Vacuum-cavity MEMS resonator

Details Vacuum-cavity MEMS resonator Vacuum-cavity MEMS resonator

2001-09-07

2004-10-26

Ghyka, Alexander (Department: 2812)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Physical stress responsive

C438S052000, C359S295000

Reexamination Certificate

active

06808954

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to microelectromechanical systems (MEMS), and in particular to vacuum cavity packaging of MEMS resonators.
BACKGROUND OF THE INVENTION
A resonator is an electronic device used for setting up and maintaining an oscillating electrical signal of a given frequency. Conventional resonators typically include electronic circuitry in combination with a mechanical oscillator element (e.g., a quartz crystal, a ceramic element or a resonance circuit). Resonators are used in many electronic devices, such wireless radio frequency (RF) equipment, for generating outgoing signals of a particular frequency, and filtering incoming signals.
In most electronic devices that require signal generation and filtering, conventional resonators are used. Such resonators have a high Q-factor (i.e., a sharp resonance peak) good frequency stability and are generally very reliable. However, conventional resonators tend to be relatively large (i.e., on the order of 1 cm), so that alternatives are preferred when trying to fabricate a compact electronic device.
One alternative to conventional crystal-based resonators is a microelectromechanical systems (MEMS) resonator. Generally, a MEMS device is a microdevice that integrates mechanical and electrical elements on a common substrate using microfabrication technology. The electrical elements are typically formed using known integrated circuit fabrication technology. The mechanical elements are typically fabricated using lithographic and other related processes to perform micromachining, wherein portions of a substrate (e.g., silicon wafer) are selectively etched away or added to with new materials and structural layers.
FIG. 1
shows one type of prior art MEMS resonator
10
formed on a substrate
12
. MEMS resonator
10
has a cantilever-type beam
16
arranged between a lower electrode
20
and an upper electrode
26
. Beam
16
is electrostatically driven by the upper and lower electrodes to oscillate at a given frequency.
FIG. 2
shows another type of prior art MEMS resonator
40
similar to MEMS resonator
10
but having a bridge-type beam
46
and an optional bridge-type upper electrode
50
. Beam
46
is anchored to the substrate at its ends so that the center portion of the beam can be driven to oscillate by being electrostatically deflected between the upper and lower electrodes.
FIG. 3
shows yet another prior art MEMS resonator
70
called a “breathing bar resonator.” MEMS resonator
70
includes a bar-type beam
76
fixed to substrate
12
with a single central support member
80
. Side electrodes
84
and
86
are located on either side of beam
76
with small gaps
88
in between. Electrodes
84
and
86
drive beam
76
to expand and contract (i.e., resonate) along its long axis in a manner that resembles breathing.
MEMS resonators are desirable for many miniaturized electronic devices because they can be made smaller than conventional resonators by an order of magnitude or more. However, because a MEMS resonator relies on the mechanical oscillation of a very small (i.e., micron-sized) beam as opposed to the vibration of a relatively large oscillation element (e.g., a centimeter-size crystal), the resonator must be packaged in a vacuum to eliminate air damping of the beam's oscillation. Vacuum packaging is also necessary to avoid the adsorption of contaminants, which can alter the resonant frequency of the beam.
A challenge in fabricating MEMS resonators is the vacuum packaging step. Various techniques for vacuum packaging a MEMS resonator are available, such as wafer bonding, flip-chip, and thick membrane transfer techniques. However, these techniques require dedicated alignment/bonding technologies that are relatively complicated to apply to MEMS packaging. Another technique for MEMS vacuum packaging involves using a permeable polysilicon release process. While conceptually simple, such a process has proven very difficult to control and has yet to lead to a manufacturable MEMS resonator vacuum packaging process.


REFERENCES:
patent: 5589082 (1996-12-01), Lin et al.
patent: 6174820 (2001-01-01), Habermehl et al.
patent: 6635509 (2003-10-01), Ouellet

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