Microinstabilities excited in the foot of a supercritical perpendicular shock wave are investigated. A two-dimensional full particle simulation with periodic boundary conditions in both directions using the physical ion to electron mass ratio is performed as a proxy for the foot region where incoming and specularly reflected ions overlap. The simulation shows that six types of different instabilities are excited in a time period shorter than ion gyroperiod of the reflected ions. The most dominant instability is the modified two-stream instability, which leads to strong parallel electron heating through a so-called two step instability and to ion phase space holes.