STUDY OF AlN/FSMA MAGNETOELECTRIC HETEROSTRUCTURE FOR MEMS APPLICATIONS

Abstract

In the current era, the progress of electronic technology is directly coupled with the advancement made in materials science. Magnetoelectric materials are the potential candidate for several multifunctional devices such as magnetic random-access memory, switches, magnetic sensors and tunable resonators. In these kinds of composites, the Magnetoelectric (ME) response is a product tensor property, i.e., outcome of the mechanical interaction between the magnetostrictive effect (magnetic/mechanical effect) occurring in the magnetic phase and the piezoelectric effect (mechanical/electrical effect) present in the piezoelectric phase. The ME coupling is observed in diverse structures such as bulk ceramic, two and three-phase ME composites and nanostructured thin films. However, the ME composite thin films have attracted enormous attention because different phases can be attached and designed at the atomic level with higher precision and sharp interface. Furthermore, nanostructured thin film composites enable the researchers to investigate the physical cause of the ME coupling effect at the nanoscale level. The strength of the ME coupling depends on many factors, such as interface quality, high anisotropy magnetostriction, lattice mismatching, and functional characteristics of the individual piezoelectric and magnetostrictive layers. The ideal candidates for magnetostrictive and piezoelectric layers are ferromagnetic shape memory alloys (like-NiMnIn, NiMnSb, etc.) and AlN, respectively. Ferromagnetic shape memory alloys (FSMAs) exhibit a large applied magnetic field and temperature induced strain in the temperature region during the martensite to austenite phase transition as compared to leading magnetostrictive materials such as Trefnol-D and metglas. The NiMnIn exhibits giant magnetostriction coefficient at room temperature with multiple degrees of freedom to tune its magnetostriction, i.e., by varying temperature, magnetic field, stress. On the other hand, Aluminium Nitride (AlN) is a lead-free complementary metal oxide-semiconductor (CMOS) compatible piezoelectric material with a high value of voltage piezoelectric coefficient (e33). The deposition process of AlN is also well reproducible, and its chemical compatibility is well suited with semiconductor fabrication technology. For strong coupling between ferromagnetic and piezoelectric layers in the artificial magnetoelectric heterostructure, it is desirable to fabricate high-quality thin films with a sharp interface that can be attained by several growth techniques such as molecular beam epitaxy, pulsed laser deposition, metal oxide chemical vapor deposition, sputtering. iii The main objectives of the thesis are (i) Fabrication of c-axis oriented AlN thin films using sputtering, and its characterization for electronics and microelectromechanical system applications (ii) Study and fabrication of strong ME response in silicon integrated FSMA and AlN based artificial thin film multiferroics structures. (iii) Synthesis and characterization of magnetic field tunable ferromagnetic shape memory alloy based piezo-resonator. A chapter-wise summary of the thesis is given below. Chapter 1 gives an overview of the magnetoelectric effect and magnetoelectric materials. Magnetoelectric multilayered heterostructure based on AlN and FSMA thin films are studied. The chapter also provides essential information to understand the magnetoelectric coupling mechanism and its application in MEMS based Magnetic Sensors. Chapter 2 presents the details of the experimental techniques employed for the synthesis and characterization of Magnetoelectric heterostructure and its layers. All thin film samples were fabricated using reactive DC magnetron sputtering technique. Various characterization techniques, such as X-rays diffraction (XRD), X-ray photoelectronic spectra (XPS), atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM), vibrating sample microscopy (VSM), and double beam laser interferometry have been discussed in details. Electrical properties of the samples were studied using Keithley 4200 semiconductor characterization system (SCS), 4294 Impedance analyzer and Vector Network Analyzer (VNA). Also, a magnetic setup attached to VNA was used to measure the frequency response of Magnetoelectric coupling based bulk acoustic wave (BAW) resonator. Chapter 3 describes the growth assessment and scrutinize dielectric reliability of c-axis oriented insulating AlN thin films in MIM structures for microelectronics applications. The effect of bottom electrodes (Al, Pt & Ti) on the texture, piezoelectric characteristics, dielectric properties and leakage current of AlN thin films have been systematically studied. X-ray diffraction results revealed that the lattice structure and texture of the bottom electrode play a vital role in c-axis wurtzite phase growth of AlN thin films. A highly (002) oriented c-axis AlN thin film was obtained over Ti as compared to Al and Pt electrodes. The piezoelectric coefficient (d33eff) of these fabricated AlN thin films were measured by piezoresponse force microscopy to evaluate their piezoelectric efficiency in resonators. AlN is a good option for passivation purpose in microelectronic devices because of high chemical stability and excellent dielectric properties. Therefore, the dielectric reliability of AlN thin films is very crucial. Among all structures, AlN/Ti iv structure exhibits the highest dielectric constant (εr)∼8.63 at 1 MHz. This work also provided a comprehensive study of leakage current conduction mechanism with an electric field. The leakage behavior depicts that all AlN film endures a transition from ohmic conduction (E < 70 kV/cm) to Space charge limited current conduction (E > 70 kV/cm). Besides, the breakdown characteristics of these fabricated films were also studied for deciding the long-term device reliability and life. This study suggests that the electrode engineering of AlN thin films has potential in electrical devices applications. Chapter 4 is divided into two sections. Section 4.1 explores the presence of strong magnetoelectric (ME) coupling in a sputtered deposited NiMnIn/AlN heterostructure on a Si substrate. The X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy results confirm the formation of a pure AlN wurtzite phase in the ME heterostructure. The magnetization vs temperature measurement shows the presence of the martensite transformation region of the NiMnIn/AlN heterostructure. The magnetic measurements exhibit the room temperature ferromagnetic nature of the NiMnIn/AlN heterostructure. The NiMnIn/AlN ME heterostructure was found to have a high ME coupling coefficient of 99.2 V/cm Oe at Hdc ~300 Oe. The induced ME coupling coefficient shows a linear dependency on Hac up to 8 Oe. In Section 4.2, a lead-free ME composite of NiMnIn (180 nm) / AlN (180 nm) / NiMnIn (50 nm) / AlN (180 nm) / NiMnIn (180 nm) structure has been fabricated using magnetron sputtering. The AlN structure with encapsulated NiMnIn layer exhibits higher dielectric permittivity, piezoelectric response and magnetodielectric response as compared to pure AlN (~360 nm) structure. The dielectric constant of the encapsulated structure was found to be about 300 % (22.3) more as compared to that of pure AlN (~ 7.24) structure. The piezoelectric coefficient of the multilayered structure was found to be about 7.49. A high value (~1268 ppm) of the magnetostriction coefficient was obtained for ferromagnetic NiMnIn layer using cantilever deflection method. The magnetic field sensing characteristics of the fabricated encapsulated structure is studied in term of magneto-dielectric effect as large as 8.5%. Such proposed multilayered structures with enhanced dielectric constant and excellent magnetodielectric characteristics are useful for futuristic magnetic field sensing and MEMS based devices applications. Chapter 5 shows the realization of a magnetic field tunable piezo-resonator. A comprehensive analysis of the influence of the magnetic field on BAW resonator consisting of a highly magnetostrictive layer and AlN thin film is systematically studied. The fundamental resonant frequency of the v fabricated BAW resonator is about ~4.22 GHz. In the presence of a magnetic field, we studied the effect on the resonator parameters such as resonant frequency, acoustic velocity, and coupling coefficient. For the magnetic field of strength 1200 Oe, the resonant frequency significantly shifts by ~360 MHz. Resonant frequency increases and electromechanical coupling coefficient ( kt) decreases with the increase in the DC magnetic field. The maximum acoustic velocity of ~7350 m/sec was observed at the magnetic field of 1500 Oe when applied parallel to the surface. Agilent Advanced Design Software (ADS) was used to extract the equivalent Modified Butterworth-Van Dyke circuit parameters (Rm, Cm, and Lm) of the resonator. Further, in the presence of the magnetic field, we obtained the variation in values of Rm, Cm, and Lm of the resonator structure. Such tunable resonators can be useful and vital in dealing with varying frequency bands for sustainable growth in wireless communication and magnetic field sensor applications. Chapter 6 describes major conclusions drawn after thorough discussion and in-depth analysis presented in individual chapters. A brief report on the scope for future work is also included.