Has the Challenge of PFCs Really Been Solved?

In the semiconductor industry, chemical vapor deposition (CVD) chambers were traditionally cleaned using perfluorocarbon (PFC) gases such as C₂F₆ and CF₄. These gases decomposed in a plasma within the process chamber, yielding highly reactive fluorine radicals that reacted with process deposits on the chamber walls and furniture. The volatile fluorinated gases thus formed (e.g., SiF₄) could then be pumped away. A relatively small fraction (typically ~30%) of the cleaning gas actually took part in the chamber cleaning reactions; the remainder simply passed through into the exhaust — and the atmosphere. The global warming effects of C₂F₆ and CF₄ are well documented.^1,2

In 1999, the World Semiconductor Council (WSC) established a global industry goal of reducing absolute PFC emissions by 10% relative to the 1995 baseline by the year 2010.³ Considering that industry growth between 1995 and the declaration in 1999 was already significant, this declaration represented a dramatic commitment. A major reduction in PFC emissions was quickly achieved by switching from C₂F₆ and CF₄ to NF₃, which decomposes very efficiently into fluorine and nitrogen in a plasma, either in or near the process chamber. In effect, this changed the abatement challenge from PFC destruction to fluorine treatment.

Although fluorine does not directly contribute to global warming, it is still a toxic gas that cannot be released indiscriminately into the atmosphere. It is difficult to remove with wet scrubbing alone, and is typically addressed by passing the CVD exhaust stream through a combustion abatement process. Heat supplied by a burning air/fuel mixture thermally reacts various components into byproducts that are easily removed. Silicon-based components from CVD gases, such as silane or TEOS, are oxidized into silica, and fluorine reacts with hydrogen from the fuel to form hydrogen fluoride, both of which can be removed from the wastewater stream in a final washing stage following the combustor. The gas that is finally released to the environment (atmosphere) is essentially free of hazardous and global warming emissions. Figure 1 illustrates the combustion abatement process.

1. Schematic representation of a chemical vapor deposition (CVD) exhaust configuration using an open-flame combustor.

The simplicity of this process is attractive, but a detailed examination of the chemical and thermal processes occurring in an open-flame configuration reveals fundamental problems. The first problem is the de novo generation of PFCs — the very same compounds the industry sought to eliminate with the switch to NF₃ — that results from the burning of the hydrocarbon fuel, typically methane, in intimate contact with fluorine from the process chamber (Fig. 2). This reaction is represented below⁴:

4F₂ + CH₄ → CF₄ + 4HF

The literature includes numerous observations of PFC creation in open-flame combustion processes.⁵

2. Comparison of NF3 gas input (blue) into an open-flame combustor to the CF4 output gas (green) as determined by FTIR, clearly illustrating that there is significant generation of CF4 in the combustor. (Source: ITRI)

The second problem is the formation of NO_x, which results from the poorly controlled thermal environment of an open flame configuration. NO_x is a collective term for several highly reactive gases⁶ composed of nitrogen and oxygen in various amounts. Many of the nitrogen oxides are colorless and odorless, although one common pollutant, nitrogen dioxide (NO₂), frequently combines with particles in the air to form a hazy, reddish-brown layer overlying urban areas. NO_x is among the main ingredients involved in the formation of ground-level ozone, which can trigger serious respiratory problems. It reacts to form nitrate particles and acid aerosols, which also cause respiratory problems, and it contributes to the formation of acid rain, to nutrient overloads that deteriorate water quality, and to atmospheric particles that impair visibility. NO_x reacts to form toxic chemicals, and contributes to global warming. Because NO_x and the pollutants formed from it are long-lived and can be transported over great distances, successful control strategies must be regional, or even global, in scope. NO_x emissions are closely regulated in many territories around the world.

Research shows that NO_x emissions do not form in significant amounts until flame temperatures reach ~1500°C (Fig. 3).⁷ Once that threshold is passed, any further rise in temperature causes a rapid increase in the rate of NO_x formation. Thermal NO_x is produced during the combustion process when nitrogen and oxygen are present at elevated temperatures as they combine to form NO and NO₂.

3. NOx formation increases rapidly as chamber temperature rises beyond 1500°C.

The formation of NO_x in open-flame combustors has also been reported extensively (Fig. 4).⁵

4. NO2 emissions observed in the exhaust from an open-flame combustor.

Inward-fired combustion

An alternative to the open-flame configuration, known as an inward-fired combustor, minimizes the formation of both PFCs and NO_x in the abatement process. In this design, the fuel and air mixture enters the chamber and combusts on the surface of a cylindrical porous ceramic pad. Process gases are introduced independently through inlets at the top of the combustion chamber. The distinguishing feature of this configuration is that the combustion of fuel, which provides the necessary heat, and the thermal oxidation/reduction of the process exhaust gases are spatially separated, eliminating any opportunity for unwanted cross-reactions between the hydrocarbon fuel and fluorine. In the inward-fired design, the fluorine encounters only water and carbon dioxide, the products of the combustion reaction. In subsequent reactions, the dynamics strongly favor the formation of hydrogen fluoride over perfluorocarbons, as the fluorine combines more readily with the hydrogen from water molecules than with the carbon now tightly bound up in carbon dioxide.

CH₄ + 2O₂ → 2H₂O + CO₂

2F₂ + 2H₂O → 4HF + O₂

The inward-fired combustor design effectively prevents the creation of PFCs in the combustion process (Fig. 5). The efficiency of this design at avoiding the unwanted cross-reaction between fuel and fluorine (from NF₃) is demonstrated in Figure 6, where CF₄ is below detectable limits, even for significant NF₃ flows.

5. Schematic diagram showing how the inward-firing combustor avoids forming CF4 by keeping fuel and process gas combustion spatially separated.

The inward-fired combustor also provides much better control of thermal conditions throughout the combustion chamber. The reaction volume is completely enclosed, and heat radiates uniformly inward from the cylindrical ceramic pad, creating isothermal conditions throughout. Exhaust gases are fully constrained to pass through the heated zone with all paths exposed to essentially equivalent thermal conditions throughout their transit. This contrasts dramatically with open-flame designs, where different paths experience quite different conditions, and even the best paths may include only brief segments at optimal temperature.

6. Comparison of NF3 gas input (green) into an inward-firing combustor to the CF4 output gas (blue), clearly illustrating that there is no generation of CF4 in the combustor.

The inward-fired configuration is both more effective and more efficient. By ensuring that all gas reaches the correct reaction temperature, it ensures effective and complete abatement. By physically confining the gas within the combustor, it permits higher flow rates through a smaller burner. By effectively capturing and distributing the heat of combustion within the isothermal region, it improves the fuel efficiency of the abatement process. Finally, the constant flow of the fuel/air mixture inward through the combustion pad prevents the accumulation of solid residues, improving overall throughput by reducing maintenance requirements and associated downtime.

The thermal uniformity provided by the inward-fired configuration avoids the formation of NO_x compounds. In an open-flame configuration, operating conditions that yield the best overall abatement effectiveness and energy efficiency will necessarily include regions within the reaction volume where temperatures are greater or less than ideal. Where the temperature is too low, abatement is incomplete; where it is too high (>1500°C), NO_x can form. Such high temperatures are completely avoided in the isothermal conditions of the inward-firing combustor, where the combustion zone is typically maintained at a temperature <800°C — sufficient to completely oxidize silane and TEOS byproducts and efficiently react the fluorine to hydrogen fluoride, but well below the temperatures required to form significant quantities of NO_x.

Other aspects of the inward-fired combustor also reduce the opportunity for NO_x formation. The physical separation of combusting fuel from the exhaust gas precludes many NO_x forming reaction pathways that would be enabled in an open-flame configuration by the availability of hydrogen from the methane fuel. Operating the inward-fired combustor at lean fuel conditions further reduces available hydrogen in the reaction volume.⁸ Finally, the extensive use of ceramics for the combustor pad and other high-temperature components eliminates hot metal surfaces that can catalyze NO_x formation reactions. It has been reported that NO_x concentrations in the exhaust of an inward-fired combustor were below instrumental detection limits, <1.4 ppm.⁹ The same report noted that another unwanted byproduct, OF₂, was also below detectable limits (<1 ppm).

The switch to NF₃-based cleaning processes ultimately resulted in a major shift in cleaning strategies within the industry. With C₂F₆-based processes, cleaning rates were limited by the relatively low efficiency at which the precursor gas molecules could be converted to fluorine radicals (typically <30%). Current generation plasma sources and the high electron attachment cross-section of NF₃ provide very high conversion rates, and engineers have been able to dramatically reduce chamber cleaning times by simply increasing the NF₃ flow rates. Recent expansions in NF₃ manufacturing capacity and resulting reductions in price have added further incentives to increase NF₃ flow to reduce cleaning downtime and increase process throughput and productivity.

With fluorine levels in cleaning exhaust typically <10%, abatement systems must cope with the challenge of effectively treating the fluorine entrained in huge flows of nitrogen. The confined flow, isothermal reaction volume and strong reducing environment of the inward-fired combustor ensure complete conversion of fluorine to hydrogen fluoride, even at high flow rates.

Summary

The use of NF₃ reduces PFC gas emissions from CVD chamber cleaning processes, but care must be taken not to generate PFCs in the combustion abatement processes used to remove fluorine from the exhaust stream. All combustors are not the same. Open-flame combustors, which mix the exhaust gas directly with the burning hydrocarbon fuel, cannot prevent unwanted reactions between carbon in the fuel and fluorine in the exhaust that form CF₄. In addition, hydrogen from the fuel, high temperatures and the presence of hot metal surfaces catalyse the formation of NO_x, an air pollutant. Inward-fired combustion solves both problems by physically separating the burning fuel from the exhaust gas and providing tightly controlled, isothermal conditions throughout the reaction volume. The inward-fired design handles higher exhaust gas flows through smaller combustors, delivers better fuel efficiency, and reduces downtime for maintenance and cleaning — ultimately reducing both the environmental impact and cost of ownership for inward-fired configurations.