Recent Advances In Silicon Carbide Chemical Mechanical Polishing Technologies

Abstract

Chemical mechanical polishing (CMP) is a well-known technology that can produce surfaces with outstanding global planarization without subsurface damage. A good CMP process for Silicon Carbide (SiC) requires a balanced interaction between SiC surface oxidation and the oxide layer removal. The oxidants in the CMP slurry control the surface oxidation efficiency, while the polishing mechanical force comes from the abrasive particles in the CMP slurry and the pad asperity, which is attributed to the unique pad structure and diamond conditioning. To date, to obtain a high-quality as-CMP SiC wafer, the material removal rate (MRR) of SiC is only a few micrometers per hour, which leads to significantly high operation costs. In comparison, conventional Si CMP has the MRR of a few micrometers per minute. To increase the MRR, improving the oxidation efficiency of SiC is essential. The higher oxidation efficiency enables the higher mechanical forces, leading to a higher MRR with better surface quality. However, the disparity on the Si-face and C-face surfaces of 4H- or 6H-SiC wafers greatly increases the CMP design complexity. On the other hand, integrating hybrid energies into the CMP system has proven to be an effective approach to enhance oxidation efficiency. In this review paper, the SiC wafering steps and their purposes are discussed. A comparison among the three configurations of SiC CMP currently used in the industry is made. Moreover, recent advances in CMP and hybrid CMP technologies, such as Tribo-CMP, electro-CMP (ECMP), Fenton-ECMP, ultrasonic-ECMP, photocatalytic CMP (PCMP), sulfate-PCMP, gas-PCMP and Fenton-PCMP are reviewed, with emphasis on their oxidation behaviors and polishing performance. Finally, we raise the importance of post-CMP cleaning and make a summary of the various SiC CMP technologies discussed in this work.

Keywords: Silicon Carbide (SiC); chemical mechanical polishing (CMP); hybrid CMP; post-CMP cleaning.

Figure 1

Illustrations of surface roughness and subsurface damage depth at the slicing, lapping, grinding and CMP steps of the SiC wafer production process [3].

Figure 2

Different configurations of SiC CMP, including (A) batch wafer and single-sided CMP, (B) batch water and double-sided CMP and (C) single wafer and single-sided CMP. (Ref. CMC Materials Co.).

Figure 3

(A) Illustration of CMP mechanism. The oxidants in the slurry oxidize the SiC surface into a softer oxide layer which can be removed by the abrasive particles in the slurry and the conditioned CMP pad simultaneously. The chemical reactions of SiC oxidation, which are initiated by either oxygens or hydroxyl radicals, are shown [7,8]. Note that asterisk represents radicals. (B) Illustration of Tribo-CMP mechanism. In addition to the global oxidation taking place at the slurry and SiC interface, localized oxidation can also occur when the solid-phase abrasive oxidants roll on the SiC wafer surface.

Figure 4

Typical setup for SiC ECMP.

Figure 5

(A) Illustration of Fenton-ECMP mechanism. The oxidants and Fe₃O₄ catalyst in the slurry oxidize the SiC surface into a softer oxide layer which can be removed by the abrasive particles in the slurry and the conditioned CMP pad simultaneously. Under acidic conditions, Fe₃O₄ generates Fe²⁺ which undergoes a Fenton reaction with H₂O₂ to generate strong oxidant hydroxyl radical, denoted as OH* [27]. The oxide layer is removed by the abrasive particles in the slurry and the conditioned CMP pad simultaneously. (B) Illustration of ultrasonic-ECMP mechanism. After the anodic oxidation of SiC, the oxide layer is removed by a vibrating grinding stone with fixed abrasives.

Figure 6

Typical setup for SiC PCMP.

Figure 7

Illustration of PCMP mechanism. Upon irradiation of UV light, the photocatalytic TiO₂ particle reacts with O₂, H₂O and OH⁻ in the slurry to generate strong oxidant hydroxyl radical, denoted as OH* [33]. The oxide layer is then removed by the abrasive particles in the slurry and the conditioned CMP pad simultaneously.

Figure 8

Typical setup for SiC gas-PCMP.