Atomic layer deposition of high-permittivity insulators from cyclopentadienyl-based precursors

Aile Tamm
Abstract & Cover

Well-controlled atomic layer deposition of high-quality thin films of ZrO2 and HfO2 can be realized using novel cyclopentadienyl-based precursors in normal laboratory conditions. Cyclopentadienyls of zirconium and hafnium, viz. (CpMe)2ZrMe. (CpMe) Zr(OME)Me, CpMe) Zr(NM22), (CpEt) Zr(NMe2), (CpMe) HfCOMeMe, and CpHf(NMe2)3 (Cp = CsHs, Me = CH3, and Et = C2H5) together with ozone as the oxygen source are thus appropriate precursors for ALD process, providing dense dielectric and insulating films on semiconductor or metallic substrates. Appreciably high conformality (step-coverage) on three-dimensional substrates can be achieved.
Both HfO, and Zro, films deposited at 300–350°C consisted predominantly of the monoclinic phase. Upon decreasing the film thickness down to 5–15 nm, the significance of metastable cubic or tetragonal phases increased, especially in Zro, films, which was promising in terms of higher dielectric permittivity and therefore increasing capacitance. The precursors containing two Cp-ligands and oxygen seemed to be more stable and yielded films with slightly higher phase homogeneity compared to precursors containing two Cp-ligands without methoxy-groups (OME). These films were also more insulating, although not necessarily superior in terms of electronic defect density. Leakage currents in HfO2 were lower than those in ZrO2, although the ZrO2 films could possess even lower interface trap densities than HfO.
Conduction mechanisms were quite alike in films grown from the different precursors, but the ultrathin films grown from (CpMe) Zr(OME) Me appeared more insulating possessing higher breakdown fields than those e.g. in the films grown from CpMe) ZrMe2. The dominant conduction mechanism was bulklimited field-assisted excitation of charge carriers, although at low voltages thermal excitation over interfacial barriers could also be taken into account.
Since the ability of metastable cubic/tetragonal phases of HfO2 and/or ZrO2 to withstand post-deposition annealing procedures without transformation to lower-permittivity monoclinic phase was a likely issue when fabricating highdensity capacitors, the films were further doped or mixed with rare earth metal oxides. In the case of HfO2, quite a recently developed monocyclopentadienylbased precursor CpHf(NMe2)3 was chosen and used in the experiments devoted to the studies on Yoz-doped HfO). In the case of Zro, a well-behaving compound (CpMe) Zr(OMe Me was chosen for the preparation of Zro, films doped or nanolaminated with Gd2O3 and Er2O3.
Doped HfO2:Y films were amorphous in as-deposited state but crystallized in the form of metastable polymorphs after heat-treatments above 500°C, possessing higher capacitance and lower equivalent oxide thickness compared to those of non-doped HfO2. The leakage currents remained significant, probably due to somewhat inhomogeneous crystallization. ZrO2:Gd and ZrO2:Er films were crystalline already in as-deposited state, and the cubic polymorph of ZrO, was retained upon annealing at 650°C in the doped films, whereas lower
permittivity monoclinic ZrO2 became apparent in the nondoped films already in the as-deposited state. The dielectric permittivity value of 31 was achieved in the ZrO2:Er2O3 films with an Er:Zr cation ratio of 0.09 and 30 in the ZrO2:Gd2O3 films with a Gd:Zr cation ratio of 0.027, whereas in non-doped films permittivity values above 20-25 could not be measured. Concerning the bottom electrode materials, the best results in terms of permittivity and leakage currents were achieved with Ru, allowing equivalent oxide thickness below 1 nm and a current density of 3x10 A/cmat 1 V. In general, at electric fields below 2-3 MV/cm, normal and trap-compensated Poole-Frenkel conduction mechanisms were competing, whereas at higher fields, Fowler-Nordheim and/or trap-assisted tunneling were to be considered. In Er2O3-ZrO2 and Gd2O3ZIO, nanolaminates the cubic ZrO, and rare earth oxide phases dominated, but the capacitance increased after annealing in the films with relatively low rare earth metal content and decreased in the case of higher rare earth content, being indicative of the sensitivity of dielectric behavior on the contribution of ZrO2 phases.
The processes and resulting films examined within the present study may well become considered as those relevant to the fabrication of high-performance capacitor dielectric materials. Further studies might become concentrated on the optimization of the dopant content and deposition (cycle) time parameters, in order to improve the structural stability, capacitance density, pre-breakdown leakage currents and conformal growth over 3D substrates. In addition, the search for even better cyclopentadienyl-based precursors for both host and dopant materials may continue.


Source of Information
Väino Sammelselg
University of Tartu
(Tartu, Estonia)
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