Secondary Ion Emission for Surface and In-Depth Analysis of Tantalum Thin Films J. M. Morabito Bell Telephone Laboratories. Incorporated, Allentown. Pa. 78703
R.
K. Lewis
Cameca. Instruments. Elmsford. N . Y . 70523
The Cameca Ion Microanalyzer has been used to analyze the surface and bulk of a number of sputtered tantalum thin films. This technique of chemical analysis has unique analytical capabilities which are based on t h e emission and subsequent mass analysis of characteristic secondary ions ejected by high energy (several K e V ) ion bombardment. The results obtained on sputtered tantalum films, reactively sputtered (argon.oxygen, argonsnitrogen mixtures) tantalum films, and oxidized tantalum films are presented to demonstrate these unique analytical capabilities. Quantitative analysis by this method is shown to d e p e n d on the accurate and reproducible measurement of characteristic parameters s u c h a s t h e secondary ion current of the impurity ( i a i ) and the secondary ionization yield of the impurity relative to t h e matrix (Krel). The magnitude of Krel is best determined by the use of standards.
This paper describes and critically evaluates a relatively new analytical technique, Secondary Ion Emission, for localized surface and bulk thin film analysis and to present a method of performing quantitative analysis by the use of standards. This technique of chemical analysis is based on the emission and subsequent mass analysis of characteristic secondary ions which are ejected from a sample due to bombardment by a high energy (several KeV) ion beam. All elements and their isotopes can be detected by mass spectroscopy with a sensitivity in the ppm range, and for some elements in the ppb range. With this technique, ion images providing spatial elemental distribution of the sample can also be obtained with approximately -1 pm resolution. In-depth analysis can also, be performed. The in-depth analysis is accomplished by a controlled and gradual sputtering of the sample, and in-depth resolutions better than 100 A have been reported ( I ) . Results have been obtained for T a films sputtered in argon, argon-nitrogen, argon-oxygen mixtures, and for thermally oxidized films which demonstrate the unique capabilities of the technique for both bulk and surface analysis. Equations for quantitative analysis have been developed. These equations require the use of standards, are empirical, and are not based on a model of secondary ion emission. In addition, the capabilities and limitations of secondary ion emission for surface analysis are presented along with complementary results on surface impurities detected on similar tantalum films by Auger Electron Spectroscopy.
MECHANISMS OF SECONDARY ION EMISSION When a surface is bombarded with a high energy ion beam, the secondary emission of neutral atoms, positive and negative ions, electrons and electro-magnetic radia(1) C. A Evans, Jr.. and J. P. Pemsler.Ana/. Chern, 42, 1130 (1970)
tion is observed (2). A major portion of this secondary emission from the surface is neutral atoms (Cathodic Sputtering). The yield of positive and negative ions emitted due to ion bombardment is a small fraction ($3) of the neutral atoms (typically to 10-5); and const,itutes the phenomenon of secondary ion emission. However, the processes or mechanisms which can lead to emission in an ionized (positive or negative) state are not completely understood but probably originate in the atomic collisions resulting from ion bombardment. In addition, the influence of parameters such as surface temperature (4-6),surface chemistry (7) (hence, vacuum conditions), primary energy (4, 8, 9) etc., on secondary ion emissions have only recently been systematically studied. Castaing and Hennequin (10 ) have described two general processes, “Kinetic” and “Chemical” which are discussed below. The “Kinetic” process in a metal occurs when the energy of the primary ion is high enough to eject one or more bound electrons which leave the atom in a metastable state due to the screening of the conduction electrons. After many collisions, the excited atom leaves the surface with a low energy and de-excites by the emission of a n Auger electron forming a positive ion. This mechanism is supported by the presence of Auger electrons in the general background of secondary electrons emitted by light metals (Al, Mg, Be, Si) during ion bombardment ( 2 1 ) . For these light metals, the Auger de-excitation takes place in vacuo, very near to the sample surface. In the “Chemical” process, direct emission occurs due to bond destruction by the high energy primary ions. For a pure metal, this process is enhanced when oxygen or other reactive gases are present (7). When negative oxygen ions are used as primary ions, the secondary ion emission observed is most probably due to the chemical process. This process also accounts for the high yield usually observed from the first few monolayers of the sample as will be discussed in the Results and Discussion Section. Andersen (7) has suggested an interpretat,ion of ion emission based on sputtered atom yields and has proposed a mechanism of positive ion emission which is based on (2) M. Kaminsky, “Atomic and Ionic Impact Phenomena on Metal Surfaces,” Academic Press, New York, N.Y.. 1965, pp 201--10. (3) J. V . P. Long, Brit. J. Appi. Phys.. 16, 1277 (1965) (4) R. C. Bradley, J. Appi. Phys.. 3 0 , 1 (1959). (5) H. E. Stanton. J Appi. Phys.. 31, 678 (1960). (6) R. Laurent and G. Slodzian, Intl. Conf. on Ion Surface InteractionSputtering-and Related Phenomena, Garching, Germany, Sept. 25-28, 1972. (7) C. A. Andersen, J . Mass Spectrom. /on Phys.. 2, 61 (1969); 3, 412 (1970) 30, (8) Y . M. Fogel and I. M. Karnaukhov. J. Tech. Phys. (U.S.S.R.), 824 ( 1960). (9) A. Benninghoven, C. Plog, and N. Treitz, lnt. Conf. on Ion Surface Interaction-Sputtering-and Related Phenomena. Garching, Gerrnany, Sept. 25-28, 1972. (10) R . Castaing and J. F. Hennequin. Advan. Mass Spectrom.. 5, 88 (1972). (11) J. F. Hennequin. J. F. Joyes, and R. Castaing, C.R Acad. SCI.. Ser. 6 . 265,312 (1967). A N A L Y T I C A L C H E M I S T R Y , VOL. 45,
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I
ION SOURCE
y PRIMARY
I
I
MAIN CHAMBER
IMAGE CONVERTER
MASS SPECTROMETER
I
ION BEAM
I
AMPLIFIER
I
IMMERSION
CONVERTER
SECONDARY
VIEWING S A M P L E AIRLOCK I N T R O D U C T I O N PORT
L!
AIRLOCK EVACUATION
Figure 1. Schematic of
IL
M A S S RESOLVING A P E R T U R E (Cp) ELECTROSTATIC MIRROR
Cameca Ion Microanalyzer
the probability of ion neutralization by the transfer of available surface electrons to the ion. This neutralization is believed to take place a t the sample surface and is therefore a strong function of its electronic properties. The probability of survival for the ions can then be correlated with surface chemistry through the bombardment of highly reactive, electronegative gases. The strongly bonded compounds formed reduce the number of electrons available for neutralization and, hence, increase the survival rate of the secondary ions. Benninghoven (12) has identified several possible processes which may be responsible for the formation of secondary ions. He also emphasizes that neutralizations in the proximity of the surface determines the yield of secondary ions and that the ionization process operative can often be determined by systematic experimental analysis.
CHEMICAL ANALYSIS BASED O N SECONDARY ION EMISSION Chemical analysis based on secondary ion emission makes use of mass spectrometric techniques. Since all of the emitted secondary ions are not of equal initial energy, a mass spectrometer design having both momentum and energy filtering is absolutely necessary for high mass resolution. In addition, the efficiency of collection and transmission of ions should be as high as possible to get maximum ion current to the detector which results in greater sensitivity for the detection of impurities. The emitted secondary ions can also be focused by a system of electrostatic lenses in series with magnetic focusing to produce ion images of the sample surface. The first ion microscope based on secondary ion emission was designed by Castaing and Slodzian (13) In the Castaing(12) A Benninghoven Z Phys 220,150(1969) (13) R Castaing and G Slodzian J M m o s (Parrs) 1, 395 (1962)
870
( x 500)
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6 , MAY 1973
Slodzian design, the ion image is formed instantaneously as in a conventional transmission electron microscope or light microscope. Ion images can also be obtained by scanning a probe type beam of approximately two micrometers diameter in a raster pattern (3, 14, 15) This latter method of image formation is identical to that used by the electron microprobe for X-ray images. In-depth chemical analysis by a continuous mass analysis with time, hence depth, is possible with the technique also. Scanning or rastering (15, 16) the beam over the area bombarded results in finer erosion control and more improved depth resolution than in the static case. Therefore, localized chemical analysis by the phenomenon of secondary ion emission is accomplished by the measurement of secondary ion mass spectra, in-depth profiles, and ion images. These, then, are the three basic modes of operation available. The performance of an instrument designed for secondary ion emission analysis can therefore be best described by its ultimate mass and depth resolution, its sensitivity, the time necessary to record an image, and the lateral resolution of the image obtained. The instrument used for our work is an improved version (16) of the original design of Castaing and Slodzian (13). The principle of operation of this particular design and information on its three modes of operation (mass resolution, lateral resolution, etc.) are given below.
PRINCIPLE OF OPERATION OF THE CAMECA ION MICROANALYZER The Cameca Ion Microanalyzer (Cameca Instruments, Elmsford, N.Y. (Figure 1) combines the features of mass 38, 5277 (1967) (15) Ion Microprobe Mass Analyzer Brochure, Applied Research Labs, Sunland. Calif (16) Cameca Ion Analyzer Brochure, Cameca Instruments, Elrnsford. N Y (14) H Liebl. J Appl Phys
spectroscopy with that of an ion emission microscope to obtain an elemental distribution map of the surface of a sample. This is accomplished by the mass analysis and bidirectional (ion optical) focusing of the positive or negative secondary ions sputtered from the sample by an impinging high energy (5-15 KeV) beam of ions (usually argon, nitrogen, or oxygen). The primary ions (positive or negative) are directed at approximately 45" incidence to the sample by means of an electrostatic lens system. The instrument has three independently pumped (liquid nitrogen trapped oil pumps) sections-namely, the ion source, main chamber. and image converter section as indicated in Figure 1. The function of these three sections will be described below. Ion Source. The ion source utilizes a Freon cooled cold cathode duoplasmatron type gun. The typical gases used in this gun are argon, nitrogen, and oxygen. Oxygen bombardment is most commonly used because of the secondary ion yield enhancement mentioned previously. Argon or nitrogen is used when it is necessary to detect oxygen or eliminate interference from oxide molecular species. Argon has the advantage of producing fewer secondary ion molecular species, while nitrogen offers higher secondary ion yield efficiencies. A double condenser electrostatic lens system allows the beam to be focused from about ten micrometers to over 300 micrometers. Typical operation requires bombarding an area larger than the imaged area which is approximately 250 micrometers in diameter. Main Chamber. The impurities present in the initial bombarding beam of oxygen or argon can be analyzed by deflecting the beam into the mass spectrometer of the Ion Microanalyzer. The ratio of the peak height of these impurities to the 1 6 0 2 - and 40A+ main peak are listed in Table I. The impurities are due to the contamination present in the gases introduced into the duoplasmatron and from the filament and walls of the source. The amount of these impurities is not significant except perhaps for the analysis of reactive gases in the low ppm range. The secondary ions emitted from the sample (sample held a t f 5 kV from ground for the acceleration of the secondary inns) are accelerated in a uniform electrostatic field by an immersion lens which forms an ionic image (Ir)independent of the m l e ratio of the ions. This image results from the superposition of many ion images, one for each isotope present in the sample. A mass spectrometer separates the ions according to their m l e ratio and, hence, a n ion image characteristic of each element is formed. An aperture (C1) placed in front of the mass spectrometer eliminates secondary ions above a minimum lateral energy ( > 1 eV). Momentum filtering is achieved by the first deflection through the mass spectrometer and by an aperture ((22) placed in front of the convex electrostatic mirror (Figure 1). The electrostatic mirror is adjusted to reflect only those ions with energy lower than some specified value (energy filtering). This design permits both the momentum dispersion and energy filtering necessary for high mass resolution, followed by a second magnetic deflection through the mass spectrometer to obtain further reduction in optical aberrations. Image Converter. The focused ions then enter the image converter section. After passing through the projection lens system, they impinge on the cathode of the image converter and the secondary electrons emitted are focused by the immersion lens of the converter into an equivalent electron image. (The immersion lens of the image converter will produce equivalent electron images of
Table I. Mass Spectrum of Oxygen from Duoplasmatron. Main Peak--1602+ Peak height Mass number
Impurity
0 3 2 + peak height
16 O+ lo-' 18 H20 8 x 10-5 1.7x 10-4 28 NP 30 NO 9 x 10-5 40 A 2.3 x 10-4 44 N20 3.3 x 10-3 46 ... 2 x 10-4 58 Ni 1.5x 10-5 84 Kr 5 x 10-5 132 Xe 4 x 10-6 Mass Spectrum of Argon from Duoplasmatron. Main Peak-40A+ Peak height Mass number
18 41 58
Impurity
H20 AH Ni
A + peak height
1 x 10-4 2 x 10-3 6X
either positive or negative secondary ions.) A movable magnetic prism placed in front of the image converter is used to deflect the secondary electrons produced. The converted ion-to-electron image can then be observed on a fluorescent screen, or recorded on film. Finally, the fluorescent screen can be replaced by a n aperture. The focused beam passes through this aperture and strikes a light scintillator-photomultiplier detector combination whose current output is proportional to the amount of a particular ion impinging on the converter cathode. Using various aperture sizes and projector lens settings, the area of the image selected for mass spectral analysis can be varied continuously from one full image diameter (-225 pm) down to a few micrometers.
MODES OF OPERATION Secondary Ion Mass Spectrometer Mode. The isotopes of all elements in the periodic table can be mass analyzed. By adjusting the entrance (C1) and exit (C2) apertures (Figure 1) and the upper limit of the energy variation on the electrostatic mirror to a minimum (1.25 eV), a mass resolution ( M l A m ) of 1000 can be obtained. This is sufficient to resolve N + (14.00307) from Si2+ (13.98846) as shown in Figure 2 but only with a considerable reduction in sensitivity. In-depth analysis of the chemical concentration of a selected isotope is also possible by tuning to a particular isotope with a mass spectrometer. For an accurate indepth analysis, all the ions must come from the same depth which requires that a constant current density over the entire beam diameter be maintained. This condition is very difficult to achieve when small beam diameters are used, since current density will have a gaussian distribution and sputtering rates from the outer portions of the beam will be lower than the center of the beam. The crater formed will then have a corresponding gaussian profile as shown in Figure 3a. Defocusing into a larger beam ( i e . , 300 pm) and decreasing primary current results in a slower sputtering rate and a crater shape shown in Figure 3b. A specific area can be selected by placing a mechanical aperture in an image plane and allowing only those ions from the flat portion of the crater to be detected. A more elegant solution is to rapidly scan (raster) the beam (15, I S ) to ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
871
The minimum volume 6(cm3) can then be calculated by the following equation:
b
4 0 0
b, z
+
I
I-
z W K E
3 V
*
W m
z
(0
z
2 > K a
4
?v)
0
z 0
w V v)
MASS CHARGE
Figure 2. Cyclic scan recording of 14N+ and 28Si2+ which demonstrates the mass resolution of the Ion Microanalyzer for N + (14.00307) and Si2' (13.98846).The primary ions were l602+
T Y P I C A L CRATER PROFILE PRODUCED BY A FOCUSED PRIMARY I O N B E A M W I T H GAUSSIAN DISTRIBUTION
(30)
DETECTOR SELECTED AREA APERTURE PRIMARY ION BEAM
OF SELECTED I O N SPECIE MASS SPECTROMETER SYSTEM SPECIMEN PROFILE
CRATER P R O F I L E PRODUCED BY L A R G E D I A M E T E R FOCUSED I O N B E A M W I T H SCHEMATIC DIAGRAM OF S E L E C T E D A R E A APERTURING O F IMAGED A R E A
(3b)
Figure
3a. Crater formed by small beam size 3b. Crater formed by larger beam and mechanical apert-
uring to eliminate signal from the crater walls
achieve constant current density (homogeneous bombardment) over the imaged area which improves the flatness a t the bottom of the crater. Rastering the primary beam is a third mode of operation after those illustrated in Figure 3a and 3b A minimum number of ions are necessary to measure an ion current or to form an ion image. Since only a fraction of the emitted particles are ionized, some finite volume of material must therefore be sputtered from the sample. The minimum volume (17) necessary will depend on the concentration ( C in ppm), absolute ionization yield (K) of the element, the transmission (7)of the instrument, and on the precision of the measurement (p). A precision of =tp% corresponds to about 104/p2 characteristic ions (1 7) or counts (18) (the standard deviation then beingp/100). (17) G Slodzian Rev Phys Appl 3 ( 4 ) 360 (1968) (18) T 0 Ziebold Anal Chem 39,858 (1967)
872
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6, M A Y 1973
where ( p N / M )the atom density (atoms/cm3), p (density), N (Avogrado's number), M (atomic weight), and (loola) the isotopic abundance correction factor. For example, in order to detect 10 ppm of pure monoisotopic ( a = 100) A1 having an absolute ionization yield ( K ) of 10-3 for argon with a precision of 370, a microvolume of approximately 100 pm3 is required. For a 100-pm beam, a thickness of 128 A must be removed (this calculation is independent of the distribution of the impurity). Ion Image Mode. The resolution of ion images obtained with the Cameca Ion Microanalyzer is limited by aberrations in the optical system, and by the minimum volume (6) of sputtered material necessary to produce an ion image (1 7). Chromatic aberrations are minimized by two deflections (Figure 1) through the mass spectrometer in opposite directions, and by use of a convex electrostatic mirror. The entrance and exit angles of the secondary ions in the magnetic field of the mass spectrometer are such that the transverse and radial focal points can be made to coincide forming a stigmatic image (13). Two-dimensional ion images corresponding to the distribution of a selected element can then be obtained instantaneously with a resolution on the order of 1Frn independent of beam size. Instantaneous imaging is not possible with a scanning probe (3, 14, 15) since there is no focusing of areas within the probe beam which is typically on the order of 2 pm. The lateral resolution by this method of image formation is limited by beam size. Application of Secondary Ion Emission to Surface and In-Depth Chemical Analysis. There are a number of factors that have to be established before applying secondary ion emission to surface or in-depth analysis. The energy (2) and nature (7) ( l 6 0 2 + , 40A+)of the primary ion beam are related in a fundamental way to the secondary ion yield (K). Knowledge of the exact relationship is essential before the measured ion current of a particular ion can be directly related to its concentration and the mechanism(s) of emission established. Sputtering rate is also of importance due to the possibilities of contamination from residual gases which adsorb on the sample and enhance emission in an unpredictable way ( 1 1). The primary current (i,) necessary to establish a specific erosion rate (ML/sec), where ML is monolayers, over a given area A can be calculated as follows: Let the atom yield/sec be defined as R
R
=
azA
(2)
where 0 = ( P N / M ) ~(atoms/cm2)/ML, /~ surface atom density; p = density of material; M = atomic weight; N = Avogrado's number, 6 x 1023 atoms/mol; t = monolayers (ML)/sec, erosion rate. The primary ion current required then is
.
R (1.6X 10-i9C0/ ion) amps S(E)
z p = __
(3)
where S ( E ) is the sputtering ratio2 (atom yield/incident ion) and a function of primary energy, mass of the incident ion, and angle of incidence. (1.6 x 10-19 C"/ion in the conversion factor to amps where C" refers to coulombs.) Substituting,
In this work S ( E ) for tantalum thin films was found to be approximately 1 for 5 KeV argon ions and 0.5 for oxygen ions a t the angle of incidence used (-45") for the measurements. Therefore, the maximum erosion rate for tantalum is estimated to be 22 ML/sec or 66 ff/sec with argon and approximately 33 A/sec with oxygen. This high erosion rate will easily prevent specimen contamination due to the residual gases present in the vacuum of the Cameca sample chamber. Chemical combination of these residual gases with the sample could result in an enhanced chemical emission of secondary ions (chemical effect). This effect is most pronounced on the periphery of the primary beam where the erosion rate is not sufficient to maintain proper surface conditions. This is illustrated in Figure 4 where the luminescent area around the periphery. of the sputtered crater illustrates the preferential emission of oxygen ions adsorbed from the ambient surrounding the sample. The secondary ion current from this portion can be prevented from reaching the detector hy mechanical aperturing (Figure 36) as discussed previously. The ratio of the number of ionized atoms (I,*) ejected to the number of neutral atoms ejected (R)is defined as the secondary ion yield, f , f
'
= -1,-
This number f , is strongly dependent on the nature of the primary gas, i.e., 10-3 for argon, lo-* for oxygen in the case of aluminum. Substituting into Equation 5 from Equation 2, we have
',I
f , =-ozAt
,i
oxygen taken on pure aluminum primary beam Consists 01 A + ions. Beam currenl was 2
The X 10.' A. The pressure in the specimen chamber was 2 X l o - ' torr. The Iuminescent aureole at the periphery of the sputtered area illustram the contaminalion of the sample and resputtering of Oxygen ions by the primary A + ions
(5)
R
defining I,+/t (1.6 x 10-19 C"/ion) as is, dary ion current,
Figure 4. Ian image of
(6) or the secon-
= ozAf,(L6 X 10-'gCo/ion)amps
and idet = i, X (lOO/ai) where a, is the abundance of a particular isotope of the impurity (i). Therefore,
i.;O6(%) C=
K,,K,varA(1.6
S(E) X 10-'sCo/ion)
(14)
where S ( E ) is the sputtering efficiency of the film for a fixed primary energy, mass of incident ion, and angle of incidence. From Equation 11we can also write,
(7)
The secondary current actually detected, i d e f . will depend on the transmission ( 7 ) of the instrument collection efficiency X mass spectrometer transmission and its magnitude will he proportional to the concentration (C/lOe) ofthe element where C is in ppm at. The transmission 7 is approximately 10%for the Ion Microanalyzer (16). Therefore,
where idetm is the measured or detected current from the matrix and a , is the isotopic abundance of the matrix. Equation 14 then relates the concentration of a given impurity to four measurable quantities i, K,, i a t , and Kre,.
id,, =
idpf
=
&,,~C/106)
uzAf,~C/1O6X1.6X 10-18C"/ion)
( 8)
(9)
Substituting for at A from Equation 4 iaet =
S(E)i,fdC/1O6)
(10)
S ( E ) f ,is the absolute secondary ion yield, K, or the numher of ionized atoms per incident primary ion. Therefore, iapt
= i&dC/l06)
(11)
Solving for C
The relative ion yield of an impurity (i) in any matrix ( m )can he defined (19) as
If the thickness of the film is known, the sputtering rate can he measured from the time i t takes to sputter through the film for a known primary current, i, and energy E. S ( E ) can then he calculated by Equation 4. K, can he measured for either positive or negative ions by the ratio of idet/i, from Equation 11, for the matrix in the pure state [(Cm/106) = 11. K, for lBITa+ with positive oxygen, i , = 45 x A and a 225-rm aperture has been measured to he 3 x and 1.3 x with argon. The factor i, is the measured secondary ion current of a particular isotope of the impurity in the matrix. Before the magnitude of,.i is substituted into Equation 14, the correct isotope ratios of the element must be measured. Improper ratios are due to interference effects from other masses or hydrocarbons which increase the measured current and lead to an incorrect calculation of its concentration. Krel can he measured by the use of standards (homogeneous alloys, compounds, etc.) or calculated from Equation 13 for the matrix and impurity in the pure state. (2)
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6. M A Y 1973
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Table II. Minimum Detectable Concentration of Some Elements in a Tantalum Matrix and the Relative Yield, Krel of These Elements with Respect to Tantalum Element
K m (oxygen)
Cmin,m (PPm at. %)
7Li
35 0.175 11.2 280 13.3 2 28 8.75 1.65 3.07 4.24 0.378 0.756 2.15 0.875 0.230 0.230 0.120 1.17 0.318 0.133 1.75
0.16 31 0.56 0.019 0.43 3.68 0.19 7.2 3.2 1.9 1.39 53 10.2 6.9 11.7 85 45 184 8 54
'2C 24Mg 2 7 ~ 1 2%
48Ti 5'V 52Cr 55Mn 56Fe 59Co 60Ni 63Cu 74Ge 9%
06Pd 07Ag ll%m lZ1Sb 184W
97Au 20981
40 3.2
A table of Krel for elements measured with respect to tanA and an talum for oxygen primary gas at i, = 45 x aperture of 225 pm is given in Table 11.
LIMITS OF DETECTION The minimum detectable concentration Cmin,i (ppm at) of an element with mass m can be calculated by the following expression 119),
where i,,, is the minimum detectable ion current by the detector which for an ion-to-electron converter (20) is reported to be A. The minimum detectable current with the Ion Microanalyzer is -1.6 x A. For tantalum the ion current of the matrix or reference element (191, idetm, using l 6 0 2 + is 3 x l O - l 3 A a t a primary curA and a selected area diameter of 225 rent of 45 x pm. a , and a, are the abundances of the impurity and the matrix, respectively. Values of K,,, were obtained by measuring the ion yield K , of the pure element (see Equation 11) for the isotope listed in column 1 of Table I1 and dividing this number by the ion yield of the matrix. This assumes that the matrix has no effect on the ion yield of the impurity and that ion yield is independent of concentration. The validity of this assumption will also depend on the mechanism of secondary ion emission. When the mechanism is Kinetic (see section on Mechanisms of Secondary Ion Emission), it is more likely to be correct than when the mechanism is Chemical, In the bulk, a superposition of mechanisms is more likely. Therefore, K,,, of most elements in a given matrix must be modified by some correction factor F .
(19) H. W. Werner, Deveiop. Appl. Spectrosc. 7 A , 239 (1969). (20) H . W. Werner and H. A. M. deGrefte, "Transactions of Third International Vacuum Congress, Stutgart. Germany, H. Adam. Ed. 2, Part 1 1 , Pergamon Press, New York, N.Y.. 1965, p 493.
874
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. L, M A Y 1973
I
a
'"
I2
Y
t
z
W
a K
3 V
z
0
H
VI
Bo
50
52
54
56
5%
60 62
64
66
68
70 72
MASS CHARGE ,TIME ( s e d
Figure 5. Secondary ion mass spectrum of 0-Ta film in the mass range of 50 to 72. The spectra in the mass range of 50-75 were recorded while removing 100 A from the surface by argon sputtering
The magnitude of this correction factor is best determined by the use of standards such as homogeneous alloys, ion implanted (21) and reactively sputtered samples (22), etc.
SAMPLE SELECTION AND PREPARATION The tantalum films used in this study have been prepared by dc sputtering with a sputtering voltage of 3.8 kV at 420 mA. Tantalum thin films produced by sputtering usually have a structure different from that of bulk tantalum which is bcc. The thin film structure has been identified (23) as a tetragonal phase, commonly referred to as p-Ta. All of the films were deposited on alumino borosilicate glass with a TazO5 underlay or on glazed ceramic. The variety of impurities (metallic, nonmetallic, gaseous) normally found in tantalum and the fact that a number of analytical measurements had already been performed on these thin films were very useful to our investigation of the unique analytical capabilities of secondary ion emission. Results (24) obtained with the technique of Auger Electron Spectroscopy on the surface impurities present on sputtered tantalum thin films helped greatly in the evaluation of the surface analytical capabilities and limitations of the secondary ion emission technique. The tantalum films studied were approximately 4000 A in thickness with a few exceptions which are indicated. Thermal oxidation of 500 A of &Ta to Taz05 was used to study the enhancement of secondary ion yield due to the presence of oxygen in the film. Samples prepared in a laboratory bell jar system were used to investigate the capabilities of the Cameca Ion Microanalyzer for oxygen and nitrogen in-depth profiles and to study the effect of the crystallographic thin film structure and concentration on the secondary ion yield of oxygen and nitrogen in tantalum. (21j J. M. Morabito and J. C. Tsai, Sur. Sci.. 33, 422 (1972). (22) J. M. Morabito, 19th National Vacuum Symposium. Chicago, Ill., Oct. 2-5. 1972. (23) M. H. Read and C. Altman, Appi. Phys. Lett.. 1, 51 (1969). (24) J. M. Morabito, Tenth National Meeting of Society for Applied Spectroscopy, St. Louis, Mo., Oct. 18-22, 1971: J. M. Morabito, Tenth Annual Varian Vacuum Technical Seminar, Palo Alto, Calif., May 9-1 1, 1972.
DEPTH
(61
Figure 6a. Depth profile of Cu in 8-Ta film. Oxygen primary ions 6b. Depth profile of Fe in 8-Ta film.Oxygen primary ions RESULTS AND DISCUSSION Surface Analysis of Ta Films. The secondary ion mass spectrum obtained on a 8-Ta film with primary argon ions in the mass range from 50 to 72 is shown in Figure 5. This spectrum was taken while sputtering through -100 A of the tantalum film. Metallic impurities are present on the surface of the film, hut the identification of these impurities is complicated by the presence of hydrocarbon C,H, interference lines. The presence of hydrocarbons is most probably due to hydrocarbon compounds and adsorbed gases on the tantalum surface prior to analysis. Hemandez et al. (25) have identified nineteen hydrocarbon lines in the surface mass spectrum obtained on aluminum. They found that the presence of CZ, CzH, and CzHz interfered with the 24, 25, 26 isotopes of Mg, making a surface analysis of this element very difficult. The silicon isotopes (28, 29, 30) had interferences (25) from CO, CzH4, CzHa, and CzHe. An example of hydrocarbon interferences observed in this work for the 50, 52, 53 isotopes of chromium is shown in Figure 5. The correct isotopic abundance (4.34, 83.8, 9.50) of these peaks was not obtained (within the first tens of angstroms helow the surface) because of mass interferences which we interpret as due to CHz+, C4H4+, C4H5+. There was also hydrocarbon interferences in the case of the iron isotopes (Figure 5). However, this interference effect disappeared and did not return after the removal of approximately 40 A from the tantalum surface, and is therefore attributed to a hydrocarbon layer on the tantalum surface prior to analysis. After the removal of this layer, the proper isotope ratios were obtained as shown for Cu in the same Figure 5. 0nly when the correct isotope abundance ratio has been oh tained for copper-63 and -65 can one have any assurance t ,-+L +hnCD cy"" are copper peaks. This is due to possible interference from .~ CsH.+ ( 1nass 6-3-)' and.CsH5+ - -- (mass ' 65). The sc!condary ion emission of these surface impurity ions (Fe, Cu, Cr, Ni) is also affected by the oxide layer &h- +--&-I..-..an--Thn f24) of U -' Ir_n l--.._ r u w r r &h:-l.---LLllLn'lGID "11 lllr C L I ' l L L l l U ' l l UUIILILT. l l l r emission mechanism operative a t the surface would then he expected to he completely chemical (surface compound dissociation) and the yields of these ions should he higher than those expected if the same impurities were present in the hulk of the metal. This hypothesis is consistent with the depth profiles obtained with primary oxygen ions for Fe and Cu (see Figures 6a and 6 b ) . The initial increase
..."
l . . ~ " l
--
(25) R. Hernandez. P. Lanusse, 0 . Slodzian. and 0 . Vidal. Me'thodes Physiques d'Analyse GAMS. 6, 411 (1970).
d(NlE1) dE
0
100
200
400 S O 0 600 700 A U G E R ENERGY [eV)
300
800
900 I 1
Figure 8. Auger spectrum prior to in Sifu ion sputtering with argonof a@-Tafilm in the two curves is due to a complicated superposition of hydrocarbon interferences and enhanced chemical emission effects. The type and extent of these interferences could only he resolved with a mass spectrometer having higher mass resolution than was available for these studies. After sputtering 60 A in depth, the secondary ion current from Fe+ and C u t stabilizes, since the chemical enhancement due to the presence of surface oxide is no longer operative and the hydrocarbon layer has been removed. From these depth profiles, we conclude that the hydrocarbon and oxide layer on this tantalum sample is 560 A, which is in good agreement with the work of Sewell et al. (26).
The distribution of iron on the surface of tantalum is shown by the ion micrograph obtained after removing the hydrocarbon layer (Figure 7). From the micrograph we see that the iron is nonhomogeneously distributed (ion images of copper showed that it is also nonhomogeneously distributed). From Equation 1 we can estimate, assuming spherical particles and a precision of lo%, that the smallest particle of pure iron detectable is 494 A in diameter. A pure iron particle with this diameter would show as a spot one micrometer in diameter on the ion micrograph (Figure 7). (26) P. 8. Sewell, D. F. Mitchell. and M. Cohen, Sur. Sci.. 29, 173 (1972).
ANALYTiCAL CHEMISTRY, VOL. 45, NO. 6. M A Y 1973 * 875
c
z W
K -
a ea
22 I
2 2
E -W
h;
2
0
u W
v)
7
J A
I t
2 0 23 2
12
3 591 60
I
93
26 27 28 29
30
MASS ,TIME (see) CHARGE
1000
4
Figure 9. Secondary ion
i
c
mass spectrum of 8-Ta film
The range shown is from 6 to 96 mass units. This spectrum was taken while sputtering through
Table Ill. Minimum Detectable Diameter (A) of Pure AI, Mg, and Si Particles. Primary Ions, Oxygen Elements
K
(ionized atoms/ incident ions)
Dmin
(A)
AI
Mg
SI
lo-’
4X
3.5 X
320A
520A
492
A
The Auger spectrum in the range from 0 eV to 975 eV obtained prior to ion cleaning on the same p-Ta film is shown in Figure 8. The major impurities on the surface of the film are sulfur, carbon, oxygen, nitrogen, iron, nickel, and a small copper peak. Iron, nickel, and copper were also detected on the surface by secondary ion emission. Bulk Analysis. A typical secondary ion mass spectrum in the mass range 6 to 96 for positive ions obtained on the same p-Ta film is shown in Figure 9. The primary ion current was 45 x 10-9 A and the aperture used was 225 pm. Primary ions were I S O z + . This spectrum was taken after sputtering through -1000 A to remove contaminants due to adsorbed gases and hydrocarbons as discussed for the surface analysis. The major metallic impurities found in the bulk were Mg, Al, Si. The isotopes of Mg (24, 25, 26) and Si (28, 29, 30) are clearly resolved (see Figure 9). These major metallic impurities (Al, Si, Mg) were not homogeneously distributed in the films (see Figure 10) and the results obtained on different areas of the same films varied by *30%. Examples of ion images for the Mg, Al, Si, and K impurities in p-Ta are shown in Figure 10. These images show that these impurities are present in particulate form which are nonhomogeneously distributed throughout the film. The smallest particles of pure Al, Si, Mg that one 076
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6, M A Y 1973
-
1000 A of the tantalum film with l60+Ions
could detect with the ion imaging mode, assuming spherical particles and a precision of lo%, are given in Table III. The lateral resolution of the micrographs is one micrometer and therefore A1 particles as small as 320 A would appear as a one-micrometer spot on the ion micrograph (Figure 10). The secondary ion yield of both A1 and K is quite high, and, hence, the volume of material (volatilized or sputtered) necessary to form an ion image is quite small. This enabled us to determine, by the direct imaging mode of the Cameca instrument, that the silicon particles in the field of view of the ion microscope also consisted of A1 and K. We therefore conclude that these particles (see Figure 10) are incorporated into the sputtered tantalum film as glass chips chemically independent of the tantalum matrix. The secondary ion yield of the constituents (Al, Si, B, etc.) from the glass matrix would then be necessary for any quantitative analysis, and these data are not available at this time. The presence of glass chips on glass slides has also been observed optically, and with the scanning electron microscopy (SEM). The SEM micrographs revealed the presence of projections on the surface of the glass substrate. The projections were not removable by rubbing and were of the order of 3000 8, in diameter. A glass particle with this diameter would also appear as a 1-pm spot on the ion micrograph (Figure 10). Evidence for the presence of Ti, Cr, Fe, C, Co, Nb, Ba, Li, B, K, Ca, Na, As is also shown in Figure 9. Aluminum, silicon, boron, arsenic, and barium are the major constituents of the alumino borosilicate glass substrate. Spark Source (27) results indicated that there were trace (27) E. L. Maim, Progr AnaiChem. 2, 148 (1969)
IVN I M A V C
OF
Ma
ION I M A Q E
FLECKS
-
ION I M A G E OF A I F L E C K S
ION I M A Q F OF K FLECKS
1 G L A S S SUBSTRATE
Figure 10. ion images of Mg, AI, Si, and K particles in a 0-Ta film on glass with 5a20s underlay
98
MASS CHARGE
Figure 11. Secondary ion !(KL.~ The increased ion Current from
spe~ilrun8Y I p- Id wrn alter
A (see Figure 9) to 1 OW" A
thermal oxidation to Ta205 is due to the enhancement 01 secondary ion emis~ianresulting from oxidation
amounts of Na, K, and Mg in the alumino borosilicate glass also. The rather large intensity of K (Figure 9) illustrates the high secondary ion yield of potassium. The large oxygen peak which is characteristic of the hombarding ion could be due to "reflected" primary ions, or sputtered ions of primary oxygen ions which were trapped a t the surface. The full width of the oxygen peak a t half maximum (FWHM) suggests a low energy spread which is more consistent with the latter mechanism (28). Iron and chromium are constituents of stainless steel fixtures in the sputtering chamber, but along with Nb, Ca, Na, and Mg (28) R. C. Bradleyand E. Ruedl. J. Appl. Phys.. 33,880 (1982).
were also detected by the Spark Source technique to be present in the tantalum cathode used to deposit the films. Nitrogen (mass 14) is believed to be incorporated during sputtering as is carbon(mass 12). Auger Spectroscopy in combination with in situ ion sputtering also found both nitrogen and carbon in the bulk of the film. Chemical Effects. Figure 11 shows the mass spectrum in the range from 6 to 100 mass units of a 0-Ta film deposited on a glazed ceramic substrate after thermal oxidation to TazO6. This spectrum was also taken after sputtering through -1000 A of the film. The constituents of the glaze are listed in Table IV. All of the glaze constituANALYTiCAL CHEMISTRY, VOL. 45. NO. 6 . M A Y 1973
877
Table I V . Glaze Composition (parts by weight) 4 8 19T
40 8
Si02 A1203
CaO BaO
5
8203
PbO Biz03 Ti02 K20
Na20
si
5 10 18 10 2 2
0
..,
dz
Coefficient of expansion X 1O6
7
I-
0.5
AS203
h-
I
P IQ
7
Ta SAMPLE a
1
Ta SAMPLE b
z W
IL:
a
3
U
z
H
i~ 2ai%
C
a ~
n
z
0
Y
l.33X10-’6A
5.41 1080
25’-525’ Firing temp. “C
s
-L
2000a-4
+-
2000
a -4
DEPTH ( % )
Figure 13a. Secondary ion emission of l60-ions as a function of depth in nonreactively sputtered @-Ta 13b. Secondary ion emission of l60-ions as a function of depth in reactively ( A - 0 2 ) sputtered @-Ta
106
100
BINDING E N E R G Y ( e v )
Figure 12. ESCA spectrum of S O 2 in the thermally oxidized p-Ta film on glazed ceramic
ents were detected in the film. P b and Bi appear in the higher mass range (204 to 209) which is not shown in Figure 11. The presence of the glaze constituents in the Taz05 could be due to the reduction of the glaze during sputtering by the tantalum film and/or thermal diffusion into the film during thermal oxidation. In either case, the glaze constituents detected in the mass spectrum are not present as a phase chemically independent of the tantalum matrix as were the alumino borosilicate glass constituents (Figure 9). Cr, Mn (Figure 11)are constituents of the stainless steel fixtures in the sputtering chamber used to deposit the film. Mo, Cu, Nb, Mg are constituents of the tantalum cathode as discussed in the preceding section. The increased total ion current from A (see Figure 9) to 1 0 - 1 4 A (Figure 11) could be due to the enhancement of secondary ion emission resulting from the thermal oxidation of the Ta film (Chemical Effect). The electron microprobe (291 detected 0.77 at % silicon in this film, but the other glaze constituents were not detected by the electron microprobe which penetrated approximately 600 A into the TaoOJ film. The secondary ion current measured for 28Si-+was A and the seconA. K,,, for sildary ion current of 1s1Ta+ was 1.4 x icon relative to tantalum for primary oxygen ions is 13.3 (Table 11). With this secondary ion emission data, we estimate by Equation 14 that the film contains -0.5 at. YO silicon. This calculation neglects matrix effects and further assumes that the silicon impurity is in metallic or “elemental” form. The chemical state of the impurity is known to have an effect on its secondary ion yield (191, 878
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6 , M A Y 1973
and it is highly probable that the silicon is in the dioxide rather than metallic form. This was confirmed by ESCA measurements (Varian, Inc., Palo Alto, Calif.) on the p-Ta film after thermal oxidation to TaZOj. Standards of elemental silicon and silicon dioxide were used to measure the binding energy of silicon in elemental and dioxide form (100 eV and 106 eV, respectively). The result obtained is shown in Figure 12 and indicated that most of the silicon is in dioxide form. A small fraction of the silicon is present (perhaps as a precipitate) with a binding energy (100 eV) equal to that of elemental silicon. This silicon, for kinetic rather than thermodynamic reasons, has not completely converted to silicon dioxide. The use of 13.3 as Krel to estimate the amount of silicon in this thermally oxidized tantalum film is therefore not valid. If we assume, however, that 0.77 at. % is correct, we can calculate Krel for silicon in this film from Equation 14 to be 10.1. The correction factor F (Equation 17) is then 0.76. Matrix Effects. Ox3gen in Sputtered Tantalum The 1 6 0 - ion depth profiles obtained on nonreactively sputtered 8-Ta and reactively sputtered (in A-02) tantalum are shown in Figures 13a and 13b. Sample b contained up to 28 at. % 02.(The electron probe X-ray intensity ratios for the nitrogen and oxygen measurements were obtained using SiSN4 and SiOz, respectively, as reference standards. The data were corrected using MAGIC IV (29). The errors introduced by the uncertainties in the high mass adsorption coefficients for these lines are currently being investigated.) The crystal structure of this film as determined by X-ray analysis was bcc (30). The crystal structure of the nonreactively sputtered film (sample a ) was identified by X-ray analysis to be p-Ta (30) Resistivity measurements confirmed that the oxygen in these reactively sputtered films was homogeneously distributed. For high oxygen concentrations (28 at. TO), electron probe results can be considered quantitatively accurate, and we can therefore estimate under the conditions used for the analysis that the relative yield of oxygen (Kre1I6) in the reactively sputtered tantalum (from Equation 14) to be 0.54. (29) J (30) L
W G
Colby. Advan X-Ray A n a / 10, 2 8 7 ( 1 9 6 7 ) Feinstein and D Gerstenberg Thin Solid f i l m s
(1972)
10, 79
r e t T a e IONS 8 4 5 x 10-9 A K, = 3 X 10-5
S(E)-1
0
1000
2000
3000 DEPTH
4000
5000
6000
Figure 14a. Secondary ion emission of i4N+ as a function of depth in p-Ta film of 4000 A thickness on a glass substrate with a 700 A Ta20s underlay
The secondary ion current of l 6 0 - detected from sample b was 1.15 x 10-15 A and 1.33 X A from sample a . If one assumes that the relative yield (Kre1)of oxygen in @-Ta(sample a ) is the same as that in bcc tantaA corresponds to 3 at. lum (sample b), then 1.33 x YO oxygen in the nonreactively sputtered @-Ta.Since 3 at. % is higher than expected (30), it appears that the yield of oxygen ions from @-Tamatrix is not the same as that from a bcc matrix. C. A. Evans (31) has, in fact, found with argon a 160+/1s1Ta+ ratio of 0.046 for nonreactively sputtered @-Ta and a ratio of 0.029 for a nonreactively sputtered film with a bcc structure, a factor of 1.6. This factor reduces the amount of oxygen corresponding to 1.33 X 10-16 A detected to 2 at. % in sample a. Nitrogen in Sputtered Tantalum. The depth profile of nitrogen in p-Ta obtained by Auger Spectroscopy indicated that the nitrogen was homogeneously distributed. The 14N+ profile obtained with the Ion Microanalyzer confirmed this result and is shown in Figure 14a along with the depth profile of IslTa+ in Figure 14b. The increased yield of 14N+ and IslTa+ a t the surface is most probably due to the presence of oxygen and carbon which enhances the yield. This is also the reason for the increase a t the film-Ta205 interface. The depth profiles obtained with both Auger Spectroscopy (22) (in combination with in situ ion sputtering) and secondary ion emission 122) confirmed that nitrogen, incorporated into the film during reactive sputtering, is homogeneously distributed. X-Ray analysis of these films indicated a structural change in the tantalum, i . e . , p-Ta p-Ta bcc T a (mixture) bcc T a with increasing nitrogen content in the film. These nitrogen doped films were prepared as standards to calibrate the Ion Microanalyzer for quantitative estimates of nitrogen in tantalum. The nitrogen content in the films was determined by the electron microprobe (29). An ultra-thin window flow proportional X-ray detector and a diffracting crystal with improved sensitivity for nitrogen was used for these quantitative measurements. The results obtained with primary oxygen ions are shown in Figure 15. The NO- secondary ion intensity (counts/second) as a function of nitrogen content was found to be nonlinear in the 4 to 20 at. % range. The NOsecondary ion intensity was, however, linear with concentration in the 8-20 at. YO range. In this concentration
-
+
i 35 x 1 0 - 1 3 ~
1
(A1
-
(31) C . A . Evans, University of Illinois. Urbana, I l l . , private communication, 1972.
0
1000 2000 3000 4000
,3000 6000
A 14b. Secondary ion emission of lslTa+ as a function of depth on the same film of 14a DEPTH (
t N
s1
40
30
*t
10
E I'
6
W
ln
z 0
8
t
21 2
1 I
4
8-Ta I I 6
-
I 8 1 0
Aromv.
I 20
I 40
ion intensity V S . Concentration of nitrogen (at. tantalum thin films reactively sputtered in (A-Nz) mixture
Figure 15. NO-
%) in
range, the structure of the film was bcc as indicated in Figure 15. From 5 to 8 at. YO, a mixture of the p (tetragonal) T a and bcc T a was present. Below 5 at. YO Nz, the film deposited had the 8-Ta structure. The yield of NO- ions is obviously different in these three phase regions (Figure 15). This effect of matrix structure on secondary ion yield must be considered in attempts to perform quantitative analysis with the data.
SUMMARY AND CONCLUSIONS Perhaps the most outstanding capabilities of secondary ion emission analysis are: 1) All elements can be detected, including isotopes with concentration for most elements in the ppm level. 2) In-depth or localized chemical analysis is possible with good depth resolution (on the order of 50-100 A). This capability makes measurement of diffusion profiles and concentration gradients possible. 3) Ion Imaging with a resolution of 1 pm provides an elemental distribution map of the impurities present which complements the in-depth analysis. In this way the lateral homogeneity or heterogeneity of the impurity can be monitored. 4) With high mass resolution, qualitative surface analysis with greater sensitivity (22) than Auger Electron Spectroscopy is possible. The above capabilities should make this technique an ideal instrument for thin film analysis. However, routine
-
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6, M A Y 1973
879
quantitative analysis is, a t present, quite difficult with the technique. Reasons for this difficulty are the existence of several undefined mechanisms in the ion formation process, and the possibility of the superposition of these mechanisms (25). In addition, ion yields vary with parameters such as primary ion energy, residual gas. pressure, nature of the primary ion, temperature, and the chemical nature of the impurity-matrix pair (chemical and matrix effects). Any model of secondary ion emission (7, 32) would have to be modified by the chemical and matrix effects present for the system under analysis. Examples of chemical and matrix effects for the Ta system have been discussed. Experience with the tantalum system suggests, that with the proper use of standards, a quantitative analysis through the use of Equation 14 can be obtained. The most critical parameter in this equation is Krel,the relative ion yield. The estimation of Krel by Equation 13 neglects both chemical and matrix effects. However, with standards of known homogeneous composition and structure (TaZN, Ta205, SiOz, etc.) this number can be measured with sufficient accuracy to perform quantitative analysis provided sputtering rates, primary energy, residual gas pressure, etc. are reproducibly controlled. An example of this approach was discussed for the Ta-02 system, for nitrogen in T a , and for silicon in Ta205. Surface analysis is also possible, but the presence of hydrocarbon compounds and adsorbed gases which cause mass interferences make reliable surface analysis even more difficult than bulk analysis. Despite difficulties due to hydrocarbon interference, we have obtained qualitative agreement between Auger Electron Spectroscopy and the Secondary Ion Emission surface analysis of sputtered tantalum films. The combination of both techniques has given new insight into the surface chemistry of sputtered tantalum and on the lateral and depth distribution of surface impurities on sputtered tantalum thin films. (32) C A Andersen. Proceedings 6th Natlonal Conference on Electron Probe Analysis, July 8.1971
NOMENCLATURE d
a
= diameter of primary ion source = spherical aberration coefficient = semi-angular aperture
i
= current in amps
C
ar
= concentration in ppm at. = absolute ionization yield of isotope i = transmission of instrument = precision = isotopic abundance
P
= density
N
= Avogrado's number =' atomic weight = atom yield/sec. = number of sputtered neutral atoms = time in seconds = number of ionized atoms = ionized atoms/neutral sputtered atoms = sputtering rate (ML/sec.)
cs
Kl
7
P
fR t Il+ fl
i lsec ldef
LP U
= secondary ion current = secondary ion current detected = primary ion current = surface atom density
e
puttered atoms
S ( E ) = sputtering coefficient incident ion Kr,l = relative ionization yield K , = absolute ionization yield of the matrix E = primary ion energy F = correction factor c/s
= counts per second
ACKNOWLEDGMENT It is a pleasure to thank D. Gerstenberg for helpful comments on the manuscript, R. H. Minetti for his help with the Auger measurements, D. Lesher for spark source measurements, and D. Wonsidler for electron probe measurements. Figures 2 and 3, measurements of K , for the respective isotopes of the pure element in Table 11, are courtesy of Cameca, France. Received for review July 31, 1972. Accepted November 30, 1972.
Determination of Lead, Uranium, Thorium, and Thallium in Silicate Glass Standard Materials by Isotope Dilution Mass Spectrometry I. L. Barnes, E. L. Garner, J. W. Gramlich, L. J. Moore, T. J . Murphy, L. A. Machlan, and W. R . Shields Analytical Chemistry Dwision, Institute for Materials Research, National Bureau of Standards, Washington, D C 20234
Mitsunobu Tatsumoto and R. J. Knight U.S. Geological Survey, Denver, Colo. 80225
A set of four standard glasses has been prepared which have been doped with 61 different elements at the 500-, 50-, 1-, and 0.02-ppm level. The concentrations of lead, uranium, thorium, and thallium have been determined by isotope dilution mass spectrometry at a number of points in each of the glasses. The results obtained from independent determinations in two laboratories demonstrate the homogeneity of the samples and that precision of the order of 0.5% (QSYOL.E.) may be obtained by the method even at the 20-ppb level for these elements. The chemical and mass spectrometric procedures necessary are presented. 880
ANALYTICAL CHEMISTRY, VOL. 4 5 , NO. 6, MAY 1973
Approximately six years ago, the staff of the Office of Standard Reference Materials and the Analytical Chemistry Division of the National Bureau of Standards became concerned about the almost complete lack of suitable standard materials for trace element analysis. Efforts involving more than 100 persons within NBS and many others who performed analyses, data comparisons, surveys, etc., have resulted in the availability of about a dozen well characterized standard materials for the areas of ceramics and glasses, clinical, biological, and mineral trace element chemistry. About an equal number are to be released in the near future.
