Do you really understand LED chips? What is the difference between the positive and negative LED chips? What is an LED chip?

A solid-state semiconductor device in which the heart of the LED is a semiconductor wafer. One end of the wafer is attached to a holder, one end is a negative electrode, and the other end is connected to the positive electrode of the power source, so that the entire wafer is encapsulated by epoxy resin. Also known as the LED light-emitting chip, it is the core component of the LED lamp, which is also referred to as the PN junction. Its main function is to convert electrical energy into light energy. The main material of the chip is monocrystalline silicon. The semiconductor wafer consists of two parts, one part is a P-type semiconductor, in which the hole dominates, and the other end is an N-type semiconductor, which is mainly electrons here. But when the two semiconductors are connected, they form a PN junction. When a current is applied to the wafer through the wire, the electrons are pushed toward the P region. In the P region, electrons recombine with the holes, and then the energy is emitted in the form of photons. This is the principle of LED illumination. The wavelength of light, which is the color of light, is determined by the material that forms the PN junction.

The reason why wafer loading is called "flip-chip" is compared to the traditional wire bonding method (WireBonding) and post-balling process. Conventionally, the electronic surface of the wafer connected to the substrate by metal wire bonding is facing upward, and the electrical surface of the flip chip is facing downward, which is equivalent to turning the former over, so it is called "flip-chip".

The essence of flip chip is that on the basis of the traditional process, when the light-emitting area and the electrode area of ​​the chip are not designed in the same plane, the electrode area is mounted toward the bottom of the lamp cup, and the process of soldering the wire can be omitted. However, the precision of the process of solid crystal is high, and it is generally difficult to achieve a high yield.

The required conditions for flip chip:

A GaN-based LED structure layer is grown on the sapphire substrate by MOCVD technology, and light emitted from the P/N junction light-emitting region is emitted through the upper P-type region. Due to the poor conductivity of P-type GaN, in order to obtain good current expansion, a metal electrode layer composed of Ni-Au is formed on the surface of the P region by evaporation. The lead of the P region is taken up through the metal film of the layer. To achieve good current spreading, the Ni-Au metal electrode layer should not be too thin. For this reason, the luminous efficiency of the device is greatly affected, and it is usually necessary to take into account both the current expansion and the light extraction efficiency. However, in any case, the presence of the metal film always deteriorates the light transmission performance. In addition, the presence of wire bond pads also affects the light extraction efficiency of the device. The structure using GaN LED flip chip can fundamentally eliminate the above problem.

In recent years, countries around the world such as European countries, the United States, Japan, South Korea and China have implemented LED lighting related projects. Among them, the “Ten Cities and Ten Thousand Miles” program promoted by China is the most eye-catching. Street lamps are an indispensable part of urban lighting. Traditional street lamps usually use high-pressure sodium lamps or metal halide lamps. The biggest characteristic of these two sources is that the arc tube is small in size, can produce a large light output, and has high light efficiency. . However, such light sources are used in road lamps. Only about 40% of the light passes directly through the glass cover to the road surface, and 60% of the light is reflected by the lamp reflector and then emitted from the lamp. Therefore, there are basically two shortcomings in the traditional lamps. One is that the illumination in the direct illumination direction is very high, and the secondary road can reach 50Lx or more. This area is obviously over-illuminated, and the illumination at the intersection of the two lamps is only It is 20%-40% of the illumination at the center of the lamp, and the light distribution uniformity is low. Second, the reflector efficiency of such lamps is generally only 50%-60%, so there is a large amount of light loss during the reflection process, so The traditional high-pressure sodium lamp or metal halide lamp street lamp has an overall efficiency of 70-80%, low uniformity, and excessive waste of illumination. In addition, the service life of high-pressure sodium lamps and metal halide lamps is usually less than 6000 hours, and the color rendering index is less than 30; LED has significant advantages such as high efficiency, energy saving, long life (50,000 hours), environmental protection, and high color rendering index ("75"). How to effectively apply LED to road lighting has become the hottest topic for LED and street lamp manufacturers. Generally speaking, according to the use environment of the street lamp, there are strict requirements on the optical design, life support, dustproof and waterproof capability, heat dissipation treatment, and light effect of the LED. As the core of LED street light, the manufacturing technology of LED chip and the corresponding packaging technology jointly determine the future application prospect of LED in the field of lighting.

The improvement of the luminous efficiency of LED chips determines the energy-saving ability of LED street lamps in the future. With the development of epitaxial growth technology and multi-quantum well structure, the internal quantum efficiency of epitaxial wafers has been greatly improved. How to meet the standard of street lamp usage depends largely on how to extract the most light from the chip with the least amount of power. Simply put, it is to reduce the driving voltage and increase the light intensity. LED chips of traditional dressing structure generally need to be coated with a translucent conductive layer on p-GaN to make the current distribution more uniform, and this conductive layer will partially absorb the light emitted by the LED, and the p electrode will block part of the light. Light, this limits the light extraction efficiency of the LED chip. The flip-chip LED chip can not only avoid the problem of light absorption and electrode pad shading on the conductive layer of the P electrode, but also guide the downward light by providing a low-ohmic contact reflective layer on the p-GaN surface. Up, this can simultaneously reduce the drive voltage and increase the light intensity. (See Figure 1.) On the other hand, the patterned sapphire substrate (PSS) technology and chip surface roughening technology can also increase the light extraction efficiency of the LED chip by more than 50%. The PSS structure is mainly to reduce the total light reflection efficiency of the photons in the device, and the chip surface roughening technology can reduce the light loss reflected at the interface when the light is emitted from the inside of the chip to the outside of the chip. At present, the LED chip adopts flip-chip structure and graphic technology. After 1W power chip white light package, the light efficiency is up to 134lm/W at 5000K color temperature.

Chip junction temperature and heat dissipation

The heat dissipation problem is a technical problem that needs to be solved by the power type white light LED. The quality of the heat dissipation effect is directly related to the life of the street lamp and the energy saving effect. LEDs rely on electrons to generate light between energy bands, and the spectrum does not contain infrared parts, so the heat of LEDs cannot be radiated by radiation. If the heat in the LED chip is not dissipated in time, it will accelerate the aging of the device. Once the temperature of the LED exceeds the maximum critical temperature (according to different epitaxy and process, the chip temperature is about 150 ° C), it will often cause permanent failure of the LED. Effectively solving the heat dissipation problem of the LED chip plays an important role in improving the reliability and life of the LED street lamp. The most straightforward way to do this is to provide a good thermal path for heat to escape from the junction. At the chip level, the vertical and flip-chip structure has better heat dissipation than the conventional dressing structure with a sapphire substrate as the heat sink. The vertical structure chip directly uses a copper alloy as a substrate, which effectively improves the heat dissipation capability of the chip. Flip-Chip technology flips the LED chip onto a silicon substrate with higher thermal conductivity by eutectic soldering (thermal conductivity is about 120W/mK, traditional packaged sapphire thermal conductivity is about 20W/mK), chip The gold bumps and the silicon substrate between the substrate and the silicon substrate simultaneously improve the heat dissipation capability of the LED chip, and ensure that the heat of the LED can be quickly extracted from the chip.

In addition, antistatic discharge (ESD) capability is another factor that affects the reliability of LED chips. The blue chip of the sapphire substrate has positive and negative electrodes on the chip with a small pitch. For the InGaN/AlGaN/GaN double heterojunction, the thickness of the InGaN active layer is only a few tens of nanometers, and the ability to withstand static electricity is limited. It is broken down by static electricity to disable the device. In order to prevent damage to the LED chip by static electricity, on the one hand, a production management method such as grounding the production equipment and isolating the human body static electricity may be employed, and on the other hand, a Zener protection circuit may be added to the LED chip. In the field of street lamps, the traditional chip structure ESDHBM is about 2000V. It is usually necessary to connect a Zener chip through a gold wire in the packaging process to improve the ESD protection capability, which not only increases the packaging cost and process difficulty, but also has greater reliability. risks of. By integrating the Zener protection circuit inside the silicon substrate, the antistatic discharge capability of the LED chip (ESDHBM=4000~8000V) can be greatly improved, the package cost can be saved, the packaging process can be simplified, and the reliability of the product can be improved.

LED street lights are usually around 60-200W. Currently, they are mainly implemented in two ways. One is to realize high-watt by means of "modules with multiple chips in series with gold series" and "multiple LEDs connected in series through PCB". number. Regardless of the implementation method, the circuit connection between the chip and the bracket is required by wire-bonding in the packaging process, and the impact of the porcelain nozzle on the LED chip during the soldering process leads to LED leakage, soldering, etc. The main reason is that the traditional dressing and vertical structure LEDs are located on the light-emitting surface of the chip. Therefore, the frontal impact of the porcelain nozzle during the wire bonding process is likely to cause damage to the light-emitting area and the electrode metal layer. In the flip-chip structure of the LED chip, the electrode Located on the silicon substrate, the chip does not impact the wire during the wire bonding process, greatly improving package reliability and production yield.

As the core device of the LED street lamp, the performance of the LED chip needs to be improved by the LED packaging process to achieve the effects of light efficiency, life, stability, optical design, and heat dissipation. Due to the different chip structure, the corresponding packaging process also has a big difference.

The positive and vertical structures of the chip are GaN in contact with phosphor and silica gel, while the flip-chip structure is sapphire in contact with phosphor and silica gel. The refractive index of GaN is about 2.4, the refractive index of sapphire is 1.8, the refractive index of phosphor is 1.7, and the refractive index of silica gel is usually 1.4-1.5. The total reflection critical angles of sapphire/(silica gel + phosphor) and GaN/(silica gel + phosphor) are 51.1-70.8° and 36.7-45.1°, respectively, and the light emitted from the sapphire surface in the package structure passes through the silica gel and phosphor interface. The critical angle of total reflection of the layer is larger, and the total reflection loss of light is greatly reduced. At the same time, the design of the chip structure is different, resulting in different current density and voltage, which has a significant effect on the light efficiency of the LED. For example, the conventional positive-loading chip usually has a voltage of more than 3.5V, and the flip-chip structure has a more uniform current distribution due to the design of the electrode structure, so that the voltage of the LED chip is greatly reduced to 2.8V-3.0V, and therefore, in the same luminous flux. In the case, the light effect of the flip chip is about 16-25% higher than that of the positive chip.

The reliability of LED is determined by LED chip, phosphor, silica gel, bracket, gold wire and other materials. The heat generated by LED chip can not be quickly exported, which will directly affect the junction temperature of LED chip and the reliability of phosphor and silica gel. At present, phosphors vary greatly depending on the system, and the high temperature resistance is also different. Generally, the phosphor starts to decay above 100-120 ° C. Therefore, how to reduce the temperature of the surface of the LED chip becomes a key factor for improving the reliability of the LED. The vertical structure chip can quickly transfer heat to the support through the metal substrate, the surface temperature of the chip is low, and the heat of the positive package chip is exported to the support through the sapphire. Since the thermal conductivity of the sapphire is low (about 20 W/mK), the heat cannot be quickly exported. Gradually accumulate, which has a greater impact on the reliability of the phosphor. Most of the heat of the flip-chip chip is quickly introduced into the silicon substrate (thermal conductivity about 120W/mK) through the gold bump, and then introduced into the bracket by the silicon substrate, and the sapphire has a low thermal conductivity, only a small portion. The heat accumulates in the sapphire, achieving heat (downward emission) and light separation (upward injection) design, while the surface temperature of the sapphire is low, which can prolong the aging cycle of the phosphor and greatly improve the reliability and life of the LED. At the same time, due to the good thermal design of the flip-chip structure, the flip-chip 1W chip can have a better LI linear relationship (see Figure 3) and saturation current tolerance and high current withstand capability. The flip-chip 1W power chip can support long-term room temperature 780mA high current aging.

LED flip chip is gradually used by most domestic lighting manufacturers in street lighting with its low voltage (below 3.0V), high luminous efficiency (100-110lm/W) and high stability. A high-pressure sodium lamp and an LED street lamp are comparatively analyzed with a street lamp installed by a customer using a flip chip as an example. Before the renovation, Gangqian Avenue uses 400W (top light) + 150W (waist light) high-pressure sodium lamp street light. The daily power consumption per pole is 6.6 degrees. After renovation, 180W (top light) + 60W (waist light) LED street light is used. The power consumption is 3.1 degrees, and the road lighting quality fully meets the requirements of the urban road lighting standard CJJ-45-2006, saving 53%. Adopting the German LM-1009 road special narrow-angle brightness meter, according to the road illumination brightness measurement method (the measuring instrument is located 60 meters away from the starting point to be measured, the instrument height is 1.2 meters, the highest and lowest brightness between the two poles is measured along the lane center line. At the point of measurement, the maximum illumination of the road is 42Lx, the minimum illumination is 8Lx, the average illumination is 30Lx, and the uniformity is 0.3. The maximum illumination of the road is 23Lx, the minimum illumination is 12Lx, the average illumination is 18Lx, and the uniformity is 0.75. .

Since the color rendering of the LED light source is above 70, the brightness distribution is uniform, and the discrimination ability for the target is much better than that of the high-pressure sodium lamp with a color rendering index of 23. Under the condition of road illumination (intermediate vision), the illumination requirement of the white LED is appropriately reduced. (Reduced by 1/3), the same lighting effect as high pressure sodium lamps can be achieved. After the replacement of LED street lights in Gangqian Avenue, the overall uniformity, longitudinal uniformity and lateral uniformity of the road surface reached 0.70 or more, achieving good lighting effects.

At present, high-power LED street lamps are mainly realized by means of "multiple chips in series and parallel connection" and "multiple LEDs are connected in series through PCB". The former has not been widely used because of the need to match the photoelectric parameters between the chips, and the process unreliability and low package yield of multiple gold wire series and parallel packages. The latter requires strict photoelectric parameter matching for multiple LEDs, and optical design is difficult. Therefore, "chip-level" modular products are an important development direction for future LED chips. The chip-level LED module realizes the serial-parallel connection between the single chips through the circuit in the substrate, solves the problem that the traditional module integration relies on the gold wire for serial-parallel connection, greatly improves the product yield rate, and greatly reduces the production of the entire packaging process. Cost, strictly control the parameter difference between the chips of the integrated module chip to ensure the reliability of the long-term use of the module chip, and the module chip can be used as a unit to perform series-parallel splicing to form a module with higher power. With flip-chip technology, multi-chip integration of different sizes, colors, shapes, and powers can be realized at the "chip level" to realize ultra-high power module products, which is an advantage that cannot be achieved by any other chip technology.

2. The LED chip is a solid-state semiconductor device, which is a PN junction, which can directly convert electricity into light.

The heart of the LED is a semiconductor wafer. One end of the wafer is attached to a holder, one end is the negative pole, and the other end is connected to the positive pole of the power supply, so that the entire wafer is encapsulated by epoxy resin.

1. Composition of led wafer: There are several kinds of elements such as arsenic (AS) aluminum (AL) gallium (Ga) indium (IN) phosphorus (P) nitrogen (N) bismuth (Si).

2, the composition of the led chip: composed of gold pad, P pole, N pole, PN junction, back gold layer (double pad chip without back gold layer).

1, led chip

1) According to the brightness of the light: A. General brightness: R, H, G, Y, E, etc.; B. High brightness: VG, VY, SR, etc.; C. Ultra high brightness: UG, UY, UR, UYS, URF , UE, etc.; D. Invisible light (infrared): IR, SIR, VIR, HIR; E. Infrared receiving tube: PT; F. Photocell: PD.

2) According to the constituent elements: A. Binary wafer (phosphorus, gallium): H, G, etc.; B. Ternary wafer (phosphorus, gallium, arsenic): SR, HR, UR, etc.; C. Quaternary wafer (phosphorus , aluminum, gallium, indium): SRF, HRF, URF, VY, HY, UY, UYS, UE, HE, UG. 2, led chip 1) according to the use of: high-power led chip, low-power led chip two;

2) According to the color points: mainly divided into three types: red, green, blue (raw materials for white light);

3) According to the shape: generally divided into square pieces, wafers;

4) According to the size: small power chips are generally divided into 8mil, 9mil, 12mil, 14mil and so on.