Why low-alpha silica is used in memory and
high-reliability semiconductor packaging

Silica is a major inorganic filler in semiconductor packaging materials such as epoxy molding compounds. The raw material can contain trace amounts of naturally occurring radioactive impurities, including uranium (U), thorium (Th), and their decay products. Alpha particles emitted by these impurities can affect the reliability of certain semiconductor devices. When alpha particles reach sensitive regions of a chip, they may cause transient changes in stored data [1][2]. Controlling alpha-particle emission from silica to a low-alpha level reduces the amount of alpha radiation released by the packaging material toward the chip while preserving silica's basic function as an inorganic filler.

Soft errors induced by alpha particles are generally single-event effects that cause a temporary change in circuit data rather than permanent structural damage to the device. As an alpha particle travels through silicon, it generates a large number of electron-hole pairs. If the resulting charge is collected by a storage node or junction and exceeds the node's critical charge, a stored "0" or "1" can flip. In their 1979 study of dynamic random-access memory (DRAM), May and Woods identified alpha particles generated by the decay of U and Th in packaging materials as an important source of single-bit soft errors [1]. For DRAM and SRAM, which are relatively sensitive to soft errors, as well as other devices with stringent data-integrity and reliability requirements, alpha-particle sources therefore need to be controlled at the packaging-material stage.

In packaging materials, silica is typically one of the main inorganic fillers in the resin system and, after encapsulation, is located close to the chip. Literature on advanced packaging notes that if the SiO₂ in an epoxy molding compound (EMC) contains radioactive impurities such as U and Th, its alpha-particle contribution needs to be reduced to meet the reliability requirements of memory devices such as DRAM [2]. Product literature published by Nippon Chemical Industrial Co., Ltd. in 2003 also described the use of low-alpha silica in resin encapsulation materials for memory devices and listed a U content below 0.1 ppb as one example of a product specification at that time [3]. This value should not be treated as a universal limit for all products, but it shows that low-alpha control had already become part of practical specification management for packaging materials in Japan.

Low-alpha control is closely related to low-uranium (low-U) control, but the two address different aspects. Uranium is one source of alpha particles, while thorium and the radionuclides in its decay chain also need to be controlled. U and Th concentrations can therefore be used to manage radioactive impurities, whereas the alpha emission rate more directly reflects the level of alpha particles released by a material under specified conditions. A low-alpha requirement can be added to an existing powder design defined by attributes such as fused structure, spherical morphology, and particle-size distribution. In addition to maintaining conventional specifications such as purity, particle size, ionic impurities, and metallic impurities, suppliers may also need to provide alpha-emission data and, where required by the customer, U and Th analytical results.

When selecting test methods, it is important to distinguish material alpha-emission measurement from device-level soft-error evaluation. JEDEC JESD221, Alpha Radiation Measurement in Electronic Materials, was released in 2011 and provides recommended methods for measuring alpha emission from materials used in semiconductor manufacturing, including guidance on instrument setup, sample size, background, detection-limit calculations, and reporting [4]. The JEITA soft-error test guideline EDR-4705, issued in June 2005, focuses mainly on semiconductor memories such as SRAM and DRAM and describes methods for predicting and testing soft errors caused by cosmic rays and radiation from semiconductor-device materials [5]. The former addresses material alpha emission, while the latter addresses device soft errors; they operate at different levels of evaluation.

Accordingly, during supplier-buyer specification alignment, material test conditions need to be matched to device reliability requirements. Suppliers should clearly define the alpha-emission measurement method, unit, detection limit, sampling conditions, and lot-control practices, and provide supporting U and Th analytical data when requested. Buyers, in turn, should define acceptance limits and confirmation frequency according to device sensitivity, package structure, and reliability targets. JESD221 specifies a measurement method; it does not establish a universal pass/fail threshold for low-alpha silica. Materials carrying the same "low-alpha" designation therefore still need to be checked for consistency in test conditions and customer specifications. Final material qualification should also consider particle-size distribution, particle morphology, and compatibility with the target resin system.

Overall, low-alpha silica is used in memory and high-reliability semiconductor packaging to reduce the risk of device soft errors caused by alpha particles emitted from packaging materials. Suppliers need to control radioactive impurities such as U and Th and maintain stable alpha-emission performance, supported by verifiable and traceable product data. Buyers need to define the appropriate test methods and acceptance limits according to device sensitivity, package structure, and reliability targets. At the same time, low-alpha requirements should be evaluated together with other silica specifications, including particle morphology, particle-size distribution, and resin compatibility. Stable mass-production conditions can be established only when these attributes collectively meet the requirements of the target device and packaging process.