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5 An introduction to ‘making the case’

Much of the T883 course is concerned with making changes to existing methods of doing something. The changes might be very radical and far-reaching in their impact, or relatively minor incremental improvements to current practices. They might involve large investments of finance and other resources over extended periods, or there might be trivial financial outlay associated with them. Some changes might be entirely within your own sphere of influence but others may involve resources outside y
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3.6 Interfaces in the supply network

Managing the internal interfaces is part of the story. Of increasing importance is the management of the processes that cross organisational boundaries between suppliers and purchasers, that is, the management of the supply network or chain. This network of suppliers, customers, government agencies and others that are necessary parts of the entire value system must be proactively managed. This includes designing the network appropriately.

A supply network is defined as:

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3.2 Business operations: function or process?

Traditionally, an enterprise's activities are organised according to a structure based on the well-known business functions: marketing, purchasing, finance, human resources, research and development (R&D), operations, and so on. The exact function title varies from organisation to organisation, but each function has its own more or less well-defined sphere of activity. It carries out its various tasks and passes on information or artefacts to other functions for them to work on. For example,
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Learning outcomes

By the end of this unit you should be able to:

  • make an effective business case for a change to an operations activity or similar using appropriate written and/or oral forms of communication;

  • show the widespread utility of operations management principles at all levels across all types of organisation;

  • introduce a transformation model of operations management, with stakeholder value as the principle output;

  • provide models, concepts and
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Introduction

This unit is adapted from the Open University course Business operations: delivering value (T883_1), which is about the essence of any enterprise – that core set of processes needed to convert various resources (such as materials, money and the effort of people) into outputs (such as manufactured goods and/or delivered services) that provide value to customers and other stakeholders. T
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Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions) and is used under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence.

Grateful acknowledgement is made to the following sources:

Figures

Figures 33, 37, 38, 40: Courtesy of Trik
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8.5 Review

We can sum up the distinctive features of wet etching with a few key points:

  • Wet etching, in general, is a simple process to operate. Wafers are immersed in a solution for a while before being taken out, rinsed, and dried. However, certain etches require more sophistication. They may need one or more of the following: heating and agitation of the solution; reflux of vapours to maintain concentrations; protection of back side of the wafer; incorporation
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8.4.3 Self-limiting etches

In practice, it is often possible to design microsystems in such a way that there is no need to pay great attention to knowing the precise moment when the etching has gone far enough. A good example is the etching of the movable structures in surface-micromachined electromechanical devices.

Figure 41 sho
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8.4.1 Open-loop control

Open-loop is the crudest way of controlling etch depth. It relies on ensuring that every aspect of the process that can affect the rate of progress of the etch is kept under tight control. This can add up to a sizeable list. Table 5 shows just some parameters that affect both wet and dry etching.

Whether
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8.4 Stopping the etch

Just as important as being able to remove material is being able to stop doing so once the intended etch depth has been reached. Success or failure in this aspect of etching determines whether or not any of the benefits of parallel processing of thousands of devices will be obtained. Uniformity of result from device to device, and repeatability from wafer to wafer, are crucial to the economic viability of the whole exercise.

There are three broad categories of approach to this problem:<
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8.3.6 Deep silicon etching

MEMS structures often require etching to a much greater depth than is needed for microelectronics. A rate of 1–2 μm min−1 may be quite sufficient for making transistors less than 1 mm deep, but to etch through 600 mm of silicon to form an accelerometer would take all day. The advent of MEMS and wafer-level packaging applications, therefore, brought a need for yet faster anisotropic etches, requiring advances both in the process and in the etching equipment.

Capacitive co
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8.3.5 Alternative plasma chamber designs: MERIE and ICP

There are several variants of the parallel-plate RIE chamber. For example:

  • The ‘magnetically enhanced’ MERIE, where magnetic fields are used to slow the leakage of plasma to the chamber walls, reducing the operating voltage and improving the power efficiency.

  • ‘Plasma mode’ operation, where the RF voltage is applied to the chamber ceiling and the platen is grounded. This reduces the ion energy at the wafer from hundreds of volts t
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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
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8.3.3 Reactive ion etching: chlorine/argon plasma etching of aluminium

In a reactive ion etch (RIE), a chemical reaction is used to weaken the bonding of the surface of the material and assist the sputtering process. This combines the high rate and selectivity of a gas-phase etch with the directionality of a sputter etch.

For example, consider aluminium etched anisotropically by a Cl2/Ar mixed-gas plasma, which etches at up to 1 μm min−1:

  • Power pumped into the plasma breaks the gases up, rel
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8.3.2 Sputter etching: argon ion etching of gold

One commercial process for cutting inkjet printer nozzles uses sandblasting. Not surprisingly, the surface finish is rather poor and there are issues with particles contaminating the devices. However, it is a physical process very like this that we need if we are to achieve a vertical etch profile.

The key is directed bombardment by highly energetic particles. When processing on the microscale, these particles are not sand grains but ions accelerated towards the surface by an electric f
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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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8.2 Wet etches: acids and bases

The simplest etches use a liquid solvent that converts the material into a soluble compound or a gas. Unfortunately, most materials used in micro-devices have few soluble compounds, so some very aggressive chemicals are needed to attack them. Here is a list of some of the most commonly used ones:

  • Hydrofluoric acid (chemical formula HF) is used to convert silicon dioxide into water-soluble H2SiF6 (plus some hydrogen and water). It
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7.4.6 Molecular beam epitaxy (MBE)

Where a suitable ALD chemistry cannot be found, or where cleanliness and high crystallinity are required, molecular beam epitaxy may be necessary. This is more akin to evaporation than to CVD, with multiple molecular beams of the separate chemical constituents each focused onto the hot wafer surface. Deposition is performed under extreme vacuum conditions (10−11 mbar) to prevent any contaminants from being incorporated, and the substrate must present a perfect cleaved crystal fac
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7.4.5 Atomic layer deposition (ALD)

For very thin conformal films, where rate is unimportant but precise thickness control is critical, a form of CVD allows deposition one monolayer at a time. One precursor gas is introduced into the chamber, which is then pumped away leaving only a monolayer adsorbed onto the wafer and chamber walls. The second precursor gas can then be supplied to complete the reaction at the surface, and then this gas is pumped away along with any gaseous reaction products. This cycle is repeated several tim
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7.4.4 Plasma-enhanced CVD (PECVD)

In PECVD a plasma is initiated in the CVD chamber, usually by supplying an RF voltage to the platen on which the wafer sits – the chamber geometry is similar to a reactive ion etch chamber. Ions are accelerated from this plasma onto the wafer surface, so that the CVD reaction is initiated not only by heating the wafer, but also by the energy imparted as the ions land. This allows high-quality film deposition at much lower wafer temperatures and higher deposition rates than unenhanced CVD, w
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