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Study of graded aluminium oxide films prepared by metal-organic chemical vapour deposition
Thin Sohd Fdms, 127 (1985) 85-91 ELECTRONICS AND OPTICS
85
STUDY OF GRADED ALUMINIUM OXIDE FILMS PREPARED BY METAL-ORGANIC CHEMICAL VAPOUR DEPOSITION C. DHANAVANTRI AND R. N. KAREKAR
Department of Phystcs, Umverszty of Poona, Pune 411007 (Indta) V. J. RAO
Physwal Chemistry Dwtston, National Chemlcal Laboratory, Pune 411008 (lndza} (Received October 10, 1984, accepted January 8, 1985)
Films of aluminium oxide with graded refractive index were successfully obtained at relatively low temperatures by the metal-organic chemical vapour deposition technique. The refractive index varied from 1.66 to 1.56 for a substrate temperature variation of 100 °C. It is also found that the deposition conditions for homogeneous films can be extrapolated to design inhomogeneous films to sufficient accuracy, as indicated by the agreement between theoretical calculations and ellipsometric results. The anomaly in the refractive index, expected at integral multiples of do (where do is the thickness multiplication factor for A and ~), is observed in ellipsometric measurements and further it is observed also in Abeles measurements.
1. INTRODUCTION
Thin films with graded refractive index play an important role as broad band antireflection coatings and also as multilayer interference filters 1. Potential applications lie in fields such as solar selective coatings and protective coatings for solar cells 2'3. The co-evaporation and sputtering techniques have been used to prepare graded index thin films of dielectric mixtures 1. The chemical vapour deposition (CVD) technique using inorganic compounds has also been used to manufacture graded index high silica optical fibres for telecommunications4. We have used the metal-organic CVD (MOCVD) technique to prepare graded index planar films, the low temperature operation of which offers an advantage compared with conventional CVD processes 5. We have prepared both homogeneous and inhomogeneous (graded index) aluminium oxide films by MOCVD. The homogeneous aluminium oxide films were prepared by a process similar to one reported earlier 6' 7 but used for the study of electrical properties. Although the basic treatment for graded refractive index films is the multilayer optical film theory of Rouard s, it is only recently, after the interest in this topic has grown for the study of inhomogeneous ion-implanted films, that the problem has been studied in detail (for SiO2 on silicon) by using simple ellipsometry at a single angle of incidence9. We also have used simple ellipsometry with these welldeveloped methods for the study of inhomogeneous aluminium oxide films. 0040-6090/85/$3 30
© ElsevierSequoia/Printedm The Netherlands
86
C. DHANAVANTRI, R. N. KAREKAR, V. J. RAO
2. EXPERIMENTAL DETAILS
The aluminium oxide films were deposited on borosilicate glass and silicon substrates (size, 1 in × 1 in) by using the M O C V D technique with a resistively heated horizontal displacement flow system. The temperature of the reaction chamber was controlled by a thyristerized linear digital temperature controller using chromel-alumel thermocouples. Alumlnium isopropoxlde was used as the reactant material. Nitrogen (at a pressure of 1 atm) was used as the carrier and diluent gas for the deposition. The following reaction takes place in the thermal decomposition of aluminium lsopropoxide 10: [ A l ( I - O C 3 H T ) 3 ] 2 ~ m1203 + 3 C 3 H v O H + 3 C 3 H 6
The homogeneous films were deposited with a stepwlse varying substrate temperature. The processing parameters for a typical homogeneous film were as follows: (1) substrate temperature, 325-450 °C with an increment of 25 °C; (2) temperatures of the reactant material, 145 and 150°C; (3) flow rate of the carrier gas, 1.51 m m - 1, (4) flow rate of the diluent, 91 m i n - 1 ; (5) deposition time, 5-15 mm. It was generally observed that the substrate temperature determined the refractive index and the reactant temperature the thickness of the film, other conditions being kept constant. Both homogeneous and inhomogeneous aluminium oxide films were transparent and adherent to glass and attained a single colour over the whole substrate. The inhomogeneous films were obtained by varying the deposihon temperature continuously. The processing parameters that were varied to obtain the mhomogeneous films were as follows: (1)substrate temperature, continuously varied from 450 to 350°C and from 415 to 325°C; (2)cooling rate, 100°C (10 min)-1; (3) reactant material kept at temperatures of 145 and 150°C (giving different thickness ranges). The other parameters were the same as for homogeneous films. Thicknesses of the alummium oxide films were measured by the method of Fizeau's interference fringes for all the films. The refractive indices were measured by Abeles' method for films with optical thicknesses near the odd multiples of 2/4. The thicknesses and refractive indices for both homogeneous and inhomogeneous films were measured with a Gaertner ellipsometer (model L 119 X) at 2 = 6328 ~. 3. CALCULATIONS FOR ELLIPSOMETRIC ANALYSIS It has been shown 9 that, when the refractive index of a surface film varies as a function n(x) along the thickness, the apparent values of the film thickness d a n d the film index ri determined ellipsometrically, assuming that the film is homogeneous, can be significantly different from the real thickness d and the mean index
1;o
( n 5 = -d
n(x) d x
respectively. The difference reaches a maximum value at a thickness do and integral
MOCVD GRADED A I 2 0 3 FILMS
87
multiples of this thickness, where do is the thickness multiplication factor for A and ~k.The ellipsometrically evaluated refractive index ~iexceeds the range of variation of the assumed distribution when the total film thickness d is approximately a multiple of do which, for our average refractive index of 1.585, is 2350 ~. The films with graded refractive index profiles normal to the substrate surface can always be treated theoretically by dividing the inhomogeneous films into a number of thinner homogeneous films 1. To achieve a result essentially independent of the number of layers 2, it is necessary to have d'/2 < 5 × 10-3, where d' is the sublayer thickness. We assumed the number of sublayers to be 200. Rouard's method s, 11 was followed for calculating the reflection coefficients to determine the value of A and ~. Tables were prepared for the A and ~b values of graded index films of aluminlum oxide on glass, assuming different linear gradients for different thicknesses. Figure 1 gives a typical plot of A versus ~k for thicknesses between 1150 and 3500/~ for a linear variation in refractive index between 1.56 and 1.66 from top to bottom of the film. The curves for the A and ~k values of homogeneous films with similar thickness ranges are also plotted in the same figure for comparison. The refractive indices chosen are 1.56, 1.66 and 1.585 (which is the mean of the first two values). 4. RESULTS AND DISCUSSION
4.1. Homogeneous films The A and ~ values for the aluminiu m oxide films, deposited under the typical conditions mentioned above, obtained by ellipsometry are given in Table I for different deposition temperatures. The thicknesses and refractive indices are obtained from these values of A and ~ by treating the films as homogeneous. The A and ~, values for glass were obtained as 179 ° 26' and 10 ° 18', giving a refractive index of 1.52. The film thicknesses obtained by the method of Fizeau's interference fringes and the refractive indices obtained by Abeles' method are also indicated in the same table for comparison. It can be seen that the thicknesses measured by ellipsometry using Fizeau's method agree to within ___100A for most of the samples. The refractive indices also match the values obtained using Abeles' method. We can also notice that there is a consistent decrease in refractive index from 1.642 to 1.561 when the temperature is reduced from 450 to 325 °C.
4.2. Inhomogeneous films After measuring the thicknesses of the inhomogeneous films by Fizeau's interference method, the A and ~ values measured by ellipsometry were examined for values of A and ~ that matched those tabulated by the method indicated above. Thus the refractive index variations were found for different thicknesses and deposition conditions and are given in Table II. It can be seen from the table that the thicknesses obtained by ellipsometry (assuming graded index films) and Fizeau's method match to within __+200.~. The apparent values of the film thickness/Tand the film index ~ determined elhpsometrically, assuming that the film is homogeneous, are also indicated in the same table for comparison, which clearly shows that these values are significantly different from the real thickness d and the mean index ( n ) as
88
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expected 9. These anomalous values of ~i and a are obtained for film thicknesses of about 4300 ~. This thickness is near an integral multiple of do (2350/~). Such an anomalous result will be possible only for inhomogeneous films. The refractive indices obtained by Abeles' method for graded index films were found to be nearly equal to the mean of the refractive index range as shown in Table II. However, these were matched only at optical thicknesses equal to odd multiples of 2/4, as required. 5. CONCLUDING REMARKS
Using the results obtained for homogeneous films (shown in Table I) where the different deposition temperatures yielded different refractive radices, inhomogeneous films were successfully prepared by varying the deposition temperature. Two temperature cycles were tried. By varying the temperature between 450 and 350 °C we obtained a refractive index variation range of 1.66-1.56 and by varying the temperature between 415 and 325 °C the refractive index variation range was slightly less, i.e. between 1.61 and 1.56. For certain thicknesses the mean refractive indices obtamed by Abeles' method and the mean refractive indices obtained by elhpsometry for graded index films agreed as expected. It is interesting to note that the anomalous values of the refractive indices were obtained by both ellipsometry and Abeles' method for film thicknesses nearly equal to an integral multiple of do. ACKNOWLEDGMENTS
The authors are indebted to the University Grants Commission, New Delhi, for a grant under the Departmental Special Assistance Programme, and to the Department of Science and Technology, New Delhi, for financial aid for the fabrication of the M O C V D set-up. One of the authors (C.D.) is grateful to the Council for Scientific and Industrial Research, New Delhi, for the award of a Senior Research Fellowship. REFERENCES
1 2 3 4 5 6 7 S 9 10 11
R A Jacobsson, Phy~ ThmFdms, 8(1975) 51 I T Rlchle and B Window, Appl. Opt, 16 (1977) 1438 D M JrotterandA J Slevers, Appl 0pt,19(1980)711, D Pearson, Appl SohdState Sct, 6 (1976) 173 L.A Ryabova, Curr Top. Mater. Sct.,7(1981)587. J A Aboaf, J Electrochem Soc, 114(1967)948 M T DuffyandW Kern, RCARev,31(1970) 154 P Rouard, Ann Phys (Parts), 7(1937)291. J N a k a t a a n d K Kajlyama, ThmSohdFdms, 80 (1981) 383 A A BarybmandV I Tomlhn, Zh Prtkl Khtm,49(1976)1699 F L McCrackm, A Fortran program for analysis of elhpsometer measurements, NBS Tech Note 479, 1969 (National Bureau of Standards, U S Department of Commerce, Washington, DC).
85
STUDY OF GRADED ALUMINIUM OXIDE FILMS PREPARED BY METAL-ORGANIC CHEMICAL VAPOUR DEPOSITION C. DHANAVANTRI AND R. N. KAREKAR
Department of Phystcs, Umverszty of Poona, Pune 411007 (Indta) V. J. RAO
Physwal Chemistry Dwtston, National Chemlcal Laboratory, Pune 411008 (lndza} (Received October 10, 1984, accepted January 8, 1985)
Films of aluminium oxide with graded refractive index were successfully obtained at relatively low temperatures by the metal-organic chemical vapour deposition technique. The refractive index varied from 1.66 to 1.56 for a substrate temperature variation of 100 °C. It is also found that the deposition conditions for homogeneous films can be extrapolated to design inhomogeneous films to sufficient accuracy, as indicated by the agreement between theoretical calculations and ellipsometric results. The anomaly in the refractive index, expected at integral multiples of do (where do is the thickness multiplication factor for A and ~), is observed in ellipsometric measurements and further it is observed also in Abeles measurements.
1. INTRODUCTION
Thin films with graded refractive index play an important role as broad band antireflection coatings and also as multilayer interference filters 1. Potential applications lie in fields such as solar selective coatings and protective coatings for solar cells 2'3. The co-evaporation and sputtering techniques have been used to prepare graded index thin films of dielectric mixtures 1. The chemical vapour deposition (CVD) technique using inorganic compounds has also been used to manufacture graded index high silica optical fibres for telecommunications4. We have used the metal-organic CVD (MOCVD) technique to prepare graded index planar films, the low temperature operation of which offers an advantage compared with conventional CVD processes 5. We have prepared both homogeneous and inhomogeneous (graded index) aluminium oxide films by MOCVD. The homogeneous aluminium oxide films were prepared by a process similar to one reported earlier 6' 7 but used for the study of electrical properties. Although the basic treatment for graded refractive index films is the multilayer optical film theory of Rouard s, it is only recently, after the interest in this topic has grown for the study of inhomogeneous ion-implanted films, that the problem has been studied in detail (for SiO2 on silicon) by using simple ellipsometry at a single angle of incidence9. We also have used simple ellipsometry with these welldeveloped methods for the study of inhomogeneous aluminium oxide films. 0040-6090/85/$3 30
© ElsevierSequoia/Printedm The Netherlands
86
C. DHANAVANTRI, R. N. KAREKAR, V. J. RAO
2. EXPERIMENTAL DETAILS
The aluminium oxide films were deposited on borosilicate glass and silicon substrates (size, 1 in × 1 in) by using the M O C V D technique with a resistively heated horizontal displacement flow system. The temperature of the reaction chamber was controlled by a thyristerized linear digital temperature controller using chromel-alumel thermocouples. Alumlnium isopropoxlde was used as the reactant material. Nitrogen (at a pressure of 1 atm) was used as the carrier and diluent gas for the deposition. The following reaction takes place in the thermal decomposition of aluminium lsopropoxide 10: [ A l ( I - O C 3 H T ) 3 ] 2 ~ m1203 + 3 C 3 H v O H + 3 C 3 H 6
The homogeneous films were deposited with a stepwlse varying substrate temperature. The processing parameters for a typical homogeneous film were as follows: (1) substrate temperature, 325-450 °C with an increment of 25 °C; (2) temperatures of the reactant material, 145 and 150°C; (3) flow rate of the carrier gas, 1.51 m m - 1, (4) flow rate of the diluent, 91 m i n - 1 ; (5) deposition time, 5-15 mm. It was generally observed that the substrate temperature determined the refractive index and the reactant temperature the thickness of the film, other conditions being kept constant. Both homogeneous and inhomogeneous aluminium oxide films were transparent and adherent to glass and attained a single colour over the whole substrate. The inhomogeneous films were obtained by varying the deposihon temperature continuously. The processing parameters that were varied to obtain the mhomogeneous films were as follows: (1)substrate temperature, continuously varied from 450 to 350°C and from 415 to 325°C; (2)cooling rate, 100°C (10 min)-1; (3) reactant material kept at temperatures of 145 and 150°C (giving different thickness ranges). The other parameters were the same as for homogeneous films. Thicknesses of the alummium oxide films were measured by the method of Fizeau's interference fringes for all the films. The refractive indices were measured by Abeles' method for films with optical thicknesses near the odd multiples of 2/4. The thicknesses and refractive indices for both homogeneous and inhomogeneous films were measured with a Gaertner ellipsometer (model L 119 X) at 2 = 6328 ~. 3. CALCULATIONS FOR ELLIPSOMETRIC ANALYSIS It has been shown 9 that, when the refractive index of a surface film varies as a function n(x) along the thickness, the apparent values of the film thickness d a n d the film index ri determined ellipsometrically, assuming that the film is homogeneous, can be significantly different from the real thickness d and the mean index
1;o
( n 5 = -d
n(x) d x
respectively. The difference reaches a maximum value at a thickness do and integral
MOCVD GRADED A I 2 0 3 FILMS
87
multiples of this thickness, where do is the thickness multiplication factor for A and ~k.The ellipsometrically evaluated refractive index ~iexceeds the range of variation of the assumed distribution when the total film thickness d is approximately a multiple of do which, for our average refractive index of 1.585, is 2350 ~. The films with graded refractive index profiles normal to the substrate surface can always be treated theoretically by dividing the inhomogeneous films into a number of thinner homogeneous films 1. To achieve a result essentially independent of the number of layers 2, it is necessary to have d'/2 < 5 × 10-3, where d' is the sublayer thickness. We assumed the number of sublayers to be 200. Rouard's method s, 11 was followed for calculating the reflection coefficients to determine the value of A and ~. Tables were prepared for the A and ~b values of graded index films of aluminlum oxide on glass, assuming different linear gradients for different thicknesses. Figure 1 gives a typical plot of A versus ~k for thicknesses between 1150 and 3500/~ for a linear variation in refractive index between 1.56 and 1.66 from top to bottom of the film. The curves for the A and ~k values of homogeneous films with similar thickness ranges are also plotted in the same figure for comparison. The refractive indices chosen are 1.56, 1.66 and 1.585 (which is the mean of the first two values). 4. RESULTS AND DISCUSSION
4.1. Homogeneous films The A and ~ values for the aluminiu m oxide films, deposited under the typical conditions mentioned above, obtained by ellipsometry are given in Table I for different deposition temperatures. The thicknesses and refractive indices are obtained from these values of A and ~ by treating the films as homogeneous. The A and ~, values for glass were obtained as 179 ° 26' and 10 ° 18', giving a refractive index of 1.52. The film thicknesses obtained by the method of Fizeau's interference fringes and the refractive indices obtained by Abeles' method are also indicated in the same table for comparison. It can be seen that the thicknesses measured by ellipsometry using Fizeau's method agree to within ___100A for most of the samples. The refractive indices also match the values obtained using Abeles' method. We can also notice that there is a consistent decrease in refractive index from 1.642 to 1.561 when the temperature is reduced from 450 to 325 °C.
4.2. Inhomogeneous films After measuring the thicknesses of the inhomogeneous films by Fizeau's interference method, the A and ~ values measured by ellipsometry were examined for values of A and ~ that matched those tabulated by the method indicated above. Thus the refractive index variations were found for different thicknesses and deposition conditions and are given in Table II. It can be seen from the table that the thicknesses obtained by ellipsometry (assuming graded index films) and Fizeau's method match to within __+200.~. The apparent values of the film thickness/Tand the film index ~ determined elhpsometrically, assuming that the film is homogeneous, are also indicated in the same table for comparison, which clearly shows that these values are significantly different from the real thickness d and the mean index ( n ) as
88
C. DHANAVANTRI,
R. N. KAREKAR,
V . J. R A O
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expected 9. These anomalous values of ~i and a are obtained for film thicknesses of about 4300 ~. This thickness is near an integral multiple of do (2350/~). Such an anomalous result will be possible only for inhomogeneous films. The refractive indices obtained by Abeles' method for graded index films were found to be nearly equal to the mean of the refractive index range as shown in Table II. However, these were matched only at optical thicknesses equal to odd multiples of 2/4, as required. 5. CONCLUDING REMARKS
Using the results obtained for homogeneous films (shown in Table I) where the different deposition temperatures yielded different refractive radices, inhomogeneous films were successfully prepared by varying the deposition temperature. Two temperature cycles were tried. By varying the temperature between 450 and 350 °C we obtained a refractive index variation range of 1.66-1.56 and by varying the temperature between 415 and 325 °C the refractive index variation range was slightly less, i.e. between 1.61 and 1.56. For certain thicknesses the mean refractive indices obtamed by Abeles' method and the mean refractive indices obtained by elhpsometry for graded index films agreed as expected. It is interesting to note that the anomalous values of the refractive indices were obtained by both ellipsometry and Abeles' method for film thicknesses nearly equal to an integral multiple of do. ACKNOWLEDGMENTS
The authors are indebted to the University Grants Commission, New Delhi, for a grant under the Departmental Special Assistance Programme, and to the Department of Science and Technology, New Delhi, for financial aid for the fabrication of the M O C V D set-up. One of the authors (C.D.) is grateful to the Council for Scientific and Industrial Research, New Delhi, for the award of a Senior Research Fellowship. REFERENCES
1 2 3 4 5 6 7 S 9 10 11
R A Jacobsson, Phy~ ThmFdms, 8(1975) 51 I T Rlchle and B Window, Appl. Opt, 16 (1977) 1438 D M JrotterandA J Slevers, Appl 0pt,19(1980)711, D Pearson, Appl SohdState Sct, 6 (1976) 173 L.A Ryabova, Curr Top. Mater. Sct.,7(1981)587. J A Aboaf, J Electrochem Soc, 114(1967)948 M T DuffyandW Kern, RCARev,31(1970) 154 P Rouard, Ann Phys (Parts), 7(1937)291. J N a k a t a a n d K Kajlyama, ThmSohdFdms, 80 (1981) 383 A A BarybmandV I Tomlhn, Zh Prtkl Khtm,49(1976)1699 F L McCrackm, A Fortran program for analysis of elhpsometer measurements, NBS Tech Note 479, 1969 (National Bureau of Standards, U S Department of Commerce, Washington, DC).