Spin Coating Film Thickness

Spin coating is usually the first step in a fabrication process and the spin coated film is typically used as a media for pattern writing. The film thickness is selected to be thick enough to survive the following etch step or enable a good lift-off. Thus the slight variation from time to time, typically less than 3%, is quite acceptable for most research projects.

Motivation

The humidity in our cleanroom is typically 55%, 10% higher than what's recommended for a nanofabrication facility. It is evident that our processing environment is different from other facilities, so it is useful to obtain the spin speed curves for common resists.

Objective

Our objective is to acquire the spin speed curve for common resist from ad hoc experiments. Data points are added as the need for a specific film thickness arises.

Design of Experiments

  1. Specify substrate
    • Chip is 12.5 x 12.5 mm square
    • 3" wafer
    • Currently use only Si wafers with oxide
  2. Spin coat a material
    • Acceleration is set to 1250 rpm/s
    • Spin time is 60 s
    • Specify spin speed
  3. Measure film thickness
    • Scratch with tweezer
    • Use profilometer or AFM

PMMA

After spin coating, PMMA is baked at 180 °C for 2 minute.

PMMA is stored in our refrigerator which is set at 7.2 °C. We tried just one experiment where PMMA is at 7.2 °C and room temperature. The difference in film thickness is not significant.

We stock PMMA A9. Dilute solutions are prepared by mixing PMMA A9 with A Thinner. For example, to get PMMA A3, we mix 10 mL of PMMA A9 with 20 mL of A Thinner. The solution is then poured into a BD Luer-Lok syringe with a 25 mm 0.2 μm PTFE syringe filter to remove particulates. The 0.2 μm filter has extremely slow flow rate, so one should confirm if 2 μm filter is acceptable.

Figure 1: Spin speed curve for PMMA.

HSQ

After spin coating, HSQ is baked at 90 °C for 1 minute.

TMAH 25% dissolves HSQ in seconds and it prefers to wet SiO2 instead of HSQ. By dipping portions of the HSQ coated chip in TMAH 25%, a sharp and clean edge can be created. For HSQ films thicker than 500 nm, it is more difficult to produce a sharp edge by dipping in TMAH 25% so the scratch method was used instead.

FOx-16 datasheet specifies that depending on the recipe, we can achieve a film thickness from 450 to 950 nm.

Figure 2: Spin speed curve for HSQ.

AZ1512

After spin coating, AZ1512 is baked at 100 °C for 1 minute.

The viscosity of AZ1512 makes it difficult to completely spread the photoresist across a wafer. Failure to cover the wafer with AZ1512 before spin coating will result in uncoated regions. We use a two step spin coating process:

  1. Spin at 200 rpm at 10 rpm/s for 60 seconds
  2. Spin at desired rpm at 1250 rpm/s for 60s
First dispense 1 mL of AZ1512 at the center of the wafer. Then run the spin program. During the first step watch for an even spread on the wafer. If the spread is not even, abort the spin program and dispense AZ1512 to cover the gaps. Then run the spin program again to completion.

Spin coating at a speed below 3000 rpm will result in thicker resist near the edge of the 3" wafer. At 1000 rpm, the edge bead is a few millimeters wide and it is ~2.18 μm thick compared to the nominal thickness of ~1.6 μm. The edge bead can be removed with a wipe and acetone.

Figure 3: Spin speed curve for AZ1512.