the influence of deposition conditions on structure and morphology of aluminum nitride films...

Thin Solid Films 434(2003) 112–120

0040-6090/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0040-6090(03)00428-0

The influence of deposition conditions on structure and morphology ofaluminum nitride films deposited by radio frequency reactive sputtering

Hao Cheng*, Yong Sun, Peter Hing

School of Materials Engineering, CPPL Lab, Nanyang Technological University, Singapore 639798, Singapore

Received 20 January 2003; received in revised form 10 February 2003; accepted 17 February 2003

Abstract

Wurtzite aluminum nitride(2H–AlN) films were deposited on(1 0 0) silicon wafers by radio frequency(RF) magnetronreactive sputtering under various deposition conditions. The evolution of structure and morphology of AlN films were studied byX-ray diffraction, and field emission scanning electron microscopy. The preferred orientation was found to be sensitive todeposition conditions such as sputtering pressure, RF power, N concentration. Gas flow rate showed no distinct influence on the2

preferred orientation, but the crystalline quality of the deposited film was improved with the increase of flow rate. Temperatureinfluenced the preferred orientation in a complex way. A correlation between preferred orientation and morphology was observed.It was found that worm-like grains are favorable in films with(1 0 0) preferred orientation. Pebble-like grains are likely to growin films with (0 0 2) preferred orientation. Pyramid cone structure prevails in films that show the existence of(1 0 1) peaks inXRD spectrum.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Aluminum nitride; Preferred orientation morphology; X-ray diffraction; Field emission scanning electron microscopy

1. Introduction

With novel thermal properties, chemical stability, highhardness, high acoustic velocity and a large electrome-chanical coupling coefficient, as well as a wide bandgap, AlN thin films have received considerable interestas a promising candidate electronic material for thermaldissipation, dielectric and passivation layers, surfaceacoustic wave devices, and photoelectric devicesw1–4x.Many techniques, such as sputteringw5,6x, chemicalvapor depositionw7x, laser chemical vapor depositionw8x, pulsed laser ablation and molecular beam epitaxyw9x, have been used to prepare AlN thin films on varioussubstrates. In most cases mentioned above, the deposi-tion temperatures are quite high. High temperature dep-osition has the disadvantage of the degradation of thesubstrate and the AlN thin films during deposition dueto thermal damage. Hence deposition of AlN thin filmsat low temperatures has become increasingly importantand valuablew10x. Sputtering technique is promising

*Corresponding author. Tel.:q65-67904614; fax:q65-67900920.E-mail address: [email protected](H. Cheng).

under circumstances where low temperature depositionand conformal coatings are neededw11x.

The performance of AlN films is greatly influencedby their microstructure. For example, films with variouspreferred orientations show different piezoelectricbehavior, and(0 0 2) preferred orientation is shown tobe betterw12x. With reduced device feature size, theheat dissipation capacity of AlN films becomes moreand more important. The microstructural factors such asmorphology, interface roughness and anisotropic struc-ture play important roles in phonon scattering procedurein AlN films at a certain temperature range, and willinfluence heat transport properties of the deposited filmsw13,14x. The fact that the microstructure of thin filmmaterials is strongly affected by the deposition condi-tions necessitates the effort to study the influence ofdeposition conditions on the microstructure and mor-phology of the deposited films. However, only limitedwork w15x had been conducted to systematically studythe structural and morphological evolution of the depos-ited AlN films, partly because of the difficulty tocharacterize these films in the sub-micro range. In this

113H. Cheng et al. / Thin Solid Films 434 (2003) 112–120

Table 1Deposition conditions

Substrate 2=2 cm (1 0 0) Si2

Substrate temperature 100–5008CSubstrate-to-target distance 14 cmTargets Al 99.99% purityGas ArqN mixtures with 0–100%N2 2

Residual pressure -5=10 Torry6

RF power 100–500 WSputtering pressure 1.4–20 mTorrGas flow rate 6–24 sccmDeposition time 3 h

Fig. 1. u–2u XRD patterns of AlN films deposited at various sput-tering pressures of 1.4–20 mTorr, RF power of 300 W, substrate tem-perature of 3508C, N concentration of 75% and flow rate of 12 sccm.2

work we present our results on microstructural evolutionof AlN films under various deposition conditions.

2. Experimental

AlN films were deposited with a 99.99% pure Altarget and in 99.9995% purity nitrogen and argonmixtures. A planar magnetron sputtering system suppliedby Coaxial Company in UK was used for film deposi-tion. The system consists of a cylinder chamber withtwo 3-inch water-cooled target holders tilted at approx-imately 308 with respect to the normal of the horizontalsubstrate holder. An impedance-matching network wasused to optimize the radio frequency(RF) power input.2=2 cm (1 0 0)"0.58 silicon wafers were used as2

substrates. The substrates were cleaned with 5% HFsolution for 3 min, followed by rinsing in acetone anddistilled water in ultrasonic bath.

High-purity Ar gas was introduced into the chamberafter the chamber was evacuated to below 5=10 Torr.y6

Then the pure Al target was pre-sputtered for 10 minwith the target shutter closed. During pre-sputtering theRF power and Ar pressure were kept at 300 W and 20mTorr, respectively. Then nitrogen gas was introducedinto the chamber and the reactive sputtering was initi-ated. Typical deposition conditions are shown in Table1. During the deposition, only the particular parameterto be studied was changed in different deposition batcheswith all the others kept constant.

A Shimadzu LabX-XRD-6000 X-ray diffraction(XRD) instrument with Cu Ka radiation was used forXRD measurement. Grazing incidence scan was usedfor phase identification and conventionalu–2u scan wasused to study preferred orientation. A JSM-6340F fieldemission scanning electron microscopy(FESEM) wasused to study the morphology of deposited films. ADimension� 3100 scanning probe microscopy was usedto study the surface roughness of AlN films. Theinstrument was used in tapping mode, wherein thecantilever is set into oscillation at its resonant frequency,then it is brought close to the sample until the etchedsingle crystal silicon tip briefly contacts the sample

surface at the bottom of the oscillation cycle. The contactreduces the amplitude of the amplitude of the cantileveroscillation. During scanning, a feedback loop consistsof a NanoScope IIIa controller is used to adjust theposition of the pizeo that moves the cantilever to keepthe cantilever amplitude constant. A topographic mapof the sample is created by recording the motion of thepizeo required to keep the amplitude constant. In thiswork the tip was scanned laterally across a 2=2 mm2

area at room temperature under air ambient. The thick-nesses of the deposited films were measured by cross-sectional FESEM images and by an Alpha-Step 500type surface profiler. The Alpha-Step 500 characterizesa surface by scanning it with a diamond stylus. It hasan inductive sensor that registers the vertical motion ofthe stylus. The resulting trace represents a cross-sectionalview of the measured sample. A 3 mg stylus force wasused throughout this work and the scanning length andspeed were chosen according to different sampleconditions.

3. Results and discussion

All the deposited AlN films were found to havewurtzite structure(2H–AlN) by grazing incidence XRD

114 H. Cheng et al. / Thin Solid Films 434 (2003) 112–120

Fig. 2. Surface morphology of AlN films deposited at various sputtering pressures with a RF power of 300 W, substrate temperature of 3508C,N concentration of 75% and flow rate of 12 sccm.(a) Ps20 mTorr; (b) Ps15 mTorr; (c) Ps7.5 mTorr;(d) Ps5 mTorr; (e) Ps1.4 mTorr;2

(f) cross-sectional micrograph(7.5 mTorr).

analysis. No cubic metastable AlN phase was found.The discussion in the following sections focuses on theevolution of preferred orientation and morphology ofAlN films produced under various deposition condition.

3.1. Influence of sputtering pressure

Fig. 1 showsu–2u XRD patterns of the AlN filmsdeposited under varying sputtering pressures of 1.4–20mTorr, at a constant RF power of 300 W, a substratetemperature of 3508C, a N concentration of 75% and2

a flow rate of 12 sccm. It is clearly shown that with thedecrease in sputtering pressure, the preferred orientationof the AlN film changed from(1 0 0) to (0 0 2). Asimilar change in preferred orientation was also foundin AlN films deposited by direct current sputtering andwas described as a result of the rotation procedure of(0 0 2) planes from perpendicular to parallel to the

substrate surfacew16x. A transition pressure of 5 mTorrwas also found where the preferred orientation was notdistinct as(1 0 0), (0 0 2) and (1 0 1) peaks appearedin the XRD pattern. It may indicate the uncompletedrotation of (0 0 2) planes towards parallel to the sub-strate surface. It is well-known that two kinds of Al–Nbond exist in wurtzite AlN, named B and B .{ 1 0 0}1 2

planes consist of only B bond, while{ 0 0 2} and1

{ 1 0 1} planes consist of B and B bonds together. The1 2

formation energy of B bond is larger than that of B .2 1

Thus adatoms with a higher energy are energeticallyfavorable for the formation of(0 0 2) and(1 0 1) surfaceplanes w17x. Decreasing the pressure will reduce thescattering effect of the reactive species in the gas phaseand increase the kinetic energy of reactive particlesw18x.This will thus promote the formation of planes containB bond. The formation of(0 0 2) instead of(1 0 1)2

preferred orientation can be further attributed to the


Fig. 3. u–2u XRD patterns of AlN films deposited at various RFpowers of 100–500 W, a sputtering pressure of 5 mTorr, substratetemperature of 3508C, N concentration of 75% and flow rate of 122

sccm.

difference in the growth rates in these two planes. Theclose-packed(0 0 2) plane grows much slower, asobserved in the deposition rate curve with a peak valueat 5 mTorr. Selected area diffraction studies showed thatthe film deposited at 1.4 mTorr was grown in a singlecrystal mode with two-domain structurew19,20x.

Fig. 2 shows the surface morphology of AlN filmsdeposited at various sputtering pressures. A correlationbetween film morphology and preferred orientation wasobserved. The films with(1 0 0) preferred orientationshowed worm-like structures that were randomly orient-ed in plane. With the decrease in the sputtering pressure,the grain size increased continuously, which can beattributed to the increased kinetic energy and surfacemobility of the adatoms under low pressures. Pyramidcone structure was found in films where(1 0 0), (0 0 2),(1 0 1) textured grains coexist. In a former reportedwork this structure was related to the existence of(1 0 1)preferred orientationw15x. This conclusion was shownto be correct as it held for all deposition conditions, asis shown in the following sections. The films with(0 0 2) preferred orientation showed a pebble-likestructure.

In fact, many factors can influence the surface mor-phology of thin film materials, such as the nucleationmode w21x, the overlap growth of neighboring grains(which induce smooth surface) w22x, the angular distri-bution of incident flux and the surface diffusion ofadatoms, as well as the difference in growth ratesbetween several crystal planes at the surface of the filmw23,24x. Recent Monte Carlo modeling on thin filmdeposition showed that in sputtering deposition wherecolumnar structure formed, the surface morphology wasdetermined by preferred orientation(plane parallel tosurface), anisotropic growth rate of different planes(which will result in facet structure at the top surface)and the adatom diffusion(between the preferred orien-tated surface planes and the facet planes) w25,26x.

For the films deposited at high pressures(07.5mTorr), the morphology was dominated by the(1 0 0)preferred orientation. Improved surface mobility of ada-toms under decreasing sputtering pressure was found toonly affect the grain size, as shown in Fig. 2a–c. Forthe film deposited at 5 mTorr, the facet grain structurecan be attributed to the existence of(1 0 1) preferredorientation. The coexistence of mixed surface planes,and the anisotropic growth rates on different latticeplanes may also contribute to the formation of facetstructure. The dense smooth pebble-like grain structurefound in the film deposited at 1.4 mTorr can be regardedas the evidence of sphere cap nucleation and 2D growthin the film, as the film was shown to grow in a two-domain single crystal modew20x. Fig. 2f shows a typicalcross-sectional micrograph of an AlN films confirmingthe columnar growth mode, which agrees well with the

prediction by Structure Zone Model for sputtered thinfilms w27x.

3.2. Influence of RF power

Fig. 3 showsu–2u XRD patterns of the AlN filmsdeposited at varying RF powers of 100–500 W, withthe sputtering pressure, substrate temperature, N con-2

centration, and flow rate kept constant. The preferredorientation evoluted in a complex way. But the trendthat (1 0 1) surface planes with higher formation energyare favorable at higher RF powers was still observed.At elevating RF power both the sputtering yield and theincident particle kinetic energy near the substrate surfacewere increasedw28x. The fact that(1 0 1) instead of(0 0 2) preferred orientation(both were shown to beenergetically favorable under conditions where adatomshave higher energies) was formed can be attributed tothe difference in growth rate on{ 1 0 1} and { 0 0 2} .According to van der Drift model, crystal grains orientedwith their slower growing direction normal to the surfaceare terminated while faster growing directions are pre-served as they intersect grain boundariesw23x. { 0 0 2}planes were found to grow in a slow ratew29x. As willbe shown later(Fig. 5), the films with(1 0 1) preferred


Fig. 4. Morphology of AlN films deposited at various RF powers of 100–500 W, a sputtering pressure of 5 mTorr, substrate temperature of 3508C, N concentration of 75% and flow rate of 12 sccm.(a) RF powers100 W; (b) RF powers200 W; (c) RF powers300 W; (d) RF powers2

450 W; (e) RF powers500 W; (f) cross-sectional micrograph(300 W).

orientation deposited at high powers grow much faster.The growth mechanism behind can be explained by thefact that as the growth rate increases, the adatoms haveless time to rearrange in low energy configurations(i.e.close-packed(0 0 2) atomic plane) before the next layerwas deposited. Instead a preferred orientation of lessdensely packed atomic planes such as(1 0 1) is devel-opedw30x.

Fig. 4 shows the morphological evolution of the AlNfilms deposited at various RF powers. A low sputteringyield and adatom kinetic energy introduced by a lowRF power result in a low deposition rate and low surfacemobility of the adatoms. Hence small grain size andsmooth surface were observed in the film deposited at100 W, as shown in Fig. 4a. Increasing the RF powerto 200 W resulted in better surface mobility and a largergrain size, as shown in Fig. 4b. Pyramid cone structurewas found in films deposited at RF powers larger than

300 W, where(1 0 1) preferred orientation was distinct,see Fig. 4c–e. A continuous growth in grain size wasfound at elevating RF power. At 500 W, surface rough-ening was observed as the facet character of grainsbecame less obvious. This can be attributed to theimproved surface mobility of adatoms at larger RFpowers, where adatoms are able to diffuse from grainboundary to lower energy positions with the introductionof vacancies and voids at the grain interfacew24x. Themeasured surface roughness confirmed this observation,as shown in Fig. 5, whereR and R are root-mean-q a

square and arithmetic mean surface roughness values,respectively.

3.3. Influence of gas flow rate

Fig. 6 showsu–2u XRD patterns of the AlN filmsdeposited at varying flow rates of 6–24 sccm with all


Fig. 5. Variation of surface roughness and deposition rate with thechange in RF power of 100–500 W, a sputtering pressure of 5 mTorr,substrate temperature of 3508C, N concentration of 75% and flow2

rate of 12 sccm.

Fig. 6. u–2u XRD patterns of AlN films deposited at various flowrates of 6–24 sccm, a RF power of 300 W, sputtering pressure of 7.5mTorr, substrate temperature of 3508C, N concentration of 75%.2

Fig. 7. Surface morphology of AlN films deposited at various flow rates of 6–24 sccm, a RF power of 300 W, sputtering pressure of 7.5 mTorr,substrate temperature of 3508C, N concentration of 75%.(a) Flow rates6 sccm;(b) flow rates12 sccm;(c) flow rates24 sccm.2

the other parameters kept constant. It is shown that flowrate played no significant role on the preferred orienta-tion. While the increased(1 0 0) peak intensity indicatedthat flow rate has some effect on crystallographic prop-erty as no significant change in deposition rates wasfound. Improved degree of crystallinity under higherflow rate can be attributed to the more clean depositioncondition resulted from high flow rates, when more


Fig. 8.u–2u XRD patterns of AlN films deposited at various substratetemperatures of 100–5008C, a flow rate of 12 sccm, RF power of300 W, sputtering pressure of 5 mTorr, N concentration of 75%.2

Fig. 9. Surface morphology of AlN films deposited at various substrate temperatures of 100–5008C, a flow rate of 12 sccm, RF power of 300W, sputtering pressure of 5 mTorr, N concentration of 75%.(a) Ts150 8C; (b) Ts250 8C; (c) Ts350 8C; (d) Ts500 8C.2

fresh gas was introduced into and more contaminationwas driven out of the deposition chamber.

Fig. 7 shows the surface morphology of AlN filmsdeposited at various flow rates. Worm-like grains werefound in all the deposited films, coinciding with theformation of(1 0 0) preferred orientation.

3.4. Influence of substrate temperature

Fig. 8 showsu–2u XRD patterns of the AlN filmsdeposited at various substrate temperatures. Althoughthe influence of substrate temperature on the formationof preferred orientation is not well established, thecoincidence of morphology and preferred orientationwas observed again. Fig. 9 shows the surface morphol-ogy of the corresponding AlN films. The films with(1 0 1) peak in XRD pattern showed pyramid conestructure. Films with strong(1 0 0) preferred orientationshowed worm-like structure. The profound effect oftemperature on the morphology and roughness of AlNfilms has also been reported previouslyw31x. Differencein surface mobility of adatoms is believed to be respon-sible for the great changes in morphology. For example,(1 0 0) preferred orientation was found in both filmsdeposited at 250 and 5008C and grains in both filmswere worm-like. While at 5008C the facet character ofindividual worms (as shown in Fig. 9b) disappeared.Instead rounded worm grains were formed, as shown inFig. 9d. This phenomenon can be attributed to theroughening effect caused by the increase in the mobility


Fig. 10. u–2u XRD patterns of AlN films deposited at various N2

concentrations of 25–100%, a substrate temperature of 3508C, flowrate of 12 sccm, RF power of 300 W, sputtering pressure of 5 mTorr.

Fig. 11. Surface morphology of AlN films deposited at various N concentrations of 25–100%, a substrate temperature of 3508C, flow rate of2

12 sccm, RF power of 300 W, sputtering pressure of 5 mTorr.(a) 25%N ; (b) 50%N ; (c) 75%N ; (d) 100%N .2 2 2 2

of adatoms at high temperatures, which facilitate thediffusion of adatoms from grain boundaries to lowerenergy positions with the introduction of vacancies andvoids at the grain interfacew24x.

3.5. Influence of N concentration2

Fig. 10 showsu–2u XRD patterns of AlN filmsdeposited at varying N concentrations of 25–100%.2

The films deposited with pure N and low N concen-2 2

trations exhibit improved crystallographic quality. It wasalso found that high N concentrations are favorable for2

(0 0 2) preferred orientation while a low N concentra-2

tion is favorable for(1 0 0) preferred orientation. Thisresult agrees well with other reported workw30x and isbelieved to be resulted from the low deposition rate ofAlN films under high N concentrationw18x. At low2

deposition rates, the adatoms have longer time to rear-range in low energy configurations, i.e. close-packed(0 0 2) atomic plane. It was the low sputtering yield ofaluminum target by N bombardment(compared to thatq

of Ar ) that introduces the low deposition rate. Againq

the coincidence of morphology with preferred orienta-tion was observed, as shown in Fig. 11. Pebble-likegrains were found in films with(0 0 2) preferred ori-entation. Worm-like structure was observed in films with(1 0 0) preferred orientation. With the existence of(1 0 1) peak in the XRD pattern, the deposited AlN filmshowed pyramid cone structure.


4. Conclusions

Wurtzite AlN films were deposited by RF reactivesputtering under various conditions. The evolution ofmorphology and preferred orientation of the depositedfilms were studied by XRD and FESEM. A correlationbetween preferred orientation and morphology wasobserved. It was found that worm-like grains are morefavorable in films with (1 0 0) preferred orientation.Pebble-like grains are likely to grow in films with(0 0 2) preferred orientation. Pyramid cone structureprevails with the existence of(1 0 1) surface atomicplane. In this work, the formation mechanism of pre-ferred orientation and morphology was also discussedin detail for each deposition parameter.

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the influence of deposition conditions on structure and morphology of aluminum nitride films...

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