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Fabrication of aluminum oxide films with high deposition rates using the activated reactive evaporation technique
Surface and Coatings Technology, 43/44 (1990) 213—222
213
FABRICATION OF ALUMINUM OXIDE FILMS WITH HIGH DEPOSITION RATES USING THE ACTIVATED REACTIVE EVAPORATION TECHNIQUE J. S. YOON, G. F. POTWIN, H. J. DOERR, C. V. DESHPANDAY and R. F. BUNSHAH Department of Materials Science and Engineering, School of Engineering and Applied Science, University of California, Los Angeles, CA 90024-1495 (U.S.A.)
Abstract Aluminum oxide films were prepared at high deposition rates using the activated reactive evaporation technique. Aluminum was evaporated in an r.f.-excited oxygen plasma. Transparent, stoichiometric and smooth films were obtained at rates as high as 8 iim h’. Process optimization and film properties, such as composition, refractive index, structure and surface morphology, will be discussed.
1. Introduction Aluminum oxide films have played an important role in various fields as an insulating layer in metal—insulator—semiconductor field effect transistors [1], gate insulators in solid state hydrogen sensors [2, 3], X-ray and accelerator neutralizer in nuclear reactors [4], tunnel barriers in Josephson tunnel junctions [5], antirefiection coatings in solar cells [6], optical waveguides [7] and protective layers for metal reflectors [8]. Various techniques such as chemical vapor deposition (CVD) [2], metal organic CVD (M0°VD) [9, 10], laser-induced CVD [1], electron beam evaporation of alumina [11, 12], ion beam sputter deposition [7, 13], glow discharge [14] and recently molecular layer epitaxy [5] have been used to synthesize aluminum oxide films. Although good quality Al2 03 films have been synthesized, the deposition rates obtained in these techniques were relatively low, such as 4.6—33.9 rim min’ 1 for for MOCVD 10], 15 nm min’ for laser-induced CVD for [1], ion 18 nm min electron beam[9,evaporation of alumina [11], 4.6 nm min’ beam sputter deposition [7] and 14 nm min’ for glow discharge [14]. Nishimura et al. [13] obtained a deposition rate as high as 90 nm min1 employing a single accelerator grid in ion beam sputter deposition process. However, the use of this technique is limited by the grid lifetime. For this study, the activated reactive evaporation (ARE) process [15] was employed to synthesize aluminum oxide films using 02 as a reactive gas. Bunshah and Schramm [16] have used the ARE technique to deposit aluElsevier Sequoia/Printed in The Netherlands
214
minum oxide films. Their experiments were performed at relatively lower oxygen pressures ranging from 2 x 10~to 3 x 10 ‘ Torr. In this study, these experiments were extended to deposit A1203 films at higher oxygen pressure using a high power electron beam with the objective of obtaining high deposition rates. Aluminum was evaporated in an oxygen gas excited by r.f. at relatively high oxygen pressures in the range 1 x 10 ~---5x 10 ~ Torr to increase the reaction between aluminum vapor and oxygen gas. The deposition process and the effect of deposition parameters on deposition rate, stoichiometry, structure and optical properties are discussed below.
2. Experimental details The deposition of aluminum oxide films was performed in a stainless steel reaction chamber with a base pressure of 10 ~6 Torr. It is shown in Fig. 1; the set-up includes a vacuum pump, an electron beam evaporation gun (Temescal model SFIH-200-2), a heating lamp and an r.f. power unit. The chamber was divided into two sections by a stainless steel plate. The electron beam gun was located in the lower section where the pressure was kept below 10 ~Torr to avoid arcing. High purity (99.999%) aluminum (Varian, 53000-13-000-500) was used as the evaporation source material. An aluminum ingot was placed
ilsubstrate holder Subs t rate
I
~enna
Oxygen
Matching tinit
___________
I
[
DL
power Supply
Pressure l)arrier
Al
___
__
Electron gun
________________
______________
-
Vacuum pump Fig. 1. Schematic diagram of the ARE system.
_______
~ Vacuum chamhci Shutter
215
in a water-cooled copper hearth whose diameter was 2 in. A hearth liner was not employed. Oxygen gas was introduced above the evaporant. The depositions were carried out at 300 °Csubstrate temperature and 100 W of r.f. power throughout this study. New aluminum ingots were used for each deposition experiment to ensure the reproducibility. The ratio of aluminum to oxygen was varied by changing the oxygen partial pressure from 1 to 5 mTorr. Polished p-type (100) silicon wafers, soda—lime glass and a molybdenum piece were used as substrates. A Norelco X-ray diffractometer with a copper target was used to obtain X-ray diffraction patterns. IR transmission spectra were recorded on a Perkin—Elmer 1330 IR spectrophotometer. Refractive indices were determined using a single-beam (628 nm) ellipsometer (Rudolf Research/Auto EL). A Dektak Il-A surface profilometer and scanning electron microscope (JEOL, JSM-T330A) were used to study the surface morphology. The deposition rate was calculated from deposition time and film thickness which was measured by the Dektak Il-A surface profilometer. The composition of the as-deposited aluminum oxide films was determined using an energy-dispersive analysis (Kevex, Delta level II) attachment to a JEOL scanning electron microscope. The actual composition of the films was determined from the difference in acquired energy-dispersive spectra between the standard Al2 03 sample and the as-deposited aluminum oxide films. The accuracy of this technique is ±2%.
3. Experimental results and discussions 3.1. Procedure for deposition of Al2 03 films The chamber was pumped down to a base pressure of 10_6 Torr before turning on the substrate heater. After the substrates reached 300 C, the electron beam gun was turned on and the electron beam current was increased very slowly to clean the surface of the evaporation source and to degas the charge. A very thin layer of aluminum oxide on the surface of the aluminum ingot was ruptured and an elliptical molten pooi was formed as the power increased. The pooi area continuously increased until the electron beam current reached about 350 mA. Beyond this point, the pooi area did not increase even though the power increased. A large portion of power delivered to the evaporation source was dissipated to the coolant in the water-cooled copper hearth because of the high thermal conductivity of aluminum. Therefore, the surface near the edge of billet could not be melted. Oxygen gas was introduced after the pooi size reached a maximum. A plasma was observed near the crucible at this point. An inductively coupled r.f. generator operating at 13.56 MHz was started after the electron beam current and the oxygen pressure reached the desired values. The r.f. power was then slowly increased. It was observed that the plasma initially localized in the vicinity of the billet spread out with increase in r.f. power. The pool size was also found to decrease with increase in r.f. power. The
216
shutter was opened after a stable operating condition was obtained. The electron beam current needed to be increased slowly to keep the pressure at a certain level as the deposition progressed. In this study, a very close relationship between the electron beam current and the oxygen pressure was observed. Oxygen pressure in the chamber could be controlled by varying the electron beam current. Moreover, it was observed that the aluminum molten pool area was governed by the combination of the electron beam current and oxygen pressure. No further discussion of these observations will be presented in this paper because of space limitations, but will be published separately. 3.2. Effect of oxygen pressure on growth of the aluminum oxide films Deposition of the aluminum oxide films was performed for 2 h at various oxygen pressures while the electron beam currents were varied from 520 to 580 mA in order to keep a certain value of oxygen pressure at a constant oxygen flow rate. The deposition rate of the films decreased from 22 to 5.8 ~tmh 1 as the oxygen pressure changed from 1 to 5 mTorr (Fig. 2). As the oxygen pressure increased, the aluminum molten pool area decreased, thus reducing the number of aluminum atoms leaving the evaporation source. Furthermore, the collisions between the evaporated aluminum atoms and oxygen atoms increased, thereby reducing the number of aluminum atoms which reach to the substrate. Therefore, the deposition rate of the film decreased as the oxygen pressure increased.
1 .5
25
•—~
/
20
Lo-e-~+~ DEPOSITION tse~e-e0/AL RATIO 0
1
2
1.4
RATE 3
I
4
I
5
6
OXYGEN PRESSURE (mTorr) Fig. 2. Dependence of deposition rate and the O:Al ratio of the ARE aluminum oxide films on oxygen pressure.
217
Dark brown colored films which indicate excess aluminum in the films were obtained at 1 mTorr whereas clear and featureless films were obtained at 5 mTorr. The color of the film prepared at 2 mTorr was light brown. The ratio of oxygen to aluminum is shown in Fig. 2. The O:Al ratio increased as the oxygen pressure increased. Aluminum-rich films were formed at lower oxygen pressure owing to an insufficiency of oxygen species at the substrate. On the contrary, the number of oxygen species increased with oxygen pressure, thereby producing near-stoichiometric films at higher oxygen pressure. The composition of the films obtained at 5 mTorr was 42 at.% Al and 58at.% 0. X-ray diffraction patterns of all the films showed a broad halo at near 22, indicating an amorphous structure (Fig. 3). An interesting observation from X-ray diffraction analysis is that this broad halo shifts to a higher scattering angle as the aluminum content increases. This is believed to be due to the change in the chemical short-range order of the films. The nearest-neighbor distances may change, resulting in change in the microscopic density of the film as the composition of the film varies. Detailed structural studies using neutron scattering or X-ray scattering measurements would be needed to understand the relationship between the composition of the aluminum oxide films and the atomic scale structure of these films. Figure 4 shows the effect of annealing on the X-ray diffraction pattern of the sample whose composition was close to stoichiometric A12O3. The sample which initially exhibited the amorphous phase was heat treated at 1200 ~C in an atmosphere of argon gas for 2 h. Several peaks appeared after annealing, which were indexed as ct-Al2 03 which has a corundum structure, except for one peak near 65~.The peak near 65~was indexed as 0-Al203. According to McArdle and Messing [17], 0-Al2 03 transforms to ct-Al203 at about 1200 ~C.
1~
~
SCATTERING ANGLE (TWO ThETA)
Fig. 3. X-ray diffraction patterns of the aluminum oxide films deposited at 100 W of r.f. power and 520—580 mA of electron beam current at oxygen pressures of 5 mTorr (spectrum a), 2 mTorr (spectrum b) and 1 mTorr (spectrum c).
218
1’~ Ii
(]~‘rI~:NI\I;ALC;l,F:
TWO ‘IEWTA)
Fig. 4. X.ray diffraction patterns of the aluminum oxide films deposited at 100 W of r.f. power, 520 580 mA of electron beam current and 5 mTorr of oxygen pressure without annealing (spectrum a) and with annealing at 1200 C for 2 h (spectrum b).
Figure 4 shows that the transformation from 0 to ~i phase had not been completed after 2 h at 1200 C. Values for refractive index are mainly determined by the O:Al ratio and the density of the film. Refractive index increases as the ratio O:Al decreases and the density of the film increases [9, 18]. A higher value for the refractive index is usually considered to be an indication of an aluminum-rich film. The results displayed in Fig. 5 show that refractive index decreases with increase
1.8
~-~-—
.---~
ijL-
~1.6
0 I
0
.-—~
4
z 1.4
1 3 0
—~~--—~—‘--—~
1
2
~_.~____~_.___1 3 4
5
‘
6
OXYGEN PRESSURE (mTorr) Fig. 5. Plot of refractive index as a function of oxygen pressure.
219
1200
1000
800
WAVENUMBER
800
400
200
1)
(cm
Fig. 6. IR transmission spectra of the ARE aluminum oxide films deposited at 1 mTorr (spectrum a) and 5 mTorr (spectrum b).
in oxygen partial pressure. The observed variation in refractive index with oxygen pressure can be attributed to the stoichiometry of the film. For a given electron beam current and r.f. excitation power, the ratio O:Al decreases as the oxygen partial pressure decreases, thereby resulting in aluminum-rich films. As the oxygen pressure decreases, values for refractive index approach a constant value, about 1.5, which is close to the refractive index of the stoichiometric Al 2 03 film. The data show that the lowest value observed, 1.47, is smaller than those of the films obtained by other techniques which are in the range 1.5—1.54 [1, 14, 19]. This may be due to the lower density of our films, which resulted from the very high deposition rate. The density of the film can probably be increased by biasing and/or heating the substrate during deposition. Typical JR transmission spectra are displayed in Fig. 6. All the IR spectra show a very broad band between 400 and 1000 cm’ which is a characteristic of an amorphous phase. An ideal Al—O octahedron is supposed to give rise to two JR-active vibration modes at 370 and 651 cm’ and these two vibration modes may split into six vibration modes at different frequencies, 330, 370, 413, 492, 651 and 770cm’, when the Al—0 coordination unit is disordered [20]. In this study, the traces of three peaks were observed at 770, 660 and 380 cm’ for the sample prepared at higher oxygen pressure (5 mTorr), which are indicated by arrows in Fig. 6. The small bump near 600 cm’ is due to the silicon substrate. The peak near 770 cm’ decreases with increasing oxygen pressure and almost disappears for the samples deposited at 1 mTorr of oxygen pressure. Even though the reason is not quite clear, we believe that this is related to the change in coordination numbers ZA1 and Z0 resulting from the composition changes.
220
3.3. Effect of electron beam current on deposition rate at higher oxygen pressure (5 mTorr) Several aluminum oxide films were deposited at 5 mTorr of oxygen pressure and various electron beam currents for 2 h. The electron beam current was changed during deposition to keep the pressure at 5 mTorr as mentioned in Section 3.1. Deposition rate increases drastically with the electron beam current as expected (Fig. 7). All the films were clear and colorless regardless of electron beam current. The maximum deposition rate obtained was 12 I.tm h However, this film cracked during deposition. The films deposited with deposition rate up to 8 tim h 1 showed a very smooth surface without any indication of cracking. A typical surface profile of the film deposited at a deposition rate of 8 I.tm h is shown in Fig. 8. The film has a local surface roughness as low as 100 A, which is excellent considering the total large thickness of the film. A scanning electron micrograph obtained for the film deposited on a silicon wafer under the same conditions as Fig. 8 is shown in Fig. 9. It ~.
lCOiiiA
I 3Cm 7OOmii\ AlbOimi \ 360imi\
7m()ii.\
(Ciii
(Ill
A
COnIA OniA
7))
40L:
700
600
700
800
C1.FCTRON 13EAM CURRENT (mA)
Fig. 7. Dependence of deposition rate of the ARE aluminum oxide films on electron beam current.
-j
600~ 400
~ 20: ~m -200 -400
I
-600 0
400 SCAN [ENGR
800
1200
(MICRONS)
Fig. 8. Surface profile of the film deposited at a deposition rate of 8 pm h
221
Fig. 9. Scanning electron micrograph of the film deposited at a deposition rate of 8 pm h’.
represents a typical surface of a homogeneous amorphous film. No visible features were observed.
4. Conclusions Deposition of the aluminum oxide film with high deposition rate has been achieved using the ARE technique. Films with a very good surface topography were prepared with near-stoichiometric composition at a deposition rate of 8 tim h~. The present authors believe that this deposition rate could be increased close up to the maximum deposition rate, 12 tim h1, without cracks by biasing and/or heating the substrates to a higher temperature than the 300 ~C used in these experiments.
References 1 2 3 4 5 6
M. Mjnakata and Y. Kurukawa, J. Electron. Mater., 15(3) (1986) 159. A. S. Wong, G. M. Michal, I. E. Locci and P. W. Cheung, J. Mater. Res., 3(5) (1988) 1002. T. H Hua and M. Armgarth, J. Electron. Mater., 16(1) (1987) 27. E. B. Graper, J. Vac. Sci. Technol A, 6(3) (1988) 1768. G. Oya, M. Yoshida and Y. Sawada, Appi. Phys. Lett., 51(15) 1987) 1143. K. I. Chang, Y. C. M. Yeh, P. A. Iles and J. M. Tracy, Con!. Rec. 1~hIEEE Photovoltaic Specialists Conf., 1987, IEEE, New York, 1987, p. 273. 7 S. M. Arnold and B. E. Cole, Thin Solid Films, 165 (1988) 1. 8 M. Viswanathan, C. L. Nagendra and G. K. M. Thutupalli, Thin Solid Films, 167(1988) 291.
222 9 J. Fournier, W. Desisto, R. Brusasco, M. Sosnowski, R. Kershaw, J. Baglio, K. Dwight and A. Wold, Mater. Res. Bull., 23 (1988) 31. 10 C. Dhanavantri and R. N. Karekar, Thin Solid Films, 160 (1989) 271. 11 J. Saraie, S. Goto, Y. Kitao and Y. Yodogawa, J. Electrochem. Soc., 134(11) (1987) 2805. 12 E. B. Graper, J. Vac. Sci. Technol. A, 5(4) (1987) 2718. 13 C. Nishimura, K. Yanagisawa, A. Tago and T. Toshima, J. Vac. Sci. Technol. A, 5(3) (1987) 343. 14 Y. Catherine and A. Talebian, J. Electron. Mater., 17(2) (1988) 127. 15 R. F. Bunshah and A. C. Raghuram, J. Vac. Sci. Technol., 9(6) (1972) 1385. 16 R. F. Bunshah and R. J. Schramm, Thin Solid Films, 40 (1977) 211. 17 J. L. McArdle and G. L. Messing, Adv. Ceram. Mater. 3(4) (1988) 387. 18 K. M. Gustin and R. G. Gordon, ,J. Electron. Mater., 17(6) (1988) 509. 19 R. S. Ehle, B. J. Baliga and W. Katz, J. Electron. Mater., 12(3) (1983) 587. 20 A. B. Kiss, G. Keresztury and L. Farkas, Spectrochim. Acta A, 36(1980) 653.
213
FABRICATION OF ALUMINUM OXIDE FILMS WITH HIGH DEPOSITION RATES USING THE ACTIVATED REACTIVE EVAPORATION TECHNIQUE J. S. YOON, G. F. POTWIN, H. J. DOERR, C. V. DESHPANDAY and R. F. BUNSHAH Department of Materials Science and Engineering, School of Engineering and Applied Science, University of California, Los Angeles, CA 90024-1495 (U.S.A.)
Abstract Aluminum oxide films were prepared at high deposition rates using the activated reactive evaporation technique. Aluminum was evaporated in an r.f.-excited oxygen plasma. Transparent, stoichiometric and smooth films were obtained at rates as high as 8 iim h’. Process optimization and film properties, such as composition, refractive index, structure and surface morphology, will be discussed.
1. Introduction Aluminum oxide films have played an important role in various fields as an insulating layer in metal—insulator—semiconductor field effect transistors [1], gate insulators in solid state hydrogen sensors [2, 3], X-ray and accelerator neutralizer in nuclear reactors [4], tunnel barriers in Josephson tunnel junctions [5], antirefiection coatings in solar cells [6], optical waveguides [7] and protective layers for metal reflectors [8]. Various techniques such as chemical vapor deposition (CVD) [2], metal organic CVD (M0°VD) [9, 10], laser-induced CVD [1], electron beam evaporation of alumina [11, 12], ion beam sputter deposition [7, 13], glow discharge [14] and recently molecular layer epitaxy [5] have been used to synthesize aluminum oxide films. Although good quality Al2 03 films have been synthesized, the deposition rates obtained in these techniques were relatively low, such as 4.6—33.9 rim min’ 1 for for MOCVD 10], 15 nm min’ for laser-induced CVD for [1], ion 18 nm min electron beam[9,evaporation of alumina [11], 4.6 nm min’ beam sputter deposition [7] and 14 nm min’ for glow discharge [14]. Nishimura et al. [13] obtained a deposition rate as high as 90 nm min1 employing a single accelerator grid in ion beam sputter deposition process. However, the use of this technique is limited by the grid lifetime. For this study, the activated reactive evaporation (ARE) process [15] was employed to synthesize aluminum oxide films using 02 as a reactive gas. Bunshah and Schramm [16] have used the ARE technique to deposit aluElsevier Sequoia/Printed in The Netherlands
214
minum oxide films. Their experiments were performed at relatively lower oxygen pressures ranging from 2 x 10~to 3 x 10 ‘ Torr. In this study, these experiments were extended to deposit A1203 films at higher oxygen pressure using a high power electron beam with the objective of obtaining high deposition rates. Aluminum was evaporated in an oxygen gas excited by r.f. at relatively high oxygen pressures in the range 1 x 10 ~---5x 10 ~ Torr to increase the reaction between aluminum vapor and oxygen gas. The deposition process and the effect of deposition parameters on deposition rate, stoichiometry, structure and optical properties are discussed below.
2. Experimental details The deposition of aluminum oxide films was performed in a stainless steel reaction chamber with a base pressure of 10 ~6 Torr. It is shown in Fig. 1; the set-up includes a vacuum pump, an electron beam evaporation gun (Temescal model SFIH-200-2), a heating lamp and an r.f. power unit. The chamber was divided into two sections by a stainless steel plate. The electron beam gun was located in the lower section where the pressure was kept below 10 ~Torr to avoid arcing. High purity (99.999%) aluminum (Varian, 53000-13-000-500) was used as the evaporation source material. An aluminum ingot was placed
ilsubstrate holder Subs t rate
I
~enna
Oxygen
Matching tinit
___________
I
[
DL
power Supply
Pressure l)arrier
Al
___
__
Electron gun
________________
______________
-
Vacuum pump Fig. 1. Schematic diagram of the ARE system.
_______
~ Vacuum chamhci Shutter
215
in a water-cooled copper hearth whose diameter was 2 in. A hearth liner was not employed. Oxygen gas was introduced above the evaporant. The depositions were carried out at 300 °Csubstrate temperature and 100 W of r.f. power throughout this study. New aluminum ingots were used for each deposition experiment to ensure the reproducibility. The ratio of aluminum to oxygen was varied by changing the oxygen partial pressure from 1 to 5 mTorr. Polished p-type (100) silicon wafers, soda—lime glass and a molybdenum piece were used as substrates. A Norelco X-ray diffractometer with a copper target was used to obtain X-ray diffraction patterns. IR transmission spectra were recorded on a Perkin—Elmer 1330 IR spectrophotometer. Refractive indices were determined using a single-beam (628 nm) ellipsometer (Rudolf Research/Auto EL). A Dektak Il-A surface profilometer and scanning electron microscope (JEOL, JSM-T330A) were used to study the surface morphology. The deposition rate was calculated from deposition time and film thickness which was measured by the Dektak Il-A surface profilometer. The composition of the as-deposited aluminum oxide films was determined using an energy-dispersive analysis (Kevex, Delta level II) attachment to a JEOL scanning electron microscope. The actual composition of the films was determined from the difference in acquired energy-dispersive spectra between the standard Al2 03 sample and the as-deposited aluminum oxide films. The accuracy of this technique is ±2%.
3. Experimental results and discussions 3.1. Procedure for deposition of Al2 03 films The chamber was pumped down to a base pressure of 10_6 Torr before turning on the substrate heater. After the substrates reached 300 C, the electron beam gun was turned on and the electron beam current was increased very slowly to clean the surface of the evaporation source and to degas the charge. A very thin layer of aluminum oxide on the surface of the aluminum ingot was ruptured and an elliptical molten pooi was formed as the power increased. The pooi area continuously increased until the electron beam current reached about 350 mA. Beyond this point, the pooi area did not increase even though the power increased. A large portion of power delivered to the evaporation source was dissipated to the coolant in the water-cooled copper hearth because of the high thermal conductivity of aluminum. Therefore, the surface near the edge of billet could not be melted. Oxygen gas was introduced after the pooi size reached a maximum. A plasma was observed near the crucible at this point. An inductively coupled r.f. generator operating at 13.56 MHz was started after the electron beam current and the oxygen pressure reached the desired values. The r.f. power was then slowly increased. It was observed that the plasma initially localized in the vicinity of the billet spread out with increase in r.f. power. The pool size was also found to decrease with increase in r.f. power. The
216
shutter was opened after a stable operating condition was obtained. The electron beam current needed to be increased slowly to keep the pressure at a certain level as the deposition progressed. In this study, a very close relationship between the electron beam current and the oxygen pressure was observed. Oxygen pressure in the chamber could be controlled by varying the electron beam current. Moreover, it was observed that the aluminum molten pool area was governed by the combination of the electron beam current and oxygen pressure. No further discussion of these observations will be presented in this paper because of space limitations, but will be published separately. 3.2. Effect of oxygen pressure on growth of the aluminum oxide films Deposition of the aluminum oxide films was performed for 2 h at various oxygen pressures while the electron beam currents were varied from 520 to 580 mA in order to keep a certain value of oxygen pressure at a constant oxygen flow rate. The deposition rate of the films decreased from 22 to 5.8 ~tmh 1 as the oxygen pressure changed from 1 to 5 mTorr (Fig. 2). As the oxygen pressure increased, the aluminum molten pool area decreased, thus reducing the number of aluminum atoms leaving the evaporation source. Furthermore, the collisions between the evaporated aluminum atoms and oxygen atoms increased, thereby reducing the number of aluminum atoms which reach to the substrate. Therefore, the deposition rate of the film decreased as the oxygen pressure increased.
1 .5
25
•—~
/
20
Lo-e-~+~ DEPOSITION tse~e-e0/AL RATIO 0
1
2
1.4
RATE 3
I
4
I
5
6
OXYGEN PRESSURE (mTorr) Fig. 2. Dependence of deposition rate and the O:Al ratio of the ARE aluminum oxide films on oxygen pressure.
217
Dark brown colored films which indicate excess aluminum in the films were obtained at 1 mTorr whereas clear and featureless films were obtained at 5 mTorr. The color of the film prepared at 2 mTorr was light brown. The ratio of oxygen to aluminum is shown in Fig. 2. The O:Al ratio increased as the oxygen pressure increased. Aluminum-rich films were formed at lower oxygen pressure owing to an insufficiency of oxygen species at the substrate. On the contrary, the number of oxygen species increased with oxygen pressure, thereby producing near-stoichiometric films at higher oxygen pressure. The composition of the films obtained at 5 mTorr was 42 at.% Al and 58at.% 0. X-ray diffraction patterns of all the films showed a broad halo at near 22, indicating an amorphous structure (Fig. 3). An interesting observation from X-ray diffraction analysis is that this broad halo shifts to a higher scattering angle as the aluminum content increases. This is believed to be due to the change in the chemical short-range order of the films. The nearest-neighbor distances may change, resulting in change in the microscopic density of the film as the composition of the film varies. Detailed structural studies using neutron scattering or X-ray scattering measurements would be needed to understand the relationship between the composition of the aluminum oxide films and the atomic scale structure of these films. Figure 4 shows the effect of annealing on the X-ray diffraction pattern of the sample whose composition was close to stoichiometric A12O3. The sample which initially exhibited the amorphous phase was heat treated at 1200 ~C in an atmosphere of argon gas for 2 h. Several peaks appeared after annealing, which were indexed as ct-Al2 03 which has a corundum structure, except for one peak near 65~.The peak near 65~was indexed as 0-Al203. According to McArdle and Messing [17], 0-Al2 03 transforms to ct-Al203 at about 1200 ~C.
1~
~
SCATTERING ANGLE (TWO ThETA)
Fig. 3. X-ray diffraction patterns of the aluminum oxide films deposited at 100 W of r.f. power and 520—580 mA of electron beam current at oxygen pressures of 5 mTorr (spectrum a), 2 mTorr (spectrum b) and 1 mTorr (spectrum c).
218
1’~ Ii
(]~‘rI~:NI\I;ALC;l,F:
TWO ‘IEWTA)
Fig. 4. X.ray diffraction patterns of the aluminum oxide films deposited at 100 W of r.f. power, 520 580 mA of electron beam current and 5 mTorr of oxygen pressure without annealing (spectrum a) and with annealing at 1200 C for 2 h (spectrum b).
Figure 4 shows that the transformation from 0 to ~i phase had not been completed after 2 h at 1200 C. Values for refractive index are mainly determined by the O:Al ratio and the density of the film. Refractive index increases as the ratio O:Al decreases and the density of the film increases [9, 18]. A higher value for the refractive index is usually considered to be an indication of an aluminum-rich film. The results displayed in Fig. 5 show that refractive index decreases with increase
1.8
~-~-—
.---~
ijL-
~1.6
0 I
0
.-—~
4
z 1.4
1 3 0
—~~--—~—‘--—~
1
2
~_.~____~_.___1 3 4
5
‘
6
OXYGEN PRESSURE (mTorr) Fig. 5. Plot of refractive index as a function of oxygen pressure.
219
1200
1000
800
WAVENUMBER
800
400
200
1)
(cm
Fig. 6. IR transmission spectra of the ARE aluminum oxide films deposited at 1 mTorr (spectrum a) and 5 mTorr (spectrum b).
in oxygen partial pressure. The observed variation in refractive index with oxygen pressure can be attributed to the stoichiometry of the film. For a given electron beam current and r.f. excitation power, the ratio O:Al decreases as the oxygen partial pressure decreases, thereby resulting in aluminum-rich films. As the oxygen pressure decreases, values for refractive index approach a constant value, about 1.5, which is close to the refractive index of the stoichiometric Al 2 03 film. The data show that the lowest value observed, 1.47, is smaller than those of the films obtained by other techniques which are in the range 1.5—1.54 [1, 14, 19]. This may be due to the lower density of our films, which resulted from the very high deposition rate. The density of the film can probably be increased by biasing and/or heating the substrate during deposition. Typical JR transmission spectra are displayed in Fig. 6. All the IR spectra show a very broad band between 400 and 1000 cm’ which is a characteristic of an amorphous phase. An ideal Al—O octahedron is supposed to give rise to two JR-active vibration modes at 370 and 651 cm’ and these two vibration modes may split into six vibration modes at different frequencies, 330, 370, 413, 492, 651 and 770cm’, when the Al—0 coordination unit is disordered [20]. In this study, the traces of three peaks were observed at 770, 660 and 380 cm’ for the sample prepared at higher oxygen pressure (5 mTorr), which are indicated by arrows in Fig. 6. The small bump near 600 cm’ is due to the silicon substrate. The peak near 770 cm’ decreases with increasing oxygen pressure and almost disappears for the samples deposited at 1 mTorr of oxygen pressure. Even though the reason is not quite clear, we believe that this is related to the change in coordination numbers ZA1 and Z0 resulting from the composition changes.
220
3.3. Effect of electron beam current on deposition rate at higher oxygen pressure (5 mTorr) Several aluminum oxide films were deposited at 5 mTorr of oxygen pressure and various electron beam currents for 2 h. The electron beam current was changed during deposition to keep the pressure at 5 mTorr as mentioned in Section 3.1. Deposition rate increases drastically with the electron beam current as expected (Fig. 7). All the films were clear and colorless regardless of electron beam current. The maximum deposition rate obtained was 12 I.tm h However, this film cracked during deposition. The films deposited with deposition rate up to 8 tim h 1 showed a very smooth surface without any indication of cracking. A typical surface profile of the film deposited at a deposition rate of 8 I.tm h is shown in Fig. 8. The film has a local surface roughness as low as 100 A, which is excellent considering the total large thickness of the film. A scanning electron micrograph obtained for the film deposited on a silicon wafer under the same conditions as Fig. 8 is shown in Fig. 9. It ~.
lCOiiiA
I 3Cm 7OOmii\ AlbOimi \ 360imi\
7m()ii.\
(Ciii
(Ill
A
COnIA OniA
7))
40L:
700
600
700
800
C1.FCTRON 13EAM CURRENT (mA)
Fig. 7. Dependence of deposition rate of the ARE aluminum oxide films on electron beam current.
-j
600~ 400
~ 20: ~m -200 -400
I
-600 0
400 SCAN [ENGR
800
1200
(MICRONS)
Fig. 8. Surface profile of the film deposited at a deposition rate of 8 pm h
221
Fig. 9. Scanning electron micrograph of the film deposited at a deposition rate of 8 pm h’.
represents a typical surface of a homogeneous amorphous film. No visible features were observed.
4. Conclusions Deposition of the aluminum oxide film with high deposition rate has been achieved using the ARE technique. Films with a very good surface topography were prepared with near-stoichiometric composition at a deposition rate of 8 tim h~. The present authors believe that this deposition rate could be increased close up to the maximum deposition rate, 12 tim h1, without cracks by biasing and/or heating the substrates to a higher temperature than the 300 ~C used in these experiments.
References 1 2 3 4 5 6
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