Microelectronics Research Center Georgia Institute of Technology

PMMA Process

    1. 60sec spincoat PMMA (see PMMA spin speed curves)
    2. 180C hotplate for 90sec
    3. expose (650uC/cm2 base dose for features larger than 1um)
    4. 2min immersion develop 1:1 MIBK:IPA
    5. rinse IPA

To remove PMMA:

    1. soak in acetone or 1165 (n-methyl pyrrolidinone)
    2. O2 plasma
    3. piranha clean

for metal liftoff process:

    either 1165 remover (n-methyl pyrrolidinone) or acetone can be used. usually 1165 (especially heated) will remove resist better than acetone or other solvents. however, sometimes 1165 is harsh for some materials (e.g. silver) and so acetone may be preferrable in those cases.

liftoff with 1165
    1. heat 1165 to 50 - 70C (measure temperature of liquid with thermometer, not just hot plate set point)
    2. place sample in heated 1165 (can place in before it comes to temperature also), the time for this step varies, usually it takes ~2 hrs
    3. optional step: can place in ultrasonic for 5 sec or more (depends on device, can damage some structures)
    4. rinse with acetone, methanol, isopropanol (1165 leaves an oily residue)

liftoff with acetone
    1. soak in acetone, the time for this step varies, depending on pattern density, can be as much as a few days or a few minutes, lower density and small nano feature take longer than high density and large features
    2. flow acetone over wafer using squeeze bottle to remove large pieces of metal
    3. place in fresh acetone in a container and float in ultrasonic bath for 5 sec or more

    sometimes it is helpful to do two sequential baths, where large pieces come off in the first bath, and the liftoff can be accelerated during this step by using a squeeze bottle of either 1165 or acetone to help break up the pieces more quickly.
    then, after most of the large pieces have been removed, the sample can be placed in a second new container of the solvent. this helps to keep pieces from redepositing onto the sample.