In situ characterization of buckling dynamics in silicon microribbon on an elastomer substrate

Abstract

Buckling of rigid thin films on elastomer substrates underlies the fabrication foundation of stretchable soft electronics. Here we demonstrate an optical approach to in situ characterize the buckling of silicon microribbon driven by releasing the pre-stretched poly-dimethylsiloxane (PDMS) substrate at a controllable strain rate. The method, based on quantitative differential interference microscopy, directly captures the space–time evolution of the surface topography at a frame rate of 100 fps in a large field of view of 50 × $195\mathrm{\mu }\mathrm{m}$ ${}^2$. The nucleation, propagation and stabilization of the buckled structure during the buckling and unbuckling processes are observed and quantified. Our experiment reveals that a sequence of partially buckled patterns are energetically stable to bridge the unbuckled and fully buckled states. This work opens a new experimental scheme for the research on stretchable soft electronics and provides new evidence for the theoretical study of the buckling dynamics.

Introduction

Micro-electro-mechanical systems (MEMS) play a profound role in semiconductor and electronics industry for its wide applications in micro devices such as strain sensors [1], [2], [3], actuators [4], [5] and microrobotics [6]. The precision of structural design and the deformability of the fabricated structure are critical to the functionality and stability of MEMS devices, especially microrobotics [7], [8] and biomedical devices [9], [10]. Such demand on precise structural formation arises dramatically in emerging foldable and stretchable soft electronics [8], [11], [12]. While there are many methods to fabricate various delicate MEMS structures [11], [12], the quantitative characterization of the structure and deformation of these micro devices is rare. Understanding how the thin film structure deforms and propagates on the soft substrate under various loading rates is not only essential for the development and design of flexible electronics, but also governs the deployment and usage of these devices in practice. By far, atomic force microscopy (AFM) has been nearly the sole actor on this stage [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. As a scanning-based probe, one can conduct the surface scans while holding and tuning the load step by step to obtain the evolution of surface structures, but AFM cannot measure how the flat micro-layers deform to their 3D functional structures at different strain rates. Some time-resolved attempts were made using optical microscope and fast scanning electron microscopy (SEM) under mechanical loading [23], [24]. But those methods cannot quantitatively resolve the topographic evolution. If the surface deformation is highly periodic, then the topography evolution can be quantified from the scattering between light and periodic structures by the optical interferometer [25]. But when the structure is very non-periodic, the intensity would be too weak for an accurate structural reconstruction. From this point of view, an in situ structural characterization in sub-micron to millimeter length scale is urgently needed.

A recent advance in quantitative optics for full-field deformation characterization [26], [27] sheds light on in situ structural characterization for MEMS. This method, based on differential interference contrast microscope (DInM), characterizes a large field of view in milliseconds, which breaks the bottleneck of the scanning-based probes for in situ experiments. The topographical gradient of surface can be analytically derived from the light path difference between two orthogonally polarized beams. Based on the localization analysis of light, the beam-shear angles can be precisely characterized [28], through which sub-micron surface structural features can be resolved by highly coherent blue LED light source [27]. Utilizing the programmable phase tuning by liquid crystal retarders and algorithmically cooperating digital image correlation, a dual beam-shear differential interference microscope (i.e. D-DInM) was designed and built for in situ characterization of deformation processes in a crystalline solid undergoing stress-induced phase transformation [27]. In this letter, we utilize the same D-DInM system with beam-shear vector $(s_1,s_2)=(588,591)$ nm to study the dynamics of silicon (Si) microribbon buckling on a polydimethylsiloxane (PDMS) substrate. For the dynamic measurement, we switched off one of the beam-shear modes to achieve sufficiently high frame rate such that the time-dependent structural parameters along the loading direction can be quantified during pre-buckling, buckling and post-buckling processes.