Effect of Soft Agglomerates on CMP Slurry Performance

Journal of Colloid and Interface Science 256, 137 142 (2002) doi:10.1006/jcis.2002.8352 Effect of Soft Agglomerates on CMP Slurry Performance G. Bahar Basim and Brij M. Moudgil 1 Engineering Research Center for Particle Science and Technology and Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 Received June 28, 2001; accepted March 15, 2002; published online June 3, 2002 The stability of the polishing slurries under extreme environments of ph, ionic strength, pressure, and temperature is required for their optimal performance in chemical mechanical polishing (CMP) operations. Agglomeration of the abrasive particles during polishing due to fluctuations in local particle or salt concentration under dynamic processing conditions may alter the slurry performance. It is known that the presence of hard and larger size particles in the CMP slurries increases the defect density and surface roughness of the polished wafers. However, the consequences of particle agglomeration or in other words soft agglomerate formation (even those transient in nature) on the polishing performance are not known. In this study, silica CMP performance was evaluated with partially dispersed slurries and it was observed that even the soft agglomerates adversely impacted the surface quality of the polished wafers, indicating that optimal slurry performance requires development of robust dispersion schemes. C 2002 Elsevier Science (USA) Key Words: chemical mechanical polishing (CMP); slurry particle size distribution; soft agglomerates; dry aggregation; polymer flocculation; salt coagulation; silica silica polishing. INTRODUCTION The rapid advances in the microelectronics industry demand significant improvements in the CMP, which is a process used to planarize the metal and dielectric layers to achieve multilayer metallization. As the microelectronic device dimensions keep on decreasing and the minimum feature size becomes smaller than 0.1 µm, a very thin layer of material removal with an atomically flat and clean surface finish has to be achieved during the polishing of wafers (1). These requirements can be met by close control of the CMP process. Past investigations suggest using monosized particles for the CMP slurries to minimize the surface deformation caused by the polishing process (2). However, in practical applications it is not always possible to remove all the oversized particles from the slurries. The coarser abrasives may be larger size (hard) particles or the agglomerates of the primary slurry particles. The latter are defined as the soft agglomerates in this study. 1 To whom correspondence should be addressed: 205 Particle Science and Technology Building, P.O. Box 116135, Gainesville, FL 32611-6135. Fax: (352) 846-1196. E-mail: bmoudgil@erc.ufl.edu. It is usually believed that the larger size particles are the main reason for the formation of scratches and pitting on the polished wafer surfaces. In a previous investigation we have demonstrated that the contamination of a commercial CMP slurry with 1 wt% larger size spherical particles resulted in increased surface roughness and a higher number of surface defects (3). Their presence also changed the material removal rate response. To remove coarser particles, filtration of CMP slurries is commonly practiced. Nevertheless, even after the slurries are filtered, the defect counts on the polished surfaces are often observed to be higher than desired (4). It has been speculated that some of the defects are caused by the transient soft agglomerate formation in the CMP slurries. Indeed, it was reported previously that the commercial CMP slurries tend to coagulate and partially disperse during the polishing process, confirming the presence of these agglomerates (5). In this study, to investigate the effects of soft agglomerates on the performance of CMP slurries, a systematic analysis was conducted on silica silica polishing. To create the soft agglomerates, a part of the 0.2-µm monosize silica slurry (at ph 10.5) was substituted with dry aggregated, polymer-flocculated, and salt-coagulated fractions. The performances of the slurries were evaluated based on the material removal rate, surface roughness, and maximum surface deformation analyses conducted on the polished wafers. EXPERIMENTAL Slurry preparation. To study the role of soft agglomerates on polishing efficiency, 0.2-µm monosize sol gel silica powder obtained from Geltech Corp. was used to prepare the baseline slurry. The ph was adjusted to 10.5 and the solids concentration was kept at 12 wt% for all the slurries. To stabilize the baseline slurry, dry silica powder was ultrasonicated in deionized water while maintaining the ph at 10.5 with NaOH addition. Preparation of the slurries from a dry powder was necessary to enable the control of system chemistry while making modifications to create some of the aggregates. Three techniques were used to prepare slurries containing soft agglomerates. Initially, dry aggregates were left partially undispersed in the baseline slurries by controlling the ultrasonication during the slurry preparation. In the second method, 137 0021-9797/02 $35.00 C 2002 Elsevier Science (USA) All rights reserved.

138 BASIM AND MOUDGIL FIG. 1. Particle size distribution of the baseline and soft agglomerated slurries. The soft agglomerates were prepared by dry aggregation, polymer flocculation, and salt coagulation methods at 5- to 10-µm-size range. and a Struers Rotopol 31 tabletop polisher was used for polishing with IC 1000/Suba IV stacked pads supplied by Rodel Inc. A grid-abrade diamond pad conditioner was utilized to abrade the pad before conducting each polishing test. The downforce was set to 7.0 psi (492 g/cm 2 ) and the rotation speed was kept at 150 rpm both for the pad and for the wafer. The slurry flow rate was 100 ml/min and polishing tests were conducted with 50-ml slurries for 30 s. The thickness of the oxide film on the wafers was measured via the spectroscopic ellipsometry method before and after polishing to calculate the material removal rate. The atomic force microscopy (AFM) technique was selected for the surface characterization of the polished wafers. For all the selected conditions, a minimum of four polishing tests were conducted and five 10 µm 10 µm size images were taken on each polished wafer to evaluate the surface roughness and maximum surface deformation responses. well-dispersed baseline silica slurry was flocculated using 0.5 mg/g, 8,000,000 molecular weight polyethylene oxide (PEO) polymer. To obtain homogeneously flocculated slurries, the slurry samples were stirred gently in 60-ml bottles in the presence of PEO and size analyses were conducted as a function of time. In the beginning, slurries were found to be highly flocculated and inhomogeneous. After 30 min of stirring, more homogeneous slurries were obtained with a main peak at 0.2 µm and a double peak at 5- to 10-µm-size range. As the stirring continued, a monosized well-dispersed slurry was achieved after 2 h of agitation and its size distribution curve overlapped with the baseline slurry size distribution. It was reported in past investigations that 8,000,000 MW PEO adsorbs onto the sol gel silica surface at ph 9.5 (6). Therefore, it is suggested that the abrasive silica particles in the slurries were flocculated by the bridging mechanism to begin with and then stabilized by breaking polymer bridges during the stirring action. Finally, in the third method, NaCl salt was added to the slurries at 0.2, 0.4, and 0.6 M concentrations to form coagulates. Particle size analyses of the slurries were conducted by a Coulter LS 230 instrument utilizing the light-scattering technique with a small volume module. The background water used to run the size analysis was prepared to have the same chemical composition as the modified slurries. Figure 1 shows the size distribution curves of the baseline, dry aggregated, polymerflocculated, and salt-coagulated slurries as a function of differential volume. It can be seen that the baseline polishing slurry had a narrow, unimodal particle size distribution with a mean size of 0.2 µm. Slurries containing soft agglomerates, on the other hand, exhibited bimodal particle size distributions with a double peak at 5- to 10-µm-size range. Polishing experiments. Polishing tests were performed on p- type silicon wafers on which a 2.0-µm- thick SiO 2 layer had been deposited by PECVD (supplied by Silicon Quest International). The 8-in. wafers were cut to square samples of 1.0 in. 1.0 in. RESULTS AND DISCUSSION Effect of Dry Aggregates Table 1 summarizes the performances of the partially dispersed/agglomerated slurries in terms of material removal rate, surface roughness, and maximum surface deformation responses. For baseline polishing, the material removal rate was 3800 Å/min and the surface roughness was 0.85 nm with 25 nm of maximum surface deformation. Figure 2a shows the AFM image of the silica wafer polished with the baseline slurry. The wafer surface was smooth with minimal deformations. When dry aggregates were present in the polishing slurries, the mean particle size increased to 0.77 µm from the original size of 0.2 µm. In the presence of dry aggregates, the material removal rate was 4300 Å/min, indicating a trend toward increased removal. The surface quality of the polished wafers, on the other hand, degraded significantly as seen in Fig. 2b. The surface roughness value increased to 2.66 nm and the maximum surface TABLE 1 Slurry Performance Summary in the Presence of Soft Agglomerates Surface Surface Agglomeration Removal rate roughness damage method (A /min) RMS (nm) R max (nm) Baseline 3800 ± 410 0.85 ± 0.24 25 Dry aggregation 4300 ± 470 2.66 ± 0.98 65 Polymer flocculation (0.5 mg/g PEO) Flocculated 3680 ± 260 1.14 ± 0.19 45 Dispersed 3090 ± 240 0.77 ± 0.09 20 Salt coagulation (NaCl) 0.2 M 4650 ± 260 0.86 ± 0.28 50 0.4 M 5210 ± 500 1.70 ± 0.86 100 0.6 M 6000 ± 120 2.76 ± 1.01 120

SOFT AGGLOMERATES AND CMP SLURRY PERFORMANCE 139 FIG. 2. AFM images of the silica wafers polished with (a) baseline 0.2 µm 12 wt% monosize silica slurry and (b) slurry with dry aggregates. deformation was detected to be 65 nm as given in Table 1. These results demonstrate the adverse effect of the dry aggregates on the polishing process. The higher magnitude of the attractive van der Waals interactions between the dry silica particles compared to the silica particles dispersed in a solution can lead to the formation of relatively rigid agglomerates (7). As a result, the aggregates of dry powders are believed to be the most rigid soft agglomerates among the ones studied in this investigation. During the CMP applications, dry aggregates are expected to sustain most of the applied load since their size is larger than the primary slurry abrasives. Therefore, they are exposed to high download during polishing that may result in their breakage. Indeed, the particle size analysis of the slurries collected from the pad after polishing with dry aggregated slurries showed a shift toward increased volume of primary size abrasives. Yet, while breaking, they may result in strong mechanical abrasion (indent) on the wafer surface due to their rigidity. It was observed in previous studies that the mechanically driven material removal created significant surface deformations during polishing similar to the damage created by the dry aggregates (8). It should be noted that the dry aggregates do not form during polishing but they may exist in the CMP slurries as a result of improper slurry preparation. Effect of Polymer Mediated Soft Agglomerates (Flocs) Figure 3 shows the size distribution of the slurries flocculated and dispersed in the presence of PEO. Polishing tests were conducted using the slurries containing PEO with and without the flocculated agglomerates. It can be seen in Fig. 4a that the flocculated slurries (mean particle size of 5.82 ± 0.67 µm) resulted in severe surface deformations during polishing with a material removal rate of 3680 Å/min as compared to the baseline slurry removal rate of 3800 Å/min. The surface roughness value increased to 1.14 nm and maximum surface deformation was found to be 45 nm in the presence of flocs. Similar to the dry aggregation case, the flocs are believed to break during the polishing operation under the applied normal and shear forces since the size distribution of the slurries collected from the pad after polishing with flocculated slurries shifted down, as can be seen in Fig. 3. Considering the flocs are made of well-dispersed particles bridged with the polymer chains, their formation should involve more flexible bonds compared to the dry adhesion case. Therefore, the rigidity of the polymer flocs is believed to be much less relative to the dry aggregates and they are expected to deform between the pad and wafer surfaces more easily. This may explain the reduced surface deformation value obtained in the presence of flocs (45 nm) as compared to the dry aggregation case (65 nm). Figure 4b shows the AFM image of the silica wafer polished with the slurry stabilized in the presence of 0.5 mg/g PEO. There were no flocs detected in this slurry with Coulter LS 230 and its size distribution curve overlapped with the baseline slurry as shown in Fig. 3. The surface quality of the wafers polished with this slurry was comparable to the baseline polishing result. The material removal rate, on the other hand, decreased to 3090 Å/min from the baseline polishing value of 3800 Å/min. This result can be explained based on the lubrication of the particle and substrate surfaces by polymer adsorption. For the slurries that are stable in the presence of PEO, each abrasive particle is expected to be surrounded by the polymer molecules. Therefore, the abrasive particles can be effectively and uniformly lubricated, resulting in a lower material removal rate but much better surface quality. FIG. 3. Particle size distribution of the baseline and polymer-flocculated slurries. The size distribution of the flocculated slurries shifted down after polishing. Slurries stirred long enough in the presence of PEO dispersed to the original size distribution of the baseline slurry.

140 BASIM AND MOUDGIL FIG. 4. AFM images of the silica wafers polished with (a) slurry flocculated using 0.5 mg/g PEO (b) slurry dispersed in the presence of 0.5 mg/g PEO. To determine the change in the frictional force with the addition of PEO, AFM measurements were conducted by attaching a 7.3-µm silica particle to the AFM tip. These measurements were performed under baseline conditions with ph 9.0 water (as the liquid AFM cell cannot be used at higher ph) and with 5 mg/g PEO solution at ph 9.0. The frictional force (F F ) was measured between the abrasive particle attached to the AFM tip and the silica wafer surface as a function of the normal force (F N ) applied to the tip. The friction coefficient (µ) was calculated from the slope of the curve based on Amonton s law (Eq. [1]). F F = µf N. [1] Figure 5 shows the AFM friction coefficient measurement in the absence and presence of PEO at a single-particle substrate interaction level. It can be seen that the addition of PEO resulted in a significant decrease in the friction coefficient (0.12) as compared to the baseline value of 0.25. The observed decrease in the friction force in the presence of PEO indicates the lubrication effect of the polymer. In agreement with this finding, the boundary lubrication studies using atomic force/friction microscopy technique have shown that the coefficient of friction of the Si(100) sample lubricated with polymeric lubricants (Z-15 and Z-DOL perfluoropolyether) was lower than that of an unlubricated sample (9). A similar trend was also reported for tungsten polishing in the presence of surfactants (10). When the alumina particles in the polishing slurries were stabilized using a mixed surfactant system (anionic and nonionic), a 30% less material removal rate was reported than that without the presence of surfactants, however, yielding a much better surface quality. Effect of Salt-Coagulated Agglomerates To delineate the effect of salt-coagulated agglomerates on polishing performance, NaCl was added into the well-dispersed baseline slurries. NaCl was selected in this investigation since it was reported in the literature that Na salts could coagulate the silica suspensions (11). The critical coagulation concentration (CCC) of NaCl for 12 wt% silica slurries at ph 10.5 was measured to be 0.25 M by conducting size analyses of the slurry as a function of salt concentration. Accordingly, three different salt concentrations were selected starting with 0.2 M, which is below CCC and 0.4 and 0.6 M concentrations above CCC. Figure 6 illustrates the particle size distribution analysis for the salt-coagulation studies. It can be seen that with the 0.2 M NaCl addition the slurries did not coagulate and the mean particle size of the polishing slurry remained at 0.2 µm. When the 400 Lateral Force (nn) 300 200 100 ph = 9.0 µ = 0.25 PEO ph=9.0 µ = 0.12 0 0 400 800 1200 1600 Loading Force (nn) FIG. 5. AFM friction coefficient measurements for the baseline (ph 9.0 DI water) and PEO containing solutions. FIG. 6. Particle size distribution of the baseline and salt-coagulated slurries. The size distribution of the slurry with 0.2 M NaCl did not change significantly. Addition of 0.6 M NaCl coagulated the slurry. Size distribution of the 0.6 M NaCl slurry before and after polishing remained the same.

SOFT AGGLOMERATES AND CMP SLURRY PERFORMANCE 141 FIG. 7. AFM images of the silica wafers polished with (a) 0.2 M NaCl containing slurry and (b) 0.6 M NaCl containing slurry. polishing tests were conducted at this salt concentration, the smoothness of the polished wafer was comparable to the baseline polishing (0.86 nm), as can be seen in Fig. 7a. The maximum surface deformation value, on the other hand, increased to 50 nm from the baseline value of 25 nm. The material removal rate response also showed a significant increase in the presence of 0.2 M NaCl. A value of 4650 Å/min was obtained as compared to the 3800 Å/min for the baseline polishing. The increase in the material removal rate can be explained based on two phenomena. First, it is well known that both the silica wafer and the silica abrasive particles exhibit large negative charge at 10.5 ph (ζ -potential of 65 mv at ph 10.5), leading to strong repulsive forces. To accomplish polishing, these repulsive forces must be overcome. As the ionic strength of the solution is increased, the additional ions introduced into the solution screen the charges on the silica surfaces, lowering the repulsion (7). The decrease in the extent of the repulsive electrostatic forces was also reported between a silica particle and silica plate at increasing salt concentrations using the AFM technique (12). The enhanced particle surface interactions in the presence of salt may lead to more effective mechanical abrasion, yielding a higher material removal rate. The second explanation relates to the wafer surface polishing pad interaction. The surface of the polishing pad was also reported to have a high negative charge (ζ -potential 25 mv) at high ph, suggesting that there are also repulsive forces between the pad and the wafer to be polished (13). As the ionic strength of the solution is increased by the addition of salt, the screening of the charges enables closer contact between the pad and the wafer surface. This phenomenon was shown to increase the frictional forces between the pad and the wafer surface by in situ dynamic force measurements (13, 14). Consequently, the pad surface interactions could also favor increased material removal. The increase in the maximum surface deformation with the addition of salt can also be related to the increasing frictional forces. Additionally, it is suggested that some transient agglomerates may be forming during polishing in the presence of 0.2 M NaCl. Theoretical calculations based on the constant charge assumption show that the magnitude of repulsive force barrier in the presence of 0.2 M NaCl is 167 kt as compared to 720 kt in the absence of salt, which is significantly lower. Thus, transient agglomerates may form in the presence of 0.2 M NaCl due to enhanced particle particle interaction when local variations occur in the particle concentration under the dynamic polishing conditions, although the size measurements did not indicate agglomeration. As the salt concentration of the polishing slurries increased to 0.4 and 0.6 M, the slurries coagulated irreversibly. The mean particle size of the 0.4 and 0.6 M NaCl containing slurries were measured as 1.3 and 3.6 µm. Figure 6 shows the size distribution of the 0.6 M NaCl containing slurry before and after polishing. Both curves are almost identical, showing bimodal distributions with a first peak around 0.2 µm and a second peak at 5- to 10-µm-size ranges. This observation indicated that the abrasive particles coagulate at high ionic strengths, and even if they break during polishing under the applied shear and normal forces, they tend to coagulate back as the pressure is released. AFM analysis conducted on the wafers polished with high ionic strength slurries showed significant degradation in the quality of the polished surfaces, suggesting that the saltmediated aggregates (coagulates) also had an adverse effect on polishing performance. Figure 7b demonstrates the AFM image of the wafer polished with 0.6 M NaCl containing slurry. The surface roughness was measured to be 2.76 nm and maximum surface deformation increased to 120 nm in the presence of 0.6 M salt. The significant increase in surface roughness is suggested to be due to the presence of stable (soft) agglomerates in the slurries at high ionic strengths. In addition, the material removal rate increased with the increase in salt concentration as summarized in Table 1. A similar observation was reported on silica CMP by Mahajan et al. (13). As explained previously, this phenomenon is believed to be occurring due to the variation in the electrostatic forces in the system. For the baseline slurries at ph 10.5, all the pad, particle, and substrate surfaces are highly negatively charged and therefore there is a strong repulsive force barrier among them. The addition of electrolyte results in neutralization of the negative charges by the counterions. Consequently, the degree of repulsion decreases and a closer interaction can take place among the pad particle substrate surfaces, resulting in higher material removal. These analyses indicate that salt addition can significantly enhance the material removal rate but results in major surface deformations.

142 BASIM AND MOUDGIL SUMMARY In this paper the effects of soft agglomerates (aggregates of the primary size abrasive particles) on CMP slurry performance were reported. The surface quality of the polished wafers degraded significantly when agglomerates were present in the slurries. The magnitude of surface defects changed depending on the rigidity of the agglomerates. A small amount of relatively harder dry aggregates deteriorated the performance of CMP surface quite dramatically. The presence of relatively softer flocs and coagulates also produced undesirable surface effects, though at a much higher concentration of aggregated particles. The material removal rate response, on the other hand, was observed to be controlled by the extent of the pad particle surface interactions that impact the magnitude of the frictional forces, enabling abrasion and material removal. These observations indicate that to design optimally performing CMP slurries interparticle and pad particle surface interaction forces must be taken into account. In addition, it became clear that not only the coarse (hard) particles but also the soft agglomerates of the primary abrasive particles must be avoided in the polishing slurries to ensure acceptable surface quality. ACKNOWLEDGMENTS The authors acknowledge the financial support of the Engineering Research Center (ERC) for Particle Science and Technology at the University of Florida, The National Science Foundation (NSF) for Grant EEC-94-02989, and the industrial partners of the ERC. Dr. Ivan Vakarelski is also acknowledged for his contributions in measurement and calculation of the friction coefficients. REFERENCES 1. Murarka, S. P., in Chemical-Mechanical Polishing Fundamentals and Challenges (S. V. Babu, S. Danyluk, M. I. Krishnan, and M. Tsujimura, Eds.), p. 3. Mater. Res. Soc. Proc. 566, Mater. Res. Soc., Warrendale, PA, 2000. 2. Cook, L. M., J. Non-Cryst. Solids 120, 152 (1990). 3. Basim, G. B., Adler, J. J., Mahajan, U., Singh, R. K., and Moudgil, B. M., J. Electrochem. Soc. 147, 3523 (2000). 4. Ewasiuk, R., Hong, S., and Desai, V., in Chemical Mechanical Polishing in IC Device Manufacturing III (Y. A. Arimoto, R. L. Opila, J. R. Simpson, K. B. Sundaram, I. Ali, and Y. Homma, Eds.), p. 408, Electrochem. Soc. Proc. 99-37. Electrochem. Soc., Pennington, NJ, 1999. 5. Material Safety Data Sheet for Semi-Sperse 12 and 25 Aqueous Dispersions, Cabot Corporation Microelectronics Division, Aurora, IL, 2000. 6. Mathur, S., Adsorption Mechanism(s) of Poly(ethylene oxide) on Oxide and Silicate Surfaces. Ph.D. thesis, University of Florida, Gainesville, FL, 1996. 7. Israelachvili, J. N., Intermolecular and Surface Forces, 2nd ed. Academic Press, New York, 1992. 8. Mahajan, U., Fundamental Studies on Silicon Dioxide Chemical Mechanical Polishing. Ph.D. thesis, University of Florida, Gainesville, FL, 2000. 9. Bhushan, B., in Handbook of Micro/Nanotribology (B. Bhushan, Ed.), p. 357. CRC Press, Boca Raton, FL, 1995. 10. Bielman, M., Mahajan, U., Singh, R. K., Shah, D. O., and Palla, B. J., Electrochem. Solid-State Lett. 2, 148 (1999). 11. Iler, R., The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry. Wiley, New York, 1979. 12. Rajan, K., Sigh, R., Adler, J., Rabinovich, Y., and Moudgil, B. M., Thin Solid Films 308 309, 529 (1997). 13. Mahajan, U., Bielman, M., and Singh, R. K., Electrochem. Solid-State Lett. 2, 80 (1999). 14. Mahajan, U., Bielman, M., and Singh, R. K., Electrochem. Solid-State Lett. 2, 46 (1999).