1582
Anal. Chem. 1001, 63,1562-1570
Investigation of Chromium, Cobalt, and.Nickel Implantation in Silicon Using Auger Electron Spectrometry, Secondary Ion Mass Spectrometry, Rutherford Backscattering Spectrometry, and Monte Carlo Simulation Henning Bubert*
Institut fur Spektrochemie und angewandte Spektroskopie, Bunsen-Kirchhoff-Strape 11, 0-4600 Dortmund 1, Germany Leopold Palmetshofer
Institut fiir Experimentalphysik, Johannes Kepler Universitat Linz, A-4040 LinzlAuhof, Austria Gerhard Stingeder
Institut f u r Analytische Chemie, Technische Unioersitat Wien, Getreidemarkt 9/151, A-1060 Vienna, Austria Marek Wielunski
Institut fiir Physik, Universitat Dortmund, Postfach 500500, 0-4600 Dortmund 50, Germany
Slllcon wafers were Implanted wlth '*Cr, 'OCo, and "NI Ions and lnvestlgated wlth regard to thelr possible quantlflcatlon as well as their us8 as reference materlal for surface analysls. The klnetlc energy of the Ions was 300 keV, the dose range extended from 0.9 X lo1* to 1.44 X lo'' lons/cm*, and the lmplantatlon was carrled out at room temperature. The lmplanatatlon proflles generated were lnvestlgated by Auger electron spectrometry (AES), secondary Ion mass spectrom etry (SIMS), and Rutherford backscatterlng spectrometry (RBS) and were accompanled by Monte Carlo (MC) slmulatlons. The posltlons of the proflle maxlma determlned (195-275 nm, dependent on the speclflc Ions and lmplantatlon dose) and the half-wldths at half-maxlmum (ca. 85 nm) sufflclently agree In Intercomparlson, whlle the mole fractions at the maxima dlffer up to 60%. An adequate determlnatlon of the mole fraction Is posslble by RBS wlthout any precondltlons; SIMS and AES also offer this posslblllty only In such cases where Implantation doses are known and used for quantlflcatlon.
INTRODUCTION Ion implantation into solids has become a technique of growing importance in many fields of modern material technology, since the physical and chemical properties can be modified in a formerly unknown manner. In the field of VLSI technology, metal silicides-ingle crystalline or polycrystalline and superficial or buried-were investigated intensively because of their significance as interconnections in electronic devices (1-10). Normally, it is absolutely essential to characterize such modifications by means of surface-analytical methods, which, in general, necessitate the availability of reference materials. Silicon implanted with metal ions can serve as such a reference material, if one succeeds in quantifying the chemical composition of the implanted silicon. Generally, this can only be ensured by applying different and independent methods to the same specimens in order to discover and avoid artifacts. In this work, silicon was implanted with chromium, cobalt, and nickel ions, and the produced specimens were investigated by Auger electron spectroscopy (AES), secondary ion mass 0003-2700/91/0363-1562$02.50/0
Table I. Implantation Dose for Wrt, uNit, and s9Cot Determined by Ion Current Measurements during Implantation specimen notation 1-Cr 2-Cr
3-Cr 4-Cr 5-Cr
1-Ni 2-Ni 3-Ni 4-Ni 5-Ni
6-Cr
dose, ions/cm2 1x0 2x0
3-CO 4-CO 5-CO 6-Co
0.9 X 10l2 0.9 x 1014 4.5 x 10'6 1.44 X 10l6
4.5 x 10'8 1.44 x 1017
spectroscopy (SIMS), and Rutherford backscattering spectroscopy (RBS). These methods were chosen because they are among the ones most often applied in the investigation of ion-implanted specimens due to their different specifications (SIMS, low detection limit; AES, high lateral resolution; RBS, no material consumption and quantitative analysis). The results obtained were compared with one another in view of the essential parameters of the implantation profiles, and they were also compared with accompanying Monte Carlo (MC) calculations, simulating the implantation process. EXPERIMENTAL SECTION Specimen Generation. The implantation was carried out with a 350-kV low-currentimplanter. The vacuum in the target station was about 2 X lo4 Pa during the implantation. Wafers of n-type
silicon (Wacker-Chemitronic GmbH, Burghausen, Germany), (100) oriented, measuring 30 X 30 mm2in size, were implanted with single-charged52Cr,W o , and -Ni ions at room temperature. The kinetic energy was 300 keV in all cases. To avoid channeling, an azimuthal angle of 14' (40' for Cr) and a polar angle of 7.5O (4.3' for Cr) were chosen. The desired implantation doses, obtained by varying the implantation current and time, extended over a range from 0.9 X 10l2to 1.44 X 10'' ions/cm2 (see Table I). The ion current measurement was performed with conventional Faraday cups (11). Three individual cups were arranged at the corners of the scanned area outside the main target area. This allowed monitoring of the dose uniformity and the determination of the implantation dose ( 1 2 , 1 3 ) . After implantation, the samples were cut into suitably large pieces for investigation with the different experimental methods. AES. Auger electron spectra and depth profiles were recorded on an Auger electron spectrometer,S A M 590 (Perkin-ElmerCorp., Physical Electronics Division), under the following conditions: 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
1563
Table 11. Comparison of the Position of the Implantation Maximum and the Relative Standard Deviation %lut
5
specimen
RBS
1-Cr
2-Cr 3-Cr 4-Cr
C
5-Cr 6-Cr
0
0 0
100
200
300 400
600
600
700
800
900 1000
kinetic energy (eV)
Figure 1. AES spectrum of the unsputteredsurface of the specimen 640indicating a thln oxide and contamination overlayer.
residual gas pressure 4 X lo+' Pa; for primary electrons, energy 3 keV, current 0.2 pA, beam diameter 1.0 pm, angle of incidence 60' with respect to the surface normal, area analysis (scanned area 100 X 200 pm2);for sputtering ions, Ar+, energy 1.5 keV, current density 78-90 pA,cm-2, angle of incidence 33.5' with respect to the surface normal, scanned area approximately 1.5 X 1.8 mm2. Alternating sputteringwith a cycle t h e of 1min was used normally. The energy resolution of the CMA was always 0.6%,and all measurements were recorded in the dN(E)/dEmode with a modulation voltage of 3 V. SIMS. SIMS depth profdea were recorded on a CAMECA IMS 3f under the following conditions: residual gas pressure 6 X lo* Pa; for primary ions, 02+, energy 5.5 keV, current 1 or 1.5 pA, angle of incidence 30°, scanned area 500 X 580 pm2,analyzed area 60 pm in diameter normally; width of the energy window 100 eV. RBS. The RBS investigations were carried out by using a standard RBS arrangement. The analyzing beam of 2.4 MeV 'He2+ was generated by a 4-MV Dynamitron Tandem accelerator at Bochum (Germany). The total energy resolution amounts to about 15 keV. The angle between the normal of the specimen surface and the incident ion beam was kept at 10' in order to avoid channeling of the incident particles. The backscattering angle was 160'. The diameter of the ion beam was 1 mm due to the aperture selected. The ion beam current was kept below 80 nA to avoid beam heating and an excessive dead time in the detection channel. The vacuum in the RBS chamber was about 2 X Pa.
SPECTRA AND EVALUATION PROCEDURES Specimens 1 and 2 (D= 0.9 X 10l2and 0.9 X 10" ions/cm2, respectively) were only produced for SIMS investigations in order to have available implanted specimens in which implantation-induced effects such as atomic mixing or amorphization should not occur or are not detectable. Due to the different detection sensitivities of the three experimental methods, only specimens 3 4 were investigated with RBS and only specimens 4-6 were subjected to AES,while all specimens were measured with SIMS. In order to estimate the influence of an undesired overlayer on the scale of a depth profile, Auger spectra were taken from the untreated surfaces of the specimens. Figure 1shows, for example, the spectrum of the surface of specimen 6-C0, indicating that the surface is covered by an oxide and contamination layer. Since the Auger peak-to-peak height (APPH) intensities of Si L3MZ3Mz3 at 76 eV (silicon oxide) and a t 92 eV (elemental silicon) were nearly identical, the thickness of this overlayer can be estimated to be about 1nm according to the formula regarding the escape depth for inorganic compounds (14). This means that the inaccuracy of depth scaling due to the SiOz overlayer can be neglected in comparison to positions of implantation maxima of 200-300 nm (see Table 11). AES. Auger depth profiles were recorded under the conditions given using the following Auger lines: Si L3MZ3Mz3
263 268 258 265 245
5-Ni
248
5-CO 6-Co
mean
RSD, %
262 264
249
1-Ni 2-Ni 3-Ni 4-Ni 140 240 3-CO 4-cO
nm
SIMS AES MC 276 268 266 273
267 254
247
229
238 240 224 229
238 222
256 255 245 228
258
2.3 2.3 4.5
242
7.2
225
237
4.4
229 227
3.5 7.0
221
217
2.7
220 225 195
229
4.6 5.2 6.9
224
210
(203) 217
210 225 230 223
219 244
225 200
225 204 192
221
203
7
Si
0
8
16
24
32
40
48
56
64
72
80
sputter time ( m i d
Flgurr 2. AES profile (APPH intensity versus sputter time)~ofthe specimen 640showlng a decrease of the APPH intensities of the Si LM ,M ,, line (92 eV) at the implantation zone.
at 92 eV, Cr L3M,J& at 529 eV, Co bM& at 775 eV, and Ni L3M6M6 a t 848 eV. Figure 2 shows, for example, the implantation profile (APPHintensity versus sputter time) of 6-Co. In order to convert the sputter time scale into a depth scale, the sputter yield was determined. For this purpose, a crater of ca. 600 nm in depth was generated by argon-ion erosion of a nonimplanted silicon wafer, and ita depth was determined by Tolanski contrast interferometry (15). The calculated sputter yield was S = 0.97 atoms/ion (f4%) (corrected for perpendicular incidence) and is in good agreement with values in the literature (16). Furthermore, the sputter craters generated during depth profiling of the implanted specimens were also determined by Tolanski contrast interferometry, and the calculated sputter yields agreed within the mentioned uncertainty of 4%. The near-surface zone is mainly characterized by partamorphization without a considerable amount of implanted atoms. This zone is followed by the implantation zone. The attempt to determine individual sputter yields for the different zones of an implanted silicon wafer failed because of the absolute error in the depth determination amounting to at least 20 nm. Therefore, all sputter times t were converted into depths z by means of the above sputter yield for silicon (index B) by using z=-
JSBt
(1) NBDDeO
with the elemental charge eo, the atomic density NB- = 5
X
1564
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
loz2atoms/cm3, the current density J of the sputtering ions, and the sputter yield SB. The conversion of the APPH intensities into mole fractions X A was carried out by using (17)
I
2 ,
1.5
1
with
0.5
0
(3) and the terms for electron backscattering, K,, for the escape depth, K,, and for the correction factor for line shape KI,
c
15
A sputtering correction factor was not introduced. Allocated symbols are as follows: NA(z),N&), atomic densities (atonis/cm3) for elements A and B of the implanted specimen, respectively; NAm,NB",atomic densities (atoms/cm3) of pure element standards A and B, respectively; ZA(z), ZB(z), intensities of elements A and B of the implanted specimen, respectively; IAm, ZBm, intensities measured on pure element standards A and B, respectively; r,(z), backscattering factor of the implanted specimen; rA,F B , backscattering factors of escape pure element standards A and B, respectively; Xz(,) depth of the implanted specimen; XA, AB, escape depths of pure element standards A and B, respectively; w A ( z ) , w ~ ( z energy ), difference between the upper and lower peak of the differentiated Auger line of elements A and B of the implanted specimen, respectively; wAm,wB-, energy difference between the upper and lower peak of the differentiated Auger line of pure element standards A and B, respectively; and EA,EB, kinetic energies (eV) of the Auger electrons of elements A and B. The ratio I A m / I B m corresponds to sA/sB, where sAand sB are the sensitivity factors taken from Davis et al. (18)and were checked by use of pure element standards. The calculation of the backscattering factors was performed by using data produced by Ichimura and Shimizu (19, 20). The escape depths Xi (i = A or B) in nanometers were calcolated by using the expression for elements given by (14)
(4) where 0 is the mean angle of emission related to the surface normal and di = 107(Nim)-1/3 is the thickness (nm) of a monolayer. When calculating rm(.z) and Xz(,) the atomic numbers Zi and the atomic densities Ni" have to be replaced by the corresponding mean values, which continuously change during the course of the depth profile. The consideration of the various backscattering terms and escape depths led to a correciion in the implantation maximum up to 20% toward h ; g h mole fractions for the implanted element. The depth profiles for the implanted element of specimens 4-Cr, 5-Ni, aiid 6-Co are presented in Figure 3. SIMS. SIMS depth profiles, normally plotted with a logarithmic intensity or mole fraction scale, are presented with a liiicar scale for better comparison with the profiles obtained
-
0
0
100
200
300
400
,
500
600
depth (nm)
Figure 3. AES depth profiles of specimens (a) 4-0, (b) 5-Ni, and (c)
6-Co.
by the other methods. Figure 4 again shows the depth profiles of specimens 4-Cr, 5-Ni, and 6-Co. In order to convert the sputter times into depths, the sputter rate was determined on all investigated specimens by measuring the depth of the sputter crater by means of profilometry (Sloan-Dektak I1 A). The accuracy of the depth measurement varied between 3 and 10% depending on the depth and the shape of the crater (focus of the primary beam). The sputter rate determined in this way will only then be correct if the sputter erosion is constant for all implanted specimens. In order to ascertain changes in the sputter rate, specimens 6-Cr and 6-Co were sputtered down to different depth levels (0.25r-, 0.5r-, r-, ...). However, the effect is actually smaller than the precision of the depth measurements, because these craters were very shallow in parts. For this reason, a constant sputter yield was used. If only one element is implanted, then XA(z)
+ XB(z) = 1
(5)
and in combination with the relative sensitivity factor RSF, which is defined by one yields a relationship for the conversion of the intensities into mole fractions
where IA(z)/zB(z) is the intensity ratio of the secondary ions
ANALYTICAL CHEMISTRY, VOL. 03,NO. 15, AUGUST 1, 1991
1565
a 1.5
.
DEPTH (nm) 5. SIMS depth profiles of specimen 8cr for the ions +3i+ and 52Cr+at different energy windows. The energy values behind the element symbors denote the lower energy limit the width of the window is approximately 100 eV.
0
100
200
300
400
500
600
depth (nm)
Figuro 4. SIMS depth profiles of speclmens (a) 4Cr, (b) 541, and (c) 640. Width of the energy window was approximately 100 eV.
of implanted elements A and silicon matrix B and fA/fB = 1.085 is the ratio of the isotopic abundances of the corresponding secondary ions (fA is always 1). The determination of RSF can be performed by the "integration method" (21, 22); i.e.,
DA =
ImxA(z) N(z)
(8)
Combining eqs 6 and 8, one obtains
RSF = ( ~ / D A ) ( ~ B~/ ~mA[ )I ~ 6 ) / I ~XB(Z) ( z ) 1N(z) dz (9) DA is the dose of implanted element A, and N(z) is again the total atomic density. RSF can be determined with eq 9 using low dose implanted samples (XB = 1and N N B m ) of wellknown dose DA. At high doses, the mole fraction XB(z) of matrix element B deviates noticeably from 1 and has to be taken into account, and the quantification was performed iteratively by using eqs 9 and 7b and starting with XB = 1 and N = NB-. During measurement of high dose implanted specimens, one has to reckon with a chemical matrix effect that normally becomes noticeable at mole fractions greater than 1%. The change of the matrix composition affects the probability to form secondary ions. This chemical effect changes mainly the intensity of secondary ions having low initial kinetic energy. If "energy filtering" is applied by setting an energy window that only permita the passing of the secondary ions possessing the corresponding energy, one can exclude the secondary ions of low energy. In order to judge whether or not the matrix
effect has been eliminated or reduced, swalled "ratio profiies" are thus formed; Le., intensity ratios of profiies measured with different energy windows were calculated. If the ratio for two different energy windows is constant, we conclude that the matrix effect is eliminated completely. Figure 5 shows five SIMS depth profiie-s, two profiles for Sgi+ and three for Wr+. Even in the logarithmic plot, the matrix effect can be clearly seen at the curve of %i+ (0 eV), which is enlarged at the maximum of the implantation profile. This is due to an increase of the generation of Y3i+ ions having low kinetic energy. This effect occurs at the presence of above some percent of Cr in the Si matrix. The reason for this finding is still unclear. The value quoted in parentheses denotes the lower energy limit of the energy window; the width of the window is approximately 100 eV. In the case of Cr, the matrix effect only becomes apparent when ratio profiles are formed. Such profiles of the ions 52Cr+(0 eV)/62Cr+(60 eV), s2Cr+(60 eV)/52Cr+ (120 eV), and 3oSi+ (0 eV)f'OSi+ (100 eV) are presented in Figure 6. The marked noise beyond approximately 550 nm is obviously due to the weak intensities of the Cr depth profiles. The ratio in Figure 6c is constant; thus, the matrix effect is eliminated by using an energy threshold of > 60 eV. The same behavior is found for Co and Ni, too. The method of energy filtering was also used in the case of the Co and Ni profiles in order to reduce or to eliminate the matrix effect and to separate the interferences with the Siz background. Proceeding in this manner, the difference in the energy distribution of atomic and molecular ions is exploited, since the atomic ions predominate at high energies. RBS. In order to perform accurate measurements of the implantation doses,we have used a relatively large solid angle of detection (Q = 8.05msr) for the backscattered a-particles that led to the above-mentioned total energy resolution of 15 keV. Thereby, the corresponding depth resolution was about 35 nm at the surface and 45 nm at a depth of about 500 nm. This relatively pure depth resolution was a compromise between exact establishment of the solid angle and high-energy resolution.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
1588
a
b
C
loo-
d:\ p’o.4
100.
P
d
d
‘
P I d
i
loo
200
gx,
400
A
loa,
(0
10 o
, 4 0 0 A A 1 L
M
a
4
o
o
m
m
2
1
o
a
)
a i
1.5
1
0.5
0 I
0
500
1000
1500
2000
energy (keV)
1-Cr 2-0 3-0 4-Cr 5-Cr 6-Cr 1-Ni 2-Ni 3-Ni 4-Ni 5-Ni
88.4 85.5 82.0 95.5
120.1 97.4 84.2 90.5 109.2
AES
b l
2
t
0
-a,
z2 0
ht, nm
SIMS
4
c
Table 111. Comparison of the Width at Half-Maximum and the Relative Standard Deviation
RBS
C
2 u
Figure 7. RBS energy spectrum of specimen 6-Co.
specimen
-s
6
MC
mean
RSD, % 15
74.2 83.0 92.9
89.0 89.0 86.9 91.3
92.9 83.2 85.6 96.4
6.9 7.6 4.5 6.7
C
10
5
79.8 86.3
88.7 98.0 87.9 83.3
82.6 71.7
82.5 82.5 82.5
88.9 84.3 80.5
14.5 3.6 7.7
0 0
100
200
300
400
500
600
depth (nm)
140
2x0 3-cO 4-CO 5-CO 6-Co
j
91.9 82.8 89.0 92.2
97.4 84.8 66.1 87.4 90.2
70.9 56.1 79.6
80.0 80.0 85.0 90.0
88.4 75.0 87.9 86.8
5.7 10.4 29.2 6.4
For exact energy calibration of the RBS spectra, use was made of a standard sample Si/Si02that was evaporated with thin layers of Au, Ag, Cu, and Ti. Figure 7 represents the energy spectrum of specimen 6-Co. In Figure 8, the depth profiles of specimens 4-Cr, 5-Ni, and 6-Co are shown, which result from the original energy spectra evaluated by means of the LORIG code (23). Subsequently, a deconvolution with regard to the low-energy resolution was carried out, which led to the half-widths at half-maximum Az in Table I11 and to
Figure 8. RBS depth profiles of speclmens (a) 4Cr, (b) 5-NL and (c) 6-Co obtained by computeraided transformation of the RBS energy
spectra using the LORIG code.
the mole fractions in Table IV. Therefore, these values deviate from the values that one would obtain from the corresponding curves (Figure 8), which are not deconvoluted. Because of the large mole fraction of the implanted atoms, the density and the stopping power of the target have to be modified for LORIG calculation. The stopping power e of the Si target (index B) implanted with element A (composition formula A,B,-,) was calculated by using the empirical Bragg’s rule E(A,BI-,) = X A ( Z ) ~+AX g ( z ) t g
(10)
ANALYTICAL CHEMISTRY, VOL. 63,NO. 15, AUGUST 1, 1991
1567
Table IV. Comparison of the Mole Fractions at the Maximum and the Relative Deviations
x,% specimen 1-Cr 2-Cr 3-Cr 4-0 5-Cr 6-Cr
RBS
SIMS
AES
MC
SIMS
0.44 1.33 4.60 14.5
6.4 X lod 6.8 X 0.41 1.34 4.64 13.8
1.43 7.12 18.9
0.4 1.28 4.47 12.4
-7 +1 +1 -5
0.46 2.34
0.53 1.71 5.16
-10
4.68
1.0 x 10-2 0.47 1.66 5.25
+12
-50
+10
0.46 1.48 5.16 15.3
9.3 x 10-8 0.60 1.95 5.32 16.4
0.67 3.59 10.8
0.53 1.71 5.30 15.0
+30 +32 +3 +7
-55 -30 -29
+15 +16 +3 -2
1-Ni 2-Ni 3-Ni 4-Ni 5-Ni 1x0 240 3-cO 4-CO 5-CO 6-Co
0.52
where t A and 6B are the elemental stopping cross sections and XA(t) and X&) are again the mole fractions of elements A and B. For mole fractions of implanted element B lower than 3%, corrections of the stopping powers are not necessary. MC Simulation. MC calculations for simulating the ion implantation process in solids are a helpful tool for a theoretical characterization of implanted specimens. Such calculations do not determine the elemental distribution of a real sample; rather they give an expected distribution based on the description of the physical processes and on the parameters used. As, on the one hand, these parameters are only known within an error of about 5% on average, the precision is determined by the number of calculated histories, by the quality of the generator of the random numbers, and/or by the algorithm used; the overall error must be estimated to about 510%. In the majority of cases, such calculationswere carried out by the two widely used programs MARLOWE (24, 25) and TRIM (26). While the TRIM code starts from an amorphous target, the MARLOWE program facilitates the calculation with crystalline as well as amorphous targets. If the azimuthal and polar angle are chosen in such a way that channeling can largely be avoided, the two programs lead to comparable implantation profiles, Le., in this case, the TRIM code can also be applied to monocrystalline targets. The advantage of TRIM is that the program runs 3-5 times faster than MARLOWE. The program chiefly used in this work does not principly differ from the TRIM code in the formulation of the stopping forces. The electronic stopping cross section was calculated by using
&(E) = c k k a k either was determined by applying the procedure described in ref 26, which makes use of the electronic stopping cross section of Si for protons, or was calculated according to the LSS theory (27). In both cases, a correction factor ck had to be introduced that was determined by comparison of calculated depth profies with those measured by AES for high dose nickel implantation of an energy of 6 MeV. By using the LSS theory, ck was determined as 1.177. Analogous findings were recently published for Ga, B, and A1 implanted into Si (28). The program used includes cascade mixing and sputter erosion using sputter yields Scr = 1.5, SC,,= 1.8, and S N=~ 1.8 atoms/ion. Furthermore, a swelling of the specimen during the calculation is introduced. Assuming that the total atomic density remains unchanged during implantation, one obtains a depth profile presented in Figure 9. This profile shows the same characteristic decrease of the Si intensity at the implantation maximum as the profiles measured with AES
E
40t 0
-9 +8 +55 +30
-4
-3 -14 +2
I
co
100
200
300
400
500
depth (nm)
Flgure 9. MC (Monte Carlo) depth profile for Si and Co for specimen 6 4 0 assuming a constant atomic density.
(Figure 2) and RBS (Figure 7). This fact hardens the assumption of a swelling of the specimen during high dose implantation. Other effeds such as enhanced diffusion and segregation or formation of chemical compounds are not involved in the program that is described in detail elsewhere (15). In order to quantify the intensity scale, the number of the histories ending in the respective depths has to be multiplied by Dm/NH, where D m is the dose value determined by X-ray fluorescence (XRF) analysis, and N H is the total number of histories used for the calculation. Figure 10 shows the result of the calculations for specimens 4-Cr, 5-Ni, and 6-Co.
RESULTS AND DISCUSSION The essential parameters that can directly be extracted from the profies are the position of the profile maximum z, the width at half-maximum Az, and the mole fraction at the maximum X., These parameters shall be discussed in the following subsections. Since some of these parameters already show a considerable discrepancy, further parameters describing the shape of the profile in more detail such as the skewness (third-order central moment) and the kurtosis (fourth-order central moment) are not considered. The values of z, Az, and X,, given in Tables 11-IV represent mean values of two to four measurements or calculations with a repeatibility of about 5% and therefore differ slightly from values that one would obtain from Figures 3,4, 8, and 10. Profile Maximum. Table I1 presents the position of the profile maxima obtained by the different methods. Also included are unweighted mean values and the relative standard deviations (RSDs), which are only given for an easier com-
1588
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991 2 1.5
1
0.5 0 0
--ae
200
300 depth (nm)
506
400
Figure 11. AES depth profiles of specimens 6Cr (full line) and 6 - 0 4
(dashed line) that were annealed at 600 "C for 2 h. The vertical arrow indicates the deviationfrom a Gaussiadike profile and the region where
C
B
100
6
4
the formation of CrSi, starts.
E
c
-0
E 2
Table V. Comparison of the Subsequent Determination of the Implantation Doses and the Relative Deviations
D,1OI6 atoms/cm2
0
15
10
I
i
J
I
1
I I I
i
1 JJ
L
0 0
100
200
300
400
500
600
ap,5%
specimen
XRF
RBS
AES
RBS
AES
3-Cr 4-Cr 5-Cr 6-Cr 3-Ni 4-Ni 5-Ni 3-CO 4-cO 5-CO 6-Co
4.0 11.8 41.6 137 4.5 14.5 43.7 4.6 14.2 45.8 138
3.9 11.8 41.8 146 4.4
17.3 65.9 192
-2.5 0 +0.5 +6.5 -2.2
+47 +58 +40
43.0 4.4 14.4 48.3 145
6.2 19.2 10.0 25.8 90.6
-1.6 -4.3 +1.4 +5.5 +5.1
-57 -56 -30 -44 -35
depth (nm)
Figure 10. MC (Monte Carlo) depth profiles for specimens (a) 4-0, (b) 5-Ni, and (c) 6-Co.
parison. At first glance, the RSDs seem to be justifiable, but the increase of RSDs at higher implantation doses is mainly due to the fact that a shift of the implantation maximum toward the surface is detected by SIMS and AES and is calculated by MC, but not by RBS. Such a shift is expected due to sputtering during implantation (e.g., ref 2) and should amount to d / 2 , where
(12) is the removed layer (D= dose, S B = sputter yield, MB= mass per mole, pB = density, F = Avogadro's number, and index B = silicon). At the moment, no explanation can be given as to why the shift of the maximum positions is not detected by RBS, and further measurements and evaluations will be necessary. W i d t h a t Half-Maximum. When material is eroded by sputtering during implantation, not only a shift of the position of the maximum is expected but also a broadening of the implantation profile. Assuming a Gaussian or Gaussian-like profile that would have a width Az at half-maximum without sputter erosion, one would obtain a width Az* with sputter erosion according to the empirical expression
Az* =
d
v
(13)
where the meaning of d is again the thickness of the eroded layer. Regarding a sputter yield of 1.5 atomslion and the highest applied dose of 1.44 X 10' ions/cm2, a layer thickness of 48 nm is calculated in the silicon matrix. Assuming Az = 85 nm, the width Az* would amount to 88.3 nm. This change
in direction of a broadening cannot be seen, because the broadening is too small compared with the measuring errors for a reliable result. A mean width of all values given in Table I11 amounts to 86 f 10 nm. The comparison of the width also shows that it is futile to calculate further central moments that would describe the profile shape in more detail. While most of the implantation profiles can be regarded to be Gaussian-like within the measuring error, the profile of specimen 6-Cr differs distinctly from a Gaussian-like profile through a bulge at the side toward larger depth (indicated by a vertical arrow in Figure 11). This was detected with both AES and SIMS. This depth region is the zone where the greatest mobility can be observed during annealing. Namavar et al. (3) have shown by means of X-ray diffractometry that only CrSi, was formed during annealing. This nucleation of CrSi2obviously starts in the region of the damage maximum, where small CrSi2 precipitates presumably act as diffusion centers for further Cr atoms and support the subsequent formation of an epitaxial CrSi, layer. Figure 11shows the AES depth profiles of specimen 6-Cr and 6-Cr-a. The last mentioned specimen is implanted with a dose of 1.44 X 10'' Cr+/cm2at 300 keV, like specimen 6-Cr, but subsequently annealed at 600 "C for 2 h in argon atmosphere. A comparison of both profiles indicates that the upper limit for the dose at which no high-dose effects occur is exceeded at 1.44 X 10" Cr+/cmz. This means that calculations simulating the implantation process for 1.44 x 10'' Cr+/cm2do not completely reflect reality. In the case of Go, a bulge could not be observed in the implantation profile. Mole F r a c t i o n at the Maximum. A comparison of the mole fractions at the positions of the maxima is listed in Table N . As one can see, the mole fraction values differ considerably from each other. Since the depths of the profile maxima as well as the doses could be determined with adequate precision
ANALYTICAL CHEMISTRY, VOL. 63,NO. 15, AUGUST 1, 1991
by RBS (see Tables 11and V),the RBS values can be regarded as the best ones. Additionally, Table IV contains the relative deviations related to the RBS values, A & m = (XSIMS XRBS)/~RBS, GAITS = ( X m- XRBS)/~RBS, and A J M C= (XMC- Xw)/XRw In the case of Cr and Ni, the mole fractional values obtained by RBS and SIMS agree within about lo%, while the SIMS values for Co are on average 19% higher than the RBS values. The reason for this is the fact that the SIMS depth profiles for Co show a conspicuously sharp maximum as compared with the AES and RBS depth profiles. The ion beam current for the Co implantation was between 2 to 3 times higher than for the Ni and Cr implantations. Therefore, beam heating occurred, leading to a sample temperature of at least 130 "C, and partly caused a change of the matrix (formationof C&i2 (4))that was only observed in SIMS measurements. The largest and also systematic deviations found for the AES values can only be caused by nonapplicability of the element sensitivity fadors taken from ref 18. One explanation is that the chemical state of the specimens has changed due to the formation of metal silicides, and thereby, the APPH intensities have increased for Cr and decreased for Co and Ni. Therefore, the use of element sensitivity factors can result in deviations from the true mole fractions, deviations that are too large for quantitative analysis. This difficulty can possibly be surmounted if the area under the Auger line (with an accurate background correction) is used or, as an interim solution, the expression hw2 is used for the intensity (29,30), where h is the peak-to-peak height and w the width of the Auger line. If, for instance, the w increases by 25% as a result of a changed chemical compound (IO), although the mole fraction remains unchanged, then h has to decrease by 36% and consequently, the calculated mole fraction, which is solely calculated from h, must deviate by exactly the same percentage. The use of the above expression for the intensity presupposes that the Auger spectra are recorded with a correspondingly high energetic resolution. Dose. After the conversions of the sputter time into a depth and of the intensity into a mole fraction for AES and SIMS, an integration can be carried out over the entire profile, whereby a determination of the implanted dose should be possible according to eq 8. The atomic density N(z) in ato ~ s / isc influenced ~ ~ by the manner in which the implanted ions are deposited on interstitial sites or on lattice sites that can be occupied after displacements of matrix atoms. If the implantation dose is large enough, these displacements lead to partial or complete lattice damage from the surface to the damage maximum, which is correlated to the maximum of the nuclear stopping. The amorphization due to this damage does not recover at room temperature and may result in a swelling of the specimen. Thereby, the density is possibly only slightly enlarged or unchanged. It is therefore difficult to say whether the atomic density N(z)is given by the density of silicon, NBm, or by the s u m of the densities, NBm + NA(z).Here, the density of silicon, NB", was used. The results of the dose determination are listed in Table V together with values obtained by XRF analysis, which were confirmed by other methods (13, 31). The AES dose values deviate considerably from the RBS and XRF dose values. This was expected since the mole fractions at the position of the implanatation maxima obtained by AES already differ significantly from the other mole fractional values. The last two columns of Table V show the relative deviations A$= (D- - Dm)/Dm and A,Dm = ( D m- D X R F ) / D X RThe ~ relative deviations of the RBS values amount to 4% on average, while the relative standard deviation for independent determinations of the XRF dose values is 5% (131,which means that the RBS measurements are of the same accuracy as the XRF determinations. An estimation of possible errors in RBS measurements (e.g., due
1568
to uncertainties in the geometry, integrator, or in the cross sections) also yields approximately 4% for the dose determination. The largest absolute deviations appear at the specimens with the highest implantation doses (6-Cr and 6-Co) that may be caused by a determination of a too large depth of the implantation profile, as already mentioned. The agreement found for the other specimens means that the mole fraction as well as the depth is determined correctly. CONCLUSIONS Silicon wafers implanted with Cr, Ni, or Co ions are suitable as reference material in surface and thin-film analysis, even in the high dose range, if certain conditions are observed: 1. The implantation dose should be determined subsequent to the implantation process by another method independent of the ion current measurement of the implanter. RBS (31) or XRF analysis (13) are suitable methods that are not material-consumptive and allow a dose determination with an accuracy of up to 5%. 2. The implanted specimens should not be subjected to an annealing process, since a considerable portion of the implanted atoms migrates during annealing to the surface where parts of it may evaporate. Such a portion varies according to the implantation dose used, even during identical annealing conditions (32). 3. The position of the implantation maximum, , , z can be determined by all four methods with an accuracy of better than 5 % . For doses equal to or above 4.5 X lo1* ions/cm2, RBS analysis yields too large values, which can possibly be corrected by increasing the depth resolution or by using a modified formula for the density (eq 10). 4. The width at half-maximum is yielded by all methods. However, outliers are possible due to incorrect determination of the sputter depth and can only be detected by repeated analysis with another method. 5. The determination of the mole fraction is possible with RBS without any preconditions; SIMS and AES also offer this possibility only if the dose values are known and used in connection with the "integration methods". In the case of AES, a quantitative analysis by use of element sensitivity factors (18)can result in large deviations from the true mole fractions. 6. Radiation damage or silicide formation in form of nuclei, islands, or complete layers do not exert any influence on the depth determination, but an effect on the conversion of the intensity scale into a mole fractional scale seems to exist in the case of AES and also in SIMS. ACKNOWLEDGMENT H.B. thanks M. Korte for carrying out all the MC calculations and most of the AES measurements, and G.S. is indepted to K. Piplits and W. Jkiger for assistance in SIMS measurements and to C. Tian for parta of the SIMS evaluation. Registry No. Cr, 7440-47-3; Co, 7440-48-4; Ni, 7440-02-0; Si, 7440-21-3.
LITERATURE CITED (1) White, A. E.; Short, K. T.; Dynes, R. C.; Garno. J. P.; Glbson, J. M. Appl. myS. Lett. 1987, 50, 95-97. (2) Campisi, 0. J.; Dietrich, H. B.: Delfino. M.; Sadana, D. K. Met. Res. SOC. S y w . ROC. 1986, 54, 747-752. (3) Namavar, F.; Sanchez, F. H.: Budnlck, J. I.; Faeihudin, A. H.; Hayden, H. C. Mt. Res. SOC. S m .R O C . 1987, 74, 407-492. (4) Petukhov, V. Yu.: KhaibuYh, I. B.; Zarlpov, M. M. mys. a m . Wch. S M . 1988, 4 , 522-529. (5) KoMhoff. K.; Mantl, S.; strlhker,B.; Sger,W. Nud. Insbum. AMtock mys. Res. 1988, 839, 276-279. (6) Saki. V. P.; Vidwans, S. V.; Rangwaia, A. A.; Arora, 8. M.; Kuldwp; Jab, A. K. Nucl. Instrum. Methcds 1987, 828, 242-246. (7) . . van O m , A. H.; Otierhbn. J. J. M.; TheMissen, A. M. L.; Mouwen, A. 0. Appl. mys. Leg. 1988, 53, 669-671. (8) Zarkrg, C.;Jiang, H.; Ostling, M.; W h b w , H. J.; Petersson, C. S.; Phil, T. V h , couches S 1987, 42 (236).55-57.
1570
Anal. Chem. 1991, 63, 1570-1573
(91 Lindner. J. K. N.: te Kaat. E. H. Met. Res. Soc. SvmD. ~. R o c . 1988. 707, 275-280. (10) Schonborn, A.; Llndner, J. K. N.; te Kaat, E. H.; Bubert. H.; Grasserbauer, M.;Frledbacher, G. Fresenius 2. Anal. Chem. 1989, 333, 51 1-5 15. (11) Dearnaley, G.; Freeman, J. H.; Nelson, R. S.; Stephen, J. Ion Implantatlon; North Holland: Amsterdam, 1973; pp 4161. (12) McKenna, C. M. I n Ion Implantation Techniques; Ryssei, H.. GlawIschnlg, H., Eds; Springer Series In Electrophyslcs 10; Springer: Berlin, I
,
'
----.
1982.
DO 731 rr .
(13) Klockenkamper, R.; Becker, M.;Bubert, H.; Burba, P.: Palmetshofer, L. Anal. Chem. 1990. 6 2 . 1674-1676. (14) Seah, M. P.; Dench, W.' A. Surf. Interface Anal. 1979, 1, 2-11. (15) Bubert, H. Mlkrochim. Acta [Wlen] 1987, 7986 I l l , 367-406. (16) Behrlsch, R.; Sputtering by Particle Bomberdmnt I ; Springer-Verkg: Berlin, 1981. (17) Seah, M. P. I n Ractical Surface Analysis; Brlggs, D., Seah. M. P., Eds; John Wiley & Sons: Chichester, 1983; pp 1811. (18) Davis, L. E.; McDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger flectron Spectroscopy; PerkinIlmer Corp.: Eden Prairie, MN, 1976. (19) Ichlmura, S.; Shimlzu, R. Surf. Sci. 1981, 112. 386-408. (20) Shlmlzu, R. Jpn. J . Appl. Phys. 1983, 22, 1631-1642. (21) Werner, H. W. Acta Electron. 1976, 79, 53-66. (22) Morrlson. G. H. I n SIMS I I I ; Bennlnghoven, A,, et ai., Eds; SprlngerVerlag: Berlin, 1982; pp 244-250. (23) Scherzer, B. M. U.; Bay, H. L.; Behrlsch, R.; Borgesen, P.; Roth, J. Nucl. Instrum. Methods 1978, 157, 75.
(24) Robinson, M. T.; Oen, 0. S. Appl. phvs. Lett. 1963, 2 . 30-32. (25) Robinson, M. T. User's Gum to MARLOWE; Radiation Shielding Information Center (RSIC), Oak RMge National Laboratory: Oak RMge, TN, 1986; Version 12. (26) Ziegler, J. F.; Biersack. J. P.; Littmark, V. The Stopping and Range of Ions in Solids; Pergamon Press: Oxford, New York, 1985; Vol. 1. (27) Llndhard, J.; Scharff, M.; Schiott, H. E. Kgl. Dan. Vid. Selsk. Met. Fys. 1983, 33 (No. 14) 1-42. (28) Dekempeneer, E. H. A.; Zalm, P. C.; Vriezema, C. J.; Polltlek, J.; Llgthart, H. J. "9.Instrum. Methods 1989, 842, 155-161. (29) Hall, P. M.; Morablto, J. M.; Conley, D. K. Swf. Sci. 1977, 62, 1-20. (30) Schonborn, A.; Bubert, H.; te Kaat, E. H. Fresenlus J . Anal. Chem., in press. (31) Bubert, H.;Burba. P.; Klockenkamper, R.: Schbnborn. A,; Wielunski, M. Fresenius J . Anal. Chem., In press. (32) Schonborn, A.; Bubert, H.; te Kaat, E. H. Verhandlungen DPG ( V I ) 1990, 2 4 , HL 3.9.
RECEIVED for review December 26,1990. Accepted April 30, 1991. This work has partly been supported financially by the Austrian National Bank and by the "Ministerium fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen" and the "Bundesministerium fiir Forschung und Technologie der Bundesrepublik Deutschland".
Perfluorosulfonate Ionomer-Phosphorus Pentoxide Composite Thin Films as Amperometric Sensors for Water Huiliang Huang and Purnendu K. Dasgupta* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Stanley Ronchinsky EG&G Inc., Environmental Equipment Division, Burlington, Massachusetts 01803
Incorporatlng HsPO4 In solutlons of perfluorosulfonate lonom81s (PFSI) slgnlfkantly Improves the Sensnlvlty of electrdytk thln-fllm PFSI molsture sensors. Stable fllms are formed wlth PFSI:H,P04 ratlos %:1, and sensors based on such fllms have been successfully used to measure moisture levels as low as 2 ppm (dew polnt -70 "C) wlth an operatlng voltage of 15 V. The sensors exhlblt fast response tlmes to moisture surge, have no slgnlflcant hysteresis or fbw rate dependence, and are Immune to prolonged exposure to very hlgh humldtty levels. At hlgh humldltles, the sensors also exhlblt unusual current-voltage characterlstlcs. I n terms of overall cost and performance, such sensors are not only cmpetltlve wlth exlstlng alternatives for bw-level mdsture measurement but are also the first such devices capable of measuring molsture from low-ppm to saturatlon levels.
INTRODUCTION In a previous paper (I), a pertluorosulfonate ionomer (PFSI) thin-film amperometric sensor for water was described. A survey of the techniques presently used to measure water also appeared therein and will not be repeated here. Briefly, PFSI films have a high affinity for water and, if such a film is in direct contact with two electrodes and sufficiently high voltage is applied across them, the water that partitions into the film from the surrounding environment is electrolytically decomposed. The magnitude of the current is used as a measure of the water content of the environment surrounding the 0003-2700/9 1/0363-1570$02.50/0
sensor. This principle of operation is quite similar to amperometric sensors coated with a P205film described by Keidel in 1959 ( 2 ) ;the Pz05film absorbs water (Pz05!@+ 2HP03 2H3P04),and the process is reversed electrolytically. Such P205-based moisture sensors are presently in wide commercial use and available from a number of manufacturers in different physical configurations for moisture measurement in gases. There are a number of disadvantages to the Pz05 sensor. The sensor is fabricated by initially coating the electrodes with syrupy H3P04and electrolytically removing the associated water. This "drying" process requires several days and sometimes weeks. Further it is uncontrollable and results in low and unpredictable sensor yield. With continuous use, the pastelike P206film tends to become nonuniform over the electrodes, causing the current output to decrease with time (3). Even more importantly, P205sensors are intolerant of high moisture surges that can occur in real measurement situations: liquid H,PO, is formed and runs off the electrodes. Obviously, it is not possible to expose such a sensor to ambient air without damage and this complicates installation procedures. Repeated exposure to relatively high moisture levels that are not high enough to cause catastrophic failure nevertheless dramatically reduce the lifetime of microfabricated sensors due to high current densities. Despite these drawbacks, the P,05film sensing technology is in wide use because of its ability to measure very low levels of moisture and relatively low cost. Sensors based on PFSI films (I)can also be inexpensive and free of the drawbacks exhibited by a P 0 sensor. However, they are significantly less sensitive; in5 0 1991 American Chemical Society
