Study of nickel silicide by X- ray diffraction
H. Fiad1, R. Ayache2, A. Bouabellou1, C.Sedrati1
1Laboratoire Couches Mince et Interface, Universityfrères MentouriConstantine, Algeria.
2 Pharmacy Department, University of Batna, 05000, Algeria.
E-mail: fiadhouda@yahoo.fr
Abstract
In order to understand fully the results from X-ray diffraction or scattering analysis, it is benefecial to understand the interactions between X-rays and matter. X-ray methods are generally non- destructive, in that sample preparation is not required, and they can provide a very appropriate route to obtain structural information on thin films and multilayers. Analysis can be performed across the whole spectrum of material types from perfect single crystals to amorphous materials.
In this work, X-ray diffraction was involved to study the formation of crystalline nickel silicide phases. For this purpose, Ni nanometric thin films were deposited on Si (100). XRD technique have shown that the Ni2Si silicide is the predominant phase at 350°C. For the annealed samples at 500°C, only NiSi monosilicide is detected.
When the annealing temperature increases to 750°C the NiSi2
disilicide is the main formed phase. The obtained samples were analyzed using scanning electron microscopy (SEM), it show a surface morphology depending strongly on the substrate orientation.
Keywords: Thin film ; nickel silicides ; X-ray; XRD.
I. INTRODUCTION
Nickel silicide is a material which finds application in both the CMOS and microsysteme devices. NiSi thin films are used in ohmic contacts in the CMOS industry due to thier low resistevity [1, 2] and comparable performance for both nMOS and pMOS devices [3-5].
The silicide materials presently used, TiSi2 and CoSi2 are limited by problems of linewidth dependent sheet resistances and high leakage current in ultra shallow junctions, respecttively. NiSi, a potential candidate to replace the above- mentioned silicides has been the subject of extensive investigation. Because of its low resistivity, low silicon consumption, low formation temperature and line – width- independent silicidation. Hawever, NiSi is thermally stable only up to 750°C, above which nucleation of the high resistivity phase NiSi2occurs.
It is therefore important to investigate the phase formation of the nickel silicide (Ni 2Si, NiSi or NiSi2) films. This article compares nickel silicide formation on (100) silicon wafers.
The composition of nickel silicide thin films has been
analysed using X-ray diffraction (XRD), Rutherford backscattering spectroscopie (RBS), and Scanning electron microscopie (SEM).
II. EXPERIMENTS
High purity Ni metal films were deposited on p-type silicon substrate (100) by thermal evaporation. The samples were then annealed in vacuum at 350°C and 500°C for 20minutes, and a time of 5 min for the sample annealed at 750°C. XRD measurements were carried out to identify the phases of the synthesized silicides by a Bruker D8-Advance with a Cu Kα source operated at 35 kV and 30 mA in a glancing incidence geometry. Scanning electron microscopy (MEB) was used to view the surface topography.
III. RESULTS AND DISCUSSION
Fig. 1 shows the XRD spectra of samples deposited and annealed at 350°C, a stronge peak at 68,8° can be clearly observed, which is attribute to Si(100) substrate. the Ni2Si is the only phase formed between nickel and Silicon.
Fig.1The the XRD spectra of Ni/Si (100) films annealed at 350°C during 20 min.
The annealing at 500°C (Fig 2) shows the decreases in the intensity of the pic (100) of nickel, this reduction is accompanied by disappearance of the Ni2Si phase, and the appearance of the peaks of the NiSi phase.
Fig.2The the XRD spectra of Ni/Si (100) films annealed at 500°C during 20 min.
At 750° C, one notices the very obvious appearance of the NiSi2 phase with a epitaxial growth on Si (100).It is also observed that the NiSi phase is not consumed yet completely at this temperature of annealing.
Fig.3The the XRD spectra of Ni/Si (100) films
annealed at 750°C during 5 min.
The results of the observation with the MEB of the samples of Ni/Si (100) are presented on the fig. 4.
In the annealing at 350°C a new structure is observed.It comes on the surface a "matrix" gray being strewn with small white tasks and at the bottom with the dark parts of varied forms but tending to polygons.These dark zones correspond, probably, with stripped silicon, there is a phenomenon "of agglomeration".
The annealing at 750°C during 5 minutes, supports of advantage the polygonal structure contrast, and one notices a kind of dark grains and grisâtres, whose form tends towards a hexagon.The latter are bordered by a very clear relief.This
"hexagonal" structure of the "grains" is, probably, revealing of a epitaxial growth of NiSi2on Si(100), like it was observed in other systems such as Cu/Si and Au/Si [6].It is finally necessary to note the presence of small white spots, especially, in the case of annealings with 350°C and 750°C.
Fig. 4 SEM images of the annealed samples at differents temperatures
Conclusions
This article reports on the situ characterisation of nickel thin films on silicon substrates. The evolution of nickel silicide from the nickel film can be observed from both the XRD and SEM in situ studies. XRD spectra also indicate that the transformation from Ni2Si to NiSi occurs only at around 500°C. XRD and SEM show that the annealing temperature play a crucial role in controlling the phase transformation of silicide.
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