Abstract: Plasma etching of thin metal and dielectric films

Abstract

Plasma etching as utilized by the microelectronics industry is discussed. Comparisons with wet etching processes show advantages of improved pattern definition and cost reductions. However, plasma etching differs in several important respects which warrant understanding for optimal use.The plasma is shown to be a source of reactant radicals, which upon interaction with the substrate produce volatile reaction products. Some radicals, e.g., CF3, have sufficient lifetimes to permit etching of substrates located outside of the glow discharge region. This offers major advantages in terms of high etching uniformity across wafers and long photoresist mask life. The quantity of radical reagent created under typical conditions (133 Pa, 250 cm3/min, 0.1 W/cm3) is shown to compare with the quantity of etched material, which leads to load dependence of the etching rate. It is also shown how narrow lines will etch more slowly than broad patterns and that this phenomena is related to pattern depth, photoresist thickness, and gas pressure. For films thinner than 0.5 μm, etching is very steep with almost vertical side walls. This also depends upon photoresist thickness and pressure.Specific etching processes are discussed. Important consideration must be given to relative etch rates for multilayered structures. For example, some important films listed in order of decreasing etch rate are polysilicon, monosilicon, Si3N4, Si2O2(CVD, chemical vapor deposition), SiO2(thermal), GaAsP, and Al. Thus etching polysilicon gates down to underlying SiO2, Si3N4 down to GaAsP, SiO2(CVD) glass down to Al bonding pads, and Si3N4 down to thermal SiO2, are all viable plasma etch processes.Chromium etches readily in Cl2–O2 plasmas, and an excellent process is in use for etching ultrafine patterns in chromium photoplates. Undercut is nil, and fractional micrometer lines are readily defined. Etching of gold is accomplished with CClF3 plasmas, placing the substrate directly in the glow region. Mo, W, Ti, and Ta all etch rapidly and cleanly. Processes for aluminum etching and preferential oxide etching are under development and should be available for general use within the next year.