Roland A. - PVD for microelectronics (779636), страница 50
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This means that at most thesputtered atoms have a few tenths of an eV in kinetic energy, which willcause only minor deviations from normal incidence at the typical ion acceleration voltages of several tens of volts. Therefore, none of these threeproblems will be significant to the ion's trajectory, and the metal ions willarrive at the sample surface at 90 ~ (from the horizontal plane) with very little, if any, divergence.8.4 Lining Trenches and ViasPurely normal-incidence directional deposition within a via or trenchwould occur from the bottom up because there would be no depositingmetal ions incident on the vertical sidewalls. However, since the relativeionization of the depositing species is always less than 100%, sidewallcoverage is observed. An example is shown in Fig.
8.16, where the bottomstep coverage is about 70% and the sidewall coverage is 15% or less.Similar work has been reported in commercially available tools, showingFIG. 8.16 (a) SEM of directional deposition of thin, 1000-]k film using I-PVD into a 3:1 aspect ratiotrench, (b) sketch of the same structure.262R. POWELL AND S. M. ROSSNAGELbottom coverage of 50 to 70% as a weak function of the aspect ratio andfeature width (Fig.
8.17 [8.16-8.19]).The bottom-layer step coverage of the I-PVD films compares to a muchlower value for collimated sputtering. Actually, this comparison can bemisleading. The degree of collimation, effectively the aspect ratio of thecollimator, directly correlates with the bottom surface step coverage.However, since the transmission of a high aspect ratio (5:1) collimator isless than 1%, it is effectively impractical to use such high levels of collimation even though the bottom-layer step coverage would be comparableto I-PVD.Sidewall coverage is important primarily for diffusion barriers or forpreferential growth surfaces.
Diffusion barriers have the more stringent requirements: The films must be hermetic seals of the underlying surface.The ideal diffusion barrier would be amorphous and as such not have anygrain boundaries that penetrate the film and allow for grain boundary diffusion. Preferential growth surfaces would be appropriate as seed layersfor plating (e.g., Cu electroplating on Cu seed layers) and may be useful inhelping to either define the orientation of the subsequently deposited filmor facilitate the surface mobility of the next layer. As an example, on planar surfaces, TiN films tend to lead to a (111) orientation on subsequentlydeposited AICu films. However, the extension of this effect into high aspect ratio features is unclear, and as such the presence of TiN on sidewallsmay or may not be needed in high aspect ratio applications.9085o~ 80--••..--__&t~ 75$o 700o.
6500 60E0550rn50650-A TiN on the Field~~,,~m-ThickOxide>450.4ARl0.355:1l0.310.257.5:110.210.1512:1I0.1I0.050.01Contact Diameter (lum)FIG. 8.17 Chart of step coverage versus aspect ratio for ionized Ti deposition [8.16] (courtesy ofApplied Materials).263IONIZED MAGNETRON SPUTTER DEPOSITION: I-PVDDirectional deposition by itself would not lead to good sidewall coverage. However, three related effects can alter the sidewall coverage andthe effective conformality of the thin films. The first effect is simply thedegree of relative ionization of the deposit: Lower levels of directionality result in deposition on the upper sidewalls of the trench or via. Second, the re-emission of deposited atoms can also lead to better conformality.
This remission can be due to either a reflection of the incidentparticle due to a highly grazing deposition angle or simply reemissionconsistent with a less-than-unity sticking coefficient for the depositingspecies. A third critical aspect of the directional deposition is the kineticenergy of the depositing ions. As the ion energy is increased, the possibility of self-sputtering of the deposited film increases. At ion energies of10 eV or so, this effect is unimportant.
However, as the ion's kinetic energy is raised (through a more negative bias voltage on the sample) tomany tens of eV, physical sputtering becomes significant. The sputteringtakes place primarily on the bottom surface of the trench or via, and themajority of the sputtered atoms land on the lower sidewalls of the feature.This is shown schematically in Fig. 8.18, where a deposition using only50% relative ionization is changed significantly as the effective sputteryield of the depositing species is increased. A comparable experimentalsituation is shown in Fig.
8.19.The degree of resputtering in these cases is expected to be only a mildfunction of aspect ratio because the primary sputtering is occurring onthe bottom surface of the feature and the step coverage at this point isonly weakly dependent on aspect ratio. Simulations by Hamaguchi andRossnagel of this case show a general decrease in the bottom step coverage and a slow increase in the sidewall step coverage as a function ofl'--"-"-!l_T~--,--9n99IarL_[99(a) Y = 0.0(b) Y = 0.4(c) Y = 1.0FIG. 8.18 Simulations of the effect of resputtering due to increases in the depositing ion kinetic energy as a function of sputter yield (Y). From left to fight: a sputter yield of 0, 0.4, and 1.0 [8.20].264R. POWELL AND S. M. ROSSNAGELFIG. 8.19 SEM measurements of trenches deposited as a function of sample bias.
The leftmost figure has a bias voltage of - 10 V, the center SEM has a bias of - 4 0 V, and the rightmost SEM has abias of - 100 V. The relative ionization of the depositing Cu species is 50% [8.20].increasing sputter yield [8.20]. Figure 8.20 shows the case for an aspectratio of 1.0, and Fig. 8.21 shows the case of an aspect ratio of 2.5:1. Itshould be noted that the ideal conformality in each case occurs at roughlythe same sputter yield, which indicates that the overall conformality on awafer with a variety of feature widths and dimensions should be uniform.By tailoring the deposition and erosion processes, nearly ideal conforreality can be obtained. Figure 8.22 is an example for a moderate aspectratio feature, and shows a uniform 600-~ layer on both tile sidewalls andbottom of the trench.
The step coverage on the bottom surface can also becompletely removed due to sputtering. In this case, the films would onlybe deposited on the wall surfaces. This may eliminate the need for the deposition of the bottom contact layer and may also prove advantageous forelectromigration resistance in devices.Another feature that begins to become important is the beveling of theupper corner of the deposit due to sputtering. Since the sputter yield isangle dependent and peaks somewhere near 45-50 degrees, a bevel can beformed in the deposited film at the upper corners.
For thin liner films, thisfeature is reactively unimportant. However, at high levels of sputtering orwith thicker films, the top bevel can become significant and deleterious tothe film topography. An extreme example is shown in Fig. 8.23, where thetop corner is completely etched away and a scalloped wall deposit is observed. The deposit on the upper sidewalls is now partly due to local redeposition from the opposing side and eventually could lead to closure of thefeature. Normally for thin liner layers, this effect is unimportant or elsevery difficult to observe.265IONIZED MAGNETRON SPUTTER DEPOSITION: I-PVDl_9C(c) Y= 1.0(b) Y= 0.4(a) Y= 0.01.0badY~Aspect Ratio = 1.00.000.51.01.5Sputtering YieldFIG.
8.20 Calculated bottom and sidewall step coverages tor a feature of aspect ratio 1.0 as a function of the sputter yield of the depositing ions [8.201.The sample bias for I-PVD can be attained in two ways. The easiest isby means of a DC potential on the wafer. However, because many wafersare configured with insulating layers or an oxided backside, it is usuallynecessary to use a front-surface, biased clamp ring to supply the potential.Once the film is continuous (50 A), the current flow is across the wafersurface to the clamp ring. An alternate embodiment is to use an RF substrate bias that is not dependent on the oxide layers on the wafer. This allows bias to be from the backside of the wafer and no longer requires theuse of front-surface clamps. The constraint with RF bias is that the potential and current flow to the wafer are more difficult to ascertain.
In thesecases, usually the RF power to the substrate chuck is measured (in watts),but this is only an indirect measure of the level of kinetic energy given tothe depositing ions. However, once the effect of the RF bias is observedR.
POWELL AND S. M. ROSSNAGEL2661.0I!Aspect Ratio = 2.5~.0.5cab"O. .N_0z,00I0.5_I1.0d....--1.5Sputtering YieldFIG. 8.21Same as Fig. 8.19 for a 2.5:1 aspect ratio [8.20].and calibrated, it is a more effective manufacturable process than the frontsurface DC clamp ring.At this point, it is worth a small digression to describe what might be thelimits of PVD or I-PVD as applied to liner, diffusion barrier, or seed layerapplications. An ideally conformal film has a uniform coverage on both thesidewalls and the bottom of the feature.
The step coverage, however, islimited by the flux of ions and atoms from the top. For a trench, the bestcase conformal step coverage, SC, is given as1SCtr~"~h = (1 + 2AR)(8.3)where AR is the aspect ratio (depth/width) of the trench. For a via, the bestcase conformality is1SC~,(1 + 4AR)(8.4)Therefore, the m a x i m u m conformal step coverage for a trench of aspectratio 4:1 is 11% and for a via of the same AR, 6%.The ideal bottom coverage, though, is not limited by the aspect ratio. Agood rule of thumb is that the bottom coverage scales directly with the relative ionization of the depositing flux: A 50% relative ionization leads to abottom step coverage of 50%, independent of the aspect ratio at least forIONIZED MAGNETRON SPUTTER DEPOSITION: I-PVDFIG.
8.22267SEM micrographs of a tailored thin film showing a highly conformal coverage inside atrench.modest aspect ratios of to 7:1 or so. These numbers can be greatly altereddue to the presence of sputtering, which can occur during I-PVD. This isprimarily because the top or field thickness can be reduced due to sputtering. This reduces the denominator in the step coverage ratio (step coverageis just the ratio of the local thickness to the top or field thickness) and canresult in step coverages greater than 100% in some cases.
In effect, thesputtering process removes atoms from the field but atoms sputteredwithin a deep feature are unlikely to leave the feature, resulting in an apparently high step coverage.268R. POWELL AND S. M. ROSSNAGELFIG. 8.23 An I-PVD deposit that had a high level of sample bias during deposition, resulting in acomplete removal of the bottom layer as well as significant beveling and build-out near the top corners of the trench.It is interesting to extend I-PVD toward extremely high aspect ratios,which might run from 7: 1 to 10: 1.
From the formulas listed above, whichare essentially just counts of the atoms incident onto the feature, the bestcase (conformal) step coverage at 10: 1 is 5% for a trench and 2.5% for avia. To form a viable film for either diffusion barrier or seed layerlsurfacediffusion would require at least 50 A, implying field deposition thicknesses of 1000 A for the trench and twice that for the via.
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