M. Reiche and S. Christiansen
Uniaxial strain in 4-inch wafers was realized by mechanically bending the wafers over a cylinder in an experimental setup as shown in Fig. 1, bonding in the bent state and subsequently thinning down of that wafer. The wafers are forced to stay bent by screws as visible in the setup and this bent state needs to be kept during an annealing step at elevated temperatures (200°C for 15 hours) which is required for increasing the bonding energy of the bonded interface. The bending direction is parallel to  flat direction of the wafers. After annealing and releasing the screws the 3 inch wafer is keeping the bow remaining bonded to the 4 inch wafer in the bent state as can be seen in Figure 3. The maximum strain that can be achieved depends on the curvature radius of the cylinder, the thicknesses of the two bonded wafers assuming that no glide at the bond interface occurs during the bonding process.
The concept of realising uniaxial strain at wafer level is shown schematically in figure 2. The thinning of one of the wafers is essential to transfer a large fraction of the strain energy that is stored in each of the wafers and exploit the resulting strain in devices. The thinning cannot easily be carried out by polishing of bent wafers, therefore hydrogen induced splitting and layer transfer techniques were used. This process relies on high dose hydrogen implantation induced splitting of a thin slice off a wafer which stays attached to another wafer when bonded.
The strain values obtained from the UV micro-Raman measurements are in good agreement with those obtained from 3D finite elements calculations .
Figure 1: Bonding setup for realization of uniaxial strain over the entire wafer area.
Figure 2: Sketch of the concept for realising uniaxial strain at wafer at wafer level.
Figure 3: Raman spectra for Si bulk and uniaxial strained Si samples corresponding to curvature radius of 1 m, 0.75 m, and 0.5 m.
 Himcinschi, C., Radu, I., Muster, F., Singh, R., Reiche, M., Petzold, M., Gösele, U. and Christiansen, S. H.:
Uniaxially strained silicon by wafer bonding and layer transfer
Proceedings EUROSOI pp 39-40 Grenoble, France (2006)