One possibility of a low temperature joining techniques relies on the bonding of atomically clean surfaces. Results on the application of this method to silicon direct bonding are being presented. Clean surfaces for bonding were prepared by ex situ chemical cleaning with ensuing hydrogen passivation and their subsequent activation by thermal desorption of the hydrogen in ultrahigh vacuum (UHV). In UHV at room temperature, the wafers were gently brought into contact to initiate the bonding process. Without any subsequent heat treatment, the adhesive strength thus achieved was equivalent to the cohesion of bulk silicon: covalent bonds join the two crystals.