Further shrinking of structures in DRAM cells requires the introduction of new materials with superior electrical properties. Especially, in future capacitors there is a need, to replace the traditional dielectrics by insulators having a significant higher dielectric constant κ. Among others, Si-stabilized HfO₂ is a promising candidate for 50 nm technology and beyond. The structure, in which HfO₂ crystallizes, plays an essential role, as the κ value of the cubic / tetragonal phase (CaF₂ structure) is significantly higher than that of the monoclinic phase, which is at room temperature the thermodynamically stable phase [1]. Using dopants (e.g. Si, Al or Y) the desired cubic / tetragonal phase can be stabilized at room temperature [2].

In this study, the influence of Si doping on the phase formation of Hf-Si-O_x dielectrics was investigated by means of high temperature grazing incidence XRD. Thin films of approx. 9nm thickness were deposited by ALD on a Si wafer with a 15nm TiN electrode film. A concentration split was deposited to adjust the Si/(Hf+Si) ratio within the range between 1-10%, what was checked with RBS and XPS. High temperature XRD measurements were done in a BTS-solid temperature chamber (mri) equipped with a Be dome under N₂ up to 700°C. In the temperature range between 150°C and 700°C the samples were heated with a constant ramp rate of 5 K/min. During the heating, grazing incidence XRD pattern (CuKα radiation, incidence angle ω = 0.65°) in the 2Θrange 26° - 38° were taken. Each scan lasted 5 minutes resulting in a temperature resolution of 25°C. Due to certain reasons (weak intensity, low particle size, stress in the film) it is not possible to decide if the (002) reflection is split, what would allow, to discriminate between cubic or tetragonal symmetry.

All samples were amorphous after deposition. Depending on the Si concentration, the HfO₂ crystallizes either in the cubic / tetragonal phase or in the monoclinic phase. The onset of the crystallization process is in the temperature range between 540°C for the sample with high Si content and 360°C for the samples with low Si content.

References:

[1] X. Zhao, d. Vanderbilt, Phys. Rev. B65 (2002) 233106.

[2] K. Tomida, K. Kita, A. Toriumi, Appl. Phys. Lett. 89 (2006) 142902.

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1. Thermal Stability of thin ALD ZrO₂ Layers as Dielectrics in Deep Trench DRAM Devices Annealed in N₂ and NH₃