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Characteristics, process parameters and performance comparison of vertical LEDs

release time:

2022-04-25 09:50

1. Introduction to LED semiconductors and substrate materials
1. The principle of electroluminescence of led
As shown in the figure above, the core part of Led is a wafer composed of p-type semiconductor and n-type semiconductor, and there is a transition layer between p-type semiconductor and n-type semiconductor, called p-n junction. When a forward voltage is applied across the pn junction, the holes in the p region are injected into the n region, and the electrons in the n region are injected into the p region. These injected electrons and holes recombine near the pn junction and convert the excess energy to light. This kind of diode made by the principle of injection electroluminescence is called light-emitting diode, commonly known as LED.   
2. Introduction of led light-emitting materials
For the selection of semiconductor light-emitting materials, first of all, there must be a suitable band gap width, that is, the band gap width of the light-emitting diode material must be greater than or equal to the photon energy of the desired light-emitting wavelength; secondly, P-type and N-type crystals with high electrical conductivity can be obtained. , used to prepare excellent PN junctions; thirdly, high-quality crystals with good integrity can be obtained, which is a necessary condition for making high-efficiency light-emitting devices; the final light-emitting recombination probability should be large.
The semiconductor materials commonly used to manufacture LEDs mainly include III-V semiconductor materials such as gallium arsenide, gallium phosphide, gallium aluminum arsenide, phosphorus arsenide, aluminum indium gallium phosphorus, indium gallium nitride, etc., and other group IV compound semiconductor silicon carbide. , II-IV group compound zinc selenide, etc.
Formulations of III-V semiconductor materials commonly used in led light-emitting:
color wavelength/nm Voltage/V Semiconductor material
red light 610<Λ<760 1.63<ΔV<2.03 AlGaAs、GaAsP
orange light 590<Λ<610 2.03<ΔV<2.10 GaAsP、AlGaInP、GaP
yellow light 570<Λ<590 2.10<ΔV<2.18 GaAsP、AlGaInP、GaP
green light 500<Λ<570 1.9<ΔV<4.0. InGaN、GaN、GaP
blue light 450<Λ<500 2.48<ΔV<3.7 ZnSe、InGaN、siC、C
purple light 400<Λ<450 2.76<ΔV<4.0 InGaN
UV light Λ<400 3.1<ΔV<4.4 AlN、AlGaN、AlGaInN
3. LED substrate material
For the production of LED chips, the selection of substrate materials is the primary consideration. The evaluation of substrate materials should comprehensively consider the following factors:
(1) The lattice matching of the substrate and the epitaxial film. Substrate material and epitaxial film lattice matching is important. Lattice matching includes two contents: lattice matching in the epitaxial growth plane, that is, the matching between the substrate and the epitaxial film in a certain direction of the plane where the growth interface is located; and the matching along the normal direction of the substrate surface.
(2) The thermal expansion coefficients of the substrate and the epitaxial film are matched. The matching of thermal expansion coefficient is also very important. Excessive difference in thermal expansion coefficient between the epitaxial film and the substrate material may not only reduce the quality of the epitaxial film, but also cause damage to the device due to heat during the operation of the device.
(3) The chemical stability of the substrate and the epitaxial film is matched. The substrate material needs to have relatively good chemical stability, and the quality of the epitaxial film cannot be degraded due to the chemical reaction with the epitaxial film.
(4) The difficulty and cost of material preparation. Considering the needs of industrial development, the preparation requirements of the substrate material are simple and the cost should not be very high. The substrate size is generally not less than 2 inches.
Comparison of the characteristics of various led substrates:
  Sapphire Silicon substrate SiC substrate GaN Molybdenum/Molybdenum Copper Alumina GaAs
Development size Current mass production is 2”~10” Suitable for developing larger sizes Suitable for developing larger sizes The current mass production is 2”~4”, if the mass production is larger, it will not be easy The current mass production is 2”~4”, if the mass production is larger, it will not be easy The current mass production is 2”~6”, if the mass production is larger, it will not be easy The current mass production is 2”~4”, if the mass production is larger, it will not be easy
Developer Hypo, Rubicon, Korean STC,
Taiwan factory and Taiwan focus, Japanese factory Kyocera,
Mainly based on semiconductor background factories, such as Toshiba, Bridgelux Cree Sorra, Seoul Semiconductor Plansee Keller's, Jiuhao Tongxin, Epistone
Number of manufacturers in the industry chain More participants, faster price drop Few participants and high prices Patented, high price Few participants and high prices Few participants and high prices Few participants and high prices Few participants and high prices
Lattice constant Lattice constant problem with GaN The difference in lattice constant from GaN is larger than that of sapphire substrate matches the lattice constant of GaN better than sapphire substrates Match the crystal structure and thermal expansion properties of the epitaxial layer Large difference Large difference Easy to form heterostructures with GaN
thermal conductivity Poor Good for high current operation Good for high current operation good, homoepitaxial good, homoepitaxial good Poor
melting point High, 2050°C center High, 2730°C center center high center
Transmittance Excellent, no need to peel off the sapphire substrate Poor, most silicon substrates need to be peeled off Good, visible light is not absorbed Poor Poor, need to go through stripping process Poor Poor
Cost Low center high high high high very tall
Extended Simple Complex Simple Complex Complex Simple Simple
2. Introduction of sapphire substrate
The composition of sapphire is aluminum oxide (Al2O3), which is formed by covalent bonding of three oxygen atoms and two aluminum atoms, and its crystal structure is a hexagonal lattice structure. Because sapphire has the characteristics of high speed of sound, high temperature resistance, corrosion resistance, high hardness, high light transmittance, high melting point (2045℃), etc., at the same time, the C-plane of sapphire (single crystal Al2O3) and the III-V and II-VI deposited films are closely related. The mismatch rate of lattice constants between them is small, making sapphire wafers a key material for making white/blue/green LEDs.
Sapphire (Al2O3) characteristic table:
Molecular formula Al2O3
Density 3.95-4.1 g/cm3
Crystal structure Hexagonal lattice
Lattice constant a=4.785Å , c=12.991Å
Mohs hardness 9      (second only to diamonds: 10)
melting point 2050℃
Boiling Point 3000℃
Coefficient of Thermal Expansion 5.8×10 -6 /K 
Specific heat 0.418W.s/g/k 
Thermal Conductivity 25.12W/m/k (@ 100℃) 
Refractive Index no =1.768 ne =1.760 
dn/dt 13x10 -6 /K(@633nm)
Light Transmission Properties T≈80% (0.3~5μm) 
Dielectric constant 11.5(∥c), 9.3(⊥c) 


1. Sapphire crystal growth method
(1) Czochralski method, referred to as CZ method. The raw material is first heated to the melting point and then melted to form a molten soup, and then a single crystal seed is used to contact the surface of the molten soup, and supercooling is formed on the solid-liquid interface between the seed crystal and the molten soup due to the temperature difference. Then the molten soup begins to solidify on the surface of the seed crystal and grow a single crystal with the same crystal structure as the seed crystal. The seed crystal is pulled up at a very slow speed at the same time, and rotates at a certain speed. Crystal Ingot. 
(2) The Kyropoulos method, referred to as the KY method, is also called the bubble method. The principle is similar to the Czochralski method. First, the raw material is heated to the melting point and then melted to form a molten soup, and then a single crystal seed (SeedCrystal, also known as a seed crystal rod) is used to contact the surface of the molten soup. A single crystal with the same crystal structure as the seed crystal begins to grow on the solid-liquid interface with the molten soup. The seed crystal is pulled up at a very slow speed, but the crystal is pulled upward for a period of time to form a crystal neck. After the solidification rate of the interface with the seed crystal is stabilized, the seed crystal will not be pulled up or rotated, and the single crystal will gradually solidify from the top down by controlling the cooling rate, and finally solidify into a whole single crystal ingot.
Development of sapphire crystal growth technology:

Comparison of various sapphire growth methods:
2. Sapphire substrate processing flow
The raw material of the sapphire substrate is a crystal rod, and the crystal rod is processed from a sapphire crystal. Its related manufacturing process is as follows:
Sapphire Crystal Ingot
Sapphire ingot manufacturing process:
1) Crystal growth: Use a crystal growth furnace to grow large and high-quality single crystal sapphire crystals
2) Orientation: Ensure the correct position of the sapphire crystal on the rod cutting machine, which is convenient for rod cutting
3) Pull out the rod: take out the sapphire crystal rod from the sapphire crystal in a specific way
4) Roll grinding: use a cylindrical grinder to perform cylindrical grinding of the ingot to obtain precise cylindrical dimensional accuracy
5) Quality inspection: to ensure the quality of the ingot and whether the size and orientation of the ingot after extraction are in line with customer specifications sapphire substrate manufacturing process:
1) Orientation: Accurately locate the position of the sapphire ingot on the slicer for precise slicing
2) Slicing: Cut the sapphire ingot into thin wafers
3) Grinding: remove the wafer cutting damage layer caused by slicing and improve the flatness of the wafer
4) Chamfering: trim the edge of the wafer into an arc shape, improve the mechanical strength of the edge of the wafer, and avoid defects caused by stress concentration
5) Polishing: Improve the roughness of the wafer and make its surface reach the epitaxial level of the epitaxial wafer
6) Cleaning: remove contaminants on the wafer surface (such as: fine dust particles, metals, organic contaminants, etc.)
7) Quality inspection: use high-precision testing instruments to inspect the quality of wafers (flatness, surface dust particles, etc.) to meet customer requirements
3. Application types of sapphire substrates
There are three types of sapphire substrates used by epitaxial wafer manufacturers:
(1) C-Plane sapphire substrate
This is the sapphire substrate surface commonly used by manufacturers for GaN growth. This is mainly because the technology for growing sapphire crystal along the C-axis is mature, the cost is relatively low, the physical and chemical properties are stable, and the technology for epitaxial growth on the C-plane is mature and stable.
(2) R-Plane or M-Plane sapphire substrate
Mainly used to grow non-polar/semi-polar GaN epitaxial films to improve luminous efficiency. Usually, GaN epitaxial films prepared on sapphire substrates are grown along the c-axis, which is the polar axis of GaN, resulting in a strong built-in electric field in the active layer quantum wells of GaN-based devices, which reduces the luminous efficiency. , to develop non-polar surface GaN epitaxy to overcome this physical phenomenon and improve the luminous efficiency.
(3) Pattern Sapphire Substrate (PSS for short)
By the way of growth (Growth) or etching (Etching), nano-scale specific regular microstructure patterns are designed on the sapphire substrate to control the output light form of the LED, and at the same time reduce the difference between the GaN grown on the sapphire substrate. Eliminate defects, improve epitaxial quality, improve the internal quantum efficiency of LEDs, and increase light extraction efficiency.
three. LED chip fabrication process and structure design
1. LED epitaxial growth process
The basic principle of epitaxial growth is: MO source and NH3 are transported from the carrier gas to the reaction chamber, and the gas flow is controlled by a mass flow meter. After the reactants enter the reaction chamber, they are transported to the surface of the substrate by the carrier gas to react to form a specific single crystal film. At present, the LED epitaxial wafer growth technology mainly adopts the organic metal chemical vapor deposition method (MOCVD).
Led epitaxial growth process:


Various LED epitaxial growth processes:
                                                                    Multi-Buffer Layers                                                                                                                                     SiN/GaN Buffer Layers
                                                Epitaxial Lateral Overgrowth (ELO)                                                                                           Patterned Sapphire Substrate (PSS)

2. Introduction of LED chip manufacturing process
The LED chip process is generally divided into three parts: pre-process, post-process, and point measurement and sorting. The main work of the pre-process is to make a grain on the epitaxial wafer. Simply put, it is the process of Chip On Wafer. Use lithography machine, mask, ICP, evaporation machine and other equipment to make graphics, and make thousands to tens of thousands of connected grains on a 2-inch wafer. The post-process is to thin the wafer containing a large number of dies made by the previous process, and then use a laser to cut into individual dies. The main work of point measurement and sorting:

(1) Measure the electrical and optical properties of each die on a large wafer or square wafer;

(2) Divide the large wafers into squares with the same specifications according to the condition table;

(3) Absorb the parts with bad appearance, and paste them
Led wafer level process technology:

  Process Steps use equipment
Pre-Process Make a cut line
Platform etching
P electrode fabrication
N electrode fabrication
Pad making
Protective Layer Fabrication
E-gun evaporator
Mask aligner
ICP(Dry etching)
Post process Test
Grinding and polishing
Cutting and Cracking
Mapping prober
Lapping Machine
Spot sorting Choose
Inspection and Packaging
Scriber, Breaker
Storing Machine
Spotting machine, sorting machine


3. Led chip photolithography process and electrode fabrication process
1) Photolithography process:
2) Electrode manufacturing process:
4. Introduction to the structural development of Led
From the structure of LED, LED can be divided into positive structure, flip structure, vertical structure and 3-dimensional vertical structure.
(1) Formal structure:
The current relatively mature III-V nitride blue light chips use sapphire material as the substrate. Due to the insulating properties of the sapphire substrate, ordinary GaN-based LEDs use a front-mounted structure. Since the p and n electrodes of the front-mounted LED are on the same side of the LED, the current must flow laterally through the n-GaN layer, resulting in current crowding and high local heat generation, which limits the driving current; and the poor thermal conductivity of the sapphire substrate seriously hinders the loss of heat.
(2) Flip-chip structure:
For the blue LED chip, the thermal conductivity of the sapphire substrate is relatively low. In order to solve the problem of heat dissipation, the flip-chip structure of the chip is proposed, and the luminous efficiency and heat dissipation effect are improved. The common LED flip-chip structure is still a lateral structure, there is still a current congestion phenomenon, and surface roughening cannot be performed.
(3) Vertical structure:
In order to solve the insufficiency of blue LED flip-chip chips, the blue vertical structure chip is proposed by drawing on the red light vertical structure LED chip. The main difference between the vertical structure and the flip-chip LED is as follows: First, the P epitaxial layer of the LED chip is flip-chip welded on the support substrate, and then the growth substrate is peeled off to form a vertical structure LED.
(4) 3-dimensional vertical structure
Usually, vertical structure LED chips that do not require gold wires are collectively referred to as "3-dimensional vertical structure LED chips". 2005: Cree proposed a 3D vertical structure LED, using the following process: firstly using wafer bonding, then forming vias on the support substrate, and forming metal pins in the vias plug), and then form the N-electrode. Due to the complexity of the process and the low yield, no products have been put on the market. February 2007: Lumileds launched a blue-light vertical structure LED chip package (Rebel) without gold wires. Its main feature is: the N electrode on the supporting substrate is connected with the middle part of the N epitaxial layer through the light emitting layer. July 2008: Qinhuangdao Pengyuan Optoelectronics, Shandong Huaguang, Shanghai Sapphire, and the Institute of Semiconductors cooperated to open samples of 3D red and blue LED chips. Compared with vertical structure LED chips, the main advantages of 3D vertical structure LED chips are as follows:
(1) Advantages brought by no need for wire bonding: such as the thickness of the 3D vertical structure LED chip package is thinner, etc.;
(2) When the LED chip is applied to transmit information, the 3D vertical structure LED chip can transmit information faster;
(3) It is easier to introduce a larger driving current.
Four, vertical structure led introduction
1. Definition of LED chip with vertical structure:
In order to facilitate the understanding of vertical structure LEDs, a definition of a vertical structure LED based on the direction of current flow in the P-type epitaxial layer is proposed: the LED chip carries the majority of the higher resistance epitaxial layer (P-epitaxial layer) Covered by a conductive layer (eg, a metal layer); each point on the conductive layer is substantially equipotential; the conductive layer is connected to the electrodes; and current flows substantially vertically through the high-resistance epitaxial layer. Based on this definition: (1) For LED chips with a vertical structure, it is not necessarily necessary to make electrodes on both sides of the chip. (2) The external power supply can be electrically connected to the epitaxial layer (N-epitaxial layer) with lower resistance at different positions, for example: in the top part of the N-epitaxial layer, or in the middle part of the N-epitaxial layer (such as , Rebel of Lumileds).
(3) The external power supply and the conductive layer can be connected in different ways, such as: eutectic welding, gold implantation, conductive glue connection.
2. Features and performance advantages of vertical structure LEDs
Compared with traditional planar structure LEDs, vertical structure LEDs have many advantages:
(1) The p and n electrodes of the planar structure LED are on the same side, and the current must flow through the n-GaN layer laterally, resulting in current crowding and high heat generation; while the two electrodes of the vertical structure LED are on both sides of the LED, the current is almost all Flowing vertically through the epitaxial layer, there is no lateral flow of current, the current distribution is uniform, and the heat generated is reduced. 
(2) The traditional front-loading structure uses a sapphire substrate. Since the sapphire substrate is non-conductive, the mesa needs to be etched, sacrificing the area of ​​the active region. In addition, due to the poor thermal conductivity of the sapphire substrate (35W/(m°C), it also limits the heat dissipation of the LED chip; the vertical structure LED uses the bonding and peeling method to remove the sapphire substrate and replace it with good electrical conductivity and high thermal conductivity. The high-efficiency substrate not only does not need to etch the mesa, but can fully utilize the active area, and can effectively dissipate heat.
(3) For front-mounted GaN-based LEDs, the p-GaN layer is the light-emitting surface. Because the layer is thin, it is not conducive to making surface microstructures. However, for vertical structure LEDs, the n-GaN layer is the light-emitting surface, and the layer has a certain thickness, which is convenient for making surface microstructures to improve the light extraction efficiency. 
In short, compared with the traditional flat structure, the vertical structure has obvious advantages in light output and heat dissipation. 
3. The process technology of vertical structure LED
The biggest difference between the GaN-based vertical structure LED process and the front-mounted structure LED process is that the vertical structure LED needs to introduce substrate transfer technology. The so-called substrate transfer technology refers to replacing the original growth substrate with a new substrate with high thermal conductivity and high electrical conductivity. The specific steps are divided into two steps. First, the new substrate and the epitaxial wafer are bonded together by the method of wafer bonding or electroplating, and then the original growth substrate is removed by methods such as laser lift-off, grinding and wet etching. .
Xuming vertical LED process technology:
(1) Grow the LED epitaxial layer on the sapphire substrate

(2) The epitaxial wafer is made into grains of a certain size
(3) Making a reflective layer

(4) LED sidewall passivation treatment
(5) Bonding the copper alloy layer as a new substrate

(6) Remove the sapphire substrate
(7) Fabrication of n-electrode

(8) Surface roughening treatment and detection of n-GaN
(9) Dicing and packaging
4. The development status of vertical structure LED
At present, several major LED manufacturers in the world, such as Cree in the United States, Osram in Germany, Philips Lumileds in the United States, and SemiLEDs in the United States all have their own GaN-based vertical structure LED products. In addition, Japan, South Korea, Taiwan and major domestic LED manufacturers are actively developing GaN-based vertical structure LED chip technology.
Cree Company of the United States is one of the professional companies in the world that uses SiC as the substrate material to manufacture epitaxial wafers and chips for blue light-emitting diodes. The problem of heat dissipation and improving the efficiency of light extraction. Cree's power LED chip product EZ series adopts thin-film chip technology and has reached the industry-leading light efficiency level. According to a report in May 2011, Cree's white light LED device research and development level has reached 231 lm/W, which is a power type white light. LED has the best results ever reported.
At the beginning of 2007, Lumileds launched the thin film flip chip (TFFC) LED product, in fact, the thin film flip chip structure is also a kind of vertical structure LED. Thanks to technological advancements, this product can perform optimally in any environment. The Luxeon K2 using TFFC technology is an LED specifically designed, binned and tested to operate at 1000mA. The thermal resistance of the packaged LED is only 5.5°C/W. The light output of the binned and tested product (minimum light output is 160lm, 1A driving current) can easily exceed 220lm at higher driving current.
The German company Osram compared the light extraction efficiency of four structures of GaN-based LEDs, of which the "ThinGaN" chip is actually a vertical structure LED. The company uses wafer bonding and lift-off technology to transfer the LED light-emitting layer to a new substrate (GaAs substrate or Ge substrate), and in 2007 began to sell the latest version of the white LED "OSTAR Lighting" that emits 1000lm of luminous flux. When the input power of this product is 27W (working current 700mA), the luminous flux of 1000lm can be obtained, and the luminous efficiency at this time is about 37lm/W. When the operating current is reduced to 350mA, the luminous efficiency can be increased to 75lm/W.
The American SemiLEDs company was established in 2004. After Osram and Cree, it uses substrate transfer technology to commercialize the production of GaN-based vertical structure LEDs. The company first introduced GaN-based LEDs on metal Cu substrates.
The research and development of GaN-based vertical structures was first carried out at Peking University in China. Since then, research institutions and major LED companies have successively carried out research and development in this area. Tongfang Optoelectronics began to engage in the research and development of vertical structure LEDs in 2008. After more than two years of hard work, it has accumulated a complete set of Cu substrate electroplating, leveling and sapphire stripping technologies. The vertical structure LED chips produced at present can achieve an efficiency of more than 100lm/W after packaging white light. At present, Tongfang Optoelectronics has more than 10 vertical structure chip manufacturing patents, three of which have been authorized by the State Intellectual Property Office, two have been authorized by the Taiwan Patent Office, and one has been authorized by the United States Patent Office, laying a solid foundation for the development and production of vertical structures. Good foundation.

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