Maximal absorption in ultrathin TiN films for microbolometer applications

Ultrathin light absorbers present a viable route toward improving the specific detectivity and response time of microbolometers. However, for an ultrathin film to absorb light efficiently, the dielectric function of the film and its thickness must satisfy strict requirements. We experimentally demonstrate an average absorptance of 48% ± 2.5% in the 8–13 μm (769–1250 cm⁻¹) spectral range for 10 nm thick titanium nitride (TiN), a value bordering on the 50% fundamental absorptance limit for a suspended thin film. The heat capacity per unit area of this absorber is only 1.2 × 10⁻⁶ J/K/cm², which is beneficial for improving the response time of a microfabricated bolometer. We also show that a sufficient condition for reaching maximal absorption in an ultrathin film is that ε″ ≫ |ε′|.