MEMs_UseOfMEMs

Use of MEMs

Benefits
As stated by Dr. Walter Merrill during a NASA tech brief in 2006, size, weight, and mass are considerations that show benefits of MEMs use. Goals of MEMs use include new functionality and new capabilities, as well as existing capabilities.

By making use of the mass-production processes, as is the case in the Integrated Chip (IC) industry, cost per unit drops. With sensors being so small, instead of just putting one sensor into a location, a whole array may be added for a small price. The benefits of MEMs are that they are smaller, cheaper, and faster than many alternatives.

MEMs and NASA

NASA employs a MEMs Technology Group that is stationed out at the Jet Propulsion Laboratory located in Pasadena, California. This group studies the use, design, implications, fabrication, and reliability testing of MEMs technologies.

NASA has studied uses for MEMs in the following areas:

  • Vibratory Micro-gyroscope
  • Micro-actuators
  • Lithographie Galvanoformung Abformung (LIGA)-based device (LIGA is a process using synchrotron radiation to expose polymethyl methacrylate. This process produces either plastic or metal parts having dimensions of millimeters and sub-micron accuracies7)
  • Micro-propulsion device
  • Micro-Valves
  • Adaptive Optics
  • Biomedical Devices
  • System on a chip
  • Micro-Instruments
  • Packaging
MEMs Technology at NASA’s Jet Propulsion Laboratory
Use of COTS MEMs

Quality and Reliability of COTS MEMs

A group has been formed at NASA to investigate the use of COTS MEMs in terms of quality and reliability assurance. With all the advantages that COTS procurement can provide, there are still issues that prevent the widespread use of COTS MEMs at NASA. Issues include reliability, packaging, compatibility and standardization (These issues are discussed further at the considerations portion of the module).

 

Pressure Sensors and Accelerometers

These two COTS MEMs in particular are a good candidate for use due to their maturity. They are standard components that have a lot of available reliability data and packaging that meets all surface mount requirements (See the Surface Mount Workbook by NASA for more information on the subject).

 

COTS MEMs Acceptance

Standardized testing based on the MEMs key failure mechanisms is an important step to COTS MEMs acceptance. Users can carry out any additional reliability testing specifically needed for their applications. A standardized test methodology when developed will also reduce unclear communication between users and suppliers thus avoiding any unnecessary expenses.

Because the fabrication cost of MEMs is relatively small, technological development should continue before realizing a significant cost savings from using COTS MEMs.

Interconnection and Packaging Issues of Microelectromechanical Systems (MEMS) and COTS MEMS

Note: Please review the COTS module on AAQ for more information on COTS.

Considerations

Packaging

Packaging of MEMS devices is more complex than that of IC devices because in some cases it needs to provide protection from the environment while in other cases it needs to allow access to the environment to measure or affect the desired physical or chemical parameters.10 MEMs have to eventually connect to the outside world while withstanding the extreme thermal and environmental conditions in which they must operate. Care should be taken for environmental protection, electrical signal conduit, mechanical support, and thermal management paths.

Packaging of MEMs is very complex. A package must also provide communication links through optimum interconnect scheme, remove heat through suitable selection of heat sinks, and provide robustness in handling and testing. Packaging includes the use of hermetic packages and vacuum systems.10 (Getters are reactive materials used for removing traces of gas from vacuum systems.11)

 

Reliability

Reliability is important in the aerospace industry where mission success can be deterred by a single small failure. NASA has established the reliability assurance group that supports the MEMs technology group as well as other groups to research reliability concerns. Reliability of MEMs depends on the materials used, sharp corners, adhesion issues, bonding, poor process capability, etc. Reliability data along with standardization and statistical analysis should be done to gain confidence in the reliability of MEMs.10

Some of the more common failure modes associated with MEMs are: Failure by Stiction and Wear, Delamination, Environmentally Induced Failures, Cyclic Mechanical Fatigue, Dampening Effect, and Packaging. Remember that traditional mechanical analysis may not directly apply to MEMs due to the large surface area to volume ratio.

Reliability tests such as those shown in “Evaluation of Thermo-Mechanical Stability of COTS Dual-Axis MEMS Accelerometers for Space Applications” in the Instructional Materials section provides an example of MEMs reliability testing.

 

Compatibility

MEMs compatibility issues with existing hardware, processes, bulk micromachining, clean room standards, etc., can all become an issue. Care should be taken when MEMs are to be designed, fabricated, and introduced into existing systems.

 

Standardization

Due to the recent explosion in MEMs, standards that are associated with these products are evolving. The emergence of standardized material characterizations, the presence of reliable material properties, processing effects and variables during design and manufacturing will be key factors in MEMs standardization.13 See "MEMs Standardization" on this module for more information.

 

Interconnect Issues

Integration into the outside world or existing systems poses problems such as reliability, functionality, durability, and compatibility. All aspects of integration should be considered before incorporating MEMs into existing systems.