Extended study of the atomic step-terrace structure on hexagonal SiC (0 0 0 1) by chemical-mechanical planarization
Introduction
Hexagonal silicon carbide materials (SiC), which are considered to be a promising candidate for electronic devices have recently attracted much attention as the third generation key materials for transistors because they can be operated in high-power, high-frequency regions that are currently inaccessible with conventional silicon-based transistor devices [1], [2], [3], [4]. In most cases, the c-plane (0 0 0 1) of hexagonal SiC substrates is used for device epitaxy. Defects on the substrate surface could be replicated into the epilayer, thus surface preparation of the substrate wafer will play a critical role in electronic integration fabrication [5]. Due to the extreme hardness and strong stability against chemicals of SiC [6], [7], it is more difficult to realize ideal planarization with high removal rate and good surface quality [3]. Chemical mechanical polishing (CMP) is applied to the surface as an efficient treatment to produce atomic level surface flatness by removing the irregularities and the damage on or near the surface due to mechanical polishing [8], [9].
Many studies have been reported to investigate the planarization of SiC [6], [10], [11], [12]. Based on the theory of crystallography and the research development of the hexagonal super-hard materials, such as gallium nitride (GaN), sapphire, 4H- and 6H-SiC [13], [14], [15], [16], [17], [18], we know that all these materials have an obvious characteristics: if the (0 0 0 1) surface of wafer is approximately 0°-off, the surface could emerge with atomic step-terrace structure after planarization [3], [19], [20], [21]. Strictly, for such hexagonal super-hard materials, besides the method of measuring the surface roughness parameter (Ra) of the wafer after planarization by surface measuring instrument, the formation surface atomic step-terrace structure is more persuasive and credible to verify the efficiency of the planarization technique, because the surface atomic step-terrace structure directly show the crystalline form of the surface. For CMP, which is an advanced and widely applied planarization technique, by using appropriate technique parameters, we can also obtain clear and complete atomic step-terrace structure, which has recently been widely recognized and accepted by academic circles.
However, in the recent study of planarization of hexagonal super-hard materials, including CMP, researchers always pay attention to the improvement of the planarization technique, which means that they only treat the appearance of surface atomic step-terrace structure as a benchmark of good result for the planarization technique, but seldom systematically and study deeply the formation rules and regulation of this surface topography, as well as the relationship between the surface topography and planarization technique. Actually, the variety of related parameters in the atomic step-terrace structure can regularly reflect the change of physical and chemical processes of the planarization. In other words, by observing the various rules of related parameters of the atomic step-terrace structure after planarization, we can indirectly or directly study and improve the theory of CMP process.
In this paper, we combine the principles of CMP tribology, the principles of AFM measurement and the crystallography theory of hexagonal super-hard materials, to introduce and discuss the formation rules of atomic step-terrace structure. We analyze the relationship between atomic step-terrace structure and surface roughness parameter Ra, and present an idea of improvement for the ultimate Ra theoretical model. We also study the relationship between the CMP process and the characteristics of the atomic step-terrace structure, and use the AFM in situ observation method to survey and mapped the step-terrace structure pattern distribution in different region of one 4H- or 6H-SiC wafer. In addition, we accidentally observe some dislocations and defects on the wafer surface, and we attempt to describe them briefly.
Section snippets
Experimental
One two-inch wafer of commercial 4H-SiC (0 0 0 1) with a 0°-off (±0.5°, on-axis oriented) Si face was used. The wafer has an original thickness of 376 μm, and had been preliminarily polished by double sides lapping. The processed wafer has a high degree of planar and parallel on the two sides.
The wafer was planarized by our CMP technique. The two-inch 4H-SiC wafer on the Si-face was processed through CETR CP-4 machine (China Shenyang Science and Technology Instrument Co., Ltd). The experiments took
Essential parameters of atomic step-terrace structure
First, we take 4H-SiC as an example to introduce the characteristic of the atomic step-terrace structure. The typical crystal structure of 4H-SiC is shown as Fig. 1.
In the hexagonal close-packed SiC structure, one layer of silicon atoms and one layer of carbon atoms form a Si-C bilayer. SiC are formed by periodic stacking sequences of bilayers that produce tetrahedral sheets. Atomic models of the six unique (fundamental) bilayers (bA, cA, aB, cB, aC, and bC) of SiC based on three principle
Conclusion
We have demonstrated CMP of 4H- and 6H-SiC with a colloidal silica slurry and successfully obtained high-definition atomic step-terrace structure by AFM. We explained the formation rules of the atomic step-terrace structure during CMP process, and found that the definition of terraces structure improved gradually and finally reached a steady status. This status could sequentially be improved by optimizing the technique of CMP and the formula of the slurry. We studied the relationship between
Acknowledgements
The authors would like to thank National Key Basic Research Program of China – 973 Program (No. 2011CB013102) for a grant to this research. The support from National Natural Science Foundation of China (No. 91223202) is also gratefully acknowledged. Meanwhile, this research has been supported by a grant from the International Science & Technology Cooperation Program of China (No. 2011DFA73410) and Tsinghua University Initiative Scientific Research Program (No. 20101081907).
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