Fabrication of Nanopores in Silicon Nitride Membranes Using Self-Assembly of PS-B-PMMA Unnati Joshi Quattrone Nanofabrication Facility, Unnatij@Seas.Upenn.Edu

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3-5-2019 Progress Report I: Fabrication of Nanopores in Silicon Nitride Membranes using Self-Assembly of PS-b-PMMA Unnati Joshi Quattrone Nanofabrication Facility, [email protected]

Vishal Venkatesh Quattrone Nanofabrication Facility, [email protected]

Hiromichi Yamamoto Quattrone Nanofabrication Facility, [email protected]

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Joshi, Unnati; Venkatesh, Vishal; and Yamamoto, Hiromichi, "Progress Report I: Fabrication of Nanopores in Silicon Nitride Membranes using Self-Assembly of PS-b-PMMA", Protocols and Reports. Paper 56. https://repository.upenn.edu/scn_protocols/56

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Abstract This progress report describes fabrication of silicon nitride membranes from Si wafers using cleanroom techniques, and of nanopore preparation via a self-assembled PS-b-PMMA film. A 36.9 µm thick membrane is successfully prepared by KOH wet etching. The membrane is a layered structure of 36.8 µm thick Si and 116 nm thick silicon nitride. It is also exhibited that in the 47 nm thick PS-b-PMMA film, the nanopore structure is observed in the vicinity of a dust particle, but most of the area indicates lamellar domain structure. The thickness of PS-b-PMMA film will be optimized to prepare a complete nanopore template in the future work.

Keywords Nanopore, Self-Assembly, PS-b-PMMA, Silicon nitride, Membrane

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This technical report is available at ScholarlyCommons: https://repository.upenn.edu/scn_protocols/56 Progess Report I: Fabrication of Nanopores in Silicon Nitride Membrane using Self-Assembly of PS-b-PMMA Unnati Joshi,1 Vishal Venkatesh,1 Hiromichi Yamamoto1, a) 1Singh Center for Nanotechnology, University of Pennsylvania 3205 Walnut St. Philadelphia, PA 19104 (Dated: Received 6 February 2019; accepted 27 February) This progress report describes fabrication of silicon nitride membranes from Si wafers using cleanroom techniques, and of nanopore preparation via a self-assembled PS-b-PMMA film. A 36.9 µm thick membrane is successfully prepared by KOH wet etching. The membrane is a layered structure of 36.8 µm thick Si and 116 nm thick silicon nitride. It is also exhibited that in the 47 nm thick (= 1.5L0, L0: polymer domain spacing) PS-b-PMMA film, the nanopore structure is observed in the vicinity of a dust particle, but most of the area indicates lamellar domain structure. The thickness of PS-b-PMMA film will be optimized to prepare a complete nanopore template in the future work.

Key Words: Nanopore, Self-Assembly, PS-b-PMMA, Silicon nitride, Membrane

I. Introduction II. Experimental Section

A. Materials A nanoporous template has been investigated for many Asymmetric PS-b-PMMA (poly-methyl methacrylate applications, such as alumina template nanowire,1 wa- rich in syndiotactic contents > 80 %) with the aver- ter filtering,2 sensors for DNA,3 magnetic memory,4 drug age molecular weight Mn of poly-styrene = 46,000, Mn delivery,5 molecular sensing,6 and molecular electronics.7 of poly-methyl methacrylate = 21,000, and polydisper- Existing techniques to fabricate nanoscale features, like sity index (PDI) =1.09 (polymer domain spacing L = E-beam lithography, are time consuming, have large en- 0 32 nm) was purchased from Polymer Source, and was ergy requirements, and require equipment and facilities used as received. Hydroxyl-terminated random copoly- with high capital cost. Block copolymer (BCP) self- mer poly(styrene-r-methyl methacrylate),α-hydroxyl-ω- assembly is emerging as an alternative to these tech- tempo moiety terminated (PS-r-PMMA-OH), was also nologies because of simplicity in their processing, con- purchased from Polymer Source, and was used as re- venient size and shape tunability of structures at the ceived. The PS content was 60 mol% (Mn = 5,400 and nanoscale by simply changing their molecular weights PDI = 1.40). Hereafter, PS-r-PMMA-OH is referred to and compositions.8–10 The grand objective of this work as a brush-OH. A CMOS grade toluene (trace impurity is to fabricate nanopores in silicon nitride membrane level, 10-200 ppb) was purchased from J. T. Baker, and using self-assembly of BCP poly(styrene-block-methyl was used as-received as solvent. methacrylate) (PS-b-PMMA), in order to meet potential demand for this process at Quattrone Nanofabrication B. Silicon nitride deposition Facility (QNF). This report details the progress of fab- A (100) Si double side polished wafer was sonicated in rication of the silicon nitride membrane from Si wafers acetone and isopropyl alcohol (IPA) for 5 min each, and using cleanroom techniques, and of nanopore prepara- was dried using a nitrogen gun. 354 and 328 nm thick sil- tion in PS-b-PMMA film using the self-assembly. Fig- icon nitride films were deposited on the side 1 and 2 of the ure 1 shows the process flow of fabrication of silicon ni- Si wafer as a hard mask upon KOH etching and mem- tride membrane using potassium hydroxide (KOH) etch- brane, respectively, using ”Test recipe” Oxford Plasma ing. Nanopore preparation in PS-b-PMMA film was con- Lab 100 (Plasma Enhanced Chemical Vapor Deposition ducted on another substrate separately. Silicon nitride (PECVD)).11 The thicknesses of silicon nitride films were membranes with nanopores will be described in the next determined using Filmetrics F40 after the depositions. report. C. UV lithography using SUSS MicroTec MA6 Gen3 Mask Aligner Side 1 was spin coated with S1813 photoresist at 4000 RPM for 60 sec, followed by baking at 115 °C for 5 min on a hot plate. The wafer was then exposed to 405 nm UV a)Electronic mail: [email protected] light using SUSS MicroTec MA6 Gen3 Mask Aligner with U. Joshi el. al. (2019) Published by Singh Center for Nanotechlogy

FIG. 1. Process ﬂow of fabrication of silicon nitride membrane using KOH etching. an exposure dose of 150 mJ/cm2. After the exposure, the coated at 1500 RPM for 1 min on Side 2, followed by wafer was developed in MF-319 (Microchem) for 1 min, baking at 205 °C for 60 sec on a hot plate to harden the and then rinsed in deionized water. Figure 2 represents primer. On top of the primer layer, Protek B3 was spin the wafer after development. coated at 1000 RPM for 60 sec, followed by baking at 120 °C for 120 sec. The wafer was again baked at 250 °C for 50 sec. The last bake ensures strong bonding between Protek B3 primer and Protek B3, and also between the entire coating and the silicon nitride layer.

F. KOH etching of Si on Side 1

FIG. 2. Side 1 of the Si wafer after exposure and development.

D. Reactive Ion Etching of Side 1 The silicon nitride ﬁlm on Side 1 was dry-etched through the developed photoresist ﬁlm, using Oxford 80 plus RIE with the following condition (the default recipe): O = 10 sccm; SF = 50 sccm; pressure = 150 2 6 FIG. 3. Wafer after KOH etching. mTorr; power = 100 W; T = 17.5 °C. The etch rate was 500-700 nm/min. The Si exposed through the window of silicon nitride E. Protective coating on Side 2 before KOH etching hard mask was wet etched using 30 wt% KOH solution In order to protect the silicon nitride layer on Side at 80 °C. The wet etching was carried out for 3 hours 2 from KOH wet etching, Protek B3 Primer was spin and 40 min. Figure 3 shows a photograph of the etched

2 U. Joshi el. al. (2019) Published by Singh Center for Nanotechlogy

FIG. 4. SEM images of membrane cross sections.

FIG. 5. SEM images of PS-b-PMMA self-assembly.

Si wafer. After the etching, a 100 nm thick silicon ni- G. Self-assembly of PS-b-PMMA block co-polymer tride film on Side 1 was still observed, while an 18 nm A single-side polished Si substrate was sonicated in thick silicon nitride film on Side 2 was found. The 18nm acetone and IPA for 5 min each, and was dried using a thick silicon nitride on Side 2 implies that the protective nitrogen gun. A 58.7 nm thick silicon oxide film was de- coating was not sufficient and a significant amount of sil- posited on the substrate using the PECVD tool described icon nitride was etched. A 98nm silicon nitride film was above. The substrate was then hydroxylated in piranha again deposited on Side 2 using PECVD to ensure mem- solution at 70-80 °C for 30 min, and rinsed with DI water. brane thickness greater than 100nm. Figure 4 exhibits A neutral brush-OH was spin coated on the hydroxylated cross-sections of the etched Si wafer, indicating that the substrate at 3000 RPM for 60 sec, and then thermally an- thickness of membrane is 36.9 µm, of which the silicon nealed at 220 °C for 5 min in air. After annealing, the nitride thickness is 116 nm. The Si thickness of 36.8 µm unreacted brush-OH polymer was removed by sonication will be etched by dry-etch on Side 1. of the substrate in toluene three times for 20 min each. The thickness of the brush-OH film was determined to be 3.6 nm using a Woollam Ellipsometer. The PS-b-

3 U. Joshi el. al. (2019) Published by Singh Center for Nanotechlogy

PMMA block co-polymer was spin coated on the brush con nitride membrane with nanopores will be described layer at 2760 RPM for 60 sec. The thickness of PS-b- in the next report. PMMA film was 47 nm (=1.5L0). The PS-b-PMMA film was then thermally annealed at 190 °C for 2 days in an IV. Summary oven in vacuum. In order to remove the PMMA portion, the sample was etched by oxygen plasma for 3 min at In this work, we have developed a process to fabri- 40 watts and 10 sccm of oxygen flow, using an Anatech cate a silicon nitride membrane using KOH wet etch- SCE-108 Barrel Asher. ing that can ensure faster and more industrially feasible Figure 5 shows SEM images of self-assembly of PS-b- membrane device fabrication. The fine adjustment of the PMMA around a dust particle. The area with a com- thickness will be carried out using dry-etching. We have mensurate antisymmetric thickness of 1.5L0 shows the also demonstrated the partial nanopore structure using lamellar domain structure, whereas the vicinity of the self-assembly of PS-b-PMMA block copolymer thermally dust particle indicates the nanopore structure. It is as- annealed at 190 °C for 2 days in vacuum oven. How- sumed that the area with nanopores has a commensurate 12 ever, the thickness of PS-b-PMMA film still needs to be thickness of 2.0L0, as reported previously. The thick- optimized to complete the nanopore template. ness of PS-b-PMMA film will be optimized to prepare a complete nanopore template in the future work. V. Acknowledgements III. Results and discussion This work was supported by the National Science A 36.9 nm thick Si and silicon nitride membrane was Foundation (Grant NNCI-1542153). successfully prepared by KOH wet etching. The Si thickness on Side 1 was 36.8 µm, while the silicon nitride 1M. P. Proenca J. Ventura A. M. Pereira C. T. Sousa, D. C. Leitao thickness on Side 2 was 18 nm. As a result, a 98 nm and J. P. Araujo. Appl. Phys. Rev., 1:031102, 2014. thick silicon nitride film had to be re-deposited on Side 2J.-C. Idrobo Y. Song J. Kong T. Laoui M. Atieh S. C. OHern, M. S. H. Boutilier and R. Karnik. Nano Lett., 14:1234, 2014. 2, to ensure that the silicon nitride thickness was 100 3C. Dekker. Nat. Nanotechnol., 2:209, 2007. nm. The 18nm thick silicon nitride on Side 2 implies that 4E. W. Edwards-Y.-H. La S. Xiao, X. M. Yang and P. F. Nealey. the protective coating was not sufficient and a significant Nanotechnology, 16:S324, 2005. amount of silicon nitride was etched in KOH solution at 5E. A. Jackson and M. A. Hillmyer. ACS Nano, 4:3548, 2010. 6 80 °C. The 100 nm thick silicon nitride membrane will be O. A. Saleh and L. L. Sohn. Nano Lett., 3:37, 2003. 7D. K. James and J. M. Tour. Chem. Mater., 16:4423, 2004. prepared by dry-etching silicon on Side 1. 8M. Gopinadhan H. Hu and C. O. Osuji. Soft Matter, 10:3867, The nanopore structure was observed in the vicinity of 2014. the dust particle, but the lamellar domain structure was 9M.A. Morris. Microelectron. Eng., 132:207, 2015. shown in the area of the commensurate anti-symmetric 10C.-C. Liu S. Ji, L. Wan and P. F. Nealey. Prog. Polym. Sci., wet thickness of 1.5L . It is assumed that the thickness of 54-55:76, 2016. 0 11I. Bajwa. https://repository.upenn.edu/scn_protocols/18/. the area with nanopores should be commensurate thick- 2016. 12 ness of 2.0L0. The thickness of PS-b-PMMA film needs 12H. Yamamoto. https://repository.upenn.edu/scn_protocols/ to be optimized to prepare a nanopore template. Sili- 27/. 2016.