Critical Dimension (CD) metrology performed using scanning electron microscopy (SEM) techniques has the problem that the very act of measuring CD in resist, i.e., after development inspection ADI, results in a change in the measured CD. This is because electrons of the electron beam used to measure in SEM are absorbed and interact with the resist (outgassing reaction), resulting in shrinkage of the resist. In particular, the CD of line features measured using SEM techniques tend to decrease (narrow) as a result of the SEM measurement and, conversely, the CD of hole or trench features lend to increase (widen) as a result of the SEM measurement. As a result, there is a fundamental difference between the position where the aerial image crosses the substrate (and determines the CD) and the CD measured by the SEM. Furthermore, it is not only photosensitive materials which are affected by SEM metrology: other soft materials, for example porous SiO_2, may also be damaged by an electron beam resulting in a measurement induced shrinkage.
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