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Why Pre-Diffusion Cleans Are Essential for Silicon Wafer Processing

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Why Pre-Diffusion Cleans Are Essential for Silicon Wafer ProcessingSilicon wafers must be completely clean before they go through the diffusion process. If contaminating particles are present on the wafer surfaces during the diffusion process, they will cause defects in the final semiconductor product. Pre-diffusion cleaning can be carried out with several methods. RCA clean and Piranha etch use chemicals to strip away wafer contamination. Megasonic cleaning uses high-frequency sound waves to dislodge surface contaminants and particles. No matter which cleaning method is chosen, cleaning must be done to reduce contaminating particle counts to a minimum. An experienced manufacturer of wet process stations can integrate the required cleaning methods into wet benches. They can then ensure that the silicon wafers are cleaned thoroughly.

RCA Clean is a Common Silicon Wafer Cleaning Method

RCA clean was originally developed at the RCA corporation and remains a popular all-round silicon wafer cleaning method. It consists of two parts: Standard Clean 1 and 2 (SC1 and SC2). SC1 removes organic material but leaves metallic contamination behind. SC2 cleans the remaining metallic particles and produces a completely clean wafer.

The SC1 cleaning bath contains a solution of ammonium hydroxide and hydrogen peroxide. The cleaning bath is heated to about 75 degrees centigrade, and the wafers are immersed for 10 to 15 minutes. All organic matter and many insoluble contaminants are removed, but some metallic ions stay attached to the wafer surface.

The metallic ions are removed during the SC2 cleaning step. The wafers are placed into a solution of hydrochloric acid and hydrogen peroxide. The solution is heated to about 75 degrees centigrade, and the wafers are immersed for about 10 minutes. Once the wafers are rinsed with deionized water and dried, they are ready for the diffusion processing steps.

Piranha Etch Quickly Cleans Heavy Contamination

When silicon wafers are heavily contaminated or need to be stripped of photoresist from previous process steps, a Piranha mixture is often used to begin the wafer cleaning process. The mix of sulfuric acid and hydrogen peroxide quickly removes large amounts of mainly organic contaminants. While it works more rapidly than RCA clean, it operates at an elevated temperature of 130 to 180 degrees centigrade and is hard to control precisely. Modutek’s proprietary “bleed and feed” process control improves process stability. The advanced controls allow for more precise temperature settings and better cleaning performance while maintaining the rapid removal of contaminants.

Megasonic Cleaning Provides Improved Removal of Contaminating Particles

Megasonic cleaning uses high-frequency sound waves in the cleaning bath to dislodge light contamination from wafer surfaces. The cleaning method features reduced use of toxic and expensive chemicals while reducing particle counts to a minimum. Even the smallest sub-micron particles can distort diffusion and cause defects in the final semiconductor product. These tiny particles are especially difficult to remove because they tightly adhere to wafer surfaces due to static charge and surface tension. Megasonic cleaning generates microscopic bubbles in the cleaning solution. When these bubbles collapse, the resulting scrubbing action removes the particles.

Modutek’s Wet Benches Support Pre-Diffusion Cleans for Specific Wafer Processing Requirements

Modutek wet process stations support all standard silicon wafer cleaning methods. Pre-diffusion cleans can be integrated into a wet bench to satisfy specific customer requirements. Since Modutek designs and builds equipment in-house, wet bench stations can be customized to meet specific customer needs. Based on its in-house expertise, Modutek can recommend solutions for wafer processing and propose equipment from its complete line of wet process stations. Once the equipment is built and delivered, Modutek can provide continuous customer support for the supplied stations. Contact Modutek for a free consultation to discuss your specific process requirements.

The Importance of Pre-Diffusion Cleans in Silicon Wafer Cleaning

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The Importance of Pre-Diffusion Cleans in Silicon Wafer CleaningWhen wafer cleaning immediately prior to diffusion is effective, semiconductor manufacturing output is of high quality and the defect rate is reduced. A major pre-occupation of pre-diffusion cleaning is the removal of microscopic particles from the surface of the silicon wafer. Particles can prevent even diffusion and may themselves be diffused into the silicon, causing defects.

Silicon wafer cleaning can be carried out in chemical baths or with megasonic cleaning systems. Several cleaning steps using different processes are sometimes required to get a specific level of cleanliness. The goal is to obtain wafers free from metallic or organic contamination and with as few surface particles as possible. As silicon microstructures are designed with smaller elements and with a higher component density, adequate wafer cleaning becomes more and more critical.

RCA Clean Uses Two Steps to Remove Organic and Metallic Contamination

RCA clean uses ammonium hydroxide, hydrogen peroxide and hydrochloric acid to remove surface contaminants from the silicon wafer. In the Standard Clean 1 (SC1) process, the wafers are placed in a mixture of ammonium hydroxide and hydrogen peroxide. The corrosive mixture removes organic matter but may leave metal ions behind.

In the Standard Clean 2 (SC2) process, the remaining metallic traces are removed by immersing the wafers in a mixture of hydrochloric acid and hydrogen peroxide. At the end of the RCA wafer cleaning process, organic and metallic contamination has been removed while as many remaining particles as possible are rinsed away as well.

Piranha Clean Removes Heavy Organic Contamination

When wafers are heavily contaminated with organic materials such as photo resist, the Piranha process cleans more quickly than RCA clean. The wafers are immersed in a mixture of sulfuric acid and hydrogen peroxide and the mixture may be heated to speed up the reaction. The piranha clean process hydroxilates the wafer surface, making it hydrophilic. This affinity for water is sometimes a useful feature for additional cleaning measures.

Megasonic Cleaning Removes Contaminants and Particles

Megasonic cleaning avoids the use of corrosive chemicals and is especially effective for dislodging microscopic particles from the wafer surface. The Megasonic process works by generating high-frequency sound waves in the cleaning bath. An ultrasonic generator produces the high-frequency electronic signal and a transducer converts the signal into sound waves that travel through the cleaning solution. The sound waves create tiny cavitation bubbles that produce a scrubbing action against the wafer surface. The action of the bubbles cleans the wafer.

Microscopic contaminating particles adhere to the wafer and are difficult to remove. With integrated circuits featuring increasingly smaller geometries, even the tiniest particles can cause defects. Megasonic silicon wafer cleaning operates in the frequency range near or above 1 MHz and the cavitation bubbles agitate the cleaning solution at the operating frequency. Such agitation breaks the bond holding the particle on the wafer surface and the floating particles can be rinsed away.

Modutek Wet Bench Equipment Supports Pre-Diffusion Cleaning Methods

RCA, Piranha and Megasonic cleans are commonly used in pre-diffusion clean, but each fabrication facility optimizes for its own sequence of semiconductor manufacturing processes. A production line may use one or several cleaning methods, and may have special requirements for production parameters such as size, temperature or timing.

Modutek designs and builds its own complete line of wet bench equipment and can advise customers on choosing the best processing stations for their applications. Ideal solutions often require extensive customization to optimize yield and reduce costs to a minimum. Modutek can provide custom solutions including pre-diffusion cleans, and can design new equipment for integration into the customer’s manufacturing line to meet specific wafer processing requirements.