wet chemical cleaning processes

Fundamentals, Processes and Applications

Wet chemical cleaning is a key process step in numerous high-tech industries.

It is used whenever surfaces must be reliably freed from contamination and the highest requirements regarding cleanliness, functionality, and process stability must be achieved.

Typical applications can be found in:

• semiconductor industry
• microelectronics
• medical technology
• optics
• glass processing
• research and development

 

Different cleaning processes are used depending on:

• material
• type of contamination
• required surface quality
• subsequent process steps

 

The most important processes include:

• RCA cleaning
• SC1 cleaning
• SC2 cleaning
• Piranha cleaning
• HF dip cleaning
• ultrasonic cleaning
• megasonic cleaning

 

Why is Industrial Cleaning Important?

Surface quality is a decisive factor for product performance and reliability.

Even the smallest contamination can:

• affect subsequent processes
• reduce adhesion
• alter electrical properties
• cause defects
• increase rejection rates

 

A professional cleaning process specifically removes:

• particles
• organic residues
• metal contamination
• oxide layers
• process residues

 

Which Types of Contamination Exist?

 

Particles

Particles can occur due to:

• mechanical processes
• handling
• manufacturing environments

They can be particularly critical for:

• semiconductor structures
• optical components
• precision parts

 

Organic Contamination

This includes:

• oils
• greases
• polymer residues
• photoresist residues

These residues can negatively affect subsequent process steps.

 

Metallic Contamination

Metal ions such as:

• iron
• copper
• nickel
• sodium

can negatively influence materials and electrical properties.

 

Oxide Layers

Many materials naturally form oxide layers on their surfaces.

Example:

Silicon forms a thin silicon oxide layer when exposed to air.

For certain processes, this oxide layer must be removed in a controlled manner.

RCA Cleaning

Standard Process for Chemical Surface Cleaning

RCA cleaning is one of the most established wet chemical cleaning processes.

Originally developed for cleaning silicon wafers, it is now considered a reference process for achieving highly clean surfaces.

The process mainly consists of:

• SC1 cleaning
• SC2 cleaning

Each step fulfills a different cleaning function.

SC1 Cleaning

Removal of Particles and Organic Contamination

SC1 stands for:

Standard Clean 1

The typical process chemistry consists of:

• ammonium hydroxide (NH₄OH)
• hydrogen peroxide (H₂O₂)
• ultrapure water (H₂O)

and is commonly referred to as:

APM (Ammonia Peroxide Mixture)

 

SC1 Operating Principle

The alkaline chemistry dissolves organic contamination and supports particle removal.

Hydrogen peroxide acts as an oxidizing agent.

Ammonium hydroxide supports:

• particle removal
• removal of organic residues
• surface activation

 

Removed Contaminants

SC1 primarily removes:

• particles
• dust
• organic residues
• process residues

 

Typical Applications

SC1 is used for:

• silicon wafers
• glass substrates
• precision components
• research substrates

 

SC2 Cleaning

Removal of Metallic Contamination

SC2 stands for:

Standard Clean 2

The typical process chemistry consists of:

• hydrochloric acid (HCl)
• hydrogen peroxide (H₂O₂)
• ultrapure water

also known as:

HPM (Hydrochloric Peroxide Mixture)

 

SC2 Operating Principle

SC2 removes metallic contamination through chemical reactions and complex formation.

Hydrochloric acid supports the dissolution and removal of metallic contaminants.

Removed Contaminants

SC2 specifically removes:

• iron
• copper
• nickel
• other metal ions

 

Importance of SC2 Cleaning

For highly sensitive applications, the removal of metallic residues is critical.

SC2 is particularly relevant for:

• semiconductor manufacturing
• sensor technology
• microelectronics

 

RCA Cleaning Process Sequence

A typical RCA cleaning process consists of:

Step 1

SC1 Cleaning

Removal of:

• particles
• organic residues

Step 2

Ultrapure Water Rinse

Removal of chemical residues

Step 3

SC2 Cleaning

Removal of:

• metal ions
• metallic contamination

Step 4

Rinsing and Drying

Creation of a defined and clean surface condition

 

HF Dip Cleaning

Removal of Oxide Layers Using Hydrofluoric Acid

The HF Dip process is a wet chemical method used to remove silicon oxide and other oxide layers from surfaces.

HF stands for:

Hydrofluoric Acid

Why is HF Dip Used?

Many materials naturally form oxide layers on their surfaces.

For silicon, for example:

Si + O₂ → SiO₂

This oxide layer can influence subsequent process steps.

HF removes this layer in a controlled manner.

Typical Applications of HF Dip

• wafer cleaning
• removal of native oxide layers
• surface preparation before coatings
• surface activation

 

Advantages of HF Dip

• defined surface condition
• effective oxide layer removal
• preparation for subsequent process steps

 

Piranha Cleaning

(H₂SO₄/H₂O₂) – Highly Effective Wet Chemical Cleaning for Organic Contamination

Piranha cleaning is an established wet chemical process for removing organic contamination.

It is particularly used in semiconductor manufacturing, medical technology, and high-purity glass and substrate applications.

Due to its extremely strong oxidizing capability, it is considered one of the most effective cleaning methods in wet bench processes.

Operating Principle of Piranha Cleaning

The Piranha solution typically consists of a mixture of:

• sulfuric acid (H₂SO₄)
• hydrogen peroxide (H₂O₂)

When mixed, a highly reactive and exothermic solution is created, generating strong oxidative radicals. These radicals attack organic residues and completely break them down into gaseous or water-soluble products.

 

Reaction Mechanism:

• oxidation of carbon compounds → CO₂, H₂O
• destruction of polymer residues, photoresist, and biofilms
• simultaneous surface activation through hydrophilization

 

Typical Process Parameters

• mixing ratio: 3:1 to 5:1 (H₂SO₄ : H₂O₂)
• temperature: 80–130 °C (self-heating reaction)
• process time: 5–20 minutes
• application: immersion bath or heated process tank

 

Applications

Semiconductor Industry

• removal of photoresist residues after lithography
• wafer cleaning before critical process steps
• preparation for oxidation or coating processes

 

Medical Technology

• cleaning of implant surfaces (e.g. titanium)
• removal of organic production residues
• creation of defined, clean surfaces for subsequent processes

 

Glass and Substrate Cleaning

• removal of extremely thin organic films
• surface activation for coatings

 

Advantages of Piranha Cleaning

• extremely high cleaning performance for organic contamination
• fast process times
• uniform cleaning effect even on complex geometries
• combination of cleaning and surface activation

 

Disadvantages and Challenges

• high safety requirements (strongly exothermic and highly reactive)
• not suitable for metallic materials (risk of corrosion)
• high chemical consumption
• complex disposal requirements and demanding equipment material specifications

 

Integration into Wet Bench Processes

Piranha cleaning is frequently used as a pre-cleaning step and combined with additional wet chemical processes:

• → RCA cleaning (SC1 / SC2)
• → HF Dip (oxide removal)
• → rinsing processes (e.g. Quick Dump Rinser)
• → drying processes (e.g. Marangoni drying)

Typical process sequence:

Piranha → DI Water Rinse → HF Dip → DI Water Rinse → Drying

 

Technical Requirements for Wet Bench Systems

For safe and stable operation, the following aspects are essential:

• chemical-resistant materials (e.g. PP, PVDF, quartz)
• temperature-resistant process tanks
• precise dosing of H₂O₂
• effective exhaust systems and safety concepts
• automated process control for high repeatability

 

Conclusion

Piranha cleaning is a key process for applications requiring the highest cleanliness standards and plays an important role in modern wet chemical manufacturing. Through the combination of strong oxidation capability and reliable integration into automated Wet Bench systems, it represents an essential process step for demanding production environments.

A combination of precise equipment design and optimized process engineering makes this technology a decisive quality factor and competitive advantage in high-tech industries.

Ultrasonic Cleaning

Mechanical Support Through Cavitation

Ultrasonic cleaning uses high-frequency sound waves to remove contamination from surfaces.

Ultrasonic Operating Principle

Through sound waves, so-called:

Cavitation bubbles

are generated within the cleaning medium.

These bubbles form and collapse repeatedly.

This creates:

• micro-streaming effects
• localized pressure pulses
• mechanical cleaning forces

These forces remove:

• particles
• deposits
• surface contamination

 

Applications of Ultrasonic Cleaning

• precision components
• medical components
• metal parts
• glass components
• electronic components

 

Advantages of Ultrasonic Cleaning

• fast cleaning process
• effective particle removal
• cleaning of complex geometries

 

Megasonic Cleaning

High-Frequency Cleaning for Sensitive Surfaces

Megasonic cleaning is an advanced development of ultrasonic cleaning technology.

It operates at significantly higher frequencies.

Typical frequency ranges:

• ultrasonic: approx. 20–100 kHz
• megasonic: approx. 500 kHz–2 MHz

 

Megasonic Operating Principle

Due to the higher frequency, smaller cavitation bubbles are generated.

This results in:

• finer cleaning forces
• more uniform energy distribution
• gentler surface treatment

 

Advantages of Megasonic Cleaning

Especially suitable for:

• sensitive structures
• micro-components
• semiconductor wafers

Advantages:

• removal of extremely small particles
• low mechanical stress on surfaces
• high process control

 

Comparison of Cleaning Processes

Process	Main Purpose
SC1	Removal of particles and organic residues
SC2	Removal of metallic contamination
RCA	Combination of SC1 + SC2 cleaning
HF Dip	Removal of native oxide layers
Piranha Cleaning	Removal of organic contamination and oxidation of organic residues
Ultrasonic Cleaning	Mechanical cleaning using cavitation effects
Megasonic Cleaning	Gentle particle removal for sensitive surfaces

 

Selecting the Right Cleaning Process

The selection of the appropriate cleaning process depends on several factors.

Material

Examples:

• silicon
• glass
• titanium
• metals

 

Contamination Type

Examples:

• particles
• metal contamination
• organic residues
• oxide layers

 

Process Requirements

Examples:

• maximum cleanliness level
• material protection
• surface activation

 

Wet Bench Systems for Cleaning Processes

For reproducible cleaning results, industrial processes require precisely engineered equipment.

A professional Wet Bench system enables:

• controlled chemical supply
• defined temperature management
• reproducible process times
• safe process media handling
• automation capabilities

 

PaceTec Expertise

PaceTec develops customized Wet Bench systems for demanding cleaning applications.

The systems are individually designed for:

• RCA cleaning processes
• SC1 / SC2 cleaning
• HF Dip applications
• Piranha cleaning
• ultrasonic cleaning
• megasonic cleaning

The following requirements are taken into consideration:

• material compatibility
• chemical requirements
• process specifications
• automation level
• safety concepts