A. M. Robinson

A thermal conductivity microstructural pressure sensor fabricated in standard complementary metal‐oxide semiconductor

We have investigated polysilicon‐based microstructural thermal conductivity pressure sensors fabricated in complementary metal‐oxide‐semiconductor technology and operated as a Pirani gauge. The thermally isolated polysilicon resistor showed a 2% change over the pressure range 0.01–100 Torr when the device was operated with a constant current of 200 μA. Larger changes were observed at higher currents. The device was tested in SF6, N2, and He, and was most sensitive in He and least sensitive in SF6. Heat losses were calculated to be mainly due to heat conduction through the support arms, especially at low pressures, with heat conduction through the gas increasing to approximately 37% at 1 atm.

High temperature absorption in the 10.4- μm band of CO 2

The absorption coefficient of pure CO(2) has been measured as a function of temperature in the 295-650 K range for the P(14)-P(24) and R(10)-R(24) laser transitions. Cubic curves fitted to the experimental results are given to allow interpolation at all temperatures within the range. The P(20) transition shows an anomolously high value of absorption. Variation of absorption coefficient with pressure was linear but small for all transitions in the 200-700-Torr pressure range at three temperatures. Calculations of the absorption coefficient are performed and compared with experimental data.

Simple resonating microstructures for gas pressure measurement

The resonance characteristics of an oscillating microcantilever are modified due to the changing damping effects as the ambient air pressure varies, and the device is used as an absolute pressure sensor. Three variations of the microcantilever device were designed, fabricated, and tested for changes in resonant frequency, quality factor, amplitude of oscillation, and actuating current required to maintain the amplitude of oscillation constant as the ambient pressure was varied from 15 to 1450 Torr. The device has been analyzed and a relationship has been derived to aid in the prediction of device behavior. Values for precision comparable to commercial pressure sensors are also presented for each method of detection.

Numerical modeling of a micromachined thermal conductivity gas pressure sensor

We have developed a software package that simulates the operation of a silicon micromachined CMOS thermal conductivity gas pressure gauge. The performance of actual devices was compared against the simulated operation and was found to be in good agreement. The 3-dimensional simulation was reduced to two 2-dimensional simulations to reduce complexity. The two equations resulting from steady state energy balance considerations were discretized and an iterative nonlinear Gauss-Seidel procedure applied to solve the system of equations. Temperature profiles and contours were calculated and the effect of geometric and materials modifications was demonstrated. >

Cantilever-in-cantilever micromachined pressure sensors fabricated in CMOS technology

The resonance characteristics of an oscillating microcantilever are modified due to the changing damping effects as the ambient air pressure varies. Such a device may be employed as a pressure sensor. We have designed modeled, and tested a CMOS micromachined magnetically actuated cantilever pressure sensor. Five variations of a cantilever-in-cantilever (CIC) device were tested for changes in amplitude of oscillation, resonant frequency, quality factor, damping relation, and actuating current required as the pressure was varied from 15 to 1450 Torr.

Thermal response of CMOS‐micromachined thermistor sensors under constant power and constant current excitation

The resistance of a micromachined polysilicon thermistor sensor has been measured under excitation by both a constant current and constant power step. The time‐varying and steady‐state response of the polysilicon resistor at ambient pressures of 1 atm and 10−4 Torr were observed under various step amplitudes. The increase of resistance above its room‐temperature value changed exponentially with time; the time constant varied inversely with the square of the current step amplitude but was constant with power step amplitude. The steady‐state change of resistance varied linearly with the applied power under both modes of excitation. The time‐varying and steady‐state responses were as predicted by a simple analytic model developed. The steady‐state operation of these devices at the two pressures permitted us to directly determine the energy losses by heat conduction through the surrounding gas and conduction through the support arms. At 1 atm, up to 90% of the heat loss is by conduction through the gas. The o...

High temperature 10.4-μm absorption in CO 2 –He–N 2 mixtures

Absorption coefficient measurements in CO(2)-He-N(2) mixtures have been performed over the 295-650-K temperature range at four wavelengths of the 10.4-microm CO(2) laser transition. Calculated values of the absorption coefficient were compared with the measured values for fifteen gas mixtures and agreed to >10%. A gas mixture interpolation scheme allows an accurate estimation of absorption by gas mixtures other than those on which measurements were made. A less accurate but simpler interpolation method, dependent only on the fraction of C0(2), is also discussed for determining the absorption coefficient.

CMOS cantilever microstructures as thin film deposition monitors

Increasing the mass of an oscillating microcantilever causes a decrease in its vibrational resonant frequency. We have deposited MgF/sub 2/ on our CMOS designed cantilever-in-cantilever microdevices and observed the resonant frequency decrease linearly with layer thickness. With initial results showing a sensitivity on the order of Angstroms, such devices demonstrate potential for novel application as thin film monitors.

Extended high temperature measurements of absorption at 10.4 μm in CO 2

Measurements of absorption by the P(14)-P(24) 10.4-microm CO(2) laser transitions have been performed at 200 Torr at temperatures from 540 to 775 K. Using these results plus earlier data, empirical curves describing the absorption coefficients as a function of temperature are determined over the temperature range from 295 to 775 K. Comparison of model predictions with experiment is made and a discussion of the divergence between them is given.

Absorption of 9.6-μm CO 2 laser radiation by CO 2 at elevated temperatures

Absorption of 9.6-μm CO2 laser radiation by CO2 at temperatures between 296 and 625 K has been measured at a pressure of 200 Torr. Experimental results for the R10—R26 and P10—P28 transitions have been obtained and compared with computed values of absorption. The relative optical broadening coefficients due to He and N2 have been measured on the R16—R22 and P16—P22 transitions over the same temperature range.