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2.3 Summary and conclusions

This topic has addressed the question ‘What is modern engineering?’ The conclusion must be drawn that, until recently, engineers were content with fairly simplistic definitions of their profession, thinking that it consisted of little other than craft skills or practical experience grafted on to a knowledge of mathematics and appropriate natural sciences. It has been methodologically naive, and definitions of the processes of engineering either lack detail (
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1.1 Introduction: what is the problem?

In late June and early July 2005 a row erupted concerning the operation of a major flagship of government social policy, the tax credit system. Introduced in 2003, it was designed to help those on low incomes and whose social circumstances prevented them from working full-time (Citizens Advice Bureau, 2005). The article reprinted in Box 1 indicates the extent of the political unrest with a system that left families relying on food parcels, and that has been variously described as being ‘in
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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
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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
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3.5.2 Non-contact (tapping) mode

If the probe is taken further away from the sample, into the region beyond the maximum on the force-separation curve of Figure 7, it can be operated in non-contact mode. Typically, the separation required is of the order of 10 nm. The attractive force is much smaller than the repulsive force of the contact mode
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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): The Open University

1 Structural devices: a static role

The superb manufacturing techniques of microelectronics enable designers of integrated circuits to exercise complete control over the electrical characteristics of each component, such as a transistor, by specifying the shapes and sizes of their active regions. Using photolithographic mask-drawing software on their workstations, they can copy and paste blocks of identical devices all over the chip, knowing that when the design is finally realised in silicon, this extreme uniformity wil
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Learning outcomes

After studying this course, you should be able to:

  • understand how to relate physical dimensions and materials properties to static and dynamic behaviour

  • demonstrate an awareness of how small features are cut out in solid materials, and how small features are built up in solid materials

  • describe the piezoelectric effect and its use for producing small-scale movement in mechanical devices

  • state the relative significance of different for
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Acknowledgements

This free course is adapted from a former Open University course called 'Engineering: mechanics, materials, design (T207).'

Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommerc
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7.4 The impact of technology on society

Engineering is apparently driven by the needs of society. The technology that results, in turn, drives other changes in our everyday lives. One of the basic needs identified in Section 2 was for shelter. There are many fine examples of long-surviving structures such as pyramids, aqueducts, bridges, walls, functional buildings, and so on. Remarkably these constructions were completed without the depth of analysis and understanding that is available today (though we don't necessarily know much
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6 A problem with sensors

The problem we will look at in this section concerns the analysis of the design of a component used in cars that are fitted with airbags. The airbag has to be inflated rapidly when an electronic circuit in the system decides that a serious collision is taking place. The crucial component in the electronics is the accelerometer, which therefore has to be extremely reliable. Motor manufacturers have turned to a technology called MEMS (micro-electromechanical systems) for these accelerometers, b
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4.6 Assess and review

Following our problem-solving map, we have reached the stage of 'assess and review solution', Figure 16.

Figur</span><br><span class=Author(s): The Open University

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4.5.2 Physical models

A physical model of an artefact or component is often built on a reduced scale, in size and/or by using materials that are cheaper and easier to manipulate than those intended for production. At this stage, we are not necessarily producing what you might think of as a prototype, but investigating particular aspects of the design. For instance, maybe we would produce a racing-bike frame to a new design but in a cheap material such as balsawood, in order to assess the air flow around it in a wi
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4.5.1 Mathematical models

Computers in the last few decades have, in many cases, made mathematical modelling a lot easier. Models that used to require hours of manual cranking through long equations can now be created on a screen using specialist software. Processes can be recreated – modelled – in the time it takes to press a few buttons.

For example, when designing a pipe network to carry a gas or fluid, such as in the village water supply problem, you might wish to know how the flow would be distributed w
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4.5 Model the best solution

In moving from the 'possible solutions' to the 'best solution' box, Figure 12, we have to assume that a certain amount of evaluation has been done in the previous loop. The solution is still on paper, and probably not much more than a sketch, but something is badly wrong if the best solution to come forward has n
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4.4.1 Selecting the best candidate

Assuming there is more than one likely looking candidate solution, we need to make a selection now so that we don't waste time taking all the candidates through the next steps, which become progressively more expensive and time-consuming. The rigour and formality of this step is very variable, but in general all schemes boil down to the same process you might use to choose some consumer item, such as a TV set. You would have a list of criteria that are important to you, and you would evaluate
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4.2 From a need to a problem

So, working from the top down, the process starts with 'need' and 'problem'; see Figure 8.

Although we usually work by identifying a need that converts to a problem, that requires a solution, don't forget the extra arrow at the side, taking this first part of the process full circle. The questions that draw
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4.1 Advancing knowledge

Over the centuries, engineers have faced and solved a huge number of problems of one sort or another. Each time a problem is solved, knowledge is advanced, something usually gets written down, and so today we have a wealth of experience to draw on. Equally, problem-solving techniques have also been developed and evolved through use and refinement, which is rather handy. Not only do we have some idea of existing solutions to similar problems, but we also have an indication of how to go
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2 Where does the need arise?

There is a rather obvious question that has to be raised at some point, so we may as well get it over with now: Why do we present ourselves with all these problems? After all, life would be easier without them and we could all go off and do jobs that don't involve them. Do we really need to know everything about the universe? Or to send people into space, at significant cost and human risk? Do we really need to send sound and pictures through space? Do we really need to communicate with peopl
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1.3 Innovation by development

Innovation by development is about changing the bit that doesn't work, or that could work better, to improve the function of the whole for reasons of cost, performance, ease of manufacture or competitive edge. You probably noticed in Box 1 'Innovation by context – an example' that Baylis had to incorporate a nu
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