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Control of relative humidity in the lithography process environment is crucial to maintaining day to day consistency in printed critical dimensions, especially when using DNQ (DiazoNaphthoQuinone) based photoresists. The dissolution rate of exposed DNQ resists varies dramatically with ambient relative humidity at time of exposure.

In the presence of water (i.e. humidity), exposure of a DNQ photoresist film converts the sensitizer to an acid which in turn increases the solubility of the exposed film in aqueous bases. With no (or insufficient) water available in the film, exposure instead creates a less soluble ester. The ester, in the presence of heat, can react with the resin and initiate cross linking thereby increasing the resin molecular weight and decreasing the dissolution rate in developer (this is the reaction responsible for the image reversal characteristics of AZ® 5200 Series photoresists).2 

A (simplified) mechanism for the reaction that occurs upon exposure of a DNQ sensitizer can be modeled as follows:

DNQReaction

Bruce et al. (IBM-Essex Junction, Vermont) have demonstrated via direct FTIR spectroscopy measurements on exposed DNQ photoresist films that the ratio of acid to ester formation is dependent on the relative humidity level at time of exposure and that humidity in excess of 40% is necessary to completely convert the sensitizer to acid.1  Their study also demonstrates a reduction in the dissolution rate of exposed DNQ films of up to 65% in response to a humidity decrease from 40 to 20%. A direct and detrimental impact on control of printed critical dimensions with variations in humidity is also documented.

When thick DNQ photoresist films (> 5.0µm) are processed, humidity levels well above 40% may still not be sufficient to fully convert the sensitizer to acid. In these cases, it may be necessary to insert a process delay between the soft bake and exposure steps to allow time for re-hydration of the film.

REFERENCES

  1. J. A. Bruce, S. R. Dupuis, R. Gleason and H.Lind,"Effect of Humidity on Photoresist Performance"
    J. Electrochemical Soc., 1997 volume 144, issue 9, 3169-3174
  2. Ralph Dammel,"Diazonaphthoquinone Based Resists"
    SPIE Optical Engineering Press, 1993, volume TT 11,  4-6, 12-26