In the whole semiconductor industry chain, semiconductor materials are in the upstream, various semiconductor components are in the midstream, and downstream applications include consumer electronics, communication, new energy, electric power, transportation and other industries. With the second and third generation compound semiconductors achieving wider applications with their unique physical properties, their upstream substrate materials GaAs, SiC and GaN are gaining more and more attention in China, and this research report will focus on these three types of substrate materials.
I. Overview of compound semiconductor materials
Compound semiconductor refers to the semiconductor materials formed by two or more elements, which can be divided into binary compound, ternary compound and quaternary compound according to the number of elements, and binary compound semiconductor can be divided into III-V group, IV-IV group and II-VI group according to the position of the constituent elements in the periodic table of chemical elements. At present, compound semiconductor materials represented by gallium arsenide (GaAs), gallium nitride (GaN) and silicon carbide (SiC) have become the fastest growing, most widely used and most productive semiconductor materials after silicon.
(A) Development stages
The field of semiconductor substrate materials has gone through three stages of development.
The first stage is from the 1950s, the first generation of semiconductor materials made of silicon Si as the representative of the diode and transistor to replace the electronic tube, mainly used in low-voltage, low-frequency, low-power transistors and detectors, such as computer CPU, GPU, memory, cell phones SoC and other devices, triggering the rapid development of the microelectronics industry with integrated circuits as the core. However, the physical properties of silicon materials limit its application in optoelectronic and high-frequency electronic devices, such as its indirect bandgap characteristics determine that it cannot obtain high electro-optical conversion efficiency, and its narrow bandgap width (1.12 eV) and low saturation electron mobility (1450 cm2/V-s) are not conducive to the development of high-frequency and high-power electronic devices.
The second stage is the beginning of the 1990s, with the development of the semiconductor industry, the physical bottleneck of silicon materials is becoming increasingly prominent, the second generation of semiconductor materials represented by GaAs and InP as GaAs, and the related device preparation technology gradually matured, so that semiconductor materials into the field of optoelectronics. good optical properties of GaAs make it widely used in optical devices, also applied in the need for High-speed devices for special occasions, is the material of most communication devices in the 4G era, such as millimeter wave devices, light-emitting devices, satellite communications, mobile communications, optical communications, GPS navigation, etc.. However, the forbidden band width (the forbidden band width reflects the degree of valence electrons being bound strong or weak, which directly determines the withstand voltage and maximum operating temperature of the device) is not large enough and the breakdown electric field is low, which limits its application in the field of high temperature, high frequency and high power devices, and arsenic is toxic.
The third stage is that in recent years, the third generation semiconductor materials represented by SiC and GaN have significant advantages in terms of forbidden band width, breakdown electric field strength, saturation electron drift rate, thermal conductivity, and radiation resistance and other key parameters, further meeting the needs of modern industry for high power, high voltage, and high frequency, as the main materials in the 5G era for high temperature, high frequency, radiation resistant, and High power devices; blue, green, violet diodes, semiconductor lasers, etc.
(B) Material properties
(C) Main applications
Currently more than 95% of the world's chips and devices are based on silicon as the substrate material, due to the great cost advantage of silicon materials, the future in various types of discrete devices and integrated circuits, silicon will still dominate, but the unique physical properties of compound semiconductor materials advantage, giving it a unique performance advantage in the field of radio frequency, optoelectronics, power devices, etc.
II,gallium arsenide (GaAs) - the second generation of semiconductor materials
(A) Material types
According to different resistances, GaAs materials can be divided into semiconductor type and semi-insulating type. Semi-insulated GaAs substrates can be used in MESFET, HEMT and HBT circuits due to their high resistivity and high frequency performance. They are mainly used in radar, satellite TV broadcasting, microwave and millimeter wave communication, wireless communication (represented by cell phone) and fiber optic communication, etc. They are mainly used to make PA components in cell phone, and occupy 85% of the market share in high frequency power amplifier market. Semiconductor-type GaAs single crystals account for about 60% of the GaAs market and are mainly used in LEDs and VCSELs (vertical resonant cavity surface emitting lasers) and other optoelectronic devices.
(B) Production process
Gallium arsenide single crystal wafer production process can be divided into
1、Polycrystal cleaning: GaAs polycrystals are put into a mixture of ammonia, hydrogen peroxide and pure water and cleaned with water in a cleaning tank; the surface impurities are removed by vibrating and washing with an ultrasonic flat oscillator, and then dehydrated with methanol; PBN crucible cleaning is the same as polycrystal cleaning.
2、Single crystal growth: After cleaning, put the cleaned GaAs polycrystal into PBN crucible, put the crucible into quartz tube, then use vacuum pump to vacuum the quartz tube, seal it and wrap quartz cotton outside (insulation) and put it into single crystal furnace, so that the crystal will finish growing in single crystal furnace and grow into single crystal bar.
3、Demoulding: After the single crystal growth is finished, the single crystal furnace will be cooled down to room temperature, and then the quartz tube will be cut open with a tube saw to separate the PBN crucible and GaAs crystal, and then the GaAs crystal will be taken out.
4、Crystal processing: The removed GaAs crystals are excised from the end cap with band saw, the outer circle is ground with external grinder, the test sample is taken with internal circular saw, and the quality of the crystal is judged according to the test sample.
5、Crystal slicing: The GaAs crystals are cut into wafers of certain thickness on the multi-wire cutting machine, and the water-based solution and cutting powder are used to cool down the treatment when cutting. After cutting, the wafers are rinsed, soaked in alcohol and then air-dried.
6、Wafer grinding: The wafer surface in the cleaning tank is pre-cleaned with a mixture of ammonia, hydrogen peroxide and pure water to clean the impurity particles on the wafer surface and make the surface cleaner; then the wafer is ground with a grinding machine to remove the damage layer of the wafer and ensure the thickness consistency.
7、Wafer polishing: After grinding, the wafer is put into the polishing machine and polished wet under the action of polishing liquid to make the surface reach a fine mirror surface, followed by surface cleaning in the cleaning tank with a mixture of ammonia, hydrogen peroxide and pure water, and then dewatered and dried with a drying machine.
8、Wafer cleaning: The wafer is cleaned with a mixture of ammonia, hydrogen peroxide and pure water to remove dust and chemical residues left on the surface of the wafer after processing in the previous process.
(C) Single crystal growth process
Since the 1950s, several GaAs single crystal growth methods have been developed. At present, the mainstream industrial growth processes include liquid sealed direct pulling (LEC), horizontal Bridgman method (HB), vertical Bridgman method (VB) and vertical gradient solidification (VGF).
(D) Global competition pattern
Compound semiconductors are mainly in foundry mode because of the overall small scale and high non-standardization of the industry. Europe and the United States dominate the GaAs industry chain, and Chinese and Taiwanese manufacturers monopolize the foundry. Sumitomo of Japan, Freiberger of Germany and AXT of the United States together account for 90% of the global market share of semi-insulated substrates. Due to the limitation of substrate size, the current production lines are mainly 4-inch and 6-inch wafers, and some companies have started to import 8-inch production lines, but they have not yet formed the mainstream. Since GaAs is mainly Emitterbase-Collector vertical structure, the number of transistors is only in the order of hundred, while silicon wafers are Source Gate Drain planar design, the number of transistors reaches tens of millions, so GaAs does not have as obvious advantage in process development as silicon wafer foundry industry.
Sumitomo is the highest level of semi-insulated GaAs monolithic wafers in the world, mainly producing GaAs by VB method, capable of mass production of 4" and 6" monolithic wafers; Germany Freiberger mainly produces 2 to 6" GaAs substrates by VGF and LEC method, and all products are used in microelectronics; half of American AXT products are used in LED and half are used as microelectronics substrates. Domestic suppliers mainly use GaAs substrates for LED chips, and a few companies such as Yunnan Germanium use GaAs substrates for RF gradually.
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