HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New MillenniumThe resist is dried and baked and the wafer is ready to begin thepatterning step.The pattern to be transferred to the wafer surface exists as a mask.The mask is a transparent support material coated with a thin layer ofopaque material. Certain portions of the opaque material are removed,leaving opaque material on the mask in the precise pattern required onthe silicon surface.The mask, with the desired pattern engraved upon it, is broughtface down into close proximity with the wafer surface. The dark areasof opaque material on the surface of the mask are the areas where it isdesired to leave silicon dioxide on the surface of the silicon. Openingsin the mask correspond to areas where it is desired to remove silicondioxide from the silicon surface.The mask is laid on the wafer and its back surface is flooded withionizing radiation, such as ultraviolet light or low-energy X-rays. Theradiation is blocked in areas where the mask has opaque material on itssurface. Where there is no opaque material on the mask surface, theionizing radiation passes through into the resist, the silicon dioxide, andsilicon.The ionizing radiation has little effect on the silicon dioxide andsilicon but it breaks down the molecular structure of the resist intosmaller molecules. This step makes it possible to dissolve exposed resistmaterial in solvents that will not dissolve the unexposed resist material.The resist is developed by immersing the silicon wafer in a solvent.The pattern originally existing as a set of opaque geometries on themask surface is transferred as a corresponding pattern into the resistmaterial on the surface of the silicon dioxide.The same pattern is transferred to the silicon dioxide by exposingthe wafer to a material that will etch silicon dioxide but will not attackthe organic resist material or the silicon wafer surface. The etching stepis usually done with hydrofluoric acid, which dissolves silicon dioxide,but is incapable of etching the surface of silicon.The final step in patterning is removal of the remaining organicresist material. The techniques used to remove resist materials involveacids, such as chromic acid and atomic oxygen, to oxidize the organicmaterials. The pattern then has been transferred into the silicon dioxideon the wafer surface. A similar sequence of steps is used to selectivelypattern each of the layers. The steps differ in the types ofetchants used.©2001 by The Fairmont Press, Inc. All rights reserved.