Inkjet-Printed Microelectromechanical Systems: Materials, Process and Devices

Patterned deposition of solution-processed materials utilizing printing technologies is a key enabler for the realization of low-cost and large-area electronics. Inkjet printing of metal nanoparticles to form patterned conductive films is one of the major features of printed electronics. During the constrained sintering of metal nanoparticles, an in-plane tensile stress builds up due to the volumetric shrinkage of the film. While this in-plane tensile stress may induce cracks and de-lamination of the film, an out-of-plane stress gradient forces the suspended structure to curl. Curling of suspended structures is a major concern in microelectromechanical systems (MEMS). Here, the mechanism of stress gradient development in inkjet-printed silver nanoparticles film is revealed. A higher surface diffusion induced non-densifying coarsening of the nanoparticles, mediated by a higher residual polymer stabilizer concentration, retards the densifying mechanism and related volume shrinkage near the bottom of the film. A lower densification rate is substantiated by a lower Young’s modulus at the bottom part of the film. Consequently, a positive stress gradient is exhibited with a lower to higher tensile stress from the bottom to the top of the film, which forces the surface-micromachined structures to curl in the upward direction. This understanding of the development of tensile stress gradient during the constrained sintering of printed nanoparticulate films constructs the way for future studies on the controlled curling of suspended structures. Moreover, having developed an understanding of stress-generation phenomena in such thin films, it is possible to propose device structures that are resistant to stress variation, or indeed, exploit this stress behavior./nThin-film transistors (TFTs) are key devices in large-area electronic systems. Micorelectromechanical (MEM) relays are an attractive alternative to TFTs due to their excellent switching characteristics. While there have been several demonstrations of printed TFTs, reports of printed MEM switches have been generally sparse due to the difficulty in realizing robust printed suspended structures. Here, a novel MEM reed relay architecture is revealed in the first demonstration of a fully inkjet-printed 3-terminal microelectromechanical reed relay offering excellent immunity to residual stress. In the reed relay architecture, the upward curling of the printed reed due to a stress gradient in the silver reed film is restricted by a printed blocking reed, thus delivering immunity to stress variations. The choice of sacrificial and dielectric materials for printed MEMS and their printability are also discussed in detail. The printed reed relays show hyper-abrupt switching with an on-state resistance of only ~15 Ω, immeasurable off-state leakage, a switching delay of 32 μs, and stable operation over 10^5 cycles. An analytical model of the reed relay turn-off voltage is developed, which is validated against the experimental results with varying reed relay geometrical parameters. The fully-printed processing capability of the demonstrated three-terminal reed relays in tandem with their stress tolerant nature and excellent device performance substantiates their promise as a new switching device for low-cost and large-area electronics. However, a four-terminal relay structure is required to realize logic circuits using MEM relays. This dissertation proposes a process flow to fabricate fully inkjet-printed stress-tolerant four-terminal relays with switching voltage tuning capability. This switching voltage tuning capability offers complementary relays for the implementation of logic operations.

Advisor: Subramanian, Vivek
Alon, Elad