Microstructure study of non-epitaxial CaF2 thin films grown on Si by transmission electron microscopy

Materials Chemistry and Physics 52 ( 1998) 66-70

ELSEVIER

Microstructure study of non-epitaxial CaF, thin films grown on Si by transmission electron microscopy K. Yu-Zhang *, Y. Leprince-Wang Dip’ptrrtement

de Physique,

Uni~~ersitb de Murne

In Vrilke,

2 me de la Butte Verte, 93166 Noisy-le-Gmnd,

Frmce

Received 27 February 1997; received in revised form 28 April 1997; accepted 30 June 1997

Abstract Non-epitaxial CaF? thin films were evaporated on Si wafer coveredwith its 2 nm native oxide 30,. The samples werepreparedwith differentthicknesses andwereinvestigatedat two depositiontemperatures. Microstmctureobservations of the depositedfilmswerecarried out usingtransmission electronmicroscopy(TEM) on bothplanarandcross-sectional views.It wasshownthat theevaporatedCaF, layers have a strongcolumnargrowthandthe meansizeof the columnargrainsincreases with the layer thickness.A closeexaminationof the interfacesCaF,/SiO,/Si, usingthe high resolutionTEM technique,revealedan interactionbetweenthem,leadingto the inhomogeneous propertiesof interface. 0 1998ElsevierScienceS.A. Keyu*ords:

Thin films; Microstructure; CaF,; Transmission electron microscopy

1. Introduction Over the last decade, epitaxial growth of CaF, film on silicon hasincited much investigation owing to its potential application in the integrated circuit industry. CaFz is an ionic insulator, whereasSi is a covalent semiconductor.Both CaF, and Si have Face-centredcubic-type lattices with only 0.6% lattice parameter mismatch at room temperature. CaF,-Si system can be consideredas a prototypical ionic-covalent heteroepitaxial system [ I]. The characterization of the quality of the depositedfilm andthe analysisof thelayer/substrate interface were largely carried out by transmissionelectron microscopy (TEM) ) reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) techniques [ 2-81. It hasbeen found that the fluoride films grew on ( 111) Si substrateswith either of two epitaxial relations: the layer lattice hadthe sameorientation asthesubstrate(type A), or it could be rotated 180” about the surface normal < 111 > axis of the substrate(type 3) [ 91. In contrast, non-epitaxial CaF, growth on silicon substrate is alsointerestingin the field of optical interferencemultilayer

* Corresponding author. 0254-0581/98/$19.00 0 1998 Elbevier Science S.A. All rights reserved PZIS0251-0554(97)02016-b

systems [ IO.111. Similar to other fluorides ( MgF,, BaF,, LaF,, LiF, etc.), CaF, is often usedaslow index material in optical coatingsin combination with high index ones (ZnS, ZnSe, TiO?, Nb205, etc.) in order to obtain materials with modulationsof refraction index. In this case,the CaF, films have been deposited either on amorphoussubstratesor on silicon waferscovered with their native oxide 1121. Contrary to the caseof an epitaxial growth of CaF, on silicon, cleaning procedure to remove the native oxide from the Si surfaceis not necessarybecauseSiO, can also be usedas a low index optical material. The film performances, such as optical reproductivity, structure homogeneity, packing density, adherenceof the films to substrates,are usually controlled by spectroscopicellipsometry, TEM, RBS, XPS, etc. [ 13-151. Nevertheless.there have beenfew publications dealingwith the depositionof non-epitaxial fluoride layers. In this paper we report an investigation by conventional and high resolution TEM techniques of the non-epitaxial CaF, thin films on Si ( 111) wafers covered with their native oxide. We focus on the influence of layer thicknessand the substratetemperatureon the growth morphology of the films. Theseresults are useful for understandingoptical behaviour of the coatingsand for establishingthe correspondingmodels fitting experimental data to the spectroscopicellipsometry measurements[ 141.

2. Experimental CaF, films have been formed by evaporation in an ultrahigh vacuum chamber (base pressure is about 10m9 Torr). equipped with a small Knudsen cell as the evaporation source and an in situ rotating polarizer spectroscopic ellipsometer. The substrates used were 1 inch silicon single crystal wafers with the ( 1 I I ) orientation, covered, as indicated earlier, with their native oxide (about 2 nm). The temperature of the substrate was maintained at about 300°C and in some cases at room temperature for comparative studies. The evaporation rate ( about 0.03 nm s- ’ ) and the deposited thickness were controlled by a calibrated quartz tnicrobalance. For growth morphology investigations, both “plan view” and “cross-sectional view” specimens were prepared. “Plan view” or “through foil” microscopy allows the observer to look through the coating. If we are looking for information about interfaces or microstructure within a single layer. then we have to observe in the direction parallel to the layers. This is called “cross-sectional” microscopy [ 161. Specimen preparation for “plan view” observation is analogous to the conventional technique used for the preparation of thin foils of metals, alloys and semiconductors. The samples were tnechanically polished starting from the substrate

Fig. I. Plan view TEM images of CaF, films deposited on a Si wafer at 300°C: (a) sample CFI. thickness= 128 nm: (b) sample CF2. thickness = 52 nm.

to 30 pm, then argon ion milled. The cross-sectional examination specimens were prepared by glueing two small chips of the CaF, coating on Si face to face and then on a copper ring. grinding the composite to 30 Frn in thickness and finally argon ion-milling at a small incident angle ( < 15”). The microstructural observations and electron diffraction studies were performed using a Topcon 200B transmission electron microscope operated at 200 kV acceleration voltage.

3. Results and discussion 3.1. Plm view, observntions of CnFz films deposited nt 300°C Two plan view TEM images of Ca.F, films deposited at 300°C with different thicknesses are shown in Fig. 1. The thicknesses of the samples CFl and CF2 are 128 nm and 52 nm, respectively. The measurement of the grain sizes undertaken from these two kinds of samples demonstrated that the mean grain size of the CFl ( b 50 nm) is larger than that of the CF2 ( m 33 nm), indicating an increase of the grain size of CaF, with film thickness. In order to investigate the possible growth orientations of the deposited layers, both conventional- and micro-electron diffraction techniques were used. Fig. 2 presents a conventional selected area diffraction (SAD) pattern taken from the CaF, layers of the sample CF 1, in which no intensity change of the rings was found. Similar SAD patterns were obtained from the sample CF2, indicating no evidence of preferred growth orientations. To confirm this result, we also did the micro-diffraction patterns from two individual grains of CaF, (Fig. 3) : one presents the (100) orientation (Fig. 3(a)) and the other (211) (Fig. 3 (b) ) . It is obvious that the growth orientations of the CaF, layers studied in this work are quite random.

Fig. 2. Conventional selected area diffraction sample CF2 in plan view observation.

(SAD)

pattern taken from the

Fig. 3. Micro-diffraction

patterns taken from the sample CF2 in plan view observation,

However, our observations are inconsistent with the investigation of Kaiser et al. [ 131, in which a ( 100) textured growth was reported. 3.2. Cross-sectiorml 300°C

obsenwtiorzs

of CciF,Jilm

deposited nt

Fig. 4 shows three cross-sectional TEM images of CaF, films deposited at 300°C with different thicknesses: 128 nm for CFI, 52 nm for CF2, as we have mentioned above, and

Fig. 1. Cross-sectional TEM images of&F, (c) sample CF3. thickorbs = 12 nm.

indicating

the different

orientations

of two CaFZ grains.

12 nm for the sample CF3. Like most thin films deposited by thermal evaporation, the CaF, layers also have columnar growth morphology and the sizes of columns increase with the film thickness. Electron diffraction patterns (see Fig. 5 for example) taken from these samples confirm once more no evidence of a preferred growth orientation of the coatings. These results accord well with our plan view observations. By examining the individual grains of CaF2, one can find that some of them grew across all the deposited film and some of the others stopped their growth in the middle of the layer.

films deposited on a Si wafer at 300°C: (a) sample CFl, thickness = 128 nm; (b) sample CF2, thickness = 52 nm;

This growth feature, especially for thicker films, was described as “pyramidal growth” by Kaiser et al. [ 131. observable when the substrate temperature is over 100°C. Using a hard disk model proposed by Hacker [ 171 with anisotropic relaxation properties, Kaiser et al. [ 131 considered that the occurrence of such a strong columnar growth up to elevated temperatures resulted from limited migration of the adatoms at the top or at the lateral surfaces of the growing columns. 3.3. Study of the interfaces tempemwes

Fig. 5. Conventional sectional obaervation the deposited layers.

SAD pattern taken from the s,unplr CF3 in crossconfirming the random characteristic of the growth of

Fig. 6. High resolution TEM images of CaF, film\ CF4 at PLOT, thickness = 40 nm.

deposited

CrrFJSiOJSi

at different

As mentioned in the Introduction, the Si ( 111) wafers covered with a native oxide layer were usedin this work as substratesfor coatings without cleaning. There are two reasonsfor this: firstly, epitaxial conditions are not necessary for optical applications;secondly,the silicon dioxide can also be used, like CaF,, as low-index dielectric medium. Our

on a Si wafer at different

temperatures:

(a) sample CF2 at 300°C. thickness

= 52 nm; (b) sample

70

K. Yu-Zhang,

Y. Lrprince-~~ang/ltrlnteriuls

previous work [ 181 showed that the native layer was normally porous but uniform in thickness ( + 2 nm). However, the homogeneity of this 2 nm amorphous oxide was found to break during the deposition of CaF,. Fig. 6 shows two high resolutionTEM images revealing thedetailed interface structures of the samples CF2 and CF4. These samples were deposited at different temperatures (300°C and 20°C) and their thicknesses were 52 nm and 40 nm, respectively, It was noted that the CaF,/SiO,/Si interface features were quite similar for both high temperature coating and low temperature coating, i.e. the evaporated CaF, always attacked the homogeneous SiO, layer owing to the known active properties of fluorine, which not only reduced the thickness of the native layer but also made the interface CaF,/SiOz in a waved form. Furthermore, the lattice fringes of the CaF, layers were found directly in contact with Si somewhere at the interfaces, allowing a partial epitaxial growth of CaF2 on the Si substrate, as indicated by the arrows A, B in Fig. 6. We presumed that a chemical interaction between CaF,, SiOz, and Si had taken place at the interfaces. Reducing deposition temperature (from 300°C to 20°C) did not seem effective in preventing this kind of chemical reaction and therefore a partial epitaxial growth of CaFz layer on Si substrate. Our results are consistent with the XPS analysis of the same kind of interfaces [ 151, in which the Si-F and Ca-0 bonds were detected instead of the exclusive Si-0 and Ca-F bonds, as expected. It should be noted that the chemical interaction complicates interface structures, leading to inhomogeneous optical properties of coatings [ 141 I

4. Conclusions Using TEM, the growth morphology of evaporated CaF, films were studied in function of the layer thickness. It was observed that the CaF, films have a strong columnar growth at a deposition temperature of 300°C and the mean grain size of the columns increases with the layer thickness. No evident crystallographic preferred growth direction was found. By means of high resolution TEM, structural details of the interfaces between the CaF, coating, SiO, native layer and Si substrate were also investigated. It was found that CaF, molecules “struck” the SiO, native layer easily owing to the

Clwmist~y

and

Physics

52 (1998j

66-70

known active properties of fluorine, and then locally the initial non-epitaxial CaF, films had an epitaxial growth with the Si substrate, leading to inhomogeneous interface properties.

Acknowledgements The samples were prepared in the “Laboratoire d’optique des Solides, URA CNRS 781, Universitt Pierre et Marie Curie”. The authors would like to thank Professor J. Rivory for useful discussions.

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