Segmented infrared detectors
Mar 29, 2023
The infrared (IR) spectral range is a carrier of numerous pieces of information. For example, temperatures or the concentrations of various gases can be determined without contact on the basis of the detected irradiance. In the special case of gas sensors, several detectors with different, spectrally limited sensitivities are often required as a reference in order to compensate for ambient light and changes in the sensor system. The number of necessary measuring channels scales with the complexity of the gas mixture to be analyzed.
In the state of the art, multiple detection channels are usually achieved by parallelizing several individual channels - i.e. instead of one IR detector chip with an optical filter mounted on top, several individual chips and filters are hybridized and placed in a common housing. This approach is technically demanding, requires relatively large installation space and incurs high costs. Here, thermopile arrays can generate advantages, where several measurement channels are housed in the same chip. A particular advantage in terms of compactness and cost is that the sensitive pixels of such an array are accommodated on one and the same sensor membrane. However, the individual thermopile segments, which react to temperature differences with a voltage signal, must be thermally isolated from each other to prevent cross-talk between the channels. In the "TPA" project, the necessary technologies were developed for this purpose, with which the spanned sensor membranes, which are only a few micrometers thick, can be almost completely separated from one another by means of separation trenches. Thermal conduction between the channels is thus almost completely suppressed. Despite the fragile structures, it was possible to develop a process chain suitable for series production with excellent yields. First demonstrators of a 4-channel sensor were developed and evaluated within the project. However, the technology also allows other designs with significantly more channels. In further follow-up projects, research is currently being conducted into solutions for setting the individual spectral sensitivities of the individual channels directly on the chip, so that in the future it will also be possible to replace the optical filters that have been hybrid-mounted up to now.