Properties of films

By GLAD

The following results are a few examples illustrating the wide panel of achievable properties produced by the GLAD process. These examples are focused on electrical conductivity, optics, mechanics, acoustics and gas sensing among others.

Electrical conductivity

Films deposited by conventional sputtering keep a typical metallic-like behavior versus temperature whereas those sputtered by GLAD (α = 85°) show a gradual transition from metal to dielectric. Such a transition is mainly assigned to the high porous structure, which favors the oxidation of titanium films to tend to the TiO₂ compound.

DC electrical conductivity vs. reciprocal temperature of titanium films sputter-deposited. Films prepared with normal incidence of the sputtered particles (α = 0°) keep a metallic-like behavior during an annealing treatment in air using incremental cycles of temperature. GLAD films obtained with an incident angle α = 85° exhibit a gradual metal-to-dielectric transition

Optics

The tilted columnar microstructure of obliquely deposited films can generate a corresponding optical anisotropy, where film properties depend on the orientation between the microstructure and the optical fields. Refractive index typically exhibits anisotropic behaviors (birefringence) due to the lateral growth of the columnar structure, which becomes significant for incidence angles higher than 50°. Some architectures (inclined columns, zigzags, triangular spirals) lead to anistropic microstructure and thus, birefringence.

Birefringence at 600 nm vs. incidence angle for WO₃ thin films sputter-deposited on glass. It is maximized near 50° for tilted columns, zigzags and triangular spirals, whereas nearly no birefringence is measured for square and continuous spirals

Mechanics

Sculptured thin films exhibiting a zigzag microstructure behave as a field of "microsprings". A theoretical approach was developed to predict hardness, Young modulus and stiffness of these zigzag films taking into account the film’s geometry (zigzag) and the bulk material properties. Such a model is based on the deformation of a single zigzag period caused by a normal force. It was successfully compared to experimental measurements performed by nanoindentation on chromium zigzag films sputter deposited with a constant thickness close to 1 µm and with a half period thickness λ/2 ranging from 50 to 1000 nm.

Theoretical and experimental evolution of Young Modulus vs. half period thickness of chromium thin films exhibiting a zigzag architecture for two column angles β = 18 and 26°

Acoustics

Tungsten thin films 300 nm thick are sputter-deposited on (100) Si wafer using a conventional sputtering process (i.e. normal incidence of the particle flux with α = 0°) and by GLAD with an incidence angle α = 80°. The resulting tungsten thin films exhibit a columnar microstructure, which is normal to the substrate for α = 0°, or tilted with a columnar angle β = 43° for α = 80°. Such an inclined columnar microstructure leads to an anisotropic propagation of the surface acoustic wave.

Imaging of the surface acoustic wave propagation after 200 ps, 1, 4 and 6 ns measured with a femtosecond heterodyne pump-probe setup. The wave amplitude clearly exhibits circular and concentric rings for conventional films (α = 0°), whereas elliptical shapes are observed for inclined films (α = 80°) due to an anisotropic propagation

Gas sensing

The GLAD technique can be implemented to transform the typical dense microstructure of WO₃ films into an inclined columnar and porous architecture. Conventional WO₃ films exhibit a significant change of electrical conductance as a function of the ozone injection. GLAD WO₃ films with a columnar angle β = 40° are even more sensitive to the ozone pulses and the speed of response or recovery is significantly enhanced. This ozone detection improvement is mainly attributed to the more porous structure of WO₃ films produced by the GLAD method.

The incidence angle α of the sputtered particles was 0 and 70° leading to a column tilt angle β of 0 and 40°, respectively. Conventional and inclined WO₃ films were sputter deposited on a commercial hot plate system. Ozone gas was periodically injected from 0 to 220 ppb and the variation of electrical conductance of the system was measured vs. time at 250°C

By RGPP

Results shown below are a few examples among the wide range of physical behaviors that can be reached by oxides, nitrides and oxynitrides.

Composition

The metalloid concentration of oxide, nitride, oxynitride ... thin films can be tuned by RGPP by adjusting the pulsing parameters. A typical example is given below in titanium oxynitride thin films

Ternary diagram showing the evolution of Ti, O and N atomic concentrations in titanium oxynitride thin films prepared by RGPP and for various duty cycles (dc = 0 to 100 % of the pulsing period).

Metal-to-dielectric transition

The metalloid concentration of titanium oxide thin films can be tuned by RGPP by adjusting the time of oxygen injection in the reactive sputtering process. A smooth transition from metal to semi-conductor, and finally to insulator is observed as a function of the oxygen content.

DC electrical conductivity vs. temperature of TiO_x thin films produced by RGPP with various injection times of the oxygen gas (t_ON time). A gradual metal-to-dielectric transition is measured as the oxygen concentration in the film increases

Optical transmittance

The metal to dielectric transition can also be measured from the optical properties of oxide and oxynitride thin films. As similarly obtained with electrical conductivity, optical transmittance in the visible range exhibit a gradual transition from metal to semi-conductor, and finally to insulator as a function of the oxygen content.

Optical transmittance spectra in the visible range of titanium oxynitride thin films deposited on glass and for various duty cycles (dc = 0 to 100 % of the pulsing period)