Solutions of electron and ion density gratings at the linear stage are given. Deep plasma gratings produced by the ponderomotive force of the interference of the two intersecting laser pulses are investigated. Dependence of the plasma grating on the plasma density, duration and intensity of the laser pulses are studied with 1D particle-in-cell (PIC) simulation. It is found that the density of peaks of such gratings can be 8 times the initial plasma density and can last a few picoseconds. We have found stable, long-lifetime electromagnetic solitons by use of these gratings. It is shown that the intensity for the formation of relativistic EM solitons with two crossed-propagating pulses is reduced to a great extent.