WO2018114902A1

WO2018114902A1 - Device for acquiring fingerprints

Info

Publication number: WO2018114902A1
Application number: PCT/EP2017/083467
Authority: WO
Inventors: Yang Ni
Original assignee: New Imaging Technologies
Priority date: 2016-12-19
Filing date: 2017-12-19
Publication date: 2018-06-28
Also published as: FR3060811A1

Abstract

The invention relates to a device for acquiring fingerprints comprising a matrix (3) of photo-detectors (4), each photo-detector (4) exhibiting a photosensitive surface (8) facing an acquisition surface (6), and comprising a propagation medium (14) extending between the photo-detectors (4) and an acquisition surface (6), said acquisition surface comprising an occultation zone (16) extending parallel to the acquisition surface (6) and adapted to block a major part of the light having passed through the acquisition surface and propagating in the propagation medium in a direction (6) normal to the acquisition surface, the occultation zone comprising transparent passages (16b) allowing light that has passed through the acquisition surface (6) and propagating in the propagation medium (14) in a direction different from a direction normal to the acquisition surface (6) to reach the photo-detectors.

Description

DEVICE FOR ACQUIRING DIGITAL IMPRESSIONS

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a fingerprint acquisition device comprising a matrix image sensor, and more particularly to a portable electronic device provided with a fingerprint acquisition device. The term fingerprint (or papillary) means the figures formed by the ridges and epidermal valleys of the palmar surface of a human finger, also called dermatoglyphs.

Many portable electronic devices provide access to digital resources. This is particularly the case of smart mobile phones of the so-called "smartphone" type. Some of the data in these digital resources is confidential, and their access must be secure. The first type of access protection historically used for telephones was to request the identification of a personal identification number (known by the acronym of PIN for "personal identification number") at four digits. However, this type of protection has proved easy to circumvent, and heavy to implement by the user, in particular because effective protection requires that this number be filled in each session of use of the phone. Thus, other means of securing telephones have been explored to allow locking and unlocking operations of the phone that are more ergonomic and simpler. The fingerprint detection of a user has proved to be one of the simplest and most effective means of protection.

Thus, portable electronic devices equipped with a fingerprint acquisition device have been proposed. These devices include a fingerprint sensor that must be both inexpensive and as compact as possible, so that it can be incorporated in a mobile device such as a smartphone. In particular, for this application, the fingerprint sensor must be thin, and have a small footprint. Currently, the thin fingerprint sensors embedded in phones mainly use the principle of capacitive fingerprint detection. In these sensors, the user's finger comes into contact with a film on the surface of the sensor, and the material differences between an underlying sensing electrode and the surface create a difference in electrical capacitance that can be measured by a sensor. active circuit of the sensor. However, capacitive sensors suffer from several limitations for this application. Thus, capacitive sensors are sensitive to electrostatic disturbances. In addition, these sensors require a complex and expensive structure, with, for example, an anisotropic single crystal sapphire blade to protect the sensor while allowing the capacitive variation of detection surface to pass.

As a result, other fingerprint sensors using the optical sensing principle have been developed. It should be noted that the need for a small thickness of the sensors does not allow the use of optical sensors total internal reflection, or TIR, acronym for "total internai reflection", whose optical elements are too bulky.

It has been proposed to directly acquire an image of the finger by means of a matrix of photodetectors constituting a sensor, typically with photodiodes in complementary metal oxide semiconductor technology, better known by the acronym CMOS for English "Complementary Metal Oxide Semi-conductor".

Figure 1 shows an example of such a configuration. A finger 100 is affixed to an acquisition surface 101 of a sensor 102 comprising a matrix 103 of photodetectors 104 on a semiconductor substrate 105 and, superimposed on the photodetectors 104, a transparent layer 106 containing in particular the metal interconnections 107 necessary for the operation of the photodetectors. The finger 100 has on its surface fingerprints including ridges 108 and valleys

109 between the ridges 108. The ridges 108 are in contact with the acquisition surface 101 of the sensor 102, while the valleys 109 are separated by air.

It therefore appears that the light coming from a valley 109, whose light path

110 is represented by a dashed arrow, must pass through air before reaching the acquisition surface 101 of the sensor 102, while the light coming from a peak 108, whose light path 112 is represented by a arrow in full line, does not cross air to reach the acquisition surface 101 of the sensor 102, the peaks 108 being in contact with the acquisition surface 101 of the sensor 102. Therefore, there is a difference in length of the light paths 110, 112 between light from a peak 108 and light from a valley 109. The loss of luminous intensity being all the greater as the path is long, the light coming from a peak108 has a higher intensity than the light coming from a valley 109.

The image acquired by such a CMOS sensor thus has contrast differences between the pixels corresponding to peaks 108 and the pixels corresponding to valleys 108 of the fingerprint 100. This difference in contrast results from:

Fresnel losses at the interface between the acquisition surface 101 of the sensor and the air, affecting the light coming from a valley 109, must pass through an air-sensor interface, while the light coming from a peak 108 passes through a finger-sensor interface, more weakly refractive since the refractive index of the transparent layer 106 of the sensor is generally more pocket of the index of refraction of the finger than the refractive index of the air; and

the difference in the length of the light path between the light coming from a peak 108 and the light coming from a valley 109. The contrast resulting mainly from this difference in the lengths of the light paths 110, 112. difference in length of the light path depends essentially on the depth of the valleys 109 and the height of the ridges 108, which is of the order of 40-60μιη in adults. However, in both cases the light must travel through the transparent layer 106 to reach the photodetectors. This transparent layer 106 typically has a thickness of about 5 μιη. Thus, the relative difference between the lengths of the light paths 110, 112 is hardly attenuated by the thickness of this transparent layer 106, since the difference in the lengths of the light paths 110, 112 then represents a difference of approximately a factor of 10. In addition, as illustrated in Figure 2, it is also common to have a glass plate 116 above the transparent layer 106 to protect the sensor from external aggression. Such a glass plate typically has a thickness of 400 μιη. The light must therefore pass through the glass plate 116 in addition to the transparent layer 106. In this case, the relative difference between the lengths of the light paths 110, 116 represents only a small portion of the lengths of the light paths (less than 10%). So there is not much difference in intensity between light from a peak 108 and the light from a valley 109. This results in a low contrast between the pixels corresponding to peaks 108 and pixels corresponding to valleys 109 of the fingerprint 100. Moreover, since glass is an isotropic medium, increasing the distance between photodetectors 104 and finger 100 implies that light from a finger point can reach several photodetectors 104 by lateral or oblique propagation, with differences light path too weak to distinguish between the light from a point of the finger facing a photodetector 104 and finger points in front of other photodetectors 108. This results in a low light resolution.

In order to improve the resolution, patent applications US2016 / 0224816 and US2016 / 0254312 propose having collimators facing photodiodes in order to let only the light propagating in a direction close to normal. This solution, however, does not improve the contrast.

PRESENTATION OF THE INVENTION

The aim of the invention is to remedy at least part of these drawbacks and preferentially to all, by proposing a simple, inexpensive and low-volume fingerprint acquisition device, in particular enabling it to be mounted on a portable electronic device such as as a smartphone, while ensuring the acquisition of fingerprint images having a resolution and a sufficient contrast to be able to implement biometric identification processes.

In this regard, there is provided a fingerprint acquisition device according to claim 1.

The blocking zone, blocking arriving in a straight line from the acquisition surface and allowing light to pass obliquely, allows light to be blocked from the finger valleys while allowing the light coming from the peaks to pass through. . The resulting image thus shows a strong contrast between the light coming from a ridge and the light coming from a valley, the latter being mainly blocked by the occultation zone. There is therefore no loss of resolution due to the increase in the distance between the photodetectors and the acquisition surface of the sensor where the finger is affixed. The occultation zone also forms a narrow passage of light which angularly limits the catch of the lateral light, that is to say that only the light whose light path is aligned with the transparent passage can pass. This increases the spatial resolution of the device even with a thick transparent medium.

This device is advantageously completed by the following features, taken alone or in any of their technically possible combinations:

the occultation zone is adapted to block the light in a direction normal to the acquisition surface for at least 80% of the photosensitive surface of each photodetector;

the occultation zone is adapted to block the light in a direction normal to the acquisition surface for the entire photosensitive surface of each photodetector;

the occultation zone comprises at least:

a first opaque layer extending parallel to the acquisition surface,

a second opaque layer extending parallel to the acquisition surface and situated between the first opaque layer and the photodetectors,

the first opaque layer and the second opaque layer having non-aligned apertures in a direction normal to the acquisition surface and forming the transparent passages of the occlusion zone;

the occultation zone is adapted to block the light in a direction normal to the acquisition surface for the entire photosensitive surface of each photodetector, and the openings of the first opaque layer face the second opaque layer; outside the openings of said second opaque layer, and the openings of the second opaque layer face the opaque first layer outside the openings of said first opaque layer;

the openings of the second opaque layer are spatially offset in a direction parallel to the acquisition surface with respect to the openings of the first opaque layer, an opening of the second opaque layer corresponding to an opening of the first opaque layer by this offset; said opening of the first layer and said opening of the corresponding second opaque layer forming a pair of apertures defining a transparent passage of the occlusion zone;

opaque interconnections connect the first opaque layer and the second opaque layer and surround each pair of openings;

the propagation medium has a refractive index n greater than 1, 3, and: the transparent passages of the propagation medium are shaped to allow a light path between the acquisition surface and a point of a photosensitive surface of a photodetector when said light path forms with the normal to the acquisition surface a higher angle to arcsin (1 / n)

the occultation zone is shaped to block a light path between the acquisition surface and a point of a photosensitive surface of a photodetector when said light path forms with the normal to the acquisition surface an angle strictly less than arcsin (1 / n). The invention also relates to a portable electronic device provided with a fingerprint acquisition device according to any one of the embodiments of the invention.

The invention also relates to a method comprising the steps of:

- manufacture of a matrix of photodetectors,

manufacturing of the propagation medium containing the occultation zone, said occultation zone comprising transparent passages,

- Setting propagation medium relative to the photodetector matrix so that the transparent passages are aligned obliquely with the photodetectors.

PRESENTATION OF FIGURES

The invention will be better understood, thanks to the following description, which refers to embodiments and variants according to the present invention, given as non-limiting examples and explained with reference to the attached schematic drawings, in which: :

- Figures 1 and 2, already commented, schematically illustrate a finger placed on the surface of a sensor according to the state of the art and light paths from this finger to photodetectors,

FIG. 3 schematically illustrates the principle of a sensor according to one possible embodiment of the invention,

- Figure 4 schematically illustrates a sensor according to a possible embodiment of the invention, wherein the occultation zone comprises two opaque layers. - Figure 5 schematically illustrates a sensor according to a possible embodiment of the invention, wherein the occultation zone comprises two opaque layers connected by interconnection partitions.

In all the figures, the similar elements are designated by the same references.

DETAILED DESCRIPTION

With reference to FIGS. 3 to 5, a fingerprint acquisition device comprises an image sensor 1, said sensor 1 comprising a matrix 3 of photodetectors 4. Such a matrix 3 typically comprises several thousand photodetectors 4 in order to present an acceptable resolution. These photodetectors 4 are formed on a semiconductor substrate and can be of any type, since these photodetectors 4 are sensitive to light and thus make it possible to acquire an image. Preferably, these photodetectors 4 are photodiodes produced by CMOS technology. However, photodetectors may be of any detector type as long as they are sensitive to light, such as photodiodes or photoresistors. The sensor 1 comprises an acquisition surface 6 on the side opposite the substrate 5 and intended to receive a finger 100. Each photodetector 4 has a photosensitive surface 8 facing the acquisition surface 6. Generally, this photosensitive surface 8 corresponds to the extent of a doped area of the substrate 5 in which charges are generated by the action of light. In order to simplify, FIGS. 3 and 4 simply show the location of a photodetector 4. FIG. 5, which is more complete, also shows a transparent layer 10 placed above the photodetectors 4 towards the acquisition surface 6, which contains the metal interconnections 12 necessary for the operation of the photodetectors 4, in particular the conductive tracks and the transistors making it possible to read the photodetectors.

In FIGS. 3 to 5, a finger 100 is affixed to the acquisition surface 6 of the sensor 1. The sensor 1 is configured to acquire at least one image of the fingerprints of the finger 100 when said finger 100 is in contact with the surface acquisition of the sensor 6, as illustrated. The finger 100 has on its surface fingerprints comprising ridges 108 and valleys 109 between the crests. The ridges 108 are in contact with the acquisition surface 6 of the sensor, while the valleys 109 are separated by air.

Between the photodetectors and the acquisition surface, the sensor 1 comprises a propagation medium 14 adapted to let the light pass. The propagation medium 14 is therefore in transparent material. The propagation medium 14 is preferably transparent at wavelengths in the sensitive strip of silicon photodetector, ie between 200 nm and 1000 nm, such as for example visible light and / or infrared light. The propagation medium 14 may for example be in simple mineral glass, plastics such as PMMA or PC, sapphire etc.

The propagation medium 14 may be homogeneous and consist of a single material, or consist of several superimposed layers of materials with different characteristics. For example, a first mineral glass layer, such as a protective glass, may cover a second layer of another transparent material commonly used in microelectronics such as silicon dioxide S102, or silicon nitride S13N4. The two layers together form the propagation medium 14. Advantageously, it is in this second layer that hosts an occultation zone. This propagation medium 14 has a refractive index strictly greater than 1, that is to say greater than the air, and is preferably greater than 1.3, in particular in order to be able to capture a fingerprint of a finger. wet.

The propagation medium 14 extends between the photodetectors 4 and the acquisition surface 6. More specifically, the propagation medium 14 is disposed on the transparent layer 10 disposed above the photodetectors 4 in the direction of the acquisition surface. 6, which contains the metal interconnections 12. It is also possible that the metal interconnections 12 of the photodetectors 4 are contained in the propagation medium 14.

The surface of the propagation medium 14 on the side opposite to the photodetectors 4 constitutes the acquisition surface 6 of the sensor on which a finger 100 can be affixed to acquire the fingerprints. Preferably, the propagation medium 14 has a thickness, between the acquisition surface 6 and the photodetectors 4, greater than at least 10 μιη, and preferably at least 50 μιη. For example, the propagation medium 14 may constitute a protective layer with a thickness of between 400 μιη and 1000 μιη.

The propagation medium 14 comprises occulting elements 16 extending parallel to the acquisition surface 6. These occulting elements 16 are adapted to block any light 120 having passed through the acquisition surface 6 and propagating in the propagation medium 14 in a direction normal to the acquisition surface 6. To do this, the occultation zone comprises an opaque part 16a of opaque material, adapted to block the light in the wavelengths of the sensitive band of a silicon photodetector, ie between 200 nm and 1000 nm, such as visible light and infrared. The opaque material may for example be metal, blackened glass or tinted plastic.

The occultation zone 16 is adapted to completely block the light whose light path 120 is in a direction normal to the acquisition surface 6 passing through a point of the photosensitive surface 8 of each photodetector 4. Thus, the opaque portion 16a of the occultation zone 16 may be shaped to completely cover (ie 100%) the photosensitive surface 8 of each photodetector 4 in a direction normal to the acquisition surface 6.

Thus, in the example of FIG. 3, the opaque portion 16a of the occulting zone 16 completely covers the photosensitive surface 8 of each photodetector 4 in a direction normal to the acquisition surface 6. By cutting the sensor according to a plane perpendicular to the acquisition surface 6, a sectional plane comprising a point of the photosensitive surface 8 of a photodetector 4 also comprises, above this point, an opaque portion 16a of the occulting zone 16.

As a result, the light rays whose light path 120 in the propagation medium 14 is normal to the acquisition surface 6, are blocked by the occultation zone 16 and do not reach the photodetectors 4.

However, the occultation zone 16 has transparent passages 16b allowing light having passed through the acquisition surface 6 and propagating in the propagation medium 14 along a light path 122 with a direction different from a direction normal to the acquisition surface 6 to reach the photodetectors. FIGS. 3 to 5 show, by full arrows 122, light paths crossing the surface 6 and reaching a sensitive surface 8 of a photodetector 4 through a transparent passage 16b, and by dashed arrows 120, 121, 125 light paths passing through the acquisition surface 6 and not reaching the sensitive surface 8 of a photodetector 4.

These transparent passages 16b extend obliquely with respect to the normal to the acquisition surface 6. Thus, only the light rays propagating along a light path 122 with an angle 0b sufficient relative to the normal can reach the sensitive surface 8 photodetectors 4, whereas the light rays propagating along a light path 121 with an angle Q _a close to the normal are blocked by the occultation zone 16. For each photodetector 4, a transparent passage 16b allows oblique radii of reaching said photodetector 4, so that there are as many transparent passages 16b as photodetectors 4. Preferably, the transparent passages 16b are defined by holes in the opaque portion 16a of the occulting zone 16.

The dimensions of these transparent passages 16b are chosen to allow sufficient light rays propagating in the propagation medium 14 from a point of the acquisition surface 6 in a given angular range to reach the sensitive surface 8 of the photodetectors 4, while blocking others. For example, these dimensions can be determined as a function of the sensitive surface 8 of a photodetector 4. The ends of the sensitive surface 8 form, with the point of the acquisition surface 6 whose acquisition is to be acquired, a triangle defining the dimensions of the transparent passage 16b which must let the light rays pass.

The occultation zone 16 thus makes it possible to geometrically limit the origin of the light rays that reach the photodetectors 4. It is then possible to maintain a good resolution even when the distance between the photodetectors 4 and the finger 100 becomes significant, which is not the case. not the case with the devices of the state of the art such as that of Figure 2.

It is particularly advantageous to shape the transparent passages 16b to allow a light path 122 between a point of a photosensitive surface 8 of a photodetector 4 and the acquisition surface 6 when said light path forms with the normal to the surface of the photodetector. acquisition 6 an angle 0b greater than the critical angle Q _c of the interface between the air and the propagation medium 14, the propagation medium side 14. More specifically, the propagation medium 1 having a refractive index n greater than 1.3, the transparent passages 16b are shaped to allow a light path 122 between a point of a photosensitive surface 8 of a photodetector 4 and the surface of acquisition 6 when said light path 122 forms with the normal to the acquisition surface 6 an angle 0b greater than arcsine (1 / n).

Concerning the upper bound of the angles with the normal formed by the light paths 122 of the light rays that can pass through a transparent passage 16b of the occultation zone, it is sufficient to consider that the limit exposed above corresponds to a luminous path 122 of the radii light reaching one end of the photosensitive surface 8 of a photodetector 4 while the light rays with the highest angles able to pass the transparent passage 16b reach the opposite end of the photosensitive surface 8 of the same photodetector 4. Similarly, the occulting zone 16 may be shaped to block any light path 120, 121 between the acquisition surface 6 and a point of a photosensitive surface 8 of a photodetector when said light path forms with the normal to the surface of acquisition 6 an angle strictly less than the critical angle Q _c of the interface between the air and the propagation medium 14, on the middle side of propagation 14, for example when the light beam propagates along a light path 120 normal to the acquisition surface 6 towards a photodetector 4. More specifically, the shading area 16 can be shaped to block a light path 120, 121 between a point of a photosensitive surface 8 of a photodetector 4 and the acquisition surface 6 when said light path 120, 121 forms with the normal to the acquisition surface 6 an angle Q _a strictly less than arcsin (1 / n).

As indicated above, the finger 100 has at its surface fingerprints including ridges 108 and valleys 109 between the ridges 108. The ridges 108 are in contact with the acquisition surface 6 of the sensor 1, while the valleys 109 are separated by air. Thus, the light from a valley 109 must pass through an air-sensor interface, while the light from a peak 108 passes through a finger-sensor interface. Thus, the light coming from a valley 109 is reflected by the acquisition surface 6 and can not pass through the interface between the air and the acquisition surface 6 when the angle of incidence of the light path 125 on the acquisition surface is too high compared to the normal to the acquisition surface 6. On the contrary, the light coming from a peak 108 can pass through the acquisition surface 6 with a much higher angle of incidence. Consequently, by shading the shading zone 16 so that it passes through the transparent passages the light paths 122 having an angle greater than the critical angle Q _c , only the light coming from the peaks 108 can reach the photodetectors 4 This results in a strong contrast in the image acquired between the pixels corresponding to valleys 109 and pixels corresponding to peaks 108 since only these are luminous.

However, it may be difficult to drill oblique holes in an opaque layer to form the transparent passages 16b of the occultation zone 16. It is possible to simplify the manufacture of the sensor 1 with an occultation zone 16 consisting of several opaque layers superimposed, having non-aligned through holes, which are not necessarily oblique.

Preferably, and as illustrated in FIGS. 4 and 5, the occultation zone thus comprises at least a first opaque layer 21 extending parallel to the acquisition surface, and a second opaque layer 22 extending in parallel to the acquisition area. The second opaque layer 22 is located between the opaque first layer

21 and the photodetectors 4. The first opaque layer 21 and the second opaque layer

22 comprise openings 23, 24 not aligned in a direction normal to the acquisition surface 6 and which form the transparent passages 16b of the occultation zone 16.

Each opaque layer 21, 22 thus comprises as many openings 23, 24 as there are transparent passages 16b and therefore photodetectors 4. The light propagating along a normal light path to the acquisition surface 6 is then blocked either by the first opaque layer 21, or by the second opaque layer 22.

As previously, the materials constituting an opaque layer 21, 22 are chosen to block the light in the wavelengths of the sensitive band of a silicon photodetector, ie between 200 nm and 1000 nm, such as visible light. and the infrared. The material may for example be metal, blackened glass or tinted plastic.

The two opaque layers 21, 22 are preferably separated, in a direction normal to the acquisition surface 6, by a distance of at least 0.4 μιη. An opaque layer 21, 22 has for example a thickness of 0.1 to 0.5 μιη. Preferably, the openings 24 of the second opaque layer 22 are spatially offset in a direction parallel to the acquisition surface 6 with respect to the openings 23 of the first opaque layer 21, an opening 24 of the second opaque layer 22 corresponding to a opening 23 of the first opaque layer 21 by this offset, said opening 23 of the first layer 21 and said opening 24 of the second opaque layer 22 corresponding to a pair of openings defining a transparent passage 16b for the light.

The offset between the respective apertures 23, 24 of the opaque layers 21, 22 determines the amount of light propagating in a direction normal to the acquisition surface 6 which is blocked, as well as the angular range with the normal to the surface of acquisition 6 allowing the light rays to reach the photodetectors 4. When the shading area 16 is adapted to block the light in a direction normal to the acquisition surface for the entire photosensitive surface 8 of each photodetector 4, these openings 23, 24 are completely offset between the two opaque layers 21, 22. Thus, the openings 23 of the first opaque layer 21 face the second opaque layer 22 outside the openings 24 of said second opaque layer 22, and the openings 24 of the second opaque layer 22 face the first opaque layer 21 outside the openings 23 of said first opaque layer 21.

It is possible, however, to tolerate an overlap between the apertures 23 of the first opaque layer 21 and the openings 24 of the second opaque layer 22. In this case, the occultation zone 16 constituted by the two opaque layers 21, 22 does not block not all the light propagating in a direction normal to the acquisition surface 6. However, the overlap of the openings 23, 24 is preferably limited to less than 50% of the area of the opening 23, 24 most small, more preferably less than 20%, and more preferably less than 10%. For example, when the opaque layers 21, 22 are sufficiently thick, it is possible that they are directly superimposed (with a zero distance between them), in which case the oblique appearance of a transparent passage 16b is obtained by a shift of respective openings 23, 24 of the first opaque layer 21 and the second opaque layer 22, with an overlap between an opening 23 of the first opaque layer 21 and an opening 24 of the second opaque layer 22. As illustrated in FIG. 5, opaque interconnecting partitions 30 can connect the first opaque layer 21 and the second opaque layer 22 and surround each pair openings 23, 24, to prevent light rays can propagate laterally between the two opaque layers 21, 22. Indeed, depending in particular on the distance between the two opaque layers 21, 22, it is possible that light rays propagating at a large angle relative to the normal, pass through an opening 23 of the first opaque layer 21, and reach an opening 24 of the second opaque layer 22 different from the opening 24 of the second opaque layer 22 which forms, with the opening 23 of the first opaque layer 21, a transparent passage 16b. The opaque interconnection partitions 30 block light rays, which would propagate between the two opaque layers 21, 22 outside the transparent passages 16b. These partitions therefore separate the different passages from each other.

To manufacture a sensor as described, a CMOS method can be used. The photodetectors 4 may be either PN junction photodiodes or phototransistors. The occultation zone 16 can be made by metallization layers which are naturally opaque to light in a layer made of transparent silicon dioxide. The occultation zone 16 thus produced is then composed of metallization layers of different levels. The opaque interconnections 30 may be made similarly to vias of contact between these different levels of metallization. In this way, the sensor of the present invention can be entirely manufactured in a standard CMOS manufacturing plant.

A layer of transparent material such as glass or plastic can be glued to the surface of the CMOS sensor to protect it mechanically and electrically. This sensor can also be glued behind the protective glass of a mobile phone to perform fingerprint applications. The propagation medium 14 is then composed of the transparent layer containing the occultation zone 16 and this protective layer.

The sensor according to the present invention can also be produced by combining a CMOS sensor and an occultation zone 16 made on a transparent medium constituting the propagation medium. This occulting zone 16 is then transferred to the CMOS sensor by obliquely aligning the transparent passages 16b with the photodetectors of the CMOS sensor. Typically this occultation zone 16 may be made on glass with metallization levels having non-aligned openings. Of course, the above methods are only non-limiting examples. In all embodiments, the fingerprint acquisition device may comprise a pressure sensitive member arranged to emit a signal controlling the acquisition of the image when the finger exerts pressure on the device. The pressure sensitive member may for example be an electromechanical switch or a pressure sensor measuring the pressure.

A fingerprint acquisition device as described herein is preferably incorporated into a portable electronic device such as a smartphone to acquire fingerprints of a user of the electronic device.

The invention is not limited to the embodiment described and shown in the accompanying figures. Modifications are possible, particularly from the point of view of the constitution of the various elements or by substitution of technical equivalents, without departing from the scope of protection of the invention.

Claims

1. Device for acquiring fingerprint images, the device comprising

An acquisition surface (6) on which a finger can be placed, a matrix (3) of photodetectors (4),

A propagation medium (1) of light extending between the acquisition surface (6) and the matrix (3) of photodetectors (4),

Occulting elements (16) extending in the medium and defining passages allowing light having penetrated into the device through the acquisition surface and propagating in the medium in a direction oblique to the surface of the device; acquisition to reach the matrix (3) of photodetectors (4),

the device being characterized in that the occulting elements (16) are arranged to prevent any light having penetrated into the device by the acquisition surface and propagating in the medium in a direction normal to the acquisition surface (6). ) to reach the matrix (3) of photodetectors (4).

2. Device according to claim 1, wherein the occulting elements define, for each passage, an input facing the acquisition surface, and an output facing the array of photodetectors, wherein an input and an output of the same passage are non-aligned in the normal direction to the acquisition surface.

3. Device according to one of claims 1 and 2, wherein an inlet and an outlet of the same passage are not superimposed in the direction normal to the acquisition surface.

4. Device according to one of claims 1 to 3, wherein the occulting elements comprise a first opaque layer (21) and a second opaque layer (22) between the first opaque layer and the photodetector matrix, wherein the opaque first layer has apertures forming passageways, and the second opaque layer (22) has openings forming passages of the passages.

5. Device according to claim 4, wherein the first layer and the second layer are parallel to the acquisition surface.

6. Device according to one of claims 4 and 5, comprising opaque interconnection partitions (30) connecting the first opaque layer (21) to the second opaque layer (22) and separating the passages from each other.

7. Device according to one of claims 1 to 3, wherein the occulting elements comprise an opaque layer having a first surface facing the acquisition surface, and a second surface opposite to the first surface relative to the layer opaque and facing the matrix of photodetectors, wherein the passages open into the first surface and the second surface.

8. Device according to claim 7, wherein the input surface and the output surface are parallel to the acquisition surface.

9. Device according to one of claims 1 to 8, wherein each passage defined by the occluding elements has only one input and one output.

10. Device according to one of claims 1 to 9, wherein the propagation medium (14) has a refractive index n greater than 1, 3, and the occulting elements (16) are arranged to prevent any light having penetrated into the device by the acquisition surface (6), and propagating in the propagation medium in a direction forming an angle strictly less than arcsine (1 / n) with respect to the normal direction, to reach the matrix of photodetectors.

1 1. A portable electronic apparatus comprising a fingerprint acquisition device according to any one of the preceding claims.