Invited Paper TF-MoM8
Combinatorial Atomic Layer Deposition of Nanolaminates

Monday, October 15, 2007, 10:20 am, Room 613/614

Nanolaminates of HfO₂ and SiO₂ were prepared using atomic layer deposition methods. Successive exposure of substrates maintained at 120 or 160°C to nitrogen flows containing Hf(NO₃)₄ and (^tBuO)₃SiOH led to typical bilayer spacings of 2.1 nm with the majority of this being SiO₂. Combining the Hf(NO₃)₄/(^tBuO)₃SiOH ALD with ALD cycles involving Hf(NO₃)₄ and H₂O allowed the systematic variation of the HfO₂ thickness within the nanolaminate structure. This provided an approach towards controlling the dielectric constant of the films. The dielectric constant was modeled by treating the nanolaminate as a stack of capacitors wired in series. The nanolaminate structure inhibited the crystallization of the HfO₂ in post-deposition annealing treatments. As the HfO₂ thickness decreased, the preference for the tetragonal HfO₂ phase increased. Nanolaminates of SiO₂ with compositionally graded mixture of HfO₂ and ZrO₂ were deposited using a combinatorial ALD process. Exposure of repeated cycles of co-dosed alkoxide precursors Hf(O^tBu)₄ and Zr(O^tBu)₄ with counter-reactant pulses of Si(O^tBu)₃(OH) formed films of uniform thickness (±5%) and uniform silicon oxide concentration. The hafnium and zirconium concentrations exhibited smooth graduation across the film from 18% - 82% (per Hf and Zr metals basis). Self-limiting deposition rates of 1.5 nm/cycle were measured, and a linear relationship of film thickness to number of deposition cycles was observed, both consistent with a true ALD process. Elemental analysis by Rutherford backscattering spectrometry, thickness measurements by ellipsometry, capacitance measurements, electron microscopy, X-ray reflectivity and X-ray diffraction results were used to map the composition and determine the film microstructure. Deposition of mixed films of SrO and HfO₂ were deposited by the related combinatorial chemical vapor deposition process using Sr(tmhd)₂[HN(CH₂CH₂NMe₂)₂], where tmhd = 2,2,6,6-¬tetramethylheptane-3,5-dionato, as the strontium precursor and Hf(O^tBu)₄ as the HfO₂ source. XRD showed that films with low Sr concentrations, e.g. < 15% Sr, exhibited a crystalline phase consistent with Sr-stabilized cubic hafnia. Films with higher Sr contents were amorphous. The dielectric constants of the films increased as the proportion of the cubic phase increased. A maximum value of 25 was obtained for the film with a Sr/(Sr + Hf) ratio of 0.07. We will report on our attempts to extend this CVD process to ALD.