MEMS Flashcards
What is a MEMS device?
A device that can be used for sensory or actuation function. At least one X, Y or Z direction is on the um scale
What does MEMS stand for?
Micro Electrical Mechanical System
What are the two functions of a MEMS device?
Sensory - used to measure a physical quantity and convert it into a discernible signal - usually converts mechanical energy to electrical energy
Actuation - converts energy in the electrical domain into the mechanical domain to generate movement
What are the market statistics for MEMS devices?
15 bill market size. In 2018, 6.6 bill was BioMEMS, 3x increase since 2012
What are the benefits of making small devices?
Increased performance
Scalable fabrication and quality
Cheaper at large volumes
What are the positive effects of scaling?
Increased forces on operation at small sizes (electrostatic)
Lower power consumption
Higher SA:VOL - increased thermal disspitation
Array technology
Reduced response time
What are the negative effects of scaling?
Higher SA:VOL - increased surface forces in action - STICTION
Increased manufacturing inaccuracy - manufacturing process should have greater tolerance
What are the MEMS functional domains?
Physical (mechanical - gyro-meter, accelerometer)
Chemical (gas/chemical sensing)
Biological (microcantiliver, medical implants)
What are the three phenomena that describe how MEMS devices work?
PZ
PR
ES
What is the PZ effect?
Mechanical stress applied to certain materials results in charge accumulation. We can measure the electrical potential change and correlate this to the stress applied. This is also reversible. E.g. crystal, ceramic, bone, DNA, proteins, PZT (lead zirconate titrate). Caused by a rearrangement of positive and negative charge in the material. Can be used in actuation and sensing
E.g. Cigarette lighter, microphone
What is the PR effect?
Mechanical stress applied to semiconductors (e.g. doped silicon) leads to change in resistance. Only used in sensing.
What is the ES effect?
Electrostatic forces generated between 2 electrically charged combs. Can be used in sensing or actuation.
Actuation - capacitive actuator. When a charge is applied across the two electrically charged combs, the static comb and moving comb are driven together. The force is dependent on the capacitance difference between the combs, the size of the voltage, the number of teeth, distance between teeth
Sensing - accelerometer. Relates capacitance change to the acceleration. Capacitance = area/distance
What is the problem with the electrostatic effect?
The electrostatic force decreases with the square of the distance between the two charged combs
What is a proof mass? What determines the movement of it?
Known quantity of mass - replicates the movement of anything it is attached to.
Movement determined by the spring constant. Z=MA/K
How do pressure sensors work?
PR effect - used in blood pressure measurements. E.g. arterial BP to measure difference across vales. Also intracranial, interocular and CSF
