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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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 also be expressed in term
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

6.3.1 Damped harmonic oscillator

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

Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

6.1 Why is resonance important?

This section aims to take you through some general ideas about vibrations, which will help you understand the principles behind the resonant behaviour of the AFM probe tip. Vibrations and oscillations crop up in many contexts. They can be modelled mathematically and form a general topic in mathematics about vibrations and oscillations in which the appropriate balances between forces and accelerations are formulated into differential equations.

Students of physics and chemistry also get
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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 t
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

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