# Investigation on Key Parameters in the Fabrication of Stamps for Transfer Printing of Micro Devices
## 2 Materials and Methods  
### Stamps Fabrication  
The whole stamps fabrication processes were performed under the air conditions, 22 \({}^{\circ}\)C, and 45% RH environment. At first, the 2-inch silicon wafer (Ke-Jing Corp., Anhui, China) was boiled at 250 \({}^{\circ}\)C on the hot plate (HT-300, Gdana Corp., Guangdong, China) for 15 min using a piranha wet etch, 98% H\({}_{2}\)SO\({}_{4}\) (Jihua Corp., Jiangsu, China): 30% H\({}_{2}\)O\({}_{2}\) (Atofina Corp., Shanghai, China) = 4:1. Then, this boiling process was followed by a deionized water rinse, as shown in Figure 1a. After the silicon wafer was dried with nitrogen (Tongwei Corp., Xiamen, China), it was baked under 135 \({}^{\circ}\)C in an oven (ESPEC Corp., Shanghai, China) for 4 h. The wafer should be cooled to the room temperature before spin coating to reduce the effect of temperature on the photoresist. For the purpose of preventing the wafer from contamination, the spin coating was performed immediately after the cooling. As shown in Figure 1b, for better uniformity, a type of negative photoresist (SU-8 2075, Micro-Chem Corp., Newton, MA, USA) was spun using a spin coater (EZ4, Analysis Corp., Hongkong, China) at 500 rpm for 12 s and then at 4000 rpm for 40 s with the ultima thickness of \(\sim\)40 \(\upmu\)m. Because of a wide range of spin coating thickness between 0.5 \(\upmu\)m to 650 \(\upmu\)m, the SU-8 photoresist was chosen. [PERSON] et al. (2018) used SU-8 50 photoresist; while we used SU-8 2075 photoresist, the thickness of which ranged from 40 \(\upmu\)m to 240 \(\upmu\)m.  
After the photoresist was spin-coated, the edge bead removal should be conducted. This removal process not only reduced the contamination of the hot plate when baking, but also enhanced the distance between the silicon wafer and the mask (Qingyi Corp., Shenzhen, China) to obtain a better effect when exposure. As shown in Figure 1c, the photoresist-coated silicon wafer was baked (soft bake) on the hot plate at 65 \({}^{\circ}\)C for 3 min and then at 95 \({}^{\circ}\)C for 9 min. Afterwards, the silicon wafer was removed from the hot plate, and was cooled to the room temperature. If the film was wrinkled, the wafer should be placed on the hot plate for a few more minutes. Heating and cooling processes should be repeated until the film did not wrinkle. [PERSON] et al. (2018) also used lithography technology to fabricate stamps. In this article, a long pass filter (Chroma Corp., Vermont, VT, USA) was used to eliminate UV radiation below 350 nm such that the verticalness of pattern sidewalls can be enhanced during the exposure, as shown in Figure 1d. Notably, multiple sets of experiments were performed to achieve the best exposure dose. Finally, the photoresist was exposed under a lithography machine (Model 200, OAI Instruments, Milpitas, CA, USA) at the power of 250 W for 10 s. At unsuitably low exposure-dose levels, the exposed photoresist would have been insufficiently cured, resulting in the severe erosion of the pattern edge when the photoresist was developed. As shown in Figure 1e, post exposure bake (PEB) should be carried out immediately after the exposure. In this step, the wafer was baked on the hot plate at 65 \({}^{\circ}\)C for 2 min and then at 95 \({}^{\circ}\)C for 10 min. If insufficient time was spent for PEB, edges of the photoresist pattern would also be eroded after the development. After 1 minute of PEB at 95 \({}^{\circ}\)C, the latent image would appear. Otherwise, the failure for the appearance of the latent image suggested insufficient exposure or heating. When PEB was finished, the temperature of the sample on the hot plate should be slowly lowered to the room temperature. As shown in Figure 1f, when thicker or larger aspect-ratio structures were developed, strong agitation should be performed. In addition, an ultrasonic bath was recommended during developing fine patterns.