Alternating Layer Chemical Vapor Deposition (ALD) of Metal Silicates and Oxides for Gate Insulators
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ALTERNATING LAYER CHEMICAL VAPOR DEPOSITION (ALD) OF METAL SILICATES AND OXIDES FOR GATE INSULATORS Roy G. Gordon, Jill Becker, Dennis Hausmann and Seigi Suh Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138
ABSTRACT A new process was developed for deposition of the silicates and oxides of metals such as zirconium and hafnium at low substrate temperatures (100-300 oC). The silicon and oxygen source is tris(tert-butoxy)silanol, (tBuO)3SiOH, and the metal precursors are metal amides. A typical reaction is ZrL4 + 2 (tBuO)3SiOH
ZrSi2O6 + 4 HL + 6 H2C=C(CH3)2 + 2 H2O
in which the ligand L is ethylmethylamide, -NEtMe. The precursor vapors were alternately pulsed into a heated reactor, yielding about 0.3 to 0.7 nm of metal silicate film for each cycle. Replacing the silanol pulses with water pulses yields pure metal oxides with a thickness of about 0.1 to 0.15 nm per cycle. The silicon content of the films can be adjusted to any desired value by replacing some of the silanol pulses by water pulses. This new process has a number of advantages over previous methods for depositing metal silicates. Uniformity of thickness and stoichiometry are readily achieved. The deposition atmosphere is non-oxidizing, so that formation of low-k interfacial oxides between the deposited layer and a silicon substrate is minimized. The new halogen-free precursors avoid halogen contamination of films and corrosion of deposition systems. This process is a promising method for forming the next generation of ultra-thin high-k gate dielectrics in silicon-based microelectronics.
INTRODUCTION Alternating layer chemical vapor deposition (ALD) is a process for forming solid materials, such as coatings, from alternating pulses of reactant vapors. (A commonly used designation, atomic layer deposition, does not describe our new silicate process, in which one of the precursors adds a layer of molecular clusters, such as Si2O6, rather than a single atomic layer.) ALD processes can have many advantages: good step coverage, dense and pinhole-free films, excellent control of thickness and stoichiometry, and scalability to larger areas. ALD of oxides has often been carried out using metal chlorides as precursors.1 However, residual chloride contamination in the films may cause problems such as loss of adhesion, electrical instability or corrosion of metal gate electrodes. Metal alkyls and metal β-diketonates may leave carbon contamination in the films. We have found that ALD using metal alkylamide precursors can produce very pure metal silicates and metal oxides under mild conditions. A novel silicon source, tris(tert-butoxy)silanol, facilitates ALD of metal silicates at high growth rates.
K2.4.1
THEORETICAL DESIGN OF THE PRECURSORS AND REACTION SEQUENCE For a reaction to function effectively in ALD, it needs two precursors with specific chemical reactivity and physical properties. When the first reagent vapor is introduced into the reactor, it must chemisorb rapidly onto the surface of the substrate, but only up to
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