ICMCTF1999 Session C2: Structure/Property Relationships

The systems we investigated are multilayers intended for laser mirrors. They consist in a stack of thin layers of a couple of brittle materials with different refrective index, which are deposited alternatively.

For these multilayers we have investigate their mechanical stability, when submitted to different sort of sollicitations.

We have determined the critical parameters which activate the degradation of these systems.

Eventhough the mechanical properties of the constitutive layers are rather similar (stiff materials, same order of magnitude of Young modulus), their mechanical response in the multilayer are rather different.

As a matter of fact, a marked difference appears in the failure mechanims: some interfaces are less resistant than others.

This behaviour is discussed with respect to the obtention conditions, and in particular with respect to the critical sizes of impurities, which act as nucleating sources of cracking and debonding.

The determination of apparent values of strain energy release rates and fracture toughnesses is discussed.

HfO2 is undoubtedly one of the most succesfull material for high power laser applications. However, the laser induced damage threshold of HfO2 coatings is usually limited due to molten oxide particles spewing out from the crucible during the evaporation process. These particles create burried nodular defects into the deposited material, known to be the ignitors of the coating damage during laser iradiation.

To reach higher laser induced damage threshold, Hf has been evaporated and oxidized in situ thanks to reactive deposition or ion beam bombardment. The oxidation process parameters have to be carefully tuned to fulfill either high densification of the coating (as high as 99 % of the bulk) or low optical absorption (lower k = 3.10-6 at 350 nm). High laser induced damage thresholds are incompatible with energetic processes (ion beam bombardment). This general trend is discussed on either HfO2 coatings or HfO2/SiO2 stacks.

A comparison with films deposited with the classical way (HfO2 evaporation) is proposed, especially concerning local absorptions induced by non stoichiometric nodular defects as determined with photothermal mapping and localized Auger spectroscopy. It gives clear indications on how injurious such defect could be for high laser flux applications.

Thanks to this optimization, SiO2/HfO2 mirrors (99,5 % reflectivity) have been achieved which exhibit laser induced damage threshold at 1,06 µm as high as 45 J.cm-2.

Recent developments in the design of unbalanced magnetrons now allow high ion currents to be drawn at the substrate (up to 20mA/cm² in multiple magnetron systems). High ion currents have generally been found beneficial for the deposition of hard wear resistant coatings on tool steels, for example. However, such magnetrons may not necessarily produce the optimum deposition conditions for all types of coatings.

Investigations have been carried out at Salford into the reactive pulsed magnetron sputtering of alumina films for use in optical applications. Two different designs of unbalanced magnetrons were used. The magnetrons were configured in the chamber in two different orientations. The use of these two variables allowed the ion current density at the substrate to be varied in the range 1 to 10mA/cm². Other deposition parameters, such as pulse frequency, target reverse voltage and reverse time were also varied. Film properties, such as adhesion, wear resistance, transmission and reflectance were measured. These measurements showed significant variations in the film properties. Cosequently, the relationship between ion current and film properties has been investigated and the initial findings are reported here.