OTS (September 2008) -- Dissolved gases have long been known to have significant impact on wet cleaning processes. Dissolved oxygen in rinse water contributes to surface oxidation and watermark formation during wafer drying [1-2]. Dissolved gases can affect megasonics processes and are often controlled in order to optimize particle removal efficiency and prevent pattern damage [3]. In addition, dissolved CO₂ is well-known to form carbonic acid, affecting the pH of liquid cleaning solutions. More recently, there is an increased concern about the effect of dissolved oxygen when cleaning surfaces with different metals exposed.

Equivalent Scaling or “Scaling by Innovation” [4], has enabled the semiconductor industry to stay on the “Moore’s Law” curve of increased circuit density and improved performance. Over the past several years, Equivalent Scaling has meant the introduction of new materials for building devices and circuits on the surface of the silicon wafer. Copper wiring and low-k dielectrics are now well established as replacements for aluminum and silicon dioxide in interconnects. We are now seeing the introduction of additional metal layers, such as CoWP [5], in the interconnect area and also the use of metal gates and high-k materials in the transistor fabrication. The presence of different metals on the substrate surface during cleaning operations makes them susceptible to galvanic corrosion [6]. One way to minimize galvanic corrosion is to minimize the amount of dissolved oxygen in the cleaning solution.

Liquid solutions in contact with the atmosphere can absorb a significant amount of dissolved oxygen. At 20°C, the saturation level of dissolved oxygen in water in contact with the atmosphere (20% oxygen) is about 9ppm [7]. This level of oxygen can lead to significant self-corrosion of copper in contact with very dilute HF solution [8]. When cleaning a structure that has exposed cobalt with dilute HF the corrosion rate of Co can be as high as 6 nm/min. Reducing the dissolved oxygen to less than 20ppb can almost eliminate the corrosion of copper and cobalt in dilute HF solutions.

The keys to wet cleaning with low dissolved oxygen are to first deoxygenate the cleaning solution and second to control the environment around the wafer surface during the cleaning process. FSI has long recognized the importance of removing dissolved gas and controlling the wafer environment during wet cleaning. FSI critical cleaning systems are equipped with degassing units to eliminate dissolved oxygen from water used for dilution and rinsing. FSI closed chamber spray systems are designed to provide a controlled environment with nitrogen purging capability to eliminate atmospheric oxygen for rinsing, drying, and chemical cleaning processes.

REFERENCES

1. Watanabe et al., “The Role of Atmospheric Oxygen and Water in the Generation of Water Marks on the Silicon Surface in Cleaning Processes,” Materials Science and Engineering B 4(1-4), 401(1989).
2. Li et al., “Effects of Ambient and Dissolve Oxygen Concentration in Ultrapure Water on Initial Growth of Native Oxide on a Silicon (100) Surface,” J. Electrochem. Soc. 152(8), G669(2005).
3. Vereecke et al., “Evaluation of Megasonic Cleaning for sub-90-nm Technologies,” Solid State Phenom. 103-104, 141(2005).
4. Butterbaugh, “Strain Engineering for Mobility Enhancement Drives Wet Processing Challenges,” FSI On The Surface e-Newsletter, July 2006.
5. Singer, “The Advantages of Capping Copper With Cobalt,” Semicon. Int., (is this how to abbr Semi Int’l?) October 2005.
6. Preusse et al., “Integration Aspects of CoWP Capping Layers for Electromigration Enhancement,” IITC 2008 Proceedings, 123(2008).
7. CRC Handbook of Chemistry and Physics.
8. Levy et al. “The Control of Dissolved Gases in Aqueous-Based Semiconductor Processing,” ECS Proceedings, Volume 99-36, 121(2000).

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