| LAPS(light addressable potentiometric sensor)is a type of potentiometric electrochemical sensor based on the principle of semiconductor field effect.It has the advantages of high detection sensitivity,good potential stability,fast response speed and compatibility with integrated circuit manufacturing process.Particularly LAPS has a great advantage and flexibility in the measurement of local signals due to the feature of "optical addressable array",and can be used in the fields of biomedicine,environmental protection,industrial and agricultural production,scientific research and so on,and is one of the research hotspots in the field of semiconductor electrochemical sensor.The key technical indexes of LAPS include spatial resolution,detection speed,detection accuracy,etc.,this paper mainly studies the problem of detection accuracy.At present,because LAPS illuminated surface has strong reflection effect on incident light,the photoelectric conversion efficiency of LAPS is rather low.In addition,there exist random noise,fluctuation of bias voltage and light intensity in the sensor and its detection circuit,so the output signal of LAPS is rather noisy.Then,there exists lateral diffusion of photon-generated carriers in substrate,which causes the signal cross talk in LAPS.These are the main factors that influence the detection performance of LAPS.This paper aims at improving LAPS detection accuracy,which is a key technical problem,through design of the substrate with light trapping structure to increase photoelectric conversion efficiency of LAPS,through light source system optimization and noise suppression algorithm to enhance the signal to noise ratio of LAPS,and adoption of meshed working electrode structure to reduce signal cross talk and stabilize output signal,the detection accuracy of LAPS is improved effectively and detection performance is also enhanced.The main innovative research achievements of this paper are as follows:(1)A dynamic circuit model of LAPS is proposed.The output signal of LAPS is very weak and mixed with a large amount of random noise,there is also existing the phenomenon of signal drift.This severely impacts the reliability of LAPS detection results,so the signal processing algorithm having the ability of suppressing signal drift and random noise is need to be studied.On the basis of the equivalent circuit model of LAPS,in this paper,a dynamic circuit model is put forward and the differential equation is established.The differential equation is solved and theexpression of LAPS output signal is obtained.Through analysis of the expression,the signal processing algorithm being used for suppressing noise of LAPS is proposed.This algorithm has two segments,including the calculation of mean square root and Kalamn filtering processing.Firstly,the mean square root of the fundamental component,second harmonic component and the third harmonic component is calculated,which can settle the problem of signal drift well;Secondly,the Kalamn filtering method is used for filtering the normalized I-V characteristic curve,in order to eliminate the random signal mixed with LAPS output signal.The researches show that,compared with conventional simply extracting fundamental component in frequency domain,after adopting the new signal processing algorithm,the signal drift and random noise are all suppressed obviously.(2)A new type of substrate structure with micro blind holes array is proposed.Research shows decreasing substrate thickness can effectively enhance the output signal of LAPS,improve the SNR(signal to noise ratio),and improve the detection sensitivity.Nevertheless,thinning large area of substrate causes significant reduction in mechanical strength of substrate and makes devices brittle.According to the principle of reducing incident light energy loss by light trapping structure,we proposed a new type of substrate with light trapping structure in this paper,by enhancing absorption of incident light in substrate to improve the detection performance of LAPS.The light trapping structure is composed by micrometre-scale blind holes array which is constructed on the light illumination area of substrate.When incident light is shone on substrate,multiple reflection of incident light on micro blind hole array surface occurs.This increases the absorption of incident light in substrate,improves the photoelectric conversion efficiency,and reduces energy loss arising from direct reflection of incident light.Research result shows that constructing light trapping structure with micro blind holes array on the illuminated surface of LAPS can enhance amplitude of LAPS output signal,improve detection sensitivity,linearity and SNR,simultaneously ensure device maintain good mechanical strength in the process of packaging and testing.(3)A new type of working electrode with mesh shape is proposed.Among the detecting points of LAPS array detection sensor,it was found that there exists serious cross talk,which affect the stability,reliability of detection signal and spatial resolution of chemical imaging.Research indicates that lateral diffusion of photongenerated carriers in substrate is the primary cause of cross talk,so it is needful tosearch effective method to suppress the lateral diffusion of photon-generated carriers.Aiming at this problem,in this paper,one type of working electrode with mesh shape surrounding each detecting point is designed.Through suppressing lateral diffusion of photon-generated carriers in substrate by the abundant majority carriers accumulating at interface area between the metal film working electrode and silicon substrate,the photon-generated carriers diffusion is more concentrated on the vertical direction,so the mutual signal cross talk among detecting points can be reduced.Research result shows that the meshed working electrode can effectively suppress signal cross talk and stabilize output signal.(4)A signal measuring method of LAPS based on area calculation is proposed.For traditional measuring method of LAPS,owing to the affection of noise,location of working points is difficult,which makes the detection performance unstable.Aiming at this problem,this paper put forward a measuring method based on area calculation.This method calculates the area between the normalized I-V curve and horizontal axis,through the relationship between chemical quantity of the measured substance and the area,to determine the current detection result.In experiments,this measuring method was used for detecting pH of electrolyte solution,and experiment result shows that the detecting sensitivity and linearity are higher,and not change with variation of voltage step length.Because this method does not need locating the working points,and also is not sensitive to the voltage step length,so it can quicken the detection progress,and is one kind of LAPS measuring method with simple operation and stable detection performance.Through above studies,the problems of LAPS,such as low photoelectric conversion efficiency,strong noise interference and signal cross talk were solved,and the LAPS system prototype machine was developed.The detection results show that the photoelectric conversion efficiency and signal to noise ratio of LAPS were enhanced,and signal cross talk was effectively suppressed.Research works of this paper is of great significance to improve LAPS detection accuracy,enhance detection performance,and extend its application scope. |
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