We present an aspect ratio dependent etching (ARDE) method, utilizing the Bosch dry etching process of silicon, to create the optimal silicon master microstructures for replication of polydimethylsiloxane (PDMS) microchannels that allow immobilizing C. elegans worms at all sizes during their full life cycle. We take advantage of the etching speed change due to a locally changing etching load, by using different openings on the wafer surface and tune it into our favor to create the ideal three-dimensional microchannel shape with only one microfabrication step.