SPIE Journal Paper | 1 April 2004
KEYWORDS: Spiral phase plates, Optical vortices, Photomasks, Cadmium, Imaging arrays, Scanning electron microscopy, Optical lithography, Critical dimension metrology, Lithography, Reticles
In an optical vortex, the wavefront spirals like a corkscrew, rather than forming planes or spheres. Since any nonzero optical amplitude must have a well-defined phase, the axis of a vortex is always dark. Printed in negative resist at 248 nm and NA0.63, optical vortices and optical vortex arrays produce contact holes with 64 nm0.6 can be patterned using a chromeless phase-edge mask composed of rectangles with nominal phases of 0, 90, 180, and 270 deg. Lithography simulation and resist exposures have demonstrated process windows with 10%Elat and ~400-nm depth of focus (DOF) for 85-nm CDs at 210-nm pitch with =0.15, but the developed contacts are somewhat elliptical. No significant surface development has appeared due to phase-edge printing. However, the spacewidth alternation phenomenon familiar from linear chromeless phase-edge lithography does cause small positional errors for vortex vias, and each of the four vortices in the repeating pattern may behave somewhat differently through focus, potentially limiting the common process window. Smaller CDs and pitches are possible with shorter wavelength and larger NA, while larger pitches give rise to larger CDs. At pitch >0.6 µm, the vortices begin to print independently for 0.3. Such "independent" vortices have a quasi-isofocal dose that gives rise to 110-nm contacts with Elat>14% and DOF >400 nm. In an actual chip design, unwanted vortices and phase step images would be erased from the resist pattern by exposing the wafer with a second, more conventional, bright-field trim mask. Compared to other ways of producing deep subwavelength contacts, the vortex via process reduces the lithography and process control challenges.
