A Review of Micromachined Thermal Accelerometers

This is an author-cre ated, un-copyedited version of an article published in Jou rnal of Micro mechanics and Microenginee ring . IOP Pub lishing Ltd is not r esponsib le for any errors or omissions in this version of the manuscript or any version deri ved from it. The Version of Record i s available online at https://doi.org /10.1088/1361-6439/aa9 64d [Journal of Micromechanics and Microengin eering 27 (1 2), 123002 (18pp), N ovember 20 17] A Review of Micromachined Thermal Accelerometers Rahul Mukherjee*, Joydeep Basu, Pradip Mandal, Prasanta K umar Guha Dept. of Electronics & Electrical Communication Engineering Indian Institute of Technology Kharag pur, Kharagpur 721302, India * E-mail: rahul10.iitkgp@gmail.com Abstract: Thermal convection based micro-electrom echanical accelerom eter is a rel atively new kind of acceleration sensor that do es no t require a solid proof mass, yielding unique benefits li ke high shock survival rating, low production cost, and integrability wit h CMOS integrated circuit t echnolog y. T his article provides a comprehensiv e survey of the research, developm ent, and current trends in the field of thermal accele ration sensors, wi th detailed enum eration on th e theory, opera tion, m odeling, and numerical simulation of such devices. Different reported varieties and structures of thermal accelerom eters have been reviewed highlighti ng key design, implementation, and performance aspects. Materials and technologies used for fabrication of such sensors have also been discussed. Further, the a dvantages and challenges for thermal accelerometers vis-à-vis other prominent accelerom eter types have been presented, followed by an overv iew of associa ted signal cond itioning circui try and potential a pplications. Keywords: Thermal a ccelerometer, convec tion, micro machined iner tial sensor, CMOS, MEMS 1. Introduction : Accelerometers have emerged as a ubiquitous sensor in recent times having high demand in the fields of consumer electronics, automotive, biomedical, defense, aerospace, navigation, and industrial applications [ 1] – [3]. The rapid progress of semiconductor fabrication technology has led to the development of predominantl y s ilicon based micro-electromechanical system (MEMS) accelerometers that are gaining popularity du e to features of small size, low power requir ement, high performance, and low cost [2]. The physical a cceleration required to be measured mi ght be stati c, like gravitational acc eleration; or dynamic, like vibration and shock. Some of the common sensor t ypes include capacitive ([4], [5]), piezoelectric ([6], [ 7]) , piezoresistive ([8], [9] ), and tunneling accelerometers [10] – most of them have a solid proof mass which changes it s position or shape du e to the applied acceleration. Owing to the mechanical movement involved, such devices h ave lower shock survival rating along with othe r issue like stiction, mechanical rin ging and h ysteresis. In contrast, thermal accelerometers sense Review of Microm achined Therm al Accelerom eters 2 acceleration b y measuring the displa cement of a tin y heated fluid bubble present within a sealed cavity. As there is no solid seism ic mass, the shock survivability o f a th ermal a ccelerometer is high. Its fabrication is also much simpler and the fabrication cost is low . Moreover, th e integration of the s ensor with complementar y metal-oxide semiconductor (CMOS) si gnal conditioning integrated circuit (IC) on the same silicon die is also convenient. Micromachined accelerometer based on convective heat transfer was first demonstrated in 1997 by Leung et al. [11] . Since then, this field has received appreciable researc h attention leading to reported devices with a number o f pe rformance aspects surpassing that of the counterparts. Nonetheless, the relative newness of the domain of MEMS thermal acceleration sensors coupled with the ever increasing scope of human-machine interaction offers ample scope for future research and innovation. This review article is meant to be an enablin g medium in this direction, providing detailed insight on the operation, modeling, and performance aspects (like sensitivity , power and b andwidth) of thermal convective accelerometers. It surveys different reported varieties of thermal accelerometers, along with the materials a nd methods used by various research groups for fabrication of such accelerometers. A brief description of the finite element simulation of such senso r design and performance have been provided using a standard software package. Furthermore, commercial developments and potential applications of thermal accelerometers and its comparative evaluation with the dominating capacitive accelerometer t ype have been summarized. The prospects of integ ration of thermal inerti al sensors with the necessary si gnal conditio ning (amplification, filtering, etc.) electronics hav e also been surveyed, followed by a concluding discussion on the future research directions in this field. 2. Workin g P rinciple : The operation of ther mal inertial sensors is based on the natural convection of fluid. A general structure of a single-ax is thermal accelerometer is illustrated in figure 1(a) which consists of a micro cavity created by front -side bulk mi cromachining of silicon wafer. An electrical resistive heater is suspended at the c enter of the cavity and a pa ir of temperature sensors (like thermistor; or thermopile made of serially connected thermocouples) are placed s ymmetrically a round the heater. The fluid (sa y air) p resent in the cavit y remains encapsulated b y an outer cover (package). Due to the he at dissipation of the heater, a hot th ermal bubble of the fluid is form ed surrounding it . In st eady state (i.e., without any acceleration), the temperature profile within the cavit y remains s ymmetrical with respect t o the heater, and the Review of Microm achined Therm al Accelerom eters 3 symmetrically placed temperature senso rs det ect identical temper atures. However, in the presence of an applied acceleration, the temp erature profile gets ske wed due to physical displacement of the ther mal bubble as shown in figure 1(b). The temperature increases on one side of the heater and d ecreases at the other side as shown in the profile in figure 1(c). The resultant differential temperature (Δ T ) is proportional to the applied acceleration and is measured by the temperature sensors. Figure 1. (a) Schematic view of thermal accelerometer, (b) cross- sectional view along AAʹ line, and (c) temperature profile along AA ʹ . 3. Modeling and Simulation : In order to predict the performance of a parti cular thermal inertial accelerometer desig n, or equivalently, to be able to tune the phy sical desig n to obtain the desirable performance, description of such devices via mathematical modeling is imperative. 3.1: Analytical Modeling: The device performance can be mathematically anal y zed b y modeling the accelerometer using simple geometri es. For the structure in figure 1( a) , which has a Review of Microm achined Therm al Accelerom eters 4 centrally pl aced he ater and working fluid enclosed b y cavit y and ou ter cover, the heater can be modeled as a c ylindrical heat source and the outer cover as a lar ger cylinder at ambient temperature [12]. The model can be even simpli fied to a spherical structur e where the heater is a spherical source, and the outer cover is a larger sphere centering the heater and kept at ambient temperature [13] , [14] . Such a simplified geometry is shown in figure 2(a). Here, r and θ are the radial distanc e and a ngle of the spheri cal coordinate s ystem respec tively. The inner sphere represents the heater wi th radius r i and surface temperature T i . On the other hand, the outer sphere represents the cavit y wall surface having radius r o and wall temperature T o (where T i > T o ). A A' represents the vertical axis along midsection. Ratio of the outer to inner sphere radius is R (= r o / r i ). The concentric c ylinder model is generall y used for single -axis accelerometers, while concentric sphere for the dual axis ones. I n three-axis acceler ometers, the x and y -axes accelerations are applied in-plane while the z-axis acceleration is applied out -of-plane and these too can be modeled with concentric spheres. Figure 2. (a) Simplified model of thermal accelerometer represented using concentric spheres. (b) Temperature contour plotted by solving the governing equation s. The governing equ ations predicting temperature profile of a th ermal a ccelerometer device are based on the principle of conservation of mass, momentum and energy [15] which are as follows: .( ) 0 t        u (1) .. p t              u u u I f (2) Review of Microm achined Therm al Accelerom eters 5 2 . p T C u T k T t           (3) Here, u is the flow velocit y vector field,  is spatial divergence operator, p is the pressure, I is the total stress tensor, f denotes the bod y forces acting on the fluid. The parameters C p , ρ and k are the specific heat, den sity and thermal conductivit y of the fluid in the cavity, respectivel y. In the realm of micro-fluidi cs, various parameters de termine the convective and conductive thermal energy flow in a fluid. For sim plification, these p arameters are clubbed together to define some non-dimensional numbers as stated below. Use of these dimensionless numbers help to simplify the governing equations as well as the overall analysis. (i) Fourier number is defined as the ratio of the rate of heat conducted through a bod y to the rate of heat stored. Fourier number: 0 2 i F r   (4) where, τ is the characteristic time , and α is the thermal diffusivity of the fluid defined as the ratio of heat conducted to heat stored which essentially represents how fast the heat diffuses through a material and is expressed as: p k C    (5) Higher value of the Fourier number indicates a faster heat propagation through the body. (ii) Prandtl number represents the ratio of diffusion of momentum to diffusion of heat in a fluid. Prandtl number:  p C Pr k  (6) Here µ is the d yna mic viscosit y of the fluid. For air, the Pandtl numbe r is around 0.7 – 0.8 and the value is higher for oils (e.g., SAE10, SAE40 etc. ). The sensitivit y of t hermal accelerometer, depends upon the Prandtl number of the fluid pres ent in the cavit y. With higher values of Prandtl number of the working fluid, the sensitivity of the device increases. (iii) Grashof number is the ratio of buoyancy force to viscous force. Review of Microm achined Therm al Accelerom eters 6 Grashof number : 2 3 0 2 () a T T Gr l     (7) where, a is the applied acceleration, β is coefficient of fluid expansion, l is the characteristic length of the devic e and 0 () TT  is the temperature diff erence between the heater and the bulk substrate. As the sensitivit y of convective a ccelerometer depends on the working fluid, characteristic length of the device and the temperature difference between the heater and the bulk substrate, the Grashof number qualitatively indicates the sensitivity of the device. (iv) Rayleigh number is the product of Prandtl and Grashof numbers. Rayleigh number: R a = P r.G r (8) The sensitivity of thermal accelerometer is directly proportional to the Rayleigh number [16]. The governing equations (1) – (3) can b e solved using the bound ary conditions of t he thermal ac celerometer under consideration to obtain the temper ature distribution and the velocit y profile. The temperature distribution ( T’ ) ca n be obtained as follows [17] , [18] :     23 0 1 2 3 ' ( . ) . . ...      T T G r Pr T G r Pr T G r Pr T (9) Where, T 0 , T 1 , T 2 , T 3 ,… are dimensionless functions of radius ratio ( R ) and the radial distance ( r ). This can be further approximated as: 01 ' ( . )  T T G r P r T (10) Here, T 0 is the temperature profile due to thermal conduction from the inner to the outer sphere , and T 1 is the temperature profile due to fluid convection, as given by: 0 11 . 11 R T R R r     (11) 2 1 1 3 2 3 4 4 4 23 1 1 1 ln cos T C R A R C C A C C R R R R R            (1 2) where, C i and A i are coefficients which are function s of R. It is to be noted that the governing equations can also b e solved using numerical co mputing tools like MATLAB [ 19] to obtain the temperature contours (e.g., in figure 2(b)) and temperature profile inside the cavity and, hence the device sensitivity. Review of Microm achi ned Thermal Accele rometers 7 3.2: Numerical Simulation: As the structure of a practical accelerometer is quite complex, it cannot be easil y anal y zed by means of the anal ytica l equations. To stud y its various performance parameters like temperature profile etc., numerical simulators are required to be used. Different software tools are commercially available to thi s end, like A NSYS [ 20], CoventorWare [21], and COMSOL Multiphysics [ 22]. Here, an example has been presented in COMSOL to illustrate the numerical simulation steps involved in Finite Element Method (FEM) anal ysis of thermal accelerometers. The tool provides an environment where effects of different intercoupled physical phenomena ca n be simulated together to predict the overall effect. Through a number of physics user interfaces , information corresponding to the physical phenomena can b e incorporated b y means of variables and equations. For a convective acc elerometer, to get the effect of acceleration on the heated fluid within th e cavity, Joule heating p hysics and laminar flow ph ysics can be used in the numerical sim ulator [22] . The devices may b e specified in 2D (two dimensions) or in 3D (three dimensions). Compared to the 2D modeling where th e simulator considers dimension in one axis (z -axis) is infinite, in 3D, all dimensions are finite. As expected, simulation in 3D provides more accurate results but at the cost of CPU time . Figure 3 illust rates an ex ample of a microm achined dual-ax is convective accelerometer havin g a planar square-shaped heater placed at the center of a cuboid cavit y. Using the same principle as a single-axis thermal accelerometer as discussed in Sec. 2, a dual -axis acceler ometer measures acceleration along two orthogonal axes using two pairs of temperature sensors placed equidistant from the central heater. The heater plate is supported b y four arms clamped from the four co rners of the cavit y. The arms may b e made of pol ysilicon or m etal (as electrical conductor), and ox ide and/or nitride la ye rs (as insulating la yer around t he conductor) . For th e p urpose of findin g the temperature distribution due to the heater within the cavit y , the t emperature sensin g structures may be omitted. The working fluid (here air) also needs to be defined with its properties set as functions of temp erature. In this example, the temperature of th e outer walls (cavity and top cover) o f the devic e has been set at 300K. Th e device has been meshed in free tetrahedral mode [22], [23]. The critical regions li ke the square h eater corner regions and tapered r egions of the supporting arms have been meshed with higher resolution (smaller size of elements) to get better accuracy . The heat flows from the heat er to th e surrounding air as well as into the supporting arms creating the temperature contour as seen in fi gure 3(a). Further, acceleration has been Review of Microm achi ned Thermal Accele rometers 8 applied towards the right side using volume force of laminar flow physics in the air volum e. This produces th e ske wed temperature contour of figure 3(b) and (c). The temperature at an y point o f the structure, or along an y straight line can be p lotted using stationary study . W ith the help of these data, the sensitivity of the device can be determined by subtracting the temperature at two opposite equidistant points from the heater (prospective location for the temperature sensors) . With the help of time dependent stud y in simulation, the transient response and hence, the -3d B bandwidth of the sensor can also be determined [24], [25]. One can play with the heater and cavity shape/size, constituent material and fluid properties, etc. for improving the sensitivi ty of the device. Apart from sensitivity and bandw idth, other relevant performance parameters (specifications) are its measurement r ange, line arity, resolution, voltage n oise densit y o r noise equivalent acceleration (NEA), overload shock limit, power consumption, size etc. [26] , [27]. Figure 3. (a) Dual-axis convective accelerometer with square plate-shaped heater with supporting arms and temperature sensing structures. Temperature contour plots due to th e heater: (b) Top view without any acceleration; (c) Top view and (d) side view with 100g acceleration (g is the acceleration due to gravity ) . Review of Microm achi ned Thermal Acce lerom eter s 9 Table 1. Summary of reported perf ormance of a few thermal convective accelerometers. Type, Technology Heater material, Shape Temp. sensor type, Shape Working fluid Sensitivity Bandwidth/ Response time Meas./ linearity range Resolution/ RMS- noise Power/ Temp. Year & Ref. Single-axis, Si MEMS Poly-Si, bridge Poly-Si thermistor , bridge Air 60mV/g 20Hz ±1g 0.5mg 20mW 1997, [11] Single-axis Si, MEMS Pt, bridge Pt thermistor, bridge Air 1bar: 2.5mV/g 25bar: 138mV/g 20Hz 3g 0.3mg 54mW 2003, [36] Single-axis Si, MEMS Pt, bridge Pt thermistor, bridge Air 0.12°C/g 120Hz ±2g 0.25mg 70mW 2008, [24] Single-axis, Si MEMS Pt, bridge Pt thermistor, bridge He 2.15bar: 0.002°C/g 320Hz ±2g — 300°C 2011, [42] Single-axis, Si MEMS Pt, bridge Pt thermistor, bridge — 0.0045°C/g — 10,000g — 200°C 2011, [39] Dual-axis, Si MEMS Poly-Si, diamond Al/Poly-Si thermopile SF 6 3.5mV/g 25Hz 5g — 7.4mW 2011, [16] Dual-axis, SOI MEMS Si, circular Si thermistor, ring-shaped Air 13mV/g 250Hz ±5g 10mg 12.5mW 2007, [35] Tri-axis, MEMS+ Polymeric Al on polyimide membran e Al thermistor on polyimide membrane Air X, Y: 8mV/g Z: 2.2mV/g 4Hz ~0.6g — 45mW 2011, [47] Single-axis, CMOS MEMS Poly-Si, bridge Poly-Si thermistor, bridge Air 375mV/g 14.5Hz 10g 30mg 35mW 2006, [25] Single-axis, CMOS MEMS Pt, bridge Pt thermistor, bridge N 2 0.034°C/g 1025Hz (closed loop) — — 70 mW 2012, [90] Dual-axis, CMOS MEMS Poly-Si, meander Al/ Poly-Si thermopile Air 0.024°C/g — 150g — 200°C 2010, [60] Tri-axis, CMOS MEMS Poly-Si, square Poly-Si thermistor Air X: 8.8mV/g Y: 12.6mV/g Z: 0.45mV/g 20Hz 3g X, Y: 2.6mg Z: 60mg 10mW 2014, [62], [63] Review of Microm achi ned Thermal Accele rometers 10 4. Varieties of Thermal Accelerometers: A sel ection of unique and representative r eports of micromachined thermal accelerometers have been reviewed here for providing the reader with an insight of the different design aspects that might influence the performance of such devices. Also, table 1 provides an outline of the performance of some of the reported accelerometers. 4.1: Thermal Accelerometer in MEMS Process : The concept of thermal convective accelerometer without soli d proof mass was first patented in 1996 by Dao et al. [28] . Subsequently, Leung’s group from Simon Fraser Universit y w as the fi rst to implement such accelerometers by custom fabrication on silicon substrate [11] , [ 29]. Although the implemented devices were single-axis ones, but , dual-axis thermal accelerometers were also conceptualized. A cavit y of dimension 1.5mm×4 mm was generated by front-side bulk micromachining of Si wafer. L ightl y dope d pol ysilicon wa s used to realize the heater and thermistor bridges (1.5µm thick, 10µ m wide and 1500µm long). Poly-Si heater is quite popular because of its higher sh eet resistance compared to metal s, and thus , suitable resistance can be achieved in a miniaturized portion. However, polysilicon shows long-term drift of electrical resistance owing to electro-migration [30] , [31]. The convection accelerometer device was sealed in ceramic D I P-16 (dual inline package). Th e output sensitivit y was hi gh (~ 60 mV/g at a heater power of 20mW ) due to a large cavity size. It was shown ex perimentally that the device sensitivity increases linearly with the heater power and the square of the air pressure within the cavity. The m easured frequenc y response was from DC to 20Hz. A relativel y lower b andwidth is characteristic of thermal accelerometers (compared to capacitive accelerometers) owing to the slower response of thermal exchange. Using silicon-on-insulat or (SOI) technology, a thermal convection based inclinometer was reported in 2001, that can also be utilized as an accelerometer [ 32], [33]. SOI is a costlier variant of conventional Si wafer, with advantages like superior electrical insulation from the bulk substrate, and excellent etch stop and sacrificial layer functions due to t he buried oxide la yer underneath the active Si la yer; hence, y ielding better process control and device performance [33] , [34]. The heater an d thermistors of the reported device were made of lightly doped silicon , and the device was sealed in a TO -8 (Transistor Outline) metal can package with either air or SF 6 as the fluid in it . It was reported that the sensitivit y increased with increase of package volume reaching maximum at a package volume of 12, 000 mm 3 . With air as th e working fluid, the Review of Microm achi ned Thermal Accele rometers 11 sensitivity wa s 132µV/ ° and the response time was 110ms at heater powe r of 45 mW . The sensitivity got enhanced to 6.6mV/° with SF 6 due to its higher densit y , though the response became slow (240ms). Another report on SOI-MEMS dual-axis accelerometer studi ed the effe ct of thermal stress (due to temperature chan ges) appearing in the Si thermi stor structures c ausing its out-of-plane deformation and change in resistance due to piezoresistive effect, reducing the sensitivity [35]. The 2mm×2 mm× 0 . 4 mm device as seen in figure 4, consists of four thermistors (of two di fferent designs ) arranged in a ring-like s hape around the c entral heater (that was heated up to 200°C using 12.5mW ). The novel shape allowed the thermistor to deform freel y with temperature, hence, reducing the th ermall y ind uced stress b y 90% in comparison to a usual clamped – clamped bridge thermistor. An off-chip circuit was employed t o condition the sensor output. When tested in a measurement range of ±5g, the sensitivi ty was ~13 mV / g (fi gure 4(c)) with a resolution of 10 mg and the total noise (thermal and 1/f ) was estimated as 0 . 33 μ V. Figure 4. Micrograph of dual-axis thermal accelerometer with thermistors (of two designs (a) and (b)) arranged in a ring-like shape around the heater. (c) Experimentally measured sensor output voltage versus applied acceleration [35]. © 2007 IOP Publishing. (Reprinted with permission.) The dependence of sensitivi ty on the cavity gas-pressure was studied ex perimentally in [ 36] . Here, platinum was used to realiz e the heater and thermistors, and the cavit y dimension was 2 mm ×2mm×0 . 4 mm . Pt while being expensive is preferred for its linear temperature coefficient (TCR) of r esistance over a wide range, better r eliability and accur acy of resistor, and hi gh er thermal resistance with respect to other commonly used m aterials like Al or Si . However, Pt is Review of Microm achi ned Thermal Accele rometers 12 not CMOS compatible as opposed to poly-Si, Al and Cu; so, can’t be used in accelerometers in a CMOS process line. The device here was packaged in TO-16 metal can w ith the gas pressure being varied through a hole. The h eater tem perature was raised to 238K above ambient temperature b y appl ying a heat er pow er of 54 mW. At atmospheric press ure, the sensit ivity was 2.5mV/g. However, it increased proportional ly to the square of gas pressure to a value of 138mV/g at 25 bar. Also, using three pairs of detectors placed at 100, 300 and 500µm from the heater, it was d emonstrated that with an increase of gas pressure, the opti mum sensitivit y position comes closer to the heater. Thermal accelerometers generally tend to become nonli near in th e case o f a lar ge characteristic length (i.e., l arge cavity size), a t elevated level s of the heater temperature, and with higher acceleration. The linearity of convective accelerometers was studied by numerical and experimental methods in [37] using a device struct ure as in figure1 . The simulation revealed that the device output was linear with acceleration for Gr in the range o f 10 − 2 to 10 3 . The sensitivity and linearity were both optimum when the temperature sensor was position ed at one-third distance between the heater and the cavit y wall. The fabricated devic e had a cavit y dimension of 3mm×2m m× 0.25mm and the sensitivity was measured as 600µV/g in a range of 0 to 10 g w ith an operating power of 87 mW [38]. The bandwidth of the device was 75Hz. The NEA (due to thermal noise of the device) was 1 mg/√Hz at 25 Hz . The resolution was found to i ncrease (due to noise reduction) along with the sensitivity with increase in heating power. Although a large cavity volume ensures higher sensitivit y due to increased heat exchange, but it leads the device towards nonlinearity region. Garraud et al. [39] were able to measure high values of acceleration with good linearity b y reducing the sensitivit y of the device to 0.0045 K/g. The sensitivit y was reduced b y m eans of reducing the cavit y width to 600 μm and by lower ing the heater temperature. The devi ce detect ed acceleration in the linear region up to 10,000g. Sensitivity of a thermal accelerometer d epends on the siz e and shape of the heater structu re. Four types of heater structures were compa red (in [40], [ 16]) and it was shown t hat a diamond-shaped heater (shown in figure 5 ) provided higher temperature gradient at the temperature sensin g point in comparison to a square-shap ed heater. The diam ond-shaped h eater was further modi fied b y using hi gh resistive material at the corners due to which the tempe rature gradient at the sensor Review of Microm achi ned Thermal Accele rometers 13 position got increased . Al /pol y- Si thermopiles were utilized here as temperature sensors. Unlike other commonl y reporte d structures, here, b ack-side etching was also use d in addition to f ront- side etchin g to create a well -defined cavit y, minimiz ing performance variation due to cavity etching defects and ensuring device reliabilit y and reproducibility . The sensitivit y and bandwidth were 3.5mV/ g and 25Hz respectively, at input power of 7.4 mW and SF 6 as the working fluid. However, the sensitivi ty reduced to 30 µV/ g with air. An illustration of the involved fabrications steps are also shown in the figure. Figure 5. Fabrication process steps and an optical image of a micromachined dual-axis thermal accelerometer with diamond-shaped heater [16]. © 2011 Elsevier. (Reprinted with permission.) To improve the b andwidth of thermal accelerometers, the cavit y size should be made sm all and thermal diffusivity of the working fluid must be high. This w as anal yzed by Courteaud et al. [ 24] who obtained a -3dB ba ndwidth of 120Hz. The sensitivit y of the device was 0.12K/g when the heater power and the characteristic dimension were 70mW and 1120µm respectivel y. It was also shown that the sensitivi ty decreased almost linearly with increase of the external ambient Review of Microm achi ned Thermal Accele rometers 14 temperature. The effect of differe nt working fluids like air, CO 2 , N 2 , water, Ar , He, and eth ylene glycol in static and d ynamic conditions were studied numericall y in [41]. Different fluids yield different Gr and Pr , hence, produ cing different heat convection behavior and temperature distribution w ithin the accelerometer. This in turn affects the frequenc y response, sensitivit y, and linearity of the device. The highest temperature difference hence, highe st sensitivity was obtained using Ar due to its low d y namic viscosity . Air, CO 2 a nd N 2 pr oduced practicall y identical results because of sim ilar thermo-ph ysical properties. The accelerometer ’s sensitivit y and frequenc y response were ex perimentally studied as a function of the nature and pressure of fluid in [42] where a 320 Hz bandwidth with He gas at 2.15bar was achieved. Bandwidth was found to increase propo rtional ly with thermal diffusivit y of the enclosed fluid, and d ecrease as the pressure of the fluid increases. Gases with lower molecular weight ( like He) improves the fre quency r esponse, but increases the sensitivi ty to the ambient temperature of the p ackage and hence, ne eds more heater power [43]. The use of inert gases is further p referred because other gases might cause the heater and detector to oxidize or age quickl y. L iquids used as working fluid would produce a relatively slow er response while requiring large heater power. An isopropanol-filled struct ure in [ 44] achieved a s ensitivit y of about 700 times of that of an air- filled accelerometer, but, having an order of magnitude greater response time. Triple-axis accelerometers that can detect acceleration along all the three directions have considerably higher design and fabrication challenges. S o, it took a while for su ch a convective device to be implemented . By means of integration of polymeric material s into MEMS process to attain the required me chanical flexibilit y , the first tri-axial accelerometer was reported in 2008 [45], [46]. Using surface micromachining on Si subst rates with pol yimide as a structural la ye r , out-of-plane/buckled sensing structures were assembled. A Cr/Au bila yer and a Ni la yer we re used to form thermo couple junctions for the thermopiles placed on the sensor plat es. The measured sensitivit y were 66, 64, and 25 μ V/g on the X, Y, and Z-axes respectively, using SF 6 as working fluid at a tot al heater power of 2.5 mW . Later, a novel design consisting of three flexible polyimide membranes (two identical Z-axis membranes on either side of a central membrane ) encapsulated within four pol ymeric microparts was reported, as seen in fi gure 6 [ 47], [48]. The central membrane included the heater and temperature senso rs for X and Y-axes, while the upper and lower membranes had sensors for Z-axis. The X and Y sensitivity was about 8mV/g and the Z-axis sensitivity was 2.2mV/g, with an overall power of 45mW and the bandwidth was 4Hz. Review of Microm achi ned Thermal Accele rometers 15 Figure 6. Tri-axial micromachined thermal accelerometer [47]. © 2011 Elsevier. (Reprinted with permission.) 4.2: Thermal Accelerometer in CMOS-MEMS Process: In contrast to the accelerometers in the previous subsection, monolithic CMOS-MEMS thermal accelerometers can have the signal conditioning circuit on the same chip as the sensor as both ar e fabricated on the same substrate (die ) usin g C MOS I C process in conjunction with some MEMS specific process steps [49] , [50]. Hence, these are compact, cost effective, and ins ensitive to parasitic compon ent effects. Moreover, the frequenc y r esponse might also improve alon g with a lower power requirement. In the custom MEMS process based acc elerometers discussed previous ly , various materials can be utili zed for implementing the heater and the temperature sensor; and the mat erial layer thickness es can als o be s et as desired. But, in a standard CMOS process, the layer materials and thi ckness es are predefined by the IC fabrication foundry. Moreover, the cavit y size of the device cannot b e made large as the devi ce cost increases with the die size. Hence, the major limitation of thermal accelerometers implemented in this process is its comparativel y low er sensitivit y due to limited cavity size and available materials. The foremost report was a single-axis convective accelerometer impleme nted in a 2µm CMOS process and packaged in ambient air b y Milanovic et al. in 1998 [51] , [ 52]. Both thermopile and thermistor types of temperature detectors w ere te sted which yielded sensitivities of 136µV/g and 146µV/g respectivel y, wit h relatively lower pow er r equirement (81mW ) with thermopile. Good Review of Microm achi ned Thermal Accele rometers 16 linearity in the range of 0 to 7g was observed for both the devices, and frequency response of up to hundreds of Hz was obtained. Sensitivit y of the devices was found to be a nearly li near function of heater power (temperature). A single-axis thermal accelerometer fabricated in AMS 0.8µm CMOS process i s shown in figure 7 [ 25], [53]. The cavity was generated by front-side bulk micromachining process , and pol y- Si resistors were used to realiz e the heater (40µm×1040µ m) and the rmistor (3 0µm×700 µm ) bridges having a separation of 200µm . The sensor output voltage was processed by a CMOS signal conditioning amplifier with controllable gain, whi ch was present on the same chip as the sensor as seen in figure 7. The h eat er temperature was 438 ° C using 35mW of power and the device sensitivity was experi mentally measured as 375mV/g (or equival entl y, 1.53 ° C/g) with a resolution of 30 mg. It had good linearity till 10g a nd th e -3dB band width was 14.5Hz. Optimum device dim ensions were formulated b y means o f FEM study of the effect of the cavit y and package width and hei ght on the device sensitivity and time constant [54] , [55]. The sensitivit y was found to increase li nearl y with the package cover height fo r a wide range. Due to etchin g defects, the cavity depth can get reduced due to which the thermal bubble size gets reduced leading to deterioration of sensitivity [55]. The temperature distribution and maximum sensitivity positions were obtained via 2D and 3D sim ulations and were compared in [56]. W ith respect to 2D, in 3D simulation, the thermal bubble got constricted and sensitivity reduced to a value closer to that obtained experimentally. Th e optimum se nsitivity position for th e temperature sensors also changed from middle to one-third distance between heater and cavity wall. Figure 7. Prototype micrograph and schematic cross-section of thermal accelerometer fabricated in CMOS process with on-chip signal conditioner [53]. © 2008 Elsevier. (Reprinted with permission.) Review of Microm achi ned Thermal Accele rometers 17 A dual-axis thermal accelerometer fabricated in TSMC 0.35µm CMOS process was designed using a micro heater and two pairs of thermopiles being used as temperature sensors [57] ‒ [59] . The cavit y dim ension w as 830µ m×830µm. The thermocouple was mad e b y n-t ype pol y- Si and Al junctions. The heater and thermopiles were co nnected b y net like structures of SiO 2 / Si 3 N 4 to enhance the mechanical stability. Th e cold junctions were placed on the Si substrate and the hot junctions we re formed over the net near to the heater. The devi ce was tested as an in clinometer from which the acceleration sensitivity was derived to be 22µ V/g at input power of 9.05mW. As the temperature is sensed at the junction of two different materials in a thermocouple, it require s less space at the hot junction where the temperature difference is meas ured. The noise volta ge was under 0.25μV and the NEA was 0.159g. Another reported dual-ax is accelerometer fabricated in AMS 0.35 µm CMOS process used meander-shaped heater o f siz e 100µm×100µm and a cavit y dimension of 600µ m×600µm [14] , [60]. The sensitivit y was found to be 0.024K/g which is somewhat low due to a low Seebeck coefficient of 6.54µV/K of the Al/pol y - Si thermopiles used, and lower cavit y siz e and heater temperature. Acceleratio n was measu red till 150g using heater temperature of 200 º C. They also utilized infrared temperature mapping to experimentall y determine the tem perature profile within the device. An optimized square-ring shap ed he ater structure was reported in [61] to improve the sensitivity. It was also s hown that the sensitivit y improved when a given peak temperature is generated at the edges of the heater rather than at its center. A monolit hic triple-axis thermal accelerometer was implemented by Maill y et al. using AMS 0.35µm CMOS process, which did n’t require th e complex assembl y op erations of the tri-ax ial sensors as discussed in the previous subsection [ 62], [63]. The principle is to sense the acceleration in the third axis b y m eans of common-mode temper ature mea surement using X and Y detectors of a 2 -axis convective accelerometer structure as illustrated in figure 8. Acceleration in the posit ive Z directio n stretches the hot bubbl e leading to temperature drop at the detectors, while reverse occurs due to negative Z-axis acceleration. Experimentally obtained sensitivity values were 8.8mV/g, 12.6 mV/g and 0.45mV/g for X, Y and Z-axes respecti vely at heater power of 8.3 mW . In comparison to the in-plan e sensitivity, the low Z-axis sensitivit y was attributed to unoptimized thermal sensor positi on and 3D thermal effects that demand proper understanding . Accelerometers based on a similar sensing metho d have also been comme rcialized b y MEMS IC [64], [65]. Review of Microm achi ned Thermal Accele rometers 18 Figure 8. Detection of out-of-plane acceleration using in-plane temperature detectors within a cavity that is asymmetric along the Z-axis [63]. © 2014 Springer. (Reprinted with permission.) 4.3: New and Innovative Devic es : A number of unique instances of reported th ermal accelerometers that in corporat e innovative materials and structur es have b een presented in t his subsection . S uch modifications are targeted towards enhancement of the different performance aspects. 4.3.1 Organic & Plastic Substrate Petropoulos et al. [66] reporte d a th ermal a ccelerometer fabricated on an organic PCB (printed circuit board ) subst rate with heater and temperature sensors made of platinum . The substrate had a much low er th ermal conductivit y (0.2Wm -1 K -1 ) than that of conventionally us ed silicon or SiO 2 , hence, providing b etter insulation for leaka ge of the he at er power t hrough th e substrate. The working fluid used was water which was covered b y a tank o f dimension 3.5 cm ×1.5cm and a depth of 850µ m. W hen 60mA cur rent was pas sed through the h eater, t he sensitivit y obtained from the device was 32mV/g. The achieved sensitivit y was also high due to large siz e o f the sensor and higher operating power. On a similar note, a flexible pol y imide substrate with low thermal conductivit y was used in [ 67]. Further, they compared CO 2 and xenon gases as working fluid and concluded tha t the sensitivit y wa s larger with CO 2 ; but with Xe, higher levels of acceleration (25g) could be measured without saturating and the response was also faster. 4.3.2 CNT Based Components The heater and temperature detectors were fabricated usin g multi-walled carbon-nanotubes ( MW CNT) by Zhang et al. on 1mm-thick glass subst rate [68] , [69] . The advantage of usin g Review of Microm achi ned Thermal Accele rometers 19 CNTs in the devi ce is a ult ra-low power requirement (in the order of tens of pW) and smaller size of the det ectors. MW CNT has negative TCR, which means a drop i n the output volta ge of the thermistors with increase in its temperature. The accelerometer’s r esponse was tested at three ranges o f the heater power. At substantiall y low value of th e heating current (< sev eral nA), the detector temperature gets reduced towards ambient temperature and measurement noise became dominant. On the contra ry, when the heating current was ver y high (several hundr ed nA), the sensor response became too low due to heating of the CNT as well as the connecting electrodes , surrounding air and the substrate. This is because the acceleration induced convection couldn’t quite affect the temperature of the CNTs which showed ver y low resistance change. F or current levels in between th ese two extremes (~0.1µ A) , the device worked pro perly and produced a linear-log relationship b etween the sensor response and applied acceleration. The sensor was specifically found to be sensitive in detec ting small acceleration va lues (~0.1 m/s 2 ) with the response getting saturated at higher acceleration. A novel thermal convective inclinometer using yarn of CNT for both the heater and thermal detectors wa s reported r ecently that also consumes significantly smaller heater powe r of 33 μW and yields a sensit ivity of 1.8 μV/g [ 70]. The CNT yarn exhib its a stable resistance over a wide range of temperatures, hence producing good device linearity. 4.3.3 Porous Silicon Based Device Instead of havin g front-side bulk micromachining to make a cavity, a 60 μ m thick porous silicon (PS) la yer was us ed for thermal isolation between the thermal accelerometer heater and th e underlying substrate in [71] . The PS lay er has a thermal conductivity of 1.2 W /mK that helps to confine the generated heat within the desired region by the rmally isolating the Si substrate. So, the device c an be operated at relatively low power. Also, due to the elimination of any freestanding structures he re, the s ensor becom es more immune to shock, aging effects, a nd calibration errors. Its hea ter was designed usin g pol y- Si and the temperature sensor using poly- Si / Al thermopiles. The die size was 1.4mm×0.9mm and the device was packaged in ceramic DIP- 8. The re ported device s ensitivit y was 13mV/ g, linearity range was up to 6 g acceleration, and the bandwidth was around 12 ‒ 70Hz. The power supp lied to the heater was 166mW . The device was also tested using different packaging of air and oil (SAE 20 ) as working flu id, namely, with non- sealed air, non -sealed oil , and sealed oil package [72] , [73]. Due to much higher viscosit y and thermal conductivit y of oil with respect to air, a higher sensitivit y was found in the latter two Review of Microm achi ned Thermal Accele rometers 20 cases; the highest bein g in the second case due to free oil surface. However, t he best linearit y was achieved by the third configuration due to the absence of fluid surface movement. 4.3.4 Novel Device Structures In a recent r epo rt , the ca vit y stru cture of accelerometer was modified to improve the sensit ivity [74]. Silicon islands were placed in between the heater and temperature sensors as in figure 9 . Since the thermal conductivit y of Si is much more than that of the fluid, presence of Si islands modulates the fluid movement and hence, the temperature profile. The sensitivity obtained from the cavity with island structure was 0.657K/g which is double of that without islands (0.335K/g). Figure 9. Silicon islands within the cavity enhance the sensitivity of the convective accelerometer. Due to the good temper ature sensitivi ty (t ypically – 2.2mV/°C) of a diode [75], it can be utilized as the temperature senso r in thermal accelerometer to improve its sensitivity. Additionally, the sensitivity can be fu rther enhanced b y connecting two or more diodes in series [76] . However, it is extremely challenging to integrate Si diode in the cavity because of the need to etch silicon t o realize the suspended membrane within the cavity . 4.4: Commercial Thermal Acce lerometers: Capacitive sensing bein g the most common and popular accelerometer variety in the market, it seems pertinent that its performance aspects be compared with that of a thermal accelerometer. While a lar ge number of manufacturers li ke Analog De vices, Bosch, NXP /Freescale, STM , TDK/InvenSense, etc. market cap acitive MEMS acceleration sensor chips, MEMS IC is currentl y the onl y major player providing commerciall y av ailable thermal accelerometers since 2002. In table 2, a tri-axial thermal accelerometer from MEMS IC manufactured in standard submicron Islands Review of Microm achi ned Thermal Accele rometers 21 CMOS process with Al /pol y- Si thermopiles [77] have been compared against an Analo g Devices capacitive sensor that us es a poly- Si surface-micromachined proof-mass b uilt on top of Si wafer [78]. The devices have been selected to have the same DOF, comparable measurement ran ge, and both provides analog output volta ges that are proportional to the applied acceleration. As evident, the thermal one has a remarkable shock survival rating while its frequency response is comparatively poor. Table 2. Comparison of commercially available thermal and capacitive acceleromet er t ypes. Thermal accelerometer Capacitive accelerometer Model number MXR9500MZ ADXL327 Sensitivity 500mV/g 420mV/g Bandwidth 17 Hz 550/1600 Hz Full scale range ±1 .5g ±2g Nonlinearity 0.5% of full scale ±0.2% of full scale RMS noise density 0.6/0.9 mg/√Hz 0.25 m g/√Hz Cross-axis sensitivity ±2% ±1% Mechanical shock survival 50,000g 10,000g Supply current 4.2mA at 3.0V 350μA at 3.0V Package dimension 7mm × 7mm × 1.8mm 4mm × 4mm × 1.45 mm A latest offering MXC4 005XC from MEMSI C uses a relatively ne w wafer-level packagin g technology that integrates the step of pa ckaging with the fabrication of the wafer which is l ater diced yielding packaged chips that are practically of the same siz e as the die. This is very efficient in terms of size and cost, with a reported shock survival of 200,000g. 5. Properties of Materials Used in Accelerometer : Diff erent materials used for manufacturing of convective inertial se nsors can be found in lit erature, with each offering distinctive b enefits and drawbacks. Proper un derstanding of the v arious properties of these materials is im perative for successful re alization of the desirable d evice performance. For ex ample, a material like pol y- Si having quite low electrical conductivit y requires much less area for making a heater or thermistor, but it s TCR can be an iss ue. For Al/poly- Si thermocouples, the Seebeck coefficient is an important factor that depends on the dopin g. If the Seebeck coefficient is high, the voltage Review of Microm achi ned Thermal Accele rometers 22 change per °C will be b etter providing higher sensit ivity. Regarding the w orking fluid present in the cavit y, as discussed previously, properties like Gr and Pr are importa nt for performance like sensitivity and response time of the accelerometer. In order to reduce the power requirement, a material with low therm al conductivit y is appropriate as the substr ate to prevent the outflow o f generated heat e nergy throug h it . Table 3 su mmarizes some rel evant ther moph y sical a nd electrical properties of materials commonly used in implementing thermal accelerometers. Table 3. Properties (at ~300K) of some common materials used in thermal accelerometer fabrication [33], [79] ‒ [84]. Material Thermal conductivity k (Wm -1 K -1 ) Electrical conductivity σ (Ω -1 m -1 ) Density ρ (kg m -3 ) Dynamic viscosity µ (Pa s) Specific heat C p (J kg -1 K -1 ) Thermal diffusivity α (m 2 s -1 ) Silicon 156 2.33×10 -4 a 2,330 — 707 97.52×10 -6 Polysilicon 31 2.6×10 - 3 b 2,330 — 707 16.5×10 -6 Silicon dioxide 1.4 3×10 - 13 2,270 — 1000 6.2×10 -7 Platinum 71.6 9.4×10 6 21,500 — 133 24×10 -6 Aluminum 235 3.7×10 7 2,710 — 904 93×10 -6 MW CNT 750 c 10 4 ‒ 10 7 1650 — 730 4.6×10 -4 Air 0.026 — 1.2 1.9×10 -5 1005 22×10 -6 Carbon dioxide 0.017 — 1.7 1.5×10 -5 850 1.1×10 -5 Argon 0.018 — 1.6 2.27×10 -5 521 21×10 -6 SF 6 0.013 — 6.14 1.6×10 -5 598 3.5×10 -6 Water 0.6 — 997 8.3×10 -4 4071 1.52×10 -7 Oil 0.145 — 900 0.3 1910 8.5×10 -8 a Depends on doping. b Depends on grain boundary structure and doping. c Effective value i s smaller due to coupling with in MWCNT bundles, s heet imperf ections, etc. 6. Sign al Conditioning for Therm al Accelerometers : The temperat ure gradient generated within a thermal convection accelerometer produces a corresponding resi stance variation of its thermistor temperature detectors du e to their pos itive temperature coefficient of resistance. As illustrated in fi gure 10, t he thermistors can be arranged in a W heatstone b ridge alon g with a pair of re ference resistors which a re present on the substrate and he nce, maintained at ambient temperature [25], [53]. Thus, the bridge generates a differential output voltage proportional to the Review of Microm achi ned Thermal Accele rometers 23 applied ac celeration. I f a pair of thermopiles are instead use d as tempera ture sensors, a differential voltage signal is directly produced [57], [14]. But, the electrical output being ver y small in magnitude, is susceptible to noise. Thus, like most other sensors , in order to ensu re an accurate me asurement, t he analog output signal requires suitable conditi oning before it can be provided to an analog- to -digital converter (ADC) which enables further digital processing and acquisition of the signal [85], [86]. For thermal inertial sensors, the signal conditioning requirements mainl y en compass amplification and filtering. One or multiple amplifiers are necessary for boosting the level of the analog signal to match the full d ynamic ran ge of the succeeding ADC , while ensuring minim al noise addition, low offset, and a proper impedance match with the sensor. F ilters might be needed to remove noise at frequencies outsi de the signal of interest, and also for anti -aliasing requirement before the signal is sampled by the ADC. Further, as th e si gnal to be acquired has a low frequenc y range (dc to ~200Hz ), hence, data converters like successive a pproximation register (SAR) and sigma- delta ( ΣΔ ) ADC s are appropriate for thermal acceleration sensor applications. Figure 10. Thermal acceleration sensor and sig nal readout electronics [25] . © 2006 Elsevier. (Reprinted with permission.) Due to low frequency of the acc elerometer si gnal, the effect of Flicker (1/f) nois e becomes significant. Also, as in any si gnal conditioning chain, the noise contribution from the first circuit block determines the noise performance o f the overall system, so, the first amplifier should preferably be a low-noise instrumentation amplifier (IA). For their r eadout interface, Chaehoi et al. [ 25] ha ve used an on-chip instrumentation amplifier implemented with a popular three operational amplifier topolog y, having programmable gain (of 10, 100 or 1000 ) by means of Review of Microm achi ned Thermal Accele rometers 24 controlling the resistor R (figure 9). Circuit techniques for minimizing 1/f noise of the IA, like chopper stabilization, correlated doubl e sampling, la rge siz ed p -type M OS transistors for the differential input pair, etc. can be applied [53], [87]. Further, instead of measuring voltage, the output current from the thermopile detectors can be sensed using a low-noise transimpedance amplifier as demonstrated b y Goustouridis et al. [73]. Process variatio ns in the tempe rature sensing resistors mi ght l ead to si gnificant values of dc -offset at the input of the instrumentation amplifier causing the output of the si gnal condition er to get saturated. To avoid this, t he signal chain may be provided with the provision of cancelling out the dc-offset presented to its input without affecting any useful input dc signal [88]. A custom desi gned si gnal conditioning analog front -end can b e used (integrated monolithically as in figure 6, or at the package-level); or, a suitable commercially available of f-chip conditioning IC ma y be utilized for the thermal acceleration sensor [46], [ 49]. Of course, the former system - on - chip solution is prefera ble as the circuit can b e tuned according to sensor specific n eeds; and also due to the aspects of assembl y and packa ging size, cost, and the effects of parasitics from interconnects and bond-pads that might impede the system’s performance . HDL (hardware description language, e.g., Verilog-A) model s of convective accelerometer have been dev eloped that enables its co-simulation with associated CMOS readout electronics usin g SPICE circuit simulators [53]. This provides opportunity to opti mize the designs of the sensor and the circuit in synchronization in order to improve the overall sy stem performance. Innovative closed-loop c onvective accelerometers have been reported where the inertial sensor has been placed within a negative thermal feedback loop. This decreases the thermal response time, consequently yielding an increased bandwidth, without reduction in sensitivity [87], [ 89] . Garraud et al. implemen ted a closed-loop design by adding two r esistors placed close to the temperature detectors as depicted in fi gure 11 [90] . The bias currents o f these resistors w ere appropriately altered (the feedback si gnal) using an electronic P ID controller, so as to rebalance any temperature differenc e produced between the two detectors from applied acceleration. By this, a closed-loop bandwidth of 1025 Hz was attained using an usual the rmal accelerometer of ~70 Hz bandwidth. Another reported closed loop implementation is a thermal sigma-delta modulator having the thermo-electrical (first-order low-pass) response of the convective accelerometer as its loop-filter [91], [92]. This again provides an improved bandwidth, and also a direct digital output. Frequency response compensation circuit to extend the frequency response Review of Microm achi ned Thermal Accele rometers 25 of thermal accelerometer was provided in [ 43]. As the sensitivity of thermal accelerometer depends on temperature dependant properties li ke densit y, specific heat, dynamic viscosit y and thermal conductivity of the fluid within its cavity, circuitry to compensate the variations o f sensitivity over a range of temperature and suppl y volt age can also be provided on-chip [93]. Further, self-test circuit for continuous che cking of the integrity of th e heater, detector and associated circuitr y of the accelerom eter c an al so be made av ailable t o incre ase the devic e reliability [ 93], [ 94]. Lin and Lin [67] achieved to ex hibit a wireless thermal accelerometer b y having an RFID (radio-frequency identification) tag flip-chip bonded to it. Figure 11. Scanning electron microscope image and measured frequency response of convective accelerometer with thermal feedback arrangement [90]. © 2012 IET. (Reprinted with permission.) 7. Applications of Th ermal Accelerometer : A ccelerometers in general are used for a wide range of s ensing applications b y automotive, avionics, mili tary, robotics, and consumer goods industries. Some of the potential application areas for thermal accelerometers are as follows: i. Automot ive applications: compute rized e lectronic stabilit y control improve s a vehicle's stability b y dete cting loss of traction, skidding, rollover, etc. and automatically regulatin g the engine throttle and braking. Conve ctive inerti al sensors are well suit ed for use in such system s [95] . Further, usage in car air-bags that are meant to protect the passengers by instantaneously inflating in the scenario of a vehicular collision is also feasible [43]. ii. P redictive drop sensor: In a computer hard-disk, the read/write header can get dama ged due to a free fall. A thermal accelerometer provided inside the disk can be u sed to detect the Review of Microm achi ned Thermal Accele rometers 26 free fall and immediatel y tri gger a me chanism by which the disk’s header can b e safel y parked preventing a crash [96]. iii. Tilt/angle detection: Along with acceleration, convective ine rtial sensors can also be used as g yrosc opes to detect orientation and rotation of devices. These are required in consumer electronics such as digital camera, virtual- reality device, gesture r ecognition, and jo ystick [32], [33], [70]. iv. Vibration detection: Constant vi bration within any ma chinery creates wear and tear on its parts like the bearing, se als and couplings. M achine-mounted inertial sensors can be used to determine the condition of the machine as well as to predict the precise cause and location of problems bef ore an y considerable da mage might occur [97]. This may also be used for structural health monitoring of bridges, buildings, and aerospace systems [98]. v. Steady thr eshold sw itch: One of the advantages of thermal inertial sensor is its high shock survival rating. Using a shocking h ammer capable of generating an acceleration of up to 50,000g, and commercia lly available dual -axis accelerometer, the effect of heavy shock was tested [99] . With such a large impact, the output of the convective accelerometer got saturated. It was concluded that such devices c ould be used to determine if the applied impact has cross ed a certain threshold level. 8. Outlook an d Conclusions: The rapid progre ss of MEMS research, manufacturing, and marketing in the past few years coupled wit h the inc reasing demand for inertial s ensors strengthen the outl ook for thermal convective sensors. This article is intended as a guide for the researchers in this domain providing a comprehensive re view of the theor y, modeling, simulation, research innovations, involved materials, applications, and critical performance aspects. Few instanc es of convective accelerometers are alread y being a ggr essively m arketed. In spite of the advant ages, thermal inertial sensors in their present form are not competitive enough in the key aspects of acceleration sensitivi ty and frequency response . As discussed, prior efforts made to improve the sensitivit y and bandwidth inc lude modifying the cavity, heater, and package geometries, using different working fluids, and optimizing the s ensor pos ition. A larger c avity size may im prove sensit ivit y while sacrificin g the bandwidth , but, will require larger area for fabrication increasing the cost and deteriorating th e mechanical st ability. Usage of some fluids to gain sensitivit y might complicate the p ackaging. Intelligentl y designed closed-loop s ystems and Review of Microm achi ned Thermal Accele rometers 27 frequency response compensation circuits are ex cellent method of enhancing the bandwidth and needs to be explored further. Improving the out-of-plane sensitivit y of triple-axis convective sensors is also needed. Thus, sensitivi ty and ba ndwidth im provement remains an are a of active research that needs to b e addressed in the near future. With regard to power consumption, the main contributors are th e heater, and the leakage through the substrate. Higher heater pow er enhances its temperature that helps to increase the sensitivit y . To reduce the power, different innovative heater, thermistor, and subst rate stru ctures and materials h ave been experimented with in literature. But, low ener gy requirement being a fund amental necessity fo r mode rn battery-operated s ystems, keen focus needs to be maintained in lowering the power. Further, design of custom temperature compensation and self-test/calibration circuitry for enhancing the device reliabilit y is an i mportant research frontier. Investigations may also be done towards amalgamating t he concept of thermal accelerometer with other innovative inertial sensing solutions to improve the overall perf ormance , like with a recently proposed liquid state accelerometer that employs a tiny el ectrolyte droplet as the sensing body over fou r electrodes (anode-cathode-cathode-anode) used for re ad -out [100] . W ith applied accel eration, the droplet moves and the electrochemically induced output current (between the anode-cathode pairs ) changes due to convective transport of ions between the ele ctrodes. Thermal convective sensors are just over two decade s old, and hence, are expected to witness mu ch m ore research attention in the near future revealing exciting developments. Its inherent features of superior shock survival, simpli stic compact structure, low cost, wide measurement range, and integrability with CMOS will certainly help in garnering the necessary attention. 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