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The Polishing Performance of Copper CMP Slurries Containing ...

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<strong>The</strong> <strong>Polishing</strong> <strong>Performance</strong> <strong>of</strong> <strong>Copper</strong> <strong>CMP</strong> <strong>Slurries</strong><strong>Containing</strong> Organic Particles as AbrasivesKiyotaka Shindo*, Setsuko Nakamura, Shigeharu FujiiElectronic & Engineered Materials Laboratory, Mitsui Chemicals, Inc.580-32 Nagaura, Sodegaura-City, Chiba, 299-0265, JapanAkinori Etho, Tomokazu, IshizukaFunctional Polymeric Materials Laboratory, Mitsui Chemicals, Inc.580-32 Nagaura, Sodegaura-City, Chiba, 299-0265, JapanE-mail : kiyotaka.shindo@mitsui-chem.co.jp<strong>The</strong> copper damascene technique with chemical mechanical planarization (<strong>CMP</strong>) is the mostsuitable approach to achieve high-performance multi-level interconnects using low-kmaterials. <strong>The</strong> primary objective <strong>of</strong> the Cu-<strong>CMP</strong> process is to reduce the defects such asdishing, scratching and erosion, which are partially due to hard inorganic abrasives or lowselectivity <strong>of</strong> copper to barrier metals and dielectric materials. Recently, we developed somenovel Cu-<strong>CMP</strong> slurries based on pure organic particles acting as abrasives to reduce thedefects during polishing using our functional polymer resin technology. <strong>The</strong>se s<strong>of</strong>t organicparticles are designed to have reactive functional groups to interact with copper surface andthey work effectively when the organic abrasives are deformed. As a result, we haveachieved high planarization and low defects.Keywords :Cu-<strong>CMP</strong>, slurry, abrasive, organic, polymer, defect, selectivity1. IntroductionChemical mechanical planarization (<strong>CMP</strong>) is considered to be one <strong>of</strong> the most capablesemiconductor device fabrications to achieve planar surfaces for very large-scale integrated circuits(VLSI). Although alumina and silica have been used as abrasives conventionally, such inorganicabrasives are so hard that they easily cause scratches. Moreover, in the case <strong>of</strong> inorganic abrasives,erosion can arise frequently because they show low selectivity in removal rates between copper andbarrier metals or silicon dioxide. Although several attempts such as downsizing <strong>of</strong> abrasives orreducing abrasives concentration have been carried out to improve post-<strong>CMP</strong> directivities [1],researchers have been struggling with a trade-<strong>of</strong>f in copper removal rates. <strong>The</strong> coexistence <strong>of</strong> highremoval rates and low defectivity has been difficult thus far. It has been challenging to develop copper<strong>CMP</strong> slurries containing organic particles as abrasives, polymer abrasive slurries, with low defectivityand high removal rates comparable to conventional slurries mostly composed <strong>of</strong> inorganic abrasives.[2]In this paper, we investigated the basic behavior <strong>of</strong> copper polishing using our newly developednovel polymer abrasive slurries for copper <strong>CMP</strong>.2. ExperimentalIn this study, we used four different kinds <strong>of</strong> polymer particles as abrasives that Mitsui Chemicals


originally synthesized. <strong>The</strong> other components we used were familiar chemical additives for copper<strong>CMP</strong> slurries. Table 1 shows surly compositions. <strong>The</strong> concentration <strong>of</strong> polymer abrasives is about 5.0wt%. We used organic acid and an inhibiter respectively. Hydrogen peroxide was added to 2.0 wt%just before polishing. <strong>The</strong> pH was adjusted from 8.0 to 9.0. <strong>The</strong> polishing behavior <strong>of</strong> copper wascarried out on an experimental polishing tool (MAT model ARW-681M) with polishing padIC1000A21/Suba400 (XY-grooved). <strong>The</strong> polishing down force was varied from 1.0 psi to 3.0 psi androtation speed <strong>of</strong> the platen/polishing head was fixed as 60/60 rpm. Slurry flow was 200 ml/min. <strong>The</strong>dependency <strong>of</strong> the down force on copper removal rates was measured by polishing copper blanketwafers for one minute. After polishing, the wafers were cleaned with DI water for 20 seconds. Dishingdepths <strong>of</strong> TEG wafers were measured using a stylus surface pr<strong>of</strong>iler ULVAC DEKTAK3030. <strong>Copper</strong>residues on the polished wafers were detected with a scanning electron microscope (SEM) HitachiS-4500. Erosion on 4.5/0.5 μm and 100/100 μm L/S regions were evaluated with SEM and atransmission electron microscope (TEM) JEOL JEM-2010.Table 1. Composition <strong>of</strong> polymer abrasive slurriesAbrasivePolymerAbrasive concentration5.0 wt%OxidizerH 2 0 2 2.0 wt%Complexing agentOrganic acidInhibitorAddedpH 8.0 – 9.03. Results3.1.Removal ratesFigure 1 shows the dependency <strong>of</strong> polishing down force on copper removal rates. We examined fourdifferent kinds <strong>of</strong> polymer abrasives that were synthesized by different synthetic conditions andmonomer species for polymerization. As shown in this figure, it is clear that the dependency <strong>of</strong> thepolishing down force changed by using different kinds <strong>of</strong> polymer abrasives. In the case <strong>of</strong> polymer A,copper removal rate is nearly equal to zero under a down force is 2.0 psi. But it shows about 600nm/min over 3.0 psi. <strong>The</strong>se results mean this slurry shows a non-linear relationship between thecopper removal rate and the polishing down force. On the other hand, with polymer B, polymer C, andpolymer D, copper removal rates rose at each polishing down force so that the down force that canpolish copper changes to the low side. In other words, it appears that we can control the dependency <strong>of</strong>the polishing down force on copper removal rates by the choice <strong>of</strong> polymer abrasives. Figure 2 showsthe removal rate pr<strong>of</strong>iles at a down force <strong>of</strong> 2.0 psi using polymer D. At the same time, we carried outthe polishing <strong>of</strong> Ta and SiO2 to investigate selectivity. As shown in this figure, the copper removalrates are almost the same for all the positions on the wafer. This slurry shows high wafernon-uniformity. Meanwhile, the removal rates <strong>of</strong> Ta and SiO 2 are zero so that this slurry shows thehigh polishing selectivity <strong>of</strong> Cu to Ta and SiO 2 .


Removal Rate(nm/min)12001000Polymer APolymer BPolymer C800 Polymer D60040020000 1 2 3 4Down Force( psi )Removal Rate(nm/min)1200Cu1000Ta、SiO28006004002000-100 -50 0 50 100Wafer Diameter(mm)Figure 1. <strong>Copper</strong> removal rates at various down forces Figure 2. Removal rates <strong>of</strong> Cu, Ta and SiO 23.2. AbrasivesIn order to investigate the stability <strong>of</strong> polymer abrasives, we carried out the mechanical stability test.In this test, we applied a constant down force <strong>of</strong> 15.5 psi and a constant shearing stress <strong>of</strong> 30 rpm topolymer abrasives for a constant time <strong>of</strong> 20 minutes. Figure 3 shows the TEM images <strong>of</strong> polymerabrasives before and after the test. Because we used a dyeing agent to make it easy to observe, theedges <strong>of</strong> the polymer abrasives look black and some abrasives seem to condense. We think eachpolymer abrasive is dispersing well. Despite applying high mechanical strength, the shapes <strong>of</strong> thepolymer abrasives are not changed. So we can think these polymer abrasives have a high stabilityagainst high mechanical strength.(a)500 nm 500 nmFigure 3. TEM images <strong>of</strong> polymer abrasives before (a) and after (b) the mechanical stability test3.3. <strong>Polishing</strong> defectsFigure 4 shows the cross-sectional TEM images after copper polishing at 50% over-polishing usingpolymer abrasives (b). We used commercially available silica abrasive slurries (c) and aluminaabrasive slurries (d) as a comparison against the polymer abrasives. <strong>The</strong> region <strong>of</strong> observation is theboundary area between the Cu line and SiO 2 insulating space that is indicated by a circle in theschematic picture (a). Although the polishing defects are easily caused around these boundary areas,there are no defects observable and the amount <strong>of</strong> dishing is less than 40 nm with our polymerabrasives. On the other hand, a part <strong>of</strong> the Ta near to the boundary area is polished perfectly whilstserious copper loss occurred with the silica-based slurries. Moreover, even the SiO 2 insulating space(b)


area is polished with alumina-based slurries. Figure 5 shows a cross-sectional SEM <strong>of</strong> L/S=4.5/0.5 μmstructure at 50% over-polishing with our polymer abrasives, where dishing depths are less than 20 nmand no erosion was detected. We believe that these excellent results with polymer abrasives are relatedto the high selectivity <strong>of</strong> Cu/Ta/SiO 2 removal rates as mentioned in section 3.1, in accordance with thebasic design <strong>of</strong> abrasives where polymers do not have both chemical and mechanical reactivity with Taand SiO 2 .4. ConclusionWe have developed Cu-<strong>CMP</strong> slurries based on polymer abrasives. We can control the dependency<strong>of</strong> polishing down force on copper removal rates by changing the polymers that have different kindsand amounts <strong>of</strong> functional groups to interact with the copper surface and thus we have achieved highcopper removal rates comparable to inorganic abrasive slurries. Polymer abrasive slurries show thehigh polishing selectivity <strong>of</strong> Cu to Ta and SiO2 so that these slurries have the ability to stop at thedesired layers. <strong>The</strong>refore, we can conclude that such high selectivity leads to a low defectiviy.We suggest that the high resistance to defects <strong>of</strong> our polymer abrasives to be a great improvementover inorganic abrasive particles in fabricating advanced multi-level interconnections comprisingultra-low-k materials with porous structures and thinner barrier metal layers.100 um 100 umTa (25 nm)OxCu100nm(a) Schematic <strong>of</strong> the cross-section(b) Polymer(c) Silica(d) AluminaFigure 4. TEM micrographs for the sectional views <strong>of</strong> L/S =100/100 μm region at up to50% over polishing with polymer abrasive slurry (b), colloidal silica slurry(c) andalumina slurry(d)


Figure 5. SEM micrograph for sectional views <strong>of</strong> L/S =4.5/0.5 μm at up to 50% over polishing withpolymer abrasive slurryREFERENCES[1]. Cristopher L et al., Mat.Res.Soc.Sympo.Proc.Vol.816 p3 (2004)[2]. Yuzhuo Li et al., Mat.Res.Soc.Sympo.Proc.Vol.816 p41 (2004)

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