Single layers and antireflection films were deposited by electron beam evaporation, ion assisted deposition and interrupted ion assisted deposition, respectively. Antireflection film of quite high laser damage threshold (18J/cm2) deposited by interrupted ion assisted deposition were got. The electric field distribution, weak absorption, and residual stress of films and their relations to damage threshold were investigated. It was shown that the laser induced damage threshold of film was the result of competition of disadvantages and advantages, and interrupted ion assisted deposition was one of the valuable methods for preparing high laser induced damage threshold films.
©2007 Optical Society of America
Preparing high laser induced damage threshold (LIDT) films is one of hot topics in optical coating community [1, 2], especially for antireflective films . For antireflection films, electric field is distributed in all layers, so every layer will be possibly damaged. To improve the LIDT of films, many preparing methods were investigated, such as electron beam evaporation, ion beam assisted deposition (IBAD) [4–6], ion sputtering , Sol-gel . Among these ways, IBAD is ever regarded as the most possible way to improve the LIDT of film, for its some advantages such as high packing density, good adhesion. However, the LIDT of films prepared by this way was not high as expected, and some investigation also indicated that IBAD is not suitable for high power laser films [5, 9]. In this work, high LIDT antireflection film deposited by interrupted ion assisted deposition (IIAD) was reported. During the IIAD process some layers were deposited using IBAD, and other layers were deposited only using electron beam evaporation. This method of preparing high LIDT films has not been reported according to our knowledge.
2. Experimental procedures
In this letter, BK7 glasses (φ30×4 mm) were used as substrates and all glasses were cleaned ultrasonically 10 minutes in alcoholic solution, washed by water and dried by high pure N2.
Single layers of HfO2 and SiO2 were deposited by IBAD and electron beam evaporation to investigate the effects of two preparing methods on the characteristics of films. The monitoring wavelength (λ) was 525nm during the deposition and the optical thickness of all single layers was 5λ/4.
Antireflection films at the center wavelength of 1064 nm were deposited by IIAD and electron beam evaporation, and the monitoring wavelength (λ) was also 525nm during all processes of preparation. The film stack was Glass/4LH2.5L/Air, which “H” was HfO2 layer and its optical thickness was λ/4, and “L” was SiO2 layer with λ/4 optical thickness too. The advantages of this film stack will be shown in section 3.1 of this letter. HfO2 layer was deposited by IBAD and SiO2 layers were deposited by electron beam evaporation during the depositon of IIAD.
Model EH1000 ion source made by Kaufman Company was used for IBAD and IIAD. High purity oxygen (99.999%) was used as working gas, and its flow rate was 8 sccm. To get the constant possible deposition rate, e-beam current and ion current were controlled strictly. E-beam current was kept 160mA when HfO2 material was deposited, and it was kept 60mA for SiO2 material depositing. Considering the conclusion about the ion current of reference , anode current of ion source was kept 2A and ion current which was decided by anode current was 100µA/cm2 during ion source working. Other parameters of preparation were shown in table.1.
The surface thermal lensing (STL) technique was employed to measure the weak absorption [10,11]. The testing apparatus and mechanism was described in reference . A single-mode, CW YAG 1064nm laser was selected to serve as the pump source, and the beam focused on the sample surface at a near-normal angle of incidence with diameter of ~100µm. The probe beam was a 50mW laser at a wavelength of 632.8nm. The beam was focused onto the sample surface coincidentally with the bump laser beam. Twenty sites on a line were selected on the sample surface to measure the absorption. STL technique permits the measurement of very low levels absorption and its uncertainty is 1ppm.
ZYGO interferometer was employed to measure the residual stress. The testing mechanism was described in reference .
LIDT was measured by 1-on-1 mode according to ISO 11254 . During the measurement, a Q-switched Nd:YAG single-mode laser with 12-ns pulse width and 1064nm wavelength was used. Ten sites of the sample were exposed at the same fluence and the fraction of sites which were damaged was recorded. This procedure was repeated for other fluence until the range of fluence was sufficiently broad to include points of zero damage probability and points of 100% damage probability to develop a plot of damage probability versus fluence. The LIDT was defined as the incident pulse’s energy density when the damage occurs at zero damage possibility (J/cm2), and it could be obtained by linear extrapolation of the damage possibility data to zero damage possibility. The more detail information was presented in reference .
3. Results and Analysis
3.1 Electric field distribution
When the LIDTs of films were investigated, it was necessary to study the electric field distribution, for electric field distribution which settles inside the films was one of the main factors influencing LIDTs. Here the electric field distributions of single layers of HfO2 and SiO2 and antireflection films were shown in Fig. 1 and Fig. 2.
It can be seen from Fig. 1 that the electric field of single SiO2 reached its peak value at the air/film interface and the electric field of single HfO2 reached its peak value at the film/substrate interface. Figure 2 shown that the electric field of antireflection film with the film stack Glass/4LH2.5L/Air reached its peak value at layer of SiO2, and reached its lowest value at HfO2/SiO2 interface. Generally speaking, the electric field peak value at the film/substrate interface, air/film interface or interfaces of high and low index layers would lead to lower LIDT of films. Later the LIDTs of samples in section 3.4 would show that all LIDTs of single layers were lower than antireflection films investigated in this letter. The electric field distribution may be one of the main factors. Furthermore, it was well known that HfO2 films deposited by e-beam or IBAD were prone to nodule formation as a result of spitting from the electron beam source . The nodule defects can result in a local enhancement of electric field that typically resulted in reduced damage thresholds. Because of the thick layer of SiO2 was deposited on top of the HfO2 layer, a beneficial smoothing effect may have occurred. It is the second factor that led to high LIDTs of antireflection films.
3.2 Weak absorption
The weak absorption of samples was shown in Fig. 3. As we can see, the weak absorption is higher for sample B than sample A, sample D is higher than sample C and sample E is higher than sample F. So it could be concluded that the absorption of samples by IBAD or IIAD is higher than samples deposited only by electron beam evaporation.
The relation between weak absorption and IBAD approach was investigated in our previous paper . In that paper, we thought that some contaminations adhered to chamber or clamps were sputtered and changed into impurity defects of films during the process of IBAD. Furthermore, the defects of crystal boundary would be introduced by ion beam impact . The results here were consistent with these previous investigations. Those defects resulted in high absorption of samples deposited by IBAD or IIAD. In fact, some investigations that opposed IBAD during the preparation of high LIDT films also took defects as main disadvantage of IBAD [1, 5, 9].
3.3 Residual stress
The residual stress of samples was shown in Fig. 4. From the data of single layers, it can be observed that:
(1) For HfO2 films, sample B deposited by IBAD had compress stress (-185MPa), while sample A deposited only by electron beam evaporation had tensile stress (298MPa).
(2) For SiO2 films, samples C (304 MPa) and D (267 MPa) had tensile stress too. Comparing with samples C, sample D deposited by IBAD had lower tensile stress.
It reveled that ions impacted films and had compress effect on the films during the process of IBAD, so films had compress stress or reduced tensile stress.
As for the data of anti-reflection films that sample E deposited by IIAD had lower compress stress (-78MPa). The most probable explanation for this is stress matching. For sample E, HfO2 layer deposited by the same conditions of sample B had compress stress, and SiO2 layers deposited by the same conditions of sample C had tensile stress. So sample E had little compress stress for inter-compensation between compress stress and tensile stress. For the same reason, sample F had lower tensile stress (165MPa) than the summation of its all layers stress as the Fig. 4 shown.
There was lower stress in sample E by comparing with the number of sample F. It demonstrated that the multilayer film would have lower residual stress if layers close to each other have opposite residual stress. The ion impaction had large effects on residual stress of films. Consequently, IIBAD is an important approach that controls the residual stress of multilayer or single layer films.
3.4 Laser induced damage threshold
The LIDTs of samples was shown in Fig. 5. By comparing LIDTs of samples A and B, C and D, it was indicated that samples deposited by IBAD had lower LIDTs than samples deposited only with electron beam evaporation. It could be explained that samples with higher absorption had more defects. Laser damage is expected to be caused by melting and evaporation processes for high-absorption defects . These defects would be the main damage factor when irradiating. Hence, samples with higher absorption would have lower LIDT.
Comparing LIDTs of samples E and F, it was found that sample E deposited by IIAD had the highest LIDT (18 J/cm2). This result was very interesting. According to the absorption of samples, sample E should have lower LIDT than sample F. The interesting result revealed that weak absorption was not the main factor that affected the LIDT of films. Residual stress is also a worth noting facto for LIDT results of films. Residual stress affected the adhesion of the film to the substrate, and enhanced the thermo-force coupling when irradiating. So, low residual stress advantages good for LIDT of film.
High weak absorption was a disadvantage for sample E but low residual stress was its advantage. However, low weak absorption was an advantage for sample F but higher residual stress was its disadvantage. The LIDT of film was the result of competition of its disadvantages and advantages. The residual stress dominated in sample E comparing weak absorption during the competition and it produced high LIDT. Conversely, weak absorption mainly controlled in sample F, and it produced low LIDT.
4. Discussion and conclusions
In general there are many factors that can contribute to determining the LIDT of optical films, such as stress, absorption, electric field distribution, roughness of surface and so on. It is difficult to say which factor is the most important. The investigation demonstrated that the LIDT of film is determined by the competition between disadvantageous factors and advantageous factors. Reducing the influence of disadvantageous factor and enhancing the influence of advantageous factor may be the reasonable way of improving the LIDTs of films. During the preparing of sample E, SiO2 layers were depositing by electron beam evaporation, so the disadvantageous influence of absorption is reduced. While HfO2 layers were deposited by IBAD, so the advantageous influence of lower residual stress is enhanced. It is the reason that sample E had higher LIDT than sample F. It can be concluded that if the advantages and disadvantages of films are controlled by IIAD, the LIDT of films will be controlled too.
In conclusion, antireflection films of quite high damage threshold (18 J/cm2) have been deposited by the IIAD technique. It is found that the LIDT of film depend on the competition between disadvantageous factors and advantageous factors, and IIAD can control the disadvantages and advantages. IIAD is a novel way of preparing the films with high LIDT.
This work is partly supported by the National Key Technologies R&D Program (Grant No. 2006BAK03A03) and the Shanghai Committee of Science & Technology (Grant No. 06DZ22016). The authors would like to thank Professor Xuanxiong Zhang of university of Shanghai for Science and Technology and Dr. Chaoyang Wei of Shanghai Institute of Optics and Fine Mechanics for helping then improve English expression and readability.
References and links
1. M. Alvisia, M. D. Giulioa, S. G. Marroneb, M. R. Perronec, M. L. Protopapac, A. Valentinib, and L. Vasanellia, “HfO2 films with high laser damage threshold,” Thin Solid Films 358, 250–258 (2000). [CrossRef]
2. Y. Zhao, Y. Wang, H. Gong, J. Shao, and Z. Fan, “Annealing effects on structure and laser-induced damage threshold of Ta2O5/SiO2 dielectric mirrors,” Appl. Surf. Sci. 210, 353–358 (2003). [CrossRef]
3. Y. Zhao, T. Wang, D. Zhang, J. Shao, and Z. Fan, “Laser conditioning and multi-shot laser damage accumulation effects of HfO2/SiO2 anti-reflective films,” Appl. Surf. Sci. 245, 335–339 (2005). [CrossRef]
4. P. J. Martin, “Review of ion-based methods for optical film deposition,” J. Mater. Sci. 21, 1–25 (1986). [CrossRef]
5. M. Alvisi, F. D. Tomasi, M. R. Perrone, M. L. Protopapa, A. Rizzo, F. Sarto, and S. Scaglione, “Laser damage dependence on structural and optical properties of ion-assisted HfO2 thin films,” Thin Solid Films 396, 44–52 (2001). [CrossRef]
6. R. R. Manory, T. Mori, I. Shimizu, S. Miyake, and G. Kimmel, “Growth and structure control of HfO2-x films with cubic and tetragonal structures obtained by ion beam assisted deposition,” J. Vac. Sci. Technol. A 20, 549–554 (2002).
7. C. J. Stolz, F. Y. Genin, and M. R. Kozlowski, “Influence of microstructure on laser damage threshold of IBS films,” Proc. SPIE 2714, 351–359 (1996). [CrossRef]
8. N. J. Bazin, J. E. Andrew, H. A. McInnes, and A. J. Morris, “Temperature effects on the LIDT of single- and multilayer sol-gel-derived thin film films,” Proc. SPIE 4347, 127–138 (2000). [CrossRef]
9. B. Andre, L. Poupinet, and G. Ravel, “Evaporation and ion assisted deposition of HfO2 films- some key points for high power laser applications,” J.Vac. Sci.Technol. A 18, 2372–2377 (2000).
10. S. Wu and N. J. Dovichi, “Fresnel diffraction theory for steady-state thermal lens measurements in thin films,” J. Appl. Phys. 67, 1170–1182 (1990). [CrossRef]
11. S. Fan, H. He, J. Shao, Z. Fan, and D. Zhang, “Absorption measurement for films using surface thermal lensing technique,” Proc. SPIE 5774, 531–534 (2004) [CrossRef]
12. S. Shao, J. Shao, H. He, and Z. Fan, “Stress analysis of ZrO2/SiO2 multilayers deposited on different substrates with different thickness periods,” Opt. Lett. 30, 2119–2121 (2005). [PubMed]
13. ISO 11254-1:2000: Lasers and laser-related equipment -- Determination of laser-induced damage threshold of optical surfaces -- Part 1: 1-on-1 test
14. W. Gao, W. Zhang, S. Fan, D. Zhang, J. Shao, and Z. Fan, “Effects of the Structure of HfO2 Thin Films on Its Laser-induced Damage Threshold,” Acta Photonica Sin. 34, 176–179 (2005).