Stresses of single and multilayer films in the range of 5 to 40 micrometers are discussed together with their spectral performance. The flatness of a substrate after coating can be less than 4 fringes (2 (lambda) ), even 1 fringe at 0.5875 micrometers . The thermal evaporated thin film materials are IIB - VI compounds (IIB being Zn etc. and VI being S and Se etc. in the periodic table), and PbTe or Ge.
"Design and fabrication of IR filters with good flatness", Proc. SPIE 2253, Optical Interference Coatings, (4 November 1994); doi: 10.1117/12.192083; https://doi.org/10.1117/12.192083