  Three-dimensional (3D) etching of materials by plasmas is an ultimate
challenge in microstructuring applications. A method is proposed to reach a
controllable 3D structure by using masks in front of the surface in a plasma
etch reactor in combination with local magnetic fields to steer the incident
ions in the plasma sheath region towards the surface to reach 3D directionality
during etching and deposition. This effect can be controlled by modifying the
magnetic field and/or plasma properties to adjust the relationship between
sheath thickness and mask feature size. Since the guiding length scale is the
plasma sheath thickness, which for typical plasma densities is at least 10s of
microns or larger, controlled directional etching and deposition target the
field of microstructuring, e.g. of solids for sensors, optics, or
microfluidics. In this proof-of-concept study, it is shown that
$\vec{E}\times\vec{B}$ drifts tailor the local sheath expansion, thereby
controlling the plasma density distribution and the transport when the plasma
penetrates the mask during an RF cycle. This modified local plasma creates a 3D
etch profile. This is shown experimentally as well as using 2d3v
Particle-In-Cell/Monte Carlo collisions simulation.
