# Investigation on Key Parameters in the Fabrication of Stamps for Transfer Printing of Micro Devices
## 3 Results and Discussion  
### Size Measurement  
The height and the depth of posts and molds measured by the step profiler show more accurate results than the width measurement. Due to the taper nature of the probe used in the step profiler, the cross section of the probe in contact with the sample varies at different heights or depths. In addition, the diameter of this cross section equals the error in measuring the width of the sample. For measurements, the laser confocal microscope adopts an optical principle, which does not affect measured samples, resulting in higher measurement accuracy. Through measurements and analysis, we obtain that, for both top and bottom faces, the average error between the post's width and the mold's width can be controlled within 5%. Similarly, the standard deviation of the height of posts fabricated in the same batch can be controlled within 0.5 \(\upmu\)m.  
We study the average deviation between the width of the post and designed mask pattern width as shown in Figure 2f and conclude that the deviation value shows relatively stable regardless of the size. Because angles of light coming from different mask patterns appear relatively consistent during exposure. The overall average deviation of the top face is positive (16.84 \(\upmu\)m), while that of the bottom face is negative (\(-\)3.23 \(\upmu\)m). Due to the expansion of PDMS during curing, the deviation of the top face stays positive. Next, the deeper the photoresist mold becomes, the harder it is to ensure the verticality of the side wall during development, resulting in a negative deviation of the bottom face of the post.  
In this set of experiments, sizes of fabricated posts range from 20 \(\upmu\)m to 180 \(\upmu\)m. The post should size closely to the chip, so that transfer printing can be better performed. Equally notably, for transferring flip chips, the top surface of the chip instead of PN pads (P-pad and N-pad of the LED chip) is attached to posts of the stamp. Therefore, flip chips with any PN gap value (the gap between P-pad and N-pad of the LED chip) can be adapted to posts. When the adhesion force between the post and the chip can overcome the force between the chip and its carrier substrate, the chip can be successfully picked up by the stamp.  
[PERSON] [18], [PERSON] [34], [PERSON] [35; 36], and [PERSON] [37] have pioneered their work by employing casting and curing method to fabricate cuboid-post stamps, the width of whose posts are over 100 \(\upmu\)m; in comparison, our posts of stamps contain the widths from 20 \(\upmu\)m to 180 \(\upmu\)m. Furthermore, we study the average error between the post's width and the mold's width. [PERSON] [39] et al. fabricate pyramidal posts, while we fabricate cuboid posts. The average error between the post's width and the mold's width can be controlled within 5% both in their and our work.  
Figure 2: Fabricated molds and stamps as well as stamps’ measurement results (**a**) fabricated molds under the optical microscope; (**b**) fabricated stamps according to the molds in Figure 2a under the laser confocal microscope; (**c**) enlarged view of the stamp in Figure 2b under the laser confocal microscope; (**d**) 3D image of posts in Figure 2b under the laser confocal microscope; (**e**) schematic illustration of a stamp; (**f**) average deviation between the width of the post and design width. Red dots represent the average deviation between the width of the top face of the post and designed width; black rectangles represent the average deviation between the width of the bottom face of the post and designed width.