We present a quantitative study of local charge injection into silicon nitride films inside dielectric stack gate structures. The charge is injected using atomic force microscope tips in direct contact with the dielectric layers. The charge distribution is imaged by measuring the contact potential difference between the atomic force microscope tip and the sample surface using Kelvin probe force microscopy. The trapped charge distribution and concentration is calculated using the two-dimensional Poisson equation. It is found that a peak trapped charge density of around 1 X 10(12) cm(-2) with a spreading of similar to250 nm is obtained using 15 V pulses of a few milliseconds in duration.