Particle-in-cell (PIC) simulations of fast particles produced by a short laser pulse with a duration of 40 fs and an intensity of 10²⁰ W/cm² interacting with a foil target are performed. The dependence of the energy of the generated fast ions on the target geometry is examined. The absorbed laser energy is transferred to fast electrons, which interact with the foil and are partially ejected from the foil surfaces. These electrons produce an electric field that causes an ion beam to be emitted from the rear surface of the foil. Three different kinds of target are considered. The optimum target plasma conditions for the maximum ion acceleration are determined.