Surface Oxidation & Chemical Etch Kinetics in SC-1/SC-2 Solutions

01Thermodynamic & Kinetic Fundamentals of SC-1 Chemistry

The Standard Clean 1 (SC-1) solution operates as a simultaneous oxidation and dissolution system. Hydrogen peroxide ( $H_{2} O_{2}$ ) acts as an oxidizing agent, while ammonium hydroxide ( $NH_{4} OH$ ) supplies hydroxyl ions ( $OH^{-}$ ) that etch the resulting oxide film.

Process schematicSchematic · source diagram

For steady-state cleaning without severe surface roughening, the oxidation rate $r_{o x}$ and oxide etch rate $r_{e t c h}$ must balance precisely.

02Chemical Reaction Kinetics in SC-1

A. Surface Oxidation Mechanism

Hydrogen peroxide decomposes on the silicon surface to yield reactive oxygen species ( $O^{*}$ or $OH^{*}$ radicals):

H_{2} O_{2} ⟶ H_{2} O + O^{*}

Si + 2 O^{*} ⟶ SiO_{2}

B. Oxide Etching Mechanism

Hydroxyl ions ( $OH^{-}$ ) attack the silicon-oxygen bonds in the chemical oxide, converting insoluble $SiO_{2}$ into soluble silicate species:

SiO_{2} + 2 OH^{-} ⟶ SiO_{3}^{2 -} + H_{2} O

If the concentration of $NH_{4} OH$ is too high relative to $H_{2} O_{2}$ , the native oxide layer becomes depleted ( $r_{e t c h} > r_{o x}$ ), exposing bare silicon directly to $OH^{-}$ ions. This leads to anisotropic pitting along ${111}$ crystal planes, drastically increasing micro-roughness ( $R_{a}$ ).

03SC-2 Metal Solubilization & Redox Thermodynamics

The SC-2 solution ( $HCl : H_{2} O_{2} : H_{2} O$ ) removes trace transition metals ( $Fe, Cu, Ni, Zn$ ) through oxidation and chloride complexation.

A. Metal Dissolution & Complexation Reactions

In an acidic peroxide medium, metallic contaminants are oxidized to their ionic states and immediately complexed by chloride ions:

Fe^{0} H_{2} O_{2} Fe^{3 +} + 3 e^{-}

Fe^{3 +} + 6 Cl^{-} ⟶ [FeCl_{6}]^{3 -}

The formation of stable, soluble chloro-complexes lowers the free metal ion activity $[M^{n +}]$ , driving the dissolution equilibrium forward according to the Nernst Equation:

E = E^{0} - \frac{RT}{n F} ln (\frac{[ M ^{n +} ]}{[ M ^{0} ]})

04Operational Parameter Optimization Matrix

Defect / Objective	Chemical Lever	Kinetic Parameter	Process Risk
Prevent Surface Roughening	Lower $NH_{4} OH$ ratio (0.05:1:5)	Maintain $r_{o x} \geq r_{e t c h}$	Reduced organic etch rate
Prevent Metal Redeposition	Maintain pH < 2 in SC-2	High redox potential ( $E_{h} > 1.0 V$ )	Acid vapor corrosion of tool
Max Particle Lift-Off	Optimize SC-1 temp ( $6 5^{\circ} C$ )	Etch depth $\sim 0.25 - 0.5 nm$	Over-etching gate oxide regions

→Prerequisites & continue reading

Prerequisites: Electrochemical Kinetics, Physical Chemistry, Thermodynamics

Also in Topic 4.1, RCA Wet Cleans & Dilute Chemistry:

Tier 1 · RCA Clean Evolution & Dilute Chemistry Fundamentals…Tier 2 · Single-Wafer Spin Tools vs. Automated Batch Immersio…