Epitaxial Nanostructured α-Quartz Films on Silicon: From the Material to New Devices
Summary
This work presents a detailed protocol for the microfabrication of nanostructured α-quartz cantilever on a Silicon-On-Insulator(SOI) technology substrate starting from the epitaxial growth of quartz film with the dip coating method and then nanostructuration of the thin film via nanoimprint lithography.
Abstract
In this work, we show a detailed engineering route of the first piezoelectric nanostructured epitaxial quartz-based microcantilever. We will explain all the steps in the process starting from the material to the device fabrication. The epitaxial growth of α-quartz film on SOI (100) substrate starts with the preparation of a strontium doped silica sol-gel and continues with the deposition of this gel into the SOI substrate in a thin film form using the dip-coating technique under atmospheric conditions at room temperature. Before crystallization of the gel film, nanostructuration is performed onto the film surface by nanoimprint lithography (NIL). Epitaxial film growth is reached at 1000 °C, inducing a perfect crystallization of the patterned gel film. Fabrication of quartz crystal cantilever devices is a four-step process based on microfabrication techniques. The process starts with shaping the quartz surface, and then metal deposition for electrodes follows it. After removing the silicone, the cantilever is released from SOI substrate eliminating SiO2 between silicon and quartz. The device performance is analyzed by non-contact laser vibrometer (LDV) and atomic force microscopy (AFM). Among the different cantilever’s dimensions included in the fabricated chip, the nanostructured cantilever analyzed in this work exhibited a dimension of 40 µm large and 100 µm long and was fabricated with a 600 nm thick patterned quartz layer (nanopillar diameter and separation distance of 400 nm and 1 µm, respectively) epitaxially grown on a 2 µm thick Si device layer. The measured resonance frequency was 267 kHz and the estimated quality factor, Q, of the whole mechanical structure was Q ~ 398 under low vacuum conditions. We observed the voltage-dependent linear displacement of cantilever with both techniques (i.e., AFM contact measurement and LDV). Therefore, proving that these devices can be activated through the indirect piezoelectric effect.
Introduction
Oxide nanomaterials with piezoelectric properties are pivotal to design devices such as MEMS sensors or micro energy harvesters or storage1,2,3. As the advances in CMOS technology increase, the monolithic integration of high-quality epitaxial piezoelectric films and nanostructures into silicon becomes a subject of interest to expand new novel devices4. In addition, greater control of miniaturization of these devices is required to achieve high performances5,6. New sensor applications in electronic, biology, and medicine are enabled by the advances in micro and nanofabrication technologies7,8.
In particular, α-quartz is widely used as a piezoelectric material and shows outstanding characteristics, which allow users to make fabrication for different applications. Although it has low electromechanical coupling factor, which limits its application area for energy harvesting, its chemical stability and high mechanical quality factor make it a good candidate for frequency control devices and sensor technologies9. However, these devices were micromachined from bulk single quartz crystals which have the desired characteristics for device fabrication10. The thickness of the quartz crystal should be configured in such a way that the highest resonance frequency can be obtained from the device, nowadays, the lowest achievable thickness is 10 μm11. So far, some techniques to micropattern the bulk crystals such as Faraday cage angled-etching11, laser interference lithography12, and focused ion beam (FIB)13 were reported.
Recently, direct and bottom-up integration of epitaxial growth of (100) α-quartz film into silicon substrate (100) was developed by chemical solution deposition (CSD)14,15. This approach opened a door to overcome the aforementioned challenges and also to develop piezoelectric-based devices for future sensor applications. Tailoring the structure of α-quartz film on silicon substrate was achieved and it allowed to control the texture, density, and the thickness of the film16. The thickness of the α-quartz film was extended from a few hundred nanometers to the micron range, which are 10 to 50 times thinner than those obtained by top-down technologies on bulk crystal. Optimizing the dip-coating deposition conditions, humidity and temperature was enabled to attain both continuous nanostructured crystalline quartz film and a perfect nanoimprinted pattern by a combination of a set of top-down lithography techniques17. Specifically, soft nanoimprint lithography (NIL) is a low-cost, large-scale fabrication and benchtop equipment-based process. Application of soft NIL, which combines top-down and bottom-up approaches, is a key to produce epitaxial quartz nanopillar arrays on silicon with a precise control of pillar diameters, height, and the interpillar distances. Furthermore, fabrication of silica nanopillar with controlled shape, diameter, and periodicity on borosilicate glass for a biological application was performed customizing soft NIL of epitaxial quartz thin film18.
Up to now, it has not been possible for on-chip integration of piezoelectric nanostructured α-quartz MEMS. Here, we draw the detailed engineering route starting from material to device fabrication. We explain all the steps for material synthesis, soft NIL, and the microfabrication of the device to release a piezoelectric quartz cantilever on SOI substrate19 and discuss its response as a piezoelectric material with some characterization results.
Protocol
Representative Results
Discussion
The presented method is a combination of bottom-up and top-down approaches to produce nanostructured piezoelectric quartz micro-cantilevers on Si. Quartz/Si-MEMS technology offers major advantages over bulk quartz in terms of size, power consumption, and integration cost. Indeed, epitaxial quartz/Si MEMS are produced with CMOS-compatible processes. This could facilitate the future fabrication of single chip solutions for multifrequency devices while preserving miniaturization and cost-effective processes. Compared to the…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (No.803004).
Materials
| Acetone | Honeywell Riedel de Haën | UN 1090 | |
| AZnLOF 2020 negative resist | Microchemicals | USAW176488-1BLO | |
| AZnLOF 2070 negative resist | Microchemicals | USAW211327-1FK6 | |
| AZ 726 MIF developer | Merck | DEAA195539 | |
| BOE (7:1) | Technic | AF 87.5-12.5 | |
| Brij-58 | Sigma | 9004-95-9 | |
| Chromium | Neyco | FCRID1T00004N-F53-062317/FC79271 | |
| Dip Coater ND-R 11/2 F | Nadetec | ND-R 11/2 F | |
| Hydrogen peroxide solution 30% | Carlo Erka Reagents DasitGroup | UN 2014 | |
| H2SO4 | Honeywell Fluka | UN 1830 | |
| Isopropyl alcohol | Honeywell Riedel de Haën | UN 1219 | |
| Mask aligner | EV Group | EVG620 | |
| PG remover | MicroChem | 18111026 | |
| Platinum | Neyco | INO272308/F14508 | |
| PTFE based container | Teflon | ||
| Reactive ion etching (RIE) | Corial | ICP Corial 200 IL | |
| SEMFEG | Hitachi | Su-70 | |
| SOI substrate | University Wafer | ID :3213 | |
| Strontium chloride hexahydrate | Sigma-Aldrich | 10025-70-4 | |
| SYLGARD TM 184 Silicone Elastomer Kit | Dow | .000000840559 | |
| SYLGARD TM 184 Silicone Elastomer Curring Agent | Dow | .000000840559 | |
| Tetraethyl orthosilicate | Aldrich | 78-10-4 | |
| Tubular Furnace | Carbolite | PTF 14/75/450 | |
| Vibrometer | Polytec | OFV-500D | |
| 2D XRD | Bruker | D8 Discover | Equipped with a Eiger2 R 500 K 2D detector |
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