  Millimeter-wave superconducting resonators are a useful tool for studying
quantum device coherence in a new frequency domain. However, improving
resonators is difficult without a robust and reliable method for coupling
millimeter-wave signals to 2D structures. We develop and characterize a tapered
transition structure coupling a rectangular waveguide to a planar slotline
waveguide with better than 0.5 dB efficiency over 14 GHz, and use it to measure
ground-shielded resonators in the W band (75 - 110 GHz). Having decoupled the
resonators from radiative losses, we consistently achieve single-photon quality
factors above $10^5$, with a two-level-system loss limit above $10^6$, and
verify the effectiveness of oxide removal treatments to reduce loss. These
values are 4-5 times higher than those previously reported in the W band, and
much closer to typical planar microwave resonators. The improved losses
demonstrated by these on-chip millimeter-wave devices shed new light on quantum
decoherence in a different frequency regime, offer increased selectivity for
high-frequency detectors, and enables new possibilities for hybrid quantum
experiments integrating millimeter-wave frequencies.
