Overcut profile

Another use of multilayer resists is in profile control. The PCM-type multilayer system is largely used for high-resolution applications with good linewidth control over complex geometries, but a further use of the multilayer system is in profile control. In this case isotropic etches are used which produce undercut or overcut profiles through suitable differential etch rates between layers. A common variant of this is two resists which are developed in mutually exclusive developers the control imparted by the multilayer system has found particular application for float-off metal deposition (Fig. 7.21). 

Figure 3. Linewidth variation caused by overcut resist profiles.

Figure 3.3 Photoresist profiles. (A) Positive resist (a) desired resist profile for lift-off i.e. exposure-controlled profile, also called overcut (b) perfect image transfer by applying a normal exposure dose and relying moderately on the developer (c) receding photoresist structure with thinning of the resist layer i.e. developer control, also called undercut. (B) Negative resist the profile is mainly determined by the exposure. Development swells the resist a little but has otherwise no influence on the wall profile.

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