Conformity - November 2008 - (Page 37) ncontrolled electrostatic charge is a significant and growing problem in semiconductor manufacturing. When a static charge builds up on sensitive products, work surfaces, equipment or personnel, the result can be destructive. Products may be damaged, processes may become degraded, and a long list of other problems may ensue. Product damage from static charge can cost manufacturers millions of dollars in scrapped materials, as well as the loss of potential revenue. In the semiconductor manufacturing industry, damage and yield losses attributed to the effects of static charges are well documented, along with the determination of many of the specific causes1-22. Particle contamination is a major issue, as charged wafers attract more particles to their surfaces than their non-charged counterparts (electrostatic attraction) 2,3,6-10,13,22 . Direct electrical damage caused by electrostatic discharge (ESD) to wafers can occur with a variety of ESD failure modes1, 12,15,16. In addition, ESD events produce electromagnetic interference (EMI) that can cause equipment malfunctions, lockups, and direct damage to products via radiated and conducted forms11, 17-19. Problems Caused Specifically by ESA When a wafer or front-opening universal pod (FOUP) becomes charged, the resulting electric field attracts airborne particulate and airborne molecular contaminants (AMCs) through electrostatic attraction (ESA) in much the same way that a television screen attracts dust. Once bonded to a charged surface, it is very difficult to remove the contamination. As line widths get smaller, the “killer particle” size needed to create a defect has decreased proportionally, and high-tech manufacturers must concern themselves with controlling increasingly smaller particles. Importantly, the ESA of particles also increases as particle sizes decrease (65, 45, 32 nm geometries), magnifying the problem. AMCs are a type of contaminant that is becoming more of a concern in semiconductor fabs. AMCs come from both organic and inorganic sources, and cannot be removed by HEPA or ULPA filters. Problems with AMCs include poisoning Deep UV photoresist processes, roughening the silicon at the pre-gate clean, and the breakdown of HEPA filter media, which can lead to the release of dopant AMCs, such as Boron. There are indications that electrostatic charge increases the attraction of AMCs to product surfaces. Recent published data in 2006 details the increased particle counts on wafers in highcharge generating containers (due to ESA) versus non-charged counterparts, leading to increased yields when ionization techniques are employed to remove the charging mechanisms. Why E78 Was Developed In 1995, SEMI (Semiconductor Equipment and Materials International) established a standards task force made up of professionals in the semiconductor industry. Their U mission was to create a guide to “minimize the impact on capital productivity due to the presence of static charge in semiconductor manufacturing environments.” The result of that group’s efforts was document E78-0998, Electrostatic Compatibility: Guide to Assess and Control Electrostatic Discharge (ESD) and Electrostatic Attraction (ESA) for Equipment. Revisions were produced as needed, the latest of which is E78-0706, published in 2006. E78 identifies static charge as a factor that can be detrimental to semiconductor manufacturing processes. To combat it, the document promotes implementing preventative measures to limit the amount of static charge generated during the handling and processing of wafers and reticles, and the handling of their FOUP carriers. It sets a limit on charge levels, so that they are not great enough to cause product or reticle damage, attract significantly more particles to surfaces, or cause equipment malfunctions to ESD-induced EMI. It also provides a matrix of recommended charge values (measured in nanocoulombs) and electric field values (measured in volts/cm), depending on the sensitivities of the products and equipment involved. The latest version of E78 expands upon ESD and presents both theory and practical voltage values that should be maintained to eliminate the effect of ESA (increased particles) on wafer surfaces. Why gamble on design? NTS Engineering Services evens the odds. As your product development partner, NTS helps ensure the successful launch and industry acceptance of your product. Our skilled engineers provide complete design solutions and integrated services for creating something totally new or redesigning an existing product. Whether you want to outsource your engineering services or enhance your own design capabilities, NTS has the insight and experience to design your product for success. With almost 50 years of experience, NTS is the best choice for your engineering and testing needs. We have served thousands of satisfied customers in a variety of industries including Aerospace, Defense, Telecommunications, and Commercial Electronics. Don’t leave your success to chance. Call NTS today for a free, no-obligation quote. Accelerating Your Time to Market 800-270-2516 www.ntscorp.com n info@ntscorp.com Integrated Engineering Solutions | Testing & Standards Compliance | Technical Resources november 2008 Conformity 37 NTS quarter page ad 10.03.08.indd 1 9/18/2008 10:09:49 AM http://www.ntscorp.com http://www.ntscorp.com
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