The percent dissociation of Cl2 was determined for two configurations of a commercial transformer-coupled plasma (TCP) reactor (LAM Research Alliance metal etcher), using Cl2 and BCl3/Cl2 feed gases, during slow etching of SiO2 covered Si wafers. Emission from Cl2 at 305 nm was recorded as a function of TCP source power, along with emission from 1% Ar and Xe, added as part of an equal mixture of the five rare gases. Absolute Cl2 number densities were determined from the Cl2-to-rare gas emission intensity ratios. The Cl2 percent dissociation increases with power, reaching 70% between 1 and 2 mTorr at the highest power (900 W, 0.080 W/cm3). The percent dissociation decreases with increasing pressure between 1 and 10 mTorr. Decreasing the gap between the TCP window and the wafer chuck from 11 to 6.5 cm decreases dissociation at pressures between 0.5 and 2 mTorr, and increases dissociation slightly at 10 mTorr. The percent dissociation as a function of power, and for the most part as a function of pressure and gap, is reproduced by a zero-dimensional model that includes electron-impact dissociation and dissociative attachment of Cl2, and diffusion-controlled recombination of Cl at the walls. Addition of BCl3 to Cl2 increases the percent dissociation of Cl2, most likely due to a passivation of the chamber walls by adsorbed BClx, lowering the Cl-atom recombination coefficient.
The passivation step used in the “Bosch” process (alternating etching and deposition steps) to perform deep anisotropic silicon etching has been examined in detail. The effect of pressure, inductively coupled plasma power, temperature, flow rate, and bias power on both deposition rate and film composition has been explored over a relatively wide range. Deposition rate was found to vary significantly as a function of temperature, power, and pressure. In contrast, only two film composition regimes were observed: high fluorine-to-carbon ratio (F:C) films (∼1.6) at low pressure∕high power versus low F:C films (∼1.2) at high pressure∕low power. Optical emission spectroscopy of the deposition plasmas also show only two regimes: C2, C3, and F emission dominated (high F:C films) and CF2 emission dominated (low F:C films). A two-step deposition mechanism is assumed: carbon deposition followed by fluorination. Low F concentration and deposition from large fluorine-deficient CxFy species in the CF2-rich plasmas result in the low F:C ratio films. Films deposited during an actual Bosch cycle generally mirror these bulk films, with slight differences. Analysis of etch:deposition rate ratios as a function of film F:C ratio indicates that, for the conditions studied here, a F:C ratio of 1.45 is optimal for Bosch processing (i.e., has the lowest etch:deposition rate ratio). Further analysis is needed to determine the effect of passivant F:C ratio on feature profiles.
Spatially resolved electron temperatures, species concentrations, and electron energy distributions in inductively coupled chlorine plasmas, measured by trace-rare gases optical emission spectroscopy Ultrahigh frequency versus inductively coupled chlorine plasmas: Comparisons of Cl and Cl 2 concentrations and electron temperatures measured by trace rare gases optical emission spectroscopy Recent advances in the interpretation of optical emission spectra from plasmas have made it possible to measure parameters such as electron temperature ͑T e ͒, relative electron density, and gas temperature ͑T g ͒ with this nonintrusive technique. Here we discuss the application of trace rare gas optical emission spectroscopy ͑TRG-OES͒, optical actinometry, and N 2 rotational spectroscopy to determine T e , relative electron density, fluorine atom concentration, and T g for fluorocarbon/Ar plasmas in an inductively coupled reactor. Various etch processes, containing mixtures of a carrier gas, C 2 F 6 , and C 4 F 8 , were evaluated as a function of pressure and flowrate. Ar, Kr, and Ne were used individually or were mixed to comprise the carrier gas. In the case of TRG-OES and optical emission actinometry, a mixture containing equal parts of He, Ne, Ar, Kr, and Xe ͑ϳ1% ea.͒ was added. A method for correcting excitation cross sections is introduced for cases when radiation trapping affects the emission of a rare gas ͑Ar͒ that is present at high concentrations. Experiments revealed that T e can be controlled through the choice of carrier gas: Ne tends to increase T e and Kr tends to decrease T e relative to Ar. This phenomenon was verified qualitatively with a simple zero-dimensional energy balance model. Additional measurements revealed that the absolute atomic fluorine concentration, determined from calibrated F-to-Ar actinometry ratios, is roughly 20% of the total gas at 5 mTorr, and decreases to 5% at 60 mTorr. The gas temperature in the Ar-carrier plasma was measured to be ϳ1200 K and was found to be insensitive to pressure whereas T g in Kr and Ne carrier gas plasmas increased from 1500-1900 K and 700-1500 K, respectively between 5 and 30 mTorr.
Previously published applications of optical emission spectroscopy as a quantitative plasma diagnostic technique are reviewed. By adding traces of rare gases to the plasma, electron temperatures (T e) and relative electron and ion densities can be determined from electron impact-induced optical emission. Excitation from both the ground state and metastable states of the rare gases must be considered. At higher rare gas partial pressures, UV radiation trapping and optical cascading must also be taken into account. Absolute species concentrations (e.g. Cl 2 , Cl, O, and F) can be derived from their optical emissions, combined with T e measurements determined from rare gas optical emission. Examples are given of neutral and ion species density measurements in chlorine, oxygen, and fluorocarbon-containing low-pressure, high charge-density plasmas. Typical results of T e measurements are also presented and compared with Langmuir probe measurements.
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