With laser direct exposure, a slight undercut of the structures can be generated: The upper part of the layer is more strongly irradiated due to the absorption of the resist and consequently more intensely crosslinked. Due to the absorption of the resist however, the lower part of the structure is exposed to a lesser extent and thus also less crosslinked. Since the developer attacks the lower layer also laterally, a small undercut is formed. A more pronounced differentiation of the crosslinking to obtain a stronger undercut is however not possible with laser light since too much energy is introduced. This fact is quite welcome for the fabrication of vertical edges. If a larger undercut is desired, this is only possible by conventional photolithography through masks at a minimum exposure dose and a wavelength of 300 – 380 nm (or UV broadband). Resist films more strongly absorbe the exposure energy in this wavelength range. Consequently, at low doses only a small amount of light “hits” the bottom and the above described process of undercut formation is enhanced (see Fig. 1 + 2).
Fig. 1 + 2: Undercut structures of AR-N 4450-10 at minimum exposure dose