Extremely Flat Metal Films Fabricated By a Surface Roughness Transfer Method for Flexible Electronics
Authors: Kim Sungjoo , Lee Jong-Lam 
Published: February 2020

1. Objective
Use of a metal substrate is ideal in flexible electronics, because it has excellent mechanical strengthy, thermal stability and chemical stability greater than that of other types of substrate.1-2 However, metals have rough surface roughness (generally Ra > 50 nm) to be used directly in the devices. Previous methods of planarization such as polishing or polymer coating do not satisfy the requirements for low price, low defect density, and low tack time.3Therefore they have not been commercialized due to their fundamental problem of high surface roughness. Here, we present an innovative approach to fabrication of extreme flat (EF) metal substrates to overcome such problems without any chemical lift-off (CLO) or planarization process. With the controlled design of peel-off, EF metal substrate can be fabricated by depositing metal on an extremely flat mother substrate, then detaching the metal from the mother substrate.
2. Experiments
After the mother substrate has been was cleaned sequentially with acetone, isopropyl alcohol, and deionized water, metal films can be formed by various techniques such as a thermal evaporation, electron-beam or evaporation. Subsequently, electroforming was used to thicken the metal films to several tens of micrometers. The flexible metal film was detached from the mother substrate. To fabricate the top-emitted OLED, glass was used as the reference substrate. EF metal and rolled steel substrate were used to test effects of surface roughness.
3. Results and Discussion
To fabricate EF metal films using a surface roughness transfer method, a flexible metal film was formed on the mother substrate using various techniques such as thermal evaporation, electron-beam evaporation, or electroforming. The flexible metal film was detached from the mother substrate. The roughness of detached surface in the flexible metals film was transferred from the mother substrate, so EF metal films could be made by using EF mother substrates. Finally, various electronic devices such as OLEDs and OPVs can be fabricated on top of the detached surface in the metal films.

Figure 1. Optical images of peeled-off surface using the 3D-profiler measurement.Peeled-off metal surfaces, and peeled-off mother substrates for Metal/Glass
We fabricated the top-emitting OLEDs to evaluate the feasibility of EF metal film, comparing other substrates. The current density-operating voltage (J-VOP) characteristics of OLEDs were measured. At the constant J, the device on glass and the device on EF metal had similar current density; neither device showed leakage current. The OLED fabricated on the rolled steel substrate did not work due to a high surface roughness; it showed high leakage current. The OLED on EF metal had generally higher luminance L at a given J than did the device on glass. Metal substrate was very effective in enhancing the heat distribution, which leads to the decreased thermal degradation of active layers during the device operation.
4. Conclusion
First of all, peel-off process, a kind of transfer printing technology, makes it possible to transfer the surface roughness from the mother substrates to the transferred materials. With the controlled design of peel-off, EF metal substrate can be fabricated by depositing metal on an extremely flat mother substrate, then detaching the metal from the mother substrate. Also, the detached metal film had surface roughness that was within 2 % of that of the mother layer, despite variations in the materials used for the layers, and methods of forming the metal film. Moreover, we fabricated the top-emitting OLEDs to evaluate the feasibility of using the transfer-fabricated EF metal films.
5. Reference
[1] S. Araki, et al. Ad. Mater. 2012, 24, OP122.
[2] David R. Rosseinsky, et al. Adv. Mater. 2001, 13, 783.
[3] Hak Ki Yu, et al. Nanoscale. 2012, 4, 6831.

Figure 1