Roland A. - PVD for microelectronics (779636), страница 51
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Modeling ofthese very high aspect ratio features is ongoing, and preliminary results areshown in Fig. 8.24.8.5 Trench and Via FillingThe filling of moderate to high aspect ratio features is more difficult thandepositing a liner or bottom contact layer. This is driven primarily by thelack of 100% ionization of the depositing species.
The neutral componentof the deposit is directionally isotopic, and this results in eventual buildout of the sidewall deposits. An example of this is shown in Fig. 8.25 fora typical, partially ionized low-energy deposition.At this point, the degree of filling is directly related to the directionalityof the deposit, which is a simple function of the relative ionization of thedepositing species. Low levels of relative ionization result in poor fillingand the eventual close-off and void formation similar to nondirectionalIONIZED MAGNETRON SPUTTER DEPOSITION: I-PVD269......zFIG. 8.24 Modeling results for 10:1 and 7:1 AR features using 75% ionization and a relative sputter yield of 0.4.PVD. An example of this is shown in Fig.
8.26 for 30, 50, and 67% relative ionization. A simulation using the Hamaguchi model of the same experimental case is shown in Fig. 8.27 [8.21].The step coverage of the bottom and sidewall surfaces has been characterized for Cu deposition at various aspect ratios and power levels.Figure 8.28 (see page 271) shows a representative feature. Figure 8.29FIG. 8.25 I-PVD deposition of AICu with approximately 60% relative ionization. The sample biasvoltage is low to suppress self-sputtering.R.
POWELL AND S. M. ROSSNAGEL270FIG. 8.26 Cross section SEMs of 4000-A trenches deposited with various relative ionizations. Fromleft to right: 33, 50, and 67% relative ionization [8.21].shows the step coverages for the bottom and sides as the inductive RFpower is increased, which results in increasing levels of metal ionization.Figure 8.30 shows similar data, now for the case of constant RF inductivepower and increasing magnetron power. This shows that as the amount ofmetal is increased due to increased magnetron sputtering, the step coverage slowly degrades due to reduced relative ionization and directionality.These results are all at room temperature and are characterized by twodifferent regions within the feature: the bottom deposit and the sidewalldeposit.
Much like collimated sputtering, the bottom deposits are denseand fine-grained. The sidewall deposits, due to the intrinsic directionalityFIG. 8.27Numerical model of the same deposition case as in Fig. 8.26 [8.21].bOFIG. 8.28Sketch and SEM of bottom and sidewall depositions [8.22]._.2.R. POWELL AND S. M. R O S S N A G E L272OR1, 0.8 AR9R1, 1.2 AR1.0I'''iI""~'9R 1, 1.5 ARO R2, 0.8 ARR2, 1.2 AR0.8[] a2, 1.5 AR-0.60.6IIrrII0.4-0.40.2-0.20.00I5009,,,I1000i.I1500,....I20009&rr--0.02500RF Power (watts)FIG. 8.29Step c o v e r a g e ratios, R I = a/c and R 2 -- b/a, s h o w n as a p p l i e d R F p o w e r is i n c r e a s e d .T h e m a g n e t r o n w a s kept at 6 0 0 W, p r e s s u r e = 45 mTorr, and w a f e r bias = - 10 V [8.22].of the deposition, tend to be columnar and underdense.
In addition, thereis usually a very noticeable seam between these two deposits, which limits the rearrangement of atoms on the surface by diffusion. Figure 8.27shows these characteristic regions as well as the seams between the sidesand the bottom. Figure 8.31 shows the results of this effect after the polishing of the top or "overburden" film, as would normally be done in thechemical mechanical polishing (CMP) step during wafer processing. Theseams result in a characteristic boundary visible from the top.
This seam isclearly undesirable in that it is indicative of an underdense deposit, whichwill have higher-than-bulk electrical resistivity and will also be quite susceptible to electromigration due to the elongated void in the direction ofcurrent flow.Room-temperature I-PVD deposition for filling is a direct function ofthe relative ionization of the depositing flux. For moderately high levels ofionization (70%), filling appears possible for aspect ratios of up to about2" 1 and feature sizes down to about 3500 ,~.
At higher aspect ratios or narrower feature size, the lateral, side deposits close off the feature prior toIONIZED MAGNETRON SPUTTER DEPOSITION: I-PVD2739 R 1, 0.8 AR1.0- r~ I"1 ............'"~ ' ..............1 ...................... " ~~ 'I- --R 1, 1.2 AR9 R 1, 1.5 ARO R2, 0.8 ARO R2, 1.2 AR13 R2, 1.5 AR0.8A 2 KW, 0.8 AR' 92 KW, 1.2 ARzx 2 KW, 1.5 AR0.6I!n-II0.40.40.20.0".....0JI..................5009.............J......................,1000.................. 115000.20.02()00M a g P o w e r (watts)FIG. 8.30 Step coverage as a function of magnetron power level at constant RF inductive power(800 W) [8.221.complete filling.
It is clear that some degree of temperature-enhanced mobility is necessary to overcome this intrinsic structure caused by the I-PVDdeposition.It is intriguing to consider what might occur if the depositing ion energywere increased sufficiently to cause self-sputtering of the film during deposition. It might be expected that the incoming ions would sputter-backthe overhanging sides of the deposit and keep the trench or contact holeopen.
In addition, it might be anticipated that the sputtering process wouldresult in a forward motion of atoms sputtered from the sidewalls, whichwould push them farther down into the feature. The answer to these expectations is both positive and negative, depending on both the featurewidth and the aspect ratio.If the ion energy of the depositing atoms is increased sufficiently tocause sputtering of the deposited film, bevels are formed on the top edgesof the trench or via. This is a good feature in that it tapers back the sidewall.
However, the atoms that are "missing" from the bevel due to sputtering are redeposited on nearby surfaces. If the feature width is large, theseredeposited atoms are spread over a large area and are unimportant. This274R. POWELL AND S. M. ROSSNAGELt i . 8 . 1 Polished-hack features showing seam formation.is shown in Fig. 8.32 for an aspect ratio of < I and in Fig. 8.33 for simulations of a low aspect ratio feature.However, as the feature width is lowered the redeposition process startsto become quite significant. This is because the redeposition can occurmostly on the opposing sidewall, which results in the build-out of thatsidewall.
This effect is quite deleterious to subsequent filling and canrapidly lead to a pinching off of the feature and void formation. This isshown in Fig. 8.34 for experimental results and in Fig. 8.35 for simulations[8.21]. The amount of sputtering necessary to see this effect for narrowfeatures is quite low, and even yields of 0.3 will result in rapid closure andvoid formation for features below 0.4 microns in width.IONIZED MAGNETRON SPUTTER DEPOSITION: I-PVDFIG. X..Z2 An I-PVD deposit for a low aspect ratio feature showing the effect of significant resputtering of the deposit.One potential solution to the redeposition problem is to use sufficientsputtering so that the entire top or field layer is completely removed.
Thereasoning here is that if there is no bevel formation because there is nofilm to bevel, then cross-trench redeposition will be reduced. An exampleof this is shown in Fig. 8.36 for a very high level of resputtering duringdeposition. The top or field layer is completely gone and the upper sidewalls are tapered back significantly. There is often a small seam formed atthe centerline of the feature, but it does not lead to void formation. Thenegative aspect of this high level of sputtering during deposition is that theoxide sidewalls are exposed by the sputtering process. They are etched(and beveled) and can also lead to impurity incorporation in the depositedFIG.
8.33Modeling studies of low aspect ratio features with significant sputtering of the deposit.FIG. 8.34The effect of increasing ion energy during I-PVD deposition (left to right) [8.21 ].278R. POWELL AND S. M. ROSSNAGELFIG. 8.35 Simulation of I-PVD deposition with high levels of ion energy in the depositing ions(8.211.metal line. In addition, the energy density on the sample is quite high inthese cases, typically several watts per square centimeter, and this resultsin significant wafer heating and potential problems.8.6 Electrical MeasurementsThe use of I-PVD for wafer applications has intrinsic advantages overcollimation and some CVD technologies.
For contact resistance, it is critical that a dense Ti layer be deposited primarily at the very bottom surfaceof a via or contact hole. I-PVD of Ti should be superior to collimation inthis application because of the increased directionality and kinetic energyof the depositing atoms. Initial studies, exploring the difference betweenI-PVD of Ti and collimated Ti have shown both lower contact resistanceand a lowered Kelvin resistance distribution (Figs. 8.37 and 8.38 [8.24]and Fig. 8.39 [8.18]). These studies, as well as others, have not shown anyincrease in damage levels due to the more energetic process [8.17].Initial results with I-PVD of TiN are also encouraging. The film structure is dense and large-grained. The sidewall coverage in trenches is alsoslightly smoother than collimation. One of the principle suppliers, AppliedMaterials, has claimed that this less porous surface is more conducive tosurface diffusion of the subsequent AlCu layers than collimated TIN[8.16].
Other reports indicate that the functional resistivity of the I-PVDTIN can be significantly lower than conventionally deposited TiN [8.10].In general, the resistivity of the TIN is related to the deposition conditions.For the I-PVD TIN, the kinetic energy of the Ti+ depositing particles(set by the sample bias) has been shown to strongly alter the resistivity ofthe resulting thin films (Fig. 8.40). One clear advantage of I-PVD TiN isthat the material is easily made at low temperatures, unlike TIN sputteredconventionally. The I-PVD TIN, as a function of sample bias voltage,IONIZED MAGNETRON SPUTTER DEPOSITION: I-PVDFIG. 8.36279I-PVD deposition with very high levels of ion bombardment of the sample.shows a strong transition into stoichiometric TiN at voltages lower than- 2 0 V [8.25]. The best-case resistivity in this case appears to be about35 micro-Ohm-cm, which is about 2 times the bulk, but significantlylower than reactively sputtered, conventional TiN deposited at temperatures of 300~ or more.280R.
POWELL AND S. M. ROSSNAGELFIG. 8.37 Resistance in contact chains, comparing I-PVD Ti (described in figure as ICP) and collimated Ti (conventional) 18.24].8.7 Materials PropertiesIn addition to the case of TiN described above, AI deposited with I-PVDtechniques is significantly different than conventionally sputtered or collimated-sputtered AI.
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