Image sensor with infrared enhancement structure

Abstract

The invention discloses an image sensor with an infrared enhancement structure and a preparation method thereof, and the image sensor with a polygonal infrared absorption enhancement structure can be obtained through the combination of multiple times of anisotropic dry etching and anisotropic wet etching. And the shape and the size of the groove of the polygonal infrared absorption enhancing structure can be optimized and adjusted according to the process parameters of each etching step, so that the polygonal infrared absorption enhancing structure can be flexibly applied to products with different design sizes, and the absorption efficiency of infrared light with different wave bands is improved.

Description

Technical Field

[0001] The present invention relates to the field of image sensors, and in particular to an image sensor with an infrared enhancement structure. Background Art

[0002] CMOS (Complementary Metal-Oxide-Semiconductor) image sensors have the advantages of simple process, easy integration with other devices, small size, light weight, low power consumption, and low cost. Therefore, with the development of technology, CMOS image sensors are increasingly replacing CCD (Charge-coupled Device) image sensors in various electronic products. At present, CMOS image sensors have been widely used in static digital cameras, camera phones, digital video cameras, medical cameras (such as gastroscopes), automotive cameras, etc. CMOS image sensors can be divided into FSI (Front Side Illumination) and BSI (Back Side Illumination).

[0003] The image sensor operates in response to image light from an external scene being irradiated on the image sensor. The image sensor includes a pixel array having a portion that absorbs incident light and a photosensitive element (e.g., a photodiode) that generates image charge after absorbing the image light, and the pixel array is composed of a plurality of pixel units arranged in an array. The photosensitive element within the pixel unit changes according to changes in the incident image light, thereby generating different images according to the incident light.

[0004] The image quality and light sensitivity of image sensors are particularly important indicators, especially in the NIR (Near Infrared) band. The level of QE (Quantum Efficiency) has a great influence on the imaging quality of image sensors. Improving the QE of image sensors in the NIR band is of great significance to improving the imaging quality of image sensors. Summary of the invention

[0005] The purpose of the present invention is to provide an image sensor with an infrared enhancement structure and a preparation method thereof, wherein at least one groove with a polygonal cross-section is formed in a pixel unit of the image sensor to increase the number of reflections of incident light in the pixel unit, thereby increasing the optical path of the incident light in the pixel area, enhancing the image sensor's ability to capture incident light, reducing light energy reflection losses, and improving the device's light absorption and conversion efficiency.

[0006] To achieve the above object, the present invention provides a method for preparing an image sensor with an infrared enhancement structure, comprising the following steps: S1 forming one or more third trenches with a wide middle and a narrow top and bottom on the substrate by dry etching; S2 wet-etching the third trench to form a fourth trench having a polygonal cross-sectional structure.

[0007] Furthermore, the S1 step is specifically as follows: performing a first dry etching on the substrate to form one or more first grooves; performing a second dry etching on the first grooves to form a water drop-shaped second groove; and performing a third dry etching on the second grooves to form a third groove which is wide in the middle and narrow at the top and bottom.

[0008] Furthermore, the wet etching in the step S2 is specifically to perform anisotropic etching on the third trench using TMAH to form the fourth trench having a polygonal cross-sectional structure.

[0009] Furthermore, the bottom of the second groove is formed into a sharp corner or a platform by controlling the time of the third dry etching.

[0010] Furthermore, by controlling the time of the second dry etching and the third dry etching, the horizontal dimension and the vertical dimension of the second groove can be adjusted respectively.

[0011] Furthermore, a cross section of the fourth groove in at least one direction is hexagonal or pentagonal.

[0012] Furthermore, the etching gas used in the first dry etching and the third dry etching is HBr and / or O2.

[0013] Furthermore, the etching gas used in the second dry etching is O2 and / or NF3.

[0014] Furthermore, the concentration of TMAH in the wet etching is 10-30%, and the temperature of the wet etching is 70-90°C.

[0015] Furthermore, the present invention also provides an image sensor with infrared enhancement function, including a substrate having a first surface and a second surface relative to each other; the substrate includes a plurality of pixel units arranged in an array; a plurality of grooves with a cross-section that is wide in the middle and narrow at both ends extend from the first surface to the inside of the substrate; at least one groove is provided in each of the pixel units; on a cross-section perpendicular to the substrate, from the first surface to the inside of the substrate, the cross-section of the groove is a polygon.

[0016] Furthermore, the polygon is a pentagon or a hexagon.

[0017] Furthermore, the angles of the sides of the pentagon or hexagon are associated with the crystal orientation of the substrate material.

[0018] Furthermore, at least part of the grooves are provided with one or more recessed units.

[0019] Furthermore, one or more recessed units are provided at adjacent positions of at least some of the grooves.

[0020] Furthermore, the recessed units include surfaces that are mutually intersecting or mutually parallel.

[0021] Furthermore, the recessed unit is one or more of a pyramid, cone, cylinder, cuboid, and cube structure.

[0022] Furthermore, the grooves and the recessed units are arranged alternately and at intervals.

[0023] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: the present invention forms a groove with a polygonal cross-section through a combination of one or more of wet etching and dry etching, thereby increasing the optical path of the incident light in the pixel area to improve the quantum efficiency of the image sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings of the present invention constitute a part of this specification and are used to further understand the present invention. The drawings illustrate embodiments of the present invention and together with the description are used to explain the principle of the present invention.

[0025] Figure 1 It is a schematic diagram comparing the optical paths of the IPA infrared enhancement structure and the infrared enhancement structure of the present invention.

[0026] Figures 2A-2H The figure is a flow chart of the method for preparing the infrared enhancement structure of the present invention.

[0027] Figure 3 This is a schematic diagram of the structure of an image sensor that combines an infrared enhancement structure with an inverted pyramid structure according to the present invention.

[0028] Figure 4 It is a schematic diagram of the structure of an image sensor with an infrared enhancement structure according to the present invention. DETAILED DESCRIPTION

[0029] The following detailed descriptions are all illustrative and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0030] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention.

[0031] In a first aspect, the present invention provides an image sensor with an infrared enhancement structure; in a second aspect, the present invention provides a method for preparing an image sensor with an infrared enhancement structure.

[0032] The following is an explanation based on some examples. It should be noted that the polygonal infrared enhancement structure examples involved in the present invention are only for illustration and are not intended to limit the scope. In particular, the description of the polygonal grooves other than pentagons and hexagons (for example, the polygonal grooves can be heptagons, octagons, etc.) is not intended to limit the scope of the invention, because they are all for the purpose of explaining some optional examples of the method for preparing the polygonal infrared enhancement structure.

[0033] Embodiment 1 As a preferred example, Figure 1 As shown, compared with the infrared absorption enhancement structure with an inverted pyramid structure, the number of reflections of the incident light therein is less than that of the polygonal infrared absorption enhancement structure of the present invention, that is to say, the polygonal infrared enhancement structure of the present invention can effectively increase the number of reflections, scatterings and refractions of infrared light in the photosensitive area, thereby improving the quantum efficiency of the image sensor for infrared light.

[0034] FIG. 2 illustrates a process for preparing an image sensor having an infrared enhancement structure. According to some embodiments, the infrared enhancement structure of the present invention may be used in an image sensor chip (eg, a front-illuminated image sensor chip or a back-illuminated image sensor chip). Figure 2A , a substrate 100 is provided. For example, the substrate 100 may include doped or undoped bulk silicon, or an active layer of a semiconductor on insulator (SOI) substrate. In general, an SOI substrate includes a semiconductor material layer, such as silicon, formed on an insulator layer. For example, the insulator layer may be a buried oxide layer or a silicon oxide layer. The insulator layer is disposed on a substrate, such as a silicon substrate or a glass substrate. Alternatively, in some embodiments, the substrate 100 may include: another element semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide and / or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP and / or GaInAsP; or a combination thereof. Other substrates, such as multilayer or gradient substrates, may also be used.

[0035] In some embodiments, one or more active and / or passive devices (not shown) are formed in the substrate 100. One or more active and / or passive devices may include N-type metal oxide semiconductor (NMOS) devices and / or P-type metal oxide semiconductor (PMOS) devices, such as transistors, capacitors, resistors, diodes, photodiodes, fuses, etc. It should be understood by those skilled in the art that the above examples are for illustrative purposes and are not intended to limit the present invention in any form. For specific applications, other circuits may also be used. Photodiodes are preferred in this embodiment.

[0036] like Figure 2A As shown, a bottom anti-reflective coating (BARC) layer 101 is formed on a substrate 100. In some embodiments, the BARC layer includes a suitable dielectric material, such as silicon oxide, silicon nitride, silicon oxynitride, hafnium oxide, tantalum oxide, or a combination thereof, but other materials may also be used. In some embodiments, the BARC layer may be formed using ALD, CVD, PECVD, PVD, metal organic chemical vapor deposition (MOCVD), etc. A photoresist layer 102 is then formed on the bottom anti-reflective layer 101.

[0037] like Figure 2B As shown, a photoresist 102 is patterned by exposing the photoresist layer 102 to ultraviolet light using a photomask (not shown) to form a plurality of openings, and then, depending on whether a positive resist or a negative resist is used, the exposed or unexposed portion of the photoresist layer 102 is removed to form the openings. The plurality of openings can be aligned with a substrate region disposed in an adjacent photosensitive region so as to subsequently form a trench of an infrared enhancement structure in the photosensitive region.

[0038] like Figure 2C As shown, the photoresist layer 102 after the opening is formed is used as an etching mask to etch the bottom anti-reflection layer 101 thereunder. In some embodiments, the etching gas is a mixed gas of O2, SO2, and He, and an endpoint detection method is used to stop the etching of the bottom anti-reflection layer 101 on the substrate 100, and finally a plurality of grooves are formed through the bottom anti-reflection layer 101 and extend to the surface of the substrate 100. The etching is usually anisotropic, so that the sidewalls of the grooves in the bottom anti-reflection layer are substantially perpendicular to the surface of the substrate 100.

[0039] like Figure 2DAs shown, the substrate 100 is further etched for the first time to form a first groove 201 in the substrate 100, wherein the etching of the substrate can be performed at a relatively low temperature to reduce the damage of the etching process to the photosensitive area or the logic area, and this step of etching is anisotropic so that the sidewall of the first groove is substantially perpendicular to the surface of the substrate 100. In some embodiments, the substrate 100 is silicon, and thus the substrate is etched using a dry etching method, including but not limited to inductively coupled plasma (ICP), transformer coupled plasma (TCP), electron cyclotron resonance (ECR), reactive ion etching (RIE), etc. The process gas includes a mixed gas containing HBr, O2, and He. The first groove reaches a specified depth by controlling the etching time.

[0040] like Figure 2E As shown, the first groove 201 is further etched for the second time. The second etching adopts an anisotropic etching process to widen the first groove laterally to form an ellipsoidal second groove 202, in preparation for the subsequent formation of a polygonal groove. The second etching adopts a mixed gas of O2, NF3, and He. By controlling the etching time, the second groove reaches a specified width. This step is also the key to determining the size of the widest part in the middle of the structure. like Figure 2F As shown, the second groove 202 is further etched for the third time to form a third groove 203. In some embodiments, the third etching uses the same process parameters as the first etching, and both are anisotropic etching, so as to further etch downward from the bottom of the second groove 202 to form a protruding groove. The width and depth of the groove can be determined by the etching time. The longer the etching time, the deeper the groove and the sharper the shape. This step is also the key to determining whether the structure will become a hexagon or a pentagon in the future. The depth of the structure is also determined by the third etching. The third etching can use the same etching mask as the first etching. In some other embodiments, an etching mask with a different critical size from the first etching can also be used.

[0041] like Figure 2G As shown, the photoresist layer 102 and the bottom anti-reflective layer 101 are further subjected to wet stripping and wet cleaning. The stripping and cleaning solutions use one or more of sulfuric acid composition (SPM), SC1, and DHF. The photoresist and BARC layer and their residues can be effectively removed by wet stripping and wet cleaning.

[0042] like Figure 2HAs shown, the third groove 203 after stripping and cleaning is further etched for the fourth time to form a fourth groove 204. The fourth etching adopts anisotropic wet etching and is carried out using an alkali solution. In this embodiment, tetramethylammonium hydroxide (TMAH) is used to etch the third groove 203 with a curved sidewall into a fourth groove 204 with a polygonal sidewall. Since the wet etching solution used is TMAH, the sidewall shape of the third groove can be adjusted according to the different etching rates of TMAH in different crystal directions, for example, the upper part of the groove is formed into a trapezoidal cross-section, and the lower half of the groove is formed into an inverted trapezoidal or inverted triangular cross-section. In some embodiments, the shape and size of the groove are related to the process parameters of the fourth etching, such as the time of TMAH wet etching. In some embodiments, the bottom of the fourth groove may also have an inverted triangular cross-section shape. In this embodiment, the bottom of the fourth groove 204 has a horizontal surface.

[0043] In some embodiments, the depth, width, and shape of the fourth groove can be adjusted by changing process parameters according to product design requirements.

[0044] like Figure 3 As shown, in some embodiments, one or more recessed units may be formed between the plurality of fourth grooves 204, for example Figure 3 The inverted pyramid structure 303 is used to further increase the optical path of the incident light in the photosensitive area, thereby improving the absorption efficiency of the photosensitive area below the groove to infrared light. In some other embodiments, the recessed unit can also be one or more of a conical, cylindrical, rectangular, or cubic structure.

[0045] like Figure 4 As shown, before forming the polygonal infrared enhancement structure of the present invention, it also includes forming a plurality of photosensitive areas 301 in the substrate 100, the substrate 100 includes a first semiconductor material; forming fourth grooves in the photosensitive areas, each photosensitive area having one or more fourth grooves 204; forming a deep trench isolation structure 302 between the plurality of photosensitive areas, the depth of the deep trench isolation structure 302 being greater than the depth of the fourth groove.

[0046] The present invention can obtain an image sensor with a polygonal infrared absorption enhancement structure through the organic combination of multiple anisotropic dry etching and anisotropic wet etching, and the shape and size of the groove of the polygonal infrared absorption enhancement structure can be optimized and adjusted according to the process parameters of each etching step to achieve different absorption efficiencies for infrared light of different bands.

[0047] In addition, the present invention also provides an image sensor having the polygonal infrared absorption enhancement structure involved in any of the aforementioned embodiments.

[0048] The basic concepts have been described above. Obviously, for those skilled in the art, the above detailed disclosure is only for example and does not constitute a limitation of the present invention. Although not explicitly stated herein, those skilled in the art may make various modifications, improvements and corrections to the present invention. Such modifications, improvements and corrections are suggested in the present invention, so such modifications, improvements and corrections still belong to the spirit and scope of the exemplary embodiments of the present invention.

[0049] It should be understood that the embodiments described in the present invention are only used to illustrate the principles of the embodiments of the present invention. Other variations may also fall within the scope of the present invention. Therefore, as examples and not limitations, alternative configurations of the embodiments of the present invention may be considered consistent with the teachings of the present invention. Accordingly, the embodiments of the present invention are not limited to the embodiments explicitly introduced and described in the present invention.

Claims

1. A method for forming an image sensor having an infrared enhancement structure, characterized in that: At least the following steps are included: S1 forming one or more third trenches with a wide middle and a narrow top and bottom on the substrate by dry etching; S2 wet-etching the first trench to form a fourth trench having a polygonal cross-sectional structure.

2. The method according to claim 1, characterized in that The S1 step is specifically as follows: Forming one or more first grooves on the substrate by first dry etching; Performing a second dry etching process on the first groove to form a water drop-shaped second groove; The second trench is subjected to a third dry etching process to form a fourth trench which is wide in the middle and narrow at the top and bottom.

3. The method according to claim 1, characterized in that The wet etching in the step S2 is specifically to perform anisotropic etching on the third trench using TMAH to form the fourth trench having a polygonal cross-sectional structure.

4. The method according to claim 2, characterized in that The bottom of the fourth trench is formed into a sharp corner or a platform by controlling the time of the second dry etching.

5. The method according to claim 2, characterized in that By controlling the time of the second dry etching and the third dry etching, the horizontal dimension and the vertical dimension of the fourth groove can be adjusted respectively.

6. The method according to claim 1, characterized in that The cross section of the fourth groove in at least one direction is hexagonal or pentagonal.

7. The method according to claim 2, characterized in that The etching gas used in the first dry etching and the third dry etching is HBr and / or O2.

8. The method according to claim 2, characterized in that The etching gas used in the second dry etching is O2 and / or NF3.

9. The method according to claim 3, characterized in that The concentration of TMAH in the wet etching is 10-30%, and the temperature of the wet etching is 70-90°C.

10. An image sensor with an infrared enhancement structure, characterized in that: The invention comprises a substrate having a first surface and a second surface opposite to each other; the substrate comprises a plurality of pixel units arranged in an array; A plurality of grooves with a cross section that is wide in the middle and narrow at both ends, extending from the first surface into the substrate; At least one groove is provided in each of the pixel units; In a cross section perpendicular to the substrate, from the first surface to the inside of the substrate, the cross section of the groove is a polygonal shape.

11. The image sensor according to claim 10, wherein: The polygon is a pentagon or a hexagon.

12. The image sensor according to claim 11, wherein: The angles of the sides of the pentagon or hexagon are associated with the crystal orientation of the substrate material.

13. The image sensor according to claim 10, wherein: One or more recessed units are also arranged in at least part of the grooves.

14. The image sensor according to claim 10, wherein: One or more recessed units are arranged at adjacent positions of at least some of the grooves.

15. The image sensor according to claim 13 or 14, characterized in that: The recessed units include surfaces that are mutually intersecting or mutually parallel.

16. The image sensor according to claim 13 or 14, characterized in that: The concave unit is one or more of an inverted pyramid, a cone, a cylinder, a cuboid, and a cube structure.

17. The image sensor according to claim 14, wherein: The grooves and the recessed units are arranged alternately and at intervals.

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