D. D. Bacon

Properties of NbN thin films deposited on ambient temperature substrates

The preparation of cubic NbN films by reactive dc magnetron sputtering is described. These superconductive films are deposited at a sufficiently low temperature (<90 °C) that photoresist liftoff techniques and can be used in fabricating Josephson junctions. The superconducting transition temperature has been measured as a function of gas composition and pressure. It reaches a maximum of 14.2 K at 15% N2–85% Ar and 1.06 Pa total pressure. The resistivity ratio of these films is close to unity. Structural studies by transmission electron microscopy and electron diffraction show that films 100‐nm thick or less are randomly oriented, and noncolumnar with a crystallite diameter of 5 nm and a lattice parameter of 4.46 A, which is significantly higher than the bulk value for cubic NbN. The films are dense with void diameters no larger than 0.7 nm. Films 300‐400 nm thick show a small degree of texturing in x‐ray studies with a Read camera. Auger analysis shows a monotonic increase in the N/Nb ratio with increase ...

Study of Ni as a barrier metal in AuSn soldering application for laser chip/submount assembly

The possibility of replacing Pt in the Ti/Pt/Au base and traditionally used metallurgical structure by Ni, while bonding InP laser chip to a submount with AuSn (80% Au) solder, has been investigated. Various Ni‐based metal alloys have been prepared by evaporation. Reflow experiments were conducted in a chamber under forming gas‐controlled ambient. The Ti/Ni/AuSn system provided much longer surface local freezing duration compared to the Ti/Pt/AuSn system. Scanning electron microscopy analysis revealed a smoother surface morphology for the Ti/Ni/AuSn system after the metal refroze. Auger electron spectroscopy depth profiles indicated the formation of a Ni‐Sn‐Au interacted layer. The interaction took place in two steps: the first stage was the dissolution of Ni into the Au‐Sn liquid followed by precipitation of a Ni‐Sn‐Au intermetallic compound; the second stage was a solid‐state interdiffusion of Sn, Au, and Ni which occured in the interacted layer and in the original Ni layer. The latter step was a diffus...

dc magnetron- and diode-sputtered polycrystalline Fe and amorphous Tb(FeCo) films: Morphology and magnetic properties

We have prepared polycrystalline Fe and amorphous Tb(FeCo) films using both dc diode‐ and magnetron‐sputtering techniques. Magnetic properties and aging characteristics of these films were measured by a vibrating sample magnetometer and an automatic torque magnetometer. Film morphologies were studied by transmission electron microscopy. The magnetic and aging characteristics are closely correlated to the film morphology. It was also found that different sputtering methods can be tailored to produce a similar film morphology. Among the sputtering parameters, the Ar pressure during deposition at room or lower temperature strongly influences the film morphology.

MnAlGe Films for Magneto‐Optic Applications

MnAlGe is a uniaxial ferromagnetic compound with a saturation 4πMs of 3600 G, and Curie temperature of 245°C. We have studied polycrystalline thin films of this compound formed on various substrates by getter sputtering. The optimum substrate temperature to obtain films with uniform magnetic properties was 500°C. Films prepared at this temperature have a substantial component of magnetization normal to the plane of the substrate, and a coercive force of approximately 2000 Oe. Information was written into films approximately 700‐A thick by Curie point writing or with a fine permanent magnet wire, and read with good optical contrast by means of the polar Faraday effect. The relatively low Curie temperature is advantageous in reducing the power required for Curie point writing for magneto‐optic memory or holographic applications.

Superconducting Tl‐Ba‐Ca‐Cu‐O films by sputtering

Superconducting Tl‐Ba‐Ca‐Cu‐O thin films 0.2–1.0 μm thick have been prepared on MgO(100) and SrTiO3(100) substrates by dc diode sputtering using a single oxide target. Films containing primarily the Tl2Ba2Ca1Cu2O8 phase were obtained with a Tc (R=0) at 102 K and a transport Jc of 104 A/cm2 at 90 K. For the nearly pure phase Tl2Ba2Ca2Cu3O10 films, the Tc’s (R=0) are higher at 116 K and the transport Jc’s at 100 K are of 105 A/cm2. Both types of films show a strong preferred orientation with the c axis perpendicular to the film plane. The rocking curve of the Tl2Ba2Ca2Cu3O10 films is 0.32° wide and the typical grain size is over 10 μm.

Superconductive tunneling into NbN deposited near room temperature

We have developed the ability to deposit niobium nitride thin films routinely having a superconducting transition temperature of 14 K by dc reactive magnetron sputtering onto substrates held near room temperature (Ts <90 °C). This allows the use of conventional photoresist liftoff techniques for patterning. Pb‐counterelectrode tunnel junctions formed on these films show excellent V‐I characteristics, suitable for device applications and offering potential advantages over conventional Pb‐alloy/Pb‐alloy and Nb/Pb‐alloy junctions. These high quality junctions are also suitable for tunneling investigations of superconductivity in the Nb‐N system.

High‐Coercive‐Force Rare‐Earth Alloy Films by Getter Sputtering

Thin films of the rare‐earth alloys of the Co–Sm system with and without the addition of Cu have been found to have exceptionally high coercive forces. A coercive force of 30 000 Oe was realized with Co3.65Cu1.35Sm films, 4000‐A thick prepared at a substrate temperature of 500°C using getter sputtering. This technique avoids troublesome oxidation problems usually encountered in preparing films of active materials such as the rare‐earth compounds.

Aging effects on amorphous Tb–transition‐metal films prepared by diode and magnetron sputtering

We have studied and compared the aging characteristics of the amorphous Tb(FeCo) films from both dc getter diode and magnetron sputtering. No protecting layer such as SiO2 or Al2O3 was put on the films. Fresh films of 1.0‐μm thickness from diode sputtering showed a perpendicular anisotropy and well‐behaved magnetic properties using a vibrating sample magnetometer and an automatic torque magnetometer. We noticed a decrease in perpendicular anisotropy when the films were exposed to the air. However, this aging phenomenon was not observed in films of the same thickness prepared by magnetron sputtering even after seven months’ exposure to the air. The reasons for the difference are due to two distinct reaction mechanisms: bulk reaction in diode‐sputtered films and surface reaction in magnetron‐sputtered films. A variation of the microstructure is believed to attribute to this. The surface reaction was studied in thinner magnetron‐sputtered films of 10 and 25 nm, where the surface‐to‐bulk ratio is larger. A de...

Bonding of InP laser diodes by Au-Sn solder and tungsten-based barrier metallization schemes

Ti/W/Au-Sn and Ti/WxMy/Au-Sn schemes were studied as alternative metallization schemes to the traditionally used Ti/Pt/Au-Sn system for the bonding of InP laser diodes to heatsinks, and in particular to CVD diamond parts. The study comprised the Ti/W, Ti/W1(Au-Sn) and Ti/W1(Ni-Sn) barrier metal schemes, co-deposited in between the Au-Sn solder and the submount. In particular, reactivity and thermodynamic stability of the systems, acid the integrity of the barrier metal to the AuSn solder interface through the thermal bonding refreezing and reflow cycles were tracked. Premature freezing of the solder through the bonding cycles was attributed to the intermixing of the underlying barrier metal and the solder, suggesting an insufficient thermodynamic stability. Dewetting of the solder from the barrier metals through the reflowing cycle, subsequent to the completion of the bonding cycle, occurs due to the excellent inert nature of the solder to the barrier system, but exhibited the deficiency of poor solder to barrier metal adhesion. The TIM, system performed as an absolute inert barrier under the Au-Sn solder, in which no premature freezing phenomena were observed through the bonding cycle, resulting, however, in a delamination of the solder from the Ti/W, while reflown both under flux and forming-gas. In order to maintain the stable nature of this system, but to improve the barrier-solder interfacial integrity, W-Au, W-AuSn and W-NiSn co-deposited intermediate adhesion layers were introduced in between the W layer and the Au-Sn solder. As a result, the adhesion of the solder to the barrier metal improved, while the most stable performance was observed while applying the W/W(NiSn) barrier system under the Au-Sn solder. The first local freezing phenomenon of the bonding solder, while using this system, was observed only after heating the sample to 320 degrees C for more than 3 min and more than 1 h was needed to completely freeze the entire solder. in addition, an excellent solder to metal interfacial integrity was observed through the gas and flux reflow cycle. Thus, the W/W(NiSn) barrier metallization is recommended as a superior scheme to replace the traditionally used Ti/Pi system for bonding laser diodes to any type of submount using Au-Sn solder.

Superconductivity in some new metastable film phases

The occurrence of new metallurgical phases and enhanced superconductivity in a number of sputtered transition‐metal alloy films has been studied as a function of substrate temperature during deposition. Results indicate that such new phases are readily produced in the temperature range 200–1000°C by this technique and that these new phases are accompanied by marked changes in the superconducting transition temperature. The findings show how easily the more complex transition‐metal alloys, particularly σ phase, can be modified at low temperatures, and demonstrate the importance of structural instability, vis‐a‐vis composition, in determining the superconducting transition temperature.