Resist stripping plasma

J Resist Stripping. After pattern transfer the resist has to be removed. For the all organic MLR systems, the resist layers can be removed with an O2 plasma if the top surface is not severely altered after an ion implantation or a substrate etching. Otherwise, a solvent treatment followed with clean-... 

In modern times, resist stripping for FEOL layers is typically accomplished with first the application of sulfuric acid and hydrogen peroxide mixture (SPM) solution, followed by the application of ammonium hydroxide and hydrogen peroxide mixture (APM) solution. This combination of solutions is able to strip off almost every kind of resist, except those that have been used in implant processes or have been hard baked or modified by plasma gases during dry etching... 

The main goal of this step is the complete removal of the resist without affecting the wafer surface. Two methods are used wet stripping (the use of either solvents, which dissolve the resists, or oxidisers that transform the resist into carbon dioxide and waste) and plasma resist stripping. 

As with many new processes, the goal of plasma etching was to find a more cost effective, environmentally sound, safer alternative to a chemically intensive procedure, in this case wet resist stripping. However, as with many new processes, new or subtle hazards may be introduced. Plasma-assisted etching was welcomed by the safety community since early plasma systems used nonhazardous gases such as oxygen and freons to perform operations that were traditionally performed in open tanks of corrosives and solvents. 

The growing trend to use plasmas created a need for resists which were compatible with dry etching but it also increased interest in their possible use in producing the primary resist image. Most process steps including ion implantation, deposition and resist stripping can already be carried out without the need for wet processing, and dry development is seen as a natural extension of this. 

The monomers used may present one or more polymerization sites. In the case of one site only, the cured material will be composed of linear chains and remain soluble in common solvents. This may lead to lower mechanical stiffness and lower plasma etching resistance, with the advantage of easy removal from the mold (resist contamination on the mold can be cleaned more easily) or wafer (resist stripping). In the case of more than one polymerization site, the material will consist of a 3D crosslinked network. 

After image transfer, the patterned resist must be readily and completely removable without substrate damage. The pattern often can be stripped from the substrate with a mild organic solvent. Proprietary stripper formulations or plasma oxidation treatments are utilized when the imaging chemistry or image transfer process has iasolubilized the pattern. 

Postdevelopment Treatment. Descum. A photoresist plasma descum step is typically but not always used after development. This step removes unwanted resist residues that were not cleared out during development and, in effect, increases the process latitude of the occasionally troublesome exposure-development sequence. Likewise, the descum step can smooth out minor irregularities of the resist side wall. This mild plasma treatment is done with 02 or 02-CF4 gases with low power and pressure settings, short process times, or both. The same plasma under more-vigorous conditions may be used later to strip the resist at the end of the masking operation. 

Finally, the stripping of residual resist is also done by a dry process. The advantages of utilizing plasma polymerization for resist coating are two fold... 

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