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Green Manufacturing

Characterizing recycled fumed silica slurries in ILD CMP applications

Ara Philipossian and Farhang Shadman, University of Arizona; Patrick Levy, Saied Tousi, and Barry Gotlinsky, Pall; and W. Scott Rader, Paul Lefevre, and Isamu Koshiyama, Fujimi

Chemical-mechanical polishing (CMP) technology has played an enabling role in attaining near-perfect planarity of interconnection and metal layers, an essential step for realizing and miniaturizing high-performance devices. To ensure stable and high-performance CMP characteristics, optimization of the slurry, the pad, and other consumables is critical. Additionally, the relatively high cost of ownership associated with CMP consumables warrants novel approaches to reduce these expenses.

Previous studies have demonstrated the feasibility of regenerating slurry.1,2 One study in particular showed that high-purity colloidal silica could be reclaimed because of its higher dispersion and lower tendency to clog than fumed silica.3 For these reasons, fumed silica slurry used in interlayer dielectric (ILD) CMP applications was chosen for the study discussed in this article. The goals of the study were to characterize the slurry, which was regenerated using filtration techniques, and to determine whether it could be rendered analytically and functionally equiv- alent to fresh slurries.

Experimental Setup

All polishing experiments were performed on 100-mm thermally grown SiO2 wafers using a scaled version of a 472 polisher from SpeedFam-IPEC (now Novellus Systems, San Jose). Details of the experimental apparatus are described elsewhere.4 To measure the shear force between the pad and the wafer during polishing, a sliding table was placed beneath the polisher. The sliding table consisted of a bottom and a top plate on which the polisher rested. As the wafer and pad were engaged, the top plate slid with respect to the bottom plate in only one direction because of friction between the pad and wafer. The degree of sliding was quantified by coupling the two plates to a load cell. The load cell was attached to a strain gauge amplifier that sent a voltage signal to a data-acquisition board.

The apparatus was calibrated to report the force associated with a particular voltage. IC-1000 perforated pads from Rodel (now Rohm and Haas Electronic Materials, Phoenix) were used. Before frictional and removal-rate data were acquired, each pad was conditioned for 30 minutes using PL-4217 slurry from Fujimi (Tualatin, OR). The silica abrasive concentration was 12.5% by weight. Pad conditioning was performed using a 100-grit diamond disk at a pressure of 0.5 psi (3.5 kPa), a rotational velocity of 30 rpm, and a disk sweep frequency of 20 per minute. Conditioning was followed by a 2-minute pad break-in step using a silicon dummy wafer. Other experimental conditions included:

• In situ conditioning at 30 rpm and 20 oscillations per minute.

• Applied wafer pressure of 6 psi (~41.5 kPa).

• A relative pad-wafer average linear velocity of 0.62 m/sec.

• A slurry flow rate of 60 cm3/min.

• Counterclockwise wafer and platen rotations.

The aggressive polishing conditions and the use of a perforated pad were selected to ensure relatively high slurry utilization efficiencies of approximately 15% and high slurry shearing. Slurry utilization in CMP applications is known to be quite inefficient, ranging from 2 to 20%.5 Since the efficiency of the process depends on such variables as the slurry flow rate, wafer pressure, linear velocities, and pad grooving, it was important to choose polishing conditions that resulted in a more efficient process. The more efficient process provided a worst-case scenario in which more of the slurry being delivered to the pad reached the wafer than would be the case under typical conditions, creating a larger quantity of slurry for regeneration.

Effluent samples were collected after multiple polishings were performed and the slurry was recycled 1, 3, and 5 times. The samples were characterized analytically to determine pH, trace-metal levels, viscosity, specific gravity, mean aggregate particle size, and large-particle counts (LPCs, or particles >1.0 Ám) before and after depth filtration.

During depth filtration, the entire depth (or thickness) of the melt-blown polymeric filtration medium is used to retain particles. As the tortuous path of the particle is increased, the probability that a filter will retain the particle also increases. When CMP slurries are filtered, the goal is to pass the native slurry particles through the filter while trapping the oversized agglomerates and foreign materials. This procedure is used more to classify particles than to filter them. Studies have shown that LPC data have been correlated to wafer defects.6 Furthermore, it has been demonstrated that filtration can reduce defects during CMP processing.5 Thus, depth filtration is used as a means of removing agglomerated particles or foreign materials that may have been introduced into the slurry as a result of ILD polishing.

CMP slurries can undergo changes in viscosity when the shear stress to the fluid is sufficient, affecting the stability of the slurry. Because of the morphology of the particles they contain, fumed silica slurries are especially prone to shearing effects. Hence, the flow rate and corresponding differential pressure chosen for this study were lower than the values in typical slurry applications.

In addition to undergoing analytical characterization, the fresh and recycled slurry samples were characterized functionally to determine their removal rate and coefficient of friction (COF) both before and after filtration. Before the polishing experiments were conducted, baseline trials were performed using fresh and "bypassed" slurry, where the slurry was allowed to flow over the rotating pad with the wafer disengaged. All filtration studies were performed utilizing a peristaltic pump to deliver the slurry to Profile Y010 (1.0-Ám) filter media from Pall (East Hills, NY). The material was cut into 1-in. segments. All tests were conducted in a single pass at a flow rate of 50 cm3/min to minimize the chances of slurry shearing during filtration.

Theoretical Approach

Coefficient of Friction. COF is defined as the ratio of shear to normal force:

In the study described here, shear force was determined experimentally using the combination of the sliding table, the load cell, and the strain gauge amplifier. Normal force was obtained by multiplying polishing pressure by the total surface area of the wafer.

Spectral Analysis. For a given polishing run, the measured total unidirectional shear force as a function of time can be divided into two components, a mean force and a fluctuating force component, as shown in the following equation:

Fshear(t) = F + f(t)

Figure 1 presents an example of the total force measurement obtained during a typical polishing run. Sampling time was 1 second and the sampling frequency was 1000 Hz. For a 75-second polishing experiment, a total of 75 such plots would be generated and analyzed for tribological attributes. The mean force F, which represents the average of all 75,000 data points, is used in calculating COF, as defined in the first equation and utilized in previous studies.1–4 For spectral analysis, the measured total unidirectional shear force function (which includes the fluctuating component) is converted into frequency domain via fast Fourier transformation.7

Figure 1: Shear force in time domain measured during a 1-second polishing run.

Figure 2 shows an example of this transformation, where the x-axis represents signal frequency (in hertz) and the y-axis indicates the amplitude of the transformed function. On a qualitative basis, this force spectrum identifies the extent and frequency at which stick-slip phenomena occur. In the tribology of the CMP process, stick-slip (i.e., hydrodynamic chattering) refers to cyclic fluctuations in the magnitudes of frictional force and relative velocity between the wafer and the pad. It is usually associated with a relaxation oscillation that depends on a decrease of COF with increasing sliding velocity. True stick-slip, in which each cycle consists of a stage of actual stick followed by a stage of overshoot (i.e., slip), requires that the kinetic COF (i.e., the parameter being measured in this study) is lower than the static COF (i.e., the maximum friction force that must be overcome to initiate macroscopic motion between the wafer and pad). In contrast, random variations in friction-force measurements do not constitute stick-slip. In this study, the stick-slip criterion was met.

Figure 2: Shear force in frequency domain measured during the same 1-second polishing run shown in Figure 1.

Another form of stick-slip can be caused by the spatial periodicity of the friction coefficient along the path of contact (i.e., pad grooves or microtrenches that are created on the surface of the pad by the conditioner, or complex films that form and are abraded on the surface of the wafer).

In Figure 2, the area under the curve is the basis of another parameter called the interfacial interaction index (γ), which is determined empirically based on transformed data in the frequency domain and is essentially a measure of the range of forces encountered during polishing. In other words, g is similar to the variance of the distribution. On a quantitative basis, the area under the curve can be viewed as representing the total amount of mechanical energy caused by stick-slip phenomena. Further studies must be conducted to distinguish among the various types of stick-slip phenomena that occur during CMP. In the meantime, this study treated all forms of stick-slip as one entity, represented by γ.

ILD Removal. The most widely adopted removal-rate equation is the one proposed by Preston, which states that the removal rate (RR) is proportional to the product of the applied pressure and the relative velocity of the substrate in contact with the pad:8

RR = kPr X p X U

Preston's constant (kPr) itself depends on the various chemical and mechanical attributes of the process.

Results and Discussion

Figures 3a and 3b show that slurry recycling had little effect on viscosity and specific gravity. Fresh filtered slurry had a viscosity of 2.41 cP while filtered slurry that had been recycled five times had a viscosity of 2.39 cP. Specific gravity was 1.081 for fresh slurry and ~1.077 for slurry that had been recycled five times. In addition, the change in specific gravity corresponded to only a 0.15% drop in solids content (from 12.50 to 12.35% by weight). Figures 4a and 4b, on the other hand, show that while slurry recycling had little effect on pH (11.00 compared with ~10.85 for fresh versus recycled slurry), it had a notable effect on trace-metal levels, as highlighted by the threefold increase shown in Figure 4b (3 ppm as compared with 1 ppm for fresh versus recycled slurry).

Figure 3: Effect of slurry recycling on (a) viscosity and (b) specific gravity.
Figure 4: Effect of slurry recycling on (a) pH and (b) metals content.

The results presented in Figure 5 show that slurry recycling had a significant impact on both mean aggregate particle size and LPCs. Mean aggregate particle size increased by 11%, from 138 to 150 nm, after filtered slurry was reused five times. However, while the use of unfiltered recycled slurry resulted in a 50-fold increase in LPCs over unfiltered fresh slurry, the use of filtered recycled slurry resulted in only a threefold increase in LPCs over filtered fresh slurry. Moreover, all of the filtered slurry samples resulted in at least three-times-lower LPC values than fresh unfiltered slurry.

Figure 5: Effect of slurry recycling on (a) mean aggregate particle size and (b) LPCs.

Slurry filtration does not have any notable impact on the above metrics except possibly on mean aggregate size (although it appears that multiple reuses of the slurry have a greater impact), and large-particle counts. In that case, filtration is quite effective in reducing the number of LPCs (especially in the case of the recycled slurries where the reduction was greater than two orders of magnitude).

Figure 6: Effect of slurry recycling on (a) ILD removal rate and (b) coefficient of friction.
Figure 7: Correlation between ILD removal rate and COF.

Figure 6 shows the effect of slurry recycling on ILD removal rate and COF. The results indicated a nearly 40% drop in removal rate as a result of recycling the fumed silica five times. The relative reduction in the COF value was also close to 40%, as shown in Figure 7, indicating a near-perfect correlation between removal rate and COF. Both ILD and COF tests compared fresh unfiltered slurry to filtered slurry that had been recycled five times. After being recycled five times, slurry is unusable without filtering. These results are consistent with a previously reported near-linear relationship between removal rate and COF.4

Figure 8: ILD removal rate as a function of slurry pH. The circles are from this study, and the squares are from the study documented in reference 9.

The drop in removal rate presented in Figure 6 is too large to account for the decrease in pH represented by the square plots shown in Figure 8, which are data derived from another study.9 However, the decrease in both removal rate and COF is quite possibly a result of the observed increase in aggregate particle size illustrated in Figure 9 (the smaller contact area between the slurry's abrasive particles and the wafer surface is associated with the larger-size abrasives).10 The reason for the increase in mean aggregate particle size with repeated slurry recycling is likely a result of the increase in trace-metal levels shown in Figure 4. It is well known that the presence of metal contamination in silica-based alkaline slurries can cause adjacent silica particles to aggregate through dehydration reactions between the hydroxyl groups on the surfaces of the silica particles and metal hydroxides, as depicted in Figure 10.

Figure 9: ILD removal rate as a function of mean aggregate particle size.

Although only about 15% of the slurry introduced onto the surface of the pad actually enters the pad-wafer interface, an analysis of the raw shear force data shown in Figure 11 indicates that the stick-slip properties of the slurry change dramatically as the slurry is recycled. This phenomenon is evident in the shear force data over time for fresh as well as recycled slurries (the top spectra in Figure 11), where the shear force associated with the fresh slurry has significantly higher variation than its recycled counterparts. This observation supports the postulation that an increase in aggregate particle size and the subsequent decrease in the contact area between the slurry's abrasive particles and the wafer surface reduce the stick-slip between the two bodies. The results of integrating the shear force spectra in the frequency domain (the bottom spectra in Figure 11) are presented in Figure 12. These results clearly indicate that the extent of the stick-slip associated with recycled slurries is 2–4 times less than that associated with fresh slurry, which reduces their effectiveness in ILD removal applications.

Figure 10: Possible mechanism accounting for aggregate particle-size growth in metal-contaminated silica-based alkaline slurries.

Conclusion

ILD CMP polishing experiments were performed to analytically and functionally characterize recycled fumed silica slurries after multiple CMP processes. The effects of multiple polishes on viscosity, specific gravity, and pH were negligible, but mean aggregate size, trace metals, and large-particle counts were notably affected. Utilizing depth filtration to regenerate the used slurry affected LPCs and possibly particle mean aggregate size. Slurry filtration reduced the large particle counts of the used slurries by more than a factor of 100. The removal rate decreased nearly 40% after the slurry was recycled five times, while filtration had only a minor impact on that decrease. Interestingly, COF data showed a nearly perfect correlation to removal rate, indicating that repeated slurry recycling and filtration causes COF to decrease significantly.

Figure 12: ILD removal rate as a function of gamma for fresh slurry, slurry recycled once, slurry recycled three times, and slurry recycled five times.

It is likely that the increase in mean aggregate particle size, which lowers the contact area between the abrasive particles and the wafer, had some impact on removal rate results. This postulation was further supported by the fact that the extent of stick-slip phenomena associated with the process decreased by a factor of 2 to 4 when recycled slurry was used.

Acknowledgments

The authors wish to express their gratitude to John Cheney, Stu Sawai, and Keishi Seki of Fujimi for their support. The work presented in this article was supported financially by the NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing.

References

1. TFA Bibby et al., "CMP COO Reduction: Slurry Reprocessing," Thin Solid Films 308–309 (1997): 538–542.

2. H-J Kim, D-H Eom, and J-G Park, "Physical and Chemical Characterization of Reused Oxide Chemical Mechanical Planarization Slurry," Japanese Journal of Applied Physics 40, part 1, no. 3a (2001): 1236–1239.

3. H Kodama, "A Reclaim Use of CMP Slurry," in Proceedings of the 29th Symposium on ULSI Ultra Clean Technology (Tokyo: Ultra Clean Society, 1996), 67–73.

4. S Olsen, "Tribological and Removal Rate Characterization of ILD CMP," master's thesis, University of Arizona, 2002.

5. A Philipossian and E Mitchell, "Performing Mean Residence Time Analysis of CMP Processes," MICRO 20, no. 7 (2002): 85–95.

6. D Capitanio et al., "Defect Reduction during Chemical Mechanical Planarization by Incorporation of Slurry Filtration" (paper presented at the Workshop on Contamination in Liquid Chemical Distribution Systems, San Francisco, CA, July 13–15, 1998).

7. E Brigham and H Oren, The Fast Fourier Transform and Its Applications (Inglewood Cliffs, NJ: Prentice-Hall, 1988).

8. F Preston, "The Theory and Design of Plate Glass Polishing Machines," Journal of the Society of Glass Technology 11 (1927), 214–256.

9. M-S Kim, "A Study on CMP Process by Regenerated Oxide Slurry Using Filter Modules," master's thesis, Hangyang University, Korea, 2002.

10. N Brown, C Baker, and R Maney, "Optical Polishing of Metals," in Proceedings of SPIE 306 (Bellingham, WA: SPIE, 1982), 42–51.


Ara Philipossian, PhD, has been the Koshiyama associate professor of planarization in the department of chemical and environmental engineering at the University of Arizona (Tucson) since January 2000. His current areas of research include lubrication and wear, fluid dynamics, consumables, equipment characterization and design, and thermal modeling as related to various aspects of planarization and postplanarization cleaning processes. From 1992 to 2000, Philipossian was a materials technology manager at Intel, where he was responsible for the development and characterization of planarization and postplanarization cleaning consumables, low-k dielectrics, and electroplating chemicals. Philipossian is the author or coauthor of more than 70 journal publications and more than 100 conference papers. He holds 12 patents in the area of semiconductor processing and device fabrication. He received BS, MS, and PhD degrees in chemical engineering from Tufts University in Medford, MA. (Philipossian can be reached at 520/621-6101 or ara@engr.arizona.edu.)

Farhang Shadman, PhD, is professor of chemical engineering with joint appointment in optical sciences at the University of Arizona and is also the director of the NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing. Before joining the University of Arizona in 1979, he was a research engineer at the General Motors Research Laboratory. Shadman specializes in chemical reaction engineering, particularly as it applies to advanced material processing and semiconductor fabrication technology. A fellow of the American Institute of Chemical Engineering, he received a PhD in chemical engineering from the University of California, Berkeley, in 1972. (Shadman can be reached at 520/621-6052 or shadman@erc.arizona.edu.)

Patrick Levy has worked at Pall for the past 15 years, most recently as a project engineer. In this role, he has provided technical support to the CMP industry. He is studying the mechanisms of particle capture and transmission in filtration media. Previously, Levy worked as a visiting research scientist at the University of Arizona, where his research centered on the characterization of fresh, spent, and reprocessed fumed silica slurries and filtration and separation technology. He received a BS in physics from the State University of New York at Albany in 1988. (Levy can be reached at 516/801-9279 or patrick_levy@pall.com.)

Saied Tousi, PhD, is a senior vp of Pall and global director of the company's scientific and laboratory services (SLS) department. He began his career with the company in 1986 as a staff scientist and has held many positions in the SLS department. Tousi helped pioneer Pall's global technical support structure, which emphasizes regional laboratories near the company's customer base. In this role, he directs scientific and technical support efforts and leads the expansion of laboratories into new areas. Additionally, he has responsibility for the quality assurance and regulatory affairs department. Tousi performed early work in aerosol monitoring for gas applications in the semiconductor industry and has been widely published, presenting papers at conferences and committees. He received BS, MS, and PhD degrees in chemical engineering from the University of Tulsa in Tulsa, OK. (Tousi can be reached at 516/801-9407 or saied_tousi@pall.com.)

Barry Gotlinsky, PhD, is vp of Pall's process technologies scientific and laboratory services group, where he is responsible for global applications support of filtration and purification technologies. He has been involved in the semiconductor industry for more than 20 years, has published numerous papers, and has given many presentations. He received a PhD in chemistry from the City University of New York. (Gotlinsky can be reached at 516/801-9260 or barry_gotlinsky @pall.com.)

W. Scott Rader, PhD, is the R&D manager at Fujimi. For the past seven years, he has been involved in the development and manufacturing of polishing slurries used in high-technology applications. Before joining the company, he worked in the R&D group at O.I. Analytical, where he focused on flow injection and continuous-flow analysis. Rader holds five patents in the area of polishing formulations. He received a BS from George Fox University in Newberg, OR, and a PhD in inorganic chemistry from the University of Nevada in Reno. (Rader can be reached at 503/972-9440 or scottr@fujimico.com.)

Paul Lefevre has been CMP business development manager at Fujimi since 2001. Before that, he worked at IBM for 10 years and at International Sematech as an IBM assignee for 2 years. A member of the Materials Research Society and the Electrochemical Society, he has contributed regularly to technical conferences on copper CMP. In 1990, he received an MS in industrial engineering from the Ecole Nationale Supérieure d'Arts et Métiers in Paris. (Lefevre can be reached at 503/579-9479 or plefevre@fujimico.com.)

Isamu Koshiyama is the former chairman and CEO of Fujimi. He joined the company in 1964 as an R&D manager and was general manager of the R&D department between 1970 and 1980. Between 1979 and 1991, he was the president of Fujimi Abrasive Sales, and between 1988 and 2003, he was chairman and CEO of Fujimi. Koshiyama received numerous prestigious awards, including an award from the minister of the national science and technology board and an R&D award from Japan Industrial Paper. He received an MS from Meiji University in 1962 and a BS from Hosei University in 1960. In 2005, he submitted a doctoral dissertation to Chubu University in Kasugai City, Japan, and is awaiting receipt of his doctoral degree. (Koshiyama can be reached at +81 52 5038181 or isamu136@pastel.ocn.ne.jp).


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