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

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... 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/Cl 2 + O 2 dry etching steps. By a variation of etching parameters regarding the Ar/Cl 2 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. Micromachines 2020, 11, 1080 2 of 18 in quantum information technologies (QITs), such as quantum memories [15, 16] and quantum communication [17] [18] [19] , SCD gained, as a host material, an ever-increasing scientific interest based on remarkable properties of different optically active point defects in its crystal lattice-the so-called color centers [20] . A long coherence time for these defect related electronic states is facilitated by the wide-bandgap material (≈5.5 eV for bulk diamond), since diamond crystals are relatively free of background nuclear spins and feature a low electron concentration as well as low phonon scattering rate [21, 22] . These color centers, such as the nitrogen-vacancy (NV) center [23, 24] , the silicon-vacancy (SiV) center [25, 26] or the germanium-vacancy center (GeV) [27, 28] , exhibit a high photostability at room temperature, polarizability and allow a practicable control of coherent single spins, hence serving as single-photon emitters. In particular, the prominent NV color center presents a promising candidate for quantum memory bits due to its long-lived and well-defined spin quantum states [29] . To yield an efficient outcoupling from the zero-phonon line (ZPL) of the color centers and improve the photon collection efficiency, various light-confining architectures can be used, such as Fabry-Pérot microcavities [30, 31] , nanopillars [32] [33] [34] or photonic crystal cavities [35] . In particular, cavities with small mode volumes and high quality factors can lead to selective and strong Purcell enhancement of the ZPL emission, an important ingredient for coherent single-photon sources, fast spin readout and an efficient creation of spin-photon entanglement. However, nanofabrication produces near-surface damage, and in particular for the NV center, it was found that this leads to a substantial degradation of the optical coherence properties. The most promising approach is to use minimally processed diamond membranes with a thickness of a few micrometers where color centers remain far away from surfaces, which are introduced into open-access microcavities by bonding to one of the cavity mirrors [25, 26, 36] . Such microcavities allow for spatial and spectral tunability in order to controllably match the geometrical and spectral overlap of a cavity mode with a selected color center. Furthermore, fiber-based microcavities allow for a direct outcoupling of photons in a single-mode fiber [24, 37, 38] . The central challenge for this approach is that additional losses introduced by the diamond can limit the efficiency of the system, and the surface roughness of the diamond membrane is the most crucial parameter. To minimize losses due to scattering and maintain a high finesse of the cavity, the surface roughness should be as low as possible, preferably in the sub-nanometer regime on a small area [39] . Our simulations of a hybrid membrane-cavity system suggest that a root mean square (rms) roughness of <0.5 nm is necessary for an effective Purcell enhancement. However, different surface defects such as pits, grooves and dislocations originating from polishing [40] and further processing of diamond substrates, e.g., structuring by dry etching procedures, can pose a challenge to reach a low surface roughness and hence fabricate high quality SCD membranes [41] . Mechanical polishing, commonly in the form of scaife polishing to remove diamond material, can introduce polishing damage extending into the bulk region under the processed surface from several hundred nanometers up to 10 µm in depth [42, 43] . Besides polishing damage, another significant source of defects causing a higher surface roughening is the micromasking effect. Due to particles on the surface, e.g., originating from the polishing procedure or sputtered material during the reactive ion etching (RIE) process, holes or etch pits can be formed at the surface [24] . In the current work, we present an approach for fabrication of thin SCD membranes with a thickness in the range of 2-5 µm, utilizing a diamond bulk mask with angled holes to withstand long etch procedures. Besides a systematic examination by varying the etching parameters to reduce micromasking and the surface roughness, the importance of a thorough cleaning procedure before structuring is highlighted. As a second step, we report on the integration and characterization of the optimized membranes in a fiber-based Fabry-Pérot microcavity. We present a bonding technique for large diamond samples that utilizes van der Waals attraction between two surfaces. We characterize the microcavity mode structure and loss in the presence of the incorporated diamond membrane. 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