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Wet etching mechanisms of ITO films in oxalic acid
Microelectronic Engineering 83 (2006) 536–541 www.elsevier.com/locate/mee
Wet etching mechanisms of ITO films in oxalic acid Tzu-Hsuan Tsai a
a,*
, Yung-Fu Wu
b
Department of Chemical and Materials Engineering, Northern Taiwan Institute of Science and Technology, No. 2, Xueyuan Road, Beitou, Taipei 112, Taiwan b Department of Chemical Engineering, MingChi University of Technology, Taipei 243, Taiwan Received 2 September 2005; accepted 10 December 2005 Available online 28 December 2005
Abstract This study examines how oxalic acid solutions affect indium tin oxide (ITO) etching. Experimental results show that the etching rate of ITO films increased linearly with the concentration of C2 O2 4 . The open circuit potentials included in the potential–pH diagrams for indium and tin in aqueous oxalic acid systems helped determine that ITO films was dissolved by the formation of InðC2 O4 Þ 2 in oxalic acid. However, the tin oxide in ITO films was difficult to dissolve, but could be removed by stripping. The EDS analysis and optical microscopic images indicate that the removal rate of SnO2 in oxalic acid etchants was slower than that of In2O3, and many residues were distributed around the ITO bars after the etching process. Moreover, kinetic experiments reveal the high activation energy of ITO etching, and it was found stirring or ultrasonic vibration of the etchants had no influence on the dissolution rate of ITO films at all. Therefore, the rate-determining step should be the chemical reaction on ITO surface. A mechanism including protonation and ligand adsorption was proposed to explain the observed results. 2005 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide; Wet etching; Oxalic acid; Mechanism; Kinetics
1. Introduction Indium tin oxide (ITO) films have been widely employed as a pixel electrode in flat panel display devices, such as liquid crystal displays (LCD) and organic light emitting diodes (OLED), due to its excellent optical transparency and good electrical conductivity. Additionally, with the rapid development of photovoltaic industry, ITO films are also an important material for fabricating solar cells. When ITO films are patterned, either dry etching or wet etching can be adopted. In general, the wet etching process is simpler than the dry method, and provides a high throughput and low cost. Therefore, wet etching is widely used to pattern ITO films in flat panel manufacturing. However, the wet etching processes might cause the problems of selective grain-boundary etching and isotropic *
Corresponding author. Tel.: +886 228927154x8041; fax: +886 228960255. E-mail address: [email protected] (T.-H. Tsai). 0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.003
etching, and the ITO deposition methods also tend to generate different ITO etching rates [1]. Small variations in the films leave ITO residues after etching and also cause the sidewall consumption of ITO films [2,3]. These non-uniform etching results would worsen the electrical property of the ITO electrode and might lead to uncontrollable sidewall shape [4]. Therefore, the wet etching process of ITO films must be studied to optimize the process for the mass production of the flat panel display. Different ITO films require different wet etching solutions. Conventional etchants are generally composed of strong acids, such as halogen acid [5–7], and aqua regia [4]. The corrosion of underlying metal films is, however, serious during patterning ITO films using strong acids [8], and the residual halide also degrades devices [4]. Therefore, weak acid etchants are increasingly used for ITO films instead of strong acids, and oxalic acid is the most frequently applied weak acid. However, the mechanisms of ITO etching using oxalic acids have seldom been investigated. Hence, this study
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
537
60 o
25 C o 35 C
Etching Rate (nm/min)
50
40
30
20
10
(a) 0 0.0
0.2
0.4
0.6
0.8
Oxalic Acid Concentration (M) 60
60 o
50
Etching Rate (nm/min)
50
Etching Rate (nm/min)
o
25 C o 35 C
40
30
20
25 C o 35 C
40
30
20
10
10
(b)
(c)
0 0.0
0.1
0.2
0.3
0.4
0.5
0 0.00
0.6
0.05
0.15
0.20
[HC2O4 ] (M)
60
60
o
o
25 C o 35 C
25 C o 35 C
50
50
Etching Rate (nm/min)
Etching Rate (nm/min)
0.10 -
[H2C2O4] (M)
40
30
20
10
40
30
20
10
(d)
(e)
0 -5
5.39x10
-5
-5
5.40x10
5.41x10
-5
5.42x10
2-
[C2O4 ] (M)
0 0.00
0.05
0.10
0.15
0.20
+
[H ] (M)
2 + Fig. 1. Etching rate of ITO films vs. (a) added oxalic acid, (b) undissociated H2C2O4, (c) dissociated HC2 O 4 , (d) dissociated C2 O4 and (e) dissociated H concentrations at 25 and 35 C.
explored the oxalic acid solutions applied to etch ITO films. Potential–pH diagrams and open circuit potentials (OCP) were used to examine the etching behaviors of
ITO films. By means of the analysis of ITO chemical reaction rate, a surface protonation and ligand-adsorption mechanism was proposed and examined.
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
2. Experimental Commercial ITO thin films deposited by dc magnetron ˚ thick on the glass were employed, sputtering with 1000 A and each specimen was prepared by dicing into 1 · 1 cm2 for experiments. Before the etching process, all specimens were immersed in 150 ppm Triton X-100 solution for 3 min with an ultrasonic bath to remove the surface impurities. Then the ITO samples were rinsed gently with deionized (DI) water and dried by nitrogen gas. The etchants were mainly prepared using oxalic acid, and its temperature was controlled with a heating bath circulator, which provided a fixed temperature jacket for the etching cell. The rotation and ultrasonic vibration of the etching cell were controlled with a magnetic stirrer and an ultrasonic bath of 43 kHz/150 W, respectively. The open circuit potentials (OCP) of the ITO-film electrodes with respect to an Ag/AgCl electrode in oxalic acid etchants were measured by Autolab PGSTAT30 potentiostat. Sodium oxalate solution was used to increase the pH of the solutions. A stable OCP can then be obtained in a period of about 50 s and was reported with respect to a standard hydrogen electrode (SHE). A resistance-measured method was adopted to determine the etching rate of ITO films. During etching, the resistance change of ITO samples was recorded with time until the resistance increased suddenly. Then, the etching duration was determined from the sudden change in resistance. The etching rate was calculated from the thickness of ITO films divided by the etching duration. The surface images of ITO films with patterns after etching were characterized using an optical microscope (OM). The ratios of indium oxide to tin oxide before or after etching were examined by an energy dispersive Xray spectroscopy (EDS). 3. Results and discussion
SnO2 and SnC2O4 are stable at pH 0–1.2, while SnO2 2 and SnðC2 O4 Þ2 are stable at pH 1.2–4. Therefore, ITO films might not dissolve fully in acidic etchants with oxalate ions owing to the insoluble SnO2 or SnC2O4. Fig. 2 also shows the measured open circuit potentials (OCP) of ITO films in 0.35 M oxalic acid solutions at low pH values. These points reveal that ITO films might form the dissolved InðC2 O4 Þ 2 and insoluble SnC2O4 in the solution containing only oxalic acid, and the insoluble SnC2O4 could dissolve 2 to SnðC2 O4 Þ2 with increasing pH. The EDS spectra were employed to analyze the relative composition of the ITO films as shown in Fig. 3, where Si, Al and Ca peaks result from the glass under ITO, In and Sn
0.35 M oxalic acid
(a) 1.0
-6
{In}=10 -4 {In}=10
+
In
InC2O4
3+
0.5
E (V)
538
-
In(C2O4)2
only 0.35 M oxalic acid 0.0
-0.5
In
+
In -1.0 0
1
2
3
4
pH (b)
0.35 M oxalic acid
1.0
-6
{Sn}=10 -4 {Sn}=10 SnO2
0.5
only 0.35 M oxalic acid E (V)
Fig. 1(a) shows the etching rates of ITO films with respect to oxalic acid concentrations at 25 and 35 C, indicating that ITO films were etched more quickly at 35 C than at 25 C, and the etching rate of ITO films increased with the concentration of oxalic acid. When the concentration exceeded 0.35 M, the etching rate started to increase very slightly. Moreover, Fig. 1 also displays the effects of 2 + [H2C2O4], ½HC2 O 4 , ½C2 O4 and [H ] on etching rates. It could be found that the approximately linear dependence was observed only for ½C2 O2 4 , showing the dissolution of ITO films was directly affected by C2 O2 4 ions. Fig. 2(a) and (b) depict the potential–pH diagrams (Pourbaix diagrams) of indium and tin, respectively, in a 0.35 M oxalic acid aqueous system from pH 0 to 4. Calculations were performed for two different activities of indium ion and tin ion, 104 and 106. For indium, the favorable oxidized species were found to be In+ and In3+ at pH 0–0.3, and InC2 Oþ 4 and InðC2 O4 Þ2 at pH 0.3–4. In other words, indium tends to dissolve in strong and medium acid solutions with oxalate ions. For tin, solid-state
0.0
SnC2O4 2-
Sn(C2O4)2 -0.5
Sn -1.0 0
1
2
3
4
pH Fig. 2. Potential–pH diagrams for (a) indium–oxalic acid–H2O and (b) tin–oxalic acid–H2O systems at indium ion or tin ion activities of 106 and 104 at 25 C. Open circuit potentials of ITO in 0.35 M oxalic acid at various pH values are also indicated by squared symbols.
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
539
6000
Si
O
Counts
4500
3000
Al Before etching 1500
After etching In Sn
Ca
0 0
100
200
300
400
Energy (keV)
Fig. 3. EDS spectra of ITO samples before and after 1 min etching with 0.35 M oxalic acid at 35 C.
peaks are from ITO films, and O peak might come from both the glass and ITO films. It was found the original counts of In-peak and Sn-peak in Fig. 3 were 1178 and 553, respectively. After ITO films were etched in 0.35 M oxalic acid for 1 min at 35 C, the counts of In-peak and Sn-peak decreased to 659 and 412, respectively. Obviously, the removal of indium oxide from ITO films was quicker than tin oxide. This result could have been explained by the discussion of Fig. 2 that in 0.35 M oxalic acid the indium oxide could dissolve but the tin oxide might not. Hence the indium oxide in ITO films was etched faster than tin oxide in oxalic acid. However, the tin oxide may still be removed as solidstate SnC2O4 or SnO2 by stripping followed with the dissolution of the indium oxide. These phenomena of stripping can be observed with an optical microscope. The OM images in Fig. 4 show ITO films on glass were patterned to 2 lm wide bars after etching with 0.35 M oxalic acid at 35 C. It was obvious that ITO films was removed by dissolution along with stripping, but significant amount of residual materials from stripping were produced and distributed around the ITO bars. Referring to the dissolution mechanisms of the general oxides, such as Fe(III) oxide and Al(III) oxide, the surface protonation behavior seems important, and their dissolution rates are promoted in the presence of proton and complex-forming ligands [9–11]. Therefore, a possible reaction scheme for ITO etching in oxalic acid was proposed as shown in Fig. 5, in which the ITO structure is represented in a simplified planar type. Fig. 5 started with a surface In atom coordinated to a neutral OH/OH2 pair (ITO0). The first step involves adsorption of a proton by the surface OH group, thus transforming into a positively charged one (ITO1). The protonation weakens the In–O and Sn– O bonds in ITO films, probably by polarizing them, and thus promotes detachment of In and Sn from the ITO films. Subsequently, the adsorption of organic ligands,
Fig. 4. OM images of 2 lm ITO bars after etching with 0.35 M oxalic acid at 35 C.
In In In In + n H+
In
H O O OH O OH
In In In Sn In
O
OH OH2
dissolution or stripping
OH
− InC2O4+
OH2
− In(C2O4 )2−
OH
− SnC2O4
OH2
− SnO2
ITO0 (fresh surface)
In In In In In
H O O OH O OH O
In In In Sn In
OH2
H O In
OH2+
In
OH2 OH2
ITO1 (protonation)
O
O
OH2
OH2
O In
In
+
+
In OH
+ n C2O42-
In
O In
O
O Sn
OH In
In O
O O
ITO2 (ligand adsorption)
Fig. 5. Schematic representation of the consecutive steps of ITO etching in oxalic acid by protonation and ligand adsorption.
C2 O2 4 , on the surface of ITO films (ITO2) further weakens In–O and Sn–O bonds and leads to detachment of InðC2 O4 Þ2 and insoluble SnC2O4. The SnO2 might be stripped during the removal of In2O3 around SnO2. Finally, a fresh ITO surface exposes after a cycle of dissolution. Fig. 5 displays this proposed cycle reactions of ITO etching in oxalic acid. The proposed mechanism seems to indicate the enough protons and ligands are important for ITO etching. Therefore, it was found in our experiment that only adding sodium oxalate could not enhance the ITO etching rate, even though Fig. 1(d) shows the linear dependence of etching rate and ½C2 O2 4 . The logarithm of etching rate vs. reciprocal temperature for each ITO sample in oxalic acid etchants is plotted in
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
Fig. 6. The linear plot demonstrates that the Arrhenius relationship was obeyed in these reactions between 25 and 50 C. The activation energy was then calculated from the slopes of these plots. The calculated average activation energy of etching ITO films with oxalic acid was found to be 68.22 kJ/mol. This activation energy is generally higher than that of the protonation steps [10]. Moreover, breaking of the In–O or Sn–O bonds during ITO etching will be the most difficult step [5]. In other words, the rate-determining step of ITO etching mechanisms would be the breaking of In–O bonds, i.e. the detachment of InðC2 O4 Þ2 described in Fig. 5. In order to derive a kinetic rate law, the steps given in Fig. 5 were described and simplified by the following reactions ½ITO0 þ Hþ $ ½ITO1
ð1Þ
½ITO1 þ C2 O2 4 $ ½ITO2 þ H2 O
ð2Þ
½ITO2 ! InðC2 O4 Þ2 þ ½ITO0
ð3Þ
In these reactions, ITO0 represents a coordinated In site by OH/OH2, and ITO1 and ITO2 are the different intermediate states formed after the adsorption of protons and ligands, respectively. Finally, the detachment of complex leads to another fresh surface, ITO0. Therefore, if the protonation and adsorption reactions (1) and (2) have low activation energies and can be achieved quickly, their equilibrium could be given by h1 ; h0 ½Hþ h2 K2 ¼ ; h1 ½C2 O2 4 K1 ¼
ð4Þ ð5Þ
where K1 and K2 are the equilibrium constants of the relevant reactions, and h0, h1 and h2 represent the surface coverage ratios of ITO0, ITO1 and ITO2, respectively. Thus, the h0 can be calculated through h0 + h1 + h2 = 1 to give
Etching Rate (nm/min)
1000
Oxalic acid 0.11 M; Ea=68.54 kJ/mol 0.35 M; Ea=67.56 kJ/mol 0.55 M; Ea=68.50 kJ/mol 0.78 M; Ea=68.26 kJ/mol
100
Ea,ave=68.22 kJ/mol
10 -3
3.1x10
-3
-3
3.2x10
3.3x10
-3
3.4x10
-1
1/T (K )
Fig. 6. Etching rate of ITO films vs. reciprocal temperature in oxalic acid. The calculation results of activation energy are also shown.
200 o
25 C o 35 C o 40 C o 45 C o 50 C Theoretical line
150
Etching Rate (nm/min)
540
100
50
0 0
50
100 +
2-
150 +
+
200 2-
k3K1K2[H ][C2O4 ]/(1+K1[H ]+K1K2[H ][C2O4 ])
Fig. 7. Etching rate of ITO films vs. the f ð½Hþ ; ½C2 O2 4 Þ of Eq. (7) in oxalic acid at various temperatures.
h0 ¼
1 . 1 þ K 1 ½H þ K 1 K 2 ½Hþ ½C2 O2 4 þ
ð6Þ
As the rate-determining step is the reaction (3), the etching rate (ER) of ITO films could be expressed as ER ¼
k 3 K 1 K 2 ½Hþ ½C2 O2 4 þ þ 1 þ K 1 ½H þ K 1 K 2 ½H ½C2 O2 4
¼ f ð½Hþ ; ½C2 O2 4 Þ
ð7Þ
In the above equations, k3 represent the rate constant of the reaction (3). The proposed mechanism could be described by the relation between ER and f ð½Hþ ; ½C2 O2 4 Þ of Eq. (7). The experimental data of ITO etching in oxalic acid etchants at 25, 35, 40, 45 and 50 C are plotted in Fig. 7. It was clear the theoretical line and experimental results match well, showing the experimental data agreed with the proposed mechanism of Fig. 5. Moreover, extensive research on dissolution kinetics shows that dissolution for simple oxides is controlled by surface reaction processes rather than solution or surface transport processes [10], which means that etching of ITO films should be the reaction-controlled type and not be influenced by mass transport of oxalic acid solutions. Therefore, the experiments with different rotation speeds were performed. The results as shown in Fig. 8 indeed demonstrate the etching rate of ITO films in oxalic acid was independent of mass transport. Even though the etching process was performed in an ultrasonic bath, the etching rate still did not change obviously. However, the rotating action helpfully decreased the residual materials around the ITO bars after etching, so Fig. 9(a) displays less spots than those in Fig. 4. Fig. 9(b) also shows ITO etching with ultrasonic vibration would further reduced the residues of ITO patterned surface due to the strong transporting action and resulted in a clear surface of ITO bars on glass. Consequently, mass transport action during etching would
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541 60
Etching Rate (nm/min)
50
40
30
20
o
25 C o 35 C
10
0 0
50
100
150
Rotation Speed (rpm)
200
250
Ultrasonic vibration
Fig. 8. Etching rate of ITO films vs. rotation speed and etching with ultrasonic vibration in 0.35 M oxalic acid.
541
potential–pH relation and EDS measurement both illustrated that the indium oxide in ITO films was etched faster than tin oxide in oxalic acid, revealing that ITO films might form InðC2 O4 Þ2 and insoluble SnC2O4. However, these insoluble species could still be removed by stripping during the dissolution of In2O3, but many solid residues would be formed after etching. A possible reaction scheme with protonation and ligand-adsorption for ITO etching in oxalic acid was proposed. It was found that the kinetics + was both related to the concentrations of C2 O2 4 and H . The consistency between the proposed mechanism and experimental results was proved, demonstrating the etching of ITO films was the reaction controlled. The experiments also indicated the etching rate of ITO films was independent of the stirring or vibration of oxalic acid solutions, but enhancing mass transport action would decrease the residual materials around ITO bars and improve the etching uniformity. Acknowledgments The authors would like to thank the Northern Taiwan Institute of Science and Technology for supporting this research. References
Fig. 9. OM images of 2 lm ITO bars after etching with 0.35 M oxalic acid at 35 C under (a) 200 rpm rotation and (b) ultrasonic vibration.
be applied to sweep the residual materials away, but not to accelerate the etching rate of ITO films. 4. Conclusions The etching kinetics of ITO films in oxalic acid solutions has been discussed in this study. The results of
[1] J.Y. Park, H.S. Kim, D.H. Lee, K.H. Kwon, G.Y. Yeom, Surf. Coat. Technol. 131 (2000) 247. [2] J.E.A.M. van den Meerakker, P.C. Baarslag, W. Walrave, T.J. Vink, J.L.C. Daams, Thin Solid Films 266 (1995) 152. [3] M. Takabatake, Y. Wakui, N. Konishi, J. Electrochem. Soc. 142 (1995) 2470. [4] C.J. Huang, Y.K. Su, S.L. Wu, Mater. Chem. Phys. 84 (2004) 146. [5] M. Scholten, J.E.A.M. van den Meerakker, J. Electrochem. Soc. 140 (1993) 471. [6] J.E.A.M. van den Meerakker, P.C. Baarslag, M. Scholten, J. Electrochem. Soc. 142 (1995) 2321. [7] F. Krannig, Annual Technical Conference Proceedings–Society of Vacuum Coaters, 1996, p. 226. [8] H. Takatsuji, T. Hiromori, K. Tsujimoto, S. Tsuji, K. Kuroda, H. Saka, Materials Research Society Symposium–Proceedings, vol. 508, 1998, p. 315. [9] R.M. Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses, second ed., Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim, 2003, pp. 297–344. [10] W. Stumm, Aquatic Surface Chemistry: Chemical Processes at the Particle–Water Interface, Wiley, New York, 1987, pp. 197–219. [11] S. Carroll-Webb, J.V. Walther, Geochim. Cosmochim. Acta 52 (1988) 2609.
Wet etching mechanisms of ITO films in oxalic acid Tzu-Hsuan Tsai a
a,*
, Yung-Fu Wu
b
Department of Chemical and Materials Engineering, Northern Taiwan Institute of Science and Technology, No. 2, Xueyuan Road, Beitou, Taipei 112, Taiwan b Department of Chemical Engineering, MingChi University of Technology, Taipei 243, Taiwan Received 2 September 2005; accepted 10 December 2005 Available online 28 December 2005
Abstract This study examines how oxalic acid solutions affect indium tin oxide (ITO) etching. Experimental results show that the etching rate of ITO films increased linearly with the concentration of C2 O2 4 . The open circuit potentials included in the potential–pH diagrams for indium and tin in aqueous oxalic acid systems helped determine that ITO films was dissolved by the formation of InðC2 O4 Þ 2 in oxalic acid. However, the tin oxide in ITO films was difficult to dissolve, but could be removed by stripping. The EDS analysis and optical microscopic images indicate that the removal rate of SnO2 in oxalic acid etchants was slower than that of In2O3, and many residues were distributed around the ITO bars after the etching process. Moreover, kinetic experiments reveal the high activation energy of ITO etching, and it was found stirring or ultrasonic vibration of the etchants had no influence on the dissolution rate of ITO films at all. Therefore, the rate-determining step should be the chemical reaction on ITO surface. A mechanism including protonation and ligand adsorption was proposed to explain the observed results. 2005 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide; Wet etching; Oxalic acid; Mechanism; Kinetics
1. Introduction Indium tin oxide (ITO) films have been widely employed as a pixel electrode in flat panel display devices, such as liquid crystal displays (LCD) and organic light emitting diodes (OLED), due to its excellent optical transparency and good electrical conductivity. Additionally, with the rapid development of photovoltaic industry, ITO films are also an important material for fabricating solar cells. When ITO films are patterned, either dry etching or wet etching can be adopted. In general, the wet etching process is simpler than the dry method, and provides a high throughput and low cost. Therefore, wet etching is widely used to pattern ITO films in flat panel manufacturing. However, the wet etching processes might cause the problems of selective grain-boundary etching and isotropic *
Corresponding author. Tel.: +886 228927154x8041; fax: +886 228960255. E-mail address: [email protected] (T.-H. Tsai). 0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.12.003
etching, and the ITO deposition methods also tend to generate different ITO etching rates [1]. Small variations in the films leave ITO residues after etching and also cause the sidewall consumption of ITO films [2,3]. These non-uniform etching results would worsen the electrical property of the ITO electrode and might lead to uncontrollable sidewall shape [4]. Therefore, the wet etching process of ITO films must be studied to optimize the process for the mass production of the flat panel display. Different ITO films require different wet etching solutions. Conventional etchants are generally composed of strong acids, such as halogen acid [5–7], and aqua regia [4]. The corrosion of underlying metal films is, however, serious during patterning ITO films using strong acids [8], and the residual halide also degrades devices [4]. Therefore, weak acid etchants are increasingly used for ITO films instead of strong acids, and oxalic acid is the most frequently applied weak acid. However, the mechanisms of ITO etching using oxalic acids have seldom been investigated. Hence, this study
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
537
60 o
25 C o 35 C
Etching Rate (nm/min)
50
40
30
20
10
(a) 0 0.0
0.2
0.4
0.6
0.8
Oxalic Acid Concentration (M) 60
60 o
50
Etching Rate (nm/min)
50
Etching Rate (nm/min)
o
25 C o 35 C
40
30
20
25 C o 35 C
40
30
20
10
10
(b)
(c)
0 0.0
0.1
0.2
0.3
0.4
0.5
0 0.00
0.6
0.05
0.15
0.20
[HC2O4 ] (M)
60
60
o
o
25 C o 35 C
25 C o 35 C
50
50
Etching Rate (nm/min)
Etching Rate (nm/min)
0.10 -
[H2C2O4] (M)
40
30
20
10
40
30
20
10
(d)
(e)
0 -5
5.39x10
-5
-5
5.40x10
5.41x10
-5
5.42x10
2-
[C2O4 ] (M)
0 0.00
0.05
0.10
0.15
0.20
+
[H ] (M)
2 + Fig. 1. Etching rate of ITO films vs. (a) added oxalic acid, (b) undissociated H2C2O4, (c) dissociated HC2 O 4 , (d) dissociated C2 O4 and (e) dissociated H concentrations at 25 and 35 C.
explored the oxalic acid solutions applied to etch ITO films. Potential–pH diagrams and open circuit potentials (OCP) were used to examine the etching behaviors of
ITO films. By means of the analysis of ITO chemical reaction rate, a surface protonation and ligand-adsorption mechanism was proposed and examined.
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
2. Experimental Commercial ITO thin films deposited by dc magnetron ˚ thick on the glass were employed, sputtering with 1000 A and each specimen was prepared by dicing into 1 · 1 cm2 for experiments. Before the etching process, all specimens were immersed in 150 ppm Triton X-100 solution for 3 min with an ultrasonic bath to remove the surface impurities. Then the ITO samples were rinsed gently with deionized (DI) water and dried by nitrogen gas. The etchants were mainly prepared using oxalic acid, and its temperature was controlled with a heating bath circulator, which provided a fixed temperature jacket for the etching cell. The rotation and ultrasonic vibration of the etching cell were controlled with a magnetic stirrer and an ultrasonic bath of 43 kHz/150 W, respectively. The open circuit potentials (OCP) of the ITO-film electrodes with respect to an Ag/AgCl electrode in oxalic acid etchants were measured by Autolab PGSTAT30 potentiostat. Sodium oxalate solution was used to increase the pH of the solutions. A stable OCP can then be obtained in a period of about 50 s and was reported with respect to a standard hydrogen electrode (SHE). A resistance-measured method was adopted to determine the etching rate of ITO films. During etching, the resistance change of ITO samples was recorded with time until the resistance increased suddenly. Then, the etching duration was determined from the sudden change in resistance. The etching rate was calculated from the thickness of ITO films divided by the etching duration. The surface images of ITO films with patterns after etching were characterized using an optical microscope (OM). The ratios of indium oxide to tin oxide before or after etching were examined by an energy dispersive Xray spectroscopy (EDS). 3. Results and discussion
SnO2 and SnC2O4 are stable at pH 0–1.2, while SnO2 2 and SnðC2 O4 Þ2 are stable at pH 1.2–4. Therefore, ITO films might not dissolve fully in acidic etchants with oxalate ions owing to the insoluble SnO2 or SnC2O4. Fig. 2 also shows the measured open circuit potentials (OCP) of ITO films in 0.35 M oxalic acid solutions at low pH values. These points reveal that ITO films might form the dissolved InðC2 O4 Þ 2 and insoluble SnC2O4 in the solution containing only oxalic acid, and the insoluble SnC2O4 could dissolve 2 to SnðC2 O4 Þ2 with increasing pH. The EDS spectra were employed to analyze the relative composition of the ITO films as shown in Fig. 3, where Si, Al and Ca peaks result from the glass under ITO, In and Sn
0.35 M oxalic acid
(a) 1.0
-6
{In}=10 -4 {In}=10
+
In
InC2O4
3+
0.5
E (V)
538
-
In(C2O4)2
only 0.35 M oxalic acid 0.0
-0.5
In
+
In -1.0 0
1
2
3
4
pH (b)
0.35 M oxalic acid
1.0
-6
{Sn}=10 -4 {Sn}=10 SnO2
0.5
only 0.35 M oxalic acid E (V)
Fig. 1(a) shows the etching rates of ITO films with respect to oxalic acid concentrations at 25 and 35 C, indicating that ITO films were etched more quickly at 35 C than at 25 C, and the etching rate of ITO films increased with the concentration of oxalic acid. When the concentration exceeded 0.35 M, the etching rate started to increase very slightly. Moreover, Fig. 1 also displays the effects of 2 + [H2C2O4], ½HC2 O 4 , ½C2 O4 and [H ] on etching rates. It could be found that the approximately linear dependence was observed only for ½C2 O2 4 , showing the dissolution of ITO films was directly affected by C2 O2 4 ions. Fig. 2(a) and (b) depict the potential–pH diagrams (Pourbaix diagrams) of indium and tin, respectively, in a 0.35 M oxalic acid aqueous system from pH 0 to 4. Calculations were performed for two different activities of indium ion and tin ion, 104 and 106. For indium, the favorable oxidized species were found to be In+ and In3+ at pH 0–0.3, and InC2 Oþ 4 and InðC2 O4 Þ2 at pH 0.3–4. In other words, indium tends to dissolve in strong and medium acid solutions with oxalate ions. For tin, solid-state
0.0
SnC2O4 2-
Sn(C2O4)2 -0.5
Sn -1.0 0
1
2
3
4
pH Fig. 2. Potential–pH diagrams for (a) indium–oxalic acid–H2O and (b) tin–oxalic acid–H2O systems at indium ion or tin ion activities of 106 and 104 at 25 C. Open circuit potentials of ITO in 0.35 M oxalic acid at various pH values are also indicated by squared symbols.
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
539
6000
Si
O
Counts
4500
3000
Al Before etching 1500
After etching In Sn
Ca
0 0
100
200
300
400
Energy (keV)
Fig. 3. EDS spectra of ITO samples before and after 1 min etching with 0.35 M oxalic acid at 35 C.
peaks are from ITO films, and O peak might come from both the glass and ITO films. It was found the original counts of In-peak and Sn-peak in Fig. 3 were 1178 and 553, respectively. After ITO films were etched in 0.35 M oxalic acid for 1 min at 35 C, the counts of In-peak and Sn-peak decreased to 659 and 412, respectively. Obviously, the removal of indium oxide from ITO films was quicker than tin oxide. This result could have been explained by the discussion of Fig. 2 that in 0.35 M oxalic acid the indium oxide could dissolve but the tin oxide might not. Hence the indium oxide in ITO films was etched faster than tin oxide in oxalic acid. However, the tin oxide may still be removed as solidstate SnC2O4 or SnO2 by stripping followed with the dissolution of the indium oxide. These phenomena of stripping can be observed with an optical microscope. The OM images in Fig. 4 show ITO films on glass were patterned to 2 lm wide bars after etching with 0.35 M oxalic acid at 35 C. It was obvious that ITO films was removed by dissolution along with stripping, but significant amount of residual materials from stripping were produced and distributed around the ITO bars. Referring to the dissolution mechanisms of the general oxides, such as Fe(III) oxide and Al(III) oxide, the surface protonation behavior seems important, and their dissolution rates are promoted in the presence of proton and complex-forming ligands [9–11]. Therefore, a possible reaction scheme for ITO etching in oxalic acid was proposed as shown in Fig. 5, in which the ITO structure is represented in a simplified planar type. Fig. 5 started with a surface In atom coordinated to a neutral OH/OH2 pair (ITO0). The first step involves adsorption of a proton by the surface OH group, thus transforming into a positively charged one (ITO1). The protonation weakens the In–O and Sn– O bonds in ITO films, probably by polarizing them, and thus promotes detachment of In and Sn from the ITO films. Subsequently, the adsorption of organic ligands,
Fig. 4. OM images of 2 lm ITO bars after etching with 0.35 M oxalic acid at 35 C.
In In In In + n H+
In
H O O OH O OH
In In In Sn In
O
OH OH2
dissolution or stripping
OH
− InC2O4+
OH2
− In(C2O4 )2−
OH
− SnC2O4
OH2
− SnO2
ITO0 (fresh surface)
In In In In In
H O O OH O OH O
In In In Sn In
OH2
H O In
OH2+
In
OH2 OH2
ITO1 (protonation)
O
O
OH2
OH2
O In
In
+
+
In OH
+ n C2O42-
In
O In
O
O Sn
OH In
In O
O O
ITO2 (ligand adsorption)
Fig. 5. Schematic representation of the consecutive steps of ITO etching in oxalic acid by protonation and ligand adsorption.
C2 O2 4 , on the surface of ITO films (ITO2) further weakens In–O and Sn–O bonds and leads to detachment of InðC2 O4 Þ2 and insoluble SnC2O4. The SnO2 might be stripped during the removal of In2O3 around SnO2. Finally, a fresh ITO surface exposes after a cycle of dissolution. Fig. 5 displays this proposed cycle reactions of ITO etching in oxalic acid. The proposed mechanism seems to indicate the enough protons and ligands are important for ITO etching. Therefore, it was found in our experiment that only adding sodium oxalate could not enhance the ITO etching rate, even though Fig. 1(d) shows the linear dependence of etching rate and ½C2 O2 4 . The logarithm of etching rate vs. reciprocal temperature for each ITO sample in oxalic acid etchants is plotted in
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541
Fig. 6. The linear plot demonstrates that the Arrhenius relationship was obeyed in these reactions between 25 and 50 C. The activation energy was then calculated from the slopes of these plots. The calculated average activation energy of etching ITO films with oxalic acid was found to be 68.22 kJ/mol. This activation energy is generally higher than that of the protonation steps [10]. Moreover, breaking of the In–O or Sn–O bonds during ITO etching will be the most difficult step [5]. In other words, the rate-determining step of ITO etching mechanisms would be the breaking of In–O bonds, i.e. the detachment of InðC2 O4 Þ2 described in Fig. 5. In order to derive a kinetic rate law, the steps given in Fig. 5 were described and simplified by the following reactions ½ITO0 þ Hþ $ ½ITO1
ð1Þ
½ITO1 þ C2 O2 4 $ ½ITO2 þ H2 O
ð2Þ
½ITO2 ! InðC2 O4 Þ2 þ ½ITO0
ð3Þ
In these reactions, ITO0 represents a coordinated In site by OH/OH2, and ITO1 and ITO2 are the different intermediate states formed after the adsorption of protons and ligands, respectively. Finally, the detachment of complex leads to another fresh surface, ITO0. Therefore, if the protonation and adsorption reactions (1) and (2) have low activation energies and can be achieved quickly, their equilibrium could be given by h1 ; h0 ½Hþ h2 K2 ¼ ; h1 ½C2 O2 4 K1 ¼
ð4Þ ð5Þ
where K1 and K2 are the equilibrium constants of the relevant reactions, and h0, h1 and h2 represent the surface coverage ratios of ITO0, ITO1 and ITO2, respectively. Thus, the h0 can be calculated through h0 + h1 + h2 = 1 to give
Etching Rate (nm/min)
1000
Oxalic acid 0.11 M; Ea=68.54 kJ/mol 0.35 M; Ea=67.56 kJ/mol 0.55 M; Ea=68.50 kJ/mol 0.78 M; Ea=68.26 kJ/mol
100
Ea,ave=68.22 kJ/mol
10 -3
3.1x10
-3
-3
3.2x10
3.3x10
-3
3.4x10
-1
1/T (K )
Fig. 6. Etching rate of ITO films vs. reciprocal temperature in oxalic acid. The calculation results of activation energy are also shown.
200 o
25 C o 35 C o 40 C o 45 C o 50 C Theoretical line
150
Etching Rate (nm/min)
540
100
50
0 0
50
100 +
2-
150 +
+
200 2-
k3K1K2[H ][C2O4 ]/(1+K1[H ]+K1K2[H ][C2O4 ])
Fig. 7. Etching rate of ITO films vs. the f ð½Hþ ; ½C2 O2 4 Þ of Eq. (7) in oxalic acid at various temperatures.
h0 ¼
1 . 1 þ K 1 ½H þ K 1 K 2 ½Hþ ½C2 O2 4 þ
ð6Þ
As the rate-determining step is the reaction (3), the etching rate (ER) of ITO films could be expressed as ER ¼
k 3 K 1 K 2 ½Hþ ½C2 O2 4 þ þ 1 þ K 1 ½H þ K 1 K 2 ½H ½C2 O2 4
¼ f ð½Hþ ; ½C2 O2 4 Þ
ð7Þ
In the above equations, k3 represent the rate constant of the reaction (3). The proposed mechanism could be described by the relation between ER and f ð½Hþ ; ½C2 O2 4 Þ of Eq. (7). The experimental data of ITO etching in oxalic acid etchants at 25, 35, 40, 45 and 50 C are plotted in Fig. 7. It was clear the theoretical line and experimental results match well, showing the experimental data agreed with the proposed mechanism of Fig. 5. Moreover, extensive research on dissolution kinetics shows that dissolution for simple oxides is controlled by surface reaction processes rather than solution or surface transport processes [10], which means that etching of ITO films should be the reaction-controlled type and not be influenced by mass transport of oxalic acid solutions. Therefore, the experiments with different rotation speeds were performed. The results as shown in Fig. 8 indeed demonstrate the etching rate of ITO films in oxalic acid was independent of mass transport. Even though the etching process was performed in an ultrasonic bath, the etching rate still did not change obviously. However, the rotating action helpfully decreased the residual materials around the ITO bars after etching, so Fig. 9(a) displays less spots than those in Fig. 4. Fig. 9(b) also shows ITO etching with ultrasonic vibration would further reduced the residues of ITO patterned surface due to the strong transporting action and resulted in a clear surface of ITO bars on glass. Consequently, mass transport action during etching would
T.-H. Tsai, Y.-F. Wu / Microelectronic Engineering 83 (2006) 536–541 60
Etching Rate (nm/min)
50
40
30
20
o
25 C o 35 C
10
0 0
50
100
150
Rotation Speed (rpm)
200
250
Ultrasonic vibration
Fig. 8. Etching rate of ITO films vs. rotation speed and etching with ultrasonic vibration in 0.35 M oxalic acid.
541
potential–pH relation and EDS measurement both illustrated that the indium oxide in ITO films was etched faster than tin oxide in oxalic acid, revealing that ITO films might form InðC2 O4 Þ2 and insoluble SnC2O4. However, these insoluble species could still be removed by stripping during the dissolution of In2O3, but many solid residues would be formed after etching. A possible reaction scheme with protonation and ligand-adsorption for ITO etching in oxalic acid was proposed. It was found that the kinetics + was both related to the concentrations of C2 O2 4 and H . The consistency between the proposed mechanism and experimental results was proved, demonstrating the etching of ITO films was the reaction controlled. The experiments also indicated the etching rate of ITO films was independent of the stirring or vibration of oxalic acid solutions, but enhancing mass transport action would decrease the residual materials around ITO bars and improve the etching uniformity. Acknowledgments The authors would like to thank the Northern Taiwan Institute of Science and Technology for supporting this research. References
Fig. 9. OM images of 2 lm ITO bars after etching with 0.35 M oxalic acid at 35 C under (a) 200 rpm rotation and (b) ultrasonic vibration.
be applied to sweep the residual materials away, but not to accelerate the etching rate of ITO films. 4. Conclusions The etching kinetics of ITO films in oxalic acid solutions has been discussed in this study. The results of
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