PHOST - Negative tone electron beam resist

Motivation

PHOST is a negative tone electron beam resist. It is possible to print isolated 100 nm patterns, but it requires dosages of upwards of 15,000 uC/cm2. This electron beam resist process was developed a long time ago as an alternative for HSQ since we needed a cheaper and more stable alternative to HSQ. The development data will be added to this page if we can find it.

Objective

Share our PHOST recipe. The development data such as contrast curve and images is not accessible.

PHOST Preparation

  1. Prepare supplies
    • PHOST powder
      • Poly(4-vinylphenol)
      • Molecular Weight of approximately 11,000
      • CAS Number: 24979-70-2
    • PGMEA solvent
      • Propylene glycol monomethyl ether acetate
      • CAS Number: 108-65-6
      • Purity greater than 99.5%
    • Scale balance
    • Weighing paper
    • Stainless steel laboratory scoop
    • Amber vial with cap - 50 mL
    • Hot plate with magnetic stir
    • Magnetic stir bar
    • Beaker - 50 mL
    • Syringe - 10 mL
      • VWR Catalog Number: BD309604
      • Syringe with Luer-Lok Tip
      • 10 mL
    • PTFE Syringe Filter - 0.45 μm
      • VWR Catalog Number: 28145-497
      • Syringe filters with polypropylene housing
      • 25 mm diameter
      • PTFE membrane
      • 0.45 μm pore size
  2. Scoop 3g of PHOST onto a weighing paper
  3. Pour 20 mL of PGMEA into a beaker
  4. Place beaker on hot place and add the magnetic stir bar
  5. Turn on magnetic stir and set it to a reasonable rate
  6. Slowly add the 3g of PHOST powder into the beaker
    • Wait for powder to dissolve before adding more
  7. Place filter on syringe
  8. Pour contents of beaker into syringe
  9. Inject the solution into an amber vial
  10. Repeat from step 8 as needed
    • Replace filter if needed

This recipe produces a 15% weight solution of PHOST in PGMEA.

Film thickness vs spin speed

The film thickness is determined by the PHOST concentration and the spin speed. The acceleration is fixed at 1250 [rpm/s] and the spin time is 60 [s]. After spin coating, the sample (a 12mm x 12 mm silicon chip with 500 nm of oxide) is baked on a hot plate at 170 C for 1 minute. Then the film at the middle of the sample is scratched with a stainless steel tweezer and a profilometer is used to measure its thickness. The table below shows the film thickness versus spin speed.

Spin speed [rpm] Film thickness [nm]
PHOST - 15%
2500 680
4000 595
5000 500

Electron beam lithography process

  1. Prepare a Si substrate
  2. Spin on PHOST
  3. Bake on a hotplate at 170 °C for 2 minutes
  4. Expose patterns at a dose of 6000 through 15000 [uC/cm2] at steps of 1000 [uC/cm2]
    • The critical dose for PHOST is approximately 10000 [uC/cm2]
  5. Develop by immersion in PGMEA for 1 minute
  6. Blow dry with nitrogen