Comparison of ultrasonic and megahertz cleaning technologies

1. Introduction

As semiconductor manufacturing process nodes continue to shrink to the nanometer level, the cleanliness requirements for silicon wafers are becoming increasingly stringent. Traditional cleaning methods can no longer meet the requirements of advanced processes for controlling surface contaminants. Ultrasonic and megasonic cleaning technologies, as physical auxiliary cleaning methods, have demonstrated unique advantages in the semiconductor manufacturing field.

2. Ultrasonic Cleaning Technology

2.1 Working Principle

Ultrasonic cleaning utilizes high-frequency sound waves (20-400kHz) to generate cavitation in liquids. When sound waves pass through the cleaning liquid, periodic compression and expansion occur, forming micron-sized bubbles. These bubbles expand rapidly during the negative pressure cycle and collapse violently during the positive pressure cycle, generating local high temperatures (approximately 5000K) and high pressures (approximately 50MPa), accompanied by strong microjets with impact speeds of 100-200m/s.

2.2 Technical Characteristics

According to the latest research, ultrasonic cleaning technology has the following advantages:

1. High cleaning efficiency: Cavitation can penetrate complex geometric structures, and is significantly effective in areas such as deep holes and blind holes that are difficult to clean with traditional methods.

2. Multifunctionality: It can be used in conjunction with various chemical cleaning agents and is suitable for removing various contaminants such as organic matter and particulate matter.

3. Process compatibility: The latest equipment can automatically adjust the frequency (20-400kHz) to adapt to different pollution types and device structures.

4. Economy: The equipment cost is relatively low, and maintenance is simple.

However, there are the following limitations:

1. Risk of surface damage: High-frequency cavitation may cause microcracks on the surface of brittle materials.

2. Particle size limitation: The removal efficiency for particles <0.5μm is significantly reduced.

3. Transducer lifespan: Under high-intensity working conditions, piezoelectric transducers are prone to aging. The latest research uses a composite transducer design to extend the lifespan to over 8000 hours.

3. Megasonic Cleaning Technology

3.1 Working Principle

Megasonic cleaning uses high-frequency sound waves (0.8-2MHz). Its mechanism of action is mainly based on:

1. Acoustic streaming: High-frequency vibrations form directional flow in the liquid, with speeds reaching 1-10cm/s.

2. Boundary layer disruption: Eliminates the stagnant layer at the solid-liquid interface, allowing the cleaning liquid to fully contact the surface.

3. Radiation pressure: Generates stable acoustic radiation force, which can directionally control particle movement.

3.2 Technical Characteristics

The latest research shows that megasonic cleaning has the following advantages:

1. Submicron particle removal: Effectively removes tiny particles of 0.05-0.2μm, meeting the requirements of processes below 10nm.

2. Zero surface damage: Avoids cavitation effects, especially suitable for cleaning fragile structures such as low-k dielectrics and FinFETs.

3. Self-cleaning effect: Linear acoustic streaming prevents recontamination, with cleaning uniformity >95%.

4. Environmentally friendly: Can reduce the amount of chemical reagents by 30-50%.

Technical challenges include:

1. Equipment cost: High-frequency generators are expensive, 2-3 times the cost of traditional ultrasonic equipment.

2. Process optimization: Requires precise control of sound intensity (usually 0.1-1W/cm²) to avoid standing wave formation.

4. Technology Comparison and Application

Comparison of Ultrasonic and Megasonic Cleaning Technology Parameters

5. Conclusion and Outlook

Ultrasonic and megasonic cleaning technologies each have their own advantages and show a complementary development trend in semiconductor manufacturing.

As the size of semiconductor devices continues to shrink, megasonic cleaning technology will play a more important role, while ultrasonic cleaning will still maintain a key position in specific application scenarios. The integrated development of the two technologies will provide more efficient and environmentally friendly cleaning solutions for semiconductor manufacturing.

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