LPCVD tungsten film has been etched using $CF_4/O_2$ plasma in a reactive ion etching(RIF) reactor having asymmetric parallel electrodes. Silicon dioxide and photo resist(PR) have also been etched to compare the etching characteristics with those of tungsten film. Laser and point detector was used to determine the etch rates of these films. CO and $CO_2$ gases are added to $CF_4/O_2$ gas system to enhance the etch rate and selectivity of W film.
Mass spectroscopy and optical emission spectroscopy(OES) were used to study the reaction and etching products of W film. From the results of mass spectroscopy and OES, it is suggested that the main role of CO is scanvenging F and O atoms while that of $CO_2$ is increasing the concentration of O atoms. It is also suggested that the main etchants of tungsten are $CF_x$ and F radicals leading to the volatile etching product of $WF_6$. While the role of oxygen atom is to remove the carbon containing polymer which might be deposited on the surface of W film during etching process.
In $CF_4/O_2$ gas system, the maximum etch rate of tungsten was obtained at about 50% oxygen, while that of silicon dioxide at about 30% oxygen in feed gas. With the addition of CO and $CO_2$ to $CF_4/O_2$ gas system, the etch rate of tungsten increased when a small amount of these gases were added. In the case of CO, tungsten etch rate increased up to the flow rate of 1 sccm. For $CO_2$. etch rate increased up to 5 sccm. It was found that the selectivity, the ratio of W etch rate to that of $SiO_2$, could be increased to 3.1 and 4.1 by adding CO and $CO_2$, respectively, from 2.0 which was the maximum value when using only $CF_4/O_2$ gas system.
