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.

Why Pre-Diffusion Cleans Are Critical in Wafer Cleaning Processing

Why Pre-Diffusion Cleans Are Critical in Wafer Cleaning ProcessingBefore silicon wafers are placed in a diffusion furnace, they must be cleaned to remove impurities and particles from their surfaces. Such pre-diffusion cleaning takes place several times during semiconductor manufacturing as microscopic structures are fabricated in the silicon. Various wafer cleaning process methods include the use of different chemical baths, ozone treatment or megasonic cleaning for ensuring that wafers are clean and free from contamination. Modutek offers a complete range of equipment that supports the various cleaning methods and helps ensure the highest quality output from semiconductor manufacturing facilities.

Cleaning Methods

Standard cleaning methods involve immersing the silicon wafers in baths of powerful chemicals that remove contaminants and particles from the surface of the wafers. Silicon is relatively inert to reactions with the chemicals used but organic and metallic impurities on the wafer surfaces are oxidized and dissolved.

In the RCA wafer cleaning process silicon wafers are placed into a solution containing ammonium hydroxide and hydrogen peroxide. This bath removes organic contaminants but may leave metallic traces. As a result, the RCA clean process is often carried out in two parts called SC1 (standard clean 1) and SC2. After the ammonium hydroxide/hydrogen peroxide (APM) bath, the SC2 step consists of a bath containing hydrochloric acid and hydrogen peroxide (HPM). SC2 removes metal ions and leaves the wafer clean and ready for further processing.

Piranha etch clean is used to remove large amounts of organic residue from silicon wafers. It consists of a bath containing sulfuric acid and hydrogen peroxide and can dissolve hard to remove organic material such as photoresist. The solution’s strong oxidizing properties hydroxilate the surfaces of the silicon wafers, making them hydrophilic or attractive to water. This property may be an advantage for subsequent cleaning steps.

Megasonic cleaning is another method for obtaining clean, particle-free silicon wafers for diffusion. The method uses cavitation bubbles produced by megasonic waves in a cleaning solution to remove particles and contaminants from wafers. The bubbles produce a scrubbing action that cleans wafer surfaces without the use of expensive and corrosive chemicals.

In some cleaning applications, ozone may be used to obtain clean, particle-free silicon wafers. The wafers are first rinsed with deionized water to remove water-miscible contaminants and are then exposed to ozone in an ozone chamber.  All organic contaminants and particles are converted to carbon dioxide leaving the wafer surface clean and particle-free.

The Importance of Effective Cleaning

The presence of particles on the silicon wafer surface affects the subsequent diffusion steps and impacts the quality of the final product. Particles may be diffused into the silicon along with the doping substances and produce unknown electrical anomalies. They may remain on the surface and block subsequent etching processes to change microscopic structures. They may interfere with the creation of tiny conductor paths on the surface of the silicon and they can create chemical patches that don’t react as expected.

With the trend to smaller size geometries in silicon wafer structures, the potential damage that a single particle can cause has increased. Particles may affect the performance of a semiconductor product or its reliability. The product lifetime may be reduced or its functionality impaired. Defective products reduce plant performance while low quality products hurt the facility’s reputation. Ideally semiconductor manufacturers need equipment that will help them produce high quality semiconductors with low reject rates by emphasizing effective wafer cleaning methods with a maximum reduction in the presence of particle contamination.

Modutek Wafer Cleaning Solutions

Modutek offers a complete line of chemical baths and wet processing equipment that supports RCA clean with SC1 and SC2, piranha clean, megasonic cleaning and ozone clean. Each wafer cleaning process is designed to provide effective pre-diffusion cleaning to produce silicon wafers with a minimum of particle contamination.

In addition to traditional chemical cleaning, Modutek has pioneered advanced ozone cleaning and is using megasonic cleaning for additional particle removal for smaller device geometries. The company can help semiconductor manufacturing facilities increase their throughput, improve their performance and reduce their costs by drawing on the extensive experience they have developed in the various silicon wafer cleaning applications.