Hi MEMS-talk,

I am interested in depositing MgF2 in either an e-beam deposition system
or by thermal evaporation. My application is for a low refractive index
layer in a Bragg mirror, and its properties seem ideal. I have noticed
in most of the literature (both in journal articles and on supplier's
websites) that in the deposition of MgF2, the substrate is heated to
about 300 degrees C. This is a problem for me, because my e-beam system
doesn't have a substrate heater, and my thermal deposition system can
only heat to about 150 degrees.

However, I do have one paper from 1975 which shows that when it is
deposited on unheated substrates the refractive index is actually lower
(good for me), although it does tend to absorb water (Ogura et al,
"Refractive index and packing density for MgF2 thin films: correlation
of temperature dependence with water sorption", Thin Solid Films, 30
(1975), pp. 3-10). Water sorption is not a problem for me, because the
germanium layers encapsulate the MgF2 whilst in vacuum.

So, to my question: has anyone had any experience in the thermal or
e-beam evaporation of MgF2 layers, and if so, do you expect me to run
into problems trying deposit onto an unheated substrate? If I must heat
the substrate, can I get away with 150 degrees in a thermal evaporation
system?

Thanks in advance!

Jason Milne
Microelectronics Research Group
The University of Western Australia