A NOVEL SENSOR USING REMOTE PLASMA EMISSION SPECTROSCOPY FOR MONITORING AND CONTROL OF VACUUM WEB COATING PROCESSES F. Papa 1, J. Brindley 2, T. Williams 2, B. Daniel 2, V. Bellido-Gonzalez 2, Dermot Monaghan 2 1 Gencoa USA, Davis, CA, USA 2 Gencoa Limited, Physics Road, Liverpool, UK Understanding the components of background and process gases during vacuum web coating processing is critical for producing high quality coatings and products. Typically, the web is the main source of contamination in the system; however, system leaks and outgassing from chamber shields may also contribute. In order to identify and track these gas components, a simple, novel sensor for the identification of residual gas in the system as well as process and contaminate gases has been developed. The sensor is based on the use of a spectrometer to analyse the optical emission of plasma created in a remote plasma generator. The sensor has the ability to work over a wide range of pressures (1e10-6 to 1 mbar) without the need for differential pumping. Species such as water vapour, hydrocarbons and air can be automatically detected and identified both during pump down and during processing. Reactive processes can also be controlled via an output signal to a closed loop controller. As the sensor uses a plasma which is generated remotely, any vacuum web process can be monitored and controlled. Introduction The cleanliness of the vacuum environment in web coating systems is critical for maintaining coating quality both during a coating campaign and from campaign to campaign. Vacuum leaks and outgassing from vacuum chamber and the web itself are the main sources of contamination. Detecting and controlling such contamination is not a trivial task. Historically, differentially pumped Residual Gas Analyzers (RGA s) have been used for such monitoring. However, they can be expensive, not so simple to use and need a fair amount of maintenance. The sensitivity of RGA s at high pressures (>1x10-3 mbar) is also limited in that the atmosphere in the vacuum chamber is sampled though an orifice and some additional vacuum components. In such cases, there is a chance for water condensation on such parts which could skew the actual results. An alternative approach has been developed in which a plasma is generated in a remote plasma generator attached directly to the vacuum chamber. By looking at the optical emission of the gas species in the plasma with a spectrometer based technology, one can determine both the types and relative amounts of the gases in the system. As this type of sensor functions over a wide range of pressures, 1x10-6 to 0.5 mbar, no differential pumping is needed. Since the monitor is directly attached to the vacuum chamber, it is less likely that the gasses such as water will be condensed before entering the monitor. Due to its wide pressure range, this type of monitor can be used in both the pre-treatment (unwind) zone as well as process zones. For web coating processes, the level of water vapour and other volatiles outgassing from the web itself can vary due to polymer type as well as storage conditions of the web before going into the vacuum coater. By
monitoring the levels of such gasses, coating quality can be improved as the conditions in the web coater can be controlled to maintain repeatability. Experimental Setup The process monitor is comprised of a spectrometer and wide range plasma generator (1x10-6 to 0.5 mbar) which is connected to a vacuum chamber via a KF25 flange. The spectrometer views the plasma created inside of the generator to generate a spectral view of the plasma. Several gases (Ar, Air, H 2, Isopropanol) have been injected into the chamber for monitoring. Evaluation of the residual gas in the system has also been done along with a comparison to a standard RGA. Automatic peak identification is done through a software algorithm. Figure 1. Schematic of new process monitor.
Figure 2. View of new plasma monitor Results When air is input to the vacuum chamber, several distinct emission lines can be seen. CO 2+, N 2+, H and O are the characteristic line for air. Figure 3. Spectrum for air.
During pump down, the two main residual gases in the system are air and water vapour. The air is quickly removed from the system, while water vapour continuously desorbs from the chamber walls and substrates. The 337 nm (nitrogen) emission line is indicative of air while the 309 nm (OH) and 656 nm (hydrogen) can be used to measure water vapour levels. Either water vapour line can be used to monitor the quality of the high vacuum. Figure 4. Pump down curves for air and water vapour. Figure 5. shows that water vapour can also be detected while running argon process gas through the system. The pressure for this measurement is 1x10-1 mbar.
Figure 5. Water vapour detection in an Ar environment. In figure 6. the process control monitor and a differentially pumped RGA were connected to a system for sputtering carbon. Hydrogen and OH emission lines were measured with the optical process monitor while the water, hydrogen and OH masses were measured with the RGA. The initial spike in the hydrogen trends occurs when the carbon magnetron is turned on. A similar response can be seen for both devices regarding hydrogen. However, no change in the water vapour partial pressure can be detected with the RGA while the optical process monitor shows a distinct spike in the OH signal followed by a gradual decrease which is to be expected.
Figure 6. Optical process monitor RGA comparison. In order to demonstrate the ability to detect outgassing from polymer substrates, a spectrum was taken with an empty vacuum chamber and with a piece of PET placed in the system. A clear increase in the OH line can be seen with the PET present.
Figure 7. Spectral results of an empty vacuum chamber and with PET. As hydrocarbons and volatile compounds may be present in web coating systems, a small amount of isopropanol was introduced into the system. Similar to air leak spectrum, many distinct and characteristic emission lines can be seen. Figure 8. Spectrum for isopropanol.
Discussion Due to the excitation and splitting of gases in the plasma generator, each gas will have its own characteristic signature. Water vapour, air and hydrocarbons can be easily detected and identified via the use of an algorithm and a database of emission lines. Even when some gases, such as water vapour and hydrocarbons have some overlapping peaks, it is possible to distinguish the relative gas amounts since the ratio of the emission peaks for each gas is different. When compared to a differentially pumped RGA, it has been shown that the sensitivity of the optical process monitor to water vapour is higher than that for the RGA. It appears that the water vapour itself is condensing on vacuum hardware on the way through the differential pumping system in this case and not reaching the detector. As the optical process monitor is connected directly to the chamber and does not need differential pumping in the typical process pressure range, the chance for this condensation is minimized. Hydrocarbons may be present in vacuum web coaters as a result of outgassing of volatile compounds from the web as well as backstreaming of oil on diffusion pumped systems. Any hydrocarbons present in the system can adversely affect the quality of the coatings being produced. Due to the unique emission signature for each type of hydrocarbon, it is possible to discern the source of the hydrocarbon contamination. Conclusion An optical process monitor has been developed for vacuum coating processes which can run at pressures from 1x10-6 to 0.5 mbar without the need for differential pumping. Water vapour, air leaks and hydrocarbon contamination can be monitored both during pump down and the pre-treatment or coating process itself. This will allow for better control of coating quality both during a coating campaign as well as from campaign to campaign.