Wataru Hattori

Superconducting digital electronics

Superconducting devices have intrinsically superior characteristics to those of semiconductor devices. Presently, we can fabricate more than twenty thousand junctions on one chip using niobium technology. We have demonstrated the operation of a network system with a superconducting interconnection chip using voltage-state logic. Single flux quantum devices are promising for future superconducting applications because the clock frequency of SFQ logic is higher than that of voltage-state. We have proposed a high-end switch based on hybrid architecture using optical devices, semiconductors and SFQ devices. To demonstrate the high-speed operation of SFQ circuits, we developed an arbiter circuit that uses SFQ components and the arbiter circuit operates at 60 GHz. We also have developed a high Tc superconducting (HTS) SFQ sampler system for observing ultra-fast signal waveforms. In addition, we will discuss the prospects of future superconducting devices based on the fabrication technologies we developed.

An HTS 21-pole microstrip filter for IMT-2000 base stations with steep attenuation

We developed a 21-pole high-temperature superconductor thin-film microstrip filter with steep attenuation. This filter is applicable in the International Mobile Telecommunications-2000 (IMT-2000) base station receiver system, where high selectivity is needed to avoid interference from signals in the personal handyphone system (PHS) band just below the IMT-2000 band. The out-of-band rejection of this filter 1 MHz below the passband edge is more than 40 dB. Owing to the improvements of the design and the fabrication processes, we also eliminated the need for a tuning process in the fabrication processes in order to reduce the production costs and the turnaround time.

Effects of copper deficiency on the structure and microwave properties of YBa2Cu3O7−δ films deposited by laser ablation

The work reported here evaluated the correlation between the copper composition and the structural and microwave properties of YBa2Cu3O7−δ (YBCO) films deposited on MgO substrates. Detailed x-ray diffraction measurements revealed that the superconducting YBCO phase in the films has a solubility range on the copper-poor composition side. The structure of the YBCO phase depends on copper composition: a lower copper composition is associated with a larger c-axis lattice constant and a decreased degree of orthorhombicity. These structural changes correlated well with increases in normal resistivity, decreases of transition temperature Tc, and increases in microwave surface resistance Rs. The films with less copper composition also show a more gradual decrease in Rs below Tc than do copper-rich films. These results suggest that changes in these properties are closely connected with copper deficiency that can occur more easily when the copper composition is low. The copper composition is, therefore, a crucial f...

Copper composition dependence of the structure and microwave properties in Y-Ba-Cu-O films

The correlation between the copper composition and the microwave properties of YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) films is studied using coplanar waveguide resonators at 5 GHz. The c-axis lattice constant increases and the axial ratio b/a-the degree of orthorhombicity-decreases as the copper composition of the YBCO films decreases. The microwave properties of these films correlate well with these structural changes. The films with copper-rich composition have a low surface resistance R/sub s/ of 60 /spl mu//spl Omega/ at 50 K and a short magnetic penetration depth /spl lambda//sub L/(0) of 200 nm. The values of these quantities increase with decreasing copper composition. The films with less copper also show a gradual decrease in R/sub s/ below T/sub c,zero/, which is different from the tendency expected from the two-fluid model. These unusual microwave properties can be explained by the inhomogeneous superconductor model in which the regions with locally lowered T/sub c/ are distributed in the YBCO phase. The copper deficiencies that occurred in the films with less copper composition cause these inhomogeneities and thus they are an important cause of high R/sub s/ values of these YBCO films.

5-μm-wide YBa2Cu3O7−δ coplanar line with low transmission loss

Narrow and low-loss YBa2Cu3O7−δ (YBCO) transmission lines for use in multichip modules have been attempted to be developed. 18-cm-long YBCO coplanar transmission lines with widths down to 5 μm were prepared by a composite patterning process combining Ar-ion milling and wet-etching. The transmission losses of the packaged 5-, 10-, and 25-μm-wide lines, respectively, were −1.57, −0.85, and −0.55 dB at 20 GHz and 55 K. These values provide similar surface resistances of 0.59–0.80 mΩ at 20 GHz and 55 K. This indicates successful fabrication of a 5-μm-wide YBCO coplanar line without notable extrinsic loss increase resulting from process damage. The attenuation constants of these lines are approximately two orders of magnitude lower than for 10-μm-wide Cu microstrip lines. These results show that the YBCO coplanar lines with widths down to 5 μm have great potential for use in multi-chip modules.

Properties of a YBCO/insulator/YBCO trilayer and its application to a multilayer Josephson junction

We have examined the properties of a trilayer high-temperature superconductor (HTS)/insulator/HTS structure and incorporated this structure with multilayer Josephson ramp edge junctions to form a SQUID with a ground plane. The lower and upper (YBCO) films in the trilayer structure showed zero-resistance temperatures of 89 K and 85 K respectively. A directly coupled ground-planed SQUID using ramp edge Josephson junctions with a normal barrier layer (PBCO) showed a voltage modulation of at 9 K and operated up to 50 K.

Correlation between Cu deficiencies and changes in the structural and electrical properties of Y-Ba-Cu-O films

The experimental results obtained in our previous work included those of the copper composition dependences of structural, dc electrical and microwave properties. From these results, we proposed the inhomogeneous superconductor model that explains the correlation between the dc electrical and the microwave properties. Our recent measurements of the Hall coefficients suggested an additional model to explain the correlation between the structural and the electrical changes. Namely, copper deficiencies are compensated by the oxygen loss due to charge invariance. This oxygen reduction transforms locally the lattice from an orthorhombic phase into tetragonal-like phase. The resultant lattice disorder causes the low mobility and high resistivity.

A reentrant delay-line memory using a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// coplanar delay-line

The rapid growth in telecommunication traffic demands a higher-speed asynchronous transfer mode (ATM) switching system. At present, the upper limit of the system clock rate is determined by the maximum clock rate of conventional semiconductor memory devices, such as the register files used in ATM cell buffer storage. This is because the maximum clock rate of these register files is restricted by the propagation delay time between each register stage. Since a reentrant superconducting delay-line memory avoids this restriction using an analogue delay given by the superconducting delay line, we have proposed that this memory should be used in high-speed ATM cell buffer storage. Recently, we fabricated a 10-/spl mu/m-wide 37-cm-long YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// coplanar delay-line. This line had a delay of approximately 2.8 ns. Using this coplanar delay-line and an assembly of commercially available integrated circuits, we successfully developed superconducting delay-line memory. This memory operates as a 32-bit buffer storage at a clock rate of 10 GHz, which is several times faster than the register files. This result shows that the superconducting delay-line memory is a powerful candidate for high-speed ATM cell buffer storage.

A 5-/spl mu/m-wide 18-cm-long low-loss YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// coplanar line for future multichip module technology

Narrow and low-loss YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coplanar lines, which can be used in multichip module technology for future high-density and high-speed digital circuits, have been developed. Etch-back planarization and a patterning process combining Ar-ion milling and wet-etching enabled us to form an 18-cm-long 5-/spl mu/m-wide YBCO coplanar line without electrical shorts, even for the narrow spacing of 2.5 /spl mu/m. The surface resistance of this line was kept at a level comparable to that of 10- or 25-/spl mu/m-wide YBCO coplanar lines and also comparable to that of unpatterned films. This indicates successful fabrication of the 5-/spl mu/m-wide YBCO coplanar line without notable loss increase resulting from process damage. The 5-/spl mu/m-wide line showed a low-transmission loss of 0.49 dB at 10 GHz and 55 K. This level of loss is similar to that in Cu coaxial cables. No significant increase in transmission loss was observed up to an input power level of 16 mW at 10 GHz and 55 K. This input power is comparable to the power-handling capability required for transmitting high-speed digital signals through the lines with characteristic impedance of 50 /spl Omega/. These results show that the narrow 5-/spl mu/m-wide YBCO coplanar line has great potential for high-density and high-speed digital circuits.

Tag-antenna-based presence sensing using near-field traveling-wave reader antenna

We propose tag-antenna-based sensing (TABS) for detecting objects and people using a low-cost near-field traveling-wave reader antenna. This technology is inspired by TABS, which enables the use of ordinary far-field radio-frequency identification (RFID) tags as extremely low-cost proximity presence sensor nodes. This allows the sensor nodes to be pervasively deployed in the Internet of Things (IoT); however, frequent reading errors have been a problem in applying practical usage for detecting objects and people. We introduce a low-cost near-field traveling-wave RFID reader antenna that significantly reduces reading errors. We successfully demonstrated the detection of items and human footprints by adopting this technology.