The parts we are making are computer generated holograms in 65 cm
diameter 1 cm thick SiO2 and are used to alter the focusing of laser
beam in fusion experiments.
Its a 4 mask lithography method that cuts 16 level aproximations
to the holographic phase only profiles.

I use sputtered chrome, 1000 angstoms thick, onto SiO2. The chrome
becomes alignment fiducials
and is also used as a resist when I 1% HF 15% NH4F etch the SiO2.
Looking at the fiducials after several processing steps I see the chrome
is chipping ( mouse nipping ) in irrigular ways. There is up to 5
microns
of chrome edge erosion after our 4 processing steps. This chrome edge
lateral migration is 10
times larger than the entire SiO2 etch depth. The mouse nipping is not
always the same
on a given part or from part to part.

I will try to tell you all I think you need to know in order for you to
write an "educated-guess" comment.

Our chrome coating processing steps are:

Supper polish the SiO2.

Clean the mountaing and transport hardware SiO2 container in a brewin
wash tank containing 50 gallon of 3-5% brewin in water.

Clean the SiO2 substrate;
rinced with di-water then
alumina polish by hand and di-water rinced, on each side.
Then FL-70 soap is hand rubbed onto one surface at a time
and rince off with di-water this gets the polishing compond off.
Now rince with 80 psi di water, then grossly
remove water with lens tissue and then acetone swipe surface to get the
water off.
Now stainless-steel sling mount it so to rince with di water then wash
with citric acid then di water rince then alcohol at 80 psi to remove
water then acetone swipe then inspect and finally mount
in the all aluminum hardware container.
The plastic wraped container containing the SiO2 substrate is then
delivered, by pickup truck, within the day, to be Chrome coated.
The plastic is removed from the container and handled with cotton gloves
Afer the container is mounted in the vacuum chamber its front cover is
removed. We next place UV-ozon lamps 1 inch beneth the SiO2 surface
and partially close the vacum door. The lamps running for about 1 hour
in atmospher
are supposed to remove the last mono layers of adsorbed organic
deposits. The lamps are then removed and the vacumm system pumps
down to 5x10^-7 range, over night. Chrome is then suptterd onto the
rotating substrate, the next morning. After the substrate and system
cools, about 2 hours, the
vacuum door is opened and the SiO2 substrate container cover is
replaced. Agin using cotton gloved hands the container is removed and
placed back in the original plastic coverings. We now Pickup truck
deliver the part to another building to be photo resist coated.

The first processing step.
The photo resist is miniscus coated to 380 nm thick with shipply 1400.
The substrate
only is now soft baked at 80 degrees centigrade for 2 hours.
Days later we then contact print our first of 4 pattern onto this photo
resist. Our exposure light is 500 watts of an uncolimated AOI lamp for 2
minutes. Then we puddle develope the resist in a 50% diluted developer
for 2 minutes. Now we hard bake the resist at 120 degrees centigrade.
Next morning we etch the chrome for 15 miniutes in CR-7 (from cyantek)
di rince and puddle etch 300 nm of the SiO2 in 1% HF 15% NH4F which
takes about 30 minutes. We now rince in DI water and acetone or Alcohol
drag wipe.

This complets the first of our four processing steps.

The next three processing steps are all the same.
The stsubstrate is now dryed at 140 degrees centigrade for 2 hours.
As it cools to 125 centigrade a petry dish of HMDS is placed in the oven
and the oven door closed and powered off. The HMDS is supposed to vapor
prime the SiO2. When cooled it repeats the first processing step.
The only difference is that the HF etch is carried out for one half the
time of the previous step.

On trying to align the mask in processing step 3 and 4 we see a
noticable failure of the chrome to stick on our SiO2.
It has receeded up to 5 microns sometimes in a smooth mannar
other times in a mouse nipped mannar. Sometimes chrome will be right
were its supposed to be, but more often its not.

Since there is some chrome that is correct I am encuraged to find out
why most other area fail.

Theories about adhesion abound. Most target cleaning. One targets
a gell like layer left behind by polishing. I would like to hear your
theory.

My feeling is chrome is more easily pealed off than pulled off.
If you want the pealing (mouse nipping)
 to have limited propogation then make the
coating thinner and more brittle.

That is evident from some test done by our coating lab. Where they
coated chrome then gold onto SiO2. They passed every adhesion pull test
but when the part was broken the chrome gold could easily be pealed
off like tape by rubbing the broken edge with your finger.


Please send comments to: [email protected]

Thanks