Effect of Intrinsic Stress

Effect of intrinsic stress on preferred orientation in AlN thin filmsB. K. Gan, M. M. M. Bilek, D. R. McKenzie, M. B. Taylor, and D. G. McCulloch

Citation: Journal of Applied Physics 95, 2130 (2004); doi: 10.1063/1.1640462 View online: http://dx.doi.org/10.1063/1.1640462 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/95/4?ver=pdfcov Published by the AIP Publishing

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Effect of intrinsic stress on preferred orientation in AlN thin filmsB. K. Gan,a) M. M. M. Bilek, and D. R. McKenzieApplied and Plasma Physics, School of Physics (A28), University of Sydney, New South Wales 2006,Australia

M. B. Taylor and D. G. McCullochDepartment of Applied Physics, RMIT University, G.P.O. Box 2476V, Melbourne 3001, Australia~Received 27 August 2003; accepted 18 November 2003!

We examine the effect of ion impact energy on the intrinsic stress and microstructure of aluminumnitride thin films deposited using a filtered cathodic arc. The dependence of intrinsic stress on ionimpact energy is studied over the range from 0 to 350 V using dc bias and up to several kV for afraction of the ions using pulse bias. For dc bias, the stress reaches a maximum at 200 V anddecreases with further increase in ion bias. The preferred orientation of the crystallites was studiedby cross-section transmission electron microscopy and diffraction. We found that there is apreference for the c crystallographic axis to lie in the plane of the film under high intrinsic stressconditions ~4 GPa!, whereas a c-axis orientation perpendicular to the plane of the film was observedfor low intrinsic stress ~0.25 GPa!. We carried out calculations of the expected distribution ofintensity in cross-sectional electron diffraction patterns to predict the effect of rotation freedom ofcrystallites with the c axis pinned. The calculated patterns agreed well with experiment. 2004American Institute of Physics. @DOI: 10.1063/1.1640462#

I. INTRODUCTION

Many researchers have studied the evolution of intrinsicresidual stress in thin filmssee selected papers.110 Thedegree of stress generated in these films is dependent on thedeposition conditions, such as the working pressure, sub-strate bias, etc.

A universal curve is observed for all materials relatingthe intrinsic stress to the ion impact energy.11 The stress infilms deposited with low thermal energies ~!1 eV! is usuallytensile. With increasing ion energy the stress becomes com-pressive, increasing to a maximum, and then decreasing.Compressive stresses for materials as a function of the ionenergy used in synthesis have been reported. This behavior isshown, for example, by chromium on silicon,4,5 titanium onsilicon,6 titanium nitride on tungsten carbide,7 and tetrahe-dral amorphous carbon on silicon.8

Aluminum nitride ~AlN! is a IIIV semiconductor com-pound with a wide direct band gap and a high refractiveindex. It is of interest for use in optoelectronic devices and asa chemically inert barrier layer. AlN crystallizes in a hexago-nal structure, and when grown as a thin film under conditionsthat do not favor epitaxy, it forms as a granular microcrys-talline material with the crystallite size determined by thegrowth conditions. In the majority of cases the crystallites ofAlN thin films are found to have a preferred crystallographicorientation with respect to the plane of the substrate.1219 Inmost cases,1316 the preferred orientation reported is with thec axis perpendicular to the plane of the film.Windischmann,13 used a nitrogen ion bombardment energyof 100 eV and reported intrinsic stresses of up to 2.8 GPa in

AlN films with the c axis perpendicular to the film. Forgrowth using energies above 100 eV,1719 a change to a caxis in the plane of the film orientation was reported forenergies between 300 and 500 eV.

In this article, we study the microstructure of films de-posited using a filtered cathodic arc under the influence ofion impact energies ranging from tens of electron volts toseveral thousand electron volts. The changes in intrinsicstress as a function of the ion energy are examined. A studyof the microstructure of the samples reveals a strong corre-lation between the stress and the preferred orientation, whichagrees with the model presented in Ref. 12.

II. EXPERIMENTAL DETAILS

All of the thin films reported in this article were depos-ited using a cathodic arc deposition system describedelsewhere.20 The cathode from which the Al plasma was ab-lated was a 50-mm-diam aluminum disk of purity 99%. Thearc current was 50 A and curved magnetic field coils wereused to steer the plasma to the substrate and eliminate mac-roparticles for the deposition of all films discussed here. Thefilms were deposited onto clean 310340-mm-thick, 20320 mm ~100! Si wafers, polished on both sides.

Prior to deposition, the chamber was evacuated to avacuum base pressure of 231025 Torr. A background gaswith pressure 1.231023 Torr, consisting of a mixture of N2and Ar gas, was admitted at flow rates of 7.0 and 2.0 sccm,respectively. The arc was then initiated to produce an Alplasma. The working pressure during deposition of AlN was531024 Torr. These conditions were found to deposit sto-ichiometric AlN. To bias the substrate, a dc power supplywas employed. Samples were fabricated using dc bias rang-ing from 0 to 350 V, in steps of 50 V, with a deposition rate

a!Author to whom correspondence should be addressed; electronic mail:[email protected]

JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 4 15 FEBRUARY 2004

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of 3 nm min21. Two lower stress samples were prepared byhigh voltage pulsing: 8 kV, 800 Hz, and 2 kV, 600 Hz, withpulse duration of 20 ms. The deposition rate was 10nm min21 and the substrate was earthed between high volt-age pulses.

Stress was measured by observing the change in the ra-dius of curvature of the Si substrate before and after deposi-tion. The intrinsic stress s f was calculated using Stoneysequation:21

s f5Es

6~12ns!ts

2

t fS 1R2 1RbD ,

where Es is the Youngs modulus for the Si~100! wafer ~125GPa!, ns is the Poissons ratio for the Si substrate ~0.28!, ts isthe thickness of the substrate, t f is the thickness of the film,R is the radius of the curvature of the film on the substrate,and Rb is the radius of curvature of the bare substrate. Mea-surements of R and t f were carried on a surface profiler,model Tencor P-10.

X-ray diffraction ~XRD! was used to detect preferredorientation in the AlN films. XRD data were collected usingCu Ka ~1.5418 ! radiation ~40 kV, 40 mA!, BraggBrentano ~u2u! geometry with 2 mm divergence and 0.05mm receiving slits, a graphite monochromator, 2u530 80, step size 0.02, and counting time of 10 s perstep. Peaks were identified using the JCPDSICDD x-raydatabase, No. 25-1133 for aluminum nitride, hexagonalstructure.

Cross-sectional transmission electron microscopy~XTEM! was carried out on selected samples. The XTEMsamples were thinned using a tripod polisher and diamondpaper followed by ion beam thinning. A combination of elec-tron diffraction and dark field imaging was used to determinethe microstructure and any preferred orientation in the film.The selected area diffraction patterns were taken using a cir-cular aperture with a diameter of approximately 100 nm.

III. RESULTS AND DISCUSSIONFigure 1 shows the measured residual stress as a func-

tion of the bias on the substrate during deposition. The actualion impact energy will be somewhat higher than the appliedbias voltage for two reasons. First, the cathodic arc plasmaplume has a natural drift velocity normal to the cathode sur-

face and the corresponding drift energy is added to the en-ergy gained by the ions in falling through the potential fromthe plasma to the substrate. This extra energy is of the orderof a few tens of electron volts. The second perturbation to theimpact energy arises from the fact that in an aluminum ca-thodic arc plasma there are some doubly, and even a fewtriply, charged ions. Although their number is reduced due tothe presence of the background nitrogen gas, we expect thata minority of ions will have higher charge states, and thuswill gain twice or three times the energy of a singly chargedion when falling through the potential to the substrate.

It is clear from Fig. 1 that AlN shows a behavior similarto most other materials, whereby residual stress initially in-creases with increasing energy and then begins to decrease.Although our power supply could only maintain a maximumdc voltage of 350 V we were able to access higher ion impactenergies by using a pulsed power supply, that could delivervoltage pulses up to 20 kV for short periods of time. Appli-cation of such high voltage pulses of duration 20 ms at fre-quencies of a few hundred Hz during the film growth re-duced the residual stress to well below 1 GPa. The stressreduction observed increased with increases in both the biasvoltage of the pulses and the frequency with which they areapplied. The lowest stress we achieved was 0.2 GPa using 7kV pulses at 500 Hz. These results are described in detailelsewhere.22

An XRD spectrum for a film deposited using pulses of 8kV, 20 ms duration, and with a repetition rate of 800 Hz, isshown in Fig. 2. We prepared this film on two different sub-strates simultaneously. The film deposited on a thin Si waferwas used to measure stress ~0.50 GPa!, while the film depos-ited on a glass slide was used to collect the XRD spectrum.Figure 2 shows that the ~0002! diffraction peak of AlN is theonly feature visible in the XRD spectrum. Its presence isindicative of a preferred orientation of crystallites with the caxis perpendicular to the plane of the film. We also observeda monotonically decreasing background due to the amor-phous glass substrate. The films with higher stress showed nodiscernible features above the noise.

In order to determine the preferred orientation, if any, ofhigh stress samples, we prepared XTEM samples of filmsgrown with a dc bias of 0 V ~earth!, 2200 V, and 2350 V.We also prepared an XTEM sample of a pulsed bias filmwith stress of 0.25 GPa, using 2 kV, 20 ms pulses at 600 Hz.The images and selected area diffraction patterns obtained

FIG. 1. Residual stress in AlN vs bias energy, E and F are AlN filmsprepared using earth and floating potentials, respectively.

FIG. 2. XRD plot of a low stress AlN sample, deposited using 8 kV 20 mspulses, 800 Hz. The film thickness is 230 nm and the film stress is 0.50 GPa.

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from these samples are shown in Figs. 36. Figures 3~a! and3~b! show a bright field transmission electron microscopy~TEM! image of the 0 V bias sample and the selected areadiffraction pattern, respectively. The diffraction pattern inFig. 3~b! is correctly aligned with respect to the image in Fig.3~a!. The cross-sectional image of the film shows the siliconsubstrate ~right!, amorphous carbon glue ~left!, and the AlNfilm, approximately 30 nm thick ~next to the carbon glue!.There is also a mixing layer, approximately 8 nm thick, be-tween the film and the substrate caused by the initial sputtercleaning in Ar of the silicon substrate using 3 kV, 20 mspulses at 1200 Hz for approximately 10 min. The diffractionpattern from the film can be indexed to the AlN crystal struc-ture with rings corresponding to $1010%, $0002%, and $1011%reflections starting from the center of the diffraction pattern.There is no clear preferred orientation in this case.

In the case of the 2200 V bias film @Figs. 4~a! and 4~b!#,the corresponding results show a broad arc corresponding to$1010%-type reflections from AlN perpendicular to the filmsurface and arcs parallel to the film surface corresponding to~0002! reflections from AlN. This type of diffraction patternstrongly suggests preferred orientation within the film. Inorder to interpret the electron diffraction patterns obtainedfrom the TEM cross sections we used a computer code writ-ten in Matlab and described elsewhere23 to calculate the

positions and intensities of the arcs expected to be observedin XTEM samples with various c-axis orientations to the filmplane and otherwise free rotation around the c axis and filmnormal. Reciprocal lattice contributions of allowed crystalliteorientations were summed, and those that intersected theEwald sphere were included in the calculated diffraction pat-tern. The intensities (I5FF*) were then modulated usingthe equation

Fhkl5 f Al@11e2pi~2h/312k/31l/2!#1 f N@e2pi~2h/312k/31$1/22u%l !1e2pi~12u !l# ,

where f Al and f N are the atomic scattering factors of Al andN, respectively, and the parameter u has a value of 0.385.24Figure 4~c! shows a calculated diffraction pattern for the caseof the c axis of the crystallites lying in the plane of the filmwith free azimuthal rotation and free rotation about their caxes. The plane of the film is located along the y direction,and the x direction corresponds to the normal to the film andsubstrate. The only rings that show arcs cutting the x axis~normal to the film plane! are those corresponding to the$1010%-, $1120%-, and the $2020%-type reflections. Thestrongest reflection along the y axis ~i.e., in the film plane! isthe c-axis reflection ~0002!. It appears as a bright spot, indi-cating that the c axis is constrained to lie in the film plane.

The preferred orientation observed in the XTEM analy-sis was compared by XRD. The c-axis in-plane orientationplaces the ~0002! planes in an orientation in which they arenot in the reflection condition for our u2u scan. Since theplanes satisfying the reflection conditions $1010%, $1120%,

FIG. 3. ~a! Cross-sectional view of a TEM sample deposited at the earthpotential. CG, carbon glue and ML, mixing layer. ~b! Selected area dif-fraction pattern of ~a!.

FIG. 4. ~a! Cross-sectional view of a TEM sample deposited at a bias of200 V. ML, mixing layer. ~b! Selected area diffraction pattern of~a!. ~c! Predicted electron diffraction pattern for a cross-sectional samplewith texture corresponding to all crystallites with c axes in the film plane~horizontal in the diagram! but unconstrained otherwise. ~d! High resolu-tion TEM image for the sample in ~a!. Fringes labeled A have a spacingof ;2.7 $1010%, and fringes labeled B have a spacing of ;2.4 $1011%.

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and $2020% give only weak reflections, the XRD scan shouldcontain no resolvable peaks above the noise level.

In addition to the results shown in Figs. 4~a! and 4~b!,we also collected a high resolution TEM image of the 2200V filmFig. 4~d!. It is clear from the lattice fringes observedin this image that the AlN microstructure consists of an as-semblage of crystal grains without evidence for any amor-phous phase, even at the grain boundaries. This conclusion issupported by the absence of any diffuse rings in the diffrac-tion pattern of Fig. 4~b!.

Figure 5 shows the TEM results for the 2350 V film. Italso reveals some evidence of preferred orientation similar tothat observed in the case of the 2200 V film. The key dif-ference, however, is the presence of many crystals with anorientation with the c axis normal to the film. This indicatesthat there are parts of the film which have reverted to thesame orientation as found in the film with no bias.

We also collected TEM images for a low stress film~0.25 GPa!, prepared using pulses of 2 kV, 20 ms durationand with a repetition rate of 600 Hz. Figure 6 shows theTEM image and selected area diffraction pattern for thissample. The strong ~0002! refractions along the axis perpen-dicular to the plane of the film in Fig. 6~b! show that thecrystallites in this sample are preferentially aligned with the~0002! direction normal to the plane of the film. The pre-dicted diffraction pattern in Fig. 6~c! for such crystallites alsosupports this result.

IV. CONCLUSIONS

Our results show that AlN deposited by cathodic arc in abackground nitrogen atmosphere shows increasing residualstress with increasing incident ion energy up to a maximumat an applied bias of around 2200 V. As the impact energyon the growth surface is increased further, the residual stressin the film starts to decrease. Pulsed biasing using very highenergies of several kV can be used to significantly relieve thestress.

We observe a strong correlation between the residualstress and the preferred orientation of the AlN film. In all ofthe cases where the stress was lower than 1 GPa, the favoredorientation has the c axis of the crystallites positioned normal

FIG. 5. ~a! Cross-sectional view of a TEM sample deposited at a bias of350 V. ML, mixing layer. ~b! Selected area diffraction pattern of ~a!.

FIG. 6. ~a! Cross-sectional view of a TEM sample deposited using 2 kV 20ms pulses, 600 Hz. Film stress is 0.25 GPa. ML, mixing layer. ~b! Selectedarea diffraction pattern of ~a!, clearly showing the ~0002! orientation. ~c!Predicted electron diffraction pattern for a cross-sectional sample with tex-ture corresponding to all crystallites with the c axis perpendicular to the filmplane.

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to the plane of the film. When the stress is high, such as3.54 GPa, the diffraction pattern obtained in XTEM is con-sistent with that predicted for a situation where the c axis ofthe crystals is constrained to lie in the plane of the film, butfree as to its orientation in that plane and its rotation aboutthe crystallite c axes. This change from c in plane to c or-thogonal to the film plane is consistent with the prediction ofthe thermodynamic theory.12 In this model the preferred ori-entations arise from the need to minimize the free energy ofthe system, which includes the strain energy associated withthe residual compressive stress. It is thus expected that athigh stresses the orientations with the lowest contributions tothe total strain energy should dominate, while at low valuesof stress surface and interface contributions will dominate.The model presented in Ref. 12 predicted that the c axis inthe plane of the film should be the one with the lowest strainenergy contribution, and thus should appear in high stressfilms. The calculation was based on elastic constants pub-lished by McNeil et al.25 The fact that the predictions of themodel were noted to be very sensitive to changes in elasticconstants is noted, and further work on the possible values ofthese constants in films deposited under various conditions isrequired.

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Effect of Intrinsic Stress