An inertial masking assembly that allows multiple thin film structures to be deposited on a single substrate by rotation of a shadow mask relative to the substrate. The assembly rotates the position of the shadow mask by the conservation of momentum. The substrate is seated on means for rotating the substrate in a confined orbit within the mask. Abruptly changing the angular velocity of the mask causes the mask to rotate under the substrate, which is mediated by a combination of friction and the substrate's inertia.