Nanolithography: Length-Scale Limitations
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Lithography is one of the fundamental technologies by which nanoscale patterns required for the fabrication and integration of nanodevices are generated. Lithographic requirements are becoming increasingly severe. In the semiconductor industry, the past three decades have seen the critical length-scales of component devices decrease by several orders of magnitude, from 15 µm in the case of the first integrated circuit, to less than 130 nm routinely obtained today. The phenomenal rate of decrease in size seems to reach the limitation of conventional optical lithography. The exposure wavelength, photoresist performance, and equipment determine the lithography limitation. The size of the constituent atoms imposes a fundamental limit on the minimum length scale that can be ultimately attained. This article first provides an overview of the fundamentals of optical lithography. It is shown how minimum attainable device dimensions are intimately related to the wavelength of light used. Then, several techniques under investigation for further enhancing the resolution of this workhorse of the microelectronics industry are described. As the options available to industry are not all “optical,” the discussions cover the various nonoptical lithographic techniques currently being explored.