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Positron annihilation spectroscopy of the defect structure of sputter-deposited TiN
Surface and Coatings Technology, 36 (1988) 593 - 603
593
POSITRON ANNIHILATION SPECTROSCOPY OF THE DEFECT STRUCTURE OF SPUTTER-DEPOSITED T1N* J. P. SCHAFFER, A. B. DEWALD, JR., and R. L. FROST Georgia Institute of Technology, Atlanta, GA 30332 (U.S.A.) A. J. PERRY GTE Valenite Corporation, Troy, MI 48084 (U.S.A.) B. NIELSEN and K. G. LYNN Brookhaven National Laboratory, Upton, NY 11973 (U.S.A.) (Received February 29, 1988)
Summary Doppler and slow positron annihilation spectroscopy (PAS) techniques were used to investigate defect structures, as a function of composition and deposition sputtering power, in TiNX (0.48 ~ x ~ 1.11) thin films fabricated by physical vapor deposition methods. The recovery of the defect structures during a 1 h anneal at 900 °Cwas also studied. The present data, together with previous positron lifetime data (for the same samples), indicate that PAS has the potential to become an extremely useful technique for the characterization of atomic-scale defects in these materials. The conventional bulk PAS techniques yield consistent data if the results are corrected for the substrate contribution to the measured S parameters and the films have thicknesses greater than a few microns. For films with thicknesses on the order of 1 2 .tm the slow PAS technique must be used. -
1. Introduction Positron annihilation spectroscopy (PAS) has been used extensively in the past few decades to characterize atomic defect structures in metals. More recently, the technique has been applied to thin films of group IVB nitrides and carbides [1, 2]. Such films, as made by physical vapor deposition (PVD) techniques, are known to contain large fractions of defects such as interstitials, vacancies and/or voids. For a recent review of this subject the reader is referred to a companion paper in this volume [3]. In the present work both Doppler and slow PAS techniques were used to investigate defect structures as a function of composition in TiNX *Paper presented at the 15th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 11 - 15, 1988. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
594
(0.48 ~ x ~ 1.11) thin films fabricated by PVB methods. The influence of deposition sputtering power and the recovery of defects during annealing were also studied. The results were compared with previous lifetime PAS studies (performed on the same samples) and with data from other more conventional defect characterization techniques. 2. Theory of positron annihilation spectroscopy Several review articles have been written on the physics of PAS [4 61. Here it suffices to say that the Doppler technique yields information concerning electron momentum distributions while the effective electron density in a material can be determined using the PAS lifetime technique. Since the presence of defects alters the annihilation characteristics, PAS can be used to determine the types of defects present and their relative densities. Doppler PAS results are often interpreted using the S parameter [7] which is defined as the number of counts in some central fixed energy window divided by the total number of counts in the annihilation energy spectrum. In general, an increase in S corresponds to an increase in the defect concentration within the specimen (as long as the electron momentum distribution at the trapping site is narrower than the corresponding distribution within the bulk). The measured value of the parameter Sm is assumed to be a linear combination of the bulk value (Sb) and the S values in the various trapping states (Se) weighted by the fraction of positrons annihilating from each state (ft), i.e. -
Sm
=
5b
lb + ~ S~f~
(1)
Lifetime PAS results are interpreted by fitting the raw data using an equation of the form N(t)
~i, exp(_~!~)
(2)
where I~and r, are respectively the measured intensities and the lifetimes. The relationship between these parameters and the defect types and densities within the specimen depends on the trapping model used to interpret the data. The majority of the PAS lifetime experiments performed to date have been interpreted using the simple trapping model [8, 91. A useful quantity for comparison with S parameter data is the mean lifetime given by f—r 111+r212
(3)
In general, an increase in the defect density will result in an increased average lifetime and an increased value of S. One of the disadvantages of conventional positron sources is that results are limited to the determination of the average defect density in a region
595
which extends from the surface down to depths of about 100 tim. This is a consequence of the spread in initial kinetic energies of positrons emitted from such sources. If depth resolution is important, such as in the study of thin films and their interfaces, then a beam characterized by a narrow energy distribution is required. The technique which utilizes such a beam is known as slow (or thermal) PAS. This technique is capable of profiling defect distributions from about 10 A to about 10 j~mbelow the surface [10 121. If conventional sources are used to characterize defects in thin films then the substrate contribution to the measured S parameter must be taken into account by using the formula 5c=5fff~5sfs (4) -
where the subscripts c, f and s refer to the (film/substrate) composite, the film, and the substrate respectively. The fraction of positrons annihilating in a film of thickness x is given by [21 A(1 +B) 1—B(1—2A) (5) where A = 1 exp(—~.ix),~ is the linear absorption coefficient of the film —
and B is the backscattering factor for the substrate. 3. Experimental procedures The TiNX samples were prepared by reactive d.c. magnetron sputtering. The details of their preparation are described elsewhere [1, 13]. The film names are of the form Y—X where Y indicates the magnetron target sputtering power in kilowatts and X gives the composition of the film with respect to the formula TiNX (e.g. film 10 1.01 was deposited at 10 kW and has a composition of TiN 101). Film compositions determined by the electron microprobe method, and X-ray measurements indicate that all films are single phase (with the possible exception of sample 8 0.43) [1]. The annealing treatment was for 1 h at 900 °Cin vacuum. Two different positron sources were onto used afor the (approximately Doppler experiments. 22Na deposited thin 5 jim) One about 30and pC the of other consisted of about 30 j.tC of 68Ge deposited sheet was of titanium onto a similar titanium foil. Identical specimens were placed on either side of one of the sources in the standard sandwich configuration. The annihilation energy spectrometer is based on an Ortec coaxial Ge(Li) detector with an energy resolution of about 1.20 keV as measured by the full width at half-maximum of the 514 keV 85Sr peak. Each Doppler spectrum contained 20000 counts in the peak channel (about 1.1 X 106 total counts). The lifetime PAS results were obtained in a previous study [1]. Each lifetime spectrum was resolved into three components without constraints on the lifetimes or intensities. A source correction was applied and the variance of fit was between 0.99 and 1.19 in all cases. Each spectrum contained approximately 106 counts. -
-
596
The variable energy (slow) positron beam has been described previously [12]. In brief, the system operates within an ultrahigh vacuum chamber and consists of a 22Na positron source moderated by a tungsten crystal with an efficiency of 5 X iO—~.The beam is magnetically focused onto the target and at each incident energy 5 X io~counts (annihilation events) are collected at a rate of 2 kHz. The slow PAS system operates in the Doppler mode.
4. Results Table 1 lists the Doppler S parameters for the TiNX thin films. Each S value is the average of at least five spectra per specimen. Six values are given for each film, three for the as-received material and three for the annealed samples. The S~values were obtained with the sample pairs in a back-to-back orientation and are characteristic of the metallurgical condition of the substrates. The S~,values were obtained in the front-to-front orientation and reflect the characteristics of the thin film/substrate composite. All the experiments summarized in Table 1 were performed using a 22Na source. The corresponding experiments using 68Ge yielded qualitatively similar results. Sf values were calculated from eqn. (4) using values for f~ obtained from eqn. (5). The accuracy of the S~values is estimated to be between 0.000 32 and 0.000 85. This error estimate reflects only the statistical uncertainty associated with the Doppler experiments and does not include errors associated with either the calculated f~ values or the thickness values. TABLE 1
S parameters for the substrates (Sq) (measured), the film/substrate composites (Se) (measured) and the TiNX thin films (Se) (calculated) Film
Xa
ffb
S~(as
0.150 0.147 0.168 0.158 0.174 0.162 0.078 0.074 0.068 0.074 0.044
0.50068 0.49122 0.49846 0.49995 0.49337 0.48953 0.49478 0.48985 0.48768 0.48649 0.49300
(jim) 10 - 0.52 10 -0.76 10 -0.84 10 - 0.94 10 - 1.01 10 - 1.05 8 - 0.48 8 - 0.72 8 - 1.00 8 - 1.07 8 - 1.11
4.5 4.4 5.1 4.8 5.3 4.9 2.2 2.1 1.9 2.1 1.2
S~(as Sf (as S~ S~ S~ received) received) received) (annealed) (annealed) (annealed) 0.50518 0.49500 0.50227 0.50314 0.49312 0.48257 0.49350 0.49033 0.48731 0.48630 0.49272
0.53064 0.51691 0.52118 0.52017 0.49190 0.44657 0.47832 0.49622 0.48219 0.48391 0.48647
0.47899 0.47835 0.47792 0.48254 0.48490 0.47842 0.47824 0.47829 0.47742 0.47833 0.47661
0.47864 0.48056 0.47937 0.48131 0.48155 0.47823 0.47865 0.47761 0.47832 0.47744 0.47753
0.47666 0.49338 0.48655 0.47476 0.46565 0.47725 0.48350c 0.47253’~ 0.49065c 0.46633c 0.49749c
a Film thickness. bFraction of positrons annihilating in the film. cThese values are highly suspect since the films are comparatively thin and the S parameters of the substrates changed significantly during the annealing process.
597 0.54-
0.52-
0.50-
---.~
0.48-
0.46-
0.44-
0.84 0.52
0.76
0.94
1.01 1.05
N:Ti Ratio Fig. 1. The correlation between S~(•) and Sf (S) for the 10 kW samples (as-received condition). Also shown are the changes in S~(0) and Sf (0) after a 1 h anneal at 900 °C.
The S values for the 10 kW samples show a correlation between S~and Sf (Fig. 1). The changes in the S values after a 1 h anneal at 900 °Care also shown in this figure. The S values for the annealed samples are lower than the corresponding values for the as-received samples (film 10 1.05 is an exception). Figure 2(a) is a comparison of the Sf values for the 10 kW and the 8 kW samples as a function of composition. The values for the 8 kW samples lie below the corresponding values for the 10 kW samples (the exception is sample 10 1.05). Figure 2(b) is an expanded view of the S parameters for the near-stoichiometric 8 kW specimens. Error bars are shown in this figure since they are larger than the data points for the expanded S scale (in the previous figures the error bars are smaller than the data points). The results of the slow PAS experiments are shown in Fig. 3 for films 8 0.72, 8 1.0 and 8 1.07 in the as-received condition. The positron implantation energy can be equated to an implantation depth if the material density and positron diffusion lengths are known. By using this technique, the influence of the substrate on the measured S parameters can be minimized. -
-
-
-
-
5. Discussion The S parameter data for the 10 kW samples show a correlation between S~,S~,and S~(see Table 1 and Fig. 1). The correlation between S3
598 0.54— .__
-
0.52—
::::~• 0.46— 0.44—
.52 ~ .48
.76 .72
(a)
1.01.11
.84 .94 N:Ti Ratio
1.05
.486— Sf
0.496 .484—
.482— .72
(b)
1.0
1.07 1.11
14:71 Ratio
Fig. 2. (a) A comparison of the calculated S parameters for a series of films sputter deposited at 10 kW (•) and a similar series deposited at 8 kW (0); (b) an enlarged view of the S parameter values for the 8 kW films near the stoichiometric composition.
and S~is not surprising since approximately 85% of the annihilation events contributing to S~occur within the substrate. However, the correlation between S~and S~is unexpected. Such a result suggests that either (1) defect structures associated with the substrate surface are being propagated into the thin films during deposition or (2) the deposition procedure is altering the structure of the substrate. An examination of the S values for the 8 kW films shows that while the S
5—S~correlation is still apparent there is no correlation between S3 and S~. Unfortunately, the substrates were not individually characterized prior to deposition; therefore it is impossible to rule out explanation (1) above. However, the observation that S~and S~are uncorrelated in the 8 kW samples suggests that the second explanation is more likely. If so, the differences in the low power samples can be attributed to the change in deposition parameters between the two sputtering powers (i.e. surface temperatures, deposition times etc.). Additional experiments are planned to investigate the cause of this correlation. Figure 1 also shows the effects of a 1 h anneal at 900 °Con the S parameters for the 10 kW samples. The S values are decreased by the anneal, indicating a decrease in the average defect concentration (the exception is film 10 1.05). Several trends are apparent in this data. First, S~is nearly constant and indicates that any differences in the metallurgical structure of the as-deposited substrates are essentially removed by the anneal. Secondly, -
599
since the higher point defect concentration in the films allows more recovery during the tempering process, the average decrease in the S values for the films is larger than the corresponding decrease for the substrates. Finally, the annealingprocess results in an increase in the S~value for specimen 10 1.05. This singular result will be discussed in more detail below. The bulk PAS measurements of the annealed 8 kW samples appear to provide contradictory S~values for three of the five films S~is increased by the annealing treatment. Since subsequent slow PAS measurements verified the bulk PAS results for the as-received films (see below), the inconsistencies in the annealed data cannot be explained solely on the basis of the small fraction of positrons annihilating in these films (i.e. 4% 8%). Several other effects complicate the simplistic positron—defect interaction model upon which eqns. (4) and (5) are based. For example, the S~values are changed significantly by the anneal and the magnitude of the change varies from film to film. As the substrate becomes less defective the positron diffusion length may increase allowing a larger fraction of positrons to annihilate at the thin film—substrate interface and/or within the thin film itself. If so, the calculated f~ values may be too low and hence the calculated Sf values may be in error. It should be noted that although the same processes would occur in the thicker 10 kW films the relative increase in f~ would be much smaller. These and other factors may result in synergistic effects which are not resolvable using the standard bulk techniques and eqns. (4) and (5). A thorough analysis of this apparent problem is currently under way. The influence of deposition sputtering power on the S~values is shown in Fig. 2. Since the S values for the 8 kW films lie below the values for the 10 kW films (again the exception is film 10 1.05) it appears that the lower deposition power results in fewer as-deposited defects. This result is in agreement with previous optical data for these films [14]. Figure 2(b) is an enlarged view of the S parameter values for the 8 kW films near the stoichiometric composition. For these films there is a minimum in the S~values for the stoichiometric composition. Thus, the defect structure in the nitrogen-rich films deposited at 8 kW appears to be fundamentally different from the defect structure in the corresponding 10 kW films. Throughout the course of this discussion film 10 1.05 has consistently been an exception to the observed trends in the data. At least two possible explanations exist. First, sample 10 1.05 may be an anomaly (additional experiments are under way to confirm the validity of the S~value for film 10 1.05). Secondly, if this result is reproducible it indicates that the defect structure of the nitrogen-rich film is substantially different than that of the titanium-rich films (and that the defect structure in the superstoichiometric films is a function of the deposition power). Since it is unlikely that an anneal would increase the defect density of the film, it must be concluded that a decrease in defect density has resulted in an increase in S~.Therefore there must be defects in the superstoichiometric film which can trap positrons but which result in a decrease in the S parameter. This is an unusual -
—
-
-
-
-
-
600
result since positron traps (such as vacancies, voids, grain boundaries and other open volume types of defects) can be expected to have momentum distributions which are narrower than the corresponding distribution within the bulk (non-defective) regions of the crystal (i.e. S(defect) > S(bulk)). The results of the previous lifetime PAS investigation [1] of these films are helpful in understanding the present Doppler results. Brunner and Perry concluded that their lifetime results could not be interpreted using the simple trapping model. The basis for their conclusion was the weak dependence of the average lifetime on the composition (defect density) of the TiNX films. The average lifetime showed a maximum at the stoichiometric composition and decreased to a relatively constant value for all other films (only the 10 kW films were studied by the lifetime technique). The authors of the lifetime paper explain their results in terms of the charge transfer from titanium to nitrogen as measured by X-ray diffraction and angle-resolved photoemission studies [15]. In their model the titanium atoms surrounding a nitrogen vacancy are positively charged and thus effectively screen the vacancy so that it is no longer an efficient positron trap. In contrast, a titanium vacancy is s strong positron trap because it is surrounded by a net negative charge. Further, a diffusion of titanium atoms into the defective nitrogen sublattice is possible at high defect concentrations. Thus, even for the titanium-rich samples there may be vacancies on the titanium sublattice which are more efficient positron traps than nitrogen vacancies. The Doppler results for both the 8 kW and the 10 kW samples are reasonably consistent with this interpretation; however, there are a few points which require further discussion. As shown in Fig. 1, the S parameter is essentially constant for all the titanium-rich 10 kW films in the as-received condition. This can be interpreted as saturation trapping, i.e. the defect density for film 10 0.94 is already high enough for every positron to be trapped by a defect. Therefore further increases in the defect concentration do not appreciably alter S~.The 8 kW data are inconclusive since there are no data points for 0.72
-
601
experimental evidence and additional trapping model calculations are required before an unambiguous interpretation of the PAS results in TiNX films can be made. In an effort to remove the substrate contribution from the Doppler S parameter data and to study the spatial distribution of defects within the TiNX films, some preliminary slow PAS experiments were performed. A quantitative analysis of the slow PAS data will be the subject of a future paper; however, the raw data will be discussed briefly in this work. Figure 3 shows the S parameter as a function of positron implantation energy for films 8 0.72, 8 1.0 and 8 1.07 in the as-received condition. The implantation energy can be equated to an implantation depth if the material density and positron diffusion lengths are known. The S values corresponding to the implantation energy range 0 20 keV are related to the defect structure of the TiNX thin films. It can be seen that the stoichiometric film appears to be less defective (i.e. have lower 5 values) than either of the other two films. This result is in agreement with the bulk PAS results. In fact, even the relative magnitude of the differences between films seems to be consistent. In both the bulk and the slow PAS measurements the difference between films 8 0.72 and 8 1.0 is greater than the difference between films 8 1.0 and 8 1.07. As the implantation energy is increased a significant fraction of the positrons is implanted into the substrate. Thus the S values at high energies reflect the metallurgical condition of the substrate rather than that of the -
-
-
-
-
-
-
-
0.47
I: : ~ 0.41 0
10
20
30
40
50
60
ENERGY (key)
Fig. 3. The S parameter values, as a function of positron implantation energy, for films 8 -0.72 (•), 8 - 1.0 (U) and 8 - 1.07 (A) in the as-received condition. In the interest of clarity, only a few of the actual data points have been marked with the appropriate symbols. Each change in slope of the lines represents an individual data point.
602
film. The inversion of the values for film 8 1.0 and film 8 1.07 should be noted. This is consistent with the bulk S~values given in Table 1. Although it is not shown in Fig. 3, slow PAS was able to resolve the change in film 8 1.0 which occurred during the anneal [3]. The S parameters for the annealed specimen were less than the corresponding values for the as-received specimen. The differences were larger in the substrate than they were within the thin film. Since the standard Doppler PAS technique was unable to resolve these changes, the success of the slow technique emphasizes its utility in the study of TiNX films with thicknesses of less than a few microns. -
-
-
6. Conclusions The present Doppler and slow PAS data, together with previous positron lifetime data, indicate that PAS has the potential to become an extremely useful technique for the characterization of atomic-scale defects in TiNX thin films deposited by PVD methods. PAS was able to resolve differences in the atomic-scale defect structures resulting from changes in
composition, differences in the deposition sputtering powers, and annealing. The conventional bulk PAS techniques were shown to yield consistent data if the results are corrected for the substrate contribution to the measured S parameters and the films have thicknesses greater than a few microns. However, both additional PAS experiments and complementary experiments using other characterization techniques will be required to yield more quantitative information concerning the defect structure of TiNX thin films. For films with thicknesses on the order of 1 2 jim the slow PAS technique was able to resolve changes in the defect structures which occurred during an anneal and which were not resolvable by the standard Doppler technique. Although consistent qualitative results have been obtained, an unambiguous and quantitative analysis will require the development of an appropriate trapping model capable of describing the interaction of positrons with the complex defect structures present in PVD thin films. -
Acknowledgments This research was supported in part by the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract DE/ACO2-76CH00016. The authors would like to thank T. C. Leung for assistance with the slow PAS measurements and J. Brunner for his help in the interpretation of the bulk PAS data. References 1 J. Brunner and A. J. Perry, Proc. 7th mt. Con!. on Thin Films, New Delhi, 1987, in Thin Solid Films, 163 (1988). 2 J. Brunner and A. J. Perry, Thin Solid Films, 153 (1987) 103.
603
3 A. B. DeWald, Jr., and J. P. Schaffer, 15th mt. Con[ on Metallurgical Coatings, San Diego, CA, 1988, in Surf. Coat. Technol. 4 R. N. West, Positron Studies of Condensed Matter, Taylor and Francis, London, 1974. 5 I. Ya. Dekhtyar, Phys. Rep., 9 (1974) 243. 6 M. Eldrup, Applications of the positron annihilation technique in studies of defects in solids. In Defects in Solids, Plenum, New York, 1985. 7 I. K. MacKenzie, J. A. Eady and R. R. Gingerich, Phys. Lett. A, 33 (1970) 279. 8 B. Bergersen and M. J. Stott, Solid State Commun., 7 (1969) 1203. 9 D. C. Conners and R. N. West, Phys. Lett. A, 30 (1969) 24. 10 K. G. Lynn and H. Lutz, Rev. Sci. Instrum., 51 (1980) 977. 11 A. Vehanen, K. G. Lynn, P. J. Schultz and M. Eldrup, AppI. Phys. A, 32 (1983) 163. 12 K. G. Lynn, B. Nielsen and J. H. Quatman, Appi. Phys. Lett., 47 (1985) 239. 13 A. J. Perry, C. Strandberg, W. D. Sproul, S. Hoffmann, C. Ernsberger, J. Nickerson and L. Chollet, Thin Solid Films, 153 (1987) 169. 14 A. J. Perry, M. Georgson and W. D. Sproul, Thin Solid Films, 157 (1988) 255. 15 K. Schwarz, CRC Crit. Rev. Solid State Mater. Sci., 13 (1987) 211.
593
POSITRON ANNIHILATION SPECTROSCOPY OF THE DEFECT STRUCTURE OF SPUTTER-DEPOSITED T1N* J. P. SCHAFFER, A. B. DEWALD, JR., and R. L. FROST Georgia Institute of Technology, Atlanta, GA 30332 (U.S.A.) A. J. PERRY GTE Valenite Corporation, Troy, MI 48084 (U.S.A.) B. NIELSEN and K. G. LYNN Brookhaven National Laboratory, Upton, NY 11973 (U.S.A.) (Received February 29, 1988)
Summary Doppler and slow positron annihilation spectroscopy (PAS) techniques were used to investigate defect structures, as a function of composition and deposition sputtering power, in TiNX (0.48 ~ x ~ 1.11) thin films fabricated by physical vapor deposition methods. The recovery of the defect structures during a 1 h anneal at 900 °Cwas also studied. The present data, together with previous positron lifetime data (for the same samples), indicate that PAS has the potential to become an extremely useful technique for the characterization of atomic-scale defects in these materials. The conventional bulk PAS techniques yield consistent data if the results are corrected for the substrate contribution to the measured S parameters and the films have thicknesses greater than a few microns. For films with thicknesses on the order of 1 2 .tm the slow PAS technique must be used. -
1. Introduction Positron annihilation spectroscopy (PAS) has been used extensively in the past few decades to characterize atomic defect structures in metals. More recently, the technique has been applied to thin films of group IVB nitrides and carbides [1, 2]. Such films, as made by physical vapor deposition (PVD) techniques, are known to contain large fractions of defects such as interstitials, vacancies and/or voids. For a recent review of this subject the reader is referred to a companion paper in this volume [3]. In the present work both Doppler and slow PAS techniques were used to investigate defect structures as a function of composition in TiNX *Paper presented at the 15th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 11 - 15, 1988. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
594
(0.48 ~ x ~ 1.11) thin films fabricated by PVB methods. The influence of deposition sputtering power and the recovery of defects during annealing were also studied. The results were compared with previous lifetime PAS studies (performed on the same samples) and with data from other more conventional defect characterization techniques. 2. Theory of positron annihilation spectroscopy Several review articles have been written on the physics of PAS [4 61. Here it suffices to say that the Doppler technique yields information concerning electron momentum distributions while the effective electron density in a material can be determined using the PAS lifetime technique. Since the presence of defects alters the annihilation characteristics, PAS can be used to determine the types of defects present and their relative densities. Doppler PAS results are often interpreted using the S parameter [7] which is defined as the number of counts in some central fixed energy window divided by the total number of counts in the annihilation energy spectrum. In general, an increase in S corresponds to an increase in the defect concentration within the specimen (as long as the electron momentum distribution at the trapping site is narrower than the corresponding distribution within the bulk). The measured value of the parameter Sm is assumed to be a linear combination of the bulk value (Sb) and the S values in the various trapping states (Se) weighted by the fraction of positrons annihilating from each state (ft), i.e. -
Sm
=
5b
lb + ~ S~f~
(1)
Lifetime PAS results are interpreted by fitting the raw data using an equation of the form N(t)
~i, exp(_~!~)
(2)
where I~and r, are respectively the measured intensities and the lifetimes. The relationship between these parameters and the defect types and densities within the specimen depends on the trapping model used to interpret the data. The majority of the PAS lifetime experiments performed to date have been interpreted using the simple trapping model [8, 91. A useful quantity for comparison with S parameter data is the mean lifetime given by f—r 111+r212
(3)
In general, an increase in the defect density will result in an increased average lifetime and an increased value of S. One of the disadvantages of conventional positron sources is that results are limited to the determination of the average defect density in a region
595
which extends from the surface down to depths of about 100 tim. This is a consequence of the spread in initial kinetic energies of positrons emitted from such sources. If depth resolution is important, such as in the study of thin films and their interfaces, then a beam characterized by a narrow energy distribution is required. The technique which utilizes such a beam is known as slow (or thermal) PAS. This technique is capable of profiling defect distributions from about 10 A to about 10 j~mbelow the surface [10 121. If conventional sources are used to characterize defects in thin films then the substrate contribution to the measured S parameter must be taken into account by using the formula 5c=5fff~5sfs (4) -
where the subscripts c, f and s refer to the (film/substrate) composite, the film, and the substrate respectively. The fraction of positrons annihilating in a film of thickness x is given by [21 A(1 +B) 1—B(1—2A) (5) where A = 1 exp(—~.ix),~ is the linear absorption coefficient of the film —
and B is the backscattering factor for the substrate. 3. Experimental procedures The TiNX samples were prepared by reactive d.c. magnetron sputtering. The details of their preparation are described elsewhere [1, 13]. The film names are of the form Y—X where Y indicates the magnetron target sputtering power in kilowatts and X gives the composition of the film with respect to the formula TiNX (e.g. film 10 1.01 was deposited at 10 kW and has a composition of TiN 101). Film compositions determined by the electron microprobe method, and X-ray measurements indicate that all films are single phase (with the possible exception of sample 8 0.43) [1]. The annealing treatment was for 1 h at 900 °Cin vacuum. Two different positron sources were onto used afor the (approximately Doppler experiments. 22Na deposited thin 5 jim) One about 30and pC the of other consisted of about 30 j.tC of 68Ge deposited sheet was of titanium onto a similar titanium foil. Identical specimens were placed on either side of one of the sources in the standard sandwich configuration. The annihilation energy spectrometer is based on an Ortec coaxial Ge(Li) detector with an energy resolution of about 1.20 keV as measured by the full width at half-maximum of the 514 keV 85Sr peak. Each Doppler spectrum contained 20000 counts in the peak channel (about 1.1 X 106 total counts). The lifetime PAS results were obtained in a previous study [1]. Each lifetime spectrum was resolved into three components without constraints on the lifetimes or intensities. A source correction was applied and the variance of fit was between 0.99 and 1.19 in all cases. Each spectrum contained approximately 106 counts. -
-
596
The variable energy (slow) positron beam has been described previously [12]. In brief, the system operates within an ultrahigh vacuum chamber and consists of a 22Na positron source moderated by a tungsten crystal with an efficiency of 5 X iO—~.The beam is magnetically focused onto the target and at each incident energy 5 X io~counts (annihilation events) are collected at a rate of 2 kHz. The slow PAS system operates in the Doppler mode.
4. Results Table 1 lists the Doppler S parameters for the TiNX thin films. Each S value is the average of at least five spectra per specimen. Six values are given for each film, three for the as-received material and three for the annealed samples. The S~values were obtained with the sample pairs in a back-to-back orientation and are characteristic of the metallurgical condition of the substrates. The S~,values were obtained in the front-to-front orientation and reflect the characteristics of the thin film/substrate composite. All the experiments summarized in Table 1 were performed using a 22Na source. The corresponding experiments using 68Ge yielded qualitatively similar results. Sf values were calculated from eqn. (4) using values for f~ obtained from eqn. (5). The accuracy of the S~values is estimated to be between 0.000 32 and 0.000 85. This error estimate reflects only the statistical uncertainty associated with the Doppler experiments and does not include errors associated with either the calculated f~ values or the thickness values. TABLE 1
S parameters for the substrates (Sq) (measured), the film/substrate composites (Se) (measured) and the TiNX thin films (Se) (calculated) Film
Xa
ffb
S~(as
0.150 0.147 0.168 0.158 0.174 0.162 0.078 0.074 0.068 0.074 0.044
0.50068 0.49122 0.49846 0.49995 0.49337 0.48953 0.49478 0.48985 0.48768 0.48649 0.49300
(jim) 10 - 0.52 10 -0.76 10 -0.84 10 - 0.94 10 - 1.01 10 - 1.05 8 - 0.48 8 - 0.72 8 - 1.00 8 - 1.07 8 - 1.11
4.5 4.4 5.1 4.8 5.3 4.9 2.2 2.1 1.9 2.1 1.2
S~(as Sf (as S~ S~ S~ received) received) received) (annealed) (annealed) (annealed) 0.50518 0.49500 0.50227 0.50314 0.49312 0.48257 0.49350 0.49033 0.48731 0.48630 0.49272
0.53064 0.51691 0.52118 0.52017 0.49190 0.44657 0.47832 0.49622 0.48219 0.48391 0.48647
0.47899 0.47835 0.47792 0.48254 0.48490 0.47842 0.47824 0.47829 0.47742 0.47833 0.47661
0.47864 0.48056 0.47937 0.48131 0.48155 0.47823 0.47865 0.47761 0.47832 0.47744 0.47753
0.47666 0.49338 0.48655 0.47476 0.46565 0.47725 0.48350c 0.47253’~ 0.49065c 0.46633c 0.49749c
a Film thickness. bFraction of positrons annihilating in the film. cThese values are highly suspect since the films are comparatively thin and the S parameters of the substrates changed significantly during the annealing process.
597 0.54-
0.52-
0.50-
---.~
0.48-
0.46-
0.44-
0.84 0.52
0.76
0.94
1.01 1.05
N:Ti Ratio Fig. 1. The correlation between S~(•) and Sf (S) for the 10 kW samples (as-received condition). Also shown are the changes in S~(0) and Sf (0) after a 1 h anneal at 900 °C.
The S values for the 10 kW samples show a correlation between S~and Sf (Fig. 1). The changes in the S values after a 1 h anneal at 900 °Care also shown in this figure. The S values for the annealed samples are lower than the corresponding values for the as-received samples (film 10 1.05 is an exception). Figure 2(a) is a comparison of the Sf values for the 10 kW and the 8 kW samples as a function of composition. The values for the 8 kW samples lie below the corresponding values for the 10 kW samples (the exception is sample 10 1.05). Figure 2(b) is an expanded view of the S parameters for the near-stoichiometric 8 kW specimens. Error bars are shown in this figure since they are larger than the data points for the expanded S scale (in the previous figures the error bars are smaller than the data points). The results of the slow PAS experiments are shown in Fig. 3 for films 8 0.72, 8 1.0 and 8 1.07 in the as-received condition. The positron implantation energy can be equated to an implantation depth if the material density and positron diffusion lengths are known. By using this technique, the influence of the substrate on the measured S parameters can be minimized. -
-
-
-
-
5. Discussion The S parameter data for the 10 kW samples show a correlation between S~,S~,and S~(see Table 1 and Fig. 1). The correlation between S3
598 0.54— .__
-
0.52—
::::~• 0.46— 0.44—
.52 ~ .48
.76 .72
(a)
1.01.11
.84 .94 N:Ti Ratio
1.05
.486— Sf
0.496 .484—
.482— .72
(b)
1.0
1.07 1.11
14:71 Ratio
Fig. 2. (a) A comparison of the calculated S parameters for a series of films sputter deposited at 10 kW (•) and a similar series deposited at 8 kW (0); (b) an enlarged view of the S parameter values for the 8 kW films near the stoichiometric composition.
and S~is not surprising since approximately 85% of the annihilation events contributing to S~occur within the substrate. However, the correlation between S~and S~is unexpected. Such a result suggests that either (1) defect structures associated with the substrate surface are being propagated into the thin films during deposition or (2) the deposition procedure is altering the structure of the substrate. An examination of the S values for the 8 kW films shows that while the S
5—S~correlation is still apparent there is no correlation between S3 and S~. Unfortunately, the substrates were not individually characterized prior to deposition; therefore it is impossible to rule out explanation (1) above. However, the observation that S~and S~are uncorrelated in the 8 kW samples suggests that the second explanation is more likely. If so, the differences in the low power samples can be attributed to the change in deposition parameters between the two sputtering powers (i.e. surface temperatures, deposition times etc.). Additional experiments are planned to investigate the cause of this correlation. Figure 1 also shows the effects of a 1 h anneal at 900 °Con the S parameters for the 10 kW samples. The S values are decreased by the anneal, indicating a decrease in the average defect concentration (the exception is film 10 1.05). Several trends are apparent in this data. First, S~is nearly constant and indicates that any differences in the metallurgical structure of the as-deposited substrates are essentially removed by the anneal. Secondly, -
599
since the higher point defect concentration in the films allows more recovery during the tempering process, the average decrease in the S values for the films is larger than the corresponding decrease for the substrates. Finally, the annealingprocess results in an increase in the S~value for specimen 10 1.05. This singular result will be discussed in more detail below. The bulk PAS measurements of the annealed 8 kW samples appear to provide contradictory S~values for three of the five films S~is increased by the annealing treatment. Since subsequent slow PAS measurements verified the bulk PAS results for the as-received films (see below), the inconsistencies in the annealed data cannot be explained solely on the basis of the small fraction of positrons annihilating in these films (i.e. 4% 8%). Several other effects complicate the simplistic positron—defect interaction model upon which eqns. (4) and (5) are based. For example, the S~values are changed significantly by the anneal and the magnitude of the change varies from film to film. As the substrate becomes less defective the positron diffusion length may increase allowing a larger fraction of positrons to annihilate at the thin film—substrate interface and/or within the thin film itself. If so, the calculated f~ values may be too low and hence the calculated Sf values may be in error. It should be noted that although the same processes would occur in the thicker 10 kW films the relative increase in f~ would be much smaller. These and other factors may result in synergistic effects which are not resolvable using the standard bulk techniques and eqns. (4) and (5). A thorough analysis of this apparent problem is currently under way. The influence of deposition sputtering power on the S~values is shown in Fig. 2. Since the S values for the 8 kW films lie below the values for the 10 kW films (again the exception is film 10 1.05) it appears that the lower deposition power results in fewer as-deposited defects. This result is in agreement with previous optical data for these films [14]. Figure 2(b) is an enlarged view of the S parameter values for the 8 kW films near the stoichiometric composition. For these films there is a minimum in the S~values for the stoichiometric composition. Thus, the defect structure in the nitrogen-rich films deposited at 8 kW appears to be fundamentally different from the defect structure in the corresponding 10 kW films. Throughout the course of this discussion film 10 1.05 has consistently been an exception to the observed trends in the data. At least two possible explanations exist. First, sample 10 1.05 may be an anomaly (additional experiments are under way to confirm the validity of the S~value for film 10 1.05). Secondly, if this result is reproducible it indicates that the defect structure of the nitrogen-rich film is substantially different than that of the titanium-rich films (and that the defect structure in the superstoichiometric films is a function of the deposition power). Since it is unlikely that an anneal would increase the defect density of the film, it must be concluded that a decrease in defect density has resulted in an increase in S~.Therefore there must be defects in the superstoichiometric film which can trap positrons but which result in a decrease in the S parameter. This is an unusual -
—
-
-
-
-
-
600
result since positron traps (such as vacancies, voids, grain boundaries and other open volume types of defects) can be expected to have momentum distributions which are narrower than the corresponding distribution within the bulk (non-defective) regions of the crystal (i.e. S(defect) > S(bulk)). The results of the previous lifetime PAS investigation [1] of these films are helpful in understanding the present Doppler results. Brunner and Perry concluded that their lifetime results could not be interpreted using the simple trapping model. The basis for their conclusion was the weak dependence of the average lifetime on the composition (defect density) of the TiNX films. The average lifetime showed a maximum at the stoichiometric composition and decreased to a relatively constant value for all other films (only the 10 kW films were studied by the lifetime technique). The authors of the lifetime paper explain their results in terms of the charge transfer from titanium to nitrogen as measured by X-ray diffraction and angle-resolved photoemission studies [15]. In their model the titanium atoms surrounding a nitrogen vacancy are positively charged and thus effectively screen the vacancy so that it is no longer an efficient positron trap. In contrast, a titanium vacancy is s strong positron trap because it is surrounded by a net negative charge. Further, a diffusion of titanium atoms into the defective nitrogen sublattice is possible at high defect concentrations. Thus, even for the titanium-rich samples there may be vacancies on the titanium sublattice which are more efficient positron traps than nitrogen vacancies. The Doppler results for both the 8 kW and the 10 kW samples are reasonably consistent with this interpretation; however, there are a few points which require further discussion. As shown in Fig. 1, the S parameter is essentially constant for all the titanium-rich 10 kW films in the as-received condition. This can be interpreted as saturation trapping, i.e. the defect density for film 10 0.94 is already high enough for every positron to be trapped by a defect. Therefore further increases in the defect concentration do not appreciably alter S~.The 8 kW data are inconclusive since there are no data points for 0.72
-
601
experimental evidence and additional trapping model calculations are required before an unambiguous interpretation of the PAS results in TiNX films can be made. In an effort to remove the substrate contribution from the Doppler S parameter data and to study the spatial distribution of defects within the TiNX films, some preliminary slow PAS experiments were performed. A quantitative analysis of the slow PAS data will be the subject of a future paper; however, the raw data will be discussed briefly in this work. Figure 3 shows the S parameter as a function of positron implantation energy for films 8 0.72, 8 1.0 and 8 1.07 in the as-received condition. The implantation energy can be equated to an implantation depth if the material density and positron diffusion lengths are known. The S values corresponding to the implantation energy range 0 20 keV are related to the defect structure of the TiNX thin films. It can be seen that the stoichiometric film appears to be less defective (i.e. have lower 5 values) than either of the other two films. This result is in agreement with the bulk PAS results. In fact, even the relative magnitude of the differences between films seems to be consistent. In both the bulk and the slow PAS measurements the difference between films 8 0.72 and 8 1.0 is greater than the difference between films 8 1.0 and 8 1.07. As the implantation energy is increased a significant fraction of the positrons is implanted into the substrate. Thus the S values at high energies reflect the metallurgical condition of the substrate rather than that of the -
-
-
-
-
-
-
-
0.47
I: : ~ 0.41 0
10
20
30
40
50
60
ENERGY (key)
Fig. 3. The S parameter values, as a function of positron implantation energy, for films 8 -0.72 (•), 8 - 1.0 (U) and 8 - 1.07 (A) in the as-received condition. In the interest of clarity, only a few of the actual data points have been marked with the appropriate symbols. Each change in slope of the lines represents an individual data point.
602
film. The inversion of the values for film 8 1.0 and film 8 1.07 should be noted. This is consistent with the bulk S~values given in Table 1. Although it is not shown in Fig. 3, slow PAS was able to resolve the change in film 8 1.0 which occurred during the anneal [3]. The S parameters for the annealed specimen were less than the corresponding values for the as-received specimen. The differences were larger in the substrate than they were within the thin film. Since the standard Doppler PAS technique was unable to resolve these changes, the success of the slow technique emphasizes its utility in the study of TiNX films with thicknesses of less than a few microns. -
-
-
6. Conclusions The present Doppler and slow PAS data, together with previous positron lifetime data, indicate that PAS has the potential to become an extremely useful technique for the characterization of atomic-scale defects in TiNX thin films deposited by PVD methods. PAS was able to resolve differences in the atomic-scale defect structures resulting from changes in
composition, differences in the deposition sputtering powers, and annealing. The conventional bulk PAS techniques were shown to yield consistent data if the results are corrected for the substrate contribution to the measured S parameters and the films have thicknesses greater than a few microns. However, both additional PAS experiments and complementary experiments using other characterization techniques will be required to yield more quantitative information concerning the defect structure of TiNX thin films. For films with thicknesses on the order of 1 2 jim the slow PAS technique was able to resolve changes in the defect structures which occurred during an anneal and which were not resolvable by the standard Doppler technique. Although consistent qualitative results have been obtained, an unambiguous and quantitative analysis will require the development of an appropriate trapping model capable of describing the interaction of positrons with the complex defect structures present in PVD thin films. -
Acknowledgments This research was supported in part by the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences under Contract DE/ACO2-76CH00016. The authors would like to thank T. C. Leung for assistance with the slow PAS measurements and J. Brunner for his help in the interpretation of the bulk PAS data. References 1 J. Brunner and A. J. Perry, Proc. 7th mt. Con!. on Thin Films, New Delhi, 1987, in Thin Solid Films, 163 (1988). 2 J. Brunner and A. J. Perry, Thin Solid Films, 153 (1987) 103.
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3 A. B. DeWald, Jr., and J. P. Schaffer, 15th mt. Con[ on Metallurgical Coatings, San Diego, CA, 1988, in Surf. Coat. Technol. 4 R. N. West, Positron Studies of Condensed Matter, Taylor and Francis, London, 1974. 5 I. Ya. Dekhtyar, Phys. Rep., 9 (1974) 243. 6 M. Eldrup, Applications of the positron annihilation technique in studies of defects in solids. In Defects in Solids, Plenum, New York, 1985. 7 I. K. MacKenzie, J. A. Eady and R. R. Gingerich, Phys. Lett. A, 33 (1970) 279. 8 B. Bergersen and M. J. Stott, Solid State Commun., 7 (1969) 1203. 9 D. C. Conners and R. N. West, Phys. Lett. A, 30 (1969) 24. 10 K. G. Lynn and H. Lutz, Rev. Sci. Instrum., 51 (1980) 977. 11 A. Vehanen, K. G. Lynn, P. J. Schultz and M. Eldrup, AppI. Phys. A, 32 (1983) 163. 12 K. G. Lynn, B. Nielsen and J. H. Quatman, Appi. Phys. Lett., 47 (1985) 239. 13 A. J. Perry, C. Strandberg, W. D. Sproul, S. Hoffmann, C. Ernsberger, J. Nickerson and L. Chollet, Thin Solid Films, 153 (1987) 169. 14 A. J. Perry, M. Georgson and W. D. Sproul, Thin Solid Films, 157 (1988) 255. 15 K. Schwarz, CRC Crit. Rev. Solid State Mater. Sci., 13 (1987) 211.