Anal. Chem. 1994,66, 1890-1896
Suppression of Cluster Ion Interferences in Glow Discharge Mass Spectrometry by Sampling High-Energy Ions from a Reversed Hollow Cathode Ion Source Ray-Chern Dengt and Peter Williams' Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287- 1604
The energy distributions of the ions produced in a reversed hollow cathode glow discharge ion source were studied using a Cameca IMS 3f mass spectrometer. Aluminum cathodes containing various impurities at concentrationsof a few hundred parts per million (ppm) were used. Two prominent peaks were found in the energy distributions, corresponding to ions with high kinetic energies (formed in the negative glow region of the discharge) and ions with low kinetic energies (formed near the cathode potential at or near the extraction aperture in the base of the hollow cathode). For AI+ ions, the high-energy peak is more intense than the low-energy peak, while for Ar+ ions the high-energy peak is much weaker than the low-energy peak due to efficient resonant charge exchange in the cathode dark space. Similar results, showing a weak high-energy peak and an intense low-energy peak, were obtained for cluster ions. These ions are collisionally dissociated in the cathode dark space, and only those ions formed near the cathode exit aperture have a significant probability of escape. Thus, the interfering signals from cluster ions and the discharge gas may be significantly reduced by adjusting the mass spectrometer to sample only the high-energy ions emanating from the negative glow. In the semiconductor and metals industries, there are continuing needs for analysis of trace impurities in semiconductors and high-purity metals. lq2 Among the techniques used for analysis of trace impurities in solids, glow discharge mass spectrometry (GDMS) has drawn increasing attention in recent year^.^,^ Characteristic features of GDMS include low detection limits, small matrix effects, and relatively uniform elemental sensitivities. Semiquantitative analysis by GDMS without calibration standards is possible; the technique is particularly useful for a fast compositional analysis of ultrapure materials. Of the ion sources used for GDMS, the hollow cathode ion source is interesting because of the large increase in excitation and ionization in the coalesced negative glow region which fills the cavity of the hollow ~ a t h o d e . However, ~,~ the hollow cathode ion source has drawn much less attention than other types of glow discharge ion source in recent years4 One obvious problem with this source has been the requirement
'
Present address: KOOO, ElectronicsResearch & Serviceorganization,Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan 31015, R.O.C. ( I ) Chu, P. K.;Huneke, J. C.; Blattner, R. J. J . Vac. Sci. Tecfinol.A 1987,5 (31, 295-301. (2) Charalambous, P. M. Steel Res. 1987, 58 (S), 197-203. (3) Harrison, W. W.; Hess, K. R.; Marcus, R. K.; King, F. L. Anal. Chem. 1986, 58, 341A-356A. (4) Harrison, W. W.; Bentz, B. L. Prog. Anal. Spectrosc. 1988, I I , 53-1 10. ( 5 ) Harrison, W. W.; Magee, C. W. Anal. Chem. 1974,46,461-467. (6) Colby, B. N.; Evans, C. A., Jr. Anal. Cfiem. 1974,46, 1236-1242.
1800 Analytical Chemistry, Vol. 66, No. 11, June 1, 1994
of machining the sample into a cylindrical, hollow geometry. This requirement makes sample preparation a lengthy process for metal samples and is not readily applicable to brittle materials, such as semiconductor wafers. Recently, we reported the development of a glow discharge hollow cathode ion source for a Cameca IMS 3f secondary ion mass ~pectrometer.~ A novel feature of the ion source is the method of easily constructing a hollow cathode from four pieces of flat samples including metal sheets and semiconductor wafer^.^,^
One of the problems encountered in glow discharge mass spectrometry is the occurrence of interferences due to cluster ions which have the same nominal masses as the analyte ions. These cluster ions arise from combinations of the sample constituents with each other and with the discharge gas. To achieve low detection limits, it is essential to reduce or eliminate the cluster ion interferences. The simplest approach is to choose an isotope of the analyte element at which no interference is found. While this method may be successful with multiisotopic elements, it is clearly inapplicable to monoisotopic elements. Cluster ion interferences may also be eliminated by operating the mass spectrometer at high mass resolution. In this way, the analyte ion signal may often be separated from the interfering ion signal and identified without any ambiguity. However, use of high mass resolution results in a significant reduction of the analyte ion signals, particularly in the case when mass resolving power exceeding -4000 is required. In other approaches, King and coworkers1° have recently reported adjusting the discharge parameters such as discharge gas pressure and cathode-toion exit-orifice distance to yield a minimum in the most abundant interferences due to metal dimers and metal argides. These authors also reported" the utilization of a collision cell in a triple quadrupole mass spectrometer system to reduce the cluster ion signals by collisional dissociation. In a preliminary in~estigation,~ we have shown that the energy distributions of the atomic ions and cluster ions extracted through an aperture in the base of the hollow cathode differ significantly. Generally, a large portion of the atomic (7) Deng, R. C.; Williams, P. In Secondary Ion Mass Spectrometery SIMS VII;
Benninghoven, A., Evans, C.A,, McKeegan, K. D., Storms, H. A,, Werner, H. W., Eds.; Wiley: New York, 1990; pp 843-846. (8) Streit, L. A. Thesis, Arizona State University, 1987. (9) Streit, L. A.; Williams, P. In Secondary Ion Mass Spectrometry SIMS VI; Benninghoven, A., H u h , A. M., Werner, H. W., Eds.; Wiley: New York, 1988; pp 201-204. (IO) King, F. L.; McC0rmack.A. L.; Harrison, W. W. J. Anal. At. Spectrom. 1988, 3, 883-886. ( I I ) King, F. L.; Harrison, W. W. I n f . J. Mass. Specrrom. Ion Processes 1989,89, 171-185. 0003-2700/94/0366-1690~04.50/0
0 1994 American Chemical Society
B
A
C
D E
Hji
.
Figure 1. Cross section of the hollow cathode ion source: A, ion extraction aperture; B, cathode base; C, sample: D, quartz tube; E, anode; F, copper wire; G,mounting flange; H, Freon inlet; I, hollow insulated feedthrough serving as anode mount and gas inlet; J, Freon outlet.
ions leaving the cathode aperture have kinetic energies near 4V, where 4 is the ion charge and Vis the discharge potential, while most cluster ions have kinetic energies near zero. The ions arising from the discharge gas also have kinetic energies near zero. We present here a method for reducing the cluster ion interferences and the ion current arising from the discharge gas by selectively sampling the higher energy ions, which are formed in the negative glow region of the discharge at a potential near that of the anode and reach the cathode aperture without undergoing collisions. EXPERIMENTAL SECTION The design of the hollow cathode ion source originated from the primary ion source developed for ion implantation in the Cameca IMS 3f by Streit and william^.^*^ A novel feature of this source is that the hollow cathode is constructed from four flat pieces of sample arranged to form a hollow square; thus, hollow cathodes could readily be constructed from brittle semiconductor wafers.g Figure 1 shows a crosssection of the ion source designed for the present work. This source was mounted in place of the regular SIMS sample stage of the Cameca IMS 3f secondary ion mass spectrometer and designed so that the external front surface of the cathode base (B) was in the plane normally occupied by the SIMS samples (5 mm from the grounded immersion lens cover). The samples used in the present study were a set of aluminum standards (Aluminium Pechiney, France). These samples were obtained as 5.5- cm-diameter, 2.5-cm-thick cylindrical disks. Sample sheets 0.050-in. thick were sliced from the disks and were further cut into rectangular pieces 0.28 in. X
0.35 in. Four such sample pieces were inserted into the grooves in the cathode base (B) and held in place by small metal spring clips to form the hollow cathode. The cathode base then was screwed into the main body of the source to complete the discharge cell. The anode (E) was a 1-in.-long, 0.25in.-diameter stainless steel rod. To prevent the stainless steel cathode base being sputtered and complicating the mass spectrum, a piece of 0.28 in. X 0.28 in. rectangular tantalum with a 0.031-in.-diameter aperture in the center was placed at the base of the cathode. The discharge was confined by a 17-mm-diameter quartz tube (D). Freon cooling was provided during operation to improve the stability of the discharge. The argon discharge gas was introduced into the discharge region through two small apertures in the anode. Ions produced in the discharge were extracted through a 400pm-diameter aperture (A) at the base of the cathode and accelerated into the mass analyzer through a 4500-V electrostatic potential drop. The discharge potential was provided by a 0-500-mA, 0-600-V power supply (Kepco HB525M), which was floated at the acceleration voltage. The discharge potential was between 250 and 550 V depending on the cathode-anode distance and the operating pressure. The pressure in the discharge cell was controlled by using a needle valve. Because of the design of the ion source, this pressure could not be measured directly. Instead, the discharge cell pressure was estimated from the pressure in the sample chamber, the known pumping speed of the sample chamber turbomolecular pump, and the diameter of the extraction aperture of the ion source. The pressure in the sample chamber was maintained between 5 X 10-5 and 2 X Torr, corresponding to an estimated pressure between -0.5 and -0.2 Torr in the discharge cell. Mass spectra were recorded by using the existing SIMS software without modification. The rather high argon pressures in the Cameca ion source region resulted in significant argon pressures in the mass spectrometer section, which is differentially pumped using ion pumps. For long-term operation with this glow discharge source, it would be advisable to replace at least one of the spectrometer ion pumps with a turbomolecular pump. In standard SIMS operation, the sample is held at 4500 V in the instrument; this potential can be varied by f125 V for recording ion energy distributions. To vary the source potential over a wider range, the 4500-V power supply was replaced by an external, programmable 0-6000-V power supply (Spellman WRM 6P/N 1500D). Energy distributions of ions produced in the ion source were determined by closing the energy window of the instrument to a width such that the maximum ion intensity of the energy spectrum could be recorded using the electron multiplier detector and scanning the ion acceleration potential, while maintaining the bandpass of the energy analyzer constant (at 4500 f 15 eV). RESULTS AND DISCUSSION Ion Energy Distributions. In the glow discharge, most ions are believed to be formed in the negativeglow region by electron impact and Penning ionization. In the present hollow cathode ion source, ions representing the constituents of the cathode are extracted through an aperture located in the base of the hollow cathode into the mass spectrometer. The ions must traverse the cathode dark space to reach this aperture, and Analytical Chemistry, Vol. 66, No. 11, June 1, 1994
1891
(a) .-
II
5 lo
-,x
VI
'1
/i
10
-100
0
100
200
300
400
-100
0
100
200
300
400
6
10
r
5., 5 10;
5 4.; 2 10; r 3' 2 101 x 2 10 VI
v
g H
I
10
I n i t i a l K i n e t i c Energy ( e V ) Flgure 2. Energy distributions of Ar+ (a) and AI+ (b) ions, obtained under discharge conditions of 290 V, 33 mA, and 0.5 Torr.
at the pressure maintained in the glow discharge (0.2-0.5 Torr), few ions can traverse the entire cathode dark space without undergoing collisions. Resonant charge-exchange and elastic and inelastic scattering collisions in the cathode dark space between ions and neutral atoms give rise to a spread of kinetic energies in the ion beam emerging from the cathode aperture, ranging from near 0 eV (i.e,, near the cathode potential) to energies corresponding to the full discharge potential. Figure 2 shows the energy distributions of the Ar+ and Al+ ions produced in the hollow cathode ion source. The energy distributions were obtained with discharge conditions of 300 V, 35 mA, and 0.5 Torr. The ions with "negative" kinetic energies were formed on the low-pressure side of the extraction aperture through charge-transfer collisions. Two peaks are prominent in the energy distributions: one located near 300 eV, and the other one located near 0 eV. Bondarenkol* has studied the energy distribution of ions of the discharge gas in a hollow cathode ion source and observed that a peak appeared at or near the discharge potential. The peak was due to the ions formed in the negative glow which reached the cathode without losing energy. Davis and Vanderslice13 found that in a glow discharge cell with planar electrodes, most ions of the discharge gas formed in the negative glow region underwent charge-transfer collisions while traversing the cathode dark space to reach the cathode surface. The initial kinetic energies of the ions finally formed were determined by the position where the last charge-transfer collision took place. In Figure 2, the peaks at 300 eV (12) Bondarenko, A. V. Sou. Phys. Tech. Phys. 1976, 21, 1497-1500. (13) Davis, W. D.; Vanderslice, T. A. Phys. Reu., 1963, 131, 219-228.
1892
Analytical Chemistry, V d . 66,No. 11, June 1, 1994
correspond to the ions formed in the negative glow region, which reach the cathode aperture with a kinetic energy corresponding to the discharge potential, i.e., without undergoing charge transfer or an elastic scattering collision, and the peaks near 0 eV correspond to ions formed by chargeexchange near the extraction aperture. In the case of Al+ ions, the high-energy peak was more intense than the lowenergy peak, while the high-energy Ar+ ions are much less intense than the low-energy ions. The difference between the energy distributions of Al+ and Ar+ ions appears to be due to the difference in charge-transfer cross sections. Ar+ ions have a relatively high charge-transfer cross section (- 3 X 10-15 cm2) in collisions with Ar atoms in the energy range under study.14 An Ar+-Ar charge transfer produces an Ar+ ion that will be accelerated through a potential dropdetermined by the point of formation of the ion in the cathode dark space. The absence of significant Ar+ signal at energies much above 0 eV indicates that the mean free path between charge transfer collisionsfor Ar+is much shorter than the extent of the cathode dark space. Al+ ions that undergo charge transfer with Ar atoms are lost from the signal. The data of Figure 2 suggest that the cross section for this asymmetric charge transfer is considerably smaller than the Ar+-Ar charge transfer cross section, as would be expected. The weak Al+ signals seen at energies between 300 and 0 eV, Le., originating in the cathode dark space, may arise from electron impact ionization in this region, from Penning ionization of A1 atoms by Ar' atoms, from Al-AP charge-transfer collisions, or from elastic scattering of the Al+ ions accelerated out of the negative glow. Chapman has shown that some ionization by secondary electrons must occur in the cathode fall region to sustain the discharge.15 The peaks near zero energy appear to result from the high probability that ions formed, or finally scattered, near the exit aperture can escape without further collisions. Figure 3 shows the energy distributions of aluminum dimer (A12+) and aluminum argide (ArAl+) ions, obtained under the same discharge conditions as those in Figure 1. Again two peaks are seen at near 0 eV and 300 eV. For both ion species, the high-energy peaks are much weaker than the lowenergy peak. Although cross sections for asymmetric charge transferinvolving cluster ions and the discharge gas are small, the high-pressure environment (0.5 Torr) favors dissociative charge-transfer or collisional dissociation. King et al.1° observed that the intensities of cluster ions decrease with increasing discharge pressure. The low intensities of the highenergy peaks indicate that dissociative collisions occur with high probability for the cluster ions travelling through the cathode dark space. The high intensities of the low-energy peaks once again could result from formation within the extraction aperture of cluster ions which escaped without further collisions in the low-pressure gas effusing from the aperture. Sampling the High-Energy Ions. Marcus et al. l6 suggested that the differing energies of the argon and metal ions in the hollow cathode plume (near zero energy in Figure 2) offered a means of discriminating against the discharge gas signal. ~~
~
(14) Hasted, J. B. Proc. R. SOC.London, A 1951, 205, 421-438. (1 5) Chapman, B. In Glow Discharge Processes; Wiley: New York, 1980. (16) Marcus, R. K.; King, F. L.; Harrison, W. W.Ana/. Chem., 1986,56972-974. (17) Huneke, J. C. Charles Evans & Associates, Redwood City, CA, personal communication.
10'j
-100
0
100
200
300
400
(a)
30
(b)
n
10'4
-100 0 100 200 300 Initial Kinetic Energy (eV)
27 40 43 54 67 80
Al+ Ar+ A10+ Alz+ ArAl+ Arz+
intensities (counts/s) 4500 V" 4255 Vb 3.62 x 5.12 x 4.83 X 2.51 X 2.66 x 1.07 X
107 109
lo2
lo6 104
108
(b)
30
Table 1. Ion Intendties at Varlous Masses When Sampled Using Different Accelerating Potentials
ions
70
400
Figure 3. Energy distributions of Ai2+ (a) and ArAi+ (b) ions, obtained under discharge conditions of 290 V, 33 mA, and 0.5 Torr.
mass
50 MASS
2.82 X 1.83 X 6.43 X 1.34 X 5.28 X 6.24 x
108 106 10' 103 lo2 103
Ions leaving the extraction aperture with a kinetic energy of 0 eV. Ions leaving the extraction aperture with a kinetic energy of 245 eV. Discharge conditions: -0.5 Torr, 250 V.
It is clear from the present work that discrimination should be much stronger when ions formed in the negative glow region are sampled and also that this technique discriminates strongly against cluster ions. From the ion energy distributions shown above, it is obvious that sampling the high-energy ions, by varying the acceleration potential of the instrument, can reduce cluster ion interferences and the discharge gas signal by 3-4 orders of magnitude. Table 1 shows the intensities of various ions sampled from a high-pressure discharge (-0.5 Torr) with acceleration potentials of 4500 and 4255 V, corresponding to ions having 0 and 245 eV kinetic energy, respectively, as they exit the cathode aperture. The intensity of the Al+ ions was higher by a factor of 8 when the high-energy ions were sampled rather than the low-energy ions; the intensities of the aluminum dimers (A129 at mass 54 and aluminum argide (ArAP) at mass 67 was lower by factors of 1900 and 50, respectively. Note that these two cluster ion species, metal dimers and argides, are the major interferences in the glow
P
50 MASS
70
Figure 4. Mass spectra of an Aluminium Pechiney standard in the low mass range using Faraday cup detection: (a) low-energy ions; (b) high-energy ions. Discharge pressure was -0.5 Torr.
discharge mass spectrum. The results in Table 1 indicate that the interference levels resulting from A12+ and ArAl+ were less than 5 and 2 ppm, respectively, when the highenergy ions were sampled; therefore, the interference level resulting from other clusters or argides, M A P or MAr+ (M is a minor constituent at a concentration less than l%), would be only a few tens of parts per billion (ppb). Figure 4 shows mass spectra of an aluminum standard 579 (Aluminium Pechiney, France) obtained from a high-pressure discharge (-0.5 Torr) with 4500 (a) and 4250 V (b) acceleration potential and Faraday cup detection (minimum signal detectable lo5 counts/s). As expected, the intensities of the Ar+ ions, ArAP, and the Ar2+ dimers were reduced in mass spectrum (b). The intensity of the ArH+ ions at mass 41 was not reduced in Figure 4b. This would be expected, because the ArH+ ion energy distribution is similar to that of the Al+ ions. Collisions in the cathode dark space are much less effective in dissociating clusters in which the constituent atoms differ greatly in mass, becausevery little of the collision energy (10% at most for ArH+) can be transferred into the internal energy of the cluster. In addition to the interferences from the dimers, polyatomic cluster ions were also present in the mass spectrum, although the intensities were much less than those of the A12+ and ArAP ions. Mass spectra of the aluminum standard 579 in the high mass range are shown in Figure 5, recorded with electron multiplier detection and 4500 V (Figure 5a) and 4200 V (Figure 5b) acceleration potentials. The mass spectra contain signals of the isotopes of tin (107 pg/g) and some polyatomic cluster ions formed from a variety
-
Analytical Chemistty, Vol. 66, No. 11, June 1, 1994
1093
a mass resolving power of -3200. In the mass spectrum obtained using low-energy ions, the A12+ ion signal was much stronger than the signal due to 54Crand 54Fe(these elements were present at concentrations of 215 and 3700 pg/g for Cr and Fe, respectively, so that the isotope concentrations were 6 and 21 3 pg/g, respectively; although the 54Crand 54Fepeaks are not resolvable, the signal is mainly due to 54Fe). In the high-energy ion mass spectrum, the intensity of the Al2+ ions decreased to 10 counts/s while the intensity of the (54Cr+ 54Fe) peak remained the same. At low mass resolution, the interference level from the aluminum matrix dimers in Figure 6b would correspond to an impurity concentration -20 ppm or 10% of the 54Fesignal under these discharge conditions, compared to an interference level corresponding to a 2% impurity in Figure 6a when the low-energy ions were sampled. Effect of the Discharge Pressure. When the discharge pressure was lowered to 0.2 Torr, the energy distributions of the Ar+ ions and ArAl+ dimers, obtained under the discharge conditions of 450 V and 40 mA, were as shown in Figure 7. The intensities of the high-energy ions, especially ArAl+, are greater at low discharge pressure. The intensities of the ions in the intermediate energy region were also increased. When operating at low discharge pressures, sputtering of the cathode by the Ar+ ions is more efficient because these ions experience fewer collisions and reach the cathode with higher energies. The production of ions from the sample is thereby enhanced, but the mean free path of ions increases as the discharge pressure is lowered, and therefore destruction of the cluster ions by collision is less probable. For ultratrace analysis, if interference is not an obstacle, it is preferable to run the discharge at low pressure to obtain the highest possible atomic ion signals. When cluster ion elimination is necessary, a high discharge pressure is preferable. Sensitivity. One disadvantage of other techniques used to eliminate cluster ion interferences in GDMS is the concurrent reduction of the analyte ion signals and, consequently, the reduction of the sensitivity. About 70-80% of the analyte signal intensity was lost when the mass resolving power of our instrument was increased from 300 to 3500, which is needed to resolve dimer signals from atomic ion signals below mass 60. In the case of metal argides, much higher resolving power is needed to obtain a resolved mass spectrum. In the present work, sampling the high-energy ions rather than ions formed at the cathode potential resulted in sensitivities for most elements a factor of 10 higher, while interferences were reduced by large factors (Table 3). The Al+/AlAr+ ratio for highenergy ions was a factor of lo4 better than for the low-energy ions and a factor of lo2 better than the Al+/AlAr+ ratio for a source using a pin-type cathode.16 Relative Ion Yields. In quantitative analysis by mass spectrometric techniques, relative sensitivity factors for calibration are often determined using standards. In the present work, the ion yields of the impurity elements in the standards, relative to the matrix Al+ ion, were calculated using the following equation:
-
1io
'
150
130 MASS
170
(b)
110
'
130 MASS
'
I50
'
170
Flgure 5. Mass spectra of an Aluminium Pechiney standard in the high mass range using electron multiplier detection: (a) iow-energy ions; (b) high-energy ions. Discharge pressure was -0.5 Torr.
of combinations of the sample constituents and the discharge gas. The intense peak at mass 133 is probably a Cs+ background signal arising from extensive use of a Cs+ primary ion beam in SIMS operation. Cesium deposited on the grounded immersion lens cover in the ion source is sputtered by the ion beam from the glow discharge source back to the vicinity of the extraction aperture. When the high-energy ions were sampled, the signals of the polyatomic ions and the Cs+ ions were reduced, while the signals of the tin isotopes remained unaffected as shown in Figure 5b. Isotope abundances provide a good measure of the extent of cluster ion interferences. Table 2 lists the isotopic abundances found for tin present in an aluminum standard sample at a concentration of 107 pg/g (-0.5 Torr discharge). For the low-energy ions, the calculated isotope abundances do not agree at all with the known isotope abundances. The interfering ions in this mass range are mostly trimers of the discharge gas, Ar, in all possible combinations of its three isotopes. The isotope abundances of tin calculated from the measured intensities of the highenergy ions are close to the true isotope ratios of the tin. The excess signals of -0.35% at masses 112 and 114 represent interference levels below 1 ppm for these isotopes. High mass resolution studies were also performed to evaluate the effectiveness of sampling the high-energy ions for elimination of interferences. At mass 54 the signals generated from an aluminum standard sample are contributed not only by the aluminum dimer signal but also by 54Crand 54Fepresent in the sample. Figure 6 shows the mass spectra of low-energy and high-energy ions at mass 54, obtained with 1094
Analytical Chemistry, Vol. 66, No. 11, June 1, 1994
-
-
-
relative ion yield = Zx/(ZM[X]j where Zxis the ion intensity of the chosen isotope of the element adjusted to 100% abundance, ZM is the ion intensity of the matrix element, and [XIis the certified concentration of the
Table 2. Comparison of
Isotope Ratios Found for 71n When Low-Energy and High-Energy 1-
mass of isotopes
112 0.97 0.79 1.33
natural abundance (%) low-energy ions high-energy ions a
114 0.65 0.40 1.00
115 0.36 0.30 0.36
116 14.53 0.98 14.38
117 7.68 5.41 7.66
Sampled’
118 24.22 1.57 24.24
119 8.58 21.7 8.55
120 32.59 14.87 32.56
122 4.63 27.92 4.52
124 5.79 26.05 5.40
Tin is 107 pg/g in Pechiney Aluminum A1 standard 579.
A v , 2
2 10 c-‘
=
I 10
54-00
53 :95
54,05
-100
0
100 200 300 400 500
-100
0
100 ZOO 300 400 500
MASS
54 ,‘OO
53:95
MASS
-
Figure 8. Mass spectra at mass 54, obtained with mass resolving power of 3200: (a) low-energy ions;(b) hlgh-energyions. The sample was Aluminium Pechiney standard 67995, which contains Cr and Fe at concentrations of 215 and 3700 pg/g, respectlvely. Discharge pressure was -0.5 Torr.
impurity element in the standard in atomic units (ppma). For elements with more than one isotope, a higher relative ion yield for one isotope indicates the presence of an interference. Table 4 lists the relative ion yields of a few elements obtained with high-energy ions and low-energy ions in a high pressure (-0.5 Torr) discharge. For low-energy ions, a large variation among the isotopes is seen for all the elements. For highenergy ions, much more uniform ion yields were obtained among different isotopes, indicating the reduction of possible interferences by sampling the high-energy ions. The relative ion yield of the 67Znisotope, which has an AlAr+ interference, was reduced from 78 to 2.7 when the high-energy ions were sampled. The elemental concentration of Zn is 740 pg/g and 67Znis 30 pg/g, so this decrease corresponds to a reduction of the interference levels from -0.2% to -50 pg/g of 67Zn for this major argide dimer. Mass 54 contains an Al2+ contribution. In the high-energy signal, this contribution is reduced by a factor of 1000 so that the relative ion yield for 54Feat a concentration of 107 ppma is within a factor of
-
I n i t i a l K i n e t i c Energy (eV) Figure 7. Energy distributions of Ar+ and ArAI+ Ions, obtained under discharge condltlons of 520 V, 17 mA, and 0.2 Torr.
2 of that for 56Fe. Only those isotopes for which there exist significant isobaric interferen~es-~~Cr/~~Fe, 58Fe/58Ni, 64Ni/64Zn-fail to have relative ion yields close to unity in the high-energy ion signal. Even here, the observed relative ion yields correspond within -30% to the predicted signals due to the interferences: the 54Fesignal should be 43 times the S4Cr signal at mass 54 (measured relative ion yield of 54Cr = 44);58Nishould be 11 times the 58Fesignal (measured relative yield of 58Fe= 8.3);64Znshould be 213 times the 64Nisignal (measured relative yield of 64Ni = 195). The average relative ion yields of the impurity elements in aluminum standards 579, 67995, and 11628 are shown in Table 5. The most abundant isotopes of each element were chosen in the calculation, and the ion yields were corrected for the isotopic abundance. Reduction of the molecular ion interferences is demonstrated by the decreases of the ion yields for most of the elements when the high-energy ions were sampled. The high value of Si may be due to interference from N2+ ions, while the high value of Cu resulted from the memory effect from SIMS analyses as mentioned before: a copper test grid is routinely used to optimize SIMS imaging performance so a high copper background signal is typically Analytical Chemistty, Vol. 66, No. 11, June 1. 1994
1895
Table 3. Sensltlvitles (counts/s/ppma) Obtalned with High-Energy Ions (H) and Low-Energy
H L 0
Ions (L)'
24Mg
28Si
4sTi
52Cr
65Mn
=Fe
59C0
60Ni
Wu
BBZn
69Ga
12oSn
121Sb
2MPb
218 20
325 103
375 32
277 27
263 191
256 23
251 40
276 23
721 85
413 71
348 127
225 213
107 86
533 390
The sample was Aluminium Pechiney standard 67995.
Table 4. Examples of Relative Ion Yields of Impurlty Elements In Aluminum Standard
Ti
Cr
Fe
Ni
Zn
mn
Hb
Lc
ma
Hb
LC
mn
Hb
LC
mn
H*
L'
mn
Hb
Le
46 47 48 49 50
0.98 0.96 0.92 0.99 1.7
0.90 0.91 0.82 1.4 38
50 52 53 54
1.8 0.68 0.67 44
40 0.69 1.3 32 860
54 56 57 58
1.1 0.62 0.64 8.26
842 0.51 0.57 5.94
58 60
0.72 0.67 0.70 0.68 195
0.52 0.50 0.89 0.78 208
64 66 67 68 70
1.0 1.0 2.7 0.98
1.1 1.9 78 9.1 18
61
62 64
1.0
m is the isotope of the element. H is the relative ion yield obtained with the high-energy ions. e L is the relative ion yield obtained with the low-energy ions. Concentration of the impurity elements in the standard (pg/g): Ti, 170; Cr, 215; Fe, 3700; Ni, 190; Zn, 740. Table 5. Average Relatlve Ion Ylelds of Impurity Elements Obtalned with High-Energy Ions (H) and Low-Energy Ions (L) Uslng Alumlnlum Pechlney Standards 579, 87095, and 11828'
H L (I
Mg
Si
Ti
Cr
Mn
Fe
co
Ni
Cu
Zn
Ga
Sn
Sb
Pb
0.85 0.53
1.0 7.6
0.81 0.67
0.71 1.1
0.53 5.3
0.62 0.47
0.56 0.97
0.77 0.50
1.6 2.4
1.1 1.5
0.80
0.76 8.6
0.23 16
0.93 14
2.7
At least two sets of data obtained in separate days from each standard were used in the calculations of these relative ion yields.
observed. It is important to point out that the signals of the impurity elements were measured with the electron multiplier, the efficiency of which is less than 100% compared to the Faraday cup used to measure the Al+ ion signals. Multiplier efficiency factors for each element have not yet been determined but are probably -70-90%. The mean of the relative ion yields in Table 5 (excluding Cu) is 0.74 0.06. The measured relative ion yields for most of the elements in Table 5 show relatively uniform sputtering and ionization efficiencies (within f30%) among elements. The results show that a standardless semiquantitative analysis by reversed hollow cathode GDMS is possible and that ion yields are sufficiently reproducible to allow quantitative analysis with the aid of standards.
*
-
CONCLUSION The reversed hollow cathode glow discharge ion source has been shown to have a number of desirable features for glow discharge mass spectrometry deriving from the ability directly to sample ions formed in the negative glow. Ion formation efficiencies in this discharge region appear to be quite uniform, allowing standardless semiquantitative analysis with a typical
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accuracy -*30%. In addition, by sampling only those ions which have traversed the cathode dark space, the ion intensity of the discharge gas species is dramatically reduced by resonant charge exchange processes, and the intensities of many interfering cluster ions are reduced by collisional dissociation. In contrast to techniques for eliminating cluster interferences outside the ion source-e.g., by high mass resolution or collisional dissociation in the mass spectrometer-the atomic ion intensity is not sacrificed in this approach. A final benefit of the approach described here is that it allows rather detailed diagnosis of the formation and destruction processes for a variety of ion species at different locations in the discharge.
ACKNOWLEDGMENT The authors would like to thank Dr. N. A. Thorne, Aluminium Pechiney, France, for providing the aluminum standards used in this study. Received for review December 14, 1993. Accepted March 11, 1994." a Abstract
published in Advance ACS Abstracts, April 15, 1994.
