A SECONDARY ION MASS SPECTROMETER

Abstract: In a secondary ion mass spectrograph, the electrical power supply voltages controlling and pertaining to the polarity of the target secondary ions are all switched between opposite polarities simultaneously while the deflec­ tion of the primary ion beam is automatically corrected for any error that may result from the polarity switch-over. This allows quick alternation of the polarity of target ions without loss of accuracy of the primary beam scan. The correction to the primary beam deflected may he predetermined under specific observing conditions or may be derived from theoretical considerations.

Description: BACKGROUND OF THE INVENTION The present invention relates to a secondary ion mass spectrometer and more particularly to an improvement in the secondary ion mass spectrometer, which irradiates a solid sample with an accelerated positive or negative primary ion beam suitable for analyzing the composition of a metal or a semiconductor material, which draws secondary ions released from the surface of the sample into a detection system by means of an electrical potential between the sample and the detection system and which analyzes the target composition by mass separation. Conventional secondary ion mass spectrometer irradiates a solid sample with an accelerated positive or negative primary ion beam and draw secondary ions released from the surface of the sample by irradiation into a detection system by means of an electrical potential between the sample and the secondary ion detection system (the extraction electrical potential), and analyzes the composition of the target on the basis of the secondary ions. Such spectrometers have been used to analyze the components and the composition of metallic or semiconductive material and particularly for analysis of microareas, trace analysis, analysis of depth distributions, and analysis of two-dimensional distributions. In the case of analyzing depth distribution by means of this secondary ion mass spectrometer, the information for a depth distribution may be obtained by continually irradiating a single point or area of the target. In the case of multielement analysis information for the multi-element may be obtained by successively measuring for each atomic mass in sample. Analysis of multielement depth distribution involves repetition of this operation. The mechanism by which secondary ions are formed in this secondary ion mass spectrometer is not well understood, but generally there is a tendency for elements having high ionization potential to become negative secondary ions, and elements having low ionization potential to become positive ions. Thus, we may measure for each element with high sensitivity as either a positive or a negative ion. However, the absolute intensity of secondary ion beam itself is meaningless because the intensity of secondary ion beam is highly sensitive to the surface state of the sample and the degree of the vacuum, etc. Usually, the intensity is expressed as the ratio of the intensities of secondary ions of the subject element to that of the matrix element, such as Fe in the case of steel material and Si in the case of a semiconductor material, etc. (for example, I(M⁺)/I(Fe⁺), I(M⁺)/I(Si⁺), I(Cl⁻)/I(Fe⁺). etc.). The secondary ion mass spectrometer consumes a very small amount of the sample during analysis, but analysis is destructive. For that reason, it constitutes a characteristic feature that it is necessary to rapidly control the mass in the mass analyzing system for the secondary ions in order to obtain good results. However, the extraction potential of secondary ion is constant by force in prior art secondary ion mass spectrometers. Therefore, in the prior art, it has not been possible to detect secondary ions of opposite polarity at the nearly same time, because the polarity of detected secondary ions was fixed to be positive or negative and the polarity of the secondary ions of matrix element such as Fe⁺ or Si⁺ or the like had to be selected so that the secondary ions can be released easily. Nevertheless, in practice it is usually important in analysis to measure the intensity of beams of the secondary ions such as Cl⁻ or F⁻ or the like which ionize in only the opposite polarity. In this case, the following problems have arisen in prior art secondary ion mass spectrometers because the polarity of the extraction potential of secondary ions or the like could only be switched manually. (1) The polarity of many operating potentials, such as the extraction potential of secondary ions, the electrical potential of the mass analyzing system of the secondary ions and the electrical potential of the detection system, etc., must occasionally be switched and switching of the polarity takes much time. Accordingly, part of the sample is continuously being destroyed by irradiation with primary ions while the polarity is being switched, and information of the measurement is interrupted during this time, or the measurement process is delayed in cases where the primary ion beam is turned off during this time. (2) The deviation of electric field which results from switching the polarity also affects the primary ion beam, and the irradiative location may shift as shown in Figs. 10 and 11. Therefore, the primary ion beam may miss a targeted microarea, the image of a two-dimensional distribution may also be distorted, and it is impossible to carry out analysis of depth distribution of both polarities at a single point. In Fig. 10 and 11, 10 represents a primary ion beam generation system which produces positive (Fig. 10) or negative (Fig. 11) primary ion beams 12, 14 and 16 represent a solid sample and a secondary ion beam released from the surface of the sample respectively. 18 represents a mass analyzing system which performs mass separation for the secondary ions comprising the secondary ion beam. The article by Gries et al in "International Journal of Mass Spectrometer and Ion Processes" Vol.59 (1984) pp 339-342 discloses in using a mass spectrometer to compensate for the deviation of the irradiative location of the primary ion beam due to switching of the polarity is to shift the sample 14 after switching, but this is time-consuming. Furthermore, it is necessary to equip the secondary ion mass spectrometer with an accurate apparatus for viewing the sample, such as an optical microscope or the like, and an accurate apparatus for slightly moving the sample. However, it has been difficult to view the sample and slightly move the sample by the conventional apparatus installed in the conventional spectrometer. It is therefore impossible to fully accurately correct the beam location. Furthermore, an additional problem has been that it was nearly impossible to switch the polarity repeatedly, because each polarity switching requires a lot of labor. On the other hand, it is known that secondary ions of both positive and negative polarities can be measured at the same time, but in this case two mass spectrometers or two mass analyzing systems for secondary ions are required. Therefore, such systems have the drawbacks of being not only exceedingly expensive, but also intricate and bulky. SUMMARY OF THE INVENTION The present invention is to eliminate the problems in the prior art mentioned above. Therefore, it is an object of the present invention to provide a secondary ion mass spectrometer which can repeatedly and rapidly switch the polarity of the extraction potential for the secondary ions, the electric potentials in the mass analyzing system for the secondary ions and in the detection system and so forth, and which can accurately detect both positive and negative secondary ions from the same analytic area. Another object of the invention is to provide a secondary ion mass spectrometer which can automatically correct the deviation of the irradiative location of the primary ion beam due to the change in electric fields resulting from the polarity switching by a compensation device which modifies the voltage applied to a deflection plate for primary beam scanning in accordance with a beforehand obtained information of the deviation, whereby other compensation device are rendered unnecessary. In order to accomplish the above mentionned and other objects, a secondary ion mass spectrometer, according to the invention, irradiates a solid sample with a primary ion beam, draws secondary ions released from the surface of the sample into a detection system by means of an electrical potential between the sample and the dectection system, and analyzes the composition of the sample by mass separation of the secondary ions. A repeating switching device is incorporated with the secondary ion mass spectrometer, which automatically switches the polarity of all electrical sources or voltages influencing the polarity of secondary ions being detected. A compensating device automatically corrects the irradiative direction of the primary ion beam in response to operation of the switching device to compensate for deviation of the irradiative location of the primary ion beam on the surface of the sample due to the polarity switching in accordance with a beforehand obtained information of the deviation. With the foregoing arrangement of the secondary ion mass spectrometer, the secondary ions produced by irradiating the sample with primary ions are of both positive and negative polarities, and the positive or negative secondary ions are introduced into a mass analyzing system for secondary ions by means of a suitable potential slope applied between the sample and the analyzing system. Accordingly, we may reverse the polarity of the secondary ions drawn into the analyzing system by reversing the polarity of this potential slope. At the same time, we also need to reverse the polarity of the potentials in the mass analyzing system for secondary ions and in a detection system. In this case, the change in the electric field between the sample and the mass analyzing system affects the electric field between the source of primary ions and the sample, and the trajectory of the primary ion beam may change slightly. Therefore, the irradiative location of the primary ion beam on the surface of the sample also shifts, but we are not able to ignore this deviation because analysis of a specific microscopic area may be conducted in the secondary ion mass spectrometer. Accordingly, it becomes possible to alternate and rapidly measure either positive or negative secondary ions originating from the same place by compensating the change in deflection of the primary ion beam. According to one aspect of the invention, a secondary ion mass spectrometer comprises: an irradiating system for irradiating a primary ion beam to a solid sample at a predetermined irradiation point for generating secondary ions, the irradiating system including a plurality of deflection plates for controlling the irradiating direction of the primary ion beam in accordance with voltage value applied thereto; a mass analyzing system for analyzing the composition of the sample by mass separation of the secondary ions; electrical power sources for supplying voltages to the deflection plates and the mass analysing system, the polarity of the secondary ions to be analyzed being switched by switching the polarity of all of the electrical power sources; and a switching circuit for switching the polarity of all of the electrical power sources for switching circuit correcting the irradiating direction of the primary ion beam by adding a predetermined correction voltage to the deflection plates synchronously with the polarity switching, the predetermined correction voltage being set so as to correspond to the previously measured deviation of the primary ion beam from said predetermined irradiation point. According to another aspect of the invention a process for performing mass spectrometry utilizing a secondary ion mass spectrometer comprises the steps of : irradiating a primary ion beam to a solid sample at a predetermined irradiation point for generating secondary ions, the irradiating direction of the primary ion beam being controlled in accordance with voltage value applied to a plurality of deflection plate; drawing the generated secondary ions to be analyzed into a mass analyzing system; analyzing the composition of the sample by mass separation of the secondary ions, the polarity of the secondary ions to be analyzed being switched by switching the polarity of all of electrical power sources which supply voltages to the deflection plates and the mass analyzing system; switching the polarity of all of the electrical power sources for switching the polarity of the secondary ions; and correcting the irradiating direction of the primary ion beam synchronously with the polarity switching, by adding a predetermined correction voltage being set so as to correspond to the previously measured deviation of the primary ion beam from the predetermined irradiation point. The predetermined correction voltage may be obtained by prior measurement of the deviation of the primary ion beam by means of observation of any one of the ion image, an optical microscope, a fluorescent plate or the like. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompnaying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment, but for explanation and understanding only. In the drawings: Figure 1 is a diagram of the overall structure of the preferred embodiment of a secondary ion mass spectrometer according to the present invention. including a perspective view of the primary ion beam generator and a section through part of a mass analyzing system for secondary ions. Figure 2 is a circuit diagram of the mechanism which switches the polarity of all electrical sources in the preferred embodiment. Figure 3 is a timing diagram of the extraction voltage, voltage on the deflection plates, the lens voltage, the acceleration voltage for secondary ions in the detection system, the electrostatic field, and the polarity of electrical source of magnetic field in an exemplary state of transition. Figure 4 is a plane figure which shows the first example of a measurement sample. Figure 5 is a plan view of the K⁺-ion image recorded by a prior art method from the first example of a measurement sample. Figure 6 is a plan view which shows a misleading Cl⁻-ion image in the same case. Figure 7 is a perspective view of a second exemplary sample. Figure 8 is a graph of the results of analysis of depth distribution in the case of prior art method applied to the second exemplary sample. Figure 9 is a graph of the results of analysis of depth distribution in the case of the present method applied to the second exemplary sample. Figure 10 and 11 are diagrams illustrating deviation of the irradiative location of the primary beam in the prior art due to change of the polarity of the extraction voltage. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figs. 1 to 9, the present invention will be described in terms of the preferred embodiments of the invention. Figure 1 shows the preferred embodiment of a secondary ion mass spectrometer according to the present invention, which comprises; a generating system 10 for the primary ion beam 12 composed of a plurality of deflection plates 20 which control the direction of propagation of the primary ion beam 12, and an electrical power source 22 for the deflection plates which applies a voltage to the plates 20; a mass analyzing system 18 for the secondary ions composed of an extraction electrode 24 used to draw secondary ions 16 released by the solid sample 14 irradiated by said primary ion beam 12 into the mass analyzing system 18, an extraction electrical power source 26 which supplies the extraction electrode 24 with an extraction potential E₀+α or E₀-α, an electrostatic lens 18 used to focus the secondary ion beam 16 drawn into the mass spectro-analysis system 18 by the extraction electrode 24, an electrical power source 30 for the lens which supplies the electrostatic lens 28 with a lens voltage Eℓ, eletrostatic field plates 32 for energy separation of the secondary ion beam 16, an electrical power source 34 for the electrostatic field plates 32 with an electrostatic field voltages ±E c , an electromagnet 36, an electrical power source 38 which supplies the magnet 36 with electrical current for generating a magnetic field, a target 40 with which the desired secondary ions collide, an electrical power source 42 which supplies the target 40 with an electric potential E t which accelerates the secondary ions in a detection system toward the target, and a detection system 46 which detects electrons 44 produced by the collision of the secondary ions with the target 40; and a switching circuit 48 which automatically switches the polarity of all of the electrical power sources and voltages influencing the polarity of the secondary ions being analyzed and in response to the switching, automatically corrects the direction of propagation of the primary ion beam 12 by adding a predetermined modified voltage corresponding to the deviation of the primary beam to the deflection plates for the primary ion beam so as to compensate the deviation of the irradiative location of the primary ion beam on the surface of the sample resulting from a polarity switch-over. The switching of the polarity by the switching circuit 48 is carried out by a relay that automatically controls switches, for example, the switch 26A for switching the polarity of the extraction electrical power source 26 shown in Figure 2, which controls the electrical supply polarity of the electrical power sources 26, 30, 34, 38 and 42. The switching circuit 48 also adjusts the voltage on the deflection plates 20 as described below. Let the mass number of positive secondary ions to be detected be labelled M1, M2 (M1, M2>0), and the mass number of negative secondary ions, M1', M2', (M1', M2'>0). Assume also that the extraction potential relative to the electrical potential of the sample, for positive and negative secondary ions respectively, is ±αV (α>0), as shown in Fig. 3. Furthermore, let the voltage on the deflection plates 20 in the x and y directions be E x and E y respectively, where the x and y directions lie in the horizontal plane essentially perpendicular to the direction of propagation of the primary ion beam, and the amplitudes of the voltages E x and E y are functions of the acceleration voltage of the primary ion beam and the extraction voltage of the secondary ions, and the polarity of the primary ions. First, to measure positive secondary ions of mass number M1, the extraction voltage is set to -α and the voltages on the deflection plates are set to -E x and -E y . The lens voltage and the acceleration voltage for the mass analyzing system 18 for secondary ions may be labelled -E ℓ and -E t respectively. The electrostatic field plate voltage E c is set in this case to a so-called positive mode, as is the polarity of the electrical source 38 for the magnet 36, and the secondary ions of mass number M1 are detected by adjusting the mass spectrometer to mass number M1 by regulating the electrical output current of the electrical power source 38 for the magnet. After measuring the ion intensity of M1, the voltages can be adjusted for measurement of the M2 ion intensity. Secondarily, to measure negative secondary ions of atomic mass M1', the extraction voltage is switched from -α to +α, the voltage on the deflection plates is switched from -E x and -E y to +E x +β x and +E y +β y respectively (β x and β y are the adjusting voltages intended to cope with deviation.), the lens voltage in the mass analyzing system and the acceleration voltage for secondary ions in the detection system are switched from -E ℓ and -E t to +E ℓ and +E t respectively, the voltage E c in the electrostatic field is switched to negative mode and the polarity of the electric source 38 for the magnetic field is also switched to negative mode. Then the secondary ion mass spectrometer can measure the ion intensity of ions of mass number M1' after adjusting the electrical output currents or voltages of the electrical source 38 for the magnetic field. The mass spectrometer can then measure the ion intensity of ions of mass number M2' after suitable field re-adjustment. Thus, it is possible to measure both positive and negative secondary ions alternatingly at the same position by simply repeating the voltage control process described above. Fig. 3 illustrated transitions between the operating states of the extraction voltage, the voltage on the deflection plates, the lens voltage, the acceleration voltage for secondary ions in the detection system, the electrostatic field voltage, and the polarity of the magnetic field for the preferred embodiment of the present invention. Examples of measurements performed by a mass spectrometer employing the present invention are described in detail below. In first example of measurement shown in Figs. 4 through 6, a sample shown in Figure 4 has KCl concentrated along its grain boundaries. In this case, K and Cl are detected as K⁺ and Cl⁻ respectively. First, when K⁺ is measured by the prior art method, the ion image shown in Fig. 5 is obtained from the scanning area A of primary beam shown in Fig. 4. Next, when the polarity of the extraction potential and so forth are all reversed and re-adjusted, and the Cl⁻ ion image is measured; the ion image shown in Fig. 6 is obtained. In this case, the irradiative location of primary beam and thus the scanning area were shifted to the area B as shown in Fig. 4. Comparing the ion images of Figs. 5 and 6, K and Cl seem to have been concentrated in different places. In order to compensate the deviation of the irradiative location of the primary beam by moving the sample itself, an apparatus to precisely move the sample by observing the ion image is required, and it is difficult and time-consuming to carry out this very accurate compensation on each switching operation. However, if the present invention is employed, the sample does not need to be shifted at all and measurement of opposite polarities is rapid and very accurate because the switching of the polarities of all of the electrical sources and voltage attendant on the conversion of the polarity of detected secondary ions and compensate for the deviation of the irradiation point of the primary ion beam are carried out automatically. In second example of measurement, analysis of the depth-direction of NaCl which separated out of Zn galvanizing is performed on the sample shown in Fig. 7. In this case, the sample set up so that the center of scanning area of the primary beam coincides with the center of the contaminant in prior art method. The primary ion is O₂⁺ and acceleration voltage is 20 KV. Na⁺ and Zn⁺ were measured in the positive secondary ion mode. Then when the system was switched manually to the negative mode to measure Cl⁻, the irradiative location of primary ion beam shifted 150 µm in the x-direction onto the Zn region. Therefore, secondary ions of Cl⁻ appearead to be completely absent. The results are shown in Fig. 8. The accuracy of the analysis of depth-direction was inferior because the measurement of the ion intensity of the secondary ion Cl⁻ was interrupted during the polarity switching, the intervals between measurement of the ion intensities of different secondary ions were relatively long, and therefore the resultant information was rough. In the case of Fig. 8, the depth distribution was about 200 nm. Furthermore, each time it was switched to negative mode, primary beam deviated from the target. Therefore, the results were that the Cl⁻ ion intensity was not measured at all and only Na⁺ appeared to have separated out of the Zn layer. This is contrary to the facts. In analysis of depth-direction, the depth of penetration is directly proportional to the time of exposure to the primary beam, provided a uniform material. However, in the prior art measurement, primary beam impinges on the Zn layer in the negative mode and destruction of NaCl is interrupted during these periods. Consequently, Na (separated material on Zn; as a matter of fact, it is NaCl, but in prior art method, it must be concluded that it is Na.) appears to condense to a relative great thickness. Then, the intensity of secondary ion of Na + was measured by using the same primary ion conditions as in the prior art example (O₂⁺, acceleration voltage 20 KV) but employing the present invention. The measurement was carried out at an extraction voltage of -1.5 KV and with the center of scan of the primary beam centered over the NaCl area in the positive mode. The Zn⁺ ion intensity was measured continuously. Then, the polarity switching was carried out automatically so that the extraction voltage was adjusted to ⁺1.5 KV. The irradiative location of primary beam, shifted 150 µm in the x-direction as a result of this switching. To compensate for this, the modification potentials β x and β y 30V in the x-direction and 0V in the y-direction (there was no shift in the y-direction) were applied to the deflection plates 20. These quantities are obtained by prior measurement of the deviation of the beam by means of observation of the ion image, an optical microscope, a fluorescent plate or the like. It is also possible to obtain the modification voltages as functions of the deviation theoretically. In the present invention, this compensation is automatically performed. Accordingly, the correcting voltages β x =30V and β y =ØV are applied to the deflection plates simultaneously with the polarity switching from positive to negative, and their inverses are applied when switching from negative to positive, so that the primary beam is held on target at all times. Thus, in the inventive example, the Cl⁻ ion intensity can be measured and a truly linear relationship between exposure time to the primary ion beam and depth is evident, in Fig. 9. In the present invention, analysis of depth-direction was very accurate, with measurements taken at intervals of about 50 nm. Thus it could be ascertained that the contaminant in the Zn was NaCl. It could also be calculated that the thickness of the NaCl was about 1 µm on the basis of the intensity of the primary beam, the scanning area, and the exposure time to the primary beam. The preferred embodiments of the present invention have been applied to secondary ion mass spectrometers of the magnetic field type, but the scope of application of the present invention is not limited to this and it should be clear that the present invention is similarly suitable for application to secondary ion mass spectrometers of the quadrapole type which have a low extraction voltage and have an easier polarity switching than magnetic types. As previously mentioned, not only can the polarities of all of the electrical sources or voltages taking part in the change of the polarity of secondary ions to be detected, be switched quickly, but also the irradiation location of the primary ion beam can be automatically corrected for any deflection accompanying the polarity switch-over when the present invention is employed. Therefore, it is not necessary to compensate for deviation by slightly moving the sample, and it becomes possible to rapidly and accurately measure the secondary ion intensities for ions of opposite polarity at the same point. Thus, it is possible to quantitatively use the intensity of negative secondary ions, and the field of application of the secondary ion mass spectrometry can expand. Further, the present invention have the additional advantage of being easily adapted to existing prior art apparatus.

Claims: A secondary ion mass spectrometer comprising : an irradiating system (10) for irradiating a primary ion beam to a solid sample at a predetermined irradiation point for generating secondary ions, said irradiating system including a plurality of deflection plates (20) for controlling the irradiating direction of the primary ion beam in accordance with voltage value applied thereto; a mass analysing system (18) for analyzing the composition of said sample by mass separation of the secondary ions; electrical power sources (22, 26, 30, 34, 38 and 42) for supplying voltages to said deflection plates (20) and said mass analyzing system (18), the polarity of said secondary ions to be analyzed being switched by switching the polarity of all of the electrical power sources; characterized in that : said spectrometer further comprises a switching circuit (48) for switching the polarity of all of the electrical power sources (22, 26, 30, 34, 38 and 42) for switching the polarity of the secondary ions, said switching circuit (48) correcting the irradiating direction of the primary ion beam by adding a predetermined correction voltage to the deflection plates (20) synchronously with the polarity switching, said predetermined correction voltage being set so as to correspond to the previously measured deviation of the primary ion beam from said predetermined irradiation point. A process for performing mass spectromety utilizing a secondary ion mass spectrometer, which process comprises the steps of : irradiating a primary ion beam to a solid sample at a predetermined irradiation point for generating secondary ions, the irradiating direction of the primary ion beam being controlled in accordance with voltage value applied to a plurality of deflection plate (20); drawing said generated secondary ions to be analyzed into a mass analyzing system (18); analyzing the composition of said sample by mass separation of said secondary ions, the polarity of the secondary ions to be analyzed being switched by switching the polarity of all of electrical power sources (22, 26, 30, 34, 38 and 42) which supply voltages to said deflection plates (20) and said mass analyzing system (18); and characterized in that : said process further comprises steps of : switching the polarity of all of said electrical power sources (22, 26, 30, 34, 38 and 42) for switching the polarity of said secondary ions; and correcting the irradiating direction of said primary ion beam synchronously with the polarity switching, by adding a predetermined correction voltage to said deflection plates (20), said predetermined correction voltage being set so as to corrrespond to the previously measured deviation of the primary ion beam from said predetermined irradiation point.

Inventors: SUZUKI, TOSHIKO C/O HIGH-TEC.RESEARCH LAB.
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1985
Main classification: H01J49
IPC codes: H01J49, H01J37, G01N23, G01Q30