Vincent M. Donnelly

The reaction of fluorine atoms with silicon

Fluorine atoms etch silicon with a rate, RF(Si) = 2.91±0.20×10−12T1/2nFe−0.108 eV/kT A/min, where nF (cm−3) is the atom concentration. This etching is accompanied by a chemiluminescent continuum in the gas phase which exhibits the same activation energy. These phenomena are described by the kinetics: (1) F(g)+Sisurf→SiF2(g), (2) SiF2(g) +F(g) →SiF*3(g), (3) SiF2(g) +F2(g) →SiF*3(g) +F(g), (4) SiF*3(g) →SiF3(g) +hνcontinuum where formation of SiF2 is the rate‐limiting step. A detailed model of silicon gasification is presented which accounts for the low atomic fluorine reaction probability (0.00168 at room temperature) and formation of SiF2 as a direct product. Previously reported etch rates of SiO2 by atomic fluorine are high by a constant factor. The etch rate of SiO2 is RF(SiO2) = (6.14±0.49)×10−13nF T1/2e−0.163/kT A/min and the ratio of Si to SiO2 etching by F atoms is (4.74±0.49)e−0.055/kT.

The design of plasma etchants

Theory and practice of plasma etching are critically reviewed. Some unifying principles are extended to explain a large body of experimental data, encompassing more than 20 substrate materials in dozens of etchant gas mixtures. These basic concepts can be used to select new etchants and plasma etching parameters.

Plasma etching: Yesterday, today, and tomorrow

The field of plasma etching is reviewed. Plasma etching, a revolutionary extension of the technique of physical sputtering, was introduced to integrated circuit manufacturing as early as the mid 1960s and more widely in the early 1970s, in an effort to reduce liquid waste disposal in manufacturing and achieve selectivities that were difficult to obtain with wet chemistry. Quickly, the ability to anisotropically etch silicon, aluminum, and silicon dioxide in plasmas became the breakthrough that allowed the features in integrated circuits to continue to shrink over the next 40 years. Some of this early history is reviewed, and a discussion of the evolution in plasma reactor design is included. Some basic principles related to plasma etching such as evaporation rates and Langmuir–Hinshelwood adsorption are introduced. Etching mechanisms of selected materials, silicon, silicon dioxide, and low dielectric-constant materials are discussed in detail. A detailed treatment is presented of applications in current s...

Comparison of XeF2 and F‐atom reactions with Si and SiO2

Silicon gasification by XeF2 is compared with F‐atom etching under conditions typical of those used in plasma etching. Temperatures ranged from −17 to 360 °C and XeF2 pressures were between 0.05 and 2 Torr. Silicon etching by XeF2 shows a sharply different etch rate/temperature dependence than the Si/F or Si/F2 reaction systems; there is no detectable reaction between XeF2 and SiO2 in contrast to the F‐atom/SiO2 system. These data indicate that physisorption can limit silicon etching by XeF2 and show that basic studies which use XeF2 as a model compound for the etching of silicon and SiO2 by F atoms should be interpreted with caution.

Anisotropic etching of SiO2 in low‐frequency CF4/O2 and NF3/Ar plasmas

Anisotropic etching of SiO2 films is reported in low frequency (∼100 kHz), moderate‐pressure (0.35 Torr) CF4/O2 and NF3/Ar plasmas. Rates up to 2000 A/min were achieved with high selectivity over GaAs and InP substrates. The etching mechanism was studied with optical spectroscopy and downstream chemical titrations. Anisotropy is attributed to ion‐enhanced reactivity of fluorine atoms with SiO2 at rates up to two hundred times larger than purely chemical etching by fluorine atoms. Damage and product sputter desorption models of this process were evaluated. These two models are nearly mathematically equivalent at steady state, and show that the effectiveness of ions in etching by enhanced reaction is roughly 15 times that in physical sputtering under these conditions.

Diagnostics of inductively coupled chlorine plasmas: Measurement of Cl2 and Cl number densities

The absolute densities of positive ions (Cl2+ and Cl+) are obtained over a 2–20 mTorr pressure range and 5–1000 W input radio-frequency rf power range in a transformer-coupled Cl2 plasma. The relative number density of Cl2+ is measured by laser-induced fluorescence. These laser-induced fluorescence data are calibrated by Langmuir-probe measurements of total positive-ion density at low powers to yield absolute values for nCl2+ and are corrected for changes in rotational temperature with rf power. In turn, the nCl2+ data are used to determine the effective-mass correction for refined Langmuir-probe measurements of the total positive-ion density. The density of Cl+ is then the difference between the total positive-ion and Cl2+ densities. For all the pressures, Cl2+ is found to be the dominant ion in the capacitively coupled regime (input powers below 100 W), while Cl+ is the dominant ion at higher powers (>300 W) of the inductively coupled regime. Experimental results are compared to those from a simple glob...

Basic chemistry and mechanisms of plasma etching

A recent review of plasma etching is extended with discussions of similarity variables governing discharges, anisotropic oxide etching in fluorine and unsaturate‐rich plasmas, surface texture, the loading effect, and gas‐surface reactions.

Plasma electron temperatures and electron energy distributions measured by trace rare gases optical emission spectroscopy

This article reviews a spectroscopic method for extracting plasma electron temperatures and electron energy distributions: trace rare gases optical emission spectroscopy. Specifically, traces of Ne, Ar, Kr, and Xe are added to the plasma and the intensities of emissions from the Paschen 2p levels are recorded. Intensities are also computed from a model that includes direct excitation from the ground state, as well as two-step excitation through the 3P2, and 3P0 metastable levels. A Maxwellian electron energy distribution function (EEDF), described by an electron temperature (Te), is assumed, and Te is extracted from the best match between the observed and calculated relative emission intensities. By choosing emission from specific sets of levels, the range of electron energies effective in exciting emission can be selected and various portions of the EEDF can be investigated. Accurate measurement of Te depends critically on accurate cross sections for electron impact excitation, and hence a large portion of this article is devoted to a critical review of this subject. Improving on previous treatments, the model for computing emission intensities and electron temperatures includes a complete analysis of the complex excitation and de-excitation of the metastable levels. Previous measurements of Te and EEDFs in chlorine and oxygen inductively coupled plasmas are re-evaluated with the current model. In general, the current version of the model yields similar results; specific differences are discussed.

Simulation of a direct current microplasma discharge in helium at atmospheric pressure

A numerical simulation of a dc microplasma discharge in helium at atmospheric pressure was performed based on a one-dimensional fluid model. The microdischarge was found to resemble a macroscopic low pressure dc glow discharge in many respects. The simulation predicted the existence of electric field reversals in the negative glow under operating conditions that favor a high electron diffusion flux emanating from the cathode sheath. The electric field adjusts to satisfy continuity of the total current. Also, the electric field in the anode layer is self adjusted to be positive or negative to satisfy the “global” particle balance in the plasma. Gas heating was found to play an important role in shaping the electric field profiles both in the negative glow and the anode layer. Basic plasma properties such as electron temperature, electron density, gas temperature, and electric field were studied. Simulation results were in good agreement with experimental observations.

Profiling nitrogen in ultrathin silicon oxynitrides with angle-resolved x-ray photoelectron spectroscopy

Angle-resolved x-ray photoelectron spectroscopy (AR–XPS) is utilized in this work to accurately and nondestructively determine the nitrogen concentration and profile in ultrathin SiOxNy films. With furnace growth at 800–850 °C using nitric oxide (NO) and oxygen, 1013–1015 cm−2 of nitrogen is incorporated in the ultrathin (⩽4 nm) oxide films. Additional nitrogen can be incorporated by low energy ion (15N2) implantation. The nitrogen profile and nitrogen chemical bonding states are analyzed as a function of the depth to understand the distribution of nitrogen incorporation during the SiOxNy thermal growth process. AR–XPS is shown to yield accurate nitrogen profiles that agree well with both medium energy ion scattering and secondary ion mass spectrometry analysis. Preferential nitrogen accumulation near the SiOxNy/Si interface is observed with a NO annealing, and nitrogen is shown to bond to both silicon and oxygen in multiple distinct chemical states, whose thermal stability bears implications on the relia...