2014 Fifth International Conference on Intelligent Systems, Modelling and Simulation

Design and Fabrication of IDT Surface Acoustic Wave Device For Biosensor Application Mohd Rosydi Zakaria, Mohd A. Farhi Shamsuddin

Uda Hashim, Tijjani Adam, Abdulmohaimen Wesam Al-mufti

School of Microelectronic Engineering Universiti Malaysia Perlis (UniMAP) Arau, Perlis, Malaysia [email protected]

Institute of Nano Electronic Engineering Universiti Malaysia Perlis (UniMAP) Kangar, Perlis Malaysia [email protected], [email protected], [email protected]

powerful and sensitive devices for the detection of Biomolecule interaction [5,6]. The surface Acoustic Wave (SAW) is generated from a combination of sensing area, interdigital transducer (IDT) that's been fabricated on a piezoelectric substrate. In developing SAW sensors, IDT is an important part of generating SAW. It functions for converting electric energy into mechanical energy and vice versa in order to generate and detecting the target. It is typically designed as a thin-film layer and placed on the piezoelectric substrate. When the signal voltage is applied, an IDT will generate acoustic waves in a piezoelectric substrate [7]. Acoustic waves generated vary depending on the applied voltage signal and the deformation of the piezoelectric such as Zinc Oxide (ZnO) substrate [8]. The interdigital transducer (IDT) can be developed in different structure according to the different characteristics that needed. This paper focused on designing the IDT for achieving higher center frequency response and reduce the insertion loss. The result of the center frequency response is compared based on standard formula and compared with the experimental result of current-Voltage (I-V) characteristics, capacitance and resistance from the fabrication process. In this paper, several IDT designs have been proposed to study the performance device in term of electrical properties and frequency response into biosensor application. The combination of SAW IDT with ZnO piezoelectric substrate will be explained in detail in this paper. Fig. 1 shows the structure of the SAW biosensor device.

Abstract— The Surface Acoustic Wave (SAW) is one of sensor types that can be used in multifunction in the sensing field. Hence, the purpose of this project is to study and investigate the design of SAW device for biosensor application. Several parameters and design has been varied and proposed to investigate the effect to the SAW Biosensor performance. The parameter such as total SAW IDT fingers and Split IDT design has successfully designed using AutoCAD software and fabricated using conventional lithography method. The Aluminum IDT was fabricated on ZnO/Si piezoelectric substrate and show a good result in term of electrical characterization and frequency response. From result, the proposed design with many fingers can increase the center frequency response while the split design control the insertion loss. It shows that these designs are suitable to apply in the SAW biosensor application. Keywords- IDT SAW design, Split IDT, SAW device

I.

INTRODUCTION

Biosensor is a well-known device that is used for various applications such as medical and food analysis. This device became famous due to a certain number of parameters that allow it to detect and identify molecules that cannot be seen by the naked eye [1,2]. With the availability of this device, it will facilitate the researchers such as scientists to examine all existing molecules in the human body as well as to be able to identify the causes of disease [2]. The researchers are more attractive to study about the development of the biosensor so that it can be used in different application as well as to fully utilize its performance limit. Nowadays, it seems that every researcher in the industry is waiting for the biosensor to become more advanced and can provide a new breakthrough so that it can be used for the good of society in terms of the management of diseases, medicine, drug deliveries as well as food analysis [3]. Many kinds of method can be used in order to develop the biosensor such as radiochemical, enzymatic, fluorescent, electrochemical, optical, and acoustic wave techniques [1,3]. However, each one of this biosensor have their own particular advantages and disadvantages in term of high complexity, separate labelling process, equipment to simulate the transducer and thus higher cost to conduct analysis [3,4]. In Biosensor device, it's very important to have a sensor with high sensitivity and have good accuracy detection. When compared to their competition, acoustic wave biosensors which is based on piezoelectric mechanism is said to more versatile, highly sensitive, reliable, reusable, and small, and easily designed. It’s also reported the acoustic wave proven 2166-0662/14 $31.00 © 2014 IEEE DOI 10.1109/ISMS.2014.139

Figure 1. Structure of conventional SAW Biosensor a) full structure and crossection of SAW Biosensor b) IDT design SAW.

II. THEORETICAL BACKGROUND In SAW device, the center frequency and insertion loss is the most important measurement. To have the most sensitive biosensor, the center frequency must be high in GHz range but, some of design with high center frequency having problem with high insertion loss [9]. To improve the high performance sensor and stability, unwanted spurious effect and insertion loss must be reduced. This reduction can be archived by using a piezoelectric 760

substrate with a high electro-mechanical coupling coefficient. Other than that, by controlling a few parameters such as the IDT type, IDT length, aperture width and delay line length [10]. Between of these, the IDT type is the most important parameter for realizing low insertion loss and improving device stability [9,10]. In order to design a SAW IDT, impulse response (IR) model is used. From the IR model, the important design parameters such as central frequency (fo), number of IDT finger (N), finger width/spacing, aperture (W), wavelength (λ), and total capacitance (Ct) can be calculates. The standard center frequency formula can be obtained using equation 1.

III. MASK DESIGN In this project, AutoCAD software is used to design the mask for the fabrication process. Sensing area is one important part of the SAW Biosensor device where this sensing area will detect the target on the sensor. This sensing area is designed in rectangular shape with the size of 4800 μm x 3000 μm. This sensing area is located in the center of IDT input and output SAW device. The design of sensing area is shown in Fig. 3. For the interdigital transducer (IDT), the masks have been designed in two types of IDT with is conventional IDT and split IDT. For the conventional IDT, it's been designed in single finger electrode and for the split IDT, it been designed in double-split of electrode fingers and triple-split electrode fingers as shown in Fig. 3.

is central frequency, Vs an acoustic velocity of ZnO Where, piezoelectric substrate and λo is the wavelength. The SAW device center frequency also can be calculated from the capacitance value. To get more accurate values of capacitance and center frequency, equation 1.2 can be applied.

= capacitance, = Center frequency, = Impedance. The interdigital transducer (IDT) can be developed in different structure according to the different characteristics that needed. These characteristics depend to the fabrication properties, frequency varying factors, geometry and etc [11]. From the study, there are three IDT structure has been developed which each of it has a characteristic differs from their geometry, phase angle and wavelength. This structure is termed as ‘single electrode’ and ‘split electrode’. For the single electrode there is a uniform spacing between the electrode fingers’ and the acoustic wavelength, λ 0 for this structure is twice the pitch of the electrode, 2p. These structures are also known as reflective transducers. Fig. 2 shows the structure of the single electrode transducer.

Figure 3. AutoCAD Mask design for specification IDT.

Table I summarize all the design that consist of four designs of biosensor devices, and it is arranged with various IDT sizes and arrangement of the electrode fingers. All the dimensions are in micrometre TABLE I. Type of (IDT) Size of finger width (µm) Number of finger Acoustic aperture

PARAMETERS FOR DESIGN SAW IDT Single Finger

Single Finger

Double Finger

Triple Finger

300

300

300

300

10

16

16

18

8

8

8

8

IV. SAW FABRICATION PROCESS There are three important process steps that need to be followed in fabricating the SAW biosensor. The component consists of preparation of piezoelectric substrate, fabricate the sensing area and Interdigital transducer electrode preparation. All these components must be fabricated in several stages.

A. Step 1: Sample Preparation For the first stage of this fabrication process, it will start with the cleaning apparatus and sample by using standard cleaning process. The oxidation processes then continue with growth the oxide on sample. The oxide will be grown on the wafer surface with the thickness around 0.3 µm by using wet oxidation furnace for about 2 hours. After the oxidation process, the wafer will be deposited with Zinc oxide (ZnO) in order to form a layer of piezoelectric substrate.

Figure 2. Example of IDT SAW design (a) Single conventional IDT, (b) Double-Split IDT, and (c) Triple-Split IDT

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B. Step 2: Growth Zno Piezoelectric Layer

V.

In order to improve sensitivity of the complete biosensor device, the silicon oxide (SiO2) substrate must be deposited with a material that will perform as piezoelectric substrate. So in this project, the zinc oxide (ZnO) has been chosen as the piezoelectric material and the interdigital transducer will be fabricated on the piezoelectric substrate layer. The ZnO is prepared by using sol-gel method and deposit using a conventional deposition process where the ZnO solution will be dropped on the silicon oxide surface using a dropper [12,13]. The substrate will be spun up using spinning to get a uniformity form. The thickness of ZnO piezoelectric is about 1 µm and the process steps are summarized in Fig. 4.

Figure 4.

RESULT AND DISCUSSION

This part discusses about the result of the SAW IDT biosenssor device. The design of the IDT finger is varied to improve the performance of the SAW Biosensor device. In the design, the size of the fingers are kept same to make the device is easier to fabricate and compared. For Design_1 and Design_2 the conventional SAW with total finger 10 and 16 respectively [14]. The SAW device was successfully fabricated and is shown in Fig. 6(a) and (b). While for Design_3 and Design_4, the finger is punctuated in same as conventional SAW design and separated into double-split and triple-split between channel negative and positive channel as shown in Fig. 6(c) and (d) respectively.

Figure 6. Conventional SAW Biosensor (a) IDT with 10 fingers, (b) IDT with 16 fingers, (c) Double Split and (d) Triple-Split IDT

Zinc Oxide piezoelectric for SAW device deposition process steps

Fig. 7 shows a result of I-V for ZnO piezoelectric substrate. From result, can be seen that there is a slightly different between the I-V curve for the IDT design that fabricated on a piezoelectric substrate (ZnO) and without piezoelectric substrate (ZnO). The result shows that the resistance of the IDT that fabricate on piezoelectric substrate is much lower compared to the IDT without piezoelectric then, it,s give effect to the current. This shows that by using piezoelectric substrate, it will affect the device where the sensitivity of the device will be increased because of the resistance is getting low. Besides that, piezoelectric substrate wills also improving device stability where SAW that propagates on piezoelectric substrate is more stabilized and it has low insertion loss. This shows that there are many advantages by using piezoelectric substrate in biosensor fabrication. The equivalent for both piezoelectric substrate are summarized in Table II

C. Step 3: IDT Formation And Fabrication In this second stage, aluminium with thickness 2000Å will be deposited on the piezoelectric substrate in order to fabricate the interdigital transducer electrode finger. After the deposition process, the photolithography process will be carried out where the positive PR will be coated on the aluminium surface and pattern transfer process for IDT mask will be done. The mask was used to transfer the pattern using conventional lithography process. Next, the PR will be developed until the IDT pattern appears and the aluminium will be etched to form the IDT electrode finger. Lastly, the PR will be stripped out by using acetone. Fig. 5 shows the fabrication process steps of IDT electrode finger formation.

Figure 5. Fabrication process of SAW IDT finger formation steps Figure 7. The I-V curve of IDT design with and without ZnO Piezoelectric

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TABLE II.

THE ELECTRICAL RESULT OF IDT FABRICATION BY USING ZNO AND WITHOUT ZNO

IDT Design

Voltage Value (V)

Current Value (A)

Resistance Value (kΩ)

Using ZnO

3.00

1.77E-05

169.5

Without ZnO

3.00

1.19E-05

252.10

value of center frequency that should get based on the IDT design or parameter. The comparison of capacitance value and the center frequency is summarized in the Table III.

To analyze the IDT-SAW characteristic, it is conducted by finding the relationship between the current and voltage (I-V) of the IDT device. All devices were measured by using equipment such as a source meter Kiethly 2400 and Alpha A high frequency (10MHz-1GHz) dielectric analyzer to find out the I-V and resistance, respectively. Based on I-V characteristic graph in Fig. 8, it can be seen that the difference between the I-V curve for conventional single SAW for Design_1 and Design_2 the current become small. It,s shows that the total of fingers will affect to the SAW device performance. Compared to the conventional and SAW with IDT split, when change the design of IDT from single IDT to double-split or triple-split the current for the designs in increases. The current for all design is 2.38x10 -5, 5.66x10-5, 1.21x10-5and 1.21x10-4 for Design_1, Design_2, Design 3 and Design_4, respectively when maintains the voltage to 3.5 V. From the result, the higher resistance value is for Design_2 with value of resistance 218.75 kΩ and the lower value of resistance is Design_4 with value of resistance 28.93 kΩ. By comparing to the resistance value of the IDTs design, Design_4 is the most suitable IDT design in order to fabricate a biosensor. It is because this design has the lowest resistance and voltage value compared to other designs and this criteria is really important in developing a biosensor.

Figure 9. Result of C-V characteristic for various IDTs design

Based on the analysis of all of the output result, it was found the total of number finger is one of the factors affecting the center frequency. This is because when increasing the number of fingers, the current and capacitance also increased. From the Table III, when the IDT was changed from 10 to 16, the center frequency for both designs is increased from 1.92 MHz to 2.40 MHz respectively. This shows that, the SAW with many IDT fingers can be improve the performance SAW biosensor and hence will improve the sensitivity of the Biosensor. TABLE III.

COMPARISON RESULT BETWEEN THE CENTER FREQUENCY VALUES BASED ON EXPERIMENT AND CALCULATION

IDT Design

Capacitance value (nF)

Resistance Value (kΩ)

Design 1

1.62

147.10

(fo) From Eq. (1) (Mhz) 3.3

Design 2

2.40

218.75

3.3

2.10 0.92 0.95

Design 3

3.85

61.83

1.6

Design 4

4.17

28.93

1.1

(fo.) From Eq. (2) (Mhz) 1.92

From the result that have been provided, Design_4 is considered as the best design among the others design. Even though, the center frequency for the Design_4 is small compared to single IDT, but the value still acceptable in term of the capacitance value and resistance. From the Table can be seen that, for the Design_4 there is just a little difference between the value of the center frequency that calculate based on the design parameter and by using capacitance equation. Based on the journal, the result may be due to the effect of capacitance and resistance value, hence reduced the insertion loss and frequency stability at the center frequency. The higher capacitance value the lowest insertion loss at the center frequency [9]. The center frequency is an important parameter in determining the sensitivity of a sensor. A higher center frequency provides better sensitivity and smaller device size, but its disadvantages are a higher reference noise, greater acoustic propagation loss, various secondary effects, and difficulty in the patterning of the IDT and reflectors. So when the insertion loss is high, the sensitivity will be reduced.

Figure 8. Result of I-V characterization of the IDTs

From the result of capacitance-voltage (C-V) in Fig. 9, the result shows there is a slight difference between the C-V curves of the IDTs for four proposed designs. The capacitance value for Design_4 has higher compared to other designs which is 4.17 nF. For the Design_3 is at 3.85nF. While for Design 1 and Design 2, it’s been designed in conventional IDT with the value of capacitance is 2.40 nF and 1.62 nF, respectively. This two design with is using a single electrode IDT, so the spacing between positive and negative electrode is become smaller. The value of the capacitance can also be gained in theoretically by using a basic equation for capacitance value as shown in Equation 2 for calculating the center frequency. The values of center frequency that gain from the calculation is then will be compared to the actual

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VI.

[4]

CONCLUSION

In this project, the IDT with different total IDT fingers and IDT with split structure are designed, fabricated and investigated for improving the sensitivity biosensor. The center frequency is calculated based on the experimental results of resistance and capacitance and compared with the basic formula of center frequency in equation 1. From the results, the total of IDT number is one of the important parameters to improve performance of SAW device based on the center frequency. From the result also, found Design_2 has a higher center frequency but also has high insertion loss and this give disadvantages to device performance. The alternative for the design is by design the SAW IDT with Split IDT in Design_4. Even the center frequency is small but the other electrical characteristic is better compared to others proposed design.

[5]

[6]

[7]

[8]

[9]

[10]

ACKNOWLEDGMENT The authors wish to thank Universiti Malaysia Perlis (UniMAP), ministry of science, technology & innovation (mosti) and ministry of higher education (mohe) for giving the opportunities to do this research in the Micro & Nano Fabrication Cleanroom. The appreciation also goes to all the team members in the Universiti Malaysia Perlis (UniMAP) and from Institute of Nanoelectronics Engineering (INEE) especially for the 2 Nanostructure Lab-on-chip Research Group

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[13]

[14]

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