阅读说明：本技术 晶硅切割方法 (Crystal silicon cutting method ) 是由 王朝成 何家冰 刘亮 于 2021-09-30 设计创作，主要内容包括：本申请实施例涉及光伏技术领域,公开了一种晶硅切割方法,在切割晶硅体之前,包括如下步骤：将辅材板通过粘胶层固定在晶硅体上；其中,辅材板包括层叠设置的第一材料板和第二材料板,第一材料板与粘胶层固定,第一材料板的邵氏硬度为85度至95度,第一材料板的厚度为0.1毫米至7毫米。本申请实施例提供的晶硅切割方法有利于提高晶硅的切割效率。(The embodiment of the application relates to the technical field of photovoltaics, and discloses a crystalline silicon cutting method, which comprises the following steps of: fixing the auxiliary material plate on the crystal silicon body through the adhesive layer; the auxiliary material plate comprises a first material plate and a second material plate which are stacked, the first material plate is fixed with the adhesive layer, the Shore hardness of the first material plate is 85 degrees to 95 degrees, and the thickness of the first material plate is 0.1 millimeter to 7 millimeters. The crystalline silicon cutting method provided by the embodiment of the application is beneficial to improving the cutting efficiency of crystalline silicon.)

1. A crystal silicon cutting method is characterized by comprising the following steps before cutting a crystal silicon body:

fixing an auxiliary material plate on the crystalline silicon body through an adhesive layer;

the auxiliary material plate comprises a first material plate and a second material plate which are stacked, the first material plate is fixed with the adhesive layer, the Shore hardness of the first material plate is 85-95 degrees, and the thickness of the first material plate is 0.1-7 mm; and/or

The viscose layer is including being located first viscose layer and two second viscose layers on the coplanar, two the second viscose layer sets up along first direction relatively the both sides on first viscose layer, two the shore hardness on second viscose layer is 80 degrees to 100 degrees.

2. The crystalline silicon dicing method according to claim 1, characterized in that:

the Shore hardness of the first material plate is larger than or equal to that of the second material plate.

3. The crystalline silicon dicing method according to claim 1, characterized in that:

the sum of the thicknesses of the first material plate and the second material plate is 10 mm to 25 mm.

4. The crystalline silicon dicing method according to claim 3, characterized in that:

the sum of the thicknesses of the first material plate and the second material plate is 18 mm to 25 mm.

5. The crystalline silicon dicing method according to claim 1, characterized in that:

the auxiliary material plate further comprises a third material plate, the third material plate is stacked on one side, away from the first material plate, of the second material plate, and the Shore hardness of the third material plate is the same as that of the first material plate.

6. The crystalline silicon dicing method according to any one of claims 1 to 5, characterized in that:

the first material plate is made of one or more of melamine, polyester resin or epoxy resin, and the second material plate is made of one or more of melamine, polyester resin or epoxy resin.

7. The crystalline silicon dicing method according to claim 1, characterized in that:

the shore hardness of the two second adhesive layers is 85 degrees to 95 degrees, and the shore hardness of the two second adhesive layers is greater than or equal to that of the first adhesive layers.

8. The crystalline silicon dicing method according to claim 1, characterized in that:

the length of each second adhesive layer along the first direction accounts for 5-15% of the length of the crystal silicon body along the first direction.

9. The crystalline silicon dicing method according to claim 1, characterized in that:

the thickness of the first adhesive layer and/or the second adhesive layer is 0.3 to 0.7 mm.

10. The crystalline silicon dicing method according to any one of claims 7 to 9, characterized in that:

the material of first viscose layer includes one or more in melamine, epoxy, polyimide or polybenzimidazole, the material of second viscose layer includes one or more in melamine, epoxy, polyimide or polybenzimidazole.

Technical Field

The embodiment of the application relates to the technical field of photovoltaics, in particular to a crystalline silicon cutting method.

Background

The current energy demand is huge, and the traditional energy is because the reserves are limited, be unfavorable for environmental protection scheduling problem, and the demand descends gradually, and the demand of clean low-cost energy increases with each day, and in the clean energy, solar photovoltaic has high-efficient low-cost advantage, and possess bigger development prospect than other wind energy, tidal energy etc..

In the field of solar photovoltaic, crystalline silicon and other materials are mainly used for manufacturing solar cells, and crystalline silicon is used as a semiconductor material with excellent performance, is widely applied to solar photovoltaic cell panels and can achieve high energy conversion efficiency. The raw silicon material can be produced in an ingot furnace as a multicrystalline silicon ingot or a single crystal is pulled in a single crystal furnace by the czochralski method, and the prepared silicon rod can then be sliced.

In the silicon crystal slicing process, a silicon rod is generally fixed to an inner top end of a slicing machine, and a diamond wire is wound around a main roller in the slicing machine. In the cutting process, the diamond wire rotates at a high speed under the drive of the main roller, and the diamond wire is extruded through the descending of the silicon rod, so that the purpose of cutting the silicon rod to produce a silicon wafer is achieved, but the cutting time is longer when the current silicon rod is cut, and the cutting efficiency of the crystalline silicon is not improved.

Disclosure of Invention

The embodiment of the application aims to provide a crystalline silicon cutting method which is beneficial to improving the cutting efficiency of crystalline silicon.

In order to solve the above technical problem, an embodiment of the present application provides a method for cutting crystalline silicon, including the following steps between the cutting of a crystalline silicon body:

fixing an auxiliary material plate on the crystalline silicon body through an adhesive layer;

the auxiliary material plate comprises a first material plate and a second material plate which are stacked, the first material plate is fixed with the adhesive layer, the Shore hardness of the first material plate is 85-95 degrees, and the thickness of the first material plate is 0.1-7 mm; and/or

The viscose layer is including being located first viscose layer and two second viscose layers on the coplanar, two the second viscose layer sets up along first direction relatively the both sides on first viscose layer, two the shore hardness on second viscose layer is 80 degrees to 100 degrees.

According to the crystalline silicon cutting method provided by the embodiment of the application, the first material plate with the Shore hardness of 85-95 degrees is adopted in the auxiliary material plate, or the second adhesive layer with the Shore hardness of 80-100 degrees is adopted in the adhesive layer, or the first material plate with the Shore hardness of 85-95 degrees is adopted in the auxiliary material plate and the second adhesive layer with the Shore hardness of 80-100 degrees is adopted in the adhesive layer, so that the Shore hardness of the first material plate and/or the Shore hardness of the second adhesive layer is close to the Shore hardness of a crystalline silicon body, the phenomenon that the diamond wire is subjected to wire arch caused by the sudden change of hardness in the cutting process is reduced, the falling speed of the crystalline silicon body is not required to be reduced when the diamond wire enters the tail receiving stage of the crystalline silicon body cutting, and the cutting efficiency of the crystalline silicon is improved.

Drawings

One or more embodiments are illustrated by way of example in the accompanying drawings, which correspond to the figures in which like reference numerals refer to similar elements and which are not to scale unless otherwise specified.

FIG. 1 is a schematic illustration of a silicon rod during slicing;

FIG. 2 is a schematic diagram of a wire bow phenomenon occurring when a diamond wire is cut to the ending stage of a silicon rod;

fig. 3 is an exploded view of a first crystalline silicon slice structure adopted in the crystalline silicon cutting method according to the embodiment of the present application;

FIG. 4 is a schematic structural view of an auxiliary material plate in the crystal silicon sliced structure shown in FIG. 3;

fig. 5 is an exploded view of a second crystalline silicon slice structure adopted in the crystalline silicon cutting method according to the embodiment of the present application;

FIG. 6 is a schematic view of the structure of the adhesive layer in the sliced crystal silicon structure of FIG. 5;

fig. 7 is an exploded view of a third crystalline silicon slice structure adopted in the crystalline silicon cutting method according to the embodiment of the present application;

fig. 8 is an exploded view of a fourth crystal silicon slice structure adopted in the crystal silicon cutting method according to the embodiment of the present application;

FIG. 9 is a schematic structural view of an auxiliary material plate in the crystal silicon sliced structure shown in FIG. 8;

fig. 10 is an exploded view of a fifth crystalline silicon slice structure adopted in the crystalline silicon cutting method according to the embodiment of the present application;

fig. 11 is an exploded view of a sixth crystalline silicon slice structure adopted in the crystalline silicon dicing method according to the embodiment of the present application.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the following describes each embodiment of the present application in detail with reference to the accompanying drawings. However, it will be appreciated by those of ordinary skill in the art that numerous technical details are set forth in various embodiments of the present application in order to provide a better understanding of the present application. However, the technical solution claimed in the present application can be implemented without these technical details and various changes and modifications based on the following embodiments. The following embodiments are divided for convenience of description, and should not constitute any limitation to the specific implementation manner of the present application, and the embodiments may be mutually incorporated and referred to without contradiction.

At present, when a silicon rod is cut by adopting diamond wires (namely, diamond cutting wires), firstly, an auxiliary material plate is bonded on the silicon rod (namely, a crystalline silicon body) through a glue material, secondly, the silicon rod is fixed at the top end of the inside of a slicing machine table through the auxiliary material plate, and the silicon rod is controlled to descend through the diamond wires rotating at a high speed, so that the slicing process of the silicon rod can be carried out.

As shown in fig. 1, the diamond wire 400 is wound around the main roller 500 and travels in the direction of arrow a of fig. 1 by the main roller 500, and the silicon rod 300 presses the diamond wire 400 in the process of descending in the direction of arrow B of fig. 1, thereby being cut by the diamond wire 400 to produce a silicon wafer.

However, as shown in fig. 2, when the diamond wire 400 is cut to the adhesive layer 200 and the auxiliary material plate 100, i.e., when the silicon rod 300 is cut in the ending stage, the diamond wire 400 has an increasingly obvious wire bow phenomenon, i.e., the diamond wire 400 changes from a straight shape to an arc shape, and the degree of curvature of the arc shape is increased. The reason why the diamond wire 400 is aggravated in the ending stage of cutting the silicon rod 300 is that the hardness of the auxiliary material plate 100 and the adhesive material is far less than that of the silicon rod 300, so that when the diamond wire 400 cuts the adhesive layer 200 and the auxiliary material plate 100 at the two sides of the silicon rod 300, the cutting resistance of the diamond wire 400 at the two sides of the silicon rod 300 is instantly reduced, the cutting resistance of the diamond wire 400 at the two sides of the silicon rod 300 is smaller, the wire bowing phenomenon of the diamond wire 400 is more obvious, the force for pulling the diamond wire 400 upwards is larger at this time, and the defect of the silicon wafer is more serious. In addition, when both sides of the silicon rod 300 are cut completely, the middle portion has a large distance without cutting completely, the diamond wire 400 continues to be cut as the silicon rod 300 continues to descend, and when the diamond wire 400 is not cut completely to the last middle point of the silicon rod 300, the wire bow of the diamond wire 400 reaches the maximum value at this time, and the force of pulling up the diamond wire 400 reaches the maximum value, in this case, the last point in the middle of the silicon rod 300 is not cut completely but is pulled up by the diamond wire 400, which easily causes a defect that the produced silicon wafer is chipped at the position, and also reduces the service life of the diamond wire 400.

Therefore, currently, in order to solve the problem of poor silicon wafers, when the diamond wire 400 is as fast as the ending stage of the cutting of the silicon rod 300, the descending speed of the silicon rod 300 must be reduced to prevent the wire bow of the diamond wire 400 from being too large, so as to ensure the quality yield of the silicon wafers, but as such, the cutting time of the silicon rod 300 is longer, which may seriously affect the cutting efficiency of the silicon rod 300, for example, in a specific case of the silicon rod 300 shown in fig. 1, the cutting time of the main body part is up to 75 minutes, but the cutting time of the ending stage is up to 25 minutes, that is, the cutting time of the ending stage of the silicon rod 300 reaches 1/3 of the cutting time of the main body part.

In the embodiment of the present application, the hardness of the auxiliary material plate 100 and/or the adhesive layer 200 is increased to make the hardness of at least one of the two approach the hardness of the silicon rod 300, so that when the adhesive layer 200 and the auxiliary material plate 100 are cut at two sides of the silicon rod 300, the situation of sudden change of the resistance to the diamond wire 400 does not occur, and the phenomenon of wire bow of the diamond wire 400 is aggravated due to the large difference between the hardness of the auxiliary material plate 100 and the adhesive layer 200 and the hardness of the silicon rod 300 is alleviated, so that the descending speed of the silicon rod 300 in the ending stage does not need to be reduced, and the cutting efficiency of the silicon rod 300 is improved. For example, also the silicon rod 300 shown in fig. 1, after the hardness of the auxiliary material plate 100 and/or the adhesive layer 200 is increased, the cutting process in the ending stage can be completed in only 5 minutes, and the cutting time is reduced by 4/5, which greatly improves the cutting efficiency of the crystalline silicon.

It should be noted that, here, the hardness of the auxiliary material plate 100 and the adhesive layer 200 is increased to be close to the hardness of the silicon rod 300 at the same time, so that it is possible to prevent the diamond wire 400 from generating a sudden change in the cutting resistance when the auxiliary material plate 100 and the adhesive layer 200 are cut on both sides of the silicon rod 300. The hardness of the auxiliary material plate 100 or the adhesive layer 200 is increased to make the hardness of one of the auxiliary material plate 100 and the adhesive layer 200 close to the hardness of the silicon rod 300, so that the purpose of preventing the diamond wire 400 from sudden change of the cutting resistance can be achieved, because the thickness of the adhesive layer 200 is far smaller than that of the auxiliary material plate 100, and the thickness of the adhesive layer 200 is usually less than 1/10 of the thickness of the auxiliary material plate 100, therefore, when the diamond wire 400 cuts the adhesive layer 200 on two sides of the silicon rod 300, the auxiliary material plate 100 can be cut in a very short time, and when the hardness of the auxiliary material plate 100 or the adhesive layer 200 is increased, the diamond wire 400 can be transited to a state of cutting the two plates at the same time in a short time, and therefore, the phenomenon of wire bow of the diamond wire 400 is not aggravated.

In addition, the hardness of the auxiliary material plate 100 and/or the adhesive layer 200 is increased here, mainly the hardness of at least one of the auxiliary material plate 100 and the adhesive layer 200 at the cutting edge is increased, so that the cutting resistance of the diamond wire 400 at both sides of the silicon rod 300 is not instantaneously reduced when the diamond wire 400 is cut through both sides of the silicon rod 300 to the adhesive layer 200 and the auxiliary material plate 100. Therefore, the wire bowing phenomenon of the diamond wire 400 is not aggravated by the instant decrease of the cutting resistance received by the diamond wire 400 at both sides of the silicon rod 300. Here, the hardness of at least one of the auxiliary material plate 100 and the adhesive layer 200 may be increased at all portions, and the wire bow of the diamond wire 400 may be reduced, so as to reduce the cutting time of the silicon rod 300 and ensure the quality of the produced silicon wafer.

The embodiment of the application provides a crystal silicon cutting method, which comprises the following steps of:

fixing the auxiliary material plate 100 on the crystal silicon body through the adhesive layer 200;

as shown in fig. 3 to 7, the auxiliary material plate 100 includes a first material plate 110 and a second material plate 120 stacked together, the first material plate 110 is fixed to the adhesive layer 200, the shore hardness of the first material plate 110 is 85 degrees to 95 degrees, and the thickness of the first material plate 110 is 0.1 mm to 7 mm; and/or the adhesive layer 200 includes a first adhesive layer 210 and two second adhesive layers 220 located on the same plane, the two second adhesive layers 220 are oppositely disposed on two sides of the first adhesive layer 210 along the first direction X, and the shore hardness of the two second adhesive layers 220 is 80 degrees to 100 degrees.

In the crystalline silicon cutting method provided in the embodiment of the application, the shore hardness of the first material plate 110 and/or the shore hardness of the second adhesive layer 220 is close to the shore hardness of the crystalline silicon body (the shore hardness of the crystalline silicon body is 100 degrees) by using the first material plate 110 with the shore hardness of 85 degrees to 95 degrees in the auxiliary material plate 100, or by using the second adhesive layer 220 with the shore hardness of 80 degrees to 100 degrees in the adhesive layer 200, or by using both the first material plate 110 with the shore hardness of 85 degrees to 95 degrees in the auxiliary material plate 100 and the second adhesive layer 220 with the shore hardness of 80 degrees to 100 degrees in the adhesive layer 200. Like this, when the viscose layer 200 and the supplementary material board 100 are cut to silicon rod 300 both sides to the diamond wire, the resistance that receives can not reduce in the twinkling of an eye to slow down the aggravation that arouses diamond wire line bow phenomenon owing to the sudden change that appears the hardness in diamond wire cutting process, just also need not through the falling speed who reduces the crystalline silicon body in crystalline silicon cutting process, and then improved the cutting efficiency of crystalline silicon. Shore hardness is one of the methods for marking hardness, and the first direction X is a traveling direction of the diamond wire.

The auxiliary material plate 100 may be in a form of a composite plate, so as to transition the change of hardness through a hierarchical form of the composite plate, thereby preventing the abrupt change of hardness from affecting the cutting of the diamond wire, and may replace a material with a specific hardness, which is not easily obtained, through a hierarchical form, thereby satisfying the cutting requirement of the silicon rod 300 in terms of hardness through the hardness transition between different materials. The auxiliary material sheet 100 may be laminated by adhering different material sheets to each other, or by spraying on both sides or one side of one material sheet, and the auxiliary material sheet 100 may comprise two, three, or more layers.

In a particular embodiment, the secondary material sheet 100 may comprise two layers, including a first material sheet 110 and a second material sheet 120, wherein the shore hardness of the first material sheet 110 may be greater than or equal to the shore hardness of the second material sheet 120. The shore hardness of the second material plate 120 is not limited, the second material plate 120 is a material plate far away from the silicon rod 300, the cutting time of the diamond wire is short in the whole cutting process of the silicon rod 300, and the influence on the quality of the produced silicon wafer can be ignored. Therefore, the second material plate 120 is made of a material with lower hardness, so that the cost can be saved.

Specifically, the sum of the thicknesses of the first material plate 110 and the second material plate 120 may be 10 mm to 25 mm, and in the case of increasing the hardness of the first material plate 110 and the hardness of the second adhesive layer 220, the thickness of the auxiliary material plate 100 (i.e., the sum of the thicknesses of the first material plate 110 and the second material plate 120) may be a lower value, such as 13 mm or 15 mm, while in other specific embodiments, the sum of the thicknesses of the first material plate 110 and the second material plate 120 may be 18 mm to 25 mm, such as in the case of increasing only the hardness of the first material plate 110, the thickness of the auxiliary material plate 100 may be a higher value, such as 20 mm or 23 mm.

To facilitate the fixation between the crystalline silicon body and the auxiliary material plate 100, as shown in fig. 8 and 9, the auxiliary material plate 100 may further include a third material plate 130, the third material plate 130 is stacked on the second material plate 120 at a side away from the first material plate 110, and the shore hardness of the third material plate 130 is the same as the shore hardness of the first material plate 110. Thus, when the auxiliary material plate 100 and the crystalline silicon body are fixed by the adhesive layer 200, since the front and back of the auxiliary material plate 100 have the same hardness, one of the surfaces of the auxiliary material plate 100 is directly selected for fixing without distinguishing the front and back of the auxiliary material plate 100, which facilitates the fixing between the crystalline silicon body and the auxiliary material plate 100.

In some embodiments, the material of the first material plate 110 may include one or more of melamine, dacron resin or epoxy resin, and the material of the second material plate 120 may also include one or more of melamine, dacron resin or epoxy resin.

In addition, the auxiliary material plate 100 may also be in a form similar to the adhesive layer 200, as shown in fig. 10, both side portions of the plate material may be reinforced and hardened, or a material having a higher hardness than that of the middle portion may be used instead of both side portions of the plate material, as shown in fig. 11, the auxiliary material plate 100 may also be in a form of a single plate, that is, a single plate having a higher hardness to accommodate the cutting at the ending stage of the crystalline silicon body. Like this, can reach the aggravation of avoiding bringing diamond wire line bow phenomenon because the sudden change that appears the hardness at crystal silicon body cutting in-process equally, improve the purpose of crystal silicon body cutting efficiency. When the two side parts of the plate are replaced by the material with higher hardness than the middle part, the middle part of the plate is easier to cut than the two side parts, so that the wire bow degree of the diamond wire can be reduced and even the wire bow phenomenon of the diamond wire disappears in the diamond wire cutting process.

The adhesive layer 200 here increases the hardness at the cut edge of the adhesive layer 200 by changing the hardness of both sides, i.e. both sides can be coated with glue with higher hardness, and the middle is coated with glue with the same hardness or lower hardness. Similar to the auxiliary material plate 100, when the middle portion is coated with the adhesive material having a lower hardness, since the middle portion (i.e., the first adhesive layer 210) of the adhesive layer 200 is easier to cut than the two side portions (i.e., the two second adhesive layers 220), the degree of wire bow of the diamond wire may be reduced, and even the wire bow phenomenon of the diamond wire may be eliminated during the cutting of the diamond wire.

In some specific embodiments, the shore hardness of the two second adhesive layers 220 may be 85 degrees to 95 degrees, and the shore hardness of the two second adhesive layers 220 is greater than or equal to the shore hardness of the first adhesive layer 210.

The length of each second adhesive layer 220 along the first direction X may be 5% to 15% of the length of the crystalline silicon body along the first direction X.

The thickness of the first adhesive layer 210 and/or the second adhesive layer 220 may be 0.3 mm to 0.7 mm, in the case of increasing the hardness of the first material plate 110 and the hardness of the second adhesive layer 220, the thickness of the first adhesive layer 210 and the second adhesive layer 220 may be a lower value, such as 0.3 mm or 0.4 mm, and in the case of increasing the hardness of the second adhesive layer 220, the thickness of the first adhesive layer 210 and the second adhesive layer 220 may be a higher value, such as 0.6 mm or 0.7 mm.

The material of the first adhesive layer 210 may include one or more of melamine, epoxy, polyimide, or polybenzimidazole, and the material of the second adhesive layer 220 may include one or more of melamine, epoxy, polyimide, or polybenzimidazole.

In other possible embodiments, the two side portions and the middle portion of the adhesive layer 200 may be made of the same adhesive, even though the shore hardness of the middle portion of the adhesive layer 200 is also 80 degrees to 100 degrees.

In the actual coating process, can choose the higher glue of hardness to paint both sides part earlier, treat 1 to 2 minutes both sides part glue after the solidification preliminarily, begin to paint the glue of mid portion promptly, because the area that the both sides part was shared can not exceed the 15% of whole viscose layer 200 area, consequently play main bonding effect be mid portion glue, even the glue of both sides part arouses because of the exposure time overlength to the bonding force of the crystalline silicon body not enough, can not influence the bonding dynamics between crystalline silicon body and viscose layer 200 yet.

Similarly, the adhesive layer 200 may also be a composite layer similar to the auxiliary board 100, i.e. the adhesive layer 200 may be formed by sequentially applying adhesives with different hardness on the board, or may be a substrate with different or the same adhesive layers on both sides.

Taking the currently cut M10 silicon rod 300 (182 mm × 182 mm) in the industry as an example, the maximum speed of the silicon rod 300 (the silicon rod 300 is currently bonded on the resin plate) is 2500 μ M/min (micrometer per minute), but when the silicon rod 300 is cut to the resin plate (the cutting depth is 150 mm), the speed of the silicon rod 300 is reduced from 2500 μ M/min to 200 μ M/min, so that the cutting time of the silicon rod 300 is greatly prolonged, the cutting efficiency of the silicon rod 300 is reduced, and the occurrence of edge breakage is also avoided. Taking the example that the auxiliary material plate 100 is the first material plate 110 with shore hardness of 80 degrees to 100 degrees, the cutting time of the silicon rod 300 is 100 minutes, and the total time consumption for cutting the silicon rod 300 can be reduced to 80 minutes, which reduces 20% of the time consumption. As shown in table 1 below, the cutting time of the ending stage of the corresponding silicon rod 300 after five first material plates 110 with different shore hardness are listed.

Table 1: cutting time consumption table of silicon rod 300 ending stages corresponding to plates with different hardness:

shore hardness (durometer) of the first material plate 110	83	87	92	95	98
						Cutting time (minutes) at the ending stage of silicon rod 300	25	18	12	8	5

As can be seen from table 1 above, when the shore hardness of the first material plate 110 is gradually brought close to the shore hardness of the silicon rod 300, the cutting time of the ending stage of the silicon rod 300 is gradually reduced. The second adhesive layers 220 having shore hardness of 80 degrees to 100 degrees are used in the adhesive layers 200, which also have a trend of reducing the cutting time consumption at the ending stage of the silicon rod 300 shown in table 1 above, and in addition, the length of the two second adhesive layers 220 in the first direction X is increased in the adhesive layers 200, so that the cutting time consumption at the ending stage of the silicon rod 300 can be further reduced.

It will be understood by those of ordinary skill in the art that the foregoing embodiments are specific examples for carrying out the present application, and that various changes in form and details may be made therein without departing from the spirit and scope of the present application in practice.