First, sapphire introduction
The composition of sapphire is alumina (Al2O3), which is formed by a combination of three oxygen atoms and two aluminum atoms in a covalent bond mode, and its crystal structure is a hexagonal lattice structure. It is often used in A-Plane, C-Plane and R-Plane. Due to the wide optical penetration of sapphire, it has good light transmission from near-ultraviolet (190nm) to mid-infrared. It is widely used in optical components, infrared devices, high-intensity laser lens materials and reticle materials. It has high sound speed, high temperature resistance, corrosion resistance, high hardness, high light transmission and high melting point (2045 ° C). It is A material that is quite difficult to process and is therefore often used as a material for photovoltaic elements. The quality of ultra-high brightness white/blue LEDs currently depends on the material quality of gallium nitride epitaxial (GaN), while the gallium nitride epitaxial quality is closely related to the surface quality of the sapphire substrate used, sapphire (single crystal Al2O3)C The lattice constant mismatch between the surface and the III-V and II-VI deposited films is small, and meets the high temperature resistance requirements of the GaN epitaxial process, making the sapphire chip a key material for making white/blue/green LEDs. .
The following figure is a cut-away view of sapphire; a top view of the crystal structure; a side view of the crystal structure; a structure diagram of Al2O3; a schematic view of the crystal surface of sapphire:
The most commonly used GaN epitaxial is the non-polar surface of C-plane (0001), so the polarity of GaN will be determined by the process.
(a) The picture is viewed from the C axis (b) The picture is viewed from the C axis side
Second, the growth method of sapphire crystal
There are two common methods for growing sapphire crystals:
1. Czochralski method, referred to as CZ method. The raw material is 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 the solid solution in the seed crystal and the molten soup. The interface is too cold due to the temperature difference. The melt then begins to solidify on the surface of the seed crystal and grow and crystallize the single crystal of the same crystal structure. The seed crystal is pulled up at a very slow speed and rotates with a certain rotation speed. As the seed crystal rises upward, the melt gradually solidifies on the liquid-solid interface of the seed crystal, thereby forming an axisymmetric single. Crystal ingots.
2, Kyropoulos method (Kyropoulos method), referred to as KY method. The mainland calls it the bubble method. The principle is similar to Czochralskimethod. The raw material is heated to the melting point and then melted to form a molten soup. Then, the seed crystal (SeedCrystal, also called seed crystal rod) is contacted to the surface of the molten soup. The single crystal with the same crystal structure as the seed crystal grows on the solid-liquid interface of the molten soup, and the seed crystal is pulled up at a very slow speed, but the crystal is pulled up for a period of time to form a crystal neck, to be melted. After the solidification rate at the interface with the seed crystal is stabilized, the seed crystal is no longer pulled up and does not rotate. The single crystal is gradually solidified from above upwards by controlling the cooling rate, and finally solidified into a whole single crystal crystal crucible.
The crystal growth diagrams of the two methods are as follows:
Schematic diagram of the Czochralski method
Third, the sapphire substrate processing process:
The raw material of the sapphire substrate is an ingot, and the ingot is processed from sapphire crystal. The relevant manufacturing process is as follows:
1. Crystal growth: The use of a crystal growth furnace to grow large-sized and high-quality single crystal sapphire crystals.
2. Orientation: Ensure the correct position of the sapphire crystal on the crowbar machine and facilitate the processing of the pry bar.
3. Crowbar: Sapphire ingots are extracted from sapphire crystals in a specific way.
4. Barrel grinding: The outer cylindrical grinding of the ingot is performed by a cylindrical grinding machine to obtain accurate outer circle dimensional accuracy.
5, quality inspection: to ensure the quality of the ingot and the size and orientation of the ingot after the picking is in line with customer specifications.
6. Orientation: Accurately position the sapphire ingot on the microtome for accurate slice processing.
7. Slice: Cut the sapphire crystal ingot into a thin chip.
8. Grinding:: The chip is cut to damage the layer caused by the removal of the chip and the flatness of the chip is improved.
9. Chamfer: Trim the edge of the chip into an arc shape to improve the mechanical strength of the edge of the sheet and avoid the defects caused by stress concentration.
10. Polishing: Improve the roughness of the chip and make the surface reach the precision of the epitaxial wafer epitaxial level.
11. Cleaning: Remove contaminants from the surface of the chip (eg, dust particles, metals, organic contaminants, etc.).
12, quality inspection:: High-precision testing equipment to verify chip quality (flatness, surface dust particles, etc.) to meet customer requirements.
Fourth, sapphire substrate application types
The sapphire substrates used by the majority of epitaxial wafer manufacturers are divided into three types:
1, C-Plane sapphire substrate
This is the sapphire substrate surface commonly used by GaN for the growth of GaN. This is mainly because the sapphire crystal grows along the C-axis, the process is mature, the cost is relatively low, the physicochemical properties are stable, and the technology for epitaxial growth on the C-plane is mature and stable.
The C-Plane sapphire substrate is a commonly used sapphire substrate. In 1993, Professor Akasaka Yoshi of Japan and Dr. Nakamura Shuji of Nichia Chemical at that time broke through the problem of lattice mismatch (buffer layer) and p-type material activation of InGaN and sapphire substrate, and finally at the end of 1993. Chemistry has been able to develop blue LEDs first. In the next few years, Nichia Chemical used sapphire as the substrate, using InGaN material, and improved the sapphire substrate and epitaxial technology through MOCVD technology to improve the luminous efficiency of blue light. At the same time, UV LED was developed in 1997, and blue-violet LED in 1999. Samples began shipping and white LEDs were available in 2001. This laid the foundation for Nichia's leading position in the LED field.
Taiwan closely follows Japan's LED technology. Taiwan's LED development first purchased epitaxial wafer processing from Japan, and then bought MOCVD machines and sapphire substrates for epitaxy. Later, local Taiwanese manufacturers studied the growth and processing technology of sapphire crystals. Production, through independent research and development, to obtain LED patent authorization and other means to achieve sapphire crystal, substrate, epitaxial wafer production, epitaxial wafer processing and other independent production technology capabilities, step by step to establish Taiwan's important position in the LED upstream business. At present, most of the blue/green/white LED products are products that are produced by MOCVD epitaxy using sapphire substrates, represented by Japan and Taiwan. The sapphire substrate is very common, and the US company Cree uses SiC as the substrate. The representative LED products follow.
2, R-Plane or M-Plane sapphire substrate
It is mainly used to grow non-polar/semi-polar GaN epitaxial films to improve luminous efficiency. The GaN epitaxial film usually prepared on a sapphire substrate is grown along the c-axis, and the c-axis is the polar axis of GaN, resulting in GaN. A strong built-in electric field appears in the active layer quantum well of the base device, and the luminous efficiency is thus reduced. The non-polar surface GaN epitaxy is developed to overcome this physical phenomenon and improve the luminous efficiency.
Designing micro- or nano-scale micro-structure-specific patterns on sapphire substrates by etching (in sapphire C-face dry etching/wet etching) to control the output light form of LEDs (on sapphire substrates) The concave-convex pattern produces a light scattering or refraction effect to increase the light extraction rate), and the GaN film grows on the patterned sapphire substrate to produce a lateral epitaxial effect, which reduces the difference in defects between GaN grown on the sapphire substrate, and improves The epitaxial quality improves the internal quantum efficiency of the LED and increases the light extraction efficiency. Compared with LEDs grown on general sapphire substrates, brightness has increased by more than 70%. At present, Taiwan produces patterned sapphire with Sino-American twin, crystal, and megacrystal. 2/4 inch of sapphire substrate is a mature product. The price is gradually stable, while the large-size (such as 6/8-inch) ordinary sapphire substrate and the 2-inch patterned sapphire substrate are in the growth stage, and the price is also high. The manufacturer also pushes the large-sized and patterned sapphire substrate. At the same time, it is also actively increasing production capacity. At present, no manufacturer in the mainland can produce patterned sapphire substrates.
Nano-patterned sapphire substrate (Source: 椿 Technology)
3, patterned sapphire substrate (Pattern Sapphire Substrate referred to as PSS)
In the form of growth or etching, a nano-specific regular microstructure pattern is 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. Discharge defects, improve epitaxial quality, and improve the internal quantum efficiency of LEDs and increase light extraction efficiency.
Generally, a GaN thin film grown on a C-plane sapphire substrate grows along its polar axis, ie, the c-axis direction, and the film has spontaneous polarization and piezoelectric polarization effects, resulting in a strong inside of the thin film (active layer quantum well). The built-in electric field (Quantum Confine Stark Effect, QCSE; history tank effect) greatly reduces the luminous efficiency of GaN films. On some non-C-plane sapphire substrates (such as R or M surface) and other special substrates ( The GaN film grown on lithium aluminum oxide; LiAlO2) is non-polar and semi-polar, and the negative effects caused by the polarization field in the light-emitting device will be partially or even completely improved. The traditional three-five nitride Semiconductors are grown on c-plane sapphire substrates. If these compounds are grown on R-plane or M-Plane, the built-in electric field can be generated parallel to the epitaxial layer to increase the probability of electron hole-to-composite. Therefore, the growth of the LED structure based on the nitride epitaxial film on the R-plane or M-Plane sapphire substrate can effectively solve the problem of low efficiency of the quantum efficiency of the LED compared to the conventional C-plane sapphire epitaxy. Increase the luminous intensity of the component. The latest news that non-polar LEDs can increase the luminous efficiency of white light by two times.
Since non-polar GaN has more potential than traditional c-axis GMN to produce high-efficiency components, many international manufacturers and research institutes have increased research and production of such epitaxial technology. Therefore, for R-plane or M- The demand and requirements of Plane sapphire substrates have also increased accordingly.
The following picture shows a simple schematic of a semi-polar and non-polar surface.
The non-polar surface refers to the surface in the normal direction of the polar surface, and the semi-polar surface is the surface between the polar surface and the non-polar surface.
Five, the main technical parameters of the sapphire substrate
Due to different technologies and processes, epitaxial wafer manufacturers have different requirements for sapphire substrates, such as thickness and crystal orientation.
Listed below are some of the basic technical parameters of sapphire substrates produced by several manufacturers (taking the mature C-plane 2 inch sapphire substrate as an example). More are the epitaxial wafer manufacturers based on their own technical characteristics and the quality of the epitaxial wafers produced. It is required to customize the sapphire substrate to meet the requirements of the sapphire substrate manufacturer, that is, customized. They are:
A: Technical parameters of C-face 2 inch sapphire substrate of Taiwan Taoyuan Zhaojing Technology Co., Ltd.
B: Technical parameters of C-face 2 inch sapphire substrate of Taiwan Hsinchu Zhongmei Jingjing Products Co., Ltd.
C: Technical parameters of C-side 2 inch sapphire substrate of Crystal Systems, USA
D: Technical parameters of C-side 2 inch sapphire substrate of Cradley Crystals, Russia
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