Experimental measurements of the etch rate and ion flux distributions on the wafer are combined with modeling to elucidate the effects of reactor wall conditions on Cl concentration and polysilicon etch rate uniformity in an inductively coupled plasma etching reactor. The spatially averaged etch rate across the wafer increases with time as etch products react with residual oxygen in the chamber and coat the reactor walls with a thin layer of silicon oxychloride film. Chlorine concentration in the plasma and the Si etch rate increase due to lower recombination probability of Cl on this film as compared to the “clean” anodized aluminum wall surface. Etch rate is highest at the wafer center when the walls are maintained in the clean state. In contrast, the etch rate peaks at the wafer edges when the walls are coated with the silicon oxychloride film. The drift in etch rate and uniformity is primarily due to a drift in Cl concentration and its spatial distribution. As the reactor walls are coated, the etch rate distribution changes from a center-fast profile to an edge-fast profile due to a change in the dominant Cl depletion mechanism from wall recombination to recombination on the wafer surface. © 2003 The Electrochemical Society. All rights reserved.

In plasma etching processes, the spatial distribution of ion flux across the wafer surface determines the uniformity and profile evolution when etching is ion limited. We have designed and built a two-dimensional array of planar Langmuir probes on a 200 mm diameter silicon wafer to measure the radial (r) and azimuthal (θ) variation of ion flux impinging on the wafer surface in plasma etching reactors. Herein we demonstrate the use of this probe array to obtain two-dimensional ion flux distributions in Ar, Cl2, and Cl2/HBr/He discharges in an inductively coupled plasma reactor. The results obtained using the probe array are in good agreement with Langmuir probe measurements but also reveal azimuthal asymmetries, due to irregularities in chamber geometry such as the pumping port and radio frequency coil configuration, that cannot be detected using radially movable Langmuir probes. The probe array can also be used to investigate the spatiotemporal fluctuations of the ion flux in the 1–100 Hz range.

Surface analysis by secondary-ion mass spectroscopy during etching with gas-cluster ion beamTransients in plasma composition and positive ion flux due to changing chamber wall conditions during Cl 2 plasma etching of Si were studied using multiple plasma and surface diagnostics. In presence of Si and O containing species in the gas phase a glassy silicon oxychloride film coats the chamber walls over a time scale determined by the concentrations of the Si and O containing deposition precursors. This time scale can be a few minutes as in the case of Si etching with Cl 2 plasma, where the concentration of silicon chloride etching products can be high, or hours as in the case of a Cl 2 plasma maintained in absence of Si wafer, where the Si and O can only come from very slow etching of a quartz window. In either case, SiCl x (1рxр4) and Cl concentrations in the gas phase and the total ion flux impinging on the wafer surface increase as the chamber walls are coated with this glassy film. The increase in SiCl x and Cl concentrations are primarily due to lower loss probability of these species by recombination on the chamber walls. The ion flux increases primarily due to higher SiCl x concentration in the discharge. During etching of Si, increases in Cl concentration and ion flux through the mechanism described above increases the etching and SiCl x production rates. This strong coupling among the discharge properties, the wall conditions, and etching rate lead to transients in plasma operation.

A two-dimensional array of planar Langmuir probes built on a 200 mm diam silicon wafer was used to measure the radial and azimuthal variation of ion flux impinging on the wafer surface in Ar/SF6 and Ar/Cl2 discharges maintained in an inductively coupled plasma etching reactor. The spatial variation of ion flux in a pure Ar discharge is approximately radially symmetric and peaks at the center of the wafer for pressures between 10 and 60 mTorr. The spatially averaged ion flux in a pure Ar discharge increases with increasing pressure and the corresponding uniformity degrades with increasing pressure within the pressure range studied. Addition of small amounts of electronegative gases to an Ar discharge flattens the radial and azimuthal ion flux distribution and accentuates azimuthal variations due to subtle asymmetries in the reactor geometry such as pumping ports. At fixed power, pressure, and flow rate, the spatially averaged ion current density decreases with increasing mole fraction of the electronegative gases in the feed gas.

A two-dimensional array of planar Langmuir probes on a 200 mm diameter silicon wafer was used in an inductively coupled plasma reactor to follow the spatial and temporal variation of ion flux impinging on the wafer in the presence of an instability. Small amplitude low frequency (∼2 Hz) oscillations superimposed on a steady state ion flux distribution were observed in SF 6 plasmas. The magnitude and phase of the oscillations depend on position on the wafer and analysis of these variations reveals that these low frequency oscillations correspond to waves that move across the wafer.