Subsections

• 2.1.1 Strained Si on SOI

2.1 Substrate Strain

Substrate strain in Si can be induced through the utilization of a virtual SiGe layer. Si and Ge having a lattice mismatch of about $4.2\%$ can be combined together to form a SiGe alloy, the lattice constant of which lies between those of Si and Ge. If a thin layer of Si is grown on a relaxed Si$_{1-y}$Ge$_{y}$ buffer, the Si layer is forced to assume the larger lattice constant of the underlying Si$_{1-y}$Ge$_{y}$ substrate. The Si layer is thus said to be under biaxial tensile strain where the amount of strain is given by the Ge content (denoted as $y$) in the substrate. This can be seen in Fig. 2.2 where the lattice structure of relaxed Si and Ge and strained Si on Si$_{1-y}$Ge$_{y}$ is shown. The strain in the plane of the interface between the strained layer and the substrate is given by the lattice mismatch,

$$\varepsilon _{\parallel} = \frac{ a_\text{SiGe} - a_\text{Si} } {a_\text{Si}}.$$ (2.1)

Here, $a_\textrm{Si}$ and $a_\textrm{SiGe}$ denote the unstrained lattice constants of Si and SiGe, respectively. Such a strain can only be sustained in thin layers, and relaxation in the form of misfit dislocations will occur for thick layers. An important criterion for growing strained Si layers without introducing misfit dislocations is that the strained layer should have a thickness below the critical value [Matthews74]. As a result of biaxial tensile strain, carriers experience a lower resistance in the strained layer and typically have 50-70% higher mobilities [Nayak93,Welser92] in the channel direction.

$\parbox{2.5in}{(a) \hfill \hfill (b)}$

Figure 2.2: Lattice structure of (a) unstrained Si and SiGe. (b) strained Si on relaxed SiGe.

Although most of the initial work on substrate-induced strain was limited by increased defect densities and threading dislocations [LeGoues92,Abstreiter85], sizable efforts were made to tackle these problems through the usage of graded SiGe buffers [Fitzgerald91] and chemical mechanical polishing techniques [Currie98]. In order to further improve device performance, it was a natural choice for the semiconductor industry to synergize the mobility enhancement of strained Si with the low power benefits of SOI technology.

2.1.1 Strained Si on SOI

The first kind of SOI based strained Si devices were fabricated by growing a SiGe layer on top of an oxide layer. A SiGe layer was formed on the oxide-capped Si substrate using different techniques such as separation-by-implanted-oxygen (SIMOX), wafer bonding and layer transfer techniques [Huang01,Sugiyama00], after which a strained Si layer was grown on top of the SiGe layer. The technology thus matured with the name SGOI (strained Si on relaxed SiGe on insulator). Due to the isolation provided by the oxide layer, the junction capacitances are reduced while strained Si results in higher mobilities, thereby increasing the overall performance. Ring oscillator circuits utilizing the SGOI technology delivering 63% drain current enhancement were reported [Mizuno03].

Although the two flavors of strained Si using SiGe, namely Si on Si$_{1-y}$Ge$_{y}$ and SGOI, increase the device performance, there are several issues due to the presence of Ge, which inhibit large scale integration. Firstly, due to the different diffusion rate of dopants (B, As) in SiGe, the source and drain extensions in these devices may cause increased short channel effects or junction resistance problems. The diffusion of Ge into the strained Si layer poses additional problems. Another major problem is the lower thermal conductivity of SiGe which causes considerable self-heating of the devices [Jenkins02].

Therefore, a second kind of SOI-based devices followed wherein the SiGe buffer layer was used only for generating strain in the Si layer but was later eliminated from the final structure. Several techniques were suggested [Rim03,Aberg04,Currie03] to bond a strained Si layer directly onto an oxide-covered Si substrate, thus forming an SSDOI (strained Si directly on insulator) device. This approach circumvents the problems associated with SiGe and opens a new front for strain-based ultra thin body and double gate devices. A schematic of the three types of strained devices utilizing a SiGe substrate, is shown in Fig. 2.3.

Despite all the progress made in strained Si technology utilizing a SiGe substrate, experimental observations suggested that this approach was probably not the best choice for introducing strain into the channel. Biaxial strain was found to give a higher mobility enhancement for electrons than for holes. Since CMOS circuits already have a large area for the pMOS devices due to the poor hole mobility, efforts to further enhance the electron mobilities could even worsen the integration densities. Furthermore, biaxial strain can cause a severe threshold voltage shift [Lim04], which also would significantly bring down the device performance.

$\parbox{3.8in}{\center(a) \hfill \hfill (b) \hfill \hfill (c)}$

Figure 2.3: Schematic of the three different types of strained Si technologies utilizing a SiGe substrate. (a) Strained Si on SiGe, (b) SGOI and (c) SSDOI.