Effect of Stiffness in Sensitivity Enhancement of MEMS Force Sensor Using Rectangular Spade Cantilever for Micromanipulation Applications

Monica Lamba, Himanshu Chaudhary, and Kulwant Singh

Manipal University Jaipur


The MEMS market is expected to witness a CAGR of 6.34% for the forecast period of 2020-2025 due to its increasing demand over the past few' years in various fields of application, including electronics such as wearable devices, smartphones, tablets, digital cameras portable navigation devices and media players, and gaming consoles [1].

MEMS sensors are mainly categorized depending upon their type, application, and geography, as indicated in Figure 18.1. Among all MEMS sensors, the force sensor shares a big portion of the total revenue. Microscale force sensors are required to protect small-scale structures as they efficiently measure forces in micro-Newton ranges and can be utilized as force feedback in various fields, including minimally invasive surgeries, material science, lifescience, and mechanobiology [2-6], as indicated in Figure 18.2.

The force feedback improves the speed and accuracy in performing micromanipulation tasks [7,8]. MEMS sensor mainly consists of mechanical and sensing structures. MEMS mechanical structures include diaphragms, cantilever beams, and electrostatic motors; however, microcantilever beams are the most preferred due to their flexibility, versatility, high sensitivity, and low' cost [9,10]. Microcantilever beams convert the applied force into displacement. The sensing structure of the MEMS force sensor is mainly categorized into electrical and optical force sensors, as shown in Figure 18.3. In this chapter, the focus will be on the piezoresistive sensing mechanism falling under the category of electrical-type MEMS force sensor ow'ing to its small size, high resolution, low' phase lag, low cost, high sensitivity, high dynamic range, easy fabrication, and easy integration.

Classification of MEMS sensors

FIGURE 18.1 Classification of MEMS sensors.

Applications of MEMS force sensors

FIGURE 18.2 Applications of MEMS force sensors.

Different types of sensing mechanisms in MEMS force sensors

FIGURE 18.3 Different types of sensing mechanisms in MEMS force sensors.

To analyze the performance of the sensor, sensitivity is one of the key parameters that need to be investigated. There are various factors, as shown in Figure 18.4, which influence the sensitivity. The sensitivity of piezoresistive microcantilever-based force sensor can be enhanced by varying the sensors’ design parameters, which include the dimension of the cantilever [11] and design, as reported by Ansari and Cho [12] and Zhang et al. [13], cantilever material, as reported by Wee et al. [14] and Nordstrom et al. [15], and by varying piezoresistor dimension and location on microcantilever, as discussed by Goericke and King [16]. The cantilever and piezoresistors are arranged in such a manner that higher stress is induced inside the piezoresistors to enhance the sensitivity of the designed sensor. Stress-concentrated region is also one of the techniques of sensitivity enhancement, as mentioned in references [17-21].

Various factors affecting sensitivity enhancement in piezoresistive microcantilever-based force sensor

FIGURE 18.4 Various factors affecting sensitivity enhancement in piezoresistive microcantilever-based force sensor.

Moreover, the above-mentioned studies related to the sensitivity enhancement of piezoresistive microcantilever-based force sensor were restricted to the geometrical parameters of cantilevers, piezoresistors, and their placement along with their material properties. To the best of the authors’ knowledge, the correlation between the stiffness and electrical sensitivity has not been reported yet. Therefore, there exists a need to analyze the effect of stiffness on a piezoresistive force sensor. In this pursuit, an investigation has been undertaken in this study by considering a unique design of a rectangular microcantilever with a rectangular spade as a mechanical structure along with different combinations of substrate and piezoresistor materials. Finite element analysis has been performed using COMSOL Multiphysics 5.3a software to examine maximum displacement and electrical sensitivity of a sensor using a varied combination of flexible and non-flexible materials operated in the range of 1-10 pN.

Theoretical Analysis and Mathematical Equations

Basic Operating Principle

When low-magnitude forces in the micro-Newton range are to be sensed for micromanipulation application, the sensor needs to be highly efficient to sense these forces effectively. The force exerted by these micromanipulation tasks on the tip of the microcantilever is indicated in the form of displacement which is further converted into an electrical signal by piezoresistive sensing mechanism. The displacement of the microcantilever due to applied force is calculated by equation (18.1) [22].

where z is the displacement of the microcantilever which depends upon the Poisson’s ratio (v), stress (a), Young’s modulus of elasticity (E), and the geometrical parameters length (L) and thickness (t).

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