The amorphous fluorinated silicon nitride thin films were fabricated on Si and fused silica substrates using inductively coupled plasma enhanced chemical vapor deposition (ICPECVD) process. The $SiH_4$, $N_2$, Ar and $NF_3$ gases were used as reaction gases, and the deposition variables for different thin films were gas flow rate ratios and RF power. The applicability as a bottom antireflective layer (BARL) in the quarter micron optical lithography was examined, and the effects of deposition variables on the absolute composition, oxidation mechanism and optical properties were investigated.
The refractive indices (n) and the extinction coefficients (k) of the fluorinated silicon nitride films at the wavelength of 248nm lay in wide ranges from 1.665 to 2.352 and from 0.007 to 0.695 respectively, depending on the gas flow rate ratio of $SiH_4$:$N_2$:$NF_3$ under the condition of 250mtorr, 250W, 300℃ and Ar=150sccm. Simulation process predicts that for a film thickness of 300Å at a wavelength of 248nm, the optimum conditions for nearly 0% reflectance are when n=2.109, k=0.68 and n=2.052, k=0.592 for Si and W-Si substrates respectively, which are included in the range of optical constants of the deposited films. The calculated reflectance values at the interface between PR and the film postulating the structures of PR/300Å SiNx:F film/c-Si and PR/300Å SiNx:F film/W-Si were less than 5% for the $NF_3$ flow rates of 0.65∼1sccm in $SiH_4$:$N_2$=2:15 (sccm) and less than 7% for the $NF_3$ flow rates of 1∼2.5sccm in $SiH_4$:$N_2$=3:20 (sccm). In addition, the reflectance is very sensitive to the film thickness, so that it could be reduced to almost 0% by changing the film thickness. As a representative example, for the film having n=2.214 and k=0.695 deposited at $SiH_4$:$N_2$:$NF_3$=3:20:2 (sccm), 0.002% and 0.19% reflectances could be obtained at film thicknesses of 270Å and 250Å for Si and W-Si substrates respectively.
Based on the above results, the BARL performance was successfully verified from the actual KrF excimer laser lithographic process. The film satisfying the optimum AR condition showed high stripping ability with a high etch rate of about 350Å/min in 160℃, 85% $H_3PO_4$ solution. Therefore, the fluorinated silicon nitride thin films are optically applicable as the bottom antireflective layer in 0.25 μm optical lithography.
The absolute quantitative analysis for all elements including hydrogen in the films was performed using the elastic recoil detection - time of flight (ERD-TOF) system. As the $NF_3$, $N_2$ flow rates and RF power increased, the fluorine content in the film continuously increased, but the hydrogen content decreased. Extraordinarily, although the oxygen gas was not fed, the oxygen content in the film dramatically increased with the increase of the variables. It was proved by in-situ deposition of SiNx film as a passivation layer on $SiN_x$:F film that the oxidation occurred during the deposition because the activated residual oxygen species in the deposition chamber went into the film with the open structure due to the fluorine element.
These oxygen and fluorine have higher electronegativities among the constituent elements, and thus it gives great influence on the film properties. As the $NF_3$, $N_2$ flow rates and RF power increase, the density of the $SiN_x$:F film decreased from 2.45 g/㎤ to 1.96 g/㎤ with the decrease of silicon content and the variation of nitrogen, oxygen and fluorine contents. The refractive index at the wavelength of 633nm decreased from 2.21 to 1.62 consistently because of the increase of the contents of oxygen and fluorine with high electronegativity. Based on the transmittance and reflectance data of the films deposited on fused silica substrate, the absorption coefficient as a function of wavelength was calculated, and then the optical energy gap ($E_{opt}$) was obtained. As the $NF_3$, $N_2$ flow rates and RF power increased, the absorption edges occurred at shorter wavelengths, and thus $E_{opt}$ increased with wide range from 2.5eV to 6.3eV continuously due to the weak band gap transition of electron by increasing the contents of oxygen and fluorine elements.
