Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Pixel-by-Pixel Calibration of a CCD Camera Based
Thermoreflectance Thermography System with
Nanometer Resolution
Mihai G. BURZO, Pavel L. KOMAROV and Peter E. RAAD*,
IEEE Conference Publishing
Southern Methodist University and TMX Scientific,
5232 Tennyson Pkwy, Bldg. 2, Plano, TX 75024, U.S.A.
Abstract- This work presents for the first time a method for
calibrating pixel-by-pixel and in-situ a CCD camera-based
thermoreflectance thermography system with nanometer
spatial resolution. Using the thermoreflectance method to
determine the temperature map of an activated device requires
two steps: first, the thermal image is acquired using a CCD
camera or laser-diode set-up and, second, the obtained thermal
image is converted to the actual temperature map by
multiplying each pixel of the thermal image with the
corresponding thermoreflectance coefficient. The critical aspect
is that even if the thermoreflectance coefficient is known, or
measured independently for each surface material, converting
the thermal image to the temperature map requires building
manually the exact corresponding map of the
thermoreflectance coefficient, which sometimes can be difficult.
In addition, the thermoreflectance coefficient is highly
dependent on the wavelength of the probing light, numerical
aperture, focus level, light uniformity, and other measurement
effects. To mitigate these issues, one must obtain the
thermoreflectance coefficient map of the same measurement
area of interest, while ensuring that the same objective lens is
used, the same focus level is maintained, and the same exact
position is kept for frame acquisition at both the low and high
temperature settings. To satisfy all of these requirements, the
position of the sample must be adjusted in 3D space with
nanometer spatial resolution.
I. INTRODUCTION
Thermal management of microelectronic microstructures
is becoming more and more crucial with the progress to
nanotechnology, increase in element power density and,
circuits being packaged closer and closer together. For large
devices and board level scales (few micrometers and above)
the technology today is at the point where anyone in need of
measuring the thermal behavior of electronic devices can
choose from an array of both scientific and commercial
measuring systems with each apparatus being best for certain
applications with most working well for most application.
Nonetheless when dealing with submicron and nanometer
scale devices the choice of thermal measurement systems is
limited with most, if not all systems, being still in R&D or
scientific/lab prototype phase at best. Among the available
technologies the thermoreflectance thermography method is
so far one of the methods that have been successfully
employed to make submicron temperature mappings [1-3]
but still has its limitations. The thermoreflectance method
has important advantages over contact and other optical
methods since it is non-contact and non-destructive, costeffective,
can produce both steady-state and transient surface
temperature, provides accurate results with excellent
submicron spatial and thermal resolutions.
Thermoreflectance microscopy is based on the physical
principle that a change in the temperature of a given surface
causes a small change in that surface reflectivity. To
measure the temperature change, ΔT, one needs to measure
the relative change in the reflectivity of the sample, ΔR/R,
and the small thermoreflectance calibration coefficient, C TR .
The thermoreflectance coefficient defines the rate of change
in the surface reflectivity as a function of the change in
surface temperature. The C TR coefficient is strongly
dependent on the material under test, the wavelength of the
probing laser [4, 5], and the composition of the sample [5].
For example, in the case of gold [6] the value of the C TR
coefficient changes from positive to zero to negative values
only by changing the wavelength from 400 to 600nm.
When using a CCD camera bases thermoreflectance
thermography system for thermal mapping of an active
electronic device, the investigator ends up with one 2D
surface map for the relative change in the reflectivity of the
sample, ΔR/R, and another 2D map of the C TR coefficient.
Combining the two maps yield the temperature map of the
device and at a quick glance it looks as a simple algebraic
task, i.e., dividing each pixel value of the ΔR/R map with the
corresponding pixel on the C TR map. Nevertheless, because
during the calibration procedure the sample itself and the
sample holder assembly is subject to thermal expansion that
causes movement in the microns to tenths of microns range,
and because the position of the sample during the
calibrations and the ΔR/R data acquisition needs to be
precisely the same, the procedure of combing the ΔR/R and
C TR maps becomes a not so trivial task.
©EDA Publishing/THERMINIC 2009 130
ISBN: 978-2-35500-010-2