Research Of Mesoscopic Mechanical Behavior Of Hydrogenated Silicon Thin Film And Fabrication Of High Temperature Pressure Sensor

This thesis gives an overview of mesoscopic mechanical behavior of hydrogenated silicon thin film and high pressure sensor fabricaton.Hydrogenated silicon thin films have been widely used in electronic and optoelectronic devices such as diodes,thin film transistors,solar cells,liquid crystal display.The research up to date has mainly been concentrated on the film electrical and optical properties and much less on the mechanical ones.However hydrogenated silicon thin films have received great attention due to their sensitive response under mechanical strain and pursued because of its potential advantages for fabrication of high sensitivity piezoresistive pressure sensors, displacement sensors and integrated tunneling sensors.So it is important to understand the mesoscopic mechanical characterization of hydrogenated silicon thin films and the intrinsic relationship with the microstructure.Hydrogenated silicon films were deposited on glass and single crystalline silicon substrate in a capacitive coupled radio-frequent（RF of 13.56MHz） PECVD system aided with direct current（dc） bias stimulation.The thickness of the thin films is measured using spectroscopic ellipsometry measuring system.X-ray diffraction was used to characterize the structure of the silicon films and grain size determination. Raman spectroscopy was performed to verify the crystalline structure and to determine the structural composition of the silicon films.Atomic force microscopy was used to investigate the topography and roughness of the films.It is shown little difference of film thickness exists and deposition rate is little affected with different substrates while microstructure of the thin films varies with different substrate.Under the same deposition condition,the films on single crystalline silicon provide better crystallinty than that on glass substrate while they have much smaller rms roughness than that deposited on single crystalline silicon.It means that the silicon atoms on glass surface have no significant diffusion.They essentially condense wherever they land on the surface of the substrate.It is also suggested that the chemical nature of the substrate surface influences the initial stage of the growth.Nanocrystalline silicon and amorphous silicon thin films are grown on the single crystalline silicon and glass substrate,respectively.Raman scattering is used on those films by three different excitation wavelengths,from red to near ultraviolet.It is found that the Raman spectrometry is different by varying the incident wavelength.We probe the incident light intensity energy,the photo absorption coefficient of silicon respectively and the Penetration depth.The experiment result is well explained and it is shown that to get further information of material microstructure optimum laser should be chosen.Mesoscopic mechanical characterization of hydrogenated silicon film is gotten by nanoindentation based on the conventional depth-sensing indentation method. The crystalline volume fraction（X_c） is obtained from the Raman spectra.An analytical relation between Xc and elastic modulus is established.It is shown the elastic modulus of the film on glass substrate is lower than that on silicon with the same Xc.The grain size of phosphorus doping thin film is smaller than that of intrinsic one and more ordered.The Xc is usually above 40%.The film with diborane doping is on the appetite side.The Xc is usually below 40%.To P-doped,intrinsic and B-doped fims,when Xc is 45%,30%and 15%respectively,the elastic modulus are lower.These results should have important impacts on the engineering of nanocrystalline silicon and related devices.There is a growing requirement for piezoresistive pressure sensor to operate in high temperature and harsh environment.Typical such areas are in the oil industry and a host of others.The conventional piezoresistive pressure sensors are made by forming diffused or ion implanted strain gauges in a Wheatstone bridge configuration on the thin silicon diaphragm.A known limitation of these silicon-based devices using isolation by reverse biased pn-junctions is the rising junction leakage current at elevated temperatures up to 100℃,which make them in unstable state.One of the promising attempts for fabrication of high temperature piezoresistive pressure sensors is to cancel pn-insulation of the piezoresistors.We use the separation by implanted oxygen（SIMOX） method,which is one of the two mainstream methods supplying commercial silicon-on-insulator（SOI） wafers,to design and fabricate the sensing chip dielectrically insulated by SiO₂.For the purposes of pressure measurement for low cost high temperature application,the mechanical structure is designed for operation between the temperature of—40～220℃and the range of 0～40MPa.Using high temperature packaging process and excitation of constant current,the sensor is testified with precise accuracy and stability.The specific works finished and main innovative contributions of this dissertation are as follows.The characteristic of silicon plane and direction is researched with micro machining and anisotropic-etching technology.For the requirement of operation in high temperature and high pressure,we choose the design of circular plane membrane.In the structure,two pairs of the strain gauges of the Wheatstone bridge are placed respectively on the direction[110]and[110]which are perpendicular each other on the （100） plane of the surface silicon layer.Four strain gauges are on the fringe of the diaphragm.Two gauges are along the direction of[110]while the other two are perpendicular to the direction of[110].For these four gauges they are parallel each other to make the Wheatstone detective bridge of four equal strains and resistances. Using SIMOX process a large dose of oxygen is implanted into the silicon wafer.The beam energy is 200keV with 1.8×10¹⁸O⁺/cm² and the target substrate is kept at 650℃to get high-quality SOI wafer.Furthermore,the sensor chip of wide range is fabricated in micro machining work bay.The size of it is 5.0mm×5.0mm×0.5mm.The electrostatic bonding of silicon and Prex7740 glass ring is done on self-made experimental instrument.The gold wire thermocompression bonding worktable is also made for wire bonding.The board covered with Cu is fabricated for leading.The soldering tin thread and lead wire is required for high temperature operation.Using high temperature packaging process,the sensor is testified with precise accuracy and excellent stability and reliability.The temperature coefficient of sensitivity（TCS） and temperature coefficient of offset（TCO） compensation circuit is selected and easily done in using a constant current of 2mA.A quantitative compensation formula is introduced in mathematics by the derivative of the equation above with respect to temperature.Using this circuit and result,the absolute value of the sensor’s TCS and TCO is easy to be less than 100×10 ⁶/℃.FSO by calibration of several temperature compensation cycles.Most non-linearity and hysteresis of the sensors are less than 0.1%FSO and 0.05%FSO,respectively.The sensitivity at the temperature of 220℃is 0.37～0.8mV/V/MPa due to different chips. Since the micro machining process methodology has been consistently proved to be reproducible,its batch production offers the advantage of low cost,reduced process time,and high yield.Thanks to high temperature packaging,the characteristics of the sensor are shown to be satisfied when operating at high temperature.This work is supported by "Six top talent" project of Jiangsu Province（Grant No. 06-D-022）,National Basic Research Program of China（Grant No.2004CB619305）,the technology development plan of Zhejiang Province（Grant No.2005C31048）,doctoral innovation plan of Jiangsu University and the fifth undergraduate scientific research project of Jiangsu University（Grant No.05A036）.