Inside composite substrates

In contrast to the current generation of bulk wafers, which typically are pure silicon, "engineered" substrates contain several materials layered one on top of the other.

The most common engineered solution currently on the market is the silicon-on-insulator (SOI) wafer.

Advanced, engineered substrates use layering, implantation and/or bonding of advanced substrate materials into and onto a silicon wafer or other substrate (such as quartz). The result is that a stack of substrate layers on some kind of "handle" wafer replaces the bulk wafer. For most applications in use now, the top layer is still silicon, so that chip manufacturers can continue to use traditional manufacturing processes and equipment in the fabrication process.

The SOI wafer is a composite substrate with an active top Si layer decoupled from the support wafer. It is the first example of an engineered substrate addressing mainstream MOSFET performance requirements. SOI now has a well-proven record. It makes a major impact on partially and fully depleted devices in terms of performance enhancement, reduction of leakage currents and power consumption, suitability for low-voltage device architectures and so forth.

Smart Cut, Soitec's approach to creating SOI wafers, is currently deployed commercially for SOI, SOQ (silicon-on-quartz or, more precisely, single crystalline Si on a fused silica substrate) and soon, strained silicon on insulator. It has also been successfully demonstrated for many other major engineered substrates, which are scheduled for commercial release in concert with customer needs and the industry road map.

A thermal approach

Smart Cut is a technique used to transfer ultrathin single-crystal layers of wafer substrate material, such as silicon, onto another surface. Differing from traditional layer-transfer techniques, which are based mainly on wafer bonding and etch-back or epitaxial liftoff, the Smart Cut approach uses a thermal activation process as an "atomic scalpel." It literally slices the wafer horizontally, lifting off a thin layer from the donor substrate and placing it onto a new substrate. A single donor substrate can be reused many times for further layer transfers.

The transferred layer thickness is predetermined by the cleavage zone created via ion implantation of hydrogen, helium, argon and the like. After the layer transfer and bonding, the cleaved surface of the thin film is treated, polished and annealed to ensure a silicon film (in the case of SOI) and surface quality comparable to silicon prime wafers.

George Celler is chief scientist and Michael Wolf is senior vice president of sales and marketing at Soitec.

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