Procedure for the Transfer of Polymer Films Onto Porous Substrates with Minimized Defects

Summary

We present a procedure for highly controlled and wrinkle-free transfer of block copolymer thin films onto porous support substrates using a 3D-printed drain chamber. The drain chamber design is of general relevance to all procedures involving transfer of macromolecular films onto porous substrates, which is normally done by hand in an irreproducible fashion.

Abstract

The fabrication of devices containing thin film composite membranes necessitates the transfer of these films onto the surfaces of arbitrary support substrates. Accomplishing this transfer in a highly controlled, mechanized, and reproducible manner can eliminate the creation of macroscale defect structures (e.g., tears, cracks, and wrinkles) within the thin film that compromise device performance and the usable area per sample. Here, we describe a general protocol for the highly controlled and mechanized transfer of a polymeric thin film onto an arbitrary porous support substrate for eventual use as a water filtration membrane device. Specifically, we fabricate a block copolymer (BCP) thin film on top of a sacrificial, water-soluble poly(acrylic acid) (PAA) layer and silicon wafer substrate. We then utilize a custom-designed, 3D-printed transfer tool and drain chamber system to deposit, lift-off, and transfer the BCP thin film onto the center of a porous anodized aluminum oxide (AAO) support disc. The transferred BCP thin film is shown to be consistently placed onto the center of the support surface due to the guidance of the meniscus formed between the water and the 3D-printed plastic drain chamber. We also compare our mechanized transfer-processed thin films to those that have been transferred by hand with the use of tweezers. Optical inspection and image analysis of the transferred thin films from the mechanized process confirm that little-to-no macroscale inhomogeneities or plastic deformations are produced, as compared to the multitude of tears and wrinkles produced from manual transfer by hand. Our results suggest that the proposed strategy for thin film transfer can reduce defects when compared to other methods across many systems and applications.

Introduction

Thin film and nanomembrane-based devices have recently garnered wide interest due to their potential use in a broad range of applications, ranging from flexible photovoltaics and photonics, foldable displays, and wearable electronics^1,2,3. A requirement for the fabrication of these various types of devices is the transfer of thin films to the surfaces of arbitrary substrates, which remains challenging due to the fragility of these films and the frequent production of macroscale defect structures, such as wrinkles, cracks, and tears, within the films after transfer^4,5,6,7. Manual transfer by hand, tweezers, and wire loops are common methods of thin film transfer, but inevitably result in structural incongruities and plastic deformation^8,9. Various types of thin film transfer methodologies have been explored such as: 1) polydimethylsiloxane (PDMS) stamp transfer, which involves the use of an elastomeric stamp to obtain the thin film from the donor substrate and subsequently transfer to the receiving substrate¹⁰, and 2) sacrificial layer transfer¹¹, in which an etchant is used to selectively dissolve a sacrificial layer between the support substrate and the thin film, thereby lifting off the thin film. However, these techniques alone do not necessarily allow for thin film transfer without incurring damage to or defect formation within the thin films¹².

Here, we present a novel, low-cost, and generalizable facile method based on sacrificial layer lift-off and meniscus-guided transfer within a custom-designed, 3D-printed drain chamber system, to mechanically place block copolymer (BCP) thin films onto the centers of porous substrates such as anodized aluminum oxide (AAO) discs with little-to-no incurred macroscale defect structures, such as wrinkles, tears, and cracks. In the present context, these transferred thin films can then be used as devices in water filtration studies, potentially after sequential infiltration synthesis (SIS) processing⁹. Image analysis of transferred films obtained from optical microscopy show that the meniscus-guided, drain-chamber system provides smooth, robust, and wrinkle-free samples. In addition, the images also demonstrate the system's ability to reliably place the thin film membranes onto the centers of the receiving substrates. Our results have significant implications for any type of device application requiring the transfer of thin film structures onto the surfaces of arbitrary porous substrates.

Protocol

1. Fabrication of the transfer tool and drain chamber system

1. Attached (Supplementary Files 1, 2) is the engineering drawing for the drain chamber assembly consisting of two parts: top and bottom. Model this device according to the specifications of the desired system (e.g., the outer diameter of the receiving substrate) and export as an STL file for 3D printing.
2. For the top part, utilize a filament printer of choice and print in the lowest resolution possible, including scaffolding wherever necessary. Adhere to the recommended parameters of the printer. It is also recommended that the top part be printed using poly(lactic acid) (PLA) to minimize material shedding.
3. For the bottom part, use an inkjet resin printer or filament printer with a build height as fine as 20 µm.
   NOTE: PLA is a suitable material which minimizes material shedding.
4. Scrub and clean both parts with deionized water, ensuring the removal of any potential shedding material from the printing process. Sonication in deionized water is also recommended. Test the threading on the two parts to ensure a good fit.
5. Complete the drain chamber with a size 117 neoprene O-ring and tubing of the parameters specified in the supporting documents (Supplementary Files 1, 2). A schematic of the entire drain chamber assembly is shown in Figure 1.
6. Print the transfer tool using any filament printer at medium to fine resolution. There are two parts: clamp and loading arm.
   NOTE: It is highly recommended that the transfer tool be printed using poly(lactic acid) (PLA), as other plastics can be poorly wetted and cause the wafer to become wet unexpectedly.
7. Complete the clamp with a size 10 screw and then attach the clamp onto a laboratory jack.

2. Initial mechanized deposition and membrane lift-off from donor substrate

1. Place a bare 25 mm-diameter AAO disc (or any arbitrary porous receiver substrate of choice) onto the bottom part of the drain chamber. Then, place the neoprene O-ring on top of the AAO disc and screw on the top part of the drain chamber.
2. Rinse and/or sonicate the setup various times with deionized (DI) water. This helps to remove any dust and/or any remaining particulates from 3D printing.
3. Place the piece of Si wafer with the transferable polymer stack (donor wafer) onto the lip of the transfer tool loading arm.
4. Fill the drain chamber with 25 mL of DI water.
5. Lower the laboratory jack so that the tool is dipped slowly into the entrance ramp of the drain chamber and that the donor silicon substrate is slowly submerged. Ensure that the wafer is submerged sufficiently for the membrane to completely delaminate and lift-off from the underlying donor substrate.
   NOTE: Using a piece of Si wafer with no dust contamination will ensure easy separation from the donor substrate.
6. Slowly raise the transfer tool out of the water and move it out of the way, making sure not to disturb the floating membrane.
7. Coax the membrane into the opening of the chamber with tweezers. Placing the tweezer in water in front of membrane will guide it due to surface tension. Touching the floating membrane itself is not necessary and should be avoided.

3. Meniscus-guided transfer to receiver substrate with the drain chamber system

1. Connect tubing to the outlet of the bottom part of the drain chamber. Attach this tubing to a 20 mL Luer-lock syringe.
2. Obtain a syringe pump with withdrawing functionality. Place the syringe onto the pump and withdraw water at a rate of 1-2.5 mL/min until all the water has been drained out.
3. After 10 min, the water should be completely removed from the drain chamber. If there is still any residual water within the chamber, reconnect the syringe and tubing and continue to withdraw any residual water.
4. After complete drainage of the water, the membrane will now be placed at the center of the receiver substrate. Disconnect the drain chamber from the syringe pump and disassemble the drain chamber to remove the receiver substrate containing the membrane.
   NOTE: The total process including set-up takes ~15 min. Reducing the working volume of water and increasing the drain rate can shorten this process.
5. Allow the sample to dry completely at room temperature before further use in any application.

Results

The BCP membrane samples were fabricated according to the previously described procedure⁹. The samples were placed onto the lip of the loading arm of the 3D-printed transfer tool (Figure 1, left) and subsequently lowered, with a laboratory jack, onto the entrance ramp of the 3D-printed drain chamber tool (Figure 1, right). A sacrificial layer of poly(acrylic acid) (PAA) between the BCP membrane and underly...

Discussion

While many of the steps listed in this protocol are crucial for the success of the thin film transfer, the nature of the custom-designed 3D printed drain chamber allows for broad flexibility, according to the user's specific requirements. For example, if the receiver substrate has a larger diameter than the 25-mm-diameter AAO discs utilized in this study, the drain chamber can be appropriately modified to fit the new specifications. However, there are certain aspects of the protocol that are necessary to ensure effective...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
35% sodium polyacrylic acid solution	Sigma Aldrich	9003-01-4
Amicon Stirred Cell model 8010 10mL	Millipore	5121
Anodized aluminum oxide, 0.2u thickness, 25mm diameter	Sigma Aldrich	WHA68096022
o ring neoprene 117	Grainger	1BUV7
Objet500 Connex3 3D Printer	Stratasys
Onshape 3D software	onshape
Polylactic acid filament	Ultimaker
ultimaker3 3d filament printer	Ultimaker
Vero Family printable materials	Stratasys