DETERMINATION OF PHYSICAL RESPONSEIN (Mo/AlN) SAW DEVICES

J. C. CAICEDO*,||, J. A. PÉREZ†,‡, H. H. CAICEDO§,¶ and H. RIASCOS‡

*Tribology Polymers,Powder Metallurgy and Processing of Solid Waste Research Group,

Universidad del Valle, Cali � Colombia

†Synchrotron Radiation and Particle Accelerators,Universidad Aut�onoma Barcelona, Spain

‡Departamento de F�{sica,Universidad Tecnol�ogica de Pereira, Grupo Plasma,

L�aser y Aplicaciones A. A 097, Colombia

§Department of Bioengineering,

University of Illinois at Chicago, IL 60612, USA

¶Department of Anatomy and Cell Biology,University of Illinois at Chicago, IL 60612, USA

||jcaicedoangulo1@gmail.com

Received 22 August 2012Revised 16 January 2013Accepted 18 January 2013Published 28 March 2013

This paper describes the experimental conditions in surface acoustic wave (SAW) designed on alu-minumnitride (AlN) ¯lms grown onSi3N4 substrates by using pulsed laser deposition.Moreover itwasstudied the dependency of optical properties with temperature of deposition. The thickness, measuredby pro¯lometry technology, was 150 nm for all ¯lms. Moreover, SAW devices with a Mo/AlN/Si3N4

con¯guration were fabricated employing AlN bu®er and Mo Channel. The morphology and compo-sition of the ¯lms were studied using atomic force microscopy (AFM), scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy analysis (XPS), respectively. The optical re°ectancespectra and color coordinates of the ¯lms were obtained by optical spectral re°ectometry technique inthe range of 400�900 cm�1. In this work, a clear dependence in morphological properties, opticalproperties, frequency response and acoustic wave velocity as function of applied deposition temper-ature was found. It was also observed a reduction in re°ectance of about 10% and an increase ofacoustic wave velocity of about 1.2% when the temperature was increased from 200�C to 630�C.

Keywords: Aluminum nitride; pulsed laser deposition; optical re°ectance; color purity frequencyresponse; surface acoustic wave.

1. Introduction

Aluminum nitride (AlN), an electrical insulating

ceramic with a wide bandgap of 6.3 eV, is a poten-

tially useful dielectric material very important in

¯elds such as optoelectronics and microelectronics.

For instance, it may serve as the dielectric gate in

high voltage and high-power electronic devices; AlN

is also being investigated as a substitute for the silicon

Surface Review and Letters, Vol. 20, No. 2 (2013) 1350017 (15 pages)°c World Scienti¯c Publishing CompanyDOI: 10.1142/S0218625X13500170

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dioxide buried layer in silicon-on-insulator (SOI)

substrates due to its low thermal expansion coe±-

cient, high breakdown dielectric strength and high

chemical and thermal stability.1 Moreover, metal

nitrides based on AlN are of high interest because of

their electrical, optical and acoustical properties.2

Their hardness and thermal coe±cients of expansion

are comparable to that of Si. AlN is used for acoustic

wave devices on Si, optical coatings for spacecraft

components, heat sinks in electronic packaging

applications, as well as electroluminescent devices in

the wavelength range from 215 nm to the blue end

of the optical spectrum.2 Some optical properties near

the fundamental bandgap of AlN have been reported

by Yamashita et al.3 since 1979. Several theoretical

studies have been well documented in the past4,5;

however, reliable experimental data2 has been repor-

ted just recently because both the sample quality and

the spectroscopic techniques have only been sub-

stantially improved in recent years. Precise knowl-

edge of the optical constants is particularly important

in view of the use of AlN thin ¯lms in optical ¯lters

and light emitting laser diodes since they also show

temperature dependence.6

Pulsed laser deposition (PLD) is largely applied

for processing thin ¯lms and other structures. The

highly no equilibrium nature of the PLD process is

attractive for the synthesis of stoichiometric thin

¯lms of various metal nitrides, carbon nitrides, oxides

and oxi-nitrides from the corresponding bulk targets.

PLD appears to be a suitable method to transfer

stoichiometric ally complex monolayer structures

from AlN ceramic structures to substrates. Thus,

from AlN stoichiometric ¯lms many infrared (IR)

measurements presented in the literature were used

to estimate the ¯lm quality. The IR spectra provided

important information about the composition, ho-

mogeneity, crystallinity and the residual stresses

present in the ¯lms. Moreover, the ellipsometric and

re°ectance measurements have been used to deter-

mine the refractive index in many AlN compounds,

since the ellipsometry and re°ectance are sensitive

and nondestructive techniques used for studies of

optical properties and microstructures of surfaces and

thin ¯lms.2 Additionally, single-chip front end RF

modules incorporating surface acoustic wave (SAW)

¯lters are a matter of intense research. At present

these modules are fabricated by bonding single crystal

piezoelectric SAW devices onto integrated circuits.

Thin ¯lms of polycrystalline AlN are promising

materials for the integration of SAW devices on Si3N4

substrates due to their good piezoelectric properties

and the possibility of deposition at low temperature

compatible with the manufacturing of Si integrated

circuits. AlN thin ¯lms of su±cient quality for SAW

applications can be obtained by the PLD technique.7

The goal of this work is to study the e®ect of the

applied deposition temperature on the chemical,

morphological properties, optical properties, fre-

quency response and acoustic wave velocity of binary

Al�N ¯lms deposited by PLD on Si3N4 (100) for use

in optical and electronic applications. Here, using

nitrogen as working gas, we report results on AlN

¯lms deposited from Al targets, their characterization

by X-ray photoelectron spectroscopy (XPS) and

scanning electron microscopy (SEM) as well as

re°ectance investigations.

2. Experimental

Experiments were carried out in standard PLD con-

¯guration consisting of a laser system, a multiport

stainless steel vacuum chamber equipped with a gas

inlet, a rotating target and a heated substrate holder.

A Nd:YAG laser that provides pulses at the wave-

length of 1064 nm with 9 ns pulse duration and repe-

tition rate 10Hz was used. The laser beamwas focused

with an f ¼ 23 cm glass lens on the target at the angle

of 45�, with respect to the normal. During deposition,

the substrate was rotated at a low speed of 2.2 rpm for

enhancing the thickness and uniformity of deposited

¯lms. The distance between the target and the sub-

strate was 6.5 cm. Before deposition, the vacuum

chamber was evacuated down to 1�10�5 mbar by using

a turbomolecular pump backed with a rotary pump.

The ¯lms were deposited in a nitrogen atmosphere as

working gas; the target was an aluminum high purity

(99.99%) target. The ¯lms were deposited with a laser

°uence of 7 J/cm2 for 10min on silicon nitride (100)

substrates (Si3N4). The working pressure was

9�10�3 mbar and the substrate temperature were

varied from 200�C to 630�C. In a previous work,8 we

showed that most of the piezoelectrically driven

devices require AlN ¯lms containing a high volume of

microcrystals oriented with the c-axis normal to the

surface and with the same crystallographic polarity, as

this ensures the highest piezoelectric response. The

J. C. Caicedo et al.

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thickness, measured by pro¯lometry technology, was

150 nm for all high-quality polycrystalline piezoelec-

tric (0002) AlN thin ¯lms, as shown in reference.8 For

each ¯lms the deposition time was 10min. XPS was

used on AlN samples to determine the chemical com-

position and the bonding of aluminum and nitrogen

atoms using ESCA-PHI 5500 monochromatic AlK�

radiation and a passing energy of 0.1 eV. The surface

sensitivity of this technique is so high that any con-

tamination can produce deviations from the real

chemical composition; therefore, the XPS analysis is

typically performed under ultra high vacuum condi-

tions with a sputter cleaning source to remove any

undesired contaminants.

Morphologic characteristics of the coatings like

grain size and roughness were obtained using an

atomic force microscopy (AFM) from Asylum

Research MFP-3Dr and calculated by a scanning

probe image processor (SPIPr) which is the standard

program for processing and presenting AFM data,

therefore, this software has become the de-facto

standard for image processing in nanoscale.

Optical re°ectance spectra and color coordinates

of the samples were obtained by spectral re°ectome-

try in the range 400�900 cm�1 by means of an Ocean

Optics 2000 spectrophotometer. The coated samples

received the white light from a halogen lamp illumi-

nator through a bundle of six optical ¯bers, and the

light re°ected on the samples was collected by a single

optical ¯ber and analyzed in the spectrophotometer.

The ¯ber was ¯xed in perpendicular direction to the

sample surface. An aluminum mirror ¯lm freshly de-

posited by rapid thermal evaporation in high vacuum

was used as the reference sample, and the experi-

mental spectra were normalized to 100% re°ectance

of the reference sample. The morphology and micro-

device structure (SAW) on AlN surface ¯lms was

analyzed by SEM (Leika 360 Cambridge Instru-

ments).

To determine the quality of the AlN piezoelectric

SAW devices this one has been fabricated on thin

¯lms of AlN with di®erent deposition temperatures.

The manufacture of the ¯lters is to de¯ne interdigi-

tated electrodes of Mo of 0.1mm thick over the layer

of piezoelectric material by a photolithography pro-

cess, from a Mo target using a RF source, as substrate

was used ¯lms AlN with 20 oscillations/min. For

electrodes of Mo a deposition time 10min with a °ow

of argon of 25 sccm and an incident power of 800W

was used. The periodicity of the interdigital trans-

ducer (IDT) and the speed of propagation of the

acoustic wave in the substrate determines the fre-

quency with which the acoustic wave can propagate.

In these devices, the IDT switch 50 pairs f ¯nger with

a width of 10�m, so that the acoustic wavelength is

40�m. The area of the electrodes is 1.45�10�2 cm�2

which agrees with the mask geometry used.

Taking into account the deposition conditions of

AlN ¯lm, the frequency response was measured by a

network analyzer (HP 8720C. From the frequency

responses, the insertion loss at center frequency was

monitored and correlated with deposition tempera-

ture and surface roughness of AlN ¯lms. The propa-

gation of the SAW was studied through the analysis

of the scattering parameters on Mo/AlN/Si trans-

mission con¯gurations.

3. Results and Discussion

3.1. XPS analysis

The XPS survey spectrum for AlN thin ¯lm grown at

300�C is shown in Fig. 1. The peaks at 534.4, 397.3,

124.4 and 73.9 eV correspond to O1s, N1s, Al2s and

Al2p binding energies, respectively. Calculation of

the peak areas from the deconvoluted spectrum (see

Fig. 2) without O1s contribution gives an atomic

ratio of Al:N ¼ 0:392:0.588, which is similar to the

stoichiometry of Al0:40N0:60.8 The core electronic

1000 800 600 400 200 00

10000

20000

30000

40000

50000

60000

70000

Inte

nsit

y (

c/s

)

Binding energy (eV)

Al-N

O (

KL

L)

O(1

s)

N(1

s)

Al(

2s)

Al(

2p

)

Fig. 1. XPS survey spectrum of Al�N coatings depositedon Si3N4 at 300�C, where di®erent elemental signals areshowed.

Determination of Physical Response in (Mo/AlN) SAW Devices

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spectra carry the information of the chemical com-

position and bonding characters of the Al�N ¯lms.

The integral of N1s, A2s and Al2p spectra corrected

by relevant sensitive factors can evaluate the con-

centrations of Al and N elements in the Al�N ¯lms.

The corresponding integral of the deconvoluted peaks

can also be used to estimate the bond contents, which

are described by the following formula9:

Ci ¼X

ðAi=SiÞ=X

ðAj=SjÞ; ð1Þ

where S is the sensitivity factor, A is the integral of

deconvoluted peaks and C is the atomic content. The

numerator is the sum of the integral of one sort of

bond; the denominator is the sum of the integral of all

types of bonds decomposed from the whole peak of

N1s, A2s and Al2p in the sample. The atomic con-

centrations of Al, N and O elements from XPS

analysis of the ¯lm deposited at 300�C are listed in

Table 1. For all the Al�N ¯lms obtained, the O con-

centration is less than 3 at.%. We can speculate that

the majority of the oxygen at the surface of the non-

treated sample (which is smooth and almost perfect) is

chemically bonded into alumina-dominated oxide-

nitride material.

The high-resolution spectra of Al2p and N1s, were

recorded from AlN ¯lms, as shown in Fig. 2. From

Fig. 2(a), the Al2p peak is composed of a shoulder

separated by 1.7 eV with intense peak. The XPS

spectrum of Al2p can be ¯tted well by two Gaussian

functions. The value of binding energy obtained for

the Al2p peak was 73.9 eV and the higher value for

Al2p was 75.6 eV, respectively. According to the lit-

erature10�12 for the Al2p peak, the ¯rst one (73.9 eV)

and the second one (75.6 eV) can be assigned to Al�N

and Al�O bonds.

82 80 78 76 74 72 70 68 66 64

0

200

400

600

800

1000

1200

1400

1600

1800

Inte

nsit

y (

c/s

)

Binding energy (eV)

Al2p Al-N

73.9 eV

Al-O

75.6 eV

(a)

408 406 404 402 400 398 396 394 392 390

0

200

400

600

800

1000

1200

1400

1600

1800

Inte

nsit

y (

c/s

)

Binding energy (eV)

N1s

N-N

400.2 eV

N-AL

397.3 eV

(b)

Fig. 2. High-resolution XPS spectrum of: (a) Al2p and (b) N1s of Al�N ¯lm is around 300�C.

Table 1. XPS analysis-derived atomic concentrations of Al and N elements in theAl�N ¯lm synthesized by PLD with 7mbar and substrate deposition temperaturesas function of atomic percentage (at.%).

Substrate temperature (�C) Pressure (�10�3 mbar) Atomic composition (at.%)Al�N ¯lms

Al N O

200 9 39.0 58.6 2.4300 9 39.4 58.8 2.0400 9 39.6 59.0 1.8500 9 39.9 59.3 1.5600 9 40.1 59.5 1.3630 9 40.3 59.7 1.1

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The appearance of the peak at 73.9 eV clearly

shows that Al has reacted with N; therefore, it can be

assigned to Al�N.11,12 In Fig. 2(b), N1s peak is

composed of spin doublets, each separated by 2.9 eV.

The XPS spectrum of N1s, well ¯tted by two

Gaussian functions, depicts the N1s spectrum with

values at 397.3 and 400.2 eV characteristic for N�N

and Al�N bonds, respectively.13

XPS results demonstrated that Al atoms bonded to

N in the form of nitride; taking into account that ele-

mental concentration of the Al�N ¯lm was obtained

by adjusting the laser incidence on Al target, and the

N2 was the working gas. In this research, it was found

that amounts of Al(N) in Al�N¯lmwere maximum at

the current establishment of process conditions and

the ratio of Al and N in the ¯lm was about 2:1. In

general, formative Al(N) phase indicates that the

aluminum and nitrogen activity and activation energy

provided by the present deposition conditions are

enough for the formation of a Al�N thin ¯lm.

Although the surface temperature of the substrate

during deposition of Al�N ¯lm is around 300�C, thesubstrate lies in a high-density plasma region and a

high ion-to-atom ratio of aluminum and nitrogen, that

behavior can promote the formation of Al�N phase at

low temperatures below 300�C.Moreover, in this work

it a correlation was determined between temperature

deposition and chemical properties. Due to the reduc-

tion of oxygen concentration with the increase of

deposition temperature, this behavior was associated

to high surface energy activity generated by high sur-

face temperatures that induce more nitrogen reaction.

In order to see the e®ects of increased substrate

temperature during thin ¯lms growth, the con-

centrations in these depth pro¯les were obtained by

curve-¯tting of high-resolution XPS spectra and

measurements of the XPS peak areas. Thus, the AlN

concentration is in accordance with the stoichiometry

necessary to act like an AlN bu®er ¯lm in the

Mo�SAW device. The dependence of aluminum and

nitrogen in the deposition temperature is shown in

Fig. 3, which represents various chemical states as

described in Table 1 and Fig. 2.

3.2. Surface analysis of AlN ¯lms viaAFM

The scanning probe image processor (SPIPr) can be

used for di®erent purposes including semiconductor

inspections, physics, chemistry and nanotechnology.

In this work, the SPIPr was used to determine

morphologic characteristics of the coatings. Grain

size and roughness were obtained using an AFM from

Asylum Research MFP-3Dr. Moreover, SPIP soft-

ware packages are considered the de facto standard

for processing 3D visualization of images taken by

scanning probe microscopes. The software can import

data through an algorithm that is able to reconstruct

images from ASCII ¯les, bitmap and jpeg images and

other general formats. Furthermore, SPIP was used

on grain size analysis and surface roughness analysis.

Nevertheless, most of the energy on the propa-

gating SAWs is contained in the region within one

wavelength below the surface, so they are very sen-

sitive to the surface state and are a®ected by any

change, such as residual stresses and the level of

roughness. Moreover, authors such as Zhuang et al.

have shown that at the same frequency and in the

small roughness limits (RMS � wavelength), the

SAW velocity increases according to the decrease in

the RMS roughness.10 Figure 4(a)�4(c) shows a

5�m� 5�m image of AlN ¯lms deposited with dif-

ferent deposition temperatures. The general trend in

the AlN ¯lms with the decreasing temperature exhi-

bits a decrease of the grain size and roughness.

Figure 4 also shows microdrops on AlN surface gen-

erated by PLD energy deposition corresponding to

high laser °uency. As this ¯gure was obtained by

AFM, observed drops are characteristic of the PLD

technique and are caused by the release of particu-

lates from the Al target upon impact with the

200 300 400 500 600 700

38,8

39,0

39,2

39,4

39,6

39,8

40,0

40,2

40,4

Al N

Aluminium Concentration (at.%)

Co

mp

os

itio

n (

at.

%)

58,6

58,8

59,0

59,2

59,4

59,6

59,8

Co

mp

os

itio

n (

at.

%)

Fig. 3. Temperature dependence for AlN ¯lms at theconcentrations of Al and N.

Determination of Physical Response in (Mo/AlN) SAW Devices

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substrate; consequently, imperfections on the surface

of the ¯lms are generated. In a previous work,14 we

found a high aluminum content of roughly 60% in this

area, thus corroborating the formation of droplets.

Moreover, the correlation between roughness and

grain size with deposition temperature is shown in

Fig. 5. The quantitative values were extracted from

the AFM images by means of statistical analysis

(SPIPr). Each data that point in the plots represents

an average over three AFM images. The corre-

sponding error bar was obtained by a standard

deviation of the values.

In Fig. 5, it is possible to compare the grain size

and roughness values for the AlN ¯lms, thus, in this

paper a decreasing tendency in surface measurements

was observed when the deposition temperature was

increased. The lower grain size and roughness values

obtained in this study by the Al�N ¯lms deposited on

silicon substrate represented a decrease of grain size

around 42% and roughness 31%, compared to that

obtained for the Al�N ¯lms deposited at 200�C. Thisfact is relevant since the continuity of the surface is

important for optical and electrical SAW devices.

The variation of grain size and roughness (see

Fig. 5) obtained in the Al�N ¯lms suggests that

thermal activation generated by the increase in the

deposition temperature on surface substrates modi¯es

the super¯cial morphology by increasing the energy

associated with atoms on the substrate surface and/

or the growing ¯lms surface which produces a relative

reduction of grain size and roughness.

3.3. Dominant wavelength and colorpurity analysis

The re°ectivity measure is the fractional amplitude of

the re°ected electromagnetic ¯eld, while re°ectance

refers to the fraction of incident electromagnetic

power that is re°ected at an interface. The re°ectance

is thus the square of the magnitude of the re°ectivity.

The re°ectivity can be expressed as a complex num-

ber as determined by the Fresnel equations for a

single layer, whereas the re°ectance is always a pos-

itive real number.15

In certain ¯elds, re°ectivity is distinguished from

re°ectance by the fact that re°ectivity is a value that

applies to thick re°ecting objects. When re°ection

200 300 400 500 600 700

35

40

45

50

55

60

65

70

75

70

80

90

100

110

120

130

140

Gra

in s

ize

(n

m)

Temperature (°C)

AlN Grain size

AlN Roughness

Ro

ug

hn

es

s (

nm

)

Fig. 5. Surface measurements obtained via AFM andSPIPr analysis for AlN ¯lms with a total thickness of150 nm: Correlation between grain size and roughness withdeposition temperature.

(a) (b) (c)

Fig. 4. AFM images for AlN ¯lms grown onto Si3N4 substrate with di®erent deposition temperatures: (a) 200�C, (b) 400�Cand (c) 630�C.

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occurs from thin layers of material, internal re°ection

e®ects can cause the re°ectance to vary with surface

thickness. Re°ectivity is the limit value of re°ectance

as the surface becomes thick; it is the intrinsic re-

°ectance of the surface, hence irrespective of other

parameters such as the re°ectance of the rear surface.

On the other hand, the dominant wavelength (of a

color stimulus) is de¯ned as: \the wavelength of the

monochromatic stimulus that, when additively mixed

in suitable proportions with the speci¯ed achromatic

stimulus, matches the color stimulus considered".15

In this work, the AlN ¯lms has been compared with

pure aluminum because the mirror ¯nish aluminum

has the highest re°ectance of any metal in the

200�500 nm and the 3000�10000 nm (far IR)

regions, while in the 500�700 nm visible range it is

slightly outdone by AlN and silver, but in the

700�3000 nm range (near IR) it is slightly outdone

by, gold and copper materials.16

The re°ectance spectrum or spectral re°ectance

curve is the plot of the re°ectivity as a function of

wavelength as can be seen in Fig. 6. Therefore, all the

deposited ¯lms appear highly re°ective with a white-

and aluminum-colored appearance. Figure 6(a) shows

the optical re°ectance spectra of the AlN single layers

obtained at di®erent deposition temperatures. The

re°ectance of the aluminum and the eye sensibility

are shown for comparison. The spectra of the samples

show high re°ectance for long wavelengths, near to

33% for the AlN ¯lms deposited with 200�C and close

to 23% for AlN ¯lms deposited with 630�C. Thesevalues of re°ectance at these wave numbers are in

good agreement with previous reports for AlN ¯lms.17

A clear decrease in re°ectivity for short wavelengths

is seen, characteristic of a system with high free-

electron density18 with a re°ectance edge below

550 nm due to a screened plasma resonance. The

white and aluminum colors of the AlN ¯lms are a

result of the steep plasma re°ection edge that occurs

in the visible region where the re°ectivity minimum is

around 560 nm.

The color purity refers to the degree of purity

present at a color; therefore in a pure color do not

exist any achromatic e®ect within its composition,

which can vary its capacity for re°ection. One of the

three dimensions of color refers to the color purity

that a surface can re°ect. When the tone is full and is

not mixed with an achromatic e®ect (brightness), this

presents its maximum saturation or purity. On the

other hand, if a particular color is combined with an

400 450 500 550 600 650 700 750 800

0

10

20

30

40

50

60

70

80

90

100

Re

fle

cta

nc

e (

%)

Wavelength (nm)

Eye sensitivity Aluminium 200ºC 300ºC 400ºC 500ºC 600ºC 630ºC

AlN films

(a)

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,70,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

AlN 630 °C

AlN 200 °C460 400470

480

485

490

495

500

505

510

515 520 530

540

550

560

570

580

590

600

610620

650

Y A

xis

X Axis

700

E

Green

Orange

Violet

Blue

Cyan

Yellow

RedAchromatic point

Line of Purples

Aluminum

(b)

Fig. 6. Dominant wavelength and color purity results: (a) Optical re°ectance of AlN ¯lms deposited onto Si3N4 (100)substrates at di®erent deposition temperature, also aluminum optical re°ectance and eye sensibility were also plotted asreferences. (b) Chromatic diagram, in the x, y coordinates, of the re°ectivity for AlN ¯lms. White coordinates of achromaticpoint are located at (1/3, 1/3) (color online).

Determination of Physical Response in (Mo/AlN) SAW Devices

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achromatic e®ect, or with its complement (luminosi-

ty), it su®ers a loss of purity or neutralization. In this

study, color purity changes with the temperature of

deposition, at ranges from 0.63 to 0.66 for AlN ¯lms,

away from color purity of pure aluminum (0.90),

hence con¯rming that the ¯lms are less white com-

pared with the aluminum. Color coordinates of AlN

single layer ¯lms are shown in Fig. 6(b).

Reference coordinates of pure aluminum are

shown for comparison in that ¯gure. The results of

color measurement indicate that all ¯lms re°ect a hue

slightly shifted far to white as compared to aluminum

re°ectivity and with similar color purity. In this work,

it was observed the high dependence on re°ectance

percentages when the deposition temperature is var-

ied from 200�C to 630�C. Figure 7(a) exhibits one

constant region for wavelengths ranged from 760 to

800 nm; in those regions it is possible to appreciate

the e®ect of deposition temperature on changes in the

re°ectance values. Figure 7(b) shows the decreasing

of re°ectance when temperature is increased, which

indicate that temperature promotes the absorbance

in the AlN deposited via PLD. The last behavior

changes in optical properties (re°ectance) can be re-

lated with changes generated by the reduction of

grain size and reduction of surface roughness when

the depositing temperature is increased producing an

activation surface energy observed form AFM results

(see Figs. 5 and 6).

From the re°ectance spectra, a weak but clear

e®ect of deposition temperature on the optical prop-

erties is seen. As the temperature is decreased, the

re°ectance of the ¯lms tends to be higher in the near

IR region. So, dominant wavelength and color purity

have been calculated from the re°ectance spectra of

all ¯lms by using the javaoptics software (see

Table 2). The dominant wavelength was varied for all

the samples, from 562 to 570 nm, and a little close to

that of pure aluminum reference (574 nm).

In Fig. 8(a), the in°uence of deposition tempera-

ture on color purity can be observed. This graph

shows the increase of purity values toward color gray

purity. Figure 8(b) shows the di®erences in the

dominant wavelength for all AlN ¯lms deposited with

di®erent deposition temperatures.

The wavelength is an important optical charac-

teristic for di®erent materials. So, when the wave-

length is changed it is possible to observe that natural

color is changed. This modi¯cation on optical prop-

erties is associated to changes generated by the re-

duction of grain size and reduction of surface

roughness observed form AFM results (see Fig. 6),

760 765 770 775 780 785 790 795 80020

22

24

26

28

30

32

34

200ºC 300ºC 400ºC 500ºC 600ºC 630ºC

Re

fle

cta

nc

e (

%)

Wavelength (nm)

AlN films

(a)

200 250 300 350 400 450 500 550 600 65020

22

24

26

28

30

32

34

36

Wavelength 760 - 800 (nm)

Re

fle

cta

nc

e (

%)

Deposition temperature (°C)

AlN films

(b)

Fig. 7. Temperature-Re°ectance dependence: (a) Re°ectances of AlN ¯lms deposited onto Si3N4 (100) substrates at dif-ferent deposition temperature from 760 to 800 nm wavelength, (b) re°ectance as function of deposition temperature (coloronline).

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moreover, the changes in chemical composition of

AlN ¯lms observed form XPS results (see Fig. 2)

shows that oxygen concentration is reduced when the

temperature deposition is increased, so, the low oxy-

gen concentration induces changes in the energy

bands generating changes at optical properties (color

purity and dominant wavelength). Finally the purity

color dependence in AlN ¯lms with temperature

obtained in this work demonstrates the possibility of

some purity color control.

3.4. SEM analysis

SEM was carried out to quantitatively study the

surface morphology of our samples in relation to an

increase in deposition temperature in AlN ¯lms

grown onto Si3N4 (100). Figure 9 shows SEM

micrographs for AlN ¯lms with random distribution

of microparticles or microdrops that were analyzed

on these surfaces in agreement with AFM results (see

Fig. 5). Accordingly, the deposition temperature

clearly a®ects the reduction of microdrops; this can

be possible due to great surface mobility when the

temperature was varied from 200�C to 630�C. Thissurface mobility reduces the possibility that the

microdrops are anchored on the surface when they

arrive with high energy on AlN ¯lm. Other possible

reason can be associated with high super¯cial tem-

perature that produces surface di®usion of micro-

particles or microdrops, which can decrease the

overall number of particles. A typical image is given

in Fig. 9(a). The magni¯cation image showed in

Fig. 10(b) exhibits that Mo lines are continuous,

without any interruption or short circuit. After

electrical measurements, it was possible to deduce an

insulator behavior for our AlN structures, a good

prerequisite for using them as SAW (Mo/AlN/SAW)

devices.

200 250 300 350 400 450 500 550 600 6500,625

0,630

0,635

0,640

0,645

0,650

0,655

0,660

0,665

Pure aluminium (0.90)

Co

lor

pu

rity

Deposition temperature (°C)

AlN films

(a)

200 250 300 350 400 450 500 550 600 650561

562

563

564

565

566

567

568

569

570

571

Pure aluminum 574 (nm)

Do

min

an

t w

av

ele

ng

th (

nm

)

Deposition temperature (°C)

AlN films

(b)

Fig. 8. Temperature dependence at color results for AlN ¯lms: (a) color purity and (b) dominant wavelength as function ofdeposition temperature.

Table 2. Optical properties for AlN ¯lms deposited onto Si3N4 substrate.

Deposition temperature (�C) Dominant wavelength (nm) Color purity Axis X Axis Y

200 562 0.63 0.234 0.121300 567 0.63 0.242 0.123400 569 0.63 0.240 0.132500 569 0.64 0.246 0.137600 569 0.65 0.262 0.158630 570 0.66 0.302 0.270Reference Aluminum 574 0.90 0.358 0.371

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3.5. Frequency response of Mo/AlNSAW devices

Hadjoub et al.19 reported an improvement in the

Rayleigh Velocity (VR) by decreasing the thickness of

the ¯lms and substrates using Si3N4 with a velocity of

5694m/s value very next to that of VR ðSiCÞ ¼6810m/s. These values are referred to substrates for

fastVR ðAl2O3Þ ¼ 5680m/s,VR ðSiÞ ¼ 4710m/s,VR

ðSiO2Þ ¼ 3410m/s, VR ðTiÞ ¼ 2960m/s and VR

ðWÞ ¼ 2670m/s, reported in the literature. It clearly

shows the importance of using substrates with good

speed of Rayleigh and compatible with existing silicon

technologies so that in our case we used Si3N4 because

VR is 20% higher than that of Si.

Recently, some reports determined the relation

between the h=� and VSAW by experiment or theory,

where h is AlN thickness, � is wavelength and VSAW is

SAW velocity. The relationship between the thick-

ness and the wavelength is h=� ¼ 0:004 the value

according to the literature.18�20 The frequency re-

sponse characteristics of Mo/AlN SAW devices fab-

ricated using the di®erent deposition conditions, such

(a) (b)

Fig. 9. SEM micrographs where is observed the microparticles or microdrops: (a) AlN ¯lms deposited with 200�C (b) AlN¯lms deposited with 630�C.

(a) (b)

Fig. 10. SEM micrographs showing the interdigital structure of Mo coating on AlN ¯lms obtained by PLD: (a) generalinterdigital structure (b) magni¯cation images where it is possible to observe the width and distribution of lines that conformthe interdigital device.

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as, 200�C, 400�C and 630�C, have been measured

and results are shown in Figs. 11(a)�11(d). By using

the AlN bu®er layers, as can be seen by comparing

the wide-frequency scan spectra of those three devices

[which are plotted in the inserts of Fig. 11(c)], the

center frequency was shifted from approximately

0.618GHz [see Fig. 11(a)] to higher frequencies of

0.621GHz [see Fig. 11(b)] and 0.625GHz [see Fig. 11

(c)]. This indicates that the AlN bu®er layer could be

used as high phase-velocity substrate in the fabrica-

tion of high-frequency SAW devices, as with Mo.

In this research using di®erent deposition tem-

peratures of AlN bu®er the insertion loss was reduced.

When using the AlN bu®er layers, the center fre-

quency was shifted from approximately 0.618 [see

Fig. 11(a)] to 0.621GHz [see Fig. 11(b)] and

0.625GHz [see Fig. 11(c)]. This indicates that the

AlN bu®er layer could be used as a high phase

velocity substrate in the fabrication of high-frequency

SAW devices. Additionally, Fig. 11(c) shows that

using AlN bu®er reduced the insertion loss. To ex-

amine in more detail the dependence of insertion loss

0,50 0,55 0,60 0,65 0,70-40

-35

-30

-25

-20

-15

-10

-5

Ins

ert

ion

Lo

ss

(d

B)

Frequency (GHz)

200 °C

-19.04 (dB)

(a)

0,50 0,55 0,60 0,65 0,70-40

-35

-30

-25

-20

-15

-10

-5

Ins

ert

ion

Lo

ss

(d

B)

Frequency (GHz)

400 °C

-12.07 (dB)

(b)

0,50 0,55 0,60 0,65 0,70-40

-35

-30

-25

-20

-15

-10

-5

Insert

ion

Lo

ss (

dB

)

Frequency (GHz)

630 °C

-5.16 (dB)

(c) (d)

Fig. 11. Frequency response characteristics of fabricated Mo/AlN SAW devices with di®erent deposition temperatures (a)200�C (b) 400�C (c) 630�C and (d) the insertion loss by the use di®erent deposition temperatures of AlN bu®er layer.

Determination of Physical Response in (Mo/AlN) SAW Devices

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on the deposition condition of AlN bu®er, the inser-

tion loss was measured for all fabricated SAW devices

as was shown for the frequency response superposi-

tion in Fig. 11(d).

The results are plotted in Fig. 12 as a function of

temperature deposition of AlN ¯lms and the esti-

mated quantitative values are also listed in Fig. 11.

As can be seen in Fig. 12, for SAW devices fabricated

using the sputtered-AlN bu®er with Mo channel the

insertion loss was exactly correlated with the corre-

sponding temperature value for all AlN ¯lms.

This can be identi¯ed by comparing the results

obtained from the device with AlN ¯lms deposited on

silicon nitride substrate. For the aforementioned

devices, the temperature deposition was increasing

systematically, as already shown in Fig. 12. More-

over, the device with AlN deposited to 630�Crevealed the relatively large insertion loss in relation

to frequency value (0.625GHz). This may be attrib-

uted to the relatively low surface roughness (90 nm)

AlN ¯lm used in Mo/AlN device considering that the

surface roughness of AlN ¯lm may also be a®ected by

the surface morphology and deposition temperature

of AlN bu®er layer,21 the results shown in Fig. 11(d)

con¯rm that frequency response characteristics (in-

sertion loss) can be a®ected by reduction of oxygen

concentration, reduction of grain size, roughness and

decreasing with re°ectance of AlN ¯lms together with

increase of color purity and dominant wavelength. In

this sense, the property of AlN bu®er may strongly

govern the characteristic of SAW device using the

Mo/AlN system.

B�en�edic et al.22 reported the direct relationship

between increased frequency and speed in SAW

devices. All this is related to the morphological

qualities of roughness and grain size favor in in-

creased rate of velocity. Therefore, the resonance

frequency of a SAW is determined by the equation

f ¼ V =k, where f is the resonance frequency, V is

the phase velocity of the acoustic wave and K is the

acoustic wavelength, which in turn is de¯ned by the

electrode pitch in the IDT. The re°ected signal of

the Rayleigh mode in AlN ¯lms deposited to highest

temperatures (630�C) is observed to be much

stronger than that of the AlN ¯lms deposited to

lowest temperatures (200�C) and the phase velocity

of AlN ¯lms deposited to (630�C) is much higher.

For The Mo/AlN SAW devices, the measured reso-

nant frequencies of the Rayleigh mode waves ranges

from 0.618 to 0.625GHz corresponding to phase

velocities of 24720 to 25000m/s, indicating good re-

peatability and reproducibility.23�25 Thus, the lower

deposition temperature of the substrate generates a

perturbation of the acoustic velocity that is related

to a change in frequency of the oscillator circuit.

Figure 13 shows improvement of velocity with in-

creased temperature of the ¯lms deposited during the

growth of AlN and was used as substrates for SAW

devices building Mo metal.

Taking into account the last behavior, it is possi-

ble to observe that the quasi-static electric ¯eld

200 300 400 500 600 700-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

0.624 (GHz)0.623 (GHz)

0.620 (GHz)

0.625 (GHz)

0.621 (GHz)

Ins

ert

ion

Lo

ss

(d

B)

Temperature (°C)

Mo/AlN SAW

0.618 (GHz)

Fig. 12. The measured insertion losses of fabricated Mo/AlN SAW devices, as a function of the temperature valuesfrom the deposited AlN ¯lms.

200 300 400 500 600 70024700

24750

24800

24850

24900

24950

25000

25050

Ve

loc

ity

(m

/s)

Temperature (°C)

0.624 (GHz)0.623 (GHz)

0.620 (GHz)

0.625 (GHz)

0.621 (GHz)

Mo/AlN SAW

0.618 (GHz)

Fig. 13. Acoustic wave velocity as a function of depositiontemperature in Mo/AlN/Si3N4 system.

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associated with the SAW interacts with the charge

carriers of the ¯lm. The SAW velocity is, therefore,

dependent on the AlN thin ¯lm conductivity changes.

These velocity changes can easily be interpreted as a

shift in the resonant frequency, when the SAW delay

line is incorporated in an oscillator con¯guration. The

e®ect of temperature is one of the easiest parameters

that can be sensed because almost all physical and

chemical e®ects depend on temperature.26

In certain applications, the e®ect of temperature

should be minimized in order to avoid any interfer-

ence with other e®ects. For acoustic wave sensors, the

temperature e®ect induces dimensional dilatations

and small changes in the mechanical and electrical

properties. Two kinds of interaction can thus be

measured: (i) shift of the acoustic wave velocity due

to intrinsic changes of material properties (elasticity,

density, etc) this is the case for almost all SAW

devices using metal-nitrides materials with low

thickness27 and (ii) for acoustic devices for which the

resonant frequency or the velocity is dependent on the

dimensional values (Lamb wave devices, thickness

shear mode, etc.), temperature can a®ect changes of

both material properties and boundary conditions.28

3.6. Correlation between opticalproperties and acoustic wave velocityin Mo/AlN SAW devices

Figure 14(a) shows the relationship between optical

properties (re°ectance), acoustic wave velocity and

deposition temperature in AlN ¯lms. In this research,

it is clearly shown that, when the deposition tem-

perature decreases, reduction of grain size (see Fig. 5),

reduction of re°ectance [see Fig. 7(b)] and the in-

crease of the color purity [see Fig. 8(a)] is associated

with an increase of frequency response (see Fig. 13)

due to the increase of acoustic wave velocity (see

Fig. 13) produced by energy activation at higher de-

position temperatures. From this correlation, it is

possible to determine that the two-merit index29

associates to an acceptable re°ectance with accept-

able acoustic wave velocity at the same temperature

deposition, and it also associates to an acceptable

roughness with acceptable acoustic wave velocity at

the same temperature deposition.

Therefore, theAlN¯lms deposited between 400 and

500�C o®er the best synergy for optical and acoustic

properties with good re°ectance and an acceptable

acoustic wave velocity. Moreover, AFM results in

Fig. 14(b) show a direct relationship between the de-

crease in roughness of the AlN thin ¯lms and the in-

crease of the deposition temperature.

In this research, is shown a clear correlation is

shown between the roughness and the Rayleigh ve-

locity. At lower roughness the highest velocity is

obtained for the samples grown at a higher temper-

ature of 630�C. Therefore, the AlN ¯lms deposited

around 450�C o®er the best synergy for optical and

acoustic properties with good re°ectance and an ac-

ceptable acoustic wave velocity. Furthermore, the

nanometric grain size and in thickness of such thin

¯lms AlN/Si3N4 a low surface roughness (� 120 nm)

200 250 300 350 400 450 500 550 600 650 70020

22

24

26

28

30

32

34

36

24650

24700

24750

24800

24850

24900

24950

25000

25050Re(%)

Wavelength 760 - 800 (nm)

Re

fle

cta

nc

e (

%)

Deposition temperature (°C)

Mo/AlN/Si SAW

V(m/s)

Ve

loc

ity

(m

/s)

(a)

200 250 300 350 400 450 500 550 600 650 700

70

80

90

100

110

120

130

140

VelocityRoughness

Deposition temperature (°C)

Ro

ug

hn

es

s (

nm

)

24700

24750

24800

24850

24900

24950

25000

25050

25100

Ve

loc

ity

(m

/s)

(b)

Fig. 14. Correlation between optical, morphological andacoustic properties with the deposition temperature: (a)correlation between optical properties (Re°ectance) andacoustic wave velocity in Mo/AlN SAW devices and (b)correlation between roughness and Rayleigh velocity as afunction of temperature deposit AlN thin ¯lms.

Determination of Physical Response in (Mo/AlN) SAW Devices

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and the nucleation side is characterized by a rela-

tively small grain size (� 70 nm), which can be de-

creased with increased substrate temperature

improving processes nucleation, hence enhancing the

propagation of elastic waves as shown in Table 3.

The morphological properties of AlN are highly

dependent on their polarity; the nitrogen-¯nished

¯lms typically have a greater roughness than the

¯nished aluminum, but instead, its crystal quality is

better than conventional AlN obtained by other de-

position techniques. Thus, initial reservoir conditions

greatly a®ect the polarity of the material obtained.30

In principle, it might be thought that the whole ma-

terial has the same polarity; however, it is possible

that there are areas in which the crystals have dif-

ferent polarities. This may explain why the lower

roughness (due to increased substrate temperature

during the AlN deposition) on AlN ¯lms has favored

an increase in the velocity of the waves in the SAW

devices.

4. Summary

In this paper, we have investigated the role of

deposition temperature in determining the optical

properties of AlN ¯lms and the frequency response

characteristics of Mo/AlN/Si3N4 SAW devices.

The temperature deposition was varied from 200 to

630�C to analyze their e®ect on the optical and

morphological properties of the ¯lms. XPS con¯rmed

the formation of the binary ¯lms. The study revealed

that temperature deposition has a marked in°uence

on the chemical and physical nature of all the AlN

thin ¯lms (oxygen concentration grain size and

roughness).

For piezoelectric ¯lms with smooth surface, a

decrease in the RMS roughness can lead to an

enhancement in its SAW velocity. Therefore, the

physical e®ect is most appreciated with changes of the

roughness and grain size when the substrate tem-

perature is increased. It was found a decrease in

the re°ectance of 10%, a increase of color purity of

4.5% and dominant wavelength around 1.5% with

temperature deposition between 200 and 630�C.Furthermore, in this work it was evidenced that the

deposition temperature clearly a®ects the reduction

of microdrops; this can be possible due to great sur-

face mobility when the temperature was varied from

200 to 630�C.The SAW propagation velocity is strongly de-

pendent on deposition conditions. The Rayleigh mode

and can be obtained with h=� ratio larger than 0.004.

The re°ection signal amplitude and phase velocity of

Rayleigh wave have been increased with increasing of

deposition temperature. Finally, it was possible to

design an interdigital structure with Mo circuit on

AlN ¯lms like an insulator, which can be used as

SAW devices.

Acknowledgments

This work was supported by \El patrimonio Autón-omo Fondo Nacional de Financiamiento para la

Ciencia, la Tecnología y la Innovación Francisco Jos�e

de Caldas" under contract RC-No. 275-2011 with

Center of Excellence for Novel Materials (CENM).

One of the authors, H. R, wishes to express his grati-

tude for the kind hospitality o®ered to him fromGrupo

de optica aplicada at Universitat de Barcelona, Cata-

lunya, Spain. Moreover, the authors acknowledge the

Table 3. In°uence of the substrate temperature of AlN thin ¯lms on the morphological properties�roughness and thevelocity of sound on the SAW.

Temperaturesubstrate (�C)

Insertion loss (dB) Frequency (Ghz) Grain size (nm) Roughness (nm) SAW velocity (m/s)

200 19.04 0.618 62 130 24720300 16.45 0.620 56 117 24788400 12.07 0.621 50 109 24840500 8.1 0.623 42 102 24932600 6.09 0.624 38 93 24964630 5.16 0.625 36 90 25000

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Serveis Cientí¯co-T�ecnics of the Universitat de Bar-

celona for and XPS, SEM and optical analysis.

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Determination of Physical Response in (Mo/AlN) SAW Devices

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