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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
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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
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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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7.1 Introduction

Micro fabrication often involves multiple layers of different materials, each following a sequence of process steps, usually deposition/lithography/etching. The microelectronics industry has been a driver for many of these techniques and that momentum has carried over into the MEMS community. The treatment that follows covers examples from both industries.

The appropriate deposition technique depends on the purpose of the layer – a thin insulating barrier layer in a transistor or capa
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6.6 Oscillators in general

Although this section has dealt only with mass-spring systems, the analysis can be extended to any system where there is an oscillating driving force acting on a mass which is located by a restoring force. In fact, the analysis is even more general than this and can be applied to electronic networks where voltages and currents oscillate in much the same way as the mass on the spring.


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6.5 Q-value

The rate at which the mass–spring system loses energy to its surroundings is referred to as the Q-value for the oscillator. The Q-value is defined as:

ΔE/E is the fractional energy loss per cycle of the oscillation. This can al
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6.4 Driven oscillations and resonance

Finally I need to consider the situation where the oscillator is driven, as in the case of the AFM cantilever. The driving force will depend on the application, but for my mass on a spring it might be a small motor driving what was the fixed end of the spring up and down. The simplest expression for an oscillating driving force FD will be something like:

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6.3.1 Damped harmonic oscillator

Starting once again with Newton's second law but including the additional damping force in the equation gives:


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6.3 Damping

In the real world, most oscillations are subject to damping and so the amplitude of the oscillation dies away over time. For example, the bell mentioned earlier would not be very effective if it did not lose some of its energy as sound waves. The oscillating cantilever of the AFM will, like the simple mass-spring system, be subject to frictional forces from the air, the material of the cantilever itself, and the fixing point.

For the mass-spring system the damping force Fd
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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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5.1.2 Dipole-dipole forces

In the case of dipole-dipole interactions, the molecules that bond together have a fixed asymmetry in their charge distributions (as is the case in Figure 22); if their orientations are favourable the two will bond together. All molecules produce London forces. The dipole-dipole interactions are in addition to
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4.3 PZT

The above requirements lead us to a range of ceramic materials with crystalline structure. One such material is lead zirconate titanate (PZT), which is an oxide alloy of lead, zirconium and titanium. It is often used in a specific composition (sometimes with additives) in order to achieve a particular crystal structure and the desired piezoelectric response. Author(s): The Open University

4.2 The piezoelectric effect at the atomic scale

It has been mentioned above that by changing the state of polarisation of a piezoelectric material we can generate movement, and vice versa. Let's examine a little more deeply what is meant by ‘state of polarisation’ and how we can maximise its effect to get the best out of electrically controlled micro-actuators.

In order to electrically polarise a material we need, by definition, to cause a separation of charges within the material. The more we can do this the greater the d
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4.1 The piezoelectric effect

The phenomenon of piezoelectricity was first predicted and demonstrated in the late nineteenth century using naturally occurring materials. It has a vast number of applications, ranging from spark ignitors to inkjet printers. It is also utilised in timing circuits, where an oscillating electric field is used to make a quartz crystal resonate at its natural frequency. In MEMS, the effect is used to generate small-scale movements in a range of devices known as micro-actuators.

The effect
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3.8 Review

In the first three sections, we have looked at devices whose usefulness is dependent on their form. In the case of the Pirani sensor, it was the dimensions of the microbridge that affected its sensitivity; in the AFM probe, its ability to resolve features on a surface is determined mostly by the form of the last few nanometres of its very tip. With devices the emphasis is not so much on the form of the structure as on how to make it move in the right way and, just as importantly, how to detec
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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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3.7.3 Oxidation sharpening

The all-important radius of curvature of the tip can be made smaller – both in the silicon nitride and pure silicon tips – by the trick of oxidation sharpening. Figure 15 shows the principle. The original tip is first heated to around 1000 °C in an oxidising atmosphere, such that the outermost micrometre
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3.6.3 Quality of resonance

As you will see when you read Section 6 Vibrations and resonance the quality of a resonance, Q, is defined as ωω. Here, ω is the resonant frequency, and Δω is the frequency range over which the amplitude of the oscillation is greater than half the maximum amplitude at resonance.

The MEMS sensors that so far have achieved the best measurement resolution are pressure sensors that rely on the pressure applied to a membrane changing the tension in
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