Solar cells : Cleaning steps for Silicon wafers

 The RCA cleaning is a standard process of wafer cleaning steps which need to be performed before high-temperature processing steps (oxidation, diffusion, CVD) of silicon wafers in semiconductor manufacturing. Werner Kern developed the basic procedure in 1965 while working for RCA, the Radio Corporation of America. It involves the following chemical processes performed in sequence:

1.     Removal of the organic contaminants (organic clean + particle clean)

2.     Removal of thin oxide layer (oxide strip, optional)

3.     Removal of metallic or ionic contamination (ionic clean)

 The wafers are prepared by soaking them in deionized water. If they are grossly contaminated (visible residues are present), they may require a preliminary cleanup in Piranha solution. The wafers are thoroughly rinsed with deionized water between each step.

 Ideally, the steps below are carried out by immersing the wafers in solutions prepared in fused silica or fused quartz vessels (borosilicate glassware must not be used, as its impurities leach out and cause contamination). Likewise it is recommended that the chemicals used are electronic grade (or "solar grade") to avoid impurities that will recontaminate the wafer.

Piranha

Many different mixture ratios are commonly used, and all are called piranha. A typical mixture is 3 parts of conc. H₂SO₄ and 1 part of 30% H₂O₂ solution; other protocols may use a 4:1 or even 7:1 mixture. A closely related mixture, sometimes called "base piranha", is a 3:1 mixture of NH₄OH with H₂O₂.

Note:- Piranha solution must be prepared with great care. It is highly corrosive and an extremely powerful oxidizer. Surfaces must be reasonably clean and completely free of organic solvents from previous wash steps before coming into contact with the solution. Piranha solution cleans by dissolving organic contaminants, and a large amount of contaminant will cause violent bubbling and a release of gas that can cause an explosion.

RCA-1

The first step (called RCA-1) is performed with a solution of (ratios may vary)

·         5 parts of deionized water (DI water)

·         1 part of NH₄OH, (27% by weight)

·         1 part of aqueous H₂O₂ (hydrogen peroxide, 30%)

at 75°C typically for 10 minutes. This base-peroxide mixture removes organic residues. Particles are also very effectively removed, even insoluble particles, since RCA-1 modifies the surface and particle zeta potentials and causes them to repel. This treatment results in the formation of a thin SiO₂ layer (about 1 nm) on the silicon surface, along with a certain degree of metallic contamination (notably Fe) that will be removed in subsequent steps.

HF and DI water rinse

 This step (for bare silicon wafers) is a short immersion in a 1% or 2% solution of aqueous HF (hydrofluoric acid) at 25 °C for about 15 seconds, in order to remove the thin oxide layer and some fraction of ionic contaminants. If this step is performed without ultra-high purity materials and ultra-clean containers, it can lead to recontamination since the bare silicon surface is very reactive. In any case, the subsequent step (RCA-2) dissolves and regrows the oxide layer.

  This step is needed almost after every chemical processing of Si wafers. In short this process may be called as HF and DI water rinse.

RCA-2

This step (called RCA-2) is performed with a solution of (ratios may vary)

·         6 parts of DI water

·         1 part of aqueous HCl (hydrochloric acid, 37% by weight)

·         1 part of aqueous H₂O₂ (hydrogen peroxide, 30%)

at 75°C, typically for 10 minutes. This treatment effectively removes the remaining traces of metallic (ionic) contaminants, some of which were introduced in the RCA-1 cleaning step. It also leaves a thin passivating layer on the wafer surface, which protects the surface from subsequent contamination (bare exposed silicon is contaminated immediately).

Rinsing and Drying

 This step is again followed by HF and DI water rinse in order to remove the thin SiO₂ layer. After this the wafers are dried using a Drier or a Nitrogen gun.