DERA Develops SiGe Virtual Substrates for New Silicon-Based Devices
Brian Dance, Contributing Editor -- Semiconductor International, 3/1/2001
 |
DERA produces its virtual substrates with surface layers of strain-relaxed SiGe by epitaxial growth. A substrate of pure silicon has an inter-atomic spacing of 0.543 nm. If a thin layer of silicon containing a proportion of the larger germanium atoms is grown onto the silicon substrate, a strained lattice is formed. When the thickness of the deposited SiGe alloy layer exceeds a certain critical value, the strain in this layer is relaxed by misfit dislocations (Figure). A thin-strained layer of silicon can then be grown onto the surface of the SiGe layer. The strain in this silicon layer, and hence the properties of the layer, can be adjusted by an appropriate choice of the atomic fractions in the underlying SiGe layer.
In the thin-strained silicon layer, the scattering of the electron and hole charge carriers are both reduced below those in unstrained silicon, since the strain reduces degeneracy at the band edges and produces changes in the band structure of the material. The reduced scattering of the charge carriers results in increased charge-carrier mobility, which is important in deep submicron CMOS devices that operate at high electric fields. In the strained silicon epitaxial layers produced at DERA, theory shows the hole mobility is increased more than the electron mobility, so there is little difference between the enhanced electron and hole mobilities. This is ideal for CMOS device design because the complementary transistor pairs can be the same size. The virtual substrate should have a low surface roughness — typically 2-6 nm.
DERA is collaborating with Applied Materials (Santa Clara, Calif.) on low-temperature epitaxy. It has developed its virtual substrate technology with an Applied Materials Epi Centura reactor with 100 and 150 mm wafers using a new proprietary low-pressure process. The SiGe material of heterojunction bipolar transistors typically contains <20% of germanium atoms (and often <10%), but virtual substrates often contain 20% or 30% of germanium atoms in their SiGe material. Conventional silicon epitaxial growth is carried out at temperatures of ~1000ºC, but the strained materials must be grown at lower temperatures, typically 600-700ºC, or the desired crystal strain will be relaxed.
The DERA growth process starts with a low germanium concentration and a high temperature, then the germanium concentration is increased up to the value required for the virtual substrate to form. When the substrate is thick enough, typically a few microns, it is fully strain-relaxed. The growth of the layer then continues at a constant composition to produce a region of very low dislocation density, relatively defect-free. The strained silicon layer is grown on this substrate at a lower temperature.
"We have developed this SiGe technology over the past 10 years to support MOD research, but it has enormous commercial potential and DERA is now offering access to this technology through its epitaxial foundry," said David Robbins, technical leader for silicon epitaxy in DERA's microsystems department.
