WO2017093252A1

WO2017093252A1 - Thermal pattern sensor comprising an upper pyroelectric portion with high thermal conductivity

Info

Publication number: WO2017093252A1
Application number: PCT/EP2016/079137
Authority: WO
Inventors: Abdelkader Aliane
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives; Safran
Priority date: 2015-11-30
Filing date: 2016-11-29
Publication date: 2017-06-08
Also published as: FR3044409B1; FR3044409A1

Abstract

The invention relates to a thermal pattern sensor (100) comprising a plurality of pixels (102) each including a pyroelectric capacitor made up of a stack including: a lower electrode (106); a lower pyroelectric portion (110) arranged on the lower electrode and including PVDF and/or a PVDF copolymer; an upper pyroelectric portion (112) arranged on the lower pyroelectric portion and comprising a compound made from PVDF and/or a PVDF copolymer, and from nanoparticles and/or microparticles of a pyroelectric material with a perovskite and/or ZnO and/or PVDF and/or a PVDF copolymer crystalline structure; a heating element (114) including a portion of electrically conductive material arranged between the pyroelectric portions such that one or more portions of the lower pyroelectric portion are directly in contact against the upper pyroelectric portion; and an upper electrode (116) arranged on the upper pyroelectric portion.

Description

THERMAL PATTERN SENSOR COMPRISING A HIGH PYROELECTRIC PORTION WITH HIGH THERMAL CONDUCTIVITY

DESCRIPTION TECHNICAL FIELD AND PRIOR ART

The invention relates to a thermal pattern sensor, or temporal temperature variation transducer in a difference of electrical potentials, comprising pyroelectric capacitors, advantageously used to perform a fingerprint capture. The invention applies to the field of active type sensors, that is to say comprising at least one heating element intended to heat the pixels.

It is known to provide a fingerprint sensor comprising thermal detection means. These thermal detection means may correspond to pyroelectric elements, diodes, thermistors or any other temperature-sensitive element that makes it possible to convert a temperature variation into a variation of potential or electrical current.

Fingerprint detection can be performed by so-called "passive" sensors exploiting a difference between the temperature of the finger and that of the sensor, as described in the documents US Pat. No. 4,394,773, US Pat. No. 4,429,413 and US Pat. No. 6,289,114. However, sensors have the disadvantage of making a measurement which depends solely on the difference between the temperature of the finger and that of the sensor. It may therefore happen that the level of the signal obtained is zero when the finger and the sensor are at the same temperature, or that the contrast of the captured images varies, which then poses problems during the subsequent processing of the images obtained (for example , a reversal of the temperatures causes a reversal of the obtained image).

To eliminate the problems raised by passive heat sensors, and especially in the case of a static acquisition where the finger does not move, so-called "active" fingerprint sensors have been proposed, such as those described in the US documents 6 091 837 and EP 2 385 486 A1. In such a sensor, each pixel comprises a pyroelectric capacitor formed of two electrodes superimposed one above the other and between which a portion of pyroelectric material is disposed, and a heating element. This heating element dissipates a certain amount of heat in the pixel, and heating of the pixel is measured after a certain acquisition time in the presence of the finger on the sensor. This makes it possible to distinguish, at the level of each pixel, the presence of a peak or a valley of the detected imprint according to whether the heat is absorbed by the skin (pixel in the presence of a crest of the impression) or preserved in the pixel (pixel in the presence of a valley of the footprint). This leads to a lower final temperature in the case of a pixel in the presence of a peak of the footprint, where the heat is absorbed by the skin, compared to a pixel in the presence of a valley of the footprint .

In the literature, pyroelectric capacitors are manufactured vertically with a heating element made around, or above and beside, electrodes of the capacitance. Such an arrangement of the heating element is not effective because the pyroelectric material, for example PVDF (polyvinylidene fluoride) or P (VDF-TrFE) (poly (vinylidene fluoride-trifluoroethylene)), is not a good thermal conductor, the transfer of heat inside the pyroelectric capacitor being very weak. The temperature difference between the peaks and the valleys in the layer of pyroelectric material, representative of the sensitivity of the sensor, is therefore also very small. Due to this small difference in temperature, it is necessary to inject a large power into the heating element, generating a significant current consumption by the sensor. Finally, when the heating element is made from the same conductive layer as that used for producing one of the electrodes, there is a problem of space between this electrode and the heating element, and a compromise must be made between the space occupied by the heating element and that occupied by the electrode.

STATEMENT OF THE INVENTION

An object of the present invention is to provide a thermal pattern sensor whose sensitivity is improved, consuming less current to heat its pixels and to overcome the understanding between the surface occupied by the electrodes and that occupied by the heating element.

For this, the invention proposes a thermal pattern sensor comprising a plurality of pixels, each pixel comprising at least one pyroelectric capacitor formed by, or comprising, at least one stack comprising at least:

a lower electrode;

a lower pyroelectric portion disposed on the lower electrode and comprising a material A corresponding to polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

an upper pyroelectric portion disposed on the lower pyroelectric portion and comprising a compound formed of or comprising a material B corresponding to polyvinylidene fluoride and / or at least one copolymer of polyvinylidene fluoride and nanoparticles and / or microparticles; a material C corresponding to a pyroelectric material of crystalline structure of the perovskite type and / or ZnO and / or polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

a heating element comprising at least one portion of electrically conductive material disposed between the lower and upper pyroelectric portions and such that one or more portions of the lower pyroelectric portion are in direct contact with one or more portions of the upper pyroelectric portion;

an upper electrode disposed on the upper pyroelectric portion.

It is proposed to achieve an improved performance pyroelectric capacitor by using two distinct portions of pyroelectric material of different compositions separated from each other by the heating element. The upper pyroelectric portion material is a compound formed of, or comprising, PVDF and / or at least one PVDF copolymer, referred to as a semi-crystalline matrix, or semicrystalline material B, in which nanoparticles are dispensed and /or some microparticles, or nanocrystals and / or microcrystals, of a material C corresponding to:

a pyroelectric material of crystalline structure of perovskite type and / or

- ZnO and / or

- PVDF and / or

at least one copolymer of PVDF.

These nanoparticles and / or microparticles added in the material B of the upper pyroelectric portion make it possible to improve the thermal conductivity of the upper pyroelectric portion. Thus, only the portion of pyroelectric material lying on the thermal pattern side to be detected has an improved thermal conductivity, which makes it possible to obtain a better heat exchange and a better sensitivity when in contact with the thermal pattern to be detected thanks to the use of this compound. This better sensitivity is also due to the fact that the pyroelectric material of the upper portion has a better thermal conductivity than that of the pyroelectric material of the lower portion, thus improving the temperature difference obtained between the upper pyroelectric portion and the lower pyroelectric portion when a measurement of a thermal pattern, and to maintain good values for the pyroelectric capacitances formed.

In addition, the arrangement of the heating element between the two portions of pyroelectric materials also contributes to this improvement of the sensitivity of the sensor. For example, when the sensor corresponds to a fingerprint sensor, the finger present on the sensor absorbs the heat energy provided by the heating element thanks to the good thermal conductivity of the upper pyroelectric portions of the pixels, and a variation of significant temperature will be present in the lower pyroelectric portions of the pixels. Compared to the sensors of the prior art, this increase in the temperature difference makes it possible to reduce the electrical power to be injected into the heating element.

In addition, the pyroelectric power of the upper pyroelectric portion is not diminished by the presence of the material C because this material has also pyroelectric properties and allows in particular to increase the pyroelectric coefficient of the upper pyroelectric portion.

Finally, the arrangement of the heating element within the pyroelectric material, between the lower and upper pyroelectric portions, makes it possible to solve the problems of space between the heating element and one of the electrodes.

Said at least one polyvinylidene fluoride copolymer of at least one of the lower and upper pyroelectric portions may correspond to poly (vinylidene-trifluoroethylene fluoride) (or P (VDF-TrFE)) and / or poly (fluoride vinylidene-trifluoroethylene-chlorofluoroethylene) (or P (VDF-TrFE-CFE)) and / or poly (vinylidene-trifluoroethylene-chlorotrifluoroethylene fluoride) (or P (VDF-TrFE-CTFE)).

The sensor can be such that:

the pyroelectric material of crystalline structure of the perovskite type comprises BaTiO 3 and / or PbZrTiO 3 and / or BaSrTiO 3 and / or CaSrTiO 3 and / or

BaTiZrO3 and / or SrTiO3, and / or

a proportion by weight of the material C in the compound of the upper pyroelectric portion is between about 5% and 20%, and / or

the nanoparticles and / or the microparticles of the material C are distributed randomly in the compound of the upper pyroelectric portion, and / or

materials A and B are similar.

The portion of electrically conductive material of the heating element may comprise several conductive segments joined one after the other forming a continuous conductive portion and between which the lower pyroelectric portion is directly in contact with the upper pyroelectric portion, and / or the portion of electrically conductive material of the heating element may be traversed by at least one opening at which the lower pyroelectric portion is directly in contact with the upper pyroelectric portion. When the portion of electrically conductive material of the element The heating element comprises a plurality of conductive segments, these segments may be arranged such that they form a coil pattern. Alternatively, the portion of electrically conductive material of the heating element may be traversed by a plurality of regularly distributed openings in the portion of electrically conductive material.

The lower electrode may comprise a titanium layer with a thickness of between approximately 50 nm and 500 nm and a TiN layer with a thickness of between approximately 10 nm and 500 nm, and / or the upper electrode may comprise a layer of titanium with a thickness between about 30 nm and 100 nm and a layer of AISi or AICu of thickness between about 100 nm and 700 nm. In such a configuration, the AISi or AICu layer passivates the titanium layer, thus protecting the titanium against oxidation. In addition, the AISi or AICu layer does not react with the pyroelectric material if it is deposited directly in contact with this pyroelectric material.

The thickness of the lower pyroelectric portion and / or the thickness of the upper pyroelectric portion may be between about 1 μιη and 5 μιη.

The pixels may be disposed on a front face of a substrate so that the lower pyroelectric portion of each pixel is disposed between the substrate and the upper pyroelectric portion of said pixel.

The pixels may be arranged forming a matrix of plural lines and columns, and the pixel heating elements of each of the pixel lines may be formed by a continuous portion of electrically conductive material. Such a configuration is advantageous when the pixels are read line by line because only one heating command can then be applied for all the pixels of the same line to be read.

One of the lower and upper electrodes of each pixel may be electrically connected to a fixed electrical potential, for example to the ground of the sensor. The other of the lower and upper electrodes of each pixel can in this case serve as read electrode of the pixel, that is to say form the electrode on which the electrical charges generated in the pyroelectric capacitor are intended to be read . The sensor may further comprise a protective layer covering said stack of each pixel and having openings forming access to electrical contacts of the lower and upper electrodes of each pixel.

The sensor may be a fingerprint sensor. In a variant, the sensor may be able to carry out a piezoelectric detection or a temperature detection.

The invention also relates to a method for producing a thermal pattern sensor comprising a plurality of pixels, each pixel comprising at least one pyroelectric capacitor formed by, or comprising, at least one stack obtained at least the implementation of the following steps:

- realization of a lower electrode;

- Making a lower pyroelectric portion disposed on the lower electrode and comprising a material A corresponding to polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

- Realization of a heating element comprising at least a portion of electrically conductive material disposed on the lower pyroelectric portion;

- producing an upper pyroelectric portion on the portion of electrically conductive material of the heating element which is such that one or more portions of the lower pyroelectric portion are directly in contact with one or more parts of the upper pyroelectric portion, the upper pyroelectric portion comprising a compound formed of or comprising a material B corresponding to polyvinylidene fluoride and / or at least one copolymer of polyvinylidene fluoride, and nanoparticles and / or microparticles of a material C corresponding to a pyroelectric material of crystalline structure of perovskite type and / or ZnO and / or polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

- Realization of an upper electrode disposed on the upper pyroelectric portion. 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:

- Figures 1 and 2 respectively show a top view and a sectional view of a pixel of a thermal pattern sensor object of the present invention, according to a particular embodiment;

FIGS. 3 to 8 show simulations of temperature difference variations between the lower and upper pyroelectric portions in thermal pattern sensor pixels;

FIGS. 9A to 91 represent steps of a method for producing a thermal pattern sensor, object of the present invention, according to a particular embodiment;

- Figure 10 shows an alternative embodiment of a heating element of a thermal pattern sensor object of the present invention.

Identical, similar or equivalent parts of the different figures described below bear the same numerical references so as to facilitate the passage from one figure to another.

The different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

The different possibilities (variants and embodiments) must be understood as not being exclusive of each other and can be combined with one another.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS Referring firstly to FIG. 1, which represents a view from above of a pixel 102 of a thermal pattern sensor 100 according to a particular embodiment, and FIG. represents a sectional view of this same pixel 102, showing in particular a pyroelectric capacitance of the pixel 102 which forms the element of thermal detection of the pixel 102. In this particular embodiment, the sensor 100 corresponds to a fingerprint sensor. In FIG. 1, the different superimposed elements of the pixel 102 are represented as being transparent in order to make them visible.

The sensor 100 comprises a substrate 104 corresponding for example to a glass substrate or a semiconductor substrate such as silicon. A substrate 104 of glass can be advantageously used when the sensor 100 comprises a read circuit made from thin film transistor (TFT) transistors, whereas a semiconductor substrate 104 can be used when the reading circuit of the sensor 100 comprises transistors made in MOS technology. The substrate 104 may also be a flexible substrate, comprising for example a plastic material such as polyimide and / or PEN (polyethylene naphthalate) and / or PET (polyethylene terephthalate), on which electronic components of the sensor 100 are made by printed electronic technology (for example via an embodiment with ink jet type writing heads, or by lithography on plastic, or in TFT technology on flexible plastic substrate or metal).

Although only one pixel 102 is shown in FIGS. 1 and 2, the sensor 100 has several pixels 102 arranged forming a matrix of several lines and several columns of pixels 102.

Each of the pixels 102 of the sensor 100 comprises measurement means, or detection, thermal forming a pyroelectric capacitance. Each pyroelectric capacitance comprises:

at least one lower electrode 106 electrically conductive and disposed on a front face 108 of the substrate 104;

at least one lower pyroelectric portion 110 comprising pyroelectric material and disposed at least on the lower electrode 106 (in FIG. 2), a portion of the pyroelectric material forming the lower pyroelectric portion 110 also extends on the front face 108 of the substrate 104, next to the lower electrode 106); at least one upper pyroelectric portion 112 comprising pyroelectric material and disposed on the lower pyroelectric portion 110;

a heating element 114 formed by at least one portion of electrically conductive material disposed between the lower and upper pyroelectric portions 110, 112;

at least one upper electrode 116 electrically conductive and disposed on the upper pyroelectric portion 112.

The pyroelectric material of the lower pyroelectric portion 110 is called material A and corresponds to PVDF and / or at least one copolymer of PVDF such as P (VDF-TrFE) and / or P (VDF-TrFE-CFE) and / or P (VDF-TrFE-CTFE).

The pyroelectric material of the upper pyroelectric portion 112 is a compound comprising on the one hand a material B corresponding to PVDF and / or at least one copolymer of PVDF such as P (VDF-TrFE) and / or P ( VDF-TrFE-CFE) and / or P (VDF-TrFE-CTFE), and secondly nanoparticles and / or microparticles of a material C corresponding to:

a pyroelectric material of crystalline structure of perovskite type and / or

- ZnO and / or

- PVDF and / or

at least one copolymer of PVDF such as P (VDF-TrFE) and / or

P (VDF-TrFE-CFE) and / or P (VDF-TrFE-CTFE).

In the particular embodiment described herein, the pyroelectric material of the upper pyroelectric portion 112 corresponds to a compound formed of P (VDF-TrFE) (material B) and nanoparticles of BaTiO 3 (material C). Advantageously, the materials A and B are similar, which facilitates the production of the pyroelectric portions 110, 112 of the sensor 100. The percentage by weight, or charge rate, of the material C in the compound of the upper pyroelectric portion 112 is for example between about 5% and 50%, and preferably between about 5% and 20% or between about 10% and 20%. Other types of pyroelectric materials of crystalline structure of the perovskite type can replace the BaTiO3 such as titanium lead zirconate (PZT) (PbZrTiOs), strontium titanate and barium (BaSrTiOs), strontium titanate and calcium (CaSrTiOs), titano-zirconate barium (BaZrTiOs), titanate strontium (SrTiOs). The nanoparticles and / or microparticles of the material C are advantageously dispersed randomly in the material B.

The thickness of each of the pyroelectric portions 110, 112 is for example between about 1 μιη and 5 μιη.

The electrodes 106, 116 each comprise at least one electrically conductive material, for example a metallic material such as Ti and / or Pt and / or Ni and / or Au and / or Al and / or Ag and / or AISi and / or AICu. In an advantageous configuration, the lower electrode 106 is formed of a Ti / TiN stack, with a titanium thickness between about 50 nm and 500 nm and a TiN thickness between about 10 nm and 500 nm, and upper electrode 116 is formed of a Ti / AISi or Ti / AICu type stack with a titanium thickness between about 30 nm and 100 nm and a thickness of AISi or AICu between about 100 nm and 700 nm . In general, the thickness of each of the electrodes 106, 116 may be between about 10 nm and 800 nm.

Each of the electrodes 106, 116 may be electrically contacted via electrical contacts, one referenced 107 corresponding to that of the lower electrode 106 and the other referenced 109 corresponding to that of the upper electrode 116, formed from the same or the same conductive layers having served for the realization of the electrodes 106, 116. One of the electrodes 106, 116, for example the lower electrode 106, is electrically connected to the mass of the sensor 100 and the other electrode, by For example, the upper electrode 116 serves as a reading electrode of the pixel 102, that is to say an electrode on which the electrical charges generated by the pyroelectric capacitance of the pixel are recovered via the electrical contact 109.

The heating element 114 is made of the pyroelectric material of the pixel 102, between the pyroelectric portions 110 and 112. In the example of FIGS. 1 and 2, the heating element 114 is formed by a portion of at least one electrically conductor, for example a metallic material such as Ti and / or Pt and / or Ni and / or Au and / or Al and / or Ag and / or AISi and / or AICu., in the form of a coil , that is to say formed of several first conductive segments arranged next to each other and parallel to each other, and electrically connected in series at their ends by second conductive segments arranged substantially perpendicular to the first segments. The width WL of the conductive segments forming the heating element 114 is for example between about 1 μιη and 20 μιη, the length L of the first conductive segments of the heating element 114 is for example between about 50 μιη and 500 μιη, and the width W of the serpentine pattern formed by these conductive segments is for example between about 25 μιη and 500 μιη.

The portion of electrically conductive material of the heating element 114 is such that portions of the lower pyroelectric portion 110 are directly in contact with portions of the upper pyroelectric portion 112. In the embodiment described with reference to FIGS. 2, this contact is obtained between the conductive segments forming the coil. Thus, at these portions of the pyroelectric portions 110, 112 in direct contact with each other, the lower and upper electrodes 106, 116 are directly opposite one another, without a conductive portion of the The heating element 114 is interposed between the electrodes 106, 116, which makes it possible to have the pyroelectric capacitance forming between the electrodes 106, 116 without the heating element 114 being seen as an electrode.

The heating element 114 is able to heat the materials of the pyroelectric portions 110 and 112 by Joule effect thanks to a current flowing from a first end 113 to a second end 115 of the heating element 114. During the operation of the sensor 100, a heating signal (voltage or constant current) is applied to the first end 113 of the heating element 114, and a current therefore flows in the heating element 114, which causes its heating as well as that of the pyroelectric material portions 110, 112.

The fact that one of the two ends 113, 115 of the heating element 114 is connected to the ground also gives the sensor 100 a protection vis-à-vis the possible electrostatic discharges, the currents related to its discharges can flow in this case to the ground via the heating element 114. This protection is obtained during the reading of the sensor 100 during which the two ends 113, 115 of the heating element 114 are connected to ground.

The arrangement of the heating element 114 within the entire pyroelectric material formed by the pyroelectric portions 110 and 112 makes it possible to improve the heating, in terms of heat distribution and heating intensity, of the pyroelectric material. relative to a heating element that would be disposed on or beside the pyroelectric material. In addition, with such a configuration, no problem of space or arrangement between this heating element 114 and the electrodes 106, 116 arises since the heating element 114 is not made from the same level of electrically conductive material than those used for producing the electrodes 106, 116.

The presence of the heating element 114 between the pyroelectric portions 110, 112 is not a problem for the reading of the pixels 102. Indeed, during the reading, the heating element 114 can be connected to the ground, the value of the pyroelectric capacitance being read by direct current. Thus, the heating element 114 does not disturb this reading.

In this configuration, the heating element 114 is in direct contact with the entire pyroelectric material of the pyroelectric capacitors. The heating of the pixels 102 carried out by this heating element 114 is therefore optimal and maximized because of the arrangement of the heating element 114 within the pyroelectric material of the pixel 102. The heat generated by the heating element 114 is thus diffused vertically and sent directly into the pyroelectric material pyroelectric portions 110, 112.

The upper electrode 116 is covered, at the portion of the pixel 102 on which a finger is intended to come to rest, by a protective layer 118, an upper face 120 corresponds to the surface on which is the thermal pattern intended to be detected by the sensor 100, for example a finger whose fingerprint is intended to be detected. This protective layer 118 may comprise preferentially ZnO and / or ΓΑΙΝ and / or IGZO ("Indium Galium Zinc Oxide") and / or ΓΙΖΟ ("I ndium Zinc Oxide") and / or ΓΑΤΟ ("Antimony Tin Oxide") and / or ΓΑΙ2Ο3 and / or SiN weakly constrained at low temperature. Other materials are also conceivable to form this protective layer 118, as for example polyimide, PVDF and / or one of its copolymers, PM MA, etc. The material or materials used as well as the thickness of the protective layer 118 are chosen to obtain good heat transfer from its front face 120 to the pyroelectric capacitance. Thus, the protective layer 118 is made such that it is neither too thermally resistive (because the heat would not cross it), nor too thermally conductive (because the heat would leave in this case on the sides, towards the other pixels, causing diathermy within the sensor), neither too thick (to have a heat transfer between the front face 120 and the pyroelectric capacitor), nor too thin (the thickness of the layer 118 is still sufficient for its protective role to be fulfilled). This layer 118 may for example comprise a PMMA-based resin having a thickness of approximately 2 μιη. According to other examples, the layer 118 may comprise ZnO of thickness equal to about 2 μιη, or IGZO of thickness equal to about 1 μιη, or alternatively PVDF of thickness equal to about 1 μιη.

FIGS. 3 and 4 represent results of simulations carried out for a pixel 102 as previously described in connection with FIGS. 1 and 2, in which the lower pyroelectric portion 110 comprises P (VDF-TrFE) and the upper pyroelectric portion 112 comprises P (VDF-TrFE) in which nanoparticles, or na-crystals, of BaTiO 3 are randomly dispensed with a loading rate of about 15%.

In FIG. 3, the abscissa represents the time during which the heating element 114 carries out the heating of the pixel 102, with a constant power of about

5 mW per pixel 102. The curve 10 of FIG. 3 represents a variation of a temperature difference between the pyroelectric portions 110, 112 (the temperatures are considered in the middle of each of the pyroelectric portions 110, 112) obtained for a pixel 102. as previously described when this pixel 102 is in contact with air, corresponding for example to the case of a pixel 102 which would be in contact with a valley of a fingerprint. Curve 12 of FIG. 3 represents the variation of the temperature difference between the pyroelectric portions 110, 112 (temperatures considered in the middle of each of the pyroelectric portions 110, 112) obtained when this pixel 102 is in contact with the skin, corresponding to for example in the case of a pixel 102 which would be in contact with a peak of a fingerprint. Curve 18 of FIG. 4 represents the difference between the values of curve 12 and those of curve 10, thus representing the temperature difference obtained between a pixel in contact with air and a pixel in contact with the skin. for the sensor 100.

By way of comparison, the curve 14 of FIG. 3 represents the variation of the temperature difference between the lower and upper pyroelectric portions 110, 112 (temperatures considered in the middle of each of the pyroelectric portions 110, 112) obtained for a pixel 102 of structure similar to that for which the curves 10 and 12 have been obtained but whose upper pyroelectric portion is composed of P (VDF-TrFE) in which nanoparticles, or nano-crystals, of P (VDF-TrFE) are distributed in a manner random, when this pixel 102 is in contact with air. The curve 16 of FIG. 3 represents the variation of the temperature difference obtained between such lower and upper pyroelectric portions 110, 112 (temperatures considered in the middle of each of the pyroelectric portions 110, 112) when this pixel is in contact with the skin. Curve 20 of FIG. 4 represents the difference between the values of curve 16 and those of curve 14, thus representing the temperature difference obtained between such a pixel 102 in contact with air and such a pixel 102 in contact with skin for this superior pyroelectric portion sensor composed of P (VDF-TrFE) and nanoparticles of P (VDF-TrFE).

The fact of using the compound P (VDF-TrFE) / BaTiO3 nanoparticles in the upper pyroelectric portion 112 increases the detection sensitivity in the pyroelectric capacitance with a ΔΤ equal to 0.93 K between two pixels, one of which is in contact with air and the other is in contact with the skin, and a ΔΤ equal to 0.72 K when the upper pyroelectric portion comprises a compound of P (VDF-TrFE) / nanoparticles of P (VDF-TrFE ), during a measurement carried out at a duration of approximately 200 μ ((values derived from curves 18 and 20). This improvement is due to the good heat transfer between the heating element 114 and the valleys / ridges of the finger via the upper pyroelectric portion 112 composed of materials B and C and disposed above the heating element 114. This good thermal conductivity is ensured by the presence of the material C, advantageously distributed randomly, in the matrix formed by the material B in the upper pyroelectric portion 112.

By way of comparison, similar measurements are made for pixels of similar structure but in which the lower and upper pyroelectric portions each comprise a compound P (VDF-TrFE) / nanoparticles of BaTiO 3. In FIG. 5, curve 22 represents the temperature variation obtained in the lower pyroelectric portion for such a pixel in contact with air, and curve 24 represents the temperature variation obtained in the upper pyroelectric portion for this same pixel. . Curve 26 represents the temperature variation obtained in the lower pyroelectric portion for a pixel in contact with the skin, and curve 28 represents the temperature variation obtained in the upper pyroelectric portion for this same pixel. In FIG. 6, curve 30 represents the difference between the values of curve 24 and those of curve 22, thus representing the difference in temperature obtained between the lower and upper pyroelectric portions for a pixel in contact with air for this sensor with lower and upper pyroelectric portions each comprising a compound P (VDF-TrFE) / BaTiO3 nanoparticles, and the curve 32 represents the difference between the values of curve 28 and those of curve 26, thus representing the difference in temperature obtained between the lower and upper pyroelectric portions for a pixel in contact with the skin for this lower and upper pyroelectric portion sensor each comprising a compound P (VDF-TrFE) / BaTiO3 nanoparticles.

The difference in temperature between two pixels one of which is in contact with air and the other is in contact with the skin, in the lower and upper pyroelectric portion sensor each comprising a compound P (VDF-TrFE) / nanoparticles of BaTi03, corresponding to the difference between curves 30 and 32, is at most 0.05 K, which is much lower than the values of curves 18 and 20 which represent the temperature difference between two pixels, one of which is in contact with air and the other is in contact with the skin, in a sensor of which only the upper pyroelectric portion comprises the addition of the material C. greater difference in temperature between pixels reflects the better sensitivity of the sensor obtained when such an addition is made only in the upper pyroelectric portion 112, and thus obtaining a good contrast for the detection, thanks to the strong temperature variation in the pyroelectric capacitance of the pixels 102 between the pyroelectric portions 110, 112, combined with the fact that the materials of the pyroelectric portions 110, 112 both have a good pyroelectric coefficient.

The curves shown in FIGS. 5 and 6 show that the sensitivity obtained for a sensor whose lower and upper pyroelectric portions each comprise a compound of materials B and C is lower than when only the upper pyroelectric portion comprises such a compound and that the lower pyroelectric portion has only P (VDF-TrFE). This is because the presence of the material C improves the thermal conductivity of the pyroelectric material, but this improvement in the thermal conductivity of the pyroelectric material is only desirable in the pyroelectric portion close to the thermal pattern to be detected in order to maintain a good difference of temperature between the lower and upper pyroelectric portions, and thus obtain a sensor 100 having a good measurement sensitivity.

Also for the sake of comparison, similar measurements are made for pixels of similar structure but in which the upper pyroelectric portion comprises only P (VDF-TrFE) and the lower pyroelectric portion comprises a compound P (VDF-TrFE) / nanoparticles of BaTi03. In FIG. 7, the curve represents the temperature variation obtained in the lower pyroelectric portion for a pixel in contact with air, and the curve represents the temperature variation obtained in the upper pyroelectric portion for this same pixel. Curve 34 also represents the temperature variation obtained in the lower pyroelectric portion for a pixel in contact with the skin, and curve 38 represents the temperature variation obtained in the pyroelectric portion. higher for this same pixel. In FIG. 8, curve 40 represents the difference between the values of curve 34 and those of curve 36, thus representing the difference in temperature obtained between the lower and upper pyroelectric portions for a pixel in contact with air for this sensor, and the curve 42 represents the difference between the values of the curve 34 and those of the curve 38, thus representing the temperature difference obtained between the lower and upper pyroelectric portions for a pixel in contact with the skin for this sensor .

These curves shown in FIGS. 7 and 8 show that the sensitivity obtained for this sensor whose lower pyroelectric portion comprises a compound of materials B and C and the upper pyroelectric portion comprises only P (VDF-TrFE) is lower than when only the upper pyroelectric portion comprises the compound of materials B and C and the lower pyroelectric portion comprises only P (VDF-TrFE). This results from the fact that the upper pyroelectric portion comprising only P (VDF-TrFE) has poor thermal conductivity, the heat transfer being then difficult between the finger present on the sensor and the heating element.

A method for producing a pyroelectric capacitance of a pixel 102 of the sensor 100 similar to that of FIGS. 1 and 2 is described with reference to FIGS. 9A to 91.

The sensor 100 is made from the substrate 104. The material of the substrate 104 (glass, semiconductor, plastic, etc.) is chosen according to the technology with which the various electronic elements of the sensor 100 are made. The substrate 104 is first cleaned to remove residues present thereon. The type of cleaning implemented is a function of the material of the substrate 104.

The second step is to deposit on the front face 108 of the substrate

104 a first electrically conductive layer 150, for example metal, from which the lower electrode 106 is intended to be made (Figure 9A). The layer 150 may comprise one or more of the materials previously described to form the lower electrode 106 of the sensor 100 shown in FIGS. 1 and 2. The layer 150 may be deposited in the PVD vapor phase by sputtering. screen printing, spraying or even by inkjet, depending on the materials to be deposited and their thicknesses. The thickness of the layer 150 corresponds to the desired thickness of the lower electrode 106.

As shown in FIG. 9B, the structure of the lower electrode 106 and of the electrical contact 107 is defined by implementing a photolithography and etching step of the layer 150 (for example by plasma or by wet etching).

Alternatively, the lower electrode 106 and the electrical contact 107 could be directly formed by a localized deposit that does not require implementation of an etching.

A first layer of pyroelectric material intended to form the lower pyroelectric portion 110 is then deposited on the entire structure previously made. This first layer of pyroelectric material (material A) is for example deposited by spin coating (spin coating), with a thickness of between about 1 μιη and 5 μιη depending on the desired thickness for the lower pyroelectric portion 110. annealing is then carried out at a temperature for example between about 80 ° C and 100 ° C for a period of time for example between about 10 and 30 minutes. This annealing makes it possible to crystallize the pyroelectric material. Photolithography and etching (for example with a plasma O ₂ or a combination SF 6 / O 2) are then implemented in order to remove a portion of the layer of pyroelectric material covering the electrical contact 107 of the lower electrode 106 (FIG. 9C ). Alternatively, a localized deposit may be implemented, such as screen printing or vaporization, or even ink jet deposition, to obtain directly the lower pyroelectric portion 110 as shown in Figure 9C.

At least one second electrically conductive layer 152 for forming the heating element 114 is then deposited on the structure obtained at this stage of the process, thus covering the lower pyroelectric portion 110 (FIG. 9D). The layer 152 may comprise one or more of the materials previously described to form the heating element 114 of the sensor 100 of FIGS. 1 and 2. Depending on the nature of the material of the layer 152, it can be deposited by PVD, inkjet, vaporization or screen printing. Parts not shown in Figure 9D of the layer 152 are intended to form the ends 113 and 115 across which the heating signal is intended to be applied.

The heating element 114 is then formed by the implementation of a photolithography step and an etching step of the layer 152 previously deposited according to the pattern of the heating element 114, for example in the form of several segments. together forming a serpentine pattern, as previously described in conjunction with Figures 1 and 2 (Figure 9E).

Again, a localized deposit of the layer 152 may be implemented to directly obtain the heating element 114 without implementation of an etching.

As shown in FIG. 9F, a second layer of pyroelectric material 154 intended to form the upper pyroelectric portion 112 is then produced over the entire structure previously obtained. The layer 154 corresponds to a layer of PVDF and / or at least one copolymer of PVDF (material B) in which nanoparticles and / or microparticles of at least one pyroelectric material of structure are advantageously dispensed in a random manner. crystalline perovskite and / or ZnO and / or PVDF and / or at least one copolymer of PVDF (material C). The layer 154 is deposited by the implementation of a spin coating, a vaporization or a screen printing, with a thickness (at the level of the heating element 114) corresponding to that desired for the upper pyroelectric portion 112. The layer 154 is annealed at a temperature for example between about 90 ° C and 120 ° C for a period of time for example between about 5 minutes and 1 hour. This annealing makes it possible to finalize the evaporation of the solvents used to form the layer 154 and to crystallize the pyroelectric material of the layer 154.

Photolithography and etching steps of the layer 154 are then implemented, forming the upper pyroelectric portion 112 (FIG. 9G). This etching may be implemented using for example a plasma type C ₄ Fs / 02, SF6 / O2 or CF _4/0 _2. During this etching, the conductive portions remaining from layers 150 and 152 previously deposited and which are not positioned under the upper pyroelectric portion 112 serve as an etch stop layer. This etching makes it possible, in particular, to reveal the electrical contact 107 previously covered by the layer 154, as well as a portion 155 of the layer 152 intended to serve for producing the electrical contact 109.

At least a third electrically conductive layer is then deposited on the entire structure previously made, and etched to form the upper electrode 116 and the electrical contact 109 of the upper electrode 116 (Figure 9H). The nature of the material or materials of this electrically conductive layer, as well as the thickness of this layer may correspond to the material and thickness examples previously described for the upper pyroelectric portion 112 of the sensor 100 of FIGS. 1 and 2. The etching implemented may correspond to plasma etching or wet etching, depending on the nature of the material to be etched.

The pixel 102 is then completed by depositing and etching the protective layer 118 (FIG. 91). The material or materials of the protective layer 118 may correspond to those previously described for the protective layer 118 of the sensor 100 of FIGS. 1 and 2. The deposition of the material of the protective layer 118 may be done physically (PVD) at low temperature or liquid (spin coating, spraying or inkjet). The etching implemented makes it possible, in particular, to form accesses to the electrical contacts 107, 109 through the protective layer 118. When the material of the protective layer 118 is deposited in a localized manner, for example by vaporization or jet-coating. ink, the protective layer 118 is formed from the deposit in a localized manner and it is not necessary to implement an etching of this material.

Since the materials of the pyroelectric portions 110, 112 comprise PVDF and / or at least one of the PVDF copolymers, it is necessary to implement, before the first use of the sensor 100, an initial polarization step of the pyroelectric materials of the pyroelectric portions 110, 112 by applying a DC voltage across these materials, via the electrodes 106, 116, to obtain a pyroelectric coefficient of these materials adapted to thermal detection. This polarization is performed only once for the entire lifetime of the pyroelectric materials. This DC bias can be at room temperature or hot (up to about 100 ° C). When the polarization is carried out at a room temperature, it is possible to apply a DC voltage up to about 120 V / μιη (Volt per micron spacing between the two electrodes 106, 116) for a duration for example between about 1 and 5 minutes. When the polarization is carried out hot, for example at a temperature of about 90 ° C, a DC voltage for example between about 50 and 80 V / μιη can be applied for a period of time for example between about 1 and 5 minutes. The temperature is then lowered until the ambient temperature is reached, then the electric field applied to the pyroelectric materials, via the applied DC voltage, is stopped. Such polarization allows the pyroelectric materials portions 110, 112 to reach pyroelectric coefficients between about 30 and 80 μ ^ (ιη ² .Κ), depending on the loading rate of the nanoparticles / microparticles of the material C in the compound of the upper pyroelectric portion 112.

The pixel array 102 of the sensor 100 is read line by line by means of selection transistors integrated in each pixel 102 and via an electronic reading circuit arranged in columns (not shown in the figures) making it possible to carry out a direct reading. charges generated by each pixel (current reading), or to perform a voltage reading of the pixels 102 via the use of a transistor mounted voltage follower.

Various calibration methods, for example using a reference pixel having a known heat capacity, can be implemented. Such methods are described for example in the document FR 2 959 814.

In the various examples described above, the sensor 100 is used as a fingerprint detector. However, the sensor 100 can be used to carry out a detection of thermal patterns other than fingerprints, because each pixel 102 of the sensor 100 reads the heat capacity placed above it and this whatever the nature of the thermal pattern detected. So, a A sensor comprising pixels 102 as previously described may be able to carry out a piezoelectric detection or to carry out a temperature detection.

For example, the sensor 100 can be used for producing a non-cooled infrared imager. The pixels 102 of the sensor 100, for example as described above in connection with FIGS. 1 and 2, are in this case integrated on a CCD or CMOS type optical sensor collecting the electrical charges generated by the sensor 100. Such an imager comprises in addition, an infrared lens filtering the light arriving on the sensor 100. In order for the sensor 100 to be subjected to a difference in temperature (necessary taking into account the measurement made by the pyroelectric capacitors), the imager comprises a device that successively block the infrared light arriving on the sensor 100 and then let this light. Such a device may correspond to a "chopper", that is to say a wheel provided with a hole and rotating in front of the sensor 100. An absorber element may be added to the pyroelectric material of the pixels 102 in order to improve absorption of the infrared radiation received.

In the previously described pixel example 102, the heating element 114 is constructed as segments of electrically conductive material serially connected to each other in a serpentine pattern. Alternatively, the heating element 114 may correspond to one or more portions of electrically conductive material of a shape other than a coil. Another embodiment of the heating element 114 is shown in FIG. 10. According to this other example, the electrically conductive portion forming the heating element 114 corresponds to a rectangular metal line of width "W" for example between about 25.degree. μιη and 500 μιη and length "L" between about 50 μιη and 500 μιη. Holes 122 of diameter d for example between about 1 μιη and 10 μιη are made through the portion of electrically conductive material forming the heating element 114.

In all cases, the heating element 114 is structured in such a way that portions of the lower and upper pyroelectric portions 110, 112 are in direct contact with each other, and thus portions of the electrodes 106, 116 are in contact with each other. look at each other without portions of the heating element 114 electrodes 106, 116 are arranged between these portions. This allows the electrodes 106, 112 and the pyroelectric material to form the pyroelectric capacitance such that the electrodes 106, 116 form conductive reinforcements between which a dielectric material formed by the materials of the pyroelectric portions 110, 112, is arranged. In the example previously described in connection with FIG. 2, the serpentine shape of the heating element 114 allows portions of the electrodes 106, 116 to be facing each other between the electrically conductive segments forming the coil. In the example previously described in connection with FIG. 10, the holes 122 also allow this formation of the pyroelectric capacitance.

Regardless of the shape of the heating element 114, the dimensions thereof and the voltage or current levels applied to effect heating of the pyroelectric material are adjusted according to the desired heating power. In order to obtain a good detection sensitivity, the electrical power injected into the heating element 114 can be between about 0.5 mW / pixel and 5 mW / pixel.

Claims

A thermal pattern sensor (100) having a plurality of pixels (102), each pixel (102) comprising at least one pyroelectric capacitance formed by at least one stack comprising at least:

a lower electrode (106);

a lower pyroelectric portion (110) disposed on the lower electrode (106) and comprising a material A corresponding to polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

an upper pyroelectric portion (112) disposed on the lower pyroelectric portion (110) and comprising a compound comprising a material B corresponding to polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer, and nanoparticles and / or microparticles of a material C corresponding to a pyroelectric material of crystalline structure of perovskite type and / or ZnO and / or polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

a heating element (114) comprising at least a portion of electrically conductive material disposed between the lower and upper pyroelectric portions (110, 112) and such that one or more portions of the lower pyroelectric portion (110) are directly in contact with each other; one or more portions of the upper pyroelectric portion (112);

an upper electrode (116) disposed on the upper pyroelectric portion (112).

The sensor (100) according to claim 1, wherein said at least one polyvinylidene fluoride copolymer of at least one of the lower and upper pyroelectric portions (110, 112) corresponds to polyvinylidene fluoride-trifluoroethylene and / or poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) and / or poly (vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene).

3. Sensor (100) according to one of the preceding claims, wherein:

the pyroelectric material of crystalline structure of perovskite type comprises BaTiO 3 and / or PbZrTiO 3 and / or BaSrTiO 3 and / or CaSrTiO 3 and / or BaZrTiO 3 and / or SrTiO 3, and / or

a proportion by weight of the material C in the compound of the upper pyroelectric portion (112) is between about 5% and 20%, and / or

the nanoparticles and / or the microparticles of the material C are distributed randomly in the compound of the upper pyroelectric portion (112), and / or

materials A and B are similar.

4. Sensor (100) according to one of the preceding claims, wherein the portion of electrically conductive material of the heating element (114) comprises a plurality of conductive segments joined one after the other forming a continuous conductive portion and between wherein the lower pyroelectric portion (110) is directly in contact with the upper pyroelectric portion (112), and / or wherein the portion of electrically conductive material of the heating element (114) is traversed by at least one opening (122) at which the lower pyroelectric portion (110) is directly in contact with the upper pyroelectric portion (112).

5. Sensor (100) according to one of the preceding claims, wherein the lower electrode (106) comprises a titanium layer with a thickness of between about 50 nm and 500 nm and a thickness of TiN layer between about 10 nm and 500 nm, and / or wherein the upper electrode (116) comprises a layer of titanium with a thickness between about 30 nm and 100 nm and a layer of AISi or AICu of thickness between about 100 nm and 700 nm.

6. Sensor (100) according to one of the preceding claims, wherein the thickness of the lower pyroelectric portion (110) and / or the thickness of the upper pyroelectric portion (112) is between about 1 μιη and 5 μιη .

The sensor (100) according to one of the preceding claims, wherein the pixels (102) are disposed on a front face (108) of a substrate (104) so that the lower pyroelectric portion (110) of each pixel (102) is disposed between the substrate (104) and the upper pyroelectric portion (112) of said pixel (102).

8. The sensor (100) according to one of the preceding claims, wherein the pixels (102) are arranged forming a matrix of several lines and several columns, and the heating elements (114) of the pixels of each of the pixel lines ( 102) are formed by a continuous portion of electrically conductive material.

9. Sensor (100) according to one of the preceding claims, wherein one of the lower and upper electrodes (106, 116) of each pixel (102) is electrically connected to a fixed electrical potential.

10. Sensor (100) according to one of the preceding claims, further comprising a protective layer covering said stack of each pixel (102) and having openings forming access to electrical contacts (107, 109) of the lower electrodes and upper (106, 116) of each pixel (102).

11. Sensor (100) according to one of the preceding claims, wherein the sensor (100) is a fingerprint sensor.

12. A method for producing a thermal pattern sensor (100) comprising a plurality of pixels (102), each pixel (102) comprising at least one capacitor pyroelectric formed by at least one stack obtained at least the implementation of the following steps:

- Realization of a lower electrode (106);

- Making a lower pyroelectric portion (110) disposed on the lower electrode (106) and comprising a material A corresponding to polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

- Realizing a heating element (114) comprising at least a portion of electrically conductive material disposed on the lower pyroelectric portion (110);

- producing an upper pyroelectric portion (112) on the portion of electrically conductive material of the heating element (114) which is such that one or more portions of the lower pyroelectric portion (110) are directly in contact with a portion or more portions of the upper pyroelectric portion (112), the upper pyroelectric portion (112) comprising a compound comprising a material B corresponding to polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer, and nanoparticles and / or microparticles of a material C corresponding to a pyroelectric material of crystalline structure of perovskite type and / or ZnO and / or polyvinylidene fluoride and / or at least one polyvinylidene fluoride copolymer;

- Making an upper electrode (116) disposed on the upper pyroelectric portion (112).