Since Ta2O5 has the characteristics of low intrinsic stress, superior layer smoothness and uniformity, and low absorption and scattering losses^[1,2], the high-reflectivity (HR) films composed of Ta2O5 and SiO2 have been widely used in various optical systems. As the output power of optical system increases steadily, HR films are expected to have a higher laser-induced damage threshold (LIDT) to ensure the stability and security characteristics of optical systems. However, the lower LIDT of Ta2O5/SiO2 HR films caused by the low laser damage resistance of Ta2O5 results in films damaged easily, which affects the stability and security of the optical system. Since damage usually occurs on the outermost layers of HR films due to the high laser energy suffered here, the LIDT of HR films can be effectively improved by replacing the outermost layers with high LIDT materials. In order to improve LIDT of Ta2O5/SiO2 HR films, Patel et al.^[3] replaced the outermost three Ta2O5 layers by HfO2 and formed HfO2/Ta2O5/SiO2 HR film, increasing the LIDT of HR film from 16J/cm2 to 25J/cm2. Schiltz et al.^[4] replaced the outermost Ta2O5 layers of Ta2O5/SiO2 HR film with HfO2, and the LIDT of HR film increased by nearly three times. Despite the great improvements made in Ta2O5/SiO2 HR film by replacing Ta2O5 with HfO2, the LIDT of HfO2/Ta2O5/SiO2 films is expected to be further improved. As one of the post-treatment technologies of film, thermal annealing can effectively improve the laser damage resistance of HR films. The LIDT is mainly affected by the oxygen vacancies, nodule, impurity, microstructural and mechanical properties of HR films in the nanosecond pulse regime. Thermal annealing can improve microstructures and reduce the oxygen vacancies by providing thermal energy^[5,6]. Zhao et al.^[7] show that thermal annealing can increase the LIDT of Ta2O5/SiO2 dielectric mirrors by improving the structure and absorbance. The roles of thermal annealing on the LIDT of HR films are different according to the annealing temperature and process. Xu et al.^[8] studied the effect of high-temperature annealing on Ta2O5 films and obtained the maximal LIDT at 873 K annealing temperature. Bananej et al.^[9] studied the effect of the annealing processes on the properties of ZrO2 thin films and found that the time–temperature gradient annealing can obtain higher LIDT. Hassanpour et al.^[10] realized the enhancement of LIDT of TiO2 by using the same method. Film stress can be changed by thermal annealing. Shen et al.^[11] showed that the residual stress of HfO2/SiO2 film is compressive at as-deposited and transforms into tensile after annealing at 200°C; the thin film even appears with microcracks at 400°C, which seriously affect the LIDT. Jena et al.^[12] found that the LIDT of HfO2/SiO2 film can be increased when thermal annealing temperature is below 400°C. The HR films annealed at 400°C show the highest LIDT and an increase of 74% compared to the as-deposited HR films.

To our best knowledge, there are few reports about the effect of thermal annealing on LIDT of HfO2/Ta2O5/SiO2 HR films. In the present study, thermal annealing is adopted to improve HfO2/Ta2O5/SiO2 HR films that are prepared by electron beam evaporation with ion beam assisting. The optical properties, microstructure, and nanosecond LIDT of HfO2/Ta2O5/SiO2 HR films annealed at different temperatures are comparatively investigated. The experimental results show that the highest LIDT of 22.4J/cm2 is obtained at 300°C annealing temperature. With the further increase of annealing temperature, the damage changes from the delamination phenomenon to the craters phenomenon, and the effect of thermal annealing on the damage mechanism of the HR film is analyzed by damage morphology.

The HR films are deposited by electron beam evaporation (Leybold Optics, ARES710, Germany) with ion beam assisting on BK7. The structure of HR film is |(LH)^8(LM)^3| air, where L, M, and H stand for SiO2 layers, HfO2 layers, and Ta2O5 layers, respectively. The thickness of the SiO2 layer is 170 nm, and the thickness of the HfO2 layer is 124 nm. For the four Ta2O5 layers near HfO2, the thickness of each layer is 124.5 nm. The thickness of the remaining Ta2O5 layer is 129 nm. In this HR film structure, the maximum electric field is located at the top HfO2 layer. A quartz crystal monitor is used to measure deposition rate and layer physical thickness. The initial pressure of the coating chamber is 8×10−6mbar, and the temperature of substrate is 100°C. The deposition rates of HfO2 and Ta2O5 layers remain at 0.2 nm/s, while that of the SiO2 layer is 0.6 nm/s. Table 1 shows the ion source parameter during deposition in detail. The thermal annealing post-process is performed at 200°C, 300°C, 400°C, and 500°C for 3 h in the furnace with the air environment. The heating rate is 5°C/min, and the temperature falls naturally to room temperature in the furnace after thermal annealing.

A spectrophotometer (Shimaszu, UV-3101, Japan) is used to measure the transmission spectra of the HR films from 800 nm to 1400 nm, and the relative uncertainty in the transmittance is 0.3%. The transmission spectra of the HR films at different annealing temperatures are shown in Fig. 1. In the range of 950 nm to 1110 nm, all HR films have a transmittance of less than 1%. With the annealing temperature increasing to 500°C, the spectra of the HR film shift to a short wavelength. The inset of Fig. 1 shows the transmission spectra from 1250 nm to 1290 nm in detail. The transmission peak of the as-deposited HR film is at 1275 nm, and it moves to 1266 nm when the annealing temperature rises to 500°C. The short wavelength shift of the transmission spectra indicates that the layers of HR films become thinner and more compact.

Fig. 1. Transmission spectra of the HR films at different annealing temperatures.

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The spectral shift to a short wavelength can be explained by the fact that the atoms of HR films are activated to fill the voids and diffuse into each layer in the process of thermal annealing^[13]. The decrease of the void density reduces the layer physical thickness, which leads to the decrease of the optical thickness, resulting in the shift of the transmission spectra to a short wavelength.

Due to the fact that the maximum electric field is located at the top HfO2 layer, the LIDT of the HR film is mainly affected by the top HfO2 layer. Electron beam evaporation introduces nonstoichiometric hafnium oxide and oxygen vacancies^[14]. The oxygen deficiency enhances the absorption of films and decreases the band gaps, which seriously decrease the LIDT^[15]. The existence of oxygen vacancies increases the concentration of free electrons, which affect the absorption coefficient. The relationship between the absorption coefficient α and the concentration of free electrons N can be expressed as^[12]α=Ne2λ2mnτε0π2c3,(1)where e, n, m, τ, λ, and ε0 are the charge of the electron, refractive index, mass of the free electron, relaxation time, absorption wavelength, and vacuum permittivity, respectively. During thermal annealing, oxygen atoms react with the nonstoichiometric hafnium oxide, thus gradually reducing the oxygen vacancies and the concentration of free electrons N.

Equation (1) indicates that the absorption coefficient α is not only related to oxygen vacancies but is also affected by the refractive index. The refractive index of HfO2 also changes during thermal annealing. In order to clarify the effect of thermal annealing on the refractive index of HfO2, the HfO2 single layer film is fabricated with the same parameter as HR films. The process of thermal annealing is the same as that of the HR film. The refractive index of HfO2 at different annealing temperatures is obtained from the transmittance spectrum by using the envelope method and Essential Macleod software^[16]; the relative uncertainty in the refractive index is about 1.3%. Figure 2 shows the influence of the annealing temperature on the refractive index of HfO2 at 550 nm. The refractive index is increased slightly with increasing annealing temperatures, which had also been observed in TiO2^[17].

Fig. 2. Influence of annealing temperature on refractive index of HfO2 at 550 nm.

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The reduction of oxygen vacancies and the increase of the refractive index both decrease the absorption coefficient of the top HfO2 layer. Since the LIDT of HR films is inversely proportional to the absorption coefficient^[18], reducing the absorption coefficient contributes to the improvement of LIDT.

Decreasing the layer physical thickness and increasing the refractive index indicate that the density (ρfilm) of the film is increased^[19]; the increasing densities are beneficial to improving the damage threshold of thin films by decreasing the porosity of film and increasing the specific heat. Based on the defect originated thermal damage mechanism, the LIDT is proportional to the 1/2 power of the density, which can be expressed as^[19]ET≈16TCπ(ρfilmcfilmkfilmτ)1/2,(2)where ET, TC, ρfilm, cfilm, τ, and kfilm are the damage threshold in joules per square centimeter, the critical temperature (e.g., melting temperature of the host film material), the density of the host, the effective specific heat at constant pressure of the host, the laser pulse length, and the thermal conductivity of the host material, respectively. In addition, thermal annealing can also remove adsorbed water, hydrocarbons, and carbon in HR films, thereby reducing the absorption laser energy ability of the HR film.

The microstructure of HR films is investigated by X-ray diffraction (XRD) with a 2θ angle in the range of 15°–65° using Cu–Kα radiation at the wavelength of 1.54 Å (1 Å = 0.1 nm). The measured XRD patterns are shown in Fig. 3. Since Ta2O5 and SiO2 are amorphous when the annealing temperature is below 500°C^[8,12], the XRD peaks in Fig. 3 can be considered to belong to HfO2.

Fig. 3. XRD spectra of HfO2/Ta2O5/SiO2 HR films at different annealing temperatures.

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For the as-deposited HR film, the most remarkable diffraction peak is at 2θ=28.36°, and there are several weak diffraction peaks nearby. According to the probability density function (PDF) card (PDF#00-006-0318), the diffraction peak at 28.36° corresponds to the m(−111) plane of monoclinic structure HfO2, and the other diffraction peaks correspond to the m(111) plane, m(020) plane, and m(220) plane, respectively. After thermal annealing, the intensity of all diffraction peaks is enhanced, and no new diffraction peak appears. The enhancement of diffraction peak intensity indicates that the crystallinity of the HfO2 is improved. The HfO2 crystallite size cHfO2 can be calculated by the HfO2 m(−111) diffraction angle θ and the full width at half-maximum (FWHM) of diffraction peaks according to Scherer’s formula^[20]: cHfO2=0.89λXRDβcosθ,(3)where β is the FWHM of the diffraction peak, and λXRD is 0.154 nm, which is dependent on the XRD equipment. The calculated results for the m(−111) plane of monoclinic structure HfO2 are shown in Fig. 4.

Fig. 4. Influence of annealing temperature on HfO2 crystallite size in the HR films.

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The HfO2 crystallite size is increased with increasing annealing temperature. When the annealing temperature is below 300°C, the crystallite size increases slightly with the increase of annealing temperature. The crystallite size increases from 12.18 nm of the as-deposited HR film to 12.74 nm at 300°C. With annealing temperature exceeding 300°C, the crystallite size increases rapidly with the increase of annealing temperature. The crystallite size increases to 14.03 nm when the HR film is annealed at 400°C and to 14.35 nm at 500°C. The change of crystallite size reflects the change of grain size. It is the fact that the process of thermal annealing can restore and recrystallize the microstructure of HR films. When the annealing temperature is below 300°C, the thermal energy offered by annealing is mainly used to restore the microstructure of HR film, and thus the grain size of HR films changes indistinctively. As the annealing temperature is further improved, in addition to repairing the HR film microstructure, thermal annealing can provide more thermal energy to promote grain size growth. Thus, when the annealing temperature is higher than 400°C, the grain size of the HR film increases significantly. The increase of grain size increases the average free path of phonon scattering at grain boundaries and decreases the thermal boundary impedance^[21,12], which helps to improve the thermal conductivity of HfO2/Ta2O5/SiO2 HR films.

Thermal conductivity is a key factor affecting the laser resistance of HR films. The improved thermal conductivity of films not only effectively suppresses temperature rises in the laser irradiation area by transferring generated heat away, but also increases the laser-induced thermal stress resistance factor^[22]. Equation (2) indicates that the LIDT is proportional to the 1/2 power of the thermal conductivity. Therefore, the LIDT can be increased by improving thermal conductivity.

The LIDTs of HR films are measured in the “one-on-one” regime according to the International Organization for Standardization (ISO) standard 11254-1^[23], where a 1064 nm pulsed laser at a pulse width of 12 ns is used to assess the LIDTs. The damage threshold error is about 11%. The influence of the annealing temperature on the LIDT of HR films is shown in Fig. 5. As can be seen from the figure, the LIDTs increase with the increase of annealing temperature when the thermal annealing temperature is below 300°C. The LIDT of HR films reaches the highest value of 22.4J/cm2 at the annealing temperature of 300°C, which is twice as high as that of the as-deposited HR film.

Fig. 5. Influence of annealing temperature on LIDT of HR films.

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However, when the annealing temperature exceeds 400°C, the LIDT decreases with the increase of annealing temperature, and the LIDT is 9.5J/cm2 at 500°C, which is lower than that of as-deposited HR film.

HfO2/Ta2O5/SiO2 HR films are irradiated by the energy density of 25.5J/cm2. The damage morphologies of the films are observed by a metallographic microscope with magnification of 500 times, and the results are shown in Fig. 6. The damage morphologies of HR film are obviously different at different annealing temperatures, which indicate that the causes of damage factors have changed.

Fig. 6. Damaged morphologies of HR films under the laser energy density of 25.5J/cm2.

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The damaged morphologies are similar when annealing is below 300°C. The damage morphologies are captured by delamination, and there are membranes warping at the edge of the damage; the boundary of damaged areas and undamaged areas is distinct. The delamination phenomenon of HR films is mainly caused by thermal stress. The temperature rises sharply at the laser spot center and forms a temperature difference with the surroundings. Thermal expansion caused by temperature difference eventually produces thermal stress in the film. The relationship between thermal stress and temperature difference can be written as^[22]σ=ηE3(1−2ν)ΔT,(4)where σ represents the thermal stress of HfO2 thin films, η is the thermal expansion coefficient of HfO2 films, E is the elastic modulus of HfO2, ν represents the Poisson ratio of HfO2, and ΔT is the temperature difference of the film produced by laser irradiation. When the thermal stress σ reaches the critical stress value, damage occurs in HR films, and delamination is formed. The delaminated damage in the HR film is observed when annealing is below 300°C. The improvement of LIDT of HfO2/Ta2O5/SiO2 can be attributed to the decrease of the absorption coefficient and the increase of thermal conductivity and density.

When the annealing temperature exceeds 400°C, the damage morphologies are the craters with broken spots, the edges of damaged areas are surrounded by ablation pattern, and the surface roughness of the damaged area is increased. The boundary of damaged areas and undamaged areas is blurred. This denotes that the damage is mainly caused by thermal explosion. Thermal explosion is caused by defects points, including inclusions, nodules, nonstoichiometric defects, and particles. For particles, when the film becomes too dense, the particles will exert pressure on the film. Under laser irradiation, the temperature of particles introduced in the preparation will increase sharply. High temperature results in vaporization of film material around particles and generates high-temperature plasma. The high temperature plasma continues to heat up and produces shock waves in subsequent laser irradiation, finally breaking through the bondage of the film layer and producing thermal explosion damage. After high temperature thermal annealing, the HfO2/Ta2O5/SiO2 HR films become denser. The thermal expansion of impurity particles in the HR film is limited, resulting in a high-pressure exertion on the film. The laser damage thus occurs easier^[24,25]. Therefore, when the annealing temperature is higher than 400°C, the LIDT of the HR film decreases with the increase of annealing temperature, and it decreases sharply to 9.5J/cm2 at 500°C.

In this Letter, the optical properties, microstructure, and LIDT of HfO2/Ta2O5/SiO2 HR films at different annealing temperatures have been studied. After thermal annealing, the spectra of the HR film shift to a short wavelength, and the grain size of HfO2 is increased. With the increase of annealing temperature, the LIDT of the HR films is increased due to the increase of thermal conductivity and decrease of the absorption coefficient. The LIDT of the HR film reaches 22.4J/cm2 at the annealing temperature of 300°C, which is twice as high as that of the films without the annealing treatment. However, when the annealing temperature exceeds 400°C, the damage changes from thermal stress to thermal explosion, resulting in a sharp decrease in LIDT, and the LIDT is only 9.5J/cm2 at 500°C, which is lower than that of the as-deposited HR film.