阅读说明：本技术 一种用于SiC晶片研磨的固相反应研磨盘及其制备方法和应用 (Solid-phase reaction grinding disc for grinding SiC wafer and preparation method and application thereof ) 是由 路家斌 曾帅 阎秋生 潘继生 聂小威 陈海阳 于 2021-07-31 设计创作，主要内容包括：本发明涉及一种用于SiC晶片研磨的固相反应研磨盘及其制备方法和应用。该固相反应研磨盘包括基盘,叠设于基盘上的填充料和嵌设于填充料中且与填充料表面齐平的若干个磨料块；所述填充料上设有贯穿填充料的气孔；所述填充料由催化剂和助研剂组成；所述磨料块由磨料、催化剂和粘接剂混合组成；所述催化剂为含铁粒子。本发明提供的固相反应研磨盘适用于SiC晶片研磨,不仅可以获得较高的材料去除率和较好的表面质量,还能够获得低损伤的SiC晶片表面,可用于表面要求较高的SiC晶片的精密研磨加工。(The invention relates to a solid-phase reaction grinding disc for grinding a SiC wafer, and a preparation method and application thereof. The solid-phase reaction grinding disc comprises a base disc, a filler stacked on the base disc and a plurality of grinding blocks which are embedded in the filler and flush with the surface of the filler; the filler is provided with air holes penetrating through the filler; the filling material consists of a catalyst and an auxiliary grinding agent; the abrasive material block is formed by mixing an abrasive material, a catalyst and a bonding agent; the catalyst is iron-containing particles. The solid-phase reaction grinding disc provided by the invention is suitable for grinding SiC wafers, can obtain higher material removal rate and better surface quality, can also obtain the SiC wafer surface with low damage, and can be used for precision grinding processing of SiC wafers with higher surface requirements.)

1. A solid phase reaction grinding disc for grinding SiC wafers is characterized by comprising a base disc, a filler and a plurality of grinding blocks, wherein the filler is stacked on the base disc, and the grinding blocks are embedded in the filler and are flush with the surface of the filler; air holes are arranged on the filling material;

the filling material consists of a catalyst and an auxiliary grinding agent;

the abrasive material block is formed by mixing an abrasive material, a catalyst and a bonding agent;

the catalyst is iron-containing particles.

2. The solid-phase reaction grinding disc of claim 1, wherein the weight percentage of the catalyst in the filler is 40-60%; the weight percentage of abrasive particles in the abrasive material block is 10-30%, and the weight percentage of the catalyst is 20-40%.

3. The solid-phase reaction grinding disk of claim 1, wherein the abrasive material blocks are uniformly dispersed; the shape of the abrasive material block is one or more of a cylinder, a cuboid or a polygonal prism.

4. The solid-phase reaction grinding disk according to claim 3, wherein the arrangement shape of the grinding blocks is one or more of a spiral shape, a circular ring shape, a honeycomb shape or a sunflower seed shape.

5. The solid-phase reaction grinding disc according to claim 1, wherein the catalyst is one or more of ferroferric oxide, carbonyl iron powder or reduced iron powder; the grinding aid is one or more of graphite powder, iron oxide powder, iron powder or stearic acid.

6. The solid-phase reaction grinding disc according to claim 1, wherein the abrasive is one or more of diamond, silicon carbide, cerium oxide or silicon oxide.

7. The solid-phase reaction grinding disk according to claim 1, wherein the binder is one or more of a resin binder, a ceramic binder or a metal binder.

8. The method for preparing the solid-phase reaction grinding disc as claimed in any one of claims 1 to 7, which is characterized by comprising the following steps:

s1: mixing an abrasive, a catalyst and a bonding agent, carrying out hot-pressing curing molding, and demolding to obtain an abrasive block;

s2: bonding the abrasive block to the base plate;

s3: and adding filling materials into the gap of the base plate, and leveling to obtain the solid-phase reaction grinding plate.

9. The method for preparing the solid-phase reaction grinding disc as claimed in claim 8, wherein the step of S1 further comprises the steps of mixing, grinding and sieving the grinding material and the catalyst before mixing.

10. Use of the solid phase reaction abrasive disc of any one of claims 1 to 7 in the grinding of SiC wafers.

Technical Field

The invention belongs to the technical field of SiC wafer processing, and particularly relates to a solid-phase reaction grinding disc for grinding a SiC wafer, and a preparation method and application thereof.

Background

Silicon carbide (SiC) wafers, which are third-generation semiconductor materials, have a large forbidden band width, high electron mobility, high breakdown electric field, high thermal conductivity, strong radiation resistance, and the like, and are one of the most promising semiconductor materials. In order to meet the requirements of epitaxial film growth of SiC wafers, the surface of the SiC wafer must be ultra-smooth, defect-free and free of surface/sub-surface damage. When a minute defect exists on the wafer surface, the minute defect is transferred to the epitaxial growth film to become a defect of the device, and therefore the surface quality and accuracy of the processed SiC wafer directly determine the performance of the SiC semiconductor device. Silicon carbide wafers need to be cut, ground, polished and the like to produce qualified SiC substrates, but high-quality and high-efficiency surface precision machining is difficult due to the high hardness, high-brittleness physical property and very stable chemical property of the silicon carbide wafers. To reduce surface/subsurface damage, the inventors have introduced a chemical reaction during processing, i.e., chemical mechanical polishing, which oxidizes the wafer surface to achieve low surface roughness and a damage-free surface. However, when a SiC wafer is processed by chemical mechanical polishing, although a damage-free, low-roughness surface can be obtained, the processing efficiency is low, and the processing can be used only for the final processing of the SiC wafer.

In order to improve the processing efficiency, the researchers have proposed adding a chemical reaction and a solid phase reaction polishing disk during the polishing process. The solid-phase reaction grinding disc combines the advantages of fixed abrasive grinding and chemical mechanical polishing, utilizes the corresponding components in the grinding disc to react with the surface of a wafer under certain conditions to generate an oxide layer, thereby avoiding the direct contact of abrasive particles with the surface of the wafer, reducing the damage of the surface of the wafer, and simultaneously ensuring the processing efficiency due to the existence of the fixed abrasive. However, due to the limitation of solid-phase reaction, the grinding disc only has related grinding tools applied to sapphire, silicon wafers and other materials in the market at present; although a method for realizing fine processing of silicon carbide by using a solid-phase reaction is reported (patent CN110355613A), the related device is complicated, the process is complicated, and the method is not suitable for industrial application and popularization.

Therefore, the development of the grinding tool related to the solid phase reaction and suitable for the silicon carbide has important research significance and economic value.

Disclosure of Invention

The invention aims to overcome the defect of the prior art that a grinding tool for finely processing SiC wafers by utilizing solid phase reaction is lacked, and provides a solid phase reaction grinding disc for grinding SiC wafers. The solid phase reaction grinding disc provided by the invention consists of a base disc, a filling material, an abrasive block and air holes, wherein a specific catalyst is distributed on any grinding surface of the whole solid phase reaction grinding disc; when the catalyst is matched with grinding fluid containing hydrogen peroxide for grinding, the catalyst can be mixed with H in the grinding fluid₂O₂The Fenton reaction generates hydroxyl free radical (. OH) with strong oxidizability, the hydroxyl free radical (. OH) reacts with the SiC wafer to generate a soft oxidation layer on the surface of the SiC, and the oxidation layer is removed by the abrasive, so that the processing efficiency can be improved, and the damage of the abrasive disk to the SiC wafer can be reduced.

The invention also aims to provide a preparation method of the solid-phase reaction grinding disc.

The invention also aims to provide the application of the solid-phase reaction grinding disc in grinding SiC wafers.

In order to achieve the purpose, the invention adopts the following technical scheme:

a solid phase reaction grinding disc for grinding SiC wafers comprises a base disc, a filler stacked on the base disc and a plurality of grinding blocks which are embedded in the filler and are flush with the surface of the filler; the filler is provided with air holes penetrating through the filler;

the filling material consists of a catalyst and an auxiliary grinding agent;

the abrasive material block is formed by mixing an abrasive material, a catalyst and a bonding agent;

the catalyst is iron-containing particles.

According to the invention, a specific catalyst is introduced into the grinding block and the filler, so that the specific catalyst is uniformly distributed on any grinding surface of the whole solid-phase reaction grinding disc, and the grinding efficiency can be improved by using the grinding aid; when the catalyst is mixed with the grinding fluid containing hydrogen peroxide for grinding, the catalyst can be mixed with H in the grinding fluid₂O₂The Fenton reaction generates hydroxyl free radical (. OH) with strong oxidizability, the hydroxyl free radical (. OH) reacts with the SiC wafer to generate a soft oxidation layer on the surface of the SiC, and the oxidation layer is removed by the abrasive, so that the processing efficiency can be improved, and the damage of the abrasive disk to the SiC wafer can be reduced.

The solid phase reaction process is as follows:

Fe²⁺+H₂O₂→Fe³⁺+·OH+OH^- (1)

Fe³⁺+H₂O₂→Fe²⁺+H⁺+·OOH (2)

SiC+4·OH+O₂→SiO₂+2H₂O+CO₂↑ (3)。

the solid-phase reaction grinding disc provided by the invention is suitable for grinding SiC wafers, can obtain higher material removal rate and better surface quality, can also obtain the SiC wafer surface with low damage, and can be used for precision grinding processing of SiC wafers with higher surface requirements.

It should be understood that the solid phase reaction grinding disc can be prepared into different size specifications according to the size of a machine tool; the content of the catalyst in the filler, the content of the abrasive in the abrasive block and the content of the catalyst can be selected and prepared according to the grinding requirement.

Preferably, the weight percentage of the catalyst in the filler is 40-60%.

Preferably, the weight percentage of the abrasive particles in the abrasive material block is 10-30%, and the weight percentage of the catalyst is 20-40%.

The content of the abrasive and the catalyst can be adjusted according to specific requirements, and the type, the particle size and the like of the abrasive can be reasonably selected.

Preferably, the abrasive material blocks are uniformly distributed.

More preferably, the shape of the abrasive material block arrangement is one or more of a spiral shape, a circular ring shape, a honeycomb shape or a sunflower seed shape.

Preferably, the shape of the abrasive material block is one or more of a cylinder, a cuboid or a polygonal prism

Preferably, the upper and lower surfaces of the abrasive block are flush with the upper and lower surfaces of the filler block, respectively.

Preferably, the catalyst is one or more of ferroferric oxide, carbonyl iron powder or reduced iron powder.

Preferably, the particle size of the catalyst is 1-300 μm.

Preferably, the grinding aid is one or more of graphite powder, iron oxide powder, iron powder or stearic acid.

Preferably, the particle size of the grinding aid is 1-300 μm.

Abrasives suitable for grinding of silicon carbide, which are conventional in the art, may be used in the present invention.

The type and the grain diameter of the abrasive can be in the conventional type and grain diameter range, and can be reasonably selected and optimized according to the grinding requirement.

Preferably, the abrasive is one or more of diamond, silicon carbide, cerium oxide or silicon oxide.

Preferably, the particle size of the abrasive is 1-300 μm.

Preferably, the adhesive is one or more of a resin bonding agent, a ceramic bonding agent or a metal bonding agent.

Preferably, an air hole (e.g., a circular hole) is provided in the center of the packing.

Preferably, the pores penetrate the filler.

The preparation method of the solid phase reaction grinding disc comprises the following steps:

s1: mixing an abrasive, a catalyst and a bonding agent, carrying out hot-pressing curing molding, and demolding to obtain an abrasive block;

s2: bonding the abrasive block to the base plate;

s3: and adding filling materials into the gap of the base plate, and leveling to obtain the solid-phase reaction grinding plate.

Preferably, S1 further comprises the steps of mixing, grinding, and sieving the abrasive and the catalyst before mixing.

The control of the particle size of the abrasive and the catalyst is realized by grinding and sieving.

The application of the solid-phase reaction grinding disc in the grinding of SiC wafers is also within the protection scope of the invention.

The invention also claims a use method of the solid-phase reaction grinding disc, which comprises the following steps: and grinding the SiC wafer by using the solid-phase reaction grinding disc by using grinding fluid containing hydrogen peroxide as grinding fluid.

Preferably, the pressure of the grinding is 10 to 30kPa (for example, 10kPa, 20kPa, 30kPa), the rotation speed of the grinding is 40 to 80r/min (for example, 40r/min, 60r/min, 80r/min), and the time of the grinding is 30 to 60min (for example, 30min, 45min, 60 min).

Preferably, the concentration of hydrogen peroxide in the hydrogen peroxide-containing grinding fluid is 5-20% wt. (for example, 5%, 10%, 15% and 20%).

Compared with the prior art, the invention has the following beneficial effects:

(1) the solid phase reaction grinding disc provided by the invention consists of a base disc, a filling material, an abrasive block and air holes, wherein a specific catalyst is distributed on any grinding surface of the whole solid phase reaction grinding disc; when the catalyst is matched with grinding fluid containing hydrogen peroxide for grinding, the catalyst can be mixed with H in the grinding fluid₂O₂The Fenton reaction is carried out to generate hydroxyl free radicals (OH) with strong oxidizability, the hydroxyl free radicals (OH) react with the SiC wafer to generate a soft oxide layer on the surface of the SiC, and then the oxide layer is removed by the abrasive, so that the processing efficiency can be improved, and the damage of the grinding disc to the SiC wafer can be reduced;

(2) the solid-phase reaction grinding disc provided by the invention is suitable for grinding SiC wafers, can obtain higher material removal rate and better surface quality, can also obtain the SiC wafer surface with low damage, and can be used for precision grinding processing of SiC wafers with higher surface requirements.

Drawings

FIG. 1 is a schematic structural diagram of a solid-phase reaction grinding disk provided in example 1;

FIG. 2 is a schematic illustration of the process for making the abrasive block provided in example 1;

FIG. 3 shows the surface topography and surface roughness of a SiC wafer ground using the solid phase reaction grinding disk provided in example 1;

FIG. 4 shows the surface topography and surface roughness of a SiC wafer ground using the solid phase reaction grinding disk provided in example 2;

FIG. 5 shows the surface topography and surface roughness of a SiC wafer ground using the solid phase reaction grinding disk provided in example 3;

FIG. 6 is a surface topography and surface roughness of a SiC wafer ground using the abrasive disk provided in comparative example 1;

fig. 7 shows the surface topography and surface roughness of a SiC wafer after grinding with the grinding disk provided in comparative example 2.

Wherein, 1 is an abrasive material block, 2 is a filling material, 3 is a basal disc, and 4 is a mould.

Detailed Description

The invention is further illustrated by the following examples. These examples are intended to illustrate the invention and are not intended to limit the scope of the invention. Experimental procedures without specific conditions noted in the examples below, generally according to conditions conventional in the art or as suggested by the manufacturer; the raw materials, reagents and the like used are, unless otherwise specified, those commercially available from the conventional markets and the like. Any insubstantial changes and substitutions made by those skilled in the art based on the present invention are intended to be covered by the claims.

In each embodiment and comparative example, a white light interferometer is adopted to detect the surface roughness, the maximum peak-valley roughness Rt (defined as the sum of absolute values of the heights of the highest peak and the lowest valley of the surface topography) and the surface topography, 5 points are taken for each sheet, the average value is taken, and an energy spectrometer is adopted to analyze the compositions of abrasive dust and surface elements of a workpiece; the mass of the workpiece was measured with an electronic analytical balance with an accuracy of 0.1mg, and the Material Removal Rate (MRR) was calculated using the mass difference and equation (4).

In the formula: MRR (nm/min) is the material removal rate; Δ m (mg) is the difference in mass of SiC before and after processing; rho is the density of the single crystal SiC and is taken as 3.2g/cm³；S(cm²) Is the area of the workpiece to be machined; t (min) is the processing time.

Example 1

The embodiment provides a solid-phase reaction grinding disc which comprises a base disc, a filler stacked on the base disc and a plurality of grinding blocks embedded in the filler and flush with the upper surface and the lower surface of the filler; the filler is provided with air holes penetrating through the filler; is prepared by the following steps:

(1) weighing 20% by mass of diamond abrasive with W5 particle size (average particle size is 5 μm, the same below), 30% by mass of ferroferric oxide with W3 particle size (average particle size is 3 μm, the same below) and 50% by mass of phenolic resin;

(2) mixing and stirring diamond grinding materials and ferroferric oxide in a roller for 30min, then placing the mixture in a ball mill for grinding for 2-4h, and then sieving through a 3000-mesh sieve to ensure the particle size of the powder; uniformly mixing the sieved mixture with phenolic resin, adding into a mold 4 (shown in figure 2), hot-pressing at 270 ℃ under 10MPa for 2h, curing, molding, and demolding to obtain an abrasive block;

3) weighing 50% by mass of ferroferric oxide with the particle size of W3, 5% by mass of graphite powder with the particle size of W3 and 45% by mass of epoxy resin liquid, mixing and stirring the ferroferric oxide and the graphite powder for 30min, then mixing the mixture with the epoxy resin liquid and ultrasonically stirring for 30min to obtain the filler;

4) bonding the abrasive blocks prepared in the step 2) on a base disc according to a circular ring arrangement mode, leaving circular air hole positions at the center of the base disc, adding filling materials into the rest gaps, solidifying and leveling to obtain the corresponding solid-phase reaction grinding disc, wherein the solid-phase reaction grinding disc is shown in figure 1.

The solid-phase reaction grinding disc of the embodiment is used for grinding single crystal SiC:

the grinding disc was used in a model KD15BX single-side grinding machine to carry out grinding experiments, and the processed workpiece was a 2-inch 4H-SiC (0001) cut piece produced by Beijing Tianke Heda blue light semiconductor Co., Ltd, and had an original surface roughness Ra of 130nm and a thickness of (400. + -. 25) μm.

During grinding, the grinding disc is driven by the driving motor to rotate around the main shaft at a constant speed. The SiC workpiece is uniformly pasted on the workpiece disc through paraffin and sleeved with the trimming ring, pressure is adjusted by the balancing weight, and the circle center of the retainer pulley and the trimming ring are on a concentric circle. In the grinding process, the dressing ring and the workpiece disc automatically rotate under the action of friction force of the retainer and the grinding disc, and the SiC workpiece is machined through relative movement.

5 wt.% of hydrogen peroxide is adopted as grinding fluid, the flow rate of the grinding fluid is 20mL/min, the grinding pressure is 10kPa, the rotating speed is 60r/min, after the SiC wafer is processed for 60min, the Ra of the SiC wafer is 10nm, and the material removal rate is 1.6 mu m/min.

The surface morphology and surface roughness of the polished wafer were examined using a white light interferometer, as shown in FIG. 3.

Example 2

The embodiment provides a solid-phase reaction grinding disc which comprises a base disc, a filler stacked on the base disc and a plurality of grinding blocks embedded in the filler and flush with the upper surface and the lower surface of the filler; the filler is provided with air holes penetrating through the filler; is prepared by the following steps:

(1) weighing 15 mass percent of diamond abrasive with the W3 particle size, 35 mass percent of carbonyl iron powder with the W1 particle size and 50 mass percent of phenolic resin;

(2) mixing and stirring the diamond grinding material and carbonyl iron powder in a roller for 30min, then placing the mixture in a ball mill for grinding for 2-4h, and then sieving with a 5000-mesh sieve to ensure the particle size of the powder; uniformly mixing the sieved mixture with phenolic resin, adding the mixture into a mold 4, carrying out hot pressing for 2h at 270 ℃ under 10MPa, carrying out curing molding, and demolding to obtain an abrasive block;

3) weighing 55 mass percent of carbonyl iron powder with a W1 (average particle size of 1 micron, the same below) particle size, 5 mass percent of graphite powder with a W1 particle size and 40 mass percent of epoxy resin liquid, mixing and stirring the carbonyl iron powder and the graphite powder for 30min, then mixing the mixture and the epoxy resin liquid and ultrasonically stirring for 30min to obtain the filler;

4) bonding the abrasive material blocks prepared in the step 2) on a base plate according to a sunflower seed type arrangement mode, leaving a circular air hole position at the center of the base plate, adding filling materials into the rest gaps, solidifying and leveling to obtain the corresponding solid-phase reaction grinding plate.

The abrasive grain-oriented solid-phase reaction grinding disc of the embodiment is used for grinding single crystal SiC:

the grinding disc was used in a model KD15BX single-side grinding machine to carry out grinding experiments, and the processed workpiece was a 2-inch 6H-SiC (0001) cut piece produced by Beijing Tianke Heda blue light semiconductor Co., Ltd, and had an original surface roughness Ra of 130nm and a thickness of (400. + -. 25) μm.

During grinding, the grinding disc is driven by the driving motor to rotate around the main shaft at a constant speed. The SiC workpiece is uniformly pasted on the workpiece disc through paraffin and sleeved with the trimming ring, pressure is adjusted by the balancing weight, and the circle center of the retainer pulley and the trimming ring are on a concentric circle. In the grinding process, the dressing ring and the workpiece disc automatically rotate under the action of friction force of the retainer and the grinding disc, and the SiC workpiece is machined through relative movement.

The method comprises the steps of adopting 10 wt.% hydrogen peroxide as a grinding fluid, wherein the flow rate of the grinding fluid is 30mL/min, the grinding pressure is 10kPa, the rotating speed is 40r/min, and the Ra of the SiC wafer is 5nm after the SiC wafer is processed for 60 min. The material removal rate was 1.1 μm/min.

The surface morphology and surface roughness of the polished wafer were examined using a white light interferometer, as shown in FIG. 4.

Example 3

The embodiment provides a solid-phase reaction grinding disc which comprises a base disc, a filler stacked on the base disc and a plurality of grinding blocks embedded in the filler and flush with the upper surface and the lower surface of the filler; the filler is provided with air holes penetrating through the filler; is prepared by the following steps:

(1) weighing 20 mass percent of silicon oxide abrasive with the W1 particle size, 30 mass percent of ferroferric oxide with the W1 particle size and 50 mass percent of phenolic resin;

(2) mixing and stirring diamond abrasive and ferroferric oxide in a roller for 30min, then placing the mixture in a ball mill for grinding for 2-4h, and then sieving with a 8000-mesh sieve to ensure the particle size of the powder; uniformly mixing the sieved mixture with phenolic resin, adding the mixture into a mold 4, carrying out hot pressing for 2h at 270 ℃ under 10MPa, carrying out curing molding, and demolding to obtain an abrasive block;

3) weighing 60% by mass of ferroferric oxide with the particle size of W1, 5% by mass of graphite powder with the particle size of W1 and 35% by mass of epoxy resin liquid, mixing and stirring the ferroferric oxide and the graphite powder for 30min, then mixing the mixture with the epoxy resin liquid and ultrasonically stirring for 30min to obtain the filler;

4) bonding the abrasive blocks prepared in the step 2) on a base disc according to an Archimedes spiral line type arrangement mode, leaving a circular air hole position at the center of the base disc, then adding a filling material into the rest gaps, and leveling after consolidation to obtain the corresponding solid phase reaction grinding disc.

The abrasive grain-oriented solid-phase reaction grinding disc of the embodiment is used for grinding single crystal SiC:

the grinding disc was used in a model KD15BX single-side grinding machine to carry out grinding experiments, and the processed workpiece was a 2-inch 6H-SiC (0001) cut piece produced by Beijing Tianke Heda blue light semiconductor Co., Ltd, and had an original surface roughness Ra of 130nm and a thickness of (400. + -. 25) μm.

During grinding, the grinding disc is driven by the driving motor to rotate around the main shaft at a constant speed. The SiC workpiece is uniformly pasted on the workpiece disc through paraffin and sleeved with the trimming ring, pressure is adjusted by the balancing weight, and the circle center of the retainer pulley and the trimming ring are on a concentric circle. In the grinding process, the dressing ring and the workpiece disc automatically rotate under the action of friction force of the retainer and the grinding disc, and the SiC workpiece is machined through relative movement.

20 wt.% of hydrogen peroxide is adopted as a grinding fluid, the flow rate of the grinding fluid is 25mL/min, the grinding pressure is 10kPa, the rotating speed is 60r/min, and the Ra of the SiC wafer is 2nm after the SiC wafer is processed for 60 min. The material removal rate was 0.05 μm/min. The surface morphology and surface roughness of the polished wafer were examined using a white light interferometer, as shown in FIG. 5.

Comparative example 1

This comparative example provides a conventional diamond abrasive disk, which is commercially available.

The SiC wafer was polished by the same polishing equipment and conditions as in example 1, and had an Ra of 98nm and a material removal rate of 1.5 μm/min. The surface morphology and surface roughness of the polished wafer were examined using a white light interferometer, as shown in FIG. 6.

Comparative example 2

This comparative example provides a polishing pad prepared by a process wherein the polishing slurry was deionized water, the remainder being identical to example 1.

The SiC wafer was polished by the same polishing equipment and conditions as in example 1, and had an Ra of 50nm and a material removal rate of 1.3 μm/min. The surface morphology and surface roughness of the polished wafer were examined using a white light interferometer, as shown in FIG. 7.

It will be appreciated by those of ordinary skill in the art that the examples provided herein are intended to assist the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and embodiments. Those skilled in the art can make various other specific changes and combinations based on the teachings of the present invention without departing from the spirit of the invention, and these changes and combinations are within the scope of the invention.