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It is clearly most efficient to sputter at the lowest practical pressureand the shortest distance.R. POWELL AND S. M. ROSSNAGEL46TABLE 2.3TRANSPORT PROBABILITY FOR PLANAR MAGNETRON SPU'ITER DEPOSITION ONTO THE SAMPLE PLANE,THE SIDE AREAS, AND BACK ONTO THE CATHODE. THE TOP GROUP Is FOR THE USE OF A CU CATHODE,THE LOWER GROUP IS FOR AL [2.27].5-cm throwKrArNe9.5-cm throwKrArNeThrow! 000 W5 cm9.5 cm14.5 cm200 W5 cm3000 W5 cmP(Pa)SampleplaneMagnetronplaneSide areas a0.72.640.72.640.72.640.520.450.380.600.460.420.800.560.520.100.180.340.120.260.320.080.160.270.160.170.130.100.120.090.050.100.110.72.640.72.640.72.640.350.270.220.440.450.360.400.420.400.180.350.390.130.350.400.100.360.340.200.240.200.100.150.170.200.180.09P(Pa)SampleplaneMagnetronplaneSide areas"0.72.640.72.640.72.640.630.490.530.480.470.450.390.350.310.0310. l I0. ! 40.0310.130.180.0450.160.180.
! 60.200.220.240.240.180.250.300.3540.530.230.1340.480.090.24_The side areas include only those areas adjacent to the magnetron cathode, parallel to the cathodesurface. It does not include all wall areas where deposition was too small to be measured.PHYSICS OF SPUTTERING472.4.3 GAS RAREFACTIONIn parallel to the thermalization process of cooling the sputtered atoms, thegas temperature can increase significantly. Since sputtering chambers arefairly open and have only modest gas flows, significant gas heating resultsin a local rarefaction of the gas density, as hot gas atoms leave the neartarget region faster than cooler gas atoms arrive from the perimeter.
Gasrarefaction effects were first observed in a dynamic mode known as thesputtering wind, in which convection-like flows were observed in thebackground gas within the chamber [2.28]. Later work showed a significant rarefaction of the average gas density m down to as low as 15% ofthe original density m due to the heating effect of the sputtered atoms(Fig. 2.17) [2.29].
Rarefaction may be important in scaling issues, in thathigh-rate sputtering (and as a result, more rarefaction) may have similar10JJiIIr162ErO,rE:)vtO(1)n-..L6tl:l4 PaE(/)a..=_4Eoffl2.6 Pa(50123456Magnetron Discharge Current (amperes)FIG. 2.17 Gas density in the region in front of the sputtering target as a function of ion (discharge)current to the target for 4 Pa (30 mTorr), 2.6 Pa (20 mTorr), and 1 Pa (7.5 mTorr). The cathode diameter was 150 mm, and the measurement position was 5.3 cm from the cathode face, on axis [2.29].48R. POWELLAND S. M. ROSSNAGELcharacteristics to low-pressure sputtering. Thus, a process developed at alow sputtering and deposition rate at a moderate pressure may be completely different as the deposition rate is scaled up and the effective gasdensity is reduced. This effect will also affect chemical effects, as seen inreactive sputtering [2.30].References2.1.
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