Research Of SPAD Device With 0.18?M CMOS Process

With the continuous development of single photon detection technology,traditional avalanche photodiodes(APDs)and photomultiplier tubes(PMTs)have been unable to accurately detect single photon signals.To this end,the researchers proposed an APD in Geiger mode,called Single Photon Avalanche Diode(SPAD).SPAD features low power consumption,high sensitivity,and low noise,and SPAD devices fabricated in CMOS technology are compatible with readout circuits.Therefore,CMOS SPAD is the best choice for making single photon detectors.This thesis designs the basic structure of a SPAD device based on a standard 0.18?m CMOS process.The structure uses a p+/center n-well junction as the avalanche region of the device,at the same time,the n~-virtual guard ring is formed at the edge of the pn junction by lateral diffusion between three adjacent n-wells to suppress premature edge breakdown of the device.Consequencely,the equivalent circuit model is established for the basic structure of SPAD and the main factors limiting the frequency bandwidth of the device are determined by analyzing the frequency response of the equivalent circuit.The simulation results are as follows:the avalanche breakdown voltage of the SPAD is 14 V when the n-well gap virtual guard ring has a width of 1.4?m and the optical window diameter is 20?m;the peak of responsivity reaches 0.22 A/W at a wavelength of 500 nm;the peak of photon detection efficiency reaches 23%at a wavelength of 480 nm;and the dark count rate is 4.1 kHz at an excess bias voltage of 1 V.Due to the high dark count rate of the designed SPAD device,two low dark count rate p-well/center n-well type SPAD and p-well/deep retrograde n-well type SPAD are proposed.The p-well/center n-well type SPAD device is based on the basic structure of the SPAD and inserts a p-well between the p+layer and the center n-well.The p-well/deep retrograde n-well type SPAD device uses a p-well as the anode of the device and a reverse-doped distributed deep n-well as the photon absorption region of the device.Both SPAD devices reduce the probability of carrier band-to-band tunneling by increasing the depletion layer thickness of the device structure,thereby essentially reducing the dark count rate of the device.The performance of the proposed two low dark count rate SPAD devices is analyzed using TCAD Silvaco simulation software.The simulation results of the p-well/center n-well type SPAD device are as follows:the device occurs avalanche breakdown when the reverse bias voltage is about 13 V,the center electric field distribution of the pn junction is uniform and the highest electric field strength reaches 6.73×10~5 V/cm when the breakdown occurs;the responsivity reaches a peak of 0.35 A/W at a light wavelength of500 nm;the photon detection efficiency is 37%at an excess bias voltage of 1 V and a light wavelength of 480 nm;the dark count rate reaches 0.82 kHz at an excess bias voltage of 1 V.The performance of the p-well/deep retrograde n-well SPAD is as follows:the device has an avalanche breakdown voltage of 9 V and a maximum electric field strength of 6.82×10~5 V/cm when avalanche breakdown occurs;the responsivity reaches a peak of0.28 A/W at a wavelength of 500 nm;the detection efficiency and dark count rate are 33%and 0.66 kHz at an excess bias voltage of 1 V,respectively.In order to reduce the influence of afterpulsing on dark count rate,the peripheral passive quenching circuit of SPAD device is designed.The transient characteristics of the quenching circuit are analyzed by Pspice circuit simulation software.The simulation results show that the dead time of SPAD is about 17 ns.