Justin R. Serrano

Reduction in the thermal conductivity of single crystalline silicon by phononic crystal patterning.

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.

Criteria for Cross-Plane Dominated Thermal Transport in Multilayer Thin Film Systems During Modulated Laser Heating

Pump-probe transient thermoreflectance (TTR) techniques are powerful tools for measuring the thermophysical properties of thin films, such as thermal conductivity A, or thermal boundary conductance, G. This paper examines the assumption of one-dimensional heating on, A 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.

Invited Article: Simultaneous mapping of temperature and stress in microdevices using micro-Raman spectroscopy.

Analysis of the Raman Stokes peak position and its shift has been frequently used to estimate either temperature or stress in microelectronics and microelectromechanical system devices. However, if both fields are evolving simultaneously, the Stokes shift represents a convolution of these effects, making it difficult to measure either quantity accurately. By using the relative independence of the Stokes linewidth to applied stress, it is possible to deconvolve the signal into an estimation of both temperature and stress. Using this property, a method is presented whereby the temperature and stress were simultaneously measured in doped polysilicon microheaters. A data collection and analysis method was developed to reduce the uncertainty in the measured stresses resulting in an accuracy of +/-40 MPa for an average applied stress of -325 MPa and temperature of 520 degrees C. Measurement results were compared to three-dimensional finite-element analysis of the microheaters and were shown to be in excellent agreement. This analysis shows that Raman spectroscopy has the potential to measure both evolving temperature and stress fields in devices using a single optical measurement.

Micro-Raman thermometry of thermal flexure actuators

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.

Raman Thermometry of Polysilicon Microelectro-mechanical Systems in the Presence of an Evolving Stress

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.

Raman Thermometry Measurements and Thermal Simulations for MEMS Bridges at Pressures From 0.05 Torr to 625 Torr

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.

Stress-induced wrinkling of sputtered SiO2 films on polymethylmethacrylate

Compressively stressed SiO2 films are deposited by rf magnetron sputtering onto polymethylmethacrylate- (PMMA) coated Si substrates. The oxide film roughens by wrinkling during deposition; wrinkling is enabled by the viscous flow of the PMMA layer. The nanoscale lateral length scale of the wrinkling, ∼120nm, is established during the first few nanometers of film deposition and is controlled by the thickness and stress of the SiO2 film at the onset of the instability. Continued deposition of SiO2 leads to a rapid increase and then saturation of the rms roughness at ∼5nm.

Single element Raman thermometry

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.

Re-examining Electron-Fermi Relaxation in Gold Films With a Nonlinear Thermoreflectance Model

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, 785 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 thermorefiectance measurements. This discrepancy arises due to thermore flectance modeling and elucidates the importance of the use of a proper the rmore flectance model for thermophysical property determination in pump-probe experiments.

Effects of mechanical stress on thermal microactuator performance

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.