As k1 factors [Eq. (2.8)] fall below 0.8, a number of effects become observable that are not seen when k1 is larger. Processes with small values of k1 began to appear in the mid-1990s, and are quite common today, as seen from Table 8.1. Some of the effects seen in patterns generated with low-k1 processes are discussed in this chapter. Several techniques that are also described-such as off-axis illumination and phase-shifting masks-have been developed to address the shortcomings of optical imaging as feature sizes become smaller than the wavelength of light. Methods to improve image contrast that involve modification of the mask or illumination are referred to collectively as resolution-enhancement techniques (RET) and are discussed in this chapter.
8.1 Off-Axis Illumination
As discussed earlier, coherent light that illuminates a grating is diffracted in very specific directions [Eq. (2.1)]. For normally incident light, sufficiently small dimensions result in situations where all beams except the zeroth order are diffracted outside the entrance pupil of the imaging optics (Fig. 8.1). In this case, no pattern is formed, because a single beam is a plane wave, containing no spatial information, as was explained in Chapter 2. For normally incident (on-axis) illumination, the grating is not imaged when the pitch is too small, because only a single beam, the zeroth-order beam, is transmitted through the lens. This illustrates the limitation to resolution imposed by diffraction.