Laser-induced chemical vapour deposition in piezoresistive pressure sensor fabrication

Abstract

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 to fabricate prototype pressure sensors of the diaphragm type from the mixed ratio of 0.1% B2H6 in the reactive gas SiH4. The pattern of aluminium conductors allowed different configurations for the location of the four resistors of the Wheatstone bridge. The strain-sensitive resistors, approximately 300 μm long, were deposited and their resistance was measured in situ on selected points of the diaphragm. The width of the deposited stripes varied from 2 to about 10 μm, and lines over 1 μm thick were written at speeds of 50 gmm/s. The resistivity of the stripes was 7–10 mω cm. The LCVD processing pressure was typically 100 mbar. In the case of silicon-on-sapphire (SOS) diaphragms (thickness of 330 μm), for a bias of 1 V, sensitivities of 0.61 to 0.71 mV/bar and gauge factors of 40 to 80 were observed when the direction of the resistors was parallel to the [110]-type epitaxial direction. A sensitivity value of 0.23 mV/bar and the gauge factor of 13 were measured when the resistor was deposited parallel to the [001]-type direction. Polycrystalline deposition on the sapphire substrate (thickness of 530 μm) gave sensitivity values of 0.13 to 0.16 mV/bar and gauge factors of 31 to 48. Temperature coefficients of resistance (in the range of 0 to 70 °C) of silicon stripes deposited on the titanium frame pressure sensor diaphragm were −50 to −2200 (SOS) and −2300 to −2900 ppm/°C (sapphire), respectively.