J Sol-Gel Sci Technol (2012) 61:206–212 DOI 10.1007/s10971-011-2615-4

ORIGINAL PAPER

A review of contamination-resistant antireflective sol–gel coatings Xiaodong Wang • Jun Shen

Received: 13 September 2011 / Accepted: 24 October 2011 / Published online: 2 November 2011 Ó Springer Science+Business Media, LLC 2011

Abstract Sol–gel derived silica antireflective (AR) coatings have been widely used as the optical components for high peak power laser systems because of their excellent optical properties and high laser-induced damage thresholds. However, the sol–gel derived coatings have a high surface area that is more susceptible to be contaminated by absorption of trace amounts of water vapor and other volatile organic compounds from the environment. In this paper, the major approaches to fabricate contamination resistant sol– gel derived silica AR coatings have been extensively reviewed. Different approaches, including solution-phase and vapor-phase silanization, ammonia–water vapor treatment and fluorine modification have been discussed. The optical properties and laser-induced damage thresholds of modified coatings have also been evaluated. The improved sol–gel AR coatings have been shown to possess superior contamination resistance to work in vacuum systems compare to the traditional sol–gel AR coatings. Keywords Sol–gel  Antireflective coating  Contamination resistant  Silanization

1 Introduction The development and construction of high peak power lasers for high energy density physics and inertial

X. Wang  J. Shen (&) Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, People’s Republic of China e-mail: [email protected] X. Wang e-mail: [email protected]

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confinement fusion (ICF), such as the National Ignition Facility (NIF), Nova and OMEGA in the United States, HiPER in European Union, Le Laser Me´gajoule (LMJ) in France, Vulcan laser in the United Kingdom, GEKKO in Japan and Shenguang in China, continues to generate strong interest in the behavior of optical components under intense laser irradiation. The design of such lasers has created significant technological challenges in the area of laser glass, KDP crystal growth and surface coating technology. To avoid the reflection losses caused by the differences between the refractive indices of air and of the optical components, various thin-film antireflective (AR) coatings are applied to the optical surface. Among the different coating techniques available, the sol–gel method is generally preferred for AR coatings in high peak power laser systems in that it offers the advantages of high laserinduced damage threshold (LIDT), relatively ease of deposition and capability of large coating size [1–6]. For many years, contamination has been known to degrade the performance of optics and LIDT. The high porosity of the sol–gel derived silica AR coating causes the effective surface area of coated optics to be at least an order of magnitude larger than the uncoated surface area. This larger surface area, combined with the polar nature of the silica surface, makes such coatings particularly susceptible to contamination by race amounts of water vapor and other volatile organic compounds such as plasticizers, silicon oil, cutting fluid, pump oil, and vacuum grease [7]. In the high peak power laser systems, hundreds of motors, slides and other mechanisms are used nearby the optics. Volatile organic compounds from these mechanisms would accelerate and aggravate the contamination to the porous AR coating. Therefore, contamination effects could become quite serious without improved technology.

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2 Basic approaches Previous work [8–10] has shown that alkylation of free hydroxyl groups on the silica with organosilicon compounds was an effective method to improve the contamination resistance of currently used sol–gel coatings. Silica-based sol is formed by alkaline hydrolysis of tetraorthosilicate (TEOS) under a base-catalyzed condition in ethanol solution. The resulting reaction cleaves off the ethoxide groups and the free hydroxyl groups on adjacent molecules combine (condense) to form a larger molecule. This condensation reaction continues in three dimensions until all the reactants are consumed to form colloidal silica particles. After removal of excess catalyst (NH3H2O), the colloidal silica particles can be deposited on a substrate by either dip-coating or spin-coating to produce an AR coating. However, the silica particle in the sol or on the coating surface has a polar hydroxyl covered surface that is susceptible to be contaminated by water vapor and volatile organics. To overcome this problem, organosilane alkylating agents are used to replace the available hydroxyl groups with trimethylsiloxyl group (Fig. 1), resulting in a hydrophobic outer shell that resists water and organic vapor, reduces the number of sites available for hydrogen bonding with polar species, and produces a more tightly woven silica particle, preventing the penetration of larger contaminants [11, 12]. 2.1 Contamination resistance characterization It has been known that the absorption of contaminants such as water vapor or other volatile organic compounds not only influences the refractive index and thickness of the sol–gel coating, which in turn reduces its AR efficiency, but also lowers the LIDT of the AR coating. Thus, it is necessary to measure both the optical performance and the LIDT to evaluate the contamination resistance capability of the modified AR coatings. Since it is not possible to do these preliminary contamination tests in the high power laser systems, the deposited coatings are generally contaminated in a simulated environment.

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The coated samples were usually kept in sealed containers (in some cases even in vacuum chambers [14, 15]) for a few days. A number of ‘‘dirty’’ substances such as decahydronaphthalene, isooctane, hexane, machine oil, deionized water and ethanol, were put into the chamber surrounding the samples to produce a saturate environment of the contaminants. The ‘‘dirty’’ substances vary in different researchers’ work due to the detailed contaminant sources are still ambiguous. But it is generally regarded as volatile organic compounds, which means the results still make sense and are still comparable. It is also important to note that the contamination testing conditions used in the study (a saturated environment of vapor contaminants) are far more severe than what any optical elements would ever be experienced in the high power laser systems over even a long time. Transmission and reflectance spectra of the coatings were both taken before and after contamination to monitor the change in optical characteristics. Furthermore, the refractive indices and thickness of the coatings could be extracted from the spectra to evaluate the optical change of the coatings directly. The single-shot laser damage testing of modified and unmodified coatings were also measured using high power laser (Nd: YAG) to see whether the LIDT of the coatings is degraded by the modification. (The laser wavelength and pulse width are different due to the different laser systems the researchers used.) A droplet of water or oil is often deposited on the surface of the coating to measure the water/oil contact angle. However, the contact angle measurements only indicate the degree of hydrophobicity or lipophilicity. It only measures the contact angle between the surface and the large water/ oil droplet, while the contaminants in the high power laser systems exist as water/oil vapor molecules. Thus, it cannot be used to evaluate the contamination resistance of the coating directly. As the organosilanes react with the silica particles, the outer shell of the particles becomes increasingly hydrophobic, tapering off to a maximum value when all of the surface hydroxyl groups have been consumed in the silanization reaction. Therefore, the contact angle can be used as a measure of the extent of silanization.

Fig. 1 Reaction of a silica particle with a reactive organosilane aklylating agent. The reactive species X can be any organosilicon compounds that is capable of reacting with surface hydroxyl groups [13]

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In general, the silanization can be done in solution-phase and vapor-phase.

Table 2 Laser-induced damage results on co-hydrolyzed sol–gel coatings (351 nm, 0.5 ns pulse) [8] Sol composition (mole %) [%]

T%

R%

T% ? R%

2.2 Solution-phase silanization

LIDT (J/cm2, 1-on-1) 19.43 ± 1.16

A solution-phase silanization reaction is usually carried out by modifying the standard silica sol solution through cohydrolysis with other organosiloxane additives, such as methyltriethoxysilane (MTES) or dimethyldiethoxysilane (DDS). In the co-hydrolysis method, organosiloxane additive was added to a standard TEOS-based sol solution at a certain molar percentage. The silica particle suspension was then modified by the organosiloxane group. Thus, the coating deposited by this modified suspension owns the capability to inhibit the water vapor and organic compounds. This process requires only minor changes to the existing TEOS sol–gel chemistry and deposition process. It is also possible to introduce any functional group such as carbon–fluorine bond to endow the final product with more properties. However, homogeneous surface morphologies and hydrolytic stability of the silane are compromised due to the lack of control over the growth process. Marshall et al. [8] reported that silica AR coatings formulated by co-hydrolysis of TEOS with MTES and DDS showed both excellent resistance to contamination by volatile organic compounds and high laser damage resistance. In this process, the organosiloxane is combined with TEOS in ethanol solution under base-catalyzed hydrolysis conditions at various molar percentages of X according to the formula, X ¼ Mx =ðMx þ MTEOS Þ where MTEOS is the number of moles of TEOS and Mx is the number of moles of the organosiloxane additive. They studied co-hydrolyzed sol–gel coatings with molar percentages of MTES and DDS ranging between 10% and Table 1 Behavior of co-hydrolyzed silica AR coating after exposed to fully saturated vapor environments of select organic liquids for 24 h at room temperature [8] Sol composition (mole %) [%]

DT% after exposure

TEOS 100

-1.0

MTES 10 DDS 10

-0.3 No change

MTES 20

-0.1

0.1

-0.1

DDS 20

-0.5

0.5

0.1

MTES 30

-0.1

0.1

0.4

DDS 30

-0.5

0.2

-0.7

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Isooctane (%)

Ethanol (%) 0.1 -0.1 -0.04

Decahydronaphthalene (%) -0.1 No change -0.1

TEOS 100

99.65

0.31

99.96

MTES 30

99.33

0.58

99.91

19.93 ± 0.46

DDS 30

99.91

0.03

99.94

18.97 ± 0.66

30%. Table 1 shows the results obtained for a representative group of contaminants. For each contamination tested, there is at least one co-hydrolyzed sol–gel composition that provides excellent contamination resistance. The reason for the increase in transmission observed in several of the coatings is due to changes in the refractive index and thickness of the coatings after contaminated under these saturated vapor exposure conditions. The LIDT results in Table 2 shows that both the MTES and DDS co-hydrolyzed AR coatings show essentially equivalent performance in both transmission and laser damage threshold to the standard TEOS-based silica AR coating. Therefore, this process is an effective method for increasing their resistance to volatile organic contaminants without compromising the AR capabilities and LIDT. 2.3 Vapor-phase silanization Vapor-phase silanization utilizes a reactive silane such as hexamethyldisilanzane (HMDS) in the vapor state to react directly with the silica coating. Compare to solution-phase silanization, vapor-phase silanization has a number of advantages [13]. Only pure vapor comes into contact with the surface to be modified, which eliminates any highermolecular-weight impurities that could compromise the LIDT of the modified coating (similar to distillation). Any excess of unreacted material that could potentially cause changes in the optical properties of the coating is avoided, as only the amount needed to fully functionalize the coating surface is deposited. The vapor-phase process is extremely simple to implement as well; the untreated sol– gel AR coating is exposed to organosilane vapor either at ambient or at an elevated temperature for an extended period of time. This method of deposition has a distinct advantage over solution deposition technique, especially for the very large optics that would be employed in high peak power laser systems. The chemical composition of the hydrocarbon layer can be easily adjusted by co-depositing several different organosilane materials simultaneously. Achieving the same results with a solution-based process requires preparation and deposition of separate sol–gel optical coatings for each composition to be evaluated, which is a much more complex and lengthy process.

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The major drawback of this approach is the unknown nature of bonding between the silane and the silica surface. Consequently, evaluating the degree of silanization or how completely the organosilanes had reacted with the surface is particularly important for vapor-phase reactions. Patil [13] and Marshall et al. [9] studied vapor-phase treatment of silica surface with hexamethyldisilazane (HMDS), tetramethyldisilazane (TMDS) and bis-(trifluoropropyl)-TMDS (FTMDS) at both room temperature and 50 °C. Contact angle measurements showed that TMDS treated sample and FTMDS treated sample possessed a contact angle of 119.2° and 114°, respectively, while HMDS had a lower contact angle (107°) than either TMDS or FTMDS. However, contamination testing suggested that HMDS treated coating was more effective than TMDS and FTMDS treated coatings (Fig. 2). It was also pointed out that none of these vapor phase techniques have provided as good protection against vacuum pump oil contamination as solution-based methods. But their resistance may still be good enough because the contamination testing condition used in this study is far more severe than the real condition.

2.4 Ammonia–water vapor treatment Ammonia–water vapor treatment of silica coatings has been used to increase their mechanical strength [16, 17]. As-prepared sol–gel silica coatings contain lots of hydroxyl groups and some unhydrolyzed ethoxyl groups on the surface of the silica particles. Ammonia–water vapor treatment can hydrolyze the remaining ethoxyl groups to

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hydroxyl groups with the formation of many Si–OH groups followed by self-condensation to siloxane linkages, as described in Fig. 3. Figure 3 shows that some of the hydroxyl groups can link up to bind together with the adjacent silica particles, thereby the average distance of the particles becomes smaller than before, which in turn makes the coating denser and improves the abrasion and contamination resistance. Several researchers [10, 14, 15, 18] had reported that the resistant capability to contamination of sol–gel AR coatings could be greatly improved by co-treatment of the coating with ammonia vapor and HMDS. The method involves initial treatment with ammonia and water vapor followed by treatment with HMDS vapor. In the first step, all residual ethoxyl groups are eliminated from the silica particle surface by hydrolysis and replaced with hydroxyl groups. This has the additional advantage of linking particles together by formation of siloxane bonds, which increases the abrasion resistance. In the second step, the HMDS firstly reacts with a surface hydroxyl group to attach one trimethylsilane group to the substrate surface and to form one trimethylaminosilane, which further reacts with the surface to deposit a second trimethylsilane group (Fig. 4). Ammonia is formed as a volatile by-product. Both the internal and external surfaces of the silica particles are finally covered by nonpolar methyl groups. In our recent work [15, 19], we employed this method to the silica AR coatings and studied the degradation of transmission of AR coatings and reduction of LIDT involving exposure of treated and untreated coatings to the oil vapor contamination.

Fig. 2 Transmission spectra for silica AR coatings treated with different vapor-phase organosilanes before and after exposure for 24 h to an environment saturated with vacuum pump oil vapor (60 °C, 80 mTorr) [13]

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Fig. 3 Condensation of silicon–hydroxyl groups to siloxane linkages

HO

OH

Si

Si SiO2

Si

OH

HO

Si

Fig. 4 The mechanism for the reaction of HMDS with a silica surface

SiO2

Si OH

Si

CH3 CH3 Si

Si CH3

N

CH3

CH3

CH3

Si O

OH Si O

Substrate

O O

Si SiO2

Si

3H2O

Si

O

CH3

CH3

Si O

H NH3

N H

Si CH3

OH

Si

HO

CH3 H CH3 CH3 Si

O

Si

NH 3 H2 O

SiO2

Si

OH Si O

Substrate

CH3 CH CH3 3 Si Si CH3 CH3 CH3 O O Si O

Si O

Substrate

Fig. 5 Transmission spectra of coated samples with different treatment before and after contamination. a Sample A, b sample B [15]

The silica sol was prepared with tetraethylorthosilicate (TEOS) as the precursor, ethanol as the solvent and aqueous ammonia as the catalyst. Polyethylene glycol with molecular weight 200 g/mol (PEG200) was added into the sol to control the structure of the silica particle. Porous silica AR coatings were then deposited on BK7 glass by dip coating method. Two kinds of samples, A and B, with different post-treatments were prepared. Sample A was heated in an oven at 120 °C for 3 h. Sample B was prepared by firstly exposing them to the aqueous ammonia solution for 24 h at 15 °C under atmospheric pressure and then to HMDS vapor for 36 h at 15 °C under atmospheric pressure and finally heating at 120 °C for 3 h. The samples were kept in a vacuum chamber with an oil diffusion pump system at 10-3–10-4 Pa for 140 h. Ten millilitre of dimethyl silicone oil (DMS) was put into the chamber to accelerate the speed of contamination. Figure 5 shows the transmission curves of the AR coatings before and after contamination. The transmission of sample A decreased continuously during the contamination exposure. Meanwhile, the peak transmission shifted gradually towards longer wavelength, indicating that the

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optical thickness (refractive index 9 physical thickness) increased. The last two spectra (after 95 and 140 h) were very close to each other indicating the coating was close to contaminant saturation. In contrast, sample B maintained its optical performance very well, as revealed in Fig. 5b. The LIDT measurement was carried out with a Nd: YAG high power laser at 1,064 nm, 1 ns pulse width. The results show that the ammonia and HMDS treatment produces an increase in the LIDT of the silica AR coatings. It is consistent with the results reported by Pareek et al. [14]. And Pareek et al. [14] also pointed out that the coatings treated by ammonia and HMDS have a higher LIDT for both before and after exposure to the oil vapor in comparison to untreated coatings. This indicates that ammonia and HMDS treated silica coatings can be very useful in improving their performance when used as AR coatings in the high power laser systems (Table 3). It is also found from Fig. 6 that the addition of PEG contributed to the increase of the LIDT of silica AR coating. After contamination, the LIDT of the coatings decreased because the absorption of impurity from the vacuum increased the thermal absorption of the coating.

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Table 3 Peak vales of the transmission before and after contamination [15], and the LIDT of the coatings before contamination Sample

T% before contamination

T% after contamination

LIDT (J/cm2, 1-on-1)

A

99.35

95.57

15.9

B

99.44

99.27

20.1

Fig. 6 LIDT values of the coating with PEG before and after contamination

However, the coating prepared from the system containing PEG still maintained a high LIDT comparing with the one without PEG. 2.5 Fluorine modification Fluorine has the highest electron affinity of any element, and for this reason it is one of the strongest oxidizing agents known. Single bond formed by Fluorine and other atoms has a shorter bond length and a higher bond energy compare with that formed by carbon and other atoms. Thus, fluorine forms stable compounds, fluorides, with all other elements for which the reaction has been attempted. Therefore, organic fluorine compounds tend to have high chemical and thermal stability, low surface free energy (water and oil-repellent properties), and low melting and boiling points. Zhang et al. [20] prepared contamination resistant AR coatings by introducing fluorine-containing organosilicon. The silica sol used for coating is modified with trimethoxy(3,3,3-trifluoropropyl)-silane(TFPTMS) and diethoxymethyl(3,3,3-trifluoropropyl)-silane(TFPMDES). Experimental results show that hydrophobic performance of the coatings is enhanced significantly. The anti-reflectivity of the coating is the best when the mass fraction of CF3–CH2–CH2–Si or CF3–CH2–CH2–Si–CH3 is between 0.40% and 1.5%. At a certain mass fraction of fluorine-

containing siloxane, the performance of high laser damage threshold is not affected obviously. A series of tests are carried out by exposing the coatings to vapor contamination in a vacuum chamber of 10-3 Pa. The results indicate that the stability and durability of AR coatings are improved greatly. Dodecafluoroheptyl-propyl-trimethoxylsilane (ActyflonG502) was selected as the fluorine source in the work reported by Zhao et al. [21]. Actyflon-G502 was doped during the synthesis of the silica sol, and the fluorine-doped silica AR coating was then prepared by the dip-coating process on K9 substrate. The results show that the peak transmission of the fluorine-doped coating is 99.7%, and its water contact angle and poly dimethyl siloxane oil contact angle are 129° and 86°, respectively. After accelerated contamination in the vacuum environment for 15 days, the transmission of the fluorine-doped AR coating was only reduced by 0.1%, which is much better than the standard sol–gel AR coating. Moreover, adding a thin protective layer on the silica coating surface, which can maintain the optical properties and resist the contamination, is also an alternative method. Teflon AF [fluorinated (ethylenic-cyclo oxyaliphatic substituted ethyl-enic) copolymer] is a family of amorphous fluoropolymers based on copolymers of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) [22]. In addition to the outstanding chemical, thermal, and surface properties, Teflon AF has unique electrical, optical, and solubility characteristics. It has the lowest refractive index [23] of any known polymer, making them very suitable as a protective layer for the silica AR coating. In our group, Wang et al. [24] had applied AF2400 layer on the silica coated KDP crystal to protect the KDP against the moisture. Though the contamination resistant testing is not carried out yet, Teflon AF is still a potential candidate for the contamination resistant silica AR coating.

3 Conclusions The effects of contamination on sol–gel derived silica AR coating are becoming increasingly noticeable as it has a high porosity and polar surface which make it easy to absorb the vapor contaminants from the environment. The methods for resisting the contamination to silica AR coatings have been reviewed in this paper. Solution-phase silanization, vapor-phase silanization, ammonia–water vapor treatment and fluorine modification are used to improve the capability of contamination resistance of the coating. These methods have significantly enhanced the contamination resistance almost without reducing the LIDT of the sol–gel derived silica AR coatings.

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In the future, more systematic study will be important to establish a more in-depth understanding of contamination mechanisms and coating structures. This understanding will be indispensable to further improve the performance and stability of sol–gel derived AR coatings for high power laser systems. Acknowledgments The authors acknowledge the organizing committee of 16th International Sol–Gel Conference (Hangzhou, People’s Republic of China) for the invitation to present this paper as an invited talk in the conference. This work was financially supported by National Natural Science Foundation of China (Grant No. 11074189), Funds for International Corporation of Shanghai Committee of Science and Technology, China (Grant No. 10520706800) and ShanghaiApplied Materials Research and Development Fund of Shanghai Committee of Science and Technology, China (Grant No. 09520714300).

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