阅读说明：本技术 具有碗型能带结构的双碱光电阴极及其制备方法 (Double-alkali photocathode with bowl-shaped energy band structure and preparation method thereof ) 是由 金睦淳 苏德坦 孙建宁 司曙光 王兴超 任玲 王亮 王从杰 侯巍 徐海洋 于 2019-11-13 设计创作，主要内容包括：本发明提供一种具有碗型能带结构的双碱光电阴极及其制备方法。所述的双碱光电阴极是以钾、铯为碱金属源的锑化物光电阴极,其峰值量子效率可达35％以上,截止波长仅为620nm；所述碗型能带结构为玻璃基底上蒸镀的宽禁带Be<Sub>3</Sub>N<Sub>2</Sub>膜层+K<Sub>2</Sub>CsSb光电阴极+Te表面掺杂层的多层膜系,从而形成的高-低-高能带结构,采用本发明的结构的光电阴极的光谱响应曲线具有峰值响应高、响应范围窄的优点。(The invention provides a double-alkali photocathode with a bowl-shaped energy band structure and a preparation method thereof. The double-alkali photocathode is an antimonide photocathode taking potassium and cesium as alkali metal sources, the peak quantum efficiency of the double-alkali photocathode can reach more than 35%, and the cut-off wavelength is only 620 nm; the bowl-shaped energy band structure is a wide forbidden band Be evaporated on a glass substrate 3 N 2 Film + K 2 The spectrum response curve of the photocathode adopting the structure of the invention has the advantages of high peak response and narrow response range.)

1. A double-alkali photocathode with a bowl-shaped energy band structure is characterized by comprising a glass substrate and a multilayer film system which is arranged on the glass substrate and has high energy band sides and low energy band middle, wherein the multilayer film system comprises the following components from inside to outside:

photoelectron reflecting layer: semiconductor thin film Be with wide bandgap₃N₂Forming;

K₂a CsSb photoelectron generating layer;

K₂sb (Te) -Cs surface doping layer;

wherein the double-alkali photocathode has a quantum efficiency of 35% or more at a peak position and a cut-off wavelength of 620 nm.

2. The double-alkali photocathode with a bowl-shaped energy band structure according to claim 1, wherein the wide bandgap semiconductor thin film Be₃N₂And K₂The CsSb surface doping layers are all of a cubic crystal structure.

3. The double-alkali photocathode with a bowl-shaped energy band structure according to claim 1, wherein the wide bandgap semiconductor thin film Be₃N₂Is greater than 1.98.

4. The double-alkali photocathode with a bowl-shaped energy band structure according to claim 1, wherein the Be₃N₂Has a wider forbidden band width of 4.05-4.47 eV.

5. A method for preparing the double-alkali photocathode with the bowl-shaped energy band structure according to claim 1, comprising:

first, at a temperature of more than 400 ℃ and above 8X 10^-4Starting to perform metal beryllium evaporation on a clean glass substrate under the condition of Pa vacuum degree;

secondly, filling a mixed gas of high-purity nitrogen and hydrogen into the vacuum of the beryllium film at the temperature of more than 400 ℃, and performing arc glow discharge to form the beryllium nitride film by the beryllium film when the vacuum degree reaches about 50 Pa;

transferring the beryllium nitride-plated glass substrate to cathode preparation equipment, and baking and degassing the vacuum equipment, the glass substrate and an alkali metal and antimony tellurium evaporation source at the temperature of more than 350 ℃;

fourthly, recording the initial reflectivity at the temperature of less than 200 ℃, and carrying out low-current evaporation degassing on a potassium source, a cesium source, an antimony source and a tellurium source;

fifthly, performing bottom potassium evaporation at the temperature of 120-190 ℃;

sixthly, evaporating potassium and antimony at the temperature of 120-190 ℃ simultaneously;

seventhly, carrying out cesium evaporation at the temperature of 110-180 ℃;

eighthly, performing antimony-cesium alternate and tellurium-cesium alternate evaporation at the temperature of 100-170 ℃;

ninth, tellurium evaporation is carried out at the temperature of 100-170 ℃.

6. The method for preparing a bialkali photoelectric cathode with a bowl-shaped energy band structure as claimed in claim 5, wherein in the first step, the evaporation method is resistance-type tantalum wire thermal evaporation, the film thickness is monitored by blue light 532nm reflectivity, the thickness is controlled to be reduced by 20% -45% of the reflectivity, and the corresponding evaporation time is shorter than 5 s.

7. The method for preparing the double-alkali photocathode with the bowl-shaped energy band structure according to claim 5, wherein in the second step, the molar ratio of nitrogen to hydrogen is higher than 9:1, the arc glow discharge voltage is higher than 2kV, and the glow discharge time is higher than 5min, so as to ensure that the metallic beryllium is fully nitrided.

8. The method for preparing a double-alkali photocathode with a bowl-shaped energy band structure according to claim 5, wherein in the eighth step, at a temperature of 100-170 ℃, the alternating cycle rule when performing antimony-cesium alternating and tellurium-cesium alternating evaporation is as follows: in the whole process, cesium evaporation current is not closed, antimony and tellurium current switches are alternated, when antimony or tellurium evaporation current is turned on, photocurrent is reduced, when the antimony or tellurium evaporation current is reduced to a half of an initial value, the antimony or tellurium evaporation current is turned off, at the moment, the photocurrent starts to rise, and when the antimony or tellurium evaporation current rises to a state that the antimony or tellurium evaporation current is not changed any more, the next cycle evaporation is carried out by turning on the antimony or tellurium evaporation current again.

9. The method for preparing a double-alkali photocathode with a bowl-shaped energy band structure according to claim 8, wherein in the eighth step, the number of alternation of antimony and cesium and the number of alternation of tellurium and cesium are controlled to be more than 6: 1.

10. The method for preparing a double-alkali photocathode with a bowl-shaped energy band structure as claimed in claim 5, wherein in the ninth step, when the maximum photocurrent value of the current cycle period in the eighth step is no longer higher than that of the previous cycle period, the last tellurium evaporation is performed, and the tellurium evaporation is finished when the photocurrent drops to 75% of the maximum value.

Technical Field

The invention relates to the technical field of double-alkali photocathodes, in particular to a bowl-shaped energy band double-alkali photocathode and a preparation method thereof.

Background

A typical representation of a bi-alkali cathode is a potassium cesium antimony photocathode whose stoichiometric ratio of potassium, cesium and antimony is 2:1:1, with a cubic crystal structure. Having a lattice constant of

The band gap is about 1eV, and the electron affinity is 1.1 eV. Potassium cesium antimony photocathodes are classified into reflective and transmissive, with the reflective photocathode substrate typically being a metal and the transmissive photocathode substrate typically being transparent glass. The response wave band of the potassium cesium antimony photocathode grown on the glass substrate is between 250nm and 700nm, and the dark current at room temperature is only 10^-17A/cm²Therefore, the cesium-antimony potassium photocathode is very suitable for being used as a photoelectric conversion material for single-photon signal detection, and taking the current popular detection of neutrino in the high-energy physical field as an example, the neutrino can excite photon signals of 380nm to 510nm after passing through a scintillator and is received by the cesium-antimony potassium photocathode, so that the neutrino can be indirectly detected. In addition, the potassium cesium antimony photocathode is very suitable for working in a strong radio frequency electric field environment, and can provide high-quality beam current for short-wavelength high-gain free electron laser devices of an energy recovery type linear accelerator and a high repetition frequency linear accelerator in a high-brightness average power light injector.

As a conventional double-alkali photocathode, Na is mentioned in patent application No. 201510438585.X₂The preparation method of the CsSb photocathode adopts the preparation flow of adding sodium by steaming cesium antimony, adding sodium by steaming cesium antimony and adding sodium by steaming antimony; manufactured in application No. 201710743036.2 patentPrepared K₂The CsSb photocathode has more and more potassium concentration and less antimony concentration in the growth process, and is beneficial to transporting electrons in the material to the surface. In the multilayer film system, the cathode structure of the application No. 200780004067.0, which uses hafnium oxide, manganese and magnesium or titanium oxide as the substrate layer, contributes to the improvement of quantum efficiency; the application No. 200710305894.5 patent adopts a film layer of beryllium oxide and multiple metal oxides mixed crystal, so that the quantum efficiency is obviously improved.

Disclosure of Invention

The invention aims to provide a double-alkali photocathode with a bowl-shaped energy band structure, which comprises a glass substrate and a multilayer film system which is arranged on the glass substrate and has high energy band at two sides and low energy band at middle, wherein the multilayer film system comprises the following components from inside to outside:

photoelectron reflecting layer: semiconductor thin film Be with wide bandgap₃N₂Forming;

K₂a CsSb photoelectron generating layer;

K₂sb (Te) -Cs surface doping layer;

wherein the double-alkali photocathode has a quantum efficiency of 35% or more at a peak position and a cut-off wavelength of 620 nm.

In a further embodiment, the wide bandgap semiconductor thin film Be₃N₂And K₂The CsSb surface doping layers are all of a cubic crystal structure.

In a further embodiment, the wide bandgap semiconductor thin film Be₃N₂Is greater than 1.98.

In a further embodiment, said Be₃N₂Has a wider forbidden band width of 4.05-4.47 eV.

According to the invention, the invention also provides a preparation method of the double-alkali photocathode with the bowl-shaped energy band structure, which comprises the following steps:

first, at a temperature of more than 400 ℃ and above 8X 10^-4Starting to perform metal beryllium evaporation on a clean glass substrate under the condition of Pa vacuum degree;

secondly, filling a mixed gas of high-purity nitrogen and hydrogen into the vacuum of the beryllium film at the temperature of more than 400 ℃, and performing arc glow discharge to form the beryllium nitride film by the beryllium film when the vacuum degree reaches about 50 Pa;

transferring the beryllium nitride-plated glass substrate to cathode preparation equipment, and baking and degassing the vacuum equipment, the glass substrate and an alkali metal and antimony tellurium evaporation source at the temperature of more than 350 ℃;

fourthly, recording the initial reflectivity at the temperature of less than 200 ℃, and carrying out low-current evaporation degassing on a potassium source, a cesium source, an antimony source and a tellurium source;

fifthly, performing bottom potassium evaporation at the temperature of 120-190 ℃;

sixthly, evaporating potassium and antimony at the temperature of 120-190 ℃ simultaneously;

seventhly, carrying out cesium evaporation at the temperature of 110-180 ℃;

eighthly, performing antimony-cesium alternate and tellurium-cesium alternate evaporation at the temperature of 100-170 ℃;

ninth, tellurium evaporation is carried out at the temperature of 100-170 ℃.

In a further embodiment, in the first step, the evaporation method is resistive tantalum wire thermal evaporation, the film thickness is monitored by a blue light 532nm reflectivity, the thickness is controlled to be reduced by 20% -45% of the reflectivity, and the corresponding evaporation time is shorter than 5 s.

In a further embodiment, in the second step, the molar ratio of nitrogen to hydrogen is higher than 9:1, the arc glow discharge voltage is higher than 2kV, and the glow discharge time is higher than 5min, so as to ensure that the metal beryllium is fully nitrided.

In a further embodiment, in the eighth step, at a temperature of 100 ℃ to 170 ℃, the alternating cycle rule when performing the antimony-cesium alternating and tellurium-cesium alternating evaporation is as follows: in the whole process, cesium evaporation current is not closed, antimony and tellurium current switches are alternated, when antimony or tellurium evaporation current is turned on, photocurrent is reduced, when the antimony or tellurium evaporation current is reduced to a half of an initial value, the antimony or tellurium evaporation current is turned off, at the moment, the photocurrent starts to rise, and when the antimony or tellurium evaporation current rises to a state that the antimony or tellurium evaporation current is not changed any more, the next cycle evaporation is carried out by turning on the antimony or tellurium evaporation current again. Preferably, the alternation times of antimony and cesium and tellurium and cesium are controlled to be more than 6: 1.

In a further embodiment, in the ninth step, when the maximum photocurrent value of the current cycle period in the eighth step is no longer higher than that of the previous cycle period, the last tellurium evaporation is performed, and the tellurium evaporation is finished when the photocurrent drops to 75% of the maximum value.

Compared with the prior art, the double-alkali photocathode with the bowl-shaped energy band structure and the preparation method thereof are characterized in that the antimonide photocathode takes potassium and cesium as alkali metal sources, the peak quantum efficiency can reach more than 35%, and the cut-off wavelength is only 620 nm; the bowl-shaped band structure is a wide forbidden band Be evaporated on a glass substrate₃N₂Film + K₂A CsSb photocathode and a Te surface doping layer, thereby forming a high-low-high energy band structure. The spectral response curve of the photocathode prepared by adopting the structure has the characteristics of high peak response and narrow response range.

Meanwhile, the invention is favorable for reducing the quantity of cathode hot electrons escaping into vacuum based on the mode of shortening the cut-off wavelength by improving the work function, and can reduce the dark noise of the cathode to a certain extent.

It should be understood that all combinations of the foregoing concepts and additional concepts described in greater detail below can be considered as part of the inventive subject matter of this disclosure unless such concepts are mutually inconsistent. In addition, all combinations of claimed subject matter are considered a part of the presently disclosed subject matter.

The foregoing and other aspects, embodiments and features of the present teachings can be more fully understood from the following description taken in conjunction with the accompanying drawings. Additional aspects of the present invention, such as features and/or advantages of exemplary embodiments, will be apparent from the description which follows, or may be learned by practice of specific embodiments in accordance with the teachings of the present invention.

Drawings

The drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures may be represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. Embodiments of various aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic structural diagram of a double-base photocathode according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a double-alkali photocathode bowl-shaped energy band structure according to an embodiment of the present invention.

Figure 3 is a graph of the double base photocathode quantum efficiency for several different membrane systems according to embodiments of the present invention.

FIG. 4 is a table showing the cut-off wavelength versus quantum efficiency for different antimony/tellurium ratios for a dibasic photocathode.

Detailed Description

In order to better understand the technical content of the present invention, specific embodiments are described below with reference to the accompanying drawings.

In this disclosure, aspects of the present invention are described with reference to the accompanying drawings, in which a number of illustrative embodiments are shown. Embodiments of the present disclosure are not necessarily intended to include all aspects of the invention. It should be appreciated that the various concepts and embodiments described above, as well as those described in greater detail below, may be implemented in any of numerous ways, and that the concepts and embodiments disclosed herein are not limited to any embodiment. In addition, some aspects of the present disclosure may be used alone, or in any suitable combination with other aspects of the present disclosure.

Referring to fig. 1-3, according to the disclosure of the present invention, a double-alkali photocathode with a bowl-shaped energy band structure is provided, which includes a glass substrate and a multi-layer film system on the glass substrate, wherein the multi-layer film system has a high energy band at two sides and a low energy band at the middle, and the multi-layer film system includes:

photoelectron reflecting layer: semiconductor thin film Be with wide bandgap₃N₂Forming;

K₂a CsSb photoelectron generating layer;

K₂sb (Te) -Cs surface doping layer;

wherein the double-alkali photocathode has a quantum efficiency of 35% or more at a peak position and a cut-off wavelength of 620 nm.

Preferably, the wide bandgap semiconductor thin film Be₃N₂And K₂The CsSb surface doping layers are all in a cubic crystal structure and preferably used as a substrate layer for growing K₂CsSb photocathode, cubic crystal Be₃N₂The structure is very stable.

Preferably, the present invention employs Be₃N₂Lattice constant of

K grown thereon₂The CsSb photocathode has low mismatching degree and is beneficial to the growth of a thin film. Be₃N₂The refractive index of crystal is more than 1.98, and K is grown on glass₂The CsSb photocathode has a certain anti-reflection effect.

In order to realize the bowl-shaped energy band, the Be of the invention₃N₂Has a wider forbidden band width of 4.05-4.47 eV, and is₃N₂-K₂Part of photoelectrons generated near the CsSb interface diffuse to the interface, and the part with the higher forbidden band width can reflect the diffused photoelectrons back to the cathode like a bowl wall, and finally emit the photoelectrons to vacuum over a surface potential barrier.

In the above embodiment, the surface doping layer is K₂Sb (Te) -Cs layer. For electron-emitting photocathodes, Cs atoms may act to reduce the surface work function. Cs atoms and atoms with negative valence in the cathode body form electron exchange, namely an Sb-Cs dipole layer is formed, and the more the Cs reacts, the more obvious the effect of the Sb-Cs dipole layer on reducing the surface work function is.

K₂Sb in the CsSb photocathode has a valence of-3, and 1 Sb atom, 2K atoms and 1 Cs atom can be used in a stoichiometric ratio. Since the radius of Te atom is equivalent to that of Sb, Te is doped into the cathode body in a displacement mode. Therefore, assuming the same number of negative-valent atoms, the number of Cs atoms that can be adsorbed on the cathode surface after doping-2-valent Te atoms is reduced, resulting in a smaller work function drop, and thus interacting with each other to form a bowl-wall-like band structure on the cathode surface.

According to the disclosure of the invention, the invention also provides a preparation method of the double-alkali photoelectric cathode with the bowl-shaped energy band structure, which comprises the following steps:

in a first step, at a temperature of more than 400 ℃ and above 8X 10^-4Under the condition of Pa vacuum degree, the clean glass substrate is subjected to beryllium evaporation. Specifically, the evaporation mode is resistance-type tantalum wire thermal evaporation, the film thickness adopts a blue light 532nm reflectivity monitoring mode, the thickness is controlled to be reduced by 20-45% of the reflectivity, and the corresponding evaporation time is shorter than 5 s;

and secondly, filling a mixed gas of high-purity nitrogen and hydrogen into the vacuum of the beryllium film at the temperature of more than 400 ℃, and performing arc glow discharge to form the beryllium nitride film by the beryllium film when the vacuum degree reaches about 50 Pa. Specifically, the molar ratio of nitrogen to hydrogen is required to be higher than 9:1, the arc glow discharge voltage is higher than 2kV, and the glow discharge time is higher than 5min to ensure that the metal beryllium is fully nitrided;

thirdly, transferring the beryllium nitride-plated glass substrate to cathode preparation equipment, and baking and degassing the vacuum equipment, the glass substrate and an alkali metal tellurium and antimony tellurium evaporation source at the temperature of more than 350 ℃; specifically, the whole time for heating, heat preservation and degassing is higher than 8 h;

fourthly, recording the reflectivity of the initial glass shell at the temperature of less than 200 ℃, and carrying out small-current evaporation degassing on a potassium source, a cesium source, an antimony source and a tellurium source;

fifthly, performing bottom potassium evaporation at the temperature of 120-190 ℃;

sixthly, evaporating potassium and antimony at the temperature of 120-190 ℃ simultaneously;

seventhly, performing cesium evaporation at the temperature of 110-180 ℃;

and eighthly, performing vapor deposition alternately on antimony and cesium and tellurium and cesium at the temperature of 100-170 ℃.

Specifically, when antimony-cesium and tellurium-cesium are alternately evaporated, the alternate circulation rule is as follows: and in the whole process, the cesium current is not switched off, the antimony and tellurium current switches are carried out, the photocurrent is reduced when the antimony or tellurium evaporation current is switched on, the antimony or tellurium evaporation current is switched off when the antimony or tellurium evaporation current is reduced to a half of the initial value, the photocurrent starts to rise at the moment, and the antimony or tellurium evaporation current is switched on again when the antimony or tellurium evaporation current is not changed any more to carry out the next cycle evaporation.

Ninth, tellurium evaporation is carried out at the temperature of 100-170 ℃.

Specifically, when the maximum photocurrent value of the current cycle period in the eighth step is no longer higher than that of the previous cycle period, the last tellurium evaporation is performed, and the tellurium evaporation is finished when the photocurrent is reduced to 75% of the maximum value.

In the aforementioned specific preparation process of the present invention, K is used in the third to seventh steps₂CsSb evaporation may be carried out using conventional double-base cathode preparation techniques, as well as the double-base cathode preparation method described in application No. 201710743036.2, which is incorporated herein by reference in its entirety.

With reference to FIG. 2, a double-alkali photocathode bowl-type band structure according to an embodiment of the present invention, in which photons are from the left side Be₃N₂Incident and electrons emerge from the cathode surface on the right, Be₃N₂The forbidden band width is obviously larger than K₂CsSb photocathode, again without Cs, vacuum level height E_vac1Is also significantly greater than K₂A CsSb photocathode.

Thus, Be₃N₂Film + K₂The CsSb photocathode and the Te surface doping layer form a high-low-high bowl-shaped energy band structure.

Combined graphic representation, E_c、E_vAnd E_fRespectively the positions of the bottom of a photoelectric cathode conduction band, the top of a valence band and a Fermi level; after Cs is fed, the energy band is bent downwards due to the n-type surface state formed by the Sb-Cs dipole layer, so that the surface work function is reduced, and the vacuum level is E_vac2And after tellurium-cesium alternation is carried out on the surface, the Sb-Cs dipoles and the Te-Cs dipoles exist on the surface, and the number of Cs atoms adsorbed on the surface of the cathode is reduced under the condition of the same number of negative valence atoms. Thus, the vacuum level is relative to E_vac2All rise to E_vac3To (3).

When incident light irradiates the multilayer film system of the invention, the energy of the incident light is transferred to electrons of a valence band and crosses a forbidden band to reach a conduction band, and because of the difference of the energy of the incident photons, the energy of the electrons which are left after overcoming the forbidden band width and the energy which is emitted into vacuum are different.

As shown in connection with FIG. 2, ① is a near Be₃N₂-K₂The electrons in CsSb interface have certain probability of diffusing to the interface due to isotropy and are subjected to wide forbidden band Be at the interface₃N₂② is the motion track of electrons with low energy, although it crosses the forbidden band width and reaches the conduction band, it will be affected by lattice collision, impurity scattering, etc. during the transportation to the surface, and finally it will be thermalized at the conduction band bottom, even if the low energy electrons without thermalization can reach the cathode surface, they still can not be successfully emitted into vacuum because of the existence of surface barrier, ③ is the motion track of electrons with high energy, and after they successfully reach the conduction band, they will be affected by the electric field possibly existing in the cathode, accelerate to the surface, and finally jump the barrier and emit into vacuum.

Figure 3 is a graph of the double base photocathode quantum efficiency for several different membrane systems according to embodiments of the present invention. In the figure, direct evaporation K on glass substrates is shown₂CsSb photocathode, manganese oxide + K₂CsSb photocathode beryllium nitride + K₂CsSb photocathode and beryllium nitride + K₂The quantum efficiency curves of the four film systems on the CsSb photocathode + Te doped surface respectively represent the typical shapes of the four film systems at present.

As can be seen from the figure, the peak quantum efficiency of the double-alkali photocathode can be improved to more than 30% by the manganese oxide film layer, the short-wave response of 300-400 nm is greatly improved by beryllium nitride, and the peak quantum efficiency is improved to more than 35%.

After tellurium doping is carried out on the surface of the cathode, the quantum efficiency is reduced overall, but the reduction speed of the quantum efficiency after 510nm is higher, and the cut-off wavelength is directly shifted to 620nm from the original 660nm to the short wavelength.

When the antimony-cesium alternation and the tellurium-cesium alternation are carried out, the alternation frequency of the antimony-cesium alternation and the tellurium-cesium alternation directly influences the antimony/tellurium ratio on the surface of the cathode, the cut-off wavelength is gradually shortened along with the gradual increase of the tellurium content, the peak quantum efficiency is also gradually reduced, and when the antimony-tellurium ratio is more than 6:1 by taking 6:1 as a limit, the quantum efficiency of the double-alkali cathode can be maintained at about 35% or even higher. However, if the tellurium content continues to increase, the quantum efficiency is greatly reduced to 60% of that of the case of no doping of tellurium, and when the ratio of antimony to tellurium is 1:1, the quantum efficiency is only about 40% of the original quantum efficiency, and at this time, even if the cut-off wavelength is smaller than 600nm and the practical significance is lost, the cathode dark noise is multiplied, which is mainly caused by the fact that the doping is too much to cause the large increase of in vivo defects.

Therefore, in the eighth step, the alternation times of antimony and cesium and the alternation times of tellurium and cesium are strictly controlled to be more than 6: 1.

The quantum efficiency of the photocathode of the above exemplary embodiment of the present invention reaches 35% or more at the peak position, and the cut-off wavelength is 620nm, which is significantly lower than that obtained by other manufacturing processes. The invention utilizes the mode of improving work function and shortening cut-off wavelength to be beneficial to reducing the quantity of cathode hot electrons escaping into vacuum, and simultaneously can reduce cathode dark noise to a certain extent, thereby being worthy of obtaining a cathode with more excellent performance.

Although the present invention has been described with reference to the preferred embodiments, it is not intended to be limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, the protection scope of the present invention should be determined by the appended claims.