A Remote Plasma Sputter Process for High Rate Web Coating of Low Temperature Plastic Film with High Quality Thin Film Metals and Insulators Dr Peter Hockley and Professor Mike Thwaites, Plasma Quest Limited (PQL) Unit 1B Rose Estate, Osborn Way, Hook, Hampshire, RG27 9UT, United Kingdom (+ 1256 740680) Remote high-density plasma generation sputter deposition technology can overcome several of the limitations inherent in other thin film deposition techniques. The benefits include the ability to sputter thick ferromagnetic materials, high rate and stable reactive sputtering for deposition of dielectrics, deposition of low stress films with properties approaching bulk, and maintenance of stoichiometry from compound targets. We have now developed this technology to operate with a 50cm x 7.5cm diameter cylindrical sputter target to provide a source for high rate, large area thin film deposition especially suited to web coating applications for flexible electronics. In addition to metal and ferromagnetic thin films, high rate stable deposition of alumina and other oxides has been demonstrated by sputtering from the metallic precursor targets in an oxygen ambient. All materials have been deposited with near zero stress and with excellent adhesion onto a range of low temperature substrates, including thin polyethylene terephthalate (PET) plastic sheet. Key Words: Plasma source Sputter deposition Optical coatings Flexible electronics INTRODUCTION Remote high density plasma generation systems have been in commercial use for many years, notably for specialised plasma etch systems and to provide a plasma assist to other deposition techniques (e.g. electron beam evaporation). However, they are yet to make a significant impact in production sputter deposition applications. This is due largely to a perceived absence of clear benefit to offset the added cost and complexity of the systems, coupled with production scale up concerns from the existing research based systems available. In the last 6 years, we have researched and developed thin film sputter deposition processes using our proprietary remote high density plasma generation system to overcome several of the limitations inherent in other thin film deposition techniques. Using circular planar targets of 10cm (4") diameter, we have demonstrated high target utilisation (>90%), ability to sputter thick ferromagnetic targets, high rate stable reactive sputtering, deposition of low stress films with properties approaching bulk, and maintenance of stoichiometry from compound targets. For a large number of applications, we have been able to demonstrate significant benefits or new capability to make the additional cost worthwhile.[1]. However, the systems used for this work are essentially single substrate coating systems, best suited to single 8 diameter wafers or multiple 4 or 6 diameter or equivalent. Our latest work aims to address these scalability concerns. THE REMOTE PLASMA SPUTTERING TECHNIQUE Standard Configuration Our basic high density plasma sputter system (figure 1) essentially comprises a standard vacuum sputter deposition chamber to which a remote plasma source is attached, usually as a sidearm unit, though alternate configurations are also used. The sputter targets are used in diode sputtering mode, i.e. with no internal magnetic elements, an additional (external) electromagnet being used in conjunction with the plasma source electromagnet to direct the remotely generated plasma to the target as indicated in the figure. The remote plasma source is an electrodeless unit with no high temperature components or vacuum feedthroughs and is therefore intrinsically compatible with UHV and reactive process environments. The unit is of similar construction to a helicon plasma source, but is less critical with regard to detailed design and operational requirements. It essentially comprises a Radio Frequency (RF) antenna (run at 13.56MHz as standard) and a DC magnetic coil positioned around a quartz tube of appropriate size to the process and targets to be used. Typically, the tube diameter is 8cm to 15cm, though we have demonstrated effective high density generation with a 20cm diameter unit for a non sputter application. In common with conventional sputter processes, argon is the primary Page 1 of 5
gas used, though oxygen, nitrogen, hydrogen, helium, carbon tetrafluoride and silane have all been shown to work effectively with the unit, either separately or in combination. SIDE ARM PLASMA SOURCE DEPOSITION CHAMBER Substrate Holder Assembly, Shutter, etc. Plasma Path in Chamber Source Electromagnet Target Electromagnet Multiple Indexable Target System Figure 1. Schematic of Standard Remote Plasma Source Sputter Deposition System. Plasma densities of in excess of 10 13 cm -3 can be achieved at the source outlet, limited by the ambient process gas pressure; OES and ion density sampling indicate that ~90% gas ionization is achievable with the source. Probe measurements also show that although high ion densities are generated (corresponding to ion current densities of up to 100mA.cm -2 ), ion energies are low, in the order of 10eV, and therefore do not cause sputtering of electrically grounded or unbiased components of the sputter system. Beyond the plasma source exit, the combination of the DC magnetic fields produced by the source and target electromagnets constrains and guides the plasma to the target. This is a cascade process of continued electron impact ionisation that, if optimised, delivers very high plasma densities at the target surface (up to 10 13 cm -3 typically). It is critical to the advantages that we have found using remote plasma generation that the target is not required to generate or sustain the plasma in any way the plasma is present at zero target voltage and essentially unaltered when bias is applied. Typically, the plasma source is operated at process gas pressures of 3x10-3 mbar and RF power of 1 to 2 kw. In a typical system configuration, this provides a sputter target current density of about 30mA.cm -2. However, the source can also produce plasma at pressures as low as 1x10-4 mbar and the source to target plasma cascade remains effective at pressures up to about 1x10-2 mbar. We have also successfully operated our systems with RF power levels from less than 100W to 5kW. In combination, this provides a plasma density range from the source of from less than 10 10 to about 5x10 13 cm -3, with a correspondingly wide range in sputter target current densities. Targets are usually circular planar i.e. standard sputter targets of 10cm to 20cm diameter and 6mm thickness. For most of our work, multiple targets are mounted on a single, rotatable circular planar holder and individually indexed into an aperture in a dark shield that otherwise blocks the plasma and thereby prevents sputtering from the non-selected targets. Multiple target units generally use 5cm to 10cm diameter circular targets, depending on system size and required target numbers (typically 4 to 6). Linear Configuration We have now further developed this technology to operate with a 50cm x 7.5cm diameter cylindrical sputter target to provide a 'linear' source for high rate, large area deposition especially suited to web coating applications. For little increase in either remote plasma power requirement or overall system size, this significantly increases the target and coating areas compared to our standard circular planar systems, whilst exceeding the high rate deposition capability. An overall potential maximum throughput gain of 50-100 times is achieved. The revised system configuration is as shown in figure 2. Rather than magnetically steering the plasma beam onto the planar target surface, it is instead led to surround the cylindrical surface. Two key characteristics of the remotely generated plasma make this possible. Firstly, although the plasma has an essentially cylindrical ion distribution (thereby providing uniform sputtering of the circular planar targets), the generation region is itself annular in cross-section due to the confinement of the ionising electrons by the DC magnetic field profile. As Page 2 of 5
such the target does not intersect or block the plasma generation region, which can thereby be continued along and around the length of the target. Secondly, it is clear that the plasma source continues to efficiently accelerate electrons within the plasma generation region even when at some distance from the source, up to 0.5m to date. Our results show that, for an appropriately optimised system, ionisation efficiency and hence plasma density is maintained along the full target length, allowing uniform high rate sputtering of the entire target surface. So far as we are able to determine, the plasma density beyond the source itself is primarily controlled by the magnetic field strength distribution, allowing some tailoring of the local sputter rates and thereby enhancement of deposition uniformity. SIDE ARM PLASMA SOURCE DEPOSITION CHAMBER Substrate Holder Assemblies, Shutters, etc. Cylindrical Target Plasma Path in Chamber Source Electromagnet Target Electromagnet Figure 2. Schematic of Linear Remote Plasma Source Sputter Deposition System (shown rotated 90degrees for comparison with figure 1). In common with the standard configuration, sputtering only occurs from the (negatively biased) target itself. As the target is not required to generate or sustain the plasma in any way, local magnetic enhancement is not required and sputtering occurs from the full target surface. The target sputters material uniformly with a 360 degree axial coverage, making it most suitable for use in web coating or similar continuous feed applications. However, multiple planar substrates can also be uniformly coated using suitable carrier plates or multiple rotating carousels. A further aspect of the system operation is that the target wall thickness is unimportant (within realistic limits); the radial geometry ensures that target erosion has no impact on deposition rate. As the plasma is also unaffected by the wall thickness, this allows the use of very thick walled targets with consequent improvement in production availability. We are currently using up to 1.5cm wall thickness routinely, but expect to increase this in later developments. This includes ferromagnetic targets such as iron. SPUTTER DEPOSITION RESULTS Initial trials have used smaller targets of 35cm x 5cm diameter and have successfully deposited high quality thin films of titanium, aluminium, stainless steel and ferromagnetic iron. High rate reactive deposition of alumina and titania has also been achieved by sputtering aluminium and titanium cylindrical targets respectively in an oxygen ambient. In all cases we can achieve the same high quality materials and process dependencies as with our standard planar target system. This indicates that the far wider range of materials we have previously demonstrated will also be achievable with the linear source including oxides, nitrides and oxy-nitrides of aluminium, silicon, titanium, tantalum and niobium, and transparent conducting oxides. The very high target sputter rates achievable using remote plasma generation give high deposition rates at target to substrate separations of 20 30cm. Combined with process optimisation, this substantially reduces the heat load on the substrates and we have been able to deposit a range of high quality dielectric, metal and ferromagnetic thin films at high rates onto 50µm thick Kapton and 25 µm thick polyethylene terephthalate (PET) sheets (see table 1). Page 3 of 5
Material Target Power Separation Rate (kw) (cm) (nm/min) Stainless steel 12 30 80 Titanium 12 26 100 Iron 6 30 45 Aluminium 11 22 100 Alumina 12 22 115 Table 1. Deposition Data for Thin Films Deposited onto Thin Organic Sheet (Kapton and PET) (Data shown is for optimum low stress processes) It should be noted that the above deposition rates are limited primarily by the system power supplies available to us at this time (12kW), which limits our work to current densities of about 30mA.cm -2. Work with smaller cylindrical targets shows a linear dependence of sputter rate with target current density to about 60mA.cm -2, increasingly sub-linear as 100mA.cm -2 is approached. This indicates that the above rates will be readily doubled, potentially tripled, in an optimally equipped system. The materials can be deposited with near zero stress and with excellent adhesion. The deposition technique is especially suited to the more difficult sputter processes for magnetic materials and dielectrics, yielding near ideal key properties without compromising process throughput. For example, figure 3 shows a BH Loop for iron sputtered onto 'Kapton' film, figure 4 shows transmission data for alumina deposited onto PET film. 30000 20000 sample KAP4 M (kilogauss) 10000 0-10000 -20000-30000 -1000-500 0 500 1000 H (Oe) Figure 3. B-H Loop for zero stress 120nm thin iron film on Kapton deposited as table 1. 110 105 Transmission (%) 100 95 90 85 80 200 400 600 800 1000 1200 Wavelength (nm) Figure 4. Optical transmission for zero stress 1µm thick alumina film on PET deposited by reactive sputtering as table 1. First experiments using the newly developed 50cm x 7.5cm diameter target are underway, designed for deposition onto a minimum of 35cm (14") wide plastic web at rates in excess of 400nm/min. over a 45cm long zone. Initial results confirm the expected scaling of rate, area coverage and uniformity for the technology. Page 4 of 5
CONCLUSION The use of a remote high density plasma source to amplify the diode sputter deposition process has previously allowed us to deposit thin films with physical properties normally only achieved by ion beam deposition, but at the very high deposition rates and coverage associated with high power magnetron sputtering. We have now demonstrated that certain key elements of the operation of the remote plasma source permit it s use with long cylindrical targets, currently up to 0.5m in length, opening up the potential for high rate, high uniformity coating of plastic web or similar large area sheets with the same high quality materials. The technology addresses the primary concern regarding production scalability, in that the sputter target size is greatly increased for a given plasma source diameter and power. In combination with a significantly improved sputter system efficiency, the new linear configuration is estimated to provide a capability to increase throughput by a factor of 50 to 100 compared to the standard configuration. In addition, our early results indicate that low stress, high adhesion films with near ideal physical properties are more readily deposited by the technique, particularly those produced by reactive sputtering, and that a wider range of substrate types can be accommodated by the process. REFERENCES 1. P. Hockley, M. Thwaites and G. Dion, High Density Plasma Deposition, 2005 Society of Vacuum Coaters 505/856-7188, 48th Annual Technical Conference Proceedings (2005) ISSN 0737-5921 Page 5 of 5
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