Quantitative Analysis of Low Alloy Steels by Secondary Ion Mass Spectrometry A. E. Morgan*' and H. W. Werner Philips Research Laboratories, Eindhoven, The Netherlands
Positive secondary ion currents from oxygen or argon ion bombardment of iron standards have been measured. In either case, the singly-charged monatomic ion currents I,+ may increase when oxygen gas is introduced into the bombardment chamber, but both the amount of the increase and the oxygen pressure dependence can vary dramatically from one element to another in the same sample. When sufficient oxygen is present to saturate every I,+, these currents can be corrected by a simple procedure based on Andersen's approach to obtain the concentrations of most elements to within a factor of 2 of their actual values; deviations of the remaining elements are discussed and remedies proposed. The parameter T, needed for this correction procedure, is independent of primary ion current density and equals 6300 K for 0 2 + or Ar+ bombardment at 5.5 keV in sufficient oxygen gas; this value apparently increases if only higher energy secondary ions are monitored. The meaning of this parameter and the origin of the oxygen enhancement effect are discussed.
Although the SIMS technique has strong potentialities for thin film and surface analysis, particularly with respect to its low detection limits, the major drawback is the difficulty of quantitative analysis without using external standards of similar chemical composition for establishing a calibration curve. This arises from the common observations t h a t t h e ion yield from one element to another can vary by orders of magnitude, and that the ion yield of a given element can differ substantially from one compound to another (matrix effect) and be severely influenced by t h e chemical nature of the bombarding species (chemical effect) ( I ) . Thus, the yield of positive secondary ions may be increased by orders of magnitude by bombarding with an electronegative gas or by merely adsorbing this gas onto t h e sample surface (2).Conversely, the choice of an electropositive bombarding species can dramatically increase the negative secondary ion yield ( 3 ) .These effects may conspire to make the relative intensities of the singly-charged atomic ions, released by bombardment of a solid, a poor reflection of the actual composition of the solid. Further, although significant yields of multiply-charged ions are generally not observed, the presence of molecular (cluster) ions often leads to coincidences in the mass spectra, which can complicate the measurements of atomic ion intensities, and may indicate t h a t consideration of the atomic ions alone is insufficient for a quantitative analysis. For our first investigations into these quantitative aspects, we chose an NBS low alloy steel standard (NBS 461) because it contains a large number of elements with an extended range of first ionization potentials. We have made attempts to relate the measured intensities of the positive, monatomic, singly-charged secondary ions to the known composition by means of an empirical correction procePermanent address, Philips Forschungslaboratorium Hamburg GmbH, 2 Hamburg 54, W. Germany.
dure, using spectra measured with various instruments, primary ion energies and current densities, bombarding species, secondary ion acceptance energies, and residual gas conditions. T h e conclusions thus derived were checked by measurements with other iron ingot standards (NBS 465 and 466).
PRINCIPLE OF QUANTITATIVE ANALYSIS T h e ion current measured with the mass spectrometer (and corrected for isotopic abundance) I M + of positive, monatomic secondary ions M+ of a certain element M present in a sample can be considered as the product of five parameters: zM+= TM+ 1,'s
*
CM
(1)
/3M+
TM+ is the overall instrumental efficiency (viz, the fraction of M+ secondary ions that emerge from the sample which is actually monitored), I , is the primary ion current and S is the sputter yield of the sample (total number of emitted particles per primary ion). From dimensional considerations, the next term should be the total number of all Mcontaining species emitted per unit time, n ~divided , by the total number of emitted particles per unit time, En. As shown in the Appendix, this ratio is the same, however, as the fractional concentration of element M present in the sample, C M ,(number of atoms per unit volume divided by the maximum possible number per unit volume) even if selective sputtering should occur. T h e final term, PM+, is the degree of ionization of the element, being the number of M+ ions emitted from the sample per unit time, n ~ +divided . by n ~This . latter term is given by the sum of the numbers of all M-bearing species emitted (ignoring the relatively small number of multiplycharged ions),
n~ =
+
~ M On
+
+
+~
~ +n ~ -I Z M M ~ O
+
M M ~ .+.
.
(2)
i.e., considering the sputtering of all neutral, positive and negative monatomic, diatomic, etc. species; MI refers to a second element, either present in the solid or introduced by the bombardment. Thus nM+ = pM+= -
nM
""'( n
MO
+ +
nM0
n ~ I -~ ~ M O
+..
I ~ M M ~ +
M M ~ O
n ~ o
.)-I
~ M O
(3) For an initial evaluation of our data, we shall assume t h a t EM
IzMO
(4)
Le., we shall consider only the first term inside the parentheses of Equation 3; the second term might become significant for easily ionizable elements or for ionic matrices, the third term for high electron affinity elements, the next terms for strong bonding between elements, etc. Various models have been proposed for estimating &+(412). We shall consider only the semi-empirical approach of Andersen ( 4 , 5 ) since the values of the physical parameters ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
699
entering into this calculation are fairly well established so as t o offer the possibility of practical utilization of the method. T h e basic assumption is t h a t the majority of singly-charged ions are actually generated inside the solid selvedge. Obviously, only a proportion, p(M+), of the total number of ions formed per unit time within t h e escape depth of t h e solid, n ' ~ +may , leave t h e solid without being neutralized: nM+ =
p(M+)n'~+
(5)
T h e escape probability of M+ is otherwise assumed to be t h e same as for MO. T o calculate the probability of ion formation, we shall explore the use of the following expression which is similar to t h a t utilized by Andersen and Hinthorne ( 4 ) ;
2) T h e escape probability of a n ion p ( M + ) is also assumed to be the same for every element present in t h e solid. However, if neutralization is effected by Auger type transitions or by resonance tunnelling of electrons from t h e solid to the departing ion, then p(M+) is given (15) by exp(-Alaa), where A is the transition rate a t t h e surface, a is a characteristic distance from the surface, and u is the velocity of the ejected ion normal to the surface. This would mean that, for a given energy, lighter ions would have a greater escape probability than heavier ions. However, quantitative use of this formula would require knowledge of the energy distributions of t h e ions within the solid. Therefore, dividing Equation 7 by Equation 8, and rearranging,
- I M + [ZMo(T)/ZM+(T)]exp(EM/hT) where Z represents an internal partition function (which is a function of temperature), E M is the first ionization potential of the element M, h is the Boltzmann constant, and T is a parameter (whose meaning will be discussed later) with the dimension of temperature ( 1 3 ) . Use of this expression attributes variations in the secondary ion yields of the elements to differences in E M and (to a lesser extent) in ZM+/ ZMO,and allows for matrix effects through the parameter T . T h e proportionality constant K (which might also contain exponential terms involving h T ) is assumed not t o differ from one element t o another; its value need not interest us here since we have not measured, and for the present purposes are not concerned with, absolute secondary ion yields. Andersen and Hinthorne ( 4 ) in fact specify a value for K by adopting t h e Saha-Eggert equation as t h e complete expression for t h e ion to neutral ratio; this equation pertains to equilibria of the form Mo + M+ e and also allows for matrix effects through a n additional parameter, viz, the electron density. Inserting Equations 3-6 into Equation 1, we arrive a t
+
IM+=?)M+.I~'S.C~I.~(M+). K [ z ~ + ( T ) /Tz) ~ ]exp( o (-Ehl/hT)
(7)
We may write an analogous expression for the reference element R of the sample:
IR+= )IR+ - I , * S * C R * p ( R + ) K[ZR+(T)/ZRO(T)I exp(-E:a/hT)
(8)
T o proceed further, we shall for simplicity adopt two further approximations. 1) T h e instrumental efficiency )IM+ is assumed to be element-independent. This quantity is determined in our apparatus (Cameca IMS 300) by three main factors: a) T h e filtering effect of t h e collecting optics, which depends on the initial energy and emission angle of a secondary ion. For initial energies below 1 eV, the collection efficiency is 100%, but, a t higher energies, t h e collection is more and more restricted t o ions emitted in directions nearer to the target normal. b) T h e energy bandpass of the mass spectrometer, which is generally limited to 20 eV to reduce peak broadening and, hence, t o improve mass resolution. c) Possible mass discrimination by the spectrometer, due t o transmission effects and to detection and measurement with an electron multiplier whose response in the DC mode utilized depends ( 1 4 ) somewhat upon the mass, energy, charge, and chemical nature of the impinging ion. In summary, we see that )I can vary from one element t o another, reflecting differences in atomic masses and in secondary ion energy distribution functions, but this variation will be ignored. 700
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
(9)
I R + [zRo(r)/zR+(T)]eXp(ER/hT) Le., correction of the measured currents of singly-charged, monatomic, positive secondary ions by the amount [ZMO(T)/ZM T+ ) ](exp(EM/hT) leads directly to relative elemental concentrations. It is our aim to test the overall validity of this equation by measuring I M +from a given standard under various bombardment conditions; a plot of log(IM+ZMO(T)/CMZ,+(T))against EM should then be a straight line whose slope would be determined by the parameter T . T h e physical constants ZMO(T),Z M + ( T )and Ehf may be found from the literature. We shall briefly recapitulate the factors which have been ignored in the derivation of Equation 9 and which may have t o be considered since they may not be the same for every element in the standard; a) instrumental efficiency, b) escape probability of ions from the solid, and c) appreciable sputtered concentrations of atomic ions and polymeric species compared to atomic neutrals. CR
EXPERIMENTAL The vast majority of the measurements were performed with the Cameca IMS 300 instrument ( 1 6 ) . The primary ions used were either Os+, Ar+ or 0-. Their nominal energy at impact was 5.5 keV for the positive species and, although the normal operating mode leads to 14.5 keV for 0 - , it was possible to adjust the primary ion accelerating voltage so that the latter energy could be varied between 10 and 15.2 keV. The nominal incident angle of 4 5 O changes somewhat with the selected primary energy, leading to more normal incidence for 14.5 keV and to more glancing incidence for 5.5 keV primary ions. The maximum primary ion current density available was several mA/cm2. The energy bandpass of the mass spectrometer was usually taken as 0-20 eV; increase of the upper energy limit had little effect on either the absolute or relative secondary ion currents. In addition, the position of a band of chosen width along the energy scale could be adjusted by means of the grid filter included in the instrument. The non-bakeable apparatus could be evacuated to the low lo-; Torr range during bombardment. Oxygen gas could be introduced via a finely-controlled leak valve up to nominal pressures in the upper Torr range; the actual pressure at the sample surface was probably somewhat higher since the gas was squirted directly onto the sample by means of a capillary tube. Specimens were sliced from 5-mm diameter rods, polished and sputter-cleaned to remove polishing contaminants. Their compositions (atomic %), as determined by chemical and chemical-spectrographic analysis, are shown in Table I. The secondary ion currents, measured by a dedicated computer (Philips P 9205) with 500-s sweeps of the 0-250 mass range, were corrected for isotopic abundances. Mass number 28 was exclusively assigned to silicon, although this could not be confirmed at the higher residual oxygen pressures because of molecular ion contributions to masses 29 and 30. A sulfur analysis was not attempted because of coincidence with 1 6 0 2 + . The slight contributions of j*Fe'H+ were distinguished from those of ssMn+ using known isotopic abundances; mass number 60 was used for nickel to separate this element from j8Fe. Cu+ was recorded after allowances for *;Til6O+ had been made. The germanium
Table I. Composition of Spectrographic Iron Standards in Atomic Percent as Quoted by NBS Element
46 1
B C 0 A1 Si P S
-0.001
Ti
0.01
V Cr Mn Fe Ni
0.026
co
cu Ge As Zr
Nb Mo Ag Sn Ta
0.69 0.07 0.01
0.093 0.095 0.04 0.14 0.36 96.06 1.64 0.25 0.30 0.001 0.021
