  A custom-built mask aligner (CBMA), which fundamentally covers all the key
features of a commercial mask aligner, while being low cost, light weight, and
having low power consumption and high accuracy is constructed. The CBMA is
comprised of a custom high fidelity LED light source, vacuum chuck and mask
holder, high-precision translation and rotation stages, and high resolution
digital microscopes. The total cost of the system is under $7,500, which is
over ten times cheaper than a comparable commercial system. It produces a
collimated ultraviolet illumination of 1.8-2.0 mW cm-2 over an area of a
standard 4-inch wafer, at the plane of the photoresist exposure; and the
alignment accuracy is characterized to be < 3 microns, which is sufficient for
most microfluidic applications. Moreover, this manuscript provides detailed
descriptions of the procedures needed to fabricate multilayered master molds
using our CBMA. Finally, the capabilities of the CBMA are demonstrated by
fabricating two and three-layer masters for micro-scale devices, commonly
encountered in biomicrofluidics applications. The former is a flow-free
chemical gradient generator and the latter is an addressable microfluidic
stencil. Scanning electron microscopy is used to confirm that the master molds
contain the intended features of different heights.
