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E-beam Resist Processing
Frequently Asked Questions:
Metal Lift-off Processing
Everything You Wanted to Know
But Were Afraid to Ask!
James Conway
Stanford Nanofabrication Facility
Stanford University
[email protected]
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.
Metal Lift-off
• Normally using PMMA
(Poly Methyl Meth
Acrylate) we will slightly
overexpose and overdevelop 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.
Typical Metal Lift-off Process:
Spray Metal Lift-Off:
1.
5 - 20 min Microchem REMOVER PG SOAK, heated to 40 – 65 degrees C.
Watch Metal Film for initial stages of metal lift-off or release from surface.
IMPORTANT: NO ULTRASONIC during the initial soaking!
2.
3 - 5 MIN. ACETONE SPRAY LIFT WITH SQUEEZE BOTTLE.
Spraying across wafer to apply shear force to the metal film.
Repeat as many times as necessary to gain full metal release.
3.
3 - 5 MIN. Microchem REMOVER PG SOAK W/ ULTRASONIC.
Works best when heated to between 40 – 65 degrees C.
4.
3 - 5 MIN ACETONE SOAK, W/O ULTRASONIC.
5.
3 - 5 MIN. ISOPROPANOL W/ ULTRASONIC.
Optical Microscope Inspection: 50 and 500 or 1000 X Magnification.
Is all excess metal removed completely? Are resist residues or contamination present
on wafer surface? Is metal adhered to substrate in patterned areas?
SEM Inspection: CD Measurements and dose determination.
Metal Lift-off
• 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.
Metal Lift-off
Important Points to Remember
• Do a relatively short initial REMOVER PG OR
ACETONE SOAK with no ultrasonic during this
soak.
• Watch the surface of the film and after about 3 - 5
minutes into this soak you should see the pattern
start to release and lift-off.
Edges and open areas normally lift first.
• You may have to soak longer to achieve full
pattern release.
Metal Lift-off
Important Points to Remember
• During your spray liftoff be sure to apply
shear force of solvent across the surface of
the wafer to get under the metal film and lift
it away and off the wafer.
• Be sure to continuously apply solvent to the
surface and to NEVER allow it to dry off.
Metal Lift-off
Important Points to Remember
• If all the metal does not lift off remove as much as
you can and then soak again in REMOVER PG
OR Acetone for an additional 3 - 5 minutes.
• You can employ gentle agitation as this point but I
have found that often this makes the problem
worse as the metal film can adhere to the now bare
surface where the resist has washed away.
• Repeat this process until all the metal that should
be removed comes off, alternating solvent soaks
with spray or squeeze bottle processing.
Metal Lift-off
Important Points to Remember
• Finally do a final Remover PG followed
with an Acetone soak for 3 - 5 minutes with
significant ultrasonic action followed with
3 - 5 minutes of Isopropanol (2 - Propanol)
with ultrasonic agitation.
• N2 blow off and inspect with a optical
microscope at high magnification.
Metal Lift – off
Final Notes:
• Be sure your total metallization thickness
does not exceed the thickness of your resist!
• Sometimes you can carefully remove
unwanted excess metal by using scotch
transparent tape and gently moving a gloved
finger across the surface of the tape and
gently pulling off the tape removing the
excess metallization. Do this very gently!
Typical Metal Liftoff Results
• Good metal
lift off
without
excess
metallization
adhering to
open areas of
substrate.
Typical Metal Liftoff Results
• Poor metal
lift-off result
displaying
metal lift off
‘hairs’ and
areas of
adhering
metallization.
Good Metal Lift-Off of 20 – 100 nm Lines
Typical Metal Liftoff Results
This sample was soaked in
acetone for too long before
spray lift began.
(Time = 20 minutes.)
The resist washed away
from underneath the
metallization and became
difficult to remove, even
with ultrasonic agitation.
This sample had to be
taped to remove all the
excess metal.
We Hope This Information Is
Useful to You and Your Work!
James Conway
Stanford Nanofabrication Facility
Stanford University
[email protected]
001-650-725-7075