Paper EM-TuA11
Processing Controlled Substrate Reactions for Deposition of Monoclinic Textured HfO₂ Thin Films on Pre-Oxidized and Nitrided Ge (001) Substrates

Tuesday, November 10, 2009, 5:20 pm, Room B1

2 nm thick films of HfO₂ have been deposited on Ge(001) substrates by remote plasma chemical vapor deposition for (i) multiple etching cycles in dilute HF followed by distilled water rinsing, and (ii) an in situ remote plasma-assisted nitridation (RPAN) process. In a second set of studies, the step(i) pre-clean was replaced by a basic re-clean using methanol and NH₄OH. Studies by X-ray absorption spectroscopy (XAS) in O and N K edge regimes, were used to monitor HfO₂ nano-grains morphologies. Previous studies indicated epitaxial textured HfO₂ and TiO₂ films were obtained on Ge(001) surfaces. However, these studies were based on a recipe that worked, and did provide insights to significant connections between pre-cleaning and post deposition annealing. We have found two aspects of processing are crucial, and these were evaluated spectroscopically. Electrically active defects in metal-oxide-semiconductor test devices indicated high-defect densities correlated directly with Ge and O reacting with HfO₂ in the interfacial transition region, resulting in mixed-morphology grains.

Interfacial and bulk film degradation are also detected in XAS O K edge measurements. The occurrence of a monoclinic (m-)HfO₂ E_g edge structure is associated with Ge-O free interfacial transition regions. Ge-O interfaces results in tetragonal (t-)HfO₂, or mixtures of t-HfO₂ and m-HfO₂ grains. Acidic and basic pre-cleans each followed by an RPAN process prevented reactions between Ge-O surface bonding and plasma-excited HfO₂ precursors, consistent with textured m-HfO₂ films. Spectroscopic ellipsometry indicated that Ge-O bonding was significantly higher for acidic pre-cleans compared with basic pre-cleans. Post deposition annealing cycles with textured m-HfO₂ films were consistent with this difference. Two step annealing after the acidic clean, the first at 550°C in Ar, and the second at 800°C in Ar, resulted in textured m-HfO₂ directly in contact with a reconstructed Ge(001) surface. N K edge XAS, had previously indicated complete release of N after annealing to 700°C in Ar. In contrast, with less Ge-O interfacial bonding in the basic pre-clean, Ge-O and Ge-N interfacial bonds were eliminated sequentially during an 800°C anneal in Ar. O K edge XAS for HfO₂ with the X-ray polarization in the direction of the dimer rows of Ge(001) wafers, or perpendicular to that direction indicated similar textured growth. This is consistent with the textured m-HfO₂ films having nano-grains aligned at ±45^o relative the dimer row direction.