Optimization of sputtered Co films

Journal of Magnetism and Magnetic Materials 102 (1991) 223-232 North-Holland

Optimization

of sputtered

Co films

Ch. Morawe, A. Stierle, N. Metoki, K. Briihl and H. Zabel Ruhr Universitiit Bochum, Fakultiit fiir Physik und Astronomie, Experimentalphysik w

4630 Bochum I, Germnvy

Received 21 May 1991; in revised form 16 July 1991

Co films in the thickness range of 120-230 nm have been sputtered on quarts glass and monocrystalline sapphire (1120) substrates. A preferential orientation of the hcp [OOOl]-and the fee [Ill]-axis, respectively, has been observed at low sputtering rates (0.05 rim/s)) and after optimization of the substrate temperature T,. For 200 ’ C I T, I 300 o C the structural coherence length normal to the film-plane reaches its maximum. The measured lattice spacings d are between the bulk values d(maa, and dot,, of the hcp- and the fee-phase, respectively, and reach a minimum in the same range of temperatures. REM investigations supplement the X-ray measurements. Furthermore, thinner Co films (lo-25 nm) were sputtered at 250 and 300 o C. X-ray patterns show superior film quality at 300 ’ C, slightly dependent on Ar pressure.

1. Introduction

Research into thin magnetic films requires samples of well-known structural properties in order to avoid confusing results due to uncontrolled preparation conditions. Highly sophisticated deposition processes like molecular beam epitaxy (MBE) are widely used to prepare semiconducting or metallic films of high fidelity [ll. Our aim was to obtain similar results using a conventional sputtering system by optimization of the sputtering parameters. It is well known that in some respect the sputtering process has advantages over the MBE technique as concerns the reproducibility of the sample quality, the smoothness of surface and interface [21, and the sample throughput. All these factors have made the sputtering process attractive for fundamental and applied research. We will show here that with this method very high quality epitaxial single crystal films can be grown by a proper choice of the substrate and growth parameters. The present work is focused on cobalt films grown on both single crystal sapphire and on quartz glass substrates. Thin films of Co [3-61 and multilayers Co/M with M = Cu [7-121, Cr 1131, Pd [14,151, Pt [16,171, Au [l&191 have recently attracted much attention because of their 0304-8853/91/$03.50

intriguing magnetic properties. They are also of interest as potential magnetic storage media and as logic components in device applications. From a structural point of view Co represents a classical example for a phonon driven martensitic transformation from a low temperature hcp to a high temperature fee phase with a transition temperature at TM = 415 o C [20,21]. It is one of the objectives of this research to analyze the effect the substrate has on the structural and magnetic propertiesof Co films. Here we will report on the structural properties, which will be followed by a report on the magnetic properties [22]. In this investigation, we have applied, in part, modern methods of X-ray thin film analysis, including X-ray total reflectivity measurements and Laue oscillations of Bragg reflections, which yield detailed information on the layer thickness, interface roughness, structural coherence length, etc. These methods will be described in some detail further below.

2. Experimental

conditions

All films were deposited by rf-sputtering (13.56 MHz) in a high vacuum environment with a base pressure of 5 X 10-*-l X 10e7 mbar after cooling

0 1991 - Elsevier Science Publishers B.V. Ah rights reserved

Ch. Morawe et al. / Optimization of sputtered Co films

224

mounted 7 cm below the substrates. The total film thickness was measured by optical interferometry according to Tolansky. The thickness of thinner films was determined by X-ray reflectivity measurements and by the Laue oscillations of Bragg reflections normal to the film plane (see further below). The growth conditions for the thicker and thinner films are listed in table 1. We have not noticed any dramatic change of the film quality upon the argon pressure in the pressure range between 2 x 10V3 and 1 x lo-’ mbar.

with liquid N,. 99.999% pure Ar was used as sputtering gas at pressures between 2 x 10e3 and 1 X lo-’ mbar. The discharge was carried out with an electric power input of 150-300 W which led to sufficiently low sputtering rates of 0.05 nmLs. Quartz glass and monocrystalline sapphire (1120) (ultrasonically cleaned in acetone and ethanol) were used as substrates. The substrate temperature was controlled by a PID controller supplying a resistivity heater on top of the substrate holder. A 99.9% pure Co target was

fee hcp JJ

1000

fee hcp

2000 T s”b=

4

3o”c

T s”b=

0 = 175nm

D =

.

200

’

’ .

. ’ . T s”b=

3ooc 175nm

. 3oooc

D =

165nm

T sub=

55ooc

1000

T sub= D =

550°c I

123nm

1000

D=

-

123nm

500

100’

’

400

450

.

n

500

2 Theta

’

c

550

‘.’

’

60°

200

400

’

I

500

450 2

-

550

60=’

Theta

Fig. 1. Series of CuK, X-ray 0:20 scans from Co films in the direction normal to the film plane. (a)-(c) are for Co films on sapphire [llzO] substrates, (d)-(f) for Co fihns on quartz glass. All intensities are plotted on a logarithmic scale. The main peak at 44.5 o is due to the Co hcp/fcc (0002)/(111) planes. The arrows indicate the Bragg positions for the pure hcp and fee phase. The weak but sharp peaks in panel (a) and (b) are from the sapphire substrate. The sharp drop of the intensity on the left side of the Bragg peaks in the panels (b) and (c) are caused by the Ni filter used in these scans. The same effect is not noticeable although present in the other scans because of the much weaker intensities.

Ch. Morawe et al. / Optimization of sputtered Cofilmr Table 1 Growth conditions and sputtering parameters for the Co films deposited on sapphire and glass substrates Series 2 Series 3 Series 1 thick films thin films thin films Argon pressure (mbar) Power(W) Deposition time (min) Deposition rate (rim/s)) Film thickness (nm) Substrate temperature ( o C)

5x10-3

5x10-3

2x10-3

300 50 0.04-0.07 120-230 30-550

150 5 0.04 12 250

150 5 0.04-0.08 12-25 300

However, pressures beyond 5 X 10e3 mbar appear to reduce the film quality slightly. The X-ray measurements were carried out in three different modes. For an overall characterization of the sample quality a Philips powder diffractometer with CuK, radiation was employed allowing 8 : 28 scans and rocking curves to be taken from the films. For a more detailed analysis of the structural coherence lengths and interface sharpness, a single crystal diffractometer equipped with a Si(l10) monochromator and radiation was used at a resolution of MoK,, AK/K = 5 X 10e4, where K is the scattering vector. Finally the epitaxial relation between the Co film and the sapphire substrate was studied using a 18 kW rotating anode machine with graphite monochromator and MoK, radiation in glancing angle configuration.

3. Thick Co films 3.1. Out-of-plane

Bragg scans

Figure 1 shows a series of CuK, X-ray scans from thick films grown at different substrate temperatures and on two types of substrates. All scans were taken at room temperature after growth and without further annealing. From these scans it is evident that sputtering at low substrate temperatures produces films of low quality. The Co/sapphire film (fig. la) grown at 30 o C exhibits polycrystalline reflections superimposed on a large diffuse (amorphous-like) background. The sharp peak at 28 = 52.5 o is due to the sapphire substrate. The Co ,film on glass substrate exhibits

225

(fig. Id) only the lowest order hexagonal (lOi and the mixed hcp (0002) and fee (111) reflections, the higher order peaks appear to be suppressed by the disorder or small crystallite size. With increasing substrate temperature most of the polycrystalline Bragg peaks disappear and a dominant peak at 28 = 44.5 o remains, corresponding to the (111) or (0002) Bragg peak of the fee and hcp phase, respectively (see figs. lb and e). The evolution of this peak indicates the development of a strong texture in the Co film on both substrates at higher temperature. The main difference is the much higher Bragg intensity for the Co/sapphire sample as compared to the Co/glass sample, indicating a much higher degree of single crystallinity in the former case. Note that all intensities are plotted on a logarithmic scale. At even higher growth temperatures of 550 o C (figs. lc and f>, which is above the martensitic temperature, the sample quality is somewhat degraded as can be noticed by the reduced peak intensities for the Co films on both substrates. The peak position at 28 = 44.5 o actually corresponds to a mixture of the hcp (0002) and fee (111) d-spacings of the two structural modifications of cobalt. In pure form, the hcp ABA.. . stacking of the cobalt lattice planes results in a slightly smaller d-spacing of &,,a,, = 0.2023 nm than the fee ABCA.. . stacking sequence with d C111j= 0.20467 nm. Their respective peak positions are shown by arrows in the scans of fig. 1. With increasing substrate temperature the peak position moves closer towards the hcp value, and for substrate temperatures above the martensitic temperature it moves back again to the fee position. This behavior is more pronounced for films grown on the sapphire substrate than on the glass substrate. Since all X-ray measurements were carried out ex situ at room temperature, it appears that the fee phase remains stable after cooling through the martensitic temperature. At the present time we believe that this stabilization is caused by grain boundaries and defects within the film rather than by the epitaxial interaction with the substrate. However, more detailed temperature and film thickness dependent measurements are required to shed more light on this issue.

Ch. Morawe et al. / Optimization of sputtered Co films

226

The dependence of the interplanar spacing on the substrate temperature is shown in fig. 2 for the thick films and for growth on both types of substrates. By the dashed lines, the d-spacings of the pure fee and hcp phases are indicated. The measured d-spacings exhibit a significant flat minimum at T, = 300 ’ C which is almost independent of the choice of the substrates. In this temperature range, however, the peak intensities of the Co/sapphire samples exceed those of Co/glass samples by two orders of magnitude as shown in fig. 1. The change of the layer spacing due to the crossing of the martensitic transformation is more pronounced for the Co/sapphire (fig. 2a) than for the Co/glass samples (fig. 2b).

‘i

:f? 2500

zl c Q

2000

r

Co

/

on

Sapphire

L 1500 Frl :

jf

2 8

1000

I

/

50~:~ 0

100

200

Substrate

300

400

Temperature

500

600

C°Cl

Fig. 3. The coherence lengths for Co/sapphire and Co/glass films normal to the film plane are plotted as a function of the growth temperature. The solid lines are guides to the eye.

3.2. Structural coherence

of the Bragg reflections in radial direction. Using the Scherrer equation [23]:

The structural coherence of the films normal to the film plane can be determined by the width

B(28) = h/L,

2.07

‘;I :,o

2.06

z 2.02

cn -b

2.01. E

where B(28) is the full width at half maximum of the Bragg peak in a 0 : 28 scan, and A the X-ray wavelength, we obtain the structural coherence lengths L,. Those are plotted as a function of the , , substrate growth temperature in fig. 3. Between a> 200 ‘C I T, I 300 ‘C the Co films on sapphire reach coherence lengths equal to the film thickfcdlll) _________________.______________ ness. Above 400 ‘C, the coherence length drops again, probably due to the martensitic transforf f mation. Over most of the temperature range studied the Co films on glass substrate show __________________________._______.~._____.-.___.~ hcp(0002) inferior quality. It is only at low and high temperatures that the coherence lengths of the Co/sap’ 8 I 8 ’ phire and Co/glass samples become comparable.

b)

2.06

II

$

2.05

2 u

Ccc(lll) .___________________________________.______

2.04

2

P

‘-j

2.03

x cn A

2.02 2.01

cos 8,

3.3. Out-of-plane texture

01 __._____._____----___---__.~._ ”1 f f

--____._

hcp(0002)

0

100

.200

Substrate

300

400

Temperature

500

600

C°Cl

Fig. 2. The interplanar spacing is plotted as a function of the growth temperature for (a) Co/sapphire and (b) Co/glass samples. The dashed lines indicate the interplanar spacings dc,sj and dolt) one would expect for the pure hcp and fee stacking sequences, respectively.

(

As mentioned above and shown in fig. 1, the Bragg intensities point to a significant texture, depending on the growth temperature. The texture was determined directly by scanning the hcp (0002)/fee (111) peak in the transverse direction, i.e. by performing a rocking scan with fixed 28 detector position and stepping the sample 8 position. Typical rocking curves are shown in fig. 4. For the Co/sapphire samples we observe a dramatic decrease of the rocking width from 13 O at c = 30 o C to less than 0.5 o at T, 2 200 o C (fig.

227

Ch. Morawe et al. / Optimization of sputtered Co fzlm Cobalt

on

Sapphire

(110)

s 1200 5 6 600

4

L 0

- 60

30

70

90

100

110

120

130

140

8

Fig. 4. Out-of-plane rocking scans of Co/sapphire samples after growth at different substrate temperatures T,. The full width at half maximum of the mosaic distribution decreases from about 2” for T,=lOO”C to 0.1” for T,=3OO”C. The intensities are normalized to the same peak hight.

5). In contrast, the mosaic distribution Co/glass samples is much broader for the temperature range studied and is order of 8 o to 10 ‘. The dip at 500 o C is due to the martensitic transformation.

of the most of on the possibly

3.4. In-plane mosaic@ and epitaxy It should be noted that the rocking scans described above only probe the texture and the mosaicity normal to the film plane. Therefore nothing can be concluded from these scans about the in-plane mosaicity. For instance, a polycrystal with one axis well aligned parallel to the film normal but random in-plane orientation such as in highly oriented pyrolytic graphite [24] would be

Co 100

200

Substrate

300

on 400

Temperature

Sapphire 500

600

PC1

Fig. 5. The widths from rocking scans plotted as a function of the growth temperature for Co/sapphire and Co/glass samples.

3D

Theta

Fig. 6. In-plane scan of the Co/sapphire sample in glancing angle configuration. The detector was fiied at the position of the in-plane CoUlO) peak. The sharp reflections are due to the sapphire substrate.

consistent with the measurements presented so far. To determine the in-plane mosaicity we performed an X-ray scan in glancing angle geometry with the film plane almost parallel to the scattering plane. This geometry is preferable over a c0 [iziol Al,Oa 100011 t

Fig. 7. Sketch of the in-plane reciprocal lattice of the sapphire substrate and the epitaxial Co film. The measured sapphire reciprocal lattice points are shown by sharp dots, whereas the Co reciprocal lattice points are represented as angular streaks corresponding roughly to the in-plane mosaic distribution of the film. The arrow indicates that the sapphire [OOO.l]direction is parallel to the Co [i2i.O] direction.

228

Ch. hforawe et al. / Optimization of sputtered Cofilms

transmission scan when the intensities are comparatively low [25]. In addition, the glancing angle geometry allows, in principle, depth resolved information by controlling the penetration depth of the X-rays into the sample at glancing angles close to the angle of total reflectivity [26,27]. Here we show only integrated intensities representative for the whole film. In fig. 6 an in-plane scan is shown with a rotating sample and a fixed detector angle at the 28 position of the in-plane Co(ll0) Bragg peak. The Co reflections appear as rather broad peaks at 35 O, 95 ’ and 155 O. The decreasing intensity is

caused by a slight misalignment of the sample and the sharp peaks at 110 o and 150 ’ are from the sapphire substrate. From this scan it can be seen that the in-plane mosaicity of the Co film is on the order of 10 O. Scans like the one shown in fig. 6 allow the mapping of the in-plane reciprocal lattices of the Co and sapphire structure, and, in particular, to determine the epitaxial relation between the Co film and the sapphire substrate. The results are shown in fig.. 7, where the sapphire in-plane Bragg peaks are indicated as small dots and the Co peaks as streaks proportional to the in-plane mosaicity of the films. According to

Fig. 8. Scanning electron micrographs of the surface morphology of the Co/sapphire (a)-(c) and Co/glass samples (d)-(f) after growth at different substrate temperatures. The length of the white bar corresponds to 1 pm.

229

Ch. Morawe et al. / Optimization of sputtered Co films

this figure, the Co [lOi.O] axis is aligned parallel to the sapphire [liO.O] axis, and the Co [i2i.O] axis is parallel to the sapphire [OOO.llaxis.

5000

,

-

2000

I

a>

T sub= D=

-

250°C 12nm

Ei

3.5. SEM-observation A series of high resolution scanning electron microscopy GEM) photographs supplements the conclusions drawn from the X-ray scans (fig. 8). The surface morphology indicates a clear temperature dependence of the crystallite size and the film roughness with pronounced differences between Co/glass and Co/sapphire samples. At low substrate temperatures T, < 100 ‘C we find on both types of substrates a low ordered film surface containing small round grains (100 nm) (figs. 8a and d). An increase of the temperature up to 300°C leads on glass substrates to larger crystallites (500 nm) with sharp edges (fig. 8e). In contrast, Co films on sapphire show at this growth temperature smooth and ordered surfaces on a scale of more than 10 pm containing only a few holes and steps (fig. 8b). A further rise of the temperature causes a coalition of the grains on glass leading to large islands (1 rJ,m) with boundaries reaching all the way down to the substrate (fig. 8f). This effect occurs for Co/glass already at lower temperatures and to a higher degree than for Co/sapphire (fig. 8~).

4. Thin Co films 4.1. Out-of-plane Bragg scam

In the thin film range from 12 to 25 nm we show only X-ray measurements for Co/sapphire samples since similar measurements on Co/glass samples gave very poor results. The poor quality is most likely due to an amorphous like growth mode for the first 10 to 20 nm of Co on glass substrates. The Bragg peaks from the Co/sapphire samples indicate again a growth direction predominantly parallel to the [OOOl]/[ 1111 direction as already described for the thick films. As can be seen from fig. 9a, there is a pronounced fee (200) reflection at 28 = 51”, which appears after growth at 250 ’ C indicating some admixture

Tsub= D =

3oooc 25nm

1: lJ?

100 t.,.,.,.,.l 400

440

48O

2

5Z”

56’=

60°

Theta

Fig. 9. Out-of-plane X-ray scans of the thin Co/sapphire grown at 250 o C (a) and 300 ’ C (b).

films

of the [lOO] growth direction. After an increase of the growth temperature by only 50 ‘C, this peak is completely gone and a scan representative for the high quality of the film appears. In addition there are oscillations in the wings on both sides of the Bragg peak which are due to the finite film thickness and which are indicative for a very high structural coherence in this thin film. These oscillations, also called Laue oscillations, were analyzed with higher resolution and are described in more detail in the next paragraph. The d-spacings are found again to be in the range between +Go,, and do,,, as for the thick films. 4.2. Laue oscillations

In thin films with a finite number of coherent lattice planes the Bragg reflections show the typical Laue oscillations given by [23]: I = sin2( KL,/2)/sin2(

m/2),

where d is the interplanar spacing, L, = Nd is the thickness of the coherently scattering planes, and K = (4a/h) sin 0 is the scattering vector.

230

Ch. Morawe et al. / Optimization of sputtered Co firms

The Laue oscillations are characteristic for a well ordered lattice and have often been observed in thin semiconducting films [29]. They are, however, not well documented for metal films. It is surprising that in the case of the sputtered thin Co films these oscillations are also visible, as can be clearly seen in fig. 10. It should be noted that the X-ray scan shown in fig. 10 was taken with MO K,, radiation. From the oscillation period the average thickness (L,) can be calculated and from the sharpness of the dips the roughness of the surface and the interface can be estimated. For the sample shown in fig. 10 (L,) is 21 nm. 4.3. X-ray reflectivity We have also performed X-ray reflectivity measurements at glancing angles on the thin Co films. The intensity of the reflected X-rays can be expressed by the Fourier transform of the electron density gradient normal to the surface 126,281:

Here R, is the Fresnel reflectivity, p(z) is the electron density and K, the scattering vector normal to the film plane. In the case of a thin film on a substrate having a different electron density, this expression leads to an oscillation in

z

*.’

/-...5.

c 3

Jj

2 \

:

102-

2

3 c

.

.

_

: ;-_

* -...

.:.._.

z

I=

:

:

101-

- . ..;\ .. ;.:

19.2'

.,I. C.

4.’

a.

:

_.

:

2:. I

19.60

. *..a. -.+

*

1, *

I

200

,

20.40

29 Fig. 11. X-ray reflectivity measurement with MoK,, radiation from the same sample as shown in fig. 10. The oscillation period indicates a total film thickness of (15,) = 22.5 nm.

the reflected intensity with a period inversely proportional to the total thickness L, of the film: AK,

= 2r/L,.

In contrast to the Laue oscillations of the Bragg peaks, the X-ray reflectivity is independent of the crystallinity of the sample and depends solely on the electron density. The difference (AL) = CL,) 7 CL,> can therefore be ascribed to some polycrystalline or amorphous part of the film. A typical X-ray reflectivity measurement from the same sample as shown in fig. 10 is reproduced in fig. 11. From the Laue oscillations we obtain (L,) = 21 nm and from the reflectivity measurement (L,) = 22.5 nm yielding a difference of only (AL) = 1.5 nm. We believe that the difference is due to a small initial polycrystalline or amorphous growth at the sapphire interface prior to a single crystal like growth. In contrast, on glass substrates the disordered growth continues throughout the film thickness.

. _..._’ : . *.’

4.4. Out-of-plane texture

20.E"

23 Fig. 10. High resolution out-of-plane Bragg scan of the (0002)/(111) peak with MoK,, radiation. The wings of the Bragg peaks exhibit Laue oscillations which are typical for a finite number of coherently scattering lattice planes. From the oscillation period follows a coherence length of (L,) = 21 nm.

The texture shows a strong change between the substrate temperatures 250 and 300°C. At 250 ’ C we find a coexistence of two growth directions: the hcp [OOOl]/fcc [Ill]-axis and the fee [lOOI-axis. An increase of the substrate temperature by only 50 o C causes a change to pure hcp

Ch. Morawe et al. / Optimization of sputtered Co films

4

L

50

*......

9.950

*...a,

.* 100

I 1o.o5o

***-e..,......,... lOlO

10.150

3

Fig. 12. Out-of-plane rocking scan of the thin Co/sapphire sample grown at 300 o C. The full width at half maximum is 0.06 o indicating a very good crystalline quality for this sputtered Co film on sapphire [llzO] substrate.

231

[ill] direction and the pure fee [ 1001 direction. It is an intriguing observation and warrants some further investigations to understand why the [OOOl]/ [ill] direction wins over the [lo01 direction, finally leading to a highly coherent layer growth. Although the in-plane mosaicity is still rather large, the in-plane Co axes have a well defined relation to the sapphire substrate on a macroscopic scale. Together with the high outof-plane coherence achieved, it appears justified to talk about epitaxial growth of Co on sapphire, carried out by conventional sputtering after optimization of the sputtering parameters.

Acknowledgements

[OOOl]/fcc [ill] growth with an alignment of the plane normal better than 0.1” (see fig. 12). This mosaicity is about as good as can be found in bulk metal single crystals.

5. Discussion The structural characterization discussed here indicates a significant dependence of the film quality on the growth temperature. All results point to a state of disorder at low substrate temperatures with amorphous-like properties. Around 300 “C, crystallinity and surface quality reach their maxima. Beyond this temperature island growth is preferred, which reduces rapidly the surface smoothness and coherence. The martensitic hcp/fcc transition around 450 o C has mainly an effect on the d-spacing of the films normal to the plane but not on the overall structural quality. An essential result from these measurements is the demonstration that the substrate has a large impact on the film quality. Coherent and highly ordered growth of Co was attainable only on monocrystalline sapphire substrates, not on amorphous quartz glass. Even on sapphire, the first few layers are probably highly disordered. In addition, the thin film studies showed that in the beginning there is some competition between two different growth directions, the hcp/fcc [OOOl]/

Assistance of W. Oswald with the SEM work is gratefully acknowledged. This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 166.

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Ch. Morawe ef al. / Optimization of sputtered Co films

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