Recording of relief structures in amorphous hydrogenated carbon (a-C:H) films for infrared diffractive optics

We demonstrate the recording of diffractive structures in amorphous hydrogenated carbon (a-C:H) films using holographic exposures and reactive ion etching (RIE). The a-C:H (diamond-like) films are a very promising material in which to record relief optical structures because they ally the properties of hardness and optical quality of the coatings. The high refractive index of the material and its transparency in the infrared (IR) part of the spectrum make such structures also very interesting for recording IR transmission diffractive optical components. Such structures were holographically recorded in photoresist, and then transferred to a thin aluminium layer, which was used as a mask for the RIE of the a-C:H films. The recorded structures were analysed by scanning electron microscopy and by diffraction measurements. The good accordance between the measurements and the calculated diffraction curves demonstrates the feasibility of the material for recording diffractive optical components.