先进的氮化镓衬底化学机械抛光技术

As the representative of substrate material, gallium nitride (GaN) has excellent mechanical properties and high thermal stability. Achieving high surface flatness is critical for subsequent epitaxial growth and device fabrication processes. Chemical mechanical polishing (CMP) technique of GaN is commonly one of the most effective ways to achieve atomically smooth surfaces. However, the current process is difficult to meet the needs of industrial development due to the characteristics of low material removal rate. Assisted enhanced CMP technique is deemed to possess significant potential due to its improved processing efficiency and surface topography quality. Herein, a variety of auxiliary enhanced CMP systems are designed and studied. In this review, recent advances both in conventional and assisted enhanced CMP of GaN are comprehensive presented, with a focus on their potential applications in various fields. The mechanism and design strategy of the process are discussed and summarized. The key issues in machining atomically flattened surface are outlined, and future strategies for sustainable development are also proposed. This review provides a novel perspective on GaN processing and offers more inspiration for future research to realize its development and commercial application.

1. Introduction 

Gallium nitride (GaN) crystals are considered to be one of the most promising third-generation semiconductor materials widely used in light-emitting diodes (LEDs), ultraviolet detectors, and other fields.Due to its exceptional physical and chemical properties, including outstanding mechanical strength, high thermal conductivity, remarkable radiation resistance, superior breakdown voltage, and stable chemical characteristics. However, the acid and alkali resistance, high hardness, and brittleness of GaN pose challenges in achieving high flatness and damagefree surface/subsurface through ultraprecision machining technologies. This difficulty has become an urgent global issue that also hampers the application prospects of GaN crystal components.

The key substrate developments of GaN can be categorized into two areas: i) bulk crystal growth techniques as substrates, and ii) substrate processing techniques to optimize the shape of bulk crystals. These are crucial for device growth and subsequent chip processing, playing a vital role in communication, display, lighting, and other fields.Significant progress has been achieved in the former area, as evidenced by recent reports. However, the development of post-processing technology lags behind that of crystal growth mainly due to the extremely challenging machinability of substrate manufacturing processes.

Figure 1. The working mechanism of traditional CMP.

Generally, to achieve an atomically flat surface on GaN chips, the fabrication process typically involves wire sawing, grinding/lapping, and chemical mechanical polishing (CMP). However, this grinding step introduces subsurface damage (SSD),which necessitates a prolonged removal time through the CMP process.Several strategies have been developed to address this problem, with a focus on enhancing material removal rates (MMR) and improving surface quality. An effective approach involves optimizing the composition ratio of the polishing fluid and adjusting processing parameters to enhance oxidation capacity and mechanical removal. Typically, the polishing liquid consists of multiple components including abrasives, surfactants, oxidants, and various additives. Furthermore, the polishing process encompasses numerous variables, such as polishing force, disc rotational velocity, abrasive concentration, pH level, and temperature.

2. Various GaN CMP Technologies 

2.1. Traditional CMP Polishing Process 

The chemical mechanical polishing (CMP) technique was initially employed in 1992 for the purpose of refining silica in semiconductors. Subsequently, Tavernier et. al team provided a detailed account of the fine processing of GaN using CMP in 2002, and Tavernier’s team also the pioneers to employ colloidal SiO2 slurry for CMP on GaN. The working mechanism of traditional CMP as shown in Figure 1. Due to significant lattice and thermal mismatch between GaN and sapphire during growth, various defects are present in GaN crystals, including threading dislocation, edge dislocation, and mixed dislocation. Conventional CMP achieves surface smoothness by chemically oxidizing GaN with surface defects into Ga2O3 and mechanically removing the oxide layer. Therefore, conventional CMP treatment can effectively eliminate large surface defects and subsurface damages generated during the growth and processing of GaN wafers.The CMP process is employed to reduce the dislocation density and stress within the wafer caused by the mechanical polishing.Furthermore, the quality of epitaxial layers regenerated by CMP is superior to the as-grown state.This section mainly reviews the primary effect factors of traditional CMP, including polishing fluid and polishing CMP, including polishing fluid and polishing process.

Lu et al.discovered that the addition of oxidant H2O2 to SiO2 abrasive resulted in an increase in its material removal rate (MRR) from 57.59 to 90.3 nm h−1 under identical conditions, while also improving its surface quality. The resulting surface roughness values were measured at 7 and 3.24 nm respectively after a polishing duration of 2 h using a polishing fluid. It was observed that the oxidizability increased with higher concentrations of H2O2. However, it should be noted that excessively high concentrations did not yield better results. More pits will appear when the H2O2 concentration is greater than 3%, which will increase the difficulty of removal. Due to the inherent instability of ·OH, it is prone to failure during the CMP process, resulting in reduced oxidation and polishing efficiency. In 2017, Pan et al. introduced a novel oxidant S2O8 2− with a higher electrode potential E0 = +2.5 = +2.5 – +3.1 V, longer half-life (≈4 s), and wider pH range (≈2–10) compared to ·OH (<1 μs). Consequently, this new oxidant exhibits a similar oxidation capacity as ·OH but offers an extended reaction time with GaN surface. The results indicate that, with Fe2+ as the catalyst, the rate of MMR is 121.1 nm h−1 and the surface roughness Ra is 0.05 nm (Figure 2A).

Fig2

The State Key Laboratory of Crystal Materials at Shandong University has also conducted research on the growth and processing of GaN single crystals. This research team has overcome the key technology of homogeneous epitaxial growth of 2-inch single crystals by the HVPE method. Figure 3A. Aiming at the GaN single crystal substrate Ga surface processing difficulties and prone to sub-surface damage layer and other difficult problems, the systematic study of the different polishing liquid systems (acidic, alkaline, and neutral) on the Ga surface of GaN single crystal substrate polishing effect, to achieve the Ga surface scratches are removed, and sub-surface damage layer is also greatly improved, to obtain the surface roughness of 0.2 nm or less of the GaN single crystal substrate. Figure 3C. The processed samples are microscopically flat and have good crystal quality, with a half-peak width of 37.59 arcsec on the GaN single crystal (0002) face and 27.54 arcsec on the face, and dislocation densities as low as 6 × 105 cm−2. Figure 3B.D.E. For the specific research paper, a detailed report is to be made in the following.

Fig3

2.2. Photo-Assisted Chemical Polishing (PCMP) 

Photochemistry is an effective pathway for oxidation. Given the robust chemical stability of GaN, efficient oxidation and removal can be achieved by combining CMP with photocatalytic oxidation technology.As an alternative approach to enhance CMP performance, PCMP primarily targets substrates exhibiting low MMR characteristics, such as SiC and GaN.The CMP system utilizing UV light-assisted polishing can be broadly categorized into two configurations: i) the UV system is positioned above the grinding pad, enabling UV light to irradiate the slurry nozzle; ii) a UV system is installed beneath the polishing device to expose the GaN wafer to UV light (Figure 4A,D).

Fig4

3. Conclusion 

Overall, this paper offers a comprehensive exploration of the recent advancements in traditional chemical mechanical polishing (CMP) and auxiliary CMP. The systematic analysis focuses on the influence of polishing fluid and process parameters on machining in traditional CMP. Additionally, the implementation of an assisted CMP strategy proves to be an effective approach for enhancing both GaN surface topography quality and material removal rate. The assisted system mainly focuses on adding optical, electrical, magnetic, and plasma to substantially improve the oxidation and mechanical removal capabilities. Since the oxidation rate is the controlling step in the polishing process, the introduction of auxiliary systems into conventional CMP can increase the oxidation rate of GaN wafer surfaces and facilitate chemical and mechanical interaction. Finally, we also discuss potential obstacles and opportunities for removing subsurface damage and achieving atomically smooth GaN wafer surfaces. This review provides novel research perspectives on GaN processing and provides additional inspiration for future research to advance its development and commercial applications.