Roland A. - PVD for microelectronics (779636), страница 39
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The stepcoverage takes a significant drop at contact hole aspect ratios roughlyequal to the collimator aspect ratio. However, even at via ARs muchgreater than the collimator AR, bottom step coverage is still significantlyincreased. This may, in part, be due to some slight forward scattering ofthe deposited atoms down the hole. Additional data for step coverage andDIRECTIONAL DEPOSITION199FIG. 6.12 Fully filled (top) and partially filled (lower) via features using collimated deposition ofCu. The aspect ratio of the collimator was 4.0, and the AR of the feature is about 2.7 [6.9].deposition rate as a function of collimator aspect ratio are shown in Fig.6.16 [6.11 ].
The step coverages shown in this figure are primarily for bottom coverage. As the aspect ratio of the collimator is increased and thedeposition becomes more directional, sidewall coverage will drop offrapidly. Therefore, high aspect ratio collimators are best used for depositing the bottom-of-the-via contact layer with low resistance (Fig. 6.17)[6.11 ] and perhaps are less valuable for conformal liner deposition.R. POWELL AND S. M. ROSSNAGEL200FIG. 6.13SEM cross section of TiN liner in deep via 16.111.Still, the use of collimated sputtering for the deposition of liners or diffusion barriers has several advantages over alternative techniques.Compared to CVD deposition, sputtered Ti and TiN are reasonably pureand stable and have low resistance.
Collimated sputtering is also compatible with the general design of the PVD manufacturing tool sets, and several tool manufacturers offer collimation in their tools. The ability to usethe existing PVD tool base is one of the intrinsic advantages of collimatedsputtering, and it allows the introduction of collimation as a simple tooloption rather than a completely new tool system, as would be necessary forCVD, for example.201DIRECTIONAL DEPOSITION,10 46l~o--~ ql,.."..9,,:9~176 %". .
. . . . . . . . . .~;Uncollimated - 7 m T\Collimated- 7 m Tooeq~o oeeeeooo4e414~Collimated - 0.7 mT32100100Depth Into Sample (nrn)FIG. 6.14 SIMS results measuring the penetration of F (from WF 6 gas, 6 min at 450~made from various processes [6.17].200in TiN filmsCollimated sputtering has several drawbacks that have limited its application to semiconductor processing. These include a slightly overhangingprofile; low rates; uniformity concerns; columnar microstructure; stress;high cost; and tool issues such as flaking, collimator-induced uniformitychanges, and target utilization; and construction and maintainence issues.Profile The collimator serves to reduce the angular divergence of thedepositing flux, but it does not make the depositing flux entirely perpendicular.
As a result, the overhang formation found with conventional sputtering is reduced significantly but not eliminated (Fig. 6.18). As seen in thefigure, the sidewall profiles are slowly undercutting, and they are thinnestat the bottom corners of the deposited film. The step coverage in this case,which is now defined as the local thickness relative to the top, flat plane(field) thickness, can be as low as a few percent. This requires, then, depositions of perhaps 1000 ]k on the top areas to reach a film of perhaps 50(5 nm) in the bottom corner.
In addition, often a crack or seam is observed in the bottom corner between the films deposited on the sidewalland on the bottom. This crack is a weak point for diffusion resistance.R. POWELL AND S. M. ROSSNAGEL2020.8I!>Q 0 . 60m*-'0.4IEo~1,o 0.2inn--'__iziL .....0.J...........12345Aspect Ratio of Feature673. .........8FIG.
6.15 Step coverage on the center-bottom of vias as a function of via aspect ratio for conventional and collimated deposition of Ti [6.18].Low Rates The collimator is a filter and as such has less than 100%transmission. In fact, the transmission of a collimator drops by roughly afactor of 3 for each unit increase in the collimator aspect ratio (see Fig.6.11). This means that a collimator of aspect ratio 1.0 has only about 30%of the open system deposition rate, and a collimator of aspect ratio 2.0 hasa transmission of about 10%.
The shape of the collimator is also fairlyunimportant to the rate fall-off (Fig. 6.19 [6.21 ]), and the rate reduction issimply due to the subtractive filtering of the wide-angle sputtered atoms.This low rate, as well as a low effective efficiency in the use of the sputtered atoms, suggests that collimated sputtering can be significantly moreexpensive than conventional deposition. The low rates may also be a concern for materials that are very sensitive to background gas contamination.For example, the grain size of deposited A1Cu has been empirically correlated with chamber base pressure: low pressures correlate with larger grainsizes.
Reducing the deposition rate by 10 times is equivalent to an increasein the effective chamber base pressure of the same magnitude in terms ofthe relative arrival rates of metal and background gas atoms.203D I R E C T I O N A L DEPOSITION1oo80~o60~40f--r- I'I'I ..... '-I.... 1'Trench Aspect RatnoISymbol2.594.56.5O9/'/O"~/-"50="40-99TiN9~~oZZ~. 2ogt"01,0.09,0.5IA1.0I,1.51L2.0Aspect Ratao of CollimatorI2.50(a)100 j,l/'I'1'I'Contact Hole Aspect Ratio-0t-~cl!'oSymbololO000510o152.0,Aspect Ratio of Collimator25(c)o"# 6o~40201000~9,I05,I1.0II1.5(b)aI20Aspect Ratio of Collimator't.259FIG.
6.16 Experimental data for (a) step coverage of lines, (b) step coverage of vias, and (c) deposition rate per unit power, all as a function of the aspect ratio of the collimator used [6. ! ! ].Uniformity Concerns The collimator functions as an array of pinholecameras for the sputtered atoms. At low pressure, each collimator cell canbe considered to "image" a small region of the cathode onto the samplesurface. The collimator itself blocks deposition onto that same area fromany other part of the cathode surface. For best deposition uniformity, then,the uniformity of the cathode emission should be as flat as possible acrossthe width of the cathode.
This is different from the noncollimated case,where typically the edge regions of the cathode are more highly eroded tocompensate for edge losses to the chamber walls.R. POWELL AND S. M. ROSSNAGEL20410 2I-IIIO No CollimationA6."Collimation1.4 x 1.4 cm,N~lxlcmB1 x 1 cm (N 2 Plasma)9 lxlcm(No Target Clean)~2 cm x 1 cmvrrr101cO0o10 ~ . . . . . .0.20.40.60.81.01.2Contact Size (lam)FIG. 6.17Contact resistance of Ti structures with and without collimation [6.11].To shape the uniformity profile, generally the magnet set behind thecathode surface is redesigned.
This is somewhat quantitative (Chapters 4& 5) and also somewhat of an art. It also depends on the aspect ratio of thecollimator used. Very high aspect ratio collimators will have more of a direct imaging effect, whereas very low aspect ratio collimators (< 0.5) willbe closer to the noncollimated case. To compensate for these geometricaldifficulties, many equipment manufacturers supply a range of magnet designs for their basic magnetron source.Columnar Microstructure Because of the directionality of the deposition, the sidewall deposits with collimated sputtering in some material sys-DIRECTIONAL DEPOSITION205Collimator:Top ThicknessDiffusion1IIILower Circuit ElementI .
6 . 1 Schemntic o f liner lilm dcpohitcd with collitnated sputtering. showing undercutting prolile and corner crack.tems are not smooth, but nodular or columnar. This is particularly an issuefor TIN, which shows columnar grains at a slight upward angle toward theopening of the via. The rough film cross section is less effective at forming a complete diffusion barrier because of the large number of grainboundaries (Fig. 6.20). There are several practical ways to overcome thisproblem.
Postdeposition annealing in nitrogen or even oxygen is one option. Other scenarios use a two-step deposition process in which the gaschemistry is altered between steps, resulting in a discontinuous graingrowth. This means that the grain boundaries from the first part of the deposition do not line up with those from the second, resulting in a better barrier 16.191.Stress One aspect of collimated sputtering intrinsic to the depositionprocess is that it is likely to be more energetic than conventional sputterdeposition. The operating pressures are low by design to reduce possiblegas-phase collisions so that the full kinetic energy of the sputtered atomsR.
POWELL AND S. M. ROSSNAGEL20616914O..Collimator12Shape A ShapeBQOr)rr|I/ -q~LLUncollimated. . . . .,,,,..-I---..8r0"~6O4o.O0'00.25l~0.50.75,__,.,. ~ 1 7 6 1 7 6 1 7 6"~176Q~176~l!t11.251.5....•~1.75Collimator Aspect Ratio (High/Width)FIG. 6.19Depositionrate d e p e n d e n c eas a f u n c t i o n o f c o l l i m a t o ra s p e c t ratio f o r r o u n d a n d s q u a r ec o l l i m a t o r h o l e s 16.211.is preserved.
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