Update: Phase Change WORM.

Optical Write-Once-Read-Many (WORM) recording is a mature technology. As shown in Table 1, Philips first exhibited this technology in 1979 and commercialized it five years later in a joint venture with Control Data Corporation.

When Plasmon purchased Philips LMS in January 1999, Philips had already introduced three generations of WORM drives and was working on the development of the fourth. The first three generations were ablative WORM products, while the fourth is based on phase change technology. This article will examine the differences between ablative and phase change recording and show that phase change is an equally, if not more, permanent recording technology.

WORM Recording

Both ablative and phase change optical recording use the same basic principle of writing data to a disk by locally changing the reflectance of a recording layer. A recording layer may be a single thin film alloy or it may be a stack of several thin film alloys deposited on top of each other. In any recording layer (MO/Phase Change/ablative) the data can be made unreadable by writing additional marks in the spaces between the original marks; however, in a well-designed WORM-drive, accidental/intentional "over-writing" is prevented by either hard-coded firmware or electrical circuits. Altering the original data in a TrueWORM recording layer is absolutely impossible, as the recording process is irreversible.

The Ablation Process

With ablative technology, the recording layer is in a crystalline state. A laser is focused to a spot and heats the material (Tellurium alloy) in the recording layer to above its melting point. The forces from the heat cause the molten material to roll back from a central point, resulting in a hole with a rim (i.e., a small crater). The rim-material first has an amorphous (darker) state, but returns within a few days to its original crystalline (brighter) state. Re-heating the area around the hole never results in back-filling the hole. The end result is a dark hole or pit with a much brighter rim and surrounding surface area. This contrast is easily detected by the laser on its read-pass and is the basis for recording and reading digital data.

If the material is heated to just under its melting point, then an unstable amorphous mark, with reduced reflectance, is created. Within a few days, this amorphous mark returns to its crystalline state with its original reflectance; hence, drives using ablative media must verify proper hole formation during the verify-pass after the write-pass. Checking the amplitude of the recorded data can do this, as holes are about four times darker than the unstable amorphous marks. Proper execution of this amplitude-checking/verify process is, of course, critical for the data integrity on ablative media.

The Phase Change Process

The unrecorded phase change layer of a WORM disk is in the amorphous state. During writing, the layer is locally heated to a temperature at which it changes to a crystalline state, thus creating permanent crystalline data marks with a different reflectance than the surrounding amorphous area.

The phase change takes place by a two step process: a nucleation step followed by a growth step. Each process occurs at a characteristic temperature and time-scale and is, therefore, very repeatable. On a sub-microsecond scale, the temperatures are in the region of 200 to 400[degrees]C for the PLMS media. The WORM Phase Change layer has no unstable intermediate state, like in the ablaive Tellurium alloy, which, of course, greatly simplifies the Data-Verify pass for drives using the Phase Change technology.

Phase Change -Permanent WORM Or Rewritable

Phase Change alloys similar to those used for WORM media can also be used to produce a rewritable version; however, the unrecorded Phase Change layers of rewritable CD or DVD disks are transformed to the crystalline state during manufacture. Amorphous data-marks are written by heating and very rapidly cooling the layer with the focused laser-beam by using short laser pulses with high write-power. For erasure, the laser power is held at an intermediate level as it passes over the recording film. This changes the amorphous marks back to the crystalline state.

In order to write the media from the crystalline state to the amorphous state, it is necessary to add a quenching layer to the recording film. This causes the film to cool very rapidly from the molten state into the amorphous condition when the laser pulse ends. In all practical disks, a thin insulating layer is formed between the recording layer and the quenching layer. This allows the drive to heat the recording layer to the melting point with an acceptable laser power. Without this laminated multilayer structure, it is not possible to re-write Phase Change media.

By contrast, the Plasmon WORM disk uses an exceptionally stable amorphous film in a single layer. This film has been used for over 12 years as an archival optical storage medium.

Resistance To Corrosion

The Tellurium alloy used in ablative products is particularly susceptible to corrosion and the sensitive layer is, therefore, protected from the atmosphere by a hermetically sealed air sandwich design. As a result, quality and reliability of the disk-seal are the dominant factors affecting the resistance of the ablative disks to corrosion. Most phase change media, on the other hand, show remarkable resistance to corrosion with accelerated life tests, indicating that it will be hundreds of years before degradation of the recorded data is seen.

Consequences For Disk Construction And Altitude Specifications

The ablation process requires a sealed air-sandwich construction. The constant pressure inside such a sealed disk causes the disk-shape to vary between concave, flat and convex when the disk is used over a range of altitudes, resulting in undesirable tilt-variations. Tilt is a major contributor to optical aberrations; hence, a sealed airsandwich has limited altitude specifications. Even within these specifications, the Read/Write margins are reduced due to the small tilt-variations.

With Phase-Change media there are more options for the disk-construction, all with unlimited altitude specifications. Construction could include a non-sealed air-sandwich, a single-sided disk with a protective lacquer, two disk-sides glued back to back, or two disk-sides glued to a spacer (i.e., a laminated disk).

Multi-Layer Recording

Current Plasmon 12-inch media uses an ablative Tellurium layer, which is not suitable for multi-layer recording. Phase Change technology opens the door for multi-layer recording in future products, which can enable much higher data densities simply due to the potential for recording on multiple layers.

It should be clear that a phase change material is not intrinsically rewritable and that a considerable amount of engineering is involved in making a product capable of supporting numerous write/erase cycles. Phase change materials for use on WORM media are engineered, instead, to give stability, durability, and longevity and, in this respect, they actually outperform the classic ablative layers. Channel-encoded data with ECC protection in a WORM Phase Change layer can never be altered. Accidental/intentional over-writing (destruction of data) is typically prevented by mechanisms designed into the drives.

Phase Change WORM recording is a mature technology that has been in use since the mid-1980s by Kodak and Panasonic. The technology has been widely accepted in the U.S. and several other countries for governmental archival applications.

John Drollinger is the director of large format optical products at Plasmon LMS (Colorado Springs, CO).

[e-mail] www.plasmon.com

1979 Philips demonstrated 12-inch recording
     technology at Briar Cliff Lab in New York
1984 Philips/CDC Joint Venture. Established
     headquarters in Colorado Springs
1985 Ship first 12-inch optical drive, capacity
     2.0GB-single head
1990 Philips LMS becomes independent
     standalone division
1991 Philips LMS ships first Dual Head 12-inch
     Optical Drive, capacity 5.6GB
1992 Philips LMS reaches milestone of 10,000
     drives shipped
1995 Philips LMS ships second generation Dual
     Head 12-inch Optical Drive (cap. 12GB)
1998 Philips LMS reaches milestone of 20,000
     drives and media shipments of 500,000
     12-inch optical platters
1999 In January, Plasmon acquires Philips LMS
1999 In July, Plasmon acquires 12-inch
     automation business from Cygnet
1999 Fourth Quarter, Plasmon LMS ships fourth
     generation 12-inch Optical Drive (third
     generation with dual head), capacity 30GB
               Life Expectancy
                                         Potential for Data
               Normal Conditions         Extreme Conditions
Phase Change
Write a ->c
unwritten thin 1mW change in write       1mW change in write
     film      sensitivity after 100     sensitivity after 2000
               years                     hours @ 70[degrees]C 85%
                                         R.H.
  data marks   Stable for [greater than] No change in any of
               100 years                 mark length or BER of
                                         initially recorded
                                         marks, CNR or BER
                                         of new recordings
                                         after 2000 hours @
                                         70[degrees]C 85% R.H. 1dB
                                         decrease of CNR of
                                         recorded marks
Abjative
unwritten thin Stable for [greater than]
     film      10 years after proper     4% reflectance drop
               curing                    after 1800 hours 60
               (if seals remain intact)  [degrees]C 95%R.H
  data holes   Stable for [greater than] increase in hole size
               30 years                  after aging (Z/AD test)
               (if seals remain intact)  0.02[micro]m at I.D.
                                         0.048[micro]m at O.D.
               Falsification
Phase Change
Write a ->c
unwritten thin
     film
  data marka   Marks cannot be
               altered.
               WORM media.
Abjative
unwritten thin
     film
  data holes   Holes cannot be
               altered. WORM media.
               NOTE: The verify-pass
               must include a data-
               amplitude check to
               insure that the reflectance
               change is indeed from a
               real hole rather than from
               an unstable amorphous
               mark.

Lifetime and Archivability Considerations

Generally, there are several different "headings," which need to be addressed when considering the lifetime/archivability of such a WORM product (Table 2). The main ones are:

1. Corrosion resistance

2. Susceptibility of the amorphous phase to spontaneously crystallize

3. The tendency of written marks to grow further

4. The change of "write performance" of the amorphous layer with aging

5. Restructuring of amorphous material

* Corrosion resistance - Samples have been studied and exposed to a Battelle nominal chloride/H2S, N02 environment for 30 days and a self protecting layer of 4nm was observed to grow. The conclusion was that, under normal conditions, the lifetime of the product due to corrosion is at least 30 years for a fully exposed surface. In fact, the venting of the media is very slight so that very little ingress of atmospheric pollutants is likely to access the disk surfaces; hence, one can expect a much longer life expectancy associated with this effect.

* The film used in the Plasmon disk has been shown to be exceedingly stable under all circumstances and the material is unlikely to transform in a time period less than 1000 years under normal storage conditions (Note: the actual estimate made from existing data is 1030 years).

* Various measures have been made of this effect. For instance, it has been observed that after 6 hours at 140[degrees]C, marks are seen to grow slightly but also get "brighter" so that written data is improved with this aging (see "e" below). It is only after an estimated 30 hours at this temperature that any loss of data quality has been observed. (The comparable figures at 120[degrees]C are 30 hours and 150 hours, respectively.) From these estimates, the life of the product at a temperature 30[degrees]C is in excess of 100 years.

* It has been observed that write characteristics improve with time at elevated temperature; thus, after heating at 120 degrees for 10 hours, the signal ampli-tudes are increased by at least 15% and, after a further 30 hours, there is a further increase of 15% in signal amplitude. The write margin for data written on the disk where the write margin is least is increased by as much as 60% due to this improvement of signal amplitude.

* The optical properties of these amorphous layers have been observed to be very stable with the reflectivity changing less than 0.2% over a period of 15 hours heating at 140[degrees]C.