Fabrication of Thin Film Devices on Curved Surfaces Using Polymeric Perforated Films
Authors: Joo Seokwon , Xiao Peng , Seo Soonmin 
Published: February 2020

We suggest two-step shadow mask method for fabricating complex metal or inorganic this film on three dimensional(3-D) substrates. In this experiment, flexible and patchable ultra-violet(UV) curable poly(urethane acrylate) (PUA) are used as a polymeric shadow mask on a curved substrate because it is patchable on a smooth surface. Metal electrode or inorganic active layer for electronic devices can be fabricated by using this two-step shadow mask method with polymeric stencils.

 According to the previous results, patchable polymeric mask was introduced for organic thin film transistors(OTFTs). The mask is very flexible and stretchable and can be patchable on a smooth substrate. This patchable nature of polymeric stencils enables it to be used as evaporation masks during deposition process. Differ from SUS masks, a polymeric mask is very soft and flexible. Thus, narrow and long line pattern can be easily deformed when the mask is aligned on the substrate. Moreover, an island pattern such as a ring can’t be fabricated by using normal shadow mask method on 3D substrates. In this work, we introduce new polymeric shadow masks for complex inorganic thin film patterns on 3D substrates by using two step evaporation process.

 Desired master patterns were firstly fabricated by photolithography. Polydimethylsiloxane(PDMS) was poured on the master and it was peeled off after curing. And flat PDMS mold was also prepared by replicating from flat silicon wafer. Few drops of UV curable PUA solution was dispensed onto the PDMS master mold. It was covered with flat PDMS mold and pressed slightly until flat and master PDMS molds are in contact. Then PUA were cured by UV light and perforated PUA film could be obtained after peeling off.

 Metal electrodes can be deposited by thermal evaporation through the hole of the polymeric masks. Due to flexible nature of polymeric thin film mask, it is very difficult to align the masks to exact position on the substrate, especially when the pattern is long and narrow. Thus, two complementary masks were prepared. One has lots of small holes and another has complementary patterns which can make lines or letters when two patterns are overlapped. This method enables pattern with a hole such as a ring pattern. Moreover, flexible polymer stencils can wrap the 3D substrate for direct fabrication of electrode on 3D substrate.

 The letter ‘A’ was fabricated on the vial. The letter ‘A’ has a hole in its shape. Thus, the figure ‘A’ was divided into two parts and two sheets of shadow masks were prepared. After two step evaporation, gold ‘A’ pattern was established on the curved glass surface. When the pattern of ‘A’ was fabricated with one shadow mask, there was no hole inside the letter ‘A’. By using the same method, long helical line could be made on the flat glass substrate. Due to soft property of polymeric thin film masks, exact helical line cannot be made if we fabricate it with one mask. In case of shadow mask for helical line, there is no supporting film in adjacent region of the line. Narrow and long parts of the film inside the mask would drooped and it makes alignment of the mask impossible. This difficulty could be solved by using the two step process.

 Electrochromic display could be also established on the curved surface. Thin ITO film was deposited both on a flexible PET film and the vial with radius of 25mm. And complex WO3 pattern was fabricated on the vial by using polymeric film mask. Then its surface with WO3 pattern was covered with ITO deposited PET film. The gap between the two ITO electrodes was filled with electrolyte solution(0.1M LiClO4 in propylene carbonate). We fabricated an emblem of our university on it and it was transparent when no voltage applied and it turned deep blue when voltage of -80V was applied to it although operating voltage of the display was high.

 In conclusion, flexible polymeric stencil mask for deposition process was introduced and very complex inorganic or metal thin film patterns including isolated structure could be fabricated on the 3D surfaces directly. This patterns can be used as electrodes of the electrical devices on the curved substrates. Various kinds of patterns could be fabricated by using polymeric masks and patterns with a hole also could be made on the 3D substrate. Furthermore, an electrochromic display on 3D surface could be realized. We anticipate that this technique will be very useful to applications such as TFTs, smart windows and photovoltaic devices on 3D substrates.