Miniature Thermopiles

As a thermal detector, the thermopile is sensitive across the entire wavelength range (i.e. from X-ray to far infrared radiation). The only requirement is that the radiation be absorbed in order to heat the detector. This effect is improved by the application of a black layer, and the spectral sensitivity is flat from the UV to the far IR range (exception: ST series). However, application is most widespread in the infrared spectral range.

The thermopile is a DC detector. That is to say, it reacts to DC light and is immune to microphony. The noise is frequency independent; it is determined by the thermal noise of the detector resistance (Johnson noise). Typical applications include non-contact temperature measurement, gas measurement, and power measurement (at low power levels).

The Working Principle

The thermopile is a thermal detector that is based on the Seebeck effect. Its predecessor was the thermocouple, which consisted of a junction between two metal wires with different thermoelectric voltage values. This effect was discovered in 1821. Both metal wires produce a voltage proportional to their temperature difference (i.e. it is always measured relatively). Through a series connection of several pairs, the sensitivity can be increased. The production of thin film thermopiles using overlapping antimony and bismuth films has been around since 1934.

Setup

The traditional Dexter thermopiles (1M, 2M, and the like) consist of antimony and bismuth contact pairs in which the radiation-sensitive "hot" contacts are applied to a blackened film. The "cold" reference contacts are located on a massive ceramic substrate, which acts as a heat sink. With this arrangement the best D* is achieved in thermopiles. Limited by manufacturing technology, one has to accept parameter variations of approx. 30% in series production.

S-line detectors are based on phosphorous-doped polysilicon and gold contacts that have a significantly higher thermal power than antimony and bismuth contacts. That is to say, they are more sensitive (by an approximate factor of 5). However, this comes at the expense of greater resistance and, therefore, also more noise. On the one hand, it makes sense to use only small detectors; in fact, the largest Si detector is currently 1.5 mm x 1.5 mm.

On the other hand, these thermopiles have the following advantages:

• They are faster, by an approximate factor of 3.


• They are less sensitive, by a factor of 3, when it comes to changes in environmental temperature.

The choice of traditional or S and ST detectors, therefore, depends on the application.