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How to improve the soft recovery characteristics of fast recovery diodes?

fast recovery diodes are a very important type of switching devices that have been widely used in various electronic devices, especially in switching power supplies. With the continuous progress of switching devices and the continuous improvement of operating frequency, the requirements for the performance of related power diodes are also increasing. Among them, the power transistor used to match the switching device has the most special requirements. The reverse recovery characteristics of this diode, such as reverse recovery time and reverse recovery softness, can significantly reduce power loss. Due to the fast switching speed of IGBT VDMOS, Therefore, it is required that the fast recovery diode not only has fast reverse recovery, but also requires soft characteristics to avoid generating high voltage spikes, radio frequency interference, and electromagnetic interference.

How to improve the soft recovery characteristics of devices has become a new topic for fast recovery diodes. Up to now, there has been no commercial mass production in China, and the fast recovery diodes used in large-scale IGBT packaging factories also rely entirely on imports. Therefore, achieving stable mass production of this product plays a crucial role in the localization of domestic energy conservation, frequency conversion, and automotive electronics industries.

The fast recovery transistor technology solutions vary greatly, but solving the distribution of silicon epitaxial materials for device soft characteristics plays a crucial role. After extensive research, it has been found that creating an n-nn+type epitaxial buffer layer structure can effectively improve the soft characteristics of reverse recovery. Type n is the buffer layer. The double base region formed by the overlay layer can significantly improve the softness of the diode. In the reverse recovery process, the expansion of the Depletion region after reaching the buffer layer is significantly slowed down. In this way, after a few carrier storage times. There are still a large number of charge carriers in the buffer layer that have not been recombined, resulting in a corresponding increase in recombination time. Thus, it is proposed that

The softness factor of high diodes makes the structure and distribution of buffer layers a challenge for the growth of epitaxial materials for fast recovery diodes; Another major factor affecting the reverse recovery of devices is material defects. After repeated experiments and analysis, it has been found that interstitial oxygen from the Czochralski monocrystalline silicon substrate material will form oxygen precipitation during the device process, which seriously affects the minority carrier lifetime of the device and affects the reverse recovery time of the device.

With the increase of silicon wafer diameter and the wide application of step Stepper. When using an 8-inch silicon substrate to grow an epitaxial layer greater than 100 micrometers, the thickness deviation on the chip can lead to changes in global and local flatness, which can lead to unfocused lithography, and the growth of polycrystalline silicon on the back can lead to poor back flatness, which ultimately results in vacuum absorption during lithography.

This article uses an 8-inch low gap oxygen resistivity<0.004 ohms, centimeter heavily doped arsenic 111 crystal oriented silicon substrate for epitaxial growth of fast recovery diodes as an epitaxial material. For the transition between two layers and the control of the transition zone between the first layer and the substrate, doping gradient technology is used to accurately control the two transition zones, and the two epitaxial layers are flat and have good repeatability; At the same time, using back processing technology, polycrystalline silicon on the back will also grow during thick layer growth, causing problems such as vacuum absorption and poor flatness. We have successfully solved this problem by using special edge treatment and back silicon slag treatment technology; By improving the setting of the air inlet of the equipment, the problem of thickness uniformity control has been successfully solved. During the material growth process, problems such as deformation and slip caused by stress between the thick epitaxial layer and the substrate material are also encountered. By controlling the base height, heating rate, growth rate, and substrate material edge, a 100% slip free material control technology has been successfully achieved. The typical result of product parameters is: thickness uniformity (EE10mm)<1.5%; Uniformity of resistivity (EE10mm)<2%; GBIR:<8um; STIR<1um, 100% slip free under strong light, and the flatness of the back and edges has also reached a level comparable to similar foreign products.