Defect-Free Silicon Crystal

A new approach to defect management has been developed at MEMC Electronic Materials (St. Peters, Mo.) resulting in silicon crystal ingots free of in-grown agglomerated or clustered defects (Fig. 1). The process suppresses the formation of two classes of defects in crystal ingots used in IC manufacturing.

Under conventional crystal growth conditions, vacancies agglomerate to form small, low-density octahedral voids, commonly called D-defects or COPs (Fig. 2). Interstitials can form distributed dislocation clusters. The void defects have been clearly associated with dielectric breakdown failures, while the dislocation defects are known to be related to certain classes of leakage current failures. The void defects have recently become a particular concern throughout the industry in spite of their extremely low density (typically 1x10⁶/cm³), noted Saeed Pirooz, director, applications engineering technology at MEMC.

Fig. 1. The crystal grown using the perfect crystal technique shows no extended defects.

Fig. 2. Vacancy region of the crystal can create voids, D-defects, causing failure of the gate oxide.

"Slow-cool" silicon is a common expression used for this general type of approach to the problem. But as device generations progress, it is less clear that such an approach will be successful in producing sufficiently low void densities to produce acceptably profitable yields. Epitaxial silicon provides an alternative but is problematic in certain cost sensitive applications, particularly DRAMs.

MEMC has developed a new crystal growth process that goes beyond controlling the rate of the defect producing reaction to integrating defect management throughout the entire growth process. Hot zone engineering ensures that the reactions that produce either the void or dislocation cluster defect are completely suppressed, not simply controlled, Pirooz said. The result is crystals completely free of both void and dislocation cluster defects. Eliminating one or the other can be readily achieved. However, crystals completely free of both classes of defects earn this material the name "perfect silicon," Pirooz said.

Fig. 3. Comparison of D-defects in 150 mm wafers indicates defect-free crystals using the perfect silicon technique.

This technique is extendable to 300 mm growth where the propagation of in-grown defects is a major challenge. Engineering work on newly developed 300 mm crystal pulling systems has already begun at MEMC.