Self-metering reservoir

Details Self-metering reservoir Self-metering reservoir

1999-09-03

2001-12-04

Walton, George L. (Department: 3753)

Fluid handling

Liquid level responsive or maintaining systems

By weight of accumulated fluid

C137S002000, C137S008000, C137S404000, C137S433000, C251S121000

Reexamination Certificate

active

06325094

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to systems for maintaining a generally constant level of fluid within a vessel and more particularly to systems for supplying vapor to a chemical process by introducing a carrier gas into a fluid column of vaporizable liquid.
2. Background Information
A common technique used in vapor generating systems for delivering chemical vapor to a process chamber is to force a carrier gas bubble through a chemical fluid in a bubbler and then to deliver the resulting vapor from the bubbler to the process chamber. Traditional bubblers, including those utilized in presently available automatic refill systems, rely on relatively large fluid volumes to intrinsically compensate for deviations in fluid level which can negatively effect the resulting vapor concentration. Since vapor sources in the fiber optics and semiconductor industries are often hazardous fluids, there has been an increasing focus on the occupational safety and health concerns resulting from use of such fluids. This has resulted in reducing the maximum allowable volumes of many of these fluids within the work place. It is therefore desirable to reduce the required fluid volume at the point of vapor generation without compromising vapor concentration control.
Typically a bubbler container is comprised of a single vessel which holds an expendable volume of vaporizable fluid. A carrier gas such as hydrogen, helium or nitrogen is introduced at the lower level of a fluid column, travels up through, and exits the fluid surface into a head space. As the carrier gas passes through the fluid column it becomes entrained with vapor which results in a corresponding reduction of the fluid volume. This reduction of the fluid level in bubbler container may be significant for several reasons. For example, the vaporization efficiency and overall vapor concentration uniformity are both affected by the fluid level and are both important elements which may affect the strict tolerance requirements of the process application. In addition, the physical fluid column in the bubbler not only determines the carrier gas contact time and resulting bubble geometry but also represents the mass to which thermal energy is either added or extracted. It also defines the head space present above and within the bubbler container which has been found to negatively effect the vapor concentration and ultimate bubbler performance when not optimized.
Inasmuch as vapor extraction from a fluid volume results in depleting the fluid volume of a bubbler, causing variations in vapor concentration, a means of replenishing this fluid is desirable. Some methods include manually replacing the bubbler ampule once the volume of fluid reaches a predetermined minimum acceptable level. Other manual methods rely on an auxiliary supply of fluid to replenish the bubbler during intermittent periods of non-use. Although such methods can result in reducing many of the concerns associated with prior art expendable bubblers, such as reducing the risk of contamination during ampule replacement or any necessary fluid replenishment, these systems typically remain idle until an interruption in vapor extraction provides a refill opportunity. With many of the advanced processes running for long periods of time before a refill opportunity is presented, the fluid level may descend considerably resulting in less than optimum vapor delivery efficiency. Although there are techniques which can be employed to compensate for the influences of a constantly descending fluid volume, such as intermittent refill in between process runs, such techniques can be complex and costly. In any event, such techniques do not satisfy the level of control achieved by the present invention.
In addition to manual replenishment of fluid, automatic bubbler refill systems are also available. However, such systems typically employ float coupled electronic devices, such as level controllers, to control the replenishment of fluid in the bubbler. Such devices are prone to failure and are generally the most common failure mechanism in the system. Other types of fluid level sensors such as optical, load cell monitoring of the contents, and resistance probes have been employed. However, the use of such devices can be costly, prone to error, and with many of the fluids being flammable, represent ignition sources if not properly rated and maintained.
SUMMARY OF THE INVENTION
Accordingly, one object of the present is to provide a cost efficient and reliable bubbler reservoir system capable of metering an external supply of fluid into itself to maintain a nearly constant liquid level while it bubbles vapor to a process.
Another object is to restrict an incoming supply of fluid to the bubbler to prevent overfill conditions without the need for electronic or optical level detection devices.
A further object of the invention is to provide a means of dramatically reducing the fluid volume at the point of vapor generation without compromising vapor concentration control.
Another object of the present invention is to prevent or reduce wear effects or particle generation from friction by the use of magnetic forces.
A further object of the invention is to provide an optimum mechanism for controlling the temperature of the inner bubbler fluid by directing makeup fluid along the outer walls of the inner bubbler vessel where thermal energy can be added or extracted, as required, prior to introducing this fluid into the inner vessel, thereby reducing potential for thermal cycling as makeup fluid mixes with inner bubbler fluid.
Another object of the invention is to provide a means of calibrating the buoyant interaction of the inner bubbler vessel relative to different fluid applications and more specifically to differences in specific gravity through the use of opposing magnetic fields, one fixed within the inner buoyant bubbler vessel and one which can be adjusted at the base of the outer vessel.
The above and other objects of the present invention are accomplished in a system that overcomes the disadvantages and limitations of the prior art by providing an apparatus and method to generate and support the delivery of vapor from a vaporizable source contained in a bubbler as described herein and to support the real-time metering of an incoming supply of makeup fluid to replace fluid lost as a result of the consumption of vapor. The system meters the introduction of the incoming fluid in direct response to the extraction of fluid being converted to and carried off as vapor. The invention's novel approach to minimizing the resident fluid volume required to support the continuous delivery of vapor is most notably due to its intrinsic control of makeup fluid in direct response to small changes in fluid within the bubbler. In a preferred form, the invention may also include one or more of the following: the use of adjustable magnetic fields to compensate for fluids of different specific gravities, the use of magnetic fields to reduce the possibility of particulate contamination otherwise resulting from surface abrasion at the contact points of moving parts within the bubbler, the use of thermoelectric converters to control the temperature of the bubbler fluid, a reduction in the overall bubbler size and temperature control type, and the ability to integrate the device much closer to the process site. The latter features not only reduce the final cost of the bubbler, and its integration into process equipment, but also further reduces the degrading influence of ambient temperature effects associated with longer vapor delivery lines running from a bubbler to the point of process.
The present invention is also particularly advantageous in applications where a plurality of bubblers are supported by a single bulk refill system. In a typical bulk refill system, a call for makeup fluid occurs when there is a discontinuation of a process due to the detection of a low liquid level in the bubbler. Due to the potential for multiple prior art bubblers to call for mat

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