Micro-masonry for 3D Additive Micromanufacturing
Summary
This paper introduces a 3D additive micromanufacturing strategy (termed ‘micro-masonry’) for the flexible fabrication of microelectromechanical system (MEMS) structures and devices. This approach involves transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.
Abstract
Transfer printing is a method to transfer solid micro/nanoscale materials (herein called ‘inks’) from a substrate where they are generated to a different substrate by utilizing elastomeric stamps. Transfer printing enables the integration of heterogeneous materials to fabricate unexampled structures or functional systems that are found in recent advanced devices such as flexible and stretchable solar cells and LED arrays. While transfer printing exhibits unique features in material assembly capability, the use of adhesive layers or the surface modification such as deposition of self-assembled monolayer (SAM) on substrates for enhancing printing processes hinders its wide adaptation in microassembly of microelectromechanical system (MEMS) structures and devices. To overcome this shortcoming, we developed an advanced mode of transfer printing which deterministically assembles individual microscale objects solely through controlling surface contact area without any surface alteration. The absence of an adhesive layer or other modification and the subsequent material bonding processes ensure not only mechanical bonding, but also thermal and electrical connection between assembled materials, which further opens various applications in adaptation in building unusual MEMS devices.
Introduction
Microelectromechanical systems (MEMS), such as the miniaturization of large scale ordinary 3D machines, are indispensable for advancing modern technologies by providing performance enhancements and manufacturing cost reduction1,2. However, the current rate of technological advancement in MEMS cannot be maintained without continuous innovations in manufacturing technologies3-6. Common monolithic microfabrication primarily relies on layer-by-layer processes developed for the manufacture of integrated circuits (IC). This method has been quite successful at enabling mass production of high performance MEMS devices. However, owing to its complex layer-by-layer and electrochemically subtractive nature, manufacturing of diversely-shaped 3D MEMS structures and devices, while easy in the macroworld, is very challenging to achieve using this monolithic microfabrication. To enable more flexible 3D microfabrication with less process complexity, we developed a 3D additive micromanufacturing strategy (termed ‘micro/nano-masonry’) which involves a transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.
Transfer printing is a method to transfer solid microscale materials (i.e., ‘solid inks’) from a substrate where they are generated or grown to a different substrate by using controlled dry adhesion of elastomeric stamps. The typical procedure of micro-masonry starts with transfer printing. Prefabricated solid inks are transfer printed using a microtip stamp that is an advanced form of elastomeric stamps and the printed structures are subsequently annealed using rapid thermal annealing (RTA) to enhance ink-ink and ink-substrate adhesion. This manufacturing approach enables the construction of unusual microscale structures and devices that cannot be accommodated using other existing methods7.
Micro-masonry provides several attractive features not present in other methods: (a) the ability to integrate functional and structural solid inks of dissimilar materials to assemble MEMS sensors and actuators all integrated within the 3D structure; (b) the interfaces of assembled solid inks can function as electrical and thermal contacts9,10; (c) the assembly spatial resolution can be high (~1 μm) by utilizing highly-scalable and well-understood lithographic processes for generating solid inks and highly-precise mechanical stages for transfer printing7; and (d) functional and structural solid inks can be integrated on both rigid and flexible substrates in planar or curvilinear geometries.
Protocol
Representative Results
Discussion
Micro-masonry, presented in Figure 4, involves silicon fusion bonding in a material bonding step. Silicon fusion bonding is achieved by placing the sample in a rapid thermal annealing furnace (RTA furnace) and heating the sample at 950 °C for 10 min. This annealing condition is both adoptable between Si – Si and Si – SiO2 bonding10,11. Alternatively, the Au bonded with a Si strip as found in Figure 5C adopts eutectic bonding, and therefore, the…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was supported by the NSF (CMMI-1351370).
Materials
| Name of Material / Equipment | Company | Comments / Description | |
| Az 5214 | Clariant | 1.5 mm thick photoresist | |
| Su8-100 | Microchem | 100 mm Photoresist used in mold | |
| Sylgard 184 | Dow Corning | PDMS mixed to fabricate stamp | |
| Hydrofluoric Acid | Honeywell | Acid to etch silicon oxide layer | |
| Silicon on insulator | Ultrasil | Donor substrate was fabricated | |
| trichlorosilane | Sigma-Aldrich | Chemical used to help pealing of PDMS from mold |
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