Exam Question Notes

Question 1

How do you calculate dynamic range?

Answer 1

Max Input / Min Input
in dB:
20log(Max/Min)

Question 2

In the case of a silicon electrostatic actuator based on a cantilever, how would you expect the stiffness to change as the electrostatic field between the cantilever and the substrate increases? Justify your answer.

Answer 2

As the voltage increases:

The electrostatic force increases nonlinearly (inversely with the square of the gap),
This causes the cantilever to deflect more,
The net restoring force becomes weaker because the electrostatic force opposes it.

This phenomenon is called electrostatic softening, and it effectively reduces the system’s stiffness.

Question 3

What causes pressure drop along a nozzle?
What affect would decreaseing the nozzle diameter have?

Answer 3

Viscous friction along the walls of the nozzle.
And converging geometry causes a velosity increase and pressure drop (conservation of mass)

decreasing the nozzle size will increase these effects and cause a greater pressure drop.

Question 4

Give an overview of the steps to manufacture a MEMS pressure sensor

Answer 4

1. Etch a thin diaphragm on wafer 1
2. Bond this to wafer 2 (the constraint wafer)

• Anodic bonding
• Glass frit seal
• Direct wafer bonding

Question 5

What is Glass frit seal bonding?

Answer 5

A glass frit (Paste) is applied using screen printing.
The parts arefitted and thermocompressed.

Question 6

What is Anodic Bonding?

Answer 6

A silicon top wafer is bonded to a glass (or silicon with deposited glass) substrate.
The glass substrate is heated by applying a voltage to bond the layers.

Question 7

What is Silicon fusion bonding?

Answer 7

Also known as Direct silicon bonding.
Bonds wafers at the atomic level without adhesive, glass or an electric field.

1) Treat wafers in solution to form hydroxyl monolayers.
2) Ft the surfaces. Theyre held by surface tension.
3) Process at high temp to drive off the hydroxyl monolayers.
4) Silicon dioxide forms fusing the layers.

Question 8

What is Striction?
What issues does it cause?

Answer 8

Striction (Static-Friction) when parts stick together due to:

• Capillary forces (from moisture or cleaning fluids)
• Van der Waals forces
• Electrostatic attraction
• Surface tension during drying

Once stuck, the parts may fail to move as intended, rendering the device non-functional.

Question 9

How can striction be mitigated?

Answer 9

1. Dry release techniques (e.g., vapor-phase etching with XeF₂)
2. Surface coatings (e.g., hydrophobic or anti-stiction layers)
3. Design changes (e.g., dimples or bumps to reduce contact area)
4. Vacuum packaging to eliminate moisture

Question 10

What MEMS packaging options are there?
Why are they required?

Answer 10

• Bonded etched wafers
• Overmoulded epoxy, metal or ceramic

They prevent contamination, protect from handling and can provide a useful atmosphere (eg a vacuum)