Tailoring of multilayer interfaces by pulsed laser irradiation

https://doi.org/10.1016/j.apsusc.2005.03.047Get rights and content

Abstract

Multilayers (MLs) consisting of a few nm thick alternating layers of two different materials are broadly used in soft X-ray optics and in giant magnetoresistance (GMR) sensors. The efficiency of ML-based devices depends on the quality and thermal stability of the interfaces, which must be sharp at the nm scale. It is shown that, using heating with excimer laser pulses of 30 ns and fluence of approximately 0.1 J cm−2, the diffusion length for one laser pulse in the above mentioned MLs is in the region of nanometers, i.e. it closely matches the thickness of the ML sublayers. Therefore, pulsed laser induced diffusion can be used for controlled manipulation and tailoring of ML interface properties. Depending on the miscibility or immiscibility of the ML material combinations, the interfaces could be intermixed or even sharpened, which is attributed to the backdiffusion process. These phenomena are demonstrated for various combinations of ML building layers, like W/Si, Co/Ag, Fe/W and Co/W. The experimental samples were analyzed by X-ray reflectivity and X-ray diffuse scattering, combined with TEM.

Introduction

Multilayers (MLs) consisting of alternating layers of two different metals or metal combined with semimetal are used mostly in soft X-ray optics [1], [2] and in giant magnetoresistance (GMR) sensors [3], [4]. The combinations of elements or compounds with high and low atomic number, e.g. Mo/Si, W/Si, and of ferromagnetic and non-ferromagnetic elements, e.g. Co/Ag, Fe/W, are examples of the first and second application, respectively. In this paper, we pay attention to laser irradiation induced diffusion in these MLs. Typical ML periods (bilayer thickness) are d = 2–20 nm and the number of periods in the stack may be up to 400 for soft X-ray mirror and a few tens for GMR sensor.

Because of small diffusion distances and high number of interfaces these structures are of interest in diffusion studies. However, the structure of MLs is complex and all diffusion driving forces and types of diffusion may be present. The most important are isothermal diffusion, thermomigration, stress-induced diffusion and electromigration [5], if the structures are electrically charged. The flux of particles can be expressed by:J=−D∇N+FNDkT,where D is the diffusion coefficient, N the concentration of particles, F the driving force and k and T have the usual meaning.

Diffusion studies in MLs have shown that the kinetics of diffusion is much faster than expected from high temperature data obtained in bulk systems. Moreover, depending on the miscibility or immiscibility of the ML material combinations [6], the interfaces could be intermixed in a controlled way or even sharpened. Sharpening is attributed to the backdiffusion (reverse diffusion, chemical cleaning) process showing negative interdiffusion coefficient, especially in immiscible combination of materials [7]. However, sharpening can be observed also in systems with complete mutual miscibility (solubility), if the diffusion coefficient is not only temperature dependent, but also concentration dependent [(D(T, c)]. In this case, the interface was reported to shift proportionally with time [8].

Section snippets

Miscibility and immiscibility of ML material combination

For fast estimation of mutual miscibility or immiscibility of pairs of materials, the heat of formation of compounds ΔH calculated by Miedema [6] can be used. For an immiscible pair, ΔH is positive, for a miscible one it is negative, for ΔH ≈ 0 low miscibility is expected. Some bimetal heat of formation are given in Table 1. In the field of GMR MLs, many immiscible combinations of ferromagnetic layers and non-magnetic spacers were used. For X-ray MLs, combinations of refractory reflectors, like

Laser heating in the diffusion studies

In the diffusion studies, materials are conventionally heated using standard furnace or rapid thermal annealing (RTA) isothermal methods. Here, the entire sample is heated. Using laser beam, the heating is restricted to specific areas of the sample. Therefore, laser heating is less inertial (Fig. 1) [9]. Both CW and pulsed lasers are used. CW diode pumped doubled YAG was used for diffusion studies of semiconductor heterostructures [10]. An additional advantage of pulsed laser is the

Diffusion length on the nanometer scale

As an example, the diffusion coefficient of Ag into Co is:D[cm2s−1]=2.5×10−6exp⁡−0.38eVkTfor T = 1000–1235 K (i.e. up to Tmelt of Ag). It was obtained by XeCl laser processing [9]. Using eq. (2), the diffusion distance l1 = (2Dτ)0.5 was calculated for one laser pulse at fluences of F = 0.15 and 0.20 J cm−2 and the pulse duration τ = 30 ns. The integration was performed for the time interval t = 0–100 ns (Fig. 1). The diffusion length results:l1=1.8nm,forF=0.15Jcm−2,andl1=3nm,forF=0.20Jcm−2.

Composition measurements

For the diffusion studies in MLs with nm periods, information on the composition with spatial resolution on nm scale is required. Therefore, if AES, SIMS, EDX and TEM are employed, nanobeam design is preferred [13]. In our works, grazing incidence (GI) XRD, X-ray reflectivity (XRR), GI diffusion scattering (GIDS) and RBS methods are employed. The depth sensitivity of GI XRD depends on the absorption of X-rays in the surface layers of MLs. For example, the absorption of Cu Kα in 10 nm thick Co

Intermixing studies in layered structures

As examples of laser irradiation induced diffusion in layered structures, the results for immiscible system Co/Ag, miscible system W/Si and partly miscible combinations Fe/W and Co/W are reported. Heat processing was performed using both XeCl and KrF lasers. The duration and repetition rate of the pulses were 30 nm and 5 Hz, respectively. All samples were deposited by e-beam evaporation in UHV onto oxidized Si wafers at temperatures up to 70 °C. Laser fluences were F = 0.05–0.60 J cm−2, and number of

Conclusion

Laser is a useful tool in the study of intermixing in layered structures in the nm thickness range. The localized heat processing and delivery of heat in certain quanta provide broad variability of experimental conditions. Fast laser melting does not deteriorate the layered structure as a rule. However, attention should be paid to further improvement of various computation procedures to extract the intermixing characteristics from the experimental profiles, because it is practically impossible

Acknowledgements

The support by Scientific Grant Agency VEGA, Bratislava under the Contract 2/3149/22 and by Slovak state order contract “Submicrometer Thin Film Technologies” No. 2003 SO 51/03R 0600/03R 0601 is acknowledged.

References (24)

  • P. Grünberg

    Acta Mater.

    (2000)
  • E. D’Anna et al.

    Thin Solid Films

    (1999)
  • E. D’Anna et al.

    Appl. Surf. Sci.

    (1996)
  • S. Luby et al.

    Mater. Sci. Eng. C

    (2002)
  • S. Luby et al.

    Thin Solid Films

    (1998)
  • M. Spasova et al.

    J. Magn. Magn. Mater.

    (1999)
  • E. Majkova et al.

    Thin Solid Films

    (1999)
  • E. Majkova et al.

    Mater. Sci. Eng. C

    (2002)
  • E. Majkova et al.

    Appl. Surf. Sci.

    (2003)
  • Y. Chushkin et al.

    Appl. Surf. Sci.

    (2005)
  • A.G. Michette

    Optical Systems for Soft X-rays

    (1986)
  • E. Spiller

    Soft X-ray Optics

    (1994)

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