Infrared-sensitive thermopile sensors are used as detectors in contactless temperature measurement systems and optical gas sensors, for example in medical technology, process monitoring and environmental sensor technology. Compared to infrared-sensitive semiconductor diodes, thermopiles can detect a significantly broader spectral range and can be produced cost-effectively using silicon technology.
In the infrared (IR) light spectrum, silicon itself can no longer be used as a light absorber and signal transmitter, as it is transparent over other spectral ranges. Additional coatings are therefore applied and structured. Here, the absorbed light leads to temperature changes. This measuring principle, which is based on local temperature changes, is susceptible to interference from other thermal influences. If, for example, a hand-held temperature measuring device (thermometer) is held in a warm hand, this heat input contributes to a drifting of the sensor signals. This can be largely compensated for by additional temperature sensors, but the complexity, design and achievable accuracy are negatively affected. The aim was therefore to develop an IR radiation sensor that is in principle independent of non-radiative heat inputs – i.e. interfering heat sources or sinks in the system or its environment.
The usual design consists of a silicon-based carrier frame and a membrane that is usually only a few micrometers thick. Due to the low thermal mass of this membrane and the low heat conduction losses, even small amounts of thermal radiation cause significant temperature differences between the illuminated membrane and the (cold) carrier frame.
With thermopile sensors, this temperature difference is recorded as a measurable electrical thermoelectric voltage. The output signal is proportional to the temperature differences. If the frame is now heated by heat input from the environment, the temperature change distorts the measured result of the detected radiation on the membrane areas.
A new approach was tested in the project. The cold ends of the thermopiles were not placed on the frame of the sensor component as before, but also on the membrane. In order to still achieve a temperature difference under illumination, the cold zone of the membrane was shielded from incoming IR radiation by a reflective metal coating. Using a suitable sensor design, thermal changes in the frame can be largely suppressed even under asymmetric thermal disturbance.
As a further innovation, the membranes were produced from the front side of the components – unlike usual. Usually, the silicon substrate is removed locally from the back of the chip, leaving a relatively wide chip frame in the process. Removing the front side of the chip allows the chip size to be reduced while maintaining the same active area, increasing the yield of chips per wafer and thus reducing costs. This also reduces the risk of breakage of the fragile membrane structures in subsequent processing steps and simplifies the assembly and housing options for the components.
The research and development work in the project “NIVo – Novel isothermal sensor with front side cavities” was funded by the Federal Ministry of Economic Affairs and Climate Action.
Funding code: 49MF200129
