Materials for Optical Disc Substrates
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Materials for Optical Disc Substrates
Friedrich-Karl Bruder, Konstantinos Douzinas, Uli Franz, Wilfried Haese, and Rafael Oser Abstract Optical data storage has become the mainstream technology during the past two decades for distributing audio, video, and software content as well as for recording and archiving personal data. The continuous demand for ever-higher storage capacities and faster data transfer has led to the development of three disc format families using infrared (compact discs), red (digital versatile discs), and blue lasers (Blu-ray discs or high-density DVDs). Substrate materials used in optical discs need to possess a complex property profile with sufficient optical, rheological, mechanical, and processing characteristics to ensure cost-efficient replication, good read/write performance, and long-term media stability. Bisphenol-A polycarbonate (BPA-PC) is the substrate material of choice and has undergone several optimization cycles to always meet new format specifications. Keywords: memory, optical, polymer.
MRS BULLETIN • VOLUME 31 • APRIL 2006
Current Optical Data Storage Technologies Using Far-Field Optics The basic physical principles for current optical disc systems have been well understood for several decades.1–3 An objective lens with a specific numerical aperture (NA) is used to focus a laser beam with wavelength λ through a transparent substrate or cover layer onto the highly reflective information layer (see Figure 1). The diffraction-limited spot radius s/2 of the laser on the information layer is given by s λ . 2 2NA
Introduction The success story of digital optical data storage (ODS) systems started in 1982 with the introduction of the compact disc (CD). Its 650-Mbyte storage capacity and superior digital sound quality were clear advantages compared with vinyl records, leading to an almost complete replacement of that technology in the market. This advance would not have been possible without polycarbonate, the substrate material of choice used in optical disc fabrication. Polycarbonate provided a unique property profile that includes high transparency, low birefringence, excellent impact resistance and stiffness, a high heat-deformation temperature, and low water uptake as well as high resin flowability and a high heat transfer coefficient. Continuing product improvement and format modifications led to the introduction of the digital versatile disc (DVD) in 1996, with storage capacities of 4.7–18 Gbyte, which can store entire movies with multilingual soundtracks and excellent picture quality. As the need for ever-increasing storage capacities continues to grow, new ODS formats have been developed and are now ready for mass commercialization: Blu-ray discs (BDs) and high-density DVDs (HD-DVDs) use blue laser diodes with a shorter wavelength that can write smaller bits and thus increase the
recordable and rewritable CD and DVD formats. Approximately 800,000 tons of polycarbonate were used for ODS production in 2004, reflecting an average yearly market growth rate of about 20%
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