A.G.R. Evans

Polycrystalline silicon strain sensors

Abstract A new theoretical model for piezoresistance in both n- and p-type polycrystalline silicon is described. This model considers piezoresistance in both the grains and in the Schottky-type barrier regions around the grain boundaries. Transport across the grain boundary is assumed to be dominated by thermionic emission. The effect of strain is to change the relative contribution to thermionic emission from each band minimum. Grain size, orientation trap density and doping dependence effects are included. By combining the bulk piezoresistive effects within the grain and the boundary effects, this model has given reasonable agreement with experimental results. This theory now enables optimum processing parameters for highest gauge factor production to be predicted.

Polycrystalline silicon as a strain gauge material

A theoretical model for piezoresistance in both n- and p-type polycrystalline silicon is described. This model considers the contribution to piezoresistance from the grain and the Schottky-type barrier regions around the grain boundaries. Comparison between theory and experiment shows reasonable agreement for both longitudinal and transverse strain measurements. The difference in magnitude between longitudinal and transverse gauge factors depends on texture and is found to be explained by the anisotropy of piezoresistance in silicon. Experimental results for the temperature coefficients of resistance and gauge factor in conjunction with the model for piezoresistance may be used to optimise sensor characteristics within the confines of available processes.

A polysilicon diaphragm based pressure sensor technology

The fabrication of capacitive pressure sensors, using silicon integrated circuit processes is described. Polysilicon diaphragms are laid down using the conventional LPCVD technique. A cavity between the polysilicon layer and silicon substrate is created by etching away an intermediate oxide layer. The subsequently sealed cavity with deformable polysilicon membrane decreases in volume with a positive differential pressure between outside and inside. Capacitance changes between an electrode on the membrane and the silicon substrate allow pressure changes to be measured. The batch fabrication technology for microminiature device manufacture is described and electrical characteristics of the devices presented.

Polysilicon strain sensors using shear piezoresistance

Abstract The conventional piezoresistive strain sensor uses either the longitudinal or transverse piezoresistive effect. However, in addition to the change in resistance, an electric field is developed perpendicular to the current flow. This yields a voltage output, similar to that found in Hall measurements, which is proportional to strain instead of magnetic field. The magnitude of this voltage is also determined by the angle of the applied strain. This paper examines the use of this effect in producing a compact pressure sensor in polysilicon films. A theoretical model is described for both the change in resistance and the perpendicular voltage, due to a shear strain, for a polysilicon device and a comparison with experimental results shows reasonable agreement.

A Scanning Electron Beam Annealer with Electrostatic Deflection Systems

A scanning electron beam annealer (SEBA) utilising readily available components and a simple electrostatic deflection system is described. As a low cost solution, the apparatus is ideal as a research tool for integrated circuit technology and an example of its application is presented.

A dual-pulse scanning electron beam annealer

Improvements to a scanning electron beam annealer are reported which allow high temperature anneals on semiconductor samples to be carried out in less than one second. These improvements will allow the study of fast annealing effects such as dopant activation, precipitation and enhanced diffusion. The wide range of temperature-time (T(t)) schedules now possible together with accurate T(t) measurements make this simple machine ideal for use in rapid thermal annealing research. An example of an enhanced diffusion study for boron and arsenic in silicon is described.