Wafer Cleaning and Rinsing Drying

1. Wafer Cleaning

During the storage, handling, and processing of wafers, it is inevitable that micro- or even nano-scale dust particles and trace impurities will be attached to them-if not completely removed, these contaminants will directly lead to circuit pattern defects, insulating film leakage, or metal wiring corrosion, ultimately leading to device failure. Therefore, the cleaning process accounts for 20%~30% of the total manufacturing man-hours and has become the core link to ensure the stability of the process.

1.1 Dust is thoroughly removed by washing: chemical and physical decomposition

From the technical path, cleaning mainly relies on the synergistic effect of chemical decomposition and physical decomposition.

For example, APM (ammonium hydroxide-hydrogen peroxide-water mixture) can effectively remove organic residues and particles, and FPM (hydrofluoric acid-hydrogen peroxide-water) has high selectivity for metal impurities on the surface of the oxide film. SPM (sulfuric acid-hydrogen peroxide, commonly known as "piranha solution") can decompose stubborn photoresist residues due to its strong oxidizing properties. Physical cleaning uses mechanical forces such as ultrasound, megasonic waves or high-pressure injection to assist chemical liquids in penetrating small gaps and improving cleaning efficiency. For the sensitive stage after metal wiring, organic solvents such as alcohol and acetone should be used to replace acidic chemical solutions to avoid the risk of metal corrosion.

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1.1 cleaning equipment

At the equipment level, wet cleaning equipment is divided into two categories: tank type and monolithic type: tank equipment realizes chemical liquid gradient cleaning through multi-tank series, which is suitable for batch processing; The monolithic equipment realizes the fine cleaning of wafer monoliths by means of rotary spraying and brushing, which is more suitable for the strict control of local contamination by advanced processes. In recent years, dry cleaning technologies such as carbon dioxide snow cleaning and ozone plasma treatment have accelerated their development under environmental protection and cost pressure due to the advantages of no wastewater discharge and low chemical pollution.

For example, low-temperature plasma cleaning can efficiently remove nanoscale particles without damaging sensitive structures by bombarding the surface with active particles, and has been widely used in cleaning scenarios between 3D NAND stacks.

At present, the cleaning process is evolving in the direction of green and intelligence. The research and development of new environmentally friendly chemical solutions such as fluorine-free APM and biodegradable chelating agents has effectively reduced the risk of heavy metal wastewater discharge. The AI-based real-time monitoring system can dynamically adjust the process parameters by analyzing the cleaning solution composition, particle concentration and surface reflectivity to achieve closed-loop optimization of cleaning effect. These technological iterations not only improve the removal of nanoscale contaminants, but also provide key guarantees for the reliability of high-density interconnect structures in emerging fields such as 3D integration and advanced packaging, and continue to promote semiconductor manufacturing towards the goal of higher yield and lower defect rate.

2. Rinse and dry the wafer after cleaning

The rinsing process is carried out with ultrapure water, which accounts for a significant proportion of the total ultrapure water consumption of the semiconductor factory, and it is necessary to ensure that the chemical solution is completely removed through multi-stage rinsing to avoid the potential impact of residues on subsequent processes and device performance. After the rinsing, the complete removal of residual moisture on the wafer surface becomes the core goal, and the drying process needs to meet multiple requirements such as no watermark, no foreign matter adhesion, and electrostatic protection.

2.1 Rotary drying method

The rotational drying method uses centrifugal force to remove moisture through high-speed rotation of the wafer, but the friction between the wafer surface and nitrogen during the rotation process is prone to static electricity, which may cause the risk of electrostatic breakdown of the device.

For this reason, it is necessary to match the electronic shower for static neutralization treatment to ensure the safety of the process. The advantages of this method are simple operation and low cost, but it requires high equipment accuracy and environmental cleanliness, and the rotation speed and nitrogen purity need to be strictly controlled to avoid secondary pollution.

2.1 Isopropyl alcohol drying method without leaving a mark

The isopropyl alcohol drying method is optimized for the watermark problem. The essence of watermark is the traces of silicon oxide hydrate and impurities formed on the surface of the wafer by residual moisture during the drying process, which is closely related to the hydrophobicity of the silicon substrate and the local water droplet retention caused by uneven drying.

Due to its low surface tension and good interdissolution with water, isopropyl alcohol can effectively replace water and reduce the probability of watermark formation. The specific implementation methods include three mainstream technologies: isopropyl alcohol steam drying by placing the rinsed wafer in an isopropyl alcohol vapor environment, using steam to replace the moisture on the surface of the wafer and drying it; Marangoni drying is to apply isopropyl alcohol vapor and nitrogen simultaneously along the interface between the wafer and water when the wafer is lifted from ultrapure water, and the water is driven to retreat rapidly through the surface tension gradient to avoid water droplets dragging and residue. Rotagoni drying combines the dual advantages of rotary drying and marangoni drying, accelerating water evaporation through rotation while using isopropyl alcohol vapor to form a surface tension gradient, achieving more efficient drying effects and further inhibiting the formation of watermarks.

In recent years, with the advancement of semiconductor process nodes to smaller sizes, higher requirements have been put forward for the cleanliness, uniformity and environmental protection of the drying process. New drying technologies such as plasma-assisted drying and supercritical carbon dioxide drying are gradually entering the research field, the former realizes contactless drying through plasma-activated surfaces, and the latter uses supercritical fluid characteristics to achieve surface-tension-free drying, effectively avoiding watermarks and static electricity problems. At the same time, environmental protection measures such as the optimization of isopropyl alcohol recycling and reuse systems and the development of low global warming potential (GWP) alternative solvents have also become the focus of the industry, promoting the development of semiconductor cleaning and drying processes in a more efficient, greener and more reliable direction.