Patent Number: 052672891
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will be described in the overall context of the implantation of ZrN or TiN onto the outer surface of a zirconium alloy tubular component of a nuclear fuel assembly. The description is based on the apparatus, method and results reported for implantation of aluminium ions into silicon, by C. Chan et al in the article identified above. The apparatus 10 for the implantation of ZrN or TiN into a tubular fuel assembly component is shown in FIG. 1 and comprises a pulsed cathodic arc metal-ion source 12, an electromagnetic duct 14 for separation of the ions from the macroparticles, and an implantation chamber 16. One suitable cathodic arc metal-ion source 12 is shown in FIG. 2. It comprises a cylindrical cathode 18 surrounded by an annular trigger ring 20 with an alumina (A1.sub.2 0.sub.3) insulator 22 between the cathode and the trigger. The cathode is made of Zr or Ti while the trigger ring 20 and the anode cylinder 24 are made of copper. The anode is located 5 cm from the cathode 18 and has a small hole 26 through which a portion of the plasma plume 28 streams out. The ring 20 is suspended around the cathode 18, from a Teflon seat 30 at the other end of the anode 24. The arc between the cathode 18 and anode 24 is driven by a capacitive discharge circuit consisting of a 0.12 ohm resistor 32 and a 2500 .mu.F capacitor 34 charged to approximately 2 kv. A spark gap circuit including gap 38, 250 kohm resistor 40, and 0.1 .mu.f capacitor 42 are connected to the trigger ring 20. Breakdown across the spark gap 38 upon full energization of capacitor 34 results in a high-voltage pulse being applied to the trigger ring 20, initiating a surface-charge breakdown across the insulator 22 to the cathode 18. This creates one or more cathode spots. Ionization of cathode material closes the main anode-cathode loop, discharging the pulse line and creating a plasma plume 28. The main arc current pulse was approximately 100 .mu.s in length with an average current of 1000 A. Approximately 5% of the arc current resulted in ions, creating 5.times.10.sup.-3 C of ion charge. Of this, approximately 10% drifted through the anode hole 26 and entered the duct 14. The duct transmits approximately 10% of the incoming ions into the implant chamber resulting in approximately 5.times.10.sup.-5 C being available for implantation. The electromagnetic duct 14 comprises a copper conduit with a 45.degree. bend, around which 150 turns of water-cooled copper tubing 46 have been wound. A current source 48 of 100 A through the tubing produces a longitudinal magnetic field within the duct with a strength of 300 G at the bend and 100 G at the entrance and exit ports 50, 52 of the duct. This magnetic field guides the ions from the source 12 around the 45.degree. bend and into the implant chamber 16 while neutral atoms and macroparticles travel in a line-of-sight into the wall 54 of the tube. A 45 V bias is applied to the duct 14 to reduce ion losses to the walls 54, resulting in a plasma flux at the exit 52 of the duct that is very highly ionized and free of macroparticles. The implant chamber 16 is a 30-cm-diam by 30-cm-long vessel pumped by a turbomolecular pump 56 to an ultimate pressure of 1.times.10.sup.-7 Torr. During the implantation process, the chamber pressure rose to 1.times.10.sup.-5 Torr at which the ion mean-free path was several meters. The chamber end walls 58, 60 (or separate fixturing means, not shown) support the fuel assembly tubular component 62 on the chamber axis, so as to be pulled and rotated under the duct exit 52 in the center of the implant chamber. The fixture or substrate tube 62 is biasable through 2 megaohm resistor 66 to -25 kv at 64. A 10 nF capacitor 68 was used to limit the voltage drop on the substrate 62 during the implant pulse. In the illustrated embodiment, the implantation chamber 16 is backfilled with nitrogen from source 53 after evacuation of ambient air, so that the zirconium ions emerging from the duct at 52, react with the nitrogen to form ZrN, which in turn enters the substrate 62. In a similar manner, other metallic elements or compounds can be implanted according to the invention, including nitrides (e.g., CrN, TiN, HfN, and TaN), carbides (e.g., TiC, CrC, and ZrC), and chromium (Cr). It would be within the skill of the ordinary practitioner to optimize the parameters for generating a metallic plasma plume and, where appropriate, a reactive gas in the chamber 16, to produce the desired implant species. In the example of aluminium implanted into silicon as reported by Chan et al, the total ion charge injected into the implant chamber was determined by scanning a Langmuir probe, biased at -90 V to repel electrons across the exit of the electromagnetic duct. The total charge of the pulse was found by a radial integration to be 5.1.times.10.sup.-8 C/pulse. The ion dose per pulse was calculated to be 1.9.times.10.sup.11 ions/pulse. During the plasma pulse, due to the discharging of the bias capacitor, the voltage of the substrate drops from approximately 2.5 to 5 kV. Electrical activation of the sample by furnace annealing in dry nitrogen for 50 min at a temperature of 1020.degree. C. resulted in the formation of a P-N junction at a depth of 1.8 .mu.m, much deeper than the maximum implant depth of 0.16 .mu.m. The sheet resistance of the diffused layer, measured by using a four-point probe technique, and the junction of the depth, measured by chemical staining of a beveled sample, was used to determine the surface concentration of the ion implanted species. By assuming a limited-source diffusion process resulting in a Gaussian distribution, the total implanted dose in the sample was determined by simple integration of the profile. The measured implanted dose increases linearly until 300,000 plasma pulses had been implanted, after which the measured dose saturated at a value of 1.times.10.sup.14 /cm.sup.2. For implant doses above 1.times.10.sup.14 /cm.sup.2, the concentration of aluminium in silicon exceeded the solid solubility limit of 2.times.10.sup.19 /cm.sup.3, the dopant is precipitated out of the crystal during the anneal cycle, and does not contribute to the measured dose utilizing this technique. Energy dispersive spectroscopy (EDS) was used to ascertain the concentration of aluminium above the precipitation limit of the anneal cycle. The relative aluminium concentrations of the samples was found by comparing the relative peak heights of the x-ray signals from aluminium to silicon between samples. Absolute calibration of the EDS signal was achieved by using a sample with an implanted dose of 0.4.times.10.sup.14 /cm.sup.2, measured with the semiconductor technique as a reference. A 5 kv probe ion beam was used so that the sampling depth of the beam was larger than the 0.16 um implant depth resulting in signal from the total implanted profile.