Department of Engineering - Annual Report 1997/98

Electronic Materials and Devices Laboratory
Nanoscale Science Group
Polysilicon Thin-Film Transistors and Circuits
Silicon Microaccelerometer Fabricated using Silicon-on-insulator and a Focused Ion Beam
Silicon Microchambers for DNA Amplification Using PCR
Silicon Nitride Microchips for Holding Optic Fibres in Silicon V-grooves
Superconducting Josephson Junctions in YBaCuO Thin Films
Scanning Electron Microscopy and Transmission Electron Microscopy
Highly Integrated Electronic Systems
Electronic Instrumentation
Power Electronics
Electrical Drives
Electroheat
Computational Electromagnetics
Photonics and Parallel Optical Systems
3D Video and Virtual Reality Displays
Semiconductor Optoelectronics
Sensor Technology

References

Electronic Materials and Devices Laboratory

Research in the group continues to be centred on the deposition, characterisation and application of thin film amorphous semiconductors.

Work on Diamond Like Carbon (DLC)(J141) has expanded once more and now includes projects on the development of Tetrahedrally Bonded Amorphous Carbon (ta-C) for cold cathode emitters for application in backlights for Active Matrix Liquid Crystal Displays and for the production of Field Emission Displays themselves. The backlight work is carried out as part of an ESPRIT project, in collaboration with Sextant Avionique and CRL (Hayes) and the FED work is carried out in close collaboration with Motorola Research, Tempe. Recent results have indicated that electron emission from carbon films is a function of their surface condition and does not depend simply upon the electron affinity of the material(J88,J89,J90,J91,J92,J93,J94,J120,J132,J133) . The work on field emission is strongly supported by effort on the optical and electrical characterisation of the taC and hydrogenated ta-C (ta-C:H)(J32,J33,J59,J60,J64,J65,J66,J67,J101), the understanding of the plasma processes during growth and the modeling of the actual growth processes themselves(J95,J96). The use of ta-C, ta-C:H and C:Si as hard coatings for wear and tribological application is also ongoing in collaboration with IBM, San Jose. Work in this area is also funded by two EPSRC grants in collaboration with the Physics and Materials Science Laboratories. Scanning Probe Microscopy of ta-C and related films has also been initiated in close collaboration with the Nanoscale Science group(J1). The interaction between the group and the School of EEE in NTU, Singapore on the electrical characterisation of DLC and related materials is also continuing and a British Council funded interaction between us and the University of Limburg, Belgium and the Politecnico di Torino on ESR and PDS measurements on DLC films has also been initiated(J31).

Philips Research, Redhill continue to support work on the growth and application of amorphous silicon (aSi:H) based thin films for displays. The main thrust at present is to investigate the deposition of both the active layer (a-Si:H and microcrystalline Si)(J52,J53,J54) and the gate insulator layer (SiO_x or SiN_x) for the manufacture of Thin Film Transistors (TFTs) at temperatures compatible with the use of plastic substrates. Currently we are investigating the use of both ECR and ECWR deposition systems for this purpose. Theoretical work on the growth and stability of a-SiH based devices is also progressing and recently a project on metal induced crystallisation has begun.

The fabrication of high voltage polycrystalline silicon (poly Si) TFTs using SIPOS field plates has been expanded to include several novel structures(J2,J21,J26) and work on vertical TFTs in collaboration with Liverpool University also continues.

High Dielectric constant oxides such as barium strontium titanate and tantalum pentoxide are to be used as the dielectric layer in DRAMs and ferroelectrics such as lead zirconate titanate (PZT) and Strontium Bismuth Tantalate are to be used in ferroelectric non-volatile memories (FeRAMS). We have calculated the Schottky Barrier heights for these oxides and used this to model leakage in such devices.

Finally an EPSRC funded project on SOI based Smart MOSFET Sensors has been initiated in collaboration with Professor Gardiner at the University of Warwick.

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 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. 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.

An important application of nanoscale engineering is in the fabrication of novel electronic optical and magnetic devices. 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. 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.

In the future development of information storage technology the role of patterned magnetic media will become increasingly important. We have been studying the fundamental properties of individual magnetic structures with dimensions less than 100 nm. In addition to allowing for the optimisation of shape of magnetic nanostructures for date storage, we have also been able to design simple logic circuits based purely on magnetics.

Sensor technology based on microelectronics is a rapidly expanding field. In addition to modelling the behaviour of such sensors, applications in electrochemistry and biochemistry are being pursued.

The research activity(J74,J75,J76) is organised along three main lines: Device characterisation; Device modelling; Circuit characterisation and modelling. The Device characterisation activity aims at developing experimental and data analysis techniques to correlate the fabrication conditions with the device performance and provide input parameters for device and circuit simulation. A new fast technique for the accurate determination of the gap Density of States (DOS), a key parameter for both process control and simulation, has been developed. The technique is based on the combination of current-voltage and high sensitivity low frequency capacitance-voltage characteristics. Transient measurements have been carried out on actual transistors and a new effect named `capacitance overshoot', which is found in excellent agreement with the predictions of our proprietory circuit simulator. Our analysis of the leakage current, which employs electrical measurements in a wide temperature range (77-500K), has shown that the current models employed in commercial device simulators are inadequate, since they neglect an important effect: the enhancement of the trap to band tunnelling due to the Poole-Frenkel Effect. This has led to the formulation of a new model and to collaboration with a commercial device simulation company for the development of new software. The new circuit simulator which we have developed has been licensed to two major European industrial laboratories and has now been adopted as the preferred design tool by Seiko Epson. The above activities have been carried out in the past within the framework of a close collaboration funded by Seiko Epson. This collaboration has now been strengthened, following the foundation of the Epson Cambridge Laboratory (ECL). Three ECL embedded researchers are working with the Polysilicon Group in the Department.

Sensors and devices using micromachining technology and MEMS (micro-electro-mechanical systems) that are now in mass production include disposable blood pressure monitors for intensive care units, and accelerometers to control automobile safety airbag activation. For navigation and other applications, accelerometers with greater sensitivities have been demonstrated using the tunnelling effect for readout. However, the fabrication process is complex, involving many masks or post-process wafer assembly. In this project a simple process is used to make a tapping mode acceleration sensor using bonded silicon-on-insulator (SOI) starting material. The single crystal silicon proof mass is free-standing and held by two silicon cantilevers. A sub-micron wide gap is cut obliquely in the readout silicon beam using a focused 30 keV gallium ion beam. (FIB processing is a well-established method in the semiconductor industry for device failure analysis and TEM sample preparation, but has not yet found widespread application in direct device fabrication.) The structure functions as a microswitch when an ac voltage is applied between the proof mass and the substrate. When the proof mass moves upwards or downwards due to an acceleration force, there are changes in the size of the readout gap and in the duty cycle of when the gap is closed. The device is operated in a feedback mode to maintain a constant duty cycle, and the dc electrostatic feedback voltage required is a measure of the acceleration. This approach to micromachining sensors using SOI wafers to make complex structures with the minimum of assembly has many other possible applications(J44,J45).

The miniaturization of analytical devices using micromachining technology is destined to have a major impact on the medical and bioanalytical field. DNA analysis has become an important analytical method for the detection of diseases, in forensics and for large scale gene mapping. The polymerase chain reaction (PCR) involves 25-30 repeated heat cycles: the double stranded target DNA is separated into single strands at high temperature (~90°C); Oligonucleotide primers that flank the DNA region to be amplified are then annealed at low temperature (~50°C); these DNA strands are extended in the presence of a polymerization enzyme and deoxynucleotide triphosphates at ~70°C. Each cycle doubles the amount of DNA. This project focuses on the construction of miniature reaction chambers and heating blocks designed to allow the temperature of PCR reagents to be changed rapidly. The use of silicon allows precise micromachining of components with well defined thermal conductance. Bulk micromachined silicon reactors and transparent silicon nitride membranes with thin film platinum heaters and temperature sensors give a system which has a thermal time constant of less than 1 second and the possibility of optical monitoring of reactions by transmission. Rapid temperature cycling and control at low power is achieved and there is close agreement between the simulated and the measured thermal properties. The fabrication process is simple, and there is potential for low cost manufacture. Rapid temperature cycling and small size are promising attributes for portable high efficiency analytical systems, and the PCR reactor could be a disposable part of a diagnostic system(J42,J43).

The use of optical fibre rather than copper wire in telecommunications has the advantage of reduced cable costs, increased data integrity, and higher bandwidth. Traditional optical device packaging, however, involves the complex assembly of miniature components, with requirements for extreme alignment accuracy. Fully integrated optoelectronic systems will only reach mass markets when production and assembly processes become cheap and reliable through novel assembly techniques. In this project silicon micromachining has been advanced to allow us to use flexible silicon nitride clips to hold optical fibres kinematically in grooves etched anisotropically into silicon substrates. Good light coupling properties between two fibres buried in the same V-groove have been demonstrated, and clipped fibres have passed the standard environmental and reliability tests for telecommunications applications. In a second development of these devices, raised clips are made with the fibre core above the silicon substrate using two masks and a two-stage etch process, and light coupling has been demonstrated between a semiconductor laser and a fibre. Potential uses of these clip technologies include fibre connect, Fabry-Perot devices, waveguide sensor and telecommunication applications(J9).

High temperature superconductors can operate in liquid nitrogen (77 K) which is a much more accessible temperature than conventional superconducting electronics based on niobium in liquid helium at 4.2 K. The aim of this EPSRC-supported project in collaboration with the University of Surrey is to make manufacturable devices in thin films of Yba₂Cu₃O₇ with potential applications in high performance circuits and sensors such as superconducting quantum interference devices (SQUID). Weak links must be made in superconductors to form a controllable Josephson junction, and the materials challenges with the YBaCuO superconductors are more severe than with conventional materials because of the short coherence length. Electron beam irradiation using 300 keV electrons focused to a five nanometre spot is very effective in producing the necessary localized damage and prototype devices, and the interferometer is defined by focused ion beam cut slots in the superconductor. The Josephson junctions are uniform in current density across the 300 nm width, they respond in the expected way according to the RSJ model to an applied external magnetic field or to excitation with microwaves, and they are stabilized for storage at room temperature by first overdosing with electrons and then annealing at 400 K before use. However, a long fabrication time is required because it is a serial process (about one minute irradiation per micrometre length of junction made), so other potentially high throughput damage methods of fabrication are being investigated, including the use of an ion implanter of the type used in the semiconductor industry(J7,J8,J108,J109,J140).

Research continues into the application of knowledge-based techniques to two key scanning electron microscopy tasks. A new knowledge-based system, XpertEze, has been developed to assist microscopists in the task of instrument control. This is showing considerable promise. Efforts have so far been concentrated on development of novel automated algorithms for assessment of image quality. Future work will involve adapting ExpertEze for intelligent control of remotely sited microscopes, and to develop an embedded version for use in production instruments(J14,J15). Research 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(J12, J13). Another recent activity centres upon the development of a `virtual' scanning electron microscope, implemented in multimedia form under software control. This is expected to have significant impact in education, training and simulation.

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 based on this system is now nearing completion. Plans are being made to explore the potential of this approach in the field of wireless optical communications.

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.

The voltages of units in the group still have to be related to the National standards and an improved way of doing this has been reported(J135). Several recent calibration visits show that the mean voltages of the units here are changing at a rate of -1.05 ppm (parts per million) per year and that the uncertainties of the group at any time in the year should be less than 0.6 ppm.

A new research programme on power and high voltage integrated circuits, supported by EPSRC, has started with the aim of producing integrated circuits for motor drives. The work on power electronics complements contributions from collaborators on devices and processing. Particular topics include the study of charge pump circuits for high side power supplies and gate drive circuits for the control of the rate of change of the output voltage of half bridge circuits.

Experimental work has continued concerning the operation of IGBTs connected in series. The sponsor is Hill Graham Controls. A new test rig is under construction and partners are being sought for power systems applications.

Computer simulation of novel vertical multiple-mode power semiconductor devices continues(J102), with the work being extended to include devices employing a single MOS gate controlling various areas of the device. The study was sponsored by Siemens ZFE. The PSPICE modelling of these devices has reached a stage where the models reproduce the terminal characteristics of the devices with great accuracy. The models employing a direct solution of the Ambiplor Diffusion Equation within PSPICE(J61) (EPSRC funded).

Measurements regarding the performance of high current multi-chip IGBT modules have continued. The non-invasive current measurement method has been used to establish the levels of non-uniform current sharing experienced by a module under various gate drive conditions(J103). The current sharing is being analysed using computer simulation of the devices. This work is EC funded, with 16 partners. Our direct collaboration is with Mitel Semiconductors and Siemens ZFE.

The research programme on drives for domestic appliances has led to a further demonstration washing machine driven by a three-phase, inverter fed motor. This machine has been subjected to acoustics tests at Hotpoint Ltd and the low noise properties of induction motor drives have been confirmed. The calorimeter constructed as part of this programme has been calibrated and the absolute accuracy is now better than 1 W and the calorimeter can measure up to 600 W(J78). Measurements of motor and inverter losses by this means are taking place.

Work has started on induction motor drives with high phase numbers, with emphasis on six, nine and twelve phase drives. The work is being pursued with Edwards High Vacuum International.

Numerical modelling in computational electromagnetics continues to be the focus of the research activity at the Electricity Utilisation Group. Software codes are nearing completion for implementing the EUG in-house microwave heating code using a parallel computer or a cluster of workstations or PC's(J77). Finite element flux corrected transport software code has been written for solving the particle continuity and Poisson equations for a needle point to plane electrode configuration in air at atmospheric pressure and at DC(J55,J56,J57,J58). The software is now being extended to 2D and to radio frequencies for investigating the deleterious effects of corona type discharges in industrial applications. The work on hybrid numerical methods for characterising industrial microwave applications and modelling of thin films continues.

Modelling of the electrical circuit used at 13.12 MHz and 27.12 MHz has been extended using an eigenvalue technique(J97). The work on coupling radio frequency to heat pump technology for drying particulate materials has been completed(J79,J80). A review paper on drying using radio frequency and microwave techniques has been presented(J87) as well as a paper which outlines the essence of unification of electroheat as an academic discipline(J86).

The Group continues to be the strategic and administrative centre for the AMPERE organisation(J82) and publishes its quarterly Newsletter(J83,J84,J85).

Computational Electromagnetics

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(J51,J68); 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; modelling and optimisation of small multiphase induction motors.

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.

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, optically addressed structures based on amorphous silicon(J165,J166) and custom SLMs for running research projects. We are particularly interested in very high speed liquid crystal modulators(J22,J23,J24,J30) and liquid crystal devices for manipulating phase(J157).

The Group has produced a range of new devices including the fast bit plane SLM (320 ´ 240 pixels with a frame rate in excess of 20,000 frames per second)(J158), a similar 640 ´ 480 pixel device and novel phase modulated SLMs, all using silicon backplane technology. They are designed for use in filtering and switching optical signals and for applications as microdisplays. These devices have been the `enabling technology' for many of the applications discussed below.

Work is widely supported by EPSRC, DTI through LINK programmes, by DERA and by industrial partners, including Nortel, British Aerospace and Thomas Swan and Co Ltd.

The EPSRC project Parallel Optoelectronics 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(J27,J28,J81,J106,J107). 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. 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(J28).

As a result of the POETS project, there was a demonstration of the role of optics in asynchronous transfer mode (ATM) telecommunications systems(J154). This follows on from the theoretical work into the way that optical interconnects might modify the architecture of electronic ATM switches and their role in telecommunications systems. An optically accessible silicon VLSI memory (optoRAM) was demonstrated in a packet switch structure in which 9,000 optical channels were incident on a single silicon VLSI chip. A version of this switch structure using vertical cavity surface emitting lasers is being investigated under the project `VCSEL based Very High Capacity Photonics Packet Switch' (VIVALDI), under the UK EPSRC Optical Systems Initiative.

A project which has been spawned by POETS is the DTI funded reconfigurable optical switches for aerospace and telecommunications systems (ROSES)(J138,J139,J159). This collaboration project has built the first operational fibre to fibre switch using holographic beam steering. This is now seen by our industrial partners as a potential solution to the problems of providing large scalable optical switches for the telecoms transmission network.

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 and how they can be applied to optical systems. Collaborative work(J112,J113) is looking at the application of smart pixel structures to signal processing using wavelet transforms.

A new type of optical correlator for pattern recognition has been demonstrated(J155,J156,J160,J161). This is also based on devices using ferroelectric liquid crystals integrated with silicon VLSI. The new architecture is an attempt to successfully combine electronic and optical processing in a compact and practical structure.

Finally, a new class of liquid crystal display has been developed within the group, known as a photo-luminescent liquid crystal display (PL-LCD)(J40,J41,J136). 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). In collaboration with Screen Technology Ltd prototype monochrome and colour displays have been demonstrated operating at video frame rates. If developments continue at the present pace they will result in major improvements in the viewability and size of liquid crystal displays to meet the increasingly stringent requirements of the new markets that are emerging for emissive flat panel displays in entertainment and information technology systems.

3D Video and Virtual Reality Displays

A video display capable of screening a three dimensional image like that of a hologram has been licensed to a local company who have now completed a display with a 42" screen for the amusement arcade market. Meanwhile work continues in the department on a flat panel version of this display which comprises a screen divided into lines. One line is selected at a time, and light projected parallel to the screen is ejected at the selected line. The three dimensional image is multiplexed line by line, and since no thin film transistors are needed it should be possible to build a large screen cheaply. A wide field of view is also anticipated.

Similar principles can be used to make a head mounted display which will be completely flat, so we are trying to make a virtual reality display which will be the same shape and dimensions as a pair of spectacles.

Work has continued on several refinements and extensions to laser models, plus experimental work concerned with both monolithic and hybrid tunable lasers. A research monograph(J16) on DFB laser modelling has collected several years' work together.

Hybrid tunable lasers using lithium niobate optical filters and large spot semiconductor laser "gain blocks" continue to look as though they may be viable for some applications: however various extra problems with these devices have been identified. In particular, a two stage optical filter is almost essential, and this increases the losses in the hybrid laser cavity. A version of the time domain laser model is being used to simulate extended cavity tunable devices, and confirmatory experimental results are being collected using a simple grating based external cavity. The modelling here is more advanced than has been attempted elsewhere, and has helped with the understanding of several effects previously thought to be experimental anomalies.

Dedicated work on laser modelling has resulted in a more self-consistent approach, with improved handling of particle and energy balances. In particular, the effects of shaped (rather than flat) absorption and recombination profiles have been investigated, and while these effects are small in standard DFB lasers, they can be significant in laser amplifiers, and possibly in some Fabry Perot lasers.

A more detailed model for Fabry Perot lasers including internal small reflections has been developed, which now includes refractive index changes via carrier injection (and Henry's alpha). This has allowed the tuning characteristics of such simple lasers to be modelled, as well as their side mode suppression ratios over a realistic and complex range of drive and temperature conditions. Work to confirm and extend these results experimentally has started.

During last year complex computer systems were developed to investigate experimentally the behaviour of multi-contact monolithic interrupted grating lasers. This work has shown that to characterise these lasers, it is probably possible to identify key points in the multi-dimensional current/power/wavelength domain, and reduce the measurements required to a feasible few thousand points, rather than the 10⁸ or so if "brute force" characterisation is used. The complex structure of these lasers makes them difficult to model, but recent work has shown promising results from using the time domain technique on these lasers: cheap memory and faster operation from personal computers are helping here, but the run time is still several hours, compared with a minute or two for standard DFB lasers.

Sensors form a bridge between the real world and electronic systems. In recent years there has been a move towards the miniaturisation of a range of sensors, including accelerometers, pressure transducers and flow sensors. The production of these devices, often on silicon and in combination with interface electronics, enables devices with improved performance at lower cost to be realised.

In this department we are working with micro-fabricated magnetic sensors for a number of applications including instrumentation, data read heads, field probes and position encoders. We are currently developing miniature flux-gate sensors(J124) with a sensitivity and response frequency well above those of standard Hall effect devices. This has been achieved through the application of state-of-the-art materials and fabrication methods together with the design of novel drive and signal processing electronics.

We have also reported work on an optical fibre sensor system for the detection of defects in long synthetic ropes for structural applications(J126). The system uses a combination of methods to monitor strain, temperature and acoustic emissions along a length of optical fibre.

Electrical Engineering References

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Last modified: October 1999