By Wulf Hofbauer
Science Centre has a new scanning electron microscope (SEM), housed at the Centre for Research and Applied Learning in Science.
In an SEM, electromagnetic fields are used to deflect a finely focused beam of electrons across a sample under investigation. There are multiple ways in which the sample can respond to this electron bombardment.
For example, the sample itself may start emitting electrons (imagine water droplets splashing off a surface hit by a water jet), which can be measured using a suitable detector. By scanning the electron beam across the sample, this ‘secondary electron’ response can be measured for many different points on the sample, one after the other, and an image constructed pixel-by-pixel.
One of the big advantages of this technique is that electron beams can be focused much more precisely than visible light, so the resolution (smallest visible detail) an electron microscope can achieve is typically much better than what a light microscope can do. On the left, is an SEM image of a high-tech item most of us will have at home: the data-carrying layer of a recordable DVD.
Data on a DVD is recorded along a single, spiralling groove (except in the case of DVD-RAM media), and one can clearly see the track pattern embossed into the thin reflective metal layer.
It is an amazing engineering feat that such delicate structures (the groove spacing is less than a micrometer), covering the considerable size of a disc with minimal defects, can be manufactured reliably at a cost of just a few cents per disc!
One also finds some interesting ‘wiggles’ scattered along the track. Given the prevalence of these wiggles, my guess is that they are not a defect, but meant to provide the read/write head of an optical drive with position information on an otherwise ‘blank’ disc.
Maybe one of our readers knows? All experiments are prone to artefacts, and scientists put a lot of effort into recognising them or ruling them out so that their results are not based on invalid data. Artefacts can also be quite interesting on their own. As the high-energy electron beam hits the sample, heat is generated, which may cause damage.
The heat caused the thin metal film coating the DVD plastic substrate to form a micrometer-sized blister (see right image). Note how the embossed tracks bulge out along with the metal layer.
An interesting feature of these blisters is that their shape follows the embossed groove pattern. It seems to be much easier to peel the metal film from the plastic surface along the direction of the track than perpendicular to it, so this is the preferred direction in which blisters grow (compare this to corrugated cardboard that is easy to bend when folded along the lines of the corrugations, but stiffer when bent perpendicular to them).
One might even think about analysing the aspect (length/width) ratio of these blisters to obtain a more detailed understanding of the micro- or nano-scale mechanics involved.
These blisters can be grown till they get quite large, but eventually, they will collapse (as evident below).
So, what is this metal film actually made of? The intuitive answer is aluminium since it is cheap and has good reflectivity. Just to be sure, we can use the SEM to figure it out using a method known as ‘energy dispersive x-ray spectroscopy’ (EDX).
As the sample is bombarded by electrons, it also emits a small amount of X-rays, and the wavelengths of these X-rays are characteristic for each chemical element. EDX measures the wavelength distribution (the spectrum) and matches it against the known ‘fingerprints’ of various elements. And here is what we get:
The carbon (C) and oxygen (O) peaks reflect the plastic substrate (polycarbonate) on which the metal film is residing, but the film itself appears to be pure silver (Ag)—not aluminium (Al) as originally expected. Who would have thought such treasure is hidden in our optical discs?