Infrared imaging methods provide unequaled ease and flexibility
for investigating micro-circuit temperatures. The researcher need
only slide the circuit under the objective lens and power it up
in order to measure the temperatures throughout the micro-circuit
at once, precisely and without physical contact. With the infrared
instruments available today, workers can measure temperature differences
as fine as 20 to 40 milliKelvin (equal to milli-degrees Celsius)
and see spatial features as fine as a few microns. Infrared measurements
are not only accurate but also the fastest available and offer the
least disturbance to the point being measured. In terms of human
bandwidth, the amount of information conveyed is astounding. The
researcher literally gets the entire picture of how the heat is
distributed over the part and how it is changing in real time. Infrared
imaging has a decided advantage for navigating under the microscope,
compared to methods which require the worker to paint an obscuring
liquid crystal or fluorescent material onto the circuit.
Micro-circuits present special problems for infrared temperature
measurement. They use liberal amounts of metal which makes them
very reflective. This reduces their infrared emissions and contaminates
their signature with stray emissions reflected from elsewhere in
the environment. However, through consistent effort and clever application
of the laws of radiation physics, these problems have largely been
solved. Practical thermal imaging microscopes are now available
with point and click operation. In this article, we discuss the
fundamentals of infrared physics as they apply to measuring micro-circuit
temperatures and we present example images from a typical measurement.
Every object whose temperature is not absolute zero emits thermal
radiation. A fireplace poker or a light bulb filament may not be
hot enough to visibly glow, but most of the energy emitted by objects
within 50 degrees C of room temperature is emitted within the 3
to 14 µm wavelength region. The fundamental description of
thermal radiation is the Planck blackbody radiation law:
|| is in photons per second per cubic centimeter and gives the
thermal photons distributed over wavelength, emitted into a
hemisphere from a given source area
||is the velocity of light, 3 x 1010 cm/s
||is Planck's constant, 6.63 x 10-34 watts/s2
||Boltzmann's constant, 1.38 x 10-23 watts s / °Kelvin
||is the wavelength in centimeters
||is the source temperature in degrees Kelvin.
Infrared microscopy makes use of this radiation by using detector
arrays, such as Indium Antimonide and Mercury Cadmium Telluride,
which are sensitive to infrared wavelengths. These detector materials
are available in the form of charged coupled device (CCD) arrays
which are two dimensional arrays, much like those used in video
cameras. An infrared microscope is constructed by assembling such
an array with appropriate infrared optics. A personal computer is
added to calculate the device temperature based on the flux received
by the detector. The detector material is usually cooled to liquid
nitrogen temperatures. Ordinary glass lenses are opaque in the mid
to longwave infrared, so infrared microscopes as well as military
thermal imagers use exotic lens materials such as silicon, zinc
selenide, or germanium. Indeed, the number of practical IR lens
materials is limited to just 6 or 8 which are not terribly expensive,
toxic or soluble in water. This compares poorly with the hundreds
of optical glasses available for visible wavelengths, which offer
the infrared designer some advantages, such as higher indices of
refraction, and it is not too difficult to obtain infrared optics.
The Concept of Emissivity
Planck's law was derived for a "blackbody" - a source
which by geometry or material is perfectly efficient in absorbing
radiation and is hence "black". The blackbody does not
reflect light so any radiation emanating from such a source is derived
solely from the thermal properties of that source. Unfortunately,
microcircuits are made of materials which make notoriously poor
infrared sources. To apply Planck's law for temperature measurement,
we must introduce the concept of emissivity. The emissivity
of a source is the ratio of its actual radiant emittance to that
of a perfect blackbody at the same temperature.
Sources which are not perfect are known as gray-bodies or selective
emitters. A gray-body is one which emits radiation in exactly the
same spectral distribution as a blackbody at the same temperature,
but with reduced intensity. A selective emitter is one in which
varies with wavelength.
Silicon is a selective emitter but, fortunately, over a small spectral
bandwidth, it behaves like a gray-body, permitting us to use classic
Planck mathematics to infer its temperature, if we know its emissivity.
Figure 1 shows a map of the emissivity of a typical micro-circuit.
The metal layers are clearly visible and the color scale shows that
the metal layers have about half the emissivity of the doped silicon.
Figure 1 - Emissivity map for a
What to do about Reflected Light?
When radiant energy is incident on a surface, three things can
happen: a fraction of the energy a may be absorbed; a fraction r
may be reflected; and a fraction
may be transmitted through the part. Since energy is conserved the
sum of these fractions must equal 1:
+ r +
= 1 Equation 3
Good absorbers are good emitters and empirically it turns out that
a material's absorbivity equals its emissivity.
Using this, we can restate Kirchoff's law as:
+ r +
= 1 Equation 4
Any part which is a poor absorber or emitter will be a good reflector
or a good transmitter. When examining a part with an infrared microscope,
some of the detected radiation inevitably comes from elsewhere,
either passing through the part or being reflected from it. This
is certainly true of integrated circuits. They are fabricated chiefly
from silicon and metal which have particularly low emissivity.
Compensating for Low Emissivities
When emissivities are very low, the instrument is as likely to
measure the temperature of the room as it is the part being tested.
What the instrument actually measures is three blackbody functions,
one for each of the emitted, reflected and transmitted component
as in equation 5.
is the integrated photon flux seen by the detector. The emitted
radiation is characteristic of Ts, the source temperature,
and the reflected and transmitted components are likely to be characteristic
of the ambient surround at temperature Ta. A typical
may be 10% for a metal layer on a micro-circuit, so that if the
part temperature and ambient temperature are close, the instrument
will detect mostly radiation from the room. Fortunately, the flux
increases approximately to the 3rd power of temperature so the researcher
can make the first term dominate by raising the temperature of the
part to be measured substantially above the room ambient. One method
of accomplishing this is to mount the part on a thermoelectric stage
and to elevate the base temperature to 70°C or more. This will
effectively mask any contribution from the ambient.
Pixel by Pixel Emissivity Correction
Most infrared radiometers allow no more than a single user estimate
for the target emissivity and they employ this single input for
all pixels in the image. Microcircuits have naturally low emissivity
and often 2 to 1 variation in emissivity between metal layers and
silicon so this could lead to serious errors. It is therefore more
accurate to measure emissivity at each pixel location. Since the
device is mounted on an opaque substage, we can assume that the
optical transmission of light through the device is zero.
Equation 1, when integrated over the detector's sensitive wavelengths,
can be rewritten as:
and again for the source at a different temperature
||is the radiance measured by the detector with the source at
a known temperature.
||is the radiance corresponding to source at a known temperature
and at emissivity
(as yet unknown) and
||is the contribution from the room ambient.
|| in these cases is the photon flux integrated over the sensitive
wavelengths of the detector N = Q(T,)d
By subtracting the two equations the ambient term drops out and
solving for :
The emissivity for each pixel is the difference between the measured
radiance for each temperature divided by the difference between
the calculated radiance at each temperature for a perfect blackbody.
By fixing the part firmly to a thermo-electrically driven substage
and measuring the radiance of the unpowered device at 2 temperatures,
we can calculate the emissivity of the circuit to a high accuracy
for each pixel.
All microscopes have a resolution limit proportional to the wavelength
and infrared microscopes utilize long wavelengths. Infrared inspection
will never offer a resolution approaching the submicron feature
size of the latest circuits. The limit of resolution for a microscope
is given by the Sparrow Criteria, which is more aggressive than
the better known Rayleigh Criteria.
d is the minimum separation between features to be resolved
is the wavelength of light
N.A. is the numerical aperture of the microscope objective.
Without oil immersion, the numerical aperture is theoretically
limited to 1, and practically limited to 0.95. In the semiconductor
industry, long working distances between the objective and the integrated
circuit further limits the N.A. It is convenient to assume an N.A.
of 0.5 for which the Sparrow Criteria is the same as the wavelength.
The Sparrow Criteria is an actual physical limit, which is independent
of magnification. Additional magnification makes the blurred area
appear larger, not less blurred. The current generation of infrared
microscopes operates at about 5 µm wavelength, which limits
the feature resolution to about the same dimension. Research is
under way to develop instruments which operate at 2 to 3 µm,
but they are not yet available. An application of Planck's law for
wavelengths shorter than 1 µm, suggests that there is too little
thermal energy at short wavelengths to make such a system practical.
Current state-of-the-art microthermal imaging microscopes operate
between 1.5 to 5.5 microns wavelength with a resolution of about
5 microns and a sensitivity of about 100 milli-Kelvin. Within those
limits there is a lot of information available in a microthermal
image, as can be seen in the following examples.
The following example images and the earlier emissivity image were
acquired using the EDO Barnes' Infrascope which is capable of resolving
5 micron features and has a sensitivity of about 0.1°C. Figure
2 shows a sequence of 4 images. The two radiance images were taken
at different substage temperatures and used to calculate the emissivity
image, also shown. The fourth image was taken with the part powered
up and it employs both the emissivity correction and a correction
to remove the background ambient. The temperature image is reproduced
in Figure 3. The instrument captured 5 separate images to produce
this result, all automated and the result of a single mouse click.
It took approximately 10 minutes to mount the previously delidded
part, and 5 minutes to set up and capture the images.
Figure 2 - Suite of images from
Microthermal Imaging System showing a sequence of two radiance images
to find emissivity, followed by power-on-the-IC temperature range.
Thanks to EDO Barnes Engineering Division, Shelton CT, for the
use of their Infrascope to make these images.
Latigo Optics, 2144 Latigo Canyon Road, Malibu, CA 90265
Tel: +1 (310) 589-1510 Fax: +1 (310) 457 2672
EDO/Barnes Engineering Division
88 Long Hill Cross Roads, Shelton, CT 06484
Tel: +1 (203) 926 4119 Fax: +1 (203) 926 1778
1. Levine, M.A., Solid State Cooling with Thermoelectrics,
Electronic Packaging & Production, Nov. 1989.
2. Melcor Corporation, Thermoelectric Handbook, Sept., 1995.
3. Rowe, D. M., CRC Handbook of Thermoelectrics, CRC Press, Inc.,
4. Smythe, Robert, Thermoelectric coolers take the heat out of today
s hot chips, Electronic Products, Aug. 1995.
For Additional Information
1. Richard D. Hudson, Jr., Infrared System Engineering,
Wiley, 1969, ISBN 471418501.
2. Warren J. Smith, Modern Optical Engineering, McGraw-Hill,1990