Adhesion

Motivation

Photoresist does not adhere to a hydrated silicon surface. A standard technique for superior adhesion of photoresist requires a specialized oven that dehydrates the wafer followed by a priming using hexamethyldisilizane (HMDS) vapor. We do not have access to such an instrument at UHNF, so we need to find a different solution.

Objective

Determine the best method to improve the adhesion of photoresist to silicon.

Design of experiment

  1. Use a 4" Silicon wafer
  2. Apply adhesion promoter
    1. Surpass 4000
      1. Immerse in Surpass 4000 for 1 minute
      2. Rinse with DI Water
      3. Rinse with IPA
      4. Dry with N2
    2. Dehydration Bake
      1. Bake on a hotplate at a specified temperature and time
      2. Place on spin coater chuck for 1 minute to cool
    3. Ozone Cleaner
      1. Process wafer in ozone cleaner for 10 minutes
      2. Spin coat immediately
    4. O2 Plasma
      1. Use RIE180 recipe named LVC: Adhesion Promoter
        • RF Power: 70 W
        • Pressure: 10 mTorr
        • O2 Flowrate: 50 sccm
        • DC Bias: 233 V
        • Chuck Temperature: 20C
        • Helium Pressure: 10 Torr
        • Time: 60s
      2. Spin coat immediately
  3. Photoresist Process
    • AZ1512
      1. Spin coat at 1000 rpm, 1250 rpms/s, 60s
      2. Bake at 100 °C for 1 minute on a hotplate
      3. Expose at 80 mJ (405 nm)
      4. Develop by immersion in AZ300 for 1 minute
    • PR1-2000A1
      1. Spin coat at 2500 rpm, 1250 rpms/s, 60s
      2. Bake at 130 °C for 2 minute on a hotplate
      3. Expose at 48 mJ (365 nm)
      4. Develop by immersion in RD6 for 1 minute
  4. Bake at 130 °C for 2 minute on a hotplate
  5. Expose pattern at 48 mJ (365 nm)
  6. Develop by immersion in RD6 for 1 minute
  7. Rinse by immersion in DI water
  8. Inspect pattern in microscope

Results

The pattern is an array of 7 μm hexagons spaced 40 μm apart covering an area that is 33 mm x 50 mm. Adhesion failure can be observed during the rinse step when the wafer is dipped into a beaker of DI water. Small turbulence in the water will cause the patterns to delaminate from the wafer and it looks like particulates washing off a surface.

Surpass 4000 photoresist adhesion promoter that modifies the surface energy of the substrate without leaving any residue. Since it is a water based solution, there is no need for a dehydration bake before immersing in Surpass. Surpass did clearly improve adhesion compared to other adhesion promotion processes we have tried, but it was not sufficient to be useful for our pattern. Table 1 is a summary of the results. The relative humidity on the day of the experiment is 33%. We performed the experiment again the next day where the humidity was at 55% and observed a similar result.

Table 1: (20181127) Cleanroom was at 19C and 33% RH

Adhesion Process Photoresist Results
Surpass AZ1512 Bad
Surpass PR1-2000A1 Bad
Dehydrate (150C,5min) PR1-2000A1 Bad
Surpass (5min) PR1-2000A1 Bad

The RIE180 was used to exposed the silicon wafer to an oxygen plasma. Exhaling on the wafer for approximately 1.5 seconds before the plasma treatment leaves a wet film which dries quickly. After the plasma treatment, the same test does not produce a visible film on the wafer. Continuously exhaling for about 10 seconds will produce a wet film again indicating that the surface treatment is temporary. In fact, for one experiment, the wafer is treated with an O2 plasma and I exhaled on a quarter of the water until there is a wet film. After patterning, the quarter of the wafer that got wet had slightly worse adhesion since some (less than 10%) of the pattern delaminated. Table 2 is a summary of the results.

Table 2: (20181211) Cleanroom was at 19C and 45% RH

Adhesion Process Photoresist Results
O2 Plasma AZ1512 Bad
O2 Plasma PR1-2000A1 Good
UV Ozone PR1-2000A1 Didn't Try

UV Ozone appears to have the same effect on the wafer as an oxygen plasma treatment; exhaling on the wafer did not produce a visible film. However, the UV Ozone treatment appears to degrade significantly when I performed the breath test 10 seconds later so I decided to abort the experiment. The UV Ozone should be tested since it is an easier and more accessible process than and oxygen plasma.

In 2016, I tested other adhesion promoters such as Ti Prime, Omnicoat and spin-on HMDS. I was not satisfied with any of them. Ti Prime contains titanium which is not compatible with many processes. Omnicoat does not appear to be compatible with AZ1512 photoresist since the adhesion is worse than doing nothing. Spin coating HMDS is not allowed at UH Nanofabrication Facility because it is known to contaminate the spin coater and has an adverse affect on photolithography. I tried it anyway and spin-on HMDS did improve adhesion. However, HMDS reacts with AZ1512 in the spin coater to form a black goo that is difficult to clean. In addition, it takes longer to develop the photoresist and the unexposed regions look bumpy in an optical microscope.

Conclusion

Photoresist adhesion to silicon is currently solved by treating a silicon wafer with an O2 plasma and using Futurrex PR1-2000A1 photoresist. The O2 plasma treatment is temporary so the photoresist must be applied as soon as possible.