Robert Gerard Schad

Repeated compressive stress increase with 400 °C thermal cycling in tantalum thin films due to increases in the oxygen content

Stresses which build up in thin films such as tantalum, during thermal processing, can cause major reliability problems in electronic and x‐ray optics applications. We demonstrate that 50–200 nm thick sputtered β‐Ta thin films undergo repeated compressive stress increases when thermally cycled to 400 °C (at a rate of 10 °C/min) and back in a purified He ambient because of small amounts of oxygen gettered by the tantalum. The oxygen contamination results from the poor quality of the atmospheric seal on the quartz annealing chamber. As‐deposited Ta thin films have a compressive stress ranging from −1 to −4 GPa. The compressive stress buildup was monitored in situ and was shown to increase −0.5 GPa on average after each thermal cycle for a final value from −6 to −7 GPa after seven cycles. After being cycled thermally seven times any perturbation of the film such as a four‐point probe resistivity measurement can cause the film to instantaneously crack in a serpentine pattern, relieving the large compressive s...

Compressive Stress Increase With Repeated Thermal Cycling in Tantalum(Oxygen) Thin Films

Stresses which build up in thin films, such as tantalum, during thermal processing, can cause major reliability problems in x-ray optics and electronic applications. We have demonstrated that 50 nm to 200 nm thick sputtered beta tantalum thin films undergo repeated compressive stress increases when thermally cycled from room temperature to 400°C (at 10°C/min) and back in a purified He ambient because of low levels of oxygen gettered by the tantalum. The oxygen contamination is a result of the poor quality of the quartz annealing chamber atmospheric seal. As-deposited stress in the sputter deposited tantalum films ranges from -1 to -4 GPa. The compressive stress build up was monitored in situ and was shown to increase -0.5 GPa on average after each thermal cycle for a final value of -6 to -7 GPa after seven cycles. After being cycled thermally seven times any perturbation of the film such as a four point probe resistivity measurement can cause the film to instantaneously crack in a serpentine pattern relieving the large compressive stress. Auger electron spectroscopy depth profiling analysis indicated that the as-deposited films contained one atomic percent oxygen which increased to eight to twelve percent after seven thermal cycles accompanied by an approximate doubling in resistivity. In conclusion, the increase in oxygen concentration in tantalum thin films which occurs upon thermal cycling leads to a repetitive increase in compressive stress which could be detrimental when the films are used in x-ray or electronic applications.

Organic Conductors as Electron Beam Resist Materials

Conducting organic π‐donor halide complexes such as tetrathiafulvalene bromide were discovered to act as electron beam resists, which display a unique combination of useful properties. Exposure of sublimed films to an electron beam generates the neutral π donor and the halogen which is subsequently lost from the film. Depending on exposure conditions, either negative (solvent developed) or positive (in‐situ developed) resist images with a resolution of the order of 0.5 μ can be generated. The strongly absorbing (UV,vis.) and highly conducting (∼10/Ω cm) films were found to become transmitting and insulating upon electron beam irradiation.

Oxygen assisted ohmic contact formation mechanism to n‐type GaAs

The presence of oxygen in the top W layer of NiGe(Au)W ohmic contacts to n‐type GaAs is found to play a critical role in reducing their contact resistance. Contacts with sputtered W containing less than 1 at. % oxygen and formed by rapid thermal annealing (RTA) yield a contact resistance (RC) greater than 0.45 Ω mm. Contacts with a reactively sputtered or electron‐beam evaporated metallic W oxide top layer, containing ∼25 at. % oxygen, yield RC’s of less than 0.15 Ω mm. Auger depth profiles of the reacted contacts show a significant outdiffusion of Ga from the GaAs substrate in the presence of the oxygenated W but not in the oxygen‐free contacts. A contact formation mechanism based on the gettering of Ga atoms by oxygen is proposed.The presence of oxygen in the top W layer of NiGe(Au)W ohmic contacts to n‐type GaAs is found to play a critical role in reducing their contact resistance. Contacts with sputtered W containing less than 1 at. % oxygen and formed by rapid thermal annealing (RTA) yield a contact resistance (RC) greater than 0.45 Ω mm. Contacts with a reactively sputtered or electron‐beam evaporated metallic W oxide top layer, containing ∼25 at. % oxygen, yield RC’s of less than 0.15 Ω mm. Auger depth profiles of the reacted contacts show a significant outdiffusion of Ga from the GaAs substrate in the presence of the oxygenated W but not in the oxygen‐free contacts. A contact formation mechanism based on the gettering of Ga atoms by oxygen is proposed.

Low Au Content Ohmic Contacts to n-GaAs Incorporating Sputtered Tungsten Oxide

Electron-beam (e-beam) evaporated low Au content NiGe(Au)W ohmic contacts with a contact resistance (R c ) as low as 0.15 Ω-mra have been reported. Due to the high melting point of W it is desirable to deposit this layer by means other than e-beam evaporation. However, the use of nearly oxygen-free sputtered W, yields contact resistances in excess of 0.7 Ω-mm. By replacing the sputtered W by a reactively sputtered metallic W oxide, containing ∼25 at. % oxygen, the low contact resistance (R c