Three‐dimensional boron impurity concentration profiles in silicon substrates have been measured with 10 nm resolution by scanning tunneling microscopy (STM) on chemically etched cleaved silicon surfaces using an impurity sensitive HF‐HNO3‐H2O solution. Comparisons to depth profiles obtained with secondary‐ion mass spectroscopy and spreading resistance methods reveal that our proposed method measures activated boron impurity concentration rather than total boron concentration. Three‐dimensional impurity profiling is demonstrated with a metal‐oxide‐silicon structure, and the lateral junction depth in the test structure is found to be 70% of the junction depth.