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head and media, random access to content, and the relative simplicity of the media structure. Building upon technology, Materials for Optical companies began developing optical discs capable of storing digital audio. In 1983, compact discs were introduced, storing approximately 74 minutes of audio. CD- ROM (read-only memory) discs, targeted for data storage and able to hold 650 Hans Coufal and Lisa Dhar, Guest Editors Mbytes, entered the market soon after, fol- lowed by recordable and rewritable ver- sions. Typically 1.2 mm thick and 120 mm in diameter, a CD-ROM consists of a single Abstract disc fabricated from polycarbonate. In the The development of recording materials has been a critical component in the read-only structure, the polycarbonate is advancement of optical data storage. The continual need for improved performance in molded with pits 500 nm wide and both consumer and business applications has pushed forward the capabilities of optical 800 nm to 3.5 μm long on its surface. A thin storage. In this issue of MRS Bulletin, we review some of the important developments in layer of metal is deposited on the surface the materials behind established technologies such as compact discs, digital versatile of the disc. A semiconductor at discs, and magneto-optical recording; the emerging technology of blue recording; and 780 nm is used to reflect off of the pit and two technologies that seek to extend the performance roadmap for , land structure of the disc to reconstruct multilayer and holographic recording. the recorded data. The early 1990s saw efforts to increase Keywords: magneto-optic, memory, optical. the storage capacity of these types of opti- cal discs, resulting in the commercial in- troduction of digital video discs—or, as they have come to be called, digital versa- Optical storage has become ubiquitous strated to the public in 1972. The technol- tile discs ()—in 1996. Unlike the as a method for distributing content, ogy, which used 30-cm-diameter discs single-disc construction of compact discs, archiving data, and managing . and stored video in analog format, was DVDs consist of two 0.6-mm-thick poly- Given the portability and interchangeabil- made commercially available in 1978. The carbonate discs coated with a thin metallic ity of the recording media, optical technolo- laserdisc was an early demonstration of layer, with each layer containing data that gies have found wide use in consumer and the power of optical storage: the absence can be read independently. The discs are business applications. Propelling the de- of physical contact between the readout bonded together to product a 1.2-mm- velopment of optical recording has been the need in consumer electronics for higher data rates and increased storage ca- pacity. For example, the progression of op- tical storage from capturing high-fidelity stereo audio to high-definition movies has required the evolution from compact discs to digital versatile discs to high-definition DVDs and Blu-ray discs (see Figure 1).1 In addition, the increasing performance re- quirements for archiving data have pushed the advancement of magneto-optical recording.2 Underlying the progress in op- tical storage has been continual advances in the recording materials to enable the higher storage capacities and faster data recording and readout rates. The era of commercial optical data storage3 began with the invention of “video disk” technology, later known as “laserdisc” technology, by David Paul Gregg in 1959. Designed for the distribu- tion of video content, the approach used discs containing surface structures called pits and lands which modulate a readout Figure 1. The continuing increase in storage capacity and complexity of multimedia objects light beam. The modulated light is cap- has driven improvements in optical storage technologies. CD ෇ , DSP ෇ tured by a pickup head and translated for digital signal processor, DVD ෇ digital versatile disc, GB ෇ gigabytes, HD-DVD ෇ playback. Initially configured using trans- high-definition DVD, KB ෇ kilobytes, MB ෇ megabytes, pix ෇ pixel, Mbps ෇ megabits per parent discs and followed by more efficient second, MPEG-1 ෇ Moving Pictures Expert Group Standard 1, MPEG-2 ෇ Moving Pictures reflective discs, were demon- Expert Group Standard 2, TB ෇ terabytes, VR ෇ virtual reality.

294 MRS BULLETIN • VOLUME 31 • APRIL 2006 Materials for Optical Data Storage

thick package. The discs can be read from that summarizes the process and require- The last article, on materials for optical one or both sides, with each side contain- ments for media standardization, the key storage, focuses on high-performance ing one or two layers of data. In addition for interchangeability in optical storage. polymers for holographic data storage. to taking advantage of the capability of The importance of optimizing the mate- Holographic storage represents a depar- optics to focus on different depths in the rials properties of substrates for the vari- ture for optical technologies from the tra- media, the increase in storage capacity of ous disc formats—CDs, DVDs, and blue ditional bitwise approach to recording; DVDs over CDs is achieved by using smaller laser discs—and issues such as media data is stored throughout the volume of feature sizes and a shorter- replication, recording, readout processes, the recording media in a parallel, rather readout laser (650 nm). A single-sided, and lifetime characteristics are discussed than serial, fashion. Dhar summarizes the single-layer DVD can hold 4.7 Gbytes of in the article by Bruder et al. in this issue. materials requirements of media for holog- information. Efforts to increase the storage In particular, the authors focus on the de- raphy and focuses on recent progress in capacity have centered on the use of blue velopment of polycarbonate materials the development of polymer recording with a shorter (405 nm) wavelength, that are not only tuned to exhibit the re- materials. which allows smaller data bits and there- quired performance profiles, but also, just Glinka ends the issue with an article on fore higher capacity. These efforts are as important, able to be cost-effectively the processes required for optical record- marked by the recent introduction of Blu- manufactured. ing media and the organizations involved ray and high-definition DVDs that enable The recent development of blue laser in standardizing those processes. For the capacities of 15–30 Gbytes. recording has extended the performance established optical storage technologies, Concurrently with the development of capabilities of the family of optical storage this standardization has been the key to CDs and DVDs, magneto-optical discs were represented by CDs and DVDs. In their ar- their broad adoption. As emerging tech- developed, primarily for professional ap- ticle, Kuiper and Pieterson write of the nologies such as multilayer recording and plications and the rewritable data market. materials used in both write-once and holographic storage reach commercializa- Introduced in the late 1980s, this technol- rewriteable blue laser materials. The au- tion and then widespread use, standardi- ogy relies on the of light by thors discuss developments in phase- zation will become a part of their the magnetic state of the material. The per- change materials, spin-coated dye systems, development as well. formance of these discs has evolved from and inorganic alloy materials and their We hope that these articles focused on capacities of 650 Mbytes on a two-sided, suitability for the smaller optical spot sizes materials for established optical storage 130-mm disc to current capacities of many enabled by blue lasers. technologies will draw attention to the gigabytes, with future products expected We then examine the rare-earth/ technological challenges involved in opti- to reach terabytes of storage capacity. transition-metal materials and structures mizing commercially viable materials. In this issue of MRS Bulletin, we have central to magneto-optical recording in The strategies used to optimize materials sought to cover areas in materials research the article by Kaneko. The author traces the with respect to issues such as performance, and engineering that span the field of op- evolution of magneto-optical recording environmental robustness, and manufac- tical storage. From the optimization of from its commercial introduction in 1988, turability have been critical in extending substrate materials for optical storage, to through the emergence of super-resolution the capabilities of established optical tech- the growing complexity of materials for techniques that enabled resolution be- nologies. These strategies serve as models magneto-optic recording, to the recent de- yond the optical limit, to current discus- for the development of materials for the velopments of materials for blue laser sions of extensions of super-resolution emerging areas of blue, multilayer, and recording, we hope to highlight the so- techniques. holographic recording. phisticated design and fabrication meth- In the article by Milster and Zhang, the ods that underpin the currently available authors examine the materials required References optical storage technologies. In addition, for multilayer recording, a technique that 1. J. Taylor, DVD Demystified (McGraw-Hill, we point to the future by including articles aims to increase the capacity of optical New York, 2000). on materials for multilayer optical record- disc recording by using multiple layers of 2. R.J. Gambino and T. Suzuki, eds., Magneto- ing and holographic data storage, two tech- bitwise-recorded data. The challenges Optical Recording Materials (IEEE Press, Piscataway, NJ, 2000). nologies that can extend the optical storage presented by the interplay between mate- 3. K. Immink, J. Audio Eng. Soc. (1998) roadmap to even higher performance. We rials properties and the optical systems p. 458. ■ conclude this issue with a contribution used for recording are summarized.

Hans Coufal, guest from the Technische in addition to his own editor for this issue of Hochschule in Munich, scientific work on ther- MRS Bulletin, is manager Germany. After serving mal and acoustic tran- of device and systems as a junior faculty mem- sients and holographic innovation in the IBM ber at the same univer- data storage. He has Research Center. He is sity and at the Freie more than 150 scientific currently on assignment Universität in Berlin, he publications, is the edi- to the Semiconductor joined the staff at the tor of eight books, and Research Corp. as the IBM San Jose Research holds 14 patents. He is a founding director of Laboratory, the precur- fellow of the Optical the Nanoelectronics sor to the Almaden Re- Society of America, the Research Corp. search Center, in 1981. American Physical Coufal earned his ap- He has held several Society of America, and plied physics degrees management positions the International Union Hans Coufal Lisa Dhar

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of Pure and Applied Physics. He is also the recipient of the Bundes-verdienstkreuz, the German counter- part of the Order of Merit. Coufal can be reached at IBM Almaden Re- search Center, KLPA/D2, IBM Re- search Division, 650 Harry Road, San Jose, CA 95120-6099, USA; Friedrich-Karl Bruder Konstantinos Douzinas Uli Franz Werner Glinka tel. 408-927-2441 and e-mail coufal@almaden. He studied physics at Konstantinos Douzinas ibm.com. the University of is head of the polymer Freiburg, where he ob- physics department Lisa Dhar, guest editor tained his PhD degree in within the polycarbon- for this issue of MRS 1992 investigating the ates business unit of Bulletin, is the vice presi- mixing and de-mixing Bayer MaterialScience dent of media develop- behavior of polymer AG in Krefeld, Germany, ment and one of the blends near surfaces a position he has held founders of InPhase and interfaces. He then since 2004. After obtain- Technologies (Long- joined Bayer’s central ing his diploma in mont, Colo.). Before research department as chemical engineering founding InPhase, she head of one of the from the National Tech- was a member of the physics laboratories. nical University in Bell Labs core team at Since then, Bruder has Athens, Greece, he re- Rafael Oser Liesbeth van Pieterson Lucent Technologies re- worked on various re- ceived his PhD degree sponsible for develop- search projects related to in chemical engineering AG in Leverkusen, tional standards com- ing the technology in structure–property rela- from MIT in 1991. The Germany. He received mittee for holographic holographic storage tions of polycarbonates, same year, he joined his PhD degree in storage systems. He re- upon which InPhase is electroconductive Bayer AG Central Re- macromolecular chem- ceived his degree in based. She holds a PhD polymers, DVD-grade search in Leverkusen, istry at the Technische electrical engineering degree from the Massa- polycarbonates, copoly- Germany, and worked Universität in Munich. from Fachhochschule chusetts Institute of carbonates with low on new materials for in- In 1994, he joined Miles Bochum in Germany Technology and a BS de- birefringence, and dustrial, medical, and (now Bayer Corp.) in and has more than 25 gree from the University hydrogenated styrene data storage applica- Pittsburgh, Pa., and years of experience in of Chicago, both in polymers and dye tions. In 2001, he joined worked on different product marketing, de- chemistry. She has au- development for record- the physics department projects in the corporate sign engineering, and thored numerous tech- able optical discs. His of Bayer AG and took polymer research group. industry standards. He nical articles in the areas current projects are over responsibility for a In 1996, he joined the was responsible for of holographic data mainly in the field of working group dealing plastics division and marketing and product storage, optical mi- optical data storage, with optical storage was responsible for the planning for optical croscopy, and pulsed such as the improve- systems. technical marketing of storage and system lasers, and she is the ment of polycarbonate Douzinas can be polycarbonate in vari- products at Sony inventor on more than for future formats such reached at Bayer ous application areas. In America, served as sen- 15 patents and patent as Blu-ray discs, high- MaterialScience AG, BU 2000, he transferred to ior director of marketing applications. density DVDs, substrate Polycarbonates, Global Bayer AG in Leverkusen, of optical storage and Dhar can be reached materials, and func- Innovations & Product taking over responsibil- multimedia products for at InPhase Technologies, tional materials for near- Management, Polymer ity as global industry America Ltd., 2000 Pike Road, Long- field recording and Physics, Bldg. R79, 47812 manager for optical data and has also held senior mont, CO 80501, USA; holographic data Krefeld, Germany; storage applications. marketing and engineer- tel. 217-369-7409 and storage. e-mail konstantinos. Franz can be reached ing positions at Maxop- e-mail lisadhar@ Bruder can be reached [email protected]. at Bayer AG, 51368 Lev- tix/Maxtor Corp., inphase-tech.com. at Bayer MaterialScience erkusen, Germany; Laserdrive Ltd., Memo- AG, BU Polycarbonates, Uli Franz is the global e-mail uli.franz@ rex, and Nixdorf Friedrich-Karl Bruder is Global Innovations & industry manager re- bayerbms.com. Computers. a senior scientist in the Product Management, sponsible for the world- Glinka was chair of polymer physics depart- Polymer Physics, Bldg. wide coordination of Werner Glinka founded the Fibre Channel In- ment within the poly- R79, 47812 Krefeld, optical data storage ac- the Glinka Company in dustry Association carbonates business unit Germany; e-mail tivities in the polycar- Woodside, Calif., in 1999 (FCIA) from 2004 to of Bayer MaterialScience friedrich-karl.bruder@ bonates business unit of and is the chairman of 2005 and founding AG in Krefeld, Germany. bayerbms.com. Bayer MaterialScience Ecma TC44, an interna- executive director of the

296 MRS BULLETIN • VOLUME 31 • APRIL 2006 Materials for Optical Data Storage

microscopy, lithography, and optical data storage. Milster is a fellow of the Optical Society of America and the Inter- national Society of Opti- cal Engineering (SPIE). His research activities have led to more than 100 papers as well as numerous contributions and invited presenta- tions at national and in- Wilfried Haese Masahiko Kaneko A.E.T. (Ton) Kuiper Tom D. Milster ternational conferences. He also designed an and has specialized in displacement detection) joined Philips Research extreme-ultraviolet optical data storage ap- media. Before beginning in 1975. His past work spectrometer which was plications since 1995. his current project, he has been related to thin part of the scientific Currently, he is respon- was a national project films, ranging from sur- package that flew in the sible for the develop- group leader for the de- face science, deposition, space shuttle with Sen. ment of polycarbonate velopment of a 100- and analysis to thin-film John Glenn in 1998. substrate materials for Gbyte multilevel magnetic heads and Milster can be optical data storage read-only memory disc. sensors. His current ac- reached at the Univer- media. Currently, he is also a tivities are in optical sity of Arizona, Optical Haese can be reached visiting professor at the storage media, with an Sciences Center, 1630 at Bayer MaterialScience Tokyo Institute of Tech- emphasis on phase- East University Blvd., AG, BU Polycarbonates, nology and is the gen- change materials. He is Tucson, AZ 85721, USA; Global Innovations & eral chair of the MORIS a member of the Dutch tel. 520-621-8280, fax Yan Zhang Product Management, (Magneto-Optical Vacuum Society and the 520-621-4358, and e-mail Polymer Physics, Bldg. Recording International editorial board of Thin [email protected]. DAFS Collaborative R79, 47812 Krefeld, Ger- Symposium) 2006 Work- Solid Films. (2002). He can be many; e-mail wilfried. shop on Thermal and Kuiper can be reached Rafael Oser is a re- reached at the Glinka [email protected]. Optical Magnetic Mate- at Philips Research search scientist in the Company, 214 Swett rials and Devices, which Eindhoven, WAG-12, polymer physics depart- Road, Woodside, CA Masahiko Kaneko has will be held in June. He HTC 04, 5656 AE Eind- ment of the polycarbon- 94062, USA; tel. 650-851- been with Sony Corpo- is a member of the hoven, Netherlands; ates business unit of 5909, fax 650-851-5987, ration since 1976 and is Magnetic Society of tel. 31-40-2747984, Bayer MaterialScience and e-mail werner. currently a principal re- Japan, the Physical fax 31-40-2746321, and AG in Krefeld, glinka@theglinkacompany. searcher and deputy Society of Japan, and the e-mail ton.kuiper@ Germany. He received com. leader of a national proj- Institute of Electrical philips.com. his diploma in physics ect for the development Engineers of Japan. from the University of Wilfried Haese is a sen- of ultrathin, small- Kaneko can be Tom D. Milster is a re- Karlsruhe in 1995. After ior scientist in the poly- diameter disc drives. He reached at Sony Corpo- search professor of opti- completing his doctor- carbonate innovations received his BS degree ration, Optical System cal sciences at the ate at the University of department of Bayer from the Tokyo Institute Development Division, University of Arizona. Freiburg in polymer MaterialScience AG in of Technology in 1970 Video Business Group, He received his BS de- physics, he joined Bayer Krefeld, Germany. He and his MS and PhD Home Electronics Net- gree in electrical engi- AG Central Research in received his PhD degree degrees in optical studies work Company, 6-7-35 neering from the 2000. His interests focus in organic chemistry of magnetic materials Kitashinagawa, University of Missouri on optical data storage at the Technische from Tokyo University Shinagawaku, Tokyo, in 1981 and his PhD de- materials and thin-film Hochschule Aachen. In in 1972 and 1975, 141-0001 Japan; tel. 81-3- gree from the University technologies. 1983, he joined Bayer respectively. 5795-6193, fax 81-3-5795- of Arizona’s Optical Sci- Oser can be reached AG in Leverkusen and Kaneko began his 6230, and e-mail ences Center in 1987. He at Bayer MaterialScience worked on a variety of work at Sony research- Masahiko.Kaneko@jp. worked as an optical en- AG, BU Polycarbonates, projects in central re- ing magnetic powders sony.com. gineer in IBM’s general Global Innovations & search. In 1989, he and LPE (liquid-phase products division before Product Management, joined the plastics divi- epitaxy) garnet films. A.E.T. (Ton) Kuiper is a joining the faculty of the Polymer Physics, Geb. sion and was responsi- He was then engaged in principal scientist at University of Arizona as R79, Raum 452, 47812 ble for developing the development of Philips Research Labo- an assistant professor. Krefeld, Germany; application technologies magneto-optical materi- ratories in Eindhoven, His major research inter- e-mail rafael.oser@ for liquid-crystalline als, especially magnetic the Netherlands. He re- est is the study of physi- bayermaterialscience.com. polyesters and multilayers for direct ceived his PhD degree cal phenomena in poly(phenylene sulfide). overwriting, magnetic in catalysis at the Uni- high-performance opti- Liesbeth van Pieterson In 1992, he joined the super-resolution, and versity of Technology in cal systems. In particu- is a senior scientist at polycarbonate group DWDD (domain-wall Eindhoven in 1974 and lar, his research involves Philips Research Labo-

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ratories in the Nether- physics and chemistry Phase-Change and 31-40-27-46505, and interest centers on lands. She received her of materials for optical Ovonic Science e-mail liesbeth.van. high-resolution three- PhD degree in chem- recording. (EPCOS), and she is [email protected]. dimensional imaging. istry of condensed In 2002, she received treasurer of the Dutch Zhang can be reached matter from Utrecht the PUG award, a prize Vacuum Society Yan Zhang is a postdoc- at Indiana University, University in 2001 and given every four years (NEVAC). toral fellow in the School of Optometry, began her work at for the best thesis in Liesbeth van Pieter- School of Optometry at 800 E. Atwater Ave., Philips Research that beta sciences at Utrecht son can be reached at Indiana University. He Bloomington, IN 47405, same year. Her research University. She is a Philips Research, WAG- graduated from the Uni- USA; e-mail yz7@ focus is in the area of member of the program 12, HTC 04, 5656 AE versity of Arizona’s indiana.edu. ■ solid-state chemistry committee of the Euro- Eindhoven, Netherlands; Optical Sciences Center with an emphasis on the pean Symposium on tel. 31-40-27-42343, fax in 2004. His research

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298 www.mrs.org/bulletin MRS BULLETIN • VOLUME 31 • APRIL 2006