| The typical emission spectrum of a mask aligner with Hg light source and without optical selective mirrors/filters contains g- (wavelength 436 nm), h- (405 nm) and i-line (365 nm), with an i-line intensity approx. 40% of the total emission between 440 and 360 nm.The absorption spectrum of AZ® and TI photo resists is matched to this Hg emission spectrum. Hereby, one has to distinguish between broadband sensitive (g-, h-, and i-line) photo resists, and resist with a narrower absorption spectrum (e.g. AZ® 9260, AZ® 5214E without g-line sensitivity, or AZ® nLOF 2000 i-line resist family).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 on the resist film.The spectral sensitivity of photo resists does not abruptly end at a certain wavelength, but smoothly drops to zero over few 10th 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. When calculating the correct exposure time from the exposure doses given in the technical data sheet for each rest, the following has to be considered: i) Does the light source emit broadband (g-, h-, and i-line) or monochromatic light? ii) Measuring the light intensity: Does the sensor only respond to g-, h-, or i-line, or to broadband intensity? iii) For which spectral range is the illumination intensity given in the mask aligner manual? iv) On which spectral range does the photo resist technical data sheet refer with respect to the necessary exposure dose, either
a certain spectral range (g-, h-, or i-line) when using broadband exposure, or i-line using monochromatic exposure, or the integrated g-, h-, and i-line intensity? v) What is the spectral sensitivity of the photo resist? Due to the many parameters listed above, we strongly recommend an exposure series as starting point for each new process (see link below). |
Literature: Exposure and Development