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Vacuum 83 (2009) 518–521

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Vacuum

journal homepage: www.elsevier .com/locate/vacuum

New target materials for innovative applications on glass

Steven Matthews a, Wilmert De Bosscher a,*, Anja Blondeel a, John Van Holsbeke a, Hilde Delrue b

a Bekaert Advanced Coatings NV, E3-laan 75–79, 9800 Deinze, Belgiumb Codelresearch, Residence Horlitin 7a, 7750 Mont-De-L’Enclus, Belgium

Keywords:Rotating cylindrical magnetronsSputter depositionSputter target manufacturingThermal sprayingSiliconTitanium oxideITO

* Corresponding author. Fax: þ32 93 800 667.E-mail address: Wilmert.DeBosscher@Bekaert.com

0042-207X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.vacuum.2008.04.065

a b s t r a c t

In the new emerging markets of flat panel display, photovoltaic and optical coating applications, theintroduction of cylindrical rotating magnetron technology can accommodate the needs for faster, betterand cheaper coating processes. Recent developments of hardware (compact end blocks, etc.) and targetmaterials for rotatable magnetron technology offer a total solution to the innovative thin filmapplications.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The arrival of the new millennium brought along an increase inour demands for a higher quality of life through products withimproved functionality at ever decreasing prices. At the same time,environmental awareness has necessitated the development andimplementation of clean and renewable forms of energy efficientproducts and power generation. In order to accommodate suchdemands, the architectural, automotive and display industries havedeveloped complex multilayer, and multifunctional thin filmcoatings for components of ever increasing dimensions.

These demands have led to rapid technological advancementswithin the thin film industry. Vacuum coating by sputtering hasbeen at the forefront of these technological accelerations. The easeof scalability to high volumes, the wide variety of coating materialsand possible coating stacks offer crucial benefits to thin film pro-ducers. Traditionally, thin film applications have developed in twomain routes: high volume, large area applications, typically in thearchitectural and automotive market, and applications involvingcomponents of smaller dimensions but greater complexity incoating functionality, typically in the electronics industry. However,the demands for larger component sizes, continuous productionand reduced manufacturing costs have seen the traditionalboundaries begin to blur. This is particularly reflected in the in-creasing incorporation of the developments from large area glassindustry into the production of flat panel displays and solar cells.

In this article, the recent developments in rotating cylindricalmagnetron technology are presented. Economic and technical

(W. De Bosscher).

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benefits of cylindrical versus planar targets observed during the last10–15 years in the large area glass coating industries are discussed.The development in manufacturing technologies of sputter targetsis presented, highlighting the attributes of target production bythermal spraying. Finally, a selection of recent developments intarget materials is presented, aimed primarily at innovativeapplications of transparent conducting oxides in flat panel displays,smart windows, touch panels and photovoltaic solar cells.

2. Benefits of rotating cylindrical magnetrons

Since their invention in the late 1960s, sputtering electrodeshave undergone dramatic development. One of the most importantadvances has been in the development of rotating cylindricalmagnetrons and advanced rotating cylindrical sputter targets. Thebenefits of this technology have been embraced by the large areaglass coating industry where rotating cylindrical magnetrons havebeen successfully used since the early 1990s (see Fig. 1).

The geometry and sputter performance of rotating cylindricalsputter targets result in several advantages relative to planarsputter targets, as discussed in the following sections [1].

2.1. Increased target material utilization

In a planar magnetron, the magnet array beneath the targetgenerates a magnetic field above the target surface. During sput-tering, this ‘‘race track’’ shaped magnetic field forces the electronsleaving the target to drift within a closed loop magnetic ‘‘bottle’’ [2].Interactions between these trapped electrons and gas atoms resultin ionization of the gas and the formation of a plasma with the samerace track shape as the magnetic field. Positive ions from theplasma are accelerated towards the negative target, leading to

Fig. 1. A dual rotating cathode system for large area glass coating applications [13].

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sputtering of the surface atoms. This race-track pattern results ingroove formation in the target, leading to localized materialthinning in some areas, while other zones remain essentiallyuntouched. This inhomogeneous rate of removal means that thepercentage of sputtered material in conventional planar targets israrely higher then 40%, and in practice is typically closer to 25% (seeFig. 2) [3,4].

Similar to planar targets, the magnetic field in a cylindricalrotating magnetron system is in a fixed location outside the target.However, because the cylindrical target is rotating through themagnetic field it continuously exposes new material to sputtering.In this way, material is homogeneously sputtered along the targetlength in a continuous, controllable manner. This results in gradualcoating thinning around the target circumference rather than thelocalized groove formation. With this approach, target utilization istypically at least 75% [1]. Not only is this a significantly moreefficient use of the target material but it also leads to less machinedown time and lower recuperation costs.

2.2. Higher target material inventory

The cylindrical nature of the coating material on a rotatablesputter target means that it may contain up to three times thevolume of material compared to a planar target (assuming anequivalent target width/diameter and the same coating thickness)[3]. This also leads to fewer target changes, thus reduced machinedown time [1].

2.3. Increased sputter rates

As demands increase for higher productivity on existingequipment, deposition rate becomes increasingly important.Higher rates are most readily achieved by increasing the powerapplied to the target, resulting in higher power densities. For planartargets, this is not always a possible solution because the bondingand/or target material may melt or crack under the high thermal

Fig. 2. Schematic representation of the race-track pattern for planar versus cylindricaltargets.

load. In contrast, the thermal load is spread homogeneously overthe complete circumference of a rotating cylindrical target. Becausematerial is only subjected to the thermal load as it is rotatedthrough the hot zone of the plasma, the peak-localized temperatureis reduced. Furthermore, the thermal energy is removed moreefficiently by the internal cooling water because of the extendedperiod of time during each rotation outside the plasma [3]. Asa result, significantly higher power densities can be used onrotating cylindrical magnetron systems. This means that the targetscan operate at a higher sputter rate, resulting in a higher coaterthroughput. Alternatively, it means that fewer targets are requiredto achieve the same sputtered layer thickness, thereby freeing upcoater positions for alternative target materials and the potential tosputter more complex stacks on the same hardware.

2.4. Improved process stability

Rotating cylindrical magnetrons have been shown to providea much more stable and reproducible sputter target surface com-pared to planar magnetrons [5]. As the cylindrical target rotates, itis continuously ‘‘cleaned’’ by the plasma, preventing build-up ofreaction products in the actively sputtered region. Therefore, thepotential for arcing associated with insulating reaction products islimited to the thin bands of material at the target ends [6]. A benefitof this is that the arc sensitive zone for rotating cylindrical targets isindependent of the target length [1]. Such attributes minimize thelevel of arcing, leading to a more stable process and reduced risk ofdefects in the sputtered layer.

2.5. Enhanced sputter process efficiency

The race-track pattern in a cylindrical magnetron system isnarrower than on a planar target. As a result, the ejection ofsputtered particles is focused in a narrower angle towards thesubstrate. This results in an increased efficiency of substrate coatingsince sputter deposition on the coater walls and shields is mini-mized, both reducing the number of required cleaning cycles andthe risk of particle contamination resulting from spallation.

During AC sputtering, use of a rotating cylindrical magnetron asan anode eliminates the disadvantage of a ‘‘magnetic anode’’thanks to the larger effective anode and field free area relative toplanar magnetrons. As a result, reductions in cathode voltage anddeposition rate are not an issue for dual magnetron sputtering withrotating cylindrical magnetrons.

3. From planar to cylindrical magnetrons

While cylindrical magnetrons are now standard in large areaglass coating applications, the display industry still relies heavily onDC sputtering of planar targets. The slow transition in technology todate has been the result of several factors [1].

Since the cost of cylindrical magnetrons does not scale linearlywith substrate or target sizes, this technology has been a costeffective alternative for large glass coaters processing substrates upto 3.2 m� 6 m. With the traditionally small dimensions of displaysin the past, this feature was of limited significance. However, withthe increasing sizes of display, photovoltaic and optical compo-nents, coupled with the greater demands on throughput, cylindri-cal magnetrons are becoming a viable alternative for planarmagnetrons in a display coater.

Until recently, the vertical orientation of planar targetscomplicated the option of retrofitting standard large area rotatingcylindrical technology. Replacement of planar cathodes with stan-dard end block hardware resulted in shorter target lengths, therebycompromising the uniform sputtered coating width. However, withthe recent introduction of compact end blocks, developed

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specifically for retrofitting smaller coaters, and new verticalmagnetrons for integration into next generation coaters for display,solar and optical applications, the advantages of cylindricalmagnetrons are now available to the display industry [1].

A final limitation has been the availability of target materials,especially transparent conducting oxides (TCOs). The portfolio ofcylindrical sputter targets has been geared primarily at therequirements of large area glass coating applications in the past.However, as with the sputter hardware, significant developmentshave been made in target materials for the cylindrical technology.The two most important TCOs, namely indium-tin-oxide (ITO) andZn-oxide, are now available as cylindrical targets and will bediscussed in the following sections.

4. Production of sputter targets

Many of the production routes for planar targets have beenadapted to the manufacture of cylindrical targets. In general, targetproduction technologies can be divided into two broad groups,namely melting/casting technology and powder consolidationtechnology (hot (isostatic) pressing, cold isostatic pressing andsintering) [4].

The target material properties largely dictate which productionroute is used. Low melting point metals and alloys are typicallyproduced by casting techniques, either directly on a backing tube oras segments subsequently bonded onto a backing tube. Ceramiccompounds and some metals/alloys are not feasible to cast due totheir high meting points and are therefore processed via powderprocessing routes. With regard to the production of cylindricaltargets, the benefits and limitations of each production techniquewith regard to purity, density and grain size are generally similar tothose observed for planar targets.

A notable exception to these techniques adapted from bulk ma-terial fabrication is thermal spraying. Target processing by thermalspraying has developed as a niche industry out of the well estab-lished thick film coatings for wear and corrosion resistance appli-cations. While not widely known in the planar target production,this has become the preferred route for several target compositions.

Thermal spraying is a generic coating technique wherebydroplets of molten or softened material are projected to a surface toform a coating. As a result, thermal spraying is able to apply anymaterial that does not sublime and has a stable melting point.Materials ranging from polymers to metals, alloys, cermets andceramics are commonly sprayed.

This material deposition method generates a characteristicmicrostructure. As the molten droplets impact the surface, theyspread out into pancake-like ‘‘splats’’ and rapidly cool. During thisprocess, the material flows in and around the surface featuresbefore shrinking onto it. In this way, the splats form a high strengthmechanical bond with the backing tube. The nature of coatingbuild-up leads to some degree of microscopic porosity which canbe reduced to a minimum by proper process control.

5. Advantages of thermal spraying as a sputtertarget manufacturing technology

The unique attributes of thermal spraying offer severaladvantages to the thin film sputtering process in relation to thealternative production techniques.

5.1. Wide range of target compositions

As highlighted above, any material with a stable melting pointand no sublimation can be applied. In addition, the fact that thefeedstock can be processed in powder form means that there are nophase diagram restrictions on the range of target compositions.

Dopants are readily incorporated in the coating and their size anddistribution can be finely tuned without limitations imposed by thesolidification process. Through appropriate process control,material combinations with widely varying melting points (likeSiþAl) can also be processed into single-piece targets. Further-more, the nature of the process enables homogeneous coatingcompositions to be readily achieved over the full target length andcoating thickness. Thanks to the high cooling rates of the sprayedmaterial, the backing tube is hardly affected by any thermalinteraction with the molten droplets. Accordingly, no diffusionlayer forms between the substrate and coating. This way, the targetcomposition is preserved over the complete layer thickness [7].

5.2. Uniform and high sputter rates

Since the target material is applied directly on to the backingtube during thermal spraying, no additional fixation with lowmelting point bond materials is required. Therefore, there is no riskof bond material related failures during sputtering. This is partic-ularly advantageous given the increasing length of sputter targets.In addition, it enables the potential for higher power densities, thushigher sputter rates, to be applied without the risk of melting thebond material.

The nature of material build-up in thermal spraying lends itselfto coating of cylindrical backing tubes and results in ‘‘continuous’’and smooth target surfaces that are presented for sputteringwithout inhomogeneities associated with edges and interfaces incase sleeves or tiles are used. This eliminates the risk of preferentialsputtering of sharp edges and the subsequent formation ofheterogeneous ‘‘bands’’ in the sputtered layer [8].

The fine size of the molten droplets in this technique ensuresthat the grain size of the coating material is maintained at a verysmall size (<100 mm) over the entire length and thickness of thetarget, irrespective of the target material. This results in uniformsputter rates over the target length [8] and potentially higherdeposition rates relative to coarser grained materials produced byother techniques.

5.3. Flexibility in target dimensions

The hardware for thermal spraying is not related to the targetdimensions. This offers significant versatility in the achievabletarget lengths and coating thicknesses. Furthermore, complextarget geometries such as the long-life ‘‘dog-bone’’ shaped targetsare readily manufactured.

Developments generated on an R&D scale are readily up-scaledto full production without the need for additional equipment andwithout the level of risk associated with larger and/or thicker targetdimensions. As the cost of thermal spray manufacturing is pre-dominantly consumable based, new developments can be achievedquickly and more economically than many competing technologies.

5.4. Recycling of spent targets

An additional benefit that is being introduced is target recycling.Thermal spraying offers the possibility for some materials to beapplied directly onto the consumed sputter targets which stillcontain significant amounts of material. In this way, both theunused material and the backing tube may be reused without theneed to strip off and refine the material.

6. Developments in sputter target materials

Recent developments in sputter target materials illustrate thesuccessful and powerful combination of thermal spraying as a tar-get manufacturing method and the rotating cylindrical magnetron

S. Matthews et al. / Vacuum 83 (2009) 518–521 521

technology, this way providing a total solution to the thin filmindustry.

6.1. Silicon targets

SiO2 and Si3N4 are widely used sputtered layers in the large areaglass industry. Pure Si targets are not commonly used as a startingmaterial for such layers due to their poor electrical conductivity atambient temperature. For this reason, Al is added to improve theelectrical conductivity, while it also increases the target toughness.By appropriate control of the powder feedstock, the Al doping levelcan be varied over a broad range, typically 0–19 wt%, whilemaintaining a strict control over the final composition and homo-geneous phase distribution. The addition of Al to the Si sputteringtarget has a minimal effect on the key thin film properties.Currently, thermal spraying is the production method of choice forsputter targets up to 15200 in length and for thicknesses up to 9 mm(with 13 mm thick dog-bone sections). For some specific applica-tions, pure Si targets may be preferred. Thermal spraying is capableof manufacturing these targets as well. Sputtering is performed instandard mid-frequent AC powering conditions.

6.2. Titanium oxide targets

The sputter process of TiO2 layers requires Ti-oxide targets thatare electrically conductive. This is most commonly achieved viathermal spraying, during which TiO2 powder undergoes partialreduction to form sub-stoichiometric, n-type semi-conductingmaterial described as TiOx (x< 2) [9]. The high cooling rate of theTiOx splats upon deposition freezes this meta-stable phase,resulting in a conductive material at room temperature [7].

The use of TiOx instead of pure metal Ti targets increases thesputter deposition speed and enhances the process stability inreactive processes substantially. The deposition rate of TiO2 ina reactive process from metallic Ti is very low. In order to depositstoichiometric TiO2 without additional control systems, very highoxygen flows are required, causing the metallic target to sputter inthe ‘‘poisoned’’ mode. Even if an advanced process control system isused (such as PEM control), the deposition speed remains limited.When sputtering from a conductive TiOx target, a high sputter ratein stable sputter conditions is obtained.

6.3. Targets for transparent conducting oxides

The market for TCOs has grown drastically over the last decade,driven by the expanding use of flat panel displays and photovoltaicsolar cells. The use of TCOs is also increasing in a wide variety ofother applications such as smart windows, touch panels, EMIshielding, etc. The TCO thin films combine a high visual trans-parency with a high electrical conductivity.

The best performing TCO is ITO. The ITO-target developmentreflects a growing trend in the transition from reactive sputtering ofmetallic targets towards the use of conductive oxide targets. Whilemetallic indium based targets are the cheapest to produce, theyrequire large amounts of oxygen during reactive sputtering andrequire complex process control to maintain consistent operation[10]. Oxidic ITO targets have been found to offer simpler processcontrol and enable a more stable sputter process.

In accommodating the increasing significance of the displayindustry, target manufacturers have recently developed cylin-drical ITO targets for display applications. While produced bydifferent manufacturing methods by different suppliers, thesetargets offer comparable sputter behavior to planar targets,while providing all the benefits of cylindrical technology. Suchbenefits have been shown to reduce the total cost of the sput-tered layer significantly relative to the use of planar targets[1,11].

Zn-oxide is a second major TCO and the material of choice forthin film photovoltaics. While not offering the same optimum layerproperties as ITO films, the cost of this material is significantlylower, making it an attractive solution. Furthermore, the Zn-oxidelayers are often applied next to Ag layers in sputtered stacks forlow-E applications. As Zn-oxide based targets do not requireoxygen addition during sputtering, they prevent the risk of poi-soning the neighboring metallic Ag targets [12].

In addition, the low vapor pressure of Zn-oxide favors the use ofcylindrical versus planar targets for such applications. Vaporizationof Zn-oxide leads to significant amounts of re-deposition duringsputtering of planar targets. This build-up increases the risk ofparticle spallation onto the sputtered layer, thereby causing failureof the film.

Zn-oxide based cylindrical targets are now available fromseveral target suppliers, under several registered trade names.

7. Conclusion

In this article, recent developments in the manufacturing ofsputter target materials are presented. Again, the cylindrical sput-ter targets manufactured by thermal spraying and the associatedrotating cylindrical magnetrons prove to be a successful combina-tion. Where the benefits of the rotating cylindrical magnetrontechnology are established in the traditional large area glasscoating industry for 10–15 years, this technology graduallyconquers the world of the transparent conducting oxides (TCOs) inflat panel displays and photovoltaic solar cells. Most innovativedevelopments are the ITO and Zn-oxide sputter targets and relatedsputter hardware.

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