Thermal and Optical Characterization Techniques
One prime aim of the CDTR is to develop and apply novel thermal characterization techniques for semiconductor materials and devices, as any dissipated heat during device operation results in elevated device temperature, which accelerates device failure. There is typically an exponential relationship between the so-called mean-time-to-failure (MTTF - the time when half of the tested devices would have failed) and the exponential of one over temperature. Detailed knowledge of the device temperature, for example the temperature near the gate contact of a transistor, is therefore an important element in predicting device lifetime. This is done by so-called accelerated lifetime testing, where MTTF is measured as a function of baseplate temperature, where the devices are operated on a hot plate.
While measuring temperature sounds easy with the plentiful availability of thermometers, it is not so simple in the case of semiconductor devices. The critical device dimensions are on the scale of microns - or even smaller. Switching times of the devices can be a millisecond, a nano-second or even faster. With power densities in semiconductor devices reaching levels approaching of those on the surface of the sun, managing device temperature has certainly become a prime challenge in ensuring device reliability.
To enable temperature measurement at sub-micron spatial and nanosecond time resolution, we pioneered the technique of Raman thermography. This technique is now regarded as the gold standard of thermal analysis for semiconductor electronics and is used widely in academia and industry for reliability testing. Our work, identifying how GaN-SiC interfaces can be grown to minimize channel temperatures in GaN transistor devices, was critical to the successful development of reliable devices. We also developed a large variety of further thermal characterization techniques, targeted for other applications like wafer mapping. These applications determine thermal conductivity and thermal boundary resistances between interfaces.
Our transient thermoreflectance mapping technique for semiconductor wafers enables the screening of wafers before any device fabrication takes place, allowing the prediction of device temperature before fabrication. This has proved very useful for the optimization of GaN-on-diamond interfaces, modifying seeding layer and diamond growth conditions, and for the development of ultra-high power microwave devices. We have also developed a 3-omega measurement setup, which can operate at much higher than usual frequencies with low noise, allowing us to study ultra-high thermal conductivity materials, such as diamond, with high accuracy.
We have recently started a spin-off company commercialising transient thermal reflectance: www.thermapsolutions.com.
Figure 1. Temperature rise in GaN HEMT determined by Raman Thermography
Figure 2. Transient reflectance thermal characterization of wafers and devices
Figure 3. 3-omega method for determination of thermal conductivities