Positive electron beam resists, molecular

Many papers have been published on positive electron-beam resists. These resists are mostly polymers which are degraded upon electron-beam irradiation. The resulting lower molecular weight polymer in the exposed area can be selectively removed by a solvent under certain developing conditions. The development is accomplished by the difference in the rate of dissolution between the exposed and unexposed areas, which is a function of the molecular weight of the polymer. Recently, Willson and his co-workers reported the new type of positive resist, poly(phthalaldehyde), the exposure of which in the presence of certain cationic photoinitiators resulted in the spontaneous formation of a relief image without any development step (/). 

From Table XI, it can be seen that polymers have spectrum of doses necessary to obtain maximum readout efficiency. We will examine in the following example the imaging of a positive electron beam resist as a function of dose and molecular weight-solubility changes. 

A positive electron beam resist image is developed by immersion in a solvent which dissolves the exposed region at a rate (Sf)which is faster (approx. lOX) than the unexposed rate (Si. The rate of dissolution of a linear polymer is related to its molecular weight by the Uberreiter function (26) ... 

Poly(methyl methacrylate) (PMMA) is the standard positive e-beam resist, usually purchased in two high molecular weight forms (495 or 950 kg moP ) in a casting solvent such as chlorobenzene or anisole. For example 950 kg mol PMMA, 4 % in anisole is a commonly used solution. Electron beam exposure breaks the polymer into fragments (as shown in Scheme 13.7) that can be dissolved in a 1 1 MIBK IPA developer (where MIBK is methyl isobutyl ketone and IPA is isopropyl alcohol). 

The photoresponsive properties of molecular glasses also have been applied in the design of resists for semiconductor lithography. In a resist, irradiation changes the solubility of the materials, making it more or less soluble (positive or negative resist, respectively). The search for new resist materials follows the development of lithographic techniques toward deep-UV and electron beam... 

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