PECVD Oxide Passivation · Youngjun Kweon

SiO₂ deposition parameters

Parameter	Value	Notes
Substrate	Si (100)
Temperature	350 °C	Supplies thermal energy for surface reactions and a dense Si–O network. Lower than thermal CVD because the plasma adds reaction energy.
Deposition rate	12.33 Å per 300 s
Refractive index	1.459 at 632.8 nm	Indicates film composition and optical quality.
Breakdown field	> 6.5 MV/cm	The field the dielectric withstands before breakdown. Higher means better dielectric integrity.
Film stress	61 MPa compressive (0°), 64 MPa compressive (90°)	The film tends to expand relative to the substrate but is held by adhesion to the wafer.
Silane source	5% SiH₄ / 95% He	SiH₄ is pyrophoric. He dilution makes handling safer, flow more controllable, and the plasma more stable.
Oxidizer	N₂O	Supplies the oxygen species that form SiO₂.

Recipe, step by step

Load the wafer onto the heated chuck in the process chamber.
Pump down to the required pressure.
Heat and stabilize at about 350 °C. A stable temperature improves rate repeatability, film density, composition, and stress uniformity.
Introduce the gases. 5% SiH₄/He and N₂O flow in through mass flow controllers: cylinder → MFC → showerhead → chamber.
Stabilize flow and pressure. Gas flows in continuously, the pump removes it continuously, and the throttle valve sets the pressure.
Apply RF power. The oscillating field accelerates free electrons, which collide with SiH₄ and N₂O to form a plasma of electrons, ions, SiHₓ radicals, H radicals, O- and N-containing species, and excited neutrals.
Plasma activates the precursors. Si-containing radicals supply silicon, and O-containing radicals supply oxygen.
Reactive species reach the wafer. Neutral radicals diffuse to the surface, and some ions accelerate through the sheath.
Si species adsorb: surface + SiH₃* → surface–SiHₓ.
Oxygen reacts with adsorbed Si: Si–H + O* → Si–O–(surface) + H-containing products. Repeated oxidation and cross-linking build the network.
The Si–O–Si network forms and expands as more species arrive.
Volatile byproducts desorb: H₂, H₂O, N₂, unreacted precursor, and other fragments.
The pump removes byproducts so they don't build up.
The film keeps growing through simultaneous activation, transport, adsorption, reaction, and desorption.
End deposition: RF off after the programmed time.
Stop gas flow: MFCs close while pumping continues.
Pump and purge residual gases before transfer or venting.
Cool the wafer, then vent and unload.

The chemistry

SiH₄ + e⁻ → SiH₃* + H* + e⁻
N₂O + e⁻ → N₂ + O* + e⁻

Silane plasma creates Si-containing radicals, and N₂O plasma creates reactive oxygen.
Neutral radicals reach the heated wafer mainly through random thermal motion, diffusion, and gas transport. Radicals are consumed at the surface, so their concentration there drops and more diffuse in.
Si radicals adsorb and attach to surface sites. Reactive oxygen strips hydrogen and forms Si–O bonds.
Repeated adsorption and oxidation build Si–O–Si links through the film, while volatile byproducts leave.

Tool anatomy: Oxford PlasmaLab

Component	Role
Process chamber	Sealed vacuum space with controlled pressure, gas, temperature, and plasma.
Heated chuck / lower electrode	Holds the wafer and heats it to the deposition temperature.
Substrate heater	Affects film density, rate, hydrogen content, stress, and quality.
RF generator	Supplies power to create and sustain the plasma.
RF electrode system	Couples RF energy into the gas as an oscillating field.
RF matching network	Matches the generator to the changing plasma load to maximize delivered power and minimize reflection.
Mass flow controllers	Regulate each gas flow for repeatable precursor ratios.
Showerhead	Spreads gas evenly over the wafer. Often part of the RF electrode.
Vacuum pump, throttle valve, gauge	Together set and measure process pressure.
Temperature control	Measures and regulates chuck temperature.
Exhaust / abatement	Safely carries away unused precursor and products. This matters because silane needs controlled handling.