The RCA clean is a standard set of wafer cleaning steps which need to be performed before high-temperature processing steps (

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...

, CVD) of

silicon wafer In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...

s in

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

manufacturing. Werner Kern developed the basic procedure in 1965 while working for RCA, the

Radio Corporation of America The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Com ...

.''RCA Clean''. Materials at Colorado School of Mines
W. Kern and D. A. Puotinen: RCA Rev. 31 (1970) 187. 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)

Standard recipe

The wafers are prepared by soaking them in

deionized water Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water was, formerly, the most common form of purified water, but, in recent years, water is more frequently puri ...

. If they are grossly contaminated (visible residues), they may require a preliminary cleanup in

piranha solution Piranha solution, also known as piranha etch, is a mixture of sulfuric acid () and hydrogen peroxide (), used to clean organic compound, organic residues off substrates. Because the mixture is a strong oxidizing agent, it will decompose most orga ...

. 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 Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...

vessels (

borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...

ware 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.

First step (SC-1): organic clean + particle clean

The first step (called SC-1, where SC stands for Standard Clean) is performed with a solution of (ratios may vary) * 5 parts of deionized water * 1 part of

ammonia water Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...

, (29% by weight of NH₃) * 1 part of aqueous H₂O₂ (

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

, 30%) at 75 or 80 °C typically for 10 minutes. 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 potential Zeta potential is the electrical potential at the slipping plane. This plane is the interface which separates mobile fluid from fluid that remains attached to the surface. Zeta potential is a scientific term for electrokinetic potential in coll ...

s and causes them to repel. This treatment results in the formation of a thin

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

layer (about 10 Angstrom) on the silicon surface, along with a certain degree of metallic contamination (notably

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

) that will be removed in subsequent steps.

Second step (optional): oxide strip

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. 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.

Third step (SC-2): ionic clean

The third and last step (called SC-2) is performed with a solution of (ratios may vary) * 6 parts of deionized water * 1 part of aqueous HCl (

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

, 37% by weight) * 1 part of aqueous H₂O₂ (

, 30%) at 75 or 80 °C, typically for 10 minutes. 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 passivating layer on the wafer surface, which protects the surface from subsequent contamination (bare exposed silicon is contaminated immediately).

Fourth step: rinsing and drying

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. 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.

Additions

The first step in the ''ex situ'' cleaning process is to ultrasonically degrease the wafer in

trichloroethylene The chemical compound trichloroethylene is a halocarbon commonly used as an industrial solvent. It is a clear, colourless non-flammable liquid with a chloroform-like sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, w ...

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscib ...

and methanol.

References

{{Reflist

External links

''RCA Clean'', School of Electrical and Computer Engineering, Georgia Institute of Technology
Semiconductor device fabrication