Ultraprecision Machining of Optical Surfaces
Hideo Takino, Masahiko Kanaoka, and Kazushi Nomura
Nikon Corporation, Sagamihara-city, Kanagawa 252-0328, Japan
Abstract – We review the features, applications, and machining of optics that have recently been receiving attention in the opticalindustry. One topic discussed in this paper is optics having more complex surfaces than conventional aspheres, such as
microstructured surfaces and freeform surfaces, which are attracting attention owing to their optical performance. We also discuss
aspheric optics with a surface accuracy of better than 100 pm rms, which are required in reduction-projection exposure systems usedin the extreme ultraviolet lithography process for integrated circuits. We give a brief overview of an extreme ultraviolet lithographysystem using accurate aspheric mirrors, and report the results of exposure using the system to demonstrate the quality of the processed
mirrors. We describe the recent progress in some ultraprecision machining techniques, such as ion beam figuring and elastic emission
machining, which enable the production of highly accurate aspheres.
Key Words: Optical fabrication, DOE, Feeform, EUV, Elastic emission machining, Ion beam, Interferometer

1. Introduction
Since the 1980s, aspheric optics have been mass–produced owing
to the progress of precision machining, profile measurement, andoptical system has various wavelengths, leading to the difficulty ofachieving high optical imaging performance over a wide range of
wavelength.

molding technologies. In the early stages of mass production using

To resolve this issue, various solutions have been proposed. One

molding, for example, pick-up lenses for compact disc players as

solution is to use blazed grating surfaces facing each other whilesmall aspheric optics and projection television lenses as large asphericmaintaining air gap between both surfaces [3]. Another solution is to

optics were produced. Nowadays, aspheric lenses are commonly used

use two types of blazed grating made from different optical materials

in single-lens reflex cameras. Such optics are manufactured by athat are joined to each other as shown in Fig. 1, which is referred to asprocess of glass molding or by a hybrid process involving thea dual-contact PF lens [4,5]. Recently, another type of dual-contact PF

application of ultraviolet (UV)-curable resin on glass optics. Theselens have been proposed as shown in Fig. 2: two types of resin having

optics are simple in shape as they have axial symmetry.

blazed gratings are placed between plastic lenses [6]. These PF lensRecently, optics with more complex surfaces than conventional

are practically used in commercial products such as camera lenses andaspheres, such as microstructured surfaces and freeform surfaces,

head-mounted displays. Dual-contact PF lenses are produced by thehave been attracting attention owing to their optical performance.

combination of various precision fabrication techniques such as theMoreover, aspheres with a surface accuracy of sub-nanometer level

ultraprecision cutting of molds, glass molding, or the application of

are required for industrial applications although their surfaces are not

UV-curable resin on grating surfaces.

complex.

In this paper, we review the features of optics that have

Freeform optics are actively used in commercial products such as

been recently receiving attention in the optical industry, their

optical communication systems, head-mounted displays, and liquid

applications, and their fabrication technologies.

crystal projectors [7-9]. Recently, a digital camera having a liquid
crystal projector has been commercialized, with freeform optics [8],2. Complex-shaped optics
Optics with subwavelength structured (SWS) surfaces, which havemicrostructures smaller than wavelengths on the surface, have been
proposed. SWS surfaces provide optical functions such as
antireflection, polarization splitting, and phase control [1]. Suchsurfaces are mainly produced through lithography. Recently, aFig. 1. Schematic of dual-contact phase Fresnel lens.

replication technique for fabricating SWS surfaces based on glass
molding has been developed to realize their mass production [2].
Optics with microstructures larger than the size of wavelengths
have also been proposed, which are referred to as diffractive opticalelements (DOEs). DOEs change the light path through diffraction,
whereas a conventional lens changes it through refraction. Phase
Fresnel (PF) lenses, which have blazed gratings, are one type of DOE,
the use of which in optical systems has been investigated for a longtime. However, DOEs are difficult to use in the imaging opticalsystem of a camera. This is because the light entering the imaging

Fig. 2. Schematic of dual-contact phase Fresnel lens.

Fig. 3. Schematic of freeform optics used in a projector installed
in a digital compact camera.

Fig. 5.㩷 Schematic of surface profile of EUV optics and power
spectral density distribution curve calculated from the surface
profile.

frequency roughness (HSRF), as shown in Fig.5. In general, thespatial frequency analysis of the surface profile is conducted to obtain
Fig. 4. Surface profile of freeform mirror used for thetransformation of laser beam intensity distributions.

power spectral density (PSD) distributions for the spatial wavelengths

one of which is shown in Fig. 3. These optics allow the downsizing of

100 pm rms for each roughness section. This means that the surface

the optical system and the enhancement of illumination efficiency.

must have atomic level accuracy and smoothness.

in each section. EUV optics require a surface accuracy of better thanFreeform surfaces also enable the transformation of laser beam

In the EUVL system, few optics are used so as to reduce the loss ofintensity distributions. It was demonstrated that the circular Gaussian

exposure light. To achieve high optical performance even using few

profile of laser beams can be converted into a rectangular flat profile

optics, most of the optics must be aspheric. Moreover, low-expansion

by using the freeform mirror shown in Fig. 4. The mirror was made ofglass is used as a material to prevent deformation by the heat load

fused silica glass, and was processed to have smooth and highly

generated by exposure. The realization of EUV optics requires

accurate surfaces by plasma-based ultraprecision machining, referred

ultraprecision machining techniques enabling surface removal thatto as plasma chemical vaporization machining (CVM) [10, 11].

can be controlled to atomic-level accuracy.

3. Highly accurate aspheric surfaces; aspheric mirrors in an

4. Fabrication of highly accurate aspheres

extreme ultraviolet lithography system

Various fabrication techniques for finishing aspheric surfaces have

In the manufacturing process of semiconductor integrated circuits,

been proposed and discussed from the viewpoints of industrial

lithography is an essential process. In the lithography process, aapplications and academic interest [10-18]. We have also been

reduction-projection exposure system is used to project circuit

investigating ultraprecision machining and profile measurement

patterns onto a silicon wafer. To precisely reduce the scale of circuit

technologies to fabricate aspheric optics with high accuracy andpatterns on wafers, highly accurate optics must be used in thesmoothness that are applicable to EUV optics.

exposure system. The use of a shorter exposure wavelength in the

In this section, we review some our efforts to fabricate EUV optics,

system results in higher resolution of the circuit patterns on the wafer.

demonstrating examples from our studies on ion beam figuring (IBF)In recent years, an exposure wavelength of 193 nm has been used in[16], elastic emission machining (EEM) [17, 18], and high-an immersion method, allowing a linewidth resolution of 32 nm. The

repeatability interferometry [19]. These studies were conducted in theuse of a shorter exposure wavelength requires optics with higher

management of the Extreme Ultraviolet Lithography Systemsurface accuracy and smoothness in the exposure system.

Development Association (EUVA) in Japan.

To further reduce the linewidth of circuit patterns, an extreme
ultraviolet lithography (EUVL) system, in which exposure is carried

4.1 Ion beam figuring

out at an extreme ultraviolet (EUV) wavelength of 13.5 nm, has

4.1.1 Removal principle and ion beam figuring machine

attracted attention and is being investigated globally. The EUVL

The bombardment of a workpiece with an ion beam results in thesystem requires extremely high quality mirrors in its optical system.

removal of its surface through sputtering, as shown in Fig. 6. An ion

The surface characteristics of optics used in the EUVL system,

beam focused to a size small relative to the workpiece is generated bywhich are referred to as EUV optics, are evaluated by dividing thean ion gun. A targeted area on the workpiece surface can be processedsurface profile into three sections: low spatial frequency roughness

by positioning the ion gun to make ions bombard the area. The(LSFR), mid spatial frequency roughness (MSFR), and high spatial

targeted area is removed at a depth of atomic order through sputtering,Fig. 6.㩷 Principle of ion beam figuring.
Fig. 9.㩷 Improvement of surface accuracy by use of IBF.

Fig. 10.㩷 Improvement of sub-nanometer surface accuracy of large
asphere by use of IBF.

Fig. 7.㩷 Ion beam figuring machine for optical finishing.

machine shown in Fig. 7. The figuring was conducted to improve theaccuracy, which corresponds to the LSFR, of a surface processed byindicating that IBF enables the machining of surfaces with the highconventional polishing.

accuracy required for EUV optics. IBF enables the removal of various

Fused silica concave spheres with a diameter of 80 mm werematerials because the removal is based on physical mechanism.

processed. Figure 8 shows a removal profile after Ar ion beam

We have developed an IBF machine and investigated its removal

bombardment for 30 min with an acceleration voltage of 800 eV whencharacteristics and figuring techniques using the machine. In the IBF

the workpiece and ion gun were stationary relative to each other, withof an optical surface, the surface is first measured with a

its ion current density measured by a Faraday cup. The figuring

three-dimensional surface profiler such as an interferometer to obtainprocess was performed by scanning the beam over the surface withan error map of the surface. Then, in accordance with the error map,the ion current density shown in Fig. 8. Figure 9 shows the surface

numerically controlled data are calculated for use when scanning the

topographies and PSD distributions before and after the process,ion beam. Since the removal depth increases with increasing

which demonstrate that surface errors corresponding to LSFR areprocessing time, the processing is conducted such that the ion beam

successfully improved, and in particular, errors with spatial

dwells over an area for a specific amount of time depending on the

wavelengths of above 10 mm are reduced.

error height. The ion gun is scanned while maintaining a specificFiguring experiments on a surface with sub-nanometer accuracy

working distance between the ion gun and the workpiece surface.

were performed to validate the effectiveness of the IBF process for

Figure 7 shows a schematic construction of the IBF machine, in which

EUV optics. In the experiment, large aspheres with a diameter of 130

an ion gun is set on a five-axis stage and a workpiece is placed above

mm made of low-expansion glass were used in consideration of thethe ion gun. This arrangement allows us to bombard ion beams

actual fabrication of EUV optics. The conditions of ion beam

normal to any point on the workpiece surface while maintaining ageneration were the same as those used for fused silica concave

constant working distance.

spheres.

4.1.2 Experiments

accuracy was improved to 180 pm rms, approaching the required

Here we discuss examples of figuring experiments using the

Figure 10 shows the experimental results. The surface

accuracy of EUV optics, demonstrating the usefulness of thistechnique for the processing of EUV optics. In addition, we have also
investigated the process characteristics for MSFR and HSFR,
although here we only give those of LSFR.
4.2 Elastic emission machining
4.2.1 Removal principle and elastic emission machining system
EEM was proposed as a polishing method by Prof. Mori of Osaka
University about 40 years ago. Since then, fundamental investigations

Fig. 8.㩷 Removal profile after ion beam bombardment for 30 min.

on EEM and its applications have been continued to be performed bymany researchers. We are interested in the smoothing capability ofFig. 11.㩷 Principle of elastic emission machining.

Fig. 12.㩷 Fabrication of aspheric surface by EEM.

Fig. 13.㩷 Surface topographies before and after processing by
EEM.

EEM and have investigated its application to the fabrication process

the process conditions of the EEM system, the maximum removal rateof EUV optics.

of 100×10-4 mm3/h was achieved. This rate is approximately 100

EEM processing is conducted in a machining slurry in whichtimes larger than that of previous EEM systems. Thus, we

powder particles are suspended in pure water. The slurry is

successfully resolved the issue of the low removal rate in EEM,

continuously supplied to a machining point by a tool, which leads toenabling the application of this system to the processing of large EUV

the powder particles attacking the surface, as shown in Fig. 11. The

optics.

powder particles chemically react with the surface, thus removing thesurface.

We processed conventionally polished surfaces to examine thesurface roughness of MSFR and HSFR. Figure 13 shows examples ofA rotating sphere is used as a tool in EEM. The rotation of thesurface topographies of fused silica plates before and after EEM

sphere close to the workpiece surface produces the flow of slurry withprocessing, observed by an optical profiler and an atomic force

dynamic pressure at the gap. This enables the powder particles tomicroscope. Examples of PSD curves are also shown in the Fig. 13.

attack the surface while maintaining the noncontact state between

The surfaces shown in Fig. 13 were obtained under the conditions forboth surfaces. Although EEM is a polishing method, it differs fromwhich the maximum removal rate is achieved. The figure shows thatconventional polishing in that the EEM tool is not in contact with theroughness with a spatial wavelength of below 100 m is reduced, thus

workpiece surface during the process.

reducing the rms roughness to below 100 pm rms. These findings

We developed an EEM system for processing aspheric surfaces

demonstrate the usefulness of EEM for the smoothing of EUV optics.

with a five-axis motion control device for the tool and a workpiece to

In addition, we have investigated the process characteristics of curved

process any desired point on the surface. In this system, five-axis

surfaces made of various materials, some of which have been alreadymotion control is conducted so that the equator of the rotating spherereported.

tool is positioned over the machining point, as shown in Fig. 12. We
performed experiments and numerical analysis to clarify the

4.3 High-repeatability interferometertribological conditions of the gap between the tool and workpiece,

To produce highly accurate aspheric surfaces, a high-repeatability

which revealed the generation of elastohydrodynamic lubricationinterferometer was developed. The principle of the interferometer isunder typical process conditions. This means that polishing using ourshown in Fig. 14; the interferometer is categorized as a Fizeau laser

EEM system is carried out under the noncontact state of the tool withinterferometer. In this interferometer, optics generating an aspheric

the workpiece surface in accordance with the principle of EEM.

wavefront are set between a light source and a test surface. A

4.2.2 Experiments

reference surface is placed in proximity to the test surface to minimize

We have investigated the removal characteristics of our EEM

the effects of air currents during measurement. In addition, various

system, such as removal rate and surface roughness. By optimizing

efforts, such as temperature control and vibration control, are made toFig. 16.㩷 Schematic of EUVL system.

Fig. 14.㩷 Schematic of high-repeatability interferometer.

Lithography processes were conducted using an EUVL system inwhich the above optical units were installed. Figure 19 shows theresulting patterns, demonstrating the successful generation of patternsof 24 nm half pitch [20]. Thus, the processed optics are sufficiently
accurate and smooth for use as EUV optics.

Fig. 15.㩷 Photograph of high-repeatability interferometer.
eliminate environmental disturbances.
Figure 15 shows a photograph of the interferometer constructed on
the basis of the above concept. Its performance was examined bymeasuring aspheres, which revealed a repeatability of 6 pm rms and a
reproducibility of 17 pm rms, demonstrating that the interferometerhas sufficient capability to measure EUV optics. The aspheric surface
shown in Fig. 10 was measured by this interferometer.

Fig. 17.㩷 Evaluation of mirror surface in projection optical unit
showing the surface topographies in each roughness section and
the PSD curve.

5. EUVL system
Figure 16 shows a schematic of an EUVL system. In this system,
an arc field of a mask on which electric circuit patterns exist isilluminated uniformly through an illumination optical unit, and thepatterns are reduced in size and imaged on a silicon wafer through a
projection optical unit. We processed the optics in the optical units ofthe EUVL system using various ultraprecision machining methods
and measurement methods, some of which are described above.
Figure 17 shows an example of the surface topographies and PSD
curve of a mirror used in the projection optical unit of the EUVL
system [20]. The surface roughnesses over entire spatial wavelengths
were found to be successfully reduced. Figure 18 shows theevaluation of the illumination optical unit, showing the illumination
intensity distribution on the mask. The results demonstrate that theunit enables uniform illumination on the desired arc field with anillumination intensity distribution of ± 0.5% [21].

Fig. 18.㩷 Evaluation of illumination optical unit.

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6. Conclusions(2002) 3971.

We have reviewed the features of optics recently receiving attention,
their applications, and their fabrication technologies. Complex-shaped
optics, such as microstructured optics and freeform optics, tend to be
widely used in optical instruments owing to their optical performance.
Moreover, optics with a surface accuracy of better than 100 pm rms
are required for industrial optical instruments. These high-quality
optics are now being successfully produced by the combination ofvarious ultraprecision machining methods and accurate measurement.
In the fields of optical science and industry, various novel optics are
being

proposed.

Thus,innovative

ultraprecision

machiningtechnologies are desired to realize such novel optics.

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We appreciate the financial support from the Risk-Taking Fund forTechnology Development from Japan Science and Technology (JST)Corporation, which enabled us to carry out research on plasma CVM.
The studies on EEM, IBF, and the high-repeatability interferometer
were performed under the management of the Extreme Ultraviolet
Lithography System Development Association (EUVA) in Japan: forthese studies, we thank the Japan Ministry of Economy, Trade andIndustry (METI) and the New Energy and Industrial Technology
Development Organization (NEDO) for their support. We thank
Selete members for their useful discussions and advices, andcooperation for EUVL exposure evaluations. We also thankKenzaburo Suzuki, Toru Nakamura, Takao Goto, and Katsuhiko
Murakami of Nikon Cooperation for giving us valuable comments
during the preparation of this manuscript.

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