Pan Pacific Symposium Conference Proceedings


LOW ALPHA PARTICLE EMITTING SILICONE MATERIALS FOR MICROELECTRONICS

Author: Ann Norris
Company: Dow Corning Corporation
Date Published: 1/25/2000   Conference: Pan Pacific Symposium


Seika Machinery, Inc.

Abstract: Silicone materials, typically filled with reinforcing silica, have a long history of successful use in the electronics industry. Some of the many attributes that make them excellent materials for adhesives, sealants or encapsulants in electronic applications are high ionic purity, low moisture absorption, and wide use temperature range. Microelectronic packaging is an exploding field in which the trend to smaller, lighter, and faster devices has led to chip scale packaging (CSP) technology. The BGA CSP, supplied by Tessera, Inc., and their licensees, requires a compliant spacer and encapsulant for maximal reliability and optimal performance, and silicones are proving to be an excellent choice for this package type. Encapsulants, die attach adhesives, and spacer materials are commercially available, being successfully used in BGA package production for all types of memory devices. In dynamic random-access memory (DRAM) chips, the effect of soft errors add additional performance requirements for the encapsulant. Soft errors are random, non-recurring single bit changes of information stored in a memory chip, caused by alpha particles penetrating the die surface and creating electron-hole pairs near a storage node. In DRAM devices, the occurrence of such soft errors is directly related to the number of bits per device, and for large memories, even small amounts of alpha particle radiation can result in unacceptably high soft-error rates. This makes it imperative for materials developed for this application to have extremely low alpha particle emissions. The main sources of these are the two radioactive elements uranium, U, and thorium, Th, naturally present in almost all materials. For DRAM chip packaging, therefore, the additional requirement of sufficiently low U and Th concentrations in these materials must be considered. We will show that silicones with very low U and Th content have been formulated that meet the requirements for DRAM chip packaging, and present the equations for calculating the material’s alpha particle flux densities from the elemental particle emission rates as a function of concentration. The paper will also discuss the thickness of a silicone encapsulant that is sufficient to prevent alpha particles originating from external sources, e.g., solder balls, from penetrating the die surface.



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