Positive resists main-chain scission

Materials that exhibit enhanced solubility after exposure to radiation are defined as positive resists. The mechanism of positive resist action in most of these materials involves either main-chain scission or a polarity change. Positive photoresists that operate on the polarity change principle have been widely used for over three decades in the fabrication of VLSI devices and they exhibit high resolution and excellent dry etching resistance. Ordinarily, the chain scission mechanism is only operable at photon wavelengths below 300 nm where the energy is sufficient to break main chain bonds. 

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

Experimental determination of the quantum efficiency of photosensitive polymers of the sort that are used in one-component positive resist systems is a more complex experimental undertaking. Here the quantum efficiency is defined as the number of main chain scissions that occurs per photon absorbed. Guillet and coworkers at the University of Toronto have... 

Positive Resists Based on Main-Chain Scission... 

In addition to the polarity change, the tone of certain resist materials may be reversed simply by changing the wavelength of exposure. For example, copolymers of methacrylonitrile and methacrylic acid undergo main-chain scission when exposed to DUV radiation to provide positive-tone im-... 

Another interesting, DUV-sensitive, planarizing layer for the exposure-PCM scheme is poly(dimethyl glutarimide) (PMGI) (structure 3.7). Exposure of PMGI to DUV light or electron beam radiation results in main-chain scission therefore, this resist is positive working. 

High sensitivity resists Table II summarizes the properties of representative positive electron resists. The polymers are classified into four groups according to the chemical structure. Almost all positive electron resists operate by main chain scission of polymer, resulting in a molecular weight decrease in exposed areas. Resist patterns are produced by development in a suitable solvent in which degraded polymer dissolve much faster than unexposed polymer. The sensitivity is determined by the scission probability and the solubility rate ratio for the degraded polymers. 

Non-chemically amplified positive resists based on main chain scission... 

Another group of positive UV resists operating on the principle of radiation-induced main chain scission utilizes the efficient photochemistry of polymeric ketones, exemplified hy poly(methyl isopropenyl ketone) PMIPK, to effect image discrimination. Scheme 7.21 shows the photolysis of this resist. ... 

PMMA was one of the first positive resists to be investigated. When subjected to electron-beam irradiation, PMMA suffers extensive main chain scission, owing to the presence of quaternary carbon atoms. PMMA is an excellent positive resist from the viewpoint of adhesion and resolution, but it exhibits rather low sensitivity (50 pC/cm at 15 kV) and poor resistance to dry etching techniques. 

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