1108
Anal. Chem. 1986, 58, 1108-1112
This is supported by the observation of ions corresponding to C,H- and C,- a t threshold energy; the intensity of C,Hions is always higher than that of C-, near threshold. These reactions also can account for the observation of ions corresponding to (M - H)- for 2,3-benzofluorene and tryptycene which have acidic hydrogens (17), as discussed earlier.
ACKNOWLEDGMENT We wish to thank K. Jordan and F. Novak for helpful discussions. Registry No. Anthracene, 120-12-7; pyrene, 129-00-0; 2,3benzofluorene, 243-17-4; triphenylene, 217-59-4; benzo[a]pyrene, 50-32-8; tryptycene, 477-75-8; 1,l-binaphthyl, 604-53-5; 3methylcholanthrene, 56-49-5; 1,12-benzoperylene, 191-24-2; 1,2:3,4-dibenzanthracene, 215-58-7; coronene, 191-07-1.
LITERATURE CITED (1) Bartle, K. D.; Lee, M. L.; Wise, S. A. Chem. SOC. Rev. 1981, 10, 113-158. (2) Stenhagen, E.; Abrahamsson, S.; McLafferty, F. W. "Registry of Mass Spectral Data"; Wiley: New York, 1974; Vol. 1-4. (3) Hunt, D. F.; Stafford, G. C.; Crow, F. W.; Russel, J. W. Anal. Chem. 1976, 48,2098-2105. (4) Barofsky, D. F.; Barofsky, E.; Held-Aigner, R. Adv. Mass Spectrom. 1978, 7 ,109-116. (5) Balasanmugam, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem. 1983, 55, 2424-2426.
(6) Scheppele, S. E.; Grizzle, P. L.; Greenwood, G. J.; Marriot, T. D.; Perreira, N. B. Anal. Chem. 1976. 48, 2105-2113. (7) Burrow, P. D.; Ashe, A. J.; Bellville, D. J.; Jordan, K. D. J. Am. Chem. SOC. 1982, 104, 425-429. (8) Buchanan, M. V.; Glerich, G. Org. Mass Spectrom. 1984, 19, 486-489. (9) Gehme, M. Anal. Chem. 1983, 55, 2290-2295. (10) Lida, Y.; Daishima, S. Chem. Lett. 1983, 273-276. (11) Kaufmann, R.; Hiilenkamp, F.; Weschung, R. G. Med. Prog. Techno/. 1979, 6 , 109-121. (12) Hercule, D. M. Pure Appl. Chem. 1983, 55, 1869-1885. (13) Lee, M. L.; Novotny, M. V.; Bartle, K. D. "Analytical Chemistry of Polycyclic Aromatic Compounds"; Academic Press: New York, 1981; pp 269-278 - - - - . -. (14) Lee, M. L.; Hites, R. A. J . Am. Chem. SOC. 1977, 99,2008-2009. (15) Arsenault, G. P. J. Am. Chem. SOC. 1972, 94,8241-8243. (16) Brocklehurst, 6.; Gibbons, W. A.; Lang, F. T.; Porter, G.; Savadatti, M. I . Trans. Faraday SOC. 1966, 62, 1793-1801. (17) Alchala, A.; Tamir, M.; Ottolenghi, M. J. Phys. Chem. 1972, 76, 2229-2235. (18) Popl, M.; Dolansky, C.; Mostecky, J. J . Chromatogr. 1974, 91, 649-658. (19) Chrlstophorou, L. G.; Grant, R. W. Adv. Chem. Phys. 1977, 36, 413-520. (20) Chrlstophorou, L. G.; Goans, R. E. J. Phys. Chem. 1974, 60, 4224-4250. (21) Knox, B. E. Oyn. Mess Specfrom. 1971, 2, 61-96. (22) Pelllzzari, E. D. J. Chromatogr. 1974, 98,323-361. (23) Sowada, U.; Hoiroyd, R. A. J . Phys. Chem. 1981, 85, 541-547. (24) Budzikieicz, H. Angew. Chem., I n t . Ed. Engl. 1981, 20, 624-637.
RECEIVED for review June 21, 1985. Resubmitted December 9, 1985. Accepted December 9, 1985.
Ion Yields of Impurites in Gallium Arsenide for Secondary Ion Mass Spectrometry Yoshikazu Homma* and Tohru Tanaka
NTT Electrical Communications Laboratories, Musashino-shi, Tokyo 180, J a p a n
Relative secondary Ion yields In GaAs are measured for posltive Ions of 11 elements and negative Ions of 10 elements Ar', and C s' bombardment. I t was found that the under 02+, Ion ylelds of negative molecular ions consisting of hnpurlty and matrix elements under Cs+ bombardment are relatively hlgh compared to the monatomlc Ions for 11, 111, and I V group elements in the periodic table. The use of these molecular ions for elements having low electron afflnity in comhatlon with monatomlc Ions for elements having hlgh electron afflnity makes it possible to obtain high sensltivity detection of a varlety of elements. For posltlve secondary ions, Impurity ion yields can be Interpreted by the local thermodynamic equllibrlum (LTE) model. The relative Ion ylelds for 44 elements were calculated by using the LTE model.
Although secondary ion mass spectrometry (SIMS) is a very high sensitivity technique for both surface analysis and bulk analysis, serious problems arise when quantitative measurements are carried out with this technique. Since secondary ion yields strongly depend on the chemical state of the ion bombarded surface, the ion yield of a given element varies from material to material. Therefore, highly accurate quantitative analysis cannot be performed without using calibration standards for each material. With the advancement of the GaAs crystal growth and device fabrication technologies, the quantitation of SIMS analysis has been required for impurities in GaAs crystals. To
perform quantitative analysis of GaAs impurities, ion-implanted GaAs samples ( 1 , 2 )or heavily impurity-doped GaAs samples (3) whose impurity concentrations are determined by chemical analysis have been used as calibration standards. In 1984 a round robin study was held in Japan with the participation of SIMS analysts, spark source mass spectrometry (SSMS) analysts, and chemical analysts using heavily doped GaAs crystals (4).Reliable standard GaAs samples for SIMS were obtained as a result of this study. From a fundamental viewpoint, accurate ion yield data are desirable for the interpretation of the ionization mechanism in semiconductors. Although the ion yields of pure elements (5) and impurity elements in steels (6) and glass and silicate (7) have been intensively studied, few systematic measurements of secondary ion yields in semiconductors have been reported. This is due to a lack of reliable standards for semiconductors. Leta and Morrison's work (1)reporting the use of the ion-implanted standards has been the most intensive report yet published with respect to semiconductors. However, their work lacks transition-metal data and information regarding negative ion yields under cesium ion bombardment, both of which are important in GaAs impurity analysis. Therefore, the round robin GaAs standard samples will offer useful data concerning secondary ionization. In this study, we will present the secondary ion yield for various elements in GaAs by using the standardy samples obtained through the round robin study. We also test the validity of the thermodynamic model based on the SahaEggert equation proposed by Andersen and Hinthorne (8,9)
0003-2700/86/0358-1108$01.50/00 1986 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
Table I. Impurity Concentrations (atoms/cm3) in Doped GaAs Samples element
D1
D2
D3
B
1.6 X l0ls
A1
1.2 x
Si Cr Mn Fe Cu Zn Se In
1.5 X lo1* 3.6 X 10l7 1.5 X 10" 2.0 X 10l6 1.8 X 10l6 3.3 X 1017 1.2 X
1017
1OI7
element
2.1 X 10l6
1.8 X lo1? 2.5 X 10l6 2.5 X 10l6 2.8 X lo1' 4.3 x 1017
5.6 x 1019
IMS 3F 10.5-keV Ozt 10.5-keV Art
B A1
7.3 X 10' 3.9 x 103
Si Cr Mn Fe
1.9 X lo2
3.0
X
Zn In
"The ion intensity, abundance.
sample
element
energy, keV
dose, ions/cm2
I1 I2 I3
C 0 S
100 150 300
1 x 1014 1 x 1014 1 x 1013
for interpretation of ion yields in GaAs. The positive ion yields for 44 elements have been calculated based on this result.
EXPERIMENTAL SECTION Impurity concentrations of doped GaAs samples used for ion yield measurements are listed in Table I. These samples were cut from the crystals grown by the liquid encapsulated Czochralski (LEC) method. The impurity concentrations in samples D1, D2, and D3 were determined by chemical analysis in the round robin study. In addition to these samples, an In-doped sample whose In concentration was determined by inductively coupled plasma emission spectrometry (ICP) was used. The accuracies of these determined concentrations in Table I are within f30% ( 4 ) . Chemical analysis for trace C, 0, and S is difficult to perform in GaAs. For these elements, ion-implantedLEC-GaAs samples were employed. The ion-implantation conditions are given in Table 11. Since the transmission efficiency of the secondary ion mass spectrometer was not known, the relative ion yields normalized to the matrix (As+ or Ga-) ion yield were measured. The secondary ion yields were determined by using the ratio of secondary ion intensity to the concentration in the doped GaAs samples. The ion intensities are corrected for natural isotopic abundances in this procedure. For the ion-implanted samples, a depth profile of the impurity was measured and the impurity ion intensity was integrated with respect to depth. The ratio of the integrated ion intensity to the ion dose was used to determine the secondary ion yield. Two types of SIMS instruments were used for secondary ion intensity measurements. One was a double-focusing ion microscope instrument, Cameca IMS 3F, and the other was a quadrupole-based scanning ion probe, A-DIDA 3000. Although measurements were mainly performed with the IMS 3F, the A-DIDA was also used for the comparison of the instrumental bombardment. factor in the positive secondary ion yield under 02+ For the positive ion measurements with the IMS 3F, primary and Ar+ with approximately ions used were mass separated 02+ 60" incidence angle with respect to the sample surface. The primary ion energy at the sample was 10.5 keV with an ion current of 1-2 pA. The primary beams were raster scanned to erode a 250 X 250 pm area. Secondary ions were detected from the center of the rastered area of 60 pm in diameter. The accelerating potential of the secondary ions was 4.5 kV. Secondary ion measurements were performed under a low-mass resolution, M / AM 300. The 400-pm contrast diaphragm was mainly used for maximum sensitivity. For the In detection, the 60-pm diaphragm was used. In the A-DIDA 3000, mass separated, 10-keV 02+primary ions were used with an approximately normal incidence angle with respect to the sample surface. The primary ion current was 0.6 FA, and the beam raster scanning area was 600 x 600 pm, Secondary ions were detected from the center region (180 x 180 pm)
lo3
1.1 x 103 lo2 lo2
3.8 X 2.8 X 3.8 X 4.4 x
cu
Table 11. Ion-Implanted GaAs Samples
-
Table 111. Relative Ion Yields of Positive Secondary Ions in GaAs (IM+/CM)/(IA.+/CA.)"
D4
5.3 X 10l6 3.1 X 10l8 3.6 X 1OIs
1109
2.5 1.0 x 103
3.0 8.4 X lo1 2.9 x 101 7.8
10'
2.1 x 102 3.1 x 103 2.7 X lo2 3.8 x 103 2.3 x 103 1.3 x 103 1.9 x 102 8.0 X 10'
5.2 x 102
103
IMt,
A-DIDA 10.0-keV 02+
is corrected for the natural isotopic
Table IV. Comparison of Relative Matrix Ion Yields and Molecular Ion Yields between 02+ and Art Bombardmentsn ion Ast Gat AsO+
AS^+
GaAst GaOt Ga2+
10.5-keV 02+ 1 1.7 x 103 6 X lo-' 1 x 10-1 1 8 X 10-1 2 x 101
10.5-keV Art 7.8 X lo-' 6.7 X lo2 2 x 10-4 4 x 10-1 2 2 x 10-4 2 x 102
"All of the yields are normalized to the Ast yield under 02+ bombardment and corrected for the natural isotopic abundance. Measurements were performed with the IMS 3F. of the rastered area with an electronic gating. The accelerating potential of the secondary ions was 200 V. Secondary ion measurements were performed under the constant AM mode of the quadrupole mass filter. For the negative ion measurements with the IMS 3F, massseparated Cs+ primary ions were used. The primary ion energy was 14.5 keV with an ion current of 0.2 FA. Other experimental conditions were the same as those for the positive ion detection. The sample chamber pressure during measurements was (2-5) X lo-' Pa in both of the instruments.
RESULTS AND DISCUSSION Ion Yields. The relative ion yields for positive secondary ions normalized to the As+ yield are given in Table I11 in comparison with the yields obtained with different primary ion species and instruments. The Ar+ bombardment shows relatively low ion yields for all of the impurities. The differences in the ion yields obtained with the two instruments under 02+ bombardment are small except for In. The relatively small In+ yield obtained with the A-DIDA may be due to the transmission efficiency reduction of the quadrupole mass spectrometer for high masses (IO). The mass discrimination of the mass spectrometers are not corrected for the ion yield data in Table 111. To compare the absolute ion yields for the 02+ and Ar+ bombardments, relative matrix ion yields corrected for the sputtering yield of each primary ion are listed in Table IV. Since Ga+ intensity was very large, simultaneous measurements of Ga+ and impurity ions cannot be performed. Thus, matrix ions were measured at a low sensitivity condition by using a 20-pm contrast diaphragm in the IMS 3F secondary ion optics. The matrix molecular ion yields are also listed in Table 1V. The Ga+ and As+ yields are almost the same value for both of the primary ions. This result i r licates that the oxygen effect on the ion yield enhancement (11)that is observed in metal or Si does not appear for the GaAs matrix ions. This is consistent with the experimental result that the matrix ion and impurity ion intensities in GaAs were not affected by
1110
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
Table V. Relative Ion Yields of Negative Secondary Ions in GaAs under 14.5-keV Cs+ Bombardment ( Z M + / C M ) / ( I G ~ - / C G ~ ) or (ZXM-/CM)/(IG~-/CG~)' element B C 0 A1 Si S Cr Mn Se In
M7.9 2.6 x 1.4 x 8.5 1.0 x 3.9 x 5.4 8.5 X 2.6 x 1.6
103 104 103 104
lo-' 104
GaMNDb 2.3 x 1.0 x 9.5 3.7 x 2.4 x 1.0 1.7 1.2 x 4.6
AsM1.1 x 3.0 x 9.8 x 1.6 x 2.8 x 7.6 x 1.6 X 3.2 X ND 1.7 x
101
104 101 104
104
104 104
ZA+ / Z A O
=Z B + / Z B O exp(-
Ei,A - Ei,B
kT
)
0-
-1
-
0
101
N
u
103 104 103 10' 10'
\
t
-2 -
-E Y
102
oxygen introduction into the sample chamber of up to 5 x Pa. On the other hand, the Si and impurity ion yields increased by 2-3 orders of magnitude when oxygen was introduced under Ar+ bombardment (12). However, the presence of the oxygen effect on the impurity ion yields in G A S cannot be denied, since the ion yields under 02+ bombardment are larger than those under Ar+ bombardment for impurity elements. This seems to contradict the result whereby no oxygen introduction effect was observed under Ar+ bombardment. One explanation for this contradiction is the difference in the sticking coefficients for oxygen between Si and GaAs. The oxygen sticking coefficient on G A S is of the order of lo4 (13), while that on Si is of the order of 10-1 (14).Thus the surface oxygen concentration on G A S is smaller than that on Si under oxygen introduction. On the other hand, the surface oxygen concentration becomes high enough to enhance the impurity ion yields because of oxygen implantation into GaAs under 02+bombardment. Relative ion yields for negative secondary ions normalized to the Ga- yield are given in Table V. The measurements were performed by using the IMS 3F under Cs+ bombardment, As is well-known (5), negative ion yields are enhanced by Cs+ bombardment. High ion yields are observed for 0, S, and Se in GaAs. Furthermore, high ion yields are also observed for molecular ions consisting of an impurity element and a matrix element. The high ion yield of molecular ions enables high-sensitivity detection of elements that have low negative ion yields. As shown in Table V, the ion yield of molecular ions for B, C, Al, Si, and In are 1-3 orders of magnitudes higher than those of atomic ions. These elements belong to the I11 and N groups in the periodic table. The covalent bond with the matrix element may contribute to the stable molecular ion formation. For transition metals such as Cr and Fe, the molecular ion yields are relatively low. From the practical viewpoint, these molecular ions are very useful for simultaneous high-sensitivity analysis of multielements. For example, an acceptor dopant, Be and a donor dopant Se can be measured a t the same time by using BeAsand Se-, while Be+ detection under 02+ bombardment and Se- detection under Cs+ bombardment are needed if monatomic ion detections are used. Interpretation of Ion Yields Using LTE Model. The positive ion yields are analyzed with the local thermodynamic equilibrium (LTE) model. The concentration ratio of neutral atoms to singly charged ions is described by the Saha-Eggert ionization equation in the LTE model. The ionization ratio of two elements, A and B, can be expressed by (6,14)
/
-:: u L
'The ion intensity, Zw or IXM-, is corrected for the natural isotopic abundance. ND, no detected.
NA+ NAo NB+/NBo
1-
(1)
AS 0
-4
I
5
1
I
6 7 8 9 1 0 IONIZATION POTENTIAL (eV)
'
Figure 1. Plots of log ( m 1 ' 2 1 + / ( ~ Z o / Zvs. + ) )ionization potential for , ' bombardment in comparison with two elements in GaAs under 0 types of instruments, IMS 3F and A-DIDA.
where NM+and Nw represent the concentrations of ions and neutral atoms for each element (M = A or B), ZM+and ZMO are the internal partition functions of the ions and neutral atoms, Ei,M is the first ionization potential, k is the Boltzmann constant, and T is the temperature parameter. NMO can be replaced by the atomic concentration, cM, provided that the ionization yield is low. For NM+, the measured ion intensity, IM+, corrected by the natural isotopic abundance can be substituted. However, it should be noted that IM+ measured by an electron multiplier depends on the mass of the measuring ions. For the correction of this mass discrimination, we multiplied I M + by rnM1l2 (15),where mM is the mass of the element M. Therefore, eq 1 can be written by
Thus, log (~A'/~IA+/(cAZAO/ZA+)) vs. Ei,A should follow a straight line. For the positive ions in GaAs, the partition functions and the parameter T were determined. The partition functions were calculated by using the polynomial approximation presented by de Galan et al. (16)for a given T. Computer iteration was performed until the T value used for the partition function calculation coincided with that derived from the least-squares fit of the eq 2. Since the behavior of impurity ions and matrix ions seemed to be different, as mentioned above, the matrix data were not used in the T value determination procedure. The In datum of the A-DIDA was also excluded in this procedure, because it is strongly affected by the mass discrimination of the quadrupole mass filter. Results for data measured by the IMS 3F and the A-DIDA are shown in Figure 1for 02+ bombardment. All of the data are normalized to the Cr value. The points of all impurity elements lie fairly well on straight-line plots. Even the largest deviations from the straight line for Cr and Mn measured by the A-DIDA are within a factor of 2. The relatively larger scatter of the A-DIDA data may be due t o the mass discrimination of the quadrupole mass filter. This result shows that the Saha-Eggert equation effectively explains the ionization of impurity elements in GaAs. However, the deviation from the straight line of As is very large compared to those of impurity elements. With respect to Ga, although the data are plotted in the figure by using the results given in Table IV, the data reliability has not been confirmed, since Ga+ and impurity elements could not be measured under the same condition.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1111
Table VI. LTE Calculation of Positive Secondary Ion Yields in GaAs element Li
Be B C N Na Mg A1
Si P
S K Ca
sc
Ti V
Cr
Mn Fe co Ni
cu
calcd yield Y, 7.7 x 1.1 x 6.7 X 6.7 X 3.0 x 7.2 x 1.9 x 4.4 x 2.0 x 9.3 x 8.0 X 2.4 X 2.9 x 3.0 x 4.6 X 7.0 x 2.6 x 9.0 x 5.9 x 3.7 x 2.5 X 2.2 x
104 102 10' lo-' 10-3 104 103
103 102
measd yield Y,
(Y, - Y,)/Y,
-0.08
Nb 3.9 x 103 1.9 x 102
Mo Ag 0.13 0.05
lob
104 103
Rb Cd In Sb Te CS
Ba Ta
lo9
102 lo2 102
Se Br
loo
102
3.7 x 2.0 x 6.2 X 1.9 x 2.4 x 1.3 x 2.7 X 5.8 X 6.2 X 4.3 x 3.2 X 1.7 X 2.6 x 7.5 x 1.3 X 7.1 X 2.7 X 7.0 X 2.7 X 5.1 x
Ge 7.3 x 10'
W 3.0 x 103 1.1 x 103 3.8 X lo2 2.8 X lo2
-0.13 -0.18 0.55 -0.21
The T values are different for the two instruments; Le., the value for the A-DIDA is larger than that for the IMS 3F. This result can be explained by the difference in the incidence angles in these instruments. The -60' incidence of the primary ion in the IMS 3F results in a higher sputtering yield compared to the normal incidence in the A-DIDA. Since the surface concentration of implanted oxygen at equilibrium is approximately inversely proportional to the sputtering yield (17), a higher oxygen concentration is expected for the ADIDA. A high oxygen concentration causes the higher T value (6,18).The ion yield difference in the impurity elements is relatively small in the A-DIDA because of the higher T value. The same plot for the data measured under Ar+ bombardment is shown in Figure 2. In this case also, the points of all impurity elements lie on a straight line, while that of As deviates from it. It should be noted that the As+ yield is large in comparison with the impurity element yields, in contrast to the relatively lower As+ yield under 02+bombardment. One explanation for the large deviation of As from the straight lines may be its tendency to form molecular ions. As shown in Table IV, the intensities of AsO+ and GaAs+ are comparable to that of As+. When oxide molecular ion intensity, IMot, is appreciable relative to atomic ion intensity, IM+, the IMt is shown to deviate from the Saha-Eggert equation (6, 18). This is due to the fact that MO+ is formed at the expense of M+. In addition, it is considered that the formation of GaAs+ also helps to reduce As+ formation. On the other hand, Icao+and I G d s t are on the order of with respect to IGat. However, it seems difficult to explain a deviation of As as large as 1order of magnitude merely on the basis of the molecular ion formation. The molecular ion to As ion ratio (IABo+ + IG&+)/Ihtis 1.6, which is not as high a value as was observed for elements such as Ti, V, Nb, and U (6,18).Besides this, it cannot explain the relatively large ion yield of As under Ar+ bombardment. Another explanation is that the ionization mechanisms are different for the matrix elements and impurity elements. This idea is based on the results which show that the absolute ion yields of the matrix elements are almost the same both for Ar+ and 02' bombardment as given in Table IV. Since the ion yield enhancement effect of oxyged was not observed for the matrix elements, in contrast to the impurity elements, a Ga-As dissociation process may be dominant in the matrix ion formation. In this situation, the same T value applied to
measd yield Y ,
calcd yield Y,
Zn
10-1
103 103 102
element
Pt AU
Hg
T1 Pb Bi
3.8
10' 102
( Y , - Y,)/ Y,
10'
-0.03
4.4 x 103
-0.02
X
loo lo-' 103 103 lo2 lo6 lo1 103 10' 10' 105 104
lo2
lo1
10' loo loo 103
1.0 x 103
1.9 x 102
--
0.
L
2
-
-1.
0
N 0
\
5
0
-2-
AS
E
Y
-
0 pl
T=4000 K
-3-
-4
I
5
I
\ \
8 9 10 IONIZATION POTENTIAL (eV)
6
7
1
Flgure 2. Plots of log (rn"21+l(~ZoIZ+)) vs. ionization potential for elements in GaAs under Ar+ bombardment in IMS 3F.
the impurity elements cannot be used for the matrix elements. From the viewpoint of quantitative analysis, the usefulness of the LTE method is reduced by the difference in behavior between matrix and impurity elements. However, if use is restricted to impurity elements, the LTE method will provide fairly good quantitative results. To provide for a quantitative data base, the iori yields of various elements in GaAs are calculated by the LTE model. The T value determined for the IMS 3F data set was used in this procedure. Table VI presents the calculated relative ion yields for 44 elements in comparison with the measured yields for nine elements shown in Table 111. For practical use, the LTE data are normalized to the As+ yield, Y e , and presented in the form of measured yield containing the mass discrimination of the electron multiplier, that is, C m ~ - " ~ ( z M + / zexp(-EM/kT)/ ~) Yh+. The proportional constant, C, w t determined so as to give the best fit for the measured ion yields presented in Table 111. For the quantitative analysis, the sensitivity can be obtained simply by multiplying the relative ion yield by the As concentration (2.25 X loz2a t m s ~ m - in ~ )GaAs. A comparison of the measured ion yields and the calculated ion yields for the nine elements shows that the agreement between these values is fairly good. The largest difference is 54% for Fe. For the other elements, the difference is within the accuracy of the present standard samples, i.e., f30%.
1112
Anal. Chem. 1988, 58,1112-1115
Therefore, it can be expected that the calculated ion yields will accurately represent the acutal ion yields for many of the elements listed in the table. It should be emphasized, however, that the LTE calculation gives impractical yields for some elements. For example, it provides too low yields for low ionization potential elements such as F and C1, and for Sn it produces a yield 2 orders of magnitude higher than that obtained by Leta and Morrison ( I ) . These apparently incorrect data have been excluded in Table VI. Other incorrect cases are naturally expected. In general, the accuracy of the LTE is considered to be within a factor of 2 (6, 7,19). However, the relative ion yields can vary depending on the instrument used and the tuning condition of the instrument transmission. Thus, the ion yield set presented in this work cannot by itself provide sufficiently accurate quantitative values. It can, however, provide foreknowledge of the ion intensities in GaAs or the semiquantitative values for elements without calibration standards. In this context, LTE calculation can be thought of as being effective for practical use.
ACKNOWLEDGMENT We are indebted to S. Hattori and S. Kurosawa for their valuable discussions apd encouragement in this work. Registry No. Li, 7439-93-2; Be, 7440-41-7;B, 7440-42-8; C, 7440-44-0; N2, 7727-37-9; Na, 7440-23-5; Mg, 7439-95-4; Al, 7429-90-5; Si, 7440-21-3;P, 7723-14-0;S, 7704-34-9;K, 7440-09-7; Ca, 7440-70-2; SC, 7440-20-2; Ti, 7440-32-6; V, 7440-62-2; Cr, 7440-47-3; Mn, 7439-96-5; Fe, 7439-89-6; Co, 7440-48-4; Ni, 7440-02-0; Cu, 7440-50-8; Zn, 7440-66-6; Ge, 7440-56-4; Se, 7782-49-2; Bra, 7726-95-6; Nb, 7440-03-1; Mo, 7439-98-7; Ag, 7440-22-4; Rb, 7440-17-7; Cd, 7440-43-9; In, 7440-74-6; Sb,
7440-36-0; Te, 13494-80-9; Cs, 7440-46-2; Ba, 7440-39-3; Ta, 7440-25-7; W, 7440-33-7; Pt, 7440-06-4; Au, 7440-57-5; Hg, 77827439-97-6;T1, 7440-28-0; Pb, 7439-92-1; Bi, 7440-69-9; 02, 44-7; GaAs, 1303-00-0.
LITERATURE CITED Leta, D. P.; Morrison, G. H. Anal. Chem. 1980, 52,514. Homma, Y.; Ishii, Y.; Kobayashi, T.; Osaka, J. J. Appl. Phys. 1985, 57,2931. Kurosawa, S.;Homma, Y.; Tanaka, T.; Yamawaki, M. I n "Proceedings of the Fourth International Conference on Secondary Ion Mass Spectrometry SIMS-IV"; Benninghoven, A., Okano, J., Shimizu, R., Werner, H. W., Eds.; Springer: Berlin, 1964; pp 107-109. Homma, Y.; Kurosawa, S.;Yoshioka, Y.; Shibata, M.; Nomura, K.; Nakamura Y. Anal. Chem. 1985, 57,2928. Storms, H. A.; Brown, K. F.;Stein, J. D. Anal. Chem. 1977, 49, 2023. Morgan, A. E.;Werner, H. W. Anal. Chem. 1976, 48, 699. Morgan, A. E.; Werner H. W. Anal. Chem. 1977, 49, 927. Andersen, C. A.; Hinthorne, J. R. Science (Washington, D.C.) 1972, 775, 853. Andersen, C. A.; Hinthorne, J. R. Anal. Chem. 1973, 45, 1421. Ehlert, T. C. J. Phys. €1970, 3, 237. Wittmaack, K. I n "Inelastic Ion-Surface Collisions"; Tolk, N. H., Tully, J. C., Heiland, W., White, C. W., Eds.; Academic Press: New York, 1977; pp 153-199. Homma, Y.; Tanaka, H.; Ishii, Y. I n "Proceedings of the Fourth International Conference on Secondary Ion Mass Spectrometry SIMS-IV"; Benninghoven, A., Okano, J., Shimizu, R., Werner, H. W., Eds.; Springer: Berlin, 1984; pp 98-100. Pretzer, D. D.; Hagstrum, H. D. Surf. Sci. 1986, 4, 265. Ibach, H.; Horn, K.; Dorn, R.; Luth, H. Surf. Sci. 1973, 38, 433. Shimizu, R.; Ishitani, T.; Ueshima, Y. Jpn. J. Appl. Phys. 1974, 73, 249. de Galan, L.; Smith, R.; Winefordner, J. D. Spectrochim. Acta, Part 6 1968, 23.521. Liebl, H. J. Vac. Sci. Technol. 1975, 72, 385. Morgan, A. E.; Werner, H. W. Surf. Sci. 1977, 65,687. Ramseyar, G. 0.; Morrison, G. H. Anal. Chem. 1984, 55,1963.
RECEIVED for review August 8,1985. Accepted December 9, 1985.
Emission Spectroscopic Studies of Sputtering on Silver-Copper Alloy Surfaces Kazuaki Wagatsuma and Kichinosuke Hirokawa*
The Research Institute for Iron, Steel, and Other Metals, Tohoku University, Sendai 980, Japan
Glow discharge Sputtering of Ag-Cu alloy surfaces was studied by optical emission spectrometry. The change in the intensity ratio between Ag and Cu emission llnes was monltored durlng the course of sputterlng. The metallurgical structures (fine partlcles of the eutectoid and coarse grains of the primary crystallltes) played an Important role In determining the length of transftion perlods before steady-state sputtering. A reduction in the overall sputtering yields wlth sputter time, probably due to the coating of Cu atoms onto the Ag grdns, was also observed.
Glow discharge emission spectrometry (GDS) using a Grimm-type light source ( I , 2 )is recognized as a useful and powerful technique for qualitative and/or quantitative surface analysis (3-6). Cathode sputtering in the discharge tube is basic for understanding how sample atoms are introduced into the glow discharge plasma. If an alloy surface is subjected to ion bombardment, the different components are sputtered
at different rates. This effect is called preferential sputtering (7). Preferential sputtering leads to a change in the surface composition and the atomic density of the sputtered elements in the plasma. Sputtering of many binary alloy systems has been studied, mainly with Auger electron spectroscopy (7-9)) and these investigations experimentally show the altered layers on the alloy surfaces. These surface modifications would also influence analytical results obtained with GDS. Therefore, observation of emission intensities from the different components in multiphase systems provides information about glow discharge sputtering mechanisms. Ion bombardment of multiphase targets shows much more complicated behavior than occurs with single-phase materials. The crystallites materials are heterogeneous. The crystallities in such structures represent different compositions, different metallurgical phases, and various sizes. While sputtering of homogeneous materials proceeds rapidly under steady-state conditions and altered layers are formed on the surfaces, sputtering of multiphase alloys is marked by a long transition time before steady-state conditions are reached ( I O ) , and such
0003-2700/86/0358~1112$01.50/0 0 1986 American Chemlcal Society
