Subwavelength Optical Lithography with Phase-Shift Photomasks

Abstract

■ Optical lithography has been the patterning method of choice for the semiconductor industry for over three decades. Through a continual decrease in exposure wavelength and increase in lens numerical aperture, this technology has kept pace with the exponentially shrinking feature sizes predicted by Moore's law. In the mid-1990s, minimum feature sizes on semiconductor chips began to drop below the available imaging wavelengths, ushering in the era of subwavelength optical lithography. Imaging in this challenging regime has been enabled by the development of resolution enhancement technologies (RETs) that work to overcome diffraction limits on imaging resolution. We have developed, as part of a Defense Advanced Research Projects Agency-sponsored program in advanced complementary metal-oxide semiconductor (CMOS) technology, phase-shift-photomask optical lithography processes capable of imaging features as small as 16% of the exposure wavelength. We have applied these processes to the development of advanced silicon-on-insulator (SOI) device technology utilizing standard commercial optical lithography equipment. We are also exploring methods of cost-effective implementation of this type of resolution enhancement technology for applications that require high-resolution imaging for relatively small quantities of wafers.