Growth, structural and electrical properties of AlN/Si (111) for futuristic MEMS applications

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Abstract

This paper presents the structural, optical, and electrical characteristics of AlN thin films on Si (111) substrate grown by DC magnetron sputtering technique for MEMS applications. The grown AlN thin films (thickness: 680, 900, and 1200 nm) exhibited polycrystalline wurtzite structure with preferred orientations along <001> directions. The mean grain size of the films varies from ~ (15–40 nm) of AlN layers. As the thickness increases, the FTIR E1 (TO) peak broadening of the films varies from 165 cm−1, 151 cm−1, and 128 cm−1, respectively. The electrical characterization of AlN/Si (111) is studied by fabricating the Metal-Insulator-Semiconductor (MIS) structure. The static dielectric constant of the AlN films varies from 6.80 to 5.94 as the films' thickness increases. The variation in dielectric constant values are due to the existence of interface trapped charges (interface trap density of states ~1010 - 1011 cm−2 eV−1) present in the AlN-silicon interface. The presented article corroborates the structural, morphological, optical, and electrical characteristics of AlN films on Si (111) with varying thickness comprehensively for piezoelectric MEMS applications.

Introduction

Aluminum nitride (AlN) thin films are being extensively used for various engineering, biological and medical applications [1,2]. In recent times, AlN is closely studied to develop different micro-electro-mechanical system (MEMS) devices, like accelerometer, micromirror, resonator, gyroscope, energy-harvester, and filters, etc. [[3], [4], [5], [6], [7], [8], [9]], due to its superior piezoelectric (non-centro symmetry along the z-axis) properties coupled with excellent CMOS process compatibility; and environment-friendly nature [10,11]. Additionally/Furthermore, AlN possesses a stable wurtzite phase, the wide bandgap of ~6.2 eV, highly chemically, thermally, and mechanically stable, high breakdown dielectric strength, low acoustic loss, high acoustic velocity (~8000 m/sec), and high electrical resistivity (ρ = 109- 1011 Ω-m) [[10], [11], [12]], can be utilized to fabricate various sensors and actuators applications [[3], [4], [5], [6],9,10]. High acoustic velocity, along with low acoustic loss in the AlN material, is preferred to make AlN based surface and bulk acoustic (SAW and BAW) sensors [13,14]. Wide bandgap property of AlN is used to fabricate deep ultraviolet detectors [15]. AlN based nanotubes are also used for various chemical sensing applications (~formaldehyde etc) [16]. Apart from this, AlN's dielectric property can be utilized as a replacement material for gate oxide in high power electronic devices fabricated on Si and SiC substrates. Due to less thermal mismatch, AlN is being investigated as a viable alternate for the silicon dioxide buried layer in the development of silicon-on-insulator (SOI) substrates [7,8].

AlN thin films have been grown on different substrates like glass, quartz, sapphire [17], silicon [18], silicon carbide [19]. Among these, silicon is extensively used because of its relatively low cost and easy availability of high-quality large area (~up to 350 mm diameters) wafers. The AlN films grown on silicon substrates can acquire both zinc-blend as well as wurtzite crystalline structure; However, for piezoelectric transducer applications, c-axis oriented wurtzite AlN phase is preferred due to its stable phase at elevated temperature and enhanced piezoelectric property. The hexagonal in-plane atomic arrangement of Si (111) substrate surface yields preferred growth of AlN <001> oriented film compared to the other Si substrate orientations. Different deposition techniques are being employed to grow AlN films such as pulsed laser deposition (PLD), sputtering, atomic layer deposition (ALD), molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) etc. [[17], [18], [19], [20], [21]]. Among them, sputtering is widely used due to its ease in operation, cost-effective, and fast growth. Oriented AlN thin films on silicon substrates are widely used in various devices [[9], [10], [11],[13], [14], [15], [16]].

In the past, several studies have been presented related to the structural, optical, and electrical characteristics of AlN thin films on silicon (100) substrate as well as other substrates for various applications [[22], [23], [24], [25], [26], [27], [28]]. In 1996, Kusaka et al. has studied the crystalline orientation and residual stress of AlN films grown on the borosilicate glass substrate by varying nitrogen gas pressure in a planar magnetron sputtering system [29]. In 2003, Cheng et al. have explained the microstructural change in AlN/Si (100) by varying the nitrogen concentration using RF sputtering [30]. In 2005, Auger et al. discussed the growth dynamics (random or textured orientation) of AlN thin films on Si (100) wafer [31] and explained two regions of growth mechanism in AlN thin films. Sah et al. presented the evolution of residual stress in AlN layers on Si and GaAs substrates with the deposition temperatures [32]. Iriarte [33] in 2010 and Vasanthi Pillay [34] in 2013 have discussed the growth of AlN layers for various microelectronics applications. Most researchers have used Si (100) for AlN thin film growth and studied the film's structural and electrical properties.

Few reports are available on AlN thin films on Si (111) substrate by sputtering technique [12,18,35,36]. Xu et al. [35] reported the impact of the surface to target distance and sputtering power for the growth of preferred oriented AlN films growth on Si (111) by sputtering. Zhang et al. [36] presented the dependence of crystallographic orientation as well as strain developed in AlN films grown on silicon (111) and (100) substrates by RF sputtering. Abdallah et al. showed the effect of trap density at the AlN and Si interface for electronic devices [37]. Few researchers have presented the electrical properties of AlN thin films by fabricating the MIM and MIS structure and studied the effect of annealing in dielectric properties as well as current leakage behaviour [[38], [39], [40]]. For futuristic MEMS devices development, the properties of the c-axis oriented AlN films need to be optimized. Moreover, a detailed understanding is required to corroborate the AlN films' optical and electrical properties with its microstructure, which is essential for improved device performance. Therefore, it is planned to study the growth and characterization of AlN on Si (111) with varying thickness and investigate its structural, microstructural, and electrical properties comprehensively.

In this report, we are presenting the growth of <001>preferred oriented AlN films on Si (111) wafer by using the reactive DC magnetron sputtering method for futuristic piezoelectric MEMS devices development. The structural, morphological, and optical properties of the films are studied. The electrical properties of the films are investigated by fabricating MIS {metal-insulator- semiconductor (Au–AlN–Si)} structure. The optical and electrical properties of the films are co-related with the structural and morphological characteristics of the films.

Section snippets

Experimental

Aluminium nitride thin films are grown on 2-inch diameter silicon (111) substrates by DC magnetron reactive ion sputtering system (Excel Instruments, India) with 99.99% pure Aluminium (Al) target in high-purity Ar and N2 mixed gases. Before loading into the deposition chamber, Si wafers were thoroughly cleaned by a standard RCA cleaning procedure. Details of RCA cleaning and deposition parameters were explained in our previous report [41,42]. After loading the substrate, first stabilized base

Results and analysis

The thicknesses of the AlN layers are found to be 680 nm, 900 nm, and 1200 nm for samples A1, A2, and A3 respectively. The thicknesses of the AlN films are determined by UV–visible reflectance fringe width and it will be discussed in the upcoming section.

Conclusions

This paper discussed growth, structural and electrical characteristics of preferred c oriented AlN layers on Si (111) substrates with the varying film thickness of 680 nm, 900 nm, and 1200 nm, respectively by reactive DC magnetron sputtering technique for MEMS applications. The average grain size of the films is found to vary in the 15–40 nm range. The dielectric constant values of the AlN films at low frequency vary from 6.7 to 5.94. The electrical parameters vary as a function of film

CRediT authorship contribution statement

Akhilesh Pandey: Conceptualization, Visualization, Writing - original draft, preparation. Shankar Dutta: Formal Analysis, Writing – Review & Editing, Visualization. Janesh Kaushik: Validation, Visualization, Reviewing and Editing. Nidhi Gupta: Visualization. Garima Gupta: Visualization. R. Raman: Reviewing and Editing, Supervision. Davinder Kaur: Supervision, Reviewing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors would like to thank Dr. Seema Vinayak, Director, Solid State Physics Laboratory (DRDO) for her guidance and provide us the permission to publish this work. We would like to acknowledge Mr. Sandeep Dalal, Ms. Monika Kumari, and Mr. Upendra Kumar for XRD, FTIR, and Electrical experiments respectively. Help from other colleagues is also acknowledged.

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      Citation Excerpt :

      In the present case of the FTIR spectra, E1(TO) peak and A1(LO) dip are obtained in all the sample’s Reflectance spectra as shown in Fig. 6 (a). The reason for obtaining peak and dip is due to crystal anisotropy and low thickness of AlN thin films (∼100 nm) [42–45]. As the V/III ratio increases in the grown AlN epi-layers, the E1(TO) peak position and its peak width (FWHM) varies.

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