Low-noise Topmetal CMOS Pixel Sensor For High Energy Physics Experiments

With the development of nuclear and particle physics,the low background(low event-rate density and low charge drifting speed)and low noise experiments are developed as research mainstream and hot spots in the future high energy physics experiments.Notable examples are neutrinoless double beta decay(Ov??)and dark matter searches.Gaseous or high pressure cryogenic liquid Time Projection Chambers(TPCs)have been considered as ideal detectors for the low-background and low-noise experiments.Therefore,low-noise,high pixel density and high integration of circuitry nature of CMOS sensors play a more and more important role on future high energy physic experiments.These sensors with higher time resolution,spatial resolution and energy resolution can be satisfied with the requirements of high energy applications.This thesis will present on pixel sensors installed with charge collection and readout circuit for the low-background and low-noise charge readout.These sensors are aimed at direct-charge collecting and measuring in TPCs without gas-avalanche gain.The detailed research content and innovation points are shown in the following:Firstly,a novel direct-charge collecting pixel sensor is proposed.The charge-collection electrode is the opening exposed most-top metal called Topmetal.And that the same layer metal called Guardring is manufactured surrounding Topmetal.The voltage difference between Guardring and Topmetal focuses electric field improving the charge-collection efficiency.This sensor structure has been applied to Topmetal-?-sensor.Topmetal-?-can record single alpha tracks emitted by 241 Am in the room-temperature air.Besides,Topmetal-?-can also collect charge in the 77 K liquid nitrogen.Secondly,a key technology on reducing the interference of Charge Sensitive Amplifier(CSA)by the row and column selection switches during the analog readout is adopted.The CSA easily oscillates under the effect of charge injection caused by switching.Hence a source follower is designed between the CSA output node and switches.This method reduces the interference by the charge injection.The input and feedback capacitances are reduced through optimizing parasitic capacitance hence increasing the charge conversion gain and signal-to-noise ratio.The technology has been applied to Topmetal-?-sensor.The Equivalent Noise Charge(ENC)is 13,9 e-and 12.6 e-in the room air and 77 K liquid nitrogen,respectively.Thirdly,a pixel-level threshold calibration method is proposed to improve the threshold uniformity.A Digital-to-Analog Convertor(DAC)is designed in each pixel.Hence the threshold voltage of each pixel can be individually adjustable reducing the threshold mismatch caused by CSA noise,comparator offset and process.The spread of threshold voltage is reduced by 7.5 times from 35.9 mV down to 4.8 mV.A good imaging result is observed during the imaging experiment using the digital readout channel of Topmetal-?-sensor.Hence the digital readout channel can work correctly.Fourthly,adjustable cut-off frequency filters are adopted to reduce the noise of CSA biases hence further improving the signal-to-noise ratio.Topmetal-?a and Topmetal-S are designed based on this method.The noise of Topmetal-?a and Topmetal-S is 12.4e-and 28.7 e-,respectively.Compared to Topmetal-?-,the analog readout noise of Topmetal-?a is reduce by 10.8%and the DC voltage variation noise 22%from 1.2 mV down to 0.936 mV.The analog readout noise of Topmetal-S is lower than the desired target of 30 e-.Topmetal-?-sensor has been installed in lossless,high position resolution and high time resolution beam monitor system of heavy ion accelerator in Lanzhou and 88-inch cyclotron in Berkeley.The beam monitor system will be applied to the next generation of cancer treatment and accelerator storage ring beam monitoring.The lower-noise Topmetal-?a will further improve the specifications of beam monitor system.The electric tests of Topmetal-S have been done.Besides,large hexagon electrode array composed of 127 Topmetal-S sensors which will be applied to Ov?? is under research.