Accelerated hermeticity testing of a glass–silicon package formed by rapid thermal processing aluminum-to-silicon nitride bonding☆
Introduction
Wafer-level hermetic sealing process provides first level MEMS device protection that may drastically reduce the expensive post-packaging cost. The common approach is to use bonding process to the topmost layer on the MEMS chip and various bonding mechanisms have been demonstrated for direct bonding with silicon, silicon dioxide or metal layers [1]. Silicon nitride has been widely used as the passivation layer in either MEMS or IC [2] but little work has been done in silicon nitride bonding because the requirements of very high bonding temperature and long processing time [3]. On the other hand, reliability and long-term stability of sealed packages are very important characteristics of hermetic sealing and there is little published work in the area of MEMS. Both issues are to be addressed in this paper.
Previously, rapid thermal processing (RTP) bonding has been demonstrated to be capable of providing low thermal budget, wafer-level processing and insensitive to surface topography to achieve excellent bonding strength [4]. In this paper, RTP bonding is applied as an foundry-compatible packaging process to bond silicon nitride with a glass cap using Al as the bonding material. The sealed packages are tested in autoclave chamber [5] to accelerate the aging and corrosion process and to predict the reliability and life time of devices under normal usage.
Section snippets
Experimental procedure and results
Fig. 1 shows the schematic illustration of Al-to-nitride RTP bonding for MEMS packaging applications. A surface-micromachined MEMS structure [2] is surrounded by an integrated sealing ring with LPCVD silicon nitride as the topmost layer. The thickness of the LPCVD silicon nitride layer is 5000 Å, and the typical sealing area ranges from 300×300 to 600×600 μm2. A Pyrex (Corning 7740) glass wafer is deposited and patterned with aluminum of 4 μm thick and 100–200 μm wide as sealing rings. The
Accelerated test and reliability analysis
In order to evaluate the reliability of the MEMS package fabricated by RTP Al-to-nitride bonding, packaged dies with or without comb-resonators are put into the autoclave chamber filled with high temperature and pressurized steam (130 °C, 2.7 atm and 100% RH) for accelerated testing. The pressurized steam can penetrate small crevasses caused by bonding defects. Moreover, the elevated temperature and humid environment can raise corrosion against the bonding interface [7].
The statistical data
Bonding mechanism
It has been reported that aluminum can react with silicon nitride at 800 °C for 5 h and form crystalline β′-sialon [3]. For RTP bonding process, X-ray diffraction (XRD) technique was used to investigate the bonding interface and it is concluded that no new crystalline structure can be detected after bonding (Fig. 8). The short pulse annealing time in RTP bonding process may be insufficient of forming any Al–N–Si crystalline compounds. Therefore, the bonding process could be dominated by volume
Conclusion
In this paper, MEMS packages formed by Al-to-nitride bonding using RTP have been demonstrated and accelerated hermeticity testing has been conducted and analyzed. The worst case of MTTF is estimated between 50 and 1700 years with 90% confidence under the jungle condition. The MTTF increases with increasing sealing ring width and decreasing bonding area. Moreover, the Al-to-nitride bonding mechanism was identified as diffusion bonding by using XRD technique and the activation energy was measured
Acknowledgements
The authors would like to thank Yonah Cho and Michael Cich of Material Science Department, UC-Berkeley for assistance on XRD analysis and Allyson L. Hartzell of Analog Devices for discussion on reliability analysis. The devices were fabricated in Microfabrication Laboratory of UCB. This work is supported in part by an NSF CAREER AWARD (ECS-0096098) and a DARPA/MTO/MEMES grant (F30602-98-20227).
Mu Chiao received the BS degree in Agricultural Machinery Engineering and MS degree in Applied Mechanics from National Taiwan University, Taiwan, in 1994 and 1996, respectively. He is currently pursuing PhD degree in the area of MEMS design, fabrication and packaging at the University of California, Berkeley.
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Mu Chiao received the BS degree in Agricultural Machinery Engineering and MS degree in Applied Mechanics from National Taiwan University, Taiwan, in 1994 and 1996, respectively. He is currently pursuing PhD degree in the area of MEMS design, fabrication and packaging at the University of California, Berkeley.
Liwei Lin received the MS and PhD degrees in Mechanical Engineering from the University of California, Berkeley, in 1991 and 1993, respectively. He joined BEI Electronics Inc. from 1993 to 1994 in research and development of microsensors. From 1994 to 1996, he was an Associate Professor in the Institute of Applied Mechanics, National Taiwan University, Taiwan. From 1996 to 1999, he was an Assistant Professor at the Mechanical Engineering and Applied Mechanics Department at the University of Michigan. He joined University of California at Berkeley in 1999 and is now an Associate Professor at the Mechanical Engineering Department and Co-Director at Berkeley Sensor and Actuator Center. His research interests are in design, modeling and fabrication of microstructures, microsensors and microactuators as well as mechanical issues in microelectromechanical systems including heat transfer, solid/fluid mechanics and dynamics. Dr. Lin is the recipient of the 1998 NSF CAREER Award for research in MEMS Packaging and the 1999 ASME Journal of Heat transfer best paper award for his work on microscale bubble formation. He led the effort in establishing the MEMS sub-division in ASME and is currently serving as the Vice Chairman of the Executive Committee for the MEMS sub-division. He holds seven US patents in the area of MEMS.
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A portion of this paper was presented in the Transducers’01/Eurosensors XV Conference at Munich, Germany, 10–14 June 2001.