Analysis of Semiconductor Wafers

Catalog

I Wafer materials

Wafer refers to the substrate for manufacturing semiconductor transistors or integrated circuits. Substrate materials include silicon, germanium, GaAs, InP, GaN, etc. Since silicon is the most commonly used, if no crystal material is specified, a wafer usually refers to a silicon wafer.

Various circuit element structures can be fabricated on silicon wafers, thereby becoming integrated circuit products with specific electrical functions. The original material of the wafer is silicon, and silica can be found everywhere on the surface of the earth's crust. Silica ore is refined through an electric arc furnace, chlorinated with hydrochloric acid, and distilled to produce high-purity polycrystalline silicon with a purity of up to 99.999999999%.

A wafer is the basic raw material for manufacturing semiconductor devices. Very high purity semiconductors are prepared into wafers through crystal pulling and slicing processes. The wafers form a very small circuit structure through a series of semiconductor manufacturing processes, and then are cut, packaged, and tested into chips, which are widely used in various electronic equipment. Wafer materials have undergone more than 60 years of technological evolution and industrial development, forming an industry situation in which silicon is the mainstay and new semiconductor materials are complementary.

In the 1950s, germanium (Ge) was the first semiconductor material used and was first used in discrete devices. The production of integrated circuits is an important step forward for the semiconductor industry. In July 1958, at Texas Instruments in Dallas, the first integrated circuit was manufactured by Jack Kilby, which was manufactured using a piece of germanium semiconductor material as a substrate.

Semiconductor industry chain process

However, germanium devices have shortcomings in high-temperature resistance and radiation resistance and were gradually replaced by silicon (Si) devices in the late 1960s. The silicon reserves are extremely rich, the purification and crystallization processes are mature, and the silicon oxide (SiO2) film formed by oxidation has good insulation properties, which greatly improves the stability and reliability of the device. Therefore, silicon has become the most widely used semiconductor material. In terms of the output value of semiconductor devices, more than 95% of semiconductor devices and more than 99% of integrated circuits worldwide use silicon as the substrate material.

In 2017, the global semiconductor market was approximately US $ 412.2 billion, while the compound semiconductor market was approximately US $ 20 billion, accounting for less than 5%. From the perspective of the wafer substrate market size, the annual sales of silicon substrates in 2017 were US $ 8.7 billion, and the annual sales of GaAs substrates were approximately US $ 800 million. The annual sales of GaN substrates are about 100 million US dollars, and the annual sales of SiC substrates are about 300 million US dollars. Silicon substrate sales accounted for 85%. In the 21st century, its dominance and core position will not be shaken. However, the physical properties of Si materials limit their application in optoelectronics and high-frequency, high-power devices.

Semiconductor market share (by material)

Since the 1990s, the second generation of semiconductor materials represented by gallium arsenide (GaAs) and indium phosphide (InP) have begun to emerge. GaAs, InP, and other materials are suitable for making high-speed, high-frequency, high-power, and light-emitting electronic devices. They are excellent materials for making high-performance microwave, millimeter-wave devices, and light-emitting devices. They are widely used in satellite communications, mobile communications, optical communications, GPS navigation, and other fields. However, GaAs and InP materials are scarce, expensive, and toxic, and can pollute the environment. InP is even considered to be a suspected carcinogen. These shortcomings have limited the application of second-generation semiconductor materials.

The third-generation semiconductor materials mainly include SiC, GaN, etc. Because their bandgap (Eg) is greater than or equal to 2.3 electron volts (eV), they are also called wide bandgap semiconductor materials. Compared with the first and second generation semiconductor materials, the third generation semiconductor materials have the advantages of high thermal conductivity, high breakdown field strength, high saturation electron drift rate, and high bonding energy, which can meet the new requirements for harsh conditions such as high temperature, high voltage, high frequency, and radiation resistance and more. It is the most promising material in the field of semiconductor materials. It has important application prospects in the fields of defense, aviation, aerospace, oil exploration, optical storage, etc. It can reduce energy losses by more than 50% in many strategic industries such as broadband communications, solar energy, automotive manufacturing, semiconductor lighting, and smart grids and the equipment volume can be reduced by more than 75%. It is a milestone for the development of human science and technology.

Compound semiconductors refer to semiconductor materials formed by two or more elements, and the second and third-generation semiconductors mostly belong to this category. According to the number of elements, it can be divided into binary compounds, ternary compounds, quaternary compounds, etc. Binary compound semiconductors can also be divided into III-V group, IV-IV group, II-VI, etc according to the position of the constituent elements in the chemical element periodic table family. Compound semiconductor materials represented by gallium arsenide (GaAs), gallium nitride (GaN), and silicon carbide (SiC) have become the fastest-growing, most widely-used, and largest-output semiconductor materials after silicon. Compound semiconductor materials have superior performance and energy band structure:

(1) High electron mobility;

(2) High frequency;

(3) Wide bandwidth;

(4) High linearity;

(5) High power;

(6) Diversity of material selection;

(7) Anti-radiation.

Therefore, compound semiconductors are mostly used in the manufacture of radiofrequency devices, optoelectronic devices, power devices, etc., and have great development potential; silicon devices are mostly used in logic devices, memories, etc., and they are irreplaceable with each other.

II Wafer Preparation: Substrate and Epitaxial Process

Wafer preparation includes substrate preparation and the epitaxy process. The substrate is a wafer made of semiconductor single crystal material. The substrate can directly enter the wafer manufacturing process to produce semiconductor devices, or it can be processed by epitaxy to produce epitaxial wafers. Epitaxy refers to the process of growing a new single crystal on a single crystal substrate. The new single crystal can be the same material as the substrate or it can be a different material. Epitaxy can produce a wider variety of materials, providing more choices for designing a device.

The basic steps of substrate preparation are as follows: semiconductor polycrystalline materials are first purified, doped, and drawn to obtain single-crystal materials. Taking silicon as an example, silica sand is first refined to a metallurgical grade coarse silicon with a purity of about 98%. After multiple purifications, electronic grade high-purity polycrystalline silicon (purity above 99.9999999%, 9 ~ 11 9s) is obtained, and single-crystal silicon rods are obtained through furnace drawing. The single-crystal material undergoes mechanical processing, chemical treatment, surface polishing, and quality inspection to obtain a single crystal polished sheet that meets certain standards (thickness, crystal orientation, flatness, parallelism, and damaged layer). The purpose of polishing is to further remove the residual damage layer on the processed surface. The polishing pad can be used directly to make devices or as an epitaxial substrate material.

Basic steps of substrate preparation

The epitaxial growth process currently includes two types of MOCVD (Chemical Vapor Deposition) technology and MBE (Molecular Beam Epitaxy) technology. For example, New Optoelectronics uses MOCVD, and Intelle uses MBE technology.

Schematic diagram of the epitaxial wafer structure

In contrast, MOCVD technology has a faster growth rate and is more suitable for industrial mass production, while MBE technology is more suitable for use in the production of PHEMT structures and Sb compound semiconductors in some cases. HVPE (Hydride Vapor Phase Epitaxy) technology is mainly used in the production of GaN substrates. LPE (liquid phase deposition) technology is mainly used for silicon wafers, which has been basically replaced by vapor deposition technology.

III Wafer size: technology development progress is different

The maximum silicon wafer size is 12 inches, and the compound semiconductor wafer size is 6 inches. The mainstream size of silicon wafer substrates is 12 inches, accounting for about 65% of global silicon wafer production capacity. 8-inch wafers are also commonly used mature process wafers, with global production capacity accounting for 25%. The mainstream sizes of GaAs substrates are 4 inches and 6 inches; the mainstream supply sizes of SiC substrates are 2 inches and 4 inches; GaN self-supporting substrates are mainly 2 inches.

The current size of the SiC substrate has reached 6 inches, 8 inches are being developed (II-VI company has produced samples). In fact, mainstream adoption is still 4-inch wafers. The main reasons are: (1) The current 6-inch SiC wafer is about 2.25 times the cost of the 4-inch, and it will be about 2 times by 2020. There is no major progress in cost reduction, and the replacement of equipment requires additional capital expenditures. The current advantage of 6-inch is only in production efficiency; (2) 6-inch SiC wafers are lower in quality than 4-inch wafers, so the current 6-inch wafers are mainly used to manufacture diodes. Manufacturing diodes on lower quality wafers is simpler than manufacturing MOSFETs.

Epitaxial growth corresponds to wafer size

GaN materials lack single crystal materials in nature, so they have been epitaxial on sapphire, SiC, Si, and other heterogeneous substrates for a long time. Currently, GaN self-supporting substrates of 2 inches, 3 inches, and 4 inches can be produced by hydride vapor phase epitaxy (HVPE) and ammonothermal method. At present, the GaN epitaxy on heterogeneous substrates is still the main business application. GaN self-supporting substrates have the largest application in lasers and can obtain higher luminous efficiency and luminous quality.

IV Silicon: Mainstream market, strong demand in subdivided fields

The pattern of suppliers from silicon wafer supply: Japanese factories control the market, and the pattern of oligarchs is stable. Japanese manufacturers account for more than 50% of the market share of silicon wafers. The top five manufacturers account for more than 90% of the global share. Among them, Shin-Etsu Chemical in Japan accounted for 27% and Japan's SUMCO accounted for 26%. The two Japanese manufacturers accounted for 53% of the total, more than half. Taiwan Global Wafer of China acquired the US SunEdison Semiconductor during the trough of the wafer industry in December 2016. The sixth promotion to third place, accounting for 17%, Germany Siltronic accounted for 13%, South Korea SK Siltron (formerly LG Siltron, acquired by SK Group in 2017) accounted for 9%. Unlike the top four manufacturers, SK Siltron only supplies Korean customers.

In addition, there are French Soitec, Taiwan Sembcorp, Hejing, Jiajing, and other companies, with relatively small shares. The types and sizes of wafers supplied by major manufacturers are different. Overall, the products of the top three manufacturers are more diverse. The top three manufacturers can supply Si annealed wafers and SOI wafers, of which only Shin-Etsu Japan can supply 12-inch SOI wafers. Siltronic in Germany and SK Siltron in South Korea do not provide SOI wafers, and SK Siltron does not supply Si annealed wafers. The sizes of Si polished wafers and Si epitaxial wafers of these companies are basically the same.

In the past 15 years, Japanese manufacturers have always occupied more than 50% of the market share of silicon wafers. There is no obvious regional transfer of silicon wafer production capacity. According to Gartner, in 2007, Shintotsu Japan (32.5%) ranked first in the market share of silicon wafers, Japan SUMCO (21.7%) second, and Germany Siltronic (14.8%) third; in 2002, Shin-Etsu, Japan (28.9%) ranked first silicon wafer market share, Japan SUMCO (23.3%) ranked second, and  Germany Siltronic (15.4%) ranked third. The most recent change in the market is the acquisition of SunEdison in the United States by Taiwan Global Wafer in December 2016, and it has promoted from the sixth to the third largest manufacturer. However, Japanese manufacturers have always occupied a 50% share.

Japan's competitiveness in the fab segment has declined while the material segment has always maintained its leading position. In the mid-1980s, the world share of the Japanese semiconductor industry exceeded 50%. Japan's advantages in the field of semiconductor materials have continued from the last century, while the competitiveness of wafer manufacturing has weakened significantly, and there has been a significant regional shift in the semiconductor fab link. The reason is that the fab link is closer to the demand side and the market changes greatly; however, the silicon wafer is highly homogenized, and new players need to have a longer time to verify it with customers. And the cost of wafers in wafer foundry accounts for less than 10%, and the foundry is not willing to risk replacing immature products for smaller price differences.

V Downstream applications for silicon wafers: Advancement in size and process

The wafer size and process are developed in parallel, and each process stage corresponds to the wafer size. (1) Process progress → transistor shrinkage → transistor density doubled → performance improved. (2) Increased wafer size → more chips produced per wafer → improved efficiency → reduced cost. At present, 6-inch and 8-inch silicon wafer production equipment are generally depreciated and the production cost is lower. It mainly produces mature processes above 90nm. Some processes have outputs on adjacent-sized wafers. From 5nm to 0.13μm, 12-inch wafers are used. Among them, 28nm is the boundary between advanced and mature processes. The main reason is that after 28nm, new designs and processes such as FinFET are introduced, which greatly increases the difficulty of wafer manufacturing.

Silicon wafer size corresponds to process

In terms of total wafer demand, the 12-inch and 8-inch NAND are the core drivers in the markets. 12-inch silicon wafers for storage accounted for the largest proportion of 35%, followed by 8-inch and 12-inch logic. In terms of product sales, memory accounts for about 27.8% of globally integrated circuit products, logic circuits account for 33%, and microprocessor chips and analog circuits account for 21.9% and 17.3%, respectively. Global demand for 12-inch silicon wafers in 2018 was about 5.86 million pieces/month, of which 1.5 million pieces/month for logic chips, 1.4 million pieces/month for DRAM, and 1.8 million pieces/month for NAND, including NORFlash, CIS and others are 1.2 million pieces/month; 8-inch silicon wafers require 5.54 million pieces/month, which is converted to 12-inch wafers by the area of about 2.45 million pieces/month.

It is estimated that the demand for 12-inch wafers including NAND and DRAM in the storage market accounts for about 35% of the total demand, the demand for 8-inch wafers accounts for about 27% of the total demand, and the demand for 12-inch wafers for logic chips is about 17%. In terms of demand, memory currently contributes the most demand for wafers, followed by 8-inch low-end applications.

In terms of specific downstream applications, advanced processes below 12 inches and 20nm have strong performance and are mainly used in mobile devices, high-performance computing, and other fields, including smartphone main chips, computer CPUs, GPUs, high-performance FPGAs, and ASICs. 14nm-32nm advanced manufacturing processes are used in applications including DRAM, NAND Flash memory chips, low-end processor chips, image processors, digital TV set-top boxes, etc.

The mature process of 12-inch 45-90nm is mainly used in areas with slightly lower performance requirements and high cost and production efficiency requirements, such as mobile phone baseband, WiFi, GPS, Bluetooth, NFC, ZigBee, NOR Flash chip, MCU, etc. 12-inch or 8-inch 90nm to 0.15μm is mainly used in MCU, fingerprint recognition chip, image sensor, power management chip, LCD driver IC, etc. 8-inch 0.18μm-0.25μm are mainly non-volatile storage such as bank cards, sim cards, etc., more than 0.35μm are mainly MOSFET, IGBT, and other power devices.

VI Compound semiconductors: key materials for electric vehicles, 5G, 3D sensing

1 The structure of compound semiconductor wafer suppliers: Japan, the United States, and Germany dominate the market

Substrate market: The high-tech threshold has led to an oligopoly in the compound semiconductor substrate market, led by Japanese, American, and German manufacturers. GaAs substrates are currently occupied by Sumitomo Electric in Japan, Freiberg in Germany, AXT in the United States, and Sumitomo Chemical in Japan, with the share of the four exceeding 90%. Sumitomo Chemical acquired the compound semiconductor business of Hitachi Cable (Hitachi Metal) in 2011 and transferred it to its subsidiary Sciocs in 2016. GaN self-supporting substrates are currently monopolized by three Japanese companies, Sumitomo Electric, Mitsubishi Chemical, and Sumitomo Chemical, accounting for a total of over 85%. The leading SiC substrate is Cree (Wolfspeed Division) in the United States, accounting for more than one-third of the market, followed by Germany's SiCrystal, the United States II-VI, and the United States Dow Corning, with a combined total of more than 90%. In recent years, SiC substrate manufacturers with certain mass production capabilities have also appeared in China, such as Beijing Tianke Heda Semiconductor Co., Ltd.

In the epitaxial growth market, the UK IQE market accounted for more than 60% as the absolute leader. The share of IQE in the UK and Visual Photonics in Taiwan reached 80%. The epitaxial growth mainly includes MOCVD (Chemical Vapor Deposition) technology and MBE (Molecular Beam Epitaxy) technology. For example, IQE and the new photoelectricity all use MOCVD, and Intelle uses MBE technology. HVPE (hydride vapor phase epitaxy) technology is mainly used in the production of GaN substrates.

The compound semiconductor industry chain presents an oligopoly competition pattern. IDM vendors include Skyworks, Broadcom (Avago), Qorvo, Anadigics, etc. In 2016, the global compound semiconductor IDM exhibited a three-oligopoly pattern. In 2016, IDM manufacturers Skyworks, Qorvo, and Broadcom accounted for 30.7%, 28%, and 7.4% of the market share in the field of gallium arsenide, respectively. The industrial chain is showing a multi-mode integration trend, and design companies' dewafer and IDM capacity outsourcing has become inevitable trends.

Global output value distribution of GaAs components (including IDM)

2 Compound semiconductor wafer downstream applications

The specific downstream applications of compound semiconductors can be divided into two major categories: optical devices and electronic equipment. Optical devices include LED light-emitting diodes, LD laser diodes, PD light receivers, and so on. Electronic devices include PA power amplifiers, LNA low noise amplifiers, radio frequency switches, digital-to-analog conversion, microwave monolithic ICs, power semiconductor devices, Hall elements, etc. For GaAs materials, SC GaAs (single crystal gallium arsenide) are mainly used in optical devices, and SI GaAs (semi-insulating gallium arsenide) is mainly used in electronic devices.

Among the optical devices, LED has the largest proportion, and LD / PD and VCSEL have a large room for growth. About 70% of Cree's revenue comes from LEDs, and the rest comes from power, RF, and SiC wafers. 80% of the market for SiC substrates comes from diodes. Among all wide bandgap semiconductor substrates, SiC materials are the most mature. LEDs made of different compound semiconductor materials correspond to different wavelengths of light: GaAs LEDs emit red light, green light, GaP emits green light, SiC emits yellow light, GaN emits blue light, and GaN blue LEDs can be used to excite yellow fluorescent materials to produce white LEDs. In addition, GaAs can manufacture infrared LEDs, which are commonly used in infrared emission of remote controllers, and GaN can manufacture ultraviolet LEDs. The red and blue laser emitters manufactured by GaAs and GaN can be used to read CDs, DVDs, and Blu-ray discs.

Electronic devices are mainly RF and power applications. GaN on SiC, GaN self-supporting substrate, GaAs substrate, GaAs on Si are mainly used in RF semiconductors (RF front-end PA, etc.); GaN on Si and SiC substrates are mainly used in power semiconductors (automotive electronics, etc.).

Because of its high power density, GaN has unique advantages in the field of base station high-power devices. Compared with silicon substrates, SiC substrates have better thermal conductivity. At present, more than 95% of GaN RF devices in the industry use SiC substrates. For example, Qorvo uses SiC substrate-based processes, while silicon-based GaN devices can be manufactured on 8-inch wafers, which has cost advantages. The GaN market is mostly in the low-voltage field, while SiC is used in the high-voltage field. 

Article Recommended:

What is a Semiconductor?

Semiconductor Materials: Types, Properties and Production Process