1. 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

2. 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.

3. Metal Lift-off
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.

4. Typical Metal Lift-off Process:
Spray Metal Lift-Off:
1.      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.      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       MIN. Microchem REMOVER PG SOAK W/ ULTRASONIC. Works best when heated to between 40 – 65 degrees C.
MIN ACETONE SOAK . 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.

5. 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.

6. 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 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.

7. 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.

8. 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 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.

9. Metal Lift-off Important Points to Remember
Finally do a final Remover PG followed with an Acetone soak for minutes with significant ultrasonic action followed with minutes of Isopropanol (2 - Propanol) with ultrasonic agitation.
N2 blow off and inspect with a optical microscope at high magnification.

10. 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!

11. Typical Metal Liftoff Results
Good metal lift off without excess metallization adhering to open areas of substrate.

12. Typical Metal Liftoff Results
Poor metal lift-off result displaying metal lift off ‘hairs’ and areas of adhering metallization.

13. Typical Metal Liftoff Results
Good Metal Lift-Off of 20 – 100 nm Lines
Typical Metal Liftoff Results

14. This sample was soaked in acetone for too long before spray lift began
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.

15. We Hope This Information Is Useful to You and Your Work
We Hope This Information Is Useful to You and Your Work! James Conway Stanford Nanofabrication Facility Stanford University