Exposure of Photoresists

Mask-Aligner and Stepper The typical emission spectrum of a mask aligner or stepper with Hg light source and without optical selective mirrors/filters contains g(wavelength 436 nm), h(405 nm) and i-line (365 nm) (fig. right-hand), with an iline intensity approx. 40 % of the total emission between 440 and 340 nm. The absorption spectrum (spectral sensitivity see next section) of AZ® and TI photoresists is matched to this Hg emission spectrum. Especially for exposure dose sensitive processes (image reversal-, thick resist processing, high resolution) a calibration of the illumination intensity (changing with bulb operating time) is strongly recommended. A measurement of the lateral intensity distribution should reveal less then 10 % deviation over the substrate size in order to allow a proper exposure dose for central and edge-near regions of the resist film. Laser Exposure of Photoresists When using a laser beam as light source for photoresist exposure in stead of the usually used Hg bulbs, one has to consider two main points: The light intensity is quite different: While in laser interference lithography, the light intensity is rather low, laser scribing causes intensities many orders of magnitude beyond the intensity of a mask aligner or stepper. The exposure wavelengths of the laser often differs from the 365, 405, or 435 nm Hg lines which are matched to the spectral sensitivity of photoresists. The spectral sensitivity of photoresists does not abruptly end at a certain wavelength, but smoothly drops to zero over few 10 nm. Therefore, using an adjusted exposure dose (e. g. laser scribing), exposure with wavelengths outside the sensitivity range given in the technical data sheet is also possible to a certain extent. The document Laser Exposure of Photoresists gives further details on this topic.