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Advances in Nano-finishing of Optical Glasses and Glass Ceramics M. Buchi Suresh, I. A. Rasheed, and Mahender Kumar Gupta Contents Introduction ....................................................................................... 2 Introduction to Glass and Glass Ceramics ....................................................... 3 Glass and Its Importance ..................................................................... 3 Glass Ceramics and Its Importance .......................................................... 3 Low Expansion Glasses and Glass Ceramics and Its Importance ........................... 4 Various Polishing and Nano-nishing Techniques .............................................. 4 Ductile Grinding .............................................................................. 5 Fresh-Feed Polishing ......................................................................... 5 Chemical Polishing ........................................................................... 6 Bowl-Feed Polishing (Float Polishing) ....................................................... 7 Elastic Emission Machining (EEM) .......................................................... 8 Magnetorheological Finishing (MRF) ........................................................ 9 Laser Polishing ................................................................................ 17 Ion Beam Polishing ........................................................................... 18 Abrasive Slurry Jet (ASJ) Machining ........................................................ 19 Plasma-Assisted Etching (PAE) .............................................................. 20 Characterization Techniques ..................................................................... 21 Test Plate Technique .......................................................................... 21 Stylus Proler Technique ..................................................................... 23 Atomic Force Microscope (AFM) ............................................................ 23 Scanning Electron Microscope (SEM) ....................................................... 23 White Light Interferometry ................................................................... 24 Phase Shift Interferometry (PSI) ............................................................. 25 Coherence Correlation Interferometry (CCI) ................................................. 25 M. B. Suresh International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad, India e-mail: [email protected] I. A. Rasheed · M. K. Gupta (*) Research Centre Imarat, Hyderabad, India e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2020 Y. Mahajan, R. Johnson (eds.), Handbook of Advanced Ceramics and Composites, https://doi.org/10.1007/978-3-319-73255-8_17-1 1

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Page 1: Advances in Nano-finishing of Optical Glasses and Glass ...link.springer.com/content/pdf/10.1007/978-3-319-73255-8_17-1.pdfGlass ceramics · Optical components · Surface roughness

Advances in Nano-finishing of OpticalGlasses and Glass Ceramics

M. Buchi Suresh, I. A. Rasheed, and Mahender Kumar Gupta

ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Introduction to Glass and Glass Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Glass and Its Importance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Glass Ceramics and Its Importance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Low Expansion Glasses and Glass Ceramics and Its Importance . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Various Polishing and Nano-finishing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Ductile Grinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Fresh-Feed Polishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Chemical Polishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Bowl-Feed Polishing (Float Polishing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Elastic Emission Machining (EEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Magnetorheological Finishing (MRF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Laser Polishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Ion Beam Polishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Abrasive Slurry Jet (ASJ) Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Plasma-Assisted Etching (PAE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Test Plate Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Stylus Profiler Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Atomic Force Microscope (AFM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Scanning Electron Microscope (SEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23White Light Interferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Phase Shift Interferometry (PSI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Coherence Correlation Interferometry (CCI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

M. B. SureshInternational Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad,Indiae-mail: [email protected]

I. A. Rasheed · M. K. Gupta (*)Research Centre Imarat, Hyderabad, Indiae-mail: [email protected]; [email protected]

© Springer Nature Switzerland AG 2020Y. Mahajan, R. Johnson (eds.), Handbook of Advanced Ceramics and Composites,https://doi.org/10.1007/978-3-319-73255-8_17-1

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Case Study for Polishing/Finishing Techniques Used for Achieving theNano-finished Optics Using Glass and Glass Ceramic Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Float Polishing Using Pitch Lap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

AbstractOptical glass and glass ceramic components with angstrom-level surface roughnessand nanometer-level dimensional accuracy are in potential demand for sophisticatedoptical fabrication. In recent years, aspherical and free-form surfaces are gainingprominence for high-performance applications. Moreover, the new optical materialsand fabrication process which exhibit superior mechanical properties are beingdeveloped to meet the stringent requirements and harsh environment. Fabricationof complex-shaped high optical finish components becomes a significant challengeas conventional finishing techniques are unable to machine aspherical or free-formsurfaces precisely. This situation demands few highly advanced and precisefinishing processes which ensure stress-free surfaces. Mostly, the optical compo-nents are fabricated by shaping or pre-finishing methods followed by final finishingprocesses. Final finishing processes include more deterministic and flexiblepolishing techniques that can achieve desired surface finish, figure accuracy andsurface integrity to make it suitable for shorter wavelength applications. In thischapter, basic principle, mechanism of various material removal processes, andprecision polishing techniques such as magnetorheological fluid-based finishingwere discussed and are compared with the convention polishing techniques.

KeywordsGlass ceramics · Optical components · Surface roughness · Material removal ·Polishing · Magnetorheological finishing

Introduction

The surface quality of glass and glass ceramics is one of the important parametersthat affects the functionality of the final product. Most of the products essentiallyrequire nano-level surface finish. The processes of nano-finishing have flexiblefinishing tools, which can be employed for such type of work pieces. This chapterprovides a comprehensive review of nano-finishing processes and their applications.

Traditional polishing processes cannot finish complex-shaped work pieces, 3Dcomponents, etc. Therefore, there is a requirement of finishing processes withcharacteristics such as deterministic, in-process externally controllable forces actingon the work pieces, capable of surface finishing complex-shaped components andalso providing surface finish superior to the traditional finishing processes. Selectionof suitable machining conditions plays an important role in the control of surfaceand/or subsurface defects [1].

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Traditionally for generating flat or spherical nano-finished optical surfaces, freelymovable abrasive as well as brittle mode grinding techniques are being adopted.These are also termed as self-correcting processes, where two initially imperfectsurfaces get into contact with slurries, pitch pad, and tool surface, so that an areaaveraging mechanism occurs resulting in low surface roughness value of the order of0.5 nm rms. These processes have an advantage in removing stresses and subsurfacecracks, which are easily generated in brittle materials like glass and glass ceramics[2, 3]. This chapter includes the following:

Introduction to Glass and Glass Ceramics

Glass and Its Importance

Glass is a form of supercooled state of silicon dioxide and various other ingredientslike calcium and other certain alkalies. It is a supercooled state containing anamorphous and noncrystalline structures. Optical glasses are high-quality glassescompared to window glasses, which are formulated specifically with requiredcharacteristics that affect the transmission of light. The quality standards have anumber of criteria, like highly transparent over the visible spectrum, opticallyhomogeneous, and inclusion-, strain-, and stress-free with defined optical constants,which have a small dependency over temperature. It should have high mechanicalstrength for easy handling of optical components during processing and assemblingand also with high resistance to chemicals. There are special-purpose glasses likelaser glass, color filter glass, and technical sheet glass, which are specially formu-lated for the use in optical systems. Laser glasses are used in optically pumped high-power lasers, and the laser rods that are made from these materials emit light in thenear-infrared region of the spectrum at about 1.06 μm. They are also used as diskamplifiers in large laser fusion experiments and also used as a source element innumber of commercially available laser systems. Color filter glasses are used inoptical systems, as they have specific spectral transmittance characteristics withinthe range of optical spectrum.

Glass Ceramics and Its Importance

Glass ceramics are also known as vitrocerams, pyrocerams, vitroceramicos,vitroceramiques, and sittals. They are processed by controlled crystallization ofcertain glasses, induced by nucleating agents. Glass ceramics always contain oneor more embedded crystalline phases along with a residual glassy phase. Thecrystallinity in the glass ceramics varies most frequently between 30% and 70%.Controlled ceramization yields an array of materials with interesting combination ofproperties which are inherently free from porosity. The first commercially viableglass ceramic was developed by US aerospace industry in the late 1950s as radomes.It was used to protect the radar equipment in the nose cones of aircraft and rockets.

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Glass ceramics used in the above applications must exhibit a combination ofproperties to withstand critical conditions like rain erosion, atmospheric reentry,homogeneity, low dielectric constant, low coefficient of thermal expansion (CTE),low dielectric loss, high mechanical strength, and high abrasion resistance. No glassor polycrystalline material or single crystal can simultaneously meet all the requiredspecifications.

Low Expansion Glasses and Glass Ceramics and Its Importance

Optical materials like fused quartz, fused silica, and crystal quartz and low expansionmaterials such as Pyrex, Cer-Vit, Sital, and Zerodur have special properties likesemitransparent and nonporous glass ceramics, which help in the grinding andpolishing process very easily and also maintain the structure over a wide range ofoperating temperatures. They are also chemically stable and mechanically strong.Fused quartz and fused silica belong to a very hard and transparent material withexcellent optical clarity. The raw material contains 100% pure silicon dioxide (SiO2),with crystalline structure, which is fused at high temperature to form amorphous orvitreous material.

Another important material is Zerodur, which is a semitransparent, nonporousglass ceramic having an extremely low coefficient of thermal expansion of0.00� 0.02 x 10�6/K between 0 �C and 50 �C. Unique characteristic of this materialis its exceptional good homogeneity and good transparency in the range 400 to 2300nanometers. This material is preferred for lightweight honeycomb mirror mounts forsatellite mirrors, manufacturing of precision optics, high-quality mirror substrates forlarge-size astronomical telescopes, X-ray telescopes, optical elements for cometprobes, ring laser gyroscopes, and optical standards for precision measurementtechnology [4–6].

Various Polishing and Nano-finishing Techniques

Optical elements of glass and glass ceramics, utilized in sophisticated instrumentslike laser-based optical rotation sensors, gradiometers, and accelerometers, instru-ments of photolithography in ultraviolet regions, and high-power laser-based equip-ment, should have:

• Shape integrity and accuracy to ensure better imaging performance• Ultrasmooth to nano-finished surfaces to avoid scattering• Minimum subsurface damage to avoid deterioration in the performance

Hence, a number of fabrication techniques have been evolved based on therequirement, and it also depends on the characteristics of the optical component.To generate the required surface on brittle material, two steps are generally followed,namely, rough shaping mode and finishing mode. The rough shaping mode is

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characterized by high material removal rate leaving a rough surface finish with ahighly stressed layer of subsurface damage approximately from 1 to 10 micron depthin loose abrasive grinding process. In finishing mode, the roughness and the sub-surface damage generated during grinding process has to be removed using fine gritabrasives or by using polishing powders whose average particle size varies frommicron to nanometer levels with different polishing techniques which has beenevolved during the late nineteenth and twentieth centuries [7, 8].

Ductile Grinding

In this process, the material removal is being accomplished by plastic flow and notby brittle fracture. It also depends on Young’s modulus and rigidity modulus of thematerial and machining depth [9]. To achieve a perfect ductile grinding finish,single-point diamond turning (SPDT) concept can be utilized instead of looseabrasive grinding. This ductile grinding helps in achieving higher material removal,i.e., 30 μm/hr., than fresh-feed polishing. Major disadvantage of this process is that itleaves a highly stressed surface. In order to achieve the machining depths ofnanometer scale, a high infeed resolution is necessary [10]. The surface roughnessachieved by this technique is approximately 2–3 nm rms in BK7 material. Figure 1shows the schematic of ductile grinding process.

Fresh-Feed Polishing

It is a chemico-mechanical process, which reduces the roughness, stress, and sub-surface cracks formed during the previous ground surface. It causes a minimumsubsurface damage, using pitch-flow effects and in-process correction of the gener-ated shape. A small amount of wear occurs during the process, and the fundamentalvariables involved are more in this type of polishing technique [11]. The surface

Ductile mode Grinding

Cutting force

Fix abrasive

Small damage layerDuctile remove

Fig. 1 Schematic of ductilegrinding process. (Courtesy:http://www.kinik.com.tw/en-us/Products/ReclaimedWafer.html)

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roughness achieved in BK7 glass material is about 0.5 nm rms. The schematic offresh-feed polishing process is shown in Fig. 2 [12].

Chemical Polishing

Chemico-mechanical polishing (CMP) is a planarization technique that includes acombination of chemical and mechanical processes, which is mainly used in thesemiconductor manufacturing industries. In CMP, the reaction takes place betweenthe abrasive-based polishing fluid and the work piece, and the abrasive removes thereaction products of the surface of the work piece by mechanical action [13]. Figure 3shows the schematic of CMP planarization process.

It is an acid and base reaction, in which the material will be removed usingreagents like hydrofluoric acid, sulfuric acid, nitric acid, and glycerine. Later the

Spindle executing simpleharmonic motion

Polyurethane Padfixed on a metal tool

Movable weight for load adjustment

Glass substrateblocked on a metal

tool

Fig. 2 Schematic of fresh-feed polishing process. (Courtesy: www.iopscience.iop.org, obtainedfrom Ref. [12])

Wafer

Head

Platen

Conditioner

Pad

Slurry

Fig. 3 Schematic of chemo-mechanical polishing process.(Courtesy: www.jss.ecsdl.org)

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components will be subjected to base solution wherein sodium hydroxide solution isused in general. Due to this chemical etching process, a kind of polished surface getsgenerated with a low level of surface roughness, and a high accuracy in shapeassociated with marginal amount of subsurface damages can be produced. Thedisadvantage of this technique is that the chemicals stick onto the surface andwere removed later [14]. Hence, this reaction is highly sensitive to inhomogeneousmaterial, and the surface roughness obtained by chemical bowl-feed polishingtechnique is 0.5 nm rms for silicon samples.

Bowl-Feed Polishing (Float Polishing)

Bowl-feed polishing is also referred to as super-polishing. Similar to the conven-tional polishing techniques, in the bowl-feed polishing technique, the sample oscil-lates with 28 number of strokes per minute over the rotating pitch lap. The basicdifference between bowl-feed polishing and traditional (fresh-feed) polishing is theuse of a bowl with a suspension of aqueous polishing material, rotating with thesame speed as the pitch lap [15]. Because of the centrifugal and gravitational forcesacting simultaneously, the particles of the polishing material in the suspension movetoward the perimeter and to the bottom of the bowl. This is confirmed by the fact thatthe polishing material sinks visibly to the bottom within an hour in the rotation bowl

Stirrer

Sample

Pitch lap

Polishingcompoundin water

Fig. 4 Top and side views ofthe bowl-feed polishing.(Adopted from Ref. [16])

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if no obstacles are encountered. Figure 4 shows the top and side views of the bowl-feed polishing [16].

The bowl-feed polishing starts with a stirrer in the bowl and provides a fairlyhomogeneous distribution of the polishing material in the bowl. Then, a continuousdeposition of new polishing material occurs on the pitch lap, because the suspensionflows over its edges. The stirrer is removed after a good contact between the rotatingpitch lap, and the surface of the work piece is obtained. At this stage, the suspendedparticles start sinking, and the quantity of polishing material falling on the pitch lapgradually decreases until pure water flows over the lap [16]. Since the polishingparticles are broken and pushed into the pitch during polishing, the surface of thepitch lap tends to become smoother in nature. Subsequently, the smooth lap surfaceenormously reduces the surface roughness of the polishing material [17].

The sample along with the lap are located in the bowl and submerged in a fluidcontaining the polishing material in powder form. The sample and the lap rotates inthe same direction and with the same speed. It generates a super smooth surface anda damage-free surface. Initially, there is a mechanical contact between the lap andsample, but later they are separated only by a layer of water. The disadvantage of thistechnique is to achieve high shape accuracy and to maintain a temperature controlover the whole surface of the sample [18]. Using this technique, the surfaceroughness of 0.2 nm rms has been achieved on the flat surface of quartz and0.13 nm rms on flat surface of BK7.

Elastic Emission Machining (EEM)

Elastic emission machining (EEM) is a precise surface finishing technique that usesthe reaction between the surfaces of the work piece and finer powder particles. It isone of the ultraprecision machining methods used to fabricate shapes with anaccuracy of 0.1 nm [19]. When the powder particle surface and the surface of thework piece come into contact with each other and during the process, the material isbeing removed from the surfaces, and there is a notable probability that the atoms onone surface will stick and move onto the other. EEM has already been applied toobtain a surface finish of the high-quality X-ray mirror optical surfaces and single-crystal Si(001) surfaces. The schematic of elastic emission machining process isshown in Fig. 5.

In this technique, a polyurethane sphere is made to float on the liquid layercontaining the abrasive particles. While polishing, the diameter of the polyurethanesphere exceeds the applied abrasive particle size, and material removal will beactivated chemically which occurs on an atomic scale. The work piece and rotatingtool are all taken in a pool of slurry. The tool is made of polyurethane rubber, whichis a soft material. EEM method makes use of the hydrodynamic effect to create alubrication film between the surface of the work piece and the rotating tool. Thearticle size should be much smaller than the thickness of the film. The abrasiveparticles are carried into a region between the rotating tool and the work surface by adrag force of the moving fluid. Because of the contacts between the particles and the

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work surface, machining occurs on an atomic scale. Till now, no super-polishedsamples have been generated whose surface roughness is less than 0.5 nm rms. Theelastic behavior and wear rate of the applied polyurethane sphere systematicallyhave to be investigated for obtaining super smooth surface finish on glass surface.Elastic emission polishing technique has been applied to silicon sample to polish tothe surface roughness of <0.5 nm using ZrO2 as a polishing abrasive material [20].

Magnetorheological Finishing (MRF)

Traditional methods of finishing high precision lenses, optical glasses, opticalceramics, and glass ceramics are very expensive and labor-intensive. To overcomethe difficulty in manufacturing high precision lenses, the Center for OpticsManufacturing (COM) in Rochester, New York, has developed a unique technologyto automate the optical lens finishing process known as magnetorheological finishing(MRF) jointly with A.V. Luikov Heat and Mass Transfer Institute (AVLHMTI),Belarus [21–25].

Figure 6 shows the position of a convex or flat or concave work piece above areference surface in the magnetorheological finishing process. A magnetorheologicalfluid material deposited on the rotating wheel is built into a form of a ribbon in the

NC tableDiaphragm pump

Flame

Fluid

Constanttemperatureunit

Work

Polyurethane sphere

Loading rod

Centre of gravity

Variable-speedmotor

NC spindle head

Cross spring

Fig. 5 Schematic diagram of the EEM process. (Adopted from Ref. [20])

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presence of magnetic field, and a finishing area is created by stiffened region. Whenthe lens rotates on a spindle at a constant speed about its radius of curvature throughthe stiffened finishing zone, it results in surface smoothing, and additionally removalof subsurface damage and figure correction are also accomplished. Converging gapbetween the work piece and carrier surface created a shear stress, which is mainlyresponsible for material removal. The zone of contact is restricted to a spot, whichconforms perfectly to the local topography of the part [26].

The magnetorheological fluid, which mainly consists of carbonyl iron (CI),abrasive particles, carrier fluids, and additives, governs the material removal inMRF. Magnetic fluid containing magnetically sensitive particles is mixed withpolishing material (abrasive), namely, cerium oxide. It is stiffened with an externalmagnetic field and is used for local polishing and shaping. These magneticallysensitive particles under the influence of magnetic field and rotating wheel providehigh shear stress to the floating abrasive particles on the polishing fluid. This causesthe friction between abrasives and surface of the work piece, which needs to bepolished [27].

The selection of the carrier fluid is based on the choice of the temperature rangesin which the MR fluid can be applied. Even though silicone oil is the most frequentlyused carrier fluid, hydrocarbon oil has an advantage due to its lower viscosity,suitability for high shear rate use, and better lubrication property. Additionally, ahydrocarbon oil-based magnetorheological fluid material has lower viscosity in theabsence of magnetic field, which is about 0.6 times less than that of siliconeoil-based fluid. On the other hand, aqueous-based MR fluid can minimize thewastage and allows the particles to be easily recycled from the fluid material (Fig. 7).

The strength of the MR fluid is restricted by the magnetic saturation of themagnetic particles. Due to remnant magnetization of the particles, undesired particleaggregation arises in concentrated MR fluids. As a result, the formation of stiffsediments, which are very difficult to re-disperse, occurred. In order to minimize theaggregation of the particles and their settling, different procedures have been pro-posed: (a) adding thixotropic agents such as carbon fibers and silica nanoparticles,

Fig. 6 Schematic of magnetorheological finishing process. (Adopted from Ref [21])

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(b) adding surfactants such as oleic/stearic acid, (c) adding magnetic nanoparticles,(d) using visco-plastic media as a continuous phase, and (e) using water-in-oilemulsions as carrier liquids. Glycerol and surfactants are used in aqueous-basedfluid as stabilizers. Alkali also helps to improve the stability and resistance to rust.This technique is also termed as sub-aperture polishing, and the surface finish thathas been achieved is 1 nm rms on glass surface [21]. The kinetic property of the fluidhas made the tool to wear out less and less occurrence of debris during the process.

Recently, a study has been done, and it was found that, in the process of materialremoval during finishing, the rheology of fluid plays an important role [28]. Therheology of MRF can get affected due to several reasons like temperature, concen-tration of magnetic particles and abrasive particles, concentration of additives, andtype of carrier fluid used to prepare MR fluid. It is observed that particle sizedistribution of abrasive particles affects the rheological behavior of MR polishingfluid [29]. It was also observed that the fluid containing abrasives having narrowparticle size distribution was found to be inducing higher shear stress and higherviscosity, whereas the fluid which contained broad particle size distribution wasfound to impart considerably lower shear rate and lower viscosity at room temper-ature. In addition to this, it is also observed that the fluid with broad particle sizedistribution of abrasive (A-1) showed near to Newtonian fluid behavior, whereas thefluid containing narrow particle size distribution (A-2) showed pseudoplasticbehavior.

In one of the case studies given here, two magnetorheological polishing fluidsMRPF-1 and MRPF-2 were prepared. The composition of these fluids includesmagnetite as magnetic particles (77 wt%) and ceria as nonmagnetic abrasive parti-cles (10 wt%) and silicone oil (13 wt%) as carrier fluid. MRPF-1 contains A-1 as

0 200 400 600 800 1000

0

5

10

15

20

25

30

Particle size (nm)

Inte

nsity

(%)

A-1 A-2

Fig. 7 Particle size distribution of abrasives A-1 and A-2 (Ref. [28])

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abrasive particles, whereas MRPF-2 contains A-2 as abrasive particles. Particlecharacterization data is given in Table 1. Two MR polishing fluids were preparedhaving abrasives with different particle size distribution but almost similar averageparticle size. The magnetic particles (magnetite) used are same in both the fluids.Particle size distribution of abrasive A-1 is broad corresponding to higher polydis-persity index, whereas A-2 depicts narrow particle size distribution corresponding tolower polydispersity index. The rheological studies on the abovementioned MRfluids were carried out using MCR 51, Anton Paar rheometer, and parallel plateconfiguration measurement system.

The particle size distribution depicts a Gaussian-type distribution curve. Theabrasives A-1 and A-2 are found to have nearly same average particle size of around600 nm. The particle size distribution of abrasive A-1 is broad having 133.30 nmdistribution width and 0.0528 as polydispersity index. The particle size distributionof abrasive A-2 is found to be narrow corresponding to 71.58 nm distribution widthand 0.0136 as polydispersity index.

Phase identification and crystal structure of abrasives were studied using BrukerD8 Advance diffractometer by using monochromatic Cu-Kα radiation(λ ¼ 1.5405 Å). X-Ray diffraction pattern of both abrasives A-1 and A-2 is shownin Fig. 8. XRD pattern of A-1 and A-2 matches with JCPDS 96-154-1461 andJCPDS 96-434-3153, respectively. The prominent peaks at 2θ ¼ 28.37�, 32.81�,

Table 1 Viscosity and shear stress data at minimum and maximum shear rate (Ref. [28])

Abrasive

Viscosity (shearrate 65.8 s�1)(Pa.s)

Viscosity (shearrate 1250 s�1)(Pa.s)

Shear stress (shearrate 65.8 s�1)(Pa)

Shear stress (shearrate 1250 s�1)(Pa)

25 �C 40 �C 25 �C 40 �C 25 �C 40 �C 25 �C 40 �CA-1 4.02 1.11 2.16 0.87 264 72.9 2700 1090

A-2 10 1.79 2.42 1.05 659 118 3020 1310

10 20 30 40 50 60 70 80 902Theta

A-1 A-2(222)

(222)

(400)

(400)

(440)

(440)

(622)

(622)

(662)

(662)

(800)

(800)

Inte

nsity

(rel

)

Fig. 8 XRD pattern of abrasives (Ref. [28])

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47.16�, and 55.87� correspond to cubic phase of cerium dioxide (CeO2). In both theabrasives, i.e., A-1 and A-2, crystal phase is found to be same having cell parametera ¼ 5.448 and 10.893 Å, respectively. The crystallite size of A-1 is found to be7.176 Å and of A-2 is found to be 7.170 Å.

EDS analysis was carried out to reveal the elemental composition of the abrasivesA-1 and A-2 as shown in Fig. 9. Analysis confirmed that cerium and oxygen werepresent in A-1 and A-2 fluids.

Figure 10a, b depicts the morphology of abrasives A-1 and A-2, which are used inMRPF-1 and MRPF-2 polishing fluids, respectively. Sample preparation for SEManalysis was done by dispersing small quantity of abrasive powder on conductivecarbon tape, which is further sputtered with gold to get an electrical conductivity onthe surface. It can be seen from the images that the particles have spherical mor-phology in both the abrasive powders. This indicates that the morphological char-acteristics of both the abrasive powders are similar to each other.

Figure 11 shows the rheological behavior of MR polishing fluids in the absence ofmagnetic field at room temperature. Results obtained from rotational shear measure-ments depict that both fluids, i.e., MRPF-1 and MRPF-2, show shear thinningbehavior. Rheology of the particle systems can be influenced by the size distributionof the dispersed particles present in it [19]. Hence, in order to study the influence ofparticle size distribution of nonmagnetic abrasive particles, off-state rheology

800

Full scale counts: 912 ARCI(2857)_pt1

600

400

200

01 2 3 4 5

keV

OP

Ce

Element

OK

PK

CeL

53.19

5.36

41.45

12.47

2.43

85.10

Atom % Weight %

A-1A-2

Ce

Ce

Ce

6 7 8 9 10

800

Full scale counts: 881 ARCI(2855)_pt1

600

400

200

01 2 3 4 5

keV

Element

OK

CeL

50.35

49.65

10.38

89.62

Atom % Weight %

Ce

Ce

6 7 8 9 10

Fig. 9 EDS spectra of abrasives A-1 and A-2 (Ref. [28])

Fig. 10 (a) SEM microstructure of abrasive A-1, (b) SEM microstructure of abrasive A-2 (Ref.[28])

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measurements were carried out. Rheological study was carried out by application ofshear rate from 0 to 1250 s�1. Shear rate vs shear stress graph shows that MRPF-1follows Newtonian fluid-like pseudoplastic behavior, whereas MRPF-2 followsnon-Newtonian fluid-like Bingham plastic behavior. MRPF-1 is composed of abra-sive A-1, which has broad particle size distribution; hence the shear stress observedis lower as compared to shear stress observed in case of MRPF-2 which suggestssignificantly higher shear stress values due to presence of abrasive A-2corresponding to narrow particle size distribution. Although XRD results haveconfirmed both A-1 and A-2 abrasives in cubic ceria phase, EDS results giveadditional information about the presence of phosphorous element in A-1 whichmay be an impurity. This presence of phosphorous in A-1 could be probably one ofthe reasons for its decreased off-state shear stress behavior and viscosity.

0 200 400 600 800 1000100

1000

She

ar s

tres

s (P

a)

Shear rate (1/s)

MRPF-1 MRPF-2

0 200 400 600 800 1000

10

Vis

cosi

ty (P

a.s)

Shear rate (1/s)

MRPF-1 MRPF-2

Fig. 11 Rheological behavior of MR polishing fluids in absence of magnetic field at 25 �C (Ref.[28])

200 400 600 800 1000

100

1000

She

ar s

tres

s (P

a)

Shear rate (1/s)

MRPF-1 MRPF-2

0 200 400 600 800 1000

1

Vis

cosi

ty (

Pa.

s)

Shear rate (1/s)

MRPF-1 MRPF-2

Fig. 12 Rheological behavior of MR polishing fluids in absence of magnetic field at 40 �C (Ref.[28])

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Rheological behavior of MR polishing fluids in the absence of magnetic field at40 �C is shown in Fig. 12. While performing MR finishing on a work piece, there is achance of increase in temperature in the work piece as well as in the fluid. Moreover,seasonal temperature variation can also induce increase in temperature in the fluid.Hence, a rheology study was carried out at slightly higher temperature (40 �C) tostudy the effect of temperature. It was observed that at 40 �C there was significantdecrease in shear stress as well as viscosity of both the fluids, i.e., MRPF-1 andMRPF-2. The initial shear stress was decreased by 2.7 times and 6 times for A-1 andA-2, respectively, whereas the initial viscosity was decreased by 3.63 times and 5.55times for A-1 and A-2 abrasives, respectively.

Figure 13 shows the rheological behavior of MR polishing fluids in presence ofmagnetic field at room temperature (25 �C). The above graph portrays magnetor-heological study of MRPF-1 and MRPF-2. The rheology of fluids in absence and inpresence of magnetic field shows similar trends. As observed earlier, the viscosity ofMRPF-2 is higher than MRPF-1. This concludes that the particle size distribution ofabrasive particles affects the rheology in presence of magnetic field as well. In thepresence of magnetic field, the magnitude of viscosity is significantly higher ascompared to viscosity in the absence of magnetic field. Viscosity variation with shearstress at different shear rates is given in Table 1.

Mechanism of MR FinishingMagnetorheological (MR) polishing fluid is composed of magnetic particles, abra-sive particles, carrier fluid, and additives in the form of a suspended liquid. Whenthis fluid is placed in the magnetic field, the apparent viscosity and yield stress of theMR polishing fluid increase by several orders of magnitude. This leads to stiffeningof the fluid. This transition from fluid to stiff nature is reversible and very rapid.When the fluid is under the influence of magnetic field, the abrasives float on thesurface of fluid and come in contact with the surface of polishing glass ceramic workpiece. This leads to material removal from the surface of work piece, hence resultingin high precision polishing. The rate of material removal during polishing in MRF iscontrolled by regulating and tuning the applied magnetic field. In Zhang et al.’s study

0 200 400 600 800 1000

100

1000

10000

Log

Vis

cosi

ty (

Pa.s

)

Shear rate (1/s)

MRPF-1, 5A MRPF-2, 5A

Fig. 13 Rheologicalbehavior of MR polishingfluids in presence of magneticfield at 25 �C (Ref. [28])

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of MRF polishing, the surface roughness was measured by an atomic force micros-copy before and after polishing as shown in Fig. 14.

From Fig. 14, it can be observed that the mean roughness (Ra) of the surfacedecreases from 1.239 to 0.684 nm and Rms (Rq) decreases from 1.857 to 1.074 nm.Since the material removal in MR polishing was on a microscale level, it is easy toeliminate the residual subsurface damage effectively. MRF has high efficiency andcan reach nanoscale surface finishing quality as compared to conventional polishingmethods. The microphotograph of glass ceramic surface is shown in Fig. 15.

Figures 14 and 15 show many protuberances on the finished surface, whichindicate crystalline grains of glass ceramic. There is an even distribution of crystal-line grains on the surface of glass ceramic, which is nothing but fundamental

50.0

Roughness Analysis

Box StatisticsZ range 72.118nm

0.004nm–5.608nm1.857nm1.239nm72.126nm2.88029.73710.0000.186nm

MeanRaw meanRms (Rq)Mean roughness (Ra)Max height (Rmax)SkewnessKurtosisArea Thresh (%rms)Area Thresh abs

(a) Before MRF

(a) After MRF

Box StatisticsZ range 12.269nm

0.006nm6.994nm1.074nm0.684nm12.278nm2.59013.65710.0000.107nm

MeanRaw meanRms (Rq)Mean roughness (Ra)Max height (Rmax)SkewnessKurtosisArea Thresh (%rms)Area Thresh abs

25.0

50.0 mm25.0number 3-f00

50.0

Roughness Analysis

25.0

50.0 mm25.0number 3-f00

Fig. 14 AFM images of glass ceramic samples (a) before MRF and (b) after MRF

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composition of glass ceramics. Glass phase and crystalline phase are the two phasespresent in glass ceramic. Crystalline phase of glass ceramic has a polycrystallinestructure. Due to the higher hardness of crystalline grains than the glassy phase,glassy phase can be easily removed from the surface of the work piece duringfinishing. The surface seems uneven due to residual crystalline grains [30].

Laser Polishing

A laser beam (CO2 laser) or a flame is utilized in this process, and the glass sample isheated approximately to its transformation point. This technique involves twoeffects: fast heating of the surface, i.e., the surface melts slightly, and it gets heatedup sufficiently to a temperature so that any subsurface damage of the previous roughshaping mode can be rectified. Hence, in this process a large amount of energy actsfor a very short duration of time onto the material, resulting in thermal stress on thesurface. There are some chances of adverse effects due to chemical reactionsbetween the glass and environment, diffusion of gases, separation of glass, andincreased corrosion. Research has already been conducted on laser polishing offused silica since 1982. Using defocused continuous wave CO2 laser radiation anda meandering scanning strategy, an initial roughness of rms ¼ 0.33 nm could bereduced to RMS 0.27 nm. Due to a fast movement of the defocused laser beam(dS > 6 mm), a homogenous temperature distribution in the quasi-line on the surfacewas achieved. The first laser polishing method with pulsed CO2 laser radiation forindustrial usage by fabricating micro optics was a starting point [31].

With laser polishing, the polishing time can be considerably reduced, and thewear of hard tooling is avoided. For laser polishing, a very thin surface layer of theglass is heated up below its evaporation temperature due to the interaction of glassmaterial and laser radiation. With an increase in temperature, the reduction in the

Digital Instruments NanoScopeScan size 5.000µm

1.000µm/div

µM 35.246 nm/divX

1

23

4Z

1.489Hz512Height35.25 nm

Scan rateNumber of samplesImage DataData scale

Fig. 15 AFM surface picture of glass ceramic after MRF

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viscosity in the surface layer leads to the reduction in the surface roughness due tothe surface tension effect. In this method, a surface roughness of the order of 1 nmrms was achieved on BK7 glass material. Figure 16 shows the schematic of laserpolishing technique [32].

Ion Beam Polishing

The optical surfaces are exposed to ion beam, and the material removal is done bysputtering technique. The sputtering rate depends upon the angle of incident beamand the local normal to the surface. The main characteristic of this technique is thatan initial coating has to be deposited onto the surface, which has to be polished. Thefilm is exposed to the ion beam, and the material is sputtered uniformly until the filmis totally removed. This results into a uniformly smooth surface. It is also termed assputtering erosion of the surface by atom-by-atom material removal process [33]. Inthis technique, there is no mechanical contact between the work piece and thepolisher and if a focused ion beam is used then the finishing of various shapes suchas aspheric shaping is also possible. Figuring of 5 μm flat ceramic surface to 2 nmrms roughness can be maintained by this technique. The schematic of ion beampolishing technique is shown in Fig. 17.

The drawback of this technique is that vacuum is needed, implantation of ionsoccurs into the material, sputtering itself is an erosion reaction, and chemicalreaction may occur between the ions and atoms of the surface and is very sensitiveto the existing subsurface damages [34].

Fig. 16 Procedural principle of laser polishing glass material. (PL, laser power; vfeed, feed speed;vscan, scan speed; bL, lL, width, length of the quasi-line; and dS, laser beam diameter) (Adopted fromRef. [32])

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Abrasive Slurry Jet (ASJ) Machining

Figure 18 shows the schematic of abrasive slurry jet micromachining process. Inabrasive slurry jet micromachining (ASJM), pressurized water is used to acceleratethe suspended abrasive particles such as garnet or aluminum oxide (Al2O3) [35]. Forthe same jet dimension and flow speed, slurry jets have a much lower divergenceangle than air jets, allowing for the micromachining of smaller features without theuse of patterned masks. High-pressure ASJM uses a slurry at a pressure of 70 MPa tocut micro-slots into metals and composite materials. In the study on the mechanismof micro-hole formation in glass with ASJM, it was concluded that ductile erosionmechanism is mainly dominated [36]. Also, it was shown that increasing thepressure and erosion time increased the hole depth, with an insignificant effect on

Fig. 17 (a) Schematic of ion beam polishing (courtesy: www.istgroup.com) and (b) experimentalfacility of ion beam polishing. (Courtesy: leica-microsystems.com)

WORKPIECEFIXTURE

PUMP

TANK

TOOL

FE

ED

ABRASIVESLURRY

ABRASIVESLURRY

TOOL CONETOOL CONE

TRANSDUCERTRANSDUCER

Fig. 18 Schematic ofabrasive slurry jetmicromachining process.(Courtesy: https://mechanicalstudents.com)

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the hole diameter. In addition, the machined channels suffer from severe wavinessdue to the mechanical vibration of the set-up in this technique, whereas slurry jetshave a more uniform distribution of impacting velocities across the jet, therebyleading to more uniform machining, much flatter bottoms, and steeper side wallswithout the use of erosion-resistant masks.

Slurry with high viscosity containing polymer abrasives is guided through anozzle with a high pressure greater than 50 bar which is used laterally onto theglass surface. As the polymer molecules whose sizes range from 10 to 200 μm hit thesurface laterally, the surface roughness also gets changed with respect to the high-pressure polymer abrasive. The requirement of high stream pressure and highviscosity slurry are the main disadvantage of this technique. Till now no data hasbeen projected after polishing the glass sample with abrasive slurry jet machiningprocess.

Plasma-Assisted Etching (PAE)

Plasma-assisted etching is an essential part of the microstructuring process infabrication of integrated semiconductor circuits and micro-mechanic devices.Thereby structures down to the submicron range are etched. Such etch processesmust show a high degree of anisotropy and a high selectivity in respect to the etchmask and the substrate [37]. These demands can be satisfied by a chemical etchreaction which is induced by impinging energetic particles such as ions, electrons, orphotons as shown in Fig. 19.

Fig. 19 Schematic ofplasma-assisted etchingprocess. (Courtesy: www.iopscience.iop.org)

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In this process, a small, confined plasma tool is moved over the surface of theglass material, and later shaping and polishing of the sample can be done locally. It isa noncontact method. Here the tool dimension and construction of the same are quitecomplicated. These two factors play a major role in finishing of curved opticalsurfaces [38]. The roughness value of 1 nm RMS on a flat silicate glass samplehas been achieved by this process.

Characterization Techniques

A critical optical surface has to be tested during the fabrication process and should beprecise in dimensions while measuring. This helps in verification of the quality ofoptical surface whether it has been achieved or not. Earlier, test plate mechanism wasutilized for verifying the flatness or sphericity measurement of optical surface duringfabrication process. Nowadays, versatile shop interferometers are being used to measurethe surface figure, surface distortion, surface topography, and radius of curvature forspherical optics. Sophisticated autocollimators, in conjunction with computer technol-ogy, have been developed to measure small angular deviations in terms of arcsecaccuracies and also help in the measurement of parallelism of plane-parallel surfaces.The qualitative determination of surface roughness measurements is also possible withadvanced technology using phase shifting interferometry technique, coherence correla-tion interferometry technique, and polarized light in the advanced interferometry toanalyze the subsurface damages that occurred during fabrication.

Test Plate Technique

It is the simplest and common method of testing the flatness or sphericity of opticallypolished surfaces. The concept of Fizeau fringes or Newton’s rings has been adoptedsince earlier years of the nineteenth century. Fraunhofer was the first person to testthe optically polished surface using test plate method. This technique is also termedas contact interferometry, where the optically polished surface and the test plate arein close contact. The affinity of the work piece with respect to the master test platecan be determined based on the formation of fringes. This helps in correcting theflatness or sphericity of the job during fabrication process in order to achieve thedesired close and regular surface fit.

Newton’s rings or Fizeau fringes represent a special case of interference wherelocalized air film is trapped between two closely fitted transparent surfaces as shownin Fig. 20. This condition exists when test plates are used on matching optically polishedsurfaces. When the light is monochromatic depending on the wavelength and thethickness of the layer, it is either extinguished or reinforced. This results in the formationof characteristic alternate light and dark bands when the thin layer is wedge shaped.

Another way of describing is when one of the rays travels a slightly shorter paththan the other, resulting in a phase difference between the two. If the phase differenceresulting from the optical path difference (OPD) is exactly equal to the wavelength of

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the monochromatic light source, then constructive interference occurs, and a brightfringe is seen. If the optical path difference is a half wave, then destructive interfer-ence occurs with the formation of a dark fringe (Fig. 21).

Testplating with white light illumination results in color fringes. The thickness of theair film between the surfaces increases from violet to red corresponding to the increase inwavelength. This provides the optician to measure the surface with high (convex) or low(concave) surface match, and he selects the blue fringes from the multicolored bands orrings as reference. The optical path difference (OPD) or λ/2 of blue fringe is about0.25 μm. Convex or concave conditions are also indicated by the way the fringes willmove when the test plate is wedged slightly relative to the test surface.

When the fringe order or fringe is counted, it should always result in a wholenumber or integer, but that is not true in real cases. There will be always a partialfringe in every test plate match. These partial fringes cannot be counted with anydegree of accuracy, and they are usually ignored. But, for very critical test platematches, these partial fringes can introduce significant errors. Modern phase inter-ferometers with sophisticated fringe analysis technique can account for these partialfringes, and the measurement is much more precise than the test plate technique.

Fig. 20 Newton rings/Fringes formation at the contact of work piece and test plate

Fig. 21 Formation of colorfringes in the work piece(http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov)

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Therefore, the use of test plate is restricted to shop floor analysis or in-process check,while the interferometers are used for final inspection and QC clearance.

Stylus Profiler Technique

In this method, especially stylus profiler, and optical methods such as scanningtunnelling microscopes (STM) and atomic force microscopes (AFM), contact mech-anism is utilized.

Atomic Force Microscope (AFM)

It is ideal for measuring features with very small lateral dimension; tip sizes aretypically 10 nm or smaller. It is limited by the maximum dimensions that can bemeasured typically 90 microns in x and y directions and 10 microns in z direction asshown in Fig. 22. Measurement time and speed is ten times greater thaninterferometry.

Scanning Electron Microscope (SEM)

This is traditionally being used for submicron analysis; however it does have somelimitations. It can give very good lateral resolution, but it is not capable of analyzingin true z direction. Normally samples in Z-dimensions have been measured bysectioning them, and because of this, measuring of steps is a problem. Any samplethat is to be measured has to be conducting in nature, and if it’s an insulator, they arenormally to be coated with a metal in thin film. Sample preparation can be

Fig. 22 Schematic of atomicforce microscope. (Courtesy:www.vu.nl)

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complicated and time-consuming issue. The schematic of the scanning electronmicroscopy is shown in Fig. 23.

White Light Interferometry

For precise components where the specifications are of the order of angstrom level,abovementioned kind of surface topography measurement techniques is not suitable.The most accurate technique for assessing surface topography is noncontact method,which includes optical methods, especially methods of interference. There have beenrapid developments in interferometry in the last two decades, as a result of the newpossibilities for digital recording and analysis of interference images. The fastestgrowing interference methods include:

• Phase shift interferometry (PSI)• Coherence correlation interferometry (CCI)

Fig. 23 Schematic ofscanning electron microscopy.(Courtesy: www.purdue.edu)

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Phase Shift Interferometry (PSI)

It is a well-established technique for surface characterization which relies on digiti-zation of interference data during controlled phase shift. It provides a complete three-dimensional image with typical height measurement with a repeatability of less than1 nm independent of field size. It consists of analysis of light phase distribution onthe measured surface and accurately measures the height of surface irregularities.The measuring range is relatively small. Tracking and analyzing the degree oftemporal coherence of interfering waves offer the possibility to measure the irregu-larities in the height of the surface over a greater range. It utilizes narrow-bandwidthfiltered light, the objective lens does not move, and it has a single focus point. Itprocesses the intensity signal to calculate the surface heights. Schematic of phaseshifting interferometer and vertical scanning interferometer is shown in Fig. 24.

Coherence Correlation Interferometry (CCI)

CCI is an ideal technique for quick measurement of surface roughness or step heightsup to 400 nm with high accuracy and repeatability. Large steps can be measured bycombining different measurements together. Lateral dimensions range from 0.5microns up to 7 mm in a single measurement. It is based on the cross-coherence oftwo low-coherence light rays, where an object light ray is being reflected from theobject while the reference light ray is reflected from reference mirror. A high-contrastinterference pattern arises, if the optical path length in the object arm is equal to theoptical path length in the reference arm.

Fig. 24 Schematic of phase shifting interferometer and vertical scanning interferometer. (Cour-tesy: www.en.wikipedia.org)

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Features of CCISchematic of coherence correlate interferometer is shown in Fig. 25. It is providedwith features of wide measuring range and high resolution, a large measurement areaon the object, short time for data acquisition and measurement, and exceptionalversatility for measuring objects made from different materials. This also helps in theanalysis of polymer materials, measurement of surface topography of the precisionmachined parts and elements, measurement of smooth and super smooth opticalelements such as micro lenses and diffractive optics.

Case Study for Polishing/Finishing Techniques Used for Achievingthe Nano-finished Optics Using Glass and Glass Ceramic Material

Optics are made not only of amorphous materials such as optical glasses and fusedquartz, but it is also made for low expansion glass ceramic materials like Cer-Vit andZerodur, ultralow expansion quartz, and single-crystal materials such as sapphire.Super smooth surfaces can be obtained on most of these materials by specialpolishing techniques like bowl-feed polishing. In bowl-feed technique, the surfaceto be polished is immersed inside the slurry. The abrasives that were used getembedded inside the lap, and the slurry tends to become in a form of a pure water.Hence the surface produced by this technique has no fine scratches. For sapphirekind of materials, polishing is done very slowly either by bowl-feed or by

Fig. 25 Schematic of coherence correlate interferometer. (Courtesy: www.npl.co.uk)

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conventional fresh-feed techniques in which a small amount of water-abrasivemixture is used and it is polished to dryness.

Similarly, other polishing techniques have been studied for special applicationssuch as substrates for microelectronic circuits. For Mn-Zn ferrite single crystal, anoncontact float polishing technique has been successfully utilized. In all the cases,the objective is to reduce the damaged layer on the surface that was usually producedby conventional polishing. In a case study, various glass materials like fused quartz,Zerodur, Corning glass ceramic, Cer-Vit, ULE quartz, and sapphire were used andobtained an approximately 2.4 Å rms roughness value. In their experimentsconducted, a tin lap with its slurry-retaining bowl mounted to the spindle wasutilized. The disadvantage of the system is that the grooves cannot be recut orsurface-cut the top of the lands, and while shutting down and while starting themachine, the parts occasionally touch the lap producing two sets of circular scuffmarks onto the lap.

Fig. 26 3D profiler

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Float Polishing Using Pitch Lap

For processing of highly precise components, the basic conventional polishingtechnique is used. As a result, the average roughness measured by Veeco 3D profileras shown in Fig. 26 (phase shift interferometer) is 7 Å.

The concept of tin lap was applied by replacing the tip lap with the 835 Burgundypolishing pitch. The polishing pitch was prepared with the burgundy, melting it and

Fig. 27 Surface roughness measurement (before polishing) (http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov)

Fig. 28 (a–g) Polishing material used and the processing steps involved in preparing polishingslurry (http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov)

28 M. B. Suresh et al.

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pouring it onto aluminum plate. It was precisely matched, and a mesh-like patternwas impregnated onto the pitch. The polishing pitch was maintained at roomtemperature and the relative humidity is 60%. The rotation speed of the spindleswas approximately between 6 and 12 rpm, and the substrate was fused quartz(Fig. 27).

The substrate was polished with nano-ceria polishing slurry in float polishingtechnique for 8 h continuously inside the float polishing drum as shown in Fig. 28(a–g). After processing the substrate was analyzed for its roughness value in Veeco3D profiler and was found to be approximately 2 Å, which was better than the resultobtained using tin lap.

The optical components such as mirror/prism, which has been polished using theconventional polishing technique, resulted in the average surface roughness value as7 Å. With the modified float polishing technique, the surface roughness was mea-sured as 2 Å. Thus, nano-finishing on glass/glass ceramics has been obtained byadopting a float polishing technique, and it was characterized by noncontact mea-suring technique by Veeco 3D optical profiler (Fig. 29).

Summary

This chapter briefly discussed few novel polishing techniques for the fabrication ofoptical materials. Magnetorheological finishing is a novel polishing technique fordeterministic subsurface damage correction.

Fig. 29 Surface roughness measurement (after polishing)

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