Method for producing a stack of layers for a matrix thermal sensor

Abstract

A method produces a matrix of pixels of a thermal sensor, suitable for passive addressing. The matrix of pixels includes a layer including a first series of electrically conducting strips, forming charge collection macro-electrodes; a layer including a pyroelectric material; and a layer including a second series of electrically conducting strips, forming heating strips. The method includes a step of transfer of one on the other of a first and a second elementary stack, the first elementary stack including the first series of strips, and the second elementary stack including the second series of strips. This method makes it possible to relax the manufacturing constraints of the series of strips.

Claims

1. A method for producing a matrix of pixels for a thermal sensor, each pixel including a pyroelectric capacitance formed by a portion including a pyroelectric material arranged between a charge collection electrode and a reference electrode, and a heating element, the heating elements of the pixels of a same line of pixels being integrally formed together into a same heating strip, the charge collection electrodes of the pixels of a same column of pixels being integrally formed together into a same charge collection macro-electrode, and the matrix of pixels being constituted of a stack of layers comprising: a layer of charge collection electrodes, including a first series of electrically conducting strips parallel with each other, forming the charge collection macro-electrodes; a layer including a pyroelectric material, comprising the pyroelectric material portions of each of the pixels; and a heating layer, including a second series of electrically conducting strips parallel with each other, forming the heating strips, wherein the layer including a pyroelectric material is located between the layer of charge collection electrodes and the heating layer; the method comprising: transferring one on the other of a first and a second elementary stack, to form said stack of layers, the first elementary stack including the layer of charge collection electrodes, and the second elementary stack including the heating layer.

2. The method according to claim 1, wherein the first and second elementary stacks are configured so that, after the transferring, the stack of layers includes, superimposed one on top of the other in this order: the layer of charge collection electrodes; the layer including a pyroelectric material; an electromagnetic shielding layer, electrically conducting; an electrical insulation layer; and the heating layer.

3. The method according to claim 2, wherein: at least one of the first and second elementary stacks includes an outer layer including an electrically insulating adhesive; and in the stack of layers, the adhesive lies between the heating layer and the electromagnetic shielding layer, and forms the electrical insulation layer.

4. The method according to claim 2, wherein: at least one of the first and second elementary stacks includes an outer layer including an electrically conducting adhesive; and in the stack of layers, the adhesive lies between the electrical insulation layer and the layer including a pyroelectric material, and forms the electromagnetic shielding layer.

5. The method according to claim 1, wherein at least one of the first and the second elementary stacks includes an outer layer comprising an adhesive, and in that the transferring is followed by a step of cross-linking the adhesive to make the first and second stacks integral with each other.

6. The method according to claim 1, wherein the transferring is carried out by folding a single substrate, the first elementary stack extending onto a first region of the single substrate, and the second elementary stack extending onto a second region of the single substrate.

7. The method according to claim 6, wherein an integrated circuit and electrically conducting lines are situated on the single substrate, the electrically conducting lines each extending between the integrated circuit and one end of a strip of the first series of strips, respectively of the second series of strips.

8. The method according to claim 6, wherein the shape of the single substrate is adapted so that, at the end of the folding, at least one end of each strip of the first series of strips is not covered by the second region of the single substrate, and at least one end of each strip of the second series of strips is not covered by the first region of the single substrate.

9. A system suitable for the implementation of the method according to claim 6, the system comprising: the first elementary stack, the second elementary stack, and the single substrate, the first elementary stack extending onto a first region of the single substrate, and the second elementary stack extending onto a second region of the single substrate.

10. The method according to claim 1, wherein the transferring is carried out by transfer of one above the other of a first substrate and a second substrate distinct from the first substrate, the first elementary stack extending onto the first substrate, and the second elementary stack extending onto the second substrate.

11. The method according to claim 10, wherein the second substrate is conserved at the end of the transferring, and forms a protective layer in the thermal sensor.

12. The method according to claim 10, wherein the second substrate is removed after the transferring.

13. A system suitable for the implementation of the method according to claim 10, the system comprising: the first elementary stack, the second elementary stack, a first substrate, and a second substrate distinct from the first substrate, the first elementary stack extending onto the first substrate, and the second elementary stack extending onto the second substrate.

14. The method according to claim 1, further comprising producing the first series of strips and the second series of strips by deposition of a metallic ink.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood on reading the description of exemplary embodiments given for purely indicative purposes and in no way limiting, while referring to the appended drawings, in which:

(2) FIGS. 1A to 1C schematically illustrate two alternatives of a first embodiment of a matrix of pixels that can be produced thanks to the method according to the invention;

(3) FIG. 2A schematically illustrates a second embodiment of a matrix of pixels that can be produced thanks to the method according to the invention;

(4) FIGS. 3A to 3C, and 4 schematically illustrate a first embodiment of a method according to the invention;

(5) FIGS. 5 and 6 illustrate different alternatives of this first embodiment;

(6) FIGS. 7A and 7B schematically illustrate a second embodiment of a method according to the invention; and

(7) FIG. 8 illustrates an alternative of this second embodiment.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(8) Hereafter, but in a non-limiting manner, examples are described of methods for producing a stack of the type of that described with reference to FIG. 2, said stack forming a matrix of pixels.

(9) Methods for producing first and second elementary stacks 302A, 302B are described hereafter. Even so, the method according to the invention does not necessarily include the implementation of these steps.

(10) The method according to the invention uses a first elementary stack, 302A (FIG. 3A), and a second elementary stack, 302B (FIG. 3B).

(11) The first elementary stack, 302A, here extends onto a first substrate, 301A.

(12) The first substrate 301A is for example made of glass, silicon, a plastic such as poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN), polyimide (Kapton film), etc.

(13) It is for example a flexible substrate.

(14) The first elementary stack 302A comprises a series of layers, superimposed one on top of the other along the axis (Oz), that is to say along an axis orthogonal to the plane of the first substrate 301A.

(15) The first elementary stack 302A here comprises, superimposed above the first substrate 301A: a layer 311 of charge collection electrodes, such as described in the introduction, including metal strips parallel with each other forming charge collection macro-electrodes; a layer 312 including a pyroelectric material, for example PVDF or AlN (aluminium nitride), or PZT (lead zirconate titanate), etc.; an electromagnetic shielding layer 316, electrically conducting, also forming a reference electrode common to all the pixels of the matrix of pixels; and a layer of adhesive 317, preferably electrically non-conducting.

(16) The charge collection macro-electrodes are spread out according to a pitch less than or equal to 150 m, preferentially less than or equal to 100 m, and even more preferentially according to a pitch less than or equal to 51 m.

(17) This pitch is for example around 80 m (i.e. a resolution of 320 dpi). For example, the width (along (Ox)) of a macro-electrode is 40 m, and the width of the interstice between two neighbouring macro-electrodes is 40 m.

(18) In an alternative, this pitch is 50.8 m (i.e. a resolution of 500 dpi). For example, the width of a macro-electrode is 25.4 m, and the width of the interstice between two neighbouring macro-electrodes is 25.4 m.

(19) Before the production of the charge collection macro-electrodes, the first substrate 301A undergoes a cleaning step (ethanol or acetone or plasma, depending on the substrate).

(20) The macro-electrodes are next produced on the first substrate 301.

(21) Different techniques for producing macro-electrodes may be implemented: lithography (lithography methods of the field of printed circuits), deposition of a full layer electrically conducting then laser ablation (for example deposition under vacuum of physical vapour deposition or chemical vapour deposition type, or deposition by cathodic sputtering, etc.), screen printing type printing (with use of a mask or stencil), printing of heliogravure type (technique in which an ink is transferred directly from an engraved cylinder onto a final surface, more particularly suitable for small distribution pitches of the metal strips), printing of offset gravure type (technique in which an ink is transferred from an engraved cylinder onto a final surface, through at least one intermediate cylinder, more particularly suitable for small distribution pitches of the metal strips), etc. ink jet type printing, etc.

(22) When the macro-electrodes are produced by printing, an electrically conducting ink, for example a metallic ink, is deposited, dried then annealed. The annealing is carried out for example in a heat chamber, at a temperature preferably comprised between 20 C. and 200 C. depending on the nature of the substrate and the ink.

(23) The macro-electrodes are for example strips of gold of 30 nm thickness, deposited by cathodic sputtering on a first substrate 301A constituted of a sheet of PEN of 125 m thickness.

(24) In an alternative, the macro-electrodes are silver strips of 1.5 m thickness, printed by offset gravure on a first substrate 301A constituted of a sheet of PEN of 125 m thickness.

(25) The layer 312 including a pyroelectric material has for example a thickness comprised between 2 m and 4 m.

(26) It is deposited for example by screen printing or by spin-coating.

(27) The pyroelectric material may be dispersed or dissolved in an ink, then deposited by a printing method. After having been deposited, the ink is dried then annealed, for example in a heating chamber.

(28) An ink based on a PVDF-TrFE copolymer is for example used. The relative quantity of TrFE (trifluoroethylene) compared to PVDF in the ink impacts the pyroelectric coefficient and the Curie temperature of the ink (temperature above which a material loses its pyroelectric properties). Depending on the type of substrate used (and thus the accessible annealing temperatures), different formulations and ratios of PVDF-TrFE may be used.

(29) In an alternative, an ink based on PZT or BaTiO.sub.3 (barium titanate) is used.

(30) The invention is not however limited to a layer 312 produced from an ink.

(31) The electromagnetic shielding layer 316 is constituted of an electrically conducting material, and preferably a poor heat conductor. The poor heat conduction makes it possible to limit thermal transfers in the layer 316, from one pixel to another of the matrix of pixels. The layer 316 has a reduced thickness, favouring heat transfers in the sense of the thickness. It is constituted for example of a layer of 0.5 m thickness made of PEDOT:PSS (mixture of poly(3,4-ethylenedioxythiophene) (PEDOT) and sodium poly(styrene sulphonate) (PSS)). If the material of the layer 316 is better electricity conductor, the layer 316 may be less thick. If the material of the layer 316 is better heat conductor, the layer 316 has to be less thick.

(32) The layer of adhesive 317 is electrically insulating.

(33) It is for example an epoxy (epoxide polymer) adhesive.

(34) The layer 317 also forms the electrical insulation layer such as described in the introduction, which makes it possible to limit the thickness of the matrix of pixels, and thereby reach greater image resolutions (see below).

(35) The thickness of the layer 317 is advantageously less than 5 m, for example 1 m.

(36) The second elementary stack, 302B, here extends onto a second substrate, 301B.

(37) The second substrate 301B may be identical to the first substrate 301A. In an alternative, it may have a lower thickness, less than or equal to 30 m, for example 15 m.

(38) The second elementary stack 302B is constituted uniquely of the heating layer 313, such as described in the introduction.

(39) The heating strips of the layer 313 are spread out preferably according to a distribution pitch identical to the distribution pitch of the charge collection macro-electrodes.

(40) Whatever the case, the heating strips of the layer 313 are spread out according to a pitch less than or equal to 150 m, preferentially less than or equal to 100 m, and even more preferentially according to a pitch less than or equal to 51 m.

(41) This pitch is for example around 80 m. For example, the width (along (Oy)) of a heating strip is 40 m, and the width of the interstice between two neighbouring heating strips is 40 m.

(42) In an alternative, this pitch is 50.8 m. For example, the width of a heating strip is 25.4 m, and the width of the interstice between two neighbouring heating strips is 25.4 m.

(43) Whatever the case, this pitch is preferably identical to the distribution pitch of the charge collection macro-electrodes (to have square pixels).

(44) Before producing the heating strips on the second substrate 301B, the latter undergoes a step of cleaning (ethanol or acetone or plasma, depending on the substrate).

(45) The heating strips of the layer 313 are next produced by methods similar to those that can be used for producing the charge collection macro-electrodes.

(46) If need be, the heating strips and the charge collection macro-electrodes may be produced jointly, in a same manufacturing step.

(47) The heating strips are for example gold strips of 30 nm thickness (along (Oz)), deposited by cathodic sputtering on a second substrate 301B constituted of a sheet of PEN of 125 m thickness.

(48) In an alternative, the heating strips are silver strips of 1.5 m thickness, printed by offset gravure on a second substrate 301B constituted of a sheet of PEN of 125 m thickness.

(49) The charge collection macro-electrodes and the heating strips may have the same characteristics (material and dimensions), which makes it possible that they are produced jointly. However, the charge collection macro-electrodes support in practice lower currents than the heating strips, and can, in an alternative, have smaller dimensions, or be constituted of materials that conduct electricity less well.

(50) In FIG. 3C is illustrated the transfer step, defining the method according to the invention.

(51) In the course of this step, the assembly constituted of the second elementary stack 302B and the second substrate 302B is turned over (180 rotation, around (Ox) or (Oy)), then deposited on the adhesive layer 317 of the first stack 302A.

(52) The transfer step is preferably followed by a step of cross-linking the adhesive, to make the first stack 302A and the second stack 302B integral with each other. The cross-linking of the adhesive may be carried out under the action of heat, or by irradiation by a beam with a predetermined wavelength (for example infrared or ultra-violet), or by irradiation by an electron beam.

(53) During the transfer step, the lateral and angular alignment of the second elementary stack 302B relative to the first elementary stack 302A is not crucial. This positioning freedom is directly linked to the solution of passive addressing of pixels, without transistors between the pixels of the matrix of pixels.

(54) FIG. 4 schematically illustrates, according to a top view, the total stack obtained at the end of the transfer step. Here, the charge collection macro-electrodes, 31, are oriented at 90 relative to the heating strips 33.

(55) In the example illustrated in FIG. 4, the dimensions of the first substrate 301A and the second substrate 302B are adapted so that, after the transfer step: each macro-electrode 31 has one end not covered by the second substrate 301B; and each heating strip 33 has one end not covered by the first substrate 301A.

(56) This configuration makes it possible to facilitate access to the heating strips and to the macro-electrodes, for connection to heating control circuits, respectively to reading circuits.

(57) The reading and heating control circuits are for example produced on a substrate distinct from the first and second substrates 301A, 301B. The connection with the macro-electrodes 31, respectively the heating strips 33, is ensured by flex connectors.

(58) In an alternative, the reading and heating control circuits are produced on one of the substrates 301A or 301B, and a flexible connector extends between the two substrates.

(59) According to another alternative, each of the substrates receives a part of the reading and heating control circuits, and a flexible connector extends between the two substrates.

(60) The idea behind the invention resides in the transfer step described above.

(61) Optionally, the method according to the invention may include the steps of producing first and second elementary stacks 302A, 302B, on the first, respectively the second, substrates 301A, 301B.

(62) The method according to the invention may also include a step of polarisation of the pyroelectric material of the layer 312. This involves subjecting the layer 312 to an intense electrical field, to modify durably the orientation of its molecules, and thereby to confer on it pyroelectric properties.

(63) The invention also covers a system suitable for the implementation of this first embodiment of a method according to the invention. The system includes in particular the first elementary stack 302A, the second elementary stack 302B, the first substrate 301A and the second substrate 301B.

(64) In a sensor as described here, the distance between the contact surface and an upper surface of the charge collection macro-electrodes, on the side of the contact surface, defines a maximum image resolution (heat propagating vertically in the matrix of pixels, but also horizontally, from one pixel to the other of the matrix of pixels).

(65) Preferably, this distance is less than or equal to the pixel pitch of the sensor.

(66) This distance is thus advantageously less than or equal to 150 m, and even less than or equal to 50 m or even 10 m.

(67) When it is wished to access smaller pixel pitches (for example when the sensor is a finger print sensor), there is thus the choice between handling a second substrate of low thickness (for example less than 15 m), or removing the second substrate after the transfer step (in this case, a protective layer may be added to the second stack, on the second substrate).

(68) When great resolution is not necessary (for example when the sensor forms a simple presence detector, to detect the presence or the absence of a body on a surface, for example the presence or the absence of a hand on a surface), the second substrate 301B may have both a large thickness (more than 100 m), and be conserved after the transfer step.

(69) FIGS. 5 and 6 illustrate different alternatives of the method described here above.

(70) In FIG. 5, the adhesive layer 516 is electrically conducting. It forms the upper layer of the first elementary stack, and also acts as electromagnetic shielding layer.

(71) An electrically conducting adhesive is for example an epoxy type adhesive, filled with metal particles, for example filled with silver particles.

(72) This alternative allows that the first and second elementary stacks each include: a layer including electrically conducting strips (the layer of charge collection electrodes or the heating layer, depending on the stack); and a layer including a material able to exhibit pyroelectric properties, depending on whether it has been subjected or not to a polarisation field (the layer including a pyroelectric material or the electrical insulation layer, depending on the stack).

(73) This alternative facilitates joint production of the first and second elementary stacks. It is particularly suitable when the second substrate is conserved after the transfer step.

(74) Numerous other alternatives may be implemented, where the adhesive layer forms the upper layer of the first elementary stack, and/or the second elementary stack, on the side opposite to the first substrate, respectively second substrate.

(75) In FIG. 6, the second substrate 601B forms a simple transfer substrate, or carrier, for example made of glass, intended to be separated from the second elementary stack at the end of the transfer step. It provides the second elementary stack 602B with the necessary mechanical stability for the implementation of the transfer step. Surface treatments, or the insertion of an intermediate adhesive tie layer, may be implemented on the second substrate 601B, before producing the second elementary stack, to facilitate a future delamination (delamination carried out, for example, with a laser).

(76) Here, a thin protective layer 614 is added to the second elementary stack 602B, between the second substrate 601B and the heating layer 613. The protective layer 614 is for example a layer made of DLC (Diamond Like Carbon), a scratch-resistant resin, an ultra-thin substrate such as a Kapton or a polyimide of 5 to 25 m thickness, a thin glass substrate, etc.

(77) Preferably, the thin protective layer 614 has a thickness less than or equal to 25 m, or even less than or equal to 10 m, and even less (for example a layer of DLC of thickness less than or equal to 1 m or a layer of scratch-resistant resin deposited by spray or by screen printing of 3 m thickness).

(78) This embodiment makes it possible to limit the distance between the contact surface of the sensor and the charge collection macro-electrodes, and thereby to improve the resolution of the sensor by limiting diathermy (or cross-talk, or heat exchanges) between neighbouring pixels.

(79) FIGS. 7A and 7B schematically illustrate a second embodiment of a method according to the invention.

(80) According to this embodiment, the first elementary stack 702A and the second elementary stack 702B each extend onto one of two regions of a same substrate 701, and the transfer step is implemented by folding this single substrate 701, such that these two regions lie one above of the other.

(81) The single substrate 701 is here a flexible substrate, for example a substrate made of polyimide of 5 m to 10 m thickness, or made of a plastic such as PET, or even glass.

(82) In FIG. 7A is schematically represented, according to a top view, the substrate 701 in the unfolded state, with the two elementary stacks 702A, 702B. The fold line 701 is represented by dotted lines.

(83) Here, the shape of the substrate 701 is adapted so that the heating strips, respectively the charge collection macro-electrodes, each have a so-called free end, which is not framed between two portions of the substrate 701 along the axis (Oz). These free ends facilitate the connection to remote reading and heating control circuits.

(84) In FIG. 7B is schematically represented, according to a sectional view, the substrate 701 partially folded with the two elementary stacks 702A, 702B.

(85) This embodiment notably makes it possible to facilitate the production, in a same step of the method, of heating strips and charge collection macro-electrodes.

(86) The invention also covers a system suitable for the implementation of this second embodiment of a method according to the invention. The system includes in particular the first elementary stack 702A, the second elementary stack 702B, and the single substrate 701.

(87) The alternatives described with reference to FIGS. 5 and 6 may be adapted to this second embodiment. It is notably possible to remove, after folding, a region of the substrate 701, initially forming a support for the second elementary stack.

(88) According to another alternative, the substrate 701 may have regions of different thicknesses, the first elementary stack 702A being situated on a region of high thickness, and the second elementary stack 702B being situated on a region of low thickness.

(89) FIG. 8 illustrates an alternative of this second embodiment, in which an integrated circuit 840 is formed on the single substrate 801.

(90) The integrated circuit here includes a silicon chip, bonded on the single substrate 801.

(91) Different electrically conducting tracks, or lines, extend onto the single substrate, in particular: heating control lines, 841, each extending between the integrated circuit 840 and a first end of each of the heating strips 83; a ground line, 842, passing through the second ends of each of the heating strips 83, and through at least one edge of the electromagnetic shielding layer 816; lines of electrodes, 843, each extending between the integrated circuit 840 and a first end of each of the charge collection macro-electrodes 81; and output lines, 844, for the electrical connection of the integrated circuit 840 with external elements.

(92) In this embodiment, the reading and heating control circuits are produced on a same chip, which simplifies synchronisation of the heating with the reading of the quantities of pyroelectric charges, and makes it possible to limit the number of output lines 844 to produce with the exterior by data multiplexing.

(93) The idea behind the invention is thus to manufacture the matrix of pixels in two parts, preferably assembled with adhesive. Thus, prints requiring precision do not suffer loss of resolution linked to the planeness of the lower layers, notably the layer comprising a pyroelectric material. These prints are not affected either by problems of adherence and/or compatibility with lower layers, notably the layer comprising a pyroelectric material. These prints may each be made directly on a substrate.

(94) The transfer step uses two substrates, or two portions of a same substrate, each receiving a part of the layers forming the matrix of pixels of the sensor.

(95) In operation, an object to image or to detect is pressed against a contact surface of the sensor. This contact surface may be situated on one of these two substrates, or one of these two portions of substrate.

(96) In the above description, it is considered that it is the elementary stack comprising the heating strips that is turned over and displaced. In an equivalent manner, it is possible to choose to turn over rather the elementary stack comprising the charge collection macro-electrodes.

(97) In the same way, the adhesive layer may be deposited rather on the elementary stack comprising the heating strips (or even on the two elementary stacks).

(98) Numerous other alternatives of the method according to the invention may be implemented without going beyond the scope of the invention, for example with different elementary stacks. In particular, the method according to the invention may also serve to produce a stack such as described with reference to FIGS. 1A to 1C.

(99) It is possible to do without an adhesive layer to make the first and second elementary stacks integral with each other. For example, it is possible to produce two elementary stacks each including an upper layer made of pyroelectric material, then to carry out a sintering to make these two layers made of pyroelectric material integral with each other, after the transfer step.

(100) A so-called collective transfer may be carried out, for producing simultaneously several matrices of pixels, from a single or two substrates receiving a plurality of first and second elementary stacks. The substrate(s) may have through openings, to facilitate the access of electrical connectors to the heating strips and/or to the charge collection macro-electrodes.

(101) The first elementary stack may extend initially onto an ultra-thin substrate. After the transfer step, it is possible to bond the assembly obtained onto a more rigid support, for example onto a chip card.

(102) It is also possible to use the method according to the invention to produce a matrix thermal sensor with passive addressing, in which each pixel comprises two superimposed pyroelectric cells, one dedicated to the measurement of a total signal, and the other to the measurement of a noise signal.

(103) The positions of the heating strips and charge collection electrodes may be exchanged. They are not necessarily constituted of a conducting ink.

(104) In the examples represented, heating strips, respectively charge collection macro-electrodes, are directly produced independently of each other. In an alternative, they can be initially connected together, to facilitate a step of polarisation of the layer including a pyroelectric material. Conducting portions connecting them together are eliminated after the transfer step, during a cutting step (for example to separate different matrices of pixels after a collective transfer).

(105) Although not represented, the sensor comprises at least one reading circuit, for measuring a quantity of charges collected by a charge collection electrode, and at least one heating control circuit, for sending electrical signals making it possible to heat the pixels of the sensor through heating strips. It may further comprise an electronic processing circuit able to construct an overall image of a thermal pattern, from measurements carried out at the level of each of the pixels of the sensor.

(106) The thermal pattern that can be imaged by the sensor may be a papillary print, or any other pattern associated with an object having a thermal capacity and a specific heat capacity.

(107) The invention is particularly suitable for the manufacture of sensors of large dimensions (surface of the matrix of pixels greater than 1 cm.sup.2). Such sensors are integrated for example on a chip card, an item of clothing, or a portable accessory.