Plasma-Assisted Atomic Layer Deposition of SrTiO₃: Stoichiometry and Crystallinity Studied by Spectroscopic Ellipsometry

Abstract

Strontium titanate (SrTiO3, STO) films were deposited by plasma-assisted ALD using cyclopentadienyl-based Sr- and Ti-precursors with O2 plasma as the oxidizing agent. Spectroscopic ellipsometry (SE) was employed to determine the thickness and the optical properties of the layers. As determined from Rutherford backscattering spectrometry (RBS), [Sr]/([Sr]+[Ti]) ratios ranging from 0.42 to 0.68 were achieved for 30–40 nm thick films by tuning the [SrO]/[TiO2] ALD cycle ratio. Films deposited at 250°C were amorphous and required post-deposition annealing to crystallize into the ultrahigh-k perovskite structure. The crystallinity of the films after rapid thermal annealing strongly depended on the film composition as observed by X-ray diffraction measurements. Using RBS data for a set of as-deposited samples, an optical constant library was built to determine the film stoichiometry from SE directly for the amorphous as-deposited films. After rapid thermal annealing the crystalline phase could be determined from the position of critical points of the measured dielectric function and the estimation of the stoichiometry was also possible for crystallized layers. These results open up a new way to use SE as a real-time characterization method to monitor and tune the STO film composition and crystallinity.

Crystallization Study by Transmission Electron Microscopy of SrTiO3Thin Films Prepared by Plasma-Assisted ALD

“…The elemental composition was extracted from the ellipsometry data by means of an optical constant library which was calibrated by means of Rutherford backscattering experiments. 21 The Table I. RTA in flowing N 2 was performed in an AST SHS100 system at temperatures ranging from 550…”

Atomic layer deposition of HfO2 using HfCp(NMe2)3 and O2 plasma

“…In their work, Niinist€ o et al showed a similar temperature dependence of the HfO 2 film crystallinity with a clear transition from amorphous to crystalline films at a deposition temperature of 300 C. The difference in temperature between this work and the one reported by Niinist€ o et al at which this transition could be observed is imputed to a $20%-25% difference between the set stage temperature value and the actual substrate temperature for the reactor employed in this work. 32 It can be concluded that the crystallization temperature of HfO 2 films is rather independent of the oxygen source used in combination with HfCp(NMe 2 ) 3 during the ALD process. However, the usage of a plasma-assisted ALD process may open the possibility for tuning of the crystalline phase by ion bombardment by employing substrate biasing during processing, 33 which is the object of further investigation.…”

The influence of non-stoichiometry on the switching kinetics of strontium-titanate ReRAM devices

“…Further details on the growth process can be found in recent publications. 23,27 For the use as bottom electrode, the Pt film is structured into 100 nm wide bars by means of nanoimprint lithography and reactive ion beam etching (RIBE). The resistive switching cell is completed by a 30 nm TiN film, deposited by sputtering, and capped with a Pt layer.…”