Lassana Cissé

Thierry Joffroy
email: thierry.joffroy@grenoble.archi.fr

EARTHEN ARCHITECTURE HELPING THE VICTIMS OF THE 2007 FLOOD IN BANDIAGARA (MALI)

Keywords: Climatic change, flood, improved traditional techniques, development

On 4 July 2007, the river crossing the city of Bandiagara overflowed due to unusually heavy rains. Within few minutes, 194 families (nearly 1,500 people), witnessed the devastation of all or part of their homes and means of subsistence. At that time, the Cultural Mission of Bandiagara and CRAterre were collaborating in the framework of the project « Renforcement des capacités locales pour une meilleure contribution du secteur de la construction au développement durable du pays dogon » (Reinforcement of local capacities for a better contribution of the construction sector to the sustainable development of Dogon country) financed by the European Union; they called upon institutions to enable them to include, as part of their activities, a contribution to the reconstruction of basic housing structures. The German organization "Misereor" and the Abbé Pierre Foundation based in France responded positively by funding the reconstruction of 20 houses for sheltering the most affected families.

The project was carried out in collaboration with the local Catholic Parish (Caritas), the municipality, the Flood Victims' Association, district chiefs, and the prefecture. A Committee was established to carefully select the 20 recipient families, which would commit to collecting materials from their affected properties and participate in the construction-training activities implemented. This reconstruction project benefited from the results of an inventory of the building cultures in Dogon country that included the study of land plots, building typologies and construction techniques. On the basis of these principles, twenty basic houses were designed and built, each with a different plan, defined by the types of materials recovered, the priorities identified by the families and the possibilities for future extensions. The reconstruction process was an opportunity to demonstrate and train workers to master innovative flood-proof building techniques and combine them with traditional practices.

INTRODUCTION

Due to perceptible climatic change in the Sahel over the past decade, localised torrential rains have generated a sudden and brutal rise of waters never before seen on the Dogon Plateau. On 4 July 2007, high waves burst the Yamé riverbed (a tributary of the River Níger, that passes through Bandiagara). In the town of Bandiagara, many families living along the Yamé River lost almost everything (homes, food stocks, materials, etc). In light of the seriousness of this catastrophe, volunteers brought emergency aid to ease the suffering of the victims. Within this framework, the Cultural Mission of Bandiagara and CRAterre launched the idea of a reconstruction programme for the most affected families, which would complement efforts by the municipality to provide the affected families with land to build on. After a pilot study (Léon house) that helped define the technical and financial references, a project proposal was developed and submitted to Misereor through the Catholic parish, and then to the Abbé Pierre Foundation. The project became a reality when funds were obtained for the construction of houses for 20 families affected by the floods, which -together with the pilot study family -brought to 21 the number of beneficiary families. The complementary activity for training workers on the building sites was financed by the European Union within the framework of the project "Reinforcement of local capacities for a better contribution of the construction sector to the sustainable development of Dogon Country", which had been running since 2007. On the ground, the project brought together the skills of the Catholic parish through Caritas/APH, the Cultural Mission of Bandiagara, CRAterre and the architectural firm Audex. In order to follow-up the implementation of the project, Misereor also engaged two technicians from the Bafoussam Professional Centre in Cameroon, which enabled a South-South exchange.

APPROACH AND STRATEGY FOR PROJECT IMPLEMENTATION

The underlying idea of the project was to use the reconstruction effort to demonstrate that the use of local materials, especially earth, can be compatible with resistance to adverse weather, and that this type of sustainable construction could be affordable and accessible to more people. In fact, with a budget of 3,000 Euros per family, it is indeed possible to achieve quality construction and to have enough space to re-settle families in decent conditions. The costs of construction to such a standard would also ensure favourable conditions for the extension of future homes, as well as duplication of these methods by the masons trained in improved construction techniques.

The project was also able to show how such reconstruction projects could be carried out in a sympathetic manner, while adapting to:

 the priority needs of the families in the construction programme,  the specificities of the land and the distinct constraints of common ownership,  the real possibilities that each beneficiary had something to contribute.

This contribution would allow us to propose certain additions to each family's construction plan (extra rooms, fencing, covered balconies, etc). Each family could choose from among:

 Participation through volunteer labour; in this case, a work-day is estimated at the same price as a labourer or mason hired for the project, and the overall is taken into account;  Contribution of materials (re-use of recovered materials, production and supply of earthen bricks, contributions in sand, stone or other local resources, etc.),  Financial contribution.

In this way, each family receives an identical 'base' financial package but can have a variety of proposed construction plans, adapted to each layout, instead of replicating any single model.

Concerning the strategy for the dissemination of knowledge, it was envisaged that the project would have three main phases; at each stage, masons would be trained to independently take charge of a site at the next phase. Phase 1: Pilot sites for two families. These site projects would support training, ie. "training project". Each of the two site projects would be executed by a team of three trainee masons, each of whom would become trainers on the next site project, with the support of technicians hired for the project. Phase 2: The idea was that the head masons of six projects would be those who were trained in Phase 1; a newly recruited mason would have to help them in their work and thus benefit from their experience to learn and in turn become the head mason in the last phase. Phase 3: This last phase of the project involved the last 12 families, and at this stage a dozen masons who, theoretically, were autonomous enough to lead the process.

This "phasing" also allowed us to put in place a training strategy that resulted in 12 masons and 24 labourers (each mason being accompanied on the site by two labourers). This strategy is illustrated in the diagram below:

SELECTION OF BENEFICIARIES

Since the available funds would not be able to assist all the victims, the organisations in charge of the implementation of the project established 12 criteria that would enable the most vulnerable families to access the resources of the reconstruction assistance project. A call for applications was launched on 28 September 2009 through posters at relevant buildings and services such as the Town Hall, the Prefecture, "Conseil de Cercle" (District chiefs), "APH", etc., as well as through announcements on local radio stations. Finally, a meeting took place with the area chiefs and the Flood Victims' Association. An application dossier was developed to facilitate the evaluation of candidates based on the established criteria.

In order to select the beneficiaries from among the 44 applicants, a selection committee was established, comprising technical services representatives from Urbanism and Settlement, Decontamination and the Fight against noise and pollution, from the Municipality of Bandiagara, Caritas/OPAH Bandiagara, the Flood Victims Association, the Cultural Mission of Bandiagara, and from the architectural firm Audex.

The establishment of the final list of beneficiaries was made in the course of three meetings and site visits facilitated by the City Council. The results were announced through posters and on local radio, following which the 20 beneficiaries were invited to the Cultural Mission to be briefed in more detail on:

1. The voting and selection process; 2. The overall principles for the works, ie.: a. A fixed financial package for each family and the possibility to enhance the construction plans depending on each family's own contribution; b. The possibility for each family to define the plan that best suits it according to their specific needs within a range of proposed features (bedrooms, living rooms, covered balconies, verandas, latrines, fencing, etc.); c. The availability of local materials (esp. earth), etc. 3. The division of the reconstruction programme into several phases and the impending construction works at the first two projects/building sites; 4. The procedures, especially how the work was to be organised with them; The meeting also had a Question and Answer session.

PERSONALISED DESIGNS This work was carried out on the basis of an inventory of building cultures in Dogon

Country, previously carried out by the Cultural Mission, NGO Radev-Mali and CRAterre within the framework of the EC-funded project "Reinforcement of local capacities for a better contribution of the construction sector to the sustainable development of Dogon Country". This enabled us to focus on the bigger elements, at the level of contemporary local practices for land (plots) use, as well as on construction typologies, including in Bandiagara. Using the results of this work, base models were conceived: one-bedroom, two or three rooms, balcony, covered veranda, bathroom unit, and kitchen. Aside from these models, numerous combinations could be offered. Concurrently, each model was costed, which also made it possible to make quick estimates on the price of planning proposals that would come out of the discussions with the beneficiary families. A table of costs was developed, allowing us to make variations based on different parameters: the amount of each component selected for the construction (eg: one, two or three rooms, one kitchen or none, one bathroom unit), as well as the type of finishings (sealant, screed, etc.). Despite the amount of time it took to put this tool in place at the beginning of the project, it was indispensable for customising the plans.

These specificities of each beneficiary family -and also of the environment and the urban framework within which the constructions will happen -have, since the formulation of the project, caused us to think about developing a participatory effort that would allow us to take into consideration:  the priority needs of each family  their capacity to contribute (financially or in kind)  the necessity to protect against strong winds (and rain) from the East  the orientation, nature (rocky or not), the slope and the characteristics of common land ownership (water outlets, buildings on the edge of plots, etc.)

To this end, the representatives of the relevant technical organisations (Cultural Mission, CRAterre and the architectural firm Audex) organised a series of individual meetings and site visits with the beneficiaries so that they could express their needs and priorities, and also give their opinions on the proposed plans and the actual implementation strategy.

Description of the process for designing plots distribution:

 A first meeting made it possible to list the particular needs of each family, to identify the priorities, to take note of some technical or aesthetical preferences (rendering, types of windows), optional additions of other details (eg. ventilation duct) and to be conscious of potential recommendations. It was also an opportunity to confirm the commitment that each family pledged in their application, so as to better assess the amount needed and the additional funding possibilities.  Working time (without the family) was then set aside to develop the proposals to be submitted to the families, keeping in mind their demonstrated needs and the financial package available to the family based on their own contribution.  A second meeting took place, whereby each family was presented with the various proposals that fit within the given budget (at least two different choices, and sometimes as many as five, given the possible combinations). The decision was then made with the help of a prepared model, allowing the families to get a better idea of their construction project and how it would fit into their plot.  The ratification of one proposal was then made by the family, either on the same day, or after consultation with other family members.  The family was invited one last time to the Cultural Mission to sign a contract.  Model and photo  Contract between the family and the partners (and, eventually, receipt for the funds)  Construction follow-up forms  Eventually: the family's budgetary participation, kept at the Cultural Mission of Bandiagara The dossier remains available for consultation at all times by the beneficiary family; especially the model, to which everyone can easily refer.

DESCRIPTION OF THE MAJOR BUILDING PHASES

We must keep in mind that the works were initially planned in such a way as to have three phases, which would allow for the training of masons and transmission of expertise during the different building stages. The order of the construction was not left to chance, and a strategic choice was made. The first two site projects and plots were selected because of their proximity to the Cultural Mission of Bandiagara and their proximity to each other, to facilitate movement by the technicians between the projects and follow-up.

Another criterion was added to this: the first two families to benefit from the programme were also among the most needy; as such, it was decided to prioritise widowed women with no resources. The second group (six projects) was established in the same way: plots located in a restricted area and families in the most difficulty. The third phase, which in theory would require fewer follow-ups, involved the remaining 12 families.

It is important to note that the training strategy was not able to completely fulfil its objectives. The masons trained in the first and second phases of the works exhibited varying levels and apprenticeship ability. As such, at the end of the second phase, those masons who were sufficiently independent and competent were allocated site projects, whereas the others stayed in their role of assistant.

First phase:

The procedure with the first two families was launched at the beginning of November 2009 and the two (2) site projects (on plots O/6 and R/1) were started soon thereafter, on 19 November. The training element was also primarily made possible through support of a technician from Audex and the Cameroonian foreman made available by Misereor. The technician from Audex, having worked closely with CRAterre since the end of 2007 within the framework of the EC-funded programme, already participated in several works of this nature (in terms of local material, esp. earth), and was thus able to become a trainer at this stage.

Through some sessions on theory by the CRAterre expert at the beginning of the works, the masons and the supervisory staff were reminded of some basic principles. In fact, the works should have begun with the professionals who had already benefited from training sessions within the framework of the EC-funded programme, and who were already skilled in improved earthen construction techniques. It was important to count on competent masons, who were able to transmit their skills.

Second phase:

The second phase of six building projects took off on 22 December 2009. At this stage, six extra masons joined the main team, bringing the team to a total of twelve masons; each builder working with two labourers. Therefore, about 36 professionals were working simultaneously, which explains the speed of the works. Mid-way through the project, a session of technical review was carried out, so as to emphasise some details that were still not being observed and, as such, may have been misunderstood. This short training, which was both theoretical and practical, was organised by leaning heavily on the technicians on site, so as to reaffirm their role as trainers of the builders, and as the ones who would ensure the proper execution of the works.

A supplementary training session was organised during this phase, in February 2010, for the construction of kitchens that were built without wood, and whose roofs were domed, made from unfired earthen bricks (adobe). The transmission of skills having been done, the local team carried out the next cupola projects independently.

Third phase:

This last phase of execution was launched on 8 February 2010, simultaneously with the second round of works on the first eight contracts. The work plan was developed with the aim of finishing the construction and handing over the keys to the beneficiaries before the onset of the rainy season. In order to allow enough time to dry before plastering the houses, the project advanced slowly. It was first of all preceded by the execution of the main works of the last houses, then the complete execution of the finishing (plasterwork, interiors and exteriors, floors, balconies, placing of doors and windows) of the first eight houses and then of all the remaining houses.

Unfortunately, at this stage, the pond from which the water was drawn ran dry. To overcome this obstacle, it was necessary to transport water from a source about 5km from the town of Bandiagara. This was made possible thanks to the support of the Catholic Mission that provided a mobile tanker, with the pick-up of the Cultural Mission. This exemplified the importance of planning the projects well from the beginning, when faced with this type of climate. In this way the building works, which used a lot of water, were carried out before embarking on the finishing touches. This was also what was done, from the beginning of the project, with the estimate of the number of bricks that would be required for all the building sites and the anticipated mass production of adobe just after the rainy season, so as to take advantage of the natural reserves of rainwater.

EPILOGUE AND CONCLUSIONS

Despite some difficulties during the implementation, and a slight delay in the anticipated delivery date, all the social partners of Bandiagara deemed the execution of this project as a success. The handing-over of the keys was done right at the beginning of the rainy season (early July 2010), which also provided shelter for the beneficiaries during that time.

On the whole, the follow-up and training programme was fruitful. It is now clear that the strategy to have the builders become progressively more responsible should have been revised based of the different levels expertise they gained; we are nonetheless aware that an investment in training allowed us to strengthen and eventually multiply the skills for the execution of a building project by young builders.

Through the promotion of local materials and practices, the project created a lot of employment for builders, labourers, transporters, etc., which resulted in increased economic benefits for the population of Bandiagara. In addition, the construction of the buildings allowed the masons, labourers and brick builders to learn new techniques. The project thus introduced ideas and skills that are today being built upon to improve the availability of basic housing.

After the rainy season, most of the project homes were the subject of new works, executed by the families to expand their basic housing (addition of extra rooms), or to finish the funds provided (equipping the kitchen, boundary wall, etc.). The evolutionary aspect of the homes, through the integration of lintels in the masonry, allowed the placement of doors in the walls. As such, the beneficiaries could easily add rooms, economising on wall space and overall space on plots that are often home to many families.

As flood victims, their first instinct would have been to build new houses out of concrete blocks and sheet metal -which would have been smaller. Based on the experience in the improvement of traditional homes (foundations, method of execution of the basement, capillary barriers, quality of bricks, etc.), the local technicians and the families are now convinced about the potential of high quality earthen construction, suited to their own means.

It is worth noting that this acceptance was especially demonstrated by some people on projects outside this programme, who adopted technical details or proposed construction typologies. The technicians have also had some initiatives and started to apply, when they deemed fit, certain construction principles learnt in the course of the reconstruction project. This was notably the case in the construction of the boarding facility for students at the Protestant parish.

In addition, the project was visited by representatives of Delegation of the European Union in Bamako, as well as the Governor of the Mopti region, and His Excellency Mohamed El Moctar, the Minister of Culture. This visit took place on 7 May 2010, with the political and administrative authorities of the District of Bandiagara. This was good publicity to show these authorities the possibilities offered by the effective use of local materials.

As a result of its success, the reconstruction programme became known to the national television station's programming editors. A news reporting team was sent to Bandiagara by the directors of the station; this resulted in a 10-minute programme broadcast in a widely-listened to one-hour slot during the course of March 2011, and several more times in the same year.

With regard to the results and the benefits, we can say that the project fulfilled the expectations of both the partners and the participants. As concerns the beneficiaries, the flood of 2007 is now but a bad memory for them. Thanks to this humanitarian action, they were able to be housed and, above all, to have a home of their own, for their children and their parents. This resettling was, above all, undertaken with a view to establishing a procedure for sustainable development, while strengthening the capacities of the building workers and technicians. It also provided a better understanding by the local and national authorities of the solutions offered by local materials. For the local population this project increased their resilience by decreasing dependence on foreign construction techniques and materials.

Brief bios:

Lassana Cissé holds a Bachelor of Philosophy. He has been working for more than 20 years with the Ministry of Culture and is the curator of Bandiagara Cliffs World Heritage Site. He is a member of ICOMOS-ISCEAH and is often contracted for provision of services to ICOMOS (expertise), UNESCO-WHC (periodic reporting) and for the elaboration of EU calls for projects.

Thierry Joffroy, architect DPLG, is specialised in earth construction and has been working in this field for the last 25 years. He is currently the Chairman of CRAterre, and the pedagogical coordinator of CRATerre-ENSAG post-Masters course. In 2010, he was awarded the Conservation medal of the Academy of Architecture for his work with CRAterre in the field of heritage conservation.

Fig. 1 -

 1 Fig.1 -The house built for the Adama Dramé family -AP4 (credit: Joffroy Thierry, 2010)

Fig. 2 -

 2 Fig.2 -Construction -training strategy (credits: Laure Cornet, 2009).

Fig. 3 -

 3 Fig.3 -Left: Analysis of urban fabric in Bandiagara and traditional organisation of a house plot. Right: Description of house structure (credits: Mathilde Chamodot, Basile Cloquet, 2008).

Fig. 4 -

 4 Fig.4 -Work sessions with the beneficiaries (Credits: Laure Cornet, 2009)

Fig. 5 -

 5 Fig.5 -Construction of cuploas (Credits: Laure Cornet, 2010)

Fig. 6 -

 6 Fig.6 -Adobes ready for the self continuation of the recovery (Credits : Thierry Joffroy, 2011)


Mariette Moevus
email: mariette.moevus@gmail.com

Romain Anger
email: romain.anger@grenoble.archi.fr

Laetitia Fontaine
email: laetitia.fontaine@grenoble.archi.fr

HYGRO-THERMO-MECHANICAL PROPERTIES OF EARTHEN MATERIALS FOR CONSTRUCTION: A LITERATURE REVIEW

Keywords: earthen materials, hygro-thermal properties, mechanical properties, literature review

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1

INTRODUCTION Building with earth requires that artisans have a very good knowledge of the material. Yet, there is limited scientific knowledge of the material. While builders develop their knowledge on the field, through direct experience, engineering consulting firms are hampered by a lack of reliable data regarding the properties of earth. Norms and standards for earth construction are few, partial and often deal with CEBs, stabilized with cement.

However, there are many other reliable construction techniques involving earth materials that do not require the use of mineral stabilizers, as demonstrated through centuries-old examples such as the buildings, mosques and skyscrapers in the towns of Ghadames, Libya and Shibam in Yemen, the castle of Baños de la Encina in Andalusia, the Mosque of Djenne in Mali, some sections of the wall of China, the tulous, large residential complexes of the Hakka in China, or more locally the many farms built with earth around the Rhône-Alpes region in France [START_REF] Fontaine | Bâtir en terre[END_REF]. These examples clearly show that it is possible to use earth for the construction of sustainable buildings.

This report is an update on the knowledge available on the hygrometric, mechanical, and thermal properties of earth, 22 years after the publication of the book of [START_REF] Houben | Traité de construction en terre CRAterre[END_REF]. It deals mainly with the intrinsic properties of the material. This update is an opportunity make an inventory of knowledge and highlight the technical data gaps that could be usefully filled. Documents and information were made available through CRAterre's documentation centre and scientific journals. Only results for which the materials and experimental protocols are described at least briefly have been included ; this significantly reduces the number of documents that were used in this survey. In addition, we were interested in nonstabilized earth exclusively, which disqualified a great portion of the scientific data available.

DRY DENSITY

The dry density of soil is the ratio between its mass and its volume measured in the dry state, after oven dying at 105°C. This property dep ends on several parameters, dealing mainly with grain size distribution, the volume and nature of the binding phase, the water content involved in the implementation of the earth materials, and the energy involved in compaction in earth materials compacted for building purposes.

Taking into account the values found through different document sources, we suggest to distinguish two families of techniques for the implementation of earth, leading to different properties of the earth material when dry: implementation by compaction (rammed earth, CEBs) or plastic state implementation (adobe, wattle and daub, cob). To these two families, a third category can be added, involving the addition of a high proportion of plant fibers (straw-earth, hemp) into the earth mix, which leads to much lighter materials. Table 1 gives typical ranges of variation in terms of the clay content, plasticity index, initial water content and dry density for these three families [START_REF] Azeredo | Mise au point de procédures d'essais mécaniques sur mortiers de terre : application à l'étude de leur rhéologie[END_REF], CSTB, 2007[START_REF] Bahar | Performance of compacted cement-stabilised soil[END_REF][START_REF] Barras | Contribution à l'élaboration d'un mélange terre-chanvre[END_REF][START_REF] Bui | Stabilité des structures en pisé: Durabilité, caractéristiques mécaniques[END_REF][START_REF] Degirmenci | The using of waste phosphogypsum and natural gypsum in adobe stabilization[END_REF][START_REF] Goodhew | Some preliminary studies of the thermal properties of Devon cob walls[END_REF][START_REF] Goodhew | Sustainable earth walls to meet the building regulations[END_REF][START_REF] Heath | Compressive strength of extruded unfired clay masonry units[END_REF][START_REF] Kleespies | Wärmeschutz und Feuchteverhalten von Lehmbaustoffen, Solararchitektur, Forschungsgruppe Lehmbau[END_REF][START_REF] Laurent | Caractérisation thermique du matériau terre. Actes de colloques -Modernité de la construction en terre[END_REF][START_REF] Laurent | Propriétés thermiques du matériau terre[END_REF][START_REF] Mueller | Preliminary Tests and Experiments on Adobe Mixtures[END_REF][START_REF] Vega | Mechanical characterisation of traditional adobes from the north of Spain[END_REF], Morel et al., 2003[START_REF] Ola | Thermal conductivity of compressed lateritic soil blocks. 9 th regional conf[END_REF][START_REF] Hakimi | Rapport: Résultats d'essais de résistance mécanique sur échantillon de terre comprimée[END_REF][START_REF] Hall | Rammed earth sample production: context, recommendations and consistency[END_REF][START_REF] Jaquin | The strength of rammed earth materials[END_REF][START_REF] Kouakou | Strength and elasto-plastic properties of non-industrial building materials manufactured with clay as a natural binder[END_REF][START_REF] Maniatidis | Structural capacity of rammed earth in compression[END_REF][START_REF] Olivier | Le matériau terre, compactage, comportement, application aux structures en blocs de terre[END_REF], P'kla et al., 2003).

HYGROMETRIC PROPERTIES 3.1 Water content

The water content of earth is a parameter of prime importance if we are interested in the mechanical and thermal properties of the material, so it is important to know the normal water content range of earth in ambient humidity. The water content of soil at equilibrium is higher if:

• relative humidity is high,

• ambient temperature is low,

• porosity is high,

• the accessible surface area of pores is important,

• the affinity of clay and water (and therefore the activity of clay) is high,

• the sample state results from dewetting from a previous higher moisture state.

In normal conditions of temperature and pressure, with a relative humidity below 70%, the water content percentage in earth walls generally varies between 0.5 and 5%. It can be higher especially in the presence of swelling clays and aggregates containing micropores and microroughness [START_REF] Hall | Analysis of the hygrothermal functional properties of stabilised rammed earth materials[END_REF][START_REF] Laurent | Contribution à la caractérisation thermique des milieux poreux granulaires : optimisation d'outils de mesure « in-situ » des paramètres thermiques, application à l'étude des propriétés thermiques du matériau terre[END_REF], Hansen and Hansen, 2002[START_REF] Heath | Compressive strength of extruded unfired clay masonry units[END_REF][START_REF] Maniatidis | Structural capacity of rammed earth in compression[END_REF][START_REF] Bourgès | Study on the physical-mechanical properties on artificial adobe and determination of the water influence[END_REF][START_REF] Holl | Vergleichende Untersuchungen zum Sorptionsverhalten von Werktrockenmörteln[END_REF].

Eckermann et al. compared vapour absorption measurements on coatings of various kinds [START_REF] Eckermann | Auswirkungen von Lehmbaustoffen auf das Raumklima[END_REF]. Although the measurements were not performed at the equilibrium moisture content, they are helpful in making qualitative comparisons between the different samples tested. It appears from this study that earth generally has a greater capacity to bind water vapor than concrete or gypsum. This large water retention capacity is linked to the porous and microporous structure of soil, but also to the physico-chemical affinity between clay and water. Swelling clays are those that have the greatest affinity to water, due to their large surface area and high cation-exchange capacity.

Shrinkage and swelling

During the drying phase, the soil undergoes a volumetric contraction or shrinkage, due to the withdrawal of water: clay platelets tighten due to the increase of capillary forces caused by the loss of water as suction increases. This shrinkage may in certain circumstances cause cracking and so it must be controlled. Conversely, when a dry soil is loaded with moisture, it expands as a result of the relaxation of capillary pressure, and the swelling of clays having a high affinity for water.

For practical purposes, it is useful for builders to know the values of shrinkage of the soil from its current state to its implementation as a building material, which is not the saturated state.

The amplitude of shrinkage is limited if:

• the water content used in the implementation of the material is low,

• the surface area and cation exchange capacity of clays are small,

• the clay content is low,

• the porosity is high (the space between the grains is not filled with binding clay phase), • the soil contains vegetal fibres: the addition of straw is an effective way to prevent or limit shrinkage.

The presence of salts in the soil can also alter the magnitude of shrinkage [START_REF] Smith | Shrinkage and Atterberg limits in relation to other properties of principal soil types in Israel[END_REF][START_REF] Bourgès | Study on the physical-mechanical properties on artificial adobe and determination of the water influence[END_REF]. The mechanisms involved are complex and dependent on the nature of the ions present.

The shrinkage of soil from its raw state to its implementation as a building material can vary between 1 and 20%. For rammed earth, the shrinkage percentage lies in the range of 1 to 3% [START_REF] Robiquet | Un nouvel essai de retrait -Facteurs influençant le phénomène de retrait[END_REF], Gray anf Allbrook, 2002[START_REF] Bourgès | Study on the physical-mechanical properties on artificial adobe and determination of the water influence[END_REF][START_REF] Bouhicha | Performance of composite soil reinforced with barley straw[END_REF][START_REF] Smith | Shrinkage and Atterberg limits in relation to other properties of principal soil types in Israel[END_REF][START_REF] Heath | Compressive strength of extruded unfired clay masonry units[END_REF][START_REF] Walker | Properties of some cement stabilised compressed earth blocks and mortars[END_REF][START_REF] Bahar | Performance of compacted cement-stabilised soil[END_REF][START_REF] Degirmenci | The using of waste phosphogypsum and natural gypsum in adobe stabilization[END_REF][START_REF] Hall | Rammed earth sample production: context, recommendations and consistency[END_REF][START_REF] Kouakou | Strength and elasto-plastic properties of non-industrial building materials manufactured with clay as a natural binder[END_REF][START_REF] Maniatidis | Mechanical and thermal characteristics of rammed earth[END_REF], P'kla et al., 2003[START_REF] Vega | Mechanical characterisation of traditional adobes from the north of Spain[END_REF].

Water vapour permeability

The permeability to water vapour in a building material allows to define its moisture exchange capacity between the inside and the outside of a building. The higher the permeability, the easier the exchange between outdoor air and indoor air. It is therefore an important property to consider regarding comfort inside the house.

For building materials, the factor of resistance to water vapour, µ, is used to characterize the permeability of a material to water vapour. It is equal to the ratio between the permeabilities of air and of the sample to water vapour. The higher the factor of resistance, the more difficult the moisture exchange between outdoor air and indoor air becomes.

For hygroscopic materials such as earth, which may fix a certain amount of air moisture, permeability increases with relative humidity. So the factor of resistance to water vapour µ decreases as the relative humidity increases. In theory, a single measure of permeability is not sufficient to fully characterize the hygrometric behaviour of earth.

For earth, µ varies between 5 and 13 and mainly depends on the pore size distribution, the nature of clays and their content. Earth has a permeability to water vapour equivalent to that of cellular concrete, lightweight aggregate concrete, gypsum and baked earth [START_REF] Kleespies | Wärmeschutz und Feuchteverhalten von Lehmbaustoffen, Solararchitektur, Forschungsgruppe Lehmbau[END_REF], CSTB, 2007[START_REF] Utz | Stabilisation of loess clay syrfaces in the archeological excavations at the example of the Terracotta army of Qin Shihuangdi[END_REF][START_REF] Bourgès | Study on the physical-mechanical properties on artificial adobe and determination of the water influence[END_REF][START_REF] Hall | Analysis of the hygrothermal functional properties of stabilised rammed earth materials[END_REF][START_REF] Volhard | Lehmbau Regeln. Begriffe -Baustoffe -Bauteile[END_REF], NF EN 1745[START_REF]Caractérisation en compression simple des blocs de terre comprimée (BTC) : application aux maçonneries[END_REF].

MECHANICAL PROPERTIES 4.1 Experimental precautions for the uniaxial compressive test

The main mechanical property of earth that is of interest to builders is its uniaxial compressive strength. To measure it, there are many available testing procedures which do not, unfortunately, lead to the same results for the same types of materials [START_REF] Morel | Compressive strength testing of compressed earth blocks[END_REF][START_REF] Fontaine | Cohésion et comportement mécanique de la terre comme matériau de construction[END_REF]. In most cases, experimental conditions influence the results and it is not the intrinsic compressive strength of the material that is being measured. Thus, these values cannot be used in a comparative manner.

The following precautions must be taken in order to properly measure the uniaxial compressive strength of earth [START_REF] Morel | Compressive strength testing of compressed earth blocks[END_REF][START_REF]Caractérisation en compression simple des blocs de terre comprimée (BTC) : application aux maçonneries[END_REF][START_REF] Walker | Strength and erosion characteristics of earth blocks and earth block masonry[END_REF][START_REF] Olivier | Mode opératoire pour la réalisation d'essais de résistance sur blocs de terre comprimée[END_REF][START_REF] Fontaine | Cohésion et comportement mécanique de la terre comme matériau de construction[END_REF]) :

• Choose sample dimensions greater than 5 times the size of larger particles.

• Choose an aspect ratio between 1.5 and 2.

• Obtain homogeneous samples.

• Let the samples stabilize in the desired hygrothermal conditions.

• Coat the samples with a material as rigid as earth (a fine earth mortar, for example) in order to obtain smooth and parallel surfaces. • Use a ball joint above the top plate of the press if the surfaces are not perfectly parallel. • Lubricate the contact between the sample and the press plates to reduce friction.

To measure the constitutive law and the elastic modulus of the specimens, care must be taken to measure the strain in the middle of the samples (using strain gauges for example) to avoid edge effects and the deformation caused by an anti-friction systems [START_REF] Mollion | Etude du comportement mécanique du pisé[END_REF].

Elastic modulus

Young's modulus for earth is difficult to measure from a compression test. Its determination requires a sufficiently precise local strain measurement. It is therefore very difficult to find written sources with reliable values of the elastic modulus of earth.

If surface roughness is imperfect, we can see a phase in which the curve increases progressively at the beginning of the testing, which corresponds to the crushing of surface ridges (Bui, 2008, Kouakou and[START_REF] Kouakou | Strength and elasto-plastic properties of non-industrial building materials manufactured with clay as a natural binder[END_REF]. This phase is not characteristic of the material and hides the elastic behavior of the original material.

To our knowledge, the only study that provides reliable measurements of the elastic modulus based on compression tests is that of Mollion, in which a measuring device adapted to strain measurement is used: three strain gauges placed around the sample measure the deformation in the middle third of the specimen [START_REF] Mollion | Etude du comportement mécanique du pisé[END_REF].

Other authors measure the elastic modulus using other techniques based on the measure of the resonance frequencies of specimens [START_REF] Fontaine | Cohésion et comportement mécanique de la terre comme matériau de construction[END_REF], or the speed of propagation of ultrasound [START_REF] Bourgès | Study on the physical-mechanical properties on artificial adobe and determination of the water influence[END_REF].

According to data from these studies, Young's modulus for raw earth is between 1 and 5.5 GPa. It gets higher depending on how low the porosity and moisture content are, and on how high the clay content and specific surface area are. However, the relationship between the composition of the earth, its microstructure and its elastic modulus is not clearly established.

The recommended modulus values vary but always lie below 1 GPa [START_REF] Walker | The Australian earth building handbook[END_REF][START_REF] Walker | Rammed Earth : Design and Construction Guidelines[END_REF], NZS 4297 :1998[START_REF] Maniatidis | A review of rammed earth construction[END_REF]. This modulus lacks a clear definition : how to measure it? Is the real Young's modulus of interest to builders, or do they just need an apparent modulus measured under certain conditions of stress?

Uniaxial compressive strength

Standards often require testing protocols that do not measure the intrinsic strength of the material: the measure is partly influenced by the test devices. This is due in part to the fact that testing procedures for earth materials are based on standard tests for cement concretes or fired clay bricks, while the mechanical properties of these materials are very different.

We have listed the most reliable results. The compressive strength of earth viable for construction can vary between 0.4 and 5 MPa. For rammed earth, the values are narrower: 0.5 to 3 MPa, the most common value being about 1.5 MPa [START_REF] Azeredo | Mise au point de procédures d'essais mécaniques sur mortiers de terre : application à l'étude de leur rhéologie[END_REF][START_REF] Barras | Contribution à l'élaboration d'un mélange terre-chanvre[END_REF][START_REF] Bui | Stabilité des structures en pisé: Durabilité, caractéristiques mécaniques[END_REF][START_REF] Bullen | Strength and durability of rammed earth walls for domestic and commercial construction[END_REF][START_REF] Fontaine | Cohésion et comportement mécanique de la terre comme matériau de construction[END_REF][START_REF] Hakimi | Rapport: Résultats d'essais de résistance mécanique sur échantillon de terre comprimée[END_REF][START_REF] Jaquin | The strength of rammed earth materials[END_REF][START_REF] Kouakou | Strength and elasto-plastic properties of non-industrial building materials manufactured with clay as a natural binder[END_REF][START_REF] Maniatidis | Mechanical and thermal characteristics of rammed earth[END_REF][START_REF] Maniatidis | Structural capacity of rammed earth in compression[END_REF][START_REF] Mollion | Etude du comportement mécanique du pisé[END_REF], Morel et al., 2003[START_REF] Olivier | Le matériau terre, compactage, comportement, application aux structures en blocs de terre[END_REF][START_REF]Caractérisation en compression simple des blocs de terre comprimée (BTC) : application aux maçonneries[END_REF], P'kla et al., 2003).

The following parameters improve compressive strength : a high density, a low water content, a high clay and silt content, a high specific surface area of the clays, good homogeneity, small grains.

Based on the present state of knowledge, it is not possible to predict the compressive strength of a given soil without making experimental tests. Knowing the density, water content or clay content of earth is not enough. Many other parameters are involved in the mechanisms of cohesion of earth, which determine its strength. The relationship between the microstructure of earth and its macroscopic mechanical properties is very complex.

Tensile strength

Tensile strength, like shear strength (for which we could find no values in the scientific literature) are considered of minor importance as compared to compressive strength in the field of construction, because earth is stressed in compression. It can be measured by indirect tests such as split testing and the three point bending flexural test.

The few values identified show that earth has a tensile strength of about 0.1 to 0.5 MPa [START_REF] Bahar | Performance of compacted cement-stabilised soil[END_REF][START_REF] Hakimi | Rapport: Résultats d'essais de résistance mécanique sur échantillon de terre comprimée[END_REF][START_REF] Jaquin | The strength of rammed earth materials[END_REF], Morel et al., 2003[START_REF] Olivier | Le matériau terre, compactage, comportement, application aux structures en blocs de terre[END_REF], P'kla et al., 2000, P'kla et al., 2003).

THERMAL PROPERTIES 5.1 Thermal inertia

Inertia is the ability to store heat and release it slowly. It allows to shift variations in temperature inside the house from the outside, and to cushion temperature changes. It depends primarily on the thermal mass of materials: the higher this capacity, the more the material may provide inertia to the building.

Another phenomenon may contribute to the thermal inertia of a wall: the latent inertia caused by the evaporation and condensation of water inside the wall. This property, specific to hygroscopic porous materials, such as earth, makes it a natural phase change material.

Heat capacity, c, is expressed in J/kg.K. It is connected to the volumetric heat capacity C in J/m3.K by the relation C = c.ρ, where ρ is the mass density of earth.

Earth's heat capacity c varies from 600 to 1000 J/kg.K with a mean value of 800 J/kg.K at 20°C [START_REF] Laurent | Caractérisation thermique du matériau terre. Actes de colloques -Modernité de la construction en terre[END_REF][START_REF] Laurent | Contribution à la caractérisation thermique des milieux poreux granulaires : optimisation d'outils de mesure « in-situ » des paramètres thermiques, application à l'étude des propriétés thermiques du matériau terre[END_REF][START_REF] Goodhew | Some preliminary studies of the thermal properties of Devon cob walls[END_REF], Goodhew and Allbrook, 2005[START_REF] Hutcheon | Thermal conductivity of rammed earth[END_REF][START_REF] Hill | Heat flow through adobe walls -part 1[END_REF][START_REF] Delgado | Earth building in Spain[END_REF][START_REF] Wessling | Thermal energy storage in adobe and in stone structures[END_REF][START_REF] Volhard | Leichtlehmbau -Alter Baustoff -neue Technik[END_REF], NF EN 1745[START_REF]Caractérisation en compression simple des blocs de terre comprimée (BTC) : application aux maçonneries[END_REF].

For an earth-straw composite, x being the amount of straw, the heat capacity is : c = (1-x).c earth + x.c straw Similarly, the heat capacity of earth depends on its moisture content following a linear relationship: C(θ) = C dry + θ.C w with θ being the volumetric water content, and C w = 4.18E6 J / m³. K.

In addition to the intrinsic heat capacity of the material, there is the contribution due to phase change of the water contained in the material: the evaporation of water causes

The thermal conductivity of dry earth depends primarily on its density and porosity. It varies between 1.5 W/m.K for dense earth (2200 kg/m3), and can drop to 0.10 W/m.K for mixtures of earth and hemp or earth and straw (500 kg/m3). Average values for several densities are shown in Table 2 [START_REF] Boussaid | Comportement thermique de la terre stabilisée au ciment[END_REF], CSTB, 2007[START_REF] Goodhew | Some preliminary studies of the thermal properties of Devon cob walls[END_REF][START_REF] Goodhew | Sustainable earth walls to meet the building regulations[END_REF][START_REF] Kleespies | Wärmeschutz und Feuchteverhalten von Lehmbaustoffen, Solararchitektur, Forschungsgruppe Lehmbau[END_REF][START_REF] Hutcheon | Thermal conductivity of rammed earth[END_REF][START_REF] Laurent | Caractérisation thermique du matériau terre. Actes de colloques -Modernité de la construction en terre[END_REF][START_REF] Laurent | Propriétés thermiques du matériau terre[END_REF][START_REF] Maniatidis | Mechanical and thermal characteristics of rammed earth[END_REF][START_REF] Ola | Thermal conductivity of compressed lateritic soil blocks. 9 th regional conf[END_REF], Wibart, 2010).

Conductivity increases with water content. For earth of about 1800 kg/m3 and suitable for rammed earth construction, it can vary from 1 to 1.2 W/m.K when the water content varies from 0 to 2%. For the same difference in moisture content, the change in conductivity can be more or less strong depending on the type of soil: not all earths have the same sensitivity to water. Earth by itself is not a good insulator, but when mixed with plant fibers and with a sufficient thickness, it can be used for the insulation of a building.

6

CONCLUSIONS In conclusion from this overview, there is little reliable experimental data on the properties of raw earth construction. This data is very fragmented as it often deals with one type of earth and focuses on only a few properties.

The main properties of raw earth updated through this review are summarized in Table 3 for the three main types of earth implementation for construction: compacted earth, mold earth and earth with added fibres. The values given in the reference [START_REF] Houben | Traité de construction en terre CRAterre[END_REF] are recalled in parentheses for comparison.

cooling and the condensation causes warming. The energy exchanged represents the latent heat of vaporization of water, which is of about 2400 kJ/kg [START_REF] Kimura | Passive cooling effect of a massive vernacular house examined by field measurements under summer conditions in Japan[END_REF].

Thermal conductivity

Thermal conductivity λ indicates the amount of heat (in W) that goes through an area of one sq meter/one meter thickness when its interior and exterior faces differ in temperature by one Kelvin. It is expressed in W/m.K. The lower λ is, the more the material is insulating. Materials with λ < 0.065 W/m.K are considered insulating. . Earth is a porous, unsaturated material. Heat transfer is related to several mechanisms: conduction in the solid, liquid and gas phases, convection, radiation, evaporation / condensation. To define the conductivity in such a material is complex. Apparent conductivity is the value of conductivity reached by measure, which results from the combination of all the mechanisms mentioned above. The equivalent conductivity is the conductivity value of a homogeneous material equivalent to the considered material, which would have the same macroscopic thermal behavior. proche de 0

Water vapour resistance factor µ 5 -13

Young Modulus E GPa 1.0 -5.5 Tab. 3 -Synthesis of the main properties of earthen materials for construction.


M Zerarka

P Austin

F Morancho

K Isoird

H Arbess

J Tasselli

Analysis Study of Sensitive Volume and Triggering Criteria of SEB in Super-Junction MOSFETs

Keywords: Power MOSFET, Super-Junction MOSFET, SEB, sensitive volume, SOA, TCAD simulations, triggering criteria

Power MOSFETs are more and more used in atmospheric and space applications. Thus, it is essential to study the influence of the natural radiation environment (NRE) on the electrical behavior of standard and Super-Junction (SJ) MOSFETs. 2D numerical simulations are performed to define the sensitive volume and triggering criteria of SEBs (Single Event Burn-out) for standard and superjunction MOSFETs for different configurations of ionizing tracks. The analysis of the results allows a better understanding of the SEB mechanism in each structure and allows the behaviour and robustness comparison for these two technologies under heavy-ion irradiation.

Introduction

Power MOSFET is a very important device in many power-electronics applications, widely used in space and atmospheric applications. Its reliability is limited by the effect of natural radiation environment (NRE). This environment is composed of particles of various nature and energy such as heavy ions which can cause the destruction of this device. In order to protect against NRE effects, many studies have been carried out to understand the failure modes. Single Event Burnout (SEB) is one of the catastrophic effects which could cause power devices failure in space systems. Heavy-ions induced destructive failures in power MOSFETs have been extensively studied and are related to the existence of a parasitic bipolar junction transistor (BJT) inherent to the device [START_REF] Keshavarz | Computer Simulations of Ionizing Radiation Burnout in Power MOSFETs[END_REF][START_REF] Roubaud | Experimental and 2D simulation Study of the Single Event Burnout[END_REF]. Destructive electrical failures have already been observed in IGBTs [START_REF] Iwamuro | Numerical Analysis of Shortcircuit Safe Operating Area for P-channel and Nchannel IGBTs[END_REF], and those induced by heavy ions were highlighted in 1992 and 1993 by Rockwell and Boeing Company [START_REF] Nichols | Observations of Single Event Failure in Power MOSFETs[END_REF]. The first adopted triggering criterion was only a critical LET (Linear Energy Transfer) before the influence of the ion penetration depth was investigated. Experimental observation of SEB in high voltage devices such MOSFET and IGBT was reported in [START_REF] Lorfevre | Heavy ion induced failures in a power IGBT[END_REF], demonstrating that avalanche conditions are not needed to trigger the IGBT, a heavy ion being able to induce latchup contrarily to power MOSFETs. SEGR (Single Event Gate Rupture) and SEB testing have been carried out by P.T. McDonald et al [START_REF] Mcdonald | Destructive heavy ion SEE investigation of 3 IGBT devices[END_REF] on three types of commercial 600-1200V planar IGBTs. These devices are all sensitive to SEB and SEGR but measurements at a reduced V CE value, from 330V to 270V indicated a significant reduction in both measured SEB/SEGR sensitivity allowing their use for space applications.

Currently, the trend is to use rather long ranges, able to cross the epitaxial layer of the classic VDMOS planartype. S. Liu, however, investigated the range effect on both SEB and SEGR phenomena [START_REF] Liu | Recommended test conditions for SEB evaluation of planar power DMOSFETs[END_REF] and recommends the use of light and short-range ions, not to be confused with the appearance of SEGR during SEB tests. Test results carried out on 600 V prototypes show no significant effect of the range on the SEB safe operating area (SOA) (for ranges from 30 to 300 μm), together with Marec et al. who suggest that the criteria is a critical deposited charge in the epitaxy, the latter also defining the sensitive volume [START_REF] Marec | Methodology to predict the SEE rate in vertical MOSFET with deep charge collection[END_REF]. On the other hand, A. Luu et al. show an influence of the ion range in commercial MOSFETs [START_REF] Luu | Sensitive Volume and Triggering Criteria of SEB in Classic Planar VDMOS[END_REF]: they define the epitaxial layer as the sensitive volume. We also have reported on the effect of the ion range on 400V-IGBTs [START_REF] Zerarka | Comparative study of sensitive volume and triggering criteria of SEB in 600 V planar and trench IGBTs[END_REF]. Few works are related to SJ-MOSFET. Huang et al showed that the presence of a horizontal electric field and a smaller vertical electric field in the SJ-device significantly reduce its vulnerability to SEB and SEGR compared to standard power MOSFETs [START_REF] Huang | Analysis of SEB and SEGR in Super-Junction MOSFETs[END_REF]. N.Ikeda et al showed experimentally that there was not much difference in SEB tolerance between the two. However, a better SEB tolerance is obtained by decreasing the die size while maintaining a low onresistance for SJ-MOSFET and by applying the hardening technique for the standard power MOSFET structure [START_REF] Ikeda | Single-Event Burnout of Super-Junction Power MOSFETs[END_REF]. In 2009 Marina et al. [START_REF] Antoniou1 | The 3.3kV Semi-SuperJunction IGBT for Increased Cosmic Ray Induced Breakdown Immunity[END_REF] propose a new structure, the 3.3 kV semi-Superjunction Insulated Gate Bipolar Transistor (semi SJ IGBT). Using 2D simulations, these devices show high immunity against cosmic radiations compared to standard FS Trench IGBTs, although they are not used in industry.

In this context, using 2D Synopsis TCAD tools, a simulation work has been conducted to define the sensitive volume and the criteria for SEB triggering induced by heavy ions in standard MOSFET and SJ-MOSFET devices. The first part of this paper presents the 2D simulation results for the definition of the sensitive volume defined by the minimum LET triggering a SEB, according to the range and depth of ion generation. The analysis of these results also allows comparing the electric behaviour of these cells for different configurations of ionizing tracks. In the second section, the drain voltage has been varied for different ranges in order to define the SOA of these devices. Finally, the broad range of temperatures that may occur on a board system necessitates an investigation of the temperature dependence of the SEB mechanism in each component.

Simulation tool and test vehicle

Simulations were carried out with 2D Sentaurus simulator using the heavy-ion model. The physics models used are:

 Mobility models: Electrical field, doping and carrier-carrier scattering effects.

 Generation /Recombination models: Auger generation and recombination, SRH and Impact Ionization models.

 Electronic Band structures: Band gap narrowing effects, Intrinsic density and Fermi stats.

Structures under study are standard MOSFET (VDMOS) and SJ-MOSFET power devices as shown in Figure 1: they are based on a flexible technology developed at LAAS laboratory and are rated for a breakdown voltage of 700~800 V. Figure 1 shows a cross-section of the half-cell structures.

Conditions of simulations

In a first step, all traces of ionization are generated vertically in the half-cell structure from different positions (x), in order to determine the most sensitive one along the x axis.

In a second step we studied the case of ions generated vertically in the volume of the half-cell of standard and SJ MOSFETs with track lengths of 10 μm generated from different depths within the epitaxial region (see In a fourth step, we simulated these structures with different drain voltages (100 V, 200 V, 300 V and 400 V) for various ranges (10 μm, 30 μm and 60 μm) while keeping the same conditions defined in the preceding second simulation step.

Finally, the temperature dependence of the model is taken into account to simulate the structures with a temperature rise from 300 K to 400 K.

Sentaurus-TCAD simulation results and discussion

Determination of the Sensitive Volume in each device

For both MOSFET structures and a 400V bias, figure 3 shows the minimum LET triggering a SEB for ions generated from different impact positions along the x axis (see arrows position). In both cases, the most sensitive areas are generally located in the intercellular region, more specifically between 30 and 38 µm in VDMOS and at 1μm in the SJ-MOSFET (see the positions of colored arrows in Figure 3). These regions are the main areas of the base current for the parasitic vertical transistor. The P+ region having a large capacity of charge collection, the sensitivity of the highly doped region decreases. These simulation results are consistent with those obtained in various studies as much for simulations as experiments [2, 11, 14 and 15]. According to our previous simulations, we have fixed the impact positions at 30 µm for the VDMOS and 1 µm for the SJ-MOSFET (see the dashed line location for each structure in Figure 3). Considering a track length of 10 μm separately, for a 400 V bias, the space charge region expands across less than 50 μm. Figure 4 clearly shows that the tracks located inside this area require lower LET values and they vary relatively little between each depth. For the standard MOSFET, the tracks positioned outside the space charge region require a much larger LET. Indeed, in this case, a single type of carrier (holes) is at the origin of the multiplication through the electric field area (see Figure 5-top) and the charges need more time to recombine before reaching the electric field region. As a result, there are likely fewer holes and electrons available to begin the impact ionization cascade.

For SJ-MOSFET, all traces are located within the space charge region that extends horizontally and which occupies the entire cell. Electrons and holes are at the origin of the impact multiplication phenomena in crossing the electric field zone whatever the depth (see Figure 5 bottom). One can see, from the analysis of these results, that the most efficient way to trigger a SEB in standard MOSFET is to deposit a quantity of charges into the space charge region. In the SJ-MOSFET the sensitive volume is located in the top half of the epitaxial region. In this case, the electric field is distributed P-homogeneously along the vertical PN junction, and the sensitive volume can be independent of its effect. We remind that electrons and holes are collected at drain and source contacts respectively. Generally, in order for the carrier density to be multiplied by the impact ionization mechanism, carriers need to cover a certain distance to acquire the required energy for creating an electron-hole pair [START_REF] Kuboyama | Improved model for single-event burnout mechanism[END_REF]. The more carriers travel into the space charge region, the more they are multiplied. Moreover, in SJ-MOSFET all carriers are at the origin of the impact multiplication phenomena in crossing the space charge region. But the tracks positioned in the bottom half of the epitaxial region require a much larger LET since the holes generated in this depth have more time to recombine before reaching the ground via the lateral pbase region.

Figure 6 shows the minimum LET triggering a SEB as a function of the ion range for both components. The simulations show that the LET required for SEB initiation decreases with increasing ion range in the N-epitaxial region. A minimum LET saturation exists for 50 μm in planar MOSFET and 80 μm in SJ-MOSFET. These ion range values correspond to the depth of the space charge region at 400 V for both cells. A high range allows the generation of carriers by avalanche in the space charge region and therefore requires a low LET. Conversely, a small range requires a large LET since the path of charges deposited in the space charge region is lower and there is less generation by the avalanche phenomenon. Therefore, the sensitive case of SEB triggering is when the ion penetrates a great part of the space charge region. A minimum charge exists for ranges lower than 50 μm in planar MOSFET, 20 μm and 80 μm in SJ-MOSFET.

With a same LET whatever the increase in range, minimum charges triggering a SEB are systematically deposited within the space charge region thus explaining the LET saturation in standard MOSFET. This is not the case for the SJ-MOSFET wherein there is no LET saturation since the space charge region expands completely into the structure. However in SJ-MOSFET, values of minimum LET vary relatively little between each range, excepted for the 10µm-range. Therefore, the range has a little effect in SJ-MOSFET compared with the standard one.

The SEB mechanism is linked to the avalanche and the forward biasing of the parasitic bipolar transistor, both providing charges to each other. For the SEB phenomenon to become irreversible, the avalanche and conduction mechanisms of the bipolar transistor must be maintained so that the avalanche has to be supplied by the electron current provided by the bipolar transistor and the parasitic transistor has to be supplied by the hole current coming from the avalanche mechanism. For a better understanding, the basic mechanisms involved in the SEB triggering are detailed, for example, in [START_REF] Hohl | Feature of the triggering mechanism for single event burnout of power MOSFETs[END_REF] for the standard power MOSFET and in [START_REF] Huang | Analysis of SEB and SEGR in Super-Junction MOSFETs[END_REF] for the SJ-MOSFET. Fig. 6. Minimum LET and quantity of charges triggering a SEB depending on the penetration depth of the ion coming from the front side in normal incidence (Vds=400 V) for standard (top) and SJ (bottom) MOSFETs Figure 7 illustrates a systematic observation of the simulated electric field evolution following a burnout, showing that the electric field is always maximum at the P + body/ N -drift junction at the end of the simulation whatever the range in the standard MOSFET. This is due to the highly localized current in the right side of each structure i.e. most of the current flows through the inherent parasitic npn transistor (n + cathode/p -body/ n - drift). The electric field is also maximum at the homojunction (n -drift/n + substrate) at the end of the simulation whatever the range in the planar and SJ-MOSFETs. This allows the establishment of a strong avalanche rate since the ionization coefficients are exponentially related to the electric field. The currents thus generated lead to thermal runaway and burnout [START_REF] Luu | Sensitive Volume and Triggering Criteria of SEB in Classic Planar VDMOS[END_REF].

The displacement of the electric field peak and its increase at the junction in each case, as shown in Figure 7, are due to the Kirk effect [START_REF] Hohl | Feature of the triggering mechanism for single event burnout of power MOSFETs[END_REF][START_REF] Johnson | A review of the techniques used for modeling single event effects in power MOSFETs[END_REF] and the build-up of a negative space charge region. Thus, a track with high range will be more susceptible to the Kirk effect for the establishment of a current than a limited and localized charge deposition. For standard MOSFET the slopes of the electric field are different between the two range values due to the collection of charges deposited in each case. While for SJ-MOSFET, the electric field level decreases before the t final only along the trace. The Range (µm) Range (µm) modification of the electric field compared to the evolution of the injection is described by the Poisson's

equation: 0 )) ( ( )) ( (                                     dx dE qv J N q y n N q dy dE MOSFET SJ qv J N q x n N q dx dE VDMOS n n d d n n d d    
These equations can explain the observations in Figure 7, i.e. the inverse of the slope of the electric field and the displacement of the peak of the P/N epi junction to the N epi / substrate junction. Indeed, with the contribution of the trace, the carrier density J n increases as the term J n /qv n becomes greater than the N d doping layer of N epi .

In the standard MOSFET, the electric field will rapidly become maximum at the N epi /substrate junction for a track crossing a large part of the epitaxial region (Figure 7, VDMOS range = 70 microns):an important range is more favorable to the Kirk effect for the establishment of a current J n than a deposit of punctual charges compared to low penetrations. For the SJ-MOSFET structure, the Kirk effect can only occur horizontally (y) because there is no variation of the electric field in the direction of the ion penetration (x), which explains the minor effect of the range in this case. Overall, for these two structures, the influence of the range can be explained only by the Kirk effect: in the case where carriers increase regeneratively until their concentrations, the displacement of the electric field (see Figure 7) with the current density flowing through the space charge region become much higher than the doping of the epitaxial region [START_REF] Luu | Sensitive Volume and Triggering Criteria of SEB in Classic Planar VDMOS[END_REF], [START_REF] Baliga | Power semiconductor device[END_REF].

For short ranges, the electric field peak moves from the P-body/N-drift junction to the substrate side. However, this does not extend to the case of long ranges where the electric field peak appears at the N-drift/substrate junction since the beginning. So the avalanche (arrow 1 in figure. 8) and burnout triggering (arrow 2 in figure. 8) appear rapidly after the first current peak (schematized by the vertical dotted line in figure. 8) for the higher penetration, while the shorter tracks take more time after this peak. Eq-1

Eq-2 Simulation results concerning the minimum LET triggering a SEB for different drain-source biases (100 V, 200 V, 300 V and 400 V) and for three ranges (10 µm, 30 µm and 60 µm) are reported on figure 9. In these cases, the space charge region proportionally extends to each bias. The threshold LET decreases accordingly with the bias increase in the standard MOSFET while the minimum LET triggering a SEB is independent of the bias voltage value for the SJ-MOSFET. Some of the observations previously made are confirmed: LET decreases with the range increase whatever the bias voltage value. For standard MOSFET, the bias voltage increase proportionally extends the space charge region within the cell thus reducing the LET discrepancy between the various ranges. However for the SJ-MOSFET, the LET discrepancy is the same regardless of the bias voltage since as we said previously, the electric field is distributed homogeneously along the vertical PN junction and the space charge region expands completely in the cell.

There is no important difference in the SEB threshold voltage as shown in figure 9. The SJ-MOSFET has a same SOA than the standard one that is consistent with the fact that these structures have the same parameters such as "Rp", the P-well resistance under the source, which is the crucial factor for increasing the SEB threshold voltage. -Under a low bias voltage (<400 V), SJ-MOSFET is more sensitive than the standard one; -At 400 V, the sensitivity is similar for both devices; -Under high bias voltage (>400V), the standard MOSFET is more sensitive than the SJ-MOSFET. This may be the reason why S. Huang et al. In [START_REF] Huang | Analysis of SEB and SEGR in Super-Junction MOSFETs[END_REF] found that the SJ-MOSFET is less vulnerable to the SEB than the standard one. But N. Ikeda et al [START_REF] Ikeda | Single-Event Burnout of Super-Junction Power MOSFETs[END_REF] demonstrated that there was no structural advantage in SEB tolerance for the SJ-MOSFET and there was not much difference in SEB tolerance between the two technologies.

Results Analysis for different temperature values

Figure 11 clearly shows that the minimum LET required to generate a SEB decreases as the temperature increases: the most critical case is found at low temperature that is consistent with previous works [START_REF] Johnson | Temperature dependence of singleevent burnout in NChannel Power MOSFETs[END_REF]. For both devices, the impact ionization rate (number of electron-hole pairs generated per unit path length) decreases with increasing temperature [START_REF] Crowell | Temperature Dependence of Avalanche Multiplication in Semiconductors[END_REF]. This is attributed to the shorter mean free path of the carriers. Since the impact ionization rate is used explicitly in the solution to the Poisson equation, the generated avalanche hole current density decreases with increasing temperature for the same injected electron current density and applied drain-source bias. In both cases the area of the fusion is located at the canal where the temperature can exceed 3000 K, because of the very high current density in this region and the high thermal resistivity of the oxide. Figure 12 shows the temperature distribution following a SEB in the end of simulation for a standard structure.

Conclusion

The sensitive volume of SEB for standard and SJ MOSFETs has been defined by delimiting its depth and thickness using 2D electrical simulations. The most critical case for triggering a SEB at 400V in standard MOSFET is related to an ion crossing the entire space charge region. However for SJ-MOSFET, the lowest LET triggering a SEB is obtained for an ion crossing the entire epitaxial region. The ion range has a little effect in SJ-MOSFET compared with the standard one. We also determined the threshold voltage of sensitivity for each structure. SOA is the same for the two studied devices, but there is a significant difference of behaviour and sensitivity as a function of the applied bias voltage. This can be an advantage for the SJ-MOSFET SEB tolerance only at high voltage. Finally, we have shown the influence of the temperature on the sensitivity revealing that the most critical case is found at low temperature in both cases.

Fig. 1 .

 1 Fig. 1. Standard (top), SJ (bottom) MOSFETs half cell layout and doping profile of each cell (right).

  Fig 2.a). The objective is to locate the sensitive volume. In a third step, we simulated the impact of ions penetrating from the front side with different track lengths (see Fig 2.b). The aim is to see from which LET value SEB triggering is initiated for each range and how each component behaves for different range values. Initially, for easier comparison of the results, all simulations are performed at the same drain polarization of 400 V in blocking state. The horizontal x-position of the generation is the one defined in the first simulation step (in this kind of studies the word "range" indicates the track length).

Fig. 2 .

 2 Fig. 2. Schematic of ionizing tracks impacting on normal incidence on the device front side

Fig. 3 .

 3 Fig. 3. Minimum LET triggering a SEB in VDMOS (a) and SJ-MOSFET (b) for various impact position (x = variable, y = 0, range = 10μm, V DS = 400 V)

Fig. 4 .

 4 Fig. 4. Minimum LET triggering a SEB of the ion coming from the front side in normal incidence generated from different depths in the epitaxial region with (range=10 μm, Vds=400 V) for standard (top) and SJ (bottom) MOSFETs

Fig. 5 .

 5 Fig. 5. Traces positioned at two different depths in a VDMOS (top) and SJ-MOSFET (bottom)

Fig. 7 .

 7 Fig. 7. Evolution of the electric field following a vertical ionizing impact with a high range (left) and small range (right) in each half cell biased at 400 V.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Evolution of the IDS current as a function of the time following an ion strike at 3 ps for short and long ranges

Fig. 10 .

 10 Fig. 10. Comparison of minimum LET triggering a SEB at different VDS for a range of 70µm Figure 10 summarizes the sensitivity variation for the two structures for an ion penetrating 70 µm into the epitaxial region. The sensitivity evolution depends on the applied bias voltage:-Under a low bias voltage (<400 V), SJ-MOSFET is more sensitive than the standard one; -At 400 V, the sensitivity is similar for both devices;

Fig. 11 .

 11 Fig. 11. Comparison of the sensitivity at different temperature value (minimum LET (T)/LET (T°)) with ion range=70µm

Fig. 12 :

 12 Fig. 12: Temperature distribution after SEB

Acknowledgements

This work has been sponsored by Fondation de Recherche pour l'Aéronautique et l'Espace (http://www.fnrae.org/) within the framework of EPOPE project.


Houssam Arbess
email: harbess@laas.fr

Karine Isoird
email: kisoird@laas.fr

Saleem Hamady
email: shamady@laas.fr

New termination architecture for 1700 V diamond schottky diode

Keywords: «Diamond», «TCAD simulation», «Schottky diode», «Field plate architecture»

New field plate architecture is applied to pseudo vertical diamond Schottky diode. Using several field plate architectures, a TCAD simulation is realized in order to reduce the electric field in the dielectric while maintaining high breakdown voltage. Firstly and after simple variations in the field plate architecture, the breakdown voltage was improved from 1632 V to 2141 V at 700 K. Concerning E max in the dielectric, we obtained a decreasing of maximum electric field from 57 to 18 MV/cm.

Introduction

Diamond is a promising material for future high power and high temperature electronics applications, where its outstanding physical properties can be fully exploited. It exhibits an extremely high band gap, very high carrier motilities, high breakdown field strength and the highest thermal conductivity of any wide band gap material [1].

Since the N-type doping diamond is not yet optimized, we do not find diamond MOS transistor or PIN diode, most of the work is focused on the improvement of the Schottky diode.

With those first batches of components, we need to develop junction termination structures that will allow the diamond devices to reach their full potential, mainly in terms of breakdown voltage. Since diamond has a huge theoretical critical electric field, the junction termination structure is going to have a major impact on the performance of the power devices. Those structures will allow us to demonstrate the efficiency of such diamond diodes.

Context and Aim of the Study

Considering the specific case of diamond, many technological steps are not available, so the solution for the device protection is limited by the field plate [START_REF]Impact of High-k Dielectrics on Breakdown Performances of SiC and Diamond Schottky Diodes[END_REF][START_REF] Mohammad | Field-plate design for edge termination in silicon carbide high-power Schottky diodes[END_REF][START_REF] Ikeda | Fabrication of a field plate structure for diamond Schottky barrier diodes[END_REF][START_REF]Diamond Vertical Schottky Barrier Diode with Al 2 O 3 Field Plate[END_REF]. The critical field of diamond is very strong which imposes severe constraints at the device termination. Since the critical field of the deposited dielectric material is less than that of diamond, we aim to find new architectures that reduce the electric field in the termination structure.

The previous structure studied at LAAS is a diamond pseudo vertical Schottky diode with Field plate as termination on the diamond surface (Fig. 1). This structure consists of two diamond layer, the first is a 7 µm P + doped diamond with a doping concentration of 3.10 20 cm -3 and the other is a 7 µm P -doped diamond with doping concentration of 8.10 15 cm -3 .To study the breakdown voltage of this structure and to look the optimum desired values of different technological parameter, we use Sentaurus TCAD simulation .The physical model used to calculate the breakdown voltage is the default Van Oversteaten model [START_REF]Sentaurus user guide A[END_REF].

Firstly we simulated an ideal schottky diode, that means one dimensional simulation and without limit condition. Fig. 2 presents the variation of the simulated breakdown voltage of one dimensional diamond schottky diode with the doping concentration of the P -layer and the thickness of the last one. In our case (8.10 15 cm -3 and 7 m as P -layer parameters), we obtain a breakdown voltage equal to 2288 V. The field plate was optimized for a length of 10 microns and dielectric thickness of 0.7 microns. The dielectric used in this optimization is the Silicon oxide SiO 2 [START_REF] Thion | Conception de protections périphériques applicables aux diodes Schottky réalisées sur diamant monocristallin[END_REF]..With these parameters, we obtained 1638 V as a breakdown voltage. However the electric field reached at the end of Field plate is higher than 19.5 MV/cm at 700 K.

All results shown in this paper were extracted at 700 K due to simulation problems at low temperatures. Other simulations have been made in order to evaluate the variation of maximum

New Field plate architecture

A new idea has been proposed in order to increase the breakdown voltage. It is to keep the whole electrode flat, with no corners, and replace the diamond below the field plate region, with an optimized thickness of dielectric material (Fig. 3). Indeed, the presence of corners on the path of the potential strengthens the electric field. In the previous architecture, the dielectric is located on the diamond surface, creating a corner in the electrode. At this corner the electric field is stronger in the diamond and then the impact ionization is greater for the first structure than the second for the same external voltage. Consequently, the breakdown voltage in our structure is higher than the one previously proposed. Fig. 4 shows the difference between the electric field distribution in the two architectures where we have plotted the electric field variation along horizontal axis passing by the surface of the dielectric and another passes through the dielectric-diamond interface. The electric field focalization in the diamond for the first one (initial architecture) increases the impact ionization and as a result, the breakdown voltage decreases. In the graph at the left (old architecture), we have two peaks. The first one is in the diamond in the beginning of the field plate and the second is at the extremity of the field plate. The graph at the right shows one peak of the electric field at the extremity of the field plate.

The highest electric field at 1700 V is 19.9 MV/cm for the initial solution and 19.8 MV/cm for the new one. The high mesh refinement in the high electric field zone to 10 nm(node-to-node distance)shows an increase in the electric field to 49 MV/cm and 48.5 MV/cm for the initial and new architecture respectively. Assuming that the oxide we used has a high quality, the maximum electric field carried by the silicon oxide is 10 MV/cm and between 5 and 7 MV/cm for silicon nitride. In this case, the oxide will break before the voltage across the diode reaches the maximum value, and consequently the field plate doesn't play its desired role.

Electric field optimization

Looking at the experimental results, the average breakdown voltage was 400 V (Fig. 4) [START_REF] Thion | Conception de protections périphériques applicables aux diodes Schottky réalisées sur diamant monocristallin[END_REF]. One of the reasons responsible for this failure may be the break of the dielectric. To improve the efficiency of the termination, we can change the dielectric as Ikeda [START_REF] Ikeda | Edge termination techniques for p-type diamond Schottky barrier diodes[END_REF], who used the Al 2 O 3 as a dielectric for the field plate, but it's not enough. We aim to find a dielectric form for the field plate able to minimize the electric field as much as possible. Our idea is to change the form of field plate. For all simulation, we used the silicon oxide as a dielectric with air above. As we have already seen, the maximum electric field is at the corner. Thus, increasing the number of corners in the potential way, reduces the electric field value at the output of the field plate (distribution of peaks). Two architectures may be able to solve this problem.

a-The pillars dielectric form. That means, making the oxide in pillars form until the end of the electrode which increase the number of corners in the path of the potential Fig. 6 (a). b-The graduated dielectric form Fig. 6 (b).

Pillars dielectric from

The termination efficiency depends on many parameters: the pillar height, the pillar width, number of pillar and finally the contact length on the last pillar (Fig. 7). Using this dielectric form we made constant the field plate length. The last oxide pillar attains a contact length of 1 micron. Fig. 8 shows the variation of the maximum electric field as a function of the pillars height. Increasing the Pillars height from 0 to 1.6 microns decreases the breakdown voltage 25 V, but we have obtained a reduction in the maximum electric field to 20 MV/cm at 1700 V. The length of the field plate on the last pillar plays its role also. Passing from 1 micron as in the previous case to a value of 0.3 microns, the maximum electric field decreases from 37 MV/cm to 27 MV/cm. Fig. 9 shows the electric field variation at the oxide-oxide interface and the oxide surface for pillars height equal to 1.6 microns at 1700 V. We can remark the electric field peaks at the corners. Fig. 7: Pillars oxide form Now, we made a simulation of the same field plate form, but this time we reduce the width of the pillars (new width = 0.5 microns and 9 pillars), that's mean increase the corner number while keeping the length of the field plate on the last pillar to its optimal value (0.3 micron). Simulation results show that by reducing the width of the pillars, the electric field drops to 24 MV/cm. Note that, if we increase the pillar height over 1.6 microns, the effect of this increase becomes negligible .

Graduated dielectric form

The second proposed field plate form is the graduated form, where the potential meets three corners before it exits the fourth one (Fig. 10). Three parameters play a role in optimizing the architecture: The step oxide height, the number of steps and the field plate length in the last step. Only three steps are made for the sake of reducing technology process. This oxide form was previously used by Brezeanu [START_REF] Brezeanu | Termination Structures for Diamond Schottky Barrier Diodes[END_REF] to increase the breakdown voltage and not to minimize the maximum electric field in the dielectric. Fig. 12 shows the maximum electric field as a function of the step height. The decrease of the breakdown voltage is now greater since the total thickness of the oxide is greater than that of the first architecture (pillars oxide form). The breakdown voltage therefore decreases from 2320 to 2306 V. At 1700 V the maximum electric field decreases from 53 MV/cm to 33 MV/cm. Even this decrease is important, electric field does not achieve the required value of 10 MV/cm.

Discussions

Each proposed solution allows to decrease the peak of electric field, but not in the target range. Also we must consider other ways.

Replacing the silicon oxide with another dielectric material with higher electric permittivity (such as silicon nitride Si 3 N 4 or aluminum oxide Al 2 O 3 ) reduces the value of the maximum electric field. However, the reached value remains above the requirement.

If we increase the total oxide thickness to over 7.5 microns when we using the silicon oxide e.g., the point of maximum electric field become at the first corner as a result of the dielectric thickness. In this paper we have given the results until 1.6 µm of dielectric thickness step. Indeed, in our clean room the maximum deposit film of silicon oxide equal to 1.6 µm.

Finally, an idea is to combine the two architectures into a new oxide form in order to obtain the maximum number of corners with minimal technological steps. With this idea, we obtained a maximum field equal to 18 MV/cm using the silicon oxide as field plate dielectric and 13 MV/cm using the aluminum oxide.

Conclusion

New field plate architecture for diamond Schottky diode was proposed which helps in increasing the breakdown voltage from 1632 V to 2141 V using the same technological parameters.

The maximum electric field in the field plate provokes a new problem since it is larger than the critical field of dielectric used. To resolve this problem, three solutions were proposed. The first is to make the field plate in a pillars form, the second is to change the field plate to graduated form and the last is making a mixed oxide form in order to increase the corners numbers. With the final solution and changing also the dielectric material (from silicon oxide to aluminum oxide) we obtained 13 MV/cm as maximum electric field.

Fig. 1 :

 1 Fig. 1: Simulated pseudo vertical diamond Schottky diode

Fig. 2 :

 2 Fig. 2: Breakdown voltage for an ideal schottky diode (one dimensional simulation). Ep present the P-thickness layer of the diode.

  NEW TERMINATION ARCHITECTURE FOR 1700 V DIAMOND SCHOTTKY DIODE ARBESS Houssam EPE'13 ECCE Europe ISBN: 978-90-75815-17-7 and 978-1-4799-0114-2 P.2 electric field with temperature shown a negligible difference.

Fig. 3 :Fig. 4 :

 34 Fig. 3: New field plate architecture for diamond Schottky diode

Fig. 5 :Fig. 6 :

 56 Fig. 5: Breakdown voltage for several diamond schottky diodes [6].

Fig. 8 :Fig. 9 :

 89 Fig. 8: Maximum electric field at 1700 V in terms of pillar height for the pillars field plate form

Fig. 10 :

 10 Fig. 10: Graduated oxide form (left), and presentation of the electrostatic potential in the structure in the dielectric structure (right).

Fig. 11 :

 11 Fig. 11: electric field distribution in the structure at the protection oxide zone (field plate) (left) and cross sectional view of the electric field for each graduation and at the oxide surface (right).

Fig. 11

 11 Fig.11shows the electric field distribution in the structure at the protection oxide zone (field plate) and cross sectional view of the electric field for each graduation and at the oxide surface. We can see in this figure the electric field peaks at each corner.

Fig. 12 :

 12 Fig. 12: Maximum electric field variation as a function of one step height.

EPE'13 ECCE EuropeISBN: 978-90-75815-17-7 and 978-1-4799-0114-2 P.5