Abstract: During the light illumination period of a monomer addition cycle in synthesizing an DNA microarray, undesirable reflections of illumination light from various interfaces that the illumination light passes through near the synthesis surface of the substrate may reduce the light-dark contrast, and negatively affect the precision and resolution of the microarray synthesis. The present invention provides an flow cell that reduces the undesired reflections by constructing certain flow cell structures with materials that have similar refractive indexes as that of the solution that is in the oligomer synthesis chamber during the illumination period and/or constructing certain flow cell structures or covering the structures with a layer of a material that has a high extinction coefficient.