Electron resists positive

Figure 5. Typical sensitivity or response curve for a positive electron resist. Positive resist response may also be plotted using other conventions (see...

A Sensitive Positive-Working Cross-Linked Methacrylate Electron Resist... 

Electron beam positive (top) and negative (bottom) resist etching rate ratios for three sets of etching conditions. 

As a general rule, the sensitivity of conventional electron beam resists is not sufficient for economic throughput in an x-ray lithographic system. This is particularly true of positive electron resists such as PMMA, the most widely used x-ray resist for experimental purposes, whose sensitivity of >500 mJ/cm2 is some 100 times too slow for practical application. Even PBS only shows a sensitivity of 94 mJ/cm2 to PdLa x-rays. Consequently, the major research effort has concentrated on negative resists because of their higher inherent sensitivity. 

Harada, K. Tamamura, T. Kogure, O. Delaited contrast (y-value) measurements of positive electron resists. J. Electrochem. Soc. 1982,129 (11), 2576-2580. 

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. 

Table II. Polymers for representative positive electron resist...

Dry-etching durable resists The real conflict requirement in positive electron resists exists in the relationship between dry-etching durability and sensitivity. Figure 2 depicts this situation. Clearly, aromatic polymers show high durability but low sensitivity (24) Poly(phenylmethacrylate) (PPhMA) withstands reactive sputter-etching more than 2 times longer than... 

Figure 2. Conflict requirement between the sensitivity and dryetching durability in positive electron resists based on methacrylate polymers. Sputter etching rates were measured under the same conditions using CF, gas. Key 1, poly(p-methoxypheny1 methacrylate) 2, poly(phenyI methacrylate) (PPhMA) 3, poly(3-phenylpropyl meyhacrylate) 4, poly(benzyl methacrylate) 5, poly(p-methoxybenzyl methacrylate) 6, poly(p-fluorophenyl methacrylate 7, poly(trichloropheny1 methacrylate) 8, poly(methyl methacrylate) (PMMA) 9, poly(tert-buty1 methacrylate) 10, poly(ethyl methacrylate) 11, poly(isobutyl methacrylate) 12, poly(n-butyl methacrylate) (PnBMA) 13, poly(dimethyltetrafluoro methacrylate) (FPM) 14, poly(trichloromethyl methacrylate) (EBR-1) and 15, poly(hexafluorobutyl methacrylate) (FBM).

The fabrication of LSI circuits, and of VLSI circuits in particular, requires patterns of micron and submicron dimensions, and consequently polymer resists with a high degree of resolution (1). So far the most frequently used positive electron resist has been poly(methyl methacrylate) (PMMA), which affords a high resolution power together with a relatively good thermal stability (2-4) A serious limitation of PMMA with respect to the efficiency of the electron lithography system is its low sensitivity to electron irradiation ( 10-5 - 10 4c/cm2). For the preparation... 

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