The first step (called SC-1, where SC stands for Standard Clean) is performed with a 1:1:5 solution of NH4OH + H2O2 + H2O at 75 or 80 degrees (Celsius). This treatments results in the formation of a thin silicon dioxide layer (about 10 Angstrom) on the silicon surface, along with a certain degree of metallic contamination (notably Iron) that shall be removed in subsequent steps.
The second step is a short immersion in a 1:50 solution of HF + H2O at 25 degrees Celsius, in order to remove the thin oxide layer and some fraction of ionic contaminants.
The third and last step (called SC-2) is performed with a 1:1:6 solution of HCl + H2O2 + H2O at 75 or 80 degrees Celsius. This treatment effectively removes the remaining traces of metallic (ionic) contaminants.
RCA cleaning (also known as SC1/SC2 etching) submits silicon wafers to oxidation by NH3:H2O2:H2O mixtures, oxide removal in diluted HF, further oxidation by HCl:H2O2:H2O mixtures, and final etching in diluted HF.