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For the next generation of fusion lasers reflecting mirrors with laser damage thresholds of at least
40 J/cm2 for 1 0 ns laser pulses at 1 .064 pm are needed. Up to now, no deposition technique has
been developed to produce such mirrors. Best R&D-values realized today are around 30 J/cm2 for
e-beam evaporated mirrors. R&D on conventional e-beam coating processes over the last 1 0 years
has come up with marginal improvements in laser damage thresholds only. However, new technologies,
like PICVD (Plasma-Impulse CVD) developed for the fabrication of ultra-low loss fiber preforms,
seem to offer the potential to solve this problem. First results have been reported already [1-3]. It
is well known that fused silica produced by CVD processes can have laser damage thresholds as
high as 80 J/cm2. However, the thickness of a single deposited film is in the pm-range for most
of the CVD-processes used for preform manufacturing; since interference optics need films in the
; /4n range (where n is the refractive index of the dielectric material) the use of preform-fabrication
processes for the purpose of interference mirror fabrication is limited to a few plasma based CVD
technologies, namely PCVD (Plasma-CVD, Philips [4]; PICVD, SCHOTT [5]). Especially PICVD is a
very powerful technology to fabricate thin film multilayers for interference mirrors, because this technique
is able to produce films down to monolayer thickness with nearly perfect stoichiometry and
morphology. In first and preliminary experiments the usual deposition in a circular tube at high temperatures
has been used for simplicity. However, to produce large area high quality laser mirrors
this principle know-how has to be transfered from circular to planar geometry. Experiments showed,
that there may be some limitations with respect to the homogeneity of a planar deposition using
microwave excitation for the plasma. Therefore experiments have been performed in parallel with
both RF and microwave excitation for comparison. In the following we will restrict ourselves to the
description and discussions of the planar processes; the principle and details of the PICVD-process
are described elsewhere [5] while RF-plasma technology is a well known process.
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