Resist patterns, Novolak resins

A novolak-based resist pattern, which tends to flow at 120 C. can hold its original shape when it is embedded and covered with an inert polymeric material. Then, it is possible to induce a thermal crosslinking of the novolak at 180-220 °C without distortion of the patterns. T wo polymeric materials have been described, i.e. PMMA and a fluorine bearing resin After the hardening bake, the mold is removed and the hardened patterns can be used in the next process step. 

Development of Resist Patterns. Development was done in AZ2401 developer diluted with 2 to 5 times its volume of water AZ2401 is an aqueous solution of KOH with a surfactant. When the resist films were exposed to electron beam doses of 5 iC/cm2 at 25 keV, it usually took 1.5 to 2.0 min for complete development of the images using a diazo-naphthoquinone sensitizer with o-chloro-cresol-formaldehyde Novolak resin in (1 3) AZ2401/water developer. With poly(2-methyl-l-pentene sulfone) the chlorinated Novolak resin exposed to I juC/cm2, it took 2.0 min in (1 4) AZ2401 developer for complete image development. 

Although the Novolak resin of the w-cresol-benzaldehyde failed to show a marked increase in CF4 plasma etching resistance, the Novolak resins of hydroxy-naphthalene-hydroxybenzaldehyde showed a remarkable increase in the plasma etching resistance. The resist films also yielded excellent patterns when used together with a diazo-naphthoquinone sensitizer almost non-diluted AZ2401 developer had to be used for image development due to the hydrophobic nature of the naphthalene group. 

After pattern delineation, the next step in a production process is e.g. etching (wet or plasma), ion implantation or metal deposition. Some of these techniques result in heating of the resist above 200 °C. Although novolak resins start to crosslink from a temperature of 120 °C, crosslinking cannot prevent thermal flow of a resist pattern because the flow occurs at a lower temperature already. Several solutions to this problem have been found, and are described below. 

Patterns of novolak-based resists can be chemically hardened by treatment with formaldehyde in an acidic medium The reaction that takes place is an inter-molecular condensation. The resin becomes highly crosslinked by reaction at its para positions. Although the method results in a high thermal flow stability (300 °C), it is not attractive for implementation in a production process because of the hazardous nature of the chemicals involved. 

Ordinary novolak, as discussed in Sect. 8.2, has a low thermal flow stability. During the bake treatment (120 °C), which is normally applied to developed resist patterns, thermal flow leads to deformation. Although this problem can be solved by hardening the pattern by chemical or physical means (see Sect. 3.2), many phenolic polymers with high T, have been studied as replacements for the novolak resin. For example poly (vinyl phenol) (T 160-180 °C) has been utilized in a commercially available... 

Formation of the resist pattern is due to the solubility change of the exposed part of the resist which results from the photochemical reaction of NQD. Figure 3 shows a change of the dissolution rate of the resist. The dissolutjon rate of a novolak film itself in an alkaline developer is approximately lOOA/sec. Once NQD is added to the novolak resin, the rate decreases drastically by the order of one thousand, which means the unexposed part scarcely dissolves in the alkaline developer. This is called the "dissolution inhibition effect" of NQD (1). And then upon exposure NQD decomposes to produce indenecarboxylic acid which makes the exposed region even more soluble than the novolak itself. 

For applications, where a high resolution is required, e.g. IC technology and optical recording, resist systems based on novolak (a cresol-formaldehyde resin) or poly(vinyl phenol) are utilized. These resists are developed in aqueous base, which causes no swelling of the polymer and therefore no loss of pattern definition. 

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