A technique based on accelerating positive nitrogen ions onto an aluminum layer has been demonstrated to be effective in forming thin (<2 nm thick) layers of aluminum nitride (AlNx) for use as tunnel barriers in Nb/Al-AlNx/Nb superconductor/ insulator/ superconductor (SIS) Josephson junctions. AlNx is the present material of choice for tunnel barriers because, to a degree greater than that of any other suitable material, it offers the required combination of low leakage current at high current density and greater thermal stability.
While ultra-thin AlN films with good thickness and stoichiometry control are easily formed using techniques such as reactive molecular beam epitaxy and chemical vapor deposition, growth temperatures of 900 ºC are necessary for the dissociative adsorption of nitrogen from either nitrogen (N2) or ammonia (NH3). These growth temperatures are prohibitively high for the formation of tunnel barriers on Nb films because interfacial reactions at temperatures as low as 200 to 300 ºC degrade device properties. Heretofore, deposition by reactive sputtering and nitridation of thin Al layers with DC and RF nitrogen plasmas have been successfully used to form AlN barriers in SIS junctions. However, precise control over critical current density Jc has proven to be a challenge, as is attaining adequate process reproducibility from system to system.
The present ion-beam technique is an alternative to the plasma or reactive sputtering techniques as it provides a highly controlled arrival of reactive species, independent of the electrical conditions of the substrate or vacuum chamber. Independent and accurate control of parameters such as ion energy, flux, species, and direction promises more precise control of film characteristics such as stoichiometry and thickness than is the case with typical plasma processes. In particular, the background pressure during ion-beam nitride growth is 2 or 3 orders of magnitude lower, minimizing the formation of compounds with contaminants, which is critical in devices the performance of which is dictated by interfacial characteristics. In addition, the flux of incoming species can be measured in situ using ion probes so that the dose can be controlled accurately.
The apparatus used in the present ionbeam technique includes a vacuum chamber containing a commercial collimated- ion-beam source, a supply of nitrogen and argon, and an ion probe for measuring the ion dose. Either argon or nitrogen can be used as the feed gases for the ion source, depending on whether cleaning of the substrate or growth of the nitride, respectively, is desired. Once the Nb base electrode and Al proximity layer have been deposited, the N2 gas line to the ion beam is vented and purged, and the ion-source is turned on until a stable discharge is obtained. The substrate is moved over the ionbeam source to expose the Al surface layer to the ion beam (see figure) for a specified duration for the formation of the nitride tunnel barrier. Next, the Nb counter-electrode layer is deposited on the nitride surface layer. The Nb/Al- AlNx/Nb-trilayer-covered substrate is then patterned into individual devices by use of conventional integrated-circuit processing techniques.
A wide parameter space was investigated over which devices were fabricated reproducibly and with high quality. The hysteretic nature of the current-voltage characteristic along with the high subgap ratio indicate the incident nitrogen ions chemically reacted with the Al layer as expected, to form a continuous AlNx barrier. Chemical analysis of the barrier performed using x-ray photoelectronspectroscopy confirmed the presence of AlNx. Critical current density Jc ranged from 550 to 9,400 A/cm2 with subgap-tonormal resistance ratios ranging from 50 to 12.6. The Jc was found to decrease with increasing dose and increasing beam energy. The run-to-run reproducibility was determined to be very good. The spatial variation of the ion current density was also measured and correlated with Jc over a 76-mm Si wafer. The junctions were also found to be stable on annealing up to temperatures of 250 ºC. This technique could be applied to form other metal nitrides at room temperatures for device applications where a high degree of control is desired.
This work was done by Anupama Kaul, Alan Kleinsasser, Bruce Bumble, Henry LeDuc, and Karen Lee of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Semiconductors & ICs category.