Reservation  -     

Center of MicroNanoTechnology CMi

Resists for Lift Off

Table of content

  1. Vistec EBPG5000 overview
  2. What is an electron beam (ebeam) lithography system?
  3. Electrons, resists and substrates
  4. Process Flow for Ebeam Lithography
  5. Data conversion
  6. Running the ebeam

III.1.a. MMA/PMMA basics

PMMA (poly methyl methacryllate) and MMA (8.5) (methyl methacryllate) are positive ebeam resists consisting of long polymer chain of carbon atoms which comes in various molecular weights. The two standard molecular weights of PMMA which we use here are of 495K and 950K. The standard use for PMMA/PMMA and MMA/PMMA bilayers in EBL is for lift-off of metallic structures. For good lift-off you need an under cut resist profile to avoid coating the sidewall of the resist when you evaporate metal. This is particularly true for high beam energies (100kV) where the sidewall profile is normally extremely vertical. The PMMA/MMA bilayer gives a larger undercut profile than PMMA/PMMA as shown below. A good rule of thumb is that you should the lower layer of resist should be 2x the thickness of the metal you want to deposit.

200nm lines after development of MMA/PMMA bilayer
200nm lines after development of MMA/PMMA bilayer.
Metalised with 15nm Ti and 45 Au
100nm lines after development of MMA/PMMA bilayer
100nm lines after development of MMA/PMMA bilayer.
Metalised with 5nm Ti and 15 Au


This larger undercut is good for liftoff, but fails to work for small dense features as the undercut causes the central MMA structure to collapse.

To achieve succesful lift-off while maintaining a good adhesion of the deposited metal it is extremely important to make sure that the area on which you will be depositing is completely free of any residue of the MMA/PMMA resist. Make sure you use properly sized beam and high enough dose. It is highly recommended to also perform a quick O2 plasma descum (10s Tepla low power) before evaporation or sputtering of your metal layer.

Lift Off Exposition
Exposition

The PMMA and MMA are developed with a mixture of MiBK and IPA
The PMMA and MMA are developed with a mixture
of MiBK and IPA

Metal evaporation
Metal evaporation

Lift off with suitable solvent like acetone to leave the metal film on the substrate
Lift off with suitable solvent like acetone
to leave the metal film on the substrate

III.1.b. Data

Polarity

Positive

Doses

For large structures (above 1µm) the clearing dose at 100kV is around 600µC/cm2, for small isolated structures its around 3000µC/cm2

Resolution

The best resolution attainable with PMMA/PMMA bilayer is around 10nm using a well focused beam and a very good process. < 20nm should be possible regularly.

Chemical capabilities