Structural and electrical properties of tantalum nitride thin films fabricated by using reactive radio-frequency magnetron sputtering

TaN thin film is an attractive interlayer as well as a diffusion barrier layer in [FeN/TaN]n multilayers for the application as potential write-head materials in high-density magnetic recording. We synthesized two series of TaN films on glass and Si substrates by using reactive radio-frequency sputtering under 5-mtorr Ar/N2 processing pressure with varied N2 partial pressure, and carried out systematic characterization analyses of the films. We observed clear changes of phases in the films from metallic bcc Ta to a mixture of bcc Ta(N) and hexagonal Ta2N, then sequentially to fcc TaN and a mixture of TaN with N-rich phases when the N2 partial pressure increased from 0.0% to 30%. The changes were associated with changes in the grain shapes as well as in the preferred crystalline orientation of the films from bcc Ta [100] to [110], then to random and finally to fcc TaN [111], correspondingly. They were also associated with a change in film resistivity from metallic to semiconductor-like behavior in the range of 77–295 K. The films showed a typical polycrystalline textured structure with small, crystallized domains and irregular grain shapes. Clear preferred (111) stacks parallel to the substrate surface with embedded amorphous regions were observed in the film. TaN film with [111]-preferred orientation and a resistivity of 6.0 mΩ cm was obtained at 25% N2 partial pressure, which may be suitable for the interlayer in [FeN/TaN]n multilayers. PACS: 81.05.Je; 81.15.Cd; 68.55.-a; 68.35.Rh Recently, exciting breakthroughs in processing magnetic thin-film media for ultrahigh recording density on a rigid disk over 25–35 Gbit/in2 have been made by Seagate and IBM [1, 2]. This area density is by far beyond the ‘theoretical limit’, i.e. 10 Gbit/in2, which was widely accepted a few years ago. With this achievement, to obtain a recording density of 40–100 Gbit/in2 using magnetic media is not just a dream in the near future. Significant progress has also been made in the new generation of read heads by using various ∗Corresponding author. (Fax: +65-777/6126, E-mail: [email protected]) ∗∗Permanent address: Research Institute of Magnetic Materials, Lanzhou University, Lanzhou 730000, P.R. China types of GMR (giant magnetoresistive) spin-valve multilayers [3–6] that fulfill the challenging requirements of reading processes at high speed/frequency on high recording density media. However, the development of write heads has not yet matched the fast development of media and read heads, and thus more intensive studies are expected in this area. In order to obtain satisfactory overwrite performance of the recording process, the write-head materials should exhibit superior properties such as very high saturation flux density Bs of around 20 kG, high-frequency permeability μ′ of order 103 at 102 to 103 MHz, very low coercivity Hc (say, < 1 Oe), thermal stability up to 400–500 ◦C and very small magnetostriction. One sort of potential candidate for the newgeneration write-head materials are the iron-based thin films such as FeN, FeTaN and FeAlN [7–12]. To reduce the thermal noise induced by eddy currents and to obtain excellent high-frequency performance up to 102 to 103 Hz, various [Fe(Ta,Al)N/M]n multilayers have been synthesized and investigated, where M represents the interlayer, e.g. TaN, Ta, SiO2, Al2O3NiFe, CoZrRe, SiN and AlN [12–22]. Among them, with TaN as the interlayer, [FeTaN/TaN]n multilayers were demonstrated to have excellent magnetic properties at high frequency up to 100 MHz and high temperature stability up to 430 ◦C [12]. TaN is a chemically inert refractory compound. Here we use TaN to represent various phases of the TaN system, such as β-Ta2N, θ-TaN, η-TaN, δ-TaN1−x , Ta5N6, Ta4N5, Ta3N5, etc. [23, 24]. Before TaN thin film was used as an interlayer in soft magnetic multilayers, it had already been applied as a stable thin-film resistor of low temperature coefficient of resistivity [25–27], an excellent diffusion barrier between silicon and metal overlayers of Ni, Al and Cu [28–33], etc. For different applications, TaN thin films have been successfully fabricated by using several kinds of techniques such as reactive sputtering [27–29, 31, 34–39], metalorganic chemical vapor deposition (MOCVD) [40], ion-beam-assisted deposition (IBAD) [41, 42] and electron-beam evaporation [43]. However, due to the complex phase diagram of the TaN system, the experimental results of different research groups are not consistent satisfactorily with each other, and the growth mechanism of TaN films has not been fully understood yet. We believe that a better understanding of the growth mech-