Material · Lithography & Materials
Wet chemicals
The high-purity acids, bases and solvents used for cleaning, etching and stripping, including sulphuric acid, hydrofluoric acid, hydrogen peroxide, ammonium hydroxide and isopropyl alcohol.
- Also known as
- Process chemicals, Wet process chemicals
- Type
- Material
- Domain
- Lithography & Materials, Process materials
Notes
Japan's 2019 export restrictions on hydrogen fluoride to Korea showed how concentrated this supply chain is.
Overview
Wet chemicals are the high-purity liquid acids, bases, oxidisers and solvents that fabs use to clean wafers, etch films and strip photoresist, such as sulphuric acid, hydrogen peroxide, hydrofluoric acid, hydrochloric acid, ammonium hydroxide and isopropyl alcohol. They are used again and again through wafer processing, so their purity and consistency affect yield directly. Industry specifications, notably SEMI's, define grades with limits on assay and trace impurities for each chemical.
How it works
Cleaning is a major use. The classic sequence is the RCA clean, developed by Werner Kern at RCA in 1965 and still a standard sequence used before high-temperature processes such as oxidation, diffusion and deposition. Its first bath, SC-1, mixes water, ammonia and hydrogen peroxide at around 75 to 80 °C to remove organic residues and particles. An optional dip in dilute hydrofluoric acid strips the thin oxide left behind. The second bath, SC-2, mixes water, hydrochloric acid and hydrogen peroxide to remove metal ions. Wafers are rinsed in deionised water between steps, and heavily contaminated wafers may first get a piranha clean.
Purity is specified by grade. SEMI C44, for example, covers two grades and three purity tiers of sulphuric acid, with assay and impurity limits and supporting test methods, and SEMI C30 does the same for hydrogen peroxide with five grades and one tier. Higher tiers exist for the most demanding uses, with test methods validated under SEMI's general guide to analysing liquid chemicals. Containers and wetted parts matter too: fused silica or quartz is preferred to borosilicate glass, and the materials of chemical distribution components are themselves tested.
Many of these chemicals are corrosive or toxic, so their storage, delivery to tools and handling are tightly controlled.
Examples
- An RCA clean run before thermal oxidation, with SC-1, a dilute hydrofluoric acid dip and SC-2.
- Sulphuric acid bought to a SEMI C44 grade and tier for wafer cleaning.
- Hydrogen peroxide supplied to a SEMI C30 grade for use in cleaning mixtures.
- A brief dilute hydrofluoric acid dip to remove a thin oxide layer before a following step.
- Quartz or fused silica tanks chosen for cleaning baths instead of borosilicate glass.
Why it matters
Wet process engineers, chemical quality engineers, delivery system engineers and safety specialists all work with wet chemicals, because a trace of metal or particles in a bath can contaminate the wafers it touches. Specifying a grade and tier gives buyers and suppliers a shared, testable definition of purity.
Related terms
Part of the Lithography & Materials glossary, with every term in the topic and the lifecycles, tools and standards that use them.
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Used on 10 pages of the Hub. In each group, the pages that mention it most come first.
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Last reviewed . Written by The Semiconductor Hub editorial team. The full glossary