Overview of Wafer Cleaning Equipment

Overview of Wafer Cleaning Equipment: Application and Development of Megasonic Cleaning Wafer Technology

After wafer cutting, a large number of particles composed of polymers, photoresist, and etching residues often adhere to its surface. If these pollutants are not effectively removed, they will have a significant impact on the geometric accuracy and electrical performance of the chips in subsequent processes. The binding force between pollutants and wafer surfaces mainly originates from physical adsorption dominated by van der Waals forces. Therefore, the industry generally adopts physical or chemical methods for “bottom cutting” treatment, gradually weakening the contact area between pollutants and substrates, and ultimately achieving their desorption.
As semiconductor manufacturing processes move towards finer nodes, the size of pollutants is increasingly approaching the nanometer level, and the difficulty of removal has sharply increased. At the same time, the sharp increase in the number of tiny particles, coupled with the continuous shrinking of wafer graphic structure dimensions, poses unprecedented complexity challenges to the cleaning process. In this context, the key role of high-performance wafer cleaning equipment is increasingly prominent.

Principle of wafer cleaning equipment

Wafer cleaning technology is mainly divided into two categories: wet cleaning and dry cleaning. Currently, wet cleaning dominates. Although the market share of wet cleaning may slightly adjust in the future development with the gradual maturity of dry cleaning technology, it is expected to maintain its mainstream position, especially in handling complex 3D NAND stack structure cleaning, copper interconnect processes and other key links with unique advantages. Wet cleaning uses liquid chemical reagents to remove metal residues, organic matter, and metal ions from the surface of wafers through oxidation, etching, and dissolution processes. It can be further divided into brush cleaning and chemical cleaning methods.

Brushing method: With a dedicated brush as the core, it can effectively remove particles of 1 micron or more on the surface of the wafer, and is commonly used for preliminary cleaning after cutting or polishing. These types of equipment are usually equipped with chemical cleaning solutions (such as ultrapure water or specific solvents). For example, when using a brush made of polyvinyl alcohol (PVA) material, a thin film of cleaning solution will form on the surface of the wafer during operation. By utilizing the hydrophobicity of the wafer surface, the solution generates a repulsive effect, thereby carrying away pollutants suspended in the liquid film.
Chemical cleaning method: mainly includes immersion cleaning tank, megasonic cleaning tank, and rotary spray cleaning equipment. Its core principle is based on the classic RCA cleaning technology and its derivative processes, using solvents, acids, surfactants, and water to remove pollutants through multiple steps such as spraying, purification, oxidation, etching, and dissolution.
Immersion cleaning tank: After placing the wafer in a dedicated flower basket, immerse it in a chemical tank for cleaning, and then transfer it to the water tank for rinsing. The material of the tank body (such as polypropylene PP, polyvinylidene fluoride PVDF or quartz glass) should be selected according to the chemical properties (acidity and alkalinity) and temperature requirements of the cleaning solution. PP material is commonly used in room temperature tanks; High temperature tanks (>180 ° C) usually use quartz tank inner tanks with PP outer tanks and heating films; Hydrofluoric acid (HF) cleaning commonly uses PVDF heating tanks, which use coil or plate immersion heating.
Megasonic cleaning tank: This technology (i.e. Megasonic cleaning) introduces high-frequency acoustic energy (usually 800kHz~1MHz) on the basis of immersion cleaning. Megasonic cleaning of wafers can significantly improve cleaning efficiency, reduce the consumption of chemical reagents and ultrapure water by more than 30%, shorten etching time, reduce the impact on fine circuit structures, and extend the service life of cleaning solutions. The core megahertz transducer is installed at the bottom of the slot and is divided into flat plate and circular arc plate types, among which the circular arc plate has better performance due to more uniform energy propagation. The tank body is often designed with a quartz inner tank water bath to prevent corrosion of the transducer; The bottom of the groove is designed with an inclination angle of 10 °~15 °, which is conducive to the upward discharge of bubbles and reduces the absorption of sound energy. The outer bath of the water bath can be made of stainless steel or quartz material.
Rotating spray cleaning equipment: As an upgraded solution for immersion cleaning, it integrates an automatic liquid dispensing system, a sealed chamber, and a waste liquid recovery system. The automatic liquid spraying system can instantly mix chemical reagents to ensure the freshness of the solution; Rotating combined with spraying makes the solution distribution more uniform; The sealed chamber effectively suppresses the evaporation of chemical liquids, reduces losses and environmental impacts, and can reduce the overall dosage of chemicals by about 30%.

Dry cleaning, as an emerging technology, does not require liquid solvents and achieves cleaning through gas-phase chemical reactions or ion bombardment. It has the advantages of no waste liquid and local selective treatment, mainly including thermal oxidation and plasma cleaning methods. The thermal oxidation method utilizes an oxidation furnace to introduce hot gas and remove reaction products through a vacuum pump; The plasma cleaning method uses plasma in a vacuum environment to cause charged pollutants to detach from the surface due to charge repulsion. In addition, other methods such as mechanical scrubbing and beam cleaning have also been applied in specific fields.

The Development of Wafer Cleaning Equipment
Looking back at the development process, in the early stages of semiconductor IC manufacturing in the 1950s, wafer cleaning mainly relied on simple brushing. In the mid-1960s, the industry developed the iconic RCA cleaning method, and in the early 1970s, nylon material brushes and supporting equipment were introduced, effectively solving the problem of removing metal particles after polishing. Afterwards, the material of the brush gradually optimized to low damage materials such as polypropylene (PP) and polyvinyl alcohol (PVA), and the structure also tended towards a loose and porous design. In the early 1970s, the first immersion wet cleaning machine capable of processing 4 wafers simultaneously was introduced. In the 1970s and 1990s, wet cleaning technology significantly improved cleaning efficiency by introducing megasonic cleaning tanks, rotary cleaning tanks, and increasing the number of chambers (advanced equipment can reach 12 chambers, with a production capacity of about 375 pieces/hour). After the 1990s, with the popularization of copper interconnect technology, dilute acid cleaning gradually replaced strong acid solvents, and the combination of ozone and megahertz ultrasonic technology further improved cleaning accuracy. Since 2010, with the process nodes entering 14 nanometers and below, dry cleaning has begun to be applied due to its low damage characteristics, but the technology is not yet fully mature, and wet cleaning is still mainstream at present. Mature cleaning equipment systems typically include wet cleaning machines, component cleaning machines, liquid supply systems, and auxiliary equipment such as drying and storage devices.
Looking ahead, the continuous miniaturization of integrated circuit processes and the widespread application of 3D structures will continue to drive the development of the cleaning equipment market. On the one hand, the reduction in line width leads to an increase in the total amount of pollutants that need to be cleaned per unit area, which puts higher demands on equipment efficiency; On the other hand, complex 3D structures require cleaning processes to penetrate deep into their interior without damaging fine structures, which poses a new challenge to equipment technology innovation. Efficient and low damage technologies such as megasonic cleaning of wafers will play an increasingly important role in addressing these challenges.

About Cheersonic

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.

Our coating solutions are environmentally-friendly, efficient and highly reliable, and enable dramatic reductions in overspray, savings in raw material, water and energy usage and provide improved process repeatability, transfer efficiency, high uniformity and reduced emissions.

Chinese Website: Cheersonic Provides Professional Coating Solutions