Gas separation: processes – Filtering
Reexamination Certificate
2000-02-29
2001-08-28
Simmons, David A. (Department: 1724)
Gas separation: processes
Filtering
C055SDIG001, C055SDIG002, C454S187000
Reexamination Certificate
active
06280507
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to airflow in controlled environments. More particularly, the present invention relates to improved process equipment exhaust and airflow management in clean room and mini-environment enclosures.
2. Description of the Related Art
Semiconductor manufacturing must be performed in a particle-free environment, due to the fact that submicron size dimensions characterize the structural elements of the electronic circuitry and such circuitry can be rendered inoperative by the presence of even a single particle. Semiconductor manufacturing process steps are therefore carried out within the confines of a clean room, a controlled environment in which the air is continuously filtered to remove dust, lint, and other particulate matter. Processes and apparatuses for managing forced airflow in clean rooms and other environmentally controlled enclosures are well known in the art.
U.S. Pat. No. 6,007,595, issued to Baik, et al. on Dec. 28, 1999, discloses the use of an isolation bar arranged across the central region of an air filtration unit in a laminar flow clean room, allowing for the placement of a partitioning panel across the central region of a filter while preventing turbulence downstream of the areas where the panels would otherwise block air flow through the filter.
U.S. Pat. No. 5,953,884, issued to Lawecki, et al. on Sep. 21, 1999, discloses an apparatus and method for manufacturing syringe bodies that are substantially free from contaminants, by forming and packaging the items in a mini-environment maintained at clean room standards of at least class 100, using horizontal and vertical laminar airflows directed into air filter units.
Many semiconductor manufacturing process steps involve the use of chemicals that are toxic or otherwise hazardous to humans, necessitating localized exhaust equipment to remove, contain, or otherwise abate fumes from such chemicals. Apparatuses and methods for local exhaust are known in the art.
U.S. Pat. No. 6,009,894, issued to Michel Trussart on Jan. 4, 2000, discloses a rapid response airflow rate-regulating device, for installation in a vacuum unit that collects contaminants from a processing station in a clean room. The device enables the vacuum unit to maintain a continuous suction of contaminant particles from the processing station when there are pressure variations in the controlled space, as for example may occur by opening a door for entry or egress of personnel.
U.S. Pat. No. 5,946,221, issued to Fish, Jr., et al. on Aug. 31, 1999, discloses a means for controlling air flow through a laboratory fume hood based on air flow face velocity. The air flow control system disclosed in Fish, Jr., et al. allows maintenance of a velocity of exhaust air flow through the fume hood sufficient to abate the hazard posed by chemical fumes, but below the level at which turbulence would interfere with the experiment or process being performed at the workstation protected by the fume hood.
Typically, semiconductor manufacturing process steps involving liquid chemical baths (“exhausted equipment and systems”) in a clean room have a local exhaust associated therewith. Efficiency of the exhaust (i.e., the exhaust energy necessary to remove hazardous chemical fumes at the exhausted equipment and systems) is a function of the interdependent relationship between the station exhaust (“pull”) and the laminar airflow from the clean room ceiling (“push”). These two motive forces of airflow form the push-pull system that is the basis for system exhaust operation.
The actual flow rate through exhausted equipment and system exhaust can be characterized by C
e
, the Coefficient of Entry. C
e
is defined by the American Conference of Governmental Industrial Hygienists as:
“The actual rate of flow caused by a given hood static pressure compared to the theoretical flow which would result if the static pressure could be converted to velocity pressure with 100% efficiency. The ratio of actual to theoretical flow.”
Maximum possible exhausted equipment and system exhaust duct velocity is achieved when C
e
=1.0. Typical C
e
values range from 0.2 to 0.7 (for highly efficient exhausted equipment and systems). In general, the greater the distance between air source and exhaust, the smaller is the value for Ce.
Unobstructed, the ceiling-to-floor laminar airflow in a clean room loses relatively little volumetric flow velocity. Typically, an 80 feet per minute (fpm) laminar airflow as measured at the ceiling will be attenuated to approximately 71 fpm at the deck level, away from exhausted equipment and systems. As air is pulled into the exhausted equipment and systems and subsequently through the exhaust located beneath the exhausted equipment and systems deck, other influences, most notably a chemical tank, reduce airflow velocity by inducing turbulence, and by bending and separating air streams. The clean room “push” is thereby reduced to a velocity insufficient to fully contain and remove chemical fumes.
At the same time, the exhausted equipment and system exhaust “pull” is insufficient to significantly increase airflow velocity over a chemical tank. Only high exhaust volumes and consistently high laminar airflow velocities from the ceiling system are able to maintain fume capture in the process area.
Because of these factors, most exhausted equipment and systems exhibit marginal flume capture capability, and many fail to consistently control contamination and hazardous fumes at the deck, resulting in occasional fumes spills into the process and operator environment.
In characterizing airflow in relation to a process tank in a clean room environment, Critical Capture Velocity (CCV) is defined as the minimum airflow velocity measured over a process tank at which fumes will be controlled below the station deck. A properly balanced system can achieve this velocity (empirically determined to be at values of 70 fpm or greater) with a combination of exhaust flow rate, laminar airflow, and minimal deck opening size, if enough exhaust capacity is available.
As wafer sizes used in the semiconductor industry grow to 300 mm or more and require larger deck openings, however, CCV becomes more difficult to achieve, and facility exhaust capacity is stretched beyond its limits. Conventional push-pull systems will not be able to produce enough combined force to achieve capture of hazardous fumes in large size wafer facilities now under design and construction.
Additionally, many 200-millimeter (mm)-exhausted equipment and systems in current use are unable to achieve CCV because of a disparity in the exhaust capacity, deck opening sizes, and laminar push. Consequently, fume spill incidents occur in many wafer fabs, resulting in yield loss, excessive rework, process disruption, environmental and regulatory noncompliance, injury, and litigation. Additionally, the economics of the semiconductor market demand a reduction in overall production costs as semiconductor fabs move to larger wafer sizes. The international semiconductor industry association SEMATECH has determined a need to reduce exhaust energy consumption by a magnitude of 35 to 40% from current average levels, even as progressively increasing wafer sizes necessitate larger, higher capacity, higher energy consumption exhaust systems.
It therefore is one object of the present invention to provide an improved exhausted equipment and systems exhaust system.
It is another object of the invention to provide such exhausted equipment and systems exhaust system, having increased efficiency, generating sufficient airflow for hazardous fume containment while consuming less energy.
It is a further object of the present invention to provide an exhausted equipment and systems exhaust system that assures maintenance of CCV at values of 70 fpm or greater.
It is still a further object of the present invention to provide an exhausted equipment and systems exhaust system that provides control of hazardous fumes outside the process envelope region of the facil
Advanced Technology & Materials Inc.
Hultquist Steven J.
McLauchlan, III Robert A.
Pham Minh-Chau T.
Simmons David A.
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