MEMS Reliability

[MEMS S&A Application] | [Chip-to-chip bonding] | [Carrier-level testing]

Micro-Electro-Mechanical Systems (MEMS) have found widespread applications in a variety of fields ranging from simple sensors to complex systems that require the integration of movable structures and active circuitry.  Among the challenges associated with transitioning MEMS from the laboratory to the marketplace is packaging. Packaging encompasses the transition of an electronic or electromechanical device from the die to the final product, so that the device is capable of functioning as part of a larger system. It includes, but is not limited to, the design of interconnections at every level of the system, the placement of the die on the carrier, removal of excessive heat generated in the case of integrated circuits (ICs) and issues of reliability associated with the system as a whole. A strong packaging background exists for microelectronic devices, but research in MEMS packaging is still in its infancy.

MEMS S&A Application

The application used in these studies is a Safety and Arming (S&A) device designed for use in undersea torpedoes in which two chips containing LIGA MEMS structures must be bonded together and operate reliably after many years of storage in extreme environmental conditions. This motivates the development of an experimental approach that can accelerate relevant failure mechanisms and allow the evaluation of the system’s reliability. As previously mentioned, the reliability concern in this paper is the integrity of the bonds. Non-destructive methods are preferred to destructive testing in most IC applications. Destructive testing often serves as a verification of the non-destructive testing techniques. This paper describes a non-destructive approach to measure delamination of the bond layers.

The requirements of the S&A system are to safely and reliably arm and detonate the weapon, but only after all safety criteria are met and the weapon has reached its intended target. The MEMS S&A system is shown in Fig. 1.


Fig. 1. MEMS S&A system showing details of various components.

The MEMS components include the S&A chip, deflection delimiter and the initiator chip. The operation and construction of the S&A system is discussed in greater detail in the following references:

The chip-level package includes the S&A, initiator chip, and delimiter. The initiator chip converts electrical energy to mechanical energy upon demand to start the explosive train. The initiator chip is also fabricated using MEMS technology. The S&A chip and the initiator chip must be reliably bonded together while maintaining precise in-plane and out-of-plane alignment. Coefficients of thermal expansion mismatch between the S&A chip and the initiator chip increases the challenge of maintaining alignment and bond integrity.

A deflection delimiter is introduced to limit out-of-plane (z-axis) compliance of several structures.  The delimiter ensures that the locks on the barrier are not violated by z-axis displacement between structures.   The deflection delimiter must allow for in-plane movement of all structures, but prevent z-axis movement of selected structures.  In addition, the delimiter must allow for wire bonding and fiber optic cable routing and mounting.

Chip-to-Chip Bonding

MEMS are finding applications in products or systems that require reliable operation over extended periods of time. The reliability requirements for the final product encompass both the mechanical behaviors and the electrical characteristics of the overall system. One critical element in many MEMS applications is chip-to-chip bonding (component bonding), for which long-term operation and storage reliability needs to be understood. MEMS packages are likely to have a large number of bond layers because of multiple interfaces inside the package. The bond layers in MEMS devices must often maintain precise chip alignment in addition to withstanding loading from the macro-environment and loading within the package. A primary indicator of failure (or impending failure) in a chip-to-chip bonded system is delamination between the chip and the material used to bond the chips together. In spite of its importance in MEMS packaging, previous work on bonding in MEMS structures is limited. Very little MEMS-specific work on the reliability of the chip-to-chip bonds exists, let alone, non-destructive methods for determining the reliability of chip-to-chip bonded MEMS.  

The Use of Acoustic Micro Imaging (AMI) in non-destructive reliability assessment of microelectronic devices is common. AMI techniques to assess the delamination in plastic encapsulated microcircuits.  AMI techniques have been used to asses the cracks in solder joints in flip-chip assemblies.  In MEMS structures bonding has been studied previously using destructive testing.  AMI has been used to assess failures in tunneling accelerometers but not applied to the assessment of bond reliability.   Others have proposed the use of ultrasonic techniques using AMI to check bond quality.  

The results of a delamination study using non-destructive techniques on chip-to-chip bonding in a MEMS-based Safety and Arming (S&A) system have been studied. A finite element model is created and results from simulation of die shear validation are compared with the experimental results in order to understand and verify the delamination measurement methodology.  

A summary of the chip-to-chip bonding work appears in the following references:

Carrier-Level Analysis

The carrier-level packaging for a MEMS-based Safety and Arming (S&A) system is currently being studied. The package houses the S&A chip, initiator chip, and a deflection delimiter. The carrier-level package provides electrical, fiber optic, pressure, and explosive interconnects. The S&A chip contains a moving “slider” that either exposes a hole through the chip or blocks the hole, as well as various environmental sensors and actuators that lock the slider into a safe position. The initiator chip converts electrical energy to mechanical energy upon demand to initiate the explosive train. The delimiter is placed between the S&A chip and the initiator chip to restrict out-of-plane movement of the MEMS structures. Various carrier approaches have been assessed. The carrier designs include hermetic, non-hermetic, and ventilated packages. The impact of moisture ingress and egress from the carrier packaging have been studied as a function of the performance of MEMS parts.

Preliminary work is summarized in:


Peter Sandborn
University of Maryland
Last Updated: January 3, 2006
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