A correlation of Auger electron spectroscopy, xray photoelectron spectroscopy, andRutherford backscattering spectrometry measurements on sputterdeposited titaniumnitride thin filmsBrad J. Burrow, Alan E. Morgan, and Russell C. Ellwanger

Citation: Journal of Vacuum Science & Technology A 4, 2463 (1986); doi: 10.1116/1.574092 View online: http://dx.doi.org/10.1116/1.574092 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/4/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Tungsten silicide composition analysis by Rutherford backscattering spectroscopy, Auger electron spectroscopy,and x-ray photoelectron spectroscopy J. Vac. Sci. Technol. A 16, 1103 (1998); 10.1116/1.581240 X-ray photoelectron spectroscopy study of TiC films grown by annealing thin Ti films on graphite J. Vac. Sci. Technol. A 15, 2029 (1997); 10.1116/1.580675 Analysis of iridium–aluminum thin films by xray photoelectron spectroscopy and Rutherford backscatteringspectroscopy J. Vac. Sci. Technol. A 8, 2251 (1990); 10.1116/1.576745 The characterization of titanium nitride by xray photoelectron spectroscopy and Rutherford backscattering J. Vac. Sci. Technol. A 8, 99 (1990); 10.1116/1.576995 Auger electron and xray photoelectron spectroscopy of sputter deposited aluminum nitride J. Appl. Phys. 55, 2935 (1984); 10.1063/1.333335

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54

A correlation of Auger electron spectroscopy, x-ray photoelectron spectroscopy, and Rutherford backscattering spectrometry measurements on sputter-deposited titanium nitride thin filmsa

)

Brad J. Burrow, Alan E. Morgan, and Russell C. Ellwanger Philips Research Laboratories-Sunnyvale. Signetics Corporation. Sunnyvale. California 94088

(Received 7 April 1986; accepted 7 July 1986)

Auger and electron spectroscopy for chemical analysis (ESCA) data ofTiNx were analyzed as a function of film composition as established by Rutherford backscattering spectrometry (RBS). The overlap of the N(KVV) and Ti(LMM) Auger transitions necessitated the assessment of two methods previously proposed for the derivative spectra. These results were compared with peak height and peak area measurements (after background subtraction) of the well-separated N ( Is) and Ti (2p) ESCA photoemissions. Neither the Auger nor the ESCA N/Ti intensity ratios scaled linearly with the N/Ti compositional ratios determined by RBS, especially for low nitrogen content. This behavior most likely results from ion-bombardment-induced losses of nitrogen in those phases with dissolved nitrogen rather than from an increased satellite emission in the Ti (2p) spectra from the near-stoichiometric nitride. In terms of precision and analysis speed, the Auger peak-to-peak quantification methods are preferred over ESCA quantification. In the near­stoichiometric phase (N/Tiz 1), RBS analysis shows higher sensitivity to nitrogen compositional changes than either ESCA or Auger.

I. INTRODUCTION

Titanium nitride belongs to a class of refractory metal ni­trides that have the unique combination of high melting points, extreme hardness, and high conductivities. I The combination of high conductivity and resistance to alumi­num migration makes titanium nitride attractive as a thin film diffusion barrier between aluminum and silicon or sili­cide contact points in semiconductor device processing.2

-5

The composition of refractory nitrides may vary over an ap­preciable range because of vacancies in the crystal lattice. For example, the composition of TiNx may range from x = 0.6 to x = 1.2 in the face-centered-cubic D phase.6 Even atx = 1.0, an appreciable number of both metallic and nitro­gen vacancies may be present. I Because of this composi­tional variation, the physical and chemical properties of the nitrides vary substantially. The resistivity of TiN x' for exam­ple, ranges from 25-800 jlfl cm, depending on the value of x and the grain size. I

•7 Thus, in characterizing the barrier

properties of thin TiN x films, it is important to measure the composition accurately.

The composition of TiN x may be obtained by electron spectroscopy, x-ray emission, or Rutherford backscattering (RBS). Although useful results have been reported for elec­tron microprobe measurements,8.9 the probability for N Ka emission is low and the Si substrate emission is high for thin films. Similarly, the backscattering cross section for low-Z elements such as N with He+ is also small. 10 Auger spectra of titanium nitrides are complicated by the overlap of the N(KVV) and Ti(L 2•3 M2,3M2•3 ) transitions. II Two meth­ods have been developed to quantify the overlapping Auger transitions. These methods both involve the separation of the overlapping transitions into a N component and a Ti component. 12.13 The Ti(2p) and N(ls) photoelectrons do not overlap in energy (ca. 58-e V separation). Therefore, quantitative analysis by electron spectroscopy for chemical

analysis (ESCA) should be more straightforward. Core-lev­el and valence-level photoemission studies relating to peak energy measurements and their dependence on composition have been reported for TiNx 14-18 and TiBx NI __ x .19 Robin­son and Sherwood analyzed the ESCA spectra of ion-plated TiN films by curve fitting with Gaussian-Lorentzian peak shapes accompanied by constant tails .20 They concluded that an oxynitride phase or phases must exist, although the correct spin doublet ratio for the Ti 2P3/2 /2PI/2 emissions of 2/1 was not obtained. No N/Ti ratios were reported. 21

This paper represents the first attempt to quantify ESCA spectra from sputter-deposited TiN x films of different com­positions. Peak heights and peak area after linear and Shirley background subtractions were used. 2o

•22 Sensitivity factors

derived from Wagner et al.,23 from two ESCA instrument manufacturers and from "pseudo-first principles" were cal­culated. These ESCA and Auger results were compared to the compositions determined by RBS. Peak energy measure­ments as a function of composition are also reported.

II. EXPERIMENTAL

ESCA and Auger analyses were conducted on a modified PHI model 590 spectrometer with two separate analyzers. The x-ray source was a dual Mg anode operated at 15 kVand 600 W. Photoelectron energies were measured in a double­pass cylindrical mirror analyzer (CMA) at a constant pass energy of 100 or 25 eV with pulse-count detection. The elec­tron gun for Auger analysis was operated at 3 kV and I jlA rastered over 200 X 200 jlm. Auger electrons were analyzed in a single-pass CMA operated in the derivative mode (mo­dulation amplitude = 3 V) without preretardation. Argon ion sputtering was done with either a single 2-kV, 28-jlA/ cm2 beam rastered over a 7.3-mm2 area or two 4-kV ion

2463 J. Vac. Sci. Technol. A 4 (6), NovlDec 1986 0734-2101/86/062463-07$01.00 ® 1986 American Vacuum Society 2463

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54

2464 Burrow, Morgan, and Ellwanger: A correlation of AES, XPS, and RBS measurements 2464

beams at 24-,uA/cm2, I 00-mm2 area. Both ion beams were at 60 0 incidence from the surface normal.

RBS results were obtained from two separate instruments. Both used He+ as the ion beam and the analyzer detected backscattered ions from both aligned and nonaligned direc­tions. In particular, the primary energy was 2.275 MeV, the total accumulated charge was 100,uC and the backscattering angles were 165" and 103°. The samples were checked for lateral inhomogeneity. The reported results are an average of the aligned spectra.

The looo-A titanium nitride films were prepared in a dc magnetron reactive sputter system on single-crystal silicon. The titanium target was sputtered in a mixture of Ar and N2 with a total constant pressure of2 X 10 \ mbar. The compo­sitions of the nitride films were varied by altering the N2 partial pressure.

III. SPECTRAL FEATURES Figure I compares the Auger line shapes of Ti and N in

both TiN 1.2 and Ti metal (excluding the L 2.3 VV transition at 451 e V). It is readily apparent that the Ti (L 2.3 M 2.3 M 2.3 )

transitions (at::::: 383 and 387 eV in Ti metal) overlap with the major N (KL L1 L 2.3 ) transition (at::::: 379 eV in Si3N4 )·

Indeed. even the minor N (KLL ) transitions (at 348 and 360 eV) also overlap with two small Ti(LMM) transitions (354 and 364 eV). Since there are no N Auger transitions above 400 eV, the Auger emission at :::::418 eV arises solely from the Ti (L ) ,M ) , V) transition. This transition therefore rep­resents th~ Ti i~tensity and chemical line shape for titanium nitride. A low-resolution ESCA spectrum of TiN in Fig. 2 also reflects these same overlapping Auger transitions at an apparent binding energy range of910-800 eV. However. the TiC2p) doublet (:::::455 eV) and the N( Is) transition (398 e V) are well separated.

Figure 3 illustrates high-resolution Ti (2p) spectra of TiN, (x = 0-1.2), which have been aligned in energy and normalized to the 2P"12 transition from Ti. The actual mea­sured peak energies of the TiC2p) and N (Is) transitions are listed in Table I. The realignment and normalization high-

w

z o

300 335 370 405 440 ELECTRON KINETIC ENERGY. eV

FIG. I. Auger spectra of Ti metal and TiN '" modulation voltage = 3 V. Note the overlap in the region 340-400 eV.

J. Vac. Sci. Technol. A, Vol. 4. No.6. Nov/Dec 1986

w

z

1000

T,(A) + N(A)

0(15) N(15)

Ar

750 500 250 ELECTRON BINDING ENERGY,

ESCA

TiN

eV o

FIG. 2. Low-resolution (pass energy = 100 eV) ESCA spectrum of TiN.

light the increased relative intensity with x at the higher binding energy of each peak. This additional intensity did not correlate with O/Ti ratios calculated from trace amounts of oxygen present in the bulk of the film. Porte et al_ 15 noted a sudden increase in intensity at ca. 2.2 eV higher energy where x> 0.8. These authors attributed this abrupt additional intensity to a depopulation and, consequently, a sudden delocalization of a vacancy band responsible for the final-state screening of the core hole. Such an abrupt. intense satellite structure was not observed here, although the inten­sity between the spin doublet does increase with N content (along with a decrease in the splitting energy). The samples in both laboratories were prepared similarly on different substrates, but Porte et al. annealed the sputter-cleaned TiN, prior to analysis, whereas we did not. The differences in relative satellite intensity may be a consequence of sput­ter-damaged crystallites.

There is a broad loss feature with a maximum appearing at 19 eV below the Ti 2P3/2 peak. This behavior has also been noted for Ti metal. It is likely that this background is due to

W

"'-

lJj

Z

N/Ti = 1.01

N/Ti = 0.72

N/Ti = 0.24

N/Ti = 0.0

ESC A Ti(2p)

469 462 455 448 441 ELECTRON BINDING ENERGY, eV

FIG. 3. High-resolution (pass energy = 25 eV) ESCA spectra of TiN, at

various compositions. These spectra have been energy aligned and normal­ized to the Ti spectrum.

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54

2465 Burrow, Morgan, and Ellwanger: A correlation of AES, XPS, and RBS measurements 2465

TABLE I. Measured ESCA binding energies ofTiNx ·

RBSN/Ti Ti(2p,d (eV) Ti(2p,/,) (eV) N( Is) (eV)

0.00' 454.3 460.4 397.1

0.12 454.6 460.5 397.4

0.24 454.8 460.8 397.4

0.72 455.1 461.0 397.4

0.96 455.1 461.0 397.2

1.01 455.3 461.1 397.3

1.19 455.3 461.2 397.3

• N below RBS detection limits.

shake-up satellites involving the closely spaced conduction bands near the Fermi edge and is typical of metal-like sam­ples (compare to shake-up losses in TiF" for example, Ref. 24.

IV. METHODS OF QUANTIFICATION

The two quantitative techniques for TiN x Auger deriva­tive spectra developed by Sundgren et al. 12 and Dawson and Tzatzov l3 are illustrated in Fig. 4. Both methods subdivide the intensity from the overlapping transitions at ;:::: 382 e V into a Ti component and a N component. Sundgren et af. derive the N component from the positive excursion of the 382-eV peak and the Ti component from the negative excur­sion of the 418-e V peak (apex to baseline). This analysis is simple, convenient, and precise, although the somewhat ar­bitrary partition does not entirely separate the two compo­nents. As shown in Fig. 1, however, the Ti contribution to the positive excursion of the 382-eV peak in Ti metal is mini­mal.

Dawson and Tzatzov calculate the Ti intensity by measur­ing the full peak-to-peak amplitude of the 418-eV peak. The N intensity is obtained by subtracting that portion of the overlapping transitions attributed to Ti. This portion is pro­portional to the Ti intensity at 418 eV calculated by measur­ing the peak-to-peak height ratio of the LMM / LMV transi­tions in Ti metal. This method assumes the absence of significant chemical line shapes, which would affect the rela­tive amplitudes of the two largest Ti transitions. The Auger

~ z o

* UJ

TiN 1.2

300

Auger

Ti + N \ Dawson + Tzauov

I Sundllfen .t a/."

/ ....... /\. 1\ ) \TI(lM'vl v

I '( !

\ Sundgren et ot.

335 370 405 440 ELECTRON KINETIC ENERGY. eY

FIG. 4. Auger spectra of TiN x illustrating the methods of Sundgren et al.

(Ref. 12) and Dawson and Tzatzov (Ref. 13).

J. Vac. Sci. Technol. A, Vol. 4, No.6, Nov/Dec 1986

~ Z o JI:

W

LMVILMM· 12 LMM

Ti

LMVlLMM • 0.7 .1\ I \

/~ /' \ '--"-"-"-"-' \/\. j \

300 335 370

\ \ I \ : \ !

405 ELECTRON KINETIC ENERGY,

440 eY

FIG. 5. Auger spectra of Ti metal, TiO, and TiSi,.

spectra of Ti metal, Ti02, and TiSi2 (Fig. 5) reveal this as­sumption to be invalid for the silicides and oxides of titan­ium. The LMM / LMVintensity ratio changes from 1.5 in Ti metal to 0.7 in Ti02 and 1.2 in TiSi2•

A plot ofN/Ti signals calculated from Auger data by the two methods decribed above versus the N/Ti composition determined by RBS is given in Fig. 6. The curve generated by the Dawson and Tzatzov method goes through the origin while the Sundgren curve does not. This is because there is a measurable positive excursion of the Ti (LMM) transition in pure Ti. This offset would dictate a detection limit of ca. 3% nitrogen by the Sundgren method if no correction was ap­plied. The larger positive excursions of the Ti (LM M) transi­tions in TiSix and Ti02 make the detection limit of TiNx

somewhat worse in these materials (see Fig. 5). Neither plot is linear throughout the composition range, especially for N/ Ti < 0.3. These plots reveal, therefore, that the Auger N/Ti peak-to-peak height ratios do not scale linearly with compo­sition over the entire range.

A similar analysis of ESCA quantification techniques is shown in Figs. 7 and 8. Three common methods of peak intensity measurements of the well-separated Ti(2p) and

10

02 04

Su .... oqrer, el .:;1 i

DCWSNl :J"d Tzot;:o. -----<il I ~~j

06 08 , 0 1 2

FIG. 6. Plot of Auger intensity ratios detennined by the two quantitation methods discussed in the text vs composition detennined by RBS.

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54

2466 Burrow, Morgan, and Ellwanger: A correlation of AES, XPS, and RBS measurements 2466

t::sc,-

w

z

505 ~88 ~71 ~5~ ~37

ELEe TRC'N B I NO I NG ENERGY. e,

FIG. 7. ESCA spectra of TiN, illustrating the peak intensity measurement

techniques mentioned in the text.

N ( Is) emissions were considered (Fig. 7). The first method required a simple peak height measurement. The other methods were digital trapezoidal integrations of the peaks after background subtraction. Therefore, these latter meth­ods differed only by the type of background. One method employed a linear background drawn between two points chosen where the elastic photoemission intensity decays into the inelastic losses at the high binding energy side, and into the Ka 3,4 satellite emission at the low binding energy side. This calculation led to a rather steep background that sub­stantially influenced peak area measurements, depending on the exact placement of the two boundary points.

The final method employed a Shirley background. 22 In this case, the low binding energy point was chosen either below the x-ray satellite structure in the Ti(2p) spectrum (Fig. 7) or at the same point defined previously for the linear background in the N ( Is) and O( Is) spectra. These distinc­tions were made because the Ka 3•4 satellites are well re­moved from the Nand 0 photoemissions. In contrast, the Ti (2p) emission consists of a doublet which overlaps the x­ray satellite emission. Therefore, the Ti(2p) background subtraction included the x-ray satellites, whereas the N ( Is)

08

o· o I.

e 2 ~

04 L

06 ~IT b, RBS

Llneor __ -2

Shlr,E',' I ,.....A.,

Shirley 2 _ --.1"

08 10 12

FIG. 8. Plot of ESCA intensity ratios determined by the four quantitation methods discussed in the text vs composition determined by RBS.

J. Vac. Sci. Technol. A, Vol. 4, No.6, Nov/Dec 1986

and O( Is) did not. [The x-ray satellites were subtracted from the Ti (2p) spectrum prior to integration.] The high binding energy point was selected at two different regions in the spectrum (see Fig. 7). The first point was the same boundary selected for the linear background (Shirley 1 background). The second point was selected where the loss structure changes shape at ;::; 490 e V, indicative of a probable change in energy loss mechanisms (Shirley 2 background). This larger area was selected to include some of the intrinsic asymmetry of the Ti(2p) transition that is lost when symme­try is imposed on the background-corrected spectrum. The choice of a high binding energy boundary has a marked ef­fect on the Ti (2p) peak area alone because the N ( Is) and O( Is) spectra do not share this same background shape. As in the linear background case, the exact choice of energy positions for the boundaries of the Shirley background sub­tractions has a substantial effect on measured peak areas. To alleviate this difficulty, an average value for the intensity was calculated over at least 5 points (1 eV).

Figure 8 compares the four ESCA intensity ratio calcula­tions versus RBS composition in the same format as Fig. 5. These plots reveal the same general shape as the Auger data. The peak areas calculated after linear background subtrac­tion showed poor agreement with the other methods. The peak height and Shirley methods appeared comparable.

V. DISCUSSION

In any quantification scheme, an overall intensity equa­tion such as those developed by Seah25 should be examined, and the approximations used should be explicitly stated. An equation relating ESCA intensities to concentration devel­oped by Seah is given below. A similar one also exists for Auger intensities, but the two are sufficiently similar (except for the backscattering factor) that only one needs to be con­sidered here.

1= a(hv)D(E) fToo f~o L(y)

xi~ _ '" i~- ooJo(xy)T(x,y,y,cP,E)

xi'" N(XyZ)exp(----Z--) z ~ [J A. m (E) cos e

Xdz dx dy d¢ dy. (1)

Here, I is the intensity measured by any of the methods de­scribed above, a is the ionization cross section (a function of x-ray energy hv), and D is the detector efficiency (a function of electron energy E). L is the angular asymmetry of emis­sion (a function of the emission angle y with respect to the incident x rays), Jo is the x-ray source intensity as a function of x and y directions on the surface plane, and T is the trans­mission function of the analyzer, which is a complicated function of x, y, y, and E as well as the azimuthal angle of emission cP. N is the concentration of atoms on the surface plane xy and with depth z, A. m is the inelastic mean free path of electrons as a function of energy, and e is the angle of emission with respect to the surface normal.

It is different to obtain absolute concentrations N(x,y,z)

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54

2467 Burrow, Morgan, and Ellwanger: A correlation of AES, XPS, and RBS measurements 2467

from Eq. (1) because not all of the variables, such as the incident x-ray Jo, are known. Furthermore, some of the func­tional relationships are not known accurately, such as the dependence of A on energy. Despite these difficulties, there are several published empirical and theoretical relationships for every functional dependence in Eq. (I). Also, by calcu­lating the intensity ratioIAIIB for atoms A and Bin the same sample, unknown variables such as Jo are removed. Given the above considerations, the following assumptions and cal­culation techniques were employed for the Ti-N system: D(E) was eliminated because the data were taken at a single pass energy. The ratio of the asymmetry parameters L is constant within the same instrument, therefore it was ig­nored. But, if one compares these ratios to theoretical mod­els or other instruments, then this parameter must be recon­sidered. The ratio ofN and Ti intensities also eliminated the instrumentalfactors Jo, the dependence of Ton x, y, y, and 4>, and the cos e factor in the electron escape depth calculation. Finally, it was also assumed that, within the sampling area and depth, the concentration ofN and Ti atoms remain con­stant, so J N(x,y,z) reduces to simply N.

With these assumptions, the ratio reduces to

IN (TN (hv)T(EN)NN J;'=oexp[ -zIAm(EN)]dz -= ITi (TTi(hv)T(ETi)NTi S~oexp[ -zIAm(ETi)]dz

(2)

The cross sections (T were taken from Scofield,26 the energy dependence of T may be assumed to be proportional to 1/ E,25 and Am may be calculated according to Seah and Dench?7 The integral J exp[ - zlA(E») dz becomes A(E) for uniform films. Equation (2), therefore, further simplifies to

-= ITi (TTiENAm (ETi ) NTi

(3)

Because the sensitivity factors in brackets are constant with composition, this equation describes a straight line. Al­though these assumptions and calculations are very com­mon and accepted, it is quite clear from the experimental evidence of Figs. 6 and 8 that this relationship does not hold for the Ti-N system.

Both ESCA and Auger data reflected a larger loss of N intensity for NITi < 0.3 than for NITi > 0.3. There are two possible explanations for this behavior; ion-bombardment­induced compositional changes, or changes in Auger and ESCA line shapes with composition. In the case of ion-in­duced damage, the sharp curvature at N/Ti;:::0.3 suggests the possibility of a phase transition near that region. The N­Ti phase diagram at N/Ti< 0.3 indicates either a-Ti with dissolved nitrogen, or a mixture of a-Ti and Ti2N.6 At NI Ti > 0.3, a mixture of Ti2N and TiN forms. This indicates that actual compound formation between Ti and N does not begin until the NlTi ratio approaches 0.3.

Since both techniques used Ar+ sputtering, the possibility of preferential sputtering of the nitrogen in solution at NI Ti < 0.3 must be considered. It seems likely that the dis­solved nitrogen is more loosely bound in the solid than the chemically bound nitrogen in TiN or Ti2N. Therefore, the sputtering rate of nitrogen could be larger for those samples

J. Vac. Sci. Technol. A, Vol. 4, No.6, Nov/Dec 1986

which contain dissolved nitrogen. Sputtering of our samples with NITi < 0.3 at 4 kV did indeed reveal a decrease in the NlTi ratio of 23%-32% as compared to the NlTi ratios from the same samples sputtered at 2 kV. In comparison, no decrease in the NlTi ratio was observed under the same con­ditions for those nitrides with NITi > 0.3. Dawson and Tzat­zov also reported similar results for TiNo.88 • 13

The existence of multielectron events in the Ti Auger and ESCA transitions may also affect quantification. As dis­cussed previously, Porte et al. discovered a large increase in the satellite emission in the ESCA spectra of TiN x as x ap­proached unity.15 If this intensity is also integrated along with the single-electron events, an apparent increase in the Ti(2p) photoemission would be detected. This would un­doubtedly affect the N iTi ratios since the N ( Is) spectra did not show similar enhanced satellite structure. The changes in transition metal spectral intensity with chemical state due to multielectron processes and the difficulties in ESCA quantification of these transitions has been indicated pre­viously.28 Moreover, the prominence of satellite structure makes curve fitting somewhat tenuous, since the extra inten­sity may be mistakenly assigned to unknown chemical forms such as titanium oxynitride.21

There are other possible explanations for the deviation from linearity, such as the dependence of CT and Am on com­position, or a systematic error at low N concentration in the RBS analysis. The RBS data have been converted to concen­tration using standard analytical techniques 10 by two inde­pendent laboratories, giving us confidence in these results. In this case, the surface height ratios ofNlTi were converted to atomic ratios by using the surface energy approximation for the stopping cross sections and scattering cross sections, and by assuming the validity of Bragg's rule. 10 For lOoo-A. films and 2.275-MeV H+ ions, the error in composition is estimated at less than 5%. The magnitude of deviation from these factors is quite small in comparison to the deviation actually observed.

It is interesting to estimate the NlTi ratio from Eqs. (1)

and (2). Only those instrumental factors which do not equal unity in the NiTi ratio need to be considered. The energy dependence of the double-pass cylindrical mirror analyzer (DPCMA) transmission function is the only relationship to be considered, since the geometrical relationships cancel. It is generally agreed that, in the constant pass energy mode, the throughput of the D PCMA is proportional to E - I, but the constant of proportionality is unknown. Fortunately, this factor cancels in the ratio as well, therefore the energy of emission is sufficient to calculate T. By similar reasoning, the cos e term drops out in the electron escape depth factor. The angular asymmetry term is a difficult one to calculate for the DPCMA because of its wide range of acceptance angles. Although there is a method to calculate L for specific values of y,25 an average value of L for all of the acceptance angles of the DPCMA must be calculated which may de­pend on the particular instrument used (e.g., the exact place­ment of support rods or an angle-resolved aperture within the analyzer). Therefore, this term is ignored but it is noted that the ratio of LT;lLN is expected to be less than one.

The net result of the above is Eq. (3) again, keeping in

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54

2468 Burrow, Morgan, and Ellwanger: A correlation of AES, XPS, and RBS measurements 2468

mind the adjustment caused by an unknown L ratio. The Scofield cross sections and Seah and Dench escape depths along with the kinetic energy of transitions were calculated for Table II. From these results, the overall sensitivity factor ratio for NITi is estimated to be

(4)

As a comparison, the same ratio calculated with published sensitivity factors produced values of 4.3 (Wagner et al. 2.1),

3.3 (from Physical Electronics software MACS, version no. 6), and 4.4 (Surface Science Laboratories, SSL). Table II lists several TiN, samples where x > 0.3 from RBS and the corresponding ESCA intensity ratios and corrected atomic ratios. This table shows that some nonlinearity remains; whereas the Physical Electronics sensitivity factor seems adequate for NITi = 0.72, the calculated sensitivity factor seems more applicable for NITi = 1.19. This result suggests that one may not use a single sensitivity factor for accurate quantification and, therefore, must resort to a calibration curve.

The question remains as to which method to use. On the basis of Figs. 6 and 8, the ESCA peak area integration fol­lowing a linear background subtraction seems the least accu­rate; the remaining techniques provide comparable results. If both ESCA and Auger are conveniently available, the choice must be Auger because of its analysis speed and small analysis area for electronic structures. Because of the con­ductive nature of TiN., charging and beam damage com­monly associated with Auger analysis do not occur. The measurement of peak-to-peak heights is also simpler and more precise than ESCA peak area measurements after background subtraction. Both Auger methods work equally well if a calibration against another technique is used. It is rather remarkable that, although the assumptions used in both Auger techniques seem tenuous (such as the peak-to­peak amplitude being an accurate measure of concentration

without correction or the relative intensities of the two major Ti Auger transitions remaining constant with composition), it is the overall precision in measuring the intensity that de­termines the final accuracy for individual measurements. Neither Auger nor ESC A analysis is as sensitive to N com­positional changes as RBS. For example, a decrease in N content from TiN I to TiNo7 (30% decrease in N) produced only a 10% decrease in the Auger or ESCA N/Ti ratio.

TABLE II. Calculated results for a (Ref. 26), T( E) [ C( E '( Ref. 25) ], and A (Ref. 27). along with corrected atomic N/Ti ratios calculated with four different sensitivity factors.

aT, = 7.90 HI, = 79geV AT, =2.7nm

ESCA Corrected RBS NlTi IN /1 Ti Calculated Wagner

0.72 0.214 0.98 0.92 0.96 0.236 1.08 1.01 1.01 0.239 1.\0 1.03 1.19 0.25\ 1.15 1.08

aN = 1.77 EN = 856 eV AN = 2.8 nm

N/Ti PHI

0.71 0.78 0.79 0.83

J. Vac. Sci. Technoi. A, Vol. 4, No.6, Nov/Dec 1986

SSL

0.94 1.04 1.05 1.10

Therefore, RBS analysis is the method of choice for TiN films> 300 A and x > 0.1. x

VI. CONCLUSIONS

Several common methods of Auger and ESCA quantifica­tion have been applied to the TiNx system and compared to RBS measurements. Both revealed nonlinearity in the cali­bration curves, particularly at N/Ti < 0.3. Consequently the use of simple sensitivity factors is inapposite. The deviation from linearity was attributed to a decrease in N concentra­tions from ion bombardment of those phases which contain dissolved nitrogen rather than an increase in satellite intensi­ty as the composition nears the I: I stoichiometry. The satel­lite intensity seems to be dependent on the degree of crystal­linity (compare Figs. 2 and 7 with Ref. 15). Consequently, the satellite emission is expected to be small. Also, since the satellite contribution to the Auger and ESCA spectra are not expected to be the same, the similar deviation from linearity for both types of analyses also indicate ion bombardment as the cause. Calculation of sensitivity factors from Seah's equation25 agreed well with published sensitivity factors, but simple application of these factors to the region where NI Ti> 0.3 showed that some nonlinearity persisted. Since the ESCA quantification of the well-separated Ti (2p) and N ( Is) peaks offered no advantages in terms of precision or accuracy, the Auger techniques are preferred on the basis of speed and small analysis area. For the near-stoichiometric nitrides, however, RBS analysis was shown to be more sensi­tive to small N compositional changes than either ESCA or Auger.

ACKNOWLEDGMENTS

The authors thank A. E. T. Kuiper of Philips Research Labs in Eindhoven, The Netherlands and Craig Hopkins of Charles Evans and Associates in Redwood City, CA for the RBS analyses, and Howard Shishido for the Auger measure­ments. We also extend thanks to Michael Strathman of Charles Evans for his helpful discussions.

"Presented in part at the 32nd A VS National Symposium in Houston, Texas on 19 November 1985.

'L. E. Toth, in Transition Metal Carbides and Nitrides (Academic, New York, 1971).

'M. Wittmer and H. Melchior, Thin Solid Films 93,397 (1982). 'M. Wittmer, J. AppJ. Phys. 53, 1007 (1982). 41. Suni, M. Maenpaa, M.-A. Nicolet, and M. Luomajarvi, J. Electrochem. Soc. 130, 1215 (1983).

'R. J. Schutz, Thin Solid Films 104, 89 (1983). "Metals Handbook, 8th ed. (American Society for Metals, Metals Park, OH, 1973), Vol. 8, p. 322.

'Y. Igasaki, H. Mitsuhashi, K. Azuma, and T. Muto, Jpn. J. App!. Phys. 17.85 (1978).

'A. N. Christensen and S. Fregerslev, Acta Chem. Scand. Ser. A 31, 861 ( 1977).

"B. O. Johansson, J.-E. Sundgren, J. E. Greene, A. Rockett, and S. A. Barnett, J. Vac. Sci. Technol. A 3,303 (1985).

lOW. K. Chu, J. W. Mayer, and M.-A. Nicolet, Backscattering Spectrometry (Academic, New York, 1974).

liS. Komiya, N. Umezu, and C. Hayashi, Thin Solid Films 63, 341 (1979). "J.-E. Sundgren, B. O. Johansson, and S.-£. Karlsson, Surf. Sci. 128, 265

(1983 ). up. T. Dawson and K. K. Tzatzov, Surf. Sci. 149,105 (1985).

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54

2469 Burrow, Morgan, and Ellwanger: A correlation of AES, XPS, and RBS measurements 2469

14L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, and C. Nordling, J. Phys. Chern. Solids 30, 1835 (1969).

ISL. Porte, L. Roux, and J. Hanus, Phys. Rev. B 28, 3214 (1983). 16L. I. Johansson, A. Callenas. P. M. Stefan. A. N. Christensen, and K.

Schwarz, Phys. Rev. B 24, 1883 (1981). 11H. Hochst, R. D. Bringans, P. Steiner, and Th. Wolf, Phys. Rev. B 25,

7183 (1982). IBN. Van Hieu and D. Lichtman, App\. Surf. Sci. 20,1186 (1984). 19A. LeBugle, R. Nyholm, and N. Martensson, J. Less Common Met. 82,

269 (1981). 20p. M. A. Sherwood, in Practical Surface Analysis by Auger and X-ray

Photoelectron Spectroscopy, edited by D. Briggs and M. P. Seah (Wiley, Chichester, England, 1983), Appendix 3.

J. Vac. Sci. Technol. A, Vol. 4, No.6, Nov/Dec 1986

"K. S. Robinson and P. M. A. Sherwood, Surf. Interface Anal. 6, 261 (1984).

22D. A. Shirley, Phys. Rev. B 5, 4709 (1972). HC. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor, R. M. Raymond,

and L. H. Gale, Surf. Interface Anal. 3, 211 (1981). "T. A. Carlson, J. C. Carver, L. J. Saethre, F. G. Santibanez, and G. A.

Vernon, J. Electron Spectrosc. Relat. Phenom. 5, 247 (1974). 2'M. P. Seah, in Practical Surface Analysis by Auger and X-ray Photoelec­

tron Spectroscopy, edited by D. Briggs and M. P. Seah (Wiley, Chichester, England, ) 983), Chap. 5.

2"J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8,129 (1976). 27M. P. Seah and W. A. Deneh, Surf. Interface Anal. 1,2 (1979). >xc. D. Wagner, Anal. Chern. 49,1282 (1977).

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.156.59.191 On: Tue, 09 Sep 2014 15:03:54