1. Field of the Invention
The present invention relates to the use of selected molybdenum, tungsten, and chromium amides as sources for the chemical vapor deposition of metal-containing films.
2. Brief Description of the Relevant Art
Chemical Vapor Deposition (CVD) is a well known method for depositing a thin film onto a substrate. As electronic device dimensions continue to shrink, CVD techniques for forming conductive thin films have increased in importance in comparison to physical deposition methods. This trend is based on the superior ability of CVD methods to conformally coat severe topography. In particular, the chemical vapor deposition of metals in Group 6 of the Periodic Table of the Elements [especially tungsten (W) and molybdenum (Mo)] has emerged as the leading method for the construction of conductive films necessary to wire microelectronic circuits in high density devices.
One of the oldest CVD methods of depositing these metals is the so-called hexafluoride system. By this method the gaseous hexafluoride of the metal to be deposited is introduced into an enclosed reaction chamber containing a heated substrate. If a free metal is desired to be deposited, a reducing gas such as hydrogen is simultaneously introduced into the chamber. If a metal carbide is desired to be deposited, a carbon source gas is simultaneously added to the chamber. The gas flows are generally effected by means of a vacuum in the reaction chamber. There are various types of physical methods for effecting the metal deposit in the reaction chamber.
In the case of tungsten, the principal chemical reaction in the CVD reaction chamber where tungsten hexafluoride (WF.sub.6) is the source gas and hydrogen is the reducing gas is shown by the following chemical equation: EQU WF.sub.6 +3H.sub.2 .fwdarw.W+6HF
The by-product hydrogen fluoride (HF) is a gas at the usual CVD temperature range at which the tungsten (W) deposit is effected; and consequently the HF is removed from the reaction chamber with the unreacted source and reducing gases under vacuum.
U.S. Pat. No. 3,565,676, which issued to Holzl on Feb. 23, 1971, describes an improvement to the above-noted standard hexafluoride CVD method. He found that minor amounts of undesirable fluoride were entrapped in the tungsten, molybdenum and rhenium.
This unwanted fluoride was believed by Holzl to be caused by side reaction which formed WF.sub.4 and impaired the strength of the metal film and allowed the formation of voids in film during later high temperature processing. Holzl found that introducing a controlled amount of oxygen into the CVD hexafluoride system controlled the quantity of residual fluoride in the metal deposited.
U.S. Pat. No. 4,150,905, which issued to Kaplan et al. on Apr. 24, 1979, teaches the chemical vapor deposition of tungsten carbide and other carbides onto a metal spherical core substrate. The resulting coated spheres were used in highest quality ball point pens. This CVD process, in the case of making tungsten carbide (WC) films, employed a fluidizing gas such as a mixture of hydrogen gas (H.sub.2) and one or more inert gases, for example, argon, helium and nitrogen. The tungsten source gas was tungsten hexafluoride and the carbon source gas was either methane (CH.sub.4), butane (C.sub.4 H.sub.10), acetylene (C.sub.2 H.sub.2) or the like. The reaction chamber was heated to a temperature of about 500.degree. C.-900.degree. C. and held under vacuum. The chemical equation for this reaction using CH.sub.4 as the carbon source is as follows: EQU CH.sub.4 +H.sub.2 +WF.sub.6 .fwdarw.WC+6HF
The HF by-product gas was removed by the vacuum.
U.S. Pat. No. 4,162,345, which issued to Holzl on July 24, 1979, teaches that CVD techniques involving tungsten carbide resulted in relatively large grain size deposits which tended to be relatively brittle and mechanically weak. Moreover, the patentee urged that prior art CVD processes required the use of relatively high substrate temperatures and had relatively slow deposition rates. Accordingly, this patent proposes a two-stage CVD reaction where a gaseous halide of tungsten (e.g. WF.sub.6) or molybdenum was reacted with an alcohol, ketone or ether to form a tungsten or molybdenum intermediate at a space away from the CVD substrate and then reacting that intermediate product with hydrogen gas and one or more gases containing oxygen and carbon. The final product, tungsten carbide or molybdenum carbide, is deposited onto the CVD substrate while the spent gases are removed.
U.S. Pat. No. 4,349,408, which issued to Tarng et al. on Sept. 14, 1982, teaches the chemical vapor deposition of a refractory metal such as tungsten or molybdenum onto a silicon substrate. This CVD process involves the reaction of the tungsten or molybdenum source gas (e.g. hexafluoride) with the silicon substrate itself. This is illustrated by the following reaction equation: EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4
The patent teaches that the reaction occurs in the range of about 600.degree. C. to about 800.degree. C. The patent further teaches that hydrogen gas may be additionally added later in the reaction to make a thicker layer of the tungsten or molybdenum metal.
U.S. Pat. No. 4,584,207, which issued to Wilson on Apr. 22, 1986, teaches a CVD process where tungsten metal is deposited onto a surface of previously deposited polycrystalline silicon surface and reacts with the polycrystalline silicon. The tungsten source gas for this CVD reaction is tungsten hexafluoride and hydrogen gas is additionally introduced. The reaction temperatures are about 225.degree.-325.degree. C.
U.S. Pat. No. 4,659,591, which issued to Gartner et al on Apr. 21, 1987, describes a CVD process for depositing tungsten metal using together tungsten hexafluoride as the tungsten source gas along with hydrogen gas, an inert gas like argon and a rare earth metal acetylacetonate hydrate.
U.S. Pat. No. 4,696,833, which issued to Moning et al on Sept. 29, 1987, teaches a hot wall CVD process wherein tungsten hexafluoride gas and hydrogen gas are the reactant gases and introduced into the reaction chamber in a specific manner.
U.S. Pat. No. 4,804,560, which issued to Shioya et al on Feb, 14, 1989, describes a CVD process of depositing tungsten metal in repeated stages using tungsten hexafluoride and hydrogen gas as the reactant gases.
U.S. Pat. No. 4,853,347, which issued to Bukhman et al on Aug. 1, 1989, describes a CVD process for depositing tungsten onto a wafer surface in a RIE reactor employing tungsten hexafluoride as the tungsten source material and subjecting it to a hydrogen plasma.
U.S. Pat. No. 4,874,642, which issued to Gary et al, teaches a CVD method of depositing a mixture of tungsten metal and tungsten carbide by employing a mixture of process gases comprised essentially of (1) tungsten hexafluoride, (2) a volatile oxygen and hydrogen-containing organic compound and (3) hydrogen.
U.S. Pat. No. 4,902,645, which issued to Ohba on Feb. 20, 1990, teaches CVD process using tungsten hexafluoride gas and a silicon hydrate in specified ratios to deposit a silicon-containing metal layer onto the CVD substrate surface.
U.S. Pat. No. 4,923,715, which issued to Matsuda et al on May 8, 1990, describes a method of preventing the deposition of tungsten metal films on the inner walls of the CVD reactor by use of a metal nitride film thereon.
Tungsten CVD is still practiced almost exclusively using tungsten hexafluoride (WF.sub.6) as the tungsten source. CVD processes using WF.sub.6 suffer from the following complications:
1. WF.sub.6 deposits films with poor adhesion to silicon dielectrics owing to formation of surface Si-F groups at the layer interface. PA0 2. Reduction of WF.sub.6 by silicon is very favorable thermodynamically, resulting in likely undesirable silicon etching. PA0 3. In the presence of hydrogen, HF is generated from WF.sub.6 which can cause wormhole defects that adversely affect long term device reliability. PA0 4. The above-stated by-products (HF and/or SiF.sub.4) are toxic and corrosive. PA0 (a) introducing a subvalent refractory metal amide as a reactant gas capable of forming said metal-containing film into a CVD reaction zone containing said substrate on which said metal-containing film is to be formed; said subvalent metal amide having formula (I): EQU [M[NRR'].sub.x ].sub.y (I) PA0 (b) maintaining the temperature of said zone and said substrate at about 200.degree. C. to about 1,000.degree. C.; PA0 (c) maintaining the pressure in said zone at about 0.001 to about 100 torr; and PA0 (d) passing said reacting gas or gases by said heated substrate for a period of time sufficient to form said metal-containing film thereon.
It should be noted that CVD process using related molybdenum and chromium halides are little used because such halide sources are less volatile than WF.sub.6.
Therefore, there is a need in the CVD art for volatile refractory metal CVD sources which do not generate toxic and/or corrosive by-products upon deposition. The present invention is believed to be a solution to that need.
Separate from the above art relating to CVD reaction, there is extensive art for making and using tungsten, molybdenum and chromium amides. For example Patent Cooperation Treaty Patent Publication No. 88/01603, filed by SRI International and published on Mar. 10, 1988, describes a process for making tungsten, molybdenum and chromium carbides by pyrolyzing the corresponding metal amides of the formula M.sub.x (NR.sub.1 R.sub.2).sub.y where M is the metal atom; R.sub.1 and R.sub.2 either hydrogen, lower alkyl, trimethylsilyl or ethylene, with the proviso that both R.sub.1 and R.sub.2 are not hydrogen, x is 2 or greater and y is an integer equal to the valence of the M.sub.x unit.
The preparation of the compounds Mo[N(CH.sub.3).sub.2 ].sub.4 and Mo[N(C.sub.2 H.sub.5).sub.2 ].sub.4 is disclosed in Bradley et al "Covalent Compounds of Quadrivalent Transition Metals Part V. Molybdenum (IV) Dialkylamides" Journal of the Chemical Society, (A) 1971, pages 2741-2744.
The preparation of the compound [W[N(CH.sub.3).sub.2 ].sub.3 ].sub.2 is disclosed in Chisholm et al, J. Am. Chem. Soc., 98, pages 4477-4485 (1976); Chisholm et al Inorg. Chem., 26, pages 3182-3186 (1987); and Chisholm et al, Inorg. Synthesis, 21, pages 51-56 (1982).
The preparation of [Mo[N(CH.sub.3).sub.2 ].sub.3 ].sub.2 is disclosed in Chisholm et al, J. Am. Chem. Soc, 98, pages 4469-4476 (1976) and in Chisholm et al Inorg. Synthesis, 21, pages 52-56 (1982). The preparation of [Mo[N(CH.sub.3).sub.2 ].sub.3 ].sub.2 is disclosed by Chisholm et al, J. Am. Chem Soc., 98, pages 4469-4476 (1976).