This paper addresses the issue of volatile organic compounds (VOCs), hazardous materials commonly present in paints, perfumes, petroleum and oil refineries, and plastic products, which can lead to health hazards. The detection of these toxic compounds has been a compelling subject for researchers. In this study, a novel approach is presented, involving the design of a sensor for VOC determination using a piezoresistive microcantilever. Microcantilevers have gained significant attention in electrochemical applications due to their enhanced sensitivity. The research explores various design factors, such as length, thickness, and material selection, to optimize the sensor’s response. Specifically, reducing the cantilever’s thickness is considered to improve the deflection sensitivity. To enhance the sensitivity of the piezoresistive sensor, particular emphasis is placed on developing the piezoresistive sensing layer to effectively address stress-induced changes. Experimental investigations involve exploring different cantilever shapes and piezolayer configurations to achieve the desired optimized condition. Ultimately, the rectangular cantilever shape is reshaped into a U-shape, which demonstrates enhanced sensitivity, offering promising possibilities for VOC detection. This study presents valuable insights into the design and optimization of piezoresistive microcantilever sensors for efficient VOC detection, its temperature effects, and monitoring applications.
I. INTRODUCTION
The evolution of silicon micro-machining technology has propelled MEMS piezoresistive pressure sensors into widespread use across diverse industrial and commercial applications.1,2 This surge in adoption is intrinsically linked to the proliferation of electronic devices, encompassing sectors such as automotive, aerospace, and compact gadgets. In addition, as concerns about air quality and environmental impact have grown, MEMS piezoresistive pressure sensors have found new applications in monitoring and controlling Volatile Organic Compounds (VOCs) in various settings.3 Consequently, there has been a resurgence of research endeavors aimed at augmenting the precision, efficiency, cost-effectiveness, and scalability of micro-pressure measurement sensors.4 These sensors operate on a fundamental principle: a thin, responsive sheet that deflects under applied pressure.5 Recent years have witnessed the establishment of various standardized membrane architectures for piezoresistive pressure sensors.6 Notably, the composition of the membrane plays a pivotal role in determining the sensor's output, especially when it comes to measuring VOC concentrations.7,8 In this paper, we embark on an exploration of the dynamic landscape of MEMS piezoresistive pressure sensors, offering insights into their indispensable role in modern technology, their contribution to VOC monitoring, and the intricate design considerations shaping their performance.
Figure 1 depicts the analyte adsorption on the sharp end of the microcantilever. The piezoresistive detection method is used in this study because it does not require any additional sensing devices. Changes in resistivity are detected as changes in current.9 Another widely used technique for detection is the optical method, which has many drawbacks, such as being non-portable, requiring external instruments, and requiring periodic alignment, but none of these drawbacks exist in the piezoresistive method. A simple piezoresistive microcantilever design is presented in this paper.10 Through various investigations, such as geometrical dimension variations and the integration of a stress concentration field, concerted efforts have been made to optimize the performance of the microcantilever. Finite Element Analysis (FEA) is widely used to design MEMS microcantilever structures to achieve stress distribution and non-linearity. For the performance analysis of the microcantilever structure, a MEMS simulation technique is used.11 The effects of different piezoresistor placements on the microcantilever are investigated, and the most susceptible area is discovered. Sensitivity is described as the combined effect of the microcantilever’s deflection and change in resistance. The above-mentioned parameters are used to analyze it.
When a cantilever beam undergoes bending, it experiences lateral tension, leading to a variation in resistance when combined with a resistor. This phenomenon is known as piezoresistance, where the electrical resistivity is influenced by pressure. Both metals and semiconductors exhibit piezoresistive properties.12 In metals, the resistance change primarily stems from geometrical variance, while in semiconductors, it arises due to modifications in bandgap energy.
i, j = direction of resistivity in the x and y plane,
R0 = initial resistance,
Δρ = change in resistivity
ΔR = R − R0 = change in resistance,
ϑ = Poisson’s ratio, and
ρ0 = electrical resistivity.
To add, Eq. (2) denotes a piezoresistive cantilever, and it should be subjected to a significant net compressive (tensile) force. The piezoresistive cantilever could be built by mounting two distinct lateral layers, structural and transducer layers, to fulfill this requirement. As a result, each substrate encounters a net stress in the opposite direction of all other layer; thereby, both tensile and compressive stresses around the cantilever beam need not balance out in that layer.
II. DESIGN AND MODELING OF A PIEZORESISTIVE MEMS MICROCANTILEVER
In this section, our primary focus is on the design and simulation of piezoresistive microcantilevers, with a specific emphasis on their potential applications in VOC (Volatile Organic Compound) sensing and detection. We delve into various critical aspects of microcantilever design, including cantilever geometry, material selection, and the strategic placement of piezoresistors. To facilitate our simulations and analyses, we employ advanced computational tools, particularly utilizing the capabilities of COMSOL Multiphysics.15 This powerful software enables us to comprehensively examine both the mechanical and electrical behaviors of these microcantilevers. By leveraging simulation techniques, our goal is to gain a deep understanding of the design process, optimize the microcantilever’s performance, and explore its potential as a highly efficient sensor in various sensing and VOC detection applications. We specifically highlight the use of COMSOL Multiphysics for Finite Element Method (FEM) simulations. Our study includes a comparative analysis between analytical and FEM simulation results for a rectangular microcantilever design.16 We explain the rationale behind selecting this particular geometry and detail the stress characteristics determined through FEM simulations.
In our research, we consider two distinct piezoresistive MEMS microcantilever designs: the double layer and single layer configurations.17 The double layer design involves the incorporation of a piezoresistive material near the top surface of the microcantilever, allowing for real-time monitoring of stress changes on the surface. As the microcantilever undergoes deflection, stress-induced strain in the piezoresistor results in a measurable change in resistance, which can be electronically detected. However, this approach necessitates embedding a piezoresistor on the microcantilever and employs two different materials (p-doped silicon and polysilicon), which can complicate the fabrication process. Alternatively, we introduce the single layer design, which simplifies the fabrication process by integrating both the piezoresistor and microcantilever into a single layer using p-doped silicon.19,20 For the simulation and modeling of both microcantilever designs, we rely on the versatile capabilities of COMSOL Multiphysics. This choice of software ensures precise and efficient analysis of the mechanical and electrical behaviors of the microcantilevers, facilitating their optimization and performance evaluation for potential applications in VOC sensing and detection.
In the design of microcantilevers, several crucial factors are considered to enable highly sensitive measurements and low detection limits for the target analyte. These factors include selecting the appropriate operating conditions, material, cantilever geometry, and array design. Most existing piezoresistive microcantilevers are designed based on surface stress loading. Nevertheless, this particular method frequently results in uniform surface stresses throughout the cantilever, which consequently leads to a deficiency in clearly delineated areas of the highest surface stress. Furthermore, it is worth noting that the influence of local surface stresses on piezoresistors that are situated at a considerable distance from the source of stress may not be substantial. In order to overcome these limitations, this research identifies the following particular design specifications for piezoresistive microcantilevers intended for the purpose of surface stress estimation:
The appropriate positioning of the piezoresistor within the area experiencing surface stress.
Maximization of the piezoresistive response (ΔR/R).
While the importance of resistor placement is acknowledged in the current research, existing cantilever devices have not fully addressed all aspects of optimizing the piezoresistive response. To achieve a large piezoresistive response, careful consideration of doping and geometry for silicon piezoresistors is necessary. By incorporating such design optimizations, microcantilevers can attain enhanced sensitivity and enable efficient detection of the selected analyte.
Figure 2 illustrates the geometric parameters of a single cantilever. In the context of piezoresistive cantilever designs, specific piezoresistor regions are created on the surface of the cantilever. As a result, the piezoresistors are confined to a distinct layer within the overall multilayered structure. This strategic placement of the piezoresistors enables precise and targeted sensing, enhancing the sensitivity and accuracy of the microcantilever’s response to external stimulations.
The “U-shape” double-layer piezoresistive cantilever shown in Fig. 3 is a specialized microcantilever design distinguished by its unique U-shaped geometry and the presence of two distinct layers. This configuration is particularly tailored for applications demanding precise sensing and detection capabilities. At its core, this cantilever functions as a sensor by capitalizing on piezoresistive materials integrated into its structure. When subjected to mechanical stress or pressure, the upper layer of the cantilever experiences deflection, inducing strain within the embedded piezoresistive elements. This strain subsequently leads to detectable alterations in electrical resistance, facilitating reliable sensing and detection mechanisms. The “U-shape” double-layer piezoresistive cantilever holds relevance across a wide range of fields, including industrial process control, automotive systems, medical devices, and environmental monitoring. In environmental applications, it can play a pivotal role in detecting VOCs and various gases, contributing to air quality assessment and safety protocols.
U-shape piezoresistive material is embedded on a rectangular cantilever beam.
Silicon’s impressive large gauge factors have positioned it as an excellent candidate for diverse stress measurement applications. Its piezoresistive properties have led to successful utilization in sensing pressure, deformation, and stress. Piezoresistance is extensively utilized in diverse domains, such as accelerometers, gyroscopes, and dynamometers. Moreover, it has been increasingly employed in atomic force microscopy (AFM) probes and surface stress sensors for micro-cantilever-based measurements. The impact of doping on the sensitivity of surface stress in piezoresistive microcantilevers has received limited attention in their design, until recently. In this study, we aim to address this gap by comparing p-type and n-type piezoresistive sensing elements for detection based on surface stress.
Through a comprehensive examination of the effects of doping on the sensitivity of surface stress, our objective is to uncover the complete capabilities of piezoresistive micro-cantilevers in accurately and effectively measuring surface stress for various applications. The dimensions of the rectangular cantilever are shown in Table I.
Dimensions used in the design of a rectangular cantilever.
Length of the cantilever (L) . | 225 µm . |
---|---|
Width of the cantilever (w) | 80 |
Thickness of the cantilever (t) | 0.65 |
Length of the cantilever (L) . | 225 µm . |
---|---|
Width of the cantilever (w) | 80 |
Thickness of the cantilever (t) | 0.65 |
The mass of the cantilever is maintained constant, and the length and thickness of the cantilever are varied to find the deflection of the cantilever.18 In addition, the width of the cantilever is kept constant, as seen in Table I. The stiffness is also calculated to study the response of the cantilever. It was seen from the literature that optimum stiffness could be attained when the width and the length are maintained at a ratio of 1:2. Table II shows the change in position vs properties of a single layer, and Table III shows the change in position vs properties of a double layer.
Change in position vs properties of a single layer. Bold denotes that the single layer cantilever properties based on the change in the position.
S. No. . | Position (μm) . | Deflection (μm) . | Stress (σmax) [pa/] . | Sensitivity (∆R/R) . |
---|---|---|---|---|
1 | 20 | 0.80 | 3.2 | 0.08 |
2 | 30 | 0.81 | 3.25 | 0.081 |
3 | 40 | 0.801 | 3.2 | 0.0801 |
4 | 50 | 0.792 | 3.2 | 0.0792 |
5 | 70 | 0.779 | 3.07 | 0.0779 |
6 | 90 | 0.755 | 2.91 | 0.0755 |
7 | 110 | 0.719 | 2.73 | 0.0719 |
8 | 130 | 0.669 | 2.48 | 0.0669 |
9 | 150 | 0.604 | 2.21 | 0.0604 |
10 | 170 | 0.521 | 1.9 | 0.0521 |
11 | 190 | 0.420 | 1.54 | 0.0420 |
12 | 210 | 0.30 | 1.13 | 0.030 |
S. No. . | Position (μm) . | Deflection (μm) . | Stress (σmax) [pa/] . | Sensitivity (∆R/R) . |
---|---|---|---|---|
1 | 20 | 0.80 | 3.2 | 0.08 |
2 | 30 | 0.81 | 3.25 | 0.081 |
3 | 40 | 0.801 | 3.2 | 0.0801 |
4 | 50 | 0.792 | 3.2 | 0.0792 |
5 | 70 | 0.779 | 3.07 | 0.0779 |
6 | 90 | 0.755 | 2.91 | 0.0755 |
7 | 110 | 0.719 | 2.73 | 0.0719 |
8 | 130 | 0.669 | 2.48 | 0.0669 |
9 | 150 | 0.604 | 2.21 | 0.0604 |
10 | 170 | 0.521 | 1.9 | 0.0521 |
11 | 190 | 0.420 | 1.54 | 0.0420 |
12 | 210 | 0.30 | 1.13 | 0.030 |
Change in position vs properties of a double layer.
S. No . | Position (μm) . | Deflection (μm) . | Stress (σmax) [pa/] . | Sensitivity (∆R/R) . |
---|---|---|---|---|
1 | 20 | 0.662 | 2.297 | 0.0662 |
2 | 30 | 0.672 | 2.991 | 0.0672 |
3 | 40 | 0.623 | 2.047 | 0.0623 |
4 | 50 | 0.592 | 1.814 | 0.0592 |
5 | 70 | 0.541 | 1.695 | 0.0541 |
6 | 90 | 0.499 | 1.692 | 0.0499 |
7 | 110 | 0.459 | 1.510 | 0.0459 |
8 | 130 | 0.417 | 1.384 | 0.0417 |
9 | 150 | 0.37 | 1.227 | 0.037 |
10 | 170 | 0.31 | 1.051 | 0.031 |
11 | 190 | 0.25 | 0.850 | 0.025 |
12 | 210 | 0.184 | 0.629 | 0.0184 |
S. No . | Position (μm) . | Deflection (μm) . | Stress (σmax) [pa/] . | Sensitivity (∆R/R) . |
---|---|---|---|---|
1 | 20 | 0.662 | 2.297 | 0.0662 |
2 | 30 | 0.672 | 2.991 | 0.0672 |
3 | 40 | 0.623 | 2.047 | 0.0623 |
4 | 50 | 0.592 | 1.814 | 0.0592 |
5 | 70 | 0.541 | 1.695 | 0.0541 |
6 | 90 | 0.499 | 1.692 | 0.0499 |
7 | 110 | 0.459 | 1.510 | 0.0459 |
8 | 130 | 0.417 | 1.384 | 0.0417 |
9 | 150 | 0.37 | 1.227 | 0.037 |
10 | 170 | 0.31 | 1.051 | 0.031 |
11 | 190 | 0.25 | 0.850 | 0.025 |
12 | 210 | 0.184 | 0.629 | 0.0184 |
In Fig. 4, a comparative analysis is presented, depicting the deflection characteristics of a single-layer cantilever in contrast to a double-layer cantilever concerning changes in position. This visual representation provides insights into how the two different cantilever configurations respond to alterations in their respective positions. The graph allows for a direct comparison of the deflection behavior, showcasing any variations or distinctions between the single-layer and double-layer cantilever designs as they undergo changes in position.
In Fig. 5, a comparative analysis is presented, illustrating the sensitivity characteristics of a single-layer cantilever in comparison to a double-layer cantilever concerning changes in position. This graphical representation offers insights into how the two different cantilever configurations respond to alterations in their respective positions in terms of sensitivity. The graph allows for a direct comparison of sensitivity behavior, showcasing any variations or distinctions between the single-layer and double-layer cantilever designs as they undergo changes in position.
In Fig. 6, a graphical representation is provided, illustrating the relationship between the applied load and deflection for a single-layer cantilever. This graph visually depicts how the deflection of the cantilever responds to varying levels of applied load. By presenting the correlation between these two parameters, Fig. 6 offers valuable insights into the structural behavior of the single-layer cantilever under different loading conditions.
In Fig. 7, a graphical representation is provided, showcasing the relationship between the position and strain of a single-layer cantilever. This graph visually illustrates how the strain, a measure of deformation, varies with different positions of the cantilever. The curve or trend in Fig. 7 reveals the strain distribution along the cantilever’s length as its position changes. Analyzing this relationship is crucial for understanding how the cantilever undergoes deformation at different locations, providing valuable insights into its mechanical behavior and strain characteristics.
In Fig. 8, a graphical representation is presented, depicting the relationship between the position and stress of a single-layer cantilever. This graph visually illustrates how the stress distribution varies along the length of the cantilever in response to different positions. The curve or trend in Fig. 8 provides insights into how stress is distributed across the cantilever’s structure as its position changes. Analyzing this relationship is crucial for understanding how external forces affect the cantilever’s structural integrity and the distribution of stress within the material.
III. TEMPERATURE EFFECTS AND LIMITATIONS
A. Temperature coefficient of resistance (TCR)
Effect: The electrical resistance of piezoresistive materials changes with temperature. This phenomenon is characterized by the Temperature Coefficient of Resistance (TCR), which represents the percentage change in resistance per degree Celsius.
Limitation: Without compensation, temperature-induced resistance changes can lead to inaccuracies in sensor readings.
B. Thermal expansion
Effect: Changes in temperature can cause the physical dimensions of the sensor, including the cantilever, to expand or contract. This alters the mechanical properties and can affect the stress experienced by the piezoresistive elements.
Limitation: Uncompensated thermal expansion may introduce errors in the sensor’s output.
C. Material properties
Effect: The mechanical properties of the piezoresistive material, such as Young’s modulus, may vary with temperature.
Limitation: These variations can impact the strain and stress characteristics of the material, influencing the overall sensor response.
Depicted in Fig. 9 is a composite structure characterized by the incorporation of a U-shaped region crafted from both p-type polycrystalline and n-type silicon polycrystalline materials. This U-shaped configuration is seamlessly embedded within a rectangular region composed of silicon. The deliberate amalgamation of p-type and n-type silicon polycrystalline materials, along with silicon, serves the purpose of mitigating temperature effects. This strategic integration capitalizes on the unique properties inherent in each material. The geometric layout, as showcased in Fig. 4, emphasizes the merging of the dual-type silicon polycrystalline U-shape onto the silicon rectangle, providing a clear illustration of the structural design.
Cantilever with n-type silicon polycrystalline and p-type silicon polycrystalline materials.
Cantilever with n-type silicon polycrystalline and p-type silicon polycrystalline materials.
Illustrated in Figs. 10 and 11 are the output voltage characteristics of the devised composite structure under room temperature conditions. The graph visually elucidates how the amalgamated p-type silicon polycrystalline, n-type silicon polycrystalline, and silicon materials work collaboratively to counteract temperature effects, thereby influencing the output voltage. This graphical representation serves as a valuable dataset, offering insights into the electrical behavior of the composite structure in the context of temperature compensation. Analyzing these data enables a comprehensive examination of the composite structure’s performance, particularly in terms of its electrical characteristics. The trends depicted in Fig. 5.24 provide crucial information about the electrical response of this specific composite design under standard room temperature conditions.
Cantilever with p-type silicon polycrystalline and n-type silicon polycrystalline materials from COMSOL’s output.
Cantilever with p-type silicon polycrystalline and n-type silicon polycrystalline materials from COMSOL’s output.
Cantilever with p-type silicon polycrystalline and n-type silicon polycrystalline materials from COMSOL’s voltage output.
Cantilever with p-type silicon polycrystalline and n-type silicon polycrystalline materials from COMSOL’s voltage output.
IV. CONCLUSION
This study focused on investigating and comparing the temperature effects in newly proposed piezoresistive cantilevers. Notably, it was observed that the temperature effect demonstrated a decrease for the n-type configuration, while conversely, an increase was noted for the p-type configuration. To address these variations, a novel structure was meticulously designed to effectively compensate for temperature fluctuations. The outcome of this optimization resulted in a noteworthy enhancement in sensitivity, measuring at an impressive 0.097 µm/pa. This marked improvement surpassed the sensitivity achieved in the previous design, which recorded a value of 0.081 µm/pa.
In conclusion, the finalized design introduces a single-layer piezoresistive cantilever with a remarkable sensitivity of 0.097 µm/pa. Specifically tailored for the detection of Volatile Organic Compounds (VOCs) at a concentration of 10 ppm, this design stands out as a promising solution. The holistic exploration and optimization undertaken throughout the various objectives contribute synergistically to the cantilever’s efficacy in both stress and temperature sensing. Altogether, this positions the optimized design as a highly effective and promising solution for VOC detection applications.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Vasagiri Suresh: Conceptualization (equal); Methodology (equal); Software (equal); Writing – original draft (equal). Rajesh Kumar Burra: Supervision (equal); Writing – review & editing (equal).
DATA AVAILABILITY
The data that support the findings of this study are available within the article.