# Investigation on Key Parameters in the Fabrication of Stamps for Transfer Printing of Micro Devices
## 1 Introduction  
As a massively parallel assembly method, the transfer printing adopts a stamp to assemble diverse micron-nanoscale devices and materials, e.g., membranes, platelets, wires among others, into a two- or three-dimensional layouts [1; 2]. It also qualifies as a highly versatile technique for heterogeneous integration on both rigid and flexible substrates [3], such as transferring concentrator photovoltaic cells [4], lasers [5; 6], silicon micro-ICs [7], GaN LEDs [8], p-n junctions [9], and quantum dots [10]. This technology can also be used to transfer highly non-planar, globular structures, such as silica microspheres and grains of pollen [11] with high accuracy, high yield of 99%, and stamp life time of 200 h [12; 13]. In general, researchers use viscoelastic polydimethylsiloxane (PDMS) as the stamp which, due to the softness of PDMS [14], can handle fragile materials without damages. In terms of procedure steps, the whole transfer printing process consists of (a) aligning and contacting the elastic stamp with devices on the donor substrate, (b) peeling off the stamp at a faster speed, and devices will be attached to the stamp by van der Waals force, (c) moving the inked stamp to the receiver substrate, and (d) peeling off the stamp at a lower speed. Afterwards, devices will be successfully printed on the receiver substrate [15]. Advantages of the transfer printing technology include: semiconductor devices can be transferred at the room temperature instead of at higher temperatures; the controllable, repeatable, and efficient transfer processes are expected to attain highly precise and large-scale rapid integration ofheterogeneous material arrays; and the natural transparency of the stamp enables the entire process to be observed through the stamp such that posts and devices-to-be-transferred can be precisely aligned. Incidentally, to make a sacrificial layer in a safer and more economical way, [PERSON] et al. anneals the crucible at high temperatures to produce cerium oxide which can be used as a water-soluble sacrificial layer [16]. Striving for higher yield, [PERSON] et al. suggest that the aspect ratio (height/width) of the post should lie between 0.5 and 5. At inappropriately low aspect ratios, the post cannot withstand the pressure during the transfer process, whereas, at overly high aspect ratios, the post tends to collapse during the transfer process [17]. Based on the method described above, there exists a surface relief assisted transfer method including micro tips [18] and gecko principle [19]: A stamp with a suitable surface relief structure enables adhesion control during retrieving and printing. Other transfer printing methods abound: load-enhanced transfer printing [20], laser-driven transfer printing [21; 22], transfer printing based on shape memory polymer [23], overlay-aligned roll-transfer printing [24], fluidic assembly method [25], electrostatic adsorption, and electromagnetic force adsorption [26]. Transfer printing features a wide range of applications, including micro-LED display technology. Micro-LED display technology refers to the integration of high-density and micro-sized LED arrays on a single chip [27; 28], with the size of the micro-LED \(<\)100 \(\upmu\)m [29; 30]. Generally, the traditional LED display endures a larger pixel size and a relatively poor display effect, whereas the micro-LED display has secured advantages of high resolution, high efficiency, high brightness, small size, more energy saving, and self-illumination without the need for a backlight [31; 32]. In addition, key technologies of micro-LED display include bonding, current drive control, detection, and repair among others. As an essential step to the mass production of micro-LED display, the mass transfer of micro-LEDs constitutes the bottleneck of the full-colored micro-LED display technology. As one of the important performances of the stamp for transfer printing, the elastic modulus exhibits the ability of stamp to resist elastic deformation under external forces.