Stylus Finds New Role Measuring Thin-film Deposition

Research into thin-film electronics is becoming increasingly reliant on the function of tools such as stylus profilometry - instrumentation-based measurement of a surface's profile, in order to quantify its roughness. Advances in the use of profilometry will be fundamental to the commercialisation of barrier films and other key elements of flexible and/or transparent electronics in future.

Versatile
A US academic has demonstrated the adaptability of stylus profilometry to analyse thin-film barriers, promoting uniformity in production and helping to characterise novel materials.
Ghassan Jabbour, director of the University of Nevada's Renewable Energy Center in Reno, has found metrology instruments central to a number of his research projects. These include work on flexible electronics, photonics and photovoltaics, optoelectronic materials and devices, and solar and voltaic engineering.

Jabbour has used stylus profilers, like the Dektak 150 made by Bruker, to measure thin film thickness and to calibrate the deposition rates in vacuum chambers after depositions on film have been done. Common applications in these vacuum chamber depositions include measuring the thickness of thin coatings, spin coatings, and roll-to-roll printing.

Jabbour says: 'We used the Dektak 150 to measure 3D step heights of depositions on some of the thin films. We tried to use optical microscopy, but these films were so transparent that we could not get good resolution or images.'

Proven Technology
The original concept of a tactical measurement is still valid and critically important many decades after the stylus profilometer was first introduced. Technical evolution is now allowing more accurate measurements to be taken. Analytical capabilities have been improved by electronics capable of providing characterisation of increasingly smaller steps and surface roughnesses, automation to speed measurements, and the incorporation 3D mapping and other software advancements.

Jabbour notes that with stylus profilometry his team is able to get a bigger picture and visualise the 3D structures that are printed. The stylus profilers have also been used in fully transparent applications like inkjet print micro-optics on glass.

Being able to accurately map the surface of a thin film rapidly within a production line is becoming an area of increasing interest for manufacturers seeking to scale up plastic electronics production. Current techniques are limited, meaning detection is carried out post-production and faulty panels may not be detected until they are integrated into the finished product. In-production detection allows for less costly waste, and the rapid reworking or repair of defective areas before they leave the factory. This and the cost associated with it is a major limiting factor on economically viable, large-scale printed electronics sheets, like those used large OLED television screens.

Vacuum Deposition
Besides adapting this proven metrology for quality control, the stylus has also demonstrated its potential in characterising the vacuum deposition new materials, where it can obtain the unknown tuning factor of via an iterative process. First a thin film is deposited, and a stylus profilometer is used to measure the film thickness. This is repeated at multiple deposition rates so that the deposition rate can be precisely programmed based on the calibrations from the stylus. In this way, deposits of any thickness can be determined precisely, down to a nanometer.

Jabbour explains: 'The stylus was critical in helping us obtain information, previously unknown about certain materials, to key into the deposition monitor.'

Long-used techniques, such as vapour deposition, spin coating, and roll-to-roll printing, are still being used today in the new generation of flexible electronics.

Multiple applications
Jabbour adds: 'Everything you see, from displays all the way to solar cells, uses one of those techniques in its fabrication. I am working on solar cells and on sensors, and I can tell you that, without a stylus profiler, the work would not be complete. You need to know the thickness of your absorber, of your transport layers, of your electrodes -- it is really critical.

'Right now the stylus is the most important tool for our work. Without it, the measuring would take a much longer time, and time is money - and innovation. If you waste time with slower techniques you lose out to your competitors. Having the right tool to measure thin film thicknesses fast and in a robust manner is paramount. For us, the stylus is unlimited.'