Towards an Ultra-Sensitive Temperature Sensor for Uncooled Infrared Sensing in CMOS–MEMS Technology

micromachines

Article Towards an Ultra-Sensitive Temperature Sensor for Uncooled Infrared Sensing in CMOS–MEMS Technology

Hasan Gökta¸s

Electrical and Electronic Engineering, Harran University, ¸Sanlıurfa63000, Turkey; [email protected]; Tel.: +90-414-318-3000 Received: 10 January 2019; Accepted: 1 February 2019; Published: 6 February 2019

Abstract: Microbolometers and photon detectors are two main technologies to address the needs in Infrared Sensing applications. While the microbolometers in both complementary metal-oxide semiconductor (CMOS) and Micro-Electro-Mechanical Systems (MEMS) technology offer many advantages over photon detectors, they still suffer from nonlinearity and relatively low temperature sensitivity. This paper not only offers a reliable solution to solve the nonlinearity problem but also demonstrate a noticeable potential to build ultra-sensitive CMOS–MEMS temperature sensor for infrared (IR) sensing applications. The possibility of a 31× improvement in the total absolute frequency shift with respect to ambient temperature change is veriﬁed via both COMSOL (multiphysics solver) and theory. Nonlinearity problem is resolved by an operating temperature sensor around the beam bending point. The effect of both pull-in force and dimensional change is analyzed in depth, and a drastic increase in performance is achieved when the applied pull-in force between adjacent beams is kept as small as possible. The optimum structure is derived with a length of 57 µm and a thickness of 1 µm while avoiding critical temperature and, consequently, device failure. Moreover, a good match between theory and COMSOL is demonstrated, and this can be used as a guidance to build state-of-the-art designs.

Keywords: CMOS; MEMS; microresonators; microelectromechanical systems; thermal detector; temperature sensor; infrared sensor; microbolometer

1. Introduction Microbolometers offer many advantages with their compact size, low power, capability of working at room temperature, small cost, reliable and simpler fabrication technique over bulky or relatively expensive detectors (liquid-nitrogen cooled HgCdTe (MCT), [1] etc.) in Infrared (IR) Sensing application. Ideal microbolometers should consist of high sensitivity temperature sensors and an IR absorbing layer. The IR absorbing layer converts the incident radiation into heat, and that heat is converted into the electrical signal via a temperature sensor (non-resonant [2,3], resonant-sensing [4–9]). The resonant-sensing type sensor has many advantages over the non-resonant type, such as smaller dimension and relatively low noise, due to a high-quality factor of 2.4 × 106 [9] and 1 million [10]. That is why resonant-sensing type sensors are also popular in mass sensing [11–13], but are mostly fabricated in Micro-Electro-Mechanical Systems (MEMS) technology (MEMS resonators) [5–9] rather than in complementary metal-oxide semiconductor (CMOS) technology (CMOS-MEMS resonators [4,14]). A high-density focal plane array (FPAs) are very demanding for high-quality thermal imaging, and this requires a high-density integrated circuit (IC). It can be achieved by either building thermal detectors and IC on the same chip (CMOS–MEMS) [15,16] or bonding a separate IC and MEMS chip together [17]; however, the one that requires bonding brings extra fabrication costs and complexity. That is why CMOS–MEMS resonant-sensing type uncooled IR detectors are becoming more attractive, as they offer all-in-one (IC + MEMS), cost-effective and high sensitivity solution together. The main

Micromachines 2019, 10, 108; doi:10.3390/mi10020108 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 108 2 of 7 performance parameter for resonant-sensing type temperature sensors (cantilever, tuning fork, free–free beam, and fixed–fixed beam) is the temperature coefficient of frequency (TCF) that represents the magnitude of frequency shift (FS) with respect to the temperature change. The wide range frequency tuning capability of a fixed–fixed beam in comparison to other resonant-sensing types was demonstrated for the first time in [14] and later used in [4] to build a high sensitivity temperature sensor in CMOS technology. Moreover, fixed–fixed beam type CMOS–MEMS resonator [4] has the potential to offer high performance with their relatively high TCF (4537 ppm/K (Table1)), while enabling a more reliable and simpler fabrication process. Despite their relatively large TCF, fixed–fixed beam type CMOS–MEMS resonators suffer from a nonlinearity problem and still need to have larger TCF for ultra-sensitive uncooled IR detection application. In this work, the nonlinearity problem of fixed–fixed type CMOS–MEMS resonator is resolved by operating the resonator around the beam bending point. In addition, at least 31× (343 kHz/11 kHz) improvement in total absolute FS with an absolute |TCF| > 589,698 ppm/K are achieved according to COMSOL and theory for 57 µm long CMOS–MEMS resonator. |TCF| increases from 589,698 ppm/K to 2178,946 ppm/K when applied Joule-heating (Vth) changes from 3.3252 V to 3.3476 V according to COMSOL. Here both Joule-heating and the change in the ambient temperature are applied together in contrast to [4], where only the ambient temperature change was used to derive |TCF|. Moreover, the effect of the pull-in force between two adjacent beams is studied in detail to find the optimum resonator working parameters for the sake of larger |TCF|. The |TCF| drastically decreases from 2,333,771 ppm/K to 16,185 ppm/K when pull-in force increases from 7 MPa to 10,000 MPa according to COMSOL for 120 µm long CMOS–MEMS resonator due to decreases in thermal stress on both fixed ends. In addition, in contrast to [4], there is no thickness effect on FS while a shorter beam results in larger FS where the beam just starts to bend. The maximum temperature around beam bending point for 57 µm long beam is calculated as 530 K via COMSOL, and that does not exceed the maximum allowable temperature in CMOS–MEMS technology [18]. According to COMSOL and theory, a significant improvement in |TCF| for 57 µm long CMOS–MEMS resonator over previous works can be achieved (Table1)

Table 1. Performance comparison between this work and literature. TCF–temperature coefﬁcient of frequency, CMOS–complementary metal-oxide semiconductor, MEMS–Micro-Electro-Mechanical Systems, NEMS–Nano Electromechanical Systems.

Resonance Absolute |TCF| Design Technology Frequency (ppm/K) This work (57 µm long CMOS–MEMS Resonator) 1.92 MHz 2,178,946 CMOS–MEMS 120 µm long CMOS–MEMS Resonator [4] 640 kHz 4537 CMOS–MEMS AIN Piezoelectric Nanomechanical Resonator [5] 161.4 MHz 30 NEMS Nanomechanical Torsional Resonator [6] 842 kHz 548 NEMS Silicon Micromechanical Resonator [7] 101 MHz 29.7 MEMS

2. Fabrication The CMOS–MEMS resonators can be fabricated via a post-process followed after a CMOS 0.6 µm process that includes a CHF3/O2 process for SiO2 etching between adjacent beams and XeF2 process for Silicon etching underneath the beams [14]. In this study, the device structures (Figure1) are slightly changed for the sake of better performance. However, the distance between devices and the silicon etching ratio is kept the same. Micromachines 2019, 10, 108 3 of 7 Micromachines 2019, 10, x FOR PEER REVIEW 3 of 7

FigureFigure 1. 1. TheThe cross cross section section for for Device Device 1 (W 1 (W = 2 = µm) 2 µm) and and for forDevice Device 2 (W 2 (W= 1 =µm), 1 µ m),where where W is W the is thickness.the thickness.

3.3. Theory Theory Modeling Modeling and and Optimization Optimization TheThe working working principle principle of of the the CMOS–MEMS resonator resonator (Figure (Figure 11)) isis basedbased onon pull-inpull-in forceforce (via(via DCDC bending bending voltage voltage (Vdc) (Vdc) applied applied between between two two adja adjacentcent beams), beams), and and the the Joule-heating Joule-heating voltage voltage (Vth) (Vth) appliedapplied on on the the embedded embedded heater heater (polysilicon (polysilicon layer) layer) through through the the resonant resonant beam. Pull-in Pull-in force force enables enables thethe softening softening effect effect on on the the resonant resonant beam beam and, and, consequently, starts the resonance operation while Joule-heatingJoule-heating increases increases the the temperature temperature throughout throughout th thee resonant resonant beam and and resultes resultes in in relatively relatively high high thermalthermal stress onon thethe fixedfixed ends. ends. This This Joule-heating Joule-heating effect effect causes causes a widea wide range range of frequencyof frequency tuning tuning and andthis this was was first first time time demonstrated demonstrated in [14 in]. [14]. The resonanceThe resonance frequency frequency with with respect respect to axial to axial load load [19] is:[19] is: 1 1 4.732 PL 2 2 EI 2 f 4.73= 1𝑃𝐿+ 𝐸𝐼 (1) (1) 𝑓 2π 1L2 EI π2 m 1 2𝜋𝐿 𝐸𝐼𝜋 𝑚 where I isis the the moment moment of inertia, L (m) is the length, m (kg/m) is is the the mass mass per per unit unit length, length, and and PP isis the the totaltotal compressive compressive axial load on fixed fixed ends [20]. [20]. More More det detailail is is given given in in [18]. [18]. In In addition addition to to Equation Equation (1), (1), COMSOLCOMSOL was was used used to to build the CMOS–MEMS resonato resonatorsrs (Figure (Figure 11)) andand calculatecalculate theirtheir resonanceresonance frequencyfrequency responses responses with with respect respect to temperature. The The simulation simulation environment environment was was selected selected as as a a vacuum, andand ambientambient temperature temperature (Tamb) (Tamb) was was set set to 273 to K.273 Solid K. Solid mechanics, mechanics, heat transfer, heat transfer, and electric and electriccurrents currents tools were tools combined were combined together together in multiphysics in multiphysics to couple to heat couple transfer heat with transfer solid with mechanics solid mechanicsand electric and currents. electric Mesh currents. study Mesh was conductedstudy was conducted to find the optimumto find the mesh optimum set up mesh for the set simulation.up for the simulation.Both the “extremely Both the “extremely fine mesh” fine and mesh” “fine and mesh” “fin weree mesh” compared were compared to decrease to decrease time budget, time budget, where wheretetrahedral tetrahedral meshing meshing was used was throughout used throughout the structure. the structure. There was There only was a slight only changea slight observed change observedbetween thebetween results. the Polysilicon results. Polysilicon conductance conductance was set as was 1.16 set× 10as5 1.16S/m × as 10 it5 wasS/m alreadyas it was measured already measuredand verified and [18 verified]. Electric [18]. current Electric was current used to was heat used the beamsto heat via the Joule-heating beams via Joule-heating while the heat while transfer the heatmodule transfer was usedmodule to model was used temperature to model distribution temperature throughout distribution the throughout beam and solidthe beam mechanics and solid was mechanicsused to model was deformationused to model and deformation mode shapes. and mode shapes. TheThe resonance resonance frequency frequency tuning range with the the a applicationpplication of of Joule-heating Joule-heating was was around around 761 761 kHz kHz when the pull-in force was 7 MPa, and it was ar aroundound 276.5 kHz when it was 10000 10,000 MPa MPa (Figure (Figure 2a).2a). This is attributed to to the the fact fact that that both both the the pull-in pull-in force force and and Joule-heating Joule-heating results results in in beam beam bending. bending. Pull-inPull-in force, force, however, however, created an ignorable stress on the fixed fixed ends in comparison to Joule-heating andand consequently consequently results results in a very small frequency tuning range [21]. [21]. In In another another words, words, the the bending bending shouldshould be be resulted resulted mainly mainly because because of of thermal thermal stre stressessses (Joule-heating) (Joule-heating) while while keeping keeping the the pull-in pull-in force force asas minimum minimum as possible to get the maximum frequency tuning range. The slope of the resonance frequency with respect to the applied Joule-heating voltage (Vth) was not constant but kept on increasing (α4 > α3 > α2 > α1) (Figure2a) with an increase in temperature. This nonlinear effect was first observed in [14], and allows better FS at higher temperatures (Figure2b) and consequently enables higher sensitivity temperature sensor design. This effect was analyzed partially in [4], and the temperature sensitivity was found as 2.98 kHz/C without any Joule-heating application.

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Figure 2. The effect of pull-in force (F) on the (a) Frequency tuning and (b) frequency shift (FS) in COMSOL simulationsimulation forfor DeviceDevice 1 1 for for a a length length of of 120 120µ mµm long long fixed–fixed fixed–fixed beam, beam, where where Fr1 Fr1 and and Fr2 Fr2 are theare resonancethe resonance frequency frequency responses responses with ambient with am temperaturebient temperature of Tamb andof Tamb +and 1 K respectively.Tamb + 1 K respectively. In contrast to [4,14], here we studied the FS in detail by combining both ambient temperature (Tamb)The change slope of and the resonance Joule-heating frequency for highly with respec sensitivityt to the temperature applied Joule-heating sensors in voltage microbolometer (Vth) was application.not constant Thisbut kept required on increasing the full analysis (α4 > α of3 the> α2 frequency > α1) (Figure response 2a) with (Figure an 2increasea) where in the temperature. resonance frequencyThis nonlinear decreases effect was until first it reaches observed the in bending [14], and point allows and better then FS starts at higher to increase. temperatures Two different (Figure resonance2b) and consequently frequency (Fr1, enables Fr2) higher responses sensitivity with respect temp toerature applied sensor Joule-heating design. This voltage effect were was calculated analyzed viapartially COMSOL in [4], at and two the different temperature environment sensitivity temperature was found (Tamb1as 2.98 kHz/C = 273 Kwithout and Tamb2 any Joule-heating = 274 K) in Figuresapplication.2–4. Hence, FS for 1 Kelvin change can be derived by subtracting resonance frequency responsesIn contrast (Fs = Fr1to [4,14],−Fr2) forhere every we studied applied the Vth FS (Figures in detail2b, 3by and combining4). The optimum both ambient device temperature operation point(Tamb) (larger change FS, and consequently Joule-heating better for sensitivity) highly sens wasitivity found temperature around the sensors bending in pointmicrobolometer (Figure2a) whereapplication. the beam This wasrequired just startingthe full analysis to have aof 0.38 the frequencyµm bending response and gives (Figure maximum 2a) where FS valuesthe resonance (X and Y).frequency The FS wasdecreases 5 kHz until when it Vth reaches = 0 V the and bending reaches uppoint to 60.6and kHzthenwhen starts Vth to increase. = 3.196 V Two and −different92 kHz whenresonance Vth wasfrequency 3.216 V.(Fr1, If Vth Fr2) is switchedresponses from with 3.196 respect V and to 3.216applied V, then Joule-heating total absolute voltage FS will were be X+|Y|calculated = 152.6 via COMSOL kHz (Figure at 2twob). Moreover,different environment the pull-in force temperature should be (Tamb1 as small = 273 as possible K and Tamb2 to create = 274 as sharpK) in Figures a bending 2–4. curve Hence, as possibleFS for 1 Kelvin (Figure change2a). This, can in be turn, derived would by result subtracting in a larger resonance X and |Y| frequency value andresponses consequently (Fs = Fr1 larger−Fr2) FSfor and every much applied better Vth sensitivity. (Figure 2b, The 3 totaland absolute4). The optimum FS was around device 152.6operation kHz (|TCF|point (larger = 2,333,771 FS, consequently ppm/K at Vthbetter = 3.216sensitivity) V) when was pull-in found force around was the 7 MPa bending whereas point it was(Figure around 2a) 16.8where kHz the (|TCF| beam was = 16,185 just starting ppm/K to at have Vth = a 3.425 0.38 µm V) when bending pull-in and force gives was maximum 10,000 MPa. FS values (X and Y). TheFurther FS was optimization 5 kHz when was Vth conducted = 0 V and byreaches analyzing up to the 60.6 dimensional kHz when Vth effect = 3.196 to find V theand optimum −92 kHz structurewhen Vth for was the 3.216 sake V. of If larger Vth is FS. switched The Joule-heating from 3.196 isV studiedand 3.216 in V, Figure then3 totab forl absolute Device1 FS to will study be theX+|Y| effect = 152.6 of thickness kHz (Figure on FS2b). and Moreover, in Figure the4b pull-in for Device force 2should to study be as the small effect as ofpossible length to on create FS by as usingsharp COMSOL.a bending curve In the as same possible way, (Figure uniform 2a). heating This, in was turn, applied would to result the beams in a larger to calculate X and the|Y| FSvalue via Equationand consequently (1) in Figures larger3a FS and and4a. much That is better why maxsensitivity. temperature The total is used absolute to plot FS FS was in Figuresaround3 b152.6 and 4kHzb in(|TCF| contrast = 2,333,771 to the uniform ppm/K temperatureat Vth = 3.216 profile V) when in Figurespull-in force3a and was4a. 7 The MPa minimum whereas pull-init was around force was 16.8 appliedkHz (|TCF| to every = 16,185 beam ppm/K in Figures at Vth3 and = 3.4254 to getV) when the largest pull-in FS. force A good was match10,000 betweenMPa. COMSOL and EquationFurther (1) isoptimization achieved for was the conducted total absolute by FS.analyzing the dimensional effect to find the optimum structureThe CMOS–MEMSfor the sake of larger resonator’s FS. The width Joule-heating can go up is studied to 6 µm in with Figure a metal-3 3b for Device layer and 1 to can study go the up toeffect 5.1 µofm thickness [14] after on post-processing. FS and in Figure That 4b isfor why Device the thickness2 to study should the effect not of exceed length 4 µonm. FS Otherwise, by using devicesCOMSOL. cannot In the resonate. same way, In this uniform study, weheating set the was width applied as 4.5 toµ mthe (Figure beams1) to and, calculate hence, onlythe FS three via differentEquation thickness(1) in Figure profiles 3a and were 4a. That used. is why The max thinner temperature the beam, is theused larger to plot the FS FS in atFigure relatively 3b and low 4b temperaturein contrast to (T the < 285uniform K) as wastemperature demonstrated profile in in [4 ].Figure However, 3a and this 4a. behavior The minimum changed pull-in with the force increase was inapplied temperature to every (Figure beam3 in). TheFigure thickness 3 and 4 has to almostget the nolargest effect FS. on A the good FS atmatch the bending between point COMSOL according and toEquation Equation (1) (1) is achieved and COMSOL. for the The total total absolute absolute FS. FS was 146.5 kHz when the beam thickness is 1 µm, and itThe was CMOS–MEMS around 142.7 kHzresonator’s when the width thickness can go was up 3 toµ m6 µm according with a tometal-3 COMSOL. layer In and the can same go way, up to it is5.1 168.4 µm kHz[14] after when post-processing. the thickness is 1Thatµm, andis why 164.8 th mkHze thickness when should the thickness not exceed is 3 µ m4 µm. according Otherwise, to (1). Althoughdevices cannot there resonate. was no noticeable In this study, change we in set the the FS, width the thinner as 4.5 beamµm (Figure was preferable 1) and, hence, due to only requiring three different thickness profiles were used. The thinner the beam, the larger the FS at relatively low temperature (T < 285 K) as was demonstrated in [4]. However, this behavior changed with the increase in temperature (Figure 3). The thickness has almost no effect on the FS at the bending point

Micromachines 2019, 10, x FOR PEER REVIEW 5 of 7

Micromachines 2019, 10, x FOR PEER REVIEW 5 of 7 according to Equation (1) and COMSOL. The total absolute FS was 146.5 kHz when the beam accordingthickness isto 1 µm,Equation and it (1) was and around COMSOL. 142.7 kHz The when total the absolute thickness FS waswas 3 146.5 µm according kHz when to COMSOL.the beam thicknessIn the same is 1way, µm, itand is 168.4 it was kHz around when 142.7 the thickneskHz whens is the 1 µm, thickness and 164.8 was mkHz 3 µm accordingwhen the thicknessto COMSOL. is 3 µm according to (1). Although there was no noticeable change in the FS, the thinner beam was MicromachinesIn the same2019 way,, 10 it, 108is 168.4 kHz when the thickness is 1 µm, and 164.8 mkHz when the thickness5 is of 3 7 µmpreferable according due to to (1). requiring Although less there temperature was no noticeablefor bending change (Tbending in thepoint FS,= 312 the K,thinner Figure beam 3b) and,was preferableconsequently, due has to smallerrequiring thermal less temperaturestresses [18]. forThat bending is why the(Tbending length point study = 312 is conductedK, Figure 3b)for 1and, µm lessconsequently,thick temperature beam (Device has for smaller 2) bending in Figure thermal (Tbending 4. stresses point = [18]. 312 K,That Figure is why3b) the and, length consequently, study is hasconducted smaller for thermal 1 µm stressesthick beam [18]. (Device That is 2) why in Figure the length 4. study is conducted for 1 µm thick beam (Device 2) in Figure4.

Figure 3. Frequency Shift (FS) with respect to 1 Kelvin (K) change by (a) Equation (1), and (b) FigureCOMSOL, 3.3.Frequency Frequency when thickness Shift Shift (FS) (W)(FS) with changeswith respect respect from to 1 Kelvin1to µm 1 Kelvin to (K) 3 changeµm (K) for change byDevice (a) Equation by1 with (a) Equationa (1), device and ( lengthb(1),) COMSOL, and of 120(b) whenCOMSOL,µm. thickness when (W) thickness changes (W) from changes 1 µm tofrom 3 µm 1 µm for Deviceto 3 µm 1 withfor Device a device 1 with length a device of 120 lengthµm. of 120 µm.

FigureFigure 4.4.Frequency Frequency Shift Shift (FS) (FS) with with respect respect to 1 Kelvinto 1 Kelvin (K) change (K) change by (a) Equation by (a) Equation (1), and ( b(1),) COMSOL, and (b) µ µ µ whenFigureCOMSOL, length 4. Frequency when (L) changes length Shift ( fromL ) (FS)changes 50 withm fromto respect 110 50 mµm to for 1to Device Kelvin110 µm 2(K) withfor changeDevi a devicece 2by with thickness(a) aEquation device of 1 thickness (1),m. and of(b 1) COMSOL,µm. when length (L) changes from 50 µm to 110 µm for Device 2 with a device thickness of 1 The minimum length was set as 50 µm and the maximum one was set as 110 µm for the following µm. reasons;The 50 minimumµm beam length already was exceeded set as 50 the µm temperature and the maximum limit (Figure one was4b, set T bending as 110 point µm for= 650 the K) following that the µ CMOSreasons;The layers 50minimum µm could beam toleratelength already was and exceeded set 110 as 50m µm beamthe and temperature was the at maximum the limit limit of one(Figure stiction was 4b,set risk asTbending in 110 post µm point fab for= 650 process the K) following that due the to sacriﬁcing low stiffness constant. FS increased with the increase in length at relatively low temperatures reasons;CMOS layers 50 µm could beam tolerate already and exceeded 110 µm the beam temperature was at the limit limit (Figure of stiction 4b, riskTbending in pointpost = fab 650 process K) that duethe (FigureCMOSto sacrificing 4layers), and could thislow is stiffnesstolerate attributed andconstant. to 110 the µm fact FS beam that increased the was longer at withthe beam limit the has ofincrease stiction higher in TCFrisk length in values post at [fab 4relatively,14 process]; however, duelow thistotemperatures sacrificing is only valid (Figurelow before stiffness 4), the and bending constant. this is point.attributed FS Onceincreased to the the beam factwith that reaches the the increase thelonger bending beamin length point,has higherat the relatively shorter TCF values beam low resultstemperatures[4,14]; however, in larger (Figure FS this and is 4), only consequently and valid this isbefore attributed better the sensitivity. bending to the factpoint. The that totalOnce the absolute thelonger beam beam FS reaches increased has higherthe from bending TCF 169.9 values point, kHz to[4,14];the 382 shorter kHzhowever, according beam this results is to only COMSOL in valid larger before and FS it the isand increased bending consequently frompoint. 184.4 Oncebetter kHz the sensitivity. tobeam 419 reaches kHz The according thetotal bending absolute to (1) point, when FS µ µ theincreased lengthshorter from decreased beam 169.9 results fromkHz toLin =382 larger 110 kHz mFS according to andL = 50consequently tom COMSOL better and it sensitivity. is increased The from total 184.4 absolute kHz to 419FS increasedkHz Thereaccording from is no 169.9to study (1) kHzwhen conducted to the 382 length kHz on theaccording decreased effect of to fromwidth COMSOL L on= 110 temperature and µm it to is Lincreased = 50 sensitivity µm from because 184.4 kHz CMOS to 419 is notkHz a accordingThere custom is no process to study (1) when and conducted the the numberlength on the decreased of effect layers of andfrom width their L =on 110 thicknesses temperature µm to L = are 50 sensitivity wellµm deﬁned. because In addition,CMOS is highlynot aThere custom sensitivity is no process study temperature andconducted the sensorsnumber on the require of effect layers material of anwiddth their with on thicknessestemperature high thermal are sensitivity expansion well defined. because constant, In CMOSaddition, such asis aluminumnothighly a custom sensitivity layers, process andtemperature and this eliminatesthe number sensors the ofrequire possibilitylayers mate and ofrial their changing with thicknesses high the thermal width. are well expansion defined. constant, In addition, such highlyas aluminumThe sensitivity optimum layers, temperature structure and this eliminates issensors shaped require the according possibility material to of thewith changing results high thermal obtainedthe width. expansion from Figures constant,3 and such4 with a length of 57 µm and a thickness of 1 µm (Device 2). The total absolute FS is 343 kHz as aluminum layers, and this eliminates the possibility of changing the width. (|TCF| = 589,698 ppm/K at Vth = 3.3252 V, |TCF| = 2,178,946 ppm/K at Vth = 3.3476 V) where the maximum temperature around bending point is 530 K with a 0.14 µm bending. The optimum structure’s working temperature is limited to 530 K in this work because the maximum allowable Micromachines 2019, 10, 108 6 of 7

temperature for the similar structure in CMOS process was found to be around 530 K when 5.7 V and 17.4 mW was applied on embedded polysilicon layer [18]. The ﬁnal structure’s mesh was set to “extremely ﬁne mesh” with a very high-density sweep of Vth (0.0004 V resolution) to get the maximum accuracy in the results. The good match is achieved between COMSOL and (1); total absolute FS is 343 kHz according to COMSOL, and it is 356 kHz according to (1). The 0.14 µm thermal bending offers the potential for a high-density thermal detector array in CMOS. The total improvement of resonator’s sensitivity with respect to temperature can be derived from the ratio of the total absolute FS with Joule-heating application (X + |Y| (Figure2b)) over the FS without any Joule-heating application (at Vth = 0 V). FS at Vth = 0 V is 11.2 kHz, and total absolute FS is 343 kHz (Figure4b) for 57 µm long beam, and this brings around a 31× improvement in the overall sensitivity.

4. Conclusions Fixed–Fixed beam type CMOS–MEMS resonator was studied in detail and optimized to build the state-of-the-art temperature sensors for high-performance uncooled microbolometers. The best performance was achieved with 57 µm long and 1 µm thick fixed–fixed beam with a maximum temperature of around 530 K, that is close but still under the critical temperature in CMOS technology [18]. The total frequency shift increased from 11 kHz to 343 kHz (31×) for 57 µm beam with much larger |TCF| (2,178,946 ppm/K) while keeping the pull-in force application as small as possible. Furthermore, the nonlinearity problem of fixed–fixed beam type CMOS–MEMS resonator was addressed by operating the device around the beam bending point. A good match between COMSOL and theory was demonstrated and can be used as guidance in future researches to build an ultra-sensitive temperature sensor for microbolometers in CMOS technology. This in return, can enable a less expensive, compact, and wider range of application compatibility such as internet of things.

Funding: This research was funded by Scientific Research Project Foundation of Turkey (grant number 18073). Acknowledgments: The author especially wishes to thank COMSOL for their support in setting up the simulation environment accurately for CMOS–MEMS resonator in this study. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Marsili, F.; Verma, V.B.; Stern, J.A.; Harrington, S.; Lita, A.E.; Gerrits, T.; Vayshenker, I.; Baek, B.; Shaw, M.D.; Mirin, R.P.; et al. Detecting single infrared photons with 93% system efﬁciency. Nat. Photonics 2013, 7, 210–214. [CrossRef]

2. Chen, C.; Yi, X.; Zhao, X.; Xiong, B. Characterization of VO2 based uncooled microbolometer linear array. Sens. Actuators A Phys. 2001, 90, 212–214. [CrossRef] 3. Kang, D.H.; Kim, K.W.; Lee, S.Y.; Kim, Y.H.; Keun Gil, S. Inﬂuencing factors on the pyroelectric properties of

Pb (Zr, Ti) O3 thin ﬁlm for uncooled infrared detector. Mater. Chem. Phys. 2005, 90, 411–416. [CrossRef] 4. Gökta¸s,H.; Turner, K.L.; Zaghloul, M.E. Enhancement in CMOS-MEMS Resonator for High Sensitive Temperature Sensing. IEEE Sens. J. 2017, 17, 598–603. [CrossRef] 5. Hui, Y.; Gomez-Diaz, J.S.; Qian, Z.; Alù, A.; Rinaldi, M. Plasmonic piezoelectric nanomechanical resonator for spectrally selective infrared sensing. Nat. Commun. 2016, 7, 11249. [CrossRef][PubMed] 6. Zhang, X.C.; Myers, E.B.; Sader, J.E.; Roukes, M.L. Nanomechanical Torsional Resonators for Frequency-Shift Infrared Thermal Sensing. ACS Nano Lett. 2013, 13, 1528–1534. [CrossRef][PubMed] 7. Gokhale, V.J.; Rais-Zadeh, M. Uncooled Infrared Detectors Using Gallium Nitride on Silicon Micromechanical Resonators. IEEE Micromech. Syst. 2014, 23, 803–810. [CrossRef] 8. Hui, Y.; Rinaldi, M. Fast and high-resolution thermal detector based on an aluminum nitride piezoelectric microelectromechanical resonator with an integrated suspended heat absorbing element. Appl. Phys. Lett. 2013, 102, 093501. [CrossRef] 9. Larsen, T.; Schmid, S.; Grönberg, L.; Niskanen, A.O.; Hassel, J.; Dohn, S.; Boisen, A. Ultrasensitive string-based temperature sensors. Appl. Phys. Lett. 2011, 98, 121901. [CrossRef] Micromachines 2019, 10, 108 7 of 7

10. Tao, Y.; Boss, J.M.; Moores, B.A.; Degen, C.L. Single-crystal diamond nanomechanical resonators with quality factors exceeding one million. Nat. Commun. 2014, 5, 3638. [CrossRef][PubMed] 11. Jensen, K.; Kim, K.; Zettl, A. An atomic-resolution nanomechanical mass sensor. Nat. Nanotechnol. 2008, 3, 533–537. [CrossRef][PubMed] 12. Yang, Y.T.; Callegari, C.; Feng, X.L.; Ekinci, K.L.; Roukes, M.L. Zeptogram-Scale Nanomechanical Mass Sensing. ACS Nano Lett. 2006, 6, 583–586. [CrossRef][PubMed] 13. Baek, I.B.; Byun, S.; Lee, B.K.; Ryu, J.H.; Kim, Y.; Yoon, Y.S.; Jang, W.I.; Lee, S.; Yu, H.Y. Attogram mass sensing based on silicon microbeam resonators. Nat. Sci. Rep. 2017, 7, 46660. [CrossRef][PubMed] 14. Gökta¸s,H.; Zaghloul, M.E. Tuning In-Plane Fixed–Fixed Beam Resonators with Embedded Heater in CMOS Technology. IEEE Electron Dev. Lett. 2015, 36, 189–191. [CrossRef] 15. Escorcia, I.; Grant, J.P.; Gough, J.; Cumming, D. Terahertz Metamaterial Absorbers Implemented in CMOS Technology for Imaging Applications: Scaling to Large Format Focal Plane Arrays. IEEE J. Sel. Top. Quantum Electron. 2017, 23, 4700508. [CrossRef] 16. Eminoglu, S.; Tanrikulu, M.Y.; Akin, T. A Low-Cost 128 × 128 Uncooled Infrared Detector Array in CMOS Process. IEEE J. Microelectromech. Syst. 2008, 17, 20–30. [CrossRef] 17. Forsberg, F. CMOS-Integrated Si/SiGe Quantum-Well Infrared Microbolometer Focal Plane Arrays Manufactured with Very Large-Scale Heterogeneous 3-D Integration. IEEE J. Sel. Top. Quantum Electron. 2015, 21, 2700111. [CrossRef] 18. Gökta¸s,H.; Zaghloul, M.E. The implementation of low-power and wide tuning range MEMS ﬁlters for communication applications. Radio Sci. 2016, 51, 1636–1644. [CrossRef] 19. Jha, C.M. Thermal and Mechanical Isolation of Ovenized MEMS Resonator. Ph.D. Thesis, Department of Mechanical Engineering, Stanford University, Palo Alto, CA, USA, 2008. 20. Abawi, A.T. The Bending of Bonded Layers Due to Thermal Stress. Available online: http://hlsresearch. com/personnel/abawi/papers/bend.pdf (accessed on 23 October 2014). 21. Hopcroft, M.A. Temperature-Stabilized Silicon Resonators for Frequency References. Ph.D. Thesis, Department of Mechanical Engineering, Stanford University, Palo Alto, CA, USA, 2007.

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