Light-field mask

The etching procedure using a negative photoresist is identical except that the lithographic mask must be opaque in the regions where metal is to be removed — this is known as a dark-field mask. Exposure of negative resist to UV light makes it insoluble in its developer. 

Fig. 1 The light field and dark field polarities of a typical mask.

Phase shift lithography is a lithographic technique to pattern sub-wavelength features by using a transparent phase shift mask and relies on the pi-optical phase-change of the exposure light field. 

Note masks are either comprised of soda-lime glass (coated with either a photographic emulsion, Pe203, or Cr films), or quartz (with a Cr film). Due to the absorption of UV light by glass, the latter is required for deep UV (DUV) photolithography. Masks may be classified as either light-field or dark-field whereas the former is mostly clear with opaque patterns, the latter is an opaque mask, with transparent features. 

Fig. 38. Diagram comparing the optical characteristics of a standard binary chrome mask with a phase-shift mask. The changes in the electric fields introduced by the phase-shift elements result in a sharper light intensity profile at the wafer surface.

These processes are considerably more complex in actual CMOS fabrication. First, the lower layers of a CMOS stmcture typically have a twin-tub design which includes both PMOS and NMOS devices adjacent to each other (see Fig. 3b). After step 1, a mask is opened such that a wide area is implanted to form the -weU, followed by a similar procedure to create the -weU. Isolation between active areas is commonly provided by local oxidation of sihcon (LOCOS), which creates a thick field oxide. A narrow strip of lightly doped drain (LDD) is formed under the edges of the gate to prevent hot-carrier induced instabiUties. Passivation sidewalls are used as etch resists. A complete sequence of fabrication from wafer to packaged unit is shown in Figure 10. 

Multi-slit systems in which the field is mapped onto the detector via a mask. This admits only the light of pre-selected objects so that a spectrum is produced at the location of each slitlet. The slit mask is custom-made for each observation. Recent examples include CMOS (Hook et al. 2003 Table 2), VIMOS (Le Fgvre et al. 2003) and DEIMOS (Faber et al. 2003). 

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