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Columnar growth in evaporated iron films
296
Journal of Magnetism and Magnetic Materials 35 (1983) 296-298 North-Holland Publishing Company COLUMNAR GROWTH IN EVAPORATED IRON FILMS H. F U J I W A R A , K. H A R A *, M. K A M I Y A *, T. H A S H I M O T O ** a n d K. O K A M O T O *** Department of Materials Science, Faculty of Science, Hiroshima University, Hiroshima 730, Japan
The columnar grain structure of iron films evaporated in the pressure range between 10 - 2 and 10-8 Torr was investigated. From electron micrographs, measurements of extinction coefficient and deviation angle of easy axis from the film plane, it was found that the movement of adatoms plays an important role for the columnar formation.
1. Introduction For the last ten years, we have been investigating the columnar growth of evaporated films from both magnetic and crystallographic observations using obliquely deposited magnetic films. Our accumulated data have revealed that the inclination angle of columnar grains, which is one of the most important characteristics of the columnar grain structure, depends on the preparatory conditions such as the substrate temperature [1] and the deposition rate [2], and we have pointed out [3] that the movement of adatoms plays an important role in the determination of the growth direction of columnar grains. For the purpose of getting definite information, the present paper gives for iron films the dependence of the columnar grain structure on the pressure during evaporation. And the movement of adatoms is discussed in connection with not only the inclination angle but also the formation of the bundling of the columnar grains, which is another important charactristic of the columnar grain structure. 2. Experimental The films were deposited at an incidence anlge of 45 ° in the pressure range between 10 -2 and 10 - s Torr. For the pressures higher than 3 x 10 -5 Torr, the most common type of vacuum system consisting of rotary and diffusion pumps was employed (referred to as system I hereafter). On the other hand, the system for the pressures lower than 2 x 10 -5 Torr consisted of sorption, getter and ion pumps (system II). The control of pressure during evaporation was carried out by opening and closing the main vacuum valve. The evaporation sources of the systems I and II were resistance heated * Faculty of General Education, Kumamoto University, Kumamoto 860, Japan. ** Faculty of Education, Tottori University, Tottori 680, Japan. *** College of Arts and Sciences, Chiba University, Chiba 260, Japan.
and electron bombarded sources, respectively. The columnar grains have been observed by replica electron microscopy. The cross section of the film has been exposed so far by breaking the substrate together with the film [4,5]. In the present work, the exposition was made not only by our usual way but also by a different method, which was newly introduced. Without breaking the film, the deposition in the new method was made all over the surface of the substrate, and the side face of the film thus prepared was observed at the edge of the substrate. The degree of the columnar growth was estimated by measuring magnetically the deviation angle of the easy direction off from the film plane in the incidence plane. The anisotropy of the extinction coefficient was obtained by ellipsometry [6] and used as a measure of the bundling of columnar grains. The optical measurement was made by means of an ellipsometer with a sensitivity of 0.01 °. A H e - N e gas laser was used as a light source. 3. Results mad discussion Fig. 1 shows electron micrographs of the films prepared in pressures of (a) 3 x 10 -5 and (b) 1 x 10 -3 Torr. The substrate temperature of both the films was 200°C and the deposition rate was about 500 .~/min. The cross section of the film, which is parallel to the incidence plane, was obtained in the usual way. The inclination angle a c in (a) is 37 ° and is larger than that in (b). As is evident from the figure, the bundling of colunmar grains in (b) is remarkable in comparison with that in (a). In fig. 2, the angle ac is plotted as a function of pressure P for the films prepared in system I. With decreasing P, the angle a c increases and approachs to the incidence angle of 45 °, which is represented by a dotted line. Fig. 3 gives as a function of P the deviation angle fl of the easy direction from the film plane towards the vapor beam direction. Since the angle reflects the shape anisotropy of the columnar grains [7], it gives the degree of the columnar growth. The fl value decreases with decreasing P, which indicates that the gap between the
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
H. Fujiwara et al. / Columnar growth in Fe films
297
IO
J 0
-O 10-7
.
{3
10-6 .
.-O "0" . , 10-5 10"4 P (Torr)
.
, 10-3
.
, IO't
Fig. 3. The angle/~ plotted as a function of P. Fig. I. Electron micro~aphs of the films prepared in (a) 3 × 10-5 and (b) 1 × 10-3 Torr.
columnar grains decreases with decreasing P. For the films prepared in system II, the magnitude of/3 with negative sign is very small. Considering the small magnitude of .fl, the columnar structure should be a densely packed one. In this case, it was difficult to observe the geometric structure by the usual way. This difficulty could be removed by introducing the new method. Preliminary electron micrographs observed at the edges of the substrate are shown in fig. 4. The edges in (a) and (b) are parallel and perpendicular to the incidence plane, respectively. The pressure was 1 × 10 -5 Torr. From the figure, it might safely be concluded that there are columnar grains which elongate towards the vapor beam direction. Previously [3], we have pointed out that the atoms adsorbed on the top surface of the columnar grain move in the direction defined by the projection of the vapor beam direction onto the surface and tilt the growth direction of the columnar grain from the vapor beam direction towards the film normal. As described above, however, the growth direction approachs the vapor beam direction with decreasing P. With respect to the movement of adatoms at low pressures, there are two possibilities. One is that the mobility of adatoms is small compared with that in high pressures. The other is that many atoms move independently of the vapor beam direction via surface diffusion and the movement of
adatoms is neglected. Judging from the fact that the packing of columnar grains becomes dense with decreasing P, the latter possibility should be accepted. When the normal of the top surface of the columnar grain is not parallel to the incidence plane, the adatoms move also in the direction perpendicular to the incidence plane. Such a movement induces the bundling of columnar grains. For the explanation of the shape of crystaUites in an early stage of growth, a similar movement has already been taken into consideration [8]. As seen in figs. 2 and 3, the effect of the movement of adatoms becomes larger with increasing P. Therefore, it is expected that the degree of the bundling becomes large with increasing P. The remarkable bundling in fig. lb supports this expectation. In order to express the degree of the bundling quantitatively, the anisotropy of the extinction coefficient k was measured. In fig. 5 is plotted A k / k as a function of P, where Ak is the difference between the extinction coefficients for the directions perpendicular and parallel to the incidence plane. Since the extinction coefficient increases with increasing packing density [9], the ratio A k / k expresses the anisotropy of the packing density, that is, the degree of the bundling perpendicular to the incidence plane. As seen in the figure, A k / k increases with the increase in P
50
~" 25 -
o o
*
I0-6
I
=
10-4
I
10-3
I
I
10-2
P (Torr)
Fig. 2. The angle a c plotted as a function of P.
Fig. 4. Electron micrographs observed at the edges of the substrate (a) parallel and (b) perpendicular to the incidence plane.
H. Fujiwaraet al. / Columnar growth in Fe films
298 08
/
"° °
o
.o---7-9----T',O"o
10-7
10-6
-
'
,
i0-II 10-4 P (Torr)
.
,,
.,
10.3
iO-I
Fig. 5. The ratio A k / k plotted as a function of P. and the dependence is quite similar to that of ft. This seems to indicate that the bundling of columnar grains is attributable to the movement of adatoms. In the low pressure region, it has been considered that the surface diffusion makes the movement of adatoms ineffective. Provided that we could restrain the surface diffusion, the movement of adatoms becomes remarmable. The substrate temperature Ts was reduced on trial. With reducing Ts, the angle fl and the ratio A k / k increase sharply from about Ts of 100°C and take the values of 2 ° and 0.10 at Ts of 50°C, indicating that there is a similar columnar structure to that observed at high pressure. For visible confirmation, fig. 6 shows electron micrographs of the film prepared at 50°C. The pressure was 8 X 10 - s Torr and the deposition rate was 160 .~/min. The cross sections in (a) and (b) are parallel and perpendicular to the incidence plane, respectively. To sum up, it is concluded that the movement of adatoms plays an important role in the formation of the columnar grain structure. References [I] T. Hashimoto, K. Okamoto, K. Hara, M. Kamiya and H. Fujiwara, Thin Solid Films 91 (1982) 145.
Fig. 6. Electron micrographs of the film prepared at 50°C. The cross sections in (a) and (b) are parallel and perpendicular to the incidence plane, respectively.
[2] T. Shigeoka, K. Hara, M. Kamiya, T. Hashimoto and H. Fujiwara, Japan. J. Appl. Phys. 19 (1980) 995. [3] T. Hashimoto, K. Hara, K. Okamoto and H. Fujiwara, J. Phys. Soc. Japan 41 (1976) 1433. [4] T. Hashimoto, K. Hara and E. Tatsumoto, J. Phys. Soc. Japan 24 (1968) 1400. [5] K. Okamoto, T. Hashimoto, K. Hara and E. Tatsumoto, J. Phys. Soc. Japan 31 (1971) 1374. [6] M. Kamiya, Mem. Fac. Gen. Educ. Kumamoto Univ. Ser. Nat. 17 (1982) 23. [7] K. Okamoto, K. Hara, H. Fujiwara and T. Hashimoto, J. Phys. So¢. Japan 40 (1976) 293. [8] J.G.W. van der Waterbeemd and G.W. van Oosterhout, Philips Res. Rep. 22 (1967) 375. [9] J.C. Maxwell Gamett, Phil. Trans. 203 (1904) 385, 205 (1906) 237.
Journal of Magnetism and Magnetic Materials 35 (1983) 296-298 North-Holland Publishing Company COLUMNAR GROWTH IN EVAPORATED IRON FILMS H. F U J I W A R A , K. H A R A *, M. K A M I Y A *, T. H A S H I M O T O ** a n d K. O K A M O T O *** Department of Materials Science, Faculty of Science, Hiroshima University, Hiroshima 730, Japan
The columnar grain structure of iron films evaporated in the pressure range between 10 - 2 and 10-8 Torr was investigated. From electron micrographs, measurements of extinction coefficient and deviation angle of easy axis from the film plane, it was found that the movement of adatoms plays an important role for the columnar formation.
1. Introduction For the last ten years, we have been investigating the columnar growth of evaporated films from both magnetic and crystallographic observations using obliquely deposited magnetic films. Our accumulated data have revealed that the inclination angle of columnar grains, which is one of the most important characteristics of the columnar grain structure, depends on the preparatory conditions such as the substrate temperature [1] and the deposition rate [2], and we have pointed out [3] that the movement of adatoms plays an important role in the determination of the growth direction of columnar grains. For the purpose of getting definite information, the present paper gives for iron films the dependence of the columnar grain structure on the pressure during evaporation. And the movement of adatoms is discussed in connection with not only the inclination angle but also the formation of the bundling of the columnar grains, which is another important charactristic of the columnar grain structure. 2. Experimental The films were deposited at an incidence anlge of 45 ° in the pressure range between 10 -2 and 10 - s Torr. For the pressures higher than 3 x 10 -5 Torr, the most common type of vacuum system consisting of rotary and diffusion pumps was employed (referred to as system I hereafter). On the other hand, the system for the pressures lower than 2 x 10 -5 Torr consisted of sorption, getter and ion pumps (system II). The control of pressure during evaporation was carried out by opening and closing the main vacuum valve. The evaporation sources of the systems I and II were resistance heated * Faculty of General Education, Kumamoto University, Kumamoto 860, Japan. ** Faculty of Education, Tottori University, Tottori 680, Japan. *** College of Arts and Sciences, Chiba University, Chiba 260, Japan.
and electron bombarded sources, respectively. The columnar grains have been observed by replica electron microscopy. The cross section of the film has been exposed so far by breaking the substrate together with the film [4,5]. In the present work, the exposition was made not only by our usual way but also by a different method, which was newly introduced. Without breaking the film, the deposition in the new method was made all over the surface of the substrate, and the side face of the film thus prepared was observed at the edge of the substrate. The degree of the columnar growth was estimated by measuring magnetically the deviation angle of the easy direction off from the film plane in the incidence plane. The anisotropy of the extinction coefficient was obtained by ellipsometry [6] and used as a measure of the bundling of columnar grains. The optical measurement was made by means of an ellipsometer with a sensitivity of 0.01 °. A H e - N e gas laser was used as a light source. 3. Results mad discussion Fig. 1 shows electron micrographs of the films prepared in pressures of (a) 3 x 10 -5 and (b) 1 x 10 -3 Torr. The substrate temperature of both the films was 200°C and the deposition rate was about 500 .~/min. The cross section of the film, which is parallel to the incidence plane, was obtained in the usual way. The inclination angle a c in (a) is 37 ° and is larger than that in (b). As is evident from the figure, the bundling of colunmar grains in (b) is remarkable in comparison with that in (a). In fig. 2, the angle ac is plotted as a function of pressure P for the films prepared in system I. With decreasing P, the angle a c increases and approachs to the incidence angle of 45 °, which is represented by a dotted line. Fig. 3 gives as a function of P the deviation angle fl of the easy direction from the film plane towards the vapor beam direction. Since the angle reflects the shape anisotropy of the columnar grains [7], it gives the degree of the columnar growth. The fl value decreases with decreasing P, which indicates that the gap between the
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
H. Fujiwara et al. / Columnar growth in Fe films
297
IO
J 0
-O 10-7
.
{3
10-6 .
.-O "0" . , 10-5 10"4 P (Torr)
.
, 10-3
.
, IO't
Fig. 3. The angle/~ plotted as a function of P. Fig. I. Electron micro~aphs of the films prepared in (a) 3 × 10-5 and (b) 1 × 10-3 Torr.
columnar grains decreases with decreasing P. For the films prepared in system II, the magnitude of/3 with negative sign is very small. Considering the small magnitude of .fl, the columnar structure should be a densely packed one. In this case, it was difficult to observe the geometric structure by the usual way. This difficulty could be removed by introducing the new method. Preliminary electron micrographs observed at the edges of the substrate are shown in fig. 4. The edges in (a) and (b) are parallel and perpendicular to the incidence plane, respectively. The pressure was 1 × 10 -5 Torr. From the figure, it might safely be concluded that there are columnar grains which elongate towards the vapor beam direction. Previously [3], we have pointed out that the atoms adsorbed on the top surface of the columnar grain move in the direction defined by the projection of the vapor beam direction onto the surface and tilt the growth direction of the columnar grain from the vapor beam direction towards the film normal. As described above, however, the growth direction approachs the vapor beam direction with decreasing P. With respect to the movement of adatoms at low pressures, there are two possibilities. One is that the mobility of adatoms is small compared with that in high pressures. The other is that many atoms move independently of the vapor beam direction via surface diffusion and the movement of
adatoms is neglected. Judging from the fact that the packing of columnar grains becomes dense with decreasing P, the latter possibility should be accepted. When the normal of the top surface of the columnar grain is not parallel to the incidence plane, the adatoms move also in the direction perpendicular to the incidence plane. Such a movement induces the bundling of columnar grains. For the explanation of the shape of crystaUites in an early stage of growth, a similar movement has already been taken into consideration [8]. As seen in figs. 2 and 3, the effect of the movement of adatoms becomes larger with increasing P. Therefore, it is expected that the degree of the bundling becomes large with increasing P. The remarkable bundling in fig. lb supports this expectation. In order to express the degree of the bundling quantitatively, the anisotropy of the extinction coefficient k was measured. In fig. 5 is plotted A k / k as a function of P, where Ak is the difference between the extinction coefficients for the directions perpendicular and parallel to the incidence plane. Since the extinction coefficient increases with increasing packing density [9], the ratio A k / k expresses the anisotropy of the packing density, that is, the degree of the bundling perpendicular to the incidence plane. As seen in the figure, A k / k increases with the increase in P
50
~" 25 -
o o
*
I0-6
I
=
10-4
I
10-3
I
I
10-2
P (Torr)
Fig. 2. The angle a c plotted as a function of P.
Fig. 4. Electron micrographs observed at the edges of the substrate (a) parallel and (b) perpendicular to the incidence plane.
H. Fujiwaraet al. / Columnar growth in Fe films
298 08
/
"° °
o
.o---7-9----T',O"o
10-7
10-6
-
'
,
i0-II 10-4 P (Torr)
.
,,
.,
10.3
iO-I
Fig. 5. The ratio A k / k plotted as a function of P. and the dependence is quite similar to that of ft. This seems to indicate that the bundling of columnar grains is attributable to the movement of adatoms. In the low pressure region, it has been considered that the surface diffusion makes the movement of adatoms ineffective. Provided that we could restrain the surface diffusion, the movement of adatoms becomes remarmable. The substrate temperature Ts was reduced on trial. With reducing Ts, the angle fl and the ratio A k / k increase sharply from about Ts of 100°C and take the values of 2 ° and 0.10 at Ts of 50°C, indicating that there is a similar columnar structure to that observed at high pressure. For visible confirmation, fig. 6 shows electron micrographs of the film prepared at 50°C. The pressure was 8 X 10 - s Torr and the deposition rate was 160 .~/min. The cross sections in (a) and (b) are parallel and perpendicular to the incidence plane, respectively. To sum up, it is concluded that the movement of adatoms plays an important role in the formation of the columnar grain structure. References [I] T. Hashimoto, K. Okamoto, K. Hara, M. Kamiya and H. Fujiwara, Thin Solid Films 91 (1982) 145.
Fig. 6. Electron micrographs of the film prepared at 50°C. The cross sections in (a) and (b) are parallel and perpendicular to the incidence plane, respectively.
[2] T. Shigeoka, K. Hara, M. Kamiya, T. Hashimoto and H. Fujiwara, Japan. J. Appl. Phys. 19 (1980) 995. [3] T. Hashimoto, K. Hara, K. Okamoto and H. Fujiwara, J. Phys. Soc. Japan 41 (1976) 1433. [4] T. Hashimoto, K. Hara and E. Tatsumoto, J. Phys. Soc. Japan 24 (1968) 1400. [5] K. Okamoto, T. Hashimoto, K. Hara and E. Tatsumoto, J. Phys. Soc. Japan 31 (1971) 1374. [6] M. Kamiya, Mem. Fac. Gen. Educ. Kumamoto Univ. Ser. Nat. 17 (1982) 23. [7] K. Okamoto, K. Hara, H. Fujiwara and T. Hashimoto, J. Phys. So¢. Japan 40 (1976) 293. [8] J.G.W. van der Waterbeemd and G.W. van Oosterhout, Philips Res. Rep. 22 (1967) 375. [9] J.C. Maxwell Gamett, Phil. Trans. 203 (1904) 385, 205 (1906) 237.