In situ phosphorus-doped polysilicon for excitation and detection in micromechanical resonators

Low-pressure chemical vapour deposited (LPCVD) in situ phosphorus-doped polysilicon films have been grown from a 60:30:300 sccm silane:phosphine (2000 ppm):nitrogen mass-flow mixture at 625°C under varied process conditions. Thickness uniformity, grain size, dopant concentration, resistivity, temperature coefficient of resistivity, longitudinal strain gauge factor and the temperature coefficient of the gauge factor are determined. A growth rate with a non-uniformity (3σ) of 5% is obtained, yielding films with a grain size of 20-30 nm and a surface roughness of 12 nm (peak-to-valley heights), both before and after annealing, and a dopant concentration of (2-3) × 1020 cm-3. Resistivities of the order of 1 mΩ cm can be obtained with a temperature coefficient close to zero after annealing at 900°C for 30 min, with a longitudinal gauge factor of -20 and a temperature coefficient of the gauge factor of -0.25%/°C. A mechanism incorporating the diffusion of dislocations during annealing is proposed to explain the observed effect. The films are appropriate for application as resistors for thermal excitation and piezoresistive detection in resonating micromechanical devices.