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SC-1 vs SC-2 vs Dilute HF: Chemistry, What Each Removes and the Surface Left Behind

RCA cleaning, the basis of silicon wafer cleaning, uses chemicals with different jobs in sequence. SC-1, a mixture of ammonia and hydrogen peroxide, removes particles and organics. SC-2, hydrochloric acid with hydrogen peroxide, removes metal contamination. Dilute hydrofluoric acid (DHF) dissolves the native oxide on the surface. After SC-1 and SC-2 the surface is covered by a thin oxide and is hydrophilic (water spreads on it); after DHF it is terminated with hydrogen and is hydrophobic (water beads up).

AspectSC-1 (APM)SC-2 (HPM)Dilute HF (DHF)
ChemicalsAmmonium hydroxide, hydrogen peroxide and water (classic ratio 1:1:5)Hydrochloric acid, hydrogen peroxide and water (classic ratio 1:1:6)Hydrofluoric acid diluted with water (about 0.5 to 1% as a guide)
pH and temperatureAlkaline; typically about 70 to 80 °CAcidic; typically about 70 to 80 °CAcidic; usually room temperature
Mainly removesParticles and organicsMetal contamination (alkali metals, Fe, Al and others)Native and chemical oxide, along with contamination trapped in it
MechanismPeroxide oxidizes the surface and ammonia slowly dissolves that oxide, lifting particles from below; surface and particles are both negatively charged, so particles do not stick backDissolves metals as soluble ions such as chloridesDissolves SiO2
Etching of silicon and oxideSlight (oxide roughly 0.1 to 1 nm per minute as a guide)Almost noneEtches oxide; silicon is barely etched
Surface after cleaningThin chemical oxide; hydrophilicThin chemical oxide; hydrophilicHydrogen-terminated; hydrophobic
Side effects and cautionsCan roughen the surface (too little peroxide, or high concentration or temperature); metals such as Fe and Al in the solution tend to deposit on the surfaceNot very effective at removing particlesThe hydrophobic surface attracts particles and water marks; noble metals such as Cu tend to plate out on the surface; oxide that should remain is also thinned
Usual position in the sequenceFirst (after SPM when there are heavy organics), to remove particles and organicsAfter SC-1, to remove metals deposited by SC-1Between SC-1 and SC-2, or last (HF-last)

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

SC-1
As the first cleaning step, to remove particles and organics. It is used wherever particles matter, such as cleaning after CMP.
SC-2
After SC-1, when metal contamination must be kept especially low, for example before gate oxidation or heat treatment.
DHF
When a bare, oxide-free silicon surface is needed, for example just before epitaxial growth, silicide formation or gate oxidation.

FAQ

Why does SC-2 follow SC-1?

Alkaline SC-1 removes particles and organics well, but metals in the solution, such as Fe and Al, tend to deposit on the surface. Acidic SC-2 then removes them by turning them into soluble chlorides and similar ions.

Why does the surface repel water after dilute HF?

Once the oxide has dissolved, the silicon surface is terminated by bonds to hydrogen. The fact that water beads up confirms that the oxide is gone. This surface attracts particles easily, though, so the waiting time before the next step is controlled.

How is surface roughening by SC-1 avoided?

The basic rule is to keep enough hydrogen peroxide so that the surface stays covered by oxide while it is etched. Today the chemicals are often diluted and used at lower temperature, combined with megasonic cleaning, to remove particles while etching less.

How is it checked that metal contamination has been removed?

Methods such as total reflection X-ray fluorescence (TXRF) measure the amount of metal on the wafer surface. Sensitivity improves further if the surface oxide is first dissolved with HF vapor and collected before analysis. Particles are counted with inspection tools that detect laser light scattered by them.

Check your understanding

What is the main purpose of SC-1 (a mixture of ammonia and hydrogen peroxide) in RCA cleaning?

  1. ARemoving metal contamination
  2. BRemoving particles and organics
  3. CRemoving native oxide
Show answer and explanation

Answer: B. Removing particles and organics

SC-1 (NH4OH:H2O2:H2O, e.g., 1:1:5 at about 70 °C) oxidizes the surface with peroxide while ammonia slowly dissolves that oxide at roughly 0.1–1 nm per minute, undercutting and lifting particles away. In the alkaline solution, particles and surface both charge negatively and repel, preventing redeposition, and organics are removed as well. SC-2 removes metal contamination, and dilute HF removes native oxide.

What is the main purpose of cleaning with dilute hydrofluoric acid (DHF)?

  1. ARemoving organics
  2. BDepositing metal films
  3. CRemoving native oxide
Show answer and explanation

Answer: C. Removing native oxide

HF dissolves SiO2, so tens of seconds in 0.5–1% dilute HF strips the roughly 1 nm native oxide from silicon. The cleaned surface is hydrogen-terminated and hydrophobic—water beading confirms removal. The treatment is often performed just before gate oxidation, epitaxy or silicide formation. SC-1 removes organics and particles, and SC-2 removes metals.

Which correctly describes the order and purposes of SC-1 and SC-2 in standard RCA cleaning?

  1. ASC-1 removes particles and organics, then SC-2 removes metal contamination
  2. BSC-2 removes particles, then SC-1 removes metal contamination
  3. CSC-1 and SC-2 are mixed and used together
Show answer and explanation

Answer: A. SC-1 removes particles and organics, then SC-2 removes metal contamination

The RCA clean, published by Kern and colleagues at RCA in 1970, first removes particles and organics with alkaline SC-1, then removes metals—which SC-1 tends to leave behind—as chlorides with acidic SC-2 (HCl:H2O2:H2O). A dilute-HF step may be inserted to strip oxide, with ultrapure-water rinses between steps. More than 50 years on, refined versions remain the foundation of wafer cleaning.

  1. What is the state of a silicon surface whose last cleaning step was dilute hydrofluoric acid?
  2. In SC-1 (a mixture of ammonia and hydrogen peroxide), what role does the hydrogen peroxide play?
  3. Which method is often used to analyze trace metal contamination on wafer surfaces?

Practice CMP and cleaning questions

Key terms explained

RCA clean
A cleaning method that removes particles and organics with SC-1 (ammonia and hydrogen peroxide) and metal contamination with SC-2 (hydrochloric acid and hydrogen peroxide).
SC-1 (APM)
A mixture of ammonia, hydrogen peroxide and water used first in RCA cleaning. It removes particles and organics while lightly etching the surface.
SC-2 (HPM)
A mixture of hydrochloric acid, hydrogen peroxide and water used after SC-1 in RCA cleaning to remove metal contamination.
Dilute hydrofluoric acid (DHF)
Hydrofluoric acid diluted with water. It removes native oxide from silicon and leaves a hydrophobic, hydrogen-terminated surface.
Native oxide
A thin oxide about 1 nm thick that forms naturally when silicon is exposed to air or water. Because it raises contact resistance and disturbs epitaxial growth, it is removed right before such steps.
SPM (sulfuric acid-hydrogen peroxide mixture)
A mixture of sulfuric acid and hydrogen peroxide. Its strong oxidizing power removes organics such as resist.
Megasonic cleaning
A cleaning method that applies ultrasound of about 1 MHz to the cleaning solution to remove fine particles from wafer surfaces.
TXRF (total reflection X-ray fluorescence)
An analysis method that shines X-rays at a grazing angle so they totally reflect, then sensitively measures the fluorescent X-rays from trace metal contamination on the wafer surface.

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