RCA clean

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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, diffusion, CVD) of silicon wafers in semiconductor manufacturing.

Diffusion Statistical movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential)

Diffusion is the net movement of molecules or atoms from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in chemical potential of the diffusing species.

Chemical vapor deposition chemical process used in the semiconductor industry to produce thin films

Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.

A semiconductor material has an electrical conductivity value falling between that of a metal, like copper, gold, etc. and an insulator, such as glass. Its resistance decreases as its temperature increases, which is behaviour opposite to that of a metal. Its conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, and gallium arsenide. After silicon, gallium arsenide is the second most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits and others. Silicon is a critical element for fabricating most electronic circuits.

Contents

Werner Kern developed the basic procedure in 1965 while working for RCA, the Radio Corporation of America. [1] [2] [3] 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 ionic contamination (ionic clean)

Standard recipe

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]

Piranha solution, also known as piranha etch, is a mixture of sulfuric acid (H2SO4) 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).

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. [2]

Fused quartz fused silica

Fused quartz or fused silica is glass consisting of silica in amorphous (non-crystalline) form. It differs from traditional glasses in containing no other ingredients, which are typically added to glass to lower the melt temperature. Fused silica, therefore, has high working and melting temperatures. Although the terms fused quartz and fused silica are used interchangeably, the optical and thermal properties of fused silica are superior to those of fused quartz and other types of glass due to its purity. For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment. It transmits ultraviolet better than other glasses, so is used to make lenses and optics for the ultraviolet spectrum. The low coefficient of thermal expansion of fused quartz makes it a useful material for precision mirror substrates.

Borosilicate glass type of glass with silica and boron trioxide as the main glass-forming constituents

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, making them resistant to thermal shock, more so than any other common glass. Such glass is less subject to thermal stress and is commonly used for the construction of reagent bottles. Borosilicate glass is sold under such trade names as Borcam, Borosil, DURAN, Suprax, Simax, BSA 60, BSC 51, Heatex, Endural, Schott, Refmex, Kimble, MG(India) and some items sold under different trade names.

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) [2]

Hydrogen peroxide is a chemical compound with the formula H
2
O
2
. In its pure form, it is a pale blue, clear liquid, slightly more viscous than water. Hydrogen peroxide is the simplest peroxide. It is used as an oxidizer, bleaching agent and antiseptic. Concentrated hydrogen peroxide, or "high-test peroxide", is a reactive oxygen species and has been used as a propellant in rocketry. Its chemistry is dominated by the nature of its unstable peroxide bond.

at 75 or 80 °C [1] 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 potentials and causes them to repel. [4] 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.

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. [2]

Third step (SC-2): ionic clean

The third and last step (called SC-2) is performed with a solution of (ratios may vary) [2]

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. [1] It also leaves a thin passivizing layer on the wafer surface, which protects the surface from subsequent contamination (bare exposed silicon is contaminated immediately). [2]

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. [2]

Additions

The first step in the ex situ cleaning process is to ultrasonically degrease the wafer in trichloroethylene, acetone and methanol. [5]

See also

Notes and references

  1. 1 2 3 RCA Clean, materials at Colorado School of Mines Archived 2000-03-05 at the Wayback Machine
  2. 1 2 3 4 5 6 7 8 Kern, W. (1990). "The Evolution of Silicon Wafer Cleaning Technology". Journal of the Electrochemical Society. 137 (6): 1887–1892. doi:10.1149/1.2086825.
  3. W. Kern and D. A. Puotinen: RCA Rev. 31 (1970) 187.
  4. Itano, M.; Kern, F. W.; Miyashita, M.; Ohmi, T. (1993). "Particle removal from silicon wafer surface in wet cleaning process". IEEE Transactions on Semiconductor Manufacturing. 6 (3): 258. doi:10.1109/66.238174.
  5. Rudder, Ronald; Thomas, Raymond; Nemanich, Robert (1993). "Chapter 8: Remote Plasma Processing for Silicon Wafer Cleaning". In Kern, Werner. Handbook of Semiconductor Wafer Cleaning Technology. Noyes Publications. pp. 356–357. ISBN   978-0-8155-1331-5.

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