How do you select catalysts for semiconductor cleanrooms?

For low-concentration ozone control in semiconductor cleanrooms, catalytic decomposition is comprehensively optimal in terms of safety, operating cost, and environmental compatibility. Catalyst selection should prioritize room-temperature decomposition efficiency, humidity tolerance, and service life, among which manganese-based mixed oxide catalysts represent a mature and reliable solution in the industry due to their high activity and stability. This article will systematically address engineering solutions for low-concentration ozone control in semiconductor cleanrooms from four aspects: ozone sources, critical hazards, comparison of abatement methods, and catalyst selection criteria.

Primary Sources of Ozone in Semiconductor Cleanrooms

Ozone in semiconductor cleanrooms primarily originates from two channels: process equipment and environmental introduction. Understanding the concentration ranges and fluctuation patterns of these sources is fundamental to designing a rational ozone control strategy.

Photolithography is one of the most significant sources of ozone. Deep ultraviolet (DUV) light sources (such as excimer lasers or mercury lamps) emit short-wavelength UV light that excites oxygen molecules in the air, causing photolysis to generate oxygen atoms, which then combine with oxygen molecules to form ozone. Inside the chamber of a lithography tool and near its exhaust, ozone concentrations can momentarily reach hundreds of ppb or even ppm levels, making this a critical contaminant source in localized areas of the cleanroom.

Corona discharge equipment is another non-negligible source of ozone. Static eliminators, certain types of plasma cleaning systems, and surface treatment devices commonly used in cleanrooms utilize high-voltage corona discharge to neutralize static charges or modify material surfaces. This discharge process simultaneously converts oxygen into ozone. Such equipment typically runs continuously, and while the ozone generated per unit is small, the cumulative effect from multiple units is significant.

Ozone brought in through HVAC systems from outside air also warrants attention. During summer or under specific climatic conditions, ambient ozone concentrations may rise and be introduced into the cleanroom via fresh air handling systems. Although HEPA/ULPA filters capture particles, they have essentially no filtration capacity for gaseous molecules like ozone, allowing outdoor ozone to enter the clean environment. For fabs located near coastal areas or industrial zones, this source can become a persistent background ozone load.

Actual Hazards of Low-Concentration Ozone to Cleanroom Processes and Equipment

In semiconductor manufacturing, even ppb-level ozone can significantly impact critical processes and device quality. The hazards primarily manifest as chemical attack on photoresist, metal layers, and polymeric materials.

Photolithography is most sensitive to ozone. As a strong oxidant, ozone reacts with organic components on the photoresist surface, altering its chemical structure. Specific manifestations include: loss of control over photoresist line width and critical dimensions (CD), roughened pattern edges, and even incomplete development or stripping of photoresist. In advanced process nodes, ozone concentrations exceeding 10-20 ppb can cause detectable lithographic defects, directly impacting chip yield.

Exposed metal layers are also susceptible to ozone corrosion. In metal interconnect or bond pad regions, materials such as copper, silver, and aluminum undergo accelerated oxidation in an ozone atmosphere, forming oxides or corrosion products on the surface. This corrosion not only increases contact resistance but may also lead to metal line断裂 or bridging shorts. For thin-film materials during deposition, ozone may alter their growth rate or compositional uniformity, causing device performance drift.

Accelerated aging of polymeric materials is the third typical hazard. Components used extensively in cleanrooms—such as seals, tubing, and cable jackets containing polymers—undergo accelerated surface cracking and pulverization under long-term low-concentration ozone exposure, releasing particulate contaminants. Once these submicron particles settle onto wafer surfaces, they create致命 defects. Additionally, ozone accelerates the loss of transmittance of certain optical components (such as pellicles for photomasks), shortening their service life.

Mainstream Technical Solutions for Ozone Abatement and Their Characteristics

For low-concentration ozone control in semiconductor cleanrooms, three technical routes are primarily used in engineering: activated carbon adsorption, thermal decomposition, and catalytic decomposition. These three approaches differ significantly in principle, operating conditions, and overall economy.

Activated carbon adsorption captures ozone through physical adsorption and surface chemisorption on porous carbon materials. Its advantages include high initial removal efficiency (up to 95% or more), simple equipment, and low capital cost. However, its drawbacks are equally prominent: saturated carbon requires replacement, and spent carbon is hazardous waste with high disposal costs; under low-humidity conditions, adsorbed saturated carbon poses a spontaneous combustion risk; and adsorption merely transfers rather than destroys ozone, failing to achieve true elimination.

Thermal decomposition heats ozone-containing gas above 300°C to thermally decompose ozone into oxygen. This method achieves complete decomposition unaffected by ozone concentration fluctuations. Nevertheless, continuous heating consumes substantial electricity, resulting in high operating costs; the slow thermal response makes it unsuitable for intermittent emissions or point-source local control; and the hot gas may adversely affect the cleanroom thermal load, typically limiting this method to centralized exhaust treatment systems.

Catalytic decomposition utilizes catalysts (such as transition metal oxides) to catalytically convert ozone into oxygen at room temperature. The catalyst itself is not consumed and can be used long-term; operation at ambient temperature and pressure consumes very little energy; the decomposition product is only oxygen, with no secondary pollution; and rapid response makes it suitable for distributed or point-of-use installation. A disadvantage is that catalysts may gradually deactivate under high temperature, high humidity, or atmospheres containing interfering species, requiring periodic evaluation and possible regeneration or replacement.

Comparison of Three Ozone Abatement Technologies

Comparison Dimension	Activated Carbon Adsorption	Thermal Decomposition	Catalytic Decomposition
Operating Temperature	Room temperature	>300°C	Room temperature
Consumable Replacement	Frequent (after saturation)	No consumables	Minimal (catalyst life 1-3 years)
Safety	Spontaneous combustion risk, hazardous waste	Burn risk from high temperature, requires insulation	Intrinsically safe, no high temperature, no hazardous waste
Applicable Ozone Concentration	Low concentration (<1 ppm)	Any concentration	Low to medium concentration (<10 ppm optimal)

In summary, catalytic decomposition offers significant comprehensive advantages for cleanroom applications: room-temperature operation adds no thermal load, no consumables means no hazardous waste generation, and distributed installation near the ozone generation point enables source control.

Core Indicators and Principles for Catalyst Selection in Catalytic Decomposition

Once catalytic decomposition is chosen as the technical route, proper catalyst selection becomes critical to engineering success. The following four core indicators must be evaluated.

Room-Temperature Activity is the primary indicator. Semiconductor cleanrooms typically operate at constant temperatures around 25°C, and the catalyst must exhibit sufficient activity at this temperature. Typical requirements: under conditions of 25-40°C, 40% relative humidity, and 100-200 ppb inlet ozone concentration, the initial ozone conversion efficiency should exceed 90%. High-quality manganese-based catalysts typically achieve initial conversion efficiencies of 95% or higher. Note that activity can vary significantly among different manufacturers' catalysts at the same room temperature, and actual measured data should prevail.

Humidity Tolerance directly affects catalyst service life under actual operating conditions. Cleanroom relative humidity is generally controlled between 30% and 70%. Water molecules compete with ozone for active sites on the catalyst surface, causing activity decline. Evaluation should focus on the decrease in conversion efficiency after 500 hours of continuous operation at 60% relative humidity. The decrease for highly tolerant catalysts should be less than 10%. Certain specially doped modified catalysts can reduce humidity effects even further.

Mechanical Strength and Pressure Drop determine the catalyst's physical stability under airflow冲击 and ventilation energy consumption. Granular or honeycomb catalysts may pulverize under long-term airflow, generating dust that contaminates the cleanroom while also increasing pressure drop. Selection should review the catalyst's crush strength or abrasion resistance指标. For applications installed in equipment exhaust ducts or at fan filter unit (FFU) returns, low-flow-resistance structures (such as honeycomb ceramic monoliths) are preferred, with initial pressure drops typically below 50 Pa.

Service Life and Regenerability impact long-term operating costs. The typical service life of catalysts for semiconductor cleanrooms is 1-2 years, depending on ozone concentration and the presence of interfering species. Some catalysts can be regenerated through thermal treatment (heating at 150-200°C in an inert atmosphere for several hours) or water washing. Selection should inquire whether suppliers offer regeneration protocols and the expected number of regeneration cycles.

Selection Principle Summary: For the specific requirements of semiconductor cleanrooms (low concentration, continuous operation, safety, cleanliness), manganese-based mixed oxide systems should be prioritized. The reasons why manganese-based mixed oxides outperform single metal oxides are: the multiple valence states of manganese ions (Mn²⁺/Mn³⁺/Mn⁴⁺) enable efficient electron transfer among them, forming a redox cycle that continuously regenerates active sites; the composite components (such as doping with copper, cerium, etc.) create lattice defects and oxygen vacancies that further enhance low-temperature activity; and compared to single MnO₂, the composite system has a higher specific surface area (up to 100-150 m²/g), providing more reaction interface.

Selection Decision Reference

Operating Condition	Recommended Catalyst Characteristics	Selection Priority
High humidity (RH>60%), continuous operation	Hydrophobically modified manganese-based or cerium-doped mixed oxide	Humidity tolerance > Room-temperature activity > Service life
Stable ozone concentration <100 ppb	Standard manganese-based mixed oxide, granular form	Room-temperature activity > Pressure drop < Service life
Point-source control, confined installation space	Honeycomb monolithic catalyst, low flow resistance	Pressure drop < Room-temperature activity < Footprint
Intermittent operation, frequent start-stop	Fast-response, high room-temperature activity catalyst	Room-temperature activity > Regenerability > Service life

Before selection, engineers are advised to measure actual fab ozone peak concentrations, background humidity, and available installation space, and to evaluate the convenience of catalyst change-out. Where conditions permit, candidate catalysts may be tested on a bench-scale apparatus for 2-4 weeks of on-site validation, using actual performance as the final selection basis.

Summary

In summary, catalytic decomposition is the preferred technology for low-concentration ozone control in semiconductor cleanrooms, offering irreplaceable advantages in room-temperature operation, no consumables, and no secondary pollution. Catalyst selection should focus on the four core indicators of room-temperature activity, humidity tolerance, mechanical strength, and service life, with manganese-based mixed oxide systems representing a mature industry solution due to their high activity and stability. Engineers should combine actual fab conditions with measurements and short-term validation to screen the most suitable catalyst product. Proper catalyst selection not only protects process yield but also serves as a key enabler for greener semiconductor manufacturing.