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In order to predict material failure accurately, it is critical to have knowledge of deformation physics. Uniquely challenging is determination of the conversion coefficient of plastic work into thermal energy. Here, we examine the heat transfer problem associated with the experimental determination of β in copper and stainless steel. A numerical model of the tensile test sample is used to estimate temperature rises across the mechanical test sample at a variety of convection coefficients, as well as to estimate heat losses to the chamber by conduction and convection. This analysis is performed for stainless steel and copper at multiple environmental conditions. These results are used to examine the relative importance of convection and conduction as heat transfer pathways. The model is additionally used to perform sensitivity analysis on the parameters that will ultimately determine β. These results underscore the importance of accurate determination of convection coefficients and will be used to inform future design of samples and experiments. Finally, an estimation of convection coefficient for an example mechanical test chamber is detailed as a point of reference for the modeling results.

Periodic porous structures offer unique material solutions to thermoelectric applications. With recent interest in phonon band gap engineering, these periodic structures can result in reduction of the phonon thermal conductivity due to coherent destruction of phonon modes characteristic in phononic crystals. In this paper, we numerically study phonon transport in periodic porous silicon phononic crystal structures. We develop a model for the thermal conductivity of phononic crystal that accounts for both coherent and incoherent phonon effects, and show that the phonon thermal conductivity is reduced to less than 4% of the bulk value for Si at room temperature. This has substantial impact on thermoelectric applications, where the efficiency of thermoelectric materials is inversely proportional to the thermal conductivity.

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.

The absorption of optical energy and subsequent thermal transport are investigated experimentally and numerically for 500 μm long, 250 μm wide, and 2.25 μm thick polycrystalline silicon microcantilevers irradiated by an 808 nm continuous-wave laser. Temperature profiles were measured using Raman thermometry at 314 and 532 mW of laser power, and the microcantilever peak temperature was measured for laser powers up to 719 mW. A modular technique for multilayered structures is used to calculate the optical absorption in polysilicon microcantilevers, and the thermal response is then calculated with a two-dimensional, finite difference model. Very good agreement is obtained between the measured and calculated temperature profiles and peak temperatures versus laser power. The abrupt increase or amplification of peak temperature for laser powers between 415 and 440 mW is shown to be a result of peaks in the temperature-dependent optical absorptance.

Optical actuators are fundamental building blocks in the development of all-optical microelectromechanical devices. Photothermally actuated devices are inevitably limited by overheating and device damage resulting from the absorption of laser power. Optimal actuator design requires an efficient use of the applied laser power while minimizing the susceptibility of device damage. Surface micromachined polycrystalline silicon flexure-style optical actuators, which are powered using an 808-nm continuous-wave laser, were evaluated for displacement performance and susceptibility to damage. Actuator displacement is linear with incident power for laser powers below those that cause damage to the irradiated surface, up to a maximum displacement of 7-9 mum. Damage of the irradiated surface causes viscous relaxation of the polysilicon film and leads to recession of the displacement during the heating and additional recession after the optical power is removed. The first spatially resolved temperature measurements during device operation were obtained using micro-Raman thermometry. The temperature measurements revealed the influence of temperature-dependent optical properties in the thermal behavior of the irradiated devices.

Adhesion failure is a significant reliability concern and a major barrier to commercializing MEMS devices. In this paper, a beam-peel-based experimental setup was improved to measure the adhesion energy between a microcantilever and a substrate at different humidity levels. The microcantilever arrays were separated from the substrate and their fixed ends were directly attached to a piezoactuator to control the beam displacement with sub-nanometer accuracy. The experimental setup was sealed in a chamber with precise humidity control. An in situ interferometer was used to measure the beam deflection and crack length during the peel test. To examine the effects of surface roughness and relative humidity on adhesion energy, four different surface pairs were measured at humidity levels ranging from 40 to 92%. Before testing, the microcantilevers and substrates were scanned using an Atomic Force Microscope. Surface roughness parameters and the exact probability density function of the asperity heights were extr...

Bent-beam, polycrystalline silicon MEMS thermal actuators were optically powered using laser illumination. The actuators were designed with a target suspended between angled legs that expand creating displacement and force when the target is heated by a laser. The laser power threshold for immediate damage and extent of the damage exhibited a strong dependence on the target design. The optically powered thermal actuators were fabricated using a five-layer surface micromachining process from combinations of 2.25 μm thick polysilicon layers (poly3 and poly4) and a 2.0 μm thick sacrificial layer. The target compositions listed in decreasing order of laser damage threshold are: the poly3–poly4 laminate targets, the poly3–poly4 targets with an intermediate 2.0 μm thick oxide layer, the poly3–poly4 targets with a 2.0 μm thick air gap, and the poly4 only targets. The laser damage thresholds for the poly3–poly4 targets with connecting posts and an air gap increases with increasing post size, which allows for better thermal transport from the irradiated poly4 layer to the underlying poly3 layer. The laser damage thresholds ranged from a maximum of 475 mW for an oval poly3–poly4 laminate actuator target to a minimum of 190 mW for a circular poly4 only actuator target.

Polycrystalline silicon (polysilicon) films are primary structural materials for microelectromechanical systems (MEMS). Due to relatively high compliance, large surface-to-volume ratio, and small separation distances, micromachined polysilicon structures are susceptible to surface forces which can result in adhesive failures. Since these forces depend on surface properties especially surface roughness, three types of microhinged flaps were fabricated to characterize their roughness and adhesive meniscus properties. The flaps enabled access to both the top and bottom surfaces of the structural polysilicon layers. Roughness measurements using an atomic force microscope revealed that MEMS surfaces primarily exhibit non-Gaussian surface height distributions, and for the release procedures studied, the bottom surface of the structural layers was significantly smoother and prone to higher adhesion compared to the top surface. A non-symmetric surface roughness model using the Pearson system of frequency curves was coupled with a capillary meniscus adhesion model to analyze the effects of surface roughness parameters (root-mean-square, skewness, and kurtosis), relative humidity, and surface contact angle on the interfacial adhesion energy. Using the measured roughness properties of the flaps, four different surface pairs were simulated and compared to investigate their effects on capillary adhesion. It was found that since the base polysilicon layer (poly0) was rougher than the base silicon nitride and the structural layer on poly0 was also rougher than that on silicon nitride, depositing MEMS devices on poly0 layer rather than directly on silicon nitride will reduce the adhesion energy.

In this work, the use of Raman Stokes peak location and linewidth broadening methods were evaluated for thermometry applications of polysilicon microheaters subjected to evolving thermal stresses. Calibrations were performed using the temperature dependence of each spectral characteristic separately, and the uncertainty of each method quantified. It was determined that the Stokes linewidth was independent of stress variation allowing for temperature determination, irrespective of stress state. However, the linewidth method is subject to greater uncertainty than the Stokes shift determination. The uncertainties for each method are observed to decrease with decreasing temperature and increasing integration times. The techniques were applied to mechanically constrained electrically active polysilicon microheaters. Results revealed temperatures in excess of 500°C could be achieved in these devices. Using the peak location method resulted in an underprediction of temperature due to the development of a relative compressive thermal stress with increasing power dissipation.

Micro-Raman spectroscopy has proven to be a valuable tool for obtaining temperature measurements in active semiconductor and MEMS devices. By using the temperature-calibrated response of the polysilicon Raman signature, we have obtained spatially resolved temperature measurements of U-shaped electro-thermal actuators. Both the peak position and the line width of the characteristic Raman peak have been used as temperature metrics. The measured thermal profiles are further compared to numerical models of the electro-thermal response of the devices as designed and fabricated. The obtained thermal profiles are in qualitative agreement with published modeled thermal profiles of similar devices and are within 15 °C of our modeled profiles. These measurements represent the first reported experimental temperature profile measurements for flexure type actuators and can be used to validate the existing models. Moreover, the comparison of line width and position-based temperatures are in good agreement, differing slightly over the flexure regions of the device.

In this paper, we report spatially resolved temperature profiles along the legs of working V-shaped electrothermal (ET) actuators using a surface Raman scattering technique. The Raman probe provides nonperturbing optical data with a spatial resolution of 1.2 /spl mu/m, which is required to observe the 3-/spl mu/m-wide actuator beams. A detailed uncertainty analysis reveals that our Raman thermometry of polycrystalline silicon is performed with fidelity of /spl plusmn/10 to 11 K when the peak location of the Stokes-shifted optical phonon signature is used as an indicator of temperature. This level of uncertainty is sufficient for temperature mapping of many working thermal MEMS devices which exhibit characteristic temperature differences of several hundred Kelvins. To our knowledge, these are the first quantitative and spatially resolved temperature data available for thermal actuator structures. This new temperature data set can be used for validation of actuator thermal design models and these new results are compared with finite-difference simulations of actuator thermal performance.

Undesirable adhesion in microelectromechanical systems (MEMS) is referred to as stiction and is a principal failure mechanism in surface-micromachined MEMS devices. Previous investigations demonstrated repairing stiction-failed polycrystalline silicon MEMS structures released from isopropyl alcohol (IPA) using Nd:YAG laser irradiation and predicting the laser repair process using a thermomechanical model. The current paper reports the effectiveness of the laser repair process and corresponding thermomechanical model predictions for microcantilevers that have failed during four release treatments: water, IPA, octadecyltrichlorosilane (OTS), and supercritical CO/sub 2/ drying. Model predictions and experimental measurements of the laser repair process are also provided for MEMS devices that failed due to contact during electrostatic actuation. The results indicate that the laser repair process is very effective for both failure modes and that the thermomechanical model predicts the laser repair of microcantilevers that failed during release much better than for microcantilevers that failed due to subsequent contact.

Adhesion during operation, referred to as in-use stiction, shortens the useful lifetimes and reliability of microelectromechanical systems (MEMS). In this paper, operational reliability tests were performed to investigate the effect of temperature on in-use stiction for surface-micromachined, polycrystalline silicon MEMS cantilevers subject to three release techniques: supercritical CO 2 drying, laser irradiation repair, and an octadecyltrichlorosilane (OTS) deposition process. The cantilevers were heated from room temperature to 300 °C and then returned to room temperature. The cantilevers were electrostatically actuated at specific temperatures between 22 and 300 °C during the heating and cooling stages, and their sticking probabilities determined. The failure probability results exhibit a slight variation with temperature for microcantilevers released using the supercritical CO 2 dry and laser irradiation repair. However, in-use stiction of microcantilevers coated with OTS is strongly dependent on temperature. The incidence of in-use stiction was highest during the heating stage, reduced during the cooling stage, and decreased further when the samples were reheated to 300 °C. The results indicate that thermal annealing of OTS coated structures in air decreases in-use stiction failures. Surface characterization of OTS coated samples was conducted using contact angle goniometry, atomic force microscopy, and X-ray photoelectron spectroscopy.

The characteristics of the materials and surfaces in microelectromechanical systems (MEMS) and microsystems technology (MST) profoundly affect the performance, reliability, and wear of MEMS and MST devices. It is critical to measure the properties of surfaces that are in contact during microstructure movement, such as the underside of a MEMS gear and the underlying substrate. However, contacting surfaces are usually inaccessible unless the MEMS device is broken and removed from the substrate. This paper presents a nondestructive method for characterizing commercially fabricated surface micromachined polycrystalline silicon (polysilicon) devices. Microhinged flaps were designed that enable access to the upper surface, the part of a structural layer deposited last; the lower surface, the part of a structural layer deposited first; and the underlying substrate. Due to the susceptibility of surface-micromachined MEMS to adhesion failures, the surface roughness is a key parameter for predicting device behavior. Using the microhinged flaps, the RMS surface roughness for polycrystalline surfaces was measured and indicated that the upper surfaces were 3.5–6.4 times rougher than the lower surfaces. The difference in the surface roughness for the upper surface, which is easily accessed and the one most commonly characterized, and that for the lower surface reveals the importance of characterizing contacting surfaces in MEMS and MST devices.

This article reports a two-dimensional, finite-difference heat transfer model for calculating the transient temperature distribution in a polycrystalline silicon cantilever during and after irradiation by a Nd:YAG laser. Results include the peak surface temperature after irradiation and the uniform temperature increase in the microcantilever following subsequent heat conduction through the thickness. The calculations reveal that the time scale after which the temperature is uniform through the thickness is on the order of hundreds of nanoseconds and that the microcantilever cools in the order of tens of milliseconds. The effects of energy transfer to the environment by convection and radiation on the cooling time are also investigated. The accuracy of the model predictions are shown through high-speed temperature measurements using a novel MEMS temperature sensor.

The thermal contact conductance G for polycrystalline silicon cantilever beams that are adhered to an underlying substrate is examined using two different optical techniques. Using time-domain thermoreflectance, we measure G=9±2 MW m−2 K−1 at 25 °C and G=4±1 MW m−2 K−1 at 150 °C. The room temperature value is confirmed using a modified Angstrom method, which establishes a lower limit of G>5 MW m−2 K−1. This contact conductance is a factor of 10–105 greater than values reported for metal–metal and ceramic–ceramic interfaces. The large interfacial conductance is consistent with the presence of a thin layer of water trapped between the cantilever and the substrate. The thermal conductivity Λ of the phosphorus doped polysilicon cantilever is nearly isotropic with Λcross plane=65 W m−1 K−1, and Λin plane=70 W m−1 K−1 at room temperature.

A technique for repairing adhered surface-micromachined polycrystalline silicon (polysilicon) structures using pulsed lasers has been developed, experimentally investigated, and modeled. Advantages of using lasers to repair adhered microfabricated structures include the ease of implementation, the ability to apply the laser energy locally to the adhered region, and the non-contact nature of the process. The amount of thermal heating is typically less than 100 °C and does not damage polysilicon structures. Pulsed laser repair has been successful for undoped and n-doped polysilicon microcantilevers that had adhered to the substrate during chemical etching of the sacrificial layers and subsequent drying. This paper reviews experiments investigating the use of Ti:sapphire and Nd:YAG laser systems to repair failed polysilicon microcantilevers. The first experiments used a Ti:sapphire laser system, and later experiments using a Nd:YAG laser produced repair yields up to 100% for 10 μm wide cantilevers that were ...

Presents a thermomechanical model that describes adhesion reduction in MEMS structures using laser heating. A fracture mechanics model is developed where the interface between the stiction-failed microcantilever and the substrate is treated as a crack, and the energy release rate is calculated using elastic theory. In order to include the effect of a temperature difference between the microcantilever and the substrate, an associated thermal strain energy is included in the fracture model. If the free length is longer than the critical buckling length, the beam buckles decreasing the strain energy of the system. For surface-micromachined polycrystalline silicon cantilevers with an initial crack length of 400 /spl mu/m, the model predicts that a temperature difference of 100 K repairs microcantilevers as long as 1300 /spl mu/m. The peeling of adhered beams from the substrate after laser irradiation is experimentally shown with measured crack lengths within 15% of predicted values indicating that the proposed model establishes the mechanism of adhesion reduction by laser irradiation.

Stiction, the adhesion of micromachined components to each other or the substrate, decreases production yields and operational lifetimes for MEMS devices. A recent study demonstrated the feasibility of using a Nd:YAG, 1063 nm laser to repair 2 /spl mu/m thick polycrystalline silicon microcantilevers with lengths up to 1000 /spl mu/m which had adhered to the substrate. This investigation examines the influence of sample age at the time of laser irradiation on the repair of stiction-failed MEMS structures. The operating conditions for the 1064 nm, Nd:YAG laser included a fluence of 70 mJ/cm/sup 2/, a repetition rate of 20 Hz, a pulse duration of 20 ns, and an exposure time of 60 s. For samples irradiated on the same day of release, repair yields were 100% for beams up to 500 /spl mu/m in length. For 1000 /spl mu/m long beams, the 10- and 30-/spl mu/m-wide cantilevers had same-day repair yields of 56% and 71%, respectively. The experimental results show that increasing the amount of time the microstructures are adhered to the substrate before laser irradiation decreases the ability to repair stiction-failed devices with delays of longer than 1 month resulting in a decreased repair yield.

Ultrashort-pulse lasers are able to recover stiction-failed microelectromechanical systems (MEMS) structures. A possible physical mechanism for this process is an electronic disruption of the bonding between the structure and the substrate, which depends strongly on the peak carrier temperatures. This article presents predicted carrier temperatures for polycrystalline silicon microcantilevers irradiated with ultrashort-pulse lasers. The effects of increased carrier-phonon coupling, reduced thermal conductivity, and increased absorption for polycrystalline silicon as compared to crystalline silicon are determined. An expression for the optimal absorption coefficient is derived for processes in which the excitation is desired at a specified depth into the material.

New methods of removing surface contaminants from microelectronic and microelectromechanical systems (MEMS) devices are needed since the decreasing size of their components is reducing the allowable contamination levels. By choosing the pulse duration and fluence to optimize electronic rather than thermal desorption in short-pulse laser processing, surface species can be removed without exceeding maximum temperature constraints. A two-temperature model for short-pulse laser heating of, and subsequent desorption from, metal surfaces is presented. A scaling analysis indicates the material properties and laser parameters on which the ratio of electronic to thermal desorption depends. Regimes of predominantly electronic and thermal desorption are identified, and predicted desorption yields from gold films show that electronic desorption is the primary desorption mechanism in certain short-pulse laser processes.

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 lm tall, and either 50 or 85 lm wide. Measurements on the 85 lm 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. [DOI: 10.1115/1.4024357]

A technique to remove moisture from microelectronic devices and improve device yield in microelectromechanical systems by reducing microstructure surface adhesion is proposed. Ultrashort-pulse laser radiation is used to create excited carriers in, and consequently desorb water from, silicon micro-structures. A theoretical model for ultrashort-pulse laser heating of silicon is presented. Calculated carrier temperatures show significant increases at short time scales, while the lattice temperatures remain almost constant, indicating the possibility for water desorption without significant device heating. A preliminary experiment confirming the feasibility of using the technique to decrease microstructure adhesion is discussed.

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.Copyright © 2012 by ASME

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. In this work, we use a vector-based thermal algorithm to solve for the temperature change and heat transfer in which a thin film subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our solution with time domain thermoreflectance measurements on glass slides and extend the thermal conductivity measurements to SiO2-based nanostructured films.

We report on the ultra-low thermal conductivity of a series of convectively assembled, anisotropic titania (TiO2 ) nanoparticle films. The TiO2 films are fabricated on aluminum coated glass substrates by flow coating a suspension of ellipsoidal colloidal nanoparticles, resulting in structured films with tailored order. Time domain thermoreflectance is used to measure the thermal conductivity of the TiO2 films. The thermal conductivities of these nanoparticle films are dependent on nanoparticle orientational order and films with more randomly oriented particles exhibit thermal conductivities less than the amorphous limit.Copyright © 2011 by ASME

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.

Due to the small component sizes in modern microdevices, surface forces can create undesirable adhesion between microstructures, which is referred to as stiction. The current study uses a 100 ns, 1064 nm, Nd:YAG laser to repair polycrystalline silicon microcantilevers stuck to the underlying substrate. The results show that a Nd:YAG, 1064 nm laser is capable of repairing failed microstructures with yields exceeding those reported in earlier studies. Yields of 100 percent for cantilevers up to 1 mm in length were demonstrated for several laser operating conditions. The yields increase strongly with increased laser fluence and increase slightly with longer exposure times.

Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.

Pump-probe transient thermoreflectance (TTR) techniques are powerful tools for measuring the 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 axially symmetric cylindrical conduction models for thermal transport 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. The 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.

This paper reports on experimental and computational investigations into the thermal performance of microelectromechanical systems (MEMS) as a function of the pressure of the surrounding gas. High spatial resolution Raman thermometry was used to measure the temperature profiles on electrically heated, polycrystalline silicon bridges that are nominally 10 μm wide, 2.25 μm thick, and either 200 μm or 400 μm long in nitrogen atmospheres with pressures ranging from 0.05 Torr to 625 Torr (6.67 Pa–83.3 kPa). Finite element modeling of the thermal behavior of the MEMS bridges is performed and compared with 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 results indicate that gas-phase heat transfer is significant for devices of this size at ambient pressures but becomes minimal as the pressure is reduced below 5 Torr. The model and experimental results are in qualitative agreement, and better quantitative agreement requires increased accuracy in the geometrical and material property values.

We report on the effects of mechanical stress on thermal microactuator performance. Packaging processes such as die attach and lid sealing usually result in stresses on the die containing microsystems devices. While this phenomenon is known, quantifying the effects systematically is difficult due to challenges in controlling the resultant stress resulting from packaging. In this study, we use a four-point bending stage to apply loads of 12 lbf (53.4 N) in tension and compression to 11.5 mm by 2.9 mm samples. Thermal microactuators and stress gauges were fabricated using the Sandia 5-layer SUMMiT surface micromaching process and diced to fit in the bending stage. At each stress level, the vernier scales on the thermal microactuator were imaged in order to determine the displacements. Thermal microactuator displacements are reported as a function of applied current up to 35 mA at varying stress levels. Increasing tensile stress decreases the initial displacement and flattens the thermal microactuator displacement versus applied current curve. Raman spectroscopy and stress gauge measurements indicate that the stress range for the fourpoint bending stage experiments extends from 200 MPa tensile to -250 MPa compressive. Numerical model predictions of thermal microactuator displacement versus current are in qualitative agreement with the experimental results. Quantitative information on the reduction in thermal microactuator performance as a function of stress provides validation data for MEMS models and can guide future designs so that they will be more robust to stresses resulting from packaging processes.

Nanocomposites offer unique capabilities of controlling thermal transport through the manipulation of various structural aspects of the material. However, measurements of the thermal properties of these composites are often difficult, especially porous nanomaterials. Optical measurements of these properties, although ideal due to the noncontact nature, are challenging due to the large surface variability of nanoporous structures. Recently, a novel pump-probe geometry was used in Time Domain Thermoreflectance (TDTR) to determine the thermal conductivity of liquids. In this work, we develop a thermal algorithm to solve for the temperature change and heat transfer in this TDTR geometry in which a thin film which is subjected to a modulated heat source is sandwiched between two thermally conductive pathways. We validate our thermal algorithm with TDTR measurements of the thermal conductivity and on a series of porous SiO2 -based nanostructured films.Copyright © 2010 by ASME

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.Copyright © 2010 by ASME

The thermal performance of microelectromechanical systems devices is governed by the structure and composition of the constituent materials as well as the geometrical design. With the continued reduction in the characteristic sizes of these devices, experimental determination of the thermal properties becomes more difficult. In this study, the thermal conductivity of polycrystalline silicon (polysilicon) microbridges are measured with the transient 3ω technique and compared with measurements on the same structures using a steady state Joule heating technique. The microbridges with lengths from 200 μm to 500 μm were designed and fabricated using the Sandia National Laboratories SUMMiT V™ surface micromachining process. The advantages and disadvantages of the two experimental methods are examined for suspended microbridge geometries. The differences between the two measurements, which arise from the geometry of the test structures and electrical contacts, are explained by bond pad heating and thermal resistance effects.

Due to the coupling of thermal and mechanical behaviors at small scales, a Campaign 6 project was created to investigate thermomechanical phenomena in microsystems. This report documents experimental measurements conducted under the auspices of this project. Since thermal and mechanical measurements for thermal microactuators were not available for a single microactuator design, a comprehensive suite of thermal and mechanical experimental data was taken and compiled for model validation purposes. Three thermal microactuator designs were selected and fabricated using the SUMMiT V{sup TM} process at Sandia National Laboratories. Thermal and mechanical measurements for the bent-beam polycrystalline silicon thermal microactuators are reported, including displacement, overall actuator electrical resistance, force, temperature profiles along microactuator legs in standard laboratory air pressures and reduced pressures down to 50 mTorr, resonant frequency, out-of-plane displacement, and dynamic displacement response to applied voltages.

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.Copyright © 2009 by ASME

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. 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 Λ 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 Λ 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

This paper reports on experimental and computational investigations into the thermal performance of microelectro-mechanical systems (MEMS) as a function of the pressure of the surrounding gas. High spatial resolution Raman thermometry was used to measure the temperature profiles on electrically heated, polycrystalline silicon bridges that are nominally 10 μm wide, 2.25 μm thick, and either 200 or 400 μm 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 results indicate that gas-phase heat transfer is significant for devices of this size at ambient pressures but becomes minimal as the pressure is reduced below 5 Torr. The model and experimental results are in qualitative agreement, and better quantitative agreement requires increased accuracy in the geometrical and material property values.Copyright © 2009 by ASME

We have developed and demonstrated a technique for optical excitation of mechanical resonance that does not require coherent, monochromatic, or time-varying light. Previous methods for optically exciting mechanical motion in microscale devices required monochromatic, coherent light or time varying light. This technology could allow sunlight (or other ambient light source) to drive a MEMS device. It could also be used to convert sunlight to mechanical energy and subsequently to electrical energy through piezoelectric or capacitive techniques, essentially a micromechanical analog to the photovoltaic cell. We have demonstrated this method of optical excitation of a MEMS cantilever using simple cantilever beam structures fabricated using Sandia National Laboratories’ SUMMiT V™ process. The bimorph structure was created with polysilicon and aluminum. The minimum power to induce resonance was 3.5–4 mW of optical power incident on the cantilever under a vacuum of less than 1 mTorr. Resonance was observed at 45.6 kHz (slightly less than the 48.5 kHz predicted by FEA).

This report documents technical work performed to complete the ASC Level 2 Milestone 2841: validation of thermal models for a prototypical MEMS thermal actuator. This effort requires completion of the following task: the comparison between calculated and measured temperature profiles of a heated stationary microbeam in air. Such heated microbeams are prototypical structures in virtually all electrically driven microscale thermal actuators. This task is divided into four major subtasks. (1) Perform validation experiments on prototypical heated stationary microbeams in which material properties such as thermal conductivity and electrical resistivity are measured if not known and temperature profiles along the beams are measured as a function of electrical power and gas pressure. (2) Develop a noncontinuum gas-phase heat-transfer model for typical MEMS situations including effects such as temperature discontinuities at gas-solid interfaces across which heat is flowing, and incorporate this model into the ASC FEM heat-conduction code Calore to enable it to simulate these effects with good accuracy. (3) Develop a noncontinuum solid-phase heat transfer model for typical MEMS situations including an effective thermal conductivity that depends on device geometry and grain size, and incorporate this model into the FEM heat-conduction code Calore to enable it to simulate these effects withmore » good accuracy. (4) Perform combined gas-solid heat-transfer simulations using Calore with these models for the experimentally investigated devices, and compare simulation and experimental temperature profiles to assess model accuracy. These subtasks have been completed successfully, thereby completing the milestone task. Model and experimental temperature profiles are found to be in reasonable agreement for all cases examined. Modest systematic differences appear to be related to uncertainties in the geometric dimensions of the test structures and in the thermal conductivity of the polycrystalline silicon test structures, as well as uncontrolled nonuniform changes in this quantity over time and during operation.« less

The thermal properties of microelectromechanical systems (MEMS) devices are governed by the structure and composition of the constituent materials as well as the geometrical design. With the continued reduction of the characteristic sizes of these devices, experimental determination of the thermal properties becomes more difficult. In this study, the thermal conductivity of polycrystalline silicon (polysilicon) microbridges are measured with the transient 3ω technique and compared to measurements on the same structures using a steady state joule heating technique. The microbridges with lengths from 200 microns to 500 microns were designed and fabricated using the Sandia National Laboratories SUMMiT™ V surface micromachining process. The differences between the two measurements, which arise from the geometry of the test structures, are explained by bond pad heating and thermal boundary resistance effects.

The reduction of nanodevices has given recent attention to nanoporous materials due to their structure and geometry. However, the thermophysical properties of these materials are relatively unknown. In this article, an expression for thermal conductivity of nanoporous structures is derived based on the assumption that the finite size of the ligaments leads to electron-ligament wall scattering. This expression is then used to analyze the thermal conductivity of nanoporous structures in the event of electron-phonon nonequilibrium.