Particle Removal Physics & Zeta Potential Manipulation

01Introduction: The Nanometer Particle Challenge

As semiconductor features scale below 2nm, a microscopic particle as small as 5 to 10 nm can bridge conductive metal lines or block lithographic light exposure, causing total device failure. Removing nanoscale particles from silicon wafer surfaces requires overcoming massive attractive surface forces.

Particle Removal Efficiency (PRE) is defined as:

PRE (%) = (\frac{N _{initial} - N _{final}}{N _{initial}}) \times 100

Where $N_{initial}$ and $N_{final}$ are particle counts measured by laser surface inspection tools before and after cleaning.

02What Holds Particles to Wafer Surfaces?

Particles adhere to silicon surfaces through van der Waals attractive forces:

F_{VDW} = \frac{A _{H} \cdot d _{p}}{12 z _{0}^{2}}

Where $A_{H}$ is the Hamaker constant ( $\sim 1 0^{- 19} J$ ), $d_{p}$ is particle diameter, and $z_{0}$ is separation distance ( $\sim 0.4 nm$ ). Because $F_{VDW}$ scales linearly with particle diameter $d_{p}$ , while cleaning removal forces (such as fluid drag) scale with $d_{p}^{2}$ or $d_{p}^{3}$ , smaller particles are exponentially harder to detach.

[ Particle (Dp) ]Schematic · source diagram

03The Solution: Zeta Potential ( $ζ$ ) & Electrostatic Repulsion

When immersed in aqueous solutions, both the silicon surface and the particle develop an electrical charge layer known as the Electrical Double Layer (EDL). The electrical potential at the slipping plane of this layer is called the Zeta Potential ( $ζ$ ).

Zeta Potential (Z_Particle)Schematic · source diagram

Controlling Zeta Potential with pH

In Alkaline Solutions (SC-1, pH ~ 10): Both silicon dioxide ( $SiO_{2}$ ) surfaces and silicon nitride/slurry particles acquire strongly negative Zeta potentials ( $ζ \approx - 30 mV$ to $- 60 mV$ ). Since like charges repel, a strong electrostatic repulsive force ( $F_{EDL}$ ) pushes particles away from the wafer surface, preventing re-deposition!
In Acidic Solutions (pH < 3): Zeta potentials shift near zero (the Isoelectric Point, IEP), eliminating repulsion and causing rapid particle aggregation and surface contamination.

04Key Rules for High PRE in Fabs

Keep pH High During Cleaning: Always use alkaline chemistries (e.g., dilute SC-1 or ammonia-DIW) when removing particles to maintain strongly negative Zeta potentials ( $ζ < - 30 mV$ ).
Combine Chemistry with Mechanical Energy: Chemical undercut plus megasonic acoustic streaming provides the detachment force needed to overcome van der Waals attraction.
Avoid Isoelectric Points (IEP): Never process wafers at liquid pH levels where the wafer or particle Zeta potential crosses zero.

→Prerequisites & continue reading

Prerequisites: General Chemistry, Basic Surface Science

Also in Topic 4.3, Particle Removal & Zeta Potential Engineering:

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