An Investigation of Materials Selection and Manufacturing Process on Resulted Device Alpha Emissivity Levels

The continued decrease in Si feature sizes has resulted in significant increase in transistor and embedded SRAM density. In the meantime, this decrease in feature size, along with the the reduction in operating voltages, has also resulted in a reduction in the charge stored for SRAMs and transistors. Both of these changes may result in an unintended increase of SER (Soft Error Rates) in IC devices. One of the major contributing factors for SER is alpha particles emitted from packaging materials, which mostly comes from the decay of Th and U. These elements typically have a concentration of a few parts per billion in materials such as silica fillers and solder alloys. Even at these levels, however, small amount of contamination on the assembly process due to mix-use of equipment or tooling can result in high and unreliability alpha emissivity near the active silicon, resulting in an increase of SER.

In this work, we investigate the effects of key process steps and equipment use on the alpha emissivity levels on flip chip packages. First, blank silicon wafers are processed through major steps of the assembly process.

Both a dedicated ULA (Ultra Low Alpha) assembly line and a mix-use line are evaluated. Contacts and handling by equipment and tooling are simulated on these wafers, and after every step, one wafer is taken out of the process for alpha emissivity measurement. Second, actual flip chip components with an ULA package material set are assembled on the same dedicated ULA and mix-use assembly lines. Alpha emissivity levels near the active silicon surface are measured for both groups of components. Alpha emissivity measurements on the blanks wafers and the

flip chip components will be reported and compared.