Paper PS1-WeM6
Ion Induced Etching Reaction of SiO₂ and Si by CF₃⁺ Irradiation

Wednesday, October 22, 2008, 9:40 am, Room 304

Fluorocarbon plasmas have been widely used to etch a silicon dioxide in the fabrication of semiconductor devices. In the view of the development of integrated semiconductor devices, more precise control of the etching process is required for further progress. In the previous work, the etching yield of SiO2 by CFx+ irradiation.¹ In the present work, we report measurement results of the desorbed products of SiO2 and Si etching due to the irradiation of CF3+ ions, which are considered to be the main ion species in fluorocarbon plasmas. Time of flight(TOF) measurements are also reported, which determines kinetic energies of the products. The low-energy mass-analyzed ion beam apparatus consists of an ion beam source, an ultra high vacuum scattering chamber. Various ions were generated in arc plasma of CF4 and were extracted from the ion source. CF3+ ions for sample irradiation were selected with a mass-analyzing magnet, passed through conventional beam optics, and decelerated to specified irradiation energy just before they reached the sample. Angular distribution of desorption products were detected through an aperture by a rotatable quadrupole mass spectrometer. To measure time of flight distributions of desorbed products, the ion beam is electronically chopped at deflectors in beam line When a CF3+ ion at 500 eV impinged on a SiO2 surface, the major desorbed product was SiF2. Angular distribution of SiF2 follows a cosine law, and the flux of SiF2 does not depend on the ion incident angle. On the other hands, When a CF3+ ion impinged on a Si surface with a large incident angle, the major desorbed product was Si and SiF. Angular distribution of desorbed Si and SiF strongly depends on the ion incident angle and energy. These results clearly show that the desorption process of etching on SiO2 differs from that on Si surface; collision cascades by incident ions on a surface hardly affect desorption of SiF2 from SiO2, unlike the desorption of Si atoms. This explanation of the desorption mechanism is also supported by the observation of TOF distribution measurements of desorbed SiF and SiF2. The TOF spectra of SiF and SiF2 are fitted well by collision cascade and Maxwell-Boltzmann distributions. These results indicate that the main desorption paths of SiF and SiF2 are different; SiF desorbs with collision cascade and SiF2 desorbs with thermal activation after collision cascade.

¹ K.Karahashi et al. J.Vac.Sci.Technol. A, 2004, A22, 1166.