Cryogenic ferrofluid sealed rotary union

A rotary union that includes a heated ferrofluid seal is disclosed. The rotary union includes an inner rotating shaft, an intermediate rotating shaft and an outer rotating shaft. The inner rotating shaft is hollow to allow the flow of cryogenic fluid in one direction. The inner rotating shaft and the intermediate shaft are spaced apart to create a channel for the return of the cryogenic fluid. The intermediate rotating shaft is separated from the outer rotating shaft by a gap so as to reduce thermal conductivity. In this way, the temperature of the outer rotating shaft is greater than the temperature of the cryogenic fluid. A heated ferrofluid seal is disposed between the outer rotating shaft and the housing.

FIELD

Embodiments of the present disclosure relate to ferrofluid sealed rotary unions, and more particularly, ferrofluid sealed rotary unions used to provide cryogenic fluid to a process chamber.

BACKGROUND

In some workpiece processing systems, the workpiece is disposed on a platen, which can be rotated. This platen may also include electrical components, such as electrodes that are in communication with a power source. Additionally, this platen may have fluid conduits to allow a fluid to pass therethrough to heat or cool the platen. These fluid conduits are in communication with an external fluid source and an external fluid sink. Because the platen rotates, a rotary union is typically used to link the platen to the external components. The rotary union provides the electrical connections, as well as fluid inlets and fluid outlets. In some embodiments, the electrical connections may be integrated, while in other embodiments, a separate electrical rotary union may be employed. Further, typically, one end of the rotary union is disposed in a process chamber, which is maintained at very low pressure, while the other side is disposed at atmospheric conditions.

The workpiece may be a semiconductor wafer, which is subjected to one or more processes while disposed on the platen. These processes may include etching, deposition and ion implantation.

In some particular embodiments, the platen is cooled to cold temperatures to enhance the process being performed on the workpiece. This may be achieved by passing a cold fluid through the fluid conduits in the platen. However, seals used in conventional rotary unions often fail to seal at cryogenic temperatures. For example, leakage may result from a shrinkage of the seal due to temperature changes, from the seal becoming brittle or less compliant due to the low temperatures, or due to other reasons. Consequently, the minimum workpiece temperature that can be attained may be based on the ability of the rotary union to continue functioning properly. In other embodiments where cryogenic temperatures are used, the rotary union may be replaced with stationary conduits, such that the platen is incapable of rotation.

Consequently, it would be beneficial if there was a rotary union that could withstand these extremely low temperatures without failing. Such a rotary union would make it possible to perform cryogenic processing of the workpiece using a rotating platen.

SUMMARY

A rotary union that includes a heated ferrofluid seal is disclosed. The rotary union includes an inner rotating shaft, an intermediate rotating shaft and an outer rotating shaft. The inner rotating shaft is hollow to allow the flow of cryogenic fluid in one direction. The inner rotating shaft and the intermediate shaft are spaced apart to create a channel for the return of the cryogenic fluid. The intermediate rotating shaft is separated from the outer rotating shaft by a gap so as to reduce thermal conductivity. In this way, the temperature of the outer rotating shaft is greater than the temperature of the cryogenic fluid. A heated ferrofluid seal is disposed between the outer rotating shaft and the housing. The ferrofluid seal may be heated using a resistive heater, a heated gas or a heated fluid.

According to one embodiment, a rotary union for carrying a cryogenic fluid is disclosed. The rotary union comprises a hollow inner rotating shaft; an intermediate rotating shaft surrounding the hollow inner rotating shaft, where a channel is created in a space between the hollow inner rotating shaft and the intermediate rotating shaft; an outer rotating shaft surrounding the intermediate rotating shaft, and separated from the intermediate rotating shaft by a gap; a housing surrounding the outer rotating shaft; a ferrofluid seal disposed between the outer rotating shaft and the housing; and a heater proximate the housing to warm the ferrofluid seal. In certain embodiments, the rotary union comprises a static seal between the intermediate rotating shaft and the outer rotating shaft. In some embodiments, the static seal hermetically seals the intermediate rotating shaft and the outer rotating shaft. In certain embodiments, the static seal comprises a ceramic ring that is brazed to the outer rotating shaft and the intermediate rotating shaft. In other embodiments, the static seal comprises a bellows. In certain embodiments, the heater comprises a resistive element. In other embodiments, a heating channel is disposed within the housing and wherein a heated fluid passes through the heating channel, wherein the heating channel and the heated fluid comprise the heater. In some embodiments, the rotary union is disposed in an isolation box, maintained at near vacuum conditions, and the gap is maintained at the near vacuum conditions.

According to another embodiment, a workpiece processing system is disclosed. The system comprises a process chamber, maintained at near vacuum conditions; a rotating platen within the process chamber, on which a workpiece is disposed; a rotating shaft assembly having an upper hollow inner shaft and return path for cryogenic fluid, wherein the upper hollow inner shaft and the return path are in communication with a conduit in the rotating platen; an isolation box having an inlet and an outlet and maintained at near vacuum conditions; a rotary union disposed within the isolation box, the rotary union comprising a lower hollow inner shaft to deliver cryogenic fluid to the rotating platen and a lower intermediate shaft spaced apart from the lower hollow inner shaft defining a channel therebetween that is in communication with the return path; a base disposed in the isolation box, having an inlet conduit in communication with the inlet and the lower hollow inner shaft, and an outlet conduit in communication with the outlet and the channel; and a heated ferrofluid seal to isolate the outlet conduit from the near vacuum conditions within the isolation box. In certain embodiments, the rotary union further comprises a lower outer shaft spaced apart from the lower intermediate shaft by a gap, wherein the gap is hermetically sealed proximate the base so that the gap is maintained at near vacuum conditions. In certain further embodiments, the rotary union further comprises a lower housing surrounding the lower outer shaft, wherein the lower housing is stationary and the heated ferrofluid seal is disposed between the lower outer shaft and the lower housing. In some embodiments, a resistive heater is disposed proximate the lower housing. In certain embodiments, a heating channel is disposed in the lower housing through which a heated fluid passes. In certain embodiments, the upper hollow inner shaft and the lower hollow inner shaft comprise one shaft.

According to another embodiment, a rotary union for carrying a cryogenic fluid is disclosed. The rotary union comprises a hollow inner rotating shaft; an intermediate rotating shaft surrounding the hollow inner rotating shaft, where a channel is created in a space between the hollow inner rotating shaft and the intermediate rotating shaft, where the channel terminates in a cavity; an outer rotating shaft surrounding the intermediate rotating shaft, and separated from the intermediate rotating shaft by a gap; a static seal to hermetically seal the gap from the cavity; a housing surrounding the outer rotating shaft; a ferrofluid seal disposed between the outer rotating shaft and the housing to seal the cavity; a heater proximate the housing to warm the ferrofluid seal; a base, having an inlet conduit in communication with the hollow inner rotating shaft, and an outlet conduit in communication with the cavity. In certain embodiments, an insulator is disposed against a bottom surface of the intermediate rotating shaft, a bottom surface of the outer rotating shaft, and a bottom surface of the housing, and is disposed between the housing and the base. In some embodiments, the static seal comprises a ceramic ring that is brazed to the outer rotating shaft and the intermediate rotating shaft. In other embodiments, the static seal comprises a bellows.

DETAILED DESCRIPTION

As described above, in certain systems, it is desirable to have a workpiece processed at very low temperatures, while disposed on a rotating platen.

FIG. 1shows a system that addresses this issue. The system includes a process chamber100, which comprises a plurality of walls101that define a sealed chamber, which is maintained at near vacuum conditions. In this disclosure, the term “near vacuum conditions” refers to a pressure of less than 50 millitorr. In some embodiments, an ion source may also be in communication with the process chamber100. In other embodiments, one or more components may be disposed between the ion source and the process chamber100. The type of processing defines the configuration. In one particular embodiment, the process is PVD (Physical Vapor Deposition). In this embodiment, gases are introduced into the process chamber100to form a film on the workpiece111. In another embodiment, the workpiece111may be subjected to an ion implantation where either a beamline system or a plasma source is in communication with the process chamber100. For example, a beam-line ion implantation system may have a mass analyzer, one or more acceleration/deceleration stages and a collimator disposed between the ion source and the process chamber100. In another embodiment, the workpiece111may be subjected to an etch process. In an etch process, a source of RF energy is typically placed above the workpiece111and the platen110may be electrically biased. Thus, the location of the ion or plasma source relative to the process chamber100is not limited by this disclosure.

In these embodiments, the platen110may be disposed in the process chamber100. A workpiece111may be disposed on the platen110. The platen110is rigidly attached to an upper inner shaft160and the upper intermediate shaft170. The upper inner shaft160is hollow. This enables the platen110to rotate about axis112. An upper outer shaft180may also enter the process chamber100. The upper inner shaft160and the upper intermediate shaft170are separated by a channel165. The upper outer shaft180is separated from the upper intermediate shaft170by a gap175. The configuration of the upper inner shaft160, the upper intermediate shaft170and the upper outer shaft180may be referred to as the rotating shaft assembly167, as all three of these shafts rotate about axis112. These three shafts are concentric. A support181may be disposed between the upper outer shaft180and the upper intermediate shaft170near the upper end of the upper outer shaft180. The support181may mechanically support the upper outer shaft180and help maintain the gap175. Further, the support181may be thermally insulating to minimize the transfer of heat between the upper intermediate shaft170and the upper outer shaft180. Additionally, the support181may have openings such that the gap175is part of the atmosphere of the process chamber100. In this way, the gap175may also be maintained at near vacuum conditions.

The process chamber100may also have a port102to which an upper housing190is attached. A seal is created between the upper housing190and the upper outer shaft180using an upper ferrofluid seal182. The upper ferrofluid seal182serves to isolate the environment within the process chamber100from the outside atmosphere. The upper ferrofluid seal182includes an inner shaft186supported by the bearings183,184. The inner shaft186of the upper ferrofluid seal182supports the upper outer shaft180, such as via use of a flange. Further, the static seals185seal the gap between the inner shaft186of the upper ferrofluid seal182and the upper outer shaft180as these components rotate together. The pair of static seals185shown serve to decrease potential leaks. The ferrofluid187in the upper ferrofluid seal182is disposed between bearings183,184. The rotating shaft assembly167may be rotated using any suitable means. For example, the rotating shaft assembly167may be moved by connection to a drive timing belt130. The drive timing belt130may be in communication with a motor135. In other embodiments, the rotating shaft assembly167may be directly driven. For example, direct drive where the motor's magnetic field drives the rotating shaft assembly167and the rotor could also be used. Thus, the mechanism used to cause the rotation of the rotating shaft assembly167is not limited by this disclosure. An electrical slip ring assembly140may also be disposed on the rotating shaft assembly167. The electrical slip ring assembly140may comprise one or more slip rings that allow electrical connection between a stationary object and a rotating object, such as the rotating shaft assembly167. The number of slip rings is not limited by this disclosure. In other embodiments, the electrical slip ring assembly140may not be employed, or may be a separate rotary union.

The rotating shaft assembly167terminates in an isolation box150. The isolation box150includes a housing155having the inlet151and the outlet152for the cryogenic fluid. The cryogenic fluid may be a gas. The hollow center of the upper inner shaft160is in fluid communication with the inlet151, while the channel165is in fluid communication with the outlet152.

A static seal179may be used to isolate the environment in the process chamber100(i.e. gap175) from the environment in the isolation box150. In certain embodiments, the static seal179may also serve to affix the upper outer shaft180to the upper intermediate shaft170. In other embodiments, support181serves as both an insulator and a mechanical support, while static seal179only acts as a sealing device. Dynamic seals178are used to isolate the environment within the isolation box150from the atmospheric pressure outside the isolation box150. These dynamic seals178may be spring loaded Teflon type seals, O-rings or any other suitable sealing device. In some embodiments, the interior of the isolation box150may be kept at sub-atmospheric pressures, such as in the millitorr range. In certain embodiments, the pressure within the isolation box150may be 50 millitorr or less. The interior of the isolation box150may be continually pumped to sustain the vacuum levels low enough to minimize heat transfer using a vacuum pump157. The vacuum pump157also removes any gases that leak through the rotary union and are not intended to enter the process chamber100. By removing air and other gasses from the isolation box150, the likelihood of icing on cold components within the isolation box150is significantly reduced.

A cross section of the isolation box150and the rotary union200are shown in more detail inFIG. 2. The rotary union200serves to provide fixed connections for the inlet151and outlet152, while allows the shafts within the isolation box150to rotate. The rotary union200has three rotating shafts; a lower hollow inner shaft210, a lower intermediate shaft220and a lower outer shaft230. These three shafts are concentric. In some embodiments, the lower hollow inner shaft210and the upper inner shaft160may be a unitary component. In certain embodiments, the lower intermediate shaft220comprises a flange221near its upper end. This flange221allows attachment of the lower intermediate shaft220to the upper intermediate shaft170.

This disclosure uses the terms “upper” and “lower” to distinguish between the rotating shaft assembly disposed near the process chamber100and the rotating shaft assembly disposed near the isolation box150. However, the rotating shaft assemblies do not have to be oriented in this manner. For example, the shafts may be aligned in the horizontal direction rather than a vertical direction. Thus, the term “upper” refers to the components located proximate the process chamber100, while the term “lower” refers to components proximate the isolation box150.

The lower hollow inner shaft210and the lower intermediate shaft220are separated by a channel215in the form of an annular cylinder. The channel215and the channel165may be in fluid communication and sealed together to form a single passageway. The dimensions of the channel215and channel165may be selected so that the hydraulic diameter defined by the size of these annular channels is roughly equal to the hydraulic diameter of the inner diameter of the lower hollow inner shaft210. The lower hollow inner shaft210and the lower intermediate shaft220are in communication with a base250. This base250contains inlet conduit153and outlet conduit154which are in communication with inlet151and outlet152, respectively. The base250is stationary within the isolation box150. In operation, cryogenic fluid flows through the inlet151and into an inlet conduit153in the base250. This inlet conduit153is in communication with the lower hollow inner shaft210. The cryogenic fluid then passes through the upper inner shaft160and through the conduit113in the platen110, as shown inFIG. 1, before returning through the channels165,215. The channel215is in communication with the outlet conduit154and the outlet152.

The lower hollow inner shaft210rotates within base250. Consequently, there may be a slight gap between these components to allow for this rotation. To prevent galling, wear and friction between the two components, a bushing, such as a plastic bushing, and more specifically a Teflon type plastic which is capable of withstanding cryogenic temperatures, may be utilized. Since there is a small gap between lower hollow inner shaft210and base250, there may be a resulting leakage path from inlet conduit153to outlet conduit154. Consequently, cryogenic fluid may leak from inlet conduit153to outlet conduit154. This leakage is internal to the rotary union200, so its only impact is to the efficiency of the rotary union200.

The lower intermediate shaft220terminates in a cavity296at the top of the base250, defined by the base250, insulator295and outer surface of the lower hollow inner shaft210. This cavity296is sealed from the vacuum environment of the isolation box150by the heated ferrofluid seal270.

Since the cryogenic fluid contacts both the lower hollow inner shaft210and the lower intermediate shaft220, both of these components will be at roughly the same temperature as the cryogenic fluid. A lower outer shaft230surrounds at least a portion of the lower intermediate shaft220. This lower outer shaft230is spaced apart from the lower intermediate shaft220by a gap225. In certain embodiments, the lower outer shaft230is disposed entirely within the isolation box150. In certain embodiments, this gap225is maintained at near vacuum conditions, such as less than 50 millitorr. A support280holds the lower outer shaft230in place and also allows the atmosphere in the gap225to be at the same pressure as the isolation box150. The support280may be made of an insulating material, such as a plastic material, including Teflon and PEEK, or a ceramic material, including alumina. This gap225reduces the transfer of heat from the lower outer shaft230to the lower intermediate shaft220. Thus, the lower outer shaft230may be at a higher temperature than the lower intermediate shaft220. In certain embodiments, the gap225may be about 0.125 inches thick, although other dimensions may be used. In certain embodiments, the lower intermediate shaft220may be at −170° C. while the lower outer shaft230is at approximately 0° C.

Each of these rotating shafts may be constructed of stainless steel, or another suitable material.

A lower housing240, which remains stationary, surrounds at least a portion of the lower outer shaft230. In certain embodiments, bearings260are disposed between the lower housing240and the lower outer shaft230and may be used to facilitate the rotation of the lower outer shaft230relative to the lower housing240.

A heated ferrofluid seal270is created between the lower housing240and the lower outer shaft230. The heated ferrofluid seal270creates a seal so as to separate the vacuum conditions that exist within the isolation box150from the cryogenic fluid in cavity296. The heated ferrofluid seal270will be described in more detail below.

Ferrofluid and bearings preferably operate at temperatures greater than about −40° C. The cryogenic fluid may be maintained at temperatures as low as −170° C.

The lower intermediate shaft220and the lower outer shaft230are held together by a static seal. In the embodiment ofFIG. 2, the static seal comprises a ceramic ring290that is brazed to both shafts. The ceramic ring290has low thermal conductivity and therefore helps maintain the temperature differential between the lower intermediate shaft220and the lower outer shaft230. The ceramic ring290also provides a hermetic seal between the two rotating shafts. The hermetic seal is employed to insure that cryogenic fluid does not penetrate the gap225. In certain embodiments, as described above, a support280may be disposed at the opposite end of the lower outer shaft230to maintain alignment between the lower intermediate shaft220and the lower outer shaft230. In certain embodiments, this support280does not create a seal, so that the vacuum conditions within the isolation box150also exist in the gap225.

An insulator295, which may in the form of an annular ring and have two different heights, may be disposed against the lower end of the rotary union200to thermally insulate the lower outer shaft230from the exiting cryogenic fluid. Specifically, the shorter inner portion of the insulator may be disposed against the bottom surface of the lower intermediate shaft220and the bottom surface of the lower outer shaft230. The taller outer portion of the insulator295may be disposed between the lower housing240and the base250. Because the taller outer portion of the insulator295is sandwiched between the base250and the lower housing240, it remains stationary and does not rotate with the rotary union.

In operation, returning cryogenic fluid flows into a cavity296defined by the outer surface of the lower hollow inner shaft210, the top surface of the base250, and insulator295. Insulator295serves two purposes:

1) to thermally isolate the base250from the lower outer shaft230; and

2) to limit thermal heat transfer via convection from the flowing cryogenic fluid from channel215to outlet conduit154to lower outer shaft230.

FIG. 3shows a second embodiment of the rotary union300disposed in the isolation box150. Components that are identical toFIG. 2have been given the same reference designators. The embodiment ofFIG. 3differs from the embodiment ofFIG. 2in the manner in which the lower intermediate shaft220is affixed to the lower outer shaft230. In this embodiment, the static seal comprises a bellows400, which may be constructed of stainless steel, that is used to affix the lower intermediate shaft220to the lower outer shaft230. The bellows400may be a very thin stainless steel web, such as between about 0.005 to 0.020 inches, that limits thermal conduction but still functions as a hermetic gas barrier.

Thus, in the embodiments ofFIG. 2andFIG. 3, the static seal performs two functions. First, it serves to minimize thermal conduction between the lower intermediate shaft220and the lower outer shaft230. Second, it serves to create a hermetic seal between the lower intermediate shaft220and the lower outer shaft230so that the gap225is isolated from the cryogenic fluid in the cavity296.

FIG. 4shows an expanded view of the heated ferrofluid seal270shown inFIGS. 2 and 3. A magnet500may be disposed in the lower housing240. The magnet500may be annular so as to surround the lower outer shaft230. On either side of the magnet may be one or more pole pieces510. The pole pieces510serve to direct the magnetic fields540in a desired orientation. The magnetic fields540may be manipulated so as to be perpendicular to the longer dimension of the rotating shafts (i.e. perpendicular to the axis of rotation). Ferrofluid530is disposed in the gap between the lower housing240and the lower outer shaft230. Because of the magnetic field540, the ferrofluid530remains in place and is unaffected by the pressure differential across the ferrofluid530.

A heater may be disposed within or proximate the lower housing240.

In one embodiment, as shown inFIG. 4, the heater550may be a resistive element that generates heat when an electrical current is passed therethrough. The resistive element may be disposed within the lower housing240, as shown inFIG. 4, or may be disposed outside the lower housing240, such as on the outer surface of the lower housing240. Electrical wires551may pass through the isolation box150to electrically connect to the resistive element. In certain embodiments, the heater550is controlled by a controller560, which provides a current to the heater550through the electrical wires551. In certain embodiments, the heater550may be part of a closed loop system where the controller560receives temperature information from a temperature sensor570, located proximate the ferrofluid530. In another embodiment, the temperature sensor570may be located in a different location to provide the controller560with a temperature that may be representative of the temperature of the ferrofluid530. In another embodiment, the controller560may utilize an algorithm based on time or other criteria to control the heater550using open loop control.

In another embodiment, shown inFIG. 5, the heater650may be constructed by disposing one or more heating channels660within the lower housing240through which a heated fluid, such as a liquid or gas, is passed. In this embodiment, a heated fluid inlet670and a heated fluid outlet680may be disposed in the isolation box150to allow the source and sink of the heated fluid to pass into and out of the isolation box150. In certain embodiments, the heater650is controlled by a controller700, which provides a heated fluid to the heated fluid inlet670. For example, the controller700may be in communication with a pump710, and a fluid heater720. In certain embodiments, the controller700may adjust the temperature of the heated fluid by controlling the fluid heater720. In certain embodiments, the controller may adjust the temperature of the lower housing240by modifying the flow rate of the heated fluid using the pump710. In certain embodiments, the controller700controls both the flow rate and the temperature of the heated fluid. In certain embodiments, the controller700may utilize a closed loop system, where the controller700received temperature information from a temperature sensor730regarding the temperature of the ferrofluid530. In other embodiments, the temperature sensor730may be located in a different location to provide the controller700with a temperature that may be representative of the temperature of the ferrofluid530. In certain embodiments, the controller700may also be in communication with a fluid sensor740to determine a temperature of the heated fluid. In certain embodiments, the controller700may adjust the pump710and the fluid heater720based on the outputs from at least one of the two temperature sensors. In other embodiments, the controller700may utilize open loop control.

In each of these embodiments, the heater serves to warm the lower housing240such that the ferrofluid is at a temperature greater than −40° C.

In summary, the present disclosure describes a rotary union that utilizes a heated ferrofluid seal270. To allow the ferrofluid530to be disposed at an acceptable temperature, the present rotary union utilizes three shafts that rotate as one. The lower hollow inner shaft210is hollow, allowing the cryogenic fluid to travel through the center of the inner rotating shaft. The lower intermediate shaft220is spaced apart from the lower hollow inner shaft210so as to create a channel215between these two shafts. The channel215serves as a return path for the cryogenic fluid. A third rotating shaft, the lower outer shaft230, then surrounds the lower intermediate shaft220and is separated from the lower intermediate shaft220by a gap225. In certain embodiments, the gap225is maintained at near vacuum conditions, which reduces the transfer of heat across the gap225, thus allowing the lower outer shaft230to be at a higher temperature than the cryogenic fluid. A heater is used to further increase the temperature of the ferrofluid530.

The system described herein have many advantages. First, using a rotary union with a cryogenic fluid is challenging because it is difficult to seal the inlet and outlet from an external environment. This is made even more difficult when the external environment is maintained at near vacuum conditions. Ferrofluid seals are able to provide seals between atmospheric pressure and vacuum, but cannot operate at very low temperatures. The present system provides a system where the ferrofluid seal is heated and can be maintained at operating temperatures within the rotary union. This rotary union with a heated ferrofluid seal allows various applications which were difficult or not previously possible. For example, cryogenic fluid can be supplied to a rotating platen within a process chamber without any leakage of cryogenic fluid.