However, if we consider one atom is shared by the bonds surrounding it, the carbon in C-C is shared by four bonds,which is considered as one-fourth of carbon; the carbon in C=C is shared by three bonds, which is considered as onethird of carbon, the oxygen in C-O is shared by two bonds which is considered as one half of oxygen. Also, assuming that the carbon contribution to etch is negative and the oxygen and hydrogen contribution to etch is positive, we can get the same parameter for C-C and C=C, C-O and C=O, as well as the absolute value of C-H and O-H. The calculation details are listed in Table 2.