Comparison of Different Sputter Processes for ITO: Planar DC versus Planar AC

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1 Comparison of Different Sputter Processes for ITO: Planar DC versus Planar AC P. Sauer, H.-G. Lotz, A. Hellmich, R. Kukla, J. Schröder Applied Films GmbH & Co. KG, Alzenau, Germany Key Words: ITO MF PET Sputter Deposition ABSTRACT The latest generation of Flexible Displays, PDAs, touch screens and mobil phones need Transparent Conductive Oxides as a reliable transparent electrode. TCO is most commonly made by sputtering ITO. Due to rising ITO material costs, the market requires low material consumption and higher target utilization. In this paper the latest developments of DC and MF planar magnetrons for ITO sputtering are reported and compared. Resulting data about target utilization, specific resistance and optical properties are discussed, supported by SEM pictures. INTRODUCTION Sputtering ITO layers from ceramic Indium Tin Oxide targets is a well established technology for coatings on glass and PET film. Commonly, DC sputtering is used with fast switching power supplies to decrease arcing. DC ITO sputtering on PET film suffers from high specific resistivity values when compared to ITO layers on substrates, which can be heated to more than C during the coating process. Currently, ITO is deposited onto PET films with thickness values between 25 mm and 180 mm; compared to the state of the art for ITO performance deposited on heated glass (180 mw*cm), typical specific sheet resistivity for ITO deposited onto PET film is between 500 and 700 mw*cm. For FPD applications sheet resistances on glass are about 15 W/sqr with high transmission (>80%). In order to achieve a corresponding resistance value on PET films, ITO layers with more than 300 nm thickness would be needed. Even when neglecting the high material and production costs, the transmission of such thick layers is too low. In addition to absorption, light scattering decreases the optical performance. Thus one of the main goals for ITO on flexible substrates is to optimize specific resistivity and transmission. Due to rising material cost, higher target utilization and less scrap is required. Machine design, web quality and coating sources are factors can impact this. In this presentation target utilization, deposition rate and life time of different sputtering sources are discussed. We compare state of the art planar DC magnetrons, having low target utilization, with DC planar magnetrons with a special shaped magnet field for high target utilization and with planar MF magnetrons /1/. RESULTS AND DISCUSSION Process Equipment Process /deposition was achieved by using different kinds of Applied Films coating machines. The coatings with DC power supply were done in a SMARTWEB TM 3-400, which was equipped with 2 planar DC magnetrons with a target to substrate distance of 100 mm. For the trials with MF power supply a lab coater with two planar cathodes, pumped by turbo molecular pumps was used. The distance between target and substrate was fixed at 130 mm. The planar ITO layers were deposited from Indium- Tin-Oxide 90/10 targets on HLK TM, Toramag TM, Smartcathode TM and TwinMag TM cathodes. Dynamic deposition rates were adjusted between 20 and 60 nm * m/min (1.7-5 nm/s). The power densities were varied between 1 and 6 W/cm 2. For the process a mixture of Argon and Argon/Oxygen (85%/15%) was used. In MF mode, the cathodes were powered by a 40 khz EMA power supply. In DC-Mode for a direct comparison, cathode 1 or cathode 2 of the two MF cathodes were connected to a DC power supply. The oxygen partial pressure was monitored by a lambda sensor. The layer thickness was controlled by power, carrier transport speed and number of oscillations underneath the sputter source. For comparison, standard PET film with 175 mm thickness was coated simultaneously together with glass test slides. The film thickness was measured with a surface profiler. Coated wafer pieces of Silicon were used for the ellipsometric evaluation of the refractive index (n) and extinction coefficient (k) AIMCAL Fall Technical Conference 1

2 Comparison of DC and MF ITO Coating With Planar Magnetrons. There are two theories regarding the possible effects of MF sputtering on the properties of ITO layers. The higher ion energies of MF-sputtering /2/ increase the surface mobility of deposited particles similar to the effect of increased surface and substrate temperature, thus decreasing the specific resistivity of the ITO film. Conversely it is known /6/ that the electronic properties of ITO layers suffer from the bombardment with higher energetic ions. This effect could increase the specific resistivity. Another positive side effect MF-sputtering technology might reduce the nodule growth on the target surface. Properties of ITO Layers Deposited by MF and DC Sputtering The process conditions for the MF and DC sputter deposition are shown in table 1 and 2. Table 1. Process conditions for MF-sputtering P(Cathode) Ar flow rate 1-3 kw sccm Ar/O 2 flow rate 0-90 sccm Total Target Surface 1400 cm 2 Effective Power Density W/cm 2 Rate (1, 2, 3 kw) 10, 20, 30 nm*m/min Figure 1. Specific Resistivity of MF-sputtered ITO layers with different rates and power levels. P=1 kw (10 nm* m/min), P=2 kw (20 nm*m/min), P=3 kw (30 nm*m /min ). The characteristics of the specific resistivity of ITO layers on glass and PET (MF) are almost identical (fig.2). Table 2. Process conditions for DC-sputtering P(Cathode on left side) P(Cathode on right side) Ar flow rate kw kw 255 sccm Ar/O 2 flow rate sccm Total Target Surface 700 cm 2 Effective Power Density 1,95 W/cm 2 Rate of Cathode on left side Rate of Cathode on right side 16 nm*m/min 16.6 nm*m/min By increasing the power level from 1 to 3 kw, the position of the specific resistivity minimum (MF) shifts to higher Ar/O 2 flow rates (fig.1). The absolute minimum values are almost identical (500 mw*cm). Figure 2. Specific Resistivity of MF-sputtered ITO layers on Glass and PET substrates (P=2 kw) Starting with low ITO layer thickness, the specific resistivity decreases until the layer forms a continuous film. Between 40 nm and 100 nm layer thickness, the specific resistivity remains almost constant. With increasing layers thickness above 100 nm, the specific resistivity starts to rise again due to increasing imperfections of the growing layer (fig.3) AIMCAL Fall Technical Conference 2

3 Table 5. Influence of Layer Thickness on the Surface Roughness of ITO layers deposited by MF sputtering on PET ITO Layer Thickness Ra in Rms in Rmax in Specific Resistivity MicroOhm *cm 41 nm 4 kw 3.38 ± ± ± nm 3 kw 3.70 ± ± ± PET 2.22 ± ± ± Figure 3. Specific Resistivity of MF-sputtered ITO layers on PET (P=2 kw). Roughness measurements of MF and DC-sputtered ITO layers on glass show an increasing RMS roughness with increasing layer thickness (table 3+4). Table 3. Influence of Layer Thickness on the Surface Roughness of ITO layers deposited by MF-sputtering on glass ITO Layer Thickness Ra in Rms in Rmax in Specific Resistivity MikroOhm *cm 80 nm 0.27 ± ± ± nm 0.43 ± ± ± nm 0.57 ± ± ± nm 1.11 ± ± ± Glass 0.17 ± ± ± Atomic force micrographs (fig.4, 5) of MF-sputtered ITO layers on PET show nodular defects in the surface structure of ITO, increasing with increasing sputter power. Figure 4. Atomic Force Micrograph of 100 nm ITO on PET (MF, Sputter Power : 1 W/cm 2, R=48 W /sqr) Table 4. Influence of Layer Thickness on the Surface Roughness of ITO layers deposited by DC-sputtering on glass ITO Layer Thickness Ra in Rms in Rmax in Specific Resistivity MikroOhm *cm 41 nm 0.20 ± ± ± nm 0.32 ± ± ± Glass 0.17 ± ± ± The surface roughness of ITO films on PET is considerably higher compared to ITO coatings on glass. This is mainly due to the higher surface roughness of uncoated PET compared to glass (table 5). Figure 5. AFM image of 101 nm ITO on PET (MF, Sputter Power : 2 W/cm 2, R=58 W /sqr) 2005 AIMCAL Fall Technical Conference 3

4 There seems to be a correlation between the specific resistivity and roughness of the MF-sputtered ITO film (fig. 6). Figure 8. Specific Resistivity and Absorption of DC sputtered ITO layers (Thickness nm, 1.37 kw, A=1-R-T) ITO layers deposited with cathode 1 on the left side require more Ar/O 2 flow to achieve a minimum in surface resistivity, compared to ITO layers deposited with the right side cathode (fig. 9). This effect might be due to different pumping conditions in the sputter compartment of the laboratory machine. Figure 6. Specific Resistivity and Roughness of MF sputtered ITO layers on PET (ITO Thickness nm, 1.0 kw) The optical absorption (A=1-R-T) of MF- and DCsputtered ITO layers, measured at 400 nm, corresponds to the characteristic curve of the specific resistivity (fig. 7+8). FIgure 9. Specific Resistivity of MF-sputtered ITO layers on Glass and PET substrates (P=1.3 kw). Rate of first Cathode: 16 nm * m / min; Rate of second Cathode = 16.6 nm * m / min Figure 7. Specific Resistivity and Absorption of MF sputtered ITO layers on PET (Thickness 40 nm, 1.0 kw, A=1-R-T) The minimum specific resistivity achieved on glass and PET was in the order of 450 mw*cm, similar to the values obtained with MF sputtering. The absorption at 400 nm (incl. PET) is also very similar. Both sputter methods created similar nodule growth on the target surface. These nodules are regions of substoichiometric indium oxide. /3,4/ 2005 AIMCAL Fall Technical Conference 4

5 The different behavior of the two cathodes regarding operating point and gas flow reveals a special problem that is specific for MF sputtering. Due to the sensitive response of ITO, small local changes in oxygen partial pressure between right and left cathode result in a dilemma: Either the left cathode or the right cathode can be run at the optimum working point. If the oxygen gas flow is correct for one cathode, the other one is out of optimum. Even with an optimum machine configuration, where both cathodes see exact the same oxygen partial pressure, the situation will not be perfect. It is nearly impossible to realize two cathodes with exactly the same strength of magnetic field and to prepare two ITO targets with exactly the same composition, grain size. Etc. It can be said that no beneficial effects could be observed which would favor the deposition of ITO using MF sputtering technology. Target Utilization and Target Lifetime of Planar Magnetrons Using Static Magnetic Fields. In general planar magnetrons with simple N/S/N magnetic field orientation are used for ITO. This results in a sharp erosion profile with a target utilization of approx %. With optimized magnetic field shape, the utilization can be increased to 40 % and higher (fig.10-11). 30 kam Magnetic field strength N/S/N magnetic field HLK optimised mag. field ToraMag depth / mm depth / mm Figure 11. Erosion profiles of HLK TM and ToraMag TM planar magnetrons. Target width 200 mm Nodule Growth and Cracking on ITO Targets. As is generally known, nodule growth on the target surface depends on ITO material quality as well as on the level of sputter power. Sputter power higher than 2 W/cm 2 raises the nodule growth on planar magnetrons with bonded ITO tiles. This also applies to ITO targets sputtered with MF magnetrons. In addition to the nodule growth, cracks occur due to stress in the ITO tiles. Optical properties n, k of ITO coated with MF and DC power Erosionprofile HLK width in mm Erosion profile ToraMag width / cm Targetwidth / mm Figure 10. Static magnetic field shapes for HLK TM and ToraMag TM planar magnetrons. Refractive Index DC 15 DC 16 DC 27 MF 79 MF 80 MF 81 MF 82 MF 83 The optimized magnetic field shows a larger tunnel (fig 10)with an evenly distributed magnetic field for the electrons and increased erosions profile width (fig. 11). That doubles the target life time and reduces the frequency of target changes Wavelength Figure 12. ITO refractive index MF coated AIMCAL Fall Technical Conference 5

6 refractive index mm mm mm mm refractive index index of extiction Figure 13. ITO refractive index DC planar. The refractive index of ITO layers, that are deposited from different sources does not show significant distinction (fig12,13). The MF coating shows slight variation depending on the working point (fig. 12). Source type Table 6.Coating rate, power density Coating rate nm m/min wavelengh (nm) Target utilization /% for ITO Power density W / cm2 Power / Kw HLK Toramag nodule formation on planar ITO targets. With optimized magnetic fields of planar cathodes, higher target utilization and lower production costs are achieved. This will increase the production and become more efficient. REFERENCES 1. H.-G. Lotz, P. Sauer, R. Kukla, M. Liehr, J. Schröder, Influence of DC and MF Sputter Technology on the Properties of ITO Layers on PET Film, Society of Vacuum Coaters, 47 th Annual Technical Conference Proceedings, 189, S. Jäger, B. Szyszka, J. Szczyrbowski, G. Bäuer, Comparison of transparent conductive oxide thin films prepared by a.c. and d.c. reactive magnetron sputtering, Surface and Coatings Technology, 98, 1304, M. Schlott, M. Kutzner, B.L. Gehman, N. Reger, F.J. Stadermann, Nodule Formation on Indium-Oxide Tin-Oxide Sputtering Targets, SID 96 Digest, 522, B.G.Lewis, R. Mohanty, D.C. Paine, Structure and Performance of ITO Sputtering Targets, Society of Vacuum Coaters, 37 th Annual Technical Conference Proceedings, 432, Data achieved on 600 mm cathode length using PET CONCLUSION ITO layers deposited on PET film with MF- and DCplanar technology show similar optical and electrical properties. In all cases, the level of absorption at 400 nm corresponds to the specific resistivity. The specific resistivity at the minimum of the characteristic curves are in the order of 500 to 650 mw*cm. This range is also typical for DC sputtered ITO layers on unheated glass substrates. The result is somewhat surprising because it was expected that the high energetic ion flux to the substrate, created by the MF sputter process /2/ would change the properties of the deposited ITO film. We conclude: There is no advantage of MF sputtering of ITO over the DC process. For ITO the higher costs of AC equipment does not pay off. Higher power levels and sputter rates strongly increase 2005 AIMCAL Fall Technical Conference 6

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