通过晶圆键合的硅光子学

Photonic integrated circuits offer the potential of realizing low-cost,  compact optical functions. Silicon-on-insulator (SOI) is a promising  material platform for this photonic integration, as one can rely on the  massive electronics processing infrastructure to process the optical  components. However, the integration of a Si laser is hampered by its  indirect bandgap. Here, we present the integration of a direct bandgap  III-V epitaxial layer on top of the SOI waveguide layer by means of a  die-to-wafer bonding process in order to realize near-infrared laser  emission on and coupled to SOI.

Two techniques for semiconductor wafer bonding have been  investigated. In the first approach, adhesive semiconductor wafer  bonding18 using the thermosetting polymer divinylsiloxanebenzocyclobutene (DVS-BCB) as a bonding agent has been developed,  while in the second approach, molecular wafer bonding has been  investigated20. Both approaches are schematically outlined in Fig. 3.

In adhesive wafer bonding, an oligomer solution of DVS-BCB is  spin coated onto the processed SOI waveguide circuit. This process  planarizes the waveguide topography. After spin coating, a baking step at 150°C removes residual solvent in the spin-coated film, while a short  baking step at 250°C transforms the liquid DVS-BCB into a sol/gel  rubber by partial polymerization. The preparation of the InP/InGaAsP  epitaxial layer surface is optimized to achieve a high bonding strength.  A HF surface treatment improves the bonding strength by chemical  modification of the III-V surface. After attachment of the III-V dies, the  wafer stack is cured at 250°C for 1 hour to polymerize the DVS-BCB  completely. A uniform pressure is applied to the wafer stack during  curing to achieve a close contact between both surfaces. DVS-BCB  was chosen as the bonding agent because of its optical transparency,  excellent planarization properties, low curing temperature, the fact  that no outgassing occurs during cure, and its high glass transition  temperature (>350°C), which determines the available post-bonding  thermal budget for the fabrication of the laser diodes.

In the molecular wafer bonding approach, the processed SOI  wafer surface is coated with an SiO2 cladding layer and planarization  is achieved by chemical mechanical polishing (CMP). Subsequently,  the III-V epitaxial layer structure is coated with SiO2 and, if  needed, a touch-polish CMP step can be used to reduce the surface  microroughness, necessary to achieve molecular bonding. This is  because of the short range of the van der Waals attraction forces on  which the molecular bonding relies. For the approach to be successful,  the surfaces of both wafers must be in contact on the atomic scale.  Typically, a root mean squared (RMS) microroughness of 0.5 nm is  required to achieve molecular adhesion. After surface planarization  and reduction of the microroughness by CMP, both wafer surfaces are  chemically activated. Hydrophilic SiO2 surfaces can be achieved by  both wet chemical treatment and plasma activation. Van der Waals  interactions between both surfaces is achieved when the two activated  wafer surfaces are attached to each other because of the presence of  a H2O interface layer, which is readily adsorbed at the wafer surface.  During annealing of the bonded stack, H2O molecules are removed  from the bonding interface, leaving a covalent bond between the  SiO2 surfaces. In other work, the processed SOI waveguide is not  planarized and the III-V die is directly bonded onto the SOI waveguide  topography, leading to air gaps in the bonded stack.

Fig3

After bonding of the III-V dies to the SOI waveguide wafer, the  InP growth substrate is removed using a combination of mechanical  grinding and wet chemical etching using 3HCl:H2O until an InGaAs  etch stop layer is reached. After substrate removal, the bonded  epitaxial layer stack is ready for device processing. An example of an  InP/InGaAsP epitaxial film transferred to an SOI waveguide circuit using  DVS-BCB is shown in Fig. 4.

While this approach requires a dedicated structure to couple light  from the III-V layer to the SOI waveguide circuit, because of the  presence of the DVS-BCB bonding layer, the use of hybrid waveguide  structures, in which the laser waveguide mode is partially located  in the Si waveguide core while it experiences gain from the III-V  layer, has been demonstrated as well in other work. This approach,  however, relies on a relatively thick Si waveguide core layer (690 nm).  Scaling this approach to Si waveguide core layers λ/2n thick as used  in this work, which are required for high density integration, is not  straightforward.