Highly Strained Silicon on Polymer Obtained By Temporary Polymer Wafer Bonding
Résumé
Strain engineering can be called upon to tune the band structures of semiconductors. It is widely used in different types of microelectronics devices and it is expected to enable unprecedented responses in optoelectronics [1]. In the case of single crystal Silicon (sc-Si), several approaches have been proposed, relying either on the lattice mismatch inherent to hetero-epitaxial growth (Si on relaxed SiGe) or on substrate expansion (porous Si) [2, 3]. However, these methods are limited to bi-axial strain and by the maximum strain achievable in sc-Si thin film. We recently evaluated a method enabling to transfer a 200nm thick sc-Si film on a stretchable polymer substrate. Tensile tests proved that applying an external mechanical load on the substrate injected significant amounts of strain into sc-Si [4]. This work is focused on studying different parameters to improve the maximum strain in a Silicon On Polymer (SOP) stack obtained from a 200mm Silicon On Insulator (SOI) wafer using temporary polymer bonding.
The overall fabrication process is described in figure 1(a). 2 × 24mm2 rectangles are defined using photolithography in a SOI thin film. The wafer is temporary bonded to a handling silicon substrate with a 40µm thick glue layer. Then, the backside of the SOI is removed with mechanical grinding and selective chemical etching. Finally, the sc-Si thin film is transferred to a 230µm thick flexible substrate thanks to a release layer between the glue and the sc-Si layer. The substrate is cut into tensile test samples as described in figure 1(a).
In a previous study we have shown that a 200nm thick sc-Si film could be uniaxialy strained up to 1.5% along the [110] direction [4]. In the present work, we quantify the impact of strain orientation, sc-Si film thickness and pattern edge roughness on maximum uniaxial strain. In order to change the sc-Si film thickness a dry etching step was used after removing the backside of the SOI ubstrate. A soft lithography mask and a standard chrome 1X lithography mask were compared for the fabrication of rectangles. To achieve even smoother pattern edges, TetraMethylAmmonium Hydroxide (TMAH) etching was also used (for 90 minutes in a 25 %wt solution at 80◦C) on a patterned SOI with a silicon oxide layer as a hard mask. Resulting samples were tested on a uniaxial tensile stage coupled to a Raman spectrometer, giving us access simultaneously to the sample macroscopic strain and the silicon local strain.
Tensile test results are summarized in figure 1(b). Reducing the thickness of the sc-Si did not improve the strain limit while better edges definitions did. Indeed, using TMAH, uniaxial strains up to 3% along [100] were reached in a 200nm thick sc-Si film on a flexible polymer substrate. These results are highly encouraging for the development of films with a wide range of strain. Furthermore, this method is easily transferable to other materials such as AlN or Ge.