MEASURING TRACE ELEMENTS IN GAS

Abstract: An apparatus for analyzing trace elements in a gas sample includes a feedback system (46, 64, 68, 70) for accurately regulating and sensing the pressure supplied to an ion chamber (32). The feedback system is capable of compensating for a wide range of input gas pressures from a source (36). The ion chamber (32) is a closed ion source which is resistant to corrosion and aids in the reduction of noise. An ion detector (34) is calibrated for accurately scaling the measurements of trace elements. A quadrupole analyser (23) receives the ions through an aperture (88) and is shielded by the ion chamber (32) from electrons from two thermionic filaments (96, 98) The ion detector (34) senses oligomers produced by the parent gas, and is so calibrated that the oligomer level measured matches the level calculated from the pressure within the ion chamber (32).

Description: The invention relates to a method of calibrating a detector in a trace analysis apparatus wherein a gas sample, defined by a parent gas having trace elements therein is supplied to and ionized in an ion chamber and thereafter introduced into an analyser, said sample then being supplied to a detector. Amongst known devices for analyzing the components in a gas sample are the mass spectrometer and the mass spectrograph, each of which includes three basic components, an ion source, an analyzer and a detector. In the operation of such a device, a gas sample is introduced into an ion chamber where molecules of the gas sample are ionized by any one of a variety of techniques, such as electron bombardment, photoionization, or surface emission. The ionized molecules emitted from the chamber are introduced into the analyzer which separates the ions on the basis of of their different masses. Since the ions are charged particles, separation can be achieved through the application of magnetic fields. Other analyzers include electrostatic and quadrupole systems. Once the ions have been separated by mass, they are detected by a sensing means, such as a multistage electron multiplier. The best of the known devices hitherto are capable of detecting elements having a concentration as small as a few parts per million in a sample. For most purposes, this sensitivity is sufficient. However, in many fields, it is desirable to achieve even greater sensitivity. For example in semiconductor fabrication, in order to maintain high circuit quality and high yields, all gases must be certified at very high purity. More specifically, during epitaxy procedures, impurities, such as hydrocarbons or oxygen, can cause formation of pits in the silicon wafer. Other applications which require reliably tested pure gases include air pollution measurements and gas chromatography systems. At the present time, the best vacuum or base pressure which can be achieved in the detector is on the order of 1.33 × 10 _6 Nm _2 (10 _8 torr). In contrast, sample gases are typically introduced into the ion chamber at a relatively low pressure of 1.33 × 10 _4 Nm _2 (10 _6 torr). This low pressure is used to prevent the sample gas molecules from interacting with themselves to create dimers and trimers, rather than the monomer ions of interest. Therefore, the ratio between the sample gas pressure (10 _6 torr) and the vacuum condition at the detector (10 _8 torr) is on the order of 10². This ratio results in fairly significant signal to noise errors, particularly when attempts are made to isolate trace elements in a parent gas. When attempting to measure trace elements in a parent gas, the detector must be very finely tuned in order to provide accurate data. Calibration based on detection of expected trace elements is difficult if not impossible. According to the present invention a method of calibrating a detector, of the kind defined hereinbefore at the beginning, is characterised by the steps of: regulating the pressure of the gas sample supplied to said ion chamber to a value at which oligomers of the parent gas are formed; calculating the expected level of oligomers of said parent gas which should be formed when said sample is introduced into said ion chamber, said calculation being based on the regulated pressure of said sample gas; measuring the actual level of oligomers of said parent gas when said sample is supplied to the detector; and adjusting the level of oligomers of the parent gas actually measured to be equal to the calculated, expected oligomer level whereby measurements to trace elements in said sample will be accurately scaled. A method according to the invention is preferably carried out in a trace analysis apparatus that includes a housing which is maintained at a base pressure on the order of 1.33 × 10 _6 Nm _2 (10 _8 torr), an ion source mounted within the housing, in communication with the input line for supplying the sample gas, and a quadrupole mass analyzer and detector mounted within the housing, downstream from the ion source, for separating and identifying the ions produced in the sample gas. The ion source is a closed system which is operated at a pressure in excess of 1.33 × 10 _2 Nm _2 (10 _4 torr). Preferably, pressures near 0.133Nm _2 (10_3) are utilized. This operating pressure is on the order of one hundred times greater than that commonly used in the prior art. This increase results in an increase in the base pressure ratio between the ion chamber and detector. The base pressure ratio, on the order of 10⁵, substantially enhances the signal to noise ratio in the detector, thereby improving the sensing of trace elements. The pressure in the ion source is to be maintained in excess of 1.33 × 10 _2 (10 _4 torr). At this level, the parent gas tends to form dimers and trimers (generically oligomers) of the parent gas molecules. In the prior art, this result was undesirable since the devices were attempting to measure the constituents of the parent gas. However, when the purpose is to analyze trace elements, these elements are present in such minute amounts that only monomer ions of the trace elements will be found, and the presence of parent gas oligomers, rather than being undesirable, is used in the calibration of the detector. As discussed more fully hereinbelow, where the pressure in the ion chamber is accurately known, the level of oligomers produced can be accurately calculated. Further, since the level of oligomers actually produced is significantly higher than the trace element ions, the oligomers can be readily measured by the detector. The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a partial cross-sectional view, including block diagrams, of the apparatus for detecting trace elements, and Figure 2 is a cross-sectional view of the housing module of a pressure regulating means. Referring to Figure 1, there is illustrated an overall view of an apparatus 10 for detecting trace elements. Briefly, the analyzer includes a sealed, main housing 20 for containing the ion source 22 and the analyzer 24. The interior of the housing 20 is evacuated by a pump (not shown) through a vacuum line 26. As discussed below, it is desirable to maintain the vacuum created therein as complete as possible. In the preferred embodiment, the base pressure within the housing is on the order of 1.33 × 10 _6 Nm _2 (10 _8 torr). The housing 20 includes a main inlet tube 30 for receiving the sample gas. The sample gas will pass through tube 30 and into the ion chamber 32. Molecules of the sample gas are then ionized and pass into the analyzer 24. While a variety of analyzers are suitable, a quadrupole mass analyzer is used. In a quadrupole mass analyzer, a DC and superimposed RF voltage field is established between a rectangular array of elongated rods in a manner to filter the ionized molecules in accordance with the charge to mass ratio of the molecules. In this manner, a mass spectrum is produced of the ionized material at the output of the analyzer. This output is supplied to a detector 34. The detector 34 is defined by a multistage electron multiplier and Faraday cup combination. The sample gas to be tested is supplied to the apparatus 10 from a source 36. Interposed between the housing 20 and the source 36 is a means 40 for accurately regulating the pressure of the sample gas introduced into the ion chamber 32. The accurate regulation of the sample gas pressure greatly facilitates the ability of the device to measure trace elements in the sample gas. A pair of valves 42 and 44 may be provided on either side of the pressure regulation means 40 for isolation purposes. The gas pressure of the source 36 will typcally vary over a range of 0.507 × 10⁵Nm _2 (.5 atmospheres) to 2.030 × 10⁵Nm _2 (2.0 atmospheres). The regulation means 40 compensates for this wide range of input pressures, and accurately measures and delivers the desired pressure of the sample gas to the ion chamber 32. As shown in Figure 2, the pressure regulation means 40 includes a housing module 46 having an open interior 48. Module 46 further includes a pair of opposed walls 50 and 52 having apertures 54 and 56, located therein. Aperture 54 is in communication with input line 58 from the gas supply source 36. Aperture 56 is in communication with output line 60 for channeling the gas to the ion chamber 32. A pump 64 is provided for regulating the pressure within the module 46 by evacuating the gas therefrom. Pump 64 is connected to the module via line 66. In addition, a pressure sensor 68 is provided for monitoring the pressure within module 46. Finally, a control means, illustrated generally at 70, is provided which is operatively connected to the sensor 68 and the pump 64. Preferably, the control 70 is connected to pump 64 via throttle valve 72. The control 70 functions to actuate throttle valve 72 enabling pump 64 to regulate the pressure within module 46 in response to sensor 68. By this arrangement, the pressure of the gas sample supplied to the ion chamber 32 can be accurately regulated. The regulation means 40 takes advantage of two physical phenomena. The first concerns the behaviour of gases when they pass through a relatively small aperture. More particularly, when a gas passes through a small aperture, it will experience a fixed pressure drop. Therefore, when the size of the aperture is known, and the pressure on the upstream side of the aperture is also known, the pressure on the downstream side of the aperture can be calculated. This principle is used to calculate the pressure leaving module 46 and supplied to ion chamber 32. Stated differently, since the size of aperture 56 is known, the pressure within module 46 can be regulated to ensure a known gas pressure in the ion chamber. As can be appreciated, in order for the above calculations to be accurate, the pressure within module 46 must be accurately determined. This result is achieved by taking advantage of the second physical principle. More particularly, and as discussed below, because the pressure in module 46 is maintained in the viscous region about 1.33 × 10²Nm _2 (1 torr)), highly accurate measurements can be taken using a relatively simple capacitance manometer sensor without having to recalibrate for each individual gas sample. A gas which is maintained at a relatively low pressure, on the order of 1.33 × 10²Nm _2 (10 _4 torr), exhibits molecular flow characteristics. At this stage, the size of the gas molecules must be considered during measurement. In contrast, measurements can more easily be made in the viscous region. However, because the conductance of each gas varies with its viscosity, measurement inaccuracies can arise, particularly where the flow path between the gas supply and the ion chamber is long. The problems are overcome by measuring the pressure at an interstage level. Since this level is in the viscous range, pressure measurements are relatively simple. In addition, the sample gas experiences a sharp pressure drop, across wall 52, down to approximately 0.133Nm _2 (10 _3 torr). This rapid transition to the molecular flow range functions to sharply reduce the length of the viscous flow path. By this arrangement, problems associated with different conductance levels are minimized, alleviating the need for frequent recalibration of the sensor. Since accurate pressure measurements may be made within the module, the pressure within the ion chamber 32 can be accurately calculated. Having described the principles of the regulation system 40, its operation will now be explained. In use, control means 70 is programmed to maintain the pressure within the module 46 at a specific level. This level is based upon the desired pressure level in the ion chamber 32. Sample gases are introduced into module 46 from source 36 and through valve 42. As discussed above, the pressure of the source will typically range between 0.507 × 10⁵Nm _2 (.5 atmospheres) to 2.030 × 10⁵Nm² (2.0 atmospheres). Pressure sensor 68 monitors the pressure within module 46. Whenever this pressure begins to exceed the desired level, control means 70 actuates throttle valve 72 in a manner to connect pump 64 to the interior of module 46. Pump 64 draws off a portion of the sample gas until the pressure within the module 46 is reduced to the desired level. The accuracy of the feedback system also enables detector 34 to be accurately calibrated, as discussed more fully hereinbelow. Once the sample gas exits aperture 56 of the chamber it will move into line 60 through probe valve 44 and into ion chamber 32 via line 30. Preferably, the pressure in ion chamber 32 is on the order of 0.133Nm _2 (10 _3 torr). The ion source 22 is relatively corrosion free and also functions to significantly reduce noise often created in prior art devices by stray electrons and photons. Ion source 22 consists of a generally cylindrical ion chamber 32 formed from stainless steel or other non-corroding material. The upstream end of ion chamber 32 is mounted to a ceramic dielectric cylinder 80. The cylinder 80 receives input line 30. The downstream end of chamber 32 is mounted to the analyzer 24. Between the analyzer and the chamber 32 there is a pair of ceramic insulating rings 82 and 84. A metallic focus plate 86 is mounted against the upstream end of analyzer 24 and includes a central aperture 88 for allowing ions into the analyzer. The ion chamber 32 includes a pair of apertures 90 for allowing electrons into the interior of the chamber. A generally cylindrical electron reflector 94 is mounted coaxially about ion chamber 32. Filaments 96 are mounted adjacent the apertures 90 in the ion chamber 32. In use, a current is passed through filaments 96 such that they emit electrons. Because of their location, a large portion of the electrons emitted from the filaments enters the ion chamber through apertures 90. To further ensure that the electrons are directed towards the chamber, reflector 94 is preferably maintained at a negative bias voltage, on the order of 50 volts. In operation, a sample gas is introduced into the ion chamber through conduit 30. The sample gas molecules are then subjected to electron bombardment from filaments 96. This bombardment ionizes a percentage of the molecules in the sample gas. Molecules of the sample gas then move through aperture 88 in focus plate 86 and into the quadrupole analyzer 24. Frequently, during the operation of the apparatus, polar gas molecules will tend to cling to the walls of the ion chamber 32. This contamination can affect the accuracy of the trace element analysis. To purge the system on a periodic basis to remove the residual contaminating molecules, a valve means 100 is provided having an outer mounting block 102 connected to the upstream end of housing 20. Block 102 includes a main channel 104 for receiving the sample gas, and an L-shaped side channel 106 which is in communication with both the main channel 104 and the interior of housing 20. When valve means 100 is in the closed position, as illustrated in Figure 1, the sample gas passes directly from module 46 along channel 104 and into the ion chamber 32. However, when the system is to be cleansed, the probe valve 44 is initally closed and valve 100 is opened. In this condition, the main channel 104 is exposed to the high vacuum contained in housing 20. The high vacuum evacuates all residual molecules clinging to the interior surface of the ion chamber 32. Preferably, the evacuation purge system is supplemented by a heating source for baking the residual contaminating molecules. The heating source (not shown) can be mounted about the reflector 94. The apparatus is intended to detect trace elements in a parent gas having a density of one part per million or less. It is extremely difficult, if not impossible, to calibrate detection equipment at these extremely low levels. In the prior art devices, sample gas pressures in the ion source were typically maintained at a level of 1.33 × 10 _4 Nm _2 (10 _6 torr). At this level, the primary ionization mode of the parent gas was through the bombardment of electrons. If the gas pressures are raised much higher, the sample gas molecules tend to interact between themselves, creating multiple dimer and trimer (generically, oligomer) ions. Since these dimers and trimers would form a significant portion of the sample, the background noise created therefrom would adversely affect measurement. However, in the present example, the pressure in the ion chamber is intended to be maintained at approximately 0.133Nm _2 (10 _3 torr). This pressure will create significant dimer and trimer ions of the parent gas, but, because of the relatively low percentage of trace elements in the sample (and therefore low partial pressure), there is almost no formation of trace element oligomers. The level of oligomer production of the parent gas within the sample chamber is directly proportional to the pressure within the chamber. Accordingly, since the pressure in the chamber can be accurately measured, the level of oligomer production can be calculated. In operation, the detector is tuned to sense the actual oligomers produced. The detector is then so calibrated that the actual level of oligomers measured matches the calculated value based on the pressure within the ion chamber. Having scaled the detector, the analyzer can then accurately isolate trace elements. Operated in accordance with this method, the apparatus is capable of detecting trace elements having a concentration of less than one part per million. Reference should also be made to European patent no. 0122006 from which the present application is divided, and to the European patent application, publication no. 0311224, which is also divided from European patent no. 0122006.

Claims: 1. A method of calibrating a detector in a trace analysis apparatus wherein a gas sample, defined by a parent gas having trace elements therein, is supplied to and ionized in an ion chamber (32) and thereafter introduced into an analyser, said sample then being supplied to a detector (34), characterised by the steps of: regulating (40) the pressure of the gas sample supplied to said ion chamber (32) to a value at which oligomers of the parent gas are formed; calculating the expected level of oligomers of said parent gas which should be formed when said sample is introduced into said ion chamber (32), said calculation being based on the regulated pressure of said sample gas; measuring the actual level of oligomers of said parent gas when said sample is supplied to the detector (34); and adjusting the level of oligomers of the parent gas actually measured to be equal to the calculated, expected oligomer level whereby measurements of trace elements in said sample will be accurately scaled. 2. A method according to claim 1, characterised in that the pressure in said ion chamber (32) is in excess of 1.33 × 10 _2 Nm _2 (10 _4 torr).

Inventors: LIN, KUO-CHIN, PICKETT, FREDERICK P.
Applicants: UTI INSTRUMENTS COMPANY
Application year: 1984
Main classification: G01N33
IPC codes: H01J49, G01N30