FR2934084A1 - IMAGER PIXEL IN REAR-BACK TECHNOLOGY AND IMAGE SENSOR - Google Patents

Abstract

The invention relates to a rear-facing type of imaging pixel which comprises a crystalline silicon absorption layer (4) in which at least one carrier-collecting electrode (7) is formed on which a first potential is applied ( V1), characterized in that it further comprises: - an amorphous silicon absorption layer (5) fixed on one side of the crystalline silicon layer (4) and in which a diode (5a, 5b) is formed ), and - a transparent electrode (6) fixed on the amorphous silicon absorption layer (5) and on which a second potential (V2) is applied, the potential difference between the second potential and the first potential leading to a reverse bias of the diode. Application to imaging.

Description

TECHNICAL FIELD AND PRIOR ART The invention relates to an imager pixel in back-type technology and an associated image sensor. BACKGROUND OF THE INVENTION

One field of application of the invention is the field of imaging. From a technological point of view, the invention applies to CMOS (CMOS for COMPLEMENTARY METAL-OXIDE SEMICONDUCTOR) technology and to hydrogenated amorphous silicon technology. At present, the production of image sensors is based on either CCD (Charge Coupled Device) technology or CMOS technology. CMOS technology is preferred, however, as CMOS manufacturing lines are more widespread than CCD manufacturing lines. The conversion of photons into electron-hole pairs is the basis for the operation of image sensors. In general, the digital acquisition of an image is achieved by the projection of an image on an imaging circuit, which causes the conversion at each point of the image of the luminous flux into electric charges, these charges being periodically measured by an electronic circuit and the value of this measurement converted into digital data. The elementary cell for converting an image sensor is the pixel. While the pixels occupy a smaller and smaller surface, with a pitch substantially less than 2 μm, the thickness of material that is needed to absorb the photons does not vary because it depends on the wavelength of the light absorbed and absorbing material such as, for example, crystalline silicon noted cSi. Thus, to absorb 90% of the radiation at a wavelength of 625 nm (red color), a thickness of about 7.5 μm of crystalline silicon cSi is required. This large thickness poses a crosstalk problem because it is important compared to the width of the pixel. Crosstalk may be optical, i.e., light radiation entering the pixel non-perpendicular to the surface may become absorbed in a neighboring pixel due to the angle of incidence. The crosstalk can also be electronic, that is, a photogenerated electron in one area of the pixel can be collected by a neighboring pixel.

To limit the effects of crosstalk, the conversion efficiency of long wavelengths that require a substantial material thickness to be absorbed is sacrificed by decreasing the active photogeneration thickness by means of deep doping which promotes recombination or by means of a layer of buried insulation at the desired depth. This layer of insulation is used with advantage in a back-side architecture which, by the absence of interconnection track on the input side of the luminous flux, advantageously allows a wide angle for access to 3 silicon by light rays. These large angles are necessary to increase the number of photons arriving on the sensor and to obtain the spatial resolution compatible with pixels of limited size.

In return, these large angles increase the optical crosstalk. A complementary technique for limiting electronic cross-talk is to create weak electric fields by producing doping gradients favoring the path of the photogenerated electrons towards the collection zone of the pixel situated closest to the photogeneration site. All of these prior art techniques provide a poor compromise between crosstalk and pixel sensitivity. The invention does not have this disadvantage.

SUMMARY OF THE INVENTION Indeed, the invention relates to a rear-type technology imager pixel which comprises a crystalline silicon absorption layer in which at least one carrier-collecting electrode is formed on which a The first potential, characterized in that it further comprises: an amorphous silicon absorption layer fixed on one side of the crystalline silicon layer and in which a diode is formed, and a transparent electrode attached to the amorphous silicon absorption layer and on which is applied a second potential, the potential difference between the second potential and the first potential leading to a reverse bias of the diode.

According to a further characteristic of the invention, the amorphous silicon absorption layer has a thickness of less than 50 nm. This feature allows carriers created by photoelectric effect in the amorphous silicon area to reach the crystalline silicon layer by diffusion. Advantageously, the crystalline silicon layer comprises, near the amorphous silicon absorption layer, a crystalline silicon zone whose doping differs from that of the rest of the crystalline silicon layer. Preferably, the crystalline silicon zone is in contact with the amorphous silicon layer.

Advantageously, the crystalline silicon zone is located at a distance from the amorphous silicon layer of less than 100 nm. Such a zone of crystalline silicon doped with a doping different from that of the crystalline silicon layer has the considerable advantage of making it possible to adjust the electronic properties of the heterostructure. More precisely, it makes it possible to reduce the residual leakage current due to the electric field created by the potential difference without substantially increasing the recombination of the carriers.

Advantageously, the amorphous silicon layer comprises, near the crystalline silicon absorption layer, and additionally to the diode, an amorphous silicon zone whose doping is modified relative to the rest of the amorphous silicon layer. Preferably, the amorphous silicon zone is located in contact with the crystalline silicon layer.

Advantageously, the amorphous silicon zone is located at a distance from the crystalline silicon layer of less than 10 nm. Such a region of doped amorphous silicon with modified doping has the considerable advantage of allowing to adjust the electronic properties of the heterostructure. More precisely, it makes it possible to reduce the residual leakage current due to the electric field created by the potential difference ΔV without substantially increasing the recombination of the carriers.

According to another additional feature of the invention, the potential difference is adjusted so that carriers created by photoelectric effect in the amorphous silicon zone reach the crystalline silicon layer by conduction under the effect of the electric field associated with the difference. potential. For imager pixels having identical semiconductor thicknesses, an imager pixel of the invention allows much larger photon absorption than an imager 6 of the prior art. The detection of images is very significantly improved.

BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will appear on reading a preferred embodiment with reference to the appended figures among which: FIG. 1 represents a first example of an imager pixel of FIG. invention; FIG. 2 represents a second example of an imager pixel of the invention; FIG. 3 represents a third example of an imager pixel of the invention; FIG. 4 represents a fourth example of an imager pixel of the invention; FIG. 5 represents a cross-sectional view of a rear-type technology imager according to the prior art; FIG. 6 represents a cross-sectional view of a first example of a rear-type technology imager according to the invention; FIG. 7 represents a cross-sectional view of a second example of a back-type technology imager according to the invention; FIG. 8 represents a cross-sectional view of a third example of a rear-type technology imager according to the invention; FIG. 9 represents a cross-sectional view of a fourth example of a rear-type technology imager according to the invention; DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION FIG. 1 represents a first exemplary imager pixel of the invention. The pixel comprises a support 1, a transfer layer 2, a PMD layer 3 (PMD for Physical Media Dependent), a crystalline silicon layer 4, an amorphous silicon layer 5, a transparent electrode 6, a carrier collection electrode 7 and, optionally, a passivation layer 8 which covers the transparent electrode 6. A diode is formed in the amorphous silicon layer 5. The amorphous silicon layer 5 thus comprises, for example, an intrinsic amorphous silicon layer 5a and a layer of P + 5b doped amorphous silicon. For a crystalline silicon layer 4 whose thickness is, for example, between 1pm and 5pm, the thickness of the layer 5a is, for example, between 5nm and 500nm and the thickness of the layer 5b between, for example for example, 5nm and 50nm. A first potential V1 is applied to the collecting electrode 7 and a second potential V2 to the transparent electrode 6. The potential difference AV (AV = V2-V1) polarizes the diode in reverse, thus creating, in the amorphous silicon, a zone devoid of free carriers. The presence of this zone devoid of free carriers, commonly called depleted zone (depletion zone in English language), aims to avoid recombinations in amorphous silicon.

The potential difference AV is, for example, equal to -2V. For a sufficient amplitude of the difference of 8 AV potential, photogenerated electrons in the area of amorphous silicon 5 can then reach the crystalline silicon layer 4 under the action of the electric field associated with the potential difference AV. Once the crystalline silicon layer has been reached, the photogenerated electrons reach the carrier collecting electrode 7 under the action of the electric field associated with the potential difference ΔV for a first part of their path and by diffusion for the remainder of their path . A larger AV potential difference lengthens the portion of the path that is performed under the action of the electric field. It should be noted that the amorphous silicon absorption layer 5 and the crystalline silicon layer 4 form a structure which must allow the photogenerated electrons in the amorphous silicon layer 5 to reach the collector electrode of carriers 7. Such structure has a conduction band. In known manner, the conduction band of a semiconductor is the energy band in which the free electrons circulate. The conduction band of the structure of the invention is therefore chosen so that there is no potential barrier preventing the photogenerated carriers from joining the crystalline silicon. In a manner known per se, this choice of the conduction band depends on the semiconductor material (amorphous or crystalline silicon), the doping of the semiconductor as well as the voltage applied to the semiconductor.

The transfer layer 2, for example a TEOS layer (TEOS for Tetra-Ethyl-Ortho-Silicate) makes it possible, in a manner known per se, to fix the layer of PMD 3 on the support 1. Also in a manner known per se , the carrier collection electrode 7 formed in the crystalline silicon layer 4 is connected to a signal reading circuit (not shown in the figure). According to the example described above, the photogenerated carriers which are collected by the electrode 7 are electrons. The invention however relates to other embodiments in which it is the photogenerated holes that are collected by the electrode 7. These other embodiments are not detailed in the present description, because the skilled person who knows a Embodiment of the invention in which the collected carriers are electrons can transpose, without particular difficulty, the invention to embodiments where they are holes that are collected. Fig. 2 shows a second exemplary imager pixel of the invention. According to this second example, the amorphous silicon layer 5 has a thickness that is sufficiently small for the carriers created there to reach the collecting electrode 7 by diffusion. The potential difference AV which creates the depleted zone in the amorphous silicon does not participate. here to a conduction of carriers to the electrode. The thickness of the layer 5 is then substantially between, for example, 2 nm and 10 nm for a crystalline silicon layer thickness 4 of between lpm and 5 pm. According to a variant as shown in Figure 3, the crystalline silicon layer 4 is P-doped, for example. The crystalline silicon layer 4 comprises, in the vicinity of the absorption layer 5a, a zone of crystalline silicon 9 whose doping differs from that of the layer 4, for example an N-type doping. The thickness of this zone 9 is for example equal to 100 nm. More specifically, the zone 9 is in contact with the amorphous silicon layer 5a, as shown in FIG. 3. According to a variant, the zone 9 is located at a distance from the layer 5a. The distance between the layer 5a and the zone 9 is less than 100 nm. The formation of such a crystalline silicon zone 9 is intended to allow the electronic properties of the heterostructure to be adjusted. In particular, it makes it possible to reduce the residual leakage current due to the electric field created by the potential difference ΔV without substantially increasing the recombination of the carriers. According to a variant as shown in Figure 4, the amorphous silicon layer 5a is intrinsic, for example. The amorphous silicon layer 5a comprises, in the vicinity of the absorption layer 4, an amorphous silicon zone 10 whose doping differs from that of the layer 5a, for example an N-type doping. The thickness of this zone 10 is for example equal to 10 nm.

More specifically, the zone 10 is in contact with the crystalline silicon layer 4, as shown in FIG. 4. It should be noted that if the crystalline silicon layer includes a zone 9, the zone 10 is then in contact with the crystalline silicon layer 4. of the zone 9. According to one variant, the zone 10 is located at a distance from the layer 4. The distance between the layer 4 and the zone 10 is less than 10 nm. The formation of such an amorphous silicon zone 10 is intended to allow the electronic properties of the heterostructure to be adjusted. In particular, it makes it possible to reduce the residual leakage current due to the electric field created by the potential difference ΔV without substantially increasing the recombination of the carriers. A rear-side image sensor (backside) of the known art is shown in FIG. 5. The image sensor comprises a support 1, a transfer layer 2, a PMD layer 3 in which are integrated metallizations ml , m2, a crystalline silicon layer 4 in which conductive electrodes 7 are formed electrically connected to the metallizations ml and m2, a passivation layer 8 placed above the crystalline silicon layer 4, FC red color filters R, green V and blue B placed above the passivation layer 8, a planarization layer P which covers the colored filters and ML micro-lenses which cover the planarization layer P. Concretely, a method of manufacturing an image sensor According to the invention, up to the non-included surface passivation step, there is a succession of steps according to those known for the manufacture of a back-side image sensor of the prior art. FIG. 6 represents an image sensor of the invention consisting of a set of pixels such as those represented in FIG. 1. Beyond the known sequence of steps mentioned previously, the image sensor manufacturing method of the invention comprises the following steps: - deposition of the amorphous silicon layer 5 with formation of diodes in the amorphous silicon layer (as previously mentioned, a diode may be formed by association of a first intrinsic zone I (5a) and a second P + doped zone (5b): It is also possible to form a diode by a first N-doped zone, a second intrinsic I zone, and a third P-doped zone; depositing a transparent electrode 6 on the amorphous silicon layer 5; depositing a passivation layer 8 on the transparent electrode 6; depositing FC: R, V, B color filters on the passivation layer 8; depositing a planarization layer P on the color filters R, V, B; deposit of micro-lenses ML on the passivation layer P. FIG. 7 represents a first improvement of the image sensor represented in FIG. 6. According to this first improvement, a lateral doping variation is produced in the thickness of the amorphous silicon layer 5. The lateral doping variation can be performed throughout the thickness of the amorphous silicon layer or only in the thickness of the intrinsic zone of the amorphous silicon layer. By lateral doping variation, it is to be understood that the doping varies in connection with the color filters: a first doping d1 is carried out under the red color filters R, a second doping d2 is carried out under the green color filters V and a third doping d3 is carried out under the blue color filters B. By way of non-limiting example, the doping is a carbon or germanium doping. Advantageously, this doping modifies the absorption capacity of the light by the amorphous silicon, which favors the wavelengths selected by the color filters and disadvantages the other wavelengths, thus increasing the selectivity of the color filters. FIG. 8 represents a second improvement of the image sensor represented in FIG. 6. According to this second improvement, the transparent electrode 6 is not formed of a single block but of three distinct electrode blocks distributed in connection with the colored filters. A first electrode block is associated with the red color filter R, a second electrode block is associated with the green color filter V and a third electrode block is associated with the blue color filter B. Different potentials can then be applied to the different electrode blocks. It advantageously follows an effect of spectral selectivity which enhances the effect of the color filter. Figure 9 shows a cross-sectional view of an image sensor according to a third improvement of the invention. According to the third improvement of the invention, there is no amorphous silicon layer under the blue B color filters whereas this layer is present, as previously described, under the green V and R red color filters. Only the layer passivation 8 then separates the blue B color filters from the crystalline silicon layer 4. Such a structure advantageously makes it possible to benefit from the weakest crystalline silicon absorption, sufficient absorption for the short wavelengths but insufficient for the lengths high waves, which consequently favors the detection of blue compared to the detection of green and red. The various image sensors of the invention advantageously operate with a maximum reflection on the lower structure which, for the low thicknesses accessible thanks to the amorphous silicon aSi, can then be optimized in thickness or index for each pixel, thus favoring the absorption according to the color detected by the pixel.

Claims (18)

REVENDICATIONS1. Imaging pixel in back-side type technology which comprises a crystalline silicon absorption layer (4) in which is formed at least one carrier-collecting electrode (7) to which a first potential is applied, characterized in that it further comprises: an amorphous silicon absorption layer (5) fixed on one side of the crystalline silicon layer (4) and in which a diode (5a, 5b) is formed, and a transparent electrode ( 6) fixed on the amorphous silicon absorption layer (5) and on which a second potential is applied, the potential difference between the second potential and the first potential leading to a reverse bias of the diode. An imaging pixel according to claim 1, wherein the amorphous silicon absorption layer (5) has a thickness such that photoelectrically generated carriers in the amorphous silicon area reach the crystalline silicon layer (4). by diffusion. An imaging pixel according to claim 1, wherein the potential difference is adjusted so that carriers created by photoelectric effect in the amorphous silicon area reach the crystalline silicon layer (4) by conduction under the effect. an electric field associated with the potential difference. An imaging pixel according to any one of claims 1 to 3, wherein the diode is formed by a first region of intrinsic amorphous silicon (5a) in contact with the crystalline silicon layer (4) and a second region of P + doped amorphous silicon (5b) above the first zone (5a). 5. Imager pixel according to any one of claims 1 to 3, wherein the diode is formed by a first region of N-doped amorphous silicon in contact with the crystalline silicon layer (4), a second region of amorphous silicon intrinsically above the first zone and a third region of P + doped amorphous silicon above the second zone. An imaging pixel according to any one of claims 1 to 5, wherein the crystalline silicon absorption layer (4) comprises, in the vicinity of the amorphous silicon absorption layer (5a), a region of crystalline silicon (9) having a doping which differs from the doping of the remainder of the crystalline silicon absorption layer (4). An imaging pixel according to claim 6, wherein the crystalline silicon area (9) is in contact with the amorphous silicon absorption layer (5a). 17 An imaging pixel according to claim 6, wherein the crystalline silicon area (9) is located at a distance from the amorphous silicon absorption layer (5a) of less than 100 nm. An imaging pixel according to any one of claims 1 to 5, wherein the amorphous silicon absorption layer (5a) has, in the vicinity of the crystalline silicon absorption layer (4), an area of amorphous silicon (10) having a doping which differs from a doping of the remainder of the amorphous silicon absorption layer (5a). An imaging pixel according to claim 9, wherein the amorphous silicon area (10) is in contact with the crystalline silicon absorption layer (4). An imaging pixel according to claim 9, wherein the amorphous silicon area (10) is located at a distance from the crystalline silicon absorption layer (4) of less than 10 nm. An imaging pixel according to claims 6 and 9, wherein the crystalline silicon area (9) is in contact with the amorphous silicon area (10). 30 An imaging pixel according to any one of claims 1 to 12, wherein a transparent passivation layer (8) is attached to the transparent electrode (6). The imaging pixel of claim 13, wherein a color filter (R, V, B) is placed on the transparent passivation layer (8). An imaging pixel according to any one of claims 1 to 12, wherein a color filter (R, V, B) is placed on the transparent electrode (6). An image sensor comprising a plurality of imager pixels according to claim 14 and / or claim 15, the color filters of the different imager pixels being red (R) filters, green (V) filters or blue filters (B), in which the amorphous silicon layer (5) of the different pixels is differently doped according to whether the color filter is a red filter, a green filter or a blue filter. An image sensor comprising a plurality of imager pixels according to claim 14 and / or claim 15, the color filters of the different imager pixels being red (R) filters, green (V) filters or blue filters (B), wherein the transparent electrodes of the imager pixels having the same color filter color are electrically interconnected and electrically isolated from the transparent electrodes of the imager pixels having a different color filter color. An image sensor comprising a plurality of imager pixels having red, green, and blue color color filters, wherein the imager pixels having red color filters and the imager pixels having green color filters are pixels according to claim 14 or claim 15, the blue color imager pixels being devoid of amorphous silicon layer, a blue color filter not being separated from the color layer. crystalline silicon (4) only by the single passivation layer (8).