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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.
References:
[1] C.M. Bates, et al., Block Copolymer Lithography, Macromolecules, 2014, 47 (1), pp 1-12.
[2] Burri, Olivier, 2D K Nearest Neighbors Python script, (2017), GitHub repository, https://gist.github.com/lacan/2643f2ce7e33d1bb07adafde9ff94101
Example Process Flow for PS-b-PMMA
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)
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- 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).
Polymer Selection
Resolution & The Flory–Huggins Parameter: The ultimate feature size of the BCP you select will be a strong function of the Flory–Huggins Parameter (χ), which is dependent on the two polymers that comprise the BCP. This is a measure of the energy of interaction between the two polymer blocks. The higher χ is, the more the two blocks want to separate. Thus, higher χ enables smaller features, but it also complicates fabrication. This is because with more disparate surface energies, the more one phase will prefer the substrate surface or air/vacuum to minimize interfacial energy, making perpendicular morphology alignment (which is desired for lithography) more difficult.
For example, PS/PMMA (χ = 0.06), considered a standard in BCPs, has a theoretical resolution of about 12 nm (~20 nm in practice), and can be processed with a neutral brush and vacuum oven anneals. PS/PDMS on the other hand (χ = 0.27) can reach sub-10nm in practice, but PDMS highly favors being in contact with air, likely resulting in a layer of PDMS at the surface, even if a perpendicular orientation of obtained at the surface with a brush. This requires additional processing to remove the upper layer. These high χ BCPs also require annealing beyond just a vacuum oven, such as solvent or microwave annealing [1].
Beyond selecting the two components of the BCP, there may also be a choice of functional end group. As long as the polymer is above ~2kg/mol, this should have little to no effect (e.g., the end group chemistry is insignificant when it is a small constituent of the polymer overall) [2].
Brushes: To make the substrate non-selective to a particular polymer phase and promote perpendicular ordering, a neutral brush layer can be used. This is typically a surface coating/monolayer whose surface energy is nearly equal for both phases of the BCP. Random co-polymers of the same blocks as the BCP to be applied work well, if they are available. Some literature reports success with homopolymer brushes. If using a homopolymer brush, it is suggested to followed established literature for your particular system [3].
Keep in mind that surface chemistry will dictate phase preference. Additionally, if you are performing grapho or chemoepitaxy, you may not need a brush.
Morphology:
Typical BCP morphologies follow the trend above. The left plot shows the theoretical phase diagram (χN vs. block/molecular weight ratio, where N is the number of monomers). On the right is an actual experimental plot for poly(isoprene-styrene) diblock copolymers. For lithography, the C/C’ (cylindrical) and L (lamellar) phases are most useful. These are relative easy phases to select using near 1/4 (or 3/4) or 1/2 block ratio, respectively [4].
A note on thickness: Approaching film thicknesses similar to the periodicity tend to yield better phase separation. This is because the boundary condition in the thickness direction enforces no phase separation in that direction. Thus, it is not unlikely for ordering to improve as the thickness is decreased. This does not necessarily mean, though, that good ordering cannot be achieved for thicker films.
Some other considerations: Typically, higher molecular weight improves phase segregation. Additionally, it has been shown that increasing molecular weight and/or the polydispersity increases the lattice parameter/domain spacing of the resulting film [5, 6].
Etch selectivity: You will want to select a BCP whose phases have good etch selectivity. For example, PS-b-PMMA has excellent etch selectivity. PS is crosslinked and PMMA is cleaved with 220 nm UV light, making development easy. Another is example is PS-b-PDMS, where PDMS is relatively unaffected by O2 plasma.
Grapho and Chemoepitaxy: For lithography purposes, it is sometimes desired to not only have an ordered array produced by the BCP, but also have some additional control over placement or orientation. For example, to have aligned lamellar structures. This can be accomplished using chemical or topological pre-patterned substrates (chemo- and grapho-epitaxy, respectively) to achieve directed self-assembly. If interested, I suggest reading the below paper for general information [7].
General reading: For general reading about BCPs, I suggest [8].
References
[1] Bates, et al., “Block Copolymer Lithography,” Macromolecules, 2014, 47 (1), pp 2-12.
[2] C Qian, S. Grigoras, and L. Kennan, “End Group Effects on the Phase Behavior of Polymer Blends: Poly(dimethylsiloxane) and Poly(methylphenylsiloxane) Blend,” Macromolecules, 1996, 29 (4), pp 1260-1265.
[3] Y. Pang, et al., “Controlling Block Copolymer−Substrate Interactions by Homopolymer Brushes/Mats,” Macromolecules, 2017, 50 (17), pp 6733-6741.
[4] F. Bates & G. H. Fredrickson, “Block Copolymers—Designer Soft Materials,” Phys. Today, 52, 2, 32, 1999, pp 32-38.
[5] N. Lynda, A. J. Meuler, M. A. Hillmyer, “Polydispersity and block copolymer self-assembly,” Prog. Polym. Sci., 33 (9), 2008, pp. 875-893.
[6] Yushu Matsushita, et al., “Molecular Weight Dependence of Lamellar Domain Spacing of Diblock Copolymers in Bulk,” Macromolecules, 1990, 23 (19), pp 4313-4316.
[7] J. Kim, et al., “Directed self-assembly of block copolymers for next generation nanolithography,” Mater. Today, 16 (12), 2013, pp 468-476.
C. Bates, et al., “Block Copolymer Lithography,” Macromolecules, 2014, 47 (1), pp 2-12.