Patent Publication Number: US-2022221426-A1

Title: Gas analyzer apparatus and method for controlling gas analyzer apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. Ser. No. 17/435,817, filed on Sep. 2, 2021, which is a national stage application of PCT/JP2020/012838, filed on Mar. 24, 2020, which claims priority of JP 2019-057147, filed on Mar. 25, 2019, the contents of all of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a gas analyzer apparatus equipped with a cleaning function. 
     BACKGROUND ART 
     Japanese Laid-open Patent Publication No. 2012-3976 discloses a technology that provides a low-cost quadrupole mass spectrometer where a grid can be effectively degassed by impacting electrons without needing a separate power supply. An apparatus includes an ion source equipped with a filament and a grid, a quadrupole unit where four columnar electrodes are disposed at predetermined intervals in the circumferential direction, and an ion detector unit that collects predetermined ions that have passed through the quadrupole, and further includes: a power supply that passes a direct current through the filament; a power supply that applies a higher potential to the grid relative to the filament; a power supply that applies a predetermined potential to the filament to create a predetermined potential difference between the grid and the filament; a power supply that applies a voltage in which positive and negative DC voltages and an AC voltage have been superimposed to facing electrodes in the quadrupole unit; and a control unit. The apparatus is configured so that a voltage corresponding to the potential difference between the positive and negative voltages can be applied to the grid via the power supply provided for the quadrupole unit. 
     SUMMARY OF INVENTION 
     For applications such as monitoring of semiconductor manufacturing processes, there is demand for a highly durable sensor that can perform monitoring in various environments including a wide variety of gases. 
     One aspect of the present invention is a gas analyzer apparatus that analyzes inflowing sample gas. This apparatus includes: a filter unit that filters the sample gas; a detector unit that detects filtered results; a housing that houses the filter unit and the detector unit; and a control unit that controls respective potentials of the filter unit, the detector unit, and the housing. The control unit includes a cleaning control unit that sets the respective potentials of the filter unit, the detector unit, and the housing to cleaning potentials for drawing in, as plasma for cleaning purposes, one of process plasma of a source that supplies the sample gas and plasma generated by a plasma generation unit. The cleaning potentials may be ground potential and/or negative potentials. By setting, at regular or appropriate timing, the gas analyzer apparatus, including the housing, at the cleaning potentials, it is possible to draw in plasma and clean the inside of the gas analyzer apparatus with the drawn plasma. 
     The gas analyzer apparatus may include the plasma generation unit. The cleaning control unit may include a unit that sets the cleaning potentials at timing when the source that supplies the sample gas generates or provides the process plasma. 
     The gas analyzer apparatus may further include an ionization unit that ionizes the sample gas, and the filter unit may include a unit that filters the ionized sample gas. A typical example of a filter unit is a unit that performs filtering of ionized sample gas according to mass-to-charge ratio. The ionization unit may include a thermion supplying unit, and the cleaning control unit may include a unit that sets a potential of the ionization unit at one of the cleaning potentials to perform cleaning including the ionization unit. 
     Another aspect of the present invention is a method for controlling (control method of) a gas analyzer apparatus that analyzes an inflowing sample gas. The gas analyzer apparatus includes a filter unit that filters the sample gas, a detector unit that detects filtered results, a housing that houses the filter unit and the detector unit, and a control unit that controls respective potentials of the filter unit, the detector unit, and the housing. The control method includes setting cleaning potentials, by the control unit, as the respective potentials of the filter unit, the detector unit, and the housing for drawing in, as plasma for cleaning purposes, one of process plasma of a source that supplies the sample gas and plasma generated by a plasma generation unit. The cleaning potentials may be selected from among ground potential and negative potentials. 
     The gas analyzer apparatus may include the plasma generation unit, and the method may further include generating the cleaning plasma, by the plasma generation unit, in parallel with the setting cleaning potentials. The setting cleaning potentials may include setting the cleaning potentials at timing when the source that supplies the sample gas provides the process plasma. When the gas analyzer apparatus includes an ionization unit that ionizes the sample gas, the setting cleaning potentials may include setting a potential of the ionization unit at one of the cleaning potentials. 
     Another aspect of the present invention is a program that controls a gas analyzer apparatus which analyzes an inflowing sample gas. The program (or program product) includes instructions that have the control unit set the respective potentials of a filter unit, a detector unit, and a housing to cleaning potentials for drawing in, as plasma for cleaning purposes, process plasma of a source that supplies the sample gas or plasma generated by a plasma generation unit. The program or program product may be provided having been recorded on a suitable recording medium. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram depicting one example of a conventional mass spectrometry apparatus. 
         FIG. 2  is a diagram depicting an example of a gas analyzer apparatus that performs plasma cleaning. 
         FIG. 3  is a flowchart depicting an overview of control of the gas analyzer apparatus. 
         FIG. 4  is a diagram depicting another example of a gas analyzer apparatus that performs plasma cleaning. 
         FIG. 5  is a diagram depicting yet another example of a gas analyzer apparatus that performs plasma cleaning. 
     
    
    
     DETAIL DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention is a gas analyzer apparatus, and one example of this is a mass spectrometry apparatus. In applications such as monitoring of semiconductor manufacturing processes, it is necessary to monitor gases that include a wide variety of components, and there is demand for sensors that can measure these gases stably and with high accuracy. 
     As one example of a gas analyzer apparatus, an overview of a quadrupole mass spectrometer will now be described with reference to  FIG. 1 . A quadrupole mass spectrometer (mass spectrometry apparatus, mass spectrometer)  91  includes an ionization apparatus (ionization unit, ionizer or ion source)  10  that ionizes gas (gas samples or sample gases)  9  to be analyzed and a gas analyzer unit (gas analyzer section, gas analyzer)  21  that analyzes ionized gas  8 . The gas analyzer unit  21  includes a quadrupole unit  20 , which is a filter unit, and a detector unit (detector, detection unit, as one example, a Faraday cup)  30  that collects gas ions  8  that have arrived after passing between the electrodes of the quadrupole. The filter unit (filter)  20  includes a plurality of, typically four, columnar electrodes that extend vertically and are disposed at predetermined intervals in the circumferential direction. The mass spectrometry apparatus  91  includes a vacuum vessel (housing)  40 , which houses or receives the ionization apparatus  10 , the filter unit  20 , and the detector unit  30 , and a vacuum pump  45  that keeps the inside of the housing  40  at negative pressure. Gas  9  that has flowed into the housing  40  is ionized by the ionization apparatus  10 . 
     The ionization apparatus  10  includes a grid  11  and a filament  12  that functions as a cathode for supplying an electron flow. One example of the grid  11  is constructed by arranging thin metal wires into a grid that is cylindrically shaped. The filament  12  is connected to metal support pins installed at predetermined intervals in the circumferential direction on a support frame, and is disposed around the outside of the grid  11 . As one example, the filament  12  is produced by coating the surface of a base material made of iridium with yttrium oxide by performing an electrodeposition treatment. A focusing electrode  25  is interposed between the filter unit  20  and the ionization unit  10  so that ions that are headed toward the filter unit  20  efficiently converge. As one example, the focusing electrode  25  is electrically connected to the support pins of the filament  12  so that the potential of the filament  12  and the potential of the focusing electrode  25  become equal. 
     A conventional mass spectrometry apparatus  91  is designed so as to operate in an environment of pure gas as a condition, that is, an environment that does not include corrosive gas. One example of a cathode material (filament material) suited to this condition is a Y 2 O 3 /Ir filament, where the core material is made of iridium (Ir) and the electron emitting layer is made of yttrium oxide (yttria, Y 2 O 3 ). Tungsten (W) materials are believed to be effective as the filament material (cathode material) for gases that contain fluorocarbons CFx as components. 
     In environments where the gas from the process includes silicon oil, the filament  12  will become coated with silicon (Si), silicon oxide (SiO 2 ), silicon nitride (SiN), or the like, especially when the mass spectrometer  91  starts or stops, which may impair functioning. 
     At present, inconel 600 is often used as the grid  11 . Part of the gas may become deposited on the grid  11  to form an insulating film, and due to this the correct potential distribution may not be created in the ionizer/ion optical region. One of examples of processes to be monitored by the gas analyzer apparatus  91  is a system that performs CVD or PVD during a semiconductor manufacturing process. The processes performed by such systems may include processes of depositing oxide or an insulator, such as silicon dioxide (SiO 2 ), silicon nitride (SiN 3 ), titanium nitride (TiN), or tantalum nitride (TaN), and in the gas analyzer apparatus  91  that monitors these systems, these oxides or insulators can become deposited on the ionization unit  10 , the filter unit  20 , the detector unit  30 , and/or the housing  40 . 
       FIG. 2  depicts one embodiment of the present invention. This gas analyzer apparatus (gas analyzing apparatus, gas analyzer)  1  is a mass spectrometry apparatus (mass spectrometer) like the gas analyzer apparatus depicted in  FIG. 1 , and is an apparatus that analyzes a sample gas  9  that flows into the housing  40  from a process  70 . The gas analyzer apparatus  1  includes an ionization unit  10  that ionizes the sample gas  9 , a filter unit (in this example, a quadrupole unit, a quadrupole filter)  20  that filters the ionized sample gas (ionized gases, gas ions)  8  according to mass-to-charge ratio, a focusing electrode  25 , a detector unit (detector)  30  that detects ions (gas components) that have passed through the filter unit  20  as a result or results of the filtering, and the housing (chamber)  40  that houses or receives the ionization unit  10 , the filter unit  20 , and the detector unit  30  and performs control so that the inside is kept at negative pressure by a vacuum pump or pumps  45 . The gas analyzer apparatus  1  further includes a control unit (controller)  60  that controls the potentials (electric or electrical potentials) of the ionization unit  10 , the focusing electrode  25 , the filter unit  20 , the detector unit  30 , and the housing  40  respectively. The potential of the filter unit  20  is controlled via a driving unit (RF/DC unit)  22  that applies a high frequency and direct current to the quadrupole. The ionization unit  10  includes a filament (cathode)  12  and the grid  11  as a thermion (thermal electrons) supplying unit  13 . 
     The gas analyzer apparatus  1  includes a plasma generation unit (plasma generator)  50  that generates plasma  55  for cleaning purposes. One example of the plasma generation unit  50  is a unit capable of generating plasma at a low pressure of around 0.01 to 1 kPa using a generation method that does not use electrodes. The plasma generation unit  50  includes a vessel  51  formed of a dielectric material with high durability against plasma, such as quartz, aluminum oxide (Al 2 O 3 ), or silicon nitride (SiN 3 ), and a mechanism  52  for generating plasma in the vessel using an electric field and/or a magnetic field. The plasma generation unit  50  can draw in gas or gases from the processes or gases with components suited for cleaning and generate plasma under a pressure of 0.01 to 1 kPa using a method such as inductively coupled plasma (ICP), dielectric barrier discharge (DBD), or electron cyclotron resonance (ECR) and others. As one example, when this plasma generation also serves for measurement purposes, the plasma may be generated under a reduced pressure of around 1 to 10 mTorr. When the plasma is generated exclusively for cleaning, the plasma may be generated under a pressure of around ten to several hundred Pa. A magnetic field and an electric field may be used together to confine the plasma inside the vessel. 
     The control unit  60  controls the potentials of each part of the gas analyzer apparatus  1 . When analyzing the sample gas  9 , the potential of the grid  11  in the ionization unit  10  is set so that an electron flow (thermion flow) of a predetermined energy (eV) with respect to the potential of the filament (cathode)  12  is obtained. As one example, the potential of the grid  11  is set at 5 to 15V and the potential of the filament  12  is set at 20 to 100V with respect to the potential of the grid  11  to produce a negative voltage. Note that these potentials are examples, and the present invention is not limited to these values. The same applies to the following description. The sample gas or gases  9  collides with the thermion flow (thermal electron flow) supplied from the filament  12  in the ionization unit  10  to become gas ions (cations)  8 , and part of the sample gas  9  is drawn by the extraction electrode (focusing electrode)  25  and supplied to the filter unit  20 . 
     As the potential of the filter unit  20 , a voltage with opposite polarity produced by superimposing a DC component and a high frequency component is applied via the driving unit  22 . The potential of the ion detector unit  30  that uses a Faraday cup or the like is set at a potential that is lower than the potential of the grid  11 , as examples, ground potential or a slightly negative potential, to detect the gas ions  8  that have passed or been led through the filter unit  20 . The potential of the housing  40  that houses these elements is set at ground potential or a positive potential to suppress the influence of the housing  40  on the ion flow  8 . 
     The control unit  60  includes a cleaning control unit (cleaning controller)  61  that draws plasma into the entire gas analyzer apparatus  1  and specifically the housing  40  to clean not only the parts housed inside the housing  40  but also the housing  40  itself. The cleaning control unit  61  sets the potential of the thermion supplying unit  13  including the grid  11  and the filament  12  of the ionization unit  10 , the potential of the filter unit  20 , the potential of the detector unit  30 , and the potential of the housing  40  at cleaning potentials Vc and maintains the respective cleaning potentials Vc during cleaning. That is, the cleaning control unit  61  includes a unit  61   a  that sets the potential of the filter unit  20 , the detector unit  30 , and the housing  40  to the cleaning potentials Vc respectively, and a unit  61   b  that sets the potential of the ionization unit  10  at one of the cleaning potentials Vc. 
     The cleaning potentials Vc may be set from among ground potential or negative potentials so as to effectively draw in the plasma, which is positively charged, and thereby raise the efficiency of the plasma cleaning. Plasma is often positively charged to several volts or so, and it is therefore desirable for the cleaning potentials Vc to be negative potentials so as to efficiently draw the plasma into the housing  40  and perform cleaning. 
     Further, the cleaning control unit  61  may set the cleaning potentials Vc having the same potential for the ionization unit  10 , the filter unit  20 , the detector unit  30 , and the housing  40 , or may set the respective potentials as cleaning potentials Vc to form a suitable voltage gradient in order to draw in plasma more effectively. As one example, the cleaning potentials Vc may be set to form a voltage gradient that aims toward the housing  40 . For purposes such as achieving concentrated cleaning effects during cleaning or controlling the intensities of the cleanings, for each section out of the ionization unit  10 , the filter unit  20 , the detector unit  30 , and the housing  40 , or for the component parts that construct respective sections, the respective cleaning potentials Vc may be controlled sequentially or randomly over time to control the one or more potentials and/or one or more potential differences at each section and/or component. The respective cleaning potentials Vc may also be changed or varied during cleaning for the system as a whole or in units of the respective sections or respective components. 
     The control unit  60  includes a plasma generation control unit (plasma generation controller)  65  that generates plasma in the plasma generation unit  50  at the timing where the cleaning plasma  55  is required. The cleaning control unit  61  includes a unit  61   c  that periodically determines the need for plasma cleaning, for example in keeping with monitoring results for the performance of the gas analyzer apparatus  1 , such as changes in the emission current from the filament  12  of the ionization unit  10 , in response to an order from an application on a higher-level than the gas analyzer apparatus  1  and in other reasons. 
     When the cleaning control unit  61  determines that plasma cleaning is necessary, the plasma generation unit  50  generates plasma  55  for cleaning purposes via the plasma generation control unit  65 . When the cleaning potentials Vc are set, the generated plasma  55  is drawn through the ionization unit  10  until the plasma  55  comes into contact with the filter unit  20 , the detector unit  30  and the housing  40 , to remove substances adhering to the surfaces of these elements, for example, insulating substances such as oxides. 
     The plasma  55  generated in the plasma generation unit  50  is charged a certain amount, around several Volts or so and as one example, a positive potential of about 5 V. This means that by keeping the gas analyzer apparatus  1  at the cleaning potentials Vc which are the ground potential and/or negative potentials, the plasma  55  can be drawn inside the gas analyzer apparatus (mass spectrometry apparatus)  1  and perform plasma cleaning. The potentials of the gas analyzer apparatus  1  during cleaning may be lower than the potential of the plasma  55  and may be a positive potential, but since it is preferable to maintain a certain potential difference with respect to the potential of the plasma  55 , the cleaning potentials Vc may be selected from among the ground potential and negative potentials below the ground potential. The potential of the plasma  55  for cleaning purposes generated in the plasma generation unit  50  may be further raised and set at a positive potential that is high with respect to that of the ionization unit  10 , the filter unit  20 , the detector unit  30 , and the housing  40 . In this case, even if the cleaning potentials Vc may be set at positive potentials during cleaning, it will still be possible to draw the plasma  55  for cleaning into the gas analyzer apparatus  1 , and more specifically, into the housing  40 , and perform cleaning. 
     The cleaning control unit  61  includes a unit (process plasma cleaning unit, process plasma cleaner)  61   d  that sets the cleaning potentials Vc at the timing when the process  70 , which is the source of the sample gas  9 , generates or provides the process plasma  75 . The process  70  to be monitored by the gas analyzer apparatus (mass spectrometry apparatus)  1  may include a system  71  that performs CVD (Chemical Vapor Deposition) and/or PVD (Physical Vapor Deposition). The system  71  may include processes of depositing an oxide or an insulator, such as silicon oxide (SiO 2 ), silicon nitride (SiN 3 ), titanium nitride (TiN), and tantalum nitride (TaN). In such processes, TEOS (tetraethyl orthosilicate) plasma is used for SiO 2  process, and plasma containing silane (SiH 4 ) and ammonia (NH 3 ) is used for SiN process. Accordingly, it is possible to clean the inside of the gas analyzer apparatus  1  by drawing these process cleaning plasmas into the gas analyzer apparatus  1 , when used or provided, as the cleaning plasma  75 . 
     When the process that is the source of the sample gas  9  for monitoring is a semiconductor manufacturing process  70 , etching plasma will be generated or provided for purposes such as cleaning the inside of the system  71  or dry etching the workpieces of the process  70 . As one example, cleaning plasma  75  including fluorocarbon (CFx), sulfur hexafluoride (SF 6 ), nitrogen trifluoride (NF 3 ), silicon tetrafluoride (SiF 4 ), or the like that generate fluorine-based radicals is generated. The process plasma cleaning unit  61   d  of the cleaning control unit  60  communicates with a process control unit (process controller)  73  that controls the system  71  of the process  70 , and acquires the timing at which the system  71  generates the plasma  75  for process cleaning or plasma for dry etching. The cleaning control unit  61  sets the cleaning potentials Vc at this timing. 
     Accordingly, it allows the gas analyzer apparatus  1  to clean itself inside by changing from the potentials set for monitoring purposes to the cleaning potentials Vc in accordance with the timing at which the cleaning plasma  75  is provided or generated by the process  70  being monitored by the gas analyzer apparatus  1 . This means that when the process  70  restarts, the gas analyzer apparatus  1  is able to start monitoring the process  70  with the gas analyzer apparatus  1  in a refreshed state. When a plurality of gas analyzer apparatuses  1  are disposed for monitoring the process  70 , it is also possible to switch between the plurality of gas analyzers  1  according to time division and draw plasma  75  from the process  70  to clean the insides of the respective gas analyzer apparatuses  1 . 
     If the time (i.e., amount of time) and/or timing required to clean the system  71  does not match the time (i.e., amount of time) and/or timing required to clean the insides of the individual gas analyzer apparatuses  1 , it is possible to provide a control valve (shutoff valve)  67   b  on the line (pipe) into which the sampling gas  9  and the cleaning plasma  75  flow and have the cleaning control unit  61  control the introduction of the cleaning plasma  75 . A control valve (shutoff valve)  67   a  may be provided on the line (pipe) that introduces the cleaning plasma  55  from the plasma generation unit  50  and the cleaning control unit  61  may control the introduction of the cleaning plasma  55 . 
     Many parts of the gas analyzer apparatus  1  are made of metal, and oxides and various other substances with insulating properties may be deposited by the gas  9  being measured. As examples, when monitoring CFx (of a low concentration), Fluorine may become separated as a gas and the carbon (C) may be deposited, while when monitoring TEOS, SiO 2  may be deposited. On the other hand, if cleaning plasma  55  or  75  such as CFx is introduced to clean the metal surfaces, the metal may corrode, which would shorten the life of the gas analyzer apparatus  1 . For this reason, it is effective to use pyrolytic carbon (pyrolytic graphite (PG)) instead of metal or to attach or coat PG onto metal surfaces, including the inner surfaces of the housing  40 , that construct the gas analyzer apparatus  1 . 
       FIG. 3  depicts one of examples of the control method for the gas analyzer apparatus  1 . In step  81 , the control unit  60  sets the respective potentials of each section and each element of the gas analyzer apparatus  1 , including the ionization unit  10 , the filter unit  20 , the detector unit  30 , and the housing  40  in a measurement state, and performs analysis of the sample gas  9  that flows in from the process  70 . When the cleaning control unit  61  has determined, in step  82 , that cleaning is required or necessary based on information such as the running time of the gas analyzer apparatus  1  or the monitoring results of the respective internal sections, in step  83 , the plasma generation unit  50  generates the cleaning plasma  55 . In step  85 , the cleaning control unit  61  sets the cleaning potentials Vc. In parallel with this, in step  85 , the plasma generation unit  50  generates the cleaning plasma  55  and the gas analyzer apparatus  1  draws in the cleaning plasma  55  to perform cleaning, including the housing  40 . 
     On the other hand, in step  84 , when the cleaning control unit  61  has communicated with the process control unit  73  and determined that it is time for the process  70  to commence cleaning and generate or provide the cleaning plasma  75 , in step  85 , the cleaning potentials Vc are set and the cleaning plasma  75  is drawn in from the process  70  to clean the gas analyzer apparatus  1 . The cleaning control unit  61  maintains the cleaning potentials Vc until the cleaning is completed in step  86 . When the cleaning has been completed, the processing returns to step  81  to commence gas analysis. 
     The functions of the control unit  60  including the cleaning control unit  61  may be provided by a program (program product)  66  that includes instructions for executing the above setting processes of the cleaning potentials Vc. The control unit  60  may include computer resources, such as a processor, a memory and others, and the program  66  may be recorded and provided on a recording medium, such as a memory. 
       FIG. 4  depicts another embodiment of the present invention. The basic configuration of this gas analyzer apparatus (mass spectrometry apparatus, mass spectrometer)  1  is the same as that of the gas analyzer apparatus shown in  FIG. 2 . The ionization unit  10  includes a filament  12  and a grid  11  as a thermion supplying unit  13 . The gas analyzer apparatus  1  of this embodiment does not include the plasma generation unit  50 , but includes a cleaning control unit  61  that performs cleaning by drawing in cleaning plasma  75  generated by the process  70  being monitored. Accordingly, the cleaning control unit  61  communicates with the process control unit  73 , and when the timing where the process  70  generates or provides the cleaning plasma  75  is reached, sets the respective cleaning potentials Vc for each section and each part, including the housing  40 , of the gas analyzer apparatus  1  and draws the cleaning plasma  75  via the sampling line  79  into the gas analyzer apparatus  1 . 
     The gas analyzer apparatus  1  may be equipped with an extraction electrode  69  whose potential is set so as to extract the cleaning plasma  75  from the process  70 . A line (pipe)  79  with a large diameter compared to the line (pipe)  79   s  for supplying the sampling gas  9  may be provided for supplying the cleaning plasma  75 . When the cleaning plasma  75  is drawn in, the cleaning control unit  61  may automatically perform an operation that closes a valve  68   a  on the sampling line  79   s  and opens a valve  68   b  on the plasma supplying line  79 . 
       FIG. 5  depicts yet another embodiment of the present invention. The basic configuration of this gas analyzer apparatus  1  (mass spectrometry apparatus, mass spectrometer)  1  is the same as that of the gas analyzer apparatus depicted in  FIG. 2 . As the ionization unit  10 , in place of the thermion supplying unit  13 , the plasma generator  50  converts the sample gas  9  from the process  70  into the plasma  55  and supplies the plasma  55  to the filter unit  20  as an ion flow  8  for gas analysis purposes. When the cleaning control unit  61  has determined that cleaning is required, the cleaning control unit  61  sets the filter unit  20 , the detector unit  30 , and the housing  40  at the cleaning potentials Vc respectively so that a large amount of plasma  55  generated by the plasma generation unit  50  is drawn into the housing  40  as the cleaning plasma  55 . 
     It is also possible to provide a control valve  59  on the plasma supply line  58  that connects the plasma generation unit  50  and the housing  40  and to control the degree of opening of the valve  59  via the cleaning control unit  61  to control the amount of plasma supplied to the housing  40 . In the same way as the gas analyzer apparatus depicted in  FIG. 4 , it is possible to switch between two lines with different diameters to control the amount of plasma supplied to the housing  40 . 
     Note that although a quadrupole-type mass spectrometer has been described above as an example, the filter unit  20  may be an ion trap or another type of filter, such as a Wien filter. The filter unit  20  is not limited to a mass spectrometry-type, and may be a filter that filters molecules or atoms of a gas or gases using other physical quantities, such as ion mobility. 
     Although specific embodiments of the present invention have been described above, various other embodiments and modifications will be conceivable to those of skill in the art without departing from the scope and spirit of the invention. Such other embodiments and modifications are addressed by the scope of the patent claims given below, and the present invention is defined by the scope of these patent claims.