Structure Design And Performance Research Of Capacitive Humidity Sensor With Three-dimensional Interdigitation

Continuously innovating semiconductor technology has promoted the continuous development of traditional sensors towards miniaturization,intelligence and multifunction,among them,the research on capacitive miniature gas/humidity sensors is also making continuous progress driven by MEMS technology and CMOS technology.This paper will focus on the sensitive structure that affects the humidity sensitive performance of the capacitive micro-humidity sensor,in view of the shortcomings of long response recovery time and complicated preparation process in current research,the threedimensional microstructure design and processing,sensor sensitive film preparation and humidity sensing characteristics are carried out.The specific research contents are as follows.Four new structures of capacitive chemical sensors are designed.(1)Perforated plate capacitive: in order to solve the problem of slow response of the plate capacitive sensor before,air holes are designed on the upper and lower plate electrodes,and combined with the integrated package of the micro air chamber,forced convection is formed to accelerate the response.(2)Three-dimensional interdigital capacitive and(3)Three-dimensional square spiral capacitive: aiming at the problem that half of the power lines in the planar electrode structure pass through the substrate and cannot be used by the sensitive film,a higher electrode structure is designed to reduce the loss of power lines,thereby improving the sensitivity of the device.(4)Suspended three-dimensional interdigital capacitive: it is a further improvement on the basis of the three-dimensional interdigital capacitive sensor,by separating the electrode structure from the substrate,the analyte can enter the sensitive film from both sides,thereby speeding up the response.Two kinds of microstructures,three-dimensional interdigital capacitive and threedimensional square spiral capacitive,were prepared by the method of thickening planar electrodes by electroplating,and the optimized experimental conditions of photolithography,evaporation and electroplating were studied.Finally,threedimensional interdigital microstructure with different copper coating thickness was successfully realized,however,the three-dimensional square spiral microstructure has not been successfully fabricated due to the uneven current density.The suspension type threedimensional interdigital microstructure adopted a preparation method combining deep silicon etching and bonding,and the suspended three-dimensional interdigital microstructure with different interdigital distance was successfully prepared through an optimized process flow.The perforated plate capacitive microstructure was designed with a process flow combining surface silicon process and bulk silicon process,but it has not been carried out due to incomplete process of the foundry.The two types of microstructures(2)and(4)that complete MEMS processing are essentially side-standing plate capacitor.In order to coat the sensitive film in the slit between the two plates,a new method of preparing the sensitive film was tried using the capillary force of the fine structure to suck the solution dripped onto the slit to form a thin film.The moisture-sensitive material used high-molecular material chitosan.The results of humidity sensitivity test show that the three-dimensional interdigital humidity sensor has good repeatability,its relative deviation is about 1%,its average response time is 33 s to 40 s,and the recovery time is 11 s to 20 s,which has a faster response recovery speed.During the experiment,it was also found that the sensitivity of the suspended three-dimensional interdigital humidity sensor will first increase and then decrease with the increase of the amount of dripping.Although the amount of each drop is much larger than the volume of the slit,the metallographic microscope observed that when the maximum amount of drop is 15?L,the sensitive solution will accumulate on the upper surface of the interdigit and seal the interdigital gap after curing.This part of the sensitive film has a more competitive advantage in adsorbing gas than the sensitive film in the interdigital gap,however,it is at the edge of the flat electric field,and the adsorption of the sensitive film cannot be effectively transformed into a change in capacitance.Moreover,even if the gap between the interdigital slits is as small as 15?m,it still does not show the ability to inhale the sensitive solution automatically,indicating that the slit too wide does not have capillary force.In addition,the electroplated thickened three-dimensional interdigital and suspended three-dimensional interdigital both show a trend that the smaller the interdigital distance,the higher the sensitivity of the sensor,which is consistent with the results of previous studies.However,the above two kinds of sensors show the change rule that the sensitivity of the sensor increases rapidly with the increase of the electrode height when the plate spacing is the same and the amount of dripping is the same.None of the three established double-layer dielectric capacitor models can explain this experimental phenomenon.