Image sensor for the visible and infrared range having an absorbing layer stack

An image sensor including a plurality of pixels, each including: a photodetector semiconductor region; a metal region arranged on a first surface of the semiconductor region; a band-pass or band-stop interference filter arranged on a second surface of the semiconductor region opposite to the first surface; and between the semiconductor region and the metal region, an absorbing stack comprising, in the order from the semiconductor region, a dielectric layer, a silicon layer, and a tungsten layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a translation of and claims the priority benefit of French patent application number 20/09549, filed on Sep. 21, 2020. The contents of which is hereby incorporated by reference in its entirety.

TECHNICAL BACKGROUND

The present disclosure generally concerns electronic devices, and more particularly aims at an image sensor.

PRIOR ART

Conventionally, an image sensor enables to obtain images of a scene in the visible and/or in the infrared range.

For example, the sensor captures the visible radiations originating from a scene to obtain a visible image. The image corresponds to radiations captured in one or a plurality of wavebands, for example, in three wavebands respectively corresponding to the blue, green, and red colors.

Certain image sensors use infrared radiations to obtain a three-dimensional image of the scene. For example, the sensor is associated with an infrared pulse emitter. The travel time of the pulses from the emitter to the scene, and then from the scene to the sensor, delivers depth information. The three-dimensional image is obtained from the depth information. Such a sensor is called time of flight sensor TOF. Such a sensor captures the radiations in a waveband corresponding to the wavelengths of the pulses emitted by the emitter. This band is typically located in near infrared, that is, infrareds having wavelengths smaller than 1,100 nm. The sensor may deliver the depth map of the scene only, or the three-dimensional image formed of the depth map combined with the visible image.

An image sensor comprises a plurality of pixels generally arranged in an array. The radiations of each waveband of interest of the sensor are specifically captured by pixels, distributed in the array, mainly sensitive to the radiations in this waveband.

In practice, when a pixel is designed to be sensitive to radiations in a waveband, this pixel is also sensitive to radiations located outside of the band. The radiations located outside of the targeted band form parasitic radiations having their detection by the pixel decreasing the quality of the image. It is thus desirable for pixels designed to be sensitive to radiations in one of the wavebands to be as little sensitive as possible to the radiations located outside of this band.

SUMMARY

An embodiment provides an image sensor comprising a plurality of pixels, each comprising:a photodetector semiconductor region;a metal region arranged on a first surface of the semiconductor region;a band-pass or band-stop interference filter arranged on a second surface of the semiconductor region opposite to the first surface; andbetween the semiconductor region and the metal region, an absorbing stack comprising, in the order from the semiconductor region, a dielectric layer, a silicon layer, and a tungsten layer.

According to an embodiment, the absorbing stack is capable of absorbing, in a single passage, more than 50% of an incident radiation at the central wavelength of the passband or of the stop band of the interference filter.

According to an embodiment, in each pixel, the semiconductor layer is made of silicon.

According to an embodiment, the silicon layer has a thickness in the range from 20 to 100 nm.

According to an embodiment, the tungsten layer has a thickness greater than or equal to 40 nm.

According to an embodiment, the dielectric layer comprises one or a plurality of dielectric materials having refraction indexes smaller than that of silicon.

According to an embodiment, in each pixel, the tungsten layer is coupled to a node of application of a bias potential.

According to an embodiment, in each pixel, the interference filter comprises a repetition of alternated layers having different optical indexes.

According to an embodiment, in each pixel, the central wavelength is in a wavelength range from 700 nm to 1,100 nm.

According to an embodiment, each pixel comprises an additional colored or infrared filter covering the first surface of the semiconductor region.

According to an embodiment, the pixels are configured to detect visible light, the sensor further comprising a plurality of depth pixels configured to detect infrared light.

According to an embodiment, the absorbing stack is present in the depth pixels.

According to an embodiment, the absorbing stack is not present in the depth pixels.

DESCRIPTION OF THE EMBODIMENTS

For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, elements of the image sensor such as circuits comprising transistors allowing the operation of pixels are not detailed, the described embodiments being compatible with usual circuits allowing the operation of the pixels of an image sensor. Further, the selection of thicknesses of layers of antireflection stacks for a given wavelength, and of band pass or band stop filters in given wavebands, are not described in detail, the described stacks and filters being compatible with usual methods enabling to select the thicknesses of the layers of antireflection stacks and of interference filters.

In the following disclosure, unless otherwise specified, when reference is made to absolute positional qualifiers, such as the terms “front”, “back”, “top”, “bottom”, “left”, “right”, etc., or to relative positional qualifiers, such as the terms “above”, “below”, “upper”, “lower”, etc., or to qualifiers of orientation, such as “horizontal”, “vertical”, etc., reference is made to the orientation shown in the figures.

Unless specified otherwise, the optical index designates the real part of the complex optical index.

FIG.1is a cross-section view schematically showing an example of a pixel100of an image sensor. The pixel is for example located in an array of pixels and has its lateral edges in contact with neighboring pixels.

In the orientation of the drawing, the sensor is intended to receive an optical radiation arriving from the top of the pixels. The term optical radiation here designates a visible radiation or a radiation located in near infrared. The optical radiation typically originates from an element of a scene located opposite the pixel.

Pixel100comprises a semiconductor photodetector region110, typically made of silicon. Semiconductor region110may be delimited by insulating trenches115filled with an electric insulator, for example, silicon oxide. As a variation, the trenches comprise conductors insulated from region110. Trenches115separate the various semiconductor regions110from the neighboring pixels of the image sensor.

As an example, the pixel extends laterally across a width in the range from 1.2 to 5 micrometers, for example, in the order of 1.4 micrometers. Region110typically has a thickness, corresponding to the height in the orientation of the drawing, in the range from 2 to 10 micrometers, for example, in the order of 6 micrometers.

Semiconductor region110has its lower surface, or front surface, covered with an insulating layer120. Metal regions122are located in layer120. Regions122are typically conductive elements such as conductive tracks and/or conductors filling vias. The conductive elements connect to one another components, not shown, such as transistors, which allow the operation of the pixel and/or of various portions of the image sensor.

In the shown example, the pixel comprises a lens130on the optical radiation incoming side. Lens130enables to focus the optical radiations towards photodetector region110.

Further, pixel100may comprise a filter140covering the surface of photodetector region110located on the side on which the optical radiations arrive. Filter140is preferably located between lens130and photodetector region110. Filter140is used to select the wavelengths of the radiations detected by the pixel. Preferably, filter140is an organic filter, for example, made of resin. Filter140may form an infrared filter, that is, giving way to optical radiations in infrared wavelengths, preferably in near infrared. Filter140may form a colored filter, that is, giving way to optical radiations in a waveband in the visible range, preferably corresponding to the red, green, or blue color.

The pixel may further comprise an antireflection layer150. Layer150may comprise one or a plurality of dielectric layers. Antireflection layer150is provided to limit the upward reflection of optical radiations at the wavelengths detected by the pixel.

Pixel100further comprises an interference filter160. An interference filter is a stack of a plurality of layers having alternated optical indexes. As an example, the interference filter comprises, preferably is formed by, an alternation of first and second layers, the first layers being identical to one another and the second layers being identical to one another. Preferably, the interference filter comprises at least two, for example, at least three, first layers and at least two, for example, at least three, second layers. As an example, the first layers are dielectric, and the second layers are dielectric and/or are metal layers sufficiently thin to be at least partially transparent. The interference filter is preferably formed by silicon oxide layers, having silicon or titanium oxide or silicon nitride layers interposed therebetween. The thickness of the interference filter is preferably smaller than the width of region110, for example, smaller than 2.0 micrometers. Interference filter160may be a band stop or band pass filter, that is, respectively block or give way to radiations in a waveband.

FIG.2is a diagram schematically illustrating, as an example, the variation according to wavelength λ (in abscissas, in nm) of the transmission T200 (in ordinates, between 0% and 100%) of the interference filter160of the pixel ofFIG.1, in the case where filter160is a band stop filter. The transmission is defined, for a radiation reaching the upper surface of filter160and substantially orthogonal to the mean plane of the layers of filter160, as being equal to the percentage of this radiation which comes out of the lower surface of the filter.

The band stop interference filter is designed to block radiations in near infrared, around a central wavelength λ0for example in the range from 700 to 1,100 nanometers (in the order of 875 nm in the shown example). Central wavelength λ0for example corresponds to a transmission minimum. Preferably, filter160gives way to less than 30% of the radiation at wavelength λ0. Preferably, the interference filter is designed so that the transmission is low, for example, lower than 50%, in a wavelength range210, and high, for example, higher than 75%, outside of a wavelength range220including range210. Range210defines the stop band of the filter. As an example, range210has a width greater than 150 nm and is entirely between 700 and 1,100 nanometers, and range220has a width greater than 250 nm. The difference between the widths of ranges220and210is for example in the range from 100 to 200 nm.

In the present example, interference filter160comprises seven alternated layers of silicon oxide and of silicon nitride. The total thickness of the filter is in the order of 1.0 micrometer.

Such a band stop interference filter is preferably used in near infrared when the pixel is provided to detect visible light. Optional filter140then is a colored filter. Although filter140is designed to only give way to radiations of a waveband in the visible range, in practice, when filter140is for example made of resin, filter140also gives way to radiations in near infrared. Interference filter160then aims at limiting the detection of radiations in near infrared.

The inventors have observed that in fact, despite the presence of interference filter160, radiations in the waveband partially cut by interference filter160remain detected by detection region110. Thereby, in the image sensor, the detection of near infrared radiations adds to the detection of visible radiations corresponding to the image, which decreases the quality of the image.

FIG.3is a diagram schematically illustrating, as an example, the variation according to wavelength λ (in abscissas, in nm) of the transmission T300 (in ordinates, between 0% and 100%) of the interference filter160of the pixel ofFIG.1, in the case where filter160is a band-pass filter.

The band-pass interference filter is designed to give way to radiations in a waveband around a central wavelength λ0. The central wavelength is preferably located in near infrared, for example, in the range from 700 to 1,100 nm (in the order of 950 nm in the shown example). The transmission preferably has the shape of a peak around the central wavelength. The transmission is for example close to 100% for central wavelength λ0and decreases on either side of wavelength λ0. The length L at mid-height of the peak is typically in the order of 35 nm. The peak may have a plurality of apexes (two apexes in the shown example) and the half-maximum is defined by half the height of the highest apex. The transmission band, or passband, of the filter is defined by the wavelengths for which the transmission is greater than the half maximum of the peak. Central wavelength λ0then corresponds to the center of the passband. The transmission is close to zero, for example, smaller than 1%, more than 50 nm away from central wavelength λ0.

Filter160comprises in the present example eighteen alternated layers of silicon oxide and of silicon. The total thickness of the filter is for example in the order of 1.5 micrometer.

Such a band-pass filter is for example used when the sensor is of time-of-flight type, associated with an infrared pulse emitter. The filter is then designed so that the wavelengths of the pulses are located in the passband of the filter. For example, the wavelength of the pulses is located in a band310centered on wavelength λ0. As an example, band310has a width in the order of 25 nm. Preferably, filter140then only gives way to infrared radiations, which enables to cut off possible secondary peaks that filter160might exhibit outside of the infrared radiation range. As a variation, filter140may be omitted.

The inventors have observed that in practice, photodetector region110detects radiations in a waveband wider than the desired transmission peak. Thereby, the pixel detects, in addition to the pulses, infrared radiations located outside of band310. The infrared radiations form parasitic radiations which adversely affect the quality of the obtained depth map.

The embodiments described hereafter enable to improve the quality of the images obtained from visible radiations, and/or the quality of the depth images.

FIG.4is a cross-section view schematically showing an embodiment of an image sensor pixel400.

Pixel400comprises elements identical or similar to those of the pixel100ofFIG.1, arranged identically or similarly, that is:a semiconductor photodetector region110, for example, delimited by insulating trenches115;one or a plurality of metal regions122;an interference filter160; andpreferably, a lens130and/or a filter140, and/or an antireflection layer150.

These elements will not be described again in detail hereafter.

Pixel400comprises, between metal region(s)122and photodetector region110, an absorbing structure410. Preferably, the pixel comprises no metal elements between photodetector region110and absorbing structure410.

Absorbing structure410is formed by a stack of layers comprising, in the order from the lower surface of photodetector region110, a dielectric layer410a, a silicon layer410b, and a tungsten layer410c. Layer410ais for example a silicon oxide layer or a silicon nitride layer. As a variant, layer410amay be formed by a stack of a plurality of layers made of dielectric materials having refraction indexes smaller than that of silicon, for example, one or a plurality of silicon oxide layers and one or a plurality of silicon nitride layers. Dielectric layer410ais for example in contact, by its upper surface, with the lower surface of photodetector region110. Layer410aparticularly enables to electrically insulate layers410b,410cand the conductive elements122of photodetector region110. Silicon layer410bis for example in contact, by its upper surface, with the lower surface of dielectric layer410a. Tungsten layer410cmay be in contact, by its upper surface, with the lower surface of silicon layer410b. As a variant, a thin bonding layer, for example, made of titanium nitride, for example, having a thickness smaller than 10 nm, may form an interface between silicon layer410band tungsten layer410c.

The thicknesses of the layers410a,410b, and410cof stack410are selected so that stack410has, for the central wavelength λ0of interference filter160, an absorption coefficient greater than that of the semiconductor material of region110. Stack410is sized so that, for the central wavelength λ0of interference filter160and for an incident radiation substantially orthogonal to the average plane of stack410, more than 50%, preferably more than 80%, preferably more than 95%, of the radiation entering into stack410is absorbed in stack410in a single passage. In other words, more than 50%, preferably more than 80%, preferably more than 95%, of a radiation entering through the upper surface of stack410is absorbed in stack410and is not reflected towards region110. For example, for wavelength λ0, approximately 90% of the radiation entering stack410is absorbed in stack410in a single passage. For example, more than 50%, preferably more than 80%, preferably more than 90% of any radiation in the wavelength range from 900 nm to 1,000 is absorbed in a single passage through stack410. Preferably, more than 90% of any radiation in the stop band or in the pass band of filter160is absorbed in a single passage through stack410.

Respectively designating with N1 the refraction index of layer410a, with N2 the refraction index of layer410b, and with N3 the refraction index of layer410c, and considering the thicknesses of layers410aand410cas infinite (or more precisely semi-infinite), it can be shown that the reflection of stack410is null if the following equation is verified:

N⁢⁢3=N⁢⁢2·N⁢⁢1-i·N⁢⁢2·tan⁢⁢δN⁢⁢2-i·N⁢⁢3·tan⁢⁢δ[Math⁢⁢1]withδ=2⁢⁢π·N⁢⁢2·dλ[Math⁢⁢2]
where d is the thickness of layer410b.

If, however, N3 comprises a non-zero imaginary part, then the transmission is null and the wave is integrally absorbed by stack410.

Studies made by the inventor have shown that by taking N1 equal to the refraction index of silicon oxide and N2 equal to the refraction index of silicon, and for a wavelength λ=940 nm, the selection of tungsten as the material of layer410c(index N3) enables to approach at closest the equality of the above equation [Math 1]. A silicon thickness d=39 nm then enables to obtain a maximum absorption (close to 100%) of the radiation by stack410.

FIG.5illustrates the relevance of the selection of tungsten over other materials currently used in integrated circuits.

Considering the above-mentioned equation [Math 1],FIG.5is a diagram illustrating the variation of the complex optical index N3 when the thickness d of silicon layer410bvaries from 0 to 130 nm. More particularly, in the diagram ofFIG.5, the axis of abscissas represents the real part Real (N3) of index N3 and the axis of ordinates represents the imaginary part Imag (N3) of index N3. The circular curve501ofFIG.5represents the variation of index N3 (real part and imaginary part) when thickness d varies from 0 to 130 nm. The point NO of curve501corresponds to the value taken by index N3 for d=0 and for d=130 nm. For these thicknesses, the imaginary part of index N3 is zero.

One has further plotted on the diagram ofFIG.5a point NWcorresponding to the complex optical index of tungsten, a point NCucorresponding to the complex optical index of copper, a point NAgcorresponding to the complex optical index of silver, and a point NAlcorresponding to the complex optical index of aluminum.

As appears in the drawing, point NWnearly coincides with a point of circle501having a non-zero imaginary part. This point corresponds to the value of index N3 for a silicon thickness d equal to 39 nm. Points NCu, NAgand NAlare very distant from circle501. This shows that, among the above-mentioned metals, tungsten is the only relevant candidate to obtain the desired absorption effect.

In practice, the thicknesses of layers410aand410care of course not semi-infinite. Further, as indicated hereabove, dielectric layer410amay comprise one or a plurality of dielectric materials other than silicon oxide. Digital simulations enable to adjust the silicon thickness d to be provided to maximize the absorption, according to the different parameters of stack410and/or to the central wavelength λ0of interference filter160. As an example, the thickness d of silicon layer410bis in the range from 20 to 100 nm, and preferably from 30 to 50 nm.

Further, to maximize the absorption in stack410, the thickness of tungsten layer410cis preferably relatively high, for example, greater than 40 nm and preferably greater than 60 nm.

In the case where interference filter is a stop-band filter of the type described in relation withFIG.2, the presence of absorbing structure410enables to decrease the detection of parasitic radiations in near infrared, and thus to improve the quality of the image.

In the case where the interference filter is a band-pass filter of the type described in relation withFIG.3, among the detected infrared radiations, the proportion of infrared radiations located outside of band310is smaller in the pixel ofFIG.4than in the pixel ofFIG.1. The presence of layer410thus decreases the proportion of detected parasitic radiations and thus enables to improve the quality of the captured depth image.

An explanation of the function of layer410is detailed hereafter in relation withFIG.6.

FIG.6is a diagram schematically illustrating, as an example, the variation according to wavelength λ (in abscissas, in nm) of the absorption T600 of the optical radiations by photodetector region110, in a single passage through photodetector region110.

The radiations located in near infrared are partially absorbed and detected by region110. For example, for the central frequency λ0of the interference filter, region110absorbs and detects in a single passage typically from 1 to 50%, preferably from 2 to 25%, of the radiations which reach region110.

In the absence of absorbing structure410, a radiation reaching region110may cross region110a first time from top to bottom almost without being absorbed, and then a second time from bottom to top after having been reflected on metal region(s)122. The radiation then performs a return travel.

For certain wavelengths, interference filter160gives way to a portion only, for example, from 1% to 80%, of the radiation. This for example occurs for the entire waveband cut by the band stop filter ofFIG.2, and in the lateral portions of the transmission peak of the band-pass filter ofFIG.3. The interference filter then reflects the non-transmitted portion of the radiation.

In the absence of absorbing layer410, for such wavelengths of partial transmission by filter160, the radiation having crossed filter160then performs multiple return travels between, at the bottom, metal regions122and, at the top, partially reflective filter160. The less the interference filter gives way to radiation, the more reflective it is and the higher the number of return travels. Thus, the decrease by filter160of the radiations reaching region110goes along with an increase in the number of passages through region110. Once it has entered region110, a radiation is all the more detected at the number of return travels is large. Accordingly, the decrease of the radiation entering through filter160does not result in a decrease of same amplitude of the detection sensitivity of the pixel.

In the presence of absorbing stack410, the number of passages of the radiation through region110is limited by stack410. This decreases the sensitivity of detection of radiations for wavelengths of partial transmission of filter160. In the presence of absorbing stack410, the partial blocking by filter160of parasitic radiations then results in a corresponding decrease in the sensitivity of detection by the pixel of parasitic radiations. The quality of the visible and/or infrared image has thus been improved.

FIG.7is a diagram schematically illustrating an example of variation, according to wavelength λ (in nm, in abscissas), of the absorption A (in ordinates, from 0 to 1) of the optical radiations by photodetector semiconductor region110in the pixel ofFIG.1(curve601ofFIG.7) and in the pixel ofFIG.4(curve603ofFIG.7).

Strong oscillations can be observed on curve601for wavelengths beyond approximately 700 nm. These oscillations illustrate the effect described hereabove in relation withFIG.6. More particularly, in the pixel ofFIG.1, semiconductor region110forms a cavity for radiations close to the central wavelength of interference filter160. This cavity has resonances at certain frequencies, which explains the oscillations of curve601. This cavity effect degrades the quality of the acquired images.

In curve603, it can be observed that the oscillations are strongly decreased. This results from the provision of absorbing structure410, which enables to avoid the above-described resonant cavity effect.

Although a silicon photodetector region, partially detecting radiations in near infrared, has been described herein, the photodetector region may be made of another semiconductor. This other semiconductor may then have a wavelength range where it only partially absorbs and detects radiations. The central wavelength of the interference filter is selected in this range. Preferably, absorbing structure410is sized to absorb in a single passage more than 50%, for example, more than 80%, of the entire radiation of this wavelength range.

FIG.8is a cross-section view schematically showing an alternative embodiment800of the pixel ofFIG.4.

In this variant, the tungsten layer410cof absorbing stack410is coupled, preferably connected, to a node of application of a bias potential VB, for example, via one of the metal regions122forming an interconnection track. The application of a bias voltage to absorbing stack410enables to discharge possible electric charges trapped in insulating layer410a. Further, the application of this bias potential may enable to avoid storing charges in layers410band/or410c, which might disturb the electric potential of region110.

As an example, insulating trenches115each comprise a conductive region810insulated from region110by an electrically-insulating material815. Regions810are coupled, preferably connected, to some of metal regions122by connections820. A capacitive element enabling to electrostatically influence region110and/or to decrease or neutralize the dark current has thus been obtained for each trench. Indeed, the decrease or the neutralization of the dark current may be obtained by accumulation of holes to store electrons, or of electrons to store holes.

Stack410has openings830at the level of connections820. Openings830enable to electrically insulate the connections820of absorbing stack410. Any electric connection crossing stack410may be insulated from layer410by an opening830. These electric connections for example form contacts towards elements of the pixel such as transistors or junctions. Openings830are preferably located at the level of the pixel edges. However, regions810, connections820, and openings830may be omitted.

FIG.9is a cross-section view schematically showing another alternative embodiment1300of the pixel ofFIG.4. In this variation, cavities1310extend in region110from its upper surface. Cavities1310are filled with a dielectric material, for example, silicon oxide. Cavities1310are preferably arranged in a grating. The pitch of the grating is for example greater than half a central wavelength of the waveband detected by the pixel. In the case where filter160is a band-pass filter, the pitch of the grating is preferably greater than half the central wavelength λ0of filter160.

The cavities1310and the portions1320of region110arranged between these cavities form a diffraction grating. The diffraction grating introduces angles in the propagation of radiations entering from the top of photodetector region110. This radiation then performs a plurality of return travels with a horizontal component in region110between the lateral walls of region110. The length of the travel of the radiation within region110, and thus the probability for the radiation to be detected by region110, are thus increased. Conversely to the return travels between the bottom and the top of the pixel, the number of which depends on the transmission of filter160, horizontal return travels increase the detection substantially in the same way for all the wavelengths of the band detected by the pixel. Thus, the diffraction grating in the upper portion of region110provides pixel1300with a sensitivity to radiations greater than that of a pixel which does not comprise this diffraction grating, while keeping the advantage of the image quality provided by limiting of the number of vertical return travels.

FIG.10is a cross-section view partially and schematically showing an embodiment of an image sensor1400comprising pixels of the type of pixel400ofFIG.4. Sensor1400may capture three-dimensional color images or color and infrared images.

More particularly, a group of four pixels400R,400G,400B, and400Z, of sensor1400has been shown. Pixels400R,400G,400B, and400Z, each correspond to the pixel400ofFIG.4where filter140is formed by a filter, respectively140R,140G,140B, and140Z, respectively giving way to blue light, green light, red light, and radiations in near infrared. Filter140Z is optional.

In each of pixels400R,400G, and400B, the interference filter160of the pixel400ofFIG.4is formed by a band-stop interference filter160BC, of the type of the filter ofFIG.2. Preferably, filter160BC is a same continuous filter common to pixels400R,400G, and400B. In pixel400Z, the interference filter160of pixel400ofFIG.4is formed by a band-pass interference filter160BP, for example, of the type of the filter ofFIG.3.

In the example, absorbing stack410and layer120are common for the various pixels of the sensor, and in particular common to the four pixels in the group of shown pixels.

As mentioned hereabove, absorbing stack410limits the number of return travels performed by light between the top and the bottom of each of the pixels, thus improving the quality of the captured image.

Further, absorbing stack410limits the quantity of optical radiation, particularly infrared, which, after having crossed from top to bottom the region110of one of the pixels, is reflected by metal regions122towards the neighboring pixels. This corresponds to an additional improvement of the image quality.

To form pixels400R,400G,400B, and400Z, preferably, regions110and the trenches115separating regions110are previously formed in a semiconductor substrate. One then successively forms, on the front surface of the substrate (lower surface), the layers410a,410b, and410cof absorbing stack410, and the insulating layers120containing metal regions122. Preferably, a handle1410, for example, a semiconductor wafer, is then glued, on the front surface of insulating layers120. All the elements from the rear surface of the substrate to a level defining the upper level of regions110are then removed, for example, by polishing.

Then, in a first step, filters160BP are formed. For this purpose, for example, all the upper surfaces of regions110are covered with a first stack of alternated layers forming filter160BP, after which the portions of the first stack of layers located at the locations of pixels400R,400G, and400B are then removed, for example, by etching.

In a second step, filters160BC are formed. To achieve this, for example, the structure obtained at the first step is covered with a second stack of layers corresponding to filters160BC. The portions of the second stack located at the location of pixel400Z are then removed, for example, by chemical mechanical polishing.

Filters140R,140G, and140B, and optional filter140Z, and then optional lenses130, are then formed.

The parasitic radiations that the filter partially lets through are all the more numerous as the filter is thin. Due to the fact that absorbing stack410decreases the detection of these parasitic radiations, one may, for a given quantity of absorbed parasitic radiations, that is, for a given image quality, decrease the thickness of the filter with respect to a sensor comprising no layer410A. Lenses130can then be brought closer to regions110, which enables to increase the viewing angle of the sensor, and this, without altering the image quality. Further, a decrease in the width, or horizontal dimension, of a region1420where the second stack, which corresponds to filters160BC, has its layers laterally stacked against the side of filter160BP, corresponds to this thickness decrease. In region1420, filter160BC has degraded filtering properties. The fact of decreasing this region enables to decrease the size of the pixels without altering the image quality as compared with a sensor comprising no absorbing layer410.

In the example ofFIG.10, absorbing stack410extends under pixels400R,400G, and400B and under pixel400Z. As a variant, stack410may extend under pixels400R,400G and400B only and not extend under pixels400Z. In this case, the manufacturing method is for example similar to what has just been described, with the difference that a step of local removal of absorbing stack410opposite pixels400Z is provided before the step of deposition of insulating layers120.

FIG.11is a cross-section view partially and schematically showing an embodiment of an image sensor1450comprising pixels of the type of the pixel400ofFIG.4. Sensor1450shows the elements of sensor1400ofFIG.10, with the difference that the manufacturing of filters160BC and160BP comprises an additional step between the first and second steps.

At this additional step, a dielectric layer, for example, made of silicon oxide, is formed at the locations of pixels400R,400G, and400B down to the upper level of filter160BP formed at the first step. To achieve this, the structure obtained at the first step is covered with the oxide layer, after which all the elements located above the upper surface of filter160BP are removed, for example, by chemical mechanical polishing.

Then, at the second step, the second stack, which corresponds to filter160BC, is deposited over the planar surface left by the polishing, and the portions of the second stack are removed from the location of pixel400Z by etching.

Advantageously, due to the fact that filters160BC are formed on a planar surface and are delimited by etching, they form a stack of planar layers of constant thicknesses over the entire surface of the filter. The filtering quality is thus better than with a filter formed of layers which are non-planar and/or of non-constant thicknesses.

At a subsequent step, a dielectric layer portion, for example, made of silicon oxide, may be formed at the location of pixel400Z, down to the upper level of filter160BC. To achieve this, for example, the structure obtained at the second step is covered with a dielectric layer, for example, made of silicon oxide, after which all the elements located above the upper surface of filter160BC are removed, for example, by chemical mechanical polishing.

Here again, although absorbing stack410has been shown as extending under pixels400R,400G, and400B and under pixel400Z, as a variant, stack410may extend under pixels400R,400G and400B only and not extend under pixels400Z.

FIG.12is a partial simplified cross-section view showing an example of an embodiment of an image sensor pixel1500. Pixel1500comprises elements identical or similar to those of the pixel400ofFIG.4. Pixel1500differs from that ofFIG.4in that it comprises, between absorbing stack410and insulating layer120, successively from the lower surface of absorbing stack410, a dielectric layer1510, a conductive region1520, an insulating layer1525, and a semiconductor region1530, for example, made of silicon. Conductive region1520and insulating layer1525may be omitted.

To form pixel1500, region110and trenches115are formed in a semiconductor substrate. The front surface of the substrate (lower surface) is then covered with absorbing stack410. The tungsten layer410cof absorbing stack410is covered with dielectric layer1510, for example, made of silicon oxide.

After dielectric layer1510, conductive region1520is formed. For example, region1520defines a ground plane and covers the entire lower surface (or front side) of the semiconductor substrate. Insulating region1525, for example, made of silicon oxide, is then formed. After this, semiconductor region1530is bonded to layer1525by molecular bonding. Components of electronic circuits of the image sensor are then formed, for example, transistors (not shown) inside and on top of semiconductor region1530, on its lower surface side (front side). Then, insulating layers and metal regions122are formed. Regions122preferably correspond to interconnection tracks between components of the image sensor circuits.

Layer150, interference filter160, optional filter140, and optional lens130, are for example formed at a subsequent step of the pixel manufacturing.

An advantage of pixel1500is that part of the image sensor circuits are formed inside and on top of region1530independently from photodetector regions110, which enables to decrease the pixel size or to integrate additional functions, as compared with an image sensor which does not comprise semiconductor region1530. Further, the presence of region1520may enable to optimize the operation of the image sensor circuits.

The presence of layer1510and of region1520in pixel1500has the same effect on the reflection of light as layer120and regions122in the pixel400ofFIG.4. In pixel1500, stack410limits the number of possible return travels of the radiations in cavity110after reflection, on region1520and on filter160. An improved image quality is obtained, in a sensor comprising pixels of the type of pixel1500, in the same way as in an image sensor comprising pixels of the type of the pixel400ofFIG.4.

FIG.13is a cross-section view partially and schematically showing an embodiment of an image sensor1600. Image sensor1600comprises elements identical or similar to those of the sensor1400ofFIG.11, arranged identically or similarly, with the difference that pixel400Z is replaced with a pixel1600Z.

Pixel1600Z comprises elements identical or similar to those of the pixel400Z ofFIG.11, arranged identically or similarly, that is, a photodetector region110Z, an absorbing stack layers410Z comprising, in the order from the lower surface of photodetector region110Z, a dielectric layer410Za, a silicon layer410Zb, and a tungsten layer410Zc, and insulating layers120Z. Photodetector region110Z, stack410Z, and insulating layers120Z are identical or similar to respectively the photodetector region110, the absorbing stack410, and the insulating layers120of the pixel400Z ofFIG.4. In pixel1600Z, semiconductor photodetector region110Z is located at a level lower than that of the photodetector regions110of pixels400R,400G, and400B. Pixel1600Z further comprises optical transmission regions1602and1604located between its photodetector region110Z and its filter160BP. It should be noted that in the shown example, in vertical projection, the contour of photodetector region110Z substantially coincides with the contour of optical transmission regions1602and1604. As a variant, photodetector region110Z may extend, laterally, beyond optical transmission regions1602and1604.

Region1602is located at the same level as the regions110of pixels140R,140G, and140B, and region1604extends vertically between region110Z and optical transmission region1602. The region110Z of pixel1600Z is located in a substrate1610which extends horizontally under the layer120common to pixels400R,400G, and400B. Absorbing stack410Z and insulating layers120Z are horizontally continued on the lower surface of substrate1610.

Components such as transistors, not shown, are formed on the lower surface, or front surface, of substrate1610. Such transistors are for example interconnected by tracks forming metal regions122Z located in layer120Z. Transmission region1602is preferably made of the same semiconductor as the regions110of pixels400R,400G,400B. Region1604is preferably made of a dielectric, for example, of silicon nitride, or of amorphous silicon, and extends from the lower surface of region1602through absorbing stack410and insulating layers120.

In operation, the optical radiations transmitted by filter160BP are guided by optical regions1602and1604all the way to photodetector region110Z.

The sensor may have densities of transistors and of metal interconnects formed by regions122and122Z between the transistors which are greater than the densities of transistors and of interconnects in a sensor comprising no substrate1610extending under pixels400R,400G, and400B. Sensor1600may be particularly compact.

To form sensor1600, regions110and trenches115separating regions110are formed in a semiconductor substrate1620, for example, made of silicon. One then forms, on the front surface of substrate1620(lower surface), optional components such as transistors, absorbing stack410, insulating layers120containing metal regions122, and region1604.

One separately forms, on the front surface of substrate1610(lower surface) comprising regions110Z, optional components such as transistors, absorbing stack410Z, and the insulating layers120Z containing metal regions122Z.

A handle1630, for example, a semiconductor substrate, is bonded to the front surface of insulating layer120Z. The portions of substrate1610located on the rear surface side are then removed down to the upper level of regions110Z, for example, by chemical mechanical polishing.

The rear surface of substrate1610is then bonded to the front surface of layer120and of region1604.

All the elements are then removed from the rear surface of substrate1620down to a level defining the upper level of regions110of pixels400R,400B, and400Z, for example, by polishing.

Filters160BC,160BP,140R,140G,140B, optional filter140Z and the possible lenses130are then formed, in the same way as that described in relation withFIG.11to form these elements.

As a variant, absorbing stack410Z may be omitted in the structure ofFIG.13.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants may be combined and other variants will occur to those skilled in the art. In particular, the variants and embodiments described in relation withFIGS.8and9are compatible with one another and may be applied, alone or in combination, to the embodiments ofFIGS.10to13.

In the above-described embodiments, the lower layer410cof absorbing stack410preferably corresponds to a specific tungsten level, that is, only deposited for the forming of absorbing stack410. However, as a variant, the lower layer410cof absorbing stack410may correspond to a first metallization level of the sensor, where are also formed interconnection metallizations at the periphery of the pixels.

Finally, the practical implementation of the embodiments and variants described herein is within the capabilities of those skilled in the art based on the functional indications provided hereinabove.