EVGNEWS Issue 2 2017

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Application Focus – Fusion Bonding for Future 3D Devices

The Gemini FB is the state of the art in wafer bonding equipment for manufacturing backside illuminated (BSI) image sensors. Sub-250 nm overlay accuracy enables production of not only image sensors but also high-accuracy optical devices, stacked memory, and advanced logic by die segmentation.

Additionally, the EVG850LT automated fusion bonder allows manufacturing of engineered substrates such as silicon on insulator (SOI), silicon carbide (SiC) and gallium nitride (GaN) for RF, power and other high-speed/high-efficiency devices.

Minimizing through silicon via (TSV) dimensions for via-last bonding, or TSV and bonding pad dimensions for hybrid bonding, are key requirements for bringing down the cost of 3D devices. Considering that the role of a TSV is essentially "only" for signal connection yet consumes valuable wafer real estate, further miniaturization has to be the logical consequence.

A first option for high-bandwidth integration is hybrid bonding, whereby a dual damascene copper and silicon oxide hybrid interface serves as both the full-area bonding mechanism and the electrical connection. A second option is the transfer of a thin processed semiconductor layer (ranging from tens to a few hundred nanometers in thickness) using a full-area dielectric bond. In contrast to hybrid bonding, the electrical connection is introduced by a via-last process between early interconnect metal levels on the bottom wafer and the second transferred transistor layer. Both hybrid bonding and full-area dielectric bonding can be achieved through aligned wafer-to-wafer fusion bonding. However, high-interconnect density along with small routing dimensions set a high bar for bond alignment precision, which is necessary for fusion bonding.

Several factors contribute to the global alignment of the wafers besides the in-plane measurement and placement of the wafers relative to each other. In fusion bonding, both wafers are aligned and a pre-bond is initiated. When bringing the device wafers together, wafer stress and/or bow can influence the formation of a bond wave. The bond wave describes the front where hydrogen bridge bonds are formed to pre-bond the wafers. Controlling the continuous wave formation and influencing parameters is key to achieving the tight alignment specifications noted above. The reason for this is that any wafer strain manifests itself in distortion of the wafer, which leads to an additional alignment shift. Process and tool optimization can minimize strain and significantly reduce local stress patterns. Typically, distortion values in production are well below 50 nm. Indeed, further optimization of distortion values is a combination of many factors, including not only the bonding process and equipment, but also previous manufacturing steps and the pattern design.

Click the thumbnails to enlarge
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GEMINI®FB process modules (SmartView®NT, cleaner module, plasma module, buffer station, robot unit)

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GEMINI®FB Side view with equipment front-end module (EFEM)

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Comparison of different 3D front-end-of-line integration schemes.

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Calculated surface overlap of metal TSVs for hybrid bonding as a function of wafer-to-wafer alignment accuracy. Comparison of ITRS roadmap relevant TSV pitches and diameters reveal, alignment accuracy of better than 200nm (3σ) is needed to achieve 60% and more TSV overlap for hybrid bonding.