Surface deformation of amorphous silicon thin film on elastomeric substrate

Background

a-Si films are used in thin film transistors, battery anodes, image sensors and photovoltaic devices, and texturing their surfaces can increase the surface-area-to-volume ratio and reduce adhesion and friction. Conventional surface modification requires adding layers or particles. Buckling of stiff films on heated elastomers offers a simpler route and had been studied for metal films and c-Si ribbons, but the buckling of a-Si films had received little attention. Large-area a-Si membranes can be released without the holes or nanoribbons needed to etch the sacrificial oxide under c-Si or III-V films.

Design and approach

A 270 nm-thick hydrogenated a-Si film was deposited by plasma-enhanced chemical vapor deposition at 280 °C on a 300 nm-thick SiO2 sacrificial layer and released by etching the oxide in 49–51% hydrofluoric acid (HF). The free film was transferred onto PDMS heated to between 105 °C and 150 °C. On cooling, the large difference in thermal expansion coefficient between a-Si (~1 × 10⁻⁶ K⁻¹) and PDMS (~3.1 × 10⁻⁴ K⁻¹) places the film under equi-biaxial compression, which relaxes by wrinkling; the heating temperature sets the pre-strain.

How it was evaluated

Pattern formation was recorded with an in situ optical microscope while the samples cooled, and wavelength and jog angle were measured from the images. Amplitudes were measured by atomic force microscopy (AFM) on 100 × 100 μm² scans and by white light interferometry, and film thickness by cross-sectional field emission scanning electron microscopy (FE-SEM). Pre-strains of approximately 2.47, 2.94, 3.40 and 3.86% at 105, 120, 135 and 150 °C were calculated from the thermal expansion mismatch, and the measured amplitudes and wavelengths were compared with analytical solutions for one-dimensional and checkerboard buckling taken from c-Si buckling models, using literature values for the moduli and Poisson's ratios.

Key results

The pattern depended on position: one-dimensional wavy wrinkles formed near the edges, disordered two-dimensional patterns at the center, and ordered herringbone structures in regions far from the edges. The authors attribute this to uniaxial stress near the traction-free edge and biaxial stress toward the interior. In situ observation at a pre-strain of approximately 3.8% showed one-dimensional wrinkles until 58 s, followed by a second, perpendicular compression that produced the herringbone; the authors concluded that sequential compression in two directions forms a herringbone, whereas simultaneous equi-biaxial strain gives random ordering.

During herringbone formation, the wavelength was approximately 38.1 μm with a jog angle of 180° after 20 s, decreased to 34.8 μm, and settled at 32.0 μm after 76 s; the jog angle fell sharply between 60 and 80 s to 82.8°. The authors presume that the built-up strain stayed below its critical limit before 60 s. AFM profiles of herringbones gave a wavelength of 30.8 μm and amplitude of 2.7 μm for PDMS heated to 105 °C, and 30.7 μm and 2.9 μm for 150 °C.

Across pre-strains from 2.47 to 3.86%, amplitudes increased linearly, from 2.1 to 2.7 μm for one-dimensional wrinkles and from 3.1 to 4.1 μm for two-dimensional random patterns, and herringbone amplitudes showed a similar linear increase. The calculated one-dimensional amplitudes were 2.1, 2.3, 2.5 and 2.7 μm, matching the measurements within experimental uncertainty, and the checkerboard solution also matched the two-dimensional amplitudes. The wavelength of the one-dimensional wrinkle was estimated at 38.8 μm against a measured 39.0 μm, whereas the two-dimensional random pattern gave 50.3 μm against a theoretical 54.9 μm. The authors conclude that buckling models developed for c-Si films can be extended to a-Si films.

Open questions

The authors did not compare herringbone parameters with theory because no analytical equations are available for that mode, and they attribute the 50.3 μm versus 54.9 μm wavelength difference for two-dimensional random patterns to applying the checkerboard equation to random ordering. The wavelength invariance applies within the tested pre-strain range up to 3.86%.

How it connects to other research

Khang and colleagues used wavy single-crystal silicon on elastomeric substrates and integrated dielectrics, dopants and electrodes to build stretchable transistors and diodes; this study examines how amorphous silicon films form wrinkle patterns and whether c-Si buckling models describe their amplitude and wavelength.