Ming Fang

Evolution of stress in evaporated silicon dioxide thin films

The evolution of stress in evaporated SiO2, used as optical coatings, is investigated experimentally through in situ stress measurement. A typical evolution pattern consisting of five subprocedures (thin film deposition, stopping deposition, cooling, venting the vacuum chamber, and exposing coated optics to the atmosphere) is put forward. Further investigations into the subprocedures reveal their features. During the deposition stage, the stresses are usually compressive and reach a stable state when the deposited film is thicker than 100 nm. An increment of compressive stress value is observed with the decrease of residual gas pressure or deposition rate. A very low stress of -20 MPa is formed in SiO2 films deposited at 3×10-2 Pa. After deposition, the stress increases slightly in the compressive direction and is subject to the stabilization in subsequent tens of minutes. In the process of venting and exposure, the compressive component increases rapidly with the admission of room air and then reaches saturation, followed by a logarithmic decrement of the compressive state in the succeeding hours. An initial discussion of these behaviors is given.

Influence of adsorption kinetics on stress evolution in magnetron-sputtered SiO2 and SiNx films

An in situ multi-beam optical sensor system was used to monitor and analyze the force per unit width (F/w) and stress evolution during several stages in magnetron-sputtered SiO2 and SiNx films. Stress was observed to relieve quickly after interrupt and recover rapidly after growth resumption in both films. Stress relief was reversible in SiO2 film but partial reversible in SiNx film. Stress relief results from both physical and chemical adsorption. Stress recovery is caused by physical desorption. And chemical adsorption results in an irreversible stress relief component. No chemical adsorption occurs in SiO2 film because of the stable chemical structure. The relationship between adsorption kinetics and films mechanical behavior is revealed.

Evolution and control of optical thin film stress

Fine control of Optical Thin Film Stress is critical to develop an expected profile parameters of large aperture coated optics. In our latest effort, the evolution of stress and force per unit width of multilayer thin film formulas, (H2L)⁶, (H2L)⁵H and (HL)⁷, consisted of HfO₂ and SiO₂ were researched. The stresses of SiO₂ single layers deposited on the HfO₂ were small than those grew on glass and get smaller as the multilayer thin film deposited. While the stresses of HfO₂ films deposited on the SiO₂ were positive correlation with the stress of SiO₂.

Stress evolution in evaporated HfO 2 /SiO 2 multilayers

Hafnium oxide/silicon dioxide (HfO₂/SiO₂) multilayers were prepared by electron-beam evaporation. The total force per unit width (F/w) during and after deposition was determined by the change of the substrate curvature measured in situ. Stress induced by water absorption is a major component of the film stress evolution in atmospheric environment. Growth stress was analyzed to understand the role of the sublayers and the influence of the underlayers’ structural features. The substrate material affects the stress evolutions in both HfO₂ films and SiO₂ films. The structural feature of the HfO₂ layer onto which SiO₂ was deposited has a significant effect on the stress evolution of the SiO₂ layer.

Properties of niobium oxide films deposited by pulsed DC reactive magnetron sputtering

Nb2O5 thin films at various cathode power and substrate bias voltages were deposited by pulsed DC reactive magnetron sputtering of a metallic Nb target in a pure oxygen atmosphere. The characteristics of the films have been studied using spectrometer, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM). Laser damage tests at 1064 nm wavelength with pulse duration of 12 ns were conducted on the single-layer systems. Results indicate that the cathode power may not be an important impact-factor of the LIDT of Nb2O5 thin films but substrate bias voltage has significant influence on the laser resistance of Niobium oxides films. The maximum laser induced damage threshold (LIDT) of 28.8 J/cm2 was obtained for the film deposited at substrate bias voltage of -60V.

Apparatus for statically testing the properties of magnetic coupling and hybrid recording thin films

We designed and built up an apparatus for measuring the properties of magnetic coupling thin films and hybrid recording media. The temperature of the tested points of the thin film samples change as irradiated by focused laser beam through adjusting laser power. By calculation, we also simulated the distribution of temperature in TbFeCo magneto-optical films irradiated by laser, in order to get the relationship between the film coercive force and the irradiating laser power. Using this apparatus, we can determine the Curie temperature and compensation temperature of the films. The apparatus might present an effective approach for studying the variation of magneto-optical characteristics of films at different temperatures and the properties of magnetic coupling multilayer films, and it could be useful for studying hybrid recording.

Analysis on the magnetic capping effect in heat assisted magnetic recording

The use of magnetic capping effect can enhance the head sensitivity of the heat assisted magnetic recording (HAMR). The principle of this effect was analyzed theoretically. The GdFeCo film was used as the capping layer and TbFeCo film was used as the recording layer. Through theoretical calculation we found that a large capping field in the same direction with the external field can be generated at the interface because of the exchange coupling effect, which had increased the effective strength of the external field. Thus, the head sensitivity was improved greatly and the data transfer rate could be faster. We expected that this idea can be used to help write the medium with much larger cocervity such as L10FePt.