What is Thermoreflectance? - Alenas Imaging
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Thermal imaging using visible light:
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The brightness of an optical reflection can depend slightly on the temperature of the mirror. But the effect is very small, and requires advanced signal processing to be detectable. |
Thermal mapping is a powerful metrology tool that enables mapping of electrical current distribution, optical power, carrier density, and subsurface material defects in addition to the obvious applications to thermal management. Many techniques are known for mapping the temperature of a surface, including thermocouples, fluorescent paints, or sensing of infrared emissions. However, only thermoreflectance offers non-contact, sub-micron thermal imaging.
Thermoreflectance depends on the fact that the optical reflectance of most materials depends slightly on temperature. This can be measured at a convenient wavelength, whether the material is transparent or opaque. Test materials or devices are illuminated by an external light source, and the reflected light is imaged onto a camera. The resulting reflectivity map is used to extract a spatially resolved image of the thermal distribution.
But reflectance variations with temperature are usually very small; about 0.01% per °K for silicon at 600 nm. To record faint thermoreflectance images, advanced signal processing methods based on lock-in averaging are required. Alenas Imaging uses patented stochastic resonanc-enhanced, pixel-by-pixel lock-in amplification techniques to extend the dynamic range of CCD cameras by as much as 44 dB, making faint thermoreflectance images visible.
The advantages of thermoreflectance are:
- Probe wavelength, typically in the visible range, can be chosen for convenience or best sensitivity
- Does not use infrared and works well with low emissivity materials such as metals or semiconductors
- In microcopy mode, thermoreflectance is the only known method to get thermal mapping with 250 nm resolution
- Temperature resolution as small as 10 mK