Demand for high density, high bandwidth, and low power Integrated Circuits (ICs) is rapidly increasing even as Moore's Law is starting to plateau. Among the new wave of technologies that are meant to continue the prevalent trend of semiconductor device scaling, 3D System Integration promises many advantages over traditional single-planar designs. This technology enables designers to stack multiple dies and unlock new functionality by reducing the footprint of the final device package. Using 3D Integration, multiple different types of chiplets (Digital IC, Analog IC, MEMS) can be integrated into a single package, potentially bypassing an expensive move to a newer process node and unlocking even more functionality from the same package.

These downscaling issues are not limited to traditional CMOS technologies but extend to Quantum Computers. Due to the highly heterogeneous nature of Quantum ICs, and especially their requirement of low qubit decoherence noise, we require high-density connections from the Qubit layer to the Microelectronics Control layer while maintaining appropriate spacial separation between the two. In this project, the qubit layer is comprised of Diamond Colour Centers in a Photonic IC with other optical components such as SNSPDs, and MEMS switches. On the other hand, the microelectronics control layer is a Cryogenic CMOS chip.

The main goal of this project is to design an Interposer and related technologies like Through Silicon Vias (TSVs) and Microbumps to successfully integrate these two chiplets in a single 3D assembly. We conduct thermal analysis of multiple 3D assembly designs using Finite Element Modelling to derive optimum fabrication specifications. Subsequently, fabrication recipes are developed and optimised for TSV etching, coating, and patterning with superconductive sidewall liners. Recipes are also developed to fabricate Indium Microbumps using techniques like evaporation, liftoff, and atmospheric reflow. Utilizing these microbumps, we conduct cold-compression flip-chip experiments. Finally, we establish cryogenic measurement setups to electrically characterize these fabricated devices.

Diamond color centers have been increasingly gaining attention in research due to their desirable optical properties that make them suitable for applications such as quantum computing, quantum sensing and quantum communication. However, the main challenge has been on-chip integration of diamond color centers with high scalability and high yield. Many diamond integration methods have been researched, such as heterogeneous wafer bonding of diamond-on-insulator photonics. While this method offers high scalability and wafer level fabrication, optical performance is not guaranteed due to the non-ideal processing which results in lower yield. Another integration method is pick and place, which demonstrated high yield as the diamond color centers can be pre-selected before integration, and thus resulting in higher yield. Here, pick and place integration of diamond nano-photonic structures is presented. For that purpose, an adiabatic coupler is designed, with simulation results showing 0.36 dB insertion loss and 3dB misalignment tolerance of 180 nm and 5 um in x and z directions respectively. A mechanical support structure is designed such that optical losses at that interface are minimized. The simulated loss is reduced from 33 % to only 4 % and the cross-talk is -11 dB. Further optical measurement is needed to determine the achieved transmission of the integrated diamond chiplets. Finally, pick and place is performed with 6 different supporting structure designs. The yield is 25 % with minimum misalignment of 50 nm which is mainly dominated by random motion of the diamond chiplet as it gets closer to the receptor chip. This random motion is attributed to the surface roughness of the receptor chip and and lower surface of the diamond chiplet. In addition, residual charges in both chips results in Coulomb forces acting on the diamond chiplet and thus contributing to the misalignment. It is expected that the magnitude of the random motion, and consequently, the misalignment, will be improved by treating the receptor chip surface with HMDS and de-ionizing both the diamond chiplet and the receptor chip.