Thin Film Laboratory
Nanoscale Science Group
Polysilicon Thin-Film Transistors and Circuits
Microsystems Design and Fabrication Technology
Josephson Junctions in Superconducting YBaCuO using Focused Electron Beam Irradiation
Scanning Electron Microscopy and Transmission Electron Microscopy
High Brightness Electron Guns
Highly Integrated Electronic Systems
Photonics and Parallel Optical Systems
Polarisation Mode Dispersion
Optical Fibre Surface Plasma Wave Chemical Sensors
Professor W.I. Milne
Dr J. Robertson
The Electronic Materials and Devices Group works primarily in the areas of diamond-like carbon, amorphous silicon and C-Si high voltage devices.
Work on diamond-like carbon has expanded to include development of electron emitting cold cathode films for field emission displays(I29,I94,I95,I96,I97,I122,I123,I127,I128,I133,I139,I140,I141) and hard coatings for applications such as magnetic disks. The films are being studied in collaboration with Motorola and recently with CRL and Sextant Avionique as part of an ESPRIT project and KIST and Kyung Hee University in South Korea. The group has shown that nitrogen doped tetrahedrally bonded amorphous carbon (ta-C) is an efficient electron emitter(I52). This is supported by considerable effort on the characterisation of the electronic(I25,I26,I51,I55,I72,I121,I132), structural(I57,I58,I59,I85,I146,I178) and photoluminescence properties of amorphous carbon(I137), the development of models(I129,I131), and understanding of the plasma beam deposition process(I36,I103), and the deposition of thin film diamond(I138). The use of ta-C as a hard coating material(I30) for wear and tribological application is carried out with numerous collaborators, including IBM San Jose. An EPSRC equipment grant has recently been awarded to purchase an ECR deposition/etching apparatus, which will allow the rapid processing of carbon and silicon films and device structures, at low substrate temperatures. Work also progresses on the aSiC:H deposition using ECR in collaboration with NTU in Singapore(I68,I77,I180,I181,I182,I183,I184). Further EPSRC funded projects on the deposition and characterisation of CNx and C-Si films have begun in collaboration with the Physics and Materials Science Departments.
The development of microcrystalline Si for TFT applications, in collaboration with Philips Research Laboratories, Redhill(I54) is still on-going. Atomic force microscopy is being used to understand the nucleation and growth processes of amorphous and microcrystalline silicon(I52, I53). Amorphous silicon has recently been grown by the filtered cathodic vacuum arc(I4,I5), capitalising on our experience with this technique for diamond-like carbon(I2,I3,I144). There is also continuing theoretical work on instability mechanisms in amorphous silicon TFTs, with Philips, and a contract to understand how such instabilities depend on the growth process.
The fabrication of high voltage polycrystalline silicon (poly-Si) TFTs using field plates and oxide layers has been demonstrated, after extensive simulations. This process allows both high voltage and low voltage TFTs to be placed on the same wafer(I21,I22,I23,I24,I31,I32).
Work in collaboration with Sandia National Labs has been initiated in order to develop ferroelectric materials for non-volatile memories(I130).
An EPSRC funded project on the development of Silicon on Insulator (SOI) based power devices has begun(I158,I159), and work on Inversion Layer Emitter Devices(I155) and trench gated IGBTs(I156,I157) is continuing in collaboration with GPS.
Dr M.E. Welland
The Nanoscale Science group is primarily concerned with developing and applying methods related to the measurement of structure and physical properties down to atomic dimensions. A substantial part of the work involves the use of scanning probe microscopy(I115,I150,I164,I165,I166) to measure surface properties down to atomic spatial resolution. A long term interest has been the use of atomic force microscopy (AFM) in tribology. Here, the aim is to make quantitative measurement of local mechanical properties. To this end we have been studying the properties of the tip-surface contact in the AFM on model surfaces(I74,175,I76,I114). Related to this is the use of forces to engineer matter at the molecular scale. In collaboration with IBM, we have developed methods to move single molecules of C60 to form patterns on the scale of 6 nm(I168).
An important application of nanoscale engineering is in the fabrication of novel electronic optical and magnetic devices(I37). By combining high
resolution detection beam lithography and AFM, electronic devices with critical dimensions less than 20 nm have been fabricated and tested. Such
measurements give an insight into electron transport through nanometre scale structures and allow for specification of material/fabrication properties
to realise a future device technology(I66,I68,I113,I163,I167). For potential optical devices, studies of nanometre scale metallic
particles have demonstrated light emission with high quantum efficiency. Such particles have potential as a new range of light emitting devices(I49,I50,I162).
Sensor technology(I104) based on microelectronics(I73) is a rapidly expanding field. In addition to modelling the behaviour of such sensors(I105,I106), applications in electrochemistry(I13,I14,I15) and biochemistry are being pursued.
Dr P. Migliorato
Dr M.J. Quinn
Dr S.W.B. Tam
This activity(I79,I92,I93,I152) is in close collaboration with, and fully supported by Seiko Epson. The aim is to develop the low temperature polycrystalline silicon technology, based on laser recrystallised material, for the next generation of displays and portable information systems to be fabricated entirely onto glass. The activity concerns three main topics: device characterisation techniques; 2-D simulation; circuit simulation. Considerable effort has been devoted during the last year to set up high sensitivity AC characteristics measurements. The TFT transient response has been investigated showing a new effect, a floating-body-induced nodal capacitance enhancement, which was also, for the first time, confirmed by 2-D simulation. The effect is important for the TFT circuit operation (see below). Based on the combination of DC and AC electrical characteristics measurements, a new method for the accurate determination of bulk and interface Density of States is under development. Hot carrier degradation effects have been studied through electrical measurements and 2-D simulation. We have developed a new technique and proved for the first time, in contrast to other authors, that the defects are generated in the bulk of the active layer rather than at the interface. Work is continuing on the validation and upgrading of PSIM, our proprietary circuit simulator for polysilicon TFTs. Excellent agreement has been obtained between the simulator and the above mentioned transient TFT measurements, as well as with circuit performance. The success of PSIM, which has been licensed to Philips, Thomson CSF and Seiko Epson, is due to its unique model, which takes correctly into account physical mechanisms such as the floating-body-induced phenomena mentioned above. The circuit design activity has resulted in the patenting of two new circuits.
Dr D.F. Moore
Dr S.C. Burgess
Dr P.J. Clarkson
Silicon micromachining is of increasing importance for devices, sensors and actuators, one recent commercial example being the capacitance based accelerometers for car airbag control(C10,I44,I46,I98,I99,I160). In a collaboration between the EDC, National Panasonic, and the University of Tokyo, novel sensors are being designed and developed including a tunnel based accelerometer using bonded silicon-on-insulator (SOI) starting material and focused ion beam etching. This should show greater sensitivity than capacitance based detection, and the use of SOI wafers greatly simplifies the fabrication and assembly. Early structures fabricated in the Engineering Department Clean Room are promising, but thin film stress effects are causing serious bending of the proof mass and other freestanding structures. The present focus of the design effort is to devise structures which compensate for the effects of stress in the SOI starting material.
Dr D.F. Moore
Dr M.E. Welland
Dr W.E. Booij
Dr M.G. Blamire
During collaborative work with the Materials Science Department and Superconductivity IRC, simple thin film quantum interference devices
(SQUID) have been prototyped with possible applications in sensors and in A/D converters(I6,I7,I8,I63,I64,I100,I101,I102,I112). Using a
combination of focused ion beam YBaCuO etching and FEBI weak link formation, SQUID loops have been made with a precision of 0.2 micron.
The Josephson junction properties using this damage approach are good compared with devices made in competing laboratories, but the serial
junction fabrication process is too slow to consider for manufacturing and is only applicable for prototyping devices. Upon the successful
completion of the ESPRIT project `SUPACT', an EPSRC project has now started with the University of Surrey, and industrial support from the
Oxford Instruments Cambridge Laboratory. One major objective is to use ion implantation to develop a high-throughput process to produce
Josephson weak links in 100 nm thick YBaCuO films using a PMMA electron beam written mask with decanano scale openings to define the device
This will involve electron beam lithography in Cambridge, ion implantation in Guildford and device and process simulation in both universities.
Dr D.M. Holburn
Research is under way into the application of knowledge-based techniques to two key scanning electron microscopy tasks. An expert system (FIRST A.I.D.) has been designed and implemented to assist with fault diagnosis, using a novel "information client" approach to integrate the system with Internet technologies for remote diagnosis and repair. A second prototype knowledge-based system, XpertEze, has been designed to assist microscopists in the task of instrument control, with current work concentrating on developing novel and fully automated image quality assessment algorithms(I16,I17). Work has continued on the development of novel techniques for remote control of the scanning electron microscope, and these are now being incorporated in production instruments. A further recent development has been the ability to share microscopical images and information among a number of users at different sites(I12,I20).
Dr C.E. Maloney
A tetrode electron gun for operation in ultra high vacuum has been contracted and initial tests are underway in order to establish the correct operating conditions. In addition, a novel method of forming limited-area electron sources using both emission enhancement and emission suppression has been developed and is giving promising results. Earlier work has been reported and further analysed.
Dr D.M. Holburn
Work has continued on the further development of Chiprack, an approach which offers the potential to reduce designs to a small number of highly integrated silicon modules linked by means of simple, regular interconnecting structures. An intelligent, reconfigurable imaging instrument is currently under development based on this system with support from industry(I69,I70).
Good progress has been made in the development of a miniaturised recording device for non invasive monitoring of human activity. Such information is of interest in epidemiological and nutritional studies investigating the apparent rise in prevalence of obesity in UK and USA populations. A key objective has been to develop sensors capable of recording heart rate or movement patterns over extended periods. This work has now been extended with the development of a multi-purpose sensor capable of recording more than one parameter at a time. Work is also under way on the development of wireless links to facilitate convenient transfer of recorded data to a host computer system. This work is being undertaken in collaboration with Department of Community Medicine and MRC Dunn Clinical Nutrition Unit.
Mr P.J. Spreadbury
To make voltage measurements at the very highest precision, a group of more than twelve 10 V standards is maintained at the Department to allow eight-digit instruments to be calibrated. The 10 V standards need to be regularly intercompared and refinements, and improvements and precautions in doing this are continuously developed.
Recently an improved way of performing this intercomparison has been used which allows the uncertainties of the process to be stated with high confidence(I149). Using this method, three new standards, constructed by 4th year project students, have been found to have rates of change with time of +0.04, -0.13 and -0.02 ppm/month with the actual measurements fitting a linear rate with RMS errors of 0.08, 0.10 and 0.05 ppm.
Dr R.A. McMahon
Dr P.R. Palmer
A further batch of integrated power switches for use in power conversion circuits at up to 13.56 MHz has been
completed. The switch comprises a power MOSFET and gate drive and level shifting circuitry. Aspects of the process and device characterization have been published(I19) and circuit details have also been reported(I18) . Work continues on applying these devices in both half bridge and class E circuits. The study of IGBTs for use in inverter drives has involved extensive measurements of switching performance. A comparison of experimental results and the predictions of the Hefner and Kraus models for the IGBT has been presented(I60).
The experimental work on the series operation of IGBTs modules continued, sponsored by Hill Graham Controls. Tests continued on a full inverter leg using the proposed gate drive strategy(I116). The general behaviour has been analysed(I117). The 10 kV, 1200A test rig is still under construction and partners are being sought for power systems applications at still higher voltages.
Samples of a novel vertical dual mode thyristor structure were designed and manufactured in collaboration with Westcode Semiconductors and Cork NMRC. The devices are 12mm square, with an expected rating of 100A and 1800V. The first samples were made in a pilot process, with a resulting low yield. However, samples have been tested at 100A in thyristor mode. Computer simulation continues. A number of companies have expressed interest.
Work on the performance of high current multi-chip IGBT modules continued. The non-invasive current measurement method has been refined and is able to quantify the individual chip currents accurately(I118). The measurements show that unmatched IGBTs share current poorly both in the steady state and in the transient. Applying various conditions to the IGBTs results in different levels of non-uniform current sharing and self heating(I119). Work has started on the computer modelling of IGBT chips operating in parallel. The objective of this is to choose parameters for matching and to set manufacturing tolerances to them. This work is EC funded, with 16 partners. Our direct collaboration is with GEC Plessey Semiconductors, Siemens ZFE and University of Dortmund.
Dr R.A. McMahon
Dr C.I. McClay
The research programme on drives for domestic appliances has led to the demonstration of a washing machine driven by a three-phase, inverter-fed motor. Experimental measurements of motor losses are being compared to computer predictions and a calorimetric test rig is under construction. A study of low cost drive options for domestic appliances has led to the publication of work on alternative topologies for two-phase motor drives(I61) and three-phase motor drives(I62).
Dr A.C. Metaxas
Numerical and experimental modelling with particular emphasis to radio frequency and microwave heating continues to be the focus of the research activity at the Electricity Utilisation Group (EUG) especially for food related products(I90). The work at microwave frequencies is supported by Eastern Electricity(I80) and software codes have been transferred to Unilever Research(I9). This software is being extended and new formulations are sought which will allow modelling of thin dielectric sheets with metallic coatings in microwave cavities. Space domain decomposition techniques are being considered for tackling large industrial systems and a code has been written to enable to use the Hitachi parallel computer facility recently installed at the University(I81). Hybrid numerical techniques are being considered for modelling special applicator structures and systems in electroheat.
The work at radio frequencies continues on several fronts(I91). Modelling of the electrical circuit has been carried out using a Saber simulation package(I108,I109). An improved method for impedance matching using a network analyser has been formulated which enhances considerably the power transfer efficiency(I107). Finite element modelling of radio frequency applicators and their tank oscillatory circuits is also under way(I110,I111). The work on coupling radio frequency to heat pump technology has been completed and the results introduced to industry. Work is also underway to model the instigation of radio frequency coronas which requires the simultaneous solution of the coupled equations of particle continuity and Poisson. Flux corrected transport techniques are used to reduce numerical diffusion.
The EUG remains the strategic and administrative centre for the AMPERE organisation and publishes its quarterly Newsletter(I86,I87,I88,I89).
Dr T.J. Flack
The work on 3-D finite-element modelling of induction motors has been completed. Models for accurately determining losses in stator duct spacers, calculating clamping plate losses and finding end-winding leakage inductance using stator phase-band models has been carried out.
Ongoing research is being carried out into: the application of new finite-element techniques, such as domain decomposition, to 2-D time-domain modelling of induction motors with a view to vastly reducing the CPU time required to apply such models; application of this new method to the determination of stray losses, and the effects of voltage supply imbalance in induction motors; modelling, design and optimisation of brushless doubly-fed induction motors; the analysis of damper bars in stand-alone diesel-generator systems with the aim of damping speed oscillations caused by the inherent torque pulsations of diesel engines; modelling and optimisation of electromechanical shakers, under an EPSRC grant and in conjunction with Ling Dynamics Ltd.
Professor S. Williamson
Dr A.C. Smith
Dr C.I. McClay
Much of the work relates to the continued development of design-orientated finite element techniques for induction motor models including the influence of skew on iron losses(I83) and the representation of rotor spiders and axial ventilation ducts(I173). Analytical models have also been developed to examine rotor casting problems and associated unbalanced magnetic pull in single-phase domestic motors(I174,I175). The work has extended into the influence of rotor bar laminar insulation on motor performance using both analytical and experimental techniques(I56).
Work on formal optimisation methods has been expanded to examine the influence of end-ring shape on cage motor performance(I147). This technique has also been used to optimise the design of pulsed coil-gun electromagnetic launchers(I176).
New developments include the analysis of brushless doubly-fed machines for applications requiring limited speed control. This type of machine/drive is generating a lot of industrial interest because it uses converters of rating less than that of the motor/generator for speed control(I171,I172). Work has also extended into the modelling of cage and line-start permanent magnet motors for single-phase domestic and three-phase industrial applications(I148).
Professor W.A. Crossland
Dr R.J. Mears
Dr T.D. Wilkinson
Dr A.B. Davey
The Photonics and Sensors Group continues its interest in highly parallel opto-electronic systems based on free-space optics, with ferroelectric liquid crystal spatial light modulators (FLC SLMs). Smart pixel systems are being designed and have been built to integrate FLC modulators on silicon VLSI backplanes(I40,I41,I169).
The photonic device fabrication facility is functioning and producing high quality devices from its combined Class 100/1000 areas. These devices include single pixel structures for high-speed modulators(I27,I28), optically addressed structures based on amorphous silicon(I179) and custom SLMs for running research projects. There is an ongoing interest in improving all aspects of liquid crystal light modulation including speed and contrast ratio.
The group has recently produced and demonstrated its first silicon backplane SLM, the fast bitplane SLM(I40,I41,I169). The device is capable of displaying a sequence of 320 by 240 pixel images at a rate in excess of 20 000 frames per second. The device has been used in projection display systems to demonstrate greyscale and colour image reproduction. The SLM is intended for use in some of the applications listed below.
Work is widely support by EPSRC, DTI through LINK programmes and by DERA and other industrial partners.
The work started on the EPSRC project parallel opto-electronics for telecommunications systems (POETS) has been completed and has spawned several new research areas.
Work is continuing to demonstrate the application of an FLC-SLM as a dynamic filter for WDM telecommunications(I124,I125). The pattern written on the SLM serves as a dynamic hologram to diffract the incident light and can filter, and equalise to the same power level, up to eight different wavelengths(I33,I34,I35). Ongoing work aims to demonstrate the additional functionality of an add-drop multiplexer (ADM) and to reduce the filter passband width to match international telecommunications standards. A pilot project is assessing the potential application of non-linear optical polymers to faster SLM devices for light modulation, switching and WDM filtering.
As a result of the POETS project, there was a demonstration of the role of optics in asynchronous transfer mode (ATM) telecommunications systems. This follows on from the theoretical work into the way that optical interconnects might modify the architecture of electronic ATM switches(I42,I43,I143) and their role in telecommunications systems. This culminated in a demonstration of optically accessible silicon VLSI memory (opto-RAM) in an ATM switch structure. There is currently interest in vertical cavity surface emitting lasers for this application(I177).
A project which has been spawned by POETS is the DTI funded reconfigurable optical switches for aerospace and telecommunications systems (ROSES). The ongoing work is to produce a working single mode to single mode optical fibre switch based on holographic beam steering(I153). This includes the design and building of a custom VLSI silicon backplane SLM.
There is also renewed interest in the role of optics and smart pixel SLMs in neural networks. A new scheme has recently been developed to improve the learning in neural networks(I10,I11) and how they can be applied to optical systems.
There is also work continuing on optical correlators based on FLC SLMs. It is hoped to demonstrate a new type of optical correlator(I170) for pattern recognition in the near future. The new architecture is an attempt to successfully combine electronic and optical processing.
A new form of liquid crystal display has been developed within the group, known as a photo-luminescent liquid crystal display (PL-LCD)(I39,I71). The PL-LCD combines all the power consumption and compactness advantages of conventional flat panel LCDs with the viewing characteristics of a cathode ray tube (CRT). This work is being carried out in conjunction with a small displays company, Screen Technology Limited.
Dr A.R.L. Travis
Development continues of a video display which shows a moving colour three dimensional image. The original version has been licensed to a local company with backing from Hollywood who are building an advanced prototype for the entertainments industry. Work has now begun on a flat panel version of the display, and we continue to investigate optically addressable spatial light modulators for high resolution 3D display.
Dr F.P. Payne
Non-linear optical processes such as second harmonic generation in frequency doubling crystals usually exploit an optical Gaussian beam focused into the crystal. The drawback of this approach is that the beam diffracts, limiting the effective interaction length in the crystal, and the resulting conversion efficiency to second harmonic light. We have recently proposed an alternative approach to frequency doubling based on the propagation of a `non-diffracting' Bessel beam through the crystal. A Bessel beam is an optical wave whose transverse amplitude is described by a Bessel function. Such a wave is a solution of the wave equation with the unusual property that it exhibits no diffraction. Experimentally, it is possible to form good approximations to such waves using suitable phase masks. We have analysed the non-linear interaction of a Bessel beam with a frequency doubling crystal, showing a marked increase in frequency doubling efficiency compared with that achieved using a Gaussian beam; the increase in efficiency is at the expense of increased sensitivity to temperature(I120).
We have also recently started a new project in collaboration with Nortel on the non-linear optical properties of liquid crystal devices, with particular reference to self-organisation effects in the liquid crystal material. The aim is to develop devices for use in optical communication systems.
Dr F.P. Payne
The rate at which data can be transmitted through an optical fibre is ultimately limited by dispersion. Chromatic dispersion, where the refractive index of the fibre varies with wavelength, can be almost eliminated by fibre design. However, inaccuracies in fibre manufacture leave residual birefringence in the fibre core which gives rise to polarisation mode dispersion - the optical pulse breaks into two orthogonally polarised pulses each travelling at slightly different speeds along the fibre. The speed of each pulse changes randomly along the fibre. This process has recently received considerable attention as it represents one of the ultimate limits of the fibre transmission rate. We are collaborating with Nortel on a new project to study the theory of this effect, using random coupled wave analysis of the pulse propagation along the fibre.
Dr F.P. Payne
We have continued work on chemical surface plasma wave sensors where the surface of a tapered optical fibre is coated with a thin layer of silver or gold. This work is in collaboration with Cambridge University Institute of Biotechnology. We have produced a detailed computer model of the sensor which has been used to optimise its design. This work will be described in detail in a forthcoming paper.
Dr R.G.S. Plumb
Professor J.E. Carroll
Various aspects of advanced semiconductor lasers and their applications have been studied analytically and by measurement, particularly aspects of high power DFB lasers and tunable semiconductor lasers.
Time domain modelling using fast Pentium PCs continues to be a most useful tool. A study optimising both uniform and partial grating DFB lasers for operation on 100 mW or more single frequency output has been completed(I142). This study has looked at length, coupling coefficient, facet reflectivities and phases as primary parameters, as well as numerous other secondary aspects. Self heating effects have been examined(I126) and another project has been started to look at this and similar effects in more detail, using the time domain model.
More extensive experimental and analytical work has been done on more general wavelength referencing in tunable optical systems. It appears that discriminator methods will be adequate for systems employing up to about 80 wavelengths across the 1.5 mm Erbium doped amplifier band.
While we have modelled and tested various monolithic tunable laser structures in recent years, and work continues in this direction, fabrication (and, even more, control) of such lasers is difficult. The availability of large spot lasers makes possible hybrid solutions using a semiconductor gain block plus lithium niobate optical filter structure. Extensive theoretical studies show that a robust tunable laser should be possible using this technique, though fully continuous tuning is difficult if narrow linewidths are required.
Dr P.A. Robertson
Sensor research has concentrated on the development of magnetic sensors for current probes, data read heads and position encoders, and an optical fibre system for condition monitoring of synthetic ropes.
The development of miniaturised fluxgate magnetic sensors is well under way; specialised laboratory equipment has been designed and built and a range of micro-fabricated devices has been produced and evaluated. Preliminary results show performance much improved over conventional Hall effect devices and work is underway to miniaturise the devices further and develop suitable interface and signal conditioning electronics(I134).
We have also been reporting on an optical fibre sensor system for the detection of incipient faults in synthetic parallel lay ropes to be used in civil engineering applications(I78,I136). The system uses a combination of intermodal coupling, Bragg grating reflectors and optical time domain reflectometry to measure temperature, strain and acoustic emissions along a fibre up to several kilometres in length.
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