Photolithography is the technology of reproducing patterns using light. Developed originally for reproducing engravings and photographs and later used to make printing plates, photolithography was found ideal in the 1960s for mass-producing integrated circuits. Projection exposure tools, which are now used routinely in the semiconductor industry, have continually improved over the past several decades in order to satisfy the insatiable demand for reduced feature size, increased chip size, improved reliability and production yield, and lower overall cost. High-numerical-aperture lenses, short-wavelength light sources, and complex photoresist chemistry have been developed to achieve fabrication of fine patterns over fairly large areas. Research and development efforts in recent years have been directed at improving the resolution and depth of focus of the photolithographic process by using phase-shifting masks (PSMs) in place of the conventional binary intensity masks (BIMs). In this chapter we describe briefly the principles of projection photolithography and explore the range of possibilities opened up by the introduction of PSMs.

Basic principles

Figure 42.1 is a diagram of a typical projection system used in optical lithography. A quasi-monochromatic, spatially incoherent light source (wavelength λ) is used to illuminate the mask. Steps are usually taken to homogenize the source, thus ensuring a highly uniform intensity distribution at the plane of the mask. The condenser stop may be controlled to adjust the degree of coherence of the illuminating beam; this control of partial coherence is especially important when PSMs are used to improve the performance of optical lithography beyond what is achievable with the traditional BIMs.