TCM coating processes - combined optical and electrical in-line testing in high volume production lines

TCM coating processes - combined optical and electrical in-line testing in high volume production lines

Marcus Klein
SURAGUS GmbH

Although many ITO and other TCO coating processes are well established, increasing coating process speed, higher deposition rates and lower coating temperatures highly affect the layer uniformity, stoichiometry of the layer and the material’s crystallinity. All these parameters have a crucial impact on the conductivity and optical transparency of the layer – the two main properties of TCO films.  Novel in-line measurement technology is required to adapt to these challenges and to assure accurate and reliable measurements of the electrical and optical properties even at high production speed in large-area coating. Therefore, non-contact sensors have been developed for monitoring sheet resistances between 0.01 and 1000 Ohm/sq in multiple-head configurations. Especially the stable measurement of high ohmic samples is challenging and requires specific adaption of the sensor concept. We combined these sheet resistance multiple heads with an optical reflectance and transmittance multiple head metrology tool. The combination of the electrical and optical methods was game changing. With the TCO film thickness monitored by Fabry-Perot analysis, the sheet resistance variations can be deconvoluted regarding film conductivity and film thickness contributions. From the spectral position of the TCO’s optical band edge and from the TCO’s IR absorbance the underlying structural variation of TCO material quality can be identified as root-cause for conductivity variations. Finally, with IR wavelength included in the optical in-line testing, the free-carrier absorption correlates to the TCO carrier concentration and enables SPC regarding carrier concentration and specific conductance of the TCO.  Examples of challenging in-line measurements and non-destructive off-line coating analysis are presented and discussed. A special focus is directed on the analysis, discussion and resolution of edge effect phenomena in sheet resistance sensing and the transformation of electrical and optical raw data into high-level quantitative data for comprehensive SPC in TCO production lines. The key target specification of our metrology development included, beyond superior metrology performance, the smooth integration into a single metrology station with an easy to use graphical user interface and a choice of standard communication interfaces to the customers MES and SPC infrastructure.