Roland A. - PVD for microelectronics (779636), страница 37
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ROSSNAGELor top areas (the "overburden") by means of a chemical-mechanical polish(CMP) process. This last process is mostly a physical abrasion process butmay also have a weak chemical etching component in some metal systems.The result of CMP is that the surface is completely planarized and that themetal-filled feature is embedded in the dielectric.D a m a s c e n e wiring gets its n a m e from a m e t h o d o f m a k i n g j e w e l r y in ancientD a m a s c u s , w h e r e b y pieces o f glass or precious stones were pressed into holesin a metal and were then polished d o w n to give an inlaid look.
In the U L S Iversion, it is the glass that is patterned and filled with precious metals.The damascene process can be repeated any number of times (12 layersis the current record) for multilevel interconnect structures because thetechnique is intrinsically self-planarizing and the deposited metal featuresare well encapsulated within the stable dielectric. In addition, it is possibleto etch two-layer features into the oxide with a sequential patterning andetch process. This typically takes the form of a trench or pad with vias extending down from the pad to a lower conductor level (Fig.
6.3). This isoften called dual damascene. Several process steps as well as at least oneor two interfaces can be eliminated by the use of a dual damascene process,resulting in significant savings as well as an increase in reliability.While this process has great promise for changing the way semiconductors are made, the metal deposition step (Fig 6.2c) is incompatible withFIG. 6.3 Schematic and SEM of a dual damascene structure consisting of a rectangular pad with twocircular vias at each end that connect to a lower conductor.DIRECTIONAL DEPOSITION189conventional sputtering for AR > 0.5. Due to the wide range of arrival angles of the depositing atoms discussed earlier, the deposition starts to coatthe upper sidewalls and corner of the feature, which shadows the lowerarea from deposition (Fig 6.4).
This has been described by some authors as"bread-loafing." Continued deposition results in a closed-off feature and aburied void, often called a keyhole void.This overall problem has been addressed in many ways, using both PVDtechnologies and non-PVD technologies such as chemical vapor deposition(CVD) and electroplating. The PVD approaches can be grouped into conventional and directional approaches (Fig. 6.5). (The non-PVD technologies will not be discussed.)The conventional PVD approaches make use of either the rearrangementof deposited atoms on the surface or the removal of atoms from the surfaceduring deposition by ion bombardment.
In the first case, either elevatedsample temperature or very high pressure on the deposited film (and elevated temperature) are used to provide sufficient surface or bulk diffusionFIG. 6.4 Conventional magnetron sputter-deposited films onto a high aspect ratio via showing theformation of a keyhole void.R. POWELL AND S. M. ROSSNAGEL190FeaturesExploitedMethod ofDepositionMobilityand/orDiffusionConventionalPVDReflow2-step, cold-hotUltra-high pressure,SurfaceEtching,,,.._Rf biasGrazing-angleion bombardmentCollimationLower pressureDirectionaIpVD]SputteredNeutralsMetalIons~I Long throw.Textured targets!,oo,,e0aone,:o:1_._~llv" ECR-deposition-'~-Cathodic arct. Ionized evaporation........|/jChart of PVD approaches to deposition into high aspect ratio features (courtesy of K.
Lai,Varian Associatcs, Palo Alto, CA).FIG. 6.5such that film atoms can move into high aspect ratio features. These topicswill be discussed in detail in Chapter 7.It is also possible to bombard and sputter the film surface during deposition. The simplest approach to this is bias sputtering, in which an RF biasis imposed on the sample during deposition and the surface is sputtered bynormal-incidence inert gas ions from the plasma. For low aspect ratio features, this can tend to preferentially resputter the film's atoms from thesharp, high-angle features. However, for aspect ratios above about 0.5, theeffect of normal-incidence bombardment is to sputter the features closed,leaving entrapped voids.A slightly different approach uses grazing-angle ion beam bombardmentduring deposition to selectively etch the deposited films from the top areasof the sample, while the shadowed areas within the deeper features are protected [6.2].
This technique overcomes the problem of sputtering shut features but requires either multiple ion sources or sample rotation to provideuniform erosion. This is shown schematically in Fig. 6.6. However, thisDIRECTIONAL DEPOSITIONMagnetron CathodeF Y tpv'F T 'Sputtered AtomsIIIIIIIIIIIIIIIIIKaufmano n SourceC o -.,]tor,Rotating Wafer HolderFIG. 6.6 Deposition system using grazing-angle ion bombardment to reduce the depos~tionontoplanar areas of the sample 16.21.technique has only been used at pressures lower than those at which mostmagnetrons operate (0. I mTorr) and has been combined with a collimator(discussed below) to help separate the magnetron plasma from the grazingangle ion beam.The directional approaches (lower half, Fig.
6.5) use either directionalneutrals or ions. Neutral sputtered atoms are not easily controlled, so themost general path toward a controlled, directional flux of neutrals is one ofsubtraction: Atoms with the wrong trajectories are removed, leaving thecorrectly directed atoms to form the deposit. The deposition of directionalneutrals uses either extended cathode-to-sample throw distances or a directional filter, or collimator, between the target and the sample. It is alsopossible to alter the sputtering process somewhat to affect the net directionof the sputtered atoms. Two examples of this are the machined target surface and the oriented-crystal target.
The ionized deposition techniques willbe discussed in Chapter 10.6.2 Long-Throw Deposition TechniquesSputter deposition at pressures below I mTorr (0.13 Pa) results in a virtually collision-free trajectory for the sputtered atoms from the target to thesample. If the throw distance is increased at these low pressures, atoms that192R. POWELL AND S. M. ROSSNAGELhave trajectories at low angles (i.e., close to parallel to the sample surface)will be deposited on the chamber sidewalls and only atoms that have closeto normal-incidence trajectories will make it to the sample.
Therefore, increasing the target-to-sample distance results in a geometrical filtering ofthe sideways-moving atoms and results in a deposition that is more vertical. This feature can be used to project a higher fraction of the sputteredatoms deposited on the sample into deep features, and it reduces the buildout characteristic of conventional sputtering (Fig. 6.4).Long-throw techniques were first attempted for an earlier patterningscheme known as lift-off, in which directional deposition is necessary toallow removal of a slightly overhanging resist mask on the sample surface[6.3].
It was necessary in that work to use a hollow cathode electron sourceto augment the magnetron discharge to allow low enough pressures. It wasfound that operating pressures in the low 10-4-Torr range were necessaryto reduce gas scattering sufficiently to have a nearly directional depositionprocess.Current-day long-throw sputter deposition tools use throw distances of25 to 30 cm, coupled with cathode diameters on the order of 30 cm [6.5].This results in relatively poor directional filtering and as such is only appropriate for features with low aspect ratios (in the range of 1.0).
It is usually necessary to alter the erosion and emission profile of the target whenusing these long throw distances. Generally, the edge regions of the targetmust be more highly etched than the center regions because significantnumbers of atoms are lost from the edge region onto the chamber walls.This is necessary to make the net deposited flux constant across the wafer.The planar uniformity may not necessarily correlate with the directionaluniformity due to the system geometry and as such may not be a good measure of the effectiveness of the directional deposition.The deposition rates in long-throw sputtering suffer significantly compared to shorter, more conventional distances. When the throw distance isincreased from a typical 30-50 mm to a long throw distance of 250-300mm, the planar deposition rate can fall 5-10 times.
This rate reduction iscoupled with increased deposition on the sidewalls and tooling within achamber.There is a fundamental geometrical problem with long-throw sputter deposition, shown schematically in Fig. 6.7. The directionality of the arriving flux is not uniform across the wafer surface [6.6]. In the center regionof the wafer, the flux is symmetric and the angular divergence is limitedby the solid angle that encloses the edges of the target source. However,near the edge regions of the sample, the arriving flux becomes asymmetric, again due to the geometrical solid angle subtended by the target. As aDIRECTIONAL DEPOSITION193FIG. 6.7 Schematic of the cross-wafer changes in directionality for the deposited flux using longthrow deposition.result, more atoms arrive incident on the outermost sidewalls and relatively few arrive incident on the inner-radius sidewalls.
SEM photos ofthis effect are shown in Fig. 6.8.This deposition asymmetry is intrinsic to the long-throw depositiongeometry and is difficult to overcome. Typically, the inner side of thetrench receives 1/3 or less of the film deposited on the outer side of thetrench, resulting in both increased deposition time necessary for completecoverage as well as a build-out of the deposition on the outer side, whichwill shadow later deposition [6.6, 6.7].To overcome this problem, it is tempting to increase the target diameterbut this results in more isotropic deposition (i.e., wider angle deposition)in the center regions. It should be noted that increasing the target diameteris geometrically equivalent to reducing the throw distance.
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