Three-dimensional packaging of superconducting quantum bits and superconducting circuits is one of the most important technologies for large scale superconducting circuits. Therefore, we proposed multi-chip 3D packaging technology to realize scalable superconducting devices. Such a multi-chip technology is used to implement to complex large circuits. In this report, we will discuss superconducting 3D packaging technology focusing on two points: flip chip and Through Silicon Via (TSV).
First, a superconducting connection using a flip chip is described. Superconducting solder bumping test chips were designed for daisy-chain connection circuit. The test devices consist of a top chip and a base chip. Nb/Ti/Au contact pads for placing were fabricated on Si substrates. To obtain electrical properties of a large number of interconnects, we design and fabricate over 10000 circular lead solder bumps from 10 to 50 um diameter on the top chip and Nb/Ti/Au-opposing-contact pads on the base chip to form a daisy chain of about 10000 chip-to-chip interconnects. We chose lead as the solder material because it has a relatively high critical temperature of 7.2 K and we can check superconducting connection using liquid He and/or conventional refrigerators.
Next, superconducting TSVs are explained. There is a method for accessing a two-dimensional array of qubits with microwaves from the vertical direction, which requires the use of TSVs to realize a waveguide-like structure. In this case, instead of embedding TSVs, the sides are homogeneously covered with a superconducting thin film. There are several ways to do this, such as long-throw sputtering and tilting the substrate, but we have used atomic layer deposition (ALD). Specifically, we evaluated the transport properties of TiN films deposited by thermal and plasma ALD, and optimized the deposition conditions. The principle is that raw material gas is injected from the upper shower head onto the TSV substrate, and the metal decomposed by the heat and plasma is transferred to the substrate and the gas is exhausted to the exhaust side. Homogeneity and crystallinity of the ALD-TiN film on the TSV substrate were evaluated.
Cross-sectional SEM and STEM images of the TSV substrate showed that TiN films with a thickness of approximately 100 nm were homogeneously deposited in the vertically formed vias to the back side of the substrate. The electron diffraction image of TiN on the TSV wall shows a clear diffraction ring, and the measured lattice spacing is consistent with that of TiN. These diffraction rings were observed at various parts of the TSV, suggesting that a homogeneous TiN film with crystallinity was deposited inside the TSV. The superconducting transition temperature of this film was measured to be 3 K. It was also found that the composition of Ti and N differs between the upper side of the TSV, which is exposed to the plasma, and the lower side, which is not. The correlation between the composition distribution and superconducting properties should be investigated to make the composition uniform.