(3He,xn) reactions. For example, our Bi + 3He reaction studies revealed that 685- and 660-keV photopeak intensities of 2osAt produced by the 209Bi(3He,4n)208At reaction are = l O times higher than the 992-keV peak intensity. In other words, bismuth-if indeed present-can clearly be determined via the 2oeAt gamma rays. No other element can produce this nuclide. Alternatively, if the analysis is made on a matrix with a high Bi/Pb concentration ratio, the Bi interference can be completely eliminated by bombardments near 25 MeV, which is lower than the thresh~ Bi + 3He reactions. old energy for production of 2 0 7 Pvia Estimate of Detection Limit. Under the following conditions: beam current 3.8 PA, length of bombardment 1.5 hr, overall detection efficiency 0.5%, we were able to detect P b concentrations of 45 ng/cm2 in an A1 matrix (Figof the ure 2). The background constituted only about total 992-keV peak area; at equal signal to background levels, under our easy conditions, we could detect 4.5 ng/ cm2 lead. Because the beam intensity can be increased by a factor of 10 (or greater, depending on the target matrix),
detection coefficients can be easily doubled, and the length of bombardment can be increased to at least one 2 0 7 P ~halflife, the detection limit can be lowered to about 50 pg/ cm2 lead. If the matrix in which lead is imbedded is 10 to 100 mg/cm2 thick, the concentration limit of detection would therefore be 5 ppb to 0.5 ppb, respectively. It should be pointed out that for the analysis of paper, the detection limit is held down to only 1 ppm, under our present target-cooling system. ACKNOWLEDGMENT The authors wish to thank Diana M. Lee for her assistance during this work. Received for review June 25, 1973. Accepted August 20, 1973. One of us (BP) would like to express his gratitude for a Senior Fulbright-Hays grant provided to him throughout this study. This research was performed under the auspices of the U.S. Atomic Energy Commission.
Quantitative Analysis of Light Elements (Nitrogen, Carbon, and Oxygen) in Sputtered Tantalum Films by Auger Electron Spectroscopy and Secondary Ion Mass Spectrometry (SIMS) J. M. Morabito Bell Telephone Laboratories, lnc., Allentown, Pa, 78703
The quantitative analysis of light elements (N, C, 0 ) in sputtered tantalum films by Auger electron spectroscopy and secondary ion mass spectrometry (SIMS) via calibration with standards and electron microprobe analysis is described. The calibration standards were pmpared by reactive sputtering, and the homogeneous distribution of the dopants (N,C, 0 ) in these standards was established by SIMS and Auger in-depth profiling measurements. Although the electron microprobe could reproducibly detect the presence of low levels ( < 5 at.%) of N, C, and 0 , the accuracy of quantitative electron microprobe measurements for these light elements in tantalum below -5 at.% was questionable due to the lack of sufficiently accurate X-ray absorption coefficients. The accuracy of electron microprobe quantitative results above - 5 at. %, however, was estimated to be within &lo%.The analysis of nitrogen, carbon, and oxygen below - 5 at. % was accomplished by an extrapolation of the normalized Auger and SIMS data. SIMS detection limits for oxygen and carbon were in the ppm range and for nitrogen in the 0.1 at. YO range depending on instrument background and amount of sample consumed. The Auger detection limits for N, C, and 0 were in the 0.3-0.4 at. % range.
The effects of varying oxygen (I, 21, nitrogen ( 2 ) , and carbon (2, 3 ) contents on the structural and electrical properties of sputtered tantalum thin films have been discussed in the literature. Both structural and electrical properties have been found to be a function of the N, C,
and 0 concentration and of sputtering parameters (voltage, current, and substrate temperature). In general, transitions from a p-Ta ( 4 ) structure to a mixture of p-Ta.and bcc-Ta, and finally to a single phase bcc (bulk) region followed by oxide, nitride, or carbide formation have been observed, These structural transformations have been studied by X-ray diffraction techniques ( I , 2, 5). Quantitative information on the amount of these light elements in tantalum, assuming a homogeneous distribution, has been provided by electron microprobe analysis ( I ) . Since the electron microprobe suffers from a lack of sufficient depth resolution to perform a localized analysis of the composition at the surface, bulk, and film-substrate interface, the quantitative analysis ( I ) provided by the electron microprobe represented an averaged or integrated concentration over a specific depth ( e . g . 600 A) of the film. The development, however, of ion sputtering-Auger (6, 7) and secondary ion mass spectrometry (SIMS) (8) has made localized analysis possible. Both techniques have unique features for localized thin film analysis, can be calibrated (9) for quantitative analysis, and are capa(1) L. G. Feinstein and D. Gerstenberg, Thin Solid Films, 10, 79 (1972). (2) D. Gerstenberg, J. Electrochem SOC.,113, No. 6 (1966). (3) D. Gerstenberg and J. Klerer, Proc. Electron. Components Conf., 1967, 77. (4) M . H. Read and C. Altman, Appl. Phys. Lett., 7, 51 (1965). (5) R. D. Burbank, J. Appl. Crystallogr., in press. ( 6 ) J. M . Morabito. Tenth National Meeting of the Society for Applied Spectroscopy, St. Louis, Mo., Oct. 18-22, 1971. (7) P. W. Palmberg, Fifth International Vacuum Congress, Boston, Mass., Oct. 1971. (8) R. Castaing and G. Slodzian, J . Microsc. (Paris). 1, 395 (1962). (9) J. M . Morabito. 19th National Vacuum Symposium, Chicago, Ill., Oct. 2-5, 1972. p 278.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
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ble of i n - d e p t h profiling measurements with better than 100-A resolution o v e r a depth of 1000 A for o p t i c a l l y flat samples.
The analytical method and p r i n c i p l e s of operation of the electron m i c r o p r o b e , s e c o n d a r y i o n mass s p e c t r o m e t r y , and i o n s p u t t e r i n g - A u g e r spectroscopy are discussed w i t h p a r t i c u l a r e m p h a s i s on q u a n t i t a t i v e a n a l y s i s . The c a l i b r a t i o n procedures d e v e l o p e d for the q u a n t i t a t i v e a n a l y s i s of N , C, 0 in tantalum b y i o n s p u t t e r i n g - A u g e r and SIMS m e a s u r e m e n t s are based on the use of electron m i c r o p r o b e q u a n t i t a t i v e a n a l y s i s of h o m o g e n e o u s standards p r e p a r e d b y r e a c t i v e s p u t t e r i n g , and the m e a s u r e m e n t of p a r a m e t e r s such as A u g e r peak h e i g h t s and seco n d a r y ion c u r r e n t s of the dopants (N, C, 0) and of the m a t r i x (Ta). The h o m o g e n e o u s d i s t r i b u t i o n of the dopants in the r e a c t i v e l y sputtered films w a s e s t a b l i s h e d b y the i n - d e p t h profiles o b t a i n e d b y a controlled and g r a d u a l in. situ ion s p u t t e r i n g of the films in c o m b i n a t i o n w i t h simultaneous s e c o n d a r y ion or A u g e r electron detection. Homog e n e o u s dopant distribution w a s necessary for accurate q u a n t i t a t i v e a n a l y s i s w i t h the e l e c t r o n m i c r o p r o b e . EXPERIMENTAL Electron Microprobe. The electron microprobe is a well established analytical technique based on the emission and analysis of characteristic X-rays produced by a focused electron beam. All elements can be detected with the exception of He, Li, and H. The escape depth for the emitted X-rays is dependent upon primary electron energy (Er,) and the density of t h e target. For tantalum (density of -15 prams/cm 3 ) , the penetration depth can be calculated to be -400 A a t a primary energy of 4 keV ( 1 0 ) . X-Ray yields are a function of factors such as X-ray ionization cross sections, primary energy, target density, and other factors discussed by h u t e r ( 2 2 ) . X-Ray a n d electron images can also be obtained which provide information on the lateral distribution of elements within the area analyzed. Quantitative analysis by the electron microprobe is based on the use of pure homogeneous standards and a model of the X-ray emission process first proposed by Castaing ( 12). Castaing's quantitative model has been refined, but remains essentially unchanged with regard to its basic concepts and procedure. For accurate quantitative information, the X-ray intensities must be corrected for t h e effects of fluorescence (ie.,secondary X-ray emission), X-ray absorption, and atomic number. Fluorescence, X-ray absorption, and atomic number corrections have been discussed in detail by Philibert (13). The particular difficulties associated with light element (2 < 10) quantitative analysis have been discussed by Baird (14) and Ong ( 1 5 ) . Both Baird ( 2 4 ) and Henke (16) have emphasized that accurate X-ray absorption correction factors are of extreme importance for the quantitative analysis of light elements. For light elements such as N , C, and 0, however, the mass absorption coefficients ( p ) are quite high and are not known with sufficient precision ( I 7) for accurate (*5-10%) quantitative analysis of light Borovskii . (18) has, in fact, found that elements below -5 at. I the available absorption correction formulas are not accurate when the primary energy (El,) used is considerably greater than the ionization energy (Ek)of the excited level ( < 1 keV for elements below atomic number 10). Yakowitz and Heinrich (19) have also emphasized the possibility of serious error in the absorption correction factor for the case of light element analysis. In addition to these complications, only those X-rays emitted in the outermost surface layers can emerge from the sample when the (10) J. W. Colby. Advan. X-Ray Anal., 10, 287-305 (1967). (11) W. Reuter, Surface Sci., 25, 80 (1971). (12) R. Castaing, These de Doctorat. Universite de Paris (1951): Publication Onera (1955). (13) J . Philibert, "Modern Analytical Techniques for Metal and Alloys," Vol. I l l , Part 2, R . F. Bunshah, Ed., lnterscience Publishers, New York, N . Y . , 1970, pp 419-531. (14) A. K . Baird, Advan. X-Ray Anal., 13, 26-48 (1969). (15) P. S. Ong, Advan. X-RayAnal.. 8. 341-51 (1964). (16) B. L. Henke. Advan. X-Ray Anal., 8, 269-84 (1964). (17) B. L. Henke and R . L. Elgin. Advan. X-Ray Anal. 13, 634-65 (1969). (18) I. B. Borovskii. Proceedings of the 7th National Conference on Electron Probe Analysis, San Francisco, Calif., July 17-21, 1972. (19) H. Yakowitz and K. F. J . Heinrich. Mikrochirn. Acta, 1968, 182.
190
absorption coefficients are high. Surface conditions [particularly carbon contamination ( 1 5 ) ] can then have a pronounced effect on the X-ray intensities observed. It is for these reasons that low primary energies (29) ( - 5 at. 70) is possible and has been demonstrated ( I O , 21). Secondary I o n Emission. The technique of secondary ion emission is based on the emission and subsequent analysis by mass spectrometry of secondary ions (positive or negative) produced by high energy ion bombardment in the range of 5-15 keV. Ion bombardment at low current densities ( - 10-7 A/cm2), under suitable vacuum conditions, allows for single monolayer (surface) analysis. Bulk analysis requires higher primary ion current densities. All elements, including isotopes, can be analyzed with a sensitivity for many elements in the ppm range and for some in the ppb range (22). Primary oxygen bombardment (IeOz+) is most commonly used, since in most cases it enhances secondary ion yields ( K ) . Argon primary ions (*OA+) are used when it is necessary to detect oxygen (e.g., 0 2 in T a ) or eliminate mass interference from oxide molecular species. Since only a small fraction to for argon bombardment) of the emitted neutral particles are ionized, a finite sample volume (22) must be volatilized or sputtered to detect a secondary ion current. This sample volume will depend on the concentration (C) of the element, secondary ion yield of the element ( K ) , instrument transmission (v), atom density, and the required precision on the measurement. The yield ( K ) of the secondary ions from the target has been found to be a function of parameters such as primary ion energy ( 2 3 ) , temperature of target ( 2 4 ) , partial pressure of reactive gases surrounding the target ( 2 5 ) , and the chemistry of the target (26). The processes resulting in sputtered atom ionization are not yet completely understood, but several mechanisms have been proposed (27, 28). A complete mass spectra (0 to 300 mass units) can be taken or a selected secondary ion(s) monitored while sputtering through the sample, L e . , in-depth profile analysis. The in-depth profiles can be obtained with good depth resolution (29). In addition, the emitted secondary ions can also be focused by a n ion optical system to produce ion images with a lateral resolution of 1 pm. The Cameca Ion Analyzer, Cameca Instruments, Elmsford, N.Y., was used for the secondary ion emission measurements discussed in this paper. A detailed description of this particular instrument can be found in reference 22. Quantitative analysis by secondary ion emission based on a thermal equilibrium model of the ion emission process and on a n internal standard (e.g., composition of the matrix) has been proposed by Andersen and Hinthorne (30). Limitations on such a model include chemical and matrix effects which are known to influence the yield of secondary ions (22). Morabito and Lewis (22) have recently described a method of quantitative analysis based on the use of suitable standards and the measurement of parameters such as the secondary ion current of the impurity ( i ( a i ) ) and the secondary ion yield of the impurity relative to the matrix (Kre1).The equation (22) which relates the concentration of the impurity in a given matrix to these parameters is,
100
c = (20) D. R. Wonsidler, unpublished Bell Telephone Laboratories data. (21) C. A. Anderssn, "The Electron Microprobe," T. D. McKinely, K. F. J. Heinrich, and D. B. Wittry, Ed. Wiley, New York, N.Y., 1966, p
58. (22) (23) (24) (25)
J . M . Morabitoand R . K . Lewis, Anal. Chem., 45, 869-80 (1973). R . C. Bradley, J. Appl. Phys.. 30, 1 (1959). H. E. Stanton, J. Appl. P h y s , 31, 678 (1960). H. Beske, International Conference on Ion Surface lnteractionSputtering and Related Phenomena, Garching, Germany, Sept. 25-
28, 1972. (26) H. W . Werner, Develop. Appl. Spectrosc.. 7 A , 239 (1969). (27) R . Castaing and J . F. Hennequin, Advan. Mass Spectrom., 5 , 88 (1972). (28) C. A. Andersen, lnt. J Mass Spectrom. / o n Phys.. 2, 61 (1969). (29) C. A. Evans, Jr., and J:P. Pemsler, Anal. Chem., 42, 1130 (1970). (30) C. A. Andersen and J. R. Hinthorne, Anal. Chem 45, 1421 (1973).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 2, F E B R U A R Y 1974
where, C = concentration of impurity, (i(al)) = secondary ion current of impurity, a , = isotopic abundance of impurity, n, = isotopic abundance of matrix, K,,, = secondary ion yield of impurity relative to the matrix, Cm = concentration of matrix, and i, = secondary ion current of matrix. Normalizing the secondary ion current ( i ( a , ) )of the impurity to the measured secondary ion current of the matrix (i,) eliminates the untractable effects of changes in sputtering rate (i),sputtering yield (S)and surface atom density ( u ) during a measurement or series of measurements (22). Normalization is of paramount importance for in-depth analysis since profile shape is particularly affected by changes in the secondary ion yield of the impurity and matrix. The observed changes in ion yields are often due to the presence of reactive elements such as oxygen (26, 27) which enhances the yield of both the matrix and impurity. Figure 1 shows the profile of nitrogen in a nitrogen doped tantalum film. The measured profile shows an apparent increase of nitrogen at the surface and interface due to the presence of oxygen (22). The in-depth profile of the matrix ( T a ) was also affected by the presence of oxygen as shown in the same figure. The true profile of nitrogen could be obtained only by normalizing the nitrogen profile to the tantalum (matrix) profile. All the secondary calibration data discussed in this paper have been normalized to the matrix. Ion Sputtering-Auger Electron Spectroscopy. Auger electron spectroscopy is based on the emission and subsequent energy analysis of secondary electrons produced by high energy (3-5 keV) electron bombardment. The energy of a small fraction of the secondary electrons emitted from the sample can be related to the core levels of the target atom and these electrons, the Auger electrons, have escape depths in the 5 - to 20-A range. This low escape depth makes Auger electron spectroscopy ideal for surface analysis and for in-depth analysis when combined with in situ ion sputtering (6, 7). All elements with the exception of He and H can be detected. Selected area Auger analysis is also possible by scanning the primary electron beam to obtain a n image of the sample ( 3 1 ) or by a recently developed ( 3 2 ) optical technique. The number of Auger electrons emitted per incident primary electron, the Auger yield. is dependent on factors such as the primary electron energy ( E " ) , the angle of incidence ( a ) ,ionization cross sections (4). the Auger transition probability ($), escape depths ( d e ) ,and on back scattering correction factors ( r ) . The dependence of the MzN&4 Auger peak height with primary energy for the tantalum atom is shown in Figure 2. For a cylindrical mirror analyzer (CMA). the Auger peak height divided by the energy of the Auger transition is a relative measure of Auger yield. For quantitative Auger analysis consider: Empirical Equation. The detected Auger current(iJ), for a fixed angle of primary electron incidence ( a ) , from a particular ( e . g . , KL23L23 nitrogen) can be expressed by Auger transition (,8) the following empirical equation: (33)
i,
=
iJWq5r$E)deR
(2)
where. i,, = primary current (FA), C = concentration (atoms/ cm3), $ = Auger transition probability, 4 = ionization cross section (cm2/atom), r = back scattering correction ( > I ) , ? ( E ) = instrument transmission corrected for CMA energy resolution, de = escape depth (A),and R = surface roughness factor. The parameters in this expression can be a function of variables such as primary electron energy, surface atom density and homogeneity. angle of incidence, etc., and are not independent of each other. If these parakneters were known. or easily measured, one could then calculate the concentration ( C ) from a n accurate measurement of the Auger current. Unfortunately. parameters such as escape depths, backscattering corrections etc.. are usually not available nor conveniently measured. Therefore, calibration by the use of homogeneous standards whose concentration can be accurately ( * l O % j measured appears a t present to be the best approach to quantitative information from the Auger data. Ca/ihrntion Method Based on Ion Sputtering-Auger Anaiysis. The calibration of Auger measurements for quantitative analysis of dopants homogeneously distributed throughout the escape depth region has become possible with the development of simultaneous in situ ion sputtering-Auger analysis (6. 7). Prior to this development, quantitative analysis by calibration methods had been restricted to sub-monolayer, uniformly distributed surface deposits prepared and analyzed under ideal vacuum conditions as (31) J. M . Morabito and D. F. Munro. Appi. Phys. Lett., 21, (12), 572 (1972). (32) Physical Electronics, inc., Instrument Brochure, Edina, M i n n . (33) ti. E. Bishop and J. C. Riviere, J . A p p / . P h y s . , 40, 1740 (1969).
/ I
0
1
I
1000
t
1
2000
I
so00
I"+
I ' L I $OW
\ I
1
60
DEPTH ( A )
Figure 1. Nitrogen (l4Ni), tantalum (18'Ta+), and normalized (14N+/'8'TaC)profiles in a nitrogen doped tantalum film
Ep:4
05
10
Ep
- PRIMARY
E
-
BEAM E N E M Y (ev) BINDING ENERGY OF IONIZED L E V E L (2194 e V )
20
1.5
2 5
3.0
EVEI,
Figure 2. Tantalum M3N4N4Auger peak height vs. €,/Ek
first described by Weber and Johnson ( 3 4 ) . This work 134) established that the peak-to-peak Auger signal was a linear function of coverage below one monolayer, provided contaminants such as carbon and oxygen had not adsorbed on the deposited surface layers prior to the Auger measurements. A similar approach has recently been used by Gerlach and Ducharme (as)to determine the ahsolute and relative ionization cross sections for C , N , 0 adsorbed on a W(100) single crystal. The results obtained were in good agreement with the absolute and relative ionization cross section measurements of C. K, 0 in the gas phase previously reported by Glupe and Mehlhorn (36).The relative ionization cross sections for N , 0, C were found to he the same (3.5. .36) within experimental error. Figure 3 is a schematic of the arrangement used in this study to ion sputter and Auger analyze a sample simultaneously. Two ion guns and a n electron gun were positioned above the sample. Primary ion energies of 1 keV A-' or Xe- at current densities of 30 FA/cm2 were used and a typical crater formed on the surface of the sample is shown in Figure 4. The electron beam ( d -0.1 m m ) was placed in the center of this crater ( d > 1 m m ) . The output from the lock-in amplifier was fed to a time base recorder or a multiplexer (32) interfaced into the Auger system (32) which allows for the monitoring of six selected Auger peaks simultaneously. The detection circuit of the multiplexer automatically measures the peak-to-peak amplitudes of the selected Auger signals. This information can then be displayed as a function of time cia a point plotter. (34) R. E . Weber and A . L. Johnson, J. Appi. Phys., 40,314 (1969). (35) R . L. Gerlach and A . R . Ducharrne, Surface Sci...32. 329 (1972) (36) G . Glupe and W . Mehlhorn, Phys. i e f f . ,25A, 274 (1967).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2. FEBRUARY 1974
191
ti I
-2
1
I
- 4 5 MIN. OF SPUTTERING
---
PRIOR 1.0 SPUTTERING
1
I
SPUTTER ION BUY
Figure 3. Experimental arrangement and electronic scheme used to ion sputter and Auger analyze a sample simultaneously
0
100
200
300 400 500 AUGER ENERGY l e v )
Figure 5. Auger spectrum situ ion sputtering
600
700
of a Ta2N film prior to and after in
the film. The actual steady state surface composition for a smooth, dense binary homogeneous film ( e . g . , TaZN) can be approximated (neglecting diffusion effects) by the product of the bulk concentration and sputtering yield ratio of the two major components (Ta, N’. Unfortunately, the sputtering yields (S)for light elements such as N, C, 0 are not accurately known. P. Palmberg (37) has found that the surface composition of vacuum cleaved MgO (50 at. % Mg, 50 at. % 0 , bulk concentration) changed to 48 at. 70 0 and 52 a t . % M g as a direct result of Xe ion O )this case sputtering. The sputtering efficiency ratio ( S X ~ ~ / Sin was 1.06.
PREPARATION OF HOMOGENEOUS STANDARDS BY REACTIVE SPUTTERING
Figure 4. Crater formed on sample by 1-keV A* ions at a current density of 30 FA/cm2. The primary electron beam is placed in the center of this crater
Figure 5 shows the Auger spectrum of a Ta2N film prior to and after in situ ion sputtering. Prior to sputtering. the tantalum (179 e\’) and nitrogen (380 eV) Auger peak heights are attenuated by surface carbon (270 eV) and oxygen (512 eV). These surface contaminants must be removed to obtain a nitrogen to tantalum Auger peak height ratio which can be related t o the known composition of this film ( - 3 3 at. 70and -67 a t . 70T a ) . This contamination is most conveniently removed by in situ ion sputtering while monitoring the Auger spectra. For this film, the in situ ion sputtering completely removed the surface carbon and reduced the oxygen Auger peak height considerably. After sputtering, the oxygen peak height was indicative of the bulk oxygen concentration in the film, and the nitrogen to tantalum Auger ratio was increased and constant throughout the film. In situ ion sputtering also prevents recontamination of the analyzed surface by contaminants such as water vapor and hydrocarbons. Under the conditions used to sputter the surface (5 X Torr A + or X e f ) , the partial pressure of these contaminants is in the Torr range. Calculations, based on the kinetic theory of gases and unit sticking probability, of the time necessary to form a monolayer of water vapor or hydrocarbon (i.e.. C4H10) on tant a l u m as a function of the partial pressure of these contaminants in the vacuum chamber indicated that sputtering rates of 10 .k/min are more than adequate to prevent this contamination from depositing on the surface. There is, however, a surface compositional change due to in situ ion sputtering. The magnitude of this compositional change for a binary system (AB) will be a function of the sputtering yield ratio (S\/SH).bulk concentration ratio ( C ,/Ce) and of surface atom density. The region of this compositional change is small ( - 5 0 A) a n d steady state conditions should be established rather rapidly. Once steady state is reached. the magnitude of the Auger peak height ratio ( e . g . , N / T a in Figure 5 ) , monitored during ion sputtering, is a relative measure of the actual bulk composition of 192
Ion implantation has been found (38, 39) to be a useful means of preparing B, P, and As doped silicon samples for the quantitative calibration of Auger spectroscopy and SIMS. This method of sample preparation is less attractive for the controlled incorporation of light elements such as N, C, 0 in tantalum because of a lack of accurate range statistics (40) in tantalum. Reactive sputtering, however, is a very convenient means of incorporating dopants such as C, N, 0, into tantalum or any metal, and is most usually accomplished by adding a varying, but wellcontrolled, partial pressure of a single reactant or reactant mixture (N2, 02, CHI) t o an inert gas such as argon during sputtering from a pure metal cathode. The sputtering parameters used for this study were 5 kV and 200 mA (single phase-unfiltered dc power supply) with a 9.5-cm cathode (35-cm diameter T a ) to anode spacing. The sputtered films were deposited on Corning 7059 glass substrates coated with a Ta205 etch stop layer ( 4 1 ) . The films (-5000 .&) were optically smooth and flat which minimized surface roughness effects on the Auger and secondary ion signal intensity. Methane + argon mixtures ( 3 ) were used to prepare the carbon doped films. The nitrogen and oxygen doped films were prepared by introducing small amounts of nitrogen or oxygen into the sputtering chamber ( I , 2), and tantalum oxynitride films were prepared by introducing N2 + 0 2 mixtures ( 3 2 ) . (37) P. W . Palmberg, Auger Spectroscopy, 1972 Fail Workshop on S u r -
face Analysis and Secondary Ion Mass Analysis, Nov. 2-3, 1972. Tarrytown, N.Y. (38) J. M . Morabitoand J. C. C. Tsai. Surface Sci.. 33, 422 (1972). (39) J. C. C. Tsai, J. M . Morabito, and R. K . Lewis, 3rd international Conference on Ion Implantation. IBM, New York (1972). to be published in the proceedings of this conference. (40) J. Lindhard, M . Scharff, and H. Schiott, Mat. Fys. Medd. Dan. Vid Selsk. 33, 1 (1963). 141) R. D. Hutternann. J: M . Morabito, and D. Gerstenberg, submitted to I € € € J. Solid-state Circuits. (42) G . I . Parisi. Proceedings of Electronic Components Conf., Washington, D.C., 1969.
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 2 , F E B R U A R Y 1974
1.o
\ I
26 A t X
z
Y
3 0 lJ
I
o
I
200
I
D
Y
I
I
400 DEPTH
I
I
600
(A 4- C H 4 )
I
I
eo0
t-
-
2 ;/mi".
I
too0
( i)
Figure 6. Secondary ion carbon profiles
reactively sputtered in at 4.5 keV
0.8
ION S P U T T E R R A T E
..
tantalum thin films mixtures. 1602+ primary ions in
20 30 60 SPUTTERED TIME IN MINUTES
10
Figure 7. Ion sputtering-Auger in-depth profiles of N/O ratio for
In-depth profiles by Auger spectroscopy and SIMS measurements showed that the nitrogen and carbon were uniformly (homogeneously) distributed throughout the films over a wide concentration range. It was not possible, however, to prepare homogeneously distributed oxygen doped films below -13 at. % with the sputtering conditions used (41). The homogeneous distribution of carbon in tantalum prepared by reactive sputtering in argon CHI mixtures is shown in Figure 6 and that of the nitrogen to oxygen ratios in tantalum oxynitride films after surface oxide (-25, A) has been removed in Figure 7 . SIMS measurements prior to electron microprobe measurements indicated that these doped films also contained hydrogen and argon. Argon was also detected by Auger analysis. The amount of hydrogen is most probably < - 7 at. 5% ( 5 ) since the temperature of the films during sputtering was as high as 400 "C. The argon content (20) by electron microprobe analyses was estimated to be 5 2 at. 70. The hydrogen and argon content did not change with increasing N, C, and 0 concentration in the films. Since the only variation in concentration for these doped films was that of the dopant (N,C, 0) and the matrix (Ta), the films could be considered to be essentially binary systems.
+
Ta O,Ny films T a b l e I. Comparative D a t a of Nitrogen C o n c e n t r a t i o n E s t i m a t e d by Indirect (Difference or Material Balance) a n d by Direct (Correction Model) Electron Microprobe Measurements Indirect, Specimen
A. SiSNI B.S&NI C. Ta-0-N D. TaHfN a
at.
59.06 58.86 25.97 53.99
% & 0.30" f 0.32 f 0.39 f 0.38
Direct, at. %
58.35 58.17 26.72 54.19
i f 0.26 f 0.48 zk 0 . 5 4
Two sigma limits for 10 analyses.
Quantitative Electron Microprobe Analysis of Homogeneous S t a n d a r d s Prepared by Reactive Sputtering. The electron microprobe analyses were performed at a primary energy of 5 keV. The X-ray emergence angle was 38.5" and the analyzed depth -600 A. The X-ray intensit y ratios for the nitrogen, carbon, and oxygen measurements were obtained using Si3N4, graphite, and SiOz, respectively, as reference standards, The data were corrected using MAGIC IV ( 4 3 ) , and all samples were oxygen plasma cleaned prior to analysis. A nitrolucid (Biodynamics Research Corporation, Rockville, Md.) detector window was used for all the analyses. The diffracting crystal used for the nitrogen analyses was lead triacontanate. A lead stearate decanoate crystal was used for the carbon analysis and either chinochlore or rubidium acid phthalate for the oxygen analyses. Since the prepared standards were essentially binary systems, the nitrogen, carbon, and oxygen compositional analysis could a h be determined indirectly by difference (material balance). The direct (MAGIC IV and suitable X-ray detectors and diffracting crystals) and indirect
methods are less accurate at low ( < 5 at. %) concentrations. However, analyses by the direct and indirect methods were in good agreement at higher concentrations (20), as shown for the case of nitrogen in Table I (20). The direct and indirect methods of analysis were also in good agreement for carbon and oxygen at higher concentrations. A comparison of the quantitative analysis provided by the electron probe with that provided by nuclear microanalysis (44)is in progress, but not yet completed. The nuclear microanalysis technique is, in principle, capable of an absolute analysis of N, C, and 0 down to low concentrations (
