lecture_15_chex42

lecture_15_chex42 - Plasmas Plasmas ! Plasma is a...

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Plasmas Plasmas ! Plasma is a nonequilibrium gas consisting of ions, electrons and neutral species such as radicals, stable molecules and atoms and molecular fragments. ! Plasma is produced by various means of electromagnetic excitation of a low pressure (0.001-10 Torr) gas. ! Typically, reactive gases such as Cl 2 , CF 4 , O 2 , SiH 4 flow through the plasma to generate radicals such as Cl, F, SiH x , O etc to either deposit or etch films. ! Nonequilibrium environment created by the plasma is advantegeous for materials processing because it provides new reaction pathways through which chemical systems can evolve. “High temperature chemistry at low temperatures.”
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Generation of a DC plasma Generation of a DC plasma ! The simplest way to create a plasma (also called electrical discharge) is to apply a potential between two metal electrodes separated by distance d under vacuum d 0 -V a d V E dx dE E a = = = 0 0 r ! Before breakdown the system looks like a capacitor. After breakdown it will behave like a resistor. ! As Va is increased starting from 0, at some critical value we will see the gas glow and plasma will be ignited.
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Generation of a DC plasma Generation of a DC plasma ! The plasma is created because of an autocatalytic reaction between the electrons in the gas and the gas atoms ! The free electrons in the gas must be accelerated to an energy at least equal to the ionization energy, ε i of the gas atoms (10-20 eV) ! Consider an electron in between the plates in electric field E. Starting at x=0 with initial velocity u=0 + + → + e Ar Ar e 2 e e e e m eEt u m eEt u eE dt x d m dt du m 2 2 2 2 = = = =
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Generation of a DC plasma Generation of a DC plasma ! Let λ mf be the mean free path between collisions of e - with a neutral atom ! The energy gained between collisions is ! Where τ mf is the mean free flight time ! Distance traveled during this time is the mean free path kT P N where N mf = = σ λ 1 2 2 2 2 2 2 1 2 1 2 1 mf e mf e e e m E e m eE m u m τ ε = = = mf mf mf e mf eE m m eE 2 2 2 2 = =
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Generation of a DC plasma Generation of a DC plasma ! Thus the energy gained by traveling in constant field for λ mf is ! The energy gained must be higher than the ionization energy to cause breakdown mf eE λ ε= i mf a Nd eV d V e ε σ > = = 1 Nd ~ Pd V plasma No plasma ! If electrons make it to the other electrode without colliding i.e., d~ λ mf we will not get a plasma ! Nd~1/ σ 1/ σ Paschen curve
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Paschen Paschen Curve Curve
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Energy gain mechanisms Energy gain mechanisms Work done on the electron by the field = energy gain from the field e mf e m E e eEx Fdx W mf 2 2 2 2 0 τ λ = = = Work done on an ion by the field = energy gain from the field + + = = = m E e eEx Fdx W mf mf 2 2 2 2 0 Since m e <<m + almost all of the energy is transferred to the electrons W e >>W +
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Energy loss mechanisms from the electrons Energy loss mechanisms from the electrons Elastic collisions: Kinetic energy is conserved and no changes in the internal energy of the atom m 2 m 1 Momentum conservation f f i u m u m u m 2 2 1 1 1 1 + = 2
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This note was uploaded on 12/29/2011 for the course CHE 142 taught by Professor Ceweb during the Fall '09 term at UCSB.

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lecture_15_chex42 - Plasmas Plasmas ! Plasma is a...

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