The application of the barrier-type anodic oxidation method to thickness testing of aluminum films Jianwen Chen, Manwen Yao, Ruihua Xiao, Pengfei Yang, Baofu Hu, and Xi Yao Citation: Review of Scientific Instruments 85, 094101 (2014); doi: 10.1063/1.4894525 View online: http://dx.doi.org/10.1063/1.4894525 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Resistive switching in aluminum/anodized aluminum film structure without forming process J. Appl. Phys. 106, 093706 (2009); 10.1063/1.3253722 In situ detection of porosity initiation during aluminum thin film anodizing Appl. Phys. Lett. 94, 074103 (2009); 10.1063/1.3081014 Observation of isolated nanopores formed by patterned anodic oxidation of aluminum thin films Appl. Phys. Lett. 88, 233112 (2006); 10.1063/1.2212535 Effects of starting material of aluminum doped zinc oxide underlayer on the electric properties of palladium doped silver film J. Vac. Sci. Technol. A 21, 1389 (2003); 10.1116/1.1560716 Bimodal spatial distribution of pores in anodically oxidized aluminum thin films J. Appl. Phys. 88, 6875 (2000); 10.1063/1.1321780
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 094101 (2014)
The application of the barrier-type anodic oxidation method to thickness testing of aluminum films Jianwen Chen, Manwen Yao,a) Ruihua Xiao, Pengfei Yang, Baofu Hu, and Xi Yao Functional Materials Research Laboratory, Tongji University, Shanghai 200092, China
(Received 20 May 2014; accepted 20 August 2014; published online 9 September 2014) The thickness of the active metal oxide film formed from a barrier-type anodizing process is directly proportional to its formation voltage. The thickness of the consumed portion of the metal film is also corresponding to the formation voltage. This principle can be applied to the thickness test of the metal films. If the metal film is growing on a dielectric substrate, when the metal film is exhausted in an anodizing process, because of the high electrical resistance of the formed oxide film, a sudden increase of the recorded voltage during the anodizing process would occur. Then, the thickness of the metal film can be determined from this voltage. As an example, aluminum films are tested and discussed in this work. This method is quite simple and is easy to perform with high precision. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4894525] I. INTRODUCTION
Thickness is a basic characteristic of thin films. Thickness test is getting more and more importance in material science and technology.1 There are many different ways to test the thickness of the film. Each of them has their own advantage and disadvantage. The testing thickness range, accuracy, and application are quite different. For example, the ellipsometry2 or the white-light interferometer3 is very good for the thickness measurement of transparent or semitransparent thin films, while they are not suitable for the opaque metal films. The x-ray reflectivity technique4 and the direct observation method by FE-SEM5 or TEM6 are two very powerful thickness test methods for metal films. However, the sophisticated instruments used in the two methods are expensive and not easy to perform. They are not inconvenient in practical applications, especially for the thickness tests of metal films in the nanometer scale. In this work, a simple thickness test method is proposed for the metal film in the nanometer scale. The principle of the method is based on the proportionality relationship between the thickness of the formed oxide film and the formation voltage in a barrier-type anodizing process.7 No special and expensive instruments are required in this method. It is also very easy to perform. As an example, the thickness measurement of aluminum films is tested by this method. Some important technical issues are discussed in this paper. II. PRINCIPLE OF ANODIZING METHOD
As a proven technology, the anodic oxidation has been widely applied to industry and daily life.8–10 There are two types of anodic aluminum oxide (AAO) films, barrier type and porous type. The barrier-type AAO film is formed in neutral electrolyte,11, 12 while the porous type AAO film is formed in corrosive electrolyte.7, 13, 14 The thickness of an AAO film is a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(9)/094101/5/$30.00
controlled solely by the formation voltage in a barrier-type anodizing process.7 There are two different control ways in a barrier-type anodizing process.7 The changes in voltage and ionic current density with respect to anodizing time would show the behavior illustrated in Fig. 1. In a current control step (region 1), the current is maintained at a constant value; thus the voltage and the thickness of formed oxide film are increasing linearly with time. In a voltage control step (region 2), the voltage is reached and maintained at a constant value. Thus, the current and the growth rate of the oxide film decrease rapidly with time. Because the current efficiency is relatively invariant in a given anodizing condition, the molar amount of a formed oxide film is proportional to the consumed metal film. For example, the thickness ratio of an evaporated aluminum film to the formed oxide film is 0.725, which is anodized in 3% ammonium tartrate.10 Hence, the formation voltage also determines the thickness of the metal film. If the formation voltage can be found out when the metal film is exhausted in a barrier-type anodizing process, the thickness of the metal film is determined and tested. Indeed, if the metal film is growing on a dielectric substrate, the recorded voltage can be easy to find out when the metal film is exhausted. As shown in Fig. 2, the anodizing process consists of three regions. At the first step (region AB in Fig. 2), the recorded voltage increases linearly with increasing time, which is the same as the current control step in a barrier-type anodizing process. Because of the electrical insulation of the formed film and the substrate, the specimen system becomes an electrical insulation system when the metal film that immersed in the electrolyte is exhausted. At this step (region BC in Fig. 2), the voltage suddenly increases to the setting upper limit voltage. Thus the recorded current is found to decrease to about zero. At last (region CD in Fig. 2), the voltage is maintained at the setting upper limit voltage, and the recorded current is slowly decreased almost to zero. Therefore, the thickness of the metal film is solely determined by the voltage VB . It can be calculated by the following
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Rev. Sci. Instrum. 85, 094101 (2014)
since no additional ions are added to the anodizing system. A pure Al plate is used as the cathodal material in this test example. The detailed study of the electrochemical electrode is referred to Ref. 17. IV. EXPERIMENTAL
FIG. 1. Formation of a barrier-type anodic oxide film.
formula: dmetal = a(bVB ) = cVB ,
(1)
where a is the thickness ratio of metal film to the formed oxide film; bis the anodizing ratio at the current control step in a barrier-type anodizing process; c is the ratio of the metal film to the formation voltage; and VB is the recorded voltage at the point B shown in Fig. 2. In addition, a natural oxide film will be formed on the surface of metal films in air. This natural oxide film is represented as the initial measurement voltage VA in the anodizing process; thus, the formula (1) should be rewired as: dmetal = ab(VB − VA ) = c(VB − VA ).
As a test example, the evaporated aluminum films were tested by this method. The aluminum films were used in this work. The aluminum films were deposited on polished quartz substrate of 10 × 10 × 1 mm3 by vacuum evaporation equipment (ZHD-400, Technol Science, Beijing, China). During the deposition, the substrates were held in a vacuum chamber with the pressure of lower than 2 × 10−4 Pa at 353 K. The rotation of the substrates was 10 revolutions per minute. 3% ammonium tartrate aqueous solution is used as an electrolyte in this study. The aluminum films are tested at 293 K. On account of electrical insulation of the quartz substrate, the tested voltage VB can be easy to find out in the anodizing process. The thickness of the aluminum film can be calculated by the formula (2), since the variables are related through a × b = c and only 2 of them have to be determined. The thickness ratio of the oxide film to the aluminum layer replaced, which is anodized in 3% ammonium tartrate aqueous solution, has been measured by Hass,10 that is a=
(2)
dAl = 0.725. dAl O 2
III. THE SETUP
The experimental setup of the test method is a typical anodizing system.15, 16 A general computer, a common source meter, a container, and clamps are used in this test system. The clamps are used to fix the sample and cathodes into the container. The source meter should be with real time communication, control and data processing functions, which make the test work automatically and avoid human disturbance. Also, the upper limit voltage of the source meter should be large enough to make sure that the voltage VB is existed. Source meter (Keithley 2410, USA) is used in this study. The good conductivity and chemical inert of the given electrolyte should be considered as the selection of cathodes’ material. Platinum and graphite would be suitable materials in some anodizing systems. Besides, the same metal material of the test sample would be also a suitable cathodal material
(3)
3
By measuring the thickness of 10 different AAO films that formed in 3% ammonium tartrate aqueous solution by the interferometer (Filmetrics F20, San Diego, CA), the ratio of formation alumina film to formation voltage at the current control step in the anodizing process is given in: b=
dAl
2
O3
Formation voltage
= (1.25 ± 0.03) nm/V.
(4)
This ratio is consistent with the literature reports.7, 10 Then, the ratio of aluminum film to the formation voltage that anodized in 3% ammonium tartrate aqueous solution is given by: c = ab = 0.725 × (1.25 ± 0.03) nm/V ≈ (0.91 ± 0.02) nm/V.
(5)
So, the formula (2) in this application can be expressed as: dAl = [(0.91 ± 0.02) · (VB − VA )] nm/V.
(6)
V. RESULTS AND DISCUSSION
Setting the upper limit voltage of 200 V and the current density of 1.25 mA/cm2 , the test voltage and current has been automatically recorded with the time by computer. As shown in Fig. 3, the initial voltage VA and the tested voltage VB of sample one are 4.0 V and 180.0 V, respectively. The thickness of oxide layers formed in air for sample one is: FIG. 2. Principle of the thickness test method in a barrier-type anodizing process.
dalumina = 1.25 × 4.0 nm = 5.0 nm.
(7)
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FIG. 3. Selecting the upper limit voltage at 200 V and the current density at 1.25 mA/cm2 , the initial voltage and the recorded voltage VB of Sample one is 4.0 V and 180.0 V, respectively.
This thickness of oxide layers formed in air is consistent with the value tested by Hass.10 So, the thickness of aluminum film of sample one can be calculated by the following formula (2): dAl = (0.91 ± 0.02) × (180.0 − 4.0) nm ≈ (159.5 ± 3.8) nm.
(8)
As shown in Fig. 4, this value is consistent with the value determined by FE-SEM within the error range. The thickness of the aluminum film of sample one is about 157 nm, where the error rate is about 5%. VI. TECHNICAL CONSIDERATIONS
Since the current efficiency of reaction occurring, the molecular weight of substance, the density of the metal film, and the density of formed oxide film are invariable in a given anodizing system, and the thickness of a formed oxide film is only dependent upon the amount of charge passed by Faraday’s laws.7 In other words, the current density value is less sensitive for the thickness test method. As shown in Fig. 5, selecting the current density of 1.25 mA/cm2 and 0.625 mA/cm2 , the thicknesses of the nat-
FIG. 4. FE-SEM of cross-section of sample one, thickness of the aluminum film is about 157 nm.
FIG. 5. Selecting current density of 1.25 mA/cm2 and 0.625 mA/cm2 , the initial voltages are 3.4 V and 3.4 V and the recorded voltages VB are 102.9 and 102.9 V, respectively.
ural oxide films are 4.3 nm and 3.8 nm, respectively. The thicknesses of the aluminum films are 90.2 nm and 90.6 nm, respectively. The thickness tested method with the different current density is consistent with each other within the margin of error. Thus it consumes a great deal of time in a low current density and the anodizing system is unstable at a too large current density. Therefore, the current density of about 1 mA/cm2 is a suitable value in this test method for aluminum film in 3% ammonium tartrate aqueous solution. Anderson and Devereux18 have discussed the importance of anodizing temperature in the anodizing process. In their study, the current is dependent upon the anodizing temperature at a given overpotential. The current efficiency decreases with the increasing of anodizing temperature, and therefore the anodizing ratio is solely dependent upon the anodizing voltage. As discussed above, the formation voltage is independent on the current density. The thickness test method is less sensitive for the anodizing temperature. Thus, the anodizing system would be unstable at a high temperature.19 Generally, the selection of anodizing temperature should be consistent with the temperature that is applied to testing the ratio c. The room temperature is suitable for anodizing temperature except in special conditions. The electrolyte applied to this thickness test method should be proven as a steady, reliable, and high current efficient electrolyte. Some useful electrolytes are shown in Table I. The anodizing ratio of aluminum or aluminum alloy with the different electrolyte is from 1.2 to 1.35. These different anodizing ratios are mainly dependent upon the different anodizing time at a given anodizing voltage. Hass10 has extensively discussed the relationship between anodizing ratio and anodizing time. Because the current efficiency is relatively invariant in a given anodizing condition, thickness of anodic oxide film is increasing linearly with the formation voltage and time at the current control step in the anodizing process. In this study, the anodizing ratio of pure aluminum is 1.25 nm/V at the current control step in a 3% weight ammonium tartrate, which is tested by the interferometer (Filmetrics F20, San Diego, CA). The anodiz-
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TABLE I. Anodizing ratio of aluminum or aluminum alloy in different electrolyte. Electrolyte (◦ C)
Anodizing ratio (nm/V) and reference
3% weight ammonium tartrate aqueous solution 0.1 mol/L ammonium pentaborate (20)
1.22 for 30 s,10 1.30 for 2 min,10 1.35 for 40 min10 1.226, 20
AGW electrolyte15 (3% aqueous solution of tartaric acid and propylene glycol at a volume ratio of 1 to 4) Molybdate and Tungstate electrolyte (20)
1.30 for 30 min21
Aqueous borate solutions
1.3516
Aluminum or aluminum alloy Evaporated aluminum Aluminium foil (trace impurities: 0.003% Fe; 0.004% Cu; 0.002 wt. % Si) Al-1% Si-0.5% Cu thin film
Aluminium foil (0.004 w/o Cu; 0.003 w/o Fe; 0.002 w/o Si) Pure aluminium sheet
ing ratio of aluminum or aluminum alloy is about 1.25 nm/V at the current control step in a barrier-type anodizing process. This thickness test method is also suitable for those metal films or semiconductors which can forma barrier-type anodic oxide film in an appropriate electrolyte. What tabulated in Table II are a number of anodizing ratios for several metals and semiconductors. The formation voltage is extremely sensitive to the thickness variation of metal or semiconductor film, from the anodizing ratios with much more than 0.1 nm/V. If the surface of the metal film is smooth enough, the test accuracy can be accurate to a single atomic layer level. However, in the practical application, the accuracy of this thickness test method is affected by the control of anodizing conditions. The determination of the parameters c in a given anodizing condition is critical in this thickness test method. In other words, the accuracy of the measurement is affected by the environmental uncertainties such as temperature, PH, and so on. To improve the accuracy, a reproducible and reliable preparation of the electrolyte under environmental uncertainties is an essential requirement. The selection of electrolyte is a critical issue in this method. The best selection of electrolyte is the electrolyte, which is not sensitive to various conditions, like the AGW electrolyte.5 The AGW electrolyte is a mixed solution of glycol and water. Hasegawa and Hartnagel15 have studied the AGW process well. In their study, the AGW process has been found to be always very stable and reliable, and the result is always re-producible even after many times of the anodization. In addition, the application of standard sample can effectively improve the testing accuracy, since the standard sample and the test sample are each anodizing in the same anodizing condition. Since this method is based on the theory of barriertype anodic oxidation, characteristics of this method are also TABLE II. Anodizing ratio of some metals and semiconductors. Metal Aluminum Tantalum Niobium Zirconium Tungsten Silicon GaAs GaSb
Anodizing ratio (nm/V) and reference 1.30,10 1.3510 1.67 2.27 2.0,7 2.1,7 2.4,7 2.77 1.87 0.387 1.3515 2.523
1.222
inherited from the anodic oxidation. Because the thickness of barrier-type oxide films is limited to
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 094101 (2014)
The application of the barrier-type anodic oxidation method to thickness testing of aluminum films Jianwen Chen, Manwen Yao,a) Ruihua Xiao, Pengfei Yang, Baofu Hu, and Xi Yao Functional Materials Research Laboratory, Tongji University, Shanghai 200092, China
(Received 20 May 2014; accepted 20 August 2014; published online 9 September 2014) The thickness of the active metal oxide film formed from a barrier-type anodizing process is directly proportional to its formation voltage. The thickness of the consumed portion of the metal film is also corresponding to the formation voltage. This principle can be applied to the thickness test of the metal films. If the metal film is growing on a dielectric substrate, when the metal film is exhausted in an anodizing process, because of the high electrical resistance of the formed oxide film, a sudden increase of the recorded voltage during the anodizing process would occur. Then, the thickness of the metal film can be determined from this voltage. As an example, aluminum films are tested and discussed in this work. This method is quite simple and is easy to perform with high precision. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4894525] I. INTRODUCTION
Thickness is a basic characteristic of thin films. Thickness test is getting more and more importance in material science and technology.1 There are many different ways to test the thickness of the film. Each of them has their own advantage and disadvantage. The testing thickness range, accuracy, and application are quite different. For example, the ellipsometry2 or the white-light interferometer3 is very good for the thickness measurement of transparent or semitransparent thin films, while they are not suitable for the opaque metal films. The x-ray reflectivity technique4 and the direct observation method by FE-SEM5 or TEM6 are two very powerful thickness test methods for metal films. However, the sophisticated instruments used in the two methods are expensive and not easy to perform. They are not inconvenient in practical applications, especially for the thickness tests of metal films in the nanometer scale. In this work, a simple thickness test method is proposed for the metal film in the nanometer scale. The principle of the method is based on the proportionality relationship between the thickness of the formed oxide film and the formation voltage in a barrier-type anodizing process.7 No special and expensive instruments are required in this method. It is also very easy to perform. As an example, the thickness measurement of aluminum films is tested by this method. Some important technical issues are discussed in this paper. II. PRINCIPLE OF ANODIZING METHOD
As a proven technology, the anodic oxidation has been widely applied to industry and daily life.8–10 There are two types of anodic aluminum oxide (AAO) films, barrier type and porous type. The barrier-type AAO film is formed in neutral electrolyte,11, 12 while the porous type AAO film is formed in corrosive electrolyte.7, 13, 14 The thickness of an AAO film is a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0034-6748/2014/85(9)/094101/5/$30.00
controlled solely by the formation voltage in a barrier-type anodizing process.7 There are two different control ways in a barrier-type anodizing process.7 The changes in voltage and ionic current density with respect to anodizing time would show the behavior illustrated in Fig. 1. In a current control step (region 1), the current is maintained at a constant value; thus the voltage and the thickness of formed oxide film are increasing linearly with time. In a voltage control step (region 2), the voltage is reached and maintained at a constant value. Thus, the current and the growth rate of the oxide film decrease rapidly with time. Because the current efficiency is relatively invariant in a given anodizing condition, the molar amount of a formed oxide film is proportional to the consumed metal film. For example, the thickness ratio of an evaporated aluminum film to the formed oxide film is 0.725, which is anodized in 3% ammonium tartrate.10 Hence, the formation voltage also determines the thickness of the metal film. If the formation voltage can be found out when the metal film is exhausted in a barrier-type anodizing process, the thickness of the metal film is determined and tested. Indeed, if the metal film is growing on a dielectric substrate, the recorded voltage can be easy to find out when the metal film is exhausted. As shown in Fig. 2, the anodizing process consists of three regions. At the first step (region AB in Fig. 2), the recorded voltage increases linearly with increasing time, which is the same as the current control step in a barrier-type anodizing process. Because of the electrical insulation of the formed film and the substrate, the specimen system becomes an electrical insulation system when the metal film that immersed in the electrolyte is exhausted. At this step (region BC in Fig. 2), the voltage suddenly increases to the setting upper limit voltage. Thus the recorded current is found to decrease to about zero. At last (region CD in Fig. 2), the voltage is maintained at the setting upper limit voltage, and the recorded current is slowly decreased almost to zero. Therefore, the thickness of the metal film is solely determined by the voltage VB . It can be calculated by the following
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Rev. Sci. Instrum. 85, 094101 (2014)
since no additional ions are added to the anodizing system. A pure Al plate is used as the cathodal material in this test example. The detailed study of the electrochemical electrode is referred to Ref. 17. IV. EXPERIMENTAL
FIG. 1. Formation of a barrier-type anodic oxide film.
formula: dmetal = a(bVB ) = cVB ,
(1)
where a is the thickness ratio of metal film to the formed oxide film; bis the anodizing ratio at the current control step in a barrier-type anodizing process; c is the ratio of the metal film to the formation voltage; and VB is the recorded voltage at the point B shown in Fig. 2. In addition, a natural oxide film will be formed on the surface of metal films in air. This natural oxide film is represented as the initial measurement voltage VA in the anodizing process; thus, the formula (1) should be rewired as: dmetal = ab(VB − VA ) = c(VB − VA ).
As a test example, the evaporated aluminum films were tested by this method. The aluminum films were used in this work. The aluminum films were deposited on polished quartz substrate of 10 × 10 × 1 mm3 by vacuum evaporation equipment (ZHD-400, Technol Science, Beijing, China). During the deposition, the substrates were held in a vacuum chamber with the pressure of lower than 2 × 10−4 Pa at 353 K. The rotation of the substrates was 10 revolutions per minute. 3% ammonium tartrate aqueous solution is used as an electrolyte in this study. The aluminum films are tested at 293 K. On account of electrical insulation of the quartz substrate, the tested voltage VB can be easy to find out in the anodizing process. The thickness of the aluminum film can be calculated by the formula (2), since the variables are related through a × b = c and only 2 of them have to be determined. The thickness ratio of the oxide film to the aluminum layer replaced, which is anodized in 3% ammonium tartrate aqueous solution, has been measured by Hass,10 that is a=
(2)
dAl = 0.725. dAl O 2
III. THE SETUP
The experimental setup of the test method is a typical anodizing system.15, 16 A general computer, a common source meter, a container, and clamps are used in this test system. The clamps are used to fix the sample and cathodes into the container. The source meter should be with real time communication, control and data processing functions, which make the test work automatically and avoid human disturbance. Also, the upper limit voltage of the source meter should be large enough to make sure that the voltage VB is existed. Source meter (Keithley 2410, USA) is used in this study. The good conductivity and chemical inert of the given electrolyte should be considered as the selection of cathodes’ material. Platinum and graphite would be suitable materials in some anodizing systems. Besides, the same metal material of the test sample would be also a suitable cathodal material
(3)
3
By measuring the thickness of 10 different AAO films that formed in 3% ammonium tartrate aqueous solution by the interferometer (Filmetrics F20, San Diego, CA), the ratio of formation alumina film to formation voltage at the current control step in the anodizing process is given in: b=
dAl
2
O3
Formation voltage
= (1.25 ± 0.03) nm/V.
(4)
This ratio is consistent with the literature reports.7, 10 Then, the ratio of aluminum film to the formation voltage that anodized in 3% ammonium tartrate aqueous solution is given by: c = ab = 0.725 × (1.25 ± 0.03) nm/V ≈ (0.91 ± 0.02) nm/V.
(5)
So, the formula (2) in this application can be expressed as: dAl = [(0.91 ± 0.02) · (VB − VA )] nm/V.
(6)
V. RESULTS AND DISCUSSION
Setting the upper limit voltage of 200 V and the current density of 1.25 mA/cm2 , the test voltage and current has been automatically recorded with the time by computer. As shown in Fig. 3, the initial voltage VA and the tested voltage VB of sample one are 4.0 V and 180.0 V, respectively. The thickness of oxide layers formed in air for sample one is: FIG. 2. Principle of the thickness test method in a barrier-type anodizing process.
dalumina = 1.25 × 4.0 nm = 5.0 nm.
(7)
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FIG. 3. Selecting the upper limit voltage at 200 V and the current density at 1.25 mA/cm2 , the initial voltage and the recorded voltage VB of Sample one is 4.0 V and 180.0 V, respectively.
This thickness of oxide layers formed in air is consistent with the value tested by Hass.10 So, the thickness of aluminum film of sample one can be calculated by the following formula (2): dAl = (0.91 ± 0.02) × (180.0 − 4.0) nm ≈ (159.5 ± 3.8) nm.
(8)
As shown in Fig. 4, this value is consistent with the value determined by FE-SEM within the error range. The thickness of the aluminum film of sample one is about 157 nm, where the error rate is about 5%. VI. TECHNICAL CONSIDERATIONS
Since the current efficiency of reaction occurring, the molecular weight of substance, the density of the metal film, and the density of formed oxide film are invariable in a given anodizing system, and the thickness of a formed oxide film is only dependent upon the amount of charge passed by Faraday’s laws.7 In other words, the current density value is less sensitive for the thickness test method. As shown in Fig. 5, selecting the current density of 1.25 mA/cm2 and 0.625 mA/cm2 , the thicknesses of the nat-
FIG. 4. FE-SEM of cross-section of sample one, thickness of the aluminum film is about 157 nm.
FIG. 5. Selecting current density of 1.25 mA/cm2 and 0.625 mA/cm2 , the initial voltages are 3.4 V and 3.4 V and the recorded voltages VB are 102.9 and 102.9 V, respectively.
ural oxide films are 4.3 nm and 3.8 nm, respectively. The thicknesses of the aluminum films are 90.2 nm and 90.6 nm, respectively. The thickness tested method with the different current density is consistent with each other within the margin of error. Thus it consumes a great deal of time in a low current density and the anodizing system is unstable at a too large current density. Therefore, the current density of about 1 mA/cm2 is a suitable value in this test method for aluminum film in 3% ammonium tartrate aqueous solution. Anderson and Devereux18 have discussed the importance of anodizing temperature in the anodizing process. In their study, the current is dependent upon the anodizing temperature at a given overpotential. The current efficiency decreases with the increasing of anodizing temperature, and therefore the anodizing ratio is solely dependent upon the anodizing voltage. As discussed above, the formation voltage is independent on the current density. The thickness test method is less sensitive for the anodizing temperature. Thus, the anodizing system would be unstable at a high temperature.19 Generally, the selection of anodizing temperature should be consistent with the temperature that is applied to testing the ratio c. The room temperature is suitable for anodizing temperature except in special conditions. The electrolyte applied to this thickness test method should be proven as a steady, reliable, and high current efficient electrolyte. Some useful electrolytes are shown in Table I. The anodizing ratio of aluminum or aluminum alloy with the different electrolyte is from 1.2 to 1.35. These different anodizing ratios are mainly dependent upon the different anodizing time at a given anodizing voltage. Hass10 has extensively discussed the relationship between anodizing ratio and anodizing time. Because the current efficiency is relatively invariant in a given anodizing condition, thickness of anodic oxide film is increasing linearly with the formation voltage and time at the current control step in the anodizing process. In this study, the anodizing ratio of pure aluminum is 1.25 nm/V at the current control step in a 3% weight ammonium tartrate, which is tested by the interferometer (Filmetrics F20, San Diego, CA). The anodiz-
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Rev. Sci. Instrum. 85, 094101 (2014)
TABLE I. Anodizing ratio of aluminum or aluminum alloy in different electrolyte. Electrolyte (◦ C)
Anodizing ratio (nm/V) and reference
3% weight ammonium tartrate aqueous solution 0.1 mol/L ammonium pentaborate (20)
1.22 for 30 s,10 1.30 for 2 min,10 1.35 for 40 min10 1.226, 20
AGW electrolyte15 (3% aqueous solution of tartaric acid and propylene glycol at a volume ratio of 1 to 4) Molybdate and Tungstate electrolyte (20)
1.30 for 30 min21
Aqueous borate solutions
1.3516
Aluminum or aluminum alloy Evaporated aluminum Aluminium foil (trace impurities: 0.003% Fe; 0.004% Cu; 0.002 wt. % Si) Al-1% Si-0.5% Cu thin film
Aluminium foil (0.004 w/o Cu; 0.003 w/o Fe; 0.002 w/o Si) Pure aluminium sheet
ing ratio of aluminum or aluminum alloy is about 1.25 nm/V at the current control step in a barrier-type anodizing process. This thickness test method is also suitable for those metal films or semiconductors which can forma barrier-type anodic oxide film in an appropriate electrolyte. What tabulated in Table II are a number of anodizing ratios for several metals and semiconductors. The formation voltage is extremely sensitive to the thickness variation of metal or semiconductor film, from the anodizing ratios with much more than 0.1 nm/V. If the surface of the metal film is smooth enough, the test accuracy can be accurate to a single atomic layer level. However, in the practical application, the accuracy of this thickness test method is affected by the control of anodizing conditions. The determination of the parameters c in a given anodizing condition is critical in this thickness test method. In other words, the accuracy of the measurement is affected by the environmental uncertainties such as temperature, PH, and so on. To improve the accuracy, a reproducible and reliable preparation of the electrolyte under environmental uncertainties is an essential requirement. The selection of electrolyte is a critical issue in this method. The best selection of electrolyte is the electrolyte, which is not sensitive to various conditions, like the AGW electrolyte.5 The AGW electrolyte is a mixed solution of glycol and water. Hasegawa and Hartnagel15 have studied the AGW process well. In their study, the AGW process has been found to be always very stable and reliable, and the result is always re-producible even after many times of the anodization. In addition, the application of standard sample can effectively improve the testing accuracy, since the standard sample and the test sample are each anodizing in the same anodizing condition. Since this method is based on the theory of barriertype anodic oxidation, characteristics of this method are also TABLE II. Anodizing ratio of some metals and semiconductors. Metal Aluminum Tantalum Niobium Zirconium Tungsten Silicon GaAs GaSb
Anodizing ratio (nm/V) and reference 1.30,10 1.3510 1.67 2.27 2.0,7 2.1,7 2.4,7 2.77 1.87 0.387 1.3515 2.523
1.222
inherited from the anodic oxidation. Because the thickness of barrier-type oxide films is limited to
