Line resolution in the sub-ten-nanometer range in SAM

Ultramicroscopy 25 (1988) 31-34 North-Holland, Amsterdam

LINE RESOLUTION IN ~

31

SUB-TEN-NANOMETER RANGE IN SAM

J. CAZAUX *, J. CHAZELAS, M.N. CHARASSE and J.P. HIRTZ Laboratoire Central de Recherche& Thomson- CSF. Domaine de Corbeoille. BP 10, F-91401 Orsay. France Received 1 October 1987; in final form 15 January. 1988

Si (KLL) Auger linescans across a GaAs/Si chemical edge (bulk sample) have been obtained at a 100 key primary, beam energy. A line resolution of 8 nm has been deduced from the FWHM measurement of the first derivative of the experimental profile. Sample preparation, experimental procedure and criteria used to obtain such a result are described and discussed.

1. Introduction Since MacDonald and Waldrop [1] demonstrated the possibility of combining Scanning Electron Microscopy (SEM) with Auger Electron Spectroscopy (AES), considerable efforts have been devoted to the development of Scanning Auger Microscopy (SAM) and to the improvement of its lateral resolution [2-5]. In this field two main questions remain, one concerning the criterion for "lateral resolution" and the other concerning the nature and preparation of the test sample. As regards the first question, despite widespread consensus on the procedure to follow (which consists in obtaining an Auger line profile by scanning the incident beam across a chemical boundary), the criterion used to express this resolution numerically varies from one author to another. The question remains, therefore, of whether the SAM lateral resolution should be determined by the incident beam spot size or not [61. The second question relates to the difficulty of obtaining a chemical edge incorporated in a totally flal surface in order to avoid topographic artefact.,, [7]. This practical difficulty explains the

* Permanent address: Laboratoire de Spectroscopie d s Electrons, Facul;.6 des Sciences, F-51062 Reims Cedex, France.

large number of papers devoted to theoretical studies compared to the very small number of papers dealing with experimental results. When such a sample is obtained, reaching the resolution limit of the technique poses an additional problem, namely the rather long time of acquisition needed to obtain a good profile. (Even for a perfect instrument, in the counting mode, the statistical fluctuations of the signal S on a background BG are greater than vrS + BG ). During this rather long time. the position of the incident beam relative to the sample must either be free from mechanical and electrical drifts and instabilities, or any drifts occurring must be far smaller than the physical resolution limit. In this report, we explain how we succeeded in overcoming these difficulties in order to achieve. for the first time, a sub-ten-nanometer line reso!Lution in SAM (a resolution criterion recently suggested by one of us [6]).

2. Experiment The instrument used in the present stud', is a VG HB 501A Scannmg Transmission Electron Microscope (STEM) v,,hich provides electron energy loss spectra and X-ray emission spectra as well as a microdiffraction pattern of a thin foil immersed in the field of the objective lens.

0304-3991/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

32

J. Cazaux et al. / Line resolution in the sub-ten-nm range in S A M

For Auger analysis, the specimen in bulk form (thickness -- 30/~m in the reported experiment) is outside the pole pieces (working distance -- 4 cm), thus enabling the emitted electrons to be collected to obtain the secondary image or to be analyzed

(Auger electrons) by a standard VG 150 o spherical spectrometer. The experiment was carried out at an oblique incidence ( i - 50 ° ), the angle between the incident beam and the detected beam being about

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Fig. 1. Top: Secondary image of the Si/GaAs interface obtained before Auger analysis. From the secondary electron profile obtained between A and B. the incident spot diameter was estimated to be inferior to 8 nm. Bottom: Wide Auger spectra uutamcu-"'-:--'~:-,, t,t'.; counting mode (CRR 4: I p - - 8 nA: E 0 = 100 keV: time of acquisition per spectrum: 20 rain) at point A and point B 170 nm asart on the two 'tides of the interface. The reference level of spectrum A corresponds to the zero of the vertical scale while the reference level for spectrum B has been shifted upward (indication 0(B) in this figure), in order to prevent overlapping of the two cu~'es. Whereas only the Si KLL line can be clearly seen in spectrum A. spectrum B sh.,-ws As and Ga (LMM) lines and also the residual contribution (backscattering effect) of the silicon substrate (Si KLL line) and the AlAs overlayer (the small signal changes between Si KLL and As LMM lines). On the higher kinetic energy side of the Si (KEL) line. the background is slightly higher in the GaAs region than b,-the silicon region (influence of Z on the backscattering effect).

33

J. Cazaux et al. / Line resolution in the sub-ten-nrn range in S A M

110 °. The chemical edge was in the incident plane so that the linescan was perpendicular to this plane (parallel mode [5]). The primary beam intensity was 8 nA and the primary beam energy was selected to give the best signal-to-background ra~io [8]: 100 keV. The sample was an MBE-grown GaAs layer (200 nm thick) on a Si substrate. This GaAs layer was then encapsulated in an AlAs layer. Next, the sample was cleaved at atmospheric pressure before being transferred to the microscope. The region of interest was the G a A s / S i interface with no visible topographic effect either in the secondary electron image or in the Auger line profile, a result in direct relation to the quality of the GaAs-on,.Si epitaxy. Fig. 1 shows the wide-scan Auger spectra obtained at points A and B on the two sides of the G a A s / S i interface: AB = 170 nm (see legend for technical details). The incident electron beam was then driven from point A to point B step by step (50 steps of 3.4 nm each) while, for each step, the signal + background intensity of the Si KLL line ('kinetic energy 1614 eV) and the background before the peak (kinetic energy 1630 eV) were measured consecutively. Several intensity profiles were obtained, and these trials demonstrated the need to increase the resident time per step to improve the statistics. The top of fig. 2 shows, as an insert, :he S + BG and BG profiles finally obtained without any kind of detectable instability. The top of fig. 2 shows the difference between the two profiles (i.e., S). As this profile showed the predicted shape, without any detectable artefacts, it was not necessary to apply the procedure suggested by Prutton et al. [9] for reduction of topographic effects. In order to express the lateral resolution numerically, we then computed the first derivative of the profile and measured the full width a: half maximum, FWHM. For this, we used the recently suggested Sparrow criterion, applying the line spread function [6] instead of dealing directly with the usual edge profile (edge spread function) and applying one of the various criteria used previously, for example: d(10%-90%), d(15%-85%), D(50%) [10]. Without the use of any kind of

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Fig. 2. Top: Si (KLL) line profile deduced from the intensity changes measured (see insert) at E K = 1614 eV (S+ BG) and at E K = 1630 eV (BG). Note the change of the background intensity due to the influence of the atomic number (,_,(GaJ~: > Z(Si)) on the backscattering effects. The experimemal conditions are: E0 = 100 keV: I o = 8 nA: beam diameter - 8 nm. CRR = 4, 170 nm and 10 rain (for the total length AB and the total acquisition time of the 50 steps). Bottom: FWHM of the first derivative gives a resolution of 8 nm without an,,' kind of smoothing procedure The doued lines only suggest the shape of the "'plateaus" (top) and the approximate shape of the halo (bottom) resulting from the contribution of the backscattcring effects. The rapid decrease (top) and the sharp peak ~,bottom* are directly related to the incident beam spot size through di-~.ct excitation of the Auger ele,.trons by the incident beam penetrating the sample. These cur~es correspond exactlx to what was expected [6].

34

J. Cazaux etal. / Line resolution in the sub-ten-nm range in S A M

smoothing procedure, the first derivative obtained is shown at the bottom of fig. 2, with a line resolution evaluated as 8 nm.

3. Discussion The above results give rise to the following remarks: (i) The test specimen for lateral resolution must have a totally fl~t surface incorporating a heterogeneous interface. The heteroet,;taxial structures grown for the semiconductor industry offer a wide variety of good candidates when they are cleaved (preferably in vacuum). Among them, the choice of silicon (Si KLL line) presents the advantage that "impact parameter" broadening, if it exists, is negligible even at high primary beam energies (see fig. 7 in ref. 10), whereas the advantage of silver is a better sensitivity factor (see fig. 1 in ref. [8]). A A g / S i interface will be very interesting to investigate in the future in order to obtain two complementary profiles instead of only one, as in our case. We have not attempted to perform the complementary profile by using As(LMM) signal because its intensity is less than one-half the intensity of Si(KLL) as shown in fig. 1 - relative to the Si profile; this will lead to an increase of the line scan duration of a factor greater than four (for a comparable S / B ~ ) . On the other hand, a complementary silver profile would be easier to obtain because of its fat better S / B G [8]. (ii) "['he lateral resolution criterion has to charactedze the ultimate performance of an instrument. The choice of the line spread function presents the advantage (over the edge spread function) of being mainly governed by the instrumental characteristics (incident intensity and spot size, stabilily) and of being far less sensitive to the physicfl effects resulting from the sample compositicn (backscattering coefficient). The use of the Sparrow criterion (FWHM) takes into account the ability of modern acquisition systems to detect very small dips between two bumps (instead of the arbitrary 26.5% dip of the Rayleigh criterion used for eye detection). From

the theoretical point of view, it would be better to deal with the point resolution in SAM. The exact value we obtained (8 nm) is not very significant; it only means that a 20-30 nm resolution can be obtained easily with our equipment in a standard experiment. It also means that dispersed catalysts can be qualitatively analyzed, grain by grain, in SAM (grain size in the 10 nm range) in the near future, even if, for many practical applications, the improvement of lateral resolution will make it increasingly difficult to quantify the experiments (intensity-concentration relationship).

4. Conclusion A line resolution in the sub-ten-nanometer range has been obtained for the first time in Auger microscopy. The experiment was carried out on a flat G a A s / S i chemical edge using the VG HB 501A STEM equipped with an Auger attachment.

Acknowledgement Thanks are due referee Dr. W, H/Ssler for useful comments.

References [1] N.C. Macdonald and J.R. Waldrop, Appl. Phys. Letters 19 (1971) 315. [2] J.G. Todd, M. Poppa, D. Moorhead and M. Bales, J. Vacuum Sci. Technol. 12 (1975) 953. [3] A.P. Janssen and J.A. Venable~, Surface Sci. 77 (1978) 351. [4] M.M. El Gomati and M. Prutton, Surface Sci. 72 (1978) 485. [5] M.M. E1 Gomati, A.P. Janssen, M. Prutton and J.A. Venables, Surface Sci. 85 (1979) 309. [6] J. Cazaux, J. Microscopy 145 (1987) 257. It I ~..t.J. I uppen and G.J. Davies, Surface interface Analysis 7 (1985) 235. [8] J. Chazelas, A. Friederich and J. Cazaux, Surface Interface Analysis 11 (1988) 36. [9] M. Prutton, L.A. Larson and H. Poppa, J. Appl. Phys. 54 (1983) 374. [10] J. Cazaux, Surface Sci. 125 (1983) 335.