PROCEEDINGS ARTICLE | February 6, 2008
Proc. SPIE. 6883, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics
KEYWORDS: Lithography, Optical lithography, Deep ultraviolet, Polymers, Photonic crystals, Scanning electron microscopy, Photomasks, Photomicroscopy, Photoresist processing, Absorption
Photonic crystals[1, 2] have stirred enormous research interest and became a growing enterprise in the last
15 years. Generally, PhCs consist of periodic structures that possess periodicity comparable with the
wavelength that the PhCs are designed to modulate. If material and periodic pattern are properly selected,
PhCs can be applied to many applications based on their unique properties, including photonic band gaps
(PBG)[3], self-collimation[4], super prism[5], etc. Strictly speaking, PhCs need to possess periodicity in
three dimensions to maximize their advantageous capabilities. However, many current research is based
on scaled two-dimensional PhCs, mainly due to the difficulty of fabrication such three-dimensional PhCs.
Many approaches have been explored for the fabrication of 3D photonic crystals, including layer-by-layer
surface micromachining[6], glancing angle deposition[7], 3D micro-sculpture method[8], self-assembly[9]
and lithographical methods[10-12]. Among them, lithographic methods became increasingly accepted due
to low costs and precise control over the photonic crystal structure. There are three mostly developed
lithographical methods, namely X-ray lithography[10], holographic lithography[11] and two-photon
polymerization[12]. Although significant progress has been made in developing these lithography-based
technologies, these approaches still suffer from significant disadvantages. X-ray lithography relies on an
expensive radiation source. Holographic lithography lacks the flexibility to create engineered defects, and
multi-photon polymerization is not suitable for parallel fabrication.
In our previous work, we developed a multi-layer photolithography processes[13, 14] that is based on
multiple resist application and enhanced absorption upon exposure. Using a negative lift-off resist (LOR)
and 254nm DUV source, we have demonstrated fabrication of 3D arbitrary structures with feature size of
several microns. However, severe intermixing problem occurred as we reduced the lattice constant for
near-IR applications. In this work, we address this problem by employing SU8. The exposure is vertically
confined by using a mismatched 220nm DUV source. Intermixing problem is eliminated due to more
densely crosslinked resist molecules. Using this method, we have demonstrated 3D "woodpile" structure
with 1.55μm lattice constant and a 2mm-by-2mm pattern area.
