Resist Memory
The DEL infrastructure is currently available on existing state of the art exposure tools. However, imaging below a k1 value of 0.25 with double exposure is impossible without the development of new materials. Conventional resists have a "memory" effect that prevents proper replication of the mask image. That is, sub-threshold exposure in the first exposure pass reduces the dose required to render the resist soluble in the second exposure pass. For example, the normalized aerial image intensities for the first exposure pass reaching the resist of equal lines and spaces can be described by the following
where A is a constant describing the amplitude and B is the minimum image intensity. For the second exposure pass, the mask and, consequently, the aerial image are translated by half pitch and lead to the following intensity function
The photochemical response of the resist results in a linear summation of the absorbed intensities from the two exposure pass. This leads to the following intensity function within the resist
= a Constant!
Consequently, the two individual mask images are not resolved when double exposed. This concept is illustrated in the following figure
The resist system converts the separate light images, intensity versus position, into chemical images, chemical composition versus position. Mathematically, this conversion of the light image into a chemical image can be represented by a translation function f(I). In the case of standard resist systems, this translation function has the linear addition property
Resolving the mask features requires a material with a nonlinear response in reciprocity to exposure dose such that
and the resist memory behavior is minimized.
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