Knowledge introduced LED epitaxial waferson December 5th, 2011 at 2:07 AM
Wafer production process is very complex production, exhibition finished wafer, then the wafer in each nine o’clock to do random testing, to meet the requirements of that yield, the other for defective products (large bias voltage, the wavelength is shorter or long side). The wafer yield will begin to do the electrode (P pole, N pole), the next wafer laser cutting, and sorting hundred percent, depending on the voltage, wavelength, brightness, full automation of sorting, which is formed LED chip (side pieces). Then also the visual, the electrode has a little wear and tear or defects, sorting out, these are behind the bulk crystal. At this point there in the blue membrane does not meet the normal requirements of the chips shipped, it naturally became the side plates or raw footage and so on. Scrap wafer (mainly some parameters do not meet the requirements), would not do square piece, directly as electrode (P pole, N pole), do not do sorting, that is, the LED bulb lights circle on the market film (there is also a good thing, such as square tablets, etc.).
Major semiconductor manufacturers with polishing Si wafer (PW) and IC epitaxial Si films as the raw material. Since the early 1980s, using epitaxial films, it has a standard PW do not have some of the electrical properties and eliminates many of the subsequent crystal growth and wafer processing introduced in the surface / near surface defects.
Historically, the Si wafer by the film manufacturer and own use, the amount is not in the IC, it needs to single crystal Si film deposited on the surface of a thin single-crystal Si layer. General epitaxial layer thickness 2 ~ 20μm, while the Si substrate thickness of 610μm (150mm diameter piece and 725μm (200mm film).
Epitaxial deposition can (also) a multi-chip processing, but also can process a single chip. Monolithic reactor can produce the best quality of epitaxial layers (thickness and resistivity uniformity, fewer defects); this wafer for 150mm “cutting edge” products and all major 200mm production.
Extension products used in four areas, CMOS complementary metal oxide semiconductor devices to support the requirements of the small size of the cutting-edge technology. CMOS wafer products is the largest application areas, and was not restored IC manufacturers for technology devices, including microprocessors and logic chips and memory applications of flash memory and DRAM (dynamic random access memory). Requirements for the manufacture of discrete semiconductor Si features with precision components. “Exotic” (exotic) contains some special semiconductor products, they use non-Si materials, many of which compound to use incorporated into the epitaxial layer of semiconductor material. Buried layer of semiconductor devices using bipolar transistors in heavily doped region to physical isolation, which is deposited in the epitaxial processing.
Currently, 200mm wafer, the wafer accounted 1/3.2000 years, including buried layer, including, for the CMOS logic wafer accounting for 69%, DRAM 11%, discrete devices accounted for 20% in 2005, CMOS logic will account for 55%, DRAM 30%, 15% of discrete devices.
LED epitaxial film – substrate
Substrate material is the development of semiconductor lighting industrial technology the cornerstone. Different substrate materials require different epitaxial growth techniques, chip processing technology and device packaging technology, the substrate material determines the development of semiconductor lighting technology line. The choice of substrate depends on the following nine areas:
1, structural characteristics, and epitaxial materials and substrates of the same or similar crystal structure, lattice constant mismatch is small, good crystallinity, defect density
2, interface characteristics, and is conducive to the nucleation and epitaxial materials and strong adhesion
3, chemical stability, in the epitaxial growth temperature and atmosphere is not easy to break down and corrosion
4, thermal performance, including good thermal conductivity and thermal mismatch is small
5, good conductivity, can be made up and down the structure
6, optical performance, making the device substrate, the emitted light is absorbed by the small
7, good mechanical properties, easy processing devices, including thinning, polishing and cutting, etc.
8, low price
9, large size, generally require no less than 2 inches in diameter.
The choice of substrate to meet the above nine areas is very difficult. Thus, the present can only be changed by epitaxial growth techniques and device processing adjustments to adapt to different substrate of semiconductor devices R & D and production. Substrate material for GaN research more, but can be used in the production of substrates present, only three, namely, Al2O3 sapphire and silicon carbide SiC substrate and Si substrate.
Evaluation of the substrate material must consider the following factors:
1 substrate and the epitaxial film structure matching: epitaxial materials and substrates of the same or similar crystal structure, lattice constant mismatch is small, good crystallinity, low defect density;
(2) substrate and the epitaxial film coefficient of thermal expansion matching: the matching coefficient of thermal expansion is very important, epitaxial film and substrate materials on the difference in thermal expansion coefficient is too large may not only decrease the quality of the epitaxial film, the device will work in process, due to fever damage caused by the device;
3 substrate and the chemical stability of epitaxial films match: substrate materials have good chemical stability, in the epitaxial growth temperature and atmosphere, not easily broken down and corrosion, not because the chemical reaction with the epitaxial film epitaxial film quality to decline ;
4 materials prepared by the level of ease and cost: taking into account the needs of industrial development, substrate preparation required simple, cost should not be very high. Substrate size is generally not less than 2 inches.
The current substrate for GaN-based LED materials are more, but can be used for commercial substrate present, only three, namely sapphire and silicon carbide and silicon substrate. Others, such as GaN, ZnO substrates is still in development stage, still some distance away from the industry.
Ideal substrate for GaN growth GaN single crystal material is, can greatly improve the crystal quality epitaxial films, reducing the dislocation density, improve the working life of the device to improve the luminous efficiency, increase the device operating current density. Preparation of single crystal GaN, but the body is very difficult, so far there has not been an effective way.
GaN epitaxial ZnO has become a candidate for the substrate, because the two have a very striking similarities. Both the same crystal structure, lattice recognition degree is very small, close to the band gap (band discontinuity is small, little contact with the barrier). However, ZnO as the Achilles heel of GaN epitaxial GaN epitaxial growth in the temperature and atmosphere easily decomposed and corrosion. Currently, ZnO semiconductor materials used to make optoelectronic devices is not yet or high-temperature electronic devices, the main device is less than the level of quality of materials and P-type doping problem is not a real solution for the growth of ZnO-based semiconductor devices have not yet developed.
The most commonly used for GaN growth substrate is Al2O3. The advantage of chemical stability, does not absorb visible light, affordable, and manufacturing technology is relatively mature. Although the poor thermal conductivity of small current in the device is not exposed to significant deficiencies in the work, but in the power-type devices working under high-current problems are acute.
SiC as a substrate material second only to the extent of application of sapphire, China’s optical crystal of Professor Jiang Fengyi grown on Si substrate that can be used to commercialize LED epitaxial wafers. Si substrate in the thermal conductivity, stability better than sapphire, the price is far lower than the sapphire, is a very promising substrate. SiC substrate has good chemical stability, good conductivity, good thermal conductivity, does not absorb visible light, etc., but also is less prominent, such as the price is too high, the quality of the crystal Al2O3 and Si so difficult to achieve good machining performance is rather poor, In addition, SiC substrate to absorb UV light below 380 nm, 380 nm is not suitable for R & D following UV LED. As the SiC substrate and good thermal conductivity properties, can solve the power problem type GaNLED cooling devices, Therefore, in the field of semiconductor lighting technology an important role.
Compared with sapphire, SiC and GaN epitaxial films lattice-matched to improve. In addition, SiC has a blue light-emitting properties, and low-resistivity material, the electrode can be made, the device epitaxial film in the packaging completely before the test as possible of SiC as a substrate material to enhance competitiveness. As the layered structure of SiC easy cleavage, between the substrate and the epitaxial film can get high-quality cleavage plane, which will greatly simplify the device structure; but also because of its layered structure, often to the surface of the substrate the introduction of a large number of defects in epitaxial films of the steps appear.
Luminous efficiency goals to achieve hopes GaN substrate of the LED, low-cost, but also through the GaN substrate lead to efficient, large-scale, single-lamp power to achieve, as well as technology-driven simplification and yield significant to improve. Semiconductor lighting were to become reality, its as important as the Edison incandescent lamp. Once the substrate to achieve a breakthrough in key technology areas, and its industrialization process will progress has been made.