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STRUCTURE PROFILE OF B+ ION IMPLANTED
IRON FILM
Yue-Lu Zhang, Si-Yun Bi, Liang-Mo Mei, Zhen-Huan Lei
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl4ment au no 12, Tome 49, dkembre 1988
STRUCTURE PROFILE OF
B+
ION IMPLANTED IRON FILM
Yue-Lu Zhang, Si-Yun Bi, Liang-Mo Mei and Zhen-Huan Lei
Department of Physics, Shandong University, Jinan, China
Abstract. -The profile of microscopic structure of a polycrystalline iron film implanted by boron ions has been investigaed using b 7 ~ e conversion electron Miissbauer spectroscopy (CEMS) with ion beam etching technique. Parameters of CEMS show minima or maxima in the middle of the implanted layer.
1. Introduction 1 1 0
i
Recently, ion implantation induced amorphization of metals has become an interesting research subject as it can offer some physical informations of signifi- cance. For example, it is providing a valuable tool for examining the range of alloy systems that are stable in the amorphous state [I]. In this paper, we will re- port the results about the structure distribution of the implanted layer formed by single energy boron ion im- plantation into a polycrystalline iron film using CEMS with ion beam etching technique.
2. Experimental results and discussion
The iron film, prepared by w u u m deposition on a polished single crystal silicon wafer, is about half a micron thick. The energy and dose of boron ions are 100 keV and 3 x 1 0 ~ ~ ~ + / c m ~ respectively. A 6 7 ~ o / ~ gamma source and a parallel plate avalanche counter with acetone vapour as ionization gas have been used in the CEMS experiments. Ion beam etching was ac- complished on the SD-3 etching machine in our lab* ratory by 300 eV argon beam. The etching depth was decided by measuring the oscillation frequency change of a qnartz crystal etched simultaneously with the iron film. The ratio between the etching rate of iron and quartz had been calibrated by a precision balance. The Mossbauer data were fitted by a standard fitting pro-
gram MOSFUN.
For the as-prepared film, the relative intensity of the second and fifth lines of the crystalline phase (a-Fe), &2.b, is quite smaller (= 0.25) than 0.666 (see Fig. la), so a quite large perpendicular anisotropy exists. There may be two reasons for this. The first is the columnar crystal structure, formed from directional growing of crystallites during deposition, which could cause shape anisotropy; Secondly, there exists an external tensile stress in the film after cooling from heated state on deposition because of the difference between the heat contraction coefficients of iron and silicon and the ne- gative saturated magnetoelastic coefficient of iron.
In figure 1, b is the CEMS after implantation and from c to f ' correspond to several different etching depths. They are all double six-line of which the group with greater linewidths and smaller splitting belongs to amorphous Fe-B phase. Bombarding ions can no doubt destroy the columnar structure of the crystallites and,
I I
- - b - 4 - 2 2 2 4 6 v ,* m m - * - - 1
Fig. 1.
-
CEMS of the iron film before (a) and after (b) boron ion implantation and at different etching depth (c) 275 A; (d) 570 A; (e) 825A;
(f) 1345A.
C8 - 1750 JOURNAL DE PHYSIQUE
through its annealing effect on the target, eliminate weighted by the total transmission function T (x) of stress in the film, thus reduce the anisotropy. But Sc2.5 the Mossbauer electrons (y : etching depth):
is raised apparently only near the surface and even de- w
creases in the middle of the implanted layer (Fig. 2). s a ( @ ) =
/
A ( ~ + x ) T ( ~ : ) & . (1) The corresponding quantity of the amorphous phase, j oSa, has similar behaviour as Sc2.6. This is because the According to the LSS theory, ion distribution in PO- implanted ions extrude the target atoms, producing an lycrystalline solids is a Gaussian curve [4]
extra stress in the film plane [2].
In the middle of the implanted layer, the amorphous N ($1 = [D/
(6
A h ) ] xfraction Sa shows a maximum while the average hyper- fine interaction field
Ha
of the amorphous phase shows a minimum (Fig. 2). Its chemical isomer shift, Ia, is also a maximum in the middle, about 0.25 mm S-Irelative to that of a-Fe. The amorphization of me- tals by light ion implantation may be explained by the critical defect density model [3]. Irradiation is consi- dered t o produce crystal defects such as dislocations and vacancy loops, until a critical defect density level is reached at which the crystalline phase (which is now also highly doped) becomes unstable and is transfor-
where N-dopent density; D-ion dose; R, and ARp-mean projected ion range and deviation. By si- mulation with our Monte-Carlo program TISM (Trans- mission of Ions in Solid Materials) for the present case, we get Rp=1240
A
and ARp=400A.
From (2), the maximum B concentration is slightly above 25 at. %.
Hence we assume that A (x) is proportional t o N (x)
and amorphizing be completed just at x = R,, namely med into G p h o u s state. As the defect density and A (a:) = exp
[-
(a:-
R , ) ~/
(%A%)].
(3) impurity concentration are maxima in the middle ofthe implanted layer, the above phenomena appear. T (x) can be approximated by an exponential func- tion 151 -
-
280
T
(4
= (l/L) exP ( - d l )80 (4)
in which L is the attenuation length. By our experi-
- 270 ments, L is about 730
A.
When combining the above- 260 equations, the following one - 250 2 Si - 240 '
sa
(y) =&T
A% x - 230d
/a Ha + + - 220 xexp(
9
+
s)
2~~ erf(%+%)
L (5) '~2.5 210 10o
-
201200 can be derived in which erf is the error function.o 2 4 6 8 10 12 1 4 1 6 18 20 Result from (5) is expressed by the dashed curve
s ($00
9)
in figure 2. If (2) had been modified for sputteringFig. 2. - CEMS parameters as functions of etching depth. effect and radiation induced diffusion, the ~ a u s s i a i Sc2& is the relative intensity of the second and fifth lines of peztk become wider and Then the
the rn-Fe. Sa and Ha are the Mijssbauer simd fraction CO-incidence between the theoretical and experimental and the average hyperfine interaction field of the amorphous results would be better.
Fe-B phase respectively.
3. Theoretical consideration [I] Al-Tarnimi, Z. Y. et al., Vacuum 34 (1984) 861. [2] Hasegawa, H. et al., J. Mater. Sci. Lett. 4 (1985) It should be pointed out that the relations between 1092.
parameters and the etching depth obtained directly [3] Linker, G., Mater. Sci. Eng. 69 (1985) 105.
from CEMS are not the real distributions yet because [4] Rauschenbach, B. et al., Phys. Status Solidi A 85
the Mossbauer electrons come from a range of depth. (1984) 473.
