Highly reflective thin film coatings for high power applications of micro scanning mirrors in the NIR-VIS-UV spectral region

This paper addresses different highly reflective optical coatings on micro scanning mirrors (MSM) for applications in the NIR-VIS-UV-spectral region to enable new applications at high optical power density like laser marking and material treatment. In the common case of MSM with an unprotected Al coating, the absorption limits the maximal power density because of induced heating. In contrast to macroscopic optics HR-micro mirror coatings have to guarantee additional demands like low-stress and CMOS compatibility. Hence, to enable novel high power applications of MSM in the NIR-VIS-UV spectral region highly reflective low-stress coatings have been developed according to a triple strategy: (a) broadband metallic reflectors, (b) dielectric multilayers and (c) enhanced hybrid coatings. For Au and Ag based NIR-coatings an excellent mirror planarity and a reflectance around 99 % (@ 1064 nm) have been achieved, whereas dielectric coatings reached 99.7 % for a (LH)4 design and thinner low-stress hybrid NIR-coatings reached up to 99.8% enabling an improved mirror planarity and excellent laser damage threshold. For the VIS and UV spectral region enhanced hybrid HR-coatings have been favored, because they enable high reflectance of up to 99.7 % @ 633 nm or 98.8 % @ 308 nm in combination with low stress, high mirror planarity and CMOS compatibility.