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Inline and Non-destructive Analysis of Epitaxial Si1-x -yGexCy R. Loo, R. Delhougne, L. Geenen, B. Brijs and W. Vandervorst, IMEC P. Meunier-Beillard, Philips Research T. Koumoto, Sony Corporation Semiconductor Network Company

The implementation of silicon germanium (SiGe) and silicon germanium with substitutional carbon incorporation (SiGe:C) in BiCMOS and CMOS technologies requires very good control of epitaxial layer thickness and layer composition. In contrast with most of the characterization methods, spectroscopic ellipsometry (SE) allows a fast, inline and non-destructive analysis, including fast wafer mapping facilities. In this paper, the existing SE measurement routine for SiGe is extended to SiGe:C with substitutional carbon (C) incorporation. The optical dispersion is described by means of the harmonic oscillator model. The extraction of the C content is based on a well-defined shift of the resonant energy of the first oscillator. The SE system is a small spot (28x14 Âľm2) spectroscopic ellipsometer, which allows the characterization of epitaxial SiGe and SiGe:C layers grown on patterned wafers, while this small window size prevents measurements

by RBS. The SE technique is therefore a very powerful tool for optimization of the layer uniformities in both thickness and composition.

Introduction

The implementation of SiGe layers in active device structures is nowadays recognized as an efficient way to improve device characteristics by band offsets and/or increased carrier mobility. Currently, chip manufacturers focus mainly on the integration of SiGe in hetero bipolar transistors (HBT) in BiCMOS technology1-3 and on the fabrication of Si/SiGe hetero CMOS devices to improve performance of n- and p-type MOS devices4-9. In a next phase, attention will go to elevated SiGe source/drain contacts to reduce short channel effects in CMOS technology10, 11. Recently, a strong interest in SiGe alloys containing carbon arises. The strong suppression of boron (B) diffusion by substitutional carbon (C), leads to an important improvement in HBT device performance12. Other publications predict 40

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greater flexibility to control strain and band-offsets, which might be beneficial for MOS applications13, 14.

All these applications contain a SiGe/Si or SiGe:C/Si heterostructure in the active device. Industrial applications require very good control of the heterostructure in terms of epitaxial thickness and composition (Ge and C content) with layer uniformities in the range of 1-2 percent. The reproducibility from one epi layer to the other has to be in the same order of magnitude. In general, Rutherford backscattering spectroscopy (RBS), secondary ion mass spectroscopy (SIMS) or photoluminescence measurements (PL) are used to measure the layer thickness and composition. These techniques are very well developed but are unsuitable as production measurement tools. RBS and SIMS are destructive, while PL requires cryogenic conditions, which makes the technique quite time-consuming. On the other hand, SE allows a fast, inline and non-destructive analysis, including fast wafer mapping facilities. Indeed, SiGe has become a routine technique to study strained epitaxial

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SiGe layers15. Recently, we demonstrated the suitability of SE as an inline analysis technique for stoichiometry and thickness determination of epitaxial SiGe and SiGe/Si-cap layers with Ge contents ranging from a few atomic percent (1-2 percent) up to 36 percent. SE is a powerful tool for process optimization towards wafer uniformity in layer thickness and Ge content, and thus for epitaxial growth on both blanket and patterned wafers15-17. RBS cannot be used to analyze patterned wafers because of the small window size. Other groups used SE to study SiGe:C, with carbon contents up to 1.8 percent18, 19. The SE technique was used to study the influence of carbon on the optical transitions, while the carbon content was measured by means of SIMS. From an integration point of view, there is still a need to determine the carbon content fast, inline and non-destructively, including wafer-mapping.

In this paper, we discuss the suitability of SE to extract the carbon content of SiGe:C layers with a fixed Ge content and C contents between 0 and 1.7 percent, and its potential for process optimization. Growth conditions are chosen in such a way that C incorporates fully substitutional for C content up to 0.5 percent. We will first give a short description of the established analysis route to determine both Ge content and thickness of epitaxial SiGe layers with uniform Ge contents. Next, we describe the observed correlation between the carbon content and optical parameters as determined by SE. Experimental

Epitaxial SiGe and SiGe:C layers were deposited on 200 mm (001) silicon (Si) wafers (n type, 1-10 Ωcm), using a horizontal cold wall, load locked reduced pressure chemical vapor deposition system (RP-CVD, ASM Epsilon-2000 Reactor), which has been developed for production applications. Deposition conditions for epitaxial growth on blanket wafer surfaces include a pressure of 40 Torr using H2 as carrier gas. For SiGe layers we used either dichlorosilane (SiH2Cl2) or silane (SiH4) as Si source gases and germane (GeH4, 1 percent diluted in H2) as Ge source gas. Substitutional C incorporation is only possible until a maximum C content and requires epitaxial growth at low temperature12. All our SiGe:C layers were grown at 600ºC by using SiH4, as Si source gas and monomethyl-silane (CH3-SiH3, MMS) as C source gas. Before deposition, the wafers received an IMEC clean20, 21 followed by an in-situ bake at 1050ºC for 30-90 seconds in H2 in order to remove the native oxide. In case of epitaxial growth on patterned wafers, the native oxide was removed by an HF dip (30 seconds

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in 2 percent HF), followed by a DI-rinse, Marangoni dry22, and an in-situ bake at 850-900 ºC. In case of selective epitaxial growth (SEG), deposition on the mask material has been avoided by adding a well-defined amount of HCl to the SiH2Cl2/GeH4 gas mixture during growth, which was 10 or 20 Torr for SEG conditions.

SE measurements were made using a commercially available KLA-Tencor ASET-F5 (advanced spectroscopic ellipsometry technology) system which is a productionoriented, completely automated, small-spot (28x14 µm2) spectroscopic ellipsometer, with an automatic wafer mapping capability. In SE the change in polarization state of light upon reflection is measured. The measurement delivers two parameters: tan Ψ (the amplitude ratio) and cos ∆ (the phase difference of the parallel and the perpendicular component of the reflected light).

The polarizer rotates continuously and the analyzer is fixed in position for each measurement. Tan Ψ and cos ∆ as function of wavelength (range 250-750 nm) are derived from the integrated intensity reaching the detector in each 45˚ octant of the polarizer’s rotation. The SE technique consists of measuring the (tan Ψ, cos ∆) spectra for the sample and performing a mathematical regression analysis between experiment and theory, so that the difference between the calculated and measured (tan Ψ, cos ∆) is minimized. By following the work of C. Ygartua and M. Liaw23, we use the harmonic oscillator model with four different oscillator levels to calculate the complex refractive index dispersion of single SiGe layers. The model used for the film stack consists of a native oxide on SiGe (or SiGe:C) on a Si substrate. The layer properties of the native oxide and the Si substrate are fixed; standard values from the literature were used. The harmonic oscillator model (used for the SiGe or SiGe:C layers) is based on the solution for the dipole moment for a harmonically bound electron acted upon by an electric field24. The oscillator parameters (En(i) resonant energies, Eg(i) damping energies, and Nosc(i) concentration of oscillators), the thickness of the native oxide, and the thickness of the epi layer are treated as variables during the regression analysis. As a starting point for the regression analysis, we use for each sample the harmonic oscillator parameters for Si0.8Ge0.2 given in Ref. 23. In this study, no explicit attempt is made to model possible surface roughness. The regression analysis is reported along with a goodness of fit value (GOF value). The GOF value, a number between 0 and 1, can be considered as representative of the confidence level of the measurement. In this work a GOF value of 0.9 or higher has been obtained for all epi layers. After the regression Summer 2004

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analysis, film properties such as refraction index and absorption coefficient can be calculated from the obtained oscillator parameters. The thickness of the native oxide was found to be in the range of 10-20 Å, except for low Ge contents (≤ 6 percent) and thin epitaxial layers (≤ 45 nm), where we found lower values for the calculated oxide thickness15.

Results

First we describe our SE analysis routine as established for strained SiGe with Ge contents up to 36 percent.

This includes the obtained variation of the oscillator parameters and the refractive index as function from the Ge content. In the second part, we describe how we use this analysis routine to extract the C content of SiGe:C layers with a fixed and predefined Ge content.

SE measurements on SiGe The correlation between the Ge content of the epitaxial layer and the fitting parameters of the harmonic oscillator model and the complex refractive index (N(λ) = n(λ) + iκ(λ)) as calculated from these parameters was obtained from a series of epitaxial SiGe layers, with Ge contents between 1 percent and 36 percent. These epi layers were grown on blanket Si substrates (except for 19 narrow oxide stripes) and analyzed by RBS, SIMS, and SE. The Ge content was kept uniform within the epitaxial layer for each sample. The layer thickness was kept below the critical thickness for layer relaxation.

Layer relaxation and the corresponding change in surface morphology influences the SE measurements as discussed in Ref. 15 and 16. The layer thickness, which is one of 42

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the fitting parameters, was also measured by means of step height measurements, after a wet chemical removal of the oxide stripes. The thickness of the SiGe layers as calculated from SE spectra agrees within 5 percent with stepheight measurements, for all samples. The harmonic oscillator resonant energy, as calculated from the SE spectra, lies in the 3-4.5 eV range (Figure 1a). This is the same energy range where interband transitions between critical points in the energy band are expected25. We obtained a clear correlation between the Ge content of the epitaxial layer and the resonant energy En(1) of the first oscillator level (Figure 1a) and the damping energies of the first (Eg(1)) and the second (Eg(2)) oscillator levels (Figure 1b).

The real part of the refractive index dispersion (n(λ)), as calculated from the oscillator parameter, shows notable features (Figure 2), which are also related to the energies of interband transitions26. The overall dispersion is influenced by the presence of the Ge in the epitaxial layer. With increasing Ge content, the peak value of the real part of the refractive index (nmax) decreases and the peak position is shifted towards longer wavelengths. The obtained correlation between the Ge content and the fitting parameters of the harmonic oscillator model and the complex refractive index serves as the basis for the stoichiometry determination using the SE technique15, 16, 23.

The Ge dependency is clear for the resonant energy of the first oscillator level En(1), and we use this curve of Figure 1a to extract the Ge content of uniform SiGe layers with an unknown stoichiometry (Figure 3). En(1) is a direct result of the regression analysis.

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obviously allows a careful fine-tuning of the deposition process conditions, resulting in an excellent thickness and Ge content uniformity over 200 mm wafers (standard deviation: 0.5 percent and 1.7 percent for the blanket and patterned wafer, respectively).

SE measurements on SiGe:C Incorporation of C into SiGe leads to a complicate modification of the electronic band structure, which cannot be described by an empirical interpolation technique based on an averaged band structure18. The carbon incorporation has a much weaker effect on band structure compared to effects on stain. Still, Kissinger et al. observed a clear correlation between the carbon

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The SE technique is very powerful to measure and optimize layer uniformity on wafer scale. First, one SE spectrum is taken on a single position on the wafer surface and the regression analysis is done in order to define the material properties. Using these material properties, a full automatic wafer mapping is possible with the SiGe layer thickness, the oxide thickness, the resonant energy and the number of oscillators of the first and second oscillator level as fitting parameters. This allows an automatic calculation of the SiGe thickness and the Ge content on each position of the wafer surface.

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Figure 5a shows the FTIR spectra of the sample with a Ge content of 15 percent and a carbon content of 0.68 percent. The presence of substitutional carbon is reflected by the vibration mode around 606 cm-1 27. For C concentrations between 0 and 0.68 percent, we observed a linear dependency of the integrated absorption as function of the substitutional C concentration (Figure 5b).

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interstitial C and/or C precipitates. This indicates that C incorporation is fully substitutional. This is not anymore the case for Si0.78Ge0.21C0.017 (1.7 percent C). For this sample the C content is above the solubility limit and interstitial C is formed as well. This is reflected in the FTIR spectra by a broad absorption band around 700-815 cm-1 (Figure 6).

For C contents up to 0.5 percent, the resonant energy as extracted by SE increases linearly with increasing amount of C (Figure 7a). This corresponds to a reduction of the effective Ge content (xeff). This reduction of xeff is less than the strain compensation. For example, a resonant energy En(1) = 3.2342 eV has been calculated 1.2E-02

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content and the energy of interband transitions between critical points18. This was our motivation, to study a possible correlation between the carbon content and En(1). For this purpose, three series of SiGe layers have been prepared with a Ge content of 7 percent, 15 percent, and 21 percent respectively and various C contents ranging from 0 to 1.7 percent. Defect free and epitaxial material quality was observed for all C contents.

According to RBS and SIMS, C incorporation has no influence on Ge incorporation. The carbon content has been measured by means of SIMS. However, the influence of the different Ge concentrations on C sensitivity in SIMS was not taken into account, because only one C implanted Si0.78Ge0.22 standard was available. It is known, however, that the Ge concentration influences the C sensitivity. The samples have also been analyzed by means of Fourier transform infrared spectroscopy (FTIR) to study the carbon substitutionality. SE spectra have been fitted with the same mathematical regression procedure as used for SiGe. The extracted layer thickness has been compared with stepheight measurements.

These data agree within the accuracy limitations of the measurement techniques.

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SEEDS FOR TOMORROW’S WORLD IMECNOLOGY

IMEC is a world leading independent research center in nanoelectronics and nanotechnology. Its research focuses on the next-generation of chips and systems, and on the enabling technologies for ambient intelligence. www.imec.be

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for Si0.85Ge0.15C0.0034, which corresponds to xeff = 13.8 percent, compared to En(1) = 3.2239 eV for the carbonfree Si0.85Ge0.15 reference layer. This corresponds to a shift in effective Ge content of 1.2 percent. The effective lattice constant is estimated to Si0.884Ge0.116 (shift 3.4 percent Ge). Apparently, substitutional C incorporation has a lower effect on band structure than on lattice parameter, which has also been observed by Kissinger et al.18 Nevertheless, the correlation between the amount of C and the shift in En(1) (Figure 7) is obvious for all series with different Ge contents. This shift in En(1) seems to saturate for C concentrations above 0.5 percent (Figure 7b). This is probably due to the limited amount of substitutional C that can be incorporated in SiGe (Figure 6). But for substitutional C incorporation, SE is suitable to determine the C content of SiGe:C layers with a known Ge content. For example, a SiGe and a SiGe:C epi layer with the same growth conditions, except for the MMS gas flow which should be zero in case of SiGe, can be used to extract both Ge and C concentration as function of the growth conditions. Similar to SiGe, the SE technique is suitable to measure and fine-tune thickness uniformity of SiGe:C layers on wafer scale.

The correlation between the C content and En(1) seems to be a function of the Ge content (Figure 7). However, it has to be taken into account that all SIMS results are based on the same C implanted Si0.78Ge0.22 standard.

For the series with a Ge content of 7 percent and 15 percent, the calculated carbon concentrations are prob46

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ably overestimated. A correct interpretation of the SIMS data would reduce the differences in En(1) for the different Ge concentrations.

Conclusions

We extended the existing SE measurement routine for SiGe to SiGe:C with substitutional C incorporation.

This permits an extraction of the carbon concentration for SiGe:C layers with a pre-defined Ge content. The optical dispersion is described by means of the harmonic oscillator model. For epitaxial SiGe, the stoichiometry determination is based on the correlation between the Ge content and the resonant energy of the first harmonic oscillator level (En(1)). The addition of substitutional C leads to a well-defined shift of the resonant energy of En(1). This shift in En(1) is a linear function of the carbon concentration as long as the carbon incorporation is fully substitutional. Substitutional C incorporation is only possible until a maximum C content, which strongly depends on the growth conditions. Above this critical value, C incorporates on interstitials. This is reflected by a saturation of the shift in En(1). Therefore, SE measurements can be used to get a rough estimation of the C solubility limit as function of growth conditions.

The investigations were done using a production-oriented completely automated small spot (28x14 Âľm2) spectroscopic ellipsometer (KLA-Tencor ASET-F5). The system allows an inline and non-destructive wafer analysis, which is not the case for RBS and SIMS measurements.

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Furthermore, the small spot allows measurements on small areas. Therefore, epitaxial layers grown on patterned wafers can easily be analyzed. The small window size prevents measurements by RBS. The automatic wafer mapping capability of the ASET-F5 was used for a careful and very efficient fine-tuning of epitaxial growth processes, with regard to growth rate, Ge and C incorporation, and wafer uniformity. Excellent thickness and Ge content uniformities over 200 mm wafers have been reached, with measured standard deviations of the layer thickness down to 0.5 percent.

Acknowledgments

The technical assistance of Ivan Peytier and Erik Sleeckx is appreciated. The authors acknowledge the IWT and Medea+ for partially supporting this work.

This paper was previously published in the ECS Proceeding Vol. 2003-03, Page 329.

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