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A polysilazane was investigated as a precursor to a spin-on glass (SOG) used for a middle-layer in a tri-level resist system. Higher film density is required for the middle-layer in order to obtain higher etch resistance during the under-resist etching and prevent the acids in the resist from diffusing to the SOG, which induces deteriorating of resist patterns. High film density of the SOG was achieved by spin-coating the polysilazane solution. The compositions of the polysilazane baked at 200 °C and 300 °C are Si42O34C4N20 and Si29O65C1N5, respectively. The polysilazane is converted to silicon-oxide likely structure by baking at 300 °C. The film density of the SOG made from the polysilazane (SGPZ) is 2.07 g/cm3, which was higher than 1.87 g/cm3 of the conventional SOG made from a polysiloxsane. The etch resistance of the SGPZ baked at 300 °C which is expected not to volatilize the under-resist is improved by 90% compared with that of the SOG made from the polysiloxsane baked at 300 °C due to the increased film density of the SGPZ. The refractive indices of the films are n=1.56, k=0.01 (ArF) and n=1.68, k=0.02 (F2). Without the stacked films of SGPZ/under-resist, reflectivities to the resist are 56.2% (ArF) and 44.2% (F2). By optimizing the SGPZ thickness, the reflectivity is reduced to less than 0.7% (ArF) and 0.4% (F2). In conclusions, the polysilazane can be as the superior material for the SOG used for the middle-layer in the tri-level resist system.
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