Interface diffusion of sputtered CoZrNb films on silicon substrate

RARE METALS Vol . 2 5 , Spec. Issue , Dec 2006, p .36

Interface diffusion of sputtered CoZrNb films on silicon substrate W

Xiaowei

, WANG b u n , YANG Jing , ZENG Fei , and PAN Feng

Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084. China (Received 2006-08-20)

Abstract: The amorphous CoZrNb films were deposited by DC magnetron sputtering. The depth distributions of the elements were analyzed by Rutherford backscattering spectrometry (RBS) . The results indicate that when the deposition time is longer than 37 min , the film composition keeps constant along the depth. When the deposition time is longer than 45 min , the Co concentration at the interface of the silicon substrate is higher than the average value in the whole film. When the deposition time is longer than 52 min, the Co atoms diffuse into the substrate during the deposition. According to the Co composition profile in the substmte , which were determined from the RBS spectra, the Co diffusion coefficients in the substrate were calculated using the solution of Fick’s second law corresponding to an infinite source with a constant diffusion coefficient. The calculated diffusion coefficients indicate an intemtitial assisted diffusion mechanism.

Key words : CoZrNb films ; interface diffusion ; RBS [This work was financially supported by the National Natural Science Foundation of China ( N o . 10405013 and No .50325105 ) . 3

1.

Woduction

As a kind of soft magnetic material, amorphous CoZrNb films exhibit high saturation magnetization, large permeability at high frequencies, low coercive force, nearly zero magnetostriction and high wear resistance [ 1-21. Due to these properties, the amorphous CoZrNb films are widely used to make the main pole head for perpendicular magnetic recording, thin film transformer, inductor and LC filter[3]. In practical applications, the interface behavior between the substrate and the film largely influences the film’ s performance. For example, due to the effect of interface pinning, the thickness of the CoZrNb film should reach a critical value to finish the large Barkhausen jump and quick domains reverse [ 4 1. However, it is known that the interface status keeps changing during the deposition process due to the techni-

cal condition influence and results in various interface behaviors. Therefore, it is necessary to find the interface effects and determine its origin in time. Rutherford backscattering spectroscopy ( RBS) is an established technique used to study the chemical composition and thickness of thin films[ 5-81. It can also be applied conveniently to study the interface mixing between the film and the substrate. In this work, CoZrNb films were prepared by DC magnetron sputtering. RBS was employed to analyze the depth distribution of the element concentration and the atomic diffusion near the interface between the substrate and film. It shows that when the film is thicker than a critical value, the interface diffusion will occur. The interface diffusion will influence the anisotropy of the CoZrNb film. The diffusion mechanism was discussed with the calculated diffusion coefficient.

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CorrrspoMting author: ZENG Fei

E-mail: zengfei @ mail. tsinghua .edu .cn

Li X . W .et d . , Interface diffusion of sputtered CoZrNb films on silicon substrate

2.

Experimental

The films were deposited in a DC magnetron sputtering system with vacuum background of 2 x Pa. The target was a Co disk on which Zr and Nb chips were disposed. Single crystal of ( 111) Si was used as the substrate. The working pressure of argon was 0.4 Pa, the input power was 155 W and the deposition rate is 7 nm. min-'. Seven samples with different sputtering time were prepared, the sequential sputtering times were set as 15, 22, 30, 37, 45, 52 and 60 min The substrate temperature before deposition was 30 9: . After deposition, the substrate temperature slightly increased to

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32-35 "c. X-ray diffraction ( XRD ) was used to check the films' structure. X-ray fluorescence (XRF) was used to determine the average composition of the films. Thickness and depth distribution of the element were analyzed by RBS. The 4He beam with the energy E = 2.02 MeV was used in the RBS experiment. The particle detector was fixed at 165". The energy resolution of the detector was 18 keV and the energy spread of incident beam was 2 . 0 keV. The RBS spectra were analyzed by using SIMNRA software[ 91

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comes from three fitting steps processed on the experimental RBS spectrum. First, the element concentration in the films is assumed to be independent on the depth. In other words, the composition is assumed to be homogeneous in the films and consistent with the average value of the whole film. Figs. 2 and 3 show the typical RBS spectrum and the corresponding simulated spectrum of samples deposited for 30 and 60 min. It can be seen that the simulated spectrum fits well with experimental spectrum for sample deposited for 30 min . That substantiates the homogeneous assumption for sample deposited for 30 min. While for film deposited for 60 min, the simulated spectrum does not fit well with the experimental spectrum in the low energy part. Two discrepancies can be found and labeled as A and B in Fig. 3( a ) . It indicates initially that the composition deviates from the average value near the interface between the film and the substrate.

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3.

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v

F * 3

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Results and discussion

3.1. Structure, thickness and composition analysis Fig. 1 shows the typical XRD spectrum of samples deposited for 15, 30 45 and 60 rnin . The broad peaks appearing in the XRD pattern indicate the amorphous nature. Other samples are also verified to be amorphous by XRD. The average composition results measured by XRF are listed in Table 1. The RBS spectra were simulated and fitted with the aid of the results in Table 1. The simulated results are listed in Tables 2 and 3 . Table 2 shows that the thickness measured by RBS is consistent with the expected value but in some samples the composition changes with the depth. Accurate knowledge of the variation of the composition with the thickness

osited for I5 mi 40

30

50

60

70

80

90

2 Ul(O)

Fig.1. X-ray dItRaction pattern of sample depositedfor 15, 30, 45, 6Omin.

Table 1. Average commition measured by XRF

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Deposited timelmin Colat %

15 22 30 37 45 52 60

80.1 81.2 79.9 81.3 79.9 81.6 79.2

.

&/at %

Nblat . %

10.0 9.5 9.9 9.4 9.9 9.3 10.3

9.9 9.3 10.2 9.3 10.2 9.1 10.5

RARE METALS, Vol. 2 5 , Spec. Issue, Dec 2006

38 Taw 2.

Smnmary of

RBS fitting results RBS

Deposited timelmin

Expectant thickness /MI

15 22 30 37

45

105 154 210 259 315

100.7 161.4 200.3 259.8 308.9

52

364

391.1

60

420

427.9

thickness

Thickness of /nm /nm layers

100.7 161.4 200.3 259.8 283.0 25.9 364.2 26.9 403.1 24.8

Co/ at. %

Zr/ at. %

Nb/ at. %

80.1 81.2 79.9 81.3 80.0 84.0 81.6 85.8 79.2 81.2

10.0 9.5 9.9 9.4 10.0 8.0 9.2 7.1 10.4 9.4

9.9 9.3 10.2 9.3 10.0 8.0 9.2 7.1 10.4 9.4

* Thr expectant thickness is the product of sputtering rate and sputtering time; * * The RBS thickness of each sample equals the sum of the thicknesses of all the delaminated layers in the sample; * * * For sample deposited for 45 to 60 niin, films are delaminated into two layers, the thickness and the composition of each layer are listed in the same row. The layer contacting with the air is above the layer contacting with the substrate. assumed that the Co atoms diffuse into the silicon substrate in the third fitting step The fit -

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15

:sk..; 150

200

250

300

350

I -Simulated

400

Channc

Fie. 2 . RBS experimental and simulated spectra of sample deposited for 30 min.

Y

3 8

m

To know the composition deviation of sample deposited for 60 min, the second fitting step was performed under the assumption that the film can be delaminated into two layers of different compositions. As shown in Table 2, the film consists of one layer of 403 nm and another of 24.8 nm near the interface. The Co concentration increases from about 79. 2 at. % to 81.2 at. % and the concentrations of Zr and Nb decrease respectively at the interface. Applying the delamination result to the simulation, the discrepancy labeled A in Fig. 3 ( a ) is found to disappear while that labeled B still exists, as shown in Fig. 3 ( b ) . To eliminate the discrepancy labeled B, considering the fact that the Co atoms accumulate at the interface, it is further

m

rr:

--Simulated 150

200

250

300

360

4C

Channel

Fig. 3. RBS experimental and simulated spectra of sample deposited for 60 min with the assumption that (a) tbe element concentration in the films is independent of the depth; (b) the film is delaminated into two layers; ( c ) the Co atoms diffbes into the sutrstrate

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Li X . W .et al . , Interface diffusion of sputtered CoZrNb films on silicon substrate ting result in Fig. 3 ( c ) shows that the discrepancy labeled B in Fig. 3 ( a ) disappears. As shown in Table 3, the Co concentration in Si substrate is about 3 at. % in initial 70 nm and 2 at. % in subsequent 370 nm. It decreases to zero in the depth of the substrate. In conclusion, the fitting process in Fig. 3 illustrates that the Co atoms accumulate at the interface and part of them diffuse into the substrate for sample deposited for 60 min. From Tables 2 and 3, no obvious composition fluctuation is found along the depth for sample deposited for 15 min to 37 min. The Co concentration slightly increases at the interface between the film and the substrate for sample deposited for 45 min to 60 min. The Co atoms are found to diiuse into the substrate only for sample deposited for 52 and 60 min . However, this diffusion does not happen for Zr or Nb atoms. That may be explained with two reasons. First, the electronegativity of Co is larger than that of Zr and Nb, hence a Co atom attracts the electrons around a Si atom more easily. Second, the Zr and Nb atoms are larger than the Co atom in size, which also hinders their mobility in silicon.

3.2. Further analysis of interface diffusion into the substrate As mentioned above, there is no apparent diffusion of Co atoms at the interface into the Table 3. Profile of Co concentration in Si substrate due to diffusion" Deposited timelmin

Depth in the substrate/nm

Co/at .%

52

60 370 > lo00 70 370 > lo00

2

60

1

0 3 2 0

* The Co concentration in Si substrate is also obtained by the delaminating method. Assuming the average cornposition of each layer approximate to the composition at the middle of that layer, the Co concentration is obtained at some depths.

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substrate except sample deposited for 52 and 60 min. It is likely that the diffusion was caused by the accumulating heat during the deposition process. The diffusion process can be analyzed using the solution of Fick ' s second law corresponding to an infinite source with a constant diffusion coefficient ( concentration and depth independent)

where C is the Co atomic concentration at a depth x after a time t , and D is the diffusion coefficient. Since there is no apparent diffusion in sample deposited for 45 min, it is assumed for sample deposited for 52 and 60 min, only until the deposition had lasted for 45 min, that the diffusion start with enough accumulated heat at the interface between the film and the substrate. With this assumption, the total diffusion times for these two samples are 7 and 15 min respectively, instead of 52 and 60 min . Using the data in Table 3, the values of D for sample deposited for 52 and 60 min are 2 . 8 8 x and 3 . 0 3 x m2 s - ' , respectively. The temperature dependence of D is generally described by D = Doexp ( - Q I k T ) (2) where Do is a temperature independent prefactor and Q is the activation energy. For diffusion in crystal silicon, Do generally ranges from 1 x 10-'to 1 x lo-* m2 * s-' [ 101. In our experiment, the temperature T is 305 K and the averagevalue of D is 2 . 9 5 ~ 1 0 - ' ~ m ~ * sThen -'. Q ranges from 0.46 to 0.52 eV, which is calculated according to Eq . ( 2 ) . Normally, the activation energies Q of interstitial migration in Si fall in the range 0.4 to 0 . 8 eV. In a vacancy assisted diffusion mechanism, Q is much larger [ 101. Therefore, the diffusion of Co in Si is regarded to be of an interstitial assisted diffusion mechanism.

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4.

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Conclusions

The deposition time and thickness of the sputtered CoZrNb film influence the composition along the depth and the corresponding magnetic properties. When the deposition time is

40 shorter than 37 min, the films are homogeneous along the depth. When the deposition time is profonged to 45 min, the Co atoms will accumulate at the interface. When the deposited time exceeds 52 min, the Co atoms diffuse into the Si substrate.

RARE METALS, Vol. 2 5 , Spec. Issue, Dec 2006

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