1. Field of the Invention
The present invention relates to an apparatus which provides for the conditioning of the surface of a substrate in a plasma-less gas phase environment. It finds particular application in etching, cleaning, or bulk stripping removal of films or contaminants from the surface of a semiconductor wafer for use in the fabrication of integrated circuits.
2. Description of the Related Art
The traditional standard for surface contamination removal in the semiconductor industrv is the RCA clean which uses liquid aqueous chemicals. Highly selective bulk film stripping is also commonly carried out with liquid aqueous chemicals. The liquid aqueous chemical processes have problems involving safety, waste disposal, cleanliness and cost, and these problems make the use of gaseous processes very attractive.
The use of plasma containing reactive gas mixtures or reactive ion etching (RIE) are an alternative to liquid aqueous chemical processes. In these type of processes, the container is filled with a low pressure gas, the substrate is inserted into the container along with a reactive etchant gas. Voltage is applied to excite the gas, which chemically reacts with the surface. These types of processes have the disadvantage of causing additional damage and contamination of the substrate surface. It is also known to use the effluent of a gas plasma having at least one reactive specie, but being substantially free of electrically charged particles. This is known in the art as a downstream plasma source and is shown in U.S. Pat. No. 4,687,544 to Bersin, entitled "Method And Apparatus For Dry Processing of Substrates".
Plasma-less UV treatments have also been disclosed before. For example, U.S. Pat. No. 2,841,477 to Hall, entitled "Photochemically Activated Gaseous Etching Method" is the earliest known reference teaching a process of using a photochemically activated gas to etch semiconductor materials. This reference shows an etching method involving the steps of immersing the semiconductor material in a photolyzable gas and directing UV light toward the portion of the material to be etched. The gas is apparently static. The UV light causes the photolyzable gas to dissociate into various chemically active species which react with the substrate surface.
U.S. Pat. No. 3,122,463 to Ligenza, entitled "Etching Technique for Fabricating Semiconductor Or Ceramic Devices" is another example showing a method of using photochemically activated F.sub.2 O gas to etch semiconductor materials. This reference shows a method of immersing the semiconductor material in a static gas and directing UV light toward the portion of the material to be etched.
Processes of the type disclosed in Hall or Ligenza have the disadvantages of non-uniform gas distribution and of a static gas regime which does not allow the transport of contaminants and etching residues out of the reactor during the etch reaction.
It has been known to use a flow of gas across the surface of a substrate in an etching process. Such processes provide excellent process control and reduce accumulation of contamination and residue at the wafer surface. For example, U.S. Pat. No. 4,749,440 to Blackwood et al, entitled "Gaseous Process And Apparatus For Removing Films From Substrates" assigned to FSI Corporation and Texas Instruments Inc. shows a device which causes anhydrous reactive gas to flow over the substrate in the presence of water vapor, to chemically react with the surface.
U.S. Pat. No. 5,022,961 to Izumi et al, entitled "Method For Removing A film On A Silicon Layer Surface" shows a device, substantially identical to the device of U.S. Pat. No. 4,749,440, which is used to etch silicon oxide using HF and alcohol gases directed across the surface of a substrate wafer.
U.S. Pat. No. 5,228,206 to Grant et al, entitled "Cluster Tool Dry Cleaning System" shows a device which directs a flow of reactive gas across the surface of a substrate and asserts that UV radiation causes the gas to photochemically react with the substrate surface. In the device of this reference the substrate is rotated to obtain more uniform UV flux on the substrate surface.
Examples of plasma-less gaseous processes include, in addition to the UV activated processes of Hall and Ligenza, the non-UV processes disclosed in the Blackwood and Izumi references and the UV-activated processes disclosed in commonly owned U.S. patent application Ser. No. 08/292,359 filed Aug. 18, 1994 and in commonly owned U.S. patent application Ser. No. 08/259,542 filed Jun. 14, 1994.
Applicants have found that systems of the type directing a flow of gas which chemically reacts, with or without photochemical activation, with the substrate as it flows across the surface provides undesirable non-uniform etching, cleaning or bulk stripping. Typically, more etching takes place on the side of the substrate where the flow starts, and less reaction occurs as the gas flows across the surface due to depletion of reactant gas. Applicants have also found that a device employing rotation of the substrate, in combination with the flow of gas across the substrate can cause a vortex or eddy effect which also can produce undesirable non-uniform effects.
All of the methods or devices discussed above have the disadvantage of either non-uniform gas distribution or non-uniform UV illumination, and the further disadvantage that none of the prior processes remove gas which has reacted with the substrate in a manner which minimizes the risk of further contamination.
In JP 57-200569 (1982) there is disclosed an apparatus for treating a wafer with a UV activated gas, the gas being activated while in a first high pressure region. The gas is passed through a single slit to a lower pressure region where it contacts a wafer carried on a belt moving under the slit.
U.S. Pat. No. 4,540,466 entitled "Method Of Fabricating Semiconductor Device By Dry Process Utilizing Photochemical Reaction, and Apparatus Therefor" to Nishizawa and assigned to Semiconductor Research Foundation, shows a device with a higher pressure gas region and a lower pressure gas region, which causes the reactive gas to flow toward the substrate surface, as opposed to directing the flow across the surface as discussed above in connection with Izumi, Blackwood or Grant. The mean free path of the gaseous particles in the high pressure region is shorter than the openings between the two regions, i.e. a viscous flow regime. The pressure in the lower pressure region is set to provide a mean free path of the gaseous particles which is greater than the diameter of the chamber, i.e. a molecular flow regime. Nishizawa does not consider the hydrodynamics at the substrate surface, but the molecular flow regime in the lower pressure region precludes a radial laminar flow of gas across the substrate surface.