1. Etch
Dry and Wet Etches

2. Index Overview Wet etch/Dry etch Etch characteristics / requirements
Oxide, Nitride, Silicon and Al
Cleaning
Summary
11/10/2018

3. Background One of the material removal techniques
Either in liquid or gaseous phase (Wet etch or Dry etch)
gas ==> plasma etch
A “lot” of 200 mm wafers in a ‘cassette”
Teflon
©IBM web site
11/10/2018

4. Wet Etch Unit: Sample Image
©IBM web site
Note: Clean room floor
“Bunny suite”
11/10/2018

5. Sample Image Wet Deck. Arizona State University semi-automatic
11/10/2018

6. Background Etching: usually after Photo Needs to be selective
To photoresist
To hard mask
To the bottom material (in most cases)
Blind Etch vs “Stop on” etch
Planarization Etch
Isotropic vs Anisotropic
should handle high Aspect Ratio
clean
cost
11/10/2018

7. Wet vs Dry etch Wet chemical etch: Usually isotropic
In some single crystals, chemical etch occurs in only one direction ==> Anisotropic
Dry etch:
Plasma : Ion impact mechanical removal, gas reaction, chemical removal
Can be anisotropic or isotropic. Controllable
11/10/2018

8. Requirements Uniformity:
Macro scale: across the wafer uniformity, plasma confinement, wet etch chemical replenishment
Micro Scale:
Micro loading or etch loading
Mass transfer/ Boundary layer controlled vs kinetics controlled
process proximity correction
11/10/2018

9. Wet Etch SiO2 Si3N4 Si Al Resist Silicon Oxide, Silicon Nitride
Aluminum, Chromium
Photoresist (develop)
Silicon Oxide: HF
Buffered HF (Buffered Oxide Etch or BOE)
NH4F + HF
BHF+Glycerin to protect aluminum
HF+HCl (increases etch rate)
Temperature as controlling element
11/10/2018

10. Wet Etch SiO2 Si3N4 Si Al Resist Etch rates in um/min
Etch rate strongly depends on type of oxide:
Thermal oxide has low etch rate
BSG has low etch rate
PSG, CVD oxide high etch rate
11/10/2018

11. Wet Etch Steps SiO2 Si3N4 Si Al Resist Soak (etch) in tank
controlled temp, composition, close loop control
double rinse (usually DI)
counter current
spin dry
11/10/2018

12. Nitride Etch SiO2 Si3N4 Si Al Resist HF or Buffered Oxide Etch HF+HCl
Hot Phosphoric acid etch
etch rate of oxide is low
of Si is lower
of resist is high!
Use oxide as a mask
11/10/2018

13. Nitride Etch SiO2 Si3N4 Si Al Resist Boiling Point of Phosphoric Acid
Replace water as necessary
Nitride edge > 30 min usually
11/10/2018

14. Silicon Etch SiO2 Si3N4 Si Al Resist KOH on single crystal
Poly crystal etches in random (i.e. All )direction
Hard Mask with silicon nitride
Or silicon oxide
thermal oxide vs CVD oxide
Use low conc and temp
otherwise oxide will also etch fast!
11/10/2018

15. Silicon Etch SiO2 Si3N4 Si Al Resist
Adding boron reduces the etch rate
Etch rate vs Conc
11/10/2018

16. Al Etch Resist Etch (Strip) SiO2 Si3N4 Si Al Resist
Phosphoric, nitric and acetic acid mixture
Al has some Si and Cu added to help electro migration; Si helps a bit in electro migration, but helps more in spiking (kind of pre-dissolves)
Si does not dissolve well in the mix
Heat it to remove non uniformities
H2CrO4, H3PO4, H2O mix also used
Resist Etch (Strip)
Resist stripping, usually < 30 mins
Chemistry depends on resist composition (organic/acetone based)
11/10/2018

17. Wet Etch / Cleaning Cleaning silicon wafers
RCA or SC1/SC2 (modified now)
SC1 : ammonia, peroxide, water :(APM, Huang A): high temp
SC2: HCl, peroxide, water (HPM, Huang B): high temperature
Gentler cleaning preferred now a days
(i.e. Lower concentration/ lower temp)
dilute HF etch added between SC1 and SC2
After etch/clean, rinse with 18 Mohm-cm DI water
SC1
Organic, complexes some metals
SC2
Sodium and other metal complexes
11/10/2018

18. Wet Etch Summary Wet etch is low temperature, high selectivity
Charge issues: ESD (electrostatic discharge) in dry etch
Increased reliability for wet etch
Of late, photochemical reactions and electrochemical dissolution are becoming important in wet etch and wet cleaning
Cheap equipment cost
Fast etch rate, Any material can be etched
Surfactant to enhance wetting
Batch processing is easy
Inline monitoring/End Point Detection is not easy
Is the only method for cleaning equipment/labware
11/10/2018

19. Wet Etch Summary Isotropic Transport limited in small trenches
“loading” in large trenches may ‘give back’ some uniformity
Poor control of dimension for sub micron, etch rate in general
Running cost high (very clean wet chemicals with out Na or other metals are very expensive)
11/10/2018