Chemical Vapor Deposition of Electroceramic Thin Films
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MRS BULLETIN/JUNE 1996
In the case of volatile memories (DRAM, static RAM [SRAM]), decreasing lateral dimensions in ICs calls for nonconventional materials to be integrated in these ICs. For example, the available chip area for storage of a given amount of charge on a capacitor is ever decreasing. One approach to maintain the necessary amount of charge on these capacitors is to use materials having higher values for the dielectric constant than the commonly used SiO2 and Si3N4. Electroceramic materials that may be used for such applications are SrTiO3 and (Ba,Sr)TiO3. The dielectric constant of such materials typically is two orders of magnitude higher than that of conventional materials. Consequently the area of the storage capacitor can be proportionally decreased. For integrated applications of electroceramic materials, obviously it is a prerequisite to synthesize them in the form
of thin films. During the last decade, "classic" deposition techniques—for example, sputtering—have been greatly improved and new ones have been developed, such as pulsed laser deposition, spin coating (sol-gel, metalorganic decomposition [MOD]), and organometallic chemical vapor deposition (OMCVD). (OM)CVD is expected to have some distinct advantages over other deposition techniques when uniform deposition over large areas is desirable. Generally good step coverage can be obtained by this technique. Chemical vapor deposition is compatible with IC technology, and many equipment suppliers are active in the field. The deposition of a number of electroceramic thin films by CVD has now been reported. These include conductors (e.g., RUO2),1 ferroelectrics (e.g., PbZrjTiT-jO;), Bi4Ti3Oi2),2 high Tc superconductors (YBa2Cu3O7-js),3 dielectrics ((Ba,Sr)TiO3),4 and several types of buffer layers.5 A limitation of the CVD of electroceramic thin films has been, and still is, the availability of suitable precursors. A precursor should have a vapor pressure sufficiently high to enable transportation via the gas phase. Decomposition of the precursor should result in the liberation of compounds (single metals, combinations of metals, or oxides) that form the desired film without incorporation of elements such as carbon or halogens. For some elements, for example barium and strontium, potential precursors have a low vapor pressure (
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