Nanopatterning with Lasers and Microspheres
Technique might allow parallel nanopatterning.
Although it concerns the very small, optical nanopatterning has some very big problems. Available techniques are slow, hard to use, expensive or limited in what they can produce. Now Princeton University researchers Euan McLeod and Craig B. Arnold have demonstrated a direct-write nano-patterning scheme that gets around such issues through the use of two lasers and a microsphere.
The technique could lead to commercially important parallel direct-write nanopatterning. Arnold, an assistant professor of mechanical and aerospace engineering, noted that optical nanopatterning methods are inherently serial and difficult to scale up. “Using our technique, we believe we can overcome this limitation,” he said.
Nanopatterning deals with features measuring 100 nm or so in size — far below the wavelength of visible light. As a result, subwavelength techniques are required for optical nanopatterning. In one approach, a probe tip or a microsphere is brought close to a surface and illuminated with a laser. The beam is focused down by the object through a near-field effect and thereby modifies the material below.
Aside from its serial nature, another problem with this approach has been controlling the spacing between the object and the surface. The near-field effect depends strongly upon distance, so maintaining this spacing is important.
The Princeton team achieved a near-constant distance in a new way — using a Bessel beam optical trap. Unlike other configurations, Bessel beam traps confine a particle in X and Y while pushing on it in Z. A micro-sphere in such a trap travels forward until the force of the beam is balanced by repulsive forces arising from a substrate. As a result, the sphere maintains a fixed distance to the surface, even if it is curved or uneven.
In this artist’s rendering, an array of microspheres — the white balls — focuses a large-area-patterning laser using a near-field effect. This modifies the material below and patterns it. The microspheres are held in place by a Bessel beam trap, generated by splitting a single laser into multiple beams. Interaction between the laser and the surface keeps the spheres a constant distance from the surface. Images courtesy of Craig B. Arnold, Princeton University.
In a demonstration, the investigators used either a 532-nm laser or a 1064-nm Spectra-Physics laser to create the Bessel beam trap and a 355-nm pulsed Coherent laser to modify the material. They spin-coated glass coverslips with polyimide films and placed water or another solution with microspheres of various sizes and types on top of the films. They then trapped a microsphere and wrote patterns of individual spots and continuous features. They monitored the process using a white light source and a Cohu CCD camera.
They wrote a minimum feature size of about 100 nm with a positioning accuracy better than 40 nm. Given the power of the trap and the nature of the liquid, these results were about as models predicted.
Using a trapping laser, a patterning laser and a microsphere, researchers demonstrated direct-write subwavelength nanopatterning. They did so in a polyimide film using polystyrene beads of 0.76 μm in diameter, patterning the film with an ultraviolet laser beam. The approach could be used for optical nanopatterning in parallel.
The results indicated also that, as might be expected, a smaller particle produces a smaller writing spot size. However, there’s also a lower limit beyond which the particle no longer concentrates enough light. In their experiments, the researchers found that to be about 0.75 μm.
Arnold noted that this limit is a function of ultraviolet transparency and of particle shape. “Further computational modeling would shed light on the ideal particle shape and size, and this is something that we are currently working on,” he said.
The technique could perform nanopatterning in parallel, with many beads, a single laser split into several trapping beams and simultaneous illumination of all beads with a large-area-patterning laser. The team members are working on that as well as on characterizing and modeling the bead, laser and substrate interaction.
Nature Nanotechnology, July 2008, pp. 413-417.
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