  Microscale resonators are fundamental and necessary building blocks for
modern radio communication filters for mobile devices. The resonator's Q factor
($Q$) determines the insertion loss while coupling ($K_t^2$) governs the
fractional bandwidth. The product $k_t^2 \times Q$ is widely recognized as the
definitive figure of merit for microresonators. Magnetostatic wave resonators
based on Yttrium Iron Garnet (YIG) are a promising technology platform for
future communication filters. They have shown considerably better performance
in terms of $Q$ when compared to the commercially successful acoustic
resonators in the $>$7 GHz range. However, the coupling coefficients of these
resonators have been limited to $<$3 %, primarily due to the restricted design
space imposed by microfabrication challenges related to the patterning of
gadolinium gallium garnet (GGG), the substrate material used for growing single
crystal YIG. This paper reports novel resonator designs enabled by breakthrough
bulk micromachining technology for anisotropic etching of GGG, leading to
coupling >8 % in the 6-20 GHz frequency range. We use the same technology
platform to show resonant enhancement of effective coupling, reaching up to 23
\% at 10.5 GHz. The frequency of resonant coupling can be tuned by design
during the fabrication process. The resonant coupling results in an
unprecedented $k_t^2 \times Q$ figure of merit of 191 at 10.5 GHz and 222 at
14.7 GHz. The technology platform presented in this paper supports both tunable
filter architecture and switched filter banks that are currently being used in
consumer mobile devices.
