Novolak resin resist solutions

The structural variations of Novolak resins also influence how well they mix or form solid solutions with a dissolution inhibitor when resist films are cast onto substrates. This is a crucial problem for resist formulation. Usually, cresol-formaldehyde Novolak resins mix well with photoactive compounds like a... 

Formulation of Resist Solutions. Forty grams of a Novolak resin was mixed with 10 g of the photoactive compound, and dissolved in 100 g of bis-2-methoxy-ethylether. After wafers were spin-coated, the samples were immediately placed on a hot plate at 82 C for 14 min. The formulation procedure of a composite resist of poly (2-methyl-1-pentene sulfone) in the Novolak resin is as follows the polysulfone was mixed with the resin (13 wt% solid), and then dissolved in 2-methoxyethyl acetate the films were spin-coated onto silicon wafers, and then baked at 100°C for 20 min prior to electron beam exposure. 

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. 

For further enhancement of electron beam sensitivity, the chlorinated Novolak resin was studied using poly (2-methyl-1-pentene sulfone) as a dissolution inhibitor. The chlorinated Novolak resin mixed well with the polysulfone, and there was no phase separation observed when the films were spin-coated. With 13 wt% of the polysulfone, the chlorinated Novolak resist cast from a cellosolve acetate solution yielded fully developed images with R/Ra = 9.2 after exposure to 2 / 2. It gave fully developed images with R/R0 = 3.2 at a dose of 1 / 2, as shown in Figure 3. There are some problems with this resist system some cracking of the developed resist images... 

In order to understand the changes, it is necessary first to know the components of the resist and the chemistry of the exposure step. For positive photoresists, the mechanism was elucidated by Pacansky and Lyerla (14). Typical resists are a solution of a naphthoquinone diazide photoactive compound (PAC) and a cresol formaldehyde novolak resin in one or more high boiling point industrial solvents. The number average molecular weight of the resin is quite low, on the order of 1000, with a polydispersity of about 10 (13). During the normal exposure step, the PAC, in the presence of water, absorbs light and is transformed into a... 

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. 

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