Dept. of Electrical Engineering
Oregon Graduate Institute of Science & Technology
Improvements in depth of focus and resolution for microlithographic projection printing systems can be achieved by illuminating the object (mask) with oblique or off axis rays. Three types of illumination as used on a waferstepper with a 0.48 numerical aperture objective lens were characterized for their effect on resolution, depth of focus, and proximity. The illumination types evaluated were partially coherent (sigma =0.64), off-axis quadrupole (inner sigma 0.26, outer sigma 0.70 and opening sigma = 0.26), and off-axis ring annular (inner sigma=0.40 and outer sigma =0.70). All projected images were recorded in a modem high contrast positive photoresist. While this type of illumination offers significant performance gains, it is not without limitations. Two shortcomings are the loss of image irradiance and defocus proximity effects. This project will evaluate original methods to improve the latter. The results show that off-axis annular and quadrupole illumination improve the depth of focus for dense 0.36 to 0.50 micron linewidths as compared to partially coherent illumination by 45% and 56% respectively. The "defocus proximity effects" with off-axis illumination mentioned above is where the depth of focus for an exterior line on a dense line-space group is significantly less than an interior line. Three potential solutions evaluated for improving this exterior linewidth proximity defocus effect were: 1. The use of special sub-resolution compensating structures on the mask (object) features, 2. Contrast enhancement photoresist processing, and 3. Anti-reflective substrate coatings. The use of a modified reticle improved the depth of focus for an exterior line, while contrast enhancement and anti-reflective coatings did not offer any improvement. The experimental results were modeled using Silvacos SOLID 3.1 optical lithography simulator (Simulation of Optical Lithography in three Dimensions). Excellent correlation between the SOLID simulation model calibrated to the specific photoresist process used here and the experimental results was achieved.
Brainerd, Steve, "Off-axis illumination for extending optical microlithographic system performance" (1993). Scholar Archive. 282.