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PLASMA AIDED DIELECTRIC ETCHING. Guoyun Tian Electrical and Computer Engineering Auburn University February 21, 2001. Outline. Introduction Components for Dielectric Etching Evaluation of Etching Processes Etch Gases Highly Anisotropic Etching Isotropic and Anisotropic Etching.

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plasma aided dielectric etching
PLASMA AIDED DIELECTRIC ETCHING

Guoyun Tian

Electrical and Computer Engineering

Auburn University

February 21, 2001

outline
Outline
  • Introduction
  • Components for Dielectric Etching
  • Evaluation of Etching Processes
  • Etch Gases
  • Highly Anisotropic Etching
  • Isotropic and Anisotropic Etching
introduction
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.
slide4
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

“Beam methods”

Ion beam methods

Glow discharge methods

Down stream etching

ECR etching

Ion milling

Reactive ion beam 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 10-4-10-1Torr

Energy of ion bombard

Low High Minimal High, but adjustable

components of dielectric etching
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:
evaluation of etching processes
Evaluation of Etching Processes
  • Uniformity:
  • Selectivity:
  • Etching Rate:
uniformity
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)
  • Bull’s 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
selectivity
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.
etching rate
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.
etching gas systems
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 (y<=2n-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

highly anisotropic etching
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

etching equipment
Etching Equipment

Wafer carrier

Electrode

Process chamber

Etchant:

Fluorine source C2F6, NF3, CHF3, and SF6with 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

cyclic high fluorocarbon system to form contact hole in sio 2
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 selectivity:4 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.

selective etching of oxide over nitride

Third electrode

Selective Etching of Oxide over Nitride

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

combination of isotropic and anisotropic etching
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)

continued
Continued

PatternPR

Dielectric

Substrate

Underlayer(Polisilicon, diffusion or conductive layer)

The low L/V ratios and  angles don’t 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 90°C.

Passivating deposit

preferred equipment for etching special feature shape
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,

processes for fabricating complex sidewalls
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.