Published: April 17, 2025
In the previous volume, as a prelude to discussing silicon (Si) wafers, I talked about how silicon, a common material, became a main player. This time, I will explain silicon wafers.
How Silicon Wafers Are Made
“Extraction of Silicon” and “Increasing Purity”
About Crystals
How Single-Crystal Silicon is Produced
Silicon Wafer Slicing
Volume 11: Shift to Larger Diameter Silicon Wafers (Part 2): How Silicon Wafers Are Made
First, let’s take a look at an overview of the silicon wafer manufacturing process.
(1) Extracting Silicon (Si) from Silicon Dioxide (SiO2)
As explained in the previous volume, silicon does not exist naturally in its pure form, so it must first be extracted from the raw material, silica rock (an ore composed primarily of SiO2). The extracted lump of silicon is called silicon metal, or metallurgical-grade silicon.
(2) Increasing Purity
Silicon metal contains too many impurities to be used for semiconductor devices. Therefore, polycrystalline silicon is produced by refining silicon metal to a purity level suitable for semiconductor applications.
(3) Growing a Single Crystal
A cylindrical single-crystal block of silicon, called an ingot, is made from molten silicon derived from high-purity polycrystalline silicon. The Czochralski (CZ) method is mainly used for this process.
(4) Producing Wafers
A wafer is cut from the ingot, its thickness and shape are formed, and its surface is polished to a mirror finish. This completes the silicon wafer manufacturing process.
Note: The terms crystal, polycrystal, and single crystal are explained in more detail later.
Producing silicon wafers is quite a challenge. Silicon wafer manufacturers usually handle the process from (3) onward.
As I mentioned previously, germanium (Ge) was used primarily in the early days of the semiconductor industry, partly because it was easier to grow high-purity single crystals of germanium than silicon. This was also because device manufacturers and research institutes grew crystals on their own at the time and germanium was primarily used because it was easier to handle.
However, determining that silicon would become the dominant material in the future, great efforts were made toward the research and development of high-purity single-crystal silicon production. By the late 1950s, the technology had been perfected, and silicon became the mainstream material. Another factor contributing to silicon becoming dominant is its abundance on Earth, whereas germanium is a rare metal.
The details of each process will be explained in the following sections.
When silica rock (an ore primarily composed of SiO2) is used as a raw material and heated in an electric arc furnace with carbon sources such as charcoal, coal, or coke, the following chemical reaction occurs, resulting in the extraction of silicon. The bond between silicon and oxygen, Si-O, is very strong, requiring extremely high temperature, approximately 1800 to 1900°C.
SiO2 + 2C → Si + 2CO (In reality, the reaction proceeds through complex intermediate steps, ultimately yielding to Si + 2CO.)
Inside the heating furnace, silicon is in a molten state and is then cooled and solidified. The lump of silicon extracted in this way is called “silicon metal,” even though it is not a metal. Since electric furnaces consume enormous amounts of electricity, it is only economically viable to produce silicon metal in regions with low electricity costs. As a result, Japan imports all of its silicon metal, most of which comes from China.
For device manufacturers, their starting point is the silicon wafer, but the production of semiconductor devices actually begins with producing silicon metal from ore in China and other countries.
Silicon metal is also used as an additive in iron and aluminum alloys, as well as a raw material for solar cells and silicone. Only a small portion is used for silicon wafers. As I mentioned earlier, the purity of silicon metal is too low for use in silicon wafers, which requires a further purification process. Silicon metal is mainly considered a raw material for producing silicon wafers used in semiconductor devices.
Note: Silica rock and quartz, ore and mineral:
Silica rock, the raw material for silicon metal, is a rock primarily composed of silicon dioxide (SiO2) and is classified as a type of “industrial use” rock. Rocks that are valuable resources for economic activities are referred to as “ores.” Naturally occurring crystals of SiO2 are called quartz, and silica rock is primarily composed of quartz. A naturally occurring solid made of a single substance, such as quartz, is called a “mineral.” Rocks, on the other hand, are typically composed of multiple minerals. For example, granite is a rock composed of minerals such as quartz and feldspar.
Next, silicon metal is used as a raw material to produce high-purity silicon suitable for semiconductor devices. Since we have obtained the lump of silicon called silicon metal, one might assume that impurities are simply removed from it. However, despite having isolated elemental silicon, it is first converted into a compound. After purifying the compound, it is then converted back to elemental silicon, which is an intricate process.
Specifically, the primary manufacturing method is the Siemens process, developed in the late 1950s.
(1) Silicon metal reacts with HCl (hydrogen chloride) to form SiHCl3 (trichlorosilane), which is a liquid at room temperature. It also contains chloride from the reaction of impurities in metallic silicon with HCl. The chemical reaction is as follows:
Si + 3HCl → SiHCl3 + H2
(2) High-purity SiHCl3 is obtained by repeatedly distilling the compound to remove impurities.
(3) The high-purity SiHCl3 is reduced with H2 (hydrogen) to precipitate high-purity silicon. This means reversing the reaction (1) to convert it back to silicon.
This process results in extremely high-purity silicon, with a purity of 99.999999999% (referred to as “eleven-nines” or 11N, owing to the eleven nines in the number), making it suitable for semiconductor devices, or semiconductor grade. Directly purifying silicon metal to this level is extremely difficult. Therefore, a more complex process is employed, in which silicon is first converted into a compound called SiHCl3.
I have used the terms “crystal,” “polycrystal,” and “single crystal” multiple times without providing an explanation, so let me briefly explain what crystals are.
A crystal is “a solid in which atoms and so on are arranged in a regular pattern.” While some solids lack a regular arrangement, most semiconductors, including silicon, are fundamentally crystal.
The most familiar example of a crystal is probably the “snowflakes.” You may have seen hexagonal-shaped quartz crystals, which are SiO2 crystals. However, external shape in a regular pattern is rather rare. Even if the atoms are arranged in a regular pattern at the atomic level, in most cases this does not appear in the external shape.
Solids are in a lower energy state compared to gases and liquids, in which atoms and molecules have very limited movement. The arrangement in a regular pattern represents the lowest energy state, which is the most stable state.
A single crystal refers to a solid piece in which atoms are perfectly arranged in a regular pattern. throughout the entire material. In contrast, a polycrystal is made up of many small single crystals, where neighboring crystals may have different orientations or slight misalignments. When a liquid solidifies, if the solidification starts from a single point, the atoms align in a perfectly ordered pattern, forming a single crystal. However, if solidification begins at multiple points within the liquid, each area forms its own crystal. Since the orientations of these crystals can slightly differ, they do not combine into single, uniform crystal. The result is a cluster of small crystals, called a polycrystal.
Semiconductor devices such as integrated circuits, which are discussed here, primarily use single crystals. Electrons and holes can flow smoothly only when the atoms are arranged in a regular pattern.
The crystal structures of silicon and germanium are called the diamond cubic crystal structure, which is the same as that of diamond, the crystalline form of carbon (C). Interestingly, semiconductor devices can also be fabricated using diamonds, which is a crystalline form of carbon. Active research is conducted on diamond-based power semiconductors. Although diamond has greater potential than SiC and GaN as a power semiconductor material, its commercialization is still expected to take some time.
At this stage, silicon wafers are produced using high-purity polycrystalline silicon. The first step is to grow a block of single-crystal silicon, called an ingot.
As I mentioned above, when a liquid gradually solidifies starting from a single point, the atoms arrange themselves in a regular pattern to form a single crystal.
Single-crystal silicon for silicon wafers is mainly produced by the Czochralski (CZ) method.
High-purity polycrystalline silicon is melted at a high temperature, around 1,400°C, in a quartz container called a crucible to form molten silicon. When a small single-crystal seed is dipped into the molten silicon and slowly pulled upward while rotating, it causes a large, nearly cylindrical single crystal to grow beneath it (see figure below).
The process begins by cutting a disc from a single-crystal ingot. The disc is then ground to a desired thickness, the damaged surface layer is removed by etching, and finally, the surface is polished to a mirror finish for flatness, completing the process.
Flatness is extremely important. Simply polishing the surface to appear flat and shiny to the naked eye is not sufficient.
As explained in Volume 2, optical equipment called an exposure system is used to transfer the circuit pattern from the mask onto the photosensitive material coated on the wafer. However, if the focus is not perfectly adjusted on the surface of a wafer, a clear pattern cannot be formed, just like a blurry photo. If the surface of the wafer is uneven, some areas will be in focus, while others will be blurry.
So, how flat does the surface need to be? In the case of the reduction projection exposure system mentioned in Volume 2, the area exposed in a single operation is approximately 1 cm square. The required flatness to avoid blurry images in that area is, for comparison, as incredibly precise as a baseball field with no more than 1 mm of unevenness.
In this volume, I have explained silicon wafers. While writing this article, I conducted extensive research online and learned many new things. Although I was somewhat familiar with the processes involved in producing single crystals and subsequent steps, this is the first time I have learned the detailed steps involved in producing high-purity silicon from silica rock. I also learned the term “silicon metal” for the first time. Around the fall of 2021, while researching the term “silicon metal,” I came across news about power supply issues in China, which led to concerns about a shortage of silicon metal and caused its price to skyrocket. It made me more aware that such issues could impact our business. I had taken the availability of silicon wafers for granted, but I was reminded once again how challenging they are to produce. I gained appreciation for silicon wafers. In the next volume, which will be the last in this series, I will discuss the shift to larger diameter silicon wafers.
Click below to read this series.
Semiconductor Miniaturization:
Volume 1: Semiconductor Miniaturization: What is Moore’s Law?
Volume 2: Semiconductor Miniaturization and Manufacturing Process
Volume 3: Semiconductor Miniaturization and International Technology Roadmap
Volume 4: Semiconductor Miniaturization and Semiconductor Business
Volume 5: Semiconductor Miniaturization and Semiconductor Business (Part 2)
Volume 6: Semiconductor Miniaturization and Semiconductor Devices
Volume 7: Semiconductor Miniaturization: What is MOSFET Scaling?
Volume 8: Semiconductor Miniaturization: Limitations of MOSFET Scaling
Volume 9: Semiconductor Miniaturization and Analog Circuits
Shift to Larger Diameter Silicon Wafers:
Volume 10: Shift to Larger Diameter Silicon Wafers: How a Common Material, Silicon, Became a Main Player