Preparation of Ce-doped colloidal SiO2 composite abrasives and their chemical mechanical polishing behavior on sapphire substrates

https://doi.org/10.1016/j.matchemphys.2015.12.026Get rights and content

Highlights

  • Novel Ce-doped colloidal SiO2 composite abrasives were prepared.

  • The chemical mechanical polishing (CMP) performances of the composite abrasives on sapphire substrate were investigated.

  • Novel composite abrasives show excellent polishing characteristics comparison with pure colloidal SiO2 abrasive.

  • We explore and report the acting mechanism of composite abrasives to sapphire CMP.

Abstract

Chemical mechanical polishing (CMP) has become a widely accepted global planarization technology. Abrasive is one of key elements during CMP process. In order to enhance removal rate and improve surface quality of sapphire substrate, a series of novel Ce-doped colloidal SiO2 composite abrasives were prepared by chemical co-precipitation method. The CMP performances of the Ce-doped colloidal SiO2 composite abrasives on sapphire substrate were investigated by using UNIPOL-1502 polishing equipment. The analyses on the surface of polished sapphire substrate indicate that slurries containing the Ce-doped colloidal SiO2 composite abrasives exhibit lower surface roughness, higher material removal rate than that of pure colloidal SiO2 abrasive under the same testing conditions. Furthermore, the acting mechanism of the Ce-doped colloidal silica in sapphire CMP was investigated. X-ray photoelectron spectroscopy analysis shows that solid-state chemical reactions between Ce-doped silica abrasives and sapphire surface occur during CMP process, which can promote the chemical effect in CMP and lead to the improvement of material removing rate.

Introduction

Sapphire, single crystal form of a-alumina, is widely used in a variety of modern high-technology applications because of its excellent optical, chemical and mechanical properties such as high hardness, great thermal stability, chemical inertness, especially used as a substrate material in the photoelectronic, microelectronic and semiconductor industries, ranging from optical windows, read/write laser sources to GaN-based light emitting diodes (LEDs), etc [1], [2], [3], [4], [5], [6]. In all of these applications, the surface flatness of sapphire substrate is a key factor that influences its performances. Undoubtedly, its planarization machining is very important [7]. However, the intrinsic nature of sapphire (great hardness and chemical inertness) poses great challenges to such machining. In order to achieve the above mentioned surface quality of sapphire, several polishing technologies are put forward by researchers [8]. At present, chemical mechanical polishing (CMP) is the only widely used global planarization technology in the manufacturing of semiconductor and sapphire substrates [9], [10], [11], [12].

In CMP, abrasives are one of key influencing factors on the polished surface quality. The sizes and distribution, dispersibility, hardness and species of abrasive are crucial for a desired CMP performance [13]. In order to improve surface planarization and material removing rate (MRR) of sapphire, several polishing abrasives have been studied. Zhu et al. [14] have studied the effect of abrasive hardness on the CMP of (0001) plane sapphire. They found that, hard abrasives (such as monocrystalline and polycrystalline diamond, ɑ- Al2O3 and γ-Al2O3) can improve MRR of sapphire, but surface arithmetic average surface roughness (Ra) which is measured at various points on the actual contour of the absolute value of the distance to the baseline of the arithmetic mean within a sampling length is higher. Nevertheless, using silica sol as polishing abrasives for sapphire can get lower Ra, but MRR of sapphire is lower. Hu. et al. [15] have researched the CMP of sapphire wafers with boron carbide and colloidal silica abrasives. They found that, B4C abrasive eliminated the uniformity in thickness within sapphire wafer, but the colloidal silica can achieve a nanoscale flatness of sapphire surface. Xiong et al. [16] adopted silicon carbide, alumina and silica sol as polishing abrasives for CMP of sapphire. The results show that, under the same condition, Ra of sapphire polished with silica sol abrasive is lower than that with silicon carbide and alumina abrasives. In general, silica sol is widely regarded as ideal polishing abrasive for sapphire CMP since it can get better surface quality [17], [18]. However, MRR of sapphire with colloidal silica as polishing abrasive is still lower and needs to be improved.

Currently, silica-based composite particles as CMP abrasives capture increasing attention, and chemical modifying of silica abrasive has been proved to be an effective way to improving its polishing performances in CMP. Zhao [19] synthesized SiO2/CeO2 abrasive for CMP of oxide wafer and Lei [20] developed porous Fe2O3/SiO2 abrasive for CMP of hard disk substrate. Zhang et al. [21] prepared silica composite abrasive by coating with a layer of polystyrene. The silica-based composite abrasives resulted in both high planarization efficiency and good planarization quality. Up to date, preparation of silica-based composite abrasive for sapphire CMP has seldom been reported. In this paper, Ce-doped colloidal SiO2 composite abrasives were prepared and their CMP performances on sapphire substrates were investigated.

Section snippets

Chemicals

Chemicals used in the synthesis were ceric ammonium nitrate ((NH4)2Ce(NO3)6), ammonia(NH3·H2O), carbamide(CO(NH2)2), colloidal SiO2, sodium hexametaphosphate and deionized(DI) water.

Preparation of Ce-doped colloidal SiO2 composite abrasives

Synthesis of Ce-doped colloidal SiO2 abrasives with different CeO2 contents: 10 wt. % crystal silica seed solution were heated to 100 °C in a four-neck flask. A certain amount of 2.5 wt.% active silicic acid solution, 0.4wt.% (NH4)2Ce(NO3)6 solution, 0.05 wt.% CO(NH2)2 solution and 1.5 wt. % ammonia were

Characterization of Ce-doped colloidal SiO2 composite abrasives

Fig. 1 is SEM spectra of pure colloidal SiO2 abrasive and Ce-doped colloidal SiO2 composite abrasives. The result shows that, by comparison with the pure colloidal SiO2 abrasive (0 wt % of CeO2), the particle sizes of Ce-doped colloidal SiO2 composite abrasives are almost no change. And the prepared Ce-doped colloidal SiO2 abrasives have inerratic spherical shape and excellent dispersibility.

TOF-SIMS is an effective means for element analysis because of its high sensitivity and mass and space

Conclusions

Novel Ce-doped colloidal SiO2 composite abrasives were prepared. The results show that the prepared Ce-doped colloidal SiO2 composite abrasives have good dispersibility and inerratic spherical shape. The slurries containing Ce-doped colloidal SiO2 composite abrasives exhibit lower Ra and higher MRR than that containing pure SiO2 abrasive in the CMP of sapphire substrates under the same conditions. The improvements of sapphire CMP may be attributed to its solid-chemical reaction between sapphire

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51475279, No. 91323302). Research Fund for Doctoral Program of Higher Education of China (Grant No. 20123108110016).

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