Bimetallic Precursors for Ferroelectric Thin Films; Molecular Control of Stoichiometry
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BIMETALLIC PRECURSORS FOR FERROELECTRIC THIN FILMS; MOLECULAR CONTROL OF'STOICHIOMETRY BRIAN A. VAARTSTRA Advanced Technology Materials Inc., 7 Commerce Dr., Danbury, CT 06810 ABSTRACT The application of ferroelectric oxides in microelectronic and optical devices is critically dependent upon methods to deposit high quality thin films. A major factor influencing the film quality is non-ideal metal stoichiometries caused by process variables. Materials such as PbTiO 3 and PbZrO 3 are well suited for stoichiometric control via molecular precursors having the required metal ratio, eliminating many process variables. Bimetallic precursors for the deposition of lead titanate and PZT thin films have been designed in order to optimize control over the metal stoichiometry. This is accomplished by the synthesis of volatile metalorganic compounds having a 1:1 ratio of Pb:Ti or Pb:Zr. Synthesis and characterization of these novel compounds are presented, and viability for metalorganic chemical vapor deposition (MOCVD) is discussed, based upon decomposition and volatility studies. INTRODUCTION The MOCVD of high dielectric PbTiO3 thin films at 5000 C has been reported using tetraethyllead and titanium(IV)isopropoxide as the Pb and Ti source reagents.1 The intra-molecular oxygen in the Ti source reagent provided sufficient oxygen for PbTiO 3 formation, as no additional oxygen gas was needed. The titanium ethoxide, 2 and butoxide3 have also been used as MOCVD source reagents. The toxicity 4of lead alkyls has also been addressed with the use of tetramethylheptanedionates as precursors. The highest priority issue for MOCVD of complex oxides is control of stoichiometry. The conventional approach for compositional control over a quaternary oxide is to introduce metalorganic precursors to the reactor via three independently controlled manifolds, each requiring accurate control of temperature, pressure, flow rates and precursor concentrations. Besides its complexity, this method makes film stoichiometry highly sensitive to inaccuracies in any of these process variables. A significant opportunity exists to reduce the sensitivity of film stoichiometry to process variations by the use of mixed-metal CVD precursors, for which correct film stoichiometry is intrinsic. A mixed-metal or heterometallic complex is one in which two or more metals are contained within the intimately bonded atoms of the molecule. This effort focused on the synthesis of new Pb/Ti and Pb/Zr heterobimetallic alkoxide precursors. Although lead titanate itself is a useful material, the ultimate goal was a viable two-component system for the more desirable lead zirconate titanate (PZT). Since Pb:(Zr+Ti) is optimally 1:1, this system is particularly amenable to heterobimetallic precursors which ensure that this ratio is constantly maintained during transport. Successful transport of two bimetallic precursors would leave only one variable, that is the Zr/Ti ratio. The structural and electronic relationship between metal alkoxides and metal oxides has been 6,7 pointed out as
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