Single Layer PMMA Liftoff for metal evaporation (ref McCord, modified)

  1. Apply 496K PMMA (4%). Spin 2500rpm for 40 to 60s. Target thickness is 300nm.
  2. Bake on hotplate for 180C for 1hr. Verify thickness is ~ 300nm
  3. Expose @ 30KeV with dose between 180 and 300 uC/cm2. Critical does ~210 uC/cm2.
  4. Develop for 1min in 1:3 MIBK:IPA. Rinse in IPA. Blow dry with nitrogen gun.
  5. Optional descum in barrel etcher. 150W, 0.6 Torr O2.
  6. Evaporate ~100nm metal
  7. Soak in methylene chloride for ~10min.
  8. Then use ultrasonic agitation for ~1min to complete liftoff.
  9. Rinse in IPA and blow dry.

-         Undesired

 

Single Layer PMMA Liftoff (ref IBM 1968, modified)

  1. Apply PMMA (9%). Spin 5000rpm for 60s. Thickness is 500nm.
  2. Bake on hotplate for 170C for 30mins. Verify thickness is ~ 500nm
  3. Expose @ 30KeV with dose between 215 uC/cm2.
  4. Develop for 1min in 1:3 MIBK:IPA.
  5. Spray with developer for 30sec. Blow dry.
  6. Evaporate target material (must be less than PMMA thickness)
  7. Soak in acetone for ~5min.
  8. Spray for 30sec. Blow dry.
  9. Rinse in IPA and blow dry.

 

Non-standard PMMA Liftoff for small feature gaps(ref Lavallee 1998, modified)

  1. Apply 900K PMMA (6%). Spin 5000rpm for 60s. Thickness ~ 270nm.
  2. Bake on hotplate for 180C for 2 hrs. Verify thickness is ~ 270nm
  3. Expose @ 30KeV. Ideally, the PMMA lines (feature gaps) would not be exposed at all but due to proximity some exposure occurs. Check fig. in ref for dosage.
  4. Develop for ~2min in 9:1 H2O:IPA @20C.
  5. Rinse in DI water 30sec. Blow dry.
  6. Evaporate target material (must be less than PMMA thickness)
  7. Use standard liftoff procedures in acetone or similar.

Single Layer General Liftoff and for thin Pd Liftoff (ref A. Javey thesis, modified)

  1. Standard exposure and development.
  2. Evaporate target material (must be less than PMMA thickness)
  3. Soak in warm (~40C) acetone for ~35-45min.
  4. Thorough rinsing in acetone.

 

  1. For ~7nm Pd films, soak in warm (~40C) acetone for ~1-2min.
  2. Spray with acetone using syringe/needle. Don’t allow to dry!
  3. Sonicate for 1-2sec.
  4. Rinse with acetone and IPA.

 

 

Consequences of Proximity Effects

  1. Overexposure of Resist (crosslinking of resist. Resist is no longer soluble in developer!)
  2. Undesired exposure of Resist in adjacent areas (increases feature size!)
  3. Charging of underlying oxide or substrate surface (oxide reliability, threshold shift)

 

Easiest ways to correct for Proximity Effects

  1. Adjust dose or beam current and use thinnest resist layers.
  2. Adjust beam energy.

 

PMMA Exposure Properties (ref McCord)

-         Critical dose ~ 200 uC/cm2 @ 30 KeV (Hitachi ebeam) for a 1:3 MIBK:IPA developer. Critical dose is roughly linear with ebeam voltage. Stronger develop such as 1:1 MIBK:IPA develops stronger with a small loss of contrast

-         Above 10x the critical dose, PMMA crosslinks and essentially becomes a negative resist, therefore, it does not develop anymore and surrounding areas due to proximity effect will develop!

 

Tricks for Good Resist Exposure/Development

-         Use minimum dose to expose resist.

-         Use less than 5:1 resist aspect ratio.

-         Use minimum resist thickness necessary.

-         Make sure # of electrons needed for exposure of feature is very large (perhaps 100s or 1000s for good S/N ratio), i.e low sensitivity especially for small features.

# of electrons = Dose(uC/cm2) x feature(cm2)/q.

-         Add ~1% methyl ethyl ketone (mek) to standard developer for ~30% contrast improvement (ref Bernstein, Hill 1992).

-         Control of temperature is necessary for contrast uniformity. Heat developer ~10% higher than room T and get ~10 to 20% improvement in contrast (ref Bernstein, Hill 1992).

For PMMA narrow traces (unexposed region, pattern will be a gap on substrate), use IPA:H2O 9:1 instead of MIBK:IPA. MIBK:IPA is apparently best for PMMA openings (pattern is metal or dielectric traces).           (ref Lavallee, Beerens 1998)