Subject: RE: [FAQ] Metal Liftoff from EBL pattern Question || EBL on Charging Substrates.
From: James Conway <jwc@snf.stanford.edu>
Date: Mon, 20 Oct 2003 10:21:18 -0700
Mon, 20 Oct 2003 10:21:18 -0700
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Hello Xiaoshan Zhu,

Thank you for your questions.  This prompted me to add these questions to a catalog of Frequently Asked Questions that I am putting together for my Ebeam Lithography and SEM classes here at Stanford Nanofabrication Facility.
I am always happy to answer any question you may have.

Thank you,

James Conway
 
 
 

METAL LIFT OFF 

What is important for getting acceptable metal lift-off from fine line EBL features is to have resist patterns that have a slight undercut or re-entrant sidewall profile.  This way the Metallization will not adhere to the side walls which would prevent easy release of the metal films covering the polymer from the metal films adhering to the substrate surface.

Normally using PMMA (Poly Methyl Meth Acrylate) we will slightly overexpose and over-develop to gain the desired undercut profile.  Other polymer blends such as ZEON's ZEP- 520 inherently render an undercut profile and greatly aid in getting clean lift-off features.
Some users may decide to employ a dual layer resist films which exhibit differing solubility's to the developer rendering a undercut film upon development.

Animated GIF attached of juts one approach to using a dual  layer resist process for Metal Lift-off:

CHARGING SUBSTRATES including Glass and Quartz substrates 

Charging is to be expected when performing lithography on insulating substrates, this is normally more severe at higher acceleration voltages and can be reduced when using acceleration voltages below 10 keV.  Most people working with insulating substrates find it useful to employ a metal layer either below or on top of the polymeric resist films.  Normally this would be a 40 - 100 Angstrom layer of a high Z metal such as Au, Pd, Cr, or Pt.  This layer would then be removed using normal wet etchants before developing out the resist film as normal.
 
 

Xiaoshan Zhu wrote:

Hi, James, This is email from Xiaoshan Zhu at University of Cincinnati. I got your email from internet and I am interested in your e-beam lithorgrapgy condition for lift-off. One small question about substrate used in e-beam lift-off: Did you try e-beam lift-off on glass or other insulated substrate? If yes, how did you treat the substrate (make it conductive) before exposure? Thanks!Xiaoshan Zhu

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