The Laser-Induced Damage Threshold (LIDT) and damage morphologies of a Ta2O5/SiO2 double cavity filter irradiated by 1064-nm, 10-ns pulses were investigated. The depths of flat bottom pits were examined by an optical profiler and then calibrated according to the Electric-Field Intensity (EFI) distributions and the cross-sectional micrographs obtained using the Focus Ion Beam (FIB) technology. The statistics for depths of 60 damage sites suggested that the Ta2O5 over SiO2 interface was more vulnerable to Laser-Induced Damage (LID) than the SiO2 over Ta2O5 interface. After examining the morphologies of interfacial delaminations carefully, we found that the Ta2O5 over SiO2 interface instead had stronger mechanical strength. So, the higher density of susceptible defects at the Ta2O5 over SiO2 interface was considered to be the reason that LID was preferentially initiated at this type of interface. Based on the above findings, a phenomenological model was proposed to describe the formation of flat bottom pits.
© 2013 Optical Society of America
Band-pass filters possessing a region of transmission bounded on either side by regions of rejection have found applications in high power laser systems [1–4]. Strong resonant electric- field forms in the band-pass filter, which makes its damage behavior quite different with that of the widely studied high-reflectance coating [5, 6] or antireflection coating [7, 8]. McInnes et al. studied the damage characteristics of a ZnS/MgF2 Fabry-Perot filter irradiated by a 1064 nm nanosecond pulsed laser . They found that the LID was initiated at the boundaries of the ZnS spacer and interpreted the interfacial damage from two aspects. First, the interfaces were speculated to be regions of high defect density and weak mechanical strength. Second, the maximal EFI came into being at both interfaces between the spacer layer and the adjacent layers. These two factors resulted in preferential LID at the spacer interfaces. They also proposed an empirical relation LIDT × EFI ≈constant, suggesting that smaller EFI at interfaces leads to higher LIDT. Subsequent study of ZnS/MgF2 Fabry-Perot filters by Hu et al. focused more on investigating the damage morphologies and the damage process . According to their temperature field calculation, the temperature peak near the interfaces of the spacer layer was greater than 2000 K during the irradiation of the 10-ns, 1064-nm laser pulse. Hu et al. then proposed that the vaporization of ZnS spacer layer played an important role in creating the observed bubble-like damage morphologies. Gao et al. investigated the damage behavior of a Ta2O5/SiO2 Fabry-Perot filter irradiated by 1064-nm, 10-ns laser pulses and also found that the damage was initiated from the Ta2O5 spacer interfaces where the EFI is maximal .
Despite the above findings of interfacial damage, two important issues were not addressed in previous researches of Fabry-Perot filters, which will be discussed in this work. The first is the neglect of distinguishing the vulnerability of the interface type. There are two types of coating interfaces which are the high-index material over low-index material (H/L) interface and the low-index material over high-index material (L/H) interface. Due to the different properties of high-index and low-index materials, the vulnerability of two types of interfaces is likely to be different. There is already a report that the H/L interface was more susceptible to LID than the L/H interface in an HfO2/ SiO2 high-reflectance coating working at 1064 nm . Considering the different growth conditions of two types of interfaces for the crystalized HfO2 layers and the amorphous SiO2 layers, it was suggested that H/L interface either had a higher density of susceptible defects or a weaker interfacial strength. In Fabry-Perot filters, the maximum EFI at two types of interfaces is always identical, so finding out which type of interface is more vulnerable to LID is critical to understand the damage initiation, the damage formation process and the final damage morphologies. The second is the absence of a detailed model describing the formation of the flat bottom pits. The morphology of interfacial damage as well as its behavior suggests that it is initiated by localized nanoabsorbing defects. Although phenomenological model based on thermal explosion has been proposed to describe the crater formation that is initiated from nanoabsorbing defects inside the film, at subsurface of the polished substrate or at the coating interface [10–12], whether these findings apply to the interfacial damage in Fabry-Perot filters needs to be verified because the EFI in a Fabry-Perot filter is more than ten times stronger than that in previously studied coatings.
In this work, a Fabry-Perot filter with high transmission at 1064 nm and rejection at 1053 nm was studied. HfO2 was not selected as high-index material, because its inhomogeneity impacted the accuracy of our optical monitoring and resulted in critical thickness errors , which severely degraded the optical performance and caused difficulty in accurately determining the EFI profile of the prepared Fabry-Perot filter. Instead, Ta2O5/SiO2 combination was used because they have reasonable high-LIDTs at near infrared region  and can be precisely fabricated . The damage characteristics of the Ta2O5/SiO2 Fabry-Perot filter was studied to explore the following issues: 1) which type of interface is more vulnerable to LID; 2) what is the main factor causing one type of interface to be more vulnerable than another; 3) what is the detailed process of the formation of damage sites. Section 2 presents the experimental design, sample preparation and damage testing conditions. Section 3 discusses the damage morphologies, damage initiation and damage evolution in the Ta2O5/SiO2 Fabry-Perot filter from the aspects of EFI distributions and interface types. Finally, section 4 presents our conclusions.
2.1 Design of the Ta2O5/SiO2 double cavity filter
A Ta2O5/SiO2 double cavity filter working at 4 degree incidence angle was used to separate two pulsed lasers operating at 1053 and 1064 nm. Either a high-index spacer layer or a low-index spacer layer can be used in a Fabry-Perot filter to achieve high reflectance at 1053 nm and high transmittance at 1064 nm, but which type of design is more damage resistant and more optimal for the study of interfacial damage is worthy of analysis. Two designs using high-index spacer layers and low-index spacer layers are first compared. The first design is [Glass:(LH)^6L2HL(HL)^6H(LH)^5L2HL(HL)^5H2L:Air] and the second design is [Glass:(HL)^5H2LH(LH)^5L(HL)^5H2LH(LH)^52L:Air], where H means quarter-wave Ta2O5 layer and L means quarter-wave SiO2 layer. These two designs have similar spectral performance but quite different EFI distributions in vicinity of the spacer layers, as shown in Fig. 1. At 4 degree incidence, the S- and P- polarized transmittance and EFI distributions are very similar, so Fig. 1 only shows the S-polarized transmittance and EFI distributions. For the design using high-index spacer layers, the maximal EFI occurs at the spacer interfaces. Whereas, for the design using low-index spacer layers, the maximal EFI comes into being in the middle of the spacer layers, and the second strongest EFI at the interfaces already reduces by more than a half. Comparing the laser damage resistance of bulk materials and interfaces, it is generally accepted that low-index materials have the highest LIDTs, followed by high-index materials and the interfaces. So the Fabry-Perot filter using low-index spacer layers must be more damage resistant, and we hardly understand why all previous studies on LID of Fabry-Perot filters used high-index spacer layers. In this work, the design having low-index spacer layers was used, which has another advantage on distinguishing which type of interface is more vulnerable to LID. As marked in Fig. 1, the spacing between the H/L and L/H interfaces for the design using low-index spacer layers is about 2.5 times wider than that of the design using high-index spacer layers, which makes it easier to judge whether the damage is initiated at the H/L or L/H interface by counting the depth of the damage sites. Using high-index spacer layers may be the reason that previous studies on Fabry-Perot filters failed to distinguish which type of interface is more vulnerable to LID.
Interfacial damage initiates from nanoabsorbing defects, where the plasma formation around nanoabsorbing defects plays an important role in forming the damage craters. It has been reported that the preferential plasma growth in the direction of the laser-beam source affected the damage characteristics . So, two irradiating directions from air-film and substrate-film sides are considered in this work. The EFI distribution in the Ta2O5/SiO2 double cavity filter is sensitive to the incident angle. An error of one degree in incident angle, plus or minus, results in a significant increase of EFI in vicinity of the spacer layer adjacent to air-film interface, as shown in Fig. 2. Due to the limited adjustment accuracy of the incident angle in the damage test, we purposely selected an incident angle that is a little bigger than 5 degree incidence angle, which leads to the preferential damage in vicinity of the Outer Spacer Layer (OSL) and makes the analysis of the depths of the damage sites easier. To further check whether the interfacial damage nearby the Inner Spacer Layer (ISL) exhibits similar behaviors as the interfacial damage in vicinity of the OSL, an incident angle from substrate-film side irradiation was selected to be a little larger than 6 degree incidence angle, which results in the preferential damage nearby the ISL according to the EFI distribution given in Fig. 2.
2.2 Fabrication of the Ta2O5/SiO2 double cavity filter
Considering that the EFI distribution in a Fabry-Perot filter is very sensitive to humidity-induced thickness shift, the Ta2O5/SiO2 double cavity filter was deposited on 30-mm diameter BK7 substrates by Ion Assisted Deposition (IAD) in an evaporation coater from the OPTORUN CO., LTD. Ta2O5 and SiO2 were used as the evaporation materials. To eliminate the influence of the growth conditions of Ta2O5 and SiO2 films on the vulnerability of interface types, the same deposition parameters were used to prepare Ta2O5 and SiO2 films. The substrate temperature was kept at 200 °C during deposition. The deposition rate was 0.4 nm/s. Oxygen was introduced into the ion source as a reactive gas. The partial pressure of oxygen was maintained at 1.2 × 10−2 Pa during coating. The ion-beam voltage was set to 1100 V and the ion-beam current was set to 900 mA . The film thickness is controlled with both optical and quartz thickness monitors. The spectral performance of the prepared Ta2O5/SiO2 double cavity filter was measured using a Cary-5000 spectrophotometer. Figure 3 shows an excellent agreement between the measured and the theoretical transmittance curves. It means that the EFI distribution in the prepared Ta2O5/SiO2 double cavity filter is almost the same with the theoretical one, which lays a good foundation for the analysis of subsequent LID studies.
2.3 Damage testing conditions and characterization techniques
The prepared Ta2O5/SiO2 double cavity filter was irradiated from the air-film and substrate-film sides by 1064-nm, 10-ns pulses from a Nd:YAG laser having a TEM00 mode, a beam diameter of 1 mm and a repetition rate of 10 Hz. The incident angles were chosen according to the analysis given in section 2.1. The 1-on-1 laser damage testing was performed according to ISO 11254 standard. The fluence corresponding to 50% damage probability was used to obtain representative damage morphologies of the prepared Ta2O5/SiO2 double cavity filter. The depths of damage sites were measured using a Bruker’s ContourGT-X 3D optical profiler. The error inherent in the physical depth obtained from an optical profiler was calibrated by the FIB cross-sectioning of several damage sites. It was believed that the accurate physical depths of the damage sites were obtained. Moreover, the detailed damage morphologies were examined by Scanning Electron Microscopy (SEM) and the interfacial quality of two types of interfaces was compared using the Transmission Electron Microscopy (TEM).
3. Results and discussion
3.1 Preference of the interface type to LID
The fluences corresponding to 50% damage probability are 4.5 J/cm2 and 9.4 J/cm2 for the substrate-film side irradiation at 6 degree incident angle and the air-film side irradiation at 5 degree incident angle, respectively. Referring to the EFI profiles in Fig. 2, it is clear that higher EFI leads to lower LIDT. For each irradiating direction, 30 damage sites were created with the same fluence corresponding to 50% damage probability. Using the ContourGT-X 3D optical profiler, the depth data of these damage sites was collected and analyzed. The depth distribution of damage sites was very narrow and it seemed that most LID initiated at some specific depths. According to our previous study , even highly absorbing 5 nm gold particle could not trigger LID at high fluence if the EFI is close to zero at its location. Based on this understanding, the LID is unlikely to be initiated from the interfaces at which the EFI is close to zero. So the depths where EFI valleys form are excluded from the following analysis, and we focus on the depths where EFI peaks occur. To make our expression more clearly, Table 1 gives the depths at which the strongest and the second strongest EFI occur for two irradiating directions. It is highly probable that most damage sites were initiated at these depths which are within the spacer layers or at interfaces below or above them. The minimal spacing between two possible depths at which the LID is initiated is about 0.3 μm. In order to give the correct depth distribution of damage sites, the depth values obtained by the ContourGT-X 3D optical profiler must be calibrated by the cross-sectional micrographs of the damage sites.
For each irradiating direction, one damage site whose depth occurs the most frequently in all damage depths and another damage site whose depth occurs rarely were selected for FIB cross-sectioning. As shown in the left column of Fig. 4, the typical damage morphology of interfacial damage is the flat bottom pit having a melting layer on the bottom. The initiating points at the center of the melting layers are more visible for shallower flat bottom pits than deeper flat bottom pits. In fact, the depth measured by the optical profiler is from the coating surface to the surface of the melting layer. To correlate this depth to the depth at which the LID is initiated, some assumptions are needed. Upon the careful examination of cross-sectional micrographs in Fig. 4, it is reasonable to assume that the difference of spacing between the points of damage initiation and the surface of the melting layers in different damage sites is much less than 0.3 μm, which therefore has negligible influence on our judgment on the depth at which the LID is initiated. Combining the information of cross-sectional micrographs and EFI profiles, the depths obtained by the optical profiler can then be corrected to the depths at which the LID was initiated. In Fig. 4, the light layers represent Ta2O5 films and the dark layers stand for SiO2 films. Even from the right column of Fig. 4, it is difficult to judge at which depth the LID is initiated. We must resort to the depths at which the EFI peaks form. The damage site in the first row of Fig. 4 was initiated within the OSL, the depth is considered to be 2.2 μm referring to the position of the EFI peak. The rest three damage sites in Fig. 4 were initiated at the H/L interfaces where the EFI peaks occur, and the corresponding depths are 2.5, 6.3 and 6.6 μm respectively.
The depths of all damage sites obtained by the optical profiler were corrected. The corrected depth distributions at which the LID was initiated are given in Fig. 5. The Ta2O5 over SiO2 interface is more vulnerable to LID than the SiO2 over Ta2O5 interface, which is similar to the case of HfO2/SiO2 high-reflectance coating . The irradiating direction or the plasma growth direction seems to have no influence on the vulnerability of interface types. For the Ta2O5 over SiO2 interface, LID happens more frequently at positions of stronger EFI. In spite of infrequency, damage initiating from within the SiO2 spacer layers has been observed. Although the defects in SiO2 film is much less than those at interfaces, LID still could be triggered by very strong EFI.
3.2 Factors causing one type of interface to be more vulnerable than another
Whether the density of susceptible defects or the mechanical strength is the reason that H/L interface is more vulnerable to LID than L/H interface has not been addressed in previous works. Here we try to distinguish which factor plays a dominant role in triggering the interfacial damage in the Ta2O5/SiO2 Fabry-Perot filter. After examining the morphologies of damage sites carefully using the SEM, a hint on which interface has stronger mechanical strength was found. Figure 6 shows the typical delaminations at the border of flat bottom pits. Comparing the contrast of the delaminated surface and the layers, we were surprised to find that all these delaminations initiated from the SiO2 over Ta2O5 interface. Such kind of interfacial damage was caused solely by mechanical stress, which indicates that the Ta2O5 over SiO2 interface has stronger mechanical strength than the SiO2 over Ta2O5 interface. To the best of our knowledge, there is only one paper dealing with the interfacial delamination in Ta2O5/SiO2 multilayers. Grigonis et al. also reported that the delaminations occurred near the SiO2 over Ta2O5 interface rather than the Ta2O5 over SiO2 interface . Therefore, we conclude that the higher density of susceptible defects at the Ta2O5 over SiO2 interface overcomes its stronger mechanical strength and results in preferential damage initiation at this type of interface.
To find out why different type of interface has different defect density and mechanical strength, the microstructure of two types of interfaces was evaluated by TEM. To achieve the best resolution, a very thin Ta2O5/SiO2 multilayer was prepared for TEM observation using the same deposition parameters given in section 2.2. The TEM micrographs have opposite contrast with the SEM micrographs, where the light layers represent SiO2 films and the dark layers stand for Ta2O5 films. Since the same deposition condition was used to prepare the amorphous Ta2O5 and SiO2 layers, the interfacial roughness and diffusion for two types of interfaces are very similar, as shown in Fig. 7. So the microstructure difference between the Ta2O5 over SiO2 interface and the SiO2 over Ta2O5 interface is not the reason that different type of interface has different properties, which is different with the case of HfO2/SiO2 multilayers. Grigonis et al. also did not discuss why the Ta2O5 over SiO2 interface has stronger interfacial strength, and it is still an open question deserving future investigations. Although interfaces have been speculated to be regions of high defect density or high absorption [20–23], it is still difficult to explain why there are more susceptible defects at the Ta2O5 over SiO2 interface than at the SiO2 over Ta2O5 interface. Our intuitive guess is that the chemical bond between tantalum and oxygen is much easier to be broken compared to the chemical bond between silicon and oxygen. The off-stoichiometric Ta2Ox is deposited on the SiO2 layer, which leads to a higher density of nanoabsorbing defects, such as off-stoichiometric oxide clusters or high-density electronic defect areas. Whereas, the stoichiometric SiO2 is deposited on the fully oxidized Ta2O5 layer, which results in a lower density of nanoabsorbing defects. The dependence of defect density on the interface type is another open question. More work is necessary to shed new light on this interesting issue.
3.3 A phenomenological description of the formation of flat bottom pits
The typical morphologies of the flat bottom pits in Fig. 4 have a cylindrical wall with a sharp edge and a melted layer at bottom. According to the thermal-explosion theory and the mechanical strength of interface, a phenomenological model is proposed to describe the formation of the flat bottom pits. The nanoabsorbing defect at the Ta2O5 over SiO2 interface is heated by the laser pulse to a very high temperature at which the defect-surrounding matrix is converted to the absorbing medium through a joint action of multiple mechanisms including photoionization by ultra violet (UV) radiation , thermionic emission of electrons  and the heat-transfer-induced band-gap collapse . Due to the narrower band gap of Ta2O5, the radiated UV energy and ejected electrons are more effectively absorbed in Ta2O5 than SiO2, which produces more free electrons in the conduction band of Ta2O5 rather than SiO2. The free electrons in Ta2O5 are strongly absorbing for 1064 nm laser irradiation, which makes the Ta2O5 layer to be the hottest layer in the Fabry-Perot filter , as marked in Fig. 8. Because the melting points of Ta2O5 (2145 K) and SiO2 (1986 K) are quite close, we propose that this Ta2O5 layer is preferentially heated to the melting temperature, which leads to the macroscopic damage. The highly heated and melted Ta2O5 layer can be treated as incompressible liquid, which creates a strong shock wave, resulting in blistering of the coating from the SiO2 over Ta2O5 interface whose mechanical strength is weaker, as shown in Fig. 8. At the edges of the blister, the coating stress is highly tensile allowing fractures and hence delamination of the coating. The morphologies of the bottom Ta2O5 layers are indicative of melted-material flow and resolidification.
The Ta2O5/SiO2 double cavity filter provides an opportunity to simultaneously expose the two types of interfaces to identical EFI to expose that the Ta2O5 over SiO2 interface is more vulnerable to LID than SiO2 over Ta2O5 interface. The higher density of susceptible defects at the Ta2O5 over SiO2 interface overcomes its stronger mechanical strength and leads to its vulnerability to LID. Moreover, a phenomenological model taking the thermal-explosion theory and the mechanical strength of interface into account can effectively describe the formation of the flat bottom pits and explain the observed damage morphologies. Unfortunately, the reason why two types of interfaces have different mechanical strength and defect density is still not found. According to the TEM micrographs, there is no structural difference between the two types of interfaces. More works are deserved to improve our understanding of the interfacial characteristics.
This work was partly supported by the National Natural Science Foundation of China (Grant Nos. 61235011, 61008030, 61108014, 61205124), and the National 863 Program.
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