TDI CCD pixel charge transfer efficiency characterization method compatible with CMOS technology

Abstract

The invention discloses a TDI CCD pixel charge transfer efficiency characterization method compatible with a CMOS technology, and the method comprises the steps: firstly, calculating the number of effective photo-generated charges and the number of photo-generated charges originally stored in a third potential well based on the same TDI CCD-ON-CMOS single-stage device under an illumination condition and a non-illumination condition; secondly, when the charges are transferred from the second potential well to a potential well of a third gate lower photodiode PD, calculating to obtain the total number of photo-generated charges after transfer; and finally, based on the effective photo-generated charge number, the photo-generated charge number originally stored in the third potential well and the total photo-generated charge number after transfer, the effective transfer charge number is obtained, and the charge transfer efficiency CTE is obtained by dividing the effective transfer charge number by the effective photo-generated charge number. According to the method, the influence of noise charges on the precision when the CTE parameters are calculated can be reduced or even eliminated, and the CTE calculation precision is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of image sensor pixels, and in particular relates to a calculation and characterization method for CTE parameters of a CCD device based on a CMOS process. Background Art

[0002] At present, CMOS (complementary metal oxide semiconductor) image sensors have been widely used in consumer electronics and scientific applications. Compared with CCD (charge coupled device) image sensors, CMOS image sensors have the advantages of low power consumption, low production cost, and high integration. CCD devices still dominate the aerospace field due to their unique characteristics of high image quality, low noise level, high sensitivity, and high photoelectric conversion efficiency, such as TDI (time delay integration) CCD devices.

[0003] With the continuous advancement of CMOS technology, it has become possible to realize CCD devices under CMOS technology. There is a strong demand for TDI CCD-ON-CMOS (CCD based on CMOS technology) devices that combine the advantages of both image sensors.

[0004] However, the CMOS process cannot achieve the gate overlapping structure of CCD devices, which results in gaps between gates, and the gate voltage will also be reduced from the high voltage of 15V applied by traditional CCD devices to a low voltage of 3.3V. These will affect the charge transfer efficiency (CTE), which is a particularly important parameter indicator in CCD devices. It represents the ratio of the charge reaching the next potential well to the charge in the original potential well after a charge transfer. The CTE parameter of traditional CCD devices can generally be maintained above 99.999%, while CCD devices based on CMOS technology are difficult to reach this level.

[0005] Therefore, improving the CTE of TDI CCD-ON-CMOS devices is of utmost importance in device design. At the same time, only when the CTE parameters are accurately characterized can they provide guidance for the physical design of the device. Therefore, how to design an accurate characterization of the CTE parameters of TDI CCD-ON-CMOS devices is also particularly important. Summary of the invention

[0006] Aiming at the problem of inaccurate CTE parameter characterization under low light conditions in the existing TDI CCD-ON-CMOS single-stage device structure (device parameters are provided by the process factory), the present invention proposes a new TDI CCD pixel charge transfer efficiency characterization method compatible with CMOS process.

[0007] In order to achieve the above purpose, the present invention provides the following technical solutions:

[0008] (1) For the same TDI CCD-ON-CMOS single-stage device, when charges are transferred to the potential well of the photodiode PD under the second gate POLY2 of the TDI CCD-ON-CMOS single-stage device under illumination conditions and no illumination conditions, the number of charges in the second potential well under illumination conditions and no illumination conditions is measured, and the effective photogenerated charge number is obtained by subtracting the number of charges under the two conditions.

[0009] (2) Based on step (1), the number of charges in the potential well of the photodiode PD under the third gate under the condition of illumination and the condition of no illumination is continued to be measured, and the number of charges in the potential well under the two conditions is subtracted to obtain the number of photogenerated charges originally existing in the third potential well.

[0010] (3) When the charge is transferred from the second potential well to the potential well of the third photodiode PD under the gate, the number of charges in the third potential well of the TDI CCD-ON-CMOS single-stage device under illumination and no illumination is measured, and the total number of photogenerated charges after the transfer is obtained by subtracting the number of charges under the two conditions.

[0011] (4) Based on the three charge numbers obtained in steps (1), (2), and (3), the effective transferred charge number is obtained by subtracting the original photogenerated charge number from the total photogenerated charge number after transfer. The effective transferred charge number is divided by the effective photogenerated charge number to obtain the accurate charge transfer efficiency (CTE).

[0012] The present invention performs parameter solution based on a buried-channel TDI CCD-ON-CMOS single-stage pixel structure. The TDI CCD-ON-CMOS single-stage device structure includes two shallow trench isolations STI1 and STI2 located at the left and right ends of an epitaxial layer EPI; a photodiode PD is located between the two shallow trench isolations STI1 and STI2; a floating diffusion node FD is adjacent to the right side of the photodiode PD; the shallow trench isolation STI2 on the right side of the epitaxial layer EPI and the floating diffusion node FD are included in a P-well P-WELL; a silicon dioxide oxide layer OXIDE is located on the upper surface of the epitaxial layer EPI, and four gates POLY1, POLY2, POLY3 and OG with the same spacing are fixed on the surface of the oxide layer, just above the photodiode PD.

[0013] The effective photogenerated charge number is calculated by measuring the charge quantity in the depletion region of the photodiode PD under the second gate of the device.

[0014] The effective transferred charge number is calculated by measuring the charge quantity before and after the transfer in the depletion region of the photodiode PD under the third gate of the device.

[0015] The beneficial effects of the TDI CCD pixel charge transfer efficiency characterization method compatible with CMOS process of the present invention are:

[0016] The present invention conducts testing experiments on the same device structure under two conditions: with light injection and without light injection. Except for the illumination condition, other conditions such as gate voltage, ambient temperature change, gate timing, etc. are all the same.

[0017] Among them, under the condition of light illumination, the charge packet of the potential well under the gate contains signal charge and noise charge, and under the condition of no light illumination, the charge in the potential well contains only noise charge.

[0018] By subtracting the number of charges in the charge packets under the two conditions, the accurate and effective signal charge number can be obtained.

[0019] Similarly, this method can be used to obtain the number of signal charges existing in different potential wells under different gates in different time periods. Then, before the charge is transferred to the potential well under the next gate, the very small number of signal charges that already exist in the next potential well can be obtained. When calculating the effective transfer amount, this part of the charge number must be subtracted, which can further improve the CTE calculation accuracy.

[0020] Therefore, compared with the general method of calculating CTE by solving residual charge, this method can reduce or even eliminate the impact of noise charge on the accuracy when calculating CTE parameters, better reflect the effect of various device parameters on CTE parameters, understand the sensitivity of CTE to different design parameters, and thus guide engineers to optimize the design to improve CTE parameters and improve the overall performance of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram of the structure of a TDI CCD-ON-CMOS single-stage pixel device provided in an embodiment of the present invention;

[0022] Figure 2 A schematic diagram of the device gate voltage and device exposure timing used in an embodiment of the present invention;

[0023] Figure 3 A schematic diagram of the device structure and data acquisition area for solving the effective light injection charge amount of the present invention;

[0024] Figure 4 A schematic diagram of the device structure and data acquisition area for solving the number of photogenerated charges existing before charge transfer in the present invention;

[0025] Figure 5 The present invention is a schematic diagram of the device structure and data acquisition area when solving the effective photogenerated charge number after charge transfer. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become clearer according to the following description. It should be noted that the accompanying drawings are all in very simplified form and are not in precise proportions, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0027] Characterization method of charge transfer efficiency of TDI CCD pixels compatible with CMOS process, such as Figure 1 As shown, the present invention is based on a schematic diagram of the structure of a TDICCD-ON-CMOS device, including a P-type substrate P-SUB (101), a P-type epitaxial layer P-EPI 102, a left shallow trench isolation STI 103, a right shallow trench isolation STI 104, a P-well P-WELL 105, a photodiode N region PD-N 106, a floating diffusion node FD 107, a gate oxide layer OXIDE 108, a gate POLY1 109, a gate POLY2 110, a gate POLY3 111 and a transmission gate OG 112.

[0028] The voltage timing applied to the device gate is as follows Figure 2 As shown, the digital units in the figure are nanoseconds, wherein the stage from 0 to 400ns is the reset stage, the stage from 500ns to 1500ns is the exposure stage, and the stage after 1500ns is the charge transfer stage.

[0029] The present invention is based on the fact that after the device has been effectively reset, the silicon wafer under the gate will form a potential well and a high potential point under the influence of voltage, and a potential barrier will be formed when the surrounding gate is not applied with voltage. The charge will always flow to the high potential point under the action of the self-induced electric field, so the CCD device can store charge. Due to the photoelectric effect, the photodiode will generate photogenerated charge after receiving a certain amount of light. As the gate voltage changes with the timing, the charge is transmitted to the last floating diffusion node and then read out by the circuit. Finally, the image is output after being processed by the circuit, completing the process from light to electricity. Traditional TDI CCD devices generally have 64 levels, 128 levels, 256 levels, etc. If it is a three-phase CCD, in the case of 128 levels, the charge will be transmitted 384 times. The CTE parameter index must reach more than 99.995% to have excellent imaging quality. However, CCD devices based on CMOS technology are currently difficult to achieve this index due to changes in gate voltage and gate structure. Improving the CTE parameters of such devices is a basic problem, which also highlights the necessity of improving the accuracy of solving the CTE parameters of such devices.

[0030] The first step in solving the CTE process of TDI CCD-ON-CMOS devices is to test the electrical characteristics of the same device structure with different exposure conditions, and measure the amount of charge in the photodiode depletion region under the second gate of the device when the transfer phase is at 1660ns and only the second gate is applied with a voltage of 3.3V, such as Figure 3 shown. Figure 3 The above is a device structure diagram for electrical characteristics testing under illumination conditions. At 1660ns, the charge packet is stored in the potential well under the second gate. At this time, the charge packet contains signal charge and noise charge. Figure 3 The following is a device structure diagram for electrical characteristic testing under no light conditions. At this time, the charge packet contains only noise charges. Take a suitable area (302) to measure the charge quantity of the device to obtain the charge number. The data acquisition area (302) must include the entire charge packet and the depletion region of the photodiode, so that all the charge numbers can be calculated. At this time, the charge number obtained under light conditions is defined as e in2-light , the charge obtained under no light conditions is defined as e in2-dark The difference between the two is the effective light injection amount, defined as e in :

[0031] e in =e in2-light -e in2-dark (1)

[0032] Calculating CTE by only calculating the effective light injection amount can avoid the order of magnitude of noise charge affecting the final calculation of CTE under low light injection conditions. This method can effectively improve the calculation accuracy.

[0033] The second step of the CTE process for TDI CCD-ON-CMOS devices is to solve the effective photogenerated charge number under the third gate at 1660ns based on the first step, as follows: Figure 4 As shown, the data acquisition area is within the dotted box (401) in the figure. Figure 4 The charge number measured under the condition of illumination is e in3-light-begin , Figure 4 The charge number measured under no light conditions is e in3-dark-begin , the difference between the two is the charge number e that existed before the transfer in3-begin :

[0034] e in3-begin =e in3-light-begin -e in3-dark-begin (2)

[0035] Before calculating the effective transfer amount, the existing photogenerated charge number is solved. This part cannot be used as the effective transfer amount to calculate the final charge transfer efficiency CTE. Through this step, the accuracy of CTE parameter calculation characterization can be further improved.

[0036] The third step in solving the CTE process of the TDI CCD-ON-CMOS device is to calculate the number of photogenerated charges transferred to the potential well under the next gate and the number of photogenerated charges in the third potential well at a time of 1720ns, such as Figure 5 As shown, the data acquisition area is within the dotted box (501) in the figure, and the definition at this time is Figure 5 The charge number measured under the condition of illumination is e in3-light-after , Figure 5 The charge number measured under no light conditions is e in3-dark-after The difference between the two is the number of photogenerated charges stored in the third potential well after transfer, e in3-after :

[0037] e in3-after =e in3-light-after -e in3-dark-after (3)

[0038] The photogenerated charge obtained at this time is the transferred amount plus the charge that existed before the transfer in the previous step. in3-after Subtract e in3-begin The precise effective transfer amount can be obtained as e out :

[0039] e out =e in3-after -e in3-begin (4)

[0040] The final charge transfer rate CTE can be characterized as the effective transfer amount divided by the effective injection amount, that is,

[0041]

[0042] At the same time, the final residual charge number can be known through the effective transfer amount and effective light injection.

[0043] Table 1

[0044]

[0045] By this method, the CTE variation trend with an accuracy of 8 decimal places can be obtained, as shown in Table 1, wherein FWC (Full Well Capacity) represents the full well capacity. The experiment uses 1%, 5%, 10%, 20%, 33%, 50% and 66% of the full well capacity FWC as variables to characterize the parameters of the charge transfer efficiency CTE. The CTE parameter variation calculated by the characterization method of the invention shows a trend of first increasing and then decreasing with the increase of light injection, and the CTE maximum value is the largest when the light injection reaches 50% of the full well capacity. This feature is consistent with the CTE variation law of known devices, indicating that this method is accurate and effective in characterizing the CTE parameters of devices.

[0046] The above are only preferred embodiments of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for characterizing the charge transfer efficiency of TDICCD pixels compatible with CMOS technology, characterized in that: The following steps are involved: S1. The effective photogenerated charge number e is calculated for the same TDI CCD-ON-CMOS single-stage device under illumination and no illumination conditions. in The number of photogenerated charges e originally stored in the third potential well in3-begin ; S2. When the charge is transferred from the second potential well to the potential well of the photodiode PD under the third gate, the total number of photogenerated charges after the transfer, e, is calculated. in3-after ; S3, based on e in 、e in3-begin and e in3-after , we can get the effective transferred charge number, which is divided by the effective photogenerated charge number e in , and the charge transfer efficiency CTE is obtained.

2. The method for characterizing the charge transfer efficiency of TDI CCD pixels compatible with CMOS process according to claim 1, characterized in that: The TDI CCD-ON-CMOS single-stage device includes two shallow trench isolations STI1 and STI2 located at the left and right ends of the epitaxial layer EPI: the photodiode PD is located between the two shallow trench isolations STI1 and STI2; the floating diffusion node FD is adjacent to the right side of the photodiode PD; the shallow trench isolation STI2 on the right side of the epitaxial layer EPI and the floating diffusion node FD are included in the P-well P-WELL; the silicon dioxide oxide layer OXIDE is located on the upper surface of the epitaxial layer EPI, and four gates POLY1, POLY2, POLY3 and OG with the same spacing are fixed on the surface of the oxide layer, just above the photodiode PD.

3. The method for characterizing the charge transfer efficiency of TDI CCD pixels compatible with CMOS process according to claim 1, characterized in that: The calculation of the effective photogenerated charge number e in The specific process is as follows: for the same TDI CCD-ON-CMOS single-stage device, when the charge is transferred to the potential well of the photodiode PD under the second gate POLY2 of the TDI CCD-ON-CMOS single-stage device under the conditions of illumination and no illumination, the number of charges in the second potential well under the conditions of illumination and no illumination is measured, and the effective photogenerated charge number e is obtained by subtracting the number of charges under the two conditions. in : And in =and in2-light -And in2-dark Among them, the charge number in the second potential well under light conditions is e in2-light , the charge in the second potential well under no light conditions is e in2-dark .

4. The method for characterizing the charge transfer efficiency of TDI CCD pixels compatible with CMOS process according to claim 1, characterized in that: The number of photogenerated charges originally stored in the third potential well is e in3-begin The specific calculation process is as follows: measure the charge number in the potential well of the photodiode PD under the third gate under the condition of illumination and the condition of no illumination, and subtract the charge number in the potential well under the two conditions to obtain the original photogenerated charge number e in the third potential well. in3-begin : And in3-begin =and in3-light-begin -And in3-dark-begin Among them, the charge number in the potential well of the photodiode PD under the third gate measured under light conditions is e in3-light-begin Under no-light conditions, the charge number in the potential well of the photodiode PD under the third gate is measured to be e in3-dark-begin .

5. The method for characterizing the charge transfer efficiency of TDI CCD pixels compatible with CMOS process according to claim 1, characterized in that: The total number of photogenerated charges after transfer is calculated as e in3-after The specific process is as follows: when the charge is transferred from the second potential well to the potential well of the third photodiode PD under the gate, the charge number in the third potential well of the TDI CCD-ON-CMOS single-stage device under illumination and no illumination is measured, and the charge number under the two conditions is subtracted to obtain the total photogenerated charge number after transfer, e in3-after : And in3-after =and in3-light-after -And in3-dark-after Among them, the charge number measured in the third potential well under light conditions is e in3-light-after , the charge measured in the third potential well under no light conditions is e in3-dark-after .

6. The method for characterizing the charge transfer efficiency of TDI CCD pixels compatible with CMOS process according to any one of claims 1 to 5, characterized in that: The specific implementation process of step S3 is as follows: The total number of photogenerated charges after transfer is e in3-after Subtract the original photogenerated charge e in the third potential well in3-begin , and the effective transferred charge number eou is obtained t : And out =and in3-after -And in3-begin The effective transferred charge number is divided by the effective photogenerated charge number e in , the charge transfer efficiency CTE is obtained:

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