Non-destructive characterization of strontium bismuth tantalate films

Materials Science in Semiconductor Processing 5 (2003) 141–145

Non-destructive characterization of strontium bismuth tantalate films P. Petrika,*, N.Q. Kha! nha, Z.E. Horva! tha, Z. Zolnaia, I. Ba! rsonya, T. Lohnera, M. Frieda, J. Gyulaia, C. Schmidtb, C. Schneiderb, H. Rysselb,c a

MFA, Research Institute for Technical Physics and Materials Science, P.O. Box 49, 1125 Budapest, Hungary b Fraunhofer-Institut fur . Integrierte Schaltungen, Schottkystrasse 10, 91058 Erlangen, Germany c Lehrstuhl fur Cauerstrasse 6, . Elektronische Bauelemente, Friedrich-Alexander Universitat . Erlangen-Nurnberg, . 91058 Erlangen, Germany

Abstract Optical, compositional, and structural properties of strontium bismuth tantalate (SBT) films deposited in a high throughput low-pressure chemical vapor deposition reactor using liquid metal-organic precursors were characterized using three non-destructive techniques, spectroscopic ellipsometry (SE), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The thicknesses and the refractive indices of the SBT layers were calculated with SE using different parametric dielectric function models. The samples were characterized with RBS using different tilt angles and probe ions to enhance the depth resolution and the mass separation. Comparison with SE measurements supports the results of Bahng et al. revealing an increasing refractive index (n) with increasing Bi/Sr ratio. The decreasing grain size measured by XRD was reflected as a decrease of n in the SE measurement. We show that RBS, XRD, and SE supply a wide range of information about the SBT layers, which can be used for qualification as well as for feedback to layer production. The results suggest that by SE, being used as in situ or in line characterization tool, the control of even complex MOCVD deposition looks feasible. r 2003 Published by Elsevier Science Ltd. Keywords: Strontium bismuth tantalate; Metal-organic chemical vapor deposition; spectroscopic ellipsometry; Rutherford backscattering spectroscopy; X-ray diffraction

1. Introduction During the past 25 years the density of dynamic random-access memory (DRAM) has been increasing by a factor of 4 every 3 years. This remarkable trend has been made possible by advances by improvements in DRAM architecture and in various areas of microelectronic processing. In recent years, a lot of work has focused on the development of ferroelectric and highpermittivity materials, because of their potential application in random-access memories (RAM). Strontium bismuth tantalate (SBT) has been recognized as a *Corresponding author. Tel.: +36-1-3922222; fax: +36-13922226. E-mail address: [email protected] (P. Petrik).

promising material for non-volatile ferroelectric RAMs due to its very low polarization fatigue, low leakage current, low switching field, and environmental safety [1]. Although the ferroelectric and dielectric properties of SBT have been extensively investigated, its optical properties have been rarely studied, especially with spectroscopic ellipsometry (SE). Optical transmission, infrared optical, and ultraviolet–visible absorption measurements were performed by Zhang et al. [2], Huang et al. [3], and Watanabe et al. [4], respectively, the latter one revealed a band gap of about 4.2 eV. Bahng et al. [5] reported the optical properties of pulsedlaser deposited SBT films for different Bi/Sr ratios measured by SE. In this paper we present measurements on SBT layers prepared by metal-organic chemical vapor deposition

1369-8001/03/$ - see front matter r 2003 Published by Elsevier Science Ltd. doi:10.1016/S1369-8001(02)00095-1

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P. Petrik et al. / Materials Science in Semiconductor Processing 5 (2003) 141–145

(MOCVD) using SE with different parametric dielectric function models, Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD) to establish a correlation between the stoichiometric and structural properties obtained by RBS and XDR on one hand, and the optical properties measured by SE on the other hand. The aim of our investigations was not to analyze a perfect SBT layer, but to compare the results of different measurement techniques laying special emphasis on the optical technique, because of its in situ capabilities. One of the main reasons of using in situ measurement is to shorten the process ramp up, i.e. fast optimization of the process by characterizing not perfect samples from the development phase. Being an indirect technique, SE requires the correlation and cross-checking with other characterization methods when measuring such complex materials as SBT deposited by MOCVD. In our case, the samples were obtained from the development phase of the process optimization to demonstrate the sensitivity of SE and the correlation of the SE results with the changes reflected by RBS and XRD and to improve parametric oscillator models for the dielectric function. The results suggest that by SE, being used as in situ or in line characterization tool, the control of even complex MOCVD deposition looks feasible.

2. Experimental details SBT films were grown on platinum-coated oxidized silicon wafers using commercially available liquid metalorganic precursors in an industrial-proven, high throughput, low-pressure chemical vapor deposition (CVD) reactor. The thickness of the samples is in the range of about 30–200 nm. The SE measurements were performed in the wavelength range of 280–840 nm using a SOPRA ES4G rotating polarizer spectroscopic ellipsometer, with a nominal angle of incidence of 751. The exact angle of incidence was determined by a measurement on an oxidized silicon sample, and was held constant in the evaluations of the SBT samples. The raw SE measurement data are the tan C and cos D values, where C and D are the ellipsometric angles that describe the reflection of the polarized light [6]. The evaluation of the measured data were carried out through the following steps: first an optical model was created, then the spectra were calculated using the optical model, followed by the fitting of the calculated spectra to the measured ones using the Levenberg–Marquardt algorithm. The best-fit model parameters were obtained in terms of their 95% confidence limits and the unbiased estimator (s) of the mean square deviation. The samples were characterized using RBS with a 3550 keV 4He+ ion beam to utilize the better heavy mass separation at high ion energies. The spectra were taken

at a scattering angle of 1651 with regard to the beam direction. A tilt angle of 71 and 751 was used. The latter one increases the depth resolution for the thin SBT layers. We also applied an N+ beam to get better mass resolution. The current of the analyzing beam was measured by a transmission Faraday cup [7]. The BS spectra were evaluated using the RBX program devel! oped by E. Kotai [8]. The XRD measurements were carried out on a Philips PW 1050 diffractometer with secondary beam monochromator using Cu Ka radiation. The powder diffraction file database of the International Center for Diffraction Data [9] was used for phase identification.

3. Results and discussion The optical properties of CVD materials depend strongly on the deposition conditions; therefore, the standard approach of using reference libraries for the material to determine the layer thickness cannot be applied. The optical properties have to be determined together with other material parameters (like layer thickness) directly (wavelength by wavelength) or by fitting whole spectra (using dispersion models). The direct determination can be used only with simple model structures, and with the multiple angle of incidence approach to increase the number of measured data and the precision using parameter fitting. With dispersion models the dielectric function of the layer is described using a limited number of parameters in a given spectral range. We used this latter approach, because it allows the measurement of complex structures (i.e. multilayer stacks), parasitic effects (like surface roughness or boundary layers [10]), and provides additional information on the sample (i.e. the band gap). Because the Pt layer is opaque in the whole wavelength range, a simple optical model was applied with a Pt substrate and an STB layer described using the Cauchy or the Adachi dispersion equation. The dielectric function of Pt was determined from a measurement on a reference sample without SBT on the Pt layer. The Cauchy model has three parameters for both the real (n) and imaginary (k) parts of the refractive index n¼Aþ

106 B 1012 C þ ; l2 l4

k ¼Dþ

106 E 1012 F þ ; l2 l4

ð1Þ

where l denotes the wavelength, A, B, C, D, E, and F are the Cauchy coefficients. The parametric dielectric function model of Adachi [11] considering only the contribution from the direct band-gap (E0) transition is eðEÞ ¼

A0 2  ð1  x0 Þ0:5  ð1  x0 Þ0:5 ; w20 E01:5

ð2Þ

where X0 ¼ ðEþiGÞ=E0 ; A0 ; E0 ; and G are the amplitude, transition energy, and broadening parameter, respectively.

P. Petrik et al. / Materials Science in Semiconductor Processing 5 (2003) 141–145

A good fit was obtained with both models as shown in Fig. 1 (a s value less than 0.09 can usually be considered as a good fit). The Adachi model provides a better fit (especially for tan C) with only three parameters, and additional information like the band-gap energy (Table 1). The Cauchy model is valid only for photon energies below the band gap, so the fit was performed in this case in a limited range of 320–840 nm (3.87– 1.48 eV). The refractive index values calculated from the fitted parameters of Table 1 are plotted in Fig. 2. The peak values of 2.6–2.7 for n and the band-gap energies of 4.00–4.48 eV (Table 1) correlate well with the values of E2.5–2.7 and 3.98–4.25, respectively, reported by Watanabe et al. [4] and Bahng et al. [5]. The RBS analysis was performed using different tilt angles and probe ions to change the depth resolution and the mass separation, respectively. Fig. 3 shows the good mass separation of Bi and Ta for the 14N+ probe beam at a tilt angle of 01 (though the Sr signal is hardly observable due to the high background arisen from Pt).

When using He+ with a tilt angle of 71, the Sr peak still appears clearly with a very low background (not shown), but the Bi and Ta signals are overlapped by the Pt signal. The tilt angle of 751 allows a good depth resolution for the very thin SBT samples. The element ratios determined from the simulation of the RBS spectra reveal stoichiometry ranging from SrBi1.1Ta3.1Ox to SrBi1.6Ta2.7Ox (Table 2). The comparison of the RBS and SE measurements supports the results of Bahng et al. [5] revealing an increasing n with increasing Bi/Sr ratio (Fig. 4). The XDR spectra show the formation of the orthorhombic SBT phase (Bi2Sr(Ta2O9)) in all samples (Fig. 5). The SBT layer is highly oriented, because mostly the (0,0,l)-type peaks are present. The relative intensities of the SBT peaks are different, which may be explained by the different average grain size. The grain size in Sample 1 must be smaller than in Samples 2 and 3, because the (0 0 8) peak is broader and the intensities are lower in spite of the thicker layer. This is reflected as

Photon Energy (eV)

Photon Energy (eV) 3

2

3

Tan Ψ

8

0

6

-0.04

measurement Cauchy (σ=0.0200) Adachi (σ=0.0019)

4 2 0 1

Difference in Tan Ψ

4

4

3

2

2.5

n 2

-0.08

ο

Angle of incidence (Φ): 75.36

1.5

0.08

0.5

Difference in Cos ∆

10

143

1

Cos ∆

0.04 0 0 -0.5

Sample 3 -1 0.3

0.4

0.5

0.6

0.7

-0.04

0.8

0.4 0.2

0.8

0

Wavelength (µm)

Fig. 1. Measured and fitted ellipsometric spectra. The dashed and dotted lines show the fitted spectra and the difference between the fitted and measured curves.

Sample 1 Sample 2 Sample 3 Sample 4

k 0.6

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Wavelength (µm)

Fig. 2. Real (n) and imaginary (k) parts of the refractive indices calculated using the parameters of Table 1.

Table 1 Fitted parameters of the Adachi model (A0 ; E0 ; and G are the amplitude, the transition energy, and the broadening parameter, respectively, see Ref. [11]; s denotes the quality of the fit)

Thickness (nm) A0 (eV1.5) E0 (eV) G s

Sample 1

Sample 2

Sample 3

Sample 4

185.377.5 107.575.2 4.4870.07 0.17870.028 0.0096

40.370.4 126.472.5 4.0270.02 0.19770.007 0.0021

37.870.2 120.371.0 4.0070.01 0.14870.007 0.0019

43.970.3 129.771.6 4.1270.01 0.14870.010 0.0013

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a significant decrease of n in the SE measurement (Fig. 2). After annealing the grains grow to a much larger size, and the layers give high intensity and welldefined XRD peaks (not shown in this article). The reflections denoted by ‘x’ in Fig. 5 cannot be identified as known oxides, carbides of strontium, bismuth, and tantalum, or mixed phases of them. Similar peaks are often present in the spectra of as deposited MOCVD SBT layers at lower angles. After annealing in oxidizing atmosphere, they always disappear, indicating that they origin from an organic surface contaminant.

4. Conclusions

determined together with the layer thicknesses using different parametric dielectric function models. Confidence limits for the thickness measurements were about 1% of the layer thickness for the thinner (30–40 nm) layers. The element ratios were determined by RBS using different tilt angles and probe ions to enhance the depth resolution and the mass separation. Comparison with SE measurements supports the results of Bahng et al. [5] revealing an increasing n with increasing Bi/Sr ratio. The decreasing grain size measured by XRD was reflected as a decrease of n in the SE measurement. The results suggest that with SE, being used as an in situ or in line characterization tool, the control of even complex MOCVD deposition looks feasible.

Spectroscopic ellipsometry (SE), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD) as non-destructive complementary techniques were used to characterize thin strontium bismuth tantalate (SBT) films. The optical properties were

Sample 4

Refractive index (λ=630 nm)

2 Sample 3

Sample 2 1.8

Sample 1

1.6 5.5

6

7

6.5

8

7.5

8.5

9

Atomic % of Bi

Fig. 3. RBS spectrum on Sample 1 with good mass separation.

Fig. 4. Correlation between the Bi-content of the SBT layers in atomic percents and the real part of the refractive index at the HeNe laser wavelength of 630 nm. The spot position was the same as for RBS and XRD. The Bi-content was obtained by RBS. The error bars for SE and RBS correspond to an error of about 73 and 75%, respectively.

Table 2 Element ratios and film thicknesses as total numbers of atoms/cm2 measured by RBS. The error of the determination of the element ratios is about 5% of the measured value. Sample

1 2 3 4

Element ratio (at%) of Sr, Ta, and Bi in the SBT layer

SBT thickness (  1017 at/cm2)

Sr

Ta

Bi

5.5 5.3 5.3 5.3

16.8 14.8 14.3 15.5

6.0 7.3 8.4 7.3

9 2.8 2.4 2.9

SiO2 layer thickness (  1017 at/cm2)

Pt layer thickness (  1017 at/cm2) Pt

H?

7 6 6.5 6.25

9 9 6.5 6.25

7 7 7 7

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from OTKA Grant No. D34594 and from the Bolyai grant.

References

Fig. 5. XRD spectra of the samples. The peak labeled with ‘Si’ belongs to the silicon substrate, peaks of the orthorhombic SBT phase are marked with their Miller-indices. Two unidentified peaks are denoted with ‘x’.

Acknowledgements The samples were prepared within the framework of the FECLAM project funded by the European Communities under Contract No. IST-2000-29352. Support from OTKA Grants No. T030441 and T033072 are greatly appreciated. P. Petrik is grateful for support

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