Answer eight questions to help you understand LED chips

　　LED chip manufacturing is mainly to manufacture effective and reliable low-ohm contact electrodes, and to meet the minimum voltage drop between contactable materials and provide pressure pads for welding wires, while emitting as much light as possible. The coating process generally uses a vacuum evaporation method, which mainly uses resistance heating or electron beam bombardment heating methods under a high vacuum of 1.33×10?4Pa to melt the material, and turn it into metal vapor under low pressure and deposit it on the surface of the semiconductor material. The P-type contact metals generally used include alloys such as AuBe and AuZn, and the contact metal on the N side often uses AuGeNi alloy. The alloy layer formed after coating also needs to expose the light-emitting area as much as possible through the photolithography process, so that the remaining alloy layer can meet the requirements of effective and reliable low-ohm contact electrodes and welding wire pressure pads. After the photolithography process is completed, it is also necessary to go through the alloying process, which is usually carried out under the protection of H2 or N2. The alloying time and temperature are usually determined by factors such as the characteristics of the semiconductor material and the form of the alloy furnace. Of course, if the chip electrode process such as blue-green is more complicated, it is necessary to add passivation film growth, plasma etching process, etc.

　　2. In the LED chip manufacturing process, which processes have a more important impact on its optoelectronic performance?

　　Generally speaking, after the LED epitaxial production is completed, its main electrical properties have been finalized, and chip manufacturing does not change its core nature, but inappropriate conditions during the coating and alloying process will cause some poor electrical parameters. For example, low or high alloying temperature will cause poor ohmic contact, which is the main reason for the high forward voltage drop VF in chip manufacturing. After cutting, if some corrosion processes are performed on the edge of the chip, it will be helpful to improve the reverse leakage of the chip. This is because after cutting with a diamond grinding wheel blade, there will be more debris powder left on the edge of the chip. If these stick to the PN junction of the LED chip, it will cause leakage and even breakdown. In addition, if the photoresist on the surface of the chip is not stripped cleanly, it will cause difficulties in front welding wires and cold welding. If it is on the back, it will also cause a high voltage drop. In the process of chip production, the light intensity can be improved by roughening the surface and dividing it into an inverted trapezoidal structure.

　　3. Why are LED chips classified into different sizes? What is the impact of size on LED optoelectronic performance?

　　LED chip size can be divided into low-power chip, medium-power chip and high-power chip according to power. According to customer requirements, it can be divided into single-tube level, digital level, dot matrix level and decorative lighting and other categories. As for the specific size of the chip, it is determined according to the actual production level of different chip manufacturers, and there is no specific requirement. As long as the process is passed, the small chip can increase the unit output and reduce the cost, and the photoelectric performance will not change fundamentally. The current used by the chip is actually related to the current density flowing through the chip. The small chip uses a small current, and the large chip uses a large current. Their unit current density is basically the same. If the current used by the 10mil chip is 20mA, then the theoretical current used by the 40mil chip can be increased by 16 times, that is, 320mA. However, considering that heat dissipation is the main problem under high current, its luminous efficiency is lower than that of low current. On the other hand, due to the increase in area, the body resistance of the chip will decrease, so the forward conduction voltage will decrease.

　　4. What is the general area of ​​a high-power LED chip? Why?

　　The high-power LED chips used for white light are generally around 40mil on the market. The so-called high-power chip power generally refers to an electrical power of more than 1W. Since the quantum efficiency is generally less than 20%, most of the electrical energy will be converted into heat energy, so the heat dissipation of high-power chips is very important, requiring the chip to have a larger area.

　　5. What are the different requirements for chip technology and processing equipment for manufacturing GaN epitaxial materials compared with GaP, GaAs, and InGaAlP? Why?

　　The substrates of ordinary LED red and yellow chips and high-brightness four-element red and yellow chips are made of compound semiconductor materials such as GaP and GaAs, which can generally be made into N-type substrates. The wet process is used for photolithography, and finally the chips are cut with diamond wheel blades. The blue-green chip of GaN material uses a sapphire substrate. Since the sapphire substrate is insulating, it cannot be used as a pole of the LED. It is necessary to use the dry etching process to make the P/N electrodes on the epitaxial surface at the same time and pass some passivation processes. Since sapphire is very hard, it is difficult to cut it into chips with diamond wheel blades. Its process is generally more complicated than that of LEDs made of GaP and GaAs materials.

　　6. What is the structure and characteristics of the “transparent electrode” chip?

　　The so-called transparent electrode must be able to conduct electricity and transmit light. This material is now most widely used in the production process of liquid crystals. Its name is indium tin oxide, abbreviated in English as ITO, but it cannot be used as a solder pad. When making it, you must first make an ohmic electrode on the surface of the chip, then cover the surface with a layer of ITO and then plate a layer of solder pad on the surface of ITO. In this way, the current coming down from the lead is evenly distributed to each ohmic contact electrode through the ITO layer. At the same time, because the refractive index of ITO is between the refractive index of air and epitaxial material, it can increase the light output angle and the luminous flux.

　　7. What is the mainstream development of chip technology used in semiconductor lighting ?

　　With the development of semiconductor LED technology, its application in the field of lighting is increasing. In particular, the emergence of white light LED has become a hot spot in semiconductor lighting. However, key chip and packaging technologies need to be improved. In terms of chips, they should be developed towards high power, high light efficiency and reduced thermal resistance. Increasing power means increasing the current used by the chip. The most direct way is to increase the chip size. The commonly seen high-power chips are about 1mm×1mm and the current used is 350mA. Due to the increase in current used, heat dissipation becomes a prominent problem. Now the chip flipping method has basically solved this problem. With the development of LED technology, its application in the field of lighting will face unprecedented opportunities and challenges.

　　8. What is a flip chip? What is its structure? What are its advantages?

　　Blue LEDs usually use Al2O3 substrates, which have high hardness and low thermal and electrical conductivity. If a positive structure is used, it will cause anti-static problems on the one hand, and heat dissipation will become the main problem under high current conditions on the other hand. At the same time, since the front electrode faces upward, it will block part of the light and reduce the luminous efficiency. High-power blue LEDs can obtain more effective light through chip flip-chip technology than traditional packaging technology.

　　The current mainstream flip-chip structure is: first prepare a large-size blue LED chip with electrodes suitable for eutectic welding, and at the same time prepare a silicon substrate slightly larger than the blue LED chip, and make a gold conductive layer and lead wire layer (ultrasonic gold wire ball solder joint) for eutectic welding on it. Then, use eutectic welding equipment to weld the high-power blue LED chip to the silicon substrate. The characteristic of this structure is that the epitaxial layer is in direct contact with the silicon substrate, and the thermal resistance of the silicon substrate is much lower than that of the sapphire substrate, so the problem of heat dissipation is well solved. Since the sapphire substrate faces upward after flipping and becomes the light-emitting surface, and the sapphire is transparent, the light emission problem is also solved.