Optical and Mass Spectrometric Diagnostic Methods for Plasma Etching
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top of the glass discharge tube), and in the downstream chamber in the region -1 cm above the 2.5 cm dia. sample. A doubly differentially pumped quadrupole mass spectrometer is mounted on the downstream chamber, line-of-sight with the volume directly above the wafer. [2] The gas (Cl 2 and 5% of an equal mixture of He, Ne, Ar, Kr, and Xe) is introduced at the top of the HR source at a total flow rate of either 5 or 10 sccm. When the flow rate was 5 sccm, the pressure at the top of the HR source was 6.4 mTorr and the pressure in the downstream region was 2.0 mTorr. These values were 10.3 and 3.5 mTorr, respectively, when the flow rate was 10 sccm, all with the plasma off. When the plasma was turned on, the pressure in the source region increased by a factor of about 1.5 and the downstream pressure increased by a factor of - 1.05.
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Fig. 1. Helical resonator plasma reactor (from ref. 1). The diagonal-line-shaded areas represent stainless steel. The shaded regions (1) and (2) represent the regions probed by OES. Region (3) represents one mean-free path at 3.6 mTorr, the effective volume probed by line-of-sight mass
spectrometry. The optical emission spectrometer is a 0.35 m focal length scanning monochromator and a GaAs photomultiplier tube. Broadband interference filter are also used to monitor continuum emission from C12 . The mass spectrometer was operated at an electron energy of 25 eV, and was mounted vertically, with the quadrupole axis perpendicular to the molecular beam axis to eliminate artifacts due to charge exchange formation of fast neutrals. 4
RESULTS AND DISCUSSION At high powers (>200 W), optical emission spectra of C12 plasmas in the source region consist principally of Cl emission in the visible and near-infrared (IR) regions (Fig. 2). In addition, weak emission is also observed from C12 + in the near-ultraviolet (UV) and visible regions, and Cl 2 at 258 and 306 nm. At low powers (e.g. 9.5 W, Fig. 2), all the emissions are of course weaker, but the Cl 2 and C12 + bands are relatively much more prominent than at high power. Emission spectra in the downstream region at high power (not shown, but see ref. 1) are similar to the low-power source spectra, but also contain additional features, ascribed to SiCIx (x=0-3) etching products. The relatively weak Cl 2 and C12 + emission in the source region at high power indicates that much less C12 is present here than in the downstream region.
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