Phase-change recording element for write once applications

Details Phase-change recording element for write once applications Phase-change recording element for write once applications

2001-11-13

2003-08-12

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S064500, C430S270130

Reexamination Certificate

active

06605330

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a phase-change optical recording element that is particularly suitable for write-once read-many times (WORM) applications.
BACKGROUND OF THE INVENTION
Optical recording has been increasingly used in recent years to publish, distribute, store, and retrieve digital information. This is done by focusing a laser beam to write and/or read information on an optical recording element usually in the form of a spinning disk. In the read-only memory (ROM) format, the information is prefabricated at the factory in the form of encoded small features on the element and the laser beam is used to read back the information. In the writeable formats, the laser beam is used to create small encoded marks through a variety of physical recording mechanisms. This permits the users to record their own data on the disk. Some recording physical mechanisms are reversible. The recorded marks can be erased and remade repeatedly. Disks that utilize these mechanisms are called erasable or rewriteable disks. Some of these physical mechanisms are one way, once the marks are made they cannot be reversed or altered without leaving a clearly identifiable trace that can be detected. Disks that utilize these mechanisms are called WORM (Write-Once-Read-Many times) disks. Each of these formats is suitable for certain practical applications.
The popularity of compact disk recordable (CD-R), which is a WORM disk, in recent years suggests the strong demand for WORM disks. WORM disks are suitable for many applications. In some of these applications, the data need to be stored in such a form that any modification to the content is not possible without leaving an easily detectable trace. For example, attempts to record over a previously recorded area may result in an increase in the read-back data jitter. An increase in data jitter of 50% is easily detectable and can be used to identify a recording element that has been modified. Recording elements that possess features that allow detection of modification attempts are hereto referred to as true-WORMs. In some other applications such publishing and data distribution, rewriteability is not necessary and the lower cost of WORM recording element makes them desirable. Yet, in some other applications some performance advantages of WORM recording elements, such as a higher writing speed, becoming the determining feature in choosing WORM over rewriteable elements.
Many physical mechanisms have been used for WORM recording. The first practical WORM optical recording element utilized ablative recording where the pulsed laser beam is used to create physical pits in the recording layer. This mechanism requires the recording elements to be in an air-sandwiched structure to leave the surface of the recording layer free from any physical obstruction during the pit formation process. This requirement not only increases the cost-but also introduces many undesirable properties that severely limit the usefulness of the recording element. Another mechanism is to use the laser beam to cause the fusing or chemical interaction of several layers into a different layer. This mechanism suffers from the requirement of relatively high laser power.
Yet another approach is to use organic dye as the recording layer. Although used successfully in CD-R disks, this mechanism suffers from its strong wavelength dependence. The optical head used in the DVD devices operating at 650 nm, for example, is not able to read the CD-R disks designed to work at the CD wavelength of 780 nm. Furthermore, a dye-based recording element tends to require more laser power for recording, and may have difficulties supporting recording at high speeds.
A more desirable approach is based on amorphous-crystalline phase-change mechanism. Phase-change material is the basis for the rewriteable DVD disks that have been introduced as DVD-RAM and DVD-RW products in the market. By properly selecting a different composition, the phase-change materials can be made WORM as well. A phase-change based DVD-WORM disk will have the best similarity in characteristics with the rewriteable DVD disks, and it can share the same manufacturing equipment with the re-writable disks. Both of these are highly desirable. Since the WORM feature requires disks that cannot be re-written, the phase-change materials for WORM needs to be different from those conventionally used for rewriteable disks. Commonly-assigned U.S. Pat. Nos. 4,904,577; 4,798,785; 4,812,386; 4,865,955; 4,960,680; 4,774,170; 4,795,695; 5,077,181 and 5,271,978, teach various alloys that can be used for write-once phase-change recording. When these alloys are used to construct a WORM optical recording element, the recording laser beam is used to change the atomic structure of the recording phase-change material from amorphous state to crystalline state. The unique feature that distinguishes these alloys from the conventional rewriteable phase-change materials is that the crystallization rate is so high at elevated temperatures just below the melting point, it is practically impossible to reverse the materials back into the amorphous phase once it is crystallized. Optical elements based on these alloys therefore possess true-WORM properties. Once the data are recorded on these elements, they cannot be altered without leaving a detectable trace. Optical recording elements based on these alloys, especially the ones using Sb
100-m-n
X
m
Sn
n
based alloys, wherein, X is an element is selected from In, Ge, Al, Zn, Mn, Cd, Ga, Ti, Si, Te, Nb, Fe, Co, W, Mo, S, Ni, O, Se, Tl, As, P, Au, Pd, Pt, Hf, or V, as taught by commonly-assigned U.S. Pat. Nos. 4,904,577; 4,798,785; 4,812,386; 4,865,955; 4,960,680; 4,774,170; 4,795,695; 5,077,181 and 5,271,978, m and n represent the concentration of X and Sn in the alloy, have further advantages over other WORM optical recording elements. They are stable, having high recording sensitivity, and can be used in a simple, single-layer construction that drastically reduces manufacturing costs. However, recording elements based on these alloys also posses some shortcomings. One of the main shortcomings is the recent discovery that the recording performance of these elements deteriorates as the recording density is increased.
With the transition into the digital age, more and more digital data are generated everyday, and the need to store these ever increasing amounts of data keeps on increasing. There is therefore a strong need to keep increasing the density of the storage devices. In optical recording elements, this increase in density is achieved mainly through a decrease in the feature size used for storing information. To accomplish this decrease in feature size, the laser wavelength is being decreased and the numerical aperture of the focusing lenses is being increased to reduce the size of the read/write laser spots. However, the capability of the storage medium to support the small feature size is not guaranteed. In the ablative type media, frequently there is a rim around the ablative marks that physically prevents small features from being made. In the Sb
100-m-n
X
m
Sn
n
phase-change alloys taught above, the noise increases when the recorded crystalline marks become smaller. The mechanism for this noise increase is not well understood. Transmission electron micrographs show the recorded marks in these alloys to generally consist of only a few crystalline grains, suggesting a low nucleation-site density in these alloy films. The low nucleation density has not presented a problem for lower density recording. When the recording density increases, however, the marks become smaller and the probability of proper nucleation during the irradiation time of the writing laser becomes smaller. Consequently, the recorded marks may become less uniform and the read back jitter increases. Adding oxygen (commonly-assigned U.S. Pat. No. 5,271,978), water, nitrogen, or methane (commonly-assigned U.S. Pat. Nos. 5,312,664 and 5,234,803) to
these
alloys improves the situation somewhat, but the small mark recording is

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