- Email: [email protected]
ITO film analyses by FTIR
INFRAREDPHYSICS &TECHNOLOGY EI~qFNIER
Infrared Phys. Technol. 36 (1995) 779-784
,
ITO film analyses by FTIR J. Henry, J. Livingstone Department of Electrical and Electronic Engineering, The University of Western Australia, Nedlands, WA 6009, Australia
Received t2 September 1994
Abstract
Indium tin oxide thin films have been the subject of extensive research due to their attractive optical and electronic properties. The effects of essential processing steps carried out on these films which are used in layered semiconductor devices have been monitored by FTIR and the different absorption spectra obtained indicate that changes are occurring in film composition. In electronics, important compositional changes are normally determined by electrical measurements which often destroy or irreversibly alter the films. Infrared measurements are rapid and non-destructive and so permit analyses through a number of fabrication steps. This technique therefore provides a means for the optimisation of these films.
1. Introduction
Thin film ITO is transparent and highly conductive and has many important uses in electronic device fabrication [!,2]. One major application is as an anti-reflective coating in photo-active devices and another as a degenerate n-type semiconductor for use in n+-p heterojunctions [3-7]. In the case of sputtered ITO films, deposition and post-deposition treatments can alter the chemical composition and it is this important feature that we have studied using FTIR. In particular we have looked at the effects of: (1) the percentage of 02 in the sputtering chamber; (2) annealing the deposited films in oxidising and inert atmospheres at temperatures ranging from 100°C to 400°C.
The results showed that marked changes occur due to some of the above effects and more importantly, gave an indication of those procedures that might be used to optimise the final film. Furthermore, we have interpreted the FTIR results to indicate that oxygen diffuses in and out of films depending on annealing conditions. Diffusion mechanisms similar to these, were proposed by other researchers [8,9] working with films being annealed in vacuum, so that some of our work substantiates their conclusions. They heated a sample firstly in air and then under vacuum and from resistivity measurements they concluded that oxygen was incorporated in air and out-diffused under vacuum. We believe, from studies similar to their work, that the interpretation of in-out diffusion mechanisms is essentially correct.
01350-44951'94/$09.50 ~) 1995 ElsevierScience B.V. All rights reserved SSD! 1350-4495(945)00013-5
780
J. Henry, J. Lirmgstone,,Infrared Phys. Technol 36 (1995) 779-784 Effects of Oxygen % on Spectral Components
2. Experimental The ITO target used for sputtering films was a disc of In,_O3 90 wt%/SnO,_ 10 wt% from Cerac Inc., Milwaukee, Wisconsin, USA. This was sputtered in argon or argon/oxygen mixtures typically for about 30mins at about 2kV and a power density of 0.5 w/cm 2. The films, of approximately 1000A, were deposited on to silicon substrates which had been cleaned by a standard degreasing procedure. The IR spectra were obtained using a Biorad FTS 7 model IR Spectrometer. The wavenumber range measured was 4000 to 400cm -~ and the range investigated with some precision was 1300 to 400 cm-'. Films were sintered in a standard diffusion furnace within the region 100'C to 400°C using high purity grade oxygen and argon. Initially, an FTIR scan was taken of the ITO target as a reference to find the wavenumbers at which the major absorbances occurred and hence which can be used to characterise ITO. While the wavenumbers at which these peaks occur were clear, we cannot compare on the same scales, the absorbance of the target spectrum with those of the deposited films owing to the great difference in their thicknesses and hence total absorptions. However, having a signature spectrum of the target's principal wavenumbers made it possible to evaluate the effects of various processes on the thin films, i.e. whether each process resulted in a film with a spectral profile more like that of the target or less so.
3. Results and discussion Typically, within a particular spectral region, a large majority of the peaks will tend to change in the same way, and we believe that the column graphs of Figs. 1-3 present the changes in the best form to give a clear picture. We have chosen the wavenumbers 740 and 880 cm-t as the wavenumbers which best illustrate the relative changes shown in Figs. 2 to 3. We have also included two of our actual spectra as illustrative of our measurements and these are shown in Figs. 4 and 5.
L-
l
0%
•
11%
122%
670
790
1050
~ ' a v e n u l l l b e / ~ el~'l4
Fig. 1. The effect of different oxygen percentages in the sputtering environment on spectral absorbance in ITO films shown at three typical wavenumbers.
3. I. Effects of oxygen partial pressure on film composition For a constant total pressure, the percentage of oxygen admitted into the sputtering ambient was varied and measured. From our results we observed negligible differences between the films sputtered in pure argon and those sputtered in a mixture with a low percentage of oxygen, however marked changes in the magnitude of the absorption were observed when oxygen content was increased to 22%, with the films' total spectral profile tending to appear more like that of the ITO target spectrum. Fig. 1 shows the effect of sputtering in ambients with varying oxygen content.
3.Z Effects of heating in an O: ambient on reactirely sputtered ITO film compositions Films sputtered in 14%, 24% and 45% oxygen were heated in a pure oxygen environment with the following results: --Sputtering ambient 14% O.,: Heating in oxygen in the range 100°C to 400°C resulted in a film that had higher peaks i.e. they had increased in magnitude to appear more like the original ITO target and this increase in relative absorption is shown in Fig. 2a. --Sputtering ambient 24% 02: Except for one measurement at 400°C, which we cannot explain, smaller changes were observed here
J. Henry, J. Liringstone/Infrared Phys. Technol. 36 (1995) 779-784
781
Effects of Heating 14% Films in Oxygen
i
•
As deposited
• lOO'C =,ux~
==
.¢ 740
880 Wavenumbers
c:m 4
Effects of Heating 24% Films in Oxygen
geq
•
As deposited
• •
lOO.C 4oooc
A~
74O
880 Wavenumbers
t~ln 4
Effects of Heating 45% Films in Oxygen 2
As deposited
I
lOOoC 4oo°¢
.< 0 740
880 W a v ~ n l l M i b e N ¢~lj 4
Fig. 2. Effects of heating ITO films which had been sputtered in varying oxygen environments. (a) shows a clear increase in absorbance (b) levels are reasonably constant and (c) shows a marked decrease in absorbance.
782
J. Henry, J. Livingstone~Infrared Phys. Technol 36 (1995) 779-784 Effects of Heating 12% Films in Argon
• As deposited • 200"C
E
i 4o0"c
740
880 Wavenumben
cm 4
Effects of Heating 41% Films in Argon
2
i,
•
As deposited
! i
20(~'C 400°C
¢1
o 740
880 Waventtmber~
aR 4
Fig. 3. Effects of heating ITO films which had been sputtered in varying oxygen environments. (a) shows a clear decrease in absorbance and (b) also shows a decrease in absorbance.
which indicate absorption levels are reasonably constant over the temperature range studied. This is shown in Fig. 2b. --Sputtering ambient 45% O2: When the as-deposited film was heated, the spectra taken after each run looked progressively less like that of the original ITO target, implying that oxygen is diffusing out of the ITO film. This is illustrated in Fig. 2c. From the above observations and the understanding that these peaks are oxygen-related, we believe that the films are being depleted or added to depending on the conditions i.e. the oxygen-rich film (45%) loses oxygen with heating whereas the 14% film adds oxygen. This agrees with other workers who have reported that annealing ITO in an oxygen ambient at temperatures higher than deposition provides oxygen to the ITO film if it is
oxygen deficient and reduces the content if it is oxygen rich [10,1l]. 3.3. Effect of heating in an argon ambient on sputtered ITO film composition Films were deposited in 0% admitted oxygen, 12% oxygen and 41% oxygen and then separately heated in flowing argon. In all cases, the spectra obtained showed a move away from that of the ITO signature, implying that oxygen is diffusing out of the films when given this treatment. These changes are summarised below: --Sputtering ambient 0% O2: The spectra obtained for the film is illustrated in Fig. 4 and shows the as-deposited curve compared to that obtained after annealing at 100:C, and then at 250°C to 350--C, indicating clear com-
J. Henry, J. Livingstone/Infrared Phys. Technol. 36 (1995) 779-784
783
A b s
o
b a
n c e
1
l
I
l
flO0
790
780
770
I
]
760 750 Wavenunbers
1
I
1
[
740
730
720
7~0
Fig. 4. Spectrum of a film in the region of 740 cm -~ deposited in a sputtering environment of0%oxygen and then sintered in argon. (a) As deposited; (b) 100:C; (c) 250-350°C.
I I
790
I
]
I
780
770
760
I
1
750 740 Wavenu~ers
I
l
I
730
720
710
Fig. 5. Spectrum of a film in the region of 740 cm-~ deposited in sputtering environment of 41% oxygen and sintered in argon. (a) As deposited; (b) 200°C; (c) 400"C.
784
J. Henry, J. Livingstone~Infrared Phys. Technol 36 (1995) 779-784
positional changes which we interpret as the out-diffusion of oxygen. - - S p u t t e r i n g ambient 12% O,: The film was sintered firstly in argon at 200-~C and then at 400°C and the curves showed a decrease in absorptions from that of the ITO target with progressive heating which is illustrated in Fig. 3a. - - S p u t t e r i n g ambient 41% O,: heating in argon at 200°C and then at 400°C produced the decrease in absorption shown in Figs. 3b and 5.
4. Conclusions The various treatments outlined above can effect marked changes in indium tin oxide films and the technological implications of these are as follows: (I) Percentage oxygen in the sputtering environment: Fig. 1 illustrates that films sputtered in a higher partial oxygen pressure, showed increased absorption and produced spectral profiles tending to appear similar to that of the indium tin oxide target. (2) Heating in an oxidising ambient: Marked changes were observed for films heated in an oxygen ambient. Films deposited with low oxygen concentrations showed clear signs of oxygen incorporation into the films, whereas
films deposited in high oxygen levels tended to lose oxygen. (3) Heating in an argon ambient: In all cases, the spectra obtained showed a move away from that of the I T O reference, and we conclude that oxygen is diffusing out of the films when given this treatment. In summary, we have shown F T I R to be a useful technique to monitor the films at various stages of processing and hence indicate possible methods for optimising film properties.
References [l] K.L. Chopra, S. Major and D.K. Pandya Thin Solid Films 102 (1983) I. [2] S. Naseemand T.J. Coutts, Thin Solid Films 138 0986) 65. [3] T.A. Gessert, X. Li, M.W. Wanlass, A.J. Nelson and T.J. Coutts, J. Vac. Sci. Technol. A 8 (1990) 1912. [4] K.S. Sree Harsha, K.J. Bachmann,. P.H. Schmidt, E.G. Spencer and F.A. Thiel, Appl. Phys. Lett. 30 (1977) 645. [5] K.J. Bachmann, H. Screiber, W.R. Sinclair, P.H. Schmidt, F.A. Thiel, E.G. Spencer, G. Pasteur, W.L. Feldmann and K.S. Sree Harsha, J. Appl. Phys. 50 3441. [6] T.J. Courts. S. Naseem. App[. Phys. Lett. 46 0985) 165. [7] S. Naseem. T.J. Coutts, H. Aharoni, J. Vac. Sci. Technol. A 4 (1986) 465. [8] L. Bardos, M. Libra, Vacuum 39 (1989) 33. [9] A.J. Steckl, G. Mohammed, J. Appl. Phys. 51 (1980) 3890. [10] T. Karasawa, Y. Miyata, Thin Solid Films 223 (1993) 135. [l 1] M. Venkatesan. S. McGee, U. Mitra, Thin Solid Films 170 (1989) 151.
Infrared Phys. Technol. 36 (1995) 779-784
,
ITO film analyses by FTIR J. Henry, J. Livingstone Department of Electrical and Electronic Engineering, The University of Western Australia, Nedlands, WA 6009, Australia
Received t2 September 1994
Abstract
Indium tin oxide thin films have been the subject of extensive research due to their attractive optical and electronic properties. The effects of essential processing steps carried out on these films which are used in layered semiconductor devices have been monitored by FTIR and the different absorption spectra obtained indicate that changes are occurring in film composition. In electronics, important compositional changes are normally determined by electrical measurements which often destroy or irreversibly alter the films. Infrared measurements are rapid and non-destructive and so permit analyses through a number of fabrication steps. This technique therefore provides a means for the optimisation of these films.
1. Introduction
Thin film ITO is transparent and highly conductive and has many important uses in electronic device fabrication [!,2]. One major application is as an anti-reflective coating in photo-active devices and another as a degenerate n-type semiconductor for use in n+-p heterojunctions [3-7]. In the case of sputtered ITO films, deposition and post-deposition treatments can alter the chemical composition and it is this important feature that we have studied using FTIR. In particular we have looked at the effects of: (1) the percentage of 02 in the sputtering chamber; (2) annealing the deposited films in oxidising and inert atmospheres at temperatures ranging from 100°C to 400°C.
The results showed that marked changes occur due to some of the above effects and more importantly, gave an indication of those procedures that might be used to optimise the final film. Furthermore, we have interpreted the FTIR results to indicate that oxygen diffuses in and out of films depending on annealing conditions. Diffusion mechanisms similar to these, were proposed by other researchers [8,9] working with films being annealed in vacuum, so that some of our work substantiates their conclusions. They heated a sample firstly in air and then under vacuum and from resistivity measurements they concluded that oxygen was incorporated in air and out-diffused under vacuum. We believe, from studies similar to their work, that the interpretation of in-out diffusion mechanisms is essentially correct.
01350-44951'94/$09.50 ~) 1995 ElsevierScience B.V. All rights reserved SSD! 1350-4495(945)00013-5
780
J. Henry, J. Lirmgstone,,Infrared Phys. Technol 36 (1995) 779-784 Effects of Oxygen % on Spectral Components
2. Experimental The ITO target used for sputtering films was a disc of In,_O3 90 wt%/SnO,_ 10 wt% from Cerac Inc., Milwaukee, Wisconsin, USA. This was sputtered in argon or argon/oxygen mixtures typically for about 30mins at about 2kV and a power density of 0.5 w/cm 2. The films, of approximately 1000A, were deposited on to silicon substrates which had been cleaned by a standard degreasing procedure. The IR spectra were obtained using a Biorad FTS 7 model IR Spectrometer. The wavenumber range measured was 4000 to 400cm -~ and the range investigated with some precision was 1300 to 400 cm-'. Films were sintered in a standard diffusion furnace within the region 100'C to 400°C using high purity grade oxygen and argon. Initially, an FTIR scan was taken of the ITO target as a reference to find the wavenumbers at which the major absorbances occurred and hence which can be used to characterise ITO. While the wavenumbers at which these peaks occur were clear, we cannot compare on the same scales, the absorbance of the target spectrum with those of the deposited films owing to the great difference in their thicknesses and hence total absorptions. However, having a signature spectrum of the target's principal wavenumbers made it possible to evaluate the effects of various processes on the thin films, i.e. whether each process resulted in a film with a spectral profile more like that of the target or less so.
3. Results and discussion Typically, within a particular spectral region, a large majority of the peaks will tend to change in the same way, and we believe that the column graphs of Figs. 1-3 present the changes in the best form to give a clear picture. We have chosen the wavenumbers 740 and 880 cm-t as the wavenumbers which best illustrate the relative changes shown in Figs. 2 to 3. We have also included two of our actual spectra as illustrative of our measurements and these are shown in Figs. 4 and 5.
L-
l
0%
•
11%
122%
670
790
1050
~ ' a v e n u l l l b e / ~ el~'l4
Fig. 1. The effect of different oxygen percentages in the sputtering environment on spectral absorbance in ITO films shown at three typical wavenumbers.
3. I. Effects of oxygen partial pressure on film composition For a constant total pressure, the percentage of oxygen admitted into the sputtering ambient was varied and measured. From our results we observed negligible differences between the films sputtered in pure argon and those sputtered in a mixture with a low percentage of oxygen, however marked changes in the magnitude of the absorption were observed when oxygen content was increased to 22%, with the films' total spectral profile tending to appear more like that of the ITO target spectrum. Fig. 1 shows the effect of sputtering in ambients with varying oxygen content.
3.Z Effects of heating in an O: ambient on reactirely sputtered ITO film compositions Films sputtered in 14%, 24% and 45% oxygen were heated in a pure oxygen environment with the following results: --Sputtering ambient 14% O.,: Heating in oxygen in the range 100°C to 400°C resulted in a film that had higher peaks i.e. they had increased in magnitude to appear more like the original ITO target and this increase in relative absorption is shown in Fig. 2a. --Sputtering ambient 24% 02: Except for one measurement at 400°C, which we cannot explain, smaller changes were observed here
J. Henry, J. Liringstone/Infrared Phys. Technol. 36 (1995) 779-784
781
Effects of Heating 14% Films in Oxygen
i
•
As deposited
• lOO'C =,ux~
==
.¢ 740
880 Wavenumbers
c:m 4
Effects of Heating 24% Films in Oxygen
geq
•
As deposited
• •
lOO.C 4oooc
A~
74O
880 Wavenumbers
t~ln 4
Effects of Heating 45% Films in Oxygen 2
As deposited
I
lOOoC 4oo°¢
.< 0 740
880 W a v ~ n l l M i b e N ¢~lj 4
Fig. 2. Effects of heating ITO films which had been sputtered in varying oxygen environments. (a) shows a clear increase in absorbance (b) levels are reasonably constant and (c) shows a marked decrease in absorbance.
782
J. Henry, J. Livingstone~Infrared Phys. Technol 36 (1995) 779-784 Effects of Heating 12% Films in Argon
• As deposited • 200"C
E
i 4o0"c
740
880 Wavenumben
cm 4
Effects of Heating 41% Films in Argon
2
i,
•
As deposited
! i
20(~'C 400°C
¢1
o 740
880 Waventtmber~
aR 4
Fig. 3. Effects of heating ITO films which had been sputtered in varying oxygen environments. (a) shows a clear decrease in absorbance and (b) also shows a decrease in absorbance.
which indicate absorption levels are reasonably constant over the temperature range studied. This is shown in Fig. 2b. --Sputtering ambient 45% O2: When the as-deposited film was heated, the spectra taken after each run looked progressively less like that of the original ITO target, implying that oxygen is diffusing out of the ITO film. This is illustrated in Fig. 2c. From the above observations and the understanding that these peaks are oxygen-related, we believe that the films are being depleted or added to depending on the conditions i.e. the oxygen-rich film (45%) loses oxygen with heating whereas the 14% film adds oxygen. This agrees with other workers who have reported that annealing ITO in an oxygen ambient at temperatures higher than deposition provides oxygen to the ITO film if it is
oxygen deficient and reduces the content if it is oxygen rich [10,1l]. 3.3. Effect of heating in an argon ambient on sputtered ITO film composition Films were deposited in 0% admitted oxygen, 12% oxygen and 41% oxygen and then separately heated in flowing argon. In all cases, the spectra obtained showed a move away from that of the ITO signature, implying that oxygen is diffusing out of the films when given this treatment. These changes are summarised below: --Sputtering ambient 0% O2: The spectra obtained for the film is illustrated in Fig. 4 and shows the as-deposited curve compared to that obtained after annealing at 100:C, and then at 250°C to 350--C, indicating clear com-
J. Henry, J. Livingstone/Infrared Phys. Technol. 36 (1995) 779-784
783
A b s
o
b a
n c e
1
l
I
l
flO0
790
780
770
I
]
760 750 Wavenunbers
1
I
1
[
740
730
720
7~0
Fig. 4. Spectrum of a film in the region of 740 cm -~ deposited in a sputtering environment of0%oxygen and then sintered in argon. (a) As deposited; (b) 100:C; (c) 250-350°C.
I I
790
I
]
I
780
770
760
I
1
750 740 Wavenu~ers
I
l
I
730
720
710
Fig. 5. Spectrum of a film in the region of 740 cm-~ deposited in sputtering environment of 41% oxygen and sintered in argon. (a) As deposited; (b) 200°C; (c) 400"C.
784
J. Henry, J. Livingstone~Infrared Phys. Technol 36 (1995) 779-784
positional changes which we interpret as the out-diffusion of oxygen. - - S p u t t e r i n g ambient 12% O,: The film was sintered firstly in argon at 200-~C and then at 400°C and the curves showed a decrease in absorptions from that of the ITO target with progressive heating which is illustrated in Fig. 3a. - - S p u t t e r i n g ambient 41% O,: heating in argon at 200°C and then at 400°C produced the decrease in absorption shown in Figs. 3b and 5.
4. Conclusions The various treatments outlined above can effect marked changes in indium tin oxide films and the technological implications of these are as follows: (I) Percentage oxygen in the sputtering environment: Fig. 1 illustrates that films sputtered in a higher partial oxygen pressure, showed increased absorption and produced spectral profiles tending to appear similar to that of the indium tin oxide target. (2) Heating in an oxidising ambient: Marked changes were observed for films heated in an oxygen ambient. Films deposited with low oxygen concentrations showed clear signs of oxygen incorporation into the films, whereas
films deposited in high oxygen levels tended to lose oxygen. (3) Heating in an argon ambient: In all cases, the spectra obtained showed a move away from that of the I T O reference, and we conclude that oxygen is diffusing out of the films when given this treatment. In summary, we have shown F T I R to be a useful technique to monitor the films at various stages of processing and hence indicate possible methods for optimising film properties.
References [l] K.L. Chopra, S. Major and D.K. Pandya Thin Solid Films 102 (1983) I. [2] S. Naseemand T.J. Coutts, Thin Solid Films 138 0986) 65. [3] T.A. Gessert, X. Li, M.W. Wanlass, A.J. Nelson and T.J. Coutts, J. Vac. Sci. Technol. A 8 (1990) 1912. [4] K.S. Sree Harsha, K.J. Bachmann,. P.H. Schmidt, E.G. Spencer and F.A. Thiel, Appl. Phys. Lett. 30 (1977) 645. [5] K.J. Bachmann, H. Screiber, W.R. Sinclair, P.H. Schmidt, F.A. Thiel, E.G. Spencer, G. Pasteur, W.L. Feldmann and K.S. Sree Harsha, J. Appl. Phys. 50 3441. [6] T.J. Courts. S. Naseem. App[. Phys. Lett. 46 0985) 165. [7] S. Naseem. T.J. Coutts, H. Aharoni, J. Vac. Sci. Technol. A 4 (1986) 465. [8] L. Bardos, M. Libra, Vacuum 39 (1989) 33. [9] A.J. Steckl, G. Mohammed, J. Appl. Phys. 51 (1980) 3890. [10] T. Karasawa, Y. Miyata, Thin Solid Films 223 (1993) 135. [l 1] M. Venkatesan. S. McGee, U. Mitra, Thin Solid Films 170 (1989) 151.