Optical storage of digital information has reached the consumer market in the form of the compact audio disk. In this technology, information is stored in the form of shallow pits embossed in a polymer surface. The surface is coated with a reflective thin metallic film, and the digital information, represented by the position and length of the pits, is read out optically with a focused, low-power (5 mW) laser beam. When used for information storage for a computer this device is called a CD-ROM, a Compact Digital-Read Only Memory. The user can only extract information (digital data) from the disk without changing or adding any data. That is, it is possible to “read” but not to “write” or “erase” information.

While it is an advantage to have permanently stored information in some cases — for example when listening to Beethoven's Ninth Symphony—in other situations the read-only feature is not appropriate. For most computer applications, it is essential that the user be able to store information on the disk and read it back at will. For example, in a word processing task—such as typing this article — it is often necessary to store the document on a disk. Optical data storage for this purpose is available in the form of a Write Once Read Many times (WORM) optical disk drive. The operating principle in a WORM drive is to use a focused laser beam (20 – 40 mW) to make a permanent mark on a thin film on a disk. The information is then read out as a change in the optical properties of the disk, e.g., reflectivity or absorbance. These changes can take various forms: “hole burning” is the removal of material (typically a thin film of tellurium) by evaporation, melting or spalling — sometime s referred to as laser ablation; bubble or pit formation involves deformation of the surface, usually of a polymer overcoat on a metal reflector.

Integrated optical recording head concepts show promise for future light-weight and compact optical heads. Several fundamental issues remain to be explored before fully integrated optical heads. become a reality. This article reviews the basic operating principles of integrated optic heads, focusing on the materials challenges inherent in developing both partially and fully integrated optical heads.

Currently available optical mass storage systems provide a feature unavailable in magnetic direct access storage devices (DASD) — removable and interchangeable media. At present, however, this feature is bought at a price, with magnetic storage devices currently outperforming their optical counterparts in access speed and volumetric storage density. The access time of optical drives is greater than 40 ms, compared to the 10–20 ms typical of magnetic DASD. This is, in large part, a consequence of the 50–100 g moving mass of the optical head. The optical head also limits the maximum volumetric storage density, with current heads ranging from about 0.5 cm to more than 2 cm in height.

Magneto-optical (MO) recording combines the advantages of optical recording and magnetic recording. It offers very high storage densities, it is a noncontact technique, and it allows an unlimited number of read/write cycles. Although the potential of magneto-optical recording was recognized nearly 25 years ago, suitable materials did not exist at that time. Since then, substantial efforts have been made optimizing existing materials and searching for new ones. In 1973 an important development was started by Chaudhari et al., who discovered amorphous GdCo with perpendicular magnetic anisotropy as a possible MO recording material. Today amorphous ternary rare-earth (RE) transition-metal (TM) alloys like GdTbFe and TbFeCo are the recording layers in the MO disks which appear on the market. Although these materials show good recording performance, they also exhibit some drawbacks, mainly caused by their susceptibility to oxidarion and corrosion. However, two new classes of suitable MO materials with good oxidation resistance are emerging.

Phase-change erasable optical recording uses a focused laser beam as a heat source to reversibly switch a micron-sized area in a thin film between the amorphous and crystalline states. A bit of information is stored as an amorphous spot in a crystalline background, and the state of the bit is determined by the differing optical properties of the amorphous and crystalline phases. This concept was first demonstrated in 1971 and then, after about a decade of exploratory work, the field accelerated throughout the 1980s at several research laboratories. Currently the subject of number of reviews, the field of phase-change materials promises to broaden and intensify in the 1990s.

The active layer, where the storage occurs, is typically a tellurium-based alloy with a variety of solute species. Early work studied the recording properties of single-layered films, but it has been clearly shown that multilayered films, where the active layer is sandwiched between two or more dielectric layers, have superior recording properties and resistance to irreversible damage caused by laser heating. The dielectric layers (typically SiO2, Si3N4, or ZnS) provide barriers to active-layer oxidation and contamination, help prevent the hole formation associated with ablative write-once storage methods, and act as crucibles and heat sinks which contain the molten spot and influence its cooling properties, respectively. A typical multilayer structure is shown in the cross-sectional transmission electron micrograph of Figure 1.

Depending upon the nature of the application, three materials currently dominate the optical substrate market today. Polycarbonate (PC), the most commonly used material, is typically used in disks with diameters up to 120 mm. For larger diameters, the inherent birefringence exhibited by PC interacts with the read laser and results in errors in the read-back signal. For 200 mm and greater, both polymethylmethacrylate (PMMA) and glass are used. Because PMMA is low in inherent birefringence, it is widely used for 200 mm video disks, an application which takes advantage of the manufacturing economies of the injection molding process. For larger diameter data storage disks, however, glass is used because it does not have the disadvantage of relatively high moisture absorption or the low heat resistance of PMMA.

This article will deal with the state-of-the-art in materials used for polymeric substrates. It will focus primarily on polycarbonate and the most significant substrate property currently confronting the field, namely birefringence. Other constraints placed by optical storage technology on the substrate material will be discussed along with new materials and research activities aimed at circumventing some of the current limitations.

The commercial life of optical disks started in the late 1970s with prerecorded, read-only video disks. The video information was stored on the disks in the form of pits with varying lengths aligned in a spiral track, a structure which lends itself to easy mass replication. The pits were recorded in a “master” disk by exposing a positive photoresist to a focused beam of blue or UV light from a gas laser and subsequently developing the resist. Now, some 15 years later, mass-replicated, read-only video and audio disks still form the most important application of optical disk technology. Today a large majority of the corresponding masters are recorded using virtually the same technique.

In view of the relative importance of prerecorded disks, it is amazing that so little seems to have changed in the mastering processes. Part of the explanation lies in the fact that the dimensions of the relief structures on the early optical disks were defined by what a player could read using relatively cheap optics, rather than by what could be made in the mastering process. If in the beginning the analog video disk system (Video Long Play or LaserVision) was rather demanding, the digital audio disk (compact disk) (see Figure 1a), introduced later, was a much easier system from a mastering point of view. In that period a trend toward cheaper and faster mastering techniques developed, resulting in a few proposals for direct-effect and direct metal mastering. However, in the meantime very high-quality mastering became necessary for the CD-video system and for preformatted data disks. In the CD-video system, the combination of digital audio and analog video leads in some cases to pits as short as 300 nm (see Figure 1b). On preformatted data disks, wide shallow grooves may have to be combined with narrow deep header pits (Figure 1c).