Mass Detection Sensor Using Cantilever Beam Engineering Essay

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In this paper we are going to discuss about a micromachined cantilever beam which is used for the detection of smaller masses. This type of cantilever beam is mainly used for the biological sensing applications. The developed biosensor was capable of rapid and ultrasensitive detection of bacteria's. The sensor was used to detect the small masses bacteria's like listeria innocua, MRSA bacteria. Biosensor of these technologies proves to be the promising solution measuring the small masses. With enhancement in the sensors type the sensitivity and the remote sensing can be increased. The reduction in the size of the sensors from micro to Nano also proves to be greater effect for measuring the smaller masses of attogram size.

The micromachined silicon cantilever is designed using merged epitaxial lateral overgrowth (MELO) method. In order to fabricate a low stress and thin cantilever chemical mechanical polishing method is used. Here two methods were used for the adhesion of the smaller mass on the cantilever beam: 1) direct adhesion of the small mass bacteria 2) depositing a layer over the silicon beam for the adhesion of mass. The sensor calculates the change in resonant frequency of the cantilever beam with the mass loading and without mass loading at normal atmospheric conditions. The vibration of the cantilever beam which was excited due to thermal and ambient noise was measured using a scanning probe microscope. In this paper we will also show the theoretical design for the cantilever beam using COMSOL which provide us with the rough idea for the resonance and deflection of the beam on the deposition of thin layers on the beam.

Introduction:

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Micro and Nanotechnology applications are vast and they have the capability to resolve the challenges of the chemical, environmental and biological analysis. One of the approaches towards the implementations of these technologies is the use of micro and Nano mechanical resonators which may function as the ultrasensitive transducers in chemical and biological sensors. Merging of silicon micro and nanofabrication with surface functionalization of biochemistry offers new exciting opportunities in developing microscopic biomedical analysis devices with unique characteristics. [1] Considering the case for cantilever beam sensors which were introduce to nanotechnology with their use in the atomic force microscope (AFM). [2] Cantilever beam links between the physical realm in micro and Nanoscale regimes. [2] Due to easy fabrication process and simple structure cantilever beams have attracted the MEMS and NEMS community attentions towards it. Micro and Nano scale cantilevers have proved to be the extremely sensitive biosensors. [2][3][4][5]

Over the past decades various methods have tried for detecting and measuring DNA-DNA, antibody-antigen and ligand-receptor interaction forces so as to aid the bimolecular reorganization. Well many methods and sensors were developed for measuring the smaller masses and forces, biosensors using cantilever beam were one of the solution to these challenges. Out of which cantilever used in AFM were the greater breakthrough for studying the bio-interaction [7], measuring antibody interaction [8-10] & hybridization of complementary DNA strands interactions. [1] [11] There have a been a rigours study made on integrating magnetic bread and scanning AFM for biosensors.[12] In past few years biosensors have attracted considerable interests in the applications from medical analysis to environmental analysis and also for industrial process. [1] In the industrial process they are used to measure the various gaseous analyte in the chips, it can used to detect the mercury vapour which are the immediate relevance applications for air pollution detection and industrial hygiene monitoring. [4][6] A biosensor mainly can be divided into three main components: [1]

Detector: It can be used to recognise the concern signal (immune sensors or enzymatic sensors)

Transducers: it can be used to convert the signal into more useful output more likely electronic signal (amperometric or piezoelectric)

Read out: it can be used to filter, amplify, display, record or transmit the transduced signal.

In the following we will focus on mechanical transduction mechanism which is based on the bending of micro-cantilevers due to the adsorption of mass on the surface of the beam. Microscale sensors have proved to be extremely sensitive biosensor. The change in resonance frequency of the beam due to mass loading is the key aspect for measuring the mass of the load. This method used for measuring the small mass is not only sensitive because of simple fabrication process and less time consuming but it is more efficient device too. These types of sensors can be used for multiple purpose mass sensitive by using the array of cantilever with different analyte according to the need of masses. Growing a magnetostrictive film over the silicon beam makes the sensors like remote sensing, self - calibration & self - testing, hence a lot development is need in the MEMS and NEMS technology.

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The main reason for using silicon cantilever is because of the fabrication advantages they give over microscale like: [2]

Dimensions of the device can be control.

Can achieve Micro to Nano dimensions

Fabrication of arrays of devices possible with silicon technology

Low cost fabrication.

Integration of various devices on the same platform

High mechanical quality factor

In this paper we will describe about a silicon based micromachined cantilever as a resonant biosensor for the detection of mass of bacteria cell and antibodies. Listeria Innocua was the bacterium that was taken into consideration as the mass. The mass of the antibody was also measure so as to measure the sensitivity and deflection of the beam so as to demonstrate the effect of interaction between the layer and the bacteria and to measure the resonant frequency of before and after the mass loading. [2] The fabrication and the measuring technique for this biosensor are described in this paper below.

Fabrication process:

In this technique we are going to fabricate the silicon cantilever using merged epitaxial lateral over growth (MELO) and chemical mechanical polishing (CMP).We are going to use the selective epitaxial growth (SEG), epitaxial lateral overgrowth (ELO) and chemical- mechanical polishing (CMP) for the microfabrication of thin single crystal silicon cantilever beam. [2][13] Various form of selective silicon growth has been also described previously. [2][14][15] This type of silicon based fabrication process can be also used for flow sensors, pressure sensor, biochemical sensor and many more. The process of fabrication using MELO and CMP results in a low stress cantilever with a sub 100nm thickness. [15]

This technique used for the fabrication of silicon cantilever doesn't include an oxide layer which reduces the mismatch between the silicon and silicon dioxide which exist when using SOI as the starting material. We need to produce maximum possible stress free cantilever, since the residual stress produce in the beam while fabrication affect the vibrational property of the beam. The present method can be used for fabricating arrays of cantilever beam by varying their dimensions. The method is also capable to produce nanometre range cantilever beam for single molecule detection applications. The thin cantilever fabrication can be shown in eight following steps: [15]

Step1 :

This steps deal with the defining of the cantilever shapes using a wet etching process. In this process the silicon dioxide layer is grown using the photolithography process. Using a buffered hydroxide (BHF) the oxide layer is wet etched and the cantilever shape is obtained. The steps for the wet etching process are shown below in the figure:

Figure 1: (Image courteous) surface micromachining of the cantilever beam. [*]

Steps involve in the process: [*]

 (a) The deposition of the sacrificial layer

(b) Defining the anchor and bushing regions on the silicon substrate.

(c) Pattering of the structural layer.

(d) Final obtained free standing cantilever beam.

Fig.2: (Image courteous) Bulk micromachined by anisotropic etching of silicon. [*]

Different views of cantilever beam: [*]

(a) View for the bottom plan of the cantilever beam with cavities, diaphragms and holes in the wafer etched figure above.

(b) Top plan view of an anisotropically etched wafer showing the fabrication of a cantilever beam using etch stop layer.

(c) Cross section, AA' in the figure shows the hole, diaphragm, and cavity of the cantilever beam.

(d) Cross section, BB', shows the cantilever beam.

In the above process we use the silicon material along <100> direction in ordered to obtain minimum density effect.

Step 2:

A thin buried layer oxide is grown over the beam in order to obtain the thinner cantilever beam using an oxidation process. The thickness of the beam can be varied using selective masking, etching and oxidation process over the buried oxide. As shown in figure 3 (a)

Step3:

In order to open the seed window a reactive ion process (RIE) is done over the oxide layer using the CHF3/O2. A sacrificial layer is grown followed by wet etching and the annealed so as to reduce the damage caused by RIE process.

Step 4: (epitaxial layer growth)

A epitaxial layer was grown using the Gemini I Pancake type reactor. The obtained output can be seen the figure 3(c) below (top view). The reactor condition maintained were T= 9700C, P= 40 Torr. HCl was used to maintain the selectivity of the oxide layer, the carier gas was hydrogen and the source was dichlorosilane (DCS).

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Step 5: (Chemical mechanical polishing)

CMP was performed in two different type of etching process over the oxide layer in a combination in order to obtain a faster and finer thin film of silicon. NALCO 2350 was used for faster etching process and NALCO2355 was used to perform the finer etching process. So as to protect the cantilever from washing away there was a need to polish the silicon down to the wafer flat surface. The following steps are given below.

FIG 3.1: (Image courteous) Micro fabrication of thin film cantilever beam using MELO and CMP (a-d) [15]

Step 6:

Tetra methyl ammonium hydroxide (TMAH) was used to etch the silicon wafer but a thin layer of 30nm oxide was grown in order to save the cantilever from the etching process. The etch window was opened with the RIE using CF3/O2.

Step 7:

The cantilever beam was released after the etching process using TMAH as shown in fig 3.2(f). The final product of the cantilever beam was obtained by etching away the oxide layer using BHF (buffered hydrofluoric).

FIG 3.2:(Image courteous) Micro fabrication of thin film cantilever beam using MELO and CMP (e-f) [15]

Step 8:

Thought the final cantilever was obtained then also the structures were deionized in the water for 10min and were treated using methanol in 3 steps for 10min. the product was finally air dried.

FIG 4: (Image courteous) cantilever beam [2]

COMSOL design: (The theoretical design for the Cantilever Beam)

A theoretical simulation on the cantilever beam was performed in order to measure it static deflection on the layer deposition and the Eigen frequency of the beam. The following consideration done for the beam is shown below in the table:

TABLE 1: Dimension of the cantilever beam taken under consideration.

The static deflection of the cantilever beam and the resonance frequency of the cantilever beam as obtained in the COMSOL software is shown below in the figure.

a)

b)

FIG 5: COMSOL DESIGN a) static deflection of the beam (8.125 e-10) b) Resonance frequency of the beam (2.310725 e5).

Measuring Technique:

To perform the mechanical characterization of the cantilever beam obtained from the MELO and CMP process a resonant frequency measurement was performed. In order to vibrate the cantilever beam thermal and ambient noise was used, since they were sufficient to vibrate the cantilever beams. In order to measure the vibration and to measure the resonance frequency of the beam dimension 3100 series (digital instruments, veeco metrology group (SPM) was used. [2][15] The power spectral density was then evaluated using the MATLAB software and the thermal data was fit to the amplitude response of the simple harmonic oscillator [13][16]

………………………………. (1)

Where f is the frequency and fo is the resonant frequency, Q is the quality factor of the silicon beam and Adc is the cantilever amplitude at zero frequency.

The resonant frequency of the cantilever beam was given by: [16]

………………………………………………….. (2)

Where m is the effective mass and k is the spring constant which is given by: [16]

…………………………………… (3)

Where fi is the resonant frequency obtained after the mass loading. While the effective mass can be calculated using: [2][16]

…………………………….. (4)

The change in mass due to change in resonant frequency can be given by: [2]

…………………………. (5)

Whereas n= 1 for the added mass distributed uniformly over the beam and n=0.24 when the mass is distributed randomly over the rectangular cantilever beam.

The planar dimensions and measured values of unloaded cantilever at the normal condition with the resonating frequency, quality factor, spring constant and mass sensitivity are shown below in the table: [2]

TABLE 1: Dimension values [2]

SAMPLE MEASURED WITH THE RESULTS:

The sample use in the process was Listeria innocua bacteria which were grown on the Luria-Bertani (LB) broth at 370C placed in a n incubator. The following parameters were considered for the Listeria Innocua bacteria growth: [1]

Initial concentration: 5*10^6 to 5*10^8 cells/ml

Buffer used: phosphate buffered saline (PBS) with pH value of 7.4

Antibody: Goat affinity-purified polyclonal

Blocking agent: BSA (Bovine Serum Albumin) for preventing nonspecific binding of bacteria cell

To remove the loosely bound bacteria PBS was used.

To obtain the effective dry mass of the bacteria non-specific binding was performed on the cantilever beam. The resonant frequencies were calculated in the air. Doppler vibrometer was used to perform the resonance measurements. In order to avoid the stiction of the beam at the substrate surface they used CPD (Critical point drying) for drying the liquid used for the bacteria adhesion. The results obtained for the frequency measurement is shown below:

FIG4: (image courteous) resonant frequency before and after the mass loading of the bacteria. [2]

The mass obtained using the mass equation (5) was around 85fg.

Another study was performed using the antibody. BSA and antibody to the bacteria was trapped onto cantilever beam by dispersing 10-15µl of solution. The resonant frequency of the cantilever beam was measured in air in order to get the change in frequency due to the antibody and BSA mass. The beam was again treated with the bacteria and the change in the frequency was observed. The result obtained can be seen in the figure below:

FIG 5: (image courteous) resonant frequency of the unloaded cantilever beam, after antibody + BSA, after the mass loading. [2]

Application of Biosensors:

Biochemical process are the most complexity process as compare to chemical and physical hence the bio-sensing are more demanding task. Normally bio-sensing devices are carried out in liquid (aqueous solution) environment. The cantilever deflections are affected by the flowing and mixing of solution caused by turbulence. Several groups have showed how bio-sensing can be implemented using micro cantilever sensors. Measuring the surface stress one can monitored unspecific mass adsorption with the micro cantilever, cantilevers can be used for force transducers to detect the presence of receptor coated magnetic beads which would stick onto the functionalized cantilever surface. [1] Cantilever based sensors are very versatile they can be used in any natural conditions. They have the capability of transducing different signals like magnetic, electric, thermal, chemical, mass and flow. [1] A micrometre transducer can be successfully implemented in the biosensor, it adds several advantages to the bio-sensors like increasing the sensitivity& limits, they are cheap and easy to fabricate and they can be integrated in CMOS (complementary metal oxide semiconductor).[1]

Using the biosensors several bio medical applications can be foreseen such as: [1]

Monitoring presence

Concentration of the substance in solutions or gases

Calculating specific binding energy

Monitoring chemical surface reactions

Studying adsorption-desorption process of substance.

Future work:

Optimization of the cantilever dimensions and shapes for maximum stress, mass or temperature sensitivity and the use of large arrays of cantilevers in parallel development are needed.

Summary:

The present study shows the use of micromachined cantilever beam for biological purpose. This present work shows about the biosensor using the MEMS technology. The present work shows that using the MEMS it is possible to measure the mass of few Femtogram or attogram. This model shows the rapid and sensitive detection of the bacterial cells. A theoretical simulation was performed for the model using the COMSOL software. Antigen-antibody was used as the thick layer for the adhesion of the bacteria. Hence it shows the micromachined cantilever as a resonant mass sensor.