The optical absorption or, respectively, spectral sensitivity of DNQ-based photoresists ranges from the blue to near-UV part of the spectrum and is matched to the emission lines (i-line = 365 nm, h-line = 405 nm, g-line = 435 nm) of Hg light sources used in typical mask aligners. This is the origin of the reddish-brownish appearance of (unexposed) resist films. Modern resists such as the AZ 5214E or AZ 9260 are not sensitive near g-line (435 nm), and the state-of-the-art AZ nLOF 2000 negative resist series is only i-line sensitive.
During exposure, DNQ-based photoresists bleach (become tranaparent) down to approx. 300 nm wavelength.
The spectral sensitivity of photoresists does not abruptly end at a certain wavelength, but smoothly drops to zero over few 10 nm. Therefore, using a sufficiently high exposure dose (e. g. for laser scribing), exposure with wavelengths outside the sensitivity range given in the technical data sheet is also possible to a certain extent.
The absorption coefficient a, the light intensity I in a depth of d in the resist film (related to the incident light intensity I0), and the extinction coefficient k depend on the wavelength l as follows:
The Cauchy constants
N1, N2 und N3 model the refractive index n as a function of the wavelength (in μm units) as follows:
Generally, the Cauchy constants are given in the unbleached and completely bleached state. It has to be considered that the Cauchy constants are fitted from values measured in the visisble part of the spectrum and therefore do not match von n(l) in the UV (such as i-line). The wavelenth-dependant Dill parameters describe the extinction coefficient of photoresist as a function of the concentration of the unexposed photo active compound PAC as follows:
The link below lists the optical parameters (Cauchy, Dill, extinction ...) of selected photoresists. |
Optical parameters of selected photoresists
Bleaching