GaN-based devices are devices based on new materials for next-generation satellite communications and radars, making them much faster and with more power than is presently possible. Knowing the temperature these devices operate at determines not only how well a device is working, but also how long it will last. To progress this work, one of the key questions being asked by the semiconductor community is how to image temperature on a length scale that is a hundred times smaller than the diameter of a human hair?

Semiconductor devices used in many of today's equipment have become smaller and smaller such that techniques previously used to measure temperature for these devices do not work anymore. Who, for example, wants a communication device in an aircraft to fail while flying? The Bristol University team have developed a unique piece of equipment which allows them to image temperature with sub-micron spatial resolution that was previously not possible. This is of great benefit not only for semiconductor technology, but further spin offs may arise in biology. How, for example, do small organs in very small animals work? That kind of question may also be addressed with the new device.

Bristol University and Quantum Focus set to revolutionise thermal imaging, by Alan Mills, III-Vs Review

The benefits of a micro-Raman analytical technique to determine the temperature of active, GaNbased, III-V devices developed by Martin Kuball et al at UK's University of Bristol, were reported in III-Vs Review a couple of years ago. This method uses the phonon frequency dependency with temperature in micro- Raman spectra, to measure the temperature of sub-micron regions of an operating device, such as a FET or HEMT gate region. Where these regions are accessible to microscopic examination, micro-Raman spectra allow the device surface temperature to be estimated to within about 5°C. The excitation source is a one-micron laser spot from a 488nm argon laser (3mW power). It provides insignificant heating of the device chips, which are transparent at this wavelength.

The use of micro-Raman spectra is a useful noninvasive technique with which to analyse thermal management and optimise design and reliability of developmental chips and commercial ICs, where junction temperature is often a critical factor for device reliability characteristics. The technique has already been used to compare the temperature resistance effects of different HFET finger layouts, MOCVD grown, on both sapphire and silicon carbide substrates, where devices on SiC substrates exhibited up to five times less thermal resistance than on sapphire.

System benefits

The system (top right) for prototype is easy to align and to operate, as well as being adaptable for manufacturing and testing environments. Raman point temperature measurements are quite rapid, taking only a few seconds to complete. However, the one disadvantage of Raman thermal imaging is its slow speed, which basically limits it to the measurement of smaller device areas (a benefit in this case). The creation of device maps takes longer, since they are built up by moving the laser beam over the sample surface from point to point.

In the initial report, a FET gate region was shown to be operating in the 170 to 225°C range, with graphical colour representation, at drain source voltages of up to 20V and sub-micron resolution. However, for the device shown (bottom left), the Raman spectrum provides another benefit of the system, the effect of a defect on local device heating can be demonstrated.

Such an accurate and non-destructive method of measuring operating device temperatures is a very important tool for power semiconductor device development. Especially compound semiconductor FETs and ICs based on AlGaN, GaN or SiC, because of the high power densities they can develop (eg. 9W/mm at X-band, or about 10 times the W/mm possible from gallium arsenide.) Additionally, because the wide gap devices can operate at much higher gate temperatures (300° C and higher) than silicon ICs, they require efficient thermal management, including the use of copper or other heat sinks for the final device package. However, an important side benefit from the future use of these wide band power devices is that they are more efficient, hence ambient cooling will be sufficient, and therefore costly air conditioning requirements for wireless base stations can be eliminated.

Thermal imaging microscope

The Bristol group has continued to develop the technique, supported in part by a three-year grant from EPSRC and DSTL. Recently, armed with the protection of a patent application, the University has announced its collaboration with Quantum Focus Instruments (QFI), located in Vista California, to develop a commercial, high spatial resolution, thermal imaging microscope. A combination of a wide field of view IR microscope (for rapid pre-alignment) and the high spatial resolution available from the Raman spectra is expected to result in a very useful analytical tool. It will soon be available to the semiconductor industry in the form of a relatively simple piece of equipment to operate and an analytical technique suitable for all accessible power device areas.

Users include QinetiQ

The final design is expected to include a QFI infrared camera combined with Renishaw's Raman microscope, with custom software to interpret the results from the two mechanical systems, for both research and development, and manufacturing environments.

Although the research programme is only at about the halfway stage, QinetiQ Ltd, UK, has already used the prototype system to measure useful thermal characteristics of their state of the art HFETs.