Showing papers by "Samuel Graham published in 2007"

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

Abstract: 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.

Thermal conduction from microcantilever heaters in partial vacuum

Abstract: This paper reports the thermal and electrical characteristics of a heated microcantilever in air and helium over a wide range of pressures. The cantilever heater size modulates thermal conductance between the cantilever and its gaseous surroundings; and the Knudsen number, Kn characterizes this thermal conductance. When Kn 1, thermal transport from the cantilever heater remains constant. This measurement of thermal conductance around Kn=1 could aid the design and analysis of Pirani sensors and other microscale thermal sensors and actuators.

Role of interface disorder on thermal boundary conductance using a virtual crystal approach

Abstract: An analytical method is presented to estimate the effects of structural disorder on the thermal boundary conductance (TBC) between two materials. The current method is an extension of the diffuse mismatch model (DMM) where the interface is modeled as a virtual crystal of finite thickness with properties derived from those of the constituent materials. Using this approximation, the TBC for a series of chromium/silicon interfaces is modeled and shown to be within 18% of experimentally obtained values. The methodology improves upon the predictive capabilities of the DMM and allows for quick estimation of the impact of interface mixing on TBC.

Thermal Metrology of Silicon Microstructures Using Raman Spectroscopy

Abstract: Thermal metrology of an electrically active silicon heated atomic force microscope cantilever and doped polysilicon microbeams was performed using Raman spectroscopy. The temperature dependence of the Stokes Raman peak location and the Stokes to anti-Stokes intensity ratio calibrated the measurements, and it was possible to assess both temperature and thermal stress behavior with resolution near 1mum. The devices can exceed 400degC with the required power depending upon thermal boundary conditions. Comparing the Stokes shift method to the intensity ratio technique, non-negligible errors in devices with mechanically fixed boundary conditions compared to freely standing structures arise due to thermally induced stress. Experimental values were compared with a finite element model, and were within 9% of the thermal response and 5% of the electrical response across the entire range measured

Stress relaxation in GaN by transfer bonding on Si substrates

Abstract: The stress state of GaN epilayers transferred onto Si substrates through a Au–Si bonding process was studied by micro-Raman scattering and photoluminescence techniques. By increasing the Au bonding thickness from 1to40μm, the high compressive stress state in GaN layer was relieved. A 10μm Au bonding layer thickness is shown to possess the maximum compressive stress relief and also the deformation potential of the quantum well was found to be ∼85meV. A nonlinear parabolic relation between luminescent bandgap and the biaxial stress of the transferred GaN epilayer in the compressive region was observed.

Thermal management methods for compact high power LED arrays

Abstract: The package and system level temperature distributions of a high power (>1W) light emitting diode (LED) array has been investigated using numerical heat flow models. For this analysis, a thermal resistor network model was combined with a 3D finite element submodel of an LED structure to predict system and die level temperatures. The impact of LED array density, LED power density, and active versus passive cooling methods on device operation were calculated. In order to help understand the role of various thermal resistances in cooling such compact arrays, the thermal resistance network was analyzed in order to estimate the contributions from materials as well as active and passive cooling schemes. An analysis of thermal stresses and residual stresses in the die are also calculated based on power dissipation and convection heat transfer coefficients. Results show that the thermal stress in the GaN layer are compressive which can impact the band gap and performance of the LEDs.

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

Abstract: 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.

Effects of composition and phonon scattering mechanisms on thermal transport in MFI zeolite films

Abstract: We report a systematic study that reveals the effect of composition (silicon-to-aluminum ratio) and the role of different phonon scattering processes on thermal transport in the nanoporous zeolite MFI. This is accomplished via synthesis of a series of films with graded compositions, thermal property measurements, and lattice dynamical modeling in the framework of the Boltzmann equation. MFI films with different Si/Al ratios (from infinity to 26) and constant (h0l) out-of-plane orientation were successfully synthesized by a seeded hydrothermal process. Three-omega measurements on these films allowed us to obtain comprehensive information on the thermal conductivity of MFI as a function of temperature (150–450 K) and Si/Al ratio. Detailed atomistic simulations (energy minimization and phonon dispersion calculations) were carried out for the MFI crystal structure with different Si/Al ratios and incorporated into a Boltzmann transport model along with approximate theoretical expressions for describing the rat...

Fabrication and Characterization of SiOx/Parylene and SiNx/Parylene Thin Film Encapsulation Layers

Abstract: Successful commercialization of flexible organic electronic devices is largely dependent on proper encapsulation that protects them from permeation of oxygen and water vapor. At present, low permeation encapsulation materials generally consist of multilayer films of organic/inorganic materials which can be deposited by plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), and vapor phase deposition. For this study, we report the effective water vapor transmission rates (WVTR) for multilayer thin films consisting of low temperature PECVD deposited SiNx and SiOx combined with a parylene organic layer. The effective WVTR was measured as a function of the number of bilayer pairs using Ca corrosion tests. The effective WVTR at 20 °C and 50% relative humidity [RH] for three bilayer pairs of SiOx/parylene ranged between 4.4–8.0 × 10−4 g/m2 /day while SiNx/parylene had a transmission rate 1.3×10−4 g/m2 /day. In general, additional layers were found to decrease the permeation rates to as low as 3.9×10−5 g/m2 /day, while the SiNx/parylene coatings performed the best overall.© 2007 ASME

Engineers Without Borders - USA, Learning Through Humanitarian Service to Underdeveloped Communities

Abstract: This paper describes the Engineers Without Borders-USA program to educate and train internationally responsible engineers and engineeri ng students. EWB-USA partners with underdeveloped and developing communities to improve their quality of life by implementing sustainable engineering projects using appropriate technology. EWB-USA continues to expand its role in the education community at the universi ty level and has started programs at the pre-college levels. These efforts stress humanitarian, service learning throu gh the application of real world projects, and allow stude nts to confront cultural as well as technological issues. Students have the opportunity to gain a different perspectiv e of their own culture and values through the lens of ot her cultures, and experience the positive and negative interaction between technological and social change .

High-speed thermal measurements of high-power diode arrays

Abstract: Thermal analysis becomes increasingly important for investigating power management issues, and the ability to obtain thermal characteristics for various packaging approaches is even more critical. Therefore, this study includes experiments that examine power loads, self-heating effects, and large area thermal characteristics of a high-power diode array using a non-contact approach. Through high-speed infrared image capture, multi-dimensional thermal data is collected noninvasively while stressing the diode array. Thermal transients recorded were found to be sensitive to ambient temperature, power cycling, and heat sink employed.