Surface Science Studies of NF 3 Plasma and Ion Beam Interactions with Silicon
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on the basics of NF 3 plasmas (see for example [I] to [11]), few studies have been done from a surface chemistry point of view [12, 13]. We have used surface analytical techniques and other diagnostics to determine the interaction of NF 3 plasma species with silicon and related compounds. The present investigation, focusing on the results of x-ray photoelectron spectroscopy, briefly addresses the interaction of NF 3 -ion beams with silicon but is devoted more toward an intriguing result obtained from NF 3 plasma-Si interaction. EXPERIMENTAL
The production of a plasma from source gases results in both ions and reactive neutrals. While the neutrals are most responsible for chemical etching, the ions may also influence the etching process through concomitant bombardment of the sample surface as a result of acceleration by an electric field which acts over a sheath region between the plasma and substrate [14]. Because the ions impinge in a direction perpendicular to the sample surface, the ion flux is beneficial in obtaining anisotropic etch profiles with high aspect ratio. As the pressure of the plasma gas decreases, however, the energy of ions incident upon the sample surface increases and may cause damage to the surface. In an attempt to account for both neutral and ion effects, we have used both actual plasmas and directed, low-energy ion beams to investigate the interaction of NF 3 with Si.
165 Mat. Res. Soc. Symp. Proc. Vol. 532 ©1998 Materials Research Society
Plasma Studies DC plasma experiments were performed in a combined ion beam/plasma processing chamber which is part of a multichamber UHV system. The ion beam/plasma processing chamber consists essentially of a 6-way cross, through the bottom flange of which is mounted a medium current feedthrough. A sample holding stage which can accommodate sample cassettes holding approximately 1 cm square Si wafers is connected to the feedthrough. The base pressure of the processing chamber, pumped by a turbomolecular pump, is in the mid-10-8 Torr range. N-type Si (100) wafers (0.01 Q cm) are first chemically cleaned in dilute HF, rinsed in deionized water and then blown dry with nitrogen before being mounted on the sample cassettes and inserted into a forechamber. All samples are characterized by XPS prior to ion beam treatment to verify surface cleanliness. Following transfer to the processing chamber, most samples are sputter-cleaned with argon (Ar) just prior to processing. As a control, some
samples are not sputter-cleaned prior to processing in order to ensure that the sputter cleaning itself does not have unintended consequences on the experimental results. Plasmas are created by first introducing NF 3 or a mixture of NF 3/X, where X is either Ar or helium (He), through a leak valve into the processing chamber until the pressure is in the mTorr range as measured by a capacitance manometer and then using a high voltage DC power supply to negatively bias the sample electrode to between 0.5 and 1.5 kV. Ion Beam Studies Ion beam studies are carried out in the sa
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