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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 or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
,
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 chemical p ...
, 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, silicium), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The ...
s in
semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
manufacturing. Werner Kern developed the basic procedure in 1965 while working for RCA, the
Radio Corporation of America RCA Corporation was a major American electronics company, which was founded in 1919 as the Radio Corporation of America. It was initially a patent pool, patent trust owned by General Electric (GE), Westinghouse Electric Corporation, Westinghou ...
.''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 An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
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 pu ...
. 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 (). The resulting mixture is used to clean organic residues off substrates, for example silicon wafers. Because the mixture is a strong o ...
. 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, such as soda-lime glass, lead glass, or borosi ...
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), ma ...
ware must not be used, as its impurities leach out and cause contamination). Likewise it is recommended that the chemicals used be of 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, (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 viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
, 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.is a scientific term for Electrokinetic phenomena, electrokinetic Electric ...
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 , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...
layer (about 10 Angstrom) on the silicon surface, along with a certain degree of metallic contamination (notably
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
) 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 Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colorless, acidic and highly corrosive. A common concentration is 49% (48–52%) but there are also stronger solutions (e.g. 70%) and pure HF has a boiling p ...
) 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 or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
, 37% by weight) * 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 viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
, 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 Trichloroethylene (TCE) is an organochloride with the formula C2HCl3, commonly used as an industrial metal-degreasing solvent. It is a clear, colourless, non-flammable, volatile liquid with a chloroform-like pleasant mild smell and sweet taste.
,
acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...
and
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...
.


See also

* Chemical-mechanical planarization *
Piranha solution Piranha solution (), also known as piranha etch, is a mixture of sulfuric acid () and hydrogen peroxide (). The resulting mixture is used to clean organic residues off substrates, for example silicon wafers. Because the mixture is a strong o ...
*
Plasma etching Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (Plasma (physics), plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, ...
*
Silicon on insulator In semiconductor manufacturing, silicon on insulator (SOI) technology is fabrication of silicon semiconductor devices in a layered silicon–insulator–silicon substrate, to reduce parasitic capacitance within the device, thereby improving perf ...
*
Wafer (electronics) In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si, silicium), used for Semiconductor device fabrication, the fabrication of integrated circuits and, in photovoltaics, ...


References

{{Reflist


External links


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