P.H. Ballentine

High temperature superconducting films by RF magnetron sputtering

The authors have produced sputtered films of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O by RF magnetron sputtering from an oxide target consisting of loose reacted powder. The use of a large 8-in stoichiometric target in the magnetron mode permits films located above the central region to be free of negative-ion resputtering effects, and hence yields reproducible, uniform stoichiometric compositions for a wide range of substrate temperatures. Superconducting YBCO films have been obtained either by sputtering at low temperatures followed by an 850 degrees C oxygen anneal, or alternatively by depositing onto substrates heated to approximately=600-650 degrees C and cooling in oxygen. Films prepared by the former method on cubic zirconia substrates consist of randomly oriented crystallites with zero resistance above 83 K. Those deposited on zirconia at medium temperatures without the high-temperature anneal contain smooth partially oriented crystallites, with a slightly depressed T/sub c/ approximately=75 K. Finally, superconducting films have been deposited on MgO using a BiSrCaCu/sub 2/O/sub x/ powder target. >

Microlithography of high-temperature superconducting films: laser ablation vs. wet etching

Narrow lines and microbridge structures have been etched in sputtered superconducting films of Y-Ba-Cu-O by variations of two methods. The first uses standard photolithography followed by wet etching in weak acid. The second uses a maskless process involving focused pulsed YAG (yttrium-aluminium-garnet) laser together with a computer-controlled x-y stage to produce local ablation of the superconducting film. Issues relating to limits of resolution, annealing of films, and degradation of superconducting properties are critically discussed for the two approaches. >

In-situ sputtering of YBCO films for microwave applications

RF magnetron sputtering from a single YBCO target onto a heated substrate ( approximately=700 degrees C) was used to obtain c-axis-oriented 1-2-3 films that are superconducting without a subsequent annealing or oxygenation step, with T/sub c/(R=0) as high as 88 K on MgO and LaAlO/sub 3/ substrates. This process uses an 8-in-diameter target in the sputter-up configuration, with a central grounded shield to eliminate negative ion bombardment. It can reproducibly and uniformly cover substrates as large as 3-in across at rates exceeding 1 AA/s. Maintaining film composition very close to stoichiometry is essential for obtaining films with good superconducting properties and surface morphology. Optimum films have critical currents of approximately=1 MA/cm/sup 2/ at 77 K. Measurements of microwave surface resistance based on a stripline resonator indicate low surface resistance for unpatterned YBCO ground planes, but excess loss and a strong power dependence in a patterned center strip.

Continued improvement of large area, in situ sputter deposition of superconducting YBCO thin films

The deposition of thin films of superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ onto substrates of up to 3-in diameter by an integrated off-axis sputtering is reported. The substrate is located above the center of an 8-in-diameter YBCO planar target, and, in conjunction with a negative ion shield, negative ion effects are avoided. A large radiant heater provides backside, noncontact heating of the bare substrates. YBCO films have been grown on polished 1-cm/sup 2/ MgO and LaAlO/sub 3/ substrates with T/sub c/>or=90 K, J/sub c/>or=2.5*10/sup 6/ A/cm/sup 2/ at 77 K, and microwave surface resistance R/sub S/<0.4 m Omega at 77 K and 10 GHz. The films have a very smooth surface morphology. Uniformity data for LaAlO/sub 3/ substrates are less than +or-5% in R/sub s/. Thickness uniformity results for 3-in substrates indicate less than 10% variation. The growth of epitaxial insulating films for use with YBCO films and application of the YBCO films in microwave devices are briefly discussed.<<ETX>>

Formation of TiO 2 Thin Films by Oxidation of TiN

TiN thin films were deposited by reactive sputtering onto Si substrates. TiO 2 films were formed by subsequent oxidation of the TiN films, using either conventional furnace heating or rapid thermalprocessing (RTP). The materials properties of the resulting films were characterized by x-ray diffraction and Rutherford backscattering, and indicate conversion of the TiN to fully-oxidized rutile TiO 2 by a diffusion-limited process. Electrical measurements of the insulating properties of the TiO 2 films indicated a relative dielectric constant greater than 100, although the leakage current was greater than optimum. A fully in-situ process for the fabrication of microcapacitors is proposed, which involves the sputter deposition of TiN, the formation of TiO 2 by RTP, and the deposition of a top TiN counterelectrode. This can be carried out under conditions that are compatible with Si microelectronic device processing.

Relaxation processes in optically excited high-Tc films

We have observed the transient electrical response of current-biased YBa 2 Cu 3 O 7 films irradiated with 150 ps pulses from a Nd:YAG laser (λ=1.06μm). The magnitude of the response always agrees quantitatively with a simple heating model, as does the speed of the response (

Preparation and properties of in-situ sputtered YBCO films

The authors have used RF magnetron sputtering from a large near-stoichiometric YBCO target onto heated substrates to produce c-axis oriented films with Tc=87 K in situ, without a subsequent post-anneal. Effects of negative ion bombardment were avoided by placing substrates above the center of the target, directly above a grounded ion shield. Film properties are rather sensitive to small changes in target composition. At 66 K, critical currents of optimum films are greater than 1MA/cm2, and microwave surface resistance at 2 GHz is less than 10 m Omega .

Thin Films of Y-Ba-Cu-O by Rf Sputtering

Thin films of Y-Ba-Cu-O have been prepared by rf diode sputtering from a single mixed-oxide powder target. The films were deposited on sapphire and cubic zirconia substrates, with the substrates either cooled, heated, or left to establish thermal equlibrium. The Cu content in the films decreased with increasing substrate temperature. Films having composition close to the desired 1–2–3 stoichiometry were fabricated either by enriching the target with CuO or by cooling the substrate. Using the latter approach, a single target was used for over 20 depositions yielding films that were uniform in composition (although slightly Cu-rich), and consistent from run to run. These films had as-deposited room temperature sheet resistances on the order of 100 kΩ. Subsequent annealing at temperatures up to 850 C resulted in films that were superconducting with an onset around 90 K and zero resistance at 50 K. The critical current was found to exceed 2000 A/cm 2 at 4.2 K.

Commerical‐scale production of high temperature superconducting thin films

The development of a commercial‐scale sputtering process for deposition of superconducitng YBa2Cu3O7 (YBCO) thin films is described.YBCO films are sputter deposited from a single 20 cm diameter powder target onto 2 inch substrates of MgO and LaAlO3 using in‐situ heating to produce high quality, c‐axis oriented films without post annealing. By locating the substrate above the center of the magnetron erosion ring and by placing a ‘‘negative ion shield’’ between the target and the substrate, negative ion resputtering is avoided. Small shifts in composition are corrected by using a slightly Y‐ and Cu‐rich target to produce films within 1% of stoichiometric composition. Films on LaAlO3 and MgO have critical temperatures up to 88 K and critical currents above 5×105 A/cm2 at 77 K. Uniformity is ±5% in thickness and ±7% in resistivity across at 2‐inch diameter substrate. Microwave surface resistance meaurements at 1.2 GHz and 4 K show RS≊0.1 mΩ for YBCO ground planes and ≊0.3 mΩ for patterned YBCO striplines on MgO.