3.1.

Absorption-Annealing Effects

Figure 1 shows the result of irradiating the ${\mathrm{HfO}}_2$ film for 8 min at a fixed location using a 355-nm, 6-mW beam focused into a submicrometer spot ($\le 0.7\mu \text{m}$, $\sim 1\text{-}\mathrm{MW}/{\mathrm{cm}}^2$ power density). Figure 1(a) is a PHI scan of a $10\times 10\text{-}{\mu \text{m}}^2$ film area centered around the location of the laser spot. A cross-sectional profile of the PHI signal [Fig. 1(b)] shows up to 70% reduction in absorption within the irradiated spot, which appeared to be permanent when confirmed by PHI scans performed after one week and then one month later. Next, absorption annealing was investigated as a function of laser power and exposure time. Figure 2(a) plots a normalized PHI signal’s dependence on exposure time for three different values of laser-beam power: 0.7, 3, and 6 mW. One can see that the main drop in signal takes place during the first minute and is then followed by a slow decline on an $\sim 10$- to 15-min time scale. The initial signal drop becomes faster and deeper with increasing laser power. Nevertheless, the temporal behavior of the 6- and 3-mW curves shows that at long exposures, the signal can eventually be stabilized at the same level. It suggests that within some range of power densities, overall absorption reduction is proportional to both power density and exposure time or, in different terms, locally deposited energy. This notion is strongly supported by the data presented in Fig. 2(b), showing the PHI signal plotted as a function of energy dose (incident power–time product). One can see that the data for different laser powers merge together, thereby confirming that the deposited energy (which is proportional to the energy dose) is a defining factor for producing absorption annealing effect.

Fig. 1

(a) A photothermal heterodyne imaging (PHI) map of a $10\times 10\text{-}{\mu \text{m}}^2$ film area. (b) The signal horizontal profile through the central spot irradiated by a 355-nm, 6-mW cw laser for 8 min.

Fig. 2

(a) Temporal behavior of absorption as a function of cw laser power on sample. (b) Normalized PHI signal for different laser powers as a function of energy dose (incident energy–time product).

In an attempt to demonstrate the possibility of annealing absorption within a sample area larger than the laser spot, we performed square raster scans with linear dimensions of several tens of micrometers. Figure 3 shows corresponding PHI images, each obtained as a result of two scans. In the first image [Fig. 3(a)], the central $20\times 20\text{-}\mu \text{m}$ part was initially scanned with a high laser power of 4.5 mW and a low scan velocity of $\sim 1\mu \text{m}/\mathrm{s}$ to achieve the annealing effect. Subsequently, the larger square area ($30\times 30{\mu \text{m}}^2$), with the same center coordinate, was imaged using a much lower laser power of 1.5 mW and a several-times-higher speed velocity, such that no, or very little, annealing effect was produced by the second scan. A cross-section horizontal signal profile revealed at least a 40% reduction in absorption within the initially scanned area. A similar procedure was used for the $20\times 20\text{-}\mu \text{m}$ image shown in Fig. 3(b), with the only difference being that the central spot was exposed for an additional 8 min using a 6-mW laser power. In this case, the horizontal signal profile shows a 70% reduction in absorption in the central spot. From a practical point of view, it should be noted that because of the scan velocity and beam size limitations (the latter defines the maximum separation between two consecutive scan lines, or minimum number of lines per scan), raster scanning for annealing purposes is very time consuming. For example, it takes at least 2 h to complete a $60\times 60\text{-}\mu \text{m}$ scan using a $1\text{-}\mu \text{m}/\mathrm{s}$ scan velocity.

Fig. 3

PHI maps resulting from 355-nm cw laser annealing by means of raster scanning with a low, $1\text{-}\mu \text{m}/\mathrm{s}$ sample velocity: (a) a $30\times 30\text{-}{\mu \text{m}}^2$ area with the central $20\times 20\text{-}{\mu \text{m}}^2$ part exposed using 4.5-mW laser power; (b) a $20\times 20\text{-}{\mu \text{m}}^2$ area with the central $10\times 10\text{-}{\mu \text{m}}^2$ part raster scan exposed, using 6 mW and, in addition, the central spot exposed for 8 min with 6-mW power; (c) and (d) horizontal signal profiles taken through the central part of (a) and (b) maps, respectively.

The question to be addressed is the possibility of scaling up the absorption-annealing process for ${\mathrm{HfO}}_2$ films used in optical parts for laser applications. In this work, we explored the possibility of producing absorption annealing in a ${\mathrm{mm}}^2$-scale area using a cw ${\mathrm{Ar}}^{+}$ laser having a maximum 351-nm output power of 1 W. In this case, a laser beam having a power of $\sim 900\mathrm{mW}$ was focused into a 50-μm-diameter ($1/e^2$) spot on the sample, which was slowly ($\sim 1\mu \text{m}/\mathrm{s}$) linearly translated for a distance of 3.6 mm. Despite a much lower power maximum density of ${46\mathrm{kW}/\mathrm{cm}}^2$ as compared to ${\sim 1\mathrm{MW}/\mathrm{cm}}^2$ in the case of a small PHI pump laser spot, a much longer (at least $50\times$) exposure time allowed us to achieve an $\sim 50\%$ absorption reduction (see Fig. 4) in the film area of $\sim 0.2{\mathrm{mm}}^2$. In this exposure regime, the energy dose per unit of film area is comparable with an $\sim 0.5\text{-}\min$ exposure using a small beam with a power of 0.7 mW (see Fig. 2).

Fig. 4

(a) A 180-μm PHI scan. (b) Corresponding horizontal signal profile of the hafnium oxide (${\mathrm{HfO}}_2$) film area irradiated by a 900-mW ${\mathrm{Ar}}^{+}$ laser. The sample vertical travel velocity was $1\mu \text{m}/\mathrm{s}$.

3.2.

Laser-Damage Performance of Annealed ${\mathsf{HfO}}_2$ Films

3.2.1.

Nanosecond-pulse damage

Laser-damage performance of thin films is usually strongly linked to the absorption properties of the film material and, therefore, can provide a true measure of absorption annealing. In this work, damage thresholds and damage morphology were investigated for cw laser–annealed film areas and then compared to the damage behavior of unexposed, as-produced film areas. As a starting point, we conducted AFM imaging of the cw laser–annealed film columnar structure, which was then compared to the columnar structure of the unexposed film. High-resolution ($\sim 7\text{-}\mathrm{nm}$) AFM images of these two areas (see Fig. 5) did not reveal any modification caused by the near-UV, cw laser exposure with power densities up to $\sim 1\mathrm{MW}/{\mathrm{cm}}^2$, implying that local heating of the material produced temperatures well below the ${\mathrm{HfO}}_2$ melting point.

Fig. 5

Atomic force microscopy (AFM) images ($2\times 2\mu \text{m}$) of cw laser, unirradiated (a) and irradiated (b) ${\mathrm{HfO}}_2$ film areas.

To evaluate the effect of absorption annealing on ${\mathrm{HfO}}_2$ film-damage resistance, a series of $20\times 20\text{-}\mu \text{m}$ cw laser–exposed areas were produced on a sample and then irradiated by a pulsed laser at fluences exceeding the damage threshold. Figure 6 depicts an optical micrograph of such a film area irradiated by a 351-nm, 0.9-ns pulse with a peak fluence ~30% above the threshold. A square-shaped unaffected area where cw-laser exposure was carried out is clearly visible inside the damaged zone. Laser-fluence estimates show an $\sim 25\%$ increase in damage threshold within the film area subjected to annealing, which unambiguously proves that absorption is reduced in cw laser–exposed film.

Fig. 6

An optical micrograph of an ${\mathrm{HfO}}_2$ film site damaged by a 351-nm, 0.9-ns single pulse. Damage morphology clearly shows a damage-free, $\sim 20\times 20\text{-}{\mu \text{m}}^2$ square area subjected to exposure by a 6-mW, 355-nm cw laser.

A high-resolution AFM map (see Fig. 7) of the sample site, shown earlier in Fig. 6, provides additional information of the impact of annealing on absorption sources in film material. Taking into account that damage morphology is represented by isolated craters, crater depth distribution provides a rough approximation for the localized absorber distribution within the sample material. The crater depth distribution was measured for the damage site area adjacent to the exposed area [Fig. 7(b)], which should provide a reasonable estimate of the initial absorber distribution within the exposed material. One can see that the crater depth does not exceed the 180-nm depth that equals the thickness of the ${\mathrm{HfO}}_2$ layer. This result indicates that annealed absorption precursors are indeed located inside the ${\mathrm{HfO}}_2$ film and not in the ${\mathrm{SiO}}_2$ film or the substrate.

Fig. 7

(a) A $30\times 30\text{-}{\mu \text{m}}^2$ AFM scan of a nanosecond-pulse damaged film site, including part of a cw laser–exposed area. (b) A higher-resolution, $8\times 8\text{-}\mu \text{m}$ scan showing damage craters bordering an undamaged cw laser–irradiated area. (c) Crater-depth distribution confirming the location of absorption sources inside the 180-nm-thick ${\mathrm{HfO}}_2$ film.

3.2.2.

Short-pulse (600-fs) damage

Previous studies⁸ suggest that electronic defects might also play a role in short-pulse (picosecond, femtosecond) laser damage. In this context, the possible impact of the cw laser annealing of the absorption precursors on the short-pulse damage performance is of interest. Similar to the nanosecond-pulse study, near-UV, cw laser irradiation of a ${\mathrm{HfO}}_2$ film was conducted utilizing both a small spot ($\le 0.7\mu \text{m}$) of a PHI laser and a 50-μm spot of an ${\mathrm{Ar}}^{+}$ laser. The results of the 1053-nm, 600-fs pulse damage testing of exposed (sites 1 and 2) and unexposed (sites 3 and 4) film areas are presented in Fig. 8, showing damage morphologies as recorded by an optical microscope. It is evident that the exposed sites show no damage at all while being irradiated by pulses with fluences above the unexposed film threshold of $3.45\mathrm{J}/{\mathrm{cm}}^2$ (compare sites 1 and 3) or show a smaller extent of damage (site 2 versus site 4) as compared to the unexposed sites. Moreover, as evidenced by the PHI image of damaged site 2 [see Fig. 8(b)], the damaging pulse has partially missed the exposed area, which, in the case of a better overlap, could show an even larger difference in the damage scale for sites 2 and 4.

Fig. 8

(a) Optical micrographs of 600-fs pulse-damage morphology. Sites 1 and 2: cw laser irradiated; sites 3 and 4: unirradiated. (b) PHI scan of site 2 showing relative positions of the cw laser–irradiated area and damaging-pulse imprint.

To summarize, near-UV, cw laser annealing improves the damage resistance of $e$-beam–deposited ${\mathrm{HfO}}_2$ films to pulsed laser radiation.

3.3.

Absorption-Annealing Mechanisms

Mechanisms of the near-UV, cw laser annealing of absorption in ${\mathrm{HfO}}_2$ films and related improvements in pulsed laser-damage resistance may be explained if one considers electronic defects as a main source of absorption and damage initiation. Numerous types of defects can exist in ${\mathrm{HfO}}_2$ bulk material with electronic energy levels located inside the bandgap⁹ and even more are expected to exist for ${\mathrm{HfO}}_2$ films caused by the columnar film structure. As suggested earlier,² in the case of near-UV nanosecond-pulse damage, some of these states—as single- and double-ionized oxygen vacancies [${\mathrm{V}}^{+}$, ${\mathrm{V}}^{2+}$, respectfully (see Fig. 9)]—are shallow enough that absorption of 351-nm photons (3.54 eV) can initiate transition of the electron into the conduction band. Further heating of these free electrons by the remaining laser pulse energy can promote electron avalanche formation and damage. The same defect energy levels might initiate multiphoton absorption and damage in the case of short, 600-fs pulses at 1053 nm. Assuming the validity of such a damage mechanism, the cw laser–induced absorption-annealing effect and linked increase in pulsed laser-damage resistance can be explained by depopulation of the absorbing states. The first possible scenario might involve cw laser–excited electron transition into the conduction band, where the electron spends time on the order of 10 ps (Ref. 8), followed by recombination with holes in the valence band or trapping into the deep defect states in the bandgap, as shown in Fig. 9. Modeling this scenario using kinetic equations may provide further clarification of the annealing mechanism. This type of study, as well as extending the investigation of cw laser annealing from monolayers to multilayer systems, should become the subject of future research.

Fig. 9

${\mathrm{HfO}}_2$ energy-level diagram illustrating the possible mechanism of near-UV absorption annealing.

The second absorption-annealing scenario may be linked to the heating of the film material resulting from the absorption of UV-laser photons. Thermal annealing is widely used to improve the mechanical and optical performance of thin films, including laser-damage resistance in the near-UV, as was recently demonstrated¹⁰ for ${\mathrm{HfO}}_2$ monolayer films at a 355-nm wavelength. In that work, the ${\mathrm{HfO}}_2$ film temperature was increased by at least 100°C above room temperature in order to observe the absorption-annealing effect, with the maximum effect obtained at an annealing temperature of 300°C. To evaluate possible thermal effects in our study, the temperature distribution in the ${\mathrm{HfO}}_2$ film was modeled with the assumption that all of the energy absorbed in the film is released in the form of heat. The energy deposition was considered homogeneous in the cylindrical film volume with a diameter equal to the FWHM diameter of the laser beam ($\sim 600\mathrm{nm}$) and a height equal to a 180-nm film thickness. The geometry of the model is shown in Fig. 10(a). The cw laser intensity was fixed at $1\mathrm{MW}/{\mathrm{cm}}^2$ (highest used in the experiment), and the energy deposited in the film was varied through variation of the film absorption in the range of 10 to 1000 ppm, with the upper absorption boundary (1000 ppm) being well above the film absorption estimated from photothermal measurements. Heat conduction was considered to be the only channel of energy dissipation, and temperature rise in the film was obtained by solving appropriate heat-conduction equations.

Fig. 10

Peak temperature rise distribution in a thin-film sample: (a) modeling geometry and distribution in $x–z$ plane ($y=0$); (b) lineout along $z$ axis ($y=x=0$); (c) lineout along $x$ axis ($y=z=0$) ; and (d) time evolution of the maximum temperature rise.

Table 1

Parameters used to calculate the rise in hafnium oxide (HfO2) film temperature.

Finally, it should be noted that irradiation of thin films by cw lasers with power densities used in this work can produce different absorption-modification effects. For instance, in similar experimental conditions,¹² irradiation of ${\mathrm{TiO}}_2$ monolayer films using an 800-nm cw laser caused an increase in absorption that was also exposure-time dependent. Consequently, absorption-modification effects are thin-film material and cw laser wavelength specific.