阅读说明：本技术 源极跟随管及cmos传感器的形成方法 (Source follower tube and CMOS sensor forming method ) 是由 田志 李娟娟 邵华 陈昊瑜 于 2020-05-29 设计创作，主要内容包括：本发明提供了一种源极跟随管及CMOS传感器的形成方法,包括：提供衬底,在所述衬底上形成有源区和与所述有源区相邻的浅沟槽隔离结构；在所述有源区内形成埋沟通道,所述埋沟通道的两侧与所述浅沟槽隔离结构接触；在所述有源区上形成栅氧化硅；部分刻蚀所述栅氧化硅的中部位置以形成中间薄两边厚的图案化的栅氧化硅；在所述栅氧化硅和所述浅槽隔离结构上形成栅极结构。通过刻蚀形成两边厚中间薄的图案化的栅氧化硅,使得埋沟通道中的大部分的载流子从图案化的栅氧化硅的中间部分传输,从而远离浅沟槽隔离结构的界面,改善浅沟槽隔离结构界面对载流子的扰动,从而改善CMOS传感器的抗噪声能力和提高CMOS传感器获得的图像的质量。(The invention provides a method for forming a source follower tube and a CMOS sensor, which comprises the following steps: providing a substrate, and forming an active area and a shallow trench isolation structure adjacent to the active area on the substrate; forming a buried channel in the active region, wherein two sides of the buried channel are in contact with the shallow trench isolation structure; forming gate oxide silicon on the active region; partially etching the middle position of the gate oxide silicon to form patterned gate oxide silicon with a thin middle and thick two sides; and forming a grid structure on the grid silicon oxide and the shallow groove isolation structure. The patterned gate silicon oxide with thick two sides and thin middle is formed by etching, so that most of current carriers in the buried channel are transmitted from the middle part of the patterned gate silicon oxide, the buried channel is far away from the interface of the shallow trench isolation structure, the disturbance of the interface of the shallow trench isolation structure to the current carriers is improved, the noise resistance of the CMOS sensor is improved, and the quality of an image obtained by the CMOS sensor is improved.)

1. A method of forming a source follower tube, comprising:

providing a substrate, and forming an active area and a shallow trench isolation structure adjacent to the active area on the substrate;

forming a buried channel in the active region, wherein two sides of the buried channel are in contact with the shallow trench isolation structure;

forming gate oxide silicon on the substrate of the active region;

partially etching the middle position of the gate oxide silicon to form patterned gate oxide silicon with a thin middle and thick two sides;

and forming a grid structure on the grid silicon oxide and the shallow groove isolation structure.

2. The method of forming a source follower tube as defined in claim 1 wherein the substrate comprises a silicon substrate and a P-type epitaxial layer grown on the silicon substrate.

3. The method for forming a source follower transistor as claimed in claim 1, wherein the active region and the shallow trench isolation structure are both plural, the active region and the shallow trench isolation structure are spaced apart, and a surface of the active region is lower than a surface of the shallow trench isolation structure.

4. The method of claim 1, wherein the ions implanted into the active region are: a P-type ion.

5. The method of forming a source follower transistor as defined in claim 1, wherein the buried channel is formed by implanting N-type doped ions into a region in the active region for forming the buried channel.

6. The method of forming a source follower tube as in claim 1, wherein the patterned gate oxide is in the shape of a reentrant pattern.

7. The method of claim 6, wherein the ratio of the thickness of the thick portion to the thickness of the thin portion of the patterned gate oxide is between 1.5 and 2.5.

8. The method of forming a source follower tube as in claim 1, wherein the shallow trench isolation structure is formed by:

etching the substrate to form a groove;

and filling silicon dioxide into the groove to form a shallow groove isolation structure.

9. The method of forming a source follower tube of claim 8, wherein the oxide is silicon dioxide.

10. A method for forming a CMOS sensor, comprising:

forming a source follower tube as claimed in any one of claims 1 to 9;

and forming a photodiode, a transfer tube, a reset tube and a row selection tube which are communicated with the source electrode following tube.

Technical Field

The invention relates to the technical field of semiconductors, in particular to a source follower tube and a method for forming a CMOS sensor.

Background

Generally, an active pixel unit of a CMOS image sensor includes a P +/N +/P-photodiode 200 (PD) located in a P-type epitaxial layer 100 and several transistors, for example, a 4T structure CMOS image sensor, four transistors specifically include a Transfer transistor 300(TX, Transfer), a Source follower transistor 400(SF, Source Follow), a Reset transistor 500(RST, Reset) and a Row Select transistor 600(RS, Row Select). Fig. 1 shows a schematic diagram of a 4T structure CMOS image sensor.

The basic operating principle of the CMOS image sensor shown in fig. 1 is as follows: before illumination, the reset tube 500 and the transfer tube 300 are opened to release the original electrons in the photodiode 200 area; in illumination, all transistors are turned off, and charges are generated in the space charge region of the photodiode 200; at the time of reading, the transfer tube 300 is opened, the charges stored in the PD region are transferred to the Floating Diffusion 700 (FD), and after the transfer, the transfer tube is closed and waits for the next entrance of light. The charge signal on the floating diffusion node 700 is then used to adjust the source follower transistor 400, convert the charge to a voltage, and output a current through the row select transistor 600 into the analog-to-digital conversion circuit. Because the size of the photodiode 200 is large, the full well capacity (the capacity of the photodiode to store charges) is improved, and more electrons can be stored, so that the dynamic range (the ratio of the brightest to darkest condition) of the pixel unit can be improved, the influence of noise on the pixel is reduced, and the signal-to-noise ratio is improved. With the demand for high pixel density, the transistor area in the pixel region and the isolation distance are reduced, wherein the intrinsic performance of the device also affects the overall pixel structure performance.

With continued reference to fig. 2, in the source follower transistor of the prior art, the buried channel 140 is located in the active region 120 and directly covers the cross section of the active region 120, and the shallow trench isolation structure 130 is located on both sides of the active region 120, and the gate oxide 150 and the gate structure 160 are located above the active region. Therefore, the edge of the buried channel 140 is in contact with the edge of the shallow trench isolation structure 130, and the oxide filled in the shallow trench isolation structure 130 is silicon dioxide, so that when the charge signal on the floating diffusion node 700 is used for adjusting the source follower transistor 400, converting the charge into a voltage, and outputting a current to the analog-to-digital conversion circuit through the row selection transistor 600, carrier electrons collide with the silicon dioxide interface of the shallow trench isolation structure 130 when passing through the buried channel 140, and the silicon dioxide interface can cause a severe scattering behavior on the carrier, thereby affecting the noise-resistant capability of the transistor, and finally affecting the quality of an image obtained by the CMOS sensor. And the impact of this scattering behavior increases as the size of CMOS sensors decreases.

Disclosure of Invention

The invention aims to provide a source follower tube and a method for forming a CMOS sensor, which can reduce the disturbance of an interface of a shallow trench isolation structure to a carrier when the carrier passes through a buried channel, thereby improving the anti-noise capability of the CMOS sensor and improving the quality of an image obtained by the CMOS sensor.

In order to achieve the above object, the present invention provides a method for forming a source follower tube, comprising:

providing a substrate, and forming an active area and a shallow trench isolation structure adjacent to the active area on the substrate;

forming a buried channel in the active region, wherein two sides of the buried channel are in contact with the shallow trench isolation structure;

forming gate oxide silicon on the substrate of the active region;

partially etching the middle position of the gate oxide silicon to form patterned gate oxide silicon with a thin middle and thick two sides;

and forming a grid structure on the grid silicon oxide and the shallow groove isolation structure.

Optionally, in the method for forming the source follower tube, the substrate includes a silicon substrate and a P-type epitaxial layer grown on the silicon substrate.

Optionally, in the method for forming the source follower transistor, the active region and the shallow trench isolation structure are both multiple, the active region and the shallow trench isolation structure are arranged at intervals, and the surface of the active region is lower than the surface of the shallow trench isolation structure.

Optionally, in the method for forming a source follower transistor, the ions implanted into the active region are: a P-type ion.

Optionally, in the method for forming the source follower transistor, the method for forming the buried channel includes implanting N-type doped ions into a region in the active region for forming the buried channel.

Optionally, in the method for forming a source follower transistor, the patterned gate oxide is in a shape of a Chinese character 'ao'.

Optionally, in the method for forming the source follower transistor, a ratio of a thickness of the thick portion to a thickness of the thin portion of the patterned gate oxide is 1.5-2.5.

Optionally, in the method for forming the source follower transistor, the method for forming the shallow trench isolation structure includes:

etching the substrate to form a groove;

and filling silicon dioxide into the groove to form a shallow groove isolation structure.

Optionally, in the method for forming a source follower transistor, the oxide is silicon dioxide.

The invention also provides a method for forming the CMOS sensor, which comprises the following steps:

forming the source follower transistor;

and forming a photodiode, a transfer tube, a reset tube and a row selection tube which are communicated with the source electrode following tube.

According to the source follower tube and the forming method of the CMOS sensor, the patterned gate silicon oxide with thick two sides and thin middle is formed through etching, so that most of current carriers in the buried channel are transmitted from the middle part of the patterned gate silicon oxide, the interface of the shallow trench isolation structure is far away, disturbance of the interface of the shallow trench isolation structure to the current carriers is improved, the noise resistance of the CMOS sensor is improved, and the quality of images obtained by the CMOS sensor is improved.

Drawings

Fig. 1 is a schematic structural diagram of a CMOS image sensor;

FIG. 2 is a schematic diagram of a prior art source follower transistor;

FIG. 3 is a flow chart of a method of forming a source follower transistor according to an embodiment of the invention;

FIG. 4 is a schematic structural diagram of a source follower transistor according to an embodiment of the present invention;

120-active region, 130-shallow trench isolation structure, 140-buried channel, 150-gate oxide silicon, 160-grid structure, 210-substrate, 220-active region, 230-shallow trench isolation structure, 240-buried channel, 250-patterned gate oxide silicon, 260-grid structure, 200-photodiode, 300-transfer tube, 400-source follower tube, 500-reset tube, and 600-row selection tube.

Detailed Description

The following describes in more detail embodiments of the present invention with reference to the schematic drawings. The advantages and features of the present invention will become more apparent from the following description. It is to be noted that the drawings are in a very simplified form and are not to precise scale, which is merely for the purpose of facilitating and distinctly claiming the embodiments of the present invention.

In the following, the terms "first," "second," and the like are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances. Similarly, if the method described herein comprises a series of steps, the order in which these steps are presented herein is not necessarily the only order in which these steps may be performed, and some of the described steps may be omitted and/or some other steps not described herein may be added to the method.

Referring to fig. 3, the present invention provides a method for forming a source follower tube, including:

s11: providing a substrate, and forming an active area and a shallow trench isolation structure adjacent to the active area on the substrate;

s12: forming a buried channel in the active region, wherein two sides of the buried channel are in contact with the shallow trench isolation structure;

s13: forming gate oxide silicon on the active region;

s14: partially etching the middle position of the gate oxide silicon to form patterned gate oxide silicon with a thin middle and thick two sides;

s15: and forming a grid structure on the grid silicon oxide and the shallow groove isolation structure.

Referring to fig. 4, first, a substrate 210 is provided, and the substrate 210 may be a silicon substrate and a P-type epitaxial layer grown on the silicon substrate. An active region 220 is formed on the substrate 210 by implanting P-type dopant ions. Next, the substrate 210 is etched to form a plurality of trenches, and the trenches are filled with oxide to form the shallow trench isolation structure 230, for example, the oxide may be silicon dioxide, and in other embodiments, other types of oxides may be filled. The active region 220 and the shallow trench isolation structure 230 are both plural, and the active region 220 and the shallow trench isolation structure 230 are formed in an interval. The surface of the active region 220 is lower than the surface of the shallow trench isolation structures 230, i.e., the active region 220 forms a trench between the plurality of shallow trench isolation structures 230.

Next, implanting ions of a type opposite to the type of the ions in the active region 220 into the active region 220 to form a buried channel 240, for example, P-type doped ions are implanted into the active region 220, and here, N-type doped ions are implanted to form the buried channel 240, the buried channel 240 is located inside the active region 220, the buried channel 240 is located at a certain depth in the active region 220, and a specific size of the depth is determined by an actual situation, and the depth position is related to the energy of the implanted ions according to a specific method in the prior art, which is not described herein again. Both sides of the buried channel 240 contact both sides of the shallow trench isolation structure 230.

Next, a gate oxide, which may be silicon dioxide, is formed on the active region 220. The gate oxide is also located within the trench. The gate oxide is partially etched to form a patterned gate oxide 250 that is thin in the middle and thick on both sides. The patterned gate oxide 250 is in the shape of a Chinese character 'ao'. The ratio of the thickness of the thick portion to the thickness of the thin portion of the patterned gate oxide 250 is 1.5-2.5, for example, the ratio may be 1.7 or 2. Polysilicon is then formed on the surfaces of the patterned gate oxide 250 and the shallow trench isolation structures 230 to serve as gate structures 260.

Next, referring to fig. 1, the present invention also provides a method for forming a CMOS, in which a photodiode 200, a transfer transistor 300, a reset transistor 500 and a row selection transistor 600 are formed on the surface of the substrate at other locations, and the forming method is the prior art and will not be described herein again.

By adopting the forming method of the source follower transistor formed by the invention, all transistors are closed when the light irradiates, and charges are generated in the space charge area of the photodiode 200; at the time of reading, the transfer tube 300 is opened, the charges stored in the PD region are transferred to the Floating Diffusion 700 (FD), and after the transfer, the transfer tube 300 is closed and waits for the next entrance of light. The charge signal on the floating diffusion node 700 is then used to adjust the source follower transistor 400, convert the charge to a voltage, and output a current through the row select transistor 600 into the analog-to-digital conversion circuit. When current passes through the source follower transistor 400, the current passes through the buried channel 240, and the patterned gate oxide 250 with thick sides and thin middle is formed by etching, so that most of the current carriers in the buried channel 240 are transmitted from the middle part of the patterned gate oxide 250, but not from the two sides of the patterned gate oxide 250. Thereby being far away from the silicon dioxide interface of the shallow trench isolation structure 230 and improving the disturbance of the silicon dioxide interface to carriers. Thereby improving the noise immunity of the CMOS sensor and improving the quality of an image obtained by the CMOS sensor.

In summary, in the method for forming the source follower transistor and the CMOS sensor provided by the embodiments of the present invention, the patterned gate oxide silicon with thick sides and thin middle is formed by etching, so that most of the carriers in the buried channel are transmitted from the middle portion of the patterned gate oxide silicon, and thus are away from the interface of the shallow trench isolation structure, and the disturbance of the interface of the shallow trench isolation structure to the carriers is improved, thereby improving the noise immunity of the CMOS sensor and improving the quality of the image obtained by the CMOS sensor.

The above description is only a preferred embodiment of the present invention, and does not limit the present invention in any way. It will be understood by those skilled in the art that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.