Nitrogen vacancy centers in diamond provide an attractive platform for quantum sensing because of their long spin coherence at room temperature and the possibility of optical spin initialization and readout. For high sensitivity, these centers need to be located close to the signal source, such as magnetic nanoparticles, while still enabling large‐area measurements. This requires thin diamond layers with controlled incorporation of nitrogen and minimal crystal defects. In this study, we report on the growth and characterization of diamond films with (100) orientation produced by microwave plasma‐assisted chemical vapor deposition. The films exhibit sharp interfaces and long spin coherence times above one hundred microseconds, despite thicknesses of only a few micrometers. By combining isotopic enrichment with controlled nitrogen incorporation, we demonstrate that the coherence times are mainly limited by the spin bath of substitutional nitrogen. The best samples reach dephasing times of 6.3 microseconds, close to the theoretical limit set by the substitutional nitrogen concentration, which indicates minimal strain in the layers. While bulk crystals can exhibit longer coherence times, our results highlight that thin, uniform films with engineered spin environments provide advantages for integration into scalable quantum sensing devices.
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