Back-illuminated image sensor and manufacturing method

Abstract

Various embodiments of this disclosure relate to back-illuminated image sensors and manufacturing methods. The integrated sensor includes a substrate made of a first semiconductor material having a first optical refractive index. The substrate includes a pixel array, wherein each pixel has a photosensitive active region formed by a refractive contrast region comprising a matrix of the first semiconductor material and a periodic structure embedded therein. The periodic structure extends from the back side of the substrate and has two-dimensional periodicity in a plane parallel to the back side. The value of the periodicity is associated with the wavelength of the optical signal and the first refractive index. Elements of the periodic structure are formed of a second optically transparent material having a second refractive index less than the first refractive index. These elements are located at positions defined by the periodicity, except at one location in the defined region, preferably at the center, where there is no corresponding element among the elements.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to French patent application number 2106486, filed on June 18, 2021, the entire contents of which are incorporated herein by reference to the fullest extent permitted by law. Technical Field

[0003] The implementation and embodiments relate to back illumination (BSI), and more specifically to the enhancement of its quantum efficiency, particularly in the infrared range. Background Technology

[0004] An integrated image sensor includes a pixel array in a substrate, which is associated with control and processing electronics regarding the signals provided by the pixels.

[0005] When an image sensor is illuminated by light signals passing through the back of a substrate, it is called a back-illuminated type.

[0006] The quantum efficiency of a pixel is defined by the ratio of the number of electrons generated to the number of photons received when the pixel absorbs a light signal.

[0007] Higher efficiency results in better sensor sensitivity and a better signal-to-noise ratio.

[0008] Therefore, it is necessary to improve the quantum efficiency of back-illuminated integrated image sensors.

[0009] Gain is also needed in terms of crosstalk caused by optical coupling between pixels (interference light signals reflected or diffracted from one pixel to its neighboring pixels) and in terms of modulation transfer function in order to obtain excellent contrast in the image. Summary of the Invention

[0010] In one embodiment, each pixel forms a photonic periodic structure that has two-dimensional periodicity in a plane parallel to the back surface, and the photonic structure has defects, for example, at the level of its central region.

[0011] Therefore, based on one aspect, a back-illuminated integrated image sensor is proposed.

[0012] The sensor is designed to be illuminated by light signals, such as (but not limited to) signals in the infrared range with a wavelength of 940 nm.

[0013] The sensor includes a substrate having a back side and containing a first semiconductor material (e.g., silicon) having a first optical refractive index.

[0014] The sensor consists of a pixel array in a substrate.

[0015] Each pixel has a photosensitive active area.

[0016] Each photosensitive active region is a refractive contrast region, which includes a matrix formed of a first semiconductor material (e.g., silicon) and a periodic structure embedded in the matrix formed of a second material (e.g., silicon dioxide) with a refractive index lower than that of the first material.

[0017] The periodic structure extends from the back side into the matrix and has two-dimensional periodicity in a plane parallel to the back side.

[0018] The two-dimensional periodicity is not necessarily the same along the two orthogonal directions of the plane.

[0019] This periodic structure is called a photonic crystal because the value of the periodicity is related to the wavelength of the light signal and the first refractive index, which guides the light signal through the photosensitive region.

[0020] Therefore, conventionally, this periodicity value is at least substantially equal to the ratio between the wavelength of the optical signal and the refractive index of the first material. That is, to further enhance performance, this periodicity value can be taken as an increase of several tens of percent, such as 30%, in the aforementioned ratio.

[0021] The periodic structure has multiple elements, such as rods, formed of a second optically transparent (i.e., non-absorbent) material and having a second refractive index less than the first refractive index.

[0022] Furthermore, the periodic structure has regions without at least one of these elements, such as the central region.

[0023] Therefore, due to the presence of this defect in the central region (the absence of at least one low-refractive-index and optically transparent element), light radiation will be confined to a specific volume of the pixel, such as at the center of the pixel, and the periodic structure will slow its propagation in the first material (e.g., silicon).

[0024] Therefore, this will lead to greater absorption in the first material, and thus to the generation of a larger number of electrons during the pixel integration time, resulting in enhanced quantum efficiency.

[0025] Periodic structures have, for example, honeycomb structures of elements in their periodic planes.

[0026] Furthermore, it is particularly advantageous that, in the periodic plane, the crystal density of all optically transparent elements occupies at least 50% of the surface area of ​​the photosensitive region.

[0027] This allows for further improvements in quantum efficiency.

[0028] As mentioned above, the element can be a rod with a cross-section of various shapes (circle, rectangle, star, etc.) in a periodic plane.

[0029] When the first material is silicon and the wavelength of the optical signal is equal to 940 nm, the spacing between optically transparent elements in the periodic plane can be on the order of 400 nm (i.e., within 400 nm + / - 30%).

[0030] The second optically transparent material can be, for example, silicon dioxide.

[0031] Each rod may have another cross section with a diameter of 200 nm (i.e., within 200 nm + / - 25%).

[0032] To ensure proper collection of the charge generated for the transistors in the processing electronic devices, it is preferable that the structure extends in the matrix to a position located at a distance from the front of the substrate.

[0033] In addition, to further improve the absorption efficiency in the first material, it is advantageous that the sensor also includes an optical mirror positioned to face each photosensitive area, which is opposite to the back surface.

[0034] When the pixel array has a periodic arrangement with a spacing of N, it is advantageous to further enhance the operation of the sensor, where the spacing N is an integer ratio to the period of the periodic structure (i.e., equal to k times the period of the periodic structure, where k is an integer).

[0035] According to another aspect, a method for manufacturing a back-illuminated sensor, the back-illuminated sensor being intended to be illuminated by an optical signal, the method includes forming a pixel array in a substrate having a back side and comprising a first semiconductor material having a first optical refractive index, the pixel array including a photosensitive active region for each pixel according to one embodiment.

[0036] According to this aspect of the method, the formation of each photosensitive active region includes the formation of a periodic structure embedded in a matrix formed of a first semiconductor material, the periodic structure extending from the back side in the matrix, the periodic structure having two-dimensional periodicity in a plane parallel to the back side, the value of which is associated with the wavelength of the optical signal and a first refractive index, the periodic structure having a plurality of elements formed of a second optically transparent material having a second refractive index less than the first refractive index, and having a region lacking at least one of these elements, preferably a central region.

[0037] According to one implementation, forming a periodic structure includes forming a mask on the back surface defining the position of the element, etching a matrix through the mask, and filling the holes created by the etching with a second material.

[0038] In one implementation, the etching of the substrate stops at a distance from the front of the substrate.

[0039] According to one implementation method, the first material is silicon, and the second material is silicon dioxide.

[0040] According to one implementation, the method also includes forming a pixel-facing optical mirror with the photosensitive area opposite the back surface. Attached Figure Description

[0041] Further advantages and features of the invention will become apparent from the detailed description of the non-limiting implementations and embodiments, as well as from the accompanying drawings, in which:

[0042] Figure 1 An integrated image sensor is schematically shown;

[0043] Figure 2 It is a cross-sectional view of the pixels;

[0044] Figure 3 It is along Figure 2 A cross-sectional view of line III-III;

[0045] Figure 4 The values ​​of periodicity associated with wavelength are schematically shown; and

[0046] Figures 5 to 8 Explain the implementation of the method used to create pixels.

[0047] Specific implementation method

[0048] exist Figure 1 In the figure, reference numeral CPT indicates an integrated image sensor, which schematically shows a substrate 1 made of, for example, silicon, having a back FAR and a front FAV.

[0049] The back-facing FAR is designed to be illuminated by a light signal SL, such as an infrared signal, typically by means of optical devices, such as lenses, optionally color filters, polarizing devices, etc., these examples are not limiting.

[0050] The sensor includes a pixel array PXAR in substrate 1.

[0051] Here, the pixel array has a periodic arrangement with a spacing N between pixels PX.

[0052] The image sensor CPT includes an interconnect portion 2 on the front side of the substrate 1, which is typically referred to by those skilled in the art as BEOL (back end of line).

[0053] This section 2 typically includes metal tracks and vias for providing interconnections between different elements of the sensor, particularly pixels, as well as transistors for control and processing electronics of the signals emitted by the pixels, which are not shown here for the sake of simplicity.

[0054] In other words, these control electronics have conventional structures that are known in themselves.

[0055] Figure 2 A schematic cross-sectional view of the pixel PX of the pixel array PXAR is shown.

[0056] The pixel PX includes a refractive contrast photosensitive region, which forms a matrix 50 formed of a first semiconductor material (typically silicon as a substrate).

[0057] Furthermore, the refractive contrast photosensitive region includes a periodic structure 5 embedded in the matrix 50, and includes a plurality of elements 51 formed of a second optically transparent (non-absorbent) material such as silicon dioxide.

[0058] When the optical refractive index of the first material (silicon) is different from the refractive index of the second material (silicon dioxide), the photosensitive region is called refractive index contrast.

[0059] The refractive index of the first material forming the matrix 50 is greater than that of the second optically transparent material.

[0060] For example, silicon has a refractive index close to 3.4, while silicon dioxide has a refractive index close to 1.46.

[0061] like Figure 2 As shown, the element 51 of the periodic structure 5 extends from the back side of the substrate FAR to a depth less than the depth of the substrate.

[0062] In other words, element 51 (typically a rod) stops at a distance d from the front surface FAV of the substrate. This distance d is, for example, on the order of 1 to 2 micrometers.

[0063] Optical devices 4, such as lenses, are set on the back of the photosensitive area FAR.

[0064] In addition to the metal track 20, the interconnect portion 2 also includes an optical metal reflector 3, which is configured to face the photosensitive area of ​​the pixel, more specifically facing the front FAV.

[0065] The metal mirror is advantageously formed from the metal layer of the interconnect portion.

[0066] like Figure 3 As shown, Figure 3 It is along Figure 2 The cross-sectional view of line III-III shows that periodic structure 5 has two-dimensional periodicity in a plane, which is... Figure 3 The plane, and the plane is parallel to the back FAR.

[0067] The periodic structure has a honeycomb arrangement of elements 51 at locations defined by periodicity / spacing and matrix in its periodic plane, and has a central region 500 with defects (i.e., a region without at least one of the elements 51).

[0068] In other words, the central region 500 includes the first material, which is silicon.

[0069] like Figure 4 As schematically shown, the periodic values ​​are associated with the wavelength of the optical signal SL and with the first refractive index of the first material (here, silicon).

[0070] To increase the density of the second material, such as in the case of a honeycomb structure, the periodicity in the planar direction can optionally differ from the periodicity in other orthogonal directions.

[0071] More specifically, for the wavelength of the light signal SL, which corresponds to 940 nm in infrared light, the spacing between the elements 51 in the periodic plane is on the order of 400 nm ± 30% (the side of the equilateral triangle connecting the centers of the three rods 51).

[0072] To further enhance the operation of the sensor, the spacing N of the pixels PX is advantageously in an integer ratio to the spacing between the elements 51.

[0073] Furthermore, the diameter of rod 51 is on the order of 200 nm ± 25%, and the crystal density of the cross-section of all rods occupies at least 50% of the surface area of ​​the photosensitive region, and for example 50%.

[0074] The defects of the periodic structure, preferably the center of the periodic structure (i.e., the absence of at least one rod at a location in a preferred central region 500, otherwise the location would have rods according to the defined spacing or periodicity) form a waveguide for optical radiation SL, and the periodic structure around the waveguide will slow down the propagation rate of the optical signal in silicon, and thus increase the absorption time and quantum efficiency of the pixel.

[0075] Therefore, a 25% increase in quantum energy can be achieved compared to pixels that do not have a periodic structure with defects in the photosensitive region.

[0076] In operation, the light signal illuminates the back of the sensor, enters the central region 500 of the photosensitive area after passing through lens 4, propagates in the aforementioned waveguide, is reflected by the metal mirror 3, and is distributed within the waveguide. This return in the waveguide further increases the absorption time in the silicon, and thus further increases the quantum energy of the pixel. Due to the presence of the waveguide (and the lack of diffraction to adjacent pixels), it also provides gains in crosstalk and modulation transfer function for the reflection quality of the same pixel.

[0077] Now for more specific reference Figures 5 to 8 This will illustrate the implementation of the method used to manufacture such pixels.

[0078] Clearly, all pixels are implemented simultaneously, but Figures 5 to 8 The steps are shown for only one pixel.

[0079] exist Figure 5 The substrate provided includes an epitaxial region 50 (matrix) of silicon with a height of, for example, 6 to 9 micrometers.

[0080] The front of the photosensitive area (the front of the matrix 50) FAV support includes the interconnect portion 2 of the metal mirror 3.

[0081] On the back side of the substrate 50, a mask 7, such as a resin mask, is formed, the apertures of which define the positions of the future silica rods (see...). Figure 6 ).

[0082] Then, as Figure 7 As shown, the silicon of the substrate 50 is etched through holes in the mask 7 using conventional etching methods known per se, and the holes thus formed are filled with silicon dioxide to form the rod 51.

[0083] It should be noted here that, as mentioned above, it is particularly advantageous that the end of the rod 51 and the end of the hole thus created by etching are located at a non-zero distance d (on the order of 1 to 2 micrometers) from the front of the substrate 50, in order to facilitate good collection of electrons generated by the absorption of optical signals in silicon.

[0084] Then, as Figure 8 As shown, mask 7 is removed, the back side is polished, and lens 4 is fixed to the back side of the pixel, thereby forming Figure 2 The pixels shown.

[0085] This invention is not limited to the described implementations and embodiments, but includes all variations.

[0086] Therefore, even if the region of a defective periodic structure is preferably the central region, it may not be the central region, especially for manufacturing reasons.

[0087] In addition, other first materials are also possible, such as germanium or silicon / germanium alloys. Further second materials are possible, such as silicon nitride or high-dielectric-constant dielectrics, known to those skilled in the art as "high-K".

[0088] Furthermore, other wavelengths of the optical signal SL (optionally within the visible range) are also possible to exchange for the adjustment of the periodic structure size.

Claims

1. A back-illuminated integrated image sensor, comprising: a substrate having a back side configured to be illuminated by a light signal and comprising a first semiconductor material having a first optical refractive index; and a pixel array in the substrate; wherein each pixel has a photosensitive active region; wherein each photosensitive active region is a refractive contrast region comprising a matrix formed of the first semiconductor material and a periodic structure embedded in the matrix, the periodic structure extending in the matrix from the back side; wherein the periodic structure has a two-dimensional periodicity in a plane parallel to the back side; wherein a value of the two-dimensional periodicity is associated with a wavelength of the light signal and with the first optical refractive index; and the periodic structure is formed of a plurality of elements made of a second optically transparent material having a second refractive index smaller than the first optical refractive index, the plurality of elements being located at positions defined by the two-dimensional periodicity; and wherein the photosensitive active region comprises a region at one of the positions defined by the two-dimensional periodicity, the region being free of a corresponding element of the elements.

2. The sensor of claim 1, wherein the region is a central region.

3. The sensor of claim 1, wherein the periodic structure has a honeycomb arrangement of the elements in the plane of the two-dimensional periodicity.

4. The sensor of claim 1, wherein a crystal density of all elements occupies at least 50% of a surface area of a photosensitive region in the plane of the two-dimensional periodicity.

5. The sensor of claim 1, wherein the elements are rods.

6. The sensor of claim 1, wherein the first semiconductor material is silicon.

7. The sensor of claim 5, wherein the wavelength of the light signal equals 940 nm and a spacing between the elements in the plane of the two-dimensional periodicity is of the order of 400 nm.

8. The sensor of claim 7, wherein the second optically transparent material is silicon dioxide.

9. The sensor of claim 8, wherein each rod has a cross-section having a diameter of the order of 200 nm.

10. The sensor of claim 1, wherein the structure extends in the matrix to a position located at a distance from a front side of the substrate.

11. The sensor of claim 1, further comprising an optical mirror positioned to face each photosensitive region opposite the back side.

12. The sensor of claim 1, wherein the pixel array has a periodic arrangement having a pitch that is an integer ratio of a period of the periodic structure. In a substrate having a back side intended to be illuminated by a light signal and comprising a first semiconductor material having a first optical refractive index: forming a pixel array comprising a photosensitive active region for each pixel; ​ 13. A method for manufacturing a backside illumination type integrated image sensor, comprising: ​ ​ forming a periodic structure in each photoactive region, the periodic structure being embedded in a matrix formed of the first semiconductor material; wherein the periodic structure extends in the matrix from the back surface and has a two-dimensional periodicity in a plane parallel to the back surface; wherein a value of the two-dimensional periodicity is associated with a wavelength of the optical signal and with the first optical refractive index; wherein forming the periodic structure comprises: generating a plurality of elements formed of a second optically transparent material having a second refractive index smaller than the first optical refractive index, the plurality of elements being located at positions defined by the two-dimensional periodicity; and defining a region in one of the positions defined by the two-dimensional periodicity, the region being free of a corresponding one of the elements.

14. The method of claim 13, wherein the region is a central region.

15. The method of claim 13, wherein forming the periodic structure comprises: forming a mask on the back surface defining the positions of the elements; etching the matrix through the mask; and filling the holes resulting from the etching with the second optically transparent material.

16. The method of claim 15, wherein the etching of the matrix stops at a distance from a front surface of the substrate.

17. The method of claim 13, wherein the first semiconductor material is silicon and the second optically transparent material is silicon dioxide.

18. The method of claim 13, further comprising: providing an optical mirror facing a photoactive region of the pixel, the photoactive region being opposite the back surface.

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