Asymmetric fluid-structure dynamics in nanoscale imprint lithography

1.2.1 Optical Lithography Process OverviewThe initial step is to have a semiconductor wafer spin-coated and bakedwith an imaging layer of photoresist. The photoresist acts as a layer ofphotosensitive organic polymers that is selectively exposed through an aerialimage of the photomask. The solubility of the photoresist is increased (positiveresist) or decreased (negative resist) upon exposure to the illumination source.Rinsing the wafer in a developer solution selectively dissolves the photoresistwhile the circuit pattern remains on the semiconductor wafer.Central to the image transfer process in optical lithography is the exposuresystem comprised of a lithographic lens, an illumination source, and a waferpositioning system. The lithographic lens is a large compound lens comprised of10 to 20 simple lens elements. The lens systems of today are designed to producea typical demagnification factor of 4×. The illumination source is typically ahigh-pressure mercury-xenon arc lamp with undesired wavelengths removed withmulti-layer dielectric filters. The remaining narrow-band light, with less than0.003 nm spectral width, is sent through a series of relay optics and uniformizingoptics and is then projected through the photomask and lithographic lens [Sheatsand Smith 1998]. Figure 1.1 illustrates the optical lithography process.The circuit pattern on the quartz plate, written by electron beamlithography, contains the master pattern at four times the size of the imagedpattern. The imaged is reduced in size through the lithographic lens and theimaging layer is exposed. The solubility of the photoresist is altered by thisradiation. The wafer is then rinsed in a developing solution to remove the highcontrastsoluble resist. The remaining resist pattern will serve as a mask forprocesses such as metal deposition, epitaxial growth, and ion implantation [Choi,Johnson, and Sreenivasan 1999].4