J Mater Sci: Mater Electron (2010) 21:562 566 DOI 10.1007/s10854-009-9957-5 Reactive nanolayers for physiologically compatible microsystem packaging Xiaotun Qiu Æ David Welch Æ Jennifer Blain Christen Æ Jie Zhu Æ Jon Oiler Æ Cunjiang Yu Æ Ziyu Wang Æ Hongyu Yu Received: 19 May 2009 / Accepted: 4 August 2009 / Published online: 15 August 2009 Ó Springer Science+Business Media, LLC 2009 Abstract This paper described a novel physiologically compatible wafer bonding technique for bio-microelectromechanical systems (bio-mems) packaging. Room temperature bonding was performed between Parylene-C and silicon wafers with a thin Parylene-C coating using reactive Ni/Al nanofilms as localized heaters. Live NIH 3T3 mouse fibroblast cells were encapsulated in the package and they survived the bonding process owing to the localization of heating. A numerical model was developed to predict the temperature evolutions in the parylene layers, silicon wafer and the encapsulated liquid during the bonding process. The simulation results were in agreement with the cell encapsulation experiment revealing that localized heating occurred in this bonding approach. This study proved the feasibility of reactive nanofilm bonding technique for broad applications in packaging bio- MEMS and microfluidic systems. 1 Introduction Bio-microelectromechanical systems (bio-mems) are designed for biomedical applications. As with other X. Qiu (&) D. Welch J. B. Christen J. Zhu H. Yu Department of Electrical Engineering, Arizona State University, Tempe, AZ 85287, USA e-mail: xqiu5@asu.edu J. Oiler Z. Wang H. Yu School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA C. Yu Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ 85287, USA MEMS, bio-mems need to be packaged to provide an interface with the macroscale world. Due to their unique applications, bio-mems have special requirements for the packaging materials and techniques compared with other MEMS [1]. First, Bio-MEMS containing biological subcomponents require the use of biocompatible materials for packaging to avoid unintentional effects on the biological substances. Second, Bio-MEMS require biocompatible technologies for assembly and packaging. For instance, the high temperatures during the standard semiconductor bonding processes such as direct bonding and intermediate layer bonding [2 5] would denature the biological substances on the wafer. In general, biological materials can only survive temperatures up to 318 K for about 2 min [6]. Therefore, new bonding processes using biocompatible packaging materials and low bonding temperature need to be investigated to address the challenges for bio-mems packaging. Parylene-C (glass transition temperature: 382 K) is a biocompatible polymer extensively employed in MEMS [7, 8], which is suitable for bio-mems assembly applications. It has been reported that parylene can be used as an intermediate layer to bond silicon wafers at 503 K for 30 min [9]. However, this global heating method will damage biological components on the wafer. Alternatively, localized heating approaches, in which high temperature can be locally generated for hermetic and strong bonds, while, at the same time, the temperature outside the bonding areas can be kept low, have been developed to solve this problem. Localized heating can be achieved using embedded micro-heaters [10], however, the use of micro-heaters can introduce complexity to the bonding design and in many cases electrical wiring is not preferred. Meanwhile, localized heating can also be accomplished by laser, microwave or ultrasound [11 13]; however, these
J Mater Sci: Mater Electron (2010) 21:562 566 563 techniques require complicated equipment and bonding facilities. In this study, we reported a novel physiologically compatible room temperature bonding technique using reactive Ni/Al nanofilms as local heat sources to bond a Parylene-C layer to silicon wafers with a thin Parylene-C coating. Reactive Ni/Al nanofilms contain thousands of nanoscale Ni and Al bilayers. With a small thermal pulse, these films can react exothermically and generate a selfpropagating reaction. Self-propagating reactions in these films are driven by a reduction in chemical bond energy. This local reduction of chemical bond energy produces a large quantity of heat that is conducted down the film and facilitates more atomic mixing and compound formation. Such exothermic reactions in nanofilms can be used as local heat sources to melt solders or brazes and thus bond components in a variety of applications, such as bonding stainless steel, aluminum, titanium, metallic glass and silicon wafers [14 20]. With localized heating, temperature sensitive components such as biological substances can be packaged without damage. Such bonding can be performed in many environments, such as in vacuum, and can be completed in a second or less. Live NIH 3T3 mouse fibroblast cells were encapsulated in the package in order to demonstrate the physiological compatibility of this bonding approach and its feasibility to be employed in bio- MEMS packaging. A numerical method was developed to simulate the temperature evolutions in the parylene layers, silicon wafer and the encapsulated liquid during the bonding process. 2 Experimental Ni/Al reactive multilayer films (Reactive NanoTechnologies Inc, Hunt Valley, MD) were fabricated by magnetron sputtering. The total thickness of the Ni/Al films used here was 80 lm, with a bilayer thickness of 40 nm. These films were used as local heat sources to bond a Parylene-C layer to silicon wafers with a thin Parylene-C coating. The geometry for Parylene-C bonding with cell encapsulation is shown schematically in Fig. 1. A Parylene-C layer (9 lm) was bonded to a silicon wafer (20 mm 9 20 mm, with cavities to hold cells) with a thin Parylene-C coating (9 lm). The cavity was formed by anisotropic potassium hydroxide (KOH) etching. Ni/Al multilayer films were placed on top of the Parylene-C layer as localized heaters. The width of the film was 2 mm. An Al substrate was used to apply pressure (about 1.1 kpa) to the bonding assembly. The reactive film was ignited by a power supply with a voltage of 2 V. After ignition, a self-propagating reaction occurred in the film and the heat released penetrated the top parylene layer and formed bond at the parylene/parylene interface. The bonding process was performed in atmosphere and it completed in less than 1 s. NIH 3T3 mouse fibroblast cells were encapsulated in the package to verify the physiological compatibility of the reactive nanofilm bonding approach. They were cultured on the bottom surface of the cavity and left in an incubator to grow for 24 h before bonding was performed on the wafer. An optical microscope was used to inspect the cell adhesion and morphology inside the cavity. Numerical simulation was conducted to predict the temperature evolutions in the parylene layers, silicon wafer and the encapsulated liquid during the bonding process using commercial finite element analysis software, Fluent [21]. The numerical model was based on a simplified description of the self-propagating reaction and the thermal transport occurring in the bonding assembly. The model assumed one-dimensional motion of the reaction front, which was described using experimentally determined heats of reaction and reaction velocities of the films [14]. More details of the numerical model can be found in a previous paper [20]. In the current study, the thermal resistance at the unbonded parylene/parylene interface was not considered. An ideal contact was assumed in the model. The simulation started when the reactive film was ignited. The physical properties of different materials used in the simulation are listed in Table 1. Water was used in the simulation instead of culture media as the encapsulated liquid. 3 Results and discussion Eighty micrometer thick Ni/Al multilayer films with a bilayer thickness of 40 nm were used in this study to bond the Parylene-C layer to silicon wafers with a thin Parylene- C coating. From a previous study, 80 lm thick films can generate enough heat to soften the parylene layers and form a uniform bond at the parylene/parylene interface [22]. For Ni/Al multilayer films, the reaction velocity decreased with increasing bilayer thickness. The heat of reaction increased as bilayer thickness increased, due to intermixing occurred during deposition [14]. In order to achieve a balance between high reaction velocity and high reaction heat, films with bilayer thickness of 40 nm were used in this study. A successfully bonded cell encapsulation package is shown in Fig. 1c. In order to inspect the cell adhesion and morphology after bonding, the package was forcefully broken. Parylene-C layer was torn in this process, while the bonding interface remained intact, indicating a strong bond was achieved. Figure 2 shows the microphotographs of NIH 3T3 cells before and after bonding. Visual inspection illustrated that there were no appreciable changes in the
564 J Mater Sci: Mater Electron (2010) 21:562 566 Fig. 1 The schematic showing of the cell encapsulation package: a top view; b cross sectional view (not drawn to scale); c a photograph of the bonding assembly Table 1 Thermophysical parameters for the reactive film, Parylene- C, Al, silicon and the encapsulated liquid used in the simulation Thermal conductivity (W/mK) Heat capacity (J/kgK) Density (kg/m 3 ) Silicon 149 707 2,330 Parylene-C 0.08 1,000 1,289 Al 202.4 871 2,719 Film (as-deposited) 160 830 5,500 Film (reacted) 25 610 5,860 Water 0.5984 4,185 998.2 Fig. 2 Microphotographs of NIH 3T3 cells: a cells encapsulated in the cavity before bonding; b cell adhesion and morphology showed no appreciable changes after packaging, indicating negligible heat exposure to the cells aspect of cell adhesion and morphology during the bonding process. The results demonstrated that the heat the cells exposed to in this bonding approach was negligible due to the localized heating nature of the reactive nanofilms. To the authors knowledge, this was the first successful attempt to encapsulate live cells in microfabricated structures using wafer bonding techniques other than epoxy bonding. Figure 3 shows the numerical prediction of temperature evolution at the parylene/parylene bonding interface within 100 ms after ignition. The highest temperature experienced at the bonding interface was 545 K, which was below the melting temperature of Parylene-C (563 K). The temperature of the interface returned to around room temperature in 100 ms, demonstrating a high cooling rate can also be achieved. Figure 4 shows the temperature distribution at the encapsulated liquid surface at the end of reaction (4 ms after ignition) (a) and 100 ms after ignition (b). The high temperature was well-confined in the bonding region and the encapsulated liquid was almost undisturbed during the packaging process. Figure 4d shows the temperature evolution at a position 10 lm above the center of the cavity bottom inside the liquid (point A in Fig. 4c). The highest temperature experienced was only 301.6 K, which posed no harm to biological substances. These results agreed with the cell encapsulation experiment, indicating that the heating during the reactive bonding process was highly Fig. 3 Temperature evolutions at the parylene/parylene bonding interface within 100 ms after ignition. The highest temperature experienced at the bonding interface was 545 K
J Mater Sci: Mater Electron (2010) 21:562 566 565 Fig. 4 Simulation results: a temperature distribution at the encapsulated liquid surface at the end of reaction (4 ms after ignition); b temperature distribution at the encapsulated liquid surface 100 ms after ignition; c cross sectional view of temperature distribution in the bonding assembly 100 ms after ignition; d temperature evolution at point A (10 lm above the center of the cavity bottom inside the liquid) within 150 s after ignition. The highest temperature experienced was only 301.6 K, which posed no harm to biological substances. The dip in the curve was due to the delay of the heat transferred to point A from the water above. Since A was close to the cavity bottom, it was heated up by the water beneath it first due to the much larger thermal conductivity of silicon compared to that of water localized and the thermal exposure to the packaging components was very limited. This was a great advantage of the reactive film bonding process, especially for bonding structures with biological materials on them. The numerical results were in agreement with previous research on reactive bonding of stainless steel specimens showing that localized heating can be achieved by both numerical prediction and experimental observation [14, 17]. 4 Conclusions The Ni/Al reactive multilayer films were successfully used as local heat sources to bond a Parylene-C layer to silicon wafers with a thin Parylene-C coating. Live NIH 3T3 cells were encapsulated in the package and they survived in the bonding process owing to the localization of heating. Numerical simulation results of the temperature evolutions in the bonding assembly were in agreement with the cell encapsulation experiment. They both revealed that localized heating occurred in this bonding approach. This study proved the feasibility of reactive nanofilm bonding technique for broad applications in packaging bio-mems and microfluidic systems. References 1. T. Velten, H.H. Ruf, D. Barrow, N. Aspragathos, P. Lazarou, E. Jung, C.K. Malek, M. Richter, J. Kruckow, M. Wackerle, IEEE Trans. Adv. Pack. 28, 533 (2005) 2. M.A. Schmidt, Proc. IEEE. 86, 1575 (1998) 3. K.M. Knowles, A.T.J. Van Helvoort, Int. Mater. Rev. 51, 273 (2006) 4. C.H. Tsau, S.M. Spearing, M.A. Martin, J. Microelectromech. Syst. 13, 963 (2004) 5. F. Niklaus, G. Stemme, J.Q. Lu, R.J. Gutmann, J. Appl. Phys. 99, 1101 (2006) 6. R.G. Craig, J.M. Powers, Restorative Dental Materials, 11th edn. (Mosby, 2002) 7. Y. Suzuki, Y.C. Tai, J. Microelectromech. Syst. 15, 1364 (2006) 8. H. Yu, L. Ai, M. Rouhanizadeh, D. Patel, E.S. Kim, T.K. Hsiai, J. Microelectromech. Syst. 17, 1178 (2008) 9. H. Kim, K. Najafi, J. Microelectromech. Syst. 14, 1347 (2005) 10. Y.C. Su, L. Lin, in Proceedings of the 14th IEEE International Conference on Micro Electro Mechanical Systems, Interlaken, Switzerland, 50 (2001) 11. A.W.Y. Tan, F.E.H. Tay, Sens. Actuators A 120, 550 (2005) 12. N.K. Budraa, H.W. Jackson, M. Barmatz, W.T. Pike, J.D. Mai, in Proceedings of the 12th IEEE International Conference on Micro Electro Mechanical Systems, Orlando, USA, 490 (1999) 13. J. Kim, M. Chiao, L. Lin, in Proceedings of the 15th IEEE International Conference on Micro Electro Mechanical Systems, Las Vegas, USA, 415 (2002) 14. J. Wang, E. Besnoin, A. Duckham, S.J. Spey, M.E. Reiss, O.M. Knio, T.P. Weihs, J. Appl. Phys. 95, 248 (2004)
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