Abstract
The outstanding material properties of single crystal diamond have been at the origin of the long-standing interest in its exploitation for engineering of high-performance micro- and nanosystems. In particular, the extreme mechanical hardness, the highest elastic modulus of any bulk material, low density, and the promise for low friction have spurred interest most notably for micro-mechanical and MEMS applications. While reactive ion etching of diamond has been reported previously, precision structuring of freestanding micro-mechanical components in single crystal diamond by deep reactive ion etching has hitherto remained elusive, related to limitations in the etch processes, such as the need of thick hard masks, micromasking effects, and limited etch rates. In this work, we report on an optimized reactive ion etching process of single crystal diamond overcoming several of these shortcomings at the same time, and present a robust and reliable method to produce fully released micro-mechanical components in single crystal diamond. Using an optimized Al/SiO2 hard mask and a high-intensity oxygen plasma etch process, we obtain etch rates exceeding 30 µm/h and hard mask selectivity better than 1:50. We demonstrate fully freestanding micro-mechanical components for mechanical watches made of pure single crystal diamond. The components with a thickness of 150 µm are defined by lithography and deep reactive ion etching, and exhibit sidewall angles of 82°–93° with surface roughness better than 200 nm rms, demonstrating the potential of this powerful technique for precision microstructuring of single crystal diamond.
Highlights
In recent years, the growth of synthetic diamond crystals has been industrialized based on high pressure high temperature (HPHT) or chemical vapor deposition (CVD) growth techniques[1,2]
Based on optical microscope inspection, patterns measured on the diamond part top surface were laterally reduced on each edge by 5.7 μm in size compared to the original photomask design. This reduction in feature size is resulting from a combination of contact lithography and hard mask recess during the etch process, and can be further reduced by including an appropriate mask bias[27]
The positive profile of the top region originates from the hard mask recess during the etching[44], while we attribute the origin of the bottom region negative profile to an isotropic component of the plasma characteristics, which can be further optimized by adjusting the plasma etch parameters
Summary
The growth of synthetic diamond crystals has been industrialized based on high pressure high temperature (HPHT) or chemical vapor deposition (CVD) growth techniques[1,2]. Several suppliers provide today commercial offerings of high-quality single crystal diamond substrates, cut and polished to plates in the size of a few tens of square millimeters and up to several hundreds of microns thick (e.g., Element Six or LakeDiamond SA). Such plates are of uniform size and free from defects typically present in natural diamonds, which make them. Traditional techniques for structuring of diamond crystals typically involve cutting and fine polishing[3] While these techniques allow for flat surfaces of optically superb quality, they can be applied to curved structures and microstructures only to a limited extent. The dimensions of these substrates and their excellent uniformity allow them to be processed using standard microfabrication techniques, which open an entirely novel
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