The 1D and 2D mapping of surface, subsurface and interfacial properties with secondary, backscattered and Auger electrons

414

Applications of Surface Science 18 (1984) 414 428 North-Holland, Amsterdam

T H E ID AND 2D M A P P I N G OF SURFACE, SUBSURFACE AND INTERFACIAL P R O P E R T I E S W I T H SECONDARY, BACKSCATTERED AND AUGER E L E C T R O N S L. C O T A - A R A I Z A and H. P O P P A

*

lnstituto de l~tsica UNAM, Laboratorio de Ensenada, Apdo. Postal 877, 22890. Ensenada, B. ( fa., Mbxico Received 17 August 1983; accepted for publication 26 March 1984

Scanning the small spot size incident electron beam of a digitized and fully computerized Auger instrument across a surface while simultaneously collecting well-defined secondary electron signals provides information on localized properties that supplement the information usually obtained during standard AES and standard sputter depth profiling studies. Subtle changes in the "' true" secondary electron spectrum may be used to produce very sensitive 1D and 2D maps of surface compositional changes and interracial oxides. When. on the other hand, using the elastically backscattered electrons one can easily generate 1D and 2D maps of subsurface features.

1. I n t r o d u c t i o n

Surface sputtering with Ar ions has become a standard procedure for preparing clean surfaces rendering them suitable for analysis in an UHV chamber equipped with surface sensitive analytical spectroscopies such as AES, XPS, UPS, SIMS, etc. [1]. Small spot size electron beams (minimum at 500 A) and scanning facilities combined with computerized signal processing allow for controlled experiments involving sputtering and the detection of its effects in a well defined quantifiable manner [2-5]. We intend to concentrate on additional applications of "crater edge line scans" [6] which make effective use of a computerized Scanning Auger Microprobe to reveal one-dimensional (1D) and two-dimensional (2D) changes in surface properties formed during localized sputter etching. As a result we can more quantitatively assess some of the surface changes seen in standard SEM (Scanning Electron Microscopy) surface images.

* Permanent address: Stanford/NASA Joint Institute for Surface and Microstructure Research, NASA Ames Research Center, Moffett Field, California 94035, USA

0378-5963/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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2. Experimental The experiments reported here were carried out in a PHI-595 Scanning Auger microprobe using a primary electron beam of 3 keV energy, of 1000 A spot size, and with 3 nA of beam current (for additional instrumental details see ref. [6]). The microprobe was also equipped with a differentially pumped ion gun. The base pressure in the analysis chamber was 2 x 10 10 Torr and stationary beam sputtering was performed at a reduced emission current of - 10 mA and with corresponding Ar pressures in the main chamber that could be varied between 5 × 10 - 9 and 3 × 10 8 Torr. The ion beam current density at the site of the specimen was measured with a Faraday cup and was varied between 4 and 12 ~ A / c m 2. The ion beam energy was 4 kV and the beam diameter at the specimen measured about 300 ~m. The data were processed with a PDP-11-04 in-line computer using the MACS software of the 595 instrument.

3. Work function mapping In the case of non-charging oxides, we have recently shown that secondary electron profiles (SE profiles), obtained by detection of the "true" secondary

/ I

z,

I

I

I

8 12 15 KINETIE ENERGY,eV

20

Fig. 1. Low energy region of the secondary electron spectrum (SE), showing changes which occur below 10 eV, for points inside (b) and outside (a) a crater produced by Ar ion sputtering on a Au covered Zr alloy sample. The electron beam energy was 3 keV, the beam current 1 nA and CMA resolution set at 0.7%.

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peak while scanning the incident electron beam along a line on the surface of the sample, are more sensitive to compositional changes then the respective Auger line scans [7]. Here we intend to show that by appropriately choosing the peak (Ep) and base (Eb) energies in the secondary electron spectra, that are used for the line scans across the surface, one can obtain information on surface properties other than or in addition to surface composition. It is well known [8-10], for instance, that changes in surface work function are reflected in changes in the secondary electron peak position. Thus, by scanning the electron beam along a line on the surface and recording the variation of the SE peak intensity, one can hope to map easily changes of the work function (AO) with good sensitivity and spatial resolution. This approach being in principle the same as that used by Janssen et al. [10] except that already existing AES instrumentation and software can be utilized. A shallow crater was produced by sputtering through the thin gold overlayer on a zirconium alloy surface. Fig. 1 shows the SE spectra corresponding to a point inside the crater and to a point on the original surface. The low energy

~ bLS -E) cA l u-AES

E ¢3

>.

O~

0

200

400 600 800 1000 MICRONS Fig. 2. C o m p a r i s o n of SE a n d c o m p o s i t i o n a l ( A u g e r ) i n f o r m a t i o n f r o m line s c a n s a c r o s s the A u / Z r crater. (a) H i g h e n e r g y side of SE p e a k ( Ep = 3.5 eV, E b = 7.5 eV). (b) L o w e n e r g y side of SE p e a k ( E p = 3.5 eV, E b = 1 eV). (c) A u - A u g e r signal ( E = 68 eV, E h = 74 eV). (d) C - A u g e r signal ( E = 269 eV, E h = 285 eV).

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p e a k of the SE s p e c t r u m o b t a i n e d at the p o i n t outside the crater is shifted by a b o u t 400 meV with respect to the p e a k from the center of the crater. C h o o s i n g a p e a k energy of E p = 3,5 eV and a base energy E h = 1 eV for line scan across the crater (S-SE line), we o b t a i n a 1D m a p (see fig. 2b), which m a y be c o r r e l a t e d with the respective w o r k function changes. S t u d y i n g the line scans of the relevant A u g e r peaks, i.e. Au, O a n d C, a d d i t i o n a l i n f o r m a t i o n is o b t a i n e d to u n d e r s t a n d the work function changes. In fig. 2c we show the v a r i a t i o n of the A u A u g e r signal across the crater. T h e decrease of the A u signal on b o t h edges of the crater correlates very well with the C increase shown in fig. 2d a s s o c i a t e d with the overlayer of surface c o n t a m i n a t i o n on the original surface. Thus, in the center of the crater, the lower work function can b e associated with the cleaned a n d / o r r o u g h e n e d gold surface e x p o s e d after sputtering. (The r e d u c t i o n of the work function of metals by surface c o n t a m i n a n t s such as C is a well k n o w n p h e n o m e n o n [9-11].) T h e question still arises as to why in the S E M m o d e the crater is seen d a r k a n d not bright due to the smaller work function. T h e answer is that the S E M picture is built up of u n d e f i n e d c o n t r i b u t i o n s of the i n t e g r a t e d SE s p e c t r u m up to a b o u t 50 eV as d e t e c t e d by the c h a n n e l t r o n SE detector. (This i n t e g r a t e d SE s p e c t r u m corres p o n d s to the video trace s u p e r i m p o s e d on the m i c r o g r a p h of the A u - Z i r c a l l o y crater in fig. 3, with the m i n i m u m at its center c o r r e s p o n d i n g to the steepest

Fig. 3. SEM micrograph of the A u / Z r crater. Superimposed is the video-SE trace which correlates with the observed contrast.

418

L. Cota-Araiza. H. Poppa / ID and 2D mapping

p a r t of the crater.) If Ep = 20 eV were chosen as peak energy for the line scan, the result would indicate a " b r i g h t " crater instead of a dark one (see fig. 1).

4. Detection of interracial oxide A similar a p p r o a c h m a y be used for the detection of different oxide phases in a heavily oxidized s a m p l e of Si (with a 1000 ,~ thick SiO2 layer). A crater was again p r o d u c e d by s t a t i o n a r y A r ion b e a m b o m b a r d m e n t : the S E M m i c r o g r a p h of the crater is shown in fig. 4. S u p e r i m p o s e d on the m i c r o g r a p h is the video trace detailing the c o n t r a s t changes of the m i c r o g r a p h which are due to changes in the integrated SE spectrum. A n A u g e r (SE) p o i n t analysis along the crater wall was p e r f o r m e d , in o r d e r to evaluate the SE spectra c o r r e s p o n d i n g to the outer oxide layer of the s a m p l e (fig. 5a), the interfacial oxide (fig. 5b) that shows up as a " h a l o " ring a r o u n d the crater in the m i c r o g r a p h , a n d the center of the crater (fig. 5c) c o r r e s p o n d i n g to the Si substrate. The SE spectra exhibit substantial differences that correlate well with some of the features observed in the m i c r o g r a p h of fig. 4. W i t h the help of these SE spectra, two peak and base energy c o m b i n a t i o n s were chosen to scan

Fig. 4. SEM micrograph of a small crater in an oxidized Si substrate with superimposed video-SE trace. Note the ""halo" feature around the crater which is associated with an interracial oxide layer.

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O--

0

8

16 2L, 32 L,0 KINETIC ENERfiY, eV

Fig. 5. SE Spectra c o r r e s p o n d i n g to different points on the crater wall of S i O 2 / S i . (a) SE s p e c t r u m t a k e n on the outer oxide surface. (b) SE spectrum at a point in the interfacial oxide or " h a l o " region. (c) SE spectrum c o r r e s p o n d i n g to the Si substrate at the center of the crater.

L-SE,~I'

rL~

H-SE I!

0 -AES l~x'~

i

i

Si-AES 68 136 20~ 272 3~,0 Ml[ RONS Fig. 6. C o m p a r i s o n of SE and c o m p o s i t i o n a l (Auger) i n f o r m a t i o n from line scans across the SiO 2 (1000 A ) / S i crater. (a) L-SE ( Ep = 17 eV, E b = 1 eV) line scan. (b) H - S E ( g p = 17 eV, E b = 40 eV) line scan. (c) O - A u g e r p e a k line scan. (d) Si-Auger peak line scan. N o t e that only the L-SE and H-SE line scans show m a x i m a at the crater edge which correlate w i t h the observed " h a l o " in the S E M micrograph, which has been associated with the oxide. The O - A u g e r signal shows no difference.

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Fig. 7. 2D maps comparing the sensitivity of mapping with the SE peak and with the O-Auger peak. (a) O-Auger ( E p = 506 eV, E b -- 5 2 0 e V ) map of the crater area. (b) SE-peak ( E p - 17 eV, E h = 1 eV) map of the crater edge, reproducing the "halo" shown in the SEM micrograph. Maps taken at 300 × magnification using 100 lines, 100 points/line, at 25 ms/point.

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across the crater. Taking the peak at 17 eV and choosing the base energy at 1 eV (L-SE) and 40 eV (H-SE), respectively, the profiles shown in figs. 6a and 6b are obtained. Both line scans clearly show the interracial oxide " h a l o " which is observed in the micrograph and video trace. However, the O (509 eV) Auger line scan (fig. 6c) contains no information about the interfacial oxide at both crater edges, and the same is true for the Si (92 eV) Auger line of fig. 6d. When comparing all line scans of fig. 6 it is also evident that the signal to noise ratio in the SE spectra is much better than in the corresponding Auger line scans. As expected from preceding 1D scans, the 2D O map of the crater area also depicts very well the crater edge but is not sensitive at all to the interfacial oxide as can be seen in fig. 7a. However, when using the SE peak for mapping, we obtain a reproduction of the interfacial oxide "halo", fig. 7b, which is comparable to the "halo" seen in the original SEM micrograph of fig. 4. This 2D map of the oxide evidently has fewer contrast levels than the 1D-SE line scan, shown in figs. 6a and 6b, however, it corroborates nicely the sensitivity of features in the SE spectrum to compositional changes of the surface.

5. Depth profiling of thin interracial oxides A thermally oxidized Si wafer with a very thin oxide layer (measured to be of the order of 12 A by extrapolating crater shapes as determined by stylus profilometry, sputtering rate estimates, and ellipsometric measurements, details

Ag

Si

0

o

Si

;.

6

;

Ag

io

12

1',.

;6

18

SPUTTERTIME

Fig. 8. Stationary ion beam sputtering depth profile of Ag/Si(O)/Si sample whch does not detect the thin 12 ,~ interfacial oxide.

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and 2D mapping

Fig. 9. SEM micrographs showing difference in topography between A g / S i ( O ) / S i sample area of (a) the outer edge of the sputtered crater and (b) the inside of the crater.

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in ref. [6], was covered in-situ by a thick evaporated Ag layer. Next, a crater was produced by stationary beam Ar ion bombardment to study the possibility of detecting the interracial oxide Si(O) between the Ag overlayer and the Si substrate [12-14]. During sputtering, the standard depth profiling technique with a stationary and small spot size electron beam positioned at the crater center was used to monitor the Ag, O and Si peaks. The results of fig. 8 show clearly that the thin Si(O) layer is not detected, and we speculate that such a thin interracial oxide layer is overlooked because the sputtering/detection conditions were not carefully enough optimized. As one consequence, strong silver island formation was encountered and appreciable knock-on mixing probably occurred. On the other hand, the corresponding Auger line scans across the left edge of the crater in fig. 10 (where the origin of the distance scale corresponds to a point outside the crater and the 200 /tm mark is a point inside the crater) display the presence of the extremely thin oxide layer between the Ag film and the Si substrate very well. This demonstrates the ease, high depth resolution, and high sensitivity of crater edge AES-line scans for interfacial oxide layer detection (as shown elsewhere [6] the crater wall angles produced here are extremely small, i.e. of the order of 10 4 10 5 rad). This interfacial oxide layer was detected in spite of sputtering induced Ag island formation which would tend to obscure the detection of the oxide. A comparative display of SEM micrographs of the crater area with the continuous Ag overlayer outside the left edge (fig. 9a) toward the center of the crater (fig. 9b) show the actual

(x2)

. . . . . Ag Si - - 0

'""~,' ~ 'v"',

II

,, (;,21

ix./ ..,,k 0

k0

80 120 160 HIERONS

I 200

Fig. lO. Auger line scans across the'left edge of the A g / S i ( O ) / S i crater; the Si(O) interfacial oxide is clearly detected.

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extent of Ag island formation. As expected, the maximum of average SEM image brightness corresponded with the maximum in secondary electron yield at the location of the interfacial oxide in the crater wall.

6. Mapping of subsurface features A small crater produced by Ar ion bombardment of a Au covered Zr alloy surface covered by a thick Ag overlayer served as a sample with well defined subsurface compositional features. The in-situ evaporated Ag layer was thick enough, - 300 A, so that the crater was practically invisible by SEM (fig. lla); a SE line scan of the same region also did not show the presence of the crater. Imaging electrons of longer escape depth [15] that can traverse the Ag overlayer and reach the detector are needed. We, therefore, tried imaging/mapping with elastically scattered electrons (a primary beam energy of - 2 . 4 keV had to be used in order to obtain the small magnification of 100 x necessitated by the relatively large sputter crater size). We then measured the energy region around the elastic peak (EP) at sample points corresponding to the center and both edges of the crater (fig. 12). The changes in the EP spectra are related to the instrumental response function of the CMA which is associated with the fact that we are analysing electrons from sample points 500 #m apart (at the two horizontal edges of the crater). For an EP line scan across the whole region, Ep was, therefore, chosen at the crossover point of the right and left edge elastic peak energy scans which corresponds to a value of Ep = 2414 eV; a base energy of E b = 2440 eV was selected for a so called H-EP line scan. In order to prevent overloading of the signal processing system the beam current was reduced to 0.5 nA and a short time for data acquisition per image point of 25 ms was chosen. Also, due to software limitations for the substraction of two line scans, 100 or 200 points per line had to be used. After performing a line scan in a region next to the crater known to be flat, in order to assess the instrumental response function (IRF) of the CMA (see fig. 13a), the horizontal scan across the region of the subsurface crater was acquired resulting in the trace shown in fig. 13b. The two scans of figs. 13a and 13b were then normalized and subtracted in order to cancel the IRF. The result is shown in fig. 13c where the crater is clearly visible, in what we call 1D-EP map of a subsurface feature. The same procedure was followed using vertical scans across a flat region of the sample and across the crater (see fig. 14). However, in order to correctly assess and minimize the IRF for vertical sample scans, the standard 60 o inclined sample holder had to be modified so that the angle of incidence of the primary electron beam could be continously changed from 30 ° to 90°; this was accomplished by using a sample "flip" mechanism activated with a simple movable feedthrough manipulator.

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Fig. 11. (a) SEM micrograph of Ag covered A u / Z r crater. Superimposed is the video-SE trace which shows practically no sensitivity to the underlying crater. (b) 2D elastic peak (H-EP) map of the same region shown in (a) demonstrating the subsurface sensitivity of elastically backscattered electrons.

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CENTER

R-edge

EpA~~2380 23'94. 2~0B 24.22 2~.36 24.50

KINETICENERGYeV ,

Fig. 12. Elastic peak energy region as measured for sample points in the center and at both edges of the A g / A u / Z r crater.

Crafer-IRF

0

200 400 600 BOO1000 HICRONS

Fig. 13. 1D EP maps produce by scanning the elastic peak on a horizontal line across the A g / A u / Z r . (a) Line scan across a flat region next to the crater (instrumental response function). (b) Horizontal EP line scan across the subsurface A g / A u / Z r crater, (c) Difference curve resulting from subtracting line scan. (d) (Background) from line scan (b) cancelling IRF.

L. Cota-Araiza, H. Poppa / ID and 2D mapping

Croter

0

2 0

427

~.

/+00 600 MICRONS

800

1000

Fig. 14. Same as fig. 13 for a vertical scan across the Ag/Au/Zr crater (sample tilt 90 o ). (a) Flat background outside the crater. (b) Vertical scan across the crater. (c) Difference between (a) and (b).

F r o m these E P line scans it was obvious that subsurface c o m p o s i t i o n a l features can be m a p p e d with very g o o d S/N. Therefore, it is only a step further to a t t e m p t to use the 2 D A u g e r m a p p i n g c a p a b i l i t y of the i n s t r u m e n t to p r o d u c e an EP m a p in the region of the crater, a n d the results of fig. 11b show the subsurface edge of the A g covered A u / Z r crater structure very well.

7. Conclusion W e have shown various new uses of a digitized scanning A u g e r m i c r o p r o b e for a p p l i e d surface analysis studies that c o m p l e m e n t those previously o b t a i n e d in a similar s t u d y [16]. W e have shown that b y observing the s e c o n d a r y electron energy d i s t r i b u t i o n (0 < E < 50 eV) at selected p o i n t s on the surface one can easily m a p changes in the work function which are due to localized changes in surface c o m p o s i t i o n , which in turn account for c o n t r a s t changes o b s e r v e d in S E M images. T h e m a p p i n g c a p a b i l i t y of the i n s t r u m e n t was use to p r o d u c e 1D a n d 2 D m a p s of the SE p e a k intensity. A n interfacial oxide not readily o b s e r v a b l e with stand a r d A u g e r analysis, b u t easily o b s e r v e d in the form of a " h a l o " a r o u n d the crater edge in s t a n d a r d S E M i m a g i n g b e c o m e s m e a s u r a b l e using this technique.

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I n o n e c a s e of a p p l i c a t i o n , a c o m p a r i s o n is m a d e b e t w e e n s t a n d a r d d e p t h p r o f i l i n g a n d A u g e r l i n e s c a n a n a l y s i s a c r o s s t h e e d g e of a l o c a l i z e d c r a t e r . O n l y t h e l a t t e r t e c h n i q u e is c a p a b l e of d e t e c t i n g a v e r y t h i n ( - 1 2 A) i n t e r f a c i a l o x i d e w i t h r e l a t i v e ease. S E M M i c r o g r a p h s o f t h e r e g i o n p r o v e t h a t p a r t o f t h e r e a s o n w h y t h e o x i d e is n o t a p p a r e n t in t h e s t a n d a r d d e p t h p r o f i l e is r o u g h n e s s i n t r o d u c e d b y s p u t t e r i n g a n d t h e f o r m a t i o n o f o v e r l a y e r i s l a n d s . F i n a l l y w e h a v e s h o w n t h a t u s i n g t h e l a r g e e s c a p e l e n g t h of e l a s t i c a l l y b a c k scattered electrons one can map subsurface features not visible with either A u g e r o r S E e l e c t r o n s . F o r t h i s to b e s u c c e s s f u l , i n s t r u m e n t a l f a c t o r s s u c h as t h e s p a t i a l i n s t r u m e n t a l r e s p o n s e f u n c t i o n o f t h e C M A for low m a g n i f i c a t i o n s t u d i e s a n d s p e c i a l i z e d s i g n a l p r o c e s s i n g p a r a m e t e r s for m a p p i n g w i t h b a c k s c a t t e r e d p r i m a r y e l e c t r o n s h a d to b e c o n s i d e r e d .

Acknowledgments W e a r e g r a t e f u l t o M r . A. G o n z ~ l e z for t e c h n i c a l a s s i s t a n c e , a n d to Dr. D. A s p n e s a n d M r . J. P e r n a s for t h e e l l i p s o m e t r i c o x i d e t h i c k n e s s m e a s u r e m e n t s .

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] Ill] [12] [13] [14] [15] [16]

A.M. Czanderna, Ed., Methods of Surface Analysis, Vol. 1 (Elsevier, Amsterdam, 1975~. N.C. MacDonald and J.R. Waldrop, Appl. Phys. Letters 19 (1971) 315. D.J. Pocker and T.W. Haas, J. Vacuum Sci. Technol. 12 (1975) 370. R.L. Gerlach and N.C. MacDonald, in: Proc. Scanning Electron Microscopy/1976, Ed. O. Johari (I1TRI, Chicago, IL, 1977) p. 199. N.J. Taylor, J.S. Johanessen and W.E. Spicer, Appl. Phys. Letters 29 (1976) 497. H. Poppa and L. Cota-Araiza, Thin Solid Films, to be published. H. Poppa and L. Cota-Araiza, in: Proc. 2nd Latin American Syrup. on Surface Physics, Puebla, Mexico, 1982. J. Schaefer and J. Hoelzl, Thin Solid Films 13 (1972) 81. J. H61zl and F.K. Schulte, Springer Tracts in Modern Physics, Vol. 85 (Springer, Berlin, 1979). A.P. Janssen, P. Akhter, C.J. Harland and J.A. Venables, Surface Sci. 93 (1980) 453. J.E. Demuth and T.N. Rhodin, Surface Sci. 45 (1974) 249. C.R. Helms, W.E. Spicer and N.M. Johnson, Solid State Commun. 25 (1978) 673. J.F. Wager and C.W. Wilmsen, J. Appl. Phys. 50 (1979) 874. S.L. Raider and R. Flitsch, IBM J. Res. Develop. 22 (1978) 294. M.P. Seah and W.A. Dench, Surface Interface Analysis 1 (1979) 2. A. van Oostrom, Surface Sci. 89 (1979) 615.