KR101298050B1 - Method for modifying surface of lithium niobate plate - Google Patents

Abstract

The present invention relates to a method of modifying the surface of a lithium niobate substrate by mixing a dry surface treatment and a wet surface treatment in order to increase surface bonding force with an organic thin film on a lithium niobate substrate and to introduce various functional groups. This surface modification method first activates the surface crystal structure of the lithium niobate substrate by a dry surface treatment method including UV, ozone, plasma, and the like, and then self-assembled monolayer (SAM) reaction or silanization on the activated surface. A wet surface treatment technique for modifying the surface of the lithium niobate substrate in a more stable and easy condition using a wet method including a) reaction.

Description

Method for modifying surface of lithium niobate plate

The present invention relates to a method of modifying the surface of a substrate on a lithium niobate substrate mainly used as a substrate for surface acoustic wave devices to increase the surface bonding force between the organic thin film and the substrate or to introduce a specific functional group.

Lithium niobate substrates have been in the spotlight in recent years for piezoelectric plate materials that exhibit excellent properties in the field of surface acoustic wave (SAW) sensors. The field of SAW sensors includes surface acoustic waves for the efficient and selective generation of surface acoustic waves in the form of longitudinal, transverse, and longitudinal waves since the first discovery of surface acoustic waves in the form of wave phenomena on solid surfaces. Research on the device has been made in earnest.

Substantial research as a surface acoustic wave sensor has only been carried out in the mid-1970s and has been widely applied as a physical sensor in the fields of mobile communication and electronic devices, and industrial applications have been spreading. The surface acoustic wave device is composed of resonator filter and single / bidirectional filter element according to the design and layout structure of IDT (Interdigital Tranducer) and used in antenna duplexer, filter, phaser, key, oscillator, transceiver, radar, sensor, etc. Recently, with the development of micro-electro-mechanical system (MEMS) and nanotechnology, the development of micro sensor, convergence with wireless technology, and application research to biosensor are being actively conducted.

Surface acoustic wave sensor detects or measures the absolute value or change of physical quantity or chemical quantity, intensity of sound, light, and radio wave from measurement object and converts it into electric signal by using surface acoustic wave traveling along base and surface of piezoelectric substrate. It can be defined as a functional device or device. The core of these surface acoustic wave sensors can be summarized in some.

First, the IDT (Interdigital Transducer) structure is efficiently made to generate and detect surface acoustic waves in a piezoelectric substrate. IDT is a metal electrode arranged on a piezoelectric substrate and is the core of surface acoustic wave devices and serves as a transmitter and a receiver as an interface between an electronic circuit and an acoustic delay line.

Second, the surface structure and composition are optimized to effectively maximize the change in physical or stoichiometry obtained on the surface. Such changes in the design and composition of the surface structure not only optimize the interaction with the detection object and the signal conversion efficiency, but also change the mode of the surface acoustic wave or adjust the measurement sensitivity. As a method for optimizing the surface structure and composition, methods such as coating of organic and inorganic thin films and activation through surface modification are mainly used.

Third, in order to meet various demands, it is applied to a multi-detection structure and research into a fusion type sensor that can read various signals simultaneously.

Simply expressing the basic structure and principle of the surface acoustic wave sensor, the mechanical surface acoustic wave is generated and propagated in both directions of the IDT by the electric signal applied to the IDT, and the surface acoustic wave signal is transmitted through the IDT and the reflector. It has a time and phase difference determined according to the distance from) and is reflected and converted into an electrical signal and output. SAW sensors vary in performance depending on the length, width, position, and thickness of the IDT and are dependent on the frequency of use. When the surface acoustic wave device is applied as a sensor, the sensing layer coats the sensor substrate on the sensor substrate, which changes its mass, elasticity, and conductivity by physicochemical stimulation, and thus pressure, rotational force, impact, tension, gravity, mass, evaporation, and biochemistry. , Temperature, humidity, freezing, viscosity, displacement, flow, photosensitization. It detects and detects wide angle, acceleration, abrasion, and contamination. In this case, the surface bonding force between the piezoelectric material and the coated thin film layer generating the surface acoustic wave acts as a very important factor in the stability and device characteristics of the thin film layer provided as a mode of converting the acoustic wave or as a movement path of the surface acoustic wave.

Various piezoelectric materials have been discovered and used to generate acoustic waves, such as crystal oscillator (crstylline quartz, SiO ₂ ), lithium tantalate (LiTaO ₃ ), lithium niobate (LiNbO ₃ ), etc. This has been the most used in recent years, in addition to gallium arsenide (GaAs), silicon carbide (SiC), angasite (LGS), zinc oxide (ZnO), aluminum nitride The commercial possibilities of various materials such as aluminum nitride (AIN), lead zirconium titanate (PZT), and polyvinylidene fluoride (PVdF) have been studied. Lithium niobate has been known as one of the best surface acoustic wave substrate materials because of its high coupling constant (K2) and fast SH velocity.

The types of seismic waves are classified according to the shape and characteristics of the seismic wave propagation to the piezoelectric substrate surface or the substrate itself. The bulk bulk acoustic wave type in which the elastic wave propagates through the substrate is in the form of a thickness shear mode resonator and a shear horizontal acoustic plate mode (SH-APM). Acoustic waves propagate along the surface of the substrate in the form of longitudinal waves (SH-SAW) and surface waves (STW). According to the purpose of use, there are Flexural Plate Wave (FPW), Love Wave, Surface Skimming Bulk Wave (SSBW), and Lamb Wave. Due to the principle of surface acoustic wave propagation, bulk acoustic wave (BAW) and lamb wave are mainly used for gas sensing, and love wave is mainly used for liquid sensing.

Analysis of recent patents related to surface acoustic wave sensors shows IDT, bio product detection, bio resonator, microfluidic device, tag, plug sensor, skin wound treatment, fragrance sensor, gel condition measurement, medical sensor, glass melting furnace measurement, synthetic substrate, film It is a sensor device, and in particular, various practical patents in the bio and medical fields have been applied, which suggests that R & D and commercialization of the field are actively progressing. For the application to the biosensor, the conversion into a love wave capable of solution phase measurement is very important. For this purpose, a stable formation of an organic-inorganic thin film layer that can act as a waveguide layer of surface acoustic waves is essential. .

As the role of layered film / coating substrates in the field of MEMS and packaging is important, research and development on the bonding structure of dielectric, piezoelectric or non-piezoelectric layers on piezoelectric plates is needed to improve the performance of SAW sensors and SAW devices. It is actively. In particular, it proves the effectiveness of the love wave, which is a shear horizontal form, and suggests the development of biosensors using characteristics as promising fields. Research on multilayered structure using films or coated substrates for surface acoustic wave sensor performance is being actively conducted, and development efforts for applying a love wave most suitable for liquid sensing to a biosensor are increasing.

In order to convert into a love wave capable of solution phase measurement, stable formation of an organic-inorganic thin film layer that can act as a waveguide layer of surface acoustic waves is essential. Thin film layers for preventing deformation and protection of metal patterns and converting them into love waves include organic thin films including PMMA, photoresistor, etc., and silicon oxide (SiOx) and nitride (SiNx) thin films. Inorganic thin film layers are mainly applied. Despite the advantages of excellent surface bonding strength and easy thickness control, the inorganic thin film layer has a cost disadvantage of including an expensive photolithography process for thin film coating. In the case of organic thin films, the thin film forming process is simple. Although it has excellent advantages in terms of cost and cost, it has a disadvantage that the surface bonding force between the piezoelectric plate and the organic thin film is weak, resulting in poor structural stability in the sawing process or additional structure forming process, and inability to maintain the organic thin film stably. . In particular, lithium niobate substrates are known to have a denser and more stable crystal structure than lithium tantalate substrates and have a higher energy required for surface activation.

For example, in the electron spectroscopy study using lithium niobate and lithium tantalate substrates conducted by Victor HR, et al., 1981, the surface binding energy (O2S: 17.4 eV) in lithium niobate crystals was lithium tantalum. It is suggested that it is much larger than the surface binding energy (O 2 S: 2.0 eV) in the rate crystal. (Physical review B 24 (10), (1981) p5559-5575)

Such relatively stable surface binding energy of lithium niobate requires higher energy for surface activation and requires a highly reactive compound or chemical reaction.

The surface modification reaction conducted by Jonathan et al. In 2008 demonstrates the characteristics of this lithium niobate substrate. Jonathan et al. (Applied surface science, 2008, 255, 1796-1800) reported that trialkoxy (alkyl) silanes and trichloro (trichloro (alkyl) silanes for the surface modification of lithium niobate substrates. Attempts have been reported to modify surfaces using self assembled monolayers (SAM) using alkyl) silane. In this method, self-assembly did not proceed in the low-reactivity trialkoxy (alkyl) silane, and self-assembly proceeded only in the highly reactive trichloro (alkyl) silane. Surface modification was done. In this case, the reaction proceeds only in a highly reactive silane compound. In addition, a highly reactive polar solvent such as anhydrous toluene may damage the metal pattern such as aluminum (Al), which is mainly used for surface acoustic wave devices. It is also showing its limitations.

Regarding surface modification on lithium niobate substrates, a method for preconditioning lithium niobate and lithium tantalate crystals using condensed compounds was filed / disclosed by Crystal Technologies in 2007. Application No. 10-2006-7027302; 2006, 12,26-withdrawal) In this method, in order to reduce the crystal of the piezoelectric plate, 1) it is coated with a condensate containing active chemicals, and 2) crystal reduction in a non-oxidizing environment. And 3) cooling to the inert temperature range of the active chemical.

However, in this method, it is necessary to coat the condensed material, which is defined as solid or liquid, on the piezoelectric plate and to heat and cool it in a non-oxidizing environment as described above. There is no choice but to use chemicals and well designed equipment and devices are required to meet the demanding conditions, such as heating in non-oxidizing environments. In addition, the present invention is only intended to prevent the accumulation of surface charges due to the piezoelectric effect and the pyroelectric effect that may occur on the surface of the piezoelectric plate due to excessive mechanical stress or temperature change during fabrication or operation of the device.

The present invention has been made in view of this point, and the object of the present invention is to overcome the disadvantage of having to use highly reactive chemicals for the activation of lithium niobate crystal surface in existing methods, It provides a new method of surface modification in an easier and relaxed process that eliminates demanding conditions such as heating and does not damage the metals used in surface acoustic wave devices.

In addition, there is another object of the present invention to effectively increase the surface bonding force between the organic thin film formed on the piezoelectric substrate and the lithium niobate substrate. Structural stability between the piezoelectric plate and the organic thin film layer having increased surface bonding force through this surface modification facilitates a sawing process or an additional process for forming a more complicated structure, and is utilized in the bio sensing field in a liquid phase. You can expect the effect to increase the possibility.

In order to solve the above problems, the present invention provides a method for surface modification of a lithium niobate substrate to activate the surface of the lithium niobate substrate by mixing the dry surface treatment and wet surface treatment.

In the dry surface treatment, any one of ultraviolet (UV), ozone (O ₃ ), ultraviolet / ozone (UV / O ₃ ), and plasma is irradiated onto the surface of the lithium niobate substrate.

The wet surface treatment is characterized in that a lithium niobate substrate having a crystal surface activated by dry surface treatment is immersed in a surface modified solution capable of forming a self assembled monolayer (SAM).

Alternatively, the wet surface treatment is characterized in that the lithium niobate substrate having the crystal surface activated by the dry surface treatment is immersed in a surface modification solution capable of causing a silanization reaction.

In this case, the surface modification solution includes a compound for introducing a functional functional group through the immobilization on the surface of the lithium niobate, the functional group is an amine group (-NH2), carboxyl group (-COOH), hydroxyl group (-OH) , An alkane group (-CH3), an alkyne group (-CH = CH2), an aldehyde group (-CHO), a thiol group (-SH) is characterized in that any one.

In addition, the surface modification solution includes a compound to make the surface of the activated lithium niobate substrate hydrophilic or hydrophobic, the compound is an amine group (-NH2), carboxyl group (-COOH), hydroxyl group (-OH) And a silane compound having any one of an alkane group (-CH3), an alkyne group (-CH = CH2), an aldehyde group (-CHO) and a thiol group (-SH) as a terminal functional group.

As the silane compound, any one of an ethoxysilane compound, a methoxysilane compound, and a chlorosilane compound may be applied.

According to the present invention, on the existing lithium niobate and lithium tantalate substrates, the activation energy of the surface crystal structure is high, so that the surface modification is not good due to the solution reaction alone, or is caused by using a compound that is too reactive for the surface modification reaction. Damage to the metal pattern of the device and complexity of the process can be excluded.

In addition, the dry activation method activates the crystal structure of the surface of the lithium niobate and lithium tantalate substrates, thereby enabling surface modification and activation in a much more stable and easier solution environment that has not been previously performed.

Such process gains can significantly increase the surface bonding force of the substrate and the organic thin film to enable stable formation of love waves and more complex structural deformation.

1 is a block diagram of dry and wet mixing process for surface modification of the present invention.
Figure 2 is an illustration of a dry and wet mixing process for surface modification of the present invention.
3 is a contact angle image of each surface treatment condition of the lithium niobate substrate of the present invention.
4 is a change in contact angle according to the surface treatment conditions of the lithium niobate substrate of the present invention.
5 is a fluorescence image according to FITC immobilization after surface treatment for each condition of the lithium niobate substrate of the present invention.
6 is a result of the desorption experiment in the solution of the coated PMMA layer after the surface treatment for each condition of the present invention.
7 is a peeling result of the PMMA layer in the Sawing process according to the dry surface treatment of the present invention.
8 is an SEM image of IDT patterns after dry and wet surface treatment of the present invention.
9 is a frequency change of SAW device before and after PMMA coating after the dry and wet surface treatment of the present invention.
10 is a time mode saw signal of the PMMA coated SAW device after the dry and wet surface treatment of the present invention.

Hereinafter, the surface modification method of the lithium niobate substrate according to the present invention will be described in detail.

The present invention mixes the dry surface treatment step (110) and the wet surface treatment step (120) on the lithium niobate substrate (112), which is mainly used as a substrate for surface acoustic wave devices, to increase the surface bonding force between the organic thin film and the substrate. To provide a method 100 for modifying the surface of a lithium niobate substrate.

In the dry surface treatment step 110, a dry surface treatment technique of irradiating a surface of a substrate using at least one dry surface treatment apparatus 111 among ultraviolet (UV), ozone, ultraviolet / ozone (UV / O ₃ ), and plasma This activates the surface crystal structure of the lithium niobate substrate.

In the wet surface treatment step 120, a wet method including a self-assembly method for forming a self-assembled monomolecular film using a solution phase self-assembly method on the surface of an activated substrate or a silanization reaction for forming a silane compound is used. To modify the surface of the substrate.

That is, a lithium niobate substrate having a crystal surface activated by dry surface treatment may be immersed in the surface modification solution 121 capable of forming a self assembled monolayer (SAM), or may cause a silanization reaction. This can be achieved by immersion in the surface modification solution.

The surface modification solution capable of forming the self-assembled monolayer includes a compound which introduces a functional functional group through immobilization on a surface of a lithium niobate substrate, and the functional functional group applied thereto is an amine group (-NH 2) or a carboxyl group (-COOH). ), A hydroxyl group (-OH), an alkane group (-CH3), an alkyne group (-CH = CH2), an aldehyde group (-CHO), a thiol group (-SH).

In addition, the surface modification solution capable of causing the silanization reaction includes a compound which makes the surface of the activated lithium niobate substrate hydrophilic or hydrophobic, and includes any one of an ethoxysilane compound, a methoxysilane compound, and a chlorosilane compound. One can be applied.

This surface modification method can modify the surface of the lithium niobate substrate with various functional groups under stable and easy conditions compared to the conventional methods, thereby forming a love wave layer that enables solution phase measurement. And stable formation of various functional organic thin films.

On the other hand, the wet surface treatment step further includes a washing process for removing the unreacted solution phase unreacted material and a drying process for drying the solvent.

When described with reference to the accompanying drawings a preferred embodiment of the surface modification method of the lithium niobate substrate as described above are as follows.

Example 1: UV / ozone (UV / O ₃₎ surface modification of the lithium niobate substrate with a treatment with APTES treated

Lithium niobate substrate of 1 x 1 cm was prepared for surface modification of the substrate, and the prepared substrate was exposed to ultraviolet rays and ozone for 5 minutes in an ultraviolet / ozone (UV / O ₃ ) irradiator to break the surface crystal structure and activate the surface. It was. The activated surface of the substrate in a dry UV / ozone method glove box (glove - box) of water and ethanol mixture from the _{(EtOH: H 2 O = 95} : 5) to 2.5% APTES (3-aminopropyltriethoxysilane) solution prepared with a solvent It was immersed in and reacted for 90 minutes. After the silanization process using 2.5% APTES solution, the mixture was washed with a mixture of water and ethanol (EtOH: H ₂ O = 95: 5), and then dried in an oven at 110 ° C. for 1 hour.

Through the above process, 1) bare lithium niobate substrate, 2) ultraviolet / ozone (UV / O ₃ ) treated substrate, 3) substrate treated with 2.5% APTES without ultraviolet / ozone (UV / O) treatment only , 4) After UV / O ₃ treatment, a substrate treated with 2.5% APTES was prepared, and contact angle measurement and FITC fluorescent molecule immobilization were performed using the prepared substrates.

In addition, after coating the PMMA organic film using the lithium niobate substrates for each condition, a test was performed to determine whether there was a change in surface bonding force by quantifying the desorption degree of the PMMA thin film after stirring for a predetermined time on a solution.

As an example, the measurement of the contact angle can show the presence or absence of surface functional groups and thus the degree of hydrophilicity or hydrophobicity of the surface. That is, as the hydrophobic property of the surface increases, the contact angle increases, and conversely, when the surface hydrophilicity increases, the contact angle decreases.

Contact angle measurement was performed using a contact angle analyzer (Contact angle analyzer, Phoenix 150, SEO Co., Ltd.), 1 μl volume of the solution was dropped on the substrate was measured four times for each substrate condition. The results of the measured contact angles are shown in FIG. 3, and when treated with APTES solution alone without UV / ozone treatment, there is no significant difference compared to bare substrates, indicating that the solution reaction alone does not facilitate surface activation. Can be. On the other hand, it was confirmed that the contact angle was significantly increased in the substrate treated with APTES after UV / ozone treatment.

In the case of the lithium niobate substrate which has successfully undergone surface modification according to this embodiment, the terminal finally has an aldehyde functional group. Therefore, it is possible to visually check whether the surface is modified by immobilization using a Fluorescein isothiocyanate (FITC) fluorescent molecule having an amine function, and may also be provided as an activation surface for immobilization of biomolecules such as proteins. Immobilization of the FITC fluorescent molecules was carried out for 4 hours in a solution of 100 ug / ml concentration, washed with distilled water ( DW ) and measured under a fluorescence microscope.

The immobilization result of the FITC fluorescent molecule is shown in FIG. 5. As can be seen from the results, the lithium niobate substrate before surface modification and the substrate treated with only 2.5% APTES solution without dry treatment did not exhibit much immobilization of FITC molecules, and thus the fluorescence amount was low. On the other hand, the substrate treated with 2.5% APTES solution after dry treatment (UV / O ₃ 5 minutes) shows that a relatively large amount of FITC fluorescent molecules were immobilized. Through this, it can be seen that the method of mixing the dry treatment and the wet treatment including ultraviolet / ozone (UV / O ₃ ) and the like is a significantly effective surface modification process compared to the wet alone or other untreated conditions.

Surface bonding force variation test results of the PMMA thin film which was processed for 3 hours on the solution using the treated lithium niobate substrates for each condition are shown in FIG. 6 and Table 1 below. In order to quantify the change in the surface binding force on the solution, five horizontal lattice rows (25 pieces of PMMA squares) were applied to the PMMA-coated lithium niobate substrate at 2 mm intervals using a blade and stirred for 3 hours on the solution. After confirming the amount of desorption. As a result of testing the change in surface contact force when exposed to the solution for a long time, it was found that the PMMA thin film had the best surface bonding force on the substrate treated with 2.5% APTES solution after dry treatment (UV / O ₃ 5 minutes).

This result is very encouraging considering that the application of the biosensor in the application of SAW devices is mainly the application of a solution phase sample. The stable formation of organic thin films, which are relatively easy and inexpensive, is expected to increase the usability of lithium niobate substrates and facilitate application to biosensors through the formation of love waves. do.

Treatment condition Bare UV / O ₃ APTES UV / O ₃ + APTES Desorption amount of PMMA layer (n / 25) 25 21 4 0 Desorption rate (%) 100 84 16 0

In FIG. 7, the lithium niobate wafers treated with APTES solution reaction after 5 minutes of UV / Ozone (UV / O ₃ ) as a dry surface treatment and only APTES without dry surface treatment were applied to the sawing process. Shows the results. As shown in FIG. 7, the PMMA layer was dropped from 56 of the 70 specimens in the Sawing process in the wafer without dry surface treatment. This is due to prolonged exposure to water jet flow in Sawing equipment for cooling blades and removing particles. On the other hand, in the case of APTES-treated wafers after dry surface treatment, it was confirmed that the PMMA layer remained stable without dropping in all 49 specimens during the cutting process.

Figure 8 shows the SEM image of the IDT pattern before and after applying the dry and wet surface treatment mixing process. Damage of the IDT pattern is an important factor in determining signal quality in the application of lithium niobate substrates to various devices, particularly SAW devices. In the results of FIG. 8, there is no pattern damage or morphological change on the IDT pattern image of the SAW device treated with 2.5% APTES solution after dry treatment (UV / O ₃ 5 minutes) according to an embodiment of the present invention. can confirm.

Example 2: After the surface modification and the PMMA coating of the SAW device a surface acoustic wave signal measured

The SAW device fabricated using a lithium niobate substrate is designed to have a structure with one IDT and two reflectors, and the surface acoustic waves oscillated in the IDT are reflected by the two reflectors so that two reflections are performed in time delay mode. It will appear as a peak. The surface acoustic wave device was fabricated by patterning aluminum (Al) on a lithium niobate substrate, followed by dry and wet surface treatment mixing process through 2.5% APTES solution reaction after dry treatment (UV / O 5 minutes) according to an embodiment of the present invention. Was used to modify the surface of the lithium niobate substrate. After surface modification, a 2 μm thick PMMA layer was coated by spin coating (5000 rpm, 90 sec).

9 and 10 show a frequency characteristic (FIG. 9) and a time delay mode signal after PMMA coating (FIG. 10) in a reflective SAW device fabricated using a lithium niobate substrate.

Figure 9 shows the frequency characteristics before and after PMMA coating. The coating of the PMMA layer can be seen to move to a lower frequency region.

FIG. 10 shows a time delay mode signal of a surface acoustic wave device having a reflective structure in a structure coated with a 2 μm thick PMMA layer. The peaks of the two reflectors are observed at 1.13 μs and 3.40 μs, indicating that the surface acoustic waves of the surface acoustic wave devices propagate normally.

110: dry surface treatment step
111; Dry Surface Treatment Equipment
112; Lithium Niobate Board
120: wet surface treatment step
121: surface modification solution
122; Oven

Claims (9)

A surface modification method of a lithium niobate substrate in which chemical functional groups are introduced to a surface through a dry surface treatment and a wet surface treatment in order to improve adhesion of a waveguide thin film made of PMMA when manufacturing a SAW sensor.
The method of claim 1,
Dry surface treatment and wet surface treatment are mixed to activate the surface of the lithium niobate substrate.
The dry surface treatment is to irradiate ultraviolet / ozone (UV / O ₃ ) to the surface of the lithium niobate substrate,
The wet surface treatment is to immerse a lithium niobate substrate having a crystal surface activated by dry surface treatment in a surface modification solution capable of causing a silanization reaction.
The surface modification solution includes APTES (3-aminopropyltriethoxysilane), which is a compound that introduces an amine group (-NH2) as a functional functional group,
The surface modification solution is a surface modification method of a lithium niobate substrate, characterized in that using a solvent mixture of water and ethanol.
The method of claim 2,
The surface modification solution is a surface modification method of a lithium niobate substrate, characterized in that it comprises a compound for introducing a functional group through the immobilization on the surface of the lithium niobate substrate.
The method of claim 2,
The surface modification solution is a surface modification method of a lithium niobate substrate, characterized in that the surface of the activated lithium niobate substrate is hydrophilic or hydrophobic. delete delete delete delete delete

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