This study examines the correlation between interface roughness and charges. Atomic force microscopy (AFM) and a newly developed area roughness function which couples two roughness parameters, root-mean-square (rms) and fractal dimension (DF), are used to reliably and accurately characterize surface roughness. Interface charges (Dit and Qf) are measured using high frequency and quasistatic capacitance–voltage methods. This study is divided into three parts where smooth, purposely roughened, and purposely smoothened Si substrates are used to make metal–oxide–semiconductor capacitors for measurements. Purposely roughened substrates are obtained using a chemical acid etch solution. Purposely smoothened substrates are initially roughened with the chemical acid etch solution and smoothened through thermal oxidation. We report that the increases of Dit and Qf with Si roughness are due entirely to the area increase and orientation changes the result from roughness. © 1999 American Vacuum Society.