- Email: [email protected]
Influence of plasma exposure in the preparation of A1N films by facing-target sputtering
182
Surface and Coatings Technology, 61 (1993) 182—186
Influence of plasma exposure in the preparation of A1N films by facingtarget sputtering Kikuo Tominaga, Hiroshi Imai and Yasuhiko Sueyoshi Faulty of Engineering, The University of Tokushima, Minamijosanjima 2-1, Tokushima 770 (Japan)
Abstract AIN films were prepared by facing-target sputtering in Ar(50%) + N
2(50%) at gas pressures below 1.33 Pa. On X-ray diffraction signal measurement, the (00.2) signal intensity decreased and its full width at half-maximum increased with decreasing gas pressure, the values for films prepared in Ar + N2 being larger than those in N2. The spacing between adjacent (00.2) planes was also extended at lower Ar + N2 gas pressures. These results were induced in film growth under plasma exposure of the AIN film. This was also confirmed by preparing AIN films in a system where a grounded net was inserted between the substrate and plasma to diminish the ion bombardment of the film. Furthermore, it was shown that AIN films prepared by facing-target sputtering were c-axis-oriented films regardless of the partial pressure of N2.
1. Introduction Polycrystalline AIN films have been used in surface acoustic wave devices operating in the higher frequency range [1, 2] because they have a higher acoustic wave velocity than other films such as ZnO. At the same time, their high resistivity and thermal stability make them suitable as insulating and passivating layers in semiconductor devices. A1N films can be prepared by various methods such as sputtering, ion plating and metal organic chemical vapour deposition (MOCVD) [3—5], but sputtering has the particular merit of allowing deposition at relatively low substrate temperatures [6—8]. We have previously prepared AIN films in pure N2 by conventional (unbalanced) d.c. planar magnetron (PM) sputtering [8] and in a facing-target (FT) sputtering system with two Al targets [9—12].In PM sputtering, we observed energetic NO atoms bombarding the film under the condition that residual H20 cannot be ignored, where the origin of the energetic NO was NO ions generated at the target surface. The region bornbarded by NO became pale yellow. However, in FT—PM sputtering, phenomena such as a decrease in c axis orientation and colouring and peeling of the substrate were observed at gas pressures below 0.1 Pa; this film degradation diminished at pressures above 0.4 Pa, although the incorporation of oxygen into the film at 0.1 Pa was less than in films prepared at higher gas pressures. These results strongly suggest film bombardment by atoms and ions such as N and N~during sputtering in pure N2. When films are bombarded by Ar~ions, the —
film degradation may become more severe, since the sputtering of films by Ar~induces deviations from the stoichiometric composition and the radius of Ar incorporated in the film is larger than that of N. Therefore, in order to examine the plasma exposure of A1N films, we prepared them in Ar + N2 by both d.c. planar magnetron sputtering with facing Al targets (FT—PM sputtering) and conventional (unbalanced) d.c. planar magnetron sputtering with one Al target and an anode.
2. Experimental apparatus A1N was prepared by FT—PM sputtering [9—il] in a system where two target holders of conventional PM sputtering were arranged parallel to each other. The internal magnets in the holders were of opposite polarities. The glass substrate was mounted beside the targets at a distance of 5 cm from the axis connecting the two targets. The energetic electrons were confined in the space between the two parallel targets. The discharge was sustained until 0.066 Pa. The present d.c. PM sputtering is unbalanced PM sputtering. The distance between the target and the substrate was 6.5 cm. In this case the discharge was sustained until 0.2 Pa. In both cases the substrate is fairly close to the plasma region and so is likely to be exposed to the plasma. An Al plate of 99.999% purity, 10 cm in diameter and 3 mm thick was used as the target. It was mounted in the water-cooled target holder by conductive Epotech 2OHS. Heat-resistant glass (BLC, Nippon Electric Glass Co.) was used for the substrate. The sputtering chamber
K. Tominaga et al.
io~Pa
was evacuated to below I x
/
Facing-target-sputtered AIN films
by a turbomolecu-
lar pump. The substrate temperature T1 was maintained at 200 °C, as measured by a thermocouple contacted mechanically on the edge of the glass substrate surface. The discharge current ‘T was fixed at 300 mA and films were prepared after 30 mm pre-sputtering. The full width at half-maximum (FWHM), 4(20), of the (00.2) X-ray diffraction peak for Cu K~x~ = 1.542 A) was estimated. Although zl(20) is determined by both the polycrystalline grain size [13, 14] and the inhomogeneous strain in the film, separation of these two factors was not done in this paper. It is also reported in previous papers that zl(20) has a good correlation with the degree of c axis orientation of the film [ii, 12]. The spacing d(00.2) between adjacent (00.2) planes was calculated from the relation 2d(00.2)sin0 = )~.
3. Experimental results 3.1. A1N film preparation in Ar(50%) + N2 (50%) All films produced by the two sputtering systems were c-axis-oriented films, i.e. the [00.2] axis (c axis) was normal to the substrate surface. The gas pressure dependence of 4(20) of the (00.2) X-ray diffraction peak for the A1N films prepared by FT—PM sputtering is shown by solid curves in Fig. 1, with filled and open circles indicating the data for film preparation in Ar(50%) + N2(50%) and N2 respectively. The data for conventional PM sputtering are shown by dashed curves, with filled and open triangles corresponding to the films prepared in Ar(50%) + N2(50%) and N2 respectively, The values of zl(20) in Fig. 1 increase rapidly below 0.133 Pa (solid curves). Furthermore, the values of 4(20) for the films prepared in Ar(50%) + N2(50%) are always larger than those in N2. These tendencies are also seen in the data for conventional PM sputtering in Fig. I
~:: FT-PM
I
183
(dashed curves). In pure N2 the film degradation at low gas pressure was assumed to be due to film bombardment by N atoms scattered at the target surface [7] or N~ ions accelerated by the potential difference between the plasma potential and the floating potential of the substrate [11, 12]. Taking into consideration that the substrate is close to the plasma region and that Ar gives a larger sputtering yield and has a larger atomic radius, the increase in zl(20) for the films prepared in Ar(50%) + N2(50%) may be attributed to the plasma exposure. Thus the incorporation of Ar and film bornbardment by Ar~ions in addition to N~ ions and N atoms will play a major role in the plasma exposure of AIN films at lower gas pressures. In Fig. 2 the plasma potential V1~,and floating potential V~at the substrate centre are shown for FT—PM sputtering, where the potentials were estimated by the Langmuir probe method. The potential difference J’-V1 increases with decreasing gas pressure, e.g it is 120 V at P= 0.133 Pa. This shows that the ions incident on the film have an energy e( V~-I/f). Since the binding energy of the compound is in general 10 eV at most, the ion energy is sufficiently high to induce film damage. In measuring the X-ray diffraction patterns, we also noticed a marked shift of the (00.2) diffraction angle for the films prepared in Ar(50%) + N2(50%) at lower gas pressures. This means that the mean spacing between adjacent (00.2) planes, d(00.2), changes considerably. For the films prepared by FT—PM sputtering at ‘T = 300 mA in N2 and Ar(50%) + N2(50%), the d(00.2) data are plotted in Fig. 3(a) as solid curves. For comparison, the value of d(00.2) for A1N powder is shown by a dashed line (2.49 A) [13, 14]. The value of d(00.2) for the films prepared in N2 is only slightly dependent on the gas pressure and is just larger than that of A1N powder. On the other hand, d(00.2) for the films prepared in Ar(50%) + N2(50%) shows a marked increase with decreasing gas pressure, e.g the value of d(00.2) at 0.106 Pa is 0.03 A (1.3%) greater than that of A1N powder, which means that the stress is tensile in this
Conventional DC PM
~~A~N+N2
I
(IT=300mA)
0
101
I 100
0
—50
-.•
Vi
Gas Pressure (Pa) I
101
Fig. 1. Gas pressure dependence of A(20) of(00.2) diffraction peak for films prepared in Ar(50%) + N2 (50%) (filled symbols) and N2 (open symbols). Solid curves represent FT—PM sputtering, dashed curves conventional PM sputtering,
100
Gas Pressure (Pa)
Fig. 2. Plasma potential 1’, and floating potential Vf at substrate centre for FT—PM sputtering.
184
K. Tominaga et al.
(a)FT—PM
2.52
/
Facing-target-sputtered AIN films
~
(300mA)
2.5
,_—J
0.6 I
Target
~0.4
2.52
N
~
K
2(100%)
4.... d~(20) ~
~ ~
-
2.5
~0.2 AIN Powder 2.48
Matsuoka et al. (N2100%)
d(00 2)
—*
2.48 I
10_i
(a)
10~
C
Gas Pressure (Pa) Gas
(b) Conventional DC PM
~‘
2 52
2.5 A
N2(50%) 101
(b)
50
0
2.48
1O~i
Pressure
10°
(Pa)
Fig. 4. zl(20) and d(00.2)data for films prepared in Ar(50%) + N2(50%) by FT—PM sputtering with a grounded mesh inserted between plasma
Ar(50%)+N2(50%)
!°“~
I
I
~o° Gas Pressure (Pa)
Fig. 3. Gas pressure dependence of d(00.2) of films prepared in Ar(50%) + N2(50%) and N2 by (a) FT—PM sputtering and (b) conventional PM sputtering,
case. For conventional PM sputtering, the gas pressure dependence of d(00.2) is shown in Fig. 3(b). The change in d(00.2) of the films prepared in N2 is again small, The value of d(00.2) of the films prepared in Ar(50%) + N2(50%) is larger than that in pure N2 and this difference becomes large at lower gas pressures. These X-ray measurements indicate that the films sputtered in N~ Ar are under tensile stress. This is consistent with the result of Este and Westwood [7] that the stress changed from compressive to tensile at 40% Ar. Although no estimation of the stress was made in the present work, it is thought to become strong with decreasing gas pressure, taking into consideration that the film prepared below 0.133 Pa peeled from the substrate. These obvious increases in d(00.2) of the films prepared at lower gas pressures in Ar(50%) + N2(50%) may be attributed to the plasma exposure of the substrate. To further confirm the influence of plasma exposure on A1N films, we set up a grounded stainless net of 4 mm mesh as shown in the inset of Fig. 4. The distance between the net and the glass substrate was 1 cm. Since the net is inserted between the plasma and the substrate, the positive ions and electrons in the plasma are likely
and substrate (see inset).
to flow to this grounded net, so the influence of plasma exposure on the A1N film will be decreased. The 4(20) and d(00.2) data of the A1N films are shown in Fig. 4. The changes in 4(20) and d(00.2) are small even at gas pressures as low as 0.133 Pa. These results indicate that isolation of the substrate from the plasma region is beneficial for obtaining films with lower stress or larger grain size. .
.
3.2. Film properties . optical transmittance and N:Al ratio The optical transmittance of A1N films prepared on fused quartz by FT—PM sputtering in Ar(50%) + N2(50%) is shown by solid curves in Fig. 5(a). At gas pressures below 0.133 Pa, slight film colouring and peeling were observed. The data for the films prepared in N2 are shown by dashed curves for comparison. The transmittance near the absorption edge, e.g. at A = 300 nm (4.15 eV), becomes smaller with decreasing gas pressure in both cases. This trend corresponds to that of 4(20) and d(00.2) in Figs. 1 and 3(a), where 4(20) and d(00.2) increased with decreasing gas pressure. The optical absorption near the absorption edge wavelength is thought to be determined by energy levels such as the band tail and defect levels in the forbidden band gap. Since these energy levels are produced by lattice disorder, the above trend is also attributed to the film bombardment by Ar + and N~ ions. Similar results were obtained for conventional PM sputtering in N2 and Ar(50%) + N2(50%) (Fig. 5(b)). Next a composition analysis was done for the films prepared by FT—PM sputtering in Ar(50%) + N2 (50%) and N2 by energy-dispersive X-ray spectroscopy (EDS). The ratios of the signal intensities of N and Al atoms obtained by EDS measurements, ‘N/’Ai~ are given in Table 1. As discussed in previous work [11], we take the film prepared at P = 1.33 Pa in N2 as a standard sample,
K. Tominaga et a!.
1OC
/
Facing-target-sputtered
A1N films
185
the film stoichiometry and the incorporation of Ar in
~‘‘I~’
1.3
the film.
:i
[15] found that it was not possible to exceed the 1:
0%)+N~(50%)
~
50
~
2OO
(a) 100
,/ .
~
06p~ Pa
400 Wavelength
composition even when the film was bombarded with 680Concerning eV N~ions under the condition of an Harper N:et Alal.arrival rate implanted ratio of N.upOn the to the composition 2.6. other Then hand, theofA1N N Cachard : rejects Al, additional et[16] al. confirmed by Rutherford backscattering that the N : Al ratio can be 1.4 when the A1N film is biased by a high
N~O.08~a
transparent and theirabove N: Al ratio r.f. voltage. and Ourcolourless films prepared 0.133isPanearly are
6Ô0 (a) FT-PM (nm)
equal to 1, which is consistent with the result of Harper et al. [15]. However, at P=0.106 Pa, N:Al is 1.33 for preparation in N2 and 1.12 for preparation in Ar~N2.
‘I’~~’
N~1.33 Pa
These
films are likely to be coloured as shown in Figs. 5(a) and 5(b) and to peel from the substrate. A large stress is incorporated in the film, which can be attributed to the inclusion of excess N and Ar atoms in it. This suggests that the sample at P=0.106 Pa is close to that observed by Cachard et a!. [16], where fairly strong ion bombardment was assumed.
o
i..s
50
2 Pa
L0%)+N,(50%) (b)
habit 3.3. Influence ofN2 partial pressure on crystal growth For A1N films by prepared by r.f. diode sputtering in
400 600 PM (b) Conventional Wavelength (nm)
Fig. 5. Optical transmittance spectra of films prepared on fused quartz by FT—PM sputtering. Solid curves represent films prepared in Ar(50%) + N2(50%), dashed curves films prepared in N2.
TABLE 1. Ratios of EDS signal intensities of N and Al, films prepared by FT—PM sputtering
‘N/IAi~for
Gas P(Pa)
1.33
0.66
0.106
________________________________________________
N2 Ar + N2
1 0.89
1.05 1.09
Ar + N2, Ohuchi and Russell [17] reported that the growth habits are strongly dependent on the partial pressure of N2 and on the deposition rate. Films with mixed (10.2), (10.1) and (00.2) orientation were grown in N2(60%—45%) at nearly the same deposition rate as that used in the present FT—PM sputtering. However, our results in Figs. 1 and 2 for FT—PM sputtering and conventional PM sputtering show that the films were always normal to to the substrate. To investigate whether the growth habit depends on the partial pressure of N2
1.33 1.12
since this film has the best optical transmittance spectrum, the highest film resistivity and the smallest 4(20). The film prepared at P=0.l06 Pa in N2 has a value of ‘N/’AI = 1.33 and is pale brown in colour. This is attributed to the implantation of N atoms into the film as a result of plasma exposure. The data for the films prepared in Ar(50%) + N2(50%) show that N atoms are deficient at P= 1.33 Pa and a slight excess of N atoms is contained in the films at P=0.67 and 0.106 Pa. However, the content of N atoms in the films prepared in Ar(50%) + N2(50%) is low compared with that in the films prepared in N2. These results can be understood by the presence of Ar~ion bombardment in Ar(50%) + N2(50%), which induces deviations from
in FT—PM sputtering, we prepared A1N films at several N2 partial pressures. The X-ray diffraction patterns of
II
AIN (002)
~ :3
.~
~ N~=100% ~‘
~ 11)
N,=50% 0
N~=20% N~’1o% I
~
lAl
I
0
3b
35
N~=0% 4b
20 (deg) Fig. 6. X-Ray diffraction patterns as a function of partial pressure of N2 gas for films prepared by FT—PM sputtering.
186
K. Tominaga et al.
/
Facing-target-sputtered
these films are shown in Fig. 6. We could not observe any clear change in crystal orientation. The difference between the results in Fig. 6 and those of Ohuchi and Russell [17] is thought to be due to the sputtering system. The reason is not clear at the present stage. In FT—PM sputtering, the large energy of the adatoms of Al and N and, as a result, the frequent rearrangement of adatoms may contribute to the enhancement of the surface mobility of adatoms on the substrate. The the most stable state is thought to be the close-packed (00.2) plane as mentioned by Ohuchi and Russell [17].
AINfilms
ratus is not complete to decrease the film bombardment at lower gas pressures by particles, probably positive ions in the plasma. In this respect our system is similar to that of unbalanced planar magnetron sputtering [18—20],where ions are intentionally conducted to the substrate along the magnetic field,
Acknowledgment This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
4. Discussion For A1N films prepared in pure N2, the excess of N atoms (Table 1) and the decrease in optical transmittance near the absorption edge (Fig. 5) can be related to N atom implantation at low gas pressures [11]. This suggests that the film is also bombarded by Ar~ions in Ar(50%) + N2(50%). Certainly the influence of Ar~ions becomes strong as shown in Fig. 1, where an increase in 4(20) of the (00.2) diffraction peak was observed at lower gas pressures. At the same time it was assumed that Ar atoms are contained in the film, although we could not estimate the Ar content by EDS analysis because of the low sensitivity for Ar atoms. The change in d(00.2) of the films prepared in pure N2 gas was only slight, but the rapid increase in d(00.2) in Fig. 3 is characteristic of the films prepared in Ar(50%) + N2(50%) and indicates strong tensile stress in the film. This may be understood by the incorporation of Ar into the film. Concerning this, the presence of film bombardment by Ar~ and N~ions was also confirmed by setting a grounded net in front of the substrate as shown in Fig. 4, because the insertion of the grounded net improved the grain growth and diminished the d(00.2) extension at lower gas pressures. The decrease in the transmittance near the absorption edge corresponds to the N content in the film in Table 1. These results show that the influence of plasma exposure on film growth cannot be ignored in A1N film preparation by FT—PM sputtering and conventional PM sputtering, especially at lower gas pressures, when the substrate is close to the plasma. This also indicates that our FT—PM sputtering appa-
References 1 A. J. Shuskus, T. M. Reeder and E. L. Paradis, App!. Phys. Lett.. 24 (1974) 155. 2 T. Shiosaki, T. Yamamoto, T. Oda and A. Kawabata, App!. Phys. Lett., 36 (1980) 643. ~ Y. Murayama, K. Kashiwagi and M. Kikuchi,J. Vac. Sci. Technol., 17(4) (1980) 796. 4 A. J. Noreika, M. H. Francombe and S. A. Zeitman, J. Vac. Sci. Technol., 6 (1969) 194. 5 M. Morita, N. Uesugi, S. Isogai, K. Tsubouchi and M. Mikoshiba, Jpn. J. AppI. Phys., 20 (1981) 17. 6 M. T. Wauk and D. K. Winslow, AppI. Phys. Lett., 13 (1968) 286. 7 G. Este and W. D. Westwood, J. Vac. Sci. Technol. A, 5(1987)1892. 8 K. Tominaga, S. Iwamura, Y. Shintani and 0. Tada, Jpn. J. App!. Phys., 22 (1983) 418. 9 K. Tominaga and Y. Shintani, Jpn. J. AppI. Phys., 28(2) (1989) 7. 10 K. Tominaga, Vacuum, 41(1990)1154. 11 K. Tominaga, H. Imai and M. Shirai, Jpn. J. App!. Phys., 30 (1991) 2574. 12 K. Tominaga, H. Imai, Y. Suueyoshi and M. Shirai, Surf. Coat. Technol., 54—55 (1992) 143. 13 M. Matsuoka, Y. Hoshi and M. Naoe, Trans. Inst. Electron., Info. Commun. Eng., J68-C (1985) 548 (in Japanese). 14 15 16 17 18 19 20
5. Maniv and A. Zangvil, J. AppI. Phys., 49(5) (1978) 2787. J. M. (1985) E. Harper, 58(1) 550. J. J. Cuomo and H. T. G. Hentzell, J. App!. Phys., A. Cachard, R. Fillit, I. Kadad and J. C. Pommier, Vacuum, 41 (1990) 1151. F. S. Ohuchi and P. E. Russell, J. Vac. Sci. Technol. A.5 (1987) 1630. R. P. Howson, H. A. J’afer and A. G. Spencer, Thin Solid Films, 193—1 94 (1990) 127. 5. Kadlec, J. Musel, W. D. Munz, G. Hakanson and J. E. Sundgren, Surf Coat. Technol., 39—40 (1989) 487. D. G. Teer, Surf Coat. Techno!. 39—40 (1989) 565.
Surface and Coatings Technology, 61 (1993) 182—186
Influence of plasma exposure in the preparation of A1N films by facingtarget sputtering Kikuo Tominaga, Hiroshi Imai and Yasuhiko Sueyoshi Faulty of Engineering, The University of Tokushima, Minamijosanjima 2-1, Tokushima 770 (Japan)
Abstract AIN films were prepared by facing-target sputtering in Ar(50%) + N
2(50%) at gas pressures below 1.33 Pa. On X-ray diffraction signal measurement, the (00.2) signal intensity decreased and its full width at half-maximum increased with decreasing gas pressure, the values for films prepared in Ar + N2 being larger than those in N2. The spacing between adjacent (00.2) planes was also extended at lower Ar + N2 gas pressures. These results were induced in film growth under plasma exposure of the AIN film. This was also confirmed by preparing AIN films in a system where a grounded net was inserted between the substrate and plasma to diminish the ion bombardment of the film. Furthermore, it was shown that AIN films prepared by facing-target sputtering were c-axis-oriented films regardless of the partial pressure of N2.
1. Introduction Polycrystalline AIN films have been used in surface acoustic wave devices operating in the higher frequency range [1, 2] because they have a higher acoustic wave velocity than other films such as ZnO. At the same time, their high resistivity and thermal stability make them suitable as insulating and passivating layers in semiconductor devices. A1N films can be prepared by various methods such as sputtering, ion plating and metal organic chemical vapour deposition (MOCVD) [3—5], but sputtering has the particular merit of allowing deposition at relatively low substrate temperatures [6—8]. We have previously prepared AIN films in pure N2 by conventional (unbalanced) d.c. planar magnetron (PM) sputtering [8] and in a facing-target (FT) sputtering system with two Al targets [9—12].In PM sputtering, we observed energetic NO atoms bombarding the film under the condition that residual H20 cannot be ignored, where the origin of the energetic NO was NO ions generated at the target surface. The region bornbarded by NO became pale yellow. However, in FT—PM sputtering, phenomena such as a decrease in c axis orientation and colouring and peeling of the substrate were observed at gas pressures below 0.1 Pa; this film degradation diminished at pressures above 0.4 Pa, although the incorporation of oxygen into the film at 0.1 Pa was less than in films prepared at higher gas pressures. These results strongly suggest film bombardment by atoms and ions such as N and N~during sputtering in pure N2. When films are bombarded by Ar~ions, the —
film degradation may become more severe, since the sputtering of films by Ar~induces deviations from the stoichiometric composition and the radius of Ar incorporated in the film is larger than that of N. Therefore, in order to examine the plasma exposure of A1N films, we prepared them in Ar + N2 by both d.c. planar magnetron sputtering with facing Al targets (FT—PM sputtering) and conventional (unbalanced) d.c. planar magnetron sputtering with one Al target and an anode.
2. Experimental apparatus A1N was prepared by FT—PM sputtering [9—il] in a system where two target holders of conventional PM sputtering were arranged parallel to each other. The internal magnets in the holders were of opposite polarities. The glass substrate was mounted beside the targets at a distance of 5 cm from the axis connecting the two targets. The energetic electrons were confined in the space between the two parallel targets. The discharge was sustained until 0.066 Pa. The present d.c. PM sputtering is unbalanced PM sputtering. The distance between the target and the substrate was 6.5 cm. In this case the discharge was sustained until 0.2 Pa. In both cases the substrate is fairly close to the plasma region and so is likely to be exposed to the plasma. An Al plate of 99.999% purity, 10 cm in diameter and 3 mm thick was used as the target. It was mounted in the water-cooled target holder by conductive Epotech 2OHS. Heat-resistant glass (BLC, Nippon Electric Glass Co.) was used for the substrate. The sputtering chamber
K. Tominaga et al.
io~Pa
was evacuated to below I x
/
Facing-target-sputtered AIN films
by a turbomolecu-
lar pump. The substrate temperature T1 was maintained at 200 °C, as measured by a thermocouple contacted mechanically on the edge of the glass substrate surface. The discharge current ‘T was fixed at 300 mA and films were prepared after 30 mm pre-sputtering. The full width at half-maximum (FWHM), 4(20), of the (00.2) X-ray diffraction peak for Cu K~x~ = 1.542 A) was estimated. Although zl(20) is determined by both the polycrystalline grain size [13, 14] and the inhomogeneous strain in the film, separation of these two factors was not done in this paper. It is also reported in previous papers that zl(20) has a good correlation with the degree of c axis orientation of the film [ii, 12]. The spacing d(00.2) between adjacent (00.2) planes was calculated from the relation 2d(00.2)sin0 = )~.
3. Experimental results 3.1. A1N film preparation in Ar(50%) + N2 (50%) All films produced by the two sputtering systems were c-axis-oriented films, i.e. the [00.2] axis (c axis) was normal to the substrate surface. The gas pressure dependence of 4(20) of the (00.2) X-ray diffraction peak for the A1N films prepared by FT—PM sputtering is shown by solid curves in Fig. 1, with filled and open circles indicating the data for film preparation in Ar(50%) + N2(50%) and N2 respectively. The data for conventional PM sputtering are shown by dashed curves, with filled and open triangles corresponding to the films prepared in Ar(50%) + N2(50%) and N2 respectively, The values of zl(20) in Fig. 1 increase rapidly below 0.133 Pa (solid curves). Furthermore, the values of 4(20) for the films prepared in Ar(50%) + N2(50%) are always larger than those in N2. These tendencies are also seen in the data for conventional PM sputtering in Fig. I
~:: FT-PM
I
183
(dashed curves). In pure N2 the film degradation at low gas pressure was assumed to be due to film bombardment by N atoms scattered at the target surface [7] or N~ ions accelerated by the potential difference between the plasma potential and the floating potential of the substrate [11, 12]. Taking into consideration that the substrate is close to the plasma region and that Ar gives a larger sputtering yield and has a larger atomic radius, the increase in zl(20) for the films prepared in Ar(50%) + N2(50%) may be attributed to the plasma exposure. Thus the incorporation of Ar and film bornbardment by Ar~ions in addition to N~ ions and N atoms will play a major role in the plasma exposure of AIN films at lower gas pressures. In Fig. 2 the plasma potential V1~,and floating potential V~at the substrate centre are shown for FT—PM sputtering, where the potentials were estimated by the Langmuir probe method. The potential difference J’-V1 increases with decreasing gas pressure, e.g it is 120 V at P= 0.133 Pa. This shows that the ions incident on the film have an energy e( V~-I/f). Since the binding energy of the compound is in general 10 eV at most, the ion energy is sufficiently high to induce film damage. In measuring the X-ray diffraction patterns, we also noticed a marked shift of the (00.2) diffraction angle for the films prepared in Ar(50%) + N2(50%) at lower gas pressures. This means that the mean spacing between adjacent (00.2) planes, d(00.2), changes considerably. For the films prepared by FT—PM sputtering at ‘T = 300 mA in N2 and Ar(50%) + N2(50%), the d(00.2) data are plotted in Fig. 3(a) as solid curves. For comparison, the value of d(00.2) for A1N powder is shown by a dashed line (2.49 A) [13, 14]. The value of d(00.2) for the films prepared in N2 is only slightly dependent on the gas pressure and is just larger than that of A1N powder. On the other hand, d(00.2) for the films prepared in Ar(50%) + N2(50%) shows a marked increase with decreasing gas pressure, e.g the value of d(00.2) at 0.106 Pa is 0.03 A (1.3%) greater than that of A1N powder, which means that the stress is tensile in this
Conventional DC PM
~~A~N+N2
I
(IT=300mA)
0
101
I 100
0
—50
-.•
Vi
Gas Pressure (Pa) I
101
Fig. 1. Gas pressure dependence of A(20) of(00.2) diffraction peak for films prepared in Ar(50%) + N2 (50%) (filled symbols) and N2 (open symbols). Solid curves represent FT—PM sputtering, dashed curves conventional PM sputtering,
100
Gas Pressure (Pa)
Fig. 2. Plasma potential 1’, and floating potential Vf at substrate centre for FT—PM sputtering.
184
K. Tominaga et al.
(a)FT—PM
2.52
/
Facing-target-sputtered AIN films
~
(300mA)
2.5
,_—J
0.6 I
Target
~0.4
2.52
N
~
K
2(100%)
4.... d~(20) ~
~ ~
-
2.5
~0.2 AIN Powder 2.48
Matsuoka et al. (N2100%)
d(00 2)
—*
2.48 I
10_i
(a)
10~
C
Gas Pressure (Pa) Gas
(b) Conventional DC PM
~‘
2 52
2.5 A
N2(50%) 101
(b)
50
0
2.48
1O~i
Pressure
10°
(Pa)
Fig. 4. zl(20) and d(00.2)data for films prepared in Ar(50%) + N2(50%) by FT—PM sputtering with a grounded mesh inserted between plasma
Ar(50%)+N2(50%)
!°“~
I
I
~o° Gas Pressure (Pa)
Fig. 3. Gas pressure dependence of d(00.2) of films prepared in Ar(50%) + N2(50%) and N2 by (a) FT—PM sputtering and (b) conventional PM sputtering,
case. For conventional PM sputtering, the gas pressure dependence of d(00.2) is shown in Fig. 3(b). The change in d(00.2) of the films prepared in N2 is again small, The value of d(00.2) of the films prepared in Ar(50%) + N2(50%) is larger than that in pure N2 and this difference becomes large at lower gas pressures. These X-ray measurements indicate that the films sputtered in N~ Ar are under tensile stress. This is consistent with the result of Este and Westwood [7] that the stress changed from compressive to tensile at 40% Ar. Although no estimation of the stress was made in the present work, it is thought to become strong with decreasing gas pressure, taking into consideration that the film prepared below 0.133 Pa peeled from the substrate. These obvious increases in d(00.2) of the films prepared at lower gas pressures in Ar(50%) + N2(50%) may be attributed to the plasma exposure of the substrate. To further confirm the influence of plasma exposure on A1N films, we set up a grounded stainless net of 4 mm mesh as shown in the inset of Fig. 4. The distance between the net and the glass substrate was 1 cm. Since the net is inserted between the plasma and the substrate, the positive ions and electrons in the plasma are likely
and substrate (see inset).
to flow to this grounded net, so the influence of plasma exposure on the A1N film will be decreased. The 4(20) and d(00.2) data of the A1N films are shown in Fig. 4. The changes in 4(20) and d(00.2) are small even at gas pressures as low as 0.133 Pa. These results indicate that isolation of the substrate from the plasma region is beneficial for obtaining films with lower stress or larger grain size. .
.
3.2. Film properties . optical transmittance and N:Al ratio The optical transmittance of A1N films prepared on fused quartz by FT—PM sputtering in Ar(50%) + N2(50%) is shown by solid curves in Fig. 5(a). At gas pressures below 0.133 Pa, slight film colouring and peeling were observed. The data for the films prepared in N2 are shown by dashed curves for comparison. The transmittance near the absorption edge, e.g. at A = 300 nm (4.15 eV), becomes smaller with decreasing gas pressure in both cases. This trend corresponds to that of 4(20) and d(00.2) in Figs. 1 and 3(a), where 4(20) and d(00.2) increased with decreasing gas pressure. The optical absorption near the absorption edge wavelength is thought to be determined by energy levels such as the band tail and defect levels in the forbidden band gap. Since these energy levels are produced by lattice disorder, the above trend is also attributed to the film bombardment by Ar + and N~ ions. Similar results were obtained for conventional PM sputtering in N2 and Ar(50%) + N2(50%) (Fig. 5(b)). Next a composition analysis was done for the films prepared by FT—PM sputtering in Ar(50%) + N2 (50%) and N2 by energy-dispersive X-ray spectroscopy (EDS). The ratios of the signal intensities of N and Al atoms obtained by EDS measurements, ‘N/’Ai~ are given in Table 1. As discussed in previous work [11], we take the film prepared at P = 1.33 Pa in N2 as a standard sample,
K. Tominaga et a!.
1OC
/
Facing-target-sputtered
A1N films
185
the film stoichiometry and the incorporation of Ar in
~‘‘I~’
1.3
the film.
:i
[15] found that it was not possible to exceed the 1:
0%)+N~(50%)
~
50
~
2OO
(a) 100
,/ .
~
06p~ Pa
400 Wavelength
composition even when the film was bombarded with 680Concerning eV N~ions under the condition of an Harper N:et Alal.arrival rate implanted ratio of N.upOn the to the composition 2.6. other Then hand, theofA1N N Cachard : rejects Al, additional et[16] al. confirmed by Rutherford backscattering that the N : Al ratio can be 1.4 when the A1N film is biased by a high
N~O.08~a
transparent and theirabove N: Al ratio r.f. voltage. and Ourcolourless films prepared 0.133isPanearly are
6Ô0 (a) FT-PM (nm)
equal to 1, which is consistent with the result of Harper et al. [15]. However, at P=0.106 Pa, N:Al is 1.33 for preparation in N2 and 1.12 for preparation in Ar~N2.
‘I’~~’
N~1.33 Pa
These
films are likely to be coloured as shown in Figs. 5(a) and 5(b) and to peel from the substrate. A large stress is incorporated in the film, which can be attributed to the inclusion of excess N and Ar atoms in it. This suggests that the sample at P=0.106 Pa is close to that observed by Cachard et a!. [16], where fairly strong ion bombardment was assumed.
o
i..s
50
2 Pa
L0%)+N,(50%) (b)
habit 3.3. Influence ofN2 partial pressure on crystal growth For A1N films by prepared by r.f. diode sputtering in
400 600 PM (b) Conventional Wavelength (nm)
Fig. 5. Optical transmittance spectra of films prepared on fused quartz by FT—PM sputtering. Solid curves represent films prepared in Ar(50%) + N2(50%), dashed curves films prepared in N2.
TABLE 1. Ratios of EDS signal intensities of N and Al, films prepared by FT—PM sputtering
‘N/IAi~for
Gas P(Pa)
1.33
0.66
0.106
________________________________________________
N2 Ar + N2
1 0.89
1.05 1.09
Ar + N2, Ohuchi and Russell [17] reported that the growth habits are strongly dependent on the partial pressure of N2 and on the deposition rate. Films with mixed (10.2), (10.1) and (00.2) orientation were grown in N2(60%—45%) at nearly the same deposition rate as that used in the present FT—PM sputtering. However, our results in Figs. 1 and 2 for FT—PM sputtering and conventional PM sputtering show that the films were always normal to to the substrate. To investigate whether the growth habit depends on the partial pressure of N2
1.33 1.12
since this film has the best optical transmittance spectrum, the highest film resistivity and the smallest 4(20). The film prepared at P=0.l06 Pa in N2 has a value of ‘N/’AI = 1.33 and is pale brown in colour. This is attributed to the implantation of N atoms into the film as a result of plasma exposure. The data for the films prepared in Ar(50%) + N2(50%) show that N atoms are deficient at P= 1.33 Pa and a slight excess of N atoms is contained in the films at P=0.67 and 0.106 Pa. However, the content of N atoms in the films prepared in Ar(50%) + N2(50%) is low compared with that in the films prepared in N2. These results can be understood by the presence of Ar~ion bombardment in Ar(50%) + N2(50%), which induces deviations from
in FT—PM sputtering, we prepared A1N films at several N2 partial pressures. The X-ray diffraction patterns of
II
AIN (002)
~ :3
.~
~ N~=100% ~‘
~ 11)
N,=50% 0
N~=20% N~’1o% I
~
lAl
I
0
3b
35
N~=0% 4b
20 (deg) Fig. 6. X-Ray diffraction patterns as a function of partial pressure of N2 gas for films prepared by FT—PM sputtering.
186
K. Tominaga et al.
/
Facing-target-sputtered
these films are shown in Fig. 6. We could not observe any clear change in crystal orientation. The difference between the results in Fig. 6 and those of Ohuchi and Russell [17] is thought to be due to the sputtering system. The reason is not clear at the present stage. In FT—PM sputtering, the large energy of the adatoms of Al and N and, as a result, the frequent rearrangement of adatoms may contribute to the enhancement of the surface mobility of adatoms on the substrate. The the most stable state is thought to be the close-packed (00.2) plane as mentioned by Ohuchi and Russell [17].
AINfilms
ratus is not complete to decrease the film bombardment at lower gas pressures by particles, probably positive ions in the plasma. In this respect our system is similar to that of unbalanced planar magnetron sputtering [18—20],where ions are intentionally conducted to the substrate along the magnetic field,
Acknowledgment This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
4. Discussion For A1N films prepared in pure N2, the excess of N atoms (Table 1) and the decrease in optical transmittance near the absorption edge (Fig. 5) can be related to N atom implantation at low gas pressures [11]. This suggests that the film is also bombarded by Ar~ions in Ar(50%) + N2(50%). Certainly the influence of Ar~ions becomes strong as shown in Fig. 1, where an increase in 4(20) of the (00.2) diffraction peak was observed at lower gas pressures. At the same time it was assumed that Ar atoms are contained in the film, although we could not estimate the Ar content by EDS analysis because of the low sensitivity for Ar atoms. The change in d(00.2) of the films prepared in pure N2 gas was only slight, but the rapid increase in d(00.2) in Fig. 3 is characteristic of the films prepared in Ar(50%) + N2(50%) and indicates strong tensile stress in the film. This may be understood by the incorporation of Ar into the film. Concerning this, the presence of film bombardment by Ar~ and N~ions was also confirmed by setting a grounded net in front of the substrate as shown in Fig. 4, because the insertion of the grounded net improved the grain growth and diminished the d(00.2) extension at lower gas pressures. The decrease in the transmittance near the absorption edge corresponds to the N content in the film in Table 1. These results show that the influence of plasma exposure on film growth cannot be ignored in A1N film preparation by FT—PM sputtering and conventional PM sputtering, especially at lower gas pressures, when the substrate is close to the plasma. This also indicates that our FT—PM sputtering appa-
References 1 A. J. Shuskus, T. M. Reeder and E. L. Paradis, App!. Phys. Lett.. 24 (1974) 155. 2 T. Shiosaki, T. Yamamoto, T. Oda and A. Kawabata, App!. Phys. Lett., 36 (1980) 643. ~ Y. Murayama, K. Kashiwagi and M. Kikuchi,J. Vac. Sci. Technol., 17(4) (1980) 796. 4 A. J. Noreika, M. H. Francombe and S. A. Zeitman, J. Vac. Sci. Technol., 6 (1969) 194. 5 M. Morita, N. Uesugi, S. Isogai, K. Tsubouchi and M. Mikoshiba, Jpn. J. AppI. Phys., 20 (1981) 17. 6 M. T. Wauk and D. K. Winslow, AppI. Phys. Lett., 13 (1968) 286. 7 G. Este and W. D. Westwood, J. Vac. Sci. Technol. A, 5(1987)1892. 8 K. Tominaga, S. Iwamura, Y. Shintani and 0. Tada, Jpn. J. App!. Phys., 22 (1983) 418. 9 K. Tominaga and Y. Shintani, Jpn. J. AppI. Phys., 28(2) (1989) 7. 10 K. Tominaga, Vacuum, 41(1990)1154. 11 K. Tominaga, H. Imai and M. Shirai, Jpn. J. App!. Phys., 30 (1991) 2574. 12 K. Tominaga, H. Imai, Y. Suueyoshi and M. Shirai, Surf. Coat. Technol., 54—55 (1992) 143. 13 M. Matsuoka, Y. Hoshi and M. Naoe, Trans. Inst. Electron., Info. Commun. Eng., J68-C (1985) 548 (in Japanese). 14 15 16 17 18 19 20
5. Maniv and A. Zangvil, J. AppI. Phys., 49(5) (1978) 2787. J. M. (1985) E. Harper, 58(1) 550. J. J. Cuomo and H. T. G. Hentzell, J. App!. Phys., A. Cachard, R. Fillit, I. Kadad and J. C. Pommier, Vacuum, 41 (1990) 1151. F. S. Ohuchi and P. E. Russell, J. Vac. Sci. Technol. A.5 (1987) 1630. R. P. Howson, H. A. J’afer and A. G. Spencer, Thin Solid Films, 193—1 94 (1990) 127. 5. Kadlec, J. Musel, W. D. Munz, G. Hakanson and J. E. Sundgren, Surf Coat. Technol., 39—40 (1989) 487. D. G. Teer, Surf Coat. Techno!. 39—40 (1989) 565.