Research Of Micro Pressure Sensor Chip Based On Boron-doped Nc-Si:H Thin Film

Hydrogenated nanocrystalline silicon(nc-Si:H) thin film is a kind of semiconductor material with novel structures,which are composed of approximate 50% nanocrystalline silicon(grain size 2-10 nm) and 50%amorphous silicon in the inteffacial region(thickness about 2-3 atomic spacing).Because of this kind of microstructures,nc-Si:H thin films have many good photoelectricity and mechanical properties.Using these properties,people have already developed many kinds of optical and electricity devices,such as light-emitting diodes,tunneling diodes and solar cells, etc.Considering the higher piezoresistivity of nc-Si:H,the aim of this paper is to develop micro-pressure sensor with higher sensitivity,including the design of chip structure of the sensor.Boron-doped nc-Si:H thin films with good properties were deposited by optimized process technique parameters at first stage of this paper.The influence of process parameters on the deposit rate and microstructures of films was studied.It was found that Radio- Frequency(RF) power has the best value for improving the deposition rate, and the deposition mechanism of nc-Si:H thin films was verified by H etching model. Substrate temperature affected the volume fraction ratio of crystalline part(X_c),grain size and the H content of films.With the silane concentration(SC) decreasing in the reactive vapour,X_c of the films increased.When the SC was changed from 1.0%to 0.5%,X_c of the films increased from 50.4%to 54.2%.We have studied the influence of process parameters on the mechanical and electricity properties.The results show that the structures of films become loose under higher RF-power,leading to the decrease of the elastic modulus and hardness of films.The electricity properties have relation with thickness of films.The resistivity of films is lower for the thicker films,but higher for the thinner ones because of the existence of amorphous crystalline thin layer on the glass substrate.Annealing process also affected the microstructures and properties of boron-doped nc-Si:H thin films.After annealing,the degree of roughness of films’ surface became larger and the mechanical properties of films were enhanced.The resistivity of films decreased after annealing process,which is ascribed to the activation of boron atom doped in the films.Based on the piezoresistivity theory of semiconductor materials,we developed the piezoresistivity model for nc-Si:H thin films,in which the special microstructures of the films are taken into consideration.When the surface of films are pressed,the interface regions of films will appear to relaxation,which leads to alteration of the tunneling path of conductive electron.Therefore,the resistivity of films will change when pressure was applied on the surface of films.Nc-Si:H thin films have larger piezoresistivity coefficient than microcrystalline silicon and polycrystalline silicon’s.With a proper X_c, its piezoresistivity may larger than that of the monocrystalline silicon.By analysing the stressed diaphragm,the sensor chip was designed according to the chip design rules.The photolithography pattern of the resistivity was designed, which was confined by some factors,such as,the square resistance of films,power loss on per area and the operability of process,etc.The manufacture process of the chip made by boron-doped nc-Si:H thin films was also introduced.