Deposition, Characterization Of Helium-containing Ti Film, And Its Annealing Behaviour

Research on helium behavior in metals is related to many important fields, and hasdrawn growing interest. When carrying out the experimental investigations, helium must beintroduced into the solid artificially. Traditional ion implantation induces the critical latticedamage, tritium decay and neutron irradiations are not suitable in the laboratories due to along half-time or requiring special safety. This thesis brings a new method of heliumintroduction into metallic films using direct current (DC) magnetron sputtering. Focusing onthe deposition and characterization of helium-containing films and thermal coarsening ofhelium bubbles, the main results can be cataloged as follows:(1) Helium can be incorporated into the growing Ti film by DC magnetron sputteringin mixture of the working gases helium and argon with the advantages of controlledcontents, uniform distribution and lower damage. Argon plays the role of sputtering ionsbombarding the target, just maintaining the stable discharge and deposition. Consequentannealing showed no signature of argon incorporation. The contribution of heliumincorporation process is recognized as two parts: one is the helium atoms bombarding thetarget, backscattered by the target and implanting the substrate; the other is helium ions nearthe plasma sheath implanting in the anode. Varying the bias voltage applied on the anodedid not change helium content notably, showing that the neutral particles entrapmentmechanism is dominating.The helium content depends on those factors: helium partial pressure in the gasmixture, the temperature on the substrate, the bias voltage on the anode and the species ofthe target. The portion of helium ion concentration in the discharge can be adjusted easilyby varying the helium partial pressure in the mixture to obtain the content required in therange of 0-49at.% (helium-to-titanium). The projectile energy of helium particles to the filmis confined about 100eV, near the sub-threshold-energy for displacement damage. So thelattice damage is rather lower, unlike the case of keV ion implantation.(2) The helium-containing Ti films were characterize systemically by ion beamanalysis (IBA), X-ray diffraction (XRD),scanning electron microscopy(SEM), transmissionelectron microscopy (TEM), slow energy positron beam analysis (PBA) and thermaldesorption spectroscopy (TDS).Helium atoms prefer occupying the vacancies and forming helium-vacancy (He-V)complexes, resulting in an expansion of the Ti lattice and the broadening of the diffractionpeaks. Further supply of helium will induce the refinement of the grains and enhance thedisorder. For certain helium content uniform blisters with the diameter of 30-40nm areobserved on the film surface and no outburst occur. Large helium content will reduce thesize of these blisters and increase their density with some simultaneous outburst.Depending on the helium content, helium present in the sputtered film as the interstitialclusters, He-V complexes, bubbles with various dimensions, etc. Helium bubbles growslowly by absorbing helium atoms and punching loops. When the helium content reachescertain value, coalescence between the small bubbles occurs, giving birth to the big bubbles. So for the case of low helium contents, high density of small intergranular bubbles andintragranular He-V complexes coexist and release at higher temperature. For the case ofmedium contents, the density of the bubbles increase with appearance of some big bubblesby the coalescence, giving the different release peaks in the middle temperature in TDS. Forthe high contents, both the mean radius and the density of various bubbles increase, smallbubble may arrange in line or aggregate into clusters, and the burst of the bubbles near thesurface zone are enabled in the lower temperature in TDS.(3) For the first time thermal desorption spectroscopy of helium in combined withDoppler broadening of PBA revealed the states of helium bubbles after one hourisochronous annealing under different temperatures. IBA determined the retained contentafter the annealing. Helium released largely when annealing above 973K, no obviousrelease happened while annealing below 973 due to the strong trapping of various bubbles.Analyzing the results of PBA and TDS, we can insight the mechanism of heliumbubble coarsening. At the low temperatures (＜680K) bubble grows gradually by punchingdislocation loop mechanism, absorbing the moving helium atoms, relaxing the internal gaspressures, controlled by helium diffusion. The bubbles state changes very little from TDS.While annealing at the moderate temperatures (680-970K), the bubble coarsens bymigration and coalescence (MC), giving different bubbles in size but no big release. TheMC mechanism is controlled by Ti atom diffusion. When annealing at high temperatures(＞970K), thermal vacancies are activated and the Ostwald ripening (OR) enlarges thediameter of bubbles and reduce the bubble density. The small bubbles dissolve into the largeones, but the larger burst out and release helium, leaving a few small bubbles eventually.The OR mechanism is limited by helium dissociation and vacancy enhanced diffusion.(4) The thermal desorption following the monoenergetic or multiple energy helium ionimplantations indicated high density of the induced vacancies trapped helium atoms tightly.No free diffusing state of helium atoms in interstitial sites was observed. The TDS ofsputtered film showed the evidence that some content of helium lie in the interstitial sites.Comparing to the high density of cascade damage by implantation, the lattice damage in thesputtered helium-containing film is rather lower. This may qualify the fundamentalinvestigations of helium behavior in the absence of the influence of the damage.The state of helium atom in the film by sputtering method is more similar with that intritides, so it can be used to simulate the accumulation of helium due to tritium decay intritides, and avoid the long term for waiting decaying.The feather of helium release from the sputtered film is related to the lattice state andhelium state, which is different from the case if ion implantation. In combination with otherrelevant techniques, subsequent thermal release to helium incorporation may give valuableadvice on the repaid evaluation of ability of helium trapping and the resist to embrittlementfor a new candidate material.