Structure-related bulk losses in ZrO2 optical thin films

Thin Solid Films, 187 (1990) 275-288 ELECTRONICS

STRUCTURE-RELATED FILMS A. DUPARRk,

275

AND OPTICS

E. WELSCH

BULK LOSSES IN ZrO, OPTICAL THIN

AND H.-G.

WALTHER

Sektion Physik, Friedrich-Schiller-Universitiit Jena, Max- Wien-Platz I, DDR-6900 Jena (G.D.R.) N. KAISER AND H. MijLLER Physikalisch-Technisches Institut der Akademie der Wissenschaften der D.D.R.. Helmholtzweg 4, DDR6900 Jena (G.D.R.) E. HACKER,

H. LAUTH,

J. MEYER

AND P. WEISSBRODT

Kombinat VEB Carl Zeiss Jena, Carl-Zeiss-Strasse 1, DDR-6900 Jena (G.D.R.) (Received

February

27, 1989; revised July 24,1989;

accepted

September

28, 1989)

Results of measurements of bulk scattering as well as absorption losses of evaporated ZrO, single-layer films are presented. A special layer design was used to eliminate the losses originating from the film interfaces. The scattering and absorption measurements are performed at 2 = 5 15 nm by means of total integrated scattering and photoacoustic techniques, respectively. Transmission electron micrographs of C-Pt replicas of cross-sections and electron diffraction studies reveal the correlations between bulk losses and morphology. The compositional depth profiles of the films were investigated by secondary neutral mass spectrometry. The observed absorption can be explained by contaminations homogeneously distributed throughout the film thickness. The results are discussed with respect to different deposition conditions and post-deposition annealing.

1.

INTRODUCTION

Zirconium dioxide (zirconia, ZrO,) is of current interest as a high index material in multilayer optical coatings since its absorption is low in a broad spectral region from the near-UV (above 240 nm) to the mid-IR (below 8 urn). Thin films of ZrO, are resistant against environmental influences. They are hard, durable and easily available. High repetitive pulse laser damage thresholds can be achieved by appropriate selection of the deposition conditions. However, the optical application of ZrO, films suffers from two serious drawbacks: variation in the refractive index with thickness’32 and high optical losses3*4,especially scattering. Thus the development of optical coatings containing ZrO, requires the localization and minimization of both absorption and scattering. Generally, the optical losses are structure- and/or impurity-related and act as performancelimiting factors 5-6. It has been shown that losses may arise from the interface as well as from the layer bulk’-“. Problems of light scattering in ZrO, films have been attributed to both crystallinity’ and the evolution of surface roughness2. Less well studied, however, is the relation between the optical losses arising from the interface and bulk regions of ZrO, films. A detailed knowledge of this 0040-6090/90/$3.50

Q Elsevier Sequoia/Printed

in The Netherlands

276

A. DUPARRB et al.

relation is of considerable interest in the design and production of high performance multilayer interference coatings. In this paper we present experimental evidence that the layer bulk can give different, in part significant, contributions to both absorption and scatter. Coating conditions and post-deposition treatment were varied to study the influences of evolutionary volume structure growth and contamination-related optical losses. The laser damage resistance and optical losses of pure and doped ZrO, films were investigated as well. These results and detailed information about the composition, crystallography and morphology of these films will be given in a forthcoming paper. 2.

EXPERIMENTAL DETAILS

2.1. Coating design andpreparation In order to separate the bulk loss components from the entire absorption and scatter, we used the layer design shown in Fig. 1 consisting of a A/2 or a 21 layer halfcoated on a high reflecting TiO,/SiO, multilayer system: air/2L(HL)112L/BK-7 substrate. Because of the electric field distribution within the half-wave or doublewave overcoat layer the optical losses arising from the interfaces should be suppressed’, ’ 2 even if the refractive index varies with the thickness as shown in Fig. 2. Thus both bulk absorption and scattering can be evaluated by subtraction of the appropriate losses of the underlying HR system. The ZrO, films were prepared by electron or 10.6pm continuous wave laser beam evaporation of monoclinic source material in a diffusion-pumped vacuum system in which a base pressure of less than 10m4 Pa was attained before the

HLHLHLHLH

Fig. I. Sample configuration Fig. 2).

for bulk absorption

and light scattering

L H L’L’&?ti*t?H”H-.‘H*‘H’

measurements

(for 2, and z2 see

Fig. 2. Computed electric field strength profile for a TiOzPSiO, quarter-wave HR stack coated with a 2 x A/4 (zl) and an 8 x A/4 (zJ single layer of ZrO, at a design wavelength I = 515 nm, - - -; influence of an example of the observed refractive index profile of an overcoat ZrO, film on the electric field distribution within the HR system, -_

STRUCTURE-RELATED

BULK LOSSES IN

ZrO,

277

evaporation began. A quartz balance the monitoring of the and reflectance-transmittance during deposition were used to control the rate of deposition and the optical thickness of the films respectively. To observe any variations in the refractive index during and after deposition scans over the 40&2800nm spectral range were made. For ex situ verification of the refractive index profiles single ZrO, layers were prepared simultaneously on separate BK-7 substrates. The residual atmosphere and the pyrometrically controlled substrate temperature were varied. The 0, partial pressure in the reactive evaporation process was 6 x 10m3 Pa. The deposition conditions and the measured bulk losses are summarized in Table I. TABLE I DEPOSITION Zr 0,

CONDITIONS.

SINGLE LAYEKS ON

Sample

a

BULK SCATTERING

HR

LOSSES s,

Preparation condirions

Electron beam evaporation, r, = 20 ‘C, amorphous Continuous wave CO, laser beam evaporation, T, = 20 C, amorphous Electron beam evaporation T, = 200 “C, without residual 0, pressure, cubicmonoclinic Electron beam evaporation, r, = 250 “C, with residual 0, pressure, cubic-monoclinic As for sample c, with post-deposition annealing at 400 “C As for sampled, with post-deposition annealing at 400 “C

b

c

d

e

f

ABSORPTION

A

AND ABSORPTION

COEFFICIENT

/&*

OF

SYSTEMS

The upper and lower loss values for each sample respectively.

s

A

(x 10-4)

(x 10-4)

/4,* (cm-‘)

1.o, 3.6 1.3 1.2

1.8, 1.9 3.1, 11.0

< 0.5, 3.5

3.9, 10.9

16.8

< 0.5,
2.6, 9.4

14.8

0.6, 10.0

0.9, 2.7

3.1

< 0.5, <0.5

3.1, 6.8

10.9

are related

to optical

7.7 17.3

thicknesses

nd = i./2 and 2E.

2.2.

of losses loss A measured at = 5 nm using photoacoustic gas a sensitivity about 5 10e6. The error cell set-up’ 3 due the detection scattered light by the walls was 10%. According Figs. 1 2 the absorption coefficient the overcoat may be from the of the (a) and (b) parts the samples: The

= A,+ = A, + P,* &* tH* where PH* = 2&,* geometrical thickness

(1) (2)

is the relative power density and tH* = kL/2n,* is the of the ZrO, film, with the averaged refractive index n,*; k = 1

278

A.

and 4 for half-wave

r=

-

and double-wave

films respectively.

A,);(ki.lrz,*‘)

I>U

PARR6 C’f U/.

Hence (3)

If a thickness-independent bulk absorption coefficient is assumed, the difference A, - A, should increase with increasing single-layer thickness. As a representative example the line-scanned lateral absorption profile for a half-wave and double-wave ZrO, film (sample b in Table I) over a sample distance of about I5 mm is shown in Fig. 3. The lateral single-layer bulk absorption signals are characterized by a steplike increase on scanning from the bare HR system to the overcoated region.

yw:-,;>,-=;

:~

I

I-

-.

i

1

‘>:‘I,.,,?
_,_,:

I rr

Fig. 3. Line-scanned lateral absorption profile of the half-wave and double-wave !aser-evaporated ZrOz single layer (see Table I. sample b) on the HR system (left-hand side) and of the bare HR system (righthand side) at i = 5 I5 nm (irradiation spot radius R = 2 mm)

The evaluation of the bulk absorption coefficient Fas just described is complicated by the real-structure-induced variation of the refractive index of the oxide layers with thickness variation. In particular, evaporated ZrO, films constitute a very troublesome example of optical inhomogeneity which can vary from run to run even if the deposition conditions are nominally identical. Using an in siru method similiar to that of Englisch and Ebert I4 modified for reflected light, we generally found a decreasing refractive index during deposition. The verification of the refractive index profiles was performed by measuring the spectral transmittance of the ZrO, films on BK-7 at normal incidence with an absolute accuracy within kO.005 and subsequently fitting (least-squares fit) a multilayer model to the experimental data by the use of a standard program based on matrix calculus. Since the measured absorption values of the films have only a negligible influence on the profile results, there was no need to take into account a complex refractive index. For example, the refractive index profiles of polycrystalline films could be approximated within the limits of uncertainty of the spectral measurements by double-layer structures with a near-substrate sublayer refractive index in the range 2.0P2.1 and 1.8-1.9 in the outer region. Ellipsometric measurements after deposition confirmed this result with slight variations within these limits. The near-substrate sublayer thickness is strongly dependent on the substrate temperature and nearly

STRUCTURE-RELATED

279

BULK LOSSES IN ZI.0,

independent of the film thickness. The refractive index of amorphous films could be approximated by homogeneous depth profiles within the range 1.7-1.8. Overcoat ZrO, films with negative refractive index profiles increase the absorption of the underlying HR system as a result of an increase in the standing electric field within the HR system (Fig. 2). For typical absorption values of the constituents of the HR system, i.e. fl= 23 cm- ’ for TiO, and B = 2 cm- ’ for SiO,, SiO,, this increase is dependent on the evaluated refractive index profiles and is in the range A = (5-8) x lo- 5 if a homogeneous distribution of the absorption within the single layers of the HR system is assumed. All values of the absorption reported in section 3 have been corrected by considering the individual refractive index profiles of the overcoat ZrO, films. In principle, similiar effects must be expected with regard to the scattering, i.e. the volume scattering values determined can be increased by a systematic error. However, in contrast to the absorption, no simple correction can be given because of the more complex mechanism of scattering. Nevertheless, as Fig. 2 and the numerical results show, the inhomogeneity has only a negligible influence on the electric field strength in the interface regions of the half-wave and double-wave ZrO, films, i.e. any interface losses are suppressed. 2.3. Determination of scattering losses Total integrated scattering measurements were carried out at E,= 5 15 nm using a conventional experimental arrangement described elsewhere15. The apparatus consists of an argon ion laser and a Coblentz sphere collecting the backscattered light within an angular range from 2” to 84” with respect to the surface normal of the sample. The least detectable scattering value is about 1 x 1O-6 if the incidence is 1. The precision of the measurement was within _+15%. The scattering measurements were also carried out by laterally shifting the sample, thus yielding diagrams with local variations over a sample distance of about 15 mm. The diameter of the laser spot was about 1 mm. Figure 4 shows the line-scanned lateral light scattering profile of the sample investigated in Fig. 3. It was empirically found that the minimum

bare

5

1

5 -

HR.system

10

sanple

shlftiq

Fig. 4. Line-scanned lateral scattering spot radius R s 1 mm).

15 I mm -

profile for the samples shown in Fig. 3. at i = 515 nm (irradiation

280

A. DUPARRk

et cd.

scattering values of the lateral scans provide the most relevant information about the intrinsic film properties. The bulk scattering values can be obtained from the measured values S, and S, in a manner similar to that described for absorption. Although scattering levels as low as 10m6 are detectable, the minimum calculable value of S,,,, was restricted to about 5 x lo- 5 since the total measured levels are of the order of lo-” and, thus, the total error is about 10 ‘. 2.4. Stwndq~ neutrul muss sprctronwtr~ nwasurrnwnt.v Depth profiling of the constituents of the ZrO, films was performed by secondary neutral mass spectrometry (SNMS) using 200eV Ar+ ions extracted from a 27.12 MHz high frequency plasma generated in an ultrahigh vacuum chamber with a base pressure p < 10m8 Pa. The depth resolution was about 10 nm. A depth profile is shown in Fig. 5.

500

1030

1500

2000

2500

time

Fig. 5. SNMS compositional

3.

RESULTS

depth profile of a ZrOz coating

ISI

Ii”

on a silicon substrate

AND DIS(‘USSION

3.1. Bulk .wutter und,jilm tnorphologl~ The acquired scattering losses are summarized in Table I. The corresponding CPt transmission electron microscopy (TEM) micrographs of the double-wave film cross-sections are shown in Fig. 6. The scattering values of layers with equal optical thicknesses exhibit significant differences which originate from different morphologies and/or crystallinities as a result of different deposition conditions. Generally, it was established that coarser intrinsic film morphologies furnish higher bulk scattering levels. ,Furthermore, if detectable losses were found they rose with increasing film thickness. In the majority of cases this rise exceeds a simple proportionality to film thickness which might be expected if the increased scattering volume only was taken into consideration. In general, light scattering by the thin film bulk is a complex phenomenon arising from any refractive index variations, i.e. from the morphological features

STRUCTURE-RELATED

BULK

LOSSES IN ZrO,

281

within the layer bulk. Thus it is useful to discuss the scattering behaviour in the light of structure zone modelsi6*” expanded by mobility and evolutionary growth development which accounts for film thickness effects”. In the case of the ZrO, films which are electron beam evaporated onto substrates at ambient temperatures (Table I, sample a, and Fig. 6(a)), the abovementioned models predict a fibrous internal structure since the adatom mobility is limited (zone I). As Fig. 6(a) shows, an evolution of volume structure is not evident. Electron diffraction patterns of these coatings revealed an amorphous structure. A characteristic feature of this type of coating is an increase in the bulk scattering that is not more than proportional to the comparable optical thicknesses. This should be expected if only the increased scattering volume is taken into consideration without any changes in the scattering conditions. In the case of laser evaporation onto low temperature substrates (TableI, sample b, and Fig. 6(b)), the fibrous amorphous structure is reminiscent of that of the electron-beam-evaporated ZrO, films. However, Fig. 6(b) reveals a coarser intrinsic structure, resulting in enhanced scattering values. We believe that this is an example ofan evolutionary growth development as assumed by Messier”. It should be noted that as in the case of pulsed laser evaporation ’ 8 the sizes of interal substructures may be dependent on the laser energy used. The films deposited by electron beam evaporation onto substrates with a temperature of 200°C without 0, reactive gas pressure revealed the polycrystalline structure shown in the TEM bright field micrograph of Fig. 7(a); the corresponding selected area electron diffraction pattern is also shown. Contrary to the results of Farabough et al.” and in agreement with the findings of others”*” we could identify the crystallographic structure as a mixture of the cubic (c) and monoclinic (m) phases. The corresponding indices are shown in Fig. 7(d). In all our experiments no tetragonal phase was found. From Fig. 7(a) and TEM dark field investigations crystallite sizes of the cubic phase content in the range from several nanometres up to 50 nm and lattice parameters ranging from 0.507 to 0.5 11 nm were obtained. The crystallite sizes of the monoclinic phase do not exceed a few nanometres. Detailed RED studies of layers of various thicknesses revealed that the monoclinic phase component is preferentially localized in the near-substrate interface region over a depth coordinate of 3&40 nm. In contrast, the cubic crystallites dominating in the layer bulk have increased grain sizes and a higher void content. Since the refractive indices of the cubic and monoclinic ZrO, bulk material are very similar, i.e. n = 2.166-2.18 for stabilized cubic materia123*24 and n = 2.17 (n, = 2.13, np = 2.19, nr = 2.20) for monoclinic materia125, we explain the observed inhomogeneities of the refractive indices by a density effect, i.e. an increase in the void content with increasing film thickness. From measurements of the waterinduced shift of the transmission lines of Fabry-Perot filters having half-wave and double-wave ZrO, spacer layers, averaged packing densities of about 0.90493 were derived. On the basis of these results, we explain the occurrence of the negative refractive indices as, for example, shown in Fig. 2 by the condensation of a more fine-grained monoclinic-cubic phase mixture of higher density in the near-substrate region and a less dense polycrystalline cubic structure in the other parts of the layer. Detailed

282

A. DUPARRh

C’tUi.

studies have shown that there is a correlation between the width of the high refractive region and the region evident in the SNMS spectra (Fig. 5) where the concentration of contaminants is drastically reduced, probably as a result of a higher density of the condensate. The cross-sections of these mixed phase coatings (Table I, sample c, and Fig. 6(c)) confirm this suggestion. They show a fibrous-columnar structure with a

Cc) Fig. 6 (continued).

STRUCTURE-RELATED

BULK

LOSSES IN i&-O,

(e) Fig. 6. TEM C-Pt micrographs bars indicate 1 pm.

283

(0 of cross-sections

of the 22 ZrO,

layers described

in Table I. The scale

distinct evolutionary growth of the internal morphology. The cross-sectional structure is more roughened in the near-surface region, leading to an increase in bulk scattering by a factor of more than 7 if the EL/2and 2i layers are compared. As shown in Fig. 6(e), post-deposition annealing resulted in the formation of a coarser intrinsic structure. Furthermore, distinct crystalline features occur on the surface because of recrystallization (Fig. 7(c)). Unfortunately, efforts to obtain structural information about the nature of these crystal grains by electron diffraction have failed. Nevertheless, detailed investigations of the layer surface by RED revealed reflections which stem in the majority of cases from slightly textured cubic crystallites exhibiting a (111) preferred orientation normal to the surface. The observed structural changes are in qualitative agreement with the increase in scattering from 3.5 x 10e4 to 10 x 10m4 measured on the 21, layers before and after annealing (see Table I, samples c and e). In ZrO, films deposited at a substrate temperature of 250 “C with an O2 partial pressure p = 6 x 10e3 Pa the crystallography is the same as described above with the main difference that the average grain size of the cubic crystallites is reduced to a few nanometres (Fig. 7(b)). The optical inhomogeneity of such a microstructure is less pronounced. Probably as a result of the finer-grained structure no scattering was detectable either before or after post-deposition annealing at 400°C (Table 1, samples d and f, and Figs. 6(d) and 6(f)). Furthermore, the polycrystalline morphology exhibits no structural evolution with growing film thickness. This seems to be a case of oxygen-induced locking of grain growth.

A. I~lJPARR~

(It LI/.

3

Fig. 7. TEM

bright

field images and corresponding

electron

diffractIon

patterns

of 30,

lilms deposlted

on silicon at (a) 200 C (see Table I, sample c) and at (b) 250 <‘(see Table I. sample d). (c) C -Pt replica of the layer surface after post-depositIon electron

diffraction

(RED)

pattern of a mixture ofcuhc

3.L’.

Bulk absorption

pattern

annealing of the layer

lsee Table

I. umple

surface. (d) Indexed

e); the corresponding

reflection

selected area electron diffraction

Cc)and monoclinic tm) phasrs.

unds~~condur~~ nrutrd

t~~u.c.~ .spc~ctrornrtry sprc’tro

Figure 5 shows a typical example of an SNMS depth profile of a ZrO, layer on silicon simultaneously deposited with the layers on the HR system. The proportion-

STRUCTURE-RELATED

BULK LOSSES IN ZrO,

285

ality of the zirconium and ZrO signals to sputtering time (depth coordinate) indicates a constant stoichiometry throughout the film thickness. Rutherford backscattering measurements of the integrated stoichiometry confirm these results. The measured oxygen-to-zirconium atomic ratios N,,/Nz, = 2.0-2.3 indicate a tendency of excess oxygen incorporation. Furthermore, the SNMS spectrum exhibits a nearly homogeneous contamination of the films of carbon and hydrocarbon species throughout the film thickness. As already mentioned, only in the film-substrate interface region do the carbon and CH signals decrease drastically over a depth region of about 30-40nm. A similar behaviour was found when observing signals of other contaminations (OH, nitrogen, CH,). Thus the aboveassumed thickness-independent bulk absorption coefficient is supported by the stoichiometry and contamination profile determination. It should be noted that in none of the examined ZrO, layers was a significant increase in contaminations in the interface regions discernible, as suggested by many researchersi0sz6. From the SNMS spectra shown in Fig. 5 it may be concluded that the main reason for absorption is the nearly homogeneous distribution of contaminations since no deviation from stoichiometry is detectable. Detailed studies of the incorporation of contaminations have revealed a very sensitive dependence on the deposition parameters and the microstructure of the zirconia films. In general, the content of carbon species can be reduced if the partial pressure of hydrocarbon species in the residual atmosphere can be lowered and/or a densification of the coating can be achieved, i.e. by substrate heating, application of ion beams during deposition or doping of the ZrO, films (detailed results will be presented in a further paper). Regarding the bulk absorption results, it was observed that the rise with increasing film thickness is generally smaller than would be expected from eqns. (lH3). As illustrated in Fig. 8, if the measured absorption values are extrapolated to zero film thickness a positive residual contribution remains, possibly originating

Fig. 8. ZrO, single-layer bulk absorption A at I = 515 nm us. film thickness (see Table I). A,, is the extrapolated zero-thickness interface absorption.

of samples c (0) and e(A)

286

A. DUPARRb

et Ul.

from an interface absorption. It should be noted that in the case of the sample e described in Table I the C-Pt replicas exhibit single defects not only in the bulk of the film but also on the surface (see Fig. 7(c)). Obviously, these defects if located within the film contribute to the bulk scattering but not to the bulk absorption. On the contrary, such considerably extended defects may disturb the zero-order electric field strength assumed on the surface. Thus they can cause both additional surface scattering and surface absorption. Absorption losses of ZrO, films were also measured by Martin et ~1.~’ at a wavelength of 550nm. Their absorption coefficient p considerably exceeds the values obtained in our investigations since no distinction between absorption and scattering was made and, furthermore, bulk as well as interface effects contribute to the measured losses. The effect of post-deposition annealing of the zirconia films prepared without 0, reactive gas pressure on the absorption is also illustrated in Fig. 8. In contrast to a strong enhancement of the bulk scattering as described above the annealing leads to a considerable decrease in the bulk absorption. Since in all the examined films the absorption decreases in baked films, we have been interested in the influence of the post-deposition annealing on the concentrations of the constituents of the films. Figure 9 shows the results of the SNMS measurements of coatings simultaneously deposited onto silicon substrates. The deposition conditions are summarized in Table I (sampled). The coatings were baked at several elevated temperatures in air for 5 h. The signals indicated in Fig. 9 are relative intensities normalized to the zirconium signals. The given bars represent the statistics of the signals (counts per second). Figure 9 shows only slight changes in the signals in films annealed up to300 “C. In films baked at 400°C a drastic decrease in the OH and N-CH, signals indicates an effective desorption probably of atmospheric species (water, nitrogen etc.) and cleaning agents. We believe that the desorption of these species is apparently one mechanism contributing to the measured reduction in absorption. Furthermore, Fig. 9 depicts the surprising result that the carbon and CH signals are not affected by annealing up to 500°C. Hence the remaining absorption can be 1

t-?g. 9. SNMS film bulk intensities normalized to the zirconium signals of various contaminations of ZrO, layers on silicon substrates as a function of post-deposition annealing in air for 5 h. The bars represent the statistics of the signals (counts per second).

STRUCTURE-RELATED

BULK

LOSSES IN ZrO,

287

attributed to the content of carbon and hydrocarbon in the film. These results emphasize the need to avoid these kinds of contaminations during deposition. Regarding the laser damage resistance of ZrO, films, low absorption values are advantageous. 4.

CONCLUDING

REMARKS

Bulk scattering and absorption in ZrO, single-layer films ofvarious thicknesses evaporated onto quarter-wave HR stacks were measured at a wavelength 3. = 515 nm. The bulk losses were found to vary considerably with the deposition conditions, with regard to both their absolute value as well as the degree of increase with increasing film thickness. This behaviour was directly related to the bulk morphology of the films observed from TEM micrographs of film cross-sections. As revealed by SNMS measurements the absorption of the films can be related to a nearly homogeneous distribution of different contaminations. To the best of our knowledge, no results have been published on SNMS analyses of ZrO, films before. Finally, post-deposition annealing has been shown to influence the scattering and absorption in very different ways. The annealing effects strongly depend on the conditions of the foregoing evaporation processs and the nature of the contaminations. ACKNOWLEDGMENTS

The authors are very grateful to Mr. B. Brauns for preparation of the laserevaporated coatings and to Mr. G. Fehlau and Mr. F. Seidel for preparation of the electron-beam-deposited films. REFERENCES

I 2 3

4 5

6 7 8 9 10 1I I2 I3 14 15 16 I7 18 I9

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