(Invited) Influence of Target-Sample Distance on Residual Stress for Sputter-Deposited Ferroelectric AlScN
Authors: Tokita Yukimura , Hoshii Takuya , Tsutsui Kazuo , Wakabayashi Hitoshi , Kakushima Kuniyuki 
Published: December 2024

Ferroelectric Al1-x
Sc
x
N films have been extensively studied as a material for next-generation non-volatile ferroelectric memories. A high remnant polarization (P
r) of over 100 µC/cm2 is attractive even under 10 nm thin films [1,2]. As the role of Sc atoms in the AlN crystal to apply tensile stress, the elongation in the lattice constant eases the displacement of N atoms by an electric field. Besides, one of the issues of ferroelectric AlScN films is the increase in E
C while thinning the thickness. The source of the change is explained by the stress applied from the bottom electrode. We have made a Raman spectroscopy study to measure the stress in AlScN films with different thicknesses, and find that higher compressive stress is presented for thinner film, especially for higher Sc composition films [3]. However, the physical source of the stress is yet to be known. In this presentation, a Raman spectroscopy measurement was conducted on AlScN films with different target-sample (T/S) distance, which can strongly affect the stress in the film, as reported in TiN case [4].
Al0.7Sc0.3N films were deposited on (0001) sapphier substrates by reactive sputtering process. Film thickness were varied from 5 to 50 nm to confirm the thickness dependence stress in the films. The T/S distance was set to 100, 130, and 150 mm.
Differetial Raman spectra obtained by subtracting the signal of sappire substrate is shown in fig. 1. We can confirm two broad and weak peaks indicated with doted line around 580 (E2
H) and 850 /cm (A1LO). The two Raman peaks show a slight blue shift with longer T/S distance. The obtained Raman shifts on thickness are summarized in fig. 2. A blue shift is observed for both peaks while thinning the thickness. With longer T/S distance, the shift in E2
H was fully relaxed even with a thickness of 5 nm. On the other hand, only a limited relaxation was obtained with A1LO.
In conclusion, we have confirmed that longer T/S distance can relax the thickness-dependent compressive stress in AlScN films. The stress relaxation can contribute to stabilize the coercive field for thin ferroelectric AlScN films.

Acknowledgement

This study was supported by MEXT Initiative to Establish Next-generation Novel Integrated Circuits Centers (X-NICS) JPJ011438

Reference

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Al0.74Sc0.26N Films at 1 V” Adv. Sci. 10, 2302296 (2023)
[3] Y. Tokita, T. Hoshii, H. Wakabayashi, K. Tsutsui, K. Kuniyuki “Identification of cpmpressive strain in thin ferroelectric Al1-xScxN filns by Raman spectrascopy” Jpn. J. Appl. Phys. 63 04SP31 (2024)
[4] A. Neidhardt, U. Reinhold, E. schroeter, W. Wuttke ” Position dependence in planar magnetron sputtering of TiN films” Thin Solid Films, 173109 127 (1989)

Figure 1