Thermopile sensors are used as infrared sensors and allow micromechanical manufacturing on a semiconductor substrate. They are used as spectroscopic gas sensors, in particular, e.g., in the automotive field, and also in infrared cameras having image resolution.
Thermopile sensors are usually designed as a series connection of thermopile pairs on a diaphragm, each thermopile pair having two thermopile legs contacted in a hot contact on the diaphragm or thermopile legs made of materials having different Seebeck coefficients. Conventional pairings of materials for the thermopile legs include silicon and aluminum or p-silicon and n-silicon (p-doped and n-doped silicon) as well as material systems based on bismuth tellurides, for example.
The contact point of the thermopile pair heats up as a function of the incident IR radiation. An absorption layer, i.e., absorber layer, is therefore usually applied to the top side of the diaphragm, which has good absorption properties in the relevant wavelength range. For example, ruthenium-containing resistance pastes, gold-black coating, silver-black coating or other absorber materials are used here for absorption.
However, such absorber materials may not be used in a CMOS production line in general.
The properties of a radiation sensor are described in general by the ratio of detector voltage Vth and incident power φ. Voltage sensitivity S of the detector is thus obtained as
  S  =                                                  V            th                                Δ            ⁢                                                  ⁢            T                                      ⁢                                            Δ            ⁢                                                  ⁢            T                    Φ                              =                            a          ·          t                G            ⁢                        N          ·                      (                                          α                                  S                  ,                  a                                            -                              α                                  S                  ,                  b                                                      )                                                1            +                                          ω                2                            ⁢                              τ                th                2                                                        where a is the absorbtivity of the thermopile, t is the transmittivity of the radiation path, N is the number of thermocouples and ΔT is the temperature difference between cold and hot contacts of the thermopile. (αS,a−αS,b) is the combined Seebeck coefficient of the material pair a b. Furthermore, τth is the thermal time constant describing the response of the detector to a radiation intensity φ modulated with frequency ω, and G is the thermal conductivity.
Absorbtivity a (λ) thus determines the sensitivity of the radiation detector, is usually a function of wavelength λ and may reach up to 99%, depending on the material used, e.g., ruthenium-containing resistance pastes. One disadvantage of ruthenium-containing resistance pastes in particular is in the manufacturing process, where the paste is first applied to the particular sensor diaphragm by dispensing individual paste drops and the paste is solidified in a heating step in which the organic components of the paste are expelled. This manufacturing process is very complex and expensive and furthermore is not compatible with the usual systems for CMOS fabrication. One disadvantage of gold-black or silver-black coatings, for example, is also that these materials are not compatible with the manufacturing operations in a CMOS line.