LiuJiajun112
PengBenxian1
YuFengqi1*
-
(Shenzhen institutes of advanced technology, Chinese academy of science, Shenzhen,
China)
-
(Guangdong power exchange center Co., Ltd., Guangzhou, China)
Copyright © The Institute of Electronics and Information Engineers(IEIE)
Index Terms
Manipulator, CMUT, CMOS-MEMS, failure analysis
I. INTRODUCTION
The number of old people with severe disability and disabled people caused by accident
is increasing. Paramedics need to regularly turn the patients over, move them around,
and massage these long-term bedridden patients to prevent pressure sores and other
diseases. Up to recently, the number of nurses per thousand people in China, the European
Union, the United States, Japan, and Norway is 3.6, 8, 9.8, 11.49 and 17.27 (1). Therefore, the study of a dexterous robot manipulator is of great significance to
reduce the burden of nurse. The robot manipulator can also help standardize the rehabilitation
process for the patients (2). Different from other robot manipulators, the safety and comfort of human limbs are
the first consideration in the design of humanoid nursing manipulator. The main parameters
affecting the safety and comfort of grasping human limbs are the stress and strain
generated by the grasping force inside the human limbs. Therefore, it is necessary
to develop robotic fingers with proximity sensors, so that it continuously adjusts
the strength to achieve soft touching and grasping. Ultrasonic transducer plays very
important role in the proximity sensors.
At present, piezoelectric ultrasonic transducer is commonly used on the market (3). However, it cannot be applied flexibly because of its large size. Due to the small
area of robotic fingers, the sensors placed in it are required to be small. Compared
with the traditional piezoelectric ultrasonic transducer, the advantages of CMUT fabricated
on CMOS-MEMS technology are as follows. Its size is only about 10 mm2. It is compatible with CMOS process and easy to do integration and mass production.
Further more it has friendly impedance matching characteristics. Because of these
advantages, CMUT becomes the preferred choice for the proximity sensors.
CMUT (Capacitive Micro-machined Ultrasonic Transducer) benefits a lot from the rapid
development of MEMS (Micro-Electro-Mechanical System). The receiver and transmitter
of CMUT share the same structure and it can be used as a receiver as well as a transmitter,
as shown in Fig. 1. In the receiving mode, the incident ultrasonic wave causes the deformation of the
membrane, thus affecting the capacitance between the top electrode and bottom one.
A suitable circuit is applied to convert the current signal caused by the change of
the capacitance into a voltage signal. In other words, the transducer realizes the
energy conversion from ultrasonic energy to mechanical energy and then to electrical
energy (4). In the transmission mode, a DC voltage is firstly applied between the top and bottom
electrode of CMUT, where the membrane is attracted by the electrostatic force. And
then an AC signal is applied. The membrane vibrates at a certain frequency, which
emits ultrasound wave at the same time. In the transmission mode, CMUT realizes the
energy conversion from electrical energy to ultrasonic energy.
The rest of the paper is as follows. In section II, a removable column is ingeniously
designed for our CMUT. In section III, the designed CMUT is fabricated and the condition
of the device is checked. In section IV, the common failure of CMUT device is studied
and the cause of the failure is determined.The last section is the conclution.
Fig. 1. Transmitting mode (left) and receiving mode (right) of CMUT.
II. Title
In this section, the integration between CMOS and MEMS process for CMUT fabrication
is discussed. A special CMUT structure with three cantilevers supporting a circular
membrane is proposed. 0.18 um CMOS standard process is adopted in our design.
1. The Design of the Air-coupled CMUT
In standard CMOS process, the materials and their distribution of each layer have
been determined already (5). Considering the feasibility of MEMS process and the availability of poly layer,
SiO2 and Al are taken as the material of CMUT membrane where Al in Metal2 is applied as
sacrificial layer.
Considering the dielectric loss in the air, the frequency of air coupled ultrasonic
transducer is generally 40-200 kHz. The low-frequency CMUT usually occupies a large
area on chip. However, under CMOS standard process, the available area in a wafer
is limited. In order to improve the area utilization on chip, we propose a novel CMUT
structure with three cantilevers supporting a circular membrane. Fig. 2 shows the design of our CMUT structure.
In CMOS standard process, the thickness of metal2 as sacrificial layer is 0.55 um
only. Due to the existence of liquid surface tension in wet etching process, the top
electrode and the bottom one often stick together, which leads to device failure.
Obviously, the increase of Metal2 thickness can solve the problem. However, the Metal2
thickness is fixed in standard process. Thus, a central column between the top electrode
and the bottom one is proposed to resolve the device failure problem. This column
will be removed by MEMS process later.
Fig. 2. The proposed CMUT structure.
2. The Structure Design of the Air-coupled CMUT
In this section, the process steps are demonstrated by using the cross-sectional view.
In MEMS process, twice dry etching and one wet etching are used to release the device.
It is worth noting that the process has self-stopping capability.
After CMOS process, we obtain the die from foundry. The chip cross-sectional view
of the CMUT structural layers is shown in Fig. 3(a). M1 is designed as the bottom electrode. M2 is designed as the gap filled with air.
M3 is as the top electrode. M4 is as the protection layer of the second dry etching,
which acts as a mask. M5 is as the protection layer for the first dry etching, which
also acts as a mask. In addition, it is noted that there are two vertical dotted lines
and a circle in the middle of the diagram, which is our proposed structure of the
central column.
Fig. 3. (a) The cross-sectional view of the CMUT structural layers. (b) After the
first ICP etching.
We use ICP (inductively coupled plasma) to do deep etching. It starts etching from
top to bottom, with its characteristics of excellent verticality and uniform rate
(6). The ICP etching has good selectivity because it can only etch non-metallic compounds
with a proper power. It stops etching when encountering metal such as Al. The etching
result is shown in
Fig. 3(b).
The aim of this process is to corrode the sacrificial layer. Here, we use a corrosive
solution mixed acetic acid and nitric acid. In MEMS process, the wet etching of a
certain material can only corrode the exposed metal without passing through the protective
layer by controlling the concentration of corrosive solution (7). The wet etching result is shown in Fig. 4(a). It can be seen that the central column provides additional support for the membrane.
After the wet etching, the samples need to be dried in supercritical dryer.
There are two purposes of the second dry etching. One is to expose the pads so that
the device can be connected with an offchip test circuit through wire bonding. On
the other hand, the central column needs to be removed so that the membrane can vibrate.
The cross section after the second etching is shown in Fig. 4(b).
So far, the complete anti-collapse, self-stopping MEMS processing for CMUT has presented
where the reliability and fault tolerance of the whole process are both considered.
Fig. 4. (a) After wet etching, (b) After second ICP etching.
III. MEMS FABRICATION AND TEST ON CMUT
Based on micro-nano scale, MEMS is a kind of precise fabrication technology. Therefore,
the whole process should be completed in an ultra-clean room. The process of the fabrication
mainly involves dry etching and wet etching. In this section, the MEMS process and
test will be presented.
1. MEMS Fabrication
After obtaining the chips from foundry, we take the microscopic picture of the die,
as shown in Fig. 5(a). The CMUT array is in the square region in deep orange. These top Layers mixed Si3N4
with SiO2 are above the CMUT structure. Because the thickness of Si3N4 is relatively thin,
it is transparent under microscope. This figure corresponds to the section of Fig. 3(a).
After dry etching, the result is shown in Fig. 5(b). SiO2 and Si3N4 on Al are etched away. The yellow is metal Al. Fig. 5(b) is consistent with the results of our model.
Fig. 5. (a) Initial profile of die, (b) After first ICP etching.
In wet etching, the metal Al is corroded away with acid solution. Then the chip is
taken out and dried with supercritical drying apparatus. As shown in
Fig. 6(a), the dark orange part is SiO
2, which meets the expectation.
Because the thickness of the layer between M2 and M3 in second dry etching is thinner
than that in first etching, the erosion period needs to be reduced with the same formula.
The etching result is shown in Fig. 6(b). After this etching, the pads are exposed. So far, MEMS process in the ultra-clean
room has been completed.
Fig. 6. (a) Wet etching, (b) Second ICP etching.
2. Device Testing
After observing the specific profile of CMUT under SEM (Scanning Electron Microscope),
the CMUT device is wire bounded to a PCB, and then connected to the impedance analyzer
for testing.
A. SEM Observation
The SEM used in the test is Zeiss Sigma 300. As shown in Fig. 7(a), the surface of CMUT is smooth and the structure of the disc and beam is complete
without any damage. As we can see in Fig. 7(b), the outer diameter of CMUT is 240 um, the inner diameter is 178 um, and the diameter
of central small disc is 14 um, which all meet the design specifications.
Fig. 7. (a) CMUT structure in SEM, (b) CMUT parameters in SEM.
B. Impedance Analysis
As shown in Fig. 8, the device is wire bounded to a PCB test board. Then, it is connected to an impedance
analyzer which is Keysight 4990A. In the test, the scanning scale is from 1 kHz to
1 MHz. Unfortunately, we cannot get any signal during the impedance test. Some MEMS
failures show in the test. We will perform failure analysis in the following section.
Fig. 8. The device is wire bonded to a PCB.
IV. FAILURE ANALYSIS ON CMUT
The goal of this section is to find out the cause of the CMUT failure, which is very
important for the design, fabrication, and production yield. The failure analysis
of the device is carried out in the two stages of CMUT processing and its final released
device.
1. Study on the Fabrication Process
In the step of dry etching, due to the top-down etching process of ICP etching, the
processing conditions of the device surface can be observed directly through the microscope,
which seems in good condition. The purpose of the wet etching stage is to form a cavity
which can help CMUT vibrate freely. The cavity is under the membrane and cannot be
observed directly. Therefore, the process quality of wet etching in MEMS processing
is our focus.
Here, we use high power ultrasonic cleaning equipment with similar frequency to peel
off the top layers of CMUT. The detailed operation is as follows. The CMUT is processed
as the same procedure as the previous one. It is placed in a small closed bottle filled
with deionized water. The bottle is placed in an ultrasonic cleaner for 60 min. As
shown in Fig. 9(a), CMUT top electrode layers were successfully peeled off, and a pit is exposed. We
use a surface profilometer to test the peeled CMUT. The profilometer measures the
depth of the groove along the red dotted line, as shown in Fig. 9(a). The measured result is shown in Fig. 9(b).
In the process of wet etching, there are two kinds of corrosion effects. Insufficient
or too long corrosion time leads to rough surface and little bumps appear in the detected
curve. If the corrosion time is appropriate, the plane will be smooth and the probe
curve will be smooth as well. Due to the edge jitter effect of the profilometer, the
double peak structure of the bump appears, which is circled in Fig. 9(b). In addition, the protrusion in the middle is caused by our proposed central column.
The curves of the rest parts are smooth, which indicates that the sacrificial layer
has been corroded enough in the wet etching.
Fig. 9. (a) CMUT after being peeled off the top electrode layers, (b) Test result
using the profilometer.
2. Analysis on Released Device
This analysis is mainly aimed at the devices after the completion of MEMS processing.
From the types of actual physical failure, device failure can be divided into the
4 types.
(a) Fracture failure. Fracture failure means that the device breaks when the load
applied on the movable structure of the device exceeds the maximum value it can bear
without significant damage precursor.
(b) Adhesion failure. When two smooth surfaces touch each other, they tend to bond
together due to some adhesive forces. For MEMS devices, the ratio of the movable surface
area to the cavity height is relatively large, which makes the capillary force and
van der Waals force relatively enhanced in the micro scale. They are the causes of
adhesion failure between micro structures.
(c) Wear failure. It usually occurs in two solid materials in contact. Under the mechanical
operation of polishing, the wear failure caused by the relative mechanical movement
between the surfaces is usually accompanied by the removal or thinning of the surface
material.
(d) Fatigue failure. Fatigue failure refers to the failure of the device for a long-term
operation under the periodic excitation of the load lower than its yield strength.
If the periodic load exists in the device for a long time, the material strength of
the device structure will be weakened gradually, resulting in small cracks on the
surface. Most MEMS micro actuators have movable structures. The movable parts often
work under periodic loads. Therefore, the fatigue failure can be delayed but cannot
be avoided.
According to Fig. 7(a), the surface of CMUT disc is flat and the cantilever is not fractured. Therefore,
fracture failure can be ruled out. From the review of fabrication process at the beginning
of this section, the top layer of the electrode is successfully peeled off. That means
there is no any adhesion. Thus, the adhesion failure can be ruled out. In the process
of MEMS, the device is not polished so that the wear failure is not the cause. In
conclusion, fatigue failure is most likely the cause.
V. FATIGUE SIMULATION ON CMUT
In this section, fatigue analysis on CMUT structure is carried out with COMSOL stress
life section. The structural fatigue life analysis method is based on the following
assumption (8). If the material of the structure is consistent, and the stress concentration factor
and load spectrum are the same, then the fatigue life is also consistent. The analysis
process is shown in Fig. 10.
The optimal operating point of CMUT is that the center displacement occupies about
1/3 of the cavity height (9). With the steady-state simulation of COMSOL, it is found that the static voltage
applied at the center position is 12.3 V. In pre-stress analysis, we need to set multi-physical
field to electro-mechanical mode. The fixed constraint is set at the edge of the beam
structure. The stress map is calculated. As shown in Fig. 11, the maximum stress is at the pining place of cantilever.
Fig. 10. Fatigue model analysis using COMSOL.
Fig. 11. The max stress in CMUT.
In order to make CMUT structure reciprocating under stress, cosine function is defined
at the boundary load, as shown in
Fig. 12(a). The amplitude is the cosine vibration with the maximum value of boundary load. Then,
the spring foundation is set to replace the fixed constraint in solid mechanics, and
the boundary conditions are consistent with the fixed constraints in steady-state
simulation. In addition, in order to calculate the fatigue life, we need to input
a suitable S-N (Stress-Life) curve. The curve takes the fatigue strength of material
standard specimen as ordinate and logarithm of fatigue life as abscissa, which represents
the relationship between fatigue strength and fatigue life of standard specimen under
certain cyclic characteristics, also known as stress life curve. According to the
article
(10), the S-N curve of CMUT thin films based on CMOS-MEMS technology can be obtained,
as shown in
Fig. 12(b).
In COMSOL, the required parameters of fatigue simulation have been prepared, and the
calculation is carried out. The operation results, the fatigue life, and stress intensity
distribution of CMUT structure are shown in Fig. 13. It can be seen from the figure that the minimum fatigue life is 6027 times, which
is located at the junction of arm end and disc of CMUT. It shows that the interface
between arm end and disc is the most vulnerable place. In our experiments, the excitation
signal is a 200 kHz square wave. The test signal is transmitted three cycles per second
and the number of excitation in each cycle is 8, according to the excitation mode
of commercial air-coupled transducer. Therefore there are 24 vibration cycles per
second. The CMUT vibrates more than 6027 times in only 252 seconds. The CMUT structure
is at the end of its stress life time.
In conclusion, the reason that the expected signal of the CMUT cannot be detected
is because of fatigue damage.
Fig. 12. (a) load-displacement curve, (b) S-N curve.
Fig. 13. (a) Results of fatigue life simulation, (b) The detailed display.
VI. SUMMARY
This paper has proposed a novel CMOS-MEMS compatible method to fabircate CMUT. The
CMUT array has been designed and fabricated using the standard 0.18um CMOS process
and post CMOS process. The adhesion of the upper and lower electrodes, which often
appears in the CMUT fabrication, has been described. A central column is proposed
to resolve the problem, by supporting the upper and lower electrodes in the wet etchings.
It can be ingeniously removed without affecting the device.
Our CMUT device is fabricated in line with the expectation. The geometry parameters
and surface morphology of CMUT are consistent with our design as well. The failure
of CMUT device has been studied. The cause of the failure has been determined to be
the fatigue damage.
ACKNOWLEDGMENTS
This work was supported in part by National key R&D program of China (Grant number
2016YFC0105002, 61674162, 2018YFB2100904, U1913601, 2018YFF010 12500), Shenzhen Key
Lab for RF Integrated Circuits, Shenzhen Shared Technology Service Center for Internet
of Things, Shenzhen government funds (Grant numbers JCYJ20180305164616316).
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Author
Jiajun Liu earned his MS degree in Shenzhen Key Laboratory for RF Integrated Circuits
at Shenzhen Institutes of Advanced Technology, Chinese academy of science.
He is currently with Guangdong power exchange center Co., Ltd.
His R&D interests are in the fields of sensors and information management system.
Benxian Peng earned his Master degree of mircoelectroinc engineering in 2008, from
Shanghai Jiao Tong University, He currently foucus on the research of the device and
IC design of ultrasound chip.
Fengqi Yu earned his Ph.D. degree in Integrated Circuits and Systems Lab (ICSL) at
UCLA.
Before joining Shenzhen Institute of Advanced Technology in June 2006 as a full professor,
he worked at Rockwell Science Center (USA) as a design engineer, Intel (USA) as an
analog circuit design engineer, Teradyne (USA) as a Sr.
IC design engineer, Valence (USA) as a Sr. principal engineer, and Suzhou CAS IC Design
Center (China) as VP and RF department director.
His R&D interests are in the fields of low power communication networks, CMOS integrated
circuits, CMOS sensors, wireless sensor networks, and RFID.