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Nickel-chromium interface resolution in Auger depth profiles
Surface Science 177 (1986) 238-252 North-Holland. Amsterdam
238
NICKEL-CHROMIUM INTERFACE IN AUGER DEPTH PROFILES D.F. MITCHELL Division of Chemistq,
RESOLUTION
and G.I. SPROULE Nutional
Reseurch Council of Cunuda, Ottuwu, Canada KlA
Received 20 May 1986; accepted for publication
OR9
1 July 1986
Auger depth resolutions for fine-grained crystalline Ni/Cr multilayers have been determined. Resolution is shown to be a function of ion species, ion energy, ion incident angle, and the presence of reactive species. Deterioration of interface resolution with depth is shown to be the result of cumulative surface roughness induced by the ion beam to an extent proportional to the ion velocity normal to the surface. The presence of a reactive species to create an amorphous surface layer is shown to inhibit the development of surface roughness. The best resolution was obtained using low energy Xe ions and large ion beam angles. The ultimate resolution is limited by the fabrication perfection of the standard, and not by the sputtering process or by the Auger mean free path length.
1. Introduction
Interface resolution in Auger depth profiles obtained by sputtering is of considerable importance in applied surface analysis, and a large number of experimental and theoretical papers dealing with resolution limitations have been published [l-5]. We take the view that the sharpest experimental interfaces are usually the best, and thus any broadening of an interface may be interpreted as an artifact originating from the sample, the instrument, or the operating conditions. With this hypothesis in mind experiments were undertaken to determine the influence of a number of variables on interface resolution. Possible limiting factors in depth resolution include: Auger electron escape depth and backscatter effects, sputter-induced mixing and transport, original sample topography and sputter-induced sample topography, ion flux uniformity in the analysis area, conversion of time profiles into non-linear depth profiles, and electron beam heating effects. It has been suggested [1,5] that resolution may appear enhanced when examining interfaces between materials of greatly differing backscatter factors (average atomic numbers) or sputter rates, or where sputter-induced compositional changes occur. It is for these reasons that we and a number of others have chosen the nickel-chromium interface as an ideal resolution test system 0039-6028/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
D. F. Mitchell, G.I. Sproule / Ni-Cr interface resolution in A DP
239
[6-lo]. Other researchers have preferred to study amorphous oxide films on metals rather than polycrystalline metal films [5,11,12], and the reader is referred to the excellent paper by Seah and Hunt [4] for a comparison of this published data.
2. Experimental Nickel-chromium
sample
The sample was a nine-layer stack of alternating Cr (50 nm thick) and Ni (63 nm thick) layers, beginning and ending with Cr (National Bureau of Standards Reference Material no. 2135). The layers were fabricated by ion beam sputtering appropriate targets onto a polished silicon wafer held at less than 130°C. The interfaces between the layers were thought to be as sharp, uniform and free of impurities as technically possible to produce. This metal sandwich system has been examined recently by one of the authors (D.F.M.) in conjunction with two other groups [S]. The conclusions from that work were felt to be in some doubt, since the data was obtained using three different instruments and operators. In addition, at the time the study was made the samples examined were thought to be identical, but this was subsequently found not to be the case. For these reasons the work has been repeated and expanded to examine the effects of the following variables: ion beam energy, ion species, interface depth, ion beam incident angle, one versus two ion gun systems, interface type (i.e. Cr --, Ni or Ni + Cr), electron beam power and sample purity. 2.2. Instrumental The instrument used in this work was a PHI 590 Auger system modified in several important aspects. Additional pumping in the form of a copper panel at 20 K was installed directly above the sample stage. This had the effect of maintaining the system pressure during sputtering (with a PHI 04-303 differentially-pumped ion gun) at 1 x 1O-9 Torr. The ion gun gas pressure control was modified using a Granville-Phillips leak valve and the pressure was stable and reproducible to k 1%. A constant overall sputter rate for the multilayers of 4.8 + 1.0 nm/min was maintained by adjusting the ion current for all variations in voltage, ion species, angle of incidence, etc. The ion beam diameter (FWHM) measured in the plane normal to the beam axis varied from 0.02 to 0.2 cm over the voltage range employed in this work. With the near grazing incidence ion beams sometimes used the sputter crater was quite large and ill-defined. At higher beam energies, i.e., smaller beam diameters, the ion beam was rastered over a small area to ensure a
240
D. F. Mitchell, G. I. Sproule / NI-Cr
inter@ce resolution m A DP
uniformly sputtered area for Auger analysis. At low energies this was not necessary. Under all ion beam conditions an increase in sputter area did not affect the results. The ion beam flux was set using a Faraday cup whose axis was parallel to the sample normal. With this limitation, and the fact that the ion beam was not energy or mass filtered, the sputter rate data obtained are considered useful only for our particular instrument. Auger excitation was, unless otherwise stated, by a stationary 2 keV, 0.3 PA beam, 8 pm in diameter at 30” with respect to the sample normal. Auger detection was provided by a derivative mode lock-in amplifier with a 6 eV modulation using a PHI 25-110 cylindrical mirror analyzer. The data to be presented were derived from unsmoothed intensity profiles. The definition of depth resolution, AZ, adopted was the interface depth over which the signal changed from 16% to 84% of the values obtained for 100% of a given element [l-3]. This is the most frequently used definition and represents 2a, assuming the signal change at the interface is of a symmetrical error function form. It has also been assumed that, from a practical stand-point, the experimental AZ is the quadrature sum of the individual effects, AZ,, such that
3. Results 3.1. Interface profiles The experimental interface profiles, except when high electron beam power was used, were found to be symmetrical when plotted against sputter time, and resembled an error function curve. Figs. 1A and 1B show the Ni experimental Auger peak-to-peak data and the range of resolution encountered in this work. Approximately six hundred measurements per element were required in each profile to obtain sufficient data resolution in the narrow interface region. Fig. 2 shows the raw peak-to-peak data points for one representative high-resolution interface (6th in fig. 1A). The curve is a computer-fitted spline between raw data points, the arrows defining the interface resolution time at 16% and 84%, and the two horizontal lines the minimum and maximum signal levels. The time measurement was converted to distance using the average sputter rate of the two elements. Interface widths were found to be statistically the same, whether the Ni (- 850 eV) or the Cr (- 525 eV) data were used.
D. F. Mitchell, G.I. Spruule / Ni-Cv interface resolutror~ rn A DP
-
-
1 i
241 --r
A
\
;
i i
-i
i-
-
80
sputty- Time
Sputter
[min]
) 91
Time [min]
Fig. 1. Range of resolution observed when sputtering a nine-layer stack of alternating Cr (50 nm thick) and Ni (53 urn thick) layers. (A) Sputtering with 1 keV Xe, 53O with respect to the surface normal. [B) Sputtering with 4.5 keV Ar, 33O with respect to the surface normal.
The effect of ion beam energy for both Ar and Xe is shown in fig. 3 for 1,2 4.5 LeV ions with the beam 53O off normal. There is an obvious d&&XZitiiO~ Of reS&uiion wii.h increasing voltage and wifh depth. The de-
and
242
D.F. Mitchell,
G. I.
64
Sproule /
65
Ni-Cr
66
interface
resolution
67
68
rn A DP
6.l9
Sputter Time [min] Fig. 2. Expanded view of one Ni/Cr interface (6th in fig. la). Points are experimental peak-to-peak measurements. and the arrows define interface resolution.
terioration with ion beam energy is approximately proportional to the square root of the voltage. The increase in AZ with depth is quite small. 3.3. Ion beam angle The effect of ion beam angle on interface resolution for 4.5 keV Xe ions is illustrated in fig. 4, showing a clear advantage to using larger sputter angles. The resolution is roughly proportional to the cosine of the sputter angle. 3.4. Two ion gun operation The data for two ion guns was obtained without cryo-pumping and with the system backfilled with Xe to 7 x 10P6 Torr. An attempt was made to maintain gas purity during profiling by continuous 2 e/s pumping with a turbomolecu-
D. F. Mitchell, G.I. Sproule / Ni-Cr
-
interface resoluiion in A DP
243
5-
Oa
I 100
I
200
I 400
500
Depth [nm]
t zo.Q 3 a8 15-
0
n
cc a,
?A
A
A
a
y
8 lo'= s E
5-
Ob 0
I 100
1 200
I 300
1 400
500
Depth [nm]
Fig. 3. Effect of ion beam energy and sputter depth on interface resolution. Ion beam respect to the surface normal. (a) For Ar ions. (b) For Xe ions.
53” with
lar pump. The guns were adjusted to give equal beam currents into a Faraday cup; when operated simultaneously the Faraday cup current was twice the single values. The effect of using two ion guns simultaneously is demonstrated in fig. 5. (The angle between the ion guns was 112” when the guns were at 61° with respect to the sample normal, and 86O when the guns were at 44O to the sample normal. This difference is assumed to be insignificant.) As with a single gun the interface resolution improves with decreasing voltage. However, a correlation with angle of incidence at constant voltage is not apparent. A saw-tooth effect in the data, dependent on the interface type (Cr + Ni or Ni + Cr) is evident in fig. 5. (An experiment was carried out using a single ion gun at 1 keV, with the system backfilled with Xe to 7 X lop6 Torr, and limited (2 L/s) turbo pumping. The result was almost the same as in fig. 5.) Carbon
244
D. F. Mitchell, G.I. Sproule / Ni-Cr
0
100
200
rnterfuce resolution in ADP
300
400
500
Depth [nm] Fig. 4. Effect of ion beam angle (measured resolution
from surface normal) for 4.5 keV Xe ions.
and sputter
depth
on interface
and oxygen were monitored during profiling and carbon (carbide peak shape) was present in significant quantities (- 30%) within the Cr layers and carbon (not carbide) at a lower level (- 5%) in the Ni layers. A very low level of oxygen was detected in both layers. Whether the source of carbon was as a component of the ion beam or is the result of direct reaction with the system I
10
1
I
I
2 Ion Guns
O= 1.0 keV 61 dsg
0 = 2.0 keV 61 dag a = 2.0 ksV 44 dsg
2
I 0
I
100
I
200
I 300
I 400
I 500
Depth [nm] Fig. 5. Interface
resolution
obtained
with two ion guns operating Torr Xe).
in a back-filled
system (7
X
10 ’
D. F. Mitchell, G.I. Sproule / Ni-Cr rnferfuce resolution rn A DP
245
background is unknown. The data in fig. 5 have been plotted using the average sputter rate of the metals. In this case, however, the Cr/Ni sputter ratio is ratio under otherwise approximately 15% less than for the “clean” Cr/Ni similar conditions. This represents the extra mass which must be removed due to carbide formation and a differing sputter efficiency for carbide. If we assume that the Cr layers are effectively thicker due to the carbon reaction and that the overlayer sputter rate is the important parameter in resolution measurement then the calculated resolution for the Cr on Ni interfaces becomes poorer but the cyclic effect remains. The overall effect of the carbon is increased resolution and reduced microroughness. 3.5. Electron beam power The effect of electron beam power on interface resolution was examined using a different Ni-Cr sandwich (not issued as an NBS standard). This sample, made in a separate production run in the same equipment, comprised alternating Cr (77 nm thick) and Ni (77 nm thick) layers. This sample always produced poorer interface resolution than the NBS standard no. 2135 under all conditions. Figs. 6A and 6B show the effect of varying the electron beam power during sputter profiling. Samples were profiled using 1 keV Xe ions at 53” off normal; the analysis area was 5 X lo-‘cm2. The electron beam power was varied by adjusting both voltage and current to give a power range of 1300 to 34000 watt/cm2. Over 2500 watt/cm2 is required before statisticallymeaningful deterioration of interface resolution occurred. The broadened interfaces produced by high beam power are asymmetrical (fig. 6A). Experiments were also conducted where 30000 watt/cm2 was applied for two h prior to analysis. The beam power was then decreased to 1000 watt/cm* and the sample profiled. No deterioration in interface resolution due to the high power pretreatment could be detected. 3.6. Relative sputter rates of nickel and chromium Although not a primary objective of this investigation, very accurate relative sputter rates (nm/s) of Cr and Ni could be determined as a function of bombardment angle, ion energy and ion species for one or two ion guns. Some results are shown in fig. 7. While these data indicate a relative sputter rate approaching unity, as expected for an average sputter condition, they more importantly show a strong dependence on ion species, voltage and angle. 3.7 Sample purity During the course of this work surfaces were examined under the normal 1 x 10e9 Torr vacuum conditions for the presence of any impurities. The Ni
D.F. Miichell, G.I. Sproule / Ni-Cr interface resolution in ADP
246
Depth
0
I
0
100
I
[nm]
I
200
I
300
I
400
500
600
Depth [nm]
Fig. 6. Effect of electron beam power on interface resolution. (A) Interface resolution deterioration with 1 keV Xe at 34000 watt/cm2 compared with 1000 watt/cm*. (B) Interface resolution at beam powers ranging from 1000 watt/cm2 to 34000 watt/cm2. N
Y-
2
; -
.s
o= xe 53&g 1gun 0 = xs 3-g lg”” A=Ar 53dsg V = Xs 65dsg 0 = Xe 61deg 2gun!,
lgun lgun
’
‘\
62 3
z b
m
._
5
0
ifi m ‘2
m_
\d b
r-
d
I
10-l
I
1111111,
I,,,,,
loo
10’
Ion Energy [keV]
Fig. 7. Effect of ion species, ion angle and ion energy on the relative sputter
rates of Cr and Ni
D.F. Mitchell, G.I.
Sproule / Ni-Cr interface resolution in A DP
241
KINETIC ENERGY [eVl Fig. 8. Auger survey showing C at the interface between Cr and underlying Ni (upper trace) and 0 at the interface between Ni and underlying Cr (lower trace).
layers were found to be free of impurities within the limits of detection. The Cr layers were also impurity-free except for the first deposited layer, which showed - 3 at% carbon throughout, and the second Cr layer which had - 1 at% carbon at one interface falling to an undetectable amount at the other. All interfaces showed detectable impurities when profiled at high resolution. Interfaces between Cr and underlying Ni showed carbon levels estimated to represent 0.5-l monolayer, while interfaces between Ni and underlying Cr showed oxygen at - 1 monolayer (fig. 8). The oxygen appeared to be on the Ni side of the interface, but this is difficult to determine with any degree of certainty due to the close proximity of the Cr and oxygen peaks. 3.8. Electron microscopy
examination
The replica electron micrographs in fig. 9 show the topography produced by ion beam sputtering under different conditions. Fig. 9a shows the smooth starting surface while figs. 9b-9d show surface roughening proportional to the observed deterioration in interface resolution. Micrographs 9b, 9c, and 9d show the surface at a depth of - 480 nm below the original surface, i.e., in the middle of the bottom Cr layer. Some of the metal film could be dry stripped from the substrate and examined in transmission. Both the Ni and the Cr layers were found to be crystalline with average grain sizes of 30-40 nm and with no preferred
248
D.F. Mitchell. G.I. Sproule / Ni-Cr
interfuce resolutron in A DP
Fig. 9. Replica electron micrographs of the original sample surface (a). and at a depth - 480 nm after sputtering with 1 keV Xe, 53” with respect to the surface normal (b), with 5 keV Xe, 53O with respect to the surface normal (c). and with two ion guns with 4.5 keV Xe, 44O with respect to the surface normal (d). The magnification spheres are 91 nm in diameter. All markers represent 1
epitaxial relationship. It was also observed that the film separated internally giving four readily observed thicknesses, indicating failure between one class of interface
(Ni on Cr or Cr on Ni).
4. Discussion It can readily be seen from the data presented that the best interface resolution is obtained by using the heaviest ion at low voltage and large sputter angle. This is consistent with suggestions and observations in the literature which consider sputter mixing to be the dominant factor in depth
D. F. Mitchell, G. I. Sproule / Ni-Cr
inierfcrce resolutm
in A DP
249
resolution of ideal samples. The present results are in many ways similar to those of Fine et al. [S], Zalar [6] and the limited data of Mathieu et al. [1,13], of gold on nickel after taking into account the difference in ion beam angle demonstrated in this work. The highest resolution obtained in the present work is greater than any published to date on similar metal multilayers [4], but is not better than that obtained for SiO, on Si, Ta,O, on Ta or Ge on Si, all of which tend to be about twice as good as the data presented here; they also show no depth dependence. The data suggest that interface resolution is a function of the cosine of the sputter angle, the square root of the ion energy, the square root of the ion mass and the depth of the interface within the sample. The first three terms could be exclusively associated with sputter mixing which would be expected to have reached equilibrium by the first interface (50 nm). The fourth term may be predominantly the result of surface roughening (fig. 9). The best data obtained with 1 keV Xe at 53” shows very little deterioration of resolution with depth; at a depth of 100 nm the resolution is 4.5 nm and at 500 nm it is 5.1 nm. This small deterioration, as seen in fig. 9, is probably exclusively the result of surface roughening. The resolution extrapolated to zero thickness is - 4.4 nm which is considered to be the result of the sample fabrication precision, Auger escape depth, a term representing the statistics of the sputtering process [3], and sputter mixing. Each of these four factors is considered in more detail below. 4. I. Sample fabrication It is hoped that during sputter deposition sufficient surface diffusion occurs to smooth out the Poisson atom deposition process resulting in layer by layer deposition. At the same time bulk diffusional-mixing normal to the surface must not occur. (Trace impurities at each interface may well affect this process.) Such fortunate circumstances are unlikely, although substantial surface smoothing must be occurring to have achieved the interface resolutions obtained and to produce crystalline films. Also, some of the depositing material may have sufficient kinetic energy to mix the interface to a minor extent. That sample fabrication is a resolution-limiting parameter is clear in that the different sample production runs produced results differing by a factor of 2 for high resolution conditions [8]. It is therefore unlikely that the best sample produced is sufficiently perfect that it is not contributing to the measured resolution. It may be that the better resolution obtained for the top interface is because the fabrication of this layer is better, for some unknown reason. 4.2. Auger escape depth The Auger escape depth for the transition energies used in this work would create an interface resolution - 1.0 nm. Assuming this is added in quadrature
250
D. F. Mitchell,
G. I. Sproule
/ Ni-Cr
interfuce
resolution
in A DP
with the other effects to produce resolutions greater than 4 nm, the resultant contribution is very small (- 0.1 nm) and thus can be dismissed. 4.3. The statistics of sputtering The sputter removal process must have a statistical nature as recognized by Benninghoven [14]: This has been used in the past to rationalize the roughly square root dependence of depth resolution and sputter depth [4,14,15]. However, in this work, and in some other investigations, resolution has been found to be less dependent on depth. To explain these observations a more sophisticated model of the sputter statistics has been developed by Seah et al. [3]. This model suggests that because the probability of sputter removal is atomic-coordination dependent, a constant resolution (atomic roughness) l-2 nm develops after removal of - 10 nm of materials. The exact value would depend on the strength of the atomic coordination function, which will be material dependent and probably ion beam parameter dependent. The 4.4 nm resolution value found in this work should reflect the statistics of the sputter process as it is a constant independent of depth. This process might be expected to contribute - 10% to the extrapolated resolution. 4.4. Sputter
mixing
Sputter mixing contributes to interface broadening by knock-on and cascade processes. This is a subject on which much has been written [16-181. The paper by Andersen [16] suggests, for example, for both Ar and Xe at 1 keV and normal incidence that a resolution - 3.0 nm is to be expected. This is not too different from the present experimental value for Xe, but our data show a strong dependence on ion species so the correspondence may be coincidental. On the other hand knock-on ranges as predicted by Schiott [17] are strongly ion species dependent but range from 0.2 nm for 1 keV xenon to 1.5 nm for 4.5 keV argon which are much smaller than our experimental resolution. Also, as the extrapolated resolution at 2 keV is less than fi times that at 1 keV (fig. 3) and if mixing is assumed to be proportional to E’j2, then sputter mixing with 1 keV Xe is contributing very little to the interface width. It is therefore concluded from the above that the best extrapolated resolutions are determined primarily by sample perfection, and to a lesser extent by sputter statistics. Sample roughness, for a given sputter depth, is proportional to the velocity component of the sputter ion normal to the surface. Thus, in this work, where surface roughness is the dominant term in interface resolution: 020:
V,aJE/m
cos 0
where Vz is the ion velocity
normal
to the surface;
E is the ion energy;
m is
D. F. Mitchell,
G.I. Sproule
/ Ni-Cr
interjuce
resolution
251
in A DP
the ion mass, and 8 is the ion beam angle measured from the sample normal. Comparing the present data with previous work, improved depth resolution has been reported in the past [19], but not always [6], when using nitrogen as the reactive gas, metallic films and backfilling the analysis chamber. The explanation for this and the good resolution obtained with amorphous oxide films is that surface roughening is greatly reduced. Surface roughening can be produced either by concerted atom migration, i.e. dislocation movement, induced in a crystalline surface by strain generated by the ion beam, or by a variation of sputter rate with crystallographic orientation. In an amorphous surface concerted movements such as slip would not occur, nor would an orientation effect. The cyclic variation in resolution for two ion guns presented in fig. 5 is the result of amorphous carbide formation in the Cr layers followed by regeneration of surface roughness in the crystalline Ni layers. Strain relief would also explain the effect of high beam power on resolution, when sputtering with high beam power the sample is heated sufficiently to effect the sputter-induced strain relief process, but without sputtering specimen heating is insufficient to generate bulk diffusion. The effect of sputter rate and interface resolution previously reported [8] may also be explained in terms of strain relief continuously affecting the sputter removal process. In order to better understand sputtering processes, future work should be performed on more perfect structures with smaller repeat distances.
Conclusions Ion beam conditions strongly influence interface resolution in Auger sputter profiles. The results reported in this paper using Ar ion sputtering are consistent with the best data in the literature on micro-crystalline metal films when corrections are made for differences in ion beam angle. In general, the use of Xe instead of Ar improves depth resolution by a factor of two for a given energy and angle. When high interface resolution is desired from smooth surfaces, low ion energies and large ion beam angles should be employed.
Acknowledgement The authors this work.
thank
Dr. M.J. Graham
for his support
and encouragement
of
References [l] H.J. Mathieu, in: Thin Film and Depth Profile Analysis, Ed. H. Oechsner (Springer, Berlin, 1984) p. 39.
Topics in Current
Physics,
Vol. 37,
252
D.F. Mitchell, G. I. Sproule / Ni-Cr
interface resolutmn in A DP
[2] S. Hofmann and J.M. Sam, in: Thin Film and Depth Profile Analysis, Topics in Current Physics, Vol. 37, Ed. H. Oechsner (Springer, Berlin, 1984) p. 141. [3] M.P. Seah, J.M. Sam and S. Hofmann, Thins Solid Films 81 (1981) 239. [4] M.P. Seak and C.P. Hunt. Surface Interface Anal. 5 (1983) 33. [S] M.P. Seah, H.J. Mathieu and C.P. Hunt, Surface Sci. 139 (1984) 547. [6] A. Zalar, Thin Solid Films 124 (1985) 223. [7] T.J. Chuang and K. Wandelt, IBM J. Res. Develop. 22 (1978) 277. [8] J. Fine, P.A. Lindfords, M.E. Gorman, R.L. Gerlack, B. Navinsek. D.F. Mitchell and G.P. Chambers, J. Vacuum Sci. Technol. A3 (1985) 1413. [9] G. Simons, M.D. Brown, J. Fine, T.D. Andreadis and B. Navinsek. Nucl. Instr. Methods 218 (1983) 585. [lo] H.J. Mathieu and D. Landolt, J. Microsc. Spectrosc. Electron 3 (1978) 113. [ll] T. Jung and W. Titel. Phys. Status Solidi 74 (1982) 85. [12] C.F. Cook, Jr., C.R. Helms and D.C. Fox. J. Vacuum Sci. Technol. 17 (1980) 44. [13] H.J. Mathieu, D.E. McClure and D. Landolt Thin Solid Films 38 (1976) 281. [14] A Benninghoven, Z. Phys. 230 (1971) 403. [15] W.O. Hofer and H. Liebl, Appl. Phys. 8 (1975) 359. [16] H.H. Andersen, Appl. Phys. 18 (1979) 131. [17] H.E. Schiott, Radiation Effects 6 (1970) 107. [18] U. Littmark and W.O. Hofer. Nucl. Instr. Methods 168 (1980) 329. [19] R.J. Blattner, S. Nadel, C.A. Evans Jr., A.J. Braundmeir, Jr. and C.W. Magee, Surface Interface Anal. 1 (1979) 32.
238
NICKEL-CHROMIUM INTERFACE IN AUGER DEPTH PROFILES D.F. MITCHELL Division of Chemistq,
RESOLUTION
and G.I. SPROULE Nutional
Reseurch Council of Cunuda, Ottuwu, Canada KlA
Received 20 May 1986; accepted for publication
OR9
1 July 1986
Auger depth resolutions for fine-grained crystalline Ni/Cr multilayers have been determined. Resolution is shown to be a function of ion species, ion energy, ion incident angle, and the presence of reactive species. Deterioration of interface resolution with depth is shown to be the result of cumulative surface roughness induced by the ion beam to an extent proportional to the ion velocity normal to the surface. The presence of a reactive species to create an amorphous surface layer is shown to inhibit the development of surface roughness. The best resolution was obtained using low energy Xe ions and large ion beam angles. The ultimate resolution is limited by the fabrication perfection of the standard, and not by the sputtering process or by the Auger mean free path length.
1. Introduction
Interface resolution in Auger depth profiles obtained by sputtering is of considerable importance in applied surface analysis, and a large number of experimental and theoretical papers dealing with resolution limitations have been published [l-5]. We take the view that the sharpest experimental interfaces are usually the best, and thus any broadening of an interface may be interpreted as an artifact originating from the sample, the instrument, or the operating conditions. With this hypothesis in mind experiments were undertaken to determine the influence of a number of variables on interface resolution. Possible limiting factors in depth resolution include: Auger electron escape depth and backscatter effects, sputter-induced mixing and transport, original sample topography and sputter-induced sample topography, ion flux uniformity in the analysis area, conversion of time profiles into non-linear depth profiles, and electron beam heating effects. It has been suggested [1,5] that resolution may appear enhanced when examining interfaces between materials of greatly differing backscatter factors (average atomic numbers) or sputter rates, or where sputter-induced compositional changes occur. It is for these reasons that we and a number of others have chosen the nickel-chromium interface as an ideal resolution test system 0039-6028/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
D. F. Mitchell, G.I. Sproule / Ni-Cr interface resolution in A DP
239
[6-lo]. Other researchers have preferred to study amorphous oxide films on metals rather than polycrystalline metal films [5,11,12], and the reader is referred to the excellent paper by Seah and Hunt [4] for a comparison of this published data.
2. Experimental Nickel-chromium
sample
The sample was a nine-layer stack of alternating Cr (50 nm thick) and Ni (63 nm thick) layers, beginning and ending with Cr (National Bureau of Standards Reference Material no. 2135). The layers were fabricated by ion beam sputtering appropriate targets onto a polished silicon wafer held at less than 130°C. The interfaces between the layers were thought to be as sharp, uniform and free of impurities as technically possible to produce. This metal sandwich system has been examined recently by one of the authors (D.F.M.) in conjunction with two other groups [S]. The conclusions from that work were felt to be in some doubt, since the data was obtained using three different instruments and operators. In addition, at the time the study was made the samples examined were thought to be identical, but this was subsequently found not to be the case. For these reasons the work has been repeated and expanded to examine the effects of the following variables: ion beam energy, ion species, interface depth, ion beam incident angle, one versus two ion gun systems, interface type (i.e. Cr --, Ni or Ni + Cr), electron beam power and sample purity. 2.2. Instrumental The instrument used in this work was a PHI 590 Auger system modified in several important aspects. Additional pumping in the form of a copper panel at 20 K was installed directly above the sample stage. This had the effect of maintaining the system pressure during sputtering (with a PHI 04-303 differentially-pumped ion gun) at 1 x 1O-9 Torr. The ion gun gas pressure control was modified using a Granville-Phillips leak valve and the pressure was stable and reproducible to k 1%. A constant overall sputter rate for the multilayers of 4.8 + 1.0 nm/min was maintained by adjusting the ion current for all variations in voltage, ion species, angle of incidence, etc. The ion beam diameter (FWHM) measured in the plane normal to the beam axis varied from 0.02 to 0.2 cm over the voltage range employed in this work. With the near grazing incidence ion beams sometimes used the sputter crater was quite large and ill-defined. At higher beam energies, i.e., smaller beam diameters, the ion beam was rastered over a small area to ensure a
240
D. F. Mitchell, G. I. Sproule / NI-Cr
inter@ce resolution m A DP
uniformly sputtered area for Auger analysis. At low energies this was not necessary. Under all ion beam conditions an increase in sputter area did not affect the results. The ion beam flux was set using a Faraday cup whose axis was parallel to the sample normal. With this limitation, and the fact that the ion beam was not energy or mass filtered, the sputter rate data obtained are considered useful only for our particular instrument. Auger excitation was, unless otherwise stated, by a stationary 2 keV, 0.3 PA beam, 8 pm in diameter at 30” with respect to the sample normal. Auger detection was provided by a derivative mode lock-in amplifier with a 6 eV modulation using a PHI 25-110 cylindrical mirror analyzer. The data to be presented were derived from unsmoothed intensity profiles. The definition of depth resolution, AZ, adopted was the interface depth over which the signal changed from 16% to 84% of the values obtained for 100% of a given element [l-3]. This is the most frequently used definition and represents 2a, assuming the signal change at the interface is of a symmetrical error function form. It has also been assumed that, from a practical stand-point, the experimental AZ is the quadrature sum of the individual effects, AZ,, such that
3. Results 3.1. Interface profiles The experimental interface profiles, except when high electron beam power was used, were found to be symmetrical when plotted against sputter time, and resembled an error function curve. Figs. 1A and 1B show the Ni experimental Auger peak-to-peak data and the range of resolution encountered in this work. Approximately six hundred measurements per element were required in each profile to obtain sufficient data resolution in the narrow interface region. Fig. 2 shows the raw peak-to-peak data points for one representative high-resolution interface (6th in fig. 1A). The curve is a computer-fitted spline between raw data points, the arrows defining the interface resolution time at 16% and 84%, and the two horizontal lines the minimum and maximum signal levels. The time measurement was converted to distance using the average sputter rate of the two elements. Interface widths were found to be statistically the same, whether the Ni (- 850 eV) or the Cr (- 525 eV) data were used.
D. F. Mitchell, G.I. Spruule / Ni-Cv interface resolutror~ rn A DP
-
-
1 i
241 --r
A
\
;
i i
-i
i-
-
80
sputty- Time
Sputter
[min]
) 91
Time [min]
Fig. 1. Range of resolution observed when sputtering a nine-layer stack of alternating Cr (50 nm thick) and Ni (53 urn thick) layers. (A) Sputtering with 1 keV Xe, 53O with respect to the surface normal. [B) Sputtering with 4.5 keV Ar, 33O with respect to the surface normal.
The effect of ion beam energy for both Ar and Xe is shown in fig. 3 for 1,2 4.5 LeV ions with the beam 53O off normal. There is an obvious d&&XZitiiO~ Of reS&uiion wii.h increasing voltage and wifh depth. The de-
and
242
D.F. Mitchell,
G. I.
64
Sproule /
65
Ni-Cr
66
interface
resolution
67
68
rn A DP
6.l9
Sputter Time [min] Fig. 2. Expanded view of one Ni/Cr interface (6th in fig. la). Points are experimental peak-to-peak measurements. and the arrows define interface resolution.
terioration with ion beam energy is approximately proportional to the square root of the voltage. The increase in AZ with depth is quite small. 3.3. Ion beam angle The effect of ion beam angle on interface resolution for 4.5 keV Xe ions is illustrated in fig. 4, showing a clear advantage to using larger sputter angles. The resolution is roughly proportional to the cosine of the sputter angle. 3.4. Two ion gun operation The data for two ion guns was obtained without cryo-pumping and with the system backfilled with Xe to 7 x 10P6 Torr. An attempt was made to maintain gas purity during profiling by continuous 2 e/s pumping with a turbomolecu-
D. F. Mitchell, G.I. Sproule / Ni-Cr
-
interface resoluiion in A DP
243
5-
Oa
I 100
I
200
I 400
500
Depth [nm]
t zo.Q 3 a8 15-
0
n
cc a,
?A
A
A
a
y
8 lo'= s E
5-
Ob 0
I 100
1 200
I 300
1 400
500
Depth [nm]
Fig. 3. Effect of ion beam energy and sputter depth on interface resolution. Ion beam respect to the surface normal. (a) For Ar ions. (b) For Xe ions.
53” with
lar pump. The guns were adjusted to give equal beam currents into a Faraday cup; when operated simultaneously the Faraday cup current was twice the single values. The effect of using two ion guns simultaneously is demonstrated in fig. 5. (The angle between the ion guns was 112” when the guns were at 61° with respect to the sample normal, and 86O when the guns were at 44O to the sample normal. This difference is assumed to be insignificant.) As with a single gun the interface resolution improves with decreasing voltage. However, a correlation with angle of incidence at constant voltage is not apparent. A saw-tooth effect in the data, dependent on the interface type (Cr + Ni or Ni + Cr) is evident in fig. 5. (An experiment was carried out using a single ion gun at 1 keV, with the system backfilled with Xe to 7 X lop6 Torr, and limited (2 L/s) turbo pumping. The result was almost the same as in fig. 5.) Carbon
244
D. F. Mitchell, G.I. Sproule / Ni-Cr
0
100
200
rnterfuce resolution in ADP
300
400
500
Depth [nm] Fig. 4. Effect of ion beam angle (measured resolution
from surface normal) for 4.5 keV Xe ions.
and sputter
depth
on interface
and oxygen were monitored during profiling and carbon (carbide peak shape) was present in significant quantities (- 30%) within the Cr layers and carbon (not carbide) at a lower level (- 5%) in the Ni layers. A very low level of oxygen was detected in both layers. Whether the source of carbon was as a component of the ion beam or is the result of direct reaction with the system I
10
1
I
I
2 Ion Guns
O= 1.0 keV 61 dsg
0 = 2.0 keV 61 dag a = 2.0 ksV 44 dsg
2
I 0
I
100
I
200
I 300
I 400
I 500
Depth [nm] Fig. 5. Interface
resolution
obtained
with two ion guns operating Torr Xe).
in a back-filled
system (7
X
10 ’
D. F. Mitchell, G.I. Sproule / Ni-Cr rnferfuce resolution rn A DP
245
background is unknown. The data in fig. 5 have been plotted using the average sputter rate of the metals. In this case, however, the Cr/Ni sputter ratio is ratio under otherwise approximately 15% less than for the “clean” Cr/Ni similar conditions. This represents the extra mass which must be removed due to carbide formation and a differing sputter efficiency for carbide. If we assume that the Cr layers are effectively thicker due to the carbon reaction and that the overlayer sputter rate is the important parameter in resolution measurement then the calculated resolution for the Cr on Ni interfaces becomes poorer but the cyclic effect remains. The overall effect of the carbon is increased resolution and reduced microroughness. 3.5. Electron beam power The effect of electron beam power on interface resolution was examined using a different Ni-Cr sandwich (not issued as an NBS standard). This sample, made in a separate production run in the same equipment, comprised alternating Cr (77 nm thick) and Ni (77 nm thick) layers. This sample always produced poorer interface resolution than the NBS standard no. 2135 under all conditions. Figs. 6A and 6B show the effect of varying the electron beam power during sputter profiling. Samples were profiled using 1 keV Xe ions at 53” off normal; the analysis area was 5 X lo-‘cm2. The electron beam power was varied by adjusting both voltage and current to give a power range of 1300 to 34000 watt/cm2. Over 2500 watt/cm2 is required before statisticallymeaningful deterioration of interface resolution occurred. The broadened interfaces produced by high beam power are asymmetrical (fig. 6A). Experiments were also conducted where 30000 watt/cm2 was applied for two h prior to analysis. The beam power was then decreased to 1000 watt/cm* and the sample profiled. No deterioration in interface resolution due to the high power pretreatment could be detected. 3.6. Relative sputter rates of nickel and chromium Although not a primary objective of this investigation, very accurate relative sputter rates (nm/s) of Cr and Ni could be determined as a function of bombardment angle, ion energy and ion species for one or two ion guns. Some results are shown in fig. 7. While these data indicate a relative sputter rate approaching unity, as expected for an average sputter condition, they more importantly show a strong dependence on ion species, voltage and angle. 3.7 Sample purity During the course of this work surfaces were examined under the normal 1 x 10e9 Torr vacuum conditions for the presence of any impurities. The Ni
D.F. Miichell, G.I. Sproule / Ni-Cr interface resolution in ADP
246
Depth
0
I
0
100
I
[nm]
I
200
I
300
I
400
500
600
Depth [nm]
Fig. 6. Effect of electron beam power on interface resolution. (A) Interface resolution deterioration with 1 keV Xe at 34000 watt/cm2 compared with 1000 watt/cm*. (B) Interface resolution at beam powers ranging from 1000 watt/cm2 to 34000 watt/cm2. N
Y-
2
; -
.s
o= xe 53&g 1gun 0 = xs 3-g lg”” A=Ar 53dsg V = Xs 65dsg 0 = Xe 61deg 2gun!,
lgun lgun
’
‘\
62 3
z b
m
._
5
0
ifi m ‘2
m_
\d b
r-
d
I
10-l
I
1111111,
I,,,,,
loo
10’
Ion Energy [keV]
Fig. 7. Effect of ion species, ion angle and ion energy on the relative sputter
rates of Cr and Ni
D.F. Mitchell, G.I.
Sproule / Ni-Cr interface resolution in A DP
241
KINETIC ENERGY [eVl Fig. 8. Auger survey showing C at the interface between Cr and underlying Ni (upper trace) and 0 at the interface between Ni and underlying Cr (lower trace).
layers were found to be free of impurities within the limits of detection. The Cr layers were also impurity-free except for the first deposited layer, which showed - 3 at% carbon throughout, and the second Cr layer which had - 1 at% carbon at one interface falling to an undetectable amount at the other. All interfaces showed detectable impurities when profiled at high resolution. Interfaces between Cr and underlying Ni showed carbon levels estimated to represent 0.5-l monolayer, while interfaces between Ni and underlying Cr showed oxygen at - 1 monolayer (fig. 8). The oxygen appeared to be on the Ni side of the interface, but this is difficult to determine with any degree of certainty due to the close proximity of the Cr and oxygen peaks. 3.8. Electron microscopy
examination
The replica electron micrographs in fig. 9 show the topography produced by ion beam sputtering under different conditions. Fig. 9a shows the smooth starting surface while figs. 9b-9d show surface roughening proportional to the observed deterioration in interface resolution. Micrographs 9b, 9c, and 9d show the surface at a depth of - 480 nm below the original surface, i.e., in the middle of the bottom Cr layer. Some of the metal film could be dry stripped from the substrate and examined in transmission. Both the Ni and the Cr layers were found to be crystalline with average grain sizes of 30-40 nm and with no preferred
248
D.F. Mitchell. G.I. Sproule / Ni-Cr
interfuce resolutron in A DP
Fig. 9. Replica electron micrographs of the original sample surface (a). and at a depth - 480 nm after sputtering with 1 keV Xe, 53” with respect to the surface normal (b), with 5 keV Xe, 53O with respect to the surface normal (c). and with two ion guns with 4.5 keV Xe, 44O with respect to the surface normal (d). The magnification spheres are 91 nm in diameter. All markers represent 1
epitaxial relationship. It was also observed that the film separated internally giving four readily observed thicknesses, indicating failure between one class of interface
(Ni on Cr or Cr on Ni).
4. Discussion It can readily be seen from the data presented that the best interface resolution is obtained by using the heaviest ion at low voltage and large sputter angle. This is consistent with suggestions and observations in the literature which consider sputter mixing to be the dominant factor in depth
D. F. Mitchell, G. I. Sproule / Ni-Cr
inierfcrce resolutm
in A DP
249
resolution of ideal samples. The present results are in many ways similar to those of Fine et al. [S], Zalar [6] and the limited data of Mathieu et al. [1,13], of gold on nickel after taking into account the difference in ion beam angle demonstrated in this work. The highest resolution obtained in the present work is greater than any published to date on similar metal multilayers [4], but is not better than that obtained for SiO, on Si, Ta,O, on Ta or Ge on Si, all of which tend to be about twice as good as the data presented here; they also show no depth dependence. The data suggest that interface resolution is a function of the cosine of the sputter angle, the square root of the ion energy, the square root of the ion mass and the depth of the interface within the sample. The first three terms could be exclusively associated with sputter mixing which would be expected to have reached equilibrium by the first interface (50 nm). The fourth term may be predominantly the result of surface roughening (fig. 9). The best data obtained with 1 keV Xe at 53” shows very little deterioration of resolution with depth; at a depth of 100 nm the resolution is 4.5 nm and at 500 nm it is 5.1 nm. This small deterioration, as seen in fig. 9, is probably exclusively the result of surface roughening. The resolution extrapolated to zero thickness is - 4.4 nm which is considered to be the result of the sample fabrication precision, Auger escape depth, a term representing the statistics of the sputtering process [3], and sputter mixing. Each of these four factors is considered in more detail below. 4. I. Sample fabrication It is hoped that during sputter deposition sufficient surface diffusion occurs to smooth out the Poisson atom deposition process resulting in layer by layer deposition. At the same time bulk diffusional-mixing normal to the surface must not occur. (Trace impurities at each interface may well affect this process.) Such fortunate circumstances are unlikely, although substantial surface smoothing must be occurring to have achieved the interface resolutions obtained and to produce crystalline films. Also, some of the depositing material may have sufficient kinetic energy to mix the interface to a minor extent. That sample fabrication is a resolution-limiting parameter is clear in that the different sample production runs produced results differing by a factor of 2 for high resolution conditions [8]. It is therefore unlikely that the best sample produced is sufficiently perfect that it is not contributing to the measured resolution. It may be that the better resolution obtained for the top interface is because the fabrication of this layer is better, for some unknown reason. 4.2. Auger escape depth The Auger escape depth for the transition energies used in this work would create an interface resolution - 1.0 nm. Assuming this is added in quadrature
250
D. F. Mitchell,
G. I. Sproule
/ Ni-Cr
interfuce
resolution
in A DP
with the other effects to produce resolutions greater than 4 nm, the resultant contribution is very small (- 0.1 nm) and thus can be dismissed. 4.3. The statistics of sputtering The sputter removal process must have a statistical nature as recognized by Benninghoven [14]: This has been used in the past to rationalize the roughly square root dependence of depth resolution and sputter depth [4,14,15]. However, in this work, and in some other investigations, resolution has been found to be less dependent on depth. To explain these observations a more sophisticated model of the sputter statistics has been developed by Seah et al. [3]. This model suggests that because the probability of sputter removal is atomic-coordination dependent, a constant resolution (atomic roughness) l-2 nm develops after removal of - 10 nm of materials. The exact value would depend on the strength of the atomic coordination function, which will be material dependent and probably ion beam parameter dependent. The 4.4 nm resolution value found in this work should reflect the statistics of the sputter process as it is a constant independent of depth. This process might be expected to contribute - 10% to the extrapolated resolution. 4.4. Sputter
mixing
Sputter mixing contributes to interface broadening by knock-on and cascade processes. This is a subject on which much has been written [16-181. The paper by Andersen [16] suggests, for example, for both Ar and Xe at 1 keV and normal incidence that a resolution - 3.0 nm is to be expected. This is not too different from the present experimental value for Xe, but our data show a strong dependence on ion species so the correspondence may be coincidental. On the other hand knock-on ranges as predicted by Schiott [17] are strongly ion species dependent but range from 0.2 nm for 1 keV xenon to 1.5 nm for 4.5 keV argon which are much smaller than our experimental resolution. Also, as the extrapolated resolution at 2 keV is less than fi times that at 1 keV (fig. 3) and if mixing is assumed to be proportional to E’j2, then sputter mixing with 1 keV Xe is contributing very little to the interface width. It is therefore concluded from the above that the best extrapolated resolutions are determined primarily by sample perfection, and to a lesser extent by sputter statistics. Sample roughness, for a given sputter depth, is proportional to the velocity component of the sputter ion normal to the surface. Thus, in this work, where surface roughness is the dominant term in interface resolution: 020:
V,aJE/m
cos 0
where Vz is the ion velocity
normal
to the surface;
E is the ion energy;
m is
D. F. Mitchell,
G.I. Sproule
/ Ni-Cr
interjuce
resolution
251
in A DP
the ion mass, and 8 is the ion beam angle measured from the sample normal. Comparing the present data with previous work, improved depth resolution has been reported in the past [19], but not always [6], when using nitrogen as the reactive gas, metallic films and backfilling the analysis chamber. The explanation for this and the good resolution obtained with amorphous oxide films is that surface roughening is greatly reduced. Surface roughening can be produced either by concerted atom migration, i.e. dislocation movement, induced in a crystalline surface by strain generated by the ion beam, or by a variation of sputter rate with crystallographic orientation. In an amorphous surface concerted movements such as slip would not occur, nor would an orientation effect. The cyclic variation in resolution for two ion guns presented in fig. 5 is the result of amorphous carbide formation in the Cr layers followed by regeneration of surface roughness in the crystalline Ni layers. Strain relief would also explain the effect of high beam power on resolution, when sputtering with high beam power the sample is heated sufficiently to effect the sputter-induced strain relief process, but without sputtering specimen heating is insufficient to generate bulk diffusion. The effect of sputter rate and interface resolution previously reported [8] may also be explained in terms of strain relief continuously affecting the sputter removal process. In order to better understand sputtering processes, future work should be performed on more perfect structures with smaller repeat distances.
Conclusions Ion beam conditions strongly influence interface resolution in Auger sputter profiles. The results reported in this paper using Ar ion sputtering are consistent with the best data in the literature on micro-crystalline metal films when corrections are made for differences in ion beam angle. In general, the use of Xe instead of Ar improves depth resolution by a factor of two for a given energy and angle. When high interface resolution is desired from smooth surfaces, low ion energies and large ion beam angles should be employed.
Acknowledgement The authors this work.
thank
Dr. M.J. Graham
for his support
and encouragement
of
References [l] H.J. Mathieu, in: Thin Film and Depth Profile Analysis, Ed. H. Oechsner (Springer, Berlin, 1984) p. 39.
Topics in Current
Physics,
Vol. 37,
252
D.F. Mitchell, G. I. Sproule / Ni-Cr
interface resolutmn in A DP
[2] S. Hofmann and J.M. Sam, in: Thin Film and Depth Profile Analysis, Topics in Current Physics, Vol. 37, Ed. H. Oechsner (Springer, Berlin, 1984) p. 141. [3] M.P. Seah, J.M. Sam and S. Hofmann, Thins Solid Films 81 (1981) 239. [4] M.P. Seak and C.P. Hunt. Surface Interface Anal. 5 (1983) 33. [S] M.P. Seah, H.J. Mathieu and C.P. Hunt, Surface Sci. 139 (1984) 547. [6] A. Zalar, Thin Solid Films 124 (1985) 223. [7] T.J. Chuang and K. Wandelt, IBM J. Res. Develop. 22 (1978) 277. [8] J. Fine, P.A. Lindfords, M.E. Gorman, R.L. Gerlack, B. Navinsek. D.F. Mitchell and G.P. Chambers, J. Vacuum Sci. Technol. A3 (1985) 1413. [9] G. Simons, M.D. Brown, J. Fine, T.D. Andreadis and B. Navinsek. Nucl. Instr. Methods 218 (1983) 585. [lo] H.J. Mathieu and D. Landolt, J. Microsc. Spectrosc. Electron 3 (1978) 113. [ll] T. Jung and W. Titel. Phys. Status Solidi 74 (1982) 85. [12] C.F. Cook, Jr., C.R. Helms and D.C. Fox. J. Vacuum Sci. Technol. 17 (1980) 44. [13] H.J. Mathieu, D.E. McClure and D. Landolt Thin Solid Films 38 (1976) 281. [14] A Benninghoven, Z. Phys. 230 (1971) 403. [15] W.O. Hofer and H. Liebl, Appl. Phys. 8 (1975) 359. [16] H.H. Andersen, Appl. Phys. 18 (1979) 131. [17] H.E. Schiott, Radiation Effects 6 (1970) 107. [18] U. Littmark and W.O. Hofer. Nucl. Instr. Methods 168 (1980) 329. [19] R.J. Blattner, S. Nadel, C.A. Evans Jr., A.J. Braundmeir, Jr. and C.W. Magee, Surface Interface Anal. 1 (1979) 32.