AlN piezoelectric films for sensing and actuation

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AlN piezoelectric films

for sensing and actuation

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AlN piezoelectric films for

sensing and actuation

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen

op woensdag 16 april 2014 om10:00 uur

door

TRAN Trong An

Master of Science, Hanoi University of Technology geboren te

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Dit proefschrift is goedgekeurd door de promotor: Prof.dr. P. M. Sarro

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter Technische Universiteit Delft Prof. dr. P. M. Sarro, promotor Technische Universiteit Delft Prof. dr. P. J. French, Technische Universiteit Delft Prof. dr. P. Steeneken, Technische Universiteit Delft Prof.dr.ir. R.A.M. Wolters, Universiteit van Twente Prof.dr. Chu Duc Trinh, Hanoi National University

Dr. G. Pandraud, Technische Universiteit Delft

Dr. O. Wunnicke, NXP Semiconductors

ISBN: 978 94 6203 566 9

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without the prior permission of the author.

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To

My grandmother My parents Ha, Kien and Mia

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I

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Introduction

In this chapter we introduce the topic of this thesis: piezoelectric aluminum nitride (AlN) thin films for sensing and actuation. A brief overview of MEMS developments in relation to piezoelectricity is given. We introduce piezoelectricity and piezoelectric materials commonly used in MEMS devices application, in order to provide a context for comparison and positioning of AlN in this field of research. Finally, the goal and structure of the thesis are outlined.

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1.1 Micro-Electro-Mechanical Systems (MEMS)

Miniaturized integrated devices or systems in which electrical and mechanical or other physical components are combined, are generally referred to as Micro-Electro-Mechanical Systems (MEMS). A large family of these devices aims at efficiently and accurately converting mechanical signals to electrical signals or vice versa. The generated signals can function as sensing, actuating or controlling elements in a system at the micro or even nano scale. A variety of application areas such as telecommunications, transportation, photonics, bio-medicine, agriculture require accurate, controllable and power efficient micro and nano sensing and actuating components.

The rising of global issues such as pollution, climate change, depletion of traditional natural resources together with the increasing human demand for information, energy, food and water, pose a continuously growing demand on the development of new devices, advanced materials and innovative technologies. Therefore, research in material science and engineering, device design and fabrication in the MEMS area is of great interest considering their potential in the abovementioned areas. In the last decades, MEMS research has been mainly focused on miniaturization of macro scale devices while preserving their functionality and improving their sensitivity, efficiency, repeatability, accuracy and reliability [1, 2].

In 1954 it was discovered that the change in the electrical resistivity when mechanical strain is applied (piezoresistive effect) is much larger for silicon than for metal films [3], highlighting its potential for the fabrication of strain gauges. The first commercial strain gauge based on Si was lanched in 1958. The integration of Si micro pressure sensors with IC was further developed when National Semiconductor introduced the first high-volume pressure sensor operated at constant temperature by a temperature controller in 1974 [4]. As indicated by Peterson in 1982, silicon, the base material for integrated circuit (IC) technology, is a material with excellent mechanical properties [5]. This makes silicon a potential candidate for MEMS applications, especially when large volume production or integration with electronics are of interest.

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Introduction

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In general, due to the similarity of the silicon-based technology used, it is possible to combine MEMS devices with IC devices, which is beneficial for mass-production and cost-effectiveness of several products. The conventional processes to manufacture IC components, complementary metal oxide semiconductor (CMOS) technologies, forms then the starting point for MEMS fabrication as well. In CMOS technology, conventional materials include polysilicon, silicon dioxide, silicon nitride, and aluminum. These materials are also of great use in MEMS devices. Other materials can often pose contamination risks in a CMOS process and should therefore be avoided or implemented with great caution.

MEMS devices, either sensors, actuators or systems, are often classified based on the physical effects applied for the device operation. So they are specifically addressed as magnetic, electrostatic, thermal, optical, piezoelectric, and so on, MEMS devices. Among these MEMS components, piezoelectric devices present several advantages, such as high speed response, repeatability of operations, low voltage, low power and linear control by a voltage supply. MEMS piezoelectric devices can also operate in both static and vibration mode for actuation. Furthermore, the influence of surrounding environmental parameters such as temperature, radiation, etc on the piezoelectric effect is limited, making these devices more reliable.

In summary, the development of CMOS compatible materials for piezoelectric MEMS applications is a significant trend in research and of large interest to industry. Research should however be focused on both material processing and device fabrication to fully benefit from the potential offered. Alternative, CMOS compatible materials and suitable for IC based fabrication processing are in demand. In this thesis we have investigated one such material, aluminum nitride (AlN) thin-films as promising candidate for piezo MEMS applications in a CMOS compatible context.

1.2 The piezoelectric effect

The accumulation of electric charge in certain solid materials (such as crystals, ceramics etc) when a mechanical stress is applied on the

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material is called piezoelectricity. This phenomenon was first discovered in 1880 by the French physicists Pierre and Jacques Curie. The piezoelectric effect is a reversible process. In fact, these materials can also be deformed by the application of an external electric field. This effect, called inverse piezoelectricity, was mathematically derived by Lippmann in 1881, using principles of fundamental thermodynamics. After that, the inverse piezoelectricity was also experimentally confirmed by the Curie brothers.

Figure 1.1. Schematic illustration of the piezoelectric effect in

quartz crystal as an example of piezoelectric effect [6].

The piezoelectric effect can be explained by the behavior of electric dipole moments that occur in solid materials. In piezoelectric crystals, the asymmetric distribution of ions in the crystal lattice creates the electric polarity as indicated in figure 1.1. Each dipole is represented as a vector. In general, the polarization P of the material is defined by the sum of dipole moments per volume. Therefore, polarization is a vector field. The nearby dipoles join together to form aligned regions called Weiss domains. In ceramic piezoelectric materials like lithium niobate (LiNbO3), barium titanate (BaTiO3) and lead zirconate titanate

(PZT) the polarization is spontaneously distributed and the domains are possible to be reoriented by applying an external electric field called poling process. This phenomenon named ferroelectricity is a

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Introduction

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particular kind of piezoelectricity. In the case of non-ferroelectricity, the polar axis of the domains cannot be reoriented. The alignment of dipoles should be supported by the growth process of materials with a textured structure [7].

Figure 1.2. The distribution of electrical domain: (1) before

polarization, (2) during polarization and (3) after polarization [8].

The behavior of the domain is depicted in figure 1.2. The orientation of domains is normally random. As a consequence, the crystal is electrically neutral as presented in figure 1.2a. On other hand, external effects can align the domains in piezoelectric materials. When a mechanical stress is applied on the piezoelectric material the polarization P is changed. This change might relate to the configuration of electrical symmetry in the crystal lattice or the re-orientation of dipole moments as illustrated in figure 1.2 b and c. The strength and direction of polarization depends on crystal symmetry, the orientation of polarization and the external stress. The change of polarization P induces charge accumulation on the surface of the material. Conversely, the misplacement of atoms in electric dipoles or the re-alignment of dipole moments causes the crystal deformation when an external electric field is applied.

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1.3 Piezoelectric materials

For piezoelectric applications, bulk materials such as quartz (SiO2),

lithium niobate (LiNbO3), barium titanate (BaTiO3) and polyvinyledene

fluoride (PVDF) etc have been used (figure 1.3). However, quartz is a single crystalline material for which no possible techniques are known to deposit it on a Si substrate. Other piezoelectric ceramics (LiNbO3,

BaTiO3 etc) are also difficult to fabricate as thin films.

Figure 1.3. Bulk materials such as quartz (SiO2) lithium niobate

(LiNbO3) barium titanate (BaTiO3) [6, 9].

Other piezoelectric materials (PZT, ZnO, AlN) have been developed in thin film form. In particular, the possibility of micro fabrication, including surface and bulk micro-machining, of structures based on these thin films enables the realization of complex structures with high sensitivity or stability of motion control. Various mechanical structures, including membranes, bridges or cantilevers, for sensing and actuation applications can be therefore fabricated by using piezoelectric thin films.

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Introduction

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The most common piezoelectric materials in macro device applications is lead zirconate titanate (PZT) as shown in figure 1.4. However, the thin films preparation of this material is very difficult. Furthermore, high temperature processing to obtain piezoelectricity and the potential contamination of Si by Pb element make its use hardly compatible with CMOS processing. PZT thin films also show a low quality factor in radio frequency (RF) MEMS applications [10]. For ZnO similar contamination problems, due to its reaction with other IC materials, can be expected. Furthermore, ZnO thin films are unstable, which males hard to reproduce their properties.

Figure 1.4 The most common piezoelectric material lead

zirconate titanate (PZT) (source: ultra-generator.com) and its crystalline structure (source: ytca.com).

Recently, AlN thin films have gained a lot of attention as promising piezoelectric material for MEMS applications, mainly due to the CMOS compatibility and the availability of equipment and techniques for a controllable, reproducible deposition. Moreover, AlN films present excellent physical properties, including wide optical band gap [11], low dielectric loss [12], high breakdown voltage (640MV/m) [13], good electrical resistance (1011_-1013_{Wcm) [14], very high thermal}

conductivity (180 W/mK at 25 °C) [15] and chemical inertness [16]. AlN films are employed to fabricate filters [17, 18], oscillators [19], sensors [20, 21] and actuators [22, 23] thanks to their good electro-mechanical coupling coefficient, high acoustic velocity and low thermal expansion coefficient.

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Figure 1.5. ZnO[24] and AlN[25] materials for piezoelectric thin

films.

1.4 Scope and outline of thesis

The scope of the research presented in this thesis is the preparation of AlN thin-films by sputtering technique and the development of IC compatible fabrication processes for piezoelectric MEMS sensors and actuators based on these AlN thin films. After a brief introduction of the AlN piezoelectric material and the techniques used to characterize this layer (chapter 2), the effect of sputtering parameters on the properties of AlN such as deposition rate, crystallinity, c-axis orientation and mechanical stress, is discussed in chapter 3. This study gives an indication of which parameter setting needs to be used for a layer with specific properties as required by the particular application envisioned. In chapter 4 optimized AlN layers are utilized to fabricate acoustic devices, namely surface acoustic wave (SAW) for sensing in liquid and shear mode resonators with ring-shape electrodes. A CMOS compatible electrode for AlN thin films, sputtered titanium (Ti), is investigated as bottom electrode and the characteristics of the Ti/AlN/Ti layer stack is treated in chapter 5, where also the influence of an AlN interlayer is investigated. Slender piezoelectric cantilevers based on the AlN/Ti/AlN stack are designed, fabricated and tested. The promising results of these novel devices are given in chapter 6. Finally, the main conclusions of this research and recommendations for future work are discussed in chapter 7.

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Introduction

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REFERENCES

[1] J. W. Judy, "Microelectromechanical systems (MEMS): fabrication, design and applications," Smart Materials & Structures, vol. 10, pp. 1115-1134, Dec 2001.

[2] D. Banks, "Microengineering, MEMS, and Interfacing. A Practical Guide," CRC Press, 2006.

[3] C. S. Smith, "Piezoresistance Effect in Germanium and Silicon,"

Physical Review, vol. 94, pp. 42-49, 1954.

[4] K. W. Nadim Maluf, "Introduction to Micro Electro Mechanical Systems Engineering," Artech House, 2004.

[5] K. E. Petersen, "Silicon as a Mechanical Material," Proceedings of the

Ieee, vol. 70, pp. 420-457, 1982.

[6] W. Brown, "In the Beginning: Compelling Evidence for Creation and the Flood," Center for Scientific Creation http://www.creationscience.com, vol. Online edition, 2013.

[7] S. Trolier-McKinstry and P. Muralt, "Thin film piezoelectrics for MEMS," Journal of Electroceramics, vol. 12, pp. 7-17, Jan-Mar 2004. [8] "The Piezoelectric Effect: Fundamentals of Piezoelectricity and

Piezoelectric Actuators," http://www.physikinstrumente.com/. [9] H. X. Xu, D. Lee, J. He, S. B. Sinnott, V. Gopalan, V. Dierolf, et al.,

"Stability of intrinsic defects and defect clusters in LiNbO3 from density functional theory calculations," Physical Review B, vol. 78, Nov 2008.

[10] M. Yamaguchi, K. Y. Hashimoto, R. Nanjo, N. Hanazawa, S. Ttsutsumi, and T. Yonezawa, "Ultrasonic properties of PZT thin films in UHF-SHF ranges - Prepared by sol gel method," Proceedings of the

1997 Ieee International Frequency Control Symposium, pp. 544-551,

1997.

[11] J. Ohta, H. Fujioka, M. Sumiya, H. Koinuma, and M. Oshima, "Epitaxial growth of AlN on (La,Sr)(Al,Ta)O-3 substrate by laser MBE," Journal of Crystal Growth, vol. 225, pp. 73-78, May 2001. [12] M. Akiyama, K. Nagao, N. Ueno, H. Tateyama, and T. Yamada,

"Influence of metal electrodes on crystal orientation of aluminum nitride thin films," Vacuum, vol. 74, pp. 699-703, Jun 7 2004.

[13] K. S. A. Butcher and T. L. Tansley, "Ultrahigh resistivity aluminum nitride grown on mercury cadmium telluride," Journal of Applied

Physics, vol. 90, pp. 6217-6221, Dec 15 2001.

[14] S. Strite and H. Morkoc, "GaN, AlN, and InN - a Review," Journal of

Vacuum Science & Technology B, vol. 10, pp. 1237-1266, Jul-Aug

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[15] L. M. Sheppard, "Aluminum Nitride - a Versatile but Challenging Material," American Ceramic Society Bulletin, vol. 69, pp. 1801-1812, Nov 1990.

[16] M. B. Assouar, O. Elmazria, M. Elhakiki, and P. Alnot, "Study of structural and microstructural properties of AIN films deposited on silicon and quartz substrates for surface acoustic wave devices,"

Journal of Vacuum Science & Technology B, vol. 22, pp. 1717-1722,

Jul-Aug 2004.

[17] M. A. Dubois and P. Muralt, "Properties of aluminum nitride thin films for piezoelectric transducers and microwave filter applications," Applied Physics Letters, vol. 74, pp. 3032-3034, May 17 1999.

[18] G. Piazza, P. J. Stephanou, and A. P. Pisano, "Single-chip multiple-frequency ALN MEMS filters based on contour-mode piezoelectric resonators," Journal of Microelectromechanical Systems, vol. 16, pp. 319-328, Apr 2007.

[19] C. J. Zuo, J. Van der Spiegel, and G. Piazza, "1.05-GHz CMOS Oscillator Based on Lateral-Field-Excited Piezoelectric AlN Contour-Mode MEMS Resonators," Ieee Transactions on Ultrasonics

Ferroelectrics and Frequency Control, vol. 57, pp. 82-87, Jan 2010.

[20] G. Wingqvist, J. Bjurstrom, L. Liljeholm, V. Yantchev, and I. Katardjiev, "Shear mode AlN thin film electro-acoustic resonant sensor operation in viscous media," Sensors and Actuators

B-Chemical, vol. 123, pp. 466-473, Apr 10 2007.

[21] M. Benetti, D. Cannata, F. Di Pietrantonio, V. Foglietti, and E. Verona, "Microbalance chemical sensor based on thin-film bulk acoustic wave resonators," Applied Physics Letters, vol. 87, Oct 24 2005.

[22] J. Olivares, E. Iborra, M. Clement, L. Vergara, J. Sangrador, and A. Sanz-Hervas, "Piezoelectric actuation of microbridges using AlN,"

Sensors and Actuators a-Physical, vol. 123-24, pp. 590-595, Sep 23

2005.

[23] N. Sinha, G. E. Wabiszewski, R. Mahameed, V. V. Felmetsger, S. M. Tanner, R. W. Carpick, et al., "Piezoelectric aluminum nitride nanoelectromechanical actuators," Applied Physics Letters, vol. 95, Aug 3 2009.

[24] "Growth and Characterization of Wide Band Gap Semiconductors "

Amazing Rust .com International Chemical Supply 2005 - 2014.

[25] "Advanced Materials Characterization Laboratory "

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2

AlN thin films for MEMS devices

In this chapter we introduce AlN as piezoelectric material for MEMS devices and the techniques implemented to characterize the thin films to determine the sputtering parameter settings for optimum piezoelectric layers.

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Chapter 2

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2.1 The structure of AlN

AlN, a group III - nitride material, can crystallize in three structures: wurtzite, zinc blend, and rock salt. Under ambient conditions, the thermodynamically stable structure of AlN is wurtzite, while rock salt crystals can be only formed at very high pressure. For zinc blende structure materials, only few reports indicate the successful deposition of AlN with this structure [1, 2].

The wurtzite structure, with a hexagonal close-packed (hcp) lattice, is formed by the tetragonal symmetry bonds for each atom. A hexagonal unit is represented by c and a lattice parameters. The lattice constant c indicates the spacing between two identical hexagonal lattice planes and the lattice constant a defines the distance between atoms in the hexagonal lattice plane. Furthermore, the length of an anion-cation bond divided by the c-lattice constant provides one more cell internal parameter representing the distortion of the unit cell. The Miller indexes (hkl) are used to indicate the crystallographic lattice planes [3] as indicated in figure 2.1. The structural parameters of AlN have also been calculated and measured as reported in literature (table 1.1).

Figure 2.1. Wurtzite structure of AlN or GaN

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AlN thin films for MEMS devices

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Table 2.1. Theoretical parameters simulated by ab-initio

method and experimental parameters of AlN structure [4].

Unit Theory Experimental

a (Å) 3.135 3.110

c (Å) 4.986 4.980

c/a 1.590 1.601

dAl-N (Å) 1.901 1.889

BE (eV) 11.03 11.6

Å is a symbol of angstrom a unit of length equal to 10−10_{m. d}

Al-N is the

equivalent length of Al-N bond. The abbreviation BE stands for Binding Energy.

By changing the deposition conditions, amorphous or crystalline AlN thin-films can be obtained. In general, thin-film AlN layers are poly-crystalline with a (002) or (100) orientation [3, 5]. Recently, AlN thin-films with high (103) orientation were also achieved [6]. The (002) orientation indicates the c-axis of hexagonal AlN to be perpendicular the substrate. Normally, the (002) lattice plane with the highest atom density in AlN crystals is the preferred growth orientation.

2.2 AlN thin-films sputtering

The growth of AlN films can be achieved using several techniques including reactive (and non-reactive) sputtering [3, 7], chemical vapor deposition (CVD) [8], pulsed laser ablation [9], and molecular beam epitaxy [10]. Among them, reactive sputtering is considered as the most promising method due to the low deposition temperature [11] and the potentially low cost aspect of volume manufacturing. High quality AlN layers have been successfully sputtered at room temperature, thus enabling the use of flexible polymer substrates

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[12]. Moreover, sputtering provides the advantage of good adherence of the films to the substrate, highly uniform composition and reduction of contaminants compared to other techniques such as chemical vapor deposition [13].

Figure 2.2. Schematic diagram of a typical sputtering system.

Sputtering is a physical vapor deposition (PVD) process where the atoms of the target are ejected by the collision between accelerated ions and the surface of the target and subsequently deposited on a substrate. The ions are generated in glow discharge plasma and accelerated by an electric field. The mode of the electric field applied for the ion acceleration process depends on the type of target material. A direct current (DC) power supply is typically chosen for a target of conductive material. On the other hand, insulating targets are sputtered using a radio frequency (RF) electric source. In some cases, the atoms from the target can react with the sputtering gas to form compound materials. This process is called reactive sputtering.

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The sputtering process conditions generally have an effect on the properties of the deposited material. In fact, parameters such as gas pressure, gas composition, power, substrate temperature, etc. have influence on the deposition rate, residual stress and structural properties of the films. Consequently, properties of the films, including residual stress and piezoelectric behavior, two of the most important properties for MEMS device applications, can be optimized by tuning the sputtering parameters.

2.3 AlN thin films characterization

In general, a number of techniques are used to characterize the deposited thin films. The main ones are here briefly introduced. 2.3.1 X-ray diffraction of AlN films

X-ray diffraction (XRD) is a non-destructive technique that can be utilized to determine the lattice parameters, strain, composition and thickness of thin films. AlN layers are normally grown on substrates such as silicon, sapphire, silicon carbide or metal coated substrates. In general, point defects, high films strain, impurities, dislocations and stacking faults can be present in the AlN films.

In the XRD technique, the crystal lattice of the layers is explored by X-ray radiation with a wavelength () in the same order of the lattice spacing. A monochromatic ray is normally filtered from the broad spectrum of X-rays generated by electron bombardment of a copper (Cu) electrode in a vacuum tube.

The interaction between the surrounding cloud of individual atoms/ions and the X-ray photons scatters the X-ray beam. Constructive interference occurs when the path length differences of scattered rays from each plane equals 2dsin as shown in figure 2.3. Based on Bragg's law, lattice spacing is calculated through the equation:

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n=2dsin (2.1)

Where n is an integer, λ is the incident X-ray wavelength, d is the spacing of lattice planes, and  is the angle between the incident ray and the lattice planes.

A schematic drawing of an X-ray diffraction measurement is shown in figure 2.3. The angle 2 is scanned by rotating the sample or the X-ray detector. In general, the crystalline samples are three dimensional (3D) gratings for X-ray diffraction. This indicates that the 2D investigation of a crystal can basically be implemented. Each diffracted spot corresponds to each plane in the crystal. The spacing of the plane and the size of the crystal are related to the location and size of diffracted spots [14]. For clearer annotations, the reciprocal lattice of the crystal can be built by using each lattice plane as a reciprocal lattice point.

Figure 2.3. Schematic view of X-ray diffraction system.

The planes of the crystals can be represented by S vectors with the magnitude of 1/d. The crystallinity of each orientation can be determined by measuring the full width half maximum (FWHM) of the rocking curve (-2 scan). In this measurement, the sample is rotated

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to bring the plane to be slightly in and out the Brag diffraction condition.

For AlN films, the -2 diffraction scans detect the preferred orientations of the film along the substrate. Typical -2 diagrams are shown in figure 2.4 [15]. A highly preferred c-axis orientation is indicated by a single (002) peak, while a low crystalline structure is represented by a wide rocking curve (7.7o_{) as for the AlN films in}

figure 2.4a. On the other hand, AlN films with very good crystallinity (rocking curve of 2.6 degrees) produce a scan as indicated in figure 2.4b. This sample contains some other grains such as (101), (102) and (103). The AlN sample in figure 2.4a presents a much higher piezoelectric response than the sample in figure 2.4b [15]. This indicates that the highly preferred c-axis is more important than film crystallinity to obtain good piezoelectricity of AlN sputtered films.

Figure 2.4. An example of RXD diagram of AlN films on

Si/SiN substrate [15]: a) sample with high c-axis orientation and low crystallinity; b) sample with high crystallinity.

2.3.2 Raman spectroscopy of AlN films

Raman spectroscopy has been utilized for the investigation of the AlN crystallinity and orientation [16, 17]. In our experiments, Raman measurements to detect the phonon frequencies of AlN films, was performed by a Renishaw InVia Raman microscope. Figure 2.5 shows

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the image (a) and the diagram (b) of the Renishaw InVia Raman microscope. The samples were excited by a 514 nm Argon gas laser with backscattering detection.

Figure 2.5. The image (a) and the diagram (b) of the Renishaw InVia Raman microscope (source: www.renishaw.com).

A confocal microscope was used to focus the monochromatic laser light bringing the beam spot size to around 1 μm. A Raman signal was formed by 1 in 1000 incident photons, due to scattering phenomenon. To collect this very small signal though a microscope objective, a filter was used to eliminate the overwhelming Rayleigh and reflected signals. Then, the Raman signal was aligned by using a dove mirror together with a slit. The dove mirror, in turn, drove the Raman signal onto a grating that disperses the Raman signal into its constituent components though Bragg diffraction. Finally, the light spectrum was detected by a multi-channel CCD collector.

According to the group theory [18], AlN material with wurtzite structure belonging to the space group C6v4 (C63mc) presents the zone-center optical modes with three Raman active modes (A1, E1 and

E2) [16]. Further, A1 and E1 modes each consists of longitudinal optical

(LO) and transverse optical (TO) components resulting in A1 (LO, TO)

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indicated in table 2.2[18]. For AlN thin-films deposited by sputtering, a major peak around 655 cm−1_{and a minor peak at approximately 610}

cm−1_{appear, corresponding to the E}

2 (high) mode and the A1 (TO)

mode, respectively. Two other modes consisting of the E2 (low) and

the A1 (LO) with very low intensity can be also observed.

Table 2.2. AlN optical phonon modes and their wave number

[18]. Mode E2 (low) A1 (TO) E2 (high) E1 (TO) A1 (LO) E1 (LO) Wavenumber (cm-1₎ 248.6 611.0 657.4 760.8 689.0 912.0

It is well known that the crystallinity of AlN films can be detected by the FWHM of the rocking curve of the E2 (high) mode that is related to

interfaces, grain size and point defects [17]. Kuball et al. reported a value of 3 cm-1_{for extremely high quality AlN bulk crystals [19] and a}

value of 50 cm−1_{was reported by Perlin et al., for very low crystalline}

AlN [20].

Figure 2.6. The relation between FWHM of E2 (high) peaks in

Raman spectra and FWHM of (002) Rocking curve (a); and between the R E2/A1 ratio and (002) orientation degree (b) [16].

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In addition, the c-axis orientation of AlN films can be deduced by the ratio of the integrated areas of the E2 (high) mode and that of the A1

(TO) mode (RE2/A1) obtained by Lorentzian function fitting, where a

higher ratio indicates a higher c-axis orientation as presented in figure 2.6 [16].

2.3.3 Stress measurements of AlN films

Residual stress (also called internal stress) of a film without an external force, is a very important factor for MEMS device fabrications and applications. High residual stress in films results in deformation or breakage of devices after the releasing step. Furthermore, increasing the film thickness leads to higher stress films and causes the cracking of the films from the substrate. In general, the value of residual stress depends on film deposition conditions.

Figure 2.7. The formation of residual stress in film: tensile stress

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The residual stress in the film is classified by the bending trend of the film after the growth or deposition process. Figure 2.7 illustrates the bending as a result of tensile or compressive stress in the film [21]. In figure 2.7a, the film is relatively smaller than the substrate. The substrate becomes concave and the film is under tensile stress. On the other hand, if the film size increases relative to the substrate, the substrate becomes convex and the film is under compressive stress. The first evaluation of film stress was presented by Stoney in 1909 with an equation which carries his name [22]:

) 1 1 ( ) 1 ( 6 2 b a f s R R t Et   





(2.2)

Where  is the residual stress in the film, Ra and Rb are the radii of

substrate curvature before and after film deposition, respectively. E is the Young's modulus and ν is the Poisson's ratio of the substrate, tf

and ts are the thicknesses of the film and the substrate, respectively.

Figure 2.8. KLA Tencor FLX-2908 Thin Film Stress Measurement

System.

We measured the curvature of the substrate by a KLA Tencor FLX-2908 Thin Film Stress Measurement System using laser scanning technique (figure 2.8). A computer is connected to the measurement

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unit for controlling and collecting the data. The value of stress is automatically calculated using equation 2.2 by software.

2.3.4 Spectroscopic ellipsometry of AlN films

The determination of a thin-film thickness by using change of light polarization was first discovered by Drude in the late 1800s. It is called “ellipsometry” because elliptical polarization of the light is utilized in this technique [30]. The change in polarization of the incident light caused by the interaction with material structure is measured by the amplitude ratio (Ψ) and the difference of phase (∆). This signal is the result of thin film properties such as thickness, optical constant, electrical conductivity etc. Thus spectroscopic ellipsometry is a non-destructive and highly sensitive measurement for the investigation of these film parameters. As a consequence, the thickness of ultra-thin films can be inspected with very good accuracy.

Figure 2.9. Schematic drawing of an ellipsometry measurement

setup [25].

In general, the polarization change of light can be detected by reflection or transmission configuration. However, spectroscopic ellipsometry is normally implemented in a reflection set up, as illustrated in figure 2.9.

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The electromagnetic radiation emitted from a light source is passed through a polarizer. The output polarized light is compensated by an optional compensator and illuminates the sample. The reflective light from the sample again travels through a second compensator and analyzer (a second polarizer). Finally, the radiation hits into a detector. In modern ellipsometry systems, the compensator is replaced by a phase modulator. The plane containing both incident and reflective beam is called plane of incidence. The electrical vector of light polarized parallel to the plane of incidence is called p-polarized light. The s-p-polarized light is p-polarized perpendicularly to this plane as indicated in figure 2.10. Therefore, the amplitude ratio (Ψ), and the difference of phase (∆) are defined by ratio of p-polarized reflectivity to s-polarized reflectivity r as presented in the equation below [25]:     i s p e r r r tan (2.3)

The value of Ψ and ∆ do not directly indicate the thickness and optical constants of the film. The relation of material properties and the parameter of ellipsometry are compared by using an optical model. For the calculation, the thickness and optical constant are varied, and Ψ and ∆ are calculated using Fresnel equation. The matching of the model and the measured data is represented by the mean squared error (MSE), defined as:

1000 ] ) ( ) ( ) [( 3 1 1 2 2 2_ _ _ _ _   

_

 n i G E G E G Ei N i C i C i S i S i N m n MSE _(2.4)

Where n is the wavelength number, m is the fit data number and N, C and S are formulated below:

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Chapter 2

24

N=Cos(2Ψ ), C=Sin(2Ψ )Cos(∆), S=Sin(2Ψ )Sin(∆) (2.5)

Figure 2.10. The electrical vector of polarized light in SE system [25].

A good fitting result is indicated by small values of MSE. In fact the measured accuracy of spectroscopic ellipsometry data is around ~ 0.001. As a result, the ideal fit between measurements and the optical model have MSE values of ~ 1. However, a much higher value can be acceptable for thick layers. In practice, the value of MSE is automatically processed by a dedicated software.

2.4 Conclusions

In order to evaluate the potential of sputtered AlN films for MEMS applications, the crystal structure and mechanical properties (residual stress, layer thickness) of the AlN films are investigated. AlN films with good crystallinity and high c-axis orientation together with low residual stress are required for piezoelectric MEMS devices. X-ray diffraction (XRD) technique is generally employed to explore the structure of AlN films. However, the drawback of this technique is the expensive equipment and long measurement time. Raman spectroscopy offers a good alternative given its very fast scan and

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25

low-cost. However, this technique cannot provide the same level of detailed information of thin film structure as X-ray diffraction measurements, but it can be used for an initial investigation. In the next chapter, the information extracted by using these techniques will be essential towards the aimed optimization of the AlN sputtered films properties.

REFERENCES

[1] D. Gerthsen, B. Neubauer, C. Dieker, R. Lantier, A. Rizzi, and H. Luth, "Molecular beam epitaxy (MBE) growth and structural properties of GaN and AlN on 3C-SiC(0 0 1) substrates," Journal of Crystal Growth, vol. 200, pp. 353-361, Apr 1999.

[2] V. Lebedev, V. Cimalla, U. Kaiser, C. Foerster, J. Pezoldt, J. Biskupek,

et al., "Effect of nanoscale surface morphology on the phase

stability of 3C-AlN films on Si(111)," Journal of Applied Physics, vol. 97, Jun 1 2005.

[3] X. H. Xu, H. S. Wu, C. J. Zhang, and Z. H. Jin, "Morphological properties of AIN piezoelectric thin films deposited by DC reactive magnetron sputtering," Thin Solid Films, vol. 388, pp. 62-67, Jun 1 2001.

[4] E. Ruiz, S. Alvarez, and P. Alemany, "Electronic-Structure and Properties of AlN," Physical Review B, vol. 49, pp. 7115-7123, Mar 15 1994.

[5] M. Ishihara, S. J. Li, H. Yumoto, K. Akashi, and Y. Ide, "Control of preferential orientation of AlN films prepared by the reactive sputtering method," Thin Solid Films, vol. 316, pp. 152-157, Mar 21 1998.

[6] S. R. Jian, G. J. Chen, J. S. C. Jang, and Y. S. Lai, "Nanomechanical properties of AlN (103) thin films by nanoindentation," Journal of

Alloys and Compounds, vol. 494, pp. 219-222, Apr 2 2010.

[7] H.P.Loebl, M. Klee, O.Wunnicke, R. Kiewitt, R. Dekker, E.V Pelt, "Piezo - electric AlN and PZT films for micro - electronic applications"

Proceedings of the IEEE Ultrasonics Symposium,Volume 2, pp1031-1036, 1999.

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Chapter 2

26

[8] Z. Chen, S. Newman, D. Brown, R. Chung, S. Keller, U. K. Mishra, et

al., "High quality AlN grown on SiC by metal organic chemical vapor

deposition," Applied Physics Letters, vol. 93, Nov 10 2008.

[9] W. T. Lin, L. C. Meng, G. J. Chen, and H. S. Liu, "Epitaxial-Growth of Cubic AlN Films on (100) Silicon and (111) Silicon by Pulsed-Laser Ablation," Applied Physics Letters, vol. 66, pp. 2066-2068, Apr 17 1995.

[10] J. D. Mackenzie, C. R. Abernathy, S. J. Pearton, V. Krishnamoorthy, S. Bharatan, K. S. Jones, et al., "Growth of Ain by Metalorganic Molecular-Beam Epitaxy," Applied Physics Letters, vol. 67, pp. 253-255, Jul 10 1995.

[11] F. Medjani, R. Sanjines, G. Allidi, and A. Karimi, "Effect of substrate temperature and bias voltage on the crystallite orientation in RF magnetron sputtered AIN thin films," Thin Solid Films, vol. 515, pp. 260-265, Sep 25 2006.

[12] H. Jin, J. Zhou, S. R. Dong, B. Feng, J. K. Luo, D. M. Wang, et al., "Deposition of c-axis orientation aluminum nitride films on flexible polymer substrates by reactive direct-current magnetron sputtering," Thin Solid Films, vol. 520, pp. 4863-4870, May 31 2012. [13] J. Sellers, "Asymmetric bipolar pulsed DC: the enabling technology

for reactive PVD," Surface & Coatings Technology, vol. 98, pp. 1245-1250, Jan 1998.

[14] M. A. Moram and M. E. Vickers, "X-ray diffraction of III-nitrides,"

Reports on Progress in Physics, vol. 72, Mar 2009.

[15] A. Sanz-Hervas, M. Clement, E. Iborra, L. Vergara, J. Olivares, and J. Sangrador, "Degradation of the piezoelectric response of sputtered c-axis AlN thin films with traces of non-(0002) x-ray diffraction peaks," Applied Physics Letters, vol. 88, Apr 17 2006.

[16] D. Chen, D. Xu, J. J. Wang, B. Zhao, and Y. F. Zhang, "Influence of the texture on Raman and X-ray diffraction characteristics of polycrystalline AlN films," Thin Solid Films, vol. 517, pp. 986-989, Nov 28 2008.

[17] V. Lughi and D. R. Clarke, "Defect and stress characterization of AIN films by Raman spectroscopy," Applied Physics Letters, vol. 89, Dec 11 2006.

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[18] V. Y. Davydov, Y. E. Kitaev, I. N. Goncharuk, A. N. Smirnov, J. Graul, O. Semchinova, et al., "Phonon dispersion and Raman scattering in hexagonal GaN and AlN," Physical Review B, vol. 58, pp. 12899-12907, Nov 15 1998.

[19] M. Kuball, J. M. Hayes, A. D. Prins, N. W. A. van Uden, D. J. Dunstan, Y. Shi, et al., "Raman scattering studies on single-crystalline bulk AlN under high pressures," Applied Physics Letters, vol. 78, pp. 724-726, Feb 5 2001.

[20] P. Perlin, A. Polian, and T. Suski, "Raman-Scattering Studies of Aluminum Nitride at High-Pressure," Physical Review B, vol. 47, pp. 2874-2877, Feb 1 1993.

[21] M. Ohring, "The Materials Science of Thin Films," Academic Pres, 1992.

[22] G. C. A. M. Janssen, M. M. Abdalla, F. van Keulen, B. R. Pujada, and B. van Venrooy, "Celebrating the 100th anniversary of the Stoney equation for film stress: Developments from polycrystalline steel strips to single crystal silicon wafers," Thin Solid Films, vol. 517, pp. 1858-1867, Jan 30 2009.

[23] J. A. Thornton, "The Microstructure of Sputter-Deposited Coatings,"

Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films, vol. 4, pp. 3059-3065, Nov-Dec 1986.

[24] L. Vergara, M. Clement, E. Iborra, A. Sanz-Hervas, J. G. Lopez, Y. Morilla, et al., "Influence of oxygen and argon on the crystal quality and piezoelectric response of AlN sputtered thin films," Diamond

and Related Materials, vol. 13, pp. 839-842, Apr-Aug 2004.

[25] H. G. T. a. E. A. Irene, "Handbook of Ellipsometry," William Andrew

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Chapter 2

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3

Optimization of AlN sputtered thin-films for

piezoelectric applications

In this chapter, we study the sputtering process for the deposition of AlN thin films on bare silicon (Si) substrates. The physical properties of the films, including residual stress and piezoelectricity, are influenced by the sputtering parameters, such as gas composition, pressure and power, and by the substrate temperature. An understanding of their effect on the layer properties is essential to optimize the process, in order to obtain layers with well-defined characteristics as required for the realization of specific MEMS devices.

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Chapter 3

30

3.1 Introduction

The most common technique used to deposit AlN thin films is sputtering. In general, sputtering conditions have a high influence on the properties of the AlN films, such as composition, structure, surface roughness, defect density, residual stress and piezoelectricity. The main sputtering parameters, gas pressure and composition, power, substrate temperature, are adjusted to obtain the required film properties. As discussed in the previous chapter, a very high c-axis orientation films is needed to obtain a good piezoelectric response.

In the literature, several studies on the relation between sputtering conditions and AlN film properties can be found. A few interesting findings are briefly summarized here. The appearance of other orientations beside (002) caused by reduction of kinetic transfer energy is explained by less densely packed configuration of other orientations in comparison to (002) [1]. For example, Cheng et al. reported that (100) oriented AlN films are produced by a low nitrogen concentration in the Ar sputtering gas. The increase of nitrogen concentration leads to the growth of other orientations, and the preferred c-axis orientation (002) is obtained at high nitrogen concentration [2]. However, in other studies also other orientations at higher nitrogen concentration are observed [3]. In particular, lower nitrogen content usually increases oxygen content that reduces the piezoelectric response [4].

Ababneh et al. studied the effect of sputtering pressure on the AlN film orientation [5]. They observed that next to the (002) peak, also other orientations are visible and they increase when the sputtering pressure increases. This can be explained considering that reduction of the kinetic energy of the arriving atoms on the substrate at higher sputtering pressure, leads to worse film quality causing lower c-axis crystallinity. Decreasing sputtering pressure (below 2mTorr) leads to the preferred (002) orientation and to higher crystallinity [6].

In general, the deposition rate is linearly proportional to the sputtering power. Furthermore, the increase of substrate temperature is expected to improve the film quality. High power deposition can also be employed to improve the quality of AlN films [7]. As reported in [8] the highly preferred (002) orientation of AlN

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Optimization of AlN sputtered thin-films for piezoelectric applications

31

films can only be obtained by providing enough kinetic energy to the film surface. This energy is normally supplied by electrically accelerating ionic species with a bias power in the plasma near the substrate [8]. By increasing the sputtering pressure, energy loss of accelerated ions occurs due to the collision rate increase in denser plasma gas. To balance the energy loss, the voltage acceleration can be increased by higher sputtering power or more bias voltage. However, further increasing of acceleration potential deteriorates the crystal quality because of the bombardment by accelerated ions [8]. The optimization of AlN by sputtering technique reported in the literature gives rather contradictory indications because the behavior of each sputtering system is somewhat different. Therefore, an optimization of the AlN thin films properties, by studying the effect of several sputtering parameters in our deposition system, is needed to determine the optimal conditions for piezoelectric AlN layers.

3.2 The pulsed DC sputtering technique

For our experiments, we used a pulsed Aviza Sigma 204 DC magnetron PVD system, available in the DIMES laboratory. AlN films are sputtered using a pure Al target in a mixture of argon and nitrogen gasses with a direct current (DC) power supply to obtain a high deposition rate. This technique is called DC reactive sputtering. The appearance of an insulating layer on the target caused by nitridation can stop the sputtering process as the rate of the formation of the insulating layer is higher than the rate of film removal by sputtering. Furthermore, this sudden breaking of the sputtering process causes many defects incorporated in the thin film. The pulsed - DC source has been employed to solve this problem as it reduces nitridation of the Al target, instead of using an expensive RF power supply. For the deposition (reactive sputtering mode) of the AlN films, the power, with a frequency of 250 kHz and a duration pulse of 1616 ns, was varied up to 4 kW. The waveform of a pulsed DC generator is shown in figure 3.1. In our system, the substrate to hold the sample can be heated up to 500 o_{C. However, the AlN deposition}

tests were carried out at lower temperatures (50 o_{C – 400}o_{C), to have}

a CMOS compatible process. The distance between target and substrate that can also influence the properties of AlN films [9, 10], is fixed at 7.5 cm. The base pressure of our deposition tool is lower than

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Chapter 3

32

5.10-8 _{Torr. High-purity (99.9999%) Ar and N}

2 gases with adjustable

flow rates (with maximum Ar and N2 flow rates of 300 sccm and 100

sccm, respectively) were let in the sputtering chamber to create a certain sputtering pressure.

Figure 3.1. Waveform of a pulsed DC generator.

The ratio of Ar and N2 flow rates is related to the nitrogen

concentration of the plasma. Nitrogen concentration is calculated by relating the N2 flow rate to the total gas flow rate. The sputtering

process can also be done by activation of the throttle valve to reduce the section of outlet pumping. As a consequence, higher sputtering pressure is obtained. However, we found that it is difficult to adjust the sputtering pressure precisely in throttle condition. Moreover, the AlN films become less uniform at high-pressure conditions. On the other hand, low pressure (less than 2 mTorr) can cause plasma instability. Therefore, our AlN layers are sputtered at a working pressure in the 2.5 - 6.5 mTorr range by changing the gas flow at constant pumping speed.

For the initial experiments, the film thickness was kept at ± 1 μm, by adjusting the deposition time. A thickness of 1 μm can be sufficient to exclude the Raman spectrum of the Si substrate [7]. The main parameters studied are nitrogen concentration, pressure, and power.

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33

In addition, dependence of AlN film quality on substrate temperature is also investigated.

In general, the AlN films can be deposited on different kinds of substrates. In our research here, (100) silicon substrate was chosen due to its IC [5] compatibility and fabrication cost consideration. The Si/AlN structure can further be used to fabricate surface acoustic wave (SAW) devices. Furthermore, computation of AlN thickness for a Si/AlN structure, based on ellipsometric spectroscopy (SE) is is more accurate than for other structures such as Si/SiO2/AlN and Si/Ti/AlN.

The native oxide present on the surface of the silicon wafer was etched by inductively coupled plasma (ICP) Ar+_{ions in an adjacent}

chamber without breaking vacuum, previous to AlN sputtering. 3.3 Effect of nitrogen concentration

Nitrogen concentration largely influences the structure of AlN films. Therefore, the first set of experiments was dedicated to the study of the effect caused by varying the nitrogen fraction in the plasma sputtering gas.

3.3.1 Sputtering parameters

AlN films were first deposited on bare silicon substrates at 300 o_{C at a}

fixed sputtering pressure of 2.5 mTorr.

Figure 3.2. A Si wafer with (a) stoichiometric and (b)

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Chapter 3

34

A low value of gas pressure is initially selected as it has been shown [11] [12] that it can promote the crystallinity and the highly preferred c-axis orientation of the AlN films. We discovered that a power of 2 kW pulsed DC power or higher produces non-stoichiometric, Al-rich films when a low nitrogen concentration (~ below 60%) is used. The films were conductive as a sheet resistivity in the order of 102_Ω.cm.

Furthermore, as clearly visible in figure 3.2, the non-stoichiometric film (figure 3.2b) is much more opaque than the stoichiometric one (figure 3.2a). This can be explained considering that high power sputtering and low nitrogen concentration lead to more free Al atoms than N atoms, resulting in the formation of Al rich films.

Table 3.1. Sputtering parameters used to study the effect of

N2 fraction.

Substrate Silicon (100)

Sputtering pressure 2.5 mTorr

Substrate temperature 300 °C

Sputtering power 1 kW

Nitrogen fraction 33 - 100%

Film thickness 1 μm

On the other hand, it is difficult to sputter at high power (1 kW or higher) and high nitrogen concentration (more than 70%). In fact, in these conditions the lower rate of film removal by sputtering in comparison to the rate of insulating layer formation on the target surface extinguishes the plasma. Consequently, the power was kept constant at 1 kW, while the nitrogen fraction was varied over a wide range. The sputtering parameters used for the first set of experiments are shown in table 3.1.

3.3.2 Deposition rate

Figure 3.3 presents the influence of nitrogen concentration in the plasma sputtering gas on the deposition rate of AlN. The deposition rate is evaluated by measuring the thickness of the sputtered AlN layer by ellipsometric spectroscopy (SE) and repeating the test to

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35

obtain the 1 m layer thickness adjusting the deposition time. The increase of nitrogen concentration (from 33% to 73%) causes a rapid reduction of the deposition rate from 0.45 nm/sec to 0.25 nm/sec. This can be explained considering the increased nitridation of the target surface [13] for higher nitrogen content in the plasma. In fact, more nitrogen content causes AlN formation rather than pure Al sputtering. On the other hand, more nitrogen in the plasma also leads to lower transfer of energy between sputtering ions and the atoms on the surface of the target due to the lower mass of N2+_{than that of}

Ar+_{. As a result, a lower deposition rate is observed [5]. For AlN films}

sputtered at high concentration of nitrogen (at 80% and 100%) similar deposition rates are obtained. This can be explained by the fact that the nitridation process reaches saturation of AlN formation, resulting in a constant rate of sputtering.

Figure 3.3. Deposition rate of sputtered AlN films vs. N2

concentration. 3.3.3 Raman spectroscopy

The films were characterized by Raman spectroscopy at room temperature to investigate the crystallinity and c-axis orientation.

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Chapter 3

36

Figure 3.4 shows the Raman spectra in the wavenumber range of 560 – 900 cm-1_{. All data presented two modes, the H2 (high) and A1 (LO)}

modes. Other modes are not clearly observed. These results indicate that all deposited AlN layers have a wurzite structure, independently of the nitrogen concentration used.

Figure 3.4. Raman spectra of 1 µm thick AlN films sputtered

with different nitrogen concentrations.

The spectra were fit by Lorentz function to determine the FWHM of the H2 (high) mode and the integrated areas ratio of the E2 (high) and

the A1 (TO) mode (RE2/A1) [7] as presented in figure 3.5. The obtained

fitted data result in a very low FWHM of H2 (high) mode from 7.3 to

8.6 cm-1_{(figure 3.5a) films. The increase of nitrogen concentration}

leads to an improvement of the film crystallinity.

However, for pure nitrogen plasma, degradation of the film crystallinity occurs. This phenomenon might be related to the lack of transferring energy provided by Ar with heavier mass than N2,which

results in lower crystalline grains in the AlN films [14]. In order to investigate the presence of c-axis orientation in the AlN films, the integrated areas ratio of the E2 (high) and the A1 (TO) mode (RE2/A1) for

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37

data show that the c-axis orientation increases gradually for intermediate N2 concentrations in the range of 50 - 73%. Lower (33%)

and higher values (80 – 100%) suffer from a decrease of c-axis orientation.

Figure 3.5. The FWHM of H2 (high) mode (a) and the integrated

areas ratio of the E2 (high) and the A1 (TO) mode (RE2/A1) (b) at

different nitrogen concentrations.

We can thus conclude that the presence of Ar plays an important role in the improvement of the crystallinity and high c-axis orientation of sputtered AlN films. The low N2 concentration (33%) results in the

deterioration of film quality because not all Al atoms are bound to a nitrogen atom.

3.3.4 X-ray diffraction

As mentioned in chapter 2, the high c-axis orientation is required to have good AlN piezoelectric films. According to Raman analysis, the good crystallinity of AlN films can be obtained at intermediate nitrogen concentration of sputtering gas. The highest c-axis orientation was acquired with 66% nitrogen fraction. Consequently, this sample was used for the XRD theta-2-theta scan. The 2-theta scan XRD pattern (figure 3.6a) contains only a very narrow (002) peak (at 36o_{) and no peak related to other grains appear. This indicates that}

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Chapter 3

38

Figure 3.6. Theta-2-theta scan XRD patterns of AlN thin films

deposited at 66% nitrogen concentrations (a) and the FWHM of the x-ray rocking curve measurements of (002) peak (b).

Figure 3.6b further shows the FWHM of the x-ray rocking curve measurements of (002) peak. The value of 2.32o_{indicates the very}

high crystalline structure of the AlN films.

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39

To obtain a better analysis of the AlN crystalline orientations, the (002) AlN peak and (101) AlN peak were measured by varying the tilt and rotation of the sample. Figure 3.7 shows the pole figures where the center presents the data of a non-tilted sample and the edge corresponds to 90o _{tilt. The very high intensity in the center of the}

figure confirmed the c-axis preferred orientation of the AlN crystallites with their {002} lattice planes parallel to the surface of the sample. Furthermore, the data of (101) peak also contain a very high intensity ring at 62o_{tilt angle indicating that the {101} lattice planes}

have an angle of 61.6° with the {002} lattice planes. The continuous rings for distribution of intensity without invisible spots (figure 3.7b) represents the in-plane orientation of the film.

3.3.5 Residual stress

The influence on film stress (a total of intrinsic and thermal stress) of the nitrogen concentration in the sputtering gas is shown in figure 3.8.

Figure 3.8. The residual stress of 1µm thick AlN films at

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Chapter 3

40

It can be seen that the lowest stress (around 350 MPa) was obtained for a N2 concentration of 50%. The films sputtered at a lower

concentration (33%) have slightly higher stress (~450 MPa). A high residual stress with value ~ 700 MPa was observed for the film deposited at the highest concentrations of nitrogen (from 66% to 100%). The thermal stress of AlN deposited on Si is estimated to be tensile (~ 195 MPa). From the values reported in figure 3.8 it can be concluded that intrinsic stress of the films is tensile. These findings are somewhat in contrast to other reports found in the literature. Lee et al. reported that the stress of the AlN films on Si (100) becomes more compressive by increasing nitrogen concentration [15]. Ababneh et al., also observed that decreasing nitrogen concentration leads to an increase of the tensile stress [5]. Tensile stress observed for sputtered films is caused by the appearance of microvoids [16]. In our case, we think that the higher crystallinity and more (002) texture in AlN films sputtered with high nitrogen concentration leads to higher tensile stress.

3.4 The effect of sputtering pressure

The first set of experiments shows that with a nitrogen concentration of 66% films with good crystallinity and high c-axis orientation were obtained. In order to determine the influence of the working pressure on the AlN thin films properties, a second set of experiments was carried out.

Table 3.2. Sputtering parameters used to study the effect of

pressure.

Substrate Silicon (100)

Sputtering pressure 2.5 - 6.5 mTorr Substrate temperature 300 °C

Sputtering power 1 kW

Nitrogen fraction 66%

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41

The pressure was varied between 2.5 and 6.5 mTorr by changing the total gas flow rate while the ratio of N2 and Ar flow rate was kept

constant. Table 3.2 indicates the sputtering parameters used for this investigation.

3.4.1 Deposition rate

The deposition rate of AlN films slightly decreases with increasing sputtering pressure, as illustrated in figure 3.9. Increasing of sputtering pressure results in a reduction of the mean free path. Consequently, more collisions are possible that cause a loss of energy, lowering the rate of the sputtering process.

Figure 3.9. Deposition rate of sputtered AlN films vs. sputtering

pressure.

3.4.2 Raman spectroscopy

Raman spectra of these AlN films (see Fig 3.10) also contain two modes, H2 (high) and A1 (LO), indicating the wurzite structure with

high c-axis orientation while other peaks are not visible. Furthermore, the FWHM of H2 (high) slightly changes with increasing sputtering

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Chapter 3

42

pressure, as presented in figure 3.11a. At gas pressures of 4.5 and 6.5 mTorr, the FWHMs of the H2 modes were around 7.3 and 6.9 cm−1,

respectively. This indicates that all of the films have very good crystallinity.

Figure 3.10. Raman spectra of 1μm thick AlN films deposited at different sputtering pressures.

In addition, RE2A1 slightly increases with increasing sputtering

pressure. This implies that the crystallinity and c-axis orientation of AlN cannot be clearly compared by simply using Raman spectroscopy. Therefore, further XRD measurements were performed for these samples.

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43

Figure 3.11. The FWHM of H2 (high) mode (a) and the RE2/A1 at

different sputtering pressures (b). 3.4.3 X-ray diffraction

X-ray diffraction measurements were performed for AlN films deposited at different sputtering pressure to circumvent the difficulty to distinguish the AlN film structures using Raman data.

Figure 3.12. Theta-2-theta scan XRD patterns of AlN thin films

AlN piezoelectric films for sensing and actuation

AlN piezoelectric films

for sensing and actuation

AlN piezoelectric films for

sensing and actuation

PROEFSCHRIFT

TRAN Trong An

Contents

1

Introduction

2

AlN thin films for MEMS devices







3

Optimization of AlN sputtered thin-films for

piezoelectric applications

_