Study On Structure Improvement Of Microstructured Si-based SnO₂ Gas Sensor

Formaldehyde was widely used in industrial, pharmaceutical and other areas of daily life. However, formaldehyde gas is very harmful to human health. Thin film Si-based micro-sensor possesses many advantages such as small volume, low-cost, easy to form cosmically and excessive function array, and so on.The structure and power consumption were developed and studied on silicon-based formaldehyde gas sensors in this paper. Design and experimental including①Separating the heating electrode and singal electrode, and the preparation of the isolation layer;②Etching of Si and SiO2, preparation of silicon cup structure;③Preparation of doped SnO2 sensitive film, study on the etching process on SnO2 sensitive film on the Pt electrodes;④Gas sensing properties investigations of the gas sensors.The process of the preparation of the Si-based gas sensor was described in this paper, including cleaning, oxidation, lithography, etching, sputtering, preparation of sensitive films etc. And each IC technique was introduced in detail.To reduce the thickness of the work area and the power consumption, lithography masks of Si cup, electrodes, isolation layer and etching SnO2 thin film were designed. SnO2 thin film and Si cup were etched by wet method and dry method. Etching mechanism and the rate of etch were studied in wet method. The surface and section of Si were photographed by Olympus STM6. Etching solution, etching mechanism and the rate of etch were studied by dry method with the characterization results. The rates of etch, the surfaces and sections of Si by Alcate ICP and ICP-2 were compared.Two kinds of heating electrode and signal electrode were designed. The temperature distributions were simulated by the ANSYS software. According to the temperature distribution, the structure of heating electrode was improved. Isolation layer SiO2 thin film and Si3N4 thin film prepared by LPD, PECVD, sputtering were compared. Finally, SiO2 thin film by PECVD was selected as isolation layer with good isolation.1.5 at.% Pd-doped SnO2 sensitive film was fabricated by Sol-gel. The thin film has been characterized by step profiler, SEM, AFM, XRD. According to the properties of SnO2, etching solution and etching conditions were studied. HF+HCl+Ti mixture was selected as the etching solution. And SnO2 sensitive film on the Pt electrode was well etched by it.The performance of new structure Si-based gas sensor was tested;the best operating temperature of the gas sensor was 175℃. The response sensitivity of the gas sensor in 0.05 ppm-10 ppm, response time and the recovery time were tested. Cross-sensitivity of the gas sensor to formaldehyde vopur with the interference gases included methanol, alcohol, methylbenzene and acetone were tested.Several factors affecting the gas sensor power consumption was studied. The power consumption of the gas sensor was down to 320 mW, when the thickness of gas sensor work area was 70μm. Compared with previous similar gas sensor (2-3 W), one order of magnitude was decreased.