Capacitive micromachined ultrasonic transducers
(CMUT) is an important research direction of MEMS
technology, its acoustic impedance is close to gas and
liquid, thus a matching layer is not needed; [4-5] It
also has good mechanical and electrical properties
and higher resonance frequencies (up to tens of MHz)
and a quality factor (up to several hundred); all of
these good properties make it possible to achieve
high sensitivity and small-scale pressure
measurements. Meanwhile, it has such advantages as:
simple structure, low cost, mass production,
integration of complexity circuits and process
compatibility with CMOS, [6] etc. So there is a wide
range of applications in underwater ultrasound
imaging, seabed resource exploitation, naval security
and other fields. However the existing capacitive
ultrasonic transducers still have the following
drawbacks: firstly, the present domestic and
international research on the capacitive ultrasonic
transducers is mainly concentrated in the above MHz
band, [7-10] which cannot meet the demand for longdistance
imaging applications, secondly, most current
capacitive micromachined ultrasound transducers
adopt surface sacrificial technology [11-12]. For
example, in one patent of application,
No. 201210068681, disclosing a low range pressure
sensor based on CMUT, the inventor devised a
capacitive transducer whose overall structure from up
to down is metal aluminum top electrode, silicon
oxide membrane, single crystal silicon substrate,
silicon oxide support, silicon nitride insulation layer
and metal aluminum bottom electrode [13]. Defects
of this structure are that the silicon oxide membrane
is formed by an oxidation process, which makes the
controllability of the membrane thickness low and the
membrane surface rough, and directly affects the
uniformity of the entire membrane deformation.
Then, a discrete metal electrode is needed to deposit
on its surface since the membrane is made of an
insulating material, silicon oxide. This will lead to an
increase in the membrane frequency, which is not
conducive to the realization of the desired frequency
sensors and limits the scope of the sensor application