1. Removing Oxygen from Quartz Stone Using Carbon
Silicon exists in quartz stone (silicon dioxide) bound to oxygen. Removing this oxygen is called "reduction," and metallic silicon is produced using carbon in this method [1]. The purity at this stage is said to be about 98 to 99%. While this number looks clean, it still has too many impurities for semiconductor use, so further purification is necessary [1]. The next section discusses that purification.
2. Turning Solid Silicon into Gas to Purify It
The method used here is called the Siemens process. Hydrogen chloride is reacted with metallic silicon at about 300°C to turn it into a gas called "trichlorosilane." This gas is distilled to reduce impurities, then reacted with hydrogen at high temperatures to extract high-purity polycrystalline silicon [1]. The reason for deliberately turning it into a gas is that distillation, a method of separation, can be used [1]. Distillation separates mixtures by using differences in boiling temperatures. It is easier to remove only impurities this way than when the material is solid.
The polycrystalline silicon made this way includes some with a purity of 99.999999999%, called "eleven nines" because there are eleven 9s in a row [2]. The ratio of impurities is about 1 in 100 billion.
3. Growing a Single Crystal by Slowly Pulling a Seed
Next, what is needed is a single crystal where the arrangement of atoms is continuous. Polycrystalline silicon is placed in a quartz crucible and melted at about 1420°C. A seed crystal is touched to the melt and slowly pulled up while rotating. This method is called the CZ (Czochralski) method [3]. A single crystal with the same atomic arrangement as the seed grows, becoming a thick rod called an ingot. For wafers with a diameter of 300 mm, the ingots become very large [3].
4. Slicing and Polishing Makes a Mirror-Like Wafer
The ingot is sliced to a thickness of about 1 mm after its outer diameter is adjusted. The cut surfaces are smoothed in order through lapping, etching, and polishing, ending with a mirror-like surface. Particles called colloidal silica are used for the polishing [4]. A wafer is defined as a disk-shaped substrate cut from silicon single crystals for semiconductors. After the wafer undergoes cleaning and inspection, it is ready for use. In current mass production of ICs, wafers with a diameter of 300 mm (12 inches) are used [5].
5. A Photo-Like Mechanism Where Light Shapes the Circuit
A liquid called "photoresist" that reacts to light is dripped onto the surface of the wafer. When the wafer is spun at high speed, centrifugal force spreads it thinly and evenly [6]. The thickness of the film is determined by the rotation speed and the viscosity of the liquid.
Next, light in the shape of the circuit is applied in a step called "exposure," and then developed. In the "positive type," the parts exposed to light are removed during development. Conversely, there is also a "negative type" where the exposed parts remain [6]. This series of processes is called "photolithography." It is repeated with etching and ion implantation to stack many layers of wiring on top of transistor layers [5][7].
6. Guiding Light Absorbed by Air with Vacuum and Mirrors
In the exposure process that creates parts of the circuits for cutting-edge chips, extreme ultraviolet (EUV) light with a wavelength of 13.5 nm is sometimes used. Because this light is absorbed by air and glass, the inside of the machine is kept in a vacuum, and light is guided by multilayer mirrors instead of lenses [8].
The light source is said to be made by hitting a droplet of tin about 30 μm in diameter with a high-output CO₂ laser to create plasma [9]. There is also an explanation that only the Dutch company ASML supplies EUV lithography equipment [9]. It is said that processes that required multiple exposures with conventional ArF immersion (wavelength 193 nm) can sometimes be completed in just one exposure with EUV [9]. This does not happen for all processes.
7. Blowing Away Dust from Clothes and Bodies with Air
In the clean room where circuits are drawn, fine particles come from people's clothes and bodies. Therefore, workers wear clean suits, hoods, and masks. At the entrance, they enter after an air shower blows dust off their clothes [10]. The air is flowed in a way that carries dust downward, such as from top to bottom [10]. Cleanliness is indicated by the international standard ISO 14644 (ISO Class 1 to 9). Specific class numbers for semiconductor factories are not listed here because the way they are written varies by document [10].
8. Cutting, Connecting with Wires, and Encasing in Resin
Wafers with circuits created in the front-end process are cut into many chips by dicing. The chips are placed on a base and connected to external terminals with thin gold or copper wires (wire bonding). After that, they are sealed with resin and proceed to inspection and testing [5][11]. The number of chips obtained from one wafer changes depending on the size of the chip.
9. Finding Dust in Your Home with Light
Try making your room a little dark and shining a flashlight (a smartphone light is fine) from the side. Dust that you usually cannot see will appear floating in the beam of light. Then, lightly shake your clothes or walk around, and compare the amount of dust in the light before and after. You can see with your own eyes how human movement affects the dust in the light. You should be able to imagine physically why semiconductor factories manage people's clothing.
For those who want to know more, you can read explanations of the actual processes on the product pages of SUMCO and Tokuyama (in Japanese) [3][2].