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Effect of Advanced Plasma Source bias voltage on properties of HfO2 films prepared by plasma ion assisted electron evaporation from metal hafnium
Thin Solid Films 540 (2013) 17–22
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effect of Advanced Plasma Source bias voltage on properties of HfO2 films prepared by plasma ion assisted electron evaporation from metal hafnium Meiping Zhu a,⁎, Kui Yi a, Detlef Arhilger b, Hongji Qi a, Jianda Shao a a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China Leybold Optics GmbH, Alzenau 63755, Germany
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 19 May 2013 Accepted 28 May 2013 Available online 11 June 2013 Keywords: Hafnium Dioxide Thin Films Metal hafnium Plasma-ion assisted electron evaporation Advanced Plasma Source Bias Voltage Stress Aging behavior Laser induced damage threshold
a b s t r a c t HfO2 films, using metal hafnium as starting material, are deposited by plasma-ion assisted electron evaporation with different Advanced Plasma Source (APS) bias voltages. The refractive index and extinction coefficient are calculated, the chemical state and composition, as well as the stress and aging behavior is investigated. Laser induced damage threshold (LIDT) and damage mechanism are also evaluated and discussed. Optical, structural, mechanical and laser induced damage properties of HfO2 films are found to be sensitive to APS bias voltage. The film stress can be tuned by varying the APS bias voltage. Damage morphologies indicate the LIDT of the HfO2 films at 1064 nm and 532 nm are dominated by the nodular-defect density in coatings, while the 355 nm LIDT is dominated by the film absorption. HfO2 films with higher 1064 nm LIDT than samples evaporated from hafnia are achieved with bias voltage of 100 V. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Among the high refractive index materials, HfO2 is one of the most commonly used one to prepare reliable interference coatings in energyrelated applications [1,2] due to its relatively high laser induced damage threshold (LIDT) [3–7], thermal and chemical stability, and large transparent range from IR down to UV region [4,5,8–11]. Considerable works have been done to investigate the properties of HfO2 films prepared by different deposition techniques [12], including electron beam evaporation [4,13,14], sputtering [4,11], ion assisted deposition (IAD) [9], and plasma ion assisted deposition (PIAD) techniques [1,15]. Among these techniques, e-beam evaporated coatings tend to experience tensilestress failures in vacuum environments, while sputtering and IAD tend to have low laser-damage resistance [16], high film stress, and/or have difficulty in manufacturing coatings for large aperture optics [17]. PIAD has the potential to be used in high energy applications due to its ability to scale up to large apertures and tune the film stress of optical coatings while maintaining high LIDT [1]. Our previous work has investigated the influence of PIAD ion assisting energy on the optical, mechanical properties and LIDT at 1064 nm of HfO2 films, using hafnia as starting material [15]. The latest technological breakthrough in manufacturing high LIDT coatings ⁎ Corresponding author. Tel.: +86 21 69918468; fax: +86 21 69918028. E-mail address: [email protected] (M. Zhu). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.05.118
is using hafnium instead of hafnia as starting material [18–20], producing HfO2 films with lower nodular-defect density, less absorption [21], improved coating interface and higher damage threshold [22]. Chow et al. [21], Khoshman et al. [23] and Andre et al. [24] have investigated the properties of HfO2 films deposited by reactive evaporation, reactive magnetron sputtering and IAD, respectively, using hafnium as starting material. However, only a limited number of studies on HfO2 films prepared by PIAD using hafnium as starting material are reported. The aim of this work is to investigate the properties of HfO2 films deposited by PIAD using metal hafnium as starting material. Several HfO2 films are deposited with different ion assisting energies. The transmittance spectrum, surface morphology, and stress of the prepared coatings are analyzed. LIDTs at 355 nm, 525 nm, and 1064 nm are evaluated as well, and the influences of nodular-defect density, chemical state, electric field distribution, and damage morphology on laser induced damage are investigated. 2. Experimental techniques HfO2 films with an optical thickness of six quarter waves at a wavelength of 510 nm are deposited by PIAD with different ion assisting energies. Fused silica substrates are used for transmittance measurement, while BK7 glass substrates are used for other investigations. The PIAD deposition is implemented by e-beam evaporation assisted by ion bombardment from an Advanced Plasma Source (APS)
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M. Zhu et al. / Thin Solid Films 540 (2013) 17–22
in a Leybold 1110 coating machine. All the HfO2 films are prepared under identical deposition parameters, except the bias voltage of APS, which is varied from 85 V to 130 V to change the ion energy, since the ion energy of this plasma source is mainly determined by self-bias voltage between anode and chamber ground [25,26]. For all the coatings, the vacuum chamber is evacuated to a base pressure of 9 × 10−4 Pa by a cryo pump, and the substrates are heated to a temperature of 140 °C. A deposition rate of about 0.12 nm/s and an Ar/O2 mixture at a pressure of about 4 × 10−2 Pa are used during deposition. The stability of the coatings is evaluated by annealing the samples for 3 h at 300 °C in atmosphere. A Perkin Elmer Lambda 1050 spectrophotometer is used to measure the transmittance of substrates and coatings at normal incident angle. Thin film design software Essential Macleod is employed to fit the measured spectra and determine the refractive index n and extinction coefficient k of each coating, using the envelope method. Absorption of coatings at 1064 nm is measured by a self-made system, which is based on surface thermal lensing technique [27]. The chemical state at the near-surface of the coating is analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) equipped with a monochromatic Al Kα (1486.6 eV) X-ray source. The Al Kα X-ray source is excited by an electron beam with beam current of 6 mA. Survey spectra are collected with pass energy of 50 eV. In order to remove the surface layer which adsorbs the water and oxygen in the air, Ar+ ion etching at 1 keV is used before XPS scans. The binding energy of the C 1 s line is taken as 284.8 eV for calibrating the obtained spectra. The XPS spectra of the HfO2 films are fitted using Gaussian-Lorentzian peak models. The chemical composition of the coating is analyzed by energy dispersive spectroscopy (EDS, Oxford X-Max, 50 mm2), operated at an accelerating voltage of 15 kV. The surface morphology is mapped using an atomic force microscope (AFM, Veeco Dimension 3100). For each sample, a 5 × 5 micron area is sampled in tapping mode and the root mean square (RMS) surface roughness is computed by the AFM software. Nodular-defect density on sample surface is investigated by a Leica optical microscope system. Stress modification caused by the HfO2 film is determined by measuring the curvature of the substrate surface before and after coating by a Techo optical interferometer. The film stress σ is described by the following equation [28]: σ¼
Es t s 2 6ð1−υs Þt f
1 1 − R2 R1
of 85 V and 100 V, confirming the presence of an open void structure which allows diffusion of water into the layer. No spectral shift is observed in films prepared at bias voltages of 115 V and 130 V, which may be attributed to a dense void-free structure or a closed void structure with empty void. As illustrated in the following section, the refractive indices of all the coatings are much lower than that of the bulk material, indicating the presence of a void structure [29]. Therefore, we suggest that a closed void structure, which does not allow the incorporation of water into the layer, exists in the films prepared at bias voltages of 115 V and 130 V. The refractive index n and extinction coefficient k of films deposited with different bias voltages are estimated from the measured transmittance spectra of the coatings using the envelope method. The maximum transmittance value in the longer wavelength region is very close to the transmittance of substrate, showing neither a positive nor a negative refractive index change. Therefore, a homogeneous film model is used. As presented in Fig. 1(a), the refractive index increases significantly as bias voltage increases from 85 V to 100 V. However, as bias voltage increases from 100 V to 130 V, the growing films might be bombarded with too-high energy ions, resulting in the formation of defects like closed void structure, which consequently causes the decrease of the refractive index. As indicated in Fig. 1(b), the film deposited with bias voltage of 85 V shows lowest extinction coefficient k. As the bias voltage increases from 85 V to 130 V, extinction coefficient k increases monotonously. As indicated in Fig. 2, film deposited with higher bias voltage shows higher absorption, which is coincident with the extinction coefficient k of the single layer coating. It is worth noting that the coatings evaporated by PIAD from hafnium suffer higher absorption than those deposited from hafnia [15]. 3.2. O/Hf ratio in hafnium dioxide films XPS is applied to investigate the chemical state of the coatings manufactured at different bias voltages. Two scans are performed, before which the Ar+ ion etching lasted 10 s and 40 s, respectively. The Hf 4f7/2 peaks of all the coatings are at approximately 17 eV with a difference of about 1.6 eV in binding energy between the Hf 4f5/2 peaks and Hf 4f7/2 peaks, while the main peaks attributed to hafnium dioxide in the O 1 s spectrum locate at approximately 530.6 eV. O/Hf
ð1Þ
where, Es and vs are the Young's modulus and the Poisson's ratio of the substrate, respectively; ts and tf are the thickness of substrate and coating, respectively; R1 and R2 are the radius of curvature of substrates measured before and after deposition of films, respectively. LIDTs at 1064 nm, 532 nm and 355 nm are evaluated under 1-on-1 test mode (according to ISO 11254–1) at normal beam incident angle with laser pulse width of 12 ns, 10 ns and 8 ns, and with the effective beam area in the sample plane of 0.12 mm2, 0.035 mm2 and 0.011 mm2, respectively. The damage morphologies are characterized by a focused ion beam-scanning electron microscope (FIB-SEM, Carl Zeiss AURIGA CrossBeam), operated at an accelerating voltage of 1 kV. 3. Results and discussion 3.1. Spectral shift, refractive index, extinction coefficient and absorption Transmittance spectra of the coatings deposited with different bias voltages are measured and compared with the spectra of the samples annealed for 3 h at 300 °C in atmosphere to evaluate the stability of the coatings. Blue shift is observed in samples prepared at bias voltage
Fig. 1. a) Refractive index n and b) extinction coefficient k of HfO2 films deposited with different bias voltages.
M. Zhu et al. / Thin Solid Films 540 (2013) 17–22
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Fig. 2. Measured absorption at 1064 nm of HfO2 films deposited with different bias voltages.
ratio is used to evaluate the stoichiometric information. As shown in Table 1, the O/Hf ratio obtained after an etching time of 40 s are slightly lower than that obtained after an etching time of 10 s, due to the preferential sputtering of oxygen by Ar+ ion bombardment. In view of the preferential sputtering speed, all the films are oxygen-deficient. As the bias voltage increases from 85 V to 115 V, the O/Hf ratio increases, which can be attributed to increasing bias voltage generates Ar+ ions with higher kinetic energy, creating more efficient atomic oxygen, and the oxidation efficiency increases. However, as the bias voltage continues to increase, the O/Hf ratio decreases instead, that can be related with a small increase on the oxygen vacancies, likely due to the damaging of the deposited films induced by the higher bias voltage used [30]. 3.3. EDS results EDS is used to analyze the chemical composition of HfO2 films. As presented in Fig. 3, the EDS scans confirmed the content of Ar in all the samples, except the one deposited with bias voltage of 85 V. The K in EDS spectrum is attributed to the BK7 glass substrate, while the C might be originated from two sources: the adsorbed hydrocarbons and the APS cathode heater which is made from graphite. It is worth noting that the amount of Ar content increases as the bias voltage increases, the reason is that the increasing bias voltage translates into higher kinetic energy for the Ar+ ions impinging normally onto the sample, which induces more Ar atoms trapped inside the films. The Ar in the film explains why the absorption in the films increases while the O/Hf ratio increases [31]. Besides, the increasing bias voltage may cause more C content originated from the APS cathode heater into the films, and consequently increases the absorption in HfO2 films.
Fig. 3. Chemical composition of HfO2 films deposited with bias voltages of a) 85 V; b) 100 V; c) 115 V; d) 130 V, respectively.
as depicted in Fig. 4. As the bias voltage increases from 85 V to 115 V, the HfO2 films appear much more fine grained, and the RMS roughness decreases, as indicated in Fig. 5. However, the columnar grain-size and RMS roughness increases when the bias voltage continues to increase.
3.4. Surface morphology
3.5. Nodular-defect density
The AFM investigation shows that the HfO2 films deposited with different bias voltages exhibit similar and columnar grain structures,
Nodular-defect density on sample surface is observed by optical microscope under 200 × magnification. The typical morphologies of nodular-defects observed by optical microscope and AFM are shown in Fig. 6(a) and (b), respectively. It is found that HfO2 coating deposited with bias voltage of 100 V has the lowest nodular-defect density, and the nodular-defect density increases when bias voltage higher than 100 V. We suggest the reason is that increasing ion energy causes more particles inside the vacuum chamber entering into the growing films, and consequently forms more nodular-defects. However, the bias voltage of 85 V is not high enough to fully ionize the oxygen, the LaB6 cathode of the APS might get a little poisoned and work unstably [32], which consequently causes more nodular-defects at bias voltage of 85 V than at 100 V.
Table 1 O/Hf Ratio of the films deposited with different bias voltages. Bias voltage (V)
85 100 115 130
O/Hf ratio Etching time: 10 s
Etching time: 40 s
1.698 1.782 1.927 1.837
1.652 1.752 1.906 1.826
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Fig. 4. AFM micrograph of HfO2 films deposited with bias voltages of a) 85 V, b) 100 V, c) 115 V and d) 130 V, respectively.
different bias voltages. After deposition, the samples are placed in a conventional clean room environment, and surface flatness is measured at controlled relative humidity and temperature of 50% and 23 °C, respectively. The surface flatness measurements are performed every several days to investigate the aging behavior. These measurement results are used to calculate the film stress using Eq. (1). As shown in Fig. 7, the stress transitions from a tensile state below 100 V to an increasingly compressive state above this bias voltage. For the coating deposited with bias voltage of 85 V, the tensile stress decreases progressively in the observed aging period, getting close to zero after 60 days' aging time, while the aging behavior of coatings deposited with higher bias voltage is approximately similar and does not change over time, which can be attributed to the closed void structure that does not absorb moisture. The small fluctuation is likely due to the accuracy of the surface flatness measurement. Fig. 5. Normalized RMS of HfO2 films deposited with different bias voltages.
3.6. Stress and aging behavior BK7 glass samples with dimensions of 50 mm in diameter and 5 mm in thickness are used to evaluate the stress in HfO2 films deposited with
3.7. LIDT, electric field distribution and damage morphology LIDTs at 1064 nm, 532 nm and 355 nm are investigated, the damage threshold are compared and shown in Fig. 8. To understand the origin of laser induced damage, the damage morphology, as well as the electric-field distribution is investigated as following.
Fig. 6. Typical morphology of the nodular defect characterized by (a) optical microscope and (b) AFM, respectively.
M. Zhu et al. / Thin Solid Films 540 (2013) 17–22
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Fig. 7. Stress and aging behavior of HfO2 films deposited with different bias voltages.
Fig. 10. Comparison of LIDT (1064 nm) of HfO2 films evaporated from hafnia and hafnium.
Fig. 8. LIDT of HfO2 films deposited with different bias voltages.
As shown in Fig. 9(a), the damage after irradiation of 1064 nm laser is initiated by defects. The 1064 nm LIDT of HfO2 films depends greatly on the nodular-defect density in coatings, as the bias voltage increases from 85 V to 100 V, the 1064 nm LIDT increases due to the reduction of the nodular-defect density; However, as the bias voltage continues to increase, the 1064 nm LIDT decreases instead because of the increase of the nodular-defect density. As reported in our previous work, the HfO2 films evaporated from hafnia at bias voltage of 90 V has higher 1064 nm LIDT [15] than films deposited with other bias voltages, of which the damage probabilities at different fluences are presented in Fig. 10. Comparing with the HfO2 films evaporated from hafnia [15], the films evaporated from hafnium suffer from higher absorption. However, the sample evaporated from hafnium with bias voltage of 100 V has higher LIDT than the HfO2 films evaporated from hafnia at bias voltage of 90 V, as shown in Fig. 10, which can be attributed to its lower nodular-defect density, indicating that using hafnium instead of hafnia as starting material can reduce the nodular-defect density in films.
Fig. 9. Typical damage morphologies examined by SEM. Irradiated by lasers at wavelength of (a) 1064 nm; (b) 532 nm; (c) 532 nm; (d) 355 nm.
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starting material instead of hafnia. Comparing the LIDT at 1064 nm, 532 nm and 355 nm, 1064 nm and 532 nm LIDT of the HfO2 coatings are dominated by the nodular-defect density in coatings, while the 355 nm LIDT is dominated by the film absorption. Acknowledgements The authors gratefully acknowledge Yun Cui and Qilin Xiao for XPS and EDS measurement. References
Fig. 11. Electric field distribution of the HfO2 films.
The damage after irradiation at 532 nm is also initiated by nodulardefects, as shown in Fig. 9(b) and (c). The damage morphology induced by 532 nm at a fluence of 8.3 J/cm2 is similar to that induced by 1064 nm laser. As the fluence increases from 8.3 J/cm2 to 11.3 J/cm2, lots of micro-defects originated from the substrate can be seen in the damaged area. Beyond our expectation, the 532 nm LIDT is significantly lower than 1064 nm LIDT and even lower than the 355 nm LIDT, which might be attributed to the higher air-film interface electric field intensity at 532 nm than 1064 nm and 355 nm, as shown in Fig. 11. Different from the damage morphology induced by 1064 nm and 532 nm lasers, the damage after irradiation of 355 nm laser is massive in nature and not clearly defect-driven, as shown in Fig. 9(d). The 355 nm LIDT of HfO2 film depends on the film absorption, as the bias voltage increases, the film absorption increases, resulting in a slight decrease of the 355 nm LIDT. According to the experimental results, we suggest an appropriate APS bias voltage and less Ar should be used to get coatings with better optical properties, lower stress, as well as higher LIDTs at 1064 nm, 532 nm and 355 nm. 4. Conclusions HfO2 single layer coatings are deposited by PIAD with different APS bias voltages, using metal hafnium as starting material. Properties including spectral shift, optical constant, absorption, O/Hf ratio, chemical composition, surface morphology, stress and aging behavior, as well as LIDTs at 1064 nm, 532 nm and 355 nm are investigated. With the increase of APS bias voltage from 85 V to 100 V, HfO2 coatings with higher refractive index, lower surface roughness, more fine grained structure, increased O/Hf ratio and higher 1064 nm LIDT are obtained. However, too-high bias voltage can cause the increase of Ar content in the films, and might cause more C content originated from the APS cathode heater into the films, which consequently increase the absorption and decrease the 355 nm LIDT. The stress of HfO2 film transitions from a tensile state below APS bias voltage of 100 V to an increasingly compressive state, which does not change over time above this bias voltage. With the APS bias voltage of 100 V, HfO2 coatings without any stress can be achieved. Although HfO2 films evaporated by PIAD from hafnium suffer higher absorption than samples evaporated from hafnia, higher 1064 nm LIDT is achieved with a bias voltage of 100 V, thanks to the reduction of nodular-defect density by using hafnium as
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Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effect of Advanced Plasma Source bias voltage on properties of HfO2 films prepared by plasma ion assisted electron evaporation from metal hafnium Meiping Zhu a,⁎, Kui Yi a, Detlef Arhilger b, Hongji Qi a, Jianda Shao a a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China Leybold Optics GmbH, Alzenau 63755, Germany
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 19 May 2013 Accepted 28 May 2013 Available online 11 June 2013 Keywords: Hafnium Dioxide Thin Films Metal hafnium Plasma-ion assisted electron evaporation Advanced Plasma Source Bias Voltage Stress Aging behavior Laser induced damage threshold
a b s t r a c t HfO2 films, using metal hafnium as starting material, are deposited by plasma-ion assisted electron evaporation with different Advanced Plasma Source (APS) bias voltages. The refractive index and extinction coefficient are calculated, the chemical state and composition, as well as the stress and aging behavior is investigated. Laser induced damage threshold (LIDT) and damage mechanism are also evaluated and discussed. Optical, structural, mechanical and laser induced damage properties of HfO2 films are found to be sensitive to APS bias voltage. The film stress can be tuned by varying the APS bias voltage. Damage morphologies indicate the LIDT of the HfO2 films at 1064 nm and 532 nm are dominated by the nodular-defect density in coatings, while the 355 nm LIDT is dominated by the film absorption. HfO2 films with higher 1064 nm LIDT than samples evaporated from hafnia are achieved with bias voltage of 100 V. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Among the high refractive index materials, HfO2 is one of the most commonly used one to prepare reliable interference coatings in energyrelated applications [1,2] due to its relatively high laser induced damage threshold (LIDT) [3–7], thermal and chemical stability, and large transparent range from IR down to UV region [4,5,8–11]. Considerable works have been done to investigate the properties of HfO2 films prepared by different deposition techniques [12], including electron beam evaporation [4,13,14], sputtering [4,11], ion assisted deposition (IAD) [9], and plasma ion assisted deposition (PIAD) techniques [1,15]. Among these techniques, e-beam evaporated coatings tend to experience tensilestress failures in vacuum environments, while sputtering and IAD tend to have low laser-damage resistance [16], high film stress, and/or have difficulty in manufacturing coatings for large aperture optics [17]. PIAD has the potential to be used in high energy applications due to its ability to scale up to large apertures and tune the film stress of optical coatings while maintaining high LIDT [1]. Our previous work has investigated the influence of PIAD ion assisting energy on the optical, mechanical properties and LIDT at 1064 nm of HfO2 films, using hafnia as starting material [15]. The latest technological breakthrough in manufacturing high LIDT coatings ⁎ Corresponding author. Tel.: +86 21 69918468; fax: +86 21 69918028. E-mail address: [email protected] (M. Zhu). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.05.118
is using hafnium instead of hafnia as starting material [18–20], producing HfO2 films with lower nodular-defect density, less absorption [21], improved coating interface and higher damage threshold [22]. Chow et al. [21], Khoshman et al. [23] and Andre et al. [24] have investigated the properties of HfO2 films deposited by reactive evaporation, reactive magnetron sputtering and IAD, respectively, using hafnium as starting material. However, only a limited number of studies on HfO2 films prepared by PIAD using hafnium as starting material are reported. The aim of this work is to investigate the properties of HfO2 films deposited by PIAD using metal hafnium as starting material. Several HfO2 films are deposited with different ion assisting energies. The transmittance spectrum, surface morphology, and stress of the prepared coatings are analyzed. LIDTs at 355 nm, 525 nm, and 1064 nm are evaluated as well, and the influences of nodular-defect density, chemical state, electric field distribution, and damage morphology on laser induced damage are investigated. 2. Experimental techniques HfO2 films with an optical thickness of six quarter waves at a wavelength of 510 nm are deposited by PIAD with different ion assisting energies. Fused silica substrates are used for transmittance measurement, while BK7 glass substrates are used for other investigations. The PIAD deposition is implemented by e-beam evaporation assisted by ion bombardment from an Advanced Plasma Source (APS)
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in a Leybold 1110 coating machine. All the HfO2 films are prepared under identical deposition parameters, except the bias voltage of APS, which is varied from 85 V to 130 V to change the ion energy, since the ion energy of this plasma source is mainly determined by self-bias voltage between anode and chamber ground [25,26]. For all the coatings, the vacuum chamber is evacuated to a base pressure of 9 × 10−4 Pa by a cryo pump, and the substrates are heated to a temperature of 140 °C. A deposition rate of about 0.12 nm/s and an Ar/O2 mixture at a pressure of about 4 × 10−2 Pa are used during deposition. The stability of the coatings is evaluated by annealing the samples for 3 h at 300 °C in atmosphere. A Perkin Elmer Lambda 1050 spectrophotometer is used to measure the transmittance of substrates and coatings at normal incident angle. Thin film design software Essential Macleod is employed to fit the measured spectra and determine the refractive index n and extinction coefficient k of each coating, using the envelope method. Absorption of coatings at 1064 nm is measured by a self-made system, which is based on surface thermal lensing technique [27]. The chemical state at the near-surface of the coating is analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) equipped with a monochromatic Al Kα (1486.6 eV) X-ray source. The Al Kα X-ray source is excited by an electron beam with beam current of 6 mA. Survey spectra are collected with pass energy of 50 eV. In order to remove the surface layer which adsorbs the water and oxygen in the air, Ar+ ion etching at 1 keV is used before XPS scans. The binding energy of the C 1 s line is taken as 284.8 eV for calibrating the obtained spectra. The XPS spectra of the HfO2 films are fitted using Gaussian-Lorentzian peak models. The chemical composition of the coating is analyzed by energy dispersive spectroscopy (EDS, Oxford X-Max, 50 mm2), operated at an accelerating voltage of 15 kV. The surface morphology is mapped using an atomic force microscope (AFM, Veeco Dimension 3100). For each sample, a 5 × 5 micron area is sampled in tapping mode and the root mean square (RMS) surface roughness is computed by the AFM software. Nodular-defect density on sample surface is investigated by a Leica optical microscope system. Stress modification caused by the HfO2 film is determined by measuring the curvature of the substrate surface before and after coating by a Techo optical interferometer. The film stress σ is described by the following equation [28]: σ¼
Es t s 2 6ð1−υs Þt f
1 1 − R2 R1
of 85 V and 100 V, confirming the presence of an open void structure which allows diffusion of water into the layer. No spectral shift is observed in films prepared at bias voltages of 115 V and 130 V, which may be attributed to a dense void-free structure or a closed void structure with empty void. As illustrated in the following section, the refractive indices of all the coatings are much lower than that of the bulk material, indicating the presence of a void structure [29]. Therefore, we suggest that a closed void structure, which does not allow the incorporation of water into the layer, exists in the films prepared at bias voltages of 115 V and 130 V. The refractive index n and extinction coefficient k of films deposited with different bias voltages are estimated from the measured transmittance spectra of the coatings using the envelope method. The maximum transmittance value in the longer wavelength region is very close to the transmittance of substrate, showing neither a positive nor a negative refractive index change. Therefore, a homogeneous film model is used. As presented in Fig. 1(a), the refractive index increases significantly as bias voltage increases from 85 V to 100 V. However, as bias voltage increases from 100 V to 130 V, the growing films might be bombarded with too-high energy ions, resulting in the formation of defects like closed void structure, which consequently causes the decrease of the refractive index. As indicated in Fig. 1(b), the film deposited with bias voltage of 85 V shows lowest extinction coefficient k. As the bias voltage increases from 85 V to 130 V, extinction coefficient k increases monotonously. As indicated in Fig. 2, film deposited with higher bias voltage shows higher absorption, which is coincident with the extinction coefficient k of the single layer coating. It is worth noting that the coatings evaporated by PIAD from hafnium suffer higher absorption than those deposited from hafnia [15]. 3.2. O/Hf ratio in hafnium dioxide films XPS is applied to investigate the chemical state of the coatings manufactured at different bias voltages. Two scans are performed, before which the Ar+ ion etching lasted 10 s and 40 s, respectively. The Hf 4f7/2 peaks of all the coatings are at approximately 17 eV with a difference of about 1.6 eV in binding energy between the Hf 4f5/2 peaks and Hf 4f7/2 peaks, while the main peaks attributed to hafnium dioxide in the O 1 s spectrum locate at approximately 530.6 eV. O/Hf
ð1Þ
where, Es and vs are the Young's modulus and the Poisson's ratio of the substrate, respectively; ts and tf are the thickness of substrate and coating, respectively; R1 and R2 are the radius of curvature of substrates measured before and after deposition of films, respectively. LIDTs at 1064 nm, 532 nm and 355 nm are evaluated under 1-on-1 test mode (according to ISO 11254–1) at normal beam incident angle with laser pulse width of 12 ns, 10 ns and 8 ns, and with the effective beam area in the sample plane of 0.12 mm2, 0.035 mm2 and 0.011 mm2, respectively. The damage morphologies are characterized by a focused ion beam-scanning electron microscope (FIB-SEM, Carl Zeiss AURIGA CrossBeam), operated at an accelerating voltage of 1 kV. 3. Results and discussion 3.1. Spectral shift, refractive index, extinction coefficient and absorption Transmittance spectra of the coatings deposited with different bias voltages are measured and compared with the spectra of the samples annealed for 3 h at 300 °C in atmosphere to evaluate the stability of the coatings. Blue shift is observed in samples prepared at bias voltage
Fig. 1. a) Refractive index n and b) extinction coefficient k of HfO2 films deposited with different bias voltages.
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Fig. 2. Measured absorption at 1064 nm of HfO2 films deposited with different bias voltages.
ratio is used to evaluate the stoichiometric information. As shown in Table 1, the O/Hf ratio obtained after an etching time of 40 s are slightly lower than that obtained after an etching time of 10 s, due to the preferential sputtering of oxygen by Ar+ ion bombardment. In view of the preferential sputtering speed, all the films are oxygen-deficient. As the bias voltage increases from 85 V to 115 V, the O/Hf ratio increases, which can be attributed to increasing bias voltage generates Ar+ ions with higher kinetic energy, creating more efficient atomic oxygen, and the oxidation efficiency increases. However, as the bias voltage continues to increase, the O/Hf ratio decreases instead, that can be related with a small increase on the oxygen vacancies, likely due to the damaging of the deposited films induced by the higher bias voltage used [30]. 3.3. EDS results EDS is used to analyze the chemical composition of HfO2 films. As presented in Fig. 3, the EDS scans confirmed the content of Ar in all the samples, except the one deposited with bias voltage of 85 V. The K in EDS spectrum is attributed to the BK7 glass substrate, while the C might be originated from two sources: the adsorbed hydrocarbons and the APS cathode heater which is made from graphite. It is worth noting that the amount of Ar content increases as the bias voltage increases, the reason is that the increasing bias voltage translates into higher kinetic energy for the Ar+ ions impinging normally onto the sample, which induces more Ar atoms trapped inside the films. The Ar in the film explains why the absorption in the films increases while the O/Hf ratio increases [31]. Besides, the increasing bias voltage may cause more C content originated from the APS cathode heater into the films, and consequently increases the absorption in HfO2 films.
Fig. 3. Chemical composition of HfO2 films deposited with bias voltages of a) 85 V; b) 100 V; c) 115 V; d) 130 V, respectively.
as depicted in Fig. 4. As the bias voltage increases from 85 V to 115 V, the HfO2 films appear much more fine grained, and the RMS roughness decreases, as indicated in Fig. 5. However, the columnar grain-size and RMS roughness increases when the bias voltage continues to increase.
3.4. Surface morphology
3.5. Nodular-defect density
The AFM investigation shows that the HfO2 films deposited with different bias voltages exhibit similar and columnar grain structures,
Nodular-defect density on sample surface is observed by optical microscope under 200 × magnification. The typical morphologies of nodular-defects observed by optical microscope and AFM are shown in Fig. 6(a) and (b), respectively. It is found that HfO2 coating deposited with bias voltage of 100 V has the lowest nodular-defect density, and the nodular-defect density increases when bias voltage higher than 100 V. We suggest the reason is that increasing ion energy causes more particles inside the vacuum chamber entering into the growing films, and consequently forms more nodular-defects. However, the bias voltage of 85 V is not high enough to fully ionize the oxygen, the LaB6 cathode of the APS might get a little poisoned and work unstably [32], which consequently causes more nodular-defects at bias voltage of 85 V than at 100 V.
Table 1 O/Hf Ratio of the films deposited with different bias voltages. Bias voltage (V)
85 100 115 130
O/Hf ratio Etching time: 10 s
Etching time: 40 s
1.698 1.782 1.927 1.837
1.652 1.752 1.906 1.826
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Fig. 4. AFM micrograph of HfO2 films deposited with bias voltages of a) 85 V, b) 100 V, c) 115 V and d) 130 V, respectively.
different bias voltages. After deposition, the samples are placed in a conventional clean room environment, and surface flatness is measured at controlled relative humidity and temperature of 50% and 23 °C, respectively. The surface flatness measurements are performed every several days to investigate the aging behavior. These measurement results are used to calculate the film stress using Eq. (1). As shown in Fig. 7, the stress transitions from a tensile state below 100 V to an increasingly compressive state above this bias voltage. For the coating deposited with bias voltage of 85 V, the tensile stress decreases progressively in the observed aging period, getting close to zero after 60 days' aging time, while the aging behavior of coatings deposited with higher bias voltage is approximately similar and does not change over time, which can be attributed to the closed void structure that does not absorb moisture. The small fluctuation is likely due to the accuracy of the surface flatness measurement. Fig. 5. Normalized RMS of HfO2 films deposited with different bias voltages.
3.6. Stress and aging behavior BK7 glass samples with dimensions of 50 mm in diameter and 5 mm in thickness are used to evaluate the stress in HfO2 films deposited with
3.7. LIDT, electric field distribution and damage morphology LIDTs at 1064 nm, 532 nm and 355 nm are investigated, the damage threshold are compared and shown in Fig. 8. To understand the origin of laser induced damage, the damage morphology, as well as the electric-field distribution is investigated as following.
Fig. 6. Typical morphology of the nodular defect characterized by (a) optical microscope and (b) AFM, respectively.
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Fig. 7. Stress and aging behavior of HfO2 films deposited with different bias voltages.
Fig. 10. Comparison of LIDT (1064 nm) of HfO2 films evaporated from hafnia and hafnium.
Fig. 8. LIDT of HfO2 films deposited with different bias voltages.
As shown in Fig. 9(a), the damage after irradiation of 1064 nm laser is initiated by defects. The 1064 nm LIDT of HfO2 films depends greatly on the nodular-defect density in coatings, as the bias voltage increases from 85 V to 100 V, the 1064 nm LIDT increases due to the reduction of the nodular-defect density; However, as the bias voltage continues to increase, the 1064 nm LIDT decreases instead because of the increase of the nodular-defect density. As reported in our previous work, the HfO2 films evaporated from hafnia at bias voltage of 90 V has higher 1064 nm LIDT [15] than films deposited with other bias voltages, of which the damage probabilities at different fluences are presented in Fig. 10. Comparing with the HfO2 films evaporated from hafnia [15], the films evaporated from hafnium suffer from higher absorption. However, the sample evaporated from hafnium with bias voltage of 100 V has higher LIDT than the HfO2 films evaporated from hafnia at bias voltage of 90 V, as shown in Fig. 10, which can be attributed to its lower nodular-defect density, indicating that using hafnium instead of hafnia as starting material can reduce the nodular-defect density in films.
Fig. 9. Typical damage morphologies examined by SEM. Irradiated by lasers at wavelength of (a) 1064 nm; (b) 532 nm; (c) 532 nm; (d) 355 nm.
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starting material instead of hafnia. Comparing the LIDT at 1064 nm, 532 nm and 355 nm, 1064 nm and 532 nm LIDT of the HfO2 coatings are dominated by the nodular-defect density in coatings, while the 355 nm LIDT is dominated by the film absorption. Acknowledgements The authors gratefully acknowledge Yun Cui and Qilin Xiao for XPS and EDS measurement. References
Fig. 11. Electric field distribution of the HfO2 films.
The damage after irradiation at 532 nm is also initiated by nodulardefects, as shown in Fig. 9(b) and (c). The damage morphology induced by 532 nm at a fluence of 8.3 J/cm2 is similar to that induced by 1064 nm laser. As the fluence increases from 8.3 J/cm2 to 11.3 J/cm2, lots of micro-defects originated from the substrate can be seen in the damaged area. Beyond our expectation, the 532 nm LIDT is significantly lower than 1064 nm LIDT and even lower than the 355 nm LIDT, which might be attributed to the higher air-film interface electric field intensity at 532 nm than 1064 nm and 355 nm, as shown in Fig. 11. Different from the damage morphology induced by 1064 nm and 532 nm lasers, the damage after irradiation of 355 nm laser is massive in nature and not clearly defect-driven, as shown in Fig. 9(d). The 355 nm LIDT of HfO2 film depends on the film absorption, as the bias voltage increases, the film absorption increases, resulting in a slight decrease of the 355 nm LIDT. According to the experimental results, we suggest an appropriate APS bias voltage and less Ar should be used to get coatings with better optical properties, lower stress, as well as higher LIDTs at 1064 nm, 532 nm and 355 nm. 4. Conclusions HfO2 single layer coatings are deposited by PIAD with different APS bias voltages, using metal hafnium as starting material. Properties including spectral shift, optical constant, absorption, O/Hf ratio, chemical composition, surface morphology, stress and aging behavior, as well as LIDTs at 1064 nm, 532 nm and 355 nm are investigated. With the increase of APS bias voltage from 85 V to 100 V, HfO2 coatings with higher refractive index, lower surface roughness, more fine grained structure, increased O/Hf ratio and higher 1064 nm LIDT are obtained. However, too-high bias voltage can cause the increase of Ar content in the films, and might cause more C content originated from the APS cathode heater into the films, which consequently increase the absorption and decrease the 355 nm LIDT. The stress of HfO2 film transitions from a tensile state below APS bias voltage of 100 V to an increasingly compressive state, which does not change over time above this bias voltage. With the APS bias voltage of 100 V, HfO2 coatings without any stress can be achieved. Although HfO2 films evaporated by PIAD from hafnium suffer higher absorption than samples evaporated from hafnia, higher 1064 nm LIDT is achieved with a bias voltage of 100 V, thanks to the reduction of nodular-defect density by using hafnium as
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