Tinjauan Mekanisme Aktuasi dan Penginderaan pada Perangkat Sensor Berbasis MEMS

Teknik Aktuasi

Teknik aktuasi adalah istilah yang diberikan untuk mekanisme yang mengubah energi input menjadi gerakan mikrostruktur. Ada berbagai prinsip dan pendekatan untuk menggerakkan perangkat MEMS [7, 50,51,52,53,54,55], yang paling penting di antaranya adalah:aktuasi elektrostatik [56,57,58,59,60,61,62, 63,64], aktuasi elektrotermal [4, 44, 65,66,67,68], aktuasi elektromagnetik [7, 69], dan aktuasi piezoelektrik [48, 70] (Gbr. 2). Perangkat sensor CMOS-MEMS logam-oksida-semikonduktor komplementer dirancang untuk bekerja dengan aktuasi elektrostatik menggunakan kapasitor pelat paralel atau aktuasi elektrotermal dengan menggunakan pemanas mikro. Di sisi lain, perangkat sensor PolyMUMPs dan MetalMUMPs dirancang untuk menggunakan aktuasi elektrotermal dengan menggunakan pemanas mikro tertanam [71].

Teknik aktuasi perangkat sensor berbasis MEMS

Perangkat berbasis MEMS dapat digerakkan dengan menerapkan gaya sinusoidal F (t ), melalui salah satu metode penggerak yang disebutkan di atas. Persamaan (1) digunakan untuk mencari persamaan umum gerak karena M adalah massa total pelat atas, b adalah koefisien redaman, k konstanta pegas balok fleksibel,

dimana F mewakili amplitudo gaya pendorong eksternal di z -arah, \(\ddot{\gamma }\)\(\ddot{\upgamma }\), \(\dot{\upgamma }\) \(\dot{\gamma }\) dan γ adalah percepatan, kecepatan dan perpindahan massa, masing-masing.

Aktuator elektrostatik cukup tersebar luas; mereka memiliki waktu respon yang cepat dan konsumsi daya yang rendah [72]. Di sisi lain, aktuator yang menggunakan prinsip elektrotermal sangat populer; mereka memiliki kekuatan pendorong yang besar dan perpindahan di bawah tegangan mengemudi yang rendah. Aktuator elektromagnetik memiliki beberapa keunggulan sebagai respons cepat, perpindahan besar. Aktuator yang menggunakan prinsip piezoelektrik memiliki sensitivitas yang tinggi meskipun gaya penggerak dan perpindahan per satuan tegangannya kecil. Oleh karena itu, pemahaman yang baik tentang prinsip yang terjadi sangat penting untuk mendapatkan perangkat berperforma tinggi dengan salah satu pendekatan aktuasi ini.

Aktuasi Elektrostatik

Sumber utama dari aktuasi elektrostatik adalah medan listrik kapasitor, dan itu bergantung pada gaya tarik menarik antara dua pelat sejajar dengan muatan yang berlawanan [12]. Aktuasi elektrostatik didasarkan pada hukum Coulomb yang menggambarkan gaya timbal balik antara dua muatan dengan jarak tertentu yang dihasilkan antara pelat tetap dan pelat bergerak. Aktuasi elektrostatik dianggap sebagai salah satu mekanisme paling populer untuk menggerakkan perangkat MEMS. Mereka memiliki desain yang sederhana, waktu respon yang cepat, kemudahan fabrikasi dan konsumsi daya yang rendah. Namun, non-linier dan peningkatan nilai tegangan penggerak adalah masalah utama dari jenis aktuator ini [72]. Beberapa jenis aktuasi elektrostatik harus dipertimbangkan dengan cermat saat mempelajari MEMS di bawah aktuasi elektrostatik. Bentuk yang paling umum didasarkan pada kapasitor yang terdiri dari dua pelat paralel sederhana atau struktur penggerak sisir yang terdiri dari beberapa jari interdigitated atau non-interdigitated [72, 73]. Sebuah metode konvensional [74] adalah aktuasi pelat paralel di mana pelat bergerak atas memiliki polaritas tertentu, dan pelat tetap bawah memiliki polaritas yang berlawanan (Gbr. 3). Perpindahan dan getaran pelat yang bergerak tergantung pada perbedaan tegangan antara kedua pelat. Bila polaritas muatannya sama, akan terjadi perpindahan tolak menolak antara pelat tetap dan pelat bergerak, tetapi bila kedua pelat memiliki muatan yang berbeda, pelat bergerak akan tertarik ke pelat tetap. Aktuasi dihasilkan ketika jenis muatan pada pelat bergerak diubah secara berkala [50, 56, 74].

Kapasitor pelat paralel untuk aktuasi elektrostatik [75]

Ketidakstabilan pull-in adalah salah satu masalah utama ketika pelat paralel digerakkan secara elektrostatik karena pelat tetap berada tepat di bawah microbeam dan gesekan statis menyebabkan perangkat runtuh [74, 76]. Aktuator penggerak sisir terdiri dari dua set sisir dari struktur jari interdigitasi (Gbr. 4). Mereka biasanya ditempatkan di pelat paralel yang sama dengan substrat di mana satu jari sisir dipasang, dan yang lainnya terhubung ke struktur fleksibel (misalnya, pegas) untuk bergerak menuju dan menjauh dari pelat tetap [72]. Ketika tegangan yang berbeda diterapkan antara sisir bergerak dan jari sisir tetap, gaya elektrostatik dari bidang pinggiran menarik kedua sisir bersama-sama oleh gaya sinusoidal eksternal tetap. Umumnya, kapasitor pelat paralel lebih kuat daripada aktuator penggerak sisir per satuan luas karena mereka memiliki kapasitansi tumpang tindih yang lebih besar antara pelat aktuator tetap dan pelat bergerak. Namun, desainer lebih memilih untuk menyisir aktuator karena dua alasan utama ini:menghasilkan perpindahan yang lebih besar (tersedia beberapa puluh mikrometer), dan gaya relatif tidak terkait dengan perpindahan. Secara keseluruhan, aktuasi elektrostatik telah menemukan aplikasi yang luas dalam aktuator mikromekanis [77], aplikasi biosensor [56], penginderaan kelembaban [57], aplikasi penginderaan partikel dan massa [50, 78], sistem nanopositioning MEMS [58], aplikasi saklar RF MEMS [ 13, 79], kebisingan loop tertutup dari osilator MEMS [60], navigasi (Global Positioning System GPS mobil) [62, 80], giroskop [81], deteksi biomarker dalam napas yang dihembuskan [82] dan sensor gas yang sensitif terhadap massa [ 63]. Selain itu, model teoritis perangkat sensor berbasis CMOS-MES yang digerakkan secara elektrostatis dan kapasitif untuk skrining diabetes noninvasif juga dilaporkan. Dimana skrining diabetes dilakukan melalui pendeteksian uap aseton pada pernafasan (EB), dimana polimer kitosan digunakan sebagai bahan penginderaan. Secara khusus, sensitivitas yang dioptimalkan dilaporkan sekitar 0,042, 0,066, 0,13, 0,18, dan 0,26 pm/ppm [83].

Skema konfigurasi penggerak sisir elektrostatik [84, 85], a aktuator sisir interdigitated longitudinal; b aktuator transversal

Untuk mengeksitasi perangkat MEMS ke kondisi resonansi atau mode getaran, penggerak alternatif harus dilakukan dengan menerapkan tegangan AC antara dua pelat stator dan rotor. Mode mengemudi standar ditampilkan sebagai berikut [49]:tegangan alternatif sederhana (V ₁ dosaωt) , tegangan bolak-balik dengan bias dc (V ₀ + V ₁ dosaωt) dan penggerak dorong-tarik (Gbr. 5). Penggerak push-pull umumnya dianggap sebagai solusi terbaik karena gaya struktural dapat diterapkan pada kedua sisi rotor. Push–pull adalah drive ideal yang sesuai untuk skema drive sisir [49].

Sisir penggerak resonator a Penggerak tegangan alternatif sederhana, b tegangan bolak-balik dengan bias dc, dan c skema mengemudi push–pull [86]

Aktuasi Elektrotermal

Teknik elektrotermal sebagian besar digunakan dalam drive MEMS sementara jarang digunakan dalam aplikasi penginderaan [72]. Teknik elektrotermal pada dasarnya sesuai untuk konfigurasi fleksibel untuk menghasilkan gerakan dengan perpindahan yang diinginkan. Didorong dilakukan oleh arus AC yang diterapkan melalui terminal pemanas mikro tertanam karena gaya termal yang dihasilkan sebagai respons terhadap perbedaan suhu dari lapisan yang berbeda [66]. Microheater terbuat dari komponen berbagai lapisan material dan beroperasi pada suhu tinggi. Ketika arus AC diterapkan, suhu perangkat naik; karena ketidaksesuaian dalam koefisien ekspansi termal (CTE) dari bahan yang berbeda, bahan memuai, menyebabkan tegangan termal yang menyebabkan perangkat bengkok [72]. Di sisi lain, pendekatan pendinginan dapat diakses melalui konduksi ke substrat dan konveksi ke udara sekitar (atau cairan).

Perubahan suhu ini mempengaruhi perangkat dalam dua cara, seperti yang dilaporkan dalam [44, 87]:(1) perubahan dimensi pada perangkat atau tegangan yang dihasilkan di dalam perangkat, dan (2) sifat material perangkat bervariasi dengan suhu. Teknik saat ini perlu dipadatkan ke microheater. Microheater menghasilkan panas yang diinginkan karena menerapkan arus tertentu ke resistor dapat menghasilkan disipasi daya yang rendah dan waktu respons yang cepat. Gaya elektrotermal dapat dijelaskan dengan (2) [88].

dimana _B adalah CTE, E adalah modulus young balok, A adalah luas penampang balok, T adalah suhu akhir, dan T ₀ adalah suhu awal sebelum pemanasan. Beberapa bahan telah digunakan dalam desain pemanas mikro. Misalnya, untuk mencapai kinerja terbaik, akurasi, cakupan dan stabilitas suhu yang luas, platinum dan emas akan menjadi keputusan terbaik untuk pemanas mikro [89]. Namun, mereka mahal [90]. Memanfaatkan pemanas mikro di sensor MEMS memerlukan sensor suhu built-in untuk mengubah panas yang dihasilkan menjadi keluaran yang berguna. Untuk sensor suhu, platinum tersedia dalam kisaran suhu 200 C hingga 800 C dan nikel tersedia dalam 100 hingga 260 C [90]. Menurut laporan, aluminium juga merupakan bahan yang baik untuk penginderaan suhu [91]. Gambar 6 menunjukkan mekanisme umum aktuasi elektrotermal yang mencakup Aktuator U-Beam (lengan panas/dingin), aktuator bimorph (aktuator Bi-material), dan aktuator tekuk (Aktuator V-Beam atau balok antar-jemput).

Skema aktuasi elektrotermal a Aktuator U-Beam (lengan panas/dingin), b aktuator bimorph (aktuator bi-material), c dan d aktuator tekuk (Aktuator V-Beam dan balok antar-jemput) [67, 92]

Aktuator balok-U juga dikenal sebagai lengan panas/dingin, di mana aktuator terbuat dari lengan sempit (panas), jangkar lengan lebar (dingin), dan bantalan kontak [67, 92]. Ini terutama digunakan untuk pembandingan in-plane atau horizontal dengan substrat. Prinsip kerja dalam teknik ini tergantung pada penerapan tegangan diferensial di atas bantalan kontak, arus mengalir melalui lengan, dan lengan panas (resistensi lebih tinggi) memanaskan kerapatan arus lebih banyak daripada lengan dingin (resistensi lebih rendah). Oleh karena itu, lengan panas yang lebih sempit lebih panas dan akan mengembang lebih dari lengan yang lebih tebal. Lengan panas menghasilkan tegangan termal yang besar. Ketegangan diferensial antara kedua lengan ini membuat seluruh perangkat membelok.

Aktuator bimorph, juga disebut sebagai bi-material, terdiri dari dua material berbeda yang mengembang secara termal yang terikat bersama. Tidak seperti dua aktuator pertama, aktuator bimorph [93] biasanya digunakan untuk aktuasi di luar bidang. Saat arus melewati balok, suhu material meningkat. Temperatur ini akan menghasilkan pemuaian yang lebih besar pada satu material dibandingkan material lainnya karena memiliki koefisien muai panas yang berbeda, sehingga mengakibatkan tegangan termal dan pembengkokan perangkat.

Aktuator tekuk termal terdiri dari balok-V atau balok antar-jemput [94], jangkar dan bantalan kontak, yang juga terutama digunakan untuk aktuasi dalam bidang atau lateral. Saat arus mengalir di antara balok, kenaikan suhu balok menyebabkan ekspansi termal karena pemanasan Joule yang terjadi, yang cenderung mengembang dan menggerakkan pesawat ulang-alik atau mendorong balok tengah perangkat ke depan dalam arah yang paling sederhana dan kurang tahan.

Dalam implementasi nyata, sebagian besar perangkat MEMS dengan penggerak termal bergetar pada frekuensi yang mendekati frekuensi alami, sehingga penting untuk menghitung frekuensi alami perangkat. Teknologi elektrotermal menjadi lebih umum karena kekuatan pendorong yang besar dan perpindahan berdasarkan ekspansi termal dari bahan lapisan penarik di bawah tegangan eksitasi yang sangat rendah, yang tidak dapat dicapai dengan strategi lain apa pun ketika pengukuran serupa digunakan [85, 95]. Aktuasi elektrotermal menunjukkan beberapa keuntungan seperti perpindahan besar [95], kemudahan fabrikasi [72], kekuatan besar, dan tegangan yang diberikan relatif rendah [52]. Sebaliknya, mereka membutuhkan sejumlah besar arus dan penguat tegangan rendah. Selain itu, ia mengkonsumsi daya tinggi karena pemanasan Joule. Satu lagi batasan adalah kepekaan terhadap suhu lingkungan. Akhirnya, suhu dan panas yang meningkat karena aktuasi elektrotermal dapat menyebabkan beberapa kelemahan untuk elektronik tetangga bersama dengan fase pengemasan sistem [72].

Berbagai jenis aktuasi elektrotermal telah dikembangkan dan dipelajari; Lengan panas/dingin [96,97,98], bi-material atau bimorph [98, 99] dan aktuator tekuk termal [98, 100]. Referensi [101], telah menyelidiki getaran kantilever mikro di bawah aktuasi termal menggunakan aktuator bimorph. Referensi [102], telah menyarankan prosedur terstruktur untuk aktuasi termal cincin mikro polisilikon (RTA) yang dibuat menggunakan proses MUMPs. Sebuah kantilever mikro berdasarkan aktuasi termal telah dirancang. Mereka mengasumsikan sistem dalam vakum atau cairan (gas) cair dan menyarankan solusi analitis [103]. Dennis dkk. [44] membuat sistem (CMOS-MEMS) dengan pemanas mikro internal yang dioperasikan pada suhu yang relatif tinggi (40–80 °C) untuk mengukur tingkat kelembapan menggunakan titanium dioksida (TiO₂ ) nanopartikel sebagai bahan penginderaan. Sensor dioperasikan dalam mode dinamis menggunakan aktuasi elektrotermal dan sinyal keluaran dinilai menggunakan sensor piezoresistif (PZR) yang terhubung ke sirkuit jembatan Wheatstone. Tegangan keluaran sensor kelembapan naik dari 0,585 menjadi 30,580 mV saat kelembapan meningkat dari 35% RH menjadi 95% RH. The sensitivity of the humidity sensor increases linearly from 0.102 mV/% RH to 0.501 mV/% RH with an increase in the temperature from 40 to 80 °C and a maximum hysteresis of 0.87% RH is found at a relative humidity of 80%. In a further notable contribution, Almur et al. [104] modelled a MetalMUMPs acetone vapour sensing system based on electrothermal actuation and capacitive sensing. The output voltage change was found to increase linearly with increasing the acetone vapor concentration from 100 to 500 ppm with a concentration sensitivity of 0.65 mV/ppm. Due to the thick nickel layer of the MetalMUMPs technology used the device has very high mass (4.7 × 10 ^–8 kg) and low mass sensitivity (0.118 MHz/pg).

Electromagnetic Actuation

The electromagnetic operation has long been utilized in the sensing and driving of the large scaling applications (such as loudspeakers). This technique demands electricity to stimulate the magnetic effect. It uses the electromagnetic force (also can be called Lorentz force after the Dutch physicist Hendrik A. Lorentz 1895) to produce a magnetic field to the resonator by applying an alternating current on the coil or inductor integrated into the resonator (Fig. 7). Lorentz Force is defined as the force generated on a point charge as a result of the combined electric and magnetic forces on the charge [72]. Electromagnetic actuation has also been utilized as a guide to other actuation techniques, for example, electrostatic [33] and electrothermal [34]. In this technique, when a conductor conveying an electric current I can be applied to an external magnetic field B , this will induce a Lorentz force F _L as stated by (3) [72].

dimana L indicted to the conductor length and × refers to the cross product operation. The current in the conductive element located within the magnetic field generates an electromagnetic force in a direction perpendicular to the current and the magnetic field. This force has a directly proportional to the current, magnetic flux density and length of the microbeam. Because of the interaction generated by the current between the magnet and the magnetic field, a mechanical displacement on the microbeam will be generated as a result of the applied current to the micro-coil. For example, electromagnetic actuation has been used to stimulate microbeam when being exposed to an external magnetic field by flowing current through the microbeam, as seen in Fig. 7a. Similarly, as alternating current flows through the device on the microbeam, this approach can be used to excite the out-of-plane resonator (Fig. 7b). Lorentz forces of equal value and opposite directions are produced if the coil is deposited on a structure of the microbeam. The opposite directions of the current flowing through the different parts of the coil correspond to these forces. Such forces balance one another out. Figure 7c, however, can induce a net motion that is used to actuate the microbeam [72].

Schematic of the electromagnetic actuation using Lorentz forces a in-plane actuator, b dan c out-of-plane actuator [72]

The electromagnetic technique involves many merits over the other kind of actuation techniques. The main advantages of the electromagnetic actuation include the large displacement without the influence of the nonlinear effect [105], lower voltages due to they based on current rather than voltage-driven [7], in addition, they have a high reproducibility rate, they also have more features like rapid response, high accuracy, and the merit for controlling easily. For MEMS applications, the electromagnetic technique is the best choice to achieve maximum driving force for device size ratio [106]. However, they still suffering from some drawbacks due to using the huge current resulting in high power consumption, also their fabrication is complicated (normally be in need of inductive parts to produce magnetic flux also the possibility of including the manufacture of coils, and the deposition of a magnetic material) [7]. This fact combined with constructive difficulties has limited magnetic actuation applications. However, there are successful application examples in the literature as it may exist in MEMS switches devices [7], Optical switches, and micro scanners [69], gyroscopes [107], or relays [108]. Many researches are also based on magnetic actuation [109]. The magnetic microsensors utilizing the commercial 0.35 μm CMOS process has been investigated [110]. In a further contribution, Barba et al. [111] designed an electromagnetically actuated MEMS cantilevers proposed in order to minimize parasitic phenomena using Boron-doped silicon. In another seminal work, an electromagnetic and Piezoelectric actuated and piezoresistive sensed CMOS- MEMS device has been modelled for humidity sensing using ZnO NRs (6 μm/chitosan SAMs) as a sensing material [83]. The sensitivity was found to be 83.3 ppm [112].

Piezoelectric Actuation

Piezoelectric effect indicates the capability of a particular material to produce an electrical voltage in response to applied the mechanical stress. Piezoelectricity was discovered by the Curie brothers in 1880, and 'Piezo' meaning is coming from the Greek word that is mean 'press' or 'push'. A piezoelectric actuator defined as transducers that converts an applied potential into a mechanical movement or strain based on the piezoelectric influence [72]. Piezoelectric materials like quartz, zinc oxide, lead zirconate titanate (piezoelectric ceramic material PZT), and polyvinylidene difluoride (PVDF) are commonly used in sensor and actuator applications on a macro scale as well as in MEMS applications in recent years, but the sensing is limited due to their lack of a DC response [49]. Piezoelectric materials act electrically as insulators and required to be placed between two conductive electrodes so as to gather charge or to apply an electric-field (using an interdigitated electrode or sandwich structure) [113].

In MEMS devices, the basic principle of the piezoelectric actuation method is based on using a thin piezoelectric layer that is deposited as a part of the MEMS beam between flexible (Fig. 8). When a voltage is applied to the piezoelectric layer, the piezoelectric material expands according to the polarization of the applied voltage, which causes an axial bending across the length of the flexible beam [114].

Schematic of the piezoelectric actuator [72]

In general, piezoelectric actuators exhibit a lot of advantages such as high output force, good operating bandwidth, very compact size, lightweight, low power consumption, and fast response [114, 115]. Their characteristics are well known and have been used for decades. Most first sensors use piezoelectric actuation and are still in use today. However, small displacements of actuators are a major drawback, and their high-temperature sensitivity [72], nonlinear working area and hysteresis limit their wide range of applications [49]. Piezoelectric materials are very brittle. They can seldom be utilized alone but are often connected to a flexible structure for actuation and sensing applications. There are several works based on piezoelectric actuation [2, 48, 114, 116] studied the prospective to use a piezoresistive microcantilever for environment application, particularly for humidity sensing. Mahdavi et al. [117] proposed a new class of accurate dew point measurements taking advantage of thin-film Piezoelectric-on-Silicon (TPoS) resonators using ZnO NRs (6 μm/chitosan SAMs). The sensitivity was found to be 16.9–83.3 ppm at the range of response time of 46 s/167 s. sensitivity to mass loading based on piezoelectric excitation is investigated for chemical sensing [118, 119] reported piezoelectric transduction of flexural-mode silicon resonators to achieve efficient temperature compensation. The resonance gas sensor using piezoelectric MEMS for defence applications was reported [120].

Actuation Techniques Discussion

Different types of MEMS actuators require different drive electronics. As per mentioned in this article, there are different principles and approaches to actuate MEMS devices. The most important of which include:electrostatic actuation, electrothermal actuation, electromagnetic actuation, and piezoelectric actuation. Electrostatic actuators are a traditional field and considered one of the most popular mechanisms for actuating MEMS devices. They have a simple design, fast response time, ease of fabrication and low power consumption. However, the non-linearity and the elevated value of the driving voltage is the major matter of this type of actuator. Electrothermal actuators provide very low resistance to their drive sources and require high currents rather than high voltages to move them. High voltage amplifiers are not particularly suitable for use in electrothermal MEMS actuators. Instead, a low voltage and high current signal source are required. Of course, this can also work if the output current of the high voltage amplifier is large enough to drive the electrothermal actuator, but it is not a particularly energy-efficient or cost-effective solution. For MEMS applications, electromagnetic technology is the best choice to achieve maximum driving force per device size ratio. However, they still suffering from some drawbacks due to using the huge current that results in high power consumption, also their fabrication is complicated (normally be in need of inductive parts to produce magnetic flux also the possibility of including the manufacture of coils, and the deposition of a magnetic material). Piezoelectric actuators also required high voltage amplifiers, but since they are typically used for resonance, the drive requirements in terms of driving voltage are typically lower than those required for other actuators. The recent research of MEMS-based sensor devices which can be actuated using the four basic actuation techniques that have been used during the last few years for many applications, as shown in Table 2. It can be clearly observed that there is a rising development pattern in the field of micro-assembly and micromanipulation for MEMS-based sensor devices that have been categorized based on actuation techniques. Nowadays, a wide range of applications has been identified for MEMS-based sensor devices that provide high sensitivity and resolution. The sensing systems are getting more robust and reliable due to the integration configuration.

Sensing Mechanisms for the Output Signal

The sensing mechanism is used to sense the output of the MEMS devices. The basic of the sensing technique is based on the use of a polymer deposited on the sensing layer of the device to absorb a particular chemical that creates a variation in the stress, mass, electrical or mechanical characteristics of the beam. To estimate the change in the mass, the resonant frequency of the device can be measured based on the detected mass increment that is one of its parameters. Then again, when the polymer absorbs the mass, piezoresistor that is situated on the surface of the sensing element can be used to estimate the mass of the beam. Another method of detecting suspected chemicals is to use thermocouples to measure temperature changes caused by the heat generated by the polymer absorbing analyte [49].

To sense the output of these MEMS devices methods such as piezoresistive, capacitive or optical mechanisms are used (Fig. 9) [123]. Different sensing mechanisms have different advantages and disadvantages at the same time.

Sensing mechanisms of MEMS-based sensor devices

For instance, optical sensing has good feasibility; however, it is very expensive to implement compared to capacitive and piezoresistive techniques [124]. Capacitive and piezoresistive techniques are the common methods used to sense the output signal. CMOS- MEMS device was designed to encompass both piezoresistive and capacitive sensing techniques due to the features of 0.35 µm CMOS technology used to design it. In contrast, PolyMUMPs and MetalMUMPs devices were designed to use only capacitive sensing techniques.

Piezoresistive Sensing

Piezoresistive is a common sensing principle used in MEMS devices. Essentially, the principle of piezoresistive materials is based on the resistance change when their strain changes in response to applied stress. This change can be observed in the electrical resistance of the material of the device as a consequence of a surface-stress change due to bending. The resistance value of the piezoresistor with the resistivity ρ of a resistor, length of l and cross-section area A is given by (4) [29].

The change ΔR in resistance is proportional to the applied strain can be defined in (5) [29, 125].

where G is the gauge factor work as a proportionality constant of the piezoresistor, ε is the strain in the material, and R is the piezoresistor resistance. A piezoresistive element behaves differently towards longitudinal and transverse strain component presented by [29]. According to (5), the resistance of a piezoresistor can vary owing to the changes of the geometry (length and cross-section) or in the property of the material (resistivity). Piezoresistive detection depends on the difference in resistivity of material when stress is applied. In a piezoresistive material, the change of resistivity because of the application of stress has a much greater effect on the resistance than the change in resistivity because of a change in the geometry (sensors rely on the change of resistance owing to the geometry are so-called strain gauges) [72]. (Fig. 10) shows different resistor orientations and external force loading directions [88].

a Longitudinal piezoresistance dominates, b dan c transverse piezoresistance dominates presented [29]

The main drawback of this sensing mechanism is the solid temperature reliance of resistivity. To limit this impact, normally, a collecting of four piezoresistive designed is utilized to shape a Wheatstone bridge. This limits the temperature reliance [72]. Furthermore, the small changes in the electric signal generated from gauges are translated and gauged as voltages utilizing this Wheatstone bridge [49]. Wheatstone bridge utilizes four resistors arranged in the bridge of the four arms (Fig. 11). Based on the quantity of these active resistors (acting as a transducer), single active bridge (single transducer), half active bridge (double transducers), and the full active bridge (quadruple transducers) of the bridge of Wheatstone might be utilized as reported [72]. The single active bridge as well called Quarter Bridge that utilizes just a single active component which changes because of a physical ambient, for example, pressure, power, temperature, while the remnants of that resistors will be passive and so on. MEMS systems have been used as temperature, calorimeter, and in combination with the Wheatstone bridge, sensors allow the base fluctuations. Thermal bridge system is one of the best-known sensing platforms among MEMS. In the AC-driven Wheatstone bridge configuration (Fig. 11b), researchers have utilized the resistance-based thermometry to analyse temperature changes with micro-Kelvin resolution. In an overview, the right half of the bridge comprises a sensing resistor on the lower branch with associated resistance on the top, while the left side is defined as the matching one. Fixed resistors with a relatively low resistance coefficient of temperature are used with resistance values, which was chosen to enhance stability and resolution based on the previous study [126].

Schematic types of a Wheatstone bridge, a quarter bridge, b half-bridge, and c full-bridge configurations [72]

Silicon piezoresistor and polysilicon patches are generally utilized in MEMS sensors. The resistivity of silicon relies upon its strategy for doping. N-type doped with silicon is less sensitive than p-type. This raises limitation to the fabrications. In this situation, an in-plane transversal and in-plane longitudinal will be the most significant elements [72]. For semiconductor materials like silicon, the longitudinal and transverse gauge factors owing to geometry change are quite tiny compared to their values due to resistivity change, hereafter the change in geometry can be ignored, and the longitudinal and transverse gauge factors due to only the resistivity change are found using (6) and (7), respectively [127].

dimana E is Young's modulus of the piezoresistive material. π _L and π _B are the longitudinal and transverse piezoresistance coefficients, respectively, for arbitrarily oriented polysilicon grains, and their values are given in Table 3 [19].

Currently, the piezoresistive effect has been utilized as a bio-or chemical detection [48], humidity sensing purpose [4], switch applications [59], biomarker detection in exhaled breath [82], micro scanners [69], pressure sensing [128], and mass-sensitive gas sensors [63]. Zope et al. [117] developed a resistively sensed thermally-driven piezo resonator composed predominantly of CMOS material for mass sensing applications. whereas mass sensitivity of 24.96 kHz/ng was reported. The extracted mass resolution of 16.3 fg have also been observed, hence showing great potential to serve as an aerosol sensor).

Capacitive Sensing

The capacitive sensing mechanism is based on measuring the changes of the capacitance between the stator and rotor fingers or between the fixed plate and movable plate [88]. Capacitive sensing is the main dominant method for micromachined applications due to its compatibility with all the fabrication approaches and stiffness [129, 130]. Capacitive sensing has many attractive features include high sensitivity, low power consumption, simplicity in design, low drift and low-temperature dependency. Furthermore, the measurement of the output signal can be easily fabricated on the PolyMUMPs die using capacitive sensing. Only a single parallel plate can be used to capacitively sensed the variations in the natural frequency or amplitude of the MEMS devices (such as a CMOS-MEMS device and a PolyMUMPs device) or by applying a comb finger (such as a MetalMUMPs device).

For CMOS-MEMS device and PolyMUMPs devices, when there is no actuation (Fig. 12a), the initial static sensed capacitance (C_s0 ) between the lower fixed plate and the movable top plate is calculated using (8) [19].

where ℇ is the relative permittivity of the dielectrics, A_s is the area of the sensed plates and z₀ is the gap between the fixed and movable plates. When the movable plate is displaced by z (Fig. 12b) the initial gap changes, and hence the capacitance will change too, and it is found by (9).

Parallel plate capacitor showing a initial condition and b after displacing the movable plate [19]

At the same time, for the MetalMUMPs devices, the output can be detected capacitively by utilizing the differential comb fingers design. As appeared in Fig. 13, there are two arrangements of fixed comb fingers to have the differential capacitance.

Comb fingers configuration for MetalMUMPs device showing a the initial condition and b the displacement of the vibrating comb finger [19]

For the initial condition, when the device is not actuated (Fig. 13a), the static detected capacitance C₀ for every set of vibrating-fixed fingers could be calculated by (10).

where l _f and w _f are length and width of the overlapping area of the vibrating and fixed comb fingers, respectively. d is the gap between the fixed and vibrating comb fingers and n is the number of the vibrating comb fingers. When the device is moved by y toward the fixed comb finger one as appeared by (Fig. 13b), the capacitances C ₁ dan C ₂ will be changed and relying upon that shift, and their values can be calculated using (11) and (12), respectively.

The output signal of the device will be measured as a voltage due to change in the sensing capacitance upon actuation using MS3110 capacitive readout circuit that translates the change in capacitance to the output voltage change in response to the temperature of the microheaters. Nowadays, capacitive effects are used in MEMS resonators [60], biosensor application [56], humidity sensor [57], diabetes screening [66], Navigation (automobile Global Positioning System (GPS)) [62], a low noise accelerometer [131], and RF MEMS capacitive switches [132].

Optical Detection

Optical sensing mechanism depends on detecting the changes in the transmitted light compared to the received light. The detection of compound species by spectral transmission strategy is broadly acknowledged. Optical sensors are notable due to their selectivity since it does not rely upon any chemical reaction or any chemical catalyst activities. Air pollutants identification is recognized by using the characteristics of the optical species (for example, absorption, refractive index, Raman scattering, fluorescence, and reflection). Optical sensors utilize emission and absorption measurements primarily through different technologies such as Fourier transform infrared spectroscopy (FTIR), surface plasma resonance (SPR), differential optical absorption spectroscopy (DOAS), laser diode absorption spectroscopy (LDAS), cavity ring-down spectroscopy (CRDS), non-dispersive infrared (NDIR) spectroscopy, light detection and ranging (LIDAR), UV fluorescence and chemiluminescence for the sensing of air pollutants in air specimens. Normally, the light will be cross through, modified or reflected by some space having the relevant medium (Fig. 14). Beer–Lambert law can be used to govern the optical detection of the air pollutants as per the following:

where the transmitted light through the specimen is I , Aku ₀ is the received light, α represent the absorptivity, l represents the length of the path.

Schematic representation of an optical MEMS sensor [133]

However, the majority of the applications for optical MEMS are in communications; there are various uses in different fields [134]. The literature covered numerous attempts to introduce the optical detection of air pollutants. A few of these sensors are used to recognize chemicals. This displays the multifunctional part of these sensors. Infrared spectrometry is a case of a technique that can be utilized for optical sensing [135]. In a further notable contribution, Huang and co-workers have also demonstrated [136] the simulation of an integrated optical MEMS accelerometer. The effective simulation results have demonstrated as follows:a resonance frequency of 562.85 Hz, a mechanical sensitivity of 781.64 nm/g, an optical system sensitivity of 1.23, a resolution of 1.037 µg, and low cross-sensitivity. The proposed accelerometer can be used for improvements in MEMS inertial navigation devices. Another model is the Fabry–Perot optical sensor [137]. Basically stated, the principles of this sensor are based on bringing gas into a chamber and a short time later going light through the chamber. Due to changes in the substance piece, the light experiencing will be not exactly equivalent to without the gas. This light is broke down, and from the results, confirmation can be made about the sort of gas being tested. The fruitful testing of an optical MEMS sensor for the location of catechol was reported [133]. The absorbance measurement of catechol oxidation was performed by blue light (472 nm) attached through a MEMS device. The light was transmitted from a free-space blue laser working in persistent wave mode and is focused on a multimode fibre utilizing manually arranged to organize. Received light was coupled by means of the optical fibre to a USB connected spectrophotometer which assisted computerized information collection utilizing the software.

Sensing Mechanisms Discussion

In view of different working principles, the above-mentioned sensing mechanisms can be divided into three types, namely piezoresistive, capacitive and optical mechanisms. There are benefits and drawbacks for different sensing mechanisms at the same time. For instance, optical sensing has good feasibility; however, it is very expensive to implement compared to capacitive and piezoresistive techniques. As a result, piezoresistive and capacitive techniques are the common methods used to sense the output signal. Any small change in the stress could be detected using a piezoresistive technique. A main disadvantage of the piezoresistive sensing technique is the high-temperature reliance of resistivity. In Table 3, present the recent work and progress (last 5 years), including actuation and sensing mechanisms, sensing materials, fabrication technology and application. The miniaturized microchip presented (in Table 4) can be utilized for gas and humidity sensing, human health screening, inertial navigation with good repeatability, high resolution, high sensitivity, and rapid response. In addition, MEMS sensors have been proven to be an effective medium for sensors combined with on-chip electronic circuitry. It can therefore function not only as a portable sensor chip but can also be linked as part of the Internet of Things (IoT) network to achieve real-time and remote high-sensitivity moisture tracking.