Process · Process Technology
Wafer cleaning
The repeated removal of particles, organics, metal contamination and native oxide between process steps. Cleans outnumber every other step type in a fab flow and directly determine defect density and yield.
- Also known as
- Wet clean, RCA clean, Pre-clean
- Type
- Process
- Domain
- Process Technology, Front-end process
Notes
The RCA clean sequence developed at RCA Laboratories in the 1960s remains the conceptual basis for modern wet cleans.
Overview
Wafer cleaning is the removal of particles, organic residues, metal traces and unwanted native oxide from a wafer's surface between process steps, so that each new layer starts on a known, contamination-free surface. Cleans are repeated many times through a fab flow, typically before high-temperature steps and after etch, resist strip and polishing. The term covers wet chemical cleans, which remain the core of most flows, as well as dry methods such as plasma-based cleans.
How it works
The classic wet recipe is the RCA clean, developed by Werner Kern at RCA in 1965. Its first step, SC-1, is a warm mixture of ammonia, hydrogen peroxide and water that removes organic residue and particles. A short dip in dilute hydrofluoric acid can then strip the thin oxide that SC-1 leaves behind, and SC-2, a warm mixture of hydrochloric acid, hydrogen peroxide and water, removes remaining metal ions. Each step is followed by a rinse in ultrapure water, and the wafer must then be dried without leaving marks.
Equipment comes in two broad styles. Wet stations move wafers through a series of chemical and rinse baths. Single-wafer cleaners treat one wafer at a time in a spinning chamber, and modern tools stack many chambers in towers to approach the throughput of batch tools. Spin scrubbers and backside cleaners, some combining chemical cleaning with brushes, handle particular jobs. Drying commonly uses isopropyl alcohol vapour, which lowers surface tension so that water marks are avoided and tall, delicate features do not collapse.
The central difficulty is removing contamination without harming the device. A clean can etch a little material, roughen a surface or leave traces of its own, and fragile three-dimensional structures limit how much force can be used. Water quality matters as much as chemistry: the ultrapure water used for rinsing is specified to very tight limits, and its purity is treated as critical to the product.
Examples
- An RCA-style sequence run before a high-temperature oxidation step, so that the new oxide grows on a surface free of organics and metals.
- A single-wafer cleaner with 24 chambers stacked in towers, rated by its maker at up to 1,200 wafers per hour.
- Low-pressure isopropyl alcohol vapour drying in a wet station, used to suppress water marks and prevent pattern collapse.
- A plasma-based clean that removes native oxide and carbon residue just before atomic layer deposition, with vacuum transfer to keep the surface protected.
- Lightly loaded rinse water from cleaning steps captured for reuse in cooling towers and scrubbers.
Related terms
Part of the Process Technology glossary, with every term in the topic and the lifecycles, tools and standards that use them.
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