OPTOELECTRONICS LETTERS Vol.7 No.6, 1 November 2011

High damage threshold HfO2/SiO2 multilayer mirrors depos-ited by novel remote plasma sputtering

XU Ying 1, CHEN Nan 2, and BU Yi-kun 2**

1. Department of Mechanical Engineering, Changchun University of Science & Technology, Changchun 130022, China2. Department of Electronic Engineering, Xiamen University, Xiamen 361005, China

(Received 17 May 2011) Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2011C

Sputtering deposition coatings offer significant advantages on electron beam (EB) deposition, including high packing density,environmental stability and extremely low losses. But the inherent high compressive stress affects its application in highpower laser system. This paper describes the technical feasibility of high damage threshold laser mirrors deposited by anovel remote plasma sputtering technique. This technique is based on generating intensive plasma remotely from the targetand then magnetically steering the plasma to the target to realize the full uniform sputtering. The pseudo-independencebetween target voltage and target current provides us very flexible parameters tuning, especially for the films stress control.Deposition conditions are optimized to yield fully oxidized and low compressive stress single layer HfO2 and SiO2. The highdamage threshold of 43.8 J/cm2 for HfO2/ SiO2 laser mirrors at 1064 nm is obtained. For the first time the remote plasmasputtering is successfully applied in depositing laser mirrors with high performance.Document code: A Article ID: 1673-1905(2011)06-0405-5DOI 10.1007/s11801-011-1058-z

The development and utilization of high power laser systemand high intensity laser-plasma interaction studies requirethe high damage threshold laser mirrors[1,2]. The significantresearches on the high damage threshold laser mirrors havebeen widely reported by conventional electron beam (EB)evaporation[3] and ion assisted deposition (IAD)[4]. Sputter-ing deposition is emerging as a preferred alternative for thedeposition of advanced optical thin films[5]. The films ob-tained by sputtering deposition have high packing densities,no surface defects and very low absorption and scatter losses.Ion beam sputtering (IBS) is the only technique, which iscapable of producing sufficiently low losses for ring lasergyro mirror. In spite of many advantages of IBS on the fabri-cation of high performance optical coatings, but IBS coat-ings have excessive compressive stresses[6], which often re-sults in crazing of the films because of the stress damage inhigh power laser application. Therefore many high damagethreshold laser mirrors are fabricated mainly by EB or IAD[3,

4], and few studies on damage threshold have been publishedfor sputtering deposition[7]. The remote high density plasmasource was developed in 2001 by Mike[8]. The technique over-

comes several limitations of conventional sputtering, and hasbeen used successfully in the field of ferromagnetic materi-als deposition[9]. In this paper, the process is developed fordepositing stable, dense, smooth and low stress metal-oxideoptical thin films for laser mirrors. Previous reports suggestedthe damage threshold of IBS coatings is strongly related tothe material,s intrinsic properties[10], therefore the materialselection is critical. We select HfO2 and SiO2 materials andadopt various deposition methods[11,12]. Damage thresholdsare measured and the damage morphologies are analyzed forsingle layer and multilayer laser mirrors at 1064 nm.

The schematic diagram of the system is shown in Fig.1.Unlike conventional magnetron sputtering, the technique re-lies on the generation of plasma remotely from the targets,and the internal magnetic elements behind the target areeliminated. An radio frequency (RF) coil antenna surround-ing a quartz tube is attached to the vacuum chamber as a sidearm. The plasma is initiated and amplified by a direct cur-rent (DC) launch electromagnet towards the plasma sourceexit. By altering the current applied to the electromagnet andin particular to steering the electromagnet, the position of

* This work has been supported by the National Natural Science Foundation of China (No. 50802080) and the Natural Science Foundation of FujianProvince of China (No.2010Jo1349).

** E-mail:buyikun139@163.com

Optoelectron. Lett. Vol.7 No.6

the plasma beam can be adjusted to ensure good beam im-pinging on the target surface. But without the DC bias on thetarget the plasma has a low ion energy that is not sufficient tosputtering from the target directly. In order to sputter on thematerial, a negative DC bias (0-800 V) needs to be appliedto the target. The system is pumped using the cryo-pumpwith a base pressure of 6×10-6 Torr. A throttle valve can beused to adjust the pump speed. Pure argon gas is introducedinto the chamber by placing a diffusion ring close to the tar-get, and the oxygen is fed into the chamber though anotherdiffusion ring placed as close as possible to the substrate.This is done to maintain the target in a metal mode. We areable to deposit good quality metal-oxide in the high growthrate region of the hysteresis curve, which is far from the run-away point. The rate of deposition by remote plasma is fourtimes greater than that obtained by conventional reactive ionbeam sputtering. The main process parameters for optimalfilm performance are RF power, DC bias voltage, work gaspressure and reactive gas pressure.

Fig.1 Schematic diagram of the remote plasma sputter-ing system

Because of the special conformation for plasma system,the remote plasma generation offers a solution to the powerconstraints of the conventional magnetron sputtering. In theremote plasma system, above approximately 90V targetvoltage, the target current is saturate which is determined bythe remote plasma RF power and system operating pressure.There is a pseudo-independence of target current and targetvoltage, as shown in Fig.2. The target current determines theplasma density, and the target voltage determines the energyof the argon ions impacting the target surface. The highertarget voltage, the higher energy of the argon ions hitting thetarget is which leads to higher kinetic energy of target material.The independence of target current and target voltage offersa great deal of flexibility with regard to the control of growthconditions and film properties. The ability to control theplasma ion density is the key to control the stress of films.The lower stress is attributed to lower ion energies and higher

ion fluxes provided by the remote plasma process, and otherparameters, such as film transparency and adhesion, can befine-tuned for the best process window. It’s a worthy researchdirection for high performance sputtering coating in laserapplication on how to ensure enough film packing density,good adhesion and low compressive stress.

Fig.2 Dependence of sputteing target current on targetvoltage with different remote plasma RF power valuesat constant process pressure 3×10-3 Torr

Another advantage is the fully uniform erosion on thesurface of the target because of eliminating the internal mag-netic elements behind the target. This is the obvious differ-ence from the conventional magnetron sputtering. It allowsfor a uniform reaction in the plasma phase when performingreactive sputtering, which leads to the formation of materialwith a uniform stoichiometry.

For researching high performance laser mirrors, oxidefilms HfO2 and SiO2 are deposited using reactive sputteringdeposition. Silicon (purity 99.999%) and hafnium (purity 99.99%) targets are used in DC sputtering mode. The BK7 glassand ZF7 glass with 30 mm in diameter, 5 mm-thick and highsurface quality of 10-5 scratch dig are used to deposit thefilms. The BK7 glass is for HfO2, and the ZF7 glass is forSiO2 because of the larger refractive index difference betweenthe SiO2 and ZF7. To achieve a better envelope fitting accu-racy and inspect the absorption at 1064 nm wavelength, thethickness of the deposited single layer is approximately oneoptical thickness at 1064 nm. Multilayer laser mirrors aredeposited onto the BK7 substrate. A multilayer stack of sub-strate/(HL)16H4L/air consisting with 16 pairs of high andlow index quarter-wave-thick layers and a half-wave-thickSiO2 overcoat at central wavelength 1064 nm is deposited bytime-power control. 4L serves as protective layer to improveresistance to laser damage. The optimal deposition param-eters for high quality HfO2 and SiO2 are listed in Tab.1

The optical properties of the single layer HfO2 and SiO2

are studied in order to deposit multilayer laser mirrors withperformance which matches design. The optical transmittance

XU et al. Optoelectron. Lett. Vol.7 No.6

of sample with uncoated substrate is measured using Perkin-Elmer Lambda 750 spectrometer from 350 nm to 1300 nm,and is shown in Fig.3. The transmittance at half-wave pointreaches the highest value, and this means that at this condi-tion the deposited film has the lowest absorption. The opti-cal constants, n , k, and physical thickness are calculated fromthe transmission spectra by the envelope method[13], which isbased on the analysis of the transmission spectra with weaklyabsorbing film that is deposited on an free-absorptionsubstrate. The dispersion data for the HfO2 and SiO2 areshown in Fig.4 and Fig.5. The refractive index is close tothat of bulk material, due to the high energy of the plasmaprocess. The index of refraction for HfO2 is 2.013 at 1064nm, and that for SiO2 is 1.462 at 1064 nm. The absorption isalso exceptionally low due to the efficiency of oxidation. Theextinction coefficient of HfO2 at 1064 nm is in 10-5 level, and

Fig.3 Transmittance curves of HfO2 and SiO2 single lay-ers under the optimal deposition conditions

Fig.4 Dispersion curve for HfO2 single layer under theoptimal deposition conditions

Fig.5 Dispersion curve for SiO2 single layer under theoptimal deposition conditions

Fig.6 Experimental and theoretical transmission spec-tra for HfO2/SiO2 laser mirror at 1064 nm

The single layer film stress is evaluated by measuringthe change of sample curvature in a Twyman-Green interfer-ometer before and after deposition, according Stoney’s singlelayer stress expression[14]:

fs

2ss

)1(61

dvdE

R , (1)

where ds and df are the thicknesses of the substrate and thefilm, respectively. Es is Young’s modulus, and s is Poisson’sratio of the substrate. R is the net change in the substrate

that of SiO2 at 1064 nm is in 10-6 level. Fig.6 shows the actu-ally measured transmission spectra of 34-layer laser mirrorand which matches well with the theoretical design result.From the measurement data, it was concluded that the re-flectance is exceeded 99.8% at 1064 nm wavelength.

Tab.1 Parameters of optimal deposition for remote plas-ma reactive sputtering

Material Work gas Reactive gas RF power Targetvoltage Deposition optical Deposition

pressure (Torr) flow (sccm) (kW) (V) thickness rate (nm/s) SiO2 8.2 10-3 4.4 1.6 300 1 at 1064 nm 0.27 HfO2 6.2 10-3 10.5 2.1 450 1 at 1064 nm 0.33

Optoelectron. Lett. Vol.7 No.6

curvature before and after deposition. The substrate used tomeasure stress is BK7, which paramters are 25.4 mm indiameter, 0.5 mm-thick, Es= 81 GPa and ns = 0.208. Thephysical thickness of the films is calculated as a part of thespectral determination of the optical constants of the films.We test the stress of HfO2 and SiO2 materials in the optimalprocess window. The stress of HfO2 is about -133 MPa, andthe stress of SiO2 is about -165 MPa, where the negative showsthe stress is compressive stress. The indicated values are sig-nificantly lower than ion beam sputtering oxide films (-300Mpa -500 Mpa)[15].

In order to investigate the damage properties of the sput-tering films, damage threshold test is performed using a setup shown in Fig.7. A Nd:YAG laser system is used as theradiation source consisting of 12 ns pulses at the wavelengthof 1064 nm with maximum pulse energy of 100 mJ, the out-put TEM00 mode laser beam is focused onto the sample us-ing a 30 cm focal length lens to generate 400 m diameterlaser beam at normal incidence. Each sample is placed in anX-Y translation stage. All the automatic capabilities are com-puterized both for the displacements and for the diagnostics,as well as laser trigger. High sensitive on-line damage detec-tion is performed with the help of a video microscope sys-tem including digital image processing. The video micro-graphs of the site under test are acquired and stored beforeand after laser trigger, and 1-on-1 testing mode is performedby the facility. All single layer and multilayer samples aretested on the same location with the increase of radiationuntil damage is observed. The laser fluence is increased inthe increments of energy density of 250 mJ/cm2. At everyradiation lever, 5-10 points are tested. After testing, we findtwo radiation levels, where no damage is observed and thelowest radiation level, at which at least 60 percent of thesites are damaged, and the threshold fluence is calculated asthe linear fit to the data between these points.

Fig.7 Schematic diagram of the laser damage thresholdtest facility

Damage morphologies for single layers and multilayermirror are shown in Fig.8-Fig.10. According to ISO11254-

1, laser damage threshold of the HfO2/SiO2 mirror is deter-mined by a linear extrapolation of the damage probabilitydata to zero damage probability as shown in Fig.11. The low-index material SiO2 has the higher damage resistance thanthe high-index material HfO2. The SiO2 single layer damagethreshold is up to 24.2 J/cm2, and the HfO2 single layer dam-age threshold is up to 16.4 J/cm2. The damage morphologiesappear weak stress damage of the coating material. The sur-face of SiO2 film appears only color change and has somefine cracks, but the breaking doesn’t happen. The surface ofHfO2 film appears some thermal melting and crapy morp-hology. The multilayer mirror damage morphology showspitting and delamination. It looks like a eye-pattern on thefilm surface and is no more the small localized defects with afew microns size, because of extrinsic absorbing centers,which is more related with the intrinsic properties of coatingmaterial. It is different from the hole damage morphology ofconventional EB evaporation. From the damage morphology,it also suggests that the breaking is likely to occur first in thehigh-index HfO2 layer. Because of the SiO2 half-wave pro-tection coating, we believe it give better contribution to in-crease the damage resistance. The damage threshold of zerodamage probability for HfO2/SiO2 laser mirror is up to 43.8J/cm2.

Fig.8 Surface damage morphology of HfO2 single layer

Fig.9 Surface damage morphology of SiO2 single layer

XU et al. Optoelectron. Lett. Vol.7 No.6

Using similar laser damage test conditions, the laser dam-age threshold of Ta2O5/SiO2 mirror by ion beam sputtering isup to 28 J/cm2 by reducing stress with dual-side coating[7].

Based on the analyses of the damage morphologies, wecan find that coating deposition by remote plasma sputteringhave few defects and very low absorption. Low compressivestress effectively improves the damage resistance. In remoteplasma process, the lower stress is achieved by lower ionenergies and higher ion fluxes using to ensure film transpar-ency and adhesion. This technology gives us a potential ad-vantage to research the high damage threshold laser mirrors.

The novel remote plasma sputtering technology for highdamage threshold laser mirrors deposition is developed. Byreactive sputtering, dense metal-oxide optical thin films (SiO2,HfO2) with a high deposition rate, excellent optical proper-ties and low compressive stress are achieved. The two mate-rials with low compressive stresses have been successfullystacked to make high reflectivity laser mirrors. The obtaineddamage threshold with 12 ns pulse width at 1064 nm is up to43.8 J/cm2 for the 34-layer HfO2/SiO2 laser mirror. The dam-age threshold of multilayer mirror is limited by the high-in-dex material, so next efforts toward the development of higherdamage threshold high-index materials are underway to ex-

Fig.10 Surface damage morphology of HfO2/SiO2 laser mirror

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