Multimode interference devices based on silicon hollow waveguides have been designed, simulated, fabricated and characterized. Adequate confinement of light into a hollow waveguide and minimization of the propagation losses require a roughness of the structure below the working wavelength. This assures to have mirror behavior at the facets that optimize the Fresnel reflections. In order to achieve this low roughness and a perfectly vertical walls, to obtain a rectangular shape, an optimization of the fabrication process, especially deep reactive ion etching process, has been made. The numerical and experimental anti-symmetrical behavior for symmetrical and anti-symmetrical out waveguides in anti-symmetrical multimode interference devices is in accordance with the theory of multimode interference effects. The excellent behavior and properties of these devices shows the silicon hollow waveguides excellent for the design of integrated optical devices.