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PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Packaging and Reliability Issues in Microelectromechanical Systems

I. Stanimirović, and Z. Stanimirović

Abstract - Packaging of microelectromechanical systems (MEMS) strongly affects the device performances. It is the core technology for the advancement of MEMS and is recognized as the next manufacturers challenge for the forthcoming years due to the growing market and stricter requirements. There is a vast diversity of applications and the most of MEMS packaging challenges are application dependent. It is the objective of this paper to review and discuss the most common MEMS packaging challenges. For that reason, five case studies on MEMS pressure sensors, MEMS accelerometers, RF MEMS, MOEMS and BioMEMS are presented for an in-depth illustration.

I. INTRODUCTION

Microelectromechanical systems (MEMS) refer to devices that integrate mechanical and electrical components and have feature sizes ranging from micrometers to millimeters. During the last decade this multidisciplinary field has witnessed explosive growth and the technology is progressing at such a rate that current understanding of the physics involved is often insufficient. New applications are emerging as the existing technology is applied to the miniaturization and integration of conventional devices. However, advancement of MEMS technology strongly depends on reliability of MEMS packaging. Pressure sensors, inertial sensors, chemical micro sensors, MOEMS, RF MEMS, microfluidics MEMS, power MEMS, microsurgical instruments are just a few examples of their diverse applications and packaging is a critical aspect of realizing most of these MEMS applications. MEMS packaging requirements increase the cost of MEMS product substantially. Their operational structure and domain make direct application of electronics packaging techniques not feasible. Stiction, fracture and fatigue, mechanical wear with respect to frequency and humidity, shock and vibration effects are the major causes of MEMS failures especially when moving parts are present – parts that interact with other components through optical, electrical, thermal, mechanical and chemical interfaces. MEMS packaging and reliability are related to the application as well as to fabrication. Standard packages for each functional interface do not exist and integration of the fabrication and packaging processes are usually application-dependent. It is the objective of this paper to review and discuss some of MEMS packaging issues that are stumbling blocks in full commercialization of MEMS.

II. RELIABILITY OF MEMS PACKAGING

MEMS packaging is considered to be the major challenge because the packaging cost is about 50% to 90% of the total cost of a MEMS product. Packaging requirements are very diverse and the lack of standardized packaging processes makes prediction of the effects of packaging on micromachined parts and performances of overall system very difficult. For that reason electronic packaging engineers are facing many MEMS packaging problems such as thermal stresses caused by die-attachment processes that may lead to inaccuracy of pressure measurements, stiction problems caused by moisture, thermal strains that affect performances of membrane devices, etc. Although reliability of MEMS packaging has been improved over the last few years [1-4], the most reliable MEMS devices are still hermetically packaged single-point contact or no-contact devices. However, novel MEMS devices demand contacts and development of standard packages for all of the MEMS applications is an urgent issue. Reliability is one of the performance measures that are strongly affected by the package as well as the device. Reliability of MEMS packaging is usually application-dependent. In order to discuss reliability issues in more detail, in the following subsections five representative MEMS packaging cases will be presented: pressure sensor, accelerometer, RF MEMS, MOEMS and BioMEMS packaging. These case studies may contribute to better understanding of MEMS packaging challenges.

A. MEMS Pressure Sensor Packaging

MEMS pressure sensors are used in great variety of applications and they come in all shapes and forms. They can be classified in terms of application, measurement type, measurement range, measurement accuracy, different functional parameters such as operational temperature range, form and material of packaging, etc. Basically, there are two commonly used types of MEMS pressure sensors: piezoresistive MEMS pressure sensors (Figure 1) and capacitive MEMS pressure sensors. Piezoresistive pressure sensor is a strain gage that experiences a change in resistivity when exposed to pressure, while capacitive pressure sensor is a strain based variable capacitor. Capacitive sensors are insensitive to temperature effects, which is a great advantage over piezoelectric sensors that are inherently sensitive to temperature changes. However, packaging materials can change mechanical properties over

I. Stanimirović, and Z. Stanimirović are with IRITEL a.d. Beograd, Batajnički put 23, 11080 Belgrade, Serbia, E-mail: inam@iritel.com

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temperature and cause mechanical stress on the sensor. MEMS pressure sensor packages come in a variety of shapes and forms. Since pressure sensors need to be physically exposed to the medium being measured, it is necessary to interface pressure sensitive micromachined part to the surrounding medium. The role of the package is to isolate all the other parts of the microsystem except the sensing membrane from the sensing environment thus protecting them from the corrosion using surface passivation layers and silicon gels. MEMS sensors are sensitive to mechanical stress. Mechanical stress is usually the result of the mismatch in thermal expansion factors between different packaging materials used. Selection of these materials (die attach material, die coating material, substrate material) is crucial for successful sensor packaging. Positioning sensor inside the package also affects sensor performances. Choosing the adequate package configuration is a difficult task especially for applications where sensor is exposed to reflow, shock, vibration, temperature excursions and sometimes hazardous environment. Overall cost of MEMS pressure sensors can be reduced by using existing packages instead of designing new packages that require new set of assembly tooling for every new sensor variety.

Fig. 1. Schematic of the stainless steel diaphragm for piezoresistive MEMS pressure sensor.

B. MEMS Accelerometers Packaging

MEMS accelerometers are one of the simplest but also most applicable micro-electromechanical systems. It is an electromechanical device from the group of inertial sensors that measures acceleration forces. They cover a broad range of applications: air bag crash sensing, seat belt tension, automobile suspension control, human activity for pacemaker control, etc. They are also used in several consumer, industrial and military applications, such as stabilization of images in camcorders and head-mounted displays, rotation of the image in mobile phones, in vibration monitoring, in real-time applications like military monitoring, missile launching, projectiles navigation and guidance systems, etc. Since MEMS accelerometers are used in such a wide range of applications their required specifications are often application dependent. MEMS accelerometer package must provide protection of the sensing element in various environments without inducing significant stress or drift. Misalignment during mounting must be calibrated in order to maintain sense direction of the device. If the die is not mounted completely parallel to the base of the package, spurious acceleration inputs in the

orthogonal directions to the desired direction can be induced resulting in extraneous signals. Also, thermal mismatch and stress hysteresis that can affect accuracy and sensitivity of the device must be taken into consideration. Therefore, mounting materials need to be chosen according to the desired operation range and shock resistance. The most common failure mode, when MEMS accelerometers are in question, is the fatigue fracture of the moving part of the accelerometer due to mechanical vibration and temperature. The moving part concentrates the stress at the weak point and, if a crack appears, it propagates and reduces the lifetime of the device. Eventually, a failure by fracture takes place. This failure mechanism is often responsible for the drift of the electrical parameters. The fatigue fracture can be accelerated by the high temperature, especially if humidity is present at the chip level, diffusing from outside through the plastic package. For that reason metal and multilayer ceramic hermetic packages are often used or wafer level packages with capping glass or silicon wafer bonded to the MEMS wafer (Figure 2). Low-noise, low-temperature sensitivity and high reliability are packaging requirements for high precision MEMS accelerometers.

Fig. 2. Schematic of wafer level MEMS accelerometer package.

C. RF MEMS Packaging

RF MEMS (Radio Frequency Microelectromechanical Systems) contain moveable and fragile components such as membranes, beams, cantilevers that provide RF functionality and must be enclosed for protection and stable performance characteristics. There are various types of RF MEMS components, such as resonators, oscillators, switches, tunable inductors, etc. The most common radio frequency devices are RF MEMS switches (Figure 3). RF MEMS switches are devices that use mechanical movement to achieve a short circuit or an open circuit in the RF transmission line. RF MEMS switches are the specific micromechanical switches that are designed to operate at RF-to-millimeter-wave frequencies (0.1 to 100 GHz). The forces required for the mechanical movement can be obtained using electrostatic, magnetostatic, piezoelectric, or thermal designs. For most applications switch reliability and a large number of switching cycles are equally important. Metal contacts are the crucial parts of RF MEMS switch both from the functionality and reliability perspective because they are the source of most dominant failure mechanisms. Typical problems are stress (buckling, incorrect movement), stiction as a result of

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humidity and insulator charging, creep, residues, etc. Besides the general MEMS packaging problems, the packaging of RF MEMS switches is additionally confronted with several other challenges. Metal sealing with isolated signal lines underneath or through the sealing ring might not be adequate to provide low signal attenuation and low reactions of electrical interconnection lines that penetrate through the hermetic package. Switch performance stability and reliability require usage of hermetic packaging (controlled atmosphere and pressure) in order to eliminate stiction, atmosphere that affects switching, particles contamination, handling problems, etc. All materials used for RF MEMS switch fabrication must have good RF properties. For some applications chip size is very important that might result in additional production steps, e.g. thinning off the substrate to allow wafer bonding. Standard packaging approaches are often inadequate for RF-MEMS switches affecting reliability and cost.

(a) (b)

Fig. 3. (a) Schematic of RF MEMS capacitive switch with designated failure modes: (1) creep, fatigue, deformation, (2) stiction, (3) dielectric degradation and charging; (b) Schematic of zero-level packaged RF MEMS capacitive switch

D. MOEMS Packaging

When optical devices are in question, optical MEMS (MOEMS) devices that integrate optical, mechanical, and electrical components on a single wafer are allowing the implementation of various key optical-network elements in a compact, low-cost form. They usually involve small moving optical parts in order to obtain more advanced functionality. Applications are various: projection mirrors, photonics switches, gratings, fiber aligners, fiber aligners, modulators, shutters, movable lenses, etc. MEMS micromirror arrays are among the most commercially successful MOEMS devices. New manufacturing processes, new materials and new testing methods allowed production of MEMS micromirror devices with remarkable performances. Such arrays exhibit particular metrology needs, and accuracy and reliability are the key factors for their successful commercialization. Performances from one device to the next can vary depending on the micro-fabrication process and the device geometry. For that reason, it is difficult to reliably ensure accuracy and repeatability of the actuator positions. MEMS micromirror array packaging issues are extremely important since they require interaction with the environment to perform their mission. One of the potential

failure mechanisms of MEMS mirror arrays is stiction. Capillary water condensation causes the landing tip of the mirror and adequate landing site to become stuck. The vulnerability to stiction can be significantly reduced by surface passivation coatings, the use of critical point (CO2) drying of MEMS devices and moisture free packaging [5]. Enclosure in a controlled atmosphere and robust hermetic packaging prevent mirrors from sticking. In that way, the presence of moisture is being greatly reduced if not completely eliminated. Also, anti-stick layers are commonly being used to lower the surface interaction energy and prevent stiction. These layers provide hydrophobic surfaces on which water cannot condense and capillary stiction will not occur. However, the reliability and reproducibility of these layers is an important issue because of the high temperatures required in MEMS packaging process steps. Similar to stiction, hinge memory poses a significant threat to MEMS micromirror device reliability. Main contributors to hinge memory failure are duty cycle and operating temperature, but the main cause of this type of failure is the creep. Macroscopically, creep is non-existent as long as temperature is kept below 0.3 times the melting temperature of the material and the mechanical stress in the material is not extreme [6]. Since it is obvious that temperature affects the lifetime of the micromirror device, thermal management is very important. In order to keep temperature in the device within the acceptable range, heatsinks are being used. Adequate thermal management significantly influences lifetime of the device allowing the mirrors to be efficiently controlled over a long period of time. Environmental robustness is a great reliability concern for all MEMS devices. Although micromirror arrays seem fragile due to their small size, their size proved to be one of their greatest assets. Small size enables their robustness. They proved to be able to sustain low-frequency vibrations and mechanical shock without mirror damage. However, dimensions of MEMS micromirrors are so small that the presence of the smallest particle during fabrication may cause non-functionality of one or more devices. During packaging the source of each contaminating particle should be detected and eliminated because particles sealed in the package may affect operation of the device during its lifetime. Because MEMS micromirror arrays interact with the environment in a certain way, they require vacuum hermetic packaging that can provide adequate protection, electronic contacts and window transparent to light (Figure 4).

Fig. 4. Schematic of sealed hermetic MOEMS micro-mirror package

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E. BioMEMS Packaging

BioMEMS stands for Biomedical Microelectromechanical Systems. BioMEMS are applications of MEMS technology in the field of Biomedical and Health Sciences. These biomedical devices that have been developed using a variety of micro-manufacturing methods can be classified as: microfluidic devices, biosensors (Figure 5) and bioelectronics, neural interface devices, chromatography/electrophoresis devices, microsurgical tools, bioreactors, tissue engineering devices, molecule/cell handling devices, implantable devices and drug delivery devices. There are three approaches for BioMEMS packaging [7]: monolithic, integration, multichip module and stacked modular systems. Simple devices or components realized by similar fabrication processes use monolithic integration approach where all components are fabricated on the same substrate and minimum assembly is required. Multichip module approach is known for its component-swapping capability. Chip is bonded to a substrate that provides electrical and fluidic connections along with mechanical support. Stacked modular system incorporates individual components stacked to form a complicated integrated system. These systems are very flexible because components can be tested individually thus improving reliability of the BioMEMS device. BioMEMS packaging is similar to MEMS packaging with addition of fluidic interfaces, remote powering and communication, short and long-term biocompatibility, life or death reliability and required approvals for real health care applications. BioMEMS packaging and reliability challenges are numerous: biocompatibility (materials used for device realization and packaging must not be toxic to living cells), clean-room assembly, shelf life of soft components, feed-throughs – at wafer, modular and/or device level, costly hermetic sealing with moisture barriers, implantability (requires sterilization, localization), mechanical stress, ease of use, serviceability, outgassing, powering and communication, precision assembly and alignment, electrical & fluidic connection (isolation between electronics and biofluids must be reliable to avoid signal interference and damage to biofluid; if unreliable it can be life-threatening),static discharge and EM shielding, thermal management (temperature control is necessary to prevent denaturation of biomolecules at high temperatures), waste management (disposability prevents transfer or interference of diseases), etc.

Fig. 5. Schematic of BioMEMS blood glucose sensor

III. CONCLUSION

MEMS packaging has two major roles: to support and protect device from mechanical or environmentally induced damages and to enable interaction with environment in order to measure the desired physical or chemical parameters. MEMS package must also provide communication links, remove heat and provide robustness necessary for safe handling. The goal is to provide reliable, economical and application-specific solutions by choosing compatible materials combinations and optimized packaging processes. In this paper five case studies have been presented representing various types of MEMS applications. It is shown that device performances are strongly affected by packaging and that it is of a great importance that design and realization of MEMS package must include all levels of reliability issues from the onset of the packaging project. MEMS packaging is an active area of research among scientists and packaging engineers and development of low-cost, high-performance and high-reliability packaging solutions is the key factor to full commercialization of MEMS, although, highly diversified functions and materials involved make this goal a challenging task.

ACKNOWLEDGEMENT

Authors are grateful for the partial support of the Ministry of Education, Science and Technological Development of Republic of Serbia (contracts III44003 and III45007).

REFERENCES [1] T.R. Hsu, “Reliability in MEMS packaging”, In Proceedings

of 44th International Reliability Physics Symposium, San Jose, CA, USA, March 26-30, 2006, IEEE International.

[2] T.R. Hsu, “Introduction to Reliability in MEMS Packaging”, In Proceedings of International Symposium for Testing and Failure Analysis, San Jose, California, November 5, 2007.

[3] A.P. Malche, C. O'Neal, S.B. Singh, W.D. Brown, W.P. Eaton, W.M. Miller, “Challenges in the Packaging of MEMS”, The International Journal of Microcircuits and Electronic Packaging 1999, 22, 3, 233-241.

[4] S.M. Spearing, “Materials Issues in Microelectromechanical Systems (MEMS)”, Acta Materialia 2000, 48, 179-196.

[5] S. Tadigapa, N. Najafi, ”Reliability of Microelectro-mechanical Systems (MEMS)”, In Proceedings of Reliability, Testing, and Characterization of MEMS/MOEMS Conference, San Francisco, CA, USA, October 22-24, 2001; SPIE, Bellingham, USA, 197-205.

[6] W.M. van Spengen, “MEMS reliability from a failure mechanisms perspective”, Microelectronics Reliability 2003, 43, 1049–1060.

[7] Y.C. Lee, B.A. Parviz, J.A. Chiou, S. Chen, “Packaging for Microelectromechanical and Nanoelectromechanical Systems”, IEEE Trans. on Advanced Packaging 2003, 26, 3, 217-226.