Cell structure of CMOS optical sensor

Abstract

The present invention discloses a cell structure of a CMOS optical sensor, comprising: providing a semiconductor substrate, in which a photodiode is formed; above the photodiode, a microlens is provided; after light passes through the microlens, it is irradiated onto the photodiode, converting the optical signal into an electrical signal; the photodiode is formed by alternating and spacing P-type semiconductor thin layers and N-type semiconductor thin layers; the arrangement direction of the alternating P-type semiconductor thin layers and N-type semiconductor thin layers is perpendicular to the incident direction of the light on the microlens, and the steps below the microlens shrink upward step by step, so that the light passing through the microlens can irradiate all the semiconductor thin layers below. The impurity distribution direction of the semiconductor thin layer is perpendicular to the incident light, which greatly reduces the process difficulty, realizes a smaller CIS cell structure, and improves the integration.

Description

Technical Field

[0001] The invention relates to the field of semiconductor device manufacturing, and in particular to a cell structure of a CMOS optical sensor. Background Art

[0002] With the improvement of CMOS process level, CMOS optical image sensor (CIS) has been widely used in consumer electronics such as tablet computers and smart phones due to its advantages such as low power consumption, low cost, small size and random reading. However, the shortcomings of CIS such as small fill factor (FF), large dark current and large crosstalk are the main reasons why it has not become a mainstream optical image sensor, especially after the mainstream process of CIS has dropped to below 0.35μm, the continuous reduction of pixel size has made the pixel pitch smaller and smaller.

[0003] like Figure 1 As shown, it is a schematic diagram of the structure of a common CIS cell structure. In the figure, 1 and 2 are photodiodes formed by alternating arrangement of semiconductor materials of different conductive types, which serve as photoelectric conversion units of pixels. Above is a microlens 3, and light passes through the microlens 3 and is incident on the photodiode.

[0004] As the number of pixels continues to increase, the size of a single pixel becomes smaller and smaller, and the size of the cell structure decreases, such as Figure 1 The aspect ratio of the columnar 1 and 2 is getting bigger and bigger, which makes the process more and more difficult, limiting the further reduction of the size. The difficulty of its process manufacturing lies in the following points:

[0005] 1. The cell spacing is getting smaller and smaller, making the photolithography process difficult to implement;

[0006] 2. As the size of a single cell decreases, the volume of the PN junction becomes smaller, and the number of photoelectrons that can be generated decreases. If the original photosensitivity of CIS is to be maintained, the doping concentration needs to be further increased, or the depth needs to be increased to maintain the total volume * doping concentration unchanged or without a significant decrease;

[0007] 3. As the photolithography size shrinks, the photoresist becomes thinner and thinner, and the injection requirements become deeper and more concentrated. The injection energy and dose are greater, and the photoresist cannot resist it. Therefore, it is difficult to produce a PN junction with a small CD, deep injection, and high concentration. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a cell structure of a CMOS optical sensor.

[0009] In order to solve the above problems, a cell structure of a CMOS optical sensor described in the present invention comprises:

[0010] A semiconductor substrate is provided, wherein a photodiode is formed in the substrate; a microlens is provided above the photodiode; light passes through the microlens and then irradiates the photodiode, converting the light signal into an electrical signal;

[0011] The photodiode is formed by alternating and spacing P-type semiconductor thin layers and N-type semiconductor thin layers; the P-type semiconductor thin layers and N-type semiconductor thin layers are both columnar, elliptical, barrel-shaped or other shapes;

[0012] The arrangement direction of the alternatingly arranged P-type semiconductor thin layers and N-type semiconductor thin layers is perpendicular to the incident direction of the light on the microlens, and gradually shortens upward below the microlens to form an illumination area, so that the light passing through the microlens can illuminate all the photodiodes below.

[0013] Furthermore, a color filter film is provided between the microlens and the photodiode to filter the wavelength of light passing through the color filter film.

[0014] Furthermore, the semiconductor substrate also includes silicide as a metal electrode of the photodiode.

[0015] Furthermore, the silicide is nickel silicide and / or cobalt silicide.

[0016] Furthermore, the doping concentration distribution direction of the semiconductor thin layer is perpendicular to the incident direction of the light.

[0017] Furthermore, the step forming area is located directly below the microlens and absorbs light passing through the microlens.

[0018] The doping concentration is highest in the illuminated area under the microlens, which is beneficial to improving the efficiency of photoelectron generation, and gradually decreases toward the non-illuminated area.

[0019] The doping concentration is evenly distributed in the direction perpendicular to the illumination direction.

[0020] The doping concentration is the lightest in the illuminated area under the microlens and the thickest at the lead-out end far from the illuminated area, which is beneficial to the accelerated absorption of photoelectrons in the electric field gradient and improves the collection efficiency.

[0021] The P-type semiconductor thin layer and the N-type semiconductor thin layer are in a step-like morphology that shrinks upward step by step, and the shrinkage is equidistant or unequal.

[0022] The cell structure of the CMOS optical sensor provided by the present invention changes the arrangement direction of the semiconductor thin layer of the photodiode to be perpendicular to the incident light, and changes the vertical PN junction into a lateral PN junction. Therefore, the PN junction can be generated without using a photolithography injection method, and P-type and N-type thin films can be grown layer by layer. Therefore, the spacing is determined by the film thickness and can be very small, and doping can be directly in situ doped during epitaxial growth, which can avoid various technical difficulties caused by injection, greatly reduce the process difficulty, realize a smaller CIS cell structure, and improve the integration. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the existing CIS cell structure, and the semiconductor distribution is consistent with the incident direction of light.

[0024] Figure 2 The invention provides a CIS cell structure, in which the semiconductor distribution is perpendicular to the incident direction of light.

[0025] Description of Reference Numerals

[0026] 1 is a first-type semiconductor, 2 is a second-type semiconductor, and 3 is a microlens. DETAILED DESCRIPTION

[0027] The following is a specific implementation of the present invention in conjunction with the accompanying drawings, and the technical solutions in the present invention are clearly and completely described, but the present invention is not limited to the following implementations. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. According to the following description and claims, the advantages and features of the present invention will be clearer. It should be noted that the drawings are all in a very simplified form and use non-precise ratios, which are only used to conveniently and clearly assist in explaining the embodiments of the present invention. All other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0028] It should be understood that the present invention can be implemented in different forms, and should not be interpreted as being limited to the embodiments proposed here. On the contrary, providing these embodiments will make disclosure thorough and complete, and the scope of the present invention will be fully conveyed to those skilled in the art. In the accompanying drawings, for clarity, the size and relative size of the layer and the zone may be exaggerated, and the same reference numerals represent the same elements from beginning to end. It should be understood that when an element or layer is referred to as "on ... ", "adjacent to ... ", "connected to " or "coupled to " other elements or layers, it can be directly on other elements or layers, adjacent to it, connected or coupled to other elements or layers, or there can be an intervening element or layer. On the contrary, when an element is referred to as "directly on ... ", "directly adjacent to ... ", "directly connected to " or "directly coupled to " other elements or layers, there is no intervening element or layer. It should be understood that although the terms first, second, third, etc. can be used to describe various elements, components, zones, layers and / or parts, these elements, components, zones, layers and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or part from another element, component, region, layer or part. Therefore, without departing from the teachings of the present invention, the first element, component, region, layer or part discussed below may be represented as a second element, component, region, layer or part.

[0029] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below with reference to specific diagrams.

[0030] In order to form a CIS cell structure of smaller size, or to reduce the process difficulty when forming a CIS cell structure of existing size, the present invention provides a new cell structure, which can significantly reduce the process difficulty.

[0031] The cell structure of the CMOS optical sensor of the present invention is as follows: Figure 2 As shown, its structure contains:

[0032] A photodiode is formed in a semiconductor substrate; a microlens is provided above the photodiode; and a color filter is provided between the microlens and the photodiode to filter the wavelength of light passing through the color filter.

[0033] After passing through the microlens, the light is irradiated onto the photodiode, converting the light signal into an electrical signal, thereby realizing the function of the image sensor.

[0034] The photodiode is formed by alternating and spacing P-type semiconductor thin layers and N-type semiconductor thin layers; the P-type semiconductor thin layers and N-type semiconductor thin layers are both columnar. As shown in the figure, 1 is a P-type semiconductor thin layer, and 2 is an N-type semiconductor thin layer. The conductivity types of 1 and 2 can be interchanged.

[0035] The arrangement direction of the alternatingly arranged P-type semiconductor thin layers and N-type semiconductor thin layers is perpendicular to the incident direction of the light on the microlens, that is, it is rotated 90 degrees compared with the traditional layout structure, and is in a step shape that gradually shortens equidistantly upwards under the microlens, so that the light passing through the microlens can illuminate all the semiconductor thin layers below. In this embodiment, an equidistantly contracted step shape is adopted, but the actual application morphology is not limited to this, and it can also be an unequally spaced morphology, as long as it is ensured that all photodiodes in the illuminated area can receive light and the conversion efficiency or expected performance of the photodiode is guaranteed.

[0036] The step formation area is located directly below the microlens and absorbs light passing through the microlens. The staggered step morphology can ensure that each semiconductor thin layer can receive light without being blocked by the upper layer.

[0037] The doping concentration distribution direction of the semiconductor thin layer is perpendicular to the incident direction of the light. Figure 2 In the structure shown, the doping concentration distribution direction is horizontal.

[0038] The doping concentration distribution of the semiconductor thin layer can have many forms, and different forms bring different technical effects. For example, if the doping concentration in the illuminated area of ​​the semiconductor thin layer is high and the doping concentration in the area far from the illuminated area is low, the generation efficiency of photoelectrons can be improved; if the doping concentration in the illuminated area is low and the doping concentration at the lead-out end far from the illuminated area is high, it is conducive to accelerating the collection of photoelectrons in the electric field gradient and improving the collection efficiency; it can also be evenly distributed in the entire thin layer. The specific form can be adjusted according to actual needs.

[0039] The cell structure of the CMOS optical sensor provided by the present invention changes the arrangement direction of the semiconductor thin layer of the photodiode to be perpendicular to the incident light, and forms a step area to receive the light passing through the microlens. The impurity distribution direction of the semiconductor thin layer is perpendicular to the incident light, which greatly reduces the process difficulty, realizes a smaller size CIS cell structure, and improves the integration. Its main measures are:

[0040] 1. In the present invention, the CIS cell spacing defined by lithography is changed to P-type and N-type epitaxial thin film growth, so the period spacing can be reduced from the current 300nm to within 100nm, and the extreme can achieve 10nm level, which is difficult to achieve with traditional graphic processes.

[0041] 2. The present invention changes the PN junction from being perpendicular to the silicon wafer to being parallel to the silicon wafer, and the photolithography-injection method can be converted into a thin film process in manufacturing, eliminating the restrictions of photolithography and injection and reducing the difficulty of manufacturing.

[0042] 3. The present invention can be further combined with the photolithography-etching-injection method to produce a 3D structure, and the photosensitive area can be changed from the planar line distribution of the present invention to a checkerboard staggered distribution, so that a smaller CIS cell structure can be achieved.

[0043] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A cell structure of a CMOS optical sensor, characterized in that: The invention comprises a semiconductor substrate, in which a photodiode is formed; a microlens is provided above the photodiode; after passing through the microlens, light is irradiated onto the photodiode, and the light signal is converted into an electrical signal; The photodiode is formed by alternating and spacing P-type semiconductor thin layers and N-type semiconductor thin layers; The arrangement direction of the P-type semiconductor thin layers and the N-type semiconductor thin layers formed by the alternating arrangement is perpendicular to the surface of the semiconductor substrate, or parallel to the main optical axis of the microlens; The P-type semiconductor thin layer and the N-type semiconductor thin layer are in a step-shaped form that shrinks upward step by step directly below the microlens to form an illumination area, so that the light passing through the microlens can illuminate all the semiconductor thin layers below.

2. The cell structure of the CMOS optical sensor according to claim 1, characterized in that: A color filter film is provided between the micro lens and the photodiode to filter the wavelength of light passing through the color filter film.

3. The cell structure of the CMOS optical sensor according to claim 1, characterized in that: The semiconductor substrate also includes silicide as a metal electrode of the photodiode.

4. The cell structure of the CMOS optical sensor according to claim 3, characterized in that: The silicide is nickel silicide and / or cobalt silicide.

5. The cell structure of the CMOS optical sensor according to claim 1, characterized in that: The light passing through the microlens is incident on the illumination area vertically, or is incident on the illumination area at a certain angle.

6. The cell structure of the CMOS optical sensor according to claim 1, characterized in that: The P-type semiconductor thin layer and the N-type semiconductor thin layer are in a step-like morphology that shrinks upward step by step, and the shrinkage is equidistant.

7. The cell structure of the CMOS optical sensor according to claim 1, characterized in that: The doping concentration of the semiconductor thin layer is the highest in the illuminated area under the microlens, which is beneficial to improving the efficiency of photoelectron generation, and gradually decreases toward the non-illuminated area.

8. The cell structure of the CMOS optical sensor according to claim 1, characterized in that: The doping concentration of the semiconductor thin layer is evenly distributed in the direction perpendicular to the illumination direction.

9. The cell structure of the CMOS optical sensor according to claim 1, characterized in that: The doping concentration of the semiconductor thin layer is the lightest in the illuminated area under the microlens and the highest at the lead-out end away from the illuminated area, which is conducive to accelerated absorption of photoelectrons in the electric field gradient and improves collection efficiency.

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