Charles Lenay

Olivier Gapenne

Sylvain Hanneton

Catherine Marque

Christelle Genouëlle

Sen

Paul Bach

SENSORY SUBSTITUTION: LIMITS AND PERSPECTIVES

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

1.

A REVIEW OF SENSORY SUBSTITUTION DEVICES

General functional principles

In general, the so-called "sensory substitution" systems transform stimuli characteristic of one sensory modality (for example, vision) into stimuli of another sensory modality (for example, touch). A sensory substitution system can be decomposed into three distinct components. A sensor permits the conversion of a certain form of energy (light, sound, mechanical or other) into signals that can be interpreted by an (electronic) coupling system which is then responsible for the coordinated activation of a stimulator. The stimulation is generally addressed to the cellular receptors of a sensory organ, and thus consumes electrical energy which can be converted into sound energy (as in the case of visual-acoustic devices) or into mechanical energy (visual-tactile devices). The sensors can be "active", that is to say they can emit and receive signals. This is the case, for example, in devices where the "sensor" component employs a telemetric principle (e.g. laser or ultrasound).

The term "sensory substitution", as such, denotes the ability of the central nervous system to integrate devices of this sort, and to constitute through learning a new "mode" of perception. It is to be noted that action on the part of the subject plays an essential role in this process, a point to which we shall return. In other words, sensory substitution can only be constituted and can only function through an ongoing exploratory activity with the sensors. Finally, the access to a mode of perception that is offered by sensory substitution devices after the requisite learning period can be described as "implicit", in other words it makes no call on conscious reasoning concerning the sensations produced by the system of stimulation.

A non-exhaustive panorama

On account of the relative independence of the three components of these systems, and the great diversity of types of sensors and stimulators which are available at the present time, there are a large number of potential sensory substitution systems, many of which have been realised. In this section, we cannot therefore give a detailed comparative inventory of all such prosthetic devices, even restricting ourselves to the tactile modality. These devices are technically very varied, since their development corresponds to diverse objectives: aids for handicapped persons, the ergonomics of interfaces, fundamental research or, again, performance in the realms of games or art. This pluridisciplinary aspect means that the literature dealing with this approach is highly dispersed, but the reader may consult several works which present a certain number of these techniques [START_REF] Barfield | Virtual environments and advanced interface design[END_REF][START_REF]Nouvelles interfaces homme-machine[END_REF][START_REF] Sampaio | Les substitutions sensorielles adaptées aux déficits visuels importants[END_REF][START_REF] Warren | Electronic spatial sensing for the blind[END_REF].

It nevertheless seems desirable to give the reader an overview of the "state of the art" in this domain. To this end, we present a table of examples of systems incorporating various sensory modalities. Some of the technologies cited are not complete sensory substitution systems (they are only single components, sensors or stimulators), but these elements are likely, in our view, to participate in the future evolution of these devices. It is to be noted that from the point of view of their technological development, many of these systems currently benefit from the miniaturisation of sensors and stimulators. For example, high-performance cameras, of very small size, are now available on the market at low cost (in particular the camera using the CMOS technology); and some research groups are working on tactile stimulation systems which provide high resolution on a very small surface (Exeter Fingertip Stimulator Array, CHANTER & SUMMERS, 1990). We shall return in what follows to the important role of miniaturisation in the appropriation of these prosthetic devices. seconds.

An essential observation is that this capacity to recognize forms is accompanied by a "projection" of the objects which are perceived as existing in an external space. Initially, the subject only feels a successions of stimulations on the skin. But after the learning process described above, the subject ends up by neglecting these tactile sensations, and is aware only of stable objects at a distance, "out there" in front of him. A number of experimental observations confirm this externalisation. For example, if the zoom of the camera is manipulated unknown to the subject, causing a sudden expansion of the tactile image, the subject takes characteristic evasive action (moving backwards and raising his arm to shield himself from what is interpreted as a dangerously approaching object5 , BACH-Y-RITA, 1972, page 98). According to the accounts given by the subjects themselves, the irritations which can be caused by the tactile matrix are clearly distinguished from the perception itself. Blind persons discover perceptive concepts which are quite new for them, such as parallax, shadows, and the interposition of objects. Certain classical optical illusions are also reproduced (BACH Y RITA, 1982;GUARNIERO, 1977).

The TVSS has also been used successfully in a work situation, by a blind person who was able to perform a certain number of delicate operations on an automatic production line for diodes (for more details, see [START_REF] Sampaio | Les substitutions sensorielles adaptées aux déficits visuels importants[END_REF].

However, once the initial flush of enthusiasm has passed, it is legitimate to ask why these devices, first developed in the 1960's, have not passed into general widespread use in the daily life of the blind community. Paradoxically, an analysis of the possible reasons for this relative failure raises some of the most interesting questions concerning these devices. One way of addressing this question is to critically discuss the very term of "sensory substitution", which carries with it the ambiguity, and even the illusory aspect, of the aim of these techniques.

SENSORY SUBSTITUTION : A DOUBLE ILLUSION

It is not a sensory substitution

It is easy to understand, as a means of publicity and marketing, the value of the term "sensory substitution" in order to describe the devices developed by Paul Bach-y-Rita: the device "enables the blind to see". Is this not an accomplishment close to a divine miracle? Nevertheless, the term is misleading and in many ways unfortunate. It implies that the essence of the innovation consists merely in a change in the sensory input, in providing a new channel for the acquisition of information about the world of light. In this case, the device would merely substitute the classical visual input via the eyes, by a hybrid input consisting of the video-camera and the matrix of tactile stimulators. If this were really all that is at stake, the work of Bach-y-Rita, while not negligeable, would have only a limited significance. If all that is to be achieved is to create artificial sensory inputs, would it not be better to use a matrix of electrodes directly implanted in the retina [START_REF] Wyatt | Ocular implant for the blind[END_REF] or even in the brain [START_REF] Brindley | Sensory effects of electrical stimulation of the visual and paravisual cortex[END_REF][START_REF] Dobelle | Artificial vision for the blind : electrical stimulation of visual cortex offers hope for a functional prosthesis[END_REF][START_REF] Dobelle | Braille » reading by a blind volunteer by visual cortex stimulation[END_REF][START_REF] Schmidt | Feasibility of a visual prosthesis for the blind based on intracortical microstimulation of the visual cortex[END_REF]? This would make it possible to short-circuit the clumsy detour by the tactile stimulation, and to increase the quantity of information delivered.

However, this would be to miss the real point of Bach-y-Rita's work. On the contrary, his major discovery is that a mere sensory substitution is of little use. The great merit of his device is actually to demonstrate this very point experimentally. If one presents a blind person with static forms on the tactile matrix (the video-camera is immobile, simply placed on the table), the subject will merely feel a vague tickling sensation or irritation ; he will be capable only of very vague distinctions.

There will be no question of recognizing or locating external, distal objects :

« The process of perceptually learning to identify these forms with the TVSS is particularly revealing. The performance of all the subjects was never better than chance, even after 60 trials, if (1) the subjects were not able to manipulate the camera, and (2) if they were not given feedback as to whether their responses were correct. » (SAMPAIO, 1994 : 9) On the other hand, if the handicapped person was able to manipulate the camera (movements from left to right, up and down, zoom back and forward, focussing, and the diaphragm), the subject rapidly developed spectacular capacities to recognize forms. He starts by learning how variations in his sensations are related to his actions. When he moves the camera from left to right, he feels on his skin that the stimuli move from right to left. When he uses the zoom, the stimuli "expand" or "contract". After having learned to point the camera in the direction of the target, he discriminates lines and volumes, and then recognizes familiar object of increasing complexity, to the point of being able to discriminate faces.

The work of Bach-y-Rita is therefore important not only because it is a useful technological innovation, but also because it provides original experimental tools for exploring fundamental mechanisms in perception. These tools make it possible to follow with precision the constitution of a new sensory modality in the adult. In particular, by providing the means to observe and reproduce the genesis of intentionality, i.e. consciousness of something as external (the "appearance" of a phenomenon in a spatial perceptive field), these tools make it possible to conduct experimental studies in an area usually restricted to philosophical speculation.

From a neurophysiological point of view, sensory substitution opens new possibilities for studying the extraordinary plasticity of the brain that the use of these prosthetic devices seem to imply. Tactile sensory input is quite different from visual input from the retina, and control of camera movements by the hands is quite different from commands to the eye muscles. Nevertheless, the brain appears to be able to organize a perceptive world with forms and events quite analogous to those given in visual perception. Moreover, if the matrix of tactile stimulators is displaced from the chest to the back, and the camera held in the hands is replaced by a miniature camera fixed to the frame of some spectacles, the adaptation is practically instantaneous. The subject recovers a distal perception in a few seconds. The functional restructuring of the brain poses fascinating problems for conceptualizing and modelling the mechanisms of the reorganization of functional neural networks.

On a functional level, these devices put into question the classical conception of perception and cognition based on parallel processing of passively received information. In that conceptual framework, the cognitive system is only a computational system which receives input information which is then processed in order to produce representations of objects and events ; reasoning on the basis of these representations then, in certain cases, leads to decisions concerning subsequent adaptive actions. It is well known that this conception of cognition is based on the computer metaphor, and carries the implication that computers could, in principle, be substituted for any other cognitive system. As a corollary, it is generally considered that the cognitive system is immersed in a world which contains information, a world which in the last resort is nothing other than information.

This conception finds a concrete technical expression in systems of "virtual reality", with the horizon of entirely substituting reality by a system which delivers sensory input in the form of information which has been calculated on the basis of a virtual "environment" which includes the effects of the actions of the subject [START_REF] Luciani | Ordinateur, geste réel et matière simulée[END_REF]. Thus, in modern technological developments, the current conception of cognition as information processing finds two sorts of concrete manifestations which serve both as a guiding analogy and as a theoretical horizon: on the side of the subject, the computer as a system of computation and decision; on the side of the object, virtual reality as a system for computing the information available to the subject. By contrast, when we consider the devices of Bach-y-Rita as they actually function in practice, it is no longer possible to conceive of cognition and perception as the simple processing of information received from the outside. This radical criticism of the computational theory of mind has hitherto been based on philosophical considerations and certain results in experimental psychology. Here, however, the empirical proof is direct: there is no perception without action.

This key idea, that there is no perception without action, finds a particularly vivid illustration in an experiment that we have carried out recently with a substitution device simplified in the extreme : a single photoelectric cell fixed on a finger is connected to a simple vibrator held in the other hand.

(LENAY, CANU & VILLON, 1997). The vibrator is activated in all-or-none fashion above a threshold in the photoelectric cell; the receptive field is quite wide, corresponding to a cone of about 20° visual angle. Thus there a single point of tactile stimulation, corresponding to a single receptive field (compared with the TVSS which has 400 points of stimulation corresponding to the same number of distinct receptive fields in the retina of the camera). Nevertheless, after several minutes of exploration, a blindfolded subject who is able to freely move the arm and the hand with the photoelectric receptor is able to succeed in localizing a light source, i.e. to indicate its direction and approximate distance. When the subject achieves greater mastery of the active production of tactile stimulations, he is conscious of the presence of an object situated in the space in front of him. It seems to the subject that the temporal succession of sensations derive from different "contacts" with a single distal object. It is to be noted that the vibrator can be moved to another region of the skin without disturbing this perception of the distal object. In fact, the subject appears to ignore the position of the tactile stimulations (unless he consciously refocusses his attention on that aspect of the situation) to the benefit of an apprehension of the spatial position of the light source. Conversely, artificial stimuli produced independently of the movements of the finger on which the photoelectric cell is placed are not associated with a distal perception, but continue to be perceived proximally at the level of the skin. Similarly, if the movements cease, the distal spatial perception disappears. If the finger is completely immobilized, the tactile stimulation is either continually present, or continually absent, but in either case nothing induces the subject to infer an external distal source. In order for perception to arise, a continual activity is necessary, consisting of oscillatory movements of the hand together with displacements of the wrist in such a way that the stimulation continually appears and disappears.

In a similar vein, in order to enlarge this field of empirical research, we have embarked on a research program concerning the perception of 2-dimensional shapes. This program involves both the technological development of simple prosthetic devices, and the elaboration of experimental protocols concerning the forms of activity which lead to successful perception and recognition of the shapes. Our preliminary results [START_REF] Hanneton | Dynamique de la reconnaissance de caractères via une interface haptique[END_REF][START_REF] Ali Ammar | Effect of bimodality on the perception of 2D forms by means of a specific assistive technology for blind persons[END_REF][START_REF] Sribunruangrit | Braille Box: Analysis of the parallelism concept to access graphic information for blind people[END_REF] clearly demonstrate that shape recognition is possible with even minimal forms of sensori-motor coupling. A point of interest in this approach is that the perception of shapes takes time, and requires the external deployment of exploratory activity. Precisely for this reason, traces of the patterns of exploration can be easily stored for subsequent analysis. Analysis of these dynamic patterns shows that experienced subjects deploy identifiable strategies, which can and must be learned in order for rapid and reliable perception to occur successfully.

The essential role of action in the progressive emergence of structured percepts strongly suggests that what is perceived, or recognized, does not derive from invariants in the sensory information, but rather from invariants in the sensori-motor cycles which are inseparable from the activity of the subject. It is by his action that the subject seeks and constructs the "rules" of constant relations between actions and subsequent sensations. Spatial localisation, as well as form recognition, correspond to temporal syntheses of successive sensations in accordance with a rule relating action and sensation6 . This conception is now quite widespread in current research on sensori-motor coupling in general [START_REF] Gibson | The senses considered as perceptual systems[END_REF][START_REF] Paillard | Les déterminants moteurs de l'organisation de l'espace[END_REF][START_REF] Berthoz | Reference frames for the perception and control of movement[END_REF]TURVEY & CARELLO, 1995) and its genesis [START_REF] Lepecq | Sensorimotor organization and cognition in infancy: some francophone contributions[END_REF].

If, as seems to be the case, perception is only possible when the subject can actively master the means of acquiring sensations, it follows that the device of Bach-y-Rita does not so much achieve a simple sensory substitution, but rather a sensori-motor substitution. The richness of the perception depends quite as much on the qualities of the actions (mobility, rapidity, zoom, etc.) as on the qualities of the sensations (sensitivity, spectral width, number of sensors, etc.). This is well illustrated by the astonishing capacity to recognize faces with devices which only give 400 points of sensory input. The face that is recognized results as a higher-level invariant on the basis of changes in sensation associated with active exploration.

It is not a sensory substitution

If one had to persist with the idea of a substitution, the notions of " sensory-motor substitution system", or " perceptual substitution", would be preferable to "sensory substitution".

However, we now wish to argue that the second reason why the phrase "sensory substitution" is misleading and unfortunate, is that what is at stake is not a substitution. The warning came from the visually handicapped persons themselves, who expressed disappointment at the very time when they began to discover this novel mode of access to objects situated at a distance in space. Certainly, these devices made it possible to carry out certain tasks which would otherwise have been impossible for them. However, this was not the fundamental desire which motivated the blind persons who lent themselves to these experiments. A blind person can well find personal fulfilment irrespective of these tasks for which vision is necessary. What a blind person who accepts to undergo the learning of a coupling device is really looking for, is rather the sort of knowledge and experience that sighted persons tell him so much about : the marvels of the visible world. What the blind person hopes for, is the joy of this experiential domain which has hitherto remained beyond his ken.

Now the problem is that this is not what the device procures. In fact, there are a large number of differences between this artificial coupling device and normal vision : there is no colour, a small number of points, a camera whose movements are limited and clumsy, all of which slows down the recognition of a situation. This novel sensori-motor coupling resembles vision in many ways, but the quality of lived experience that it procures is quite different -as can be readily appreciated by sighted subjects who are blindfolded for the purposes of the experiment. The device of Bach-y-Rita does not produce a sensory substitution, but rather an addition, the creation of a new space of coupling between a human being and the world. The sensory substitution devices upset the classical definitions of the diverse sensory modalities.

It would be vain to believe that one has alleviated the suffering of a blind person just by giving him access to a sort of information. What is always at stake, is the insertion of the person in a world of shared meanings, which depend on a personal history whose coherence must not be brutally shows a person blind from birth an image of his wife, or if one shows some students pictures of nude women, the disappointment is complete : their perception does not convey any emotion. But after the event, it is clear that it is the reverse which would have been astonishing. Meaning or emotional significance are not things that are already there, in the world, just waiting to be picked up like a piece of information. Here again, by the failure of its initial ambition, the device of Bach-y-Rita provides a crucial empirical proof: an isolated subject cannot attribute an existential meaning to objects and events that he perceives simply on the basis of a new perception. Does it follow that something essential is lacking in these devices? Unable to give a "content" to the perception (colour, value), they demonstrate what distinguishes natural perception from a simple capacity to discriminate and categorize. There is a striking similarity between these observations, and reports of the absence of emotion and meaning felt by persons blind from birth who recover sight by removal of a cataract.

In other words, it is not the principle of sensory substitution as such which is responsible for the impossibility of gaining access to qualia [START_REF] Gregory | Recovery from blindness[END_REF].

It is remarkable that in all the observations reported in the literature, it is always a question of a purely individual use of these devices. The user is surrounded by sighted persons, but is isolated in his particular mode of perception. Now it is plausible to suppose that perceptual values are closely linked to the existence of a shared history and collective memory, a memory which can only emerge in the course of interactions between several subjects in a common environment. This suggests possible experiments in the future. In any event, it seems to us that the term "perceptual supplementation" is more appropriate than "sensory substitution". This new term implies that these devices do not exactly remedy a deficit, but rather that they introduce perceptual modalities that are quite original.

The sensori-motor coupling devices thus give rise to experimental research into a deep problem, classically restricted to philosophy and psychology, concerning the origin and the nature of the value attached to things. A purely intellectual link inferred between a perceived form, and a feeling of pleasure or pain in another sensory modality, does not seem to be immediately sufficient to confer an emotional value to the form in question.

DEFINING THE CONDITIONS OF APPROPRIATION

Ergonomic constraints

The ergonomic properties, in a wide sense, play an important role in determining the acceptability of these devices. The necessary qualities are in large part dictated by the principles just referred to which condition the success of "sensory substitution". Thus, given the importance of the role of action in the deployment of these new modes of perception, a prosthetic device which hinders the exploratory movements of the subject is to be avoided. This constraint leads to a requirement for developing systems which are light and autonomous. The systems of tactile stimulation which employ electromagnetic transducers do not currently fulfil this requirement. Even though they are very effective and simple to use, in their present state of development they are heavy and require a large amount of energy, so that in order to be autonomous they require batteries which are also heavy. Of course, the difficulties involved in producing a device which is light and autonomous increase with the resolution of the tactile stimulation. In addition, in order to function well as a prosthetic device, the ideal system should be easy to "put on" or to "take off", like the spectacles used by sighted persons.

However, the device should also be robust. Since the aim is that the device should integrate the daily life of a person who already has to cope with the anxiety and stress of a partial inadaptation to the environment, it would not be reasonable to propose a system which is fragile and liable to break down.

When it comes to aesthetic criteria, an apparent contradiction arises: the device should lend itself to being "forgotten" by the user and those around him, yet at the same time it should "advertise itself" when the situation requires. A sensory device should be sufficiently discreet so as not to reinforce the handicap, and to avoid the user being perceived as a "technological monster".

Miniaturisation is not the only solution to this problem; it is quite possible to follow the shape of usual objects present on the site where the device is to be used. For example, a device aimed at providing access to graphic information on a computer screen could advantageously take the form of a mouse.

At the same time, even though it may seem to contradict the previous principle, the appearance of the device can also fulfil a symbolic function. The traditional white colour of a blind person's cane has the advantage of signalling to others the fragility of this person while undertaking a journey. Finally, another aspect which is important even if it is not strictly ergonomic is the cost of the device. Systems analogous to the TVSS currently available on the market employ advanced technology, and are produced in small numbers, with the result that their cost is virtually prohibitive (45000 $ for VideoTact, the electrotactile stimulation system with 768 points produced by the Unitech company).

Adaptation to real expectations

It seems to us that the appropriation of a sensory substitution device depends on it corresponding to a real need of the relevant population. We have identified two main preoccupations. The first need concerns the population of visually handicapped persons using computer technology to study, work or simply communicate. The availability of vocal synthesis, Braille keyboards and systems for optical character recognition has until now at least partially fulfilled this need in the case of operating systems (Dos, Unix) which code the information in the form of asci characters. However, the rapid and inexorable development of operating systems which employ mainly a graphical access to computers functionalities is leading to serious discouragement in this population which has already made a tremendous effort to adapt. To the extent that the transformation of graphic or iconical information into the form of tactile stimulation does not present a major technological difficulty, the application of sensory substitution techniques to this area ought to be fairly easy (for example, ee the Internet site of the Unitech company: http://www.execpc.com/~unitech/winhapt.html).

The second need concerns a wider population. The traditional white cane gives precious assistance in locomotion and avoiding obstacles. However, it does not give access to a distal perception, without contact, of the global scene. A mode of distal perception, ideally coupled with the cane without replacing it, would greatly facilitate the anticipation of movements, for example by making it possible to extract from the environment static or mobile silhouettes of obstacles to be avoided. Other potential applications, such as the access to reading or writing in black and white (such as the Optacon) are less crucial, particularly since they are in competition with proven systems such as Braille. It is quite possible to imagine a system which would fulfil conjointly the two needs just described, the reading of graphical information on a computer screen and the distal perception of the structure of a scene. Such a system would have the immense advantage of authorizing a perceptual continuity in the daily life of a visually handicapped person, and would be all the more easily accepted.

The importance of the mode and protocols of learning

Even if the observations reported by Paul Bach-y-Rita indicate that adaptation to the TVSS can be surprisingly rapid, it is important to remain realistically lucid and to recognize that even the most user-friendly device will inevitably require a substantial learning process. It is revealing that the Optacon device is delivered with an instruction manual which includes a set of progressive exercises which are essential for learning to use it effectively. This manual associates instructions in Braille and a corresponding "black" figure. The pedagogical quality of the instructions is clearly a key factor in successful appropriation, and is in itself a subject for further research. However, it seems to us that it is also important to take into account the conditions under which the learning is conducted. It is indeed possible that one of the reasons for the relative failure of sensory substitution systems to date lies in the nature of the relation which arises, in the laboratory, between the blind person and the experimentalist who "tests" the capacity of the subject to "see" the objects that are presented to him.

When designing a learning protocol, it is important to bear in mind that a sensory device does not give rise to a sort of "degraded vision", but rather to an entirely new mode of perception which should be shared by the instructor and the pupil. Using the system should give rise to a shared experience, and it is only in this context that one can hope that learning will also produce the attribution of qualia to the percepts.

Finally, we also consider that it would be vain and pretentious to imagine that sighted persons could know, in advance, the best way of learning how to use a sensory device. Thus, the immersion of the system in an appropriate environment will not only make it easier to take into account the experience and the suggestions of the people for whom the system is designed, but may well lead to the emergence of modes of use and appropriation which were not foreseen by the designers.

The "intrinsic" effectiveness of sensory substitution

When the conditions of appropriation are properly taken into account as just discussed, the intrinsic effectiveness of sensory sensation is, perhaps paradoxically, the point which leaves the least room for discussion. The basic possibility of "sensory substitution" seems to be a very general principle, having its source in the plasticity of the central nervous system and for that reason being relatively independent of sensory modalities involved. A question does arise as to the incapacity of the scientific community to mobilize for a project which is fascinating and useful (both from the point of view of fundamental research and technological innovation), but which does disturb conventional barriers between academic disciplines.

TOUCH AND SENSORY SUBSTITUTION

To start with, we will note some of the advantages of a tactile sensory input, rather than auditory input as in a number of existing devices (Sonic Guide, The Voice). The advantages of touch are of three types. Tactile stimulators can be easily dissimulated ; the person using the device is thus the only one with access to the stimulation. Secondly, if the tactile stimulations are situated on regions of the skin that are rarely used (the chest, the back of the hand...), they do not interfere with other sensory modalities that are inevitably strongly exploited by visually handicapped persons. Finally, stimulation of the cellular receptors which contribute to the sense of touch make it possible to transmit information in parallel to the central nervous system. This parallelism of the sense of touch has been the object of some discussion (see for example BACH Y RITA, 1972, page 15). However, a number of experiments (including those with the TVSS)

have shown that this parallelism can be exploited, and that sensory substitution devices are able, via the sensory-motor coupling, to give rise to perceptive resolutions superior to those of the material resolution of the matrix of stimulators. This property, shared by "natural" perceptive systems, is called "hyperacuity". By comparison, in the case of an auditory stimulation, it is more complicated to use the parallel properties of the internal ear. The conversion of an image into an auditory event requires the invention of a coupling device capable of recoding into two sound waves (if stereophonic sound is used) the spatio-temporal properties of an image [START_REF] Meijer | An Experimental System for Auditory Image Representations[END_REF]. On the other hand, it is theoretically possible to reproduce the topography of a visual image directly on the surface of the skin.

It must nevertheless be noted that, even if these three properties mean that tactile stimulation is probably the best solution for sensory substitution, certain technological obstacles remain to be overcome. Indeed, the two techniques of stimulation that have been used to date have certain defects that are almost prohibitive. Electromagnetic stimulatory systems are heavy and consume a large amount of energy. The electrotactile systems produce stimulations that are often felt as "disagreeable itching" on the skin, and furthermore require the use of a conducting gel.

However, we may take the risk of sharing the optimism of Paul Bach-y-Rita; recent developments give reason to hope that these obstacles will soon be overcome. Bach-y-Rita et al (1998) are currently experimenting with a system of stimulation in the mouth, which make it possible in particular to dispense with a conducting gel. In addition, there are new alternative technologies with great promise, involving piezoelectric transductors (CHANTER & SUMMERS, 1998) or alloys with shape-memory (GRANT & HAYWARD, 1997). It must be emphasized, however, that the development and optimisation of effective stimulators with low energy requirements will only be possible if the physiological characteristics of the sensory receptors involved in touch are taken into account. Thus the electromagnetic systems, usually tuned to a frequency of about 250 to 300 Hz, mainly stimulate in a diffuse fashion sensory receptors with wide receptor fields on the skin. There would be a great advantage in exploiting the great variety in the sensitivity of the different cell-types involved in touch, in order to enrich the transductive capacities of these tactile interfaces (see [START_REF] Shinoda | Tactile Feeling Display Based on Selective Stimulation to Skin Mechanoreceptors[END_REF].

CONCLUSION

As long as one holds fast to a classical conception of perception in terms of the acquisition of information, one will be stuck with the principle that it is always better to have access to more information. In this framework, persons with sensory handicaps will inevitably be considered as defective. We have proposed an alternative conception, in which "sensory substitution systems" are rather thought of as supplementation devices which bring about new modes of coupling with the environment. They do not make a difference disappear; rather, they create new differences -and they have applications which are not exclusively reserved for handicapped persons (for example, artistic applications, games, augmented reality, the development of portable and intuitive systems for the detection of heat, radioactivity....). In spite of appearances, it is the classical perception which carries the germ of exclusion since it considers that the problem of handicapped persons lies in a quantitative difference. By contrast, true respect for the world of handicapped persons lies with better knowledge and understanding of the qualitative difference of possible perceptual modes.

  shattered. Now what is cruelly missing in this new perceptual modality is what Bach-y-Rita calls the qualia, i.e. the values and the quality of lived experience associated with perceived entities. If one

Table 1 . A non-exhaustive list of various sensory substitution systems. This table also includes single components (sensors or stimulators ) which are available for incorporation in this context. Abbreviations: (EM) electromagnétic, (ET) electrotactile, (SMA) shape memory alloys, (PE) piezo-electric, (CA) compressed air, (CG) constraint gauge. 1.3. The results obtained with the TVSS

 1

	The TVSS (« tactile vision substitution system ») makes it possible to convert the image
	captured by a video camera into a "tactile image". In the standard version, the tactile image is
	produced by a matrix of 400 activators (20 rows and 20 columns of solenoids of one millimeter
	diameter). The matrix is placed either on the back (first version), or on the chest, or on the brow
	(COLLINS & BACH Y RITA, 1973). Many technical improvements have been introduced
	(miniaturisation, image definition, electrical stimulation, etc), and this device is still commercially
	available under the label VideoTact Equipped with the TVSS, blind (or blindfolded) subjects are almost immediately able to
	detect simple targets and to orient themselves. They are also rapidly able to discriminate vertical
	and horizontal lines, and to indicate the direction of movement of mobile targets. The recognition
	of simple geometric shapes requires some learning (around 50 trials to achieve 100% correct
	recognition). More extensive learning is required in order to identify ordinary objects in different
	orientations. The latter task requires 10 hours of learning in order to achieve recognition within 5

TM (Unitech Research Inc)

. Finally, it has recently been adapted with a view to being used by young babies (current research by E. SAMPAIO).

ETA : « electronic travel aid ».

Whereas in fact the matrix of tactile stimulators is placed on the back of the subject.

A proper definition and/or modelling of the notion of "sensori-motor invariant" remains however to be formulated.


O L Krutkin

A B Altukhov

A D Gurchenko

E Z Gusakov

S Heuraux

M A Irzak

L A Esipov

T P Kiviniemi

C Lechte

S Leerink

P Niskala

G Zadvitskiy

Investigation of nonlinear effects in Doppler reflectometry using full-wave synthetic diagnostics

In this work, Doppler reflectometry (DR) and radial correlation DR (RCDR) nonlinear scattering effects were studied using full-wave modeling and a set of representative FT-2 tokamak turbulences as inputs. Narrowing of the RDCR correlation function and widening of the DR poloidal wavenumber spectrum are demonstrated. An effect on the dependence of the DR signal frequency shift on the probing wavenumber is found, namely, this dependence "linearizing" in the nonlinear scattering regime. Nonlinear effects are shown to be weaker for O-mode probing than for X-mode probing, while a faster transition to nonlinear regime is demonstrated for RCDR compared to DR in both probing scenarios.

Introduction

Turbulent transport plays a major role in plasma confinement, which makes understanding and control of plasma turbulence one of the main goals of fusion research. The tools for the turbulence characterization include Doppler reflectometry (DR) and radial correlation Doppler reflectometry (RCDR) [1], the two diagnostics based on probing the plasma with a microwave beam at an oblique incidence with respect to magnetic surfaces and measuring the characteristics of the scattering signal to extract information on the turbulence that induced the scattering. RCDR in particular utilizes simultaneous probing with two microwaves at different frequencies incident obliquely onto the magnetic surface in the presence of the cutoff. As it was shown in [START_REF] Gusakov | Popov 38th EPS Conference on Plasma Physics P[END_REF][START_REF] Gusakov | [END_REF], at a large enough incidence angle by performing correlation analysis of backscattering signals, information about turbulence properties, such as the radial correlation length can be extracted, while the conventional DR provides information on frequency and poloidal wavenumber spectra of the turbulence, as well as on the fluctuation rotation velocity in poloidal direction.

However, for both the RCDR and the conventional DR, the analytical theory predicts direct relation of measured quantities to the turbulence characteristics only in the linear regime of scattering [START_REF] Gusakov | [END_REF], corresponding to low turbulence amplitudes. Some analytical results for the nonlinear regime, such as criteria for the onset of the nonlinear behavior [4,5] and for the transition to fully nonlinear regime corresponding to the probing wave extinction due to its strong phase modulation by the turbulence and strong wave-front distortion were derived. A saturation of the scattering signal power growth with the turbulence level was obtained in this regime for the radial correlation reflectometry [6,7] and for DR [8]. Nevertheless, the interpretation of experimental DR and RCDR measurements is challenging in the nonlinear regime and in the transition to it. Moreover, even in the linear regime, due to a substantial contribution of the small-angle scattering off long-scale fluctuations in the reflected signal, the turbulence radial correlation length could be overestimated [START_REF] Gusakov | [END_REF]9]. Methods were, however, suggested to amend this issue and to reconstruct the turbulence radial wavenumber spectrum based on RCDR data [10,11].

Overall, the mentioned difficulties of the reflectometry data interpretation make full-wave numerical modeling one of the main tools of analysis of RCDR and DR. Numerical studies of RCDR and DR were performed [12] and recently, synthetic diagnostics which allow comparing the gyrokinetic modeling results directly to the experimental measurements were developed [13-[16]. One such diagnostic was developed for FT-2 tokamak [13] and demonstrated a good agreement with the experimental data. As this synthetic diagnostic was created, extensive full-wave computations using two-dimensional arrays of density values computed by a Gyrokinetic code (GK) were carried out. This work highlights the results of these computations focusing on the effects of the nonlinear scattering on DR and RCDR measurements.

The computation approach

Parameters of the FT-2 ohmic discharge used for the gyrokinetic modeling are as follows: B 0 =1.7 T, I p =19 kA, n e max =4.2⋅10 19 m -3 , while the major and minor radii of the device are 0.55 m and 0.08 m respectively. More detailed information on the discharge as well as on experimental profiles can be found in [13], [15]- [17], as well as schematics and illustrations of the DR configuration used. Same as in experimental cases [13], [15] [16] the X-mode probing was performed by a horizontal antenna located at the high-field side (HFS) of the tokamak, which could be shifted vertically from the equatorial plane by up to 2 cm, making the probing oblique and allowing a scan over poloidal wavenumbers of the probing wave. The central probing frequency used was f 0 =70 GHz, which corresponds to cutoff located at r=5.5 cm.

Such a setup enabled the probing of the turbulence with poloidal wavenumbers of up to

k θ =12 cm -1 .
For O-mode probing, a vertically aligned antenna shifted from the chamber axis along the major radius by 5 cm towards the HFS was used (once again reproducing the experimental configuration in [17]). The probing was performed at the central frequency f 0 =30 GHz with the cutoff also being located in the vicinity of r=5 cm. The turbulence wavenumber probed in this case takes the value k θ =9 cm -1 . Beam diameter of 2 cm was utilized for both polarizations.

This discharge was modeled by ELMFIRE GK code [18] and the resulting density profile was used for the synthetic RCDR diagnostic. ELMFIRE is a global particle-in-cell code that computes the full distribution function of gyro-kinetic ions and drift-kinetic electrons. to the measured radial profile of neutral hydrogen density and at the wall temperature. The specifics of the modeling approach, such as implementation of sinks, sources and collisions can be found in [20]. The temporal dependence available has the duration of 150 μs with the step of 0,15 μs (after being averaged over ELMFIRE intrinsic time step).

The main computation used for further full-wave modelling was performed on a cylindrical grid with 100x8 radial (r) and toroidal (φ) cells respectively, while number of poloidal (θ) cells, which adjusts the grid size to local Larmor radius, scaled with r up to 950 at r=7 cm.

This allowed for the cell size of 0.07 cm (in radial and poloidal direction) in the probing area.

Such poloidal resolution allows for the value k θmax =47 cm -1 to be resolved by the code at the studied minor radius, which in turn allows for the value of k θ ρ s = 2.6, where

/ 2 s e i i T T ρ ρ = .
and ρ i is ion Larmor radius. These parameters mean that ELMFIRE can be used to model trapped electron mode (TEM) instability, which was shown to be typical for FT-2 tokamak [21]. On the other hand, small-scale electron temperature gradient (ETG)

instability cannot be correctly resolved by ELMFIRE, which is not a problem as the wavenumbers probed by DR are limited by k θ ρ s = 0.7, placing ETG mode outside of the accessible wavenumber range. Further details on the FT-2 tokamak turbulence and its modeling with ELMFIRE can be found in previous papers [13], [15], [16].

Aside from an output of the computation based on experimental profiles, we also used the output of another ELMFIRE run with a temperature profile, which did not correspond to the experimental profile as input. This non-experimental profile, while possessing the same peak value, had a flatter shape resulting in an overestimation of the electron temperature in the probing wave cut-off region (220 eV instead of actual experimental value of 100 eV). The grid in the poloidal direction had 150 points at any minor radius whereas the turbulence obtained in this case possesses a steeper poloidal wavenumber spectrum. While less relevant for the FT-2 tokamak comparison to experiment, this case was expected to provide clearer indication of nonlinear effects influencing the DR frequency spectra compared to the experimental one while still using a realistic turbulence. This is connected to the fact that the linear signal is proportional to the components of spectrum with higher k θ while nonlinear one is caused by multiple scattering off the lower k θ turbulence [4,5].

The realistic density profile, produced with ELMFIRE code for two different profiles of electron temperature was then separated into the background density and the density perturbations by averaging over the window of 30 μs. Those perturbations were multiplied by a constant factor a to perform the scan over the turbulence amplitude and observe previously mentioned nonlinear effects coming into play.

Obtained background density along with density fluctuations were used as the input for the full-wave code IPF-FD3D [14]. The code was used in its 2D configuration to compute the scattered signal received by the antenna. The configuration used was copied from the experiment. The DR signal calculation was performed separately for each time step provided by ELMFIRE (assuming the turbulence to be frozen), which seems appropriate considering the slow scale in time of the drift-wave (µs <) compared to wave propagation timescale (~ns).

As a result of the computation for every 0.15 μs time step obtained from ELMFIRE, the temporal dependence of the complex backscattering signal was obtained. From this dependence a cross-correlation function (CCF) of scattered signals corresponding to the probing at different frequencies was computed. DR spectra were also computed by performing Fourier transform in the windows of 12 μs averaged over the full 150 μs of the computation and each toroidal cross-section. The synthetic CCF and DR spectra as well as the total signal power were used for the analysis.

X-mode DR modeling results and analysis

Due to the strong dependence of its refractive index on the plasma density, X-mode is more susceptible to nonlinearity, which made it the prime object of this study. We will start by presenting the non-experimental temperature profile case, extensive computations for which were performed in the X-mode case, as it exhibits the nonlinear effects much clearer.

An example of the DR frequency spectra for a number of different turbulence amplitudes (with Δf corresponding to the frequency shift from f 0 ) is plotted in the Fig. 1, while Fig. 2 demonstrates the dependence of the total scattered power received by the antenna. Fig. 2 is plotted against the dimensionless factor a by which the amplitude of the turbulence produced by ELMFIRE code is multiplied. That means that a=1 corresponds to the original ELMFIRE turbulence, while a=2 means that the turbulence was artificially enhanced twofold for the fullwave computation.

In this scenario, which corresponds to the non-experimental electron temperature profile, it can be seen that with the growth of the turbulence amplitude, DR spectra shift to higher frequencies. The notion of this shift being caused by a nonlinear effect is further confirmed by the signal power growing at a rate different from quadratic for the higher a-values in the Fig. 2. On the top half of the figure the scattered signal power is plotted. At about a=0.5 the growth becomes faster than linear, which corresponds to the nonlinear regime, described in [4,5], while for avalues of 1 and higher the nonlinear saturation described by the analytical theory [6,7] and shown in the computations [5] can be observed. To make the nonlinearity more noticeable, an exponential factor defined for two values of a as (lg(P2)-lg(P1))/(lg(a2)-lg(a1)) where P is the scattering signal power is plotted at the bottom half of Fig. 2. In the case of quadratic dependence corresponding to the linear scattering regime this factor has the value of 2, for the nonlinear regime this value is higher than 2, while in the saturated nonlinear regime the value becomes lower than 2.

Nonlinear scattering, however, does not by itself explain the shift of the scattering spectrum to higher frequencies. The explanation we propose also includes nonlinear dispersion of turbulent fluctuations leading to a lower phase velocity at the higher fluctuation poloidal wavenumbers observed in the GK computations, shown as green solid curve in Fig. 3. That curve on the figure corresponds to the direct computation of the density fluctuations poloidal spectrum from ELMFIRE data with consecutive computation of the mean frequency of each wavenumber spectral component. Mean frequency for all curves is calculated as the first moment of the spectrum ( ) ( )

mean s s f f p f d f p f d f = Δ ⋅ Δ Δ Δ Δ ∫ ∫
, where p s (Δf) corresponds to the spectral power at specific frequency. For the solid curve it can be seen that for the higher poloidal wavenumber values this mean frequency saturates and stop increasing. This behavior is consistent with the theory expectations [22] providing the following dependence of the drift wave phase velocity in the plasma reference system: It should also be mentioned, that, strictly speaking, the points on the Fig. 3 correspond to different radial positions of the turning point, which depend on the probing poloidal wavenumber. However, in the previous work [15] with the means of ray-tracing and the weighting function, the difference in radial position was shown to be up to 0.3 cm which is less than the weighting function localization and should not impact results significantly.

In the nonlinear regime characterized by multiple scattering, the backscattering signal is provided in part by fluctuations possessing a factor of 2 smaller poloidal wavenumbers than those satisfying the Bragg condition [5]. Therefore for the nonlinear turbulence dispersion law shown in Fig. 3 the multiple scattering off the lower-k fluctuations will result in a larger frequency shift than a single scattering induced by the fluctuation possessing a high poloidal wavenumber and satisfying the Bragg condition.

To confirm this idea, we considered the average DR signal spectrum frequency shift corresponding to the different antenna positions and consequently, different poloidal wavenumbers of the probing wave. If the proposed explanation is correct, in the linear regime we would see a saturation of the Doppler frequency shift with growing k θ due to the turbulence dispersion law. However in the nonlinear regime of scattering we would obtain the dependence much closer to a linear one. This indeed turned out to be the case and the corresponding plot is shown in the Fig. 3, where the average frequency shift of scattered signal dependence are plotted along with the dispersion calculated directly from the turbulence. The quantities are plotted against the double poloidal wavenumber of the probing X-mode in the turning point (corresponding to poloidal wavenumber of the turbulence) as well as against the antenna vertical shift y. Next we present a modeling of DR signals utilizing the turbulence computed for the experimental profile of electron temperature [13]. This case is characterized by a slower decay of the poloidal wavenumber spectrum of the turbulence, which in turn means that nonlinear regime of scattering will now dominate signal at higher turbulence amplitudes, as predicted by the analytical theory [4], [5]. The spectra, obtained in this case for different turbulence levels and presented on Fig. 4, exhibit a substantially smaller frequency shift, while the DR signal power dependence on the turbulence level transits straight into the nonlinear saturation from the linear regime as it is seen in Fig. 5, which is a possibility ! predicted by the analytical theory and shown in the numerical computations [4], [5] for the case when turbulence spectrum is not steep enough.

The reason why the difference of spectra in this case is milder is probably the larger value of the linear Bragg backscattering signal, which dominates over the nonlinear one. Also, during the transition from a single to multiple scattering, the frequency shift does not change significantly (as seen in Fig. 6), likely due to the turbulence dispersion law, the saturation It should be mentioned that in this work we do not analyze the meaning of the obtained mean frequency shifts and their connection to Doppler shift or turbulence phase velocity. An extensive analysis of the mean frequency shift including rotation velocities, GAM influence [13] and impact of parallel velocity [15] is presented in previous works [16] and we refer the readers to these works for more information.

Overall it seems the main effect nonlinear regime has on the DR frequency spectrum is "linearization" of the Doppler frequency shift dependency on the probing wave poloidal wavenumber caused by the multiple scattering off the longer-scale turbulence dominating DR signal. The backscattering spectrum frequency shift in this regime is determined by the longscale turbulence dispersion law, as displayed at Fig. 3 and Fig. 6. This in turn means that the saturation of a linear dependence of the frequency shift could be considered as an indicator of the diagnostic operating in linear regime.

The effect of nonlinear scattering on the DR turbulence poloidal wavenumber spectrum measurements and on RCDR is well described in [6], and was reproduced in the computations [12,14]. In the case of DR signal power dependence on the antenna vertical shift y , which in the linear scattering regime is related to the turbulence poloidal wavenumber spectrum, the nonlinear effects lead to broadening of the dependence due to the nonlinear saturation of the spectral components corresponding to lower k θ as it is shown on Fig. 7. In case of the RCDR CCF, analytical theory predicts that in linear regime the correlation decay is slow due to the nonlocal small-angle scattering [START_REF] Gusakov | Popov 38th EPS Conference on Plasma Physics P[END_REF], [START_REF] Gusakov | [END_REF] while in the nonlinear regime CCF does not directly correspond to the turbulence spatial correlation [6], [7] and was shown to be as steep or steeper than the turbulence correlation function [12]. A same effect was observed in this paper and is presented on the Both of the nonlinear effects observed are in agreement with the previous results [6], [7], [12],

[14] and complicate the interpretation of experimental data. Moreover, obtained results indicate that RCDR diagnostic transits into the nonlinear regime at lower amplitudes of the turbulence compared to DR. Overall, is seems that to obtain reach? the correct interpretation of experimental data numerical modeling is necessary.

O-mode DR modeling results and analysis

For O-mode computation, Fig. 9 illustrates the nonlinear narrowing of the RCDR CCF with the turbulence amplitude growth, suggesting that the transition from the linear to the nonlinear regime is happening. This in turn means that The interpretation of the results coming from the original ELMFIRE data says that the RCDR operates in linear regime. This interpretation conclusion is further confirmed by the scattering signal power dependence on the turbulence amplitude shown in the Fig. 10.

As for the DR frequency spectrum, in the case of O-mode it still demonstrates the aforementioned shift to the higher frequencies in the nonlinear regime, but not a significant one, as can be seen at Fig. 11. This is appropriate for "experimental" ELMFIRE case, where the turbulence dispersion is closer to linear regime.

Nonlinear effects for O-mode start to play a role at much higher amplitudes compared to Xmode.

! ! Overall it seems that in the cases of O-mode for the experimental situation both DR and RCDR operate within the linear regime, which makes the linear numerical modeling relevant and means that O-mode measurements are more suitable for a direct interpretation. While the results obtained have demonstrated a qualitative agreement with the analytical predictions, the subject of the future work would be to perform quantitative analysis on the nonlinear scattering regime offset. Analytical criteria provided by [4]- [7] would need to be adapted for the parameters domain of FT-2 experiments, possibly including modifications due to the cylindrical geometry effects and compared against the modeling results as well as other nonlinearity interpretations, such as those presented in [15] and [16].

Conclusions

Within this work the full-wave computations of synthetic DR and RCDR signals are performed using a realistic FT-2 tokamak turbulence obtained with the help of GK ELMFIRE code. Nonlinear effects are demonstrated and found to be in agreement with the theoretical expectations [4] [6] [7]. An effect of "linearization" of the DR signal frequency shift dependence on the probing wavenumber is found in the nonlinear scattering regime.

Nonlinear effects are shown to be weaker for O-mode probing in the studied cases, while a faster transition to the nonlinear regime is demonstrated for RCDR compared to DR in both probing scenarios.

  Neumann and Dirichlet boundary conditions are used at the boundary of the simulation domain for the electrostatic potential. Particles passing the boundary are returned to the simulation domain as an electron-ion pair according to a probability distribution proportional

!Fig. 1 .

 1 Fig. 1. DR signal power spectra for X-mode probing at antenna vertical shift +2cm.

Fig. 2 .

 2 Fig. 2. DR signal power dependence on the turbulence amplitude and exponential power factor.

  such saturation in comparison with what other curves (corresponding to synthetic signals) display is explained by the fact that due to the turbulence amplitude getting smaller at higher k θ the numerical noise plays a bigger role and adds a systematic error shifting the value of mean frequency closer to zero.

!Fig. 3 .

 3 Fig. 3. Average frequency shift of scattering signal for X-mode against the double probing poloidal wavenumber on top and the vertical position of antenna y. Lines with markers correspond to synthetic DR signal for different amplitudes of the turbulence. Solid line -the turbulence dispersion provided directly by ELMFIRE plotted against the turbulence poloidal wavenumber.

Fig. 4 .

 4 Fig. 4. Power spectra probing at antenna vertical shift +2cm. Different lines correspond to different amplitudes of the turbulence.

!Fig. 5 .Fig. 6 .

 56 Fig. 5. DR signal power dependence on the turbulence amplitude and exponential power factor.Fig. 6. Average frequency shift of scattered signal for X-mode. Different lines correspond to different amplitudes of the turbulence.

Fig 8 .!Fig. 7 .Fig. 8 .

 878 Fig. 7. DR power dependence on antenna vertical shift. Different lines correspond to different amplitudes of the turbulence.Fig. 8. Normalized RCDR CCFs. Different lines correspond to different amplitudes of the turbulence.

Fig. 10 .

 10 Fig. 10. DR signal power dependence on the turbulence amplitude.

Fig. 11 .

 11 Fig. 11. DR signal power spectra for probing at 34 GHz and different amplitudes of the turbulence.

Fig. 9 .

 9 Fig. 9. CCF for -5 cm antenna shift. Different lines correspond to different amplitudes of the turbulence.

Fig. 9 .

 9 Fig. 9. CCF for -5 cm antenna shift. Different lines correspond to different amplitudes of the turbulence.

Acknowledgements

Modeling of the O-mode DR signals was performed with the financial support of RFBR grant 18-32-00031. Modeling of the X-mode DR signals was performed with the financial support of the RSF grant 17-12-01110. Measurements at the FT-2 tokamak were supported by the state contract of the Ioffe Institute. This work has been partially carried out within the framework of the French Federation for Magnetic Fusion Studies (FR-FCM) and the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement № 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.


Pierre Roumanille

Valérie Baco-Carles

Corine Bonningue

Michel Gougeon

Philippe Tailhades

Philippe Monfraix

Developing new joining materials for low-temperature electronics assembly

  L'archive ouverte pluridisciplinaire

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible Any correspondence concerning this service should be sent to the repository administrator: tech-oatao@listes-diff.inp-toulouse.fr Official URL: https://doi.org/10.1109/NMDC.2016.7777169

New regulations due to environmental and health concerns including the European RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electric and Electrical Equipment) directives have led to a shift towards tin-based, lead-free solders in electric and electronic equipment. For the assembly of electronic devices on printed circuit boards (2 nd level packaging), the most widely used lead-free alloy is near eutectic tin-silver-copper (Sn-Ag-Cu or SAC). Compared to tin-lead based solders, the higher melting points of SAC solders can result in additional stress, accelerated ageing and a global decrease in reliability for components and polymer-based printed circuit boards during reflow processing. These recent concerns have led to new research and development in lead-free solders area [START_REF] Kotadia | A review: On the development of low melting temperature Pb-free solders[END_REF]. Numerous new alloy compositions are being investigated as well as new approaches like manufacturing alloy nanoparticles or improving mechanical properties with nanoparticle reinforcements in conventional micrometresized solders.

The present work focuses on a new kind of lead-free joining method for surface-mount technology based on precursor chemistry. The interest of metal oxalates as new soldering materials for die attachment (1 st level packaging) was previously demonstrated with silver oxalate [START_REF] Kiryukhina | Silver oxalate-based solders: New materials for high thermal conductivity microjoining[END_REF], [START_REF] Kiryukhina | Silver oxalate: towards a new solder material for highly dissipative electronic assemblies[END_REF]. The thermal decomposition of metal oxalates under controlled atmosphere can be used to produce small metal particles below their melting point. These particles are found to be in a highly active particulate form [START_REF] Dollimore | The production of metals and alloys by the decomposition of oxysalts[END_REF]. First experimental studies are focusing on several metal oxalates (tin oxalate and bismuth oxalate) to assess their suitability for lowtemperature metal particle production. The main work is dealing with controlled chemical precipitation synthesis and characterization of the compounds (Figures 1 and2) as well as study of the properties of decomposition solid products (powder X-Ray diffraction, Fourier-transform infrared spectroscopy, Scanning Electron Microscopy and thermal analyses under different atmospheres).

*Research supported by IRT Saint Exupéry, Toulouse, France. P. Roumanille is with IRT Saint Exupéry, Toulouse, France and CIRIMAT, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France (e-mail : pierre.roumanille@irt-saintexupery.com).

V. Baco, C. Bonningue, M. Gougeon and P. Tailhades are with CIRIMAT P. Monfraix is with IRT Saint Exupéry First results tend to show that tin oxalate starts to decompose at about 573K. This temperature is too high to use asprecipitated tin oxalate for a soldering application at low temperature. Moreover, tin oxalate spontaneously decomposes into tin oxide even if the heat treatment is carried out under pure hydrogen. Small tin particles were though obtained from tin oxide under such reducing conditions close to 673K (Figure 3). The oxalate route could then be a way to produce morphology-controlled tin particles. Concerning bismuth oxalates, the first part of the work implied the precise control of the synthesis. Indeed, several hydrated oxalates and hydroxyl oxalates are known and their formation is strongly dependent on the synthesis Developing new joining materials for low-temperature electronics assembly* P. Roumanille, V. Baco, C. Bonningue, M. Gougeon, P. Tailhades, P. Monfraix parameters. Thermal analysis showed that dehydration of hydrated bismuth oxalate Bi2(C2O4)3,7H2O occurs in two steps between 303K and 473K. Then, under an inert atmosphere, bismuth oxalate decomposes into temporary nanometric bismuth metallic particles in between 483K and 523K (Figures 4 and5). This range of temperature is below the melting point of bulk metallic bismuth (Tm(Bi) = 544K).

It is then possible to measure by differential thermal analysis the melting point of the nanometric bismuth particles formed. This characteristic and the sintering propensity of the nanometric metallic particles are currently being finely studied with the aim of potential use of a bismuth oxalate for soldering applications at low temperature.

  This is an author's version published in: http://oatao.univ-toulouse.fr/24458 To cite this version: Roumanille, Pierre and Baco-Carles, Valérie and Bonningue, Corine and Gougeon, Michel and Tailhades, Philippe and Monfraix, Philippe Developing new joining materials for low-temperature electronics assembly. (2016) In: 11th IEEE Nanotechnology Materials and Devices Conference (NMDC), 9 October 2016 -12 October 2016 (Toulouse, France).

Figure 1 :Figure 2 :

 12 Figure 1 : Tin oxalate particles (SEM image)

Figure 3 :Figure 4 :Figure 5 :

 345 Figure 3 : Tin particles resulting from tin oxalate decomposition under hydrogen atmosphere