The production process of the epitaxial wafer is very complicated, the extension of the extension film, the next in each epitaxial film free to take nine points to do the test, meet the requirements is good, the other for the defective product (the voltage deviation is large, the wavelength is short or Long, etc.). (P pole, N pole), then use the laser cutting epitaxial wafers, and then percentage percent picking, according to the different voltage, wavelength, brightness, fully automated sorting, that is, the formation of the electrode LED chip (square). And then also to visual, to have a little flaw or electrode wear, sort out, these are the back of the scattered crystal. At this time in the blue film does not meet the normal requirements of the wafer, also naturally become a side film or hair pieces. Bad pieces of the wafer (mainly some parameters do not meet the requirements), do not have to do square film, directly to the electrode (P pole, N pole), do not do the seizure, which is currently on the market LED large round (There are also good things inside, such as square films, etc.).
Semiconductor manufacturers mainly use polished Si (PW) and epitaxial Si as IC raw materials. The use of epitaxial wafers began in the early 1980s, with some electrical properties that the standard PW did not have and eliminated many of the surface / near surface defects introduced during crystal growth and subsequent wafer fabrication.
Historically, epitaxial wafers are manufactured and used by Si chip manufacturers and are of little use in ICs. They require the deposition of a thin monocrystalline Si layer on the surface of monocrystalline Si wafers. The thickness of the general epitaxial layer is 2 to 20 m, and the thickness of the substrate Si is 610 m (150 mm diameter sheet and 725 m (200 mm sheet).
Epitaxial deposition can be (at the same time) a multi-chip processing, but also processing monolithic. Monolithic reactors produce the best quality epitaxial layer (thickness, good resistivity uniformity and less defects); this epitaxial wafer is used for 150mm "leading" products and all important 200mm products.
Epitaxial products
Epitaxial products are used in four areas, CMOS complementary metal oxide semiconductor support for small device size requirements of the cutting-edge technology. CMOS products are the largest application area for epitaxial wafers and are used by IC manufacturers for non-recoverable device processes, including flash memory and DRAM (dynamic random access memory) for microprocessors and logic chips and memory applications. Discrete semiconductors are used to fabricate components that require precision Si characteristics. "Exotic" (semiconductor) contains some special products, they use non-Si materials, many of which use compound semiconductor materials into the epitaxial layer. The buried layer semiconductor utilizes bipolar transistor elements within the heavily doped region for physical isolation, which is also deposited in epitaxial processing.
At present, the 200mm wafer, the epitaxial wafer accounted for 1 / 3. In 2000, including the buried layer, for logic devices CMOS accounted for 69% of all epitaxial wafers, DRAM accounted for 11%, discrete devices accounted for 20% .2005, Logic will account for 55%, DRAM accounted for 30%, discrete devices accounted for 15%.
LED epitaxial wafers - substrate materials
Substrate material is the cornerstone of the semiconductor lighting industry technology development. Different substrate materials, the need for different epitaxial growth technology, chip processing technology and device packaging technology, the substrate material determines the development of semiconductor lighting technology. The choice of substrate material depends mainly on the following nine aspects:
1, the structural characteristics are good, the epitaxial material and the substrate crystal structure of the same or similar, lattice constant mismatch degree is small, good crystallinity, defect density is small;
2, the interface characteristics are good, is conducive to epitaxial material nucleation and strong adhesion;
3, good chemical stability, in the epitaxial growth of the temperature and atmosphere is not easy to break down and corrosion;
4, good thermal performance, including good thermal conductivity and thermal resistance is small;
5, good conductivity, can be made up and down structure;
6, optical performance is good, the device produced by the light emitted by the substrate is small;
7, good mechanical properties, easy processing of the device, including thinning, polishing and cutting;
8, low prices;
9, large size, the general requirements of not less than 2 inches in diameter.
The choice of the substrate to meet the above nine aspects is very difficult. Therefore, at present only through the epitaxial growth technology changes and device processing technology to adapt to different substrates on the semiconductor light-emitting device research and development and production. There are many substrates for gallium nitride, but there are only three substrates that can be used for production, namely sapphire Al2O3 and silicon carbide SiC substrates and Si substrates.
Evaluation of the substrate material must take into account the following factors:
1. The structure of the substrate and the epitaxial film match: epitaxial material and the substrate material crystal structure of the same or similar, lattice constant mismatch small, good crystallinity, defect density is low;
2. The thermal expansion coefficient of the substrate and the epitaxial film match: the thermal expansion coefficient of the match is very important, epitaxial film and the substrate material in the thermal expansion coefficient difference is not only possible to reduce the quality of epitaxial film, but also in the device work process, Resulting in damage to the device;
3. Substrate and epitaxial film chemical stability of the match: the substrate material to have good chemical stability, in the epitaxial growth temperature and atmosphere is not easy to break down and corrosion, not because of the chemical reaction with the epitaxial film to reduce the quality of epitaxial ;
4. The difficulty of the preparation of materials and the level of cost: taking into account the needs of industrial development, substrate material preparation requirements simple, the cost should not be high. The substrate size is generally not less than 2 inches.
LED substrate material summary
There are currently more substrates for GaN-based LEDs, but there are currently only three substrates that can be used for commercialization, namely sapphire and silicon carbide and silicon substrates. Other such as GaN, ZnO substrate is still in the development stage, there is still some distance from the industrialization.
Gallium nitride
The ideal substrate for GaN growth is GaN single crystal material, which can greatly improve the crystal quality of epitaxial film, reduce the dislocation density, improve the working life of the device, improve the luminous efficiency and improve the device working current density. However, the preparation of GaN single crystal is very difficult, so far there is no effective way.
Zinc oxide
ZnO has been able to become GaN epitaxial candidate substrate, because the two have a very striking resemblance. Both crystal structures are the same, the lattice recognition is very small, the forbidden band width is close (band with discontinuous value is small, contact barrier is small). However, the fatal weakness of ZnO as a GaN epitaxial substrate is easy to decompose and corrode at the temperature and atmosphere of GaN epitaxial growth. At present, ZnO semiconductor materials can not be used to manufacture optoelectronic devices or high-temperature electronic devices, mainly the quality of the material does not reach the device level and P-type doping problems have not been truly resolved, suitable for ZnO-based semiconductor material growth equipment has not yet developed successfully.
Sapphire
The most common substrate for GaN growth is Al2O3, which has the advantage of good chemical stability, no absorption of visible light, moderate price, and relatively mature manufacturing technology. Poor thermal conductivity Although the device is not exposed in the small current work is not obvious enough, but in the power of high-current device under the work of the problem is very prominent.
Silicon carbide
SiC as a substrate material after the application of a wide range of sapphire, the current China's crystal photoelectric Professor Jiang Fengyi Si substrate in the growth can be used to commercialize the LED epitaxial wafers. Si substrate in thermal conductivity, stability is better than sapphire, the price is far lower than the sapphire, is a very promising substrate. SiC substrate has good chemical stability, good electrical conductivity, good thermal conductivity, do not absorb visible light, but the lack of aspects is also very prominent, such as the price is too high, the crystal quality is difficult to achieve Al2O3 and Si so good, mechanical processing performance is poor, In addition, SiC substrate absorbs ultraviolet light below 380 nm, which is not suitable for the development of UV LED below 380 nm.As a result of the beneficial conductivity and thermal conductivity of SiC substrate, it can solve the heat dissipation problem of power GaNLED device, So in the field of semiconductor lighting technology occupies an important position.
Compared with sapphire, SiC and GaN epitaxial film lattice matching is improved. In addition, SiC has a blue luminescent properties, and a low resistance material, can make electrodes, so that the device before the packaging of the epitaxial film is fully tested to enhance the SiC as a substrate material competitiveness. Since the layered structure of SiC is easily cleaved, a high quality cleavage surface can be obtained between the substrate and the epitaxial film, which greatly simplifies the structure of the device; but at the same time, due to its layered structure, The epitaxial film introduces a large number of defective steps.
The goal of achieving luminous efficiency is to hope for the GaN of the GaN substrate, to achieve low cost, but also through the GaN substrate to lead to efficient, large area, single lamp high power to achieve, as well as driven technology simplification and yield improve. Once the semiconductor lighting has become a reality, its significance as much as Edison invented incandescent. Once in the substrate and other key technology areas to achieve a breakthrough, its industrialization process will be made rapid development.