Sapphire wafer application research and analysis in the LED industry

LED (Light Emitting Diode) has been widely recognized as the next generation of major lighting tools, and has advantages such as long life, energy saving, and safety compared to incandescent lamps currently in use. At present, LED mainly uses sapphire as the main substrate material, and chemical mechanical polishing (CMP) is used as the final processing program.

The preparation of blue LEDs relies on GaN thin films, and there are many kinds of substrate materials used for growing GaN thin films. The most commonly used ones are sapphire and SiC. In recent years, sapphire has become the most important substrate material due to its small lattice mismatch coefficient with GaN and good light transmittance. It can be seen from the application trend table of sapphire and SiC substrate in LED that since 2005, the proportion of SiC substrate is less than 10%, and the proportion of sapphire substrate is as high as 90% or more. Ultra-high brightness LEDs require the sapphire substrate material to be ideal, ie the crystal lattice is complete without any processing defects. Cracks and chipping defects in domestic sapphire wafer production generally account for 5%-8% of the total. At the same time, the polishing rate of the sapphire wafer is also very low, the processing takes several hours, and about 20% of the wafer surface has deep marks after processing, resulting in rework or even scrapping, thereby greatly increasing the processing cost. This situation not only leads to low processing efficiency of the sapphire wafer, but also affects the subsequent growth of the GaN film on the sapphire wafer, which in turn affects the manufacture and illumination of the LED. Most of the ultra-high brightness LEDs currently used in China are imported from the United States, Japan and other countries. Increasing the material removal rate during sapphire wafer processing and improving the surface quality after processing have become the main research targets of sapphire processing.

Selection of substrate materials for LEDs: SiC is a very important substrate material. Compared with sapphire, SiC is a low-resistance material that can be fabricated with electrodes whose lattice constant and thermal expansion coefficient of the material are closer to those of GaN materials. It is easy to dissolve and has blue light-emitting characteristics. However, SiC materials also have their disadvantages, mainly because their thermal expansion coefficients are relatively different from those of GaN, which easily lead to cracks in the epitaxial GaN layer, are not suitable for large-scale use, and are relatively expensive. The sapphire substrate is the best and most common substrate material currently used. The single crystal sapphire substrate and the GaN lattice can match each other, have good high temperature stability and mechanical properties, and meet the high temperature resistance during the growth of the GaN film. The requirement is that the single crystal sapphire substrate has good light transmittance in the visible light range, and the research on it is also more, the production technology is relatively mature, and the price is relatively cheap, so it becomes white, blue, green and blue-green light. A key substrate material for GaN substrates.

In general, the following factors must be considered when evaluating substrate materials:

1. The degree of structural matching between 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 property is good, and the defect density is low;

2. The degree of thermal expansion coefficient matching between 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 may not only reduce the quality of the epitaxial film, but also during the operation of the device. Heat caused damage to the device;

3. The degree of chemical stability matching between the substrate and the epitaxial film: the substrate material should have good chemical stability, and 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 made due to chemical reaction with the epitaxial film. decline;

4. The difficulty of material preparation and the cost: In consideration of the needs of industrial development, the preparation of substrate materials is simple and the cost is not high, and the substrate size is generally not less than 2 inches.

Characteristics of sapphire: sapphire (α-Al2O3) crystals have excellent properties such as high hardness (9 grades), high melting point (2045 ° C), good light permeability, good thermal stability, and stable chemical properties. It is widely used in aerospace, industrial and life fields, and is particularly suitable as an LED substrate material.

Although sapphire has the above-mentioned excellent optical properties and mechanical properties, it is because of these special properties that the surface processing is very difficult. When GaN is grown on a sapphire substrate, the sapphire surface is required to achieve ultra-smooth damage-free. Surface, but because its hardness is second only to diamond, grinding and polishing technology is difficult, processing time is long, sapphire single crystal substrate is a typical brittle hard material, if there is high tensile stress during processing and application, it will crack. Or surface and subsurface damage, and the traditional method of processing single crystal sapphire substrate causes surface scratches and subsurface damage on the workpiece, and these surface scratches and subsurface damage will affect the optical properties and cause stress concentration on the workpiece surface. The life and reliability of the workpiece are affected.

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