Developing a Standard for Valid Methodology for the Characterisation of Functional Alloy Thin Films

The EU aims to be carbon neutral by 2050 and it is predicted that solar power will generate 60 % of electricity in Europe by this date. Many modern high-efficiency devices rely on thin films such as LEDs, energy efficient windows and thin film solar cells composed of copper-indium-gallium-selenide, or ‘CIGS’. Ranging in depth from fractions of a nanometre to several micrometres in thickness these layers can impart novel properties, but even small changes in them can affect device performance. Thus, manufacturers need to precisely control composition, interface properties and thickness during production. Key to this is accurate depth profiling techniques using calibration samples to ensure measuring instruments are sufficiently accurate. To minimise uncertainty, calibration samples must be similar to the layers under investigation, but characterisation methods relied on only a few particular calibration samples or reference materials, and the results showed significant discrepancies. A key output of EMRP project Metrology for the manufacturing of thin films was a SI traceable methodology based on reference free X-Ray Fluorescence (XRF) analysis of CIGS. This enabled the composition and element in-depth distribution of non-homogeneous thin film samples to be analysed, but the project highlighted the lack of appropriate calibration samples and reference materials.

This EMPIR project built upon this work to establish traceable reference methodologies for analysing thin film CIGS materials. Together with Helmut Fischer GmbH, a leading manufacturer of XRF instruments, certified calibration samples were generated which were used in an extended study on CIGS. Significant differences were found in elemental composition between measurements made with or without the certified calibration samples, especially in the Indium element of the CIGS, and the thickness was about 20 % thinner when measurements were made in conjunction with the samples. Two standard operation procedures were then established, one for XRF and a second for another chemical characterisation method, Sputter-assisted Auger Electron Spectrometry (AES). Both techniques were then used in the International CCQM key comparison CCQM-K129, comparing measurements of mole fractions of Cu, In, Ga and Se in CIGS thin films. Through this PTB gained a calibration measurement capability (CMC) for determining CIGS chemistry using XRF and now has a new calibration service for characterising CIGS using this instrument. BAM has also launched a depth profiling service on films using sputter-assisted AES. Project outputs have fed into the relevant standards developing organisations in this field, such as ISO/TC 201 and DIN and is expected to be progressed to the ISO committee covering characterisation of functional alloy thin films. New, standardised methodologies for thin film materials will help the development of new CIGS. In addition, since many applications involving thin films are associated with renewable energies, low-cost high-efficiency products based on advanced thin films will bring a positive environmental effect from energy saving and reduced CO₂ production.