Effects of Slurry Particles on Silicon Dioxide CMP

Transcription

1 G /2004/151 8 /G512/11/$7.00 The Electrochemical Society, Inc. Effects of Slurry Particles on Silicon Dioxide CMP Wonseop Choi, Jeremiah Abiade, Seung-Mahn Lee,* and Rajiv K. Singh**,z Department of Materials Science and Engineering and Particle Engineering Research Center, University of Florida, Gainesville, Florida , USA The performance of chemical mechanical polishing CMP is determined by the dynamic contact behavior sliding or rolling of the slurry particles during polishing. The dynamic contact behavior of slurry particles is dependent on the shape, size, and concentration of particles. In this paper, the dynamic contact characteristics and their effects on the CMP performance were investigated through in situ friction force measurements, atomic force microscopy images, and polishing experiments. For slidingdominated contact, the friction force remains unchanged within the specified testing period yielding scratch-type surface topography. For rolling-dominated contact, the friction force decreases with time under a constant solids loading, producing shot-blast surface images. The polishing rate increases with solids loading of sliding particles and decreases with solids loading of rolling particles. Further optimization of the CMP process is possible by tailoring the dynamic contact of abrasive particles The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted October 28, 2003; revised manuscript received January 12, Available electronically July 20, As the minimum feature size of microelectronic devices reaches the 100 nm technology node, chemical mechanical polishing CMP has become the best choice for global and local planarization. 1-4 Global planarization which is essential to produce a multilevel integrated circuit IC device, is achieved by reducing the topography variation at the wafer scale. Without CMP, it would be impossible to fabricate complex, dense, and miniaturized IC devices. During the past decades, CMP has significantly advanced both in the development of sophisticated processing tools and in the formulation of slurries. Despite these achievements, CMP remains one of the least understood areas in microelectronic device fabrication processing. Even though the characteristics of slurry particles i.e., shape, size and concentration are critical in determining CMP performance, the effect that the slurry particles have on polishing performance is not clear. For polishing of copper or ferrite, it was suggested that the polishing rate is proportional to particle size and solids loading. 5,6 Cook 7 presented data suggesting that the polishing rate is independent of particle size for glass polishing. Izumitani 8 suggested that the polishing rate decreases with increasing particle size. Our previous work suggests two polishing mechanisms in silica CMP. 9 One is a contact-area-based mechanism by which A C 1/3 1/3 0 1 where A is the contact area, C 0 is the particle concentration, and is the particle diameter. In this model, the polishing rate increases with an increase in particle concentration and a decrease in particle size, which was observed during tungsten CMP. 10 The other is an indentation-volume-based mechanism by which V C 1/3 0 4/3 2 * Electrochemical Society Student Member. ** Electrochemical Society Active Member. z rsing@mse.ufl.edu where V is the indentation volume. According to this indentationvolume-based mechanism, the polishing rate increases with decreasing particle concentration and increasing particle size. This mechanism was observed via silica polishing experiments. 9 On the micro- and nanoscales, the polishing rate is significantly dependent on the dynamic contact of particles between the pad and the wafer. To examine the dynamic interfacial contact of particles during CMP, friction force measurement techniques and surface finish analysis are considered the most applicable methods because of the possibility of direct observation of contact phenomena. 11 Abrahamson et al. 12 compared the material removal mechanism for loose abrasive particles i.e., two-body contact and for bonded abrasive particles i.e., three-body contact. As seen in Fig. 1, it has been reported that three-body contact produced a shot-blast appearance, whereas two-body contact yielded continuous scratches. Macmillan et al. 13 studied the sliding frictional behavior under various chemical environments using pin-on-disk friction force measurements. Scratches by particles undergoing sliding motion were observed on the MgO surface. Previous results showed that the electrostatic repulsion force between particle and wafer and wafer surface roughness play important roles in determining the friction force during polishing. 14,15 This paper presents further validation of the polishing mechanism using sol-gel silica slurries. The effects of particle size and solids loading on friction force and surface finish were studied to delineate the dynamic contact of particles during polishing. Experimental CMP slurries were formulated by dispersing sol-gel silica particles from Geltech Corporation in deionized DI water. The average particle diameters provided by Geltech were 0.2, 0.5, 1.0, and 1.5 m. The solids loadings were varied from 0.5 to 30 wt %. Slurries composed of sol-gel silica particles were dispersed using an ultrasonic bath and were stabilized by adjusting ph values to 10.5 with 0.1 M NaOH and 0.1 M HNO 3. The average size and distribution of stabilized sol-gel silica particles were analyzed with a Coulter LS 230 instrument that uses a dynamic light scattering technique. In addition, the shape and size of the particles were determined by scanning electron microscopy SEM analysis. Polishing samples were clipped to in. from an 8 in. p-type silicon wafer on which silica of 2.0 m thick was deposited by plasma-enhanced chemical vapor deposition. IC 1000/Suba IV stacked pads supplied by Rodel Inc. were utilized as CMP pads, and a Grid-Abrade diamond pad conditioner was used to abrade the pad before each test. A Struers RotoPol-31 tabletop polisher was used to polish samples. The following conditions were applied to each run: down pressure of 3.5 psi kpa, polishing time of 30 s, slurry flow rate of 100 ml/min, and rotation speed of 150 rpm 110 cm/s. The polishing tests were repeated three times to ensure reliable results. The polishing rates were determined by measuring oxide film thickness on the samples before and after the polishing experiments, using a J. A. Woollam variable angle spectrometry ellipsometry. Atomic force microscopy AFM with a Digital Instruments Nanoscope III was used to characterize the surface finish of the polished samples. In situ friction force measurements were carried out to investigate the contact behavior at the pad-particles-wafer interface. As seen in Fig. 2, an in situ friction force measurement tool was mounted on the Struers RotoPol-31 tabletop polisher. A Sensotec model 31 load cell was used to measure the friction force, which was connected to a data acquisition system, with a data point recorded every 250 ms. Experimental conditions of in situ friction force measurements were the same as the process conditions of pol-

2 G513 Figure 1. Wear patterns for a fused silica and b 1018 steel using two-body abrasion top and three-body abrasion bottom. 12 ishing experiments. The friction force was measured during 1 min for each run. During the initial 15 s, DI water was utilized as a baseline condition. For the next 45 s, the variation of friction forces due to the presence of slurry particles was measured. Figure 2. Schematic illustration of friction force measurement tool. Results and Discussion Analysis of shape and size of sol-gel silica particles. We directly observed the shape and size of sol-gel silica particles with SEM and statistically detected the size and distribution of the silica particles with light scattering particle size analysis. As shown in Fig. 3, the primary particles are spherical in shape and are similar in size. The average particle sizes measured from SEM micrographs are a 0.22, b 0.59, c 1.13, and d 1.62 m, respectively. These direct observations are statistically confirmed by light scattering particle size analysis. As shown in Fig. 4, particle size distribution curves show single peaks with average particle diameters standard deviation of particle size of 0.20 m ( 0.05 m), 0.55 m ( 0.08 m), 1.09 m ( 0.16 m), and 1.53 m ( 0.22 m), respectively. The average particle size measured by SEM is analogous to that measured by light scattering methods. Both observations agree well with the average particle size that Geltech has provided. The spherical shape and uniform size distribution of these silica particles make it possible to distribute equally the particles in contact with the pad and wafer during CMP.

3 G514 Figure 3. SEM micrograph of silica particles: a 0.2, b 0.5, c 1.0, and d 1.5 m. Figure 4. Size distribution of silica particles measured by dynamic light scattering. Analysis of in situ friction force as a function of particle size and solids loading. In situ monitoring of interfacial interactions between pad, particles, and wafer is essential to analyze the polishing mechanism. The interfacial contact of pad, particles, and wafer can be altered as a function of particle size and solids loading, resulting in a variation of friction force. Therefore, in situ friction force measurements can provide an insight into the polishing mechanisms. Figure 5 shows the friction force as a function of time for various solids loading of 0.2 m silica particles. During the initial 15 s, the friction force was measured with just DI water between the wafer and the pad. Particle-free slurries were utilized as a baseline for comparison with the friction force measured for abrasive-containing slurries. During the following 15 s, the abrasive-containing slurry flowed into the pad-wafer interface, elevating the friction force from the baseline to higher levels. Particles inserted into the interface between pad and wafer form three-body contact pad, particles, and wafer, enhancing the friction force. Finally, average friction forces were saturated with time in the range of s. Particles embedded in a soft pad slide on the wafer surface at a constant speed, causing a constant friction force. While particles are in contact with the wafer, the friction force increases with the increase of solids loading. The increase of solids loading leads to an increase in the number of particles in contact with the wafer, enlarging the total contact area of particles between the wafer and pad. Therefore, the increase in friction force due to an increase in solids loading may be attributed to

4 G515 Figure 5. Friction force as a function of time for various solids loading of 0.2 m silica particles. an increase in contact area between the wafer and particles embedded in the pad for sliding conditions. Figure 6 shows the friction force as a function of time for various solids loadings of 0.5 m sol-gel silica particles. As shown in Fig. 6a, the friction force increased and then remained unchanged with time for low solids loading less than 7 wt %. The friction force also increased with the increase of solids loading. This indicates that particles in contact with the wafer are sliding, and the number of particles in contact with the wafer increases with an increase in solids loading. The observed increase in friction force was in agreement with our previous results, in which the friction force was enhanced when the solids loading was increased. 16 As shown in Fig. 6b, the friction force initially increased and then decreased with time for high solids loading more than 8 wt %. When slurry particles flow into the pad-wafer interface, the fraction of the wafer surface covered with slurry particles increases to the point of saturation, at which point lubrication begins. The observed decrease in friction force in the presence of spherical silica particles indicates the lubricating effect of rolling silica particles. The frictional forces produced by rolling are much lower than those produced by sliding, which is in agreement with theoretical principles concerning rolling. 17 Lubrication due to rolling motion in the presence of a layer of Bucky ball bearings was also reported. 18 When spherical ball bearings were rolled between two flat surfaces, the friction force decreased with time. We also observed Fig. 6b that the friction force decreased with increasing solids loading. This decrease in friction force is in opposition to the frictional behavior shown in Fig. 6a. This demonstrates an alteration in the characteristics of the interfacial dynamic contact due to the change in solids loading and particle size. Figure 7 shows the friction force as a function of time for various solids loading of 1.0 m silica particles. As shown in Fig. 7a, for low solids loading less than 4 wt %, the friction force remained unchanged with time in the presence of slurry particles. Compared to frictional forces produced by 0.5 m silica particles, the transition between the sliding and the rolling behavior decreases from 7 to 4 wt %, which is due to an increase in rolling probability as the particle size increases. 17 Otherwise, when the small silica particles e.g., 0.2 m were used, evidence of rolling behavior was not observed using our in situ friction force measurements. Figure 7b shows the friction force as a function of time with high solids loading more than 5 wt %. The friction force initially increased due to the addition of slurry into the pad-wafer interface and then gradually decreased with time. This frictional behavior is analogous to that produced by 0.5 m silica particles of high solids loading. Friction force measurements at 5 and 8 wt % silica particles showed two Figure 6. Friction force as a function of time for 0.5 m silica particles: a low solids loading less than 7 wt % and b high solids loading more than 8wt%. frictional regions: one is a high frictional force region i.e., sliding region between 20 and 30 s, and the other is the low frictional force region i.e., rolling region between 30 and 60 s. These two regions indicate that a transition occurs from sliding to rolling of particles between the pad and the wafer. The sliding region decreases with increasing solids loading. This suggests that sliding is a main contact motion for low solids loading and that rolling is a dominant contact motion for high solids loading. Figure 8 shows the friction force as a function of time for different solids loading of 1.5 m size silica particles. As seen in Fig. 8a, the friction force remains unchanged with time for the initial stage of slurry flowing, which indicates that particles are undergoing sliding motion for the solids loading less than3wt%. The transition from sliding to rolling occurred at 3 wt % for 1.5 m particles as opposed to 4 wt % for 1.0 m particles. This result indicates that larger particles start rolling motion at lower solids loading conditions. Figure 8b shows the friction force as a function of time for high solids loading more than 4 wt % of 1.5 m silica particles. The friction force obtained at 4 wt % showed the combined frictional behavior of sliding and rolling. For high solids loading more than 5 wt %, the friction force decreased with time, indicating that particles are undergoing predominantly rolling motion. Figure 9 shows the friction force as a function of solids loading for different particle sizes, which was redrawn from the graph of friction force vs. time Fig For sliding conditions, the friction force increased with an increase in solids loading and particle size;

5 G516 Figure 7. Friction force as a function of time for 1.0 m silica particles: a low solids loading less than 4 wt % and b high solids loading more than 5wt%. Figure 8. Friction force as a function of time for 1.5 m silica particles: a low solids loading less than 3 wt % and b high solids loading more than 5wt%. whereas, in rolling conditions, the friction force decreased with an increase in solids loading and particle size. Clearly, the response of friction force to solids loading and particle size is dependent on the dynamic condition of slurry particles. For sliding slurry particles, the effect of solids loading and particle size on friction force can be explained as follows. The increase of solids loading leads to an increase in the number of particles contacting the pad and the wafer. Increasing the amount of particles at the pad-wafer interface results in a more uniform pressure distribution at the wafer surface for constant down pressure. 19,20 The friction force as measured by us has been assumed to arise only from regions of pad-wafer contact and particle on pad-wafer contact. 21 For smaller particles 0.2 m, the increase in solids loading results in an increase in total contact area Eq. 1. This increase in the number of particles available for polishing at the wafer surface results in a more uniform distribution of pressure, which is evidenced by a reduction in surface roughness as solids loading increases see Fig. 10, 14a. However, the pressure distributed on the abrasive particles remains sufficient to ensure that particles remain embedded in the polishing pad. As evidenced by the AFM images Fig. 10, particles embedded in the polishing pad produced scratches on the surface due to sliding motion. In the particle size range m, wherein a transition from sliding to rolling motion occurred, the contact area scales inversely with particle size Eq. 1. The transition from rolling to sliding oc- Figure 9. Friction force as a function of solids loading for different particle sizes.

6 G517 Figure 10. AFM overview images for wafer polished with 0.2 m Geltech silica particles of different solids loading: a 0.5, b 5, c 15, and d 30 wt %. curs at lower solids loading conditions as the particle size increases. As discussed for 0.2 m particles, for constant down pressure, increasing the amount of slurry particles at the pad-wafer interface results in a reduction in the pressure per particle at the wafer surface. Zhao and Shi have produced a theoretical CMP model in which the material removal mechanism is dependent on the existence of a threshold pressure. 22 This threshold pressure marks the onset of either rolling or sliding motion. According to their work, material removal at the wafer surface should be negligible for rolling conditions. Appreciable removal rates are expected when the friction between the pad and particle exceeds that between particle and wafer surface. The surface area of the particles may be utilized to explain the appearance of the sliding-to-rolling transition at lower solids loading conditions as particle size increases. Surface area decreases as particle size increases, resulting in less interaction area between particle and opposing surfaces pad and/or wafer. Viain situ friction force measurements, we were able to give experimental validation of the prediction of threshold pressure during CMP. Effect of particle size and solids loading on surface finishes. The surface finish of polished samples can be considered as the surface topography produced during CMP. The surface topography of polished samples was measured by AFM, in which surface images are taken by scanning the sample relative to the probing tip and measuring the deflection of the cantilever as a function of lateral position at the atomic level. 23 Figure 10 shows overview images of wafers polished with 0.2 m silica particles for different solids loading: a 0.5, b 5, c 15, and d 30 wt %. Randomly distributed scratches have been detected in all the images investigated. As evidenced by the in situ friction force measurements in Fig. 5, material removal was achieved by sliding of slurry particles. Therefore, scratches were produced due to the sliding of slurry particles along the wafer surface. These observed scratches on the surface are also in agreement with those produced by two-body contact, as shown in the top images of Fig This indicates that slurry particles remove material at the wafer surface by sliding while embedded in the pad. It was also observed that the number of scratches produced on the wafer decreased with increasing solids loading. From the difference of surface topography between 0.5 and 30 wt %, it can be easily detected that the increase in solids loading leads to the formation of a smoother surface. Figure 11 shows overview images of wafers polished with 0.5 m silica particles for different solids loading: a 0.5, b 5, c 15, and d 30 wt %. When solids loadings were low, many scratches were produced on the wafer surface. These observed scratches were similar to the images detected on wafers polished with 0.2 m silica particles. However, when solids loadings were high 15 and 30 wt %, shot-blast appearances were found on the wafer surface instead of scratches. These shot-blast images were in accordance with the images produced by three-body contact shown

7 G518 Figure 11. AFM overview images for the wafer polished with 0.5 m Geltech silica particles of different solids loading: a 0.5, b 5, c 15, and d 30 wt %. in the bottom of Fig Similar contact images were observed on surfaces produced by a rolling sphere for three-body contact. 23 In addition, as seen in Fig. 6b, the decrease of friction force due to rolling of slurry particles was observed. This indicates that rolling particles move freely between the pad and the wafer, yielding shotblast surfaces. It is interesting to observe the phenomena at intermediate solids loading conditions i.e., 5wt%. For the intermediate condition of solids loading, scratch images were observed together with a shot-blast appearance. This indicates that material is removed by two different dynamic motions i.e., sliding and rolling during polishing. As seen in Fig. 6b, in situ friction force measurements detected the coexistence of two dynamic contact motions at 8 wt %. Despite the different critical solids loading at which two dynamic contact motions coexist, it is possible to detect the transition between predominantly sliding contact and predominantly rolling contact. Figure 12 shows overview images of a wafer polished with 1.0 m silica particles for different solids loading: a 0.5, b 2, c 15, and d 30 wt %. An overview image of the wafers polished with 0.5 wt % particles shows numerous scratches. As mentioned previously, we assume that scratches are produced due to the sliding motion of particles. Both scratches and shot-blast features appear together on the surface of wafers polished with 2 wt % particles Fig. 12b. Significant plastic deformation is shown along a band m wide. These tracks also exhibit considerable cracking and cratering. These plastic deformations, which were not shown on wafers polished with small particles 0.2 and 0.5 m, may only be produced by large silica particles. Macmillan et al. 13 also showed a similar SEM picture of the track that was produced by a sliding sapphire ball. An overview image of a wafer polished with 15 and 30 wt % particles clearly showed a series of shot-blast regions, which indicates the rolling of particles observed during three-body contact. Figure 13 shows overview images of a wafer polished with 1.5 m silica particles for different solids loading: a 0.5, b 2, c 15, and d 30 wt %. For 0.5 wt %, polished surfaces with numerous scratches are apparent. However, overview images of a wafer polished with 2 wt % particles shows a predominance of shot-blast regions rather than scratches. For 15 wt %, instead of scratches, a mostly shot-blast appearance is observed on the wafer surface. Also, overview images of a wafer polished at 30 wt % show a smooth wafer surface with a shallow shot-blast appearance. Figure 14 shows the root-mean-square rms surface roughness as a function of solids loading for various particle sizes: a 0.2, b 0.5, c 1.0, and d 1.5 m. For 0.2 m silica particles, the surface roughness decreased with an increase in solids loading. However, for low solids loading

8 G519 Figure 12. AFM overview images for the wafer polished with 1.0 m Geltech silica particles of different solids loading: a 0.5, b 2, c 15, and d 30 wt %. of 0.5, 1.0, and 1.5 m silica particles, the surface roughness increased with an increase in solids loading. The aforementioned roughness results were obtained during the sliding conditions. For sliding conditions, the surface roughness produced due to polishing with small particles is different from that of large particles. The increase in solids loading of small particles i.e., 0.2 m leads to an increase in the total contact area. It has been noted that local bonding during contact results in weakening of bonding forces at the surface, which allows material to be removed at the atomic level. 24 Along with the redistribution of pressure at the wafer surface, the increased contact area is expected to enhance the kinetics of the chemical reaction by enhancing the solubility of the silica species, resulting in low surface roughness. 8 The increase in solids loading of large particles 0.5, 1.0, and 1.5 m results in an increase in surface roughness. For low solids loading of large particles, the increase of surface roughness may be due to the small number of particles. It is thought that the number of scratches increases up to critical solids loading, leading to an increase in surface roughness. For rolling conditions more than 5 wt % of 0.5, 1.0, and 1.5 m silica particles, the surface roughness decreased with an increase in solids loading, which is in agreement with the response of friction force to solids loading, as seen in Fig. 9. Therefore, lubrication due to rolling particles is thought to reduce the surface roughness as solids loading increases. Effect of particle size and solids loading on polishing rate. To further elucidate the mechanism of silica polishing, the polishing rate was investigated as a function of particle size and solids loading. Figure 15 shows the polishing rate as a function of solids loading for various particle sizes at 3.5 psi. The polishing rate increased with the increase of solids loading for the 0.2 m silica particles. For 0.5 m silica particles, the polishing rate shows the transition behavior at 5 wt % of particles. The 1.0 and 1.5 m silica particles induced the same transition behavior at lower solids loading of particles i.e., 2wt%. According to in situ friction force measurements and AFM images of polished wafer surfaces, the 0.2 m silica particles and the larger particles 0.5, 1.0, and 1.5 m at low solids loading achieved material removal via sliding of slurry particles. The increase in the contact area of sliding particles leads to an increase in friction force and polishing rate, which yields no evidence of lubrication effects such as evidenced during rolling of slurry particles. For large particles 0.5, 1.0, and 1.5 m at high

9 G520 Figure 13. AFM overview images for the wafer polished with 1.5 m Geltech silica particles of different solids loading: a 0.5, b 2, c 15, and d 30 wt %. solids loading, a negligible amount of material is removed due to the rolling motions of slurry particles, which was confirmed by in situ friction force measurements and AFM images of the polished wafers. The polishing rate decreased with increasing solids loading, suggesting that lubrication effects due to rolling of slurry particles dominated as solids loading increased. For low solids loading less than 2 wt %, the increase in particle size leads to an increase in the polishing rate, which was achieved due to the sliding motion of slurry particles. In situ friction force measurements provided an insight into the interfacial contact of sliding particles. The increase in particle size results in an increase in the indentation depth per particle, which was confirmed by surface roughness rms values. The increased indentation depth per particles may be responsible for the increase in friction force and polishing rate observed. For high solids loading more than 15 wt %, the increase of particle size leads to a decrease in polishing rate, which was due to rolling motions of slurry particles. Based on in situ friction force measurements, the lubrication effects of rolling particles increased with increasing particle size, thus leading to the reduction of friction force and polishing rate in agreement with theoretical predictions. 22 These observations of varying polishing rate with particle size and solids loading suggested that various polishing mechanisms are involved in silicon dioxide CMP. It has been shown that, by delineating dynamic motions of slurry particles at the padwafer interface, appropriate criteria may be selected for robust slurry design for optimized CMP performance. Conclusions The investigation into the interfacial dynamic motion of silica particles during CMP provides critical information about silica polishing mechanisms. The polishing mechanisms of silica were elucidated via in situ friction force measurements and AFM images. The dynamic motion sliding and/or rolling of silica particles was determined by the friction force behaviors. AFM images demonstrated that the effect of sliding motion of silica particles was scratch-type surfaces and that rolling motion of silica particles induced a shotblast appearance. From in situ friction force measurements and analysis of AFM images, it was found that the polishing rate in silica CMP was dependent on the characteristics of the dynamic motion of silica particles. For the sliding motion of particles, the polishing rate is directly proportional to solids loading and particle size. For the rolling motion of particles, the polishing rate is inversely proportional to solids loading and particle size. We conclude that these phenomena are results of dynamic motions of silica particles. The

10 G521 Figure 14. Surface roughness rms as a function of solids loading for various particle sizes: a 0.2, b 0.5, c 1.0, and d 1.5 m. observed polishing behavior provides further insight into the polishing mechanism while confirming theoretical predictions of the existence of a threshold pressure that determines dynamic particle motion sliding and/or rolling at the pad-wafer interface. Further work is needed to quantitatively determine the threshold pressure that marks the onset of sliding or rolling motion. Figure 15. Polishing rate as a function of solids loading for different particle size. Acknowledgments The authors acknowledge the financial support of the Particle Engineering Research Center PERC at the University of Florida, the National Science Foundation NSF grant EEC , and the Industrial Partners of the PERC. The University of Florida assisted in meeting the publication costs of this article. References 1. J. M. Steigerwald, S. P. Murarka, and R. J. Gutmann, Chemical-Mechanical Planarization of Microelectronic Materials, John Wiley & Sons, New York K. D. Beyer, IBM Micronews, 5, L. Shon-Roy, Solid State Technol., 43, R. K. Singh, R. Bajaj, M. Moinpour, and M. Meuris, Mater. Res. Soc. Symp. Proc., R. Jairath, M. Desai, M. Stell, R. Tolles, and D. Scherber-Brewer, Mater. Res. Soc. Symp. Proc., 337, Y. Xie and B. Bhushan, Wear, 200, L. M. Cook, J. Non-Cryst. Solids, 120,

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