The Chemistry of Optical Discs

Chemistry for Everyone edited by

Products of Chemistry

George B. Kauffman California State University Fresno, CA 93740

The Chemistry of Optical Discs David Birkett Loctite RD&E, Tallaght Business Park, Dublin 24, Ireland; [email protected]

Optical data storage is rapidly becoming ubiquitous. Compact discs (CDs), laser discs and minidiscs, with capacities of hundreds of megabytes, have been with us for some time. In recent years Digital Versatile Discs (DVDs) with capacities up to 18 gigabytes have started to supplant prerecorded VHS tapes in the video stores. Recordable DVDs are beginning to appear for home video recording, digital video cameras, and computer applications. Suppliers are already proposing “next generation” DVDs and even new multi-layer holographic or fluorescent discs capable of storing terabytes of information. Clearly, this information revolution has been driven chiefly by physicists and engineers, but without parallel chemical developments, none of this would have been possible. As far as the chemist is concerned optical data storage, as currently practiced commercially, breaks down into four technologies: CDs and DVDs in prerecorded, write-once, and erasable formats; and magneto-optical (MO) discs and the related minidiscs. I will deal with each of these in turn. Prerecorded CDs and DVDs Figure 1 shows how prerecorded CDs and DVDs relate to one another. The data are encoded as a series of pits that are physically stamped into a polycarbonate substrate. A reflective metallic layer is sputtered 1 over the substrate, so that the data can be read by a small laser: the wavelength of which is a function of the size of the pits. The incident beam and the reflected beam interfere, and it is the interference that the reader picks up. CDs have a monolithic structure, with the information on the “outside”. The reflective layer is protected from oxidation by a thin lacquer layer. DVDs, in con-

Figure 1. Prerecorded CDs and DVDs.

trast, have a sandwich construction, composed of two half discs, each half the thickness of a CD. The layers of information are on the “inside” and the two halves are held together by an adhesive layer. As Figure 2 shows, there are various different subformats that vary in the number of layers of information (one to four). There is interesting chemistry involved in every stage of the process: the initial creation of the mold, the molding of the polycarbonate, the deposition of the reflective layers, the lacquering of CDs, and the bonding of DVDs.

Mastering Before a CD or DVD can be mass-produced it is necessary to make an extremely accurate mold. This is known as mastering. The first step is to coat a clean, polished glass disc with a photoresist compound. Photoresists are photosensitive polymers in which areas exposed to light change chemically to become more soluble (positive photoresists) or less soluble (negative photoresists) in a developing solution. In the case of optical discs the digital information that encodes the music or video switches a laser on and off. Those parts of the photoresist that have been exposed to the laser light become more soluble in an alkaline developing solution. There are various ways to achieve this, but one common method is to incorporate diazoketone groups into an acrylic or styrenic polymer. On exposure to light these groups decompose and rearrange to ketenes, which in aqueous alkali hydrolyze to carboxylate groups (see Fig. 3). The etched photoresist is baked to eliminate any moisture or solvents, and then has a very thin, electrically conductive coating (nickel) applied by sputtering. This can then have a much thicker nickel coating laid down by electroplat-

single sided, single layer

DVD 5 4.7 Gigabyte

double sided, single layer

DVD 10 9.4 Gigabyte

single sided, dual layer

DVD 9 8.5 Gigabyte

double sided, dual layer

DVD 18 17 Gigabyte

Figure 2. DVD constructions.

JChemEd.chem.wisc.edu • Vol. 79 No. 9 September 2002 • Journal of Chemical Education

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Chemistry for Everyone

CH CH2 C CH2 O

CH CH2

C CH 2 O

OMe

O

n

n HN O

SO2

SO2

N2

N2 O

O

m

m

NaOH(aq)

hν N2

+ N2

O

C

O

CO2 N a

O

Figure 3. Positive photoresists.

NaO

ONa

C O Cl 2

O

O

Cl

O

O

+

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+

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y

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O

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x = 1 - 1 2 , y ,z = 0 - 1 2

NR3

O + C l R3N

H O

O n

Figure 4. Polycarbonate manufacture.

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Chemistry for Everyone reading wavelength

% Transmission (3mm sample)

bonding wavelength 100

80

60

standard polycarbonate optical grade polycarbonate

40

polymethylmethacrylate 20

0 350

400

450

500

550

600

650

Wavelength / nm Figure 5. UV-visible transmission of polycarbonate vs polymethylmethacrylate polymers.

ing.2 This nickel disc (the “father”), when removed from the glass plate, is essentially the negative template, in terms of the pit structure, of the polycarbonate CD or DVD. It is not used directly to stamp the final discs: it would not last long enough. Rather, the technicians make a metal “mother” disc from this, which in turn is used to make the final stamper.

The Bulk Plastic To date, all CDs and DVDs are molded in polycarbonate. Polycarbonate has an excellent combination of properties in comparison with other thermoplastics: it can readily be made optically clear; it has a sufficiently high (ca. 160 o C) glass transition temperature (Tg , an indication of when the polymer softens) for discs to be able to withstand climatic aging tests at, for instance, 80 oC, 95% relative humidity; and has suitable melt-flow properties (the ease with which the polymer melt will flow through narrow gaps) to allow molding at a reasonable temperature.

The grade of polycarbonate is important. Particularly good melt-flow properties are clearly vital to successful molding of such detailed pit structure. However, its optical properties are likewise critical; high transmission (of the order of 90% for a 3 mm layer) at the reading wavelengths of 650 and 780 nm is obviously essential. The resin manufacturers achieve this through washing out the salt, mostly sodium chloride impurities produced during manufacture, and reducing the moisture content. The chemistry of polycarbonate manufacture is shown in Figure 4. It is an interfacial process: the sodium salt of bisphenol A in aqueous alkali forming one phase and phosgene in an inert organic solvent the other. The resulting oligomers are then polymerized in the presence of tertiary amine catalysts. Most DVDs are bonded, as we shall see later, using ultraviolet curing adhesives; therefore, as high a transmission as possible at the bonding wavelength (typically 365 nm) is desirable. This is a little more problematic as polycarbonate, like all plastics, begins to absorb more strongly as the UV wavelength shortens. Special “optical” grades have this “shoulder” pushed down by a few nanometers (see Fig. 5) by trade secret modifications, which possibly comprise incorporating aliphatic comonomers. Linking the melt-flow, optical properties and stresses during the molding process can force the polymer molecules into extended conformations, which in turn introduces birefringence (different refractive indices in different directions). The producers minimize this by introducing bulky side groups in comonomers with bisphenol A (see Fig. 6 for one possibility suggested in the trade literature). Finally, residual moisture and chlorine in the resin can promote the corrosion of the reflective layer of a disc, so that both need to be tightly controlled. The industry is already attempting to put even more information into a layer of pits. The still smaller pit size puts increased demands on the melt-flow properties of the bulk resin. Furthermore, a shorter wavelength (blue-violet, 405 nm) laser is required to read the data, but as we can see from Figure 5, the transmission of polycarbonate is already begin-

O

O

Figure 6. Use of bulky side groups to reduce birefringence.

O

conventional polycarbonate

polyethylene

O O

O

low birefringence modification

PCHE


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OH

hν

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+ X

OH

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typical photoinitiator O R + n

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(CHMeCH2O) n

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a "urethane acrylate" resin

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.

+ O

2

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.

O . ROO + n

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p o ly m e r

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isobornyl acrylate

oxygen inhibition

Figure 7. UV acrylic chemistry.

ning to fall at such wavelengths. Resin suppliers are proposing new resins for this application. Poly(methylmethacrylate) is one option, having melt-flow, UV transmission, and birefringence properties superior to polycarbonate, but unfortunately a lower Tg, which can shorten the durability of the disc. A much newer candidate material is polycyclohexylethylene (PCHE) from the Dow Chemical Company, shown in Figure 6. This polymer exhibits much less birefringence than polyethylene because of the bulky side groups.

Reflective and Semi-Reflective Layers Once the disc has been properly molded, the next stage is to apply, usually by sputtering, a metallic reflective layer. For CDs this layer has traditionally been aluminium, so the sputtering conditions and optimum layer thickness for this material are well established. The simplest DVD formats (DVD-5 and DVD-10) also use aluminium. To ensure a long disc lifetime, there are some severe durability tests, such as performance integrity when exposed to 70–85 oC temperatures, 50–95% relative humidity, for 96–720 hours. If unprotected in such an environment, or if in contact with an aggressive adhesive layer, the thin aluminium layer corrodes 1084

rapidly. Lacquers and adhesives have been available for some time that provide adequate protection for these relatively simple formats. For DVD-9’s and the related DVD-18’s the situation is more complicated. With these formats the laser reads two layers of information from the same side. The first of these layers, “layer 0”, can not be coated with a fully reflective layer; rather, it needs a semireflective coating, so that the laser can focus on either layer 0 or the fully reflective coating on “layer 1”. It has proved difficult to obtain the desired balance between transmission, reflection, and absorbance with aluminium, but there are a number of alternative materials. Gold is easy to sputter, and can give excellent optical properties, but of course it is relatively expensive. Furthermore, gold is much less electropositive than aluminium, so that an electrochemical potential in the region of 3 volts is set up across the adhesive layer. Under the warm humid conditions of the durability test the aluminium can act as a sacrificial anode and corrode away, unless the adhesive is carefully formulated to prevent this. A second, less expensive, option for the semireflective layer is silicon. Use of this element too has implications for



R

R

N N n

R'

X

R'

cyanines N

R

N

N

Pd N

N R

R N

N

R

N

phthalocyanines

Figure 8. Dye layers for CD-R or DVD-R.

the adhesive layer, in addition to the galvanic effect. Silicon is much less ductile than aluminium or gold, so that stresses caused by differential thermal expansion of the substrates, or by shrinkage in the adhesive, are transmitted through to the polycarbonate, and could cause stress cracking during the durability test. Finally, silicon is a “dirty” material to sputter, requiring frequent down times to clean the sputterer. The industry is therefore also investigating special alloys of silver (with palladium and platinum). These are less sensitive to corrosion than pure silver (which tarnishes rapidly under the durability test conditions, irrespective of any anodic attack on the aluminium). The alloys are less expensive than gold (although still much more expensive than silicon or even pure silver) and are much cleaner to sputter than silicon.

Adhesive Chemistry The reflective layer of CDs has traditionally been protected by acrylic lacquer that is cured using ultraviolet light. This was therefore the chemistry that was first turned to for bonding DVDs, and which is currently used to bond the vast majority of prerecorded DVDs. Such formulations typically comprise a “urethane acrylate” or “epoxy acrylate” resin, diluted with an acrylic monomer such as isobornyl acrylate, and a photoinitiator, generally of the hydroxyalkylphenylketone type (see Fig. 7). The adhesive is dispensed in a ring around the center of a disc, and then spreads through capillarity, wetting, the weight of the upper disc half, and applied centripetal forces. It is important, particularly for DVD-9’s, that the adhesive layer thickness is very even (in the region of 50 µm), requiring the use of sophisticated spinning profiles. The physics of this coating process is surprisingly complicated: the rheology of the adhesive and its temperature dependence must be carefully controlled, as must the humidity and static charge in the bonding station, for these can influence the contact angle that the adhesive makes with the polycarbonate surface. Once in place the adhesive must cure rapidly, within a few seconds. However, the reflective layers in DVD-10’s, and even semireflective layers in DVD-9’s, are highly absorbing in the ultraviolet region, so that very high powered lamps are needed as are highly reactive adhesives. On the other hand, this approach has its limitations, since both the infrared energy also emitted by the UV lamp, plus the exotherm of the curing reaction, heat the disc. Too much heat will distort the disc, rendering it unplayable. At the edge of the disc there is also a risk of oxygen inhibition slowing down the cure, and giving a tacky feel to the disc. To eliminate this the lamp should be designed to give as high an intensity as possible at the edges, while the adhesive must be formulated to be relatively insensitive to oxygen inhibition. It is beneficial if the degree of shrinkage on cure is not too high, to avoid introducing stresses into the disc, while the cured adhesive should be sufficiently flexible to absorb

Write Once reflective layer dye layer

write (250 °C)

polycarbonate

Figure 9. Recordable discs.

Erasable reflective layer dielectric layer phase change layer (amorphous) dielectric layer phase change layer (crystalline) polycarbonate

write (600 °C then quench)

erase (200 °C)


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+

A

O

+ n R

+

A(OCHRCHR')n R' O O

O

O

A cycloaliphatic epoxy resin. Typically this would be flexibilised by prereacting 2 moles of the above with one mole of a polycaprolactone diol.

+

−

Ar3S SbF6

+ HY( any abstractable hydrogen) from resin, fillers, etc

UV

−

Ar2S + ArY + H+SbF6 (H+ is the active cation)

Figure 10. UV cationic adhesive chemistry.

such stresses. The percent conversion of the resins and reactive diluents must be high for two reasons: unreacted monomers can cloud the reflective layer and any residual unsaturation can lead to problems if the disc is later printed with ultraviolet curing inks (which is normally the case). If the decorative pattern is uneven in absorbance then varying intensities of UV light will penetrate through to the adhesive. If the adhesive is not fully cured then this in turn can lead to further, uneven shrinkage in the adhesive layer and distortion of the disc. The properties of the cured adhesive are clearly important. The role it plays in preventing corrosion of the reflective layers has been discussed above, but there are numerous other performance parameters. The obvious function of any adhesive is to hold two substrates together with sufficient strength. For DVDs the key manifestation of this requirement is that the adhesive strength must at least be of an order with the adhesion of the sputter layer to the polycarbonate. This is to prevent piracy: if a disc could be cleanly split, then it would, in principle, be possible to remaster a perfect digital copy. The ideal case is if the strength of the bond is comparable to the cohesive strength of the polycarbonate, for then it is impossible to split the disc without destroying the information content. For DVD-9’s, where one layer of information is read through the cured adhesive, the optical properties of this layer become significant. Obviously it must have high transparency at the reading wavelength of 650 nm, but it must also have a refractive index as close as possible to that of polycarbonate at the same wavelength to minimize scattering at the interface. Write-Once Discs For recordable discs, whether write-once or erasable, there are different problems for the chemist. The base plastic is the same as for prerecorded discs, and there is no mastering stage necessary, but there is the recording layer to consider, and the reflective layer and the lacquer or adhesive have distinct requirements. For both types of recordable discs the polycarbonate has a concentric or spiral ridge-groove structure molded into it (instead of the pits in a prerecorded disc) 1086

with a reflective layer sputtered over this, and then for writeonce discs (CD-Rs or DVD-Rs) the recording layer is applied. The recording layer is a dye, either a cyanine or a phthalocyanine (see Fig. 8) applied from solution by spin-coating followed by carefully controlled drying. Cyanines are easier to apply and dry; phthalocyanines are more stable. To write to such a disc a laser heats a spot up to 250 oC, decomposing the dye and partially exposing the reflective layer (see Fig. 9). This pattern of reflective spots is the information content of the disc. The groove-ridge structure aids in separating the spots and preventing overlap. Because the reflective layer may not be fully exposed, a higher reflectivity than aluminium provides is necessary. Furthermore, the dye could chemically attack aluminium, so that gold or silver are the materials of choice for this application. CD-Rs can be lacquered in the normal way, while singlesided DVD-Rs (e.g., DVD-5Rs) can use a UV acrylic adhesive. However, for double-sided recordable discs (e.g., DVD10Rs) insufficient UV radiation could penetrate the recording layer to cure the adhesive without damaging the recording layer itself. A different adhesive chemistry is necessary. One option is to use hot-melt adhesives: hydrocarbon resins, applied in the molten state by roller-coating and then allowed to cool and solidify. These have the advantage of low cost, important given that such discs will eventually become commodity items. However, it has proved difficult to pass the more severe durability tests with this chemistry while keeping the melt viscosity low enough for easy dispensing, and such products are only now finding acceptance. Another approach is to use “UV cationic” (also known as UV epoxy) adhesives (see Fig. 10). These make use of the fact that the oxirane ring, particularly if further strained by fusing it onto a cyclohexane ring, can undergo a cationically catalyzed ring opening polymerization. The cation (in fact the proton of an extremely strong acid such as HSbF6) comes from the photochemical decomposition of a triaryl sulphonium or diaryl iodonium species. The key feature of this type of polymerization, in contrast to free-radical induced polymerization of vinylic monomers, is that the chain growth can continue after the light source has been turned off. Therefore one can screen print such adhesives onto the disc halves, irradiate them to start the reaction, then assemble the disc and allow the reaction to proceed off-line. Once again, however, there are drawbacks. Because of the strong acids generated, the reflective layer (even silver) must be protected from the adhesive by a thin acrylic lacquer. The adhesives, themselves, are expensive, and the adhesive/lacquer combination makes a large contribution to the overall cost of a disc. Disc manufacturers have also experimented with pressure sensitive acrylic tapes, but to date there is no clear favorite technology for bonding such formats. Erasable Discs There is a family of rewriteable formats (CD-RW, DVD+RW, DVD–RW, or DVD–RAM) the members of which differ in how the recorded data are addressed. The different formats will be incompatible, but are identical as far as the chemist is concerned. In all these discs the recording layer is a low-melting alloy of various B-metals (for example,



Notes

N S

laser Figure 11. Magneto-optic discs and minidiscs.

1. Sputtering is a vapor deposition method. A target of a metal or other material is bombarded with positive ions of a rare gas, generated by a glow discharge. The target would typically be negatively biased by hundreds or thousands of volts. 2. In this process the part to be plated forms the cathode of a galvanic cell, while nickel metal is the anode. The electrolyte is an aqueous solution of nickel sulphate, nickel chloride, and boric acid.

Bibliography tellurium, antimony and germanium), known as a “phase change material”. These alloys exist in a reflective, crystalline state and a non-reflective amorphous state. The alloy arrives on the disc from the sputterer in the glassy state, so that the whole disc has to be annealed by tracking over it with a laser. As Figure 9 shows, writing then takes the form of heating a spot quickly to 600 oC with the laser, and then quenching, so that the spot returns to the amorphous state. Erasing is simply a matter of annealing the spot briefly at 200 oC again. Because during writing the spot reaches high temperatures the polycarbonate requires protection, in the form of dielectric layers, for example zinc sulfide. Magneto-Optical Discs and Minidiscs One other category of discs exists that is sometimes classified with the optical discs. These discs use a laser to heat a spot of a magnetic material above its Curie temperature in the presence of a magnetic field. Cooling it down freezes in the magnetic orientation (see Fig. 11). The data are then read magnetically. For conventional magneto-optical discs the laser switches on and off while the magnetic field remains constant. With minidiscs the laser remains on all the time, while the magnetic field switches orientation. Of interest to the chemist is the recording layer, which needs just the right Curie temperature, plus a rapid response to a varying magnetic field. Terbium-cobalt ferrites are the current materials of choice.

General Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Eckroth, D., Eds.; Wiley & Sons: New York, 1978–1984. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F.; Bikales, N.; Overberger, C. G.; Menges, G.; Kroschwitz, J. I., Eds.; Wiley & Sons: New York, 1985–1990. Birkett, D. P. Chem. Ind. 2000, 5, 178–181. Hatcher, M. Education in Chemistry, 1998, 6, supplement between pp 154–155. Chilton, J. A.; Goosey, M. T. Special Polymers for Electronics and Optoelectronics; Chapman & Hall: London, 1995.

Photopolymers University of Rochester Department of Chemistry; http:// www.chem.rochester.edu/~chem421/polymod2.htm (accessed June 2002)

Sputtering Unaxis Materials; http://www.materials.unaxis.com/en/opticdata.htm (accessed June 2002)

Polycarbonates and other Base Plastics Legrand, D. G., Bendler, J. T., Eds. Handbook of Polycarbonate Science and Technology: Marcel Dekker: New York, 1999. Bayer Plastics; http://www.makrolon.com/produkt.htm (accessed June 2002) The Dow Chemical Company; http://www.dow.com/plastics/news/ optical_media.htm (accessed June 2002)

UV Acrylic Adhesives

The Future The pace of change in optical disc technology is astonishing. The DVD is barely established and already prototypes of even higher density discs (20 gigabyte per layer of information) are being demonstrated. These use still smaller pits than current DVDs, and are read with a correspondingly shorter wavelength (blue) laser. New plastics, such as PCHE or special grades of polymethylmethacrylate will be necessary. Even more advanced multilayer fluorescent or holographic discs are being developed, which could in principle carry terabytes of information. These discs use completely different recording layers and will require putting down layers of adhesive with thicknesses of the order of 20 nm rather than 50 µm. This article is therefore something of a snapshot; the information technologists will be keeping the chemists busy for some time to come.

Dainippon Ink and Chemicals, Inc.; http://www.dic.co.jp/eng/products/envfprod/0701.html (accessed June 2002)

Dye Layers for Write-Once Discs Optical Storage Technology Association; http://www.osta.org/technology/cdqa6.htm (accessed June 2002)

Rewriteable Discs Sony Corporation; http://www.sony.co.jp/en/Products/DataMedia/features/phase_change/phase_change2.html (accessed June 2002) Tech TV, Inc.; http://www.techtv.com/screensavers/showtell/jump/ 0,24331,2275803,00.html (accessed June 2002)

MO Discs and Minidiscs Sony Electronics Inc.; http://www.sel.sony.com/SEL/rmeg/mediatech/ techspec/techMD.html (accessed June 2002)


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