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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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3.7.2 The hybrid probe

Where the benefit to be gained from combining the narrow-angled silicon tip with the super-light, any-shape silicon nitride cantilever outweighs the expense and difficulty of the more complex process sequence, AFM probes can be made with silicon tips on silicon nitride cantilevers. Figure 14 shows one such pro
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3.7.1 The machined-at-once tip and cantilever

Just as in conventional manufacturing, micro engineering is cheapest to do if as few different materials as possible are used and if the number of separate processes involved is minimised. Therefore, the idea of making the cantilever and the probe out of the same material and in the same process step is a very attractive one.

When silicon nitride is deposited onto a silicon surface, it produces a thin film that coats the whole of the material to an equal thickness. We have already seen
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3.6.4 Materials selection for cantilevers

Table 1 shows some of the physical and mechanical properties of materials that can be deposited and etched in thin-film form. One of the consequences of manufacturing these materials in thin-film form is that properties that in the bulk material can be determined to within a few per cent are much less easy to
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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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3.6.2 Resonant frequency

There are two very good reasons for wanting the resonant frequency of the AFM cantilever to be as high as possible: to minimise the effect of vibrations from the surroundings, and to obtain a high image acquisition rate. Given the very high resolution of the measurements they are intended for, atomic force microscopes are bound to be susceptible to the effects of air movements and vibrations in the buildings where they are sited. Building vibrations are most significant in a frequency range f
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3.5.4 Other modes

The investigation of new scanning modes for the AFM has been something of a playground for researchers: think of any interaction between materials in which a force plays a part and you have a potential scanning mode. Coating the probe with a magnetic material, appropriately magnetised, enables samples to be scanned in magnetic-force mode. An obvious industrial use for this technique is the investigation of the structure of magnetic storage media. Electrostatic forces too have been used. Using
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3.5.1 Contact mode

Contact mode produces images with the highest resolution. This is because when the probe tip is as close as it can be to the surface, the influence of atoms other than the one directly under the probe tip is relatively small. This is a simple geometrical effect – if the tip were withdrawn a large distance from the surface, a large number of atoms would be at a very similar distance from the tip, and therefore would have a similar contribution to the overall force. In contact mode, the repul
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3.5 Scanning modes of the AFM

One of the interesting effects of scale is the answer to the question of whether the probe needs to come into contact with the surface of the sample being scanned. The cantilever on which the probe tip is mounted is a very compliant structure. The control system of the AFM ensures that the deflection of the cantilever, and hence the force it exerts on the surface, is maintained within very strict limits. Author(s): The Open University

3.4 The atomic force microscope

The most commonly used scanning probe microscope is the AFM – the atomic force microscope. It works in a way much more similar to the gramophone stylus, but instead of detecting the movement of the probe tip electromagnetically, it usually does so optically. As the probe tip is drawn across the sample, a laser beam is reflected off the cantilever on which the tip is mounted. A position-sensitive optical detector picks up the deflection of the beam, converting the angle of bending into a vol
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3.3 The scanning tunnelling microscope

The first scanning probe microscope, the scanning tunnelling microscope (STM), was invented by Heinrich Rohrer and Gerd Binnig in 1981, and used the quantum-mechanical effect of electron tunnelling (in which electrons ‘tunnel’ through an energy barrier that classical physics would suggest is too high to cross). In this instance, the energy barrier is the tendency of the metal of the probe tip to want to hang on to its electrons. In effect, as you try to remove an electron from the surface
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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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3.1 Introduction

The atomic force microscope is a key visualisation tool for the ‘invisible’ world of micro and nano technology. Within it, right at the heart, is a probe tip that is itself a triumph of nanotechnology.

This section is going to begin with a fair amount of detail about how scanning probe microscopes of various types work, starting with a description of the scanning tunnelling microscope (STM). After that I want to concentrate on its close relative, the atomic force microscope. Then we
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2.2 The challenge for innovation

For a Pirani sensor, the basic task is to provide a reliable measurement of pressure in a vacuum system as it varies from atmospheric pressure down to a value at least as low as 1 Pa. This statement can be further qualified by saying that unless its performance or cost is a fantastic improvement on the existing type, the micromachined sensor must be compatible with existing interface electronics, such that only minor modifications to its design are needed. This implies an electrically resisti
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2.1 Introduction

One aspect of micro and nano scale engineering that distinguishes it from many other forms of manufacturing is the way it involves building both the devices themselves and the very materials from which they are made, in one place and at more or less the same time. In general, MEMS are made from thin layers of new material produced, and then shaped in some way, on the surface of a silicon wafer. The devices contain several different materials, and have a three-dimensional structu
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Introduction

This unit examines how small features can be etched and cut out of solid materials at a very small scale.

This unit is an adapted extract from the Open University course Engineering small worlds: micro and nano technologies (T356).


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