reactive electron beam evaporation of gadolinium oxide optical

Thin Solid Films 440(2003) 155–168

0040-6090/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0040-6090(03)00678-3

Reactive electron beam evaporation of gadolinium oxide optical thinfilms for ultraviolet and deep ultraviolet laser wavelengths

N.K. Sahoo*, S. Thakur, M. Senthilkumar, D. Bhattacharyya, N.C. Das

Spectroscopy Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

Received 23 August 2002; received in revised form 17 March 2003; accepted 14 April 2003

Abstract

Availability of ultraviolet optical thin film materials, especially high index refractory oxides that transmit down to deepultraviolet (approx. 0.2mm) is very much limited. The present article discusses some of the research optimization studies ongadolinium oxide(Gd O ), a novel lanthanide sesquioxide material, for such challenging spectral requirements. Optical and2 3

topographical properties of single layer films have been studied using phase modulated spectroscopic ellipsometry, spectrophotom-etry and multimode atomic force microscopy. Films deposited at lower substrate temperatures have shown higher band gaps andhigher substrate temperatures yielded higher refractive indices. Multilayer high reflection filters have been developed for ultravioletlaser wavelengths such as ArF(193 nm), KrCl (222 nm), KrF (248 nm) and Nd-Yag-III(355 nm), using this material at lowersubstrate temperature conditions and successfully tested for their performances.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Atomic force microscopy; Optical coating; Optical properties; Surface morphology; Multilayers; Ellipsometry; Gadolinium oxide;Microstructural properties; Viscoelastic property; Force modulation; E-beam evaporation

1. Introduction

One of the most important regions of the electromag-netic spectra, which represents state-of-the-art in bothlasers and coatings are the ultraviolet(UV) from 0.2 to0.35 mm. There are very few thin film materials withsuitable optical and mechanical properties which cansatisfy the coating requirements in this wavelengthregion. Most of the coating materials, which are trans-parent in the visible region, become highly absorbing inthe UV region. Thus, this spectral region has beenposing considerable challenges with respect to newmaterials and their combinations. There are a few UV-transmitting high index materials-like Al O , HfO ,2 3 2

Sc O and ceramic composites, which have been tried2 3

out by several thin film experimentalists for developingmultilayer optical coatingsw1–5x. However, most of theresearch studies have indicated that such materials forma durable coating only when deposited at higher sub-strate temperatures. In the course of our present optim-ization studies, Gadolinium oxide is found to be a

*Corresponding author. Fax:q91-22-5505151.E-mail address: [email protected](N.K. Sahoo).

potential candidate for such requirements with its trans-mitting region starting approximately near 0.19mm andextending beyond 16mm. This makes it an attractivehigh index optical film material not only for ultravioletcoatings but also for multilayer devices with extendedspectral characteristics for lasers and related applica-tions. This novel material also forms durable coatingseven when deposited at ambient substrate temperatures,which makes it as an attractive choice for a variety ofsubstrates and low index coating materials for develop-ing multilayers.Technologically, UV-laser coatings and materials have

applications in laser ablation, laser spectroscopy, fluo-rescence stimulation in bio-medical applications andmore recently, in photolithography of ever-decreasingdimensions in microprocessor manufacturing. Typicallaser wavelengths for such applications include 355 and266 nm(third and fourth harmonics of Nd-YAG), 351nm (XeF), 308 nm (XeCl), 248 nm (KrF), 222 nm(KrCl) and 193 nm(ArF). The shorter wavelengthsbelonging to excimer gas lasers have applications indeep UV microlithography for generating feature sizes-0.25mm for data storage and microelectronics fabri-

156 N.K. Sahoo et al. / Thin Solid Films 440 (2003) 155–168

cation. Most importantly, the pulse widths for excimerlasers vary from;30 ns to sub-ns with powers exceed-ing 500 MW and repetition rates approaching 1 kHz.For such specific high power laser applications laserinduced damage has also been putting additional limitingfactors and constraintsw3x. This necessitates havingdesigns and coating materials with maximum stabilityat such high laser powers. In this work, process para-metric studies of a novel thin film optical coatingmaterial using gadolinium oxide(Gd O ), a lanthanide2 3

sesquioxide, as a high index material are reported. Someadvanced characterization techniques such as phasemodulated spectroscopic ellipsometry, multimode scan-ning-probe microscopy and uv–vis-nir spectrophotome-try has been used in investigating optical andmicrostructural properties of the electron beam depositedfilms. Some multilayer devices for ultraviolet laserssuch as ArF(193 nm), KrCl (222 nm), KrF (248 nm),XeF (351 nm) and Nd-Yag-III (355 nm) have beensuccessfully developed and tested using this novel mate-rial along with SiO as the low index counterpart.2

2. Previous studies on gadolinium oxide

Gadolinium oxide(Gd O ) has in recent days drawn2 3

considerable attention in microelectronics, magneticrecording media, new superconducting components, andsemiconductor and optoelectronic technologies. GdOx

has been found to produce layers with excellent surfacemorphologies as evidenced by surface roughness of lessthan 1 nm. Both good interfacial electrical characteristicsand good thermal stability of Gadolinium oxide arebeing utilized for reproducible fabrication of high per-formance metal oxide semiconductor field effect transis-tors (MOSFETs) from compound semiconductorsw6,7x.Gd O is one of the prominent rare earth oxides used to2 3

stabilize ZrO because its cubic phase appears at a lower2

dopant contentw8x. Steiner et al.w9x have studied,epitaxially grown crystalline Gd O on GaAs substrate2 3

and its structural modifications with heat exposures andmild ion bombardmentw9x. Like most other lanthanidesesquioxide single crystals, Gd O crystallizes in mon-2 3

oclinic, hexagonal and cubic structuresw9x. Garcıa-´Murillo et al. have recently studied some of the opticaland structural aspects of the films prepared by the sol–gel technique for high-resolution X-ray imaging appli-cationsw10x. It is well known that the resolution in suchimaging applications can be improved using a thin anddense scintillating film. Therefore, their primary aimwas to produce a highly dense lanthanide sesquioxideLn O (LnsGd, Lu) with good scintillation perform-2 3

ances and they observed Gd O to be the most appro-2 3

priate candidate.Recently, Dakhel has reported some of the optical

properties of Gd O films, prepared by evaporation from2 3

resistively heated crucible in the wavelength region of

300–700 nmw11x. He observed that the amorphousstructure for his film of approximately 180 nm thick-nesses and also predicted its band gap to be more than6 eV. In spite of several attractive properties of such ahigh band gap lanthanide sesquioxide material, there isno such detailed process optimization study reported sofar related to optical coating applications, which is theprime objective of our present investigations.

3. Experimental aspects

The gadolinium oxide films have been evaporated byreactive electron beam evaporation using VERA 902automated optical coating system. The gadolinium oxidegranules of Cerac’s product-G-1076 with the purity)99.9% have been used for this thin film evaporation.The real time rate of deposition and total physicalthickness were controlled and determined by Leybold’sXTCy2 quartz crystal monitors. A residual gas analyzerof Pfeiffer’s Prisma 200 has been used to analyze thepartial pressure of reacting gases. Leybold’s OMS 2000optical monitoring system has been used to record theoptical thickness using quarterwave-monitoringapproach. For controlling the total pressure during thereactive evaporation process, MKS mass flow controllershave been suitably interfaced with the coating system.More than 20 samples with various process parametershave been prepared for the present investigations. Theoptical transmittance and reflectance spectra of eachfilm were recorded using Shimadzu’s UV-3101PC uv–vis-nir spectrophotometer as well as Cary 2390 spectro-photometer systems(wavelength range: 185–3150 nm).The former spectrophotometric system uses an integrat-ing sphere for determination of both specular and dif-fused reflectance characteristics of thin film samples.Subsequently, the samples were analyzed using phasemodulated spectroscopic ellipsometric technique withthe help of Jobin Yvon’s UVSEL Ellipsometer(wave-length range: 175–1700 nm). The description of thissystem has been discussed elsewherew12,13x. It is wellknown that most often the optical and physical propertiesof the films vary along the growth direction in a verycomplex manner. For such films spectrophotometric andellipsometric measurements, yield a very different kindof spectral characteristics as compared to simplified andideal homogeneous structures. Various realistic thin film-modeling approaches have been adopted to extract thedispersive optical constants, void fractions, surfaceroughness and physical thickness of such filmsw14,15x.Topographic and surface viscoelasticity measurementswere carried out using a multimode scanning probemicroscope of NT-MDT’s Solver P-47H. The details ofthese mesurements and analyses are presented in thesubsequent sections.

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4. Analysis by phase modulated ellipsometry (PME)

During the last few years, it is well known amongstthe thin film researchers that the PME technique can beconveniently extended to non-destructively detect andcharacterize the distribution of inhomogeneities in trans-parent or weakly absorbing films, interfaces and canalso determine the optical function of the film materialat the same timew16x. Essential parameters of a slightlyinhomogeneous thin film are the mean dispersive refrac-tive index, physical thickness and the degree of inhom-ogeneity w17x. An important role in the theoreticalanalysis of spectroscopic ellipsometric data is played bythe so-called, quarter-wave(QW) and half-wave(HW)wavelength pointsw18x. The consideration of films withlinear refractive-index profiles predicts that the valuesof the ellipsometric angleC at the QW points areindependent of the degree of inhomogeneity, whereasthose values at the HW points are most sensitive to thefilm inhomogeneities and can be used to determine thedegree of the inhomogeneity. This approximation isbased on the assumption that the contribution of theinterference effect inside the film can be ignored. At thesame time, it incorporates exactly the reflections con-nected with abrupt changes of the refractive indexprofiles at the film boundaries. This approximation,however, is only a first approximation to the propertiesof inhomogeneous films. It is valid for films with alinear refractive index profile, but it does not entirely fitwith the properties of ellipsometric spectra in the caseof complicated refractive index profiles. Subsequently,it has been demonstrated that thec values at the QWpoints are independent of film inhomogeneity in onlysome of the special cases. Charmet and Gennes laterderived a more precise approximation for the case ofinhomogeneous layer bounded by the interface withhomogeneous substratew19x. In this case, the effect ofthe interface was incorporated exactly, whereas for theinhomogeneous layer the first Born approximation wasused. Such an approach was later extended to includethe local perturbations at both sides of the substrate.Subsequently, Tikhonravov and coworkers derived amore precise approximation for the inhomogeneous filmwith two abrupt changes of the refractive index at thesubstrate and the ambient interfacesw20,21x.

During the present investigation, spectroscopic phasemodulated ellipsometric technique has been extensivelyused to characterize the thin films for their refractiveindex, extinction coefficient, physical thickness andrelative compositions. Since some of the present filmsshowed index non-linearity in their characteristics, suchbehaviour was modeled using discrete-layer-based anal-ysis techniques. Among others, the roughness of surfacesor interfaces has long been known to influence theellipsometric data, and many groups have investigatedthis issue both theoretically and experimentally. It is

widely believed today that the effect of surface rough-ness on ellipsometric measurement can be quantitativelysimulated by treating the roughness as an imaginarylayer and can be involved in the theory of effectivemedium approximationsw22x. The effective mediumapproximation(EMA) is mostly accepted as a standardprocedure for treating mixed thin film layers includingphysical roughness in the optical analysis, when it isassumed, the lateral characteristic length of the rough-ness is much smaller than the wavelength. Recentsophisticated use of EMA has shown to be capable ofimplicitly taking into account of the specific surfacestructures with defined distance and radiusw22x. Thegeneral equation that describes the optical properties ofthe mixture ofm materials in EMA is given as follows:

m´y´ ´ y´h j hs f (1)j8´qK´ ´ qK´h j hjs1

where´ and f are the dielectric function and volumej j

fraction of phasej. The quantity´ is the dielectrich

function of the host material that has to be definedchoosing one of EMA sub-models.K is the screeningydepolarization factor and it is used to describe themicrostructure of the mixtures in the material. Forspherical inclusions,K takes the value of 2, which isthe case for the gadolinium thin film samples used underpresent investigations. The topographical measurementsof such samples using AFM technique have confirmedsuch compositional modeling.It is always more convenient to analyze the inhomo-

geneity of a layer by transforming it into a multilayersub-structure composed of many thin homogeneouslayers of different dispersive refractive indices. Theaccuracy of this technique depends on the number ofappropriate sub-layers with related index variation.Sometimes it is also possible to approximate the char-acteristics of such multilayer thin film systems to acontinuously varying inhomogeneous index profile. Dur-ing the present studies, we have adopted this approachfor a qualitative as well as quantitative analysis of thestructural as well as optical inhomogeneities in thinfilms w23x. These analyses have been carried out usingthree sub-layer systems with distinct optical and struc-tural properties, which are very similar to the modelproposed by Chindaudom et al.w24x. The dispersion forrefractive indices as well as extinction coefficients hasbeen defined for each sub-layer with the appropriatefunctions. As shown in Fig. 1, the model consists of asubstrate film interface layer with some voids(thicknessd1, void fractions vf1), below a nearly void-free middlelayer (thickness d2, void fractions vf2) and a micro-rough surface dense over layer(thickness d3, voidfractions vf3). The optical constants have been suitablyfitted using Cauchy’s absorbent and transparent disper-sion models in the appropriate spectral region of the


Fig. 1. Three-layer model of cross-section of thin film showing growthfeatures and it has been used for the present phase modulated spec-troscopic ellipsometric analysis.

measurement. The generalized form, which has beenused during this work for the refractive indexn andabsorption coefficientk, is given by:

4 9B=10 C=10n(l)sAq q (2)2 4l l

and,

4 9E=10 F=10y5k(l)sD=10 q q (3)2 4l l

wherel is the wavelength in nm and A, B, C, D, E, Fare coefficients for fitting. The ellipsometric spectra(cand D) of each sample are allowed to fit on basis ofthis modelw25x.

During the analysis and optimization it was observedthat the three-layer growth models of most of theprocess-optimized samples converged to a two-layerstructure and the thickness of the rough surface over-layer closely matches with the AFM measurements.Such a two-layer convergence has indicated a fairlyhomogeneous structure in these optimized films, whichhave been subsequently used for our multilayer devel-opment experiments. Some of the sample fittings in theellipsometric data for the gadolinium oxide films havebeen shown in the Fig. 2a and b.In the Fig. 2a fitting of ellipsometric parametersc

andD have been depicted with both experimental andsimulated characteristics. Fig. 2b depicts index disper-sion of individual sub-layers used in the ellipsometricgrowth modeling of this sample. It is worth mentioningthat for this measurement the single layer modelingcould not be fitted to the ellipsometric spectral datawithin the acceptable tolerance value, since each film

has at least a roughness component. Although three-layer model has fitted our data reasonably well, there isa further scope of improving the results by incorporatingmore number of sub-layers in the modeling, which canlead to a more detailed structure. After the sub-layergrowth analysis, the mean optical constants were com-puted in order to compare the variation in propertieswith respect to process parametersw23x.

4.1. Deposition parameter dependence of refractiveindex

The deposition parameters for PVD processes thatrequire close controls are: oxygen partial pressure, rateof deposition and substrate temperature. Conventionally,electron beam and resistance heated evaporation pro-cesses require optimized values in these parameters toachieve a high and desired quality thin film structureand properties. Like most thin films refractory oxidematerials, optical properties of Gd O have a strong2 3

dependency on these process parameters used during thepresent electron beam deposition process. The paramet-ric dependences were systematically studied by varyinga particular parameter, while keeping all others constant.In the present studies, it has been observed that theoxygen and substrate temperature are two dominantparameters in comparison to rate of deposition as pre-sented below. Substantial differences in the opticalproperties have been observed in the samples preparedat ambient and elevated substrate temperatures. Themost significant observation is the stable structure in thesingle layer thin films as well as multilayer devicesdeveloped at lower and even at ambient substrate tem-peratures. It opens up a wide range of possibilities forthis well-behaved high index lanthanide sesquioxide filmmaterial in using it with varieties of substrate materials.

4.1.1. Substrate temperature dependenceSubstrate temperature has played a very prominent

role in controlling the mean refractive index and bandgap of the present Gd O films. Variation of spectral2 3

index profile with respect to temperature has beendepicted in the Fig. 3. As it can be seen from this plotthat the refractive index profile has a steady increase intheir values as the substrate temperature increases. Thehighest temperature is restricted to 3508C in consideringthe practical aspects of the deposition system. Most ofthe commercial coating systems like our present one(VERA 902) have the limitation in going beyond thistemperature value. With this maximum limit to temper-ature, influence of other parameters for these films hasbeen studied in detail. Most interestingly, the coatingsat lower substrate temperatures those including theambient have shown most promising results with respectto stability. Such a property will make these films moreattractive in developing coatings on polymers, metal-dielectric filters and several other applications where


Fig. 2.(a) Spectroscopic ellipsometric analysis of Gd O film using three layer model and(b) Corresponding computed refractive index variation.2 3

In this plotc , D are experimental values andc , D are simulated ellipsometric parameters.E E S S

room temperature conditions are required. In our presentstudy, several high reflection filters have been developedfor various ultraviolet and deep ultraviolet laser systemsand the developmental task has been carried out at anambient and low-substrate temperature conditions.

4.1.2. Oxygen pressure dependenceOxygen pressure has very distinctive influences at

different rate of depositions. The film deposited withrate of deposition of 1 nmys, optimum oxygen pressure

of 1=10 mbar has produced the maximum value iny4

the refractive index. Below and above this pressure itproduces lower index profile as shown in the Fig. 4aand b. At a lower rate of 0.5 nmys the extent ofvariation is relatively less as shown in Fig. 4a. In thisfigure, the variation of oxygen pressure has been shownfor two extreme values. Similarly in Fig. 4b this varia-tion is recorded for the rate of evaporation of 1 nmys.It can be seen from this plot that the films deposited at1=10 mbar have shown better spectral profile.y4


Fig. 3. Spectral variation of refractive index with respect to the sub-strate temperature. The deposition rate and oxygen pressures weremaintained at 0.5 nmys and 1=10 mbar, respectively, during thisy4

experiment.

Fig. 4. Spectral variation of refractive index with respect to oxygenpressures(a) at substrate temperature of 3508C and deposition rateof 0.5 nmys and(b) at substrate temperature of 3508C and depositionrate of 1.0 nmys.

4.1.3. Rate dependenceRate of deposition has also different influences on

refractive index at different substrate temperatures. Inour experiment, the deposition rate has been varied from0.5 to 2.0 nmys at different substrate temperatures andoxygen pressures. It has been observed that at ambientsubstrate temperatures the variation in the rate of depo-sition has an opposite trend on the refractive indexprofile than at elevated temperatures. Fig. 5a and b haveshown the variation of spectral index profile with respectto rate of deposition under two different substratetemperatures. It has been clearly noticed that ambienttemperatures at lower rates give rise to higher spectralindex where as the effect is opposite at higher substratetemperature.

4.2. Band gap analysis

Band gap analysis of the films deposited under vari-ous process conditions has shown very interesting andinformative results not only for obtaining the qualitativeassessment but also in using them in the appropriatemultilayer devices. Films prepared under certain depo-sition conditions have shown some tailoring in theabsorption edges similar to indirect band gaps. Fig. 6shows a plot of the absorption coefficient, alpha(a) vs.energy (in eV) for a film deposited under highersubstrate temperature of 3508C, oxygen pressure of2=10 mbar and the rate of evaporation of 1.0 nmys.y4

As it can be seen from this plot, there are two distinctregions showing the features of both the direct andindirect band gaps. In order to determine the band gapsprecisely, the following equation has been used with

respect to the appropriate energy dependence. It is wellknown that for high absorbing region wherea exceeds10 cm ,a obeys4 y1

rasB(hnyE ) yhn (4)opt

whereE is the optical band gap energy and B is aopt

constant, andrs1y2, 3y2, 2, 3 depending upon thetype of the electronic transitions. For allowed directtransitions rs1y2 while rs3y2 for forbidden directtransition. For allowed indirect transitionrs2 and forforbidden indirect transitionrs3. The range withinwhich this equation is valid is very small and hencesometimes it becomes too difficult to determine theexact value of the exponentr.


Fig. 5. Spectral variation of refractive index with rate of depositionat (a) substrate temperature of 3508C and oxygen pressure of1=10 mbar and(b) at ambient substrate temperature and oxygeny4

pressure of 1=10 mbar.y4

Fig. 6. Absorption co-efficient of Gd O film deposited under sub-2 3

strate temperature of 3508C, deposition rate of 1.0 nmys and oxygenpressure of 2=10 mbar indicating the presence of both direct andy4

indirect band gaps.

The simplest way to deduce the type of transition isto examine the value ofr, which relateshn to a withr

a linear relationship. As shown in Fig. 7a and b, in therespective small energy ranges, the linear dependenceof a anda on hn have been identified and plotted1y2 2

in the graphs. For this sample the direct band gap is5.65 eV and the value of the indirect band gap is 5.07eV. During the course of our process-optimizing studies,it was noticed that under several distinct parametricconditions, mostly direct transitions have been observedin the band gaps. One noteworthy observation is thesubstrate temperature dependence of these band gapsduring preparation. It was quite distinctly noticed thatfilms prepared at lower substrate temperatures showedhigher band gaps. One of such results has been depictedin Fig. 8a. It can be seen from this plot that while other

parameters being the same(Rs1.5 nmys and Os21=10 mbar), the film prepared at ambient substratey4

temperature has a higher band gap of 6.3 eV than a filmprepared at a substrate temperature of 3508C having asmaller band gap of 5.86 eV. It is interesting to notethat there is a strong interdependence of depositionparameters in achieving a wide range of optical prop-erties and band gaps in these films. Fig. 8b shows thata different rate of deposition(1 nmys) can give rise toa higher band gap value(6.36 eV) even on a slightlyheated substrate(70 8C). At this deposition rate, thefilms deposited under the substrate temperature of 2508C have shown band gap value of 5.37 eV, which ismuch smaller than the film deposited at higher rate(1.5nmys) and higher substrate temperature(350 8C) (asshown in Fig. 8a). During our investigations, we couldnotice an excellent microstructural compatibilitybetween Gd O and SiO films deposited at the substrate2 3 2

temperature of 708C, which is very close to ambientvalues. Scanning probe viscoelasticity measurementshave indicated a similar type of matching surface stiff-ness property in these films at this temperature condition.Such deposition conditions are also highly favourable

while making filters and coatings on metallic, polymerand plastic substrates. All these studies indicate thatcontrary to other oxide materials, which invariably needhigher substrate temperature for achieving high qualityand stable films, gadolinium oxide can yield a stablefilm structure at lower and even at ambient conditions.The more detailed aspects of these stability criteria arepresented in the following sections. Fig. 9 depicts theband gaps of the films deposited at ambient substratetemperature and at two different rates. As it can be seen


Fig. 7. Graphical determination of optical band gaps(a) Indirect bandgap,(b) Direct band gap of Gadolinium oxide thin film deposited atsubstrate temperature of 3508C, deposition rate of 1.0 nmys and theoxygen pressure of 2=10 mbar.y4

Fig. 8. Plot ofa with photon energy for thin films deposited(a) at2

substrate temperatures of ambient and 3508C with deposition rate of1.5 nmys and(b) at substrate temperatures of 708C and 2508C butwith a different rate of 1 nmys.

from this figure, the lower rate of 0.5 nmys has yieldeda higher band gap value of 6.32 eV under ambientsubstrate temperature condition.

5. Analysis by atomic force microscopy

The surface properties and topography of the thinfilm samples have been investigated by a multimodescanning probe microscope system of NT-MDT’s SolverP-47H using its AFM head. The capabilities with thisscanning head are related to the measurement of surfacetopography, hardness distribution, friction forces, adhe-sion characteristics, viscoelasticity, electrical potentialconductivity and surface capacitancew26,27x. In thepresent studies all the AFM measurements were carriedout in ambient environment using contact mode opera-tion. Silicon cantilevers having a typical spring constantof 0.6 Nym and resonant frequency of 75 kHz were

used for both topographic and viscoelastic measure-ments. Gadolinium oxide films deposited under variousprocess parameters have shown a variety of interestingtopographic and other microstructural properties. In thiswork, some of the very prominent aspects of the filmproperties are presented. Viscoelasticity and long-termspectral stability measurements have established a verygood correlation in the experimental results.

5.1. Topographic measurements

Using this measurement sample films prepared undertwo different temperatures have been characterized fortheir topographies. Fig. 10a and b portray the surfacetopographies of Gadolinium oxide thin films depositedat substrate temperatures of 3508C and 708C, respec-tively. As it can be seen from the measurements, the


Fig. 9. Plot ofa with photon energy for two different thin films deposited at rate of deposition of 0.5 nmys and 1.0 nmys, respectively. The2

films were deposited at ambient conditions.

Fig. 10. AFM scans of Gd O films deposited at(a) 350 8C and (b) 70 8C showing two-dimensional topographic variations. Corresponding2 3

roughness values are 5.76 nm and 12.63 nm, respectively.

film deposited at lower substrate temperature is domi-nated by large grain structures with an average rough-ness value of 12.63 nm. The root mean square(RMS)value of the roughness for such a sample is 16.25 nm.However, the film deposited at higher substrate temper-atures has shown more regular and small features in thetopography. It has an overall roughness value of 5.76nm and RMS value of 7.2 nm. This clearly indicatesimproved topographic properties of the films depositedat elevated temperature at the expense of smaller opticalband gaps. Appearance of smaller grain size at higher

temperatures may be attributed to favorable adatommobility in yielding better film density and index. Theroughness values obtained by these measurements arecomparable with the information obtained through ellip-sometric modeling.

5.2. Force modulation (Local Viscoelasticity Imaging)results

This mode is usually done simultaneously with con-ventional AFM topographic imaging and helps in iden-


Fig. 11. Two-dimensional(a) Topography and(b) Force modulation AFM images of Gd O film deposited at substrate temperature of 708C,2 3

deposition rate of 1.0 nmys and oxygen pressure of 1=10 mbar.y4

Fig. 12. Two-dimensional(a) Topography and(b) Force modulation images of Gd O film deposited at a substrate temperature of 708C, deposition2 3

rate of 2.0 nmys and oxygen pressure of 1=10 mbar.y4

tifying and mapping the differences in surface stiffnessor elasticity. Force modulation technique is particularlyuseful for detecting soft and stiff areas on substrates orfilms, which exhibit overall uniform topographyw28–30x. With the force modulation technique, the probe orthe cantilever usually moves with a small vertical(z)oscillation (modulation), which is significantly fasterthan the scan-rate. It has important applications wheresurface features must be differentiated, and also ininvestigative studies of relative surface elasticity. Thesetechniques use a variety of surface properties to differ-entiate among different component materials on hetero-geneous surfaces.

The force on the sample is modulated about the setpoint scanning force such that the average force on thesample remains equal to that in simple contact mode.When the probe is modulated into contact with a sample,the sample surface resists the oscillation and the canti-lever bends. Under the same applied force, a stiff areaon the sample will deform less than a soft area; i.e.stiffer areas put up greater resistance to the verticaloscillation and consequently, greater bending of thecantilever. The variation in cantilever deflection ampli-tude at the frequency of modulation is a measure of therelative stiffness of the surface. Topographical informa-tion (DC, or non-oscillatory deflection) is collected


Fig. 13. Two-dimensional(a) Topography and(b) Force modulation images of Gd OqSiO multilayer reflecting filter developed for KrF(2482 3 2

nm) laser applications. The surface viscoelastic image shows a uniform contrast over the surface implying an appreciable multilayer stability.

Fig. 14. Experimental and computed reflection spectra of Nd-YAG-III (355 nm) reflection filter, consisting of 21 alternative layers ofGd O and SiO . The quarterwave design adopted here is(HL) H,10

2 3 2

where H is Gd O and L is SiO .2 3 2

simultaneously with the force modulation data(AC, oroscillatory deflection).The force modulation images as depicted in Figs. 11b

and Fig. 12b clearly differentiates the stiffer black areafrom the softer white zones and with some regularitiesin their positions. As it has been shown in Figs. 11aand 12a, the topographical images hint at their presencesonly. This information is extremely useful when devel-oping a stable multilayer structure combining two ormore different thin film materials(such as Gd Oq2 3

SiO multilayers). One can achieve a better stability in2

the multilayer device by choosing appropriate propertiesin surface elasticity in the component films. In thepresent case, appropriate deposition parameters forGd O films have chosen(Ts70 8C, Rs0.8 nmys,2 3

O s0.8=10 mbar) so that its viscoelasticity has ay42

good match with that of SiO films(Ts70 8C, Rs1.02

nmys, Os1.0=10 mbar). A surface with uniformy42

viscoelastic property shows minimum contrast variationin the acquired force modulation image. In fact thisaspect has been experimentally verified with the AFMmeasurement results of 25-layer 248 nm(KrF) reflectingfilter as shown in Fig. 13a and b. The viscoelasticimaging of such a multilayer(Fig. 13b) has shown auniform elasticity over the multilayer surface, which hasultimately demonstrated a very stable spectral character-istic in our spectrophotometric measurements.

6. Development of multilayer UV laser filters

Using gadolinium oxide(Gd O ) (Substrate temper-2 3

ature: 708C, Rate: 0.8 nmys, Oxygen partial pressure:0.8=10 mbar) as high index material along withy4

silicon dioxide (SiO ) (Substrate Temperature: 708C,2

Rate: 1.0 nmys, Oxygen partial pressure: 1.0=10y4

mbar) as the low index counterpart, several multilayerreflectors and filters have been successfully developedand tested for various laser applications. Out of theseprecision multilayer devices, experimental and computedspectral characteristics of four reflectors have beenpresented in Figs. 14–17, which have been developedfor Nd-Yag-III (355 nm), KrF (248 nm), KrCl (222nm) and ArF (193 nm) laser applications, respectively.Amongst these, the reflection filters for KrF, KrCl and


Fig. 15. Experimental and computed reflection spectra of 248 nmreflection filter for KrF Laser, consisting of 25 alternative layers ofGd O and SiO (Stack design:(HL) 0.7 H).12

2 3 2

Fig. 16. Experimental and computed reflection spectra of 222 nmreflection filter for KrCl Laser, consisting of 31 alternative layers ofGd O and SiO (Stack design:(HL) 0.3 H).15

2 3 2

Fig. 17. Experimental and computed reflection spectra of 193 nmreflection filter for ArF Laser, consisting of 41 alternative layers ofGd O and SiO (Stack design:(HL) 0.5 H).20

2 3 2

ArF lasers are highly challenging, and demonstrate theusefulness of this material for deep ultraviolet applica-tions. We have used 41 alternative layers of Gd O and2 3

SiO to achieve the required reflectance spectra for ArF2

(193 nm) laser application and the experimental andcomputed characteristics are shown in Fig. 17. The lastlayer is chosen as half of a quarterwave to reduce theripples in the pass band as per the experimental requi-rements. For KrCl(222 nm) and KrF (248 nm)-lasersreflecting filters, 31 and 25 layers of alternative Gd O2 3

and SiO have been used, respectively. The last layers2

(Gd O ) in these deigns were appropriately chosen to2 3

minimize the ripples in the pass band to the desiredlimit. For the development of these extreme ultravioletdevices, the quartz crystal monitor XTCy2 has beensuitably calibrated using OMS 2000 optical monitorsystem. The post deposition reflection spectra wererecorded using Shimadzu UV3101PC spectrophotometersystem fitted with a specular reflection attachment. Anintegrating sphere accessory in this instrument has beenused to record the diffused reflectance values of thedeveloped multilayer filters. It is worth mentioning thatin spectrophotometric measurements, transmittancespectra is better calibrated than the reflectance spectraand so also the accuracies. Besides, in Shimadzu 3101pcspectrophotometer, the specular reflectance attachmentuses an incidence angle of 58 during the measurement.For such reasons, the experimentalRqT of some mul-tilayer devices have slightly exceeded 100% in certainspectral regions due to a small calibration error in thereflectance-measuring mode of the instrument. The dif-ferences in the computed and experimental spectralreflectance characteristics of these developed filters can

be attributed to the quartz crystal calibration and moni-toring errors in detecting the specified layer thicknesses.Besides, the dispersive complex refractive indices ofeach sub-layer in the multilayer devices might havediffered from each other affecting both the bandwidthsas well as amplitudes of the spectral characteristics.Dispersion being more dominant in this lower wave-length region of the spectrum introduces larger errors inthe experimental characteristics with respect to theircomputed values.


Fig. 18. Spectral transmittance characteristics of 248 nm(KrF) reflec-tion filter measured in October 2001 and then in July 2002; i.e., at atime interval of approximately 10 months after repetitive use with thelaser. Both the mesurements have shown almost no shift in the spectralcharacteristic, which indicate a very appropriate multilayer temporalstability.

All these multilayer devices have shown quite stablepost-depositional spectral characteristics with respect toaging. The transmittance spectra of KrF(248 nm)reflector measured at an interval of 10 months of usewith the laser have been depicted in Fig. 18. Bothspectra have shown a very negligible shift in theircharacteristics implying an extremely good temporalspectral stability with respect to aging. This device isbeing used effectively for a spectroscopic experimentwith a KrF laser system(248 nm) operating at a pulseenergy of 400 mJ, pulse width of 25 ns and repetitionrate of 100 Hz. Similarly the device developed for Nd-Yag-III laser application(355 nm) has been successfullyused in another experiments with the laser pulse energyof 200 mJ, pulse width of 6 ns and a repetition rate of10 Hz.

7. Conclusions

Optical and microstructural properties of electronbeam deposited gadolinium oxide films, a novel lantha-nide sesquioxide optical material, have been studied indetail for the application in multilayer optical coatingswith our present requirements to the ultraviolet and deepultraviolet regions. The films have been characterizedby phase modulated spectroscopic ellipsometry, multi-mode atomic force microscopy and uv–vis-nir spectro-photometric techniques. Interestingly, the films depositedat lower and ambient substrate temperatures have shownhigher band gaps making them attractive for excimerlaser applications as well as for a variety of substrate

materials. During our experiments, band gap valueshave been observed in the range of 6.36 eV for thefilms deposited under lower values of substrate temper-atures. However, films deposited at elevated substratetemperatures have shown smooth topographies withhigher values in refractive indices and smaller RMSroughness. Surface viscoelasticity measurements throughforce modulation AFM technique have shown a nearregular distribution of soft and stiff zones on the surfacesof most sample films. Using process optimized films ashigh index layers several high reflection filters forvarious ultraviolet laser applications, such as KrCl, KrF,XeF and Nd-YAG-III wavelengths, have been success-fully developed and used for our spectroscopic experi-ments with a high degree of long-term spectral stabilityand reliability.

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