# Investigation on Key Parameters in the Fabrication of Stamps for Transfer Printing of Micro Devices
## 3 Results and Discussion
### Elastic Modulus Measurement  
To measure the elastic modulus of posts, [PERSON] et al. use AFM and the Hertz model [17]. In our study, the AFM and DMT model have been used instead, as shown by the inset in Figure 5a. Our rationale lies in the fact that the Hertz model suits measurements for the elastic modulus of a rigid sample,  
Figure 4: The relationship between the process parameters of the mold (**a**) results of a linear fit of \(c\) with \(r\); (**b**) the relationship among \(r\) and development time duration \(t\), the designed width \(w\) of the mold; (**c**) the relationship among \(c\) and development time duration \(t\), the designed width \(w\) of the mold. whereas DMT does measurements for the elastic modulus of a viscous sample. [PERSON] et al. primarily study the effect of the temperature on the elastic modulus of PDMS. In addition, we also study the effect of the width on the elastic modulus [40]. Through AFM, we obtain the force-displacement curve of the cantilever with respect to the post during both inserting and retrieval processes. Then, we use the DMT model to fit the retrieval curve to acquire the value of the elastic modulus. The reason why we choose the retrieval curve instead of the inserting curve lies in the fact that, when the probe approaches the stamp, the atomic force between the probe and stamp will affect measurement results during inserting. At the width of 130 \(\upmu\)m, the elastic modulus reaches the maximum, as shown in Figure 5a. Figure 5b shows the elastic modules of stamps with different curing temperatures. The elastic modulus of the stamp reaches the maximum of 25 Mpa and 38 Mpa at the curing temperature of 62 \({}^{\circ}\)C and 142 \({}^{\circ}\)C, respectively. However, the elastic modulus of PDMS is related to the proportion of curing agent, curing temperature, curing time, and the precision of the measuring instrument. According to experimental results, within a certain range, the more the PDMS curing agent exists, the greater the elastic modulus becomes; the longer the curing time elapses, the greater the elastic modulus also becomes. Consequently, the deviation between measurement dots and the fitting curve may originate from the measuring deviation between different experiments and fitting deviation with the DMT model. The average measuring deviation between different experiments and fitting deviation with DMT model is \(\sim\)3 Mpa (\(\sigma\)). Figure 5: Elastic modulus measurement; (**a**) elastic modulus of posts with different widths (curing at 60 \({}^{\circ}\)C, height: 46 \(\upmu\)m, inset figure: an AFM-based method for measuring the elastic modulus of a post); (**b**) elastic modulus of posts with different curing temperatures (height: 46 \(\upmu\)m, width: 130 \(\upmu\)m). We conduct transfer printing experiments of LEDs with stamps fabricated in our lab. Prior to the experiment, GaN-based flip-chip LEDs are supplied by Xiamen Changelight Co. Ltd., Xiamen, China. The LED wafer is grown on the sapphire substrate with InGaN/GaN multiple quantum wells and at the emission wavelength of 460 nm. In addition, flip-chip LED chips are fabricated and packaged by regular chip process with the size of 100 \(\times\) 200 \(\upmu\)m\({}^{2}\). After several hundred times of transfer printing experiments, we have demonstrated that lab-made stamps have managed to transfer LEDs at a high yield. Incidentally, transfer printing experiments carried out using our stamps have been published as independent work [41]. [PERSON] et al. [42] acquire the pressure and peeling speed of the stamp in the printing step, while we do the pressure and peeling speed of the stamp in the picking-up step. In our study, the best picking-up pressure ranges from 5 to 50 mN, and the best picking-up speed ranges from 17 to 21 mm/s. The pressure and the peeling speed of the stamp in the picking up step in [PERSON] et al. 's [18] study are 1 mN and 1 mm/s, respectively.