ABSTRACT
Reactive Sputtering of oxides by means of Dual Magnetron cathodes in mid frequency mode (MF) offers the best pre-conditions for long term stability of optical antireflective (AR) coatings. Features of reactive sputtering processes for deposition of SiO 2, TiO 2, SnO 2 and ITO will be discussed with regard to the measured intensity of a spectral line of the indi-vidual target material sputtered.
Highest stability at high deposition rates of the processes is obtained by application of Plasma Emission Monitor (PEM)control circuits. This way of controlling the reactive gas in-let depending on in situ  measured spectral line intensity of-fers the possibility for optimization of both high film quality and high productivity. A comparison of dynamic deposition rates will be made for high and low index oxide materials.New approaches for in situ  process monitoring and balance control to improve both uniformity and reproducibility of working points for large area multilayer AR coating on PET foils will be presented.1
INTRODUCTION
Optical multilayers on large-area glass panes or long webs can suitably be produced by reactive magnetron sputtering.In this field of applications there is an ever increasing de-mand for highly insulating layers such as SiO 2 and Al 2O 3 the long-term stable production of which is a particular challenge to thi
n film technology. The large-area coating calls for high and stable deposition rates as well as for reproducible opti-cal layer properties especially for TiO 2. An adequate design
of magnetron sources and their surroundings yields outstand-ing values with respect to coating uniformity. Here the mag-netic field configuration and the implemented reactive gas inlet near the target are of decisive importance.
A new approach for the long-term stable production espe-cially of SiO 2 layers is given by Dual Magnetron sputter sources. They operate in bipolar mode, using pulsed DC or MF power supplies for the purpose. The latter increasingly claim a firm place in production plants and make use of a sine-wave voltage curve. In either case the cathode and anode functions on the target surface change periodically.2
PROCESS STABILIZATION FOR REACTIVE SPUTTERING
For deposition of top grade oxide layers either in production plants or on a laboratory scale it is first necessary to over-come the basic problem of spontaneous target coverage with compound layers. This results in a drastic rate reduction that becomes evident in the hysteresis behavior of the reactive sputter process (Fig. 1). Stable working points will be attained in the ranges I and III only. The region b
etween the limiting points A and B can be utilized for reactive processing in pro-duction plants only conditionally or not at all.
All measures for process stabilization have been devised to overcome the said hysteresis behavior and to obtain the high-est possible rate for the defined layer property requested.The marked ranges of an existing basic stability (range I and range III) must unconditionally be extended into the transi-tion mode to ensure the satisfactory operation of production plants. Technological measures developed to approach this goal are discussed below.
Reactive Dual Magnetron Sputtering of Oxides for Large Area Production of
Optical Multilayers
J. Strumpfel, G. Beister, D. Schulze, M. Kammer and St. Rehn, V on Ardenne Anlagentechnik
GmbH, Germany Keywords: Pulsed & periodic deposition; Process monitoring - in situ ; AR coatings;
Dual magnetron deposition
Figure 1.  Schematic behavior of sputter rate versus constant Oxygen flow during reactive sputtering.
Stable conditions for depostion can be provided within the metallic and reactive mode only (range I + III). It can be extended within the transi-tion mode of range II by special PEM control modes.2.1CONTROL VIA PLASMA EMISSION MONITOR The disadvantageous behavior illustrated in Fig. 1 has been defeated successfully with the aid of the Plasma Emission Monitor (PEM) [1, 2]. The corresponding control loops make use of the in situ  intensity measurement of a spectral line of the sputtered target material as a measure of the actual metal sputter rate above the target. Referenced to the nominal value,the reactive gas inlet is controlled as a function of this sig-nal. Gas admission preferably occurs near the target. The PEM control loop regulates the ratio of collision numbers between the sputtered metal particles and the admitted reactive gas.On the working point desired this ratio is kept constant. The rate increase obtained by means of the PEM is related to the layer property required and, for a great many materials,amounts to a factor 2-3. In addition, this method permits a rate stabilization to ≤± 3%.
The unfavorable hysteresis behavior also exists in the case of reactive sputtering with Dual Magnetrons in the MF mode.As known from the DC mode, proper matching of the PEM control loops causes a substantial improvement of the pro-cess stability also in the MF mode especiallay for materials like TiO 2, SnO 2 and ITO. The features of such a PEM control loop on a Dual Magnetron sp
utter source are evident from Fig. 2. Inlet of the reactive gas takes place parallel to the symmetry line between the targets.
Figure 2.  Intensity Control Mode via Plasma Emission Moni-tor (PEM) control circuit by dosed O 2 inlet for stabilizing of Reactive Dual Magnetron Sputtering 2.2IMPEDANCE CONTROL
For target materials such as silicon and aluminium - that ex-hibit a strong impedance reduction of the discharge upon the admission of oxygen - the impedance control mode is pref-erable in practice [3]. Instead of the intensity of a spectral line the discharge voltage needed to maintain a given nomi-nal power that will be rated (see Fig. 3). This means, how-ever, that the power supply always gives a constant power.The discharge voltage required will be determined by dosed O 2 admission. This control loop also results in an efficient process stabilization. However, such an impedance control proves to be suitable for certain materials only; i.e. SiO 2 and Al 2O 3 layers.
2.3COMPARISON OF V ARIANTS FOR PROCESS STA-BILIZATION Based on the Fig. 4 the variants of
•mass flow control,•impedance control and •PEM control
shall be compared for silicon oxide coatings. All specified points A through M correspond with those in Fig. 1. This non monotonic behavior of the intensity versus MF-voltage is typically but exceptionally valid for silicon only. The reason for these behavior is the strong impedance shift.
Figure 3.  Impendance Control Mode via PEM control cir-cuit by dosed O 2 inlet for stabilizing of Reactive Dual Mag-netron Sputtering
To compensate these influences, at least one of the relevant layer properties must be measured in situ  when working with this control variant. The point A is the limiting point with maximum O 2 inlet. For lower voltages the required O 2 flow reduces accordingly. In range III the dynamic deposition rates are very low; they amount to 5 nm·m/min only. With growing operating time of the silicon target this value drops continu-ously.
The Impedance Control  allows to utilize a particularly wide operating range. Independent of the monotony behavior of the reactive gas flow the wanted target state can be adjusted via the control of t
he discharge impedance. As already men-tioned it is a prerequisite that the discharge power of the MF power supply remains constant on all working points chosen.In the case of SiO 2 layers it is recommended to select a work-ing point between 700-750 V because here very good layer properties can be obtained at very high dynamic rates of 45nm·m/min or more (cf. Table 1). In this voltage range the nec-essary oxygen flow reaches its maximum. Compared to the variant with Mass Flow Controller (MFC) the impedance control allows to overcome the hysteresis behavior during SiO 2 coating in full. Hence the PEM controller can be fully utilized as impedance control unit.
The Control Loop with Plasma Emission Monitor  can also be employed to advantage. Compared to the MFC control -performed in range III - the PEM control resulted in a rate increase by a factor 4; i.e. to 21 nm·m/min. Because of the changing monotony behavior of the intensity, however, reli-able operation is ensured only up to a voltage of about 520 V.It should be explicitly stated here that this restriction for PEM holds only with respect to the SiO 2 coating in the P = de.3
BALANCE CONTROL FOR LARGE-AREA COAT-ING
Additional measures are required to improve the uniformity of thickness distributions for large-area high-rate coating of glass or plastic webs by means of reactive sputtering. In the available gas inlet sys
tems, gradients of the reactive gas par-tial pressure are likely to occur as a function of the working point chosen and due to internal degassing processes in the deposition chamber. Very sensitive but efficient control fa-cilities are needed to implement a layer thickness unifor-mity ≤± 2% in order to compensate these minor differences also from the outside. It has been shown again that, in the case of high-rate reactive sputtering, a long-stretched
Figure 4.  Complete behavior of Silicon spectral line inten-sity  λ = 251 nm versus MF discharge voltage during Reactive Dual Magnetron Sputtering of SiO x  at different working points The control of the O 2 flow via Mass Flow Control (MFC)can be carried out in the ranges I and III only (cf. Fig. 1). Here it should be taken into account that, due to changed degassing conditions of the substrate, the different levels of the reac-tive gas partial pressure cannot  be equalized.
magnetron source can work in a long-term stable mode only if dynamic gas inlet corrections are performed in a position of equilibrium [4].
3.1TWO CHANNEL PEM CONTROL
An appropriate system was created for a Dual Magnetron sput-ter source. Two PEM control loops are operating on a cath-ode independent of each other. Gas inlet takes place via two piezo valves through which the gas flow enters on the end of the magnetron so that dosing occurs according to a two-chan-nel intensity measurement (Fig. 5). This active system for balance control turned out to be sound in practice for the deposition of both ITO and TiO 2 layers. All additional passive inlet systems via MFC or needle valves are inferior to this two-channel solution with the PEM.3.2NEW IMBAL CONTROL
A new PC version of the Plasma Emission Monitor PEM combines the two necessary features especially for; i.e.•reliable process stabilization at high rates for SiO 2 as
well as
•additional balance control for the impedance mode (cf.
item 2.2).The features are shown in Fig. 6. By means of only two PEM channels the impedance control and the balance control will be performed. This new concept for balance control is pos-sible by comparison of two input signals of the silicon spec-tral line intensity at right and left side of the Dual magnetron.The right side one acts as a master to provide the setpoint for the left side independently from the setpoint of the impedance control loop [5].
Figure 5.  Arrangement of two channel PEM control for Reac-tive Dual Magnetron Sputtering
Figure 6.  Arrangement of new IMBAL-PEM control for Reac-tive Dual Magnetron Sputtering 4
RESUL TS OF LARGE-AREA COA TING
Given in the following table are the coating rates obtained in a production plant for web coating with dual magnetron length of 1500 mm (Table 1).
Table 1.Comparison of Deposition Rates for Several Oxides Sputtered with Dual Magnetron 1500 mm Length
Material
Mode MF Power
Rate Thickness Specific Rate stationary at 1m / min
at 1m / min AC, bipolar
kW nm / s nm (nm / kW) · m
SiO 2MF, PEM
reactive to183,75457,5SiO 2MF, Impedance 123,754511,25ITO MF, PEM 64,65527SnO 2
MF, PEM
5,5
4,6
55
30
Employed for all reactive processes are Dual Magnetron sput-ter sources. Such an equipment allows t
o obtain an optical multilayer stack that excels in its high layer thickness uni-formity with is essential for AR coating. A precondition to approach this goal are the plasma pretreatment and the in situ measurement of the spectral optical properties of the layer system or of the individual layers. The result of such a high specified AR multilayer stack is shown in Fig. 7. This AR coating was produced in an industrial sputter roll coater on PET webs at high speed. The small oscillations of the mea-sured curve are typically for hard coated PET substrates. The optical performance was obtained by a stack of four active optical layers of high and low index material. The high index material was a transparent conductive oxide.
The cathodes are arranged around cooling drums. Each step for such a optical single layer will be performed in a com-partment which is pumped separately.REFERENCES
[1]S. Schiller, U. Heisig, Chr. Korndörfer, G. Beister,
J. Reschke, K. Steinfelder, J. Strümpfel,
“Reactive D. C. high rate sputtering ...”
Surf. Coat. Technol. 33 (1987) 405
[2]J. Strümpfel
“Prozeßstabilisierung beim reaktiven Hochratezer-
stä”
Thesis, Technical University Chemnitz Germany,
February 1991
[3]Chr. Korndörfer
“Untersuchungen zum reaktiven Hochratezerstäuben von Cr-Si Basismaterial”
Thesis, Technical University Chemnitz, 1990
[4]S. Schiller, U. Heisig, Chr. Korndörfer, J. Strümpfel,
V. Kirchhoff
“Plasma Emission Monitor in web coating”
Bakish, R.: Proceedings of 2nd Int. Conf. on Vacuum web coating,
Ford Lauderdale, USA, Oct. 1988,
[5]J. Strümpfel, St. Rehn, W. Lang
Patent pending
Figure 7.  Plot of an AR coating with antistatic properties on hard coated PET deposited by reactive Dual magnetron sput-tering in mid frequency mode.
5CONCLUSIONS
Reactive sputtering with long-stretched Dual Magnetron cath-odes allows to coat large areas with optical multilayer stacks at high precision in a long-term stable mode. The design of the cathodes as well as the outfit with MF power supplies, matched gas inlet systems and appropriately rated control loops for process stabilization are prerequisites for a pro-duction technology adapted to a given application of web or glass coating.

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