Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1998-12-15
2004-07-06
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S24000R, C423S235000, C423S248000, C095S223000, C095S224000, C095S199000, C261S021000
Reexamination Certificate
active
06759018
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to abatement of undesirable components such as fluorine, silane, gaseous fluorides, acid gases, hydride gases and halide gases from effluent streams containing same, and more specifically to the use of systems employing a wet scrubber apparatus and method for abating undesirable components of the aforementioned type in semiconductor manufacturing processes.
2. Description of the Related Art
In point of use wet scrubbing abatement of semiconductor off-gases, various applications require the removal of hydride gas, acid gas, and entrained solids. This is especially true in processes that use or produce SiH
4
(silane), NH
3
(ammonia), F
2
(fluorine), HF (hydrogen fluoride), SiF
4
(silicon tetrafluoride), or COF
2
(carbonyl fluoride), such as certain CVD (chemical vapor deposition) processes.
In these effluent gas stream treatment applications, the art has typically employed a multiple component scrubbing system. In such a device, the silane and optionally the ammonia are thermally oxidized in one module of the abatement system, and the HF, F
2
, SiF
4
, COF
2
, and optionally NH
3
are scrubbed using water in another, separate module. Disadvantages of thermal oxidation include (i) high energy consumption, and (ii) the generation of NOx resulting from the oxidation of ammonia. In addition, high temperature heated modules may accelerate corrosion downstream of the thermal module because the acid gases (F
2
and HF) are heated, but not abated in the thermal unit. Typically, a water scrubbing module is located directly downstream from the thermal module. It is in the hot, moist interface region between the water scrubbing unit and the thermal unit that the hot acid gases typically cause corrosion.
There is therefore a compelling need for a simple, reliable abatement device that can effectively treat effluent streams containing gas species of the type mentioned above.
More specifically, concerning fluorocompounds as effluent gas species that are desirably abated in treatment of effluent gas streams containing same, perfluorinated gases are widely used in chip manufacturing to generate in-situ F
2
and fluorine radicals using plasma-assisted reactions. These highly reactive species are produced to remove silica from tool chambers or to etch materials such as nitrides, oxides, or polysilicon from wafers. The most commonly used carbon-based perfluorinated species include CF
4
, C
2
F
6
, and C
3
F
8
. Nitrogen trifluoride (NF
3
) and sulfur hexafluoride (SF
6
) are also widely used.
Perfluorinated compounds (PFCs) are also among the strongest greenhouse gases with global warming potentials (GWPs) three and four orders of magnitude higher than CO
2
. Moreover, PFCs are extremely stable molecules having lifetimes in the atmosphere of thousands of years. Even though the semiconductor industry is not the largest source of PFC emissions, the industry is actively pursuing strategies to reduce PFC emissions and to protect the environment.
Ongoing research to reduce PFC emission levels falls into four categories: optimization, use of alternative chemicals, recovery/recycle techniques, and abatement processes.
Process optimization involves adjusting the operating conditions in the reactor to achieve enhanced PFC conversion within the semiconductor manufacturing tool. Existing non-optimized conditions in the semiconductor manufacturing process result in PFC utilization that varies depending on the specific gas and process used. For instance, oxide etches using a combination of CF
4
and CHF
3
rank lowest with 15% process efficiency. Tungsten deposition processes are reported to utilize up to 68% of NF
3
. Recent developments in optimized plasma clean technologies were demonstrated to provide up to 99% NF
3
utilization within the semiconductor manufacturing tool.
High PFC conversions inevitably result in the formation of hazardous air pollutants (HAPs). Breakdown products include mostly fluorine (F
2
) and silicon tetrafluoride (SiF
4
) gases and to a lesser extent HF and COF
2
. Destruction of fully fluorinated gases generates considerably augmented HAP yields compared to the initial PFC volumes delivered to the semiconductor manufacturing tool. For instance, assuming stoichiometric conversion of PFCs into F
2
, a 1 liter per minute (lpm) flow rate of NF
3
could potentially produce 1.5 liters per minute (lpm) of F
2
. The combined exhaust stream of four chambers in a semiconductor manufacturing process system could potentially generate up to 6 standard liters per minute (slm) of fluorine gas resulting in a post-pump effluent concentration of 3% F
2
(50 lpm ballast N
2
per pump).
These estimated values double with hexafluorinated PFCs (compared to NF
3
) and are likely to increase in the future with the projected throughputs of 300 mm wafer manufacturing. These estimates represent worse case scenarios and do not account for the short duration and periodic nature of processes using PFCs, the lower concentrations of F
2
emissions during initial cleaning stages, and the reduced probability that two or more chambers run PFC cycles synchronized. Nonetheless, such estimates indicate the serious and worsening character of the PFC problem associated with semiconductor manufacturing operations.
The toxic and corrosive nature of fluorinated HAPs pose considerable health and environmental hazards in addition to jeopardizing the integrity of exhaust systems. In particular, the oxidizing power of F
2
is unmatched by any other compound used or generated in the semiconductor manufacturing facility, and is far more reactive than other halogens. The large volumes of F
2
and other fluorinated hazardous inorganic gases released during optimized plasma processing require the utilization of point of use (POU) abatement technologies in order to minimize potential dangers and to prolong tool operating life.
There are several potential alternative methods for point of use F
2
abatement. At high concentrations, fluorine reacts exothermically with all elements except O
2
, N
2
, and noble gases. Consequently, a reasonable approach to F
2
abatement is to remove this highly active gas using naturally-occurring reactions without adding energy to the system. The main challenges to this potential approach are heat dissipation and forming acceptable by-products.
Alternative fluorine abatement techniques affording potential solutions to the fluorine abatement problem include wet as well as dry reaction techniques, and thermal reaction techniques.
In dry processing, the fluorine gas stream is flowed through a dry bed filled with a reactive material. Suitable dry chemicals would convert F
2
into innocuous solids or benign gases without generating excessive heat. This last condition could be a limiting factor especially when large volumes of F
2
are involved.
In a thermal reaction approach, thermal abatement units combine reactive materials and F
2
inside a reactor heated using fuel or electrical energy. The by-products generated by the thermal abatement of F
2
typically include hot acids requiring the use of a post-reaction water scrubber. The removal efficiencies in these post-reaction water scrubber beds are often compromised, inasmuch as the scrubbing efficiency of most acid gases decrease as a function of temperature. In addition, containment of hot concentrated acids requires expensive materials and construction to prevent temperature-enhanced corrosion attack.
In wet processing techniques, advantage is taken of the fact that fluorine gas reacts quickly and efficiently with H
2
O. The main products of the reaction between water and F
2
are HF, O
2
, and H
2
O
2
. Objections to using water scrubbers include concerns over the formation of unwanted OF
2
, and the water consumption necessary to achieve acceptable removal efficiencies at high fluorine challenges.
Comparison of the foregoing treatment options shows that wet scrubbing techniques are potentially the most attractive, provided that the OF
2
by-product formation an
Arno Jose I.
Deseve Jason
Holst Mark
Lorelli Jeff
Sweeney Joseph D.
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