Reactive Ion Etching

SiO₂ RIE parameters

Recipe

1. Load the wafer through the load lock into the chamber.
2. Pump down from atmosphere to vacuum. Low pressure increases the ions' MFP, making the etch directional.
3. Introduce CF₄ and Ar and stabilize near 9 mTorr.
4. Top RF on to create the plasma. Higher source power raises plasma density and reactive-species concentration. Electrons collide with CF₄:

   CF₄ + e⁻ → CFₓ + F* + e⁻

   F radicals do the chemical etching, CFₓ radicals drive surface chemistry and possible passivation, and CFₓ⁺ and Ar⁺ ions plus electrons fill out the plasma.
5. Bottom RF on to bias the wafer. The bottom electrode develops a negative DC self-bias, which forms a plasma sheath above the wafer. Positive ions accelerate through the sheath toward the wafer, giving anisotropic etching.
6. Plasma off, purge, vent, and remove the wafer through the load lock.

Etch chemistry

Reactive ion etching combines chemical reaction with ion bombardment.

1. Fluorine radicals adsorb on exposed SiO₂ and fluorinate the surface. Fluorine forms strong Si–F bonds.
2. Positive ions bombard the surface. Ar⁺ and CFₓ⁺ ions accelerate vertically to break or weaken Si–O bonds, activate the surface, remove reaction layers, increase fluorination, and help products desorb.
3. Volatile products form. Once Si–O bonds are disrupted, fluorine reacts with Si to form volatile species such as SiF₄.
4. Products leave the wafer: SiF₄(surface) → SiF₄(g).

Why it's anisotropic

• Fluorine radicals alone would etch fairly isotropically, attacking the bottom and sidewalls equally.
• Ion bombardment adds direction because the bottom of a feature gets much stronger bombardment than the sidewalls. The bottom etches faster, giving an anisotropic profile.
• CFₓ species may also coat the sidewalls and further support anisotropy.

Running the Trion ICP-RIE

1. Load the wafer. It goes into the load lock, a robot arm moves it to the chuck, and the clamp secures it.
2. Seal the chamber.
3. Pump down from atmospheric pressure to vacuum.
4. Set wafer temperature. The chuck comes to process temperature while pumping, and backside He fills the gap for better heat transfer.
5. Introduce gases through the delivery system: cylinder → MFC → inlet → chamber.
6. Stabilize flow and pressure. Gas flow, pumping, and the throttle valve together set chamber pressure.
7. Turn on the ICP source. The oscillating field gives energy to free electrons, which collide with gas molecules and turn the gas into plasma.
8. The matching network stabilizes power transfer. Plasma impedance changes with pressure, density, and gas composition, so the network adjusts to deliver more power and reflect less.
9. The plasma stabilizes. Density, matching, species concentration, pressure, and gas composition settle, giving a repeatable starting point.
10. Apply bottom RF. The wafer develops a negative self-bias and a sheath forms above it.
11. Ions accelerate toward the wafer. The sheath field is mostly perpendicular to the wafer, so ions strike vertically.
12. Radicals reach the wafer at the same time, supplying the chemical part of the etch.
13. Ion-assisted reactions occur while the mask protects selected areas.
14. Volatile products form and desorb.
15. The pump removes products and lets fresh reactive species reach the wafer.
16. The etch runs for the programmed time.
17. End of etch. ICP off extinguishes the plasma, and bias off removes the self-bias.
18. Stop gas flow while pumping continues.
19. Pump and purge (pump → purge → pump) to clear residual gases.
20. Stop backside He and release the clamp.
21. Transfer the wafer back to the load lock.
22. Remove the wafer.

Tool anatomy: Trion ICP-RIE