This chapter discusses the design, fabrication, characterization, modeling, and reliability of thermal microactuators. Microelectromechanical systems (MEMS) devices contain both electrical and mechanical components and are in use and under development for applications in the consumer products, automotive, environmental sensing, defense, and health care industries. Thermal microactuators are standard components in microsystems and can be powered electrically through Joule heating or optically with a laser. Examples of MEMS designs containing thermal microactuators include optical switches (Cochran et al., 2004; Sassen et al., 2008) and nanopositioners (Bergna et al., 2005). Advantages of thermal microactuators include higher force generation, lower operating voltages, and less susceptibility to adhesion failures compared to electrostatic actuators. Thermal microactuators do require more power and their switching speeds are limited by cooling times. Extensive work has been performed designing, fabricating, testing, and modeling thermal microactuators. Howell et al. (2007) has reviewed the fundamentals of thermal microactuator design. Designs of electrically powered MEMS thermal actuators include actuators fabricated from a single material (Comtois et al., 1998; Park et al., 2001; Que et al., 2001) and bimorphs (Ataka et al., 1993). Thermal actuator designs using a single material are both symmetric, referred to as bent-beam or V-shaped, structures (Baker et al., 2004; Park et al., 2001; Phinney et al., 2009) and asymmetric (Comtois et al., 1998), which have a hot arm and a cold arm. Asymmetric actuators are also referred to as flexure actuators. Some studies investigated both bent-beam and flexure actuators (Hickey et al., 2003; Oliver et al., 2003). In addition to electrical heating, powering thermal microactuators optically using laser irradiation has been demonstrated (Oliver et al., 2003; Phinney & Serrano, 2007; Serrano & Phinney, 2008). Modeling efforts have focused on bent-beam microactuators (Baker et al., 2004; Enikov et al., 2005; Howell et al., 2007; Lott et al., 2002; Wong and Phinney, 2007) and flexure actuators (Mankame and Ananthasuresh, 2001). This chapter focuses on bent-beam and flexure microactuators. In order for thermal actuators to operate, sufficient heating and thermal expansion of the components must occur. However, device temperatures that are too high result in permanent deformation, damage, and degradation in performance. In addition, packaging processes and conditions affect the performance and reliability of microsystems devices motivating studies on the effects of surrounding gas pressure and mechanical stress on thermal MEMS.

Thermal interface materials (TIMs) serve a critical role in the thermal management of electronic systems by enhancing the flow of heat from source to sink. Nanostructured materials, such as arrays of carbon nanotubes (CNTs) have been shown to outperform many commercially available TIMs due to their low intrinsic resistance and large compliance that enables them to conform to rough surfaces. These characteristics, combined with their low density and ability to withstand vacuum environments and extreme temperatures, make CNT-based TIMs very suitable for space applications. In space, materials are exposed to high doses of gamma radiation due to the lack of an atmosphere to serve as an absorbing medium. With typical design lifetimes of 5 to 10 years or even more, total radiation exposure can be significant and can affect the structure and performance of the TIM. In this work, the potentially adverse effects on the thermal performance of CNT TIMs of gamma-ray irradiation is reported. CNT TIMs were irradiated in a gamma cell at a rate of 250 rad/s to total doses of 50 and 100 Mrad. The thermal interface resistance was measured before and after gamma-ray irradiation using a transient photoacoustic (PA) method at room temperature and a contact pressure of 134 kPa and indicated no adverse effects of gamma-ray exposure on thermal performance.

Removal of waste heat generated via Joule heating during the operation of electronic devices is critical to overall system performance and reliability. A significant fraction of the overall thermal budget is consumed by heat transfer across the interface of contacting materials. To enhance the flow of heat from source to sink, thermal interface materials (TIMs) are used to reduce thermal contact resistance (TCR) by increasing real contact area at the interface. In space systems, TIMs are exposed to high doses of gamma radiation not encountered in typical terrestrial applications. With typical design lifetimes of 5 years or more, total radiation exposure can be significant and can affect the structure and performance of the TIM. Here, we report measurements of the pressure-dependent TCR of metallic foils and carbon nanotube TIMs (CNT-TIMs) in both vacuum and ambient air environments. The TIMs were irradiated in a gamma cell at a rate of 200 rad/s to a total dose of 50 Mrad. TCR was measured before and afte...

Accurate thermal conductivity values are essential for the successful modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure the thermal conductivity of these systems, as well as the thermal conductivity itself, varies with the device materials, fabrication processes, geometry, and operating conditions. In this study, the thermal conductivities of boron doped single-crystal silicon microbridges fabricated using silicon-on-insulator (SOI) wafers are measured over the temperature range from 80 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and either 50 or 85 μm wide. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry (SSERT) and optical time-domain thermoreflectance (TDTR). A thermal conductivity of 77 Wm−1 K−1 is measured for both microbridge widths at room temperature, where the results of both experimental techniques agree. However, increasing discrepancies between the thermal conductivities measured by each technique are found with decreasing temperatures below 300 K. The reduction in thermal conductivity measured by TDTR is primarily attributed to a ballistic thermal resistance contributed by phonons with mean free paths larger than the TDTR pump beam diameter. Boltzmann transport equation (BTE) modeling under the relaxation time approximation (RTA) is used to investigate the discrepancies and emphasizes the role of different interaction volumes in explaining the underprediction of TDTR measurements.

Despite a larger sensitivity to temperature as compared to other microscale thermometry methods, Raman based measurements typically have greater uncertainty. In response, a new implementation of Raman thermometry is presented having lower uncertainty while also reducing the time and hardware needed to perform the experiment. Using a modulated laser to excite the Raman response, the intensity of only a portion of the total Raman signal is leveraged as the thermometer by using a single element detector monitored with a lock-in amplifier. Implementation of the lock-in amplifier removes many sources of noise that are present in traditional Raman thermometry where the use of cameras preclude a modulated approach. To demonstrate, the portion of the Raman spectrum that is most advantageous for thermometry is first identified by highlighting, via both numerical prediction and experiment, those spectral windows having the largest linear dependence on temperature. Using such windows, the new technique, termed single element Raman thermometry (SERT), is utilized to measure the thermal profile of an operating microelectromechanical systems (MEMS) device and compared to results obtained with a traditional Raman approach. The SERT method is shown to reduce temperature measurement uncertainty by greater than a factor of 2 while enabling 3 times as many data points to be taken in an equal amount of time as compared to traditional Raman thermometry.

Laser machining is frequently utilized in the manufacture of photovoltaics. A natural by-product of these fabrication processes, heat, not only serves as a means of material removal but also modifies the material in an extended region beyond that ideally intended for alteration. This modified region, termed the heat affected zone, is detrimental to performance and should therefore be minimized. While undoubtedly thermal in origin, it is unclear exactly how the thermal environment during laser machining correlates to changes in the PN-junction that reduce performance. In response, we combine in-situ Raman based thermometry measurements with post-event failure analysis to identify the physical mechanisms damaging the junction during laser machining. From this approach, damage is shown to initiate prior to melting and be driven primarily by the diffusion of dopants for fluences that do not induce ablation. Additionally, comparatively small regions of damage are shown to have a large impact on operation. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4798382]

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.

This paper reports on experimental and numerical investigations of electrically powered MEMS structures operated under different gas pressure and electrical power conditions. The structures studied are boron-doped single crystal silicon-on-insulator (SOT) microbridges that are heated by an electrical current. The microbridges are 85 mu m wide, 125 mu m tall and 5.5 mm long and lie 2 mu m above the substrate. The impact of the narrow gap in the gas phase thermal transport is evaluated by operating the devices under various nitrogen gas pressure conditions, ranging from 625 Torr to similar to 1 mTorr - spanning the continuum to noncontinuum gas heat transfer regimes. Raman thermometry is used to obtain spatiallyresolved temperature measurements along the length of the device under the various operating conditions. The large dopant concentration (similar to 4 x 10(19) cm(-3)) within the active silicon layer is found to affect the Raman spectrum used for thermometry via Fano-type interactions, resulting in an asymmetric Raman line shape. With large Raman peak asymmetries, use of the Raman line width as the temperature metric is less reliable as it shows decreased sensitivity to temperature. However, the asymmetry itself, when considered as a fitting parameter, was found to be a reliable indicator of sample temperature. The measured device temperatures are compared to finite element simulations of the structures. Noncontinuum gas phase heat transfer effects are incorporated into the continuum simulations via temperature discontinuities at the solid-gas interface, provided by a model developed from noncontinuum simulation results. Additionally, the impact of the large dopant concentrations is incorporated into the thermal models via a modified thermal conductivity model which considers impurity scattering effects on thermal transport. The simulation and experimental results show reasonable agreement.

Accurate thermal conductivity values are essential to the modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure thermal conductivity, as well as thermal conductivity itself, varies with the device materials, fabrication conditions, geometry, and operating conditions. In this study, the thermal conductivity of boron doped single-crystal silicon-on-insulator (SOI) microbridges is measured over the temperature range from 77 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and two widths, 50 or 85 μm. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry and optical time-domain thermoreflectance. A thermal conductivity of ∼ 77 W/mK is measured for both microbridge widths at room temperature, where both experimental techniques agree. However, a discrepancy at lower temperatures is attributed to differences in the interaction volumes and in turn, material properties, probed by each technique. This finding is qualitatively explained through Boltzmann transport equation modeling under the relaxation time approximation.

In this work, we examine Fermi relaxation in 20 nm Au films with pump-probe themoreflectance using a thin film, intraband thermoreflectance model. Our results indicate that the Fermi relaxation of a perturbed electron system occurs approximately 1.10±0.05 ps after absorption of a 785 nm, 185 fs laser pulse. This is in agreement with reported values from electron emission experiments but is higher than the Fermi relaxation time determined from previous thermoreflectance measurements. This discrepancy arises due to thermoreflectance modeling and elucidates the importance of the use of a proper thermoreflectance model for thermophysical property determination in pump-probe experiments.

We discuss recent experiments for the characterization of our femtosecond pure rotational CARS facility for observation of Raman transients in N{sub 2} and atmospheric air. The construction of a simplified femtosecond four-wave mixing system with only a single laser source is presented. Pure-rotational Raman transients reveal well-ordered time-domain recurrence peaks associated with the near-uniform spacing of rotational Raman peaks in the spectral domain. Long-time, 100-ps duration observations of the transient Raman polarization are presented, and the observed transients are compared to simulated results. Fourier transformation of the transients reveals two distinct sets of beat frequencies. Simulation results for temperatures from 300-700 K are used to illustrate the temperature sensitivity of the time-domain transients and their Fourier-transform counterparts. And strategies for diagnostics are briefly discussed. These results are being utilized to develop gas-phase measurement strategies for temperature and species concentration.

Operating temperatures are known to directly affect the performance and reliability of a range of modern microdevices, including light-emitting diodes (LEDs), microelectromechanical systems (MEMS), and high-power electronics. As a consequence, accurate temperature measurements have become imperative in the development of these technologies. Such measurements are complicated, however, by the complex multimaterial stacks typically used and by the fact that traditional probes (thermocouples) have a size and thermal mass on the order of the device being interrogated. In response to these difficulties, Raman thermometry is frequently implemented, because it is a non-contact, material-specific measurement largely benign to device operation that is capable of comparatively small spatial (similar to 500 nm) and thermal (similar to 1 degrees C) resolutions. Practically, these measurements can be made using a variety of spectral features including the position, linewidth, and intensity of the Raman signal associated with specific optical phonon modes. Each of these spectral characteristics offers particular advantages, depending on the type of device and its operational conditions. Here, the practical implementation of Raman thermometry using each of these spectral characteristics is reviewed to highlight the assumptions implicit with their use and to compare their effectiveness in measuring temperature.

We study the electron relaxation processes in nanoporous Au structures using pump-probe thermoreflectance. Using a modified two temperature model, we determine that the electron Fermi relaxation time is unaffected by the ligament size and is an athermal process that is constant with laser pulse excitation, yet the electron-phonon relaxation is affected by boundary scattering and is dependent on electron temperature. The increased, temperature dependent electron-phonon coupling measurements can be explained by electron-boundary scattering and electron-electron scattering that alters the rate of electron equilibration with the surrounding media. These results lend insight into electron relaxation processes in high surface area nanostructures.

In nanosystems, the primary scattering mechanisms occur at the interfaces between the material layers. As such, the structure and composition around these interfaces can affect scattering rates and, therefore, thermal resistances. In this work, we measure the room-temperature thermal boundary conductance of aluminum films grown on silicon substrates subjected to various pre-Al-deposition surface treatments with a pump-probe thermoreflectance technique. The Si surfaces are characterized with atomic force microscopy to determine mean surface roughness. The measured thermal boundary conductances decrease as Si surface roughness increases. In addition, stripping of the native oxide layer from the surface of the Si substrate immediately prior to Al film deposition causes the thermal boundary conductance to increase. The measured data are compared to an extension of the diffuse mismatch model that accounts for interfacial mixing and structure around the interface in order to better elucidate the thermal scattering processes affecting thermal boundary conductance at rough interfaces.

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.more » 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.« less

Pump-probe transient thermoreflectance (TTR) techniques are powerful tools for measuring thermophysical properties of thin films, such as thermal conductivity, Λ, or thermal boundary conductance, G. This paper examines the assumption of one-dimensional heating on Λ and G determination in nanostructures using a pump-probe transient thermoreflectance technique. The traditionally used one dimensional and radial (3D) models are reviewed. To test the assumptions of the thermal models, experimental data from Al films on bulk substrates (Si and glass) are taken with the TTR technique. This analysis is extended to thin film multilayer structures. Results show that at 11 MHz modulation frequency, thermal transport is indeed one dimensional. Error among the various models arises due to pulse accumulation and not accounting for residual heating.