High-end optical lithography is purely dedicated to printing lines with the smallest possible pitch. For this, a mask of lines is imaged onto a substrate with only two off-axis illuminations. The imaging is then due to the interference between two diffraction orders from the grating, and is known as two-beam imaging. This complex procedure produces an array of lines, the simplest 1D lattice possible. Anything else on the mask other than this dense set of lines is hardly printed, for example an isolated line. The 1D lattice can be done without a mask, by simply using two-beam interference, and is hence referred to as two-beam interference lithography. If more than two beams are used (multibeam interference lithography) all of the 4 2D Bravais lattices and all of the 14 3D Bravais lattices can be created. The problem in multiple interference lithography arises when a large-scale post-patterning is needed for the 1D, 2D or 3D lattices.

In a series of papers, this group from Georgia Institute of Technology introduced pattern-integrated interference lithography (PIIL), which essentially solves the above problem. They describe an optical apparatus which produces multiple beam interference lithography, but has a placeholder for a mask. The mask defines, either in dark or bright fields, where the lattice will be printed, thus printing the lattice only in the areas defined by the mask.

In this specific paper, the authors take PIIL a step forward by showing how this method is capable of producing patterned- 3D lattices. The ability to create 3D lattices is equivalent to that of creating a new material. Such lattices or crystals are referred to as synthetic materials, and if the lattice constant is in the optical region they are termed photonic crystals, which can act as optical insulators. PIIL is extremely applicable for photonic crystals. The authors’ method can be used to write optical circuits, where the photonic crystal serves as the insulators and the voids are the light conducting medium. In this paper the authors present as an example a hollow cavity surrounded by a photonic crystal.

To the best of my knowledge, this is the only method that can be used to both create and pattern 3D periodic structure in a single step. The end result is a beautiful periodic 3D lattice, bounded by voids in the case of negative photo-resist or solid in the case of positive photo-resist. As in many branches of physics, the devil awaits at the interface of God’s made crystal, and the perfect periodic structure suffers for a few cycles. However, the well established optimization tools used for projection lithograph, such as surface mask optimization and optical assist features, can be deployed here to swindle the devil, and make this aperiodic region very small.

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