preferred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings . referring to fig1 , a vacuum chamber 1 is equipped with a gas supply system 2 and a vacuum pump 3 serving as an evacuation system . a substrate 6 to be subjected to etching is placed upon a lower electrode 5 within the vacuum chamber 1 . the lower electrode 5 is connected to an rf power source 4 . an optical filter 14 is provided to a window formed in a side wall of the vacuum chamber 1 . in the side wall is also provided a photo - diode 15 for detecting plasma light intensity within the vacuum chamber 1 through the optical filter 14 . for performing etching , while a predetermined amount of processing gas is supplied into the vacuum chamber 1 by the gas supply system 2 , the vacuum chamber 1 is evacuated by the vacuum pump 3 so as to adjust the pressure inside of the vacuum chamber 1 to a predetermined level . rf power is applied to the lower electrode 5 from the rf power source 4 , whereupon plasma is generated in the vacuum chamber 1 and the substrate . 6 on the lower electrode 5 is etched . the intensity of plasma light at this time is detected by the photo - diode 15 through the optical filter 14 , which passes light having a predetermined wavelength , to monitor the etching process . specific examples of embodiment of the method of etching according to the invention will be described below . gaseous mixture containing cl 2 and bcl 3 was supplied at a predetermined flow rate into the vacuum chamber 1 , while evacuating the vacuum chamber 1 to adjust the pressure therein to 15 mtorr . rf power was applied to the lower electrode 5 from the rf power source 4 , for generating plasma in the vacuum chamber 1 to etch the substrate 6 on the lower electrode 5 . the substrate 6 had a multilayer structure including layers of 1000 angstrom thick titanium , 1800 angstrom thick aluminum , 800 angstrom titanium , and glass . the etching process was monitored by detecting plasma light intensity with the photo - diode 15 using the optical filter 14 for passing light having a wavelength of 396 nm . at the point when it was ascertained that the aluminum layer had been etched away , chf 3 gas was added as a deposition gas , wherein the proportion of chf 3 gas is about 35 % with respect to the total flow rate of the processing gas , from the time point when the lowermost titanium layer was etched until the etching was completed . according to the above - described etching method , no crosswise etching into the etch shape was observed , nor any opaque spots were found on the resultant substrate 6 . however , when the proportion of chf 3 gas added as a deposition gas was 40 % or more with respect to the total flow rate of the processing gas under the identical circumstance to the above - described etching method , the experimentation resulted with an etch stop on the substrate . gaseous mixture containing cl 2 and bcl 3 was supplied at a predetermined flow rate into the vacuum chamber 1 , while evacuating the vacuum chamber 1 to adjust the pressure therein to 15 mtorr to 50 mtorr . rf power was applied to the lower electrode 5 from the rf power source 4 , for generating plasma in the vacuum chamber 1 to etch the substrate 6 on the lower electrode 5 . the substrate 6 had a multilayer structure including layers of 1000 angstrom thick titanium , 1800 angstrom thick aluminum , 800 angstrom titanium , 3600 angstrom thick a - si , and glass . the etching process was monitored by detecting plasma light intensity with the photo - diode 15 using the optical filter 14 for passing light having a wavelength of 396 nm . at the point when it was ascertained that the lower titanium layer had been etched away , chf 3 gas was added as a deposition gas gas from the time point when the lowermost a - si layer was , etched until the etching was completed . fig2 shows the profile of plasma light intensity with respect to the etching process time observed in this example . chf 3 gas was added in a proportion of 10 % with respect to the total flow rate of the processing gas during the period from g to h in fig2 . in this specific example , the discharge is stopped shortly before adding chf 3 gas , but it may be continued . according to the above - described etching method , no crosswise etching into the etch shape was observed , nor any opaque spots were found on the resultant substrate 6 . also , a further experimentation resulted with less crosswise etching into the etch shape observed when the proportion of chf 3 gas was 10 % or less with respect to the total flow rate of the processing gas , and better result was observed when the proportion of chf 3 gas was from 5 to 10 % of the total flow rate of the processing gas . in the above - described embodiments , detection of plasma light intensity through an optical filter for passing light having a predetermined wavelength was used for monitoring etching process , however , various methods for monitoring etching process can be implemented by detection of , for example , electric potential of plasma , plasma impedance , ion flux and mass analysis . also , it is possible to determine when to supply chf 3 gas by obtaining a sample data respect to the progression and the time period of the etching process instead of the above - described monitoring methods . although the present invention has been fully described in connection with the preferred embodiment thereof , it is to be noted that various changes and modifications apparent to those skilled in the art are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom .