A High-Yield Microfabrication Process for Sapphire Substrate Capacitive Pressure Sensors Providing 70 MPa Range and 0.5 kPa Resolution

We report a high yield fabrication process based on surface micromachining of sapphire substrates and plasma-deposited thin films for pressure sensors with extraordinarily high output swing relative to baseline and parasitic capacitance. This feature allows the use of massively parallel diaphragms arrays in a single sensor chip that is read out with a single interface circuit, providing a superior full-scale range and resolution in a small footprint. Five types of sensor chips are reported, incorporating: a) a single diaphragm (ø
$100 ~\mu \text{m}$
diameter); b) 18 diaphragms in parallel (ø
$100 ~\mu \text{m}$
diameter); c) 85 diaphragms (ø56-ø
$92 ~\mu \text{m}$
diameter); d) 8 diaphragms in parallel (ø
$200 ~\mu \text{m}$
diameter); and e) 32 diaphragms in parallel (ø110-ø
$150 ~\mu \text{m}$
diameter). The respective maximum gauge pressures are 30 MPa, 30 MPa, 70 MPa, 70 kPa, and 110 kPa; the respective resolutions are 1.0 kPa, 0.2 kPa, 0.5 kPa, 5.7 Pa, and 2.6 Pa. Full-scale range is defined with the upper boundary as the applied pressure at which the incremental response falls below 30% of the incremental response at the lower boundary (which is approximated as atmospheric pressure). In two of these cases, varying diameters are used to reduce the inherent non-linearity of the sensor response over the full-scale pressure range. The 85-diaphragm sensor, intended for downhole pressure sensing, provides a capacitance response of 0.46 fF/kPa and 7.1 pF
${C}_{0}$
. The 32-diaphragm sensor, intended for lower operating pressures, provides capacitance response of 29 fF/kPa and 11.5 pF
${C}_{0}$
. The fabrication process, design features, and experimental results are presented.