FABRICATION OF MICRO-HEATERS EMBEDDED IN PDMS USING A DRY PEEL-OFF PROCESS

Ikjoo Byun, Ryohei Ueno, and Beomjoon Kim Institute of Industrial Science, The University of Tokyo, Tokyo, JAPAN

ABSTRACT

This paper describes a reliable fabrication method of micro-heaters embedded in polydimethylsiloxane (PDMS). Gold patterns are transferred and embedded to the PDMS from a silicon substrate, by peeling off. The surface adhesion among silicon substrate, Au patterns, and PDMS is modified with self-assembled monolayers. Therefore, micro-heaters embedded in PDMS can be fabricated by a simpler and easier way compared to conventional methods. The thermal characterization of micro-heaters and PDMS device is carried out by electrical and infrared thermo-microscopic measurements. The experimental results well agree with the numerical analysis performed by finite element method. INTRODUCTION

The lab-on-a-chip (LoC) systems have been one of the dominant themes in analytical instrumentation for chemical and biomedical applications over the past decade [1]. Thermal control is one of the most critical factors for precise control of chemical reaction [2], protein synthesis [3], and polymerase chain reaction (PCR) [4], etc. Micro-heaters [3] enables more precise temperature control compared to heating blocks [4] or heating wires [5].

For LoC devices, polydimethylsiloxane (PDMS) is one of the most popular polymer because it is bio-compatible, chemically resistant, optically transparent, inexpensive and easy to be fabricated [2,3]. Usually, heaters for LoC devices are fabricated on a glass substrate, and assembled with PDMS micro-fluidic channels [3]. On the other hand, micro-heaters embedded in PDMS have the advantages such as flexibility, rapid prototyping, and greater compatibility with existing PDMS chips [6].

Some research groups have been trying to produce a conducting PDMS by mixing various fillers (e.g. carbon black powder or metallic powder) and PDMS prepolymers [6,7]. To pattern the conducting PDMS, gel-state conducting PDMS is filled into photoresist (PR) patterns, then the extra mixture is removed using a razor blade. However, this method suffers from two reasons: (1) a razor blade can damage PR patterns mechanically and (2) a conducting PDMS is sensitive to the change of ambient temperature due to a large coefficient of thermal expansion. Thus, it is difficult to control the temperature of micro-heater reliably even though it is calibrated. Recently, it has been demonstrated that (3-mercaptopropyl)trimethoxysilane (MPTMS) can considerably promote the adhesion between Au and PDMS using a liquid deposition method [8]. Also, Au micro-patterns can be embedded in PDMS using a dry peel-off process [9,10].

In this paper, we show a simple and reliable fabrication method of Au micro-heaters embedded in PDMS.

Evaluations including temperature with respect to applied voltages, spatial distribution and transient response were conducted. Finally, simulation results are compared to the experimental ones.

FABRICATION

The key point in fabricating the Au micro-heaters is the direct transfer of metal layer into PDMS carried out using a “dry peel-off” process, which involves modifying the surface properties of the substrate and metal patterns through self-assembled monolayer (SAM) treatment and peeling off the PDMS with embedded metal layers [10]. The fabrication process was based on the “dry peel-off” process, but improved compared to the previous research [10], as shown in Figure 1. A Si wafer was treated with a piranha solution for 10 min, followed by dehydration at 150 °C for 10 min. Then, a sparse MPTMS layer was formed on the Si substrate to make a moderate adhesion using a vapor deposition for 10 min. A thin Au layer (thickness: 100 nm) deposited by thermal evaporation onto the substrate was lithographically patterned. The substrate with both Au patterns and photoresist (PR) patterns were immersed in perfluorodecyl -trichlorosilane (FDTS) solution (5 mM in hexane) for 5 min for anti-adhesion between Si and PDMS. After PR removal, the substrate with Au patterns was treated with an ethanolic solution of 20 mM MPTMS for 2 h as a molecular adhesive between Au and PDMS. A 10:1 (by weight) mixture of PDMS base/curing agent was poured on the substrate, then heat-cured in an oven at 60 °C for 3 h, then maintained at room temperature for 12 h. The thickness of PDMS substrate was controlled to 3 mm. Finally, the PDMS with Au patterns was peeled off from the Si substrate.

Figure 1: (a–f) Schematic illustration of the fabrication process, and optical images of Au micro-heaters (g) before and (h) after peeling off the PDMS from the Si substrate.

978-1-4799-3509-3/14/$31.00 ©2014 IEEE 514 MEMS 2014, San Francisco, CA, USA, January 26 - 30, 2014

CHARACTERIZATION The micro-heaters were designed with different

geometric shapes (width: 40, 80 and 160 μm, length: 15 mm), as shown in Figure 2(a). Pads for electrical connection to power supply were designed for 3 × 3 mm. Joule heating was applied to the micro-heaters operations. To apply the voltage and measure the electrical resistance, DC voltage current source/monitor (6240A, ADCMT, Japan) was used. Copper wires and Au micro-heaters were electrically connected with silver paste (Electroconductives, D-362, Fujikura Kasei Dotite, Japan).

A simple setup was employed to characterize the micro-heaters, as shown in Figure 2(b). The temperature of micro-heaters and PDMS were observed by infrared (IR) thermo-microscopy (FSV-GX7700, Apiste). The emissivity for the IR imaging was determined to 0.86 for the PDMS [11]. It was difficult to measure the temperature both Au micro-heaters and circumferential PDMS, simultaneously because of a large difference of emissivity between Au (0.02) and PDMS (0.86). Thus, the highest temperature of the target area was measured and recorded (i.e. the temperature of circumferential PDMS adjacent to the Au micro-heaters). To investigate the transient response of heating and cooling, the power source was turned on for 100 s, then turned off with natural convective cooling. The temperature of Au micro -heaters was measured with 15 Hz of IR imaging.

Figure 2: (a) Design of the Au micro-heaters and (b) schematic illustration of the experimental setup for measuring the temperature during Joule heating.

SIMULATION

Electro-thermo-mechanical models for steady-state as well as transient analysis were developed to investigate the temperature at certain applied voltages, spatial distribution, and transient response of temperature [12]. The simulation was conducted by finite element method using the commercial software FEMLAB® (COMSOL, version 4.3). The material properties applied to the simulation are shown in Table 1.

Table 1: Material properties for the simulation. Materials Thermal

conductivity, k, W m-1K-1

Specific heat, Cp, J kg-1K-1

Density, ρ, kg m-3

Au [13] 315 130 19320

PDMS [14] 0.18 1100 1030

RESULTS AND DISCUSSION Temperature versus Applied Voltage

The temperature of micro-heaters was measured during the voltage applied-periods. Micro-heaters with wider width required lower voltage to be heated up compared to those with narrower width. The temperature of Au micro-heaters was proportional to the square of the applied voltage, as shown in Figure 3. The simulation results (dotted lines) well agreed with the experimental ones (solid lines).

Figure 3: Experimental and simulation results of temperature of micro-heaters embedded in PDMS versus applied voltage.

Heating Efficiency

The temperature of micro-heaters was proportional to the power consumption. In order to simulate the temperature with respect to the power consumption, the power consumption was calculated based on the applied voltage and the theoretical resistance of Au micro-heaters. The resistivity (ρ) of Au micro-heaters was calculated by the equation (1) where ρ0 is the resistivity of Au at 20 °C (23.5 nΩm), α is the temperature coefficient of resistance of Au (0.0034 K-1) [15], T is the temperature of Au, and T0 is 20 °C .

0 (1 ( ))0T Tρ ρ α= + − (1)

The heating efficiency (H), the temperature heated up by

the unit power, can be defined as the equation (2) where T is the temperature of micro-heaters, T0 is the initial temperature, and P is the power consumption.

0T T

HP−

= (2)

The experimental and simulation results of the heating

efficiency are shown in Table 2. The heating efficiency can be increased by decreasing the length of the micro-heaters. Approximately, a power of 1.3–1.5 mW was necessary to increase the 1 °C of temperature of micro-heaters. Interestingly, the heating efficiency was decreased at the

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micro-heaters with wider width. It can be assumed that the heat dissipation was increased as the surface area of the heater was increased. It was observed that the heating efficiencies estimated by simulation were higher than those measured by the experiment. The reason is assumed that the electrical properties of thin film Au are different with bulk state [15].

Table 2: Experimental and simulation results of heating efficiency in the temperature range from 25 °C to 100 °C when the length of micro-heaters is 15 mm. Width (μm) 40 80 160

Experiment (K mW-1) 0.76 0.67 0.66

Simulation (K mW-1) 0.81 0.77 0.71 Resistivity and Temperature Coefficient of Resistance

From the experimental data of resistance with respect to the temperature, the empirical resistivity (ρ0E) and temperature coefficient of resistance (αE) of Au thin film (100 nm) embedded in PDMS can be calculated, as shown in Table 3. The average values are 22.9 nΩm for ρ0E at 25 °C and 0.0024 K-1 for temperature coefficient of resistance. These values obtained from the thin film differ from the reported bulk value (23.5 nΩm for ρ0 at 20 °C and 0.0034 K-1 for α), but similar to the property of the Au thin film reported previously (0.0017 K-1 [15] and 0.0022 K-1 [16] for α).

Table 3: The empirical electric properties of thin film Au (100 nm): resistivity at 25 °C and temperature coefficient of resistance. Width (μm) 40 80 160

Resistivity (nΩm) 22.1 21.8 24.8 Temperature coefficient of resistance (K-1) 0.0029 0.0020 0.0023

Spatial Distribution

The spatial temperature distribution was investigated by measuring the surface temperature of Au micro-heaters and PDMS substrates, as shown in Figure 4. At the same applied voltage, Au micro-heaters with wider width consumed more electrical power, so that the temperature was increased more. It was observed that the temperature was decreased with increasing distance from the heaters. The temperature difference between the heater and the distant point about 1 mm from the heater was about 20 °C when the voltage of 1.2 V was applied to the micro-heater (width: 160 μm). At the position 0 mm, the temperature of experimental results was ~35 °C even though the temperature of peripheral PDMS was more than 60 °C that was caused by a mismatch of the emissivity between Au and PDMS. Also, the simulation of surface temperature distribution well agreed with the experimental results.

Figure 4: (a–c) IR thermo-microscopic images of Au micro-heaters embedded in PDMS during Joule heating by applying the voltage of 1.2 V, (d–f) experimental and simulation results of spatial temperature distribution. Transient Response

The transient response of temperature rise and fall is an important characteristic to most thermal system. To investigate the transient response, a square signal of voltage was applied during the IR imaging of micro-heaters, as shown in Figure 5. When the signal (3 V) to the Au micro-heaters (width: 40 μm), the temperature was increased rapidly from the room temperature (25 °C) to 80 °C within 10 s, then slowly saturated to 90 °C. Finally, the temperature was saturated 60 s after the signal was applied. When the signal was turned off, the temperature was rapidly decreased to 35 °C within 10 s, then slowly decreased to the room temperature.

However, the simulation results showed 10 °C lower temperature compared to the experimental results (Simulation 1, applied voltage: 3 V). When the maximum temperature was set to 90 °C at the micro-heaters for the simulation (Simulation 2), the results are much similar to the experimental ones. Even though the maximum temperature was same at the 60 s after the signal on, the simulation results (Simulation 2) showed slower transient response compared to the experimental results. Furthermore, the reason of these mismatches will be investigated.

Figure 5: Experimental and simulation results of transient responses of Au micro-heaters embedded in PDMS. Signal duration was 60 s.

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CONCLUSION Micro-heaters patterned on a Si wafer were transferred

and embedded in PDMS using a dry peel-off process. The fabrication method was simple including only conventional photolithography, metal deposition and etching, and surface modification through SAM. Moreover, the process did not cause any chemical swelling, contamination, or mechanical damage by a razor.

The temperature of micro-heaters depend on the power consumption, but does not depend on the width of micro-heaters. The temperature was saturated within 1 min, which is reasonable for many applications. The simulation results well agreed with the experimental results in terms of temperature at certain voltage or power consumption, surface distribution, and transient response.

As further works, peel-off transfer technique will be investigated to be applied to other polymers (e.g. polyimide or polyethylene terephthalate). This study would broaden not only the LoC, but also the fields of optical and electrical applications with low cost.

ACKNOWLEDGEMENTS

This work has been, partially, supported by the JSPS Core-to-Core Program A (Advanced Research Networks).

REFERENCES [1] J. El-Ali, P. K. Sorger, K. F. Jensen, “Cells on Chips”,

Nature, vol. 442, pp. 403-411, 2006. [2] J. W. Ha, A. Kundu, J. H. Hang, “Poly-dimethylsiloxane

(PDMS) Based Micro-reactors for Steam Reforming of Methanol”, Fuel Process. Technol., vol. 91, pp. 1725-1730, 2010.

[3] T. Yamamoto, T. Nojima, T. Fujii, “PDMS-glass Hybrid Microreactor Array with Embedded Temperature Control Device. Application to Cell-free Protein Synthesis”, Lab Chip, vol. 2, pp. 197-202, 2002.

[4] Y. S. Shin, et al., “PDMS-based Micro PCR Chip with Parylene Coating”, J. Micromech. Microeng., vol. 13, pp. 768-774, 2003.

[5] N. G. Wilson, T. McCreedy, “On-chip Catalysis using a Lithographically Fabricated Glass Microreactor—The Dehydration of Alcohols Using Sulfated Zirconia”, Chem. Commun., pp. 733-734, 2000.

[6] H. S. Chuang, S. Wereley, “Design, Fabrication and Characterization of a Conducting PDMS for Microheaters and Temperature Sensors”, J. Micromech. Microeng., vol. 19, pp. 045010, 2009.

[7] X. Niu, S. Peng, L. Liu, W. Wen, P. Sheng, “Characterizing and Patterning of PDMS-based Conducting Composites”, Adv. Mater., vol. 19, pp. 2682-2686, 2007.

[8] I. Byun, A. W. Coleman, B. Kim, “Transfer of Thin Au Films to Polydimethylsiloxane (PDMS) with Reliable Bonding Using (3-mercaptopropyl)trimethoxysilane (MPTMS) as a Molecular Adhesive”, J. Micromech. Microeng., vol. 23, pp. 085016, 2013.

[9] I. Byun, A. W. Coleman, B. Kim, “SAM Meets MEMS: Reliable Fabrication of Stable Au-patterns Embedded in PDMS Using Dry Peel-off Process”, Microsyst. Technol., 2013, DOI: 10.1007/s00542-013- 1923-8.

[10] I. Byun, A. W. Coleman, B. Kim, “Microcontact Printing Using a Flat Metal-embedded Stamp Fabricated Using a Dry Peel-off Process”, RSC Adv., vol. 3, pp. 24872-24876, 2013.

[11] J. L. Lin, M. H. Wu, C. Y. Kuo, K. D. Lee, Y. L. Shen, “Application of Indium Tin Oxide (ITO)-based Microheater Chip with Uniform Thermal Distribution for Perfusion Cell Culture Outside a Cell Incubator”, Biomed. Microdevices, vol. 12, pp. 389-398, 2010.

[12] H. F. Arata, P. Low, K. Ishizuka, C. Bergaud, B. Kim, H. Noji, H. Fujita, “Temperature Distribution Measurement on Microfabricated Thermodevice for Single Biomolecular Observation using Fluorescent Dye”, Sens. Actuators B, vol. 117, pp. 339-345, 2006.

[13] S. M. Lee, D. C. Dyer, J. W. Gardner, “Design and Optimisation of a High-temperature Silicon Micro-hotplate for Nanoporous Palladium Pellistors”, Microelectronic Journal, vol. 34, pp. 115-126, 2003.

[14] D. Erickson, D. Sinton, D. Li, “Joule Heating and Heat Transfer in Poly(dimethylsiloxane) Microfluidic Systems”, Lab Chip, vol. 3, pp. 141-149, 2003.

[15] F. Aviles, O. Ceh, A. I. Oliva, “Physical Properties of Au and Al Thin Films Measured by Resistive Heating”, Surf. Rev. Lett., vol. 12, pp. 101-106, 2005.

[16] R. B. Belser, W. H. Hicklin, “Temperature Coefficients of Resistance of Metallic Films in the Temperature Range 25° to 600°C”, J. Appl. Phys., vol. 30, pp. 313-322, 1959.

CONTACT

*Beomjoon Kim, tel: +81-3-5452-6224; bjoonkim@iis.u-tokyo.ac.jp

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