Fabrication and Characterization of Single-Crystal Diamond Membranes for Quantum Photonics with Tunable Microcavities

Abstract

The development of quantum technologies is one of the big challenges in modern research. A crucial component for many applications is an efficient, coherent spin-photon interface, and coupling single-color centers in thin diamond membranes to a microcavity is a promising approach. To structure such micrometer thin single-crystal diamond (SCD) membranes with a good quality, it is important to minimize defects originating from polishing or etching procedures. Here, we report on the fabrication of SCD membranes, with various diameters, exhibiting a low surface roughness down to 0.4 nm on a small area scale, by etching through a diamond bulk mask with angled holes. A significant reduction in pits induced by micromasking and polishing damages was accomplished by the application of alternating Ar/Cl₂ + O₂ dry etching steps. By a variation of etching parameters regarding the Ar/Cl₂ step, an enhanced planarization of the surface was obtained, in particular, for surfaces with a higher initial surface roughness of several nanometers. Furthermore, we present the successful bonding of an SCD membrane via van der Waals forces on a cavity mirror and perform finesse measurements which yielded values between 500 and 5000, depending on the position and hence on the membrane thickness. Our results are promising for, e.g., an efficient spin-photon interface.

Keywords: fiber-based microcavity; membranes; micromasking; nanophotonics; roughness reduction; single-crystal diamond.

Figure 1

Fabrication process of single-crystal diamond (SCD) membranes and characterization in a fiber-based microcavity: in dark blue the surface with a higher roughness is indicated, while in yellow the bulk diamond mask with angled holes is depicted. Since the “soft” etch was applied only for samples with a higher roughness (>1 nm on a 4 × 4 µm² area), it is highlighted in grey.

Figure 2

A tunable fiber-based microcavity: (a) schematic drawing of the system. The mirror can be moved laterally for coarse positioning while the fiber can be precision-positioned both laterally and longitudinally; (b) microscopic image of the cavity, taken from behind the plane mirror with a CMOS camera. One can see the fiber which is glued onto a needle for stability and the bonded diamond sample. Interference fringes that appeared after bonding the sample are visible.

Figure 3

Atomic force microscopy (AFM) images of SCD surfaces after cleaning, XPS C1s core spectra and in comparison before and after strain relief etch and “soft” etch: (a) after first cleaning procedure (rms ~ 20 nm) and (b) after second cleaning procedure with piranha solution (rms ~ 4.4 nm); (c) after oxygen plasma asher treatment (rms ~ 4 nm); (d) XPS C1s core spectra measured for general grade SCD sample as-received from the manufacturer and after treatment in the oxygen plasma asher, with an inset showing the lower binding energy site. (e) General grade SCD with an initial low surface roughness (rms ~ 0.6 nm) before and (f) after strain relief etch (50 min Ar/Cl₂ + 90 min O₂), including some hole formation due to micromasking and a similar roughness without considering the holes (rms ~ 0.7 nm); (g) electronic grade SCD exhibiting distinct polishing grooves (rms ~ 4 nm) before and (h) after strain relief etch with a cyclic Ar/Cl₂ + O₂ recipe, which leads to an improved smoothening of the surface (rms ~ 1.6 nm); (i) electronic grade SCD surface before (rms ~ 1.2 nm) and (j) after (rms ~ 0.76 nm) a “soft” etching with Ar/Cl₂ (40 min). The white line in the upper left corner of (i) depicts an AFM artifact, which was not considered in the height plot and hence in the surface roughness calculation.

Figure 4

Comparison of the morphological characterization of SCD membranes, from left to the right: overview via optical micrograph, white light interferometer (WLI) measurement from the membrane center, AFM image from the center: (a) general grade SCD membrane (∅ = 470 μm, thickness 2–3 μm, rms ~ 0.7 nm over a 5 × 5 μm² area) structured with one long Ar/Cl₂ cleaning step and one long O₂ structuring step (in total three etch processes); (b) general grade SCD membrane (∅ = 510 μm, thickness 5 μm, rms ~ 0.4 nm over a 4 × 4 μm² area) structured with cyclic Ar/Cl₂ + O₂ recipe (in total three etch processes).

Figure 5

Microscopic images showing the bonding process: (a) the electronic grade SCD before and (b) after the bonding process. Before the bonding process has finished, one can clearly see the residual water around the sample edges as well as the interference fringes that occur due to the gap between both surfaces; (c) microscopic image of the processed general grade SCD sample after the bond. As the sample was broken in half during the cleaning process, the membrane is now located on the edge of the sample.

Figure 6

Numerical simulations of a 1D model of the cavity and related finesse measurements: (a) The electrical field intensity of a hybrid cavity system (blue). The orange lines show the refractive indices in the respective areas (from left to right: fiber mirror, air gap, diamond membrane, planar mirror); (b) the same simulation for a different diamond thickness, leading to a diamond-like mode. Both simulations are performed with the same mirror transmission without diamond which leads to a higher field in the air-like case. This occurs because the diamond membrane alters the transmission of the planar mirror, depending on the diamond thickness; (c) longitudinal cavity modes for determining the cavity finesse: a narrow linewidth laser, running at a wavelength of 639.7 nm, is coupled in the cavity and the length is scanned periodically. Blue and orange curves show measured voltage over oscilloscope time base for two different positions on the membrane. We attribute the two positions to diamond-like (orange) and air-like (blue) modes. The inset shows a centered zoom into the air-like and the diamond-like fundamental modes. Fits to the data reveal the expected Lorentzian shape of cavity modes. The FWHM at the diamond-like position on the sample is much larger than at the air-like position, yielding a much lower finesse.

Figure 7

Hybridized mode structure at different positions on the membrane: the upper panel shows the measured mode dispersion on two different positions. The blue dashed line indicates the wavelength of the laser used for finesse measurements. The first position (left panels) is very close to a diamond-like mode whereas the second position (right panels) approaches an air-like mode. The lower panels show transmission simulations using a matrix transfer method. Diamond thicknesses of 5.995 (air-like) and 5.900 µm (diamond-like) yield the best agreement with the measured data.