In the semiconductor industry, wafer cleaning is a critical process to ensure the removal of any contaminants that could interfere with the fabrication of microchips. Particles, organic residues, and metal ions must be effectively removed from the wafer surface to ensure the precision and reliability of these intricate devices. A variety of techniques are used, including RCA Clean, which combines multiple steps to thoroughly prepare wafers for fabrication.
In this blog, we’ll explore the key wafer cleaning techniques including chemistries like piranha solutions, sulfuric peroxide mixtures (SPM), hydrochloric peroxide mixtures (HPMs), along with how manufacturers manage the waste from these processes.
RCA Clean: A Two-Step Process for Comprehensive Cleaning
RCA Clean is one of the most widely used processes for cleaning semiconductor wafers. It is a two-step procedure designed to remove both organic and inorganic contaminants, ensuring that the wafers are free from particles, organic matter, and metal ions. The RCA cleaning process was developed in 1965 by the RCA corporation. The cleaning steps are carried out in heated inert tanks, such as quartz baths, at a temperature of around 80 degrees centigrade. Each step takes about 10 minutes. After the cleaning process, the wafers are rinsed with deionized water and dried.
Step 1: Standard Clean 1 (SC1)
SC1, also known as Standard Clean 1, involves a solution composed of ammonium hydroxide (NH₄OH), hydrogen peroxide (H₂O₂), and deionized water. This mixture is referred to as Ammonium Peroxide Mixture (APM). SC1 is highly effective at removing organic particles, light metals, and general contaminants from the wafer surface. The ammonium hydroxide loosens the contaminants, while hydrogen peroxide oxidizes and breaks down organic residues, making SC1 an essential cleaning step in semiconductor fabrication.
Step 2: Standard Clean 2 (SC2)
After SC1, the wafer undergoes Standard Clean 2 (SC2), which uses a mixture of hydrochloric acid (HCl), hydrogen peroxide (H₂O₂), and deionized water. This solution is commonly called Hydrochloric Peroxide Mixture (HPM). SC2 targets metallic contaminants that SC1 may have left behind, ensuring the removal of metal ions. The combination of hydrochloric acid’s ability to dissolve metals and hydrogen peroxide’s strong oxidizing properties ensures that even trace levels of metals are thoroughly eliminated from the wafer surface.
Piranha Solution and SPM in Wafer Cleaning
Piranha is a hot solution made from sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) used for removing organic residues and metals from substrates. Due to its highly exothermic reaction and aggressive cleaning power, it is a preferred choice for deep cleaning in semiconductor manufacturing. However, Piranha is extremely energetic and must be handled with caution due to its corrosive properties and potential dangers to both personnel and equipment.
Sulfuric Peroxide Mixture (SPM)
SPM is another sulfuric acid and hydrogen peroxide mixture used to clean wafers. This solution is known for its ability to strip and clean wafers effectively. The temperature of SPM is critical for maintaining optimal performance: too high a temperature accelerates decomposition, while too low a temperature slows down removal rates.
Waste Disposal: Managing Hazardous By-Products
The cleaning processes in semiconductor fabrication generate significant amounts of hazardous waste. The residues left from SC1 and SC2 include chemicals such as hydrogen peroxide, sulfuric acid, ammonium hydroxide, and hydrochloric acid, along with the contaminants removed from the wafer surfaces.
Improper disposal of these chemicals can cause environmental damage, water contamination, and pose safety risks. Strict regulations govern the disposal of hazardous by-products, requiring manufacturers to neutralize and treat their waste before discharge. Common methods include diluting the waste or applying chemical neutralizers to reduce the acidity and reactivity of the solutions. However, these treatments often generate additional waste streams that must also be managed, adding complexity and cost to the process.
Some semiconductor manufacturers have moved to resell their concentrated acid waste or manufacture products like sodium sulfate (Na2SO4) and calcium sulfate (CaSO4). Regardless, inefficient peroxide abatement can limit the process, increase holding time and space prior to discharge, or compromise the manufactured product quality or consistency.
Why Traditional Treatments Fail
Solutions like catalase, activated carbon, and other chemistries fail in treating concentrated peroxide solutions mentioned previously because they lack reaction kinetics and/or stability at elevated acidic and/or peroxide concentrations.
How Can ElectraMet Support Peroxide Abatement?
ElectraMet’s Gamma system offers an innovative solution for dealing with hazardous chemistries from wafer cleaning without presenting additional waste challenges or compromising the integrity of the secondary products like sodium sulfate (Na2SO4) and calcium sulfate (CaSO4). The Gamma system uses high efficiency, stabilized catalytic media to break down concentrated hydrogen peroxide solutions into harmless water and oxygen. The remaining solution, sulfuric (H2SO4) and other chemistries, are unaltered. This revolutionary technology eliminates the need for chemical neutralizers and reduces the environmental footprint of semiconductor manufacturers. Roadblocks to recycling and upcycling piranha and sulfuric peroxide mixtures into microelectronics-grade sulfuric are now removed, presenting an new opportunity to eliminate waste while producing unequaled return on investment for Gamma systems.
By integrating ElectraMet’s Gamma technology into their wastewater treatment systems, manufacturers can simplify their waste management processes, minimize their environmental impact, and ensure compliance with regulatory requirements. The Gamma system not only supports the safe disposal of hazardous chemicals but also reduces the long-term costs associated with managing complex waste streams.