PE-MOCVD of Dielectric Thin Films: Challenges and Opportunities
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netron, rf-diode, and electron-cyclotronresonance [ECR] plasma-assisted), excimer laser ablation, evaporation (thermal, e-beam, and flash), mist deposition, metalorganic decomposition (MOD), sol-gel, and chemical vapor deposition (thermal, plasma, and photo-enhanced). The need for conformality over device topography makes metalorganic chemical vapor deposition (MOCVD) an appealing technique. Additionally the emphasis on forming multicomponent thin-film oxides of Sr, Ba, Bi, and Pb below 500°C makes plasma-enhanced CVD, in particular electron-cyclotronresonance (ECR) CVD, an appropriate choice among the various CVD methods. Moreover, juxtaposing the emerging methods of intelligent processing with MOCVD will become a necessity in order for it to remain competitive among alternative technologies.
Plasma-Enhanced Metalorganic Chemical Vapor Deposition Metalorganic CVD of BaTiO3, SrTiO3, PbTiO 3 (Ba,Sr)TiO 3 , Pb(Sc,Ta)O 3 , (Pb,La)(Zr,Ti)O3, and SrBi2(Ta,Nb)2O9 thin films at temperatures of 600-700°C have been reported using various precursor systems. Although such films have acceptable electrical properties, development of ULSI circuits requires low deposition temperatures and conformal step coverage. Although the latter is achievable by thermal MOCVD, the former requirement must be met by nonthermal modes of enhancement—for example by plasma or photons. Some advantages of low-temperature plasmaenhanced MOCVD (PE-MOCVD) are as follows: (a) Interfaces or doping profiles and previously deposited layers are not perturbed, (b) Solid-state diffusion and defect formation are m i n i m i z e d .
(c) Compatibility with low-melting-point substrates/films is maintained, (d) Complex and metastable compounds may be synthesized. Although the chemistry and physics of a plasma glow discharge are complex, the plasma performs a couple of key functions: (1) Reactive chemical species generated by electron impact overcome kinetic (activation) barriers for a particular reaction. (2) Surface processes are stimulated by bombardment of activated gas atoms and molecules, energetic neutrals and ions, metastable species, and electrons. These effects are equivalent to raising the temperature of the reaction without raising the temperature of the substrate substantially. The combination of physical processes with chemical reactions results in material properties, and etching profiles and rates, unachievable by either process operating alone. The important characteristics in solidstate chemistry and processing parameters that control the formation of the ferroelectric perovskite PZT phase below 500°C were discussed previously on the basis of calculated thermodynamic phase stability diagrams.1 In the case of PZT, it was found that films existing initially as a pyrochlore phase could be converted completely to perovskite below 500°C for lead-rich compositions. The generation of reactive chemical species translated into an increase in the deposition rate with radio-frequency (RF) plasma power. For example the deposition rates for thermal and PE-CVD
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