IR microscopy helps boost 3-D IC yields

IR microscopy makes it possible to capture images of all device layers through full-thickness wafers.

By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 10/1/2010 12:00:00 AM

IR (infrared) microscopy, which has long played a role in failure analysis and R&D labs, is a relatively new imaging tool in semiconductor fabs, where it is being employed in automated systems for verifying the alignment of wafer-to-wafer bonding in 3-D stacked-IC manufacturing, said Greg Baker, president of Olympus Integrated Technologies America.

Fig. 1  IR (infrared) microscopy image of particle, void, and dendrite defects on a bonded wafer pair. Courtesy of SEMATECH. More from the October 2010 Machine-Vision & Inspection Test Report.

Baker said that manufacturers must verify the alignment accuracy of bonded wafer pairs immediately after bonding to determine if devices are electrically connected and functioning optimally before they move to the next processing step. The overlaid alignment marks of the bonded wafers must be imaged and measured, but visible light observation methods can't penetrate silicon, let alone see through a full-thickness 775-micron wafer to the alignment marks at the bond interface. Destructive analysis methods require taking wafers offline, which is expensive and slow.

Since IR wavelengths are transmitted through silicon, IR microscopy makes it possible to capture images of all device layers through full-thickness wafers. By measuring the alignment offset on the images, manufacturers can predict the electrical yield of bonded wafer pairs.

"There are different types and sources of offset, and the amount will vary with location on the wafer," Baker said. "By imaging a sample of alignment marks around the wafer, you can characterize the wafer pair and create a vector map or offset table. By correlating this data with electrical test data, you can then associate a certain amount of alignment offset with yield, so this information can function as an early yield indicator. This has been clearly demonstrated by SEMATECH in their 3D-TSV [through-silicon via] Interconnect program and was presented earlier this year at SPIE."

Fig. 2  These IR microscopy images show (top) a particle on a wafer before bonding and (bottom) the same particle after bonding. Courtesy of Olympus Integrated Technologies America.

Besides inspection and overlay metrology, IR microscopy is applicable to other stacking applications such as defect detection and defect review, said Baker. The bonding process can create new defect types such as voids, dewetting, and dendrites (Figure 1). It can also change the characteristics of existing pre-bond defects (Figure 2).

The IR inspection and overlay metrology is also useful in MEMS (microelectromechanical systems) and PV (photovoltaic) solar wafers. "In both MEMS and PV, manufacturers are patterning front side and back side of a single wafer and using TSVs to connect the patterns," he said. "These front-side and back-side patterns must be aligned properly."

The Olympus 3D-IC Automated Metrology System adds IR microscopy and metrology technology to an existing Olympus platform, said Baker. The system's confocal laser-scanning microscope provides the precise x-, y-, and z-axis data required by 3-D metrology applications. The confocal laser-scanning observation method gives the advantage of optical sectioning to eliminate out-of-focus image information, producing an overall higher-resolution image than is available with conventional microscopy. "The technology can be used for inspection and metrology in any multiple-level lithography process," said Baker. The system has a total magnification of up to 14,400X and can resolve 0.5 microns through a full-thickness 775-micron wafer.