Thermal Annealing and Laser Treatment of Sol-gel Derived Zirconia Thin Films

The preparation of inorganic thin films by sol-gel processing has proven to be a flexible and cost-efficient alternative to vacuum-based technologies such as sputtering [1, 2]. The development of wet-chemical coating solutions with a sufficient stability under industrial conditions, however, is a challenging task. In this context the use of soluble precursor powders has been established for film compositions ranging from Al₂O₃ [3], TiO₂ [4], ZrO₂ [5] to lead zirconate titanate (PZT) [6]. As usual for such inorganic compositions film densification and crystallization were performed by thermal annealing.

As an alternative to oven treatment, sol-gel derived films may be annealed by irradiation with a laser source. Different lasers such as ArF [7, 8], KrF [7, 9‐14], HeCd [15] and frequency tripled NdYAG [16, 17] operating at wavelengths below 1 µm have been used. In these cases, selective energy transfer to the film material is assumed which may enable the use of polymer substrates that may not withstand thermal sintering above 150°C. Even though this objective is commonly claimed, in most of the above studies inorganic substrates were used. This is so because organic residues were removed from the films by thermal treatment up to 500°C before the final laser irradiation.

For sintering of sol-gel films on glass and silica substrates the use of CO₂ lasers with a wavelength of 10,6 µm has been reported [18‐21]. As these glasses strongly absorb in this spectral range [22, 23], an indirect heating of the above sol-gel films can be assumed. The local temperatures may well exceed the respective softening point of the glass substrate. Therefore, crystal phases that are not accessible by oven treatment may become accessible [19, 24‐26].

In a previous publication [27] we systematically compared the CO₂ laser treatment of TiO₂ films derived from soluble precursor powders to parallel thermal annealing experiments. Laser power density could well be correlated to furnace temperatures in terms of film thickness and refractive index. However, whereas conventional thermal heating resulted in the anatase phase, exclusively rutile was found in films after laser treatment. In this manuscript we extend these systematic investigations to sol-gel derived coatings of zirconia (ZrO₂) composition.

Thin films in ZrO₂ composition were deposited on borosilicate glass substrates by dip coating. After thermal annealing their thickness and refractive index was measured. From Fig. 1 it can be seen that up to 450 ° a steady consolidation from 350 nm to 125 nm is induced, up to 600°C a constant level is maintained. In parallel the refractive index of the film rises from 1,55 to 1,95 as the annealing temperature is increased.

In Fig. 1b the SEM cross-sectional view of a ZrO₂ coating that had been thermally annealed at 600°C is given. The granular microstructure is very similar to those of its alumina [3] and titania [27, 29] counterparts. Obviously, despite their different compositions and crystallinity sol-gel processing of soluble precursor powders results in comparable textural film features.

With a constant feed rate of 500 mm/s the laser power density was varied for the irradiation of ZrO₂ films that only had undergone drying at 80°C. In Fig. 2a the resulting film thickness is given along with the values for the parallel thermal treatment experiments. The course of the respective data can well be correlated with each other: For a laser power density of 65 W/mm² the shrinkage is similar to samples that only had been dried at 80°C. For 415 W/mm² a densification equal to thermal annealing at 600°C is observed. For the evaluation of the refractive index (Fig. 2b) a similar good correspondence between the two experimental series can be seen: Values of 1,95 are measured for a laser power density of 415 W/mm² corresponding to annealing temperatures of 500 to 600°C.

It has to be emphasized that the laser irradiation in our study is done on as-dried sol-gel coatings and the removal of residual organics as well as densification/crystallization is done in a single irradiation step. In contrast to that most of the previous reports in the literature perform laser sintering on coatings that had experienced pre-treatment in a furnace at temperatures between 300 and 500°C [7, 10, 15, 17, 19, 20, 25, 26].

X-ray diffraction (XRD) experiments were performed on the films in order to determine the crystal phases present. After furnace treatment (Fig. 3a) up to 400°C the coatings remain amorphous, at 450°C first diffraction pattern corresponding to the tetragonal phase of ZrO₂ become apparent. At 500°C the peak intensity increases, above 550°C a saturation is observed. No monoclinic ZrO₂ is observed which corresponds to other sol-gel derived coatings after annealing at 500°C [30]. In contrast to that report, however, our films remain free from cracks throughout thermal processing.

Laser treatment at 64 W/mm² results in the first appearance of tetragonal ZrO₂ (Fig. 3b), above that laser power density this phase coexists with monoclinic ZrO₂. The appearance of this thermodynamically more stable modification takes place in sol-gel derived films at temperatures exceeding 600°C [31, 32]. It therefore can be concluded that the local temperatures during laser sintering well exceed 600°C as suggested by theoretical simulations [19, 23] where surface temperatures exceeding 1000°C were calculated. In addition, it has been found that the crystallization of monoclinic ZrO₂ is favored by high heating rates [33].

As an intermediate result, it has to be noted, that different crystalline phases and thus differences in microstructure result from thermal annealing and CO₂ laser treatment, even though the film thickness and refractive index (Fig. 2) on first sight suggested matchable trends.

In our previous investigation on TiO₂ films, a similar behavior was observed: Laser treatment resulted solely in formation of the thermodynamically stable rutile phase, whereas in conventional thermal annealing only phase-pure anatase was found [27].

It appears from Fig. 3b that the peak intensity decreases when the laser power density is raised to 605 W/mm². This observation may indicate the removal of film material by ablation at high energies as previously reported [34].

The crystal sizes as calculated from the peak width of Fig. 3 by the Scherrer equation are given in Fig. 4. Thermal annealing between 500 and 600°C results in crystallites of tetragonal ZrO₂ with sizes between 15 nm and 20 nm. These diameters basically conform to the size of granular structures found in Fig. 1b. For laser annealing with power densities below 300 W/mm² tetragonal grains in the same order of magnitude are found. Above 300 W/mm² tetragonal and monoclinic ZrO₂ coexist with grain sizes exceeding the values measured for films that had undergone thermal treatment. This observation supports the assumption that the local temperatures during laser irradiation significantly exceed 600°C.

For a given laser power density the beam feed rate determines the photon fluence, in a manner of speaking the local energy input to the substrate: The faster a given beam traverses, the smaller the power delivered to the surface.

With a power density of 414 W/mm² and a feed rate of 500 mm/s a film densification similar to that of thermal sintering at 600°C was obtained (Fig. 2). With this given power output the feed rate of the laser beam was varied from 500 mm/s to 4000 mm/s. In Fig. 5a the resulting ZrO₂ film thickness is compared to that resulting from conventional furnace processing. It can be seen that the laser irradiation can precisely emulate the thermal film densification. The same applies for the refractive index of the ZrO₂ coatings (Fig. 5b).

XRD phase analysis was performed on this sample series. From Fig. 6a it can be seen that a feed rate of 500 mm/s results in a certain ratio of monoclinic to tetragonal reflex intensity, that is shifted to tetragonal ZrO₂ at higher feed rates. At 1500 mm/s the tetragonal phase is detected almost exclusively. Obviously, the energy input under these conditions does not facilitate the formation of the thermodynamically more stable monoclinic phase any more. When the feed rate is raised to 3000 mm/s, the diffraction signal corresponding to tetragonal ZrO₂ is transformed to a broad hump indicating a low level of crystallinity.

Crystallite size analysis reveals that for feed rates of 500 mm/s and 1000 mm/s tetragonal and monoclinic grains with diameters exceeding 30 nm coexist. For thermal treatment between 500 and 600°C only tetragonal crystallites with diameters between 15 nm and 20 nm were found.

Structural uniformity is a key feature for the technical application of thin films. In Fig. 7a representative SEM image for a laser-processed ZrO₂ film is given. In general, no distinctive border between adjacent laser lines (hatch 200 µm) can be distinguished. Within the beam center, some defects are located inside a zone of 70 µm width. The cross-sectional view reveals that in this region the ZrO₂ film spans bubbles that extend into the substrate. The defects obviously result from partial flaking of such ZrO₂ film domes. This observation suggests that the glass substrate is substantially liquefied by the high temperatures that is induced by laser processing. The bubbles are presumably generated by gaseous products that result during the pyrolysis of the sol-gel film. Outside the central defective zone, however, the ZrO₂ film appears homogeneous and uniform similar to samples that have been thermally annealed (Fig. 1b).

Some similar features have been observed for laser-treated TiO₂ films previously studied [27]. In contrast to that report, however, no evidence for partial ZrO₂ removal by ablation could be observed here even though identical laser parameters have been applied. In the whole the ZrO₂ material seems to be more damage resistant than TiO₂ [27].

ZrO₂ sol-gel coatings can successfully be processed by conventional thermal annealing and CO₂ laser treatment. Film properties such as film thickness, refractive index and grain size systematically correlate with laser power density and beam feed rate. Results indicate that laser irradiation locally induces substrate temperatures by far exceeding 600°C that lead to the coexistence of the monoclinic and tetragonal phase of ZrO₂.