High-Strength Amorphous Silicon Carbide for Nanomechanics

Abstract

For decades, mechanical resonators with high sensitivity have been realized using thin-film materials under high tensile loads. Although there have been remarkable strides in achieving low-dissipation mechanical sensors by utilizing high tensile stress, the performance of even the best strategy is limited by the tensile fracture strength of the resonator materials. In this study, a wafer-scale amorphous thin film is uncovered, which has the highest ultimate tensile strength ever measured for a nanostructured amorphous material. This silicon carbide (SiC) material exhibits an ultimate tensile strength of over 10 GPa, reaching the regime reserved for strong crystalline materials and approaching levels experimentally shown in graphene nanoribbons. Amorphous SiC strings with high aspect ratios are fabricated, with mechanical modes exceeding quality factors 10^8 at room temperature, the highest value achieved among SiC resonators. These performances are demonstrated faithfully after characterizing the mechanical properties of the thin film using the resonance behaviors of free-standing resonators. This robust thin-film material has significant potential for applications in nanomechanical sensors, solar cells, biological applications, space exploration and other areas requiring strength and stability in dynamic environments. The findings of this study open up new possibilities for the use of amorphous thin-film materials in high-performance applications.

Figures (21)

• Figure 1: (a) Schematic of dry under-cut processes with cryogenic SF$_6$ plasma isotropic etching (top) and vapor hydrofluoric acid etching (bottom), to suspend a-SiC nanomechanical resonators (green) on silicon (orange) and fused silica (blue) substrates, respectively. (b) Etch rates of cryogenic SF$_6$ plasma isotropic etching (log scale) and vapor hydrofluoric acid etching (linear scale), on four commonly used Si-based materials: Si, SiO$_2$, a-Si$_3$N$_4$, and a-SiC. (c) Ultimate tensile strength comparison between LPCVD a-SiC, crystalline (blue), and amorphous (red) materials. Reference: SiO$_2$Chu2009, steel Imran2018, Kevlar Zhou2004, InGaP Manjeshwar2022, a-Si$_3$N$_4$Norte2016Bereyhi2019, c-Si Sementilli2022Chen2004, a-SiC Cui2019, 6H-SiC (crystalline) Kwon2015, 3C-SiC (crystalline) Kwon2015, graphene Goldsche2018Rasool2013. The solid and transparent colors of bars represent the lower and upper limits of the materials' ultimate tensile strengths, respectively.
• Figure 2: (a) Schematic of systematically characterizing mechanical properties of a tensile stress thin film material. (a-i): Measuring film thickness with ellipsometry. (a-ii): Measuring the film stress via wafer bending technique after film deposition. (a-iii) Extracting material density / (a-iv) Young's modulus / (a-v) Poisson ratio of a-SiC thin films by fitting resonant frequencies of square membranes / cantilevers / strings of different sizes, respectively. (a-vi): Designing a-SiC nano-mechanical resonators with desired performance. (b) Characterizing the mechanical properties of LPCVD a-SiC thin film (a-SiCR2) by numerically fitting the measured resonant frequencies of suspended resonators with different geometries and dimensions, including squared membranes (top), cantilevers (middle), strings (bottom). The resonant frequencies of the resonators mentioned above are measured with Laser Doppler Vibrometer (LDV, Polytec PSV-4).
• Figure 3: Tensile test experiment to measure the ultimate tensile strength of a-SiC thin films (a) Simulated stress profile of a hourglass-shaped geometry of 50 um tether length made with a-SiCR2, the tensile stress is concentrated at the middle narrow tether up to 10 GPa, different maximum stress can be obtained with different tether lengths. (b) SEM image of a pad of a-SiCR2 hourglass-shaped structures with 18 different tether lengths, from 30 to 115 um. Below a certain tether length, the maximum stress surpass the ultimate tensile strength of the material, and the tethers break in the middle region after under-cutting, indicating the ultimate tensile strength of a-SiC. A zoom-in view of the hourglass-shaped geometry with a 50 um tether length is shown on the left. (c) The stress profiles along the hourglass-shaped geometries with different tether lengths. (d) Survival ratios of hourglass-shaped geometries with maximum stress correspond to different stress interval (orange columns) for a-SiCR2 (top), a-SiC170 (middle) and a-SiCR3 (bottom). The maximum stresses shown for unbroken devices fabricated with a-SiCR2/a-SiC170/a-SiCR3 are 12.04/10.27/11.12 GPa, respectively.
• Figure 4: Intrinsic quality factor characterization and high-Q factor a-SiC nanomechanical resonators optimized using Bayesian optimization. (a) The geometry of the defect mode, of a 20 unit-cell (UC) PnC nanostring with unit cell length L$_\mathrm{uc}$ and defect length L$_\mathrm{def}$. (b) The measured frequency spectrum of a 20 UC PnC nanostring made with a-SiCR2. The yellow shaded area represents the engineered phononic band gap. (c) The intrinsic quality factor $Q_0$ of a-SiCR2 is measured with 20 to 44 UC PnC nanostrings. (d) Ringdown measurements of 20 UC PnC nanostring made with a-SiCR2 (orange) and with a-SiCR2FS (blue). (e) Comparison of the intrinsic quality factors $Q_0$ of a-SiCR2 (orange) and a-SiCR2FS (blue) with PnC nanostrings of 20 to 26 UC. The hollow rings and the solid line represent the measured Q factors and the numerical fittings respectively. (f) The stress distribution (top) and mode shape of the defect mode (bottom) of the 6 mm tapered a-SiCR2 PnC nanostring optimized by Bayesian optimization. The optimized tapered PnC nanostring has 24 unit cells and a maximum stress of 1.2 GPa concentrated on the middle. (g) Ringdown measurement of the optimized tapered PnC nanostring, high-Q factor up to $Q_{exp}=1.98\times10^8$ is measured.
• Figure S5: Mechanical properties characterizations. After identifying the film stress and thickness with wafer bending and ellipsometry methods, the density $\rho$ / Young's modulus $E$ / Poisson ratio $\nu$ of a-SiC films deposited at different conditions are systematically measured with the resonant frequency of the a-SiC membranes (left column)/cantilevers (middle column)/strings (right column) made of the specific a-SiC film, as shown with images on the top. From top- to bottom of each row, resonant frequency data (hollow dots) and analytically fitting (solid line) for a-SiCR2/a-SiC170/a-SiCR3/a-SiCR4/a-Si$_3$N$_4$ thin films are shown respectively. The bottom row are measurement and fitting for the widely used material stoichiometric a-Si$_3$N$_4$ for reference purpose, whose fitted mechanical properties are close to the ones reported Shin2021Ghadimi2018, validating the generality of the method.