PLASMA AIDED DIELECTRIC ETCHING

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PLASMA AIDED DIELECTRIC ETCHING

• Guoyun Tian
• Electrical and Computer Engineering
• Auburn University
• February 21, 2001

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Outline

• Introduction
• Components for Dielectric Etching
• Evaluation of Etching Processes
• Etch Gases
• Highly Anisotropic Etching
• Isotropic and Anisotropic Etching

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Introduction

• Plasma etching is a dry etching method.
• It overcomes problems such as contamination,
  waste disposal caused by wet chemical etching.
• It can result in highly anisotropic etching.
• A desired etch feature can be achieved by a
  combination of isotropic and anisotropic etching.
• High etch rate and selectivity can be achieved by
  manipulating the etch gas mixtures.

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Categories of Dry Etching Methods( John L.
Vossen, Aerner Kern, Thin Film Processes II,
Academic Press Inc.)
Laser assisted etching
Reactive gas etching
Dry etching
Ion beam methods
Beam methods
Glow discharge methods
Down stream etching
Ion milling
Reactive ion beam etching
ECR etching
Sputter etching RIE
Barrel
Ion beam assisted chem. etch.
Parallel plate plasma etching
Etching mainly by reactive neutrals

Pressure range 0.2 2 Torr 0.01 - 0.2
Torr 0.1 - 1Torr 10-4-10-3
Torr 10-4-10-1 Torr
Energy of ion
bombard Low High
Minimal
High, but adjustable
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Components of Dielectric Etching

• Power Supply RF and/or MW
• Etch Gases Halogen-containing gases.
  Fluorocarbon gases are preferred
• Dielectric Materials oxides, nitrides, polymers,
  low-k dielectrics etc.
• Chamber Systems

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Evaluation of Etching Processes

• Uniformity
• Selectivity
• Etching Rate

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Uniformity

• Gas Flow location (the relative position of gas
  inlet and pumping port has to be optimized for
  the given reactor geometry) rate (at certain
  minimum level with respect to the desired etch
  rate)
• Bulls Eye Effect corresponding appearance of
  interference colors of reflectivity on an
  incompletely etched wafer because the relative
  reactivity of wafer surface with respect to
  electrode.
• Edge Effect wafer should be placed far away from
  the edges of electrode. Edge effect results in
  non-uniformity by changing sheath thickness as
  well as varying angles of incidence.
• Loading Effect use dummy wafer increase flow
  rate promoting wall reaction

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Selectivity

• Carbon Blocking The increased selectivity is
  mainly due to carbon-containing species
  originating in the fluorocarbon glow discharges
  and accumulating on the surface. Using gas
  mixtures with H2 to form HF to prevent reactions
  with carbonaceous species Using gas mixtures
  with higher C/F ratio Using a scavenger for
  fluorine such as third silicon electrode or
  silicon-containing gas to form volatile
  silicon-fluorine compounds that can be removed.
• Volatility Materials that form volatile
  compounds in a glow discharge etch much faster
  than the materials that are converted to
  involatile compounds, such as etching
  photoresist on Si, SiO2, Al in a O2 plasma.
• Thermodynamics Etch processes with large
  negative free energy of reaction ?H are usually
  much faster, such as Al etches much faster than
  SiO2 in a Cl2 plasma.

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Etching Rate

• Gas Flow Rate Etching rate first increases with
  gas flow rate, reaches maximum, and then
  decreases.
• Gas Additives Add gases such as O2 to react with
  free radicals in the fluorocarbon gas system to
  release fluorine and increase etch rate.

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Etching Gas Systems

• Straight-chain low fluorocarbon gas system such
  as CHF3, CF4/O2, CF4/NF3, or CHF3/CF4/He
• Straight-chain higher fluorocarbon gas system
  such as C2F6, C3F8, C4F10, or C4F8
• Cyclic high fluorocarbon system saturated
  fluorocarbon compound c-CnF2n such as c- C3F6, or
  c- C4F8 and non-saturated fluorocarbon compound
  c-CnFy (ylt2n-2) such as c-C3F3 or c-C4F6. The
  schematic view of c- C3F6 and c-C4F6, F entered
  at the center of a carbon ring indicates that the
  hydrogen atoms of each of the hydrocarbon
  compounds having the same carbon skeletons are
  unanimously substituted by fluorine atoms.

c- C3F6 all bond saturated
c-C4F6 containing one unsaturated bond
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Highly Anisotropic Etching
Silicon Nitride
Patterned Photoresist
Remote and In-situ Plasma Etching
Oxide
After Remote Plasma Etching
Lowenstein, U.S. Patent 4857140(1989) Method for
Etching Silicon Nitride
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Etching Equipment
Wafer carrier
Electrode
Process chamber
Etchant Fluorine source C2F6, NF3, CHF3, and SF6
with He, H2
Gas distributor
Pipe from remote plasma to chamber
Second gas distributor
Remote plasma generator (MW)
Electrode (RF)
H2 bypass can increase selectivity but will
reduce the etch rate to the directly passing
through the generator
In-situ plasma combining with remote plasma
increase etching rate
Vacuum pump
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Cyclic high fluorocarbon system to Form Contact
Hole in SiO2
Yanagida, U.S. Patent 5338399 (1994) Dry Etching
Method
Patterned resist
Dielectric
Substrate
Diffusion layer
c- C4F8 vs C3F8 Resist selectivity 3.5 vs
1.5 Silicon selectivity 7.2 vs 3.9
c-C4F6 vs c- C4F8 Resist selectivity4 vs
3.5 Silicon selectivity 12 vs 7.2 Higher C/F
ratio
The C3F8 and c- C4F8 mixed gas composed mainly
straight chain C3F8 were also used to achieve
high etching rate and high selectivity. RF with 2
MHz frequency and Magnetic strength field about
150 Gauss. The proper gas pressure and flow rate
were used.
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Selective Etching of Oxide over Nitride
Third electrode
Matching network
Gas inlet
Antenna tuned to resonance for inductively
coupling
Top wall
Chamber housing
Plasma source
Side wall(anode)
Substrate processing
Bottom wall
Substrate support electrode (cathode)
Vacuum system
Pressure 5 mtorr 50 mtorr, Etchant CF4, C2F6,
C3F8
Reduce the amount of fluorine in the plasma so
that reducing the decomposition of ploymer to
increase the selectivity
Marks et al. U.S. Patent 5423945
(1995) Selectivity for Etching an Oxide over a
Nitride
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Combination of Isotropic and Anisotropic Etching
Contact hole
Patterned PR
Oxide
Sloped walls
Ploy-crystalline Si
Etchants Isotropic carbon tetrafluoride(CF4),
and/or ammonium trifluoride(NF3) mixed with O2
Anisotriopic carbon tetrafluoride mixed with
trifluoromethane and argon or helium or nitrogen.
As the etching proceeds, the content of fluorine
atoms is adjusted to in favor of the formation of
free radicals and ions with simultaneously
reducing the spacing of electrode.
Generating contact holes with beveled sidewalls
in intermediate oxide layer, PN4764245 (1988)
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Continued
PatternPR
Dielectric
Substrate
Underlayer(Polisilicon, diffusion or conductive
layer)
The low L/V ratios and ? angles dont allow the
uniform filling of the etched features, resulting
in formation of overhangs at the edges and
corners of the etched features. Voids and gaps
can be formed during the following deposition.
Preferred L/V ratio is 0.6-1.4, average ? angles
of less than about 90C.
Passivating deposit
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Preferred Equipment for Etching Special Feature
Shape
Impedance matching to plasma zone
Microwave applicator
Plasma zone
Gas distributor
Process zone
Substrate
Merry et al., U.S. Patent 6015761 (2000)
Microwave-activated etching of dielectric
layers,
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Processes for fabricating complex sidewalls

• Process gas composition and process conditions
  to etch features having particular shape depend
  on the composition of dielectric layer.
• Process gases comprises (i) fluorocarbon gases
  (all those straight chain gases and/or their
  mixtures) for providing fluroride-containing
  dissociated species that etch the dielectric
  layer. (ii) inorganic fluorinated gases (NF3,
  SF6, HF), that enhances dissociation of the
  flurocarbon gas, and/or reduces the recombination
  of dissociated fluorine-containing species during
  transport of plasma.(iii) O2 for controlling the
  the amount of passivating deposits formed on the
  substrate to provide highly isotropic etching.
  Their volumetric flow ratio are primary factors
  in controlling the shape of etched features
• The recombination of dissociated species F to
  non- dissociated F2 results in slower dielectric
  etching rates and reduced the isotropic etching.