Dave,
If your ion beam system heats the wafer, you'll get the classic Si/Ti/Au
failure, where Au diffuses into the Si through the Ti. One way to check on this
is to get a temperature dot or high vacuum temperature sensitive lacquer from
Omega and put it on the backside of your wafer during the deposition. If the
temperature starts to climb over 120 C-150C, you're probably interdiffusing
your Au Si. On our system for some deps, I have seen temperatures as high as
200 C.

If it turns out this is the cause, you have several ways to solve this:
1. Keep the temperature down. Lowering the power isn't necessarily the
solution, because as you lower the power the dep time goes up. There's probably
some analytical solution for the minimal heat transfer vs. dep power, but that
would be your challenge. One sure way is to stagger your dep. Deposit it in 3
runs of 0.1 um, with a 1-2 hour cooling off period in between. You can't break
vacuum, which is why it takes so long to cool.
2. Use a different tacking layer. Cr-Au is anecdotally a little better for
preventing Au-Si diffusion than Ti.
3. Thicken your tacking layer by 100 A. It might buy you some protection.

None of the above will protect your device from Au-Si interdiffusion in
operation if it gets hot. There are many good papers on this subject in the
literature. The only longer term solution is ...

4. Last and best solution is to insert a diffusion barrier. Two most popular
are Ti-TiN-Au or Ti-Pt-Au. Generally greater than 1000 A of a diffusion barrier
is desireable. Stress becomes a control challenge depending upon your system.

Good luck,
Tony Flannery



dave.c.murray@baesystems.com wrote:

> Dear all,
> We are experiencing problems with high electrical resistivity of Cr/Au thin
> films.
> The films consist of 0.03um Cr and 0.3um Au, deposited by ion beam
> sputtering, with no subsequent heat treatment.
>
> The resitivity we achieve is ~x7 that of bulk gold, we know that  be achieved with magnetron sputtering.
> AAS spectroscopy suggests an impurities level below 0.1%.
>
> The 2 questions we ask are:
> 1. Is there any reason why ion beam sputtered layers should not match the
> resistance of magnetron sputtered layers?
> 2. Assuming that we don't have an impurity problem, what else can go wrong
> with a thin gold layer to increase its resistivity?
>
> I thank you for help in advance
>
> Dave
>
> Dr David Murray
> BAE SYSTEMS Infra-Red Limited
> PO Box 217
> Southampton
> SO15 OEG   UK
> tel +44(0) 2380 316842
> e-mail: dave.c.murray@baesystems.com
>
> ********************************************************************
> This email and any attachments are confidential to the intended
> recipient and may also be privileged. If you are not the intended
> recipient please delete it from your system and notify the sender.
> You should not copy it or use it for any purpose nor disclose or
> distribute its contents to any other person.
> ********************************************************************
>
> _______________________________________________
> mems-talk mailing list
> mems-talk@memsnet.org
> http://fab.mems-exchange.org/mailman/listinfo/mems-talk