The thickness of gate dielectrics for complementary metal–oxide–semiconductor (CMOS) devices with 0.1 to 0.35 μm gate dimensions is typically 4 to 7 nm. Besides serving as an insulator, the dielectric, usually SiO2, also acts as a diffusion barrier between the doped polycrystalline silicon gate material and the Si substrate (channel). Dopant diffusion from the poly‐gate into the device channel may cause undesirable shifts in the transistor threshold voltage. The determination of dopant diffusion through the gate dielectric into the channel region of the transistor is necessary to optimized device processing. This paper reports a method, resonance ionization mass spectrometry (RIMS), to measure dopant distribution profiles near the gate oxide. The pertinent spectroscopy of sputtered B and P atoms is demonstrated. Matrix effects are shown to be diminished. RIMS depth profiles of CMOS test structures show both dopant penetration into the channel and blockage by the gate oxide depending upon annealing conditions.