Direct writing of piezoresistive silicon resistors using laser-induced CVD

Abstract

Laser-induced chemical vapour deposition (LCVD), using a movable focused Ar + laser beam (1 W, 514.5 nm), was used for direct, maskless writing of strain sensitive silicon resistors for the fabrication of prototype pressure sensors on a silicon on sapphire (SOS) and sapphire substrates from the mixed ratio of 0,1% B 2 H 6 in the reactive gas SiH 4 . The strain sensitive resistors, approximately 300 μm long, were deposited, with in situ measurement of their resistance. The width of the deposited stripes were about 10 μm, and lines over 1 μm thick were written at speeds of 50 μm/s. The resistivity of the stripes was 7-10 mΩcm. The LCVD processing pressure was typically 100 mbar. In the case of silicon-on-sapphire (SOS) diaphragms (thickness of 330 μm) longitudinal gauge factors of 40-80 were observed when the direction of the resistors was parallel to the [110]-type epitaxial direction. The gauge factor of 13 was measured when the resistor was deposited parallel to the [100]-type direction. Polycrystalline deposition on the sapphire substrate (thickness of 530 μm) gave gauge factors of 31-48. Temperature coefficients of resistance (0 o C...70 o C) of silicon stripes were -500...-2200 (SOS) and -2300...-2900 ppm/ o C (sapphire), respectively