Optimization Of collapsed mode CMUT receiver for maximum off-resonance sensitivity
Author
Khan, Mansoor
Advisor
Köymen, Hayrettin
Date
2018-07Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Capacitive Micromachined Ultrasonic Transducer (CMUT) has a flexural conductive
plate suspended over a fixed substrate with metal electrode deposited
over it. This circular suspended plate is fixed at the rim but is free to move under
ambient medium loading or applied bias between CMUT plate and bottom
electrode. In presence of an acoustic medium, when the bias is applied the center
of the plate de
ects more towards the substrate owing to the electric field which
is established in the cavity structure. This electric field and ambient force are
balanced by plate restoring force in stable region of un-collapsed mode of plate
operation. As bias is increased further, the electrostatic forces overwhelms the
plate restoring force and the center of the plate contacts with the substrate with
a thin insulation layer in between. We call this state, collapse mode of CMUT
operation as long as the center of the plate stays in contact with the substrate.
In this work, we etch small cavities and employ thin CMUT plate which is easily
depressed by the atmospheric force over an evacuated cavity to produce a stable
contact with the bottom electrode without any bias.
CMUTs have widely been used as sensors in a wide range of applications such
as underwater
ow metering sensing, airborne applications, medical ultrasound
imaging where CMUTs have been characterized for wide bandwidth and high
sensitivity than piezoelectric ceramics. Commercial scanners using 1-D CMUT
arrays are also reported to have produced clinical-quality images. Despite the
success of CMUTs in medical ultrasound over the past three decades, to this day
no effort has been made to optimize the receive sensitivity of a collapsed-mode
CMUT.
Traditional practice is to bias the CMUT plate close to collapse voltage to acheive
higher coupling coe cent and sense the incoming ultrasound as an open circuit
receive voltage (OCRV) signal of the transducer using a voltage pre-amplifier
or short circuit receive current (SCRC) employing a transimpedance amplifier.
Maintaining plate in the vicinity of collapse threshold is rather di cult as any
mechanical disturbance may collapse the plate and compromise the sensitivity
of the transducer. In this work we propose ambient pressure collapse CMUT
transducer capable of providing a stable performance at off-resonance frequencies.
When the plate of a collapsed CMUT is subjected to incoming ultrasound
its receive signal becomes a strong function of bias. The electric field sustained between
the biased collapsed plate and the substrate is larger than an un-collapsed
plate owing to small insulation layer gap at the contact center. This results in
an increased input capacitance of collapsed CMUT which together with higher
electromechanical turns ratio makes collapsed CMUT a viable choice for a higher
acoustic output than its coventional counterpart.
First, we derive and use a linear-equivalent circuit model under small signal
conditions to assess the performance of collapsed CMUT as a function of bias.
We derive both OCRV and SCRC normalized to incident pressure for a collapsed
CMUT in terms of lumped circuit elements. The performance curves as a function
of DC bias for varying CMUT operating conditions are obtained. We compare
SCRC performance with OCRV and show that simulated SCRC performance with
a transimpedance amplifier is not impaired by electrical losses. We then optimize
the SCRC performance with bias for any given CMUT operating conditions or
geometry.
To characterize the model sensitivity we design and fabricate CMUT cells based
on anodic wafer technology. We employ a transimpedance amplifier to measure
and verify experimentally through fabricated CMUT cells, -60 dB V/Pa sensitivity
at 100 kHz when the CMUT is biased between 50 to 65 Volts.
Keywords
Open Circuit Receive VoltageShort Circuit Receive Current
Coupling Conditions
Collapse mode CMUT
Off-Resonance Sensitivity
Anodic Bonding