Effects of Hydroxyethyl Cellulose and Colloidal Silica Abrasive on the Hydrophilicity of Polished Si Wafer Surfaces

Final polishing processes are important to produce a clean surface in Si semiconductor wafers. Final polishing is carried out using a slurry that typically comprises colloidal silica, alkaline and a water-soluble polymer. Hydroxyethyl cellulose (HEC) has been widely used as a water-soluble polymer to impart hydrophilic properties to the polished wafer surface in order to reduce defects. In this study, we examined the effects of HEC concentration on the hydrophilicity of the polished wafer surface. We show correlations between the free HEC concentration in slurry liquid phase, friction during polishing, and hydrophilicity of the polished wafer surface. Furthermore, we show that the residual silica abrasive on the wafer enhances its hydrophilicity. These results show that HEC in the slurry not only acts as a hydrophilizing agent but also as an adsorptive medium between the wafer and silica. Silica has a highly hydrophilic surface owing to the presence of Si-O − groups on its surface. Therefore, a highly hydrophilic surface of polished wafer is achieved by adsorptive HEC as well as silica adsorbed on the wafer, bound by HEC. This mechanism is useful to develop new ﬁnal polishing slurries to produce ultra-clean Si wafers.

As device fabrication nodes have become smaller, the importance of ultra-clean Si wafers for semiconductor applications has increased. Chemical-mechanical polishing (CMP) 1 is one of the main processes applied to achieve a high quality Si wafer surface. CMP typically involves a multi-step polishing process with a suitable polishing slurry in each step. 2 The final polishing step is important to reduce the number of nanoscale defects, down to 26 nm or smaller, therefore both functions to remove defects caused by previous steps in the polishing process and to prevent further defects are necessary. In addition, it is important to prevent the wafer surface from adsorbing particles, which are hard to remove in the subsequent cleaning steps. It has been shown that wafer surface protection using a polymer is effective for reducing defects and adsorption of particles. 3 The slurry creates a protective layer on the wafer surface and has been widely used in the final polishing step. Typical final polishing slurries contain watersoluble polymers that make a protective layer on the wafer surface and introduce hydrophilicity to the surface to avoid the adsorption of dried particles, which may originate from many substances, such as silica abrasives, silicon debris, and foreign contamination from environment. Once particles are adsorbed on a dried wafer surface, it is difficult to remove them in the cleaning processes as they become strongly adsorbed. Therefore, most of the available commercial final slurries contain water-soluble polymers. Hydroxyethyl cellulose (HEC) is usually preferred as a hydrophilizing agent and many studies exploring the effect of HEC on wafer properties have been reported. [3][4][5][6][7][8] However, the hydrophilic properties imparted on the wafer by HEC during the polishing process have not been well documented.
The aim of this study is to understand the mechanism of hydrophilicity introduced by the final polishing slurry comprising HEC as a hydrophilizing agent, colloidal silica, and NH 4 OH. We focus on the correlation between the free HEC concentration, the wafer friction during polishing, and the hydrophilicity of the polished wafer surface. Furthermore, we investigate the behavior and effect of colloidal silica as a so-called second wetting agent.

Experimental
We prepared several types of final polishing slurries that comprised colloidal silica abrasive, HEC and NH 4 OH at several different concentrations. The size of colloidal silica was measured by dynamic light scattering methods using MicrotracBEL Nanotrac UPA-UT 151, * Electrochemical Society Member. z E-mail: tsuchiyako@fujimiinc.co.jp and the volume average particle size was found to be 66 nm in diameter. We obtained concentrated slurries by the addition of aqueous ammonia solution into the colloidal silica slurry; then, a HEC (M w = 250,000) solution diluted by deionized water (DIW) was added. The concentrated slurries were diluted by 20 times using DIW and the test polishing slurries (named as SiO 2 /HEC slurry) were prepared. The concentration of HEC ranged from 0.02 to 0.51 g/L and the silica concentration was 5 g/L in the diluted polishing slurries. In addition, HEC solutions (named as HEC-aq) with a concentration ranging from 0.02 to 0.51 g/L and silica slurry (SiO 2 -aq) with a concentration of 5 g/L were prepared as reference polishing slurries. The pH was maintained at 10.2 for all slurries by adjusting the NH 4 OH concentration.
In order to determine the amount of adsorbed HEC on the silica abrasive surface, two types of solutions were prepared. The first contained the liquid part of SiO 2 /HEC slurry, which was prepared by a centrifugal separation method at 20,000 rpm for 30 min. The second was HEC-aq, which had the same HEC concentration as the SiO 2 /HEC slurry but did not include any silica. The organic carbon concentration of these two solutions was measured by using a total carbon analyzer (SHIMADZU TOC-5000A). The free HEC concentration was calculated by dividing the carbon concentration of the liquid part of the SiO 2 /HEC slurry by 0.51, the molar carbon ratio in HEC. The amount of HEC adsorbed per gram of silica surface was also calculated by dividing the difference in the organic carbon concentrations of the two solutions by 0.51, and then by the silica concentration. The variation in the measurements was 11%.
The polished wafer samples for evaluating the wafer surface hydrophilicity were prepared as follows. A P-type <100> bare Si wafer was cut into 3.2 cm square pieces and cleaned in ammonium hydrogenperoxide mixture (APM) solution (consisting of NH 4 OH (aq) (at 29%), H 2 O 2(aq) (at 31%) and DIW in a 1:1:8 volume ratio) for 30 s and dipped into a 5% hydrofluoric acid (HF) bath for 30 s to remove the natural oxide layer from the wafer surface before polishing. The wafers were polished using an ENGIS EJ-380IN with the SiO 2 /HEC, HEC-aq and SiO 2 -aq slurries. The polishing pad was made of suede, with a polishing pressure of 1.9 psi, a head/platen rotation speed of 30 rpm, polishing time of 60 s, and a slurry flow rate of 1 mL/s. After polishing, the wafers were rinsed by flowing DIW for 3 s and dipped into a DIW bath for 30 s. Afterwards, the wafers were placed in an upright position until dry.
The polishing friction was determined from measurements of the platen motor current during polishing. The difference in the value of the platen motor current with and without the polishing pressure can be used to evaluate the polishing friction. The variation in the measurements of the platen motor current was 13%. We evaluated the hydrophilicity of polished wafers by measuring the contact angle of a sessile drop using a contact angle meter (KY-OWA CAX-200). The evaluated contact angles were obtained from the average of five measurements per polished wafer surface. The contact angle measurements had variation of 5%.
We also observed the polished wafer surface in detail using a scanning electron microscope (SEM) (HITACH FE-SEM SU-8000) and the surface coverage ratio of the silica abrasive on the wafer was calculated by dividing the adsorbed silica area by the total surface area of the silicon wafer determined from the SEM images. The variation of the surface coverage was 11%.
Finally, we evaluated the wafer surface coverage ratio and the contact angles after the polished wafers were dipped into HF, in order to understand the effect of the silica abrasive on the hydrophilicity. Sample preparation procedures did not change from those mentioned above, except that the wafers were dipped into 5% HF bath for 10-60 s after polishing to dissolve the silica on the wafer surface. These wafers were again rinsed with flowing DIW for 3 s and were stood in an upright position to dry.

Results and Discussion
The adsorptivity of HEC onto the abrasive surface of silica in the SiO 2 /HEC slurry is shown in Figure 1, with the amount of adsorbed HEC per gram of silica surface shown as a function of the concentration of free HEC, which did not adsorb onto silica but remained in the liquid phase of the slurry. From these results, we confirm that HEC may exist both on the silica surface and in the liquid phase of the slurry as free HEC. The amount of HEC adsorbed onto the silica surface increased with the increase of free HEC concentration in the slurry, until the adsorbed amount became saturated at around 0.024 g/1 g SiO 2 with the free HEC concentration at over 0.2 g/L. These results suggest that at low HEC concentrations, the HEC is selectively adsorbed onto the silica surface rather than remaining in the liquid phase, as evidenced by the steep increase in adsorbed HEC with the increase of free HEC concentration. With a further increase in the HEC concentration of the slurry, the rate of increase in the amount of adsorbed HEC diminished. This behavior arises due to the adsorbed HEC preventing further adsorption owing to steric repulsion forces. 9 At higher HEC concentrations, the amount of HEC adsorbed onto the silica surface reached a maximum, and this saturation concentration significantly increased the free HEC concentration in the liquid phase of slurry.
We next investigated the effect of free HEC in the liquid phase on the polishing friction in order to understand how HEC influences the wafer surface during polishing. Figure 2 shows the platen motor current (i.e., polishing friction) as a function of free HEC concentra- tion. The platen motor current decreased with increasing free HEC concentration in the HEC-aq and became saturated at around 0.029 g/L, as indicated by the arrow in Figure 2. After this saturation point, the polishing friction remained stable. The polishing friction arises between the wafer surface and the polishing pad during polishing. If the wafer surface is hydrophobic, it is difficult for the hydrophilic slurry to flow between the wafer and the pad, resulting in a high polishing friction. On the other hand, if the free HEC adsorbs onto the wafer surface and increases the hydrophilicity of the wafer surface, the slurry may flow between the wafer and the pad more easily. In this investigation, the polishing friction decreased and became stable with the increase in the free HEC concentration. This could indicate that the free HEC adsorbed onto the wafer surface and increased its hydrophilicity. Furthermore, the platen motor current of SiO 2 /HEC slurry shown in Figure 2 also decreased with the increase in free HEC concentration, saturating at over 0.029 g/L. According to these results, the effect of free HEC on the polishing friction did not change either with or without silica, but the platen motor current of SiO 2 /HEC slurry was higher than that of the HEC-aq once both motor currents were stable. This suggests that the free HEC could be more effective in bringing the silica on the wafer surface, but the silica increases the mechanical energy during polishing. 10,11 To explore the effect of free HEC on the hydrophilicity of polished wafer surface, we measured the sessile drop contact angle on a wafer  surface after polishing, shown in Figure 3. It shows that the contact angle decreased with the increase of free HEC in the HEC-aq. Hence, we can confirm that an increase in free HEC increases the hydrophilicity of the wafer surface, as suggested above. On a closer analysis of Figure 3, it is clear that the contact angle shows a significant decrease until a concentration of 0.029 g/L, which is shown by the arrow, and reaches a plateau. This behavior is also shown in Figure 2. These two facts are in good agreement to support the above mentioned model of the free HEC behavior. The SiO 2 /HEC slurry in Figure 3 also shows a significant decrease and a gradual change of contact angle as the free HEC increases, but the concentration also reached a plateau at 0.029 g/L, as in Figure 2. H. S. Hwang et. al also reported that increase of total HEC concentration in the slurry enhanced the hydrophilicity on the wafer surface. 3 In contrast, we found that especially free HEC in liquid phase was important to act as a hydrophilizing agent on the wafer surface. In addition, free HEC acts not only as a hydrophilizing agent, but also as a polishing friction stabilizer by improving the slurry flow between the wafer and the pad. Furthermore, we show the possibility of the silica effect on increasing the hydrophilicity of the polished wafer surface, since the contact angle of SiO 2 /HEC was lower than that of the HEC-aq. This fact could lead to modification of the previously proposed model 3 that only HEC increases the hydrophilicity of the polished wafer surface.
To investigate the effect of silica abrasive on the hydrophilicity, we imaged the polished wafer surface. Figure 4 shows SEM images of the wafer surface polished with SiO 2 -aq slurry without any HEC (Figure 4a) and the SiO 2 /HEC slurry with various free HEC concentrations between 0.002 and 0.4 g/L (Figures 4b to 4f). As seen in the images, the silica (white dots in the pictures) remains on the polished wafer surface and the number of remaining silica particles increased with the increase of free HEC concentration. This result is difference from the previous report that HEC prevented the adsorption of silica and reduced the residual particles on the wafer. 3 In order to express the silica effect clearly, the surface coverage ratio by the silica on the polished wafer was calculated from the SEM images. A steep increase in the coverage of residual silica abrasives from 0.06% to 0.31% was observed with an increase of free HEC concentration from 0.002 and 0.009 g/L. Further increases in free HEC concentration resulted in only slight increases to the residual silica ratio. These observations may be explained by the increased hydrophilicity of the wafer surface due to the free HEC, which allows for a smooth slurry flow over the wafer surface. Furthermore, HEC can adsorb onto the silica surface, as shown in Figure 1, such that adsorbed HEC on the wafer surface may increase the adhesion of the silica. Thus, HEC may act as an adsorption medium between the wafer and silica.
Furthermore, we show the correlation between the hydrophilicity of the polished wafer and the surface coverage of silica abrasive on the polished wafer. Figure 5 shows the sessile drop contact angle dependence on the surface coverage of silica. The contact angle decreases with increasing of SiO 2 surface coverage. These results show that the hydrophilicity increase of the polished wafer surface was affected by the residual silica adsorbed on the surface.
On the basis these findings, we next conducted an examination of the HF treatment used for the dissolution of the silica abrasive, in order to understand the effect of silica on the hydrophilicity of polished wafer surface. Figure 6 shows the silica surface coverage ratio, on the wafers polished with the SiO 2 -aq and SiO 2 /HEC slurries, as a function of HF treatment time. The surface coverage ratio decreased with HF treatment time, and it was clear that the silica was almost completely dissolved (from a visual analysis) after 30 s in both SiO 2 -aq and SiO 2 /HEC slurries.
In order to further investigate this behavior, Figure 7 shows the sessile drop contact angle, as a function of HF treatment time, on the wafer surface polished with the HEC-aq, SiO 2 -aq and SiO 2 /HEC slurry. The contact angle of the HEC-aq treated sample was stable regardless the HF treatment time, suggesting that the adsorbed HEC on the polished wafer surface was not removed by the HF treatment. On the other hand, from Figure 7, the contact angle of the SiO 2 -aq treated sample reached almost the same base-line value of the water   in 10 s. In addition, SiO 2 /HEC treated sample also increased at 10 s and then finally reached the same value of HEC-aq treated sample. From the data in Figures 6 and 7, we confirm the relationship between the decrease of surface coverage ratio by silica and the deterioration of hydrophilicity. Since the silica can introduce hydrophilicity to the wafer surface, it is possible to say that adsorption of both HEC and silica onto the wafer surface during polishing is necessary to generate high hydrophilicity.
Based on these results, we interpret that the hydrophilicity expression mechanism on wafer surfaces polished with the final polishing slurry, typically comprising HEC, colloidal silica abrasive and NH 4 OH, as follows. HEC can remain in the slurry in two different locations: one is adsorbed onto the silica surface and the other is in the free liquid phase. Under these conditions, the free HEC could adsorb onto the wafer surface and introduce hydrophilicity to the wafer during polishing. This hydrophilicity was effective in allowing a smooth slurry flow between the wafer and the pad; hence, the polishing friction became stable. Furthermore, silica remained adsorbed on the wafer surface through the adsorptive properties of HEC. As silica has a highly hydrophilic surface, derived from the Si-O − polar group, the remaining silica on the wafer surface produced a high hydrophilicity along with the HEC. In this study, we found that HEC had three different roles in producing a hydrophilic wafer surface, acting as 1) a hydrophilizing agent, 2) a polishing friction stabilizer, and 3) an adsorption medium between the wafer and silica. Moreover, the silica also acts as the hydrophilizing agent and this effect may enhance the hydrophilicity conferred by HEC. We considered that high hydrophilicity on the polished wafer surface by the adsorption of silica and HEC is significantly effective to avoid the adsorption of dried particles, therefore we can expect to achieve a high quality wafer surface after final cleaning.

Conclusions
We investigated the polishing friction and hydrophilicity as a function of free HEC concentration. The contact angle measurements show that the hydrophilicity of polished wafer surface improves with increasing free HEC concentration, which did not adsorb onto silica but existed in the liquid phase. This hydrophilicity improvement may be linked to the stabilization of polishing friction. From this, we determined that the free HEC acts not only as a well-known hydrophilizing agent, but also acts to stabilize the polishing friction. Besides this, the free HEC concentration was correlated with the residual silica abrasive adsorbed on the polished wafer surface. This indicated a new role for the free HEC as an adsorption medium between the wafer and silica. We also observed that the residual silica increased the hydrophilicity on the polished wafer more effectively. Hence, it can be concluded that silica acts as a hydrophilizing agent together with HEC, and is effective in improving the hydrophilicity of Si wafer surfaces in the final polishing processes.