WO2017144455A1 - Sensor of thermal patterns with thermo-resistive capsules - Google Patents

Abstract

The invention relates to a sensor (300) of thermal patterns of an object, in particular a papillary print, comprising a contact surface for applying the object thereto, and configured to allow thermal exchanges between this object and sensitive elements situated above a substrate and under said contact surface. These sensitive elements are formed by a series of three-dimensional structures (310), termed capsules, formed of thermo-resistive material. Each capsule (310) delimits the lateral and upper boundaries of a gas-filled or vacuum-sealed cavity, and is connected to means for reading its electrical resistance, by two first connection pads situated in or on the substrate. Such a sensor offers an excellent rate of filling of the detection surface by the sensitive elements, namely the capsules. Furthermore, the air or the vacuum contained in the capsules insulates them from the substrate, thereby contributing to the quality of an image of the object, formed with the aid of the sensor.

Description

 SENSOR OF THERMAL PATTERNS WITH THERMO-RESISTIVE CAPSULES

DESCRIPTION

TECHNICAL AREA

The invention relates to the field of thermal pattern sensors or detectors, or sensors of the thermal impression of an object, for imaging the thermal patterns of an object, said object to be imaged.

 Such sensors form transducers of a temporal variation of temperature, in a difference of potentials or currents. They measure in particular a two-dimensional distribution of the thermal mass of an object with which they are in direct physical contact, and even of its thermal capacity and / or its thermal conductivity.

 Such a sensor can form a mass spectrometer, or flow meter, type of analyzer (by heating the object in one location and measuring how far the heat is propagating). It can in particular form measuring means at various depths in an object, by varying injected power to heat the object, and measurement times.

 It can form in particular a papillary impression sensor, for imaging an imprint related to the particular folds of the skin, in particular a fingerprint, but also a palm footprint, plantar, or phalangeal. These various imprints are collectively referred to as papillary impressions.

Such a papillary impression sensor uses a difference in thermal impact, on a contact surface, between regions in direct physical contact with the finger, at the level of the crests of the impression, and regions that are not in direct physical contact with the finger. finger, at the valleys of the footprint. STATE OF THE PRIOR ART

Various types of thermal pattern sensor sensors, in particular papillary impression sensors, are known in the prior art.

 For example, sensors based on the pyroelectric properties of a material such as PVDF are known. Such a material, however, only measures temperature variations as a function of time. After a very short time interval, the temperatures stabilize and the image obtained is insufficiently contrasted.

 Sensors based on the thermo-resistive properties of a material whose resistance is a function of temperature are also known.

 When you put your finger on the sensor, the contact with the finger heats the thermo-resistive material. The temperature of the thermoresistive material varies, depending on whether it is covered by a region of the finger corresponding to a crest of the fingerprint, or a valley of the fingerprint. It is thus possible to form an image of the fingerprint.

US 6,633,656 describes an example of such a sensor. The thermoresistive material is vanadium oxide (VO _x ). It is deposited in the form of a pixellated layer, directly on a substrate, or isolated therefrom by a layer of an insulating material.

Despite the possible presence of the insulating material, the heat is transmitted rapidly from the VO _x layer to the substrate, which adversely affects the contrast of the image obtained.

 The fingerprint sensor described in the Ji-Song article is also known.

Han, "Thermal Analysis of Fingerprint Sensor Having a Microheater Array," International Symposium on Micromechatronics and Human Science, 1999 IEEE.

 In this article, the authors study the characteristics of a thermal sensor in which each pixel consists of a silicon bar.

 The silicon bar has a central region, forming the sensitive zone of a pixel of the sensor, flanked by two lateral regions receiving electrodes.

The silicon bar is heated, the detection exploiting the heat transfer properties between the silicon bar and the skin. The sensitive zone of the silicon bar is also framed by two cavities etched in the substrate. These two cavities meet under the sensitive zone, and improve a thermal insulation between the silicon bar and the substrate.

 A disadvantage of this embodiment is that the surface filling ratio of the sensitive elements, here the central regions of the silicon bars, is limited, in particular by the lateral size of the etched cavities.

 An object of the present invention is to provide a thermal pattern sensor such as a papillary impression, having both an optimized thermal insulation between the sensitive elements and a substrate, and a high degree of filling of the sensitive elements.

STATEMENT OF THE INVENTION

This objective is achieved with a thermal pattern sensor of an object, in particular a papillary impression, comprising a contact surface for applying the object, and configured to allow heat exchange between this object, and sensitive elements located at the object. above a substrate, under said contact surface.

 According to the invention, the sensor comprises a series of three-dimensional structures, called capsules, formed of thermo-resistive material and forming said sensitive elements.

 Preferably, this material is more particularly constituted by amorphous silicon or an alloy comprising amorphous silicon. The term "amorphous silicon" is also used to denote a hydrogenated amorphous silicon, doped with germanium (Ge) and / or with boron (B).

 Each capsule delineates the lateral and upper boundaries of a cavity filled with gas or sealed under vacuum.

 Each capsule is connected to means for reading its electrical resistance, by two first connection pads located in or on the substrate.

The configuration of the sensitive elements, in the form of three-dimensional structures located above the substrate, and delimiting the lateral and upper of a cavity filled with a gas or sealed under vacuum, provides excellent thermal insulation between the substrate and these sensitive elements.

 This configuration does not require digging the substrate, which makes it possible to achieve a high surface coverage rate of the detection surface by the sensitive elements, for example greater than 0.9.

 The first two connection pads extend at the level of the substrate, under the capsules. Consequently, the filling ratio is also not limited by a lateral connection to the means for reading the electrical resistances, the capsules being connected vertically to the reading means.

 In the prior art mentioned in the introduction, the substrate is hollowed under a bar made of thermo-resistive material. Such etching is difficult to control, especially in depth, so that the thickness of air between the substrate and the bar, and the corresponding thermal insulation, are also difficult to control. The invention does not impose such an etching. The capsules according to the invention on the contrary offer good control of the thickness of gas or vacuum, filling the cavity and thermally insulating the substrate and an upper wall of the sensitive elements.

 In addition, the capsules being able to be sealed under vacuum, better thermal insulation can be obtained than with a simple thickness of air.

 Finally, capsules offer better mechanical stability than a suspended bar as described in the prior art.

 Preferably, the sensor according to the invention has means for heating a capsule.

 At least one capsule may be connected to a current or voltage source forming means for heating a capsule, for injecting a current or a bias voltage adapted to directly heat said capsule.

In addition or alternatively, the heating means of a capsule may comprise thermo-resistive components arranged between two adjacent capsules, and connected to a source of current or voltage, for the injection of a current or a current. bias voltage adapted to heat said thermoresistive components. The capsules are then advantageously divided into lines to form a capsule matrix, and the thermoresistive components extend along lines parallel to the capsule lines.

 In addition or alternatively, the heating means of a capsule may comprise at least one metal filament disposed above a capsule, and connected to a source of current or voltage, for the injection of a current or a bias voltage adapted to heat the filament.

 An electrical insulation layer may extend directly between the capsule and the metal filament, except above two second bond pads located in or on the substrate, and connected to the current or voltage source.

 Alternatively, the metallic filament may extend in a straight line, in direct physical contact with the capsule, the straight line being orthogonal to a vertical plane connecting the two first connection pads.

 Preferably, the sensor according to the invention comprises piloting means arranged to actuate the heating means of a capsule during a predetermined time interval, and the reading means are connected to comparison means to determine a variation. of the electrical resistance of the capsule between two predetermined moments.

 Each capsule advantageously has a bonnet-shaped, of which an upper wall is traversed by at least one orifice, and whose side and upper walls cooperate with a lower layer on which the capsule rests, to delimit the cavity.

 Preferably, the capsules extend over a thermal insulation layer covering the substrate, each capsule having two first feet which pass through the thermal insulation layer, to come into contact with the first two connection pads.

 The capsules are advantageously separated from each other, without direct physical contact between them.

The substrate may be of silicon, and the electrical resistance reading means may be made in CMOS technology. In a variant, the substrate may be made of glass, and the electrical resistance reading means may be made using TFT technology.

 According to a particularly advantageous embodiment, the sensor further comprises a plurality of photodiodes and light sources distributed under and / or between the capsules.

 Preferably, each capsule has a bonnet shape, an upper wall of which is traversed by at least one orifice, and each photodiode and / or each light source extends under the top wall of a capsule.

 The sensor according to this embodiment preferably has several types of light sources, which differ by a central transmission wavelength.

 The invention also covers a use of a sensor according to this embodiment, to determine whether or not a papillary impression consists of human living tissue.

 The invention also relates to a method for manufacturing a sensor according to the invention, the method comprising the following steps:

 depositing and etching a sacrificial layer above the substrate, to form sacrificial pads;

 depositing the capsule material on and between the sacrificial pads; evacuation of the sacrificial blocks through at least one open orifice in each capsule.

Preferably, the substrate is made of glass, and the sacrificial layer is a mineral layer, evacuated by etching with hydrofluoric acid.

 In a variant, the substrate is made of silicon, and the sacrificial layer is an organic layer, evacuated by oxygen plasma etching.

The invention finally relates to a method of using a sensor according to the invention, in which the capsules together form a capsule matrix in which the capsules are distributed in parallel lines which each extend over the entire width of the capsule. matrix of capsules, the reading of the electrical resistance of the capsules being carried out line of capsules per line of capsules, and the heating of the capsules being also realized line of capsules by line of capsules and of synchronous way with the reading of the electric resistances.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood on reading the description of exemplary embodiments given purely by way of indication and in no way limiting, with reference to the appended drawings in which:

FIGS. 1A and 1B illustrate a first embodiment of a fingerprint sensor according to the invention; FIG. 2 illustrates a second embodiment of the impression sensor according to the invention, with better thermal insulation between the capsules and the substrate; Figure 3 illustrates a variant of the embodiment of Figure 2; FIGS. 4A and 4B illustrate a third embodiment of the impression sensor according to the invention, with means for heating the capsules; FIG. 4C illustrates a variant of the embodiment of FIGS. 4A and 4B; Figs. 5A and 5B illustrate another variation of the embodiment of Figs. 4A and 4B; FIGS. 6A and 6B illustrate a fourth embodiment of the impression sensor according to the invention, with means for heating the capsules; FIGS. 7A and 7B illustrate a fifth embodiment of a fingerprint sensor according to the invention, for implementing an active thermal type detection; FIGS. 8A and 8B schematically illustrate a sixth embodiment of a fingerprint sensor according to the invention, further incorporating the elements of an optical sensor; and Figure 9 illustrates in detail a fingerprint sensor of the type illustrated in Figures 8A and 8B.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

 In the following, but in a nonlimiting manner, a sensor according to the invention of the fingerprint sensor type is described more particularly, the object to be imaged being constituted by a fingerprint.

 Figure 1A schematically illustrates a first embodiment of a papillary impression sensor 100 according to the invention, in a sectional view.

 In the following, we will consider, by way of example and without limitation, that it is a fingerprint sensor 100.

 The impression sensor 100 is of matrix type, that is to say composed of a plurality of sensitive elements, distributed for example in rows and columns.

 The sensor 100 is configured to allow heat exchange between an imprint to be imaged (not shown), and sensitive elements disposed between a substrate 120 and a contact surface 131.

 Throughout the text, the terms "upper" and "lower" refer to a vertical axis (Oz), orthogonal to the substrate, and oriented from the substrate 120 (lower side) to the contact surface 131 (upper side).

 According to the invention, each sensitive element consists of a three-dimensional structure, called capsule 110.

 The capsules 110 are made of a thermoresistive material, that is to say whose electrical resistivity is a function of temperature. Therefore, for each capsule, an electrical resistance is measured which varies according to its temperature.

 In particular, the electrical resistance R and the temperature can be connected by:

R (T ₀ + 9) = R (T ₀ ) * e ^{TCR * e}

with T _Q the initial temperature, Θ the temperature variation, and TCR the thermal coefficient of resistance of the material. The thermoresistive material is for example made of amorphous silicon (a-Si), in particular hydrogenated amorphous silicon (a-Si: H) which can be doped, for example by boron or germanium atoms. This material has in particular a very good TCR, and good mechanical strength. In addition, it is a material with high compliance during deposition, allowing the geometric discontinuities related to the capsule shape do not involve electrical discontinuity. It is compatible with standard TFT and CMOS technologies (see below). Finally, it has a resistivity compatible with the standard reading circuits of thermal type impression sensors.

Alternatively, the amorphous silicon is mixed with another component within an alloy. For example, an alloy comprising amorphous silicon, in particular hydrogenated amorphous silicon (a-Si: H), for example a-Si _x Ge _y B _z : H, may be used.

 Each capsule 110, has a bonnet-like shape surrounding a cavity 112. The cover consists of side walls 110B, capped by an upper wall 110A.

 The upper wall 110A is pierced by at least one orifice 111 or vent, allowing to evacuate a sacrificial layer used to manufacture the capsule (see below).

 In the figures, the orifice 111 is represented in the center of the upper wall 110A.

The orifice may, however, be positioned elsewhere on the capsule. There may also be several orifices per capsule.

 According to a preferred embodiment, the side walls 110B extend substantially vertically, parallel to the axis (Oz). They have a constant height according to (Oz). The upper wall 110A is a flat wall, which extends in a horizontal plane, orthogonal to the axis (Oz). It is in direct physical contact throughout its perimeter, with the upper edges of the side walls 110B.

 The upper wall 110A here has a square shape, the side walls have a square-based cylinder shape, and the capsule surrounds a rectangular parallelepiped-shaped cavity 112.

In the example illustrated in FIG. 1A, all the capsules are formed in one piece, connected to each other on the lower side of their respective lateral walls, by peripheral regions that extend directly on the substrate. The walls 110A and 110B of the capsules 110 may have a thickness E of about 1 μιη, for example between 0.5 and 2 μιη, which is sufficient to confer good mechanical strength to the capsule.

 In any event, the production of the sensitive elements in the form of capsules gives them great mechanical stability, especially with regard to the pressure stresses that can be exerted when a user presses the finger on the contact surface 131 .

 The upper walls of the capsules together define a detection surface of the impression sensor.

 Each capsule may have reduced dimensions, so that the capsules are distributed in a step of reduced repetition, especially less than 51 μιη, for example 50.8 μιη or 25.4 μιη. The impression sensor 100 thus provides an image having excellent resolution.

 The capsules can be arranged very close to each other. For example, two neighboring capsules may be spaced only a few hundred nanometers apart.

 It is thus possible to produce a matrix having a very good surface coverage of the detection surface, by the capsules, and more particularly by the upper walls of the capsules. This filling ratio is for example greater than 0.8 or even 0.9.

 Capsules enclosing a cavity 112 cubic, or rectangular parallelepiped shape, offer an optimal filling rate.

 In the example illustrated in FIG. 1A, each capsule 110 rests directly on the substrate 120.

 The capsules are therefore in contact with the substrate only at a lower peripheral surface, on a lower face of the side walls 110B of the capsules.

The cavity defined by each capsule is delimited laterally by the side walls 110B of the capsule. On the top, it is delimited by the upper wall 110A of the capsule. Bottom side, it is delimited by a surface on which the capsules rest, here the substrate 120. Each cavity 112 may be filled with a gas such as air, having a low coefficient of thermal conductivity, for example less than 1 Wm -1 .K ^-1 and even less than 0.03 Wm ^-1 .K ^-1 , at 20 ° C and atmospheric pressure In this case, each cavity 112 does not necessarily form a closed volume.

 Alternatively, each cavity 112 is sealed and evacuated during a sealing process. If necessary, the cavity 112 can then enclose a getter material, to maintain the vacuum quality over time. A getter material, or gas trap, limits the appearance of gas in an enclosure. It can act of an easily oxidizable metal such as titanium, or vanadium, zirconium, cobalt, iron, manganese, aluminum or an alloy of these metals.

 In this case, the cavity 112 forms a closed volume.

 This cavity, filled with a gas such as air or a vacuum, provides an excellent thermal insulation between the capsule and the substrate, and more particularly between the upper wall 110A of the capsule and the substrate 120.

 The substrate 120 is for example a substrate compatible with CMOS technology

(for "Com m enta ry Metal Oxide Semiconductor"), including silicon. This variant is particularly suitable for a fingerprint sensor, in which the capsules are distributed in a surface matrix ranging from a few mm ² to a few cm ² .

 Alternatively, the substrate 120 may be a substrate compatible with TFT technology

(for "Thin-film transistor"), especially glass. This variant is particularly suitable for a palm impression sensor, in which the capsules are distributed in a large matrix.

 The substrate receives a circuit for reading the electrical resistance of each capsule. In particular, each capsule is electrically connected to two first connection pads 150, here flush with the upper surface of the substrate 120, under the capsules, here under the side walls 110B of the capsules.

The first connection pads 150 are made of metal, in particular copper or aluminum. Each first connection pad is here in direct physical contact with a capsule.

 The first two connection pads 150 are connected to means 160 for reading an electrical resistance, symbolized in the figures by an ohm-meter. The reading of the electrical resistance implements a bias of the capsule in current, respectively in voltage, and a voltage measurement respectively current.

 The sensor according to the invention thus provides a vertical connection between the capsules 110, and the connection pads connected to the means 160 for reading an electrical resistance.

 This vertical connection makes it possible to bring the capsules closer to each other, without being limited by the bulk of electrodes which would extend laterally, next to and not below the capsules.

 Figure 1B illustrates a capsule 110 in a sectional view in a plane parallel to the substrate. The first connection pads 150, shown in transparency, are connected here by a vertical plane, parallel to a diagonal of the upper surface of the capsule.

 The invention is not limited to this arrangement. Many other arrangements can be envisaged. A rule linking the dimensions of a capsule and the positioning of these pads is described below, ensuring that an electric resistance measuring current flows in the upper walls of the capsules.

 Figure 1B also illustrates the plane AA ', passing through the first connection pads 150, and corresponding to the plane of the sectional view shown in Figure 1A.

 The capsules are covered by a protective layer 130, the outer face of which forms the contact surface 131 of the impression sensor.

 This protective layer provides protection for sensor elements with respect to repeated contact with human tissue.

 Here, the protective layer 130 also extends in the free space between two neighboring capsules. Alternatively, it extends only above the capsules. It has for example a thickness of about 30 μιη, or less.

The protective layer 130 is preferably electrically insulating (of electrical resistivity greater than at least a factor of 10 with respect to the capsules), to prevent the finger from shorting the capsules when pressed against the contact surface 131. The protective layer 130 is also thermally conductive. Its thermal conductivity, for example is at least 20 Wm ^_1 -K ¹ at 20 ° C at atmospheric pressure. It allows efficient and limited heat transfer between the contact surface and the capsules.

 Concretely, it can be a thick oxide layer, a layer of an epoxy polymer (epoxy paint), a layer of amorphous carbon called DLC (for "Diamond-Like Carbon"), etc. .

 The protective layer may contribute to hermetically sealing the capsules, covering the orifices 111. In a variant, the thickness of this very thin protective layer is not sufficient for this hermetic seal, the capsules then being filled, for example, with air.

 In operation, the user places his finger directly on the fingerprint sensor, in direct physical contact with the contact surface 131.

 In a first mode of detection, called "passive", the skin is then in direct physical contact with the contact surface 131, at the level of the crests of the fingerprint. The body heat is transferred by conduction from the contact surface 131 to the capsules.

 At the valleys of the fingerprint, there is no direct physical contact between the skin tissues and the contact surface. The body heat is thus little or not transferred to the capsules, through the contact surface 131.

 Thus, the temperature of a capsule indicates whether it is surmounted by a ridge or valley of the footprint. The temperature of the capsules is measured by the means 160 for reading an electrical resistance.

 The electrical resistance of the capsule is generally between 1 kΩ and 10 ΜΩ.

 Each resistance value of a capsule can be converted to gray level, by unrepresented conversion means, to form an image of the fingerprint.

A second detection mode, called "active", will be described in the following. The quality of the image formed using the sensor according to the invention, in particular its contrast, depends in particular on the quality of the thermal insulation between the capsules 110 and the substrate 120.

 The invention makes it possible to obtain optimized thermal insulation between the capsules and the substrate, thanks to the configuration of the sensitive elements as described above.

Each cavity 112 under the capsule has a height H1 which may be less than a few micrometers, for example between 0.2 and 5 μιη, in particular 2 μιη.

 In any event, the higher this height Hl, the better the thermal insulation between the upper wall of the capsule and the substrate.

 For a given arrangement of the first connection pads, there is a relation between the height H2 of a capsule, and its width a (see FIG. 1B), allowing the current to flow from one first connection pad 150 to the other , passing through the upper wall 110A of the capsule, and not only through its side walls 110B.

 An electrical resistance, and therefore a temperature, is thus measured at the level of the upper wall of the capsule, ie under the finger, where the thermal exchanges with the finger are the most important.

 This relation defines a threshold value for H2, the height H2 being preferentially lower than this threshold value. It also defines a threshold value for the height H1 under the capsule, which is equal to the height H2 minus the thickness E of the walls of the capsule.

 For example, for a capsule whose upper wall is a square of side a, and for first connection pads 150 defining a diagonal of this square (see Figure 1B), H2 preferably satisfies: (a * 2 + 2 * H2 ) <2 * a.

 Alternatively, for a capsule whose upper wall is a square of side a, and for first connection pads 150 defining a width of this square, passing through its center, H2 preferably satisfies: (a + 2 * H2) < 2 * a.

The impression sensor 100 can be realized using the following steps

depositing a sacrificial layer on the substrate 120; local etchings (for example by photolithography) of the sacrificial layer, up to the substrate, to form sacrificial pads;

 depositing thermoresistive material on and between the sacrificial pads to form the capsule matrix;

 local etching of the thermoresistive material, to form the orifices 111;

 evacuation of the sacrificial blocks, through the orifices 111;

 if necessary, deposition of the protective layer 130.

This manufacturing method provides excellent control of the shape of a capsule and the corresponding cavity, this cavity defining the thermal insulation between the substrate 120 and the top wall 110A of each capsule.

 It is thus possible to realize a cavity sensor having a controlled thermal insulation, and stable in space, between the capsules and the substrate.

 This manufacturing process is particularly simple, especially since it is directly the capsules that form the sensitive elements of the impression sensor.

 Here, the sacrificial layer is etched everywhere, except at the locations intended to form cavities 112.

 The thermoresistive material is deposited, at substantially constant thickness, over all of the lateral and upper surfaces of the sacrificial pads, and between two sacrificial pads.

 The thermoresistive material forming the capsules can be deposited by chemical vapor deposition (CVD), in particular when the impression sensor is made in CMOS technology, or by plasma-enhanced chemical vapor deposition (PECVD), in particularly when the impression sensor is made in TFT technology, or by physical vapor deposition (PVD).

 The sacrificial layer may be organic material. It may be a polymer, especially an organic polymer such as polyimide. The sacrificial pads can then be removed by oxygen plasma etching.

In particular, it is possible to realize a fingerprint sensor compatible with TFT technology, on a glass substrate, by producing the capsules using a layer sacrificial polymer (for example polyimide), then removed by oxygen plasma etching, and a PECVD deposit of thermo-resistive material.

A passivation layer, intrinsic to the substrate, protects it and serves as a barrier to oxygen plasma etching. This passivation layer, not shown in the figures, is for example SiN _x or SiO _x . In a variant, the substrate is protected by a specific layer, called the etch stop layer, not shown in FIG. 1A, and disposed on the substrate.

 In a variant, the sacrificial layer may be made of mineral material. It may be an oxide, for example silicon oxide. The sacrificial pads can then be removed by HF etching (hydrofluoric acid).

 In this case, the substrate must be protected by a stop layer of etching, not shown in Figure 1A, and disposed between the substrate and the capsules. It can be a SiC layer, or AIN, or Al2O3, etc. The thickness of this layer is between 20 and 200 nm, preferably 50 nm. This layer is open at the first connection pads.

 In particular, a fingerprint sensor compatible with CMOS technology can be produced by producing the capsules using a mineral sacrificial layer, subsequently removed by HF etching, and a CVD deposit of thermo-resistive material.

 EP 2 743 659 describes, in another context, an example of a method using a sacrificial layer subsequently removed by HF etching.

 FIG. 2 schematically illustrates a second embodiment of a cavity sensor 200 according to the invention, making it possible to improve the thermal insulation of the capsule vis-à-vis the substrate.

 The embodiment of FIG. 2 will only be described for its differences with respect to the embodiment of FIG. 1A. The reference numbers of FIG. 2 correspond to those of FIG. 1A, the first digit being replaced by a 2.

In this second embodiment, the thermal insulation between the capsules 210 and the substrate 220 is further improved, since the side walls of the capsule are thermally insulated from the substrate by a thermal insulation layer 270, which extends here under these side walls. Each capsule here rests directly on the thermal insulation layer 270, on an entire lower face of the side walls of said capsule. The thermal insulation layer 270 itself extends directly onto the substrate.

 In order to ensure electrical contact between the first connection pads 250 and the capsule 210, the latter also has two first legs 213, or vias, which each extend vertically from the top wall of the capsule to a first connection pad 250, through the thermal insulation layer 270. Each first foot 213 has a section (dimension in a plane orthogonal to (Oz)) reduced, for example between 0.5μιη and 2μιη, preferably 1 μιη.

 The first legs 213 can be positioned anywhere on the capsule, from its upper wall 210A to the substrate. The first legs 213 pass through the cavity 212, spaced from the side walls 210B of the capsule.

 The distance between the first two feet 213 of a capsule defines a range of electrical resistances measured between the two first connection pads 250 corresponding (associated with a temperature range of the capsule).

 This distance can be adjusted to adapt this range of electrical resistances to the parameters of a read circuit forming the means for reading an electrical resistance (see FIG. 1A).

 This distance can in particular be adjusted so as to minimize the noise on electric currents read by these means for reading an electrical resistance.

 It is therefore possible to adjust a range of electrical resistances measured by the reading means of an electrical resistance, without affecting the main structure of the capsule, in particular the geometry of its side walls and its upper wall.

 Here, the first legs 213 extend inside the cavity 212, at two corners associated with a first diagonal of the upper surface of the capsule.

 The sensor of FIG. 2 can be produced by a method similar to the method described above, and using the following steps:

deposition of the thermal insulation layer 270 on the substrate 120, this thermal insulation layer also having a stop function of the etching (to protect the lower layers during the evacuation of the sacrificial pads); depositing a sacrificial layer on the layer 270;

 local etches of the sacrificial layer to layer 270 to form sacrificial pads, and local etching of the sacrificial layer and layer 270 to form channels;

 depositing thermoresistive material on and between the sacrificial pads, and in the channels, to form the capsule matrix with the feet 213;

 local etching of the thermoresistive material to form the orifices; evacuation of the sacrificial blocks, passing through the orifices;

 if necessary, deposit of the protective layer.

For a mineral sacrificial layer (for example SiO ₂ ) with HF etching, the layer 270 may be AIN, ΓΑΙ2Ο3, SiC, amorphous carbon, DLC and optionally polyimide.

For a polyimide sacrificial layer with oxygen plasma etching, the layer 270 may be SiN, SiO 2 or SiON.

FIG. 3 schematically illustrates a variant of this second embodiment of a fingerprint sensor.

 The embodiment of FIG. 3 will only be described for its differences with respect to the embodiment of FIG. 2. The numerical references of FIG. 3 correspond to those of FIG. 2, the first digit being replaced by a 3.

 Here, the capsules are physically isolated from each other, without direct physical contact between them. They are thus thermally and electrically isolated from each other, which improves the contrast of an image of the impression obtained using the impression sensor 300 according to the invention.

Here, the thermo-resistive material of the capsules does not extend between the capsules, on the substrate side. On the other hand, the thermo-resistive material of the capsules also extends between the capsules, at the same height and the same thickness as their respective upper walls. Trenches 314 extend into this thermoresistive material, between the capsules, to isolate the capsules from each other. Preferably, the trenches 314 separating the capsules together form a grid consisting of a first series of parallel trenches intersecting with a second series of parallel trenches.

 According to this embodiment, the stop functions of the thermal etching and insulation are filled by two different layers. A etch stop layer 340 extends above the thermal insulation layer 370, which itself extends on the substrate 320. Here, the layer 370 is deposited directly on the substrate, without a layer. intermediate, and the layer 340 is deposited directly on the layer 370, without intermediate layer.

 The sensor of FIG. 3 can be produced by a method similar to the method described above, and using the following steps:

 depositing on the substrate 320 the thermal insulation layer 370; deposition on the layer 370 of the etching stop layer 340;

 depositing a sacrificial layer on the layer 340;

 local etchings of the sacrificial layer to the layer 340 to form sacrificial pads, and local etching of the sacrificial layer and layers 340 and 370 to form channels;

 depositing thermoresistive material in the interstices between the sacrificial pads, and above them, and in the channels, to form the matrix of capsules with the feet;

 local etching of the thermoresistive material, to form the orifices 311 in the capsules 310, and the trenches 314 between the capsules;

 evacuation of the sacrificial pads, through the orifices 111, respectively the trenches 314;

 - if necessary, deposit of the protective layer.

The sacrificial pads at locations for forming the separation spaces between the capsules may together form a grid, with a sacrificial pad associated with a capsule in each hole of the grid. This method is particularly advantageous since the separation of the capsules is carried out in the same technological step as the etching of the orifices in the capsules.

 In a variant, the impression sensor differs from the sensor of FIG. 1A or 2 only in that the portions of the thermo-resistive material between the capsules, on the substrate side, are opened by trenches separating adjacent capsules.

 In the following, various embodiments of a fingerprint sensor according to the invention, having means for heating the capsules, are described.

 As detailed above, in a passive detection mode, when the finger is placed on the sensor, each capsule heats more or less, or not at all, depending on whether it is surmounted by a ridge or by a valley of the impression. However, after a certain time, the temperature of the sensor can be homogenized, so that the temperature difference between a capsule associated with the peaks and a capsule associated with valleys of the footprint decreases. This results in a loss of contrast in the image of the print. The same problem arises if from the beginning, the temperature of the capsules and the temperature of the finger are equal.

 To avoid this homogenization, one can slide the finger on the surface of the impression sensor.

 We are interested here in another solution, consisting of heating the capsules.

 There is heat exchange between the finger and the capsules.

 It can be conductive heat exchange, when there is direct contact between the skin tissues and the contact surface, at the crests of the impression.

 Alternatively, it is a convective heat exchange at the valleys of the footprint.

 The heat exchange being more efficient by conduction than by convection, the variation of the temperature of each capsule varies, according to whether it is under a crest or under a valley of the imprint.

By measuring a variation of the electrical resistance of a capsule, during a predetermined time interval, it is possible to know whether it is surmounted by a peak or a valley of a fingerprint. This type of detection can be called "active detection mode". It uses measurements of electrical resistance variations, coupled with heating of the sensitive elements.

 The heating of the capsules makes it possible to break the thermodynamic equilibrium that can be established within the impression sensor, in order to preserve a contrasting image of the impression.

 This heating can also improve a contrast of the image of the impression, in a passive detection mode, when the initial temperature of the capsules is too close to the temperature of the finger, and / or to avoid saturation of the sensor, if this temperature difference is too high. In practice, one can then implement the heating for a second acquisition, if the first acquisition does not provide sufficient contrast.

 The heating of a capsule can be achieved by heating the capsule directly, thanks to its thermo-resistivity properties, or by heating an external element that will heat the capsule by thermal conduction.

 Figures 4A and 4B illustrate a third embodiment of the impression sensor according to the invention, for heating the capsules.

 The embodiment of FIG. 4A will only be described for its differences with respect to the embodiment of FIG. 3. The reference numerals of FIG. 4A correspond to those of FIG. 3, the first digit being replaced by a 4.

 In FIG. 4A, the thermal insulation layer 470 also has a stop function of etching, so that there is no stop layer of the specific etching.

 FIG. 4B is a view from above of a capsule 410 as represented in FIG.

4A.

FIG. 4B illustrates in particular the plane BB ', corresponding to the plane of the sectional view represented in FIG. 4A. The plane BB 'is orthogonal to a plane AA', passing through the first legs 413 as described with reference to FIG. 2. FIG. 4A thus differs from FIG. 3 in that it corresponds to the section plane BB '. and not at the cutting plane AA '. Each capsule 410 is heated by a metallic filament, shown in thick black line, and forming a heating element on the capsule (see Figures 4A and 4B). The metal filaments are for example Ti, TiN, Al, etc.

 As shown in FIG. 4B, each metal filament of a capsule is formed in one piece, and distinct from the metal filaments of the other capsules.

 Each metal filament is electrically connected to a power source

481.

 Again, it is a vertical electrical connection, through two second feet 483 extending vertically from the top wall of the capsule to the substrate, through the cavity 412, and two seconds connection pads 482, flush with the upper surface of the substrate 420.

 In each capsule 410, each of the two second legs 483 extends to a second connection pad 482, passing through the thermal insulation layer 470.

 In each capsule, an electrical insulation layer 485 extends directly between the upper surface of the capsule 483 and the metal filament 480, except above the second legs, in two regions 486 of the metal filament.

 A so-called heating polarization current is injected.

 The current flows successively in a second connection pad 482, then a second foot 483, then in the metal filament, then in the other second foot 483 and to the other second connection pad 482.

 The metal filament has an electrical resistance of the order of 100 Ω, for example between 50 Ω and 200 Ω.

 The intensity of the current is sufficient to heat the metal filament, for example to increase its temperature by several degrees Celsius.

 Each metal filament 480 thus heated will in turn heat the capsule by conduction through the electrical insulation layer 485. The metal filaments can increase the average temperature of the capsules by several degrees Celsius.

The electrical insulation layer 485 can close an orifice passing through the upper wall of the corresponding capsule. Alternatively, it is engraved at the same time as the capsule, the orifice passing through both the electrical insulation layer and the capsule. In a variant, it does not extend above an orifice of the capsule.

 The electrical insulation layer 485 may be an alloy comprising silicon, such as SiN or SiC, or an oxide such as SiO.

 Figure 4B illustrates more particularly the shape of a metal filament, above a capsule.

 The metallic filament 480 is in the form of an assembly of straight lines and curved lines. In particular, two circles extend concentrically in the center of the upper surface of the capsule. On the side of each region 486, a segment extends from said region to the small circle, crossing the large circle. This segment is aligned on an axis connecting the two regions 486, or in other words on a diagonal of the upper surface of the capsule.

 The regions 486 as described above here respectively extend at two corners of the upper surface of the capsule, on a diagonal thereof.

 The invention is not limited to this embodiment, and many other embodiments of metal lines can be implemented without departing from the scope of the invention. Figure 4C illustrates an example of another embodiment, in which the metal filament 480 'extends in a single coil.

 In these two exemplary embodiments, the metal filament does not extend in a straight line, to occupy a large surface area on the capsule and thus optimize heating of the capsule.

 FIGS. 5A and 5B illustrate a variant of the embodiment of FIGS. 4A and

4B.

 The embodiment of FIG. 5A will only be described for its differences with respect to the embodiment of FIG. 4A. The reference numbers of FIG. 5A correspond to those of FIG. 4A, the first digit being replaced by a 5.

According to this embodiment, there is no electrical insulation layer separating a capsule 510 and a metal filament 580. The metal filament 580 is in direct physical contact with the capsule 510 throughout its entire extent. Figure 5B corresponds to Figure 4B. It can be seen that the metal filament 580 extends in a straight line. This straight line extends orthogonal to a vertical plane 587, shown in phantom, connecting the first two feet 513 (shown in transparency in Figure 5B).

 The plane 587 corresponds to the cutting plane AA 'as described above.

 This arrangement allows the current lines from a first end region 586 of the metal filament to flow only to the other end 586, so that it is no longer necessary to electrically isolate the metal filament 580 and the capsule .

 The at least one orifice (not shown), passing through the upper surface of the matrix, is spaced from the metal line 580.

 FIGS. 6A and 6B illustrate a fourth embodiment of impression sensor 600 according to the invention, making it possible to heat the capsules.

 The embodiment of FIG. 6A corresponds substantially to the embodiment of FIG. 3, except in particular the location of the first legs 613. These first legs 613 are connected here by a vertical plane passing through the midpoints of two opposite sides of the upper wall.

 In addition, a single thermal insulation layer 670 also acts as a stop layer for etching.

 Figure 6B is a top view of the impression sensor. Figure 6B illustrates the plane CC of the section shown in Figure 6A.

 According to this embodiment, each capsule 610 is heated by thermo-resistive components 690, or resistors, forming by Joule effect heating dipoles between the capsules.

 The thermoresistive components 690 may consist of the same material as the capsules, or more preferably of a metal.

Each thermo-resistive component 690 extends between two adjacent capsules, preferably directly on the substrate 620, or even optionally on a thin layer of hooked, not shown, disposed directly between the substrate and said component. If necessary, the thermoresistive component 690 extends under the thermal insulation layer 670. Each thermo-resistive component 690 extends near a first foot 613 of a capsule, here between two first legs 613 of two neighboring capsules.

 Preferably, the thermoresistive components are not in direct physical contact with the feet 613, to avoid electrically shorting the capsules, especially when these components are metallic.

 Especially when the thermoresistive components are made of the same material as the capsules, they can be in direct physical contact with the capsules.

 Each thermoresistive component 690 is electrically connected to a current source 691, via two second bond pads 692, flush with the upper surface of the substrate 620.

 As described with reference to FIG. 5A, the current source 691 supplies a bias current, which here heats the thermo-resistive component by Joule effect.

 The heat travels by conduction from a thermoresistive component to the top wall of the capsule through a first leg 613 thereof.

 Alternatively, it is possible to realize a foot extending from the top wall of the capsule to the substrate, and specifically dedicated to the conduction of heat, from such a thermoresistive component to the upper wall of the capsule.

 Figure 6B illustrates the matrix of capsules 610, distributed in a square mesh, and spaced apart from each other.

 The width of the capsule matrix designates one of its dimensions in a plane parallel to the substrate. This is not necessarily the biggest dimension.

 The thermoresistive components extend in parallel lines, called heating lines, which each extend over the entire width of the capsule matrix. Each heating line may consist of several thermo-resistive components connected in series, one after the other, or preferably of a single thermoresistive component which extends from one end to the other of the matrix of capsules. Each row of capsules corresponds to a heating line.

In operation, not all heating lines are operated at the same time. This limits the maximum electrical power to be supplied to the fingerprint sensor.

 In addition, it is possible to successively integrate the electrical signals of the different lines of capsules, and to heat only the capsule line whose signals are integrated. Each line of capsules is then read while the signal is integrated on the next line.

 This allows to restrict an energy consumption of the impression sensor

600.

 In particular, in the case of a "rolling shutter" type reading, the electrical signals of the capsule lines of the capsule matrix are integrated one after the other, from bottom to top (or from top to bottom). In the same way, the heating lines are activated one after the other, from bottom to top (or from top to bottom), and synchronously with the integration of the signals of the capsule lines. Only the capsule line whose signal is integrated is heated by the adjacent heating line (s).

 Here, the same heating line heats the two lines of adjacent capsules. According to a variant not shown, the feet 613 are arranged on a capsule so that only one of them is located near a heating line, so that the activation of a heating line only heats a single line of capsules.

 FIG. 7A illustrates a capsule of a fifth embodiment of a fingerprint sensor according to the invention, in a view from above.

 This embodiment is adapted to an active detection mode.

 According to this embodiment, each capsule 710 is heated directly, here by a current source 701, by exploiting the thermo-resistivity properties of the material forming the capsules.

The sectional view of this embodiment, in the vertical sectional plane AA 'illustrated in FIG. 7A, corresponds for example to FIG. 3. The plane AA' passes through the first connection pads 750, and, if appropriate, by the first feet of the capsule. The first connection pads are dedicated to the connection to the electrical resistance reading means 760, as described above. The sectional view of this embodiment, in the section plane BB "illustrated in FIG. 7A, corresponds to FIG. 4A or 5A, without the metal filaments and any electrical insulation layers between these filaments and the capsules.

 Each capsule therefore has two second feet, not shown, each of which extends vertically to the substrate, up to two respective connection pads 701 for connection to a current source 781.

 The current source 701 provides a bias current, which heats the capsules by Joule effect.

 The spacing D1 between the two second vertical feet is adapted to the value of the bias current, and to an energy that is desired to dissipate in the capsule, for example between 0.1 and 1 mW.

 To dissipate 1 mW in each capsule, under 1 mA of bias current, a resistance of 1 kΩ is required, much lower than a resistance measured between two corners of the capsule, of the order of one Ω.

 Consequently, the spacing D1 between the two second feet is less than a spacing D2 between the first two connection pads 750, and where appropriate between the first two feet.

 In any case, the intensity of the current delivered by the current source 701 is sufficient to heat the capsule, for example to increase its average temperature by several degrees Celsius.

 The current source 701 forms heating means. It is connected to control means 704 for operating these heating means for a predetermined time interval.

FIG. 7B illustrates a current pulse supplied by the current source 701 (constant current lo between times t ₁ and t ₂ , and zero anywhere else).

This current pulse provides the capsule with constant power in the form of heat, between times t ₁ and t ₂ (see FIG. 7B).

When the capsule is covered by a valley of the impression, the heat is transmitted to the finger by convection. The efficiency of this heat transfer is lower, so that the temperature of the capsule greatly increases between the instants ti and t ₂ (temperature variation _ΔΤ ν, see Figure 7B).

When the capsule is covered by a crest of the impression, the heat is transmitted to the finger by conduction. The efficiency of this heat transfer is large, so that the temperature of the capsule increases slightly between times t ₁ and t ₂ (temperature variation ΔΤ ₀ <ΔΤ _ν ).

 These temperature variations correspond to variations in electrical resistance.

The means 760 for reading the electrical resistance of the capsule are therefore connected to comparison means 705, for measuring a variation of the electrical resistance of the capsule between two instants, in particular between the instant of starting of the heating of the capsule, and the instant of end of this heating, here between ti and t ₂ .

 The variation of electrical resistance can be converted to gray level to form an image of the fingerprint.

 According to a variant not shown, a single pair of connection pads connects the capsule to the current source 701 or the means 760 for reading the electrical resistance, depending on the state of a switch. The capsule is first heated, then its resistance is read. For example, a large current is injected to heat the capsule, and a specific current to read its resistance.

 According to another variant, a single pair of connection pads connects the capsule to the means 760 for reading the electrical resistance, also forming a current source for heating the capsule. A current is injected to heat the capsule (by applying a fixed voltage for example), then the value of the resistance is read by measuring the injected current.

In each of the embodiments of FIGS. 4A to 7B, several capsules can share the same current source, for example being simultaneously connected to the same current source, or alternatively depending on the state of a switch. For example, all the capsules of the same line of capsules of a matrix sensor are advantageously connected simultaneously to the same current source. All the embodiments and variants described with reference to FIGS. 4A to 7B implement heating of the capsules by means of a bias current and one or more current sources.

 According to variants not shown, the heating uses a bias voltage, thanks to one or more voltage sources.

 Each of the embodiments providing means for heating the capsules, see in particular FIGS. 4A to 6B, may comprise control means and comparison means as described in FIGS. 7A and 7B, for an active detection mode.

 In addition, the line-by-line reading of the resistances of the capsules, and the line-by-line heating of the capsules, can be implemented in each of these embodiments.

 The capsules can be divided into a single line of capsules, the finger (respectively the hand) being moved (e) above this line of capsules to detect the entire footprint.

 The different embodiments can be combined with each other, for example to provide, in the same impression sensor, different means for heating the capsules.

 In each of the embodiments, several capsules may share the same electrical resistance measuring means, being connected thereto alternately, depending on the state of a switch.

 In each of the embodiments, a thin layer of hooking may extend directly under the sidewalls of each capsule, ensuring that it does not short circuit the capsule. For this, one can for example choose an electrically non-conductive hung layer, or texturing the hooked layer so that it does not extend above the first connection pads.

In each of the embodiments, a thin layer of electrical contact recovery can be deposited on the first and / or second connection pads, to facilitate the resumption of electrical contact with the capsule, in particular at a first and / or a second foot of the capsule. This electrical contact recovery layer comprises, for example, titanium or tantalum. It is for example Ti, Ta, TiN, Ti / TiN, TaN, or Ta / TaN. It is advantageously deposited directly on the substrate, then etched everywhere except above the connection pads.

 Finally, a sixth embodiment of the invention is described, integrating the elements of an optical imager on the same substrate as that receiving the capsules.

 In particular, photodiodes and light sources extend in, or above, this substrate.

 Each photodiode and each light source can extend under a capsule, or between neighboring capsules.

 The light sources emit towards the contact surface of the sensor. The spatial distribution of the capsules, photodiodes and light sources corresponds to three interwoven matrices, so that optical and thermal measurements relate to the same region of a studied object.

 This combines a thermal sensor and an optical sensor that can each be associated with the same detection surface.

 It is thus possible to consolidate and / or complete and / or correct thermal measurements by optical measurements, and vice versa. For example, the effect of temperature homogenization on the contact surface can be compensated by optical measurements.

 Preferably, it is cleverly exploited orifices, or events, present in each capsule, and for discharging a sacrificial material used to build the capsules.

 Since the latter let the light pass, the photodiodes and / or the light sources are placed under capsules, that is to say between the substrate and the upper wall of a capsule.

 This combines a thermal sensor and an optical sensor with minimal space.

 The thermal and optical measurements are advantageously carried out simultaneously.

Preferably, the photodiodes each extend under a capsule, so that there is no spatial shift between thermal pixels formed by the capsules and optical pixels formed by photodiodes. If the orifices are covered by one or more layers such as a protective layer, or a thin layer of electrical insulation (see FIG. 4A), this layer is then transparent, over all or part of the spectral sensitivity spectrum of the photodiodes, and / or all or part of the emission spectrum of the light sources (transmission rate greater than 80%). DLC, for example, is transparent in the visible.

 The photodiodes may each consist of a diode PiN (for the English "Positive Intrinsic Negative Diode") formed above the substrate, or a photodiode formed directly in the substrate material (in particular in a semiconductor substrate receiving connection circuits made in CMOS technology, hereinafter referred to as "CMOS substrate").

 The same capsule may cover a single photodiode, or a light source alone, or both.

 When a photodiode and a light source are arranged under the same capsule, it is advantageous to provide means to avoid blinding the photodiode when the light source is turned on.

 For example, an opaque wall may extend inside the capsule, between the light source and the photodiode, to stop light radiation at the emission wavelength of the light source. This wall may be made of the same material as the capsule. This wall may extend over the entire height of the cavity defined by the capsule.

 Alternatively, the light source is surrounded by a metal wall, to block light rays propagating laterally.

 As a variant, it is the ignition sequence of the light sources and the integration of the photodiodes that makes it possible to integrate only photodiodes located at a distance from a light source that is on.

The light sources may be light emitting diodes (LEDs), in particular LEDs based on gallium nitride (GaN). Each LED is advantageously disposed between the substrate and the upper wall of a capsule. As a variant, the light sources may be organic light-emitting diodes (OLEDs), produced on the substrate, outside each capsule, between neighboring capsules.

 Each OLED advantageously has an elongate shape, parallel to capsule lines and lines of photodiodes. Each OLED can in particular extend over the entire width of an optical sensor formed by the photodiodes.

 The same sensor may have several types of light sources, each having emission spectra centered on different wavelengths. These several types of light sources may differ only in the nature of a higher wavelength conversion layer.

 The light sources may for example together form a matrix RGB, consisting of light sources emitting respectively in red, blue or green.

 When a photodiode extends under the upper wall of a capsule, this upper wall being traversed by at least one orifice, the position of this orifice, respectively of one of these orifices, is advantageously aligned with the position of the photodiode, along a vertical axis orthogonal to the substrate.

 The orifice may have a spatial filtering function, thanks to a suitable shape (this shape may be circular, square, or any other arbitrary shape).

 In the same way, the position of this orifice, respectively of one of these orifices, can be aligned with the position of a light source under a capsule, along a vertical axis orthogonal to the substrate. The orifice can thus have a collimation function.

 The upper wall of each capsule may have two orifices exactly, one being aligned with a photodiode under the capsule and the other with a light source under the capsule.

 Alternatively, the upper wall of a capsule may have more than two orifices, to increase the luminous flux passing between the inside and the outside of the capsule.

Alternatively, a capsule receiving both a photodiode and a light source may however have a single orifice. Each orifice may have a circular section. However, other forms can be implemented, for example a slot shape, square shape, or any other arbitrary form.

 When the light sources are LEDs each located under a capsule, they can furthermore form means for heating a capsule, for the implementation of an active detection mode as described above.

 It is easily understood that the characteristics relating to the introduction of light sources and photodiodes can be combined with each of the embodiments of a thermal sensor as described above.

 Figures 8A and 8B illustrate an example of this embodiment of the invention.

FIGS. 8A and 8B more particularly illustrate a pixel of a sensor according to this embodiment of the invention, each pixel comprising a capsule 810 covering a light source 8010 and a photodiode 8020.

 Figure 8A is a top view of this pixel. Figure 8A illustrates more particularly the upper wall of the capsule 810, pierced by two orifices 811, or vents.

 Figure 8B illustrates this pixel schematically, and in sectional view.

 As illustrated by FIGS. 8A and 8B, the center of one of the orifices 811 is aligned with the center of the light source 8010, and the center of the other of the orifices 811 is aligned with the center of the photodiode 8020, according to respective vertical axes parallel to (Oz).

 A wall 8030 extends inside the capsule 810, and delimits two distinct cavities, one of which receives the photodiode 8020 and the other receives a light source 8010.

 The wall 8030 makes it possible to prevent light rays emitted by the light source 8010 from being received directly by the photodiode 8020.

 The wall 8030 is constituted here of the same material as the capsule 810.

 In manufacturing, it is formed at the same time as the capsules.

 Each orifice 811 allows the evacuation of the material of the sacrificial layer on one side or the other of the wall 8030.

The light source 8010 is here a GaN-based LED. In operation, the light source 8010 emits light radiation towards the contact surface 831 of the sensor according to the invention. The light rays propagate to a footprint pressed against this contact surface. They are backscattered on this footprint and return to the photodiode 8020. The photodiode thus allows the acquisition of an image of the imprint.

 The sensor illustrated in FIGS. 8A and 8B corresponds otherwise to the embodiment of FIGS. 4A and 4B.

 If the light sources are LEDs, in particular GaN-based LEDs, these are formed on the substrate, before the photodiodes are made when the photodiodes are PiN diodes, if the photodiodes are formed directly in the substrate ( preferably a CMOS substrate).

 If the substrate intended to receive the capsules is made of sapphire, LEDs (preferably GaN based LEDs) can be produced directly on this substrate.

 If the substrate for receiving the capsules is a CMOS substrate, the LEDs (preferably GaN-based LEDs) can be formed first on an original substrate and then hybridized to the CMOS substrate.

 For example, LEDs (preferably GaN-based LEDs) can be made on a CMOS substrate, then PiN diodes can be made on the same substrate, and finally capsules can be formed (see Figure 9 and description below). Preferably, each capsule then covers a set of an LED and a photodiode.

 As a variant, PiN diodes are produced on a CMOS substrate (at the locations etched with an electrical insulator), and then capsules are formed on this substrate (after deposition of a stop layer of etching on the structure comprising the diodes PiN), and finally OLEDs are made between the capsules (after resumption of contact to the CMOS substrate). Here, only the photodiodes are each located under a capsule. For technological reasons, OLEDs are performed after the capsules.

As a variant, LEDs (preferably GaN based LEDs) are produced on a sapphire substrate, then PiN diodes are made on top of an electrical insulator covering the LEDs, and finally a barrier layer is deposited. engraving and form the capsules. This mode embodiment involves the production of TFT type driving transistors. Preferably, each capsule then covers a set of an LED and a photodiode.

 As a variant, PiN diodes are produced on a glass substrate (after making TFT control transistors), then the capsules are formed, and finally OLEDs are produced between the capsules. PiN diodes can also extend outside capsules.

 As a variant, the PiN diodes are produced on a glass substrate (after making TFT control transistors), then the capsules are formed, and LEDs (preferably GaN based LEDs) are transferred between the capsules. PiN diodes can also extend outside capsules.

 FIG. 9 illustrates in detail a pixel of the type of that of FIGS. 8A and 8B, made by GaN-based LED transfer on a CMOS substrate.

 The CMOS substrate 920 has a series of conductive tracks 921.

 In order not to overload FIG. 9, the source of current and the wall between a light source and a photodiode have not been represented.

 In a first manufacturing step, the GaN-based LEDs are produced on the CMOS substrate 920.

 For this, active stacks of LED 9010 are formed, each of which extends above a respective conductive track 921 of the CMOS substrate. Here, a respective metal pad 9011 extends between each active LED stack and the corresponding conductive track 921, in direct physical contact therewith. Then, a first layer 9041 of electrical insulation is deposited between the active stacks of LEDs, a metal deposit 9012 forming a cathode (or anode) is produced on each active stack of LEDs, and the whole is covered with a second layer 9042 of FIG. electrical insulation. The metal deposits 9012 are connected to the CMOS circuits through vias, not shown.

Next, a matrix of photodiodes, interleaved with the LED array, is formed on top of the same CMOS substrate. For this, the two layers 9041, 9042 of electrical insulation are etched locally. The deposition and etching of a PiN 9020 amorphous silicon structure is then performed at the etched locations. Each PiN structure is made of a metal deposit 9022 forming anode or cathode, and the whole is covered with a third layer 9043 of electrical insulation. The metal deposits are connected to CMOS circuits thanks to vias, not shown. Each PiN structure forms a photodiode.

 The third layer 9043 of electrical insulation is covered with a stop layer of etching, not shown, to protect the structure thus produced during the production of the capsules, and in particular during the evacuation of the material of the layer sacrificial implementation to build the capsules.

 Electrical insulators 9041, 9042, 9043 can also fulfill a thermal insulation function.

 The capsules 910 are then made as described above.

 The term "detection of the living" is used to determine whether or not a papillary impression consists of living human tissue.

 The inventors have noticed that the detection of living presents better results with optical measurements than with thermal measurements.

 The nesting of an optical sensor with the thermal sensor thus makes it possible to identify imitation of impression with greater reliability, and to improve the degree of security of a fingerprint test.

 Preferably, detection of the living is carried out by measurements of the response of a fingerprint to a predetermined illumination (DRS measurements, for "Diffuse Reflectance Spectroscopy").

 We work preferably on only part of the pixels of an image acquired by the photodiodes, corresponding to the valleys, respectively to the peaks of the footprint.

 Classically, DRS measurements are made using a central light point, and looking at the footprint response at various distances. In other words, an attenuation curve of the light intensity returned by the imprint is measured, preferably for different wavelengths.

 Living detection may involve the estimation of an absorption coefficient and a reduced diffusion coefficient.

The patent application FR 1563180, filed on December 23, 2015, describes such a method of recognizing the living, in which a luminous point is not used, but illuminating figures consisting of dark bands and light bands. The absorption and reduced diffusion coefficients are determined by minimizing a difference between a predictive model of the response of a fingerprint to a known illumination function (sine, step, slot, lines, etc.), and the response of the footprint actually measured in response to this illumination.

 The predictive model can consist of the convolution of the illumination function, with the impulse response of a medium whose values of said coefficients are known. The values of said coefficients associated with the predictive model are then sought which minimizes the difference between the predictive model and the measured response.

 The illumination function is preferably a function of a single dimension of space, the light sources being switched on to form light lines.

 A light line may consist of a series of LEDs based on GaN, each disposed under a capsule, advantageously with respect to a slot-shaped orifice. Alternatively, a light line may consist of one or more elongated OLEDs.

 The illumination function is advantageously a sinus function. The response of the imprint to this function can be obtained using three images acquired in response to three respective illumination figures, each consisting of light bands and dark bands distributed over the same period and in three different phases ( 0 °, 120 °, 240 °).

 In each of the embodiments described above, the first two feet, providing the electrical connection between the top wall of the capsule and the means for reading an electrical resistance, may be formed of metal, rather than made of the same material as capsules. It is the same for the possible two second feet.

 Although a papillary impression sensor has been described in each of the examples, the object to be imaged is not limited to a papillary impression, as indicated in the introduction. The object to be imaged may especially consist of an object that does not have the same temperature everywhere, or whose contours are simply to be identified on the contact surface.

Claims

A sensor (100; 200; 300; 400; 500; 600) of thermal patterns of an object, in particular a papillary impression, comprising a contact surface (131; 831) for applying the object thereto, and configured to allowing heat exchange between this object and sensitive elements located above a substrate (120; 220; 420; 620; 820; 920) under said contact surface (131; 831), characterized in that it comprises a series of three-dimensional structures (110; 210; 310; 410; 510; 610; 710; 810; 910), said capsules, formed of a thermoresistive material made of amorphous silicon or an alloy comprising amorphous silicon , and forming said sensitive elements, each capsule defining the lateral and upper boundaries of a cavity filled with a gas or sealed under vacuum, and being connected to means for reading (160, 760) its electrical resistance, by first two connection pads (150; 750) located in or on the ubstrat.

The sensor (400; 500; 600) according to claim 1, characterized in that it has means (701; 690; 691; 480; 481; 480 '; 481; 580; 8010; a capsule.

3. Sensor according to claim 2, characterized in that at least one capsule (710) is connected to a current source (701) or voltage forming means for heating a capsule, for the injection of a polishing current or voltage adapted to directly heat said capsule.

4. Sensor (600) according to claim 2 or 3, characterized in that the heating means of a capsule comprise thermo-resistive components (690) disposed between two capsules (610) adjacent, and connected to a current source (691) or voltage, for the injection of a bias current or voltage suitable for heating said thermoresistive components.

5. Sensor (600) according to claim 4, characterized in that the capsules (610) are distributed in rows to form a matrix of capsules, and in that the thermoresistive components (690) extend along parallel lines. to the lines of capsules.

6. Sensor (400; 500) according to any one of claims 2 to 5, characterized in that the heating means of a capsule comprise at least one metal filament disposed above a capsule (410; ), and connected to a current source (481) or voltage, for the injection of a bias current or voltage suitable for heating the filament.

7. The sensor (400) according to claim 6, characterized in that an electrical insulation layer (485) extends directly between the capsule and the metal filament (480; 480 '), except above two seconds. connection pads (482) located in or on the substrate, and connected to the current or voltage source (481).

Sensor (500) according to claim 6, characterized in that the metal filament (580) extends in a straight line, in direct physical contact with the capsule (510), the straight line being orthogonal to a vertical plane (587). ) connecting the first two connection pads (513).

9. Sensor according to any one of claims 2 to 8, characterized in that it comprises control means (704), arranged to actuate the heating means (701) of a capsule for a predetermined period of time, and in that the reading means (760) are connected to comparison means (705) for determining a change in the electrical resistance of the capsule (710) between two predetermined times.

The sensor (200; 300; 400; 500; 600) according to any one of claims 1 to 9, characterized in that the capsules (210; 310; 410; 510; 610; 710; 810; 910) are extend over a thermal insulation layer (270; 470; 670; 9041; 9042; 9043) covering the substrate, each capsule having two first legs (213; 613) which pass through the thermal insulation layer, to come into contact with the first two connection pads (750).

11. Sensor according to any one of claims 1 to 10, characterized in that it further comprises a plurality of photodiodes (8020; 9020) and light sources.

(8010; 9010) distributed under and / or between the capsules (810; 910).

12. A sensor according to claim 11, characterized in that each capsule (810; 910) has a bonnet-like shape, of which an upper wall is traversed by at least one orifice (811), and in that each photodiode (8020; 9020 and / or each light source (8010; 9010) extends under the top wall of a capsule.

13. Sensor according to claim 11 or 12, characterized in that it has several types of light sources (8010; 9010), which differ by a central transmission wavelength.

A method of using a sensor (400; 500; 600) according to any one of claims 2 to 13, characterized in that the capsules (410; 510; 610; 710; 810; 910) together form a capsule matrix in which the capsules are distributed in parallel lines which each extend over the entire width of the capsule matrix, in that the reading of the electrical resistance of the capsules is carried out line of capsules per line of capsules, and in that the heating of the capsules is also carried out line of capsules per line of capsules and synchronously with the reading of the electrical resistances.

15. Use of a sensor according to any one of claims 11 to 13 for determining whether or not a papillary impression consists of human living tissue.