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ScienceWise - Jan/Feb 2006

Cracking Under Stress

Article Illustration
The researchers at Electronic Materials Engineering have observed many bizarre variants of the cracking behaviour. Pictured above is a form they have dubbed ‘chain-mail’ created from a network of interlinked wave cracks.
Article Illustration
In the image above, cracks in the thin silicon films are clearly following an alignment corresponding to specific crystallographic planes in the underlying silicon crystal that makes up the substrate. The wavy cracks have never been observed before and the exact mechanism by which they are created is still being investigated.
Article Illustration
The researchers call the cracking pattern shown above the ‘snake skin’ for obvious reasons. It’s formed by two parallel straight cracks with repeated curved cracks in between. This form of cracking occured relatively slowly, over several seconds. It could be observed happening by eye. The wavy cracks, in contrast, grow very quickly, too fast to observe.

As scientists build increasingly complex structures from deposited thin films they’re discovering a host of new problems relating to the physical nature of these layered systems. Stresses building up inside the layers can lead to cracking and distortion of films, and to failure of devices based on these films. Professor Rob Elliman, Dr Tessica Dall, Mr Marc Spooner and Mr Taehyun Kim from the Department of Electronic Materials Engineering have recently witnessed a particularly amazing form of cracking behaviour in amorphous silicon-rich oxide films deposited on silicon wafers.

“We’ve observed two novel modes of crack propagation, one that produces straight cracks and a second that produces near-perfect sinusoidal, or wave-like, cracks aligned along different directions,” says Professor Rob Elliman, who heads the Department. “The sinusoidal cracks have a wavelength of around a hundred micrometres and can propagate over centimetre distances with near constant form. This long-range periodicity suggests a simple interplay between two competing processes and we are trying to understand these in detail.”

The silicon-rich oxide layers are being deposited on silicon substrates by plasma-enhanced chemical vapour deposition. Individual layers are between 100-1500 nm thick. After the film has been deposited it is then heated to 1100ºC to precipitate out silicon nanocrystals. It’s during this heating that the cracking occurs.

“It appears that straight cracks begin to form first followed by the wavy or oscillating cracks,” says Elliman. “Both sets of cracks, straight and wavy, lie parallel to particular crystallographic directions in the underlying silicon substrate. Interestingly, the wavy cracks do not appear to have been observed before in such thin films. However, similar cracks have been observed in rubber stretched one way more than another.”

To understand what’s happening, Professor Elliman’s team has been studying several aspects of the process, specifically the effect of heating on the film stress and the amount of hydrogen the film contains.

“Understanding how stress develops in thin films and why and how cracks form is fundamental to the successful application of thin film technologies,” says Professor Elliman. “Our results show that hydrogen release can be used to tailor the stress in thin films to produce stress-free films or films with a particular stress. This is particularly important to the builders of micro-electro-mechanical systems, or MEMs, where film distortion or failures caused by internal stresses can quickly destroy a device.”

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