Roland A. - PVD for microelectronics (779636), страница 42
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Using Eq. (7.1 ), we can then write the diffusiondistance of the mobile adatom as=(D,,?)I/' expwhere we have used the fact that the activation energy of a metal belowits melting point can be empirically approximated as E , = 5kT, [7.3].Eq. (7.2) shows the quantitative influence on reflow of higher temperature (larger ratio of TITJn).longer time at high temperature, and conditions that reduce the effective value of Do (such as surface oxidation). Inaddition, the relatively low eutectic point of alloys such as Al-Cu(577°C) or Al-Si (548°C) can lead to more flow at a given temperaturethan pure Al, which melts at 660°C.PLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE219Figure 7.3 shows a simulation of elevated-temperature A1 deposition atfour different surface diffusion lengths from L = 0.06 p m to 0.6 pm.Perfect wetting of the film was assumed, so that a continuous film withoutvoids developed.
In spite of this simplification, Fig. 7.3 graphically showshow even a factor-of-10 change in surface diffusion length can greatly impact the profile. The diffusion lengths indicated in Fig. 7.3 were not calculated ub initio but were determined as the value of L giving the best fit ofthe modeled profile with the experimentally measured profile.To make this more quantitative, consider the hot A1 profile (T = 523 K)in Fig.
7.3b for which a value of L = 0.18 p m gave the best agreementwith experiment. Since T,/T = (933 Kl523 K) = 1.78, we can useEq. (7.2) to calculate that D"T= 240 pm2 = 2.4 Xcm2. An educatedguess for T can then be made by considering T to be the time to immobilize a surface A1 adatom by burial with incident A1 or reaction with a reactive residual gas. For example, if the Al deposition rate were 1 pmlmin(167 A/sec) and burial occurred after 5 A were deposited followingadatom arrival, then T = 5 msec. Alternatively, assuming that residual gascontamination pressure (e.g., water vapor) is .= lo-' Torr, then the time toform a monolayer of contamination is 0.02 sec. If a coverage of 10% onthe surface is sufficient to immobilize the Al atom with reasonable probability, then T-- 10% X 0.02 sec = 2 msec.
Based on the agreement of theseFIG. 7.3 SIMBADIM simulation of T = 250°C (523 K) A1 deposition at four different diffusionlengths: (a) 0.06 pm, (b) 0.18 pm, (c) 0.6 pm, and (d) 1.2 p m .220R. POWELL AND S. M. ROSSNAGELadmittedly hand-waving arguments, a value of T .= 3 msec seems reasonable.
Since we calculated that DOr = 2.4 Xcm2 for the profile inFig. 7.3b, taking r = 3 msec then leads to an order-of-magnitude estimatefor Do of = 8 X l o p 4 cm2/sec.7.2 Elevated-Temperature PVD AlIn general, one expects hot PVD processes to be most effective with materials that have high surface and/or bulk diffusivity at typical PVD processtemperature, which turns out to be the case for Al. M. Inoue et al.
ofFujitsu reported in 1988 that sputtering A1 onto very hot patterned substrates (> 500°C) could be used to produce smooth, planarized metal films[7.4], and in 199 1 a refinement involving "aluminum planarization by postheating" was presented by C. S. Park er al. of Samsung. The latter method,now commonly called reflow, involved the deposition of Al at room temperature followed by a vacuum-integrated, high-temperature annealingstep at ^- 500°C 17.51. Submicron contacts with aspect ratio > I : 1 couldbe completely filled with Al alloys by this process. Figure 7.4 shows typical process conditions for a single reflow (room-temperature PVD AI +high-temperature anneal) and a double reflow in which the single reflowpnlceas is applied twice in succession without a vacuurn break.
In thedouble reflow case. the thickness of the initial Al deposition is typically= 30-5070of the desired final film thickness. Both viscous flow of A1 andbulk diffusion are relatively small at 500°C, and material transport is primarily by surface diffusion.Reflow requires an extremely clean system because both surface contamination on the wafer and gas-phase impurities such as oxygen or watervapor that oxidize the surface will significantly reduce the diffusion of A1adatoms (see Eq. (7.2)).
particularly at high temperature where the reactivity of A1 is greater. Surface diffusion is suppressed even if aluminumoxide only forms in islands. Therefore, providing a proper wafer degas before reflow and maintaining an uItralow partial pressure of oxidants duringreflow are essential to success. For example, exposure of A1 to oxidizinggases above that needed to form a monolayer of A1,0, (= 5-10 Langmuirs) can suppress the reflow process.
For an oxidizing gas partial pressure of lops Torr, this would take about 10-20 minutes. In this regard, thetime between the initial Al deposition and the reflow must be kept shortenough to prevent oxidation of the A1 surface that could poison thePLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE221Single Step Reflow ProcessDegasPreclean Sputter Etch (450~Collimated PVD Ti (300A, 100~PVD AI-Cu (8000A, 50~Reflow Anneal (525~ 1.5 mTorr)Double Step Reflow ProcessDegasPreclean Sputter Etch (25~Collimated PVD Ti (300A, 25~PVD AI-Cu (1670A, 25~Reflow (580~ 1.5 mTorr)PVD AI-Cu (3330A, 25~Reflow (580~ 1.5 mTorr)FIG. 7.4Typical process conditions for single and double A! retlow.process. Therefore, prompt transfer of the wafer between the PVD AI module and annealing module is required.
Also, since the self-diffusion constant of A1 is exponentially dependent on temperature (D oc e x p ( - E , / k T ) ) ,the uniformity of the reflow over the wafer surface will be strongly affectedby the uniformity of the wafer temperature.It has also been found that the reflow process is enhanced by depositionin vacuum of a thin underlayer of Ti (e.g., ~ 300 fi~) immediately prior tothe AI deposition, although an underlayer of TiN deposited without vacuum break has also been found effective for this purpose [7.4, 7.6, 7.7].The underlayer serves as both a wetting layer and adhesion layer for the A1and is sometimes deposited by collimated PVD to improve its conformality in high aspect ratio structures.
A Ti wetting layer is particularly important to use if a TiN barrier is present. Since TiN is often air-exposed to improve its barrier properties by oxygen-stuffing of grain boundaries, theoxygenated TiN surface is readily reduced by the A1, and the resulting oxygen contamination can then poison the reflow. The Ti layer prevents this222R. POWELL AND S. M.
ROSSNAGELproblem by gettering oxygen at the surface of the TiN and forming loweroxides of Ti that are not easily reduced by A1. At the completion of thehigh-temperature AI reflow, the Ti wetting layer has typically been converted into a refractory Ti-aluminide (TiA13) that serves to prevent stressand/or electromigration-induced voiding of the A1 lines.
On the other hand,TiA13 has a high resistivity ( ~ 3 5 / x l l - c m ) and takes up volume in the plugthat could otherwise have been occupied by the much lower resistivity A1or AI alloy ( ~ 3 / x ~ - c m ) .Assuming a gas has a sticking coefficient of unity on a given surface and adsorbs uniformly, a monolayer of gas will cover the surface after about 2 secat 10-6 Torr. The Langmuir is a unit of gas exposure defined such that 1 L =10 -6 Torr-sec, and therefore corresponds to an exposure of about 0.5 monolayer. Because the units of a Langmuir are pressure X time, 1 L correspondsto 1 sec exposure at 10 -6 Torr, 100 sec at 10-8 Torr, etc.While reflow A1 processing has been implemented for 0.5-/xm devices,there are difficulties in making it work in production at much smallergeometries.
Since reflow relies on the mobility of A1 over an underlyingwetting layer, conformal coverage of this thin layer is desired, with particular attention to the sidewalls. Sidewall coverage can be facilitated byusing sloped or even champagne-glass-shaped hole profiles; however, thehigh packing density of sub-0.5-/xm devices requires straight-walled contacts and vias that are much more difficult to coat and fill.Another concern about reflow AI is the relatively high wafer temperature, which is contrary to the trend toward lower process temperature( < 400~in advanced device fabrication. Clearly, heating the waferabove the melting point of A1 (660~is to be avoided because of potential interactions with other materials; however, even temperatures of500-550~ can produce films with large grains and grain boundary grooving with resulting high surface roughness and poor reflectivity. Such filmsare difficult to optically align for submicron lithographic patterning.
In addition, because the reflow process is extremely sensitive to oxidation,wafer degas is required, which in turn requires subjecting the wafer to temperatures 50~ hotter than the reflow process itself (i.e., > 550~The effect of repeated high-temperature cycling on lower levels of metal raisesconcerns about stress voiding and could limit the use of high-temperaturereflow to the lowermost l e v e l s - i.e., the contact hole and lowest-levelv i a - of a multilevel metal interconnect. Consequently, the application ofa hot A1 PVD process to the upper via levels of advanced devices willPLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE223probably require wafer temperature < 400~ In addition, reflow temperature must also be compatible with the increased thermal sensitivity ofpolymeric insulators that are being considered as a low-k interlevel dielectric replacement for CVD silicon oxides [7.8].7.2.2 TwO-STEP PROCESS (TSP) AL PVDReflow Al involves the high-temperature annealing of an A1 film depositedcold, i.e., at room temperature.
It is natural to ask if the process could beimproved if the sputter deposition of AI were done hot. This turns out notto be the case. In the absence of perfect wetting of the A1 to the substrate,the increased mobility of the A1 adatoms at high temperature quickly leadsto agglomeration of the A1 into islands. As the AI continues to deposit andnucleate on this discontinuous seed layer, the high mobility of the A1 atomsleads to the nuclei growing larger and more widely spaced before they coalesce to form a continuous film. Large nuclei forming at the top edge ofthe via can shadow the walls, thereby preventing deposition deeper into thestructure and exacerbating the situation. The net result is incomplete filling or complete filling but with a buried void (see Fig.
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