Give the steps in a standard RCA cycle during wafer cleaning.
1 Answer

Contamination on a wafer consists of particles which must be removed by wafer cleaning. In order to remove metallic impurities, organic contaminants etc. wafers were subjected standard RCA cleaning procedure.

The purpose of the RCA clean is to remove organic contaminants (such as dust particles, grease or silica gel) from the wafer surface; then remove any oxide layer that may have built up; and finally remove any ionic or heavy metal contaminants.

First method uses an $H_2 \space SO_4$ and a strong oxidant (usually $H_2O_2$) to decompose the resist Into $CO_2$ and $H_2O$.

The second method uses oxygen plasma to convert resist to gaseous byproducts (again $CO_2$ and $H_2O$. This reduces pollution problems. Used in removal of photoresist in backend processes. Wafer cleaning is usually accomplished today either using immersion of cassettes of wafers into cleaning baths or through chemical sprays.

SC-1(standard cleaning 1 or APM) and SC-2(standard cleaning 2 or HPM) are used for the metal removal.

The SC-1 solution is high- pH solution consisting of water, hydrogen peroxide and $NH_4OH$ (in ratio 5:1:1), in which wafers are placed at $70-80^0C$ for 10 minutes. Then the wafer is rinsed under running DI water to remove the solution. In this process it oxidizes the silicon and leaves a thin oxide on the surface of wafer, which should be removed if pure silicon is needed.

The SC-2 solution is a low – pH solution consisting of water, hydrogen peroxide and HCl. It is designed to dissolve and remove from the Si surface alkali residues and any residual trace metals (such as Au and Ag), as well as metal hydroxides, including $Al(OH)_3 , Fe(OH)_3 , Mg(OH)_2$ ,and $Zn(OH)_2$.

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