A Few Thoughts about Reliability Challenges to MEMS

Had some relaxing days by the beach in Bohol after the Alabang Course last week.

Panglao

Thought I would share a few thoughts about reliability challenges to MEMS (Micro Electro Mechanical Systems) this week. In the last couple of decades MEMS have been quietly infiltrating all aspects of our lives. Where are they?

In cars:

• Air Bag Sensors
• Pressure sensors for engine management and tyres
• Inertial sensors and gyroscopes for navigation
• Anti skid braking systems
• Headlight Levelling
• Automatic Door Locks
• Active Suspension

In your phone:

• Microphone
• Inertial and tilt sensors for screen rotation, motion sensing etc
• Gyroscope and magnetometer for navigation
• Autofocus actuator for camera
• RF switches
• Filters and oscillators

Elsewhere:

• Infra red image sensors for security and military applications
• Bio MEMS, Lab on a chip for health care diagnostics

and the list goes on...

From a reliability aspect, a failure in a phone may be a big inconvenience but the failure of an air bag to operate could kill someone. MEMS devices suffer from all the same challenges to reliability that most integrated circuits suffer from but because of their unique construction and applications MEMS have some extra challenges. Here are some of the main ones.

Problem	Description	Reason for Problem
In-use stiction	Surface that should move freely get stuck together.	Capillary forces, electrostatic attraction, or chemical bonding. Influenced by surface treatment, surface contamination, and humidity
Mechanical fatigue	Repeated mechanical flexing of parts causes micro cracks which eventually  lead to failure	Conventional mechanical effects plus local oxidation
Wear	Sliding or impacting surfaces cause wear damage and debris which eventually leads to mechanical failure or sticking	Micro abrasion and cold welding. Conventional lubrication approaches usually not suitable.
Corrosion	Corrosion of metal parts due to ingress of moisture and/or ionic contaminants.	Many MEMS structures cannot be completely isolated from ambient
Electrostatic charging	Charge accumulation in dielectric layers causes hysteresis effect and possible sticking of surfaces	Charge trapping mechanisms in dielectrics
Change of shape with temperature	Bending or warping of components as temperature changes	Thermal expansion mismatch
Stress changes over life	Bending or warping of components over time caused by build up or relaxation of stress	Thin films deposited at low temperatures may anneal during operation. E.g. electroplated Ni transforms to tensile stressed  film with annealing.

SEM picture of a capacitive RF-MEMS switch with two bridge beams, showing stiction of the front beam

From Reliability and failure analysis issues in MEMS Ingrid De Wolf (IMEC) in Microsystems Technology:  Fabrication, Test and Reliability Kogan Page Science,  2003

Wear between moving parts

From Brad Waterson , “Failure Mechanisms in Microelectromechanical Systems(MEMS)” Proc ISTFA 2002