Hybrid polishing mechanism of single crystal SiC using mixed abrasive slurry (MAS)

doi:10.1016/j.cirp.2010.03.114

CIRP Annals

Volume 59, Issue 1, 2010, Pages 333-336

https://doi.org/10.1016/j.cirp.2010.03.114 Get rights and content

Abstract

Single crystal SiC is a mechanically hard and chemically inert material used in optical and power devices. This work proposes the development of a hybrid polishing technique using a mixed abrasive slurry (MAS) with colloidal silica and nano-diamond. Hybrid removal mechanism of the MAS on the SiC is investigated by polishing results, chemical analyses and AFM studies. Each role of two abrasives is distinguished by scratching tests with AFM contact mode on the chemically reacted SiC surface. Finally, this paper provides an optimum MAS condition to realize highly efficient material removal rate (MRR) keeping defect-free surface.

Introduction

Silicon carbide (SiC) is considered as a leading semiconductor material for next generation semiconductors, microelectromechanical systems (MEMS) and high power and high temperature electric applications due to its excellent mechanical properties, wide energy bandgap, good carrier mobility, and excellent thermal properties [1], [2]. Since high level commercial substrates require a defect-free surface, the chemical mechanical planarization (CMP) process has been regarded as a key technology for the final wafer processing of SiC substrate fabrication. Although single crystal SiC has many attractive properties, the outstanding chemical, mechanical and wear properties of SiC make it hard to remove its surface with CMP.

Before CMP process, SiC substrates undergo sawing, lapping, and mechanical polishing process for surface preparation. These mechanical processes unavoidably leave many surface (or sub-surface) damages such as scratches and dislocations. These surface defects are mainly originated from fracturing process in lapping and mechanical polishing [3], [4]. Even though low surface roughness can be achieved with mechanical polishing, a dense network of scratches and defects can be revealed after high temperature thermal process [5]. Therefore, the purpose of CMP process is to achieve high quality surfaces by reducing its roughness and removing mechanically damaged layer after lapping and mechanical polishing.

Researches on SiC CMP have been focused on increasing material removal rate and verifying polishing mechanisms of SiC. Zhou et al. [6] suggested a CMP process of SiC using concentrated colloidal silica slurries at high pH values and at elevated temperature. They said that a higher pH value increases chemical reaction rate by increasing the concentration of the –OH groups which weaken the Si–C bonds, and the chemical reactions are thermally activated on the basis of Arrhenius relationship. The maximum material removal rate (MRR) (<0.2 μm/h) in their study was observed in the CMP using colloidal silica slurry with pH 11 at 55 °C. Neslen et al. [7] studied the effect of process parameter variations on the MRR of SiC by using commercial colloidal silica slurry. The maximum MRR in their study was 0.25 μm/h. Generally, most researches on SiC CMP have been conducted with alkaline colloidal silica slurries for reducing surface roughness. The results showed that SiC CMP with colloidal silica slurry took extremely long time for removing surface damages. Recently, some researchers suggested a hybrid polishing with submicron diamond and colloidal silica slurry to enhance the MRR in SiC CMP. Kuo and Currier [8] augmented the MRR in SiC CMP with an oxidizer enriched colloidal silica slurry and a hybrid of specially treated submicron diamond and colloidal silica slurry (referred to as a CMP-D slurry). Lee and Jeong [9] also used a mixture of colloidal silica slurry and nano-diamond abrasive (referred to as a mixed abrasive slurry (MAS)) for high MRR and fine surface. Recent studies show that the hybrid CMP using mixed abrasive has high MRR and outstanding scratch reduction ability.

In spite of their efforts to verify the polishing mechanism of SiC CMP, it is still vague. Therefore, in this article, hybrid removal mechanism of the MAS on the SiC is investigated by polishing results, chemical analyses and AFM studies to understand the CMP mechanism and achieve high MRR.

Section snippets

AFM study

To distinguish each role of two abrasives (colloidal silica and diamond) in MAS, scratching tests using AFM contact mode on the chemically reacted SiC surface were conducted with the MultiMode Scanning Probe Microscope (Vecco). The generation of reacted layers on SiC surface could be detected with X-ray photoelectron spectroscopy (XPS). Two types of AFM tips were used to investigate the abrasion characteristics during CMP. One was a silicon tip, and the other one was a diamond-coated tip.

AFM study on SiC

To confirm the generation of a reacted layer on SiC with a KOH-based slurry used in this experiment, a SiC wafer was immersed into the KOH-based slurry for 24 h. The generation of SiO₂ reacted layer with KOH solution was confirmed with XPS analysis. Fig. 1 shows the XPS spectra of two SiC wafers; one was an as-received sample, another was a sample which was immersed into KOH-based slurry. In the as-received sample, a Si peak was observed at 99.5 eV in XPS spectra. After the SiC wafer was dipped

Conclusions

In this paper, we focused on investigating the removal mechanism of MAS CMP for SiC by using AFM studies and CMP results. According to AFM scratching results and XPS analysis, the chemically reacted (oxidized) layer is generated with KOH-based CMP slurry. Chemically reacted SiC has a higher wear amount than the as-received one. The scratching results with a diamond-coated tip show higher MRR than the results with silicon tip in both chemically reacted and as-received samples. The CMP results

Acknowledgments

This research was supported by NCRC (National Core Research Center) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2010-0001-226).

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Citation Excerpt :
CMP using free abrasives in slurries is the most widely used ultra-precision process for making electronic grade SiC wafers by delicately balancing mechanical friction with chemical softening. Many scholars have accelerated the SiC material removal rate (MRR) by optimizing abrasive selections, such as colloidal silica, Al2O3, CeO2, etc. [15–17]. Others attempted refinements on chemical softening contributions.
In pursuing high precision and high-quality surface of silicon carbide (SiC) substrates for electronic applications, however, the processing efficiency on Si-face has long been a difficult challenge compared to that of the C-face. Conventional polishing slurries are aqueous-based due to its unparalleled compatibility with most of chemical reagents, and have been widely studied and well understood. The chemical reactivity of all added accelerating agents in aqueous polishing systems is constrained by the prevalent presence of water as a solvent. On the other hand, non-aqueous polishing systems have not been well explored. In this report, reactive non-aqueous organic systems containing polar hydroxyl groups are evaluated as a high efficiency processing vehicle for Si-face polishing of SiC on fixed abrasive pads (FAPs). The surface quality of SiC is surprisingly improved while the polishing efficiency is accelerated on a diamond FAP in presence of methanol compared to those using water, and are further improved when organic acids are introduced to the reactive non-aqueous fluids. Contact angle measurement and X-ray photoelectron spectroscopy (XPS) analysis on the polished SiC surface are completed to assist us to shed light on the removal behavior and to explore and elucidate the removal mechanism of non-aqueous system fluids on FAPs. The initial positive results from this reactive non-aqueous polishing system serves our community as a promising approach for the ultra-precision polishing on other hard-to-process semiconductor and ceramic materials.

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Hybrid polishing mechanism of single crystal SiC using mixed abrasive slurry (MAS)

Abstract

Introduction

Section snippets

AFM study

AFM study on SiC

Conclusions

Acknowledgments

Annals of the CIRP

Annals of the CIRP

Annals of the CIRP

Annals of the CIRP

Annals of the CIRP

Annals of the CIRP