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CMOS-based resistive and FET devices for smart gas sensors
CHAPTER 7
CMOS-based resistive and FET devices for smart gas sensors Julian William Gardnera, Prasanta Kumar Guhab a
School of Engineering, University of Warwick, Coventry, United Kingdom E&ECE Department, IIT Kharagpur, Kharagpur, India
b
7.1 Introduction to CMOS gas sensors There is an increasing demand for low-cost, low-power, handheld, compact gas or volatile organic compound (VOC) sensors that can be implemented within the Internet of Things (IoT). There exist many different transduction principles to detect hazardous gases and VOCs, for example, surface or bulk acoustic wave (SAW/BAW), electrochemical (EC), infrared (IR), calorimetric, and resistive. Among these, traditional IR gas sensors are perhaps the most accurate ones but limited to higher concentrations of methane (1%–4%) and carbon dioxide (100s ppm). They work on the principle of infrared absorption. They contain an infrared source (i.e., hot-wire bulb or diode) that passes through the target gas (within a gas chamber system) and an infrared filter and detector (usually pyroelectric). The gas attenuates certain infrared wavelengths (based on molecular vibrational bands) as the light passes through it, while other wavelengths pass through it unattenuated. Current IR gas sensors are relatively expensive today ($50–$200) and cannot detect toxic gases such as CO, NO2, and formaldehyde at the low levels (parts per billionparts per million) needed. It is not easy to make IR gas sensors using low-cost complementary metal-oxide-semiconductor (CMOS) platform. However, there have been recent reports of microhotplate-based IR emitters (i.e., IR source), which will reduce the cost drastically and improve compactness of the entire system [1]. Among traditional gas sensors, electrochemical (EC) sensors currently occupy the majority of the market share for accurate and stable monitors. The basic components of such sensors are two electrodes ([i] working electrode, also known as the sensing electrode and [ii] counter electrode) and an ion conductor electrolyte (e.g., mineral acid and organic electrolyte) in between them. When a target gas comes in contact with the working electrode, a reduction/oxidation (REDOX) reaction takes place. This electrochemical reaction results in an electrical current that passes through the external circuit and sensed hence creating an amperometric sensor. The current is proportional to the concentration of the gas. As the electrodes have a finite catalytic activity, it is necessary to limit the rate of diffusion of target gas into the sensor (using a barrier) to ensure the Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00007-4
Copyright © 2020 Elsevier Inc. All rights reserved.
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gas is efficiently reacted. This barrier takes the form of a small hole or capillary in the sensor housing. EC sensors usually have a shelf life of 6 months to 1 year, depending on the gas to be detected and the environment in which they are used. When such sensors are used to detect low concentrations (parts per billion), they are sensitive to changes in ambient temperature and also cross sensitive to humidity and other gases. EC sensors are much cheaper than that of IR sensors; however, a typical electrochemical sensor will still cost around $20, and they need a separate interface board with a high-gain amplifier that will take the cost closer to $100 or more. This high cost prevents the penetration into truly mass markets that are needed today for the IoT. The high cost is primarily because of the use of a semiautomated manufacturing process and requirement of expensive catalytic material, and it is difficult to integrate them with CMOS platforms due to the presence of the electrolyte. For the reasons outlined earlier, resistive gas sensors have a major advantage over IR and EC gas sensors, because they can be integrated with CMOS platforms and manufactured in very high volumes (>10 M units per annum) at ultralow cost (<$2). In the literature, many researchers have claimed their sensors are “CMOS compatible,” where in most of the cases, CMOS compatible means that CMOS materials have been used in the fabrication, but no standard CMOS process has been used to produce their respective devices. However, if sensors are fabricated using a commercial CMOS foundry, then not only batch fabrication of the sensors is possible (i.e., reliable, reproducible, large-scale production of devices with good matching and identical performance) but also integration of interface circuitry with the sensors on the same silicon die, hence, reducing the total cost of a sensor module and making them small enough for use in smart digital devices (e.g., wearables and smart phone). However, there are some limitations in using a standard CMOS process. First of all, commercial CMOS foundries only offer certain materials (e.g., silicon, polysilicon, silicon dioxide, silicon nitride, and aluminum/copper; in some cases, they also provide high-temperature interconnects like tungsten for metallization). Also, silicon doping levels (i.e., sheet resistance) and thickness of each CMOS layer are fixed for a particular process node, whereas gas-sensing devices often require materials that are not within the standard CMOS layers. For example, in the case of resistive gas sensors, the most popular choice of sensing material is a metal oxide (usually doped tin dioxide). But preprocessed wafer is allowed by CMOS foundry (popularly known as pre-CMOS), even the CMOS process steps can be stopped to incorporate some additional sensor fabrication steps on partially processed wafers before they take back to CMOS process line again (popularly known as intra-CMOS). However, these are not for all customers (due to risk of contamination in processed wafer); only companies that have their own in-house CMOS foundry can afford pre-CMOS or intraCMOS. In the last decade, some of the microelectromechanical systems (MEMS) steps have been included in a standard CMOS process line; for example, some CMOS foundry
CMOS-based resistive and FET devices for smart gas sensors
supports wafer level back etching of silicon wafer; this step is key to realize small silicon microhotplates for use in metal oxide (MOX) gas sensors. However, post-CMOS is the only viable option to include any non-CMOS material for sensor fabrication (if required). In the case of post-CMOS, high temperatures and harsh chemical process steps cannot be used to synthesize the gas-sensing layer, because these can damage aluminumbased metal tracks and alter circuit performance [2]. Thus, the fabricated sensor performance from CMOS foundry is limited by tight restrictions on the available process technology. However, researchers are very keen to accommodate microsensors in the chip while maintaining most of the standard CMOS process because of the multiple advantages as mentioned earlier. Particularly, the concept of a sensor on a chip can provide a low-voltage, low-cost battery-operated solution that enables sensor integration even in small modern devices such as wearables. In this book chapter, we focus on two micro gas sensors, namely, resistive sensors and field-effect transistor (FET) sensors, which are easier to integrate within a CMOS technology and hence suitable for the IoT. The corresponding analogue front-end and signal conditioning circuits will be considered. Finally, issues associated with sensor packaging are outlined as well as some commercial chips available today for toxic gas and total VOC (tVOC) detection for air quality.
7.2 Fabrication of microheaters Resistive gas sensors are sensing devices whose electrical resistance is a function of the concentration of the target gas. One of the first resistive gas sensor was fabricated by Naoyoshi Taguchi, and later on (in 1969), Figaro Engineering Inc. was established by him to commercialize the so-called Taguchi gas sensor (TGS) [3]. The original TGS is composed of a platinum heater coil inside a ceramic tube. A manually painted thick layer of porous tin oxide film is deposited over platinum electrodes on a ceramic cylinder and sintered at a high temperature to achieve appropriate crystalline structure. The resistance of the tin oxide coating changes in the presence of a redox gas at high temperature (typically 200°C–400°C), and this is measured via gold electrodes. A platinum heater is used to take the sensing layer to this elevated temperature. Such sensors are bulky and consumed large power (100s of mW) but are still made today in Japan (e.g., FiS) and China (e.g., Huawei) and sold in high numbers for c.$10 each. It is extremely challenging to integrate high-temperature resistive sensors into a CMOS platform. One of the main challenges is to isolate the hot sensor from its surroundings and the interface electronics. The performance of silicon-based electronics degrades above 125°C—not to mention issues with the die attach and packaging. Hence, it is absolutely necessary to isolate sensor and circuit even though they are present on the same die. In this respect, MEMS-based microhotplate plays an important role. Work carried out in the United Kingdom (e.g., University of Cambridge and University of
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Warwick) and Europe (e.g., ETH Zurich) in the last two decades has been instrumental in this respect [2, 4, 5]. A microhotplate is usually a thin membrane (few micrometer thick) that keeps the heater thermally isolated to achieve high temperatures at low power loss. This is the heart of resistive sensors on a CMOS-MEMS platform. Microhotplates are usually very fast (i.e., high temperature can be reached within milliseconds) due to their low thermal mass. A microhotplate is power efficient, because the small heated area is surrounded by a low thermal conductivity material (e.g., silicon dioxide, silicon nitride, and air). A microhotplate contains a microheater, temperature sensor, and electrode (often interdigitated electrodes (IDEs)). The microheater is used to heat up the sensing layer and is usually made of high-temperature metal (e.g. tungsten), doped single crystal silicon, or doped polysilicon [4]. There are many reports of platinum-based microheater, that is, see [2]. Platinum is a highly stable metal even at high temperature (up to 700°C) and thus appropriate as a microheater. However, a CMOS foundry does not support platinum, so one needs to deposit platinum as a post-CMOS process. This negate its use compared with tungsten layer. Conventional CMOS metals, like aluminum, are not appropriate because of the possibility of electromigration [2]. Stability of polysilicon is also not very good for long-term use [2]. There are reports of MOSFET-based microheater [2]. MOSFET can switch on/off easily, and also, it is the main active device in IC technology. However, at high temperatures, carrier mobility in the MOSFET channel degrades and can even result in breakdown. In the case of a CMOS temperature sensor, the usual choice is a p-n junction diode or CMOS resistor (silicon or polysilicon layer). This is located on the membrane to know the temperature of the gas-sensing layer. The voltage across the temperature sensor changes with operating temperature (in the case of diode, voltage decreases with increase in temperature, whereas in the case of resistor, it increases with increase in temperature). The temperature sensor needs to be calibrated properly. It is also possible to use the microheater as the temperature sensor. A microhotplate can be fabricated by etching silicon from the rear side of the wafer. This can be done either using a wet etching (anisotropic) or dry etching (isotropic) technique. Wet etching is a cheaper option (especially using KOH), but it consumes more silicon area because of the anisotropic etching profile, whereas dry etching (such as deep reactive ion etching [DRIE]) gives near-vertical walls and hence consumes perhaps 30% less silicon floor plan. A buried oxide (BOX) layer of silicon-on-insulator (SOI) wafer acts as etch stop layer [4]. One can also use highly doped (p+ or n+ region) silicon region as an etch stop [5]. Such a membrane structure is also known as a closed membrane. Microhotplates can also be realized by etching from the top side (by using combination of wet and dry etching). This will give a suspended membrane [6]. Here, the membrane is kept hanging from different sides by oxide arms. A suspended membrane is more power efficient than a closed membrane, because it is isolated from circuit region by air and heat
CMOS-based resistive and FET devices for smart gas sensors
Fig. 7.1 Schematic of microhotplate.
dissipation through the oxide arms is much lower compared with closed membrane. However, a closed membrane is more stable than its suspended counterpart. A schematic of a typical closed membrane is shown in Fig. 7.1.
7.3 Fabrication of resistive and FET sensing elements Modern solid-state (resistive) gas sensors are generally based on wide-bandgap semiconducting metal oxide sensing materials. Resistive gas sensors available in the market (e.g., Figaro Engineering TGS) generally use doped tin oxide as the sensing material. Conventional bulk metal oxides are not effective (the smaller surface area causes lower sensitivity) for microsensors. Thus, nanomaterials are suitable candidates for gas detection, because of their high surface area in small volume. So, there is a possible increase in sensitivity even for a miniaturized sensor. Two types of metal oxides are found in literature that have been extensively studied, for example, n type (e.g., SnO2, ZnO, and WO3) and p type (e.g., NiO and CuO) [7–10]. The n-type materials have an abundance of electrons, whereas the p-type materials have an abundance of holes. The oxygen molecules present in the atmosphere interact with these materials and adsorb electrons to form oxygen adsorbates (e.g., O , O2 , and O2 ). Thus, a depletion layer is formed on the oxide surface, which narrows the conduction channel resulting in a high resistance (in the case of n-type material). In the case of p-type material due to reduction of electrons, a hole accumulation layer forms, which results in decrease in resistance of the sensing layer [10]. The targeted gas molecules when come in contact with the oxygen adsorbates attached to the sensing layer surface change the conductivity of the sensing layer. For example, in the case of reducing gas, after interaction with oxygen adsorbates, electrons will be released back to the sensing layer; thus, its resistance decreases (for n type) or increases (for p type). Such interaction often takes place between 200°C and 400°C. The working principle is shown in Fig. 7.2. Apart from metal oxides, other key materials used for sensing purpose are carbon nanomaterials, for example, carbon nanotubes (CNTs), and graphene. Contrary to metal oxides, carbon materials sense gases near room temperature; however, the response from them is usually poor [11]. Pure graphene, although a promising candidate for future
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Fig. 7.2 Working principle of metal oxide as gas-sensing layer.
electronic devices due to its very high mobility, is not very effective for gas sensing due to the lack of defects [12]. In this respect, reduced graphene oxide (RGO) and other twodimensional (2-D) layered materials, for example, MoS2 and WS2, show better performance [12–14]. RGO often contains different functional groups (epoxy, carbonyl, hydroxyl, etc.), which act as active sites for gas molecule attachment. Liquid exfoliation technique is popular to synthesize layered materials; this can create sulfur vacancies (in MoS2 and WS2), which act as active sites for gas molecules’ attachment [15]. However, growing nanomaterials directly on CMOS wafer is very challenging; the CMOS substrate cannot sustain prolonged heating (because of aluminum electromigration), which is often necessary during growth process and annealing of the nanomaterials. Again, it is recommended not to use any harsh chemicals and environments (e.g., long
CMOS-based resistive and FET devices for smart gas sensors
plasma exposure might damage fragile microhotplate structure) for nanomaterial synthesis on CMOS. However, direct growth on the sensor region gives better adhesion. A typical direct growth technique is hydrothermal technique where reasonably low temperature is used (<200°C) [16]. Also, such chemical route can create nanostructure with defects, which are useful for sensing. A SEM image of ZnO nanowires grown on an SOI microhotplate is shown in Fig. 7.3A. Chemical vapor deposition (CVD) is also a very popular technique to synthesize nanomaterial and often used to grow metal oxide and CNTs. However, CVD is a high-temperature process and not suitable for CMOS wafer. There are reports of CNT growth on microhotplate directly using local growth technique [11]. Here, microheater was used to heat up to form metal catalyst islands so that CNTs only grow on the sensor area without damaging the circuit region. But due to high-temperature process, these CNTs have very little defects and hence give poor response in the presence of gases. There are also reports of depositing thin sensing film using RF sputter [17]. This has the advantage of batch level fabrication, which will ensure uniformity of sensing layer over the wafer; however, one needs to buy specific targets for this. Also, for metal-functionalized sensing layer, say Pt-functionalized metal oxides, Pt target is hugely expensive. Again, it is difficult to achieve controlled defect through sputtering, which is much easier to get through chemical route. Instead of growing directly, one can synthesize the nanomaterial ex situ and then deposit it on to the microhotplate region. In this case, drop casting or spin coating can be used. However, more sophisticated approach will be inkjet printing or dip pen
1 mm
Mag = 27.83 K X WD = 5 mm
EHT = 5.00 kV Gun Vacuum = 2.41e-009 mBar
10 mm
EHT = 3.00 kV WD = 6.2 mm
Signal A = SE2 Mag = 1.13 K X
Fig. 7.3 Nanomaterial integration with microhotplate. (A) Hydrothermally grown ZnONW on microhotplate. (B) Deposition of ZnO nanorod with dip pen nanolithography on microhotplate. (Part A: Adapted from S. Santra, P.K. Guha, S.Z. Ali, P. Hiralal, H.E. Unalan, J.A. Covington, G.A.J. Amaratunga, W.I. Milne, J.W. Gardner, F. Udrea, ZnO nanowires grown on SOI CMOS substrate for ethanol sensing, Sensors Actuat. B 146 (2010) 559–565. Part B: Adapted from S. Santra, A. De Luca, S. Bhaumik, Z. Ali, F. Udrea, J.W. Gardner, S.K. Ray, P.K. Guha, Dip pen nanolithographydeposited zinc oxide nanorods on a CMOS MEMS platform for ethanol sensing, RSC Adv. 5 (2015) 47609–47616.)
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nanolithography [9]. These are computer-controlled techniques for nanomaterial deposition and ideal for batch fabrication. A SEM image of ZnO nanorods deposited on a microhotplate using dip pen nanolithography is shown in Fig. 7.3B. The Chem-FET is another sensing device that can be integrated with CMOS platform. The FET-based sensor can be top gate or back gate. In the case of top gate technology, FETs can be made sensitive to some gases or ions if the gate is exposed. This is formed by using a catalytic gate (Pd, Pt, and Ir), polymer gate, a suspended gate, or a porous gate [18, 19]. The most studied case for gases is a FET with a Pd gate used for H2 detection. According to Lundstr€ om (who was among the first to study this kind of gas sensor) and his coworkers, hydrogen dissolved in the Pd gate (at approximately 150°C) moves to the Pd/SiO2 interface and forms a dipole layer [20]. The dipole changes the work function difference between the metal and the SiO2, which in turn changes the applied gate potential and hence shifts the I–V characteristics of MOSFET device in the range of a few millivolt. This process is reversible, since the dipole molecule disappears in the absence of the gas and the threshold voltage of the sensor returns to its initial level. But the recovery rate can be very slow and might take several hours. A suspended gate FET is based on an air gap sandwiched between a layer of metal and the insulator, with an inlet to allow the molecules of gases to interact with both surfaces and thus change the surface potential [21]. However, the complexity of the fabrication process may be an issue in this case. Polymer chem-FETs are attractive because they can sense the gases at or close to room temperature (hence the very low power consumption) and also they can be deposited very easily; however, baseline drift and sensitivity with time and temperature remain issues to be addressed. The back-gate FET is a much simpler structure (as shown in Fig. 7.4). Here, a highly doped silicon substrate is used and oxide is grown on top as gate oxide. On top of the gate oxide, a nanomaterial is deposited, which acts as FET channel and sensing layer. So, this layer is exposed to the target gas. In this case, large gate voltage is required because of the thicker silicon substrate.
Fig. 7.4 Back-gate FET schematic.
CMOS-based resistive and FET devices for smart gas sensors
7.4 Interface circuitry for resistive gas sensors There are two possible ways of interfacing gas sensors with their associated interface circuitry: (a) hybrid approach and (b) monolithic approach. In the first approach, sensor and circuit can be fabricated from separate foundries, so sensor design has less constraints, whereas in the second approach, both sensor and circuit are fabricated on the same silicon die. Chemoresistive sensors are based on a change in resistance at high operating temperatures, so the main circuit blocks required for interfacing these sensors are (i) driving circuit for heater and temperature sensor, (ii) temperature controlling unit, and (iii) interfacing with sensing element. (i) The typical driving circuit of the resistive microheater is a constant current source. Such a current source is usually controllable, so that different levels of current can be passed through the heater to get different temperatures. Usually, the current source should be capable of driving at least few 10s of mA using low-cost CMOS technology. Similar circuit configuration can also be used for temperature sensors, but in this case, current level will be lower so that it will not heat up the membrane. In the case of a FET-based heater, voltage is used between drain and source (VDS) and also between gate and source (VGS) to control the current through the FET heater. (ii) The temperature control of a sensing layer is very important. This is because operating at optimum temperature of the sensing layer improves sensitivity and selectivity of sensors toward a particular gas. Starting from a simple on-off technique to the more sophisticated on-chip proportional-integral-derivative (PID) controller can be used for this purpose [22]. The on-off (or bang-bang) controller is the simplest form, where if the microhotplate is cooler than a set-point temperature, the heater is turned on at maximum power, and once it is hotter than the set-point temperature, the heater is switched off completely. However, on-off control can give rise to instability and might damage the heater (unless controlled carefully) due to very rapid changes in voltage (hence temperature). A simple temperature control circuit is shown in Fig. 7.5, where the heater on-off is controlled by comparing with the temperature sensor voltage [23]. There are also reports of pulse width modulation (PWM) for controlling temperature. However, PID control is best because smooth temperature control is possible. (iii) The front end (i.e., circuit interfacing the sensing layer) is one of the very important circuit blocks of the sensor system. There are several challenges involved to design the front end: • A common problem with any microsensors is that they tend to generate small electrical signals at the sensor output, so the front-end circuitry needs to be of very low noise, that is, a good analogue design is required. Often, a gain control
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VDD
Microhotplate
Current source Temperature sensor
Heater Membrane
Vcontrol
+ –
Fig. 7.5 Heater temperature control scheme by comparing temperature sensor voltage. (Adapted from D. Barrettino, M. Graf, M. Zimmermann, C. Hagleitner, A. Hierlemann, H. Baltes, A smart single-chip microhotplate-based chemical sensor system in CMOS-technology, Analog Integr. Circ. Sig. Process 39 (2004) 275–287.)
instrumentation amplifier is used in this case. Sometimes, a chopper amplifier is also used to shift the noise signal at high frequency and then filter it out. • The resistance of the sensing layer (metal oxide) is very high. The baseline resistance (resistance of sensing layer in the absence of any gas) varies enormously (typically sheet resistances varies from 1 kΩ per square up to even 10 GΩ per square). Even the same metal oxide can show very different sheet resistances due to variation in synthesis process parameters, morphology, and thickness of the sensing layer. One way to reduce resistance is to use interdigitated electrode, rather than ordinary electrode. However, it is challenging to accommodate this wide resistance range through the front-end circuit. On top of this large baseline resistance, the circuit needs to amplify the change in resistance due to the presence of gas. To amplify the resistance change, one needs a variable gain amplifier. This is because if the sensor response is poor, then large gain is required, whereas if the sensor response is large, then gain from amplifier should be small to avoid output clipping. • There can be significant drift of sensor signal with time. This may be due to aging of the nanomaterial or poisoning of sensing layer due to incomplete desorption of gas analytes. There should be provision of drift cancelation either through circuit or through softwire (lookup table). The literature reports many different solutions to the aforementioned challenges. A very simple scheme to measure resistance is using a resistor divider circuit or a Wheatstone bridge. This scheme is more suitable for developing PCB level electronics. In one of
CMOS-based resistive and FET devices for smart gas sensors
the reports, a bank of resistors was used in a voltage divider mode with the sensing element [24]. The voltage across sensing element is kept close to the mid of the supply rail (in the absence of the gas) by choosing the appropriate resistor from the bank through microcontroller (as shown in Fig. 7.6). Through this simple approach, a wide sensing element resistance range can be covered. But this is not suitable when seeking full on-chip integration as it needs either trimming or variable resistors from outside the chip and also large resistance occupies lot of silicon area. A simple scheme has been reported in Ref. [23], where large dynamic range of sensing material was compressed by using two diodes (popularly known as log converter approach). The schematic is shown in Fig. 7.7. Here, two current mirror circuits are used: one diode is with sensor, and the other one is connected to a fix known resistor (connected through current mirror). The difference of the two diode voltages contains the information of sensor resistor. Another popular approach found in the literature is the resistance to frequency conversion. One example of the scheme is shown in Fig. 7.8. Here, a cascode current source is used to charge a capacitor, whereas current sink is used to discharge it. This current is controlled by the sensing material resistor (connected to the reference arm). Thus, the resulting triangular wave contains both the resistor and capacitor term, which can be found out from the frequency of the wave [25]. (Adapted from [25].)
Fig. 7.6 Voltage divider sensor interfacing circuit. (Adapted from D. Burman, D. Choudhary, P.K. Guha, ZnO/MoS2 based enhanced humidity sensor prototype with Android App interface for mobile platform, IEEE Sensors J. 19 (11) (2019) 3993–3998.)
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VDD M1
Vcontrol
M2
M3
Vout1
+
M4
Vout2
D1
D2
–
Sensing material
Fig. 7.7 Logarithmic compression circuit for sensing material interface. VDD
VH Vref
CNTy CNTx
+
C
+ –
–
–
CNTy CNTx
Sensing material
Q
CNTx
+ + VL
CNTy
– FlipFlop
Fig. 7.8 Resistance-controlled oscillator circuit for sensing material interface.
7.5 Integration of temperature and humidity sensors 7.5.1 Temperature sensor The response of a gas sensor is generally sensitive to its operating temperature. The response (change resistance) may increase, decrease, or even be relatively insensitive to its operating temperature, but the baseline signal generally has a strong temperature dependence. Therefore, it is important to know the operating temperature of the gassensing layer. Most microheaters operate by constant current drive, and so, the temperature of the heater will vary with the ambient temperature. In this case, it is also useful to
CMOS-based resistive and FET devices for smart gas sensors
know the ambient temperature as well so that the sensor response and heater resistance can be compensated. The ideal microheater drive circuit would be a constant resistive one in which the resistance of the heater is set and a closed loop control system is designed. This can be implemented using a microcontroller. However, in practice, the resistance of the microheater can slowly drift, and so the accuracy of such a closed loop system can be negated. In addition, the response and recovery times also depend upon the operating temperature, and so if dynamic information is used, then again, it is important to know the exact operating temperature of the gas-sensing layer and compensate for changes in it from ambient conditions.
7.5.2 Humidity sensor Humidity is perhaps the most important parameter that affects the response as MOX gas sensors and being important for indoor air for human comfort. Apart from this, humidity sensors find application in a number of fields like meteorology, biology, medicine, agriculture, industry, and geology. So, there is a strong need to also measure humidity and compensate the gas sensor response, as increasing humidity generally lowers the response of MOX gas sensors by 5%–50%. Various types of humidity sensors have already been reported in the literature, that is capacitive, resistive, surface acoustic wave, and quartz crystal microbalance [26–28]. Among these, polymeric or β-alumina capacitive humidity sensors are widely used for low-cost chips. Here, a hygroscopic dielectric material (e.g., polymer) sandwiched between a pair of electrodes forming a small capacitor. The change in device capacitance due to a change in dielectric permittivity with humidity is exploited as the sensing mechanism. However, their hysteretic behavior is often a serious drawback in practical applications. On the other hand, resistive humidity sensors are recently gaining popularity because of their simpler structure, and they are easy to integrate on a CMOS platform. Here, nanomaterial is deposited (or grown) on top of the interdigitated electrodes. Different metal oxides (e.g., SnO2, ZnO, and WO3) and polymer composites have been explored in literature as humidity sensing layer [29–30]. Recently, there have been some reports of 2-D layered material-based humidity sensors, for example, graphene oxide (GO)-, MoS2-, and WS2-based humidity sensors [31–33]. At low RH, water molecules are primarily physisorbed onto the available active sites (hydrophilic groups and vacancies) of the GO surface through double hydrogen bonding, which is called the first-layer physisorption of water. In this regime, the water molecules are thus unable to move freely because of the restriction from double hydrogen bonding. As the RH increases, the multilayer physical adsorption of water molecules occurs. From the second physisorbed layer, water molecules are physisorbed through single hydrogen bonding on the hydroxyl groups. Thereafter, the water molecules become mobile and progressively more identical to those in the bulk liquid. As the multilayer physical adsorption progresses,
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the physisorbed water can be ionized under an electrostatic field to produce a large number of hydronium ions (H3O+) as charge carriers. So in high RH, proton hopping takes place between adjacent water molecules that gives rise to a high conductivity and hence gives large responses [31]. In practice, polymer-based (polyaniline) capacitive humidity sensors are the most common choice and have been commercialized for many years. Now, integrated chips are available as described in a later section and allow more accurate measurement of gases using low-cost MOX materials, such as SnO2 and WO3.
7.6 Packaging of CMOS gas sensors Research generally focuses upon the silicon die, but the commercial world needs to consider the assembly of the silicon die in a package and then interface to a PCB or microcontroller. In practice, the die attach, wirebonding, and packaging can cost much more than the silicon die itself. The simplest solution is to attach the die directly to a PCB, and in these days, surface mount technology is almost universal. Hence, the use of a flip-chip arrangement can allow the use of a low-temperature oven to attach the silicon to a PCB. However, due to the control of vapors arising from the glue and also the PCB itself, most manufacturers opt for low-cost small outline plastic packages that can then be surface mounted onto a PCB in high volumes at low cost (a few cents per device). In similar cases, the system in a package approach has been used. However, this adds additional costs, and now, fully integrated chips are appearing on the market—often referred to as COMBO chips. The flow of the gas or VOC into and out of the package also has to be considered, and so, packages are usually designed specifically for the application in hand. The tooling up of the packaging can cost $50k, but such on-off costs are small when high volumes of sensors are sold, that is, millions per year. Finally, most gas sensor packages have a relative small number of pins to save cost, and it is increasingly rare to have an analogue output. The default now is a digital interface such as i2c that can be directly fed into a low-cost micro for use in digital devices such as smart home appliances, smart wearables, and mobile phones.
7.7 Commercial CMOS gas sensors The last few years have seen a significant rise in the companies selling MEMS-based gas sensors for a range of applications. The traditional markets have been industrial and medical, but the volumes are relatively modest, and so, manufacturers are chasing the high volume markets offered by consumer electronics and the IoT. In this new and emerging space, there are perhaps three market leaders offering COMBO chips for toxic gas and
CMOS-based resistive and FET devices for smart gas sensors
tVOC sensing. Fig. 7.9 shows the leading smart chips manufactured by Bosch, Sensirion, and ams for CMOS-based gas sensing. These chips have been sold in high volumes and employ MEMS sensors manufactured using CMOS or SOI CMOS technology. Table 7.1 summarizes their technical performance, and they usher in a new era of low-cost CMOS gas sensors for the rapidly emerging consumer electronics industry. The application domain targets by these CMOS chips are commonly stated as indoor air quality, air cleaners and purifiers, smart thermostats, home controllers, smart accessories, smart phones, and IoT devices. The potential sales of these chips is 100 million to low billions of units per year, so the future market could be worth $10B per annum.
Fig. 7.9 Some commercial smart CMOS gas sensors manufactured today: (A) Bosch BME680, (B) Sensirion Multi-Pixel SGP30, and (C) ams.
Table 7.1 Some technical specifications of commercial gas and tVOC MEMS chips from technical datasheets of manufacturers Chip
Measurands
Bosch BME680
Gas (air quality), pressure, temperature, humidity
Sensirion SGP30
Gas (tVOC), ethanol, hydrogen
Ams/ CCS 811b
Gas (tVOC)
Size (mm)/ package
Supply voltage (V)/power (mW)
Output
Cost ($)
331 Metal lid LGA 2.5 2.5 1 6-pin DFN 2.7 4 1 10-pin LGA package
1.7–3.6 <1
I2C/ SPI
10
1.6–2.0 < 90
I2C
9
1.8–3.6 < 60
I2C
8
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References [1] S.Z. Ali, A. De Luca, R. Hopper, S. Boual, J. Gardner, F. Udrea, A low-power, low-cost infra-red emitter in CMOS technology, IEEE Sensors J. 15 (12) (2015) 6775–6782. [2] P.K. Guha, S.Z. Ali, C.C.C. Lee, F. Udrea, W.I. Milne, T. Iwaki, J.A. Covington, J.W. Gardner, Novel design and characterisation of SOI CMOS micro-hotplate for high temperature gas sensors, Sensors Actuat. B 127 (2007) 260–266. [3] https://www.figaro.co.jp/en/company/history.html. [4] S.Z. Ali, F. Udrea, W.I. Milne, J.W. Gardner, Tungsten-based SOI microhotplates for smart gas sensors, J. Microelectromech. Syst. 17 (6) (2008) 1408–1417. [5] D. Barrettino, M. Graf, W.H. Song, K.-U. Kirstein, A. Hierlemann, H. Baltes, Hotplate-based monolithic CMOS microsystems for gas detection and material characterization for operating temperatures up to 500°C, IEEE J. Solid State Circ. 39 (7) (2004) 1202–1207. [6] M.Y. Afridi, J.S. Suehle, M.E. Zaghloul, D.W. Berning, A.R. Hefner, R.E. Cavicchi, S. Semancik, C.B. Montgomery, C.J. Taylor, A monolithic CMOS microhotplate-based gas sensor system, IEEE Sensors J. 2 (6) (2002) 644–655. [7] S. Santra, A.K. Sinha, A. De Luca, S.Z. Ali, F. Udrea, P.K. Guha, S.K. Ray, J.W. Gardner, Mask-less deposition of Au-SnO2 nanocomposites on CMOS MEMS platform for ethanol detection, Nanotechnology 27 (12) (2016)125502. [8] A.K. Nayak, R. Ghosh, S. Santra, P.K. Guha, D. Pradhan, Hierarchical nanostructured WO3-SnO2 for selective sensing of volatile organic compounds, Nanoscale 7 (2015) 12460–12473. [9] S. Santra, A. De Luca, S. Bhaumik, Z. Ali, F. Udrea, J.W. Gardner, S.K. Ray, P.K. Guha, Dip pen nanolithography-deposited zinc oxide nanorods on a CMOS MEMS platform for ethanol sensing, RSC Adv. 5 (2015) 47609–47616. [10] S. Dey, S. Santra, S. Sen, D. Burman, S. Ray, P. Guha, Photon assisted ultra-selective formaldehyde sensing by defect induced NiO based resistive sensor, IEEE Sensors J. 18 (14) (2018) 5656–5661. [11] S. Santra, S.Z. Ali, P.K. Guha, G. Zhong, J. Robertson, J.A. Covington, W.I. Milne, J.W. Gardner, F. Udrea, Post-CMOS wafer level growth of carbon nanotubes for low cost microsensors—a proof of concept, Nanotechnology 21 (48) (2010) 7. [12] R. Ghosh, A. Midya, S. Santra, S.K. Ray, P.K. Guha, Chemically reduced graphene oxide for ammonia detection at room temperature, ACS Appl. Mater. Interfaces 5 (15) (2013) 7599–7603. [13] D. Burman, S. Santra, P. Pramanik, P.K. Guha, Pt decorated MoS2 nanoflakes for ultrasensitive resistive humidity sensor, Nanotechnology 29 (11) (2018) 115504. [14] R. Jha, P.K. Guha, Liquid exfoliated pristine WS2 nanosheets-for ultrasensitive and highly stable chemiresistive humidity sensors, Nanotechnology 27 (47) (2016) 475503. [15] D. Burman, R. Ghosh, S. Santra, S.K. Ray, P.K. Guha, Role of vacancy sites and UV-ozone treatment on few layered MoS2 nanoflakes for toxic gas detection, Nanotechnology 28 (43) (2017) 435502. [16] S. Santra, P.K. Guha, S.Z. Ali, P. Hiralal, H.E. Unalan, J.A. Covington, G.A.J. Amaratunga, W.I. Milne, J.W. Gardner, F. Udrea, ZnO nanowires grown on SOI CMOS substrate for ethanol sensing, Sensors Actuat. B 146 (2010) 559–565. [17] P. Dwivedi, S. Dhanekar, S. Das, S. Chandra, Effect of TiO2 functionalization on nano-porous silicon for selective alcohol sensing at room temperature, J. Mater. Sci. Technol. 33 (6) (2017) 516–522. [18] M. Liess, D. Chinn, D. Petelenz, J. Hanata, Properties of insulated gate field effect transistors with polyaniline gate electrode, Thin Solid Films 132 (1996) 152–155. [19] J.A. Covington, J.W. Gardner, D. Briand, N.F. de Rooji, A polymer gate FET sensor array for detecting organic vapours, Sensors Actuat. B 3853 (2001) 1–8. [20] I. Lundstorm, M.S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogen-sensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55. [21] I. Eisele, T. Doll, M. Burgmair, Low power gas detection with FET sensors, Sensors Actuat. B 77 (2001) 19–25. [22] D. Barrettino, M. Graf, S. Hafizovic, S. Taschini, C. Hagleitner, A. Hierlemann, H. Baltes, A singlechip CMOS micro-hotplate array for hazardous-gas detection and material characterization, in: L. C. Fujino (Ed.), International Solid-State Circuit Conference, San Francisco, CA, 2004, pp. 312–313.
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[23] D. Barrettino, M. Graf, M. Zimmermann, C. Hagleitner, A. Hierlemann, H. Baltes, A smart singlechip micro-hotplate-based chemical sensor system in CMOS-technology, Analog Integr. Circ. Sig. Process 39 (2004) 275–287. [24] D. Burman, D. Choudhary, P.K. Guha, ZnO/MoS2 based enhanced humidity sensor prototype with Android App interface for mobile platform, IEEE Sensors J. 19 (11) (2019) 3993–3998. [25] M. Grassi, P. Malcovati, A. Baschirotto, A 141-dB dynamic range CMOS gas-sensor interface circuit without calibration with 16-bit digital output word, IEEE J. Solid State Circuits 42 (7) (2007) 1543–1554. [26] S. Borini, R. White, D. Wei, M. Astley, S. Haque, E. Spigone, N. Harris, J. Kivioja, T. Ryh€anen, Ultrafast graphene oxide humidity sensors, ACS Nano 7 (12) (2013) 11166–11173. [27] Y. Yao, X. Chen, W. Ma, W. Ling, Quartz crystal microbalance humidity sensors based on nanodiamond sensing films, IEEE Trans. Nanotechnol. 13 (2) (2014) 386–393. [28] Y. Tang, Z. Li, J. Ma, Highly sensitive surface acoustic wave (SAW) humidity sensors based on sol-gel SiO2 films: investigations on the sensing property and mechanism, Sensors Actuat. B Chem. 215 (2015) 283–291. [29] Q. Kuang, C. Lao, Z.L. Wang, Z. Xie, L. Zheng, High-sensitivity humidity sensor based on a single SnO2 nanowire, J. Am. Chem. Soc. 129 (19) (2007) 6070–6071. [30] H. Li, B. Liu, D. Cai, Y. Wang, Y. Liu, L. Mei, L. Wang, D. Wang, Q. Lia, T. Wang, Hightemperature humidity sensors based on WO3-SnO2 composite hollow nanospheres, J. Mater. Chem. A 2 (2014) 6854–6862. [31] S. Ghosh, R. Ghosh, P.K. Guha, T.K. Bhattacharyya, Humidity sensor based on high proton conductivity of graphene oxide, IEEE Trans. Nanotechnol. 14 (5) (2015) 931–937. [32] R. Jha, P.K. Guha, Humidity sensing properties of co-exfoliated heterogeneous WS2/WSe2 nanohybrids, IEEE Trans. Nanotechnol. 17 (3) (2018) 582–589. [33] D. Burman, R. Ghosh, S. Santra, P.K. Guha, Highly proton conducting MoS2/graphene oxide nanocomposite based chemoresistive humidity sensor, RSC Adv. 6 (2016) 57424–57433.
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CMOS-based resistive and FET devices for smart gas sensors Julian William Gardnera, Prasanta Kumar Guhab a
School of Engineering, University of Warwick, Coventry, United Kingdom E&ECE Department, IIT Kharagpur, Kharagpur, India
b
7.1 Introduction to CMOS gas sensors There is an increasing demand for low-cost, low-power, handheld, compact gas or volatile organic compound (VOC) sensors that can be implemented within the Internet of Things (IoT). There exist many different transduction principles to detect hazardous gases and VOCs, for example, surface or bulk acoustic wave (SAW/BAW), electrochemical (EC), infrared (IR), calorimetric, and resistive. Among these, traditional IR gas sensors are perhaps the most accurate ones but limited to higher concentrations of methane (1%–4%) and carbon dioxide (100s ppm). They work on the principle of infrared absorption. They contain an infrared source (i.e., hot-wire bulb or diode) that passes through the target gas (within a gas chamber system) and an infrared filter and detector (usually pyroelectric). The gas attenuates certain infrared wavelengths (based on molecular vibrational bands) as the light passes through it, while other wavelengths pass through it unattenuated. Current IR gas sensors are relatively expensive today ($50–$200) and cannot detect toxic gases such as CO, NO2, and formaldehyde at the low levels (parts per billionparts per million) needed. It is not easy to make IR gas sensors using low-cost complementary metal-oxide-semiconductor (CMOS) platform. However, there have been recent reports of microhotplate-based IR emitters (i.e., IR source), which will reduce the cost drastically and improve compactness of the entire system [1]. Among traditional gas sensors, electrochemical (EC) sensors currently occupy the majority of the market share for accurate and stable monitors. The basic components of such sensors are two electrodes ([i] working electrode, also known as the sensing electrode and [ii] counter electrode) and an ion conductor electrolyte (e.g., mineral acid and organic electrolyte) in between them. When a target gas comes in contact with the working electrode, a reduction/oxidation (REDOX) reaction takes place. This electrochemical reaction results in an electrical current that passes through the external circuit and sensed hence creating an amperometric sensor. The current is proportional to the concentration of the gas. As the electrodes have a finite catalytic activity, it is necessary to limit the rate of diffusion of target gas into the sensor (using a barrier) to ensure the Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00007-4
Copyright © 2020 Elsevier Inc. All rights reserved.
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gas is efficiently reacted. This barrier takes the form of a small hole or capillary in the sensor housing. EC sensors usually have a shelf life of 6 months to 1 year, depending on the gas to be detected and the environment in which they are used. When such sensors are used to detect low concentrations (parts per billion), they are sensitive to changes in ambient temperature and also cross sensitive to humidity and other gases. EC sensors are much cheaper than that of IR sensors; however, a typical electrochemical sensor will still cost around $20, and they need a separate interface board with a high-gain amplifier that will take the cost closer to $100 or more. This high cost prevents the penetration into truly mass markets that are needed today for the IoT. The high cost is primarily because of the use of a semiautomated manufacturing process and requirement of expensive catalytic material, and it is difficult to integrate them with CMOS platforms due to the presence of the electrolyte. For the reasons outlined earlier, resistive gas sensors have a major advantage over IR and EC gas sensors, because they can be integrated with CMOS platforms and manufactured in very high volumes (>10 M units per annum) at ultralow cost (<$2). In the literature, many researchers have claimed their sensors are “CMOS compatible,” where in most of the cases, CMOS compatible means that CMOS materials have been used in the fabrication, but no standard CMOS process has been used to produce their respective devices. However, if sensors are fabricated using a commercial CMOS foundry, then not only batch fabrication of the sensors is possible (i.e., reliable, reproducible, large-scale production of devices with good matching and identical performance) but also integration of interface circuitry with the sensors on the same silicon die, hence, reducing the total cost of a sensor module and making them small enough for use in smart digital devices (e.g., wearables and smart phone). However, there are some limitations in using a standard CMOS process. First of all, commercial CMOS foundries only offer certain materials (e.g., silicon, polysilicon, silicon dioxide, silicon nitride, and aluminum/copper; in some cases, they also provide high-temperature interconnects like tungsten for metallization). Also, silicon doping levels (i.e., sheet resistance) and thickness of each CMOS layer are fixed for a particular process node, whereas gas-sensing devices often require materials that are not within the standard CMOS layers. For example, in the case of resistive gas sensors, the most popular choice of sensing material is a metal oxide (usually doped tin dioxide). But preprocessed wafer is allowed by CMOS foundry (popularly known as pre-CMOS), even the CMOS process steps can be stopped to incorporate some additional sensor fabrication steps on partially processed wafers before they take back to CMOS process line again (popularly known as intra-CMOS). However, these are not for all customers (due to risk of contamination in processed wafer); only companies that have their own in-house CMOS foundry can afford pre-CMOS or intraCMOS. In the last decade, some of the microelectromechanical systems (MEMS) steps have been included in a standard CMOS process line; for example, some CMOS foundry
CMOS-based resistive and FET devices for smart gas sensors
supports wafer level back etching of silicon wafer; this step is key to realize small silicon microhotplates for use in metal oxide (MOX) gas sensors. However, post-CMOS is the only viable option to include any non-CMOS material for sensor fabrication (if required). In the case of post-CMOS, high temperatures and harsh chemical process steps cannot be used to synthesize the gas-sensing layer, because these can damage aluminumbased metal tracks and alter circuit performance [2]. Thus, the fabricated sensor performance from CMOS foundry is limited by tight restrictions on the available process technology. However, researchers are very keen to accommodate microsensors in the chip while maintaining most of the standard CMOS process because of the multiple advantages as mentioned earlier. Particularly, the concept of a sensor on a chip can provide a low-voltage, low-cost battery-operated solution that enables sensor integration even in small modern devices such as wearables. In this book chapter, we focus on two micro gas sensors, namely, resistive sensors and field-effect transistor (FET) sensors, which are easier to integrate within a CMOS technology and hence suitable for the IoT. The corresponding analogue front-end and signal conditioning circuits will be considered. Finally, issues associated with sensor packaging are outlined as well as some commercial chips available today for toxic gas and total VOC (tVOC) detection for air quality.
7.2 Fabrication of microheaters Resistive gas sensors are sensing devices whose electrical resistance is a function of the concentration of the target gas. One of the first resistive gas sensor was fabricated by Naoyoshi Taguchi, and later on (in 1969), Figaro Engineering Inc. was established by him to commercialize the so-called Taguchi gas sensor (TGS) [3]. The original TGS is composed of a platinum heater coil inside a ceramic tube. A manually painted thick layer of porous tin oxide film is deposited over platinum electrodes on a ceramic cylinder and sintered at a high temperature to achieve appropriate crystalline structure. The resistance of the tin oxide coating changes in the presence of a redox gas at high temperature (typically 200°C–400°C), and this is measured via gold electrodes. A platinum heater is used to take the sensing layer to this elevated temperature. Such sensors are bulky and consumed large power (100s of mW) but are still made today in Japan (e.g., FiS) and China (e.g., Huawei) and sold in high numbers for c.$10 each. It is extremely challenging to integrate high-temperature resistive sensors into a CMOS platform. One of the main challenges is to isolate the hot sensor from its surroundings and the interface electronics. The performance of silicon-based electronics degrades above 125°C—not to mention issues with the die attach and packaging. Hence, it is absolutely necessary to isolate sensor and circuit even though they are present on the same die. In this respect, MEMS-based microhotplate plays an important role. Work carried out in the United Kingdom (e.g., University of Cambridge and University of
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Warwick) and Europe (e.g., ETH Zurich) in the last two decades has been instrumental in this respect [2, 4, 5]. A microhotplate is usually a thin membrane (few micrometer thick) that keeps the heater thermally isolated to achieve high temperatures at low power loss. This is the heart of resistive sensors on a CMOS-MEMS platform. Microhotplates are usually very fast (i.e., high temperature can be reached within milliseconds) due to their low thermal mass. A microhotplate is power efficient, because the small heated area is surrounded by a low thermal conductivity material (e.g., silicon dioxide, silicon nitride, and air). A microhotplate contains a microheater, temperature sensor, and electrode (often interdigitated electrodes (IDEs)). The microheater is used to heat up the sensing layer and is usually made of high-temperature metal (e.g. tungsten), doped single crystal silicon, or doped polysilicon [4]. There are many reports of platinum-based microheater, that is, see [2]. Platinum is a highly stable metal even at high temperature (up to 700°C) and thus appropriate as a microheater. However, a CMOS foundry does not support platinum, so one needs to deposit platinum as a post-CMOS process. This negate its use compared with tungsten layer. Conventional CMOS metals, like aluminum, are not appropriate because of the possibility of electromigration [2]. Stability of polysilicon is also not very good for long-term use [2]. There are reports of MOSFET-based microheater [2]. MOSFET can switch on/off easily, and also, it is the main active device in IC technology. However, at high temperatures, carrier mobility in the MOSFET channel degrades and can even result in breakdown. In the case of a CMOS temperature sensor, the usual choice is a p-n junction diode or CMOS resistor (silicon or polysilicon layer). This is located on the membrane to know the temperature of the gas-sensing layer. The voltage across the temperature sensor changes with operating temperature (in the case of diode, voltage decreases with increase in temperature, whereas in the case of resistor, it increases with increase in temperature). The temperature sensor needs to be calibrated properly. It is also possible to use the microheater as the temperature sensor. A microhotplate can be fabricated by etching silicon from the rear side of the wafer. This can be done either using a wet etching (anisotropic) or dry etching (isotropic) technique. Wet etching is a cheaper option (especially using KOH), but it consumes more silicon area because of the anisotropic etching profile, whereas dry etching (such as deep reactive ion etching [DRIE]) gives near-vertical walls and hence consumes perhaps 30% less silicon floor plan. A buried oxide (BOX) layer of silicon-on-insulator (SOI) wafer acts as etch stop layer [4]. One can also use highly doped (p+ or n+ region) silicon region as an etch stop [5]. Such a membrane structure is also known as a closed membrane. Microhotplates can also be realized by etching from the top side (by using combination of wet and dry etching). This will give a suspended membrane [6]. Here, the membrane is kept hanging from different sides by oxide arms. A suspended membrane is more power efficient than a closed membrane, because it is isolated from circuit region by air and heat
CMOS-based resistive and FET devices for smart gas sensors
Fig. 7.1 Schematic of microhotplate.
dissipation through the oxide arms is much lower compared with closed membrane. However, a closed membrane is more stable than its suspended counterpart. A schematic of a typical closed membrane is shown in Fig. 7.1.
7.3 Fabrication of resistive and FET sensing elements Modern solid-state (resistive) gas sensors are generally based on wide-bandgap semiconducting metal oxide sensing materials. Resistive gas sensors available in the market (e.g., Figaro Engineering TGS) generally use doped tin oxide as the sensing material. Conventional bulk metal oxides are not effective (the smaller surface area causes lower sensitivity) for microsensors. Thus, nanomaterials are suitable candidates for gas detection, because of their high surface area in small volume. So, there is a possible increase in sensitivity even for a miniaturized sensor. Two types of metal oxides are found in literature that have been extensively studied, for example, n type (e.g., SnO2, ZnO, and WO3) and p type (e.g., NiO and CuO) [7–10]. The n-type materials have an abundance of electrons, whereas the p-type materials have an abundance of holes. The oxygen molecules present in the atmosphere interact with these materials and adsorb electrons to form oxygen adsorbates (e.g., O , O2 , and O2 ). Thus, a depletion layer is formed on the oxide surface, which narrows the conduction channel resulting in a high resistance (in the case of n-type material). In the case of p-type material due to reduction of electrons, a hole accumulation layer forms, which results in decrease in resistance of the sensing layer [10]. The targeted gas molecules when come in contact with the oxygen adsorbates attached to the sensing layer surface change the conductivity of the sensing layer. For example, in the case of reducing gas, after interaction with oxygen adsorbates, electrons will be released back to the sensing layer; thus, its resistance decreases (for n type) or increases (for p type). Such interaction often takes place between 200°C and 400°C. The working principle is shown in Fig. 7.2. Apart from metal oxides, other key materials used for sensing purpose are carbon nanomaterials, for example, carbon nanotubes (CNTs), and graphene. Contrary to metal oxides, carbon materials sense gases near room temperature; however, the response from them is usually poor [11]. Pure graphene, although a promising candidate for future
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Fig. 7.2 Working principle of metal oxide as gas-sensing layer.
electronic devices due to its very high mobility, is not very effective for gas sensing due to the lack of defects [12]. In this respect, reduced graphene oxide (RGO) and other twodimensional (2-D) layered materials, for example, MoS2 and WS2, show better performance [12–14]. RGO often contains different functional groups (epoxy, carbonyl, hydroxyl, etc.), which act as active sites for gas molecule attachment. Liquid exfoliation technique is popular to synthesize layered materials; this can create sulfur vacancies (in MoS2 and WS2), which act as active sites for gas molecules’ attachment [15]. However, growing nanomaterials directly on CMOS wafer is very challenging; the CMOS substrate cannot sustain prolonged heating (because of aluminum electromigration), which is often necessary during growth process and annealing of the nanomaterials. Again, it is recommended not to use any harsh chemicals and environments (e.g., long
CMOS-based resistive and FET devices for smart gas sensors
plasma exposure might damage fragile microhotplate structure) for nanomaterial synthesis on CMOS. However, direct growth on the sensor region gives better adhesion. A typical direct growth technique is hydrothermal technique where reasonably low temperature is used (<200°C) [16]. Also, such chemical route can create nanostructure with defects, which are useful for sensing. A SEM image of ZnO nanowires grown on an SOI microhotplate is shown in Fig. 7.3A. Chemical vapor deposition (CVD) is also a very popular technique to synthesize nanomaterial and often used to grow metal oxide and CNTs. However, CVD is a high-temperature process and not suitable for CMOS wafer. There are reports of CNT growth on microhotplate directly using local growth technique [11]. Here, microheater was used to heat up to form metal catalyst islands so that CNTs only grow on the sensor area without damaging the circuit region. But due to high-temperature process, these CNTs have very little defects and hence give poor response in the presence of gases. There are also reports of depositing thin sensing film using RF sputter [17]. This has the advantage of batch level fabrication, which will ensure uniformity of sensing layer over the wafer; however, one needs to buy specific targets for this. Also, for metal-functionalized sensing layer, say Pt-functionalized metal oxides, Pt target is hugely expensive. Again, it is difficult to achieve controlled defect through sputtering, which is much easier to get through chemical route. Instead of growing directly, one can synthesize the nanomaterial ex situ and then deposit it on to the microhotplate region. In this case, drop casting or spin coating can be used. However, more sophisticated approach will be inkjet printing or dip pen
1 mm
Mag = 27.83 K X WD = 5 mm
EHT = 5.00 kV Gun Vacuum = 2.41e-009 mBar
10 mm
EHT = 3.00 kV WD = 6.2 mm
Signal A = SE2 Mag = 1.13 K X
Fig. 7.3 Nanomaterial integration with microhotplate. (A) Hydrothermally grown ZnONW on microhotplate. (B) Deposition of ZnO nanorod with dip pen nanolithography on microhotplate. (Part A: Adapted from S. Santra, P.K. Guha, S.Z. Ali, P. Hiralal, H.E. Unalan, J.A. Covington, G.A.J. Amaratunga, W.I. Milne, J.W. Gardner, F. Udrea, ZnO nanowires grown on SOI CMOS substrate for ethanol sensing, Sensors Actuat. B 146 (2010) 559–565. Part B: Adapted from S. Santra, A. De Luca, S. Bhaumik, Z. Ali, F. Udrea, J.W. Gardner, S.K. Ray, P.K. Guha, Dip pen nanolithographydeposited zinc oxide nanorods on a CMOS MEMS platform for ethanol sensing, RSC Adv. 5 (2015) 47609–47616.)
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nanolithography [9]. These are computer-controlled techniques for nanomaterial deposition and ideal for batch fabrication. A SEM image of ZnO nanorods deposited on a microhotplate using dip pen nanolithography is shown in Fig. 7.3B. The Chem-FET is another sensing device that can be integrated with CMOS platform. The FET-based sensor can be top gate or back gate. In the case of top gate technology, FETs can be made sensitive to some gases or ions if the gate is exposed. This is formed by using a catalytic gate (Pd, Pt, and Ir), polymer gate, a suspended gate, or a porous gate [18, 19]. The most studied case for gases is a FET with a Pd gate used for H2 detection. According to Lundstr€ om (who was among the first to study this kind of gas sensor) and his coworkers, hydrogen dissolved in the Pd gate (at approximately 150°C) moves to the Pd/SiO2 interface and forms a dipole layer [20]. The dipole changes the work function difference between the metal and the SiO2, which in turn changes the applied gate potential and hence shifts the I–V characteristics of MOSFET device in the range of a few millivolt. This process is reversible, since the dipole molecule disappears in the absence of the gas and the threshold voltage of the sensor returns to its initial level. But the recovery rate can be very slow and might take several hours. A suspended gate FET is based on an air gap sandwiched between a layer of metal and the insulator, with an inlet to allow the molecules of gases to interact with both surfaces and thus change the surface potential [21]. However, the complexity of the fabrication process may be an issue in this case. Polymer chem-FETs are attractive because they can sense the gases at or close to room temperature (hence the very low power consumption) and also they can be deposited very easily; however, baseline drift and sensitivity with time and temperature remain issues to be addressed. The back-gate FET is a much simpler structure (as shown in Fig. 7.4). Here, a highly doped silicon substrate is used and oxide is grown on top as gate oxide. On top of the gate oxide, a nanomaterial is deposited, which acts as FET channel and sensing layer. So, this layer is exposed to the target gas. In this case, large gate voltage is required because of the thicker silicon substrate.
Fig. 7.4 Back-gate FET schematic.
CMOS-based resistive and FET devices for smart gas sensors
7.4 Interface circuitry for resistive gas sensors There are two possible ways of interfacing gas sensors with their associated interface circuitry: (a) hybrid approach and (b) monolithic approach. In the first approach, sensor and circuit can be fabricated from separate foundries, so sensor design has less constraints, whereas in the second approach, both sensor and circuit are fabricated on the same silicon die. Chemoresistive sensors are based on a change in resistance at high operating temperatures, so the main circuit blocks required for interfacing these sensors are (i) driving circuit for heater and temperature sensor, (ii) temperature controlling unit, and (iii) interfacing with sensing element. (i) The typical driving circuit of the resistive microheater is a constant current source. Such a current source is usually controllable, so that different levels of current can be passed through the heater to get different temperatures. Usually, the current source should be capable of driving at least few 10s of mA using low-cost CMOS technology. Similar circuit configuration can also be used for temperature sensors, but in this case, current level will be lower so that it will not heat up the membrane. In the case of a FET-based heater, voltage is used between drain and source (VDS) and also between gate and source (VGS) to control the current through the FET heater. (ii) The temperature control of a sensing layer is very important. This is because operating at optimum temperature of the sensing layer improves sensitivity and selectivity of sensors toward a particular gas. Starting from a simple on-off technique to the more sophisticated on-chip proportional-integral-derivative (PID) controller can be used for this purpose [22]. The on-off (or bang-bang) controller is the simplest form, where if the microhotplate is cooler than a set-point temperature, the heater is turned on at maximum power, and once it is hotter than the set-point temperature, the heater is switched off completely. However, on-off control can give rise to instability and might damage the heater (unless controlled carefully) due to very rapid changes in voltage (hence temperature). A simple temperature control circuit is shown in Fig. 7.5, where the heater on-off is controlled by comparing with the temperature sensor voltage [23]. There are also reports of pulse width modulation (PWM) for controlling temperature. However, PID control is best because smooth temperature control is possible. (iii) The front end (i.e., circuit interfacing the sensing layer) is one of the very important circuit blocks of the sensor system. There are several challenges involved to design the front end: • A common problem with any microsensors is that they tend to generate small electrical signals at the sensor output, so the front-end circuitry needs to be of very low noise, that is, a good analogue design is required. Often, a gain control
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VDD
Microhotplate
Current source Temperature sensor
Heater Membrane
Vcontrol
+ –
Fig. 7.5 Heater temperature control scheme by comparing temperature sensor voltage. (Adapted from D. Barrettino, M. Graf, M. Zimmermann, C. Hagleitner, A. Hierlemann, H. Baltes, A smart single-chip microhotplate-based chemical sensor system in CMOS-technology, Analog Integr. Circ. Sig. Process 39 (2004) 275–287.)
instrumentation amplifier is used in this case. Sometimes, a chopper amplifier is also used to shift the noise signal at high frequency and then filter it out. • The resistance of the sensing layer (metal oxide) is very high. The baseline resistance (resistance of sensing layer in the absence of any gas) varies enormously (typically sheet resistances varies from 1 kΩ per square up to even 10 GΩ per square). Even the same metal oxide can show very different sheet resistances due to variation in synthesis process parameters, morphology, and thickness of the sensing layer. One way to reduce resistance is to use interdigitated electrode, rather than ordinary electrode. However, it is challenging to accommodate this wide resistance range through the front-end circuit. On top of this large baseline resistance, the circuit needs to amplify the change in resistance due to the presence of gas. To amplify the resistance change, one needs a variable gain amplifier. This is because if the sensor response is poor, then large gain is required, whereas if the sensor response is large, then gain from amplifier should be small to avoid output clipping. • There can be significant drift of sensor signal with time. This may be due to aging of the nanomaterial or poisoning of sensing layer due to incomplete desorption of gas analytes. There should be provision of drift cancelation either through circuit or through softwire (lookup table). The literature reports many different solutions to the aforementioned challenges. A very simple scheme to measure resistance is using a resistor divider circuit or a Wheatstone bridge. This scheme is more suitable for developing PCB level electronics. In one of
CMOS-based resistive and FET devices for smart gas sensors
the reports, a bank of resistors was used in a voltage divider mode with the sensing element [24]. The voltage across sensing element is kept close to the mid of the supply rail (in the absence of the gas) by choosing the appropriate resistor from the bank through microcontroller (as shown in Fig. 7.6). Through this simple approach, a wide sensing element resistance range can be covered. But this is not suitable when seeking full on-chip integration as it needs either trimming or variable resistors from outside the chip and also large resistance occupies lot of silicon area. A simple scheme has been reported in Ref. [23], where large dynamic range of sensing material was compressed by using two diodes (popularly known as log converter approach). The schematic is shown in Fig. 7.7. Here, two current mirror circuits are used: one diode is with sensor, and the other one is connected to a fix known resistor (connected through current mirror). The difference of the two diode voltages contains the information of sensor resistor. Another popular approach found in the literature is the resistance to frequency conversion. One example of the scheme is shown in Fig. 7.8. Here, a cascode current source is used to charge a capacitor, whereas current sink is used to discharge it. This current is controlled by the sensing material resistor (connected to the reference arm). Thus, the resulting triangular wave contains both the resistor and capacitor term, which can be found out from the frequency of the wave [25]. (Adapted from [25].)
Fig. 7.6 Voltage divider sensor interfacing circuit. (Adapted from D. Burman, D. Choudhary, P.K. Guha, ZnO/MoS2 based enhanced humidity sensor prototype with Android App interface for mobile platform, IEEE Sensors J. 19 (11) (2019) 3993–3998.)
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VDD M1
Vcontrol
M2
M3
Vout1
+
M4
Vout2
D1
D2
–
Sensing material
Fig. 7.7 Logarithmic compression circuit for sensing material interface. VDD
VH Vref
CNTy CNTx
+
C
+ –
–
–
CNTy CNTx
Sensing material
Q
CNTx
+ + VL
CNTy
– FlipFlop
Fig. 7.8 Resistance-controlled oscillator circuit for sensing material interface.
7.5 Integration of temperature and humidity sensors 7.5.1 Temperature sensor The response of a gas sensor is generally sensitive to its operating temperature. The response (change resistance) may increase, decrease, or even be relatively insensitive to its operating temperature, but the baseline signal generally has a strong temperature dependence. Therefore, it is important to know the operating temperature of the gassensing layer. Most microheaters operate by constant current drive, and so, the temperature of the heater will vary with the ambient temperature. In this case, it is also useful to
CMOS-based resistive and FET devices for smart gas sensors
know the ambient temperature as well so that the sensor response and heater resistance can be compensated. The ideal microheater drive circuit would be a constant resistive one in which the resistance of the heater is set and a closed loop control system is designed. This can be implemented using a microcontroller. However, in practice, the resistance of the microheater can slowly drift, and so the accuracy of such a closed loop system can be negated. In addition, the response and recovery times also depend upon the operating temperature, and so if dynamic information is used, then again, it is important to know the exact operating temperature of the gas-sensing layer and compensate for changes in it from ambient conditions.
7.5.2 Humidity sensor Humidity is perhaps the most important parameter that affects the response as MOX gas sensors and being important for indoor air for human comfort. Apart from this, humidity sensors find application in a number of fields like meteorology, biology, medicine, agriculture, industry, and geology. So, there is a strong need to also measure humidity and compensate the gas sensor response, as increasing humidity generally lowers the response of MOX gas sensors by 5%–50%. Various types of humidity sensors have already been reported in the literature, that is capacitive, resistive, surface acoustic wave, and quartz crystal microbalance [26–28]. Among these, polymeric or β-alumina capacitive humidity sensors are widely used for low-cost chips. Here, a hygroscopic dielectric material (e.g., polymer) sandwiched between a pair of electrodes forming a small capacitor. The change in device capacitance due to a change in dielectric permittivity with humidity is exploited as the sensing mechanism. However, their hysteretic behavior is often a serious drawback in practical applications. On the other hand, resistive humidity sensors are recently gaining popularity because of their simpler structure, and they are easy to integrate on a CMOS platform. Here, nanomaterial is deposited (or grown) on top of the interdigitated electrodes. Different metal oxides (e.g., SnO2, ZnO, and WO3) and polymer composites have been explored in literature as humidity sensing layer [29–30]. Recently, there have been some reports of 2-D layered material-based humidity sensors, for example, graphene oxide (GO)-, MoS2-, and WS2-based humidity sensors [31–33]. At low RH, water molecules are primarily physisorbed onto the available active sites (hydrophilic groups and vacancies) of the GO surface through double hydrogen bonding, which is called the first-layer physisorption of water. In this regime, the water molecules are thus unable to move freely because of the restriction from double hydrogen bonding. As the RH increases, the multilayer physical adsorption of water molecules occurs. From the second physisorbed layer, water molecules are physisorbed through single hydrogen bonding on the hydroxyl groups. Thereafter, the water molecules become mobile and progressively more identical to those in the bulk liquid. As the multilayer physical adsorption progresses,
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the physisorbed water can be ionized under an electrostatic field to produce a large number of hydronium ions (H3O+) as charge carriers. So in high RH, proton hopping takes place between adjacent water molecules that gives rise to a high conductivity and hence gives large responses [31]. In practice, polymer-based (polyaniline) capacitive humidity sensors are the most common choice and have been commercialized for many years. Now, integrated chips are available as described in a later section and allow more accurate measurement of gases using low-cost MOX materials, such as SnO2 and WO3.
7.6 Packaging of CMOS gas sensors Research generally focuses upon the silicon die, but the commercial world needs to consider the assembly of the silicon die in a package and then interface to a PCB or microcontroller. In practice, the die attach, wirebonding, and packaging can cost much more than the silicon die itself. The simplest solution is to attach the die directly to a PCB, and in these days, surface mount technology is almost universal. Hence, the use of a flip-chip arrangement can allow the use of a low-temperature oven to attach the silicon to a PCB. However, due to the control of vapors arising from the glue and also the PCB itself, most manufacturers opt for low-cost small outline plastic packages that can then be surface mounted onto a PCB in high volumes at low cost (a few cents per device). In similar cases, the system in a package approach has been used. However, this adds additional costs, and now, fully integrated chips are appearing on the market—often referred to as COMBO chips. The flow of the gas or VOC into and out of the package also has to be considered, and so, packages are usually designed specifically for the application in hand. The tooling up of the packaging can cost $50k, but such on-off costs are small when high volumes of sensors are sold, that is, millions per year. Finally, most gas sensor packages have a relative small number of pins to save cost, and it is increasingly rare to have an analogue output. The default now is a digital interface such as i2c that can be directly fed into a low-cost micro for use in digital devices such as smart home appliances, smart wearables, and mobile phones.
7.7 Commercial CMOS gas sensors The last few years have seen a significant rise in the companies selling MEMS-based gas sensors for a range of applications. The traditional markets have been industrial and medical, but the volumes are relatively modest, and so, manufacturers are chasing the high volume markets offered by consumer electronics and the IoT. In this new and emerging space, there are perhaps three market leaders offering COMBO chips for toxic gas and
CMOS-based resistive and FET devices for smart gas sensors
tVOC sensing. Fig. 7.9 shows the leading smart chips manufactured by Bosch, Sensirion, and ams for CMOS-based gas sensing. These chips have been sold in high volumes and employ MEMS sensors manufactured using CMOS or SOI CMOS technology. Table 7.1 summarizes their technical performance, and they usher in a new era of low-cost CMOS gas sensors for the rapidly emerging consumer electronics industry. The application domain targets by these CMOS chips are commonly stated as indoor air quality, air cleaners and purifiers, smart thermostats, home controllers, smart accessories, smart phones, and IoT devices. The potential sales of these chips is 100 million to low billions of units per year, so the future market could be worth $10B per annum.
Fig. 7.9 Some commercial smart CMOS gas sensors manufactured today: (A) Bosch BME680, (B) Sensirion Multi-Pixel SGP30, and (C) ams.
Table 7.1 Some technical specifications of commercial gas and tVOC MEMS chips from technical datasheets of manufacturers Chip
Measurands
Bosch BME680
Gas (air quality), pressure, temperature, humidity
Sensirion SGP30
Gas (tVOC), ethanol, hydrogen
Ams/ CCS 811b
Gas (tVOC)
Size (mm)/ package
Supply voltage (V)/power (mW)
Output
Cost ($)
331 Metal lid LGA 2.5 2.5 1 6-pin DFN 2.7 4 1 10-pin LGA package
1.7–3.6 <1
I2C/ SPI
10
1.6–2.0 < 90
I2C
9
1.8–3.6 < 60
I2C
8
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CMOS-based resistive and FET devices for smart gas sensors
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