CN114900687B - Simulation method for influence of different light intensities on full well capacity of pixel unit of CMOS image sensor - Google Patents
Simulation method for influence of different light intensities on full well capacity of pixel unit of CMOS image sensor Download PDFInfo
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- CN114900687B CN114900687B CN202210538335.3A CN202210538335A CN114900687B CN 114900687 B CN114900687 B CN 114900687B CN 202210538335 A CN202210538335 A CN 202210538335A CN 114900687 B CN114900687 B CN 114900687B
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- H—ELECTRICITY
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- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N17/00—Diagnosis, testing or measuring for television systems or their details
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
- H04N25/70—SSIS architectures; Circuits associated therewith
- H04N25/76—Addressed sensors, e.g. MOS or CMOS sensors
Abstract
The invention relates to a simulation method of the full well capacity influence of different light intensities on a CMOS image sensor pixel unit, which comprises the steps of resetting a clamping photodiode before the exposure stage of the CMOS image sensor pixel unit starts, setting the grid voltage of a transmission grid to be 0V, enabling the photodiode to be in an accumulation state, then adopting a collimation light source to enter in a simulation environment, calling a light injection model to realize the simulation of a light generation process, setting fixed light intensity, adopting transient simulation, extracting the change curve of the number of photo-generated carriers in the clamping photodiode along with the integral time to obtain the full well capacity, finally changing the light intensity, repeatedly extracting the change curve of the number of photo-generated carriers along with the integral time until the full well capacity is the same as the full well capacity under the previous light intensity condition, and thus realizing the simulation of the influence of different light intensities on the full well capacity of the CMOS image sensor pixel unit. The invention can intuitively see the influence of different light intensities on the capacity of the full trap.
Description
Technical Field
The invention relates to the technical field of image sensor pixel unit simulation, in particular to a simulation method for the influence of different light intensities on the full well capacity of a CMOS image sensor pixel unit.
Background
The full well capacity of a complementary metal oxide semiconductor (Complementary Metal Oxide Semiconductor, CMOS) image sensor, which represents the maximum amount of charge that can be stored in the photodiode potential well, is an important indicator for measuring the photo-sensing capability of a CMOS image sensor. For multi-bit digital image sensors, the gray level of the image always reaches a constant value, i.e., full well capacity, when the light intensity is large enough or the exposure time is long enough.
In previous studies on full well capacity, the relationship between full well capacity and different light intensities was rarely considered. However, from recent studies, it was found that there is a great influence of the light intensity on the full well capacity, and it is considered that the full well capacity cannot be defined unless the light intensity is specified. These conclusions are critical to the characterization and design of the CMOS image sensor, and if different light intensities correspond to different full well capacities, there may be significant errors in the full well capacities obtained without considering the light intensities and the evaluation results of the dynamic range, signal-to-noise ratio, sensitivity, and other parameters determined by the full well capacities.
Disclosure of Invention
The invention aims to provide a simulation method for the influence of different light intensities on the full-well capacity of a CMOS image sensor pixel unit, which comprises the steps of resetting a clamp photodiode before the exposure stage of the CMOS image sensor pixel unit starts, setting the grid voltage of a transmission grid to be 0V, enabling the photodiode to be in an accumulation state, then adopting a collimation light source to be incident in a simulation environment, calling a light injection model, realizing the simulation of a light generation process, setting fixed light intensity, adopting transient simulation, extracting the change curve of the number of photo-generated carriers in the clamp photodiode along with the integration time to obtain the full-well capacity, finally changing the light intensity, repeatedly extracting the change curve of the number of photo-generated carriers along with the integration time until the full-well capacity is the same as the full-well capacity under the previous light intensity condition, and thus realizing the simulation of the influence of different light intensities on the full-well capacity of the CMOS image sensor pixel unit. The invention can intuitively see the influence of different light intensities on the capacity of the full trap.
The invention discloses a simulation method for the influence of different light intensities on the full well capacity of a pixel unit of a CMOS image sensor, which comprises the following steps:
a. resetting the clamping photodiode before the exposure stage of the pixel unit of the CMOS image sensor begins, and setting the transmission gate voltage to be 0V so that the photodiode is in an accumulation state;
b. adopting a collimation light source to make incidence in a TCAD software simulation environment, and calling a Ray tracking light injection model in TCAD software to realize simulation of a light generation process;
c. setting fixed light intensity, adopting transient simulation, and extracting a change curve of the number of photo-generated carriers in the clamping photodiode along with the integration time to obtain full-well capacity;
d. and c, changing the light intensity, repeating the step c until the full well capacity is the same as the full well capacity under the previous light intensity condition, and stopping extracting the change curve of the number of the photo-generated carriers in the clamping photodiode along with the integration time to obtain the simulation of the influence of different light intensities on the full well capacity of the pixel unit of the CMOS image sensor.
The invention discloses a simulation method for the influence of different light intensities on the full well capacity of a pixel unit of a CMOS image sensor, which comprises the following steps:
a. resetting the clamping photodiode before the exposure stage of the pixel unit of the CMOS image sensor begins, and setting the transmission gate voltage to be 0V so that the photodiode is in an accumulation state;
b. adopting a collimation light source to enter in a TCAD software simulation environment, setting parameters X.ORIGIN and Y.ORIGIN, and determining the position of the light source; a parameter ANGLE specifying a propagation direction of the beam with respect to the X-axis; parameters MIN.WINDOW and MAX.WINDOW jointly specify the incidence range of light rays, in an Atlas module, the light rays can be automatically divided into a series of light rays within the specified incidence range according to the geometric shape of a device, after the light rays are incident, a light injection model (Ray tracking model) is called, and the simulation of the light generation process is realized through formulas (1) - (5);
the light generation rate G in the model, i.e. the generation rate of electron-hole pairs in silicon, is expressed as:
p is the radiation intensity factor, and includes reflection, transmission, and damage caused by absorption of materials in the path of the light. η (eta) 0 Is the internal quantum efficiency, which represents the number of carrier pairs generated per photon. y is the relative distance of the rays, h is the Planck constant, λ is the wavelength, and c is the speed of light. Alpha is the absorption coefficient, and k is the imaginary part of the refractive index of light;
in the simulation process, G is coupled to a Poisson equation, and in a current continuity equation, the light intensity and carrier concentration at corresponding grid points are subjected to transient solution:
where ψ is the electrostatic potential, ε is the Si dielectric constant, ρ is the local space charge density. The local space charge density is the sum of all mobile and fixed charges, including electrons, holes, and ionized impurities. n and p are electron and hole concentrations, q is the unit charge amount, J n 、J p Is the electron and hole current density, mu n Sum mu p For electron and hole mobility, D n And D p The electron and hole diffusion coefficients, respectively. n is n ie T is the effective intrinsic carrier concentration L Is crystallineA cell temperature;
c. setting fixed light intensity, adopting transient simulation, and extracting a change curve of the number of photo-generated carriers in the clamping photodiode along with the integration time to obtain full-well capacity;
d. and c, changing the light intensity, repeating the step c until the full well capacity is the same as the full well capacity under the previous light intensity condition, and stopping extracting the change curve of the number of the photo-generated carriers in the clamping photodiode along with the integration time, thereby realizing the simulation of the influence of different light intensities on the full well capacity of the pixel unit of the CMOS image sensor.
The simulation method for the influence of different light intensities on the full well capacity of the pixel unit of the CMOS image sensor is suitable for testing the full well capacity of the CMOS image sensor of any model before and after irradiation. The invention can intuitively see the influence of different light intensities on the capacity of the full trap.
Therefore, the invention is suitable for device development units, scientific research institutes and space load units needing to master the performance of the CMOS image sensor.
Drawings
FIG. 1 is a graph showing the number of photo-generated carriers with integration time at different light intensities.
Detailed Description
Examples
a. Resetting the clamping photodiode before the exposure stage of the pixel unit of the CMOS image sensor begins, and setting the transmission gate voltage to be 0V so that the photodiode is in an accumulation state;
b. adopting a collimation light source to enter in a TCAD software simulation environment, setting parameters X.ORIGIN and Y.ORIGIN, and determining the position of the light source; a parameter ANGLE specifying a propagation direction of the beam with respect to the X-axis; parameters MIN.WINDOW and MAX.WINDOW jointly specify the incidence range of light rays, in an Atlas module, the light rays can be automatically divided into a series of light rays within the specified incidence range according to the geometric shape of a device, after the light rays are incident, a light injection model (Ray tracking model) is called, and the simulation of the light generation process is realized through formulas (1) - (5); the light generation rate G in the model, i.e. the generation rate of electron-hole pairs in silicon, is expressed as:
p is the radiation intensity factor, and includes reflection, transmission, and damage caused by absorption of materials in the path of the light. η (eta) 0 Is the internal quantum efficiency, which represents the number of carrier pairs generated by each photon, y is the relative distance of the rays, h is the Planck constant, λ is the wavelength, c is the speed of light, α is the absorption coefficient, and k is the imaginary part of the refractive index of the light;
in the simulation process, G is coupled to a Poisson equation, and in a current continuity equation, the light intensity and carrier concentration at corresponding grid points are subjected to transient solution:
where ψ is the electrostatic potential, ε is the Si dielectric constant, ρ is the local space charge density. The local space charge density is the sum of all mobile and fixed charges, including electrons, holes and ionized impurities, n and p are the electron and hole concentrations, q is the unit charge amount, J n 、J p Is the electron and hole current density, mu n Sum mu p For electron and hole mobility, D n And D p The electron and hole diffusion coefficients, respectively. n is n ie T is the effective intrinsic carrier concentration L Is the lattice temperature;
c. setting and fixingThe light intensity was 10mW/cm 2 By transient simulation, the change curve of the number of photo-generated carriers in the clamping photodiode along with the integration time is extracted, and as shown in a curve a in fig. 1, the full-well capacity is 10580e - ；
d. Varying the light intensity to 30mW/cm respectively 2 、70mW/cm 2 、90mW/cm 2 、100mW/cm 2 Repeating step c, and extracting the variation curve of the number of photo-generated carriers in the clamped photodiode along with the integration time, as shown in curve b, curve c, curve d and curve e in fig. 1, respectively, to obtain a full-well capacity of 11000e - 、11500e - 、11700e - 、11700e - The method comprises the steps of carrying out a first treatment on the surface of the From FIG. 1, it can be seen that the light intensity (100 mW/cm 2 ) Full well capacity and light intensity at the time (90 mW/cm 2 ) The full well capacity of the pixel units of the CMOS image sensor is the same, and the change curve of the number of photo-generated carriers in the clamping photodiode along with the integration time is stopped to be extracted, namely the simulation of the influence of different light intensities on the full well capacity of the pixel units of the CMOS image sensor is realized, and the same full well capacity 11700e - The full well capacity is accurate.
Because different light intensities correspond to different full well capacities, the full well capacities obtained under the condition of not considering the light intensities and the evaluation results of parameters such as the dynamic range, the signal-to-noise ratio, the sensitivity and the like of the CMOS image sensor determined by the full well capacities can have great errors. In the partial light intensity range and the integral time range adopted by the experiment, the full trap capacity is found to be unchanged with the change of the light intensity, which is inconsistent with the report in the literature, and the analysis reason is probably due to the limitation of the working band of the spectrometer on the illumination intensity range, so that the full trap capacity is not changed with the change of the light intensity. Therefore, the invention expands the exposure intensity range through simulation, and can intuitively see the influence of different light intensities on the full well capacity of the pixel unit of the CMOS image sensor. The simulation method provided by the invention can show that the full well capacity is changed along with the light intensity in a low light intensity range, and the full well capacity is not changed in a high light intensity range, so that the simulation method plays an important role in revealing the mechanism of the influence of the light intensity on the full well capacity, and provides a powerful theoretical basis for the performance improvement design of the CMOS image sensor.
The above is only a specific embodiment of the simulation method for the effect of different light intensities on the full well capacity of the pixel unit of the CMOS image sensor according to the present invention, but the protection scope of the present invention is not limited thereto, and any person skilled in the art will understand that the substitution or addition and the subtraction are included in the scope of the present invention.
Claims (1)
1. The simulation method for the influence of different light intensities on the full well capacity of the pixel unit of the CMOS image sensor is characterized by comprising the following steps of:
a. resetting the clamping photodiode before the exposure stage of the pixel unit of the CMOS image sensor begins, and setting the transmission gate voltage to be 0V so that the photodiode is in an accumulation state;
b. adopting a collimation light source to make incidence in a TCAD software simulation environment, and calling a Ray tracking light injection model in TCAD software to realize simulation of a light generation process;
c. setting fixed light intensity, adopting transient simulation, and extracting a change curve of the number of photo-generated carriers in the clamping photodiode along with the integration time to obtain full-well capacity;
d. and c, changing the light intensity, repeating the step c until the full well capacity is the same as the full well capacity under the previous light intensity condition, and stopping extracting the change curve of the number of the photo-generated carriers in the clamping photodiode along with the integration time to obtain the simulation of the influence of different light intensities on the full well capacity of the pixel unit of the CMOS image sensor.
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