1. Signal Processing Methods of Thermoreflectance Imaging
 Christopher Wegemer, Kerry Maize,
James Christofferson, Ali Shakouri
Jack Baskin School of Engineering
University of California – Santa Cruz
Thermal Imaging Results
Abstract
Heterodyne Signal Processing
The Queenstown embayment was divided into 8 separate basins based on the natural geographic shape of the creek. These basins were incorporated into the model.
The basic principle of heterodyning is the generation of new frequencies by combining two or more signals. For our application, a light source is used as a carrier signal, modulated at a frequency f2, and the device under test is modulated at a lower frequency f1.
f f1
Combining these two signals results in a waveform with sidebands at f2-f1 and f2+f1 where the signal can be recovered.
Using simple mathematical formulas, the camera can “lock in” to a specific frequency by oversampling at a rate of four times the desired beating frequency.
Using a heterodyne “stroboscope” method1, thermal images were obtained for different electronic devices both at high and low frequencies. Down sampling was used at high frequencies to capture the response of devices operating above the range of the camera, and up sampling was used to characterize large devices with low frequencies, thus reducing noise.
Motivation
Each sector had the following variables necessary for the functioning of the model.
As microelectronic devices rapidly get smaller, the need to characterize their performance becomes increasingly more difficult. Small devices that operate at high frequencies cannot be directly imaged by practical means. By utilizing a simple signal processing technique, the device can be operated in the MHz range while the signal is extracted at a tunable beating frequency within the limit of a basic thermal camera. Larger devices that take a significant amount of time to heat and cool can use the same technique to be viewed at a higher frequency, thus eliminating the 1/f noise…..
Future Work
To increase the accuracy of concentration approximation, tidal delay between each sector was incorporated into the model. Because of this, the rate of change of concentration in each sector could not be governed by a single equation. For each type of sector, a different design was required.
1. Ensure the correct functioning of the model
2. Test the model against data from Queenstown
3. Draw conclusions and formalize results for publishing
Thermoreflectance Theory
Design applied to end sectors 1, 7, and 8
Design applied to middle sector 3
Design applied to sector 2
This sector was the one where dye was injected, as shown.
Every material reflects and absorbs certain amounts of different wavelengths of light. When the material heats, the “reflectivity” also changes. For small temperature variations (above extremely low temperatures), the relationship can be considered linear according to the equation:
Thus, if the reflectivity constant is known, the change in temperature can be measured by measuring the change in reflectivity. With a sensitive CCD camera, a temperature resolution of 10 mK can be achieved with nano-scale spatial pixels.
References
1. Grauby, S., Forget, B.C., Hole, S., Fournier, D. High resolution
photothermal imaging of high frequency phenomena using a visible charge
coupled device camera associated with a multichannel lock-in scheme, Review
of Scientific Instruments, Volume 70, number 9.
Design applied to sector 6
This sector was the one connected to the outer bay.
Design applied to central sectors 4 and 5