Paper PS1-WeA9
Analysis of Run-to-Run Variability in the Bosch Process using rf Probe and Emission Spectroscopy Measurements

Wednesday, October 31, 2012, 4:40 pm, Room 24

Deep silicon etching is a crucial step for several emerging and evolving technologies, including micro-electromechanical systems (MEMS) and CMOS image sensors. Among etching methods available, the Bosch process is widely used because it allows fast etching rates together with high aspect ratios. This process relies on rapidswitches between isotropic etching in SF₆-containing plasmas and polymer deposition for sidewall passivation in C₄F₈-containing plasmas. As a result of reactor wall conditioning, run-to-run variability in etching rates and uniformity is commonly observed. In order to understand the origin of such drifts and thus to develop new strategies to reduce these effects, we have started investigations of the temporal evolution of the properties of SF₆ and C₄F₈ plasmas in the Bosch process on an industrial, inductively-coupled plasma reactor (ICP) at Teledyne Dalsa. In this context, a rf probe was installed on the transmission line connecting the generator to the substrate holder. This system measures the amplitude of the current and voltage as well as the phase between them, allowing determination of the real and imaginary parts of the impedance. During one etch cycle in Ar/SF₆ (total duration of 10s), the absolute value of the reactance, |X|, decreased sharply within the first second and then reached a plateau. For one sample exposed to many etch and deposition cycles, the cycle-averaged value of |X| slowly decreased between the first and the 70^th cycle. For many samples etched in the same chamber with a 2 minutes O₂ plasma clean between each run, the run-averaged value of |X| decreased with the sample number. Over this whole range of experimental conditions, the resistance remained fairly constant. From optical profilometry measurements, the etching rate was found to decrease from 340 to 300 nm/s between run #1 and #5. This decrease matched relatively well the observed decrease of |X|, suggesting that the reactance determined from rf probe measurements is a good parameter to examine in a non-intrusive way run-to-run variability in deep silicon etching. More recently, preliminary analysis of the plasma emission in the Ar/SF₆ plasma revealed that the F (703.7 nm)-to-Ar (750.4 nm) line-intensity ratio (related to the F density) remained constant within the first few seconds and then decreased mid-way in the etch cycle due to strong F uptake. Future experiments will attempt to determine whether the observed decrease of the Si etching rate due to wall conditioning can be attributed to a reduction of the fluorine concentration during the etch cycle or to a higher density of fluorocarbon radicals during the passivation cycle.