James J. Spates

Resonator/Oscillator response to liquid loading.

The resonant frequency of a thickness-shear mode resonator operated in contact with a fluid was measured with a network analyzer and with an oscillator circuit. The network analyzer measures changes in the devices intrinsic resonant frequency, which varies linearly with (ρη)(1/2), where ρ and η are liquid density and viscosity, respectively. The resonator/oscillator combination, however, responds differently to liquid loading than the resonator alone. By applying the operating constraints of the oscillator to an equivalent-circuit model for the liquid-loaded resonator, the response of the resonator/oscillator pair can be determined. By properly tuning the resonator/oscillator pair, the dynamic range of the response can be extended and made more linear, closely tracking the response of the resonator alone. This allows the system to measure higher viscosity and higher density liquids with greater accuracy.

Gas sensing with acoustic devices

A survey is made of acoustic devices that are suitable as gas and vapor sensors. This survey focuses on attributes such as operating frequency, mass sensitivity, quality factor (Q), and their ability to be fabricated on a semiconductor substrate to allow integration with electronic circuitry. The treatment of the device surface with chemically-sensitive films to detect species of interest is discussed. Strategies for improving discrimination are described, including sensor arrays and species concentration and separation schemes. The advantages and disadvantages of integrating sensors with microelectronics are considered, along with the effect on sensitivity of scaling acoustic gas sensors to smaller size.

Flexural plate wave resonator excited with Lorentz forces

A flexural plate wave resonator was constructed by patterning current lines on a silicon nitride membrane suspended on a rectangular silicon frame. Eigenmodes of the rectangular membrane were excited using Lorentz forces generated between alternating surface currents and a static in-plane magnetic field. Preferential coupling to a particular membrane mode was achieved by positioning current lines along longitudinal mode antinodes. An equivalent-circuit model was derived which characterizes the input impedance of a one-port device and the transmission response of a two-port device over a range of frequencies near a single membrane resonance. Experiments were performed to characterize the device’s response to changes in dc magnetic field strength, ambient gas composition, gas pressure, and input power.

Magnetically-excited flexural plate wave resonator

A flexural plate wave (FPW) resonator was constructed by patterning current lines on a silicon nitride membrane suspended on a rectangular silicon frame. Eigenmodes of the rectangular membrane were excited using Lorentz forces generated between alternating surface currents and a static in-plane magnetic field. The magnetic field strength required for these devices can be achieved with small permanent magnets (/spl ap/1 cm/sup 3/). Preferential coupling to a particular membrane mode was achieved by positioning current lines along longitudinal mode antinodes. An equivalent-circuit model was derived that characterizes the input impedance of a one-port device and the transmission response of a two-port device over a range of frequencies near a single membrane resonance. Experiments were performed to characterize the effects of varying magnetic field, ambient gas, gas pressure, and input power. To our knowledge, this is the first experimental demonstration of a resonant FPW device.A flexural plate wave (FPW) resonator was constructed by patterning current lines on a silicon nitride membrane suspended on a rectangular silicon frame. Eigenmodes of the rectangular membrane were excited using Lorentz forces generated between alternating surface currents and a static in-plane magnetic field. The magnetic field strength required for these devices can be achieved with small permanent magnets (/spl ap/1 cm/sup 3/). Preferential coupling to a particular membrane mode was achieved by positioning current lines along longitudinal mode antinodes. An equivalent-circuit model was derived that characterizes the input impedance of a one-port device and the transmission response of a two-port device over a range of frequencies near a single membrane resonance. Experiments were performed to characterize the effects of varying magnetic field, ambient gas, gas pressure, and input power. To our knowledge, this is the first experimental demonstration of a resonant FPW device.

Magnetically-excited flexural plate wave device

Novel devices have been designed, fabricated, and tested that use the Lorentz force to excite flexural plate waves in a silicon nitride membrane. Single and dual port devices have been fabricated and the excitation spectra measured. Eigenmodes of the membrane are excited in this resonant device and non-linear effects, due to amplitude stiffening of the membrane, are observed. The effects of temperature and ambient gas on resonant frequency and Q have been explored. These devices have the advantage of material compatibility with silicon process technology and are capable of operating at very high temperatures.

Investigation of the quartz resonator as an industrial cleaning monitor

The evaluation of cleaning processes can be performed in real time using the quartz resonator (QR) cleaning monitor as an in situ probe to monitor surface contamination. Viscous properties of thin films are modeled to determine the QRs limitation to film thickness in contact with liquids of varying properties. The demonstrated ability to provide submonolayer mass detection during a cleaning process allows this monitor to be used to probe the kinetics and effectiveness of a cleaning process. This capability can rapidly evaluate the total effectiveness of environmentally friendly alternative cleaning processes needed to replace cleaners such as chlorofluorocarbons (CFCs) and chlorinated hydrocarbons. These monitors could also be used to verify cleaning bath integrity so that bath cleaning solution changes are optimized. This condition would maintain high product quality and yield while minimizing waste from disposal.

Pressure sensing with a flexural plate wave resonator

A silicon nitride membrane supported on a silicon substrate and having a sealed air cavity underneath has been used as a pressure sensor. Membrane resonances are excited and detected using conductor lines patterned on the membrane in combination with an applied magnetic field. The resonant frequency depends strongly upon membrane tension and thus the differential pressure across the membrane.