Angular dependence of SiO2 etching in a fluorocarbon plasma

SiO2 etch rates in a CF4 plasma were obtained at various surface angles usi ng a Faraday cage with pinholes on the upper plane through which ions are i ncident on the substrate fixed at various angles inside the cage. The react ive ion etching experiments were performed at 5 mTorr in a wide bias-voltag e range from -100 to -800 V. The normalized etch-yield curves showed virtua lly the same angular dependence regardless of the ion incident energy. The curve shape was similar to that of physical sputtering except that the rati o of the maximum yield to that at 0 degrees was as low as about 1.3. This i s the deviation from the cosine dependence characteristic of chemical sputt ering, which is the main mechanism of SiO2 etching in a fluorocarbon plasma . This deviation is partly attributed to the fluorocarbon polymer film, whi ch existed as a few monolayers-thick film on the substrate surface at low a ngles near 0 degrees but as a submonolayer at high angles between 45 degree s and 75 degrees. The effect of the film-coverage difference on the deviati on is explained in terms of reactant chemisorption, product desorption, ene rgy deposited by ions causing the collision cascade, and incorporation of p rojectiles and recoils leading to the mixing effect. The deviation is also attributed to physical sputtering, which became comparable to chemical sput tering as the ion energy increased. Both factors, i.e., the formation of th e thin fluorocarbon polymer film and physical sputtering, are supposed to c ontribute to the curve deviation, although their relative importance change d with the ion incident energy. (C) 2000 American Vacuum Society. [S0734-21 01(00)04406-7].