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Plasma Etching - Thin Film Materials Processing - Lecture Slides, Slides of Material Engineering

These are the Lecture Slides of Thin Film Materials Processing which includes Vaporization, Vapor Pressure Curves, Thermal Desorption, Molecular Binding Energy, First Order Desorption, Desorption Rate, Real Surfaces, Diffusion of Gas Particles etc. Key important points are: Plasma Etching, Anisotropic Etch, Ideal Etching Process, Directionality of Etching, Wet Etching, Wet Etching Problems, Wet Etching Characteristics, Wet Etch Chemistries, Dry Etching Characteristics

Typology: Slides

2012/2013

Uploaded on 03/21/2013

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Plasma Etching
Outline
Plasma vs. Wet Etching
The Plasma State
-Plasma composition, DC & RF Plasma
Plasma Etching Principles and Processes
Equipment
Advanced Plasma Systems
Terminology
Etching - the process by which material is removed from a surface
Mask Layer - Used to protect regions of the wafer surface. Examples are photoresist or an
oxide layer
Wet Etching - substrates are immersed in a reactive solution (etchant). The layer to be
etched is removed by chemical reaction or by dissolution. The reaction products must be
soluble and are carried away by the etchant solution.
Dry Etching - Substrates are immersed in a reactive gas (plasma). The layer to be etched is
removed by chemical reactions and/or physical means (ion bombardment). The reaction
products must be volatile and are carried away in the gas stream.
Anisotropic - etch rate is not equal in all directions.
Isotropic - etch rate is equal in all directions.
Selectivity - the ratio of etch rate of film to etch rate of substrate or mask.
Aspect Ratio - ratio of depth to width of an etched feature.
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Plasma Etching

Outline

  • Plasma vs. Wet Etching
  • The Plasma State - Plasma composition, DC & RF Plasma
  • Plasma Etching Principles and Processes
  • Equipment
  • Advanced Plasma Systems

Terminology

Etching - the process by which material is removed from a surface

Mask Layer - Used to protect regions of the wafer surface. Examples are photoresist or an oxide layer

Wet Etching - substrates are immersed in a reactive solution (etchant). The layer to be etched is removed by chemical reaction or by dissolution. The reaction products must be soluble and are carried away by the etchant solution.

Dry Etching - Substrates are immersed in a reactive gas (plasma). The layer to be etched is removed by chemical reactions and/or physical means (ion bombardment). The reaction products must be volatile and are carried away in the gas stream.

Anisotropic - etch rate is not equal in all directions.

Isotropic - etch rate is equal in all directions.

Selectivity - the ratio of etch rate of film to etch rate of substrate or mask.

Aspect Ratio - ratio of depth to width of an etched feature.

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Why Plasma Etching?

Advanced IC Fabrication with small geometries requires precise pattern transfer

Geometry in the < 1.0 micrometer range is common

Line widths comparable to film thickness

Some applications require high aspect ratio

Some materials wet etch with difficulty

Disposal of wastes is less costly

Anisotropic Etch, High Aspect Ratio

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Wet Etching

Probe with^ Thermometer glass cover

Wafer Boat

Teflon Stirrer & Guide Plate

Wafers

Controller

Hot Plate

Holes

Plastic Screw for Handle

Teflon Cover

50°C

Generally Wet Etching is Isotropic

Wet Etching Problems

poor adhesion resist

After Isotropic Etch

Before Etch

substrate

resist

isotropic Blocking undercut

Mask

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Wet Etching Characteristics

  • Advantages:
    • Simple equipment
    • High throughput (batch process)
    • High selectivity
  • Disadvantages:
    • Isotropic etching leads to undercutting
    • Uses relatively large quantities of etch chemicals, must immerse wafer boats, must discard partially used etch to maintain etch rate
    • Hot chemicals create photoresist adhesion problems
    • Small geometries difficult, line with > thickness, etch block caused by surface tension
    • Critical Etch time, dimensions change with etch time, bias develops
    • Chemical costs are high
    • Disposal costs are high

Wet Etch Chemistries

  • Silicon (Nitric Acid and Hydrofluoric Acid and water)
    • Si +HNO3 + H2O --> SiO2 + HNO2 +H2 (+6HF) --> H2SiF6 + HNO2 + 2H2O + H
  • SiO2 (HF Water and NH4F)
    • SiO2 + 6HF --> H2 +SiF6 +2H2O
  • Si3N4 (Dilute Hot Phosphoric (180C) H3PO4)
  • Al (HPO4) +HNO2 +Acedic CH3COOH + H2O
    • Nitric oxidizes Al --> Al2O3 and HPO4 dissolves Al2O

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Dry Etching Methods

  • Dry Etching -- uses gas reactant species to etch film
    • Plasma Etching -- typically high pressure, no ion bombardment (substrate placed on grounded electrode)
    • Reactive ion etching -- typically lower pressures, ion bombardment (substrate placed on powered electrode
    • Ion beam methods -- plasma is generated in a separate chamber and ions are accelerated towards the substrate (independently control flux of radicals and ions)
    • Beam methods -- plasma is generated in a separate chamber and mainly neutrals active species (radicals) are directed towards substrate.

Dry Ethcing Nomenclature

Barrel Sputter Etching RIE Parallel Plate Plasma Etching

Glow Discharge Methods Downstream Etching ECR Etching

"Beam" Methods

Ion Milling Reactive Ion Beam Etching Ion Beam Assisted Chemical Etching

Ion Beam Methods

Dry Etching

Pressure Range

0.2-2.0 Torr 0.01-0.2 Torr 0.1-1.0 Torr 10 -3-10-4^ Torr 10 -4-10-1^ Torr

Ion Energy

low high minimal High but adjustable

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Definitions

  • Plasma - partially ionized gas containing an equal number of positive and negative charges, as well as some other number of none ionized gas particles
  • Glow discharge - globally neutral, but contains regions of net positive and negative charge
  • Most thin film processes utilize glow discharges, but “plasmas” and “glow discharges” are often used interchangeably

DC Glow Discharge

  • Before application of the potential, gas molecules are electrically neutral and the gas at room temperature will contain very few if any charged particles. Occasionally however, a free electron may be released from a molecule by the interaction of, for example, a cosmic ray or other natural radiation, a photon, or a random high energy collision with another particle.

0V

A → A+^ + e -

hv

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DC Glow Discharge

  • Newly produced electrons are accelerated toward the anode and the process cascades (Breakdown).
100V

e -^ A + e - e -

A +

e - e - A + e - e -

Ae +^

e - Ae +^

e - Ae +^

e - Ae +^

e -

Cathode Anode

DC Glow Discharge

  • With sufficient voltage, the gas rapidly becomes filled with positive and negative particles throughout its volume, i.e. it becomes ionized.
100V

Cathode Anode

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DC Glow Discharge

  • Positive ions are accelerated toward the negative electrode (cathode). Collision with the cathode causes the emission of secondary electrons which are emitted from the cathode into the plasma.
100V
A +

e -

Cathode Anode

Secondary Electron Coefficient

  • Secondary Electron Coefficient ( δ ) vs Incident Electron Energy - Secondary Electron Coefficient ( γ i ) vs Incident Ion Energy

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DC Glow Discharge Regions

  • The glow discharge, overall, must always remain neutral, although portions of it may be charged negatively or positively.
  • Glow Discharge Regions
    • 1 -- Cathode Dark Space (Crooke’s Dark Space)
    • 2 -- Negative Glow
    • 3 -- Faraday Dark Space
    • 4 -- Positive Column
    • 5 -- Anode Dark Space
100V

Cathode Anode

Current and Potential Distributions in a DC Glow Discharge

100V

Cathode Anode

V

~10V

Most of the voltage drop is across the cathode dark space

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DC versus RF Plasmas

  • Insulating materials will not sustain a plasma
    • Ion current charges the insulator positively and ultimately extinguishes the plasma (ie. Can not bleed off charge)
  • Use rf power to deposit insulating materials

RF Plasma

  • At frequencies > 100kHz electrons respond and ions do not
    • Typical rf frequency - 13.56 MHz (designated by FCC)
  • High mobility of electrons causes a dc “self bias” to develop on target after the first ac cycles(~1/2 rf peak-to-peak)

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Magnetrons

  • Magnetic fields change trajectory of electrons in a magnetic field
    • Imposing a magnetic field effectively increases the distance an electron travels, this in turn increases the ionization rate (and subsequently the sputtering rate)

Various Magnetron Configurations

  • Planar Magetron
  • Enhanced rate in high ion region

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Various Magnetron Configurations

  • S-gun

Collision Processes

High Energy Electrons Collide with Gas Molecules and Create New Species

Ionization - An electron is completely removed from a gas molecule or atom, to make a positive ion. (Negative ions are quite rare in plasmas.)

Electronic Excitation - This accounts for the glow. When electrons collide with atoms or molecules, they excite or energize electrons to higher energy levels. When these electrons fall back to lower levels, they emit energy usually in the form of photons of visible light.

Molecular Fragmentation - Electron collisions break up molecules into fragments which as a result have unsatisfied chemical bonding and are chemically reactive. These are called radicals.

Radicals - have no net charge, and therefore are not accelerated by the field or are not attracted by charged particles. They have a long lifetime compared to charged particles.

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Loss Mechanisms

In a plasma, unstable particles are continuously generated. The concentrations of ions, radicals, active atoms, & electrons increase until their loss rate is equal to the generation rate, forming a steady-state plasma.

Recombination of ions and electrons: They attract each other and are annihilated.

Drift, diffusion to walls : Electrons are lost at conductive surfaces, chamber walls or electrodes. Ions are lost (converted to neutral particles) by contact with conductive surfaces, especially positive electrode.

Recombination of radicals: 2O => O 2

Chemical reaction: 4F + Si => SiF4 (Fluorine radical combines with silicon wafer to produce silicon tetrafluoride gas. This is a typical dry etching process.)

Ion Bombardment at Surfaces

CF

- -

+ CF 3

+ ion

  • Electron

Electron collisions

create ions

Negatively charged surface

F Fluorine radical

F

F

F

F

Ions bombard

surfaces

Radicals react

with surfaces

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GEC Cell

SF

CF

CHF

O

H

Dry Etching Spectrum

Physical (Sputtering) Directional Poor Selectivity Radiation Damage Possible

Reactive Ion Etching Physical and Chemical Variable Anisotropy Variable Selectivity

Chemical Plasma Etching Fast, Isotropic High Selectivity Low radiation Damage

Pressure Ion Energy

Low

<50 mTorr

100 mTorr

High

Low

400 mTorr

High

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