Together in electric themes
Paul Farrington, managing director of Haden Drysys Environmental, describes an innovation to ensure emissions to air from semiconductor manufacture meet stringent environmental consents.In today's electronics industry the semiconductor is ubiquitous. As the industry grows, greater attention to being environmentally friendly is clearly evident. In addition to the introduction of environmental management systems, the industry also uses innovative technology to ensure its emissions meet environmental legislation.
When designing a treatment system for air emissions from a semiconductor manufacturing plant it is important to recognise not only the types of volatile compounds likely to be present in the air stream but also the complications that silicon presents. Semiconductor manufacture is a silicon based industry. While the precise process differs according to the manufacturer and semiconductor type, there are six basic steps, several creating volatile organic compounds (VOCs). An ingot of pure silicon is pulled from a seed crystal immersed in a bath of molten silicon and formed into wafers. The wafers are coated with a dielectric, normally silicone dioxide, often using a technique called chemical vapour deposition. Multiple layers of circuit patterns are created by a process called photolithography during which the wafer surface is coated with a light sensitive chemical called a photoresist. The wafer is then etched. A number of photolithography and etching stages produce the multiple layers of circuit patterns.
The chemicals and gases used in chemical vapour disposition, photolithography and the etching process account for the majority of the VOCs present in the gas stream to be treated prior to release to atmosphere. Good environmental management ensures that there are small quantities of VOCs in the air stream to be treated. Often there are 20 to 30 different VOCs to be treated, some common and others complex. The problem for most treatment processes is the silicon inherent in the manufacturing process. This leads to silicon-based VOCs and potential blockages caused by silicon dioxide.
Haden's approach has been to design a system that addresses both the capture and economical treatment of the low concentrations of VOCs while accounting for potential blockage of the system with silicon dioxide and silicon-based VOCs. The system incorporates a concentration phase and subsequent oxidation of the VOC laden air stream. Haden's system uses a rotary concentrator with a microporous hydrophobic zeolite carrier that adsorbs the VOCs. This process concentrates the VOCs, effectively reducing the air volume to be treated and significantly reducing operating costs.
The concentrated VOCs are desorbed into an air stream that is treated by a recuperative oxidiser. The recuperative oxidiser offers reliable, efficient destruction of the VOCs with no inhibition of performance characteristics by silicon dioxide. The recuperative oxidiser uses a shell and tube heat exchanger to recover heat from the exhaust gases for use heating the incoming VOC laden air stream. A secondary heat exchanger on the exhaust outlet is used to recover heat that powers the desorption of VOCs from the concentrator.
"As WEEE continues to drive improved environmental performance in the electronics industry there is increasing interest in reliable, cost effective technology that can help meet compliance standards. The system brings a high removal efficiency and can comfortably achieve less than 10mgC/Nm3 emission level. We have developed a treatment configuration that has proven successful in the industry and we envisage this system will become the preferred technology in the ever growing semiconductor manufacturing market," concludes Paul Farrington.