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A novel submicron-gap electrode fabrication technology using thermal oxidation
Materials Science and Engineering C 32 (2012) 369–374
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
A novel submicron-gap electrode fabrication technology using thermal oxidation Xuejiao Chen, Jian Zhang ⁎, Huhua Xu, Shichao Hui, Meiguang Zhu Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electrical Engineering, East China Normal University, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 28 March 2011 Received in revised form 21 October 2011 Accepted 13 November 2011 Available online 20 November 2011 Keywords: Submicron-gap electrode Thermal oxidation Deep reactive ion etching Photolithography Metallization
a b s t r a c t A novel and reproducible method to fabricate submicron-gap electrodes using thermal oxidation has been presented. In this method, oxidation process determines the gap distance. The micron-level silicon electrode gaps with different shapes were first generated on the silicon wafer by conventional photolithography followed by deep reactive ion etching process. Then thermal oxidation was conducted to realize the transition from silicon to silicon dioxide, i.e. reduce the gap width. Finally, the planar electrodes with sub-micron spacing were formed by metallization and photolithography. Scanning electron microscopy (SEM) was used to examine the electrode configuration and the electrical properties of as-prepared electrode pairs were also characterized. The results showed that using the method investigated in this work, Au electrodes with a submicron-sized gap could be easily fabricated, with good uniformity and reproducibility. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the fabrication of metal electrodes with micro- or even nano-scale gap has received considerable attention, since these electrodes offer some advantages such as the decreased size, the improved performance and the capability to develop new-concept devices such as molecule devices [1–5]. They can be used as the manipulation tool of nano-materials, the platform of nano-sensors, and the basic element of nano-circuits. For example, biological probe can be introduced into electrodes' gap to construct biosensors. So far lots of methods have been investigated to fabricate the electrode pairs, including chemical–mechanical polishing [6], electroplating technique [4], high-resolution electron beam lithography [7], focused ion beam [8] and plasma ashing technique [9], etc., However, due to the rigorous requirements for fabrication condition, plus the high cost and the low yield, most of these methods are not suitable for batch-fabrication. In this work, a simple, reproducible and ingenious fabrication technique using thermal oxidation to fabricate submicron-gap electrodes was investigated. This method enables the reliable mass production of submicron-gap electrodes, which may have great application in bio-electronic devices and single-molecule devices [10–12]. The design mechanism of submicron-gap electrodes is that the gap width will decrease due to the volume variation during the transition from silicon to silicon dioxide. In another word, if a bare silicon surface is oxidized, 44% of the oxide thickness will lie below the original surface, and 56% above it [13].
⁎ Corresponding author. Tel./fax: + 86 21 54345203. E-mail address: [email protected] (J. Zhang). 0928-4931/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2011.11.008
We consider a silicon slit or trench formed on silicon substrate. Supposing D0 is the original designed gap width and D(T,t) is the final width after oxidation as shown in Fig. 1, the final gap width, D (T,t), which changes with the oxide thickness XSiO2, can be estimated as, DðT; t Þ ¼ D0 −2 0:56X SiO2 ðT; t Þ
ð1Þ
According to the linear parabolic model developed for wet oxidation, proposed by Deal and Grove, the thickness of oxide layer is related to oxidation time through 2
X SiO2 ðT; t Þ þ AX SiO2 ðT; t Þ ¼ Bt
ð2Þ
Where T is the wet-oxygen oxidation temperature, A is the linear rate constant, B is the parabolic rate constant which is related to the oxidation parameters (temperature, pressure, water content, etc.),
Fig. 1. Sketch map of electrode gap evolution before and after thermal oxidation.
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X. Chen et al. / Materials Science and Engineering C 32 (2012) 369–374
Fig. 4. The morphology of the electrodes characterized by a metallurgical microscope.
(DRIE). Then the thermal oxidation was used to oxidize the silicon, which can realize the gap width transition from micron-meter to submicron-meter.
Fig. 2. Fabrication sequence of submicron-gap electrodes: (a) Photolithography and DRIE; (b) Thermal Oxidation; (c) Metallization.
and t is the oxidation time, respectively. Once the oxidation condition is defined, the constants A and B are known [14]. Combining Eqs. (1) and (2), it can be found that the gap width, D (T,t), is controlled by choosing the proper oxidation time. And the gap width will decrease while the oxidation time increases. If the oxidation time is chosen properly, the gap can be controlled below micrometer. Here, we present a simple method to fabricate the submicron-gap electrodes. The as-developed submicron-gap electrodes were tested by I–V characterization to examine the isolation properties. 2. Experimental The schematic diagram of electrode fabrication is shown in Fig. 2. In this study, the silicon gap with micron-level width and different shapes were first designed and generated on the silicon wafer by conventional photolithography followed by deep reactive ion etching
Fig. 3. Electrode pattern design (a) flat to flat; (b) triangle to flat; (c) interdigitated.
Fig. 5. SEM images of electrode gap after (a) DRIE, (b) Oxidation and (c) Metallization.
X. Chen et al. / Materials Science and Engineering C 32 (2012) 369–374
2.1. Electrode configuration design In our work, the oxidation temperature is fixed at ~ 1000 °C. Referring to Deal and Grove's results [14], A and B in our study are supposed to be 0.226 μm and 0.287 μm 2/h, respectively. So the oxidation layer thickness can be estimated from Eq. (2) once the oxidation time, t, is determined. To consider the limitation of photolithography, the original gap width, D0, was designed ranging from 1.4 μm to 2.6 μm in our study. According to Eq. (2), if the oxidation time, t, is 7 h, it is predicted that the designed gaps can reach sub-micron level. In order to examine the oxidation effect on the final gap, three kinds of electrode configuration have been designed, including flat to flat, triangle to flat, and interdigitated. The configurations of as-designed electrodes are demonstrated as Fig. 3, in which the blue part represents the metallic electrodes and the red one represents the gap between electrode pairs. In all configurations, the depth of the electrode gaps is fixed at ~ 10 μm.
371
Table 1 The changes of the electrode gap distance: same angle to flat shape. Electrode gap
Angle (°)
D0 (μm)
D(T, t) (μm)
Δ = D0 − D(T, t) (μm)
a b c d e f
65 65 65 65 65 65
2.6 2.3 2.0 1.8 1.6 1.4
1.8 1.5 1.3 1.1 0.8 0.6
0.8 0.8 0.7 0.7 0.8 0.8
2.2. Electrode fabrication The electrode fabrication process is depicted schematically in Fig. 2. The polished silicon wafers (100 orientated, 300 μm thickness, Boron-doped) were used. The trench-like structures with 1.4–2.6 μm width and 10 μm depth were first obtained by photolithography and DRIE, just as shown in Fig. 2(a). Wet thermal oxidation with temperature of 1000 °C for 7 h was then performed as shown in Fig. 2(b).
Fig. 6. For the electrodes with same angle to flat shape, after oxidation, the gap distance decreases (a) from 2.6 μm to 1.8 μm; (b) from 2.3 μm to 1.5 μm; (c) from 2.0 μm to 1.3 μm; (d) from 1.8 μm to 1.1 μm; (e) from 1.6 μm to 0.8 μm; (f) from 1.4 μm to 0.6 μm.
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Fig. 7. For the electrodes with different angle to flat shape, after oxidation, the gap distance decreases (a) from 1.4 μm to 1.1 μm; (b) from 1.4 μm to 1.0 μm; (c) from 1.4 μm to 0.9 μm; (d) from 1.4 μm to 0.7 μm; (e) from 1.4 μm to 0.6 μm; (f) from 1.4 μm to 0.5 μm.
Finally, 300 Å Ti/2000 Å Au layers were deposited via magnetron sputtering. And the final electrodes were formed by photolithography as shown in Fig. 2(c). In our work, to simplify the mask design and the fabrication process, the metallization process was introduced to deposit Au films to both the planar electrodes pairs and gaps simultaneously. The Au film in the gaps is intended for the immobilization of biological probe in our future work, due to gold nanoparticles' large surface area and biocompatibility with bio-system [15], and then we can construct bio-electronic devices to detect target solution. Moreover,
Table 2 The changes of the electrode gap distance: different angle to flat shape. Electrode gap
Angle (°)
D0 (μm)
D(T, t) (μm)
Δ = D0 − D(T, t) (μm)
a b c d e f
30 40 50 65 95 180
1.4 1.4 1.4 1.4 1.4 1.4
1.1 1.0 0.9 0.7 0.6 0.5
0.3 0.4 0.5 0.7 0.8 0.9
during Ti/Au layer deposition, since the gaps were ~10 μm in depth, the isolation between the electrode pairs can be realized automatically, as shown in Fig. 2(c). It is demonstrated in our study that this process can avoid the short circuit and simplify the fabrication steps. In this study, scanning electron microscopy (SEM) is used to monitor the electrodes' fabrication. 3. Results and discussion 3.1. Characterization of submicron-gap electrodes In our study, the morphology of the electrodes was first characterized by a metallurgical microscope (Caikon DMM-220C). And as shown in Fig. 4, it can be found that both electrode pairs and gaps have been deposited with Au films by metallization. By scanning electron microscopy, the oxidation effect on the electrodes can be studied. Fig. 5 showed the electrode gap change with triangle-to-flat structure during different process step (a) DRIE, (b) Oxidation and (c) Metallization, respectively. It can be found that the gap width becomes narrower, which indicated that the transition from silicon to silicon dioxide during the wet oxidation process
X. Chen et al. / Materials Science and Engineering C 32 (2012) 369–374
Fig. 8. The response curve of gap distance change with angle and its exponentially fitted result.
can effectively reduce the gap width and lead to the generation of submicron gaps. It is found that the original pattern design can affect the final electrode gap width remarkably. Fig. 6 showed the oxidation results of triangle-to-flat electrode pairs with different original gap width design. The triangle structure is with the identical angle, ~ 65°. The results of gap width evolution during different process steps were reviewed in Table 1. From Table 1, it can be seen that the gap width decreases almost for 0.8 μm for all after oxidation, i.e. gap width change Δ = D0 − D(T, t) = ~ 0.8 μm. For the patterns with identical angles, the electrodes with different original gap width have almost the same volume expansion rate. It is noted that according to the initial design, the decreased values of the gap width should be ~ 1.4 μm for flat-to-flat configuration. For the triangle-to-flat one, the value was ~ 0.8 μm in our work. This difference should be attributed to the electrode configurations. In order to further study the influence of the electrode configuration on the final gap width, the triangle-to-flat structure with different angles, were studied. The SEM results were shown in Fig. 7. The gap distance changes of each electrode configuration were reviewed in Table 2. It is found that the gap distance variation is related to the angle closely. The shaper the angle is, the less the gap width changes. The flat-to-flat structure has the smallest gap distance, D(T, t) = ~ 500 nm. This is because more oxygen is locally available on the flat regions than the triangle ones [16]. According to the results in Table 2, the response curve of gap distance change with angle was achieved and also exponentially fitted, as presented in Fig. 8. The fitted result is Y = − 1.38*exp(−X/39.23) + 0.92, R 2 = 0.95, where Y represents the gap distance change value, X represents the electrode's angle, and R is the correlation coefficient. It can be concluded that as long as the electrodes' design is appropriate, i.e. the designed gap's distance is between 1 μm and 2 μm, the
373
Fig. 10. Current–voltage (I-V) behavior of the fabricated submicron-gap electrodes.
submicron- or nano-gap electrodes can be achieved by conventional photolithography technology combined with thermal oxidization. As discussed above, among different configurations of electrode pairs, the flat-to-flat configuration had the optimal oxidization effect and the smallest final gap distance. Hence, the flat-to-flat electrode arrays, i.e. the interdigitated electrodes (IDTs) as shown in Fig. 9, will be the optimal candidate for the small gap-width construction, since this kind of electrodes can increase the contribution of sensing interface to enhance the detected electrical signal, i.e. to increase the sensitivity [17]. In our future work, IDTs will be applied for bioelectronic devices. 3.2. Electrode insulation testing Three interdigitated electrodes with gap distance of 1.8 μm, 1.3 μm and 600 nm, respectively, were performed by the insulation testing. Only the electrodes with high insulation capability can be selected for further application. For the electrode insulation testing, the system that consists of one digital source meter Keithley 6517A, one probe station with 2 probes and one optical microscope was used. The DC voltage ranging from 0 V to 1 V was applied with 0.1 V step and the corresponding current values were recorded by PC. For all the electrodes selected, the measured current is very low, b 0.5 pA, just as shown in Fig. 10. The DC insulation resistance calculated is > 1012 Ω, implying the good isolation of the as-prepared electrodes. The high insulation resistance can be attributed to two aspects: the long oxidation process leads to the thick oxide layer, which can isolate Ti/Au electrode from Si substrate and decrease the leakage current remarkably. The second reason is that during the metallization process, the gap is deep enough to cut off the Au film, and then avoid the short circuit.
Fig. 9. SEM images of the interdigitated electrodes after (a) DRIE, (b) oxidation and (c) metallization.
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Based on the electrode's design in our work, there is a layer of Au film in gaps as shown in Fig. 2(c). It cannot only simplify the experiment process, but also act as a bridge to graft probe biologic molecule into gaps to construct molecule devices. In addition, since narrow gaps lead to high sensitivity [18], the as-fabricated submicron-gap electrodes will be a good candidate for bio-sensors. 4. Conclusion A new fabrication method of submicron-gap electrodes with lowcost and batch-fabrication properties was discussed in our study. The influence of the electrode configurations on the final electrode gap distance was investigated. The I–V characteristics showed the asprepared electrodes with Au films in gaps have high insulating property and are potential for further application in bio-electronic devices. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant nos. 60672002, 61076070) and Innovation Program of Shanghai Municipal Education Commission (Grant no. 09ZZ46). We deeply appreciate the financial support.
References [1] S.J. Park, T.A. Taton, C.A. Mirkin, Science 295 (2002) 1503–1506. [2] Y. Lee, Y.-S. Jo, Y. Roh, Mater. Sci. Eng. C 23 (2003) 833–839. [3] T.L. Chang, C.Y. Tsai, C.C. Sun, C.C. Chen, Biosens. Bioelectron. 22 (2007) 3139–3145. [4] Y. Kashimura, H. Nakashima, K. Furukawa, K. Torimitsu, Thin Solid Films 438–439 (2003) 317–321. [5] K. Tsutsui, M. Nakata, M. Morita, M. Tokuda, K. Nagatsuma, H. Onozato, T. Kaneko, T. Edura, Curr. Appl. Phys. 7 (2007) 329–333. [6] C.S. Lee, C.H. Han, Sens. Actuators A 97–98 (2002) 739–743. [7] F. Carcenac, L. Malaquin, C. Vieu, Microelectron. Eng. 61–62 (2002) 657–663. [8] T. Nagase, K. Gamo, T. Kubota, S. Mashiko, Thin Solid Films 499 (2006) 279–284. [9] Y. Lee, Y. Roh, K.S. Kim, Thin Solid Films 515 (2006) 744–747. [10] F. Persson, L.H. Thamdrup, M.B. LMikkelsen, Nanotechnology 18 (2007) 245301. [11] C.J. Wu, Z.H. Jin, H.Q. Wang, H.L. Ma, Y.L. Wang, J. Micromech. Microeng. 17 (2007) 2393–2397. [12] J. Park, L.-H. Chen, D. Hong, C. Choi, M. Loya, K. Brammer, P. Bandaru, S. Jin, Nanotechnology 20 (2009) 015303. [13] S.Y. Lee, Encyclopedia of chemical processing, CRC Press, 2006. [14] B.E. Deal, A.S. Grove, J. Appl. Phys. 36 (1965) 3770–3778. [15] P. Du, H.X. Li, Z.H. Mei, S.F. Liu, Bioelectrochemistry 75 (2009) 37–43. [16] T.S. Ravi, R.B. Marcus, J. Vac. Sci. Technol. B 9 (1992) 2733–2737. [17] J. Tamaki, T. Hashishin, Y. Uno, D.V. Dao, S. Sugiyama, Sens. Actuators B 132 (2008) 234–238. [18] L.Y. Wang, N.N. Kariuki, M. Schadt, D. Mott, J. Luo, C.J. Zhong, X.J. Shi, C. Zhang, W. Hao, S. Lu, N. Kim, J.Q. Wang, Sensors 6 (2006) 667–679.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
A novel submicron-gap electrode fabrication technology using thermal oxidation Xuejiao Chen, Jian Zhang ⁎, Huhua Xu, Shichao Hui, Meiguang Zhu Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electrical Engineering, East China Normal University, Shanghai 200241, China
a r t i c l e
i n f o
Article history: Received 28 March 2011 Received in revised form 21 October 2011 Accepted 13 November 2011 Available online 20 November 2011 Keywords: Submicron-gap electrode Thermal oxidation Deep reactive ion etching Photolithography Metallization
a b s t r a c t A novel and reproducible method to fabricate submicron-gap electrodes using thermal oxidation has been presented. In this method, oxidation process determines the gap distance. The micron-level silicon electrode gaps with different shapes were first generated on the silicon wafer by conventional photolithography followed by deep reactive ion etching process. Then thermal oxidation was conducted to realize the transition from silicon to silicon dioxide, i.e. reduce the gap width. Finally, the planar electrodes with sub-micron spacing were formed by metallization and photolithography. Scanning electron microscopy (SEM) was used to examine the electrode configuration and the electrical properties of as-prepared electrode pairs were also characterized. The results showed that using the method investigated in this work, Au electrodes with a submicron-sized gap could be easily fabricated, with good uniformity and reproducibility. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the fabrication of metal electrodes with micro- or even nano-scale gap has received considerable attention, since these electrodes offer some advantages such as the decreased size, the improved performance and the capability to develop new-concept devices such as molecule devices [1–5]. They can be used as the manipulation tool of nano-materials, the platform of nano-sensors, and the basic element of nano-circuits. For example, biological probe can be introduced into electrodes' gap to construct biosensors. So far lots of methods have been investigated to fabricate the electrode pairs, including chemical–mechanical polishing [6], electroplating technique [4], high-resolution electron beam lithography [7], focused ion beam [8] and plasma ashing technique [9], etc., However, due to the rigorous requirements for fabrication condition, plus the high cost and the low yield, most of these methods are not suitable for batch-fabrication. In this work, a simple, reproducible and ingenious fabrication technique using thermal oxidation to fabricate submicron-gap electrodes was investigated. This method enables the reliable mass production of submicron-gap electrodes, which may have great application in bio-electronic devices and single-molecule devices [10–12]. The design mechanism of submicron-gap electrodes is that the gap width will decrease due to the volume variation during the transition from silicon to silicon dioxide. In another word, if a bare silicon surface is oxidized, 44% of the oxide thickness will lie below the original surface, and 56% above it [13].
⁎ Corresponding author. Tel./fax: + 86 21 54345203. E-mail address: [email protected] (J. Zhang). 0928-4931/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2011.11.008
We consider a silicon slit or trench formed on silicon substrate. Supposing D0 is the original designed gap width and D(T,t) is the final width after oxidation as shown in Fig. 1, the final gap width, D (T,t), which changes with the oxide thickness XSiO2, can be estimated as, DðT; t Þ ¼ D0 −2 0:56X SiO2 ðT; t Þ
ð1Þ
According to the linear parabolic model developed for wet oxidation, proposed by Deal and Grove, the thickness of oxide layer is related to oxidation time through 2
X SiO2 ðT; t Þ þ AX SiO2 ðT; t Þ ¼ Bt
ð2Þ
Where T is the wet-oxygen oxidation temperature, A is the linear rate constant, B is the parabolic rate constant which is related to the oxidation parameters (temperature, pressure, water content, etc.),
Fig. 1. Sketch map of electrode gap evolution before and after thermal oxidation.
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X. Chen et al. / Materials Science and Engineering C 32 (2012) 369–374
Fig. 4. The morphology of the electrodes characterized by a metallurgical microscope.
(DRIE). Then the thermal oxidation was used to oxidize the silicon, which can realize the gap width transition from micron-meter to submicron-meter.
Fig. 2. Fabrication sequence of submicron-gap electrodes: (a) Photolithography and DRIE; (b) Thermal Oxidation; (c) Metallization.
and t is the oxidation time, respectively. Once the oxidation condition is defined, the constants A and B are known [14]. Combining Eqs. (1) and (2), it can be found that the gap width, D (T,t), is controlled by choosing the proper oxidation time. And the gap width will decrease while the oxidation time increases. If the oxidation time is chosen properly, the gap can be controlled below micrometer. Here, we present a simple method to fabricate the submicron-gap electrodes. The as-developed submicron-gap electrodes were tested by I–V characterization to examine the isolation properties. 2. Experimental The schematic diagram of electrode fabrication is shown in Fig. 2. In this study, the silicon gap with micron-level width and different shapes were first designed and generated on the silicon wafer by conventional photolithography followed by deep reactive ion etching
Fig. 3. Electrode pattern design (a) flat to flat; (b) triangle to flat; (c) interdigitated.
Fig. 5. SEM images of electrode gap after (a) DRIE, (b) Oxidation and (c) Metallization.
X. Chen et al. / Materials Science and Engineering C 32 (2012) 369–374
2.1. Electrode configuration design In our work, the oxidation temperature is fixed at ~ 1000 °C. Referring to Deal and Grove's results [14], A and B in our study are supposed to be 0.226 μm and 0.287 μm 2/h, respectively. So the oxidation layer thickness can be estimated from Eq. (2) once the oxidation time, t, is determined. To consider the limitation of photolithography, the original gap width, D0, was designed ranging from 1.4 μm to 2.6 μm in our study. According to Eq. (2), if the oxidation time, t, is 7 h, it is predicted that the designed gaps can reach sub-micron level. In order to examine the oxidation effect on the final gap, three kinds of electrode configuration have been designed, including flat to flat, triangle to flat, and interdigitated. The configurations of as-designed electrodes are demonstrated as Fig. 3, in which the blue part represents the metallic electrodes and the red one represents the gap between electrode pairs. In all configurations, the depth of the electrode gaps is fixed at ~ 10 μm.
371
Table 1 The changes of the electrode gap distance: same angle to flat shape. Electrode gap
Angle (°)
D0 (μm)
D(T, t) (μm)
Δ = D0 − D(T, t) (μm)
a b c d e f
65 65 65 65 65 65
2.6 2.3 2.0 1.8 1.6 1.4
1.8 1.5 1.3 1.1 0.8 0.6
0.8 0.8 0.7 0.7 0.8 0.8
2.2. Electrode fabrication The electrode fabrication process is depicted schematically in Fig. 2. The polished silicon wafers (100 orientated, 300 μm thickness, Boron-doped) were used. The trench-like structures with 1.4–2.6 μm width and 10 μm depth were first obtained by photolithography and DRIE, just as shown in Fig. 2(a). Wet thermal oxidation with temperature of 1000 °C for 7 h was then performed as shown in Fig. 2(b).
Fig. 6. For the electrodes with same angle to flat shape, after oxidation, the gap distance decreases (a) from 2.6 μm to 1.8 μm; (b) from 2.3 μm to 1.5 μm; (c) from 2.0 μm to 1.3 μm; (d) from 1.8 μm to 1.1 μm; (e) from 1.6 μm to 0.8 μm; (f) from 1.4 μm to 0.6 μm.
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Fig. 7. For the electrodes with different angle to flat shape, after oxidation, the gap distance decreases (a) from 1.4 μm to 1.1 μm; (b) from 1.4 μm to 1.0 μm; (c) from 1.4 μm to 0.9 μm; (d) from 1.4 μm to 0.7 μm; (e) from 1.4 μm to 0.6 μm; (f) from 1.4 μm to 0.5 μm.
Finally, 300 Å Ti/2000 Å Au layers were deposited via magnetron sputtering. And the final electrodes were formed by photolithography as shown in Fig. 2(c). In our work, to simplify the mask design and the fabrication process, the metallization process was introduced to deposit Au films to both the planar electrodes pairs and gaps simultaneously. The Au film in the gaps is intended for the immobilization of biological probe in our future work, due to gold nanoparticles' large surface area and biocompatibility with bio-system [15], and then we can construct bio-electronic devices to detect target solution. Moreover,
Table 2 The changes of the electrode gap distance: different angle to flat shape. Electrode gap
Angle (°)
D0 (μm)
D(T, t) (μm)
Δ = D0 − D(T, t) (μm)
a b c d e f
30 40 50 65 95 180
1.4 1.4 1.4 1.4 1.4 1.4
1.1 1.0 0.9 0.7 0.6 0.5
0.3 0.4 0.5 0.7 0.8 0.9
during Ti/Au layer deposition, since the gaps were ~10 μm in depth, the isolation between the electrode pairs can be realized automatically, as shown in Fig. 2(c). It is demonstrated in our study that this process can avoid the short circuit and simplify the fabrication steps. In this study, scanning electron microscopy (SEM) is used to monitor the electrodes' fabrication. 3. Results and discussion 3.1. Characterization of submicron-gap electrodes In our study, the morphology of the electrodes was first characterized by a metallurgical microscope (Caikon DMM-220C). And as shown in Fig. 4, it can be found that both electrode pairs and gaps have been deposited with Au films by metallization. By scanning electron microscopy, the oxidation effect on the electrodes can be studied. Fig. 5 showed the electrode gap change with triangle-to-flat structure during different process step (a) DRIE, (b) Oxidation and (c) Metallization, respectively. It can be found that the gap width becomes narrower, which indicated that the transition from silicon to silicon dioxide during the wet oxidation process
X. Chen et al. / Materials Science and Engineering C 32 (2012) 369–374
Fig. 8. The response curve of gap distance change with angle and its exponentially fitted result.
can effectively reduce the gap width and lead to the generation of submicron gaps. It is found that the original pattern design can affect the final electrode gap width remarkably. Fig. 6 showed the oxidation results of triangle-to-flat electrode pairs with different original gap width design. The triangle structure is with the identical angle, ~ 65°. The results of gap width evolution during different process steps were reviewed in Table 1. From Table 1, it can be seen that the gap width decreases almost for 0.8 μm for all after oxidation, i.e. gap width change Δ = D0 − D(T, t) = ~ 0.8 μm. For the patterns with identical angles, the electrodes with different original gap width have almost the same volume expansion rate. It is noted that according to the initial design, the decreased values of the gap width should be ~ 1.4 μm for flat-to-flat configuration. For the triangle-to-flat one, the value was ~ 0.8 μm in our work. This difference should be attributed to the electrode configurations. In order to further study the influence of the electrode configuration on the final gap width, the triangle-to-flat structure with different angles, were studied. The SEM results were shown in Fig. 7. The gap distance changes of each electrode configuration were reviewed in Table 2. It is found that the gap distance variation is related to the angle closely. The shaper the angle is, the less the gap width changes. The flat-to-flat structure has the smallest gap distance, D(T, t) = ~ 500 nm. This is because more oxygen is locally available on the flat regions than the triangle ones [16]. According to the results in Table 2, the response curve of gap distance change with angle was achieved and also exponentially fitted, as presented in Fig. 8. The fitted result is Y = − 1.38*exp(−X/39.23) + 0.92, R 2 = 0.95, where Y represents the gap distance change value, X represents the electrode's angle, and R is the correlation coefficient. It can be concluded that as long as the electrodes' design is appropriate, i.e. the designed gap's distance is between 1 μm and 2 μm, the
373
Fig. 10. Current–voltage (I-V) behavior of the fabricated submicron-gap electrodes.
submicron- or nano-gap electrodes can be achieved by conventional photolithography technology combined with thermal oxidization. As discussed above, among different configurations of electrode pairs, the flat-to-flat configuration had the optimal oxidization effect and the smallest final gap distance. Hence, the flat-to-flat electrode arrays, i.e. the interdigitated electrodes (IDTs) as shown in Fig. 9, will be the optimal candidate for the small gap-width construction, since this kind of electrodes can increase the contribution of sensing interface to enhance the detected electrical signal, i.e. to increase the sensitivity [17]. In our future work, IDTs will be applied for bioelectronic devices. 3.2. Electrode insulation testing Three interdigitated electrodes with gap distance of 1.8 μm, 1.3 μm and 600 nm, respectively, were performed by the insulation testing. Only the electrodes with high insulation capability can be selected for further application. For the electrode insulation testing, the system that consists of one digital source meter Keithley 6517A, one probe station with 2 probes and one optical microscope was used. The DC voltage ranging from 0 V to 1 V was applied with 0.1 V step and the corresponding current values were recorded by PC. For all the electrodes selected, the measured current is very low, b 0.5 pA, just as shown in Fig. 10. The DC insulation resistance calculated is > 1012 Ω, implying the good isolation of the as-prepared electrodes. The high insulation resistance can be attributed to two aspects: the long oxidation process leads to the thick oxide layer, which can isolate Ti/Au electrode from Si substrate and decrease the leakage current remarkably. The second reason is that during the metallization process, the gap is deep enough to cut off the Au film, and then avoid the short circuit.
Fig. 9. SEM images of the interdigitated electrodes after (a) DRIE, (b) oxidation and (c) metallization.
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Based on the electrode's design in our work, there is a layer of Au film in gaps as shown in Fig. 2(c). It cannot only simplify the experiment process, but also act as a bridge to graft probe biologic molecule into gaps to construct molecule devices. In addition, since narrow gaps lead to high sensitivity [18], the as-fabricated submicron-gap electrodes will be a good candidate for bio-sensors. 4. Conclusion A new fabrication method of submicron-gap electrodes with lowcost and batch-fabrication properties was discussed in our study. The influence of the electrode configurations on the final electrode gap distance was investigated. The I–V characteristics showed the asprepared electrodes with Au films in gaps have high insulating property and are potential for further application in bio-electronic devices. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant nos. 60672002, 61076070) and Innovation Program of Shanghai Municipal Education Commission (Grant no. 09ZZ46). We deeply appreciate the financial support.
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