Resolution enhancement of lithography

RELACS resolution enhancement of lithography assisted by chemical shrink... 

The postexposure-based techniques are grouped into three broad categories, namely, reflow-based. shrink techniques, chemical-based shrink techniques, and plasma-assisted shrink techniques. The reflow-based shrink techniques comprise thermally induced reflow and electron-beam heating-induced reflow of patterned resist features. The chemical-based shrink techniques comprise those techniques that either increase or decrease the sidewall thickness of already patterned resist features, thus effectively altering their critical dimension. Examples of chemical-based shrink techniques that result in an increase in the sidewall of the patterned features include techniques based on RELACS (resolution enhancement of lithography assisted by chemical shrink) and CARL (chemical amplification of resist lines).Examples of chemical-base shrink techniques that result in decrease... 

As the name implies, this technique increases the thickness of the sidewall, resulting in a decrease in the diameter of the hole or trench opening. It is used in reducing the CD of contact hole and trench features. The resolution enhancement of lithography assisted by chemical shrink (RELACS) is a good example of this technique. 

MUV lithography where exposure is based on the 313 nm mercury emission, is a relatively mature technology. The resolution enhancement that accrues from a shift to the MUV region has been carefully documented (69) and MUV projection printers are commercially available at this time. These include the Perkin Elmer Micralign series 300 and 500 both of which have MUV capability. 

MLR systems offer many advantages in optical, e-beam, x-ray, and ion-beam lithography. An advantage common to all imaging methods is in enhancement of resist sensitivity. As the resolution and the aspect ratio requirements are separated in an MLR system, faster resists that are usable only for low aspect ratio images can now be candidates for the top layer. Other advantages of MLR systems differ from one imaging method to the other. They will be discussed separately. 

A major consequence of these considerations is that new exposure sources and/or very sensitive resist materials must be developed in order to realize the resolution enhancement offered by deep-UV lithography without the penalty of extremely long exposure times. Considerable advances have been made on both fronts. 

For further enhancement of the resolution at 193 nm, immersion lithography employing water as an optical element between the last lens and resist has emerged recently as a new technology in competition with 157 nm lithography. In immersion lithography the effective exposure wavelength is reduced by the refractive index of the immersion fluid (193/1.44= 134 nm for 193 nm water immersion). 

Levinson, Principles of Lithography, 2nd ed., p. 243, SPIE Press, Bellingham, WA (2005). A.K. K. Wong, Resolution Enhancement Techniques in Optical Lithography, 2nd ed., p. 10, SPIE Press, Bellingham, WA (2001). 

In general, the single-exposure techniques rely on the use of either hyper-NA >1.0 as implemented in immersion lithography (see Chapter 13) or exposure wavelength reduction as implemented in EUV lithography (see Chapter 14) and electron-beam lithography (see Chapter 15). These techniques may be complemented with reticle-based resolution-enhancement techniques such as phase-shifting masks and the like. 

In the last decade, several techniques have been developed in order to improve the resolution that can be obtained with optical lithography. This review describes a number of these techniques. Some techniques make use of a photosensitive layer on top of the photoresist, which results in enhancement of the contrast of the aerial image (Sects. 3.1.1 and 3.1.2). Other techniques use the possibility of slope control during development (Sects. 3.1.3 and 3.1.4). Also discussed are some techniques involving anisotropic plasma etching. 

Figure 14 shows the process flow of the Resolution Enhanced Lithography (REL) process (10). In this process, after the image exposure the resist is baked at around 100 0 and exposed to deep-UV light to make the resist crosslink. This temperature is too low to decompose NQD rapidly, so the reaction needs to be induced by light exposure to take place. And at this temperature, there is less water in the resist and the novolak resin is in the rubbery state, which means the resin has higher reactivity than in the glassy state at room temperature. Therefore, upon deep-UV exposure, NQD may tend to react with the novolak resin to crosslink. 

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