Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Fig. 4 Thermoreflectance thermography results for a polysilicon
resistor: left – calibration map, C TR x 10 -4 (1/K) at 460 nm
wavelength of probing light; right – contours
of temperature change, ΔT, °C
Fig. 3 Thermoreflectance thermography results for a polysilicon
microresistor: top – image of the DUT (light area is C-shaped poly
resistor; bottom – 2D map of the relative change in reflectivity,
ΔR/R × 10 4 , induced by the change in temperature ΔT, at 460nm
wavelength of probing light
could be more appropriate. In Fig. 2, the maximum value of
the C TR is obtained at 485 nm for gold and 640 nm for
polysilicon. However, in the case of the device containing
gold and oxide covered polysilicon, as evident from Fig. 2,
there is a large spectrum of wavelengths that will offer very
good C TR values for both materials, one of such wavelengths
being 485 nm. It is also worth mentioning that if one desires
to obtain the absolute best measurement uncertainties the
better way to proceed is to use the zone-by-zone calibration
and measure and calibrate twice at 485 nm (for best Au
region results) and 640 nm (for best polysilicon results) and
then combine the results to obtain the temperature change for
the device under test.
B. Temperature Results for a Representative Device
The temperature map of a polysilicon microresistor,
shown in the top part of Fig. 3, was measured and calibrated
and the results are presented here. The temperature was
probed through a layer of both field oxide and passivation
layer with a probing light wavelength of 460 nm. The value
of 460 nm was chosen because it produces good results for
all of the probed regions of the device.
The device was pulse modulated and a differential scheme
was used to obtain the map of relative change in reflectivity
of the device, ΔR/R. The ΔR/R results are presented on the
bottom part of Fig. 3. The electrical power applied to the
device was 250 mW, and 1,000 frames were averaged at a
mean frame rate of 10 frames/sec. As evident from Fig. 3
the value of the C TR coefficient is negative for the poly
region and positive for the rest of the materials. In other
words the device temperature increase caused by the Joule
heating produces a decrease in the reflectivity of the poly
region and an increase in reflectivity for the rest of the
regions.
Next, the calibration map of the device was obtained by
measuring the change in the reflectivity of the device at the
low temperature (T L = 30°C) and at the high temperature (T H
= 50°C) and then the C TR coefficient was computed using (1)
and the results are presented on the left side of Fig. 4. As
expected based on the results shown in Fig. 3 the
thermoreflectance coefficient of the poly is negative (-1.15 ×
10 -4 K -1 ) while its value is positive for the region containing
the diode leads with a value of 1.46 × 10 -4 K -1 .
Finally, the temperature map is obtained based on (2)
using the ΔR/R map (shown in the bottom side of Fig. 3) and
the thermoreflectance coefficient (shown in the left side of
Fig. 4) and the temperature change results are shown in the
right side of Fig. 4. The results indicate that the corners of
the poly-resistor are slightly cooler than the rest of the poly.
That is probably caused not only by the way that the device
cools but also the path that the current takes through the
resistor. The data shown in Fig. 4 also shows that the area
found at the center of the device, occupied by the diode,
appears to be much cooler than the microresistor area. More
on this aspect will be presented in the next section.
©EDA Publishing/THERMINIC 2009 133
ISBN: 978-2-35500-010-2