Chapter 2. City Gas Consumption in Energy-Intensive Industries
3. City Gas Consumption in the Fabricated Metal Industry
3.2. City Gas Consumption in Semiconductor Fabrication
The semiconductor fabrication process can be roughly divided into three stages: wafer production, semiconductor manufacturing, and semiconductor assembly. A wafer is a semiconductor circuit board made of silicon. To make wafers, a single crystal silicon ingot is cut into thin slices, which are then shaped, polished, and cleaned to make them flat and remove any defects. When fabricating a wafer, a semiconductor circuit is designed together and drawn on a glass plate to produce a photomask.
Figure 2-15. Flow Chart of Wafer Fabrication
Source: BM Energy website (http://www.bmenergy.co.kr/sub/sub_0301.php, last accessed on December 5, 2019).
실리콘 원석 Silicon metal
결정 성장로 Crystal growth chamber
다결정 실리콘 덩어리로 채워진 석형도가니 Quartz crucible full of polycrystalline silicon
결정 성장 Crystal growth
성장된 단결 정봉 (잉곳) Grown single crystal ingot
결합 시험 Binding test
잉곳 절단 Ingot shaping
테두리 연마 Edge polishing
웨이퍼 평탄화 Wafer flattening
웨이퍼 식각 Wafer etching
웨이퍼 세정 (먼지/금속 불순물) Wafer cleaning (dust/metal impurities)
웨이퍼 완성 Finished wafer
Once the wafer is fabricated, it moves on to the semiconductor fabrication stage. Semiconductor fabrication includes a series of processes: oxidation, photoresist application, exposure, development, etching, ion implantation, chemical vapor deposition, and metallization.
Figure 2-16. Flow Chart of Semiconductor Production
Source: Inside data from the Korea Semiconductor Industry Association.
웨이퍼 생산단계 Wafer production
반도체 제조단계 Semiconductor fabrication
2) 실리콘 표면
- 산화막
- 실리콘 표면노출
- 실리콘 표면보호
2) Silicon surface - Oxide film
- Silicon surface exposure - Silicon surface protection 8) 연마공정 8) Polishing
9) 클리닝 공정 9) Cleaning
반도체 조립 단계 Semiconductor assembly
10) 불량품 선별 10) Defect screening
11) 웨이퍼 절단 11) Wafer cutting
12) Chip 부착 12) Chip attachment 13) 화학수지 밀봉 13) Chemical resin sealing 14) 최종검사 및 출구 14) Final test and release
F가스 제거시설 F-gas removal facility
대기 방지시설 Air pollution prevention facility
배출 Emission
First, an oxide film is formed by spraying oxygen or water vapor on the wafer surface. The oxide film prevents short circuits in the semiconductor wiring. Next, a photoresist is applied to the surface of the wafer, and a photomask containing the pattern of an integrated circuit is placed on top of it. Using strong ultraviolet rays, the circuit pattern is drawn onto the wafer. In the developing stage, a developer is sprayed onto the wafer, removing the photoresist from the areas that were exposed to UV rays but not from the unexposed areas. In the etching stage, the circuit pattern is etched onto the surface of the wafer, and a process gas, such as hydrogen fluoride, is sprayed onto the wafer surface to cause corrosion of the areas where the photoresist was removed. In the ion implantation stage, impurities in the form of gas particles are injected into the parts connected to the circuit to change the electrical properties of the target. In the deposition stage, a process gas is sprayed onto the wafer surface to form an insulating film that serves as a protective layer. In the metallization stage, metal wires made of gold, silver, and aluminum are connected according to the circuit pattern drawn on the wafer surface. When metallization is completed, the surface of the semiconductor is polished and cleaned to finish.
In the semiconductor assembly process, defective products are screened by testing each semiconductor made out of a wafer, and the ones without defects are cut into chips. Next, centipede- like metal wires are connected to the semiconductor chip, which is then sealed with plastic for protection. The finished product undergoes a final performance test before it is released.
3.2.2. City Gas Consumption in Semiconductor Fabrication
There are two main uses of city gas in semiconductor fabrication. First, city gas is used as boiler fuel for maintaining the temperature and humidity in the clean room. Semiconductor fabrication facilities operate 24 hours a day, 365 days a year. They are never shut down, even when the semiconductor market is in decline, because it takes considerable time and money to restart them. Therefore, the industry’s city gas consumption remains relatively steady despite the fluctuations in the production volume of semiconductors. In addition, new facilities begin operation in phases, leading to a high initial energy level, since most of the basic energy is used even if the initial production level is low.
Second, city gas is used to incinerate the process gases, such as hydrogen fluoride, used for semiconductor fabrication. As explained in the section on semiconductor fabrication, process gases such as hydrogen fluoride are used in the etching and chemical vapor deposition stages. If released into the atmosphere after use, these process gases cause air pollution. To prevent this, they are subjected to a pollutant removal process in the incineration facility, which is fueled by city gas.
Of these two uses, the amount of city gas consumed to maintain constant temperature and humidity in a clean room is determined by the size of the production facility, while the consumption of city gas used for the incineration of air pollutants is determined by the production volume. An examination of the proportions of city gas consumed for both purposes shows that the volume of city gas consumed by a boiler in a clean room is much greater. Therefore, the overall consumption of city gas in semiconductor fabrication is mostly determined by the size of the production facility.