CRYOPUMPS AND INLET FLOW RESTRICTORS FOR CRYOPUMPS

A flow restrictor for restricting a flow rate of gas flowing into a cryopump and the cryopump are disclosed. The flow restrictor is configured to be mounted in an inlet of the cryopump, the flow restrictor comprising: an inlet component for providing a gas flow path into the cryopump; a shielding plate mounted to at least partially obscure the gas flow path though the inlet component; and an intermediate component linking the shielding plate to the inlet component, the intermediate component comprising at least one aperture, the at least one aperture defining at least one gas flow path into the cryopump The shielding plate is configured to shield the gas flow path through the inlet component such that when mounted on the cryopump there is no direct line of sight path through the inlet component to a cryopanel within the cryopump.

FIELD

The field of the invention relates to cryopumps and to an inlet flow restrictor for cryopumps.

BACKGROUND

Flow restrictors or throttle plates may be used to restrict the flow of gas into a cryopump in order to limit the pumping speed of the pump and maintain a desired pressure in the process chamber in for example PVD physical vapour deposition processes. These flow restrictor plates have conventionally been provided with multiple orifices of different geometric shapes through which type II and type III gases enter the pump and whose size and number control the flow rate or speed. A potential problem with plates having holes or orifices is that during viscous or continuous flow the orifices have a line of sight view of the second stage cryopanel in the pump and this increases radiant heat loads and can cause preferential gas pumping at these sites. Preferential gas pumping can cause columns of gas molecules condensed as “frost” to grow from the second stage cryopanel up towards the plate openings particularly during high gas flow rates. As the columns become more distant from the cryopanel they become warmer, receive an increased radiant load and may start outgassing thus raising the pressure in the chamber. Increased second stage temperature due to the radiant loads and/or warmer gas columns cause the vapour pressure of the type II gases to rise and pressure in the pump/chamber to rise with it. Type II gases are gases such as nitrogen that condense at the temperatures of the second stage cryopanels of a cryopump, while type III gases do not condense at these temperature and are generally captured by an adsorbent on the cryopanels.

FIG.1shows an example of a cryopump having a throttle or flow restrictor plate5according to the prior art. Flow restrictor plate5sits across the inlet of the pump and comprises a plurality of orifices7through which gas flows into the pump. The size of the orifices is selected for the desired pumping speed of the pump. The pump is a cryopump and has a refrigerator unit15with a first refrigerator heat station10connected to the inner housing of the pump which is insulated from the outer housing of the vacuum vessel9. It also has a second stage heat station11which connects to second stage cryopanel12and other adsorbent cryopanels13. The upper cryopanel12will experience frost build-up14over upper surface of the cryopanel with particular build-up forming spires at locations corresponding to orifices7. A potential problem with the non uniform build-up of frost and with radiant heat loads is that the pressure recovery inside the chamber takes longer and regeneration may be required sooner.

It would be desirable to provide a cryopump where the time between regenerations is increased and where pressure recovery times inside the chamber are decreased.

SUMMARY

A first aspect provides a flow restrictor for restricting a flow rate of gas flowing into a cryopump, said flow restrictor being configured to be mounted in an inlet of said cryopump, said flow restrictor comprising: an inlet component for providing a gas flow path into said cryopump; a shielding plate mounted to at least partially obscure said gas flow path though said inlet component; and an intermediate component linking said shielding plate to said inlet component, said intermediate component comprising at least one aperture, said at least one aperture defining at least one gas flow path into said cryopump; wherein said shielding plate is configured to shield said gas flow path through said inlet component such that when mounted on said cryopump there is not a direct line of sight path through said inlet component to a cryopanel within said cryopump.

In some embodiments the inlet component lies in a plane parallel to and offset from said shielding plate, such that when mounted on said cryopump said inlet component lies between said pumping chamber of said cryopump and said shielding plate.

The inventor recognised the problems associated with a conventional flow restrictor plate mounted on the inlet of a cryopump and addressed these problems by providing a two stage flow restrictor having an inlet component that is axially displaced from a shielding plate. The shielding plate shields the inlet component from gases entering the pump through the inlet forcing the gases around the shielding plate and via an intermediate component to the gas flow path through the inlet component. Thus, gases entering the cryopump are diverted around the shielding component through the at least one aperture in the intermediate component and into the flow path through the inlet component. In this way, direct line of sight of the inlet is shielded from the cyropanels by the shielding element and the preferential pumping paths provided by orifices that look directly at the cyropanels are avoided.

In some embodiments, said inlet component has an annular form delimiting an orifice, said orifice defining said gas flow path. An annular form of the inlet component forming a single orifice provides for more uniform flow across the cross sectional area of the inlet and helps inhibit preferential build-up of frost on the cyropanels at particular sites.

In some embodiments, said intermediate component comprises a plurality of apertures.

The intermediate component may be provided with a single aperture running around the surface linking the shielding plate and inlet component or it may have multiple apertures. The size of the apertures and/or the number of apertures may be selected to limit the flow to a desired amount depending on the requirements of the cryopump. Where there are multiple apertures selecting both the size and number of apertures may allow for accurate control of flow rates.

In some embodiments, a surface of said intermediate component comprising said at least one aperture lies at an angle of between 120° and 60° to said shielding plate.

It is advantageous if the intermediate component is angled with respect to the shielding plate and the inlet component such that the apertures do not look directly at the cyropanels. In this regard, may be advantageous if it is at an angle of between 60° and 120° to the plane of the shielding plate and in some embodiments if it is substantially perpendicular to the shielding plate.

In some embodiments, said intermediate component comprises a cylinder.

In some embodiments, an outer periphery of said inlet component extends beyond an outer periphery of said shielding plate.

An advantageous geometry of the shielding plate and the inlet component may be for the shielding plate not to extend as far as the outer perimeter of the inlet component but to extend further than the outer perimeter of the orifice of the inlet component. In this way, the orifice is directly shielded by the shielding plate but a path for gas to enter the pump around the edge of the shielding plate and then through the intermediate component is provided.

Although the geometry of the shielding plate and inlet component may have a number of forms such as rectangular, square or oval, in some embodiments, said shielding plate and said inner component have a substantially circular outer perimeter.

A circular cross section for a cryopump is generally advantageous for more uniform flow.

In some embodiments, said at least one aperture of said intermediate component is configured to restrict flow into said cryopump to a predetermined flow rate.

The size and/or number of the aperture(s) of the intermediate component may be selected according to the desired flow rate of the process being performed in the chamber evacuated by the cryopump.

A second aspect provides a cryopump comprising: a pump inlet; a refrigeration unit; a cryopanel configured to be cooled by said refrigeration unit; and comprising a flow restrictor according to a first aspect, said flow restrictor being mounted in an inlet of said cryopump such that said flow restrictor restricts a flow of gas into said inlet.

A third aspect provides a method of upgrading a cryopump comprising: removing a throttle plate mounted across an inlet of said cryopump for limiting flow into said cryopump; and replacing said throttle plate with a flow restrictor according to a first aspect.

DETAILED DESCRIPTION

Before discussing the embodiments in any more detail, first an overview will be provided.

Embodiments provide a throttle plate, sputter plate or a flow restrictor that utilizes an indirect orifice/opening scheme arranged to inhibit preferential pumping of type II gasses or radiation from negatively affecting the second stage frost or cryopanel. The idea is to allow monolithic type II gas storage on the second stage cryopanel increasing the amount of type II gas that can be stored. In this regard if the gas is pumped uniformly more can be stored, if one area builds condensed gas up more quickly than the others the gas partial pressure within the pump will rise.

Flow restrictors of an embodiment act as an orifice (with one or multiple openings) and allow type II and type III gasses to enter the pump at a rate or speed that is controlled by the size and number of orifices. They are arranged so that the orifices are at an angle to the pump inlet and no direct line of sight to the cryopanel is provided.

The cryopump is configured so that the flow restrictor operates at a temperature of 45K to 110K, that is it is connected to the first stage of a two-stage refrigeration device. In embodiments the flow restrictor is mounted on the inner wall of the pumping chamber which is cooled by the first stage heat station. The second stage cryopanel operates between 8K to 16K and may have charcoal or similar adsorbent material for type III gas pumping. The second stage cryopanel may or may not be configured to shield type II gasses from the adsorbent material for type III gas. In this regard panels13in the embodiment ofFIG.4may comprise charcoal covered cryopanels that are shielded by the upper cryopanel12.

The flow restrictor throttles the speed of the pump for type II and type III gasses so as to match the (PVD) process gas flow rate and achieve a predicted pressure within the vacuum chamber. The amount of openings or orifices should be carefully designed so that the pumps provide a predictable and consistent pumping speed. When a flow restrictor is used in a pump inlet a higher vacuum is achieved below the flow restrictor opening and a lower vacuum on the chamber side.

Flow restrictor or throttle plate pumps are mainly used in a viscous or continuous flow regime, such as in physical vapour deposition PVD processes. With viscous flow to the flow restrictor any opening/orifice that has line of sight or “looks” directly at the pump's second stage cryopanels can cause preferential gas pumping. Radiant heat loads are always present with cryogenic vacuum pumps, but proper shielding can mitigate the effect on the second stage. The first stage of most cryopumps have much higher refrigeration capabilities so intercepting the heat load at the flow restrictor is advantageous. The second stage of most cryopumps have less refrigeration capabilities and should be shielded from high heat loads and uneven gas loading. Where preferential gas pumping through the inlet openings/orifices is allowed, the line of sight path grows “spires” and these hamper the pump's performance over time. Therefore inhibiting the gasses' viscous flow to enter through the flow restrictor in a direct line to the second stage is advantageous for increased pumping performance.

In this regard, cryopumps are “capture” pumps, so any gasses that are cryopumped (below their vapor pressure) onto its surfaces will stay trapped until the cryopanel is “regenerated” or warmed to release them to the relief valve/rough valve. All cryopumps have a finite amount of gas that can be pumped before the pressure degradation of the pump makes it unusable for the desired process. When a particular process has a very high flow rate of type II gasses the pump stores these gasses as frost on the second stage plate and when the frost reaches the flow restrictor, or the condensed gasses (frost) get too warm the pump will not operate as intended. The time between regenerations is mainly controlled by the pump's maximum storage capacity for type II gasses and the gas flow rate. Capture is very important to the pump user as increased capture reduces the frequency of regenerations. Providing a more uniform capture increases the amount of gas that can be captured before the effects of the condensed gas inhibits the performance of the pump to a degree that a regeneration is required.

The flow restrictor or throttle plate of embodiments has a top or shielding plate that may be circular but can be any shape. The shielding plate may be smaller than the inlet plate and there is a “spacer” or intermediate component between the two that separates the two plates by a sufficient amount to allow type II gasses to enter the cryopump. The flow path through the spacer may be substantially at 90° to the pump inlet. The “spacer” or intermediate piece separates the shielding plate and inlet plate and provides a path for gas diverted by the shielding plate to flow between the two plates and across the outer edge of the inlet plate into the pump through the opening in the inlet plate. The opening may be circular but it could be any shape. The top or shielding plate may overhang the “spacer” to further shield radiation and unwanted gasses from entering the cryopump.

A flow restrictor configured in this way allows for reduced radiant heat loading and provides monolithic frost pumping without the drawbacks of line of sight or preferential gas pumping. This reduction of uneven gas loading and radiation on the second stage cryopanel keeps the second stage cryopanel colder and increases gas capture capabilities.

This flow restrictor is designed so that the gas is routed through openings/orifices that may be any shape in the intermediate spacer. This arrangement allows for randomized gas pumping and provides a monolithic frost build-up substantially uniformally over the entire surface area of the second stage cryopanel. Such monolithic gas pumping is advantageous inhibiting frost on the second stage cryopanel from contacting the warmer first stage radiation shield or flow restrictor.

The shielding plate is important to maximize or at least increase the type II gas frost pumping and radiation abatement allowing for longer times between regenerations.

FIG.2shows a flow restrictor40according to an embodiment. Flow restrictor40has in this embodiment a circular cross section and comprises a shielding plate1mounted via lower surface1A on an intermediate component3which is in the form of a cylinder with apertures3A around the longitudinal surface. The intermediate component3is mounted on inlet component2which comprises protrusions4for mounting the flow restrictor40on the inner walls of a cryopump.

The flow restrictor40is mounted at the inlet of the cryopump and restricts flow into the pump. The shielding plate1shields the cryopanel within the cryopump from line of sight view of the inlet to the pump while the apertures3A provide a route for gas to flow into the pump through a orifice in the middle of inlet component2. This provides substantially uniform flow across the cross section of the gas flow path provided by the orifice in component2. The size and number of apertures3A can be selected to restrict the flow of gas into the pump to a speed that may be required to maintain pressure within the pump at a desired rate.

FIG.3shows a view from the top of the flow restrictor looking into the pump with the shielding plate1removed. This shows inlet component2with orifice2A that provides the gas flow path into the pump. The upper surface of intermediate spacer component3is shown.

FIG.4shows a cryopump according to an embodiment on which the flow restrictor is mounted. Flow restrictor is also shown as a side view and from above with the shielding plate1in place.

The cryopump has a refrigerator unit15which cools a first stage refrigerator heat station10which is used to cool the housing on which the flow restrictor40is mounted and second stage refrigerator heat station11which is used to cool the upper cyropannel12and other cryopanels13. These lower cryopanels may be coated with an adsorbent material for adsorbing type III gases. The upper panel12shield the lower panels13from type II gases, which are condensed on the upper panel12.

This figure shows the build-up of the condensed type II gases in the form of frost14formed from gas molecules captured on cryopanel12. This illustrates how with the flow restrictor in place there is a uniform build-up of frost allowing significantly more gas to be captured before the frost reaches the flow restrictor. Having a more uniform gas flow and corresponding uniform capture and frost build-up allows the time between regenerations to increase, in some embodiments by up to 50%.

FIG.5shows a view from below the flow restrictor, showing the cryogenic bosses or feet4which are used to mount the flow restrictor on the inner housing of the cryopump.FIG.5also showing the shielding plate viewed through the orifice2A in the inlet component2. As can be seen the shielding plate1completely shields the orifice2A from direct line of sight paths between the inside and outside of the pump.

The flow restrictor40of embodiments is adapted to be mounted within the inlet of a cryopump, in some embodiments on the inner wall of the pump housing. in embodiments a cryopump may be upgraded by removing any existing throttle plate and placing a flow restrictor40of an embodiment in the inlet, such that gas flow into the pump is diverted around the shielding plate via the intermediate component to the orifice in the inlet component, thereby providing uniform gas flow across the cross section of the pump inlet.