Through intense concentration on cause-and-effect, and careful yield analysis, fabs are getting a much better handle on defect reduction. A wide range of approaches and solutions, and projections for the future in contamination and defect control, were explored recently in Santa Clara, CA, at a Materials Integrity Symposium sponsored by Entegris, Inc., Chaska, MN.
“Gone are the days when revolutionary ‘solutions’ were justified based solely on paranoia control,” stated David Jensen of Advanced Micro Devices in discussing AMD’s approach. Anything to cut contamination or reduce defects that can be justified in terms of cost of ownership is fundamental to materials handling decisions, he said.
Jansen stressed the importance of reducing variability. Cutting down on variability is often more important than just lowering a specification, he pointed out. A chemical with about 5 ppb +/- x% may be preferable, for example, to one with purity of less than 1 ppb. Such choices are process-related and cost needs to enter the equation.
Decisions like this are worked out much more often today through partnerships with suppliers, according to Jensen. Fabs will need a lot of help as new materials are introduced into chipmaking. Fundamental understanding of the impact of trace impurities will be needed to make sure specifications are adequate but not excessive. Cleanliness must approximately double with each generation, but measurements of critical particle sizes may not always be possible.
Particle requirements must be referred to measurements at the point of use, to avoid excessive demand for filtration in delivery systems. Contamination modeling is needed, and efforts should include particle avoidance as well as active particle control. The increasing value of wafers will drive fabs toward in situ measurement and control of contamination and process parameters, using integrated sensors and feedback.
IBM’s methods for materials control and yield improvement were described by William Miller, IBM Microelectronics. He cited the negative binomial yield model specified by the latest Roadmap, including the critical area of a device on the wafer and the defect density. When this analysis is applied to different product types, it provides a convenient tool for fast learning on process improvement and identifying problems. These might include out-of-control tools, materials batch change effects on tools and processes, and process shifts. Normalized yield analysis data can be correlated to in-line metrics and controls to find trouble spots like these.
IBM tracks fab productivity with multiple metrics, Miller explained, including product cycle time, daily going rate, serviceability, and yields for both test and process wafers. The yield is the key metric.
When a problem is identified, root cause analysis is essential. Miller gave a few examples, including:
? An impurity in a CMP process that led to over-polishing and shorts from dishing.
? Transport temperature variation that resulted in materials changes.
? Supplier shift in particle size for CMP slurry resulting in scratches.
Transporting thinned wafers is becoming a more common problem, and an approach to developing standards for an effective packaging scheme was described by Herve Deshayes of STMicroelectronics.
An international group including engineers from Europe, the US, and Japan are contributing to the strategy for a wafer box and packaging suitable for wafers thinned down to as little as 140 microns. The goal is a Cybernetix WPC (wafer packing container) compatible with boxes from Achilles, Entegris, ITW, and SPI, for functions such as split, merge, transfer or kitting of wafers.
Preventing wet process contamination from fluoropolymers was a topic explored by J. F. Unbalzano of DuPont, Wilmington, DE. Fully fluoronated polymers should be chosen over partially fluoronated ones, but if partial versions are used the least reactive types should be used. He illustrated the advantages of greater smoothness in Teflon designated as PFA HP Plus versus PFA HP, and also described PTFE NXT and AF amorphous fluoropolymers which can further drive contamination reduction.
A wide range of issues were probed at the two-day conference, including wafers, packaging materials, chemicals, slurries, wafer handling systems and carriers, fluid processing and handling, and gases. There was also a talk on handling data storage disks by John Mitchell of Komag.
– Bob Haavind, WaferNews Editorial Director