Dustin Kendig

Thermal imaging of encapsulated LEDs

Thermoreflectance imaging is used to obtain 2D temperature maps of encapsulated LED arrays and elements with sub-micron spatial resolution. Typical LED encapsulation is opaque for infrared light, which prevents direct measurement of the semiconductor die with infrared cameras and thermocouples. A lock-in transient imaging technique with a megapixel silicon CCD is used to obtain the thermoreflectance and electroluminescence signals simultaneously. Transient thermal response in different locations of the die is characterized. Different thermal time constants are observed which correspond to various heat transfer mechanisms.

Stable thermoreflectance thermal imaging microscopy with piezoelectric position control

Thermoreflectance thermal imaging microscopy is based on very small change in the surface reflection as a function of temperature. Image shift and instrument drift are limiting factors to obtain accurate and reproducible thermal images. Under large magnification and for devices with sizes on the order of hundreds of nanometers, image registration could significantly encumber accurate thermoreflectance measurements. Additionally, image blurring is an issue because of small sample movements during the measurement. The problem of image registration and defocusing is particularly important during the calibration process to extract the thermoreflectance coefficient of the materials under study. Calibration requires changing the sample temperature with an external stage, which causes significant movement due to heat expansion from the stage. In this work, we discuss how the image registration and defocusing affect accurate measurement and calibration in thermoreflectance thermal imaging. We also show that by incorporating and controlling the position of the sample under test with a closed-loop piezo-stage one can perform accurate and reproducible thermal measurement. Using this setup, we measured the coefficient of thermoreflectance (CTR) of gold at several wavelengths and under different magnifications. We present for the first time thermoreflectance calibration of feature sizes below the diffraction limit, on the order of 200nm.

High Resolution Thermal Characterization and Simulation of Power AlGaN/GaN HEMTs Using Micro-Raman Thermography and 800 Picosecond Transient Thermoreflectance Imaging

Self-heating in gallium nitride based high frequency, high electron mobility power transistors (GaN HEMTs) is inspected using micro-Raman thermography and 800 picosecond transient thermoreflectance imaging. The two methods provide complementary temperature information inside the semiconductor and on top metal layers of the GaN HEMT. Self heating is measured under both steady-state and ultra-fast pulsed transient operation with submicron spatial resolution, 50 milliKelvin temperature resolution, and nanosecond time resolution. Fine grain electrothermal modeling of the HEMT steady state and transient self-heating are presented alongside measurements. Large spatial and temporal temperature gradients are quantified. Deviations due to unknown parameters are discussed.

Thermoreflectance imaging of defects in thin-film solar cells

We have identified and characterized various defects in thin-film a-Si and CIGS solar cells with sub-micron spatial resolution using thermoreflectance imaging. A megapixel silicon-based CCD was used to obtain noncontact thermal images simultaneously with visible electroluminescence (EL) images. EL can be indicative of pre-breakdown sites due to trap assisted tunneling and stress induced leakage currents. Physical defects appear at reverse bias voltages of 8V in a-Si samples. Linear and nonlinear shunt defects are investigated as well as electroluminescent breakdown regions at reverse biases as low as 4.5V. Pre-breakdown sites with electroluminescence are investigated.

Nanosecond transient thermoreflectance imaging of snapback in semiconductor controlled rectifiers

Transient thermoreflectance imaging method has been applied for the first time to reveal current distribution in ESD protection devices through the surface temperature change due to self heating. Experimentally calibrated temperature images are obtained of a multiple finger, 80 square micron 100V NLDMOS-SCR device in snapback operation regimes for different current levels (1.15–1.47A) and at different times ranging between 100 nanoseconds to one millisecond after the ESD pulse. The novel applied methodology demonstrates a practical and straightforward way to characterize non-uniform temperature and current distribution in ESD structures, revealing effects of non-simultaneous triggering of individual fingers on the multiple finger SCR device.

Sub-diffraction limit thermal imaging for HEMT devices

Thermal characterization of high-speed switching power transistors, such as high electron mobility transistors (HEMT), is critical for the evaluation of their performance as well as their long-term reliability. Unlike IR thermal imaging, thermoreflectance thermal imaging (TRI) uses LED lights in the visible range and therefore can be used to measure thermal response of these nanoscale devices under operating condition. However, TRI is also limited in terms of spatial resolution by optical diffraction as we reach device sizes on the order of hundreds of nanometer. We carried out a series of thermoreflectance thermal imaging experiments on the metal heater lines with widths ranging from 100 nm to 1 μm fabricated on InGaAs semiconductor film. Analytical and finite element numerical modeling are used to compare experimental data with theoretical temperature profiles. We demonstrate that thermoreflectance thermal imaging is capable of detecting temperature rise in devices with sub-diffraction feature sizes. We show that optical diffraction leads to underestimation of the magnitude of small scale hot spots. We also present a combined analytical-numerical model to reproduce the experimental results, and finally propose an approach that can be utilized to compensate for this diffraction artifact and acquire the correct temperature response from thermoreflectance thermal imaging results.

Transient thermal characterization of HEMT devices

Hetero junction Bipolar Transistor (HBT) and High Electron Mobility Transistor (HEMT) arrays are commonly used for RF and microwave high speed and high power applications. Thermoreflectance imaging can be utilized to understand the transient thermal characteristics of a GaN HEMT device. A time resolution of 50 ns clearly shows the thermal location-dependent time constants for the device, which could be used for further analysis of the thermal structure. An array of GaAs HEMT devices on a Monolithic Microwave Integrated Circuit (MMIC) is also characterized to gain an understanding of the local thermal resistance distribution in comparison to a finite element analysis. Since thermoreflectance is sensing the light reflectance of the surface, hotspots underneath opaque layer(s) is discussed to illustrate the utilization of this method for such devices.

Application of thermoreflectance imaging to identify defects in photovoltaic solar cells

Thermoreflectance imaging is used to identify various defects in solar cells with sub-micrometer spatial resolution. Lock-in transient and four-bucket imaging techniques in a megapixel silicon-based CCD are used to obtain the thermoreflectance and electroluminescence signals simultaneously. Linear and non-linear shunts are discovered in thin-film a-Si, poly-Si, and CdTe solar cells. Electroluminescent defects are found in poly-Si solar cells at reverse biases of 5V. Thermal images of micrometer-size defects are taken through 3mm of glass encapsulation.

Thermal characterization of high power transistor arrays

Thermal performance is an important factor in the design of power devices. Previous studies have shown that nonuniform temperature distributions occur in both small transistors [1,2,3] and in large area power transistor devices [4,5,6]. We present extensive thermal characterization of large scale transistor arrays that are typical in power applications. Thermal images using the thermoreflectance technique as well as thermocouple data are presented for the device under both low and high current conditions. Thermal characterization is obtained for load currents up to 4 amperes and current densities up to 45A/mm2 in the power arrays. Temperature nonuniformity in the arrays is studied as a function of array size, bias level, and ambient temperature. Increased heating is shown to develop near the source contact region in the arrays.

Side-by-side comparison between infrared and thermoreflectance imaging using a thermal test chip with embedded diode temperature sensors

A side-by-side comparison between thermoreflectance imaging (TR) and infrared (IR) imaging is made using a specially designed thermal test chip with an embedded diode sensor array. IR thermal imaging is commonly used in industry. However, due to the infrared wavelength and the diffraction limit, IR has limited spatial resolution for chip level thermal characterization. In this paper we compare the spatial, thermal, and temporal resolutions of IR and TR methods and verify the results with integrated diode temperature sensors in the test chip. Thermoreflectance imaging showed higher spatial resolution, temporal resolution, and temperature accuracy on the metal heater. Infrared imaging showed to be less accurate on the metal without any coating to improve the emissivity. The TR measurement on the diode was within 1.7% of the diode reading, while the IR measurement was within 6%.