*Corresponding author: Tel.: #44-161-295-5713; fax: #44-161-295-5903.

E-mail address: n.telling@physics.salford.ac.uk (N.D. Telling)

Journal of Magnetism and Magnetic Materials 198}199 (1999) 731}733

Multilayer growth by low energy ion beam deposition

D.E. Joyce, N.D. Telling*, J.A. Van den Berg, D.G. Lord, P.J. Grundy

Joule Laboratory, Department of Physics, University of Salford, Salford M5 4WT, UK

Abstract

We report our progress in the development of a relatively new deposition technique, that of direct low energy ion beamdeposition. We describe this technique, which is based on ion implanter technology similar to that used in thesemiconductor industry, where slow deposition rates, of order 0.005 nm/s, potentially allow for better control of the "lmproperties as a function of depth. We present preliminary data from "lms grown on this system with deposition energiesranging from several tens of eV to several hundred eV with particular reference to the e!ect of deposition energy onmorphology of Co based "lms and multilayers. We also explore possibilities for this technique which include ultra thin"lm growth and isotopic deposition. ( 1999 Elsevier Science B.V. All rights reserved.

Keywords: Multilayer growth; Low energy ion beam deposition

1. Introduction and instrumentation

In recent years there has been a huge expansion ofinterest in two dimensional nanostructures, particularlywith reference to micro chip fabrication and, in mag-netics, recording media and spin electronics. As a resultof this the demands on thin "lm engineering have becomeever greater. There is always a desire for improved con-trol over the "lm growth processes a!ecting the "lmmicrostructure, surface, and interfacial characteristics.The deposition system used in this work is of a typesimilar to that used in the semiconductor industry [1].The principal components are outlined in Fig. 1.

In essence, a solid, liquid or gas precursor is ionised ina plasma. The ions are then extracted and accelerated to15 kV, mass selected, decelerated to 0 V and deposited ona substrate. There are two ion beam sources (A and B)each of which is a Freeman type discharge chamber withthe electric and magnetic "eld conditions as well as theelectron current being manually adjustable. The species

to be ionised may be introduced into the system froma liquid or from a gas via three #ow controlled gas linesor, alternatively, a solid precursor may be sublimated ineither of the two ovens. Typical source materials used inthis work include gaseous Ar and CO, liquid CCl

4, and

the metallic salt CoCl2. In this instrument a combination

of a powerful electromagnet and a set of slits may be usedto provide a beam with a speci"ed and well de"ned ionmass. Further re"nement of the beam is provided by theneutral trap which ensures that it is only ionised particlesof a well de"ned mass which impinge on the substrate.The beam may be scanned across the substrate by ap-plying an oscillating voltage to the neutral trap magnet.The target chamber itself is under an ultra high vacuumof order 10~10 mbar and the substrate holder allows fortemperaure control of the substrate between 77 and1500 K. Since the substrate holder, transport tube andthe source chambers are all electrically isolated, and thesubstrate is grounded, the incident ion energy is set bythe electrical potential energy at which the sources areheld. Substrates are loaded into the sample chamber Fig.1(1) via a load lock chamber Fig. 1(2) and a samplepreparation chamber Fig. 1(3). In situ analysis techniquesavailable include scanning tunneling microscopy (STM)(Fig. 1(4)) and X-ray photoelectron spectroscopy (XPS)(Fig. 1(5)).

0304-8853/99/$ } see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 0 2 1 - X

Fig. 1. Dual source, dual magnet ion beam deposition systemschematic.

Fig. 2. X-ray re#ectivity of cobalt "lms on silicon substrates(o!set for clarity).

Fig. 3. AFM (a) and MFM (b) images of a 40 nm Co "lm onsilicon. The AFM scan size is (0.5 lm)2 and the MFM scan sizeis (50 lm)2.

Since the mass resolving slits have a resolution suchthat individual isotopes may be selected, a large numberof possibilities for research are opened up. For instance,in MoK ssbauer spectroscopy, the MoK ssbauer hyper"ne"eld is closely related to the spontaneous magnetisationof the whole specimen. Thus, depositing isotopically pureFe57 layers (the isotope most studied) will greatly en-hance the sensitivity of the technique in ultra thin "lmapplications. Nuclear magnetic resonance measures thesame hyper"ne "eld as that in the MoK ssbauer experi-ment. By tuning the measurements to detect particularisotopes one can view the local environment at wellde"ned sites. Neutron scattering is a powerful techniquefor the examination of layered structures and isotopicsubstitution. It is well known as a method by whichcontrast is enhanced. For example, hydrogen atoms onspeci"c sites in a polymer or a surfactant are replaced bydeuterium or even tritium atoms to obtain a more de-tailed picture of the structure. In each of these techniquesisotopic monolayers may be deposited such that inter-facial or bulk interactions may be individually probed.

The availability of STM and XPS in situ characterisa-tion techniques allows for detailed study of the depend-

ence of growth and surface structures on the depositionconditions. The control over the near surface energyduring deposition is such that MBE like epitaxial growthmay be achieved at low temperatures. This could im-prove the quality of micro-fabrication techniques con-siderably and allow for greater integration in microchips.Also, phase transitions in ultra thin "lm alloys and atmultilayer interfaces could be followed as a function ofannealing by XPS.

2. Results

The equipment has been tested initially by growingthin cobalt "lms. Fig. 2 shows X-ray re#ectivity dataclearly showing the presence of thin "lms on the siliconsubstrates. X-ray di!raction experiments have been car-ried out and con"rm that they are Co "lms with a ran-domly oriented polycrystalline texture. However, sincethe "lms are so thin, the di!racted intensity is quite lowand not su$ciently above the noise level to be reliable.Over the deposition energy range investigated (130 to40 V) neither X-ray di!raction nor X-ray re#ectivitymeasurements showed any signi"cant change in themicrostructure. However, oxidation of the cobalt surfacewould tend to disguise any characteristic surface features.TEM investigations currently underway should elucidatethe microstructure. Atomic force microscopy (Fig. 3a) ofthe Co "lms shows grain sizes of the order of the "lmthickness and magnetic force microscopy images (Fig. 3b)are typical of thin Co "lms with an easy axis predomi-nantly in the plane of the "lm. Co/C and Co/Si multilayer"lms have also been deposited. Fig. 4 shows X-ray re#ec-tivity data from two 4 nm carbon "lms deposited on Si at60 V, one of which is bisected by an ultra thin cobaltlayer. This layer tends to reduce the surface roughness ofthe multilayer although it is not yet clear why this shouldbe so. In the Co/Si system layer contrast is lost for thin"lms because of the high inter-di!usion rate.

732 D.E. Joyce et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 731}733

Fig. 4. X-ray re#ectivity of a 4 nm carbon "lm and a 4 nmcarbon "lm bisected by an ultra thin cobalt layer. Scans areo!set for clarity.

3. Conclusions

The novel technique of direct low energy ion beamdeposition has been demonstrated and shown to be ap-plicable to magnetic multilayers. Further development isrequired but early indications promise a powerful re-search tool.

References

[1] D.G. Armour, Nucl. Instr. and Meth. B 89 (1994) 325.

D.E. Joyce et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 731}733 733