Cleaning

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 Isopropanol. Photoresist can also be removed using commercially available NMP 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, Hydrofluoric acid, Buffered HF, and RCA Clean. Piranha is a strongly oxidizing mixture of sulfuric acid and hydrogen peroxide, and Nanostrip is a commercially available, stabilized version of piranha. The RCA clean (Radio Corporation of America) technique is a cleaning process which is effective at removing both organic and metallic contaminants. The RCA1 Clean is also known as Standard Clean 1 and is used for cleaning organic contaminants. The RCA2 Clean is also known as Standard Clean 2 and is used for removing metallic or ionic 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. An Acid is defined as a solution having a greater hydrogen-ion concentration. The pH will be below 7. Strong acids (which ionize completely) include Sulfuric acid and Nitric acid, while weak acids (which only ionize partially) include things like Acetic acid. Even water can be considered an acid if it is used as an etching solution. A basic solution should have a higher concentration of (OH^-) hydroxyls. The pH will be greater than 7.
   • 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

• Solvent cleaning can be performed at Solvent Bench 14, Solvent Bench 83, Solvent Bench 84, and Solvent Bench 94. Solvent Bench 14 and Solvent Bench 84 have ultrasonic tanks where sonication can be performed.
• Overnight soaking can be done on Solvent Bench 84 in the cleanroom and Solvent Bench 94 in the ROBIN lab.

Acid/Base

• RCA cleaning can be performed at PFC Bench 01 and RCA Bench 81.
• Nanostrip and piranha cleaning can be performed at Acid Bench 02, Acid Bench 12, Acid Bench 72, Acid Bench 73, and Acid Bench 82, Acid Bench 92.
• Overnight soaking can be done on Acid Bench 73 in the cleanroom and Acid Bench 92 in the ROBIN lab.

Plasma

• Plasma cleaning can be done using the YES Plasma Stripper and the Glen Plasma Cleaner.

Mechanical

• Combined brush and chemical cleaning is available using the SSEC Wafer and Mask Cleaner. This tool is used primarily for cleaning wafers after chemical mechanical polishing in the Strasbaugh CMP tool.
• Sonication can be done using the ultrasonic tanks in Solvent Bench 14 and Solvent Bench 84 or the megasonic tank in Solvent Bench 14.
• CO₂ snow cleaning is available in the LNF ROBIN lab.

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).

Please see Material restrictions

Further reading

• LNF Tech Talk for Wafer cleaning