1. Microelectronics Processing
Etching
E. Finkman – Microelectronics Processing

2. The Etch Process The properties of the etch process should be:
Follow the photolithography step
Remove selected materials from wafer surface
Clean the wafer surface
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3. Goal of Etch Process
Permanently transfer the mask pattern from photolithography into the surface layer of the wafer
Etching can be done in either “wet” or “dry” environment:
Wet etch – liquid etchants
Dry etch – gas phase etchants in a plasma
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4. Etching Process
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5. Basic concepts • Etching process consists of three steps:
1. Mass transport of reactants (through a boundary layer) to the surface to be etched
2. Reaction between reactants and the film(s) to be etched at the surface
3. Mass transport of reaction products from the surface through the boundary layer
• Etching is usually done using liquid phase or gas phase reactants
– liquid phase (wet) etching —reaction products soluble in solvent or gaseous
– gas phase etching — reaction products gaseous / sublimation temperature

6. Etch Parameters Etch Rate
Rate at which material is removed from the wafer
Uniformity
Etch rate constant across the wafer
Throughput
Amount of wafers etched during one process cycle
Directional Control
Controlling the horizontal and vertical etch rate: isotropic or anisotropic
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7. Etch Parameters Selectivity Controlled by gas formula
Controlled by etch rate
Selectivity = Surface Layer Material Etch Rate Underlying Layer Material Etch Rate
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8. Figures of merit

9. Etch Parameters Directional Control – Isotropic Etch
Etches equally in all directions
Results in undercutting and sloping walls
Isotropic Etch
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10. Etch Parameters Directional Control – Anisotropic Etch
The vertical etch rate is faster than the horizontal etch rate
Produces straight sidewalls
Anisotropic Etch
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11. Bias
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12. Degree of Anisotropy
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13. Process Techniques : Wet Etch
Chemical reaction between liquid etchants and surface
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14. Wet Etch Process Wet Etch Process Performed in an immersion tank
Wafers in wafer carrier are lowered into etchant solution
Wafers remain in solution for specified amount of time .referred to as etch duration
Wafer carrier is removed from solution
Wafers are rinsed in deionized water
Wafers are dried
Etch Duration
Etch Duration = Thickness of Layer (Å) Etch Rate of etchant
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15. Cascade DI water rinse

16. Wet Etch Process - Examples
End point
Detection!
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17. Wet Etch Process – Examples - HF
End point detection
SiO2 wetts
Si hydrofobic
Safety: HF inert in contact with skin
Attacks bone!
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18. Wet Etch Process - Examples
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19. Wet Etch Process - Examples
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20. Anisotropic Silicon Etching – Effect of Slow {111} Etching
[110] Oriented Silicon
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21. Anisotropic wet etching of Si
{111} planes
(110) surface orientation
Heavily boron
doped region
Hole
Diaphragm
V-groove
54.74
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22. Anisotropic Silicon Etching: Applications
(3) Field-Emission Tips
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23. Anisotropic Silicon Etching: Applications
(4) MicroElectroMechanical Systems - MEMS
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24. Etching selectively by doping

25. Wet Etch Characteristics
Characteristics of Wet Etch Process
Highly selective
Isotropic etch
For use on features larger than 3 microns
High throughput
Low cost equipment
Hazardous liquid chemicals
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26. Wet Etch Limitations
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27. Wet Etch Process Wet Etch Process Steps Etch Rinse Dry
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28. Wet Etch Process Key Wet Etch Parameters Etchant concentration
Temperature of etchant
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29. Wet Etch Process Wet Etch in an Immersion Tank
Rinse in an Immersion Tank
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30. Wet Etch Process Dry in Spin-Rinse-Dryer
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31. Wet Etch Process Wet Etchants Acetic Acid HC2H3O2 Hydrofluoric Acid HF
Ammonium Fluoride NH4F
Phosphoric Acid H3PO4
Nitric Acid HNO3
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32. Wet Chemical Etchants for VLSI Fab.
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33. Chemical Mechanical Polishing (CMP)

34. Dry Etch
Chemical reaction or physical etch between gas.etchants and surface material on wafer:
Dry etching methods
Glow discharge methods
Dry physical etching (Sputter etching)
Plasma assisted etching
Dry chemical etching (Plasma etching)
Reactive ion etching (RIE)
Ion beam methods
Ion mlling
Reactive ion beam etching
Chemical assisted ion milling
Common materials to dry etch
Si, SiO2, Si3N4, Al, W, Ti, TiN, TiSi2, Photoresist
Difficult materials to dry etch
Fe, Ni, Co, Cu, Al2O3, LiNbO3, etc.
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35. Dry Etch Process Characteristics of Dry Etch Process Highly selective
Anisotropic etch
For use of features smaller than 3 microns
Expensive equipment
Limited human exposure to hazardous chemicals
RF Power safety risks
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36. Dry Etch Techniques Physical etch Reactive Ion etch
Chemical (plasma) etch
Dry
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37. Gas (plasma) etching advantages

38. Plasma: the 4th state of the matter
On earth we live upon an island of "ordinary" matter. The different states of matter generally found on earth are solid, liquid, and gas. We have learned to work, play, and rest using these familiar states of matter. Sir William Crookes, an English physicist, identified a fourth state of matter, now called plasma, in Plasma is by far the most common form of matter. Plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that which is not visible.
Natural plasma
Man-made plasma
Sources:
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39. Basic properties of plasma

44. Dry Etch Process Physical Etch
Also referred to as ion beam etching, sputtering, ion .milling
Ions bombard wafer surface causing molecules to .sputter off the surface - Argon introduced into RF Power Chamber
Advantages - Low level of undercutting - Anisotropic etch
Disadvantages - Low selectivity rate - Requires high level of RF Power
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45. Sputter etching
In this process all of the electrical energy, usually RF, is applied to the substrate.
Physically bombard the films to be etched with energized chemically inert ions or atoms.
Material is removed by ion bombardment of the substrate. This process is most often used to pre-clean substrates prior to deposition.
Glow discharge is used to energize chemically inert ions or atoms (e.g., Ar)
Highly anisotropic etching
Damage to underlying material => may change device properties
Rarely used in VLSI
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46. Dry Etch Process Ion Beam Etcher
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47. Dry chemical etching (Plasma etching)
Purely chemical etching.
RF energy is applied to a separate electrode with the substrates grounded. Chemical reaction between etchant gas and surface layer of wafer etches the wafer.
Glow discharge is used to produce chemically reactive species (atoms, radicals, or ions).
Advantage - High selectivity rate
Disadvantage - Isotropic etch
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48. Plasma assisted etching
Plasma assisted etching sequence
Take a molecular gas
CF4
Establish a glow discharge
CF4+e  CF3 + F + e
Radicals react with solid films to form volatile product
Si + 4F  SiF4 
Pump away volatile product (SiF4 )
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49. Etchants and etch products (different sources)
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50. RF-powered Plasma Etch System
Steady-state voltage distribution in
RF-powered plasma etch system
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51. Plasma Etching Mechanisms
Chemical etching
Due to their incomplete bonding,
free radicals are highly reactive
chemical species.
Various reactions and species
present in a plasma
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52. Reactive Ion Etcing (RIE)
Combines both physical and chemical etching techniques
If RF energy is applied to the substrates in a low pressure halogen-rich environment, material can be removed by both chemical means and ion bombardment of the substrate surface. Greater control over line widths and edge profiles is possible with oxides, nitrides, polysilicon and aluminum.
Accomplished by replacing the neutral gas in a r.f. sputtering system by one or more chemical species
Glow discharge is used to produce chemically reactive species (atoms, radicals, or ions) and chemically inert ions
Widely used in VLSI fabrication
Advantages - High selectivity rate - Anisotropic
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53. Dry Etch Process : RIE
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54. Ion Energy vs. Pressure for a Plasma
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55. Chemical vs. chemical/physical etching
(using reactive neutral species
and ionic species)
Anisotropic etching
Purely chemical etching
(using only reactive
neutral species)
Isotropic etching
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56. Effect of the inhibitor
w/o inhibitor
=> Isotropic
w/ inhibitor
=> Anisotropic
fast inhibitor
deposition
=> Bevelled walls
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57. Importance of RIE Importance of reactive ion etching
Highly anisotropic etching
Precise pattern transfer
High resolution
Less consumption of chemicals
Cost effective
Environmentally benign
Clean process
Vacuum
Ease of automation
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58. Barrel plasma system Quartz tube
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59. High density plasma (HDP) system
Electron Cyclotron
Resonance (ECR)
Inductively coupled
plasma (ICP)
ECR vs ICP
ECR was introduced at OPT in 1985.
ICP was introduced much later ( ) for plasma processing.
most important with both: independent control of ion energy and ion current density
lower (substrate) electrode grounded, RF driving opt.
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60. Sputter etch and ion milling system
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61. RIE: Examples
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62. How to Control Anisotropy?
Ionic bombardment damage exposed surfaces.
Sidewall coating by inhibitor prevents sidewall etching.
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63. How to Control Selectivity?
Example 1:
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64. How to Control Selectivity?
Example 2: Si etching in CF4 + O2 mixture.
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65. How to Control Smoothness?
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66. Temperature Dependence of Selectivity
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67. In General: Etching Process Includes Several Sequential Steps
Example 1: RIE of Al lines.
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68. Sequential Steps in Etching process
Example 2: Etching of deep trenches.
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69. Sequential Steps in Etching deep trenches
Solution: Multiple step RIE sequence.
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70. Summary of Plasma Systems & Mechanisms
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71. The Four Classes of Plasma Etching Mechanisms
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72. Dry Etch Process Summary
Factors Influencing Dry Etch Process
Etch rate - RF Power level - Gas formula - Etch Temperature
Pressure - Extremely high pressure results in an isotropic etch - Low pressure with high energy can damage wafer
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73. Dry Etch Process Summary
Factors Influencing Dry Etch Process
Micro-loading - Different etch rates across wafer surface - Ashing can occur
Post-etch corrosion - Due to residual etchant left on wafer after final rinse - Using a none Chlorine based etchant like Fluorine …..eliminates the problem.
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74. Plasma-Etching Gases Used in VLSI Fabrication
PSG
BPSG
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75. Wafer Cleaning Resist Stripping
Removes residual resist after etch process
Wet stripping - For use on wafers that have not been plasma …. …..etched - For non-metallic surfaces an acid solvent is used - For metallic surfaces an organic solvent is used
Plasma Stripping - For use on wafers that have been etched by plasma - Uses oxygen as stripping plasma to remove …..photoresist
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76. Photoresist Stripping
Key components of a microwave plasma asher include a 2.45-GHz microwave generator and process chamber.
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77. Photoresist Stripping
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78. Wafer Inspection and Metrology
Etch check
Final check
Critical dimension measurement
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