电容式微超声传感器技术与应用综述

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"Capacitive Micromachined Ultrasonic Transducers: Technology and Application" 这篇综述文章详细探讨了电容式微机械超声换能器（CMUTs）的技术及其应用。CMUTs是近年来发展起来的一种替代传统压电换能器的新型技术，具有诸多优势，包括更宽的带宽、层叠阵列的制造能力、更高的效率和灵敏度。首先，文章阐述了CMUT的工作原理。电容式微超声传感器主要基于电容变化来实现声波到电信号或反之的转换。当声波作用于CMUT的薄膜时，薄膜会发生微小的位移，导致电容值的变化，进而将机械振动转化为电信号。反之，通过施加电压于电容，也可以使薄膜振动，产生超声波。接着，文章详细介绍了CMUT的制造工艺步骤。这通常涉及微电子机械系统（MEMS）技术，包括硅片的微加工、薄膜沉积、光刻和蚀刻等步骤。这些过程确保了CMUT的微型化和精确的结构设计，使得在微小尺度上实现高性能的超声换能成为可能。文章还深入讨论了CMUT膜的坍塌电压设计参数。这个参数对于优化CMUT的性能至关重要，因为它直接影响到器件的稳定性、工作频率和输出功率。通过对膜的厚度、材料选择以及电介质层的优化，可以调整坍塌电压，从而实现最佳工作状态。此外，文章列举了几种重要的应用实例，展示了CMUTs的实用性和潜在价值。这些应用可能包括医学成像（如超声波成像）、无损检测、生物传感器以及微流体控制等领域。在医学超声成像中，CMUTs能够提供更高分辨率的图像，有助于疾病的早期诊断和治疗。在无损检测中，由于其优良的频率响应，CMUTs能够用于检测材料内部的缺陷。 CMUT技术是一项具有广阔前景的技术，它在多个领域都有潜力替代传统的压电换能器，推动超声技术的发展。随着对CMUTs的理解和制造工艺的不断改进，它们在未来的应用将会更加广泛和深入。

deposition and patterning of a silicon nitride or a silicon

dioxide layer as a mask, then (100)-oriented silicon wafers

are etched in KOH aqueous solution, obtaining pits with

triangular or trapezoidal cross-section, and with depth

depending on the mask-opening dimension (Fig. 7). The

so-micro machined silicon back plate is coated by a metallic

layer and, subsequently, a polymer ﬁlm with a thickness

ranging from 2e13 mm, metalized on one side, is stretched

over it and kept in tension by a metallic frame. Thanks to

the small dimension of the micro membranes, suspended

by the rails, the working frequency of these transducers

can reach 1 or 2 MHz in air [17,19]. The membrane is forced

to stay in contact with the rails due to the electrostatic

pressure caused by the polarization voltage, and this

constraint prevents down displacement of the membrane

towards the substrate. But, as soon as the frequency

increases, upward displacements are allowed. Due to this

degree of freedom, and to the mutual mechanical coupling

of the micro-membranes, regions in phase and out of phase

appear on the surface of the membrane, destroying the

coherence of the acoustic radiation [17], and limiting the

range of frequency these transducers can operate.

Surface-micromachined capacitive ultrasonic

transducers

Wafer bonding, considered as a bulk process, is widely used

in micromachining, and older than surface micromachining

techniques. There are three basic wafer-bonding tech-

niques: anodic bonding, fusion bonding, and adhesive

bonding. Among these techniques, silicon fusion bonding

has earned a stable position in today’s technology, and is

now used for various applications: bond-and-etch back

silicon-on-insulator (SOI) wafers, SMART-CUT SOI wafers,

power devices, and many silicon microstructures, such as

pressure sensors and accelerometers. Silicon fusion bonding

is a direct bond between two silicon surfaces. It takes

place at high temperatures, forming strong covalent bonds

between the silicon wafers.

Most of these techniques are based on surface micro-

machining techniques [2,20], others are modiﬁed BiCMOS

processes [21], and others combine bulk and surface

micromachining for the realization of the membranes and

of the back plate [22].

In a surface-micromachining process, the micromechanical

structures are fabricated by deposition on the silicon wafer

of thin structural layers over thin sacriﬁcial layers that can

be selectively removed. Thin ﬁlm deposition techniques,

able to control the ﬁlm’s mechanical properties, such as

internal stress or stress gradients, and dry and wet etchings

that remove sacriﬁcial layers without damaging the struc-

tural ones, are essential for surface micromachining as

discussed below in this section.

The micromachining processes developed for the fabri-

cation of cMUTs can be classiﬁed according to the maximum

temperature used, which limits the degree of compatibility

with IC processing and hence the possibility of integration

of transducers and electronics on the same wafer.

The ﬁrst process for the fabrication of cMUTs, based on

surface micromachining techniques, was developed at

Stanford University in Palo Alto, CA, USA [2]. The trans-

ducer is fabricated on a highly doped silicon wafer that

acts as the ﬁxed electrode. The membrane’s structural

material is silicon nitride deposited by LPCVD over a sacri-

ﬁcial layer of thermal oxide. Both the silicon nitride layer

and the backside of the silicon wafer are coated with

evaporated gold. Then a pattern of 3-mm diameter holes

on a bidimensional grid with 100-mm period is transferred

by lithography and etching through the gold and the

silicon nitride layer to reach the sacriﬁcial oxide. Then

the membranes are released by wet etching of oxide in

pure hydroﬂuoridric acid. The membranes’ diameter is

controlled by timed etch. The major manufacturing issue of

this process consists in the membranes’ release. The risk of

fracture of the thin membranes and their sticking to the

substrate can be avoided by properly choosing layer thick-

nesses, thin ﬁlms’ stresses, and membranes’ dimensions.

During drying after the sacriﬁcial etching, capillary forces,

proportional to the membrane area, which can be balanced

by internal stress in the membrane, attract the membrane

toward the substrate [23]. A thin membrane separated

by a narrow gap from the back plate is the basic feature of

the cMUT, but a membrane that is too thin easily collapses

onto the substrate, and a thin gap limits the membrane’s

displacement during operation. A large value of the internal

stress in silicon nitride ﬁlm increases the resistance to

collapse, but it can cause the membrane to break, resulting

in yield loss; furthermore, it affects the resonance vibra-

tion frequency of the membrane in air. As a consequence,

the cMUT presented by Haller and Khuri-Yakub [2] had

a 0.75-mm thick membrane suspended on a 1-mm thick gap.

The silicon nitride layer was deposited by LPCVD using

a mixture of ammonia and dichlorosilane at 800



C in order

to tune the residual stress of the membrane at a value of

280 MPa.

Another issue of the process is the timed etch of the

sacriﬁcial layer. The etching rate decreases with time as

the etchant and the etch products must diffuse longer

through the small holes in the structural layer. The

membrane’s diameter depends on the etching duration and

only circular membranes can be obtained, as the etch

Fig. 7 Scanning Electron Microscopy (SEM) of a portion of

a CMUT silicon back plate fabricated by anisotropic etching of

silicon in KOH aqueous solution [68]. cMUT Z capacitive

micromachined ultrasonic transducer; KOH Z potassium

hydroxide.

12 M.S. Salim et al.

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