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Particle Adhesion Studies Relevant to Chemical Mechanical Polishing† Zhenyu Lu,‡ Niels P. Ryde,§ S. V. Babu, and Egon Matijevic´* Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5814 Received March 8, 2005. In Final Form: June 22, 2005 This study describes particle adhesion experiments carried out to elucidate interactions between particles in slurries used for polishing of wafers and disks. For this purpose the packed column technique was employed, which simulated chemical mechanical polishing of copper with silica and alumina, as well as of silicic oxide with ceria. The model systems consisted of uniform copper and glass beads as collectors, representing the wafers, and colloidal dispersions of silica, alumia, and silica coated with nanosize ceria, all of well-defined properties that are used as abrasives. It was shown that a strong correlation exists between deposition and detachment results of the adhesion studies and the polish rates measured using actual substrates with the same or similar slurries.
Introduction The current generation of advanced semiconductor devices contains a billion or more active and passive electrical elements. They need to be interconnected before they become operational. Because of their immense numbers, only multilevel metallization structures can provide all the necessary wiring. Nanoscale planarization of metal (Cu, Ta/TaN, etc.) and of dielectric films (SiO2, Si3N4, etc.), that act as conductors and insulators, respectively, is of great technical importance for the fabrication of such multilevel metallization wiring structures.1-4 During the planarization process, the wafers containing the films are pressed face down and rotated on a soft porous polyurethane pad, mounted on a rotating table. The removal of excess metal or dielectric material and planarization of the remainder is achieved by a synergistic combination of chemical and mechanical forces (hence the name chemical mechanical planarization or CMP for this process), using slurries containing different chemical reagents and abrasives. The latter are colloidal dispersions which flow between the pad and the wafer.3,5 Depending on the film being planarized, the dispersion may consist of several different chemicals. For example, slurries used for planarizing Cu films contain H2O2 as an oxidizer, an amino acid (such as glycine) acting as a chelating agent, an inhibitor to passivate the film and control the dissolution, a surfactant, pH controlling agents, etc., as well as typically 50-200 nm sized abrasives.1-3 * Corresponding author. Telephone: 315-268-2392. Fax: 315268-6656. † Part of the Bob Rowell Festschrift special issue. ‡ Present address: Micron Technology, Inc., Boise, ID 83707. § Present address: Elan NanoSystems, King of Prussia, PA 19406. (1) Chemical-Mechanical Polishing 2001: Advances and Future Challenges; Babu, S. V., Cadien, K. C., Yano, H., Eds.; Materials Research Society: Warrendale, PA, 2001. (2) Chemical-Mechanical Polishing 2002; Babu, S. V.; Singh, R.; Hayasaka, N.; Oliver, M. Materials Research Society: Warrendale, PA, 2002. (3) Zanye, A.; Kumar, A.; Sikder, A. K. Mater. Sci. Eng. 2004, R45, 89. (4) Advances in chemical-mechanical planarization. Singh, R. K.; Bajaj, R. MRS Bull. 2002, 27. (5) Lu, Z.; Lee, S.-H.; Babu, S. V.; Matijevic´, E. J. Colloid Interface Sci. 2003, 55, 261.
While silica and alumina particles are commonly used for polishing copper, silica and ceria are used to polish SiO2 films. In a typical planarization operation, the metal or dielectric film is modified and/or softened by the chemical activity of the suspension. The particles abrade the modified film from only the elevated regions, while the passivating agent present in the slurry protects the lowlying areas from dissolution and erosion. Material removal from the elevated portions continues until the entire film is planar.1-4 There are three essential components in the CMP, all interdependent in some fashion, which are as follows: 1. Solid surfaces (wafers and pads). 2. Dispersed particles (abrasives in slurries). 3. Solutes (various chemical additives). The planarization process is affected by the mutual interactions of these three constituents of the CMP as described earlier. More specifically, these are as follows: 1. Solid/solute interactions, which refer to the effects of different dissolved additives on all solid surfaces, i.e., of wafers, adhesive particles, and pads. 2. Particle/particle interactions, which deal with the stability of slurries or the properties in mixtures of abrasives. 3. Film/particle interactions, which account for adhesion phenomena, i.e., attachment of abrasives to the wafers and the removal of the deposited particles from these surfaces. Elucidation of these interactions requires comprehensive studies using different approaches and techniques, which should, when combined, produce conditions for optimum planarization. This study’s focus is on particle abrasive phenomena that are of fundamental importance in the planarization process. Depending on the system, efficient attachment of slurry particles on wafers and their subsequent removal may be beneficial in terms of enhancing polish rates. On the other hand, such attachments may cause scratches on the surface, which are highly undesirable. Excessive adhesion of particles on pads may diminish their concentration having a detrimental effect on the slurry performance. Finally, the particle detachment process is of great significance, especially in the post-planarization cleanup of wafers and pads from deposited abrasives.
10.1021/la058006v CCC: $30.25 © 2005 American Chemical Society Published on Web 08/10/2005
Particle Adhesion Studies
The problems involved in these structures can be considered in terms of particle deposition in porous media, which has been reported in the literature over the last four decades. Notably, C. R. O’Melia6 pointed out that an important mechanism of deep bed filtration is the colloid interaction between suspended particles and the porous substrate granules.7-9 Early on, particle capture was discussed in terms of column filtration efficiency (or collision efficiency), dealing either with very dilute suspensions10 or early adsorption on granule surfaces which were, in essence, all unoccupied. Numerous subsequent investigators discussed the kinetics of adsorption in terms of rate constants applied to specific adsorption isotherms. Studies by Elimelech et al.19-22 and Adamczyk et al.23-29 have provided a plethora of information regarding kinetics of deposition and detachment from collector surfaces. The introduction of a multilayer kinetic model by the group at Clarkson provided the ability to separate colloid forces due to the same type of particles (particleparticle) from unlike particles (particle-collector) as described originally in ref 30 and compared to other kinetic models in ref 31. Any quantitative evaluation of adhesive phenomena in the CMP process is difficult, if not impossible, to do under operating conditions, since the narrow gap between the pad and the wafer is hard to access. Instead, it is necessary to resort to indirect experimental and theoretical modeling to develop a better understanding of the problems involved, and in order to optimize the polish process and to eliminate the undesirable effects. These goals were achieved in this study by utilizing the so-called packed column technique originally introduced by Clayfield et al.32,33 and later modified and used for different purposes 31,34,35 The experiments were designed to simulate adhesion and (6) O’Melia, C. R. Proc. ASCE 1965, 91, 92. (7) Ruckenstein, E.; Prieve, D. C. J. Chem. Soc., Faraday Trans. 1973, 69, 1522. (8) Spielman, L. A.; Friedlander, S. K. J. Colloid Interface Sci. 1974, 46, 22. (9) Dahneke, B. J. Colloid Interface Sci. 1974, 48, 520. (10) Fitzpatrick, J. A.; Spielman, L. A. J. Colloid Interface Sci. 1973, 43, 350. (11) Elimelech, M.; O’Melia, C. R. Environ. Sci. Technol. 1990, 24, 1528. (12) Elimelech, M.; O’Melia, C. R. Langmuir 1990, 6, 1153. (13) Elimelech, M. J. Colloid Interface Sci. 1991, 146, 337. (14) Elimelech, M. Water. Res. 1992, 26, 1. (15) Song, L.; Elimelech, M. Colloids Surf. A 1993, 73, 49. (16) Song, L.; Elimelech, M. J. Colloid Interface Sci. 1994, 167, 301. (17) Elimelech, M. J. Colloid Interface Sci. 1994, 164, 190. (18) Elimelech, M. Sep. Technol. 1994, 4, 186. (19) Song, L.; Johnson, P. R.; Elimelech, M. Environ. Sci. Technol. 1994, 28, 1164. (20) Song, L.; Elimelech, M. J. Colloid Interface Sci. 1995, 173, 165. (21) Liu, D.; Johnson, P. R.; Elimelech, M. Environ. Sci. Technol. 1995, 29, 2963. (22) Grolimund, D.; Elimelech, M.; Borkovec, M.; Barmettler, K.; Kretzchmar, R.; Sticher, H. Environ. Sci. Techol. 1998, 32, 3562. (23) Adamczyk, Z.; Dabros, T.; Czarnecki, J.; van de Ven, T. G. M. Adv. Colloid Interface Sci. 1983, 19, 183. (24) Adamczyk, Z.; van de Ven, T. G. M. J. Colloid Interface Sci. 1984, 97, 68. (25) Adamczyk, Z.; Petlicki, J. J. Colloid Interface Sci. 1987, 118, 20. (26) Warszynski, P.; Czarnecki, J. J. Colloid Interface Sci. 1989, 128, 127. (27) Adamczyk, Z. Colloids Surf. 1989, 39, 1. (28) Adamczyk, Z.; Zembala, M.; Siwek, B.; Warszynski, P. J. Colloid Interface Sci. 1990, 140, 123. (29) Adamczyk, Z.; Siwek, B.; Zembala, M.; Belouchek, P. Adv. Colloid Interface Sci. 1994, 48, 151. (30) Privman, V.; Frisch, H. L.; Ryde, N.; Matijevic´, E. J. Chem. Soc., Faraday Trans. 1 1991, 87, 1371. (31) Ryde, N. P.; Matijevic´, E. Colloids Surf. 2000, 165, 59. (32) Clayfield, E. J.; Lumb, E. C. Discuss. Faraday Soc. 1966, 42, 285. (33) Clayfield, E. J.; Smith, A. C. Envision Sci. Technol. 1970, 4, 413. (34) Ryde, N.; Kallay, N.; Matijevic´, E. J. Chem. Soc., Faraday Trans. 1 1991, 87, 1377.
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detachment conditions in several different CMP systems, which were then compared with the observed polish rates. The procedure consists of passing a dispersion of uniform particles, used as abrasives, through a column packed with collector beads representative of materials to be polished. When the latter are much larger (>50 times) than the particles, no removal will take place due to particle entrapment. Indeed, any particle adhesion will be governed by the colloidal interactions with the beads. The result of such experiments can be theoretically interpreted based on the models of interactions of unlike particles.30,36-38 It is shown that the adhesion effects identified with this technique correlate well with the actual polish results for the systems simulated in this study. Theoretical Section The most accessible experimental quantity in a column adhesion experiment is the concentration of suspended particles in the effluent, which is monitored as a function of time. This information is used to produce the so-called column breakthrough curve, which display the change in the effluent concentration (C) as a function of time. Theoretically computed column breakthrough curves may be used in comparison with experimental data for the purpose of obtaining colloidal interaction parameters for studied systems. A multilayer deposition theory is used to model the theoretically calculated curves. The equations shown below are derived elsewhere30,39,40 and only the ones necessary to perform the numerical analysis are given here. The multilayer adsorption model can be characterized by three phenomenological parameters: (i) a mass transfer coefficient for deposition into a mononlayer (Kn)1), (ii) a mass transfer coefficient for deposition into subsequent layers (Kn>1), and (iii) an excluded area per adsorbed particle, a, which is related to the maximum surface concentration of particles in the first layer by a ) 1/Γmax. For convenience, the three parameters are normalized to produce the dimensionless quantities R, β, and γ by
R)
Kn)1 Kth
(1)
β)
Kn>1 Kth
(2)
γ)
a πrp2
(3)
The deposition efficiencies R and β are related to the colloid stability of the collector-particle interaction (i.e., to heterocoagulation) and the particle-particle interaction (i.e., homocoagulation). Specifically, they can be related to the stability ratios W-1. The normalization factor Kth represents a calculated mass transfer coefficient for a bed of packed spheres,31 which implicitly contains the convective diffusion properties in the suspension. The third parameter, γ, is normalized against the projected area of a suspended particle, πrp2. (35) Kallay, N.; Barouch, E.; Matijevic´, E. Adv. Colloid Interface Sci. 1987, 27, 1. (36) Kallay, N.; Matijevic´, E. J. Colloid Interface Sci. 1981, 83, 289. (37) Kallay, N.; Nelligan, J. D.; Matijevic´, E. J. Chem. Soc., Faraday Trans. 1 1983, 79, 65. (38) Ryde, N.; Kihira, H.; Matijevic´, E. J. Colloid Interface Sci. 1992, 151, 421. (39) Ryde. N.; Matijevic´, E. J Colloid Interface Sci. 1995, 169, 468. (40) Pfeffer, R.; Happel, J. AlChE J. 1961, 10 605.
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The column breakthrough curve can be calculated from
(R - β)[1 - exp(-RaKthC0G)] + RβaKthC0G C ) (4) C0 (R - β)[1 - exp(-RaKthC0τ)] + RβaKthC0τ where as stated before, C is the particle number concentration in the effluent stream and C0 the initial particle concentration. The quantity G incorporates the mass balance between the suspended and adhered particles in the column and can be calculated by numerical integration of the equation
(β - R) ∂G ) -βfKthG [exp(-RaKthC0G) - 1] ∂x RaC0
(5)
Figure 1. Left: illustration of silica particle covered with nanosized ceria attached to a collector bead. Right: the scanning electron micrograph of such particles used in this study.42
Equations 4 and 5 contain the column breakthrough variables x and τ, which are defined as
τ ) t - z/u
(6)
x ) z/u
(7)
The time, t, is counted from the moment the first particles of the suspension enter the packed bed; z is the coordinate along the column cylinder, whereby the top corresponds to z ) 0 and the bottom to z ) z0. The quantity u is the superficial velocity of the fluid, defined as
u)
v Sφ
(8)
in which v denotes the volumetric flow rate, S is the column cross-section area, and φ the column bed porosity. The parameter f is the ratio of bead surface area to void volume in the column
Figure 2. Schematic drawing of the packed column.
Materials. Monodispersed silica spheres of 100-200 nm in diameter, respectively, were supplied by the Nissan Co. A slurry of calcined alumina, having a mean diameter of 200 nm and a
density of 3.7 g cm-3, was provided by Ferro Corp. Silica particles of 400 nm were prepared as described5 and were coated with ceria particles of 20 nm.42 A scanning electron micrograph and a schematic presentation of such particles is given in Figure 1. Spherical copper beads (99% purity) of 80 µm mean diameter were purchased from Goodfellow Co., while spherical glass beads of 60 µm were provided by La Pine Co. Packed Column Technique. The packed column used in this study is shown schematically in Figure 2, had an inside diameter of 1 cm. The height of the bed, consisting either of copper or glass collector beads, was 1 cm in all experiments. The size of suspended particles used in this study was small enough to avoid any retention by entrapment. The flow rate of the dispersion was kept constant at 5.1 cm3min-1 in experiments with Cu beads and at 1.6 cm3min-1 with glass beads and was controlled by peristaltic pumps all at 25 °C. The particle content in the dispersions was evaluated by light scattering intensity, which was shown to be linearly dependent on their concentration for all investigated systems. Experimentally, particle retention was determined by the difference of C0 - C and then integrated at given time intervals. The resulting breakthrough curves can be used to distinguish between monoand multilayer adhesion cases. The detachment process was followed by loading the collector beads with a known amount of particles and then rinsing the column with solutions of different compositions (pH, ionic strength). The amount of released particles at different time was determined by their concentration in the effluent. Chemical Mechanical Polishing. Polishing was carried out with 1.25-in. copper disks (99.999% pure), using IC-1400 K Groove pads, made of polyurethane (Rodel Inc.). The polishing experiments were performed on a benchtop Struers DAP-V polisher. The rotation speed of the pad was set at 90 rpm and the disk holder was kept stationary, at the applied pressure of 6.3 psi and the slurry flow rate of 60 cm3min-1. During the polishing, the slurry was agitated continuously with a magnetic stirrer to maintain a good dispersion. The polish rates were determined
(41) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Wetterling, W. T. Numerical Recipes; Cambridge University Press: New York, 1986.
(42) Lu, Z.; Lee, S.-H.; Gorantla, R. K.; Babu, S. V.; Matijevic´, E. J. Mater. Res. 2003, 16, 2323.
f)
3(1 - φ) φ‚rb
(9)
in which rb is the radius of the beads. The mass transfer coefficient, Kth, is calculated from40
Kth ) 0.624
() D rb
2/3
(Asu)1/3
(10)
where D is the particle diffusion coefficient, which is evaluated from the Stokes-Einstein equation, and As is a geometric factor defined by
As )
2[1 - (1 - φ)5/3] [2 - 3(1 - φ)1/3 + 3(1 - φ)5/3 - 2(1 - φ)2]
(11)
Given the experimental details, eqs 6-11 can be calculated explicitly. The parameters, R, β, and γ, can be obtained by fitting eq 4 to the experimental data using a nonlinear fitting scheme.41 In the process, the quantity G is computed numerically from eq 5 for each iteration step. The concentration of detached particles can be derived from the same set of basic equations.39 The expression reads
C)
fkrz0Γmax exp[-krτ] u
(12)
Experimental Section
Particle Adhesion Studies
Langmuir, Vol. 21, No. 22, 2005 9869 Table 2. ζ Potentials of Copper and Copper Oxide as a Function of the pHa copper
Figure 3. Breakthrough curves at 25 °C for aqueous dispersions of 200 nm silica particles, flowing through a bed packed with 80 µm copper beads, at different pH values. The influent concentration, C0, was 0.01 wt % or 1.3 × 1016 particles per m3 of slurry. Symbols represent experimental data points, while solid lines are fitted using the multilayer model with parameters listed in Table 1.
copper oxide
pH
ζ/mV
pH
ζ/mV
3.4 5.9 6.8 9.2 11.2
-5 -10 -34 -38 -32
3.6 4.8 5.8 6.9 8.9 10.2 11.5
23 17 23 -10 -16 -17 -37
a The measurements on copper were carried out using 150 nm particles suspended in a 2.0 × 10-3 mL dm-3 NaClO4. Data for copper oxide were obtained with 600 nm particles suspended in a 1 × 10-2 NaClO4 solution. In both cases, the pH was adjusted using HCIO4 or KOH.
Table 1. Multilayer Deposition Parameters and Calculated Inverse Stability Ratios for the Experimental Data Shown in Figure 3 pH
R
β
γ
W-1
2 3 4 5 6
0.054 0.047 0.09 0.10 0.08
0.0010 0.0005 0.002 0.002 0.0002
86 182 106 104 119
1.00 1.00 0.53 0.00 0.00
by the mean weight loss of the disk, using a Mettler balance AB54 with an accuracy of 10-4 g. Electrophoresis. Electrokinetic experiments were carried out in a Brookhaven Zeta PALS instrument, which yields ζ potentials from the electrophoretic mobilities, as calculated from the Smoluchowski equation.
Figure 4. Same system as in Figure 3, modified by the addition of H2O2 in different concentrations, all at pH 4. The lines are fitted with parameters listed in Table 3. Table 3. Multilayer Deposition Parameters for the Data Shown in Figure 4
Results and Discussion
% H2O2
R
β
γ
Deposition of Silica Particles on Copper Beads. Colloidal silica is commonly used as the abrasive in CMP, especially in polishing of copper. To relate the phanarization results to particle adhesion, experiments were carried out with aqueous silica dispersions (200 nm in diameter) passing through a column of copper beads (80 µm) at several pH values and at different concentrations of H2O2. Figure 3 shows that very few silica particles adhere to the copper beads over the studied pH range. The symbols represent experimental data, while full lines are fitted using the described multilayer model with parameters listed in Table 1. As expected, the negligible values for β clearly indicate that no multilayers are formed. Electrokinetic measurements with copper beads show negative ζ potentials (Table 2), which should facilitate attachment over the pH range of 2-4. However, the listed ζ-potentials did not reflect electric conduction of copper particles, which would produce higher surface potentials, resulting in sufficiently strong electrostatic repulsion between silica particles and copper beads to prevent any significant uptake. The addition of H2O2 at pH ) 4 resulted in increased deposition of silica particles on copper beads, most likely due to the formation of an oxide layer on the copper (Figure 4). The corresponding multilayer deposition parameters in Table 3 show a larger value for R than the one in the absence of H2O2. Again, the negligible values of β indicate that no multilayers are formed, which is corroborated by the high values of γ, also signifying that the attached particles are widely separated. The adhesion data for the same system in the presence of 0.25% H2O2 as a function of the pH (Figure 5, Table 4)
0.00 0.25 0.50 1.00 3.00 5.00 7.00
0.20 0.63 0.52 0.55 0.61 0.42 0.40
0.000 0.055 0.017 0.003 0.005 0.003 0.002
175 10 9 13 22 31 34
indicate strong multilayer deposition at pH ) 2, while at other pH values only monolayers are formed. The attachment efficiency is readily understood if one considers the ζ-potentials (Table 2) of the oxide layer on copper produced by H2O2, which is responsible for the electrostatic attraction between interacting surfaces. In the system at pH 2 multilayer deposition is then caused by high ionic strength, due to the concentration of the added acid. This effect was documented earlier on several systems.43,44
Figure 5. Same system as in Figure 3 containing 0.25 wt % of H2O2.at different pH values. The lines are fitted with parameters listed in Table 4.
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Table 4. Multilayer Deposition Parameters of the Data Shown in Figure 5 pH
R
β
γ
2 4 6 8 10
2.0 1.6 1.1 0.94 0.38
1.0 0.10 0.013 0.0003 0.0002
2 5 7 23 116
Experimentally determined polish rates for silica on copper as a function of added H2O2 5,45 are plotted in Figure 6 (0). Superimposed in the same diagram are the highest numbers of adsorbed particles obtained from Γmax ) 1/(γπrp2), as determined in the packed column, all as a function of the H2O2 concentration. Analogous data for these effects of the pH are given in Figure 7. A strong correlation between polish rates and the number of attached particles is clearly indicated by these results. Deposition of Alumina Particles on Copper Beads. Alumina particles are also commonly used as abrasive in the CMP of metals, which is exemplified in this study using copper, for which polish data are available. To evaluate the pH effect on adhesion, aqueous dispersions of calcined alumina (200 nm in diameter), containing different amounts of acid or base, were passed through a column loaded with copper beads, and the results are displayed in Figure 8. The electrokinetic potentials reported for copper (Table 2) and the fact that the isoelectic point of this aluminum oxide is at pH ∼ 8,46,47 readily explain the results, because at lower pH values the
Figure 8. Breakthrough curves at 25 °C for aqueous dispersions of 200 nm alumina particles flowing through a bed packed with 80 µm copper beads, at different pH values. The lines are fitted using parameters listed in Table 5.
Figure 9. Plots of the numbers of deposited alumina particles on copper beads (shaded), and the polish rates of copper disks with these particles at different pH values (0).
Figure 6. Plots of the maximum numbers of deposited silica particles on copper beads (shaded) and the polish rates of copper disks with fumed silica (Aerosil 90) (0) at different concentrations of H2O2.5
Figure 7. Plots of the maximum numbers of deposited silica particles on copper beads (shaded) and of polish rates of copper disks with colloidal silica (50 nm in diameter) (0), containing 0.25 wt % of H2O2 at different pH values.
particles and beads bear charges of opposite signs, causing attachment due to electrostatic attraction. The exception at pH 2 is due to higher ionic strength of the dispersion. In basic solutions, both interacting surfaces of the same sign of charge are repulsive and no deposition takes place. The maximum in the β value at pH ∼ 8 is near the isoelectric point for alumina, which would cause multilayer formation (see Table 5). As in previous cases the polish rates coincide with the deposition data, except at the lowest pH value, which again may be due to the aggregation of the adhered particles (see Figure 9). Another set of experiments was carried out to evaluate the detachment of adhered particles by washing the loaded column with a basic solution. Specifically, the alumina particles were deposited on copper beads at four different pH values for 15 min, followed by washing with DI water for 1 min, and then their removal was evaluated by passing through the bed an aqueous solution of pH 9.2. The release of the particles was significantly affected by the pH at which the particles were attached to the beds (Figure 10). Thus, alumina deposited at pH 2 and 4 was present on copper beads in monolayers due to electrostatic attraction. Rinsing the column at pH 9.2 caused a change of sign in the charge on alumna particles to negative, and conse(43) Kuo, R. J.; Matijevic´, E. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2014. (44) Kuo, R. J.; Matijevic´, E. J Colloid Interface Sci. 1980, 78, 407. (45) Li, Y.; Hariharaputhiran, M.; Babu, S. V. J Mater. Res. 2001, 16, 1066. (46) Matijevic´, E. Interfacial Electrokinet. Electrophoresis 2002, 199. (47) Jindal, A.; Hegde, S.; Babu, S. V. J. Electrochem. Soc. 2003, 150, 5.
Particle Adhesion Studies
Figure 10. Fraction of released alumina particles in the same system shown in Figure 9 as a function of rinsing times with an aqueous solution of pH 9.2. The pH in the figure refers to values used in particle deposition.
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Figure 12. Breakthrough curves at 25 °C. for aqueous dispersions of colloidal silica coated with nanosized ceria (shown in Figure 1) containing1 1.5 × 1015 particles per m3, flowing through a bed packed with glass beads of 60 µm at pH 2 (0), 4 (O), and 6 (4), respectively. The solid lines are fitted using parameters listed in Table 6. Table 5. Multilayer Deposition Parameters and Calculated Inverse Stability Ratios for Experimental Data Shown in Figure 8 pH
R
β
γ
2 4 6 8 10 12
1.1 0.9 1.4 0.23 0.05 0.12
2.2 2.2 0.6 30 0.04 0.02
2.6 7.8 1.8 49 500 110
Table 6. Multilayer Deposition Parameters and Inverse Stability Ratios for Data Shown in Figure 12 coated Si/Ce
Figure 11. Plots of numbers of remaining alumina particles on copper disk after 10 min of washing the loaded column with an aqueous solution of pH 9.2 (0), and polish rates of copper disks carried out with the same alumina abrasives deposited at different pHs values (shaded).
quently their release. In contrast, in systems in which the alumna uptake was carried out in pH 6 and 8, the particle release was insignificant. Both parameters R and β have high values, suggesting multilayer formation and strong bonds with the metal surface through several contact points. It is noteworthy that there is also a good correlation between polish rates and the particles retention of particles (Figure 11). These results further confirm that the attachment of abrasive to disks (or wafers) is a critical condition controlling polish rates. Deposition and Detachment of Silica Particles Coated with Ceria in Contact with Glass Beads. It was demonstrated previously48 that silica particles coated with nanosize ceria (illustrated in Figure 1) exhibit a high polish rate on silicon oxide wafers. For this reason, column experiments using such coated particles were carried out on a model system, consisting of glass beads as collectors at pH 2, 4, and 6. Figure 12 displays the experimental column breakthrough data, together with the fitted curves computed with parameters listed in Table 6. These plots indicate monolayer deposition at pH 2 and 4 and multilayer formation at pH ) 6. It may appear that the latter is the result of a low surface potential, as the pH is close to the isoelectric point. A more intricate analysis, based on the inverse stability ratios W-1, reveals that surface heterogeneity of particles plays a key role. The inverse stability ratios were calculated using the expressions of Hogg, (48) Lee, S.-H.; Lu, Z.; Babu, S. V.; Matijevic´, E. J. Mater. Res. 2002, 17, 2744.
Ce alone
pH
R
β
γ
WHet-1
WHom-1
WHet-1
WHom-1
2 4 6
0.19 0.25 0.42
0.01 0.0 1.5
13 18 1.8
1 1 0
0 0 0.012
1 1 1
0 0 0.0011
Healy, and Fuerstenau,49 with the values of the surface potentials for silica particles coated with ceria of 43, 40, and -8 mV at pH 2, 4, and 6, respectively48 and that of the glass beads of -85 mV for all pH values.31 The inverse heterostability ratio at pH ) 6 shown in Table 6 suggests that no particles should adhere to the glass beads, whereas the value of R clearly shows the opposite. This discrepancy can be readily understood by considering the heterogeneous nature of the surfaces of coated particles. According to Figure 1, ceria should be in contact with the glass bead; consequently, it is not surprising that a calculation of W-1, based on the average surface potential, as measured by the electrophoresis of composite particles, would yield an incorrect result. Should the potential characteristic of pure ceria be used,31 one would anticipate adhesion at this pH. The agreement between the multilayer deposition efficiency, β and W -1, is better when the surface potential of the composite particle is employed, although the magnitude of β suggests an even stronger attraction. This result is likely due to the segregration of the charge on the composite particle, since oppositely charged surface groups align at a close distance of separation, producing net electrostatic attraction.50 To study the detachment effects in the same system, the column was first loaded with particles at pH 4, after which an aqueous solution of pH ) 10 was flown through the column, causing adsorbed particles to spontaneously detach. In separate experiments, the release at three (49) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. J. Chem. Soc., Faraday Trans. 1 1966, 62, 1638. (50) Kihira, H.; Matijevic´, E. Adv. Colloid Interface Sci. 1992, 42, 1.
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Figure 13. Effects of the flow rate of the rinsing solution of pH 10 on the attachment of silica particles on coated with ceria attached to glass beads.
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Figure 15. Detachment rate constants at different times for the different flow rates for the system shown in Figure 13.
Figure 14. (C/C0) × u vs time curves for the same data shown in Figure 13.
different flow rates was determined as displayed in Figure 13. The removal of particles is not a function of C0, but the normalization is done for convenience. Equation 12 requires for the concentration of detached particles to be inversely related to the flow rate. The flow dependence can be factored out by plotting C × u, or more convenient (C/C0) × u, as shown in Figure 14. The detachment rate constant can then be evaluated from the negative slopes of such plots. The curvature of the latter indicates that multiple detachment rates are needed to characterize the system. The slopes, therefore, need to be determined from the tangents at various times. The results shown in Figure 15 corroborate the notion that the detachment rate constant is independent of the flow rate. (Note that the semilog slopes do not depend on whether C/C0sor (C/C0) × usvs time is considered). The detachment rate constant can itself be related to the energy barrier that needs to be overcome in order for release to occur,35 and it is influenced by charge heterogeneity in both the ceria-coated silica particles and glass collector beads. In post cleanup of CMP, it is important to know how easy it is to remove abrasive particles from the polished surface. The result shown in Figure 13 can readily be converted to percent removed as shown in Figure 16, which shows that up to 90% of deposited particles can be released under these conditions. The fact that the detachment rate depends inversely on the flow rate allows one to calculate the removal rate for different flow rates for a given system.
Figure 16. Fraction of released particles in the same system as in Figure 13, as a function of elution time with a rinse solution of pH 10 at these different flow rates.
Conclusions The column adhesion method allows one to evaluate the colloidal interactions relevant to the chemical mechanical polishing process. The deposition kinetics of the three studied systems was found to be consistent with the behavior expected from electrostatic interactions between the collectors in the particles. The reported deposition results clearly show that the polish rates are strongly dependent on the ability of the substrate to attract abrasive particles. Obviously, the latter need to be at close distance from the wafer or the disk surface to achieve efficient planarzation. On the other hand, the adhered particles should stick to the surface by physical rather than chemical forces in order to allow for their removal at the post!CMP stage, as also documented in this study. Acknowledgment. This work was supported by Intel Corp. through a contract from the Semiconductor Research Corp. (SRC). LA058006V
