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Abstract
As a material that exhibits a high refractive index and an excellent chemical stability, Ta2O5 is widely used in the microelectronics and chemical industries. In this study, we analyzed the effect of plasma shock waves on Ta2O5 thin films. The experimental results showed that the damage characteristics of the film include thermal ablation, surface cracks, and peeling off from the substrate owing to the repeated action of the pulse. Coating characterization before and after exposure to laser plasma was performed using UV-Vis spectroscopy, energy dispersive spectroscopy (EDS), Raman spectroscopy, and X-ray diffraction. With the increase in the light absorbed by the ablated products, the transmittance of the film decreased after damage, and the Raman spectrum and X-ray diffraction pattern of the sample indicated that the film changes from an amorphous to a crystalline structure owing to the action of the high-temperature and high-pressure plasma. The EDS analysis results indicated that the oxygen content in the film decreases because of the damage inflicted to its structure.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Owing to its excellent chemical stability, high dielectric constant, and high refractive index, Ta2O5 is widely used in the chemical and microelectronics industries and has great potential applications in optical components with high damage threshold [1–3]. Ta2O5 is a semiconductor, exhibiting a wide optical band gap (∼4 eV). Owing to these excellent characteristics, Ta2O5 is widely used in solar cells and optical waveguides [4,5]. Most studies on Ta2O5 films reported different deposition methods such as chemical vapor deposition, ion beam sputtering (IBS) deposition, and physical evaporation deposition [6–8], along with pretreatment methods, such as ultraviolet (UV) annealing, pure oxygen thermal annealing, and combinations of the above methods, in preparing high-quality Ta2O5 films [9,10]. Research on the laser-induced damage characteristics of Ta2O5 films mainly focused on the effect of film defects on the damage threshold under laser irradiation, followed by damage mechanism analysis.
Typically, plasma is generated when local defects on the surface of optical elements cause laser breakdown, and subsequently, the plasma damages the adjacent films. The mechanism involved in the laser plasma damage of the adjacent films lacks systematic understanding. Therefore, to understand the influence of plasma shock waves on Ta2O5 films, experimental and theoretical studies were carried out on the laser plasma-induced damage of Ta2O5 films. This study provides a reference for the evolution of the damage inflicted to films by multi-pulse lasers.
2. Experimental procedure
We used an electro-optical Q-switched Nd:YAG pulsed laser (f = 1 Hz, λ = 1064 nm, pulse width = 12 ns). The pulsed laser beam passed through a beam splitter (exhibiting a transmission-to-reflection-energy ratio of 8:2). This ensured that a part of the laser beam reached the energy meter, which monitored the laser energy, and the other part was focused using a focusing mirror with a focal length (f) of 200 mm. Laser plasma was generated at the focus of the lens, located on top of the sample, and the sample was placed on a three-dimensional moving platform for multiple experiments. Figure 1 shows the experimental setup.
2.1. Sample preparation
A Ta2O5 (99.99% purity) film sample was deposited on a K9 glass, after the substrate was properly cleaned by placing it in a 3% NH4HF2 aqueous solution for 8 min, followed by ultrasonication using petroleum ether. A coating machine (NPBF series, OPTORUN, Japan) and a radiofrequency (RF) ion source (VEECO, USA) were used to excite the plasma. During the deposition process, the energy of the RF ion source was precisely controlled, and pure oxygen was used as the working gas. During the deposition of the film, the vacuum level was measured and controlled using a HY9940C composite pressure controller. The thickness of the film was monitored using an optical monitoring system. The purity of the Ta2O5 film was 99.99%, and the base pressure of the chamber was in the range of 4.6–6 mbar. Vacuum baking was carried out at 200 °C for 40 min. The thickness of the film was 4.7 µm, and the radius of the RF source was 8 cm. The corresponding operating parameters were as follows: bias voltage (800 V), acceleration voltage (340 V), and bombardment current (400 mA).
2.2. Laser plasma-induced damage
To study the damage characteristics of the Ta2O5 film, which gradually changed under multiple pulses, we performed multi-pulse experiments in different regions of the same sample. Figure 2 shows the damage morphology of the film due to the laser plasma when the distance between the plasma core and the surface of the sample (d) was 0.2 mm. Figure 2(a) shows the damage morphology corresponding to a laser energy of 159 mJ after one pulse. A small black point can be observed below the plasma core, which represented the damage due to the high-temperature plasma core. The small point is surrounded by a largely circular area where the film melted owing to the high temperature. Moreover, the damage morphology of the film varies with the number of pulses. When the number of laser pulses was three or higher, the damage morphology of the film can be divided into three regions: A, B, and C, as shown in Figs. 2(b)–(f). The degree of ablation of regions A, B, and C gradually reduced. Unevenly distributed black dots can be observed in the three regions, and the number of small dots increased with the increase in the number of pulses. When the number of laser pulses was 8 or 10, a shape similar to a water ripple can be observed in the A region. We suspected that this was due to the shock wave front reaching different positions on the surface of the film after multiple pulses. When the number of pulses increased to 15, cracks were observed in the B and C regions. Most cracks were observed at both ends of the laser incident direction, with fewer cracks in the direction perpendicular to the laser incident direction.
Fig. 2. Laser plasma-induced damage morphology of Ta2O5 film after: (a) 1, (b) 3, (c) 5, (d) 8, (e) 10, and (f) 15 pulses.
If the number of laser pulses is greater than 15, the overall morphology of a damaged film cannot be observed using a high-power optical microscope. In this study, we used a low-resolution optical microscope to observe the damage morphology of the film, which is shown in Fig. 3. The overall damage can be characterized by an irregular elliptical shape, and the film on both sides of the ellipse exhibited obvious signs of peeling off. We divided the damaged area into three regions: A, B, and C. The A region was mainly an ablated region, the B region included a partially ablated region and an unablated region, and the C region was mainly a transition region. Several ablation points could be observed in area A, as shown in Figs. 4(a) and (b). These points were mainly due to the high-temperature plasma melting the thin film material. At the same time, because of the rapid decrease in the plasma temperature with the propagation time of the shock wave, the molten material cooled and solidified on the surface of the film to form ablation points of different sizes. Cracks were also observed in the A region. The damage in the B region can be largely characterized by disordered cracks, as shown in Fig. 4(c). As shown in Fig. 4(d), the film starts to peel off from the substrate. The region C was mainly characterized by partial thermal ablation and a few cracks.
Fig. 4. Microstructure of the damaged areas of the Ta2O5 film: (a) and (b) region A in Fig. 3, and (c) and (d) region B in Fig. 3.
3. Results
3.1. Film characterization methods
To study the microscopic changes in film after being exposed to plasma, we used various methods to test the changes in the physical state and crystallization of film before and after damage. The obtained results could lay a foundation for further theoretical research. To compare the absorption of light in the smooth and damaged areas, the transmission spectra of film were measured using an UV-Vis spectrophotometer (Shimadzu, Japan) in the 190-to-1100 nm range. A Raman spectrometer system (LabRam HR, Horiba-Jobin Yvon, France) equipped with a 532 nm laser excitation source was used to determine the fracture characteristics of the film. The X-ray diffraction (XRD) data were recorded using an automated diffractometer (Rigaku D/M-2200 T, Japan). The chemical composition of film was obtained using an Octane Plus (EDAX, USA) attachment.
3.2. UV-Vis spectroscopy
Figure 5(a) shows the transmission spectrum of Ta2O5 film before and after damage. When the surface of film was damaged, the transmittance of the film in the visible light range was significantly reduced.
The band gap energy (Eg) of the materials can be calculated using the Tauc formula [11]:
Here, A is a constant, α is the absorption coefficient, h is the Planck’s constant, v is the frequency of the incident photons, and hv is the photon energy. The absorption coefficient α of the film can be calculated using the thickness d and transmittance T of the sample, as follows: The band gap of film could be obtained by plotting (ahv)1/2 as a function of hv, as shown in Fig. 5(b). The band gap of film was obtained by fitting and extending the linear portion of the spectrum for α = 0. The optical band gaps of the smooth and damaged areas of the film were 4.02 and 3.99 eV, respectively. When the surface of film was ablated, the optical band gap slightly reduced. This change could be attributed to an increase in the surface defects after film damage, resulting in a decrease in the optical band gap.3.3. Energy dispersive spectroscopy analysis
We performed an energy dispersive spectroscopy (EDS) analysis to study the variations in the chemical composition of the film before and after the damage. According to the deposition conditions of the film, the film mainly consisted of O and Ta. Figure 6 shows the variations in the elemental composition of the film before and after the damage. In addition to O and Ta, the presence of C was observed in the film, attributed to the adsorption of organic matter in air on the surface of the film. The data in Table 1 indicate that the O content of the damaged film is lower than that of the undamaged film. Therefore, we concluded that during plasma exposure, the original structure of the film breaks, resulting in the loss of oxygen.
Table 1. Elemental composition of the Ta2O5 film before and after damage.
3.4. Raman spectroscopy
Raman spectroscopy was used to characterize the typical damaged and smooth regions. Figure 7 shows the results. According to Perez et al., the spectral range (0–1000 cm−1) can be divided into three regions [12]. In the low-energy range (v < 100 cm−1), phonon modes originate from the interactions between different Ta polyhedral and TaOn5−2n and/or Ta6O126+ clusters. The mid-energy Raman bands (100 < v < 450 cm−1) in such oxide compounds generally correspond to O–Ta–O bending coordinates in different types of polyhedral might contribute to several of these bending modes. The higher energy bands (450 < v < 900 cm−1) could be associated with coupled modes that mainly involved the stretching of various Ta–O bonds present in the structure, with different bond order magnitudes [13,14]. The peak at 240 cm−1 can be assigned to the bending mode of Ta-O-Ta in TaO6 and the peak at 676 cm−1 in the middle energy band to the vibration of Ta-O in TaO6 [15]. The peaks at 806, 797, and 794 cm−1 correspond to different bond strengths for stretching that may be assigned to the Ta-O bond. As the film was deposited on a K9 glass (contains approximately 13% B2O3) substrate [16], some features of the spectrum are similar to those of the spectrum of silica glass. Here, we describe some characteristic peaks by Raman spectroscopy of fused silica and K9 glass substrate. The 478 cm−1 band can be assigned to a vibrationally isolated Si-O-Si, Si-O-B mode of the four-membered rings, which have small inter-tetrahedral angles [16], and the intensity of the Si-Si stretching modes of the substrates may have contributed to the presence of the 490 cm−1 band [11].
3.5. Crystalline structure
Figure 8 shows the XRD patterns of the as-deposited and damaged Ta2O5 films. The deposited film did not present any significant peaks, indicating that the structure of the deposited film was not crystalline. In contrast, the Ta2O5 film presented significant peaks in the damaged area after being exposed to laser plasma (2θ = 23.103, 28.521, 36.885, 50.914, 55.886, and 59.043°), indicating that the damaged film presented a crystalline structure. The XRD pattern of the damaged film could be ascribed to the hexagonal δ-Ta2O5 phase (PDF 19-1299, lattice parameters: a = b = 3.6240 Å, c = 3.88 Å, α = β = 90°, and γ = 120°) or hexagonal δ-Ta2O5 phase (PDF 18-1304, lattice parameters: a = 7.24 Å, b = 7.24 Å, c = 11.61 Å, and α = β = 90°, and γ=120°).
To identify the crystal phases after damage, we compared the experimental diffraction pattern of the damaged area of the film with the reference PDF 19-1299 and PDF 18-1304 patterns (Fig. 9), and observed that the peaks of the hexagonal Ta2O5 phase (PDF 18-1304) are in good agreement with the experimental data in the strong peak range. However, for the entire region of the spectrum, the low-intensity diffraction peaks did not exactly match the results in this study. Conversely, the test results for the PDF 19-1299 were a better match throughout the entire spectral range, particularly for some small diffraction peaks. Therefore, we concluded that the crystal phase of the damaged Ta2O5 film is mainly a hexagonal Ta2O5 phase (PDF 19-1299), though PDF 18-1304 was also present.
Fig. 9. XRD patterns of the damaged area of the Ta2O5 film compared with reference Ta2O5 (PDF 18-1304) and Ta2O5 (PDF 19-1299) patterns.
According to the classical and solid phase nucleation theories, unstable amorphous structures change into stable states during heating, because the free energy of the amorphous phase is higher than that of the crystalline phase. Compared with crystalline structures, amorphous structures are metastable structures, which tend to crystallize, and high-pressure heating processes contribute to reducing the critical free energy required to form a stable core. A certain energy barrier must be overcome when amorphous materials convert into crystalline ones, as a certain amount of diffusion activation energy is required for the conversion. According to many reports [7,17,18], the temperature required for crystallizing amorphous Ta2O5 films in air is approximately 873 K. In this study, however, when the shock wave front of the laser plasma reached the surface of the film, the temperature was much higher than 873 K, and therefore, it was enough to overcome the potential energy required for an amorphous-to-crystalline phase transition. Moreover, the critical free energy required to form a stable core was reduced owing to the high pressure [17]. Therefore, although the plasma shock wave acted only briefly on the surface of the film, the film could still be crystallized.
4. Theoretical analysis
The microscopic changes in the film were closely related to the effects of the laser plasma. We systematically studied the main action characteristics of the plasma and combined them with the changes in the physical state of the film to analyze the effect. This contributed to a deeper understanding of the effect of laser plasma.
4.1. Effect of shock wave
The pulsed laser was focused at the focus of the convex lens. When the energy of the laser was high enough (intensity level was in the order of hundreds of MV/cm2 or higher) [19], the air at the focus broke down, generating a plasma core. The plasma core expanded to the limit of the expandable diameter and subsequently saturated. The hot compressed air layer around the plasma core separated after a certain time. When the plasma core stopped expanding, shock waves were generated. This process was similar to the point explosion model. However, in actual situations, the velocity of the shock wave would decrease to the speed of sound as the propagation distance increased. According to the Taylor point explosion model, the velocity of the shock wave would eventually become zero; this would contradict the actual situation. In this study, we used the modified point explosion model to determine the variations in the radius and velocity of the plasma shock wave with time, as follows [20].
In Eqs. (5) and (6), Q is the absorbed energy, ω is a constant (which was approximately 2 in air) used to solve the aerodynamic equation using the numerical method, ${\rho _0}$ is the undisturbed gas density, N represents the dimensions of the system (N = 3, 2, and 1 represent spherical, cylindrical, and planar shock waves, respectively), and M is the Mach number. Ignoring the loss of energy, the pressure of the shock wave front can be expressed as:
As shown in Fig. 10, the initial pressure of the plasma shock wave is approximately 50 MPa, and the pressure of the shock wave front rapidly decreases with the increase in the transmission distance. As the velocity of the shock wave in the axial direction (laser incident direction) was approximately 1.6 times the normal velocity [21], the plasma shock wave exhibited an ellipsoidal shape with the increase in the transmission time, forming a long elliptical shape on the surface of the film, as shown in Fig. 3. According to the relationship between the velocity of the shock wave and the pressure, the pressure in the axial direction was approximately 2.56 times the normal pressure. The shedding of the film occurred at the edges of the damaged area rather than at its center (Fig. 3). When the shock wave propagated along the axial (laser incident direction) and normal directions, it came into contact with the surface of the film under certain angles in the non-axial and non-normal directions. Therefore, the film under the plasma core could not be detached from the substrate, whereas the film at the edges of the damaged area could detach from the substrate much easier owing to the combined force of the axial and normal pressures.
4.2. Shock wave propagation in multilayer structures
When shock waves propagate from air into films and substrates, they are reflected and transmitted at the interfaces of the two media owing to the differences in the material of the air-film-substrate system. According to the continuity condition and Newton’s third law, the relationship between the reflected (R) and transmitted (T) waves on both sides of the interface can be expressed as follows [23]:
Figure 11(a) shows the propagation diagram of a shock wave in an air-film-substrate system. Figure 11(b) shows the partial enlarged view thereof. When the shock wave reached the air/film interface, reflection and transmission occurred. According to the acoustic impedance ratio of the material, the reflected wave was a compression wave, and the transmitted compression wave continued to reflect and transmit at the film/substrate interface. As the acoustic impedance of the film was greater than that of the substrate, the reflected wave was a stretch wave. Ignoring the attenuation of the shock waves in the film, the intensity of the tensile wave was calculated to be approximately 28 MPa. As the number of pulses increased, the film structure gradually softened, and therefore, a large number of cracks could be observed on the surface of the film. At the same time, owing to the relationship between the plasma shock wave and the propagation distance in air, the shock wave that reached the film under a certain angle generated a horizontal component on the surface of the film; therefore, the film peeled off from the substrate (Fig. 4(d)).
4.3. Thermal effect
When a high-temperature and high-pressure plasma core is formed, a plasma shock wave is formed as the plasma core expands. The pressure of the shock wave and the temperature of the wave front drop rapidly as the propagation distance increases. The gas density of the shock wave front can be expressed as [22]:
In the local region, plasma expanded adiabatically. Hence, the temperature(T) at the shock wave front can be expressed as:Here, R is the gas constant, and P is the pressure of the shock wave front. The temperature of the shock wave front varies with the propagation distance, as shown in Fig. 12.
Figure 12 shows the relationship between the temperature of the shock wave front and the propagation distance. The initial temperature of the plasma shock wave front could reach 104 K, but decreased rapidly with the increase in the transmission radius. The shock wave with a high temperature and pressure reached the region of the film directly under the plasma core; the temperature and pressure then rapidly decreased along the radial direction of the film surface. Thus, the temperature and degree of ablation of the region directly below the plasma core were the highest, as depicted in Fig. 2. The melting area of the film gradually increased under the action of repeated pulses. The ablative depth was the largest in the direction perpendicular to the plasma core, it gradually decreased in the radial direction, and it was funnel-shaped. Regions of different colors were formed owing to the interference of light under the optical microscope used to analyze the sample. In addition, the plasma shock wave only existed for a very short period of time, the melted film rapidly solidified after the shock wave acted upon it, and afterward a large number of damage points were formed on the surface of the film (Fig. 4).
4.4. Influences of oxygen vacancy in the film on the band gap
According to the results of the reduction in the oxygen content of the damaged film in EDS. To investigate the band gap of the Ta2O5 film with and without oxygen vacancy, a first-principle analysis was carried out based on the density functional theory (DFT) and Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA). As crystalline Ta2O5 is often used to calculate the band gap, hexagonal Ta2O5 (ICSD 18-1304) was first chosen for the calculation. The space group was P6/mmm, and the lattice constant a = b = 7.24 Å, c = 11.61 Å, α = β = 90, γ = 120. The cutoff energy was set to 340 eV, and the K-points grid was set to 4*4*2. The total energy converges to 2 e−6 eV/atom. An oxygen vacancy was created by removing an oxygen atom closest to the cell center. Figure 13 shows the calculation results.
The band gaps with and without oxygen vacancy were found to be 1.03 and 1.92 eV, respectively. These values are significantly lower than the experiment values, because of the well-known intrinsic factor of the DFT. However, it does not affect the analysis of the characteristics of the film before and after the damage. According to the results, when oxygen vacancies are present in the film, the microscopic band gap of the film reduces to some extent. At the macro level, the transmittance of the film is also reduced. The results are consistent with those reported by Xu et al [25].
5. Conclusions
In this study, the damage characteristics of Ta2O5 film induced by laser plasma were studied using UV-Vis spectroscopy, Raman spectroscopy diffraction, and EDS. According to the experimental results, the film was damaged because of multiple effects including shock and thermal effects. As the plasma expanded the compressed air to generate a plasma shock wave, the shock wave adiabatically expanded within a certain range, and consequently, the high-temperature and high-pressure shock wave front reached the surface of the film and caused the film to melt, break, and peel off from the substrate. During the process, the increase in the ablation products led to an increase in the defects on the surface of the film, thereby increasing the absorption of light and decreasing the transmittance. However, the critical potential energy required for the film to transition from an amorphous to a crystalline state decreased because of the high temperature and high pressure. Thus, even when the plasma shock wave had a short action time with respect to the film, the film could still be crystallized. Moreover, the EDS test results showed that the oxygen content in the film decreases after the plasma shock wave. According to the first-principle calculation, when an oxygen vacancy exists in the film, the energy band decreases. This result is consistent with the results measured via UV-Vis spectroscopy. This study provides a better understanding of the characteristics of Ta2O5 films during laser plasma-induced damage.
Funding
Department of Science and Technology of Sichuan Province (2018FZ0032); National Natural Science Foundation of China (NSFC) (U1730141).
Acknowledgments
We would like to thank Sheng Jing for the useful discussion on numerical simulation.
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