Introduction to Plasma Etch: Concepts and Constructs

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. O O ATOMS 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 F H N O O O oo O The Hydrogen Atom The Nitrogen Atom The Fluorine Atom

3. I (1 electron in outer orbit) III IV V VI oo VII VIII O H F O (3 electron in outer orbit) (4 electron in outer orbit) (5 electron in outer orbit) (6 electron in outer orbit) (7 electron in outer orbit) (8 electron in outer orbit) O O oo The Hydrogen Atom oo o oo o oo N O O The Nitrogen Atom O The Fluorine Atom O O ATOMS O O B O C O O O O O The Boron Atom The Carbon Atom The Oxygen Atom oo O o o Ar Si 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 oo oo oo H oo N oo H o o O oo Si oo O o o O oo oo O oo H H oo O H oo O oo C O O O o O oo oo oo oo oo oo o O F F F F F F O O O O O O H H H H H H O O O O O O O O O O O O O O O O O O oo oo oo oo oo oo O Si o oo oo O O o o oo F oo Si oo F o o O oo oo O o oo oo oo O C O oo O 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 Hydrogen Atom Fluorine Atom Hydrogen Fluoride Molecule Water Molecule Silicon Difluoride Ammonia Molecule Carbon Monoxide Silicon Dioxide

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

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

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

8. O O O O H + O O O O oo oo H oo N oo H O oo O O H oo F oo + H Ions One less electron in Outer Orbit One less electron in Outer Orbit One extra electron in Outer Orbit Two extra electron in Outer Orbit 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)

10. V + (Argon Positive Ion) Ar 0 Ar 0 Ar (Argon Atoms) Voltage Difference Across the Electrodes (Argon Atom) (Argon Atom) Cosmic Ray

11. V + (Argon Positive Ion) Ar 0 Ar 0 Ar (Argon Atoms) (Argon Atom) Cosmic Ray

12. V + (Argon Positive Ion) Ar Argon Excitation 0 Ar Argon Ionization 0 Ar (Argon Atoms) Anode Cathode 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. Do not forget that V + (Argon Positive Ion) Ar Argon Excitation 0 Ar Anode Cathode Argon Ionization 0 Ar (Argon Atoms) Cosmic Ray 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. All the charged species (ions and electrons) stay between the anode and the cathode. O O O O F F F F O O O O C C C C O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O F F F F O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O H H H H H O O O O O O o C F O F F F F F O O O O O O O O O O O O O O O F O O O O O Add molecules to plasma Region An Argon Plasma Anode Cathode let 4 separate atoms come together ( 4 zero charged entities) Carbon Trichloride Molecule

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

16. All the charged species (ions and electrons) stay between the anode and the cathode. + + H H O O O + O O F F O C O O O O O O O O O O O O O O O F O O O O O O F O C O O O O O O O O O O O O O O O O O O F O O O F O O Add molecules to plasma Region An Argon Plasma O Anode Cathode 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. F O O O O O O O What are these unusual entities produced in Plasmas called? 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. Strips of coating (mask) that will not react with the free radicals. Deposited Silicon Dioxide Film F F F F F F F F O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O Silicon Wafer Add a Free Radical that reacts with the silicon dioxide but not the protective coating strips.

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

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

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

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

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

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

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

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

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

30. Plasma etch a bit more detail

31. Plasma Concept

32. Apply Voltage Across Anode and Cathode. Provide a Steady State Stream of Gas at Reduced Pressure Momentary Increase in Voltage Across Electrodes to Induce Ar Ionization. Allow Time for Free Electron to Collide with other Ar Atoms. 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. Summary for Discharge Creation Preliminary Activities Discharge Initiation. Ionization Process (Electron given off) Self-Sustaining the Discharge 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. Positive Column Higher Density of Ar Positive Ions. (Glow space) Increased “Local” Electron Field Strength. Higher Kinetic Energy in Electron Moving Through the Region. Electron/Argon Collision Results in Argon Ion Formation NOT Faraday Dark Space Argon Excitation, (the Light Source of the Plasma) . (No glow, no argon excitation) Negative glow space Cathode Dark Space Simple Discharge Tube Discharge Vocabulary and Geography Cathode Dark Space (Crooks Dark Space)

34. (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. (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. Steps for Successful Plasma Etch (Same steps as in CVD) (a) Reactive Species are Produced. Glow Discharge Gas for Region Glow Discharge (c)

35. Reaction with an electron from the plasma 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 A carbon trifluoride molecule (a free radical)

36. Si or SiO Si or SiO 2 2 D) CF and 0 in plasma E) CF and H in plasma 4 4 2 2 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) 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 Etch rate for Si surface goes down Etch rate for Silicon dioxide surface remains almost constant Etch rate for both increases

37. A silicon surface Silicon tetrafluoride A silicon dioxide surface Etch rate decreases but decreased is not dramatic Etch rate increases 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 F/C ratio 4 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. A silicon surface Silicon tetrafluoride A silicon dioxide surface Etch rate decreases but decreased is not dramatic Etch rate increases 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 F/C ratio 4

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 Low Etch Rate (Lattice Damage)

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

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

43. 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) Carbon Tetrafluoride Ions for Plasma Etch Options 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.