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Abstract
Optical properties and ultraviolet laser damage of single-layer atomic layer deposition (ALD) SiO2 films were investigated. ALD SiO2 films of high transparency shows weak absorption at 355nm. The absorption at 355 nm measured by laser calorimeter varies linearly with the film thickness with absorption coefficient of ∼0.76 ppm/nm. Such absorption is considered originating from various point defects in ALD SiO2 film. Fourier transform infrared (FTIR) spectra confirm the presence of point defects in ALD SiO2 films including non-bridging oxygen atoms and residual OH groups. Nanosecond laser-induced damage of ALD SiO2 film at 355 nm was investigated. The damage threshold and damage morphology suggest that laser-induced damage of ALD film is associated with point defect clusters which can absorb enough laser energy to initiate micro-explosion in ALD films. Furthermore, the ALD films were conditioned with sub-nanosecond ultraviolet laser. Significant improvement in damage resistance has been demonstrated after sub-nanosecond laser conditioning. After laser conditioning to 3 J/cm2, the damage threshold of 535 nm thick ALD film increased from 5.5 J/cm2 to 14.9 J/cm2 and improved about 170%. Annealing of point defects by sub-nanosecond ultraviolet laser is supposed to be the reason for the improvement of the damage resistance.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Atomic layer deposition (ALD) is a coating technology based on sequential reaction of gaseous reactants limited at the solid-state surface of the substrate [1,2]. It has been widely used to fabricate functional coatings in a variety of fields including semiconductor, solar energy and optics. Unlike traditional physical vapor deposition methods such as e-beam evaporation, ALD enables optical film fabrication with super-high uniformity and excellent thickness accuracy even over large-area or strongly-curved optical surfaces [3]. In addition, ALD has a particular advantage in production of nanoscale structures on surface that can provide novel optical properties [4]. The inherent characteristics of ALD make it a potential coating technology to fabricate optical devices in high-power laser systems, where the optics usually have large aperture and require high surface quality.
Silica (SiO2) is a common dielectric material in silicon microelectronic devices. Atomic layer deposited SiO2 can act as an interlayer between high-κ materials and Si substrate in field effect transistor as SiO2 possesses the possibility of preventing chemical reactions of high-κ dielectric with Si [5]. Besides, SiO2 is one of the most important low refractive index materials in optical application. To fabricate mirrors or anti-reflectors with multi-layer coatings in laser system, low refractive and high refractive index oxide materials are deposited alternatively. Owing to its large energy bandgap (∼ 9 eV), SiO2 is highly transparent over a wide spectral range of wavelength from near-infrared to ultraviolet. However, in conventional multi-layer stacks, the bandgap of high refractive index material such as TiO2 (∼3.2 eV) or HfO2 (∼5.6 eV) is much narrower than SiO2. When the multi-layer coating is irradiated with high-power laser in ultraviolet (UV) region, high refractive index material could be damaged more easily than SiO2. On the other hand, nanoporous SiO2 thin film has been successfully prepared with porosity gradient by ALD, which is expected suitable for application in antireflection films [6]. Such functional optics fabricated by only one materiel (SiO2) with wide bandgap might have better laser damage performance for high-power laser application [7].
In early years, SiO2 ALD was difficult due to the lack of appropriate precursors. Inorganic silicon precursors (SiCl4, SiCl3H and etc.) were firstly put into use for SiO2 ALD, which had disadvantages of low reactivity, generating corrosive by-product and introducing chlorine impurities in film [8,9]. Aminosilanes were then proposed as more suitable Si precursors for the ALD process of SiO2. Different kinds of aminosilanes, such as bis-dimethylaminosilane (SiH2[N(CH3)2]2, BDMAS), bis-tert-butylaminosilane (SiH2[NH(C4H9)]2, BTBAS), bisdiethylaminosilane (SiH2[N(C2H5)2]2, BDEAS), tris-dimethylaminosilane (SiH[N(CH3)2]3, TDMAS) and etc., have been developed and used for the deposition of SiO2 in a wide range of temperature [10,11]. Using commercial silicon precursors, Putkonen and et al. established thermal and plasma SiO2 ALD processes with very low impurity levels [12]. Later, various properties of ALD SiO2 for optical application were studied including mechanical, structural and optical properties [13,14]. For high power laser systems, laser damage performance of optics is one of the important issues to be addressed. In previous studies, UV laser damage properties of different kinds of SiO2 film have been investigated, where the SiO2 films were prepared through e-beam evaporation, plasma-enhanced chemical vaper deposition or sol-gel coating [15–19]. But for ALD film, most work focused on the damage behavior under near-infrared laser irradiation (1064 nm) [20–25]. To the best of our knowledge, damage properties of ALD SiO2 by intense laser UV irradiation have not yet been investigated extensively, even for single-layer ALD SiO2. In this work, we present an investigation into laser-induced damage of single-layer ALD SiO2 under nanosecond UV laser irradiation at 355 nm. Optical properties, defect characterization and UV laser-induced damage performance of ALD SiO2 with different thickness were studied.
2. Experiment details
2.1 Sample preparation
The ALD SiO2 film samples in this work were prepared on fused silica substrate which were double-side polished with surface roughness of ∼0.8 nm (RMS). All the experiments were conducted on 5 mm thick, 50 mm diameter samples except for laser absorption test which were conducted on 1 mm thick, 25 mm diameter samples. Before ALD coating, all the samples were ultrasonically rinsed with detergent and deionized water. Then the samples were chemically etched in hydrofluoric (HF) acid solution with concentration of ∼5% for 80 mins to remove the absorbent impurities (Ce, Fe and etc.) and passivate the subsurface cracks that were introduced on surface of fused silica optics during the manufacturing process [26]. After that, the samples were rinsed again by deionized water to make the surface clean. The etched depth was about 4 μm which was calculated by measuring the mass reduction of fused silica substrate due to chemical etching. Surface roughness of the etched samples increased slightly to ∼1.1 nm (RMS). Single-layer SiO2 thin films were deposited onto the HF-etched samples by thermal ALD in Beneq TFS500 at substrate temperature of 300 ℃ using BTBAS as Si precursor and O3 as oxygen source. In a typical cycle during SiO2 ALD, the BTBAS pulse time was 0.5 s, followed by a purge pulse of 3 s, an O3 exposure pulse of 2 s and another 3 s purge. Film thickness and growth rate were measured on silicon wafers by the ellipsometer co-deposited with fused silica samples. In our experiment, the growth rate of ALD SiO2 was ∼0.1 nm/cycle. Three sets of fused silica samples were coated by ALD SiO2 film with different thickness (108 nm, 404 nm, and 535 nm) for optical characterization and laser-induced damage test.
2.2 Characterization
Optical properties were measured on the ALD SiO2 film samples, including UV-Vis spectroscopy, laser absorption, and infrared spectroscopy. Optical transparency of ALD SiO2 samples on 50 mm diameter samples in a wavelength range from 200 nm to 1000 nm was examined by transmission spectra conducted on Perkin-Elmer Lambda 950 spectrophotometer. Laser absorption experiment was carried out on 25 mm diameter samples by laser calorimeter (Laser Zentrum Hannover, Germany) working at 355 nm, which can measure absolute surface absorption of optics with high sensitivity under intense laser irradiation. Infrared spectra from 400 cm−1 to 4000 cm−1 were measured on reflection mode by a Fourier Transform Infrared (FTIR) Spectrometer (Bruker VERTEX 80V, Germany) with resolution down to 0.2 cm−1, which can provide information about silica bonding of ALD film. Element analysis on surface of ALD SiO2 film and bare fused silica substrate was performed by energy dispersive spectrometer (EDS) in a scanning electron microscope (JEOL, Japan).
A nanosecond Nd:YAG laser with third harmonic output (355 nm, ∼8.3 ns) was used to conduct laser-induced damage test. P-polarized laser beam was focused onto the sample to form a Gaussian beam spot with effective diameter of ∼424 μm. The samples were placed nearly perpendicular to the beam direction with ALD film at the exit surface of laser beam path. R-on-1 damage test was employed to examine the damage threshold and damage probability of ALD SiO2 samples, in which laser fluence was incrementally raised until damage occurring. Laser-induced damage was monitored by detecting the scattering signal from damage site illuminated by a 532 nm laser diode. After damage test, the damage morphology was captured by a Nomarski microscopy and the depth of damage craters were measured by an atomic force microscope (Park system, Korea).
We also explored laser conditioning effect of ALD SiO2 film. Another sub-nanosecond laser (355 nm, ∼500 ps) was used for laser conditioning of ALD film. P-polarized laser beam was focused onto the sample with beam direction nearly normal to the sample surface. The beam spot on sample was Gaussian shape with effective diameter of ∼500 μm. Laser conditioning (LC) of specific square areas on sample was performed by laser pulse irradiation with raster scan mode. The pulse repetition rate was 50 Hz and the spatial pulse-to-pulse overlap was ∼90%. During each raster scan, the laser fluence maintains constant. After one raster scan cycle is done, the laser fluence was increased and another cycle was started.
3. Results and discussion
Figure 1 shows the optical transmission spectra of ALD SiO2 films with different thickness on fused silica substrates. All the ALD film samples show excellent transparency from 200 nm to 1000 nm as compared to pristine substrates. The transmission spectra of ALD samples have little difference from the spectrum measured on bare fused silica. In transmission spectra, no obvious absorption peak can be observed in the UV range from 200 nm to 400 nm. Further, we checked the weak absorption of ALD SiO2 films by laser calorimetric measurements. In Fig. 2, the absorption of etched fused silica substrate without any ALD films is only about 7.44 ppm (parts per million). In contrast, the absorption on ALD samples were 101 ppm (108 nm thick sample), 306 ppm (404 nm thick sample) and 422 ppm (535 nm thick sample), respectively. The significant difference on absorption result implies that the micro-structure of ALD film is different from that of fused silica. Absorption of ALD film might result from some kinds of defects existing in the ALD deposited SiO2 film. Additionally, it is found that the absorption of ALD films shows nearly a linear relationship with film thickness, which means that the aforementioned defects are distributed homogeneously in the ALD film. The fitted absorption coefficient is about 0.76 ppm/nm.
Fig. 1. Transmission spectra from 200 nm to 1000 nm on bare fused silica and ALD samples with different thicknesses.
Fig. 2. The relationship between film thickness and measured absorption at laser wavelength of 355 nm.
Since ALD film deposition is based on chemical reaction between precursors, examining the chemical bonding status of the deposited film can provide important information about the origin of absorptive defects. Figure 3 shows the IR absorption spectra of ALD SiO2 films. Two main bands associated with Si-O-Si bonding are revealed on the spectra, which are typically assigned as Si-O-Si rocking mode (∼477 cm−1) and Si-O-Si asymmetric stretching mode (∼1120 cm−1) [18,27]. Peaks of Si-O-Si rocking mode and Si-O-Si stretching mode both show reduced intensities and exhibit red-shift compared with the IR absorption of bare fused silica. For instance, on the 535 nm thick ALD film, peak shifts of Si-O-Si rocking mode and stretching mode are 2.3 cm−1 (shift from 479.2 cm−1 to 476.9 cm−1) and 13 cm−1 (shift from 1122.9 cm−1 to 1109.9 cm−1). As shown in Fig. 3(b), peak shifts of the two modes are both linearly correlated with ALD film thickness. As shown in Fig. 3(c), another weak IR absorption band associated with Si-O-Si bending mode at ∼ 785 cm−1 also shows very slight peak shift. The observed features on IR spectra are probably due to the slightly non-stoichiometric nature of ALD SiO2 film. Previous study has shown that ALD SiO2 film is always oxygen-rich with Si:O atom ratio below 0.5 [12]. In our experiment, the atomic ratio between Si and O of ALD film was estimated by EDS measurement. EDS spectra were taken from the 535 nm thick ALD SiO2 film and bare fused silica substrate, respectively. Since EDS is a semiquantitative method to reveal the element content. A relative stoichiometry parameter was introduced to estimate the sub-stoichiometry of SiO2 films, which was defined as (Si:O)ALD film/(Si:O)fused silica. The calculated value was 0.87 (smaller than 1), which means that the ALD SiO2 film is indeed oxygen rich compared with bare fused silica substrate. It means that there could be more oxygen atoms that are not well bridged between Si atoms in ALD SiO2 than in pristine fused silica. The existence of Si-O or Si-O-H non-bridging modes will reduce the proportion of Si-O-Si bridging modes, making the Si-O-Si rocking and stretching mode exhibit peak shift and reduced intensity. As the ALD film thickness increasing, another IR absorption band starts to appear at ∼925 cm−1 [shown in Fig. 3(c)], which corresponds to the SiO-H bending mode [27]. In Fig. 4(d), the FTIR data of ALD film exhibit a wide but weak absorption band from 2500 cm−1 to 3600 cm−1. Such absorption can be assigned to the SiO-H stretching mode [27]. It is noted that the absorption of SiO-H stretching mode on bare fused silica is higher than some of the ALD film samples. That is because before ALD coating, substrate should be warmed up to the reaction temperature (300 ℃). During heating, the OH content on surface will decrease. But on the ALD film, the absorption intensity of SiO-H stretching mode seems increase along with the film thickness. These results indicate that residual OH groups could exist in ALD SiO2 film. In every deposition cycle, the first-half reaction takes place between Si-OH and BTBAS. During the deposition, very small amount of residual Si-OH species will exist in the film and cannot be removed completely because of the steric hindrance effect that NH(C4H9) ligands around the Si center in BTBAS [11]. The steric hindrance effect will cause spatial repulsion, which means that other BTBAS molecule can hardly adsorbed at Si-OH site just under the NH(C4H9) ligand. On this account, OH residuals in ALD film is inevitable whose amount is dependent on the thickness of deposited film. The presence of non-bridging oxygen atoms and residual OH groups imply that there could be more optically activated defects in ALD SiO2 film than in pure fused silica, such as non-bridging oxygen hole centers (NBOHC), E’ centers, atomic vacancies and so on. These intrinsic point defects in ALD SiO2 film usually have sub-bandgap energy levels and can attribute to optical absorption when irradiated by ultraviolet laser at 355 nm. Clusters of such point defects (either called damage precursors) could absorb enough laser energy to initiate laser damage on optics surface.
Fig. 3. (a) FTIR absorption spectra (400 cm−1–1300 cm−1) of ALD SiO2 with different film thicknesses. (b) Peak positions of the Si-O-Si rocking mode and stretching mode in ALD SiO2 films with different thickness. (c) Enlarged FTIR spectra (700 cm−1–1000 cm−1) in the dashed rectangle shown in Fig. 3(a). (d) FTIR absorption spectra (2000cm−1–4000 cm−1) of ALD SiO2 with different film thicknesses.
Fig. 4. (a) Damage probability curves of ALD SiO2 films and bare fused silica. (b) The relationship between laser absorption and damage threshold of ALD films. The red fitting curve is a guide for eyes.
Figure 4(a) shows the R-on-1 damage probability curves of ALD SiO2 film samples performed by nanosecond laser irradiation at 355 nm. Damage thresholds of ALD SiO2 films are 7.92 J/cm2 (108 nm sample), 5.20 J/cm2 (404 nm sample), 5.02 J/cm2 (535 nm sample), respectively, which are significantly lower than the result on bare fused silica substrate (∼13.4 J/cm2). Such result is in line with expectation if we link the damage result with the measured absorption as shown in Fig. 2. In Fig. 4(b), the damage thresholds show strong inverse correlation with the absorption intensity.
Damage morphologies of ALD SiO2 film samples were further investigated by Nomarski microscope and atomic force microscope (AFM). Figure 5 exhibits typical morphologies of initial damage on the three sets of ALD films. On the sample with ALD thickness of 108 nm [shown in Fig. 5(a)], the damage sites are almost pit-like and dispersed separately on surface. The size of the damage pit is mostly around 1 μm. But for samples with thicker ALD SiO2 (404 nm and 535 nm), damage morphologies developed into two types, which are shown in Fig. 5(b) and Fig. 5(c). One type of them is still small-sized (below several micrometers) with nearly round shape. The average size of such damage seems to increase along with the deposited film thickness. As shown in Fig. 5(d), typical depth of such small-sized damage (∼400 nm) is shown less than the film thickness (∼535 nm). So, it is supposed to be a film damage. The film damage results from micro-explosion induced by nanometer point defect clusters located in the deposited film without affecting the substrate. Size variation of such damage reflects that the absorptivity of damage precursors varies with ALD film thickness. Thick ALD film may contain more point defects and has propensity to form larger damage precursors. The larger damage precursors can absorb laser energy more efficiently and induce higher local temperature. Materials surrounding the damage precursor will be heated and melted to initiate damage. The damage size is dependent on the local temperature of micro-explosion. The other type of damage observed on thick film samples exhibits much more large dimensions (typically > 10 μm). Peeling off of substrate materials can be seen at the edge of damage craters. The depth of such damage crater is much larger than the film thickness. However, the damage is also believed to originate from defects in ALD film because laser fluence for initiating such damage is much lower than the damage threshold of bare fused silica substrate. The large-sized damage craters were suggested to be created by strong mechanical shocking after initial micro-explosion of damage precursors [28,29].
Fig. 5. (a) Nomarski microscope photograph of damage initiated at 9.1 J/cm2 on 108 nm film; (b) Nomarski microscope photograph of damage initiated at 6.7 J/cm2 on 404 nm film; (c) Nomarski microscope photograph of damage initiated at 6.1 J/cm2 on 535 nm film; (d) AFM image and height profile of the film damage shown in the red square in (c). (e) AFM image and height profile of the damage crater shown in the blue square in (c).
The above presented results show that damage performance of as-deposited ALD SiO2 film is not so satisfactory as compared to fused silica substrate. To overcome this problem, UV laser conditioning (LC) was employed to improve damage resistance of ALD film. LC is a pre-illumination process of optics where the laser fluence is raised step by step. It has been successfully applied on optical films and can increase their UV laser damage threshold remarkably [30,31]. Here, we used a sub-nanosecond laser (355 nm, ∼500 ps) to perform LC experiment on another 535 nm thick ALD SiO2 film sample. Two 15 mm × 15 mm areas on the sample were pre-irradiated by different laser fluence sequences. One area was laser conditioned to 1 J/cm2, which was raster-scanned with two fluence steps (0.5 J/cm2 and 1 J/cm2). The other 15 mm×15 mm area was laser conditioned to 3 J/cm2 with fluence steps of 0.5 J/cm2, 1 J/cm2, 2 J/cm2, and 3 J/cm2. It is noted that damage sites induced by sub-nanosecond laser would start to appear during laser conditioning if the fluence were further increased to 4 J/cm2. Figure 6(a) shows the nanosecond laser damage probability curves measured on areas laser conditioned to 1 J/cm2, laser conditioned to 3 J/cm2 and without LC, respectively. It is found that surface roughness was almost unchanged after LC. The measured roughness values (RMS) were 0.99nm, 0.98nm and 1.04nm, corresponding to the area without LC, LC to 1 J/cm2 and LC to 3 J/cm2, respectively. Significant improvement of damage resistance can be observed on the area where the ALD SiO2 film was pre-irradiated by sub-nanosecond laser. After LC to 3 J/cm2, nanosecond laser damage threshold of ALD film increased from 5.5 J/cm2 to 14.9 J/cm2 (improved ∼170%), which is almost as well as that of bare fused silica substrate as shown in Fig. 4(a). However, the physical mechanism of LC is still not very clear. Explanation of LC effect was firstly given by Feit et al. [32], suggesting that the increase of damage threshold after LC is due to size decreasing of damage precursor. Duchateau proposed two physical mechanisms corresponding to different types of precursors [33]. In the model, defect annihilation due to thermal-activated mitigation and temperature-induced phase transition are suggested to explain the LC effect. Another explanation for laser conditioning believes that depopulation of absorption states associated with electronic defects via laser-annealing was the main reason of damage resistance improvement [34]. To understand this problem, we examined the FTIR absorption of ALD SiO2 film processed by different LC conditions. As shown in Fig. 6(b), the IR spectra on ALD SiO2 film conditioned to 3 J/cm2, 1 J/cm2 and without conditioning are nearly identical. IR absorption peaks associated with Si-O-Si bonding are at 477 cm−1 and 1110 cm−1, still red-shifted compared with fused silica substrate. It indicates that the structural features in ALD film was probably not changed by sub-nanosecond UV LC. The result is different from that on CVD SiO2 film treated by CO2 laser heating, where structural change was observed on the FTIR spectra [18]. FTIR result shows that UV LC has little effect on the chemical bonding structure of ALD SiO2 film. So, the temperature-induced phase transition mechanism can be rule out. As described before, laser-induced damage threshold is determined by absorptance of the damage precursors (point defect clusters) in the film. When the film is irradiated by sub-nanosecond UV laser, electrons from point defect states in the damage precursor can be excited more easily to conduction band due to the higher peak power density if compared with the situation of nanosecond laser irradiation. Then the excited electrons will experience recombination with holes in valence band or trapping into deep defect states in bandgap. Also, local temperature at the damage precursor can be increased appropriately due to the absorption of sub-nanosecond UV laser energy. The temperature rise can induce thermal-activated annihilation of point defects in the damage precursor. These processes can decrease the density of point defects in the damage precursor, reduce the absorbance of the damage precursor, and finally improve the damage resistance of the ALD SiO2 film. Thus, the mechanism of sub-nanosecond UV LC can be attributed to laser-induced annealing of point defects in the damage precursor.
Fig. 6. (a) Damage probability curves of ALD film with different LC conditions (solid lines). A damage probability curve of fused silica substrate is presented for comparison (dashed line). (b) The FTIR spectra on ALD film with different LC conditions (solid lines). The dashed curve is the FTIR spectrum on fused silica substrate for comparison.
4. Conclusion
In summary, we have demonstrated optical properties and UV laser damage performance of single-layer ALD SiO2 films thermal deposited by reaction between BTBAS and O3. The deposited ALD film on fused silica substrate shows good transparency from 200 nm to 1000 nm. When the ALD film was irradiated by UV laser at 355 nm, weak absorption can be observed by Laser calorimeter. Laser absorption at 355 nm varies linearly with the film thickness, which is considered originate from various microscopic point defects in film. FTIR absorption result confirms the presence of defects such as non-bridging oxygen atoms and residual OH groups in ALD SiO2 films. Damage performance of ALD film including damage threshold and morphology were investigated. The results suggest that laser-induced damage of ALD film is associated with point defect clusters which can provide enough laser energy absorption to initiate micro-explosion in film. Significant improvement of damage resistance on ALD SiO2 film has been obtained by sub-nanosecond UV laser conditioning. The mechanism of LC may be contributed to laser-annealing of point defects in ALD film.
Funding
National Natural Science Foundation of China (61705207, 61905227); Science Challenge Project (TZ2016006-0503-02); Foundation for Scientific & Technological Innovations of CAEP (CX2019025); Foundation for Youth Talents of LFRC, CAEP (LFRC-PD012).
Disclosures
The authors declare no conflicts of interest.
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