Electron Beam Nanolithography
Focused Electron Beam Irradiation Josephson Junctions
Micromachined Sensors using Bonded SOI Silicon Wafers
Scanning Probe Microscopy
Thin Film Laboratory
Polysilicon Thin-Film Transistors and Circuits
Scanning Electron Microscopy and Transmission Electron Microscopy
Highly Integrated Electronic Systems
High Brightness Electron Guns
Low Temperature Measurement of Dielectrics
Photonics and Parallel Optical Systems
Prof. A.N. Broers
The ESPRIT programme investigating processing techniques on the nanoscale has continued during this year. Work has been done on the potential resolution limits of electron beam lithography on self-assembled monolayers as well as using novel phase-separated layers as resist layers. The study and use of more conventional resists in the fabrication of nanoscale device structures has also continued. High resolution patterns have also been prepared for IBM Zurich to enable an understanding of the limits of a new lithography process to be gained.
The group is also moving towards being able to do in-situ testing of nanoscale devices while observing the effects both in electron microscopes and atomic force microscopes.
The EPSRC project on SiGe has entered its last year with a number of interesting insights into the properties of electron-beam-irradiated SiGe heterostructures being gained(H7,H8,H9,H10,H49,H109,H146).
Dr D.F. Moore
Dr A.J. Pauza
Dr M.G. Blamire
Collaborative work between the Engineering Department, Materials Science and the Superconductivity Interdisciplinary Research Centre has seen progress in understanding and controlling the properties of focused electron beam irradiation (FEBI) at around 350 keV to fabricate weak link Josephson junctions in YBaCuO thin films and so prototype electronic circuits operating at 60 K(H74,H89). In collaboration with GEC Marconi, Siemens, CEA Grenoble, and the University of Ilmenau, the Engineering Department and the IRC Cambridge have just completed an ESPRIT project on high-temperature superconducting devices and performance comparisons with conventional devices. Recent device investigations include producing a pair of FEBI junctions 50 nm apart with a control electrode between them for current injection(H5). A related conference on Future Nanometer-scale Electron Devices (FNED) was hosted in Trinity Hall 1-4 July 1996 concentrating on potentially manufacturable devices. The indications are that Si circuits will dominate the device technology field for at least the next decade but superconductors and single electron transistors may have a future role.
Dr S.C. Burgess
Dr N. Shibaike
Silicon microfabricated devices are being developed for reliable low cost sensors including accelerometers with proposed tunnel junction readout(H51,H73). Using bonded silicon on insulator (SOI) wafers as the starting material, novel structures are made by conventional surface micromachining followed by focused ion beam etching through 3 micron thick Si cantilevers at oblique angles to form submicron gaps to be closed by electrostatic actuation(H89). The collaborative work between National Panasonic, the University of Tokyo and the Engineering Design Centre, Cambridge, has concentrated on the design, materials and fabrication of microsystems. The major challenge at present for the accelerometer is to compensate for the effect of stress in thin films, including those in commercially available SOI wafers.
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 this involves the use of scanning probe microscopy; a technique which combines atomic spatial resolution with measurement of local electronic, optical, magnetic and tribological properties. Typical current applications include semiconductor growth, high temperature diffusion and kinetic studies, plasmon-enhanced photoemission, molecular manipulation and nano-tribology. There is also a substantial effort into developing silicon based micromechanical sensors. A recent demonstration of femtojoule energy detection in isolated systems has led to further growth in research in this important technological area(H11,H12,H36,H56,H58,H78,H79,H80,H81,H122,H123,H131,H132,H133,H134.H135) .
Dr W.I. Milne
Dr J. Robertson
The Thin Film Group works primarily in the areas of diamond-like carbon, amorphous silicon and Si high voltage devices.
Work on diamond-like carbon is expanding, to develop electron emitting cold cathode films for field emission displays and hard coatings for applications such as magnetic disks. The electron emission films are being developed in collaboration with Motorola and recently with CRL, Feltham. The group has shown that nitrogen doped tetrahedrally bonded amorphous carbon (ta-C) is an efficient electron emitter(H70,H71,H84,H85,H104,H105). This is supported by considerable effort on the characterisation of the electronic, structural and photoluminescence properties of amorphous carbon(H17,H30,H72,H100,H107,H108,H111,H112,H114-H117,H130,H142,H145) , the development of models(H99,H101), and understanding of the plasma beam deposition process(H4,H18), and the deposition of thin film diamond(H40,H41,H112). The first ta-C thin film transistor (TFT) was fabricated(H20,H21). The use of ta-C as a hard coating material is carried out with numerous collaborators, including IBM San Jose(H92). 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 continues on the development of microcrystalline Si for TFT applications, in collaboration with Philips Research Laboratories, Redhill. Atomic force microscopy is being used to understand the nucleation and growth processes of amorphous and microcrystalline silicon(H36,H106). Amorphous silicon has recently been grown by the filtered cathodic vacuum arc(H1,H2,H3,H4), capitalising on our experience with this technique for diamond-like carbon. There is continuing theoretical work on instability mechanisms in amorphous silicon TFTs, with Philips(H25), and a contract to understand how amorphous silicon 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(H16,H110). This process allows both high voltage and low voltage TFTs to be placed on the same wafer. The project with Ford UK to develop high power MOSFETs for car electronics has been very successful, with the production of two patents, and ends in March 1997.
Work in collaboration with Sandia National Labs has been to develop ferroelectric materials for non-volatile memories(H102,H103,H128,H129).
Dr P. Migliorato
This activity(H22,H68,H69,H90,H94) is in close collaboration with Seiko Epson and is fully supported by this company. 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. In addition to measurements of the static I-V characteristics in the range 77-500 K, transient I-V measurements have been set up and applied to the study of trap kinetics. Hot carrier degradation effects are under investigation through a combination of electrical measurements and 2-D simulation. The simulations make use of MEDICI software, supplied free to the team by TMA. A new model for phonon assisted tunnelling from traps has been developed, resulting in accurate simulation of the transistor leakage current. The distinguishing feature of the model is its ability to account for the Poole-Frenkel lowering of the tunnelling barrier, which is a particularly important effect in materials with a continuous density-of-states. Inclusion of the model into MEDICI is being organised with TMA. Work is continuing on the validation and upgrading of PSIM, our proprietary circuit simulator for polysilicon TFTs. Excellent agreement has been obtained between simulation and experiment by using circuits fabricated by Seiko Epson. The software has been licensed to Philips, Thomson CSF and Seiko Epson.
Dr D.M. Holburn
Work has continued on the development of novel control techniques for the scanning electron microscope. Considerable progress has now been made in remote control of the SEM by means of the Internet, with successful control of the microscope in CUED carried out from sites in the USA and elsewhere(H6).
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. The European Union has recently designated the disposal and re-use of end-of-life electronic products as a matter of priority. Investigations have indicated that Chiprack may offer significant advantages in the design for disassembly and re-use of electronic components or sub-assemblies(H48).
A new initiative is now under way to investigate techniques for non invasive monitoring of human activity. A key objective is to develop a compact device capable of recording heart rate and movement patterns over extended periods. This information is to be used in epidemiological studies including coronary heart disease and nutritional studies investigating the apparent rise in prevalence of obesity in UK and USA populations. This work is being undertaken in collaboration with Department of Community Medicine and MRC Dunn Clinical Nutrition Unit.
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.
Prof. S. Williamson
Dr A.C. Smith
Work on electrical machines continues to be focused on the induction motor and its variants. Much of that work relates to the further development of a design-office orientated finite-element model(H140) which has been extended to allow for double-cage rotors(H137) and frame losses(H120). The double-cage work led to a modification to the usual equivalent circuit(H42). Other finite-element work makes use of a time-stepping model, and has been used to investigate the effect of closed stator slots on acoustic noise(H33), and the distribution of high frequency iron losses(H59).
Formal optimisation techniques have been coupled with motor models to optimise the design of a cageless reluctance synchronous machine(H50) and the shape of a rotor slot(H139). The same techniques have been used to explore the performance limits of a pulsed electromagnetic projectile launcher(H141).
Unbalanced magnetic pull has been investigated using analytical models. Work on three-phase motors(H29,H118) focused on machines in which the rotor position is asymmetric, whereas work on single-phase motors dealt with symmetrically positioned rotors with electrically unbalanced cages.
Dr A.C. Metaxas
Numerical modelling in computational electromagnetics continues to be the focus of the research activity at the Electricity Utilisation Group. The work is being supported by Eastern Electricity(H41,H63) and software codes are being written for Unilever Research(H26,H27,H28). The work is being extended so that the existing codes can be used in conjunction with the Hitachi parallel computer facility recently installed at the University. This will enable the group to tackle industrial scale problems of radio frequency and microwave heating at the Industrial Scientific and Industrial frequencies of 13.56 MHz, 27.12 MHz, 900 MHz and 2450 MHz respectively. Further, this will also enable the group to model the instigation of radio frequency coronas which require the simultaneous solution of particle continuity, Poisson and Maxwell equations.
Modelling of the electrical circuit used at radio frequencies has been carried out using a Saber simulation package to determine the electrical operating parameters of the system(H76). A new method has also been put forward using a network analyser in preliminary matching experiments involving radio frequency heating systems to ensure the attainment of very high power transfer efficiencies. The work on coupling radio frequency to heat pump technology is nearing completion with extended trials under way on various drying scenarios.
The Group continues to liaise with external companies, it publishes the quarterly Newsletter AMPERE(H63,H64,H65) and has formulated its ideas of teaching electroheat using a unified approach(H66,H77).
Dr T.J. Flack
Work on the three-dimensional finite-element modelling of induction motors, under an EPSRC grant, is nearing completion. A three-dimensional finite-element frequency domain model has been developed to compute the losses in stator duct spacers(H32). This work has highlighted their significance on stray load losses in induction motors. The model has been extended to enable the determination of clamping plate losses, and the calculation of stator end-winding leakage reactance.
A feasibility study on the use of SQUID sensors for the electromagnetic non-destructive evaluation of reinforced concrete is being undertaken. The aim of the work is to determine whether the degree and location of corrosion damage to the reinforcing steel bars can be found using electromagnetic measurements. Preliminary results using an idealised test-rig involving a single steel bar are promising.
Dr P.R. Palmer
Dr R.A. McMahon
The experimental work on the series operation of IGBTs has achieved three 1200 V, 400 A modules working in series and is now sponsored by Hill Graham Controls. To reach these levels in practice, some modifications were made to the approach published earlier(H43), which resulted in a Patent Application. Turn off and turn on at near rated currents was demonstrated(H82). The 10 kV, 1200 A test rig is under construction and partners are sought for the application to power systems at higher voltages.
First samples of a novel dual-gated thyristor structure, the MBGT, were received and tested. The results confirmed the earlier simulation results and proved a significant step in the analysis of the operation of these devices. The 350 V MBGT exhibits the low on-state voltage of a thyristor and the fast switching capabilities of an IGBT(H47,H83). Computer simulation of full scale devices continues and vertical test structures of around 50 A capability are in process with Westcode Semiconductors and Cork NMRC. A Patent Application was made for the MBGT and further commercial partners are being sought.
Work on the performance of high current multi-chip IGBT modules commenced, using the non-invasive methods applied earlier to high current GTOs. The method produced very satisfactory results and a programme of developing the method is under way. This work forms a part of a very large EC project, with 16 partners. Our direct collaboration is with GEC Plessey Semiconductors, Siemens ZFE and University of Dortmund.
An integrated power switch, comprising a power MOSFET with monolithically integrated drive and level shifting circuitry has been shown to work satisfactorily under conditions found in half bridge converters operating at 13.56 MHz and supplied from rectified mains. The level shifting circuitry has been shown to function as designed(H14). A further batch of devices is being fabricated. The class E circuit has been developed and increased power output has been obtained by parallel operation(H124). A new state-space analysis of the circuit has greatly aided design. Work has begun on characterising IGBTs which have been specially prepared for use in inverters for small drives. The use of IGBTs and MOSFETs in parallel as a switch is also being explored.
Dr R.C. Healey
Prof. S. Williamson
Dr R.A. McMahon
Work on electrical drives has centred on the development of an advanced induction motor model(H119) which has been incorporated in a vector controller to achieve improved performance(H138). This motor model is now being further enhanced by including a thermal model to update machine parameters as the machine changes temperature. A spin-off of this research has been the development of an accurate method for measuring transient torques using a piezo-electric force table(H46).
The relative merits of the universal motor and three-phase induction motor for service in domestic appliances have been assessed(H95). Alternative winding and inverter arrangements for the induction motor offering the promise of lower cost are being investigated. The performance of one option, employing a bifilar wound motor has been reported(H44). An EPSRC grant has been awarded to study the induction motor and inverter as a package for domestic appliances.
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, improvements and precautions in doing this are continuously developed.
The values of the 10 V standards are traced to the volt maintained at the National Physical Laboratory (NPL) by taking four units to be calibrated there each year. Most recently the method of doing this comparison has been refined and a way of removing the error due to any unit being affects by travel, temperature change, etc, is now used(H121).
Dr P.A. Robertson
Research is continuing into the development of an optical fibre sensor for the distributed measurement of temperature, strain and acoustic emission along synthetic ropes and within structures, where a single optical fibre can extend to over a kilometre. The detection method utilises a combination of inter-modal coupling and reflections from Bragg gratings written into the fibre.
Work is also continuing on the modelling and fabrication of miniaturised electromagnetic sensors using non-linear magnetic thin films and photolithographic processing techniques. A demonstration sensor has been fabricated with sensitivity and bandwidth values exceeding those of a conventional Hall effect sensor by a significant margin. Potential applications for these devices include magnetic field probes, electronic navigation equipment and current transducers.
Dr S. Evans
The dielectric properties of five different CPA (cryoprotectant agents) used in medical cryobiology have been measured in aqueous solution concentrations from 15% to 80%. Measurements cover 27 MHz to 3 GHz and temperatures from -80°C to +30°C. Work continues on complete perfurates containing ionic components at the appropriate level for clinical use. It is clear that if higher concentrations of CPA than are used at present could be tolerated, then perfurates could supercool without ice crystal formation. This is advantageous in the electrical properties as they affect the stability of rapid electromagnetic heating and advantageous in avoiding cell damage due to ice crystal formation(H31,H67).
Dr R.G.S. Plumb
Prof. J.E. Carroll
Time domain modelling using fast Pentium PC computers has continued to evolve as a most valuable tool, with advanced work on low chirp push-pull lasers, very high power single frequency lasers at 1.5 mm, and effects of spurious injected power into DFB and tuneable lasers.
High power "standard" DFB lasers have been modelled extensively, with a full range of facet phases and reflectivities and including longitudinal variations of carrier and gain profiles. The results show significantly better agreement in the predicted "yield maps" and those obtained from real manufactured devices. Yet further improvement should theoretically be obtained for very high power lasers with diffraction gratings over only part of their length and work is continuing to resolve differences between observed and experimental results. Time domain modelling has also been used to model and explain the sensitivity of DFB lasers to single and multiple wavelength signals injected close to the modes of the laser. This has important implications for wavelength division multiplexed (WDM) systems.
Experimental work on a 4-section "super-structure" grating tuneable lasers has been extended to include their modulation characteristics: essentially, low speed amplitude modulation is possible, but conditions and limits are critical. Algorithms for computer control of the wavelength and power of these lasers have been written, although present devices need individual adaptation of their control parameters. It was found possible to excited 1.3 mm and/or 1.5 mm light simultaneously from the same waveguide in suitably ion-beam etched versions of these lasers. This work is being patented. The control electronics for tuneable lasers has been modified to allow measurement and evaluation of Grating Assisted Vertically Coupled Filter (GAVCF) type tuneable lasers supplied to us by BT labs.
Buried heterostructure lasers used as high power pump lasers for erbium doped fibre amplifiers have been subjected to electrostatic discharge (ESD) pulses while operating, and analysed for signs of damage. These lasers proved to be surprisingly resistant to ESD, improving yet further when biased than when cold. The nature of damage observed indicated the presence of optically reflecting and absorbing defects after ESD pulses at 6 - 12 kV(H34,H35,H38,H39,H52,H53,H54,H55,H57,H93,H113).
Prof. W.A. Crossland
Dr R.J. Mears
A continuing group interest is in highly parallel opto-electronic systems based on free-space optics, with spatial light modulators using ferroelectric liquid crystals. Smart pixel systems are being designed and built which integrate ferroelectric liquid crystal light modulators into silicon VLSI circuitry(H19,H23,H24,H61,H62,H86,H87,H88,H91,H127,H136) .
The photonic device fabricating clean room facility is functioning to allow the fabrication of spatial light modulators based on ferroelectric liquid crystals and silicon VLSI chips, and amorphous silicon photoconductors and is extending to cover processing on semiconductor lasers and related opto-electronic integrated circuits. Ferroelectric liquid crystal devices are being investigated for applications on autostereoscopic displays and space switching of optical beams in optical systems described below.
Work is widely supported by EPSRC, by DTI through LINK programmes and by ESPRIT, DRA and Industry.
(i) A space-wavelength testbed network has been set up under the EPSRC project "Optical Switching Testbed", in a collaboration with the Computer Laboratory. A space-wavelength crossbar employing novel liquid-crystal switches was constructed and evaluated over the University optical fibre network. Work under the EPSRC project "Parallel Optoelectronic Telecommunication Systems" has included the demonstration of a new tunable holographic filter and laser based on reconfigurable holograms written in ferroelectric liquid crystals.
(ii) An opto-electronic chip set using silicon VLSI and ferroelectric liquid crystals was designed and built on the LINK project "Smart and Advanced Spatial Light Modulators" for an analogue neural network incorporating learning, and work continues to implement these systems. Another LINK project "Optically Connected Parallel Machines", a 64 x 64 fibre optic switch with a switch bandwidth of 64 Gbit/s has been completed during last year. A new LINK project entitled "Fast Switching in Chiral Smectic Liquid Crystals" has been developed.
(iii) An ESPRIT project has been concerned with an optical correlator for road sign or face recognition, for example. Novel methods of achieving scale invariance and of discriminating against closely correlated false targets using computer-generated phase-only filters have been devised and demonstrated.
(iv) An ultra-fast image-generating spatial light modulator called the fast bit-plane device is being built under a contract with DRA Malvern. It aims to produce several hundred 320 x 240 pixel planes per video frame.
(v) A novel flat panel display technology is being developed in partnership with industrial sponsors. This uses liquid crystals, but has the advantages of an emissive display technology.
Dr A.R.L. Travis
Development continues of a video display which shows a moving colour three dimensional image(H75,H125). 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 continues on miniaturising the display by designing liquid crystal modulators with high frame rates(H143,H144). A frame rate of at least 500 Hz has been demonstrated and we hope to reach 2 kHz: enough for a VGA 3D image with 60 degrees field of view.
A technique based on confocal microscopy has been shown to be able to compress a 3D image by a factor 5 to 20 times greater than MPEG, and demonstrations of this in real time are the next step.
Electrical Engineering References