Patent Publication Number: US-2021190079-A1

Title: Gas supply apparatus

Description:
This application is a divisional of U.S. application Ser. No. 15/552,835, filed Aug. 23, 2017 which is a national stage entry under 35 U.S.C. § 371 of International Application No. PCT/GB2016/050229, filed Feb. 2, 2016, which claims the benefit of G.B. Application 1502993.7, filed Feb. 23, 2015. The entire contents of U.S. application Ser. No. 15/552,835, International Application No. PCT/GB2016/050229, and G.B. Application 1502993.7 are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a purge gas apparatus for supplying purge gas to a multistage vacuum pump. The present disclosure also relates to a multistage vacuum pump purge gas supply apparatus and a multistage vacuum pump comprising a purge gas supply apparatus. 
     BACKGROUND 
     Vacuum pumps are used in many industrial applications, such as steel manufacture, refining, scientific instruments and semiconductor or electronic component manufacture (including, but not limited to Li-ion battery production, solar cell, transistor, integrated circuits and flat panel display manufacture). The present disclosure is described below with reference to semiconductor manufacture, but it is understood that the disclosure is not limited to vacuum pumps used exclusively in this field. 
     Vacuum pumps used to pump gaseous fluid from semiconductor tools typically employ, as a backing pump, a multistage positive displacement pump. Such a semiconductor vacuum pump generally has a number of pumping stages that progressively compress the process gas until exhausting it at slightly above atmospheric pressure. Purge gas is also progressively introduced into the pump stages, typically in proportion to the compression ratio of the particular pump stage. 
     During semiconductor processes such as chemical vapour deposition processing, deposition gases are supplied to a process chamber to form a deposition layer on the surface of a substrate. As the residence time in the chamber of the deposition gas is relatively short, only a proportion of the gas supplied to the chamber is consumed during the deposition process. Consequently, unconsumed gas molecules pumped from the chamber by a vacuum pump can pass through the pump in a highly reactive state. As a result, pump components can be subjected to damage due to corrosion and degradation resulting from the pumping of the aggressive, unconsumed gas molecules. 
     To dilute process gases as they pass through the pump, an inert purge gas, such as nitrogen (or clean dry air (CDA) if appropriate) can be supplied to the pump. Nitrogen purge is recommended for many vacuum dry pumps used in the flat panel and semiconductor industry for reasons of safety, reliability and performance, whereas CDA can be used if the species of gases passing through the pump have a relatively low reactive state. Introducing the purge progressively through the pump mechanism stages provides the optimum combination of effectiveness and performance. The quantity of purge gas must be carefully controlled to avoid both under-dilution of the process gases, as this could lead to pumping reliability problems, and over-dilution of the process gases, as this could lead to unnecessary costs and loss of pumping performance. 
     It is known to have flow selector apparatus that typically consist of a pressurised manifold or pipe arrangement with a number of outlets through which the purge gas can enter the pump mechanism. Additionally, WO 2007/107781 describes a flow selector capable of varying the flow rate of gas into and from the flow selector. The outlets each have an aperture sized to provide a known flow rate dependant on the manifold pressure. The manifold is rotated to couple a manifold inlet to different sets of outlets. Such a device supplies a fixed amount of purge according the selected amount. 
     Moreover, there are occasions when for environmental and economic reasons it is desirable to minimise the nitrogen consumption. Step changes in the gas purge flow rates can be achieved by opening or closing actuation valves in various combinations. For a multistage pump this could require a large number of valves. It is also likely that the resulting purge rate combination would not be ideally matched to the process gas flow. Alternatively, manual adjustment of the restrictors can be carried out. This is time consuming and requires individual measurement of each stage purge to ensure that the required purge level had been achieved. WO2013/144581 describes a purge gas supply for a multistage vacuum pump wherein the supply apparatus comprises a plurality of outlets for supplying gas to respective ports at proportionally fixed gas flow rates. A control module is provided for controlling the gas flow to the inlet in response to a control signal thereby providing efficient purge gas distribution. 
     SUMMARY 
     There is now a need to improve upon the known systems. Presently, purge systems can suffer from backflow of gases into the purge supply means as the vacuum pump cycles between operational states. For instance, the purge gas supply might be optimised for a condition when the pump operates at its ultimate pressure state. However, during roughing operations, the pressure of gas inside the vacuum pump&#39;s pumping chambers may increase above a threshold pressure resulting in ineffectual purge supply to the pump. 
     The present disclosure seeks to address or ameliorate some of the short-comings associated with known systems and processes. 
     In the broadest terms, the present disclosure aims to provide an improved purge gas supply that is less susceptible to the shortcomings of known systems. The purge supply of the present disclosure aims to reduce backflow or recirculation of pumped gases into the purge supply apparatus by providing an arrangement where the purge ports on a multistage vacuum pump contains purge gas at sufficient pressure to resist the backflow when the vacuum pump is operating outside of its ultimate pressure regime. We have found that the embodiments described below do not necessarily need a non-return valve to resist the unwanted pump gas back-flow. 
     More specifically, when viewed from a first aspect, the present disclosure provides a multistage vacuum pump purge gas supply apparatus, comprising: a gas inlet in fluid communication with a plurality of gas outlets for supplying gas to respective purge ports of a multistage vacuum pump; and a flow controller disposed immediately upstream of the gas outlets, the flow controller comprising an input for receiving gas, a volume for containing gas at a given pressure, and a variable flow restrictor disposed between the volume and each of the outlets, wherein the variable flow restrictor is moveable with respect to the volume to facilitate continuous variance of the flow of gas in proportion to each of the outlets, between a first and second flow rate. 
     When viewed from a second aspect, the present disclosure provides a multistage vacuum pump purge gas supply apparatus, comprising: a gas inlet in fluid communication with a plurality of gas outlets for supplying gas to respective ports of the multistage vacuum pump; and a flow controller disposed immediately upstream of the gas outlets, the flow controller comprising an input for receiving gas, a volume for containing gas at a given pressure, and a variable flow restrictor disposed between the volume and the outlets, wherein during operation, the pressure of the gas in the volume is higher than the pressure of the gas within any pump chambers of the multistage vacuum pump at any one of the ports of the multistage pump. 
     Both the first and second aspects offer an improved purge gas supply apparatus for a multistage vacuum pump that enables the purge supply to resist backflow or recirculation of gases being pumped by the vacuum pump during high pressure operations outside of the ultimate pressure operating characteristics. 
     Control orifices can be disposed before the outlets and can be arranged so that gas can be supplied to the respective purge ports at proportionally fixed gas flow rates. 
     Furthermore the variable flow restrictor can be arranged to provide a continuous range of rates of flow between a first and second flow rate. The first flow rate can be a predetermined maximum flow rate through the outlet. In other words, the maximum flow rate can be determined by the size of the outlet or other orifice. The second flow rate can be a predetermined minimum flow rate, or the second flow rate can be zero. Thus, a continuously variable flow rate is provided between to extremes in a given range of flow, as required for the specific multiple staged vacuum pump. 
     Additionally, the variable flow restrictor can comprise a first section arranged to connect the volume with each of the outlets, wherein the first section is moveable with respect to the volume. By moveable it is meant that the first section is either rotatable or linearly moveable with respect to the volume. The movement allows the flow to be adjusted by restricting or opening the restrictions, and thus the flow of gas. The first section can comprise a drum-like feature that is rotatable about a longitudinal axis, said drum can comprise a series of through-holes each arranged to connect an outlet with the volume and thus allow gas to flow to the purge ports. In other words, each through-hole cooperates with an aperture in the volume to allow gas to exit the volume such that movement of the first section relative to the volume enables provision of a continuous range of rates of flow between a first and second flow rate. 
     Alternatively, the first section can comprise a series of needle valves each arranged to connect an outlet with the volume. The rate of flow of gas through each needle valve can be set independently of one another to allow relative flow of gas through each outlet to be controlled independently and respectively to each other. The series of needle valves can comprise a plate section having a series of apertures to allow gas to pass to the outlet, and a series of adjustable stops or tapered needles, each arranged to cooperate with an aperture to restrict the flow of gas through the aperture. The plate and stops can be moveable with respect to one another to allow adjustment of the rate of flow of gas through each or every aperture. In other words, each needle valve can be pre-set to a given flow rate such that the flow through each needle valve is predetermined and proportional to the other needle valves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the present disclosure will now be described, by way of example only, with reference to the accompanying Figures. 
         FIG. 1  is a schematic diagram of a known pump system comprising a purge gas supply. 
         FIG. 2  is a schematic diagram of an embodiment of a purge gas supply apparatus embodying the present disclosure. 
         FIG. 3  is another schematic diagram of an embodiment of the present disclosure. 
         FIG. 4  is a schematic diagram of an alternative embodiment of the present disclosure. 
         FIG. 5  is a schematic flow diagram of another alternative embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic overview of a pump system  1  in fluid communication with a processing tool  3  is shown schematically in  FIG. 1 . The pump system  1  is coupled to a variable purge system  5  operable in accordance with the present disclosure. 
     The vacuum pump system  1  comprises a multistage vacuum pump  7  having a pump inlet  9 , a pump (exhaust) outlet  11  and a plurality of purge ports  13 . An electric pump motor  15  drives the vacuum pump  7  in response to control signals from a system controller  17 . The system controller  17  comprises a digital processor (not shown) configured to measure the operating current of the pump motor  15  to monitor the status of the vacuum pump  7 . 
     In the present embodiment, the processing tool  3  is a chemical vapour deposition apparatus comprising a vacuum chamber (not shown), but of course the disclosure is not limited to CVD apparatuses and can be applied to other processes or applications. The vacuum chamber is in fluid communication with the pump inlet  9  via a foreline  19 . A gas sensor  21  can be provided in the foreline  19  to detect the type of gas present in the pump inlet  9  and to output a corresponding gas detection signal to the system controller  17 . 
     The variable purge system  5  comprises a manifold  23  having a purge gas inlet  25  and a plurality of purge gas outlets, each connected to a port  13 . Operation of the manifold  23  is controlled by the system controller  17 . In this example purge ports  13  are provided for the ¾ interstage purge, ⅘ interstage purge, low vacuum (LV), shaft seal (SS) and exhaust (Exh) stage of the vacuum pump  7 . The purge gas is typically nitrogen (N2). 
     Typically, in a known system, the rate of flow into the multi-stage vacuum pump is set to customer requirements during the manufacture and commissioning phases. It is usual to set the purge gas supply rate according to the requirements of the pump whilst it is running on a process cycle at its ultimate attainable pressure (the condition known as “running at ultimate”). Running in this condition reactive process gases from the process chamber evacuated by the pump pass through the pump, hence the need for a purge gas to improve the service lifetime of the pump. 
     However, the purge module might not be suitable for supplying purge gas to the pump during running conditions outside of the ultimate condition, for instance during a pump down phase (so-called “roughing”) when the pressures of gases inside the pump chambers can be greatly increased. As a result, the pressure of purge gas can be overcome at the purge port and pumped gases might escape the pump volume and enter the purge gas module with undesirable consequences. Embodiments of the present inventive concept aim to reduce the risk of unwanted purge gas module contamination without the need to use additional non-return valves (which are considered to be an expensive solution to this problem). 
     Once set, a variable flow restrictor on a known system is left in position and not adjusted further. Thus, the predetermined flow of gas is determined by the size of these orifices and the pressure drop across them. If the gas pressure upstream of the orifices is insufficient then process gas passing through the pump can enter the purge gas supply system and recirculate within the purge apparatus. This is highly undesirable for many reasons, not least because particles in the process gas might block or restrict flow orifices, or corrosive gas can enter the purge system causing damage or malfunctioning of the purge supply. 
       FIG. 2  shows an embodiment of purge gas supply module  35  that embodies the present inventive concept. The module comprises an inlet  36  for receiving a supply of purge gas, typically nitrogen from a suitable source. A non-return valve  38  ensures gas cannot leak from the module inlet. A regulator  40  is used to measure and control flow of gas into the module and regulate the pressure within the system. The flow line then splits into two lines  41  and  42 , whereby a first purge line  41  is used to supply gas to the inter-stage purge ports of the multi-stage vacuum pump and a second purge line  42  supplies gas to the shaft seal (SS) and exhaust port (Exh). In this example, there are three inter-stage purge ports, located between the fourth and fifth stage, fifth and sixth stage, and the six and seventh stages respectively. A plurality of fixed flow restrictor orifices  43  are used on each of the module to enable proportional flow of gas at different rates to the purge ports according to the requirements stipulated for efficient purge supply. Valves  45  and  47  are disposed on each line of the gas supply to enable the line to isolated from the supply if needs be. 
     The gas supply line  41  splits into three branches  41 A,  41 B and  41 C upstream of a variable flow restrictor  50 . The variable flow restrictor  50  is used to vary the flow of gas to all of the fixed restrictors  43  that supply purge gas to the inter-stage purge ports and the variable flow restrictor is located immediately upstream of fixed restrictor orifices and the inter-stage purge ports. The variable restrictor  50  comprises adjustment device  51  that is linked to all of the flow valves within the variable restrictor  50  such that a single adjustment is used to adjust the flow of gas to all the inter-stage purge ports supplied by the variable restrictor. This adjustment device can be manually or computer operated according to the desired needs of the purge system. 
     This arrangement provides a pressure drop across the flow restrictor arrangement such that the pressure of gas upstream of the fixed restrictors  43  (that is, immediately downstream of the variable restrictor  50 ) is sufficient to limit recirculation of gases being pumped by the vacuum pump. Furthermore, the pressure of gas upstream of the variable restrictor  50  is determined at least in part by the regulator and can be arranged to limit or prevent backflow of process gas from the pump into the purge system to a location within the purge system that could facilitate recirculation of process gases. In other words, the provision of a system according to the arrangement shown in  FIG. 2  uses the purge supply pressure to reduce the risk process gas entering the purge system and passing through the orifices  43 , variable controller  50  and three supply lines  41 A-C to a location (single supply line  41 ) where the process gas could mix and recirculate with gas along a different line to the one that had allowed the process gas ingression to occur. The flow can be deduced from pressure gauge readings taken from gauges  52  and  54  disposed either side of the variable flow restrictor, although these gauges are not essential to the operation of the system but do provide useful readings for evaluation of the system&#39;s performance. 
       FIG. 3  shows a first embodiment  100  of the present inventive concept, comprising a gas supply assembly  102 . The assembly comprises a single inlet  104  to receive purge gas from the mass flow controller  40 , and three outlets  106 ,  108  and  110 , respectively. Each of the outlets is coupled to the appropriate respective purge port of the multi-stage vacuum pump. The fixed orifices  43  are located upstream of the outlets. The inlet opens into a manifold volume  112  where the purge gas can be maintained at a given purge supply pressure. The flow of gas to the ports is controlled by a flow adjustor barrel  114  that comprises three through holes  116 . Each through hole is supplied by lines  41 A,  41 B and  41 C respectively, which are in gas communication with the manifold  112 . The through holes pass through the barrel in a radial direction and each is arranged to cooperate with one of three apertures provided by the opening of the respective supply line  41 A-C. 
     The flow is controlled by rotating the adjustor barrel relative to the volume. Full flow is achieved when each of the through holes co-axially aligned with a respective aperture or supply line. The flow can be reduced by twisting the barrel so that the through holes and supply lines are now misaligned slightly thereby restricting gas flow through the through hole. The flow can be stopped completely if the barrel is twisted to an angle where the through holes no longer align with the supply lines and the passage for the gas is completely obstructed as a result. Thus, a continually variable flow of gas can be achieved through the restrictor, across a range of flows having a maximum flow rate and zero. 
     This embodiment has certain advantages over the known systems because the manifold provides a volume of pressurised gas close to the purge port on the pump. As a result, the purge gas supply module can be set up to provide optimal flow rates of purge gas when the pump is running at ultimate (that is, during low pressure operation) and to reduce or prevent ingression of pumped gas into the purge module when the pump is operating during roughing or pump down (that is during high pressure operation). The volume  112  can be arranged to hold gas at sufficient pressure such that purge gas can flow into the pump ports at the required rates during ultimate operation, but gas from the pump cannot flow into the purge module unless it overcomes the pressure of gas within the volume during other operating conditions. 
       FIG. 4  shows an alternative embodiment  145  of the present disclosure. In this embodiment, the volume  112  is provided as before, upstream of a series of flow restrictors  146 . In this embodiment the restrictors comprise a needle valve arrangement consisting of an aperture  148  arranged to cooperate with a pin or stop  150  having a tapered point section. The effective size of the aperture (and hence the flow rate of gas through it) is controlled by varying how far the tapered section is inserted into the aperture. When the tapered section is outside of the aperture, then the flow of purge gas through the aperture is effectively determined by the size of the aperture. However, as the tapered section is moved into the aperture, so the effective size of the aperture is reduced in a continuous manner until the aperture physically engages with the tapered section and is closed thereby restricting flow through the aperture to zero. 
     In the embodiment shown there are four needle valve arrangements, each being connected to a purge port respectively (B, C, D and E). The valves can be pre-adjusted independently to determine the respective flow rates with respect to the two valve arrangements. A frame or mechanism  155  is provided to allow relative movement of the apertures and needles so that the overall flow of gas through to the outlets and purge ports can also be controlled. Hence, once set, the flow of gas through the system can be controlled by moving the needles relative to the apertures as indicated by arrows A or Z. This can be achieved by providing a manual control means, such as a twistable ( 156 ) threaded jack, or an electromechanical arrangement comprising a servo with appropriate drive and control means. 
       FIG. 5  is a schematic diagram of a purge system  200  incorporating the second embodiment described above, or any alternative system that embodies the present inventive concept. In this arrangement, the features common to other systems described above have been provided with the same reference numerals. The system in  FIG. 5  is similar to one shown in  FIG. 2 , however the fixed orifices  43  disposed between the purge port and variable regulator  50  in  FIG. 2  are not required in the  FIG. 5  embodiment. The function of the fixed orifices has been incorporated into the orifices  51 A,  51 B and  51 C used in the variable flow regulator arrangement. The second embodiment described above and shown in  FIG. 4  can easily incorporate this arrangement whereby the apertures  148  are designed to provide the same function as the fixed aperture orifices  43  used in other embodiments. 
     Alternative embodiments of the present disclosure will be envisaged by the person skilled in the technical field without departing from the general inventive concept. For example, the first embodiment can comprise a sliding barrel as an alternative to the rotating barrel. As the barrel is slid in direction along its major axis the through-holes are no longer coaxially aligned with the apertures and so flow is restricted or reduced from a maximum amount. The amount of misalignment can be adjusted continuously and smoothly through a range of flow rates from zero to a maximum flow when coaxial alignment is achieved.