Roland A. - PVD for microelectronics (779636), страница 43
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7.5).What is required is a fine-grained, continuous seed layer of A1 uponwhich the hot A1 can flow. This concept is the basis of the two-step process(TSP) for PVD AI illustrated in Fig. 7.6. TSP consists of cold deposition ofa fine-grained AI seed layer upon which hot AI deposition and flow proceeds; therefore TSP is also referred to as the cold-hot A1 process. Technically, the two-step process actually consists of three steps since, as withreflow, a wetting layer is deposited immediately prior to the cold A1 stepto reduce the surface tension of the cold AI and prevent voiding at the sidewalls during the hot AI step.
In addition, the cold and hot steps are conveniently done in the same PVD module by using backside gas to "switchon" the hot A1 PVD step.In this case, the TSP AI process can be summarized as follows. After deposition of a PVD Ti wetting layer, the wafer is handed off without vacuum break into a separate process chamber where both cold and hot PVDA1 take place. The heater table in the PVD AI module is maintained at elevated temperature (e.g., 4 7 5 ~with the gas turned off.
The cold A1seed layer is then deposited under conditions of high power (e.g., 10 kW),which can be done sufficiently fast to limit the rise in wafer temperature.At this point the backside gas is switched on, and the wafer quickly increases in temperature due to the gas-assisted heat transfer between waferand heater table. At the same time, the sputtering power to the target is alsoR. POWELL AND S. M . ROSSNACELWafer Hot at StartWafer Cold at StartTT .-.
7m 7wThroughFinal ProfileF'IG. 7.5 1)epositlon of Al onto a hot substrate can lead to Islanding of the Al. which in turn g i v erise to a discontinuous film that prevents void-frce lill~ng.Reprinted from the March 1990 e d ~ t i o no fSolid State Technology (copyright I990 by PennWcll).decreased (e.g., I kW) and the remainder of the Al thickness is slowly deposited onto the hot wafer.This slow deposition allows sufficient time for thermal diffusion to planarize the surface and fill the structure. Since the as-deposited cold Allayer has a large degree of structural disorder, it provides a ready source ofvacancies and point defects that act to enhance the bulk transport of material from the surface.
Ideally, one wants to prevent the top of the featurefrom closing during deposition since this "chokes off' surface diffusion ofmaterial down into the feature and creates buried voids that can only befilled by the slower process of bulk diffusion. Slow deposition helps prevent this closure by allowing time for material that would otherwise growlaterally at the top of the feature to diffuse deeper into the structure.Unfortunately, formation of voids, albeit small ones, cannot always be prevented before complete filling of very high aspect ratio holes.
In this case,PLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSUREFIG. 7.6225(a) Two-step process (TSP) for AI, (b) illustration of how TSP might be implemented on aPVD cluster tool.R. POWELL AND S. M. ROSSNAGEL226the slow deposition then allows time for the buried voids to fill prior to thedesired A1 field thickness being deposited.
Because the total time of thecold-hot A1 step can be relatively long, two modules are sometimes dedicated to this step to balance the throughput of the overall cluster tool (seeFig. 7.6b).It should be noted that the concepts underlying two-step processing (andmultistep PVD processing in general) can and have been implemented inmany different ways, so there is really no such thing as the two-stepprocess. With this understanding, a representative time-temperature profilefor a TSP process is shown in Fig. 7.7.
Note that the wafer chuck temperature was held constant through the process (heater set point ~ 480~sothat the changes in wafer temperature are the result of radiative couplingand, during the hot A1 step, gas-assisted heat transfer to the chuck. Also,the steady-state temperature of the wafer is ~ 50~ cooler than the chuck,so when discussing TSP it is important to indicate whether the temperatureis that of the wafer or the heater table.Modeling of the single-chamber TSP process using the SIMBAD TM codeis shown in Fig.
7.8, where simulated filling of a 0.6-~m • l-/~m contacthole is compared with experiment. Initially, a 0.2-/~m-thick layer of coldA1 (20~ was deposited for 10 sec at 11 kW, followed by 0.35 ~m of hot1st Step2nd Step11 kW1 kWI~,r-"~~~1v I425~vAtsecII SRateDep"200100~TempII|,.,II02020~25-50 A/sec.... 180 - 1101200Time (sec)FIG. 7.7Representative time-temperature profile for a two-step AI process.PLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE227FIG. 7.8 SIMBAD TM simulation (above) and experimental SEMs (below) of a single-chamber TSPAI process used to lill a 0.6-~m • l-/xm contact hole.
Initially, a 0.2-~m-thick layer of cold AI (20~was deposited lbr 10 sec at 11 kW, tollowed by 0.35/.tin of hot AI (530~ tor 180 sec at 0.9 kW.AI (530~for 180 sec at 0.9 kW. Figure 7.8 provides a snapshot of theTSP process after 60, 120, and 180 sec of hot AI deposition (left to right)progressing from voided to filled contact. The model is in excellent agreement with experiment and shows how the combined result of low-anglesputtered A1 adatoms and surface diffusion leads to the formation of a largeburied void 120 sec into the hot AI step, with the void being completelyfilled by vacancy-assisted bulk diffusion at the end of the 180-sec process.7.2.3 IMPROVEMENTSTO TSP AL PVDThe extension of TSP A1 technology to higher aspect ratio structures and/orlower temperatures has concentrated primarily on vacuum quality, the microstructure and coverage of the Ti wetting layer, the conformality of theA1 layer, and advanced AI alloys.R.
POWELL AND S. M. ROSSNAGEL228Vacuum Quality As discussed earlier, high partial pressures of oxidants during sputtering and reflow are to be avoided since this can suppress the TSP process by surface oxidation of the A1. However, it has alsobeen found that ultralow partial pressures of oxidants allow TSP temperature to be reduced. For example, data by Kikuta et al. [7.9] on filling ofa 3:1 aspect ratio contact hole with pure A1 (Fig. 7.9) show that the temperature needed for complete filling could be reduced from 450~ to430~ by lowering the partial pressure of water vapor during the PVD A1step from 10 -7 Torr to 10 - 9 Torr.
Using even lower partial pressures (2 •10 -1~ Torr of 02) and UHV conditions, Mukai et al. showed that evaporated AI can be reflowed at temperatures as low as 250~ Although diffusive transport of A1 proceeds more slowly at low temperature, the use oflower temperature can actually facilitate the process both because filmcontamination caused by wafer and hardware outgassing (e.g., the substrate heater) is less likely and because the A1 film is less reactive withgases such as 02, H20, and N 2 at lower temperatures.The Ti Wetting Layer Wetting describes the extent to which two dissimilar materials are attracted at their interface. Wetting is represented in100-'!'I'IlibI(~L800m~60c"-40H20" 1 x-0-20Aspect Ratio: 3=;I400~I42010 .9 TorrH20" 1 x 10 -7 Torr,I440,I460,480Sputtering Temperature (~FIG.
7.9 Filling ratio of a 3:1 contact hole for hot Al PVD as a function of wafer temperature fordifferent partial pressures of H20 (taken from Kikuta et al., ref. 7.9).PLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE229Fig. 7.10, which shows a vapor of sputtered A1 atoms forming a dropletshaped film on a substrate, where "substrate" is taken in the most generalt e r m s - e.g., bare Si, an interlevel dielectric such a s SiO 2, a coating of Tion the dielectric, etc. The contact, or wetting angle qS, between the film andsubstrate depends on the interfacial tensions between vapor and film (Tf),film and substrate (Tfs), and substrate and vapor (Tv).
In equilibrium, thesevector tensions are balanced as shown in Fig. 7.10 ( T v - Tes + T f cos oh)so that the wetting angle can be expressed as4 ' - c~[ (Lv-Lf Tf~,)](7.3)FIG. 7.10 The relative values of the three interfacial tensions for a film-substrate combination determine the wetting angle ~b of the film on the substrate. A substrate with a small wetting angle for AIis desired to facilitate the PVD AI cold-hot process.230R. POWELL AND S. M. ROSSNAGELThe surface tension T f of A1 is ~ 1.5 J/m 2, while the other two tensionshave values that depend on the specific substrate but are typically in therange of about 0.2-2 J/m 2.
Figure 7.10 represents the wetting of an A1 filmfor several values of T v and Tfs. To facilitate TSP A1 processing, a smallwetting angle is preferred (e.g., th = 10 ~ since this results in the A1adatoms spreading out as a thin film instead of bailing up (e.g., th = 135~The choice of a proper wetting layer is therefore a key to successful TSPprocessing - - analogous to the addition of a surfactant to laundry water toimprove its ability to wet the fine structures created by microscopic clothing fibers.Ti turns out to be a good wetting layer primarily because the Ti-A1 bondstrength is relatively strong compared to either A1-A1 or Ti-Ti. Use of Ti asa wetting layer has been further enhanced by controlling Ti directionalityand deposition temperature. Namely, to ensure that this Ti film is continuous over steep structures, deposition by directional PVD methods such ascollimation and long-throw sputtering have been used (see Chapter 6).
Inaddition, it has been found that depositing the Ti with the wafer cold (e.g.,at room temperature) forms a finer-grained nucleation/wetting layer for theAI that facilitates later reflow. Other materials such as TiN have also beenexplored as improved wetting layers for AI flow. Unlike Ti, TiN does notconvert to high resistivity TiA13 during the hot cycle of the cold-hot TSPprocess. It has also been reported that TiN films deposited by ionized PVDmethods can have a very smooth surface morphology that enables the twostep hot AI process to be done at lower temperature and/or in higher aspectratio features.Conformality of the Al Layer Conformality of the AI layer is equallyimportant to avoid significant bread-loafing that could bridge the top of thestructure with metal and form a large, buried void. The only way to fill aburied void is by bulk diffusion, which is in general much slower than surface diffusion.
Therefore, the goal is to prevent a void from forming or, atleast, to prevent its formation until late in the TSP process when it will besmaller and easier to remove. By using lower-pressure PVD ( ~ 0.5 mTorr)to reduce gas-phase scattering and improve conformality, cold AI layershave been deposited for this purpose. This reduces the amount of AI migration needed for complete hole fill and metal layer planarization, and thelow pressure helps maintain a clean environment. Another low-pressurePVD method - - long-throw sputtering m has also been used to deposit AIfor TSP applications. Although long-throw deposition generally leads toasymmetric sidewall coverage, this is compensated for to some extent bythe smoothing effect of the reflow process.
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