Influence of substrate temperature on properties of MgF2 coatings

doi:10.1016/j.apsusc.2007.01.037

Applied Surface Science

Volume 253, Issue 14, 15 May 2007, Pages 6113-6117

https://doi.org/10.1016/j.apsusc.2007.01.037 Get rights and content

Abstract

Thermal boat evaporation was employed to prepare MgF₂ single-layer coatings upon both JGS1 and UBK7 substrates at different substrate temperatures. Microstructure, transmittance and residual stress of these coatings were measured by X-ray diffraction, spectrophotometer, and optical interferometer, respectively. Measurement of laser induced damage threshold (LIDT) of the samples was performed at 355 nm, 8 ns pulses. The results showed that high substrate temperature was beneficial to crystallization of the film. Above 244 °C, the refractive index increased gradually with the substrate temperature rising. Whereas, it was exceptional at 210 °C that the refractive index was higher than those deposited at 244 and 277 °C. The tensile residual stresses were exhibited in all MgF₂ films, but not well correlated with the substrate temperature. In addition, the stresses were comparatively smaller upon JGS1 substrates. A tendency could be seen that the LIDTs reached the highest values at about 244 °C, and the films upon JGS1 had higher LIDTs than those upon UBK7 substrates at the same temperature. Meanwhile, the damage morphologies showed that the laser damage of the coating resulted from an absorbing center at the film–substrate interface. The features of the damages were displayed by an absorbing center dominated model. Furthermore, the reason of the difference in LIDT values was discussed in detail.

Introduction

Lasers in the UV range are leading in the wide field of precision material-processing [1] and microlithography. The economic efficiency of the modern applications is directly dependent on the quality of the dielectric coatings deposited upon the optical components used in the beam lines and the laser systems. Even though the important experimental and theoretical progress has been made, the problem of laser damage of dielectric coatings has not yet been completely solved [2]. Much more efforts are required to realize optical components providing low optical losses, high stability, high irradiation resistance and good optical performance for the UV laser radiation [3].

Moreover, the number of available materials in the UV range is limited because of strong absorption in this region for many materials. The commonly used coating materials in this region include metal oxides (Al₂O₃, Sc₂O₃, HfO₂ and SiO₂) and wide band-gap fluorides (MgF₂, YF₃, LaF₃, AlF₃, LiF, and CeF₃) [4]. Among them, MgF₂ is a widely used material with low refractive index and prefect damage resistance especially in the UV region. Although it has been actively studied during the last few decades [5], [6], the effect of the substrate temperature on the structural, optical, mechanical properties and damage resistance of MgF₂ coating has not been systemically investigated in the reported literature as yet. Therefore, it is necessary to go deep into it for the sake of more effective employment of MgF₂.

In this article, MgF₂ single-layer coatings were deposited upon two kinds of substrates by thermal boat evaporation at different substrate temperatures. The microstructure, optical property and mechanical property of the coatings were characterized by several methods and the damage resistances of the films were also tested. The effect of the substrate temperature on them was investigated in detail.

Section snippets

Experiment

MgF₂ coatings were deposited upon selected superpolished JGS1 and UBK7 substrates with the dimension of Ø 30 mm × 3 mm at the same time. The purification of the initial material was 99.99%. The coating material was evaporated from thermal boat in the same coating chamber at different substrate temperatures: 210, 244, 277, 310, and 343 °C. Before deposition the chamber was pumped down to a base pressure of 1.5 × 10⁻⁵ Torr below. The thickness of the film was of about 990 nm and monitored optically during

XRD analysis

The XRD patterns of the samples deposited at different substrate temperatures are shown in Fig. 2. A broad hump is observed at 2θ between 20° and 40°, which is attributed to amorphous substrate. The diffraction peaks of the coatings match well with MgF₂ tetragonal structure (JCPDS 70-2269). The peaks appear at 2θ = 27.2°, 40.4°, 43.7°, 53.5°, 56.2°, and 64.6° which are well indexed to the corresponding crystal plane of tetragonal (1 1 0), (1 1 1), (2 1 0), (2 1 1), (2 2 0), and (2 2 1). As the substrate

Conclusion

MgF₂ single-layer coatings were deposited upon JGS1 and UBK7 substrates by thermal boat evaporation at different substrate temperatures. As the substrate temperature rose, the crystallization status of the film became better and the refractive index increased gradually. But it was exceptional at 210 °C that the refractive index was higher than those deposited at 244 and 277 °C. The residual stresses were tensile stresses in all MgF₂ coatings and had no obvious corresponding relation with the

Acknowledgement

The authors would like to thank Y. Cui for LIDT measurements.

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Influence of substrate temperature on properties of MgF2 coatings

Abstract

Introduction

Section snippets

Experiment

XRD analysis

Conclusion

Acknowledgement

Thin Solid Films

Proc. SPIE

Proc. SPIE

Proc. SPIE

Appl. Opt.

Appl. Opt.

Influence of substrate temperature on properties of MgF₂ coatings