Existing Processes for Cleaning

The existing cleaning methods can be divided into wet and dry cleaning. The wet cleaning process uses a combination of solvents, acids, surfactants and deionized (DI) water to spray and dissolve contaminants from the surface area. The Dl water is used to rinse after each chemical use. The oxidation of the wafer surface is sometimes integrated into the cleaning 

Dry cleaning, also called gas-phase cleaning, is based on excitation energy such as plasma, radiation or thermal excitation. This section will concentrate on the wet processes, the area where ozone is of interest. 

To better illustrate the purpose behind the cleaning steps, the conventional RCA cleaning process will first be examined in detail. It was developed for wafer cleaning in 1965, published in 1970 (Kern and Puotinen, 1970) and is still of importance. The following Table 6-1 shows the steps of the preliminary cleaning and the conventional RCA cleaning, as well as the purpose of each step. 

The RCA cleaning was developed at a time where the semiconductor industry was much smaller and the environmental restrictions were not as strict as today. Since then the goal in the development of new processes was to reduce the number of necessary cleaning steps, chemical consumption and waste disposal. Recent improvements in wet cleaning have been very successful in further reducing costs, chemical and water usage. Many advances are based on the use of ozonated ultra pure water (UPW) as a replacement for hydrogen peroxide or even sulfuric based mixtures (Heyns et al., 1999). 

The so-called IMEC clean (from the Interuniversity Microelectronic Center, set-up in 1984 by the Belgian Government) is one possible improvement. It is a simple two step process with an optional third step (Heyns et al., 1997). 

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