Precision micro-mechanical components in single crystal diamond by deep reactive ion etching

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.


Section I. Components fabrication
The single crystal diamond substrate with dimensions of 5.5 mm x 5.5 mm x 0.15 mm (High Pressure High Temperature (HPHT) raw crystal, grown, cut and polished by LakeDiamond SA) are first cleaned using a Piranha solution (H2SO4(96%):H2O2(30%) (3:1)). The substrate is then subjected to additional cleaning in an oxygen plasma (600 W, 400 sccm O2 flow, 0.8 mbar, PVA TePla GIGAbatch ) for 2 minutes, and a 200 nm thick Al layer is sputtered on both sides of the substrate immediately afterwards (200 W, 15  The substrate is attached on a silicon handling wafer using QuickStick 135, followed by an Hexamethyldisilazane (HMDS) vapor deposition at 130°C. A 2.5 µm thick layer of ECI 3027 photoresist is spin coated at 1750 rpm, followed by a 5 minutes softbake at 100°C. A first exposure of the photoresist is performed (600 mJ/cm 2 , SUSS MicroTec MA6 Gen3) on the edge-bead affected region (from the substrate edge to 0.5 mm inside the substrate), followed by development in AZ 726 MIF developer for 137 seconds. A second exposure (225 mJ/cm 2 , SUSS MicroTec MA6 Gen3) is performed on the central region of the substrate, with the pattern of the parts to be fabricated, followed by a development in AZ 726 MIF for 108 seconds.
The SiO2 is etched in a He/H2/C4F8 based plasma in steps with a duration of less than 4 minutes each during a total time of 30 minutes and 20 seconds (1200 W ICP Power, 300 W bias power, 175 sccm He, 30 sccm H2, 10 sccm C4F8, 4 mTorr chamber pressure, SPTS APS). The photoresist is stripped using an O2 plasma (600 W, 400 sccm O2 flow, 0.8 mbar, PVA TePla GIGAbatch) for 2 minutes, a 5 minutes immersion in a MICROPOSIT REMOVER 1165 solution heated at 75°C followed by a DI water rinsing and drying under N2 flow, and a second O2 plasma (600 W, 400 sccm O2 flow, 0.8 mbar, PVA TePla GIGAbatch) for 2 minutes. In order to smoothen the SiO2 sidewalls, the substrate is dipped for 15 seconds in a buffered hydrofluoric acid solution (NH4F(40%):HF(50%) (7:1)), followed by DI water cleaning and N2 blow-drying. The aluminum layer is etched in a Cl2/BCl3 based plasma for 1 min (800 W coil power, 150 W platen power, 10 sccm Cl2 flow, 10 sccm BCl3 flow, 3 mTorr chamber pressure, STS Multiplex ICP), immediately followed by a DI water rinsing and drying under N2 flow to remove any chlorine residues. The single crystal diamond substrate is etched for approximately 5 hours in an O2 based plasma (

Section III. Edge Bead Removal
A substantial edge bead is formed after the photoresist spin coating ( Figure S2a). This edge bead has to be removed in order to allow a close contact between the photomask and the photoresist during the exposure of the component patterns. Therefore, a first photolithography is performed by exposing the photoresist in a region ranging from the diamond edges to 0.5 mm towards the diamond center, followed by the development of the resist ( Figure S2b).

Section IV. Hard Mask Sidewall Smoothening
After etching the SiO2 hard mask, the sidewalls exhibit an important roughness and semidetached sheets ( Figure S3a). As the hard mask topography is transferred to the diamond sidewalls during the etching, it is beneficial to reduce the hard mask roughness before the diamond etching step. This is done by dipping the diamond-on-wafer assembly in a buffered HF bath for 15 seconds, followed by rinsing and drying. The resulting smoothened sidewall is shown in Figure S3b. The exposed Al adhesion layer next to the SiO2 hard mask is also slightly etched, however this is not problematic since this Al layer has to be removed in the next step by dry etching.

Figure S3
Sidewall of the SiO2 hard mask before (a) and after (b) the 15 seconds HF dip smoothening step.  Figure S4f, 148 µm), corresponding to an average etch rate of 29.8 µm/hour. In order to use this smaller diamond, a dedicated photomask with smaller dimensions and patterns slightly different from those shown in Figure S1 was fabricated.

Figure S4
Scanning Electron Microscope recordings of (a) 7 µm thick SiO2 hard mask patterned on a 2.6 mm x 2.6 mm x 0.3 mm single crystalline diamond substrate, and evolution of the deep reactive ion etch after (b) 1h30, (c) 2h, (d) 2h30, (e) 4h and (f) 5 hours.

Section VI. Sidewall Angle Determination
The slopes of the two sidewall regions were determined by measuring the corresponding angles on the SEM recordings of two sidewalls of a fabricated part ( Figure S5), and correcting them to compensate for the 45° tilted stage (e.g. in Figure S5a