Nanomeshed Si nanomembranes

Xun Han, Kyung Jin Seo, Yi Qiang, Zeping Li, Sandra Vinnikova, Yiding Zhong, Xuanyi Zhao, Peijie Hao, Shuodao Wang & Hui Fang
npj Flexible Electronicsvolume 3, Article number: 9 (2019) | Download Citation

Abstract
One of the main challenges in stretchable electronics is to achieve high-performance stretchable semiconductors. Here, we introduce an innovative concept of nanomeshed semiconductor nanomembrane which can be regarded almost as intrinsically stretchable to conventional microelectronic layouts. By making a silicon film into homogeneous nanomeshes with spring-like nano traces, we demonstrated a high electron mobility of 50?cm2/V·s, and moderate stretchability with a one-time strain of 25% and cyclic strain of 14% after stretching for 1000 cycles, further improvable with optimized nanomesh designs. A simple analytic model covering both fractional material and trace sidewall surfaces well predicted the transport properties of the normally on silicon nanomesh transistors, enabling future design and optimizations. Besides potential applications in stretchable electronics, this semiconductor nanomesh concept provides a new platform for materials engineering and is expected to yield a new family of stretchable inorganic materials having tunable electronic and optoelectronic properties with customized nanostructures.

Introduction
Stretchable electronics have emerged as promising platforms for many important areas such as bio-mimetics, health monitoring, biomedical therapeutics, and soft robotics.1,2,3,4,5,6 Due to their low modulus, these stretchable platforms can either serve as artificial electronic organs or form more conformal and compatible interface with irregular, shape-evolving or soft objects.7,8,9 Compelling examples of such applications include electronic skin demonstrations from various stretchable active matrices10,11,12 and multifunctional balloon catheters for cardiac electrophysiological mapping and ablation therapy.13,14,15,16 Historically, core material elements in conventional high-performance electronics are inorganic single crystals such as silicon (Si) or compound semiconductors, which arguably laid the foundation for modern society.17,18,19 However, those materials are usually rigid. Great efforts have made these semiconductors in thin film structures and render them flexible, but they are still brittle, making it difficult to achieve stretchable devices. Seeking stretchable electronic components, especially stretchable semiconductors with high performance, therefore, has been one of the critical challenges in achieving next-generation stretchable electronics.

In the past decade, there has been significant progress in realizing stretchable semiconductors, mainly from two complementary ways. One approach involves designing novel microstructural layouts in standard materials, as exemplified by configuring inorganic semiconductor based circuits into microscale island-bridge layout.20,21,22,23 The other centers in developing intrinsically stretchable components such as organic semiconductors24,25,26,27 and nano-wire or nano-tube networks.28,29,30 Among all existing approaches, microscale structuring often lacks the high-density advantage in modern microelectronics, which has been exploited to the extreme level under what’s commonly known as Moore’s law. On the other hand, intrinsically stretchable semiconductors currently are still quite limited in their mobility and/or reliability, with typical electron mobility still less than 10?cm2/V·s,31,32,33 while bottom up assembled networks such as carbon nanotube webs are limited by their uniformity.34,35 Existing approaches are still incompetent when high-density, high-performance stretchable electronics are needed.

Here, using Si as a model system we conceptualize and demonstrate a new stretchable semiconductor platform, namely nanomeshed Si nanomembranes. The nanomeshed nanomembrane is a dense network of fully connected, single-crystalline Si traces of nanoscale line-width and thickness. Due to their spring-like traces, the semiconductor nanomeshes possess promising stretchability while with high electrical performance and high scalability to microscale footprints. This latter property renders semiconductor nanomeshes almost as intrinsically stretchable for microelectronic layouts. As a proof of concept, we achieved nanomeshes of Si with tunable trace widths from a lift-off and transfer process using a Si-on-insulator (SOI) wafer, with nanomesh pattern defined by two different soft lithography methods. To facilitate the nanomesh transfer and provide additional mechanical support, we further introduced a bilayer structure of Polyimide (PI)/Si nanomeshes on elastomer substrates to improve the stretchability. The Si nanomesh film with a peak effective mobility of 50?cm2/V·s has demonstrated a one-time stretchability of up to 25% strain, and robust cyclic stretchability with less than 10% fatigue after 1000 stretching cycles with a constant strain of 14%. An analytical model coupling factors from both the fractional material and trace sidewall surfaces predicted the mobility trend of the Si-nanomesh transistors as a function of fractional Si, indicating that the mobility can be enhanced with future surface passivation. Together these results demonstrate nanomeshing semiconductors as a unique and promising pathway towards high-density, high-performance stretchable microelectronics.

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