Steven T. Cho

An ultraminiature CMOS pressure sensor for a multiplexed cardiovascular catheter

A multiplexed ultraminiature pressure sensor designed for use in a cardiovascular catheter is described. The sensor operates from only two loads, which are shared by two sensors per catheter. The sensing chip is 350 mu m wide by 1.4 mm long by 100 mu m thick. CMOS readout circuitry at the sensing site converts applied pressure to a frequency variation in the supply current, which is detected at the end of the catheter by a microprocessor-controlled interface. The nominal pressure sensitivity is 2 kHz/fF about a zero-pressure output frequency of 2.7 MHz. This on-site circuitry contains two reference capacitors which allow external compensation for nonlinearity and temperature sensitivity and has an idle-state power dissipation of less than 50 mu W. With the transducer sealed at ambient pressure, the device can resolve pressure variations of about 3 mmHg, while vacuum-sealed devices do considerably better and should permit >

An ultrasensitive silicon pressure-based flowmeter

An ultrasensitive silicon pressure-based flowmeter has been developed for use in measuring sub-SCCM gas flows in semiconductor process equipment. The device utilizes a capacitive pressure sensor to measure the pressure drop induced by flow across a micromachined silicon microchannel. The flowmeter is fabricated using a single-sided dissolved-wafer process and requires only six masks. The processed silicon wafer is electrostatically bonded to a glass substrate and the wafer is selectively etched away, using boron etch-stops to define the final silicon-on-glass device structure. The capacitive pressure sensor uses a thin (2.2- mu m) stress-compensated membrane, which enables the sensor to monitor differential pressure as low as 1 mtorr while withstanding overpressures greater than 700 torr. The flowmeter is capable of measuring flows in the 10/sup -5/ SCCM range.<<ETX>>

Scaling and dielectric stress compensation of ultrasensitive boron-doped silicon microstructure

The scaling of boron-doped silicon membranes based on diaphragm dimensions and stress compensation is characterized. Devices with varying edge length and plate thickness are fabricated and tested for sensitivity. The stress for p++ silicon, LPCVD silicon dioxide, and LPCVD silicon nitride is measured using an electrostatic technique, that uses silicon microbridges. Silicon membranes with varying thickness of oxide and nitride are characterized for sensitivity. The results confirm a previously reported analytical scaling theory for the structures. Based on this theory, scaled experimental devices are found to show sensitivities within three percent of the calculated design targets.<<ETX>>

Secondary sensitivities and stability of ultrasensitive silicon pressure sensors

Experimental results on the characterization of secondary effects in boron-doped silicon membranes are presented. Capacitive pressure sensors were fabricated with and without a variety of dielectrics on the diaphragms and were tested over pressure and temperature. The temperature coefficient of sensitivity (TCS) and zero pressure temperature coefficient of offset (TCO) were found to be as much as an order of magnitude higher than previously reported values; this is due to the strong dependence of pressure sensitivity on internal stress and the large mismatches in thermal expansion coefficients between silicon and dielectrics. Creep and fatigue affected the offset by <0.2% full scale, and the change in pressure sensitivity was insignificant; hysteresis observed on all devices was also <0.2% full scale. The results indicate that it may be possible to extend the pressure range by an order of magnitude, which would increase the resolution from 10 to 16 b.<<ETX>>