A versatile maskless microscope projection photolithography system and its application in light-directed fabrication of DNA microarrays

摘要

We present a maskless microscope projection lithography system (MPLS), inwhich photomasks have been replaced by a Digital Micromirror Device typespatial light modulator (DMD, Texas Instruments). Employing video projectortechnology high resolution patterns, designed as bitmap images on the computer,are displayed using a micromirror array consisting of about 786000 tinyindividually addressable tilting mirrors. The DMD, which is located in theimage plane of an infinity corrected microscope, is projected onto a substrateplaced in the focal plane of the microscope objective. With a 5x(0.25 NA) Fluarmicroscope objective, a fivefold reduction of the image to a total size of 9mm2 and a minimum feature size of 3.5 microns is achieved. Our system can beused in the visible range as well as in the near UV (with a light intensity ofup to 76 mW/cm2 around the 365 nm Hg-line). We developed an inexpensive andsimple method to enable exact focusing and controlling of the image quality ofthe projected patterns. Our MPLS has originally been designed for thelight-directed in situ synthesis of DNA microarrays. One requirement is a highUV intensity to keep the fabrication process reasonably short. Another demandis a sufficient contrast ratio over small distances (of about 5 microns). Thisis necessary to achieve a high density of features (i.e. separated sites on thesubstrate at which different DNA sequences are synthesized in parallel fashion)while at the same time the number of stray light induced DNA sequence errors iskept reasonably small. We demonstrate the performance of the apparatus inlight-directed DNA chip synthesis and discuss its advantages and limitations.