Microelectronic mechanical system and methods

DETAILED DESCRIPTION OF THE INVENTION In general, the present invention provides a method to make devices with encapsulated release structures.
The current invention is particularly useful for fabricating MEMS oscillators, optical display devices, optical transmission devices, RF devices and related devices.
MEMS oscillators can have any number or simple or complex configurations, but they all operate on the basic principle of using the fundamental oscillation frequency of the structure to provide a timing signal to a coupled circuit.
Referring to FIG.
1, a resonator structure 102 has a set of movable comb features 101 and 101′ that vibrate between a set of matched transducer combs 105 and 105′.
The resonator structure 102, like a pendulum, has a fundamental resonance frequency.
The comb features 101 and 101′ are secured to a ground plate 109 through anchor features 103 and 103′.
In operation, a dc-bias is applied between the resonator 102 and a ground plate 109.
An ac-excitation frequency is applied to the comb transducers 105 and 105′ causing the movable comb features 101 and 101′ to vibrate and generate a motional output current.
The motional output current is amplified by the current to-voltage amplifier 107 and fed back to the resonator structure 102.
This positive feed-back loop destabilizes the oscillator 100 and leads to sustained oscillations of the resonator structure 102.
A second motional output current is generated to the connection 108, which is coupled to a circuit for receiving a timing signal generated by the oscillator 100.
Referring now to FIG.
2a showing a plan view of a wafer, a wafer structure 200 preferably comprises a silicon substrate 201 and a first etch-stop layer 203.
The first etch-stop layer 203 may not be required to perform the methods of the instant invention, especially when the silicon substrate 201 is sufficiently thick to allow sacrificial layers to be etched without completely etching away the silicon substrate 201