Al Barrier Layer Research Articles

Minimizing Film Stress and Degradation in Thin-Film Niobium Superconducting Cables

The future of superconducting and cryogenic electronic systems can significantly benefit from densely integrated superconducting multi-layer and multi-signal flexible cables due to the massive number of electrical interconnects needed in systems such as superconducting quantum computers and cryogenic detector arrays. In order to maintain superconductivity in niobium (Nb) thin films, film stress and degradation must be minimized. We are working towards configurations with embedded traces, where it is expected that the superconductor material will be subjected to subsequent fabrication steps that must not degrade the properties of the superconductor. We previously observed degradation of the superconducting properties of Nb, such as reduction of both transition temperature and critical current, as a result of curing a polyimide passivation layer at supplier recommended curing temperature (350 oC). The deterioration in the superconducting properties may be due to mechanical stress in the film or diffusion of impurities into the Nb during the curing process Film stress plays a vital role in the superconducting properties of Nb. Previous research by other groups has focused on in situ ion bombardment, substrate fixturing and wafer preparation in order to minimize film stress. In this work, we discuss the role of argon (Ar) pressure and power during Nb sputtering on the quality of Nb and Nb/Al thin films. By varying the Ar pressure and applied power during sputter deposition, we have produced both tensile and compressive films on flexible substrates in order to find the pressure that yields a near zero stress Nb and Nb/Al thin film at room temperature. A low stress Nb film was tested with a thin Al barrier layer (of the order of 10's of nm) between Nb and polyimide to protect the Nb superconductivity during the PI curing step. Nb traces with a thickness of roughly 250nm and a width of 50um were used for this work. Nb films deposited at different Ar pressures and power levels were tested for critical transition temperature (Tc), critical current (Ic), and sheet resistance (Ω/□), to compare the superconducting behavior of different Nb films. Details of the fabrication processes, experimental procedures and performance results will be presented. This work will help determine materials stacks-ups that may be useful for future multi-layer Nb-based flexible superconducting cables. Acknowledgment: We gratefully acknowledge financial support and technical guidance from Microsoft Research for this work.

Aluminium and tantalum nitride barriers against copper diffusion in solar absorbers

The thermal stability of magnetron sputtered solar absorber coatings was investigated at temperatures of 200–500°C. Diffusion barriers of aluminium (Al) and tantalum nitride (TaNx) were studied against thermal diffusion of copper substrate atoms. The diffusion barriers studied were experimental DC magnetron sputtered Al and TaNx layers between a copper substrate and an absorber coating. The absorbers were aged by means of heat treatment in air. The influence of diffusion barriers was analysed by optical and microstructural analyses before and after the ageing procedures. Without a barrier coating, copper substrate atoms diffused through the absorber to the surface, which degraded optical selectivity. The diffusion can be prevented or retarded with a dense diffusion barrier layer. The 500-nm-thick Al barrier layer prevented Cu diffusion and retained optical selectivity at 300°C. At higher temperatures, diffusion was observed but the Al barrier retarded the degradation when compared to an absorber without the barrier layer.

INVESTIGATION OF Ta/Ni–Al INTEGRATED FILM USED AS A DIFFUSION BARRIER LAYER BETWEEN Cu AND Si

Ta (3.3 nm)/ Ni – Al (3.3 nm) integrated films deposited on Si substrates by magnetron sputtering, annealed at various temperatures in a ultra-high vacuum, have been studied as diffusion barrier layers between Cu and Si for application in Cu interconnection. The images of transmission electron microscopy (TEM) prove that the cross-sectional interfaces of Cu / Ta / Ni – Al / Si sample annealed at 600°C are clear and sharp. No Cu –silicide peaks can be found from the X-ray diffraction (XRD) patterns of the 850°C annealed sample, but the sheet resistance of the sample increases abruptly. Moreover, large grooves are found from the image of atomic force microscopy (AFM) for the 850°C annealed sample, implying the failure of the diffusion barrier. The integrated Ta / Ni – Al barrier layer retains thermally stable nature up to at least 800°C, indicating that the Ta / Ni – Al integrated film is an excellent diffusion barrier between Cu and Si .

Microstructural evolution of the interfacial layer in the Ti–Al/Yb0.6Co4Sb12 thermoelectric joints at high temperature

A thermoelectric (TE) device is basically fabricated by joining p- and n-type thermoelectric materials with electrodes. For skutterudite (SKD) based TE devices, Ti and alloys like Co–Fe–Ni are widely used as the barrier layer joining the SKD and the electrode. In this paper, TE joints composed of Yb0.6Co4Sb12 (Yb-SKD) and a Ti–Al barrier layer with various Al contents (0–15at.%) were fabricated by a one-step SPS process. The influence of the Al content on the evolution of the interfacial microstructure during accelerated aging at 600°C was studied. During sintering and aging, Ti and Al diffused into each other and formed inert Ti–Al alloys, which hindered the interfacial diffusion and resulted in a lower growth rate of the diffusion layers. On the other hand, excess Al at the Ti–Al/Yb-SKD interface encouraged the interfacial diffusion. The interplay of the above two mechanisms decides the inter-diffusion behaviors at the interface. In this work, the two mechanisms reached balanced when the Al content was 6at.%, which resulted in the best interfacial stability and the highest shear strength after aging. At the same time, with the increase of the Al content in the Ti–Al barrier layer, the CTE mismatch between the Yb-SKD and the Ti–Al barrier layer was reduced, consequently the interfacial integrity after aging was improved. As a result, the contact resistivity of the Ti–Al/Yb-SKD joints (Al: 6–12at.%) remained below 12μΩcm2 after 16days of aging.

Low-temperature integration of lead-based ferroelectric capacitors on Si with diffusion barrier layer

Ferroelectric lead zirconate titanate thin films have been integrated on silicon substrates using Ti–Al-based conducting diffusion barriers produced by sputter deposition. The microstructure of the Ti–Al barrier layer was systematically altered through changes in the sputtering conditions, specifically the power density and deposition pressure. We find that the crystallinity of the Ti–Al film strongly correlates with sputtering power density and ambient i.e., it is amorphous at low power density and/or high deposition pressure, and polycrystalline at high power density and/or low deposition pressure. Electron energy loss spectroscopy studies demonstrate that the amorphous Ti–Al (a-Ti–Al) films contain a higher concentration of dissolved oxygen than crystalline Ti–Al. A low temperature sol–gel process has been used to prepare Pb(Zr,Ti)O3 PZT films at 450 °C on conducting Si wafers with a-Ti–Al conducting barrier layer and La–Sr–Co–O top and bottom electrodes. The excellent ferroelectric properties obtained with the a-Ti–Al barrier provide a promising approach for integration of PZT-based capacitors with silicon transistor technology for the fabrication of nonvolatile ferroelectric memories.

The effect of optical prepulse on direct-drive inertial confinement fusion target performance

The effect of optical prepulses on the performance of imploding inertial confinement fusion (ICF) targets has been measured. The neutron yields from nearly identical spherical targets imploded by the OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser were measured as a function of prepulse fluences in the drive laser. These experiments indicate that the cumulative prepulse fluence on target must be less than ∼0.2 J/cm2, which corresponds to a prepulse intensity contrast of ⩾107. This effect is the result of damage to the Al barrier layer used on OMEGA ICF targets. To verify this, the effect of low-intensity irradiation on these Al layers was observed on planar targets using the time-resolved reflectivity and the time-integrated transmission measurements. These experiments show that damage to the Al layers begins at ∼0.1 J/cm2, consistent with the implosion performance results.

Nb/Al-AlOx/Nb edge junctions for distributed mixers

We have fabricated high quality Nb/Al-oxide/Al/Nb edge junctions using a Nb/SiO/sub 2/ bi-layer film as the base electrode, suitable for use as traveling wave mixers. An edge is cut in the bi-layer with an ion gun at a 45 degree angle using a photoresist mask. The wafer is then cleaned in-situ with a physical ion gun clean followed by the deposition of a thin Al (a1) film, which is then thermally oxidized, an optional second Al (a2) layer, and a Nb counter electrode. It was found that devices with an a2 layer resulted in superior electrical characteristics, though proximity effects increased strongly with a2 thickness. The counter electrode is defined with an SF/sub 6/+N/sub 2/ reactive ion etch, using the Al barrier layer as an etch stop. The Al barrier layer is then either removed with an Al wet etch to isolate the individual devices, or the devices are separated with an anodization process. Various ion gun cleaning conditions have been examined; in addition, both wet and plasma etch bi-layer edge surface pre-treatments were investigated. It was found that edge junctions with large widths (i.e., those more suitable for traveling wave mixers) typically benefited more from such treatments. Initial receiver results at 260 GHz have yielded a DSB noise temperature of 60 K.

Microfield emitter array triodes with electron bombarded semiconductor anode

The Schottky diode with a thin Al barrier layer is examined as an electron bombarded semiconductor (EBS) anode for the microfield emitter array (micro-FEA) triode. The electrons from FEA accelerated to 4 keV can be multiplied over 1000 times at an anode. The high frequency performances of the micro-FEA triode with EBS anode are estimated. It is shown that the planer triode with lateral thin film emitter and EBS anode promises over 100 GHz operation.