Radical surface interactions in industrial silicon plasma etch reactors

Abstract

Silicon etching in Cl 2 -based plasmas is an important step for the fabrication of IC circuits but the plasma surface interactions involved in this process remain poorly understood. Based on the developments in plasma and reactor wall diagnostics, this paper reviews the recent progress in the understanding of radicals' interactions with surfaces during silicon etching processes. X-ray photoelectron spectroscopy analysis of the reactor walls shows that during Si etching in Cl 2 /O 2 plasmas, the initial Al 2 O 3 chamber walls are coated with a thin SiOCl layer. Broadband absorption spectroscopy with UV light emitting diodes is used to measure the densities of SiCl X radicals (X = 0-2) and Cl 2 molecules in steady state plasmas running with the chamber walls coated with different materials. To estimate the surface sticking/recombination probability of these radicals on different surfaces, we have performed time-resolved absorption measurements in the afterglow of pulsed discharges. Our work, in agreement with previous results, shows that the Cl 2 /Cl density ratio in the discharge is driven mainly by the chemical nature of the chamber walls explaining why process drifts are often observed in Cl 2 /O 2 plasmas. The recombination coefficient of Cl atoms on SiOCl surfaces is about 0.007, while it is about 0.1 on clean walls (AlF 3 ). Based on these results, we discuss the best strategy leading to reproducible process control, the present strategy being a systematic reactor cleaning/conditioning between wafers. The SiOCl layer deposition mechanism is then discussed in detail. The sticking coefficient of SiCl on this surface is near unity, while SiCl 2 appears to be weakly reactive toward it. Therefore, SiCl (and SiCl + ions) are the main vectors of Si deposition on the reactor walls, where their subsequent oxidization by O atoms leads to the formation of a SiOCl deposit. Furthermore, we show that SiCl reaction in the plasma volume with Cl 2 , through the exchange reaction SiCl + Cl 2 → SiCl 2 + Cl, is very important for the control of radical densities. Finally, time-resolved measurements of the gas temperature by laser-absorption Doppler spectroscopy show that in the afterglow of pulsed discharges the transport of particles is affected by the convection phenomenon. The very fast electron temperature cooling in the early afterglow generates a pressure gradient in the reactor which induces a rapid gas movement from the cold regions of the reactor toward the reactor center, where the plasma was generated. This leads to an increase in the density of stable molecules in the volume previously occupied by the plasma.

Measured density of copper atoms in the ground and metastable states in argon magnetron discharge correlated with the deposition rate

“…5, the neutral atoms temperature of the magnetron plasma T a , which corresponds to the L r T value deduced from the two temperature fitting 25,26 , is 320 K; the 6 rotational distribution in the high J levels being represented by H r T =1500 K. For gas pressures of 0.6, 4.5 and 14 µbar, the T a deduced from the recorded nitrogen spectra at different discharge powers are plotted in figure 6 and range between 300 and 380 K. Values at intermediate pressures can be deduced by the interpolation of these data. As expected, at fixed pressure, T a increases with the dissipated power, but also slightly with the gas pressure 21 . From the measurement of the Cu emission linewidth with a Fabry-Perot interferometer, Ball et al have reported a comparable gas temperature of 350 K in a dc copper magnetron discharge running with argon gas 27 .…”

“…A very reliable method to obtain the gas temperature is from the linewidth of the Doppler-broadened absorption profile of a specific line, measured with a single-mode tunable diode laser. Absorption from argon metastable atoms is particularly popular in pure argon plasmas 17,18,19,20 or by adding a small amount of argon to the halogen based plasmas 21 . Another commonly used technique consists of adding a small amount of a diatomic molecule, very often N 2 , to the plasma and on the measurement of its rotational temperature.…”

On the heterogeneous character of the heartbeat instability in complex (dusty) plasmas

“…21,22 The latter is equal to the neutral gas temperature. To perform this measurement, the laser beam is attenuated to the power of a few lW and its wavelength is scanned in the vicinity of the transition center.…”

Global (volume-averaged) model of inductively coupled chlorine plasma: Influence of Cl wall recombination and external heating on continuous and pulse-modulated plasmas

“…The gas temperature identifies the flow-out rate. If it is not properly defined in the pulse off region, the mass flow drains excessive amount of species from the chamber with the pulse on value of 800 K. Regarding this, we estimate a cooling time-scale about 20 ms that is based on the observations by Cunge et al [61,62]. In this respect, we assume that the gas temperature drops to the ambient temperature in 20 ms.…”