ultra-precision: enabling our...

Phil. Trans. R. Soc. A (2012) 370, 3993–4014doi:10.1098/rsta.2011.0638

REVIEW

Ultra-precision: enabling our futureBY PAUL SHORE* AND PAUL MORANTZ

Precision Engineering Institute, Cranfield University, Cranfield, Bedfordshire,MK43 0AL, UK

This paper provides a perspective on the development of ultra-precision technologies:What drove their evolution and what do they now promise for the future as we facethe consequences of consumption of the Earth’s finite resources? Improved applicationof measurement is introduced as a major enabler of mass production, and its resultantimpact on wealth generation is considered. This paper identifies the ambitions of thedefence, automotive and microelectronics sectors as important drivers of improvedmanufacturing accuracy capability and ever smaller feature creation. It then describeshow science fields such as astronomy have presented significant precision engineeringchallenges, illustrating how these fields of science have achieved unprecedented levelsof accuracy, sensitivity and sheer scale. Notwithstanding their importance to scienceunderstanding, many science-driven ultra-precision technologies became key enablersfor wealth generation and other well-being issues. Specific ultra-precision machine toolsimportant to major astronomy programmes are discussed, as well as the way in whichsubsequently evolved machine tools made at the beginning of the twenty-first century,now provide much wider benefits.

Keywords: ultra-precision; telescopes; machine tools; measurement

1. Introduction

‘Measure what is measurable, make measurable what is not’

—ascribed to Galileo1.

Many twentieth-century drivers can be identified behind the improvement ofmanufacturing accuracy and fine feature fabrication. These include demandsset by defence and weapons programmes, automotive and aerospace massproduction, microelectronics, telecoms, medical imaging and, perhaps mostsignificantly, large science programmes. This paper details some specific*Author for correspondence ([email protected]).1Noted mathematician Hermann Weyl attributed the maxim as a quotation taken from the1890–1909 edition of Galileo’s collected works: ‘Galileo enunciates the principle, “to measure whatis measurable and to try to render measurable what is not so as yet”’ [1, p. 4].

One contribution of 16 to a Discussion Meeting Issue ‘Ultra-precision engineering: from physics tomanufacturing’.

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astronomy particle physics gravity environment

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Figure 1. Drivers of achievable accuracy capability in precision machining.

ultra-precision developments, initially devised to meet demands of largescience projects, which have now become key enabling technologies forrenewable energy generation and environmental monitoring in the twenty-firstcentury (figure 1).

Prior to looking at specific manufacturing advancements, it is perhapsappropriate to consider a number of important advances in the field ofmeasurement science. At the end of the nineteenth century, Carl EdvardJohansson, a Swedish armourer inspector, invented the so-called combinationgauge block set [2]. In the UK, these are referred to as slip gauges (‘slip’ meaning‘ground’ in Swedish), while in the USA they are referred to as ‘Jo blocks’after the inventor himself. Johansson had been concerned with the cost andthe effectiveness of measurement tooling supporting the fabrication of riflesand ammunition. His concept was very simple: a set of accurate blocks ofspecific differing thicknesses that could be combined to make up any requiredmeasurement distance. The flatness and roughness of the gauge blocks were of avery high standard, typically 100 nm r.m.s. and 25 nm r.m.s., respectively. Thehigh quality of the surfaces enabled the blocks to be wrung together, meaningthe blocks would effectively stick to each other through a combination of surfacetension, molecular attraction and air pressure as a consequence of their high-quality surfaces. The application of slip gauges to the calibration of micrometrecallipers significantly advanced inspection for weapons manufacture (figure 2).

The benefits were soon recognized by Henry Ford, who engaged Johanssonto support development of automobile production. Ford’s assembly concept wasitself based on the so-called American System of Manufacture pioneered inarmouries and exhibited with great impact at the Crystal Palace Great Exhibitionin 1851 [3]. The concept demanded interchangeability of components for effectiveand efficient production. The impact of Johansson’s invention has since beenglobally significant and even today slip gauges remain a measurement artefactproviding traceability for many manufacturers. Johansson was posthumouslyawarded a gold medal by the Royal Swedish Academy of Engineering Sciences in1943 for his ‘simple’ invention of the combination gauge block set.

Phil. Trans. R. Soc. A (2012)



Review. Ultra-precision: our future 3995

Figure 2. From top left anticlockwise: early Johansson combination gauge set (courtesy of CEJohansson), 1959 Ferranti coordinate measuring machine [4], 1970s laser interferometer [5] andmodern touch-trigger probe [6].

Subsequent measurement technologies have enabled higher manufacturingaccuracy to be achieved with greater speed and ever more complex shapecomponents. Critical measuring inventions included: the laser interferometer, thecoordinate measuring machine (CMM) and the touch-trigger probe (figure 2).

When the first working laser was reported [7], Charles Townes, whosefundamental patent had anticipated its development, was famously told withoutirony it was ‘a solution looking for a problem’ [8]. The following half century,however, has revealed the laser to be a ubiquitous and necessary componentin numerous diverse applications, and not least in the field of precisionmeasurement. Its highly monochromatic and coherent output permits (throughoptical interference) very high-resolution measurement over extremes of distancescales; its low divergence and high intensity enable long reach, wide optical fieldsand fast measurement; its achievable stability of beam geometry and frequencyallows extremely low-uncertainty measurements in a wide range of metrologicalapplications. It has become the cornerstone of displacement, angle and formmetrology, based on a number of principles—interferometry underlying the mostsensitive, with sub-nanometre precision possible [9].

Whereas the CMM may arguably find origins in a nineteenth-centuryboatyard [10] or conceivably antecedents in eighteenth-century or earliersculptors’ pointing machines used to ‘reverse engineer’ three-dimensional





forms [11], realistically its birth was in the mid-twentieth-century combinationof electronic measurement scales and kinematic machine design. What is nowcalled the CMM was invented in the Ferranti company in 1956 and commerciallyintroduced as a manually operated machine in 1959 and around the same timeat the Digital Electronic Automation (DEA) company [4]. The later additionof sensitive electronic probes and automatic machine control had created fastautomatic machines by the mid-1970s. Ultimately, the introduction of specificmetrological software completed a recognizably modern CMM, which is nowcapable of sophisticated general dimensional metrology, in some cases achievingdemonstrable dynamic ranges (maximum dimension : resolution) exceeding 109

with measurement uncertainty ratios above 106.A specific invention that ushered in the age of automatic precision measurement

was the touch-trigger probe [12]. It was initially conceived in Rolls-Roycespecifically for low-force probing of delicate components of the Olympus jetengine. Its simple application to automatic control and its consistency of probetrigger force ultimately permitted high-speed precise probing of a range ofcomponents—even while the CMM itself was still in motion. This dynamiccapability markedly improved the efficiency of CMMs, permitting measurementrepeat rates at one per second or more.

(a) Relationship of ultra-precision technologies and nanotechnology

The ability to produce manufactured components to ever more demandinglevels of accuracy has been identified by numerous researchers. In 1974, theJapanese precision engineering researcher, Taniguchi, defined nanotechnology asthe production technology to achieve high accuracy and ultra-fine dimensions ofthe order of a nanometre [13]. His charts recorded and proposed improvementsin machining accuracy capability (figure 3). These charts can be compared withMoore’s law, the mid-1960s prediction [14] based on contemporary trends thatthe integrated circuit (IC) transistor count-per-chip would double every 2 years;it is expected to hold more or less true until at least 2020 [15]. Taniguchistated that the production technology required for nanotechnology must includemachine systems for processing, together with appropriate measurement andcontrol techniques. He highlighted that enabling of effective measurement waspivotal to the development of new manufacturing processes and practices. It isclear, though, that the recognition of the fundamental need for measurementcan be traced through, for instance, Lord Kelvin: his fuller statement [16] oftenparaphrased as ‘to measure is to know’ harks back to the maxim attributed earlierto Galileo.

The field of nanotechnology was in practice established before Taniguchi coinedthe phrase itself. The Nobel Prize winner Richard Feynman advocated atom-by-atom construction and predicted the development of both the transistor andatom-scale microscopy in the late 1950s. Feynman’s widely cited lecture ‘There’splenty of room at the bottom’ [17] has provided inspiration to physicists andengineers alike.

Feynman effectively predicted the direction of the microelectronics sector priorto its actual creation. He also highlighted the need for lubrication media of verylow viscosity in order to enable nanometre-scale motions. As we shall see laterwith regard to ultra-precision machine developments that this point has been





100 (0.1 mm)

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normal machining

turning and milling machines

grinding machines

CNC machineslapping and honing machinesjig boring and grinding machines

step and repeat camerasoptical lens grinding machinesprecision grinding machinessuper-finishing machinesdiamond grinding machines

high-precision mask alignersultra-precision diamondturning machinesdiffraction grating ruling enginesfree abrasive machiningelectron beam lithographysoft X-ray lithographyion beam machiningSTM/AFM molecular manipulation

precision machining

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Figure 3. An interpretation of the Taniguchi curves, depicting the general improvement of machineaccuracy capability with time during much of the twentieth century.

very important. Just like Johansson, Feynman spent his early career within thedefence sector, as a theoretical physicist at the Los Alamos Laboratory workingwith the Manhattan Project [18].

There is perhaps no comprehensive means, nor is it necessarily important,to differentiate the fields of ultra-precision engineering and nanotechnology.Optics fabricators can claim to have been applying ultra-precision technologies toproduce surfaces having form accuracy of tens of nanometres for decades, if notcenturies. Today the importance of the precise manipulation of light remains amajor concern across many fields of science, including astronomy, gravity andfusion research. For established commercial sectors, such as microelectronics,telecommunication and displays, the ability to handle light to a similar levelof precision remains a critical technical area affording products a competitiveadvantage. As we shall see later, that same ability has now become a prioritytopic in renewable energy generation and environmental monitoring.

Ultra-precision technologies include machine, process and metrologytechnologies that enable the manufacture or function of parts having nanometretolerance surfaces or features applied to artefacts of differing scale. In thismanner, we encompass small-scale components having nanometre-tolerancedfeatures. Such features would not necessarily comply with generally accepteddefinitions for precision engineering [19,20], which tend to be based on the relativeratio between the scale (size) and the accuracy of components. This precisionengineering ratio is clearly some measure of difficulty of fabrication but perhapsdoes not reflect all advances in the manufacturing fields demanding ultra-precisionattributes.

Of the many activities that have advanced ultra-precision manufacturing, thispaper highlights some that have been facilitated by large science and defencesector programmes during the twentieth century and which are now in the





twenty-first century being applied to address concerns that are critical in thecontext of growing population, consumption of resources and expectations ofquality of life.

2. The development of ultra-precision machine processes beforethe twenty-first century

Ultra-precision processes include those which were based on conventionalprocesses (such as turning, milling and grinding) and developed to becomehigher accuracy capability technologies. Other processes often referred to as non-conventional processes are based on energy beams and chemical mechanisms.In this section, we highlight the development of three important ultra-precision processing technologies: two of a conventional type and one considerednon-conventional.

(a) Single-point diamond machining

Machining of components using lathes can be traced back to antiquity—a depiction of lathe turning exists in the tomb of Petosiris from around 300BC [21]—although the technique is undoubtedly much older. The poweredmachine lathe was a key advance in the nineteenth-century’s IndustrialRevolution [22] developed from the much earlier water-powered wood turninglathe [23]. Accuracy capability improvement of the turning process can belinked to numerous machines and cutting tool advancements, many of whichare twentieth century in origin. The diamond turning process is presently at thepinnacle of turning processes with regard to machining accuracy capability andsurface finish.

Single-point diamond machining employs an ultra-sharp high-purity diamondtool used with ultra-precision lathes and milling machines. These have smooth,precise motions founded on stiff guideways and fluid film bearings that control therelative movement of the workpiece and tool to nanometre levels of controland achievable component part accuracy [24]. Diamond turning was initiallydeveloped using modified measuring machines such as those made by MooreSpecial Tool (figure 4) in the USA and Taylor Hobson in the UK. In the1960s and 1970s, the US Lawrence Livermore National Laboratory (LLNL) builtdedicated large-scale diamond turning machines (LDTM) such as the LargeOptics Diamond Turning Machine (LODTM), under defence programmes [25,26].Similarly in the UK, the Science and Engineering Research Council commissionedthe Cranfield Institute of Technology (later, Cranfield University) to design andbuild a LDTM (figure 4) [27] although this was for astronomy purposes. Themachine established the UK capability to fabricate X-ray grazing incidence spacemirrors [28]. Through the development of the LLNL and Cranfield machines,a number of high-precision machine techniques were firmly established. Furtherrefinement for ultra-precision applications has improved these technologies, whichinclude: pressurized oil and air bearings, smooth drive actuation, active thermalstabilization methods, application of laser interferometers for machine tool motioncontrol, laser interferometer-referenced metrology frames, vibration isolation and





Figure 4. From top left anticlockwise: ca 1975 Moore M18 DTM, Cranfield LDTM (1979), CranfieldNION DTM (1989) and Moore 500FG (1999). Courtesy of Moore, CUPE, CPE Ltd and MooreNanotechnology Systems.

responsive numerical control. Such techniques ensured that these large machineshad a form machining accuracy capability in the range 50–1000 nm on componentsup to 1 m scale or more.

Of the many concepts shared between the contemporary LLNL LODTMand Cranfield LDTM, perhaps the most important was the application oflaser interferometers operating at 10 nm resolution, referenced to a masterstraight edge. Error data collected through the lasers were used to modifymotional tool paths. In 1982, the Rosat space telescope wide-field camera mirrors,of metre scale, having circularity and axial profile accuracies significantly betterthan 1 mm total error envelope, were made using LDTM.

By the mid-1980s, Cranfield had produced several classified high-precisionand ultra-precision diamond turning machines for the defence sector. Throughthe UK’s National Initiative on Nanotechnology (NION, figure 4), new diamondturning machines were also built having 1 nm resolution-controlled motions, usingactive laser interferometer-referenced metrology frames, with highly sophisticatedthermal control systems operating at milli-kelvin resolution. The approach tothermal stabilization advanced the earlier work by Bryan at LLNL. By the mid-1980s, commercial ultra-precision aspheric generator machines were produced





in the USA, UK, Germany and Japan. These machines firmly establishedconventional diamond turning machines having approximately 100 nm machiningaccuracy capability for 350 mm sized parts.

The ability to fabricate natural and synthetic diamonds into cutting toolswith near atomically sharp edges was finessed. Understanding of the effectsof diamond type, crystal orientation, edge preparation and trace elements onthe cutting performance was greatly advanced [29]. Important research into thediamond machining of hard and brittle materials such as silicon and germaniumwas undertaken by many [30–33]. Collectively, their work established today’sproduction methods for the fabrication of infrared (IR) optics as applied in missileguidance systems, heat-seeking detection cameras and night vision sights [34].Materials ostensibly considered brittle, such as silicon, can today readily beturned without brittle fracture owing to the quality of control of the cut chipthickness using high-quality lathes. The resultant surfaces approach 1 nm r.m.s.roughness with a limited amount of subsurface atomic bond dislocation, typicallyapproximately 200 nm deep.

Recent technical advancements of commercial diamond turning machinesrelate to the development of high-frequency response motions and additionalaxes. Technically speaking, in general application, linear motor technologyand higher-resolution Moiré fringe gratings have replaced capstan or ballscrewdrives and laser interferometers, respectively. Additionally, high-frequency piezo-actuated tool holders providing kilohertz bandwidth position control of the toolenabling complex structured surface generation with small feature size at realisticproduction rates.

(b) Abrasive machining technologies

Abrasive machining has long been established. Assyrian ground optics nearly3000 years old have been found [35]. Abrasive machining of optics in the form ofgrinding and polishing were the concern of many leading scientists from Galileoand Huygens forward [36].

Today’s major science programmes continue to fund abrasive processingresearch [37]. The development of precision grinding and precision polishingmachines followed quite distinct routes as they depend on different technologies.Grinding is fundamentally a position-controlled process (akin to diamondturning), where the motions of the machine dictate the quality of the partproduced through a direct influence on its accuracy, since the abrasives are fixedinto a tool; whereas polishing is a force-controlled process, where abrasives arefree to move within a fluid between the part and a moving tool. In the caseof polishing, the applied pressure between tool and part, and the duration ofengagement, are the dominant controlling material removal parameters, althoughthere are others, in what is often a multi-step iterative process to converge on thedesired component shape, as with the oldest manual craft processes.

The development of ultra-precision grinding machines closely followed thetechnical progression of single-point diamond turning machines. In the early1980s, IBM funded the development of the so-called lap-grinding machines forgrinding ceramic read/write heads, where roughnesses of 10 nm r.m.s. weredemanded. This lap-grinding process removed the need for ‘messy’ polishingprocesses. The lap-grinding machines employed air bearing spindles and linear





Figure 5. From top left anticlockwise: read/write head grinding machine (CUPE 1982), asphericoptics grinding machine (Rank Pneumo, 1988), large off-axis grinding machine (CUPE 1989) andCompact Nanogrinder (Lidköping Machine Tools 2000).

slideways and provided greater accuracy in read/write head fabrication, therebyenabling lower flying heights that provide faster data exchange rates and higherdata density, this technology being later applied directly to optical grinding [38].The US company Pneumo produced aspheric optics grinding machines adaptedfrom their successful aspheric optics diamond turning machines from the mid-1980s (figure 5). These machines also incorporated air bearing grinding spindlesand permitted aspheric shape glass optics to be ground to form an accuracylevel of 200–300 nm peak to valley. They enabled effective fabrication of asphericoptics since their output quality permitted much shorter subsequent polishingoperations than previously possible.

A large special-purpose grinding machine sold to Eastman Kodak in 1988 andmade by Cranfield Precision Engineering Limited embodies an ultra-precisiongrinding machine possessing many technologies refined from earlier LDTMprojects. The OAGM 2500 was purchased to deliver numbers of large mirrorsegments for US astronomy projects such as the Keck telescopes.

This large-scale Cartesian machine (figure 5) has a working volume of 2.5 ×2.5 × 0.6 mm with a claimed volumetric accuracy in the range 3–5 mm [39]. Thisunprecedented accuracy level and scale combination, beyond that of commercialCMMs of similar size, even today, was achieved through application of a fullthree-dimensional metrology frame operating at 1 nm resolution through multiplelaser interferometers. By incorporating a coordinate measuring capability, also





laser referenced to the metrology frame, the opportunity exists to measureand compensate for grinding process issues such as tool wear and forcevariations.

Ultra-precision grinding machines were developed to support silicon waferprocessing. As the size of silicon wafers increased up to 300 mm by the year 2000,the grinding machines became larger and demanded improved relative accuracy toensure subsequent post-grinding processes would be effective on larger size wafers.Many ultra-precision silicon wafer grinding machines employ the technologiesfound in diamond turning machines.

In contrast to the scale-up of silicon wafers, many precision automotivecomponents such as fuel injectors and bearings have decreased in size. However,their accuracy demands have also become ever more stringent. The driver forhigher accuracy in these automotive components is a requirement for reducedemissions and improved efficiency from internal combustion engines, throughreduced friction and longer life from bearing systems. This is an example of howenvironmental and sustainability issues are becoming increasingly significant inrequiring ultra-precision technologies.

The Nanogrinder developed by Lidköping Machine Tools in Sweden representsan important technical indicator of future small-scale ultra-precision machines(figure 5). This machine [40] was under development at the end of the millennium.The Nanogrinder employed large air bearings with integrated direct drivemotions held within the main rotary systems. This approach significantly reducedmachine size and energy demands, while delivering high accuracy capabilityand increased output. The approach was made possible through adopting airbearings, high-power density direct drive motors (linear and rotary) and sub-nanometre resolution cylindrical Moiré fringe gratings, technologies similar tothose in modern diamond turning machines.

Free abrasive glass polishing has been researched by many leading scientistsduring the late nineteenth and twentieth centuries. This list includes keycontributions that advanced the purely abrasive Newtonian model [41] to thepartial ‘flow’ model [42–46]. Rayleigh considered the grinding process to bedominated by microfracturing, whereas he considered polishing to be dominatedby molecular or near molecular flow. These scientists had differing views as to thecontribution of material flow mechanisms, debating the likely flow layer depth tobe between 25 and 1000 nm. Understanding of both the physics and chemistry offree abrasive processes remains a major research area [47] to advance productionefficiency in the microelectronics and optics sectors alike [48].

Whereas the progress of fixed abrasive grinding can be traced through machinetool technology developments, the progress of polishing should be charted againstprocess and measuring technology developments. The range of modern polishingprocesses includes: magnetorheological finishing (MRF), which carries abrasivesin a fluid that can be stiffened using electromagnetic influence [49]; elasticemission finishing [50], which drives very fine abrasives into surfaces using a high-speed moving tool; and variants of sub-aperture polishing [51,52], examples ofwhich are shown in figure 6. Many of these free abrasive polishing techniques areapplied using sub-aperture tools. Tool path calculations are derived from errormeasurement maps of the component surface. An iterative convergence processfrom measurement to polishing to measurement and so on is applied. The errormaps allow a time dwell tool path to be calculated so that the tool resides longer





Figure 6. From top left anticlockwise: REOSC sub-aperture robotic polishing [53], elastic emissionfree-form optics polishing facility [54], QED Q22 MRF polishing machine [55] and MRFpolishing [56].

on high regions and less so on relatively low regions of the surface. In so doing,the desired surface shape is approached through a series of process–measurementiterations.

There is no doubt that the capability of sub-aperture grinding and polishinghas advanced through numerous projects; however, the astronomy demand for aleading high-performance X-ray space telescope can be identified as stimulatingthe achievement of exceptional accuracy in large and delicate mirror optics.

The NASA Chandra X-ray space telescope was launched in 1999 and is the mostsensitive X-ray space telescope ever deployed. It provides an 800 cm2 collectingarea (equivalent to a 0.3 m diameter optical telescope) and provides a resolutionof 0.5 arcsec (compared with the earlier Rosat telescope at 3 arcsec). The largestChandra (AXAF) mirror element is 1.2 m in diameter and 0.83 m long; a mancan stand inside it, cleaning its surface (figure 7).

The Chandra mirrors were made in the glass ceramic Zerodur and producedto exacting quality by Hughes Danbury in the USA. Chandra’s mirrors wereprocessed using small tool (sub-aperture) grinding and polishing techniques. Themirrors were carefully supported on aluminium rings fitted at each end of thetube-like mirrors during processing. These support rings were removed duringpost-grind and post-polish measurements so that their distortion influence didnot affect the measurements of the freely supported mirrors.





Figure 7. From top left anticlockwise: ca 1992, sub-aperture polishing of Chandra mirror; ca 1993,Chandra X-ray mirrors (Zerodur) being cleaned by man standing inside the mirror; 1991, CIDSmeasurement system built at Cranfield for Hughes Danbury. (Courtesy of Hughes, Hughes andCPE Ltd, respectively.)

Hughes Danbury achieved significantly greater accuracy and at a larger scaleon the Chandra mirrors compared with earlier X-ray space telescopes. Thishigher accuracy capability was principally achieved through the development ofimproved measurement applied during manufacture. This improved measuringcapability required new special-purpose measuring machines. Data from thesemeasuring machines ensured that subsequent polishing runs were effective inconverging towards required final shape. The special-purpose measuring machinesincluded a Precision Metrology Station (PMS) and a Circularity and InnerDiameter Station (CIDS). PMS provided axial figure errors, at specific azimuthalpositions, by interferometrically measuring the separation between the mirrorsurface and a calibrated reference surface. CIDS, built at Cranfield in the UK(figure 7), determined the inner diameters and roundness at the near-end regions





of the mirrors. CIDS also used interferometers to collect data from the mirrorsurface with respect to known reference artefacts. In operation, the Chandramirrors were loaded over the top of CIDS.

CIDS rotated inside the Chandra mirror carrying two sets of opposedmeasuring probes [57]. By fusing the data from the PMS and CIDS instruments,Hughes Danbury could create low-frequency error maps to drive their time dwellsub-aperture polishing process. Typically, it took four polishing cycles per elementto achieve an axial form error of 5 nm r.m.s. It was necessary to use polishingtools of differing size to reduce profile error features of differing wavelength.Roughness of the Chandra mirrors was 1.8–3.4 Å r.m.s. as measured over distancesof 0.01–1 mm.

Other polishing techniques employ a chemical mechanism and these are referredto as chemical–mechanical polishing or CMP. CMP has been widely developedespecially within the microelectronic sector [58].

(c) Ion beam technologies

The most precise surfaces produced at the turn of the twentieth century wereundoubtedly finalized using ion beam figuring (IBF). These surfaces includethe optical surfaces for telescopes and perhaps most significantly, at least ina commercial sense, the lenses that form the optical towers of lithographysteppers: these being the machine tools that in fact enable compliance withMoore’s law.

IBF, as employed for optical surface figure correction, was pioneered byWilson & McNeill [59]. The process uses a beam of accelerated ions operating in avacuum environment, causing the ions to bombard previously polished surfaces.The kinetic energy causes atoms within the optical substrate to be ‘knocked out’in a process often called sputtering. In IBF, the ion beam is formed so that it offersa Gaussian-shaped removal ‘footprint’. IBF is used with a time dwell strategy akinto that employed with sub-aperture computer-controlled polishing, where slowertransit of the IBF ‘head’ over the surface causes more material to be removed, thusallowing controlled change of surface shape through controlled speed variations.The significant advantage of IBF over abrasive-based polishing is in the realizationof atomic levels of material removal sensitivity of this non-contact process.The IBF process offers nanometre levels of fabrication finesse without the needfor ultra-precise machine motions, although costly vacuum-based operation isrequired (figure 8).

Realization of IBF was perhaps most significantly demonstrated through thecreation of 2.5 m scale IBF systems at Eastman Kodak and their application to thefinal shaping of the Keck telescopes’ primary mirror components; a combined totalof 72 mirror segments of 1.8 m scale. It was reported that each mirror was figuredin approximately 80 h with a material removal rate in the region of 0.1 mm3 min−1,improving form accuracy from approximately 500 nm r.m.s. to 10–30 nmr.m.s. [64]. Production data for lithography optics fabrication are commerciallysensitive and in the most part unpublished. However, the IBF systems areidentifiable and claims of reliable sub-nanometre form accuracy production arein evidence [63]. Representative systems and outputs are shown in figure 9.

At the conclusion of the twentieth century, it remained clear that the demandsof science projects remained a key driver of ultra-precision manufacturing





Figure 8. From top left anticlockwise: ca 1990, view inside the 2.5 m capacity ion beam figuring(IBF) facility [60]; ca 1999, GranTecan telescope mirror segments [61]; commercial IBF machine[62]; and IBF facility [63].

technology advancement and that these provided enormous benefit in high-technology sectors.

3. Twenty-first-century ultra-precision demands

At the start of the twenty-first century, the requirement for ultra-precisionprocessing has broader significance. The demands of astronomy programmes forlarger, more accurate mirrors, fabricated much more quickly, nevertheless remainfully aligned with another pressing demand: for lithography optics required toensure future adherence to Moore’s law.

This point is most clearly illustrated by comparing the optical demands ofthe European Extremely Large Telescope (E-ELT) with those proposed for thefuture extreme ultraviolet (EUV) lithography steppers, operating at increasinglyshorter optical wavelengths, in order to achieve 13 nm (even 6 nm) feature sizecreation capability; final figure accuracy and physical scale demands of individualmirrors are similar (and extremely challenging), despite the different applications(figure 9).

The broader importance of ultra-precision technologies at the start of thetwenty-first century is driven by a number of pressing requirements, including:sustainable or ideal energy generation, energy efficiency in manufacture/





Figure 9. From top left anticlockwise: E-ELT 42 m telescope [65], EUV lithography system [66],National Ignition Fusion chamber [67] and high-power solar concentrator (2011) [68].

consumption and the demand for advanced products. Example products includedisplays, animated packaging, programmable plastic newspapers and wallcoverings.

For much of the twentieth century, the promise of a clean, almost carbon-free,energy supply based on the principle of nuclear fusion was simply a science ideal.At the start of the twenty-first century, the prospect of a fusion energy source hasbeen transformed into arguably the most important engineering challenge for thelong-term sustainability of the human population on the planet. The NationalIgnition Facility at the Lawrence Livermore National Laboratories [69] and theLaser Mega Joule facility in Bordeaux [70] represent pinnacle science facilitiesfor laser-induced inertial fusion confinement. These essentially defence-drivenfacilities are yielding results that will gain support for key fusion energy-producingresearch facilities such as the US LIFE project [71] and the European HiPERprogrammes [72]. These laser-induced fusion confinement research facilitiesrepresent enormous ultra-precision machines in that the key enabling componentsin themselves are feats of ultra-precision engineering. Example challenges include:the manufacture of the high-fluence damage threshold large optics (these are wearcomponents requiring regular replacement) and the target fuel pellets, togetherwith the numerous demands of the control, steerage and tracking of multiplehigh-power laser beams (figure 9).

The surface and subsurface requirements of high-power laser optics posedemands beyond those driven previously by astronomy projects and the micro-electronics sector. As at the beginning of the seventeenth century, for





Figure 10. From top left anticlockwise: BoX ultra-precision free-form grinding machine (courtesyof Loxham Precision, 2009), Helios 1200 RAP figuring system (courtesy of Cranfield University,2010), compact six-axes Integ-m4 machine (courtesy of Cranfield University, 2012)—inset NiF fusiontarget [75].

the twenty-first century, new surface creation capabilities are required toenable science understanding and economic wealth and energy generation,with the added constraint of environmental sustainability. New ultra-precisionmachine tools are being established for rapid manufacture of optics for fusionenergy systems.

The Cranfield BoX ultra-precision free-form grinding machine was created toproduce E-ELT mirror segments in 10 h [73]. Another next-generation machinetool for large optics fabrication is the Helios 1200. It is a joint developmentbetween Cranfield University and a spin-out from LLNL, RAPT Industries. Heliosis a rapid plasma beam surface figuring system capable of reducing the finalfiguring time (and cost) of large-scale ultra-precision optics for fusion, astronomyand lithography applications [74]. These machines are shown in figure 10.

A microengineering next-generation ultra-precision machine tool underdevelopment is the Cranfield m4 machine. It is an advanced mass-productioncompliant multi-axis diamond machining system for rapid fabrication of fusionfuel pellets [76]. This highly compact ultra-precision six-axes diamond micro-machining machine (figure 10) will offer rapid fabrication of fusion fuel shapecomponents through high-response dynamic motions of nanometre accuracy.

Manufacture of large optical surfaces is a demand for solar energy systems.For concentrated solar thermal (CST) systems such as those of SouthwestSolar Technologies (figure 9), the optical demand is rather obvious. Mirrorquality is a parameter influencing energy efficiency. Less obvious but more





demanding is the required accuracy of optical surfaces within concentrated solarphotovoltaic (CSP) systems. Here, ultra-precision diamond turning is a keyenabling technology for high energy efficiency and low manufacturing cost. Inorder to concentrate solar light onto the photovoltaic (PV) junction, diffractiveoptical elements can be employed. The cost-effective mass production of large-scale high-quality concentrating diffractive films/lenses demands a reel-to-reelproduction process. Although these diffractive optical parts need to be producedin huge quantity and at low cost, they require ultra-precision diffractive featurestypically seen in high-end IR systems as used in missile guidance systems.

It is perhaps therefore not surprising that the diamond turning process usedfor making IR optics for the defence sector is the core enabling technologyfor the mass production of CSP diffractive film mould tools. The diamondturning process is used to fabricate large embossing drums that are employedwithin a reel-to-reel process producing low-cost high-quality diffractive optics.The capacity and technology base for the diamond turning machines used tomanufacture these large-scale ultra-precision structured drums are very similarto the machines used previously to fabricate X-ray space optics. The machinetool suppliers who previously delivered machine tools for astronomy and defenceprogrammes during the twentieth century are, in the twenty-first century,producing ultra-precision machine tools for machining diffractive film embossingtools—an example of machine, process and output is shown in figure 11.

The complexity of the surface structures is driven by the need for very complex-shaped features to avoid light loss/absorption together with the need for featuresto have sub-micrometre placement accuracy or registration over distances up to2 m or more. The machining duration to cut a structured diffractive drum ofsuch size having 10–50 mm features can be as much as 100–200 h. Any significantuntreated thermal distortion of the machine tool during this processing timewould cause a failure to produce useful product. Consequently, the most advancedstructured drum diamond turning machines employ active temperature controloperating in the milli-kelvin range and use metrology frames as developedpreviously for the large machines for astronomy optics manufacture.

The economic importance of display technologies has become ever moresignificant; displays are now a main feature of our daily lives, and provideproduct differentiation spanning children’s toys, mobile phones, computers, TVsand large advertising displays. The fabrication of advanced displays is enabledby ultra-precision fabrication technologies, for example, reel-to-reel fabricationof brightness enhancement films for displays. These reduce energy consumptionwhile providing greater illumination and longer battery life. Such optical filmscan also provide greater quality viewing, three-dimensional and security features.The manufacture of active elements for flat panel displays represents complexultra-precision manufacture where accretion, removal and joining processes mustwork in combination for effective mass production of these hard (non-flexible)panel items.

The future of displays, and potentially cost-effective large-area solar energyproduction, is associated with low-cost organic (plastic)-based substrates andtheir reel-to-reel fabrication. Significant progress in plastic-based displayshas already been made through a combination of innovations in ultra-precision engineering and chemistry (arguably nanotechnology). Active organic-based substrate manufacture demands new ultra-precision processes operating





roll ofclear film

roll ofstructured

film

coating UV curingstructured

drumUV curing

lacquer

UV light

structuredfilm

Figure 11. From top left anticlockwise: drum diamond turning facility at Cranfield-led UPS2

(courtesy of Cranfield, 2009), reel-to-reel structure film fabrication (courtesy of Microsharp, 2007),SEM image of structured surface replica (courtesy of UPS2 IKC, 2010) and high-complexitystructured drum (courtesy of UPS2 IKC, 2010).

over large areas at high accuracy and with high speed. By moving awayfrom inorganic (metal conductive)-based electronics on non-flexible materialsubstrates, a significant opportunity exists to reduce the capital costs ofestablishing manufacturing facilities for electronics-based products. As such, themanufacture of next-generation organic-based electronics can be made moreaccessible to manufacturing companies. Organic electronics and ultra-precisionengineering provide a key opportunity for companies, and even countries,to re-engage in high-technology product fabrication, avoiding prohibitivecapital barriers.

4. Conclusions

This paper has introduced the historical drivers of increasing manufacturingaccuracy exemplified by the long-established demands of science programmes.The authors have attempted to illustrate that the ultra-precision engineeringcreated in support of many large-scale science projects has subsequentlytranslated into core manufacturing processes (or product systems) of today’sadvanced processes and products. The paper also highlights the changing focus





of the major science (and engineering) programmes in the twenty-first centurycompared with those of the twentieth century. We have also sought to highlightsustainable energy creation (along with wealth creation) as the major drivers ofultra-precision engineering for the future.

In conclusion, we emphasize that ultra-precision engineers enable major sciencesuccesses. In the twenty-first century, it is ultra-precision engineers who hold apractical responsibility for the continued sustainability of the human population.

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