Figure 1: Spectrum of a 36-layer 30deg 633nm mirror
Written by Dr Sebastien Pochon, Applications Engineer, Oxford Instruments Plasma Technology
A high quality optical coating should have low optical loss. Losses inside an optical coating arise from scatter and absorption.
Ion beam sputtering produces films with total losses so low that sophisticated devices are needed to measure them. Data for the measurement of mirrors with losses less than 2ppm have been published by two independent groups. Both groups made their mirrors on Oxford Instruments Ion beam sputter deposition systems – the IonFab500Plus.
A major feature of an optical coating is its surface quality. The quality of the surface determines the performance of the optical device itself. High quality optical coatings start with a smooth super-polished optical substrate with roughness typically of 0.05 nm rms.
A conventionally deposited film will add roughness to the surface of the optical substrate, the degree of the roughness dependant upon the technique used. For example a film deposited by evaporative techniques produces a surface roughness of typically 1 nm rms, while ion assisted deposition techniques produce a surface roughness of typically 0.4 nm rms. Ion beam sputter deposition produces films with a surface roughness equal to that of the super-polished substrate, 0.05 nm rms.
The figure above shows the spectrum centred at 633nm of a 36 layers mirror obtained with Ionfab500Plus tool. Total losses for SiO2/Ta2O5 mirrors are less than 40ppm with repeatability of ±2%, <±0.001 R.I. and uniformity of less than ±2% across 10” planet and <±0.0005 R.I. Surface roughness increase <0.02nm RMS for initial substrates <0.07nm RMS. Loss readings are subject to suitable substrate and clean room conditions being of a suitably high quality. Oxford Instruments’ customers have reported achieving <20ppm mirrors in production.
Ionfab500Plus enables excellent refractive index control and uniformity that provides high wafer yield.