Hello Andy,

Large scale silicon chip processes use the etch process for a few reasons.
Primarily because the metal must be sputtered and sputtered metal is not
conducive to a good lift-off process.

Contributing causes:

Sputtered Al with 1% Si (and other alloys) is typical to avoid Al spiking and
compounds like that can't be evaporated.

Sputtering gives good low stress step coverage. This same conformality which is
good for step coverage is bad for lift-off.

Sputtering can fill without keyholes, large aspect ratio holes.

Low stress step coverage is critical for reliability and is specified in all
Mil. and space specs.

A robust deposition, lithography and RIE etch are easier in high volume (for Si)
than lift-off is (process control and logistics).

The contact areas can be meticulously chemically cleaned, sputter cleaned and
covered with metal before any chance of contamination (low contact resistance).

Good luck,

Vince


-----Original Message-----
From: [email protected] [mailto:mems-talk-
[email protected]] On Behalf Of Xiaochen Sun
Sent: Monday, February 28, 2011 11:55 AM
To: [email protected]
Subject: [mems-talk] why no lift-off for Si processing

Hi,

People use metal lift-off for III-V (GaAs, InP etc.) metallisation
everyday. Could anyone let me know the reason why people rarely use
lift-off in Si processing? We usually do Ti/Al sputtering/evaporation
and then dry etch metal. Is it something to do with adhesion? Thanks!

Andy