The Ohnishi parameter, the most popular model, states that etch rate is proportional to the effective carbon content in the polymer as a function of NT/(NC-NO), where NT is the total number of atoms, NC is the number of carbon atoms, and NO is the number of oxygen atoms. The Ohnishi parameter generally describes the basic trend of polymer etching, and most polymers follow the trend well. Later researchers did many modifications based on the Ohnishi model and tried to make the equation more accurate, for example, by combining the carbon and oxygen content with the ring parameter [6] and by introducing the number of fluorine atoms, NF, in the Ohnishi equation for the polymers containing fluorine [7, 8, 9].These modifications fit experimental data better mathematically, but they all lack an explanation of physical meaning.