ALUMINUM NITRIDE THIN FILMS FOR OR SAW DEVICES DEPOSITED BY PULSED LASER DEPOSITION J.A. Pérez1,2, J. J. Romero2, M. S. Martín-González2 1
Engineering Physics, Technological University of Pereira, Laser and Plasma Applications Group – Colombia 2 Thermoelectric Group, Microelectronics Institute of Madrid, CSIC 28760 Tres Cantos, Madrid, Spain jaimeandres.perez@csic.es
Introduction Aluminium nitride (AlN) thin films are widespread applied because they had some excellent proper•es such as chemical stability, high thermal conduc•vity, low electric conduc•vity and wide band gap (6.2 eV). Moreover, it presents a thermal expansion coefficient similar to that of GaAs, and a higher acous•c velocity, making it excellent for op•cal devices in the ultraviolet spectral region, acous•c op•c devices, and surface acous•c wave (SAW) devices. Polycrystalline films exhibit piezoelectric proper•es and can be used for the transduc•on of both bulk and surface acous•c waves. If compared to other piezoelectric film, such as the well known ZnO, AlN shows a slightly lower piezoelectric coupling; its Rayleigh wave velocity is close to the maximum in the range of values of most materials, being that of ZnO close to the minimum. The Rayleigh sound velocity in c-cut AlN, 5607m/s, is much higher than that of most substrates of prac•cal interest in SAW devices technology. This suggests that AlN and ZnO, rather than alterna•ves, have to be considered each with its own field of applica•on, with a preference for AlN in high frequency applica•ons. The PLD growth of AlN films is rather cri•cal because of its tendency to present microcracking. This tendency is more evident with increasing the thickness of the film and when using silicon substrates, par•cularly in the (100) orienta•on, while using Si3N4 substrates has been shown to improve the films growth. In this work, we will study the reac•ve PLD deposi•on of AlN at different substrate temperatures, in order to find the best condi•ons for the fabrica•on of thin films for the development of SAW devices. A surface acous•c wave (SAW) is a type of mechanical wave which travels along the surface of a solid material. It was discovered in 1885 by Lord Rayleigh, and is o!en named a!er him. Nowadays, these acous"c waves are o!en used in electronic devices. At first sight it seems odd to use an acous"c wave for an electronic applica"on, but acous"c waves have some par"cular proper"es that make them very a$rac"ve for specialized purposes. Much research has been done in the last 20 years in the area of surface acous"c wave sensors. These applica"ons include all areas of sensing (such as chemical, op"cal, thermal, pressure, accelera"on, torque and biological). SAW sensors have seen rela•vely modest commercial success to date, but are commonly commercially available for some applica•ons such as touchscreen displays.
Experimental Setup
Plume of AlN during the deposi•on process
Opera•ng principle of a Surface Acous•c Wave.
Frequency response SAW devices
0,9
510
0.623 (GHz)
24850 0.621 (GHz)
0.620 (GHz)
100
570 500
0,5
600
E 490
24800
0,3
24750
0,2
610 620 650
Aluminum Red
200
300
-4
400
500
600
700
Temperature 0.625 (°C) (GHz)
Insertion Loss (dB)
-8
700
AlN 630 °C les AlN 500 °C urp fP AlN 400 °C eo Lin AlN 300 °C
0.624 (GHz)
0.623 (GHz)
X Axis Chromatic diagram, in the x, y coordinates, of the reflectivity for AlN films. White coordinates of achromatic point are located at (1/3, 1/3).
0.621 (GHz)
AlN films
(b)
34 Eye sensitivity Aluminium 200ºC 300ºC 400ºC 500ºC 600ºC 630ºC
80 70 60 50 40 30
32 30 28 26 24
20
22
10 0 400
(a) 450
500
550
600
650
700
750
800
Wavelength 760 - 800 (nm) 20 200
Wavelength (nm)
AlN 200 °C
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7
-10 -12
470 Violet 460 400
0,0
Mo/AlN SAW
Blue
480
0,1
-6
-14
485
0.618 (GHz)
24700
Orange
590
495
0,4
AlN films
90
Yellow
580
Y Axis
24900
560
505
0,6
0.624 (GHz)
36
Cyan
0,7
24950
550
Achromatic point
Frequency response characteristics of fabricated Mo/AlN SAW devices with different deposition temperatures. The sound velocity and the insertion losses increase with the film deposition temperature. The Rayleigh mode and can be obtained with h/λ ratio larger than 0.00415 with SEM micrographs showing the interdigital struc- speeds higher than 24000 m /s. ture of Mo coating on AlN films obtained by PLD: (a) general interdigital structure, (b) magnification images where the width and distribution of lines that conform the interdigital device are observed.
Reflectivity Analysis
530 Green
0,8
0.625 (GHz)
Reflectance (%)
Mo/AlN SAW
25000
Velocity (m/s)
515 520
540
25050
Reflectance (%)
Optical emission spectroscopy of the plasma rof a nitrogen pressure of A 9·10-3 mbar. In 509.985 nm observed emission band of AlN (0.0). second emission band, weaker, is analyzed in 523.060 nm for AlN (1.0)
The experiments were made in usual PLD configura•on consis•ng of a laser system, a mul•port stainless steel vacuum chamber equipped with a rota•ng target and a heated substrate holder. A Nd:YAG laser that provides pulses at the wavelength of 1064 nm with 9 ns pulse dura•on and repe••on rate 10 Hz was used. Before deposi•on the vacuum chamber was evacuated down to 1·10-5 mbar . The films were deposited in a nitrogen atmosphere as working gas (9·10-3 mbar ); the target was high purity aluminum (99.99%). The films were deposited with a laser fluence of 7 J/cm2 for 10 minutes on silicon nitride (100) substrates heated to temperatures between 200°C and 630 °C.
250 300
350
400
450
500 550
600
650
Deposition temperature (°C)
Temperature-Reflectance dependence: (a) Reflectances of AlN films deposited onto Si3N4 (100) substrates at different deposition temperature also aluminum optical reflectance and eye sensibility were also plotted as references. (b) Optical reflectance spectra and color coordinates of the samples were obtained by spectral reflectometry in the range 400–900 cm-1 by means of an Ocean Optics 2000 spectrophotometer as function of deposition temperature.
0.620 (GHz)
-16 -18 -20
0.618 (GHz)
-22 200
300
400
500
600
700
Temperature (°C)
AlN Roughness
Grain size (nm)
65
130 120
60 110 55 100
50 45
90
40
80
35 200
300
400
500
600
20
30
40
73,7
70
76,3
200
0 80
Binding energy (eV)
78
76
Binding energy (eV)
74
72
60 55 55 50 50 45 45 40 40
XPS survey spectrum of Al-N coatings deposited on Si3N4/Si at 300 °C. Oxygen is the only observed contamination in these films.
Si-422
AlN-222
Si-331 AlN-311
400
After changing the substrate temperatures between 200 ºC and 630 °C the best condi-3 tions of deposit have been for a N2 pressure of 9·10 mbar and temperatures above 300 ºC.
·
X-ray photoelectron spectroscopy (XPS) confirmed the formation of the binary films AlN at substrate temperatures over 300 ºC.
·
The SAW propagation velocity is strongly dependent on deposition conditions. The reflection losses and phase velocity of Rayleigh wave increased with the increase of deposition temperature.
·
It was found a decrease in the reflectance of 10%, a increasing of color purity of 4.5% and a variation of the dominant wavelength around 1.5% with temperature deposition between 200 - 630 ºC.
35 35 30 30 0
High-resolution XPS spectrum of: (a) Al2p and (b) N1s, where the formation of oxynitride N-Al-O and Al-N bonds are observed to occur at different temperatures.
Composition (at.%)
400
Composition (at.%)
73,8 76,95
Al(2s)
60 400 ºC
200 ºC
600
·
65
0 800
In this work, we have investigated the role of deposition temperature on the optical properties of PLD-grown AlN thin films as well as on the frequency response characteristics of Mo/AlN/Si3N4Si SAW devices fabricated with these AlN films on Si3N4Si (100) substrates.
N O
Al2p
N(1s) O(1s)
10000
1000
90
65
Intensity (arb.unit)
20000
80
70
Al(2p)
30000
70
o
50000 40000
60
X-ray diffraction (GIXRD) for AlN films deposited at 300 °C and 9·10-3 mbar. AlN film show a high texture with preferential (hexagonal) orientation (0002)
600 ºC
O (KLL)
Intensity (c/s)
76,25
Al-N Al-O
Al-N
50
2q ( )
o
Temperature ( C)
60000
Si-400
Summary 10
70 700
AFM images for AlN films grown onto Si3N4 substrate with different deposition temperatures: (a) 200 ºC, (b) 400 ºC and (c) 630 ºC. Both the average grain size and the roughness of the films are observed to decrease with the substrate temperature.
70000
Si-311
AlN-200 Si-220
AlN-0002
Intensity (arb. unit.)
70
Optical properties for AlN films deposited onto Si3N4 substrate.
140
AlN Grain size
Roughness (nm)
75
Si-111
AlN-111
Analysis of AlN thin films
100
200
300
400
500
600
Substrate Temperature (ºC) Temperature dependence at the concentrations of N and O in the AlN films.
Acknowledgements: The authors thank Dr. L. Vergara Herrero for the manufacture of SAW devices. This work has been financed by the projects: Nano-structured High-efficiency ThermoElectric Converters (nanoHITEC) and PHOtoacoustic MEasurements of Nanostructures for Thermoelectric Applications (PHOMENTA). J.A.Pérez. acknowledges FPI grant from MINECO.