WO2021174471A1

WO2021174471A1 - Microfluidic chip and fabricating method thereof

Info

Publication number: WO2021174471A1
Application number: PCT/CN2020/077879
Authority: WO
Inventors: Mingyang LV; Yue Li; Yanchen LI; Dong Wang
Original assignee: Boe Technology Group Co., Ltd.; Beijing Boe Optoelectronics Technology Co., Ltd.
Priority date: 2020-03-05
Filing date: 2020-03-05
Publication date: 2021-09-10
Also published as: CN113811390A

Abstract

A microfluidic chip and a fabricating method thereof are provided. The microfluidic chip may include a first substrate and a first dielectric layer (16) on the first substrate. A surface of the first dielectric layer opposite from the first substrate may include a plurality of first concave portions and a plurality of first convex portions. The microfluidic chip may further include a layer of first metal nanoparticles (18) on the surface of the first dielectric layer opposite from the first substrate.

Description

MICROFLUIDIC CHIP AND FABRICATING METHOD THEREOF

TECHNICAL FIELD

This disclosure relates to microfluidic technology, in particular, to a microfluidic chip and a fabricating method thereof.

BACKGROUND

Electrowetting on dielectric (EWOD) digital microfluidic chips do not use syringe pumps, micro valves and other components required by traditional continuous microfluidic chips, and are more integrated and easily automated. With increasing requirement on detection flux, detection efficiency, and detection cost control, an area of individual driving electrode of the chip is continuously decreasing, and a volume of a used droplet is correspondingly reduced. In the related art, in order to maintain or improve detection sensitivity, electrophoresis is usually used to enrich molecules to be tested in a fixed region of the chip, and positive or negative molecules in a solution are brought together by action of an electric field. However, this method has obvious disadvantages. For example, the enrichment process takes a long time, the chip area increases, and it is difficult to ensure uniformity of the degree of the enrichment, which usually leads to quantitative fluctuation. Furthermore, additional required components also increase difficulty of manufacturing the chip.

BRIEF SUMMARY

One embodiment of the present disclosure is a microfluidic chip. The microfluidic chip may include a first substrate and a first dielectric layer on the first substrate. A surface of the first dielectric layer opposite from the first substrate may include a plurality of first concave portions and a plurality of first convex portions. The microfluidic chip may further include a layer of first metal nanoparticles on the surface of the first dielectric layer opposite from the first substrate. As such, the surface of the first dielectric layer having the plurality of concave portions and the plurality of convex portions forms a three-dimensional periodic structure attached with a layer of first metal particles, thereby acting as a hydrophobic or superhydrophobic surface and a surface-enhancing substrate for the microfluidic chip.

Another embodiment of the present disclosure is a method for fabricating a microfluidic chip. The method for fabricating the microfluidic chip may include forming a first dielectric layer on a first substrate; forming a plurality of first concave portions and a plurality of first convex portions on a surface of the first dielectric layer opposite from the first substrate; and coating the surface of the first dielectric layer opposite from the first substrate with a layer of first metal nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

Fig. 1 shows a schematic diagram of a microfluidic chip according to some embodiments of the present disclosure;

Fig. 2 shows a schematic diagram of a microfluidic chip according to some embodiments of the present disclosure;

Figs 3a-3c show schematic illustration of principle of quantitative standard curve establishment and concentration detection by a microfluidic chip according to some embodiments of the present disclosure;

Figs 4a-4j show schematic illustration of a method of manufacturing a microfluidic chip according to some embodiments of the present disclosure; and

Figs 5a-5e show schematic illustration of a method of manufacturing a three dimensional periodic surface by a nanoimprinting technique according to some embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure will be described in further detail with reference to the accompanying drawings and embodiments in order to provide a better understanding by those skilled in the art of the technical solutions of the present disclosure. Throughout the description of the disclosure, reference is made to Figs. 1-5e. When referring to the figures, like structures and elements shown throughout are indicated with like reference numerals.

In this specification, the terms “first, ” “second, ” etc. may be added as prefixes. These prefixes, however, are only added in order to distinguish the terms and do not have specific meaning such as order and relative merits. In the description of the present disclosure, the meaning of "plural" is two or more unless otherwise specifically defined.

In the description of the specification, references made to the term “some embodiments, ” “one embodiment, ” “exemplary embodiments, ” “example, ” “specific example, ” “some examples” and the like are intended to refer that specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least some embodiments or examples of the present disclosure. The schematic expression of the terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be included in any suitable manner in any one or more embodiments or examples. A number modified by “about” herein means that the number can vary by 10%thereof.

EWOD digital microfluidic chip technology has become very popular in recent years. It integrates driving electrodes into the structure itself, and can realize separation, movement and fusion of individual microdroplet by manipulating a voltage of each driving electrode. EWOD digital microfluidic chip is an ideal workstation for large-volume, high-precision biochemical analysis such as vaccine screening. The amount of sample required is extremely small, and a few microliters of reagent is enough to complete an experiment, greatly reducing cost of development and testing. Compared with the continuous microfluidic chip in the field of microfluidics, the EWOD digital microfluidic chip is free from additional components such as syringe pumps and microvalves, and is more integrated and easier to automate. EWOD digital microfluidic chip has made great progress in the field of detection. For example, people have begun to use it to detect glucose concentration, specific antigen concentration, etc. . The detection method mostly uses Raman, infrared or other means, and the detection equipment is mostly external. Among them, Raman, a non-destructive, fast and specific spectroscopy technique, is often used in a variety of biochemical tests.

In the microfluidic chips according to some embodiments of the present disclosure, an originally flat polytetrafluoroethylene hydrophobic surface in the related art is changed to a surface of a periodic three-dimensional structure attached with a layer of metal nanoparticles such as gold or copper nanoparticles. Experiments have shown that the surface of the three-dimensional structure attached with the gold nanoparticles forms a large number of tiny air cells due to existence of gaps. When a liquid droplet is placed thereon, the action of the air cells can keep the liquid droplet in a hydrophobic or superhydrophobic state, and a contacting angle of the water droplet may reach 130° or more. At the same time, the hydrophobic or superhydrophobic surface attached with the layer of gold nanoparticles may be used as a reinforcing substrate for surface enhanced Raman scattering (SERS) . Due to the excellent surface plasmon resonance effect of gold nanoparticles, the SERS effect can be generated, and a plurality of hot spots may be formed on the surface of the substrate, thereby greatly increasing the electric field strength. As a result, the intensity of Raman spectrum scattered by molecules of an analyte may be significantly increased by, for example, more than 104 times. That is, the detection sensitivity may be significantly increased by more than 104 times. In addition, gold has good biocompatibility, can better maintain biological activity, and is more suitable for the detection of antigens, antibodies, cells, DNA, and the like. In view of cost, copper can also be used instead of gold.

Fig. 1 shows a schematic diagram of a microfluidic chip according to some embodiments of the present disclosure. The microfluidic chip may include a three-dimensional structure 10 formed by a glass substrate, driving electrodes 12, common electrodes 14, a dielectric layer 16, and noble metal particles 18. In one embodiment, as shown in Fig. 1, the microfluidic chip may include a first substrate and a first dielectric layer on the first substrate. A surface of the first dielectric layer opposite from the first substrate may include a plurality of first concave portions and a plurality of first convex portions. The first substrate may be made of a glass.

In one embodiment, the first dielectric layer 16 may be made of silicon nitride, silicon dioxide, silicon oxynitride or an organic resin, or a combination thereof. The surface of the first dielectric layer opposite from the first substrate may be configured to exhibit a static contact angle equal to or greater than about 130° for deionized water at room temperature.

In one embodiment, the plurality of first concave portions and the plurality of first convex portions are arranged alternately in both a first direction and a second direction on the surface of the first dielectric layer opposite from the first substrate. The first direction may be perpendicular to the second direction. In one embodiment, a cross-section of one of the plurality of first convex portions in a plane perpendicular to the first substrate may be in a rectangular or square shape, and each side of the rectangular or square shape may be in a range of about 3 μm to about 5 μm. A cross-section of one of the plurality of first concave portions in a plane perpendicular to the first substrate is in a rectangular or square shape, and each side of the rectangular or square shape may be in a range of about 3 μm to about 5 μm.

In one embodiment, the microfluidic chip may further include a layer of first metal nanoparticles 18 on the surface of the first dielectric layer opposite from the first substrate. The first metal nanoparticles 18 may be gold or copper metal nanoparticles. Each of the first metal nanoparticles may have a diameter in a range of about 15 nm to about 500 nm, preferably about 30 nm to about 100 nm. Orthographic projection of the layer of first metal nanoparticles on the first substrate may cover orthographic projection of the plurality of first concave portions and the plurality of first convex portions on the first substrate. As such, the surface of the first dielectric layer having the plurality of concave portions and the plurality of convex portions forms a three-dimensional periodic surface with attached metal particles, thereby acting as a hydrophobic surface and a surface-enhancing substrate.

In one embodiment, as shown in Fig. 1, the microfluidic chip may further include a plurality of driving electrodes and a plurality of common electrodes between the first dielectric layer and the first substrate. The plurality of driving electrodes may be on a surface of the first substrate facing the first dielectric layer. The plurality of common electrodes may be on a side of the driving electrodes opposite from the first substrate.

In one embodiment, as shown in Fig. 2, the microfluidic chip may further include a second substrate opposite the first substrate, a second dielectric layer on a side of the second substrate facing the first substrate, and a layer of second metal nanoparticles on the surface of the second dielectric layer opposite from the second substrate. A surface of the second dielectric layer opposite from the second substrate may include a plurality of second concave portions and a plurality of second convex portions. The surface of the second dielectric layer having the layer of second metal nanoparticles and the surface of the first dielectric layer having the layer of first metal nanoparticles are apart and opposite from each other. The second substrate may be made of the same or different material as the first substrate, and the second substrate may be made of silicon nitride, silicon dioxide, silicon oxynitride or an organic resin, or a combination thereof.

According to some embodiments of the present disclosure, the surface structure of the microfluidic chip is changed from the originally flat polytetrafluoroethylene hydrophobic surface to a periodic three-dimensional structure of gold nanoparticle surface. Experiments have shown that the surface of the three-dimensional structure attached with the gold nanoparticles forms a large number of tiny air cells due to existence of gaps. When a liquid droplet is placed thereon, the action of the air cells can keep the liquid droplet in a higher hydrophobic state, and the hydrophobic angle may reach 130° or more. While ensuring high hydrophobicity, due to excellent surface plasmon resonance effect of gold nanoparticles, the surface electric field may be greatly increased. As a result, the detection sensitivity of the object to be measured may also be significantly increased, for example, by more than 104 times. In addition, gold has excellent biocompatibility and is more conducive to the detection of antigens, antibodies, cells, DNA, etc. Thus, some embodiments of the present disclosure innovatively abandons the ultra-flat surface in the related art, and uses a surface of a periodic three-dimensional array structure attached with noble metal nanoparticles, thereby meeting requirements of high hydrophobic state and high detection sensitivity.

One embodiment of the present disclosure is a microfluidic device, comprising the microfluidic chip according to one embodiment of the present disclosure. The microfluidic device may be a digital microfluidic device such as a EWOD digital microfluidic device.

Qualitative and quantitative detection standards may be established for a microfluidic detection device comprising the microfluidic chip. It is easy to implement qualitative detection for the detection device because each molecule has its own unique peak spectrum. The peak spectrum can be detected to determine the presence of the substance. For quantitative detection, a quantitative curve of integrated intensity of a characteristic peak of Raman spectrum of an analyte and concentrations of the analyte may be established. First, in each batch of vapor-deposited samples, one test sample including the analyte is added, and the test sample is divided into 10 regions. After the evaporation deposition, the samples to be tested with different concentrations of the analyte were subjected to Raman testing to obtain the corresponding Raman spectrums, as shown in Fig. 3a. A quantitative relationship between integrated intensity of the characteristic peak and the concentration of the analyte to be detected was fitted, as shown in Fig. 3b, and used as the quantitative curve of the batch of chips. Each batch of samples is retested to fit the quantitative curve, which can eliminate difference in enhancement performance due to fluctuation in the evaporation process, and thus provide more accurate quantitative detection.

For the microfluidic chips according to some embodiment of the present disclosure, the principle and method of droplet transport is the same as those of the general EWOD digital microfluidic chips. The droplet movement is realized by controlling switches of two driving electrodes that are connected by the droplet. The difference is that if the general microfluidic chip wants to detect an analyte with high detection sensitivity, it is necessary to transport the droplet to a fixed site and perform the detection after a special treatment. In some embodiments of the present disclosure, it is not necessary to fix the detection site, and the metal particles on the chip are all detection sites, and the detection sensitivity is high without any special treatment. During the detection, a laser head may be directed to the droplet under a dark room, and the Raman spectrum of the analyte is obtained by focusing on the surface of the metal nanoparticles. After calculating the integrated intensity of the characteristic peak in the Raman spectrum of the analyte by software, the concentration of the analyte can be obtained by substituting it into the batch quantitative curve, as shown in Fig. 3c.

One embodiment of the present disclosure is a method for fabricating a microfluidic chip. The method for fabricating the microfluidic chip may include forming a plurality of driving electrodes on a first substrate. In one embodiment, first, a first metal layer 22 and a layer of photoresist 24 may be sequentially formed on a first substrate 20, as shown in Fig. 4a. Then, as shown in Fig. 4b, a patterning or photolithography technique is performed on the first metal layer to form the plurality of driving electrodes, which are arranged at intervals from one another. Then, as shown in Fig. 4c, the layer of photoresist is removed from the plurality of driving electrodes. The patterning process may be a typical patterning process, which includes at least exposure, development, and etching steps. The first metal layer may be made of metal Al, Mo, Cu, Ti, Ni, Ag, or ITO or a combination thereof.

After forming the driving electrodes, the method for fabricating the microfluidic chip may further include forming a plurality of common electrodes on the first substrate, wherein the plurality of common electrodes may be on a side of the driving electrodes opposite from the first substrate. In one embodiment, as shown in Fig. 4d, first, a first layer of a first dielectric material 26 covering the plurality of common electrodes is formed on the first substrate. Then, as shown in Fig. 4e, a second metal layer 28 and a layer of photoresist 30 may be sequentially formed on a surface of the first layer of the first dielectric material opposite from the first substrate. Then, as shown in Fig. 4f, a patterning or photolithography technique is performed on the second metal layer to form the plurality of common electrodes, which are arranged at intervals from one another and insulated from the driving electrodes, on the first layer of first dielectric material. Then, as shown in Fig. 4g, the layer of photoresist is removed from the plurality of common electrodes. The patterning process in this step may also be a typical patterning process, which includes at least exposure, development, and etching steps. The second metal layer may be made of metal Al, Mo, Cu, Ti, Ni, Ag, or ITO or a combination thereof.

After forming the common electrodes, the method for fabricating the microfluidic chip may further include forming a first dielectric layer on the first substrate, and forming a plurality of first concave portions and a plurality of first convex portions on a surface of the first dielectric layer opposite from the first substrate. In one embodiment, as shown in Fig. 4h, a second layer of the first dielectric material 32 covering the plurality of common electrodes is formed on the first layer of the first dielectric material. The first layer of the first dielectric material 26 and the second layer of the first dielectric material 32 are combined to form the first dielectric layer 40. Then, the plurality of first concave portions and the plurality of first convex portions may be formed on the surface of the first dielectric layer opposite from the first substrate by a photolithography technique or a nanoimprinting technique. In one embodiment, the plurality of first concave portions and the plurality of first convex portions are formed on the surface of the first dielectric layer opposite from the first substrate by a photolithography technique. As shown in Fig. 4h, a layer of photoresist 34 is formed on a surface of the first dielectric layer opposite from the first substrate. Then, as shown in Fig. 4i, a patterning or photolithography technique is performed on the first dielectric layer to form the plurality of first concave portions 36 and the plurality of first convex portions 38 on the surface of the first dielectric layer opposite from the first substrate. Then, as shown in Fig. 4j, the layer of photoresist is removed from the plurality of first convex portions. As such, a three-dimensional periodic surface of the first dielectric layer having the plurality of concave portions and the plurality of convex portions is formed, and acts as a hydrophobic or superhydrophobic surface for the microfluidic chip.

In one embodiment, the plurality of first concave portions and the plurality of first convex portions are formed on the surface of the first dielectric layer opposite from the first substrate by a nanoimprinting technique. As shown in Fig. 5a, a layer of photoresist 34 is formed on a surface of the first dielectric layer 40 opposite from the first substrate 20 by, for example, a spin coating technique. Then, as shown in Fig. 5b, a nanoimprinting mold 42 is pressed into the layer of photoresist 34. The surface of the nanoimprinting mold pressing into the layer of photoresist includes a plurality of concave portions and a plurality of convex portions. Each of the convex portions of the nanoimprinting mold may have a height of about 50 nm to about 1000 nm. A length and a width of a cross-section of each of the convex portions of the nanoimprinting mold in a plane parallel to first substrate may be in a range of about 50 nm to about 1000 nm. Then, as shown in Fig. 5c, UV light is used to irradiate the layer of photoresist from a side of the nanoimprinting mold to solidify the layer of photoresist. Then, as shown in Fig. 5d, the nanoimprinting mold is removed from the surface of the first dielectric layer, thereby forming a plurality of convex portions composed of the layer of photoresist on the surface of the first dielectric layer. Then, as shown in Fig. 5e, an etching process is performed on the first dielectric layer using the plurality of convex portions composed of the layer of photoresist as a mask. As such, a plurality of first concave portions 36 and a plurality of first convex portions 38 are formed on the surface of the first dielectric layer 40 opposite from the first substrate. Then, the plurality of convex portions composed of the layer of photoresist is removed from the plurality of first convex portions composed of the first dielectric layer. Accordingly, a three-dimensional periodic surface of the first dielectric layer having the plurality of concave portions and the plurality of convex portions is formed, and acts as a hydrophobic or superhydrophobic surface for the microfluidic chip.

In one embodiment, after forming the three-dimensional periodic surface of the first dielectric layer, the method for fabricating the microfluidic chip may further include coating the surface of the first dielectric layer opposite from the first substrate with a layer of first metal nanoparticles.

In one embodiment, the surface of the first dielectric layer opposite from the first substrate is coated with a layer of first metal nanoparticles by a low temperature evaporation deposition technique. The low temperature evaporation deposition technique presents certain advantages such as high purity and controllable particle size. In this technique, the gold or copper nanoparticles may be formed on the three-dimensional periodic surface, which is at a low temperature. For example, the substrate temperature may be at about 30 ℃ during the low temperature evaporation, and the completed microfluidic chip will not be negatively affected by the low temperature. Furthermore, high temperature metal ions will be more easily deposited on the low temperature dielectric layer. The size of metal particles can be precisely controlled, for example, at about 50 nm.

In one embodiment, during the low temperature evaporation deposition, a chamber is first pumped into vacuum, and then filled with a pure inert protective gas, and the regulating pressure is about 103 Pa. The arc between a tungsten electrode and a nozzle is ignited by a high frequency igniter. Under action of gas pressure and arc current, ionized gas with high temperature and activity rushes out of the nozzle to form stable plasma. High purity bulk metal material such as gold is evaporated to gasification by high temperature plasma heating. The evaporated metal atoms diffuse in inert gas and collide fiercely with inert gas atoms. By means of flame boundary and quenching device, the metal atoms rapidly lose energy and cool down. In this effective cooling process, supersaturation of a very large area in the metal vapor occurs, and accordingly, ultrafine metal particles are formed by spontaneous nucleation and condensation from the gas phase. Then, loose metal nanoparticles can be obtained by stabilizing passivation treatment of the ultrafine metal particles for a certain period of time.

In one embodiment, the method for fabricating the microfluidic chip may further include forming a second dielectric layer on a second substrate, forming a plurality of second concave portions and a plurality of second convex portions on a surface of the second dielectric layer opposite from the second substrate; and coating the surface of the second dielectric layer opposite from the second substrate with a layer of second metal nanoparticles. The surface of the second dielectric layer coated with the layer of second metal nanoparticles and the surface of the first dielectric layer coated with the layer of first metal nanoparticles are apart and opposite from each other. The process of forming the plurality of second concave portions and the plurality of second convex portions on the surface of the second dielectric layer may be similar to that of forming the plurality of first concave portions and the plurality of first convex portions on the surface of the first dielectric layer as discussed previously, and the details thereof are not repeated herein. The process of coating the surface of the second dielectric layer opposite from the second substrate with second metal nanoparticles may be similar to that of coating the surface of the first dielectric layer opposite from the first substrate with first metal nanoparticles as discussed above, and the details thereof are not repeated herein.

The principle and the embodiment of the disclosure are set forth in the specification. The description of the embodiments of the present disclosure is only used to help understand the method of the present disclosure and the core idea thereof. Meanwhile, for a person of ordinary skill in the art, the disclosure relates to the scope of the disclosure, and the embodiment is not limited to the specific combination of the technical features, and also should covered other embodiments which are formed by combining the technical features or the equivalent features of the technical features without departing from the inventive concept. For example, embodiments may be obtained by replacing the features described above as disclosed in this disclosure (but not limited to) with similar features.

Claims

A microfluidic chip, comprising:

a first substrate; and

a first dielectric layer on the first substrate,

wherein a surface of the first dielectric layer opposite from the first substrate comprises a plurality of first concave portions and a plurality of first convex portions.
The microfluidic chip of claim 1, further comprising a layer of first metal nanoparticles on the surface of the first dielectric layer opposite from the first substrate.
The microfluidic chip of claim 1, wherein the plurality of first concave portions and the plurality of first convex portions are arranged alternately in both a first direction and a second direction on the surface of the first dielectric layer opposite from the first substrate, the first direction being perpendicular to the second direction.
The microfluidic chip of claim 1, wherein a cross-section of one of the plurality of first convex portions in a plane perpendicular to the first substrate is in a rectangular or square shape, and each side thereof is in a range of about 3 μm to about 5 μm.
The microfluidic chip of claim 1, wherein a cross-section of one of the plurality of first concave portions in a plane perpendicular to the first substrate is in a rectangular or square shape, each side thereof is in a range of about 3 μm to about 5 μm.
The microfluidic chip of any one of claims 1-5, wherein the first dielectric layer comprises silicon nitride, silicondioxide, silicon oxynitride or an organic resin.
The microfluidic chip of claim 1, wherein the surface of the first dielectric layer opposite from the first substrate is configured to exhibit a static contact angle equal to or greater than about 130° for deionized water at room temperature.
The microfluidic chip of claim 2, wherein the first metal nanoparticles comprise gold or copper.
The microfluidic chip of claim 2 or 8, wherein each of the first metal nanoparticles has a diameter in a range of about 30 nm to about 100 nm.
The microfluidic chip of any one of claims 2, 8, and 9, wherein orthographic projection of the layer of first metal nanoparticles on the first substrate covers orthographic projection of the plurality of first concave portions and the plurality of first convex portions on the first substrate.
The microfluidic chip of any one of claims 1-10, further comprising a plurality of driving electrodes and a plurality of common electrodes between the first dielectric layer and the first substrate,

wherein the plurality of driving electrodes are on a surface of the first substrate facing the first dielectric layer, and the plurality of common electrodes are on a side of the driving electrodes opposite from the first substrate.
The microfluidic chip of any one of claims 2, 8, 9, and 10, further comprising:

a second substrate opposite the first substrate;

a second dielectric layer on a side of the second substrate facing the first substrate, a surface of the second dielectric layer opposite from the second substrate comprising a plurality of second concave portions and a plurality of second convex portions; and

a layer of second metal nanoparticles on the surface of the second dielectric layer opposite from the second substrate.
The microfluidic chip of claim 12, wherein the surface of the second dielectric layer having the layer of second metal nanoparticles and the surface of the first dielectric layer having the layer of first metal nanoparticles are apart and opposite from each other.
A microfluidic device, comprising the microfluidic chip of any one of claims 1-13.
A method for fabricating a microfluidic chip, comprising:

forming a first dielectric layer on a first substrate;

forming a plurality of first concave portions and a plurality of first convex portions on a surface of the first dielectric layer opposite from the first substrate; and

coating the surface of the first dielectric layer opposite from the first substrate with a layer of first metal nanoparticles.
The method of claim 15, wherein the plurality of first concave portions and the plurality of first convex portions are formed on the surface of the first dielectric layer opposite from the first substrate by a photolithography technique or an nanoimprinting technique.
The method of claim 15, wherein the surface of the first dielectric layer opposite from the first substrate is coated with the layer of first metal nanoparticles by a low temperature evaporation deposition technique.
The method of claim 15, before forming the plurality of first concave portions and the plurality of first convex portions, further comprising:

forming a plurality of driving electrodes and a plurality of common electrodes between

the first dielectric layer and the first substrate,

wherein the plurality of driving electrodes are on a surface of the first substrate facing the first dielectric layer, and the plurality of common electrodes are on a side of the driving electrodes opposite from the first substrate.
The method of claim 15, further comprising:

providing a second substrate opposite the first substrate;

forming a second dielectric layer on the second substrate,

forming a plurality of second concave portions and a plurality of second convex portions on a surface of the second dielectric layer opposite from the second substrate; and

coating the surface of the second dielectric layer opposite from the second substrate with a layer of second metal nanoparticles;

wherein the surface of the second dielectric layer coated with the layer of second metal nanoparticles and the surface of the first dielectric layer coated with the layer of first metal nanoparticles are apart and opposite from each other.