A method and apparatus for reducing the pressure at a vacuum pump exhaust

Abstract

An ejector device 100 for reducing a pressure at a vacuum pump exhaust 110 by introducing a flow of gas into a process gas exhaust flow between the vacuum pump and a process gas treatment system. The

Description

FIELD OF THE INVENTION The field of the invention relates to a method and apparatus for reducing the pressure at a vacuum pump exhaust. BACKGROUND Vacuum pumps may be used for evacuating vacuum chambers, such as those used for semiconductor processing. Semiconductor processing may generate gaseous byproducts, some of which may condense readily and in particular may be prone to condensing as the pressure increases towards the exhaust of the vacuum pump and / or in the pipes connecting the vacuum pump to a gas treatment system. This can lead to particles condensing and collecting on the walls of the pump, on the connecting pipes and / or at the inlet to the gas treatment system. This can lead to the pump sticking and in some cases to the triggering of a high pressure exhaust alarm. This may lead to increased power consumption, increased frequency of the high pressure alarm triggering and / or to more frequent maintenance requirements. SUMMARY One aspect provides an ejector device configured for reducing a pressure at a vacuum pump exhaust by introducing a flow of gas into a process gas exhaust flow between said vacuum pump exhaust and a gas treatment system for treating said process gas, said ejector device comprising: an inlet portion comprising an inlet for coupling to said exhaust of said vacuum pump; an outlet for coupling to a conduit providing a fluid communication path to said gas treatment system; a reduced diameter throat portion for accelerating a gas flow from said inlet to said outlet; and a gas flow injector for introducing a flow of injection gas into said reduced diameter throat section. It was recognised that one way to reduce condensation at the exhaust of a vacuum pump exhausting a process gas flow such as that generated by semiconductor processing might be to reduce the pressure at the exhaust. It was also recognised that an ejector device could be used to reduce the pressure and that this would have an advantage of no moving parts and thus, low maintenance. Although ejectors are known for reducing pressure in the fields of internal combustion engines for example, by injecting a fluid into the flow, there is a technical prejudice against introducing additional gas flow into the exhaust of a vacuum pump pumping process gas as this leads to an increased amount of gas that needs treatment. Furthermore, the leak requirements for many semiconductor process gases are stringent, such that adding additional devices in the exhaust flow may not be straightforward. In some embodiments, said ejector device comprises an outer wall forming a flow path between said inlet and said outlet, said gas flow injector comprising a conduit extending into said flow path and comprising a nozzle arranged to output said flow of injection gas into a central region of said flow path. The ejector may be configured such that the gas flow injector is formed by a conduit that extends into the flow path to output the gas flow substantially axially into the central region of the flow path. This allows the gas to be added efficiently without inducing an undue amount of turbulence as the additional gas flow is in substantially the same direction as the exhaust gas flow. It also allows the injected gas to be towards the central region of the flow path where the gas flow is faster. In some embodiments, the conduit extends through the outer wall. The outer wall may be an outer annular wall. In some embodiments, said gas flow injector conduit extends through an outer wall of said inlet portion and is configured such that said nozzle is located within a decreasing diameter region, between said inlet portion and said throat. The nozzle of the gas flow injection may be arranged in the decreasing diameter area of the inlet portion of the ejector device upstream of the throat section. Having the nozzle within the decreasing diameter region is beneficial for the efficiency of the process as the inlet gas flow is focused in a region that encourages acceleration of the fluid. In some embodiments, said ejector device comprises an outer annular wall forming a flow path between said inlet and said outlet. In some embodiments, said inlet and said outlet are axially aligned such that a central region of the inlet and the central region of the outlet are joined by a straight line. This allows the fluid flow to flow substantially in a straight direction and reduces eddy currents which may arise at comers and trigger condensation. In some embodiments, a wall of said decreasing diameter portion is sloped and is angled at an obtuse angle to a wall of said inlet portion. The angle is chosen so as to avoid sharp changes in a direction of the wall containing the flow path and provide a smooth transition area to reduce pressure loss and inhibit powder accumulation. In some embodiments, said outlet of said nozzle faces towards said throat. This again allows the injected gas to be in the direction of the fluid flow reducing energy dissipation and increasing efficiency. In some embodiments, said ejector comprises an adaptor for coupling said vacuum pump exhaust and said ejector inlet. In some embodiments, said adapter is frustoconical with an increasing diameter between said exhaust and said ejector device inlet. The exhaust of the vacuum pump may be of a standard size and may have a smaller cross-sectional area than would be desirable in an ejector device and thus, a frustoconical adaptor with increasing diameter from its inlet towards the ejector device may be used to couple the two components together. In some embodiments, said nozzle is a circular or multi-lobed nozzle. In some embodiments, said nozzle comprises a diameter of between 1 and 5mm. The diameter, position and distance between nozzle and throat are chosen to meet the required differential pressure. The nozzle diameter size allows for a fast injection gas flow into the process gas flow path. In some embodiments, said gas flow injector is mounted such that said nozzle is towards an axis of said ejector device, that is close to a centre of a cross section, within 20%, preferably within 5% of the length of the diameter from the central point. By mounting the nozzle within the gas flow towards the axis, gas is injected into the faster flowing region of the fluid flow, improving symmetry of the flow and increasing efficiency. In some embodiments, said gas flow injector is mounted on a support frame comprising an outer annular wall for mounting in said inlet portion and struts extending from said outer annular wall and configured to support said conduit such that said nozzle is within said central region of said gas flow path. Where the injector device is mounted within the fluid flow and particularly where there is a significant fluid flow, then it may be desirable to mount the conduit of the injector device on a support frame to hold it stably in a central position away from the walls. In some embodiments, the conduit is mounted on a support frame that has T-shaped struts that is a straight strut extending across the diameter of the annular outer wall and with a central strut extending perpendicularly from the middle of the straight strut and attaching to and supporting the conduit. In other embodiments, there are Y shaped struts, that is three struts extending from the outer wall with a central strut extending perpendicularly from the middle position and supporting the conduit. The Y shaped struts provide for a firmer support but there is a potential for additional powder deposition and this may lead to an increase in blockages. In some embodiments, the ejector device further comprises a diffuser portion between said reduced diameter throat portion and said gas flow outlet, said diffuser portion having a diameter increasing from a diameter of said reduced diameter portion to a diameter of said gas outlet. The ejector device may have a diffuser portion in which the injection gas and process gas flow expands and mixes which provides a reduction in gas velocity and a return of the pressure of the gas flow to a pressure close to but slightly below ambient. In some embodiments, the injector device comprises a flow controller for controlling a flow of said injection gas introduced to said ejector device. The amount of gas injected into the ejector device that is required to inhibit condensation may vary depending on the process gas being pumped and thus, it may be desirable to have a flow controller for controlling the flow. In this regard, where it is being used at the exhaust of a vacuum pump evacuating a semiconductor processing chamber then as the process within the chamber changes the amount and type of gas generated may change and thus, the required of amount of injection gas may also change. In some embodiments, said flow controller comprises a throttle valve such as a controllable aperture valve, a proportional valve or a needle valve. In some embodiments, said flow controller is configured to receive signals from a pressure sensor sensing a pressure indicative of an exhaust pressure of said vacuum pump, and to control said injection gas flow in response to said signals. In some embodiments, there may be a feedback control of the injection gas such that the flow of gas injected is controlled in dependence upon the sensed pressure at or close to the exhaust, this allows the pressure to be maintained at or close to a desired value selected to inhibit condensation. The pressure sensor may be a part of the pump or a part of the ejector or a separate unit. In some cases, the pressure sensor may have a display for displaying the measured pressure and the flow control may be done manually by an operator adjusting the variable aperture valve to vary the flow such that the pressure is maintained at or close to a desired value. In some embodiments, said ejector device is configured for a differential pressure between said inlet and said outlet of more than 1000Pa and for a Nitrogen injection gas flow rate of less than 100 standard litres per minute. In some embodiments the process gas flow rate is below 130 slm. In some embodiments, the injection flow rate of nitrogen is less than 100 standard litres per minute. Too much gas flow is hard to handle for many abatement systems, so a smaller amount of injection gas flow may be applied, preferably <100slm. The shape of the nozzle its diameter, position and the distance between nozzle and throat are designed to meet required differential pressure requirements. In some embodiments, said ejector device comprises a length of less than 500mm, a largest diameter of less than 100mm and said reduced diameter throat section comprises a diameter of more than 20mm In some embodiments, the reduced diameter throat section is between 20 and 35mm diameter with a length of between 90 and 110mm. In some embodiments, the throat section is formed of a straight pipe that provides space for process gas and injection gas coming together and mixing, such that during the mixing process, the process gas receives energy from the injection gas and the injection gas loses energy. The velocity of the injection gas as it enters the flow of the process gas is greater than the flow of the process gas. In some embodiments more than five times greater. The injection gas velocity may in some cases be between 180 and 220 m / s while the process gas velocity prior to injection of the injection gas may be between 8 and 12 m / s. The diameter of the throat influences the gas mixing and ejector performance. If the throat is too narrow, the gas flow rate is mainly limited by the pipe conductance and this restricts the amount of gas flow that can go through the throat, and the pressure will increase. If the diameter is too wide, the high speed effect of injection flow will dissipate in a short distance. For effective inhibiting of powder accumulation in pumping exhaust gases from a semiconductor process, the throat should be between 20~35mm. The length of the throat also affects the ejector performance and is preferably between 90~110mm. In some embodiments, said ejector device is configured with a leak rate of less than 5.4 X 10 -5 mbarl / s. Where the ejector device is being used in a vacuum pump and abatement system for abating semiconductor processing gases then it may be important that the leak rate is kept very low as the gases evacuated from the semiconductor processing chamber may be hazardous, flammable and / or explosive. Such a leak rate requires careful design and precludes the use of conventional ejector devices. In some embodiments, the ejector device comprises an outer wall forming a flow channel comprising the inlet portion, the throat portion and the diffuser, the outer wall being formed of a single unit, with the gas injector conduit passing through, and being sealed to, an aperture in the outer wall. This allows the leak rate to be kept to a low level. In some embodiments, the material of the ejector device is a metal. Metal is a robust material with a high thermal conductivity. Furthermore, the exhaust of a vacuum pump and conduits of an abatement system may also be metal allowing the ejector device to seal effectively to them. In some embodiments, the ejector device is formed of stainless steel, preferably 306 or 316 stainless steel. In some embodiments, said ejector device further comprises temperature control means for heating said ejector device to a predetermined temperature. An ejector device is used to reduce pressure at the exhaust of a vacuum pump pumping process gases in order to reduce condensation of these gases. In order to reduce condensation of these gases within the ejector device itself, it may be advantageous to provide some temperature control of the ejector device. The controlled temperature may depend on the gases being pumped. In this regard, a semiconductor process may generate different byproducts at different stages in the process and thus, the condensable gases being pumped will vary along with their condensation temperatures. Thus, the preferred temperature for the ejector operation may also vary. In some cases the predetermined temperature may be between 100 and 250°C. The ejector device may be an inert gas in some cases nitrogen. A further aspect provides a vacuum pump for evacuating a process gas chamber, said vacuum pump comprising an ejector device according to one aspect, connected to an exhaust part of said vacuum pump. A yet further aspect provides a vacuum system comprising at least one vacuum pump and a gas treatment system said vacuum system further comprising an ejector device according to one aspect, configured to receive process gas evacuated from a process chamber by said at least one vacuum pump and to output said process gas and an injection gas to said gas treatment system. A yet further aspect provides a method of reducing the exhaust pressure of a vacuum pump evacuating a semiconductor process chamber, said method comprising: attaching an ejector device according to one aspect between an exhaust of said vacuum pump and an inlet of a gas treatment system. In some embodiments, said gas treatment system comprises a gas scrubber. In some embodiments, said gas treatment system comprises an abatement system. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 schematically shows a vacuum pump with condensed particles; Figure 2 schematically shows a vacuum pump, ejector device and gas treatment system according to an embodiment; Figure 3 schematically shows an ejector device coupled to a vacuum pump exhaust according to an embodiment; Figure 4 schematically shows a portion of an ejector device according to an embodiment; Figure 5 schematically shows gas flow through an ejector device according to an embodiment; Figures 6A schematically shows an inlet adaptor according to an embodiment; Figures 6B schematically shows a conduit support according to an embodiment; Figures 6c schematically shows a conduit support according to a further embodiment; and Figures 7 schematically shows a flow diagram illustrating steps in a method according to an embodiment. DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. Ejectors may be used to introduce a low pressure area into a fluid flow by injecting a fluid into the fluid flow and causing acceleration by passing it through a venturi, thereby reducing the pressure in the flow without using moving parts. Vacuum pumps, particularly dry vacuum pumps that are evacuating process gases to be treated by a gas treatment apparatus such as an abatement system may suffer from condensation of particles as the pressure rises within the pump towards the exhaust and this may lead to the pump sticking (as shown in figure 1) and / or to a high exhaust pressure alarm. The high pressure exhaust alarm may be due to one or more of: a high outlet pressure; and / or pressure fluctuation at an inlet to, or within a connecting pipeline to, a gas treatment apparatus such as a scrubber and may be triggered by byproduct condensation towards the exhaust of the pump. Where the pump is a multi-stage pump this may be in the final stage. High exhaust pressure will lead to more electricity consumption and trigger dry pump system warnings or alarms. Generally, there is one exhaust pressure sensor installed at the pump to read out pump exhaust pressure and send the signal to the main pump controller to protect its mechanism. Figure 1 schematically shows byproduct condensation 32 within the final stage of a multi-stage pump where the pressure is at its highest, the condensation causing sticking of the pump mechanism. For example, where the vacuum pump is evacuating a vacuum chamber used in an ACL etch process, byproducts are formed such as -[CS]n- and H2SO4, and -[CS]n- is very sticky and condenses easily. H2SO4 may also be generated and can cause pump corrosion. This may lead to the pump getting stuck and triggering an alarm. ACL(a-CxHy)+O2+COS->CO2+-[CS]n-+C(s, graphite)+C2(g) -[CS]n-+H2O->H2SO4 An ejector according to an embodiment may be used to reduce local pressure and address this problem. The ejector device is configured to reduce pressure within a defined region, such that with appropriate positioning it may be used to effectively reduce pump exhaust pressure and indirectly reduce the last stage pressure within a multi-stage pump. Furthermore, installing the ejector at the pump exhaust, or between the pump and gas treatment apparatus, closer to the pump than the gas treatment apparatus, may render the exhaust pressure of the dry pump to be substantially independent of the pipeline and gas treatment apparatus such as a scrubber connected to it. In effect, the ejector device provides pressure regulation and mitigates for pressure variations. Furthermore, partial pressure of byproducts will decrease as total pressure towards the pump exhaust decreases and byproducts will not condense easily, allowing the condensation and associated problems, such as the pump sticking, to be mitigated. However, providing a suitable ejector with the required properties for dry pump exhaust use is challenging. Conventional ejectors, may be made of plastic material, making them susceptible to failure. Their leak rates are also generally too high for the requirements of semiconductor processing use. They may also be large and be difficult to install at exhaust pipelines of a vacuum system, particularly one in the confined space of a sub-fab and may have configurations such as corners in the flow path which make them unsuitable for the demands of semiconductor process gases with solid byproduct. Furthermore, the amount of injected gas used in ejectors of embodiments should be limited as a large amount of injection gas flow is hard for a gas treatment apparatus such as a scrubber to handle. Embodiments provide a design of an ejector that addresses these challenges and is suitable for dry pump use. Embodiments provide an ejector formed of 304 or 316 stainless steel, making it robust and hard to damage. Furthermore, the use of stainless steel allows for a better connection between the ejector and associated apparatus and pipeline, which may also be stainless steel and this in turn allows for a lower leak rate. Embodiments provide a compact ejector that is small and short. Example dimensions comprise a diameter<100mm and a length<500mm, making it easy to install and remove . The throat diameter may be greater than or equal to 20mm. The amount of injection gas flow is relatively low for an ejector device allowing it to be treated by the scrubber without unduly affecting this operation. Generally this is kept below 100 standard litres per minute. In effect the ejector is designed specifically to reduce pump exhaust pressure and pump internal pressure to help the process gas exit the pump more easily and one or more of its material, length, the injection gas flow, throat diameter, operational pressure and temperature control and other local dimensions are designed to make it suitable for use in the exhaust flow of a vacuum pump of a semiconductor processing system. Figure 2 shows one possible arrangement for an ejector device 100 according to an embodiment. The ejector device 100 is located in a conduit between vacuum pump 10 and gas scrubber 20. The ejector device 100 is significantly closer to the vacuum pump 10 than it is to the scrubber 20. Figure 3 shows an ejector device attached to an exhaust 110 of a vacuum pump via an inlet adapter 115. In this embodiment the exhaust of the vacuum pump has a pressure sensor 170 for measuring the pressure of the gas flow exiting the vacuum pump. In other embodiments the pressure sensor may be on a process gas inlet of the ejector device, the process gas inlet may have the same diameter as the pump exhaust. The ejector device 100 comprises an inlet or gas process portion 125, a narrow throat portion 120 and an intermediate portion 126 between the inlet portion 125 and the throat portion 120. The intermediate portion 126 is frustoconical in shape and reduces in diameter from the inlet portion to the throat portion. There is then a diffuser portion 150 where the flow channel widens from the throat section 120 and mixing of the process and injection gas occurs. The inlet portion 125, intermediate portion 126, throat portion 120 and diffuser 150 are formed from a unitary piece of stainless steel that forms a flow path from the inlet to the outlet of the ejector device. This flow path has a central axis that lies at the centre of the inlet and the outlet such that there is a line of sight from the inlet to the outlet for the gas flow albeit the width of the channel varies with the diameter of the outer wall as it travels from the inlet to the reduced section throat and then to the diffuser. The size of the ejector may vary depending on the application. The inlet portion or process gas chamber of the ejector may have an inlet with a diameter selected according to a particular process being evacuated. An inlet adaptor is provided to connect the inlet portion to the pump exhaust pipe. The inlet adaptor size is dictated by the pump exhaust pipe and the inlet portion or process gas chamber of the ejector. Connection methods can be thread or flange but are not limited to these two examples. The diameter and the length of the throat section 120 may be selected according to the required differential pressure and the process being evacuated. If the required differential pressure is high, the diameter can be smaller. Otherwise, the size can be wider to provide more space to inhibit powder accumulation. Generally, the diameter of the throat portion is greater than 20mm. The length of the diffuser portion 150 can also be selected according to the system and installation constraints. The inlet and outlet diameters should match the pump exhaust and the pipeline connecting to the abatement system. Within the inlet portion there is a conduit 130 that feeds an injection gas that is an inert gas such as nitrogen or argon into the fluid flow of process gas from the vacuum pump. This conduit extends through the outer wall and is sealed thereto. The conduit may be made of metal and in some cases of stainless steel matching that of the outer wall. The injection gas flow is adjustable and there is a variable aperture or throttle valve 160 for doing this. There is a nozzle 140 that outputs the injection gas into the ejector device just upstream of the throat in the intermediate portion 126. The amount of flow can be adjusted to a higher flow rate when high differential pressure is needed and to a lower flow rate for reducing energy consumption where a lower differential pressure is acceptable. Several types of throttle valve can be chosen to adjust the gas flow, for example a proportional valve or needle valve. In this embodiment, there is a flow controller 180 that receives signals from pressure sensor 170 and transmits a control signal to the variable aperture valve 160 and thereby controls the rate of flow of the injection gas into the ejector 100. In other embodiments, this control may be done manually and the display on the pressure sensor 170 may be used to determine the pressure and an operator may control the valve themselves in dependence on the reading. Figure 4 schematically shows the conduit 130 extending into the inlet portion 125 and the position of the nozzle 140 within the intermediate portion 126 of the ejector device. As can be seen, the nozzle is upstream of the throat and the angle between the inlet portion and the intermediate portion is an obtuse angle that is greater than 90°. This angle and intermediate section helps provide a smooth transition area to reduce pressure loss and inhibit powder accumulation. Figure 5 schematically shows the flow of gas through an ejector device 100 according to an embodiment. Process gas enters the device at process gas inlet 40 and flows through the inlet or process gas section past the injection gas conduit. The injection gas is input via injection gas inlet 60 and exits into the process gas flow via nozzle 140. The mixed gases then flow through the throat portion and into the diffuser and then out at the mixed gas outlet 50 towards an abatement system. In some embodiments the ejector device 100 comprises a temperature management system or heater 190 that is configured to provide heat to the outer surface of the ejector device and impede any condensation of gases within the device. The temperature management system 190 is generally a heating system and it is particularly advantageous for processes that generate a lot of easily-condensable byproducts. Its heating type can be resistance wire or other heating types. It may have a display for displaying the temperature allowing an operator to control the temperature in an open loop system. Alternatively, it may be an automatic closeloop system that uses a temperature sensor to help maintain the required temperature. Such a heating device is optional and may operate effectively where the ejector device outer surface is metal such as 304 or 316 stainless steel. Figure 6A shows the frustoconical inlet adapter 115. Figures 6B and 6C show two embodiments of the support frame 135 which is an optional feature for supporting the gas injector conduit in position. The support frame may be a Y-shaped structure such as that shown in Figure 6B or a T-shaped structure such as that shown in Figure 6C. The Y-shaped structure of Figure 6B may be used for processes which need a large amount of gas flow and require a solid frame to support the conduit. The T- shaped structure of Figure 6C may be used where powder handling is an issue and a lot of solid biproducts are condensed making the blocking of the pipeline a possibility such that this structure is preferable to the Y-shaped structure of Figure 6B. Figure 7 schematically shows a flow diagram illustrating steps in a method according to an embodiment. In a step S10 an inlet adaptor is attached to the exhaust of a vacuum pump. In a further step S20 the inlet of the ejector device is coupled to an outlet of the inlet adaptor. The diffuser of the ejector device is coupled at step S30 to a conduit providing a fluid flow communication path to a gas scrubber. These steps may be performed in any order. At step S40 the injection gas conduit is connected to a supply of inert gas. Where the flow control of the injection gas is a closed loop control system then at step S50 the flow control valve on the injection gas conduit is coupled to a feedback controller that receives signals from a pressure sensor sensing the pressure at or close to the exhaust of the vacuum pump and controls the aperture size of the control valve. At step S60 where the ejector device comprises a temperature management system such as a heater, the temperature management system or heater may be connected to a power supply allowing the ejector device to be heated as required. In summary the structure of embodiments of the ejector may comprise a process gas inlet, pressure sensor or pressure transducer, inlet adapter, support frame for supporting the injection gas flow pipeline, injection gas flow pipeline, flow controller, throat, diffuser and temperature management system. The inner and outer surfaces of the ejector can be coated with a corrosion resistant coating such as a polymer or zinc. The ejector can be coupled to a purge flow pipeline and / or some grille to remove byproduct powder. It can be formed as a monolithic structure or may be formed of constituent portions sealed together. Monolithic structures can be more integrated and reduce leakage and may be cheaper to manufacture, while constituent structures provide convenience for part replacement. In operation the process gas flow enters the ejector through a process gas inlet 5 port, and injection gas flow enters the system through injection port. The gases mix together in the throat and exit the ejector via the ejector diffuser to the gas outlet. A performance of the ejector may be evaluated by determining the differential pressure between the inlet and outlet. Generally speaking differential pressure between process gas inlet and outlet of this ejector is higher than 10 1000pa where process gas flow is below 130 slm and injection gas flow is below 100slm. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the 15 invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS 10 vacuum pump 20 gas scrubber 32 condensed particles 40 process gas flow inlet 50 mixing gas flow outlet 60 injection gas flow inlet 100 ejector device 110 pump exhaust 115 inlet adaptor 120 throat 125 inlet or process gas portion 126 intermediate portion 130 injection gas conduit 135 conduit support 140 nozzle 150 diffuser portion 160 throttle valve 170 pressure sensor 180 flow controller 190 heater

Claims

1. An ejector device configured for reducing a pressure at a vacuum pump exhaust by introducing a flow of gas into a process gas exhaust flow between said vacuum pump exhaust and a gas treatment system for treating said process gas, said ejector device comprising:an inlet portion comprising an inlet for coupling to said exhaust of said vacuum pump;an outlet for coupling to a conduit providing a fluid communication path to said gas treatment system;a reduced diameter throat portion for accelerating a gas flow from said inlet to said outlet; anda gas flow injector for introducing a flow of injection gas into said reduced diameter throat section.

2. An ejector device according to claim 1, wherein said ejector device comprises an outer wall forming a flow path between said inlet and said outlet, said gas flow injector comprising a conduit extending into said flow path and comprising a nozzle arranged to output said flow of injection gas axially into a central region of said flow path.

3. An ejector device according to claim 2, wherein said gas flow injector conduit extends through an outer wall of said inlet portion and is configured such that said nozzle is located within a decreasing diameter region, between said inlet portion and said throat.

4. An ejector device according to any preceding claim, wherein said gas flow injector is mounted on a support frame comprising an outer annular wall for mounting within said inlet portion and struts extending from said outer annular wall configured to support said conduit such that said nozzle is within said central region of said gas flow path.

5. An ejector device according to any preceding claim, further comprising a diffuser portion between said reduced diameter throat portion and said gas flow outlet, said diffuser portion having a diameter increasing from a diameter of said reduced diameter portion to a diameter of said gas flow outlet.

6. An ejector device according to any preceding claim, comprising a flow controller for controlling a flow of said injection gas introduced to said ejector device.

7. An ejector device according to claim 6, wherein said flow controller is configured to receive signals from a pressure sensor sensing a pressure indicative of an exhaust pressure of said vacuum pump, and to control said injection gas flow in response to said signals.

8. An ejector device according to any preceding claim, wherein said ejector device is configured for a differential pressure between said inlet and said outlet of more than 1000Pa and for an injection nitrogen gas flow rate of less than 100 standard litres per minute and a process gas flow rate of less than 130slm.

9. An ejector device according to any preceding claim, wherein said ejector device comprises a length of less than 500mm, a largest diameter of less than 100mm and said reduced diameter throat section comprises a diameter of more than 20mm.

10. An ejector device according to any preceding claim, wherein said ejector device is configured with a leak rate of less than 5.4 X 10-5 mbarl / s.

11. An ejector device according to any preceding claim, wherein said ejector device is formed of a metal.

12. An ejector device according to any preceding claim, wherein said ejector device further comprises temperature control means for heating said ejector device to a predetermined temperature.

13. A vacuum pump for evacuating a process gas chamber, said vacuum pump comprising an ejector device according to any preceding claim, connected to an exhaust port of said vacuum pump.

14. A vacuum system comprising at least one vacuum pump and a gas treatment system, said vacuum system further comprising an ejector device according to any one of claims 1 to 12, configured to receive process gas evacuated from a processor chamber by said at least vacuum pump and to output said process gas and an injection gas to said gas treatment system.

15. A method of reducing the exhaust pressure of a vacuum pump evacuating a semiconductor process chamber, said method comprising:attaching an ejector device according to any one of claims 1 to 12 between an exhaust of said vacuum pump and an inlet of a gas treatment system.

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