Progress toward superconductor electronics fabrication process with planarized NbN and NbN/Nb layers

To increase density of superconductor digital and neuromorphic circuits by 10x and reach integration scale of $10^8$ Josephson junctions (JJs) per chip, we developed a new fabrication process on 200-mm wafers, using self-shunted Nb/Al-AlOx/Nb JJs and kinetic inductors. The process has a layer of JJs, a layer of resistors, and 10 fully planarized superconducting layers: 8 Nb layers and 2 layers of high kinetic inductance materials, Mo$_2$N and NbN, with sheet inductance of 8 pH/sq and 3 pH/sq, respectively. NbN films were deposited by two methods: with $T_c$=15.5 K by reactive sputtering of a Nb target in Ar+N$_2$ mixture; with $T_c$ in the range from 9 K to 13 K by plasma-enhanced chemical vapor deposition (PECVD) using Tris(diethylamido)(tert-butylimido)niobium(V) metalorganic precursor. PECVD of NbN was investigated to obtain conformal deposition and filling narrow trenches and vias with high depth-to-width ratios, which was not possible to achieve using sputtering and other physical vapor deposition (PVD) methods at temperatures below $200 ^oC$ required to prevent degradation of Nb/Al-AlOx/Nb junctions. Nb layers with 200 nm thickness are used in the process layer stack as ground planes to maintain a high level of interlayer shielding and low intralayer mutual coupling, for passive transmission lines with wave impedances matching impedances of JJs, typically <=50 $\Omega$, and for low-value inductors. NbN and NbN/Nb bilayer are used for cell inductors. Using NbN/Nb bilayers and individual pattering of both layers to form inductors allowed us to minimize parasitic kinetic inductance associated with interlayer vias and connections to JJs as well as to increase critical currents of the vias. Fabrication details and results of electrical characterization of NbN films, wires, and vias, and comparison with Nb properties are given.Comment: 12 pages, 16 figures, 4 tables, 49 references. Submitted to IEEE TAS on Nov. 10, 202

Characterization of superconducting through-silicon vias as capacitive elements in quantum circuits

The large physical size of superconducting qubits and their associated on-chip control structures presents a practical challenge towards building a large-scale quantum computer. In particular, transmons require a high-quality-factor shunting capacitance that is typically achieved by using a large coplanar capacitor. Other components, such as superconducting microwave resonators used for qubit state readout, are typically constructed from coplanar waveguides which are millimeters in length. Here we use compact superconducting through-silicon vias to realize lumped element capacitors in both qubits and readout resonators to significantly reduce the on-chip footprint of both of these circuit elements. We measure two types of devices to show that TSVs are of sufficient quality to be used as capacitive circuit elements and provide a significant reductions in size over existing approaches

Volatile metal borohydride complexes: synthesis and characterization of new chemical vapor deposition precursors

The complex sodium methylborohydride, Na(H3BCH3), can be prepared in high yield (83%) by the addition of trimethylboroxine, (H3C)3B3O3, to sodium aluminum hydride, NaAlH4. The subsequent reaction of two equivalents of sodium methylborohydride with the alkaline earth bromides; MgBr2, CaBr2, SrBr2, and BaBr2 in 1,2-dimethoxyethane, DME, affords the new alkaline earth methylborohydride DME adducts: [Mg(H3BCH3)2(DME)]2, Ca(H3BCH3)2(DME)2, Sr(H3BCH3)2(DME)3, and Ba(H3BCH3)2(DME)3. [Mg(H3BCH3)2(DME)]2 sublimes between 80 and 90 °C at 10 mTorr while the larger alkaline earth methylborohydrides do not sublime up to 120 °C. [Mg(H3BCH3)2(DME)]2 is an asymmetrically bridged dimer in the solid state where each Mg center has a terminal κ2H-methylborohydride, a bridging κ2H-methylborohydride, a bridging κ1H-methylborohydride, and a chelating DME. The other alkaline earth methyborohydrides have two κ3H-methylborohydrides and two chelating DME for the Ca complex and three chelating DME for the Sr and Ba complexes. Rare earth methylborohydride THF adducts are prepared by the reaction of a rare earth chloride (Sc, Y, Nd, Gd, Er) with 3 to 4 equivalents of sodium methylborohydride in THF. The scandium and yttrium complexes are isolated by sublimation at 50 °C while the neodymium, gadolinium, and erbium complexes are isolated by sublimation at 60 °C. In the solid state, scandium methylborohydride has three κ3H-methylborohydrides and one coordinated THF. The yttrium, gadolinium, and erbium complexes crystallize as charge separated ion pairs: [RE(H3BCH3)2(THF)4] [RE(H3BCH3)4], where the cation has two κ3H-methylborohydrides and four coordinated THF and the anion consists of four κ3H-methylborohydrides. The neodymium complex is a methylborohydride bridged dimer, [Nd(H3BCH3)3(THF)2]2, where each Nd center has two κ3H-methylborohydrides, two bridging κ2H,κ2H-methylborohydrides and two THF. In addition to the THF adducts, the neodymium DME adduct, Nd(H3BCH3)3(DME)1.5, has also been synthesized by a similar method. This complex can be sublimed under vacuum at 115 °C. The Er complex has been used in preliminary CVD experiments which demonstrate the ability to grow thin films between 250 and 350 °C using this new precursor. The synthesis of sodium aminodiboranates with sterically bulky or electron withdrawing substituents on nitrogen has been achieved by the treating amine-borane with either BH3•THF or by thermolysis at elevated temperatures followed by the addition of BH3•THF, which produced µ-aminodiborane. The µ-aminodiborane can then be ring opened with NaH, similar to what has been reported by Keller for the synthesis of sodium N,N-dimethylaminodiboranate. Implementing this method, the sterically bulky aminodiboranates: sodium N-isopropyl-N-methylaminodiboranate, sodium N,N-diisopropylaminodiboranate, sodium cis-2,6-dimethylpiperidinyldiboranate, sodium tert-butylaminodiboranate, and sodium N-isopropylaminodiboranate have been prepared. The aminodiboranates with electron withdrawing substituents on nitrogen: sodium N-benzylaminodiboranate, sodium N-benzyl-N-methylaminodiboranate, and sodium 2,2-difluoroethylaminodiboranate were also able to be prepared by the addition of BH3•THF to the appropriate amine-borane followed by treatment with sodium hydride. Unfortunately, these aminodiboranates decompose at room temperature. Magnesium cis-2,6-dimethylpiperidinyldiboranate was able to be synthesized by treatment of MgBr2 with two equivalents of sodium cis-2,6-dimethylpiperidinyldiboranate in diethyl ether followed by sublimation at 50 °C under vacuum. The hydrolysis/thermolysis product µ-(cis-2,6-dimethylpiperidinyl)diborane is, however, present in the sublimate due to similar volatility to the desired magnesium product. Synthesis of magnesium N,N-diisopropylaminodiboranate was attempted by ball milling MgBr2 and sodium N,N-diisopropylaminodiboranate followed by sublimation at 65 °C. Interestingly, primarily decomposition products, N,N-dimethylimine and magnesium borohydride, Mg(BH4)4, were observed by 11B NMR in the reaction mixture. Static chemical vapor deposition (CVD) has been successfully used to deposit conformal thin films of hafnium diboride, HfB2, and iron metal from hafnium borohydride, Hf(BH4)4, and iron pentacarbonyl, Fe(CO)5, respectively. Microtrenches with aspect ratios greater than 10:1 were able to be completely infilled with HfB2 or iron and macrotrenches were able to be coated with thin films of HfB2 which has a 40% step coverage at an aspect ratio of 1000:1. HfB2 thin films deposited by static CVD have a Hf:B ratio similar to films deposited using Hf(BH4)4 in an actively pumped, low pressure CVD system; although the relative hydrogen content of the film deposited by static CVD was greater. Iron thin films deposited by static CVD have an iron composition as high as 97% with approximately 1.5% carbon and oxygen each

Nanosoldering Carbon Nanotube Junctions by Local Chemical Vapor Deposition for Improved Device Performance

The performance of carbon nanotube network (CNN) devices is usually limited by the high resistance of individual nanotube junctions (NJs). We present a novel method to reduce this resistance through a nanoscale chemical vapor deposition (CVD) process. By passing current through the devices in the presence of a gaseous CVD precursor, localized nanoscale Joule heating induced at the NJs stimulates the selective and self-limiting deposition of metallic nanosolder. The effectiveness of this nanosoldering process depends on the work function of the deposited metal (here Pd or HfB₂), and it can improve the on/off current ratio of a CNN device by nearly an order of magnitude. This nanosoldering technique could also be applied to other device types where nanoscale resistance components limit overall device performance