Bulk gas system: several gases with large consumption, including nitrogen (N2), oxygen (O2), hydrogen (H2), argon (AR), helium (He) and compressed air (CDA).
I. Overview of Bulk Gas System
Bulk gas system is mainly composed of gas supply system and pipeline system. The gas supply system includes gas source, purification and quality monitoring. The gas source is generally set in the gas field independent of the production plant, and the gas purification is generally set in the special purification room in the production plant, which can ensure the reduction of the transportation distance of high-purity gas, not only ensure the gas quality, but also save the cost. The pipeline for conveying high-purity gas generally adopts SUS 3 16L EP grade pipeline. The purified high-purity gas is transported from the purification room to the overhead floor of the sub-factory or production workshop to form a pipe network, and finally sent to various production equipment by hooks.
Among them, nitrogen is most used in the whole Fab manufacturing, and it can be divided into ordinary nitrogen (GN2) and process purified nitrogen (PN2) according to the different quality requirements of the use point.
Schematic diagram of N2 power supply system
Second, the bulk gas supply system
2. 1 gas station
2. 1. 1 First of all, the most reasonable and economical gas supply mode must be selected according to the gas consumption required by the factory.
In the overview, it has been explained that nitrogen is the most commonly used. According to its different uses, the following gas supply modes can be considered:
1) Liquid nitrogen storage tanks are regularly filled with tank trucks, and high-pressure liquefied gas is evaporated into gas by vaporizer for factory use. This method is suitable for moderate gas consumption in general semiconductor factories, and it is also the most widely used method at present.
Carburetor carburetor
2) Nitrogen is produced on site by air separation unit. This is suitable for the case of a large amount of N2. Integrated circuit chip manufacturers often use this method to supply gas, and also set up liquid nitrogen storage tanks as a backup.
3) Oxygen and argon are often supplied by ultra-low temperature liquid oxygen storage tank with evaporator.
Schematic diagram of oxygen and argon supply
4) Hydrogen is supplied in gaseous state, and generally one steel cylinder bundle can meet the production requirements. If the gas consumption is large, the pipe trailer can be used to supply gas, but it is rarely used in China at present due to factors such as road fire safety approval. It is believed that with the rapid development of China's microelectronics industry, the relevant safety regulations will be more perfect, and more pipe trailers will be used for gas supply. If the amount of hydrogen is quite large, it is necessary to produce hydrogen on site, such as using a water electrolysis device.
Schematic diagram of hydrogen and helium supply
5) Helium gas supply, generally a steel tube bundle can meet the production requirements. If the gas consumption is large, the pipe trailer /ISO tank car can be used for gas supply.
6) Compressed air is mainly produced by the compressor in the gas field, which is absorbed by the dryer to obtain dry and clean compressed air, which is called CDA for short. CDA is generally used in all industries, and the pressure of semiconductor Fab is slightly higher than that of general industries, usually above 8.5bar.
In the whole gas station, we need to pay special attention to several issues:
First of all, we must attach great importance to the safety of hydrogen supply system and oxygen supply system. For example, the plane layout of gas stations must meet the relevant national and industrial safety standards.
Secondly, when designing gas supply pressure, we should not only refer to the pressure demand of end users, but also consider the pressure drop of purifiers, filters and pipeline systems.
2.2 gas purification and filtration
2.2. 1 gas quality requirements
At present, the purity requirement of bulk gas often reaches ppb level, and the production process line requires higher quality of bulk gas.
Therefore, it is necessary to use stainless steel pipes to transport bulk gas from the gas filling station to the purification room of the production plant for purification. Impurities in the gas are removed by a purifier, and then particles are removed by a filter. For safety reasons, the hydrogen purification room is generally designed as a single room with explosion-proof requirements.
purifier
At present, all gas purifiers used in China are imported, and the main manufacturers are SAES, Sun, Toyo, JPC, ATTO, etc. The purifier can purify different gases according to its different working principles.
Generally speaking, N2 and O2 purifiers mostly adopt catalytic adsorption, while getter is the best for Ar, H2 and he purifiers, and H2 purifier also adopts catalytic adsorption combined with getter.
CDA often uses a heated adsorption dryer to dry compressed air. Adsorption dryer uses the characteristics of adsorbents (activated alumina, silica gel and molecular sieve) to reduce the moisture content in compressed air. Generally speaking, the dew point of the outlet air can reach above -40 degrees.
It should be noted in the design that different gas purifiers need different utilities to match. For example, catalytic adsorption N2 purifier requires high purity hydrogen for regeneration; The catalyst adsorption purifier needs cooling water. Therefore, the relevant utility pipelines must have interfaces in the gas purification room.
Attention should be paid to the problem of dew condensation in the indoor pipeline at the back end of CDA dryer. Many of them were not fully considered in the design, and some pipelines were seriously dewed in the later operation, especially those near the wall, which had a certain impact on the wall.
2.3 Gas quality monitoring
After purification and filtration, the quality of feed gas should be monitored to see whether its purity and particle size are higher than the actual process requirements. At present, online continuous monitoring of oxygen content, water content and particle size in gas is emphasized, while intermittent monitoring of CO, CO2 and THC impurities is adopted, and conditional Fab will also be continuously monitored. Test results and other test parameters (such as pressure and flow) will be sent to SCADA (Monitoring and Data Acquisition) system in the control room. At the same time, these data can also be shared with necessary units or organizations.
2.4 Reliability of gas supply system
Because the input and output of microelectronics industry are very large, any interruption of gas supply will bring huge economic losses. Therefore, the safety and reliability of the gas supply system must be fully considered in the design. If on-site gas production is adopted, it is often necessary to set up a storage gas supply system for this gas as a backup.
Every gas purifier needs a spare one. Prevent one purifier from switching to the standby purifier in time when it is abnormal. Ensure the stability of gas supply.