Undercut profile of ER in nanofabrication

Lift off using (single layer) negative resist is very challenging for two reasons.

1. The resist profile is tapered with wider opening due to electron forward scattering, which leads to film coating on the side wall and thus makes a clean lift off difficult.
2. Common negative resists such as polystyrene and SU-8 becomes cross-linked and insoluble upon exposure, and thus a strong chemical must be used to dissolve it, which may attack the metal or sublayer.

How about double layer? → PMMA + HSQ = good (H. Yang et al)

The undercut profiles are highly important in the case of lift-off of isolated features.

In principle, since the sensitivity $\left(\mu C/cm^{2}\right)$ for negative resists like polystyrene is inversely proportional to its number averaged molecular weight, one can achieve an undercut profile using a bilayer with the bottom layer and having a lower molecular wieght (thus less sensitive)

$$ MW \propto \cfrac{1}{sensitivity} $$

If we use bilayer, trilayer

→ The bottom layer is developed faster and it returns in the formation of undercut profile.

Undercut length depends on

1. resist thickness
2. pre-baking temperature
3. concentration and dissolution time of developer

LOR? → Dissolvable in alkali chemicals, and inert to most organic solvents, and solids. (when being baked around its glass transition temperature $189\degree C$