The electric field distribution for an electron cyclotron resonance (ECR) cavity was measured using microcoaxial probes. Typically, two to three resonant modes are found for each plasma condition and longer cavity length provides a more stable plasma. A 30 degrees periodicity is seen in the angular dependence of the electric field, and the peak right across the microwave input probe becomes more dominant at shorter cavity length. The longitudinal dependence of the electric field has a peak close to the middle of the cavity, and the measured field patterns agree well with the theoretical prediction. The resonant modes were identified by fitting the experimental measurements to linear combinations of electromagnetic resonant modes with characteristic length close to the cavity height. Using this method, all the modes in this ECR cavity were found to be multimode, and the resonant mode with the longest cavity length was found to be a mixing of TE(314) and TE(611) modes. Significant changes in the electric field patterns are seen when the ECR cavity is out of tune. The sensitivity of electric field distribution to ECR cavity tuning allows process control to be implemented by monitoring the changes in the electric field intensity of just a single probe position. Despite the variations in electric field distribution for different resonant modes, uniformity of photoresist etching across 15-cm-diam wafer is better than 5%. The good uniformity of the photoresist etching suggests that the plasma density at the wafer is uniform, probably due to the diffusion of the reactive species from the ECR source.