Growth and patterning of amorphous FeSiBC films

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Journal of Magnetism and Magnetic Materials 162 (1996) 147-154

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Growth and patterning of amorphous FeSiBC films C. Shearwood a,*, A.D. Mattingley b, M.R.J. Gibbs b " MEMS Unit. Department of Electronic and Electrical Engineering, Mappin Street. Unil.ersit3' of Sheffield, Sheffield, S! 3JD. UK b Sheffield Centre for Advanced Magnetic Materials and Del'ices, Department of Physics. Unicersity of Sheffield. Sheffield. $3 7RH. UK Received 5 October 1995; revised 11 March 1996

Abstract We report the growth of amorphous FeSiBC films on a range of substrates including oriented semiconductor, polycrystalline metal, and polyimide. The films vary in thickness from 2.6 txm down to 20 nm and were characterised by the Magneto-Optic Kerr Effect (MOKE), and for the thicker films a combination of MOKE and a high resolution inductive magnetometer. These measurements imply a skin-depth of 30 nm, at the frequency of the MOKE laser. A representative amorphous FeSiBC film of thickness 0.3 txm was patterned into wires of nominal width 10 Ixm, separation 5 Ixm, and length 10 mm by conventional microelectronic fabrication techniques. The switching of the wire-array was investigated by MOKE. Keywords: Amorphous systems - films; FeSiBC; Magnetoelastic; Microfabrication; MOKE; Sputtering

1. Introduction Amorphous ferromagnetic films may offer advantages in a wide range of actuator and sensor applications [1-3], particularly in view of research directed at improving their piezomagnetic characteristics [4]. In this study, our aim is to demonstrate the sputtergrowth of amorphous films on a representative number of substrates, with a view to developing integrated sensors and fabricating mesoscopic magnetic model structures [5-10]. Growth on substrates including oriented semiconductor (GaAs(001) and Si(001)), polycrystalline metal (AI), SixN v, and polyimide (Kapton ®) have been investigated, i.e. substrates currently used in microelectronic fabrica-

Corresponding author.

tion and those used as membrane material for actuator elements. A film composition based on METGLAS ® 2605SC (Fe81Si3.sBI3.sC 2) was chosen since, in the ribbon form, it has a large magnetomechanical coupling constant, lack of Barkhausen noise and good low frequency characteristics which make it ideal for device applications [11]. The high saturation magnetostriction of the ribbon (A~ = 41 ppm [12]) may be mirrored in the FeSiBC films when stoichiometry has been achieved [4]. For thinner films grown on single crystal substrates, induced magneto-crystalline anisotropy effects can be significant. However, these effects are expected to be small for amorphous films. Other factors, contributing to modifications of the magnetic reversal process, include diffusion at the interface [13] and surface degradation [14]. We investigate the influence of the above effects on the magnetic prop-

0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 8 8 5 3 ( 9 6 ) 0 0 2 5 7 - 0

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erties of variable thickness amorphous FeSiBC films, grown on GaAs(001) substrates, The control of shape anisotropy can be achieved using modern processing techniques, producing well defined uniaxial anisotropy [5-10]. Shape induced anisotropies have previously been observed in patterned polycrystalline Co wires [5] and epitaxial Fe wires [6,7]. In this paper we use standard microelectronic fabrication techniques to pattern an array of amorphous wires. The wire dimensions are chosen such that the fabrication is accessible by optical lithography. Amorphous films are appropriate, as typical magnetic domain sizes of 10 txm have been reported [15], enabling the possibility of single domain structures. A combination of a high resolution inductive and a Magneto-Optic Kerr Effect (MOKE) magnetometer is used to characterise the magnetic properties of the films and wires. The volume of magnetic material interrogated by MOKE is restricted to the skin-depth and therefore MOKE measurements enable surface magnetisation to be isolated and compared to the bulk magnetisation of the film, as measured by an inductive magnetometer. We demonstrate the applicability of this procedure by initially comparing the magnetic behaviour of METGLAS ® 2605SC ribbons measured by these different techniques.

A 0.4 p,m FeSiBC film was deposited on all the substrates simultaneously. Reproducibility, within the same run and run-to-run, was verified by the grown of eight 90 nm films on GaAs(001) substrates. Variations in the measured coercivity and magnetisation were found to be typically less than 10%. A series of FeSiBC films, ranging in thickness from 0.22 Izm down to 20 nm, were grown on GaAs(001) substrates. This substrate was chosen due to its technological importance and to investigate substrate/film interdiffusion [ 14]. All the substrates were scribed to sizes of 5 mm X 5 mm, with the exception of the Si(001): 10 mm x 10 ram. No film delamination was observed with these films, contrary to earlier research [1]. This is extremely important for successful device fabrication.

3. Fabrication An FeSiBC film (thickness t ~ 0.3 Ixm) was patterned into wires by a combination of photolithography, metallisation, and lift-off. The wires were patterned such that their long-axis was made parallel to the easy-axis of the unpatterned film. Wires of width, w ~ 10 Ixm, separation, s ~ 5 I~m, and length, l ~ 10 mm were chosen. The dimensions were chosen such

2. Film growth The films were grown by radio frequency sputtering using a 4 inch diameter METGLAS ® 2605SC target cut from ribbon. All substrates were initially cleaned in-situ by sputter-bias etching followed by the immediate deposition of the film. The targetsubstrate separation was set to 6 cm. The target was sputtered at a power density of 4 W cm -2 and an argon pressure of 20 mTorr. In a previous investigation this argon pressure was found to correspond to minimum coercivity [4]. The films were grown at a deposition rate of 3.3 n m / m i n with the thickness confirmed ex-situ using a TALYSTEP ® profilometer. A uniaxial anisotropy developed in the films during growth. The origin of this anisotropy is believed to be related to the columnar growth of the FeSiBC film. A range of substrates was chosen; GaAs(001), Coming glass (7059), Si(001)/SixNy, and Si(001).

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Fig. 1. A t o m i c force m i c r o s c o p e image o f a m o r p h o u s FeSiBC wires with nominal separation s ~ 5 p.m, wire-width w ~ 10 ixm, and thickness t ~ 0.3 tzm, fabricated on a C o m i n g - g l a s s substrate. The scan-size is 60 p ~ m × 6 0 p~m.

C. Shearwood et a l . / Journal of Magnetism and Magnetic Materials 162 (1996) 147-154

that l >> w >> t, to restrict the magnetisation vector to lie in-plane and parallel to the long-axis of the wires. In this instance, the substrate was Coming glass • After patterning the 1 Ixm layer of positive Photoresist by conventional photolithography, the substrate was loaded into the sputter growth chamber prior to film growth. The Photoresist mask and film combination were pre-sputtered for 30 s to promote adhesion of the film to the underlying Coming-glass. Excessive pre-sputter times were found to deteriorate the Photoresist mask. The pattern transfer was achieved by lift-off in a warm acetone solution. Fig. 1 shows an atomic force microscope image of the amorphous FeSiBC wire-array. The scan-size and scan-rate were set to 60 txm × 60 p~m and 0.78 Hz respectively. Generally the wires are seen to be of a regular geometry although there are some edge features. These features are generated during lift-off as a consequence of the conformal coverage of FeSiBC over the Photoresist mask during film growth.

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4. Structural characterisation A M E T G L A S ® 2605SC sample (cut from the target ribbon) and the FeSiBC films were examined by a 0 - 2 0 X-ray diffractometer, with C u K ~ radiation. The full width at half maximum (FWHM) and centroids ( 2 0 ) of the peaks were determined by fitting with two half Lorentzians. Fig. 2A presents an X-ray diffraction (XRD) trace of the amorphous ribbon (target material) together with a representative trace of FeSiBC films deposited onto Si(001) and polycrystalline Al substrates (Fig. 2B,C). The intensity scales on the XRD traces are arbitrary. Comparisons of the XRD traces of the film/substrate combination with the bare substrate accounts for all the observed crystalline peaks. Table l presents a summary of the centroid positions (2 0) and the F W H M of the FeSiBC films along with a M E T G L A S ® 2605SC ribbon. Values for the film grown on Coming glass are not included due to the large overlap between the amorphous peak of the glass and FeSiBC film. There was no evidence of crystallisation in any of the FeSiBC films. Similarly, XRD measurements of the wire-array showed no signs of crystallisation, despite the constraint of be-

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ing grown as wires defined by a Photoresist mask. High resolution transmission electron microscopy on thinner films ( ~ 40 nm) confirmed the FeSiBC as amorphous [15]. The films were also examined by wavelength dispersive electron probe microanalysis using a JEOL-JXA 8600 microscope. The composition of the M E T G L A S ® 2605SC target ribbon was measured as Fe81.9Si3ABI0.6C3.700.7, which compares well with the expected values of Fe 81Si 3.5B 13.5C 2. By contrast, the sputtered FeSiBC films showed departures from stoichiometry. For example, the composition o f the F e S i B C film on G a A s ( 0 0 1 ) was Fe76 S i 3.0 B 9 . 3 C 5 . 1 0 6 . 6 • In other instances, the amount of oxygen remained constant at about 10 at% while the Fe level varied between 67 and 76 at%. The

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C. Shearwoodet al. / Journal of Magnetism and Magnetic Materials 162 (1996) 147-154

Table 1 Summary of the XRD centroid position and FWHM taken on a METGLAS® 2605SC ribbon and FeSiBC films. Substrates include polycrystalline AI, Si(001), GaAs(001), Si(001)/Si~N~, and Kapton®. The coercivity (easy-axis) is given, together with the film thickness Substrate 20 FWHM Hc t (°) (°) (kA/m) (p.m) METGLAS~ 2605SC ribbon Polycrystalline AI Kapton® GaAs(001) Coming glass Si(001)/SixNy Si(001)

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diode-laser beam was left unfocused to enable an average to be made of regions in the film which might switch at different coercive fields. The laser sampling area was 1 mm × 2 ram. Complementary measurements were also taken on all the specimens using a high resolution inductive magnetometer [ 17]. A direct comparison between the two techniques is expected to give good agreement if the magnetic material is homogeneous on a scale length much greater than the optical skin-depth, 6ski,. There are no examples of MOKE measurements performed on FeSiBC films and few on ribbons [18,19].

6. Results and discussion deviations from stoichiometry, of the sputtered films compared to the parent ribbon, are due to the different sputtering rates of the various elements in the target, The origin of the oxygen is believed to be due to water vapour, present as the main partial pressure component of the chamber pressure.

5, Magnetic measurements All the magnetisation measurements were made at room temperature, with the applied field in the plane of the film. The variation of the magnetisation as a function of the applied magnetic field was measured by MOKE in the longitudinal geometry [16]. The film was positioned between the poles of an electromagnet, such that the applied magnetic field was in the plane of the film and in the plane of incidence of the laser light. An internally stabilised 3 m W diodelaser (A = 675 nm) was plane polarised and incident onto the film. On reflection, after passing through an analyser, the beam was detected using a large area photodiode. A setting of the analyser at 5 ° from extinction was found to correspond to the maximum signal to noise ratio. The incident and reflection angles were maintained at 15 ° throughout the experiment, limited by the geometry of the electromagnet. The field was swept at a frequency of 3 Hz, with field strengths of up to 1000 k A / m from a programmable power amplifier. Typically, 50 loops were taken and averaged to form each hysteresis loop. The

X-ray measurements indicate that all the films deposited were amorphous, irrespective of thickness or substrate choice (see Table 1). Note in particular the narrow range of F W H M of the sputtered films compared to the amorphous ribbon, indicating a similar state of disorder. The peak-position ( 2 0 ) of the films varied by ~ 0.5% compared to the METGLAS ribbon. This variation suggests different levels of homogeneous stress which may be important for these highly magnetostrictive films. Sample to sample stress and thickness variations primarily account for the measured coercivity variation. No film delamination was observed, even for the thicker films.All the hysteresis loops have been normalised to saturation and then displaced from the origin. Fig. 3A is an inductive plot of the ribbon with the H-field perpendicular to the ribbons original long-axis. The almost identical measurement with H-field parallel to the ribbons original long-axis is omitted for clarity. The coercivity ( H c) for perpendicular and parallel configurations are 9 A / m and 24 A / m respectively. Fig. 3B shows the hysteresis loops obtained by M O K E on the free and contact side of the same M E T G L A S ribbon. There is excellent agreement between the inductive and M O K E measurements. This suggests that the surface properties of the ribbon (both the free side and contact side) are representative of the bulk of the ribbon. The poorer signal-to-noise ratio of the MOKE measurements is due to non-specular reflection from the surface of the ribbon.

C. Shearwood et al. / Journal of Magnetism and Magnetic Materials 162 (1996) 147-154 3

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C. Shearwood et al. // Journal of Magnetism and Magnetic Materials 162 (1996) 147-154

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clude interface strain, local diffusion, and substrate induced magneto-crystalline anisotropy. Fig. 5 presents a summary of the magnetisation measurements made by both magnetometers. Below 50 nm the coercivity is seen to increase with decreasing FeSiBC thickness (Fig. 5A). Both MOKE and inductive measurements are in agreement. However, below 50 nm the apparent magnetisation determined by MOKE falls since 8~ki, is no longer shorter than the film thickness, and therefore a reduced amount of magnetic material is sampled (Fig. 5B). The skin-depth is estimated to be (~skin ~ 30 nm, a value consistent with ~skin ~ 25 nm reported for amorphous CoFeB ribbons [19]. In a recent study of the growth of Fe films on GaAs substrates, intermixing of As with Fe was observed to give rise to a magnetic dead-layer [13].

Low concentrations of Ga mixed into amorphous FeB films has been shown to lead to modifications to the magnetic properties of the film [20]. Our measurements imply that diffusion of either Ga or As into the FeSiBC films is not significant for films with a thickness of 20 nm and above. Clearly, the magnetisation loops of the ribbon and the sputtered films are very different. In this case, deviations from stoichiometry and the inclusion of oxygen into the films increase the coercivity. Also, coercivity scales with the reciprocal of thickness (see Fig. 5A) and there may be residual and inhomogeneous stress giving pinning of domain walls, all contributing towards harder films. In a parallel investigation, near stoichiometry of the sputtered films has been realised, compared to the parent ribbon. Coercivities as low as 7 A / m have now been recorded for thicker films grown on Si(001) [3]. Fig. 6 is a plot of in-plane, orthogonal magnetisation measurements made on (A) an unpatterned and (B) a patterned FeSiBC film of thickness 0.3 txm. The unpatterned film has a growth induced anisotropy field of H k ~ 2 k A / m . During fabrication of the wires their long-axes were aligned parallel to the easy-axis of the unpatterned film. Patterning the film into wires, alters the relative strength of the magnetic anisotropy through manipulation of the shape factor.

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C. Shearwood et al. / Journal of Magnetism and Magnetic Materials 162 (1996) 147-154

The patterned film is seen to have an easy-axis in the direction of the long-axis of the wires. The coercivity of the wires along their long-axis is now 1.4 k A / m compared to 1 k A / m for the unpatterned film. This minor change reflects the change in domain-pinning and domain-size associated with the specific wire geometry. However, the hard-axis now has a far greater anisotropy field of H k ~ 7 k A / m , due to the demagnetising effect associated with the small wirewidth. It is reasonable to assume that the magnetisation reversal process along the hard-axis is by coherent moment rotation. However, two mechanisms contribute towards corrupting this behaviour; uneven switching across the wire-width and long-range dipolar coupling between neighbouring wires in the array. If a single wire is imagined to be composed of narrow strips, then a central strip experiences a smaller demagnetising field compared to a strip near the edge of the wire. Consequently, the central strip undergoes rotation before the outer ones. The combined effect of all the strips is to give rise to a broader hysteresis loop. This so called twisted domain configuration has previously been observed in sub-micron Fe wires [6,7]. Similar observations were made with computer simulations of the switching fields of interacting cubic particles [21]. The switching characteristics of wires depend critically on an interplay between shape, surface roughness, intrinsic anisotropy, as well as internal stress. Fig. 1 shows irregularities along the edges of the fabricated wires. It is expected that effects due to these irregularities are not insignificant in determining the precise switching characteristics.

7. Summary We have shown that a range of variable thickness FeSiBC films can be grown on a wide variety of both crystalline and non-crystalline substrates. The films all remain amorphous and as such retain their low coercivity. Furthermore, the thicker films (2.6 ~m) show no signs of delamination. Amorphous wires have been produced by standard microfabrication techniques, enabling the magnetic reversal mechanism of structured films to be investigated. These measurements have highlighted

153

the need to carefully model the behaviour of magnetic elements integrated together on the same substrate where the effect of device geometry and separation have a far from trivial contribution to the magnetic reversal process. Combinations of a bulk and surface sensitive magnetometer can give important information about the role of surface magnetisation. Furthermore, techniques such as MOKE, have an extremely important role to play in characterising the magnetic properties of magnetic material, especially fabricated in the geometry in which they are to function.

Acknowledgements Measurements made by Mr R.M. Abraham and S. Dean are acknowledged, together with the X-ray diffraction and electron-probe micro-analysis performed respectively by Mr B. Chapman and Mr H.R. Perrott of Bath University. The financial support of the ESPRC is acknowledged.

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[12] A.P. Thomas and M.R.J. Gibbs, J. Magn. Magn. Mater. 103 (1992) 97. [13] J.A.C. Bland, R.D. Bateson, P.C. Riedi, R.G. Graham, H.J. Lauter, J. Penfold and C. Shackleton, J. Appl. Phys. 69 (1991) 4989. [14] L. Smartz. U. KiSbler, and W. Zinn, J. Appl. Phys. 71 (1992) 5199. [15] L.J. Heyderman, J.N. Chapman, M.R.J. Gibbs and C. Shearwood, J. Magn. Magn. Mater. 148 (1995) 433. [16] J.A.C. Bland, M.J. Padgett, R.J. Butcher and N. Bett, J. Phys. E 22 (1989) 308.

[17] P.T. Squire, S.M. Sheard, C.H. Carter and M.R.J. Gibbs, J. Phys. E 21 (1988) [18] J.M. Gonzalez and J.L. Vicent, J. Appl. Phys. 57 (1985) 5400. [19] J. Woods, A. Ushioda, M.M. Donovan, S.W. Sun, M. Tobise and R.C. O'Handley, J. Appl. Phys. 63 (1988) 3669. [20] Y.N. Chen, D.R. Huang and Y.D. Chuang, IEEE. Trans. Magn. 19 (1983) 1919. [21] Y. Uesaka, Y. Nakatani and N. Hayashi, J. Magn. Magn, Mater. 123 (1993) 209.