Roland A. - PVD for microelectronics (779636), страница 38
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A second possibility is to increase the erosion rate at the cathode edge to increase the inward-moving flux. This results in a less uniform planar deposition (i.e., onthe top surface), which is exaggerated toward the wafer edge. Althoughthis is often undesirable from a process-control point of view (i.e., the edge194R. POWELL AND S. M. ROSSNAGELFIG. 6.8 SEM photos of trenches near the outermost region of a 200-mm wafer: (a) the depositioncross section for a trench that runs tangent to the edge of the wafer, (b) the deposition cross sectionfor a trench that is parallel to the radial direction on the wafer [6.4].of the wafer gets a thicker planar deposit than the center), it may result ina roughly uniform deposition within a feature.
It should be noted that in adamascene process, the films deposited on the top, planar areas are completely removed with the CMP process. Nevertheless, CMP works bestwith uniform, planar films, and it is preferable not to have largecenter-edge asymmetries.A final possibility for overcoming the deposition asymmetry problem isto increase the throw distance even more. As the wafer is moved fartherand farther away from the target, the depositing flux that arrives at thewafer becomes more nearly normal incidence. As a practical matter,though, the increased throw distance results in a much-reduced depositionrate, increased gas scattering, and potentially a less directional deposition.The mean free path for gas atoms is approximately given for most speciesby:A-1no"~5cmPwhere n is the gas density, o is the cross section for a momentum-transfercollision [6.8], and P is the pressure in mTorr.
Although sputtered atomsare more energetic than background gas atoms and have a 50% smallercross section, significant increases in throw distance require reductions inDIRECTIONAL DEPOSITION195the operating pressure of the system. Conventional magnetrons can be operated well in the 0.5-mTorr range but require some degree of enhancement (e.g., hollow cathode) to reach the 0.1-mTorr region or below thatwould be required for throw distances much beyond 25 cm.Another technical concern related to long-throw sputtering is in therealm of manufacturing tool operation.
Because the long-throw chambercan be typically 2 times the height of a conventional chamber, the volumeof the chamber is increased as is the interior surface area. This results inslower pump-down times following a chamber vent and adds a minor expense to the cost of operating these tools. Conversely, though, since thechamber is much larger than conventional sputter chambers, there is moreroom for additional flanges on the chamber, allowing increased pumpingcapability and/or diagnostic access.Given these rather practical, yet fundamental, problems with long-throwsputter deposition, the process is used somewhat sparingly.
It will be appropriate for aspect ratios of < 1.0, but will not work well with higher ratios. In addition, this technique scales poorly to 300-mm wafer diametersand beyond. For a 300-mm wafer system, the cathode diameter would needto be on the order of 45 cm, requiring a 45-cm throw distance to providedirectionality equal to the 200-mm wafer case.
This puts significant additional stress on the need to attain a low working pressure, which wouldneed to be now in the high 10-5-Torr range. This pressure range requiressignificant modification of the magnetron to use auxiliary electron sources,and these modifications are mostly incompatible with manufacturing applications.6.3 Collimated Sputter DepositionA collimator, simply a directional filter, may consist of a simple aperturefor a particle beam application to an array of aligned tubes for a plasma application.
For sputter deposition applications, collimators are typicallyclose-packed arrays of tubes or cylinders configured in the space betweena sputter target and a sample [6.9]. The tubes are oriented such that theyare perpendicular to the plane of the target and sample (Fig. 6.9).In general, the collimator is positioned a few centimeters from thecathode surface ( > 3 cm) so that it does not interact directly with themagnetron plasma. In addition, the energy deposited on the collimatorfrom the plasma can be many tens to a hundred or more watts, and increasing the cathode-to-collimator distance reduces the heating. On thelower side of the collimator, it is necessary to locate the sample at leastR.
POWELL AND S. M. ROSSNAGEL196FIG. 6.9General configuration used for collimated sputter deposition.one hole diameter away for collimator aspect ratios of < 2, and perhapstwo hole diameters away for higher aspect ratios. This reduces shadowing of the sample by the walls of the collimator. (This shadowing effecthas been modeled and measured experimentally by the Alberta group[6.101.)A collimator cell has an aspect ratio defined as the length of the tube divided by its diameter.
For practical sputtering systems, this aspect ratioranges from about 1/2 to 4. The aspect ratio of the collimator limits the fluxof sputtered atoms ejected from the target by simply absorbing the atomsthat impinge on the collimator walls. This selectively filters atoms that arenot moving along the axis of the collimator cell. The amount of filtering isaspect-ratio-dependent; the transmitted solid angles for the deposited fluxas a function of aspect ratio are given in Fig. 6.10.
As is obvious from thefigure, increasing the aspect ratio narrows the divergence of the transmitted flux, but at the expense of the net deposition rate. Geometrically, thiscan be viewed as a small cone drawn within the emission "sphere" of sputtered atoms (Fig. 6.10). The higher the aspect ratio the smaller the cone,but also the smaller the volume enclosed by the cone. This relative volumewould correlate directly with deposition rate.The net deposition rate at the sample is strongly reduced due to this filtering. Figure 6.11 shows the effect of both collimator aspect ratio as wellDIRECTIONAL DEPOSITION197For a 2-cm high collimator located 2 cm from cathode:Aspect ratio1:12:13:14:1Emission width (degrees)28 i.e., +/- 14)14117FIG.
6.10 Geometrical representation of the filtering effect of a collimator. The area within the arrows is the range of angles that are transmitted through the collimator. The solid angles (actually, inthree dimensions this is a solid cone) transmitted as a function of aspect ratio are given in the table.as chamber pressure on the net, planar deposition rate below a collimator.Generally, for each increase in the collimator aspect ratio of 1.0, the deposition rate is reduced about 3 times.
In addition, the effect of increasingpressure is such that gas scattering at higher pressures reduces the rateeven more, as atoms are scattered within the collimator itself and land onthe collimator cell walls.Collimated sputtering was first applied to wafer patterning in the mid1980s, again pointed toward lift-off processing [6.9]. As with the earlylong-throw work, it was necessary to augment the discharge with electronsfrom a hollow cathode to allow low-pressure operation. In the late 1980s,magnetrons became available that operated well at 1 mTorr and below, andit was no longer necessary to use the hollow cathode enhancement. Earlywork also showed the capabilities for filling moderate aspect ratio features(Fig. 6.12).Collimated sputter deposition has been used on a wide scale for the deposition of thin diffusion barriers or "liners" within vias or trenches.First shown by Joshi and Brodsky [6.11] (Fig.
6.13), this has been widely198R. POWELL AND S. M. ROSSNAGEL1E+04"~"-6-"ip-2.2-co,,.iI4:4co,,:IIIi1E+03~ i6 =.4"3-~g2-.~ 1E+020~a64a2 ~0FIG. 6. l lIi',tl-- 1 mTorrI|. . . . . . . . . . . .i5J"!I~--!10152025Chamber Pressure ( m T o r r )...... . . . .i30!35D e p o s i t i o n rate t h r o u g h a c o l l i m a t o r as a function o f c o l l i m a t o r a s p e c t ratio and s y s t e mpressure.described in various references [6.12-6.16]. The most c o m m o n applications are for the deposition of Ti layers at the bottom of vias that are usedto decrease the contact resistance of the subsequent metal used to fill thevia (typically W). Ti and primarily TiN are also valuable as diffusion barriers, which are then used within a via to provide a barrier for the interaction of Si and A1, and also to provide a nucleation layer for W-CVD.The TiN also functions to protect the SiO 2 walls from attack by the WF 6gas used for W-CVD [6.17] (Fig.
6.14). These materials will be discussedin much greater detail in Chapter 10.The step coverage for interconnect metallization has been functionallyredefined in the recent past to mean the relative thickness at a specificpoint compared to the thickness of the film on the top areas. The bottomcoverage in a contact hole or via can be significantly enhanced by the useof collimated sputter deposition. Figure 6.15 shows data measuring thebottom step coverage as a function of the aspect ratio of the contact holefor 1" 1 and 1.5"1 AR collimated sputter deposition of Ti [6.18].
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