A Robust Fully-Integrated Digital-Output Inductive CMOS-MEMS Accelerometer with Improved Inductor Quality Factor

micromachines

Article A Robust Fully-Integrated Digital-Output Inductive CMOS-MEMS Accelerometer with Improved Inductor Quality Factor

Yi Chiu 1,* , Hsuan-Wu Liu 2 and Hao-Chiao Hong 1

1 Department of Electrical and Computer Engineering, National Chiao Tung University, Hsin Chu 300, Taiwan; [email protected] 2 Taiwan Semiconductor Manufacturing Company, Hsin Chu 300, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-3573-1838

Received: 26 October 2019; Accepted: 11 November 2019; Published: 18 November 2019

Abstract: This paper presents the design, fabrication, and characterization of an inductive complementary metal oxide semiconductor micro-electromechanical systems (CMOS-MEMS) accelerometer with on-chip digital output based on LC oscillators. While most MEMS accelerometers employ capacitive detection schemes, the proposed inductive detection scheme is less susceptible to the stress-induced structural curling and deformation that are commonly seen in CMOS-MEMS devices. Oscillator-based frequency readout does not need analog to digital conversion and thus can simplify the overall system design. In this paper, a high-Q CMOS inductor was connected in series with the low-Q MEMS sensing inductor to improve its quality factor. Measurement results showed the proposed device had an oﬀset frequency of 85.5 MHz, sensitivity of 41.6 kHz/g, noise ﬂoor of 8.2 mg/√Hz, bias instability of 0.94 kHz (11 ppm) at an average time of 2.16 s, and nonlinearity of 1.5% full-scale.

Keywords: CMOS-MEMS; accelerometer; inductive; inductor; sprinductor; LC tank; oscillator; quality factor; digital output

1. Introduction Micro-electromechanical systems (MEMS) accelerometers have been widely used in consumer electronic devices and automobiles. Typical MEMS accelerometers can be classified as capacitive [1–4], piezoresistive [5,6], convective [7], and inductive [8–10] types. Among these different types, the capacitive accelerometers have been attracting the most focus in both industry and academia due to their high sensitivity, insensitivity to temperature, and compatibility to the MEMS and complementary metal oxide semiconductor (CMOS) fabrication processes. Due to their miniature dimensions and, thus, their small signal levels, it is desirable to place the readout circuits close to the sensors to reduce the adverse effects of parasitics and noise. Monolithic integration of sensors and readout circuits on the same chip by using commercial CMOS processes is one of the best techniques to integrate CMOS circuits and MEMS sensors. In a monolithically integrated CMOS-MEMS capacitive accelerometer, the sensing capacitor is composed of a movable and a fixed electrode. Both electrodes are typically realized by stacking multiple metal and oxide layers of the backend CMOS processes [1–4]. Such structures are prone to deformation and curling due to the residual stress in the deposited films. The deformation of electrodes will change either the overlapping area or the gap between the electrodes and introduce uncertainty in its capacitance value and sensitivity. To overcome this problem, curl compensation frames [1] or pure oxide structures [2] were proposed. However, the deformation and curling of the sensor structures still could not be fully eliminated [2].

Micromachines 2019, 10, 792; doi:10.3390/mi10110792 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 792 2 of 12

To alleviate the effect of residual stress on the sensor structure and its performance in the presence of structural deformation, this paper presents an inductive CMOS-MEMS accelerometer employing on-chip planar variable inductors as sensing elements. The main difference between inductors and capacitors as sensing elements is that capacitors have two physically distinct electrodes whereas inductors have only one coil loop. Therefore, stress-induced deformation changes only the curvature of the planar inductor coils, while the inductor areas and wire spacing remain largely constant. Therefore, the inductance of the planar coils and thus the characteristics of the sensor are more robust and consistent even in the presence of deformation. CMOS-based inductive sensing has been applied to proximity sensing [11–13], magnetic biosensing [14], and tactile sensing [15]. The sensing inductors in these studies were either on-chip CMOS inductors [12–14], on-chip electroplated inductors [11], or off-chip printed circuit board (PCB) inductors [12,13]. The readout circuits were custom-designed on-chip CMOS circuitry [11–13,15] or a commercially available off-chip inductance conversion integrated circuit (IC) [14]. All of the above sensors are some forms of proximity sensors, i.e., they detect the distance between the sensing coil and another conducting or ferromagnetic object while the sensing coil remains fixed and undeformed. The requirement of two objects (coil and another conductor) or additional ferromagnetic materials in the sensors reduces the compatibility for monolithic CMOS integration. In Liao’s research [8], bond wires and their parasitic inductance were used with on-chip circuits for acceleration detection. Although bond wires are readily available in standard packaged ICs, and no special materials are needed, the wire bonding process is much less precise than the CMOS process. Therefore, the variation of the sensor characteristics can be quite large. Inductive CMOS-MEMS accelerometers with integrated on-chip deformable inductors were first demonstrated in our previous works [9,10]. These are true monolithically integrated devices with no additional non-CMOS materials. The deformable sensing inductors are fabricated using standard CMOS processes with much higher precision compared to the bond wire inductors. Oscillator-based frequency readout was employed so that analog to digital conversion was not needed, and, thus, the overall system design could be simplified. However, it was found that the sensor performance could be limited by the low quality factors (Q factors) of the single-turn MEMS inductors in the LC-oscillator based readout design. This paper presents a new circuit design where a high-Q CMOS inductor is connected in series with the MEMS sensing inductor to improve the overall quality factor and, thus, the sensor performance. The proposed device was designed and fabricated by using a commercial 0.18 µm CMOS process plus post-CMOS release processing. The operation principle of the inductive accelerometer, the analysis of the effects of residual stress on the MEMS structures, the MEMS structure and CMOS circuit design, the CMOS-MEMS fabrication, and the sensor characterization are discussed in the following sections.

2. Operation Principle The schematic of the inductive CMOS-MEMS accelerometer is shown in Figure1[ 10]. A proof mass, m, is suspended by four springs, k, and anchored to the substrate (Figure1a). Embedded in the springs are metal routings forming closed electrical loops and effective inductors, L. Therefore, the springs and inductors share the same physical structures, and they are thus termed ‘sprinductors.’ Upon the action of an external acceleration, a, the inertial force, ma, deforms the sprinductors. In Figure1b, the sprinductor is compressed and the surface area enclosed by the planar inductor coil is reduced. Since the inductance of a coil is related to the total enclosed magnetic flux when a current passes through it, the planar sprinductor with reduced area thus has a reduced inductance. Similarly, when the sprinductor is stretched by the inertial force, as shown in Figure1c, its area and inductance are increased. If the deformation is small, the inductance change, ∆L, is approximately proportional to the external acceleration, a. Such sprinductors can be arranged on the two sides of the proof mass so that they undergo differential change of inductance, ∆L, when acted upon by the external acceleration. ± Such differential inductors can be used in differential readout circuits to enhance signals and suppress common-mode noise. Micromachines 2019, 10, 792 3 of 12 Micromachines 2019, 10, x 3 of 12

L L-L L+L

sprinductor

ma ma (a) (b) (c) mass and spring anchor inductor coil

FigureFigure 1. Schematic 1. Schematic of proposed of complementary proposed metalcomplementary oxide semiconductor metal micro-electromechanical oxide semiconductor systemsmicro-electromechanical (CMOS-MEMS) inductivesystems (CMOS accelerometer-MEMS) inductive showing suspendedaccelerometer mass showing and sprinductor: suspended mass (a) Withoutand sprinductor acceleration,: (a) ( bW) compressedithout acceleration, sprinductor (b) andcompressed reduced inductance sprinductor under and acceleration, reduced inductance and (c) stretchedunder acceleration sprinductor, and and (c increased) stretched inductance sprinductor under and acceleration.increased inductance under acceleration. The principle of the readout circuit is shown in Figure2. The inductance values of the sensing The principle of the readout circuit is shown in Figure 2. The inductance values of the sensing sprinductors are converted to frequency outputs by two LC tank oscillators with frequencies: sprinductors are converted to frequency outputs by two LC tank oscillators with frequencies: q f =f f f0 ∆ f=f 11//22π ( L  ∆ LL))CC ((1)1) 0 ∓ ±

where f0 1/ 2 LC is the nominal oscillation frequency without acceleration and where f0 = 1/2π √LC is the nominal oscillation frequency without acceleration and ∆ f = (∆L/2L) f0 is f ( LL/2 ) f the frequency shift0 is due the to frequency external acceleration.shift due to Asexternal shown acceleration. in Figure2, the As twoshown oscillators in Figure are designed2, the two C C suchoscillators that the are capacitances, designed suchC1 andthat Cthe2, incapacitances the LC tanks, and,1 and thus, 2 , the in the two LC oscillation tanks and frequencies,, thus, the twof10 andoscillationf10, are difrequenciesfferent. The, outputsf10 and of the, are two different. oscillators The are outputs mixed of and the low-pass two oscillators filtered. are Therefore, mixed theand frequencylow-pass filtered. at the mixer Therefore, output the is: frequency at the mixer output is: f f ( f  f )  (ff f )  ( f  f ) 2  f f 1= ( 2f 10f ) 20+ (∆ f 1+ ∆ f 2) ( 10f 20f ) + 2∆ f ((2)2) 1 − 2 10 − 20 1 2 ≈ 10 − 20 where f10 f 20 is the frequency offset at the output for distinguishing between positive and where f10 f20 is the frequency offset at the output for distinguishing between positive and negative negative − input acceleration and 2f is the frequency output signal. An on-chip counter further input acceleration and 2∆ f is the frequency output signal. An on-chip counter further converts the frequencyconverts the output, frequency 2∆ f , intooutput digital, codes., into digital codes. ForFor adequate adequate mechanical mechanical spring spring constants, constantk,s the, k, meandering the meandering sprinductors sprinductors are realized are realized by narrow by andnarrow long andmetal long/oxide metal/oxide composite beamscomposite that formbeams single-turn that form inductors, single-turnL, as inductors shown in, L Figure, as shown1. Such in MEMSFigure inductors1. Such MEMS have low inductors quality factors have low due quality to low inductance factors dueand to low high inductance series resistance, and highRs, of series the loops.resistance For, example,Rs , of the the loops. length For and example, width of the the segment length and beams width in the of sprinductor the segment in this beams study in are the 395sprinductorµm and 5inµ m,this respectively. study are 395 The μm inductance and 5 μm, andrespectively. resistance The of suchinductance inductors and obtained resistance from of such the Coventorwareinductors obtained (Coventor from Inc., the Raleigh, Coventorware NC, USA) finite-element-method(Coventor Inc, Raleigh, (FEM) North simulation Carolina are, listed U.S.A.) in Tablefinite1-.element The Q factor-method of the (FEM) MEMS simulation sprinductor are at the listed 1.2 GHz in Table oscillation 1. The frequency Q factor is only: of the MEMS sprinductor at the 1.2 GHz oscillation frequency is only: QL = ωLL/RsL = 0.88 (3) QL LR L/ sL  0.88 (3) Such a low Q factor makes the oscillator design difficult and degrades its frequency stability and Such a low Q factor makes the oscillator design difficult and degrades its frequency stability and sensing resolution. Although it is possible to lower the series resistance and improve the Q factor by sensing resolution. Although it is possible to lower the series resistance and improve the Q factor by increasing the line width, W, of the metal routing, such a design simultaneously increases the width of increasing the line width, W, of the metal routing, such a design simultaneously increases the width the sprinductors and their spring constants, k. Furthermore, the series resistance is proportional to 1/W of the sprinductors and their spring constants, k. Furthermore, the series resistance is proportional whereas the spring constant is proportional to W3. Since the sensitivity of a spring-mass accelerometer to 1/W whereas the spring constant is proportional to W3. Since the sensitivity of a spring-mass is inversely proportional to k, the improvement of Q by widening the sprinductor width is quickly accelerometer is inversely proportional to k, the improvement of Q by widening the sprinductor diminished by the reduced sensitivity. In the current study, the width of the sprinductor is 5 µm width is quickly diminished by the reduced sensitivity. In the current study, the width of the according to the trade-off between inductance, spring constants, and fabrication yield. Instead of sprinductor is 5 μm according to the trade-off between inductance, spring constants, and fabrication further increasing the sprinductor width, we propose to connect a high-Q CMOS inductor in series yield. Instead of further increasing the sprinductor width, we propose to connect a high-Q CMOS

inductor in series with the low-Q MEMS sprinductor to improve the total quality factor, Qt . The high-Q inductor is implemented by a multi-turn coil in the top metal (M6) layer provided by the

Micromachines 2019, 10, 792 4 of 12

accelerometer is inversely proportional to k, the improvement of Q by widening the sprinductor Micromachineswidth is 2019quickly, 10, 792 diminished by the reduced sensitivity. In the current study, the width of the4 of 12 sprinductor is 5 μm according to the trade-off between inductance, spring constants, and fabrication yield. Instead of further increasing the sprinductor width, we propose to connect a high-Q CMOS withinductor the low-Q in series MEMS with sprinductor the low-Q MEMS to improve sprinductor the total to qualityimprove factor, the totalQt .quality The high-Q factor, inductorQt . The is implementedhigh-Q inductor by a multi-turn is implemented coil in by the a top multi-turn metal (M6) coil layerin the provided top metal by (M6) the processlayer provided design kits by (PDK)the fromprocess the CMOS design manufacturer. kits (PDK) from The the inductance, CMOS manufa resistance,cturer. The and inductance, quality factor resistance, of the low-Qand quality MEMS sprinductorfactor of the and low-Q the high-Q MEMS CMOS sprinductor inductor and used the high in this-Q researchCMOS inductor are listed used in Tablein this1. research It can be are seen thatlisted the totalin Table Q factor 1. It can when be seen the twothat inductorsthe total Q are factor connected when the in two series inductors is: are connected in series is: Qt = ω=+(ωLL + LH)/(RsL+=+ RsH) = 4.26 (4) QLRRtLHsLsH(4.26L )/( ) (4) at 1.2at 1.2 GHz, GHz, showing showing a five-fold ﬁve-fold enhancement enhancement compared compared with with the original the original low-Q low-Qsprinductor. sprinductor. The Theenhanced enhanced Q Qfactor factor is expected is expected to improve to improve the theoscillation oscillation stability stability and sensing and sensing resolution resolution of the of theaccelerometer. accelerometer.

OSC1 OSC: oscillator =+Δ ff1101f LPF: low pass filter

sprinductor L −ΔL C1 ± − f12f f12f

proof mass × LPF counter a

L +ΔL C sprinductor 2 analog output

=−Δ ff2220 f OSC2

FigureFigure 2. 2.Readout Readout circuit circuit of of proposedproposed inductive accelerometer accelerometer with with digital digital output. output. Table 1. Inductor parameters at 1.2 GHz. Table 1. Inductor parameters at 1.2 GHz.

Item L (nH) Rs (Ω) Q Item L (nH) Rs (Ω) Q Low-Q MEMS sprinductor LL = 0.92 RsL = 7.92 QL = 0.88 Low-Q MEMS sprinductor LL = 0.92 RsL = 7.92 QL = 0.88 High-Q CMOS inductor LH = 8.91 RsH = 9.34 QH = 7.19 High-Q CMOS inductor LH = 8.91 RsH = 9.34 QH = 7.19 Total inductor Lt = 9.83 Rst = 17.26 Qt = 4.29 Total inductor Lt = 9.83 Rst = 17.26 Qt = 4.29 3. Design and Simulation 3. Design and Simulation The eﬀect of residual stress on MEMS sensing structures such as comb ﬁngers and sprinductors The effect of residual stress on MEMS sensing structures such as comb fingers and weresprinductors investigated were to manifest investigated the advantageto manifest of the inductive advantage sensing of inductive over conventional sensing over capacitive conventional sensing. Thecapacitive detailed sensing. design ofThe the detailed proposed design accelerometer, of the proposed including accelerometer, the MEMS including sensing the structureMEMS sensing and the readoutstructure circuit, and arethe discussedreadout circuit, in the are following discussed sections. in the following sections.

3.1.3.1. Eff ectEffect of Residuralof Residural Stress Stress on on MEMS MEMS Sensing Sensing StructuresStructures Finite-elementFinite-element analysis analysis by usingby using Comsol Comsol 4.3b (COMSOL4.3b (COMSOL Inc, Stockholm, Inc, Stockholm, Sweden) Sweden) was conducted was to investigateconducted to the investigate effect of structuralthe effect of deformation structural deformation due to residual due to stress residu onal the stress MEMS on the comb MEMS finger capacitancecomb finger and capacitance meandering and sprinductormeandering sprinducto inductance.r inductance. To emulate To theemulate effect the of effect residual of residual stress in CMOS-MEMS,stress in CMOS-MEMS, the comb fingersthe comb and fingers sprinductors and sprind wereuctors modeled were modeled as bimorph as bimorph structures, structures, as shown as in Figureshown3 ina,b. Figure The top3a,b. layer The wastop layer a conductor was a cond (aluminum)uctor (aluminum) and the bottomand the layerbottom was layer a dielectric was a (SiOdielectric2). Both (SiO layers2). wereBoth layers 5 µm thick.were 5 The μm length, thick. The width, length, and width, gaps of and the gaps comb of fingers the comb or coil fingers segments or werecoil 230 segmentsµm, 4 µwerem and 230 4 μµm,m, 4 respectively. μm and 4 μm, A respectively. residual stress A residual was applied stress to was the applied bottom to layer the to induce the deformation, as shown in Figure3c,d. The capacitance and inductance of the deformed sensing structures were calculated using the FEM tool (Figure3e,f). As shown in Figure3c, the curling of the comb fingers significantly reduces the finger overlap area. Therefore, the capacitance is reduced by 34% at 400 MPa residual stress (Figure3e). On the contrary, the sprinductor has a similar − level of deformation (Figure3d) but its inductance is only reduced by 0.2% at 400 MPa (Figure3f). − Micromachines 2019, 10, 792 5 of 12

bottom layer to induce the deformation, as shown in Figure 3c,d. The capacitance and inductance of the deformed sensing structures were calculated using the FEM tool (Figure 3e,f). As shown in Figure 3c, the curling of the comb fingers significantly reduces the finger overlap area. Therefore, Micromachinesthe capacitance2019, 10 ,is 792 reduced by 34% at −400 MPa residual stress (Figure 3e). On the contrary, the5 of 12 sprinductor has a similar level of deformation (Figure 3d) but its inductance is only reduced by 0.2% at −400 MPa (Figure 3f). This confirms that, as a sensing mechanism, the inductance is much Thisless conﬁrms sensitive that, and asmore a sensing robust mechanism,when subjected the to inductance stress induced is much deformation less sensitive as compared and more to robustthe whencapacitance. subjected to stress induced deformation as compared to the capacitance.

top metal electrodes or coil bottom dielectric layer + V −

200

100 100 0 -100 -100 I

-200 (a) (b)

30 0.75

20 0.50

10 0.25 Capacitance (fF) (fF) Capacitance Inductance (nH) (nH) Inductance 0 0 0 −100 −200 −300 −400 0 −100 −200 −300 −400 Stress (MPa) Stress (MPa) (e) (f)

FigureFigure 3. 3.Finite-element-method Finite-element-method (FEM) (FEM) simulation simulation of ofcapacitance capacitance and and inductance inductance when when subjected subjected to toresidual residual stress stress induced induced deformation: deformation: (a) Comb (a) Comb finger ﬁnger capacitor, capacitor, (b) meandering (b) meandering sprinductor, sprinductor, (c) (c)deformed deformed capacitor capacitor at at−200200 MPa MPa residual residual stress, stress, (d) deformed (d) deformed sprinductor sprinductor at −200 at MPa200 stress, MPa ( stress,e) − − (e)capacitance capacitance of of comb comb finger ﬁnger capacitance capacitance vs. vs. stress, stress, (f) ( inductancef) inductance of meandering of meandering sprinductor sprinductor vs. stress. vs. stress. Note:Note: (a)–(d) (a)–(d) are are shown shown to to scale, scale, (c)(c) andand (d) are sh shownown with with same same color color scale scale of oftotal total displacement. displacement.

3.2.3.2. Accelerometer Accelerometer Structure Structure Design Design TheThe solid solid model model of of the the MEMS MEMS sensingsensing structurestructure is is shown shown in in Figure Figure 4a.4a. The The proof proof mass mass and and sprinductorssprinductors in thein MEMSthe MEMS structure structure are composed are compos of polySi-M1-M6ed of polySi-M1-M6 polysilicon-metal-oxide polysilicon-metal-oxide composite stackscomposite with embedded stacks with tungsten embedded via withtungsten a total via thickness with a total 10.14 thicknessµm. The area10.14 ofμm. the The central area proof of the mass is 800central900 proofµm. mass The high-Qis 800 ×CMOS 900 μm. inductor The high-Q is embedded CMOS inductor in the proof is embedded mass to savein the the proof chip mass area. to Such × a designsave the also chip improves area. Such the Qa design factor ofalso the improves CMOS inductor the Q factor when of the the underneath CMOS inductor silicon when substrate the is removedunderneath in the silicon release substrate process. Theis removed mass is suspendedin the release by 4process. sprinductors The mass at the is corners. suspended Mechanically, by 4 eachsprinductors sprinductor at is the a folded corners. spring Mechanically, composed ofeach 3 segment sprinductor beams, is asa shownfolded inspring Figure composed4b, with aof length 3 (l)segment of 395 µ mbeams, and aas width shown of in 5 Figureµm. The 4b, totalwith springa length constant (l) of 395 of μ them and sprinductors a width of was5 μm. calculated The total by using the ﬁxed-guided beam equation and was found to be 1.92 µN/µm.

Micromachines 2019, 10, 792 6 of 12

spring constant of the sprinductors was calculated by using the fixed-guided beam equation and Micromachineswas found to2019 be, 10 1.92, 792 μ N/μm. 6 of 12

Micromachinesspring constant2019, 10 ,of 792 the sprinductors was calculated by using the fixed-guided beam equation and6 of 12 was found ptoroof be mass1.92 μN/μm.

proof mass l sprinductor

high-Q CMOS l sprinductor sprinductor inductor proof mass high-Q CMOS sprinductor proof mass (a) inductor (b)

(a) (b) Figure 4. Solid model of MEMS structure: (a) whole device, (b) sprinductor. Figure 4. Solid model of MEMS structure: (a) whole device, (b) sprinductor.

The inductance of the sprinductor under deformation due to external force was obtained from FEMThe simulation inductanceFigure using of the 4.Coventorware. Solid sprinductor model of MEMSFigure under structure: 5 deformation shows (thata) whole the due sensing device, to external ( binductan) sprinductor. forcece of was the obtainedsprinductor from FEMhas simulation a sensitivity using of Coventorware.2.5 × 10−2 nH/ FigureμN with5 shows respect that to the force. sensing The inductancesensitivity ofwith the respect sprinductor to The inductance of the2 sprinductor under deformation due to external force was obtained from hasacceleration a sensitivity was of 2.5found10 by− tanHking/µN into with account respect the to mass, force. m The, to sensitivitybe 0.003 nH/g, with or respect equivalently to acceleration 3.2 × FEM simulation using× Coventorware. Figure 5 shows that the sensing inductance of the sprinductor3 was10 found−3 (∆L/L by)/g. taking into account the mass, m, to be 0.003 nH/g, or equivalently 3.2 10− (∆L/L)/g. has a sensitivity of 2.5 × 10−2 nH/μN with respect to force. The sensitivity with× respect to acceleration was found by ta0.97king into account the mass, m, to be 0.003 nH/g, or equivalently 3.2 × 10−3 (∆L/L)/g.

0.95 0.97

0.93 0.95

Inductance (nH) 0.91 0.93

0.89

Inductance (nH) 0.91 −1.5 −1.0 −0.5 0 0.5 1.0 1.5 μ Force ( N) 0.89 FigureFigure 5. 5.FEM FEM simulation simulation−1.5 of of− inductance,1.0 inductance, −0.5 L, under0 external external0.5 1.0force force and and1.5 deformation. deformation. μ Force ( N) 3.3.3.3. Oscillator Oscillator Design Design Figure 5. FEM simulation of inductance, L, under external force and deformation. FigureFigure6a shows6a shows the the LC LC oscillator oscillator that that converts converts thethe sensingsensing inductance value value into into frequency frequency variation.3.3.variation. Oscillator The The parallelDesign parallel LC LC tank tank thatthat containscontains a a series series coil coil resistance, resistance, RRs ,s ,in in the the inductor inductor equivalent equivalent circuitcircuit has has a resonance a resonance frequency frequency of: of: Figure 6a shows the LC oscillator that converts the sensing inductance value into frequency 2 variation. The parallel LC tank that containsr a seriesRC coil resistance,s Rs , in the inductor equivalent =−1112s = − f0 1 11Rs C 1 2 1 (5) circuit has a resonance frequencyf0 = of:22 ππLC1 LQ= LC 1 (5) 2π √LC − L 2π √LC − Q2 RC2 A MIM capacitor C = 1.67 pF was=− chosen111 so thats the= oscillation frequency − was about 1.2 GHz. The f0 11 (5) A MIMpositive-feedback capacitor C transistors,= 1.67 pF M1~M4, was22ππ chosen provideLC so effect thatLQ theive negative oscillation LC resistance2 frequency in parallel was about with the 1.2 LC GHz. Thetank positive-feedback to compensate transistors,for the energy M1~M4, loss due provide to the eff seriesective coil negative resistance, resistance Rs. The in parallelbias currents with theandLC A MIM capacitor C = 1.67 pF was chosen so that the oscillation frequency was about 1.2 GHz. The tankaspect to compensate ratios of M1~M4 for the energywere designed loss due toaccording the series to coil the resistance, total resistiveRs. The impedance bias currents at resonance and aspect positive-feedback transistors, M1~M4, provide effective negative resistance in parallel with the LC ratiosfound of M1~M4from post-layout were designed extraction. according to the total resistive impedance at resonance found from tank to compensate for the energy loss due to the series coil resistance, Rs. The bias currents and post-layoutThe extraction.outputs of the oscillators were connected to common-sources buffers before the mixing. aspect ratios of M1~M4 were designed according to the total resistive impedance at resonance SinceThe the outputs two inputs of the of oscillators the mixer were need connected to be biased to common-sourcesat different levels, bu theff ersoutput before common the mixing. mode Sinceof found from post-layout extraction. the two inputs of the mixer need to be biased at different levels, the output common mode of the buffers The outputs of the oscillators were connected to common-sources buffers before the mixing. forSince the two the ditwofferential inputs oscillatorsof the mixer were need biased to be atbiased 0.9 V at and different 1.4 V, accordingly.levels, the output The dicommonfferent outputmode of bias of the buffers intrinsically results in different load capacitance for the two oscillators. Therefore, the two oscillators will automatically oscillate at slightly different frequencies, f10 and f20, as required by Equation (2). It is noted that additional buffers were implemented and connected in parallel to the main Micromachines 2019, 10, 792 7 of 12

the buffers for the two differential oscillators were biased at 0.9 V and 1.4 V, accordingly. The Micromachinesdifferent2019 output, 10, 792bias of the buffers intrinsically results in different load capacitance for the two7 of 12 oscillators. Therefore, the two oscillators will automatically oscillate at slightly different frequencies,

f10 and f20 , as required by Equation (2). It is noted that additional buffers were implemented and signalconnected path so in that parallel the analog to the oscillationmain signal signals path so could that the be observedanalog oscillation directly signals by external could instruments, be observed as showndirectly in Figure by external2. instruments, as shown in Figure 2.

3.4.3.4. Mixer, Mixer, Low-Pass Low-Pass Filter, Filter, and and Counter Counter Design Design FigureFigure6b shows6b shows the Gilbertthe Gilbert cell mixer cell mixer in this in study. this Thestudy. di ffTheerential differential outputs outputs of the two of the oscillators two areoscillators connected are to co M1nnected/M2 and to M1/M2 M3~M5, and respectively. M3~M5, respectively. The differential The differential output currentoutput current of the mixerof the is convertedmixer is by converted M7 and M8by M7 to a and single-ended M8 to a single-ended signal. It is furthersignal. It amplified is further to amplified a rail-to-rail to a signalrail-to-rail by the common-sourcesignal by the amplifier,common-source M9 and amplifier, M10. The M9 gate-drain and M10. capacitance,The gate-drain Cgd capacitance,, of M9 is amplified Cgd, of M9 by is the Miller’samplified effect by and the presents Miller’s a effect large capacitiveand presents load a atlarge the capacitive mixer output load that at the limits mixer the eoutputffective that bandwidth limits andthe removes effective the bandwidth sum-frequency and removes component the sum-frequency in the mixer. component Thus, only in thethe dimixer.fference Thus, frequency only the in Equationdifference (2) passesfrequency through in Equation the output (2) bupassesffer. Thethrough buff er,the M11~M14, output buffer. further The amplifies buffer, theM11~M14, signal to drivefurther the external amplifies instruments the signal orto next-stagedrive the external circuits. instruments The output ofor thenext-stage buffer iscircuits. fed to anThe 18-bit output on-chip of synchronousthe buffer is counter fed to foran 18-bit a typical on-chip mixer synchronou output frequencys counter offor 100a typical MHz mixer and sampling output frequency frequency of of 100 MHz and sampling frequency of 500 Hz. 500 Hz.

M7 M8 M9 M3 M4

inductor Cgd M11 M13 equivalent V2n Rs L V2p M3 M4 M5 M6 circuit Vout C M12 M14 M1 M2 V1n M1 V1p

MB M10 MB

(a) (b)

FigureFigure 6. 6.(a ()a LC) LC tank tank oscillator, oscillator, ( b(b)) GilbertGilbert cellcell and single-end single-end output output buffer. buﬀer.

4. Results4. Results and and Discussion Discussion TheThe proposed proposed accelerometer accelerometer was was fabricatedfabricated usingusing a standard CMOS CMOS process process followed followed by by post-CMOSpost-CMOS release release processing. processing. AfterAfter thethe MEMSMEMS structure was was released, released, its its resonance resonance frequency frequency waswas measured measured to ensureto ensure successful successful releasing. releasing. The The oscillators oscillators were were tested tested ﬁrstfirst whenwhen thethe devicedevice waswas at rest.at The rest. accelerometer The accelerometer was then was testedthen tested by using by aus rotationing a rotation tableand table a shakerand a shaker for static for and static dynamic and characterization,dynamic characterization, respectively. respectively.

4.1.4.1. CMOS CMOS and and Post-CMOS Post-CMOS Fabrication Fabrication TheThe device device was was first first fabricated fabricated by by a a 0.18 0.18 µμmm 1P6M1P6M commercial CMOS CMOS process process followed followed by by post-CMOSpost-CMOS dry-etching dry-etching releasing releasing provided provided byby thethe TaiwanTaiwan Semiconductor Research Research Institute Institute (TSRI), (TSRI), Taiwan,Taiwan, R.O.C. R.O.C. [16 ].[16]. Figure Figure7a depicts 7a depicts the crossthe cross section section of a CMOSof a CMOS device device as received as received from thefrom CMOS the foundry.CMOS Infoundry. the post-CMOS In the post-CMOS release process, release aprocess, photoresist a photoresist (PR) layer (PR) was layer first was applied first applied to cover to the CMOScover circuit the CMOS areas socircuit that areas they so would that they not bewould affected not be by affected the following by the etchingfollowing processes etching processes (Figure7b). (Figure 7b). In Figure 7b, CF4-based anisotropic reactive ion etching (RIE) was applied to remove In Figure7b, CF 4-based anisotropic reactive ion etching (RIE) was applied to remove the sacrificial the sacrificial oxide around the sensor structures. Subsequently, SF6-based isotropic RIE was used to oxide around the sensor structures. Subsequently, SF6-based isotropic RIE was used to remove the remove the silicon substrate and release the sensor structures (Figure 7c). Figure 7c also shows the silicon substrate and release the sensor structures (Figure7c). Figure7c also shows the released sensor structures, such as the proof mass and the sprinductors, which are typically composed of the materials in the backend CMOS process, including oxide, aluminum metal, and tungsten via. Figure8 shows the micrographs of a fabricated and released device. Figure8c shows that the sprinductors are deformed due to the residual stress. The tip deformation was measured by a WKYO NT-1100 (Bruker Inc, Micromachines 2019, 10, 792 8 of 12

Micromachines 2019, 10, 792 8 of 12 released sensor structures, such as the proof mass and the sprinductors, which are typically composedreleased sensor of the structures,materials in such the backendas the proof CMOS mass process, and includingthe sprinductors, oxide, aluminum which are metal, typically and tungstencomposed via. of Figurethe materials 8 shows in the the micrographs backend CMOS of a fabricatedprocess, including and released oxide, device. aluminum Figure metal, 8c shows and that the sprinductors are deformed due to the residual stress. The tip deformation was measured by Micromachinestungsten 2019via., Figure10, 792 8 shows the micrographs of a fabricated and released device. Figure 8c shows8 of 12 athat WKYO the sprinductors NT-1100 (Bruker are deformed Inc, Tucson, due Arizona,to the resi U.dualS.A.) stress. white The light tip interferometer, deformation was and measured was found by μ toa WKYOhave an NT-1100 average (Brukervalue of Inc, 10.3 Tucson, m, corresponding Arizona, U.S.A.) to a whiteradius light of curvature interferometer, of 8 mm. and Such was a found large Tucson,deformationto have AZ, anUSA) average would white valueseriously light of 10.3 interferometer,affect μm, the corresponding capacitance and was tovalues a found radius and to ofthe have curvature sensor an average characteristics of 8 mm. value Such ofif a 10.3comblargeµ m, correspondingfingerdeformation sensing towould capacitors a radius seriouslyof were curvature affectemployed. the of 8capacitance The mm. in-plane Such values a resonance large and deformation the frequency sensor would characteristicsof the seriouslyreleased ifMEMS acombﬀect the structure was measured by a PSM-1000 planar motion analyzer (Polytec GmbH, Waldbronn, capacitancefinger sensing values capacitors and the sensor were employed. characteristics The in-plane if comb ﬁngerresonance sensing frequency capacitors of the were released employed. MEMS The Germany)). Figure 9 shows that the resonance frequency is 4.45 kHz with a mechanical Q factor of in-planestructure resonance was measured frequency by of a thePSM-1000 released planar MEMS motion structure analyzer was measured (Polytec GmbH, by a PSM-1000 Waldbronn, planar about 37. motionGermany)). analyzer Figure (Polytec 9 shows GmbH, that Waldbronn, the resonance Germany)). frequency Figure is 4.459 kHzshows with that a mechanical the resonance Q factor frequency of about 37. is 4.45 kHz with a mechanical Q factor of about 37. circuit sensor

circuit sensor

(a) (b) (c) Si sub oxide source/drain gate contact/via metal PR (a) (b) (c) Si sub oxide source/drain gate contact/via metal PR Figure 7. Fabrication process of proposed CMOS-MEMS accelerometer. FigureFigure 7. 7.Fabrication Fabrication processprocess ofof proposedproposed CMOS-MEMS CMOS-MEMS accelerometer. accelerometer.

OSC2 counter OSC2 counter

mixer

MEMS sensor MEMS mixer

MEMS sensor MEMS OSC1 OSC1 (a) (b) (a) (b)

deformed sprinductor deformed sprinductor (c) (d)

(c) (d) FigureFigure 8. 8.(a )(a Chip) Chip micrograph micrograph before before release,release, (b) scanning electron electron micrograph micrograph of ofMEMS MEMS structure structure afterafter release, release, (c )(c high-Q) high-Q CMOS CMOS inductor inductor embedded embedded in proof mass, mass, ( (dd) )deformed deformed sprinductor. sprinductor. Figure 8. (a) Chip micrograph before release, (b) scanning electron micrograph of MEMS structure after release, (c) high-Q CMOS inductor embedded in proof mass, (d) deformed sprinductor.

Micromachines 2019, 10, 792 9 of 12 Micromachines 2019, 10, 792 9 of 12 Micromachines 2019, 10, 792 9 of 12

150 150

100 100

50 50 Displacement (nm) Displacement (nm)

0 0 1000 2000 3000 4000 5000 6000 1000 2000 3000 4000 5000 6000 Frequency (Hz) Frequency (Hz) FigureFigure 9. 9.Resonance Resonance frequencyfrequency measurement of of release release MEMS MEMS structure. structure. 4.2. Accelerometer Characterization 4.2. Accelerometer Characterization TheThe released released CMOS-MEMS CMOS-MEMS accelerometer accelerometer was mounte mountedd in in the the center center of of a printed a printed circuit circuit board board (PCB),(PCB), as as shown shown in in Figure Figure 10 10,, and and tested tested for for bothboth circuitcircuit functions and and acceleration acceleration sensitivity. sensitivity.

θ θ

Figure 10. CMOS-MEMS accelerometer die mounted on a printed circuit board (PCB) for electrical testing. Figure 10. CMOS-MEMS accelerometer die mounted on a printedinted circuitcircuit boardboard (PCB)(PCB) forfor electricalelectrical testing. 4.2.1. Oscillatortesting. Test

4.2.1.The Oscillator oscillation Test frequencies of the two oscillators were measured by a Keysight N9030B signal source analyzer (Keysight Inc, Santa Rosa, CA, USA) and were found to be 1.484 GHz and 1.382 GHz, The oscillation frequencies of the two oscillators were measured by a Keysight N9030B signal respectively, with a frequency difference of 102 MHz. The measured frequency at the analog output of source analyzer (Keysight Inc, Santa Rosa, California, U.S.A.) and were found to be 1.484 GHz and the1.382 mixer GHz, was respectively, 92.3 MHz, as with shown a frequency in Figure 11differea. Thence measured of 102 MHz. frequencies The measured agree well frequency with the at designthe value.analogMicromachines The output slight 2019 of discrepancy ,the 10, 792mixer was between 92.3 MHz, the oscillator as shown frequencyin Figure 11a. diff erenceThe measured and the frequencies mixer frequency agree10 of 12 is attributedwell with to the the design different value. buff Theers thatslight drive discrepanc the mixery between and the the external oscillator instruments. frequency difference and the mixer frequency is attributed to the different buffers that drive the mixer and the external instruments. The frequency output of the on-chip counter was capture by a Keysight 16902B logic analyzer (LA) (Keysight Inc, Santa Rosa, California, U.S.A.) at a sampling rate of fs = 500 Hz for 16 s. The (LA) (Keysight Inc, Santa Rosa, California, U.S.A.) at a sampling rate of fs = 500 Hz for 16 s. The frequency stability of the counter output was analyzed for Allan’s deviation. AsAs shownshown inin FigureFigure 11b, the frequency bias instability is 0.94 kHz (11 ppm) at average time 𝜏 == 2.162.16 s.s.

(a) (b)

FigureFigure 11. 11.(a) ( Mixera) Mixer output output waveform, waveform, (b ()b Allan’s) Allan’s deviation deviation atat countercounter output. output.

4.2.2. Static Acceleration Test The PCB in Figure 10 was mounted on a rotation stage to measure the component of the gravitational acceleration projected onto the sensing axis, gcos𝜃. Figure 12 shows the measured

counter frequency, f(𝜃), vs. rotation angle, 𝜃, at a sampling rate of fs = 500 Hz. Curve fitting shows:

f (0θθ)=× 4.16 1047 cos( ) +× 8.58 1 ， R2 = 0.9889 (6)

Therefore, the offset frequency, f0 , and sensitivity, S f , of the accelerometer are 85.8 MHz and 41.6 σ kHz/g, respectively. The average standard deviation of frequency in Figure 12 is f = 5.4 kHz. Therefore, the noise floor of the accelerometer is: σ / S ff= noise floor = 8.2 mg / Hz (7) fs /2

×107 8.588

8.586

8.584

8.582

Frequency (Hz) Frequency data 8.580 curve fitting

8.578 −180 −120 −60 0 60 120 180 θ (Deg) Figure 12. Static acceleration test.

4.2.3. Dynamic Acceleration Test The dynamic acceleration test was performed by mounting the PCB in Figure 10 on an electrodynamic shaker (2007E, The Modal Shop, Inc, Cincinnati, Ohio, U.S.A.). A reference accelerometer (PCB 352C65, PCB Piezotronics, Inc, Depew, New York, U.S.A.) was mounted coaxially to calibrate the vibration levels. The excitation vibration frequency was 50 Hz, and the

sensor output was recorded at fs = 500 Hz. Figure 13 shows the signal amplitude vs. vibration

Micromachines 2019, 10, 792 10 of 12

The frequency output of the on-chip counter was capture by a Keysight 16902B logic analyzer (LA)

(Keysight Inc., Santa Rosa, CA, USA) at a sampling rate of fs = 500 Hz for 16 s. The frequency stability of the counter output was( analyzeda) for Allan’s deviation. As shown in Figure(b) 11b, the frequency bias instability is 0.94Figure kHz (1111. ( ppm)a) Mixer at output average waveform, time τ = (b2.16) Allan’s s. deviation at counter output.

4.2.2.4.2.2. Static Static Acceleration Acceleration Test TheThe PCB PCB in in Figure Figure 10 10 was was mountedmounted on a rotati rotationon stage stage to to measure measure the the component component of the of the gravitationalgravitational acceleration acceleration projected projected onto onto the the sensing sensing axis, axis,gcos gθcos. Figure𝜃. Figure 12 shows 12 shows the measured the measured counter frequency,counter ffrequency,(θ), vs. rotation f(𝜃), vs. angle, rotationθ, at angle, a sampling 𝜃, at a rate sampling of fs = 500rate Hz.of Curvefs = 500 ﬁtting Hz. shows:Curve fitting shows: f (θ) = 4.16 104 cos(θ) + 8.58 107, R2 = 0.9889 (6) f (0θθ)=× 4.16× 1047 cos( ) +× 8.58 1× ， R2 = 0.9889 (6)

Therefore, the offset frequency, f0, and sensitivity, S f , of the accelerometer are 85.8 MHz and 41.6 kHz/g, Therefore, the offset frequency, f0 , and sensitivity, S f , of the accelerometer are 85.8 MHz and 41.6 respectively. The average standard deviation of frequency in Figure 12 is σ = 5.4 kHz. Therefore, the kHz/g, respectively. The average standard deviation of frequency in Figuref 12 is σ = 5.4 kHz. noise floor of the accelerometer is: f Therefore, the noise floor of the accelerometer is: σ f /S f = ff/ S = √ noisenoise floor floor =p = 8.28.2 mg mg // HzHz (7) fs/2 (7) fs /2

×107 8.588

8.586

8.584

8.582

Frequency (Hz) Frequency data 8.580 curve fitting

8.578 −180 −120 −60 0 60 120 180 θ (Deg) FigureFigure 12.12. Static acceleration acceleration test. test.

4.2.3.4.2.3. Dynamic Dynamic Acceleration Acceleration Test Test TheThe dynamic dynamic acceleration acceleration testtest waswas performed by by mounting mounting the the PCB PCB in inFigure Figure 10 10on onan an electrodynamicelectrodynamic shaker shaker (2007E, (2007E, The The Modal Modal Shop, Shop, Inc., Cincinnati,Inc, Cincinnati, OH, USA). Ohio, A U.S.A.). reference A accelerometer reference (PCBaccelerometer 352C65, PCB (PCB Piezotronics, 352C65, PCB Inc., Depew,Piezotronics, New York,Inc, Depew, NY, USA) New was York, mounted U.S.A.) coaxially was mounted to calibrate thecoaxially vibration to levels. calibrate The the excitation vibration vibration levels. The frequency excitation was vibration 50 Hz, and frequency the sensor was output 50 Hz, was and recorded the sensor output was recorded at f = 500 Hz. Figure 13 shows the signal amplitude vs. vibration at fs = 500 Hz. Figure 13 shows thes signal amplitude vs. vibration levels. The sensitivity is 37.8 kH /g, and the nonlinearity is 1.5% full-scale (FS). The discrepancy between the static and dynamic sensitivities is attributed partially to the diﬀerent test setup. The performance comparison of the proposed device and other CMOS or CMOS-MEMS accelerometers in the literature is shown in Table2. It can be seen that the current device has much improved performance as compared with our prior work without the series high-Q inductor [9]. The other performance parameters are also comparable to similar devices in the literature. Micromachines 2019, 10, 792 11 of 12

levels. The sensitivity is 37.8 kH/g, and the nonlinearity is 1.5% full-scale (FS). The discrepancy between the static and dynamic sensitivities is attributed partially to the different test setup. The performance comparison of the proposed device and other CMOS or CMOS-MEMS accelerometers in the literature is shown in Table 2. It can be seen that the current device has much Micromachinesimproved2019 performance, 10, 792 as compared with our prior work without the series high-Q inductor [9].11 of 12 The other performance parameters are also comparable to similar devices in the literature.

200

) y = 37.8x−1.36 R² = 0.9992 kHz

( 150 ut p 100 Out y uenc

q 50 Fre

0 0 1 2 3 4 5 Acceleration (g) FigureFigure 13.13. Dynamic acceleration acceleration test. test.

TableTable 2. 2.Performance Performance comparisoncomparison of monolithic monolithic CMOS CMOS accelerometers. accelerometers.

CMOSCMOS Tech. 1 Sensing 2 3 3 Sensitivity Noise Floor Nonlinearity 4 Type Sensing f0mech (kHZ) f f0 elec (GHz)Sensitivity Noise Floor Nonlinearity Output Reference (µm) 1 Element 0mech 0elec (/g) (mg/√Hz) (%FS) 4 Tech. Type √ Output Reference Element (kHZ) 5 (/g) (mg/ Hz) (%FS) 0.35(𝛍m) C on-chip 8.8 (GHz) NA 105 mV 0.4 1.0 A [2] 0.18 C on-chip 4.7 5 191 mV 0.35 1.0 A [3] 5 NA 0.180.35 C on-chip on-chip 8.8 7.0 NA 1.9105 mV 3.6 MHz0.4 0.21.0 1.2A A[2] [4] 0.130.18 C I bondon-chip wire 4.7 3.1 NA 5 2.1191 mV 10 kHz 0.35 80 1.0 - A A [3] [8] 0.180.18 C I on-chip on-chip 7.0 6.6 1.9 2.03.6 MHz 22 kHz 0.2 460 1.2 11 A A [4] [9] 0.180.13 I bond on-chip wire 3.1 4.5 2.1 1.4 10 kHz 42 kHz 80 8 - 1.5A A/D[8] This work 1 C:0.18 capacitive, I I: inductive;on-chip 2 mechanical6.6 2.0 resonance 22 frequency;kHz 3 460electrical oscillation 11 frequency; A 4 A: analog, [9] D: 5 digital;0.18 chopper-basedI on-chip readout. 4.5 1.4 42 kHz 8 1.5 A/D This work 1 C: capacitive, I: inductive; 2 mechanical resonance frequency; 3 electrical oscillation frequency; 4 A: 5. Conclusionsanalog, D: digital; 5 chopper-based readout.

5. ConclusionsThis paper presents the design, fabrication, and characterization of an inductive CMOS-MEMS accelerometer with on-chip digital output based on LC oscillators. FEM analysis confirms that the inductiveThis detection paper presents scheme the is muchdesign, more fabrication, robust in and the characterization presence of residual of an stressinductive induced CMOS-MEMS deformation asaccelerometer compared with with capacitive on-chip digital detection. output A high-Qbased on CMOS LC oscillators. inductor FEM was connectedanalysis confirms in series that with the the inductive detection scheme is much more robust in the presence of residual stress induced low-Q MEMS sprinductor to improve its quality factor. The measurement showed that the proposed deformation as compared with capacitive detection. A high-Q CMOS inductor was connected in device had an output offset frequency of 85.5 MHz, sensitivity of 41.6 kHz/g, noise floor of 8.2 mg/√Hz, series with the low-Q MEMS sprinductor to improve its quality factor. The measurement showed bias instability of 0.94 kHz (11 ppm) at average time 2.16 s, and nonlinearity of 1.5% full-scale. The that the proposed device had an output offset frequency of 85.5 MHz, sensitivity of 41.6 kHz/g, noise performancefloor of 8.2 comparesmg/√Hz, bias well instability with similar of 0.94 CMOS kHz (11 or CMOS-MEMSppm) at average accelerometers time 2.16 s, and in nonlinearity the literature. of 1.5% full-scale. The performance compares well with similar CMOS or CMOS-MEMS accelerometers Author Contributions: Conceptualization, Y.C. and H.-C.H.; chip design and implementation, H.-W.L., Y.C., and H.-C.H.;in the measurement,literature. H.-W.L.; writing—review and editing, Y.C. and H.-C.H. Funding:Author ThisContributions: work was supportedConceptualization, in part byY.C. the and grants H.-C.H.; MOST-106-2221-E-009-091-MY2, chip design and implementation, MOST H.-W.L., 106-2221-E-009- Y.C., 166-MY3,and H.-C.H.; and the measurement, Research of H.-W.L.; Excellence writing—review (RoE) Program and (MOST editing, 108-2633-E-009-001) Y.C. and H.-C.H. of the Ministry of Science and Technology, Taiwan, R.O.C. Funding: This work was supported in part by the grants MOST-106-2221-E-009-091-MY2, MOST Acknowledgments:106-2221-E-009-166-MY3,The authorsand the acknowledgeResearch of Excelle the supportnce (RoE) of Program the National (MOST Center 108-2633-E-009-001) for High-performance of the Computing,Ministry of the Science Taiwan and SemiconductorTechnology, Taiwan, Research R.O.C. Institute, the National Chiao Tung University Nano Facility Center, Taiwan, R.O.C., for various simulation, fabrication, and characterization tools. Acknowledgments: The authors acknowledge the support of the National Center for High-performance Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study;Computing, in the collection, the Taiwan analyses, Semiconductor or interpretation Research ofInstitut data;e, in the the National writing Chiao of the Tung manuscript, University or inNano the decisionFacility to publishCenter, the Taiwan, results. R.O.C., for various simulation, fabrication, and characterization tools.

References

1. Zhang, G.; Xie, H.; de Rosset, L.E.; Fedder, G.K. A lateral capacitive CMOS accelerometer with structural curl compensation. In Proceedings of the 12th IEEE International Conference on Micro Electro Mechanical Systems, Orlando, FL, USA, 21 January 1999. 2. Tsai, M.-H.; Liu, Y.-C.; Liang, K.-C.; Fang, W. Monolithic CMOS-MEMS pure oxide tri-axis accelerometers for temperature stabilization and performance enhancement. J. Microelectromech. Syst. 2015, 24, 1916–1927. [CrossRef] Micromachines 2019, 10, 792 12 of 12

3. Tseng, S.-H.; Lu, M.S.; Wu, P.-C.; Teng, Y.-C.; Tsai, H.-H.; Juang, Y.-Z. Implementation of a monolithic capacitive accelerometer in a wafer-level 0.18 µm CMOS MEMS process. J. Micromech. Microeng. 2012, 22, 055010. [CrossRef] 4. Chiu, Y.; Hong, H.-C.; Wu, P.-C. Development and characterization of a CMOS-MEMS accelerometer with diﬀerential LC-tank oscillators. J. Microelectromech. Syst. 2013, 22, 1285–1295. [CrossRef] 5. Haris, M.; Qu, H. A CMOS-MEMS piezoresistive accelerometer with large proof mass. In Proceedings of the 5th IEEE International Conference on Nano/Micro Engineered and Molecular Systems, Xiamen, China, 20–23 January 2010. 6. Chiu, Y.; Huang, T.-C.; Hong, H.-C. A three-axis single-proof-mass CMOS-MEMS piezoresistive accelerometer with frequency output. Sens. Mater. 2014, 26, 95–108. 7. Liao, K.-M.; Chen, R.; Chou, B.C.-S. A novel thermal-bubble-based micromachined accelerometer. Sens. Actuators A Phys. 2006, 130, 282–289. [CrossRef] 8. Liao, Y.-T.; Biederman, W.J.; Otis, B.P. A fully integrated CMOS accelerometer using bondwire inertial sensing. IEEE Sens. J. 2011, 11, 114–122. [CrossRef] 9. Chiu, Y.; Hong, H.-C.; Chang, C.-M. Three-axis CMOS MEMS inductive accelerometer with novel Z-axis sensing scheme. In Proceedings of the 19th International Conference on Solid-State Sensors, Actuators and Microsystems, Kaohsiung, Taiwan, 18–22 June 2017. 10. Chiu, Y.; Hong, H.-C.; Lin, C.-W. Inductive CMOS MEMS accelerometer with integrated variable inductors. In Proceedings of the 29th International Conference on Micro Electro Mechanical Systems, Shanghai, China, 24–28 January 2016. 11. Passeraub, P.A.; Besse, P.A.; Bayadroun, A.; Hediger, S.; Bernasconi, E.; Popovic, R.S. First integrated inductive proximity sensor with on-chip CMOS readout circuit and electrodeposited 1 mm ﬂat coil. Sens. Actuators A Phys. 1999, 76, 273–278. [CrossRef] 12. Matheoud, A.V.; Solmaz, N.S.; Frehner, L.; Boero, G. Microwave inductive proximity sensors with sub-pm/Hz resolution. Sens. Actuators A Phys. 2019, 295, 259–265. [CrossRef] 13. Wang, H.; Chen, Y.; Hassibi, A.; Scherer, A.; Hajimiri, A. A frequency-shift CMOS magnetic biosensor array with single-bead sensitivity and no external magnet. In Proceedings of the International Solid-State Circuits Conference, San Francisco, CA, USA, 8–12 February 2009. 14. Yeh, S.K.; Fang, W. Inductive micro tri-axial tactile sensor using a CMOS chip with a coil array. IEEE Electron Device Lett. 2019, 40, 620–623. [CrossRef] 15. Vogel, J.G.; Chaturvedi, V.; Nihtianov, S. Eddy-current sensing principle in inertial sensors. IEEE Sens. Lett. 2017, 1, 2500504. [CrossRef] 16. Tseng, S.-H.; Hung, Y.-J.; Juang, Y.-Z.; Lu, M.S.-C. A 5.8-GHz VCO with CMOS-compatible MEMS inductors. Sens. Actuators A Phys. 2007, 139, 187–193. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).