Cleaning

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Wafer and sample cleanliness is an important part of processing in the clean room. Residue or particles on a sample can cause a variety of problems. The best method for improving sample cleanliness and device performance is to use Good Cleanroom Practices.

Types of contamination

  1. Particles: These can be dust, hair, dried photoresist etc. These can get stuck to masks, and a single particle can affect many samples.
  2. Fingerprints: Touching a wafer with a gloved hand can leave behind oils, salts, or other contaminants. These can cause subsequently deposited materials to not adhere to the wafer, or defects in lithography, changes in electrical properties, etc.
  3. Processing particles: Metal, oxide or other processing contaminants can adhere to the wafer and cause damage and defects in lithography.
  4. Organic residues: Organic residues from incompletely removed photoresist, solvents, or grease and other industrial pollutants if the sample was exposed to conditions outside the clean room can cause errors in lithography, adhesion problems, or problems with etching.

Types of cleaning

  1. Chemical cleaning utilizes chemicals to remove particles or contaminants from the surface of the sample.
    • Solvents are often used to dissolve photoresist, a common source of particles and residue. They are also marginally effective in removing other organic residue. Solvent cleaning can be as simple as soaking/rinsing samples in acetone and IPA. Photoresist can also be removed using commercially available solvent mixtures such as PRS 2000 or Remover PG, which typically are heated. Solvent cleaning can also be combined with sonication for greater effectiveness. Solvent cleaning can be used to clean most substrates (silicon, compound semiconductors, metals, glasses), with the exception of plastics or other polymers, which may be dissolved.
    • Acids or Bases in combination with an oxidizer such as hydrogen peroxide are also commonly used to remove organic and other contaminants from the surface. The most common of these methods for cleaning the surface are Nanostrip, Piranha Etch, RCA1, and RCA2. Piranha is a strongly oxidizing mixture of sulfuric acid and hydrogen peroxide, and Nanostrip is a commercially available, stabilized version of piranha. The RCA cleaning technique is a three-stage cleaning process which is effective at removing organic and metallic contaminants. These cleaning methods can be used on silicon and some other semiconductors, but many metals and some semiconductor materials will be attacked and are therefore incompatible with these chemicals.
    • UV-Ozone utilizes ozone generated from a high power ultra violet (UV) source to remove anything organic from the surface. The ozone will react with the organic molecules on the surface and vaporize the product (CO2).
  2. Mechanical cleaning is used alone or in conjunction with chemical cleaning
    • Sonication is a high frequency agitation of the sample mechanically removing particles from the surface. This is often done in conjunction with a chemical cleaning such as solvents, or RCA1. Ultrasonic agitation will affect larger particles and areas, Megasonic agitation will affect smaller (<1µm) particles and areas.
    • CO2 snow cleaning process is based upon the controlled expansion of either liquid or gaseous carbon dioxide that generates dry ice particles. Upon impact with the sample surface, the dry ice removes particles by momentum transfer, even micron and submicron particulates, and hydrocarbons via a transient solvent or a freeze fracture mechanism. The gas pressure blows the contaminants away.
    • Brush cleaning is often on a tool with a brush. This spinning brush with a soft head is used to remove particles from the surface, however, caution must be taken to not damage other layers on the surface.
  3. Plasma cleaning typically uses an oxygen plasma to remove organic contamination from the surface. At times Argon or CF4 is added to the cleaning to improve removal of material. One common application of plasma cleaning is to remove photoresist which has been hardened by exposure to RIE. Hardened resist often can not be removed by solvents alone; in this case, O2 plasma "ashing" of the photoresist, possibly followed by solvent cleaning, is an effective method of removing photoresist. Another application is when O2 plasma is used to "descum" a wafer, i.e., when it is used to remove photoresist residue left in trenches after development. A descum step helps ensures that the open areas in the resist pattern are entirely clear and unobstructed.

Equipment

Solvent

Acid/Base

Plasma

Mechanical

References

  • W. Kern, "The Evolution of Silicon Wafer Cleaning Technology," J. Electrochem. Soc., vol. 137, no. 6, (1990).
  • Handbook of Silicon Wafer Cleaning Technology, 2nd Ed., ed. Karen A. Reinhardt & Werner Kern, William Andrew Inc., NY (2008).