Method for improving signal-to-noise ratio of LOFIC pixel conversion gain switching point, pixel structure and image sensor

Abstract

The invention discloses a method for improving the signal-to-noise ratio of a LOFIC pixel conversion gain switching point, a pixel structure and an image sensor. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point is realized by changing a pixel read-out time sequence and an overflow tube gate voltage, after a frame is read out, the overflow tube gate voltage is changed, a next frame is read out again, and then the read-out frames are spliced. According to the method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point, the working time sequence of the LOFIC pixel is changed, and compared with a traditional LOFIC working time sequence, the effects that at the high-low gain switching position, space domain noise is reduced due to pixel non-consistency overflow, and the imaging quality under the light intensity at the switching position can be improved can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of CMOS image sensor pixels, and in particular to image sensor pixels based on lateral overflow integrated capacitor technology, and in particular to a method for improving the signal-to-noise ratio of a LOFIC pixel conversion gain switching point, a pixel structure and an image sensor. Background Art

[0002] LOFIC (Lateral Overflow Integration Capacitor) technology is a high dynamic range imaging technology. It is designed with a lateral overflow tube inside the pixel, which can overflow the excess charge into the LOFIC capacitor inside the pixel under strong light conditions, thereby increasing the full well capacity of the pixel. Figure 1 As shown. The photodiode in the LOFIC pixel adopts a PPD structure as an element for the conversion and accumulation of photoelectrons; the TG (Transfer Gate) transfer gate controls the transfer of charges in the PPD to the FD region; the FD (Floating Diffusion) floating diffusion region is a charge-voltage conversion node, which is in a floating state when the signal is read out, and its capacitance value determines the conversion gain of the LOFIC active pixel. The SG transistor is responsible for the connection between the FD node and the Cs capacitor. The high and low conversion gains can be switched through the SG switch. The RST transistor is responsible for resetting the FD, Cs capacitor and PPD. The source follower SF and the row selection transistor SEL are combined to realize the readout of the pixel signal. The Cs capacitor is responsible for collecting the saturated photogenerated charge from the PPD and providing a low conversion gain node for the pixel. The overflow of the saturated photogenerated charge first passes through the TG transistor to enter the FD node. When the FD node is filled to the full well capacity, the subsequent saturated photogenerated charge is collected by the Cs capacitor through the SG transistor. Compared with 4T active pixels, the LOFIC active pixel structure integrates the switch transistor SG and the Cs capacitor, so that the pixel has two conversion gains, high and low. In low light conditions, the high conversion gain of the FD node is used to obtain a high-sensitivity signal; in strong light conditions, SG is turned on, and the FD and Cs capacitors are connected to reduce the conversion gain and realize the quantization of strong light signals.

[0003] The working sequence of LOFIC active pixel is as follows: Figure 2As shown. Before exposure, first turn on the RST, SG and TG transistors to reset the PPD, FD node and Cs capacitor, and then enter the exposure stage. In order to ensure the overflow channel, the TG and SG transistors can apply a certain voltage during the pixel exposure. After the exposure is over, the SEL tube is turned on and the pixel signal readout stage is entered. The SG transistor is turned off, and the reset voltage HR of the FD node is read out. Then the transfer transistor TG is turned on to transfer the photogenerated charge accumulated in the PPD to the FD node, and the signal voltage HS is read out. At this time, the readout of the high conversion gain signal is completed. Subsequently, the TG and SG transistors are turned on to turn on the FD node and the Cs capacitor, switch to the low conversion gain state, read the signal voltage LS, and finally turn on the RST transistor to reset the FD node and the Cs capacitor, and read the reset voltage LR.

[0004] The high conversion gain signal can be obtained by performing a Correlated Double Sampling (CDS) operation on HR and HS, and the low conversion gain signal can be obtained by performing a Delta Reset Sampling (DRS) operation on LR and LS.

[0005] The dynamic range enhancement signal is composed by selecting either of the two linear signals in low light and high light. The dynamic range enhanced CMOS image sensor maintains high sensitivity, high signal-to-noise ratio and excellent mobile image quality. As the light intensity continues to increase, the PD of the LOFIC pixel accumulates more electrons until saturation begins to overflow into Cs. During the LOFIC pixel overflow process, due to process deviations and the influence of IR-drop, there is pixel inconsistency, which causes the LOFIC pixel to overflow inconsistently, which will lead to an increase in spatial noise at the high and low conversion gain switching points and a deterioration in imaging quality. An array SNR curve of a LOFIC pixel is shown below: Figure 3 As shown in the figure, at the high and low conversion gain switching point, due to pixel inconsistency and overflow inconsistency, the spatial noise increases and the overall SNR decreases. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings and defects of the prior art and provide a method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point, a pixel structure and an image sensor. The LOFIC readout method with dual gate voltage of the overflow tube is adopted. The dual gate voltage readout method reads out the different gate voltages of the overflow tubes of the front and rear frames. Through the mutual splicing of data, the influence of the non-uniform overflow between the high and low conversion node pixels on the array SNR can be reduced, the imaging quality of the conversion node is improved, and the dynamic range is also improved.

[0007] The first purpose of the present invention is to provide a method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point, which is achieved by changing the pixel readout timing and the overflow tube gate voltage. After reading out a frame, the overflow tube gate voltage is changed, the next frame is read out again, and then the read frames are spliced.

[0008] The external input voltage is used as the input reference voltage, and the timing control circuit is used to select the access of different voltages in different time periods to control the change of the overflow tube gate voltage.

[0009] Wherein, the external input voltage is 1.5V.

[0010] Among them, for the first frame, before exposure, the RST, SG and TG transistors are turned on to reset the PPD, FD node and Cs capacitor, and then the exposure stage is entered, and the gate voltage of the SG tube is set to VSG1; after the exposure is completed, the SEL tube is turned on, and the pixel signal readout stage is entered; the SG transistor is turned off, and the reset voltage HR1 of the FD node is read out, and then the transfer transistor TG is turned on to transfer the photogenerated charge accumulated in the PPD to the FD node, and the signal voltage HS1 is read out, and the readout of the high conversion gain signal is completed at this time; subsequently, the TG and SG transistors are turned on, the FD node and the Cs capacitor are turned on, and the signal voltage LS1 is read out, and finally the RST transistor is turned on to reset the FD node and the Cs capacitor, and the reset voltage LR1 is read out.

[0011] Among them, for the second frame, before exposure, the RST, SG and TG transistors are turned on to reset the PPD, FD node and Cs capacitor, and then the exposure stage is entered, and the SG tube gate voltage is set to VSG2; after the exposure is completed, the SEL tube is turned on, and the pixel signal readout stage is entered; the SG transistor is turned off, and the reset voltage HR2 of the FD node is read out, and then the transfer transistor TG is turned on to transfer the photogenerated charge accumulated in the PPD to the FD node, and the signal voltage HS2 is read out. At this time, the readout of the high conversion gain signal is completed; subsequently, the TG and SG transistors are turned on, the FD node and the Cs capacitor are turned on, and the signal voltage LS2 is read out, and finally the RST transistor is turned on to reset the FD node and the Cs capacitor, and the reset voltage LR2 is read out.

[0012] Among them, the VSG1 is 0V, and the VSG2 is 1.5V.

[0013] The step of splicing the read frames includes:

[0014] Select preset data from HCG1, HCG2, LCG1, LCG2 to synthesize HDR images:

[0015] When HCG1 is less than the first threshold, the HDR image uses HCG1 data;

[0016] When HCG2 is greater than the second threshold and less than the third threshold, the HDR image uses HCG2 data;

[0017] When LCG1 is greater than the fourth threshold and less than the fifth threshold, the HDR image uses LCG1 data;

[0018] When LCG2 is greater than the sixth threshold, the HDR image adopts LCG2 data;

[0019] Among them, HCG1 and HCG2 are first high conversion gain data in the first frame and the second frame respectively, and LCG1 and LCG2 are low conversion gain data in the first frame and the second frame respectively.

[0020] The first threshold, the second threshold, the third threshold, the fourth threshold, the fifth threshold and the sixth threshold are obtained through actual testing.

[0021] The second object of the present invention is to provide a pixel structure that uses the method of improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point to read out data.

[0022] The third object of the present invention is to provide an image sensor including the pixel structure.

[0023] The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point of the present invention changes the working timing of the LOFIC pixel. Compared with the traditional LOFIC working timing, it can achieve a reduction in spatial noise caused by pixel inconsistency overflow at the high and low gain switching point, thereby improving the imaging quality under the light intensity at the switching point. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of the LOFIC active pixel structure;

[0025] Figure 2 This is the working timing diagram of LOFIC active pixel;

[0026] Figure 3 is the LOFIC active pixel array SNR;

[0027] Figure 4 It is the working timing diagram of the dual-gate voltage readout LOFIC active pixel of the present invention.

[0028] Figure 5 It is the dual gate voltage readout LOFIC active pixel array SNR of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] The present invention is achieved by changing the pixel readout timing and the overflow tube gate voltage. After reading out a frame, the overflow tube gate voltage is changed and read out again, thereby reducing the spatial noise caused by pixel inconsistency at the high and low conversion gain switching point.

[0031] Among them, the setting of the overflow tube gate voltage is closely related to the PD full well capacity, Cs capacitance, and FD node capacitance.

[0032] Specifically, the gate voltage is inputted with an external 1.5V voltage as an input reference voltage, and the timing control circuit selects different voltages to be connected at different time periods to control the change of the overflow tube gate voltage.

[0033] After the change, the dual gate voltage readout LOFIC active pixel working timing is as follows Figure 4 As shown in Figure 2, different overflow tube gate voltages are used in the two frames, such as Figure 4 Specifically, during the processing, the following steps are adopted:

[0034] In the first frame, before exposure, the RST, SG, and TG transistors are first turned on to reset the PPD, FD node, and Cs capacitor, and then the exposure phase is entered, and the gate voltage of the SG tube is set to VSG1. After the exposure is completed, the SEL tube is turned on and the pixel signal readout phase is entered. The SG transistor is turned off, and the reset voltage HR1 of the FD node is read out. Then the transfer transistor TG is turned on to transfer the photogenerated charge accumulated in the PPD to the FD node, and the signal voltage HS1 is read out. At this time, the readout of the high conversion gain signal is completed. Subsequently, the TG and SG transistors are turned on to turn on the FD node and the Cs capacitor, switch to the low conversion gain state, read the signal voltage LS1, and finally turn on the RST transistor to reset the FD node and the Cs capacitor, and read the reset voltage LR1.

[0035] In the second frame, before exposure, the RST, SG, and TG transistors are first turned on to reset the PPD, FD node, and Cs capacitor, and then the exposure phase is entered, and the gate voltage of the SG tube is set to VSG2. After the exposure is completed, the SEL tube is turned on and the pixel signal readout phase is entered. The reset voltage HR2 of the FD node is read out, and then the transfer transistor TG is turned on to transfer the photogenerated charge accumulated in the PPD to the FD node, and the signal voltage HS2 is read out. At this time, the readout of the high conversion gain signal is completed. The SG transistor is turned off, and then the TG and SG transistors are turned on to turn on the FD node and the Cs capacitor, switch to the low conversion gain state, read the signal voltage LS2, and finally turn on the RST transistor to reset the FD node and the Cs capacitor, and read the reset voltage LR2.

[0036] For example, VSG1 can be 0V and VSG2 can be 1.5V.

[0037] The array SNR curve of a dual-gate voltage readout LOFIC active pixel is shown in Figure 5 As shown, the setting of the overflow tube gate voltage affects the selection of data under different light intensities when the subsequent HDR image is synthesized. The data of HCG1, HCG2, LCG1, and LCG2 are used in turn to synthesize the HDR image, especially the high and low conversion gain switching points, and the data that is not affected by pixel inconsistency overflow is used to compensate each other.

[0038] The black line in the figure is the SRN curve of HCG1 and LCG1 in the first frame. The SNR decreases near the switching point of HCG1 and LCG1. This is because in the LOFIC pixel overflow process, due to process deviations and the influence of IR-drop, there is pixel inconsistency, which causes the LOFIC pixel inconsistency overflow, which will lead to an increase in spatial noise at the high and low conversion gain switching point and a decrease in SNR. The red line is the SNR curve of HCG2 and LCG2 in the second frame. The reason for the decrease in SNR near the switching point of HCG2 and LCG2 is the same as before.

[0039] Specifically, the data of HCG1, HCG2, LCG1, and LCG2 are used to synthesize the HDR image. The HDR image synthesis is to select appropriate data from HCG1, HCG2, LCG1, and LCG2. For example, when HCG1 is less than the first threshold, the HDR image uses HCG1 data; when HCG2 is greater than the first threshold and less than the third threshold, HCG2 data is used; when LCG1 is greater than the fourth threshold and less than the fifth threshold, LCG1 data is used; when LCG2 is greater than the sixth threshold, LCG2 data is used.

[0040] Among them, the above-mentioned thresholds need to be obtained through actual tests.

[0041] An embodiment of the present invention further provides a pixel structure, which uses the method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point to perform data readout.

[0042] An embodiment of the present invention further provides an image sensor, comprising the pixel structure.

[0043] The basic principles and main features of the present invention and the advantages of the present invention are shown and described above. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0044] Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is therefore intended that all changes falling within the meaning and range of equivalent elements of the claims are included in the present invention.

[0045] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for improving the signal-to-noise ratio of a LOFIC pixel conversion gain switching point, characterized in that: This is achieved by changing the pixel readout timing and the overflow tube gate voltage. After reading out one frame, the overflow tube gate voltage is changed, the next frame is read out again, and then the readout frames are spliced.

2. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point according to claim 1, characterized in that: An external input voltage is used as an input reference voltage, and a timing control circuit is used to select different voltages to be connected in different time periods to control the change of the overflow tube gate voltage.

3. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point according to claim 2, characterized in that: The external input voltage is 1.5V.

4. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point according to claim 1, characterized in that: For the first frame, before exposure, the RST, SG and TG transistors are turned on to reset the PPD, FD node and Cs capacitor, and then the exposure stage is entered, and the gate voltage of the SG tube is set to VSG1; after the exposure is completed, the SEL tube is turned on, and the pixel signal readout stage is entered; the SG transistor is turned off, and the reset voltage HR1 of the FD node is read out, and then the transfer transistor TG is turned on to transfer the photogenerated charge accumulated in the PPD to the FD node, and the signal voltage HS1 is read out. At this time, the readout of the high conversion gain signal is completed; subsequently, the TG and SG transistors are turned on to turn on the FD node and the Cs capacitor, switch to the low conversion gain state, read the signal voltage LS1, and finally turn on the RST transistor to reset the FD node and the Cs capacitor, and read the reset voltage LR1.

5. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point according to claim 4, characterized in that: For the second frame, before exposure, the RST, SG and TG transistors are turned on to reset the PPD, FD node and Cs capacitor, and then the exposure stage is entered, and the gate voltage of the SG tube is set to VSG2; after the exposure is completed, the SEL tube is turned on, and the pixel signal readout stage is entered; the SG transistor is turned off, and the reset voltage HR2 of the FD node is read out, and then the transfer transistor TG is turned on to transfer the photogenerated charges accumulated in the PPD to the FD node, and the signal voltage HS2 is read out. At this time, the readout of the high conversion gain signal is completed; subsequently, the TG and SG transistors are turned on to turn on the FD node and the Cs capacitor, switch to the low conversion gain state, read the signal voltage LS2, and finally turn on the RST transistor to reset the FD node and the Cs capacitor, and read the reset voltage LR2.

6. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point according to claim 5, characterized in that: The VSG1 is 0V, and the VSG2 is 1.5V.

7. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point according to claim 1, characterized in that: The splicing of the read frames includes: Select preset data from HCG1, HCG2, LCG1, LCG2 to synthesize HDR images: When HCG1 is less than the first threshold, the HDR image uses HCG1 data; When HCG2 is greater than the second threshold and less than the third threshold, the HDR image uses HCG2 data; When LCG1 is greater than the fourth threshold and less than the fifth threshold, the HDR image uses LCG1 data; When LCG2 is greater than the sixth threshold, the HDR image adopts LCG2 data; Among them, HCG1 and HCG2 are first high conversion gain data in the first frame and the second frame respectively, and LCG1 and LCG2 are low conversion gain data in the first frame and the second frame respectively.

8. The method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point according to claim 1, characterized in that: The first threshold, the second threshold, the third threshold, the fourth threshold, the fifth threshold and the sixth threshold are obtained through actual tests.

9. A pixel structure, characterized in that: Data readout is performed using the method for improving the signal-to-noise ratio of the LOFIC pixel conversion gain switching point as described in any one of claims 1 to 8.

10. An image sensor, characterized in that Comprising the pixel structure as claimed in claim 9.

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