Roland A. - PVD for microelectronics (779636), страница 44
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As long as a sufficiently thick,PLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE231continuous A1 seed layer is deposited (e.g., ~ 5 0 0 / ~ in a 0.35-/xm hole),successful reflow can be carried out. By employing a collimated Ti wettinglayer deposited at room temperature and carrying out A1 PVD at low pressure, it has been possible to use TSP for void-free filling of 4:1 aspect ratio0.5-~m plugs at wafer temperature ~ 380~ [7.10].Advanced Al Alloys Although A1 alloys based on the A1-Si-Cu systemhave dominated IC metallization, alternative alloys have been explored forimproved performance. With respect to hot A1 PVD, there is strong interest in alloys whose eutectic point is lower than that of either A1-Si (577~or A1-Cu (548~and that are therefore likely to flow more readily atlower temperature.
Since A1-Si-Cu is well established in IC processing, A1Ge-Cu alloys are of particular interest since Ge is isolectronic to Si, bothbeing Group IV elements of the periodic table. Also, only a few weightpercent of Ge is needed to significantly lower the reflow temperature[7.11-7.13]. For example, Kikawa et al. report that the addition of 0.5 % ofGe to AI-0.5%Cu allowed the reflow temperature needed to fill a via to bereduced by ~ 50~ from 460~ to 410~reflecting a comparable decrease in the bulk melting point of the alloy [7.14]. With regard to electrical performance, A1-1%Ge-0.5%Cu alloys have exhibited electromigrationreliability similar to that of conventional A1-1%Si-0.5%Cu alloys [7.12].While the basic TSP process and its subsequent improvements have addressed many of the early concerns about hot AI PVD processing, susceptibility to geometry remains an issue.
For example, a wide via fills moreslowly than a small one, and the outermost features in an array of structures may fill faster than the innermost ones because they have more surrounding real estate to draw upon as a source of AI. This can lead toprocess variability whenever feature size and/or aspect ratio change over agiven wafer or from wafer to wafer.7.3 Elevated-Temperature PVD CuAlthough reflow, TSP, and Forcefill TM processing (to be discussed inSection 7.4) of AI alloys containing up to a few weight percent Cu havebeen well documented, much less has been reported on the hot PVD processing of pure Cu.
This has begun to change based on the desire to replaceor augment A1 alloy interconnects in ULSI devices with lower-resistivity,more electromigration-resistant Cu wiring. Compared to A1, though, Cuhas a much higher melting point (T = 1356 K for Cu and 933 K for AI).Therefore, one might expect that ho~ PVD of Cu would be more difficult232R. POWELL .4ND S. M. ROSSNAGELto implement than hot PVD of A1 since the ratio of TITmpat typical reflowor TSP temperature (e.g., T = 450°C = 723 K) is so much smaller for Cu(= 0.5) than for A1 (= 0.8). On the other hand.
Cu exhibits a surface diffusion coefficient D = Do exp(-EJkT) with a relatively Iow activationenergy El, = 0.83 eV, allowing thermal flow to be exploited - althoughthe flow is not as pronounced as with Al. Also. the choice of a suitablebarrier and/or wetting layer for hot PVD Cu processing is still an openissue. Ti has worked well with A1 and Al alloys but is not suitable for Cu.Other possibilities such as TiN, Ta, TaN, and PVD W or CVD W over TiNhave not been sufficiently explored to make a clear recommendation, although work with Ta wetting layers has been quite promising [7.15]. Forexample, Cu has successfully been reflowed into high aspect ratio features (e.g., 0.1-pm X 0 .
5 - p m trenches) at moderate times and temperatures (e.g., 5 min at 400°C). In this regard, it has been reported that anatomic hydrogen-enriched H/H, ambient during reflow annealing of Cucan enhance surface mobility and thereby lower the required process temperature [7.16]. The effect appears to be a combination of surface cleaning by active H (e.g., removal of 0 and C contamination) and reductionof the Cu-Cu binding energy of the top layer of Cu atoms to the bulk. Asa result, void-free reflow of PVD Cu in 0.15-pm-wide X 0.5-pm-deeptrenches could be achieved i n 30 min at 320°C.One of the most interesting aspects of hot Cu PVD relates to microstructural effects that have been found to have a strong influence on reflow process variability 17.17-7.191. The evolution of the microstructureof Cu films is quite different than that of A1 and may relate to fundamental differences in their metallurgy and oxidation kinetics.
For example, theas-deposited grain size distribution of PVD Cu is usually not log-normal,but often contains a bimodal distribution of large and small grains. Underelevated-temperature annealing, the growth of small Cu grains occurs before that of the larger grains, and the microstructure and topography of thefilm evolves in a random way that may be very different from one via tothe next. As a consequence. microstructural effects can influence theglobal uniformity of the process.
For example, the deformation known asgrain boundary grooving is a response to interfacial tension between twograins and serves to lower the surface free energy of the Cu below that ofa free surface with the same curvature. As a result, surface diffusion slowsin the vicinity of the groove and filling is held back.
Since filling then depends on details of the local film microstructure, i t can proceed at different rates in structures that are geometrically identical.Figure 7.1 1 shows experimental data and simulations for PVD Cu ona 0.35-pm-wide. 2: 1 aspect ratio trench with a Ta wetting/adhesionPLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE233FIG.
7.11 Experimental SEM data (top) and G R O F I L M S TM simulation (bottom) of PVD Cu on a0.35-/~m wide, 2:1 aspect ratio trench with a Ta wetting/adhesion layer, before and after a reflow anneal (25 min at 450~The simulation took into account microstructural effects such as grain boundary grooving and faceting [7.17 ].layer, both before and after a reflow anneal (25 min at 450~TheG R O F I L M S TM simulation took into account microstructural effects suchas grain boundary grooving and faceting, and is in good agreement withexperiment. Figure 7.12 shows the same G R O F I L M S simulation of Cureflow over a multiple-trench topography.
When microstructural effectswere omitted from the model, the time to completely fill each trenchwas the same. However, the process showed large variability when these234R. POWELL AND S. M. ROSSNAGELFIG. 7.12 G R O F I L M S TM simulation of PVD Cu reflow over a multiple trench topography: (1) assputtered, (2) reflowed without microstructural effects taken into account, and (3) reflowed with microstructural effects taken into account [7.17].realistic effects were included, which is consistent with experimentaldata (Fig. 7.13). Other than engineering the PVD Cu grain structure orwetting layer for improved process uniformity, one should probablykeep the reflow time sufficiently long and/or temperature high to accommodate the worst-case configuration.PLANARIZED PVD: USE OF ELEVATED TEMPERATURE AND/OR HIGH PRESSURE235FIG.
7.13 Cross-sectional TEM image of PVD Cu reflow in 0.5-/.~m, 1.5: 1 aspect ratio trenches witha Ta liner, showing variability of the process due to local differences in Cu microstructure [7.17].7.4 Application of High PressureThe simultaneous application of ultrahigh pressure (= 60 MPa or 600 atmospheres) and moderate heat (= 400°C) after PVD have been exploitedto enhance the filling of sub-0.5-pm geometries without the need forexcessive temperature [7.20-7.221.
This novel process was introducedby Electrotech (now a part of Trikon Technologies, Inc.) and is calledF o r ~ e f i l l ' "or~ "Hi-Fill." The process is sometimes referred to as "highpressure sputtering"; however, this is not accurate since the sputtering stepis done at conventional mTorr-type pressures and is physically separatedfrom the subsequent application o f ultrahigh pressure.Figure 7.14 shows a schematic diagram of the Forcefill process for an Alvia-fill application. The wafer would typically be degassed, via cleaned,and have a TiITiN barrier deposited. An Al layer is then deposited at400-450°C in a conventional sputtering system under high-deposition-rateconditions to generate rapid grain growth in the plane of the film. Whenthe final Al thickness is greater than the hole diameter, it then forms a continuous bridge over the entrance of the via hole, resulting in a sealed gascavity at the process pressure (= 3 mTorr).
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