Wafer-Scale MgB2 Superconducting Devices

Abstract

Progress in superconducting device and detector technologies over the past decade has realized practical applications in quantum computers, detectors for far-infrared telescopes, and optical communications. Superconducting thin-film materials, however, have remained largely unchanged, with aluminum still being the material of choice for superconducting qubits and niobium compounds for high-frequency/high kinetic inductance devices. Magnesium diboride (MgB₂), known for its highest transition temperature (T_c = 39 K) among metallic superconductors, is a viable material for elevated temperature and higher frequency superconducting devices moving toward THz frequencies. However, difficulty in synthesizing wafer-scale thin films has prevented implementation of MgB₂ devices into the application base of superconducting electronics. Here, we report ultrasmooth (<0.5 nm root-mean-square roughness) and uniform MgB₂ thin (<100 nm) films over 100 mm in diameter and present prototype devices fabricated with these films demonstrating key superconducting properties including an internal quality factor over 10⁴ at 4.5 K and high tunable kinetic inductance in the order of tens of pH/sq in a 40 nm thick film. This advancement will enable development of elevated temperature, high-frequency superconducting quantum circuits, and devices.

Keywords: MgB2; high frequency; high-Tc; kinetic inductance; superconducting devices; thin films; wafer-scale.

Figure 1

Schematic illustration of superconducting MgB₂ thin-film fabrication process flow. (a) Left: magnesium and boron are cosputtered onto a rotating substrate to a desired thickness (e.g., 50 nm). A small substrate bias (e.g., 15 W) is applied to achieve a smooth surface. Center: a thin boron capping layer is deposited on top of the cosputtered film. Right: the wafer sample is annealed at around 600 °C in a 100% nitrogen environment for 2–10 min in a rapid thermal processor. (b) The final product is a superconducting thin MgB₂ film with a boron cap on a substrate. (c) MgB₂ thin film on a 100 mm diameter single-crystal silicon substrate with a thin (30 nm) silicon nitride buffer layer. (d) MgB₂ thin film on a 100 mm diameter single-crystal sapphire substrate.

Figure 2

DC superconducting properties of MgB₂ thin films. (a) Resistivity versus temperature plot of a MgB₂ thin film showing superconducting transition with T_c,0 = 32 K and (b) shown across a wider temperature range from 4.2 to 300 K. (c) Critical current density (J_c) of two superconducting MgB₂ thin films at different temperatures (J_c > 10⁷ MA·cm^–2 at 4.2 K) showing reproducibility of the films. (d) Resistivity versus temperature plot of MgB₂ film with T_c,0 = 37.2 K, highest ever reported for sputtered MgB₂ film, and (e) shown across a wider temperature range from 4.2 to 300 K.

Figure 3

Morphological characterizations of postannealed MgB₂ thin films. (a) Atomic force microscopy of the MgB₂ thin film with T_c,0 of 32 K (Figure 2a) and the surface roughness of 0.476 nm rms. (b) High-angle annular dark-field (HAADF) STEM image of 40 nm thick superconducting MgB₂ thin film with a 30 nm boron cap layer on a high-resistivity silicon wafer with a 30 nm silicon nitride buffer layer, showing sharp interfaces. (c) Deviation from the median magnesium to boron ratio in 40 nm thick boron-rich MgB₂ film samples on the silicon nitride buffer layer (red) and sapphire (blue) analyzed by depth-profile X-ray photoelectron spectroscopy. The magnesium to boron ratio stays within 10% of the median for samples on silicon nitride. Significant migration of magnesium from the MgB₂ layer to sapphire results in huge deviation of −30% from the median ratio near the interface. The range of deviation from the median Mg:B ratio for the sample on silicon nitride is shaded in gray.

Figure 4

Eddy current maps of (a) 50 nm thick as-deposited Mg–B composite film with 85 W RF power on magnesium target, (b) 40 nm thick postannealed MgB₂ film after annealing at 600 °C for 10 min, (c) 100 nm thick as-deposited Mg–B composite film with 95 W RF power on magnesium target, and (d) 80 nm thick postannealed MgB₂ film after annealing at 585–590 °C for 2 min followed by 600–615 °C for 4 min. Sheet resistances from eddy current measurements directly correspond to magnesium (as-deposited) and MgB₂ (postannealed) distributions. The sheet resistance wafer uniformities (% of wafer within 1−σ of the average sheet resistance) are between 96.49% and 97.73%. (e) T_c,0 distribution across the 100 mm diameter postannealed MgB₂ film measured at 15 different points. The T_c,0 wafer uniformity is 94.50%, calculated by 1 – (T_c,0 max – T_c,0 min)/(2 × T_c,0 average) as the number of sample points is much smaller than 30.

Figure 5

Resistivity versus temperature plots of MgB₂ films of different thicknesses. The unetched 80 ± 4 nm thick film (red) has the highest superconducting transition temperature, T_c,0 = 33.3 K, followed by the film etched back to 38.5 ± 2.1 nm in thickness (orange) with T_c,0 = 30.1 K, followed by the film etched back to 22.4 ± 1.1 nm in thickness (green) with T_c,0 = 25.9 K, followed by the film etched back to 11.4 ± 0.6 nm in thickness (blue) with T_c,0 = 25.7 K, and followed by the film etched back to 6.9 ± 0.4 nm in thickness (navy) with T_c,0 = 21.4 K.

Figure 6

Postannealed resistivity of smooth (roughness <0.5 nm rms) MgB₂ films (y axis) controlled by initial magnesium content in the as-deposited Mg–B composite films (resistivity along the x axis). Reducing the magnesium content (i.e., decreasing the relative RF power on the magnesium target) increases as-deposited resistivity as well as postannealed resistivity by forming boron-rich MgB₂ films.

Figure 7

Schematic illustration of a superconducting MgB₂ coplanar waveguide device fabrication process flow.

Figure 8

Wafer-scale MgB₂ superconducting device demonstration. (a) Coplanar waveguide microwave resonators and other test chips patterned in MgB₂ film deposited on a 100 mm sapphire substrate. (b) Optical image and (c, d) high-magnification SEM images (false color) for a resonator chip on the Si substrate. MgB₂ film is denoted by the red area.

Figure 9

MgB₂ RF device measurements. (a) S21 transmission of a MgB₂ resonator at different temperatures. Inset shows the five resonances corresponding to a perfect yield on this chip. (b) Alpha (α)—fraction of total inductance that originates from the superconducting kinetic inductance—for two different chips (plotted in triangles and squares) at different CPW conductor widths. The resonant frequency for each measurement is represented by the color fill. (c) Fractional frequency shift as a function of the temperature for two different resonator chips. Higher temperature data were not possible due to the high coupling factor to the resonators. In the future, we plan to design some low coupling resonators for better measurements up to the critical temperature. The fit is a crude two-gap model used by Yang et al. The gap values were highly constrained in the model to show the participation of the larger gap (about 12%) even for the polycrystalline films. (d) Internal quality factor (Q_i) as a function of temperature. We performed interface cleaning processes to achieve Q_i ≥ 10⁴ and expect it to improve further as we mature the fabrication process. Inset shows the reduction in Q_i seen for different cpw geometry (plotted as the center conductor width for Z₀ = 50 ohms).