A mathematical model is applied to build a compact reactor for uniform thickness deposition in the molecular beam regime. The injector system uses a gas source and is compatible with perpendicular light illumination of the substrate for patterned deposition (etching). Titanium dioxide deposition is achieved and experimental results are compared to the mathematical model. Thickness uniformity better than 2% is experimentally achieved on a 150-mm diameter substrate and is compared to the 1 % calculated. This approach allows a compact reactor design and an easy up grading in deposition area size and illumination optics design for light assisted processes.