ABSTRACT The Tessera µBGA chip-scale package is currently in mass production for flash memories, static random-access memories (SRAM) and dynamic random-access memories (DRAM). However, in DRAM devices, the issue of soft errors, caused by a particles emitted from the packaging material, adds another performance requirement to the materials. The main sources of alpha particles are the two radioactive elements uranium, U, and thorium, Th, which are naturally present in almost all materials in small concentrations. For silicones to be used in the packaging of DRAM chips, requires that the silicone employed contain sufficiently low levels of U and Th. By Drs. Ann W. Norris and Udo Pernisz, Dow Corning Corp., Midland, Michigan Silicones have a long history of successful use in the electronics industry. They offer many attributes that make them excellent materials for applications in electronics, including high ionic purity, low moisture absorption and consistent performance over a wide temperature range. While the many available cure chemistries give silicones the properties required for use in electronics, an addition cure is typically employed. This cure is attractive because it contains no cure by-products, and because heat accelerates the cure process. Furthermore, an addition cure system can be delivered in either a one-part or a two-part formulation. Primerless Adhesives Silicones can be made into excellent primerless adhesives by utilizing adhesion promoter technology. When used as adhesives, sealants or encapsulants, silicones are typically filled with reinforcing silica. Microelectronics packaging is an expanding field, in which the recent trend to smaller, lighter and faster devices has led to the adoption of CSPs like the Tessera µBGA package. This package requires a compliant spacer and encapsulant for optimal reliability and performance. Silicones are proving to be a material of choice for this package1. Encapsulants, die attach adhesives and spacer materials are commercially available and are being successfully employed in the production of µBGA packages. Figure 1 shows a diagram of the package design with compliant spacers and encapsulant separating the silicon die and the flexible TAB tape with the solder ball array. Figure 2 shows the encapsulant being dispensed during package assembly.
DRAM Requirements In the late 1970s and early 1980s, concern and evidence of "soft" errors in DRAM memory chips had already been discussed and documented. Soft errors are defined as random, non-recurring changes of single bits of the information stored in a memory chip, leading to data errors. Researchers have observed that a particles, when allowed to penetrate the die surface, can create enough electron-hole pairs near a storage node to cause single-bit errors,2-3 at random, in the device. In DRAMs, the occurrence of soft errors is directly related to the number of bits per device, and for large memories, even small amounts of a particle radiation can produce a high soft-error rate. Thus, it is imperative that the materials developed for DRAM use possess very low alpha particle emission rates. The main sources of alpha particles are uranium, U, and thorium, Th, which are naturally present in almost all materials in small concentrations. For silicones to be used in packaging DRAM chips depends, therefore, on these materials meeting the requirement of sufficiently low levels of U and Th concentration. Experimental Measurements Samples of Dow Corning 6810, 6811, 6910, and 7910 silicones were submitted to an outside test laboratory for the analysis of U and Th. Duplicate samples were measured by inductively coupled plasma-mass spectrometry (ICP-MS) using a Seiko SPQ6500 instrument. The results (average values) obtained are shown in Table 1.
The 6810 encapsulant was not developed for DRAM applications but was included as a reference material. The 6811, 7910 and 6910 products were developed for DRAM applications, making it critical that they contain a low U and Th content. When these materials, in 75-mil thick sheets (1.9 mm) were sent to an outside lab for a particle emission measurements, the 6810 encapsulant was measured to have a flux density of 0.012 cm-2 h-1; the three other products, however, were below the detection limits of the instrument (<5x10-3 cm-2 h-1)4. In discussing the requirements for DRAM-grade material, White et al. stated that a particle counts below 0.001 cm-2 h-1 are required for applications in DRAM packaging.5 The last three products shown in Table 1 are believed to comply with industry standards, however, the alpha flux density was below the detection limit and therefore, a theoredical calculation needed to be conducted. More recent information on the effects of a particles on DRAM devices suggests lower values for current chip design may be required, because smaller design rules lower the critical charge acceptable in an a particle event. Emission Flux Density For the application of elastomers as IC packaging material, the a particle flux density at the interface between chip and elastomer is relevant. Flux density measures the number of particles emitted through the elastomer interface per unit area and unit time in [cm-2 h-1]. The goal is to derive this number from the original emission rates of U and Th which emit a particles in their decay process. The alpha particle emission rates are given as the number of particles/ second from one gram of the substance. For naturally occurring uranium (which consists of 99.27% of 238U) and for thorium (232Th) these values for 1 g of material are6 U: Ep = 4.2 MeV R0 = 2.51x104
s-1 T1/2 = 4.5x109 a Also given are the density p and half-life T1/2 of the two elements, and the kinetic energy Ep of the a particles; the values for uranium apply to the 238U isotope. (Natural uranium contains 0.72% 235U and a trace amount of 234U). The a particles are radiated into the full 4¶ steradian of surrounding space, with the emitter taken to be a point source. At the energy emitted, their mean range L in typical Si-based polymeric materials is on the order of 20 µm; exact values for materials of known atomic composition can be calculated from data for a standard reference substance (such as air in which L = 2.4 cm)6.
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