Pre-diffusion cleans are carried out several times during the semiconductor manufacturing process, and each time the chemical residues and particles on the wafers have to be removed as completely as possible. If silicon wafers still have surface contamination when they are placed in the diffusion oven, diffusion may be uneven and the resulting semiconductor products may be defective.
The several methods available for wafer cleaning either use chemical baths or Megasonic cleaners to remove surface contaminants from the silicon wafers. Modutek offers a complete line of wet bench stations that support pre-diffusion wafer cleaning as well as other semiconductor fabrication processes.
The Importance of the Pre-Diffusion Wafer Cleaning Process
The contamination with particles on silicon wafer surfaces prior to diffusion is a key factor in reducing the yield of electronic components in semiconductor fabrication plants and research facilities. Particles may be diffused into the silicon and create electric defects or they may block diffusion behind the particle. After diffusion, particles may affect etching and interfere with microscopic conductors. The resulting electronic components may have to be scrapped.
As microscopic structures in silicon become smaller and more detailed, large particles become more disruptive on both an electrical and the physical scale. Even tiny particles that were previously considered unimportant can affect the smallest silicon structures. When electronic products are defective, it affects facility productivity and reduces throughput. Performance is lower and profitability is reduced. In some cases, the products are not defective but their life expectancy may be reduced and the manufacturer’s reputation is affected. An effective pre-diffusion wafer cleaning process results in fewer defects and a higher quality output.
How Pre-Diffusion Cleaning is Carried Out
Before being placed in the diffusion oven, silicon wafers are immersed in chemical or Megasonic baths. Surface impurities are dislodged, dissolved or chemically neutralized. Complete cleaning may take more than one process and the following methods are commonly used.
RCA cleaning is a popular method consisting of two components, SC1 and SC2, where the SC stands for standard clean. For SC1, the wafers are immersed in a solution of ammonium hydroxide and hydrogen peroxide. The aggressive chemicals remove all organic compounds from the wafer surfaces but may leave traces of metallic contamination. As a result, during SC2, the wafers are placed in a mixture of hydrochloric acid and hydrogen peroxide that removes any remaining metal ions.
An alternative to the RCA standard clean methods is the Piranha clean, especially useful for applications with heavy wafer contamination. It consists of a sulfuric and hydrogen peroxide mixture that rapidly removes material such as photo resist. A further feature of Piranha clean is the hydroxylation of the silicon wafer surface, making it hydrophilic or attractive to water, a characteristic useful for some subsequent process steps.
Megasonic cleaning uses high-frequency sound waves in a cleaning solution to dislodge contaminants from the surface of the silicon wafer. This cleaning method does not use harsh chemicals, and a generator is used to produce a signal in the MHz range at a specified power level. A transducer in the cleaning tank converts the signal to sound waves in water. The sound waves create microscopic cavitation bubbles with a strong scrubbing action that removes particles. Depending on the manufacturing process, semiconductor manufacturing facilities use one of these methods for pre-diffusion cleaning.
Modutek Pre-Diffusion Cleaning Equipment
Modutek’s wet bench chemical stations provide a complete range of pre-diffusion cleaning solutions. The company can offer equipment that supports each of the above cleaning methods as well as other semiconductor fabrication processes. All of Modutek’s pre-diffusion cleaning baths produce silicon wafers with extremely low particle counts while all of the company’s equipment is designed for high quality output, fast processing and efficient operation resulting in superior manufacturing facility performance. Contact Modutek for a free consultation or quote on equipment designed to meet specific manufacturing requirements.
Reviewed and Approved by Douglas Wagner
President & CEO, Modutek Corporation