technology IR microscopy
Emissivity issues This inherent weakness, a diffraction-limited spatial resolution, is impossible to address. However, there are steps that can be taken to tackle the other drawback of IR microscopy - poor accuracy of temperature measurements. This problem stems from uncertainties associated with the emissivity of the local surface (emissivity is a measure of how efficient the surface is at emitting radiation, and is highest for a perfect blackbody).
Figure 2: Radiance vs. temperature calibration curve (10 µm diameter micro-particle)
can extract material temperature from the shifts in the wavelength of monochromatic light interacting with crystal vibrations. The great strength of this technique is its high spatial resolution – sub-micron (~500 nm) measurements are possible. However, building-up a temperature profile of the device demands raster scanning of the monochromatic source, a laser spot, across the chip’s semiconductor surface.
Obtaining a good temperature map takes a long time, because Raman signals are notoriously weak. Individual measurements require several seconds or more, and mapping out the temperature of an entire device can be impractical. Further, if temperature profiles are required across gold contact and semiconductor areas, then measurements have to be made from the back surface of the device.
While faster measurements are possible by turning to infrared (IR) microscopy, the technique has two major drawbacks: inferior spatial resolution and greater uncertainty in the local temperature. Like Raman thermography, diffraction defines the fundamental limit of the spatial resolution. However, while Raman thermography often uses excitation from a 532 nm argon ion laser or 632 nm HeNe laser, IR emissions are passive and the microscope collects radiation typically spanning the 2-5 µm range.
The conventional approach for catering for variations in emissivity begins by placing the device on a heated stage under the IR microscope objective and bringing it up it to a known temperature, which is measured by a calibrated thermocouple.
An IR microscope collects radiation from different parts of the heated electronic device. By knowing the radiation emitted from a blackbody at the same temperature and over an identical range of wavelengths, it is possible to compute the surface emissivity across the device.
The next step is to power up the device, measure the radiation it emits, and then calculate its temperature profile using known emissivity values. With this approach, hot surface areas can be identified very quickly.
However, the accuracy of these temperature measurements relies on accurately knowing surface emissivity, which can be a challenge. Materials employed in many III-Vs, including nitride devices, have low emissivity, high reflectance and/or high transparency to infrared radiation.
For example, gold, which in many instances is used for contacts and interconnections, has an incredibly low emissivity (it is about 2 percent of that of a blackbody) and strongly reflects background radiation, which interferes with the surface emissivity measurement. The upshot is an ‘apparent’ higher measured surface emissivity.
Another issue is that semiconductor materials have different degrees of transparency to IR radiation. This means that radiation is not just collected from the front surface – it can also come from material interfaces and the back surface. The type of bond (eutectic, epoxy etc) to the package tends to govern the intensity of radiation stemming from the back surface.
Figure 3: Imaging showing the position of the micro-particle sensor on the HEMT
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www.compoundsemiconductor.net January / February 2011
The interfering IR radiation from these sub-layers also gives rise to an apparently higher surface emissivity, leading to subsequent temperature calculations that are lower than the actual temperature.
Traditionally, this problem is addressed by coating the device with a high emissivity coating, but this can visually obscure the device and cause heat spreading. In addition, spatial temperature resolution suffers, and there is also a greater likelihood of device damage.
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