With the generation of high order harmonics (HHG) on the plasma surface it is possible to turn the laser pulse into a train of attosecond or even zeptosecond pulses in the back radiation. These attosecond pulses may have amplitude several orders of magnitude larger than that of the laser pulse under appropriate conditions. We study this process in detail, especially the nanobunching of the plasma electron density. We derive the analytical expression that describes the electron density profile and obtain a good agreement with particle-in-cell simulations. We investigate the most efficient case of HHG at moderate laser intensity (a0 = 10) on the over dense plasma slab with an exponential profile per-plasma. Subsequently we calculate the spectra of single attosecond pulses from back radiation using our expression for density shape in combination with the equation for spectrum of nanobunch radiation.