High-resolution RBS investigation of LaLuO3 as candidate for a second-generation high-k material


High-resolution RBS investigation of LaLuO3 as candidate for a second-generation high-k material

Kosmata, M.; Zier, M.; Munnik, F.

The ever-shrinking MOSFET (metal–oxide–semiconductor field-effect transistor) requires new materials that exhibit a higher dielectric constant compared to SiO2, for the gate dielectric [1]. Currently underway is the search for second-generation high-k materials [2] with higher permittivity, superior thermodynamic stability in contact with Si [3], matching band alignment with Si [4] and processing compatibility with poly-Si and metal gate electrodes. A suitable candidate material is lanthanum lutetium oxide (LaLuO3, LLO). During the so-called “gate first” manufacturing process, the gate oxide stack is subjected to thermal treatment. It is, therefore, important, to investigate the thermal stability of the deposited layer stack.

The samples under investigation are stacks of Si/LaLuO3/Si made by FZ Jülich, before and after annealing at 900°C. The samples have been studied with standard RBS (Rutherford Backscattering Spectrometry), high-resolution RBS and high-resolution TEM (Transmission Electron Microscopy). Standard RBS measurements have been performed under a scattering angle of 170° and two different angles of incidence to the surface normal (0° and 70°), where the first provides good separation of La and Lu in the spectrum and the latter higher depth resolution. These two measurements give the total amount of La, Lu and O and to a minor degree the depth profile of these elements. However, the results indicate some mixing of Si into the LLO, but the depth resolution was insufficient to obtain unambiguous results. High-resolution TEM could not provide a definitive answer either. Therefore, high-resolution RBS has been employed to investigate this question.

The high-resolution RBS set-up [5] consists of a Browne-Buechner type magnetic spectrometer and a position sensitive detector coupled to a 3 MV Tandetron accelerator. A 2.024 MeV C2+ ion beam has been used. The spectrometer is located at a forward scattering angle of 35° to maximise the depth resolution and scattering cross-section, achieving an energy resolution of < 0.1%. Due to the limited length of the position sensitive detector, only a narrow energy window of < 0.1 E0 can be analysed. Therefore, not all elements and also often not the whole width of one layer in the sample can be measured in one run and several measurements with a shifted energy window have to be performed. These individual spectra can be analysed simultaneously using WiNDF [6] or in a more intuitive approach, all partial spectra are combined in one complete spectrum. For this purpose, the counts were re-binned into new energy bins of 1 keV width, considering a different channel to energy calibration for each spectrum. For an internal self-consistency check the partial spectra overlap a certain amount and the final spectrum is obtained by averaging the overlapping data.

The resulting spectra for a sample before and after thermal treatment clearly show a redistribution of Si during annealing. This work will be continued with an investigation of the annealing temperature dependence and different annealing processes on the intermixing of Si.

References
[1] http://www.itrs.net/links/2009itrs/home2009.htm.
[2] M. Li et al. Adv. in Sci. and Techn. 45 (2006) 1342.
[3] E.P. Gusev et al. Microelectronic Engineering 59 (2001) 341.
[4] J. Robertson. Appl. Surf. Sci. 190 (2002) 2.
[5] R. Grötzschel et al. Nucl. Instr. Meth. B219 (2004) 344.
[6] N.P. Barradas, C. Jeynes, R.P. Webb, Appl. Phys. Lett. 71 (1997) 291.

Keywords: High resolution RBS; Rutherford Backscattering Spectrometry; high-k Material; Browne-Buchner

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