  Supercritical collisionless perpendicular shocks have an average
macrostructure determined primarily by the dynamics of ions specularly
reflected at the magnetic ramp. Within the overall macrostructure,
instabilities, both linear and nonlinear, generate fluctuations and
microstructure. To identify the sources of such microstructure, high-resolution
two- and three-dimensional simulations have been carried out using the hybrid
method, wherein the ions are treated as particles and the electron response is
modelled as a massless fluid. We confirm the results of earlier 2-D simulations
showing both field-parallel aligned propagating fluctuations and fluctuations
carried by the reflected-gyrating ions. In addition, it is shown that, for 2-D
simulations of the shock coplanarity plane, the presence of short-wavelength
fluctuations in all magnetic components is associated with the ion Weibel
instability driven at the upstream edge of the foot by the reflected-gyrating
ions. In 3-D simulations we show for the first time that the dominant
microstructure is due to a coupling between field-parallel propagating
fluctuations in the ramp and the motion of the reflected ions. This results in
a pattern of fluctuations counter-propagating across the surface of the shock
at an angle inclined to the magnetic field direction, due to a combination of
field-parallel motion at the Alfv\'en speed of the ramp, and motion in the
sense of gyration of the reflected ions.
