No More Dummy Wafers to be Provided

Jim McVittie mcvittie at cis.Stanford.EDU
Thu Jan 25 18:02:39 PST 2001


Lampoly and P5000etch Users,

Up until now we have provided dummy wafers for Lam and P5000 users to
season the plasma etch chambers before use. Except for the AMT hexode
system, we do not provide such wafers for the other etchers. This has
become has burden on the staff. Since the use of these dummies is a user
issue and does not affect these etchers for the most part, we have decided
to let the users provide their own dummies. Below are some guidelines to
help with you condition or season an etch chamber before use.

Why is the running of dummies needed?

In seasoning an etch chamber, one is trying to get the coating on the
chamber walls to be same as what occurs during the etching of the actual
device wafers. The reason for doing this is that your etch results depend
on the reactive species concentrations in the plasma. These reactive   
species depend not only on the source gases and other process settings but
also on the wall reactions which depend on what is on the surface of the
walls. For most etch processes a equilibrium surface composition develops   
on the chamber walls after a few minutes. The dummies are used to let the
chamber get to this equilibrium condition before the real etching begins.
Hopefully,
the result is that each of your device wafers etch same this time as the
time before.

What type of dummies are needed?

Dummies need to reflect what the real device wafers look like at the
particular etch step. If one is etching a typical poly-Si wafer where the
resist coverage is about 50 %, one should use alternating resist and bare
Si wafers for the dummies. If one is etching wafers that are mainly bare
Si, then of course one should use just use bare Si dummies. In the case of
oxide contact etching where the surface of the device wafers are usually >
90% resist, resist dummies should be used.  Al etching is similar to
poly-Si in that the resist coverage is usual around 50% so alternating
resist and bare Al wafers are the best dummies.

How many dummies should be run?

The number of dummies depends on how long it has been since the last time
the chamber was conditioned, if a different process has been run in the
chamber, if the chamber has been open to air, or if the chamber has be
giving a plasma clean. In industry, up to 25 dummies wafers can be run
before starting to run production. For our use where our chambers tend to
be dedicated, I typically recommend running 4 dummies each for 60 seconds.
If your run is not critical, running no dummies may work ok for you.

How many times can I reuse a dummy?

This depends on the process. Obviously, a resist wafer with no resist left 
needs to be re-coated before further use. For bare Si wafers, the limits
are black silicon formation and wafer thickness. Black silicon is when
there is lots of micro-masking on the surface and the surface looks black.
The problem with black silicon is that the wafer can start giving off
particles. Wafer thickness is an issue because thinned wafer can break
during clamping. I recommend replacing Si dummies after they have loss 1/3
of their thickness. 


			Jim 	
--------------------------------------------------------------
James P. McVittie	                Senior Research Scientist
Allen Center for Integrated Systems     jmcvittie at stanford.edu
Stanford University             	Tel: (650) 725-3640	
Rm. 336, 330 Serra Mall			Fax: (650) 723-4659
Stanford, CA 94305-4075	




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