Microstructure of iron thin films evaporated at oblique incidence

Journal of Magnetism and Magnetic Materials 40 (1983) 175-184 North-Holland Publishing Company

175

MICROSTRUCTURE OF IRON THIN FILMS EVAPORATED AT OBLIQUE INCIDENCE K. OZAWA R&D Division, Magnetic Products Group, Sony Corp., Tagajo-shi, Miyagi-ken 985, Japan

H. MASUYA Sony Corp. Research Center, Hodogaya, Yokohama 240, Japan

and M. TAKAHASHI Department of Applied Physics, Tohoku University, Sendal 980, Japan Received 12 May 1983

Evaporated iron thin films of thickness in the range 250 to 3400 ,~ were deposited on plastic films at oblique incident beam angles ¢p. Using a microtome technique, the microstructure of the films were directly observed by TEM. The nature of the surfaces have been investigated using the RHEED method. At incidence angles of 55 and 65 ° an elongated wall lies in the direction perpendicular to the incidence angle. At incidence angles of 75 and 80 ° the structure varies along the thickness direction of the film and columns are approximately oriented to the [001] axis. Although the columnar angle as does not follow the tangent rule 2 tan a s ,= tan q0, the packing density of the film is as expected from the columnar angle. TEM observations and the RHEED method also reveal the process of crystal growth during deposition. The growth of columns appears to be determined by the geometric shadowing effect and the diffusion of adatoms.

1. Introduction It is no exaggeration to say that progress in magnetic recording has come along with the improvement of the magnetic recording medium [1]. Magnetic metal thin films have been considered to be the ultimate form of magnetic recording medium, with their high output at short wavelength and the ease of replay equalization [2]. Recently it is becoming commonly understood that thin film tapes can be formed by evaporation at an oblique incident beam angle. [3]. Evaporated thin films of cobalt or cobalt-nickel are usually considered as the most appropriate material for consumer magnetic products. However, iron thin films are most promising as well because of their high saturation magnetization and coercive force [4,5]. We have investigated iron thin films evaporated at various incident beam angles. Any crystalline thin film produced by vapor deposition at an oblique incidence angle generally

exhibits a columnar structure and a textural structure. Since the magnetic properties of iron films are mainly determined by the structure, a study of the microstructure is very important for optimizing the magnetic recording medium. However, despite the large number of experiments and discussions on the columnar structure which have appeared in the literature of the last two decades [6,7], the structure of evaporated iron thin films is still somewhat uncertain. In particular, it appears that there is no report which directly observes and clearly explains the microstructures of thin films of thickness in the range 1000 to 2000 A, which is suggested to be the optimum for high recording density and good head-tape contact [8]. In the present study, the microstructures of evaporated iron films of thickness from 250 to 3400 ,~ were directly observed by transmission electron microscopy (TEM) on the film planes and on cross-sectional planes which were prepared in the thickness of about 1000 ,~ using an ultra-mi-

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176

K. Ozawa et aL / Microstructure of oblique incidence iron films

crotome technique. The crystallographic structure was analyzed using a reflection high energy electron diffraction (RHEED) method. The microstructure differs both with the incident beam angle and along the thickness direction. At high incidence angles the grain growth occurs nearly in the direction of the incidence beam and the columnar structure is approximately oriented to the [001] axis.

2. Experimental procedure Iron films were deposited on plastic films (KAPTON) at various oblique incidence angles from 0 to 80 ° in a vacuum of 8 × 10 -6 Torr. The substrate was washed with neutral detergent and rinsed in an ultrasonic bath of methyl alcohol. The substrate was at room temperature at the start of evaporation and its temperature was not controlled during deposition. Iron (99.9% pure) was heated in a tungsten conical basket heater in the vacuum chamber, and the vapor beam was deposited on the substrate at a distance of 30 cm. The rate of evaporation was kept the same in all cases. Therefore the deposition rate ranged from 1000 to 10 fi,/min at incidence angles of 0 to 80 °. Although it was attempted to deposit films of

Electron Beam for RHEED Incidence Beam / 9~. 2" .-...~ncidence Angle/ w4 p,ane

identical thickness, this could not be strictly achieved because the packing density of the iron film changed with the incidence angle and the thickness. Therefore the thickness ranged from 900 to 3000 ,~. On the other hand, the thickness of the samples evaporated at high oblique incidence angles was systematically altered from 250 to 3400 ,~ in order to examine the crystal growth during deposition. The evaporated thin films on the plastic films were used for direct observations of their microstructure. Observations were made on the film plane (A plane) and the cross-sectional planes (B and C planes, which are parallel and perpendicular to the incidence plane, respectively). These planes are illustrated in fig. 1. The A plane was observed after dipping the deposited film in a hydrazine ( H 2 N N H 2 • H 2 0 ) solution and then removing the plastic film. The samples for the observation of the cross-sectional B and C planes were prepared by slicing, the deposited film in a thickness of about 1000 A with an ultra-microtome. Direct observations of bright and dark field images were made on the A, B and C planes by T E M [9]. TEM was used not only for direct observations of the microstructure, but also for the analysis of the crystallographic structure using a selected area electron diffraction (SAED) method. The nature of the iron surface was examined using the R H E E D method, by which the crystallographic axis of the columnar structure can be determined. The electron beam for the R H E E D analysis was introduced in the direction as shown in fig. 1. The surfaces of these planes were also observed by scanning electron microscopy (SEM).

3. Results

/ B plane

Fig. 1. Illustration of the observed planes of a thin film evaporated at an oblique incident beam angle rp and the incidence direction of the electron beam for the RHEED analysis.

TEM micrographs of the A planes are shown in figs. 2a-d. The SAED pattern is inserted in the T E M micrograph. At incidence angles less than 40 ° , no special morphology was recognized and the iron film did not seem to be oxidized. At incidence angles of 55 ° and 60 ° elongated walls in the direction perpendicular to the incidence plane were recognized and a weak crystallographic

K. Ozawa et al. / Microstructure of oblique incidence iron films

(a)

(c)

So = 0 .

=55"

(b)

177

~ = /40*

(d) 9'= 75*

Fig. 2. TEM micrographs of the film planes of films deposited at several incidence angles. Diffraction patterns are also shown: (a) q~ = 0 °, aFe only; (b) q~ = 40*, aFe (unoriented); (c) qp = 55", aF¢ (slightly oriented), F¢304 (unoriented); (d) ¢# = 75 °, aFe (strongly oriented), Fe304 (unoriented).

orientation was detected (fig. 2c). The film was slightly oxidized and the oxide was analyzed to b e Fe304 by the SAED pattern. At incidence angles greater than 75 ° a colurrmar structure parallel to the incidence plane was recognized and habits perpendicular to the incidence beam appeared (fig. 2d). The SAED pattern of fig. 2d shows that the film was oriented crystallographically and was strongly oxidized to Fe304. Figs. 3a-e show TEM micrographs of the

cross-sectional B planes. The RHEED patterns and the analysis of the textural structure are also shown in these figures. Fig. 3a'-e' show the dark field images taken by TEM. At an incidence angle of 40 ° the dark field image showed a weakly oriented structure. The RHEED pattern, however, did not clearly show the orientation. At incidence angles greater than 55 ° it is recognizable that the grain growth occurred nearly in the direction of the incidence beam. At incidence angles of 75 and

K. Ozawa et aL / Microstructure of oblique incidence iron films

178

(a)

(b)

(c)

SO = 0 °

(9 = ~0 °

~

=

55 °

(a)

Dark Fie[d

(b)

(c )'

Image

K. Ozawa et a L / Microstructure of oblique incidence iron films

(d)

(e)

~=65

~ :

O

75 °

179

(d)'

(e)'

Fig. 3. The brigth and dark field images of the cross-sectional B planes of films deposited at several incidence angles. RHEED patterns of the thin film surfaces are also shown: (a) and (b) no special orientation; (c) ~ = 55 °, the pattern shows aFe and Fe304, the texture axis a t is mainly [111], a t [ i l l ] = ( t 0 + 5 ) °, at[001 ] = ( - 5 0 4 - 5 ) 0 ; (d) q0 ~ 65 °, aFe and Fe304, the texture axis is mainly [001], at[001 ] = (35+5)0; (¢) ~ = 75 °, aFe and Fe304, at[001 ] = (554-7) °.

80 ° the columnar growths became needle-like and had the habits at the top of the columns (fig. 3e). The dark field image for an the incidence angle of 75 ° revealed that some of the columns formed appeared to be single-crystals, but that the structure near the suhstrate was fine and polycrystalline. At an incidence angle of 55 ° the texture axis appeared to be a mixture of the [111] main axis and the [001] weak axis. At incidence angles greater than 65 ° the columnar axis approximately coincided with the texture axis which was oriented to the [001] axis.

Since it was recognized that the structure differed along the thickness direction of the film at incidence angles greater than 75 ° as shown in fig. 3e, the deposition time was varied at an incidence angle of 80 ° and the microstructures of films of different thicknesses were examined. Fig. 4 a - c show TEM micrographs of the cross-sectional B planes and the R H E E D patterns of films of several thicknesses at an incidence angle of 80 ° and a deposition rate of 80 /k/rain. Fig. 4c' shows the darkfield image of fig. 4c. For the film of thickness

180

K. Ozawa et al. / Microstructure of oblique incidence iron films

Ca)

(b)

(c)

(c)'

Fig. 4. TEM micrographs and RI-IEED patterns of the cross-sectional B planes of films of various thicknesses deposited at an incidence angle of 80°; (a) 250 A; (b) 1100/~; (c) 3400 A; (c)' the dark-field image of (c). RHEED pattern shows aFe and Fe~O4, and

a,[001] = (55 + 7)°.

of about 250 A the columnar structure was uncertain. However, the texture axis a t was found weakly in the direction of the [001] axis. The angle of a t is 66 ° (fig. 4a). With an increase of the deposition thickness, the columnar structure became clearer and the columnar angle % smaller. Films of thickness greater than 1000 A showed two-layered structures, The layer near the surface contained interstices and the column was strongly oriented. These results are summarized in table 1.

Observations of the surface using the replica technique were also attempted. Although the existence of columnar structures could be recognized b y this method also, no further information could be obtained. SEM observation of the iron thin film removed from the plastic substrate gave more information about the columnar structure than that obtained with the replica sample. Some examples are shown in fig. 5a and b. At an incidence angle of 55 ° the

K. Ozawa et al. / Microstructure of obfique incidence iron films

181

Table 1 The structures of films of various thicknesses evaporated at an incidence angle of 80 ° Film thickness (A)

Angle of the columnar axis as

Angle of the texture axis at[001]

Crystal quality

Structure

250

50 to 60 °

66 °

bad

one layer very fine no interstices

50 to 60 °

60 °

poor

upper layer: 45 °

60 °

good

58 °

good

two layers upper: 300 ,~; interstices lower: 100 .~; no interstices

58 °

very good

two layers upper: 300 .~; interstices lower: 100/~; no interstices

58 °

very good

two layers upper: 300 A; many interstices lower: 100 A; no interstices

800

1100

lower layer:

60 °

upper layer: 45 ° 1800 lower layer

60 °

upper layer: 40 ° 2300 lower layer:

60 °

upper layer: 400 3400 lower layer:

columns

appeared

60 °

like walls elongated

in the di-

rection perpendicular to the incidence plane. Fig. 5 b s h o w s t h a t a t a n i n c i d e n c e a n g l e o f 80 ° t h e t o p

(a)

So = 55"

(b)

one layer columnar width of 100 .g, no interstices two layers (weak) upper: columnar width of 300 A; a few interstices lower: columnar width of 100 A; no interstices

of the needle-like column had a trianglar morphology.

~=

80"

Fig. 5. SEM micrographs of the film planes of samples deposited at incidence angles of 55 and 80 °. The tilt angle of the samples is 0%

K. Ozawa et aL / Microstructure of oblique incidence iron films

182

4. Discussion

un~[

In our work the c o l u m n a r structure was clearly recognized in films deposited at incidence angles greater than 55 ° . At incidence angles of 55 and 65 ° the columns appear like walls elongated in the direction perpendicular to the incidence plane. At incidence angles of 75 and 80 ° the columns become needle-like. However, even at an incidence angle of 80 ° a very weak elongated wall structure can be recognized. There are interstices between these columns or elongated walls. The higher the incidence angle, the more interstitial space is contained as shown in fig. 3a-e. This fact is also confirmed by measurements of the saturation magnetization of the film which are shown in fig. 6, where Ms(cp ) is the saturation magnetization of a thin film evaporated at the incidence angle tp and M s is the bulk value for iron ( M s = 1710 emu/cm3). At an incidence angle of 80 ° the saturation magnetization of the thin film is less than a quarter of that of bulk iron. Decrease of the saturation magnetization is also caused by oxidation of the iron film. As already mentioned, the thin film is slightly oxidized to Fe304. In the case of thick films deposited at high incidence angles it was recognized that Fe304 was sometimes formed around the colunln.

I

I

I

I

0.8

t~

0.6

1[ \ A

0.4

~r

0.2 0

0

I

I

I

I

20

40

60

80

Incidence Angle (deg.)

Fig. 6. The effectivespontaneous magnetizationM~(~) of the iron thin films plotted against the incidence angle. M~ is the bulk value (1710 emu/cm3).

a

5o= 0*

b

tl00]

9'= 55*

1"1003 I [lOxO] I

c

T = 65*

d

So= 80*

Fig. 7. Schematic views of the cross-sectional planes of iron thin films evaporatedat various incidenceangles. Fig. 7 schematically shows the microstructures of the iron thin films evaporated at several incidence angles. The texture axis changes from the [111] axis to the [001] axis with the increase of the incidence angle. However, these two crystallographic axes should be understood to be mixed. At incidence angles of 75 and 80 ° some of the columns appear to be single-crystals and the direction of easy growth nearly corresponds to the crystallographic axis. It appears that the direction of growth is determined by the self-shadowing effect and the growth rate. The relation of the columnar axis a, and the texture axis a t to the incidence angle is shown in fig. 8. Nieuwenhuizen and Haanstra first found that the relation of the columnar axis could be well described by 2 tan % ffi tan ¢p, which is known a s the "tangent rule" [10]. Fig. 8 shows that iron films do not follow the tangent rulei This result suggests that the column formation occurs not only by the geometric shadowing effect postulated by KSnig and Helwig [11], but also by the "evolutionary selection" of specific favored growth orientation pointed out by Van der Drift [12]. Although the rnicrostructure of the iron films

K. Ozawa et al. / Microstructure of oblique incidence iron films i

i

I

183

I

~80 A

~

ett

[1111

Inci

/

ence

Beam

60

ca

b

c

d

r,- I ~ 0

~

,40

..6-620

o t- t'¢'~

0 0

40

20

60

80

Incidence Angle ~ (deg. I

Fig. 8. The angles of the columnar axis and the texture axis plotted against the incidence angle.

does not follow the tangent rule, the packing density of the film should be influenced by the angle of the columnar structure if the material is one in which column formation occurs. Dirks and Leamy proposed the relationship that the packing ratio p ( ~ p ) / p ( O ) is proportional to 1 - A tan a s, where p(q0) is the packing density of the film at incidence angle q0 and A is a constant [13]. Assuming that the magnetization saturation Ms(q0) is proportional to p(~), M s ( ~ p ) / M s ( O ) = 1 - A tan a s is obtained. This relationship provides a reasonable description of the micro-structure as shown in fig. 9. 1.0

'

'

'

'

l

0.8

~o0.6 o XOA 0.2 0

~ 0

,

,

,

I ' 0.5

, tan

,

J

,

I 1.0

gs

Fig. 9. The packing density of iron thin films plotted against tan as. The data illustrate the fit with M s ( ~ ) / M s ( o ) = l A t a n a s.

e Fig. 10. Illustrations of the crystal growth during deposition at an incidence angle of 800 .

At the high incidence angles of 75 and 80 ° the thin film shows a two-layered structure. The cross-sectional micrographs of films of various thicknesses shown in fig. 4 suggest the following sequence of the growth of the iron film during deposition. (1) In the fist stage of deposition the crystallographic grain is very small as shown in fig. 10a. However, a blurred crystallographic orientation already exists. (2) Up to a thickness of 1000 A, the lower layer is formed as shown in fig. 10b, and on continued deposition an upper layer consisting of triangular cones as well as columns begins to grow as shown in fig. 10c. (3) Because of the growth of these columns and triangular cones, in the upper layer the growth of the columnar structures of the lower layer are sometimes disturbed by the shadowing effect as shown in fig. 10d.

184

K. Ozawa et aL / Microstructure of oblique incidence iron films

(4) Several triangular cones continue to grow and become triangular columns as shown in fig. 10e. The growth of the columns appears to be explained b y the shadowing effect and the diffusion of adatoms. The triangular cone appears to have special crystallographic morphology. However, individual covered planes of the triangular cones could not be clearly determined because the columns were too small to be analyzed using an ordinary electron beam.

5. Conclusions The columnar structure of iron thin films evaporated at oblique incidence angles was directly observed b y T E M and the nature of the surface was analyzed using the R H E E D method. T h e process of crystal growth during the deposition was also investigated and it was revealed that some of the columns have a texture axis and a crystallographic morphology. Whereas at incidence angles less than 40 ° the thin film did not show any orientation, it was observed by the diffraction pattern of the cross-section that the microstructure had a very weak crystallographic orientation at an incidence angle of 40 ° . At incidence angles of 55 and 65 ° an elongated wall structure was recognized in the direction perpendicular to the incidence plane. At incidence angles of 75 and 80 ° it was recognized that the structure differs along the thickness direction of the film and the column is approximately oriented to the [001] axis. A l t h o u g h the c o l u m n a r angle a s does not follow the tangent rule 2 tan a s = tan ~, the packing density of the film is explained by the columnar angle. T h e interstitial space between these columns increases with the increase of incidence angle. It was observed that the surface of the columns was sometimes covered with F e 3 0 4.

The structure which we have investigated well explains the magnetic properties and the anisotropy of electrical resistance of the evaporated iron thin films [14].

Acknowledgments The authors wish to express their special thanks to Dr. N. T a m a g a w a of Sony Corp. for his valuable advice. They are also indebted to Mr. K. Yazawa of Sony Corp. for experimental assistance and useful discussions and to Mr. M. Sato of T o h o k u University of preperation of the samples.

References [1] G. Bate, J. Appl. Phys. 52 (1981) 2447. [2] D.E. Speliotis and C.S. Chi, IEEE Trans. Magn. MAG-13 (1977) 1287. [3] Y. Maezawa, M. Katao, H. Hibino, M. Odagiri and K. Shinohara, Proc. IERE 54 (1982) 1. [4] D.E. Speliotis, G. Bate, J.K. Alsted and J.R. Morrison, J. Appl. Phys. 36 (1965) 972. [5] R.H. Welch, Jr. and D.E. Speliotis, J. Appl. Phys. 41 (1970) 1254. [6] K. Okamoto, T. Hashimoto, K. Hara and E. Tatsumoto, J. Phys. Soc. Japan 31 (1971) 1374. [7] K. Hara, H. Fujiwara, T. Hashimoto, K. Okamoto and T. Hashimoto, J. Phys. Soc. Japan 39 (1975) 1252. [8] K. Ozawa and T. Eguchi, Recent Developments in Tape Technology for the Digital VTR (submitted to the SMPTE Study Group of Digital Television Tape Recording, 7 Jan. 1982, Miami, Florida). [9] P.B. Hirseh, A. Howie, R.B. Nicholson, D.W. Pashley and M.L Whelan, Electron Microscopy of Thin Crystals (Butterworth, London, 1965) p. 295. [10] J.M. Nieuwenhuizen and H.B. Haarnstra, Philips Tech. Rev. 27 (1966) 87. [11] H. Krnig and G. Helwig, Optik 6 (1950) 111. [12] A. van der Drift, Philips Res. Rep. 22 (1967) 267. [13] A.G. Dirks and H.J. Leamy, Thin Solid Films 47 (1977) 219. [14] K. Ozawa, T. Yanada, H. Masuya, M. Sato, S. Ishio and M. Takahashi, J. Magn. Magn. Mat. 35 (1983) 289.