Refrigeration – Low pressure cold trap process and apparatus
Reexamination Certificate
2000-04-05
2001-07-24
Capossela, Ronald (Department: 3744)
Refrigeration
Low pressure cold trap process and apparatus
C277S630000, C277S650000
Reexamination Certificate
active
06263679
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of cryogenic vacuum pumps, commonly referred to as “cryopumps.” Cryopumps utilize pumping surfaces cooled to cryogenic temperatures by a cryogenic refrigerator to condense and absorb gases.
A known vertical-mount cryopump design is illustrated in FIG.
1
. The cryopump
10
is joined at a flange
14
to a process chamber, such as a process chamber of a cluster tool for semiconductor wafer fabrication. The cryopump
10
is then used to remove gases from the process chamber. As is typical, the illustrated cryopump
10
features a pair of pumping surfaces (use of the singular term, “pumping surfaces,” hereafter, is to be understood to include both a single surface and any number of additional surfaces). In the illustrated embodiment, the primary, lower-temperature pumping surface is in the form of an array of baffles
34
. The array of baffles
34
is cooled to a temperature of about 4K to about 25K by the second stage
32
of a two-stage cryogenic refrigerator. The higher-temperature pumping surface includes a radiation shield
36
and a frontal array
38
in thermal contact with the radiation shield
36
. The radiation shield
36
and frontal array
38
surround the lower-temperature array of baffles
34
and are cooled by the first stage
29
of the cryogenic refrigerator. Both pumping surfaces are contained within a vacuum vessel
12
.
When the cryopump
10
is operating, gases with higher boiling points (e.g., water vapor) are condensed on the higher-temperature pumping surface. Gases with lower boiling points (e.g., nitrogen) pass through the frontal array
38
of the higher-temperature pumping surface to the lower-temperature pumping surface where they are condensed. Further, an adsorbent, such as charcoal or a molecular sieve, is typically attached to the lower-temperature surface (e.g., to the underside of the baffles
34
) to remove gases with very low boiling points, such as hydrogen, helium and neon. The above-described condensation and adsorption produce a high vacuum in the vacuum vellel
11
and in the process chamber to which the cryopump
10
is mounted.
Once a high vacuum is established, a workpiece (e.g., a semiconductor wafer) can be moved into and out of the process chamber through partially-evacuated load locks. Each time the process chamber is opened, additional gases enter there through. These gases are then condensed onto the pumping surfaces, thereby maintaining the low-pressure conditions needed for processing the workpiece. In addition, processing gases that are introduced in the process chamber are also condensed onto the pumping surfaces.
After several days or weeks of continued processing, the gases that condense and absorb on the pumping surfaces begin to saturate the cryopump
10
. The trapped gases are then released from the pumping surfaces via a regeneration procedure, whereby the cryopump
10
is temporarily shut down to allow the pumping surfaces to warm. As the surfaces warm, so do the gases condensed thereon, thereby facilitating the release of these gases. The released gases are then purged from the vacuum vessel, and cooldown of the cryopump
10
is repeated.
DISCLOSURE OF THE INVENTION
Over the course of cryopump operation and, in particular, during regeneration, particulates of the adsorbent (e.g., charcoal) may detach from the baffles and circulate within the radiation shield. In existing systems, when captured species are released during regeneration, they may pass through a liquid phase, and before the species becomes gaseous, the liquid can wash the adsorbent particulates from the cryopump to the gate valve of a process chamber. Adsorbent particles can also be deposited via vapor transport at the gate valve during a purge of the cryopump. Adsorbent particles that reach the gate valve can be trapped in the seal of the gate valve and ground up with subsequent opening, and closing of the gate valve. Trapping of adsorbent particles in the seal of the gate valve also interferes with the ability to form a gas-tight barrier when the gate valve is closed. In accordance with this invention, the transport of particulates to the gate valve is prevented by using a dam for trapping particulates at or within the flange of the gate valve.
A dam of this invention is sized and shaped to stop the flow of particulates from the cryopump to the seal and valve member of the gate valve. The dam can be part of a gasket mounted at the junction of the flange of a cryopump and the flange of a gate valve. Alternatively, the dam can be a separate component from the gasket.
A gasket of this invention includes a ring, and a dam extending from the ring into an interior volume defined by the ring The gasket is sized and shaped to be mounted at the junction of the flanges of a cryopump and a gate valve. The gasket is particularly well suited for use with cryopumps that are horizontally mounted to a gate valve at a port of a process chamber, in which case the dam is positioned toward the bottom of the flange.
Though the ring and dam are separately recited for clarity of description, the two can jointly form a seamless, unlitaly assembly. Where the gasket ring and the dam are separate elements, both can mounted at the interface of the flanges of the cryopump and a gate valve on a process chamber, or the dam can be separately mounted via a secured spring within a corridor of the gate valve extending from the flange, in which case, the dam is still considered to be mounted “at the flange.” In either case, the ring is preferably circular in shape, though a ring of this invention need not be precisely circular.
The dam can have the shape of a disc section and can inwardly extend from the arch-shaped edge between about 5% to about 15% of the diameter of the passage between the crypump and gate valve. In further preferred embodiments, the dam has a height of at least about 1 cm, more preferably between about 0.5 inches (about 1.3 cm) and about 1 inch (2.5 cm). Further, the gasket can be formed of copper, with the ring having an inner diameter of about 20 cm.
In a method of this invention, a cyropump can be mounted to the gate valve by positioning the dam, either as part of the gasket or separate therefrom, between a flange on the cryopump and a flange on the gate valve and compressing the gasket between the flanges. Preferably, the gasket is in a substantially-vertical plane with the dam positioned at the bottom of the gasket. When the cryopump is put into operation, wherein the cryopump is cooled to generate a vacuum in the process chamber with intermittent regeneration procedures and coordinate opening and closing of the gate valve, the gasket prevents loose particulates in the cryopump from reaching the gate valve. In alternative embodiments of the method, the dam can be positioned within the flange (i.e., within the corridor defined by an inner surface of the flange and the tubing that extends therefrom to the valve member).
An advantage of this invention is that the dam can be used to reduce or prevent transport of adsorbent particulates to the gate valve. As a consequence, the cleanliness of the process chamber, which is of extreme importance in fields such as semiconductor wafer processing, can be better insured. Further, the gasket and dam of this invention will reduce or eliminate problems associated with an inability to fully seal the gate valve due to contamination of the seal by adsorbent particulates. Further still, the dam of this invention is not only effective as a barrier against transport of adsorbent particulates, but also as a barrier against the transport of any other debris originating from or entering into the cryopump.
REFERENCES:
patent: 5043148 (1991-08-01), Yasue
patent: 5062271 (1991-11-01), Okurmura et al.
patent: 5261244 (1993-11-01), Lessard et al.
patent: 5333466 (1994-08-01), Harrington et al.
patent: 5357760 (1994-10-01), Higham
patent: 5483803 (1996-01-01), Matte et al.
patent: 5542257 (1996-08-01), Mattern-Klossen et al.
patent: 5548964 (1996-08-01), Jinbo et al.
Capossela Ronald
Hamilton Brook Smith & Reynolds PC
Helix Technology Corporation
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