The production process of epitaxial wafers is very complicated. After the epitaxial wafers are exhibited, nine points are randomly selected from each epitaxial wafer for testing. Those that meet the requirements are good products, others are defective products (large voltage deviation, short wavelength or Longer, etc.). Good quality epitaxial wafers will start to be electrodes (P pole, N pole), then cut the epitaxial wafer with laser, and then pick it up 100%, according to different voltages, wavelengths, brightness, fully automated sorting, which is formed LED chip (square piece). Then, visual inspection is needed to sort out any defects or worn electrodes. These are the scattered crystals behind. At this time, there is a wafer on the blue film that does not meet the normal shipping requirements, and it naturally becomes an edge piece or a wool piece. Defective epitaxial wafers (mainly because some parameters do not meet the requirements), they are not used for square wafers, they are directly used as electrodes (P-pole, N-pole), and no sorting is done, which is the LED circle on the market. Film (there are also good things in it, such as square films, etc.).
Semiconductor manufacturers mainly use polished Si wafers (PW) and epitaxial Si wafers as raw materials for ICs. In the early 1980s, epitaxial wafers were used, which had certain electrical characteristics not found in standard PWs and eliminated many surface / near-surface defects introduced in crystal growth and subsequent wafer processing.
Historically, epitaxial wafers have been produced and used by Si wafer manufacturers for their own use. They are not used in large amounts in ICs. It requires the deposition of a thin single crystal Si layer on the surface of single crystal Si wafers. Generally, the thickness of the epitaxial layer is 2-20 μm, and the thickness of the substrate Si is 610 μm (150 mm diameter plate and 725 μm (200 mm plate).
The epitaxial deposition can process multiple pieces at the same time (simultaneously) or single pieces. The monolithic reactor can produce the best quality epitaxial layer (thickness, resistivity uniformity, and few defects); this epitaxial wafer is used for the production of 150mm "frontier" products and all important 200mm products.
Extension products
The epitaxial products are used in four aspects, and CMOS complementary metal oxide semiconductors support cutting-edge processes that require small device sizes. CMOS products are the largest application area of ​​epitaxial wafers, and are used by IC manufacturers for non-recoverable device processes, including microprocessors and logic chips, and flash memory and DRAM (dynamic random access memory) for memory applications. Discrete semiconductors are used to manufacture components that require precise Si characteristics. The "exoTIc" semiconductors include some special products that use non-Si materials, many of which are compound semiconductor materials incorporated into the epitaxial layer. The buried layer semiconductor is physically isolated using heavily doped regions in the bipolar transistor element, which is also deposited in the epitaxy process.
At present, among 200mm wafers, epitaxial wafers accounted for 1 / 3.2000, including the buried layer, CMOS for logic devices accounted for 69% of all epitaxial wafers, DRAM accounted for 11%, discrete devices accounted for 20%. By 2005, CMOS Logic will account for 55%, DRAM for 30%, and discrete devices for 15%.
LED epitaxial wafer--substrate material
Substrate materials are the cornerstone of technological development in the semiconductor lighting industry. Different substrate materials require different epitaxial growth technology, chip processing technology and device packaging technology. The substrate material determines the development route of semiconductor lighting technology. The choice of substrate material mainly depends on the following nine aspects:
1. Good structural characteristics, the crystal structure of the epitaxial material and the substrate is the same or similar, the lattice constant mismatch is small, the crystal performance is good, and the defect density is small
2. Good interface characteristics, good for nucleation of epitaxial materials and strong adhesion
3. Good chemical stability, not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth
4. Good thermal performance, including good thermal conductivity and low thermal mismatch
5. Good conductivity, can be made up and down structure
6. The optical performance is good, the light emitted by the fabricated device is absorbed by the substrate little
7. Good mechanical properties, easy to process devices, including thinning, polishing and cutting, etc.
8. Low price
9. Large size, generally require a diameter of not less than 2 inches.
It is very difficult to choose the substrate to satisfy the above nine aspects at the same time. Therefore, at present, it is only possible to adapt to the development and production of semiconductor light-emitting devices on different substrates by changing the epitaxial growth technology and adjusting the device processing technology. There are many substrate materials for GaN research, but there are currently only three substrates that can be used for production, namely sapphire Al2O3 and silicon carbide SiC substrates and Si substrates.
The following factors must be considered comprehensively when evaluating substrate materials:
1. The structure matching of the substrate and the epitaxial film: the crystal structure of the epitaxial material and the substrate material are the same or similar, the lattice constant mismatch is small, the crystallization performance is good, and the defect density is low;
2. The thermal expansion coefficient matching of the substrate and the epitaxial film: the matching of the thermal expansion coefficient is very important. The difference between the thermal expansion coefficient of the epitaxial film and the substrate material is not only possible to reduce the quality of the epitaxial film, but also due to heat generation during device operation And cause damage to the device;
3. The chemical stability of the substrate and the epitaxial film: the substrate material should have good chemical stability, it is not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth, and the quality of the epitaxial film cannot be reduced due to the chemical reaction with the epitaxial film ;
4. The difficulty of material preparation and the level of cost: Considering the needs of industrial development, the preparation requirements of substrate materials are simple, and the cost should not be very high. The substrate size is generally not less than 2 inches.
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