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etch the exposed Cr film, and is then removed with solvent leaving a patterned layer
of Cr on the substrate.
a)
b)
c)
E-beam
Figure 2.1: Conventional photomask process. (a) A latent image is irradiated by
electron beam exposure; (b) developing the resist yields physical resist image; (c)
the resist pattern is used as an etch mask to pattern the Cr, and the resist is removed.
Patterning the Cr layer, as in Figure 2.1c, may be done with liquid etchants
or in a reactive ion etch (RIE) process. Liquid phase etching is known to be very
isotropic, which results in Cr loss under the resist features and leads to a loss of Cr
feature dimension. Because of this, liquid-phase etching is not adequate for high-
resolution applications. RIE also has an isotropic component, 38,39 although it is
much less pronounced than in wet processing. It is not unusual, for example to see
etching of the Cr layer underneath the e-beam resist, which may result in a CD bias
as large as 100 nm in standard photomask production. 38
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