Thermomechanical analysis of emerging microsystems using Raman spectroscopy
Open Access
- Author:
- Lundh, James Spencer
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 07, 2021
- Committee Members:
- Sukwon Choi, Chair & Dissertation Advisor
Suzanne Mohney, Outside Unit & Field Member
Stephen Lynch, Major Field Member
Md Haque, Major Field Member
Daniel Haworth, Professor in Charge/Director of Graduate Studies - Keywords:
- Raman spectroscopy
temperature
stress
thermal transport
thermal management
wide bandgap semiconductors
ultrawide bandgap semiconductors
gallium nitride
aluminum gallium nitride
aluminum nitride
gallium oxide
diamond
lead zirconate titanate
2D materials
semiconductor devices
HEMT
FET
MODFET
pMUT
MEMS
RF electronics
power electronics
optical metrology
2DRT
nanoparticle Raman
thermoreflectance thermal imaging
infrared thermography - Abstract:
- To meet the demands for device miniaturization and enhanced performance capabilities, new material systems and devices are constantly being investigated with implications for a plethora of military and civilian applications. However, reducing device size while simultaneously increasing device performance leads to significantly greater operational heat fluxes and increased peak operating temperatures. As a result, thermal obstacles are becoming more common and more severe in emerging microsystems. Additionally, materials processing and device fabrication lead to residual stresses in the thin films upon which these devices are integrated. Since both temperature and stress degrade device performance, lead to reliability issues, and reduce device lifetime, it is crucial to probe the temperature and stress state of thin films and the associated device technologies. In this dissertation, Raman spectroscopy is used to study and understand thermal and mechanical phenomena in next generation microelectronics, including power electronics, radio frequency (RF) electronics, and microelectromechanical systems (MEMS), in order to resolve thermal obstacles limiting the full potential of emerging microsystems and to prevent reliability issues. For power and RF electronics, the thermal dynamics of GaN high electron mobility transistors (HEMTs) were studied to understand the relationship between electrical pulse width and thermal penetration depth as it relates to thermal transport for transient switching applications. From this transient thermal analysis, it is suggested that passive thermal management utilizing substrate engineering may be insufficient for fast switching applications because the thermal penetration depth is less than the GaN channel/buffer layer thickness. Thermal analysis is then extended to ultrawide bandgap (UWBG) semiconductors, including the first thermal analyses of AlxGa1-xN (x is Al content) channel HEMTs and diamond field-effect transistors (FETs). Thermal management solutions incorporating both bottom-side and top-side approaches were devised for an AlxGa1-xN channel HEMT, and it was shown that the thermally optimized AlxGa1-xN channel HEMT could excel in extreme environments compared to a standard GaN-on-SiC HEMT. However, the longer thermal time constants, due to the low thermal conductivity of AlxGa1-xN, further reduce the role of the substrate in thermal management for fast switching applications. Diamond has been suggested as a potential pathway for thermally robust electronics; however, the thermal performance of these devices had not yet been quantified. For the first time, the device-level thermal resistance of hydrogen-terminated diamond FETs was quantified (~ 1 mm·K/W), and a comparative thermal analysis demonstrated that diamond has up to ~50× and ~10× lower thermal resistance than lateral FETs based on other UWBG and WBG semiconductors, respectively. For MEMS, microscale thermal analysis of lead zirconate titanate (PZT)-based piezoelectric MEMS actuators allowed an understanding of energy loss mechanisms and quantification of their relative contributions to self-heating under various biasing regimes. Significant energy loss was observed under bipolar operation, which was attributed to energy loss via domain wall motion. Joule heating was also investigated under both direct current (DC) and alternating current (AC) biasing. While Joule heating was negligible under DC biasing, it became a significant source of heat generation under AC bias conditions due to high instantaneous capacitor charging currents. Thermal analysis concludes with the development of a novel technique, 2D Raman thermography (2DRT), that utilizes 2D materials for universal thermal imaging of micro/nanodevices. Considerations for further exploration and study were discussed, namely, using tip-enhanced Raman scattering (TERS) or 2D materials with nanoscale domain sizes to potentially overcome diffraction limited spatial resolution and enable nanoscale thermometry. Raman spectroscopy was subsequently used for mechanical analysis of aluminum nitride (AlN) thin films and devices at the wafer-level and device-level, respectively. Using Raman spectroscopy and spectroscopic ellipsometry, radially dependent residual stress distributions and film thicknesses were respectively quantified at the wafer-level and provided insight into the growth kinetics for magnetron sputtering of AlN films. At the device-level, Raman spectroscopy was used to relate device processing conditions to the residual stress state and distribution in individual piezoelectric micromachined ultrasonic transducers (pMUTs). Large stress gradients (~100 MPa across 20 μm) were observed outside metallization structures, and film release led to stress relaxation which can have implications for quality factor. The relationship between residual stress and resonant frequency of the pMUTs was investigated; however, due to other factors, including film thickness and processing conditions, no strong correlation was observed. The results presented within establish the framework for thermomechanical analysis of emerging microsystems using Raman spectroscopy. Specifically, this dissertation demonstrates metrology development, including 2DRT, and application to UWBG semiconductors for RF/power electronics and piezoelectric/ferroelectric materials for MEMS. This has allowed physical insight into thermal transport phenomena, energy loss mechanisms, growth kinetics, reliability physics, and electronic transport phenomena in these emerging microsystems. This work contributes to the continued advancement of (i) high power and high frequency electronics for applications including power conversion and transmission, communication systems, and electronic warfare and (ii) MEMS with increased functionality and sensitivity for applications including medical imaging, energy harvesting, and sensing.