8.3.4 Etchants and protectants: sulphur hexafluoride/oxygen plasma etching of siliconL A high etch rate requires a highly reactive etchant, forming a gaseous reaction product that we don't have to remove in a separate process. We have considered chlorine and bromine as etchants, but the reactivity series for simple radicals is F > O > Cl > N > Br > H, so we would prefer to use fluorine or oxygen. Oxides are almost always solids, with the notable exception of carbon dioxide. This makes O2 the plasma etchant of choice for carbon compounds, as the rapid etch selec
8.3.1 Fluorine-based etching of silicon Given the noxious chemistry needed to etch silicon with a liquid, it is perhaps surprising that a gas can do the job at all. However, both xenon fluoride (XeF2) and chlorine trifluoride (ClF3) gases have been used successfully for just this purpose. Each acts as a source of fluorine atoms, which are just barely bound together into molecules and are easily rearranged around silicon atoms with which they form strong bonds, turning them into inert SiF4 gas. These
7.4.3 Chemical vapour deposition (CVD) If step coverage or equipment cost is more critical than purity, then PVD is supplanted by CVD. There are many variants on the chemical vapour deposition technique, but the concept is simple: gases adsorb onto the wafer surface where a chemical reaction forms a solid product. Any other products are gases, or at least volatile liquids, and are pumped away. There is one obvious restriction: the wafer surface must be the only place where the reaction can occur. If it is not, particle
7.3 Depositing metals and alloys Metal layers are used extensively in device fabrication: to carry current for both power and signals, to apply the voltages that control transistors and generate forces for MEMS, as mirrors and optical coatings, and in magnetic devices for recording media. Different applications might require a continuous film, a long track, multiple thin layers or a plug filling a ‘via hole’ through to a buried layer. The electrical properties resulting from micro structure and composition must be contro
3.7.4 The carbon-nanotube tip A way of escaping the issues affecting process compatibility that arise from the use of techniques such as oxidation sharpening is simply to assemble the probe from separate parts – and this has been successfully done using carbon nanotubes. Single-walled carbon nanotubes can have diameters as small as 0.4 nm, but more typically they are of the order of 1 to 2 nm. This represents a great improvement on the radii of curvature achieved with oxidation sharpening. One might have thought that it
3.2 The principles of scanning probe microscopes Scanning probe microscopy is a term that is applied to a set of imaging methods based on a common element: a fine stylus. In many ways, what scanning probe microscopes do is similar to what a gramophone does. A gramophone stylus scans a spiral groove (by travelling along it) on which information has been encoded in the form of undulations in the groove wall. Side-to-side and up-and-down movements of the stylus (which is mounted on one end of a rod supported and pivoted at its centre) as it fo
2.3 The fabrication process for a MEMS Pirani sensor This section is fairly long, but is best read in one go. If you run out of time, reschedule your study to allow you to start again from here. Thin layers of material are added to the surface by a variety of means, depending on the material to be deposited, and what is already on the wafer. The sensor starts off, as so many microsensors do, with a silicon wafer, shown in cross section in Author(s):
7.2 The professional engineer It has been suggested that there are four main criteria that identify a profession: Custody of a clearly definable and valuable body of knowledge and understanding associated with a long period of training. A strong unitary organization which ensures that the profession generally speaks with ‘one voice’. Clearly defined and rigorous entry standards, backed up by a requirement to register with the profes 7.1 The engineer and society Section 2 outlined some of the needs for engineering. Society relies on engineers to create solutions to the problems involved in meeting those needs. This is a good time to pause and point out that inevitably, in return for all this fun and power, engineers have a responsibility to society. The people who employ our services, directly or indirectly, have to have an assurance that we are working within certain social, safety and ethical boundaries. Particularly given the increasing tren Learning outcomes By the end of this unit you should be able to: critically evaluate disasters and their causes, especially from mechanical or material failures; demonstrate the importance of systematic and rigorous analysis of disasters, so that future failures can be avoided or prevented. 3.3 Magnetic tape recorders Experiments showed that the use of paper tape coated with iron oxide particles significantly improved the signal-to-noise ratio and enabled a lower tape speed to be used. A plastic-based version of this magnetic tape, developed by the German company BASF, led to the development of a commercial tape recorder with audio characteristics that could nearly match those of the gramophone record, but not at an economical price. Secret work on tape recorders was undertaken by the Germans throug 3.1 Introduction I've an opera here you shan't escape – on miles and miles of recording tape. Flanders, M. and Swann, D. (1977) ‘The Song of Reproduction’ from The Songs of Michael Flanders and Donald Swann, London, Elm Tree Books and St George's Press, p. 99 Sounds, pictures, measurement data, financial statistics, personal details, etc. can all be recorded and stored on magnetic media, i.e. m 2.3 Berliner experiments with plates Emile Berliner was a young German immigrant to the USA with an interest in science. Whilst working in several menial jobs he educated himself in basic physics and chemistry, eventually building a small laboratory at his boarding house. Experiments with electricity and acoustics led to his invention of a new telephone transmitter, which he sold, enabling him to set up as a full-time inventor. He became interested in recording sound through studying a device called the phonoautograph. This appa 1 Biological materials Materials engineers have long recognised the impressive range and combination of properties offered by biological materials. Figure 1 shows some representative examples of the combination of tensile strength and toughness (measured by Young's modulus, or elastic modulus for polymers) offered by natural mat Introduction This unit examines how self-assembled structures based on lipids and proteins provide a framework for cellular processes. This unit is an adapted extract from the Open University course Engineering small worlds: micro and nano technologies
(T356). Acknowledgements Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: 5.2 Cancellation and reinforcement I have shown that a phase difference between two points in space arises as a natural consequence of the finite time it takes a pressure wave to travel between two points in space. This is not the only way in which a phase difference can arise. A phase difference can arise between two sine waves if one is delayed relative to the other. Also, almost any form of electronic sound-processing equipment affects the phase of the signal it is processing, so that what comes out is not in phase with wha 5.4 Methodology, method, technique, and tools As you engage with systems thinking and practice you will become aware how different authors refer to systems methodologies, methods, techniques, and tools, as well as systems approaches. Having just spent some time explaining what I mean by a systems approach, I now want to distinguish between methodology, method, technique and tool. Several authors and practitioners have emphasised the significance of the term methodologies rather than methods in relation to Systems. A method i 1.1 Making sense of the metaphor The metaphor of the juggler keeping the four balls in the air is a powerful way for me to think about what I do when I try to be effective in my practice. It matches with my experience: it takes concentration and skill to do it well. But metaphors conceal features of experience, as well as calling them to attention. The juggler metaphor conceals that the four elements of effective practice often seem to be related. I cannot juggle them as if they were independent of each other. I can imagine 9.9 Perspectives review Just as you were completing your rich picture, I asked you to identify and record any stakeholdings, thinking, feelings, and views about what to do. In the next activity, I invite you to do a similar exercise based on where you are now. I then want you to re-examine the notes and compare the earlier perspective against your current perspective. Expect to spend about half an hour on this activity.
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