The Principle of Silicon Carbide Crystal Growth
In nature, crystals are everywhere, and their distribution and application are very extensive. Salt, sugar, diamond and snowflakes are all crystals that can be seen in daily life; Besides, crystal materials such as semiconductor crystal, laser crystal, scintillation crystal, and super-hard crystal also play an important role in industry, medical treatment, semiconductor, and many scientific research fields.
Different crystals have different structures, properties and preparation methods. But their common feature is that the atoms in the crystal are regularly arranged, and the lattice with a specific structure is then formed through periodic stacking in three-dimensional space. Therefore, the appearance of crystal materials usually presents a regular geometric shape.
Silicon Carbide Single Crystal Substrate Material (hereinafter referred to as SiC Substrate) is also a kind of crystalline materials. It belongs to wide bandgap semiconductor material, and has the advantages of high voltage resistance, high temperature resistance, high frequency, low loss, etc. It is a basic material for preparing high-power electronic devices and microwave RF devices.
SiC is a IV-IV compound semiconductor material composed of Carbon and Silicon in a stoichiometric ratio of 1:1, and its hardness is second only to diamond.
Both Carbon and Silicon atoms have 4 valence electrons, which can form 4 covalent bonds. The basic structural unit of SiC crystal, SiC tetrahedron, arises out of the tetrahedral bonding between Silicon and Carbon atoms. The coordination number of both Silicon and Carbon atoms is 4, i.e. each Carbon atom has 4 Silicon atoms around it and each Silicon atom also has 4 Carbon atoms around it.
As a crystal material, SiC Substrate also has the characteristic of periodic stacking of atomic layers. The Si-C diatomic layers are stacked along [0001] direction.Due to the small difference in bond energy between layers, different connection modes are easily generated between atomic layers, leading to over 200 SiC polytypes. Common polytypes include 2H-SiC, 3C-SiC, 4H-SiC, 6H-SiC, 15R-SiC, etc. Among them, the stacking sequence in the order of "ABCB" is called the 4H polytype. Although different polytypes of SiC have the same chemical composition, their physical properties, especially the bandgap width, carrier mobility and other characteristics are quite different. And the properties of 4H polytype are more suitable for semiconductor applications.
The growth parameters such as temperature and pressure significantly influence the stability of 4H-SiC during growth process. Therefore, in order to obtain the single crystal material with high quality and uniformity, the parameters such as growth temperature, growth pressure and growth rate must be precisely controlled during the preparation.
At present, the preparation methods of Silicon Carbide are Physical Vapor Transport Method (PVT)，High Temperature Chemical Vapor Deposition Method (HTCVD), and Liquid Phase Method (LPE). And PVT is a mainstream method that is suitable for industrial mass production.
During PVT growth, SiC seed crystal is placed on the top of crucible while the source material (SiC powder) is placed in the bottom. In an enclosed environment with high temperature and low pressure, SiC powder sublimates, and then transports upward to the space near the seed under the effect of temperature gradient and concentration difference. And it will recrystallize after reaching the supersaturated state. Through this method, the size and polytype of SiC crystal can be controlled.
However, the PVT method requires maintaining appropriate growth conditions throughout the entire growth process, otherwise it will lead to lattice disorder and form undesired defects. Besides, the SiC crystal growth is completed in an enclosed space with limited monitoring methods and many variables, thus the control of the process is difficult.
In the process of growing SiC crystal by PVT method, step flow growth is considered as the main mechanism to form single crystals. The vaporized Si and C atoms will preferentially bond with the atoms on the crystal surface at steps and kinks, where they will nucleate and grow, so that each step flows forward in parallel. When the width between each step on the growth surface is far greater than the diffusion free path of the adsorbed atoms, a large number of adsorbed atoms may agglomerate, and form the two-dimensional island, which will destroy the step flow growth mode, resulting in the formation of other polytypes instead of 4H. Therefore, the adjustment of process parameters aims to control the step structure on the growth surface, so as to prevent the formation of undesired polytypes, and achieve the goal of obtaining 4H single crystal structure, and finally preparing high-quality crystals.
The growth of the crystal is just the first step to prepare high quality SiC Substrate. Before being used, 4H-SiC ingot needs to go through a series of processes such as slicing, lapping, beveling, polishing, cleaning and inspecting. As a hard but brittle material, SiC single crystal also has high technical requirements for the wafering steps. Any damage generated in each process may have certain heredity, transfers to the next process and finally affect the product quality. Therefore, the efficient wafering technology for SiC Substrate also attracts the attention of the industry.
