Since the world's first GaN-based blue LED was introduced in 1993, the development of LED manufacturing technology has been remarkable. The GaN-based LEDs currently commercialized internationally are manufactured on sapphire substrates or SiC substrates. But sapphire due to high hardness, conductivity and thermal conductivity and other reasons, the later device processing and application brings a lot of inconvenience, SiC also exists high hardness and expensive shortcomings, while the relatively inexpensive Si substrate due to excellent thermal conductivity and conductivity and mature device processing technology and other advantages, so the Si substrate GaN-based LED manufacturing technology by the industry's general concern.
At present, Japan's Nichia company monopolizes the patent technology of GaN-based LED on sapphire substrate, and the U.S. CREE company monopolizes the patent technology of GaN-based LED on SiC substrate. Therefore, the research and development of GaN-based LED production technology on other substrates has become a hot spot in the international arena. Nanchang University and Xiamen Hualian Electronics Co., Ltd. have undertaken the national 863 plan project "Si substrate based power GaN-based LED manufacturing technology", after nearly three years of research and development, has passed the Ministry of Science and Technology project acceptance.
1 Si substrate LED chip manufacturing
1.1 Technology route
Grow GaN on Si substrate to make LED blue light chip.
Process flow: grow AlN buffer layer on Si substrate → grow n-type GaN → grow InGaN/GaN multi-quantum well light-emitting layer → grow p-type AIGaN layer → grow p-type GaN layer → bond with Ag reflective layer and form p-type ohmic contact electrode → peel off the substrate and remove the buffer layer → make n-type si-doped layer of ohmic contact electrode → alloy → passivation → scribe → test → packaging.
1.2 Main manufacturing process
A Thomas Swan CCS low pressure MOCVD system was used to grow GaN-based MQW structures on a 50 mm si(111) substrate. Trimethyl gallium (TMGa) was used as the Ga source, trimethyl aluminum (TMAI) as the Al source, trimethyl indium (TMIn) as the In source, ammonia (NH3) as the N source, and silane (SiH4) and magnesium dichloride (CP2Mg) were used as n-type and p-type dopants, respectively. The AlN buffer layer was first epitaxially grown on the Si(111) substrate, then the n-type GaN layer, InGaN/GaN multi-quantum well light-emitting layer, p-type AlGaN layer, and p-type GaN layer were grown sequentially, followed by the Ag reflector on the p-side and the formation of p-type ohmic contact, then the epitaxial layer was transferred to the conductive substrate by hot-press soldering method, and the Si substrate was removed by etching with Si etching solution and expose the n-type GaN layer, use alkali etching solution to roughen the n-type surface and then form n-type ohmic contact, so that the production of vertical structure LED chip is completed.
As seen in the structure diagram, the Si substrate chip is flip-film structure, from bottom to top for the back Au electrode, Si substrate, bonded metal, metal reflector (p-ohm electrode), GaN epitaxial layer, roughened surface and Au electrode. This structure chip current vertical distribution, substrate thermal conductivity, high reliability; light-emitting layer backside metal reflector, surface roughening structure, high light extraction efficiency.
1.3 Key technology and innovation
The use of Si as GaN light-emitting diode substrate, although the LED manufacturing cost is greatly reduced, but also to solve the problem of patent monopoly, however, compared with sapphire and SiC, the growth of GaN on Si substrate is more difficult, because the thermal mismatch between the two and lattice mismatch is greater, Si and GaN thermal expansion coefficient difference will also lead to GaN film cracking, lattice constant difference will be in the GaN epitaxial In addition, the Si substrate LED may also have a 0.5 V heterogeneous barrier between Si and GaN to increase the turn-on voltage and poor crystal integrity resulting in low p-type doping efficiency, resulting in increased series resistance, and Si absorption of visible light will reduce the external quantum efficiency of the LED. Therefore, in view of the above problems, the luminescent layer dislocation density control technology, chemical stripping substrate transfer technology, p-type GaN ohmic electrode preparation technology with high reliability and high reflective characteristics and bonding technology, epitaxial material surface roughening technology with high optical efficiency, substrate patterning technology, optimized vertical structure chip design technology were studied and used in depth, and many technical problems were solved in a lot of experiments and explorations. Finally, the blue light-emitting chip with the size of 1 mm×1 mm, light output power of more than 380 mW at 350 mA, light-emitting wavelength of 451 nm, and operating voltage of 3.2 V was successfully prepared, fulfilling the targets specified in the project. The key technologies and technical innovations adopted are as follows.
(1) Using a variety of online control techniques, the epitaxial materials in the reduction of edge dislocation and spiral dislocation, improve the thermal mismatch and lattice mismatch between Si and GaN, solve the problem of GaN single crystal film cracking, and obtain the thickness of more than 4 μm crack-free GaN epitaxial film.
(2) By introducing AIN, AlGaN multilayer buffer layer, the stress of epitaxial GaN material on Si substrate is greatly relieved, and the crystal quality is improved, thus improving the luminescence efficiency.
(3) By optimizing the Si concentration structure in the n-GaN layer and the growth conditions of the interface between quantum wells/barriers, the reverse leakage current of the chip is reduced and the antistatic performance of the chip is improved.
(4) By adjusting the magnesium concentration structure of the p-type layer, the operating voltage of the device is reduced; by optimizing the thickness of the p-type GaN, the light extraction efficiency of the chip is improved.
(5) By optimizing the epitaxial layer structure and doping distribution, the series resistance is reduced, the operating voltage is lowered, and the thermal generation rate is reduced, which enhances the efficiency of the LED and improves the reliability of the device.
(6) The multilayer metal structure is adopted, taking into account ohmic contact, reflective characteristics, bonding characteristics and reliability at the same time, and the welding technology is optimized to solve the problem of poor adhesion of silver reflector and p-GaN and large contact resistance.
(7) A variety of welding metals were selected and the welding conditions were optimized to successfully obtain a firm bond between the GaN film and the conductive Si substrate, solving the problem of cracks generated in the process.
(8) The surface roughening by a combination of wet and dry methods reduces the light loss caused by internal total reflection and waveguide effects, improves the external quantum efficiency of the LED, and enables the device to obtain a high light output efficiency.
(9) The problem of insufficient depth and uneven roughening of GaN surface is solved, the problem of unclean cleaning of the roughened surface is solved and the metal structure of N electrode is optimized, and a low resistance and stable ohmic contact is obtained on the roughened N-polar n-GaN surface.
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