Plasma Etching

1
Plasma Etching

• Josh Gentle
• Matthew Grund
• ELEC 6570

2
Overview

• What is etching?
• Two types of etching
• Application of Plasma in etching
• Process
• Mechanisms
• Reactive Ion Etching (RIE) System
• Problems

3
Questions

• What are the three types of grass like cones and
  ways to stop the forming of cones?
• What are two benefits of the laser inferometer
  has compared to the optical emission
  spectrometer?

4
Introduction

• What is etching?
• the removal of material from the surface by
  physical and or chemical process
• Two classes of etching processes
• Wet etching is where the material is dissolved
  when immersed in a chemical solution
• Dry etching is where the material is sputtered or
  dissolved using reactive ions or a vapor phase
  etchant

5
Types of Etching

• Anisotropic Etching
• means different etch rates in different
  directions in the material.

• Isotropic (wet) Etching
• means uniform etch rates in all directions in the
  material.

Figure 1 Difference between anisotropic and
isotropic wet etching.
http//www.memsnet.org/mems/beginner/etch.html
6
Comparison

• Review of wet etching characteristics
• Wet etching is limited to pattern sizes of 3
  microns or less.
• Wet etching is isotropic (equally in all
  directions), can result in sloped walls.
• Wet etching requires rinse and dry steps.
• Wet etching has an intrinsic contamination
  potential.
• Wet etching can undercut if the resist-wafer bond
  fails.
• Review of dry etching characteristics
• Dry etching has lower cost of chemicals
• Dry etching has lower impact on the environment
• Dry etching has higher cost of equipment
• Dry etching has variable selectivity

http//planet.tvi.cc/fabian/newpage7.htm
7
Application of Plasma in Etching

• Four step process
• Formation of active gas species
• Transport of the active species to the surface
• Reaction at the surface
• Pump out the reaction products

http//www.gs68.de/tutorials/plasma/node12.html
8
Mechanisms of Plasma Etching

• Sputter Etching
• Ions created by the plasma and propelled by the
  sheath potentials to knock loose particles
• Pressure has to be low

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Mechanisms of Plasma Etching

• Chemical Etching
• Plasma generates neutral species
• Neutral species spontaneously react with
  substrate material

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Mechanisms of Plasma Etching

• Accelerated Ion-assisted Etching
• Ion are created by the plasma and accelerated by
  the sheath potentials which damage the surface
• Neutral etchant are now able to react more easily
  with surface

http//web.ask.com/redir?bpghttp3a2f2fweb.ask.
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11
Application of Plasma in Etching

• Reactive Ion Etching (RIE)

Typical parallel-plate reactive ion etching
system
http//www.memsnet.org/mems/beginner/etch.html
12
Aluminium and Tungsten Metal Etching

• Most popular materials used for interconnnection
• Chlorine is used for etching and bromine is being
  investigated
• Tungsten has excellent deposition conformability

http//web.ask.com/redir?bpghttp3a2f2fweb.ask.
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1qplasmaetchinguhttp3a2f2ftm.wc.ask.com2f
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13
Polymer Etching

• Cheaper Fabrication
• Lower Processing Temperatures
• Good vertical profile mask selectivity is
  important
• Hydrogen generally used for etching
• UHF-ECR etchers

http//www.oxinst.com/pdf/PolymerEtchArticle.pdf
Katasuya Watanabe, Kenetsu Yokogawa, Yutaka
Omoto, Hiroyuki Makino, UHF-ECR Plasma Etching
System for Dielectric Films of Next-generation
Semiconductor Devices, Hitachi Review 166-170,
Vol.52 (2003).
14
Ashing

• Requirements
• Ash rate 1-2um/min
• Remove resist without contamination
• High Temperature Oxygen Process
• Uses pure
• C
• Infinite Selectivity to Silicon
• Low Temperature Process with Fluorine
• Carried out at room temperature
• Inorganic materials can be removed
• Low k material ashing

http//www.gs68.de/tutorials/plasma/node19.html
15
Selectivity

• Is the ability to etch one material and refrain
  from etching another
• Crucial for avoiding line width loss
• depends on both chemistry and charged particle
  bombardment
• most demanding in etching gates for MOSFETs

Commission on Physical Sciences, Mathematics, and
Applications, Plasma Processing of Materials
Scientific Opportunities and Technological
Challenges, 13-36, (1991)
16
End Point Detection

• Laser interferometer
• In-situ etch rate monitoring
• Endpoint does not require etch stop layer
• Endpoint can be chosen anywhere within the layer
  (onnce etch rate has been established)
• Optical Emission
• Monitoring of reactive species or etch
  by-products provides endpoint signal.
• Endpoint relies on etch stop layer.
• Scanned monochromator allows full spectrum
  analysis.

http//www.oxfordplasma.de/technols/rie.htm
17
Uniformity

• Uniformity is the preservation of both the
  profile and critical dimension (CD) of the etch
  across the entire wafer
• Reaction by-products are a cause of
  Irregularities in the etch
• To achieve uniformity
• Uniform plasma and etching gas distribution in
  the etching chamber
• uniform distribution of etching by-products on
  the wafer.
• Little or no charge damage must occur

Shinji Kawamura, Naoshi Itabashi, Akitaka Makino,
Masamichi Sakaguchi., UHF-ECR Plasma Etching
System for Gate Electrode Processing, Hitachi
Review Vol. 51 (2002), No. 4
18
Deposits

• (a) Trapezoidally shaped deposits
• (b) Horns
• (c) Bowed Walls
• (d) Rounded Bottoms

http//users.ece.gatech.edu/alan/Lecture11-Etchin
g20especially20Plasma20Etching.pdf
19
Grass

• Formation of Cones
• Several Types
• Aluminum
• Sputtered
• Polymer
• Solutions
• Gold Coating
• Eliminating O2
• Reducing Gas Pressure

Aluminum Grass
Sputtered Grass
Polymer Grass
http//www.triontech.com/techPapers/RIEGrass.html
20
Trenching

• Etch attack at the edge of the gate
• Usually observed at low gas pressure
• Solved by increasing etch selectivity

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21
Answers to Questions

• Three types of grass and solutions.
• Aluminum- eliminating O2
• Sputtered- gold coating
• Polymer- Reducing gas pressure
• Two benefits Laser interferometer has compared to
  the Optical emission spectrometer.
• Does not require etch stop layer
• Endpoint can be chosen anywhere within the layer