Publications

An optical sensor system has been developed for the autonomous monitoring of NO{sub 2} evolution in energetic material aging studies. The system is minimally invasive, requiring only the presence of a small sensor film within the aging chamber. The sensor material is a perylene/PMMA film that is excited by a blue LED light source and the fluorescence detected with a CCD spectrometer. Detection of NO{sub 2} gas is done remotely through the glass window of the aging chamber. Irreversible reaction of NO{sub 2} with perylene, producing the non-fluorescent nitroperylene, provides the optical sensing scheme. The rate of fluorescence intensity loss over time can be modeled using a numerical solution to the coupled diffusion and a nonlinear chemical reaction problem to evaluate NO{sub 2} concentration levels. The light source, spectrometer, spectral acquisition, and data processing were controlled through a Labivew program run by a laptop PC. Due to the long times involved with materials aging studies the system was designed to turn on, warm up, acquire data, power itself off, then recycle at a specific time interval. This allowed the monitoring of aging HE material over the period of several weeks with minimal power consumption and stable LED light output. Despite inherent problems with gas leakage of the aging chamber they were able to test the sensor system in the field under an accelerated aging study of rocket propellant. They found that the propellant evolved NO{sub 2} at a rate that yielded a concentration of between 10 and 100 ppm. The sensor system further revealed that the propellant, over an aging period of 25 days, evolves NO{sub 2} with cyclic behavior between active and dormant periods.

A new instrument to accurately and verifiably measure mechanical properties across an entire MEMS wafer is under development. We have modified the optics on a conventional microelectronics probe station to enable three-dimensional imaging while maintaining the full working distance of a long working distance objective. This allows standard probes or probe cards to be used. We have proceeded to map out mechanical properties of polycrystalline silicon along a wafer column by the Interferometry for Material Property Measurement (IMaP) methodology. From interferograms of simple actuated cantilevers, out-of-plane deflection profiles at the nanometer scale are obtained. These are analyzed by integrated software routines that extract basic mechanical properties such as cantilever curvature and Young's modulus. Non-idealities such as support post compliance and beam take off angle are simultaneously quantified. Curvature and residual stress are found to depend on wafer position. Although deflections of cantilevers varied across the wafer, Young's modulus E - 161 GPa is independent of wafer position as expected. This result is achieved because the non-idealities have been taken into account.

The authors report here the design, fabrication and demonstration of an electrostatically actuated MEMS diffractive optical device, the Polychromator grating. The Polychromator grating enables a new type of correlation spectrometer for remote detection of a wide range of chemical species, offering electronic programmability, high specificity and sensitivity, fast response and ruggedness. Significant results include: (1) The first demonstrations of user-defined synthetic spectra in the 3-5 {micro}m wavelength regime based upon controlled deflection of individual grating elements in the Polychromator grating; (2) The first demonstration of gas detection by correlation spectroscopy using synthetic spectra generated by the Polychromator grating.