1. Introduction To Plasma Etch
“Clear cornerstone constructs
can create complicated concepts”
B.J. Moose, 1968
Remember three sets of three’s
If it isn’t an atom, ion or molecule it isn’t of interest to 98% of the people 99% of the time
If it isn’t in the gas, liquid or solid state it isn’t of interest to 90% of the people 91% of the time
If you focus on the nucleus, the protons or the
neutrons, you miss the view 92% of the time.

2. ATOMS Lewis Diagrams are very useful!
One nucleus; number of protons in it gives atom its name
Set of electrons; equal to number of protons; located somewhere outside nucleus
Electrons furthest from nucleus are of primary importance 92.5% of the time
Lewis Diagrams are very useful!
Show the atom’s protons, neutrons
and most of its electrons as a symbol.
Show the atom’s outer most electrons
as dots or circles.
Memorize the Lewis Diagrams of
the first 20 Atoms. oo oo O H N F O O O O oo O
The Hydrogen
Atom
The Nitrogen
Atom
The Fluorine
Atom

3. ATOMS I III IV V VI VII VIII H N F B C O Si Ar
(1 electron
in outer orbit)
III
(3 electron
in outer orbit) IV
(4 electron
in outer orbit) V
(5 electron
in outer orbit) VI
(6 electron
in outer orbit)
VII
(7 electron
in outer orbit)
VIII
(8 electron
in outer orbit) H
The Hydrogen
Atom O N O oo
The Nitrogen
Atom oo o oo F O
The Fluorine
Atom O O B C O O O O O O
The Boron
Atom
The Carbon
Atom
The Oxygen
Atom oo O o o Si Ar O o o oo O
The Silicon
Atom
The Argon
Atom
All atoms have equal number of protons and electrons.
All atoms have a neutral charge (i.e. a charge of zero).
An atom will gain or lose electrons but not gain or lose protons

4. Molecules All molecules have two or more nucleus
All molecules have the same number of protons and electrons.
All molecules have a neutral charge (i.e. a charge of zero).
A molecule will gain or lose electrons but not gain or lose protons O oo Si O H O oo F H O oo F oo F O oo F O H O H O oo F O H O
Hydrogen Atom
Fluorine Atom oo F O Si oo F o H O O oo H
Hydrogen Fluoride
Molecule
Water Molecule oo N O H
Silicon Difluoride oo C O o C O Si oo O o
Ammonia
Molecule
Carbon Monoxide
Silicon Dioxide

5. 1) What is the overall charge on this entity?
1 Zero Charged entity (a molecule) oo N O H
4 Zero Charged entities H O N
(4 separate atoms)
1) What is the overall charge on this entity?
Pre-plasma puzzlers
Jot down the answers to these questions

6. + H N H 2) What is the overall charge on this entity? + N H N Hoo
4 Zero Charged entities
1 Zero Charged entity (a molecule)
(4 separate atoms) H
2) What is the overall charge on this entity? O + + oo N O H O
3 Zero Charged entities O N H O
(3 separate atoms)
But 1 electron went away!
3) What is the overall charge on this entity? H + H + H + H + + oo N O H
Now this entity
has a positive
charge o N O oo H o H o
But 1 hydrogen ion
showed up!
4 Zero Charged entities O H

7. IONS 4) What do we call these two different entities? N H + N H +oo N O H +
IONS oo N O H +
Any entity that does not have the same number of electrons as it has protons.

8. Ions N H + F O H + Structures when in the gas state oo oo oo
One less electron
in Outer Orbit
One less electron
in Outer Orbit
One extra electron
in Outer Orbit
Two extra electron
in Outer Orbit oo N O H + O F oo O oo H +
Oxide
Ion
Hydrogen
ion
Ammonium
Ion
Fluoride
ion
Structures when in the gas state

9. Subtractive Process Plasma Etching (Dry Etch) Chemical Etching
(Wet Etch)
Plasma Etching
(Dry Etch)

10. + Voltage Difference Across the Electrodes V (Argon Positive Ion)
(Argon Atom) Ar
(Argon Atom) Ar
(Argon Atoms)
Cosmic Ray

11. + V (Argon Positive Ion) (Argon Atom) Ar Ar Ar (Argon Atoms)Ar
(Argon Atoms)
Cosmic Ray

12. + Anode Cathode V Plasma Formation:
(Argon Positive Ion) Ar +
Argon Ionization
Argon Excitation Ar
Anode
Cathode Ar
(Argon Atoms)
Plasma Formation:
Partial Ionization of atoms or molecules in the gas state that makes equal number of positive and negative entities.
Cosmic Ray
An Environment where excitation and ionization occur.

13. + Cathode Anode Anode Cathode Plasma Region V Do not forget that
Cosmic Ray Ar
(Argon Atoms) V
(Argon Positive Ion) +
Argon Ionization
Argon Excitation
Cathode
Anode
All the charged species (ions and electrons) stay between the anode and the cathode.
Anode
Cathode
Plasma Region
An Environment where excitation and ionization occur.
Partial Ionization of atoms or molecules in the gas state that produces equal number of positive and negative entities.

14. An Argon Plasma H H Anode F C F C Cathode H F C H H F C F C F F
All the charged species (ions and electrons) stay between the anode and the cathode.
Add molecules to plasma Region
An Argon Plasma C O F H C O F H
Anode
Cathode C O F H H O C O F H
let 4 separate atoms come together
( 4 zero charged entities) O F o C O O F
Carbon Trichloride
Molecule O F

15. Positive ions tend to move to cathode!
All the charged species (ions and electrons) stay between the anode and the cathode.
Add molecules to plasma Region
An Argon Plasma H + C O F H H + H + O O O O H + O
Anode C O F + O O C O F
Cathode O C O F + O F
Positive ions tend to move to cathode!
Electrons tend to move to anode!
Light is emitted from excited atoms! C O F
What about?
What about? O F

16. Neutral entities can leave the plasma boundary
All the charged species (ions and electrons) stay between the anode and the cathode.
Add molecules to plasma Region
An Argon Plasma O H + C F O
Anode C O F
Cathode O F
Neutral entities can leave the plasma boundary
The technology significance of plasmas is the fact that unusual entities are produced that can leave the plasma and be used.
15-Feb-02

17. What are these unusual entities produced in Plasmas called?F
Free Radicals!
Any atom or molecule that has an unpaired electron is know as a Free Radicals!
Any atom or molecule that has an unpaired electron is know as a Free Radicals!
What are Free Radicals used for?

18. What are Free Radicals used for?
One application is to etch a silicon dioxide film that has been deposited onto a silicon wafer
Strips of coating (mask) that will
not react with the free radicals.
Deposited Silicon Dioxide Film
Silicon Wafer

19. Deposited Silicon Dioxide Film Strips of coating (mask) that will
Add a Free Radical that reacts with the silicon dioxide but not the protective coating strips. O F O F
Silicon Wafer
Deposited Silicon Dioxide Film
Strips of coating (mask) that will
not react with the free radicals. O F O F O F O F O F O F

21. Cathode Anode An Argon Plasma C F F F Si O
All the charged species (ions and electrons) stay between the anode and the cathode.
Add molecules to plasma Region
Cathode
Anode
An Argon Plasma C O F
Fluorine Radical does not react with mask material O F O F
Mask Material Si oo O o

22. Edge of plasma region O F O F O F O F Si oo O o Si oo F o

23. Edge of plasma region Si F F Si F F Si F F F Si O Si F Si F oo o oo o

24. Edge of plasma region Si F F Si F F Si F F F Si O Si F Si F oo o oo o

25. Edge of plasma region Si F F Si F F Si F F F Si O Si F Si F oo o oo o

26. Edge of plasma region Si F F Si F F Si F F F Si O Si F Si F oo o oo o

27. Edge of plasma region F F F F F F F F F F F F F F Si F Si Si o o o o o

28. Edge of plasma region F F F F F F F F F F F F F F Si F Si Si o o o o o

29. Edge of plasma region F F F F F F F F F F F F F F Si F Si Si o o o o o

30. Plasma etch a bit more detail

31. Plasma Concept

32. Summary for Discharge Creation Preliminary Activities
Apply Voltage Across Anode and Cathode.
Provide a Steady State Stream of Gas at Reduced Pressure
Discharge Initiation.
Momentary Increase in Voltage Across Electrodes to Induce
Ar Ionization.
Allow Time for Free Electron to Collide with other Ar
Atoms.
Ionization Process
(Electron given off)
Self-Sustaining the Discharge
For Electron/Argon Collisions, Elastic Collisions Most
Probable.
The Electric Mean Free Path for Ar and Electrons is about
133 microns when the Argon is near 133 Pascal Pressure.
Keep Electron Kinetic Energy High to Encourage Inelastic
Collisions.
Electron needs to transfer 15.7 ev
Electrons accelerated by the electric field between the plasma electrodes
Excitation Process
Electron needs to transfer between 11.7 ev and 15.7 ev
Photon given off

33. Simple Discharge Tube Positive Column (Glow space) Faraday Dark Space
NOT
Faraday Dark Space
(No glow, no argon excitation)
(Glow space)
Cathode Dark Space
Positive Column
Negative glow space
Discharge Vocabulary and Geography
Cathode Dark Space
(Crooks Dark Space)
Higher Density of Ar Positive Ions.
Increased “Local” Electron Field Strength.
Higher Kinetic Energy in Electron Moving Through the Region.
Electron/Argon Collision Results in Argon Ion Formation
Argon Excitation, (the Light Source of the Plasma) .

34. (c) Steps for Successful Plasma Etch
(Same steps as in CVD)
(a) Reactive Species are Produced.
(a) Reactive Species are Produced.
(b) Species Transport to the Wafer Surface.
(c) Species Adsorption onto Wafer Surface.
(d) Reaction with Surface Component to Make
Volatile Product.
(e) Desorption from the Surface.
(b) Species Transport to the Wafer Surface.
(c) Species Adsorption onto Wafer Surface.
(d) Reaction with Surface Component to Make
Volatile Product.
(e) Desorption from the Surface.
(f) Transport from Surface into Exit
Gas Stream.
Glow
Discharge
Gas for
Region
Glow Discharge
(c)

35. (a free radical) (a free radical)
Simple Reactive Plasma Etch using Carbon Tetrafluoride
(gas molecules introduced into the plasma state.)
A carbon trifluoride positive ion
(a free radical)
A Fluorine atom
An electron that is accelerated in the plasma and can collide with more carbon tetrafluoride molecules
Reaction with an electron from the plasma
A carbon trifluoride molecule
(a free radical)

36. C) Fluorine gas does etch Si
Radical/Ion Ratio
Radicals Exist in Higher Concentrations
(a) They Survive Longer (not Influenced by Plasma Field Forces).
(b) They are Generated Faster.
Ion species enhance etching that occurs by triggering events that facilitate process
Carbon Tetrafluoride Reactive Plasma Etch
C) Fluorine gas does etch Si
Si or SiO 2
D) CF and 0 in plasma 4 2
E) CF and H in plasma 4 2
Etch rate for Si surface goes down
Si or SiO 2
Etch rate for Silicon dioxide surface remains almost constant
Etch rate for both increases
Observations
(D.L. Flamm. V.M. Donnelly, Plasma Chem. Plasma Process, 1,317,(1981)
(L.M. Ephrath, E.J. Petrillo, J. Electrochem. Soc., 129, 2282, (1982)

37. F/C ratio 4 F/C ratio 4 A silicon surface A silicon dioxide surface
Silicon tetrafluoride
A silicon surface
F/C ratio
Hydrogen Fluoride is formed 4
Etch rate increases
(Add hydrogen to react with fluorine free radicals before they reach the etching surface.)
(Add oxygen help remove carbon film as carbon dioxide from reaction surface)
Etch rate decreases
A silicon dioxide surface
F/C ratio
Etch rate decreases but decreased is not dramatic 4
Etch rate increases
As the silicon dioxide surface is etched, oxygen is released which reacts with the carbon on the surface. This action compensates for the lower number of free radicals because of the increase in hydrogen.

38. F/C ratio 4 F/C ratio 4 A silicon surface A silicon dioxide surface
Silicon tetrafluoride
A silicon surface
F/C ratio
Hydrogen Fluoride is formed 4
Etch rate increases
(Add hydrogen to react with fluorine free radicals before they reach the etching surface.)
(Add oxygen help remove carbon film as carbon dioxide from reaction surface)
Etch rate decreases
A silicon dioxide surface
F/C ratio
Etch rate decreases but decreased is not dramatic 4
Etch rate increases

39. Overview :
Mixing Various Components into Plasma Changes Etch Rate
Overtones:
Fluorine to Carbon Ration Provides Concept Rule of Thumb
Etch Profile Background
Observations
(J.W. Colburn, H.F. Winters,. J. Vacuum Society Technol., 16,397,(1979)
Low Etch Rate
Low Etch Rate

40. Low Etch Rate
(Lattice Damage)

41. Overtones
Etch Profile Option
a) Depends on Bombardment of Etch Surface
with Energetic Ions.
b) Unless Energetic Particles Strike the Surface .
Plasma Etch Options
Etch Action Orientation
No ion bombardment on sidewall therefore, minimal sidewall etch.
Negative bias on sample influences etch rate of bottom surface of the etched feature
Ion bombardment disturbs and facilitates thin carbon film reaction with oxygen to make carbon dioxide.

42. Ion bombardment removes material and facilitates surface etch while sidewalls have some non-enhanced isotropic etch

43. F/C Ratio 4 Carbon Tetrafluoride Ions for Plasma Etch Options
Adjust F/C ration ( i.e. add H for example 2
(That is to say, increased ion bombardment will
increase surface disruption and thus increase
Anisotropic etch rate)
F/C Ratio
Facilitates removal of carbon film so the etch rate increases
Decrease in the number of free radicals so that etch rate decreases 4
(Add hydrogen)
(Add oxygen)
(As time passes with fixed amount of plasma components)
Etch rate decreases with time because the amount of carbon on surface increases with time, (F/C ratio goes down.