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Journal of Crystal Growth 275 (2005) e1269–e1273

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Hexagonal to cubic crystal structure transformation duringaerosol deposition of aluminum nitride

Atsushi Iwata�, Jun Akedo

Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology,

Namiki 1-2-1, Tsukuba-shi, Ibaraki-ken 305-8564, Japan

Available online 8 December 2004

Abstract

Aluminum nitride film is deposited on soda-lime glass by aerosol deposition method with helium carrier gas. The

deposition rate becomes more than 10 times larger by the pre-processing of the primary powder with ball milling. By

adding the heat treatment as the pre-processing, the deposition rate further increases. During the aerosol deposition

some of the particles transform from wurtzite structure (hexagonal) to rock salt structure (cubic). With the ball milling

as pre-processing, the peaks of rock salt structure in X-ray diffraction pattern are mostly higher than that of wurtzite

structure. However with the ball milling and heat treatment as pre-processing, the peaks of wurtzite structure become

higher. This means portion of rock salt structured crystallites becomes less by the pre-processing of heating.

r 2004 Published by Elsevier B.V.

PACS: 81.15.Ef; 81.20.Ev; 81.05.Je

Keywords: A1. Crystal structure; A3. Polycrystalline deposition; B1. Nitrides; B2. Semiconducting aluminum compounds

1. Introduction

Aerosol deposition method is a very promisingmethod to make ceramic films with several tens ofmicrometer thick or more. It is a process carriedout at room temperature inside a reduced pressurechamber. The method consists of dispersingprimary ceramic powder in the carrier gas andblowing the produced aerosol and hence the

e front matter r 2004 Published by Elsevier B.V.

ysgro.2004.11.082

ng author. Fax: +8129 861 7091.

ss: iwata.a@aist.go.jp (A. Iwata).

floating primary ceramic powder particles ontothe substrate accelerated by a nozzle [1]. Theparticles crash onto the substrate and form a solidfilm that adheres to the substrate firmly. Noheating is necessary. Very wide range of ceramicsis successfully deposited with this method. Thesubstrates can be metals, glasses or ceramics.In the case of aluminum nitride (AlN), an

intriguing fact was found [2]. Even though theprimary aluminum nitride powder has purelywurtzite (hexagonal) structure as usual, most ofthe deposited films have both crystallites with

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A. Iwata, J. Akedo / Journal of Crystal Growth 275 (2005) e1269–e1273e1270

wurtzite structure and crystallites with rock salt(cubic) structure. This was confirmed through X-ray diffraction and transmission electron micro-scopy. This apparently means that the crystalstructure of a part of the primary powder particlestransforms from wurtzite to rock salt during thedeposition. The crystal structure that is stable inthe ambient temperature and pressure is wurtzite,and rock salt structure is stable in high-pressureenvironment. As a usual case in aerosol depositionmethod, the crystallite size in the deposited layer ismuch less than that of the primary powder crystal.This also applies to AlN and the crystallite size ofthe deposited films is less than 100 nm. In thesesmall crystallites only very small crystallites withthe size of less than 20 nm transforms fromwurtzite to rock salt.AlN is used as electronic substrates, heat sinks

and electronic packaging because of its highthermal conductivity, thermal expansion similarto that of silicon and good dielectrical strength. Asthe power required to electronic circuits increases,higher thermal conductivity is demanded. In thecubic structures decreased phonon scattering, andhence higher thermal conductivity are expecteddue to their higher symmetry [3]. However, cubicAlN films with the thickness and size used aselectronic substrates and heat sinks are difficult tomake. There is no way to synthesize cubic AlNfilms of this size directly. Either static or shockcompression over 14GPa pressure is required totransform wurtzite to cubic [4]. Here the advan-tages of aerosol deposition, namely room tem-perature processing, reduced pressureenvironment, and high deposition rate are highlyexpected. However, the relationships of the por-tion of cubic crystallites in the deposited films andthe processing conditions in aerosol deposition arenot yet studied. Therefore processing conditionssuch as pre-treatment of the primary powder andcarrier gas flow rate are studied.

2. Experimental procedure

Aerosol deposition is conducted in the machinedeveloped in our laboratory. The carrier gas ishelium. The flow rate of the gas is controlled in the

range from 2 to 30 l/min. Helium gas blows up theprimary powder in the mechanically shaken bottle.This generates the aerosol inside the bottle. At thesame time the gas carries the blown-up tinyparticles of AlN into the vacuum chamberevacuated by a mechanical booster pump and oilrotary pump combination. The gas and theparticles are accelerated by the nozzle that has10mm by 0.4mm opening. Then they impinge onthe substrate and form the solid AlN film. Thevacuum pressure of the chamber varies with theflow rate of the helium gas, and is the order of100 Pa during the deposition.The substrates are the soda-lime glass plates of

1.3mm thickness, known as slide glasses. They areplaced 10mm from the nozzle. They are moved ina reciprocating motion of 10mm amplitude at thevelocity of 1.2mm/s. With the 10mm nozzleopening and 10mm displacement, the resultingsize of the deposited AlN film is around10mm� 10mm.The primary AlN powder is bought in the

commercial market (Tokuyama AlN powder Fgrade). It has nominal average powder size of1.29 mm. This powder is used either as purchased,pre-processed by ball milling or pre-processed byball milling and heat treatment combination. Pre-processing intends to improve deposition rate [5].Ball milling is conducted with Fritch P-5 ballmilling machine at 400 rpm with zirconia balls for1, 3, 5 and 7 h. Heat treatment is carried out in themuffle furnaces at 800 1C for 4 h.The deposition time is 2min. The film thickness

is measured with Tokyo Seimitsu diamond stylusprofilometer, Surfcom 480A. Standard X-raydiffraction scan of the deposited AlN films isperformed to estimate the portion of rock salt AlNin the films using Cu Ka radiation with RigakuRINT 2100V/PC diffractometer.

3. Results and discussion

The deposited AlN films are white to graycolored. Fig. 1 is the SEM picture of the surface ofthe deposited film. Many dimples characterize thesurface. These are thought to be made by thecollision of the primary particles that did not

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Fig. 1. SEM picture of the surface of aerosol deposited AlN

film.

0

10

20

30

40

00 2 4 6 8

0.5

1

1.5

2

2.5

3

Milling time h

Film

thic

knes

s �m

Film

thic

knes

s �m

2 l/min 6 l/min 10 l/min

15 l/min 20 l/min 30 l/min

0 2 4 6 8Milling time h

2 l/min 6 l/min 10 l/min

15 l/min 20 l/min 30 l/min

(a)

(b)

Fig. 2. Thickness of the films aerosol deposited for 2min

related to ball milling time of the primary powders: (a) the

powders are ball milled only; (b) the powders are ball milled

and heat treated at 800 1C for 4 h.

A. Iwata, J. Akedo / Journal of Crystal Growth 275 (2005) e1269–e1273 e1271

become the part of the film. The films have micro-Vickers hardness number of 600–1200. The hard-ness of the AlN block that is sintered by hotpressing from the similar primary powder isHv1000. Therefore the aerosol deposited AlNfilms can have the similar hardness as theconventionally sintered AlN, and sometimes canbe harder. Thin films are semi-transparent at thevisible range. The substrates of thick films aresometimes broken during or after the deposition.The films are firmly fixed with the fragments of thesubstrates, which indicate the adherence is verystrong. The relationships of pre-processing of theprimary powder and the deposition rate areinvestigated. Fig. 2(a) shows the relationshipbetween the ball milling time and the film thicknessof 2min deposition, which is an index of thedeposition rate. The milling time of 0 h means noball milling is applied to the primary powder.Namely, only very thin films are deposited at anygas flow rate with the original powder. Longer ballmilling time results in larger deposition rate exceptfor the small gas flow rate. One hour ball millingincreases the deposition rate roughly 10 timescompared with no milling. However, the 2 h ballmilling only doubles the deposition rate comparedwith 1 h ball milling, and the longer milling timesdo not effectively increase the deposition rate.Fig. 2(b) also shows the relationship of the film

thickness of 2min deposition with the ball milling

time. However, the primary powders are ballmilled and heat treated at 800 1C for 4 h. Themilling time of 0 h means the primary powder isnot ball milled, but heat treated. The heattreatment without ball milling does not affect thedeposition rate significantly. However, the heattreatment with ball milling greatly increases thedeposition rate.

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0.01

0.1

10 2 4 6 8

10

Milling time h

Cub

ic to

hex

agon

al r

atio

6 l/min 10 l/min

A. Iwata, J. Akedo / Journal of Crystal Growth 275 (2005) e1269–e1273e1272

As the index of the portion of rock salt AlNcrystallites in the aerosol deposited film, the ratioof the strongest peak heights of rock salt andwurtzite AlN in XRD patterns is calculated. Thestrongest peak of rock salt AlN lies at around 441of 2y value and of wurtzite AlN lies at around 331as shown in Fig. 3. Fig. 4 shows the relationshipbetween the ball milling time and the cubic (rocksalt) to hexagonal (wurtzite) peak height ratio.Solid symbols denote the powders are pre-pro-cessed with ball milling only, and open symbolsdenote the powders are pre-processed with ballmilling and heat treatment. Clearly seen is thecubic to hexagonal ratio differs about ten timesbetween the milled powders and milled and heattreated powders. The crystal structure transforma-tion from wurtzite to rock salt is significantlysuppressed by pre-processing the primary powderwith heat treatment.Ball milling of the primary powder would inflict

strain and defects on the powder particles. Theparticles with high strain and many defects tend tobe unstable and are easier to induce the transfor-

0

600

0

600

AlN - Aluminum Nitride (Wurtzite)

AlN - Aluminum Nitride (Rock salt)

30 32 34 36 38 40 42 44 46 48 502-Theta(°)

Inte

nsity

(C

PS)

(a)

(b)

Fig. 3. X-ray diffraction patterns of aerosol deposited AlN

films: (a) powder pre-processed with ball milling and heat

treatment. Thickness 7.7 mm; and (b) powder pre-processed

with ball milling. Thickness 2.2mm.

20 l/min 30 l/min

heated, 10 l/min heated, 20 l/min

heated, 30 l/min

Fig. 4. Cubic (rock salt) to hexagonal (wurtzite) peak height

ratio related to ball milling time. Solid symbols: the powders are

pre-processed with ball milling only. Open symbols: the

powders are pre-processed with ball milling and heat treatment.

mation. The heat treatment reduces these strainand defects and makes the transformation difficultto occur. Therefore the number of particles thatexperience wurtzite to rock salt transformationduring the aerosol deposition is reduced with theheat treatment as pre-processing.

4. Conclusions

Aluminum nitride film is deposited on soda-limeglass by aerosol deposition method with heliumcarrier gas. The deposition rate becomes morethan 10 times larger with the pre-processing of theprimary powder by ball milling. By adding theheat treatment as the pre-processing, the deposi-tion rate further increases. During the aerosol

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A. Iwata, J. Akedo / Journal of Crystal Growth 275 (2005) e1269–e1273 e1273

deposition some of the particles transform fromwurtzite structure to rock salt structure. With theball milling as pre-processing, the XRD peaks ofrock salt structure are mostly higher than that ofwurtzite structure. However, with the ball millingand heat treatment as pre-processing, the peaks ofwurtzite structure become higher. This means theportion of rock salt crystallites becomes less by thepre-processing of heating.

Acknowledgements

This study is partly supported by New Energyand Industrial Technology Development Organi-

zation (NEDO) in Nano Structure Forming forAdvanced Ceramic Integration Technology Pro-ject.

References

[1] J. Akedo, et al., in: S.G. Pandalai (Ed.), Recent

Research Developments in Materials Science, vol. 2-2001

Part 1, Research Signpost, Trivandrum, India, 2001,

pp. 51–77.

[2] A. Iwata, et al., Trans. MRS-J, 29 (2004) 1097.

[3] I. Petrov, et al., Appl. Phys. Lett. 60 (1992) 2491.

[4] T. Mashimo, et al., J. Appl. Phys. 86 (1999) 6710.

[5] J. Akedo, et al., Jpn. J. Appl. Phys. 41 (2002) 3344.