Metrology is consistently one of the more challenging areas in the semiconductor field. Whether it’s the monitoring of ultra-small features and defects, or inspecting the backside or bevel of a wafer, the challenges are ever growing. The situation is further complicated by the need for in-line metrology that can be used in the fab during device manufacturing as distinct from out-of-line failure analysis. For 3D integration, the issue of monitoring image alignment on bonded wafers is a particularly challenging one. This stems from the difficulty of effectively transmitting conventional wavelength light through silicon in order to analyze the bond interface region between the wafers.
During a Semicon West meeting with Greg Baker, President and COO of Olympus Integrated Technologies America, and a subsequent presentation given at the SEMATECH 3D Metrology Workshop there, we were introduced to Olympus’ latest IR confocal microscopy technology. Confocal or laser scanning microscopy has been around for a while and its advantages over conventional optical microscopy are well understood. By using a pin-hole aperture and selective spatial filtering it is possible to control depth-of-focus and eliminate out-of-focus light. The result is an increase in the contrast and quality of 2D images. Furthermore, the technology has the advantage of being able to collect serial optical section data thereby enabling the development of 3D images.
The key to addressing the poor penetration of light through silicon, and in particular in bonded structures is the tuning of the laser light source. Olympus reports that by using an IR laser with a wavelength in the region of 1310 nm they can penetrate in excess of 775 μm of silicon and clearly delineate structures. Furthermore, it is then possible to form a 3D reconstruction of the features being examined, an example of which is shown below. This offers a non-destructive method of doing in-line monitoring to identify process variation associated with defects and misalignment of structures.
Alignment offset in particular is critical. Excessive structural offset can greatly impact yield and can lead to characteristics like high electrical resistance (that can be verified subsequently by e-test). It is possible to quantitatively measure the offset whether it be linear or rotational.
Particle defects can also adversely affect yield. The confocal IR technique allows for the resolution of light point defects that would otherwise be unidentifiable in an integrated 3D structure. By adjusting focus and magnification it is possible to classify and categorize defects effectively. Both known and random defects can be monitored with or without 3D reconstruction and it is possible to characterize whether a defect is due to a particle or a recessed area associated with corrosion.
Olympus appears to have some momentum with their new technology. In April they installed an IR confocal system at SEMATECH’s global consortium of 3D R&D Center at the College of Nanoscale Science and Engineering’s (CNSE) Albany NanoTech location. This tool will focus on monitoring alignment of bonded wafers and through-silicon via (TSV) structures with a view to enabling 300 mm high volume TSV manufacturing.