Author: jiangwei68
1 Overview
GaN has great application prospects in the field of optoelectronics and microelectronics, but due to the lack of large-sized substrates, heteroepitaxial growth can only be performed on other substrates. Among various growth techniques, MOCVD is most used. During the growth of GaN thin films, various growth parameters such as substrate temperature, microwave power, growth time, gas flow rate, etc. have a great influence on the final crystal quality. The growth of GaN thin films requires an optimal growth process, which necessitates good repeatability of the process. Therefore, an automatic monitoring system is required to monitor the entire growth process in real time. The optical monitoring system provides real-time information on the reflectivity and temperature of the wafer during GaN epitaxy. This monitoring system is integrated with the MOCVD system to provide all data and analytical data for epitaxial growth. In order to obtain more information, it is necessary to develop a dual-wavelength monitoring system, and many characteristics such as epitaxial growth rate, compound composition, roughness, and wafer surface state can be calculated by using 633 nm and 950 nm dual wavelength measurements. These are not possible with a single wavelength, and there is also a 405nm wavelength option, which provides more information on the front of the growth buffer layer and the quantum well layer.
2, temperature
How to accurately control the temperature of epitaxial growth during epitaxial process is very important. This parameter can affect the content of indium in the quantum well, and the amount of the content will cause the wavelength of the LED to shift. In the past, most of the design of the reaction chamber used hot spots. Coupling or pyrometers measure the temperature on the back side. In fact, we don't really notice the temperature variability during epitaxial growth, but in fact we actually measured the surface temperature of the wafer with a slight difference, because the upper quartz plate has different thicknesses. Coverage and the difference in the rotational speed of the carrier, if the actual temperature cannot be measured, then no good results can be made.
3, warpage
In today's MOCVD mass production of MOCVD, the main focus is on 2" sapphire substrates. The future trend will be toward large diameters, but the bending effect due to material lattice mismatch will change from 2" to large diameter in the substrate. The violent effect is caused. The stress caused by the lattice mismatch between GaN and sapphire substrate increases the warpage with the increase of thickness and wafer size. When the wafer is warped, the surface temperature will be uneven, and the center and edge temperature will be different. When the quantum well is grown, the yield is lowered due to the temperature difference, and the difference in thermal expansion coefficient of GaN on the sapphire substrate is used to calculate different stress structures, and the laser reflection measures the warpage of the wafer during the epitaxial growth process. It is very important to monitor the warpage of the wafer in real time during mass production. The multilayer quantum well is very sensitive to temperature and requires very precise control during epitaxial growth. Although the warpage of the 2" wafer is relatively small, it still plagues the future development of large diameters. The estimation is as follows. It is assumed that a 2" wafer will have a warp curvature of 20 km-1 if it is in the LED of a long multilayer quantum well structure. Producing an error of 2nm wavelength, there will be an error of 8nm wavelength on the 4" substrate. If the 2" wafer warps 10 km-1, it will lose 3% yield, then the 4" substrate will have 10%. The yield loss, if the real-time wafer warpage is not monitored during the large-diameter epitaxial growth, will not yield good yield.
4, window material
In addition, the optical window of the measuring system also has a great influence on the measurement results. The window material must be suitable for measuring the necessary requirements, such as geometric size and material. The window must reflect the best measurement effect and have no interference to the epitaxial growth. The diameter of the window must be large enough for the incident and reflected light to pass. The angle of incidence, the distance of the sample, the angle of the beam, and the diameter of the beam determine the size of the window. For a properly functioning optical inspection system, the diameter of the window can be small and the rotation of the sample can cause the beam. Deviation, so a larger window is required. For most commercial MOCVD systems, a diameter of 2" is usually sufficient. The window material must be transparent in the spectral range of 200 - 800 nm. In addition to low absorbance, the material requires low reflectivity. Lower refractive index materials have low reflectivity. Another key issue is material deposition on the inner surface of the window. Any deposition reduces transparency (especially in the UV range) and affects the test results. In addition, deposition can also cause scattering loss. To weaken the measurement signal, in order to prevent the deposition window from constantly ventilating, nitrogen is usually used. When the window is ventilated, the flow of the ventilation and the reaction gas of the furnace body should be balanced. Any reaction gas flow caused by the ventilation may affect the growth result. Generally, in addition to the MBE system, the MOCVD system cannot be heated to prevent it. The product, heated (and unshielded) window is very close to the optical detection system, and the radiation from the heater may cause measurement errors. It is especially necessary to suggest that the window of the anti-reflective coating is not recommended because it causes the window function to work in one Very small wavelength range, not the entire spectrum.
Original connection: http://blog.ofweek.com/jiangwei68/DiaryDetail.do?diaryid=12657b240045bbf6
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