technology IR microscopy
Figure 6 The temperature of the junction was monitored with increasing DC power to the diode and compared with a reading from miniature (50 µm diameter) thermocouple bonded onto the package next to the diode
microscope and any radiation in the 2 to 5 µm band emitted by the LED will be seen as weak, out-of-focus background radiation. Thermal mapping of the front-face of the LED will provide an improved measurement of the maximum junction temperature and will assist in identifying hotspots and therefore potential failure points.
Our tool also outperforms the conventional IR microscope in delivering a more accurate temperature profile of a MEMS micro-heater. While conventional IR measurements mistakenly record lower temperatures on both the gold metal heater and the optically transparent silicon dioxide layer, our technique offers a more realistic thermal profile, with an exponential fall-off in temperature recorded from the heater element to the cooler region of the device.
Validation of the measurement (Figure 7) was carried out by knowing the thermal coefficient of the resistor heater. This enabled plotting the average temperature of the heater and comparing this with the micro-particle sensor measurement (Figure 8) at a single point on the heater and over a range of DC input powers. These measurements have enabled us to make more accurate thermal maps of the sensor region of MEMS devices used as high performance gas sensors.
The metal surface and delicate nature of the heated membrane precludes Raman thermography and thermal probing using a miniature thermocouple. We have made similar measurements to assist in the optimisation of thermal heaters used in electron microscopy. We believe this work will pioneer the way forward in the
Figure 8: Comparison between micro-particle at a single point and electrical measurement on a MEMS device
development of pseudo-contactless micro-sensors which can be positioned and used to scan across the surface of a structure to enable 2D temperature mapping. The sensors described are already much smaller than miniature thermocouples, which inherit the thermal mass of external connections to measurement equipment and the problems of attachment to the device. The essence of the technique may enable the development of moveable nano-scale thermal sensors to temperature probe the coming generations of nano-scale devices.
The authors acknowledge EPSRC (EP/C511085/1) and emda for partially funding this work. They also thank many organisations for samples, including Bristol, Sheffield, Cambridge Universities ,e2v (Lincoln), Silson Ltd, and the National Physical Laboratory (NPL).
© 2011 Angel Business Communications. Permission required.
Further reading P.W Webb IEE Proc, 138, 390 (1981) C. H. Oxley, et al SSE, 54 63 (2010) C. H. Oxley et al KTN workshop on nanotechnology in European Semiconductor Conference in Cardiff, 2009. R. Hopper et al Measurement and Science Technology 21 045107 (2010)
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www.compoundsemiconductor.net January / February 2011
Figure 7: Comparison between micro-particle and conventional IR measurement on a MEMS device
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