Abstract. We present a homogenization technique for rarefied gas flow over a microstructured surface consisting of patterns of periodic features. The length scale of the model domain is comparable to the mean free path of the molecules, while the scale of the surface patterns is much smaller. The flow is modeled by a system of linear Boltzmann equations with a diffusive boundary condition at the patterned surface. The resulting homogenized boundary condition holds at a virtual flat surface and incorporates the microscopic geometry information about the surface structure on the macroscopic level. Numerical results validate the approach. The setup models low pressure chemical vapor deposition processes in the manufacturing of integrated circuits. 1. Introduction. Low pressure chemical vapor deposition is used in the manufacturing of integrated circuits to deposit a thin layer of material onto the surface of a silicon wafer. The deposition surface necessarily involves a microstructure given by the electrical components of the future microchip. Classical models for this process include reactor scale models [17] with a typical length scale of over 10 cm, which model the gas flow throughout the chemical reactor, and feature scale models [3] with a typical length scale of under 1 µm, which focus on the evolution of the film profile inside an individual feature.In more detail, the process works as follows. Molecules of the species to be deposited are carried inside the chemical reactor by an inert carrier gas to a microstructured surface, where they are partially absorbed and partially reflected at a certain rate. The length scale of the surface structure is several orders of magnitude smaller than that of the reactor and therefore cannot be reasonably resolved on the reactor scale. On the other hand, this structure will influence the gas flow through the boundary conditions; i.e., adsorption on the microstructured surface will result in a different behavior of the gas flow than adsorption on a flat surface, even on the macroscopic reactor scale. We therefore have to solve a homogenization problem at the boundary by deriving a boundary condition for the flow problem which incorporates the microscopic geometric information about the surface structure into the macroscopic flow picture.