Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Fig. 1 Thermoreflectance thermography system
The goal of this work is to present a thorough procedure
for pixel-by-pixel calibration of a CCD based
thermoreflectance thermography system, i.e., successfully
acquiring and combing the ΔR/R and CTR maps to obtain
the thermal map of the activated device. A brief introduction
to the measurement methodology is presented, followed by
the new calibration system and advantages of using the
pixel-by-pixel calibration. The power of the method is
demonstrated for a microelectronic CMOS technology
device supplied by TIMA and manufactured by Austria
Microsystems. The data are checked against the results of a
verified and validated numerical simulation. The temperature
data obtained using a built in diode are also discussed.
II. METHODOLOGY
The temperature measurements using the
thermoreflectance thermography system are carried out in
several stages. First, the thermal image (change in
reflectivity) of a device is acquired. In this step the changes
in the surface reflectivity ΔR/R as a function of the changes
in temperature are measured at each point of interest with
submicron spatial resolution. Second, the calibration map is
obtained using the procedure that will be described in the
next paragraph. In this step, the thermoreflectance
coefficient, C TR , is determined at each point on the surface of
the DUT. Finally, in the third step, the resulting C TR and
ΔR/R maps are combined to obtain the surface temperature
map of the DUT. Details about each step are provided next.
The schematic of the TRTG is shown in Fig. 1. The
system is capable of acquiring temperature fields with a
512×512 point frame resolution, at 10 to 30 frames per
second, and with up to 0.2 µm spatial resolution. In the first
stage (mapping) the DUT is activated (pulse modulated),
then the change in the reflectance ΔR/R of the DUT is
captured as a change in the intensity of the reflected light on
each element (pixel) of the CCD camera and the captured
image is acquired and processed using the NI Labview on
a PC. In the second stage (calibration), the reflectivity map
(intensity of the reflected light) of the device is acquired at a
low temperature (T L ) as I L and then at a high temperature
(T H ) as I H . The thermoreflectance coefficient is then
computed for each pixel as,
C TR = [(I H – I L ) / I L ] / (T H –T L ) (1)
Finally the temperature map of each pixel is computed as,
ΔT = (ΔR / R) / C TR (2)
For the pixel-by-pixel calibration method to work
properly, it is essential that both the focus and the horizontal
position of the sample be maintained during the entire
process of acquiring the three sets of images needed for the
final temperature map: (i) thermal image, (ii) low
temperature image (calibration), and (iii) high temperature
image (calibration). The thermal expansion present during
the calibration stage of the measurement causes not only z-
axis (out of focus) movement but also horizontal movement
of the sample.
One way of controlling the z-axis movement is to reduce
the movement of the top surface of the sample by
constructing a heating stage/sample holder system where the
thermal expansion is directed away from the top area of the
device, such as the one presented by Vairac et al. at
Therminic 2005 [7]. However, since the system is a passive
system there are still going to be uncontrolled differences in
the elevation of the sample due to thermal expansion. Vairac
et al. reported a 50 nm/K movement due to the thermal
expansion while using an innovative way of controlling the
thermal expansion. For the typical values of temperature
difference, ΔT, of 30-50K generally used for the
thermoreflectance calibration process, thermal expansion
©EDA Publishing/THERMINIC 2009 131
ISBN: 978-2-35500-010-2