# Investigation on Key Parameters in the Fabrication of Stamps for Transfer Printing of Micro Devices
## 2 Materials and Methods
### Stamps Fabrication
In this experiment, the development was performed using the development solution (SU-8, Micro-Chem Corp., Newton, MA, USA) for 10 min and then an ultrasonic bath under 53 kHz in an ultrasonic cleaning machine (DTC-15J, Dingtai Corp., Hubei, China) for another 10 min. Incidentally, it was preferable to use containers which would not react with the developing solution, such as glass, during the development. After silicon-photoresist models were developed in the development solution, they were rinsed with fresh development solution for 10 s and subsequently rinsed with isopropanol solution for 10 s and subsequently rinsed with isopropanol solution for 10 s. The results of the experiments were presented in Fig. 1.  
Figure 1: Schematic illustration of stamp fabrication processes (**a**) cleaning and drying; (**b**) spin coating; (**c**) soft bake; (**d**) exposure; (**e**) post exposure bake; (**f**) development; (**g**) rinsing and drying; (**h**) hard bake; (**i**) the cross section of the mold; (**j**) pouring PDMS; (**k**) curing; (**l**) peeling off. (Qixiangtengda Corp., Shandong, China) for another 10 s, then silicon-photoresist molds were dried using a nitrogen gun as shown in Figure 1g. [PERSON] et al. (2018) also used lithography technology to fabricate stamps. Here, we used isopropanol solution to rinse off the residue on silicon-photoresist molds. If a white film appeared during the rinsing with isopropyl alcohol, this appearance indicated insufficient development. Under this circumstance, appropriate development operations should be performed, and were followed by the rinse and dry step. Properties of SU-8 2000 photoresist would change after encountering higher temperatures. Therefore, the photoresist ought to be baked at a temperature of 10 \({}^{\circ}\)C higher than the ultima operating temperature, usually at 150 \({}^{\circ}\)C to 250 \({}^{\circ}\)C for 5 to 30 min (hard bake), as shown in Figure 1h. The hard bake step helped improve the photoresist surface cracking that occurred after the development. In this experiment, the soft bake was optimized as 3 min under 65 \({}^{\circ}\)C and as 9 min under 95 \({}^{\circ}\)C. Likewise, for a better exposure, the optimal dose was determined as 250 W for 10 s. The best time for PEB was determined as 2 min under 65 \({}^{\circ}\)C and 10 min under 95 \({}^{\circ}\)C.  
After steps aforementioned, the stamp mold was completed. For the fabrication of stamps, the 184-silicone elastomer and the curing agent (Sylgard 184, Dow Corning Corp., Midland, MI, USA) were mixed in a ratio of 10 : 1 and were stirred evenly. After stirring, the mixture was placed in a vacuum chamber (SLZK-40, Shengli Corp., Shanghai, China) for 30 min, so that bubbles would escape.Then, the mixture (PDMS) was cast into the mold, and a piece of 2-inch glass (Fuyao Corp., Fujian, China) was covered on it as a carrier. Later, all of them were placed in the vacuum chamber (\(-\)72 cmHg) for 30 min again to remove bubbles and then were baked at 60 \({}^{\circ}\)C for 24 h, as shown in Figure 1j,k. When the PDMS was cured, it was peeled off from the mold and the PDMS was formed on the glass carrier, as shown in Figure 1l. Note that, during baking, samples should be placed in the oven after the temperature rose to the desired point to ensure that the PDMS was cured at the precise temperature. In addition, the more highly the temperature reaches, the more easily the silicon wafer would be broken, leading to the problem that stamps would not be easily peeled off. [PERSON] et al. also used casting and curing method to fabricate stamps, and they transferred PDMS patterns on the glass after the PDMS was cured ([PERSON] et al., 2018). However, prior to curing PDMS, we covered a piece of glass on PDMS; after PDMS was cured, it would be formed on the glass carrier directly. Fewer steps and less time in our method were required.