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| Dateline: October 29, 2000 | |
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 Figure 1 - A Picasso drawing reproduced on the nanoscale with SNOL [1]. |
Exposing optically active surfaces to patterns of light (photolithography) is the primary method used for fabricating microelectronic (and mechanical) circuits. Requirements for smaller and faster circuits are resulting in several novel means for using light to fabricate nanoscale features. Due to the far-field diffraction limit, light can only be focused to half of its wavelength by traditional means. However, researchers are finding new ways to surpass this barrier in order to allow the fabrication of the next generation of computer chips.
Several methods are now available to the chip designer who wishes to use optical means for fabricating circuits (or other devices) with features smaller than half of the wavelength of light used. Stimulated emission depletion is one technique that has effectively broken the diffraction barrier by exciting localized fluorescence. Another promising method is quantum interferometry. This method takes advantage of the quantum mechanical principle known as entanglement. When two photons are entangled, they behave as though they have half the wavelength, and can thus be focused twice as efficiently. Entangling more photons results in an even smaller effective wavelength.
 Figure 2 - Scanning Near-field Optical Lithography (SNOL) was used to pattern a standard photoresist to create minimum features sizes around 80 nm [1]. |
SNOM is perhaps the most established method for breaking the diffraction barrier. Using a sharp tip to guide light into localized areas of a surface has been used to pattern standard photoresists with feature sizes as low as 100 nm as early as 1996 (figures 1 & 2) [1]. This resolution could be substantially improved by using thinner photoresist films and better SNOM tips. When only one tip is used this method is too slow to be used in industry. However, if it were operated in parallel by using an array of tips, Scanning Near-field Optical Lithograpy (SNOL) could be used to effectively extend photolithography to the nanoscale.
Another option for surpassing size limits placed on circuitry by the fabrication method is to select a photosensitive material which changes its nanoscale properties in response to light. For instance, Naber and coworkers used SNOL to pattern monolayers (films only one molecule thick) of dye molecules with feature sizes around 100 nm [2]. Zeolites are crystals with small holes that have industrial applications for separating materials. Brinker et. al. has developed thin films of zeolite materials in which the size of the holes can be tuned by exposure to light [3]. In addition, the local refractive index, surface area and wetting behavior of this material can be optically defined. Photosensitive resins are another promising material [4]. Maruo and Ikuta found that these resins polymerize when exposed to light intensities above a certain threshold. Furthermore, use of stereolithography allows these materials to be patterned in three dimensions, while traditional optical lithography is limited to two dimensional surfaces.
Acknowledgements: Thank you Andreas Naber for the images and enlightening discussion.
References:
[1] A. Naber, H. Kock, and H. Fuchs, "High-Resolution Lithography with Near-Field Optical Microscopy" (1996) Scanning 18, 567-571.
[2] A. Naber, T. Dziomba, U.C. Fischer, H.-J. Maas, and H. Fuchs "Photopatterning of a monomolecular dye film by means of scanning near-field optical microscopy" (2000) Appl. Phys. A 70, 227-230.
[3] D. A. Doshi, N. K. Huesing, M. Lu, H. Fan, Y. Lu, K. Simmons-Potter, B. G. Potter Jr., A. J. Hurd, and C. J. Brinker "Optically Defined Multifunctional Patterning of Photosensitive Thin-Film Silica Mesophases" (2000) Science 290, 5489, 107-111. Abstract
[4] S. Maruo, and K. Ikuta "Three-dimensional microfabrication by use of single-photon-absorbed polymerization" (2000) Appl. Phys. Letters 76, 19, 2656-2658.