Characteristics Research of a High Sensitivity Piezoelectric MOSFET Acceleration Sensor
sensors
Letter Characteristics Research of a High Sensitivity Piezoelectric MOSFET Acceleration Sensor
Chunpeng Ai, Xiaofeng Zhao and Dianzhong Wen *
Key Laboratory of Electronics Engineering College of Heilongjiang Province, Heilongjiang University, Harbin 150006, China; [email protected] (C.A.); [email protected] (X.Z.) * Correspondence: [email protected]; Tel.: +86-0451-8660-8413
Received: 29 July 2020; Accepted: 1 September 2020; Published: 3 September 2020
Abstract: In order to improve the output sensitivity of the piezoelectric acceleration sensor, this paper proposed a high sensitivity acceleration sensor based on a piezoelectric metal oxide semiconductor field effect transistor (MOSFET). It is constituted by a piezoelectric beam and an N-channel depletion MOSFET. A silicon cantilever beam with Pt/ZnO/Pt/Ti multilayer structure is used as a piezoelectric beam. Based on the piezoelectric effect, the piezoelectric beam generates charges when it is subjected to acceleration. Due to the large input impedance of the MOSFET, the charge generated by the piezoelectric beam can be used as a gate control signal to achieve the purpose of converting the output charge of the piezoelectric beam into current. The test results show that when the external excitation acceleration increases from 0.2 g to 1.5 g with an increment of 0.1 g, the peak-to-peak value of the output voltage of the proposed sensors increases from 0.327 V to 2.774 V at a frequency of 1075 Hz. The voltage sensitivity of the piezoelectric beam is 0.85 V/g and that of the proposed acceleration sensor was 2.05 V/g, which is 2.41 times higher than the piezoelectric beam. The proposed sensor can effectively improve the voltage output sensitivity and can be used in the field of structural health monitoring.
Keywords: piezoelectric MOSFET; piezoelectric beam; acceleration sensor; high sensitivity
1. Introduction Micro-electro-mechanical system (MEMS) acceleration sensors, with the advantages of low cost, low power consumption, high compatibility with integrated circuit (IC) process and high integration [1–4], have a wide range of applications in automotive electronics, structural health monitoring, navigation and other fields [5–8]. MEMS acceleration sensors usually include piezoresistive [9,10], capacitive [11] and piezoelectric acceleration sensors [12,13]. Piezoresistive accelerometer usually consists of a deformable structure and varistors. Under the action of external acceleration, the deformation of the deformable structure causes the resistance of the varistor to change, thereby measuring the acceleration. The varistors are usually made into a Wheatstone bridge structure to improve the output sensitivity. The piezoresistive accelerometer has the advantages of good stability, wide measurement range, and is also limited by ambient temperature [14–16]. A capacitive acceleration sensor is composed of fixed plates and movable plates. The gap or area of the plate capacitor changes while the external acceleration is applied. The applied acceleration is obtained by measuring the change of capacitance. High sensitivity and zero frequency response are its advantages, while its disadvantages are high impedance and nonlinearity [17–19]. The working principle of the piezoelectric acceleration sensor is similar to that of the piezoresistive type, but the difference is that the piezoresistive material is replaced by piezoelectric material. Compared with piezoresistive and capacitive MEMS acceleration sensors, piezoelectric MEMS acceleration sensors have advantages of low power cost and high range
Sensors 2020, 20, 4988; doi:10.3390/s20174988 www.mdpi.com/journal/sensors Sensors 2020, 20, 4988 2 of 14 of applying frequency; at the same time, they are also limited by high output impedance, weak output signal and so on [20–23]. SensorsThe researchers 2020, 20, x FORare PEER committed REVIEW to improving the structure of the piezoelectric acceleration2 of 14 sensor so as to improve its performance, especially its sensitivity. For example, Jin Xie et al. present a have advantages of low power cost and high range of applying frequency; at the same time, they are MEMS piezoelectric in-plane resonant accelerometer with a two-stage microleverage mechanism. also limited by high output impedance, weak output signal and so on [20–23]. The sensitivityThe researchers of the device are committed is 28.4 Hz to/g improving and the relative the structure sensitivity of the piezoelectric is 201 ppm/ accelerationg at the base sensor frequency aroundso as 140.7 to improve kHz, which its performance, are 57% and especially 268% higher its sens thanitivity. previously For example, reported Jin Xie dataet al. present [24]. Qiang a MEMS Zou et al. reportedpiezoelectric novel single-in-plane and resonant tri-axis accelerometer piezoelectric-bimorph with a two-stage accelerometers microleverage that aremechanism. built on The parylene beamssensitivity with ZnO of thinthe device films. is A 28.4 highly Hz/g symmetric and the rela quad-beamtive sensitivity bimorph is 201 structure ppm/g at withthe base a single frequency proof mass is usedaround for tri-axis140.7 kHz, acceleration which are 57% sensing. and 268% The higher unamplified than previously sensitivities reported of the datax-axis, [24]. Qiangy-axis, Zou and etz -axis are 0.93,al. reported 1.13, and novel 0.88 single- mV and/g, respectivelytri-axis piezoelectric- [13]. Atbimorph the same accelerometers time, the researchersthat are built alsoon parylene studied the dopedbeams piezoelectric with ZnO thin materials films. A in highly order symmetric to improve quad-beam the piezoelectric bimorph structure properties with so a as single to improve proof the mass is used for tri-axis acceleration sensing. The unamplified sensitivities of the x-axis, y-axis, and sensitivity. Ramany et al. presented a nano-electro-mechanical systems accelerometer using undoped z-axis are 0.93, 1.13, and 0.88 mV/g, respectively [13]. At the same time, the researchers also studied zinc oxide nanorods and 1 wt. (Weight) %, 3 wt.% and 5 wt.% of vanadium-doped zinc oxide nanorods the doped piezoelectric materials in order to improve the piezoelectric properties so as to improve as anthe active sensitivity. layer. TheRamany highest et al. sensitivity presented ofa 3.528nano-electro-mechanical V/g was acquired systems for 5 wt.% accelerometer of vanadium-doped using zincundoped oxide with zinc maximum oxide nanorods output and voltages 1 wt. (Weight) of 2.30 %, V 3 and wt.% 2.9 and V at5 wt.% 9 Hz of resonant vanadium-doped frequency zinc and 1 g acceleration,oxide nanorods respectively as an active [25]. layer. The highest sensitivity of 3.528 V/g was acquired for 5 wt.% of vanadium-dopedIn this work, by zinc taking oxide advantage with maximum of the high output gate voltages sensitivity of 2.30 of MOSFET,V and 2.9 V in at order 9 Hz to resonant improve the outputfrequency sensitivity and 1 and g acceleration, reduce the respectively output impedance [25]. of piezoelectric acceleration sensors, we designed a piezoelectricIn this MOSFETwork, by taking acceleration advantage sensor of the (PMAS) high gate structure. sensitivity The of MOSFET, PMAS with in order high to sensitivity improve can be usedthe output in acceleration sensitivity monitoring and reduce underthe output special impedance frequency of piezoelectric vibration environments, acceleration sensors, such aswe health designed a piezoelectric MOSFET acceleration sensor (PMAS) structure. The PMAS with high monitoring on turning tools. sensitivity can be used in acceleration monitoring under special frequency vibration environments, 2. Basicsuch Structureas health monitoring and Operating on turning Principle tools.
2.1. Basic2. Basic Structure Structure and Operating Principle 2.1.The Basic structure Structure of PMAS is shown in Figure1a. It consists of a piezoelectric beam and an N-channel depletionThe MOSFET. structure The of PMAS piezoelectric is shown beam in Figure is a two-ended1a. It consists device: of a piezoelectric one endis beam connected and an withN- the sourcechannel of MOSFET depletion as MOSFET. the ground The piezoelectric terminal of beam PMAS, is a two-ended the other device: end is one connected end is connected with the with gate of MOSFETthe source to control of MOSFET the output as the ground current terminal of MOSFET, of PMAS, and the the other drain end of is MOSFET connected is with the the current gate of output terminalMOSFET of PMAS. to control The the direction output current of the of measured MOSFET, accelerationand the drain isof parallelMOSFET to is thethe currentz-axis asoutput shown in Figureterminal1a. The of accelerationPMAS. The direction applied of by the the measured vibration acceleration system is is reciprocating parallel to the up z-axis and as down shown along in the z-axis.Figure Therefore, 1a. The acceleration the output applied signal by of the the vibration piezoelectric system beam is reciprocating is a sinusoidal up and signal down withalong athe certain z-axis. Therefore, the output signal of the piezoelectric beam is a sinusoidal signal with a certain frequency. The N-channel depletion MOSFET is chosen to ensure that the MOSFET works in the triode frequency. The N-channel depletion MOSFET is chosen to ensure that the MOSFET works in the region. As shown in Figure1b, a load resistance RL is used in series with PMAS to convert the current triode region. As shown in Figure 1b, a load resistance RL is used in series with PMAS to convert the signalcurrent into asignal voltage into signal a voltage for signal output. for output.
(a) PMAS (b) VDD
Piezoelectric Beam MOSFET RL a
S G D z N+ N+ x PMAS y o
Current Output GND GND FigureFigure 1. Basic 1. Basic structure structure and testand circuittest circuit of piezoelectric of piezoelectric metal metal oxide oxide semiconductor semiconductor field field effect effect transistor (MOSFET)transistor acceleration (MOSFET) acceleration sensor (PMAS): sensor (a (PMAS):) basic structure; (a) basic structure; (b) test circuit.(b) test circuit.
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Sensors 2020, 20, x FOR PEER REVIEW 3 of 14 The structure of the piezoelectric beam is shown in Figure2a, which consists of a silicon cantilever beam substrateThe with structure a proof of mass the andpiezoelectric a piezoelectric beam multilayer is shown structure.in Figure Silicon2a, which cantilever consists beam of a substratesilicon is fabricatedcantilever bybeam lithography substrate and with inductive a proof coupled mass and plasma a piezoelectric (ICP) etching multilayer technology. structure. The piezoelectric Silicon multilayercantilever structure, beam substrate including is electrodesfabricated by (Pt lith topography electrodes and and inductive Pt/Ti composite coupled plasma bottom (ICP) electrode) etching and a ZnOtechnology. piezoelectric The piezoelectric layer, was deposited multilayer by structure, direct-current including (DC) electrodes and radio (Pt frequency top electrodes (RF) magnetronand Pt/Ti sputteringcomposite under bottom the electrode) optimized and parameters a ZnO piezoelectric [26], respectively. layer, was Then, deposited 5 wt% by Li-doped direct-current ZnO was (DC) used and as radio frequency (RF) magnetron sputtering under the optimized parameters [26], respectively. Then, the piezoelectric layer. In general, ZnO is an n-type semiconductor due to the defects of oxygen vacancy 5 wt% Li-doped ZnO was used as the piezoelectric layer. In general, ZnO is an n-type semiconductor and zinc interstitial atoms in the process of fabrication. After doping lithium as an acceptor impurity, due to the defects of oxygen vacancy and zinc interstitial atoms in the process of fabrication. After the resistivity of ZnO is increased, thereby enhancing its piezoelectric properties. The piezoelectric doping lithium as an acceptor impurity, the resistivity of ZnO is increased, thereby enhancing its beam is designed to be 9800 5800 500 µm3 in size. Figure2c shows the dimension of the cantilever piezoelectric properties. The× piezoelectric× beam is designed to be 9800 × 5800 × 500 μm3 in size. Figure beam. l , w and h are the length, width and height of the cantilever beam, respectively. lm, wm and 2c showsb b the dimensionb of the cantilever beam. lb, wb and hb are the length, width and height of the h are the length, width and height of the proof mass, respectively. l w h was designed to be m cantilever beam, respectively. lm, wm and hm are the length, width andb heightb b of the proof mass, 3 × ×3 6000 2400 80 µm , lm wm hm was designed to be 1000 27003 395 µm . After the piezoelectric respectively.× × lb × wb × hb× was designed× to be 6000 × 2400 × 80 μ×m , lm ×× wm × hm was designed to be 1000 beam× 2700 was × manufactured, 395 μm3. After it the was piezoelectric rigidly pasted beam on was the customizedmanufactured, test it printedwas rigidly circuit pasted board on (PCB), the andcustomized the piezoelectric test printed beam’s circuit electrodes board (PCB), were connected and the piezoelectric with the PCB beam’s plate electrodes electrodes were by aconnected chip press welderwith (KNS4526,the PCB plate Kullicke electrodes & So byffa, a Haifa,chip press Israel). welder (KNS4526, Kullicke & Soffa, Haifa, Israel).
(a) Top (b) (c) Electrode
Piezoelectric Layer lb Composite Front Bottom Electrode hb Isolated Layer wb Back Substrate hm Pt ZnO Ti wm
SiO2 Si lm
FigureFigure 2. Basic 2. Basic structure structure of piezoelectric of piezoelectric beam: beam: (a) explosion (a) explosion view; (view;b) front (b and) front back and side, back (c) dimensionside, (c) of thedimension cantilever of the beam. cantilever beam.
2.2.2.2. Operating Operating Principle Principle AsAs shown shown in Figurein Figure3a, the 3a, piezoelectric the piezoelectric beam beam does notdoes deform not deform without without external external force. Theforce. centers The of positivecenters of and positive negative and negative charges charges in the in piezoelectric the piezoelectric layer layer coincide coincide with with each each other;other; the the whole whole piezoelectricpiezoelectric layer layer is electricallyis electrically neutral neutral and and there there are are no no chargecharge outputs.outputs. The The PMAS PMAS also also has has a acertain certain outputoutput when when it it is is not not a ffaffectedected byby accelerationacceleration du duee to to the the existence existence of ofa conductive a conductive channel channel in the in N- the N-channelchannel depletiondepletion MOSFET even when when the the gate gate voltag voltagee is is 0. 0. When When external external acceleration acceleration is isapplied, applied, accordingaccording to to Newton’s Newton’s first first and and second second laws, laws, thethe proofproof massmass will produce forces forces opposite opposite to to the the accelerationacceleration direction direction due due to theto the inertia, inertia, which which can ca causen cause the the piezoelectric piezoelectric beam beam to beto be deformed. deformed. At At this this time, the bending creates the electric dipole moment in the piezoelectric layer, forcing the positive time, the bending creates the electric dipole moment in the piezoelectric layer, forcing the positive and negative charge centers to separate. The same amount of charges of different signs are generated and negative charge centers to separate. The same amount of charges of different signs are generated on the upper and lower surfaces of the piezoelectric layer. The generated charge is used as the gate on the upper and lower surfaces of the piezoelectric layer. The generated charge is used as the gate signal of the MOSFET, which can directly control the width of the channel, thereby controlling the signal of the MOSFET, which can directly control the width of the channel, thereby controlling the drain current of the MOSFET. drain current of the MOSFET. The stress analysis of the piezoelectric beam is shown in Figure 4. dZnO is the distance from the The stress analysis of the piezoelectric beam is shown in Figure4. d is the distance from ZnO thin film to the bottom of the Si substrate, dn is the distance from theZnO neutral plane of the d themultilayer ZnO thin structure film to the to bottomthe bottom of theof the Si substrate,Si substrate,n his1, h the2, …, distance h6 represent from the the thickness neutral planeof Si, SiO of the2, multilayerTi, Pt, ZnO structure and Pt tolayers, the bottom respectively. of the Si substrate, h1, h2, ... , h6 represent the thickness of Si, SiO2, Ti, Pt, ZnO and Pt layers, respectively.
Sensors 2020, 20, x FOR PEER REVIEW 4 of 14
Electrode
Piezoelectric layer
Sensors 2020, 20, 4988 Electrode 4 of 14 Sensors 2020, 20, x FOR PEER REVIEWV DD VDD 4 of 14
RL RL Vout Vout Electrode SDG SDG Piezoelectric (a) layer (b) N+ N+ z N+ N+ Electronics Electrode Electronics Depletion layerVDD x Depletion layer DD y V P-type substrate P-type substrate RL RL Vout Vout B B N-channel depletion N-channel depletion GND SDG GND SDG MOSFET MOSFET (a) (b) Figure 3. WorkingN principle+ of piezoelectricN+ PMAS:z (a) without external Nacceleration;+ (b) Nwith+ external acceleration. Electronics Electronics
Depletion layer x The main assumptions are as follows: (1) ysince the length andDepletion width layer are far greater than the thickness of the cantileverP-type substratebeam, it is considered that the cantileverP-type beam substrate has pure bending deformation and ignores theB shear stress; (2) the beam bending caused byB the residual stress is ignored; (3) the beamN-channel bending depletion caused by the residual stress is ignored;N-channel (4) it is depletion assumed that there is GND MOSFET GND MOSFET no relative sliding between each thin films; (5) the cantilever beam is in an open environment and thereFigure are no 3. 3. upperWorkingWorking and principle principlelower offixed piezoelectric of piezoelectricplates, so PMAS: the PMAS: influence (a) without (a) withoutof external air damping external acceleration; is acceleration; ignored (b) with [27–29]. (externalb) with acceleration.external acceleration.
The main assumptions are as follows: (1) since the length and width are far greater than the Cross Section A F thickness of the cantilever beam, it is xconsidered that the cantilever beam has pure bending deformation and ignores the shear stress; (2) the beam bending caused by the residualPt stress is ignored; (3) the beam bending caused by the residual stress is ignored; (4) it ish6 assumed that there is h5 no relative sliding between each thin films; (5) the cantilever beam is in an open environmentZnO and there are no upper and lower fixed plates, so the influence of air damping is ignoredh4 [27–29]. h3 dZnO Ti h2
Cross Section A Fh1 SiO2 dn x l z Pt h6 Si y x h5 ZnO FigureFigure 4.4. StressStress analysisanalysis diagramdiagram ofof piezoelectricpiezoelectric beambeam structure.structure.h4 h3 dZnO Ti TheAccording main assumptions to the Euler-Bernoulli’s are as follows: equation, (1) since when the length the free and end width of h arethe2 farcantilever greater thanbeam the is thicknesssubjected of to the a concentrated cantilever beam, transverse it is considered force F, thatthe stress the cantilever on the cross beam section has pure A of bending ZnO thin deformation film is: h1 SiO2 and ignores thed shearn stress; (2) the beam bending causedl by the residual stress is ignored; (3) the beam MFx⋅ bending caused by the residualσ stress=−=− isA z ignored;dd (4) it is assumed dd that there is no relative sliding(1) AII ZnO n ZnO n Si between each thin films; (5) the cantilevery beam isx in an open environment and there are no upper and lower fixed plates, so the influence of air damping is ignored [27–29]. where MA is the bending moment, I is the moment of inertia of the multilayer structure. AccordingThe equivalent to the Figurewidth Euler-Bernoulli’s 4.of Stress layer analysisi relative equation, diagram to Si when substrate of pi theezoelectric free is: end beam of the structure. cantilever beam is subjected to a concentrated transverse force F, the stress on the cross section A of ZnO thin film is: According to the Euler-Bernoulli’s equation, when the free end of the cantilever beam is MA F x subjected to a concentrated transverseσ = forced F, thed nstress= on· dthe crossdn section A of ZnO thin film is:(1) A I | ZnO − | I | ZnO − | ⋅ MFxA where MA is the bending moment,σ =−=−I is thedd moment of inertia dd of the multilayer structure. (1) AII ZnO n ZnO n where MA is the bending moment, I is the moment of inertia of the multilayer structure. The equivalent width of layer i relative to Si substrate is:
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The equivalent width of layer i relative to Si substrate is:
E = i = wi0 wSi, i 1, 2, ... , 6 (2) ESi where w0 i is the width of each thin film, Ei is the young’s modulus, i represents Si, SiO2, Ti, Pt, ZnO and Pt layers, respectively. Therefore, the equivalent sectional area Si can be expressed as:
S = w h , i = 1, 2, ... , 6 (3) i i · i The equivalent second moment of inertia of layer i is:
3 hi I0 = w , i = 1, 2, ... , 6 (4) i 12 i The distance from the i layer to the bottom of Si substrate is:
6 X h z = h i (5) i i − 2 i=1
The distance from the neutral plane of the multilayer structure to the Si substrate is:
P6 Sidi, i=1 z0 = (6) P6 Si i=1
The second moment of inertia is:
6 Xh 2i I0 = I + S (z z0) (7) i i i − i=1
Substituting Equations (6) and (7) into (1), the cross-section stress of ZnO is:
F x dZnO dn σ = · · | − | , 0 x l (8) P6 ≤ ≤ 6 3 6 ! Sidi, h wi P i + P hi i=1 Si hi 6 12 − 2 − P i=1 i=1 Si i=1
According to the Hooke’s law and the theories in mechanics of materials, a deflection δ will appear when a force F is applied at the free end of the cantilever beam, and the deflection δ of the free end can be expressed as [30,31]: F Fl 3 4Fl 3 δ = = b = b (9) k 3EI 3 Ewbhb where k is the stiffness of the cantilever beam. The fundamental resonant frequency is [32]: r α2 EI f = n (10) 2π mL4 Sensors 2020, 20, 4988 6 of 14
where E is the modulus of elasticity, I is the area moment of inertia, αn is a constant which its value depends on the mode of cantilever beam’s vibration, m is the mass of the cantilever beam, L is the length of the cantilever beam. Considering the influence of the mass proof on the resonant frequency of cantilever beam, the first mode resonant frequency of the cantilever beam can be expressed as [33–35]: s 1.875 2 0.236EI f = (11) 3 lm lm 2π (l lm/2) 0.236ρh w l + ρh w + ρhmwmlm b − b b b − 2 b b 2 where ρ is the density of Si. The moment of inertia of multilayer structure is ignored due to the thickness of the multilayer structure being much less than that of the cantilever beam. It can be seen that lc is inversely proportional to the f frequency of the cantilever beam, and the change of lc has a greater impact on resonant frequency. Based on the piezoelectric effect, the surface charge density of ZnO is [26]: D3 = d31σ (12) Therefore, under the external acceleration, the charges generated by the piezoelectric beam are:
R w R l q = b b D dxdy 0 0 3 R w R l = b b d dxdy 0 0 31σ R wb R lb d Fx d dn 31 ZnO− = | | 6 dxdy 0 0 P 6 3 6 ! Sidi, P h wi P h = i +S h i i 1 12 i i 2 6 i=1 i=1 − − P (13) Si i=1 2 w l d F d dn b b 31 ZnO− = | | 6 P 6 3 6 ! Sidi, P h wi P h = i +S h i i 1 2 12 i i 2 6 i=1 i=1 − − P Si i=1
The piezoelectric beam can be approximately considered as a parallel plate capacitor with a dielectric inside, so the output voltage VB of the piezoelectric beam is: q V = (14) B C In this case, due to the MOSFET works in the triode region, the relationship between source drain current IDS and gate voltage VGS can be expressed as follows [36]:
µnWCox 1 I = (V V )V V2 (15) DS L GS − T DS − 2 DS where, µn is dependent effective mobility, W is the channel width, L is the effective channel length, Cox is the insulation capacitance, VGS is the gate voltage which is equal to VB,VT is the threshold voltage. The output voltage of PMAS is:
Vout = V V = V I R (16) DD − R DD − DS L Therefore, when the piezoelectric beam is connected to the gate of MOSFET, the output voltage of the piezoelectric beam is equal to VGS. With Equations (13)–(16), the relationship between Vout and F is: µnWCox q 1 2 Vout = V V V V R (17) DD − L C − T DS − 2 DS L It can be seen from Equation (17) that the output voltage of the PMAS is proportional to the channel width-to-length ratio of the MOSFET. Sensors 2020, 20, x FOR PEER REVIEW 7 of 14
Therefore, when the piezoelectric beam is connected to the gate of MOSFET, the output voltage of the piezoelectric beam is equal to VGS. With Equations (13)–(16), the relationship between Vout and F is: μ WC =−noxq − −1 2 VVout DD VVVR T DS DS L (17) LC2
SensorsIt2020 can, 20 be, 4988 seen from Equation (17) that the output voltage of the PMAS is proportional to7 ofthe 14 channel width-to-length ratio of the MOSFET.
3. Fabrication Technology Figure5 5 shows shows the the fabrication fabrication process process of of the the piezoelectric piezoelectric beam. beam. The The n-type n-type< <100>100> orientation silicon wafer was cleaned by the standard Radio Cooperation of America (RCA) process (Figure 55a),a), and the silicon dioxide layer was grown by the thermalthermal oxidation method as the isolation layer (Figure(Figure5 5b).b).In In the the manufacturing manufacturing process process of of Pt Pt/ZnO/P/ZnO/Pt/Tit/Ti piezoelectric piezoelectric multilayer multilayer structure, structure, the the lift-o lift-ff processoff process is selected is selected to complete to complete the the fabrication fabrication of theof the piezoelectric piezoelectric multilayer multilayer structure. structure. As As shown shown in Figurein Figure5c–h, 5c–h, firstly, firstly, the photoresist the photoresist is evenly is evenly coated oncoated the surfaceon the ofsurface the substrate of the substrate layer, then layer, patterned then bypatterned photolithography. by photolithography. The Pt/Ti compositeThe Pt/Ti composite electrode electrode is coated is by coated RF magnetron by RF magnetron sputtering, sputtering, and the photoresistand the photoresist is removed is removed by the stripping by the stripping solution toso completelution to complete the fabrication the fabrication of the bottom of the electrode. bottom ZnOelectrode. layer ZnO and toplayer electrode and top wereelectrode also preparedwere also byprepared the lift-o byff theprocess lift-off as process the Pt/Ti as bottom the Pt/Ti electrode. bottom Afterelectrode. that, After the cantilever that, the cantilever structure isstructure released is byreleased twice photolithographyby twice photolithography and inductively and inductively coupled plasmacoupled (ICP)plasma etching (ICP) toetching complete to complete the fabrication the fabrication of the piezoelectric of the piezoelectric beam (Figure beam (Figure5i–j). In 5i–j). order In toorder improve to improve the piezoelectric the piezoelectric properties properties of the ZnOof the piezoelectric ZnO piezoelectric layer, we laye dopedr, we ZnO doped with ZnO lithium. with Lilithium. atoms Li with atoms a small with atomica small radiusatomic asradius acceptor as acceptor impurities impurities can increase can increase the resistivity the resistivity of the of ZnO the piezoelectricZnO piezoelectric thin film thin and film increase and increase the output the impedance,output impedance, thereby achieving thereby achieving the purpose the of purpose improving of theimproving piezoelectric the piezoelectric properties of properti ZnO. Duringes of ZnO. the preparation During the process, preparat mostion process, of the doped most lithium of the doped atoms willlithium replace atoms the will positions replace of thethe zincpositions atoms of and the cause zinc the atoms decrease and ofcause the latticethe decrease constant. of Asthe a result,lattice theconstant. residual As stressa result, in ZnOthe residual thin film stress is compressive in ZnO thin stress film [is37 compressive]. stress [37].
(a) (b) (c) (d)
(e) (f) (g) (h)
Si SiO2 Photoresist z y x Ti Pt ZnO (i) (j) o
Figure 5.5. Fabrication process of the piezoelectricpiezoelectric beam: ( a) Si wafer;wafer; ((b)) growinggrowing SiOSiO22;(; (c) coatingcoating photoresist; (d) patterningpatterning photoresist; (e) depositing PtPt/Ti;/Ti; (ff)) removingremoving photoresist;photoresist; ((g)) depositingdepositing ZnO; (h) depositing Pt;Pt; ((ii)) etchingetching onon thethe toptop side;side; ((jj)) releasingreleasing cantilevercantilever beam.beam. 4. Results and Discussion 4. Results and Discussion 4.1. Test System 4.1. Test System Figure6 shows the test system for the proposed acceleration sensor. It consisted of a standard vibrator Figure 6 shows the test system for the proposed acceleration sensor. It consisted of a standard (Dongling ESS-050, Dongling Vibration Test Instrument Co., Ltd., Suzhou, China), an oscilloscope (DSO-X vibrator (Dongling ESS-050, Dongling Vibration Test Instrument Co., Ltd., Suzhou, China), an 4154A, Agilent Technologies Inc., Santa Clara, CA, USA), a semiconductor characteristic analysis system (4200SCS, KEITHLEY 4200, Keithley, Cleveland, OH, USA) and a control computer. The system can apply acceleration from 0 to 30 G; the lower limit of the frequency is 50 Hz, and the upper limit is 20,000 Hz. Sensors 2020, 20, x FOR PEER REVIEW 8 of 14 oscilloscope (DSO-X 4154A, Agilent Technologies Inc., Santa Clara, CA, USA), a semiconductor characteristic analysis system (4200SCS, KEITHLEY 4200, Keithley, Cleveland, OH, USA) and a Sensorscontrol2020 computer., 20, 4988 The system can apply acceleration from 0 to 30 G; the lower limit of the frequency8 of 14 is 50 Hz, and the upper limit is 20,000 Hz.
Figure 6. Testing system of acceleration sensor.sensor. 4.2. Frequency Characteristic of the Piezoelectric Beam 4.2. Frequency Characteristic of the Piezoelectric Beam The frequency characteristic of the piezoelectric beam was analyzed by the sweep mode of the The frequency characteristic of the piezoelectric beam was analyzed by the sweep mode of the vibrator. The range of the excitation frequency was set from 20 to 2000 Hz, and applied acceleration vibrator. The range of the excitation frequency was set from 20 to 2000 Hz, and applied acceleration was 1 g constant along the z-axis direction of the piezoelectric beam. The piezoelectric beam was rigidly was 1 g constant along the z-axis direction of the piezoelectric beam. The piezoelectric beam was connected with the vibration table by a customized fixture. When the excitation frequency reached a rigidly connected with the vibration table by a customized fixture. When the excitation frequency certain value, the output of the piezoelectric beam reached its maximum value for the first time. At this reached a certain value, the output of the piezoelectric beam reached its maximum value for the first time, the excitation frequency was the first resonance frequency of the piezoelectric beam. Figure7a time. At this time, the excitation frequency was the first resonance frequency of the piezoelectric shows the relationship between the output voltage and the excitation frequency of the piezoelectric beam. Figure 7a shows the relationship between the output voltage and the excitation frequency of beam. When the excitation frequency reached 1072 Hz, the output of the piezoelectric beam reached the piezoelectric beam. When the excitation frequency reached 1072 Hz, the output of the theSensors maximum 2020, 20, x value FOR PEER of 0.649 REVIEW V. There is only one peak within 50–2000 Hz, which proves that 10729 of Hz 14 piezoelectric beam reached the maximum value of 0.649 V. There is only one peak within 50–2000 is the first-order resonance frequency of the piezoelectric beam. Hz, which proves that 1072 Hz is the first-order resonance frequency of the piezoelectric beam. According to Figure 7a, the quality factor of the piezoelectric beam can be roughly analyzed as (a) @ 1 g (b) f =1072 Hz@ 1 g shown in Figure0.6 7b. The quality factor1072 Hz Q represents the0.6 energy dissipated by the body in overcoming the internal friction in resonance. -3 dB 0.4 Es 0.4 f Q ==2π f f2 − 1 (18) Ec ff12
0.2 where Es is 0.2the mechanical energy stored by the oscillator in the resonant state and Ec is the energy Output voltage (V) voltage Output dissipated Output voltage (V) by the oscillator in the resonant state every cycle, f is the resonance frequency and f1, f2 is the half power point frequency (−3 dB). From Figure 5b we can estimate that Q is 529.1. 0.0 0.0 Table 1 shows0 the 500design 1000 dimensions 1500 2000of piezoelectric1060 beams. 1065 By 1070 substituting 1075 1080the data 1085 in Table 1 Frequency (Hz) Frequency (Hz) into Equation (10), the theoretical resonance freque ncy of the piezoelectric beam is 995 Hz, which is lowerFigure than 7.theRelationship test result curve1072 betweenHz. This output is mainly voltage due and to excitation the deviation frequency: of the (a )thickness resonant frequency; and length of Figure 7. Relationship curve between output voltage and excitation frequency: (a) resonant frequency; the cantilever(b) quality beam factor. from the design value during the manufacturing process. (b) quality factor.
According to FigureTable7 1.a, Theoretical the quality calculation factor of paramete the piezoelectricrs of resonance beam frequency. can be roughly analyzed as 4.3. IDS-VDS Characteristic of MOSFET with Piezoelectric Beam shown in Figure7b. The quality factor Q represents the energy dissipated by the body in overcoming E ρ lb wb hb lm wm hm the internalThe I-V friction characteristic in resonance. and transferπ characteristic curves of the MOSFET were tested by 4200SCS, (GPa) (Kg/m3) (μm) E(μm) f(μm) (μm) (μm) (μm) as shown in Figure 8a. Considering that Qthe= maximum2π s = limited current of 4200SCS is 0.1 A, the voltage(18) 190 2330 3.14 6000 Ec2400 f 80f 1000 2700 395 range of VGS was set from −2.5 V to −0.5 V with an increment1 − 2 of −0.5 V. Figure 8b shows the transition wherecharacteristicEs is the curve mechanical of MOSFET, energy V storedDS is 5 V by in the testing. oscillator It can in be the concluded resonant statethat the and pinch-offEc is the energyvoltage dissipatedof MOSFET by (V theGS(off) oscillator) is −2.5 inV; thewith resonant the increase state everyof VGS cycle,, the IDSf isalso the increases. resonance When frequency VGS reaches and f 1, −f0.452 is theV, the half limit power current point of frequency the test instrument ( 3 dB). From 0.1 A Figure is reached.5b we can estimate that Q is 529.1. − Table1 shows the design dimensions of piezoelectric beams. By substituting the data in Table1 into Equation (10), the theoretical resonance frequency of the piezoelectric beam is 995 Hz, which is V = - 0.5 V 0.10 lower than0.10 the(a) test result 1072 Hz. ThisGS is mainly due to the deviation(b) of the thickness and length of the cantilever beam from the design value during the manufacturing@V = 5 V process. 0.08 0.08 DS limited current = 0.1 A
0.06 0.06 (A) (A)
VGS = - 1.0 V DS DS
I 0.04 I 0.04
0.02 V = - 1.5 V 0.02 GS VGS(off) = -2.5 V VGS = - 2.0 V 0.00 0.00 VGS = - 2.5 V 0.0 1.0 2.0 3.0 4.0 5.0 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 VDS (V) V (V) GS
Figure 8. Characteristics curves of the MOSFET: (a) IDS-VDS characteristics curves; (b) transition characteristic curve.
The test circuit shown in Figure 1b, 4200SCS, is connected with two output terminals of PMAS for data acquisition. The output voltage of the piezoelectric beam was used as gate voltage of MOSFET. In order to ensure that the output voltage of the piezoelectric beam can reach the maximum value, the test frequency was set at 1072 Hz, which is the resonance frequency of the piezoelectric beam. The I-V characteristic curves of the MOSFET are shown in Figure 9 (VDD = 5 V, RL = 10 kΩ and applied acceleration was 1.5 g). Under the external acceleration generated by the stander vibration system, the proof mass drove the piezoelectric beam to vibrate up and down, which makes the IDS of MOSFET fluctuate in a certain range. The maximum and minimum values of the wave range correspond to the maximum deformation of upward and downward bending of the piezoelectric beam, respectively. When VDS is a certain value, the difference between the upper and the lower limit is ΔIDS. With the increase of external excitation acceleration, the vibration amplitude of the piezoelectric beam increases and further increases the width of the curves. That means ΔIDS will increase with as acceleration increases. Because the signal produced by the piezoelectric beam is
Sensors 2020, 20, 4988 9 of 14
Table 1. Theoretical calculation parameters of resonance frequency.
E ρ l w h l w h π b b b m m m (GPa) (Kg/m3) (µm) (µm) (µm) (µm) (µm) (µm) 190 2330 3.14 6000 2400 80 1000 2700 395
4.3. IDS-VDS Characteristic of MOSFET with Piezoelectric Beam The I-V characteristic and transfer characteristic curves of the MOSFET were tested by 4200SCS, as shown in Figure8a. Considering that the maximum limited current of 4200SCS is 0.1 A, the voltage range of V was set from 2.5 V to 0.5 V with an increment of 0.5 V. Figure8b shows the transition GS − − − characteristic curve of MOSFET, VDS is 5 V in testing. It can be concluded that the pinch-off voltage of MOSFET (V ) is 2.5 V; with the increase of V , the I also increases. When V reaches GS(off) − GS DS GS 0.45 V, the limit current of the test instrument 0.1 A is reached. −
0.10 V = - 0.5 V (b) 0.10 (a) GS @V = 5 V 0.08 DS limited current = 0.1 A 0.08
0.06 0.06 (A) (A)
VGS = - 1.0 V DS DS
I 0.04 I 0.04
0.02 V = - 1.5 V GS 0.02 V = -2.5 V V = - 2.0 V GS(off) 0.00 GS VGS = - 2.5 V 0.00 0.0 1.0 2.0 3.0 4.0 5.0 V (V) -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 DS V (V) GS
Figure 8. Characteristics curves of the MOSFET: (a) IDS-VDS characteristics curves; (b) transition
characteristic curve.
The test circuit shown in Figure1b, 4200SCS, is connected with two output terminals of PMAS for data acquisition. The output voltage of the piezoelectric beam was used as gate voltage of MOSFET. In order to ensure that the output voltage of the piezoelectric beam can reach the maximum value, the test frequency was set at 1072 Hz, which is the resonance frequency of the piezoelectric beam. The I-V characteristic curves of the MOSFET are shown in Figure9( V = 5 V, R = 10 kΩ and applied DD L acceleration was 1.5 g). Under the external acceleration generated by the stander vibration system, the proof mass drove the piezoelectric beam to vibrate up and down, which makes the IDS of MOSFET fluctuate in a certain range. The maximum and minimum values of the wave range correspond to the maximum deformation of upward and downward bending of the piezoelectric beam, respectively. When VDS is a certain value, the difference between the upper and the lower limit is ∆IDS. With the increase of external excitation acceleration, the vibration amplitude of the piezoelectric beam increases and further increases the width of the curves. That means ∆IDS will increase with as acceleration increases. Because the signal produced by the piezoelectric beam is sinusoidal under the action of the stander vibration system, ∆IDS can be used as an important parameter to measure the output sensitivity of PMAS. In the test circuit of Figure1b, by selecting appropriate load resistance RL, PMAS can amplify the signal of the piezoelectric beam so as to improve the sensitivity of output voltage.
1
Sensors 2020, 20, x FOR PEER REVIEW 10 of 14 sinusoidalSensors 2020 under, 20, x FOR the PEER action REVIEW of the stander vibration system, ΔIDS can be used as an important10 of 14 parameter to measure the output sensitivity of PMAS. In the test circuit of Figure 1b, by selecting appropriatesinusoidal loadunder resistance the action R Lof, PMASthe stander can amplifyvibration the system, signal Δ ofIDS thecan piezoelectricbe used as an beam important so as to improveparameter the sensitivityto measure ofthe outpu outputt voltage. sensitivity of PMAS. In the test circuit of Figure 1b, by selecting Sensorsappropriate2020, 20, 4988load resistance RL, PMAS can amplify the signal of the piezoelectric beam so as10 to of 14 improve the sensitivity of output voltage. 0.10 Maximum deformation of @1.5g IDS upward bending 0.080.10 Maximum deformation of @1.5g ΔI upward bending 0.060.08 DS
(A) Piezoelectric device DS 0.06 Δ I 0.04 IDS (A) Piezoelectric device
DS Δ Δ I 0.020.04 I DS(Max)IDS I I Maximum deformation of 0.000.02 DS(Min)DS(Max) I Maximumdownward deformation bending of 0.000.0 0.5 1.0 1.5DS(Min)2.0 2.5 3.0 downward bending 0.0 0.5 1.0VDS 1.5(V) 2.0 2.5 3.0 V (V) Figure 9. IDS-VDSD characteristicsS curves of the MOSFET with the piezoelectric beam.
FigureFigure 9. 9.I IDS-V-VDS characteristics curves curves of of the the MOSFET MOSFET with with the the piezoelectric piezoelectric beam. beam. 4.4. Sensitivity CharacteristicDS DS of PMAS 4.4.4.4. Sensitivity Sensitivity Characteristic Characteristic ofof PMASPMAS Figure 10a shows the output voltage curves of the piezoelectric beam and PMAS, where the peak-FiguretoFigure-peak 10 10avaluea shows shows of the the the output outputoutput sinusoidal voltagevoltage curves curves of of arethe the piezoelectricdefined piezoelectric as output beam beam andvoltage and PMAS, PMAS, Vpiezo where and where VthePMAS the . piezo PMAS Vpeak-to-peakpiezopeak-to-peak increased value valuefrom of 0.287of the the outputV output to 1.314 sinusoidal sinusoid V at theal curvescurves excitation areare defined definedfrequency asas output outputof 1072 voltagevoltage Hz, and V piezotheand accelerationand VVPMAS. . Vpiezo increased from 0.287 V to 1.314 V at the excitation frequency of 1072 Hz, and the acceleration Vrangepiezo waincreaseds from 0.2 from to 0.2871.4 g. VIt tocan 1.314 be concluded V at the excitation that Vpiezo frequency increases ofwith 1072 the Hz, increment and the of acceleration excitation range was from 0.2 to 1.4 g. It can be concluded that Vpiezo increases with the increment of excitation acceleration,range was from and 0.2 the to relationship 1.4 g. It can between be concluded them is that approximatelyVpiezo increases linear. with Figure the increment 10b shows of the excitation output acceleration, and the relationship between them is approximately linear. Figure 10b shows the output voltageacceleration, curve and of the the PMAS relationship (VPMAS between). In the conditions them is approximately of excitation linear. frequency Figure of 101072b shows Hz and the applied output voltage curve of the PMAS (VPMAS). In the conditions of excitation frequency of 1072 Hz and applied accelevoltageration curve range of the from PMAS 0.2 (toV PMAS1.4 g,). V InPMAS the increased conditions from of excitation0.327 V to frequency 2.744 V. Compared of 1072 Hz with and V appliedpiezo, at acceleration range from 0.2 to 1.4 g, VPMAS increased from 0.327 V to 2.744 V. Compared with Vpiezo, at theacceleration same condition range from VPMAS 0.2 toincrease 1.4 g, dV PMASsignificantlyincreased and from VPMAS 0.327 was V to approximately 2.744 V. Compared linear with withVpiezo the, the same condition VPMAS increased significantly and VPMAS was approximately linear with the excitationat the same acceleration. condition V increased significantly and V was approximately linear with the excitation acceleration. PMAS PMAS excitation acceleration.
2.0 2.0 2.0 1.4 g (a) 2.0 1.4 g (b) 2.744 V 1.4 g (a) 1.3141.314 V 1.4 g (b) 2.744 V 0.00.0 0.00.0
2.02.0 2.02.0 1.1 g 1.1 g 2.104 V 1.1 g 1.1411.141 V 1.1 g 2.104 V 0.00.0 0.00.0
2.02.0 2.02.0 0.8 0.8 g g 0.8 0.8 g g 0.9120.912 V 1.5181.518 V V 0.00.0 0.00.0
2.02.0 2.02.0 0.5 0.5 g g 0.5 0.5 g g 0.8820.882 V V 0.6080.608 V Output Voltage (V) Voltage Output
Output Voltage (V) Voltage Output Output Voltage (V) Voltage Output 0.00.0 (V) Voltage Output 0.00.0
2.02.0 2.02.0 0.2 0.2 g g 0.2 g 0.327 V 0.287 V 0.2 g 0.327 V 0.0 0.287 V 0.0 0.00.0
-0.005 0.000 0.005 -0.005 0.000 0.005 -0.005 Time0.000 (s) 0.005 -0.005 0.000 0.005 Time (s) Time (S) Time (S) FigureFigure 10. 10.Output Output characteristics characteristics of of the the piezoelectric piezoelectric beam beam and and PMA PMA under under the the acceleration acceleration range range from Figure 10. Output characteristics of the piezoelectric beam and PMA under the acceleration range 0.2from g to 0.2 1.4 g g: to ( a1.4) output g: (a) output voltage voltage curves curves of piezoelectric of piezoelectric beam; beam; (b) output (b) output voltage voltage of PMAS. of PMAS. from 0.2 g to 1.4 g: (a) output voltage curves of piezoelectric beam; (b) output voltage of PMAS. According to Equation (13), the theoretical output voltage of the piezoelectric beam is 2.110 V under the acceleration condition of 1 g, which is higher than the actual output voltage of 1.141 V. The reason is that the thickness of the cantilever beam deviates from the theoretical value due to the uniformity error of ICP etching. The thickness of the cantilever beam is not uniform, which leads to the uneven distribution of stress when the cantilever beam is forced to bend. At the same time, there will be defects inside the ZnO piezoelectric thin film during the manufacturing process, which causes the charge generated by the stress to be less than the theoretical value. Sensors 2020, 20, x FOR PEER REVIEW 11 of 14
According to Equation (13), the theoretical output voltage of the piezoelectric beam is 2.110 V under the acceleration condition of 1 g, which is higher than the actual output voltage of 1.141 V. The reason is that the thickness of the cantilever beam deviates from the theoretical value due to the uniformity error of ICP etching. The thickness of the cantilever beam is not uniform, which leads to Sensorsthe uneven2020, 20 distribution, 4988 of stress when the cantilever beam is forced to bend. At the same time,11 there of 14 will be defects inside the ZnO piezoelectric thin film during the manufacturing process, which causes the charge generated by the stress to be less than the theoretical value. Figure 1111 showsshows thethe comparisoncomparison of ofV VPMASPMAS and VVpiezo withwith the the increase increase of of acceleration. acceleration. It It can be seen thatthat underunder the the same same external external acceleration, acceleration, the th outpute output voltage voltage of PMAS of PMAS is significantly is significantly higher higher than thatthan ofthat piezoelectric of piezoelectric beam. beam. It means It means that MOSFETthat MOSFET in PMAS in PMAS plays plays a role a ofrole amplification. of amplification. After After that, twothat, groups two groups of output of output curves curves are fitted are linearly,fitted linearl andy, it canand beit can concluded be concluded that the that two the groups two groups of curves of havecurves good have linearity good linearity and the and slopes the ofslopes the fitting of the curves fitting arecurves the outputare thevoltage output sensitivityvoltage sensitivity of the two of devices.the two Thedevices. sensitivity The sensitivity of PMAS of is 2.05PMAS V/g, is which 2.05 V/g, is 2.41 which times is higher2.41 times than higher that of thethan piezoelectric that of the beampiezoelectric at 0.85 Vbeam/g. It at proved 0.85 V/g. that It the proved PMAS that structure the PMAS can eff ectivelystructure improve can effectively the sensitivity improve of the piezoelectricsensitivity of beam.the piezoelectric The charge beam. output The of charge the piezoelectric output of the beam piezoelectric as the gate beam control as the voltage gate control of the MOSFETvoltage of can the e MOSFETffectively convertcan effectively the output convert charge the into output an output charge current, into an andoutput it can current, also increase and it can the loadalso increase capacity the of theload piezoelectric capacity of the acceleration piezoelectri sensor.c acceleration However, sensor. the proposed However, sensor the proposed has a relatively sensor narrowhas a relatively frequency narrow range, frequency which limits rang itse, which application limits field. its application In addition, field. it can In addition, be seen from it can Equation be seen (14)from that Equation the width-to-length (14) that the width-to-length ratio of the MOSFET ratio isof directlythe MOSFET proportional is directly to theproportional output current. to the The outputIDS cancurrent. be improved The IDS bycan adjusting be improved the aspect by adjusting ratio of the the MOSFET aspect ratio during of thethe manufacturing MOSFET during process, the therebymanufacturing further process, improving thereby the output further sensitivity improving of the PMAS.output sensitivity of the PMAS.
3.0 PMAS piezoelectric device 2.5 linear fitting of red dots linear fitting of black dots 2.0
1.5 Slope=2.05
1.0 Slope=0.85 Output Voltage (V) Output 0.5
0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Acceleration (g)
FigureFigure 11. Relationship11. Relationship curves curves between between output voltageoutput and volt accelerationage and acceleration of PMAS and of the PMAS piezoelectric and beam.the piezoelectric beam. Table2 shows the performance comparison of piezoelectric acceleration sensors. The proposed sensorTable has 2 certain shows advantages the performance in sensitivity, comparison but doesof piez notoelectric dominate acceleration in terms ofsensors. chip size. The Itproposed can also besensor seen has from certain the comparison advantages that in se piezoelectricnsitivity, but acceleration does not dominate sensors in have terms higher of chip sensitivity, size. It can but also at a disadvantagebe seen from the in terms comparison of measurement that piezoelectric range and acceleration load capacity. sensors have higher sensitivity, but at a disadvantage in terms of measurement range and load capacity. Table 2. Performance comparison of the proposed sensor with others.
TransductionTable Material 2. PerformanceDie comparison Area (mm of2) the proposedSensitivity sensor with (V/g) others. Reference
Piezoelectric Li doped ZnO 9.8 5.82 2.05 at resonance frequency this work Transduction Material Die Area× (mm ) Sensitivity (V/g) Reference Piezoelectric AlN 2.3 2.3 0.355 [38] Piezoelectric Li doped ZnO 9.8 × 5.8 2.05 at resonance frequency this work Piezoelectric V doped ZnO - 3.528 at resonance frequency [25] PiezoelectricPiezoelectric V doped AlN ZnO 2.3 × - 2.3 1.9 at resonance 0.355 frequency [ [38]39] piezoresistivePiezoelectric V doped Si ZnO 3.5 - 3.5 3.528 at resonance 0.004 frequency [ [25]40] × piezoresistivePiezoelectric V doped Si ZnO 2 - 2 1.9 at resonance 0.106 frequency [ [39]41] × piezoresistive Si 3.5 × 3.5 0.004 [40] 5. Conclusionspiezoresistive Si 2 × 2 0.106 [41]
In summary, this paper proposed a high sensitivity piezoelectric acceleration sensor. It consisted of a piezoelectric beam and an N-channel depletion MOSFET. Utilizing the advantage of MOSFET’s high input impedance, the output signal of the piezoelectric beam was used to drive the MOSFET, thereby converting the output charge of the piezoelectric beam into output current, and improving the sensitivity of the piezoelectric acceleration sensor. The results show that the resonance frequency Sensors 2020, 20, 4988 12 of 14 of the piezoelectric beam was 1072 Hz and the sensitivity of the proposed sensor was 2.05 V/g at the resonance frequency, which is 2.41-times higher than that of the piezoelectric beam. This research provides a good foundation for the integration of piezoelectric MOSFETs in the future. In future work, continuing to improve the sensitivity of piezoelectric acceleration sensors is an important research direction for us. We will improve the output sensitivity of the PMAS by optimizing the width-to-length ratio of MOSFETs and improving the manufacturing process of piezoelectric materials, while at the same time optimizing the cantilever beam structure to increase its frequency range.
Author Contributions: C.A. and X.Z. wrote the manuscript; X.Z. and D.W. designed the project; C.A. performed the experiments; C.A. and X.Z. contributed to the data analysis. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the National Natural Science Foundation of China (No. 61971180), project of the central government supporting the reform and development of local colleges and universities. Basic Scientific Research Project of the Provincial Higher University in Heilongjiang Province (KJCXZD201705). Conflicts of Interest: The authors declare no conflict of interest.
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