Micro adhesion controllable vacuum assisted transfer printing for fabrication of deformable electronics

Abstract

With the advent of the Internet of things era, portable electronic devices, one of the most important information processing devices, have become an indispensable part of human life. Wearable electronics are a type of portable electronic device that integrates with clothing or that directly touches the skin. Through the integrated design, processing information will become more convenient and your lifestyle will change significantly. The research of wearable electronic devices to meet the growing demand of vast consumers has become an irresistible trend. Traditional electronics are mainly based on integrated circuits that are manufactured from solid flat semiconductor wafers and are not suitable for irregular, soft or moving objects such as wrinkled clothing or human skin. The dilemma of traditional electronics leads to the birth of flexible electronics. In this regard, transfer printing technology has drawn great interest throughout the world due to its capability to go beyond the limits of conventional CMOS manufacturing such as high temperature and harsh chemical etching. Transfer printing allows micro and nano-sized devices to be deterministically assembled, allowing material grades to be heterogeneously integrated into the desired functional layout. Not only does it offer the same performance as traditional wafer-based devices, but it also creates engineering opportunities in the field of flexible and flexible inorganic electronics. To date, various methods have been proposed for transfer printing techniques, such as micro-contact printing with elastomeric stamps, laser ablation transfer, electro-static/-magnetic transfer, fluidic self-assembly, and stress layer exfoliation transfer. However, none of these methods have completely solved some serious drawbacks such as fast processing time, misalignment, chip damage, complicated fabrication methods and expensive cost. Therefore, in order to solve the enemies mentioned above, a new type of transfer printing method must be developedTo solve above mentioned limitations, we suggest vacuum transfer printing to integrate microchips on deformable substrates for the future wearable electronics. The pressure difference can generate suction force that reaches a pressure of up to 1 atm, so it can be applied to the transfer printing process with simple vacuum controllable system. By only utilizing the pressure based force, the microchips are not physically or electrically damaged during the transfer printing process. In the following paragraphs, the manufacturing method for vacuum transfer printing module, the chip preparation method, and the result of the transfer process are mentioned.In Chapter 2, fabrication methods of vacuum transfer printing module are described with conventional photolithographic technique. To induce vacuum suction force directly on targeted microchips, micro channel paths connected to external pumping unit is the most crucial point in the vacuum module fabrication process. Also, the micro channel containing chamber structure need to maintain its’ original shape during pressure change process. To make the concrete channel structure, conventional Polydimethylsiloxane (PDMS) demolding process was carried out by using hard PDMS (h-PDMS). After demolding process, a thin glass containing micro hole array was attached H-PDMS where the concave channel pattern exists. Ring-shaped column patterns around each micro-holes are fabricated by a photolithography process to extend and protrude a portion that can generate suction force.Chapter 3 describes the fabrication of free-standing microchips supported by a single bridged structure. For easier and precise pick up procedure, the number of sustaining bridge structure needs to be small as possible. In order to achieve 100% yield free standing chips, etching depth and bridge width was optimized. Also, FEM simulation analysis was carried out to manifest vacuum suction force is large enough for selective pick up from the donor wafer.In Chapter 4, the result of vacuum transfer printing is represented with numerous scanning electron microscope (SEM) and optical images. To demonstrate versatility of vacuum transfer printing, disparate types, thickness, shapes, sizes, and processed microchips are integrated on a single substrate. Also microchips are transferred on diverse substrate such as Polyimide (PI), paper, and PDMS. The result clearly exhibits that heterogeneous integration is possible with the vacuum transfer technique. Finally, 10×10 μLED array and transistor array were successfully transferred on flexible substrate followed by thermo-compressive bonding procedure for the electrical/physical interconnection with the target substrate. The transferred μLED array showed no significant electrical/optical degradation compared to the epitaxial film on the GaAs as-grown wafer. Also, flexible transistor array was fabricated in the similar method as μLED array.