Alain Bernard
email: alain.bernard@irccyn.ec-nantes.fr

Florent Laroche
email: florent.laroche@irccyn.ec-nantes.fr

Catherine Da-Cunha
email: catherine.da-cunha@irccyn.ec-nantes.fr

Models and methods for knowledge formalisation in a PLM context

Keywords: Integrated design and Manufacturing, Knowledge-based engineering, Knowledge value, Virtual Engineering

Knowledge management is one of the critical issues for the improvement of enterprise performances. Different methods exist for extracting and formalizing knowledge in order to create knowledge-based applications. Most of integrated design and manufacturing technological processes are more or less based on knowledge that has been translated into models and then into Knowledge-Based Computer-Aided Applications. This paper presents some examples of bases and applications concerning the issues of knowledge-based engineering for integrated design and manufacturing

Introduction

Recent evolutions have transformed the key issues of enterprises performance. Knowledge-based digital (virtual) engineering increasingly constitutes a strategic purpose, in particular for mechanical engineering companies. The actual strategic need is the definition of an integrated design and manufacturing environment, built from knowledgebased applications, that constitutes an efficient PLM system. This strategic need mainly comes from the new relationship that subcontractors have with their OEMs in an extended or virtual enterprise context. So, this is both useful and necessary that small and medium-size enterprises (SME) in the field of mechanical engineering integrate new methods and tools for PLM integration. SMEs need specific PLM environments but the main difficulty is to succeed in defining the real needs and to formalize the knowledge required for a global efficiency. The required approach should provide assistance for costing, development and industrialization of products, manufacturing evaluation and the same issues related to the complete life-cycle of the product. It is based on the capitalization, reuse and extension of experts' knowledge. In order to have a realistic definition of the requirements of such system, one efficient way is "to go inside" the SMEs to extract the main needs in terms of collaborative work, technical processes, technical data management and more generally for PLM needs definition. Based on representative case-studies that constitute the panel to start the definition and the formalization of the needs, a generic solution is built at the conceptual level. But such proposition is possible because of a jointly characterization of a generic enterprise object model and also of a generic method for expertise extraction and formalization. Such expertise is modeled and the models are used for the specification and the development of a knowledgebased PLM environment that fits the requirements of each expert. In this paper we present and illustrate a global approach based on a generic model for enterprise objects and processes, also based on a generic extended approach for expertise formalization and finally on the use of these model and method for an inductive process based on three immersive case studies, and, finally, some first synthesis for the specification of a generic PLM system dedicated to a framework of mechanical engineering field organized as an extended enterprise..

A generic model for Enterprise objects

The first need is to propose a model dedicated to the representation of any enterprise object. One of the main goals is to fit the requirement of being able to model and manage the whole information of the whole product lifecycle. To achieve this goal, the FBS-PPRE model is proposed and defined in brief hereafter. In [START_REF] Labrousse | FBS-PPRE, an Enterprise Knowledge Lifecycle Model[END_REF], a brief comparison between the views included in the different methodologies known in the literature is proposed. This shows that there is a lack of completeness in all these approaches. For example, the external effects are not modeled and often not taken into account. These approaches do not tend to be universal. Nevertheless, they are of great interest in their respective application area. Their use can also be complementary. The worst thing is that the integration of these approaches remains quite uneasy, with a consequence that making a view of the whole enterprise is quite difficult. Another observation is that the dynamic of the views remains partial, restricted to the product view: in most of these models, the resources or the processes structures being not able to evolve.

Different views of an Enterprise object

It is assumed [START_REF] Labrousse | FBS-PPRE, an Enterprise Knowledge Lifecycle Model[END_REF] that "Enterprise object" is a generic concept dedicated to encapsulate the PPRE concepts: Process, Product, Resource and External effect (even if this last concept is also dependant of the context in relation with the enterprise). The PPRE concepts represent in fact the four roles that a given enterprise object can play during its lifecycle. Based on this assumption, this means that each enterprise object could be described with the same model. The "Enterprise Object" generic concept is thus used and defined as "an entity playing a role in the company". As a given object can play different roles successively, this concept aims at managing the different objects independently of their roles [START_REF] Labrousse | Generic FBS concept for Process/Product/Resource integration[END_REF]. The process concept is a sequential, spatial and hierarchical organization of activities using resources to make products or services (also generally called outputs). The product concept is the result of the process, the object that the process intends to modify, to which the process is supposed to add value. The resource concept is an object contributing to the process without being its purpose. It could be a consumable resource. The external concept is an object acting as a constraint (positive or negative, even neutral) on the process / product / resource system. It is a part of the context that can disturb the process progress. It is also assumed [START_REF] Labrousse | FBS-PPRE, an Enterprise Knowledge Lifecycle Model[END_REF] that each enterprise object can be characterized by three views: functional, behavioral and structural views which are the basic concepts of the FBS (Function-Behavior-Structure) approach [START_REF] Labrousse | Generic FBS concept for Process/Product/Resource integration[END_REF] [3] [4] [START_REF] Umeda | Function, Behavior and Structure[END_REF]. In the functional view, the functions describe the aim of an object. The operational functions are formulated independently of any particular solution.

On the opposite, the technical functions depend on the technical choices. In the behavioral view, the behavior describes the dynamic aspect of an object. It includes a set of rules (continuous models) and sequential states graph (discrete model) representing the transformation of an object stimulated during a process. In the structural view, the structure defines the elements that are parts of the object. It defines also the attributes of these parts.

Natures of an Enterprise object

The Enterprise objects cannot be classified by their roles (they change during their lifecycle and, consequently, they are circumstantial). At the opposite, the Enterprise objects can be classified by their nature. It has been assumed that, in coherency with the systemic approach, five object natures can be used:

• Material (object that can have a concrete form); • Organizational; • Temporal (object that can play a process role); • Software (including information, decisions, the generated documents);

• Energy. "Object nature" is a concept linked to intrinsic parameters of the object.

Behaviour of an Enterprise object

The behavior management is an originality of the FBS-PPRE model. It is constituted on one hand of "processes" and on the other hand of a description that will be the same for the products, the resources and the external effects. The process concept (in the center of the representation) plays a major role in the FBS-PPRE model:

• It links the different objects playing a role in the activity. It helps at defining the context;

• It also helps at managing the behavior of the product, of the resources and of the external effects. The states are representative of the structure changes. They are the inputs and outputs of the process element. They represent the discontinuous behavior of the objects.

Use and prospective issues about the FBS-PPRE model

Knowledge formalization for PLM is one of the main industrial challenges. It highlights the role of knowledge capitalization and management. But, the development of the knowledge-based applications of of knowledge bases content implies a real integration: of all the view points of Enterprise objects, depending on the different ontologies they refer to. So, on a general point of view, the main difficulty remains the representation of the knowledge in order to insure its interpretation, its sharing and its keeping.

The FBS-PPRE model allows advances principally in three domains: the completeness of the modeling, the management of the dynamic of the objects, the conceptual unification.

As regards completeness, the model offers a wider view than the usual approaches. Each object having an influence on the enterprise processes (defined as a temporal, spatial and hierarchical organization of activities) is taken into account through its role of product (the product is the result of the process), of resource (the resource is contributing to the process without being its purpose) or of external effect (the external effect is then acting as a constraintpositive or not -on the process). Furthermore, each object is modeled with the same views: the functional, the behavioral and the structural views. Due to its definition, the behavioral view of an object is always linked to a process. But, different behaviors can be taken into account: the model makes it possible to characterize the behavior of an object during its use process (the object has then a role of resource), but it also makes it possible to keep the evolution of an object during a process of design, of realization, of dismantling, of recycling, etc (the object has then a role of product). Moreover, this modeling of the dynamic applies whatever the nature of the object: it also makes it possible to manage natively the transformation of the temporal objects. This strongly increased completeness, in particular with the management of the dynamic of all the objects, Due to the generic views of the FBS-PPRE modeling, the enterprise objects can be described according to the same formalism independently of their circumstantial roles of process, of product, of resource, or of external effect. Furthermore, the FBS-PPRE model constitutes a real conceptual unification independently of the nature of the Enterprise objects (temporal, material, software, organizational or energy). The model is thus very compact and easier to apprehend: its implementation and its maintenance are easy. These conceptual elements thus constitute an essential support for the representation of knowledge.

The adoption and the deployment of the FBS-PPRE model contribute to the analysis, the specification and the follow-up of the enterprise processes. They can lie with an ISO 9001 quality certification process: the objectives of this standard are indeed in perfect adequacy with FBS-PPRE.

To use this model in the company, a reliable and effective information system should be configured and implemented. This has already been successful in several particular enterprises, mainly in the context of our research collaborations. This contribution is the base of our investigation about knowledge formalization. Anyone can speak about knowledge-based application but the core representation of knowledge needs a structured modeling that enables to support knowledge encapsulation and the model transformation phases that are required for the definition of the knowledge-based applications. So, based on this generic and unified Enterprise model and on its ability to formalize knowledge content, it has been proposed a methodological contribution. It consists in the proposition of an extended knowledge extraction and formalization method that can be used for the definition of knowledge-based applications for technical expertise mainly used during the design phases, in order to be able to evaluate the performances of the product (manufacturability, maintainability, etc…).

Proposition of a generic method for knowledge capitalization

This proposition is mainly based on the MOKA (Methodology and tools Oriented to Knowledge based engineering Applications) [START_REF] Callot | Proceedings of the Conference on Integration in Manufacturing[END_REF] [7] approach and its ontology that has been extended in order to be able to fit the technological expertise formalization [START_REF] Ammar-Khodja | An overview on Knowledge-Management, in Methods and Tools for Effective Knowledge Life-Cycle-Management[END_REF]. This was necessary because of the need to develop industrial knowledge-based engineering applications. The context of the experiments is a French national project (USIQUICK : www.usiquick.com) dedicated to 5axis machining of complex aeronautic parts.

USIQUICK: a KBE application for 5-axis machining of complex parts

The project was started with the aim of developing a knowledge-based engineering system to help experts to define the process planning related to complex mechanical parts that need to use 5-axis machining.

The project involved eight partners: An aircraft manufacturer, who proposed expertise about process planning for complex parts, a CAD/CAM leader for the industrialization of the software solution, five laboratories ensuring the scientific coherence of the project and proposing innovative solutions to solve strategic locks and a Frenchgovernment institute (Cetim) who analyzed the possible use of the project results in other fields and proposed extra test cases and tool databases.

Globally, the project focuses on the definition of milling process plans mainly in complex aeronautic part manufacturing with a high amount of reengineering and this implies particular geometries and processes.

In order to optimize the information flow from design to production, a three-step method is proposed [START_REF] Candlot | Deployment of an Innovative Resource Choice Method for Process Planning[END_REF]: 1) Transformation phase which consists in an analysis of the part to compute a maximum of information registered in an appropriate level of feature and to compute the machinability of faces, 2) Preparation phase where the synthesis templates of the previous phase are presented to the user and, with appropriate tools, the process plan skeleton can be built and constrained, 3) Automation phase where the unconstrained choices are automatically optimized and a complete documentation is proposed by the system. These phases became the three major modules of the developed engineering tool based on the formalization and the integration of expert knowledge.

One of the project's challenges was to translate knowledge that has been expressed in form of legacy specifications for the development of the system into a computerized form so that the computer can use it. The difficulty is thus to select the right methods and tools for supporting and structuring such transfer. One solution could be to structure the knowledge within a knowledge base (KB). The building of this KB implies the deployment of a capitalization process to help and guide the knowledge treatments. Capitalizing knowledge consists in processing and treating knowledge to prepare it for management activities. This capitalization will enable knowledge to be shared through a specific form making it understandable by each actor of the project. Knowledge capitalization is the process of capturing and formalizing expertise before its implementation in a system. The aim of knowledge capitalization is to develop methods and tools that make the task of capturing and validating expert knowledge as efficient and effective as possible. Experts tend to be important and busy people; hence it is vital that the methods used minimize the time each expert spends off the job taking part in knowledge acquisition sessions [START_REF] Fasth | Knowledge Based Engineering for SMEs[END_REF].

To reach the multi-experts collaboration, the knowledge sharing and reuse within the USIQUICK context, we propose to capitalize the knowledge in two major phases: a capture phase and a formalization phase.

Capitalization process proposal

According to the KBE systems development principle, knowledge must be identified, captured, analyzed, structured and formalized in a way that it could be accessible and reusable by each one. However, this principle does not allow any distinction between the activities handling the knowledge content (this means knowledge itself) and those handling its form.

What we are proposing is not completely different or contradictory with KBE development principle.

Our aim is to structure all these activities according to the knowledge aspect addressed at each stage of the capitalization process. This structuring consists in separating the activities that handle the knowledge itself from those handling its form. This distinction tends to help knowledge engineers during capitalization activities deployment. This structuring can also be considered as working on the knowledge's state. Indeed, working on the knowledge content consists in transforming its state from a raw state (independently of being explicit or tacit) to a structured one. And working on the form, deals with the representation of the knowledge to go from a structured state to a formalized state. The transition between the two states is based on the design of a knowledge base. This base constitutes a knowledge repository that can be accessible and which will be the knowledge reference So, the first step is Knowledge capture. This is the process that transforms the human experts' knowledge into formulated knowledge that can be used directly by an expert system or by a computer system. This process can be refined in three major steps: the elicitation step, the analysis step and the structuring step.

Several elicitation techniques exist depending on knowledge source's type. Within the USIQUICK project, the elicitation had to be done from documents that represent legacy specifications for the development of the final system. Consequently, among the existing methodologies for KBS and KBE development, the only one that can meet our needs is MOKA. This is because it offers the possibilities of eliciting knowledge from documents within engineering domains through its ontology.

An ontology is a set of different interrelated concepts that describe a given domain [START_REF] Gruber | A translation approach to portable ontologies[END_REF]. MOKA describes in terms of rules, processes, modeling techniques and definitions the necessary stages for the specification of KBE systems. MOKA provides a framework both for capturing and for representing knowledge. This framework works at two levels: informal level and formal level. The first one is relatively simple and oriented to represent and formalize knowledge in language that can be understood by experts without being specialist in formalization languages. The advantage of this level is that it makes the validation of the acquired knowledge possible. This level makes also the communication between the expert, the knowledge engineer and the software developer easier. The second level is more formal and aims to represent and store knowledge in an encoding form in order to plug it into computers. The MOKA spirit is not different from the approaches proposed within the other knowledge management methodologies, the difference lies in the deployment strategy. The other point that differentiates it from the other methods is about the concepts it proposes to analyze related to the application domain. MOKA proposes the ICARE ontology based on five generic knowledge object types (Illustrations, Constraints, Activities, Rules, Entities) and relations among them to describe the domain. These objects are also defined as well as their use constraints. Starting from this ontology, our first step was the identification of the knowledge objects. The identification step is a preliminary domain investigation and analysis that aims to recognize the knowledge elements or objects that must be acquired. The specifications we got consist of texts, tables and images in MS Word format. The domain library, which approximates domain ontology, consists of technical sentences condensed from legacy specifications. The use of the MOKA ontology allowed us to identify a great number of knowledge objects. However, there is some knowledge related to, for example, resources or functions, that have been missed. Facing this insufficiency, the concept of resource (R) has been introduced to encapsulate knowledge about the different tools and machines used by manufacturing processes (or operations) to realize geometries. The concept of function (F) was also introduced to identify what is the objective of the reasoning activities. According to these two new concepts, the ICARREF ontology has been proposed in order to better fit the manufacturing domain requirements [START_REF] Ammar-Khodja | An overview on Knowledge-Management, in Methods and Tools for Effective Knowledge Life-Cycle-Management[END_REF]. At this stage the knowledge to be kept has been identified and the elicitation can be done completely by an extraction strategy. The extraction consists of recognizing a subset of knowledge objects and their relationships, and then associating them with applicable fragments of the specifications. The eventual output of extraction can be in plain text, in XML, or in Excel form, depending on the application of the supported software. Once the knowledge extracted it must be analyzed. This analysis has two objectives: its analysis and its structuring. The structuring step is achieved using trees and diagrams according to the MOKA approach. Knowledge objects having the same type are linked using trees with "Is a" and/or "Is composed of" relation types. Knowledge objects having different types are linked using diagrams. For diagrams building the relations are defined according to the objects they link. It can be "Has a rule", "Has a constraint", "Has a function", etc. At this stage, the three steps of the capture phase have been done and a first representation of the knowledge is built. This representation will enable the evaluation of the knowledge. The evaluation consists in analyzing the knowledge according to two criteria: completeness and feasibility.

The completeness indicates if, as transmitted by the expert in the specifications, this knowledge is enough to define the process planning for specified geometries. It allows also identifying if for specified utilization of the application, context for each knowledge object is well described. This criterion highlights the additional knowledge to capture or to explain more that it has already been done. Each one knows that there is a gap between the "real world" and the "computer's world". The analysis of the feasibility aims to point out the knowledge that cannot be coded as specified by the expert and that requires to develop additional algorithms to make its automation possible.

The USIQUICK experience has shown that considering the two knowledge aspects separately, the content and the form, helps to decrease the complexity of knowledge based engineering system development. In summary, the capitalization process we propose aims to structure knowledge engineering activities deployment (Figure 1). It has to be highlighted that the generic concepts of the FBS-PPRE model constitute the foundations of the new extended ICARREF ontology. The generic concepts have been adapted to the need of elicitation and representation of knowledge.

Figure 1. Synthetic view of the global process

Based on such a model, a process and a method, we actually focus on the generalization of our approach for a generic definition of PLM systems for SMEs. The content of our actual results and perspectives in such a way are described in the following sections.

Specification of a generic PLM system for SME's extended enterprise

Mechanical engineering industries are faced with growing challenges. After having refocused on their core business in order to increase efficiency, they are now encouraged to acquire more diversified skills. Market globalization and an increase of customer demand have forced companies to produce more complex and customized products in a shorter lead time. To solve this paradox (refocus on the core business and need of multiple specific skills), companies have setup networks in order to pool and share their mutual skills. These networks are built for a specific project which is called 'virtual enterprise', or 'extended enterprise' when it is done over a longer period. One of the key points of the success of such a structure is the ability to communicate on the target product. Products that generate a large amount of information, the classical communication system (phone/fax/email used by 90% of companies) is not structured enough to enable efficient cooperation. For many years, software has been developed to pool all this information. From the EDM (Electronic Document Management) in the 80's to the PDM (Product Data Management) and the PLM (Product Life cycle Management) nowadays, the companies and particularly the contractors understand the benefit of such approaches and software. Within this context Cetim (an industrial technical centre that represents 7800 French mechanical industry companies) has performed a survey on digital and collaborative engineering for the mechanical engineering companies [START_REF] Cetim | Enquête de besoin sur le travail collaboratif[END_REF]. This survey showed that only 5% of SMEs of fewer than 100 people use a PLM system to manage their technical data. However, more than 70% of these same SMEs consider as important the reuse of knowledge, the share of information within the company and outside, the security of information access and storage and the follow-up of modifications [START_REF] Cetim | Etude sur les besoins en PLM[END_REF]. And these are the exact functionalities offered by the PLM software tools. After many visits to SMEs, the contractors see the main obstacles to a PLM deployment as being not only the cost but also the complex nature of the setting, use, and maintenance. It seems, however, that the SMEs in the mechanical engineering industry have very specific needs in terms of PLM. 70% of these companies have customers in various business fields (mainly in the automobile and aeronautic sectors). As a consequence, they have a lot of technical skills to manage at the same time because the contractors use different CAD and PLM systems. There is a true need for PLM in SMEs in the mechanical industry, but there are some obstacles that stand in the way of this development.

-PLM: from concept to meta-modeling

An acceptable definition of PLM is: "A strategic business approach that applies a consistent set of business solutions in support of the collaborative creation, management, dissemination and use of Annotation product definition information across the extended enterprise from concept to end of life -integrating people, processes, business systems, and information" [START_REF] Inc | Product Lifecycle Management[END_REF]. Much work has been done in this field, especially in the aeronautic and automobile sectors in order to propose technical data management methods [15] [16]. Some others try to address the SME specificities and propose solutions such as Delplace for sand casting foundries [START_REF] Delplace | L'Ingénierie numérique pour l'amélioration des processus décisionnels et opérationnels en fonderie[END_REF]. PLM relies on a data model being composed of business objects that intervene in the business processes. Several modelling methods and languages have been developed so as to model these objects. Many modeling languages enable these objects and related activities to be represented such as SADT, IDEF3, BPMN [START_REF] White | Introduction to BPMN[END_REF] or FBS-PPRE. Nevertheless, in an extended enterprise context, it is necessary to be able to communicate together. Therefore standardized models are required to enable sharing or communication between companies that have not made the same choice of modeling.

Much work has been done in different sectors to increase the interoperability of data models with the standards, using mainly the STEP standard (STandards for the Exchange of Product model data) [19] [20].

Hence the creation of PDM Schema [START_REF]PDM Schema v1[END_REF] tried to unify the different information models of STEP APs using their common objects. It seems that if many methods exist for modeling business objects, their use for the creation and maintenance of a data model that supports PLM is not detailed enough for industrial exploitation. We will keep the FBS-PPRE modeling method [START_REF] Labrousse | FBS-PPRE, an Enterprise Knowledge Lifecycle Model[END_REF] which enables the dynamic representation of enterprise objects Moreover we can notice that in spite of the existence of standards, it is still difficult to get a data model both adapted to the company and interoperable with the standards.

-Research approach

Our approach aims to propose a methodology in order to structure and manage the technical data of companies in the mechanical industry in an extended enterprise context. To define this methodology and to build a common data model for the different companies, we have implemented an inductive research/action practice, based on a spiral cycle approach, typically consistent in terms of scientific experimentation:

The first phase has been to interview companies to extract the present practices in terms of digital and collaborative engineering, and the best practices to implement. Benchmarking has also been done on existing software tools to list the functionalities and their ability to meet SME needs.

Then we created a typology [22] [23] to choose pilot companies representative of the mechanical industry. We selected different companies to cover the whole mechanical industry.

Finally we went into the different companies so as to directly and inductively integrate the technical data structuring and managing methods. This phase is coupled with the implementation of the methods with real data in the companies to verify the gap between our proposal and the objectives. From the initial situation of each of the companies, we act by inductive research/action to arrive at the final state that we defined during the audit phase.

During this phase, we have formalized a general method of managing technical data, and created a meta-model. This approach is compatible with standards and may be used in an extended enterprise context.

-Immersion in companies

The first immersion enabled us to put into practice methods of technical data structuring and management, adapted to the chosen companies. In order to do this, we analyzed the needs of the company in terms of PLM.. The PSL CONCEPT company produces and sells equipment for ships. Among these products, there are reserve rudders, pulleys and tackles, sheaves, jam cleats and various accessories. This company organizes the main part of its products into families. The main needs of this company are as follows: Knowledge capitalization, BoM management, Reference management, Quotation, Archive management. We applied the axiomatic design principle [START_REF] Suh | The Principle of Design[END_REF] [25] [START_REF] Suh | Axiomatic Design of Software Systems[END_REF] to link the functional parameters to the design parameters in order to decline it into the technical data structuring and management of a product family [START_REF] Bernard | Global approach for product data management, application to ship equipment part families[END_REF]. We broke down the different functions of the family, and then we created a set of functional dimensioning parameters for the products in order to link those parameters to the different functions. In summary, we propose a global approach to meet the specific PLM needs of this company [START_REF] Duigou | Approche globale pour la gestion de données techniques, application à une famille de poulies d'accastillage[END_REF] [START_REF] Duigou | Global Approach for Technical Data Management[END_REF]. The implementation of the software based on this approach and the results that we have obtained (the design time for a new pulley has gone from hours to minutes) prove that the proposed approach corresponds to the needs of this kind of company (type II), in the mechanical industry. The second pilot company is named CAPRICORN. This company manufactures crankshafts, connecting rods and pistons for the up market automotive industry and racing cars. It is a type III company in our typology: "elementary parts manufacturer". This kind of company has problematic manufacturing technical data. The elementary parts manufacturers directly receive their drawings and CAD models from their customers. Then they add their expertise to draw up the plan of procedure of the product and produce it. The audit phase makes us focus on the following initial problems: External exchange, Knowledge capitalization, Documentation, Internal exchange: Once the documentation made it has to be sent to the manufacturing department. If a modification occurs, the right version must be sent to this department. To respond to those needs, we chose to structure the technical data in three groups. The information from the product, directly extracted from the STEP file, the information from the work centers and the craft rules, both collected during the audit phase. This case study enables us to identify the different technical data used by the planning department during the industrialization phase. It also enables us to extract the knowledge to use those data internally as well as externally via the exchange with the customer and the production department. The third case study is SMP, a grinding machine producer. This company is a type I, "machine producer". Due to the high number of components of its products and its high customization, this type of company often encounters bill of material problems. After the audit phase we select the following initial needs: BoM creation, BoM management, BoM structuring for the production department, BoM integration in the ERP: The integration of the BoM in the ERP of the company is done manually, which is time consuming and source of error. The approach proposed here is based on the double view of the bill of material. This case study enables us to identify the technical data that is transferred between the engineering department and the planning department in this company. It also allows us to extract the knowledge useful to their transfer and especially concerning the multi view of a product. We applied this multiview notion using a buffer file that contains all the information of the product, the structuring of the product being specific to each view. During this phase of immersion we integrated a knowledge management approach, based on the one exposed in the preceding sections of this paper, with first of all an extraction of fundamental knowledge, then a structuring of that knowledge and finally its integration into the software. The validation of this work was done by software tests carried out by the expert. Obviously the specific application integrated in the pilot companies are too specialized to be directly integrated into a generic model for the extended enterprise. Some of those data and some processes are really specific to the products manufactured by the company or to its production processes. Nevertheless some other can be processed in a general way and have been synthesized to create requirements related to the future generic PLM system.

Conclusions and perspectives

In this paper, we have presented a generic model for enterprise objects modeling that supports the encapsulation of knowledge [START_REF] Bernard | Methods and Tools for Effective Knowledge Life-Cycle-Management[END_REF]. This encapsulation is possible thanks to an extended generic method, based on an extended MOKA ontology (ICARREF) that aims to give a solution for the definition and the realization of knowledgebased applications. An original inductive immersive approach lets us getting some very practical inputs and feedbacks on the application of these model and method that contribute to the improvement of the general approach we would like to propose to SMEs for the definition of their specific PLM needs.

6

Acknowledgements

All our gratitude to those of the researchers of my team who contributed to the results described in this paper, Dr M. Labrousse, Dr. S. Ammar-Khodja, Dr. N. Perry, Dr. A. Candlot, Mr J. Le Duigou (PhD candidate), and the PSL CONCEPT, CAPRICORN and SMP companies for allowing us to carry out our studies in their companies and for their technical support. All our thanks to the Cetim, the French Ministry of Higher Education and Research and the partners of the USIQUICK project.


Clément Carlé

Shervin Keshavarzi

Andrei Mursa

Petri Karvinen

Ravinder Kumar Chutani

Sylwester Bargiel

Samuel Queste

Rémy Vicarini

Moustafa Philippe Abbé

Abdel Hafiz

M Abdel Hafiz

V Maurice

R Boudot

N Passilly
email: nicolas.passilly@femto-st.fr

Reduction of helium permeation in microfabricated cells using aluminosilicate glass substrates and Al2O3 coatings

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.

Alkali vapor cells are core elements of compact atomic devices such as clocks 1,2 , magnetometers 3,4 , gyroscopes 5 , microwave electrometers 6 , or THz-imaging systems 7 which can reach outstanding levels of accuracy and sensitivity. Vapor cells are also key components of a wide variety of quantum physics experiments, for the implementation of lasercooling platforms 8 , quantum teleportation demonstrations 9 or the achievement of spin-squeezed 10 and entangled 11 states. The last twenty years have witnessed a renewed interest in vapor cells with the advent of their microfabricated version [12][13][14][15][16][17] . The latter, when combined with integrated photonics, has stimulated the development of low-power and easy-to-deploy chip-scale atomic devices 18 , with real-world applications and tremendous prospects.

In an atomic cell device, it is critical to ensure that the probed atomic sample evolves in a well-controlled and stable environment. Variations of the cell inner pressure resulting either from background gas entrance or buffer gas leakage have to be avoided to prevent any perturbation of the probed atomic transition. In microwave atomic clocks, a buffer gas is typically added to extend the interrogation duration past the transit time. While adding this gas is beneficial in terms of short-term stability, it imparts a frequency shift proportional to the buffer gas pressure on the clock 19,20 and imposes this pressure to be constant to ensure long-term stability [21][START_REF] Camparo | Proceedings of the 39 th Annual Precise Time and Time Interval Meeting[END_REF][START_REF] Abdullah | [END_REF] . In microcell optical frequency references [24][25][26][27] , the presence of contaminants in the cell induces a broadening of the optical resonance 28 that prevents benefiting from the thin natural linewidth of the transition. In trapped-ion or cold atom systems, degradation of the cell background pressure reduces the trapped ion or magnetooptical trap lifetime and jeopardizes the signal-to-noise ratio of the detected signal 29,30 .

The cell atmosphere stability is then likely affected from gas permeation, especially of helium due to its low density, its volatility (smallest mono-atomic gas) and its significant natural concentration in the earth atmosphere (5.24 ppm) 29,[31][32][33] .

For instance, the limitation of the long-term stability of an optical clock based on the Rb 778 nm two-photon transition has been attributed to He permeation 34 . In standard cold atom or trapped-ion systems, where ultra-high vacuum background pressure levels are required, preventing the science cell from being polluted by helium gas often requires using of an active external ion pump. This approach, suitable for the demonstration of laser cooling in lab-scale platforms, compact transportable systems 35 or even micro-electro-mechanical-systems (MEMS) cells 36 , is not compatible with the demonstration of a fully-miniaturized boardable atomic device. In this context, chip-scale ion pumps are being developed 37 , but, along with increasing their complexity, the intrinsic generation of large magnetic field close to the atomic sample may limit the development of high-precision instruments.

Since passive getter pumps do not interact with helium because of its noble character 38,39 , the main solution to prevent helium from entering the cell while keeping compact devices consists in constructing the cell or the science chamber with low-permeation materials. Several remarkable studies have recently been reported in this domain including a miniaturized vacuum package based on a metal-ceramic package and sapphire windows for a trapped ion atomic clock 40 , a compact and passively pumped vacuum package combining titanium body and sapphire windows, sustaining cold atoms for more than 200 days 41 or the development of a stand-alone vacuum ceramic cell maintaining a cold atom cloud for more than one year 42 . Yet, the above-mentioned studies are all based on standard cells, and do not benefit from the collective fabrication and mass-production capabilities offered by MEMS technologies.

Microfabricated alkali vapor cells consist typically of a silicon-etched cavity sandwiched between two anodicallybonded glass wafers [12][13][14][15][START_REF] Abdullah | [END_REF]25,43,44 . BSG is well-adapted for anodic bonding to silicon and is widely used due to its waferform availability. However, if silicon is rather hermetic 29 , BSG is known to exhibit a non-negligible gas permeation rate, for noble gases such as Ne [START_REF] Abdullah | [END_REF]45 , and especially He 46,47 . Microfabricated cells with significantly reduced He permeability have then been reported where the cell windows were made of ASG substrates 47,48 . In Ref. 47, helium permeation rate was derived from the frequency drift of a coherent population trapping (CPT) clock, while the physics package with a cell filled with Ar and N 2 was placed inside a chamber filled with He. The positive frequency drift was then consistent with a progressive entrance of helium into the cell.

In the present study, we investigate helium permeation in microfabricated alkali vapor cells, with two types of glass substrates (BSG or ASG), through long-term measurements of the pressure-shifted hyperfine transition frequency of a CPT atomic clock based on cells filled with He. Then, He permeation is monitored as a progressive decrease of the clock frequency along time, evidence of He going out of the cell. Moreover, we propose to add coatings onto the cell windows, as complementary permeation barriers. For this purpose, thinfilm Al 2 O 3 coatings, usually implemented in MEMS cells to avoid alkali disappearance with time 49,50 , are deposited onto the glass substrate prior to the bonding with the silicon preform. We show that Al 2 O 3 layer can reduce He leak rates up to a factor of 110, when cell windows are made of BSG. For cells using ASG, which already exhibits He permeation rates about 450 times smaller than those obtained with BSG, alumina coatings bring a further improvement by a factor of nearly 6 leading to a reduction of the He permeation rate by more than three orders of magnitude in comparison with standard cells made of bare BSG.

Let us remind first the basic formalism used in this manuscript to study He permeation from microfabricated cells. The evolution of the buffer gas pressure P(t) in the microcell can be described by an exponential decay, such as:

P(t) = P ext -(P ext -P in )e -t/τ , (1)
with t the time, P in the initial buffer gas pressure in the cell at t = 0 s, P ext = 3.98 mTorr the partial pressure of He in the atmosphere, and τ the time constant described as:

τ = V d K A P re f , (2)
where V is the volume of the cell cavities and P re f = 760 Torr is the atmospheric reference pressure. Since, as mentioned above, permeation of the cell silicon body is negligible, we assume that He permeates only through the glass windows. Consequently, A and d are the surface and the thickness, respectively, of the cell windows. Finally, K is the glass permeation rate.

The latter can then be derived from the change of He pressure in the cell which in turn induces an evolution of the clock transition frequency due to the temperature-dependent buffer gas pressure shift ∆ν bg , given by:

∆ν bg = P[β + δ (T -T 0 ) + γ(T -T 0 ) 2 ], (3)
with P the buffer gas pressure (at a reference temperature T 0 of 0 • C), β the pressure coefficient, δ the linear temperature coefficient, γ the quadratic temperature coefficient and T the cell temperature (set at 70 • C in the first part of this study). For Eq. (3), we use He coefficients reported in Ref. 19 (β = 1185±46 Hz/Torr, δ = 1.49±0.15 Hz/Torr.K). Hence, the permeation rate K through the cell windows made of different glasses and eventually coated with Al 2 O 3 can be determined by deriving the buffer gas pressure along time from the monitoring of the frequency of an atomic clock using the different cell configurations.

For this purpose, we implemented a table-top CPT clock featuring an automated rotatable platform carrying 6 cell physics packages. Such a configuration allows investigating a significant number of cells in a regular manner during extended measurement campaigns. Figure 1 shows a schematic of the multi-cells CPT clock. It is based on a single and fixed vertical optical axis in front of which 6 different cell physics packages, mounted on a rotation platform, can be sequentially positioned. The laser source is a commercially-available vertical-cavity surface-emitting laser (VCSEL) tuned on the Cs D 1 line at λ = 895 nm 51 . The VCSEL is bonded onto a dedicated card that embeds a bias-tee through which a 4.596 GHz signal is injected to modulate the laser and produce CPT optical sidebands. A lens and a quarter wave plate are used to collimate and circularly polarize the laser beam. The rotation platform is actuated by a stepper motor in order to switch periodically the cell under test. Each package consists of a MEMS cell inserted in a duralumin piece temperaturestabilized at 70 • C, surrounded by a static magnetic field to raise the Zeeman degeneracy and protected by a mu-metal magnetic shielding. The laser light is detected at the output of the cell under test by a photodiode. The acquired signal is used in a first servo loop that acts on the laser dc current to stabilize the laser frequency to the center of the Dopplerbroadened optical resonance. A second servo loop is used to lock the microwave interrogation signal frequency to the CPT resonance. In this experiment, the 4.596 GHz signal is generated by a commercial microwave synthesizer, referenced to an active hydrogen maser used as a reference for frequency shifts and stability measurements.

The cell technology is comparable to the one described in Refs 43 and 44. Atom-light interaction takes place in a cylindrical cavity (shown in the inset of Fig. 1) with a diameter of 2000 ± 20 µm and a length of 1500 ± 10 µm. The side of the squared dispenser cavity is 1600 ± 20 µm long. The two glass substrates are 510 ± 10 µm thick and are made either of BSG Borofloat ® 33 from SCHOTT or of ASG SD2 from HOYA. Each wafer typically gathers 215 cells, from which between 10 to 20 are filled with Cs dispensers. For this study, eight wafers of Cs-He cells have been fabricated in total.

Figure 2(a) shows typical temporal traces of the clock frequency, for two different Cs-He microfabricated cells, one with BSG and one with ASG, resulting from data acquisitions longer than 40 days. In here, the reduction of the clock frequency with time is compliant with He atoms exiting pro-gressively the cell. In that sense, this experiment is different than the one reported in Ref. 47 where a positive slope was observed, due to He entering into the cell placed in a pressurized chamber under He atmosphere. For comparison, a similar measurement was performed on a Cs microcell with BSG windows and filled with about 70 Torr of Ne. Since Ne permeation rate is known to be more than three orders of magnitude lower than He [START_REF] Abdullah | [END_REF] , the BSG-Ne cell is used here as a reference and sets the measurement background of our setup. Focusing onto the first 10 days, linear fitting of clock frequency data yields frequency variations of -1182, -3.5 and -0.3 Hz/day for the BSG, ASG and BSG-Ne reference cell, respectively. These results show that ASG drastically reduces He permeation, leading here during the first days to a clock frequency drift reduction by a factor of nearly 350 compared to BSGbased cells. Following such a measurement, the clock frequency is converted into buffer gas pressure using Eq. ( 3). The corresponding evolution of the buffer gas pressure is shown in Fig. 2(b). Pressure data are then fitted with Eq. ( 1) to estimate the time constant τ, whereas the permeation constant K is extracted using Eq. ( 2). In such measurements of several tens of days, the exponential behavior of the buffer gas pressure evolution is clearly visible for the BSG-based cells. The corresponding time constant τ is 75 ± 6 days, whereas it is extended to 35000 ± 3000 days in ASG-based cells. Hence, derived permeation constant K for BSG and ASG, are K = 5.9±0.7 × 10 -19 , and 1.3±0.2 × 10 -21 m 2 .s -1 .Pa -1 , respectively. Concerning ASG, our measurements are in good agreement with the ones of Dellis et al. 47 despite a different cell temperature and confirm its significant capabilities for reduction of He permeation. In addition, note that permeation constant for BSG-Ne cells is found to be K = 3.5 ±0.2× 10 -22 at 70 • C, close to the value of K = 5.7±0.7 × 10 -22 measured at a higher temperature of 81 • C by Abdullah et al. [START_REF] Abdullah | [END_REF] In this study, we propose to investigate the further permeation reduction obtained with additional thin-film coatings. Al 2 O 3 layers, for instance, have been known and employed to avoid alkali consumption in vapor cells, notably for Cs 49 , Rb 50 , or more recently Sr 52 . The deposition of Al 2 O 3 layers used in our experiments was performed by atomic layer deposition (ALD) with a commercial Beneq TFS-200 system. 20 nm-thick films have been coated directly on the glass wafers prior to the bonding with silicon and thus cover the inner side of the glass windows. Among the eight cell-wafers developed for this study, five were coated with Al 2 O 3 . Figure 3(a)-(d) reports clock frequency measurements performed on several typical cells for each of the four tested configurations, i.e. BSG, ASG as well as BSG and ASG coated with Al 2 O 3 . All these clock measurements were performed at a cell temperature of nearly 70 • C. As shown, the frequency variations strongly differ from one configuration to another. Standard BSG cells (Fig. 3(a)), characterized by an initial He pressure of 72.5 ± 0.5 Torr exhibit a rapid decrease of the clock frequency, measured to be about -1200 Hz/day during the first 15 days. We then measure that about 50 Torr of He escaped the cell after 100 days. These results yield an estimated permeation constant K in the range of 5.5 to Cells with ASG (Fig. 3(b)) featuring an initial He pressure of 84±2 Torr lead to a drastic reduction of the clock frequency change with a slope of about -3.4 Hz/day. This corresponds to a He pressure loss of about 120 mTorr after 40 days. In comparison with BSG, this corresponds to a reduction of the permeation constant K by about 450, yielding K in the range of 1.09 to 1.43±0.14 × 10 -21 m 2 .s -1 .Pa -1 and a time constant increased from 75 to 35000 days. This improvement is in the same order than the one observed in Ref. 47.

Figure 3(c) displays measurements on BSG cells coated with Al 2 O 3 . Reported frequency shifts were measured on 4 different cells taken out of three different wafers. A slight dispersion is experienced, resulting first from different initial pressures, and that may be also attributed to a lack of homogeneity in the deposition of Al 2 O 3 layers or their partial damaging during subsequent cell fabrication or dispenser activation. Nevertheless, the shift rate is significantly lower, i.e. nearly 100× lower than for BSG cells, which corresponds to an improvement of the permeation constant of 110, leading to values ranging from 4.6 to 6.3±0.5 × 10 -21 m 2 .s -1 .Pa -1 . Note that such permeation is only 4× larger than the one of ASG and demonstrates that Al 2 O 3 contributes to reduce He permeation through the cell glass windows. Finally, Fig. 3(d) shows the clock frequency as a function of time for ASGbased cells coated with Al 2 O 3 . For e.g. D3 cell, whose pressure is initially measured at 39.6 ± 0.4 Torr, the clock frequency shift is reduced by a factor of 10 (-0.3 Hz/day), leading to a loss of 4 mTorr of He after more than two weeks at 70 • C. The corresponding permeation constant is equal to 2.2±0.2 × 10 -22 m 2 .s -1 .Pa -1 , i.e. 5.8× smaller than for uncoated ASG. It can be underlined that such permeation value for He is even better than the one for Ne in BSG and allow us to envision using He as a buffer gas in microfabricated cells.

Figure 4 summarizes of the permeation constant K derived for all of the 13 measured cells. Some values extracted from the literature 32,33 for BSG are also indicated. Note that all the above measurements have been performed at nearly 70 • C. Nevertheless, in order to predict and eventually simulate the behavior of microfabricated cells with time and at different temperatures, it is important to determine the activation energy E D corresponding to a specific glass substrate. If, in the literature, numerous data can be found on BSG, and in particular on Pyrex, including E D (equal to, e.g. -0.29 eV [31][32][33] ), it seems that no measurement of SD2 ASG has been reported yet. In Ref. 47, E D of SD2 ASG was assumed to be compa- rable to the one measured for another ASG, the Corning 1720 glass having close chemical composition (but lower amount of Al 2 O 3 , i.e. 18% instead of 25%) and for which the corresponding E D was measured equal to -0.52 eV in Refs. 32 and 33. In this study, we aim at estimating the value of E D for SD2 ASG by measuring the clock frequency evolution of the B2 cell at 3 different temperatures. The energy of activation E D can then be extracted through linearization of the dependence ln(K) = ln(K 0 ) -E D k B T , with T the cell temperature and k B the Boltzmann constant. Figure 5 shows the corresponding result. The fit of experimental data yields -E D /k B = 5680 K and K 0 = 1×10 -14 m 2 .s -1 .Pa -1 from which is derived the activation energy E D ≃ -0.49 eV. This value is slightly smaller than the one measured for the Corning 1720 glass (-0.52 eV).

According to this result, we can, as perspectives, estimate the lifetime of ASG-based microfabricated cells in two examples of applications: the first one concerns an atomic clock where He would be employed in a gas mixture along with Ne to shift the cell operational temperature toward higher values 46 . In this case, the relatively small required amount of He (e.g. 4.5% of the mixture for 100 • C, thus around 3 Torr) is favorable so that the inversion temperature should not change by more than 1 • C for nearly 500 days. The second application could be cells dedicated to host magneto-optical traps placed in earth atmosphere at room temperature. For such applications, the inner pressure should remain below 1 × 10 -6 mbar 42 , value which would be achieved after 85 days if only influenced by He permeation. Note that such estimated lifetime should be significantly extended with Al 2 O 3 coatings, and reach almost 500 days provided that a similar enhancement (×5.8 at 70 • C) is observed at room temperature.

In conclusion, we have investigated the permeation of he-lium through the glass windows of microfabricated alkali vapor cells by measuring the frequency shifts encountered by a CPT-based atomic clock. A total of 13 cells have been tested, divided in four configurations, either relying on BSG or ASG windows, or on BSG or ASG coated with Al 2 O 3 layers. We have demonstrated that Al 2 O 3 coatings, usually employed to prevent the consumption of alkali in vapor cells, can also significantly reduce He permeation. Cells using BSG coated with Al 2 O 3 feature permeation constants about 110 times smaller than those based on BSG only. These performances are only 4 times lower than the ones measured for ASG for which permeation constants of about 1.1 × 10 -21 m 2 .s -1 .Pa -1 were found, at a temperature of 70 • C. Al 2 O 3 coatings seem to be also efficient when deposited on ASG, further improving the hermeticity of cells with permeation constant reaching about 2.2 × 10 -22 m 2 .s -1 .Pa -1 , i.e. almost twice less than for BSG-Ne cells employed in miniature atomic clocks. Finally, the activation energy for ASG was estimated to be about -0.49 eV from measurements of the permeation constant at three different cell temperatures.

1 FIG. 1

 11 FIG. 1. (Color online) Multi-cell CPT clock experimental setup. 6 cell physics packages are embedded into mu-metal shields and mounted onto a rotation platform allowing their sequential probing. CL: Collimation Lens, λ /2: Half-Wave Plate, λ /4: Quarter-Wave Plate, LO: Local Oscillator. PD1 and PD2 are photodiodes. The figure inset shows the cell architecture, based on a structured Si spacer sandwiched between two glass substrates.

FIG. 2 .

 2 FIG. 2. (Color online) (a) Temporal trace of the clock frequency using microfabricated Cs-He cells based on BSG or ASG. In the inset, a clock frequency measurement performed on the same bench with a Cs-Ne cell is reported for reference since Neon permeation is supposed to be much lower. Windows with missing data correspond to breaks or crashes of the clock software. The clock frequency at t = 0 is 9192 736 114 Hz and 9 192 715 170 Hz, for ASG and BSG cells, respectively. (b) Evolution of the buffer gas pressure in the cell derived (Eq. (3)) from frequency measurements shown in (a), where extrapolated data follow an exponential model based on Eq. (1).

FIG. 3 .

 3 FIG. 3. (Color online) Temporal trace of the clock frequency for the 4 different configurations of Cs-He cells based on: (a) BSG, (b) ASG, (c) BSG coated with Al 2 O 3 (d) ASG coated with Al 2 O 3 . The right vertical axis converts the clock frequency evolution into the actual buffer gas pressure decrease. For clarity and easier comparison between all the cells, the initial pressure value was offset to 0 at t = 0. Actual initial buffer gas pressure values are given in the text. Light-colored zones indicate the size of error bars attributed to the buffer gas pressure estimation.

FIG. 5 .

 5 FIG. 5. Permeation constant K (logarithm) versus the inverse of the cell temperature T (1000/T ), for an ASG-based cell.

  FIG. 4. (Color online) Distribution of He permeation constants K measured for the 13 tested cells, split in 4 different windows configurations.

		ASG + Al 2 O 3	ASG	BSG + Al 2 O 3	[32]	[31]	BSG
		10 22	10 21	10 20 K (m 2 .s 1 .Pa 1 )	10 19		10 18
		47.2
		47.4
	ln (K)	47.8 47.6
		48.0
		48.2
		2.750 2.775 2.800 2.825 2.850 2.875 2.900 2.925 1000/T (K 1 )

ACKNOWLEDGMENTS

This work was supported partially by the Direction Générale de l'Armement (DGA) and by the Agence Nationale de la Recherche (ANR) in the frame of the ASTRID project named PULSACION under Grant ANR-19-ASTR-0013-01 as well as by the Région Franche-Comté (NOUGE-CELL project). It was also supported by the Agence Nationale de la Recherche (ANR) in the frame of the LabeX FIRST-TF (Grant ANR 10-LABX-48-01), the EquipX Oscillator-IMP (Grant ANR 11-EQPX-0033) and the EIPHI Graduate school (Grant ANR-17-EURE-0002). The work of Clément Carlé was funded by the Centre National d'Etudes Spatiales (CNES) and the Agence Innovation Défense (AID) and the one of Petri Karvinen by the Photonics Research and Innovation (PREIN) Programme of the Academy of Finland, decision 320166. This work was partly supported by the french RENATECH network and its FEMTO-ST technological facility (MIMENTO).

DATA AVAILABILITY STATEMENT

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

AUTHOR DECLARATIONS

Conflict of Interest

The authors have no conflicts to disclose.


Axelle Mabo

Raphael Borie

Lidwine Wemeau-Stervinou

Yurdagül Uzunhan

Gomez

Gregoire Prevot

Martine Reynaud-Gaubert

Julie Traclet

Emmanuel Bergot

Jacques Cadranel

Sylvain Marchand-Adam

Anne Bergeron

Blanchard

Benjamin Bondue

Philippe Bonniaud

Arnaud Bourdin

Pierre Regis Burgel

Sandrine Hirschi

Charles Hugo Marquette

Sébastien Quétant

Hilario Nunes

Cécile Chenivesse

Bruno Crestani

Yoann Guirriec

Delphine Monnier

Cédric Ménard

Pierre Tattevin

Vincent Cottin

Dr David Luque Paz
email: david.luque.paz@chu-rennes.fr

Stéphane Jouneau

Orphalung Network

Infections in autoimmune pulmonary alveolar proteinosis : a large retrospective cohort

Background

Autoimmune pulmonary alveolar proteinosis (aPAP) is a rare disease, predisposing to an increased risk of infection. A complete picture of these infections is lacking.

Research question

Describe the characteristics and clinical outcomes of patients diagnosed with aPAP, and to identify risk factors associated with opportunistic infections.

Methods

We conducted a retrospective cohort including all patients diagnosed with aPAP between 2008 and 2018 in France and Belgium. Data were collected using a standardised questionnaire including demographics, comorbidities, imaging features, outcomes and microbiological data.

Results

We included 104 patients, 2/3 were men and median age at diagnosis was 45 years. With a median follow-up of 3.4 years (IQR 1.7-6.6 years), 60 patients (58%), developed at least one infection, including 23 (22%) with opportunistic infections. Nocardia spp was the main pathogen identified (n=10). Thirty-five (34%) patients were hospitalised due to infection. In univariate analysis, male gender was associated with opportunistic infections (p=0.04, OR=3.88; 95% CI (1.02 to 22.06)). Anti-granulocyte macrophage colony-stimulating factor antibody titre at diagnosis was significantly higher among patients who developed nocardiosis (1058 (316-1591) vs 580 (200-1190), p=0.01). Nine patients had died (9%), but only one death was related to infection.

Interpretation

Patients with aPAP often presented with opportunistic infections, especially nocardiosis, which highlights the importance of systematic search for slow-growing bacteria in bronchoalveolar lavage or whole lung lavage.

INTRODUCTION

Pulmonary alveolar proteinosis (PAP) is a rare disease, with a prevalence ranging from 4 to 40 cases per million inhabitants.1 2 PAP is characterised by an altered activity of alveolar macrophages that inhibits their physiological role of alveolar recycling of pulmonary surfactant and cell debris. This impairment results in an accumulation of intra-alveolar surfactant and can lead to progressive hypoxaemia.

Different classifications of PAP exist, and PAP can be divided into autoimmune PAP (aPAP)-caused by anti-granulocyte macrophage colony-stimulating factor (GM-CSF) antibody-mediated disruption of GM-CSF signalling-secondary PAP (due to haematological diseases, chronic infections, etc), hereditary or congenital.3-5 aPAP represents more than 90% of all PAP in adults and is characterised by the presence of autoantibodies neutralizing GM-CSF biological activity. [START_REF] Stanley | Granulocyte/macrophage colony-stimulating factordeficient mice show no major perturbation of hematopoiesis but develop a characteristic pulmonary pathology[END_REF] The diagnosis of PAP is challenging, essentially suggested by bronchoalveolar lavage (BAL), described as 'milky', and its cytological analysis reveals periodic acid-Schiff (PAS) positive acellular eosinophilic lipoprotein and foamy alveolar macrophages with PAS-positive intracellular inclusions.7 8 aPAP diagnosis is then made on the presence of anti-GM-CSF antibodies. The antibodies testing can be performed by ELISA, which is the gold-standard technique, or by a functional method.9 10 GM-CSF has a pivotal role in the PAP pathophysiology by controlling alveolar macrophage and neutrophil functions.11 Through its transcription factor PU.1, GM-CSF stimulates differentiation, proliferation and survival of alveolar macrophages.12 13 Autoantibodies blocking GM-CSF activity result in an impairment of surfactant clearance by alveolar macrophages, which can impact gas exchanges. By affecting macrophages differentiation, GM-CSF antibodies also impair their immune functions such as adhesion, pathogen recognition, chemokine secretion and microbial phagocytosis. 14 15 Neutrophils functions are also altered with GM-CSF inhibition, by decreasing adhesive, phagocytic and bactericidal capacities.11 Therefore, patients with PAP are at risk for infections.2 4 16 In PAP, infections occur in 5%-13% of patients and account for 20% of attributable mortality,16 17 with a majority of opportunistic pathogens, such as Nocardia spp, mycobacteria and fungi.18-22 No predictors of opportunistic infections have been identified in literature so far. Available data on aPAP are scarce, mostly based on small series. A large cohort of aPAP in Japan has increased our knowledge of this disease but very few infections were reported (n=12/223, 5.7%), and more generally, data on non-Asian population are still lacking. [START_REF] Inoue | Characteristics of a large cohort of patients with autoimmune pulmonary alveolar Proteinosis in Japan[END_REF] In this study, we aimed to describe the characteristics and clinical outcomes of patients diagnosed with aPAP, and to identify risk factors associated with opportunistic infections through a retrospective cohort.

METHODS

Study population and data collection

We conducted a multicentre retrospective cohort study including all patients diagnosed with aPAP between 2008 and 2018 in 25 centres in France and Belgium (available in online supplemental appendix 1). To build this cohort, we systematically screened the medical record of all patients who had detectable anti-GM-CSF antibody in serum. All the anti-GM-CSF antibody testing in serum had been previously performed at the Immunology Department of the Rennes University Hospital, that is, the unique referral centre for this dosage for participating countries. Then, we included only adults (age ≥18 years) who fulfilled the diagnostic criteria for PAP defined by the presence of PAS-positive acellular eosinophilic lipoprotein and foamy alveolar macrophages with PAS-positive intracellular inclusions in the BAL or lung biopsy. Data were collected using a standardised questionnaire including demographics, comorbidities, imaging features, outcomes and microbiological data. All imaging features were reviewed by two physicians in a blinded fashion. For survival data, the follow-up was censored in October 2021.

Infection was defined as the combination of compatible symptoms, imaging findings and/or microbiological data. Infections were categorised as opportunistic when related to the following micro-organisms: Nocardia spp, Actinomyces spp, Mycobacterium spp, Pneumocystis jirovecii, Cryptococcus spp, Aspergillus spp or Mucorales. Other microorganisms were categorised as non-opportunistic.

Diagnosis of pneumonia was defined as the combination of pulmonary or systemic symptoms of infection, new opacities on chest X-ray or CT scan and/or proven pathogen on respiratory sample. If the suspected pneumonia occurred at the same time as the aPAP diagnosis, or in the absence of previous radiologic examination performed, the identification of a pathogen was mandatory. Bacteria identified in BAL or sputum samples were considered as causative pathogens by the physicians in charge of the patients. We reported any infectious event mentioned in the patient's medical file, even if prior to aPAP diagnosis.

Complete resolution of PAP was defined as disappearance of any symptom, and normalisation of CT and functional tests. Spontaneous resolution was defined as resolution without any aPAP treatment.

Anti-GM-CSF antibody assessment

Anti-GM-CSF antibodies titre was determined by a functional assay, which is the reference one in France and Belgium, measuring the ability of antibodies to neutralise the effect of recombinant cytokine GM-CSF on the growth of GM-CSF-dependent erythroblastic cell line TF1.11 23 Briefly, serial dilutions of sera were incubated with 1 ng/mL GM-CSF (Cellgenix, Freiburg, Germany) and TF1 cells. The proliferation of TF1 cells was measured by incorporation of tritiated thymidine for each dilution. The resulting count-per-minute curve followed a sigmoidal one which allowed for the calculation of a standardised antibody titre inhibiting 50% of TF1 cell proliferation (IC50). The limit of quantification for this technique was a titre of 5. All sera with detectable anti-GM-CSF antibodies using the functional method were also positive with the ELISA technique.

Statistical analysis

We first described the characteristics of patients with and without opportunistic infections. Results were expressed as median and IQR or mean and SD for quantitative variables, and relative frequencies for qualitative variables. Missing data for each variable were excluded from the denominator. The Student's t-test or Mann-Whitney test and Fisher's exact test were used, as appropriate. To assess the risk factors of developing opportunistic infections in aPAP, univariate analysis was performed using logistic regression model. Two-tailed p values were reported, with p<0.05 considered as statistically significant. Statistical analysis was carried out using R-Studio 2020, Integrated Development for R (R-Studio, Boston, Massachusetts, USA).

RESULTS

Baseline characteristics

Of the 375 patients for whom an anti-GM-CSF antibodies dosage was performed in our institution between 2008 and 2018, 141 tested positive. Among them, 104 patients from 25 centres had a confirmed diagnosis of aPAP and were included in the analysis (figure 1). Iterative measurements of the anti-GM-CSF antibodies titre was available for 57% of patients (n=59/104). The diagnosis was obtained by BAL in 71% (n=75/104) or lung biopsy in 25% (n=26/104) of cases. Data were missing in the three remaining cases.

Baseline characteristics of the cohort are summarised in table 1 and online supplemental table S1. Almost two-thirds of patients were men and median age at diagnosis was 45 years. Fiftynine (58%) patients were smokers and 36 (35%) were exposed to dust inhalation. Twenty-two patients (21%) were immunocompromised, due to immunosuppressant treatments or underlying disease other than aPAP. HIV status was negative for all patients. At the time of diagnosis, 89% of patients were symptomatic and the most frequent symptoms were dyspnoea (77%) and cough (46%). Twenty patients (19%) presented weight loss.

Other symptoms, such as fever (7%) or digital clubbing (7%) were rare. Five out of seven patients with fever at aPAP diagnosis had non-opportunistic pneumonia with proven microbiological documentation in 4 cases (Staphylococcus aureus, n=1; Escherichia coli, n=1; Chlamydia pneumoniae, n=1; Haemophilus influenzae, n=1; not documented, n=1). For the two other patients, fever was not related to an infection.

Median lung volumes on pulmonary function test, such as forced vital capacity, forced expiratory volume in 1 s and total lung capacity, were not impaired. However, median diffusing capacity for carbon monoxide was 56% of predicted values, confirming the gas exchange alteration.

Characteristics of infections

We identified 74 infections in 60 patients during the follow-up (median follow-up 4.3 years; IQR 1.7-10.8). Twenty-three patients had opportunistic infections (table 2). Sixty-five Per cent of them (n=15/23) occurred at the time of diagnosis of aPAP. Two opportunistic infections (one tuberculosis and one nocardiosis) and seven non-opportunistic infections occurred before aPAP diagnosis.

Nocardia spp was the main pathogen identified (n=10). Only three of these patients required intensive care unit (ICU) admission, and no death related to an opportunistic infection was reported at the end of follow-up. Nocardiosis mostly involved Nocardia abscessus (n=4), Nocardia pneumoniae (n=2) and Nocardia paucivorans (n=2). Pneumonia (n=6) and brain abscesses (n=5) were the two most frequent form of nocardiosis, and only one patient had both lung and cerebral nocardiosis. None of them was immunocompromised. All patients with nocardiosis were hospitalised, including two patients admitted in ICU.

For non-opportunistic infections, 84% of patients (n=31/37) had pneumonia (online supplemental table S2). In less than half of cases, there was no microbiological pathogen identified (46%, n=17/37). Eleven patients presented at least two infectious events. Among them, four presented one opportunistic infection and at least one non-opportunistic infection; five other patients had at least two non-opportunistic infections reported. Two patients developed two different opportunistic infections (nocardiosis and atypical mycobacteriosis; nocardiosis and pneumocystis).

Thirty-five of all patients (34%) were hospitalised due to infection, primarily nocardiosis (n=10) and pneumococcal pneumonia (n=6). Three patients presented an opportunistic infection (two mycobacteriosis and one nocardiosis) and received subcutaneous GM-CSF in addition to anti-infective therapy. One patient died of Pseudomonas aeruginosa pneumonia.

Risk factors associated with infections

The comparison of patients who developed opportunistic infections and those who did not is summarised in table 1. In univariate analysis, male gender was associated with opportunistic infections (p=0.04, OR=3.88; 95% CI (1.02 to 22.06)). Patients with opportunistic infection tended to have lower body mass index than the rest of the population, but it was not statistically significant. All other characteristics, including age, comorbidities or symptoms at diagnosis, were similar between the two groups. The risk of opportunistic infections did not differ in patients previously treated by rituximab, corticosteroids or other immunosuppressive therapy.

We found no correlation between the serum anti-GM-CSF titre and the occurrence of infection (p=0.49), nor with the occurrence of opportunistic infection (p=0.51). Patients presenting infections requiring hospitalisation tend to have a higher anti-GM-CSF titre, without reaching statistical significance (p=0.051). When focusing on patients with aPAP who had developed nocardiosis, anti-GM-CSF titre at diagnosis was significantly higher (1058 (316-1591) vs 580 (200-1190), p=0.01).

Treatments and outcomes

For the whole cohort, the median follow-up was 3.4 years (IQR 1.7-6.6 years). Spontaneous improvement of the disease was observed in 28% of patients (n=29/104), among which 11% presented complete resolution (n=11/104). Seventy patients (67%) received specific treatment of aPAP, primarily whole lung lavage (WLL) (n=55/70, 79%), subcutaneous GM-CSF (n=26/70, 37%) and rituximab (n=21/70, 30%). Outcome was favourable in 81 patients (78%), including 30 complete resolutions. Nine patients died, with a 5-year overall survival of 95% (online supplemental appendix 2). Only one death was related to the progression of aPAP due to a fibrotic evolution. The other causes of death were: malignancy (n=3), P. aeruginosa pneumonia (n=1), terminal respiratory failure secondary to chronic thromboembolic pulmonary hypertension (n=1) or to chronic obstructive pulmonary disease (n=1) and unknown (n=2).

DISCUSSION

The main findings of this large cohort of patients diagnosed with aPAP are as follows: (1) opportunistic infections are common in patients with aPAP (22%, with a median follow-up of 3.4 years), primarily due to Nocardia spp (n=10); (2) despite important rates of opportunistic infections, the survival remains high; (3) male gender is an independent risk factor for opportunistic infections and (4) a high GM-CSF antibody titre at diagnosis is associated with the risk of nocardiosis.

To the best of our knowledge, this cohort study of aPAP patients is the largest in non-Asian population so far. In this work, population characteristics were consistent with previous studies. [START_REF] Inoue | Characteristics of a large cohort of patients with autoimmune pulmonary alveolar Proteinosis in Japan[END_REF] [START_REF] Long | Clinical characteristics of pulmonary alveolar Proteinosis[END_REF][START_REF] Bonella | Pulmonary alveolar Proteinosis: new insights from a single-center cohort of 70 patients[END_REF][START_REF] Briens | Pulmonary alveolar Proteinosis[END_REF] The increased susceptibility to infections in aPAP has been reported and is supported by a strong rationale. [START_REF] Ataya | The role of GM-CSF Autoantibodies in infection and autoimmune pulmonary alveolar Proteinosis: A concise review[END_REF] The loss of GM-CSF signalling results in altered functions of alveolar macrophages: (1) impaired differentiation, (2) decreased bacterial recognition adhesion and killing and (3) reduced phagocytosis. 3 12 18 27 We hypothesise that impairment of GM-CSF signalling could increase the susceptibility to ubiquitous environmental pathogens, such as Nocardia or non-tuberculous mycobacteria which were the most frequent pathogens identified in our cohort. Human monocytes are able to produce GM-CSF in presence of Nocardia in vitro, suggesting a key role of GM-CSF pathway in nocardiosis pathogenesis. [START_REF] Rosen | Nocardia-induced granulocyte macrophage colony-stimulating factor is neutralized by Autoantibodies in disseminated/Extrapulmonary Nocardiosis[END_REF] In addition, the accumulation of cholesterol into foamy macrophages could represent a source of energy for some pathogens, especially mycobacteria, facilitating persistent infection. 29 30 Only small sample size case series has highlighted the risk of opportunistic infections for aPAP patients. [START_REF] Saijo | Anti-granulocyte-macrophage colony-stimulating factor Autoantibodies are a risk factor for central nervous system infection by Cryptococcus Gattii in otherwise immunocompetent patients[END_REF] 20 31 32 Nevertheless, a large Japanese cohort of 223 patients with aPAP only reported 12 infections (5.6%) over a 6-year follow-up, including four aspergillosis and five mycobacteria pulmonary diseases. This low rate could be explained by ethnic differences or by a selection bias, which has been suggested by the authors themselves. Indeed, their study mainly assessed risk factors for disease severity and progression, while our study focused on infections. As 65% of opportunistic infections occurred at diagnosis, we consider it unlikely that results be affected by the time of follow-up. A literature review from 1950 to 2010 reported 75 cases of opportunistic infections among aPAP patients.18 Conversely to our study, Punatar et al found a larger proportion of tuberculosis and several fungal infections due to Histoplasma capsulatum-which is not encountered in Western Europe-or Cryptococcus spp. In our work focusing on infectious diseases, the proportion of patients developing opportunistic infections during the follow-up was high (22%), which suggests a causal link between aPAP, lung immune deficiency and respiratory tract infections.27 Although most infections are not eligible for preventive therapy, this study emphasises the need for comprehensive microbiological extensive investigations in these patients-including slowgrowing bacteria-in BAL or WLL.

Nocardia was the main opportunistic pathogen reported in our cohort (9%). Nocardiosis is a rare opportunistic infection that generally occurs in immunocompromised patients, particularly in those with phagocyte defects.33 GM-CSF pathway is involved in the phagocytosis abilities of alveolar macrophages. Rosen et al reported seven cases of nocardiosis in patients not known to be immunocompromised, and anti-GM-CSF antibodies were found among five of them.31 Interestingly, we found that anti-GM-CSF antibody titre at diagnosis was higher among patients who developed nocardiosis during follow-up. None of the aPAP patients died from nocardiosis, contrasting with the poor outcomes reported in immunocompromised hosts. 34 35 The high proportion of nocardiosis found in our cohort, and the association with the GM-CSF antibody titre, are in line with a previous study suggesting a tight relationship between this pathogen and the GM-CSF pathway. [START_REF] Rosen | Nocardia-induced granulocyte macrophage colony-stimulating factor is neutralized by Autoantibodies in disseminated/Extrapulmonary Nocardiosis[END_REF] We found a low mortality rate of 9% in the whole cohort, associated with almost 80% of patients with clinical, partial or complete, improvement confirming the good prognosis of aPAP. Only two deaths were related to aPAP: one chronic respiratory failure and one death from P. aeruginosa pneumonia. In the literature, the 5-year survival is currently around 95%.1 2 16

Our study has several limitations. First, this study suffers from potential biases inherent to its observational retrospective design. For example, non-opportunistic infections were poorly documented, and some other infections might have been treated on an outpatient basis, without being reported in patient medical records. However, the proportion of missing data remains low in this cohort. Second, due to the long study period and the numerous participating sites, the management of aPAP was not standardised and may differ. Anyway, this limitation did not affect the assessment of risk factors of opportunistic infections. Third, anti-GM-CSF antibodies titres were quantified using a functional method, whereas ELISA technique is considered as the gold standard. All sera with detectable anti-GM-CSF antibodies using our functional method were subsequently confirmed with the ELISA technique. However, as ELISA assay quantifies antibody concentration against whatever GM-CSF epitope, it might detect antibodies with no pathogenic activity, while our functional technique selects patients with antibodies blocking GM-CSF activity, which may be more relevant and accurate. However, this would make our result of a higher titre of anti-GM-CSF antibodies at diagnosis in patients who developed nocardiosis difficult to confirm worldwide. Fourth, our study was not powered to perform subgroup analysis, especially to assess the impact of additional immunosuppressive therapies on the occurrence of opportunistic infections.

In this large multicentre cohort, the rate of opportunistic infection, especially nocardiosis, was high. These infections did not impact the long-term overall survival rate. This study highlights the importance of systematically searching for slow-growing bacteria when BAL or WLL is performed. Further studies are needed to improve our knowledge on the pathophysiology of aPAP and its infectious complications.

Acknowledgements

We are very grateful to all investigators who contributed to this study: RB, BC, LW-S, YU, HN, EG, GP, MR-G,VC, E Bergot, JC, SM-A, A Bergeron, E Blanchard, BB, PB, A Bourdin, PRB, SH, CHM, SQ, DM, CM, PT, SJ, Daniel Bonnet, Loïc Grandon, Anne Gondouin, Marie Jouvenot, Audrey Creachcadec, Luc Thiberville.

Funding

The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

Data availability statement

Data can be made available upon reasonable request.

Contributors

AM fulfilled the role of guarantor. DLP did the analysis. AM, DLP, PT and SJ cowrote the original draft. AM, RB, LW-S, YU, EG, GP, MR-G, JT, E Bergot, JC, SM-A, A Bergeron, E Blanchard, BB, PB, A Bourdin, PRB, SH, CHM, SQ, HN, CC, BC, YG, DM, CM, PT, VC, DLP and SJ contributed substantially to the investigation and data collection, as well to reviewing and editing the manuscript.

Competing interests

The authors reported no conflict of interest related to this work. SMA reports having received consultancy for board membership, consultancy or speaker fees from AstraZeneca, Boehringer Ingelheim, Novartis and Roche, GSK, BMS, Chiesi and Pfizer; and travel support from Boehringer Ingelheim. Other authors have nothing to disclose.

Patient consent for publication

Not applicable.

Ethics approval

The study was approved by the Ethics Committee of Rennes University Hospital (approval number 19.104). Patients were informed about the study and could refuse to be enrolled. Written informed consent was waived.

Provenance and peer review

Not commissioned; externally peer reviewed.


Clément Carlé
email: clement.carle@femto-st.fr

Shervin Keshavarzi

Andrei Mursa

Petri Karvinen

Ravinder Kumar Chutani

Sylwester Bargiel

Rémy Vicarini

Moustafa Philippe Abbé

Abdel Hafiz

Vincent Maurice

M Abdel Hafiz

R Boudot

N Passilly

Mitigation of He permeation in microfabricated vapor cells with alumino-silicate glass and Al2O3 coatings

Keywords: Microfabricated cells, alumino-silicate glass, 𝑨𝒍 𝟐 𝑶 𝟑 coatings, permeation rate, buffer gas, frequency stability

I. INTRODUCTION

Vapor cell atomic clocks, magnetometers or gyroscopes permit the measurement of physical quantities with extreme precision [START_REF] Budker | Optical magnetometry[END_REF][START_REF] Kitching | Atomic sensors: a review[END_REF]. With the progress of microfabrication technologies and integrated photonics, small-size, low-power atomic devices have become real and are now deployed in a large spectrum of applications [START_REF] Kitching | Chip-scale atomic devices[END_REF]. Such chip-scale atomic devices relies on an alkali vapor micro-fabricated cell usually made of a silicon-etched cavity sandwiched between two anodically-bonded glass wafers. In atomic clocks, variations of the cell inner atmosphere can be responsible for variations of the clock transition frequency through the pressure shift [START_REF] Kozlova | Temperature and pressure shift of the Cs clock transition in the presence of buffer gases: Ne, N2, Ar[END_REF], leading to clock frequency instabilities. It is then of crucial importance to maintain alkali atoms in a well-controlled and stable cell inner atmosphere for stability performances.

Gas permeation through the cell glass windows was identified among the possible causes of the cell inner atmosphere evolution, especially when light buffer gases, such as helium or neon, are used [START_REF] Rushton | Contributed review: The feasibility of a fully miniaturized magneto-optical trap for portable ultracold quantum technology[END_REF][START_REF] Norton | Permeation of gases through solids[END_REF]. In [START_REF] Abdullah | Aging studies on micro-fabricated alkali buffer-gas cells for miniature atomic clocks[END_REF], the leak of Ne through the BSG windows of a micro-fabricated cell was estimated to limit the clock fractional frequency stability at a level of about 5x10 -11 at 1 day, at a cell temperature of 80°C. In [START_REF] Dellis | Low helium permeation cells for atomic microsystems technology[END_REF], the use of alumino-silicate glass (ASG) for microfabricated cells was demonstrated to reduce He permeation by more than two orders of magnitude, in comparison with borosilicate glass (BSG). In [START_REF] Dellis | Low helium permeation cells for atomic microsystems technology[END_REF], helium permeation rate was extracted by measuring the positive drift of the frequency of a CPT clock due to the progressive entrance of helium into the cell (placed inside a chamber filled with He). In [START_REF] Carlé | A pulsed-CPT Cs-Ne microcell atomic clock with frequency stability below 2x10 -12 at 10 5 s[END_REF], the midlong-term fractional frequency stability of a pulsed Cs-Ne microcell atomic clock was improved by the use of a MEMS cell featuring ASG windows. In this study, we report on measurements of helium permeation rate of glass windows of more than 25 microfabricated alkali vapor cells filled with He, and built with ASG or BSG windows, combined or not with Al2O3 layers. The permeation rate is extracted from routine measurements of the pressure-shifted clock frequency over weeks to months.

II. METHODS AND RESULTS

Helium permeation through the glass windows of microfabricated cells is measured with a CPT clock setup. The typical fractional frequency stability at one day of the clock is about 10 -10 , leading to a resolution on the gas pressure variation of about 1 mtorr of helium per day [START_REF] Beverini | Buffer gas pressure shift in the mF=0->mF=0 ground state hyperfine line in Cs[END_REF]. In order to extend the number of tested cells, we have developed an experimental setup that embeds six cells packages on an motorized rotation platform so that each cell can be interrogated sequentially in clock configuration. The setup uses a unique and fixed optical path provided by a verticalcavity surface-emitting laser (VCSEL) directly modulated at 4.596 GHz. The MEMS cell technology is comparable to the one described in [START_REF] Vicarini | Demonstration of the mass-producible feature of a Cs vapor microcell technology for miniature atomic clocks[END_REF]. Various cell wafers were developed with different glass substrate configurations. Helium permeation rate is extracted from the fit with an exponential decay function of the clock frequency experimental data, using the gas permeation law [START_REF] Abdullah | Aging studies on micro-fabricated alkali buffer-gas cells for miniature atomic clocks[END_REF][START_REF] Dellis | Low helium permeation cells for atomic microsystems technology[END_REF]. Figure 1 shows an example of a clock frequency measurement of a Cs-He cell using BSG, at a cell temperature of 70°C, over about 40 days. In this example, the extracted permeation rate K is about 5.8x10 -19 m 2 .s -1 .Pa -1 . Our measurements show that the deposition of Al2O3 coatings onto BSG reduces He permeation rate by a factor up to 100 in comparison with bare BSG. Despite some dispersion in the measured performances for BSG cells coated with Al2O3 layers, attributed to the different atomic layer deposition (ALD) processes used, our results tend to demonstrate that Al2O3 coatings, known to slow down alkali consumption and then its disappearance in MEMS cells [START_REF] Woetzel | Lifetime improvement of micro-fabricated alkali vapor cells by atomic layer deposited wall coatings[END_REF][START_REF] Karlen | Lifetime assessment of RbN3-filled MEMS atomic vapor cells with Al2O3 coating[END_REF], are also able to mitigate He permeation in MEMS cells. ASG is found to reduce He permeation with a permeation rate lower than 2x10 -21 m 2 .s -1 .Pa -1 at 70°C. In comparison with BSG, the He permeation is reduced by more than two orders of magnitude, in correct agreement with results reported in [START_REF] Dellis | Low helium permeation cells for atomic microsystems technology[END_REF]. Our studies also allow the extraction of the energy of activation of ASG, not reported in the literature to our knowledge.

Figure 1 :

 1 Figure 1: Temporal trace of the clock frequency for a Cs-He cell with BSG windows.