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Metal-free ozone in the fab

OZONE IS BECOMING WIDELY USED IN the semiconductor industry for many purposes, including disinfection, reduction of total organic carbon (TOC) in rinsing water, wafer cleaning and Si02 formation. The sterilization ability of ozone (due to its strong oxidizing power and the disruption of the bacteria’s membrane permeability) is well known and has been used for the sterilization of drinking water for many years. In the semiconductor industry, the sterili¬zation ability is used for the disinfection of process water to reduce the formation of contaminants on tubes and for the disinfection of rinsing water to pre¬vent defects on the wafer. Organic contaminants in the rinsing water could also cause a haze over the wafer surface that contaminates the wafer and leads to wafer defects. The ability of ozone to reduce these organic contaminants can be enhanced by the combination of ozone and UV light called Advanced Oxidation Process (AOP), in which hydroxyl radicals are formed with a higher oxidation potential than ozone alone.

The main application of ozone in the semicon-ductor industry is wet cleaning in combination with diluted hydrofluoric acid (DHF). The main advantages compared to the traditional RCA clean are the reduc¬tion of the consumption of hazardous chemicals and the saving of deionized (DI) water due to much shorter rinsing times. The main cleaning techniques IMEC clean, the Diluted Dynamic Clean (DDC) and the Ultra Clean Technology (UCT) – all first use a combination of DI water and ozone (DI03) for the removal of organic contamination and noble metals. The following steps for the removal of particles, metals and the oxide layer are different. For example, in IMEC cleaning DI water containing hydrofluoric and hydrochloric acid is used. A hydrophobic H-stabilized surface is obtained and if desired only rinsed with DI water. If a hydrophilic surface is desired, DI03 with HCL or another ozone mixture is used to re-grow a thin oxide layer. An advancement of the DHF/DI03 based wet cleaning technique is the single-wafer spin. cleaning (SCROD). DHF and DI03 are alternatively sprayed onto a rotating wafer until the wafer fits the purity requirements. In this case no impurities of other wafers can be transferred through the bath. A similar process using only DI03 can also be used for photoresist removal. DI03 directly oxidizes the resist

from the wafer surface, forming carbon dioxide and water. However, the process should take place under pressure and the ozone concentration should be as high as possible. An improvement in the ozone con¬centration at the wafer surface can be achieved by injecting ozone vapor into the process chamber. Ozone can diffuse through a controlled water boundary layer on the wafer surface built through the rotation, result¬ing in a surface ozone concentration and subsequent oxidation rates significandy higher than what can be achieved using ozone dissolved in water.

All these applications of ozone in the semiconductor industry require a high purity of ozone. Usually ozone is generated by a high-voltage volume discharge in which the metal electrode is in contact with the tracing gas. Therefore metal will be sputtered into the ozone gas and can contaminate the wafer surface. The high voltage electrode of a surface discharge ozone gen¬erator is generally made of tungsten, which is more resistant than the stainless steel generally used in the volume discharge generators. However, caused by the high density of energy at the electrode comers, the dielectricum will be eroded over time and particle filters should be installed before using the ozone in order to avoid contamination of the wafers. To avoid fouling of the electrodes and dropping of ozone capacity in both cases additional gases have to be added which remain in the process and have to be degassed with the excess ozone. Solutions (from ANSEROS, for one) exist today to meet semiconductor industry requirements. Ozone can be generated in clean quartz modules with¬out contact by the metal electrodes, and therefore no metals can contaminate the wafer surfaces. The ozone capacity remains constant with highest reliability.

The market for ozone generators is significant, with approximately 500 ozone generators sold each year in the US, 100 in Europe, and 200 in Asia. Ozone processing technology can reduce water, chemical, and energy consumption in semiconductor processing facilities, and a greener future looks possible as ozone gas supplants some chemicals in the production of semiconductors.

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