Gas separation – Combined or convertible – In environmental air enclosure
Reexamination Certificate
2001-04-04
2003-02-11
Simmons, David A. (Department: 1724)
Gas separation
Combined or convertible
In environmental air enclosure
C055S385100, C055SDIG001, C055SDIG002, C095S273000, C454S187000
Reexamination Certificate
active
06517594
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to environmental control in semiconductor manufacturing operations, including management of airflow in controlled environments and containment and capture of hazardous fume species. More particularly, the present invention relates to improved chemical containment and contamination reduction apparatuses, systems, and processes in semiconductor manufacturing facilities. In a specific aspect, the invention relates to a chemical containment apparatus for capturing and reducing hazardous chemical fumes released by chemical tanks used in a semiconductor wet processing system.
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 that such circuitry can be rendered inoperative by the presence of even a single particle of dust. Semiconductor manufacturing process steps are therefore carried out within the confines of a clean room, which is a controlled environment through which an exhaust system continuously flows large amount of filtered air to remove dust, lint, and other particulate matter.
On the other hand, many semiconductor manufacturing process steps involve the use of chemicals that are toxic or otherwise hazardous to humans, which also necessitates use of localized exhaust equipment to contain, remove, or otherwise abate fumes from such chemicals.
Typically, efficiency of an exhaust system (i.e., the exhaust energy necessary to remove hazardous chemical fumes in a particular environment) is a function of two independent factors: the station exhaust (“pull” force) and the laminar airflow from the clean room ceiling (“push” force). These two motive forces of airflow form a push-pull system, which is the basis for system exhaust operation.
Exhaust efficiency can in one aspect be characterized by C
e
(Coefficient of Entry), which 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 exhaust efficiency is achieved when C
e
=1.0. Typical C
e
values range from 0.2 to 0.7 (for highly efficient exhaust systems). In general, the greater the distance between the air inlet and exhaust outlet, the smaller is the value for C
e
.
If unobstructed, ceiling-to-floor laminar airflow in a clean room loses relatively little volumetric flow velocity and therefore has relatively higher C
e
value. Typically, an 80 feet per minute (fpm) laminar air stream as measured at the ceiling is slightly attenuated to approximately 71 fpm at the deck level, away from semiconductor processing equipment and systems below the deck. When such air stream is pulled further down to beneath the deck level, semiconductor processing equipment, most notably chemical tanks, obstruct flow path for such air stream and induce air turbulence in their vicinity. The air stream is bent or separated by the obstructing objects, and volumetric flow velocity of such air stream is reduced to a level that is too low to fully contain and remove chemical fumes.
Because of these factors, conventional exhaust systems exhibit marginal fume capture capability, and many fail to consistently control contamination and hazardous fumes under the deck, resulting in occasional fumes spills into the process and operator environment.
Critical Capture Velocity (CCV) is generally used in characterizing minimum fume capture capacity required for an exhaust system in a clean room environment. 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, the exhaust systems in many 200-millimeter (mm)-facilities 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 would therefore be a significant advance in the art: (i) to provide an improved air management system, with increased efficiency for containing hazardous chemical fumes and reducing fume spill, while consuming less energy; (ii) to provide an air management system for chemical containment and contamination reduction, which assures maintenance of CCV at values of 70 fpm or greater; and (iii) to increase the C
e
of exhaust equipment toward a value of unity.
In the fabrication of semiconductor wafers, a multitude of cleaning steps is required to remove impurities from the surface of the wafer prior to subsequent processing. Generally, a batch of wafers is dipped into one or more chemical tanks that contain chemicals that are needed for clean or etch functions.
A serious problem associated with such immersion wet cleaning process is that the liquid chemicals contained by the chemical tanks release hazardous fumes at their surfaces. Such fumes tend to migrate to the environment surrounding the tanks and pose environmental hazards or cause worker injuries.
Containment of hazardous chemicals in semiconductor wet processing systems is done today using below-deck exhaust systems, which function to capture chemical fumes or keep them at or below the deck.
In order to effectively keep hazardous chemicals under control and out of the workspace above the deck, the below-deck exhaust systems have to effectuate flow of large volumes of filtered air through the wet cleaning tools at very high rates, e.g., on the order of 150 cubic feet per minute (cfm). High capacity exhaust systems are expensive, energy-consuming, and difficult to install and maintain.
Moreover, pressure fluctuations in the exhaust air stream or in the semiconductor processing system overall still cause deleterious turbulence and loss of fume control at certain localities in the system, resulting in formation of “plumes” of hazardous vapors.
Some wet processing systems employ safety lids as supplemental means for fume control. The safety lids isolate each chemical tank from the environment to reduce migration of hazardous chemicals into workplaces. They also function to reduce dissipation of useful chemicals due to migration of fumes.
Commonly used safety lids include clamshell type and drawbridge type lids. Such lids significantly reduce the flexibility of wet processing systems as well as the wafer throughput. The opening and closing of such lids still results in local turbulence. Accordingly, such lids cannot completely eliminate escape of chemical fumes from the chemical tank. Moreover, currently available safety lids are mechanically complex, expensive, and difficult to maintain. Furthe
Olander W. Karl
Walker Bruce G.
Advanced Technology & Materials Inc.
Hultquist Steven J.
Pham Minh-Chau T.
Ryann William
Simmons David A.
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