The current method of semiconductor device fabrication involves the use of a light sensitive polymer known as photoresist. A layer of the photoresist material is spun onto a semiconductor substrate surface at a thickness of about 1 micron (10,000.ANG.) and a patterned exposure performed through a previously prepared photomask. The wafer is then developed as in normal photography. After developing, the unexposed areas of photoresist are washed from the surface leaving selected areas of the wafer exposed or covered, depending on the intent of the fabricator. The wafer is now ready for a process step, such as the bombardment of the surface with boron ions to introduce a doping atom into the silicon wafer to change electrical properties. After this process step is completed, the developed areas of photoresist must be removed to allow the sequence to be repeated with a different pattern.
The current semiconductor industry standard stripping and cleaning method involves the use of at least three liquid chemical solutions known as "SPM", "SC-1" and "SC-2", in use since 1970. "SPM" stands for "sulfuric acid peroxide mix," and "SC" stands for "standard clean". SPM is a solution of concentrated sulfuric acid and 30% hydrogen peroxide, and is used to remove heavy organics, such as photoresist. SC-1 is a solution of 29 wt/wt % ammonium hydroxide, 30% hydrogen peroxide and deionized water. It is used at approximately 70.degree. C. to 80.degree. C. to oxidize surface organic films and remove some metal ions. SC-2 is a final rinse solution of 37 wt/wt % hydrochloric acid and 30% hydrogen peroxide and deionized water. It is used at approximately 75.degree. C. to 80.degree. C. These solutions were first developed at the RCA corporation during the 1960's and are sometimes known as "RCA cleans". This approach may be accomplished at temperatures less than 100.degree. C., which is an important consideration as uncontrolled dopant diffusion in the wafer itself will occur if temperatures of approximately 150.degree. C. to 200.degree. C. are reached for any extended period of time.
The liquid process is deficient in that safety, environmental considerations of disposal and water availability are major drawbacks. In addition, a more important limitation comes from the inherent surface tension of these materials. The liquids have difficulty in entering features smaller than approximately 0.3 micrometers. Finally, the liquid process is relatively time consuming because there is a drying step required to remove the liquid cleaning agents. This results in low throughput. As device and feature size continue to decrease in size, new methods of stripping and cleaning must be found.
The most straightforward approach to dealing with the problems associated with liquid cleaning is to develop gas phase methods. Gases are easier to dispose of by scrubbing, have less volume, do not require a drying step and do not have the same surface tension drawbacks. This approach is known as "dry cleaning".
Initially, gas phase methods similar to those used to remove contaminants such as dust were applied. These techniques use applied thermal or UV energy for contaminant excitation in an air, oxygen or inert gas atmosphere. (See U.S. Pat. Nos. 5,024,968; 5,099,557 to Engelsberg). Unfortunately, these systems lack sufficient energy to remove photoresist or very heavy contamination.
This problem was partially solved by the use of various excimer laser photoresist stripping processes such as those disclosed in U.S. Pat. No. 5,114,834 to Nachshon, and in WO9507152 to Elliott et al. Nachshon teaches the application of a laser at an angle perpendicular to the semiconductor surface to remove photoresist via ablation. A reactive gas, such as oxygen or ozone may be provided to react with the ablated material. Elliott teaches the application of a laser at an angle which is preferably 15.degree. to the semiconductor surface to remove feature edges of photoresist via ablation. It is recommended that two applications should be used, wherein the disc is rotated 90.degree. between the first and second. As with Nachshon, a reactive gas such as oxygen or ozone may be provided to react with the ablated material.
These processes are deficient however, in that they leave a residue of carbon on the wafer surface which is on the order of 100.ANG. to 200.ANG. in depth (for a 1; micron photoresist layer). Srinivasan et al (J. Appl. Phys. 61(1) January, 1987) teach that the source of the carbon does not appear to be from redeposition of partially combusted ablated carbon, but from a type of ashing process produced from the instantaneous high (&gt;1000 K) temperatures and pressures (&gt;100 atm) of the laser itself. This residue must be reduced to a thickness of less than about 4 .ANG. before the next fabrication step can be performed. Attempts to remove the carbon residue with a UV light and ozone treatment have not been successful in part because the UV laser light is absorbed by the ozone. In addition, metallic contaminant residues, such as Al or Fe may also result from the semiconductor fabrication processes. Activated chlorine gas has been used to remove these residues, but this results in damage to the wafer.