Publications

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This paper compares measurements made by Raman and infrared thermometry on a SOI (silicon on insulator) bent-beam thermal microactuator. Both techniques are noncontact and used to experimentally measure temperatures along the legs and on the shuttle of the thermal microactuators. Raman thermometry offers micron spatial resolution and measurement uncertainties of ±10 K; however, typical data collection times are a minute per location leading to measurement times on the order of hours for a complete temperature profile. Infrared thermometry obtains a full-field measurement so the data collection time is much shorter; however, the spatial resolution is lower and calibrating the system for quantitative measurements is challenging. By obtaining thermal profiles on the same SOI thermal microactuator, the relative strengths and weaknesses of the two techniques are assessed. Copyright © 2011 by ASME.

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This paper compares measurements made by Raman and infrared thermometry on a SOI (silicon on insulator) bent-beam thermal microactuator. Both techniques are noncontact and used to experimentally measure temperatures along the legs and on the shuttle of the thermal microactuators. Raman thermometry offers micron spatial resolution and measurement uncertainties of {+-}10 K; however, typical data collection times are a minute per location leading to measurement times on the order of hours for a complete temperature profile. Infrared thermometry obtains a full-field measurement so the data collection time is much shorter; however, the spatial resolution is lower and calibrating the system for quantitative measurements is challenging. By obtaining thermal profiles on the same SOI thermal microactuator, the relative strengths and weaknesses of the two techniques are assessed.

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Deep reactive ion etching (DRIE) of silicon enables high aspect ratio, deep silicon features that can be incorporated into the fabrication of microelectromechanical systems (MEMS) sensors and actuators. The DRIE process creates silicon structures and consists of three steps: conformal polymer deposition, ion sputtering, and chemical etching. The sequential three step process results in sidewalls with roughness that varies with processing conditions. This paper reports the sidewall roughness for DRIE etched MEMS as a function of trench width from 5 μm to 500 μm for a 125 μm thick device layer corresponding to aspect ratios from 25 to 0.25. Using a scanning electron microscope (SEM), the surfaces were imaged detecting an upper region exhibiting a scalloping morphology and a rougher lower region exhibiting a curtaining morphology. The height of rougher curtaining region increases linearly with aspect ratio when the etch cleared the entire device layer. The surface roughness for two trench widths: 15 μm and 100 μm were further characterized using an atomic force microscope (AFM), and RMS roughness values are reported as a function of height along the surface. The sidewall roughness varies with height and depends on the trench width. Copyright © 2011 by ASME.

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Thermoreflectance techniques are powerful tools for measuring thermophysical properties of thin film systems, such as thermal conductivity, Λ, of individual layers, or thermal boundary conductance across thin film interfaces (G). Thermoreflectance pump-probe experiments monitor the thermoreflectance change on the surface of a sample, which is related to the thermal properties in the sample of interest. Thermoreflectance setups have been designed with both continuous wave (cw) and pulsed laser systems. In cw systems, the phase of the heating event is monitored, and its response to the heating modulation frequency is related to the thermophysical properties; this technique is commonly termed a phase sensitive thermoreflectance (PSTR) technique. In pulsed laser systems, pump and probe pulses are temporally delayed relative to each other, and the decay in the thermoreflectance signal in response to the heating event is related to the thermophysical properties; this technique is commonly termed a transient thermoreflectance (TTR) technique. In this work, mathematical models are presented to be used with PSTR and TTR techniques to determine the Λ and G of thin films on substrate structures. The sensitivities of the models to various thermal and sample parameters are discussed, and the advantages and disadvantages of each technique are elucidated from the results of the model analyses. © 2010 Springer Science+Business Media, LLC.

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We will present experimental and computational investigations of the thermal performance of microelectromechanical systems (MEMS) as a function of the surrounding gas pressure. Lowering the pressure in MEMS packages reduces gas damping, providing increased sensitivity for certain MEMS sensors; however, such packaging also dramatically affects their thermal performance since energy transfer to the environment is substantially reduced. High-spatial-resolution Raman thermometry was used to measure the temperature profiles on electrically heated, polycrystalline silicon bridges that are nominally 10 microns wide, 2.25 microns thick, 12 microns above the substrate, and either 200 or 400 microns long in nitrogen atmospheres with pressures ranging from 0.05 to 625 Torr. Finite element modeling of the thermal behavior of the MEMS bridges is performed and compared to the experimental results. Noncontinuum gas effects are incorporated into the continuum finite element model by imposing temperature discontinuities at gas-solid interfaces that are determined from noncontinuum simulations. The experimental and simulation results indicate that at pressures below 0.5 Torr the gas-phase heat transfer is negligible compared to heat conduction through the thermal actuator legs. As the pressure increases above 0.5 Torr, the gas-phase heat transfer becomes more significant. At ambient pressures, gas-phase heat transfer drastically impacts the thermal performance. The measured and simulated temperature profiles are in qualitative agreement in the present study. Quantitative agreement between experimental and simulated temperature profiles requires accurate knowledge of temperature-dependent thermophysical properties, the device geometry, and the thermal accommodation coefficient.

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Mechanical stresses on microsystems die induced by packaging processes and varying environmental conditions can affect the performance and reliability of microsystems devices. Thermal microactuators and stress gauges were fabricated using the Sandia five-layer SUMMiT surface micromachining process and diced to fit in a four-point bending stage. The sample dies were tested under tension and compression at stresses varying from ?250 MPa, compressive, to 200 MPa, tensile. Stress values were validated by both on-die stress gauges and micro-Raman spectroscopy measurements. Thermal microactuator displacement is measured for applied currents up to 35 mA as the mechanical stress is systematically varied. Increasing tensile stress decreases the initial actuator displacement. In most cases, the incremental thermal microactuator displacement from the zero current value for a given applied current decreases when the die is stressed. Numerical model predictions of thermal microactuator displacement versus current agree with the experimental results. Quantitative information on the reduction in thermal microactuator displacement as a function of stress provides validation data for MEMS models and can guide future designs to be more robust to mechanical stresses. © 2010 IOP Publishing Ltd.

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Thermal boundary resistance dominates the thermal resistance in nanosystems since material length scales are comparable to material mean free paths. The primary scattering mechanism in nanosystems is interface scattering, and the structure and composition around these interfaces can affect scattering rates and, therefore, device thermal resistances. In this work, the thermal boundary conductance (the inverse of the thermal boundary resistance) is measured using a pump-probe thermoreflectance technique on aluminum films grown on silicon substrates that are subjected to various pre-Al-deposition surface treatments. The Si surfaces are characterized with Atomic Force Microscopy (AFM) to determine mean surface roughness. The measured thermal boundary conductance decreases as Si surface roughness increases. In addition, stripping the native oxide layer on the surface of the Si substrate immediately prior to Al film deposition causes the thermal boundary conductance to increase. The measured data are then compared to an extension of the diffuse mismatch model that accounts for interfacial mixing and structure around the interface. © 2010 by ASME.

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Pump-probe transient thermoreflectance (TTR) techniques are powerful tools for measuring thermophysical properties of thin films, such as thermal conductivity, A, or thermal boundary conductance, G. TTR experimental setups rely on lock-in techniques to detect the response of the probe signal relative to the pump heating event. The temporal decays of the lock-in signal are then compared to thermal models to deduce the A and G in and across various materials. There are currently two thermal models that are used to relate the measured signals from the lock-in to the A and G in the sample of interest. In this work, the thermal models, their assumptions, and their ranges of applicability are compared. The advantages and disadvantages of each technique are elucidated from the results of the thermophysical property measurements. Copyright © 2009 by ASME.

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