阅读说明：本技术 一种基于热电堆的无线温度传感器 (Wireless temperature sensor based on thermopile ) 是由 张巧杏 张国珠 袁* 姜岩峰 于 2020-03-28 设计创作，主要内容包括：本发明涉及一种基于热电堆的无线温度传感器,其特征在于包括基体,所述基体的上下表面分别设置有上层氮化硅和下层氮化硅,所述上层氮化硅的设置有四排多晶硅条组,四排多晶硅条组形成口字形结构,每排多晶硅条组包括并排布置的多根多晶硅条,所述上层氮化硅的表面设置有多根金属条,多根金属条将所有的多晶硅条串联,在上层氮化硅的表面设置有热阻层,热阻层的中心为矩形的空腔,在该空腔处露出矩形的上层氮化硅,该处的上层氮化硅形成基于热电堆的无线温度传感器的红外辐射区。本发明得到了较为稳定的介质膜支撑方案,显著提高了成品率,并且通过该工艺得到的热电堆红外探测器,其性能相对于传统的热电堆探测器得到了较为明显的提高。(The invention relates to a thermopile-based wireless temperature sensor which is characterized by comprising a substrate, wherein the upper surface and the lower surface of the substrate are respectively provided with an upper layer of silicon nitride and a lower layer of silicon nitride, the upper layer of silicon nitride is provided with four rows of polycrystalline silicon strip groups, the four rows of polycrystalline silicon strip groups form a square structure, each row of polycrystalline silicon strip group comprises a plurality of polycrystalline silicon strips which are arranged side by side, the surface of the upper layer of silicon nitride is provided with a plurality of metal strips, all the polycrystalline silicon strips are connected in series by the plurality of metal strips, the surface of the upper layer of silicon nitride is provided with a thermal resistance layer, the center of the thermal resistance layer is a rectangular cavity, the rectangular upper layer of silicon nitride is exposed at the cavity, and the upper layer of silicon nitride at the cavity forms an infrared radiation region of the thermopile-based wireless temperature sensor. The invention obtains a stable dielectric film supporting scheme, obviously improves the yield, and compared with the traditional thermopile detector, the performance of the thermopile infrared detector obtained by the process is obviously improved.)

1. A wireless temperature sensor based on a thermopile is characterized by comprising a base body (1), wherein the base body (1) is a rectangular P-type silicon substrate, the upper surface and the lower surface of the base body (1) are respectively provided with an upper layer silicon nitride (2) and a lower layer silicon nitride (3), the upper layer silicon nitride (2) is provided with four rows of polysilicon strip groups, the polysilicon strip groups form P-type electrodes, the four rows of polysilicon strip groups form a square structure, each row of polysilicon strip group comprises a plurality of polysilicon strips (4) which are arranged side by side, the surface of the upper layer silicon nitride (2) is provided with a plurality of metal strips (5), the metal strips (5) form metal electrodes, the plurality of metal strips (5) are used for serially connecting all the polysilicon strips (4), the input ends and the output ends of the metal strips (5) are formed at one corner of the whole chip, the surface of the upper layer silicon nitride (2) is provided with a thermal resistance layer (6), the thermal resistance layer (6) is of a rectangular structure, all the polycrystalline silicon strips (4) and the metal strips (5) are covered by four sides of the rectangular structure, a rectangular cavity is formed in the center of the thermal resistance layer (6), the rectangular upper layer silicon nitride (2) is exposed at the cavity, and the upper layer silicon nitride (2) at the cavity forms an infrared radiation area (9) of the thermopile-based wireless temperature sensor.

2. A thermopile-based wireless temperature sensor according to claim 1, in which each polysilicon strip (4) is perpendicular to its respective near-by edge of the ir-radiating region (9).

3. The thermopile-based wireless temperature sensor according to claim 2, wherein the end of the polysilicon strip (4) close to the infrared radiation region (9) is a hot end, the end of the polysilicon strip (4) far away from the infrared radiation region (9) is a cold end, and the surface of the thermal resistance layer (6) is provided with a reflective layer covering the position right above the cold end.

4. A thermopile-based wireless temperature sensor according to claim 1, in which the reflective layer comprises four reflective strips (7), the four reflective strips (7) being located directly above the cold ends of the polysilicon strips (4) of the four rows of polysilicon strip groups (40).

5. A thermopile-based wireless temperature sensor according to claim 4, in which the reflecting strip (7) is formed by sputtering titanium on the thermal resistance layer (6).

6. A thermopile-based wireless temperature sensor in accordance with claim 1, wherein the substrate (1) and the middle section of the underlying silicon nitride (3) are etched to form a window (8) disposed longitudinally therethrough, the window (8) being located below the infrared radiation region (9).

7. A thermopile-based wireless temperature sensor according to claim 1, in which the window (8) has an isosceles trapezoid cross-section.

8. A thermopile-based wireless temperature sensor according to claim 1, in which each row of polysilicon strip groups comprises four to fifty polysilicon strips (4) arranged side by side.

9. A thermopile-based wireless temperature sensor according to claim 1, in which the metal strip (5) is of aluminium.

10. A thermopile-based wireless temperature sensor according to claim 1, characterized in that the material of the base body (1) is a P-type silicon substrate with a crystal orientation <100 >.

Technical Field

The invention belongs to the technical field of detectors, and relates to a process design of a thermopile infrared detector.

Background

The infrared detector can detect electromagnetic waves in an infrared region, has a wavelength range of 0.75-1000 microns, and has wide civil and military values. The infrared thermopile detector converts an infrared radiation signal into an electric signal by utilizing a Seebeck effect and outputs the electric signal, and compared with a common infrared detector, the micro-mechanical infrared thermopile detector has the advantages of small volume, light weight and capability of working at room temperature; has high sensitivity and very wide spectral response; compatible with standard IC process, low cost and suitable for batch production. The device can be widely applied to non-contact temperature measurement, power meters, infrared alarms, frequency spectrometers, automatic switches, medical gas analyzers and the like.

Referring to fig. 1, the thermopile infrared detector is designed according to the seebeck effect, and is formed by forming a thermocouple pair by using materials having large seebeck coefficient differences, wherein one end of the thermocouple pair is a hot end and has a seebeck coefficient of α a for receiving infrared radiation capability, and the other end of the thermocouple pair is a cold end and has a seebeck coefficient of α B for producing temperature difference and generating voltage.

The resulting potential difference is therefore related to temperature by:

V_out＝(α_A-α_B)ΔT

thermopile infrared detectors generally consist of three parts: thermocouple, medium supporting layer, radiating substrate. There are generally three configurations of thermopile infrared detectors:

1. a closed membrane structure;

2. a cantilever beam structure;

3. a suspended structure.

Compared with the other two structures, the thermopile infrared detector with the closed membrane structure has the minimum thermal resistance and the response time is also the shortest; the thermopile infrared detector of the suspension structure has the highest thermal resistance and the longest response time; the performance of the cantilever beam structure is between the two structures. From the process point of view, the detector with the closed membrane structure is easiest to prepare, and the preparation process of the detector with the cantilever beam structure is most complicated.

Although the thermopile infrared detector with a closed membrane structure in the current market has a simple structure, the yield is low, the process cost and the design difficulty of the detector adopting a cantilever beam structure can be increased greatly, and the obtaining of high performance on the thermopile infrared detector with the closed membrane structure through improving the process on the premise of not increasing the process cost is a main direction of the current design.

Disclosure of Invention

The invention aims to overcome the problems and provides a wireless temperature sensor based on a thermopile, which adopts a closed membrane structure, the closed membrane structure is relatively simple, and the production yield is improved by optimizing the process.

In order to solve the technical problems, the invention provides the following technical scheme:

a wireless temperature sensor based on a thermopile is characterized by comprising a base body, wherein the base body is a rectangular P-type silicon substrate, the upper surface and the lower surface of the base body are respectively provided with upper-layer silicon nitride and lower-layer silicon nitride, the upper-layer silicon nitride is provided with four rows of polycrystalline silicon strip groups, the polycrystalline silicon strip groups form P-type electrodes, the four rows of polycrystalline silicon strip groups form a square structure, each row of polycrystalline silicon strip group comprises a plurality of polycrystalline silicon strips which are arranged side by side, the surface of the upper-layer silicon nitride is provided with a plurality of metal strips, the metal strips form metal electrodes, the plurality of metal strips connect all the polycrystalline silicon strips in series, an input end and an output end of each metal strip are formed at one corner of a whole chip, a thermal resistance layer is arranged on the surface of the upper-layer silicon nitride, the thermal resistance layer is of a rectangular structure, all the polycrystalline silicon strips and the metal strips are covered by four sides of the rectangular structure, a rectangular upper layer of silicon nitride is exposed at the cavity, where the upper layer of silicon nitride forms the infrared radiation region of the thermopile-based wireless temperature sensor.

Preferably, each polysilicon strip is perpendicular to the edge of the infrared radiation region adjacent to the polysilicon strip.

Preferably, one end, close to the infrared radiation region, of the polycrystalline silicon strip is a hot end, one end, far away from the infrared radiation region, of the polycrystalline silicon strip is a cold end, a reflecting layer is arranged on the surface of the thermal resistance layer, and the reflecting layer covers the position right above the cold end.

Preferably, the reflective layer comprises four reflective strips, and the four reflective strips are positioned right above the cold ends of the polysilicon strips of the four-row polysilicon strip group.

Preferably, the reflective strips are formed by sputtering titanium on the thermal resistance layer.

Preferably, the middle sections of the substrate and the lower layer of silicon nitride are etched to form a window which is longitudinally arranged in a penetrating mode, and the window is located below the infrared radiation region.

Preferably, the window has an isosceles trapezoid cross section.

Preferably, each row of polysilicon strip groups includes four to fifty polysilicon strips arranged side-by-side.

Preferably, the metal strip is made of aluminum.

Preferably, the material of the base is a P-type silicon substrate with a crystal orientation of <100 >.

The invention has the beneficial effects that:

according to the invention, a stable dielectric film supporting scheme is obtained by optimizing the process, and a series of process optimization schemes such as a wax sealing scheme is adopted to protect the silicon substrate during back cavity corrosion, so that the yield is obviously improved, and compared with the traditional thermopile detector, the performance of the thermopile infrared detector obtained by the process is obviously improved.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention.

Fig. 1 is a schematic diagram of the seebeck effect.

Fig. 2 is a schematic diagram of a thermopile-based wireless temperature sensor in accordance with the present invention.

FIG. 3 is a cross-sectional view of a semi-finished product of a first step of the thermopile-based wireless temperature sensor fabrication of the present invention.

FIG. 4 is a top view of a semi-finished product of a first manufacturing step of a thermopile-based wireless temperature sensor of the present invention.

FIG. 5 is a cross-sectional view of a semi-finished product of a second step of manufacturing a thermopile-based wireless temperature sensor in accordance with the present invention.

FIG. 6 is a top view of a semi-finished product of a second step of manufacturing a thermopile-based wireless temperature sensor in accordance with the present invention.

Fig. 7 is a cross-sectional view of a semi-finished product of a third production step (cross-sectional view at the polysilicon strip) of a thermopile-based wireless temperature sensor of the present invention.

FIG. 8 is a top view of a semi-finished product of a third step of the thermopile-based wireless temperature sensor fabrication of the present invention.

Fig. 9 is a cross-sectional view (cross-sectional view at the metal strip) of a semi-finished product of a production step four of a thermopile-based wireless temperature sensor of the present invention.

FIG. 10 is a top view of a semi-finished product of a fourth step of the manufacturing of a thermopile-based wireless temperature sensor of the present invention.

Fig. 11 is a cross-sectional view (cross-sectional view at the metal strip) of a semi-finished product of a production step five of a thermopile-based wireless temperature sensor of the present invention.

FIG. 12 is a top plan view (not actually visible in phantom) of a semi-finished product of manufacturing step five of a thermopile-based wireless temperature sensor of the present invention.

Fig. 13 is a cross-sectional view (cross-sectional view at the metal strip) of a semi-finished product of a manufacturing step six of a thermopile-based wireless temperature sensor of the present invention.

FIG. 14 is a top plan view (not actually visible in phantom) of a semi-finished product of a sixth manufacturing step of a thermopile-based wireless temperature sensor of the present invention.

Fig. 15 is a sectional view (sectional view at a metal bar) of a semi-finished product of a production step seven of a thermopile-based wireless temperature sensor of the present invention.

FIG. 16 is a top plan view (not actually visible in phantom) of a semi-finished product of a seventh manufacturing step of a thermopile-based wireless temperature sensor of the present invention.

Wherein:

the solar cell comprises a substrate 1, an upper layer of silicon nitride 2, a lower layer of silicon nitride 3, a polycrystalline silicon strip 4, a metal strip 5, a thermal resistance layer 6, a reflecting strip 7, a window 8, an infrared radiation region 9 and a polycrystalline thin film 40.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments, and all other embodiments obtained by a person of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the protection scope of the present invention.

Referring to fig. 2, a thermopile-based wireless temperature sensor includes a base 1, the base 1 is a rectangular P-type silicon substrate, silicon nitride grown on the upper and lower surfaces of the base 1 respectively form an upper layer silicon nitride 2 and a lower layer silicon nitride 3, polysilicon grown on the surface of the upper layer silicon nitride 2 is then subjected to boron implantation and etching to form four rows of polysilicon strip groups, the polysilicon strip groups form P-type electrodes, the four rows of polysilicon strip groups form a square-shaped structure, each row of polysilicon strip group includes four to fifty polysilicon strips 4 arranged side by side, metal sputtered on the surface of the upper layer silicon nitride 2 is then etched to form a plurality of metal strips 5, the metal strips 5 form metal electrodes, the metal strips 5 are preferably made of aluminum, the plurality of metal strips 5 connect all the polysilicon strips 4 in series, and form input ends and output ends of the metal strips 5 at one corner of the whole chip, the surface of the upper layer silicon nitride 2 is provided with a thermal resistance layer 6, the thermal resistance layer 6 is of a rectangular structure, four sides of the rectangular structure cover all the polysilicon strips 4 and the metal strips 5, the center of the thermal resistance layer 6 is of a rectangular cavity structure, the rectangular upper layer silicon nitride 2 is exposed at the cavity, the upper layer silicon nitride 2 at the position forms an infrared radiation area 9 of the thermopile-based wireless temperature sensor, each polysilicon strip 4 is respectively vertical to the edge of the infrared radiation area 9 close to the polysilicon strip 4, therefore, one end of the polysilicon strip 4 close to the infrared radiation area 9 is a hot end, one end of the polysilicon strip 4 far away from the infrared radiation area 9 is a cold end, the surface of the thermal resistance layer 6 is provided with a reflection layer, the reflection layer comprises four reflection strips 7, the reflection strips 7 are formed by sputtering titanium on the thermal resistance layer 6, the four reflection strips 7 are positioned right above the cold ends of the polysilicon strips 4 of the four rows of polysilicon strip groups, the middle sections of the substrate 1 and the lower silicon nitride 3 are etched to form a window 8 which is longitudinally arranged in a penetrating manner, the cross section of the window 8 is isosceles trapezoid, and the window 8 is positioned below the infrared radiation area 9.

Preferably, the material of the substrate 1 has a crystal orientation of<100>The size of the base body 1 is 3 × 3 mm, the thickness of the base body 1 is about 300-400 microns, and the doping concentration is 10¹⁴/cm³To 10¹⁶/cm³The P-type electrode is made of a P-type polycrystalline silicon semiconductor, the thickness of the polycrystalline silicon strip 4 is 0.5-2 micrometers, the typical value is 1.5 micrometers, the thickness of the metal strip 5 is 0.1-1 micrometer, the typical value is 0.5 micrometer, the thermal resistance layer 6 is a silicon oxide layer with the thickness of 0.1-0.5 micrometer, the typical value is 0.2 micrometer, and the P-type electrode is used for isolating the influence of the ambient temperature on the cold end.

A production process of the thermopile-based wireless temperature sensor comprises the following steps:

growing silicon nitride on the upper surface and the lower surface of a substrate;

cleaning a substrate before growing silicon nitride;

when cleaning the substrate, cleaning the substrate with the third liquid and the first liquid in a cleaning room, and then rinsing the substrate with diluted hydrofluoric acid to remove impurities on the surface; the third liquid is prepared from the following components in percentage by volume H₂SO₄∶H₂O₂The ratio of NH to the first liquid is 4: 1₃·H₂O∶H₂O₂∶H₂O is 1: 4, and the volume ratio of the diluted hydrofluoric acid is HF: H₂O＝1∶20；

When growing silicon nitride on the upper surface and the lower surface of the substrate, growing silicon nitride by L PCVD (low pressure chemical vapor deposition method) to form an upper layer of silicon nitride and a lower layer of silicon nitride, wherein the thicknesses of the upper layer of silicon nitride and the lower layer of silicon nitride are both 5000 angstroms, the upper layer of silicon nitride provides a dielectric film support for an infrared radiation area, and the dielectric film of the upper layer of silicon nitride forms a closed film;

the semi-finished product formed in the first step is shown in fig. 3-4;

growing polycrystalline silicon, injecting boron and annealing;

continuously growing polysilicon on the upper silicon nitride surface by L PCVD, wherein the thickness of the polysilicon is 1.5 microns, and simultaneously doping boron on the polysilicon to obtain a P-type polycrystalline film 40, wherein the implantation dose is 1.5E16, and the implantation energy is 50 KeV;

then cleaning the silicon wafer by using the third liquid and the first liquid;

then annealing in a high-temperature furnace, repairing the lattice defects and activating impurities, wherein the annealing temperature is 1000 ℃, and the annealing time is 20 minutes;

the semi-finished product formed in the second step is shown in FIGS. 5-6;

step three, photoetching a first plate to form four rows of polycrystalline silicon strip groups, wherein the four rows of polycrystalline silicon strip groups form a square structure, and each row of polycrystalline silicon strip group comprises a plurality of polycrystalline silicon strips which are arranged side by side;

coating a tackifier before photoetching a first plate to ensure that photoresist is better attached to polycrystalline silicon, and baking the wafer for 1 hour at 180 ℃; removing moisture adsorbed on the surface, fully drying and keeping clean;

then, photoetching a first plate, and rotationally coating a photoresist, wherein the photoresist is 9920 in type;

pre-baking for 1 minute at 95 ℃ on a hot plate;

adopting proximity exposure of a positive plate, wherein the dosage is 10;

developing for 60 seconds;

scanning the basement membrane for 8 minutes in an oxygen environment;

etching (reactive ion etching) polysilicon by using RIE (reactive ion etching), wherein the thickness of the polysilicon is 1.5 microns, each row of polysilicon strip group comprises a plurality of polysilicon strips arranged side by side, preferably each row of polysilicon strip group comprises four to fifty polysilicon strips arranged side by side, and more preferably, 4 × 8 polysilicon strips are formed in figure 6, the length of each polysilicon strip is 900 microns, and the width of each polysilicon strip is 35 microns;

the semi-finished product formed in the third step is shown in fig. 7-8;

sputtering metal, photoetching a second plate, and etching the metal by a wet method;

removing glue and cleaning before metal sputtering, sequentially adopting a third liquid, a third liquid and a first liquid to carry out three times of cleaning, and then adopting diluted hydrofluoric acid to rinse to remove a natural oxide layer;

then generating a 5000 angstrom metal layer on the upper surface of the semi-finished product in the last step by magnetron sputtering, wherein the sputtered metal is preferably metal aluminum;

then, photoetching a second plate, rotationally coating No. 9920 photoresist, and carrying out exposure and development under the same conditions in the steps;

the metal layer was then wet etched at 50 ℃ for 2 minutes to yield 4 × 8 metal strips 900 microns in length and 35 microns in width, the number of which is shown in FIG. 6;

then sequentially cleaning by adopting positive photoresist film removing agents; cleaning with acetone and ethanol; washing with deionized water;

then heating at 400 ℃ for 40 minutes to form ohmic contact between the metal strips and the polysilicon strips;

the semi-finished product formed in the fourth step is shown in fig. 9-10;

step five, passivation treatment, and photoetching of a third plate;

firstly, carrying out passivation treatment by PECVD (plasma enhanced chemical vapor deposition), and growing a 2000 angstrom thermal resistance layer on the upper surface of the semi-finished product in the last step, wherein the thermal resistance layer is preferably a silicon oxide layer;

then, photoetching a third plate, rotationally coating No. 9920 photoresist, and carrying out exposure and development under the same conditions as the steps;

then RIE is adopted to etch the thermal resistance layer, and the central infrared radiation area and the peripheral bonding pads are exposed; the side length of the infrared radiation area is 800 micrometers;

the semi-finished product formed in the fifth step is shown in fig. 11-12;

sputtering titanium, stripping, and forming a reflecting layer on the cold end;

carrying out magnetron sputtering on titanium and then stripping, wherein the thickness is 1000 angstroms, and a reflecting layer is formed at the cold end, so that the influence of the ambient temperature on the cold end is further prevented;

the semi-finished product formed in the sixth step is shown in fig. 13-14;

step seven, wet etching is carried out to etch out a window

The width of the lower edge of the window formed by etching on the back surface of the chip is 2600 micrometers; the width of the upper edge of the window is slightly larger than the transverse dimension of the infrared radiation area;

firstly, setting the temperature of a hot plate to 160 ℃, then placing a 5-inch glass plate above the glass plate, and coating CRY sealing wax on the glass plate, wherein the sealing wax has the advantages that the sealing wax can be removed by using acetone and ethanol, and the sealing wax has the defect of weak alkali resistance;

then, the whole 4-inch silicon wafer is placed on a glass plate with the front side facing downwards, so that the front side of the silicon wafer is fully contacted with CRY sealing wax, bubbles between the silicon wafer and the sealing wax are removed, and the protection effect is enhanced;

after the position of the silicon wafer is determined, a large amount of APIE sealing wax is smeared on the periphery of the silicon wafer, so that the side surface of the silicon wafer is further protected to avoid underetching;

putting the silicon wafer into a KOH solution with the temperature of 70 ℃ and the concentration of 33.3 percent, carrying out wet etching, fishing out the silicon wafer for a plurality of times in the process, and supplementing wax;

after the corrosion is finished, removing APIE sealing wax on the periphery of the silicon wafer by using trichloroethane (the wax has the defect that the wax is not easy to remove completely), then placing a glass plate on a hot plate, taking down the silicon wafer at 160 ℃, and placing the silicon wafer in acetone ethanol for soaking;

the chip is taken out to complete the preparation, and the final product is shown in figure 2.

The following steps can be further carried out between the fifth step and the sixth step:

photoetching a fourth plate;

firstly, sequentially carrying out acetone ultrasonic treatment, ethanol ultrasonic treatment and deionized water cleaning to remove the photoresist;

baking at 180 deg.C for one hour before slicing;

photoetching a fourth plate, and spin-coating 5214 photoresist;

pre-baking for 3.5 minutes at 85 ℃ on a hot plate;

adopting proximity exposure of a positive plate, wherein the dosage is 3;

developing for 5 minutes;

between step six and step seven there may be the following steps:

photolithography fifth edition

Firstly, ultrasonically stripping the photoresist;

and photoetching a fifth plate, coating 9920 # photoresist on the back surface of the chip in a rotating mode, and carrying out exposure and development under the same conditions of the three steps.

According to the thermocouple strip pair, the Seebeck principle is utilized, the polycrystalline silicon strip and the metal strip are used as thermocouple strip pairs to form a plurality of pairs of thermocouple strips, and high detection rate, high response rate and low corresponding time are obtained under the optimized process.

The response rate, detection rate and response time of the detector are the criteria that usually measure the performance of the detector.

1. Response rate R

R＝U/P(V/W)

The responsivity is defined as the ratio of the output voltage to the input infrared radiation capability.

2. Detection rate D

The detection rate is defined as the signal-to-noise ratio obtained per unit bandwidth of the amplification circuit, with a unit radiation power acting per unit area of the detector.

Wherein S is_aIs the area of the infrared absorption region of the detector, U_noiseFor the noise of the detector, Δ f is the equivalent noise bandwidth of the test system.

3. Response time tau

T＝C/G

The response time of the detector is inversely proportional to the total thermal conductance G of the detector and directly proportional to the heat capacity C.

According to the invention, a stable dielectric film supporting scheme is obtained by optimizing the process, and a series of process optimization schemes such as a wax sealing scheme is adopted to protect the silicon substrate during back cavity corrosion, so that the yield is obviously improved, and compared with the traditional thermopile detector, the performance of the thermopile infrared detector obtained by the process is obviously improved.

The above embodiments are preferred embodiments of the present invention, and those skilled in the art can make variations and modifications to the above embodiments, therefore, the present invention is not limited to the above embodiments, and any obvious improvements, substitutions or modifications made by those skilled in the art based on the present invention are within the protection scope of the present invention.