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

Abstract

The manuscript has been published in Review of Scientific Instruments (http://rsi.aip.org/rsi/) Rev. Sci. Instrum. 77, 063711 (2006) We present a maskless microscope projection lithography system (MPLS), in which photomasks have been replaced by a Digital Micromirror Device type spatial light modulator (DMD TM , Texas Instruments). Employing video projector technology high resolution patterns, designed as bitmap images on the computer, are displayed using a micromirror array consisting of about 786000 tiny individually addressable tilting mirrors. The DMD, which is located in the image plane of an infinity corrected microscope, is projected onto a substrate placed in the focal plane of the microscope objective. With a 5× (0.25 NA) Fluar microscope objective, a fivefold reduction of the image to a total size of 9 mm 2 and a minimum feature size of 3.5 µm is achieved. The ultra high pressure (UHP) lamp of a video projector is a cheap, durable and powerful alternative to the mercury arc lamps commonly used in lithography applications. The MPLS may be employed in standard photolithography, we have successfully produced patterns in 40 µm films of SU-8 photoresist, with an aspect ratio of about 1:10. Our system can be used in the visible range as well as in the near UV (with a light intensity of up to 76 mW/cm 2 around the 365 nm Hg-line). We developed an inexpensive and simple method to enable exact focusing and controlling of the image quality of the projected patterns. Our MPLS has originally been designed for the light-directed in situ synthesis of DNA microarrays. One requirement is a high UV intensity to keep the fabrication process reasonably short. Another demand is a sufficient contrast ratio over small distances (of about 5 µm). This is necessary to achieve a high density of features (i.e. separated sites on the substrate at which different DNA sequences are synthesized in parallel fashion) while at the same time the number of stray light induced DNA sequence errors is kept reasonably small. We demonstrate the performance of the apparatus in light-directed DNA chip synthesis and discuss its advantages and limitations.

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