  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which photomasks have been replaced by a Digital Micromirror Device type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, 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 5x(0.25 NA) Fluar
microscope objective, a fivefold reduction of the image to a total size of 9
mm2 and a minimum feature size of 3.5 microns is achieved. 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/cm2 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 microns). 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.
