Traditionally, (dark field) imaging based, particle detection systems rely on identifying a particle based on its irradiance. It can be shown that for a very smooth wafer with 0.1 nm surface roughness (rms) this approach results in a particle detection limit larger than 20 nm. By carefully studying the physical mechanism behind this practical limit, we have developed an alternative interferometric measurement technique that is able to improve upon this limit. This technique is based on the interferometric amplification of the particle signal, while choosing the phase of the reference beam carefully as not to amplify the coherent background speckle. Although this allows to detect particles that are 30% smaller, compared to irradiance based detection this technique poses much more stringent requirements on the wavefront errors of the imaging optics.