Sample Screening

Materials World magazine
,
1 Mar 2009

Graeme Purdy of Ilika Technologies Ltd, Southampton, UK, describes accelerated product development with high throughput for (opto)electronics.

We live in an increasingly demanding society where a seemingly insatiable consumer appetite for new products and technologies is offset by concerns about environmental sustainability. The solution to this challenge can often be found through innovation in materials science – discovering and optimising novel materials and devices which can perform more efficiently.

The traditional innovation process has, however, been rather long. The ten years generally taken to turn a twinkle in a scientist’s eye into the first commercial prototype is longer than most of us are prepared to wait. It’s certainly longer than most sources of finance are prepared to wait. So the question is, can we realistically accelerate materials research to come up with solutions faster than has historically been achieved? An increasingly adopted approach is the use of high throughput techniques. Instead of making one material at a time, these automated technologies are designed to make ‘libraries’ of hundreds or thousands of systematically varying materials in one experiment.

The libraries are laid out so that they can be characterised and tested using accepted scientific techniques which have been modified for high throughput operation. Typically, a technique is automated to be able to scan across a library of materials, making measurements for each of the samples and storing the data in an addressable database. Perhaps the most important aspect of such an approach is to make sure that the characterisation measurements correlate closely with the performance of the material in the final product. This feature of ‘scalability’ is one of the key benefits of the approach developed by Ilika Technologies Ltd, based in Southampton, UK.

Material of choice

The first step in using high throughput techniques is to credibly demonstrate that the process finds the accepted ‘material of choice’ if used in place of traditional materials development. Such benchmarking is the starting point for almost all programmes.

While the range of materials systems which Ilika can address is broad, this article will focus on those with electronics or optoelectronics properties. Piezoelectrics are materials that generate an electric potential in response to an applied mechanical stress (and vice versa). The main material of choice with this property is the ceramic lead zirconate titanate (PZT). Trying to predict which ceramics might exhibit a piezoelectric property is difficult. No one has convincingly done so to date, and making thousands of different ceramic formulations, which may or may not form stable crystalline structures, can be tedious. So Ilika has tried to apply its high throughput methods to systematically make and test libraries of these materials.

To establish a benchmark for the programme, Ilika made films of PZT, varying the composition across a substrate. The film, shown is about 30x30mm in size. Using a modified atomic force microscope tip, piezo-response measurements can be taken across the film showing the effect of varying crystal composition.

Whether we realise it or not, most of us look through transparent conducting oxides (TCOs) every day. This is because these materials are used in the fabrication of flat screens for our televisions, computers and mobile phones. They are used in the manufacture of photovoltaic panels which are fuelling the solar revolution. Transparent conducting oxides form the top contact in thin-film PV panels, allowing sunlight through, but at the same time enabling current to be drawn off the photovoltaic stack.

There are a few TCO materials used according to the intended application, including indium tin oxide (ITO) and doped zinc oxide. The key challenge is to select the right TCO that balances performance with cost. The image above shows a continuous oxide film varying in composition from one side to the other. The optical transparency and conductivity have been systematically measured across the film to locate the composition at which the optimum properties can be found. The colour of the film is an interference effect.

Memory matters

Phase change memory is a type of non-volatile computer memory – it does not lose its information when the power is turned off. Such memories can store information optically and electronically. The optical version is familiar to all of us in the form of CDs, DVDs and Blu-Ray discs. The electronic version is still in the prototype stage, but is expected by many analysts to take over from flash memory, which is currently usedin memory sticks and chips and is even embedded in electronic circuits.

Phase change memory materials generally belong to the class of materials known as chalcogenides. The key to information storage lies in the material’s ability to readily switch between the amorphous and crystalline phases. The amorphous phase is less reflective and more resistive, while the crystalline phase is more reflective and conductive. On an optical disk, digital data is stored by writing amorphous marks with a laser. The marks are subsequently read by measuring reflected light. In an electronic data storage system, the phase change memory material is switched in miniature cells, which store data by virtue of being in either the amorphous or crystalline states.

Ilika is optimising materials for their use in electronic phase change memory applications (often called PCM, PCRAM or PRAM). The benchmark system for electronic memory is the germanium-antimony-tellurium (GST) alloy, which has been synthesised and tested. Key parameters of interest are the threshold voltage at which the reversible phase change occurs, the speed at which the transition takes place, the relative conductivity of the phases and the reversibility of the transition (number of cycles to failure).

Such measurements can be made rapidly using high throughput techniques, allowing the materials to be optimised before more expensive tests are conducted with chip sets. Much is still to be understood about this versatile and enigmatic family of materials, although these high throughput studies have offered a glimpse of some of the useful properties the alloys can demonstrate.