Multilevel resists

The formulation of a single-layer resist that can meet beyond-state-of-the-art demands is an arduous task. To date, very few such materials have been advertized, and their field performance is yet to be proven. The difficulty lies in the fact that requirements of sensitivity, etch resistance, and planarization are mutually exclusive. For example, thinner resists capable of higher resolution sacrifice substrate etching protection and planarization. Consequently, the focus of lithographers lately has centered upon multilevel-resist processes that distribute desirable resist properties among several different organic and inorganic layers. 

X-ray lithography also takes advantage of the increased resist sensitivity due to the thinner imaging films of multilayer systems. Thinner imaging films further improve X-ray resolution by minimizing the penumbra effect, a problem associated with an uncollimated X-ray beam. Consequently, the oblique exposure of features near pattern edges are minimized by multilevel resist processes, thereby restoring the desired profile. 

A Single-Layer, Multilevel Resist Limited-Penetration Electron-Beam Lithography... 

One technique for reducing this proximity effect is to replace the single layer of resist with a multilevel resist (MLR) system. In this case, a thin radiation sensitive resist is separated from the substrate by a thick layer of organic material (1). The utility of this approach has been demonstrated experimentally in several laboratories and also investigated by various computer modeling studies. 

Table 1.1. Organosilicon Resists for Bilayer Multilevel Resist Systems...

The practice of multilevel resist technology has evolved rapidly since its introduction in 1973 (39) largely through resist materials innovation. The first applications of this technology involved use of DQN-type resists over poly(methyl methacrylate) (PMMA) as the planarizing layer. The pattern was... 

The current technological competition for practical fabrication of 0.5 - 1.0 pm feature is in between photolithography and electron beam direct writing, and between single layer and multi-level resist. For less than 0.5 pm, the use of electron beam writing with multilevel resist will be inevitable. Further developments in electron resists from the standpoint of both resist chemistry and process development will be necessary to establish the electron beam lithography. 

SO multilevel resists have not yet seen widespread manufacturing use. The limited selection of polymeric materials suitable for 157-nm resist formulation has brought renewed interest in multilevel systems. 

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