Roland A. - PVD for microelectronics (779636), страница 10
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At pressures in the tens of mTorr and above, thesputtered atoms are typically stopped by gas-phase collisions someplacebetween the target and the sample, and effectively become like any othergas atom and undergo diffusive transport. Another common term used todescribe these slowed-down sputtered atoms is thermalized, implying thatthey have cooled down to the point of matching the gas temperature, whichis typically a few hundred degrees C.2.4.1 BALLISTICTRANSPORTIn ballistic transport, the sputtered atoms have virtually no in-flight collisions and arrive at the film deposition surface with their original kineticenergy intact. This provides for a rather energetic deposition process, asthe average kinetic energy can be 10 or more times the local thermal energy of the atoms at the film.
The ballistically deposited atom can almostbe thought of as implanting itself in the top layer or two of the film surface, and the deposition can be accompanied by the formation of defectsand/or a sort of local annealing. Films deposited in the ballistic transportregime tend to be small-grained, dense, and often have relatively good adhesion. In addition, since the deposition is kinetic and not thermal, it is42R.
POWELL AND S. M. ROSSNAGELpossible to manufacture unusual materials that might not be stable thermally simply by depositing the correct flux ratios. Ballistic depositions aregenerally characterized by small grain size and compressive stress.Ballistic transport is also directional, at least within geometrical limits.Since there are no gas-phase collisions, the sputtered atoms travel in astraight line from the target to the sample (line-of-sight). This will allowfor various means of directional filtering, such as collimation or longthrow sputtering, which will be discussed in Chapter 6.
Ion beam sputtering, which will only be briefly discussed in Chapter 3, is also characteristic of ballistic sputter deposition because the operating pressures are wellbelow 1 mTorr.2.4.2DIFFUSIVE TRANSPORTAs the pressure is increased, it becomes more likely that a sputtered atommay have a gas-phase collision with a background gas atom during transport. This starts to become significant at pressures above a few mTorr.
Inthese collisions, significant kinetic energy (up to 50%) can be shared withthe gas atom, resulting in both cooling of the sputtered atom and heating ofthe background gas. In addition, since the momentum- and energy-transfercross sections are strongly energy dependent [2.24] (Fig. 2.15), as the sputtered atom slows down due to collisions, it becomes effectively larger. Thiseffect is not physical (the atom does not grow); rather, it is related to theeffective interaction time between the electron clouds of the two collidingatoms. As the atom slows down, there is more time for the electrostatic interaction and it is as though the particle increases in its effective size.The end result is that as the sputtered atom slows down it becomes evenmore likely to have collisions, and it can quickly lose its initial kinetic energy in perhaps 5-10 collisions.
This is known as thermalization and resultsin an effective temperature for the sputtered atom that is the same as the gastemperature, perhaps 1000~ or less. Measurements of average sputteredatom temperature show a strong drop as the pressure is increased (Fig. 2.16)[2.25].
Thermalized deposition can be much different from ballistic deposition because the depositing atoms have virtually no kinetic energy [2.26].In fact, thermalized depositions are much more similar to evaporated depositions, both in grain size and in stress. The grain size is typically larger andthe stress more tensile. Since the deposition is more thermal, it may be lesslikely to deposit homogeneous alloys of unusual or immiscible materials.Sputter deposition systems are usually intended for the rapid depositionof thin films, so it is relevant to see how efficient the transport process is.PHYSICS OF SPUTTERING18m16vs-O-XeKr~3~8LU201iI25iiiiiii10 20 50 100 200 5001000E n e r g y (eV)1211v9o')0~37-XeKr6~4"~3oE21II25IIIIIII10 20 50 100 200 5001000E n e r g y (eV)FIG.
2.15 Cross sections for energy (top) and momentum (bottom) transfer for various materials asa function of energy for Ar, Kr, and Xe [2.24].44R. POWELL AND S. M. ROSSNAGEL10.0>D1.0vtg0.1+\10.001I10.010.11.0P (torr)FIG. 2.16 The effective average kinetic energy of sputtered Cu as a function of system pressure.
Thedifferent data points relate to changes in magnetic tield and measurement position, and the solid linesare the result of modeling [2.25].An atom sputtered from the cathode in a typical magnetron sputter tool(see Chapter 5) is likely to be deposited on one of three surfaces: the sample, the surrounding shields or fixtures, or potentially right back on thecathode itself. Obviously, the first case is most important.
Deposition onthe fixtures (shutters, shields, windows, etc.) results in lower net efficiencyas well as cleaning concerns over time. Initially, deposition back to thecathode (redeposition) might not be considered that bad, in that the atomsare simply recycled by later sputtering. However, as will be seen inChapters 4 and 5, most sputtering cathodes have "dead" areas, or areaswhere the erosion rate is fairly low.
In these areas, there can be a netbuildup of the sputtered material, which can result in either topographicalproblems on the source (bumps, nodules) or even peeling and flaking.Sputtered-atom transport is rarely measured in direct terms. In manycases, systems are characterized by practical units, such as deposition ratePHYSICS OF SPUTTERING45per watt, which, if the exact sputter yield and cathode dimensions areknown, may be extended back to more fundamental units. Generally this isnot done, simply because users are interested in the actual performance ofthe system rather than the absolute units.Transport can be defined as a probability of deposition, ranging from 0to 1.0.
A transport probability of 1.0 would mean that all of the atoms sputtered from some location arrived at the intended destination, and obviouslya transport probability of 0.0 means that none of the sputtered atoms weredeposited. Most practical systems will no doubt be someplace in between,simply because of the difficulty in managing either the emission or trajectory of the sputtered atoms.Measurements have been published of sputtered-atom transport for asimple magnetron system in an open chamber [2.27]. The open chamberis necessary to remove the complication of the various shields, shutters,and fixtures that are usually present in most systems and act as collectionpoints for material.
The measurements used a magnetron sputteringsource of diameter 20 cm mounted in a chamber of diameter 50 cm andlength 30 cm. Samples were configured on the side areas (beside the cathode and on the walls) and also on a full-diameter sample plane that couldbe moved to different throw distances. It is necessary in this case to use afull-diameter sample plane to collect all the atoms that reach the samplelocation.The results for several throw distances (5 to 14.5 cm), for pressures upto 30 mTorr, for AI and Cu, and for some different gases are shown in Table2.3. The redeposition back to the cathode was inferred by locating sampleson the various dead areas of the cathode and averaging the net depositionrate there over the entire cathode surface.
This implies that the depositedatoms into the etch track (which cannot be measured) are recycled.The results show several interesting points. As might be expected, theshortest throw distances result in the best transport, as do the lowest pressures. In general, though, the transport tends to be only moderately efficient: No more than 50% of the sputtered atoms typically make it to thesample plane.
The best case is the sputtering of A1 with Ne at low pressureand short throw distance. In this case, the mass of the gas is less than themass of the sputtered atom, so gas scattering is reduced. It should be notedthat the difference between A1 transport in Ar and its transport in Ne at5 mTorr even for the short throw distance of 5 cm (from 0.8 to 0.6) can entirely be associated with gas scattering. The results also show the significant impact of either increased sample throw distance or increased pressure.
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