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Alex Ruyack, CNF Fellow - Jan. 2017 - Parts re-purposed from 2017 issue of the Nanometer

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In the future, we are working on various path forwards for BCP lithography implementation at the CNF. One path is further process tuning to reduce defects in the film and improve uniformity and periodicity. Beyond this, we are also looking into additional processing steps required to alter the BCP film morphology. In particular, we are working on a graphoepitaxy process that will result in parallelly aligned domains, rather than pores. Finally, we are also investigating other BCP systems for smaller features sizes (<10 nm), such as PS-b-PDMS.

Example Process Flow

Materials/notes:

• Solvent: Toluene (PGMEA can also be used)
• Brush/Surface treatment: P9085-SMMAranOHT 2% in toluene
• Copolymer: P8205-SMMA 1% or 2% in in toluene
  • Lower wt. % will yield thinner films, but the polymer separation is typically better defined
• Keep stir bar in bottle, mix before spinning a few hours (helps reduce non-uniformies in the finished film)
• Work in e-beam room, spinners and oven
• Line spinner with beta wipes and discard when done
• Keep substrates very clean (tested substrates include Si with thermal, PECVD, and ALD oxide)
• Pieces or full wafers will work. Consider pieces for testing since the polymers are not inexpensive (~$500/g)

Recipe

1. Treatment with brush - P9085-SMMAranOHT
   1. Mix solution before spinning!
   2. Spin chips and rinse with toluene
   3. Coat sample with the surface treatment (much like P20). It’s very low viscosity
   4. Spin using 1600 rpm, 10krpm/s ramp, 30s¹
   5. Anneal overnight in vacuum oven at lowest pressure possible, T = 170°C²
2. Coating with P8205-SMMA
   1. Mix solution before spinning!
   2. Rinse excess surface treatment with toluene. Spin the chip and drop toluene from a pipette.
   3. Coat sample with the BCP solution
   4. Spin using 2000 rpm, 10krpm/s ramp, 30s (for 1% solution this will yield about 30nm thickness)
   5. Anneal in vacuum oven, everything the same as above except T=180°C
3. Expose and Develop
   1. 1 min 15s flood exposure on ABM - 220 nm (You must change the mirror to the 220 nm from the standard - replace when done!)
4. 5 min acetic acid etch to remove cleaved PMMA
5. Pattern transfer
   1. Oxide etch – Oxford 100, CH₂F₂/He: 20/80 sccm, RIE/ICP = 60W/3000W, 4 mtorr, 30 sec per 50 nm.
   2. Si etch – Cobra, HBr/Ar: 20/4 sccm, RIE/ICP = 18W/800W, 8mtorr.

Comments:

¹1600 rpm seems good for 2% solution viscosity (too fast and then treatment becomes too thin, leaving uncovered areas, too slow and you end up with pooled solution at the corner/edges of square chips – full/circular substrates may prevent this)

²Literature shows T_g of about 110C for both PS and PMMA and T_d of 220C for PS and 300 for PMMA, so this is a good spot between them – Harris, Appel, Ade, 2002. Going higher is probably fine as long as you are below T_d. Thicker films might benefit from this.

³Time is not critical as long as it above a threshold – the light crosslinks PS and cleaves PMMA. I would not recommend below 1 min.

Characterization

• AFM will not work with high aspect ratio structures. For example, 2% BCP solution produces about 20-30 nm diameter pores. AFM trip dimensions are ~ 10nm tip with 15-degree slope. This means that only a pore depth up to about 19-37 nm can be read out (and you will get significant effects due to tip profile). Anything more than that will not work for measuring thickness. While you can’t get good depth measurements, you can still see pores and get a feel for your samples (but honestly, SEM is a lot faster and easier).

• SEM works well with 2 kV and the InLens detector. There is decent contrast difference between the two phases for PS-PMMA polmyers. Once developed, you can go to 20kV with no charging issues, even on oxide. This gives much nicer images at high magnifications since it bumps up your ultimate resolution (Abbe’s Equation).

• Thickness measurement can be done either with Filmetrics (using n for PS or PMMA, 1.59/1.45 respectively, works fine. PS is slightly more accurate since it is the majority polymer in our composition) or profilometry. Profilometry may yield better results since the index issue will not be present, but results between the two typically agree within about 20%. You must scratch your surface free of any BCP to use the profilometer.

• To strip baked BCP use toluene or 1165 (both tested to work before any sort of pattern transfer). Post pattern transfer resist may be harder to get rid of. The PT72 Sidewall clean recipe has worked: CF₄/O₂: 5/30 sccm, 150W, 60 mtorr. The rate is listed at 100nm/min. It worked to strip remaining BCP after a Si etch in the cobra on plain Si (no O₂ etch).