Manufacturing of Optimized Ge Substrates Using a Covalent Bonding Process
Authors: Abadie Karine , Fournel Frank , Morales Christophe , Vignoud Lionel , Colona Jean-Philippe , Widiez Julie 
Published: December 2020

Germanium is an interesting material for applications ranging from nanoelectronics to solar cells, near infrared detectors, gamma spectroscopy detectors or optical components [1]. Previous studies demonstrated the interest of thick germanium-on-insulator (GeOI) substrates for the manufacturing of advanced photonics devices [2].
Two methods are commonly used to manufacture GeOI substrates for photonic applications. First, a rather thick, slightly tensily strained Germanium layer can be grown on top of a 200mm Si(001) wafer and the top part transferred, thanks to the Smart CutTM approach, onto oxidized Si. The threading dislocation density in that Ge layer will be of the order of a few 106 cm-2, then, which could be a drawback for future applications. To manufacture GeOI with, on top, a higher quality Ge film, another approach is to start from bulk Ge, which has a much better crystalline quality [3, 4]. The drawback of this approach is that the thin Ge film on top of the GeOI substrate does not have any internal strain, then. In order to fabricate high quality strained GeOI substrates, in the end, one could start from Ge films coming from bulk Ge donors and strain while bonding them on top of 200mm Si(001) substrates.
Covalent bonding is an interesting candidate in order to obtain high quality Ge films on Si while managing, at the same time, the internal stress. Indeed, stress can be tailored with the proper temperature control during the bonding process itself. The EVG®Combond® equipment used in this study enables to bond at “high” temperature Si/Ge heterostructures with a very high bond strength. In this paper, the bonding process performed will be described and characterized.
A bonding at temperatures higher than Room Temperature is adopted to generate tensily strained Ge films. The choice of the right temperature depends on two criteria, however: (i) have the highest stress possible in the future Ge film and (ii) allow the Si/Ge heterostructure with thick substrates to withstand the stress without breaking at room temperature just after bonding. Point (i) requires high temperatures, while low temperatures are better for point (ii); a tradeoff will thus have to be identified. Simulations were performed in order to evaluate the bow of the structure after bonding and will be presented in the paper.
Si/Ge heterostructures were fabricated at 100°C. A very high bond strength (> 5 j/m2) was measured using the classical double cantilever beam (DCB) technique on an equivalent Si/Ge/Si structure. Indeed, the bow with a Si/Ge heterostructure was too important to allow a correct bond strength measurement. Thus, an epitaxy of a few µm of Ge was performed on top of a Si wafer and the Si/Ge stack bonded to another Si wafer with the same covalent bonding process prior to measurements of the bonding interface strength. Bow in temperature of the Si/Ge heterostructure was also measured. A heating ramp and then a cool down phase were applied to the bonded pair in an atmospheric pressure chamber with a curvature radius measurement done with a laser beam. The bonded pair reached an equilibrium point with a bow of 0 µm for a temperature close to 100°C, in line with the bonding temperature adopted (figure 1).
Using this newly qualified bonding process, a strained Ge film was transferred onto an Si wafer. The process flow will be described in the paper, as well as the characterization of the thin Ge film transferred at the end of the process. The quality of the Ge film will be characterized using a SP2 equipment in haze mode. Transmission Electron Microscopy will be performed to observe the amorphous zones at the bonding interface, and Focused-Ion-Beam Scanning Electron Microscopy will yield the thickness of the Ge film transferred.

Figure 1: Bow measurement in temperature of a Si/Ge heterostructure, bonded using a covalent bonding process at 100°C

[1] R. King et al., ‘Pathways to 40% efficient concentrator photovoltaics’. 20th European Photovoltaic Solar Energy Conf., Barcelona, Spain, 2005, p. 118
[2] V. Reboud et al., “Structural and optical properties of 200 mm germanium-on- insulator (GeOI) substrates for silicon photonics applications,” Proc. SPIE 936714, 1–6 (2015).
[3] F. Letertre et al., ‘Germanium-on-insulator (GEOI) structure realised by the Smart CutTM technology’, Mater. Res. Soc. Proc., 2004, 809, (B4.4), p. 153
[4] C. Deguet et al., ‘200 mm germanium-on-insulator (GEOI) structures realized from epitaxial wafers using the smart cut technology’, Electrochem. Soc. Proc., 2005, 2005-06, p. 78

Figure 1