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LED epitaxial wafer introduction and identify quality method - (AddTime - 013-01-26)

Epitaxial wafer production process is very complicated, show the epitaxial wafer, then in each epitaxial wafer random extraction nine to do the test, and to meet the requirements of good quality, the other is for the defective products (voltage deviation is very big, wavelength partial short or partial long, etc.). Good article of epitaxial wafer will begin to do electrode (P pole, N pole), then use laser cutting epitaxial wafer, and then, according to the different working voltage, wavelength, intensity for automatic HuaFen inspection, also is the formation of LED chips (square piece). Then for visual inspection, a little defects or electrode wear and tear, sorting out, these are the back of the bulk crystal. At this time in blue membrane have does not conform to the normal delivery requirements of the wafer, it naturally became the edge piece or rushes, etc. Defective products of epitaxial wafer (mainly some parameters not meet the requirements), don't have to do the square piece, directly do electrode (P pole, N pole), also don't do sorting, is currently on the market led wafer (this is also a good thing, such as square piece, etc.).

Semiconductor manufacturers mainly use polishing Si tablet (PW) and epitaxial Si piece as IC raw materials. In the early 1980 s began to use epitaxial wafer, it has standard PW don't have some of the electrical characteristics and eliminate a lot of in the crystal growth and subsequent wafer processing introduced by the surface/near surface defects.

Historically, epitaxial wafer is by Si tablet manufacturers and private, in the IC dosage is not big, it needs to be in single crystal Si wafer surface deposition on a thin single crystal Si layer. General epitaxial layer thickness for 2 ~ and μ m, and substrate Si thickness for 610 μ m (150 mm diameter tablet and 725 μ m (200 mm tablet).

Epitaxial deposit can be (and) a processing chip, can also be processed single chip. Monolithic reactors can produce the best quality epitaxial layer (thickness, resistivity uniformity, good less vulnerable); This kind of epitaxial wafer used for 150 mm "frontier" products and all the important 200 mm in the production of the products.

Augmented product
Augmented product applied in four aspects, CMOS complementary metal oxide semiconductor support for small device size in the forefront of the technology. CMOS product is epitaxial wafer the biggest application fields, and is used for IC manufacturers do not recovery device process, including microprocessor and logic chip and storage application of flash memory and DRAM (dynamic random access memory). Discrete semiconductor used in the manufacture of requirements with precision Si characteristics of the element. "Strange" (exotic) semiconductor class contains some special products, they want to use non Si materials, many of them want to use compound semiconductor materials into the epitaxial layer. Buried layer semiconductor using bipolar transistor components inside heavily doped region physical isolation, this is also in the extension process of deposition. At present, 200 mm chip, epitaxial wafer accounted for a third. In 2000, including buried layer, and used in the CMOS logic devices accounted for 69% of all epitaxial wafer, DRAM 11%, discrete device accounted for 20%. By 2005, CMOS logic will account for 55%, DRAM 30%, discrete device accounted for 15%. LED epitaxial wafer - substrate materials

Substrate material is semiconductor 200W led Flood light industry the cornerstone of the development of the technology. Different substrate materials, need different epitaxial growth technology, chip processing technology and device packaging technology, substrate material determines the semiconductor 200W led Flood light technology development route. Substrate material choice depends mainly on the following nine aspects:

[1] structure characteristic, epitaxial material and the substrate of the same or similar crystal structure, lattice constant mismatch is small, crystallization performance is good, defect density is small;
[2] interface properties is good, be helpful for epitaxial material nucleation and adhesion property is strong; [3], chemistry stability is good, in the epitaxial growth temperature and atmosphere it is not easy to break down and corrosion;
[4] thermal performance is good, including thermal conductivity and thermal mismatch small;
[5] conductivity is good, can be made into fluctuation structure;
[6] optical performance is good, making devices from light to be substrate absorb small;
[7] mechanical performance is good, easy processing devices, including reduced thin, polishing and cutting, etc.;
[8] low price;
[9] big size, general requirements diameter not less than 2 inches. Substrate should be chosen at the same time satisfy the above nine aspects is very difficult. So, at present only through epitaxial growth technology changes and device process adjustment to adapt to different substrate of semiconductor luminescence device research and development and the production. Used for gallium nitride research substrate material is more, but can be used in the production of substrate at present only three kinds, namely sapphire Al2O3 and silicon carbide SiC substrate and Si substrate. Evaluation of substrate material must be comprehensive consideration of the following factors:

1. The substrate and denotation membrane structure matching: epitaxial material and the substrate material of the same or similar crystal structure, lattice constant mismatch is small, crystallization performance is good, defect density is low;
2. Substrate and epitaxial film coefficient of thermal expansion matching: thermal expansion coefficient of the match is very important, epitaxial film and substrate materials in the coefficient of thermal expansion are not only too may make epitaxial film quality decline, will be in the working process of the device, the fever caused by the damage of the device; 3. Substrate and epitaxial film chemical stability matching: substrate material to have a good chemical stability, in the epitaxial growth temperature and atmosphere not easy decomposition and corrosion, not because of the membrane and denotation chemical reaction to the decrease of the quality of the epitaxial film; 4. The material preparation difficulty and the cost of the high and low: considering the needs of the development of industrialization, the preparation of substrate material for concise, cost should not be high. Substrate size generally not less than 2 inches. The current used in GaN base LED the substrate material is more, but can be used to the commercialization of substrate at present only three kinds, namely sapphire and silicon carbide and silicon substrate. Other such as GaN, ZnO substrate is still in the development stage, there is a distance from industrialization.

Gallium nitride:
GaN growth for the most ideal substrate is GaN single crystal materials, can greatly improve the epitaxial film crystal quality and reduce the dislocation density, improve the device working life, improve the luminous efficiency, improve the device working current density. But preparation GaN body single crystal is very difficult, so far there are no effective measures.

Zinc oxide:
ZnO can become GaN epitaxial candidate substrate, because both have very striking similarities. Both the same crystal structure, lattice recognition is very small, the forbidden band width close (band discrete value is small, the contact barrier small). However, ZnO as GaN epitaxial substrate fatal weakness is in the GaN epitaxial growth temperature and atmosphere easy decomposition and corrosion. At present, the ZnO semiconductor material is not used to make optoelectronic devices or high temperature electronic devices, mainly is the material quality can not reach the device level and P type doping problems are not really solve, suitable for the growth of ZnO base semiconductor materials equipment has not been successfully developed.

GaN growth for the most common substrate is Al2O3. Its advantage is, chemistry stability is good, do not absorb the visible light, moderate price, manufacturing technology relatively mature. Thermal conductivity difference though in device small current work no exposure is not enough, but in power type device large current problems under the work is very outstanding.

Silicon carbide:
SiC as substrate material applications of degree after sapphire, at present China's crystal can photoelectric JiangFengYi professor in the Si substrate on growth out can be used to commercial LED epitaxial wafer. Si substrate in the thermal conductivity and stability is superior to sapphire, the price is far below the sapphire, is a very promising substrate. SiC substrate chemical stability, conductive performance is good, good thermal conductivity, do not absorb the visible light, etc., but also very outstanding lack, such as the price is too high, it is difficult to achieve the quality of crystal Al2O3 and Si so good, mechanical processing performance is poor, in addition, SiC substrate absorb ultraviolet light under 380 nm, not suitable for used to research the following uv LED 380 nm. Due to the SiC substrate good conductive properties and thermal conductivity, which can solve power type GaN LED device heat dissipation problems, therefore, in the semiconductor 200W led Flood light technology, which occupies an important position.

Compared with sapphire, SiC and GaN epitaxial film lattice matching improved. In addition, SiC has blue luminescence characteristics, but also for low resistance materials, can make electrode, make the device in the packaging before epitaxial film completely test possible, enhance the SiC as substrate material competitiveness. Due to the layer structure of SiC to cleavage, substrate and denotation between membrane can get high quality cleavage plane, this will greatly simplify the structure of the device; But at the same time because of its layer structure, in the substrate surface often give epitaxial film into a large number of defects in steps.

To realize the goal of luminous efficiency to hope that GaN substrate LED, and realize low cost, also want to through the GaN substrate lead to high efficiency, large area, single lamp power realization, and drive technology to simplify the yield and greatly improved. Semiconductor 200W led Flood light once become a reality, its significance as Edison's invention of the incandescent lamp. Once the substrate, and other key technology breakthroughs, the industrialization process will be achieved rapid development.