Future Trends in Optical Recording

Philips Tech. Rev. 44, No. 2, 51-57, April1988 51

Future trends in optical recording

G. E. Thomas

The text of the article below is largely taken from the invited paper that the author presented at the International Symposium on Optical Memory 1987, held in Tokyo from the 16th to the 18th of September 1987. In this article he describes the most important trends that can be rec- ognized today in the field of optical recording. Besides the well-known audio Compact Disc ('CD'), the Laser Vision disc and the Digital Optical Recording ('DOR') disc for data storage, there will be new applications in the future, especially if erasable optical recording becomes a practical reality.

Introduetion It is undoubtedly only a coincidence that the symposium for which this material was originally put together coincided with the fifteenth anniversary of the first public demonstration of optical recording, in September 1972 at Philips Research Laboratories in Eindhoven. To resurrect a cliché, it will be clear to all who have been involved in optical recording [1] that the field has been characterized during its brief history by turbulent and, at times, explosive growth. Indus- trially, the field of optical recording is a curious mix- ture of extremely. successful consumer applications, motor arm optical disc such as the video disc and - in particular - the Com- pact Disc, and professional data-storage applications in which the major breakthrough in the market is Fig. I. An optical recording system consists of three main compo- avidly predicted but still awaited. The word 'curious' nents: the optical disc, the arm and the motor. The information on has been deliberately chosen here, since the two out- the optical disc is contained in microscopically small optical details arranged on the disc in a spiral path, rather as in a conventional standing advantages of optical disc recording - the audio disc. These details are 'scanned' by a beam of light, from a high storage capacity per unit area and the random- semiconductor laser, which is reflected from the surface of the disc on to a detector. The arm contains all the facilities necessary for the access capabilities - which could be expected to en- light beam to follow the information track in the radial direction sure its breakthrough in the professional area, have and for focusing the beam on to exactly the right position. The motor must always rotate the disc at the correct speed; in most optical not yet led to that goal. On the other hand, these ad- recording systems this speed depends on the radial position of the vantages have been quite successfully exploited in the arm. The unit containing the optical components is usually referred to as the 'light pen' of the system. products for consumer entertainment. Against this background, it is clear that it is very difficult to pre- [I] G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Ros- malen and K. Schouhamer lmmink, Principles of Optical Disc Dr G. E. Thomas is with Philips Research Laboratories, Eindhoven. Systems, Adam Hilger, Bristol 1985. 52 G.E. THOMAS Philips Tech. Rev. 44, No. 2

and digital (CD) read-only systems [1]- [3]. The fol- lowing major surge in activity dealt with the development of media that were suitable for recording [4]- [6] and the associated investigation of higher-power laser technology and modulation methods adapted to these media. The current widespread efforts in the technology of erasable recording (based on both magneto- optics and amorphous/crystalline phase-change strategies) represent the third major development phase in optical recording [7] [8]. lt is safe to predict that future developments will mainly centre on improvement of the existing systems, both in terms of a significantly higher storage density and of higher data rates. An improvement in the total performance assumes an improvement in the individual optical recording subsystems, from the laser to the disc itself; see figs 1 and 2.

Coding Fig.3 summarizes the current state of affairs with respect to storage density on optical media. In this figure, the bit density (in Mbit/cm'') for a number of current systems is plotted as a function of the spatial Fig. 2. Schematic drawing showing the most important components optical cut-off frequency (Je = 2NA/À.) for the optics of the 'light pen' in an optical recording system. 1 semiconductor laser. 2 half-silvered mirror. 3 collimator lens, which makes the and wavelengths used. As can be seen from the cluster beam parallel. 4 objective lens for focusing the beam. 5 actuator, of points in the lower left-hand corner, only one area which determines the position of the objective lens. 6 optical disc. 7 beam-splitter. 8 detector for the beam reflected from the disc. The of this diagram has been exploited to any degree. It is detector is in fact a multiple sensing device, and information about clear that the current situation is determined by the focusing and positioning of the beam with respect to the optical track on the disc is also obtained during the read-out. The varia- availability of semiconductor lasers in the wavelength tions in the light beam reflected from the disc can depend on various range from 720 to 820 nm and by the relative ease of physical principles: differences in level ('pits') in a reflecting layer can give differences in intensity on reflection, differences in direc- mass-producing diffraction-limited optics with a nu- tion of magnetization of a recording film can affect the polarization merical aperture of 0.4 to 0.5. The spread in bit density of the reflected light ('magneto-optic recording'), or local differences in crystal structure can give differences in intensity on reflec- is a measure of the effectiveness of various modulation tion ('phase-change recording'). methods in exploiting the characteristics of the optical recording channel. It is also interesting to note that the current CD system [2], in which pulse-length modula- diet future trends in optical recording. It is appropriate tion is used, has already attained a respectable data at this symposium to let the perceived requirements of density. If, in a hypothetical experiment, we go from sophisticated professional systems guide the predic- the analog frequency-modulation methods like those tions. However, it may well turn out that future adap- used in video-disc systems (e.g. for PAL signals) to the tations of optical recording in the consumer area will digital domain via a conversion with 8 bits per pixel, we provide much of the stimulus for new developments. obtain a similar relatively high storage density. Both In particular, the demands placed on storage systems systems exploit the high accuracy with which the lead- for high-definition video signals are fairly stringent. ing or trailing edges (or both) of optical effects on a disc There will certainly be a need for increased storage can be located to encode the data stream. capacities and higher data-transfer rates. Detailed investigations of the signal characteristics As we look back on the brief history of optical re- of the optical recording channel have only been made cording systems, we can distinguish a number of re- comparatively recently [9]. Such investigations require search and development phases. The first phase was sophisticated optical recorders and signal-processing obviously concerned with investigating and defining facilities. The channel capacity is ultimately limited by the basic strategies to be followed in designing the noise. For the present, however, the capacity is mainly optics, servomechanisms, coding and modulation limited by intersymbol interference and crosstalk. As methods, electronics and media for both analog (video) better quantitative measurement techniques become Philips Tech. Rev. 44, No. 2 OPTICAL RECORDING 53

available, so that the properties of various media (replicated, write-once and erasable) can be characterized in NA À î um ) J 0.45 0.8 more detail, it is becoming increasingly probable that IJ 0.65 0.8 advanced coding and modulation strategies willlead to ID 0.65 0.53 an increase in the bit density. This could amount to a IF 0.65 0.4 factor of 4 to 5 with respect to the present situation. B IJ JJJ 10' An indication of some future trends is given in fig. 3. Î Starting with the characteristics of the CD system, for example, and assuming that noise is not the chief capacity-limiting factor of the channel, then the achievable bit density should increase with the square of the cut- off frequency. A number of reference lines (dashed) are shown for optics with a larger numerical aperture and for shorter recording wavelengths (possibly obtained by frequency doubling). The feasibility of the various combinations will be discussed briefly below. Finally, fig. 3 shows the bit density resulting from a 10' ~~--~--~~--~--~~ model calculation of the performance of the optical 1 2 3 4 Sum" recording channel, based on the assumption that ----. fe Gaussian white noise forms the ultimate limitation. Fig. 3.Storage density B as a function of the spatial optical cut-off 3 2 frequency Je = 2NA/À, where NA is the numerical aperture and À This ultimate performance (it is proportional tO/e / ) is the wavelength of the light (in urn). The points represent the cur- is the theoretical limit for diffraction-limited optics. rent state of the technology for a number of existing systems, such As can be seen, a considerable gap exists with respect as Compact Disc (CD), LaserVision (LV) and Digital Optical Re- cording (DOR). The vertical dashed lines indicate Je for particular to the current systems. It is unrealistic to presume that combinations of NA and À. The two sloping lines represent extrap- this gap can ever be completely eliminated, since prac- olations, one based on the CD system, assuming a quadratic rela- tion between Je and B, the other on a model calculation of the tical considerations such as manufacturing tolerances noise-limited situation in which B is proportional to Je3/2. and the like remain. Nevertheless, a combination of tactics (i.e. improving the coding techniques in combination with a higher /e) will undoubtedly offer pros- [2] Special issue on the Compact Disc, Philips Tech. Rev. 40,149- pects of achieving recording densities that are higher 180, 1982. by at least an order of magnitude. [3] H. C. Haverkorn van Rijsewijk , P. E. J. Legierse and G. E. Thomas, Manufacture of LaserVision video discs by a photo- polymerization process, Philips Tech. Rev. 40, 287-297, 1982; J. G. Kloosterboer, G. J. M. Lippits and H. C. Meinders, Lasers Photopolymerizable lacquers for LaserVision video discs, Philips Tech. Rev. 40, 298-309, 1982. The future trends in the development of semicon- [4] G. C. Kenney et ai., An optical disk replaces 25 mag tapes, IEEE Spectrum 16, No. 2 (February), 33-38, 1979; ductor lasers for optical recording are fairly easy to K. Bulthuis et al., Ten billion bits on a disk, IEEE Spectrum predict. A subject of great importance at the moment 16, No. 8 (August), 26-33, 1979; L. Vriens and B. A. J. Jacobs, Digital optical recording with is the development of lasers in the wavelength range tellurium alloys, Philips Tech. Rev. 41, 313-324, 1983/84. around 800 nm with a high useful power output. The [51 D. J. Gravesteijn and J. van der Veen, Organic-dye films for optical recording, Philips Tech. Rev. 41, 325-333, 1983/84. term 'useful' here relates to the power in a beam of [6] H. van Tongeren and M. Sens, Write-once phase-change re- sufficiently high quality for application in the light cording in GaSb, Topical Meeting on Optical Data Storage, Technical Digest Series 1987, Vol. 10, (Optical Society of pen of an optical recording system. The lifetime and America, Washington D.C., 1987), pp. 50-53; D. J. Gravesteijn, H. M. van Tongeren, M. Sens, T. Bertens the feedback characteristics of these lasers [10] are and C. J. van der Poel, Phase-change optical data storage in also critical parameters. The trend is clearly towards GaSb, Appl. Opt. 26, 4772-4776, 1987. the use of higher powers for advanced write-once and [7] M. Hartrnann, B. A. J. Jacobs and J. J. M. Braat, Erasable magneto-optical recording, Philips Tech. Rev. 42, 37-47,1985. erasable materials. This is a natural consequence of [8] D. J. Gravesteijn, New material developments for write-once the general desire to attain higher data rates in optical and erasable phase-change optical recording, Topical Meeting on Optical Data Storage, Technical Digest Series 1987, Vol. 10, recording. Here, of course, developments in the field (Optical Society of America, Washington D.C., 1987), pp. 40- of laser arrays (see jig. 4) in which each laser can be 43; also published in Appl. Opt. 27, 736-738, 1988. [9] C. M. J. van Uijen, Measured performance of modulation independently modulated are eagerly awaited, since strategies for digital optical recording media, Topical Meeting the possibilities of achieving higher data rates through on Optical Data Storage, Technical Digest Series 1987, Vol. 10, (Optical Society of America, Washington D.C., 1987), the use of parallel recording are obvious. pp. 102-105. A second hoped-for trend in lasers is the reduction [10] B. H. Verbeek, D. Lenstra and A. J. den Boef, Noise due to optical feedback in semiconductor lasers, Philips Tech. Rev. of the wavelength. This can be achieved by making 43, 292-302, 1987. 54 G.E. THOMAS Philips Tech. Rev. 44, No. 2

use of new materials for semiconductor lasers [Ill. cal elements with a varying refractive index ('gradient Another way of doing this is to use the method of fre- index') has emerged as a viable technology. These new quency doubling briefly mentioned earlier. A large technologies have permitted the design and construc- world-wide effort is being devoted to frequency- tion of light paths that are not only less expensive to doubled laser systems in which second harmonics are manufacture, but are also relatively simple and light generated in nonlinear materials. The relative ease of in weight. More recent developments in the area of producing Ill-V laser-pumped Nd: YAG lasers, in light paths incorporating beam splitters based on grat- which frequency-doubling is applied to give a wave- ing operation will strengthen this trend. Finally, there length of 530 nm, has stimulated the search for simi- are also the fascinating developments in integrated lar techniques for versions of the 800-nm semiconduc- planar light paths and the use of holographic focusing tor laser with direct-doubling and high power (at techniques and optical-fibre technology. These are the 400 nm). In view of the power densities that this forerunners of optical components that can easily be would require from the III-V laser, it would be neces- incorporated in high-performance actuators for use in

n - AI045 Goo SsAs p-Alo"GOOB6As p- AiO's Gao.ssAs n-GoAs

p- GaAs

Fig.4. a) Cross-section through the layer structure in a double-heterojunction semiconductor laser. On a substrate S of p-GaAs there are, in order, a current-isolating layer B, a confinement layer Gp, an active layer A, in which the laser operation takes place, a confinement layer Gn and a top layer T, to which contacts can be applied. The V groove etched into the current-isolating layer ensures that the current injected into the structure in the y-direction is confined in the x-direction. Dimensions and compositions of the various layers are indicated in the figure. The laser light is generated in the grey region and propagates in the z-direction. The length of the layer structure in the a-direction is much greater than the transverse dimension of the structure. b) A number of such lasers placed next to one another form a laser array. If the spacing L is large, the lasers can operate independently of one another, and at a spacing of say l Sûurn they can in principle be modulated independently, which is useful for parallel recording. If L is small (e.g. 5 urn), a composite laser with an increased power is formed because of phase coupling. sary to consider the application of phase-coupled systems in which the speed of rotation of the disc is in- laser arrays (fig.4). Success in this area would mean a creased for high-data-rate applications and in which breakthrough in achievable storage densities, al- the random-access facility is greatly improved. though such a breakthrough would have to be accom- Another challenge is that of the investigation of the panied by a great deal of development activity in re- strengths and weaknesses of the various modern opti- corder optics and on media that can be used in the cal technologies when they are combined with the pre- 400-nm range. dicted trends noted earlier towards higher numerical apertures and shorter laser wavelengths. In the past the servo technology required for dynam- Optics and actuators . ie positioning to submicron accuracy has often been The growth of optical recording into the mass mar- called the Achilles heel of optical recording. But now ket in the consumer area has given an enormous stim- many authorities in this field agree that new servo ulus to the optical industry. It has led to the develop- techniques being developed, new actuator con- ment of various techniques for the mass production cepts [13] (see jig. 6) and new electronic possibilities of inexpensive, high-quality optics. These techniques will allow this area to keep pace with developments in include the hybrid polymer-on-glass replication pro- the other subsystems. In other words, the control cess and the precision moulding of glass and plastics systems for optical recorders should not form a bottle- for the production of aspheric collimator and object- neck for foreseeable developments in high-perfor- tive lenses [12]; see jig. 5. In addition, the use of opti- mance systems. Philips Tech. Rev. 44, No. 2 OPTICAL RECORDING 55

Recording media Of the developments in the various subsystems associated with optical-recording technology, those in the recording media are the most diverse. Using the obvious subdivision of replicated, write-once and erasable discs, some general trends can be predicted. Replicated media [3] will undoubtedly maintain their current status as carriers of information intended for large-scale distribution. In view of this, the

6 5

Fig.6. Like any other rigid body, an objective lens has six degrees of freedom: three orthogonal translations and three rotations. In the actuators now in use only two or three degrees of freedom are normally employed (they are called 2D or 3D actuators). A recent development has led to the 5D actuator shown here, in which the objective lens 'floats' freely surrounded by six separate banana- shaped coils that can control all five of the relevant degrees of freedom (rotation about the axis of symmetry of the lens is irrelevant). The actuallens has a diameter of 3 mm, and is fixed in a permanent- magnet ring with a diameter of 5.5 mm. It can be moved 2 mm in 1em the vertical direction and 1 mm in two perpendicular horizontal f++1 directions. The complete actuator is mounted on an arm that can be Q b used to give larger displacements.

necessity to define widely accepted system standards for this area will preclude unlimited diversification in the types of replicated discs. The pattern of development here will tend towards reduction of costs and improvement in quality - where quality means fewer inherent errors on the disc itself. The development of simpler mastering techniques and cost-effective replication technologies for the production of limited series of discs would improve market penetration in the areas of CD-ROM and CD-Audio. The development of new disc formats, which now range from 9 to 30 cm in diameter, will undoubtedly stabilize, since most application areas would now seem to be covered. Fig. S. In optical recording an objective lens focuses the laser beam on to the recording layer (i) on the 'reverse' side of the transparent The prediction of the future trends in write-once optical disc (2). If the consequences of the lens aberrations are to media and systems is uncertain. Although there seems be kept within acceptable limits, something other than a single sphericallens must be used. a) One known solution is to use a num- to be a niche in the marketplace for truly permanent ber of lenses (3 to 6), as in the classical microscope objective. The objective shown here has a numerical aperture of 0.45, and with its [11) See for example: Semiconductor laser for visible light, Philips mount it weighs about 1.5 grams. b) If an aspheric lens is used it is Tech. Rev. 44, 23, 1988. possible to obtain an optically equivalent objective made in one piece [12) J. Haisma, T. G. Gijsbers, J. J. M. Braat, W. Mesman, J. M. and weighing only 20 mg. A lens of this type can be made by the Oom en and J. C. Wijn, Aspherics, Philips Tech. Rev. 41,285- 'polymer-an-glass' replication technology. A thin layer of polymer 303, 1983/84. is applied to a spherical glass preform (7)[11). The thickness of the [13) G. E. van Rosmalen, A floating-lens actuator, paper TA7, Int. polymer coating (8) is not constant, but is varied in an accurately Symp. on Optical Memory (ISOM '87), Tokyo, Japan, Sept. predetermined way from 0 to 14 urn to produce a high-quality 1987; also published in Jap. J. Appl. Phys. 26 (supp!. 26-4), aspheric lens. c) Photograph of an aspheric lens made in this way. 195-197, 1987. 56 G.E. THOMAS Philips Tech. Rev. 44, No. 2

archival information storage - a type of storage for materials [5] and an amorphous-to-crystalline phase- which optical recording technology [4] offers unique change [6] - are so good that they rival replicated possibilities - the further development of this market discs in quality. Although in the past the properties of will become uncertain once erasable systems are wide- some of the 'ablative' (hole-forming) write-once sys- ly available. Nevertheless, it should be mentioned that tems could deteriorate because of the inherent chemi- the signal characteristics of a number of proposed cal instability of the materials, a number of the more write-once systems - e.g. those using organic-dye recently developed systems no longer suffer from this problem, so that truly archival recordings can be made (fig. 7). The coming years will see the introduetion of erasable optical-recording products (fig. 8). It is generally conceded that magneto-optics [7] is in a very advanced state of development and that this technology will lead the way into the marketplace. This introduetion has become a possibility because of the development in recent years of procedures for direct overwriting, of simpler systems for polarization read-out of the signals and of optical-disc technologies that circum- vent the problems of the limited chemical stability of the magneto-optical thin films. A great deal of work world-wide is now being concentrated on assessing the limits of magneto-optics in terms of signal character- a istics, compatibility with various special modulation methods and useful lifetime. In addition to magneto-optics, considerable pro- gress has been made in the past year in the development of recording systems in which writing and erasure depend on a reversible amorphous/crystalline phase change [8]; fig.8b. It is now clear that phase-change recording is feasible - this technology is attractive because of its relative simplicity and reasonable compatibility with the recording and read-out methods for replicated and write-once discs - even though it is not as mature a technology as magneto-optics. A great deal of comparative assessment of the various proposed phase-change systems still has to be done. In some quarters, however, predictions are being made that erasable phase-change systems will one day b match the signal quality of known advanced write- once systems and thus become the choice for future generations of high-performance optical recording.

digital recording shown here the individual crystalline domains are so close together that they overlap. (In this figure and the next one c the spacing between adjacent tracks is always about 1.6 urn.) Philips Tech. Rev. 44, No. 2 OPTICAL RECORDING 57

a b

Fig. 8. a) Magneto-optic effects can be used for erasable optical recording. A magnetic recording layer is initially magnetized perpendicular to the surface; a laser can then be used in conjunction with a constant external magnetic field to reverse the direction of magnetization locally. Read-out depends on the rotation of the plane of polarization of incident laser light. The information can be erased by irradiating with a laser beam in the presence of a constant external magnetic field, but now with the same direction as the original magnetization. b) A more recent development is erasable optical recording based on a reversible amorphous/crystalline phase change. Here the recording layer is of gallium antimonide or indium antimonide, doped with other elements. Initially the recording layer is in the crystalline state, and small amorphous domains can be created in it by heating rapidly with a laser beam to just above the melting point (the three 'black' areas in the up- per right-hand corner of the photograph are amorphous domains). These have different reflection properties from the surrounding crystalline material. The crystalli ne structure can be restored by heating again with a laser beam to just below the melting point: this erases the stored information. Many erased domains can be identified in the photograph; their reflection properties are again the same as for the surrounding material.

As the demands for extremely high storage densities has been on the evolution of current systems to higher- increase, the search for new optical recording systems performance systems. But in a field as young as opti- that meet the requirements will continue. In addition cal recording we should not discount the probability to the developments mentioned above - which we of a revolution leading to entirely new possibilities. can assess reasonably well in the light of the knowl- edge we now have - entirely new methods will undoubtedly emerge. It is almost certain that older ideas Summary. The relatively short history of optical recording can be such as multilevel recording will be reinvestigated. divided into a number of periods. First there is the development of New techniques such as 'spectral hole burning', in the basic read-only systems that are intended for playing replicated discs, then come the systems and media that can also be used for re- which the optical properties of a recording layer are cording, and finally there is the appearance of erasable optical re- modified wavelength-selectively and the bit density cording. The future challenge will be to develop systems with higher densities and higher data rates. This article looks at the resultant can increase by several orders of magnitude, offer ex- consequences for the subsystems of optical recording - from laser citing prospects. In this short summary the emphasis to disc. 58 Philips Tech. Rev. 44, No. 2, 58-60, April 1988

Scientific publications

These publications are contributed by staff from the laboratories and other establishments that form part of or are associated with the Philips group of companies. Many of the articles originate from the research laboratories named below. The publications are listed alphabetically by journal title.

Philips GmbH Forschungslaboratorium Aachen, A WeiJ3hausstraJ3e, 5100 Aachen, Germany Philips Research Laboratory, Brussels, B 2 avenue Van Becelaere, 1170 Brussels, Belgium Philips Natuurkundig Laboratorium, E Postbus 80000, 5600 JA Eindhoven, The Netherlands Philips GmbH Forschungslaboratorium Hamburg, H Vogt-Kölln-Stral3e 30, 2000 Hamburg 54, Germany Laboratoires d'Electronique et de Physique Appliquée, L 3 avenue Descartes, 94450 Limeil-Brévannes, France Philips Laboratories, N.A.P.C., N 345 Scarborough Road, Briarcliff Manor, N.Y. 10510, U.S.A. Philips Research Laboratories, R Cross Oak Lane, RedhilI, Surrey RHI 5HA, England Philips Research Laboratories, Sunnyvale, S P.O. Box 9052, Sunnyvale, CA 94086, U.S.A.

N. J. van Veen E Influence of the detector response function in the Anal. Chem. 59 2088-2091 1987 determination of concentration depth profiles from angle-resolved electron spectroscopy

P. Dolizy & F. Groliere L Optical method for heterogeneity analysis in thin Appl. Opt. 26 2401-2406 1987 absorbing films

R. L. S. Devine, C. T. Foxon, B. A. Beryllium diffusion across GaAs/(Al,Ga)As hetero- Appl. Phys, A 44 195-200 1987 Joyce, J. B. Clegg& J. P. Gowers R junctions and GaAslAlAs superlattices during MBE growth

J. J. Kelly&A. C. Reynders E A study of GaAs etching in alkaline H202 solutions Appl. Surf. Sci. 29 149-164 1987

S. van Lerberghe E Adaptivity in parallel all-pole lattice filters Digital Signal Proces- 125-128 1987 sing - 87, V. Cappel- lini & A. G. Constan- tinides (eds), Elsevier Science, Amsterdam

S. Stotz A Shift of the morphotropic phase boundary in the Ferroelectrics 76 123-132 1987 PZT system under the influence of electric fields and uniaxial stresses

J. L. W. Kessels E PHILAN: A LAN providing a reliable message High Speed LocalArea 201-212 1987 service Networks, O. Spaniol & A. Danthine (eds), Elsevier Science, Amsterdam

E. Delhaye, C. Rocher, J.-C. Baelde, A 2.5-ns, 40-mW, 4x4 GaAs multiplier in two's IEEE J. SC-22 409-414 1987 J .-M. Gibereau & M. Rocchi L complement mode

M. J. M. Pelgrom & H. A. H. A 32-kbit variable-length shift register for digital ibid. 415-422 1987 Termeer E audio application

M. J. M. Pelgrom, H. E. J. Wulms, FEBRIS: A chip for pattern recognition ibid. 423-429 1987 P. van der Stokker & R. A. Bergamaschi E

P. Sheridan & C. M. Huizer E An expression for the propagation delay of a differ- ibid. 457-459 1987 ential split-level (DSL) CMOS logic gate

P. Jean (TR T, Le PIessis Robinson), Wide-band monolithic GaAs phase detector for IEEE MTT-S Int. 169-171 1987 V. Pauker & P. Dautriche L homodyne reception Microwave Symp. Digest, Las Vegas, NV, 1987

J. H. M. Neijzen & J. H. A. Fretting corrosion of tin-coated electrical contacts IEEE Trans. 68-74 1987 Glashörster (Philips Centrefor Manu- CHMT-IO facturing TechnoI., Eindhoven) E Philips Tech. Rev. 44, No. 2 SCIENTIFIC PUBLICATIONS 59

J. J. Goedbloed E Transients in low-voltage supply networks IEEE Trans. EMC-29 104-115 1987

K. A. Schouhamer Immink & Binary transmission codes with higher order spectral IEEE Trans. IT-33 452-454 1987 G. F. M. Beenker E zeros at zero frequency

P. N. Kuin E The interpretation of rheograms of magnetic IEEE Trans. MAG-23 97-99 1987 lacquers measured with a couette apparatus

J. P. C. Bernards, C. P. G. Material and recording properties of perpendicular ibid. 125-127 1987 Schrauwen, S. B. Luitjens, V. Zieren CoCr media &R. W. de Bie E

J. J. Brondijk, P. E. Wierenga, E. E. Roughness and deformation aspects in calendering ibid. 146-149 1987 Feekes" & W. J. J. M. Sprangers* of particulate magnetic tape (*P.D. Magnetics, Oosterhout) E

M. H. Azarian & P. N. Kuin E Determination of the particle size distribution of ibid. 192-194 1987 magnetic pigments

D. Hennings A Barium titanate based ceramic materials for Int. J. High Techno!. 91-111 1987 dielectric use Ceram.3 J. H. Streng E Calculation of the surface pressure on a vibrating J. Acoust. Soc. Am. 679-686 1987 circular stretched membrane in free space 82

P. W. J. M. Boumans & J. J. A. M. Inductively coupled plasmas: Line widths and J. Anal. At. Spectrqm. 513-525 1987 Vrakking E shapes, detection limits and spectral interferences. 2 An integrated picture

P. Suchet, M. Duseaux, 1. Maluenda Effects of dislocations on threshold voltage of GaAs J. App!. Phys. 62 1097-1101 1987 & G. M. Martin L field-effect transistors E. P. Honig, P. E. Wierenga& J. H. Theory of elastic behavior of composite materials ibid. 1610-1612 1987 M. van der Linden E

O. Boser N Statistical theory of hysteresis in ferroelectric ibid. 1344-1348 1987 materials

D. Moroni, J. P. André, E. P. Menu, Photoluminescence investigation of InGaAs-InP ibid. 2003-2008 1987 P. Gentric & J. N. Patillon L quantum wells

C. van Opdorp & M. R. Leys E On the factors impairing the compositional transi- J. Cryst. Growth 84 271-288 1987 tion abruptness in heterojunctions grown by vapour- phase epitaxy

P. F. Bordui, J. C. Jacco, G. M. Growth of large single crystals of KTiOP04 (KTP) ibid. 403-408 1987 Loiacono, R. A. Stolzenberger & from high-temperature solution using heat pipe J. J. Zola N based furnace system

M. Delfino, M. Jaczynski, A. E. Gettering of copper in silicon-on-insulator structures J. Electrochem. Soc. 2027-2030 1987 Morgan, C. Vorst, M. E. Lunnon & formed by oxygen ion implantation 134 P. Maillot S

M. F. H. Schuurmans, H. W. A. M. Possibility of green light emission from GaP/AlP J. Lumin. 37 269-273 1987 Rompa & R. Eppenga E (001) superlattices

P. C. M. Gubbens", A. M. van der 169Tm Mössbauer effect in Tm2Te14B J. Magn. & Magn. 238-242 1987 Kraan* (*Interuniv. Reactor Inst., Mater.68 Delft), R. P. van Stapele & K. H. J. Buschow E

F. J. J .. Blommaert* & J. J. Neve* Reading fields of magnifying loupes J. Opt. Soc. Am. A4 1820-1830 1987 (*Inst. for Perception Res., Eindhoven)

J. W. C. de Vries E Resistivity of thin Au films as a function of grain J. Phys. F 17 1945-1952 1987 diameter and temperature

M. J. J. Theunissen, H. J. J. Jaspers, Neutron activation analysis of the process of metal J. Radioana!. & Nuc!. 391-396 1987 J. M. G. Hanssen & M. L. Verheijke gettering in silicon slices Chem.113 E

M. L. Verheijke, H. J. J. Jaspers, J. Application of neutron activation analysis in the ibid. 397-403 1987 M. G. Hanssen & M. J. J. Theunissen field of silicon technology for the modern electronic E industry

P. K. Larsen, G. Meyer-Ehmsen Surface disorder induced Kikuchi features in reflec- J. Vac. Sci. & Technol. 611-614 1987 ( Univ. Osnabrück), B. Bölger &A.-J. tion high-energy electron diffraction patterns of A5 Hoeven E static and growing GaAs(OOI) films 60 SCIENTIFIC PUBLICATIONS Philips Tech. Rev. 44, No. 2

G. G. P. van Gorkom & A. M. E. Silicon cold cathodes as possible sources in electron J. Vac. Sci. & Technol. 1544-1548 1987 Hoeberechts E lithography systems A5

D. R. Wolters & A. T. A. Zegers-van On dielectric breakdown in oxidized silicon ibid. 1563-1568 1987 Duijnhoven E

G.E. Thomas E Thin films for optical recording applications ibid. 1965-1966 1987

D. R. Wolters & A. T. A. Zegers-van On the initial regime of silicon oxidation Phil. Mag. B 55 669-672 1987 Duynhoven E

P. Friedel & J .-P. Landesman L Photoemission study of the passivation of GaAs in a ibid. 711-719 1987 nitrogen multipolar plasma

J. S. Cohen & A. G. Schlijper (Shell Long-range order in AlxGal_xAs Phys. Rev. B 36 1526-1530 1987 Lab., Rijswijk) E

G. E. W. Bauer & T. Ando (Univ. Comment on 'Excitonic coupling in GaAs/GaAIAs Phys. Rev. Lett. 59 601 1987 Tokyo) E quantum wells in an electric field'

H. H. A. Smit", R. C. Thiel* (*Univ. 166Gd Mössbauer effect in Gd2C014B Physica 145B 329-334 1987 Leiden) & K. H. J. Buschow E

P. Gamand, M. Fairburn, C. Varin 1 to 20 GHZ monolithic distributed amplifier using Proc. 17thEur. Micro- 249-254 1987 & J .-C. Meunier L,R GaAs MESFET's or HEMT's wave Conf., Rome 1987

M. Levent- Villegas, M. Soulard & C. S-band GaAs monolithic linear dual phase modulator ibid. 421-426 1987 Villalon L R. H. Coursant, P. Byraud", L. Preparation and characterization of lead titanate Proc. 6th lEE Int. 442-447 1986 Eyraud* (*INSA, Villeurbanneï, M. and lead metaniobate piezoceramics for ultrasonic Symp, on Applica- Fink (Univ. Paris) & J. M. Tellier L transducer design tions of ferroelectrics, Bethlehem, PA, 1986

R. H. Coursant, C. Méquio & J. M. Automatic method of intrinsic characterization of ibid. 730-734 1986 Tellier L lossy piezoelectric structures

P. Rabinzohn, J. Bellaiche, C. Reactive ion etching of tungsten nitride in CHFa- Proc. 8th Int. Syrnp. 1044-1049 1987 Roeher & S. Gourrier L SF 6 based gas mixtures using a statistical design on Plasmas chemistry, approach Vol. 2, Tokyo 1987

M. Rocchi L GaAs digital IC's for multigigabit's systems Proc. ESSCIRC '87, 149-156 1987 Bad Soden 1987

A. J. Guest R Modelling microchannel plate (MCP) performance Proc. Eurodisplay '87 208-211 1987 in thin, flat CRTs

P. Houdy, V. Bodart, 1. Névot*, D. Capability of kinetic ellipsometry as an in situ Proc. Spie 688 122-128 1987 Corno", B. Pardo* (*Inst. d'Optlque control system for ultra thin layers stacks growth: Théorique et Appliqée, Orsayï, M. Application to the realization of performant soft x- Arbaoui", N. Alehyane", R. rayoptics Barchewitz" (+Univ. Paris), Y. Lepêtre" & E. ZieglerO (OArgonne Nat. Lab., Argonne, IL) L

P. Friedel, J. P. Landesman & R. Etude par photoémission de la passivation de GaAs Rev. Phys. Appl. 22 797-802 1987 Mabon . L en plasmas multipolaires d'azote et d'hydrogène

J. B. Clegg, I.G. Gale, G. Blackmore A SIMS calibration exercise using multi-element Surf. & Interface 338-342 1987 (R. S.R. E., Great Malvern), M. G. (Cr, Fe and Zn) implanted GaAs Anal. 10 Dowsett*, D. S. McPhail* (*City of London Polytech., London), G. D. T. Spiller (British Telecom Res., Martlesham Heath) & D. E. Sykes (Loughborough Consultants, Lough- borough) R G. Delmas, J.-P. Hazan, M. Steers& La domotiqueou unemaison au service de l'habitant Télécom No. 72 21-22 1987 C. Piaget L

P. C. P. Bouten E Lifetime of pristine optical fibers Thesis, Eindhoven 1-140 1987

W. J. van Gils E Design of error-control coding schemes for "three ibid. 1-231 1988 problems of noisy information transmission, storage and processing

P. L. Holster & J. A. H. J acobs E Theoretical analysis and experimental verification Triboloby Int. 20 276-289 1987 on the static properties of externally pressurized air- bearing pads with load compensation

Volume 44, No. 2 pages 33-60 Published 20th April 1988 e PHILIPS

J. J. J. Bastiaens and W. C. H. Gubbels, The 256-kbit SRAM: G. ~. Thomas, Future trends in optical recording, an important step on the way to submicron IC technology, PhilipsTech. Rev. 44, No. 2, 51-57,April 1988. Philips Tech. Rev. 44, No. 2, 33-42,April 1988. The relatively short history of optical recording can be divided into a number The static random-access memory (SRAM) with a capacity of 256 kbit, devel- of periods. First there is the development of the basic read-only systems that oped at Philips Research Laboratories, contains details of 1.2 urn. The cells in are intended for playing replicated discs, then come the systems and media that this memory, made in full CMOS technology (complementary metal-oxide can also be used for recording, and finally there is the appearance of erasable semiconductor), consist of four transistors, plus two connecting transistors. optical recording. The future challenge will be to develop systems with higher The current requirement and access time of the SRAM are both small, because densities and higher data rates. This article looks at the resultant consequences of the application of the 'dynamic double-word-Iine technique'. Scaling down for the subsystems of optical recording - from laser to disc. the details in an integrated circuit entails various problems. Shortening the transistor channel causes the current leakage known as 'punch-through' and premature failure of the transistor due to 'hot' electrons. Narrowing the transistor channel leads to difficulties because of the 'bird's-beak' effect, which raises the threshold voltage. These problems have been solved by adapting the technology, including the change to LDD transistors (LDD: lightly doped drain). Other problems can arise from contacts between the interconnection layers and the resistance of the conductors. -

.. ------_. -- - . - - T. Blaffert, EXPERTISE: an expert system for infrared spectrum evaluation, I wish to subscribe to Philips Tech. Rev. 44, No. 2,44-50, April 1988. Infrared spectra 'are exceptionally useful for providing information about chemical structure. Analytical chemists who use these spectra to identify the PHILIPS TECHNICAL REVIEW chemical structure of unknown or newly synthesized compounds will find the EXPERTISE expert system a great help. Besides identifying complete spectra by comparing them with the spectra in an extensive library, this system can to some extent 'reason' as the chemist does. This is achieved by subdividing the structures in the library into substructures, establishing the spectral features re- lated to these substructures, recognizing these features in spectra of unidenti- (date) (signature) fied compounds and combining the substructures to form a complete structure. The procedure for identifying the substructures from the spectral features in- eludes a safety fa_ctorto prevent substructures from being excluded in error because of statistical variations in the spectra. This is done with the aid of fuzzy- set theory. The system may provide more than one result, in which case the Pleasê tick the appropriate box chemist has to make the final decision. The great advantage of such systems is that all the possible results are found relatively quickly. Regular subscription- 80 guilders or U.S. $ 35.00 per volume o

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Contents

Wet-chemical etching of m-v semiconductors 61 J. J. Kelly, J. E. A. M. van den Meerakker, P. H. L. Notten and R. P. Tijburg Removing material by wet-chemical etching is an important process in the fabrication of Ill- V semiconductor components

Then and Now (1938-1988) 75

Bistability in quantum-welliasers 76 A. 1. Kucharska, P. Blood and E. D. Fletcher Special semiconductor-laser structures allow rapid switching between high and low light-output levels

Chemical modification of surfaces 81 J. J. Ponjeé and P. N. T. van Velzen . Surface modification - an effective new chemical technology

Striations in a gas discharge 89 F. C. van den Heuvel The electrical circuit helsan effect on optical instabilities in a gas discharge

Scientific publications 96

e PHILlPS