Thin Layer Transfer Using Room Temperature Wafer-Level Bonding Process
Authors: Abadie Karine , Fournel Frank , Morales Christophe , Moriceau Hubert , Wimplinger Markus 
Published: February 2020

Direct wafer bonding has already been demonstrated as an efficient solution to create a linkage between two different surfaces. One of the main limitation is the temperature required to stabilize the bonding interface and the strength. For example, to completely stabilize a Si/Si bonding realized at room pressure, an annealing step done at 1000°C or above is required. In the past, alternative low temperature processes, using plasma activation of the surfaces before contact between them, were developed for SiO2 to Si and SiO2 to SiO2 bondings, lowering the annealing temperature at 300°C and enabling enough bond strength for many applications [1]. Another possible option to realize a room temperature Si/Si bonding is to use a covalent bonding process [2]. The EVG®580 ComBond® was developed with this aim by bringing the wafers in contact in an ultra-high vacuum environment, after activation in vacuum of both surfaces in the ComBond® Activation Module (CAM®) [3,4]. This paper aims to present observations made after the transfer of a thin layer using this novel direct covalent Si/Si bonding process. The bonding process was realized in an EVG®580 ComBond® equipment installed at CEA-Leti.
At first, the equipment and the process window were qualified in terms of metal and particle contaminations. Measurements of particles with a threshold at 90nm were performed, and detection of metals contamination demonstrated that the whole bonding process was metal free with a specification of 1.1010 at/cm² for all metallic elements. The activation process, performed in the CAM®, was also characterized, especially in terms of etching rate, roughness and amorphous thickness measurement done by ellipsometry. Then, an appropriate bonding process, demonstrating a high bond strength (>5 J/m²) was developed for Si/Si 200 mm wafers. Using this process, the bonding of a SOI (Si 100nm - SiO2 200nm) with a Si wafer was performed, in order to transfer the SOI Si layer onto a new substrate and to characterize both the bonding and the transfer qualities.
In this study, bonding quality is assessed using C-mode Scanning Acoustic Microscopy (C-SAM) which is performed just after bonding, as shown on figure 1a, where no defects have been observed. More generally, in order to control the bonding and the layer transfer processes, particle contamination measurements deserve to be performed on the SOI surface before bonding and after layer transfer. It may be done using a SurfScan equipment (SP2) from KLA-Tencor with a detection threshold value of 90 nm for instance. If large particles are deposited at the bonding interface during activation process, they lead to non-transferred areas (i.e. holes) visible after layer transfer. As we can see from figure 1b, no holes or additional defects are detected after such a layer transfer. Thus, perfect quality of transferred layers on 200 mm full wafers is shown. Additional measurements may be performed in order to characterize the bonding interface. Ellipsometry is used to measure the thickness of the amorphous area created during the surface activation step. TEM observations, made before and after recrystallization, allow to confirm the thickness of the amorphous layer, as well as to study the temperature at which the Si interface is fully recrystallized. Finally, SIMS measurements is used to characterize the other species than Si which could be found at the bonding interface like oxygen, carbon, and metallic elements for instance.
The successful transfer of a thin Si layer onto a new substrate shown in this work demonstrates that such a bonding process would deserve to be applied to many other thin layer transfer techniques.
REFERENCE
[1] T. Plach, K. Hingerl, S. Tollabimazraehno, G. Hesser, V. Dragoi and M. Wimplinger, J. Appl. Phys., 113, 094905 (2013)
[2] H. Takagi, K. Kikuchi, R. Maeda, T. R. Chung and T. Suga, "Surface Activated Bonding of Silicon Wafers at Room Temperature", Appl. Phys. Lett. Vol. 68, No.16 (1996), pp. 2222-2224.
[3] C. Flötgen, N. Razek, V. Dragoi, M. Wimplinger, “Novel Surface Preparation Methods for Covalent and Conductive Bonded Interfaces Fabrication”, ECS Trans. 64(5), The Electrochemical Society, pp. 103-110, 2014
[4] C. Flötgen, N. Razek, V. Dragoi, “Functional Interfaces Fabrication Through Room Temperature Wafer-Level Bonding”, Proc. of WaferBond Conference, 2015, pp. 55-56
EVG®580 is a Trade Mark of EVGroup
ComBond® is a Trade Mark of EVGroup
CAM® is a Trade Mark of EVGroup

Figure 1