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Progress Report I: Fabrication of Nanopores inSilicon Nitride Membranes using Self-Assembly ofPS-b-PMMAUnnati JoshiQuattrone Nanofabrication Facility, unnatij@seas.upenn.edu

Vishal VenkateshQuattrone Nanofabrication Facility, vishalv@seas.upenn.edu

Hiromichi YamamotoQuattrone Nanofabrication Facility, hyam@seas.upenn.edu

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

Progress Report I: Fabrication of Nanopores in Silicon NitrideMembranes using Self-Assembly of PS-b-PMMA

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

KeywordsNanopore, Self-Assembly, PS-b-PMMA, Silicon nitride, Membrane

DisciplinesChemical Engineering | Electrical and Computer Engineering | Engineering | Engineering Science andMaterials | Materials Science and Engineering | Nanoscience and Nanotechnology | Physical Sciences andMathematics

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Progess Report I: Fabrication of Nanopores in Silicon Nitride Membraneusing Self-Assembly of PS-b-PMMA

Unnati Joshi,1 Vishal Venkatesh,1 Hiromichi Yamamoto1, a)1Singh Center for Nanotechnology, University of Pennsylvania3205 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 cleanroomtechniques, and of nanopore preparation via a self-assembled PS-b-PMMA film. A 36.9 µm thick membraneis successfully prepared by KOH wet etching. The membrane is a layered structure of 36.8 µm thick Si and116 nm thick silicon nitride. It is also exhibited that in the 47 nm thick (= 1.5L0, L0: polymer domainspacing) PS-b-PMMA film, the nanopore structure is observed in the vicinity of a dust particle, but mostof the area indicates lamellar domain structure. The thickness of PS-b-PMMA film will be optimized toprepare a complete nanopore template in the future work.

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

I. Introduction

A nanoporous template has been investigated for manyapplications, such as alumina template nanowire,1 wa-ter filtering,2 sensors for DNA,3 magnetic memory,4 drugdelivery,5 molecular sensing,6 and molecular electronics.7

Existing techniques to fabricate nanoscale features, likeE-beam lithography, are time consuming, have large en-ergy requirements, and require equipment and facilitieswith high capital cost. Block copolymer (BCP) self-assembly is emerging as an alternative to these tech-nologies because of simplicity in their processing, con-venient size and shape tunability of structures at thenanoscale by simply changing their molecular weightsand compositions.8–10 The grand objective of this workis to fabricate nanopores in silicon nitride membraneusing self-assembly of BCP poly(styrene-block-methylmethacrylate) (PS-b-PMMA), in order to meet potentialdemand for this process at Quattrone NanofabricationFacility (QNF). This report details the progress of fab-rication of the silicon nitride membrane from Si wafersusing cleanroom techniques, and of nanopore prepara-tion in PS-b-PMMA film using the self-assembly. Fig-ure 1 shows the process flow of fabrication of silicon ni-tride membrane using potassium hydroxide (KOH) etch-ing. Nanopore preparation in PS-b-PMMA film was con-ducted on another substrate separately. Silicon nitridemembranes with nanopores will be described in the nextreport.

a)Electronic mail: hyam@seas.upenn.edu

II. Experimental Section

A. Materials

Asymmetric PS-b-PMMA (poly-methyl methacrylaterich in syndiotactic contents > 80 %) with the aver-age molecular weight Mn of poly-styrene = 46,000, Mnof poly-methyl methacrylate = 21,000, and polydisper-sity index (PDI) =1.09 (polymer domain spacing L0 =32 nm) was purchased from Polymer Source, and wasused as received. Hydroxyl-terminated random copoly-mer poly(styrene-r -methyl methacrylate),α-hydroxyl-ω-tempo moiety terminated (PS-r -PMMA-OH), was alsopurchased from Polymer Source, and was used as re-ceived. The PS content was 60 mol% (Mn = 5,400 andPDI = 1.40). Hereafter, PS-r -PMMA-OH is referred toas a brush-OH. A CMOS grade toluene (trace impuritylevel, 10-200 ppb) was purchased from J. T. Baker, andwas used as-received as solvent.

B. Silicon nitride deposition

A (100) Si double side polished wafer was sonicated inacetone and isopropyl alcohol (IPA) for 5 min each, andwas dried using a nitrogen gun. 354 and 328 nm thick sil-icon nitride films were deposited on the side 1 and 2 of theSi wafer as a hard mask upon KOH etching and mem-brane, respectively, using ”Test recipe” Oxford PlasmaLab 100 (Plasma Enhanced Chemical Vapor Deposition(PECVD)).11 The thicknesses of silicon nitride films weredetermined using Filmetrics F40 after the depositions.

C. UV lithography using SUSS MicroTec MA6 Gen3Mask Aligner

Side 1 was spin coated with S1813 photoresist at 4000RPM for 60 sec, followed by baking at 115 °C for 5 min ona hot plate. The wafer was then exposed to 405 nm UVlight 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, thewafer was developed in MF-319 (Microchem) for 1 min,and then rinsed in deionized water. Figure 2 representsthe wafer after development.

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

D. Reactive Ion Etching of Side 1The silicon nitride film on Side 1 was dry-etched

through the developed photoresist film, using Oxford80 plus RIE with the following condition (the defaultrecipe): O2 = 10 sccm; SF6 = 50 sccm; pressure = 150mTorr; power = 100 W; T = 17.5 °C. The etch rate was500-700 nm/min.

E. Protective coating on Side 2 before KOH etchingIn order to protect the silicon nitride layer on Side

2 from KOH wet etching, Protek B3 Primer was spin

coated at 1500 RPM for 1 min on Side 2, followed bybaking at 205 °C for 60 sec on a hot plate to harden theprimer. On top of the primer layer, Protek B3 was spincoated at 1000 RPM for 60 sec, followed by baking at120 °C for 120 sec. The wafer was again baked at 250 °Cfor 50 sec. The last bake ensures strong bonding betweenProtek B3 primer and Protek B3, and also between theentire coating and the silicon nitride layer.

F. KOH etching of Si on Side 1

FIG. 3. Wafer after KOH etching.

The Si exposed through the window of silicon nitridehard mask was wet etched using 30 wt% KOH solutionat 80 °C. The wet etching was carried out for 3 hoursand 40 min. Figure 3 shows a photograph of the etched

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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-tride film on Side 1 was still observed, while an 18 nmthick silicon nitride film on Side 2 was found. The 18nmthick silicon nitride on Side 2 implies that the protectivecoating was not sufficient and a significant amount of sil-icon nitride was etched. A 98nm silicon nitride film wasagain deposited on Side 2 using PECVD to ensure mem-brane thickness greater than 100nm. Figure 4 exhibitscross-sections of the etched Si wafer, indicating that thethickness of membrane is 36.9 µm, of which the siliconnitride thickness is 116 nm. The Si thickness of 36.8 µmwill be etched by dry-etch on Side 1.

G. Self-assembly of PS-b-PMMA block co-polymer

A single-side polished Si substrate was sonicated inacetone and IPA for 5 min each, and was dried using anitrogen gun. A 58.7 nm thick silicon oxide film was de-posited on the substrate using the PECVD tool describedabove. The substrate was then hydroxylated in piranhasolution at 70-80 °C for 30 min, and rinsed with DI water.A neutral brush-OH was spin coated on the hydroxylatedsubstrate at 3000 RPM for 60 sec, and then thermally an-nealed at 220 °C for 5 min in air. After annealing, theunreacted brush-OH polymer was removed by sonicationof the substrate in toluene three times for 20 min each.The thickness of the brush-OH film was determined tobe 3.6 nm using a Woollam Ellipsometer. The PS-b-

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

PMMA block co-polymer was spin coated on the brushlayer at 2760 RPM for 60 sec. The thickness of PS-b-PMMA film was 47 nm (=1.5L0). The PS-b-PMMA filmwas then thermally annealed at 190 °C for 2 days in anoven in vacuum. In order to remove the PMMA portion,the sample was etched by oxygen plasma for 3 min at40 watts and 10 sccm of oxygen flow, using an AnatechSCE-108 Barrel Asher.

Figure 5 shows SEM images of self-assembly of PS-b-PMMA around a dust particle. The area with a com-mensurate antisymmetric thickness of 1.5L0 shows thelamellar domain structure, whereas the vicinity of thedust particle indicates the nanopore structure. It is as-sumed that the area with nanopores has a commensuratethickness of 2.0L0, as reported previously.12 The thick-ness of PS-b-PMMA film will be optimized to prepare acomplete nanopore template in the future work.

III. Results and discussion

A 36.9 nm thick Si and silicon nitride membrane wassuccessfully prepared by KOH wet etching. The Si thick-ness on Side 1 was 36.8 µm, while the silicon nitridethickness on Side 2 was 18 nm. As a result, a 98 nmthick silicon nitride film had to be re-deposited on Side2, to ensure that the silicon nitride thickness was 100nm. The 18nm thick silicon nitride on Side 2 implies thatthe protective coating was not sufficient and a significantamount of silicon nitride was etched in KOH solution at80 °C. The 100 nm thick silicon nitride membrane will beprepared by dry-etching silicon on Side 1.

The nanopore structure was observed in the vicinity ofthe dust particle, but the lamellar domain structure wasshown in the area of the commensurate anti-symmetricwet thickness of 1.5L0. It is assumed that the thickness ofthe area with nanopores should be commensurate thick-ness of 2.0L0.12 The thickness of PS-b-PMMA film needsto be optimized to prepare a nanopore template. Sili-

con nitride membrane with nanopores will be describedin the next report.

IV. Summary

In this work, we have developed a process to fabri-cate a silicon nitride membrane using KOH wet etch-ing that can ensure faster and more industrially feasiblemembrane device fabrication. The fine adjustment of thethickness will be carried out using dry-etching. We havealso demonstrated the partial nanopore structure usingself-assembly of PS-b-PMMA block copolymer thermallyannealed at 190 °C for 2 days in vacuum oven. How-ever, the thickness of PS-b-PMMA film still needs to beoptimized to complete the nanopore template.

V. Acknowledgements

This work was supported by the National ScienceFoundation (Grant NNCI-1542153).

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