With the development of the first generation of silicon semiconductors and the second generation of gallium arsenide semiconductor materials, their device applications are also tending to the limit. More and more fields of modern science and technology require materials with high working frequency, high power density, high temperature resistance, good chemical stability and can work in strong radiation environments, so the third generation of semiconductors (i.e., wide bandgap semiconductors, bandgap width greater than 2.2eV) has received great attention, these materials include SiC, AlN, GaN, ZnO, diamond, etc., among which the most mature technology is silicon carbide (SiC).
Among the three key links (substrates, epitaxy and devices) in the silicon carbide (SiC) semiconductor industry, substrates are the core of the silicon carbide industry chain, accounting for the highest value in the industrial chain, close to 50%. The development of the substrate industry is also the main driving force for cost reduction and large-scale industrialization of the silicon carbide industry in the future.
Crystal structure of SiC
SiC single crystal is an IV.-IV compound semiconductor material composed of two elements, Si and C, according to a stoichiometric ratio of 1:1, and its hardness is second only to diamond. Both C atom and Si atom are 4 valence electrons that can form 4 covalent bonds to form the basic building blocks of SiC – the Si-C tetrahedron, with 4 Si atoms around each C atom and 4 C atoms around each Si atom.
As a crystalline material, SiC substrate also has the property of periodic stacking of atomic layers. The Si-C diatomic layer is stacked in the direction of [0001], and due to the small difference in bond energy between the layers, it is easy to produce different connection methods between the atomic layers, which leads to a large variety of crystal forms in SiC. Common crystal forms include 2H-SiC, 3C-SiC, 4H-SiC, 6H-SiC, 15R-SiC, etc., among which, the structure stacked in the order of "ABCB" is called 4H crystal form. Although SiC crystals with different crystal forms have the same chemical composition, their physical properties, especially the bandgap width and carrier mobility, are quite different. Among them, the performance of 4H crystal form in all aspects is more suitable for applications in the semiconductor field.
A variety of factors such as growth temperature and pressure will affect the crystalline stability of SiC substrates, so in order to obtain high-quality and crystalline single-crystal materials, it is necessary to accurately control various process parameters such as growth temperature, growth pressure, and growth rate during the preparation process.
4H-SiC 衬底的加工步骤
1. Crystal plane orientation
Ingot orientation is used by X-ray diffraction, and when an X-ray beam is incident on the crystal plane to be oriented, the angle of the diffracted beam is used to determine the orientation of the crystal plane.
2. Cylindrical grinding
The diameter of the single crystal grown in the graphite crucible is larger than the standard size, which is reduced to the standard size by cylindrical tumbling.
3. End face grinding
4-inch 4H-SiC substrates generally have two locating edges, the primary locating edge and the secondary locating edge, which are ground out by the end face.
4. Line cutting
Wire cutting is one of the more important processes in the processing of 4H-SiC substrates. Crack damage and residual subsurface damage caused by the wire cutting process will adversely affect the subsequent process, on the one hand, it will prolong the time required for the subsequent process, and on the other hand, it will cause the loss of the wafer itself. At present, the most commonly used silicon carbide wire cutting process is reciprocating diamond consolidated abrasive multi-wire cutting. 4H-SiC ingots are cut primarily by the reciprocating motion of metal wires consolidated with diamond abrasives. The thickness of the wafer is about 500 μm, and there are a large number of wire scratches and deep subsurface damage on the wafer surface.
5. Chamfering
In order to prevent chipping cracks and other phenomena on the edge of the wafer during subsequent processing, and to reduce the loss of grinding pads and polishing pads in the subsequent process, it is necessary to grind the edge of the wafer with sharp edge after wire cutting into the specified shape.
6. Downgauging
The wire cutting process of 4H-SiC ingots leaves a large number of scratches and sub-surface damage on the surface of the wafer, and the main purpose of thinning is to remove these scratches and damage as much as possible by using diamond grinding wheel feeding.
7, Polishing
The grinding process is divided into coarse grinding and fine grinding, and the specific process is similar to thinning, but using boron carbide or diamond abrasives with smaller particle sizes, lower removal rates, mainly to remove damage that cannot be removed in the downgauging process and newly introduced damage.
8. Polishing
Polishing is the final step in the processing of 4H-SiC substrates, which is also divided into rough and fine polishing. The surface of the wafer is subjected to a soft oxide layer that is removed by the mechanical action of alumina or silicon oxide abrasive grains. After the completion of this process, the surface of the substrate is virtually free of scratches and sub-surface damage, and has extremely low surface roughness, which is the key process to achieve ultra-smooth and damage-free surface of 4H-SiC substrate.
9. Cleaning
Removes contaminants such as particles, metals, oxide films, and organics left over from the process.
Classification of SiC substrates
From the perspective of electrochemical property differences, silicon carbide substrate materials can be divided into conductive substrates (resistivity range 15~30mΩ·cm) and semi-insulating substrates (resistivity higher than 105Ω·cm).
● Semi-insulating silicon carbide substrates are mainly used in the manufacture of radio frequency devices, optoelectronic devices, etc.
● Conductive silicon carbide substrates are mainly used in the manufacture of Schottky diodes, MOSFETs, IGBTs and other power devices.
Production process of silicon carbide single crystal substrates
1. Preparation of raw materials
The physical vapor phase transfer method (PVT) requires Si and C to be synthesized into SiC polycrystalline granular powder at a ratio of 1:1, and its particle size and purity will directly affect the crystal quality, especially the semi-insulating substrate, which has extremely high purity requirements for the powder (impurity content less than 0.5ppm).
2. Seed crystals
Silicon carbide seed crystal is the substrate for crystal growth, providing the basic lattice structure for crystal growth, and also the core raw material that determines the crystal quality. The seed crystals are located inside the reactor or above the feedstock.
3. Crystal growth
SiC晶体生长是SiC衬底生产的核心工艺，核心难点在于提升良率。 目前SiC晶体的生长方法主要有物理气相传输法(Physical Vapor Transport Method, PVT法)、高温化学气相沉积法(High Temperature Chemical Vapor Deposition, HTCVD法)、液相法(Liquid Phase Epitaxy)等。
Physical Gas Transport (PVT)
Physical vapor phase transfer (PVT) consists of three main steps: sublimation of the SiC source, transport of the sublimated substance, surface reaction, and crystallization.
● When the crystal grows, by changing the size and shape of the heat dissipation hole of the insulation material on the graphite crucible, a temperature gradient of 15-35°C/cm is formed in the growth chamber, the SiC raw material is in the high temperature zone, the seed crystal is in the low temperature zone, and the inert gas with a pressure of 50-5000Pa will be retained in the furnace to increase the convection;
● Then the temperature in the crucible is raised to 2000-2500°C by induction heating or resistance heating, and the gas phase Si2C, SiC2 and Si produced by the sublimation of SiC raw materials are transported from the surface of the raw material to the low-temperature seed crystals under the action of temperature gradient and crystallized into bulk crystals.
This method has low requirements for growth equipment, simple process, strong controllability, relatively mature technology development, and China has begun to gradually realize the mass production of 8-inch substrates.
However, it is difficult to prepare P-type substrates by PVT method, and there are two intrinsic characteristics that are not pure enough:
●The raw material is silicon carbide solid, so the purity is not easy to control;
●In the process of converting powder into gas, a variety of gases can be generated.
High Temperature Chemical Vapor Deposition (HTCVD)
● The high-temperature vapor deposition method uses the principle of electromagnetic coupling;
● During growth, the growth chamber is heated to 1800°C-2300°C through the induction coil;
● SiH4+C3H8 or SiH4+C2H4 gas is stably introduced into the growth chamber, and transported upward to the direction of seed crystals carried by He and H2, providing Si sources and C sources for crystal growth, and realizing the growth of SiC crystals at the seed crystals;
● The temperature at the seed crystal is lower than the evaporation point of SiC, so that the gas-phase silicon carbide can be condensed on the lower surface of the seed crystal to obtain pure silicon carbide ingots.
The HTCVD method can achieve a more accurate Si/C ratio by controlling the proportion of input gas from the source, and then obtain high-quality and high-purity silicon carbide crystals, but due to the high cost of crystal growth of gas as a raw material, this method is mainly used for the growth of semi-insulating crystals.
Liquid Phase (LPE)
Liquid phase SiC crystal production process:
● The liquid phase method uses graphite crucible to provide C source for crystal growth;
● The temperature at the crucible wall is high, and a large amount of C will be dissolved to form a saturated solution of C;
● The saturated solution is transported to the bottom of the seed crystals with the convection in the co-solvent;
● The temperature of the seed end is low, and the solubility of C decreases accordingly, forming a supersaturated solution of C;
● The supersaturated C in the solution binds to the Si in the co-solvent to grow SiC crystals on the seed crystal;
● After the C of the supersaturated part is precipitated, the solution returns to the high temperature end of the wall with convection, and dissolves the C again to form a saturated solution.
The liquid phase method has relatively low growth temperature, high crystallization quality, fast growth rate, easy to grow thick, easy to expand, and p-type low-resistance substrates can be obtained. At present, 4-6 inch silicon carbide crystals can be produced by liquid phase method in China. With the advantages of energy saving and cost reduction, the liquid phase method may be further industrialized in the future.
4. Ingot processing
The prepared silicon carbide ingots are oriented using an X-ray single crystal orientation instrument, and then ground and rounded, the seed crystal plane is removed, the dome surface is removed, and the silicon carbide crystal is processed into a standard diameter size.
5. Crystal cutting
The growing crystals are cut into flakes, and because the hardness of silicon carbide is second only to diamond, it is a highly hard and brittle material, so the cutting process takes a long time and is easy to crack. The cutting methods mainly include mortar wire cutting, diamond wire multi-wire cutting and laser irradiation stripping.
6. Wafer grinding and polishing
Grinding and polishing is to process the surface of the substrate to an atomic-level smooth plane, and the surface state of the substrate, such as surface roughness and thickness uniformity, will directly affect the quality of the epitaxial process. Silicon carbide has the characteristics of high hardness, and the commonly used abrasives suitable for silicon carbide are boron carbide, diamond and other high-hardness abrasives. Polishing materials generally include alumina, cerium oxide, silicon oxide, etc.
7. Wafer cleaning detection
This step is used to remove residual particulate matter and metal impurities during processing, and the final inspection can obtain comprehensive quality information such as substrate surface, surface shape, crystal quality, etc., to help downstream processes trace.
Wafer inspection includes:
● Crystal integrity: microtubule density, crystallization quality, hexagonal voids and cracks, dislocation density, polymorphism;
● Crystal form: Determination of crystal form;
● Impurities: body impurity content;
● Electrical test: resistivity.
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