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Rudolph introduces new acoustic metrology and defect inspection technology

Rudolph Technologies has introduced its new SONUS Technology for measuring thick films and film stacks used in copper pillar bumps and for detecting defects, such as voids, in through silicon vias (TSVs). Copper pillar bumps are a critical component of many advanced packaging technologies and TSVs provide a means for signals to pass through multiple vertically stacked chips in three dimensional integrated circuits (3DIC). The new SONUS Technology is non-contact and non-destructive, and is designed to provide faster, less costly measurements and greater sensitivity to smaller defects than existing alternatives such as X-ray tomography and acoustic microscopy.

“SONUS Technology meets a critical need for measuring and inspecting the structures used to connect chips to each other and to the outside world,” said Tim Kryman, Rudolph’s director of metrology product management. “Copper pillar bumps and TSVs are critical interconnect technologies enabling 2.5D and 3D packaging. The mechanical integrity of the interconnect and final device performance are directly dependent on tight control of the plating processes used to create copper pillar bumps. Likewise, the quality of the TSV fill is critical to the electrical performance of stacked devices. This new technology allows us to measure individual films and film stacks with thicknesses up to 100µm, and detect voids as small as 0.5µm in TSVs with aspect ratios of 10:1 or greater.”

Kryman added, “SONUS Technology builds on the expertise we developed in acoustic metrology for our industry-standard MetaPULSE systems, which are widely used for front-end metal film metrology. By offering similar improvements in yield and time-to-profitability in high volume manufacturing (HVM), SONUS offers a compelling value proposition to advanced packaging customers.”

Both MetaPULSE and SONUS systems use a laser to initiate an acoustic disturbance at the surface of the sample. As the acoustic wave travels down through the film stack, it is partially reflected at interfaces between different materials. Although the detection schemes are different, the reflected waves are detected when they return to the surface and the elapsed time is used to calculate the thickness of each layer. In the case of SONUS Technology, two lasers are used. The first laser excites the sample and the second probes for the returning acoustics. This decouples excitation and detection allowing SONUS to continuously probe the sample resulting in a much larger film thickness range. So, where MetaPULSE can measure metal films and stacks to ~10 microns, SONUS can measure films in excess of 100 microns. In addition, SONUS Technology’s use of interferometry to characterize the surface displacement provides a rich data set that can be analyzed to not only characterize film thickness, but perform defect detection.

The primary alternatives for such measurements are X-ray based tomographic analysis and acoustic microscopy. SONUS Technology’s ability to detect voids as small as half a micrometer is approximately twice as good as current X-ray techniques, which have a spatial resolution of about 1 micrometer. Acoustic microscopy can make similar measurements, but the sample must be immersed in water, which, though not strictly destructive, does effectively preclude the return of the sample to production. SONUS is both non-contact and non-destructive and is designed for R&D and high-volume manufacturing.

In the run up to the product introduction, Rudolph worked closely with TEL NEXX to develop SONUS-based process control for pillar bump and TSV plating processes. Arthur Keigler, chief technology officer of TEL NEXX, said, “We are attracted by the opportunity SONUS Technology offers our mutual customers in the advanced packaging market. The ability to measure multi-metal film stacks for Cu pillar, and then continue to use the same tool for TSV void detection offers immediate productivity and cost benefits to manufacturing and development groups alike.”

While Rudolph is initially focused on using the technology for copper pillar bump process metrology and TSV inspection, they are also investigating other applications, ranging from detecting film delamination to metrology and process control for MEMS fabrication processes.

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