David R. Boris

Controlling the electron energy distribution function of electron beam generated plasmas with molecular gas concentration: II. Numerical modeling

In this work, the second in a series of two, a spatially averaged model of an electron beam generated Ar?N2 plasma is developed to identify the processes behind the measured influence of trace amounts of N2 on the development of the electron energy distribution function. The model is based on the numerical solution of the electron Boltzmann equation self-consistently coupled to a set of rate balance equations for electrons, argon and nitrogen species. Like the experiments, the calculations cover only the low-energy portion (<50?eV) of the electron energy distribution, and therefore a source term is added to the Boltzmann equation to represent ionization by the beam. Similarly, terms representing ambipolar diffusion along and across the magnetic field are added to allow for particle loss and electrostatic cooling from the ambipolar electric field. This work focuses on the changes introduced by adding a small admixture of nitrogen to an argon background. The model predictions for the electron energy distribution function, electron density and temperature are in good agreement with the experimentally measured data reported in part I, where it was found that the electron and ion energy distributions can be controlled by adjusting the fraction of nitrogen in the gas composition.

Controlling the electron energy distribution function of electron beam generated plasmas with molecular gas concentration: I. Experimental results

This paper, the first in a series of two, presents experimental results demonstrating the control of electron and ion energy distribution functions in electron beam generated processing plasmas by adding trace concentrations of N2 to an Ar background. Measurements of the electron energy distribution function, f0(E), are performed using a Langmuir probe while measurements of the Ar ion energy distribution function are performed using an energy-resolved mass spectrometer. The experimental results agree with modeling results, described in part II of this work, which indicate that inelastic electron collisions with nitrogen molecules provide an energy sink that can be exploited to control the electron energy distribution function.

Electron beam-generated Ar/N2 plasmas: The effect of nitrogen addition on the brightest argon emission lines

The effect of nitrogen addition on the emission intensities of the brightest argon lines produced in a low pressure argon/nitrogen electron beam-generated plasmas is characterized using optical emission spectroscopy. In particular, a decrease in the intensities of the 811.5 nm and 763.5 nm lines is observed, while the intensity of the 750.4 nm line remains unchanged as nitrogen is added. To explain this phenomenon, a non-equilibrium collisional-radiative model is developed and used to compute the population of argon excited states and line intensities as a function of gas composition. The results show that the addition of nitrogen to argon modifies the electron energy distribution function, reduces the electron temperature, and depopulates Ar metastables in exchange reactions with electrons and N2 molecules, all of which lead to changes in argon excited states population and thus the emission originating from the Ar 4p levels.

Initial evaluation and comparison of plasma damage to atomic layer carbon materials using conventional and low Te plasma sources

The ability to achieve atomic layer precision is the utmost goal in the implementation of atomic layer etch technology. Carbon-based materials such as carbon nanotubes (CNTs) and graphene are single atomic layers of carbon with unique properties and, as such, represent the ultimate candidates to study the ability to process with atomic layer precision and assess impact of plasma damage to atomic layer materials. In this work, the authors use these materials to evaluate the atomic layer processing capabilities of electron beam generated plasmas. First, the authors evaluate damage to semiconducting CNTs when exposed to beam-generated plasmas and compare these results against the results using typical plasma used in semiconductor processing. The authors find that the beam generated plasma resulted in significantly lower current degradation in comparison to typical plasmas. Next, the authors evaluated the use of electron beam generated plasmas to process graphene-based devices by functionalizing graphene with...

Measuring the electron density, temperature, and electronegativity in electron beam-generated plasmas produced in argon/SF 6 mixtures

This paper presents measurements of electron density (ne0), electron temperature (Te), and electronegativity (α) in electron beam-generated plasmas produced in mixtures of argon and SF6 using Langmuir probes and plasma resonance spectroscopy. Langmuir probe measurements are analyzed using a model capable of handling multi-component plasmas with both positive and negative ions. Verification of the model is provided through plasma frequency resonance measurements of ne0. The results suggest a simple approach to ascertaining α in negative-ioncontaining plasmas using Langmuir probes alone. In addition, modest amounts of SF6 are shown to produce sharp increases in both Te and α in electron beam generated plasmas.

The influence of magnetic field on electron beam generated plasmas

Magnetically confined argon plasma in a long cylindrical tube driven by an electron beam is studied experimentally and theoretically. Langmuir probes are used to measure the electron energy distribution function, electron density and temperature in plasmas generated by 2 keV, 10 mA electron beams in a 25 mTorr argon background for magnetic field strengths of up to 200 Gauss. The experimental results agree with simulations done using a spatially averaged Boltzmann model adapted to treat an electron beam generated plasma immersed in a constant magnetic field. The confining effect of the magnetic field is studied theoretically using fluid and kinetic approaches. The fluid approach leads to two regimes of operation: weakly and strongly magnetized. The former is similar to the magnetic field-free case, while in the latter the ambipolar diffusion coefficient and electron density depend quadratically on the magnetic field strength. Finally, a more rigorous kinetic treatment, which accounts for the impact of the magnetic field over the whole distribution of electrons, is used for accurate description of the plasma.

One-dimensional Ar-SF6 hydromodel at low-pressure in e-beam generated plasmas

A one-dimensional steady-state hydrodynamic model of electron beam generated plasmas produced in Ar-SF6 mixtures at low pressure in a constant magnetic field was developed. Simulations were performed for a range of SF6 partial pressures at constant 30 mTorr total gas pressure to determine the spatial distribution of species densities and fluxes. With the addition of small amount of SF6 (∼1%), the confining electrostatic field sharply decreases with respect to the pure argon case. This effect is due to the applied magnetic field inhibiting electron diffusion. The hallmark of electronegative discharge plasmas, positive ion—negative ion core and positive ion—electron edge, was not observed. Instead, a plasma with large electronegativity (∼100) is formed throughout the volume, and only a small fraction (≈30%) of the parent SF6 molecules were dissociated to F2, SF2, and SF4. Importantly, F radical densities were found to be very low, on the order of the ion density. Model predictions for the electron density, ...

Atomic fluorine densities in electron beam generated plasmas: A high ion to radical ratio source for etching with atomic level precision

Electron beam generated plasmas are generally characterized by a high plasma density (>1010 cm−3), and very low electron temperatures (<1 eV), making them well-suited for next generation processing techniques where high fluxes of low energy ions are desirable. In addition, both modeling and optical emission spectroscopy indicate relatively low concentrations of atomic radicals compared to discharges. Due to their relevance to industrial etching applications, this work focuses on the characteristics of electron beam generated plasmas produced in fluorine-containing chemistries (SF6, CF4, F2), with particular attention paid to atomic fluorine densities. Atomic F* emission is measured in Ar/SF6, Ar/CF4, and Ar/F2 mixtures and the Ar 750 nm/F 704 nm line ratios are then used to calculate the F atom densities as a function of reactive gas concentration, the first radical density measurement in this type of plasma to date. These results are compared with F atom density calculations performed using a zero dimens...

A comparative study of dc biased RF impedance probes and Langmuir probes for the measurement of processing plasma parameters

In this work, we compare measurements of electron density (ne), plasma potential (p), electron temperature (Te), and electron energy distribution function (EEDF or f(E)) made with a dc biased RF probe and a Langmuir probe. The measurements show good agreement between the two probes across an order of magnitude in plasma density.

Non-equilibrium steady-state kinetics of He-air atmospheric pressure plasmas

A non-equilibrium, steady-state collisional-radiative kinetics model is developed to study atmospheric pressure discharges produced in He mixed with dry air (79% N2 and 21% O2). The model is based on a self-consistent solution of the Boltzmann equation for the electron energy distribution function coupled to a system of non-linear equations for species that govern plasma chemistry (electrons, ions, radicals, atoms, and molecules in ground and excited states). The main plasma parameters, including the maintaining electric field and species densities, are obtained as a function of He-to-air ratio. The results indicate that the concentration of air strongly influences the plasma. Notably, the He metastables and ion densities collapse at air concentrations above 0.1%, while the power required to maintain the plasma sharply increases as the concentration of air exceeds 1%. The model is applied to study the plasma characteristics along the length of an atmospheric pressure plasma jet using He as a carrier gas.