ref. 043-06P2) • Two using a paste based on silver nano particles (NBE Tech, Nanotach) Each TV is made with a silicon carbide diode chip (SICED 1200V/15A, 2.7x2.7 mm²) and DCB substrate (Al2O3 with bare, 300 µm-thick copper metallization). The silver paste is deposited on the DCB using a 100 micron-thick stainless steel screen. In addition to the variation of the paste (micro or nano) two other parameters are changed between the TVs: the pressure applied to the dies (1.3 or 6.4 MPa), and the moment the die is placed on the paste deposit (before or after drying). These parameters are summarized in Table 1, and the practical assembly process is described in [START_REF] Buttay | Die Attach of Power Devices Using Silver Sintering -Bonding Process Optimisation and Characterization Proceedings of the European Conference on Power Electronics[END_REF]. After assembly, each chip is wire-bonded with 2 aluminium wires (Figure 1).

Thermal Characterization

The thermal resistance measurement technique described here can be applied to most power semiconductor devices (MOSFETs, Thyristors, Diodes, IGBT). It is based on thermally sensitive electrical parameter used as a junction temperature indicator [START_REF] Dr | Fundamentals of thermal resistance measurement[END_REF].

The measurements are performed using a thermal analyzer located at the IMS laboratory (Figure 2).

The test is divided into two steps: the first is the calibration, followed by the thermal resistance measurement. The calibration for each TV is necessary and consists in immersing the TV, which is connected to a low value current generator, in a temperature controlled dielectric bath (Figure 3). Several measurements of the resulting forward voltage are obtained at 5°C increments from 90°C down to 45°C. The initial temperature was the maximum temperature (90°C) then the temperature was decreased for the following measurement (Figure 4). It is more difficult to get an accurate measurement due to thermal overshoot in the opposite case (from cold to hot). A constant current "i0" of 1mA is injected into the diode producing a negligible self-heating of the chip. At the thermal equilibrium, the voltage drop across the diode and the junction temperature (TJ) are measured. The relationship between the voltage and temperature is linear, with a slope of -2mV / °C.

Once the calibration is obtained, the sample is cleaned with acetone and dried in air before being connected to the thermal analyser to measure the RTh and attached to a cold plate as depicted in Figure 5 to control its thermal environment. A thermocouple is placed between the controlled cold plate and a Thermal Interface Material (TIM) layer in order to measure the cold plate temperature Tcold plate. Adding a TIM allows to reduce the contact thermal resistance between the case and the cold plate. Different TIM are used to attach the test vehicle to the cold plate (Table 2). A double-sided TIM adhesive (ref-2 sided 3M 467 PSA), a thermal grease layer and a non-adhesive TIM layer were tested for the measurement. In the case of the TIM adhes- ive layer, an un-controlled pressure is applied by pressing manually on the entire sample. For the two others layers, we use a torque screwdriver to fix the sample. The adhesive TIM was chosen because its constant thickness and homogeneous bonding capability makes it possible to achieve a controlled thermal performance without applying a pressure during the measurement (this is not the case with the two other TIMs).

Table 2: Thermal properties of the TIM layers

The measurement is performed automatically by the Anatech Phase 10 analyzer, and can be explained with two steps (Figure 6) :

• In the first step, the device under test is connected to a high (several amps) current source (left source in Figure 6). This results is an increase of the junction temperature of the diode, • The power source is then rapidly (a few microseconds) disconnected, and a low value current source is connected to the diode. This current value is identical to that used during calibration, which means the calibration curve in Figure 4 can be used to determine the junction temperature reached by the diode from its forward voltage.

The dissipated power in the component is calculated from the voltage and current measurements:

Q = P = V x I [W] With:
• Q: the heat dissipated by the chip • P: the dissipated power • V: the voltage across the diode • the injected current

As the most of the heat is transferred by conduction, the convection (and radiation) mechanisms can be neglected . The thermal resistance of the assembly is the sum of all thermal resistances Figure 7: -R Th-JC is the thermal resistance between the junction of the die and bottom side of the DCB substrate -R Thcoldplate is the thermal resistance within the cold plate -R th-TIM is the thermal resistance of the TIM layer -RTh-CA, the thermal resistance between the junction and the air, due to convection, which is considered negligible here.

The equation for measuring the thermal resistance in steady state is written as follows:

R Th = Δ T Q = T J -T A Q [K/W]

Experimental Results

The thermal equilibrium is achieved with a 3.8 amps injected current load, resulting in heat dissipation of about 4.5 Watts (Table 3). TV1 has the highest thermal resistance value which can be explained by the location of the chip on the DCB substrate. (Table 1)

The R Th measured includes the R Th of the TIM layer that we used to fix TV on the cold plate. Another set of measurements was made with a thermal equilibrium achieved with a 5 amps current load. The results are summarized in Table 4, showing that the previous applied power (4.5 W) was high enough for the measurement. In most cases, the results in Table 3 and Table 4 are comparable (less than 10% variation). However, it must be noticed that for a given TV, the thermal resistance changes a lot with the nature of the TIM. And for a given TIM, the results are widely different between the TVs. In fact, the adhesive TIM gives the most consistent result.

The grease gives very different results between the TVs. This is due to the un-controlled thickness of the grease deposit, and a better dispersing solution should be used (screen printing). In Table 3, for an adhesive TIM, TV1 exhibits a much higher Rth value (2.83 K/W vs 2.06, 2.07, 2.08 K/W for TV2, 3, and 4 respectively). This is because the die in TV1 was bonded close to the edges of the DCB, whereas it was bonded in the center for all other TVs. As a consequence, when comparing TV2, 3 and 4, we can consider that the effect of the silver on the total Rth is negligible, even if the silver joints were obtained for very different process parameters. A thermal dissipation simulation was made with ANSYS multiphysics software. The junction temperature obtained by simulation is close to the experimental test. To simulate the junction temperature evolution, we used the ambient temperature and power dissipation we already obtained from the experimental test as inputs. We can see in Figure 9 and Figure 10 two simulation results obtained for two different locations of the chip (on the edge, as with TV1, and on the center of the substrate, as with all other TVs).

Using the simulation tool, we were able to confirm that the TIM layer is the main component of the total Rth (Figure 11). The simulation also showed that the location of the chip is much more important than the silver joint thickness (Figure 12).

3

Thermal stability of the sintered silver

Description of the issue

Although sintered silver is a very attractive material for high temperature applications, due to its very high melting point, some authors have highlighted the risk of silver migration [START_REF] Mei | Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate at High Temperatures[END_REF][START_REF] Yanga | Initial Stage of Silver Electrochemical Migration Degradation Microelectronics Reliability[END_REF][START_REF] Mei | Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate Between 250°C and 400[END_REF]. This phenomenon is the migration of silver ions from the anode to the cathode causing the formation of a conductive dendrite (Figure 13). This is an area where the random presence of silver can cause a short circuit between the electrodes. In particular, this was demonstrated in the case of silver electrodes on a ceramic substrate [START_REF] Mei | Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate Between 250°C and 400[END_REF]. 4)

If we define the lifetime of a sample as the operating time before the short circuit appears [START_REF] Mei | Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate Between 250°C and 400[END_REF], this lifetime is sensitive to three parameters :

• An increase in the electric field applied to the sample decreases his lifetime. Tests conducted by Yunhui Mei et al. showed that migration occured at electric fields as low as 10 V/mm, with a strong reduction in lifetime for fields reaching 100 V/mm. • The lifetime of the sample decreases with increasing temperature. In the same way as for the potential, Yunhui et al. investigated the effect of temperature from 250 to 400°C. Silver migration was observed for all temperature, with lifetime following an Arrhenius law. • The lifetime of the sample decreases with increasing oxygen partial pressure. Oxygen is shown to be required for the migration to occur [START_REF] Mei | Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate Between 250°C and 400[END_REF]. The results presented in the literature use electrodes entirely made out of silver. In the case of our system, silver is only used as the die-attach material. Therefore, there is a need to assess the risk of silver migration in this case.

Description of the test vehicle

Three sets of test vehicles have been built:

• a set has been etched in DCB, with the copper metallization as is (no plating applied to the copper), using the layout visible in Figure 14. A 3x3 mm² silver pad (heraeus material) has then been screen-printed on the copper, and sintered without pressure during sintering nor die on top of the silver deposit. • another identical, but with a nickel/gold finish on top of the copper, to prevent oxidation • the last set was obtained by screen-printing silver paste on an alumina substrate (Figure 15).

Operating mode

The test vehicles are connected to a high-voltage Source and Measure Unit (SMU Keithley 2410). This equipement can source up to 1100V, and measure current with a resolution of 10 pA. Care has been taken in the connexion to avoid current leakage. In particular, we used a triaxial cable for the high potential (Figure 16). The cable is connected to the test vehicle using HMP (High The test vehicle is placed in a forced-convection oven, at an ambient temperature of 300°C. Such a high temperature made the use of standard triax cable impossible. In consequence, we built a high-temperature triax cable by using a high-temperature coax (Omerin Siliflam THS1200) shielded with a copper braid. As a result, the leakage current is found to be around 70 pA at 300°C.

During the test, a voltage bias of 1100 V is applied to the test vehicles using the SMU.

Results and discussion

The results are summarized in Table 5. It can be seen that the leakage current measurements vary widely between test vehicles (from 186.6 to 1.7 nA). This is probably due to the sample preparation: for both DCB test vehicles, the electrodes were etched in the copper layer of a plain DCB board. Any residual copper left after the etching step would increase the leakage current. However, for a given test vehicle, we found no evidence of silver migration: the leakage current only changed by a few nano-amps, over periods reaching 10 days.

For the bare-copper sample, the tests were stopped after a few days, as we suspected that the oxide layer that formed over the copper would form an electrical insulation.

For the gold-plated test vehicle, although no silver dendrite can be observed, it must be noticed that a black deposit developed around the cathode (Figure 17). The composition of this deposit is unknown, but it has no effect on the leakage current (Table 5). Investigation is ongoing to determine its nature.

In order to have an estimation of the electrical field experienced by the silver deposit on the DCB test vehicles, a finite-elements simulation has been performed (Figure 18). It shows that the copper electrodes provide a shielding effect, with an electrical field decreasing rapidly a few hundreds of microns away from the gap. However, for a silver deposit located as close to the gap as in Figure 17, the silver deposit experiences electrical fields in excess of 1 kV/mm (Figure 18). This is much more intense than studied in [START_REF] Mei | Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate Between 250°C and 400[END_REF] and should result in the migration of silver particles.

Regarding the screen-printed silver test vehicle, no migration has been detected yet. This is surprizing, as the model proposed in [START_REF] Mei | Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate Between 250°C and 400[END_REF] would predict a lifetime of 8 hours in this configuration (2 mm gap between electrodes, 550 V/m electrical field).

Conclusion

For the study of the thermal resistance, and although die attaches were sintered in very different conditions, no difference could be demonstrated among the silver joints. This proves that the silver sintering technology produces die-attaches with excellent thermal performances, well above that of the TIM layers or of the alumina substrate. New test vehicles are being assembled to follow the thermal resistance evolution during thermal cycling ageing.

No evidence of silver migration was found, which is in contradiction with the results available in the literature. This is surprizing, and further investigations are needed. Future tests will include test vehicles with a narrower gap, to increase the electrical field and reduce the dendrite length required to produce a short circuit. This will constitute a worst-case configuration that will help proving (or disproving) the risk of silver migration. In any case, the present results show that power modules should be able to operate for relatively long periods of time at milder temperatures (up to 200°C) without any risk of silver migration.

Figure 4 :Figure 3 :

 43 Figure 4: The junction calibration of the chip

Figure 2 :

 2 Figure 2: test bench for thermal resistance measurement

Figure 5 :

 5 Figure 5 : Test vehicle #4 fixed on the cold plate with thermal grease (left) and test vehicle #3, attached using the double adhesive TIM layer (right)

Figure 8 Figure 7 : 8 Figure 6 :

 8786 Figure 8 Schematic description of the thermal resistance measurement method (R th )

Figure 10 :

 10 Figure 10: Heat dissipation of the test vehicle 2 with a 50 microns silver joint thickness and 6,29 Watts power dissipation in the chip localisated in the middle of the DCB substrate and the double sided thermal adhesive properties.

Figure 9 :

 9 Figure 9: Heat dissipation of the test vehicle 1 with a 50 µm silver joint thickness and 6.29 Watts power dissipation in the chip and the double sided thermal adhesive properties. (Corresponding to the first line in Table4)

Figure 15 :

 15 Figure 15: Image of the screen-printed silver electrodes after sintering, with a 2 mm gap.

Figure 14 :

 14 Figure 14: The test vehicle etched in a DCB substrate with a nickel finish (before gold plating). The gap between electrodes is 2 mm, and the sample size is 34x10 mm².

Figure 13 :

 13 Figure13: Classical process of silver migration[START_REF] Yanga | Initial Stage of Silver Electrochemical Migration Degradation Microelectronics Reliability[END_REF]

Figure 12 :Figure 11

 1211 Figure 12: The junction temperature is influenced by the location of the chip on the DCB substrate

Figure 17 :

 17 Figure 17: Pictures of the inter-electrodes space, for the gold-plated sample, after 0, 113 and 257 h at 300°C. The clear square corresponds to the silver deposit. The apparent change in color for the gold is due to the camera settings, and is not an actual change. A black deposit formed around the cathode (left electrode) during the test.

Figure 16 :

 16 Figure 16: Schematic description of the silver migration test bench. A garded measurement is used to prevent current leakage in the interconnections.

Figure 18 :

 18 Figure 18: 2-D Simulation (Comsol Multiphysics) of the electrical field experienced by the test vehicle (Figure 14), with a 2 mm gap. The picture represents two copper conductors (dark blue), one with the silver deposit (left), on an alumina substrate, in air.

Table 1 :

 1

	TV #	Silver paste	Chip location	Pressure [MPa]	Chip deposit
	1	Micro	Side	6.4	Before paste drying
	2	Micro	Center	6.4	After paste drying
	3	Nano	Center	6.4	Before paste drying
	4	Nano	Center	1.3	Before paste drying

sintering process data. Micro: Heraeus paste; Nano: NBE Tech paste; side: die located near the edge of the substrate; center: die located at the center of the substrate

Table 3 :

 3

Thermal resistance measurement of the test vehicles wirebonding, with 3.8 amps current load.

Table 4 :

 4 Thermal resistance measurement of two types of test vehicle using the micro-and nano-silver particles for the die-attach and aluminium wirebonding, with 5 amps current load.

Table 5 :

 5 Leakage current measured on the test vehicles with time, for a 1100 V bias and 300°C ambient temperature. Current values are expressed in nano-amps.

	Test vehicle	Initial	1 day	3 days	10 days Remark
	DCB/bare copper	186.6	180.0	160.0	Test stopped due to copper oxidation
	DCB Ni/Au plating	15.8		9.8	9.8
	Screen-printed Silver	1.7	2.4	2.3	Test ongoing


B Mouawad
email: bassem.mouawad@insa-lyon.fr

Maher Soueidan
email: maher.soueidan@insa-lyon.fr

D Fabrègue

C Buttay
email: cyril.buttay@insa-lyon.fr

V Bley

B Allard
email: bruno.allard@insa-lyon.fr

H Morel

Full densification of Molybdenum powders using Spark Plasma Sintering

Keywords: Molybdenum, Spark Plasma Sintering, densification, microstructure

Pure molybdenum powder was sintered using Spark Plasma Sintering under various temperatures, and holding times, under a pressure of 77 MPa and a heating rate at 700°C/min.

After sintering, a carbide layer was observed at the surface. The carbide layer thickness, the relative density of the sample as well as the microhardness and the grain size of Mo were measured. The carbide thickness depends on time and temperature and it was found that the carbide layer grows in a parabolic manner, with the activation energy of carbon diffusion in Mo being equal to 34 Kcal/mol. The densification of Mo is controlled mainly by the sintering temperature and the holding time. The molybdenum powder was successfully consolidated by SPS in short times. A relative density of 100% is achieved at a sintering temperature of 1850°C and a holding time of 30 minutes. It was shown that the hardness decreases slightly with temperature and time. It should be related to the increase in grain size with the sintering temperature and time.

I. Introduction

Molybdenum has a body-centered cubic structure with a melting point of 2610°C and a density of 10.22 g/cm 3 and presents a great potential to become an important refractory metal.

The refractory properties of molybdenum reflect the high strength of its inter-atomic bonding [1]. This material has been used for high temperature applications in a variety of industries. In addition, the relatively low thermal neutron cross-section of Mo makes it suitable for nuclear applications. The unique combination of physical, chemical and mechanical properties of Mo makes it an ideal material for a variety of engineering applications where high temperature resistance, and ductility are key issues [1][2][3][4][5].

Molybdenum has been used in many electronic applications from the earlier days of the technology. This metal combines both satisfying thermal conductivity (142W/m.K at 20°C) and a coefficient of thermal Expansion (CTE) which offers a good match with that of silicon (4.8 and 2.6 ppm/°C at 25°C respectively). Thanks to that large area silicon devices (up to 10 cm in diameter) can be directly bonded onto a molybdenum disk to improve their mechanical strength in high power, press-pack packages [6]. Such components have been manufactured for several decades, using molybdenum disks directly stamped from fully dense wrought sheet. In this way, the deleterious effects of residual porosity on the thermal conductivity are eliminated.

Generally, powder metallurgy (P/M) has been used for the production of bulk Mo components [3][4][5]. High sintering temperatures, in the range of 1800 -2000°C, with long sintering m qu f fic on above 90% of the theoretical density for refractory metals. Because of the difficulty in achieving full-fic of Mo, much research focused on the enhancement of the sinterability. In this regard, one of the suggested processes for enhancing sinterability is the activated sintering process in which a metal such as Ni, Pt, Pd or Co is added to Mo [7][8][9][10][11][12][13]. However, these elements can degrade the electrical and thermal properties of the Mo. To achieve nearly full density in short processing time intervals, while concurrently minimizing grain growth-related degradation of mechanical properties, a few non-conventional techniques have been reported. Explosive consolidation [START_REF] Murr | Shock Waves and High Strain-Rate Phenomena: Concepts and Applications[END_REF][START_REF] Meyers | [END_REF][16][START_REF] Staver | Proc. Int. Cogerence Shock Waves High Strain Rate Phenom. Met[END_REF], ceracon rapid omni-directional ROC [START_REF] Kasiraj | [END_REF][19][20], plasma-activated sintering PAS [21,22], microwave sintering [23,24], are a few of the rapid consolidation techniques reported in the literature. First studies on short-time sintering of molybdenum powders by Plasma Pressure Compaction (P 2 C) are reported in [25,26]. Micro-sized powders (average size 47 µm) were consolidated at 1650°C, 48 MPa, for 1-2 min, up to a relative density of 97%.

Nano-sized powders (average size 0.1 µm) were sintered at lower temperature (1400°C), 48

MPa, for 3 min, with a relative density of 97%. Nano-and micro-sized consolidated samples showed a microhardness varying from 2.18 GPa (222 Hv) to 2.16 GPa (220 Hv) respectively.

Spark plasma sintering (SPS) is a technique developed h 50' h b c m c

or metal powder to be fully densified at relatively low temperature and in very short time [27,28]. It is similar to conventional hot-pressing, but the heating is obtained by applying an intense pulsed current through electrodes located on top and bottom of a sintering die. This current was first thought to generate spark discharge at the interface between particles, activating and purifying the particles surface [29] but now, the absence of plasma seems to be generally admitted [30]. Anyway the presence of current seems to lead heat and mass transfer to be completed in extremely short time. Further advantages are sintering of powder without any additives, no need for cold compaction and lower sensitivity to the initial powders characteristics. SPS is therefore an economical alternative to conventional sintering. Recently, Ohser-Wiedemann et al. have achieved a relative density of 95% of a sintered bulk of micropowder molybdenum using SPS at 1600°C, an external pressure of 67 MPa and a holding time at maximum temperature of 3 min. Moreover, the hardness of all investigated samples are in the same range, independent from the sintering conditions. The average hardness is 2.05 GPa (209 Hv) [31].

In this research paper, we investigate the consolidation of Molybdenum powders by Spark Plasma Sintering. We study the influence of SPS processing conditions on the density, hardness and grain size of bulk molybdenum specimens. In certain conditions the final density obtained was 100%.

II. Experimental procedures

Commercial Molybdenum powder (Goodfellow, Cambridge, UK, 99.99+% purity) with a particle size ranging from 55µm to 355 µm was consolidated by the SPS technique. The as-received powder was found to have an average grain size equal to 16 µm (see figure 1). In this technique, molybdenum powders were poured directly, with no special treatment into a cylindrical graphite die and without any additive or binder. The experiments were carried out using the SPS equipment (FCT HPD 25) available at the MATEIS laboratory.

In each experiment, the molybdenum powders are introduced in a carbon die of 20 mm inner diameter. Samples 20 mm in diameter and 2 mm high were sintered in vacuum (10 -2 Torr), with temperatures ranging from 1200 to 1950°C. The uniaxial pressure was fixed at 77

MPa during the consolidation and maintained during the cooling down to room temperature.

This pressure corresponds to the maximum pressure that the graphite die can sustain. The temperature was measured with an optical pyrometer focused on the surface of the graphite die (see figure 2) and automatically regulated from 600 °C to the final sintering temperature.

The temperature of 600 °C was reached via a preset heating program as the pyrometer could not sense low temperature (below 600 °C). The heating rate was fixed to 700 °C/min and the sintering time was varied from 5 to 30 minutes. It is important to note that carbon foils were placed under and below the powder to assure an easy dismantling of the sintered samples.

To study the microstructure, the samples were prepared for examination in optical microscopy to identify: 1) the grain size and morphology, and 2) the presence and distribution of processing related defects. The defect features include: a) irregular surface cracks, b)

string-like and angular features, and c) microscopic pores and voids. The surfaces and cross sections (sections parallel to the sintering pressure) of the sintered samples were ground and polished. An initial wet grind and coarse polish on progressively finer grades f Bu h ' Ultra Plan, Ultra Pad and Texmet polishing cloths. Finish polishing was achieved using 1 µm diamond paste. The samples were then cleaned to remove the lubricant. The cleaning process found to be successful consisted of (i) ultrasonic cleaning in acetone to remove the lubricant, (ii) ultrasonic cleaning in trichloroethylene, (iii) ultrasonic cleaning in microelectronic grade ethanol, (iv) followed by rinsing in DI water and blow drying with nitrogen gas. The aspolished samples were wet chemically etched in H 2 SO 4 :HNO 3 :H 2 0 (1:1:1) for about 1 minute.

The polished and etched samples were also examined by scanning electron microscope (SEM), with the objective of determining the morphology and distribution of sintering defects porosity. The SEM study was performed on a Philips XL20.

Precise density measurements of the consolidated bulk molybdenum samples were m u g h ch m ' p c p cc g T B328-94. The relative density was calculated based on the theoretical density of Mo (10.22 g/cm 3 ) listed in [1].

The microhardness and hardness measurements have been realized on a Vickers machine Wolpert (V-testor VDT 11) using a 0.3 kg charge and a Shimadzu HSV 20 using a 5

Kg charge. The hardness value given for each sample is the average of at least 10 measurements taken randomly.

The grain size was measured using an optical microscope coupled to the image analysis software Grani. The method used to calculate the grain size is the intercept one (ASTM 112).

For elementary analyses, Secondary Ion Mass Spectroscopy (SIMS) measurements were done using a Cameca IMS 4f spectrometer with O 2+ primary ion source. The impact energy was 15 keV.

For structural analysis, the samples were characterized by X-ray diffraction (XRD:

X'Pert Pro MPD from Panalytical, equipped with an X'Celerator detector monochromator, CuKalpha radiation, Almelo, The Netherlands).

III. Experimental results and discussion

III.1. Molybdenum carbide formation

In order to check the SPS consolidated samples, the cross-sectional microstructure was imaged using an optical microscope with bright field illumination for low magnifications and One other peak was detected and can be attributed to molybdenum. The results presented in this figure show, as expected, a linear increase in product thickness with time. In that case, only the sintering time is taken into account since the heating rate as well as the cooling rate is high. This assumption could involve some errors for shorter times.

Fig. 7 presents the evolution of the growth rate as a function of the inverse of the temperature. One can see that the growth rate increases when the temperature increases. From the Arrhenius plot of Fig. 7 the apparent activation energy (E A ) of the growth process was calculated. E A was estimated to be equal to 34 Kcal/mol (142.3 KJ/mol), which is close to the values found in the literature [31,35,36].

III.2. Molybdenum consolidation III.2.1. Density

To measure the actual density of our Mo samples, the Mo 2 C layers were removed from both sides by polishing. The calculated relative densities with and without carbide layers are listed in table 1. We can see that the relative density without carbide is higher than the one with carbide at a given sintering parameter. This can be explained by the difference in density between Mo 2 C (8.9 g/cm 3 ) and Mo (10,22 g/cm 3 ). ). In this case, the maximum relative density of 99.5% can be reached at a sintering temperature of 1950°C. Moreover, our results are in good agreement with previous results obtained by P 2 C [25,26] and SPS [31], where a relative density of 98% was measured at 1650°C for P 2 C and 95% at 1600°C for SPS. Fig. 8b shows the relationship between the relative density and the holding time at 1850°C and 77 MPa. This demonstrates that when the holding time is increased from 5 to 30 minutes, the relative density increases from 98.6 % to 100%. To our knowledge, a relative density of 100% for molybdenum has never been reported yet using a sintering technique.

III.2.2. Hardness

Vickers hardness measurements made at different locations on the surface of each samples are summarized in table 1. The samples were polished to a mirror finish before the hardness measurement. The measured hardness was observed to be near-uniform throughout each sintered sample, indicating uniform densification. Polishing of the sample surface reduces the spread in measured hardness values. Fig. 8 (a and b) shows the variation of the hardness as a function of temperature and holding time respectively. At 1200 and 1350°C, the samples were not totally densified, so the hardness measurement are not representative. The hardness decreases very slightly when increasing the sintering temperature (it varies from 160 at 1450°C to 149 Hv5 at 1950°C, see fig. 8a) as well as when increasing holding time. This decrease in hardness could be explained by the increase of grain size with temperature (Hall and Petch relationship). This will be explored in the next section. In both cases, the variation of hardness is very slight and could be simply due to experimental scattering.

The hardness values obtained in this study could be compared to the ones already published.

However, the literature concerning the hardness exhibits high variations according to the initial state of the powders and to the process parameters used for their sintering. For example, Tuminen and Dahl [START_REF] Tuominen | 108th AIME Annual Meeting[END_REF] reported microhardness values of about 270 Hv 10 for unsintered Mo powder extrusions, and 250 Hv 10 for conventionally sintered Mo bars. These high values are easily understandable since the powder come from extrusion and thus must be highly deformed leading to a high hardness value. Kim et al. [START_REF] Kim | [END_REF] obtained hardness values equal to 218 Hv 0.1 for nanopowder and 191 Hv0.1 for commercial powder, which is in agreement with our results taking into account the measurement errors. At last, Srivatsan et al. [26,39] reported hardness values of about 236 Hv10 by Plasma Pressure Compact with consolidation at a temperature of 1650°C for Mo powder (grain size of about 47 µm). Finaly, using SPS techniques, R. Ohser-Wiedemann [31] reported an average hardness value of about 209 Hv 0.1, independently on sintering condition, which is consistent with the values obtained in this study.

III.2.3. Grain size

The cross-sectional microstructure of molybdenum sintered at 1850 °C for 5 min, and with a pressure of 77 MPa is shown in Fig. 9a after wet chemical etching in H 2 SO 4 :HNO 3 :H 2 0

(1:1:3). No pores were observed between the grains. It was found that the average grain size (82 μm) g h h g g z (16μm). Fig. 9b shows the grain size distribution and we can also see that the grains grow discontinuously. From fig. 9a andb could suggest that secondary recrystallization occurs during sintering. Fig. 9 (c andd) shows the variation of average grain size as a function of temperature and time respectively. The sintering temperature has a significant effect on grain size. When the sintering temperature is under 1450°C, the grain growth is slow and limited, the average grain size remains equal to h g z (16 μm). Wh h g mp u f m 1450 1950°C h grains grow significan w h g g z g g f m 33 μm 108 μm respectively (shown in fig. 9c). Comparing to those in the literature, the same phenomena was obtained with the ref [31]: the grain size start increasing at high temperature (higher than 1400). This result was obtained by SPS. In contrast, when using a conventional technique such as isothermal sintering the grain growth starts at temperature lower than 1100°C [START_REF] Kim | [END_REF].

This big difference may be due to the fast heating ramp of the SPS. Note that the comparison of the average grain values is very difficult because the as-received Mo powder and grain size are not the same.

The relationship between the grain size and the holding time is presented in fig. 9d. In this series of experiments, the temperature was set to 1850°C, the pressure to 77 MPa and the sintering time ranged from 5 to 30 minutes. This demonstrates that when the holding time is

c f m 5 30 m h g z c f m 82 102 μm. Th ff c
grain size growth can be explained by the fact that grain grows with an exponential function of the temperature but follows a logarithmic law according to the time.

Considering the maximal sintering temperature is 1950°C, It can be concluded that the main temperature range to affect the grain growth is 1450-1950°C. Below 1450°C, the sample exhibits a low density (less than 95%). Thus the heat generated by Joule Effect is mainly used to increase the density of the sample, accelerating the diffusion of species to enlarge the bridges between particles. However, at higher densities, when the sample is close to be fully dense, grain growth can occur like in bulk samples. Considering the effect of time at a fixed temperature, as the sample exhibits already a high density at the temperature considered, the grain growth occurs. Anyway the effect of time is lower than that of temperature, which is well known in homogenization heat treatment of steels.

IV. Conclusion:

In

a

  figure 4 clearly shows that the layer contains only two elements (Mo and C). This means that the M1 layer is composed by Mo and C, which corresponds to molybdenum carbide.

  not observed at lower temperature, but it could be due to the removing of the thin carbide layer at the same time as the carbon foil. In order to clarify the formation of molybdenum carbide, it is important to identify the source of carbon involved in the formation of this molybdenum carbide. The only possible C source in contact with the Mo powders is the graphite die and the carbon foil because they are the only elements in contact with Mo powders.The growth of the molybdenum carbide layer formed at 1850°C and at 77 MPa is shown in Fig.6. In this figure, the square of the thickness is plotted against time. The thickness values plotted are the average values of the Mo 2 C formed over more than 10 measurements.

Fig 8 (

 8 Fig 8 (a and b) shows the relative densities of the sintered samples without the carbide layer as a function of temperature and holding times respectively. It was shown that the relative density increases with sintering temperature for a given time (5 min) and pressure (77 MPa) (see fig 8a). In this case, the maximum relative density of 99.5% can be reached at a

Figure 2 :

 2 Figure 2: Schematic drawing of Spark Plasma Sintering apparatus.

Figure 3 :

 3 Figure 3: Cross sectional view of molybdenum sintered by SPS at 1850°C for 5 min, and with a pressure of 77 MPa (a) optical microscope and (b) Scaning Electron Micrograph. M1 and M2 represent the two different microstructures obtained in the same sample.

Figure 4 :

 4 Figure 4: SIMS spectra collected on the superficial layer (M1) of the sample presented in figure 3.

Figure 5 :

 5 Figure 5: XRD pattern of the molybdenum of the sample present in figure 3.

Figure 6 :

 6 Figure 6: The square of layer thickness of the Mo 2 C as a function of time. The dotted line is plotted to guide the eye. The growth rate is then parabolic in time.

Figure 7 :

 7 Figure 7: Evolution of the Mo 2 C layer growth rate as a function of the temperature. From the Arrhenius plot the apparent activation energy (E A ) was estimated to be at 34 Kcal/mol.

Figure 8 :

 8 Figure 8: Evolution of relative density and hardness a) as function of sintering temperature for a given time at 5 min and b) as function of holding time for a given temperature at 1850 °C.

Figure 9 :

 9 Figure 9: a) Cross-sectional optical micrograph of bulk molybdenum after wet chemical etching at 1850°C for 5 min, and with a pressure of 77 MPa, b) grain size distribution, c) effect of sintering temperature on average grain size at a given time (5min) and c) effect of holding time on average grain size at 1850°C.

Figure 1 :

 1 Figure 1: SEM micrograph of commercial Mo Powder before consolidation and after wet ch m c ch g. Th g g z b u 16 μm.

Figure 2 :

 2 Figure 2: Schematic drawing of Spark Plasma Sintering apparatus.

Figure 3 :

 3 Figure 3: Cross sectional view of molybdenum sintered by SPS at 1850°C for 5 min, and with a pressure of 77 MPa (a) optical microscope and (b) Scaning Electron Micrograph. M1 and M2 represent the two different microstructures obtained in the same sample.

Figure 4 :Figure 5 :

 45 Figure 4: SIMS spectra collected on the superficial layer (M1) of the sample presented in fig.3.

Figure 6 :

 6 Figure 6: The square of layer thickness of the Mo2C as a function of time. The dotted line is plotted to guide the eye. The growth rate is then parabolic in time.

Figure 7 :

 7 Figure 7: Evolution of the Mo 2 C layer growth rate as a function of the temperature. From the Arrhenius plot the apparent activation energy (E A ) was estimated to be at 34 Kcal/mol.

Figure 8 :Figure 9 :

 89 Figure 8: Evolution of relative density and hardness a) as function of sintering temperature for a given time at 5 min and b) as function of holding time for a given temperature at 1850 °C.

Table 1

 1

gathers all the results from the sintering experiments in terms of carbide thickness, densities, hardness and grain size. From table 1, one can see that the molybdenum carbide is obtained during sintering at high temperature (>1500°C). This mean that the carbide layer was

  this work, the molybdenum powder was sintered by SPS. Pressure and heating rate were fixed at 77 MPa and 700°C/min respectively. The sintering temperature was varied from 1200 to 1950°C and the holding time was varied from 5 to 30 minutes. The results of this investigation demonstrate that molybdenum powders can be successfully consolidated by SPS in very short times. The contact of the Mo powders with the graphite die and the carbon foil leads to the diffusion of carbon inside the samples to form a molybdenum carbide layer. The thickness of this layer depends on temperature and time and can reach 211 µm at 1950°C. The carbide layer grows in a linear manner, with an activation energy of carbon diffusion in Mo equal to 34 Kcal/mol.A relative density of 100% was achieved at a sintering temperature of 1850°C and a holding time of 30 minute. It can be concluded that the relative density is controlled by temperature and time. Hardness measurements were made at different locations on the surface of each samples. It was shown that the hardness of Mo bulk decrease very slightly with temperature and time. The grain growth during SPS was controlled by the sintering temperature and time. The sintering temperature has a significant effect on grain size. When the sintering temperature is under 1450°C, the grain grow is slow and limited, while sintering temperature exceeding 1450°C result in the grain growing significantly. Moreover, it was also shown that the grain size increases with time.

Table Captions :Table 1 :

 Captions1 Sintering parameters and results of density, hardness measurements and grain size

	Figures Captions
	Figure 1: SEM micrograph of commercial Mo Powder before consolidation and after wet
	chemical etching. The average grain size is about 16 µm.

Table 1 :

 1 Sintering parameters and results of density, hardness measurements and grain size

	Sample	Sintering	Holding	Carbide	Relative	Relative	Mo	Grain
		temperature	Time	Layer	density with	density	Hardness	size
		(°C)	(min)	Thickness	carbide layer	without	(Hv5)	(µm)
				(µm)	(%)	carbide layer
						(%)
	1	1200	5	-	84.4	84.4	133	16
	2	1350	5	-	91.2	91.2	132	16
	3	1450	5	-	94.8	94.8	160	33
	4	1550	5	39	96.1	96.5	157	36
	5	1650	5	56	96.1	97.0	161	42
	6	1750	5	79	97.7	98.4	150	63
	7	1850	5	132	97.7	98.6	150	82
	8	1950	5	211	96.9	99.5	149	108
	9	1850	10	187	97.3	99.6	157	83
	10	1850	20	362	95.7	99.8	156	92
	11	1850	30	391	94.5	100	156	102

Acknowledgement

The authors would are pleased to acknowledge important contributions used in the body of this work from G. Bonnefont and F. Mercier from MATEIS for SPS processing, as well as to thank M. Perez from MATEIS for fruitful discussions.


Bassem Mouawad
email: bassem.mouawad@insa-lyon.fr

Cyril Buttay
email: cyril.buttay@insa-lyon.fr

Maher Soueidan
email: maher.soueidan@insa-lyon.fr

Morel Hervé
email: herve.morel@insa-lyon.fr

Ampère

Vincent Bley
email: vincent.bley@laplace.univ-tlse.fr

Damien Fabregue
email: damien.fabregue@insa-lyon.fr

Sintered molybdenum for a metallized ceramic substrate packaging for the widebandgap devices and high temperature applications

Keywords: Wide-bandgap devices, aluminium nitride, molybdenum, Spark Plasma Sintering I

Silicon Carbide (SiC) is a good candidate for high temperature power electronic applications. To ensure good reliability, packaging materials with a coefficient of thermal expansion (CTE) matching that of SiC are needed. A metallized ceramic substrate based on aluminium nitride (AlN) and molybdenum (Mo) is reported in this paper. This substrate is built using a spark plasma sintering equipment. Results show that a dense Mo layer can be sintered on an AlN plate, with good adhesion, forming a Mo/AlN/Mo structure with well-matched CTEs.

INTRODUCTION

Wide-bandgap semiconductors devices, such as silicon carbide (SiC), have pushed the boundaries in terms of power density, conversion efficiency or switching speed compared to silicon [START_REF] Raynaud | Comparison of high voltage and high temperature performances of wide bandgap semiconductors for vertical power devices[END_REF]. In particular, SiC devices can operate at much higher temperature [START_REF] Buttay | State of the art of high temperature power electronics[END_REF]. These devices are assembled into modules, which provide electrical interconnects, thermal management (by using a metallized ceramic substrate, a heatspreader and a heat sink) and electrical insulation (provided by the ceramic layer of the substrate). However, this package, which comprises many different materials (ceramic, metals, organic materials…) with vastly different Coefficients of Thermal Expansion (CTE), is very sensitive to thermal cycling. Indeed, any change in the operating temperature will result in a variation of the mechanical stresses experiences at the various interfaces. Repetitive temperature variations (thermal cycling conditions), and the associated stress changes, are a source of mechanical fatigue, which can lead to failure of the package.

With the extended temperature range (and therefore with the extended thermal cycling) permitted by SiC devices, the package becomes a limiting factor, and new technologies must be proposed to achieve more robust modules [START_REF] Neudeck | High-temperature electronics -a role for wide bandgap semiconductors?[END_REF].

II. PACKAGING

A. Metallized ceramic substrate

In this paper, we focus on a specific part of the package, namely on the metallized ceramic substrate. This substrate concentrates many of the challenges described above: it has a large surface (usually in the order of around 10~cm²), associates materials with different CTEs (a ceramic and a metal), and must provide good electrical conductivity (of the metal layer) and thermal conductivity (of the metal and ceramic layers).

The most substrate used in conventional packaging is the Direct Bonded Copper (DBC). The DBC is formed by a ceramic layer (Aluminium oxide -Al 2 O 3 -or aluminium nitride -AlN-) between two copper layers. This DBC structure leads to a high differential stress due to the materials CTE mismatch that may cause failure at high temperature [START_REF] Dupont | Evaluation of substrate technologies under high temperature cycling[END_REF] (see table below).

B. Materials

Some of the materials found in a high temperature power module are listed in Table I, along with their thermal properties Recently, aluminium nitride, which has CTE properties very close to that of SiC, was proposed as a substrate material for high-temperature device packaging [START_REF] Bratcher | Aluminium nitride package for high-temperature applications[END_REF][START_REF] Martin | High-temperature aluminium nitride packaging[END_REF][START_REF] Chasserio | Ceramic substrates for high-temperature electronic integration[END_REF]. A gold thick-film metallization on AlN ceramic-based 500 °C packaging for SiC devices has been demonstrated [START_REF] Chena | Material system for packaging 500°C SiC microsystems[END_REF]. The electrical interconnection system of this approach has been tested in air environment for over 5000h at 500 °C and a packaged SiC test device operated for over 1000h. Other approaches found in the literature include a molybdenum (Mo) thick film metallization performed by screen printing of molybdenum-manganese paste on AlN and tungsten (W) thin film metallization sputtered on AlN with a proprietary adhesion enhancer (i.e. Titanium (Ti)) since W does not adhere to AlN [START_REF] Savrun | An aluminum nitride package for 600 °C and beyond[END_REF]. The adhesive strength for both, Mo and W metallizations, was found to be 87 MPa and 84 MPa respectively after a heat treatment of 1100 °C for 30 min.

As with these technologies, the work described here focuses on replacing the metallized ceramic substrate used in conventional packaging systems to a new one that will be able to withstand thermal cycling.

C. Molybdenum / AlN

As discussed in the last section, for the high temperature operation (and wide-range thermal cycling), both the ceramic and the metal layers need to have a CTE close to that of the SiC devices. Mo was found to be a promising candidate for high temperature metallized ceramic substrate and has many good advantages: (i) similar CTE to AlN and SiC (4.8x10 -6 /°C for Mo vs. 4.1x10 -6 /°C for AlN vs. 3.7x10 -6 /°C for SiC, see table I), (ii) good adhesion material, (iii) relatively low cost, (iv) it is widely used in electrical contact for high temperature. The disadvantages are: (i) it is very difficult in achieving full densification and (ii) very difficult to assembly with AlN without any additives using standard solution [START_REF] Savrun | An aluminum nitride package for 600 °C and beyond[END_REF]. Preliminary results in the densification of pure Molybdenum shows that a full densification can be obtained using a new sintering technique such as Spark Plasma Sintering [START_REF] Mouawad | Full densification of Molybdenum powders using Spark Plasma Sintering[END_REF]. We suggest to use the same sintering technique to realize both molybdenum powder sintering, and Mo / AlN layers bonding simultaneously.

III. EXPERIMENTAL PROCEDURE

A. Spark plasma sintering

As with a classical sintering press, a Spark Plasma Sintering (SPS) system applies both pressure and heat to samples. The difference is that the heat is provided by a pulsed DC-current circulating through a graphite die (or through the sample if it is conductive), whereas the conventional sintering press uses external heating elements. SPS offers rapid sintering, with ramps of up to 700 °C/min using current pulses of several kA. This has great potential for achieving fast densification results with minimal grain growth in a short sintering time.

The experiments were carried out in a SPS FCT systeme GmbH, HP D 25 model. This apparatus consists of a uniaxial press with a maximum force of 250 kN and a power supply capable of producing a pulsed current DC-current with a maximum of 8000 A at 8 V.

B. Samples preparation

Commercial Molybdenum powder (Goodfellow, Cambridge, UK, 99.99+% purity) with a particle size ranging from 55 µm to 355 µm was consolidated using SPS technique. In this technique, molybdenum powders were poured directly, with no special treatment into the cylindrical graphite die of SPS without any additive or binder.

For the ceramic layer, disks of 19,8 mm in diameter were laser-cut into commercial plates of AlN with two different thicknesses (633 µm and 1270 µm). The disks were cleaned using (i) ultrasonic cleaning in acetone, (ii) ultrasonic cleaning in microelectronic grade ethanol, (iii) immersion in H 2 SO 4 :H 2 O 2 (1:1) to ensure non-existent of any metal particles (iv) followed by rinsing in DI water and blow drying with nitrogen gas. Two sets of experiments were performed. The first set focussed on the molybdenum sintering parameters, and no AlN disks were used, Molybdenum powder samples of 20 mm in diameter and around 2 mm-thick were sintered in vacuum (10 -2 Torr), without any additives. The second set of experiments was targeted at the Mo/AlN assembly. The 19,8 mm disks of AlN were put in 20 or 40 mm diameter graphite dies, and covered with a 1 mm-thick layer Mo powder. These samples were also sintered in vacuum (10 -2 Torr).

Precise density measurements of the consolidated bulk molybdenum samples were made using the Archimedes' principle according to ASTM Standard B328-94. The relative density was calculated based on the theoretical density of Mo (10.22 g/cm 3 ).

At the end of each experiment, the samples were crosssectioned, mounted in a hot mounting resin (Presi Phenolic resin) and then prepared metallographically (mechanical a. Molybdenum sintering parameters reported in [START_REF] Mouawad | Full densification of Molybdenum powders using Spark Plasma Sintering[END_REF] grinding and polishing down to 0,05 µm silica slurry). The samples were examined using optical and scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDX) was used to identify the interface materials reaction.

C. Mo powder Sintering

The sintering of the molybdenum powder using a SPS is presented in details in [START_REF] Mouawad | Full densification of Molybdenum powders using Spark Plasma Sintering[END_REF]. Table II presents the relative density in function of sintering temperature, ranging from 1200 °C to 1950 °C with a holding (sintering) time ranging from 5 to 30 min, under a fixed pressure of 77 MPa and a fixed heating rate of 700 °C/min. A relative density of 100% is achieved at a sintering temperature of 1850 °C hold for 30 min.

D. Molybdenum / AlN assembly

First, Molybdenum powder was sintered on a disk of AlN with thickness of 633 µm using a 20 mm diameter's graphite die. We achieved a good assembly at the Mo/AlN interface for all samples. However, we noticed that the AlN layer fissured during the process. We attributed these cracks to the pressure applied by the SPS press. However, even at the lowest force setting (5 kN, or 16 MPa for a 20 mm sample diameter), cracks occurred. To avoid cracks, we used a disk of AlN with a higher thickness (1.27 mm), and placed it in a 40 mmdiameter graphite die. Fig. 2 shows a cross-sectional SEM photo of a Mo/AlN interface, for a sample assembled at 1850 °C with a force of 5 kN applied by the press (which corresponds to 4 MPa in the 40 mm diameter die) with a holding time of 5 min. It shows the sublimation of a layer of AlN occurring during the sintering. Moreover, small cracks can also be seen, but they are limited to the neighbourhood of the interface. A close-up is given in fig. 3. Some EDX characterization were performed and, shows that at the x marks, we have an area in the ceramic layer rich in aluminium. This means that some reaction occurred at the AlN/Mo interface.

IV. DISCUSSION

SPS allows for molybdenum sintering at "low" temperature (much lower that the melting point of molybdenum, at 2623°C), with a high density (up to 100%). Using the same equipment, a bond between molybdenum and aluminium nitride was successfully achieved, although some sublimation of AlN occurred. To reduce this effect, we can reduce the sintering temperature. The microstructure at the interface between Mo and AlN must be conducted in order to get a better insight of the phenomena that have taken place. Moreover, a bending test will now be carried out to ensure that the bonding strength is good enough for our applications. At last but not least an electrical characterization (dielectric strength) is required as well as a thermal cycling to study the reliability of the substrate.

V. CONCLUSION

Molybdenum powder was sintered to bulk using a Spark Plasma Sintering technique. This technique allows to achieve full densification in a few minutes. Molybdenum powder was sintered on disks of AlN, at 1850 °C for 5 min with a pressure of 4 MPa, showing a chemical reaction at interface level. Small cracks can also be seen, but they are limited to the neighbourhood of the interface.

Fig. 1

 1 shows a cross-sectional optical micrograph of a bulk of molybdenum sintered at 1850 °C for 5 min. The sample was chemically etched in H 2 SO 4 :HNO 3 :H 2 0 (1:1:3) solution. It shows that there are almost no pores between grains of Mo.

Figure 1 .

 1 Figure 1. Cross-sectional optical micrograph of bulk molybdenum

Figure 2 .Figure 3 .

 23 Figure 2. Cross-sectional SEM photo of Mo/AlN interface

TABLE I .

 I THERMAL PROPERTIES OF DIFFERENT MATERIALS OF APOWER ELECTRONIC MODULE[START_REF] Savrun | An aluminum nitride package for 600 °C and beyond[END_REF]

		Thermal properties
	Materials	CTE (x10 -6 / °C)	Thermal conductivity (W/m.K)
	Copper (Cu)	17	398
	Molybdenum (Mo)	4.8	138
	Tungsten (W)	4.5	178
	Gold (Au)	14	315
	Aluminium Oxide (Al2O3)	7.2	25
	Silicon Carbide (SiC)	3.7	300
	Aluminium Nitride (AlN)	4.1	200

TABLE II

 II

		.	MOLYBDENUM'S SPS SINTERING PARAMETERS
			Molybdenum sintering parameters
	Sample	Temperature	Holding Time	Relative Density
			(°C)	(min)	(%)
	1		1200	5	84.4
	2		1350	5	91.2
	3		1450	5	94.8
	4		1550	5	96.5
	5		1650	5	97.0
	6		1750	5	98.4
	7		1850	5	98.6
	8		1950	5	99.5
	9		1850	10	99.6
	10		1850	20	99.8
	11		1850	30	100

ACKNOWLEDGMENT

The authors would like to thank the INSA de Lyon for their financial support of this work under the BQR grant. They are also pleased to acknowledge important contributions used in the body of this work from G. Bonnefont and F. Mercier from MATEIS for SPS processing.