FR2792089A1 - PHOTOSENSITIVE DEVICE, ESPECIALLY FOR USE IN FINGERPRINT SENSORS OR IMAGING DEVICES - Google Patents

Abstract

The present invention relates to a photosensitive device such as a fingerprint detector. This device consists of a transparent substrate 10, a two-dimensional matrix 11 deposited on the substrate and comprising at least one image point formed of an opaque zone 13 comprising a photodetector element 14 associated with an optical window, lighting means 17 , 18 of the substrate at an angle allowing total internal reflection and a transparent layer 12 deposited on the matrix and returning by total internal reflection the light emitted by the lighting means towards the photodetector element.

Description

The present invention relates to photosensitive devices or retinas.

  Photosensitive devices are used in many fields, including imaging or fingerprint recognition. In the case of fingerprint recognition, different methods of fingerprint detection are used. In particular, optical detection methods are known. A generally used method is shown in Figure 1. It consists of a glass prism 1 operating in total internal reflection. This prism is associated with a light source comprising a lamp 2 and a reflector 3 directing the light rays towards a face a of the prism. The light rays are reflected on the large side b of the prism and are returned through the side c to an optic 4 or system of lenses and a photosensitive retina 5. If a finger 6 is placed on the large side b of the prism 1, the areas of the finger in contact with the prism break the total reflection of the light which then enters the skin where it is absorbed. A dark zone is then imaged on the retina 5 while the non-contact zones make it possible to totally reflect the light creating clear image zones on the retina 5. This fingerprint detection device according to the prior art has the drawback of 'to be

particularly bulky.

  Consequently, the aim of the present invention is to provide an improved photosensitive device having the advantage of being compact and able to be produced using layered deposition techniques.

  thin, well known to those skilled in the art.

  The present invention therefore relates to a photosensitive device, characterized in that it consists of - a transparent substrate, - a two-dimensional matrix deposited on the substrate and comprising at least one image point formed by an opaque zone comprising an element photodetector, associated with an optical window, - means for illuminating the substrate at an angle allowing total internal reflection,

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  - a transparent layer deposited on the matrix and reflecting by internal total reflection the light emitted by the lighting means

towards the photodetector element.

  According to a preferred embodiment, the transparent substrate is made of a transparent material such as glass or quartz. The matrix is made of a material such as amorphous silicon, low temperature polycrystalline silicon with a glass substrate, high temperature polycrystalline silicon with a quartz substrate, or monocrystalline silicon. Preferably, the photodetector element is chosen from photodiodes or phototransistors. In addition, the transparent layer deposited on the matrix consists of a glass plate or a plastic material and has a thickness less than or

  equal to the spacing between photodetector elements.

  According to another characteristic of the present invention, the transparent layer can be coated with a conductive layer

  transparent used to screen external electric fields.

  According to yet another characteristic of the present invention, the means for illuminating the substrate comprises a lamp, a reflector sending the light emitted by the lamp towards the substrate and means allowing the light to enter the substrate at an angle.

allowing total reflection.

  According to a first embodiment, the means allowing the light to enter at an angle allowing the total internal reflection is

  consisting of a network of prisms or a Fresnel network.

  According to a second embodiment, the means for illuminating the substrate is positioned so as to use said substrate as a waveguide directing the light towards the two-dimensional matrix and the opaque areas of the two-dimensional matrix are coated on the substrate side with a layer. reflective. This results in a more compact system

  avoiding the use of a Fresnel network or a network of prisms.

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  Preferably, the image points of the two-dimensional matrix are arranged in rows and columns, the photodetector element of each image point being connected by a switching circuit to a selection line and to a data line. The selection lines are connected to a control circuit sequentially selecting each selection line and the data lines are connected to a detection circuit capable of reading the signal emitted by the element.

photodetector selected.

  Thus, the photosensitive device described above can be used in devices such as fingerprint sensors,

  conventional or contact imaging devices.

  Other characteristics and advantages of the present invention will become apparent on reading the various embodiments described below with reference to the attached drawings, in which: FIG. 1 already described is a schematic view of a

  fingerprint detector according to the prior art.

  Figure 2 is a schematic sectional view of a photosensitive device used as a fingerprint detector according to a mode of

  realization of the present invention.

  Figure 3 is a schematic sectional view of another embodiment of the photosensitive device according to the present invention. Figures 4a and 4b respectively represent a

  the two-dimensional matrix and its associated electrical circuit.

  FIGS. 5a and 5b schematically represent another embodiment of a cell of the two-dimensional matrix with its

associated electrical circuit.

  To simplify the description, in particular in FIGS. 2 and

  3, the same elements have the same references. On the other hand, by cell in Figures 4 and 5 is meant the image point of the matrix

  two-dimensional and its associated switching circuit.

  A first embodiment of a photosensitive device according to the present invention is shown in Figure 2. This

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  photosensitive device essentially comprises a substrate 10 made of a transparent material such as glass or quartz. On this substrate is produced a two-dimensional matrix 11 carrying opaque zones 13 in which photodetector elements 14 are produced such as photodiodes or phototransistors and windows 15 associated with each photodetector. The two-dimensional matrix is produced by thin layer techniques using amorphous silicon, low temperature polycrystalline silicon, high temperature polycrystalline silicon, monocrystalline silicon. However, if the silicon used is low temperature polycrystalline silicon, the transparent substrate 1 0 is made of glass; if the silicon is high temperature polycrystalline silicon, the transparent substrate 10 is made of quartz and if the silicon is monocrystalline silicon, it is deposited on a transparent insulator, which can be glass or quartz. On this two-dimensional matrix 1 5, the complete structure of which will be described in more detail below, a transparent layer 12 is deposited, the thickness of which is substantially equal to or less than the spacing between the photodetector elements 14. This layer is produced either by a glass plate or by a layer of plastic material. It can be carried out directly on the two-dimensional matrix or bonded to this said matrix. According to an additional characteristic not shown, this layer 12 can be covered with a transparent conductive layer such as a layer of lTO (indium tin oxide)

  used to screen external electric fields.

  As shown in FIG. 2, the photosensitive device described above is illuminated by a lighting system comprising in a known manner a lamp 17 and a reflector 18 returning the light rays rl, r'l to the external face of the substrate 10. The system lighting is positioned so as to allow the light rays to enter the photosensitive device at an angle greater than or equal to that of the total internal reflection at the parallel faces of the substrate 10 and of the layer 12. Therefore on the external face on the lighting system side of the substrate 10 is therefore provided a means allowing the light to enter said substrate at an angle allowing the total internal reflection. This means 16 is preferably formed by an array of prisms or a Fresnel array. In this case, if a finger 19

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  is placed on the external face of the layer 12, the light rays rl, r'1 which have passed through the substrate 10 and have passed through the windows 15 of the two-dimensional matrix come to be reflected by total reflection on the external face of the layer 12. If the finger 19 is placed on the layer 12, the light r'l enters the areas of the finger in contact and no longer illuminates the corresponding photodetector elements 14, while in the areas where there is no no contact, the light rl is

  reflects by total reflection and is detected by the photodetectors 14.

  Another embodiment of a photosensitive device according to the present invention will now be described with reference to FIG. 3. In this case, to limit the overall size of the device, the lighting system is positioned on the side of the assembly formed by the substrate 10, the two-dimensional matrix 11 and the layer 12, as shown in FIG. 3. The lighting system comprises in known manner a lamp 17 'and a reflector 18'. The lamp is positioned so as to send the light rays r2, r'2 to the external face 10a of the substrate 10 which, in this case, functions as a waveguide. To allow the photosensitive device to operate, the surface on the substrate side 10 of the opaque zones 13 of the two-dimensional matrix is covered with a reflective layer 13 ′ such as a metallic layer. Thus, as shown in FIG. 3, the rays r2, r'2 after being reflected on the face 10a of the substrate 10, are reflected either on the surfaces 13 'or pass through the windows 15. The rays r'2 having passed through the windows 15 are reflected by total internal reflection on the external face of the layer 12 to be returned to the photodetectors 14 if there are no fingerprints in contact with the external face of the layer 12. The device described above works from

  equivalent to the device in Figure 2.

  We will now describe with reference to FIGS. 4 and 5 different embodiments of the two-dimensional matrix in which the image points each comprising at least one opaque zone 13, a window 15 and a photodetector 14 are arranged in rows and columns and are each connected by through a circuit

  switching to selection lines and data lines, they

  same connected to line selection control devices

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  and to devices for reading the data supplied by the

  photodetectors 14 on the data lines.

  As shown in FIGS. 4a and 4b, the photodetector 14 of FIGS. 2 and 3 is constituted by a phototransistor TP1 whose gate is connected to a polarization line LC and whose source is connected to ground while the drain is connected to a storage capacity Ca1 mounted in parallel on the phototransistor TP1. The drain of the phototransistor TPl1 is also connected to the source of a switching device constituted by a transistor T1 of the TFT type for example, the gate of which is connected to a selection line L1 and the other electrode of which, namely the drain , is connected to a data line or columns C1. The cell, immediately adjacent in the direction of

  columns, has a structure identical to the cell described above.

  It consists, as shown in FIG. 4b, of a phototransistor TP2, the grid of which is connected to a line of polarization LC common to the line of polarization LC of the phototransistor TP1. The source of transistor TP2 is connected to ground. The other electrode, namely "the drain" in the embodiment shown, is connected to a terminal of a storage capacity Ca2 the other terminal of which is connected to ground and to one of the electrodes of the transistor.

  switching T2, the other electrode of which is connected to column C1.

  On the other hand, as shown in FIG. 4b, the column Cl1 is connected via a switch S either to a reference potential V, or to a load reading circuit comprising essentially at each column C1 an amplifier A, the output of which is connected via a capacitor C on the input. The operation of the device in FIGS. 4a and 4b will now be explained. First of all, the columns Cl ... are preloaded by positioning the switch S of each column on the

  reference potential V. If all the lines L1, L2, etc. are addressed.

  of the two-dimensional matrix, the switching transistors T1, T2 are turned on and the capacitors Ca1, Ca2, ... are loaded at potential V. The transistors TP1 and TP2 are in the off state (voltage of

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  Negative LC) and partially discharge the capacities Cal1 and Ca2

  in proportion to the light flux that each of them receives.

  During a read operation, the switch S is switched to a load read circuit consisting, as mentioned above, of an amplifier A whose output is connected to the input via a capacitor C. During the successive selection of the lines L1, L2, ..., the transistors T1, T2 are successively turned on and the residual charges stored on the capacitors Ca1, Ca2 are read by the reading circuits giving

  information on the presence or absence of a fingerprint to be detected.

  We will now describe, with reference to FIGS. 5a and 5b, another embodiment of a two-dimensional matrix according to the present invention. In this case, the photosensitive transistor TP'1 of a cell at its gate connected to the selection line L'2 of the next cell. Its source is connected to a reference voltage, while its drain is connected to one of the electrodes with a charge storage capacity Ca'1 and to the source of a switching transistor T'1 whose gate is connected to the line L'l and the drain of which is connected to the column C'1. As previously described, each column or data line C'1 ... is connected via a switch S ', either to a positive reference voltage V', or to a charge reader Le constituted in known manner an amplifier and a capacitor mounted between the output of the amplifier and its input. On the other hand, as shown in Figure 5a, the capacity C'al has a second

  electrode connected to the reference voltage V '.

  The device described above also operates in two passes. During a first pass, the data lines or columns C'1 are loaded at the reference potential V '. Each selection line L'2, L'1 ... is activated from the bottom and up to the top of the device. When the line L'2 is activated, the transistor T'P1 is on and the capacitor C'al is discharged. When the line L'1 is activated, the transistor T'1 turns on and the capacitor C'al is charged to the

potential V '.

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  During the second pass, the charges are read in the capacitors C'al .... In this case the switching device S 'is positioned so that the read amplifier Le is connected to the columns C'1. ... When selecting the different selection lines L'l1, L'2, the transistors T'1, ... are successively on and the charges of the capacitors C'al, C'a2 are successively read using Le amplifiers. The values of the charges read depend on the leakage current of the transistors T'P1, themselves proportional to the illumination that they receive through the windows 5 and after total reflection

  possible on the outlet face of the blades 12.

  Line scanning circuits can be made on glass using the same transistor technology as that of the

matrix (Tl, TP1, T'1, T'P1).

  In the same way, the end of a packet of columns can be multiplexed towards a single amplifier using registers and switches made on the insulating substrate using the technology of

matrix transistors.

  It is obvious to those skilled in the art that the embodiments described above are only examples and can be

changed in many ways.

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Claims (19)

  1. Photosensitive device, characterized in that it consists of: a transparent substrate (10), a two-dimensional matrix (11) deposited on the substrate and comprising at least one image point formed by an opaque zone (13) comprising a photodetector element (14), associated with an optical window, - lighting means (17, 18, 17 ', 18') of the substrate at an angle allowing total internal reflection, - a transparent layer (12) deposited on the matrix and returning by total internal reflection the light emitted by the means   1 5 lighting towards the photodetector element.   2. Device according to claim 1, characterized in that the means for lighting the substrate comprises a lamp (17), a reflector (18) returning the light emitted by the lamp to the substrate and means (1 6) allowing light entering the substrate at an angle   allowing total internal reflection.   3. Device according to claim 2, characterized in that the means (16) allowing the light to enter at an angle allowing the total internal reflection is constituted by an array of prisms or a Fresnel network.   4. Device according to claim 1, characterized in that the substrate lighting means comprises a lamp (17 ') and a reflector (18') returning the light emitted by the lamp towards the substrate, said lighting means being positioned so as to use said substrate as a waveguide directing light towards the two-dimensional matrix. 5. Device according to claim 4, characterized in that the opaque areas of the two-dimensional matrix are coated, side   substrate, a reflective layer (13 '). 2792089   6. Device according to any one of claims 1 to 5,   characterized in that the image elements of the two-dimensional matrix are arranged in rows and columns, the photodetector element of each image element being connected by a switching circuit to   a selection line and a data line.   7. Device according to claim 6, characterized in that the photodetector element is connected to the terminals of a storage capacity (Cal1, Ca2), itself connected to the switching circuit,   the photodetector element being connected to a polarization line.   8. Device according to claim 6, characterized in that the photodetector element is connected to the terminals of a storage capacity 1 5 (Cal1, Ca2) of which an electrode is connected to the switching circuit, the photodetector element being connected to the line of next selection.   9. Device according to claims 6 to 8, characterized in that   that the selection lines are connected to a control circuit   sequentially selecting each selection line.   10. Device according to claims 6 to 8, characterized in that   that the data lines are connected to a detection circuit capable of reading the signal emitted by the selected photodetector element.   1 1. Device according to any one of claims 1 to   , characterized in that the transparent substrate is produced in a   transparent material such as glass or quartz.   12. Device according to any one of claims 1 to   11, characterized in that the control circuits are made on the transparent substrate. 1 1 2792089   13. Device according to any one of claims 1 to 11,   characterized in that the matrix is made of a material such as   amorphous silicon, polycrystalline silicon, monocrystalline silicon.   14. Device according to claim 13, characterized in that, if the polycrystalline silicon is low temperature polycrystalline silicon, the substrate is made of glass and if the polycrystalline silicon is polycrystalline silicon   high temperature, the substrate is quartz.   15. Device according to any one of claims 1 to 14,   characterized in that the photodetector element is chosen from among the   photodiodes or phototransistors.   16. Device according to any one of claims 1 to 15,   characterized in that the transparent layer consists of a plate glass or plastic material.   17. Device according to claim 15, characterized in that the transparent layer has a thickness less than or equal to   The spacing between photodetector elements.   18. Device according to claim 16, characterized in that the   transparent layer is coated with a transparent conductive layer.   19. Use of the device according to any one of   claims 1 to 18, in devices such as fingerprint sensors,   conventional or contact imaging devices.   20. Method for reading a photosensitive matrix according to the   Claims 6 and 7, characterized in that:   - during a precharging step, the switching device is turned on and the storage capacities are loaded at a reference voltage, then the photodetector elements being in the blocked state, the capacities are discharged in proportion to the light flux received by said photodetector elements, and 12 2792089   - during a reading step, the switching transistors are turned on and the residual charges stored in the storage capacitors.   21. Method for reading a photosensitive matrix according to the   Claims 6 and 8, characterized in that:   - during a precharging step, the selection lines are successively activated from the bottom to the top so as to charge the storage capacities at a reference potential, - during a reading step, the charges of the capacitors are read storage as a function of the leakage current of the photodetector elements   proportional to the illumination received.