Minimum ion-beam exposure-dose determination for chemically amplified resist from printed dot matrices

Matrices of 90 nm dots have been printed into high-sensitivity positive resist (UVII HS, Shipley) with an ion projection system (IMS Vienna) to investigate the influence of shot noise on the printing probability of dots. Dot defect probability increases with diminishing ion dose following a Poisson distribution which demonstrates that shot noise is the dominating effect. The minimum ion numbers per dot to generate 50% defect probability are Ncrit=115 for standard resist treatment. This corresponds to N=165 for smaller defect probabilities of 10-4. Resist sensitivity was decreased with postexposure bake temperatures of 110 degrees C instead of 140 degrees C to improve the resolution capability of the resist. Under these conditions, and an additional resist top coat, Ncrit=130 ions per dot have been measured. The article demonstrates that ion projection lithography is not limited by shot noise at minimum resolution elements of 90 nm diam. The corresponding exposure doses are 0.3 mu C/cm2.