Ultra-high aspect ratio pores milled in diamond via laser, ion and electron beam mediated processes
Graphical abstract
Introduction
Diamond is a material renowned for its unique combination of chemical, physical, electrical and optical properties. In recent years, considerable interest in diamond has been driven by quantum photonic [1,2] and magnetometry [3] applications afforded by diamonds wide bandgap and the electronic spin system of color center defects, and increased availability of high quality chemical vapor deposition (CVD) grown diamond [4]. An increasing number of applications requiring high aspect ratio features have also arisen including pore based particle sensors for application in biosensing [5] and targets for inertial confinement fusion (ICF) [6]. Solid-state sensors with pores that span a membrane can provide unique information about an analyte such as size and relative population, with the dimension of the smallest constriction of the pore controlling the maximum particle size that can be detected [5]. A major driver for this study, ICF target fabrication, requires the drilling of a pore into a diamond shell to remove the underlying silicon substrate material via chemical etching [6]. Decreasing the pore diameter reduces growth of hydrodynamic instabilities during an implosion and increases the neutron yield [7]. These new applications, however, have required an advancement in micro machining capabilities to overcome the chemical inertness and exceptional hardness of diamond. Further understanding of conventional micron-scale milling techniques and exploration of new capabilities for the sculpting of diamond is therefore required to expand its range of uses and to optimize existing applications for diamond.
For micron-scale feature sizes the hardness of diamond rules out conventional tooling removal processes and more advanced methods such as micro-electrical discharge machining [8]. Reactive ion etching (RIE) is the most widely used, large-area, micron-scale diamond processing technique. In RIE, a mask is applied to the substrate and material removed by bombarding the surface with reactive ions derived by plasma dissociation of gases such as hydrogen and oxygen [9,10]. The aspect ratio (depth:width) of features produced by RIE of diamond is typically on the order of 8:1 and up to 25:1 under specific conditions, with etch rates on the order of 9.5 μm hour-1 [11]. Compared to mask-based methods such as RIE, direct-write methodologies enable rapid prototyping and in situ repair of fabrication defects. Laser ablation [12,13], focused ion beam (FIB) milling [[14], [15], [16]] and gas-mediated electron beam induced etching (EBIE) [[17], [18], [19], [20], [21], [22], [23], [24]] have previously been investigated for the sculpting of diamond for various applications. Laser ablation of diamond proceeds via vaporization of surface diamond and laser-induced graphitized material, and formation of volatile COx material that desorbs from the surface [25]. Processing via laser ablation can achieve fast, sub-wavelength resolution removal of surface material [12], and for diamond the multi-photon ablation process has been exploited for creation of polarization-selective etching of emergent nano-structures [13]. Previous laser processing studies have revealed carbon removal rates of 0.0031 atoms per photon using a 1064 nm, Q-switched Nd:YAG laser [26], and a double pulse ablation methodology has enabled >10:1 aspect ratio milling of high quality, smooth wall pores in metal [27]. The application of the double pulse ablation method, however, has not been reported for fabrication of pores in diamond. The double pulse technique is highly desirable as single pulse methods can produce considerable roughness during processing of diamond [5,26].
The higher resolution micro-machining technique, FIB milling, is conventionally performed via sputtering of surface atoms using gallium ions, however recent advances in oxygen plasma focused ion beam (OP-FIB) milling technology has enabled the use of chemically reactive ions [15]. A drawback of the FIB techniques is the unwanted staining and defect generation caused by ion implantation from the primary ion beam [15,28]. Previous studies have reported aspect ratios of up to 20:1 using chemical assisted gallium FIB milling [29], however, aspect ratios on the order of 10:1 are more typical [30]. High aspect ratio pores also are typically reported with structures that taper significantly reaching a point at the base of the structure [31]. To negate staining of material, EBIE has been implemented for the fabrication of optically active diamond structures [20]. EBIE of diamond proceeds via the adsorption of oxygen species and subsequent electron induced removal of the resultant COx species from the surface of diamond [17,19], and can produce features on the order of 10's of nanometers. The EBIE experiments performed here extend a previous study where 42, 0.5–1 μm diameter pores with an aspect ratio of ~12:1 were produced in a thin diamond membrane. Remarkably, the pores displayed minimal tapering over their ~10 μm span into the membrane [5]. A recent advance in the EBIE of diamond has also revealed the kinetics of oxygen-mediated EBIE of diamond and its faster, isotropic etch rate which could increase the throughput of pore fabrication [17].
While research has been conducted for these three respective direct-write techniques, direct comparison between them for specific outcomes are limited. In this article, double-pulse laser ablation [27], OP-FIB and O2-mediated EBIE methodologies are investigated for the fabrication of micropores spanning >60 μm in depth. The fabrication of micropores with sub-10 μm diameter with these techniques is reported with aspect ratios and milling rates between the techniques directly compared. The results provide insights into the mechanisms of milling and reveal the fundamental physical mechanisms that limit the aspect ratio of the few micron diameter pores. Fabricated pore morphology is characterized by X-ray radiography and scanning electron microscope (SEM) imaging. Numerical simulations are performed to study the interaction of electrons of varying energies with the wall and gaseous contents of the pore and inform the experimental methodology to produce high aspect ratio morphologies. These findings are critical for developing an understanding of the physical mechanisms governing these direct-write processes, and to identify processing methodologies for fabrication of desired feature morphologies.
Section snippets
Methods and materials
Micropore fabrication using all milling methodologies was performed on a flat polycrystalline diamond substrate (3 × 3 mm, CVD grown, Diamond Materials GmbH) or a nanocrystalline diamond film deposited on a 2 mm diameter silicon sphere (W-doped, CVD grown, Diamond Materials GmbH). Nanocrystalline diamond film deposited on a 2 mm diameter silicon spheres were used for laser ablation and OP-FIB experiments to determine performance on actual ICF target material and as they are much easier to
Double pulse laser ablation
Fig. 2 reveals the morphology of pores produced by double pulse laser ablation in a 65 μm thick diamond on silicon sphere substrate. The milling rate is approximately 1250 μm3 of material removed in 150 ms for a pore produced by irradiation with 1500 total pulse pairs. The variation of the total number of pulse pairs and energy per pulse have minimal effect on the pore morphology with no clear trend in width or surface expansion. Pores have a funnel-like taper at the top surface where the
Conclusions
In summary, capabilities for producing high-aspect ratio pores in diamond by double pulse laser-ablation, OP-FIB milling and O2-mediated EBIE have been reported. Double-pulse laser ablation provides rapid removal of diamond material at aspect ratios of approximately 10:1, featuring a tapered funnel-like pore profile. OP-FIB milling is revealed to be three orders of magnitude slower than laser ablation, however it displays great benefits in terms of pore morphology. The chemically activated
Acknowledgements
This work was performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory (LLNL) under Contract No. DE-AC52–07NA27344. This research was partially funded by the Australian Government through the Australian Research Council (Grant No. DP160101301) and Thermo Fisher Scientific. The authors wish to thank A. Nikroo, J. Biener, L. Berzak-Hopkins, C. Weber and S. Le Pape (LLNL) for fruitful discussions regarding diamond microsphere technology.
Author contributions
N. Alfonso, C. Kong, A. Forsman, L. Carlson and N. Rice performed the laser ablation experiments. A. Martin, W. Burnett, M. Stadermann and T. Bunn designed the OP-FIB experiments which were performed by W. Burnett. J. Bishop, A. Martin and M. Toth designed the EBIE experiments which were performed by J. Bishop. N. Alfonso and C. Kong performed the X-ray radiography measurements. J. Bishop and W. Burnett performed the SEM imaging. A. Martin performed the numerical simulations. A. Martin, J.
Declaration of competing interest
W. Burnett is employed by Oregon Physics, a manufacturer of plasma focused beam ion columns.
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