Local crystal structure and mechanical properties of sputtered Ti-doped AlN thin films.

In this article, we predominantly report the investigation of the local crystal structure around a Ti dopant by X-ray absorption spectroscopy (XAS) and the nano-mechanical properties of co-sputtered Al1-xTixN (x = 0 to 4%) thin films. Grazing incidence X-ray diffraction (GIXRD) results show that these films are crystallized with the hexagonal wurtzite structure of AlN. Surface chemical analysis and morphology analysis of Al1-xTixN films are executed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques, respectively. X-ray absorption near-edge structure (XANES) shows that a Ti atom replaces Al in the AlN crystal and forms localized distorted tetrahedral TiN species, leading to a tensile strain. The bond lengths (Ti-N)ax and (Ti-N)bs are found to be moderately decreased with increasing Ti concentration, which is extracted from the extended X-ray absorption fine structure (EXAFS) analysis. However, the Ti-Al bond length in the second coordination sphere having Al vacancies is unaffected by Ti concentration. The hardness (H) and modulus (E) of Al1-xTixN films are measured by the nano-indentation technique, and increase from 17.5 to 27.6 GPa and 231 to 293 GPa, respectively with x = 0 to 4 at% because of the improvement of p-d hybridization between the Ti and N atoms.

Local crystal structure in the vicinity of Cr in doped AlN thin films studied by X-ray absorption spectroscopy.

This article reports the detailed X-ray absorption spectroscopy (XAS) study of Al1-xCrxN (x = 4, 6, 11%) thin films synthesized by the reactive magnetron co-sputtering technique. All these films were crystallized with a hexagonal wurtzite structure with preferential orientation along the a-axis without the formation of any secondary phases. Surface chemical analysis to evaluate the Cr concentration was carried out using X-ray photoelectron spectroscopy. The study confirmed the presence of AlN and Cr in bonding with N. The local crystal structure around the Cr dopant in the as-synthesized and annealed thin films has been analyzed by both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. From XAS, it was found that Cr replaced the Al atom in the AlN lattice and led to a localized CrN species with distorted tetrahedral AlN in the absence of Cr clustering. The bond lengths of (Cr-N)ax, (Cr-N)bs and Cr-Al, extracted from the EXAFS fitting, were found to decrease with the Cr concentration for both the as-synthesized and annealed thin films due to the enhancement of p-d hybridization between the dopant and the host atoms. However, in the annealed 11% Cr film, the bond lengths are larger than the other and tend to match the Cr-N geometry in CrN.

Tunable transport property of oxygen ion in metal oxide thin film: Impact of electrolyte orientation on conductivity.

Quest for efficient ion conducting electrolyte thin film operating at intermediate temperature (~600 °C) holds promise for the real-world utilization of solid oxide fuel cells. Here, we report the correlation between mixed as well as preferentially oriented samarium doped cerium oxide electrolyte films fabricated by varying the substrate temperatures (100, 300 and 500 °C) over anode/ quartz by electron beam physical vapor deposition. Pole figure analysis of films deposited at 300 °C demonstrated a preferential (111) orientation in out-off plane direction, while a mixed orientation was observed at 100 and 500 °C. As per extended structural zone model, the growth mechanism of film differs with surface mobility of adatom. Preferential orientation resulted in higher ionic conductivity than the films with mixed orientation, demonstrating the role of growth on electrochemical properties. The superior ionic conductivity upon preferential orientation arises from the effective reduction of anisotropic nature and grain boundary density in highly oriented thin films in out-of-plane direction, which facilitates the hopping of oxygen ion at a lower activation energy. This unique feature of growing an oriented electrolyte over the anode material opens a new approach to solving the grain boundary limitation and makes it as a promising solution for efficient power generation.

Nanomechanical and optical properties of yttrium thin films by magnetron sputtering.

This Letter reports on nanomechanical and optical properties of yttrium thin films deposited on an Si (100) wafer. Elemental depth profiling by a secondary ion mass spectrometer revealed absence of formation of yttrium hydride, both on the surface and beneath. The optical properties were investigated by spectroscopic ellipsometry, and the refractive indices extracted after suitable modeling were found to be 2.51 at 546 nm. Hardness and elastic modulus of these films were found to be 7 and 142 GPa, respectively. These studies indicate that yttrium thin films are suitable for x-ray mirrors, photocathode emitters in e-beam lithography, electron microscopes, and free-electron lasers.

Poster - Thur Eve - 57: Craniospinal irradiation with jagged-junction IMRT approach without beam edge matching for field junctions.

PURPOSE: Craniospinal irradiation were traditionally treated the central nervous system using two or three adjacent field sets. A intensity-modulated radiotherapy (IMRT) plan (Jagged-Junction IMRT) which overcomes problems associated with field junctions and beam edge matching, improves planning and treatment setup efficiencies with homogenous target dose distribution was developed. METHODS AND MATERIALS: Jagged-Junction IMRT was retrospectively planned on three patients with prescription of 36 Gy in 20 fractions and compared to conventional treatment plans. Planning target volume (PTV) included the whole brain and spinal canal to the S3 vertebral level. The plan employed three field sets, each with a unique isocentre. One field set with seven fields treated the cranium. Two field sets treated the spine, each set using three fields. Fields from adjacent sets were overlapped and the optimization process smoothly integrated the dose inside the overlapped junction. RESULTS: For the Jagged-Junction IMRT plans vs conventional technique, average homogeneity index equaled 0.08±0.01 vs 0.12±0.02, and conformity number equaled 0.79±0.01 vs 0.47±0.12. The 95% isodose surface covered (99.5±0.3)% of the PTV vs (98.1±2.0)%. Both Jagged-Junction IMRT plans and the conventional plans had good sparing of the organs at risk. CONCLUSIONS: Jagged-Junction IMRT planning provided good dose homogeneity and conformity to the target while maintaining a low dose to the organs at risk. Jagged-Junction IMRT optimization smoothly distributed dose in the junction between field sets. Since there was no beam matching, this treatment technique is less likely to produce hot or cold spots at the junction in contrast to conventional techniques.

Poster - Thur Eve - 47: Evaluation of the ArcCHECK device for commissioning and patient-specific QA.

The most promising method of accurately verifying VMAT treatments is by direct dose measurement over the three dimensions of irradiated volume. ArcCHECK device (Sun Nuclear, Melbourne, FL) have the potential to detect delivery errors on the treatment machine due to mechanical problems resulting from gantry and MLC motion. The estimation of the dosimetric leaf gap (DLG) parameter for Varian MLC (Varian Medical Systems, Palo Alto, CA) was attempted using ArcCHECK. Finding the optimal DLG value for use in TPS requires a measuring device like ArcCHECK to be employed especially in highly intensity modulated fields. In addition, ArcCHECK was used to assess the effect of positional error of MLC leaf in a given VMAT plan. Patient-specific QA tests were performed using the ArcCHECK device. QA results of patient plans that failed considering portal dosimetry technique were reassessed with ArcCHECK measurements for IMRT plans. The preliminary test results and performance of the ArcCHECK device were very encouraging. VMAT plans for head and neck cases were generated and their delivery was evaluated using ArcCHECK. Results have shown a success rate greater than 90% in the quality assurance of individual plans. Optimal DLG value was detected using ArcCHECK. Also, the device showed enough sensitivity to identify failed QA plans. Moreover, MLC central leaf pair position offset in a VMAT plan of the order of 1mm was fairly distinguished by ArcCHECK measurements.

Synthesis and characterization of nanocrystalline Ti(1-x)Al(x)N by reactive magnetron sputtering.

Ti(1-x)Al(x)N metastable films were synthesized by reactive magnetron co-sputtering with different Ti to Al ratios. XRD studies showed that as-deposited films were crystalline for concentrations of Al (35, 40, 55 and 64%) and become amorphous at 81% Al. These films were annealed at 1073 K to study the phase separation. Films up to 55% Al did not show any phase separation after annealing. But films with 64% Al splits into c-TiAIN and c-AIN, whereas films with 81% Al split into cubic-TiN and hex-AIN. Distribution of crystallites and their size were analyzed by TEM. High density of crystallites with dimensions between 3-11 nm was dispersed in amorphous matrix for 64% Al films. Nanoindentation technique was used to determine the mechanical properties of these films without substrate effect. Maximum hardness obtained for as deposited and annealed films (64% Al) were 35 GPa and 38 GPa, respectively.

Implantation induced hardening of nanocrystalline titanium thin films.

Formation of nanocrystalline TiN at low temperatures was demonstrated by combining Pulsed Laser Deposition (PLD) and ion implantation techniques. The Ti films of nominal thickness approximatly 250 nm were deposited at a substrate temperature of 200 degrees C by ablating a high pure titanium target in UHV conditions using a nanosecond pulsed Nd:YAG laser operating at 1064 nm. These films were implanted with 100 keV N+ ions with fluence ranging from 1.0 x 10(16) ions/cm2 to 1.0 x 10(17) ions/cm2 The structural, compositional and morphological evolutions were tracked using Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectrometry (SIMS) and Atomic Force Microscopy (AFM), respectively. TEM analysis revealed that the as-deposited titanium film is an fcc phase. With increasing ion fluence, its structure becomes amorphous phase before precipitation of nanocrystalline fcc TIN phase. Compositional depth profiles obtained from SIMS have shown the extent of nitrogen concentration gradient in the implantation zone. Both as-deposited and ion implanted films showed much higher hardness as compared to the bulk titanium. AFM studies revealed a gradual increase in surface roughness leading to surface patterning with increase in ion fluence.

Nanoindentation and scratch studies on magnetron sputtered Ti thin films.

Ti thin films sputter deposited on D9 steel at two different temperatures were studied for their mechanical behavior under static and sliding contact conditions using nanoindentation and scratch tests. The film hardness measured at the surface of the coatings exhibited a value of 2.5 GPa, for both conditions. From the scratch test, it is understood that coatings deposited at 200 degrees C substrate temperature showed superior adhesion strength. Critical load to failure for these coatings was evaluated at 2 N.

Microstructural studies of nanocomposite thin films of Ni/CrN prepared by reactive magnetron sputtering.

Synthesis and characterization of nanocomposites of Ni/CrN thin films prepared by DC magnetron sputtering from a target of 50 wt.%Ni-50 wt.%Cr is investigated. The films prepared as a function of nitrogen flow rate and substrate temperature showed that the films contained Ni and CrN phases with crystallite sizes in the nanometer range. Measurement of nanomechanical properties of the composite films exhibited a significant decrease in the values of hardness and Young's modulus than those of pure CrN films.

Performance characteristics of a microMOSFET as an in vivo dosimeter in radiation therapy.

The commercially available microMOSFET dosimeter was characterized for its dosimetric properties in radiotherapy treatments. The MOSFET exhibited excellent correlation with the dose and was linear in the range of 5-500 cGy. No measurable effect in response was observed in the temperature range of 20-40 degrees C. No significant change in response was observed by changing the dose rate between 100 and 600 monitor units (MU) min(-1) or change in the dose per pulse. A 3% post-irradiation fading was observed within the first 5 h of exposure and thereafter it remained stable up to 60 h. A uniform energy response was observed in the therapy range between 4 MV and 18 MV. However, below 0.6 MeV (Cs-132), the MOSFET response increased with the decrease in energy. The MOSFET also had a uniform dose response in 6-20 MeV electron beams. The directional dependence of MOSFET was within +/-2% for all the energies studied. The inherent build-up of the MOSFET was evaluated dosimetrically and found to have varying water equivalent thickness, depending on the energy and the side of the beam entry. At depth, a single calibration factor obtained by averaging the MOSFET response over different field sizes, energies, orientation and depths reproduced the ion chamber measured dose to within 5%. The stereotactic and the penumbral measurements demonstrated that the MOSFET could be used in a high gradient field such as IMRT. The study showed that the microMOSFET dosimeter could be used as an in vivo dosimeter to verify the dose delivery to the patient to within +/-5%.

Comprehensive quality assurance for stereotactic radiosurgery treatments.

We have used a commercially available high precision Lucy phantom to perform comprehensive quality assurance for stereotactic radiosurgery treatments. The quantitative evaluation of system uncertainties included imaging, planning and treatment delivery systems. The quality assurance tests showed that the well-defined targets were identified to within +/-1 mm in all the imaging modalities. The pre-known target volumes were reproduced within 2 cm3 in both MR and CT. The planned target was delivered within 2% of the prescribed dose and to within 2 mm accuracy. The inaccuracy in the isocentre position at the Linac was less than 1.2 mm. The maximum error observed in the depth helmet was 0.5 mm and the overall uncertainty was within 0.23 mm. We have also established a quality assurance program based on the study and proposed the tolerance and the frequency of the tests required to be carried out. The tests were carried out using a Radionics planning system and delivered on a Varian Clinac 2100 linear accelerator machine. These tests also established a base line for future comparisons.

Stereotactic radiotherapy for AVMs: the University of Toronto experience.

Since July 1989, 66 patients have received stereotactic radiosurgery for arteriovenous malformations of the brain. All cases were reviewed by our multidisciplinary group. As result of our treatment algorithms these patients underwent stereotactic radiosurgery, either as the sole therapy or as part of combined modality treatment. Using a 6 MV linear accelerator, we have usually employed doses of either 15 or 20 Gy to the edge of the lesion, ensuring that critical normal structures do not receive a dose in excess of 15 Gy. Of the initial 24 patients followed for a minimum of 2 years, 12 have complete obliteration documented by angiography; 8 have > 90% obliteration (several have deferred further angiographic follow-up which may show progression to complete obliteration); 3 have had the nidus diminish; and one has had no change. Within this cohort, one patient experienced a transient acute effect; one patient has developed a minor late effect; one suffered a fatal hemorrhage despite ongoing response to radiosurgery; one has recently undergone retreatment.