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Determination of layer thickness by Auger electron spectroscopy
Surface Science i 22 (1982) L619-L621 North-Holland Publishing Company
L619
SURFACE SCIENCE LETTERS s~ETERMINATION OF LAYER THICKNESS BY AUGER ELECTRON CTROSCOPY J.F. SMITH and H.N. SOUTHWORTH Department of Metallurgy and Materials, University of Birmingham, P.O. Box 363, Birmingham B15 2TT, UK Received 17 June 1982
Seah [1] has devised a non-destructive technique for determining the thickness of an adsorbed layer which requires the mean free paths and intensities of both high and low energy Auger electrons from an element within the layer. The relative intensities of the high and low energy peaks are compared with the relative intensities of the same peaks from the pure element. Clearly, since the high energy electrons normally have a greater mean free path, this peak will be affected more by changes in layer thickness. In the following, it is shown that the layer thickness can be deduced from observations of low and high energy peaks from the substrate material; this is more practicable, since the spectrum of the pure adsorbate is not required. Fig. 1 depicts an adsorbed layer, or segregated layer, of thickness x on a substrate which produces Auger electrons with escape depths m and n. The total Auger electron intensity at the adsorbate-matrix interface is obtained by integrating all contributions from the semi-infinite volume; for example, making the reasonable assumption of an exponential decrease in the number of Auger electrons emerging from atoms deeper and deeper within the matrix [2], one obtains for the low energy electrons of escape depth n
f,
lid I.~EI;t
Figi 1. Layer thickness x is calculated from knowledge o f the escape depths n and m of Auger electrons produced by matrix atoms.
0039-6028/82/0000-0000/$02.75 © 1982 North-Holland
J.F Smith, H.N. Southworth / Determination of layer thickness by AES
L620
I~ =
I
off
e x p ( - z / n ) dz
=
I°n,
(1)
where I ° is a constant. Escape depths are basically a function of electron energy, and are not strongly dependent upon matrix properties [3]; since no further contribution to the Auger yield given by eq. (1) will take place in the adsorbed layer, therefore, the current will simply decay exponentially according to
I',,~ = I°n exp( - x / n ),
(2)
where Iff is the Auger current actually measured. After the surface has been subjected to prolonged sputtering, x = 0 and eq. (2) becomes eq. (1), i.e., if the Auger currents are measured before and after sputtering, all the terms in eq. (2) are known apart from x. In practice, Auger currents are rarely measured; instead, and justifiably [4], the peak-to-peak amplitudes in the d N ( E ) / d E curve are measured and assumed to be proportional to the Auger signal. Therefore, if px is this peak-to-peak height,
px = alOn exp( - x / n ) ,
(3)
in which a is a constant of proportionality. To obviate errors arising from a change in experimental conditions, particularly variations in primary beam current and sample position, all the peaks in a spectrum are'usually normalised to one chosen peak, the assumption being that the whole spectrum will be equally affected. To accomplish this in the present example, eq. (3) must be divided by the corresponding expression for the peak-to-peak height of the high energy Auger peak, i.e.,
P:=aI°nexp,-x/n) = (1_1) n P~x b l ° m e x p ( - x / / m ) Kexp
x,
(4)
where the constant K is the peak-to-peak height ratio at x = 0. Convenient spectra With which to test the applicability of eq. (4) were published by Seah [1]. These consist of analyses of tin and sulphur enriched grain boundaries in Fe-Sn and Fe-S binary alloys. Upon inserting the 47 and 702 eV iron peak-to-peak heights in eq. (4), together with values of escape depth from ref. [3], the equation may be solved for x: the calculated thicknesses are 2.0 and 2.4 A for the sulphur and tin layers, respectively. The corresponding values given by Seah are 0.53 and 1.92 monolayers. Translating thickness in Angstrbms into monolayer equivalents is bound to be somewhat dubious in the absence of information on the chemical state of the adsorbed species; the effective radius of an atom, of course, depends strongly upon its ionic condition. However, if the common valencies of sulphur and tin of - 2 and + 4 are assumed, Williams [5] gives ionic radii of 1.84 and 0.71 ~,, respectively. When these values are used to convert the sulphur and tin
J.F. Smith, H.N. Southworth / Determination of layer thickness by AES
L621
thicknesses to monolayers, 0.54 of a m o n o l a y e r is predicted for sulphur and 1.7 monolayers for tin, in good agreement with the m e a s u r e d levels of 0.53 and 1.92 monolayers, respectively.
References [1] M.P. Seah, J. Phys. F3 (1973) 1538. [2] C.C. Chang, in: Characterization of Solid Surfaces, Eds. P.F. Kane and G.B. Larrabee (Plenum, 1974). [3] J.C. Rivi~re, Contemp. Phys. 14 (1973) 513. [4] R.E. Weber and A.L. Johnson, J. Appl. Phys. 40 (1969) 314. [5] R.A. Williams, Handbook of the Atomic Elements (Vision Press, 1970).
L619
SURFACE SCIENCE LETTERS s~ETERMINATION OF LAYER THICKNESS BY AUGER ELECTRON CTROSCOPY J.F. SMITH and H.N. SOUTHWORTH Department of Metallurgy and Materials, University of Birmingham, P.O. Box 363, Birmingham B15 2TT, UK Received 17 June 1982
Seah [1] has devised a non-destructive technique for determining the thickness of an adsorbed layer which requires the mean free paths and intensities of both high and low energy Auger electrons from an element within the layer. The relative intensities of the high and low energy peaks are compared with the relative intensities of the same peaks from the pure element. Clearly, since the high energy electrons normally have a greater mean free path, this peak will be affected more by changes in layer thickness. In the following, it is shown that the layer thickness can be deduced from observations of low and high energy peaks from the substrate material; this is more practicable, since the spectrum of the pure adsorbate is not required. Fig. 1 depicts an adsorbed layer, or segregated layer, of thickness x on a substrate which produces Auger electrons with escape depths m and n. The total Auger electron intensity at the adsorbate-matrix interface is obtained by integrating all contributions from the semi-infinite volume; for example, making the reasonable assumption of an exponential decrease in the number of Auger electrons emerging from atoms deeper and deeper within the matrix [2], one obtains for the low energy electrons of escape depth n
f,
lid I.~EI;t
Figi 1. Layer thickness x is calculated from knowledge o f the escape depths n and m of Auger electrons produced by matrix atoms.
0039-6028/82/0000-0000/$02.75 © 1982 North-Holland
J.F Smith, H.N. Southworth / Determination of layer thickness by AES
L620
I~ =
I
off
e x p ( - z / n ) dz
=
I°n,
(1)
where I ° is a constant. Escape depths are basically a function of electron energy, and are not strongly dependent upon matrix properties [3]; since no further contribution to the Auger yield given by eq. (1) will take place in the adsorbed layer, therefore, the current will simply decay exponentially according to
I',,~ = I°n exp( - x / n ),
(2)
where Iff is the Auger current actually measured. After the surface has been subjected to prolonged sputtering, x = 0 and eq. (2) becomes eq. (1), i.e., if the Auger currents are measured before and after sputtering, all the terms in eq. (2) are known apart from x. In practice, Auger currents are rarely measured; instead, and justifiably [4], the peak-to-peak amplitudes in the d N ( E ) / d E curve are measured and assumed to be proportional to the Auger signal. Therefore, if px is this peak-to-peak height,
px = alOn exp( - x / n ) ,
(3)
in which a is a constant of proportionality. To obviate errors arising from a change in experimental conditions, particularly variations in primary beam current and sample position, all the peaks in a spectrum are'usually normalised to one chosen peak, the assumption being that the whole spectrum will be equally affected. To accomplish this in the present example, eq. (3) must be divided by the corresponding expression for the peak-to-peak height of the high energy Auger peak, i.e.,
P:=aI°nexp,-x/n) = (1_1) n P~x b l ° m e x p ( - x / / m ) Kexp
x,
(4)
where the constant K is the peak-to-peak height ratio at x = 0. Convenient spectra With which to test the applicability of eq. (4) were published by Seah [1]. These consist of analyses of tin and sulphur enriched grain boundaries in Fe-Sn and Fe-S binary alloys. Upon inserting the 47 and 702 eV iron peak-to-peak heights in eq. (4), together with values of escape depth from ref. [3], the equation may be solved for x: the calculated thicknesses are 2.0 and 2.4 A for the sulphur and tin layers, respectively. The corresponding values given by Seah are 0.53 and 1.92 monolayers. Translating thickness in Angstrbms into monolayer equivalents is bound to be somewhat dubious in the absence of information on the chemical state of the adsorbed species; the effective radius of an atom, of course, depends strongly upon its ionic condition. However, if the common valencies of sulphur and tin of - 2 and + 4 are assumed, Williams [5] gives ionic radii of 1.84 and 0.71 ~,, respectively. When these values are used to convert the sulphur and tin
J.F. Smith, H.N. Southworth / Determination of layer thickness by AES
L621
thicknesses to monolayers, 0.54 of a m o n o l a y e r is predicted for sulphur and 1.7 monolayers for tin, in good agreement with the m e a s u r e d levels of 0.53 and 1.92 monolayers, respectively.
References [1] M.P. Seah, J. Phys. F3 (1973) 1538. [2] C.C. Chang, in: Characterization of Solid Surfaces, Eds. P.F. Kane and G.B. Larrabee (Plenum, 1974). [3] J.C. Rivi~re, Contemp. Phys. 14 (1973) 513. [4] R.E. Weber and A.L. Johnson, J. Appl. Phys. 40 (1969) 314. [5] R.A. Williams, Handbook of the Atomic Elements (Vision Press, 1970).