System pressure, sputter rate

Top spray systems During top-spray cooling of an overheated core, the wall temperature is usually higher than the Leidenfrost temperature, which causes water to be sputtered away from the wall by violent vapor formation and then pushed upward by the chimney effect of the steam flow generated at lower elevations (as shown in Fig. 4.25). A spray-cooling heat transfer test with BWR bundles was reported by Riedle et al. (1976). They found the dryout heat flux to be a function of spray rate and system pressure. The collapsed level required to keep the bundle at saturation for various pressures compared reasonably well with that in the literature (Duncan and Leonard, 1971 Ogasawara et al., 1973). 

Figure 17.30 The dependence of the sputtering rate distribution on system pressure 8//max = 1550 G, a = 1.5 cm, b = 8.0cm. Power = 10 w, Ar flow rate = 1.0 seem.

The sputtering rate does not change much with the increase of cathode surface area at low system pressure p= lOmtorr). The plasma treatment is localized and effectively confined. However, at a relatively high system pressure (/) = SOmtorr), the sputtering rate is almost zero for the larger cathode. This case reiterates the importance of system pressure in AMT Ar sputtering treatment. 

RF- or DC-driven planar magnetron discharges are applied for spntter depositiom The pressures in the discharges are usually quite low, in this case 1-10 mTorr. The pressure is hmited in the physical sputter deposition system by the requirement that the sputtered atoms mean free path be larger than the separation between the sputtered target and substrate where the atoms should be deposited. The physical sputter deposition rate can be estimated by assuming that all sputtered material is deposited on the substrate ... 

Pumping in plasma systems can be done with any pump that can operate at the desired flow rate and pressure, that is compatible with the gases being used, and that can handle the contaminants generated. Typical flow rates for plasma cleaning, sputter deposition, and ion plating are about 200 seem. 

The most widely used vacuum deposition techniques are evaporation and sputtering, often employed for smaller substrates. In the evaporation process, heating the metal by an electron beam or by direct resistance produces the vapours. The system is operated at a very high vacuum (between 10-5 and 10 6 Torr) to allow a free path for the evaporant to reach the substrate. The rate of metal deposition by evaporation processes varies from 100 to 250,000 A min h These processes can be operated on a batch or a continuous scale. On the other hand, in the case of the sputtering technique, the reaction chamber is first evacuated to a pressure of about 10-5 Torr and then back-filled with an inert gas up to a pressure of 100 mTorr. A strong electric field in the chamber renders ionisation of the inert gas. These inert gas ions... 

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