Continuous Improvements and Future Challenges of Air Pollution Control at an Advanced Semiconductor Fab

This study reviews the air pollution control strategy at an advanced semiconductor fab focusing on its continuous improvements and future challenges. A wide range of air pollutants is emitted from various sources classified as organic solvents, corrosive, toxic and combustible gases. This effective strategy employs a two-stage treatment method to comply with national emission regulations. Eight different types of local scrubbers (typ. gas flow rate: 0.3–2.0 CMM for the dry type or 60–83.3 CMM for the wet type) are used as pre-treatment devices at the first stage to remove specific target pollutants with high concentrations emitted from process chambers. Exhaust gases from local scrubbers are then grouped and further treated by central control facilities at the second stage, including the dual zeolite rotor-concentrator plus the thermal oxidizer for VOCs (typ. gas flow rate: 2500 CMM), the dual-central wet scrubbers (CWS) and alkaline CWS (typ. gas flow rate: 2000 CMM) for acid and alkaline gases, respectively. After the two-stage treatment, the removal efficiency of the VOCs can reach higher than 98.4%, surpassing the emission standard of 90%. The design parameters and operating conditions of the CWSs meet the criteria set in the emission standard for the semiconductor industry. In the future, CWS performance can further be improved by using advanced structured packing materials with larger specific surface areas to shorten the residence time and lower the chemical dosing amount and pressure drop while achieving higher removal efficiency for acid and alkaline gases at a reduced operating cost. The challenges to removing derived fine PM and white smoke still remain which can be resolved by using efficient control devices in the pre-and post-treatment stages, such as wet electrostatic precipitators. Finally, the by-product NO x can be minimized by using low-NO x burners or de-NO x control technologies in the future.


INTRODUCTION
Modern technologies such as artificial intelligence, the Internet of Things, and 5G have revolutionized our daily lives in which semiconductor chips have played a critical role.Taiwan is known to be the leader in semiconductor manufacturing with a comprehensive semiconductor chip production infrastructure and supply chain.In 2020, the global market shares in the semiconductor industry and semiconductor foundry of Taiwan reached 30% and 70%, respectively (Chang et al., 2021).Taiwan Semiconductor Manufacturing Company Limited (TSMC), the subject of this article, has become the world's largest semiconductor manufacturing company for advanced chips which creates a great impact on the world's economy (Wang and Chiu, 2014).TSMC was founded in 1987 with its headquarters located in Hsinchu Science Park in Northern Taiwan.Currently, nine semiconductor manufacturing fabs have been established in Taiwan including four 12-inch GIGAFAB fabs, four 8-inch wafer fabs, and one 6-inch wafer fab.In 2021, the total capacity was around 13 million equivalent 12-inch wafers.TSMC would like to establish more fabs by 2025 to meet the high demand for semiconductor chips (TSMC, 2021).
With the continuous expansion of semiconductor manufacturing capacity and the new construction of fabs in TSMC and other companies, care should be taken since the environmental and human health impacts of the manufacturing processes are obvious (Liu et al., 2010).Among all environmental issues, air pollution control is one of the critical issues that has drawn the greatest attention of the TSMC stakeholders besides waste management, water management and climate change (TSMC, 2020).It was found that semiconductor fabs faced white smoke issues (Tsai et al., 1997;Liu et al., 2010), adverse health effects on workers and nearby residents exposure to potential hazardous air pollutants (Liu et al., 2010;Chang et al., 2022), pollution of Ga/As ultrafine particles (Chen et al., 2016) and stinking odor (Chang et al., 2022), etc.
Air pollutants in the advanced semiconductor fab such as TSMC are emitted from many different fabrication processes, during which many chemicals (such as metal, chlorinated, fluorinated, halogenated and organic solvents, inorganic acids, perfluoro-compounds (PFCs)) are used (Tsai et al., 2004;Huang et al., 2005;Eom et al., 2006;Choi et al., 2018).As shown in Fig. 1, there are five fundamental steps to complete one mask layer process, including (1) wafer cleaning; (2) thin film deposition; (3) photolithography; (4) etching; and (5) stripping.The manufacturing procedure starts with the wafer cleaning step by the wet bench, which removes contaminants on the wafer surface.After the cleaning step, the thin film deposition process is chosen from one of the furnaces, plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition (APCVD), or sputter tool.Then the Fig. 1.A fundamental semiconductor manufacturing process of one mask layer production, process chemicals, and corresponding gaseous pollutants.process proceeds with the photoresist (PR) coating, exposure, and development to transfer the pattern from the mask to the wafer.The etching process is to form the pattern of the device by dry plasma etch or wet etch, followed by the photoresist stripping process to remove the photoresist by the dry plasma asher or wet bench.Ion implant is an optional process to adjust the concentration of N-type or P-type dopant to get the designed film resistance and depth junction (Wolf and Tauber, 1986).The toxic and combustible gases are mainly used in the processes of oxidation, diffusion, ion implantation and CVD.Acid, alkaline, and organic solvent gases are mainly used in the etching, cleaning, and organic solvents such as hexamethyldisilazane (HMDS), 1-methoxy-2-propanol (PGME), and tetramethyl ammonium hydroxide (TMAH) are used in photolithography processes.The gases exhausted from these pollutant sources, which have various toxic, corrosive or flammable properties, need to be classified well before the exhaust tubes are grouped and connected to the corresponding specific local scrubbers (LSCs) (typ.gas flow rate: 0.3 to 2.0 CMM for the dry type or 60-83.3CMM for the wet type) for the pre-treatment.After that, the pre-treated gas pollutant concentration is reduced substantially but some pollutants including volatile organic compounds (VOCs), acid (such as H2SO4, HF, HCl, HNO3, and H3PO4) and alkaline (such as NH3) gases, heavy metals and particulate matters are remained (Chein et al., 2004;Hung et al., 2005;Eom et al., 2006), which are required to be treated further in different terminal central facilities (typ.gas flow rate is 2000 to 2500 CMM) before being discharged to the atmosphere to meet the emission regulations.
To reduce the environmental impact, the air pollution emission standard for the semiconductor industry was promulgated by the Taiwan Environmental Protection Agency (TWEPA) for the semiconductor industry (TWEPA, 2002).VOCs in the exhaust gas need to meet either a total emission rate of less than 0.6 kg hr -1 (based on methane) or a removal efficiency of higher than 90%.Additionally, individual acid gases (HF, HCl, HNO3, and H3PO4) need to meet either an emission rate of less than 0.6 kg hr -1 or a removal efficiency of higher than 95%.A stricter emission rate of 0.1 kg hr -1 but a lower removal efficiency of 90% is regulated for H2SO4 acid gas.The removal efficiency of acid gases is normally applied to wet washing devices (for example, wet scrubbers, WSs).If the efficiency can't meet the standards, the control devices just need to meet the design criteria and operating conditions including the pH value > 7, wetting factor > 0.1 m 2 hr -1 , residence time > 0.5 s, and specific surface area > 90 m 2 m -3 (TWEPA, 2002).It indicates that the emission standards are not very stringent for both VOC and acid gas control for the semiconductor industry which are now being revised to be more stringent.

Central VOC Control Device
Control of VOCs such as isopropyl alcohol (IPA), toluene, benzene, methanol, ethylbenzene, and xylene is considered the top priority since they account for 34% of the total emitted air pollutants in TSMC (TSMC, 2020).VOC abatement methods can be categorized as destruction methods, capture methods, and hybrid methods, and their advantages and disadvantages are shown in Table 1 (Krishnamurthy et al., 2020;Chang et al., 2003;Cooper and Alley, 2010).Although these methods are well developed, not all of them are suitable for treating the exhaust flow from a semiconductor fab containing different organic species with low concentrations and high flow rates.Among them, the hybrid honeycomb zeolite rotor concentrator (HZRC) combined with a thermal oxidation (TO) system has been proven to be an effective physical-chemical method for treating the VOC in the semiconductor exhaust flow with a flow rate as high as 10 4 to 5 × 10 5 m 3 h -1 (Chang et al., 2003;Yang et al., 2012;Guo et al., 2022).In addition, the hybrid method is economical and yields fewer secondary emissions compared with other single oxidation processes.However, to achieve very high removal efficiency of VOCs (i.e., > 95%), the design and operating parameters such as rotation speed, concentration ratio, process flow rate, inlet concentration of VOCs, and the type of zeolite coated on the concentrator need to be optimized for a specific semiconductor fab.For instance, the optimal operating conditions of the HZRC to achieve high removal efficiency are: the air inlet velocity of HZRC is < 1.5 m s -1 , the rotation speed is 6.5 m s -1 , and a desorption temperature is 200-225°C (Lin and Chang, 2009).The temperature of the oxidation process also needs to be controlled well (i.e., lower than 1000°C) to avoid the formation of secondary pollutants such as NOx (Kim et al., 2018).In the hybrid method, regenerative thermal oxidation (RTO) was suggested to replace the TO for thermal decomposition (Warahena and Chuah, 2009)  proven to achieve low power consumption and high energy saving.However, some practical application issues of the RTO were found in the study company such as pressure fluctuation and VOC cross-contamination, which needs to be further improved if it is to be adopted (Please see the details in Section 2.3).

Central Acid and Alkaline Gas Control Devices
There are many different acid and alkaline gas removal control technologies with their advantages and disadvantages (Cooper and Alley, 2010).A wet scrubber is a widely used abatement device to control the emission of acid and alkaline gases (Chein et al., 2005;Li et al., 2021).The advantages of the WSs are their small footprint, low capital cost, and simple structure.However, it was reported that the conventional WSs using conventional packing material could not achieve high removal efficiency for the acid and alkaline gas pollutant concentrations less than 1.0 ppm (Tsai et al., 2004) and 3.0 ppm (Chein et al., 2005), respectively.Thus, to keep the high removal efficiency of acid and alkaline gases, the parameters including the contact area of liquid/gas, pH value, residence time, and chemical dosing amount are essential for design considerations (Li et al., 2021).
In addition, it was found that using structured packing materials can help to reduce the operation cost with less amount of dosing chemicals and shorter residence time (Le et al., 2021).Also, for the semiconductor exhaust flow treatment, the WSs need to be feasible for mixed acid or alkaline gases with a wide range of concentrations from ppm levels to ppb levels, which could not be handled effectively by existing conventional WSs (Chien et al., 2015a).

Central PM and NOx Control Devices
The other pollutants, such as derivative particulate matter (PM) or NOx, are known to be emitted from an advanced semiconductor fab (Huang et al., 2005;Kim et al., 2018) and are only regulated in the less stringent emission standard for all stationary sources but not the special standard for the semiconductor industry.Typically, PMs and NOx are the by-products formed in the LSCs installed behind process tools.The formation of condensable PMs (CPMs) can result in white smoke issues (Tsai et al., 1997;Li et al., 2021) and the NOx emission can lead to ozone formation (Kim et al., 2018).Note that metals such as Ga, As, In and Cr used in semiconductor manufacturing can be also emitted and present in the particulate phase resulting in metal particulate pollution (Chen et al., 2016).It was found that PMs are mainly in the range of 0.1-1.0µm with the number and mass percentages of up to 99.67% and 96.43% (Choi et al., 2018), respectively, which have to be removed by more efficient control devices, such as the wet electrostatic precipitator (WEP) (Lin et al., 2013).In addition, when some new chemicals are used for semiconductor manufacturing processes, unexpected and unregulated gaseous pollutants can be released.For instance, N2O is known to be an important greenhouse gas at an advanced semiconductor fab which also needs to be treated (TSMC, 2019).Thus, an integrated approach should be implemented to deal with many different pollutant emissions with complex waste gas compositions from different sources which are discharged at high flow rates at an advanced semiconductor fab.
The air pollution control technology at the study company, which employs LSCs for pre-treatment and central HZRC plus TO and central WS for terminal treatment of VOCs and acid and alkaline gases, has proved to meet the emission standards and achieve the sustainability goals in 2030.However, the current standards are not stringent enough.In addition, the study company needs to deal with the total emission control issue since many more fabs will be constructed in the future.Therefore, it is imperative to develop new technologies to reduce emissions further.This study aims to review the development of air pollution control technologies at the advanced semiconductor fab over the past years and address the future challenges to future reduce emissions.The need to remove PM, NOx and N2O is also identified.Hopefully, this study can provide comprehensive information for establishing an effective air pollution control technology for other semiconductor fabs.

THE CURRENT AIR POLLUTION CONTROL TECHNOLOGY
TSMC has established a comprehensive air pollution prevention strategy as shown in Fig. 2, in which the best available technology (BAT) is adopted, including source classification and multistage processing for effective air pollution control to meet the TWEPA regulation (TSMC, 2016).First, the pollutants emitted from different manufacturing processes are identified, categorized, collected and abated by many different types of LSCs.After that, the pre-treated exhaust flows of different pollutants are sent to central treatment facilities.After two-stage treatment, the exhaust gases in the stack are monitored by the continuous emission monitoring system to ensure the normal operation of these control devices with good control efficiency.In addition, the backup system is applied to prevent abnormal events and ensure continuous operation for 24 hours per day and 365 days per year to achieve the removal efficiency for any pollutant to be as high as 95% (TSMC, 2021).This air pollution prevention strategy is currently being applied in different semiconductor fabs owned by the study company, located in Taiwan and oversea countries such as China, and will be adopted in new facilities in the U.S. and Japan in the near future.

Performance of Local Scrubbers
For the abatement of many different pollutants emitted from various process tools, eight different types of LSCs are used, which are burn-wet, plasma-wet, thermal-wet, and high-temperature thermal-wet, wet-chemical dosage, adsorption, condensation, and wet scrubbing followed by wet electrostatic precipitator (Wet-WEP).For the wet etching process, the wet-type LSCs operate at a flow rate of 60-83.3CMM while for the other dry processes, dry-type LSCs such as adsorption and condensation operate at a flow rate of 0.3-2.0CMM.Table 2 shows the target pollutant sources and performance of these LSCs.

Burn-wet LSC
The burn-wet LSC (Fig. 3(a)) is the most well-developed device compared to other LSCs and is generally used to abate the PFCs emitted from the etching process.The waste gas is first treated in a combustion chamber at a temperature higher than 1000°C where CH4 fuel gas is burned.Some byproducts generated from the burning process, such as HF and HCl are then removed in the subsequent packed column with NaOH solution.To obtain the high efficiency of > 99% for acid gases, the pH of the absorption liquid is controlled at 10-11.The scrubbing liquid flow is 20-25 L min -1 and the makeup water is about 0.5-2 L min -1 .Since high temperature is required for the effective decomposition of PFCs, NOx could be formed and become the secondary pollutants.

Plasma-wet LSC
Plasma-wet LSC (Fig. 3(b)) is also used to treat some PFCs such as C4F6, CF4, and C5F8 released from the dry etching process.The plasma-wet LSC needs to increase the temperature higher than 2000°C by using a torch to achieve high decomposition efficiency for these PFCs.Then, the generated acid gases (such as HF) can be washed out in the packed column with a removal efficiency of > 95%.Another by-product, NOx, can be generated as well.Since this type of LSC also has a limited operation flow rate, it is only used for treating waste gases from a specific process.

Thermal-wet LSC
The thermal-wet LSC (Fig. 3(c)) is widely used in treating some bulk gases and special gases which has a thermal decomposition temperature lower than 850°C such as SiH4, WF6, NF3, NH3, HF, and Cl2.This type of LSC consists of a spray chamber followed by a heated chamber by heating elements and a second spray (or scrubbing) chamber.The first chamber is to wash out the powder emitted from the processes.The heated chamber is to oxidize and decompose the target gases, in which metallic or ceramic materials are used for the heating elements.By-product acid gases (i.e., HCl, HBr, Cl2, and HF) are abated in the spray column with a high removal efficiency of > 95% due to their high solubility.The SiO2 powder generated during the thermal decomposition process is also scrubbed in the second washing chamber into the buffer tank and then drained to the wastewater treatment unit.Due to low-temperature operation compared to the burn-wet and plasma-wet LSCs, NOx generation is limited in this local treatment device.

High-temperature thermal-wet LSC
Compared to thermal-wet LSC, the high-temperature thermal-wet LSC uses similar technology but is operated at a higher temperature, such as 1000-1100°C to decompose nitrous oxide (N2O) effectively, which is a new chemical gas used with a large amount in the thin film process in addition to ammonia (NH3) and fluorinated greenhouse gas (F-GHG).Since N2O is also categorized as GHG and causes the formation of photochemical smog, therefore it must be treated with high efficiency.This method has a lower operating cost, shorter residence time (i.e., less than two seconds) and higher overall removal efficiency for all gases than other methods such as catalytic decomposition (Javed et al., 2007).After high-temperature thermal-wet decomposition, the removal efficiencies of N2O, NH3 and F-GHG are higher than 90% and 99% (TSMC, 2019), respectively.Note that the by-products such as NOx can be an issue of this treatment method since the temperature is high at 1000-1100°C.Thermal decomposition is selected but not burn or plasma decomposition since the former uses fuels such as CH4, which can produce complex by-products including NOx, HCN, HNO, etc. and the latter has flow rate limitation.

Wet scrubber type LSC
Wet scrubber-type LSC is used to control the emission containing acid or alkaline gases emitted from the wet bench, wastewater and chemical storage tanks which don't contain combustible and organic solvent gases.It can be named as process site or facility site LSC depending on where it is used.In this packed-bed wet scrubber, NaOH (45%) and spent H2SO4 chemical solution are used as absorbents to abate acid and alkaline gases, respectively with the removal efficiency of higher than 95%.

Dry-adsorption LSC
The dry-adsorption LSC is mainly used to treat toxic gases containing some non-water-soluble metals (i.e., PH3, As, Co).This treatment method is an irreversible chemical reaction and adsorption method using normal-temperature or high-temperature catalysts.The waste gas is channeled to the chamber containing the adsorption catalyst for toxic gas removal and then the exhaust is further introduced into the central WS as the second stage.A gas sensor is used to monitor whether the catalyst is still functional.The dry-absorption LSC is commonly used in the process having low exhaust gas flow rates such as the ion implantation process.

Wet-WEP (wet scrubbing and wet electrostatic precipitator)
Wet-WEP is used to remove acid, alkaline gases, IPA and H2SO4 -containing particles generated from the wet-bench cleaning process (Fig. 3(d)).The H2SO4 -containing particles are formed in the exhaust gas of the high-temperature sulfuric peroxide mixture (HT-SPM).The Wet-WEP includes a stage of wet scrubber for acid, and alkaline gas removal and a stage of wet electrostatic precipitator for particle removal.The waste gas first enters the scrubbing chamber from the top to the bottom and then passes through the WEP from the bottom to the top.In the WEP stage, particles are charged by corona discharge and then collected on the grounded electrodes on which water is sprayed continuously to remove the deposited particles.Water mist is also sprayed on the top of the WEP to clean the discharge electrodes when necessary.As a result, the removal efficiency of particles is greatly improved (> 95%).However, the spark-over phenomenon occurs easily due to the existence of water droplets.Therefore, obtaining the optimal operating conditions for this LSC is very important.This Wet-WEP LSC is limited to the exhaust gas which has H2SO4 -containing particles but not other non-soluble particles, and it also needs to be stopped periodically for cleaning the discharge electrodes to maintain high removal efficiency.

Condensation LSC
The condensation LSC is used to deal with the organic gases having high boiling points (> 150°C) in the photoresist stripping process such as N-methyl-pyrrolidinone (NMP) and Dimethyl sulfoxide (DMSO).Generally, condensation involves a cooling process to lower the temperature of the exhaust gas below the dew point, which consists of a dehumidification system and a refrigerator.Dehumidification is conducted to reduce the moisture and prevent the icing effects when the waste gas is cooled to a temperature below 0°C in the refrigerator.Currently, the removal efficiency of the condensation LSC is greater than 95%.
In summary, the above LSCs help to abate the pollutant emission greatly with an overall removal efficiency of > 90%.However, some challenges need to be overcome.For instance, the formation of secondary pollutants such as NOx in the burn-wet, plasma-wet, and high-temperature thermal-wet LSCs needs to be removed.In addition, the formation of derivative PMs from the other manufacturing and combustion processes is possible which needs further treatment.The safety of Wet-WEP also needs to be considered since the explosion hazard happened in the WEP in a semiconductor fab in the past (Kim et al., 2019).In the future, a better design of the Wet-WEP can be studied for stable operation for long-term use without frequent maintenance needs.
After the pollutants are treated by LSCs in the first stage, the exhaust gases containing low concentrations of inorganic acid gases, alkaline gases, and VOCs were classified, grouped and introduced separately to the corresponding central control facilities in the second stage, including the acid exhaust systems (SEX), ammonia exhaust systems (AEX), and VOC exhaust systems (VEX), respectively, for further abatement to meet the emission standards.The SEX and AEX systems are central WSs for acid and alkaline gas control, respectively, while VEX systems use HZRC-TO for VOC control.The acid and alkaline gases are treated separately in SEX and AEX systems, respectively, to avoid white smoke generated due to the chemical reaction between mixed acid and alkaline gases.

Performance of Central WSs for Acid and Alkaline Gas Abatement
Two types of central WSs for acid and alkaline gas control are currently used in the advanced semiconductor fabs of the study company.The schematic diagram of the first type of the central WSs, the so-called single central WSs is shown in Fig. 4(a).The single central WS is for acid gas removal, solid-liquid particle capture, and water droplet removal.It consists of an air distributor, packing material (i.e., raschig rings), pre-filter layer, water sprinkler, demister 2, self-cleaning system, and demister 1, which are assembled in series from the bottom to the top.The air distributor helps to create a uniform air flow pass through the packing material to enhance the diffusion effectiveness.The pre-filter and demister 2 are to remove particles while demister 1 is to remove droplets.The self-cleaning is conducted by recycling condensed water from the make-up air unit (MAU) to wash out particles collected in the demister 2. The acid gases are removed in the packing material by diffusion and chemical reaction on the surface of raschig rings.The exhaust gas is introduced into the device from the bottom to the top.NaOH solution is used as the scrubbing liquid for the acid gas deduction.
To achieve high removal efficiency of the acid gases, the central WS is designed to have a residence time of > 0.73 s, a wetting factor of > 0.155 m 2 h -1 , the pH of the scrubbing liquid is 8-11, and the specific surface area of > 128 m 2 m -3 .It is seen that these operation parameters of the central WSs far surpass the regulatory requirements as shown in Table 3.A similar design is used for alkaline gas control.But the scrubbing liquid is the spent H2SO4 to control the pH value at 2-6 (Table 3).It is also seen that the design parameters and operating conditions of the central WSs for alkaline gas control meet the criteria regulated by the study company (TSMC, 2016).The criteria of the design and operation parameters can be applied to design effective central scrubbers operating at a flow rate of less than 2000 CMM at other fabs as well.
Another type of central WS is the dual-stage WS (TSMC, 2019).The dual-stage WS is used for the wet-bench exhaust gas which has co-existing acid gas with a small amount of alkaline and organic gases.The dual-stage WS consists of two towers which are connected in series as shown in Fig. 4(b).The first tower is for acid gas removal, solid-liquid particle capture, and water droplet removal similar to the single central WS.The second tower, which is a spray tower without packing material, consists of three water-spaying systems and a demister at the top.The second tower is to remove alkaline and organic gases and water droplets.In the first tower, NaOH solution is used as the scrubbing liquid for acid gas removal while only water is spayed for scrubbing un-removed gases in the second tower.

Performance of Central VOCs Control Facility
The study company uses the hybrid system for VOCs abatement consisting of two HZRC in series (so-called dual HZRC) and one TO as shown in Fig. 5.The VOCs exhaust gases from the LSCs are combined and introduced to the first HZRC to concentrate VOCs and then introduced to the second HZRC for further adsorption to reduce the emitted VOC concentration effectively.After the first HZRC, the VOC concentration in the exhaust gas can be reduced to as low as 2-5 ppm and is further reduced to 0.5-1.5 ppm after passing through the second HZRC, which is near the ambient VOC levels.
To adsorb VOCs effectively, the exhaust gas is first introduced into the cooling section, where the temperature of the exhaust gas is reduced to below the dew point of the VOCs.However, cooling gas can result in the condensation of water vapor when the relative humidity of the exhaust gas is higher than 80% (Yamauchi et al., 2007).Thus, a condenser may be needed for pre-treatment of the humid gas before the HZRC to help enhance the removal efficiency of the VOCs having high solubility (Lin et al., 2005).Instead of employing the condenser, the study company uses a so-called wet-impregnation zeolite material to resolve this issue and help enhance the adsorption efficiency (TSMC, 2019).
The adsorbed VOC in two HZRCs is desorbed and sent to the TO for thermal oxidation and decomposition.The desorption temperature is set at 220-230°C to desorb VOCs with high boiling points completely.The burning temperature is set at 730°C to oxidize successfully the VOCs and minimize the by-product formation (Kim et al., 2018).The overall removal efficiency of the VOCs is as high as 98.4%, which meets the TWEPA standard of > 90%.Additionally, the TO which uses a 3-channel heat recovery system can achieve a heat recovery efficiency of over 70% to reduce CH4 consumption and GHG emission.
The study company has spent lots of efforts to enhance the effectiveness of the VOCs control system over the years by using the dual HZRC method which has been proven to be very effective compared to the single HZRC used in the previous years as shown in Table 4. Using dual HZRC not only can reduce the VOC emission concentration but also helps to achieve a higher concentration  3.5 4 ratio of 16 leading to energy-saving in the TO.That is, this HZRC-TO hybrid method helps to treat exhaust gas with a high flow rate but less energy consumption based on the optimal operation conditions of the HZRC-TO (shown in Table 4) in previous studies (Chang et al., 2003;Lin and Chang, 2009).The operating parameters of the HZRC-TO can be applied to design the effective VOC control devices operating at the flow rate of less than 2500 CMM at the other fabs.
Although the RTO method used in the study company in previous years (2016-2017) has higher energy recovery efficiency of 95% than the TO method, which is 73%, the TO method is now adopted because the cross-contamination problem was found in the RTO system.In the RTO system, during exhaust gas and post-treated gas switching in the ceramic heat exchange materials by the poppet valves once every 3 min, the VOC removal efficiency is reduced due to residual VOC gas existence in the system (TSMC, 2020).In addition, a high pressure fluctuation of up to 50 Pa in the piping system is created during the valve switching process.The clogging by high boiling point VOCs in the ceramic media is also another issue in the RTO but not the TO system.Therefore, for practical application in the advanced semiconductor fab, the RTO method needs to be further improved.

Improvements
With continuous efforts, the air pollution control systems of the study company have been improved for better control performance from 2016 to 2021 as shown in Fig. 6.It shows a continuous decline of unit air pollution emission or emission factor from 0.87 to 0.43 g per 12" mask layer while the total emission amount of all air pollutants: VOCs, acid gases and alkaline gases, remain nearly the same from 295.2 to 309.3 tons.Note that the emission factor was calculated as the ratio of the total emission amount of all air pollutants divided by the total number of 12-inch equivalent wafer mask layers produced.The results indicate that while more fabs have been constructed with higher production capacity, the effectiveness of the BAT air pollution control systems is increased at the same time, especially for the VOC control.After the improvement of the control technology, the VOCs emission accounts for ~30% while acid and alkaline gases account for ~70% of total emission amount in 2021 in comparison to the much higher VOC percentage before improvement.
As shown in Fig. 7, the average VOC removal efficiency increases from 95.5% in 2016 to 98.4% in 2021, leading to the reduction of total VOC emission amount from 123.6 to 104.5 tons yr -1 during the period despite the increase in the production capacity.The emission amount of the acid gas is also slightly reduced from 107.2 to 100.1 tons yr -1 in the same period.In contrast, the emission amount of the alkaline gas is not reduced but increased from 61.4 to 91.5 tons yr -1 .Although the study company has done very good work in reducing the emission factor, the total emission amount is still high at ca. 300 tons and needs more new technologies to achieve the goal  of "zero pollution" for air emission (TSMC, 2015), and new sustainable environmental management, green manufacturing, pollution reduction goals set by 2030 (TSMC, 2019).In particular, alkaline gas reduction is urgently needed to ease the increase in the total emission amount.

Challenges of Acid and Alkaline Gas Control
It is mentioned in Section 2 that the design parameters and operating conditions of the central WS well meet the TWEPA criteria in the emission standard.The exhaust gas sent to the central WSs has low pollutant concentrations of acid and alkaline gases and contains pollutants in particle and droplet phases, which come from the process chambers and the pre-treatment process in the LSCs in the first stage with mixed gases.The current design of the central WSs has been improved over the years to enhance the removal efficiency of these low concentration pollutants.As compared to the previous design, the tower height was increased from 8 to 9.3 m and the packing material height also increased by two times with the specific area increased by 1.4 times to increase the exhaust gas retention time in the control equipment.With better contact between exhaust gas and absorption liquid, the pollutant removal efficiency is 10% higher than with traditional treatment and the removal efficiency for all acid gases is improved from 71 to 89%.
It is to be noted that the removal efficiency of the acid gases is not high enough and the increase in the residence time and packing height can lead to an increase in the pressure drop, power consumption and operation cost when the traditional packing materials such as rachig rings are used.Some WSs packed with structured packing materials such as honeycomb wet scrubber (HWS) or parallel-plate WS (PPWS) (Chien et al., 2015a(Chien et al., , 2015c;;Le et al., 2021) can be the better alternative control methods.The PPWS was proved to achieve a very high removal efficiency of > 99% for HF, HCl and CH3COOH with very low concentration (sub-ppm level) in a short residence time (0.5 s).But it is costly to scale up the PPWS with the nano-TiO2 coated plates for industrial applications.The HWS, which uses water-absorbing polypropylene (PP) fabric with a higher specific surface area of 483 m 2 m -3 as packing material, can achieve very high efficiency (> 95%) for the mixed acid gases of HF, CH3COOH, HCl, HNO3, HNO2, and H2SO4 with the inlet concentrations ranging from supper-ppmv to sub-ppmv during a long operation period of 3.5-yr period (2017-2021) (Le et al., 2021).The pressure drop of HWS is also very small ranging from 0.5-0.8cm H2O.In the future, this high-efficiency HWS can be employed as the central and local devices to further enhance the removal efficiency of acid and alkaline gases while reducing the power consumption, carbon emission, and size of the control devices.

Challenges of PM and White Smoke Control
Particulate matter formation and white smoke are the existing problems at the study company, which are due to the chemical reactions or mixing of acid and alkaline gases in the waste gas streams.At the process tools, which use both acid and base chemicals in the process, a switching box is installed in the common exhaust duct to separate gases to the SEX or AEX.However, the switching box fails to separate gases to the target exhaust system owing to the delayed time of airflow from the upstream to downstream of the duct, two waste gas flows can be mixed to form particle pollutants thereby leading to white smoke and PM2.5 issues.Additionally, during the single wafer wet cleaning process, the exhaust gas mixing of acid and alkaline gases can occur which creates particles.To resolve these problems, the common duct after tools and switching boxes were removed and replaced by the specific exhaust ducts to pass the specific waste gases separately to the SEX, AEX, or VEX depending on the recipe of the process.
The formation of particles owing to the reaction of gaseous compounds during the local treatment processes at high temperatures is also observed.Fig. 8 shows the particle mass concentration distributions measured by the NCTU micro-orifice cascade impactor (NMCI) (Chien et al., 2015b) at the exhaust of some LSCs such as burn-wet (Fig. 8(a), plasma-wet (Fig. 8(b)), and thermal-wet (Fig. 8(c)) installed after different process tools.It is found that although the sizes of particles emitted from these LSCs are different, the mass median aerodynamic diameters (MMADs) fall in a similar range of 100-300 nm.This particle size range normally corresponds to the lowest removal efficiency of PM control devices (Lin et al., 2013;Chen et al., 2014).The total suspended particle mass concentrations (TSP) are also different among different local treatment methods and pollutant sources, which vary from 2.5-42.8mg m -3 .To deal with PM removal, control devices of cyclones, centrifugal devices and wet-WEP (Fig. 3(d)) for PMs are installed after some types of LSCs in the study company (TSMC, 2017(TSMC, , 2019)).Cyclones and centrifugal devices are not efficient for fine particle removal but only feasible for large droplet removal while the wet-WEP are only used in the wet-bench cleaning process but not other processes due to safety reasons in the study company.In the future, some control devices showing better performance for fine PM removal can be considered as an additional local stage after the existing LSCs.An efficient venturi scrubber assisted with particle condensation growth developed by Tsai et al. (2005) was proved to be effective for the treatment of the fine PMs in the exhaust of the LSCs with a removal efficiency of 80-90% for particles larger than 100 nm.In this type of venturi scrubber, the fine-water mist is generated to cool down the hot exhaust gas from the LSCs to supper-saturation conditions resulting in the effective condensation growth of fine particles to micro-sized particles which are then removed efficiently.A similar device developed by Huang et al. (2007), in which a hot stream was used to replace the fine-water mist to enhance the particle condensation growth, can achieve a higher removal efficiency of > 90% for fine particles.For the practical application, the saturated steam can be generated by utilizing the heat from the combustion process in the LSCs.When the particle condensation growth was combined with a wire-to-plate single-stage WEP, a very high removal efficiency of 99.2-99.7%for nanoparticles (< 100 nm) can also be obtained (Chen et al., 2014).That is, these advanced technologies after LSCs are available for efficient PM control.
Since fine PMs cannot be treated well in the LSCs, further treatment by the central control device is desirable to reduce the emission further.The WEP is a feasible method in the central treatment phase when the organic gases can be separated effectively.A very high collection efficiency of 91-99.9%can be achieved for particles ranging from 30-1870 nm in a wire-on-plate ESP (Li et al., 2015).However, the particle accumulation on the ground plate needs to be cleaned regularly to avoid the gradual decrease in removal efficiency due to the loading effect (Lin et al., 2013;Chen et al., 2014).A wire-to-plate single-stage WEP was proven to last for 22-25 operation days when the collection plates were cleaned in 10 s for every 10 min (Chen et al., 2014).But the operation of the WEP needed to be stopped temporarily during cleaning owing to safety issues.Thus, further improvement of the WEP for continuous operation is required.In addition, many other types of WEP or hybrid WEP have been studied to deal with some typical characteristics of the semiconductor exhaust gas.For instance, a wet electroscrubber which is the combination of a WS and a WEP was developed for controlling both acid gas and PMs (Jaworek et al., 2006;Su et al., 2019) or an integrated WEP-WS was established for PM and NOx control (Sung et al., 2020).To prevent the failure caused by explosive gases, the non-explosive WEP was also proposed, in which the charging stage is designed to be outside of the main flow duct to prevent contact of explosive gas with sparks (Kim et al., 2019).Also, when the exhaust gas contains corrosive gas, the WEP using anticorrosive metallic materials was studied to achieve a high charging effect and low corrosion problem (Kim et al., 2012).That is, the WEP technique has been improved substantially for particle removal as the local device, and can be a good choice as the central device in the future if it is scaled up.

Challenges of NOx Emission Amount Control
NOx is a secondary pollutant generated from high-temperature burning processes in control devices such as the burn-wet LSC, the plasma-wet LSC, and the TO of the central control device for VOC thermal oxidation.The formation of NOx is due to the chemical reactions of nitrogen in air/fuel with oxygen, hydroxyl and radicals in burning air, which happens at high temperatures and long residence time.For the LSCs in the study company, the ratio of gas-fuel was reduced from 2:1 to 1.4:1 to limit the oxygen content, and the temperature of the combustion zone was forced by N2 sheath air to drop very quickly from 2000°C to 800°C in 0.25-0.5 seconds to reduce NOx concentration.As a result, the amount of NOx emission was reduced from 126 tons in 2020 to 66.8 tons in 2022 (i.e., 53% removal amount).In the future, the NOx emission target is less than 40 tons, and the NOx emission concentration is less than 1.0 ppm, which warrants the installation of low-NOx burners in the RTO/TO or wet scrubbers for NO2 removal at the LSC stage (Kim et al., 2018;Sung et al., 2020).An integrated pilot device consisting of an indirect dielectric-barrierdischarge (DBD) plasma system and WS was reported to achieve the removal efficiency of NOx over 80%.In another WS method, NO was first oxidized to NO2 by O3 and then reduced to N2 by Na2S solution to achieve 90% removal efficiency (Sung et al., 2020).In the future, these WSs and low-NOx burners can further be tested in the study company to achieve the NOx emission goals.

CONCLUSION
The comprehensive air pollution control strategy at the advanced semiconductor fab with good robustness has been reviewed, which can be used as a reference for the other fabs in the future.The pollutant source characteristics emitted from different manufacturing processes determine the choice of specific LSCs as the first-stage treatment facilities followed by central devices as the second stage at the study company.The improvements and challenges of the current air pollution technology have been discussed highlighting the continuous reduction of emissions, and the needs and possibilities for even better control of acid and alkaline gases, PM, white smoke, and NOx in the future for further reduction of emissions.

Fig. 2 .
Fig. 2. The compressive air pollution control strategy at an advanced semiconductor fab.

Table 4 .
Comparison of the operation parameters of single HZRC+TO and dual HZRC + TO for VOC control at the advanced semiconductor fab.

Fig. 6 .
Fig. 6.The total emission amount of VOCs, acid and alkaline gas pollutants (bar chart), and the emission factor per unit of production (pink line) from 2016 to 2021 (TSMC, 2021).

Fig. 8 .
Fig. 8. Particle mass concentration distribution at the outlet of the LSCs outlet including (a) burn-wet, (b) plasma-wet, and (c) thermal-wet at the advanced semiconductor fab.

Table 1 .
since it is Comparison of different abatement methods for central VOC control devices.

Table 2 .
The target pollutant source and performance of the LSC (TSMC, 2020).