Fabrication of Large-area Free-standing Ultrathin Polymer Films

Summary

We describe a method for the fabrication of large-area (up to 13 cm diameter) and ultrathin (as thin as 8 nm) polymer films. Instead of using a sacrificial interlayer to delaminate the film from its substrate, we use a self-limiting surface treatment suitable for arbitrarily large areas.

Abstract

This procedure describes a method for the fabrication of large-area and ultrathin free-standing polymer films. Typically, ultrathin films are prepared using either sacrificial layers, which may damage the film or affect its mechanical properties, or they are made on freshly cleaved mica, a substrate that is difficult to scale. Further, the size of ultrathin film is typically limited to a few square millimeters. In this method, we modify a surface with a polyelectrolyte that alters the strength of adhesion between polymer and deposition substrate. The polyelectrolyte can be shown to remain on the wafer using spectroscopy, and a treated wafer can be used to produce multiple films, indicating that at best minimal amounts of the polyelectrolyte are added to the film. The process has thus far been shown to be limited in scalability only by the size of the coating equipment, and is expected to be readily scalable to industrial processes. In this study, the protocol for making the solutions, preparing the deposition surface, and producing the films is described.

Introduction

Free-standing thin polymer films are used in a variety of applications including sensors, ^1-3 MEMs, catalysis or filtration, ⁴ and tissue engineering. ^5-8 They are also used for fundamental studies exploring the behavior of polymers under confinement. ^9-13 A free-standing film is one that is supported on a non-continuous substrate such as an annular ring or hoop as opposed to a silicon wafer or glass slide. This work describes a simple, repeatable fabrication procedure for ultrathin free-standing polymer films that is suitable for large-area films or high-throughput production. It is compatible with a variety of different po....

Protocol

1. Solution Preparation

1. Filter 60 g of ethyl lactate using a syringe and a 0.20 µm syringe filter. Add 0.3 g of polyvinyl formal to the ethyl lactate. Place the solution into the oven at 50 °C for 4 hr. Shake the vial gently to see if the polymer has dissolved completely.
   1. If the solution is cloudy or still shows optical inhomogeneities, return the vial to the oven for another 2 hr. This recipe is for a 0.5 wt% polymer solution, which is typically used for film thicknesses around 30 nm. Solutions with higher polymer weight content can be used for thicker films.
2. Prepare a PDAC solution by weighing 1.0 g of P....

Results

Figure 1 shows an example of a free-standing thin polymer film over a large area. This 55 nm thick polyvinyl formal film was fabricated using the procedure described here and is mounted on a 13-cm diameter steel hoop. The delamination occurs over large areas without introducing defects that lead to tearing of the film. Thus, the intrinsic strength of polyvinylformal can be exploited even for very thin films. Figure 2 shows a 22 nm thick free-standing film that is strong enough to be load.......

Discussion

The PDAC substrate treatment is based on self-limiting electrostatic interactions, meaning substrates of any size can be easily treated provided that they are negatively charged (e.g., silicon or glass). Figures 1-2 shows very large thin films (up to 13 cm in diameter) fabricated using this protocol, with the only change being the volume of reagents used. The ultimate achievable size appears to be limited only by the deposition and delamination equipment or the ultimate strength of the polymer u.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Vinylec E	SPI
ethyl lactate, >98%, FCC, FG	Sigma-Aldrich	W244007-1KG-K
4" silicon wafers <100>, Single side polished	International Wafer Service
sulfuric acid, 98%, ACS reagent grade	Sigma-Aldrich	320501-6X500ML
hydrogen peroxide, 30%, semiconductor grade	Sigma-Aldrich	316989-3.7L
isopropanol, ACS grade, 4 L	Fisher Scientific	A464-4
dichloromethane, ACS grade	Alfa-Aesar	22917
deionized water, distilled
PDAC reagent (Sigma-Aldrich 409014)	Sigma-Aldrich	409014
Spin Coater	Laurell Technologies	WS-650-23
Barnstead/Thermolyne Super Nuova explosion-proof hot plate
explosion-proof forced air oven	VWR	1330 FMS
balance with a range of 1 mg to 1,020 g	Mettler Toledo	MS1003S
reflectance spectrometer	Filmetrics	F20-UV
manipulator consisting of a Klinger tilt stage, a Brinkman rack-and-pinion and a lab jack
Cutting tool/template, LLNL-built, no drawings
straight edge, LLNL, no drawings
Tent hoop, LLNL
culture dish 190 mm x 100 mm, Pyrex	VWR
20 ml beaker, Pyrex	VWR
250 ml beaker, Pyrex	VWR
1,000 ml beaker, Pyrex	VWR
60 ml glass vial with plastic stopper	VWR
Petri dish, 150 mm diameter x2, Pyrex	VWR
600 ml beaker x2, Pyrex	VWR
tweezers, stainless steel
cutting blade	Exacto
clean room wipes	Contec	PNHS-99
polyester knit 9/91 IPA/DI water wipes	Contec	Prosat
Fluoroware wafer trays	Ted Pella	1395-40
Nylon Micro fiber (camel hair)
Disposable BD 3-ml plastic syringe	VWR
0.2 μm Luer-lock PTFE filters	Acrodisc
0.45 μm Luer-lock PTFE filters	Acrodisc