3.3 Optical amplifiers

Figure 22 shows in outline one possible structure for an Erbium-doped fibre amplifier (EDFA).

Figure 22
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2.4.3 Polarization mode distortion

Because light is an electromagnetic wave, it has a ‘state of polarization’, which, for light in single-mode fibre, is at right angles to the path of the fibre. If you've not encountered electromagnetic waves before, all you need to appreciate is that as light travels down the fibre the electromagnetic field has an orientation across the fibre (Author(s): The Open University

7.4.2 Reactive PVD

For a simple compound, such as a metal oxide or nitride, a modification to the sputter deposition technique can be used. The plasma is formed from a reactive gas, such as nitrogen, instead of the inert argon used for conventional PVD. A side effect of the plasma excitation is that chemical bonds are broken in the source gas to release copious unpaired atoms (radicals), which are extremely reactive and form a monolayer coating of nitride on the target surface. These can then be sputtere
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7.3.1 Electroplating

Electroplating is a relatively fast process, inexpensive and simple, although fairly messy and limited in applicability. The wafer is dipped into a solution with dissolved salts of the metal (e.g. CuSO4 + H2SO4) and is connected to a negative voltage. A positive metal electrode (anode), also in the solution, completes the circuit. Anywhere that current can flow into the wafer surface, metal will be deposited. Plating has several advantages: it will deposit met
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7.2.3 Chemical composition

As outlined in Table 2, some deposition techniques are best suited to conducting materials, whereas others come into their own only for chemical compounds. In either case, chemical composition may be an important consideration. Impurities can interfere with the conduction properties of the material (notably in t
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7.2.2 Step coverage (conformality)

Not all layers require precise thickness control. Sometimes all that matters is that the film coats the entire surface, including vertical walls and (most difficult) the corners at the bottom of deep holes.

For example:

  • We may require an insulating layer of oxide between two conductors.

  • A protective titanium nitride barrier layer prevents aluminium from diffusing into silicon, and an underlying titanium adhesion layer ensures that
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7.2.1 Thickness control and uniformity

Often, final device characteristics, such as the value of a capacitor, the threshold voltage of a transistor, the resistance per square in a thin-film resistor and the resonant frequency of an acoustic wave filter, depend strongly on the thickness of a deposited layer. Therefore it must be ensured that the layer thickness is the same at all points on the wafer, and on every wafer that comes off the production line. Specifications of ±1% uniformity and reproducibility are not uncommon, and so
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7.2 Film properties

In practice, we can hardly ever use just the fastest technique to put some material down onto the wafer. Before deciding how to deposit a particular layer, we must consider which film properties are important for the function of the device. The commonest requirements relate to uniformity, step coverage, composition, micro structure and stress. We shall consider each of these in detail.


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6.2 Natural frequency of free oscillations

Most of us have a fairly accurate understanding of what is meant by resonance – it's what causes a bell to continue to make a sound long after it has been struck. Yet this is just one example of resonance, a phenomenon that occurs in nature in a surprisingly large number of places.

It is all to do with the reversible transfer of energy from one form to another in a system. The common feature associated with mechanical systems that are able to store energy by oscillating is that they h
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5.1.1 London forces

The distribution of charge in one atom or molecule occurs naturally as the electrons move around the nucleus. If a second atom or molecule is introduced, the charge distribution from the first will induce a complementary charge distribution in the second. Looking at Figure 22 you can see that the negative bias o
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5.1 Stickiness

Stickiness is something that we take for granted in the macroscopic world; it's what happens when you put your elbow in some jam, or spill a sugary drink then put a paper on the drying surface. It's also responsible for friction: everyone knows that gripping a sliding rope will produce enough heat to cause severe burns. But where does this heat come from, and why is stickiness important in the microscopic world?

Atomic and molecular bonds come with a wide range of strengths: from very s
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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
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3.6 Design considerations for AFM probes

AFMs have proved so useful in so many areas of science and engineering that they are now to be found in most universities and many manufacturing companies. The making of probes for these instruments is no longer a cottage industry, partly because of the sheer numbers involved, but also because of the need for consistency of performance from probe to probe. This requires precise shaping, both of the tip itself and of the cantilever on which it is mounted. The quality of image obtained from an
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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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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
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10.2 Adding decibels

A feature of decibels is that adding two decibel values is equivalent to multiplying the ratios they represent. To see how this comes about, consider another context in which a decibel measurement is often used, that of signal amplification.

In Figure 28, the triangular symbol represents
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1 Part 1 Investigating the innovation process

In Part 1 I invite you to look around at the technological products in your home or at work and consider their development history and their impact on the lives of you and your family. I then define the key concepts associated with the process of invention, design, innovation and diffusion.


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6.4 Summary

After passing through a treatment works, water flows via transmission mains to service reservoirs which act to even out variations in consumption during a 24-hour demand period. It is then taken by distribution mains from the service reservoirs to the consumers. The head of water necessary for flow is provided by positioning the service reservoir at the correct level or by pumping. A pumped pipeline is called a rising main. The size of mains pipelines depends on the quantity and flow rate of
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5.14.3 Electrodialysis

Electrodialysis is an electrochemical process in which ion transfer separates salt from water. It is effective only for substances that can be ionized: for example, salt (NaCl) becomes, in solution, a mixture of Na+ and Cl ions. (Silica, on the other hand, does not ionize and hence is not removed by electrodialysis. It could, however, be removed by reverse osmosis.) When electrodes, connected to a suitable direct current supply, are immersed in a salt solution, curren
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5.14.1 Multistage flash distillation

In this process (Figure 34) saline water (screened first, if it is sea water) is distilled under reduced pressure in a series of sealed tanks. Due to the reduced pressure, the water evaporates suddenly or ‘flashes’ at a temperature lower than 100°C, typically 80°C. Pure water condenses on cooling coils in the tanks and is collected. As the temperature of the feed water falls in each succeeding tank (as the latent heat of evaporation is extracted from it) a correspondingly lower pressure
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