and typically will use an etch gas of carbon tetra fluoride cf 4 having a gas flow rate of about between 20 to 75 sccm preferably 75 sccm the rf power of the etching process is selected to be between about 50 300 watts preferably 300 watts and the frequency of the power supplied is about 13 56 mhz but could of course vary from this frequency the temperature of the chamber is maintained at between about 5 u00b0 30 u00b0 c and preferably at about 7 u00b0 c during the process the surface of wafer 14 is continuously monitored about every 0 15 seconds by ellipsometer 18a and 18b to determine the thickness of the residual silicon dioxide layer 30 39 until the silicon dioxide layer 30 is completely removed referring now to fig4 there is a plot of actual results showing the thickness of the silicon dioxide layer 30 39 versus etching time obtained in real time and using the in situ ellipsometer for etching 6 147 u00e5 or 615 nanometers as shown initially the measured thickness remains constant as shown by reference number 34 as the etching gases are turned on and the chamber pressure rises to its necessary operating level the rf power is then turned on and the thickness immediately begins to decrease as shown by reference number 36 in the example shown in fig4 all of the film is removed and overetching of the next layer or substrate would commence at reference point 38 however as can be seen because of the extreme predictability of the curve instead of completely removing.