High Accuracy Thickness Control With Direct Monochromatic

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Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND® High Accuracy Thickness Control with Direct Monochromatic Monitoring Alfons Zöller, Detlef Arhilger, and Harro Hagedorn Bühler Alzenau GmbH (formerly Leybold Optics), Alzenau, Germany Contributed Technical Article D irect monochromatic monitoring in intermittent mode has been widely used in production of optical coatings since 2005. It is used in classical box coaters and in high precision sputtering systems for many different layer systems such as dielectric mirrors, antireflection coatings, sophisticated edge filters, polarizer coatings, beam splitters, multiple cavity band-pass filters, and notch filters. The film thicknesses and properties on the monitor glass or witness are identical or at least very close to those of the substrates. Computer simulation and pre-production analysis virtually eliminates the need for test and calibration runs. A repeatability experiment using a UV-IR cut filter, and the first run result of a challenging notch filter coating, show clearly the advantages of this monitoring technique. the monitor glass crosses the light beam. The measured transmittance value is then stored and refreshed at each rotation. Introduction Figure 1. Schematic diagram of PIAD process with intermittent monitoring on dome. The principle of monochromatic monitoring is well described in the literature [1-4]. During film growth the variation of the transmittance or reflectance at a selected single wavelength is measured with high accuracy. The layer cut-off condition is coupled to turning-points and therefore linked to the optical thickness. Variations of the refractive index are compensated which makes this monitoring technique very robust in a production environment. For many layer systems, errors made during the deposition of a layer are compensated in subsequent layers. However, in some cases error accumulation may take place. A simple method to prevent this is to change the monitor glass between the layers. For each layer system a monitoring strategy has to be determined, which includes selection of wavelengths and potential test glass changes. Substantial progress with direct optical monitoring in intermittent mode on rotating substrate holders was reported in 2005 [5]. Since that time this monitoring technique has been successfully applied in a large number of box coaters with substrate holders up to 1500mm in diameter. It is also established in plasma assisted magnetron sputtering (PARMS) systems [6]. In the case of multilayer designs with irregular and widely varying thicknesses and large numbers of layers, it is an extensive task to select a stable monitoring strategy. Computer simulation for optimization of the monitoring strategies and pre- production analysis was therefore introduced and is used extensively [7,8]. However, for multilayer systems with broad blocking ranges it is sometimes difficult to find an adequate monitoring strategy on a single test glass. A test glass changer for direct optical monitoring was therefore developed and introduced [9]. A quartz halogen lamp is used as the light source. Illumination of the monitor glass is either via a fiber optic with collimator, or light source assembled directly onto the chamber. The transmitted light is fed into the entrance slit of a grating monochromator by use of a fiber optic. A photo detector is mounted onto the exit slit of the monochromator. In case of planetary substrate motion the substrates are placed on planets while the monitor glass is located on a holder between 2 planets. Figure 2 shows the top view of a box coater with such a planetary arrangement. The planets are moved with the sun wheel operating with single rotation at around 20 rpm. In addition, each planet rotates around its own axis and is driven by the sun wheel using a special gear system. The double rotation is used to improve the thickness uniformity on the substrate holders. The monitor glass, located in a holder, fixed on the sun wheel, moves in single rotation. The monitor glass is located at the same circumference as the center of the planets. With this arrangement the film thickness on the monitor glass is representative of the thickness in the center of the planets. The measurement is synchronized with the drive axis of the central rotation. Hardware Set-Up Monochromatic monitoring in intermittent mode is frequently applied in box coaters and in high precision magnetron sputtering systems. The performance of the optical monitoring system employed is described elsewhere [15]. PIAD Box Coater Figure 1 shows the schematic of an intermittent measuring arrangement in a typical box coater. A substrate or a witness, which is located out of center on the dome, acts as the monitor glass. The control electronics of the optical monitor is synchronized with the substrate drive, and during each rotation the transmittance is measured for a few milliseconds while Figure 2. Top view of a box coater with planetary substrate holders. Magnetron Sputtering In high precision magnetron sputtering systems the intermittent monitoring is done in the same way as described above. Figure 3 shows the schematic of a HELIOS sputtering system. A high speed, planar substrate holder moves the substrates below the sputtering cathode. One of the substrates, or a witness located on a substrate holder, is used as the monitor glass. At each rotation a small thickness increment is deposited. Like the monitoring, the deposition is also intermittent. The 34 | SVC Bulletin Spring 2015 Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND® Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND® light beam of the monitoring system is guided with fiber optics. The typical rotation speed is in the range of 200 rpm. the wavelength is kept constant. In addition to thickness monitoring at a single wavelength, the optical monitor is also able to measure spectra by scanning the monochromator. This function is useful for comparison of the optical performance between vacuum and air, while for R&D purposes it may also useful to measure spectra between the depositions of individual layers. Applications Figure 3. Schematic diagram of a HELIOS sputtering with intermittent monitoring. Monitoring System Depending on the required wavelength range the monitor system is equipped with a silicon (Si), indium gallium arsenide (InGaAs), photo multiplier (PMT) or led sulphide (PbS) detector. It covers the wavelength range from 330 nm to 2400 nm in combination with quartz halogen lamp. A deuterium lamp is used for the UV spectral range from 200nm to 330nm. The noise to signal ratio of the measured transmittance is typically kept below 0.02% with Si and InGaAs detector. This low signal noise allows a very accurate turning-point and trigger-point detection. Extensive software algorithms were used for signal processing, turningpoint detection and online correction of trigger-points. The selection of the monitor wavelengths, which may vary from layer to layer, depends on the monitoring strategy employed. During the deposition of a layer Intermittent monitoring is now used successfully in production for many different layer systems such as dielectric mirrors with a simple λ/4 design, anti-reflection coatings for the VIS, NIR and mid IR spectral range, sophisticated edge filters, polarizer coatings, beam splitters, multiple cavity band-pass filters and notch filters [10-15]. Film thicknesses and properties on the monitor glass or witness are equal or at least very close to that of the substrates. This is the huge advantage, as illustrated clearly by the application examples described below. UV-IR Cut Filter An experiment using a UV-IR cut filter with alternating Nb2O5/SiO2 layers was made using a Syruspro 1500 (Leybold Optics GmbH) box coater with APSpro plasma source. The monitor wavelengths were selected in order to apply online corrected trigger-point cut offs. The deposition rates were 0.5 nm/s for both materials. Five consecutive production runs were made, each with run five substrates distributed over the radius of the 1450 mm diameter dome. The monitor glass was located 550mm out of center and the rotation speed was 33 rpm. The optical performance of all 25 substrates is shown in Figure 4a, which includes uniformity and reproducibility. Figure 4b shows the analysis of the reproducibility of the different continues on page 36 ,72'HSRVLWLRQ 6ROXWLRQV 'RQ·WVHWWOHIRUOHVVWKDQ2ULJLQDO*HQHUDO3ODVPD3URGXFWV Rotary Magnetron 8OWUD6WUHQJWK0DJQHW%DU 0RYLQJ0DJQHW 3ODQDU0DJQHWURQ0030TM) The Leader in Advanced Thin Film & Source Technology! 5RWDU\0DJQHWURQV‡3ODQDU0DJQHWURQV‡/DPEGD0D[‡/LQHDU,RQ6RXUFHV (WK6W‡7XFVRQ‡$=‡‡86$‡7HO‡)D[‡VDOHV#JHQHUDOSODVPDFRP __________________ Spring 2015 SVC Bulletin | 35 Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND® Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND® High Accuracy Thickness Control with Direct Monochromatic Monitoring continued from page 35 radial substrate positions on the dome. Position 1 was located close to the center while position 5 was placed on the outer position of the large dome, with the other positions in between. Each bar is equivalent to the total deviation of the wavelengths of the long wave transmittance edge at the respective position. The monitor glass, which was located at position 4, shows the best repeatability (<+/- 0.1%). The reproducibility of the outer position 5 is approximately +/- 0.2 %. Position 4 and 5 represent around. 50% of the useful substrate area. This shows clearly the advantage of direct monitoring. Further details of this experiment are described elsewhere [12]. Figure 4a. Reproducibility and uniformity of ive runs with ive substrates distributed over the dome. Figure 5a. Theoretical monitoring curves for text-glass 2 (– H-index material; – L-index material). Figure 5b. Optical performance of a 210 layer notch ilter in comparison to theory. Conclusions Direct monitoring in intermittent mode is well established for PIAD and sputtering processes. The results achieved for dielectric mirrors, anti-reflection coatings, sophisticated edge filters, multiple cavity bandpass filters and notch filters have been published in various papers over the last decade. The repeatability experiment using a UV-IR cut filter and the first run result of a challenging notch filter coating show clearly the advantages of this monitoring technique. The monitoring reproducibility is negligible in comparison to the uniformity over large area substrate holders. Computer simulation and pre-production analysis mostly eliminates the need for test and calibration runs. References Figure 4b. Reproducibility of the individual positions on the dome. 1. H. A. MacLeod Thin-Film Optical Filters, IOP Publishing ISBN 0- 7503- 0688 -2, 1986, pp. 499-522 Notch Filter Coating 2. Ronald R. Willey Practical Design and Production of Optical Thin Films, Marcel Dekker ISBN: 0-8247-9428-1, 1996 3. S. Larouche, A. Amassian, B. Baloukas, L. Martinu, “Turning-point monitoring is not simply optical thickness compensation”, conference paper in Optical interference coatings (Optical Society of America, Washington, DC, 2004), paper TuE8 4. A.V. Tikhonravov, M. K. Trubetskov, “Eliminating of cumulative effect of thickness errors in monochromatic monitoring of optical coating production: theory”, Appl. Opt. doc. ID 74972 (2006) 5. A. Zöller, M. Boos, R. Götzelmann, H. Hagedorn, W. Klug, “ Substantial progress in optical monitoring by intermittent measurement technique”, SPIE Vol. 5963, 2005 6. M. Scherer, H. Hagedorn, W. Lehnert, J. Pistner, “Innovative production of thin film laser components”, SPIE Vol. 5963-45, 2005 7. A. Zöller, M. Boos, H. Hagedorn, B. Romanov, “Computer simulation of coating processes with monochromatic monitoring”, SPIE Vol. 7101, 2008 8. A.V. Tikhonravov, M. K. Trubetskov, “Computational manufacturing as a bridge between design and production”, Appl. Opt. 44, 6877-6884 (2005) 9. A. Zöller, H. Hagedorn, W. Weinrich, E. Wirth, “Testglass changer for direct optical monitoring”, SPIE Vol. 8168, 2011 A notch filter coating for the NIR spectral range was produced in a HELIOS sputtering system. The coating design consists of 210 alternating layers of Nb2O5/SiO2 with a total layer thickness of approximately 38 μm. The individual layer thicknesses vary between 5nm and 350nm. Three different test-glasses were used for the monitoring. Figure 5a shows the theoretical monitoring curves for testglass number two at 1595nm. The rotation speed was 180 rpm while the monitor glass was located 450mm out of center. Figure 5b shows the measured optical performance of the first run in comparison to the theory. The FWHM of the reflection band is app. 90nm while the rejection bandwidth with optical density 4 is > 60nm. This result demonstrates both, the outstanding monitoring performance and the high process stability of the magnetron sputtering system. 10. A. Zöller, M. Boos, R. Goetzelmann, H. Hagedorn, W. Klug, “Direct Optical Monitoring in Intermittent Mode”, 49th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, pp. 229-234, 2006 36 | SVC Bulletin Spring 2015 Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND® Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND® 11. M. Scherer, U. Schallenberg, H. Hagedorn, W. Lehnert, B. Romanov, A. Zöller, “High performance notch filter coatings produced with PIAD and magnetron sputtering”, SPIE Vol. 7101, 2008 12. A. Zöller, M. Boos, H. Hagedorn, A. Kobiak, H. Reus, B. Romanov, “Direct Optical Monitoring Enables High Performance Applications in Mass Production”, conference paper in Optical interference coatings (Optical Society of America, Washington, DC, 2007), paper WC3 13. A. Zöller, J. Williams, S. Hartlaub, “Precision Filter Manufacture Using Direct Optical Monitoring”, in conference paper Optical interference coatings (Optical Society of America, Washington, DC, 2010), paper TuC8 14. T. Begou, C. Hequet, F. Lemarchand, M. Lequime. “All dielectric broadband mirror for Fabry-Perot interferometer”, in conference paper Optical interference coatings (Optical Society of America, Washington, DC, 2013), paper PTE.6 15. A. Zöller, M. Boos, R. Goetzelmann, H. Hagedorn, B. Romanov, M. Viet, “Accuracy and error compensation with direct monochromatic monitoring”, conference paper in Optical interference coatings (Optical Society of America, Washington, DC, 2013), paper WB5 About the Author Alfons Zöller Alfons Zöller received his Diploma in Electrical Engineering in 1973 from the University of Applied Science, Frankfurt, Main, Germany. In 1976 he joined the Leybold Group, and spent 13 years engaged in the development of optical monitors and automatic control systems. Later on he was responsible for the development of the APS Plasma Source and plasma ion assisted coating processes, including precision optics and ophthalmic applications. From 2000 to 2002 he was Plant Manager and Managing Director of Corning NetOptix’s German operation. In this position he drove a start up facility for development and production of DWDM thin film filters for fibre optic applications. In 2003 he re-joined Leybold Optics as Manager of R&D, and is responsible for R&D of optical monitoring systems and processes for precision optics applications. ____________ For further information, contact Alfons Zöller, Bühler Alzenau GmbH, at [email protected]. ________________  2015 SPRING MEETING & EXHIBIT April 6 – 10, 2015 | San Francisco, California REGISTER NOW Preregistration Deadline – March 20, 2015 Ź Energy Meeting Chairs Ź Nanomaterials Artur Braun Swiss Federal Laboratories for Materials Science and Technology (EMPA) Ź Electronics and Photonics Ź Soft and Biomaterials Ź General – Fabrication and Characterization Hongyou Fan Sandia National Laboratories Ken Haenen Hasselt University & IMEC vzw Lia Stanciu Purdue University Jeremy A. Theil Quantumscape, Inc.  www.mrs.org/spring2015 Spring 2015 SVC Bulletin | 37 Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | Next Page M q M q M q M q MQmags q THE WORLD’S NEWSSTAND®