ELECTRON PROBE MICROANALYSIS OF SUBMICRON ALLOY FILMS

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ELECTRON PROBE MICROANALYSIS OF SUBMICRON ALLOY FILMS

P. Willich

To cite this version:

P. Willich. ELECTRON PROBE MICROANALYSIS OF SUBMICRON ALLOY FILMS. Journal de

Physique Colloques, 1984, 45 (C2), pp.C2-621-C2-624. �10.1051/jphyscol:19842145�. �jpa-00223816�

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ELECTRON PROBE MICROANALYSIS OF SUBMICRON ALLOY FILMS

P. Willich

Philips GmbH Forsahungslaboratovium Hamburg, D-2000 Hamburg 54, F.R.G.

Résumé - Des couches minces de l'ordre de 200 ng/cm2, obtenues par évaporation et pulvérisation ont été étudiées à la micro- sonde électronique, incluant l'analyse d'C>2 et Ar piégés en concentration de 0,2 à^10^%. Pour E„ de 5 à 10 KV, la profondeur d'émission X est estimée à 10 nm près: avec différents modèles de correction, les erreurs en analyse quantitative ne dépassent pas 3,5 %.

Abstract - EPMA has been applied to electron beam evaporated and r.f. sputtered layers of about 200 tig/cm2. This includes analysis of trapped argon and oxygen ranging from 0.2 to

10 wt%. For primary electron energies of 5 to 15 keV depth range of X-ray emission may be predicted with an accuracy of ± 10 nm.

Errors of quantitative analysis are discussed for various correction models and should not exceed 3.5 %.

I - INTRODUCTION

Electron probe microanalysis (EPMA) may be performed at a sufficiently low primary electron energy (Eg) for which the depth range of X-ray emission is reduced to about 0.2 to 0.5 um. This letter considers some practical aspects of EPMA under conditions of thin film analysis: Ex- perimental depth ranges compared to the theoretical predictions, eval- uation of analytical sensitivities, accuracy provided by various models of matrix correction, and simultaneous quantitative analysis of incorporated argon and oxygen.

II - DEPTH RANGE OF X-RAY EMISSION

Experimental depth ranges have been studied for FeLa, AuMa, FeKa, and AuLa X-ray lines covering the range of critical excitation energies

(Ec). from 0.7 keV to 12 keV. Pure element films of 0.05 to 0.5 nm were deposited on carbon by electron beam evaporation. The exact mass depo- sition was determined by chemical analysis. X-ray intensities normaliz- ed to a thick film standard were measured as a function of primary electron energy. The extrapolation of this curve to the saturation level at low E Q yields the primary electron energy for which the film thickness represents the depth range of X-ray emission. In Figure 1 experimental depth ranges are compared to those obtained by the range re- lation of Castaing [1]. The influence of mass absorption was calculated by the absorption factor of Philibert. For AuLa and FeKa this model gives an accurate representation of the measured depth ranges. For the X-ray lines with Ec < 5 keV, Figure 1 shows larger deviations from the experimental data which may be explained by the strong influence of X- ray absorption. Alternatively, theoretical depth ranges have been de- rived from depth distribution functions 0 (pz) calculated by the formula of Parobek and Brown [2]. Figure 1 shows an accurate fit to the experimental data for FeLa and AuMa, but significant deviations for

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842145

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C2-622 JOURNAL DE _PHYSIQUE

- Castaing's formula

---

i'arobek,Brown $,pz,

0 experimental' ,

electron energy I keV 1 Fig. 1

-

Experimental and calculated depth range of X-ray emission for the element targets of Fe and Au as a function of primary electron energy.

lL' Ib io io h

40

$0 $0 do

atomic number

Fig. 2

-

Overvoltage and counting rate (crystal spectrometers) of some elements from C to Bi corresponding to a depth range of 0.3 pm. Data refer to a concentration of 30 wt% in a matrix of Fe.

FeXa and AuLa (Ec > 5 keV). Within the limits pointed out, both models enable one to predict depth ranges of 0.2 to 0.5 pm with an error of about

+

10 nm correspondinq to the error estimated for the determination of experimental depth ranges. A comparable depth range for X-ray lines separated by a large difference of the critical excitation energy may be obtained by measurements at two electron energies.

I11

-

ANALYTICAL SENSITIVITY

Analysis of thin films at low overvoltages (EO/Ec<2) or by low energy X-rays combined with low electron energies (EO<10 keV) conflicts with the requirements of analytical sensitivity. This concerns the ability to distinguish between two compositions that are nearly equal. The X- ray intensity of 17 elements from carbon to bismuth was calculated by ZAF correction for the electron energy representing a linear depth range of 0.3 pm in a matrix of 70 wt% of iron (Fig. 2 ) \ This was based on measurements of the pure element or a well defined cbmpound by crystal spectrometers. A suitable choice of the X-ray line results in at least 1000 cps (beam current 100 nA) corresponding to 3 measurements, each at a counting time of 30 seconds, to obtain a sensitivity of 0.8%.

This should be acceptable even for a large series of samples consider- ing the stability of the applied computer-controlled instrument

(CAMEBAX)

.

IV

-

ACCURACY OF QUANTITATIVE ANALYSIS

Atomic number correction may be a significant limitation to the accuracy of quantitative thin film analysis. This was studied for An-Cu alloys (NBS SRM 482) simulating a large difference of atomic number between specimen and pure element standard (AZ = 40). Conventional ZAF correction was performed by a commercial on-line program [ 3 ] using backscattering factors of Duncumb combined with the Bethe formula for the calculation of stopping power factors. Analytical errors of the applied ZAF model show a tendency towards overcorrection with decreas- ing EO (Fig. 3). Alternatively, the atomic number correction was performed by modified expressions of Love and Scott [ 4 ] for the backscat- ter factor R and the stopping power factor S. Figure 3 indicates im- proved accuracy for AuMa at electron energies of 5 to 10 keV. Correc-

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10 _{20 wt %}Au

.\

20wt% Cu

0

t 2 -

-

0

-

-2 -

convent~onol Z A F .i . '. ,

Love I Scott .A..- -.-___

Brown 1 Packwood -'a

Fig. 3

-

Quantitative analysis of Fig. 4

-

Concentration of oxygen Au-Cu alloys. Error of various depending on the percentage of correction models as a function OKa X-rays emitted from a surface of primary electron energy. layer of 0.1 wm for primary ener-

gies of 5 to 15 keV.

tion proved to be also effective for CuKa at very low overvoltages with a tendency towards undercorrection with increasing Eo. The model of Brown and Packwood [5] is based on a Gaussian expression to calculate @

(pz) curves. Figure 3 shows relative errors of 2% for AuMa at low values of Eo. However, the applied version of this program [ 6 1 results in significant overcorrection for CuKa. Practical application of thin film analysis often requires operation at a low overvoltage and measurement of a low energy X-ray line simultaneously. From this point of view the conventional ZAF model proved to be a useful compromise. For a depth range of 0.3 bm, e.g. AuMa at 12 keV and CuKa at 15 keV, errors of ZAF correction may be as high as 3.5% when the difference of atomic number between specimen and standard is large (AZ = 40).

V

-

ANALYSIS OF TRAPPED ARGON AND OXYGEN

Quantitative analysis was completed by simultaneous determination of incorporated gases. Argon was calibrated by extrapolation of the 'pure argon' counting rate from measurements of Si to C1 [71. By this method a sputtered layer of Si containing 10.2 wt% of Ar was defined as a com- pound standard. Calibration of oxygen was accomplished by a single crystal of Y3Fe5OT2. OKa was measured for carbon coatings of various thicknesses to extrapolate the counting rate of the uncoated material.

The analytical error caused by the ZAF correction was estimated by EPMA of 15 oxide materials using mass absorption coefficients of Henke [ 8 1 and should not exceed 7.5% for Eo = 10 keV. Oxide layers up to a thick- ness of 0.1 wm were detected even on freshly prepared metal layers by SIMS depth profiling. Figure 4 shows concentrations of 0 measured at E O of 5 to 15 keV plotted versus the percentage of OKa X-rays emitted from a surface region of 0.1 wm. These values were calculated from @

(pz) depth distribution functions [3]. Consequently, the concentration of incorporated oxygen

-

corresponding to an 'infinitely' high value of

-

may be extrapolated from measurements at several electron ener- t ~ e s . The accuracy of this method was tested by pure iron (hulk) for which no incorporated oxygen was detected by SIMS measurements. Correc-

tion for the influence of surface oxydation proved to be essential to the analysis of low concentrations of trapped oxygen in metal layers.

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C2-624 JOURNAL DE PHYSIQUE

Tab. 1

-

Electron probe microanalysis of films prepared by r.f. sputtering. Film thicknesses of 0.5 to 0.6 fim on A1203.

SAMPLE C O M P O S I T I O N [ w T X I

Cr Si Ar 0 TOTAL

Tab. 2 sis of beam ev of 0.3

-

Electron probe microanaly- films prepared by electron .aporation. Film thicknesses

to 0.4 pm on silicon.

SAMPLE

1 2 3 4 5 6 7 8 9 1 0 11

TOTAL

100.0 100.1 100.0 99.4 99.9 99.1 1 0 0 . 3 100.5 100.4 101.3 101.4

VI

-

DISCUSSION OF RESULTS

Thin film materials may be prepared by bombarding a target with argon ions, generated in a radio frequency glow discharge (ref. sputtering).

Compositions of Cr/Si thin film resistor materials prepared by this method are given in Table 1. Incorporation of gases may be correlated to certain parameters of preparation, e.g. composition of the residual gas and substrate bias voltage. Analyses were performed at 7 keV (0, Si, Ar) and 9 keV (Cr) corresponding to a depth range of 0.4 pm (150 pg/cm2). Sums of concentrations close to 100 wt% (ZAF) indicate accurate calibration of argon and no significant errors for quantitative analysis of oxygen. Table 2 shows compositions (ZAF) of layers prepared by electron beam evaporation. Depth range of X-ray emission is about 0.3 pm (300 pg/cm2) for 8 keV (AuMa) and 10 keV (GdLa, FeKa)

.

^{OKa was}

measured at 5, 8 and 10 keV. At Eo = 8 keV oxide surface layers corre- spond to about 0.6 wt% of oxygen which is comparable to the level of incorporated oxygen. Table 2 indicates a correlation between the concentration of iron and trapped oxygen rising up to 2.6 wt% for a layer of pure iron.

REFERENCES

[I] Castaing R., Adv. in Electronics and Electron Physics, Morton L., Ed., Academic Press, New York (1960) 317

[2] Parobek L., Brown J. D. , X-ray Spectrom. 7 ( 1978) 26

[3] Henoc J., Conty C., and Tong M., ~icrobeam Analysis (1979) 281 [4] Love G., Scott V.D., J. Phys. D: Appl. Phys. 1 1 (1978) 1369 [5] Brown J.D., Packwood R.H., X-ray Spectrom. -1982) 187

[6] Von Rosenstiel A.P., Metaalinstituut TNO, Apeldoorn, The Nether- lands, private communication

[7] Willich P. , Obertop D., X-ray Spectrom. 1 1 (1 982) 32 .

[8] Henke B.L., Lee P., Tanaka T.J., ~ h i m a b u G r o R.L., Fujikawa B.K., Atomic Data and Nuclear Data Tables, Academic Press, New York, 27 (1982) 1