  Scaling trapped-ion quantum computing will require robust trapping of at
least hundreds of ions over long periods, while increasing the complexity and
functionality of the trap itself. Symmetric 3D structures enable high trap
depth, but microfabrication techniques are generally better suited to planar
structures that produce less ideal conditions for trapping. We present an ion
trap fabricated on stacked 8-inch wafers in a large-scale MEMS microfabrication
process that provides reproducible traps at a large volume. Electrodes are
patterned on the surfaces of two opposing wafers bonded to a spacer, forming a
3D structure with 2.5 micrometer standard deviation in alignment across the
stack. We implement a design achieving a trap depth of 1 eV for a calcium-40
ion held at 200 micrometers from either electrode plane. We characterize traps,
achieving measurement agreement with simulations to within +/-5% for mode
frequencies spanning 0.6--3.8 MHz, and evaluate stray electric field across
multiple trapping sites. We measure motional heating rates over an extensive
range of trap frequencies, and temperatures, observing 40 phonons/s at 1 MHz
and 185 K. This fabrication method provides a highly scalable approach for
producing a new generation of 3D ion traps.
