ScienceWise - Jan/Feb 2007

Stamping Silicon at RSPE

Article Illustration
Professor Jim Williams and Dr jodie Bradby with the indenter
Article Illustration
Article Illustration

Scientists have known for some time that when you apply very high pressure to  silicon it deforms by changing its phase from one crystal structure to another. However, a group of researchers at ANU are in the process of turning a curiosity driven investigation into this intriguing phenomena into what may form the basis for an important new technology.

The silicon found in silicon chips is known as silicon I (Si-I) which has a diamond like crystal structure. When you press down on this silicon at quite a high pressure 109Pa (around one tonne per square millimeter) the material is compressed and its density increases by over 20%. Under this pressure it changes phase from semiconducting Si-I to metalic silicon II (Si-II) with a tetragonal crystal structure. While such pressures sound extreme, they're easily achieved using a sharp diamond point in a device known as a nanoindenter. The pressure formed Si-II has the electronic and mechanical properties of a metal and not a semiconductor. However, this phase only persists for as long as the pressure is maintained. As the pressure is released or unloaded (as the diamond tip is raised), the silicon turns into one of two other phases depending on how quickly this is done.

If the pressure is released rapidly you end up with insulating amorphous silicon. If done slowly it transforms into a mixture of two other crystal phases  silicon-III (a body-centred-cubic structure) and silicon-XII (a rhombohedral structure). One is a semiconductor and the other is a semi-metal. The researchers have also found that if you have certain dopant atoms in the silicon to begin with, the two end phases can be made highly conducting. This offers a completely new way of working with and modifying silicon.

Rather than conventional chip manufacture which uses different dopants applied via a pattern mask to alter the electrical properties of the silicon, this method allows you to write the pattern directly into the pure material using only pressure. This is unlikely to ever become the dominant method of chip manufacture but it does have some very useful niche applications.
Because stamping eliminates the need for complex and expensive pattern making, it offers the possibility of cost effective low quantity runs rather like digital printing for semiconductors. This idea may also find application in smart cards where it offers an easy way of writing information. The card might have a conventionally formed chip embedded in it and utilise stamping technology to connect or disconnect particular points thus embedding unique information on a card by card basis.

Another exciting possibility for stamping technology is in nanoscale memory. The laws of physics set a lower limit on the size of a conventional transistor because it relys on impurity atoms. If you need one given impurity atom per million silicon atoms, you simply cant make a transistor smaller than a million atoms. But with stamping technology it is possible to create much smaller devices only a few nanometers across because the process uses pure silicon.

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