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The enormousness of the ocean offers great challenges for little and compact underwater platforms development. With assorted exploratory and communicating related applications, the demand for high preciseness acoustic and non-acoustic detectors are spread outing. This paper proposed two miniaturize supersonic transducer ( MUT ) designs attacks following micro-electro mechanical system ( MEMS ) -based engineering. Two feeling mechanisms were studied, viz. , piezoelectric ( pMUT ) and capacitive ( cMUT ) . Responses of each feeling mechanism were identified utilizing Finite Element Method ( FEM ) for different stuffs which were ZnO, PZT and vitreous silica for pMUT and Si3N4, Teflon and PDMS for cMUT. Analysiss conducted including mechanical, piezoelectric and harmonic frequence. PZT were found to be the most antiphonal stuff with 1.3A-10-3 Aµm/V and 2750.2 pC/V of response but carry the lowest resonating frequence at 85.9 kilohertz. For cMUT design, PDMS carry the highest sensitiveness at 1.6 I?m/Pa, with regard to Si3N4 and Teflon. However, farther analysis revealed that resonance for PDMS occur at 515.9 kilohertz.

Keywords: Micro-acoustic, piezoelectric, capacitive, submerged

Introductions

Compact and little underwater platforms can be found in many demanding submerged applications such as for pilotage, imaging and communications. It includes robotic-based platform such as the independent or remotely operated submerged vehicles ( AUV/ROV ) , every bit good as stationary platforms including ground tackle and buoy for detector webs. Multiple detectors have been utilised affecting acoustic and non-acoustic, with illumination designs following MEMS and microelectronics fiction engineering forecasted to transport immense possible these coming old ages [ 1 ] . Many advantages offered by illumination transducers including low power ingestions, higher sensitiveness, lightweight, embedded electronics and bio-compatibility. Depth sounding, force per unit area measuring, obstruction turning away, pilotage, imaging and communicating are several applications utilize acoustic moving ridge, with micro-ultrasonic transducer ( MUT ) design. Presently, merely two feeling mechanisms are available at the micro-structure detector design degree, i.e. piezoelectric, known as pMUT and capacitive or cMUT [ 2-5 ] .

In rule, pMUT membrane construction contains functional and structural beds. Functional bed was formed by a bed of piezoelectric movie, sandwich between two electrodes, sputtered on both side of piezoceramic. Electrostriction procedure occurs each clip the piezoceramic bed is deformed by inward acoustic signal every bit good as when electromotive force is applied across two electrodes. Structural beds provide support and normally are placed under the functional bed. Silicon based stuffs are popular pick for its superior snap with high output and tensile strength, supplying good support and damping for the whole vibrating construction. Assorted fiction techniques have been reported from conventional layer-by-layer deposition on Si or glass substrate [ 6 ] to adhesive wafer adhering utilizing polymer adhesives [ 7 ] . Huge choices of piezoelectric stuff available include zinc oxide ( ZnO ) , lead zirconate titanate ( PZT ) , aluminum nitride ( AlN ) and quartz. This paper intends to look into the pMUT device response utilizing different piezoelectric stuffs. ZnO, PZT and vitreous silicas were selected for this survey, sing good documented deposition processs and lower cost solutions.

On the other manus, cMUT operates based on capacitive rules, where the distortion of the membrane bed caused by the contact acoustic signal creates a quiver in conformity to the received signal. CMUT has a basic construction of a top electrode fabricated on the membrane bed and separated from the bottom electrode by a specific thickness of spread. This construction therefore making a electrical capacity construction with one electrode is allowed to travel while the other one is fixed. The quiver of the membrane will modulate the separation distance between both electrodes, therefore making a fluctuation of electrical capacity value. The effects of electrodes, membrane dimension, array constellation and device construction [ 8-11 ] have been antecedently studied. In order to optimize the public presentation of this sort of micro graduated table transducer, the stuff used for the membrane component in cMUT is Silicon Nitride, Si3N4. This is because of its stuff belongingss and easiness of fiction factor until to day of the month. However, in theory, other polymer based stuff such as polydimethylsiloxane ( PDMS ) and Teflon are besides suited as the membrane component based on their application in microfluidic environment [ 12-13 ] . In add-on, the elastomic belongingss of such stuffs can increase the sensing sensitiveness due to big warp per unit force per unit area in acoustic sensing application. This survey investigates this issue ( suitableness of PDMS and Teflon for cMUT application ) and includes the Finite Element Analysis ( FEA ) response of PDMS and Teflon, along with bing stuff, Si3N4, as the vibrating component in cMUT.

Two miniaturize transducer design were proposed in this survey viz. pMUT and cMUT. Both designs are projected to be utilized on compact submerged platforms for assorted applications. Responses of each transducer design were determined utilizing finite component methods. Key stuffs choice issues for pMUT and cMUT have been investigated and comparative attack is utilized throughout this work. However, the range of probe is limited to a individual component design alternatively of transducer array.

Theory

A pMUT

pMUT design under probes is shown in Fig. 1. It consists of six stuff beds, with piezoelectric movie sandwiched between 0.5 Aµm of aluminium electrodes each. Top electrode is partly sputtered on top of the piezoelectric movie. Structural wafer beds consist of bottom-etched silicon-on-insulator ( SOI ) , go forthing silicon oxide and Si beds as portion of vibrating membrane after bottom Si bed was etched. The 6th bed is a polymer adhesive, Cytop, placed between functional and structural wafer bed, adhering those two organizing a stop. A bed of PDMS polymer used to box the device that consist multiple elements of pMUT in an array. Finally, nickel aluminium bronze was sputtered to encapsulate pMUT component from saltwater. Box pMUT was so placed inside nickel aluminium bronze lodging. Element constellation in an array and lodging analysis are non within the range of this paper.

Nickel aluminium

bronze lodging

Epoxy

Nickel aluminium bronze

movie encapsulation ( 1Aµm )

PDMS ( 2Aµm )

Piezoelectric movie ( 40Aµm )

Silicon ( 10Aµm )

Cytop ( 5Aµm )

Silica ( 2Aµm )

Silicon

Figure 1: pMUT cross subdivision with nickel aluminium bronze lodging

For a individual bed round form stop with a fixed border, magnitudes of warp correspond to the sum of applied force per unit area as given in Equation 1:

( 1 )

Where P is applied force per unit area in Pascal, a is surface country, H is device thickness, E is Young modulus and m is 1/I… , with I… is Poisson ratio. Plus and minus mark indicates diaphragm warp upward and downward severally. The system was represented with Butterworth-VanDyke ( BVD ) equivalent electric resistance theoretical account, as shown in Fig. 2, having a series of motional induction, Lm, motional electrical capacity, Cm and motional opposition, Rm in analogue with stack electrical capacity, Co ; stand foring vibrating mass, piezo-elasticity, mechanical losingss and inactive analogue home base capacitance severally.

Lumen

Co

Rm

Centimeter

Figure 2: BVD tantamount electric resistance theoretical account

Stack electrical capacity, Co was observed from DC analysis, while frequence analyses produced series resonant frequence, degree Fahrenheit every bit good as parallel resonating frequence, fp. Additionally, motional electrical capacity, Cm and motional induction, Lm can be calculated as follows:

( 2 )

( 3 )

B. cMUT

The basic construction of cMUT is depicted in Fig. 3.

Substrate

Top Electrode

Passivation bed

Membrane

Vacuum Gap

Insulation Layer

Bottom Electrode

Figure 3: Conventional diagram of a cMUT.

The map of each bed is summarized in Table 1.

Table 1: The structural bed of cMUT

Structural bed

Common Material

Passivation bed

Parylene

Top electrode

Aluminum

Membrane

Silicon Nitride

Gap

Vacuum

Insulation bed

Silicon Oxide

Bottom Electrode

Aluminum

Substrate

Silicon

Normally, passivation bed is added on the top construction as a preventative stuff and/or as an electric resistance fiting bed. The insularity bed is added between substrate and spread to avoid an electrical shorting during its operation. The warp of the membrane depends on several factors such as the sum of bias electromotive force, the acting force per unit area on the membrane, the construction dimension and the flexural rigidness of the membrane stuff. The bias electromotive force is required to bring forth an electrostatic status needed for cMUT operation, either in transmittal or response and find the inactive warp of the membrane construction. The sensitiveness increases with the prejudice electromotive force. In response to the force per unit area wave signal, the warp of the membrane, w will be maximal at the Centre and can be represented by Equation 4:

( 4 )

where Po is the entire force per unit area, including the electrostatic force ensuing from the DC prejudice, a is the membrane radius and D is the stuff dependent flexural rigidness. D is given by:

( 5 )

Tocopherol, T and I… are the Young modulus, Poisson ratio and membrane thickness, severally.

Methodology

A pMUT

All analyses and word pictures were done utilizing Analyzer tools bundle within Coventora„? package. For pMUT design, it is assumed that the theoretical account to be a multilayered home base with all outer surfaces clamped at the fixed border. For finite component engagement, all theoretical accounts were simplified with merely the vibrating portion left as shown in Fig. 4.

Sfix

Stop

Spzt

Spzb

Sbot

Piezoelectric bed

Figure 4: Side position of vibrating membrane with surfaces boundary status for FEA

There are five of import surfaces, defined on the vibrating membrane of pMUT. First is the outer border surface, Sfix. Following surface is on the top of the theoretical account, Stop whom will have the inbound acoustic signal. Another two surfaces located on top and underside of piezoelectric bed and being in contact with top and bottom electrodes, denoted with Spzt and Spzb severally. And in conclusion, the lowest surface which is the bottom portion of the stop, Sbot. These simplified theoretical accounts were so split into two separate parts with different mesh scene. Both parts nevertheless have undergone the same additive tetrahedron engagement. Piezoelectric coefficients of all material beds were set to be zero except for piezoelectric stuff. Stiffness matrix, piezoelectric strain coefficients and dielectric coefficients for ZnO, PZT and vitreous silicas are gettable from Coventor mention usher. Young modulus, E and Poisson ratio, I… were taken as the step of elastic coefficients of all isotropous stuffs as in Table 2.

Table 2: Mechanical belongingss of isotropic stuffs

Material

Density

( A-10-15kg/Aµm3 )

Young ‘s modulus

( MPa )

Poisson ‘s ratio

Aluminum movie

2.30

7.70A-104

0.30

PDMS

970

0.36-0.87

0.50

Cytop

2.03

1.40A-103

0.42

Nickel aluminium bronze

7.58

1.15A-105

0.33

Silica

2.70

7.30A-104

0.17

Silicon

2.50

1.69A-105

0.30

During piezoelectric analysis, DC electromotive force was applied across the electrodes and magnitude of membrane warp is observed. Harmonizing to Fig. 4, positive electromotive force is applied when Spzt is supplied with possible with mention to Spzb and frailty versa for negative electromotive force. Extra question revealed the sum of charge, Q generated at Spzt and Spzb therefore stack electrical capacity, Co of the device at supplied electromotive force, V can be calculated based on Co=Q/V. By utilizing mechanical convergent thinker, force per unit areas were applied on Stop and Sbottom to mime the response and projection of sound moving ridge. Deflection that occur on +Z and -Z way of the membrane was observed. For both piezoelectric and mechanical analyses, Sfix is assumed to be impersonal electrically and repair automatically. Finally, frequence analysis was carried out to find resonating frequence of the pMUT. Closed-circuit status was applied where zero potency was supplied on both electrodes. Same analysis rhythm were carried out, utilizing three different piezoelectric stuffs with other structural parametric quantities were kept changeless.

B cMUT

For cMUT mold, a construction with specification given in Table 3 was realized by utilizing ANSYS package. The chief focal point was to look into the feature of three different stuffs ; Si3N4, PDMS and Teflon as membrane component. The warp behaviour of the membrane would be plotted as a comparing on the sensitiveness of feeling mechanism between these stuffs.

Table 3: Model specifications

Dimension

Selected value

Size of Membrane ( Aµm )

700 x 700

Thickness of Membrane ( Aµm )

2

Gap Thickness ( Aµm )

50

The mold was performed in 3D and AµMKSV unit. The membrane form was selected to be in square form. SOLID 95 and SOLID226 component are selected to pattern the membrane and spread, severally. For simpleness, the top electrode, where the electrical potency was to be applied, was modeled by delegating the nodes located at the outermost bed of the membrane component. Using the same attack, the nodes at the lower spread stuff was assigned as underside electrode where the 0 V potency will be applied. These attacks save computational undertakings particularly in 3D environment, and at the mean clip still continuing the original construction.

For this structural analysis, three stuff belongingss that contribute to structural distortion are Young Modulus, Poisson ratio and denseness. All belongingss for each stuff of involvements are given in Table 4. The membrane and the spread were glued together utilizing ‘vglued ‘ bid to guarantee the continuity for work outing couple field job [ 14 ] . The engagement procedure was executed utilizing tetrahedral component. By puting the zero supplanting boundary status at every borders, the processing is run to obtain the parametric quantity of involvements. In cMUT, the inactive warp demands to be analyzed to look into the initial warp when the prejudice electromotive force is applied.

This is done by imitating the Centre node supplanting resulted from different value of bias electromotive force applied onto the top electrode. The scope of bias electromotive force is between 10 V to 300 V. Then the mechanical response of the construction is studied by imitating the relationship between the applied force per unit area and the resulted Centre node warp in the presence of specific bias electromotive force. A scope of 10 to 50 Pa is used with the prejudice electromotive force is fixed to 200 V. Then the analysis is continued by executing the average analysis on the construction with different membrane stuff. This analysis is intended to pull out the quiver manners of the membrane with their several natural frequence. However, merely the first average frequence is of our involvement as it produces the maximal warp at the Centre point. This average analysis is besides used to expect the resonance extremum frequence in harmonic analysis. Harmonic analysis is so performed to look into the harmonic response of the construction at certain scope of operating frequence. Based on the performed modal analysis, two set of frequence scope are used as the frequence scope for harmonic analysis. For Si3N4 and Teflon, the analysis is carried out for a frequence scope between 10 to 100 kilohertzs while for PDMS, 10 to 1MHz is used.

Table 4: Mechanical belongingss of cMUT membrane stuffs

Material

Density

( A-10-15kg/Aµm3 )

Young ‘s modulus

( MPa )

Poisson ‘s ratio

Si3N4

3270

320A-103

0.26

PDMS

970

0.36-0.87

0.50

Polytetrafluoroethylene

2200

0.5A-103

0.46

Results & A ; Discussions

A pMUT

Linear responses have been observed due to applied electromotive force on pMUT utilizing ZnO, PZT and Quartz piezoelectric stuff as illustrated in Fig. 5. Positive electromotive force has ensuing maximal upward warp at the centre of the membrane. At the same thickness, PZT has the highest warp with 1.3A-10-3 Aµm/V of response, followed by ZnO and Quartz with 3.0A-10-5 Aµm/V and 5.0A-10-7 Aµm/V of responses severally. The analyses were extended to gauge the sum of charge on the top surface of the piezo, Spzt. The charge response curve is shown in Fig. 6 with PZT carries 2750.2 pC/V. ZnO and Quartz piezo bed carry somewhat different sum of charge responses with 19.45 pC/V and 7.92 pC/V. Based on the electromotive force applied across the electrodes, stack electrical capacity Co of the pMUT harmonizing to BVD theoretical account can be calculated utilizing Co = Q/V, with premise that supplanting is negligible. The FEA conducted merely take into history the anisotropic insulator in piezo bed and disregard fringing field through the air.

Z ( Aµm )

Z ( Aµm )

Z ( Aµm )

-V

+V

( B ) ( degree Celsius )

Figure 5: Responses of pMUT to applied electromotive force for ( a ) Quartz ( B ) ZnO and ( degree Celsius ) PZT

Applied electromotive force ( V )

Charge ( personal computer )

Figure 6: Charge response of pMUT in conveying manner

In mechanical analysis, series of mechanical force per unit area were applied on top and bottom surfaces of the pMUT which is Stop and Sbot. Positive warp is observed when upward force per unit area was applied on Sbot while negative warp occurs due to downward force per unit area on Stop. Downward and upward motions of the membrane miming the quiver rhythm during conveying and having acoustic signal. The thickness of the structural bed dwelling of silicon oxide and Si have been optimized and balanced with functional bed of sputtered piezoelectric stuff. Encapsulation beds dwelling PDMS polymer and thin movie Ni aluminium bronze provide extra mass and muffling so that conveying and having responses of the pMUT is equal. Fig. 7 shows that inbound and outward force per unit area responses of pMUT are equal with PZT are the most antiphonal at 2A-10-5 Aµm/Pa followed by ZnO and Quartz with 4A-10-6 Aµm/Pa and 8A-10-6 Aµm/Pa of responses severally.

-Z ( Aµm )

Pressure ( Pa )

Pressure ( Pa )

+Z ( Aµm )

( a ) Inbound ( B ) Outbound

Figure 7: pMUT responses to mechanical force per unit area

With both aluminium top and underside electrodes are grounded or supplied with zero possible, closed-circuit resonance analysis was conducted to find resonating frequence of pMUT with Cm and Lm of the BVD equivalent circuit are in series. By shorten the electrodes and neglecting losingss, Rm the piezoelectric consequence is assumed to be eliminated with lone structural effects are taken into history for resonating frequence derivation. Resonant frequence of the pMUT utilizing three different piezoelectric stuffs is shown in Fig. 8.

log Z

degree Fahrenheit ( kilohertz )

Figure 8: Frequency analysis of pMUT utilizing ZnO, PZT and Quartz.

Transducer with PZT piezoceramic was found to be in resonance manner at 85.9 kilohertzs while pMUT with ZnO and Quartz piezo bed have 193.6 kilohertzs and 188.5 kilohertz of resonating frequences severally. The sum of warp during closed-circuit resonating analysis is invalid, and average analysis should be conducted to find the sum of warp at the resonance. In average analysis, the resonating frequence is computed as the characteristic root of a square matrixs of the undamped and homogeneous equation of gesture of the vibrating construction of system.

B cMUT

The response of the designed construction is measured from the warp profile obtained from the finite component analysis. Fig. 9 illustrates the inactive warp of three different membrane stuffs for cMUT application.

Figure 9: Inactive warp of Si3N4, Teflon and PDMS

PDMS and Teflon both show a big distortion under the consequence of prejudice electromotive force status. This behavior is due to low Young Modulus particularly for PDMS, which makes the material displaying more warp upon specific force. Si3N4 has the least inactive warp, even under high prejudice electromotive force. The same phenomenon observed in the relationship between the external force per unit area and Centre point warp. This relationship represents the behaviour of the device during its operation under external frequence changing force per unit area signal. The graph from Fig. 10 is used to mensurate the sensitiveness of the device by agencies of how much it deflects per unit force per unit area.

Pressure ( Pa )

Deflection ( Aµm )

Figure 10: Relationship between external force per unit area and Centre point warp

The more it is deflected, the larger the fluctuation in the spread separation, so the consequence it has on electrical capacity changed besides becomes more discernible. From Fig. 10 the sensitivenesss of Si3N4, Teflon and PDMS are found to be 0.034 I?m/Pa, 0.67 I?m/Pa, and 1.6 I?m/Pa severally.

The average analysis outputs a consequence as in Table 4. The consequence is merely confined to the first quiver manner ( cardinal frequence ) in order to gauge the cardinal resonance frequence of the construction during operation.

Table 4: First manner frequence of different membrane stuffs

at design dimension ( Modal Analysis )

Si3N4

PDMS

Polytetrafluoroethylene

70.6 kilohertz

515.9 kilohertz

11.5 kilohertz

Fig. 11 represents the harmonic analysis of three different membrane stuffs. Two scope of frequence scope are used for elucidation intent. For comparing intent, merely the resonance frequences of the first manner quiver are considered.

Freq. ( Hz )

Deflection ( Aµm )

( a )

Freq. ( Hz )

Deflection ( Aµm )

( B )

Figure 11: First manner frequence utilizing harmonic analysis for

( a ) Si3N4 and Teflon ( 0-100kHz ) ( B ) PDMS ( 0-1MHz )

The resonance frequence between the modal and harmonic analysis output an mistake of 2 % , 6 % and 8 % for Si3N4, Teflon and PDMS severally. This is due to step size trying frequence in harmonic analysis. As can be seen, at this design constellation, the resonance frequence of Si3N4 and Teflon are below 100kHz, but the PDMS vibrates with a wider bandwidth with the resonance occurs at MHz scope.

Decisions

pMUT design with polymer adhesive bed have been proposed specifically for wafer adhering fiction technique. Three piezoelectric stuffs have been successfully studied viz. PZT, ZnO and Quartz. PZT carry the highest charge response, nevertheless mechanical analysis revealed that PZT deficiency of stiffness ensuing in major warp when force per unit area is applied comparison to ZnO and Quartz. By holding the least charge response, quartz piezoelectric crystal besides less stiff compared to ZnO, ensuing in higher resonant frequence. In the center of it, ZnO produced adequate charge sums and vibrate reasonably due to applied cyclic force per unit area with mention to PZT and quartz. From frequence analysis, three resonant frequences were gettable from three different piezoelectric stuffs. At less than 100 kilohertzs, many underwater applications chasing such as deep incursion deepness sounding and low resolution-long scope imagination. For ZnO and Quartz with resonating frequences higher than 150 kilohertzs, high declaration imagination and obstruction turning away echo sounder seems the right applications. Furthermore, this survey has featured the use of thin Ni aluminium bronze movie as encapsulation bed, protecting the transducer from caustic sea H2O. Previously we have suggested the use of Ni aluminium bronzy metal as top electrodes, covering the pMUT from sea H2O [ 3 ] .

Meanwhile, the response of cMUT analysis concludes that there are advantages and disadvantages associated with each stuff to be implemented as a membrane. The usage of rubber-like stuff such as PDMS and Teflon increases the sensitiveness to external force per unit area signal, but unluckily these stuffs besides suffer from big inactive warp that significantly change the original construction of cMUT. So at high DC prejudice electromotive force, Si3N4 is still superior albeit the sensitiveness to the external force per unit area reduced by the order of two compared two PDMS. In order to obtain a good trade-off between the inactive and dynamic warp, other factors such as application and sensitiveness tolerance besides need to be considered. In frequence scope position, high resonance frequence of PDMS membrane makes it suited for higher frequence operation such as in imaging application. Si3N4 and Teflon ( at the design geometry ) are more suited for lower frequence operation such as in acoustic communicating and pilotage.

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