🚀 go-pugleaf

Some one must have done this - low substrate temperature measurment for MBE.

We have a much modified (and now very old) Varian 360 MBE which we use for
growth of InGaAs/GaAs layers (MQWs for modulators and 980 nm lasers).
Currently we measure the substrate temperature in two ways -

1) We have a thermocouple attached to a sapphire bead which is supposed
to be in contact with the back of the Mo block (we still indium solder
our wafers but put silicon nitride or dioxide on the back which overcomes
a lot of the problems).  In practice, the sapphire bead doesn't touch the
back and we rely on radiative coupling to the bead (which isn't too bad at
high temps).

2) We have an optical pyrometer which looks at the side of the Mo block
but which has limited temperature response and depends critically on
what the condition of the Mo block is (ie coated with As, shinny, ...)
and how accurately you can point the pyrometer.

The system is calibrated using the Al/Si eutectic temperature.  We have
looked at using the oxide desorption temperature but seem to have a problem
with our substrate cleaning so that the desorption temperature appears
to vary by about 50 C from run to run.  (Anyone willing to tell me 
EXACTLY what their favourite cleaning recipe is?? - I've seen lots of
very brief descriptions but no real details.  We currently use a sulphuric
acid / hydrogen peroxide etch.)

Anyway, the problem (at last) is that we need to measure lowish temperatures
now (of the order of 300-500 C) and we would like some recommendations of how
to do this using optical pyrometry.  What have been peoples' experiences, how
do you calibrate the system, what products are available, how accurate
are they, how precise are they, do you look at the block or the semiconductor,
have you had to worry about the window (ie do you need a sapphire window
or will a standard window do and simply change the effective emissivity), 
do you need to heat the window to prevent condensation of As, how do you
limit the field of view .....? 

I hope someone somewhere can shed some light on the problem.  By the way, I'm 
sick an tired of trying to fix the sapphire bead thermocouple so as to
guarantee contact so please don't suggest that as a fix - unless you know how
to do it :-)

-- 
John Dell,Telecom Res Labs, PO Box 249, Clayton,  Victoria, 3168,  AUSTRALIA
Internet: j.dell@trl.oz.au				TEL: (+613) 253 6623 
UUCP:	  {uunet,hplabs,ukc}!munnari!trl.oz.au!j.dell	FAX: (+613) 253 6665