Why Pre-Diffusion Cleans Are Essential for Silicon Wafer Processing

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.

How Precise Control of the SPM Process Improves Processing Results

How Precise Control of the SPM Process Improves Processing ResultsWhile the SPM (Sulfuric acid Peroxide Mix) or Piranha process quickly removes organic contaminants such as photoresist, it is difficult to control. The cleaning action depends on both the temperature and the concentration of the mixture. Both vary if the process is left to carry on without intervention. Better controls can improve cleaning performance, reliability, and repeatability while increasing the lifespan of the mixture. Modutek has developed advanced controls that maintain temperature and concentration precisely at their desired levels to improve cleaning results.

The SPM Process Suffers from Decreasing Concentration and Slower Cleaning

The SPM mixture is typically about three parts sulfuric acid to one part hydrogen peroxide. The preparation of the mixture is highly exothermic. Once the mixture stabilizes, the wafer cleaning process occurs in a heated tank at 130 to 180 degrees centigrade. The hydrogen peroxide is unstable and decomposes to form water and oxygen. As the amount of water in the mixture increases and the concentration of hydrogen peroxide goes down, the cleaning effectiveness of the mixture deteriorates.

Small amounts of hydrogen peroxide are periodically added to the mixture to correct the problem of a decreasing hydrogen peroxide concentration. This spiking with hydrogen peroxide causes a sudden temperature increase due to the exothermic nature of the mixing process. While spiking solves the problem of a decreasing hydrogen peroxide concentration, the temperature can’t be kept constant. An SPM mixture that is not spiked is useful for about two hours, but spiking with hydrogen peroxide increases the lifespan of the mixture to about one day.

Modutek Has Developed Advanced Controls to Improve Process Results

Modutek’s “Bleed and Feed” process control strategy keeps the concentration of hydrogen peroxide and the mixture temperature within narrow limits. The temperature variations due to spiking and the need for spiking are eliminated. With concentration and temperature controlled accurately, the strip rate of the mixture remains constant and predictable. Reliable timing and repeatability of the process are excellent.

Modutek achieves these results by using a two-tank control system. The process tank is the “dirty” tank, while the second tank is the “clean” tank. At periodic intervals, a small amount of the mixture is drained from the dirty tank. It is replaced with a fresh mixture from the clean tank. The clean tank mixture is then replenished with new chemicals. A programmable controller controls the process, so the dosage intervals and the amount of mixture to be drained are adjustable and recorded by the programmable controller. As a result, once the process controls are optimized, the SPM wafer cleaning process can be run exactly the same way each time.

Modutek’s “Bleed and Feed” Reduces Costs and Provides Benefits

Customers who have switched to Modutek’s “Bleed and Feed” controls find that their costs are reduced, and the process line performance is better. Modutek’s control strategy reduces chemical use, and the mixture lasts longer when the temperature variations caused by spiking are eliminated. Downtime due to having to replace the bath chemicals is reduced as well. The risk of accidents from operators adding too much hydrogen peroxide or from spills is eliminated. In addition to savings from lower chemical purchases, reduced costs include lower chemical handling, storage, and disposal expenses.

Support for the “Bleed and Feed” Process

Modutek offers the “Bleed and Feed” process controls for the SPM process on new wet process stations. As one of the leading semiconductor equipment manufacturers, Modutek continues to work on improving its silicon wafer cleaning equipment to meet customer needs. Since Modutek designs and builds its equipment in-house, innovative features can be developed to support unique customer requirements. Wet bench processes are often specialized, and Modutek can utilize in-house expertise to customize equipment as needed. Innovation and customization make Modutek a valuable partner for wet bench technology.