100 when the pressure of gas flowing in the plasma chamber was increased to 2 5 torr the etch rate of the doped silicon oxide dropped to 3800 angstroms per minute with a standard deviation of plus or minus 175 point 24 on the graph represents such reading the etch rate of polysilicon at the same pressure was approximately 80 angstroms plus or minus 20 this is represented in the graph by point 28 when the pressure was increased to 3 torr the etch rate of doped silicon dioxide became 3610 angstroms plus or minus a standard deviation of 150 this is represented at 26 there was at the same time essentially no etching of polysilicon the selectivity provided by the above at 3 torr was approximately 50 1 this selectivity is quite adequate the line 29 connecting the points 23 through 26 represents in essence the selectivity achievable at various pressures with a 3 to 1 relationship between the fluoroform and sulphur hexafluoride the flow rate of fluoroform was decreased to 25 cubic centimeters per minute whereas the flow rate of sulphur hexafluoride was retained at 48 cubic centimeters per minute with the result that the flow ratio of sulphur hexafluoride to fluoroform became approximately 2 to 1 etch rate at various pressures were then measured which etch rates are represented in fig4 by the circles 31 33 and 34 36 at a pressure of 2 torr it was found that 5109 angstroms per.