Reclamation and separation of perfluorocarbons using...

Details Reclamation and separation of perfluorocarbons using... Reclamation and separation of perfluorocarbons using...

2000-04-04

2002-05-07

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Compressing and indirect cooling of gaseous fluid mixture to...

And solid sorption

C062S616000, C062S640000, C062S655000, C095S042000, C095S045000, C095S050000, C095S090000, C095S232000, C095S288000, C096S004000, C096S134000, C096S243000

Reexamination Certificate

active

06383257

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
Perfluorocarbons (PFCs) have many favorable properties such as good stability, non-flammability, low surface tension and low solubility. These favorable properties make perfluorocarbons useful in many applications such as lubricants, solvents, refrigerants, hydraulic fluids, fire extinguishers, insulators and cleaning agents. In the past, perfluorocarbons were manufactured, used and discharged to the environment freely. However, it was recognized some years ago that their inertness, besides making them materials of choice for the uses above, also causes them to break down extremely slowly in the upper atmosphere, giving them the potential to create global warming.
For example, many processes involved in the fabrication of semiconductors, e.g., etching, deposition, oxidation, are carried out within closed reaction chambers and involve the use of perfluorocarbons. These chambers are also referred to as tools. Typically, silicon wafers that are fabricated into semiconductors are placed into these chambers, or tools, one-by-one, or in some cases, more than one at a time. After a processing step is completed, the silicon wafers are removed from one chamber, usually by robotic means, and passed to the next chamber (or tool) where they undergo the next step in the process. Over time, as a result of these processing steps, the internal walls of the chambers become contaminated as a result of the various gases and other materials that are present in the chamber during the processing steps. Thus, periodically, the internal walls of these chambers must be cleaned. One manner for cleaning these chambers is through the use of perfluorocarbons (PFCs). PFCs are introduced into the chamber for cleaning purposes while the chamber is devoid of silicon wafers and is between processing steps. The cleaning process includes exposing the chamber to PFCs, sometimes in combination with high temperatures. At the end of the cleaning process, an effluent stream containing contaminant material and PFC cleaning agents is removed from the processing chamber by a vacuum pump located downstream of the chamber and pumped away.
These effluent streams can contain mixtures of global warming perfluorocarbon compounds such as C
2
F
6
, CF
4
, NF
3
, SF
6
, CHF
3
, C
3
H
8
, CH
3
F and C
2
HF
5
, reactive compounds such as HF, F
2
, COF
2
, SiF
4
and SiH
4
, and environmentally benign atmospheric gases such as N
2
, O
2
, CO
2
, H
2
O and N
2
O. Many of these reactive compounds result from reaction between the PFCs used in the cleaning process and the contaminants deposited on the walls of the reaction chamber. Typical effluent streams exiting the reaction chamber during cleaning contain approximately 50% PFCs, and 50% nitrogen and other atmospheric gases and flow at a typical rate of 2,000 standard cm
3
per minute. The stream is typically under a vacuum at a pressure of approximately 10 torr to 100 torr. Since these effluent streams usually include reactive compounds that can damage the vacuum pump during removal from the chamber, the effluent stream is usually diluted to a proportion of approximately 1% PFCs by mole and approximately 99% N
2
and other atmospheric gases prior to entering the vacuum pump. Dilution enables the vacuum pump to operate more efficiently.
Dilution is accomplished by adding approximately 50,000 to 100,000 standard cm
3
per minute (50 to 100 standard liters per minute) of N
2
to the effluent stream. This heavy dilution with nitrogen minimizes the damaging effects of reactive compounds, and is necessary for efficient operation of the vacuum pump. The diluted stream exits the vacuum pump at an approximate pressure of 760 torr.
Prior to atmospheric venting, the vacuum pump effluent stream must be treated. Environmental considerations require removal of damaging compounds from the semiconductor effluent stream prior to venting to the atmosphere. However, efficient separation of the low quantities of damaging compounds, e.g., 1%, from the comparatively high quantities, e.g., approximately 99%, of environmentally benign nitrogen and other atmospheric gases becomes more difficult after the dilution step. Corrosive components can be removed from these effluent streams using well established scrubbing technology. Reactive components, such as SiH
4
, can be removed using combustion into less harmful compounds. However, until recently, PFC compounds were frequently vented without emission control. Recent efforts to reduce emission of these global warming compounds have led to the use of emission control methods such as adsorption, membrane separation or cryogenic distillation technologies and devices for carrying out these methods. Such emission control methods and devices provide means to separate the PFCs from the atmospheric gases (e.g., N
2
). As shown in the sequence below, these emission control devices are typically located after the corrosive gas scrubber, at which point atmospheric gases typically comprise approximately 99% of the mixture, the balance consisting largely of PFC compounds.
REACTION CHAMBER (TOOL)→VACUUM PUMP→SCRUBBER→(99% Atmospheric Gases, 1% PFCS)→EMISSION CONTROL DEVICE (Adsorption Device or Membrane Separation Device)→VENT.
As a typical example, a single-stage membrane separation device can provide a PFC-enriched mixture containing approximately 80% to 95% atmospheric gases. Re-use of the PFCs requires additional steps of membrane separation for increased purity. Such additional treatment can be accomplished using additional membrane separation stages to reduce atmospheric gases levels to approximately 1%, followed by cryogenic distillation. The distillation process can then separate valuable and reusable PFC compounds, such as C
2
F
6
from the other PFC compounds and atmospheric gases to produce a high purity level, e.g., approximately 99.999%. In many instances, this level of purity is necessary for reuse in semiconductor processing.
However, membrane and distillation separation methods require expensive equipment and may entail high operating costs. A reduction or elimination of such expensive membrane and distillation separation equipment should improve the economic viability of PFC abatement and recovery.
Under prior art inventions, membrane separation devices have been combined with cold trap condensers to provide separation of condensable components from atmospheric gases. For example, U.S. Pat. Nos. 5,779,763 (Pinnau et al.); U.S. Pat. No. 5,199,962 (Wijmans); U.S. Pat. No. 5,089,033 (Wijmans); U.S. Pat. No. 5,205,843 (Kaschemekat et al.); and, U.S. Pat. No. 5,374,300 (Kaschemekat et al.), all assigned to Membrane Technology and Research, Inc. (MTR) of Menlo Park, Calif., teach methods for separating or recovering condensable components from a gas stream using a condensation step in combination with one or more membrane separation stages. U.S. Pat. No. 5,779,763 (Pinnau et al.) in particular teaches methods for separating perfluorinated compounds, including perfluorocarbons, from atmospheric gas streams.
Separation by condensation methods requires the gas stream to be cooled to the dew point temperature of the condensable components to effectuate condensation of these components. The dew point temperature for the mixture tends to increase as the pressure of the gas mixture increases. In order to reduce cooling requirements, the processes discussed in the MTR patents all involve condensation at temperatures above −100° C. (−148° F.). U.S. Pat. No. 5,779,763 (Pinnau et al.) which is specifically directed towards recovery of PFCs from semiconductor tool effluent streams, claims condensation at temperatures above −30° C. (−22° F.). These temperatures are considerably above the normal condensation temperatures of pure PFC compounds. In order to accomplish condensation of PFCs at this relatively high temperature, the gas stream must first be compressed to

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