A method for process monitoring includes receiving a sample having a first layer that is at least partially conductive and a second layer formed over the first layer, following production of contact openings in the second layer by an etch process, the contact openings including a plurality of test openings having different, respective transverse dimensions. A beam of charged particles is directed to irradiate the test openings. In response to the beam, at least one of a specimen current flowing through the first layer and a total yield of electrons emitted from a surface of the sample is measured, thus producing an etch indicator signal. The etch indicator signal is analyzed as a function of the transverse dimensions of the test openings so as to assess a characteristic of the etch process.