RCA Cleaning The RCA Cleaning Process by Scott Clark, MSCE   Overview A standard RCA clean consists of a combination of twoclassic chemistries. These chemistries are: SC-1, and SC-2 also known as RCA-1 and RCA-2. The functions of these chemistries, in the order which they occur, are:   1. Particle Removal   2. Metals Removal These steps ensure that all foreign substances from the surface of the silicon oxide which enhances the even distribution of the dopant into the underlying silicon lattice. This process is commonly added to an organic removal and an oxide etch process to form a pre-diffusion clean process. Particle Removal Background Particles are removed in this step by undercutting the oxide layer upon which the particles rest. That is, the native oxide layer is slowly dissolved which removes the particles by dislodging them and a new oxide layer is formed by oxidation of the “cleaned” surface. Chemistry and Application The chemistry used in this step is commonly called SC-1 (for Standard Clean-1) and is comprised of ammonium hydroxide, hydrogen peroxide, and DI water. This mixture removes any remaining organics by oxidative dissolution. Further, many metal contaminants (Au, Ag, Cu, Ni, Cd, Co, and Cr) are dissolved, complexed, and removed from the surface. The SC-1 compounds are mixed the volume ratio of 1:1:5; respectively, NH4OH (29 wt% as NH3), 30% H2O2, and DI water. The ammonia (NH3) serves as a mild oxide etchant, the hydrogen peroxide serves as a powerful silicon oxidizer. The peroxide continuously grows oxide only in areas in which the silicon is “bare” which results in a continual availability of oxide for the ammonia to remove. Wafers are usually held for 5-10 minutes in this solution which is typically at ca. 70°C, followed by quench and rinse with cold ultra-filtered DI water. Higher temperatures lead to more aggressive particle removal due to higher oxide etch rates. Care must be taken not to use excessive temperatures--in excess of 80°C--since this can cause hydrogen peroxide to rapidly evolve gas which can accumulate in the recirculation filter housing. This would lead to dewetting of the filter and, consequently a sharp decrease in the recirculation rate. High temperatures would also result in a loss of NH3. To assist in the removal of surface particles megasonic action is used. The position of the transducers is critical; they must be positioned so that the energetic waves travel parallel to the wafer surface. With this geometry the force vector generated by the megasonics radio frequency waves pushes the particles in a direction that carries them away from the wafer surface. This energy also keeps the particles moving so that they do not re-adhere to the surface of the wafer. Excessive amounts of ammonium hydroxide will lead to the etching of silicon this result is termed microroughening. Microroughening has detrimental effects on the quality and breakdown voltage characteristics of thin, thermally grown gate oxide films. It has been reported that a reduction of the NH4OH concentration in the 1:1:5 SC-1 mixture down to 0.1:1:5 or 0.01:1:5 not only eliminates roughening but also enhances the removal of particles.1 It has been suggested that a good compromise mixture in terms of particle removal efficiency and avoidance of microroughening of the silicon would have a volume ratio 0.25:1:5; NH4OH (29 wt% as NH3), 30% H2O2, and DI water, respectively. Metals Removal Background The next step is to remove any residual metals that are on the wafers at this point. Metals will be relatively abundant on the otherwise clean wafer surface. The impurities in the chemicals used in the preceding process steps are the primary source of these metal contaminants. The presence of metals can severely degrade the quality of the oxide layer; parts per billion (ppb) grade chemicals in the previous steps should be used to minimize metal contamination. Chemistry and Application The chemistry used in this step is known by the common name of SC-2 it is a mixture of 37 wt% HCl, H2O2, and DI water. These chemicals are generally mixed in a 1:1:6 ratio by volume. The wafers are immersed in the solution which is at 70°C for 5 - 10 minutes, followed by quenching and rinsing in cold ultra-filtered DI water. This process removes alkali ions, NH4OH-insoluble hydroxides such as Al(OH)3, Mg(OH)3, and Zn(OH)3, and any residual trace metals such as gold and copper that were not completely desorbed by SC-1 are readily complexed by chlorine to form molecules that do not adhere to the wafer surface. Furthermore, SC-2 does not etch oxide or silicon. References 1. Kern, W., in “Handbook of Semiconductor Wafer Cleaning Technology” (Kern, W. ed.), p. 49. Noyes Publications, New Jersey, 1993.

 

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