Image Sensor

Abstract

The present application relates to image sensors. A multispectral image sensor includes a semiconductor layer and a plurality of pixels formed inside and on top of the semiconductor layer. Each pixel includes an active photosensitive region formed in a portion of the semiconductor layer laterally bounded by a peripheral insulating wall. The pixels include a first pixel of a first type and a second pixel of a second type. The portion of the semiconductor layer of the first pixel has a first lateral dimension, the first lateral dimension is selected to define a lateral cavity that resonates at a first wavelength, and the portion of the semiconductor layer of the second pixel has a second lateral dimension different from the first lateral dimension, the second lateral dimension is selected to define a lateral cavity that resonates at a second wavelength different from the first wavelength.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from French Patent Application No. 1856285 filed on July 9, 2018, which is incorporated herein by reference. Technical Field

[0003] The present disclosure relates to the field of image sensors. Background Art

[0004] Traditionally, an image sensor includes a semiconductor layer with a plurality of pixels formed inside and on top of the semiconductor layer. Each pixel specifically includes an active photosensitive region formed in the semiconductor layer, typically corresponding to a photodiode, and may also include one or more control transistors formed inside and on top of the semiconductor layer.

[0005] In a multispectral image sensor, such as a color image sensor, each pixel conventionally comprises a filter element arranged on one side of the illuminated surface of the semiconductor layer, opposite the photosensitive area of ​​the pixel, capable of filtering the incident radiation so that only light radiation received within a pixel-specific limited wavelength range corresponding to the measurement range of the pixel is transmitted to the photosensitive area of ​​the pixel. In other words, pixels of different types, i.e. pixels with different measurement wavelength ranges, differ from each other substantially due to the properties of their respective filter elements.

[0006] Traditionally, the filter elements of the pixels of a multispectral image sensor are formed by a colored resin, such as an organic resin. A specific resin should be provided specifically for each pixel type. This makes the manufacture of such sensors relatively complex, and becomes more complex as the number of sensor pixel types increases and the lateral size of the pixels decreases. Summary of the invention

[0007] The present disclosure relates to the field of image sensors. Particular embodiments relate to multispectral image sensors, ie, sensors comprising multiple pixels, where different types of pixels are capable of measuring light of different wavelength ranges.

[0008] Embodiments may overcome all or some of the disadvantages of known multispectral image sensors.

[0009] An embodiment provides a multispectral image sensor, the multispectral image sensor comprising a plurality of pixels formed within and on top of a semiconductor layer. Each pixel comprises an active photosensitive region formed in a portion of the semiconductor layer laterally defined by a peripheral insulating wall. The plurality of pixels comprises at least one pixel of a first type, wherein the portion of the semiconductor layer of the pixel has a first lateral dimension selected to define a lateral cavity resonating at a first wavelength. The plurality of pixels also comprises at least one pixel of a second type, wherein the portion of the semiconductor layer of the pixel has a second lateral dimension different from the first lateral dimension, the second lateral dimension being selected to define a lateral cavity resonating at a second wavelength different from the first wavelength.

[0010] According to an embodiment, each pixel comprises a diffractive structure arranged on the rear surface of a portion of the semiconductor layer of the pixel.

[0011] According to an embodiment, the diffractive structure is formed by a cavity formed in a portion of the semiconductor layer of the pixel, the cavity being filled with a material having a refractive index lower than the refractive index of the material of the semiconductor layer.

[0012] According to an embodiment, the sensor is designed to be back-side illuminated.

[0013] According to an embodiment, the front surface of the semiconductor layer is coated with a stack of an insulating layer and a metal layer having metallization of interconnections of the sensor pixels formed therein.

[0014] According to an embodiment, the peripheral insulating wall is formed by a metal wall sandwiched between two walls made of insulating material.

[0015] According to an embodiment, the sensor comprises two or more n different types of pixels formed in and on top of portions of the semiconductor layer having different lateral dimensions selected to define lateral cavities resonating at different wavelengths.

[0016] According to an embodiment, n is greater than or equal to 4.

[0017] According to an embodiment, portions of the semiconductor layer of pixels of the first type and the second type have substantially the same thickness.

[0018] According to an embodiment, portions of the semiconductor layer of the first type and the second type pixels have different thicknesses.

[0019] Another embodiment provides a multispectral image sensor including a plurality of pixels formed within and on top of a semiconductor layer. The plurality of pixels includes at least one pixel of a first type formed within and on top of a first portion of the semiconductor layer, the first portion of the semiconductor layer being selected to define a first thickness of a vertical cavity that resonates at a first wavelength. The plurality of pixels also includes at least one pixel of a second type formed within and on top of a second portion of the semiconductor layer, the second portion of the semiconductor layer having a second thickness different from the first thickness, the second thickness being selected to define a vertical cavity that resonates at a second wavelength different from the first wavelength.

[0020] According to an embodiment, the first pixel includes a first active photosensitive area formed in a first portion of the semiconductor layer, and the second pixel includes a second active photosensitive area formed in a second portion of the semiconductor layer.

[0021] According to an embodiment, the first portion and the second portion of the semiconductor layer are laterally bounded by vertical insulating walls.

[0022] According to an embodiment, the front and rear surfaces of the semiconductor layer are coated with an insulating material having a refractive index smaller than that of a material of the semiconductor layer.

[0023] According to an embodiment, the front surfaces of the first and second portions of the semiconductor layer are substantially coplanar.

[0024] According to an embodiment, the front surface of the semiconductor layer is coated with a stack of an insulating layer and a metal layer having metallization of interconnections of the sensor pixels formed therein.

[0025] According to an embodiment, the sensor is designed to be back-side illuminated.

[0026] According to an embodiment, the sensor comprises two or more n different types of pixels formed in and on top of portions of the semiconductor layer having different thicknesses selected to define vertical cavities resonating at different wavelengths.

[0027] According to an embodiment, n is greater than or equal to 4.

[0028] According to an embodiment, the first portion and the second portion of the semiconductor layer have the same lateral dimensions.

[0029] According to an embodiment, the first portion and the second portion of the semiconductor layer have different lateral dimensions.

[0030] The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1is a cross-sectional view schematically and partially showing an example of the multispectral image sensor according to the first embodiment.

[0032] Figure 2 is a cross-sectional view schematically showing an example structure of an image sensor pixel according to the first embodiment.

[0033] Figure 3 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the first embodiment.

[0034] Figure 4 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the first embodiment.

[0035] Figure 5 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the first embodiment.

[0036] Figure 6 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the first embodiment.

[0037] Figure 7 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the first embodiment.

[0038] Figure 8 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the first embodiment.

[0039] Fig. 9 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the first embodiment.

[0040] Fig.10 is a cross-sectional view schematically and partially showing an example of a multispectral image sensor according to the second embodiment.

[0041] Fig.11 are cross-sectional views schematically showing steps of an example of a method of manufacturing an image sensor according to the second embodiment.

[0042] Fig.12 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the second embodiment.

[0043] Fig.13 is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the second embodiment.

[0044] Fig.14is a cross-sectional view schematically showing another step of the example of the method of manufacturing the image sensor according to the second embodiment.

[0045] Fig.15 is a cross-sectional view schematically showing another step of the method of manufacturing the image sensor according to the second embodiment.

[0046] Fig.16 is a cross-sectional view schematically and partially illustrating an alternative embodiment of the image sensor according to the second embodiment. DETAILED DESCRIPTION

[0047] In different drawings, the same elements are marked with the same reference numerals. In particular, the common structural and / or functional elements of different embodiments may be marked with the same reference numerals and may have the same structure, size and material properties.

[0048] For the sake of clarity, only those steps and elements useful for understanding the described embodiments are shown and described in detail. In particular, in the examples described below, the internal structure of the pixel, in particular the internal structure of the active photosensitive area and possible control transistors, is not described in detail, and the described examples are compatible with all or most known pixel structures and are provided for possible adjustments based on the teachings of the present disclosure within the capabilities of those skilled in the art.

[0049] Throughout this disclosure, the term "connected" is used to indicate a direct electrical connection between circuit elements without intervening elements other than conductors, while the term "coupled" is used to indicate an electrical connection between circuit elements that may be direct, or that may be through one or more intervening elements.

[0050] In the following description, when referring to terms defining absolute positions, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or terms defining relative positions, such as the terms "above", "below", "upper", "lower", etc., or terms defining directions, such as the terms "horizontal", "vertical", etc., unless otherwise specified, these terms refer to the orientation in the figures, and it is understood that the sensors described in practice may be oriented differently.

[0051] The terms "about," "substantially," and "approximately" are used herein to indicate a tolerance of plus or minus 10%, preferably plus or minus 5%, of the value in question.

[0052] Figure 1 is a cross-sectional view schematically and partially showing an example of the multispectral image sensor according to the first embodiment.

[0053] Figure 1The sensor consists of multiple pixels, where different types of pixels are able to measure light radiation in different wavelength ranges. Figure 1 , four adjacent pixels P1, P2, P3 and P4 of different types are shown. More generally, the number of different pixels of the sensor may be any number greater than or equal to 2, it being understood that the described embodiments are particularly advantageous for sensors having a large number of pixel types, for example greater than or equal to 4, greater than or equal to 5, or greater than or equal to 6. For example, the sensor may include multiple pixels of each type, for example distributed in an array of rows and columns, or a single pixel of each type, for example to form a spectrometer.

[0054] Figure 1 The sensor is formed inside and around a semiconductor layer 101, such as a single crystal silicon layer in CMOS technology. The semiconductor layer 101 includes: a front surface (which is coated with a stack of insulating layers and metal layers) 103. Figure 1 an upper surface in the orientation), wherein in particular various interconnect metallizations of the sensor components are formed; and a rear surface ( Figure 1 lower surface in orientation). Figure 1 The sensor is a back-side illuminated sensor, that is to say is designed for illumination on the rear surface of the semiconductor layer 101 .

[0055] Figure 1 Each pixel of the sensor comprises a portion of a semiconductor layer 101 which is laterally delimited, along substantially the entire height of the layer 101, by a peripheral insulating wall 105 extending vertically along the semiconductor layer 101. By way of example, the peripheral insulating wall 105 is made of an electrically insulating material, such as silicon oxide. As a variant, the wall 105 comprises a central conductive wall, for example made of polycrystalline silicon, sandwiched between two insulating walls, for example made of silicon oxide. The central conductive wall can be biased so as to form a MOS capacitor with the semiconductor layer 101. As a variant, the central conductive wall is made of a metal, for example tungsten, which makes it possible to limit the light received by each pixel and thus to limit crosstalk phenomena, i.e. to avoid that photons received by a sensor pixel lead to the generation of electron-hole pairs in a neighboring pixel. By way of example, in a top view, the peripheral insulating wall 105 forms a continuous gate having dimensions substantially equal to the dimensions of the pixel array of the sensor.

[0056] In each pixel, the portion of the semiconductor layer 101 inside and around which the pixel is formed includes a photosensitive active area corresponding to, for example, a photodiode. The pixel may also include one or more control transistors on the front surface side of layer 101 formed inside and on top of the portion of the semiconductor layer 101 of the pixel.

[0057] According to one aspect of the first embodiment, portions of the semiconductor layer 101 having different types of sensor pixels formed therein and around have different thicknesses (in the vertical direction). Portions of the semiconductor layer 101 having the same type of pixels formed therein and around have substantially the same thickness.

[0058] More specifically, for each type of pixel of the sensor, the thickness of the portion of the semiconductor layer 101 forming the pixel of the type under consideration within and around it is selected to define a vertical optical cavity in each pixel of the type under consideration, the vertical optical cavity resonating at one or more wavelengths specific to the type of pixel under consideration. In other words, the thickness of the portion of the semiconductor layer 101 forming the sensor pixel is selected so that the vertical optical cavities of pixels of the same type resonate at substantially the same wavelength or multiple same wavelengths, and the vertical optical cavities of pixels of different types resonate at different wavelengths.

[0059] It should be noted that in each pixel, so that the portion of the semiconductor layer 101 inside and around which the pixel is formed defines a vertical optical cavity, the front and back surfaces of the considered portion of the semiconductor layer 101 should be reflective for light propagating inside the portion. Figure 1 In the sensor, this is obtained by the fact that each front and back surface of the semiconductor layer 101 is coated with (and in contact with) a layer of insulating material whose refractive index is lower than the refractive index of the material of layer 101, for example silicon oxide in the case where layer 101 is made of silicon.

[0060] exist Figure 1 In the example of , the front surface of the semiconductor layer 101 is substantially flat, i.e., the front surfaces of the portions of the layer 101 in and around which the sensor pixels are formed are substantially in the same plane. However, the back surface of the semiconductor layer 101 is structured. More specifically, the back surfaces of the portions of the layer 101 in and around which the sensor pixels are formed are substantially flat and coplanar for pixels of the same type, and non-coplanar parallel planes for pixels of different types.

[0061] exist Figure 1In the example of , each pixel also comprises a transparent filling element 107 coating the rear surface of the portion of the semiconductor layer 101 forming the pixel inside and around it. The filling element 107 has the function of compensating for the thickness differences between the portions of the layer 101 of the pixels of different types of sensors. In other words, for each pixel of the sensor, the thickness of the filling element 107 is small while the thickness of the layer portion 101 of the pixel is significant, and vice versa, so that the rear surface of the filling element 107 of the sensor pixel is substantially in the same plane parallel to the front surface of the semiconductor layer 101. The filling elements 107 of the pixels are all made of the same material, for example silicon oxide.

[0062] exist Figure 1 In the example of FIG. 1 , on the rear side of the sensor, the peripheral insulating wall 105 of the pixel is flush with the rear surface of the filling element 107 to define a substantially flat continuous surface together with the rear surface of the filling element 107. Furthermore, in this example, an additional planarization layer 109, for example made of silicon oxide, coats the rear surface of the filling element 107 and the rear surface of the peripheral insulating wall 105.

[0063] exist Figure 1 In the example of , each pixel further comprises a microlens 111, which is arranged on the back side of the sensor, opposite to the portion of the layer 101 of the pixel, capable of focusing the incident light towards the active photosensitive area of ​​the pixel. More specifically, in this example, the microlens 111 is arranged on the back surface of the planarization layer 109.

[0064] In operation, in each pixel of the sensor, incident radiation is transmitted in the semiconductor layer 101 via the microlens 111, the planarization layer 109 and the transparent filling element 107. In this example, the elements of the sensor that coat the rear surface of the semiconductor layer 101 have substantially the same frequency response for all sensor pixels. The incident light penetrates into the semiconductor layer 101 through the rear surface of the semiconductor layer 101 and then reflects alternately on its front and rear surfaces. The vertical optical cavity defined in this way has one or more resonant wavelengths within the wavelength range transmitted to the semiconductor layer 101. This results in one or more light absorption peaks of the semiconductor layer 101 at the resonant wavelength. Therefore, although a wide wavelength range is transmitted to the semiconductor layer 101, the amount of photogenerated charge collected in the active photosensitive area of ​​each pixel mainly represents the light intensity received at the resonant wavelength of the pixel. Therefore, different types of pixels with different resonant wavelengths measure light intensities of different wavelengths.

[0065] It should be noted that Figure 1 In the example of , the sensor pixels all have substantially the same lateral dimensions. In other words, Figure 1In a sensor, pixels of the same type are all identical or similar, and pixels of different types differ from each other only by the thickness of the portion of the semiconductor layer 101 forming them respectively inside and around them (and therefore the thickness of their respective filling elements 107).

[0066] Figure 1 An advantage of an embodiment of the invention is that it enables the formation of a multi-spectral image sensor without having to form filter elements made of different materials on different types of pixels (ie, for measuring light of different wavelength ranges).

[0067] Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Fig. 9 is a schematic diagram showing the manufacturing process of Figure 1 A cross-sectional view of an example of the steps of a method of a multispectral image sensor of the type described. It should be noted that Figure 1 Compared to the representation of Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Fig. 9 In the view of , the front surface corresponds to the lower surface and the rear surface corresponds to the upper surface. Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Fig. 9 In the example, for simplicity, only two different types of pixels are formed, corresponding to Figure 1 The pixel P1 of the sensor (at Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Fig. 9 in the right hand part of Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 and Fig. 9In practice, if the sensor comprises a plurality of pixels of the same type, the steps specific to the pixel type under consideration are performed simultaneously for all pixels of this type.

[0068] Figure 2 The initial structure is shown, which comprises a semiconductor layer 101 of uniform thickness, inside and on top of which the active photosensitive area for each pixel of the sensor and possibly one or more pixel control transistors (not detailed in the figure) have been formed. An interconnect stack 103 coats the front surface of layer 101 (at Figure 2 the lower surface in the orientation).

[0069] form Figure 2 The process for forming the structure of the sensor may for example start from a relatively thick semiconductor substrate (for example a thickness greater than 100 μm), after which the active photosensitive areas and possible sensor pixel control transistors are formed on the front surface side of the substrate, inside and on top of the substrate. Then, the interconnect stack 103 is formed on the front surface side of the substrate, after which the substrate is thinned from the back surface of the substrate to leave only the semiconductor layer 101, for example with a thickness of less than 10 μm. In order to ensure the mechanical resistance of the sensor, a supporting base (not shown) may be placed against the front surface of the interconnect stack 103 before the semiconductor substrate thinning step. At this stage, the parts of the semiconductor layer 101 inside and around which the different sensor pixels are formed are formed not to be insulated from each other.

[0070] Figure 3 The step of etching vertical trenches 201 from the rear surface of the semiconductor layer 101 is shown, the vertical trenches 201 insulating the parts of the semiconductor layer 101 inside and around which different sensor pixels are formed from each other. In the top view, in each pixel of the sensor, the parts of the semiconductor layer 101 inside and around which the pixel is formed are completely surrounded by the trenches 201. In the present example, the trenches 201 extend vertically over the entire thickness of the semiconductor layer 101 and are interrupted at the level of the rear surface of the interconnect stack 103.

[0071] Figure 4 The step of filling the trench 201 to form the vertical walls 105 to insulate the sensor pixels is shown. As an example, the trench 201 may be completely filled with an insulating material, such as silicon oxide. As a variant, only the walls of the trench are coated with an insulating material, such as silicon oxide, and then the trench is filled with a conductive material, such as polysilicon or a metal such as tungsten. Before filling the trench, a protective coating, such as made of silicon nitride, may be deposited over the trench walls. One or more planarization steps may be provided, such as by chemical mechanical polishing (CMP), so that the rear surface of the peripheral insulating wall 105 is flush with the rear surface of the semiconductor layer 101.

[0072] As a variation, the trench 201 is etched ( Figure 3) and fill the groove 201 ( Figure 4 ) can be implemented from the front surface of the substrate 101 before forming the interconnect stack 103 and thinning the substrate 101 from the back surface. In this case, the depth of the trench 201 can be selected to be substantially equal to or greater than the final thickness of the substrate 101 after thinning, so that at the end of the step of thinning the substrate 101, the peripheral insulating wall 105 is flush with the back surface of the substrate 101.

[0073] Figure 5 A step of thinning the semiconductor layer 101 from its rear surface at a level located at the pixel P1 is shown. More specifically, during this step, the portion of the semiconductor layer 101 inside and around which the pixel P1 is formed is etched substantially uniformly from its rear surface over its entire surface to obtain a target thickness so that a cavity resonating at the desired wavelength of the pixel P1 can be formed. During this step, a mask (not shown) protects the pixel P2 from being etched, and more generally, protects all pixels of a type different from the pixel P1 from being etched. Pixels of the same type as the pixel P1 are etched at the same time as the pixel P1. The etching method is selected to selectively etch the semiconductor layer 101 above the peripheral insulating wall 105. Thus, at Figure 5 At the end of the etching step, a recess 203 is obtained in the pixel P1 (and possibly in all pixels of the same type as the pixel P1 ) which is laterally delimited by the insulating wall 105 , opposite the rear surface of the semiconductor layer 101 .

[0074] Figure 6 A step of filling the recess 203 with a filling element 107 is shown, the filling element 107 being made of a material transparent to the radiation to be measured, such as silicon oxide. As an example, the filling material is deposited on the back side of the entire sensor with a thickness sufficient to fill the recess 203, after which a planarization step, such as by chemical mechanical polishing (CMP), is provided to remove excess filling material. At the end of this step, the rear surface of the filling element 107 of the pixel P1 is flush with the rear surface of the peripheral insulating wall 105 and is at the same level as the rear surface of the semiconductor layer 101 in the pixel P2.

[0075] Figure 7A step of thinning the semiconductor layer 101 from its rear surface at a level located at the pixel P2 is shown. More specifically, during this step, the portion of the semiconductor layer 101 inside and around which the pixel P2 is formed is etched substantially uniformly from its rear surface over its entire surface to obtain a target thickness so that a cavity resonating at the desired wavelength of the pixel P2 can be formed. During this step, a mask (not shown) protects the pixel P1 from being etched, and more generally, protects all types of pixels different from the pixel P2 from being etched. Pixels of the same type as the pixel P2 are etched simultaneously with the pixel P2. The etching method is selected to selectively etch the semiconductor layer 101 above the peripheral insulating wall 105. Thus, at Figure 7 At the end of the etching step, a recess 205 is obtained in the pixel P2 (and possibly in all pixels of the same type as the pixel P2) which is laterally delimited by the insulating wall 105, opposite the rear surface of the semiconductor layer 101.

[0076] Figure 8 A step of filling the recess 205 with a filling element 107 is shown, the filling element 107 being made of a material transparent to the radiation to be measured, for example of silicon oxide. As an example, the filling material is deposited on the back side of the entire sensor with a thickness sufficient to fill the recess 203, after which a planarization step is provided, for example by chemical mechanical polishing (CMP), in order to remove excess filling material. At the end of this step, the rear surface of the filling element 107 of the pixel P2 is flush with the rear surface of the peripheral insulating wall 105 and is at the same level as the rear surface of the filling element 107 of the pixel P1.

[0077] Since the sensor comprises different pixel types, by adjusting the etching depth of layer 101 each time according to the resonance wavelength required for each pixel type, Figure 7 and Figure 8 The steps can be repeated multiple times.

[0078] Fig. 9 A subsequent step of depositing a planarization layer 109 on the back side of the structure obtained at the end of the previous steps is shown. Layer 109 is for example a continuous layer of substantially uniform thickness which extends over the entire surface of the sensor. Fig. 9 Also shown is the step of forming microlenses 111 on the rear surface of the planarization layer 109. As an example, the sensor comprises one microlens 111 per pixel arranged opposite the photosensitive area of ​​the pixel.

[0079] Fig.10 : is a cross-sectional view schematically and partially showing an example of a multispectral image sensor according to the second embodiment. It should be noted that Fig.10 The sensors include Figure 1Hereinafter, the common elements will not be described, and only the common elements related to Figure 1 The difference between the sensors.

[0080] As in Figure 1 In the example, Fig.10 The sensor consists of multiple pixels, where different types of pixels are able to measure light radiation in different wavelength ranges. Fig.10 , four adjacent pixels P1 , P2 , P3 and P4 of different types are shown.

[0081] Fig.10 The sensor and Figure 1 The difference between the sensors is that Fig.10 In the example of FIG. 1 , the semiconductor layer 101 in and around which the sensor pixels are formed has substantially the same thickness in all the sensor pixels.

[0082] As in Figure 1 In the example of , each sensor pixel comprises a portion of the semiconductor layer 101 which is laterally delimited along substantially the entire height of the layer 101 by a peripheral insulating wall 105 extending vertically in the semiconductor layer 101 , in which the active photosensitive area of ​​the pixel is formed.

[0083] According to one aspect of the second embodiment, the portions of the semiconductor layer 101 in which the active photosensitive regions of pixels of different types are formed have different lateral dimensions (in the horizontal direction). The portions of the semiconductor layer 101 in which the active photosensitive regions of pixels of the same type are formed have substantially the same lateral dimensions. It should be noted that, as an example, the lateral dimensions here refer to: the diameter in the case where the semiconductor layer portion 101 of the pixel has a circular shape (in the top view), or the width and / or length in the case where the semiconductor layer portion 101 of the pixel has a square or rectangular shape (in the top view).

[0084] More specifically, for each type of pixel of the sensor, the lateral dimensions of the portion of the semiconductor layer 101 within and around which the pixel of the type under consideration is formed are selected so as to define in each pixel of the type under consideration a vertical optical cavity resonating at one or more wavelengths specific to the type of pixel under consideration. In other words, the lateral dimensions of the portion of the semiconductor layer 101 in which the sensor pixels are formed are selected so that the lateral optical cavities of pixels of the same type resonate at substantially the same wavelength or multiple same wavelengths, and lateral optical cavities of different types resonate at different wavelengths.

[0085] exist Fig.10In the example of the embodiment of the present invention, the pitch between pixels, i.e. the center-to-center distance between two adjacent pixels of the sensor, is substantially the same for all sensor pixels. Therefore, in order to define portions of the semiconductor layer 101 having different lateral dimensions in different pixels of the sensor, the width of the peripheral insulating wall 105 is varied. More specifically, in this example, in each pixel of the sensor, the width of the peripheral insulating wall 105 of the pixel is small while the lateral dimensions of the portion of the layer 101 of the pixel are significant, and vice versa.

[0086] exist Fig.10 In a sensor of , each pixel comprises a diffractive structure 301 on the rear surface of the semiconductor layer 101, the diffractive structure 301 being capable of deviating light entering the portion of the semiconductor layer 101 of the pixel so that the main part of the light received by the pixel propagates in a substantially horizontal direction within the portion of the semiconductor layer 101 of the pixel. The diffractive structure 301 comprises, for example, a single or multiple cavities formed in the portion of the semiconductor layer 101 of the pixel on the rear surface of the semiconductor layer 101 and filled with a material having a refractive index different from that of the semiconductor layer 101, for example, a material having a refractive index less than that of the semiconductor layer 101, for example, silicon oxide in the case where the layer 101 is made of silicon. For example, one or more cavities of the diffractive structure 301 have a depth less than one tenth of the thickness of the semiconductor layer 101, for example, a depth less than 250 nm. In the example shown, in each pixel, the diffractive structure 301 comprises a single cavity of a cone or pyramid shape, which is arranged substantially at the center of the pixel, with its base flush with the rear surface of the layer 101 and its top facing the front surface of the layer 101. However, the described embodiments are not limited to this particular shape. The diffractive structure 301 is, for example, the same in all sensor pixels. As a variant (not shown), in each pixel, the rear surface of the semiconductor layer 101 may be substantially planar, with the diffractive structure 301 then being formed in the insulating layer 109 below the rear surface of the layer 101. In this case, the diffractive structure 301 may include one or more cavities formed in the insulating layer 109 and filled with a material having a refractive index different from the refractive index of the layer 109, for example a material having a refractive index greater than the refractive index of the layer 109, such as polysilicon.

[0087] It should be noted that in each pixel, the lateral sides of the considered portion of the semiconductor layer 101 should reflect light propagating within the portion of the semiconductor layer 101, so that the portion of the semiconductor layer 101 inside and around which the pixel is formed defines a resonant lateral optical cavity. Fig.10 In the sensor of , this is obtained by the fact that in each pixel, each side surface of the portion of the semiconductor layer 101 of the pixel is coated with a layer of insulating material having a refractive index less than that of the material of the layer 101, for example silicon oxide in the case where the layer 101 is made of silicon. Figure 1 As in the embodiment of FIG. 5 , the peripheral insulating wall may include a central conductive wall, such as a metal, which facilitates confinement and lateral reflection of light within the optical cavity.

[0088] exist Fig.10 In the example of , an additional planarization layer 109 , for example made of silicon oxide, coats the back side of the sensor, in particular the rear surface of parts of the semiconductor layer 101 and the rear surfaces of the peripheral insulating walls 105 of the sensor pixels.

[0089] exist Fig.10 In the example of , each pixel further comprises a microlens 111, which is arranged on the back side of the sensor, opposite to the portion of the layer 101 of the pixel, capable of focusing the incident light towards the active photosensitive area of ​​the pixel. More specifically, in this example, the microlens 111 is arranged on the back surface of the planarization layer 109.

[0090] In operation, in each pixel of the sensor, incident radiation is transmitted in the semiconductor layer 101 via the microlens 111, the planarization layer 109 and the diffractive structure 301. Figure 1 In the example of FIG. 1 , the sensor elements coating the rear surface of the semiconductor layer 101 have substantially the same frequency response for all sensor pixels.

[0091] The incident light penetrates the semiconductor layer 101 from the rear surface of the semiconductor layer 101, is deviated by the diffraction structure 301, and then is alternately reflected on the side surface of the semiconductor layer 101. The lateral optical cavity defined in this way has one or more resonant wavelengths. This results in one or more light absorption peaks of the semiconductor layer 101 at the resonant wavelength. Therefore, although a wide range of wavelengths are transmitted to the semiconductor layer 101, the amount of photogenerated charge measured by each pixel mainly represents the light intensity received at the resonant wavelength of the pixel. Therefore, different types of pixels with different resonant wavelengths measure light intensities at different wavelengths.

[0092] It should be noted that Fig.10 In the example, pixels of the same type are all identical or similar, and pixels of different types differ from each other only by the lateral dimensions of the portions of the semiconductor layer 101 within and around which they are respectively formed.

[0093] Fig.10 An advantage of an embodiment of the invention is that it enables the formation of a multispectral image sensor without having to form filter elements made of different materials on different types of pixels (ie, for measuring light in different wavelength ranges).

[0094] Fig.11 , Fig.12 , Fig.13 , Fig.14 and Fig.15 is a schematic diagram showing the manufacturing process of Fig.10 A cross-sectional view of an example of steps in a method of a multispectral image sensor of the type described. Fig.10 Compared to the representation of Fig.11 , Fig.12 , Fig.13 , Fig.14 and Fig.15 In the view of , the front surface corresponds to the lower surface, and the rear surface corresponds to the upper surface. Fig.11 , Fig.12 , Fig.13 , Fig.14 and Fig.15 In the example, for simplicity, only two different types of pixels are formed, corresponding to Fig.10 The pixel P1 of the sensor (at Figures 11 to 15 in the right hand part of Figures 11 to 15 in the left-hand part of the ).

[0095] Fig.11 Shown with Figure 2 The structure is the same or similar to the initial structure.

[0096] Fig.12 Shows something like Figure 3 The step of etching a vertical trench 201 from the rear surface of the semiconductor layer 101 is performed, the vertical trench 201 insulating the portions of the semiconductor layer 101 forming different pixels of the sensor from each other inside and around the vertical trench 201. Figure 3 The main difference compared to the steps is that Fig.12 In the step of , the trench 201 has a width that varies according to the type of pixel considered.

[0097] Fig.13 Shows something like Figure 4 The step of filling the trench 201 to form the vertical insulating wall 105 of the sensor pixel.

[0098] Fig.14 There is shown the step of etching one or more cavities 401 in each pixel on the rear surface of the portion of the semiconductor layer 101 of the pixel to form the diffractive input structure 301 of the pixel.

[0099] Fig.15 The subsequent step of depositing a planarization layer 109 on the back side of the structure obtained at the end of the previous step is shown. Layer 109 is for example a continuous layer extending over the entire sensor surface. In this example, the material of the planarization layer 109 (for example silicon oxide) also fills the cavity 401, thereby forming a diffractive structure 301. Fig.15 Also shown is a method similar to forming a microlens 111 on the rear surface of the planarization layer 109. Fig. 9steps.

[0100] Fig.16 is schematically and partially shown Fig.10 A cross-sectional view of an alternative embodiment of an image sensor.

[0101] Fig.16 The sensor and Fig.10 The difference between the sensors is mainly that Fig.16 In the example of FIG. 1 , in each pixel, the portion of the semiconductor layer 101 containing the active photosensitive area of ​​the pixel is completely specific to the pixel (and unlike Fig.10 In other words, in this example, the active photosensitive areas of two adjacent pixels are laterally separated by two peripheral insulating walls 105 extending between them, the two peripheral insulating walls 105 having a portion 501 of the semiconductor layer 101 that does not contain the active photosensitive area. As an example, in a top view, the areas 501 form a continuous grid, the size of which is substantially equal to the size of the pixel array of the sensor, separating different pixels of the sensor from each other.

[0102] exist Fig.16 In the example of , the pitch between pixels, i.e. the center-to-center distance between two adjacent pixels of the sensor, is substantially the same for all sensor pixels. Furthermore, the width (in the horizontal direction) of the peripheral insulating wall 105 is substantially the same for all pixels of the sensor. Thus, in order to define portions of the semiconductor layer 101 having different lateral dimensions in different pixels of the sensor, the width of the inactive portions 501 of the semiconductor layer 101 separating the pixels is varied. More specifically, in each pixel of the sensor, the width of the separating portions 501 of the layer 101 is smaller while the lateral dimensions of the portion of the layer 101 containing the active photosensitive area of ​​the pixel are larger, and vice versa.

[0103] and Fig.10 Compared with the sensor, Fig.16 An advantage of the sensor is that all peripheral insulating walls 105 of the sensor have substantially the same width, which makes forming the sensor easier.

[0104] Various embodiments and variations have been described. Those skilled in the art will appreciate that certain features of these various embodiments and variations may be combined, and that other variations will occur to those skilled in the art. In particular, the first embodiment and the second embodiment may be combined, i.e., a pixel (e.g., about a pixel arranged to have vertical resonant operation) may be provided. Figure 1 described) and arranged to have different types of pixels with lateral resonant operation (e.g. with respect to Fig.10 and Fig.16It is possible in particular to provide a sensor having different types of pixels which differ from one another both by their lateral dimensions and by the thickness of the portion of the semiconductor layer 101 containing the active photosensitive area of ​​the pixel.

[0105] Furthermore, the described embodiments are not limited to the examples of materials and dimensions mentioned in this disclosure.

[0106] These changes, modifications and improvements are intended to be part of this disclosure and are intended to fall within the spirit and scope of the present invention. Therefore, the foregoing description is only exemplary and not restrictive. The present invention is limited only by the following claims and their equivalents.

Claims

1. A multispectral image sensor, comprising: Semiconductor layer; and A plurality of pixels are formed within and on top of the semiconductor layer, each pixel comprising an active photosensitive region formed in a portion of the semiconductor layer laterally bounded by a peripheral insulating wall, the plurality of pixels comprising first pixels of a first type and second pixels of a second type, wherein the portion of the semiconductor layer of the first pixel has a first lateral dimension, the first lateral dimension being selected to define a lateral cavity that resonates at a first wavelength, and wherein the portion of the semiconductor layer of the second pixel has a second lateral dimension different from the first lateral dimension, the second lateral dimension being selected to define a lateral cavity that resonates at a second wavelength different from the first wavelength. 2 . The sensor of claim 1 , wherein the first pixel and the second pixel each comprise a diffractive structure arranged on a rear surface of a portion of a semiconductor layer of the pixel.

3. The sensor according to claim 2, wherein: The diffractive structure is formed by a cavity formed in part of the semiconductor layer of each pixel, the cavity being filled with a material having a refractive index less than the refractive index of the material of the semiconductor layer. The sensor of claim 2 , wherein the sensor is configured for backside illumination.

5. The sensor of claim 2, further comprising an insulating region adjacent to the front surface of the semiconductor layer, the insulating region having a metal layer having metallization of interconnections of the pixels.

6. The sensor according to claim 1, wherein the peripheral insulating wall is formed by a metal wall sandwiched between two walls made of insulating material.

7. A sensor according to claim 1, wherein the sensor has more than two different types of pixels formed inside and on top of a portion of the semiconductor layer, each different type of pixel having a different lateral dimension, and the different lateral dimensions are selected to define a lateral cavity that resonates at different wavelengths.

8. The sensor of claim 7, wherein the sensor has at least four different types of pixels.

9. The sensor of claim 1, wherein portions of the semiconductor layer of pixels of the first type and the second type have substantially the same thickness. 10 . The sensor of claim 9 , wherein the first pixel and the second pixel each include a diffractive structure arranged on a rear surface of a portion of a semiconductor layer of the pixel.

11. The sensor of claim 1, wherein portions of the semiconductor layer of the first type and the second type of pixels are different.

12. A method of forming an image sensor, the method comprising: etching a trench structure in a semiconductor substrate to separate a first portion of the semiconductor substrate from a second portion of the semiconductor substrate, the first portion and the second portion having different lateral dimensions; filling the trench structure, wherein the first portion defines a first lateral cavity resonating at a first wavelength and the second portion defines a second lateral cavity resonating at a second wavelength different from the first wavelength; forming a first diffraction structure at a rear surface of a first portion of the semiconductor substrate, and forming a second diffraction structure at a rear surface of a second portion of the semiconductor substrate; as well as A microlens layer is formed over rear side surfaces of the first and second portions of the semiconductor substrate.

13. The method of claim 12, wherein the first portion of the semiconductor substrate includes a first active photosensitive region forming a first pixel of the image sensor, and wherein the second portion of the semiconductor substrate includes a second active photosensitive region forming a second pixel of the image sensor, the first pixel being configured to sense a different wavelength of light than the second pixel. The method of claim 12 , wherein filling the trench structure comprises filling the trench structure with an insulating material. 15 . The method of claim 12 , wherein filling the trench structure comprises lining sidewalls of the trench structure with an insulating material and filling the trench structure with a metal material.

16. The method according to claim 12, wherein: The first portion and the second portion are etched at a backside surface of the semiconductor substrate, and the method further includes forming a plurality of interconnect regions over a frontside surface of the semiconductor substrate. 17 . The method of claim 12 , wherein the first portion and the second portion of the semiconductor substrate have the same thickness.

18. A multispectral sensor, comprising: a semiconductor substrate having a front side and a back side; a plurality of interconnect layers at a front side of the semiconductor substrate; a microlens layer at the back side of the semiconductor substrate; as well as a plurality of photosensitive regions within the semiconductor substrate between the interconnect layer and the microlens layer, each of the photosensitive regions having a lateral dimension associated with a color of light to be detected, wherein the multispectral sensor comprises photosensitive regions having at least four different lateral dimensions, such that the multispectral sensor is capable of detecting at least four different colors of light.

19. The sensor according to claim 18, wherein: The photosensitive areas are separated from each other by a peripheral insulating wall formed by a metal wall sandwiched between two walls made of insulating material.

20. The sensor of claim 18, wherein the front side and the back side of the semiconductor substrate are coated with an insulating material having a refractive index less than the refractive index of the semiconductor material of the photosensitive region.

21. The sensor of claim 18, wherein each of the photosensitive regions has the same thickness.

Images