CMOS Image Sensor and Method for Forming the Same

Abstract

The present invention provides a CMOS image sensor and a method for forming the same. The CMOS image sensor includes: a plurality of pixel units, and a microlens structure located above the pixel units; the microlens structure includes: a microlens core stacked by multiple light-transmitting laying layers, and a microlens outer layer covering the microlens core, wherein the multiple light-transmitting laying layers have different refractive indexes. By using the light-transmitting laying layer as a hard mask to form the microlens core composed of multiple light-transmitting laying layers, the present invention can improve the stability of the lithography and etching processes, and the formed image sensor has better light sensitivity and relative illuminance, and less crosstalk between pixels.

Description

Technical Field

[0001] The present invention relates to the field of image processing, and more particularly to a CMOS image sensor and a method for forming the same. Background Art

[0002] CMOS 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 image sensors and being applied to various electronic products. Currently, CMOS image sensors have been widely used in static digital cameras, camera phones, digital video cameras, medical imaging devices (such as gastroscopes), vehicle-mounted imaging devices, etc. CMOS image sensor products can be divided into FSI (Front Side Illumination) and BSI (Back Side Illumination).

[0003] The image sensor irradiates incident light on the image sensor through a microlens. The microlens is a main factor for improving the light concentration efficiency and determining the characteristics of the image sensor. There are usually two methods for preparing the microlens: the thermal reflow process and the reverse etching process. In the thermal reflow process, the columnar photoresist is heated above the glass transition temperature, so that the photoresist is in a molten state. Under the action of the surface tension of the photoresist, the photoresist will automatically form a hemispherical microlens. Since the photoresist is prone to adhesion after melting, once the adjacent molten photoresists come into contact, the lens surface shape cannot be formed, resulting in abnormal microlens structures finally formed, so that the incident light cannot be fully utilized and background noise will be caused. Therefore, the thermal reflow process needs to ensure a certain distance between the molten photoresists, which ultimately leads to too small a fill factor of the image sensor; the reverse etching process is based on the thermal reflow process with an additional reverse etching process, so that the material at the bottom of the photoresist is etched reversely according to the shape of the molten hemispherical photoresist to finally form a microlens with almost no gap, which can achieve the effect of improving the fill factor of the image sensor. However, the currently commonly used thermal reflow process and reverse etching process need to be processed in a dedicated color filter foundry (OCF FAB), so additional logistics time and complex processing processes are required, resulting in increased costs.

[0004] The material and structure of the microlens are the key factors determining the light concentration efficiency. In the traditional microlens structure of the image sensor, most of the microlens structures only use one material and have only one refractive index, which cannot adjust the incident light path and cannot significantly improve the light performance of the image sensor.

[0005] On the one hand, it is necessary to ensure the optical performance of the image sensor while reducing the processing cost, thereby improving the competitiveness of the product; on the other hand, by setting a suitable microlens structure, the formation process of the microlens is made more stable and the optical path is more optimized. The present invention is proposed to solve the above two problems. Summary of the Invention

[0006] The purpose of the present invention is to provide a CMOS image sensor and a forming method thereof, which have better photosensitivity and relative illuminance, less crosstalk between pixels, and a more stable process.

[0007] Based on the above considerations, the present invention provides a CMOS image sensor, including:

[0008] A plurality of pixel units, and a microlens structure located above the pixel units;

[0009] The microlens structure includes: a microlens core stacked by multiple light-transmitting laying layers, and a microlens outer layer covering the microlens core, wherein the multiple light-transmitting laying layers have different refractive indexes.

[0010] Optionally, the microlens core is: the first light-transmitting laying layer to the (N-1)th light-transmitting laying layer stacked from bottom to top along the optical path direction, and the refractive indexes of the first light-transmitting laying layer to the (N-1)th light-transmitting laying layer are n 1 to n N-1 ;

[0011] The refractive index of the microlens outer layer is n N ; where N is a natural number greater than or equal to 3.

[0012] Optionally, the material of the light-transmitting laying layer or the microlens outer layer is selected from SiN, SiO 2 or SiON, and the n 1 to n N first increases and then decreases.

[0013] Optionally, the material of the (N-1)th light-transmitting laying layer is SiN or SiON, and the (N-1)th light-transmitting laying layer is a hard mask layer for forming the microlens core.

[0014] Optionally, the microlens core is integrally in an island shape, a column shape, or an arc shape.

[0015] Optionally, the cross-sectional shape of the island shape, column shape, or arc shape is: circular or triangular or polygonal.

[0016] Optionally, the n 1 is equal to n N .

[0017] Optionally, when N is an even number, nN / 2 With n N / 2+1 is (n 1 , n 2 , …… n N ), and n N / 2 to n 1 decreases in sequence, and n N / 2+1 to n N decreases in sequence.

[0018] Optionally, when N is odd, n （N+1） / 2 is (n 1 , n 2 , …… n N ), and n （N+1） / 2 to n 1 decreases in sequence, and n ( N+1） / 2 to n N decreases in sequence.

[0019] Optionally, the microlens structure is applied to the inner lens of a color or black-and-white image sensor or the outer lens of a black-and-white image sensor.

[0020] The present invention also provides a method for forming a CMOS image sensor, including the following steps:

[0021] Form a microlens core stacked by multiple light-transmitting laying layers along the optical path direction above the pixel unit;

[0022] Deposit a microlens outer layer to coat the microlens core to form a microlens;

[0023] Among them, the multiple light-transmitting laying layers have different refractive indexes.

[0024] Optionally, the step of forming the microlens core includes: sequentially forming multiple light-transmitting laying layers, and forming a patterned topmost light-transmitting laying layer through a patterning process;

[0025] Using the patterned topmost light-transmitting laying layer as a hard mask, etch the light-transmitting laying layer under the hard mask, and retain the hard mask and jointly form a stacked microlens core with the light-transmitting laying layer under the hard mask.

[0026] Optionally, the step of forming the microlens core is: sequentially forming the first light-transmitting laying layer to the (N-1)th light-transmitting laying layer, and the refractive indexes of the first light-transmitting laying layer to the (N-1)th light-transmitting laying layer are n 1 to n N-1 ;

[0027] Form a photoresist layer on the surface of the (N-1)th light-transmitting laying layer, and form a patterned (N-1)th light-transmitting laying layer through photolithography and etching processes;

[0028] Using the patterned (N-1)th layer of light-transmissive paving layer as a hard mask, continue to etch the (N-2)th layer of light-transmissive paving layer to form microlens nuclei with island-shaped, column-shaped, or arc-shaped structures;

[0029] Deposit the outer layer of the microlens to coat the microlens nuclei, forming microlenses, where the refractive index of the outer layer of the microlens is n N , and N is a natural number greater than or equal to 3.

[0030] Optionally, deposit the outer layer of the microlens to coat the microlens nuclei, and the adjacent outer layers of the microlenses overlap to form overlapping microlenses.

[0031] Optionally, the maximum width of the cross-section of the microlens nuclei is 1 / 6 to 3 / 4 of the width of the pixel unit.

[0032] Optionally, the cross-section of the island-shaped, column-shaped, or arc-shaped structure is: circular or triangular or polygonal.

[0033] The CMOS image sensor and its forming method of the present invention have the following beneficial effects:

[0034] 1) By using the light-transmissive paving layer as a hard mask to form microlens nuclei, the stability of the lithography and etching processes can be greatly improved, thereby avoiding the missing of microlenses due to process instability and ultimately causing white screen black dots;

[0035] 2) Having microlens nuclei with multiple layers of different refractive indices, for the pixels at the edges (four corners), light is more likely to converge into the pixels, thus reducing the difference between the four corners and the center, with a relatively high relative illuminance. In particular, light is more likely to converge to the center of the pixel, preventing light from reaching the P-type isolation region and being lost;

[0036] 3) Having microlens nuclei with multiple layers of different refractive indices, for the pixels at the edges (four corners), light is more likely to converge into the pixels, and light or generated electrons are not easily introduced into adjacent pixels, with better anti-optical crosstalk performance;

[0037] 4) Having microlens nuclei with multiple layers of different refractive indices, for the pixels at the edges (four corners), light is more likely to converge into the pixels, and the influence of the brightness channel difference is small. Therefore, GR / GB, R / G, and B / G are closer to 1. In particular, light is easily converged to the center of the pixel, and the possibility of light being blocked in some areas due to the asymmetric design is small;

[0038] 5) The microlenses with multiple layers of different refractive indices and devices such as pixel units are all fabricated in the foundry. Their alignment accuracy with the previous layer is higher, and the additional logistics cost is saved, thereby improving the competitiveness of the product. Description of the Drawings

[0039] Other features, objects, and advantages of the present invention will become more apparent by reading the following detailed description of non-limiting embodiments with reference to the accompanying drawings.

[0040] Figures 1-6 FIG. 4 is a process schematic diagram of a method for forming an image sensor chip according to Embodiment 1 of the present invention;

[0041] Figures 7-12 FIG. 8 is a process schematic diagram of a method for forming an image sensor chip according to Embodiment 2 of the present invention.

[0042] In the figures, throughout the different views, the same or similar reference numerals denote the same or similar devices (modules) or steps. Detailed Embodiments

[0043] To solve the above problems in the prior art, the present invention provides a CMOS image sensor and a method for forming the same. Using a light-transmitting laying layer as a hard mask to form a microlens core composed of multiple light-transmitting laying layers can improve the stability of lithography and etching processes, and the formed image sensor has better light sensitivity and relative illuminance, and less crosstalk between pixels.

[0044] In the following detailed description of the preferred embodiments, reference will be made to the accompanying drawings that form a part of the present invention. The accompanying drawings illustrate, by way of example, specific embodiments that can implement the present invention. The example embodiments are not intended to exhaust all embodiments according to the present invention. It can be understood that other embodiments can be utilized without departing from the scope of the present invention, and structural or logical modifications can also be made. Therefore, the following detailed description is not restrictive, and the scope of the present invention is defined by the appended claims.

[0045] The present invention will be further described below in conjunction with specific embodiments.

[0046] Embodiment 1

[0047] As Figure 6 shown, an embodiment of the present invention provides a CMOS image sensor. The CMOS image sensor includes:

[0048] A plurality of pixel units and a microlens structure located above the pixel units; the microlens structure includes: a microlens core 12 stacked by multiple light-transmitting laying layers, and a microlens outer layer 14 covering the microlens core, wherein the multiple light-transmitting laying layers have different refractive indexes.

[0049] Specifically, as Figure 2As shown, in this embodiment, the pixel units are separated by dashed lines, taking the adjacent first pixel unit 101 and second pixel unit 102 as an example. The pixel unit includes a photodiode, as well as a transfer transistor, a reset transistor, a source follower transistor, a row selection transistor (not shown), etc. In this embodiment, a passivation layer 11 is further provided on the pixel unit. On the one hand, it is used to protect the chip from environmental factors such as water vapor. On the other hand, it serves as a stop layer for the subsequent etching of the light-transmissive laying layer under the hard mask.

[0050] As an example, such as Figure 6 shown, the microlens core 12 includes a first light-transmissive laying layer 121 and a second light-transmissive laying layer 122 laid from bottom to top along the optical path direction. Different materials are selected for the first light-transmissive laying layer 121 and the second light-transmissive laying layer 122. The refractive index of the first light-transmissive laying layer 121 is set to n 1 , and the refractive index of the second light-transmissive laying layer 122 is set to n 2 . The microlens outer layer 14 covers the microlens core 12, and the refractive index of the microlens outer layer 14 is set to n 3 . In other embodiments, the microlens core includes a first light-transmissive laying layer to an (N - 1)th light-transmissive laying layer stacked in sequence from bottom to top along the optical path direction. The refractive indices of the first light-transmissive laying layer to the (N - 1)th light-transmissive laying layer are n 1 to n N-1 in sequence, where N is a natural number greater than or equal to 3. The refractive indices of the light-transmissive laying layers are not completely the same, and the refractive index of the microlens outer layer is set to n N .

[0051] In the present invention, the microlens core is set to be stacked by multiple light-transmissive laying layers with incomplete same refractive indices. The microlens core set in this way can adjust the incident path of light. For the pixels at the edges (four corners), light is more likely to converge into the pixels, the difference between the four corners and the center is reduced, and the relative illuminance is high. Especially, light is particularly likely to converge to the pixel center and is not easily lost by reaching the isolation area. Moreover, light or generated electrons are not easily introduced into adjacent pixels, and the anti-crosstalk performance is better. At the same time, light is more likely to converge into the pixels, and the difference in the brightness channels is small. Therefore, GR / GB, R / G, B / G are closer to 1. Especially, light is likely to converge to the pixel center, which can reduce the possibility of light blocking in some areas due to the asymmetry of the design.

[0052] For the microlens structure, that is, the material of the microlens core and the microlens outer layer covering the microlens core, a transparent or translucent material that can function as a lens can be selected without special limitation. Preferably, the materials of the light-transmissive laying layer and the microlens outer layer can be selected as SiN, SiO 2Materials with good light transmittance such as SiON can also be appropriately selected according to their uses, and are not limited here. Preferably, the material of the outermost light-transmitting laying layer of the microlens core is selected as harder SiN or SiON. The outermost light-transmitting laying layer serves as a hard mask layer for forming the microlens core, which can greatly improve the stability of the photolithography and etching processes, thereby avoiding the missing of microlenses due to process instability and ultimately causing white screen black dots.

[0053] In this embodiment, the material of the first light-transmitting laying layer 121 is selected as silicon oxide, the material of the second light-transmitting laying layer 122 is selected as silicon nitride, and the material of the microlens outer layer 14 is silicon oxide, which is the same as the material of the first light-transmitting laying layer 121.

[0054] The refractive index settings in the multi-layer light-transmitting laying layers of the microlens structure, especially the microlens core, can be set according to actual needs to adjust the light path. In other embodiments, the refractive index of the microlens structure is set such that the refractive index of the middle layer is high, and the refractive indices of the layers on both sides decrease in sequence. Specifically, when N is an even number, n N / 2 and n N / 2+1 are the maximum numbers in (n 1 , n 2 , …… n N ), and n N / 2 to n 1 decrease in sequence, n N / 2+1 to n N decrease in sequence; when N is an odd number, n （N+1） / 2 is the maximum number in (n 1 , n 2 , …… n N ), and n （N+1） / 2 to n 1 decrease in sequence, n （N+1） / 2 to n N decrease in sequence. In another embodiment, the refractive index in the microlens structure, i.e., n 1 to n N , is set to increase first and then decrease. Of course, the refractive indices of the light-transmitting laying layers of the microlens core can also be set to other combinations according to actual needs, and are not specifically limited.

[0055] As an example, as Figure 6 shown, the shape of the microlens core 12 can be selected as an island shape, a column shape, an arc shape or other shapes. The cross-sectional shapes of the island shape, column shape, and arc shape are: circular, triangular or polygonal. In this embodiment, the shape of the microlens core 12 is columnar. The width w2 of the microlens core 12 can be set according to actual needs. Preferably, the width w2 of the microlens core 12 is 1 / 6 to 3 / 4 of the width w1 of the pixel unit.

[0056] In addition, as Figure 6As shown, the outer layers of adjacent microlenses overlap and are set as overlapping microlenses, which can improve the filling rate of the microlenses, thereby improving the fill factor of the image sensor.

[0057] This embodiment also provides a method for forming the above image sensor, including:

[0058] Form a microlens core stacked by multiple light-transmitting laying layers along the optical path direction above the pixel unit;

[0059] Deposit the outer layer of the microlens to coat the microlens core and form a microlens;

[0060] Among them, the multiple light-transmitting laying layers have different refractive indexes.

[0061] The step of forming the microlens core includes: sequentially forming multiple light-transmitting laying layers, and forming a patterned topmost light-transmitting laying layer through a patterning process;

[0062] Using the patterned topmost light-transmitting laying layer as a hard mask, etch the light-transmitting laying layer under the hard mask, and retain the hard mask and jointly form a stacked microlens core with the light-transmitting laying layer under the hard mask.

[0063] For the specific forming method, refer to Figures 1-6 for detailed description.

[0064] As Figure 1 shown, provide a semiconductor substrate (not shown), and several pixel units are provided on the semiconductor substrate. Taking the adjacent first pixel unit 101 and the second pixel unit 102 as examples. In this embodiment, a passivation layer 11 is also provided above the pixel unit.

[0065] As Figure 2 shown, sequentially form a light-transmitting laying layer 12 above the pixel unit. The forming method of the light-transmitting laying layer includes chemical vapor deposition method, evaporation method, sputtering method, etc. In this embodiment, the light-transmitting laying layer 12 includes a first laying layer 121 and a second light-transmitting laying layer 122 located above the first light-transmitting laying layer 121.

[0066] As Figures 3-5 shown, form a microlens core 12 above the pixel unit. Form a patterned photoresist layer 13 on the surface of the second light-transmitting laying layer 122, and through photolithography and etching processes, form a patterned second light-transmitting laying layer 122; using the patterned second light-transmitting laying layer 122 as a hard mask, continue to etch the remaining light-transmitting laying layers to form a microlens core 12.

[0067] The present invention creatively adopts a method of using the outermost light-transmitting laying layer as a hard mask to etch the remaining light-transmitting laying layer, which can improve the process stability of photolithography and etching, thereby avoiding the missing of microlenses caused by process instability and finally resulting in white screen black dots.

[0068] The width w2 of the microlens core 12 can be set as needed. Preferably, the width w2 of the microlens core is 1 / 6 to 3 / 4 of the width w1 of a single pixel unit.

[0069] As Figure 6 shown, deposit the material of the microlens outer layer to form the microlens outer layer 14. In this embodiment, deposit a thick enough material of the microlens outer layer to make the adjacent microlens outer layers 14 overlap, improve the filling rate of the microlenses, and thus improve the fill factor of the image sensor.

[0070] It should be noted that the microlens structure and its forming method provided in this embodiment can be applied to the inner lens of a color or black-and-white image sensor or the outer lens of a black-and-white image sensor.

[0071] In this embodiment, by using the light-transmitting laying layer as a hard mask, a microlens core with a stack of multiple light-transmitting laying layers that are not completely the same is formed, which not only improves the performance of the microlenses and the process stability during the preparation process, but also improves the photosensitivity and relative illumination, and reduces the crosstalk between pixels.

[0072] Embodiment 2

[0073] As Figure 12 shown, an embodiment of the present invention provides a CMOS image sensor. The CMOS image sensor includes:

[0074] A plurality of pixel units and a microlens structure located above the pixel units;

[0075] The microlens structure includes: a microlens core 22 stacked by multiple light-transmitting laying layers, and a microlens outer layer 24 covering the microlens core, wherein the multiple light-transmitting laying layers have different refractive indexes.

[0076] Specifically, as Figure 12 shown, in this embodiment, the pixel units are separated by dotted lines, taking the adjacent first pixel unit 201 and second pixel unit 202 as an example. The pixel unit includes a photodiode and a transmission transistor, a reset transistor, a source follower transistor, a row selection transistor (not shown), etc. In this embodiment, a passivation layer 21 is further provided above the pixel unit. On the one hand, the passivation layer 21 is used to protect the chip from environmental factors such as water vapor, and on the other hand, it serves as a stop layer for etching the light-transmitting laying layer under the hard mask in the subsequent process.

[0077] As an example, asFigure 12 As shown, the microlens core 22 includes a first light-transmitting laying layer 221, a second light-transmitting laying layer 222, a third light-transmitting laying layer 223, and a fourth light-transmitting laying layer 224 laid from bottom to top along the optical path direction, and the corresponding refractive indices are n 1 , n 2 , n 3 , n 4 , and the microlens outer layer 24 covers the microlens core 22. The refractive index of the microlens outer layer 24 is set as n 5 . In other embodiments, the microlens core includes a first light-transmitting laying layer to an (N - 1)th light-transmitting laying layer stacked in sequence from bottom to top along the optical path direction. The refractive indices of the first light-transmitting laying layer to the (N - 1)th light-transmitting laying layer are n 1 to n N-1 in sequence, N is a natural number greater than or equal to 3, the refractive indices of the light-transmitting laying layers are not completely the same, and the refractive index of the microlens outer layer is set as n N .

[0078] In the present invention, the microlens core is set to be stacked by multiple light-transmitting laying layers with incomplete same refractive indices. The microlens core set in this way can adjust the incident path of light. For the pixels at the edges (four corners), light is more likely to converge into the pixels, the difference between the four corners and the center is reduced, and the relative illumination is high. In particular, light is very likely to converge into the pixel center, is not easy to reach the isolation area and be lost. Moreover, light or generated electrons are not easy to enter adjacent pixels, and the anti-crosstalk performance is better. At the same time, light is more likely to converge into the pixels, and the difference in the brightness channels is small. Therefore, GR / GB, R / G, and B / G are closer to 1. In particular, light is very likely to converge into the pixel center, which can reduce the possibility of light shielding in some areas due to the asymmetry of the design.

[0079] For the material of the microlens structure, that is, the microlens core and the microlens outer layer covering the microlens core, a transparent or translucent material that can function as a lens can be selected without special limitation. Preferably, the materials of the light-transmitting laying layer and the microlens outer layer can be selected from light-transmitting materials such as SiN, SiO 2 or SiON, etc., and can also be appropriately selected according to their uses, which is not limited here. Preferably, the material of the outermost light-transmitting laying layer of the microlens core is selected as harder SiN or SiON. The outermost light-transmitting laying layer serves as a hard mask layer for forming the microlens core, which can greatly improve the stability of the lithography and etching processes, thereby avoiding the missing of microlenses caused by process instability and finally resulting in white screen black dots. In this embodiment, the material of the first light-transmitting laying layer 221 is selected as silicon oxide, the material of the second light-transmitting laying layer 222 is selected as silicon oxynitride, the material of the third light-transmitting laying layer 223 is selected as silicon nitride, the material of the fourth light-transmitting laying layer 224 is selected as silicon oxynitride, and the material of the microlens outer layer 24 is selected as silicon oxide, which is the same as the material of the first light-transmitting laying layer 221.

[0080] The refractive index setting in the multi-layer light-transmitting laying layer of the microlens core structure, especially the microlens core, can be set according to actual needs, so as to realize the adjustment of the light path. In other embodiments, the refractive index of the microlens core structure is set such that the refractive index of the middle light-transmitting laying layer is high, and the refractive indices of the laying layers on both sides decrease in sequence. Specifically, when N is an even number, n N / 2 and n N / 2+1 are the maximum numbers in (n 1 , n 2 , …… n N ), and n N / 2 to n1 decrease in sequence, n N / 2+1 to n N decrease in sequence; when N is an odd number, n （N+1） / 2 is the maximum number in (n 1 , n 2 , …… n N ), and n （N+1） / 2 to n 1 decrease in sequence, n （N+1） / 2 to n N decrease in sequence. In another embodiment, the refractive indices in the microlens structure, namely n 1 to n N , are set to increase first and then decrease. Of course, the refractive indices of the light-transmitting laying layers of the microlens core can also be set to other combinations according to actual needs, and no specific limitation is made.

[0081] As an example, as Figure 12 described, the shape of the microlens core 22 can be selected as an island shape, a column shape, an arc shape or other shapes. The cross-sectional shapes of the island shape, the column shape and the arc shape are: circular, triangular or polygonal. In this embodiment, the shape of the microlens core 22 is columnar. The width w2 of the microlens core 22 can be set according to actual needs. Preferably, the width w2 of the microlens core 22 is 1 / 6 to 3 / 4 of the width w1 of the pixel unit.

[0082] In addition, as Figure 12 shown, adjacent microlens outer layers overlap and are set as overlapping microlenses, which can improve the filling rate of the microlenses, thereby improving the fill factor of the image sensor.

[0083] This embodiment also provides a method for forming the above-mentioned image sensor, including:

[0084] Forming a microlens core stacked with multi-layer light-transmitting laying layers along the light path direction above the pixel unit;

[0085] Depositing a microlens outer layer to coat the microlens core to form a microlens;

[0086] Among them, the multi-layer light-transmitting laying layer has different refractive indexes.

[0087] The steps of forming the microlens core include: sequentially forming a multi-layer light-transmitting laying layer, and forming a patterned uppermost light-transmitting laying layer through a patterning process;

[0088] Using the patterned uppermost light-transmitting laying layer as a hard mask, etching the light-transmitting laying layer under the hard mask, and retaining the hard mask to form a stacked microlens core together with the light-transmitting laying layer under the hard mask.

[0089] For the specific forming method, refer to Figures 7-12 for detailed description.

[0090] As Figure 7 shown, a semiconductor substrate (not shown) is provided, and several pixel units are provided on the semiconductor substrate. The adjacent first pixel unit 201 and the second pixel unit 202 are taken as examples. In this embodiment, a passivation layer 22 is further provided on the pixel unit.

[0091] As Figure 8 shown, a light-transmitting laying layer 22 is formed on the pixel unit. The forming methods of the light-transmitting laying layer include chemical vapor deposition method, evaporation method, sputtering method, etc. In this embodiment, the light-transmitting laying layer 22 includes a first laying layer 221, a second light-transmitting laying layer 222, a third light-transmitting laying layer 223, and a fourth light-transmitting laying layer 224 which are sequentially arranged.

[0092] As Figures 9-11 shown, a microlens core 22 is formed on the pixel unit. A patterned photoresist layer 23 is formed on the surface of the fourth light-transmitting laying layer 224, and a patterned fourth light-transmitting laying layer 224 is formed through photolithography and etching processes; using the patterned fourth light-transmitting laying layer 224 as a hard mask, etching the remaining light-transmitting laying layers to form the microlens core 22.

[0093] The present invention creatively adopts the method of using the outermost light-transmitting laying layer as a hard mask to etch the remaining light-transmitting laying layers, which can improve the process stability of photolithography and etching, and thus can avoid the missing of microlenses caused by process instability and finally lead to white screen black dots.

[0094] The width w2 of the microlens core 22 can be set as required. Preferably, the width w2 of the microlens core is 1 / 6 to 3 / 4 of the width w1 of a single pixel unit.

[0095] As Figure 12 shown, deposit the microlens outer layer material to form the microlens outer layer 24. In this embodiment, deposit a thick enough microlens outer layer material to make the adjacent microlens outer layers 24 coincide and improve the filling rate of the microlenses.

[0096] It should be noted that the microlens structure and its forming method provided in this embodiment can be applied to the inner lens of a color or black-and-white image sensor or the outer lens of a black-and-white image sensor.

[0097] In this embodiment, by using a light-transmitting laying layer as a hard mask, a microlens core with a stack of multiple light-transmitting laying layers that are not completely the same is formed, which not only improves the performance of the microlens and the process stability during preparation, but also can improve the photosensitivity and relative illumination, and reduce the crosstalk between pixels.

[0098] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, in any case, the embodiments should be regarded as exemplary and non-limiting. In addition, obviously, the word "including" does not exclude other elements and steps, and the word "one" does not exclude a plurality. Multiple elements stated in the apparatus claims can also be implemented by one element. The words first, second, etc. are used to denote names and do not denote any particular order.

Claims

1. A CMOS image sensor, characterized in that, it comprises: a plurality of pixel units, and a microlens structure located above the pixel units; the microlens structure includes: a microlens core stacked by multiple light-transmitting laying layers, and a microlens outer layer covering the microlens core, wherein the multiple light-transmitting laying layers have different refractive indexes; The microlens core is: the first light-transmitting laying layer to the (N-1)th light-transmitting laying layer stacked in sequence from bottom to top along the optical path direction, and the refractive indices of the first light-transmitting laying layer to the (N-1)th light-transmitting laying layer are n 1 to n N-1 ; The refractive index of the outer layer of the microlens is n N ; where N is a natural number greater than or equal to 3; The material of the light-transmitting laying layer or the outer layer of the microlens is selected from SiN, SiO 2 or SiON, and the n 1 to n N first increases and then decreases; wherein, the material of the (N-1)th light-transmitting laying layer is SiN or SiON, and the (N-1)th light-transmitting laying layer is a hard mask layer for forming the microlens core.

2. The CMOS image sensor according to claim 1, characterized in that, the microlens core is integrally in an island shape, column shape or arc shape.

3. The CMOS image sensor according to claim 2, characterized in that, the cross-sectional shape of the island shape, column shape or arc shape is: circular or triangular or polygonal.

4. The CMOS image sensor according to claim 1, characterized in that, The said n 1 equals n N .

5. The CMOS image sensor according to claim 1, characterized in that, When N is an even number, n N / 2 and n N / 2+1 is the largest number in (n 1 , n 2 , …… n N ), and n N / 2 decreases in sequence from n 1 to n N / 2+1 to n N decreases in sequence.

6. The CMOS image sensor according to claim 1, characterized in that, When N is odd, n （N+1） / 2 is the largest number in (n 1 , n 2 , …… n N ), and n （N+1） / 2 decreases successively from n 1 to n （N+1） / 2 to n N decreases successively.

7. The CMOS image sensor according to claim 1, characterized in that, the microlens structure is applied to an inner lens of a color or black-and-white image sensor or an outer lens for a black-and-white image sensor.

8. A method for forming a CMOS image sensor, characterized in that, it includes the following steps: forming a microlens core stacked by multiple light-transmitting laying layers above the pixel units along the optical path direction; depositing a microlens outer layer to cover the microlens core to form a microlens; wherein, the multiple light-transmitting laying layers have different refractive indexes; the step of forming the microlens core includes: sequentially forming multiple light-transmitting laying layers, and forming a patterned topmost light-transmitting laying layer through a patterning process; using the patterned topmost light-transmitting laying layer as a hard mask to etch the light-transmitting laying layer under the hard mask, and retaining the hard mask and jointly forming a stacked microlens core with the light-transmitting laying layer under the hard mask; The steps of forming the microlens core are as follows: successively forming a first light-transmitting laying layer to an (N - 1)th light-transmitting laying layer, and the refractive indices of the first light-transmitting laying layer to the (N - 1)th light-transmitting laying layer are n 1 to n N-1 ; forming a photoresist layer on the surface of the (N-1)th light-transmitting laying layer, and forming a patterned (N-1)th light-transmitting laying layer through a photolithography and etching process; using the patterned (N-1)th light-transmitting laying layer as a hard mask, and continuously etching the (N-2)th light-transmitting laying layer to form a microlens core in an island shape, column shape or arc shape; Deposit the outer layer of the microlens to coat the microlens core, forming a microlens, wherein the refractive index of the outer layer of the microlens is n N , and N is a natural number greater than or equal to 3.

9. The method for forming a CMOS image sensor according to claim 8, characterized in that, depositing a microlens outer layer to cover the microlens core, and overlapping adjacent microlens outer layers to form overlapping microlenses.

10. The method for forming a CMOS image sensor according to claim 8, characterized in that, the maximum width of the cross-section of the microlens core is 1 / 6 to 3 / 4 of the width of the pixel unit.

11. The method for forming a CMOS image sensor according to claim 8, characterized in that, the cross-section of the island shape, column shape or arc shape is: circular or triangular or polygonal.

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