If you are going to wire bond you need a gold layer at least 1um thick, the
ultrasonic power is ripping the metal right of the surface. Your 100nm of Au
is not nearly enough to wire bond.

Rick

Rick Morrison
Radant Mems Inc.

-----Original Message-----
From: Jonathan Griffiths
Sent: Friday, October 14, 2005 02:05 PM
To: [email protected]
Subject: [mems-talk] TiAu to GaAs adhesion

I am encountering problems with adhesion of Ti/Au layers to GaAs
substrates after electron-beam lithography with PMMA.  My process runs as
follows:

* Spin, expose and develop etch pattern with Shipley S1813 optical resist
system.

* Etch substrate in buffered HF/peroxide etchant followed by soak in DI
water.

* Spin double layer 495k/950k PMMA (180nm, 50nm) with 150degC pre-,
intermediate and post-bakes.

* Expose with 100kV e-beam writer.  Broad area clearing dose has proven
to be ~500uC/cm^2 (this seems high perhaps?).

* Dip develop in 5:15:1 MIBK:IPA:MEK for 10s and rinse in IPA for 1
minute.

* HCl dip for 90s.  I've considered brief O2 ashing at this stage to
remove resist residue, but am concerned about the effects on fine features
(~40nm).

* Evaporate 20nm Ti, 100nm Au and lift-off in hot acetone.


Adhesion problems become apparent with subsequent ultrasonic ball bonding.
Unprocessed GaAs chips in the same evaporation have no adhesion problems.

Can anyone suggest a likely cause?  Could a small residue of cross-linked
PMMA be forming in the exposed areas?