Method for reducing noise of image sensor and image sensor

Abstract

The invention provides a method for reducing the noise of an image sensor and the image sensor, and the method comprises the steps: dividing a pixel array of the image sensor into a plurality of pixel sub-columns, forming pixel column isolation structures at the two sides of each pixel sub-column, enabling the semiconductor substrates of the pixel sub-columns to be completely electrically isolated, and enabling the pixel sub-columns to be electrically isolated from each other; therefore, the substrate noise interference between the pixel sub-columns is reduced. According to the scheme, the noise input by the pixel array from the outside can be effectively suppressed, and meanwhile, the influence range of the noise generated in the pixel working process is controlled, so that the noise level of the image sensor is remarkably reduced, and the dynamic range of the image sensor is widened.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and particularly relates to a method for reducing noise of an image sensor and an image sensor. Background Art

[0002] Among the performance indicators of an image sensor, the noise level is one of the very important indicators. The existence of noise will interfere with the image signal, producing a mottled superimposed effect in the image and affecting the resolvability of image details. Therefore, it is necessary to reduce noise in the entire signal path within the image sensor. With the continuous development of image sensor technology, the pixel size is continuously decreasing, and the available space of the peripheral circuit and the pixel itself is continuously decreasing, making the noise interference between electronic components more serious.

[0003] Common noises in existing image sensors include shot noise, thermal noise, 1 / f noise, etc. Among them, shot noise comes from the quantization process of the optical signal and cannot be eliminated. Currently, there are mainly three directions for the common noise elimination methods for the rest of the noises: reducing the generation of noise in the sensor, preventing the propagation of noise in the sensor circuit, and reducing the coupling degree of the signal to the noise signal. For example, optimizing the working state of the device to reduce the electric field strength and temperature, reducing the coupling capacitance between different electrical signals, and using special circuit structures to separate or cancel the noise, etc.

[0004] However, in the current conventional noise reduction methods, the noise propagated through the substrate well is not directly controlled. The circuit modules of traditional image sensors are often fabricated on a continuous silicon substrate, and different circuit potentials are separated by different well injections. This scheme is very effective in relatively large-scale circuit designs. However, as the pixel size gradually decreases, the well injection range for dividing the device must also be reduced. At this time, the distance between different circuits decreases, and the resistance brought by the well injection separation decreases; at the same time, to maintain the threshold voltage of small-sized devices, the well injection concentration increases, further increasing the capacitance of the well division. The resistance-capacitance (RC) coupling between different well regions becomes more significant, causing mutual interference during device operation and introducing obvious noise to the circuit. In addition, for the well injection range, in order to maintain the uniformity of the threshold voltage of the device itself, it is often necessary to keep the well injection boundary away from the device, which has a more intractable impact on the manufacturing process of the image sensor. Summary of the Invention

[0005] The object of the present invention is to provide a method for reducing noise of an image sensor, which is characterized by including: Dividing the pixel array of the image sensor into a plurality of pixel sub-columns, and forming pixel column isolation structures on both sides of each pixel sub-column, so that the semiconductor substrates of each pixel sub-column are completely electrically isolated from each other, so as to reduce the substrate noise interference between each pixel sub-column.

[0006] Further, when dividing the pixel array of the image sensor into a plurality of pixel sub-columns, correspondingly divide the column circuit module of the image sensor into a plurality of column circuit sub-columns by column, and form a circuit column isolation structure on both sides of each column circuit sub-column, so that the semiconductor substrates of each column circuit sub-column are completely electrically isolated from each other.

[0007] Further, when dividing the pixel array of the image sensor into a plurality of pixel sub-columns, each pixel sub-column includes at least one source follower transistor output column of the image sensor.

[0008] Further, forming a pixel column isolation structure on both sides of each pixel sub-column includes: Etch the semiconductor substrate on both sides of the pixel sub-column according to a preset lithography pattern to form a first trench; Form a dielectric layer on the surface of the first trench; Form a conductive gate in the dielectric layer using a conductive material; Apply a voltage to the conductive gate to form a pinning on the side of the first trench, so that the semiconductor substrates of each pixel sub-column are completely electrically isolated from each other.

[0009] Further, during the process of forming the image sensor, before forming the gate, through at least one epitaxial process, form an epitaxial layer on the surface of the pixel unit, or perform plasma implantation to form a pinning on the surface of the semiconductor substrate of the gate.

[0010] Further, it further includes: Divide the electronic components in the column circuit module into several functional areas according to functions. While forming the circuit column isolation structure on both sides of each column circuit sub-column, also form a functional area isolation structure between each functional area, so that the semiconductor substrates of each functional area are completely electrically isolated from each other.

[0011] Further, the conductive gate is formed at least using a polycrystalline semiconductor material and / or a metal material.

[0012] Further, it further includes: Arrange the power supply line and the ground line of the image sensor inside each pixel sub-column, so that the power supply and the ground of each pixel sub-column are independent of each other.

[0013] Further, it further includes: adopting a differential circuit in the image sensor column circuit sub-column to suppress external input noise.

[0014] The present invention also provides an image sensor, which suppresses the noise in the image sensor by using the method for reducing the noise of the image sensor as described above.

[0015] Through the above solution, the present invention proposes a new method for reducing the noise of an image sensor, which can effectively isolate the substrate along with the reduction of the pixel size of the image sensor, and proposes a circuit design for use in combination to significantly reduce the noise level of the image signal. The adjusted design can effectively suppress the noise input from the outside to the pixel array, and at the same time control the influence range of the noise generated during the operation of the pixels, thereby significantly reducing the noise level of the image sensor and improving the dynamic range of the image sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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.

[0017] Figure 1 is a schematic diagram of the module structure of an image sensor in the prior art; Figure 2 is a schematic diagram of the pixel sub-column and pixel column isolation structure in the present invention; Figure 3 is a schematic diagram of the circuit sub-column and circuit column isolation structure in the present invention; Figure 4 is a schematic cross-sectional structure diagram of the circuit column isolation structure in the present invention.

[0018] In the drawings, throughout the different views, the same or similar reference numerals denote the same or similar devices (modules) or steps. EMBODIMENTS

[0019] The object of the present invention is to provide a method for reducing the noise of an image sensor, which is characterized by including: Dividing the pixel array of the image sensor into a plurality of pixel sub-columns, and forming a pixel column isolation structure on both sides of each pixel sub-column, so that the semiconductor substrates between the pixel sub-columns are completely electrically isolated, to reduce the substrate noise interference between the pixel sub-columns.

[0020] As Figure 1 shows an example distribution of a pixel array, a row circuit module, and a column circuit module on a currently common image sensor. In the present invention, the pixel array region of the image sensor is divided into a plurality of pixel sub-columns, as shown by the dotted area A in Figure 1 . A pixel column isolation structure 100 is formed on both sides of each pixel sub-column, as shown in Figure 2 , so that the semiconductor substrates 110 between the pixel sub-columns are completely electrically isolated, thereby preventing substrate noise from spreading between the pixel sub-columns and reducing the noise level of the output signal.

[0021] Preferably, when dividing the pixel array of the image sensor into a plurality of pixel sub-columns, each of the pixel sub-columns includes at least one source follower output column of the image sensor. As Figure 2 Shown is an alternative embodiment. In this embodiment, each pixel unit includes a photodiode 105 and a transfer transistor 104. Every four adjacent pixel units are arranged in a 2×2 array and share a floating diffusion region 103, a reset transistor 106, and a source follower 107. Therefore, in this embodiment, when setting the pixel sub-columns, it is necessary to ensure that there is one source follower output column, that is, it is not possible to set only one column of pixel units as a pixel sub-column, but at least two columns of pixel units must be set as a pixel sub-column.

[0022] In an alternative embodiment, the pixel column isolation structure formed on both sides of each of the pixel sub-columns can be formed through the following steps: Step S100: According to a preset lithography pattern, etch the semiconductor substrate 110 on both sides of the pixel sub-column to form a first trench 111 (not shown in the figure); Step S200: Form a dielectric layer 101 on the surface of the first trench 111, as Figure 2 shown; Step S300: Use a conductive material to form a conductive gate 102 in the dielectric layer 101, as Figure 2 shown; Step S400: Apply a voltage to the conductive gate 102 to form a pinning on the side of the first trench 111, so that the semiconductor substrates 110 of each of the pixel sub-columns are completely electrically isolated from each other.

[0023] Preferably, the conductive gate 102 is formed at least by using a polycrystalline semiconductor material and / or a metal material. By applying a fixed bias voltage to the conductive gate 102, the substrate noise is prevented from propagating through the conductive gate 102, so as to achieve the electrical isolation effect of the pixel column isolation structure 100.

[0024] On this basis, preferably, when dividing the pixel array of the image sensor into a plurality of pixel sub-columns, the column circuit module of the image sensor can be correspondingly divided into a plurality of column circuit sub-columns by column, as Figure 1 shown by the dashed area B in the figure, and a circuit column isolation structure 200 is formed on both sides of each of the column circuit sub-columns, so that the semiconductor substrates 110 of each of the column circuit sub-columns are completely electrically isolated from each other, as Figure 3As shown, where 210 is the column control module and 220 is the digital-to-analog conversion module. In existing designs, for the convenience of the well injection process preparation, the same type of wells in adjacent columns are often connected into one piece, and thus it is impossible to avoid the noise interference between different components within the same well area. In this embodiment, the electronic components in the module are completely substrate-isolated by the circuit column isolation structure 200 column by column in a manner similar to that of the pixel array part, as Figure 4 shown. Since there is a design of shared wells in the column circuit module, isolating the semiconductor substrate 110 column by column can divide the wells between different columns from each other, prevent noise feedback between columns, and thus reduce the noise level.

[0025] Further, in an alternative embodiment, the electronic components in the column circuit module can be divided into several functional regions according to their functions. While forming the circuit column isolation structure 200 on both sides of each column circuit sub-column, a functional region isolation structure 300 is also formed between each functional region, so that the semiconductor substrates 110 of each functional region are completely electrically isolated from each other.

[0026] As Figure 4 shown is a cross-sectional view of an alternative embodiment in the column circuit module. Among them, the polysilicon gate 201, the dielectric layer 202, and the interface hole pinning layer 203 together form the circuit column isolation structure 200, which completely cuts off the electrical connection of the semiconductor substrate 110 of the column circuit module column by column (including the deep P-type implantation region 207 and the deep N-type implantation region 208 inside the semiconductor substrate). At this time, 204a (PMOS transistor inside the N well) and 205a (NMOS transistor inside the P well) located in the same column, and the substrate contact 206 are also isolated from each other through the PN junction formed by the well itself, while the devices in different columns (such as 204a / 204b, 205a / 205b) have no substrate path at all, thus greatly reducing the possibility of noise propagation.

[0027] In an alternative embodiment, during the process of forming the image sensor, before the gate is formed, an epitaxial layer can be formed on the surface of the pixel unit through at least one epitaxial process to form pinning on the surface of the substrate gate, or, alternatively, plasma implantation can be used to form pinning on the surface of the substrate gate. Optionally, no special pinning doping distribution can also be done at all, and by applying a relatively negative voltage to the substrate gate, holes can be attracted to complete pinning.

[0028] Optionally, in the present invention, the power supply line and the ground line of the image sensor can be arranged inside each pixel sub-column, so that the power supply and the ground of each pixel sub-column are independent of each other through the pixel column isolation structure 100.

[0029] Preferably, a differential circuit can also be adopted in the sub-column of the image sensor column circuit to suppress external input noise.

[0030] The present invention also provides an image sensor, which suppresses the noise in the image sensor by using the method for reducing the noise of the image sensor as described above.

[0031] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and that 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 "comprising" does not exclude other elements and steps, and the phrase "a" does not exclude a plurality. A plurality of elements recited in the apparatus claims can also be implemented by one element. The terms first, second, etc. are used to denote names and do not denote any particular order.

Claims

1. A method for reducing noise in an image sensor, characterized in that, Including: Dividing the pixel array of the image sensor into a plurality of pixel sub-columns, forming pixel column isolation structures on both sides of each of the pixel sub-columns, so that the semiconductor substrates of each of the pixel sub-columns are completely electrically isolated from each other, to reduce the substrate noise interference between each of the pixel sub-columns.

2. The method for reducing noise of an image sensor according to claim 1, characterized in that, When dividing the pixel array of the image sensor into a plurality of pixel sub-columns, correspondingly dividing the column circuit module of the image sensor into a plurality of column circuit sub-columns by column, and forming circuit column isolation structures on both sides of each of the column circuit sub-columns, so that the semiconductor substrates of each of the column circuit sub-columns are completely electrically isolated from each other.

3. The method for reducing noise of an image sensor according to claim 1, characterized in that, When dividing the pixel array of the image sensor into a plurality of pixel sub-columns, each of the pixel sub-columns includes at least one source follower output column of the image sensor.

4. The method for reducing noise of an image sensor according to claim 1, wherein The forming of the pixel column isolation structures on both sides of each of the pixel sub-columns includes: Etching the semiconductor substrates on both sides of the pixel sub-columns according to a preset lithography pattern to form first trenches; Forming a dielectric layer on the surface of the first trenches; Forming a conductive gate in the dielectric layer by using a conductive material; Applying a voltage to the conductive gate to form a pinning on the side surfaces of the first trenches, so that the semiconductor substrates of each of the pixel sub-columns are completely electrically isolated from each other.

5. The method for reducing noise of an image sensor according to claim 1, wherein, During the process of forming the image sensor, before the formation of the gate, through at least one epitaxial process, an epitaxial layer is formed on the surface of the pixel unit, or plasma implantation is performed to form a pinning on the surface of the semiconductor substrate gate.

6. The method for reducing noise of an image sensor according to claim 2, wherein, Also including: Dividing the electronic components in the column circuit module into a plurality of functional regions according to functions, and while forming the circuit column isolation structures on both sides of each of the column circuit sub-columns, also forming functional region isolation structures between each of the functional regions, so that the semiconductor substrates of each of the functional regions are completely electrically isolated from each other.

7. The method for reducing noise of an image sensor according to claim 4, wherein The conductive gate is formed at least by using a polycrystalline semiconductor material and / or a metal material.

8. The method for reducing noise of an image sensor according to claim 1, wherein, Also including: Arranging the power supply line and the ground line of the image sensor inside each of the pixel sub-columns, so that the power supply and the ground of each of the pixel sub-columns are independent of each other.

9. The method for reducing noise of an image sensor according to claim 1, wherein, Also including: Adopting a differential circuit in the column circuit sub-columns of the image sensor to suppress external input noise.

10. An image sensor, characterized in that, Suppressing the noise in the image sensor by using the method for reducing the noise of the image sensor as claimed in claims 1 to 9.

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