How Pre-Diffusion Cleans Are Used in the Wafer Cleaning Process

The silicon wafers processed in semiconductor manufacturing facilities and research centers are repeatedly cleaned in aggressive chemical cleaning baths used to remove surface contaminants from the wafers. The cleaning is important because surface impurities affect the diffusion of dopants in the diffusion ovens. Depending on the next diffusion step, wafers have to be completely clean and several types of material may have to be removed. The wafer cleaning process can include multiple cleaning methods to prepare the wafers for the subsequent diffusion step.

Organic Material and Photoresist

When the previous manufacturing step included masking, wafers may still have photoresist adhering to their surfaces. Any such materials have to be removed because they will interfere with the doping process. If contamination is heavy, Piranha etch, a mixture of sulfuric acid and hydrogen peroxide is often used. The sulfuric acid dissolves the carbon-based compounds and the peroxide oxidizes the carbon to carbon dioxide, which is given off as a reaction product.

General Cleaning and Metallic Particles

For lighter organic contamination the two-step RCA clean process (originally developed by RCA Corporation) is often used. The first or SC1 step cleans organic contamination from the wafers but not metallic particles. The second or SC2 step removes the metallic particles and creates a protective layer on the silicon surface.

SC1 clean involves immersing the wafers in an aqueous mixture of ammonium hydroxide and hydrogen peroxide. This removes traces of organic matter and non-metallic contaminants but it leaves metallic particles on the wafer surfaces. These particles have to be removed before diffusion takes place because they may otherwise be embedded into the wafer in addition to the desired doping and cause defects in the semiconductor material.

In SC2, a hydrochloric acid and hydrogen peroxide solution in deionized water dissolves the metallic particles and leaves a clean wafer surface. In addition, a passivation layer is created that protects the silicon from further contamination. The wafers are now ready for further processing.

Native Oxide Layer Removal

The SC1 clean results in an oxide layer forming on the silicon wafers, and even if SC1 is not used, silicon oxide forms naturally on the clean, bare wafers. For some semiconductor fabrication steps, the native silicon oxide layer has to be removed. The silicon oxide layer is stable and very few chemicals can attack and remove it. Hydrofluoric acid is often used but it is extremely dangerous to handle and a safe set-up is important. Usually the wafers are dipped briefly into the acid, which acts very quickly. After a rinse with deionized water and drying, the wafers are ready for the diffusion oven.

Other Cleaning Methods

For some semiconductor fabrication processes, Modutek has pioneered wafer cleaning methods that avoid the use of harsh chemicals. Megasonic cleaning uses megasonic sound waves in a cleaning solution to dislodge particles and clean wafers. Vibrations and the cavitation bubbles of the high-frequency waves dislodge submicron particles and remove contaminating films. Modutek’s ozone treatment is a fast and cost-effective alternative to chemical cleaning for some applications. The ozone converts organic residue to carbon dioxide and does not leave metallic particles behind.

With its extensive experience in wet processing applications and its expertise in providing high-quality wafer cleaning process equipment, Modutek can advise customers on solutions for their semiconductor fabrication needs. The company can evaluate specific fabrication processes and suggest equipment from its complete line, ensuring that the proposed stations fulfill requirements safely and effectively. Modutek’s wet benches support the pre-diffusion cleaning processes as well as general cleaning, stripping and etching and they are available in manual, semi-automated and fully automated stations. Contact Modutek at 866-803-1533 for a free quote or consultation to discuss which process equipment will work best to support your application.

How Wet Bench Stations Are Tailored for Specific Manufacturing Process Requirements

The fabrication of semiconductor wafers requires that an elaborate set of chemical etching, stripping and cleaning steps be followed in a precise procedure that can be consistently repeated. The semiconductor manufacturing process requires that the use of hydrofluoric acid and other chemicals which must precisely controlled and monitored. These high-tech processes are critically important to industries that fabricate semiconductors, medical equipment and photovoltaic cells. They require the use of custom-made wet benches designed to assist in the safe and precise control of corrosive chemicals at high temperatures.

Wet bench equipment

Modutek provides a full range of wet bench equipment to address the specialized needs of semiconductor manufacturing facilities and research centers. Wet bench stations can be tailor made according to the specific manufacturing process requirements of a customer.

Citric-nitric passivation wet benches are critical to the passivation of titanium and stainless-steel parts in various industries. Electro-polishing wet bench equipment is deployed in the medical device industry in the manufacture of precision steel, cobalt chrome and titanium components.

Multistage wet process fume hoods with circulation pumps are used in aerospace component manufacture, printed circuit board fabrication, and the semiconductor manufacturing industry. Modutek’s wet bench equipment for the semiconductor industry meets UL2360/FM4910 standards for fire and contamination safety. This enables fabricators to remain compliant with applicable environmental laws.

Titanium anodizing equipment, agitated immersion washers, in-line conveyor washers, spray cabinet washers and other precision equipment are an integral part of the semiconductor industry, as well. Modutek is an industry leader in the design, development and deployment of these and many other kinds of wet bench equipment.

The precision needed in manufacturing equipment

Sulfuric acid-peroxide is a powerful chemical that is critical to the cleaning processes applied to semiconductor wafers. Effective application of this chemical mixture is inherently unstable and requires constant rebalancing. The rebalancing and control is needed to maintain efficiency. The mixture needs to be constantly replenished with hydrogen peroxide at high temperatures, something that can be costly even in simple material costs. Modutek’s high-tech systems not only ensure the correct concentration levels, they also help fabricators contain material costs through the use of an exclusive chemical re-use system to contain material costs. Every part of the process is maintained and monitored by equipment that is software controlled.

The silicon nitride wet etching process is another semiconductor fabrication stage that requires a high level of precision and control. Silicon nitride is used to mask chemical etchings on silicon wafers. Once an etching successfully completes, the masking material requires removal. Typically, a mixture of phosphoric acid and the ionized water is applied. Precise control over the chemical mixture is needed, however, if only to prevent the risk of dangerous heat levels and explosions. Modutek’s wet bench station design addresses this challenge through the use of high-tech safety features and the use of precision software.

Modutek’s in-house design process

From manual to semi-automated and fully automated wet bench stations, Modutek’s products are designed and developed in-house right down to the software. From servo-controlled robots and automated equipment to low-cost manual stations, Modutek’s equipment covers the needs of the industries they serve. With their in house expertise and 35+ years of operation, the company is well equipped to meet and support the requirements of semiconductor manufacturers and research centers. Developing equipment that provides thorough and precise control of the manufacturing process is what sets Modutek apart from other semiconductor equipment manufacturers. Contact Modutek today at 866-803-1533 for a free consultation or quote on wet bench equipment that can be tailor made to support your specific manufacturing requirements.