Image sensor forming method and image sensor

Abstract

The invention provides an image sensor forming method, which comprises the following steps of: after an image sensor thinning process and before manufacturing a micro lens, forming a hole array of which the inner diameters of holes are periodically changed on the back surface of an image sensor so as to form a light condensing layer of which the effective refractive index is regularly changed outwards along the radial direction from the center of a pixel unit, therefore, the focusing effect of the incident light in the pixel unit is improved. According to the scheme, the condensation layer with the effective refractive index changing regularly is formed, light at the edge of the photodiode can be gathered to the middle of the photodiode, light loss is reduced, the quantum efficiency of the image sensor is improved, the imaging quality is improved, crosstalk between adjacent pixels is reduced, and the process steps are simple.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and in particular to an image sensor forming method and an image sensor. Background Technology

[0002] As image sensors are applied in increasingly wider fields, the performance requirements for image sensors are constantly improving. Quantum efficiency (QE) is an important parameter describing the photoelectric conversion capability of optoelectronic devices. It is a measure of the electro-photosensitivity of photosensitive devices, referring to the ratio of photocurrent generated per unit number of photons to incident photocurrent. Quantum efficiency can quantify its photoelectric conversion capability. Generally, the higher the quantum efficiency, the higher the imaging quality of the CMOS sensor. Existing back-illuminated CMOS sensors have better quantum efficiency than traditional front-illuminated CMOS sensors. However, as the size of sensor pixels continues to shrink, it is still necessary to improve the quantum efficiency of back-illuminated image sensors to obtain better imaging quality. Summary of the Invention

[0003] The purpose of this invention is to provide an image sensor forming method, comprising: After the image sensor thinning process and before the microlens are fabricated, an array of holes with periodically varying inner diameters is formed on the back of the image sensor to form a light-collecting layer with an effective refractive index that changes regularly from the center of the pixel unit outwards, thereby improving the focusing effect of incident light in the pixel unit.

[0004] Furthermore, the light-concentrating layer that forms an effective refractive index that changes regularly from the center of the pixel unit outwards includes: filling the hole array with a material that has a different effective refractive index from that of the image sensor semiconductor substrate, thereby changing the effective refractive index at different positions of the hole array.

[0005] Furthermore, the formation of an array of holes with periodically varying inner diameters on the back surface of the image sensor includes: A neutral layer and a directional self-assembled material layer are alternately formed on the back side of the image sensor, and an array of holes with periodically varying inner diameters is formed on the back side of the image sensor through at least two etching processes.

[0006] Furthermore, before alternately forming a neutral layer and a oriented self-assembled material layer on the back side of the image sensor, the method further includes: A first spin-coated mask layer is formed on the back of the image sensor; A first mask layer and a second mask layer are sequentially grown on the surface of the first spin-coated mask layer.

[0007] Furthermore, the alternating formation of a neutral layer and a oriented self-assembled material layer on the back surface of the image sensor includes: A first neutral layer is formed on the surface of the second mask layer; A first oriented self-assembly material layer is formed on the surface of the first neutral layer; The first oriented self-assembled material layer is separated into phases by baking or ultraviolet baking processes to form a first material layer and a second material layer respectively. By controlling the composition ratio of the self-assembled material, the first material layer forms a preset pore inner diameter size and arrangement period. The first material layer is removed by etching or developing processes, so that the second material layer forms a porous structure. Using the second material layer as a mask, etching is performed through the first neutral layer to form a hole array in the second mask layer; Remove the second material layer and the first neutral layer, and refill to form a second spin-coating mask layer, and perform a planarization process.

[0008] Furthermore, the formation of an array of holes with periodically varying inner diameters on the back surface of the image sensor includes: A third mask layer is formed on the second spin-coating mask layer; According to the preset photomask pattern, the third mask layer is photolithographically formed into a ring structure and transferred to the second spin-coated mask layer, thereby opening a portion of the second mask layer; The second mask layer is etched open to selectively change the inner diameter of the holes in a portion of the aperture array; Remove the third mask layer and the second spin-coated mask layer, and use the remaining second mask layer to continue etching the first mask layer, the first spin-coated mask layer, and the semiconductor substrate on the back of the image sensor to form the hole array with periodically changing hole inner diameter.

[0009] Furthermore, after forming the second spin-coating mask layer, the process further includes: A second neutral layer is formed on the second spin-coated mask layer; Based on the preset photomask pattern, the second neutral layer is photolithographically formed to create a surrounding structure; A second oriented self-assembled material layer is formed. The second oriented self-assembled material layer is then phase-separated according to the second neutral layer by a baking process or a UV baking process to form a third material layer and a fourth material layer, respectively, so that the third material layer forms a ring shape.

[0010] Furthermore, the formation of an array of holes with periodically varying inner diameters on the back surface of the image sensor includes: The third material layer is removed by an etching process or a development process, and the fourth material layer is used as a subsequent etching mask. The second neutral layer and the second spin-coated mask layer are etched, thereby opening a portion of the second mask layer; The second mask layer is etched open to selectively change the size of some of the holes; Remove the second spin-coated mask layer and the second neutral layer, and use the remaining second mask layer to continue etching the first mask layer, the first spin-coated mask layer, and the semiconductor substrate on the back of the image sensor to form the hole array with periodically changing hole inner diameter.

[0011] Furthermore, after forming an array of holes with periodically varying inner diameters on the back of the image sensor, the method further includes: Remove the remaining first mask layer and the first spin-coated mask layer, and fill the hole array with a dielectric material having a specific refractive index; Continue to form the deep trench isolation structure and high dielectric constant layer on the back of the image sensor.

[0012] The present invention also provides an image sensor formed using the image sensor forming method described above.

[0013] This invention utilizes a directional self-assembly process to create apertures of varying sizes, forming a light-gathering layer with an effective refractive index that changes radially outward from the pixel unit center. This allows light from the edges of the photodiode to converge towards the center, reducing light loss and enabling more light to be absorbed by the photodiode, thus improving the quantum efficiency of the image sensor and achieving better image quality. It also reduces crosstalk between adjacent pixels. Furthermore, this solution achieves the specific light-gathering effect of a microlens simply by using apertures of different sizes, and the fabrication of these apertures requires only a few directional self-assembly processes, simplifying the process and solving the light-gathering problem when the color filter array and pixel unit are mismatched. Attached Figure Description

[0014] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments, taken in conjunction with the accompanying drawings.

[0015] Figure 1 This is a schematic diagram of the structure of an array of holes with periodically changing inner diameters formed during the formation of an image sensor in an embodiment of the present invention; Figures 2-4 , Figure 15 These are schematic diagrams illustrating the structure of the image sensor formation process in embodiments of the present invention. Figures 5-10 This is a schematic diagram of the structure during the formation process of an image sensor in a specific embodiment of the present invention; Figures 11-14This is a schematic diagram of the image sensor formation process in another specific embodiment of the present invention.

[0016] Throughout the figures, the same or similar reference numerals denote the same or similar devices (modules) or steps. Detailed Implementation

[0017] This invention provides a method for forming an image sensor. In one specific embodiment, after the image sensor thinning process and before fabricating microlenses, an array of holes with periodically varying inner diameters is formed on the back surface of the image sensor, such as... Figure 1 As shown, a light-collecting layer is further formed on this basis, with an effective refractive index that changes regularly from the center of the pixel unit outward along the radial direction, thereby improving the focusing effect of incident light in the pixel unit.

[0018] In one alternative embodiment, after forming an array of holes with periodically varying inner diameters, a material with an effective refractive index different from that of the image sensor semiconductor substrate can be filled inside the array of holes to change the effective refractive index at different positions of the array of holes, thereby forming a light-collecting layer in which the effective refractive index changes regularly from the center of the pixel unit outwards.

[0019] Preferably, in this invention, a hole array with periodically varying inner diameters can be achieved on the back of the image sensor using directional self-assembly technology. Specifically, it includes: Step S200: Alternately form a neutral layer and a oriented self-assembly material layer on the back side of the image sensor; Based on this, proceed as follows: Step S300: Form an array of holes with periodically varying inner diameters on the back of the image sensor through at least two etching processes.

[0020] In one alternative embodiment, prior to step S200, the following steps may be performed: Step S110: Form a first spin-coated mask layer 210 on the back of the image sensor; Step S120: A first mask layer 220 and a second mask layer 230 are sequentially grown on the surface of the first spin-coated mask layer 210, such as... Figure 2 As shown.

[0021] As shown in the figure, 100 represents the semiconductor substrate used to form the image sensor. Preferably, the first spin-coated mask layer 210 can be made of a photoresist material (SOC), and the first mask layer 220 and the second mask layer 230 can be made of dielectric materials, such as low-temperature oxide (LTO) and silicon nitride (SiN), respectively. Before growing the first mask layer 220 and the second mask layer 230, the first spin-coated mask layer 210 can be planarized.

[0022] In this embodiment, step S200 can be performed by alternately forming the neutral layer and the oriented self-assembled material layer and then further processing them: Step S210: A first neutral layer 310 is formed on the surface of the second mask layer 230; Step S220: A first oriented self-assembly material layer 320 is formed on the surface of the first neutral layer 310, such as... Figure 3 As shown; Step S230: The first oriented self-assembled material layer 320 is separated into two phases by a baking process or a UV baking process, forming a first material layer 321 and a second material layer 322, respectively. Figure 3 As shown. By controlling the composition ratio of the self-assembly material, the first material layer 321 is made to form a preset pore inner diameter size and arrangement period; Directed self-assembly of block copolymer lithography (DSA) uses block copolymers, formed by polymerizing two monomers with different chemical properties, as raw materials. These copolymers undergo phase separation during thermal annealing to form nanoscale patterns. These patterns are then induced to become regularized using specific methods. For example, the neutral layer material in this invention has a certain inducing effect on the DSA material. Currently, mass-produced DSA materials use long-chain polymers of PS (polystyrene):PMMA (polymethyl methacrylate). Due to the significant difference in chemical affinity at the two ends of the long chains, the materials tend to aggregate molecules with the same affinity during curing. In this invention, by changing the composition ratio of the first material layer 321 (PS) or the second material layer 322 (PMMA) in the DSA material, the proportion of specific phase precipitation can be controlled, thereby forming different periodic structures.

[0023] Next, proceed with the following steps: Step S310: Remove the first material layer 321 by etching or developing process, so that the second material layer 322 forms a porous structure; Step S320: Using the second material layer 322 as a mask, etching is performed through the first neutral layer 310 to form a hole array in the second mask layer 230, such as... Figure 4 As shown; Step S330: Remove the second material layer 322 and the first neutral layer 310, and refill to form the second spin-coating mask layer 410, and perform planarization treatment, such as... Figure 5 As shown.

[0024] Optionally in this embodiment, after step S330 refilling to form the second spin-coating mask layer 410, another DSA phase separation can be performed to form a surrounding hole structure, and the following steps can be performed: Step S331: A second neutral layer 411 is formed on the second spin-coated mask layer 410, such as... Figure 5 As shown; Step S332: Based on the preset photomask pattern, photolithographically etch the second neutral layer 411 to form a surrounding structure, such as... Figure 6 As shown in (a) and (b), where Figure 6 (a) is a side sectional view. Figure 6 (b) is a top sectional view. Preferably, the surrounding structure can be a concentric ring, such as... Figure 6 (b) shows the rectangular ring; Step S333: Form a second oriented self-assembled material layer 412. Through a baking process or a UV baking process, the second oriented self-assembled material layer 412 is phase-separated according to the second neutral layer 411 to form a third material layer 413 and a fourth material layer 414, respectively. The third material layer 413 is then formed into a surrounding shape, such as... Figure 7 As shown in (a) and (b), where Figure 7 (a) is a side sectional view. Figure 7 (b) is a top-view cross-sectional view.

[0025] Based on this, optionally, the array of holes with periodically varying inner diameters can be further formed through the following steps: Step S431: Remove the third material layer 413 by etching or developing, using the fourth material layer 414 as a subsequent etching mask. For example, in this embodiment, the third material layer 413 is PMMA material, which is removed in this step. Figure 8 As shown; Step S432: Etch the second neutral layer 411 and the second spin-coating mask layer 410, thereby opening a portion of the second mask layer 230, such as... Figure 8 As shown; Step S433: Etch the opened portion of the second mask layer 230, selectively changing the size of a portion of the aperture, such as... Figure 9 As shown; Step S434: Remove the second spin-coating mask layer 410 and the second neutral layer 411. Using the remaining second mask layer 230, continue etching the first mask layer 220, the first spin-coating mask layer 210, and the semiconductor substrate 100 on the back of the image sensor to form the hole array with periodically varying hole inner diameters, such as... Figure 10 As shown in (a) and (b), where Figure 10 (a) is a side sectional view. Figure 10 (b) is a top view of the cross section, which shows that the hole array has two apertures, large and small, which alternate periodically.

[0026] In the above embodiments, this solution achieves a relatively dense effect of varying aperture sizes through at least two DSA processes. Similarly, the periodic density of aperture variations can be further increased by increasing the number of DSA processes; this can be achieved simply by adjusting the number and thickness of the corresponding mask layers as needed, which will not be elaborated further here.

[0027] In another embodiment, after forming the second spin-coated mask layer 410 in step S330, smaller holes can be formed using a DSA process. Then, a periodic ring structure can be created on the spin-coated mask layer using photolithography, and the opening size of some holes can be increased to form a hole array with periodically varying inner diameters. The specific steps are as follows: Step S410: Form a third mask layer 420 on the second spin-coating mask layer 410, such as... Figure 11 As shown, preferably, the material of the third mask layer 420 can be the same as that of the first mask layer 220, for example, a low-temperature oxide material.

[0028] Step S420: According to the preset photomask pattern, the third mask layer 420 is photolithographically formed into a ring structure and transferred to the second spin-coated mask layer 410, thereby opening a portion of the second mask layer 230, as shown below. Figure 12 As shown; Step S430: Etch the opened portion of the second mask layer 230, selectively changing the inner diameter of the holes in a portion of the aperture array, such as... Figure 13 As shown, this is to cause the hole size to change periodically; Step S440: Remove the third mask layer 420 and the second spin-coating mask layer 410. Using the remaining second mask layer 230, continue etching the first mask layer 220, the first spin-coating mask layer 210, and the semiconductor substrate 100 on the back of the image sensor to form the hole array with periodically varying hole inner diameters. The side cross-sectional view is shown below. Figure 14 As shown, and formed as Figure 1 The top-view cross-sectional view shown indicates that the hole array has two aperture sizes, large and small, which alternate periodically.

[0029] In this embodiment, due to the limitation of the minimum linewidth of photolithography, the size of the annular structure produced has a certain lower limit. Therefore, the number of large holes formed is smaller, the period of alternation between large and small holes is larger, and the light-gathering effect is inferior to the previous optional embodiment, but it can still achieve the technical effect required by the present invention.

[0030] In an optional implementation, after step S300 forms an array of holes with periodically varying inner diameters, the following steps may also be performed: Step S510: Remove the remaining first mask layer 220 and the first spin-coated mask layer 210, and fill the hole array with a dielectric material 510 having a specific refractive index. Preferably, a dielectric material with a refractive index lower than that of the substrate material can be selected. For example, for a silicon substrate, alumina or other materials can be used for filling, which can reduce light reflection and increase light transmittance. Step S520: Continue forming the deep trench isolation structure and high dielectric constant layer 520 on the back of the image sensor, as follows: Figure 15 As shown.

[0031] The present invention also provides an image sensor formed using the image sensor forming method described above.

[0032] After the front-side process of the image sensor is completed, the present invention forms a porous structure with different aperture sizes on the back side using directional self-assembly technology, and then continues the back-side process after filling the porous structure with a medium whose refractive index is lower than that of the substrate. On the one hand, since the refractive index of the medium filling the holes is smaller than that of silicon, when light enters the substrate from the medium, the light is deflected towards the center of the photodiode, so that light that could not be collected by the photodiode at the edge can now be collected, which plays a better role in anti-reflection and anti-reflection. On the other hand, incident light farther away from the center of the photodiode usually needs to be deflected at a larger angle to be focused at the center of the photodiode. In this scheme, due to the difference in the inner diameter of the holes, the loading effect during the etching process makes the etching depth of the larger holes deeper than that of the smaller holes. Therefore, more medium material can be filled, and the average refractive index is smaller than that of the smaller holes. That is, when light passes through the holes, the deflection angle of the larger holes is greater. Therefore, this scheme designs an array of holes with a trend of decreasing refractive index from the center of the photodiode outwards, with at least one set of larger holes far away from the center of the photodiode, so that incident light farther away from the center of the photodiode can be focused at the center of the photodiode.

[0033] In summary, the present invention adds a light-concentrating layer between the color filter and the semiconductor substrate in a typical image sensor. This allows light from the edge of the photodiode to be focused towards the center, reducing light loss and enabling more light to be absorbed by the photodiode, thus improving the quantum efficiency of the image sensor and achieving better image quality. It also reduces crosstalk between adjacent pixels. Furthermore, this solution uses only holes of different sizes to achieve the specific light-concentrating effect of a microlens, and the fabrication of different hole sizes only requires a few directional self-assembly processes. The process is simple and can solve the light-concentrating problem when the color filter array and pixel units are mismatched.

[0034] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered exemplary and not restrictive in any way. Furthermore, it is clear that the word "comprising" does not exclude other elements and steps, and the word "a" does not exclude a plural. Multiple elements recited in the apparatus claims may also be implemented by a single element. The terms "first," "second," etc., are used to denote names and do not indicate any particular order.

Claims

1. A method for forming an image sensor, characterized in that, include: After the image sensor thinning process and before the microlens are fabricated, an array of holes with periodically varying inner diameters is formed on the back of the image sensor to form a light-collecting layer with an effective refractive index that changes regularly from the center of the pixel unit outwards, thereby improving the focusing effect of incident light in the pixel unit.

2. The image sensor forming method as described in claim 1, characterized in that, The light-collecting layer that forms an effective refractive index that changes regularly from the center of the pixel unit outwards includes: filling the hole array with a material that has a different effective refractive index from that of the image sensor semiconductor substrate, thereby changing the effective refractive index at different positions of the hole array.

3. The image sensor forming method as described in claim 1, characterized in that, The array of holes with periodically varying inner diameters formed on the back of the image sensor includes: A neutral layer and a directional self-assembled material layer are alternately formed on the back side of the image sensor, and an array of holes with periodically varying inner diameters is formed on the back side of the image sensor through at least two etching processes.

4. The image sensor forming method as described in claim 3, characterized in that, Before alternately forming a neutral layer and a oriented self-assembled material layer on the back of the image sensor, the method further includes: A first spin-coated mask layer is formed on the back of the image sensor; A first mask layer and a second mask layer are sequentially grown on the surface of the first spin-coated mask layer.

5. The image sensor forming method as described in claim 3, characterized in that, The alternating formation of a neutral layer and a directional self-assembled material layer on the back side of the image sensor includes: A first neutral layer is formed on the surface of the second mask layer; A first oriented self-assembly material layer is formed on the surface of the first neutral layer; The first oriented self-assembled material layer is separated into phases by baking or ultraviolet baking processes to form a first material layer and a second material layer respectively. By controlling the composition ratio of the self-assembled material, the first material layer forms a preset pore inner diameter size and arrangement period. The first material layer is removed by etching or developing processes, so that the second material layer forms a porous structure. Using the second material layer as a mask, etching is performed through the first neutral layer to form a hole array in the second mask layer; Remove the second material layer and the first neutral layer, and refill to form a second spin-coating mask layer, and perform a planarization process.

6. The image sensor forming method as described in claim 5, characterized in that, The array of holes with periodically varying inner diameters formed on the back of the image sensor includes: A third mask layer is formed on the second spin-coating mask layer; According to the preset photomask pattern, the third mask layer is photolithographically formed into a ring structure and transferred to the second spin-coated mask layer, thereby opening a portion of the second mask layer; The second mask layer is etched open to selectively change the inner diameter of the holes in a portion of the aperture array; Remove the third mask layer and the second spin-coated mask layer, and use the remaining second mask layer to continue etching the first mask layer, the first spin-coated mask layer, and the semiconductor substrate on the back of the image sensor to form the hole array with periodically changing hole inner diameter.

7. The image sensor forming method as described in claim 5, characterized in that, After forming the second spin-coating mask layer, the method further includes: A second neutral layer is formed on the second spin-coated mask layer; Based on the preset photomask pattern, the second neutral layer is photolithographically formed to create a surrounding structure; A second oriented self-assembled material layer is formed. The second oriented self-assembled material layer is then phase-separated according to the second neutral layer by a baking process or a UV baking process to form a third material layer and a fourth material layer, respectively, and the third material layer is formed into a ring shape.

8. The image sensor forming method as described in claim 7, characterized in that, The array of holes with periodically varying inner diameters formed on the back of the image sensor includes: The third material layer is removed by an etching process or a development process, and the fourth material layer is used as a subsequent etching mask. The second neutral layer and the second spin-coated mask layer are etched, thereby opening a portion of the second mask layer; The second mask layer is etched open to selectively change the size of some of the holes; Remove the second spin-coated mask layer and the second neutral layer, and use the remaining second mask layer to continue etching the first mask layer, the first spin-coated mask layer, and the semiconductor substrate on the back of the image sensor to form the hole array with periodically changing hole inner diameter.

9. The image sensor forming method as described in claim 4, characterized in that, After forming an array of holes with periodically varying inner diameters on the back of the image sensor, the method further includes: Remove the remaining first mask layer and the first spin-coated mask layer, and fill the hole array with a dielectric material having a specific refractive index; Continue to form the deep trench isolation structure and high dielectric constant layer on the back of the image sensor.

10. An image sensor, characterized in that, Formed using the forming method described in claims 1 to 9.

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