Figure 3.69: Plan view of a microactuator design.
nating voltages on each pair of the metal pads can drive the movable structure
in both directions.
In the straight beam suspension design, the Y direction stiffness k y of a
suspension beam is expressed as
k y = Eh 3 w
where E is the Young's modulus, h is the height of the spring, w is the spring
width and l is the spring length along the Z direction. On the other hand, the
X direction (operational direction) stiffness k x is expressed as
k x = Ehw 3
The stiffness ratio is
k x =
Theratiobetweenthestiffness in the Y direction and the stiffness in the X
direction must be maximized to make the structure very stiff in the Y direction
(normal to XZ plane). If the spring is designed with beam width of 2 µmin
the X direction and 60 µmintheY direction, then the aspect ratio (h/w)of
the spring is 30, and the beam is very stiff in the Y direction. The aspect ratio
is somehow limited by the fabrication process.
The electrostatic driving force F is expressed as
F = hnV 2
where is the permittivity of air, h is the height of the structure, n is the
number of the electrode pairs in the comb drives, V is the driving voltage, and
g is the gap width of the comb-drive fingers.