1. Field of the Invention
This invention is generally related to vapor deposition reactors that are useful for chemical vapor deposition procedures. More specifically, the present invention concerns an improved reactor which is designed to eliminate snowing of gas phase growth reactants on a workpiece to eliminate the non-uniform deposition of films on substrates and to have balanced injection of reactive gases to form films of uniform thickness.
2. Description of the Prior Art
A thin film material can be deposited from a vapor. Such chemical vapor deposition processes include the forming of film, such as silicon, silicon dioxide, silicon nitride, and aluminum oxide on a substrate. As a typical example, dielectric films of silocon dioxide are widely used in the fabrication of planar microelectronics, particularly in MOS technology for selective masking, electric field isolation, and gate insulation. Several processes are available for growing silicon dioxide films on silicon. The thermally grown process produces excellent silicon dioxide films, but is only suitable for growing thin gate oxides.
Previous efforts at chemical vapor deposition processes involved the thermal decomposition of silane into free silicon for oxidation by free oxygen gas mixed with silane. This reaction can be carried out at much lower temperatures than the thermally grown process, but as presently practiced, the oxide films are often defective. The defects often result from inclusions of large particles in an otherwise fine grained film. Furthermore, such films also have variations in such properties as thickness, etch rate, and charge distribution properties.
In a chemical vapor deposition process for the deposition of silicon dioxide, the reactive gases typically are a mixture of 0.003 percent to 0.8 percent silane and 0.01 percent to 10.0 percent oxygen in a non-reactive carrier gas such as nitrogen, which makes up the balance of the processing gas. This gas mixture is introduced into a reactor containing a substrate upon which the silicon dioxide is to be deposited. The substrate is heated to cause the reaction. When the heated silane dissociates, silicon atoms, which are released from the hydrogen bonds, are in a very active state in which they seek to reestablish other bonds. Since there is free oxygen present, the silicon atoms combine with the oxygen atoms to form silicon dioxide molecules. Ideally, the newly formed silicon dioxide molecules come into contact with the surface of the substrate, and unsatisfied oxygen atom bonds attach to atoms already on the surface of the substrate to produce a stable structure. Deposited atoms form a base upon which other silicon dioxide molecules can deposit so that a fine grained structure is built up. However, if the newly formed silicon dioxide molecules come into contact with other silicon dioxide molecules in the gas before reaching the surface of the substrate, the molecules in the gas join up. As more are encountered, the molecules continue to attach and grow to form large particles. Such particles land on the surface like snow and form a coarse, porous and granular structure. The result is that 10 percent nonuniformity is presently considered acceptable and as being the best than can be accomplished. Such results are not appropriate for purposes which require uniform film thickness.
One of the causes of such growth in the gas phase into large particles is a premature gas phase reaction. If the silane gas stream has been sufficiently heated before reaching the vicinity of the substrate, dissociation occurs followed by formation of silicon dioxide molecules which attach to each other rather than deposit on the substrate because the silicon dioxide molecules are not directly adjacent the substrate. Accordingly, for the foregoing and other reasons related to the character of the reaction, it is desirable that a chemical vapor deposition reactor be created to permit uniform deposition so that improved characteristics result.