Abstract:
A leak rate measuring device contains a strip spectrometer in which the ion path of the respective gas is influenced by at least one variable influencing quantity. When a gas having a predetermined mass is detected, and leakages of a gas having other predetermined masses interfere with this detection due to lack of selectivity of the spectrometer, the influencing quantity is modulated in a sinusoidal manner, and the wanted signal is subsequently selected in a lock-in amplifier. This modulation enables, for example, the elimination of the interfering influence of underground water during the leak rate measurement while using helium as a test gas.

Description:
FIELD OF THE INVENTION 
   The invention relates to a method for leak rate measurement and to a leak rate measuring device comprising a vacuum pump means for pumping gas out of a container, and a strip spectrometer (e.g. a sector-field mass spectrometer) for the mass-dependent deflecting of ions of the gas by variation of an influencing quantity (e.g. the anode voltage) and for the determining of the quantity of ions impinging on an ion catcher. 
   BACKGROUND OF THE INVENTION 
   Using the vacuum method, even minimum leakage rates of a container can be reliably detected. The smaller the leakage rate is, the higher will be the demands posed on cleanliness and end vacuum. When searching for leaks in situ, the container is evacuated by use of a leak detector until the test pressure required for the leak detector has been reached. Then, suspected leak sites on the container are sprayed from outside with a fine test gas jet. Test gas entering the container will be pumped off by the vacuum pump means and be detected by a mass spectrometer. A leak rate measuring device of this type is described in the brochure “Industrielle Dichtheits-Prüfung” of Leybold-Heraus GmbH dated 1987. 
   The mass spectrometers used in leak rate measurement are strip spectrometers such as e.g. the sector-field mass spectrometer wherein gas ions are caused to follow a curved path and then to pass an opening arranged within a shutter so as to have the gas ions impinge on an ion catcher electrode. The latter is connected to a highly sensitive electrometer amplifier by which the very small stream of ions will be sufficiently amplified for supplying the ions to a follower amplifier. An anode is fed with a mass-specific anode voltage. This voltage will cause a specific speed of the ions. In this manner, depending on the respective value of the anode voltage, ionized particles with different specific masses can travel along the ion path and impinge on the ion catcher. By suitable selection of the anode voltage, one can determine the respective specific mass which is to be examined. 
   Mass spectrometers of the modulating type wherein offset and disturbance effects are suppressed by use of modulation technique are already known. These spectrometers modulate the disturbance effects in such a manner that the measurement signal is caused to change between a largest possible sensitivity and a smallest possible sensitivity. Thus, a large modulation depth is obtained, and disturbance effects can be optimally eliminated. 
   Various options for modulation are available, notably:
         a. acceleration voltage (in the present case, anode voltage)   b. magnetic field   c. direction/site of ion entrance, by use of modulated deflection voltages       

   As a test gas for leak detection devices, frequent use is made of helium. Helium has the specific mass M 4 . A difference resides in the restricted selectivity of the mass spectrometer. Due to this restriction, the signal peaks of the integral specific masses, which actually should be distinct from each other, are caused to merge into each other. For instance, one component of the M 3  signal will spread into the range of the M 4  signal so that, if a large quantity of an M 3  gas exists, a gas of the specific mass M 4  (e.g. helium) cannot be measured with sufficient selectivity. In practice, this is indeed the case. On containers for leak measurement, water will deposit both on the outside and on the inside of the container. The H 2  component of water includes also M 3  portions whose existence considerably disturbs the measurement of an M 4  gas. Although one could perform the measurement under vacuum conditions long enough to allow the water components to be pumped off sufficiently, this approach would require a very long pumping period and thus cause a long time to pass until a stable indication of the leak rate is possible. The influence of the water on the measurement result is referred to as “water underground”. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a method for leak rate measurement and a leak rate measuring device wherein, while a selective measurement is performed in the range of the specific mass of the test gases, the influences of adjacent masses are eliminated without the need to accept overly long measurement times. 
   In the method described herein, the above object is achieved by the features of claim  1 , and in the device described herein, by the features of claim  4 . 
   According to the invention, the modulation is performed about the point of the highest sensitivity for the mass to be detected. By observing the individual frequency portions, an optimum separation can be effected between the current signal of the mass to be detected and the current signal of the adjacent masses. In the process, the peak (local maximum value along the mass axis) of the mass to be measured is separated from the flanks caused by adjacent masses, and from other disturbing DC variables. 
   In this manner, according to the invention, the wanted signal generated by the test gas is freed from disturbing signals, particularly from the slow drift of the water underground during or after the pump-off process. The influence of the M 3  component on the measurement result of the M 4  component is annihilated. The water underground is eliminated. Thus, even minimum quantities of helium can be detected in spite of the presence of water. 
   The leak rate measuring device according to the invention is distinguished in that, in a selected measurement range which corresponds to the peak of the mass spectrum occurring at a mass number, there is applied a periodically varying modulation quantity and that a filter device extracts from the generated signal a measurement signal of twice the frequency of the modulation quantity for evaluation. 
   In this regard, use is made of the circumstance that the peak region of the M 4  curve represents a non-linearity. In this region, the influencing quantity which influences the deflection is preferably modulated sinusoidally. This will result in a first modulation product which, because of the non-linearity, has twice the modulation frequency, and in a further modulation product which, because of the linear extension of the adjacent M 3  curve, has the same frequency as the modulation voltage. With respect to their frequency and/or their phase relationship, the two voltages can be compared to the modulation frequency so as to separate them from each other. In this manner, the M 4  signal can be effectively separated from the influences of the adjacent M 3  signal. 
   The leak rate measuring device of the invention can be utilized in various manners:
     1. The measurement object (the container) is evacuated. Helium is sprayed on from the outside. The gas sucked from the container is examined for traces of helium. In doing so, time will be gained by performing the measurement as provided by the invention.   2. The measurement object is arranged in a large recipient which is emptied by pumping. The measurement object is then filled with helium. In doing so, time is gained because, after e.g. about 3 minutes instead of the usual 10 minutes, a useful quantitative statement can be made on possible leaks. (The actual periods will depend on the volume.)   3. The measurement object is arranged in a large recipient and already contains helium. The recipient is evacuated. Also here, time is gained since a useful quantitative statement on possible leaks can be made at an earlier point (after e.g. 3 minutes). In this case, however, the measurement can be performed with higher accuracy because one has no possibility to determine the zero point. (The helium cannot be “switched off”). Without the measurement arrangement of the invention, one has no possibility to distinguish the helium from the residual water.   

   The invention allows for a very sensitive leak rate measurement wherein the leak indicator can be extended to cover a range up to e.g. 1E-10 mbar liters/sec. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will be explained in greater detail hereunder with reference to the drawings. 
       FIG. 1  illustrates the general configuration of the leak rate measuring device, 
       FIG. 2  shows a schematic representation of the function of the sector-field mass spectrometer, 
       FIG. 3  shows a characteristic development of a scan of the anode voltage represented by the signal curve provided by the mass spectrometer, 
       FIG. 4  shows the essential part of the function development according to  FIG. 3 , subdivided by function portions of the mass M 3  and the mass M 4 , 
       FIG. 5  show representations of the modulation voltage of the mass spectrometer and the resulting signal voltages for the mass M 4  and the mass M 3 , and 
       FIG. 6  show the developments over time of the influence of water and helium on the measurement result. 
   

   DETAILED DESCRIPTION 
   The general configuration of a leak rate measuring device according to the counterflow principle is illustrated in  FIG. 1 . A test sample  1  to be subjected to a leakage test is connected via a valve  2  to a test gas source  3  delivering helium. The test sample  1  is accommodated in a gas-tight test chamber  4 . From test chamber  4 , a conduit including a valve  5  extends to the test apparatus  6 . This conduit is connected to a turbo molecular pump  7  having its entrance side connected to a mass spectrometer  8  and having its exit side connected to a forepump  9 . Molecular pump  7  generates a high vacuum whereby helium which has entered the test chamber  4  through a leak of test sample  1 , is sucked in. Internally of molecular pump  7 , the helium, while flowing opposite to the conveying direction, will move into the mass spectrometer  8  in order to be identified. 
   The mass spectrometer  8  is a strip spectrometer, particularly a sector-field mass spectrometer as schematically illustrated in  FIG. 2 . The spectrometer comprises an ion source  17  with a cathode  18  and a heated anode  19 . Ion source  17  is surrounded by a shield  16  with an aperture  20  formed therein for allowing an ion beam  21  to exit. Within a magnetic field  22 , the ion beam  21  is deflected. The deflected ion beam impinges onto an ion catcher  25  connected to a highly sensitive electrometer amplifier  26 . This amplifier will amplify the very small ion current. Normally, amplifier  26  is a DC amplifier arranged to operate up to the femto-ampere region (10 −15  A). 
   The configuration of mass spectrometer  8  as described so far is already known. Using such a mass spectrometer in the measurement arrangement shown in  FIG. 1 , one will obtain e.g. the curve shown in  FIG. 1  when performing a scan across the anode voltage. In  FIG. 3 , the anode voltage is represented along the abscissa, and the current I measured by amplifier  26  is represented along the ordinate. In this Figure, E-2 denotes 10 −2 , E-8 denotes 10 −8 , etc. 
   When the anode voltage U A  of anode  19  is continuously increased, ions with a respective different specific mass will reach the ion catcher  25  via the provided ion path  21 . Helium has the mass M 4 . Thus, on the site corresponding to mass M 4 ; a peak  30  is generated, with its amount depending on the quantity of the detected helium. 
   At the anode voltage corresponding to the specific mass M 3 , a peak  32  is generated which is much higher than peak  30 . Peak  32  is to be attributed to the presence of water (H 2 O). Therefrom, H 2  ions are generated which include a portion of the specific mass M 3 . As long as water exists in the test chamber  4 , the peaks of the masses  3  ( 32 ) and  2  dominate the whole diagram. 
   The sector-field mass spectrometer  8  has a limited selectivity. This means that the peaks  30  and  32  are not very narrow but do have a certain width. Peak  32  has extensions  33 , in  FIG. 3  marked by interrupted lines, which extend into the region of mass M 4  and overlap with the helium component. This causes the already mentioned water underground. The influences of the very strong components M 2  and M 3  influence the component M 4  and adulterate the height of the component. 
   In  FIG. 6 , the leak rate, which has been obtained from the current I measured by amplifier  26 , is represented in logarithmic scale along the ordinate, and the pumping time t in seconds is represented, likewise in logarithmic scale, along the abscissa. The measurement was performed at the specific mass M 4 . The part  35  of the curve represents the case wherein the anode voltage U a  is kept constant in the usual manner and the leak rate is calculated from the DC current. The curve  36  represents the case wherein the anode voltage is modulated and the leak rate is calculated from a frequency component (twice the modulation frequency) of the current. The curve  35  shows the water effect beginning at a leak rate of about 1×10 −8  mbar l /sec. and then decreasing due to the vanishing of the water. Underlying the water effect is a flat course of the signal represented by curve  36  in the range of several 10 −10  mbar liters/sec. 
   The evaluation of the sole frequency component eliminates the M 3  and M 2  extensions and thus the slow drift of curve  35 . While the desired useful signal would normally be available only after about 5,000 seconds, it is now obtained already after about 200 seconds. This results in a considerable reduction of the measurement time. The slowly vanishing influence of the M 3  component, i.e. the water component, is eliminated. 
   For filtering out the M 4  component, an influencing parameter of the mass spectrometer—e.g. the anode voltage—exerting an influence on the deflection, is modulated with a periodic modulation voltage U M . The modulation voltage U M  is a sinusoidal voltage having a relatively low modulation frequency in the range of e.g. about 1 Hz. The modulation voltage U M  is used to modulate the anode voltage in the peak  30  ( FIG. 4 ) so that the modulation voltage U M  periodically follows the curved course  37  shown in scaled-up representation in  FIG. 4 . In the region of the modulation voltage, the extension  33  of the influence of mass M 3  takes a substantially linear course  38 . 
   The curved course  37  forms a non-linearity to the effect that the modulation voltage U M  will cause a signal  40  to be generated with the frequency  2  f M , i.e. twice the frequency of the modulation signal. The course  38 , on the other hand, will result in a signal  41  having a frequency and a phase relationship corresponding to those of the modulation signal. 
   For separating the mutually overlying signals  40  and  41  existing across amplifier  26 , the signals are supplied e.g. to a lock-in amplifier which receives the modulation signal U M  as a reference signal. The lock-in amplifier is a phase-selective amplifier which will separate the signals  40  and  41  from each other. In this manner, the signal  40  which has been generated exclusively under the influence of mass M 4 , can be selected. Thus, the influence of the water underground is eliminated. 
   The method described with reference to  FIGS. 4 and 5  is applicable both for the case that the amplifier  26  is an AC amplifier and the case that this amplifier is a DC amplifier. By the phase selection of signal  40 , also the DC portion and other offset effects are eliminated.