Midterm Presentation for a class

1. 按一下以編輯母片標題樣式
NATIONAL TSING HUA UNIVERSITY
National Tsing Hua University
HsinChu, Taiwan
Presenter : Wan-Cheng Chiu (邱萬誠)
Instructor : Cheng-Hsien Liu (劉承賢)
Midterm Project Presentation
-Presentation I-
April 22, 2014
NATIONAL TSING HUA UNIVERSITY
Capacitive Silicon Resonator Structure with Movable
Electrodes to Reduce Capacitive Gap Widths Based on
Electrostatic Parallel Plate Actuation

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• Introduction & Motivation
• Principle
• Purpose
• Method
• Design
• Experiment Results
• Conclusion
Outline

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•Resonator is a fundamental component in many MEMS devices.
•Small electromechanical structure that vibrate at high frequencies
Introduction
Mode Shape Mode Shape

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•These devices can be used as timing references, filters, sensors, and
so on.
•It has the possibility to replace quartz due to the capability for further
miniaturization.
Introduction & Motivation
Medical Ultrasonography
GPS
Smartphone

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•For two parallel plate:
Principle
+
V
-
g
Capacitance = εA/g
If one of the plate is free to move up
and down, then the capacitance can
vary.

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•For a single beam capacitive resonator, we can measure the response
of the device.
Principle
When a small signal vi
is applied, we will get a
frequency response.
Resonant
frequency
vi
Rm im
Vp
vo
Driving
electrode
Sensing
electrode
Resonator
body

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Principle
•Electrical System• Mechanical System
Lm = mr / ηe
2
Rm = cr / ηe
2
Cm = ηe
2 /kr
mr
kr = ωn
2mr
cr = (√krmr)/Q

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Principle
• Pull-In Effect
g0
2/3g0
•Spring Force:
Fspring = kδ
•Electrostatic Force:
Fe = εAV2/2(g- δ)2
Fnet = εAV2/2(g- δ)2 - kδ
•Pull-In Voltage:

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Purpose
•Electrical System
Lm = mr / ηe
2
Rm = cr / ηe
2
Cm = ηe
2 /kr
Rm is also known as motional impedance,
a low Rm device has a low insertion loss
and lower phase noise.
In this paper, the author focus on reducing
gap width to reduce the motional
impedance.

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Method
By making the electrodes movable,
when voltage is applied the electrodes
will move towards the resonator body
due to electrostatic force. Thus the gap
widths become smaller.
d Rm

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Design
•The electrodes are attached to spring,
which will make the electrodes movable.
•In the design stoppers are added to
prevent pull-in effect from occurring.
•The new gap width is equal to:
greduced = gB-B’ – gA-A’

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Experiment Results
•The image below is a SEM image of silicon resonator with movable electrodes.
A-A’ is the stopper gap = 400nm
B-B’ is the capacitive gap = 500nm
Final gap = 100nm
＊A same version of the silicon resonator without movable electrodes is also fabricated
for measurement comparison.

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Experiment Results
•The measurement was done inside a vacuum chamber with pressure of 0.01 Pa.
The resonator with movable
electrode shows lower insertion loss
and Rm.

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Experiment Results
• Loading Effect Qloaded:
Originally, the Q of the device can be calculated by:
However, when we measure a device, other resistances needs to be included:
Rm
R1 R2
Since the Rm of the movable electrode resonator is
smaller, the loading effect affects the Q more than
the other resonator.

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Experiment Results
•By applying different bias voltage, VDC, to the resonator we can observe the frequency
shift.
•The resonator with movable
electrode has a better tuning
capability than the other.
•This is an advantage because
due to fabrication process or
operation temperature the
resonant frequency may shift
from our desired frequency.

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Conclusion
• In this paper, capacitive silicon resonators with
movable electrodes was designed, fabricated,
and evaluated.
• The insertion loss increased by 21 dB, and the
motional impedance was reduced.
• The frequency tuning capability is also 7 times
better.

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~Thank you for your attention~