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Manufacture of multi-level encoded subwavelength optical data storage media [Invited Paper - 2005 European Optical Society] Hubert Kostal1, Jian Jim Wang1 and Fred Thomas2 1 NanoOpto Corporation, 1600 Cottontail Lane, Somerset, NJ 08873-5117 Phone: 732-627-0808 Email: hkostal@nanoopto.com; jwang@nanoopto.com 2 Iomega Corporation, 1821 West Iomega Way, Roy, Utah 84067 Phone: 801-332-4662 Email: thomasf@iomega.com Summary The concept of storing information in multilevel formats using sub-optical wavelength structures to alter the reflected properties of an interrogating laser stylus has been a subject of interest/research for the past few years in industry and academia. A significant challenge to commercial viability of these data storage schemes is the low-cost replication of the data storage media. Presented is the state-of-the art relative to future low-cost replication of such data storage media. Introduction Nanostructures— structures with one or more dimensions measured in less than a hundred nanometers—produce a broad range of important and often unexpected optical effects. By operating in the subwavelength realm, nanostructure-based optical structures can reach, and sometimes cross, the boundary between classical and quantum optics. These effects include reflected orientation, polarization, phase, wavelength, amplitude and refractive index filtering or modification. Thus using these mechanisms, nano- optical structures offer the capability to create ROM and potentially WORM optical data elements (ODEs) for which data is encoded in a massively multi-level format. The term multi-level6 refers to the encoding of more than a binary (2) number of data states in an area smaller than a single diffraction-limited optical stylus’ spot size. A few papers1-3 have been published, and some public notice4, has been garnered in recent years on the topic of subwavelength optical data storage. These publications make note of computer simulations2 and empirical data3 which support the feasibility of these approaches. The physical realization of such structures is just emerging as an area of investigation; Figures 1 and 2 show micro-graphs of lithographically produced structures, fabricated by the authors, which are capable of encoding information in this manner. Capacities in the terabyte range for 120 mm optical discs (DVD/CD sized) have been noted as within the potential of this new data storage technology3-4. This paper examines if and how these subwavelength structures might be replicated in formats as large as 120 mms at low-cost. The potential for this technology to become a commercially viable path for future content distribution is technically contingent on such capability. Fig 1. AO-DVD (Articulated Optical – DVD) morphology demonstrated in photoresist. Fig 2. NG-DVD (Nano-Grating – DVD) replicated morphology demonstrated in dielectric material. Subwavelength Manufacturing Process for Nano-grating Structures The high volume, high fidelity replication of fine scale nano-structure arrays is a topic of interest for a broad range of optical applications, spanning optical switching for telecommunications, digital imaging for both consumer electronics and security, and displays. A number of nano-lithography methodologies have been investigated, including both direct lithography methods – such as e-beam lithography – and indirect lithography methods – such as nano-embossing and nano-imprinting. Indirect methods generally utilize a direct lithography method to produce a mold that is then used repeatedly as a master in volume production – similar in a sense to type setting for printing. Key factors for commercial application include fidelity, ease of replication, speed of replication, and support for fine scale arrays. Figure 3 illustrates a indirect method, referred to as “nano-pattern transfer”, that has been applied by the authors5,6 to replicate, in volume, a range of optical devices and functions, including polarization elements, phase retarders, optical filters, and focal length and location changing optics (e.g., lenses, mirrors). Key factors for commercial application include fidelity, ease of replication, speed of replication, and support for fine scale arrays. Nano-pattern transfer utilizes direct lithography methods to write a master mold into a durable material, such as silicon, which is patterned with the complement of the actual pattern that is desired. Molds of up to 150mm in diameter have demonstrated. The master mold, or more likely a “clone” of the master (to extend the life of the master) is then used with a combination of printing and semi-conductor- like fabrication steps to replicate the patterns in volume. Volume replication then prepares a substrate – both glass wafers and poly-carbonate have been demonstrated – with a thin layer of resist on its surface. The production mold is brought into contact with the resist layer and, under appropriate pressure to ensure complete patterning, is set, either thermally or using UV light. The mold is then removed and the patterned resist is used as an etching mask to transfer the pattern to the underlying substrate. Finishing operations include coating deposition and cutting the wafer to its desired size. Figure 4 illustrates a cross section of a general nano-structure design that can be created using this method; note that both sides of the substrate can be individually processed. General capabilities of this method relevant to the production of optical storage media are: (1) ability to replicate sub-100nm features – needed to support sub-micron beam diameters in reading the media; (2) ability to create fine scale variations in optical functionality by adjusting nano-structure dimensions or alignment relative to the path of the laser stylus – needed to provide multi-level optical response; (3) ability to reproduce tightly spaced optical arrays to enable variation of optical response; (4) ability to integrate the nano-structure in a stack with thin film anti-reflection and other coatings to optimize optical response; (5) ability to pattern both sides of a substrate, doubling the media capacity; and (6) ability to create shaped structures in addition to gratings, allowing more flexibility in optical coding. Fig 3. Nano-pattern transfer manufacturing is a combination of printing and semi-conductor manufacturing that results in high accuracy and volume in the fabrication of nano-optic components at a wafer scale. 1. Mold 2. Prepared substrate 3. Impression & Separation 4. Reactive Ion Etching 5. Post-processing: Coating and dicing Fig. 4 A general optical nano-structure is a combination of optical thin films and nano-structure gratings. For optical data storage, these structures are constructed on a reflective surface. Conclusions Nano-optic structures, in both their discrete and integrated forms, are all manufactured in single uniform process: nano-imprint manufacturing. This process is combination of printing and semi-conductor manufacturing – both of which are high volume, highly repeatable, and highly scalable processes. Specifically, this capability for manufacture of both nano-grating and gray-scale7 lithographically mastered, massively multi-level, subwavelength optical data storage media is shown. Mass replicated and shipping optical elements with tolerances less than 10 nm are presented, with optical control over reflected phase, polarization, and amplitude. The extension to pixilated structures is described, along with initial observations on optical coding. References 1. ISOM/ODS: F. Thomas, "AO-DVD (Articulated Optical - Digital Versatile Disk) A 20X to 100X Performance Enhancement Path for DVD-ROM ," presented at ISOM/ODS 2002, 7-11 July, Waikoloa, Hawaii. 2. OSA/ODS: F. Thomas, "Exploring optical multi-level information storage using subwavelength-sized media structures," in Optical Data Storage 2003, N. Miyagawa and M. O’Neill, Proc. SPIE Vol. 5096, (Optical Society of America, Washington, D.C., 1900), pp. 391-399. 3. Submited to: ISOM/ODS 2005: F. Thomas, H. Kostal and J. Wang, "Massively multi-level optical data storage using subwavelength-sized nano-grating structures," in Optical Data Storage 2005, 10-14 July, Honolulu, Hawaii. 4. J.R. Minkel, “More Bits in Pits,” Scientific American, pp. 30, February 2005. 5. J. Wang, X. Deng, L. Chen, P. Sciortino, J. Deng, F. Liu, A. Nikolov, A. Graham, and Y. Huang, “Innovative nano-optical devices, integration and nano-fabrication technologies (invited paper)”, Proc. of SPIE (Passive components and fiber-based devices, edited by Y Sun, S. Jian, S. Lee, K. Okamoto), Vol. 5623, pp. 259 – 273, (2005). 6. J. Wang, L. Chen, S. Tai, D. Deng, P. Sciortino, J. Deng, and F. Liu, “Wafer based nano-structure manufacturing for integrated nano- optic devices,” J. Lightwave Technology, Vol. 23, No. 2, February 2005. 7. C. Wu, “HEBS Glass Gray Scale Lithography”, Canyon Materials, Inc.: Product Information No. 01-88.
