In Section 2.1 the importance of the depth of
DOF was theoretically discussed with the help of
(2.2). However, in practical applications the definition of the
DOF is more involved since a change in focus
not only alters the final resist profile but also increases the sensitivity
to other processing errors, whereby the dependence on exposure dose is
of primary concern. Resist profiles are often described
by three parameters related to a trapezoidal approximation of the
profile: the linewidth or critical dimension (CD), the
sidewall angle, and the final resist thickness.
Since the effect of focus depends on exposure, the only way to judge the
response of the process is to simultaneously vary both focus
and exposure. The focus-exposure matrix obtained this way can easily
be visualized by a Bossung plot .
Figure 3.2 shows two
examples using first linewidth and then exposure as the response.
The two other profile parameters of sidewall angle and resist loss can
be plotted in the same graph with the linewidth. The result is a process
window, i.e., the region of focus and exposure that keeps the final resist
profile within prescribed specifications. An example is given in
The process window describes the process capability that must match the process requirements to guarantee an acceptable yield. A simple graphical analysis of the process window is based on fitting all ``maximum'' rectangles or ellipses inside the shaded area representing systematic and random errors respectively. Plotting their height versus their width, i.e., exposure latitude versus depth of focus, provides the most concise representation of the coupled effects of focus and exposure on the lithography process (cf. Figure 3.3).