IR microscopy technology
The method is complementary to Raman thermography, as temperature measurements can be extended across metal regions. We have found that the peak surface temperature measurements on a TLM AlGaN/GaN heterostructure agreed well with Raman measurements made on the same device under similar conditions.
Figure 4: Peak temperature rise in the channel region of an AlGaN/GaN HEMT measured using micro- particle and conventional IR techniques. Conventional IR results were measured at Position 2
A tiny probe
At De Montfort University, which is based in Leicester, UK, we have developed a novel IR micro-particle sensor technology that overcomes the problems of performing IR temperature measurements on low emissivity and highly transparent materials. The IR micro-particle carbon-based sensor, which has a high and known emissivity, is placed in isothermal contact with the surface of the device. The IR radiation emitted by the micro-particle sensor is collected by the microscope (Figure 1) and used to obtain a more accurate indication of the surface temperature, which is now independent of the material properties of the device under test.
The micro-particle sensor heats up very rapidly, enabling temperature measurements to be made without resorting to lengthy acquisition times. Three-dimensional thermal calculations indicate that the thermal time constant for a 3 µm-diameter micro-particle sensor is in the microsecond region, which is three orders of magnitude less than the millisecond sampling rate of the IR microscope. The size of the micro-particle has very little impact on the level of emitted radiation density, so long as its diameter exceeds approximately 8 µm. If the particle is smaller, the radiation level is lower – it is 25 percent less when the particle’s diameter is 3 µm. This fall in intensity probably stems from the combination of the onset of the diffraction limit of the microscope and quantum-like effects within the micro- particle.
We have directly calibrated our micro-particle sensor by measuring its emission over a range of temperatures (Figure 2 shows a typical calibration curve for a 10 µm diameter IR micro-particle sensor). The micro-particle sensor can be viewed as a “pseudo contact-less thermal probe” that can be moved in essence by controlled steps across the front-face of a device (metal and semiconductor areas) to build up its surface temperature profile. The device being measured remains in a circuit/package configuration required for the application.
Our IR microscope with a micro-particle probe has mapped the temperature profile of various devices. This includes the channel region of an AlGaN/GaN HEMT (see Figure 3), which was biased with a drain-source voltage of 10 V and a gate-source voltage ranging from 0 to –5 V. The results obtained by this method have been compared with those produced by conventional IR temperature measurements (see Figure 4).
The technique shows the potential of being able to map the surface temperature inside the 5 µm channel region without having to coat the device with a high emissivity coating. The problems in uniformly coating such a small channel will be huge, with non-uniformity giving rise to anomalous temperature measurements. To make matters worse, the coating may substantially change performance of the transistor. Further, the micro-sensor can be removed whereas the coating cannot.
Another class of device we have studied is a white- emitting, AlGaN phosphor LED. This device produces strong optical emission in the 0.4 - 0.8 µm spectral range, and a very weak output in the 2 - 5 µm IR waveband. Conventional IR is unsuited to this type of measurement, because the phosphor material is semi-transparent to IR radiation. Turning to our ‘micro-particle’ sensor approach (see Figure 5) sidesteps this issue, providing a direct measurement of the LED’s surface temperature (see Figure 6).
Our approach measurement offers the feasibility of thermal mapping the front-face of the LED, as the measured emission in the 2 to 5 µm waveband will be maximised by the high emissivity ‘micro-particle’ sensor. Also, we believe this radiation will be focused by our
Figure 5: Micro-particle placed on the junction area of the diode(Inset) Optical image of the diode, the lens has been removed for access to the diode.
January / February 2011
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