  Diamond offers good optical properties and hosts bright color centers with
long spin coherence times. Recent advances in angled-etching of diamond,
specifically with reactive ion beam angled etching (RIBAE), have led to
successful demonstration of quantum photonic devices operating at visible
wavelengths. However, larger devices operating at telecommunication wavelengths
have been difficult to fabricate due to the increased mask erosion, arising
from the increased size of devices requiring longer etch times. We evaluated
different mask materials for RIBAE of diamond photonic crystal nanobeams and
waveguides, and how their thickness, selectivity, aspect ratio and sidewall
smoothness affected the resultant etch profiles and optical performance. We
found that a thick hydrogen silesquioxane (HSQ) layer on a thin alumina
adhesion layer provided the best etch profile and optical performance. The
techniques explored in this work can also be adapted to other bulk materials
that are not available heteroepitaxially or as thin films-on-insulator.
