- Email: [email protected]
Morphological stability of AgAu multilayer on Si
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applied
‘.
surface science ELSEVIER
Applied Surface Science 113/l
14 (1997) 768-772
Morphological stability of Ag-Au multilayer on Si C.R. Chen, Y.C. Peng, L.J. Chen
*
Department of Materials Science and Engineering, National Tsing Hau Uniuersi& Hsinchu, Taiwan, ROC
Abstract The morphological stability of Ag-Au transmission
multilayers on Si has been studied by conventional and high-resolution X-ray diffraction and Auger electron spectroscopy. In samples annealed at 300 and 4OO”C,
electron microscopy,
multilayered Ag-Au films form a continuous textured layer on Si. In samples annealed at 5OO”C,pinholes were found to develop in the Ag-Au films. Island structures were formed in samples annealed at 600°C. The improved stability is attributed to the intermixing of Au and Ag so that the (Au-Ag)/Si interface is more stable than that of either the Au/% or Ag/Si interface. PAC.? 68.55.Jk;
61.16.Bg;
68.65.+
g
1. Introduction
a result, Ag tends to form island structure
The morphological stability of the metal-semiconductor interface is of great importance for the design and the application of layered structures. In spite of the fact that both Au/Si and Ag/Si systems have been the subjects of extensive studies [l-4], little attention has been directed towards the deposition of Ag-Au alloys onto silicon. The binary phase diagram shows that the eutectic point is low (363°C) for the Au/% system. As the annealing temperature is higher than the Au/Si eutectic point, Au grains were found to agglomerate on (0Ol)Si. The films became very uneven in thickness. A high density of pinholes was observed to be present in thin regions. After high temperature annealing, Au islands were formed on Si [2]. On the other hand, for the Ag/Si system, the adhesion for Ag on Si is rather poor. As
on silicon at a temperature as low as 200°C [1,3,4]. In addition, the electromigration resistance for Ag is very low. The morphological stability can be improved by the deposition of a thin Au layer (about 3-nm-thick) at the interface of Ag/Si [4]. To overcome the problems encountered for both Au/Si and Ag/Si systems, a mixture of Ag and Au was thought to be a good alternative to replace either Au or Ag. The eutectic point of Au-Ag/Si is higher than the Au/Si [5]; the adhesion ability to Si for the Au-Ag film is higher than for the Ag film; and the electromigration resistance of Au-Ag alloy is higher than for Ag [6]. The purpose of this work is to investigate the morphological stability of Ag-Au multilayers on Si.
2. Experimental * Corresponding author. Tel. + 886-3-573 1166; fax: + 886-35718328; e-mail: [email protected]. 0169-4332/97/$17,00 Copyright f’II SO169-4332(96)00821-S
Single-crystal, 2-inch-diameter n-type (OOl)-oriented silicon wafers with a resistivity of I-10 R cm
0 1997 Elsevier Science B.V. All rights reserved.
CR. Chen et al./Applied
Sur$ace Science 113/ 114 (1997) 768-772
were used in the present study. The wafers were cleaned chemically by a standard procedure. The samples were then dipped in a dilute HF solution (HF:H,O = 150) immediately before being loaded into an ultrahigh vacuum (UHV) dual electron beam evaporation chamber with a base pressure of 1 X lo-” Torr. The wafers were first heated to 900°C for 10 min to remove the native oxide on the silicon surface (Shiraki clean). Six periods of Au(3 nm)Ag(3 nm) layers were deposited with the Au layer directly on top of (0Ol)Si. The deposition rate was 0.06 rim/s.. Heat treatments were performed in a three-zone diffusion furnace at 300-600°C in N, ambient. High-purity nitrogen gas was passed through a titanium getter tube maintained at 800°C to minimize the oxygen content in the ambient gas. The titanium getter was used as a sacrificial purifier. The accuracy in the temperature measurement was estimated to be within +2”C. Unless otherwise mentioned, the annealing time at each temperature was 1 h. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used for structural analysis. Auger electron spectroscopy (AES) with ion beam sputtering was utilized to determine the depth profile of the chemical composition. Both planview and cross-sectional TEM samples were prepared and examined. A JEOL-200CX scanning transmission EM operating at 200 kV was used for TEM examinations. High-resolution TEM (HRTEM) imaging was carried out in a JEOL 4000EX electron microscope with a point-to-point resolution of 0.18 nm. No objective aperture was inserted into the beam path during imaging. Most of the cross-sectional TEM micrographs were taken along the [l lo] zone axis of single-crystal Si substrate.
Fig. 1. XTEM. bright field (BF) image of an as-deposited Au) X 6/(001 )Si sample.
ture Au,Si confirmed in Fig. 3. 2b, the d (022) and
(Ag-
phase [7]. The presence of Au,Si was by the analysis of XRD spectrum as shown From the HRTEM image, shown in Fig. spacings of 0.248 and 0.210 nm match (222) Au,Si planes, respectively. The
3. Results and discussion Fig. 1 shows a XTEM image of an as-deposited sample. The Ag-Au interfaces are seen to be rather flat. Fig. 2a and b show the planview diffraction pattern and high-resolution cross-sectional image of an as-deposited sample, respectively. The appearance of an additional ring located inside the {200} ring of the Ag-Au structure, as shown in Fig. 2a, is attributed to the formation of an orthorhombic struc-
Fig. 2. (a) Diffraction as-deposited (Ag-Au) respectively.
pattern (DP) and (b) HRTEM image of an X 6/(001)Si, planview and cross-sectional,
770
CR. Chen et al./Applied
Surface Science 113/ 114 (1997) 768-772
20 Fig. 3. XRD spectrum sample.
of an as-deposited
(Ag-Au)X
6/(OOl)Si
measured intersecting angle of 82” is consistent with the interplanar angle of these planes. In samples annealed at 300 and 400°C multilayered Ag-Au films became a continuous textured layer on Si. Au and Ag atomes were found to intermix almost completely. Fig. 4a and b show the AES depth profiles of as-deposited and annealed samples, respectively. For an as-deposited sample the ripples in composition modulation are clearly seen. In annealed samples, the Au and Ag components are much less distinguishable. The AuAg/(OOl)Si interface is very sharp. An example is shown in Fig. 5. Fig. 6a and b show the SEM images of samples annealed at 500 and 600°C, respectively. In samples annealed at 500°C pinholes were found to develop in the Ag-Au films. Island structure was formed in samples annealed at 600°C. The epitaxial relationship for the multilayered Ag-Au films on (0Ol)Si was found to be different
Fig. 5. XTEM. BF image of (Ag-Au)x6/(OOl)Si nealed at 400°C for I h, cross-sectional.
sample
an-
from that of either Au or Ag on (0Ol)Si. For the Au/(OOl)Si system, a diffraction ring pattern is seen as shown in Fig. 7a. Very strong {220} Au rings and rather weak (111) and {002} Au rings are seen in the diffraction pattern suggesting that Au grains were largely oriented in the [ 1111Au direction close to the normal to the surface, i.e., the gold grains underwent a textured growth on silicon with [l 1l]Au//[OOl]Si. For the Ag/Si(OOl) system, diffraction pattern anal-
75
8 ‘;13 50 b 8 25 w
Sputter time (min) Fig. 4. AES depth profiles of (a) as-deposited 400°C samples.
and (b) annealed at
Fig. 6. SEM images of (Ag-Au) (a) 500 and (b) 600°C.
x 6/(001 )Si samples annealed at
C.R. Chen et aI./Applied
771
Surface Science 113/ 114 flY971 768-772
the Au,Si phase near the (Ag-AuI/Si interface. From the diffraction pattern of Fig. 2a, it is seen that the textured (022) Au,Si plane is parallel to (1OO)Si rather than the (220&i plane. It suggests that Au,Si serves as a seed layer for the growth of (Ag-Au) films rotated by 15” along the [l 111 axis. The lattice mismatch between the (220)Ag-Au plane and (4OO)Si plane is 6% which is much smaller than the mismatch of 25% between (220)Ag-Au plane and (22O)Si plane. The morphological stability of Ag-Au muitilayers on Si was found to be much improved compared to those of either Au/Si or Ag/Si system [8]. The eutectic point of Au-Ag/Si system is higher than that of the Au/Si system [5]. The temperature for forming island structures was increased to about 600°C. In addition, the adhesion of the Ag-Au films on Si was also found to be superior to that of the Ag films on silicon, Au thin layer can act as a adhesion layer in the Ag-Au multilayer in as-deposited samples. In samples annealed at higher temperature, the
Fig. 7. DP of as-deposited samples.
(a) Au/(OOl)Si
and (b) Ag/(OOl)Si
ysis of planview samples revealed also the almost completely epitaxial growth of Ag thin films with [lll]Ag//[OOl]Si as shown in Fig. 7b. The bright spots marked in the micrograph indicate that many Ag grains were oriented epitaxially with [ll l]Ag//[OOl]Si and (220)Ag//(220)Si. In the Ag-Au/Si(OOl) system, textured growth of Ag-Au thin films with [ 11 l]Ag-Au/[OOl]Si was also found. But compared to the Ag/(OOl)Si system, the bright (220)Ag-Au spots are rotated by 15” along the [l 1 I] axis from (22O)Si to (OlONi, i.e., [2jO](AgAu)/[lOO]Si, as shown in Fig. 8. For XTEM samples, no epitaxial Ag-Au spots were found in the diffraction pattern along the [ 1 lo]-orientation. For samples viewed along the [ lOO]-direction revealed the epitaxial relationship of [liO]Ag-Au/[ lOO]Si. The diffraction pattern viewed from Si[lOO] direction is shown in Fig. 9. The growth of the epitaxial multilayered Ag-Au layer rotated by 15” is attributed to the presence of
Fig. 8. DP of a planview (Ag-Au)X at 500°C. (b) Indexed pattern of (a).
6/(OOl)Si
sample annealed
772
C.R. Chen et al./Applied
(b)
Si(O22)
.O
h’
l F
t-b
0 y.
Surface Science 113/114
0
Au-Ag( 1il)
0
*. o ~
The research was supported by the Republic of China National Science Council through a grant No. NSC84-2215E007-011.
0 0
t3
0.
Fig. 9. (a) DP of a cross-section of the (Ag-Au)X sample annealed at 500°C. (b) Indexed pattern of (a).
were found to develop in the Ag-Au films. Island structure was formed in samples annealed at 600°C. The epitaxial relationship for the multilayered Ag-Au on (0Ol)Si is different from those of Au and Ag on (001)Si. In the Ag-Au/(OOl)Si system, textured growth of Ag-Au thin films with [I 1l]AgAu/[OOl]Si was also found. Compared with the Ag/Si(OOl) system, (220)Ag-Au is rotated by 15” along the [l 1 l]Ag-Au axis from (22O)Si to (OlOlSi, i.e., [2ZOXAg-Au)/[lOO]Si. The rotation by 15” of the Ag-Au multilayer epitaxy is attributed to the presence of Au,Si phase near the Ag-Au/Si interface in which Au,Si serves as a seed layer for the growth of Ag-Au films.
Acknowledgements
Si(022) 0 0
(19971 768-772
6/(001&i
References Ag-Au alloy also possessed good adhesion ability in contact with Si [9].
4. Summary and conclusions The morphological stability of Ag-Au multilayers on Si has been studied by both conventional and high-resolution transmission electron microscopy, X-ray diffraction and Auger electron spectroscopy. In samples annealed at 300 and 4OO”C,multilayered Ag-Au films formed a continuous textured layer on Si. In samples annealed at 500°C pinholes
[l] A. Cros and P. Muret, Mater. Sci. Reports 8 (1992) 271. [2] C.R. Chen and L.J. Chen, J. Appl. Phys. 78 (1995) 919. [3] C. Calandra, 0. Bisi and G. Ottaviani, Surf, Sci. Reports 4 (1985) 271. (41 Y.C. Peng, C.R. Chen and L.J. Chen, unpublished results. [5] S. Hassam, J. Agren, M.G. Escard and J. P. Bros, Metall. Trans. A 21 (1990) 1877. [6] H. Yasunaga and S. Yoda, Jpn. J. Appl. Phys. Part I 30 (1991) 1882. [7] L. Hultman, A. Robertsson, H.T.G. Hentzell, I. Engstrom and P.A. Psaras, J. Appl. Phys. 62 (1987) 3647. [s] J. Yuhara, M. moue and K. Morita. J. Vat. Sci. Technol. A 11 (1993) 2714. [9] S.K. Wonell, J.M. Delaye, M. Bibole and Y. Limoge, J. Appt. Phys. 72 (1995) 5195.
xr.:.:x:\:::l$.&::::+
‘,:..I.% ‘“...:.:.:.:.:.::;.:.+:.:.:.:.: .‘.” ‘i:“‘“‘.:.:.:.::;.:.:.::::::: .,...,i ‘:‘:‘::w> 2::: :::,:“‘.,.,.../. .. :::,::.:.:.:.>.... ::
“‘“..i’~.......:(. .:,:.,,_,, ,,.,
:.:.:.~L
,:,:,
~~,~
,:::::::
.,./. .,.,..~.:.,,,,i
.:.:_yz
applied
‘.
surface science ELSEVIER
Applied Surface Science 113/l
14 (1997) 768-772
Morphological stability of Ag-Au multilayer on Si C.R. Chen, Y.C. Peng, L.J. Chen
*
Department of Materials Science and Engineering, National Tsing Hau Uniuersi& Hsinchu, Taiwan, ROC
Abstract The morphological stability of Ag-Au transmission
multilayers on Si has been studied by conventional and high-resolution X-ray diffraction and Auger electron spectroscopy. In samples annealed at 300 and 4OO”C,
electron microscopy,
multilayered Ag-Au films form a continuous textured layer on Si. In samples annealed at 5OO”C,pinholes were found to develop in the Ag-Au films. Island structures were formed in samples annealed at 600°C. The improved stability is attributed to the intermixing of Au and Ag so that the (Au-Ag)/Si interface is more stable than that of either the Au/% or Ag/Si interface. PAC.? 68.55.Jk;
61.16.Bg;
68.65.+
g
1. Introduction
a result, Ag tends to form island structure
The morphological stability of the metal-semiconductor interface is of great importance for the design and the application of layered structures. In spite of the fact that both Au/Si and Ag/Si systems have been the subjects of extensive studies [l-4], little attention has been directed towards the deposition of Ag-Au alloys onto silicon. The binary phase diagram shows that the eutectic point is low (363°C) for the Au/% system. As the annealing temperature is higher than the Au/Si eutectic point, Au grains were found to agglomerate on (0Ol)Si. The films became very uneven in thickness. A high density of pinholes was observed to be present in thin regions. After high temperature annealing, Au islands were formed on Si [2]. On the other hand, for the Ag/Si system, the adhesion for Ag on Si is rather poor. As
on silicon at a temperature as low as 200°C [1,3,4]. In addition, the electromigration resistance for Ag is very low. The morphological stability can be improved by the deposition of a thin Au layer (about 3-nm-thick) at the interface of Ag/Si [4]. To overcome the problems encountered for both Au/Si and Ag/Si systems, a mixture of Ag and Au was thought to be a good alternative to replace either Au or Ag. The eutectic point of Au-Ag/Si is higher than the Au/Si [5]; the adhesion ability to Si for the Au-Ag film is higher than for the Ag film; and the electromigration resistance of Au-Ag alloy is higher than for Ag [6]. The purpose of this work is to investigate the morphological stability of Ag-Au multilayers on Si.
2. Experimental * Corresponding author. Tel. + 886-3-573 1166; fax: + 886-35718328; e-mail: [email protected]. 0169-4332/97/$17,00 Copyright f’II SO169-4332(96)00821-S
Single-crystal, 2-inch-diameter n-type (OOl)-oriented silicon wafers with a resistivity of I-10 R cm
0 1997 Elsevier Science B.V. All rights reserved.
CR. Chen et al./Applied
Sur$ace Science 113/ 114 (1997) 768-772
were used in the present study. The wafers were cleaned chemically by a standard procedure. The samples were then dipped in a dilute HF solution (HF:H,O = 150) immediately before being loaded into an ultrahigh vacuum (UHV) dual electron beam evaporation chamber with a base pressure of 1 X lo-” Torr. The wafers were first heated to 900°C for 10 min to remove the native oxide on the silicon surface (Shiraki clean). Six periods of Au(3 nm)Ag(3 nm) layers were deposited with the Au layer directly on top of (0Ol)Si. The deposition rate was 0.06 rim/s.. Heat treatments were performed in a three-zone diffusion furnace at 300-600°C in N, ambient. High-purity nitrogen gas was passed through a titanium getter tube maintained at 800°C to minimize the oxygen content in the ambient gas. The titanium getter was used as a sacrificial purifier. The accuracy in the temperature measurement was estimated to be within +2”C. Unless otherwise mentioned, the annealing time at each temperature was 1 h. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used for structural analysis. Auger electron spectroscopy (AES) with ion beam sputtering was utilized to determine the depth profile of the chemical composition. Both planview and cross-sectional TEM samples were prepared and examined. A JEOL-200CX scanning transmission EM operating at 200 kV was used for TEM examinations. High-resolution TEM (HRTEM) imaging was carried out in a JEOL 4000EX electron microscope with a point-to-point resolution of 0.18 nm. No objective aperture was inserted into the beam path during imaging. Most of the cross-sectional TEM micrographs were taken along the [l lo] zone axis of single-crystal Si substrate.
Fig. 1. XTEM. bright field (BF) image of an as-deposited Au) X 6/(001 )Si sample.
ture Au,Si confirmed in Fig. 3. 2b, the d (022) and
(Ag-
phase [7]. The presence of Au,Si was by the analysis of XRD spectrum as shown From the HRTEM image, shown in Fig. spacings of 0.248 and 0.210 nm match (222) Au,Si planes, respectively. The
3. Results and discussion Fig. 1 shows a XTEM image of an as-deposited sample. The Ag-Au interfaces are seen to be rather flat. Fig. 2a and b show the planview diffraction pattern and high-resolution cross-sectional image of an as-deposited sample, respectively. The appearance of an additional ring located inside the {200} ring of the Ag-Au structure, as shown in Fig. 2a, is attributed to the formation of an orthorhombic struc-
Fig. 2. (a) Diffraction as-deposited (Ag-Au) respectively.
pattern (DP) and (b) HRTEM image of an X 6/(001)Si, planview and cross-sectional,
770
CR. Chen et al./Applied
Surface Science 113/ 114 (1997) 768-772
20 Fig. 3. XRD spectrum sample.
of an as-deposited
(Ag-Au)X
6/(OOl)Si
measured intersecting angle of 82” is consistent with the interplanar angle of these planes. In samples annealed at 300 and 400°C multilayered Ag-Au films became a continuous textured layer on Si. Au and Ag atomes were found to intermix almost completely. Fig. 4a and b show the AES depth profiles of as-deposited and annealed samples, respectively. For an as-deposited sample the ripples in composition modulation are clearly seen. In annealed samples, the Au and Ag components are much less distinguishable. The AuAg/(OOl)Si interface is very sharp. An example is shown in Fig. 5. Fig. 6a and b show the SEM images of samples annealed at 500 and 600°C, respectively. In samples annealed at 500°C pinholes were found to develop in the Ag-Au films. Island structure was formed in samples annealed at 600°C. The epitaxial relationship for the multilayered Ag-Au films on (0Ol)Si was found to be different
Fig. 5. XTEM. BF image of (Ag-Au)x6/(OOl)Si nealed at 400°C for I h, cross-sectional.
sample
an-
from that of either Au or Ag on (0Ol)Si. For the Au/(OOl)Si system, a diffraction ring pattern is seen as shown in Fig. 7a. Very strong {220} Au rings and rather weak (111) and {002} Au rings are seen in the diffraction pattern suggesting that Au grains were largely oriented in the [ 1111Au direction close to the normal to the surface, i.e., the gold grains underwent a textured growth on silicon with [l 1l]Au//[OOl]Si. For the Ag/Si(OOl) system, diffraction pattern anal-
75
8 ‘;13 50 b 8 25 w
Sputter time (min) Fig. 4. AES depth profiles of (a) as-deposited 400°C samples.
and (b) annealed at
Fig. 6. SEM images of (Ag-Au) (a) 500 and (b) 600°C.
x 6/(001 )Si samples annealed at
C.R. Chen et aI./Applied
771
Surface Science 113/ 114 flY971 768-772
the Au,Si phase near the (Ag-AuI/Si interface. From the diffraction pattern of Fig. 2a, it is seen that the textured (022) Au,Si plane is parallel to (1OO)Si rather than the (220&i plane. It suggests that Au,Si serves as a seed layer for the growth of (Ag-Au) films rotated by 15” along the [l 111 axis. The lattice mismatch between the (220)Ag-Au plane and (4OO)Si plane is 6% which is much smaller than the mismatch of 25% between (220)Ag-Au plane and (22O)Si plane. The morphological stability of Ag-Au muitilayers on Si was found to be much improved compared to those of either Au/Si or Ag/Si system [8]. The eutectic point of Au-Ag/Si system is higher than that of the Au/Si system [5]. The temperature for forming island structures was increased to about 600°C. In addition, the adhesion of the Ag-Au films on Si was also found to be superior to that of the Ag films on silicon, Au thin layer can act as a adhesion layer in the Ag-Au multilayer in as-deposited samples. In samples annealed at higher temperature, the
Fig. 7. DP of as-deposited samples.
(a) Au/(OOl)Si
and (b) Ag/(OOl)Si
ysis of planview samples revealed also the almost completely epitaxial growth of Ag thin films with [lll]Ag//[OOl]Si as shown in Fig. 7b. The bright spots marked in the micrograph indicate that many Ag grains were oriented epitaxially with [ll l]Ag//[OOl]Si and (220)Ag//(220)Si. In the Ag-Au/Si(OOl) system, textured growth of Ag-Au thin films with [ 11 l]Ag-Au/[OOl]Si was also found. But compared to the Ag/(OOl)Si system, the bright (220)Ag-Au spots are rotated by 15” along the [l 1 I] axis from (22O)Si to (OlONi, i.e., [2jO](AgAu)/[lOO]Si, as shown in Fig. 8. For XTEM samples, no epitaxial Ag-Au spots were found in the diffraction pattern along the [ 1 lo]-orientation. For samples viewed along the [ lOO]-direction revealed the epitaxial relationship of [liO]Ag-Au/[ lOO]Si. The diffraction pattern viewed from Si[lOO] direction is shown in Fig. 9. The growth of the epitaxial multilayered Ag-Au layer rotated by 15” is attributed to the presence of
Fig. 8. DP of a planview (Ag-Au)X at 500°C. (b) Indexed pattern of (a).
6/(OOl)Si
sample annealed
772
C.R. Chen et al./Applied
(b)
Si(O22)
.O
h’
l F
t-b
0 y.
Surface Science 113/114
0
Au-Ag( 1il)
0
*. o ~
The research was supported by the Republic of China National Science Council through a grant No. NSC84-2215E007-011.
0 0
t3
0.
Fig. 9. (a) DP of a cross-section of the (Ag-Au)X sample annealed at 500°C. (b) Indexed pattern of (a).
were found to develop in the Ag-Au films. Island structure was formed in samples annealed at 600°C. The epitaxial relationship for the multilayered Ag-Au on (0Ol)Si is different from those of Au and Ag on (001)Si. In the Ag-Au/(OOl)Si system, textured growth of Ag-Au thin films with [I 1l]AgAu/[OOl]Si was also found. Compared with the Ag/Si(OOl) system, (220)Ag-Au is rotated by 15” along the [l 1 l]Ag-Au axis from (22O)Si to (OlOlSi, i.e., [2ZOXAg-Au)/[lOO]Si. The rotation by 15” of the Ag-Au multilayer epitaxy is attributed to the presence of Au,Si phase near the Ag-Au/Si interface in which Au,Si serves as a seed layer for the growth of Ag-Au films.
Acknowledgements
Si(022) 0 0
(19971 768-772
6/(001&i
References Ag-Au alloy also possessed good adhesion ability in contact with Si [9].
4. Summary and conclusions The morphological stability of Ag-Au multilayers on Si has been studied by both conventional and high-resolution transmission electron microscopy, X-ray diffraction and Auger electron spectroscopy. In samples annealed at 300 and 4OO”C,multilayered Ag-Au films formed a continuous textured layer on Si. In samples annealed at 500°C pinholes
[l] A. Cros and P. Muret, Mater. Sci. Reports 8 (1992) 271. [2] C.R. Chen and L.J. Chen, J. Appl. Phys. 78 (1995) 919. [3] C. Calandra, 0. Bisi and G. Ottaviani, Surf, Sci. Reports 4 (1985) 271. (41 Y.C. Peng, C.R. Chen and L.J. Chen, unpublished results. [5] S. Hassam, J. Agren, M.G. Escard and J. P. Bros, Metall. Trans. A 21 (1990) 1877. [6] H. Yasunaga and S. Yoda, Jpn. J. Appl. Phys. Part I 30 (1991) 1882. [7] L. Hultman, A. Robertsson, H.T.G. Hentzell, I. Engstrom and P.A. Psaras, J. Appl. Phys. 62 (1987) 3647. [s] J. Yuhara, M. moue and K. Morita. J. Vat. Sci. Technol. A 11 (1993) 2714. [9] S.K. Wonell, J.M. Delaye, M. Bibole and Y. Limoge, J. Appt. Phys. 72 (1995) 5195.