Application of Megasonic Cleaning Technology Throughout the Entire Wafer Manufacturing Process

1. Introduction

As integrated circuit manufacturing continues to advance toward 7 nm and below, control of surface contaminants on wafers has become one of the critical factors determining device performance and reliability. Conventional ultrasonic cleaning—operating at frequencies of approximately 20–100 kHz—can damage delicate patterned structures due to the intense shock waves generated by cavitation bubble collapse, rendering it unsuitable for advanced process nodes. In contrast, megasonic cleaning, with its higher frequency range (typically 800 kHz to 2 MHz) and gentler physical mechanisms, significantly reduces surface‑damage risks while maintaining high cleanliness, making it an indispensable cleaning technique for processes at 28 nm and below. This paper systematically examines the value and evolving trends of megasonic cleaning across the entire front‑end wafer‑fabrication flow, grounded in underlying physical principles and supported by typical application scenarios and equipment architectures.

2. Principles and Technical Characteristics of Megasonic Cleaning

Megasonic cleaning employs piezoelectric transducers to convert electrical energy into high‑frequency mechanical vibrations, which are transmitted through a coupling medium—deionized water or a chemical solution—to the wafer surface. Its cleaning effectiveness is primarily attributed to two mechanisms:

Acoustic Streaming: When high-frequency sound waves propagate through a liquid, they induce rapid macroscopic fluid motion, generating micro‑vortices that produce shear stress to detach adhered particles.

Stable Cavitation: At megahertz frequencies, cavitation bubbles oscillate nonlinearly without undergoing violent collapse. The resulting microjets and shock waves are significantly weaker than those produced by ultrasonic cavitation, enabling the gentle loosening of submicron particles without damaging delicate structures.

Compared with ultrasound, megasonic waves have shorter wavelengths (approximately 0.7–1.5 mm in water) and thinner boundary layers, achieving a particle removal efficiency (PRE) exceeding 90% for particles smaller than 0.2 μm. Moreover, megasonic cleaning exhibits excellent compatibility with chemical solutions and is often used in conjunction with standard cleaning baths such as SC1 (NH₄OH/H₂O₂/H₂O) and SPM (H₂SO₄/H₂O₂).

3. Application Scenarios in Typical Process Nodes

Zhaosheng cleaning is integrated throughout the front-end process of wafer fabrication, spanning multiple critical steps from post-CMP to before thin-film deposition, as follows:

• 3.1 Post-CMP Cleaning

After chemical mechanical polishing (CMP), the wafer surface retains silica or cerium oxide particles from the slurry, metal ions such as Cu²⁺ and Fe³⁺, and organic additives. Megasonic cleaning, in combination with SC1 solution, dissolves metal ions in an alkaline environment while acoustic streaming and cavitation work synergistically to remove sub‑micron particles. Experimental results demonstrate that this approach can reduce particle defect density by more than 90% without causing significant corrosion to copper interconnect structures.

• 3.2 Pre-Lithography Cleaning

Prior to photoresist coating, the wafer surface must achieve an exceptionally high level of cleanliness and uniform hydrophobicity. Megasonic cleaning effectively removes organic contaminants introduced during previous process steps—such as residues from anti-reflective layers and condensates from solvent vapors—as well as environmental particulates, thereby preventing defects like pinholes, bridging, or focus errors in the lithographic pattern. This step typically employs a diluted HF–megasonic combination, balancing the removal of native oxide layers with the elimination of organic contamination.

• 3.3 Post-Ion Implantation Cleaning

After high-energy ion implantation, the photoresist undergoes carbonization and hardening, while implanted metallic impurities (such as As, P, B, etc.) may remain on the surface. Megasonic cleaning is typically performed in conjunction with a high‑temperature SPM solution (a mixture of sulfuric acid and hydrogen peroxide) to remove the hardened resist residues through strong oxidizing action, while the acoustic field facilitates the diffusion of reaction products, thereby enhancing cleaning uniformity. This step is critical for the quality of subsequent annealing‑induced activation and the formation of the source–drain regions.

• 3.4 Pre-Deposition Cleaning of the Film

Prior to the deposition of the gate oxide, metal interconnects, or dielectric layers, it is essential to thoroughly remove the native oxide (chemically formed silicon dioxide) and adsorbed particles in order to reduce interface state density and contact resistance. In this process, megasonic cleaning is often combined with dilute hydrofluoric acid (dHF) or ozone‑water to achieve precise removal of oxide layers on the 1‑nm scale while preventing any increase in surface roughness of the underlying silicon substrate; this approach is particularly effective for pre‑gate‑oxide interface treatment in high‑k dielectric stacks.

4 Device Form Factors and Technology Comparison

Currently, megasonic cleaning equipment is primarily categorized into the following four types, each tailored to meet the specific requirements of different production lines and processes:

5. Technical Challenges and Development Trends

Although megasonic cleaning has been widely deployed in mainstream fabs, it still faces several key challenges:

Standing wave effect: Under normal incidence, the superposition of the reflected and incident waves creates standing-wave nodes, resulting in uneven cleaning. An oblique-incidence approach has been shown to be an effective solution.

Risk of structural damage: In FinFET or nanosheet architectures, high-frequency acoustic waves may induce mechanical resonance, necessitating further optimization of frequency and power parameters.

Extending acoustic field uniformity for large‑size wafers (300 mm → 450 mm) requires novel multi‑frequency transducer arrays and dynamic scanning techniques.

Future development directions include: intelligent frequency modulation (which continuously monitors reflected power and adaptively adjusts it), hybrid processes combining megasonic cleaning with low‑temperature ozone or supercritical CO₂, and AI‑based endpoint detection systems, all aimed at achieving precise, end-to-end digital control of the cleaning process.

6. Conclusion

Thanks to its gentle yet highly effective particle‑removal capabilities, megasonic cleaning has evolved from a standalone RCA‑compliant alternative into a core process‑step‑level cleaning technology that spans the entire flow—from post‑CMP through pre‑lithography, post‑ion implantation, and pre‑thin‑film deposition. In terms of equipment architecture, single‑wafer systems dominate advanced nodes, tank‑type systems balance high‑volume production, while spray‑nozzle and oblique‑incidence technologies continue to expand their application horizons. As process scaling advances and new materials are introduced, megasonic cleaning will keep evolving toward higher uniformity, reduced damage, and greater intelligence, solidifying its role as a critical enabler for maintaining chip yield.

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