1. The RCA clean is a standard set of wafer cleaning steps which need to be performed before high-temperature processing steps (oxidation, diffusion, CVD) of silicon wafers in semiconductor manufacturing.
2. It involves the following chemical processes performed in sequence:
- Removal of the organic contaminants (organic clean + particle clean)
- Removal of thin oxide layer (oxide strip, optional)
- Removal of ionic contamination (ionic clean)
RECIPE
1. The wafers are prepared by soaking them in deionized water. If they are grossly contaminated (visible residues), they may require a preliminary cleanup in piranha solution. The wafers are thoroughly rinsed with deionized water between each step.
2. 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 "CMOS grade") to avoid impurities that will recontaminate the wafer.
STEP 1 - ORGANIC CLEAN + PARTICLE CLEAN (SC-1)
1. The first step (called SC-1, where SC stands for Standard Clean) is performed with a solution of below at 75 or 80 °C[1] typically for 10 minutes.
- 5 parts of deionized water
- 1 part of ammonia water, (29% by weight of NH3)
- 1 part of aqueous H2O2 (hydrogen peroxide, 30%)
2. This base-peroxide mixture removes organic residues. Particles are also very effectively removed, even insoluble particles, since SC-1 modifies the surface and particle zeta potentials and causes them to repel.This treatment 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 will be removed in subsequent steps.
STEP 2 (OPTIONAL) : OXIDE STRIP
1. The optional second step (for bare silicon wafers) is a short immersion in a 1:100 or 1:50 solution of aqueous HF (hydrofluoric acid) at 25 °C for about fifteen seconds, in order to remove the thin oxide layer and some fraction of ionic contaminants.
2. 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 (SC-2) dissolves and regrows the oxide layer.
STEP 3 : IONIC CLEAN (SC-2)
1. The third and last step (called SC-2) is performed with a solution of (ratios may vary) below at 75 or 80 °C, typically for 10 minutes.
6 parts of deionized water
1 part of aqueous HCl (hydrochloric acid, 37% by weight)
1 part of aqueous H2O2 (hydrogen peroxide, 30%)
2. This treatment effectively removes the remaining traces of metallic (ionic) contaminants, some of which were introduced in the SC-1 cleaning step. It also leaves a thin passivizing layer on the wafer surface, which protects the surface from subsequent contamination (bare exposed silicon is contaminated immediately).
STEP 4: RINSING AND DRYING
1. Provided the RCA clean is performed with high-purity chemicals and clean glassware, it results in a very clean wafer surface while the wafer is still submersed in water.
2. The rinsing and drying steps must be performed correctly (e.g., with flowing water) since the surface can be easily recontaminated by organics and particulates floating on the surface of water. A variety of procedures can be used to rinse and dry the wafer effectively.
PIRANHA SOLUTION
1. Piranha solution, also known as piranha etch, is a mixture of sulfuric acid (H2SO4), water, and hydrogen peroxide (H2O2), used to clean organic residues off substrates. Because the mixture is a strong oxidizing agent, it will remove most organic matter, and it will also hydroxylate most surfaces (add OH groups), making them highly hydrophilic (water-compatible).
2. Many different mixture ratios are commonly used, and all are called piranha. A typical mixture is 3 parts of concentrated sulfuric acid and 1 part of 30% hydrogen peroxide 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 ammonia water with hydrogen peroxide.
3. 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.
4. The solution may be mixed before application or directly applied to the material, applying the sulfuric acid first, followed by the peroxide. Due to the self-decomposition of hydrogen peroxide, piranha solution should be used freshly prepared. The solution should not be stored, as it generates gas and therefore cannot be kept in a closed container.[2] As the solution reacts violently with many items commonly disposed of as chemical waste if the solution has not neutralised, it must be left in clearly marked containers.
PIRANHA SOLUTION APPLICATION
1. Piranha solution is used frequently in the microelectronics industry, e.g. to clean photoresist residue from silicon wafers. Hobbyist electronics enthusiasts use the solution to etch home made circuit boards. A mask is applied to a blank copper board, and the piranha solution rapidly removes the exposed copper not covered by the mask.
2. Piranha solution is particularly useful when cleaning sintered (or "fritted") glassware. The size of the pores of sintered glassware is critical for its function, so it should not be cleaned with strong bases, which gradually dissolve the sinter. Sintered glass also tends to capture material deep within the structure, making it difficult to remove. Where less aggressive cleaning methods fail, piranha solution can be used to return the sinter to a pristine white, free-flowing form without excessive damage to the pore dimensions.
3. This is usually accomplished by allowing the solution to percolate backward through the sintered glass. Although cleaning sintered glassware will leave it as clean as possible without damaging the glassware it is not recommend due to the risk of explosion.
4. Piranha solution is used to make glass hydrophilic by hydroxylating the surface, thus increasing the number of silanol groups on the surface.
CHEMICAL REACTION
1. The dehydration process exhibits itself as the rapid carbonisation of common organic materials, especially carbohydrates, when immersed in piranha solution. Piranha solution was named in part for the vigour of this first process, since large quantities of organic residues immersed in the solution are dehydrated so violently that the process resembles a piranha feeding frenzy. The second and more definitive rationale for the name, however, is the ability of piranha solution to "eat anything", in particular, elemental carbon in the form of soot or char.
2. This second and far more interesting process can be understood as the sulfuric-acid boosted conversion of hydrogen peroxide from a relatively mild oxidizing agent into one sufficiently aggressive to dissolve elemental carbon, a material that is notoriously resistant to room-temperature aqueous reactions. This transformation can be viewed as the energetically favourable dehydration of hydrogen peroxide to form hydronium ions, bisulfate ions, and, transiently, atomic oxygen:
H2SO4 + H2O2 → H3O+ + HSO4− + O
3. It is this extremely reactive atomic oxygen species that allows piranha solution to dissolve elemental carbon.
(Source: Radio Corporation of America)
