Performance Analysis of MEMS Piezoresistive …Performance Analysis of MEMS Piezoresistive Cantilever Based Sensor for Tuberculosis Detection Using Coventorware FEA Neethu Karayil

Performance Analysis of MEMS Piezoresistive

Cantilever Based Sensor for Tuberculosis

Detection Using Coventorware FEA

Neethu Karayil and K. J. Suja Electronics & Communication Engineering Department, National Institute of Technology, Calicut, Kerala, India

Email: [email protected], [email protected]

Abstract—Micro machined cantilever platform offers an

opportunity for the development and mass production of

extremely sensitive low-cost sensors for real- time in situ

sensing of many biological species. The merits of MEMS

micro cantilever sensors are its high sensitivity, design

simplicity, portability and high speed. In this paper, a micro

cantilever sensor is designed to meet the requirements of a

biosensor that detects tuberculosis. The addition of mass on

the micro cantilever surface makes it to bend and vibrate

with a resonance frequency. The advantages of

incorporating stress concentrated region (SCR) on the

piezoresistive micro cantilever and its optimal position for

maximum sensitivity is also studied in this paper. Different

shaped SCRs were modeled and it is found that rectangular

shaped SCR is found to be suitable structure showing

maximum sensitivity. FEA simulation tool Coventorware©

has been used for the simulation.

Index Terms—MEMS, cantilever, piezoresistive mechanism,

SCR

I. INTRODUCTION

Micro cantilever beams are being used for fabricating

high performance biological sensors for the detection of

explosives and harmful chemical and biological species.

These sensors have a wide range of applicability in

defense and medical fields [1]-[2]. These micro-scale

sensors utilize a receptor, which is specific to a single

chemical or biological target, for immobilizing the

species of interest and then using a wide variety of

physical and chemical mechanisms for detection and

transduction, leading to a recordable signal response [3].

A cantilever is a simplest mechanical structure, which is

clamped at one end and free at the other end. Micro

cantilever is a micro fabricated rectangular bar- shaped

structure, longer as compared to width, and has a

thickness much smaller than its length or width. To serve

as a sensor, cantilever has to be coated with a sensing

layer, which should be specific, i.e. able to recognize

target molecules. Stress is created on the sensitive end of

the microcantilever owing to the adsorption of

biomolecules. The adsorption of analyte on the sensitive

end of microcantilever is shown in Fig. 1. Finite Element

Manuscript received March 1, 2017; revised June 3, 2017.

Method is used for the simulation of MEMS cantilever

structure.

(a)

(b)

Figure 1. (a) Microcantilever-based biosensor (b) Stress-free cantilever and bending of the cantilever due to the generated surface stress by

interaction with analyte.

II. CANTILEVER BASED SENSING

A microcantilever beam is fixed firmly at one end and

is free at the other end. Resonant frequency being one of

the important parameters is the frequency of

microcantilever at which it oscillates with maximum

amplitude. It undergoes deflection when mass is adsorbed

301© 2017 Int. J. Mech. Eng. Rob. Res.

International Journal of Mechanical Engineering and Robotics Research Vol. 6, No. 4, July 2017

doi: 10.18178/ijmerr.6.4.301-304

onto it. So the microcantilever needs to be designed so as

to attain the maximum sensitivity by maximizing

resonance frequency and deflection of the beam. For a

rectangular profile microcantilever, the surface stress σ

and deflection z are related by Stony's Equation as given

by equation (1) [3].

2

2

4 1 LZ

Et

(1)

Here L and t are the length and thickness of the

microcantilever, E and ν are the Elastic modulus and

Poisson’s ratio of the microcantilever material. Resonant

frequency ‘f’ for a rectangular profile microcantilever

with mass density ρ is given as in equation (2)

2 22

Etf

L (2)

From the above equations, it is seen that attempts to

increase the deflection by increasing length or decreasing

thickness will decrease the natural frequency. Therefore

considering overall sensitivity which is defined in terms

of the deflection and frequency can be expressed as in

equation (3)

2

2(1 ).z f

t E

(3)

At the microcantilever center line the stress becomes

zero and it increases linearly with distance away from it.

So highest sensitivity is attained with a piezoresistor

placed on the base of the microcantilever.

A. Piezoresistive Detection Mechanism

When a piezoresistive material such as doped silicon is

strained, its electrical conductivity changes and also its

resistance. So, by incorporating the piezoresistive

material into a microcantilever, stress can be monitored

and also the deflection of the microcantilever [4]. When a

piezoresistive microcantilever is exposed to the target

molecules, there is an interaction between the probe and

target molecules. This interaction induces a stress which

causes microcantilever bending and therefore

piezoresistive material undergoes strain. A resistance

change is seen due to straining of the piezoresistive

material and this resistivity change can be measured

easily by using Wheatstone bridge. The piezoresistors

connected in a Wheatstone bridge configuration is

depicted in Fig. 2. Piezoresistivity is a very common

mechanism for microelectromechanical systems and

when the cantilever bends due to adsorption of molecules,

the piezoresistors that are integrated with the cantilever

will experience a strain [5]. Another commonly used

method called the optical method for detection has

several disadvantages on its side as being not portable,

the requirement of external devices, the need of periodic

alignment etc., but all these drawbacks are not present in

the piezoresistive method. The fractional change is given

as in equation (4).

1 3R

R t

(4)

Figure 2. Piezoresistors placed on a microcantilever structure in a Wheatstone bridge configuration.

Fig. 3 (a) shows the current change in piezoresistors

for various applied loads, the higher the load applied, the

higher the value of electric current obtained.

(a)

(b)

Figure 3. (a) Current change for various applied load (b) Fractional change of resistance for various applied loads

Fig. 3 (b) shows resistance change as a function of

various loads applied. It is seen that fractional resistance

change increases with increase in load applied. The

sensitivity of piezoresistive microcantilever is defined in

terms of fractional resistance change that occurs due to

the applied loads. The resistance change in the

piezoresistive microcantilever is calculated between

resistance at no load and resistance at a particular load

value. Thus the optimization of electrical responses of

piezoresistive microcantilever biosensor can be realized



by translating effects of applied loads to the current

change and sensitivity of the device.

B. Optimal Position of Piezoresistors

The applied surface stress on the piezoresistive

microcantilever are measured directly with the

mechanical energy transduced into a measurable

electrical signal. The electrical resistance will change

according to the strain developed, and the resistance

depends on the difference in lateral and transverse stress

of the microcantilever.

When piezoresistive microcantilever is used in

biomedical applications, it should be highly sensitive and

the detection mechanism used should properly sense the

stress gradient. During biological and chemical sensing

applications when the surface is applied to

microcantilever, the stress difference has its highest

magnitude near the base of the microcantilever. Therefore,

the placement of the piezoresistors within this region

where there is a maximum stress is important for

attaining a sensitive detection scheme. The piezoresistive

micrcantilever was incorporated with a stress

concentrated region that showed larger sensitivity in

terms of deflection. Square hole is used as the stress

concentrated region. The maximum induced stress when

SCR is present is on higher side is compared to its value

when there is no SCR is as shown in Fig. 4 (a). By

simulating the Mises stress values the microcantilever's

length as depicted in Fig. 4, the significance of the

incorporated SCR to the overall stress values of the PRM

structure is verified. At an applied load of 10 µN an

observation of the distribution of the Mises stress value is

depicted in Fig. 4(b).

The structure with SCR shows higher stress value at

the SCR area with a difference of 20MPa compares to the

one without SCR. This observation proves the

significance of the SCR presence to the Mises stress

values of the investigated sensor. Although the difference

is significantly small, it is worth noted that displacement

value for microcantilever with a square hole is relatively

higher than the microcantilever without a square hole.

This observations caused by the surface area reduction

due to the presence of the square hole which yield higher

deflection

(a)

(b)

Figrue 4. (a) Mises stress distribution with various applied loads (b) Mises stress along the length

C. Different Shapes of SCR

Different shapes of the SCR as shown in Fig. 5 have

been analysed namely rectangular, diamond shaped and

circular shaped holes.

(a)

(b)





(c)

Figure 5. Different shapes of SCR (a) Circular (b) Rectangular (c) Diamond shaped holes

From the analysis it is clearly observed that from the

point of sensitivity, the rectangular hole can increase

sensitivity and the other shaped holes decrease sensitivity.

For a piezoresistive microcantilever the sensitivity is

related to a change in resistance of the piezoresistors

when it is strained by the application of load. On the

other hand, from the point of deflection, the circular hole

decreases deflection whereas remaining structures

increases deflection. When considered from both the

sensitivity and deflection point, of view, only the

rectangular hole shows an increase in both sensitivity and

deflection.

A comparison of sensitivity for various shaped SCRs is

shown in Fig. 6. Hence we suggest the use of rectangular

hole on microcantilever biosensors.

Figure 6. Comparison of sensitivity for various shaped SCRs

III. CONCLUSION

The microcantilever is one of the popular MEMS

structures available in market which is utilized as

biosensor. It should be highly reliable and sensitive. The

piezoresistive sensing mechanism is used to sense the

stress change, which involves embedding of the

piezoresistors on the top surface of the microcantilever to

record the stress change occurring at the surface. The

factors affecting the sensitivity aspect of microcantilever

design include its shape and incorporation of stress

concentrated region. There is a considerable increase in

the sensitivity when SCRs were incorporated on the beam,

different shapes of SCRs are employed and analyzed for

sensitivity. It was found that the rectangular shaped SCR

gives better sensitivity if compared to circular or diamond

shaped SCR.

REFERENCES

[1] R. Raiteri, et al., “Micromechanical cantilever-based biosensors,”

Sensors and Actuators B: Chemical, vol. 79, no. 2, pp. 115-126,

2001. [2] N. Lobontiu and E. Garcia, “Two microcantilever designs:

lumped-parameter model for static and modal analysis,” Journal

of Microelectromechanical Systems, vol. 13, no. 1, pp. 41-50, 2004.

[3] R. A. Rahim, et al., “Design optimization of MEMS dual-leg shaped piezoresistive microcantilever,” in Proc. IEEE Regional

Symposium on Micro and Nanoelectronics, 2013, pp. 379-382.

[4] P. N. Patel, R. Yadav, and M. Adhvaryu, “Design and analysis of

diversified micro-cantilever structure for sensor applications,” in

Proc. 2nd International Conference on Emerging Technology Trends in Electronics, Communication and Networking, 2014, pp.

1-5.

[5] S. J. Park, J. C. Doll, and B. L. Pruitt, “Piezoresistive cantilever performance—Part I: Analytical model for sensitivity,” Journal of

Microelectromechanical Systems: A Joint IEEE and ASME Publication on Microstructures, Microactuators, Microsensors,

and Microsystems, vol. 19, no. 1, p. 137, 2010.

Neethu Karayil has obtained her M Tech

Degree in Electronic Design and Technolgy

from National Institute of Technoloigy, Calicut, Kerala, India and this work is a part

of her M Tech Thesis. Her area of interest is Design, analysis and modeling of miniaturized

devices.

Dr. Suja K J is working as Assistant Professor in the Department of Electronics and

Communication, NIT Calicut, Kerala. She has done her Ph D research in the field of Micro

Electro Mechanical Systems (MEMS) at

National Institute of Technology and graduated in 2015. She is working with the

National MEMS Design center at her Institute. She has five publications in referred

International Journals. Also she has presented

her work in 4 International Conferences. She has attended the ICMENS Conference at Hong Kong and presented her research work in the

conference. She has obtained her M Tech Degree in Optoelectronics from Kerala University, Kariavattom Campus, Kerala She is currently

working on design, fabrication and mathematical modeling of MEMS

devices. Her main area of interest involves Simulation and modeling of MEMS devices and VLSI.

Documents

Performance Analysis of MEMS Piezoresistive …Performance Analysis of MEMS Piezoresistive Cantilever Based Sensor for Tuberculosis Detection Using Coventorware FEA Neethu Karayil