A pixel structure, a working method, and an image sensor

Abstract

The present invention provides a pixel structure, a working method, and an image sensor. The pixel structure includes a photodiode disposed in a substrate. A groove is provided in the photodiode, and an insulating film layer is provided on the inner wall of the groove. A metal material layer is provided in the groove inside the insulating film layer. The metal material layer, the insulating film layer, and the photodiode form a charging capacitor, and the photodiode can be charged by controlling the voltage of the metal material layer. By forming the charging capacitor structure, the pixel structure of the present invention can fill the photodiode with electrons under the condition of no light for CTE testing. Moreover, this CTE testing does not require the use of complex and expensive optoelectronic testing equipment in a dedicated testing stage, but can be carried out in the WAT testing stage and the CP testing stage. Thereby, the testing complexity is reduced, the testing efficiency is improved, and it also provides convenience for the characteristic testing of other transistors in the pixel structure except for the photodiode.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor integration technologies, and particularly relates to a pixel structure, a working method, and an image sensor. Background Art

[0002] A solid state image sensor is a semiconductor device that converts an optical image into an electrical signal, and is divided into two types: an image sensor based on a charge coupled device (CCD) and a CMOS image sensor (CIS). The working processes of CCD and CIS are generally the same. Both convert an optical signal into photo-generated charges through a photosensitive device, then collect and process the photo-generated charges to convert them into voltage or current signals, and finally output the optoelectronic signals in the form of digital signals. Among them, the pixel structure of a CMOS image sensor is composed of a photodiode and an MOS transistor.

[0003] CMOS image sensors can be divided into three types according to different pixel structures: a CMOS image sensor with a passive pixel structure (Passive Pixel Sensor, PPS), a CMOS image sensor with an active pixel structure (Active Pixel Sensor, APS), and a CMOS image sensor with a digital pixel structure (Digital Pixel Sensor, DPS). The active 4T pixel structure with correlated double sampling operation has extremely low noise. Therefore, it has become a commonly used pixel structure in current CMOS image sensors. For example Figure 1As shown, the active 4T pixel structure consists of a photodiode and four N-type transistors (transfer transistor TG, select transistor Select, reset transistor Reset, and source follower SF). The charge storage region PD of the photodiode serves as the area for generating and storing photo-induced electrons. It is surrounded by P-type doping (i.e., P-well PW) and has no external electrodes, with its potential in a floating state. The structural characteristics of the charge storage region PD of the photodiode determine key parameter indicators such as the full well capacity FWC and quantum efficiency in the CIS evaluation system. In the charge transfer efficiency (CTE) test, the charge storage region PD of the photodiode needs to be exposed to light first. After the charge storage region PD of the photodiode is filled with electrons, the transfer transistor TG is turned on to transfer the electrons in the charge storage region PD of the photodiode to the floating diffusion point FD for reading. Finally, the ratio of the electrons entering the floating diffusion point FD to the electrons collected by the charge storage region PD of the photodiode is calculated. In the CTE test, a light source control device is required to provide light energy to the charge storage region PD of the photodiode, and the CTE test can only be carried out in the CP stage, which increases the test complexity and test time. Summary of the Invention

[0004] The object of the present invention is to provide a pixel structure, an operating method, and an image sensor, which can reduce the test complexity and improve the test efficiency.

[0005] To solve the above technical problems, the present invention provides a pixel structure, including a photodiode disposed in a substrate. A groove is provided in the photodiode, and an insulating film layer is provided on the inner wall of the groove. A metal material layer is provided in the groove inside the insulating film layer. The metal material layer, the insulating film layer, and the photodiode form a charging capacitor, and the photodiode can be charged by controlling the voltage of the metal material layer.

[0006] Optionally, a well region and a floating diffusion point are provided in the substrate. The floating diffusion point and the photodiode are spaced apart. Among them, the doping type of the well region is p-type, and the doping types of the photodiode are both n-type;

[0007] A gate structure is provided on the substrate between the floating diffusion point and the photodiode;

[0008] A reset transistor, a select transistor, and a source follower are provided on the substrate. The floating diffusion point serves as the source of the reset transistor. The floating diffusion point is connected to the gate of the source follower through a metal wire. The source of the source follower is connected to the drain of the select transistor. The drain of the source follower and the drain of the reset transistor are both connected to a first power supply.

[0009] Further, the photodiode includes a clamping layer and a charge storage region. The charge storage region is disposed in contact below the clamping layer. The charge storage region, the insulating film layer, and the metal material layer form the charging capacitor, and the gate structure, the floating diffusion point, and the charge storage region together form the transfer transistor.

[0010] Further, the bottom of the groove is located in the charge storage region.

[0011] Further, the thickness of the insulating film layer is greater than the thickness of the gate oxide layer of the transfer transistor.

[0012] Optionally, the insulating film layer is an oxide layer, and the metal material layer includes, from outside to inside, a metal film layer, a metal compound layer, and a metal filling layer in sequence;

[0013] Among them, the material of the metal film layer is titanium metal, the material of the metal compound layer is titanium nitride, and the material of the metal filling layer is tungsten metal.

[0014] On the other hand, the present invention provides a working method for a pixel structure. Using the pixel structure described above, it includes the following steps:

[0015] Divide one frame period of the pixel structure working into a reset stage, a charge generation stage, and a charge transfer stage;

[0016] Execute the reset stage;

[0017] Execute the charge generation stage, and charge the photodiode by controlling the voltage of the metal material layer;

[0018] Execute the charge transfer stage.

[0019] Optionally, the specific steps of executing the charge generation stage are:

[0020] The metal material layer is connected to a positive voltage, the selection transistor and the transfer transistor are both turned off, and the reset transistor is turned on. At this time, electrons accumulate on the capacitor plate of the charging capacitor and quickly reach saturation;

[0021] The metal material layer is grounded, the selection transistor and the transfer transistor are both turned off, and the reset transistor is turned on. At this time, the charges on the capacitor plate of the charging capacitor on the side of the insulating film layer close to the charge storage region are released and retained in the charge storage region.

[0022] Optionally, the specific steps of executing the charge generation stage are:

[0023] The metal material layer is connected to a negative voltage, the selection transistor and the transfer transistor are both turned off, and the reset transistor is turned on. At this time, electrons transfer from the metal material layer and accumulate in the charge storage region.

[0024] In another aspect, the present invention further provides an image sensor, which includes the pixel structure described above. The pixel structure includes a photodiode disposed in a substrate, a groove is provided in the photodiode, an insulating film layer is provided on the inner wall of the groove, and a metal material layer is provided in the groove inside the insulating film layer. The metal material layer, the insulating film layer and the photodiode form a charging capacitor, and the photodiode can be charged by controlling the voltage of the metal material layer.

[0025] Compared with the prior art, the present invention has the following unexpected technical effects:

[0026] The present invention provides a pixel structure, a working method and an image sensor. The pixel structure includes a photodiode disposed in a substrate, a groove is provided in the photodiode, an insulating film layer is provided on the inner wall of the groove, and a metal material layer is provided in the groove inside the insulating film layer. The metal material layer, the insulating film layer and the photodiode form a charging capacitor, and the photodiode can be charged by controlling the voltage of the metal material layer. By forming the charging capacitor structure, the pixel structure can fill the photodiode with electrons under the condition of no light for CTE testing. And this CTE testing does not require the use of complex and expensive optoelectronic testing equipment in a dedicated testing stage, but can be carried out in the WAT testing stage and the CP testing stage, thereby reducing the testing complexity, improving the testing efficiency, and also facilitating the characteristic testing of other transistors in the pixel structure except the photodiode. Description of the Drawings

[0027] Figure 1 It is a schematic structural diagram of an active 4T pixel structure.

[0028] Figure 2 It is a timing diagram of an active 4T pixel structure.

[0029] Figure 3 It is a schematic structural diagram of the pixel structure provided by an embodiment of the present invention.

[0030] Figure 4 It is a timing diagram of the pixel structure provided by an embodiment of the present invention.

[0031] Description of the Reference Numerals:

[0032] Figure 3 In which:

[0033] 100 - Substrate; 101 - Well region; 102 - Shallow trench isolation structure; 103 - Charge storage region; 104 - Clamping layer; 105 - Floating diffusion point; 110 - Insulating film layer; 120 - Metal material layer; 201 - Gate oxide layer; 202 - Polysilicon gate; 203 - Sidewall. Detailed implementation manners

[0034] A pixel structure, a working method, and an image sensor of the present invention will be further described in detail below. The present invention will be described in more detail with reference to the accompanying drawings, in which preferred embodiments of the present invention are shown. It should be understood that those skilled in the art can modify the present invention described herein and still achieve the advantageous effects of the present invention. Therefore, the following description should be understood as broad knowledge for those skilled in the art and not as a limitation on the present invention.

[0035] For clarity, not all features of the actual embodiments are described. In the following description, well-known functions and structures are not described in detail because they would obscure the present invention with unnecessary details. It should be recognized that in the development of any actual embodiment, numerous implementation details must be made to achieve the specific goals of the developer, such as changes from one embodiment to another in accordance with system-related or business-related constraints. Additionally, it should be recognized that such development work may be complex and time-consuming, but is merely routine work for those skilled in the art.

[0036] To make the objectives and features of the present invention more obvious and understandable, the specific implementation manners of the present invention will be further described below with reference to the accompanying drawings. It should be noted that the accompanying drawings are all in a very simplified form and use non-precise ratios, only for conveniently and clearly assisting in explaining the objectives of the embodiments of the present invention.

[0037] CMOS image sensors can be divided into front-side illumination (FSI) pixel structures and back-side illumination (BSI) pixel structures according to the photosensitive manner of the pixel structure. As Figure 1 - Figure 2 shown, the timing of a conventional BSI pixel structure is divided into a reset stage, an exposure stage, and a charge transfer stage. In the reset stage, the select transistor Select is turned off, and the reset transistor Reset and the transfer transistor TG are both turned on. The power supply Vdd raises the potentials of the floating diffusion point FD and the charge storage region PD of the photodiode, and the charge storage region PD of the photodiode and the P-well PW are in a reverse-biased fully depleted state. In the exposure stage, the select transistor Select and the transfer transistor TG are both turned off, and the reset transistor Reset is turned on. At this time, the charge storage region PD of the photodiode is exposed to light, and electron-hole pairs are generated in the depletion region between the charge storage region PD of the photodiode and the P-well PW under light. Among them, electrons will move towards the center of the charge storage region PD of the photodiode under the action of the electric field. With the accumulation of electrons, the potential of the charge storage region PD of the photodiode will gradually decrease and finally reach the full-well state.

[0038] In the first region of the charge transfer stage (i.e., after the reset transistor Reset is turned off and before the transfer transistor TG is turned on), the source follower SF is in the saturation state, and the output voltage at this time is Vout1. In the second region of the charge transfer stage, the selection transistor Select and the transfer transistor TG are both turned on, the reset transistor Reset is turned off, and a low-resistance conductive channel appears between the floating diffusion point FD and the charge storage region PD of the photodiode. The charge storage region PD of the photodiode is filled with electrons and at a low potential, while the floating diffusion point FD is at a high potential. Under the action of the electric field force, all the electrons in the charge storage region PD of the photodiode will be transferred to the floating diffusion point FD, pulling down the potential of the floating diffusion point FD. As the potential of the floating diffusion point FD decreases, the gate potential of the source follower SF will also decrease synchronously, thereby changing the conduction state of the source follower SF. The output voltage at this time is Vout2.

[0039] The above BSI pixel structure requires a light source control device to provide light energy to the charge storage region PD of the photodiode during the CTE test, and the CTE test can only be carried out during the CP stage, which increases the test complexity and test time.

[0040] To solve the above problems, as Figure 3 shown, this embodiment provides an image sensor, and the image sensor includes a pixel structure. The pixel structure includes a photodiode disposed in the substrate 100, a groove is provided in the photodiode, an insulating film layer 110 is provided on the inner wall of the groove, and a metal material layer 120 is provided in the groove inside the insulating film layer 110. The metal material layer 120, the insulating film layer 110, and the photodiode form a charging capacitor, and the photodiode can be charged by controlling the voltage of the metal material layer 120.

[0041] In this embodiment, by forming a charging capacitor structure, the pixel structure can be filled with electrons for the photodiode under the condition of no light for CTE testing. And this CTE test does not need to use complex and expensive optoelectronic testing equipment in a dedicated test stage (i.e., the CP test stage), so it can be carried out in the WAT test stage and the CP test stage, thereby reducing the test complexity, improving the test efficiency, and also facilitating the characteristic testing of other transistors in the pixel structure except the photodiode.

[0042] In this embodiment, the pixel structure can be an active pixel structure such as a 4T active pixel structure or a 6T active pixel structure.

[0043] The following takes the pixel structure as a 4T active pixel structure for detailed description.

[0044] The pixel structure includes a substrate 100, which is a p-type silicon substrate 100. The substrate 100 includes a pixel unit region, and a shallow trench isolation structure 102 is disposed around the pixel unit region. A well region 101 is formed in the substrate 100. The doping type of the well region 101 is p-type. The well region 101 extends from one surface (e.g., the front surface) of the substrate 100 into the substrate 100, and the shallow trench isolation structure 102 is located in the well region 101. Among them, the ion doping concentration of the well region 101 is greater than that of the substrate 100.

[0045] A photodiode is disposed in the substrate 100. The photodiode extends from the front surface of the substrate 100 into the substrate 100, and the depth of the photodiode is greater than the depth of the well region 101.

[0046] Among them, the photodiode includes a clamping layer 104 and a charge storage region 103. The charge storage region 103 is disposed in contact below the clamping layer 104, and the front surface of the substrate 100 exposes the clamping layer 104. The doping type of the clamping layer 104 is p-type, and the ion doping concentration of the clamping layer 104 is greater than that of the well region 101. The doping type of the charge storage region 103 is n-type.

[0047] A floating diffusion point 105 is also disposed in the substrate 100. The floating diffusion point 105 and the photodiode are spaced apart in the well region 101. The floating diffusion point 105 extends from the surface of the substrate 100 into the substrate 100, and the depth of the well region 101 is greater than the depth of the floating diffusion point 105. Among them, the doping type of the floating diffusion point 105 is n-type.

[0048] A gate structure is disposed on the substrate 100 between the floating diffusion point 105 and the photodiode. The gate structure covers part of the floating diffusion point 105 and the photodiode. At the same time, the gate structure, the floating diffusion point 105, and the charge storage region 103 together form a transfer transistor. The floating diffusion point 105 serves as the drain of the transfer transistor, and the charge storage region 103 serves as the source of the transfer transistor.

[0049] The gate structure includes a gate oxide layer 201, a polysilicon gate 202, and sidewalls 203. The gate oxide layer 201 is disposed on the front surface of the substrate 100. The polysilicon gate 202 is located on the gate oxide layer 201. The sidewalls 203 are located outside the polysilicon gate 202 and are disposed on the sidewalls of the gate oxide layer 201 and the polysilicon gate 202.

[0050] A reset transistor Reset, a select transistor Select, and a source follower SF are disposed on the substrate 100. The floating diffusion point 105 serves as the source of the reset transistor Reset. The floating diffusion point 105 is connected to the gate of the source follower SF through a metal wire. The source of the source follower SF is connected to the drain of the select transistor Select. The drain of the source follower SF and the drain of the reset transistor Reset are both connected to the first power supply Vdd. The source of the select transistor Select outputs an output voltage.

[0051] A groove is provided in the photodiode. The groove is disposed adjacent to the gate structure of the transfer transistor, and a gap may be provided between the projection of the gate structure of the transfer transistor on the front surface of the substrate 100 and the opening of the groove (that is, there is a spacing between the opening of the groove and the sidewall 203 of the gate structure close to the groove side), or they may be adjacent and without a gap (that is, the opening of the groove is adjacent to the sidewall 203 of the gate structure close to the groove side). The opening size of the groove can be adjusted according to the size requirements of the actual preparation of the metal material layer.

[0052] The opening of the groove is located on the front surface of the substrate 100, and the bottom of the groove is located in the charge storage region 103, that is, the depth of the groove is greater than the depth of the clamping layer 104 and less than the total depth of the clamping layer 104 and the charge storage region 103. In this embodiment, the depth of the groove is the same as the height of the shallow trench isolation structure 102, so that the charging capacitor first forms an insulating material in the photodiode by forming the shallow trench isolation structure 102, and then forms a hole in the shallow trench isolation structure 102 and fills it in the form of a plug to form the insulating film layer 110 and the metal material layer 120 of the charging capacitor.

[0053] An insulating film layer 110 is provided on the inner wall of the groove. The insulating film layer 110 may be an oxide layer. The thickness of the insulating film layer 110 is greater than the thickness of the gate oxide layer 201 of the transfer transistor to prevent leakage between the metal material layer 120 of the charging capacitor and the charge storage region 103 of the photodiode.

[0054] A metal material layer 120 is provided in the groove inside the insulating film layer 110. The metal material layer 120 includes a metal film layer, a metal compound layer, and a metal filling layer from outside to inside. The material of the metal film layer is, for example, metal titanium, the material of the metal compound layer is, for example, titanium nitride, and the material of the metal filling layer is, for example, metal tungsten. The charge storage region 103, the insulating film layer 110, and the metal material layer 120 constitute the charging capacitor.

[0055] The metal material layer 120 is connected to a second power supply Vp, and the second power supply Vp can be +Vdd or -Vdd.

[0056] As Figure 4 shown, the working method of the pixel structure in this embodiment is as follows:

[0057] Step S1: Divide one frame period of the pixel structure operation into a reset stage, a charge generation stage, and a charge transfer stage;

[0058] Step S2: Execute the reset stage A;

[0059] Step S3: Execute the charge generation stage B, and charge the photodiode by controlling the voltage of the metal material layer 120;

[0060] Step S4: Execute the charge transfer stage C.

[0061] Step S2 specifically includes: In the reset stage A, the metal material layer 120 is grounded, the select transistor Select is turned off, and the reset transistor Reset and the transfer transistor are both turned on. At this time, the first power supply Vdd raises the potentials of the charge storage region 103 of the photodiode and the floating diffusion point 105, and the charge storage region 103 of the photodiode and the well region 101 are in a reverse-biased fully depleted state to achieve the reset of the photodiode and the floating diffusion point 105.

[0062] Step S3 specifically includes:

[0063] In one embodiment, in the charge generation stage B, the metal material layer 120 is connected to the voltage Vdd, the select transistor Select and the transfer transistor are both turned off, and the reset transistor Reset is turned on. At this time, on the basis of the reverse-biased fully depleted state between the charge storage region 103 of the photodiode and the well region 101, the voltage Vdd is additionally increased, so that the potential of the charge storage region 103 of the photodiode is further raised, which will cause a significant increase in the reverse-biased leakage current between the charge storage region 103 of the photodiode and the well region 101, and even avalanche breakdown may occur. At the same time, since a capacitor is formed between the metal material layer 120 and the charge storage region 103 of the photodiode, the electrons in the reverse-biased leakage current between the charge storage region 103 of the photodiode and the well region 101 accumulate on the capacitor plate of the insulating film layer 110 close to the charge storage region 103 side and quickly reach saturation. After the metal material layer 120 is switched from the voltage Vdd to the ground, the charge on the capacitor plate of the insulating film layer 110 close to the charge storage region 103 side releases to the charge storage region 103. At this time, the charge storage region 103 has a negative potential, and the charge storage region 103 and the well region 101 are still in a reverse-biased state. At this time, the electrons will remain in the charge storage region 103.

[0064] In another embodiment, during the charge generation phase B, the metal material layer 120 is connected to the voltage -Vdd, the select transistor Select and the transfer transistor are both turned off, and the reset transistor Reset is turned on. At this time, since a capacitor is formed between the metal material layer 120 and the charge storage region 103 of the photodiode, when the negative potential of the metal material layer 120 is large enough, F-N tunneling occurs between the metal material layer 120 and the charge storage region 103 of the photodiode, and electrons are transferred from the metal material layer 120 and accumulated in the charge storage region 103.

[0065] In step S4, the charge transfer phase C includes a first phase I and a second phase II. The first phase I is when the metal material layer 120 is grounded, after the reset transistor Reset is turned off and before the transfer transistor is turned on; the second phase II is when the metal material layer 120 is grounded, the reset transistor Reset is turned off, and both the select transistor Select and the transfer transistor are turned on.

[0066] In the first phase I, the source follower SF is in the saturation state, and the output voltage is the output voltage Vout1 at this time.

[0067] In the second phase II, a low-resistance conductive channel appears between the charge storage region 103 of the photodiode and the floating diffusion point 105. The charge storage region 103 is filled with electrons, and the charge storage region 103 is at a low potential while the floating diffusion point 105 is at a high potential. Under the action of the electric field force, all the electrons in the charge storage region 103 will be transferred to the floating diffusion point 105, pulling down the potential of the floating diffusion point 105. As the potential of the floating diffusion point 105 decreases, the gate potential of the source follower SF will also decrease synchronously, thereby changing the conduction state of the source follower SF, and the output voltage outputs the voltage Vout2 at this time.

[0068] When the pixel structure provided in this embodiment is subjected to the CTE test, the charge storage region 103 of the photodiode is filled with electrons by the charging capacitor, read out, and the electrons in the charge storage region 103 are transferred to the floating diffusion point 105 for reading. Finally, the ratio of the number of electrons entering the floating diffusion point 105 to the number of electrons collected by the charge storage region 103 is calculated to evaluate the performance of the transfer transistor.

[0069] In summary, the present invention provides a pixel structure, a working method, and an image sensor. The pixel structure includes a photodiode disposed in a substrate. A groove is provided in the photodiode, and an insulating film layer is provided on the inner wall of the groove. A metal material layer is provided in the groove inside the insulating film layer. The metal material layer, the insulating film layer, and the photodiode form a charging capacitor, and the photodiode can be charged by controlling the voltage of the metal material layer. By forming the charging capacitor structure, the pixel structure can fill the photodiode with electrons under the condition of no light for CTE testing. Moreover, this CTE testing does not require the use of complex and expensive optoelectronic testing equipment in a dedicated testing stage, but can be carried out in the WAT testing stage and the CP testing stage, thereby reducing the testing complexity, improving the testing efficiency, and also facilitating the characteristic testing of other transistors in the pixel structure except the photodiode.

[0070] In addition, it should be noted that unless otherwise specified or indicated, the descriptions of the terms "first" and "second" in the specification are only used to distinguish each component, element, step, etc. in the specification, rather than to represent the logical relationship or sequential relationship between each component, element, step, etc.

[0071] It can be understood that although the present invention has been disclosed above with preferred embodiments, the above embodiments are not intended to limit the present invention. For any person skilled in the art, without departing from the scope of the technical solution of the present invention, many possible changes and modifications can be made to the technical solution of the present invention by using the disclosed technical content above, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the protection of the technical solution of the present invention.

Claims

1. A pixel structure, characterized in that: A photodiode is provided in a substrate, a groove is provided in the photodiode, an insulating film layer is provided on the inner wall of the groove, a metal material layer is provided in the groove inside the insulating film layer, the metal material layer, the insulating film layer and the photodiode constitute a charging capacitor, and the charging capacitor can charge the photodiode in the absence of light by controlling the voltage of the metal material layer to perform a charge transfer efficiency test; Wherein, the insulating film layer is an oxide layer, and the metal material layer includes a metal film layer, a metal compound layer and a metal filling layer in order from the outside to the inside; The material of the metal film layer is metal titanium, the material of the metal compound layer is titanium nitride, and the material of the metal filling layer is metal tungsten.

2. The pixel structure according to claim 1, characterized in that: A well region and a floating diffusion point are arranged in the substrate, and the floating diffusion point and the photodiode are arranged at intervals, wherein the doping type of the well region is p-type, and the doping type of the photodiode and the photodiode are both n-type; A gate structure is provided on the substrate between the floating diffusion point and the photodiode; A reset transistor, a selection transistor and a source follower are arranged on the substrate. The floating diffusion point serves as the source of the reset transistor. The floating diffusion point is connected to the gate of the source follower through a metal wire. The source of the source follower is connected to the drain of the selection transistor. The drain of the source follower and the drain of the reset transistor are both connected to a first power supply.

3. The pixel structure according to claim 2, characterized in that: The photodiode includes a clamping layer and a charge storage area, wherein the charge storage area contact is arranged below the clamping layer, the charge storage area, the insulating film layer and the metal material layer constitute the charging capacitor, and the gate structure, the floating diffusion point and the charge storage area together constitute the transfer transistor.

4. The pixel structure according to claim 3, characterized in that: The bottom of the groove is located in the charge storage area.

5. The pixel structure according to claim 3, characterized in that: The thickness of the insulating film layer is greater than the thickness of the gate oxide layer of the transfer transistor.

6. A method for operating a pixel structure, using the pixel structure as claimed in any one of claims 1 to 5, characterized in that: The following steps are involved: A frame period of the pixel structure operation is divided into a reset phase, a charge generation phase and a charge transfer phase; Execute the reset phase; performing a charge generation phase and charging the photodiode in the absence of light by controlling the voltage of the metal material layer; The charge transfer phase is performed.

7. The operating method of the pixel structure according to claim 6, characterized in that: The charge generation phase is performed as follows: The metal material layer is connected to a positive voltage, the selection transistor and the transmission transistor are both turned off, and the reset transistor is turned on. At this time, electrons accumulate on the plates of the charging capacitor and quickly reach saturation; The metal material layer is grounded, the selection transistor and the transfer transistor are both turned off, and the reset transistor is turned on. At this time, the charge on the capacitor plate of the charging capacitor on the insulating film layer close to the charge storage area is released and retained in the charge storage area.

8. The operating method of the pixel structure according to claim 6, characterized in that: The charge generation phase is performed as follows: The metal material layer is connected to a negative voltage, the selection transistor and the transfer transistor are both turned off, and the reset transistor is turned on. At this time, electrons are transferred from the metal material layer and accumulated in the charge storage area.

9. An image sensor, characterized in that: The method comprises adopting a pixel structure as claimed in any one of claims 1 to 5, wherein the pixel structure comprises a photodiode arranged in a substrate, wherein a groove is arranged in the photodiode, an insulating film layer is arranged on the inner wall of the groove, a metal material layer is arranged in the groove inside the insulating film layer, the metal material layer, the insulating film layer and the photodiode constitute a charging capacitor, and the charging capacitor can charge the photodiode in the absence of light by controlling the voltage of the metal material layer to perform a charge transfer efficiency test; Wherein, the insulating film layer is an oxide layer, and the metal material layer includes a metal film layer, a metal compound layer and a metal filling layer in order from the outside to the inside; The material of the metal film layer is metal titanium, the material of the metal compound layer is titanium nitride, and the material of the metal filling layer is metal tungsten.

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