IE84509B1 - Porous abrasive segments and tools incorporating the same - Google Patents

Abstract

ABSTRACT Abrasive segments (10) for use in a segmented grinding wheel are disclosed. Each is made of a composite including a plurality of diamond and/or cubic boron nitride superabrasive grains with an average particle size of less than 300 microns and a nonmetallic bond matrix cured together. The composite has a plurality of interconnected pores disposed therein, and the volume distribution is from 0.5 to 25 volume percent abrasive grain, from 19.5 to 65 volume percent non-metallic bond and from 40 to 80 volume percent interconnected porosity. Also shown is a segmented grinding wheel comprising the abrasive segments.

Description

POROUS ABRASIVE SEGMENTS AND TOOLS INCORPORATING THE SAME The present invention relates generally to porous abrasive segments for use . in abrasive tools suitable for surface grinding and polishing of hard and/or brittle materials. This invention more particularly relates to highly porous, bonded abrasive articles with segments having an interconnected pore structure and methods for making same. The tools are useful in high performance grinding operations, such as backgrinding silicon, alumina titanium carbide and silicon carbide wafers, which are typically used in the manufacture of electronic components.

The use of porous abrasives to improve mechanical grinding processes is generally well known. Pores typically provide access to grinding fluids, such as Coolants and lubricants, which tend to promote more efficient cutting, minimise metallurgical damage (e.g. surface burn), and maximise tool life.

Pores also permit the clearance of material (eg. chips or swarf) removed from an object being ground, which is important especially when the object being ground is relatively soft or when surface finish requirements are demanding (eg. when backgrinding silicon wafers).

Previous attempts to fabricate abrasive articles and/or tools including porosity may generally be classified into one of two categories. in the first category, a pore structure is created by the addition of organic pore inducing media (such as ground walnut shells) into the abrasive article. These media themfally decompose upon firing, leaving voids or pores in the cured abrasive tool.

Examples of this category are US Patents 5221294 to Carmen, et al, and 5429648 to Wu, and Japanese Patents A161273 to Grotoh, et al, A 281174 to Satoh, et al. In the second category, a pore structure may be created by the addition of closed cell materials, such as bubble alumina, into an abrasive article. See for example US Patent 5203886 to Sheldon, et al. in an alternative approach, Wu et al, in US Patents 5738696 and 5738697, each of which is fully incorporated herein by reference, disclose an abrasive article and method for fabricating the same including fibre-like abrasive grains having a length to diameter aspect ratio of at least 5:1. The poor packing characteristics of the elongated abrasive grains resulted in an abrasive article including increased porosity and permeability and suitable for relatively high-performance grinding.

As market demand has grown for precision components in products such as engines, refractory equipment, and electronic devices (e.g. silicon and silicon carbide wafers, magnetic heads, and display windows) the need has grown for improved abrasive tools for line precision grinding and polishing of ceramics and other relatively hard and/or brittle materials. The abrasive tools known in the art have not proven entirely satisfactory in meeting the above stated needs. Therefore, there exists a need for improved abrasive articles and abrasive tools, and in particular, those including a relatively high degree of porosity.

The present invention provides an abrasive segment for a segmented grinding wheel, the abrasive segment comprising a composite including a plurality of super-abrasive grains selected from the group consisting of diamond and cubic boron nitride and having an average particle size of less than 300 microns and a non-metallic bond matrix cured together, the composite having a plurality of interconnected pores disposed therein, and including from 0.5 to 25 volume percent abrasive grain, from 19.5 to 65 percent non-metallic bond and from 40 to 80 volume percent interconnected porosity.

The method forfabricating such abrasive segments may include forming the interconnected porosity by a) adding a dispersoid to the grains and non-metallic bond prior to curing the composite; and b) immersing the cured composite into a solvent and dissolving the dispersoid to leave the abrasive segment substantially free of dispersoid particles.

In particular, the composite may be formed by a) blending a mixture including from 0.5 to 25 volume percent abrasive grain, from 19.5 to 65 volume percent non-metallic bond material and from to 80 volume percent dispersoid particles; and b) pressing the mixture into an abrasive laden composite.

The method for fabricating the segments may include the steps of thermally processing the composite.

The non-metallic bond material may be an organic bond material. The dispersoid particles may include granular sugar and the solvent used may be boiling water.

The segments may be used to form a segmented grinding wheel. The grinding wheel may include a core having a minimum specific strength of 2.4 MPa—cm3/g, a core density of 0.5 to 8.0 g/cm3, and a circular perimeter, an abrasive rim including a plurality of the segments, and a thermally stable adhesive bond between the core and each of the plurality of segments.

Referring to the accompanying drawings: Figure 1 is a schematic representation of one embodiment of an abrasive segment of this invention; Figure 2A is a partial schematic representation of one embodiment of a grinding wheel including sixteen of the abrasive segments of Figure 1; Figure 2B is a cross-sectional view taken along Line"A"—"A" of‘Figure 2A; and Figure 2C is a partially enlarged view showing the Portion 110 of Figure 2B.

The porous abrasive segments according to the invention may be useful in precision grinding, polishing, or cutting applications, for example as an abrasive grinding wheel with abrasive segments 10 fonning a segmented grinding wheel 100 (see, for example, Figures 1 and 2, which are described in further detail hereinbelow with respect to the Example).

The abrasive segments includes a non-metallic bond, such as an organic bond material (eg. phenolic resin) and includes from about 40 to about 80 volume percent interconnected porosity. Grinding wheels (e.g. grinding wheel 100) including one or more of the abrasive segments 10 of this invention are potentially advantageous for mirror finish grinding of hard and/or brittle materials, such as silicon wafers, silicon carbide, alumina titanium carbide, and the like. These grinding wheels may be further advantageous in that they may eliminate the need for dressing (or othenivise conditioning) the grinding face of the grinding wheel during mirror finish grinding of the above materials. Other potential advantages of this invention will become apparent in the discussion and examples that follow.

The super-abrasive grains, selected trom diamond and cubic boron nitride (CBN), and may be with or without a metal coating. Abrasive grain size and type selection typically vary depending on the nature of the workpiece and the type of grinding process. For fine finish (i.e. ‘mirror finish‘) grinding, super- abrasive grains having a smaller particle size, such as ranging from about 0.5 to about 120 microns or even from about 0.5 to about 75 microns may be desirable. in general, smaller (i.e. finer) grain sizes are preferred for fine grinding and surface finishing/polishing operations, while larger (i.e. coarser) grain sizes are preferred for shaping, thinning, and other operations in which a relatively large amount of material removal is required.

An example of a suitable non-metallic bond material is an organic bond material such as a thermosetting resin, but other types of resins may be used. Preferably, the resin is either an epoxy resin or a phenolic resin, and it may be used in liquid or powder form. Specific examples of suitable thermosetting resins include phenolic resins (e.g. novolak and resole), epoxy, unsaturated polyester, bismaleimide, polyimide, cyanate ester, melamines, and the like. I The abrasive segments according to the invention include from about 40 to about 80 volume percent interconnected porosity in which the average pore size ranges from about 150 to about 500 microns. These embodiments further include from about 0.5 to about 25 volume percent super-abrasive and from about 19.5 to about 65 volume percent organic bond cured together at temperatures ranging from about 100 to about 200‘C (or 400 to about 450"C for polyimide resins) at pressures ranging from about 20 to about 33 MPa. (Dispersoids having an acicular shape, e.g. having an aspect ratio of > or = 2:1 may be desirably used to achieve about 40 to 50 volume percent interconnected porosity). After cooling, the abrasive laden composites, including dispersoids that are substantially in contact with one another, are immersed in a solvent in order to selectively remove (i.e. dissolve) the dispersoids. The resultant abrasive article has a foam-like structure including a mixture of abrasive and bond matrix, and having a network of effectively randomly distributed interconnected pores (i.e. voids from which the dispersoid was dissolved).

Substantially any dispersoid that may be readily dissolved in a solvent such as water, alcohol, acetone, and the like, may be used. In general, dispersoids that are soluble in water, such as sodium chloride, potassium chloride, magnesium chloride, calcium chloride, sodium silicate. sodium carbonate, sodium sulfate, potassium sulfate, magnesium sulfate, and the like, and mixtures thereof are preferred. For use in some grinding applications (such as silicon wafers and other electronics components), the use of a non-ionic (i.e. non-salt) dispersoid, such as sugar, dextrin, polysaccharide oligomers, may be desirable. Most preferred are dispersoids having a relatively high solubility in water and relatively rapid dissolution kinetics, such as sodium chloride or sugar.

The particle size of the dispersoids is typically in the range from about 25 to about 500 microns. In one desirable embodiment the dispersoids include a particle size distribution from about 74 to about 210 microns (i.e. including dispersoid particles finer than U. S. Mesh (Standard Sieve) 70 and coarser than US Mesh 200). In another desirable embodiment, the dispersoids include a particle size distribution from about 210 to about 300 microns (i.e. including dispersoid particles finer than US Mesh 50 and coarser than US Mesh 70). In yet another desirable embodiment, in which sugar is used as a dispersoid, particle size distributions ranging from about 150 to about 500 microns may be used (i.e. including dispersoid particles finer than US Mesh and coarser than US Mesh 100).

The abrasive segments described hereinabove may be used to fabricate substantially any type of grinding tool. Generally desirable tools include surface grinding wheels (e.g. ANSI Type 2A2T or Type 2A2TS abrasive wheels and Type 1A and IA1 abrasive wheels) as well as cup wheels (e.g.

ANSI Type 2 or Type 6 wheels, or Type 1 19V bell-shaped cup wheels). The abrasive grinding wheels may include a core (e.g. core 20 of Figures 2A to 2C) having a central bore for mounting the wheel on a grinding machine, the core being designed to support a porous abrasive rim disposed along its periphery (see for example grinding wheel 100 in Figure 2A, which is discussed in more detail hereinbelow with respect to the Example). These two portions of the wheel are typically held together with an adhesive bond that is thermally stable under grinding conditions, and the wheel and its components are designed to tolerate stresses generated at wheel peripheral speeds of up to at least 80m/sec, and desirably up to 160m/sec or more.

In one embodiment the core is substantially circular in shape. The core may comprise substantially any material having a minimum specific strength of 2.4 MPa-cm°/g and, more desirably, in the range from about 40 to about 185 MPa—cm°/g. The core material has a density of 0.5 to 8.0 g/cm°, and preferably from about 2.0 to about 8.0 g/cm3. Examples of suitable materials are steel, aluminum, titanium, bronze, their composites and alloys, and combinations thereof. Reinforced plastics having the designated minimum specific strength may also be used to construct the core. Composites and reinforced core materials typically include a continuous phase of a metal or a plastic matrix, often initially provided in powder form, to which fibres or grains or particles of harder, more resilient, and/or less dense, material is addedas a discontinuous phase. Examples of reinforcing materials suitable for use in the core of the tools of this invention are glass fibre, carbon fibre, aramid fibre, ceramic fibre, ceramic particles and grains, and hollow filler materials such as glass, mullite, alumina, and Z—Light spheres. Generally desirable metallic core materials include ANSI 4140 steel and aluminum alloys 2024, 6065 and 7178. Further detail regarding suitable core materials, properties, and the like is provided in the Ramanath patents, U8-A-6093092 and US-A- .

A grinding wheel (e.g. grinding wheel 100 shown in Figure 2A) may be fabricated by first forming individual segments of a preselected dimension, composition and porosity, as described hereinabove (see for example segment 10 shown in Figure 1, which is discussed in more detail hereinbelow with respect to the Example). Grinding wheels may be moulded and cured by a variety of processes known in the art. Among these processes are hot pressing (at pressures of about 14-28 MPa), cold pressing (at pressures of about 400-500 MPa or more), and hot coining in a steel mould (at pressures of about 90-1 10 MPa). The skilled artisan will readily recognise that cold pressing (and to a lesser extent hot coining) are useful only for dispersoid particles having a high compressive strength (i.e. resistance to crushing).

For organic bond abrasive articles in which a sugar containing dispersoid is used, cold or "warm" pressing (at temperatures less than about 160°C) may be desirable. Additional details regarding pressing and thermal processing techniques are provided in US Patent 5827337, which is fully incorporated herein by reference.

Following pressing, thermal processing, and immersing into a solvent, the segments are typically finished by conventional techniques, such as by grinding or cutting using vitrified grinding wheels or carbide cutting wheels, to yield an abrasive rim segment having the desired dimensions and tolerances.

The segments may then be attached to the periphery of the core with a suitable adhesive (see, for example, Figures 2A to 2C, which is also discussed hereinbelow). Desirable adhesives include 353-NDT epoxy resin (EPO-TEK, Billerica, MA) at a 10:1 weight ratio of resin to hardener, and Technodyne® HT-18 epoxy resin (obtained from Taoka Chemicals, Japan) and its modified amine hardener mixed in a ratio of about 100 parts by weight resin to about 19 parts by weight hardener. Further detail regarding adhesives, their properties, and the application thereof to metal bond grinding wheels is provided in the Ramanath patents referred to above.

The abrasive articles and tools of this invention (e.g. grinding wheel 100 shown in Figure 2A and discussed in more detail hereinbelow) are desirable for grinding ceramic materials including, various oxides, carbides, nitrides and silicides, such as silicon nitride, silicon dioxide, and silicon oxynitride, stabilised zirconia, aluminum oxide (e.g. sapphire), boron carbide, boron nitride, titanium diboride and aluminum nitride and composites of these ceramics, as well as certain metal matrix composites, such as cemented carbides, polycrystalline diamond and polycrystalline cubic boron nitride.

Either single crystal or polycrystalline ceramics may be ground with these abrasive tools. Further, the abrasive articles and tools of this invention are particularly well suited for grinding materials used in electronics applications, such as silicon wafers (used in semiconductor manufacturing), alumina titanium carbide (used in magnetic head manufacturing), and other substrate materials.

The following example illustrates the present invention. Unless otherwise indicated, all parts and percentages in this example are by weight.

EXAMPLE Segmented grinding wheels, each including sixteen segments, were assembled to construct a Type 2A2TS face-grinding type grinding wheel 100, as shown in Figure 2A. Grinding wheel 100 includes sixteen symmetrically spaced segments 10 bonded to an aluminum core 20, yielding a grinding wheel 100 having an outer diameter 102 of about 282 millimetres and a slotted rim 104. As shown at 110 the segmented rim protrudes a distance 112 from the face of aluminum core 20 of about 3.9 millimetres. The abrasive segments 10 and the aluminum core 20 were assembled with an epoxy resin/amine hardener cement system (T echnodyne HT-18 adhesive, obtained from Taoka Chemicals, Japan) to make grinding wheels having a slotted rim 104 consisting of sixteen abrasive segments 10. The contact surfaces of the core and the segments 10 were degreased and sandblasted to insure adequate adhesion. The segments were fabricated as described below: Granular sugar (obtained from Shaw's, lnc., Worcester, MA) was shaken in a -10. .5 litre paint can for approximately 2 hours using a paint shaker (made by Red Devils, lnc., Union, NJ), in order to break off the sharp corners and edges, thereby effectively "rounding" the sugar granules. The granular sugar was then screened to obtain a particle size distribution from about 250 to about 500 microns (i.e. -35/+60 US Mesh).

Powdered resin bond was pre-screened through a US Mesh 200 screen in order to remove agglomerates. Fine diamond abrasive powder, particle size distribution from about 3‘to about 6 microns obtained from Amplex® Corporation (Olyphant, Pennsylvania) as BB3/6 was added to the powdered resin and mixed until substantially homogeneous. The mixture, including approximately 80 volume percent resin and about 20 volume percent abrasive, was screened three times through a US Mesh 165 screen and was then added to granular sugar (prepared as described above). The resin/abrasive/sugar mixture was then mixed until substantially homogeneous and screened twice through a US Mesh 24 screen.

Three composite mixtures were fabricated. The first mixture (used in the fabrication of wheel 7-A) included about 4 volume percent diamond abrasive, about 20 volume percent 33-344 resin bond (a bisphenol-A modified phenolic resole resin obtained from Durez Corporation of Dallas, TX), and about 76 volume percent granular sugar. The second mixture (used in the fabrication of wheel 7-B) included about 6 volume percent diamond abrasive, about 30 volume percent 29-346 resin bond (a long flow phenolic novolac resin obtained from Durez Corporation of Dallas, TX), and about 64 volume percent granular sugar. The third mixture (used in the fabrication of wheel 7- 0) included about 6 volume percent diamond abrasive, about 30 volume percent 29-108 resin bond (a very long flow bisphenol-A modified resole obtained from Durez® Corporation of Dallas, TX), and about 64 volume percent granular sugar.

The resin/abrasive/sugar mixtures were paired into disk shaped steel moulds, levelled, and pressed at a temperature of about 135°C at a pressure of about 4100 psi (28 MPa) for about 30 minutes until a matrix with approximately 99% theoretical density is achieved. After cooling the disksvwere lightly sanded with 180 grit sandpaper to remove the mould skin and the sugar dispersoid removed by immersing in boiling water for approximately 2 hours.

After sugar removal the disks were dried and baked to complete the curing of the resin. The drying and baking cycle was as follows. The disks were first ramped to 60°C with a ramp time of about 5 minutes and held thereat for about 25 minutes. The disks were then ramped to 90'C with a ramp time of about 30 minutes and held thereat for 5 hours. Finally, the disks were ramped to t60'C with a ramp time of about 4 hours and held thereat for about 5 hours. After baking the disks were cooled to room temperature and milled into segments for use in assembling grinding wheels.

Three organic bonded segmented wheels were tested for fine backgrinding performance on silicon wafers. The grinding testing conditions were: Grinding Test Conditions: Machine: Strasbaugh 7AF Model Wheel Specifications: Coarse spindle: Norton #3-Fl7B69 Fine Spindle: Wheel 7-A Wheel 7-B Wheel 7-C Wheel Size: Type 2A2TSSA: 280 x 29 X 229 mm (11 x1 1/8 x 9 inch) Grinding Mode: Dual grind: Coarse grind followed by fine grind Fine Grinding Process: Wheel Speed: Coolant: rpm Deionised water Coolant Flow Rate: 3 gal/min (11 litres/min) Work Material: Silicon wafers, N type 1 O0 orientation, 150 mm diameter (6 inch), 0.66 mm (0.026 in.) starting thickness (obtained from Silicon Quest, CA) Material Removed: step 1: 10 um, step 2: 5 pm, step 3: 5 pm, lift: 2 pm Feed rate: step 1: 1 um/s, step 2: 0.7 pm/s, step 3: 0.5 pm/s, lift: 0.5 pm/s Work Speed: 590 rpm, constant Dwell: 100 rev Coarse Grinding Process: Wheel Speed: 3,400 rpm Coolant: Deionised water Coolant Flow Rate: 3 gal/min (11 litres/min) Work Material: Silicon wafers, N type 100 orientation, 150 mm diameter (6 inch), 0.66 mm (0.026 in.) starting thickness (obtained from Silicon Quest, CA) Material Removed: step 1: 10 pm, step 2: 5 pm, step 3: 5 pm. lift: 10 um Feed rate: step 1:3 um/s, step 2: 2 um/s, step 3: 1 pm/s, lift: 5 um/s Work Speed: 590 rpm, constant Dwell: 50 rev Where abrasive tools required truing and dressing, the conditions established for this test were as follows: Truing and Dressing Operation: Coarse Wheel: using 150 mm (6 inch) diameter Strasbaugh coarse dressing pad Wheel Speed: 1200 rpm Dwell: 25 rev Material removed: step 1: 190 pm, step 2: 10 um, lift: 20 um Feed rate: step 1: 5 um/s, step 2: 0.2 um/s, lift: 2 um/s Work Speed: 50 rpm, constant Fine Wheel: using 150 mm (6 inch) diameter Slrasbaugh extra fine dressing pad Wheel Speed: 1200 rpm Dwell: 25 rev Material removed: step 1: 150 um, step 2: 10 um, lift: 20 pm Feed rate: step 1: 5 pm/s, step 2: 0.2 um/s, lift: 2 um/s Work Speed: 50 rpm, constant Results for the grinding test are shown below in the Table. Two hundred wafers were fine ground using the porous, resin-bonded wheels of this invention (wheels 7-A, 7-B. and 7-C). Each of the inventive wheels exhibited relatively stable peak normal force of about 90 N (i.e. about 20 lbs) for at least two-hundred wafers. This type of grinding performance is highly desirable in backgrinding silicon wafers because these relatively low force, steady state conditions minimise thermal and mechanical damage to the workpiece. Further, the porous wheel of this invention provided for the highly desirable grinding performance described above for at least two hundred wafers without the need for dressing of the wheel.

Additionally, the resin-type was observed to effect the wear rate of the grinding wheel. Wheels 7-A and 7-C exhibited relatively high wear rates of 2.2 and 1.7 microns per wafer, respectively, while wheel 7-B (including the long flow phenolic novolac resin) exhibited a relatively low (and desirable) wear rate of 0.5 microns per water. in summary, this Example shows that the wheels including abrasive segments according to the invention produce highly desirable backgrinding performance on silicon wafers.

Table Wheel Specification Peak Normal Force (N) Wear Rate (micron/wafer) Wheel 7—A 90 2.2 (DZ 33-344) Wheel 7-B 90 0.5 (IZ 29-346) Wheel 7-C 90 1-7 (IZ 19-108)

Claims (18)

1. An abrasive segment for a segmented grinding wheel, the abrasive segment comprising a composite including a plurality of super-abrasive grains selected from the group consisting of diamond and cubic boron nitride and having an average particle size of less than 300 microns and a non- metallic bond matrix cured together, the composite having a plurality of interconnected pores disposed therein, and including from 0.5 to 25 volume percent abrasive grain, from 19.5 to 65 percent non-metallic bond and from 40 to 80 volume percent interconnected porosity.

2. An abrasive segment according to Claim 1 wherein the plurality of super- abrasive grains comprise diamond and have an average particle size ranging from 0.5 to 75 microns.

3. An abrasive segment according to Claim 1 or 2 wherein the non—metallic bond matrix comprises an organic bond material.

4. An abrasive segment according to Claim 3 wherein the organic bond material comprises a resin selected from the group consisting of phenolic resins, epoxy resins, unsaturated polyester resins, bismaleimide resins, polyamide resins, cyanate resins, melamine polymers and mixtures thereof.

5. An abrasive segment according to Claim 3 wherein the organic bond material comprises a phenolic resin.

6. An abrasive segment according to Claim 3 wherein the organic bond material comprises a phenolic novolac resin.

7. An abrasive segment according to Claim 3 wherein the organic bond material comprises a phenolic resole resin. . 15 -

8. An abrasive segment according to any one of Claims 1 to 7 wherein the interconnected porosity is formed by a) adding a dispersoid to the grains and non-metallic bond prior to curing the composite; and b) immersing the cured composite into a solvent and dissolving the dispersoid to leave the abrasive segment substantially free of dispersoid particles.

9. An abrasive segment according to Claim 8 wherein the composite is formed by a) blending a mixture including from 0.5 to 25 volume percent abrasive grain, from 19.5 to 65 volume percent non-metallic bond material and from 40 to 80 volume percent dispersoid particles; b) pressing the mixture into an abrasive laden composite and wherein the method of fabricating the segment includes the step of thennally processing the composite.

10. An abrasive segment according to Claim 1 or 9 wherein the dispersoid particles are substantially non-ionic.

11. An abrasive segment according to Claim 9 or 10 wherein the dispersoid particles comprise sugar.

12. An abrasive segment according to any one of Claims 9 to 11 wherein the pressing (b) comprises pressing for at least five minutes at a temperature ranging from 100 to 200'C at pressures ranging from 20 to 33 megaPascal.

13. An abrasive segment according to any one of Claims 9 to 12 wherein the themial processing is curing at a temperature ranging from 100 to 200'C. _ 17 _

14. An abrasive segment according to any one of Claims 9 to 13 wherein the thermal processing (c) is performed after the immersing and comprises baking for at least one hour at a temperature ranging from 100 to 200'C.

15. An abrasive segment according to any one of Claims 8 to 14 wherein at least one surface of the composite is abraded prior to the immersing (b).

16. A segmented grinding wheel comprising: a core having a minimum specific strength of 2.4 MPa-cm°/g, a core density of 0.5 to 8.0 g/cm3 and a circular perimeter, an abrasive rim including a plurality of segments according to any one of Claims 1 to 15 and a thermally stable adhesive bond between the core and each of the plurality of segments.

17. A segmented grinding wheel according to Claim 16 wherein the organic bond matrix comprises a phenolic resin; the abrasive grain comprises diamond having an average particle size ranging from 0.5 to 300 microns; the thermally stable adhesive bond comprises an epoxy adhesive bond; and the interconnected porosity is formed by adding a granular sugar dispersoid to the abrasive grains and organic bond prior to curing the composite and immersing the cured composite into water solvent and dissolving the dispersoid.

18. An abrasive segment for a segmented grinding wheel according to Claim 1 substantially as herein described in the Examples with reference to and as shown in