Abrasive-free surface finishing of glass using a Ce film

Accepted Manuscript Title: Abrasive-free surface finishing of glass using a Ce film Authors: Junji Murata, Kazuki Goda PII: DOI: Reference:

S0924-0136(18)30445-X https://doi.org/10.1016/j.jmatprotec.2018.10.009 PROTEC 15962

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

10-7-2018 19-9-2018 11-10-2018

Please cite this article as: Murata J, Goda K, Abrasive-free surface finishing of glass using a Ce film, Journal of Materials Processing Tech. (2018), https://doi.org/10.1016/j.jmatprotec.2018.10.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Title Abrasive-free surface finishing of glass using a Ce film

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Author Junji Murata * and Kazuki Goda

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Department of Mechanical Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka

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author

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*Corresponding

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577-8502, Japan

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Dr. Junji Murata

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Department of Mechanical Engineering, Kindai University

Tel: +81-6-4307-3474

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3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan

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E-mail address: [email protected]

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Highlights

Ce film was deposited on a polishing pad by vacuum evaporation

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Adding deionized (DI) water gives scratch-free surface and good material removal rate

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DI is ejected at high pad speed, significantly lowering polishing performance

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Supplying potassium hydroxide (KOH) to the pad gives good MRR and ultra-smooth finish

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Approximately 0.53 g of Ce is deposited on the pad, reducing CeO2 usage by 94%

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Abstract

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A novel form of abrasive-free polishing is described, using Ce film deposited on a polishing pad by

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vacuum evaporation. Polishing using metallic films other than Ce, such as Al and Cu, generates

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defects on the glass surface with almost negligible material removal rates (MRRs), whereas Ce film

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with deionized (DI) water achieves smooth, scratch-free surfaces with noticeable MRR. If DI water is not supplied to the pad, there is significant deterioration in polishing performance. MRR increases

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with greater thickness of Ce film, and extremely smooth surface with sub-nanometer roughness is

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achieved using Ce film of thickness 2.5 μm. Polishing performance is dependent on conditions such as pad rotation rate and polishing pressure. By adding potassium hydroxide (KOH) to the polishing

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solution, the MRR almost matches that obtained with conventional abrasive polishing, but achieves ultra-smooth surface finish (Ra: 0.388 nm). The polishing method presented here uses approximately 94% less Ce compared with conventional abrasive polishing, thereby dramatically reducing consumption of this valuable rare earth element.

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Keywords

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Cerium oxide, Finishing, Abrasive, Vacuum evaporation, Thin film, Glass

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1. Introduction

Cerium oxide (CeO2) provides superior polishing performance compared with other materials, and is

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the most widely used polishing medium for SiO2-based glass materials. Compared with typical

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abrasive materials such as SiO2 and Al2O3, the mechanical strength of CeO2 is less than that of glass

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materials. Nevertheless, it achieves higher material removal rate (MRR) combined with excellent

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surface quality of material, because CeO2 is believed to remove SiO2 surface by means of a chemical

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reaction between CeO2 and SiO2. However, commercially available CeO2 powder for glass finishing

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contains several rare-earth elements such as La and Pr as well as Ce. After being reused for a certain duration of polishing time, the resulting slurry containing CeO2 particles is wasted because the

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polishing performance of CeO2 becomes degraded with the wear of abrasives and the accumulation in the slurry of chips removed from the material surface. As a result, the glass finishing process becomes

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more costly, and large amounts of rare-earth elements are wasted. There is currently a strong demand for technologies to reduce the use of CeO2 abrasives in glass finishing processes, because of the rapidly increasing demand for rare-earth materials. Janos et al. (2015) proposed a method for recovering CeO2 abrasives from waste polishing slurry. 3

However, their proposed method involves complex procedures, including high-temperature thermal treatment and chemical immersion, which increase the cost of the polishing process. Honma et al. (2012) investigated the glass polishing performance using ZrO2 particles as an alternative abrasive

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material to CeO2, but the MRRs obtained using ZrO2 are generally much lower than those using CeO2. Zhou and Zhu (2018) reported CeO2/CeF3 composite particles and Yoshida et al. (2007) developed

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CeTi2O6 particles to enhance the polishing performance of glass. However, preparation of composite particles requires complex and high-cost synthesis procedures to incorporate other elements with

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CeO2. Murata et al. (2016) proposed magnetic-assisted polishing for high-efficiency polishing of

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optical components. However, owing to the limited size of the polishing machine tools, it is difficult to

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apply their magnetic-assisted polishing process to large-scale production processes, especially for the

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polishing process of flat panel display substrates several meters in size.

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This paper proposes a novel method of polishing glass that does not employ abrasive particles. As shown in Figs. 1 (a) and (b), in the present polishing method, cerium thin film deposited on a polishing pad was employed for polishing a glass surface. No abrasive particles were used in the process. The

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surface of the deposited Ce film is readily oxidized in water to form a native oxide or hydroxide (CeOxHy) layer on the surface. The native oxide or hydroxide layer, which has an analogous chemical

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state to the surface of CeO2 particles, is considered to remove glass surface at the contact point between glass and Ce film surfaces. Consequently, the surface of the glass is flattened. Isohashi et al.

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(2017) proposed an abrasive-free polishing process, called catalyst-referred etching (CARE), for SiC

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using Pt thin film formed on polymer substrates. Murata et al. (2012) also studied the CARE polishing

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process for producing a smooth GaN surface. However, along with the high cost of Pt film in the CARE

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process, there are no reports on the CARE process flattening the glass surface. Compared with

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Fig. 1. Schematics of the polishing process. (a) overview, and (b) close-up of the polishing area. (c) Glass samples attached to the jig. (d) Sampling points for measuring roughness of glass sample conventional loose-abrasive polishing, the proposed method of abrasive-free polishing for glass is cost-effective because it requires no waste treatment of slurry, and produces a clean, polished surface

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with only a small amount of particle contamination. This paper describes the preparation of the Ce-deposited polishing pad and observation of the

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pad using electron microscopy with X-ray analysis. The glass polishing characteristics are investigated with different film thickness, polishing conditions, and pH of solution, and are compared

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with conventional forms of abrasive polishing.

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2. Materials and Methods

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2.1. Preparation and characterization of Ce-deposited polishing pad Ce films were deposited onto polishing pads by vacuum evaporation (DEPOX VTR-350M/ERH,

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ULVAC, Inc.) with resistive heating. Polyurethane-impregnated polyester felt pads (SUBA800, Nitta

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Haas, Inc.) of diameter 200 mm and thickness 1.5 mm were used. The pure (99.9%) metallic Ce fractions (CEE01GB, Kojundo Chemical Laboratory Co., Ltd.) on a heat source of tungsten filament

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(B-2, The Nilaco Corp.) were evaporated with an electric current of 50 A flowing through the filament for 30 min under vacuum pressure on the order of 10-3 Pa. The evaporation distance between the heat source and the polishing pad, defined as dev, was 100 or 180 mm. The Ce film thickness, TCe, was determined by eq. 1:

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𝑇Ce =

∆𝑚 𝜌𝑆

(1)

where Δm is the weight increase of the sample after deposition, ρ is the density of Ce, and S is the area of the sample. The sample were prepared with the different weight of Ce fraction and were

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characterized using a scanning electron microscope (SEM; SU1510, Hitachi High-Technologies Corp.)

2.2. Polishing experimental apparatus and operating conditions

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with a built-in energy-dispersive X-ray (EDX) spectrometer.

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The glass samples used in this study were soda-lime glass of diameter 20 mm and thickness 5

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mm. Prior to the polishing experiment, the sample surface was lapped using SiC abrasives (GC #8000,

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lapping typically ranged from 8 to 10 nm.

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Fujimi, Inc.) with a cast iron plate. The arithmetic average roughness (Ra) of the glass surfaces after

The polishing experiments were conducted using a single-sided polishing machine (FACT200,

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Nano Factor Co., Ltd) under the conditions summarized in Table 1. The Ce-deposited polishing pad

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underwent no surface treatments (such as truing, dressing, or brushing) prior to the polishing experiments. As shown in Fig. 1 (c), three glass samples were attached to the jig using a hot-melt wax

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and were polished simultaneously.

Table 1 Polishing experimental conditions Polishing table

200 mm in diameter

Polishing fluid

Deionized water, KOH aq.

Polishing pressure 10, 30, 40 kPa

Supply rate

10 mL/min

Rotational rate

Polishing time

10 min × 3

60, 200, 300 min-1

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After polishing, the material surfaces were rinsed with deionized (DI) water and evaluated using three-dimensional (3-D) white light interferometry (WLI; NewView 5032, Zygo Corp.) over an area of

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720 × 540 μm2. As shown in Fig. 1 (d), five different points on the each three simultaneously polished samples (fifteen points in total) were measured to obtain the average roughness (Ra).

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The thickness of material removal (MR) was calculated by measuring the weight loss of samples before and after polishing using an electronic balance as shown in eq. 2: ∆𝑚 𝜌𝑆

(2)

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MR =

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where ∆m is the weight loss of the sample, ρ is the density of soda-lime glass, and S is the area of the

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sample. The material removal rate (MRR) was obtained by dividing MR by polishing time.

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b

a

200 mm

c

200 mm

d

e

g

h

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500 μm

f

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×50

5 μm

Ce 0.8 μm

Ce 2.5 μm

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Before deposition

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×5000

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Fig. 2. (a, b) Photographs of the polishing pad following deposition at evaporation distance of (a) d

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3. Results and Discussion

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(d-h) deposition. (c-e):×50, (d-f): ×5000

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= 100 mm and (b) 180 mm. (c-h) SEM images of the polishing pad surfaces before (c, f) and after

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3.1. Fabrication of Ce-deposited polishing pad As shown in Fig. 2 (a), the polishing pad was thermally deformed after the evaporation, due to

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the high temperature radiated from the heat source at evaporation distance dev of 100 mm. When the

dev was set to 180 mm, the thin film was uniformly deposited on the entire surface of the polishing pad without thermal deformation (Fig. 2 (b)). The higher electronic current more than 50 A flowing through the filament also caused thermal deformation of the pad. As shown in Fig. 2 (c-e), although the pores of the polishing pad were partly filled with the deposited film when the film thickness was 9

increased, the porous structure was maintained on the polishing pad and has an important role in retaining the polishing solution. The magnified SEM images of the pads shown in Fig. 2 (f-h) show that the granular film structure, of which the particle diameter is several micrometers, was observed

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on the deposited film. The cracks, which are believed to be caused by the film stress, were observed on the film of thickness 2.5 μm (Fig. 2 (h)). The films of thickness >2.5 μm showed poor adhesion

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properties, and could not be used for polishing because the film was easily removed from the pad by the abrasion with the glass. The hardness of the Ce-deposited pad was measured to be 83 (using a

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type-A durometer hardness tester), which is almost equivalent to that of the uncoated polishing pad.

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The EDX spectra emitted from polishing pads before and after deposition, shown in Fig. 3 (a),

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clearly show that Ce (0.88, 4.84, 5.26, and 5.61 keV) element is contained in the polishing pad after

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Ce evaporation. Along with the strong O peak (0.53 keV) observed in the Ce-deposited polishing pad, the elemental maps (Fig. 2 (c-e)) of the pad showed that O element has the same distribution as Ce

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elements, which suggests that the native oxide layer was formed on the deposited Ce film. W peak,

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a

b

c

(arb.unit) unit) (arb. Intensity強度

A

C

O

Ce

Ce

W

500 μm

d

After deposition

e

Ce

Before deposition 0 0

1 1

2 2

3 3

4 4

5 5

6 6

Energy (keV)

Energy (keV)

C

O

Fig. 3. (a) EDX spectra from polishing pad surfaces before and after deposition. (b-e) SEM/EDX micrograph for Ce-deposited polishing pad. (b) electron image, (c) cerium, (d) carbon, and (e) oxygen elemental maps. 10

which derives from the filament heat source of the evaporation, was also detected in the spectrum of the Ce-deposited pad; however, the peak intensity was much smaller than the Ce peaks.

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3.2. Polishing characteristics 3.2.1 Film material

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The polishing characteristics of the prepared Ce-deposited pad was evaluated and compared with the different film materials. Fig. 4 (a) demonstrates that a ground glass surface was polished to

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a mirror surface by the Ce-deposited polishing pad over 60 min. As shown in Fig. 4 (a), the Ce-

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deposited pad showed a linear increase in material removal thickness with time, and showed a MRR

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of 75.7 nm/min; in comparison, the MRRs obtained using Cu (5.15 nm/min) or Al (0.94 nm/min)

算術平均粗さ roughness,(nm) Surface Ra (nm)

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c 15 15

Ce 75.7 nm/min

2 2.0

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Material removal thickness, MR (μm)

b 2.5 2.5

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a

1.5 1.5

Al 0.94 nm/min

Cu 5.15 nm/min Ce (in dry) 3.44 nm/min

1 1.0

0.5 0.5

10 10

Al Cu

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Ce (in dry) Ce

00

00 00

10 10

20 30 20 30 研磨時間 (min) Polishing time (min)

0 0

40

10 10

20 30 20 30 研磨時間 (min) Polishing time (min)

40 40

Fig. 4. (a) Photograph of glass material before (left) and after (right) polishing using Ce-deposited polishing pad. Effect of metal film materials deposited on the polishing pad on material removal thickness (b), and surface roughness (c). The error bars in (c) denote standard errors (SE), obtained from five measured points for three different glass surfaces (15 measured points in total) 11

deposited pad are negligible. Using the Al or Cu deposited pad, the surface roughness of the glass was almost constant (Fig. 2 (c)). Fig. 5 (a) shows that the Al-deposited pad introduced numerous scratches of ~20 nm depth.

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As shown in Fig. 5 (d), the strong oxygen peaks are observed on the EDX spectra emitted from Al film before and after polishing, which is evidence of a native oxide layer being formed on the Al film. The

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native oxide layer on Al film (e.g. Al2O3), which is widely used as a polishing abrasive, likely caused the scratches on the glass surface. Using the Cu-deposited pad, few scratches are newly introduced

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during polishing; however, the scratches on the initial surface generated by lapping process are not

Al

b

Cu

c

Ce

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a

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+20 nm 200 μm -20

Before polishing

10 min

20 min

C O

e Al

After polishing As-deposited

0

0

2

2

4

4

C O

Intensity (arb. unit)

Intensity (arb. unit)

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d

30 min

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**

After polishing

* * * As-deposited

0

6

0

Energy (keV)

* Ce

2

2

4 4

6

6

Energy (keV)

Fig. 5. (a-c) Change in surface morphology of glass with polishing time obtained by WLI. The film material deposited on the polishing pads is Al (a), Cu (b), and Ce (c). (d, e) EDX spectra emitted from (d) Al and (e) Ce film before (as-deposited) and after polishing. 12

removed (Fig. 5 (b)). The Ce-deposited pad achieved a sub-nanometer surface roughness, with the Ra of 0.83 nm by 30 min of polishing (Fig. 4 (c)). Furthermore, as shown in Fig. 5 (c), the scratches on the initial

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surface were gradually removed from the glass surface and an ultra-smooth and scratch-free polished surface was obtained by the Ce-deposited pad. However, using the Ce-deposited pad for dry polishing

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(i.e., without any polishing solution) achieved only 4.5% of the MRR that was achieved with the wet polishing, and a slight change of surface roughness even after 30 min of polishing. In conventional

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CeO2 abrasive polishing, CeO2 act as a catalyst to form a hydration layer on the glass surface, and

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this layer is then removed by the action of the abrasive. Assuming that the material removal

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mechanism of the Ce-deposited pad is comparable to that of conventional CeO2 abrasive polishing,

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the DI water is essential to material removal because it generates the hydration layer on the glass surface. EDX analysis of the deposited film (Fig. 5 (d) and (e)) shows that the intensity of the Al and

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Ce peak decreased after polishing, indicating that the films were worn by the polishing. However, the

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intensity of the oxygen peak was almost constant even after polishing, which suggests that the surface of Ce and Al film is believed to be easily oxidized by the oxidizing species such as dissolved oxygen

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and OH- in the polishing liquid.

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3.2.2 Film thickness The thickness of the deposited Ce film was evaluated for its effect on polishing characteristics. As shown in Fig. 6 (a), MRR increased with greater film thickness (TCe). Note that the MRRs were

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determined from the slope of the regression line of MR plots (inset in Fig. 6 (a)). No material removal and change in surface roughness was observed without Ce-film deposition. Increased TCe achieved

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lower surface roughness in less polishing time. The polishing pad with TCe of 2.5 μm achieved surface roughness <1 nm Ra. The error bar of MRRs (Fig. 6 (a)), which indicates the maximum and minimum

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values, becomes larger with increase in TCe. As shown in the inset of Fig. 6(a), while the MRs for TCe

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of 0.5 and 0.8 μm increase linearly with polishing time, the MR increment for TCe of 2.5 μm was

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slightly decreased after 30 min polishing. This is probably due to wear of the Ce film caused by its

a

100 100

20 20

0

0

A

00

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40 40

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MR (μm)

11.0 2.5 μm 0.75 80 0.50.5 80 0.5 μm 0.8 μm 0.25 60 0 00 60 10 20 30 (min) 0 研磨時間 5Polishing 10 15time 20(min) 25 30

0.5

0.5

1.0

1

1.5

1.5

b 算術平均粗さ roughness, (nm) Surface Ra (nm)

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Material removal rate, MRR (nm/min)

abrasion with the glass surface. As mentioned earlier, the Ce film with TCe of 2.5 μm showed surface

12 12

88

2

2.5

2.5

3.0

3

0.1 μm 0.5 μm

66

0.8 μm

44

1.7 μm

22 00

2.0

TCe 0 μm

10 10

2.5 μm

00

10 10

20 20

30 30

40 40

Polishing time (min)

研磨時間 (min)

Ce film thickness, TCe (μm)

Fig. 6. Dependence of material removal characteristics on Ce film thickness. (a) Relationship between Ce film thickness and material removal rate (MRR). The inset shows the time dependence of material removal thickness (MR) for TCe=0.5, 0.8, 2.5 μm. (b) Change in surface roughness with polishing time using different Ce film thickness. The error bars in (a) denote max. and min. values for the three continuous polishing samples, and in (b) denote SE. 14

cracking (Fig. 2 (h)), which subsequently reduced the wear resistance of the Ce film.

3.2.3 Effect of polishing conditions

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The material removal characteristics were evaluated for differing polishing pressure and pad rotation rates. For conventional abrasive polishing, MRR can be predicted by Preston’s equation

MRR = k‧P‧V

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of polishing: (3)

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where k (the “Preston coefficient”) is determined experimentally, P is the polishing pressure applied

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between material and polishing pad surfaces, and V is the relative velocity between material and

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polishing pad.

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As shown in Fig. 7 (a), MRR is increased with higher rotation rate below 200 min-1; however, MRR subsequently shows dramatic decline at rotation rate of 300 min-1. At the higher rotation rate,

Ra

100 100

200 200

研磨時間 Rotation rate(min) (min-1)

44 22

300 300

10 10

MRR (nm/min)

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MRR (nm/min)

0 0

Material removal rate, MRR (nm/min)

00

66

MRR

A

3030

88

400 400

400400 300 40 kPa 200 300 300 200 30 kPa 10 kPa 100 00 10 20 30 time30 (min) 0Polishing 10 20 40 200 200 研磨時間 (min)

88 66 44

100 100

00

00 00

22

Ra

MRR

Surface roughness, Ra (nm)

100100 200 min-1 75 120 120 5050 60 min-1 25 300 min -1 00 9090 10 20 30 5研磨時間 101520253035 Polishing time (min) (min)

6060

b

1010

Surface roughness, Ra (nm)

Material removal rate, MRR (nm/min)

a 150 150

00

10 10

20 20

30 30

研磨時間 (min) Pressure (kPa)

40 40

50 50

Fig. 7. Dependence of material removal characteristics on polishing conditions. Effect of rotation rate of polishing pad (a) and polishing pressure (b) on MRR and surface roughness. The insets show the time-dependence of MRR. The error bars for MRR and Ra denote max. and min. values, and SE, respectively. 15

the polishing solution (DI water) is thrown off the pad surface by centrifugal force, resulting in a lack of polishing solution in the area between the glass and pad surface. As mentioned earlier, the polishing solution is essential to achieving sufficiently high-quality polishing characteristics (Fig. 4).

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The lack of polishing solution leads to inferior MRR at rotation rate of 300 min -1 compared to those achieved at 60 and 200 min-1. Furthermore, as shown in the inset of Fig. 7 (a), at rotation rate of 300

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min-1, MRR gradually decreased with polishing time and became almost negligible after 30 min. The higher relative velocity between the glass and polishing pad surfaces at rotation rate of 300 min -1

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exacerbates wear of the Ce film, resulting in decreased MRR with long polishing duration.

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As shown in Fig. 7 (b), MRR is increased with increased polishing pressure, and was

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approximately three times higher at pressure of 40 kPa than that achieved at 30 kPa. Such high MRR

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at 40 kPa is believed to be primarily associated with the increased contact area between the glass and Ce film surface. Okamoto et al. (2012) reported that, in the CARE polishing process, the real area of

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contact between a metallic thin film and a material, as determined by the Hertz contact theory, is

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proportional to the load applied between the film and material, which enhances MRR with increased polishing pressure. Furthermore, increased polishing pressure and rotation rate result in increased

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temperature of the polishing pad, which is believed to enhance the chemical reaction between the oxide layer (CeO2) on the Ce film and the material, thereby enabling high MRR to be achieved at pressure of 40 kPa and rotation rate of 200 min-1. As seen in the relationship between rotation rate and MRR, the higher polishing pressure causes a significant decrease in MRR with extended polishing

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time (inset in Fig. 7 (b)), which is also associated with wear of the Ce film.

3.2.4 Effect of solution pH and comparison with abrasive polishing

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The polishing characteristics obtained by the proposed abrasive-free polishing method were compared with the conventional abrasive polishing method using CeO2 particles. CeO2 slurry was

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prepared by adding commercially available CeO2 abrasives (SHOROX A-10, SHOWA DENKO K.K.) with an average particle diameter of 1.3 μm to DI water at 3 wt.% concentration. The polishing pad

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used was the SUBA800 without any film coating. The polishing pressure and the rotation rate were

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30 kPa and 200 min-1, respectively. Unless otherwise noted, the polishing conditions match those

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summarized in Table 1.

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As shown in Fig. 8 (a), the surface roughness obtained using Ce film for 30 min is almost equal to that obtained by CeO2 abrasive polishing; however, the MRR of Ce film polishing is approximately

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one-third that of CeO2 abrasive polishing. In order to improve the MRR of the Ce film, potassium

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hydroxide (KOH) was added to the polishing solution. Wang et al. (2011) reported that alkaline solution can improve the MRR in the chemical mechanical polishing (CMP) of Si wafer using CeO2.

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Fig. 8 (a) demonstrates that, compared with DI water of pH 7, alkaline solution of pH 11 can achieve approximately four times higher MRR of Ce-film polishing without deterioration in surface roughness. The improved MRR when using alkaline solution of pH of 11 is probably due to the change of surface state of the oxide layer on the Ce film, from Ce4+-rich to Ce3+-rich state by adding KOH. Wang and

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Golden (2003) reported that CeO2 film is chemically stable at pH range 7–9, but is unstable and forms Ce(OH)3 layer at pH higher than 11. Considering the mechanism of glass polishing by CeO2 particles reported by Wang et al. (2011), in which Ce3+ reduce SiO2 to remove oxygen atoms from glass, the

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Ce3+-rich surface (Ce(OH)3) is preferable to obtain higher MRR compared to the Ce4+-rich surface (CeO2).

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Compared with the conventional abrasive polishing method using CeO2, the proposed polishing with Ce-film and alkaline solution achieves sufficient polishing performance in terms of MRR and

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surface roughness. As shown in Fig. 8 (b) and (c), Ce-film polishing shows larger errors in surface

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roughness compared with CeO2 abrasive, but produces an extremely smooth surface (0.388 nm Ra)

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without scratches or cracks. SEM images of CeO2 abrasives, shown in the inset of Fig. 8(a),

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demonstrate that the particles are more angular than the rounded and granular surface of Ce film

300 300

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MRR

200 200

表面粗さ

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研磨レ ート

Ra

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1.5 1.5 1.0 1

Ra

100 100

00

2 2.0

0.5 0.5 00

DI water (pH7)

KOH aq (pH11)

b Ra 0.388 nm

表面粗さ (nm Ra)

a

400 400

Surface roughness, Ra (nm)

ートrate, /m in) 研磨レ (nmMRR (nm/min) removal Material

(Fig. 2 (g)). The sharp edge of CeO2 particles result in the greater surface roughness compared with

c Ra 0.722 nm

200 μm

CeO2 abrasive

Ce film

Fig. 8. (a) Effect of polishing liquid pH on material removal characteristics, and comparison with conventional abrasive polishing using CeO2 particles. The error bars denote SE. (b-c) Surface morphology (upper) and section profile (lower) of glass surface polished for 10 min by Ce-film (b) or CeO2 particles (c). The inset in (a) shows SEM imagery of CeO2 particle (Scale bar: 2 μm). 18

the Ce-film method. Zhou et al. (2009) reported a fixed abrasive polishing of glass using CeO2 abrasives and they showed that the MRR and surface roughness was approximately 20-180 nm/min and 0.6-0.8 nm Ra, respectively. The results obtained in this study showed a superior MRR and

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surface roughness to those obtained using the fixed abrasive process. Using the proposed method, approximately 0.53 g of Ce is deposited on the pad, which represents

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a 94% decrease in the use of CeO2 abrasives (9 g) for 30 min polishing duration.

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4. Conclusion

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A novel, abrasive-free method of polishing glass, in which Ce film is deposited on a polishing pad, was

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described in this paper. The Ce film is formed on the pad using vacuum evaporation in combination

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with a resistive heating system. The appropriate thickness of Ce film was determined as <2.5 μm to ensure its durability during polishing. EDX elemental maps suggested that the deposited film is

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easily oxidized in the atmosphere to form an oxide layer on the Ce film. The use of Al and Cu film to

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polish glass resulted in MRRs that were almost negligible, and also generated scratches on the glass surfaces, whereas polishing using Ce film produced smooth, scratch-free surfaces and achieved

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superior MRR. Much lower performance was observed when polishing without supplying water to the Ce film. MRR increased with increase in Ce film thickness, and extremely smooth surface of subnanometer roughness was achieved using Ce film of thickness 2.5 μm. Although MRR increased with faster pad rotation, it decreased dramatically at rotation rate of 300 min-1, owing to displacement of

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the polishing solution by high centrifugal force. High polishing pressure (40 kPa) and rotation rate (300 min-1) caused increased wear of Ce film, resulting in the MRR declining with polishing time. The use of alkaline solution (pH 11) during polishing achieved approximately four times higher MRR

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compared with polishing with DI water. Compared with conventional abrasive polishing using CeO2 particles, the proposed polishing method uses 94% less Ce and achieves superior surface quality. The

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life of the Ce-deposited pad is very important and should be prolonged for practical use of the proposed polishing method. In the future, long-life Ce-deposited pads should be developed by using in situ

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deposition of Ce film applying an electrochemical plating.

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Acknowledgements

This research is partly supported by a research grant from the Advanced Machining Technology

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and Development Association.

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References

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Figure captions

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Fig. 1. Schematics of the polishing process. (a) overview, and (b) close-up of the polishing area. (c)

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Glass samples attached to the jig. (d) Sampling points for measuring roughness of glass sample

Fig. 2. (a, b) Photographs of the polishing pad following deposition at evaporation distance of (a) d =

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100 mm and (b) 180 mm. (c-h) SEM images of the polishing pad surfaces before (c, f) and after (d-h)

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deposition. (c-e):×50, (d-f): ×5000

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Fig. 3. (a) EDX spectra from polishing pad surfaces before and after deposition. (b-e) SEM/EDX

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elemental maps.

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micrograph for Ce-deposited polishing pad. (b) electron image, (c) cerium, (d) carbon, and (e) oxygen

Fig. 4. (a) Photograph of glass material before (left) and after (right) polishing using Ce-deposited

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polishing pad. Effect of metal film materials deposited on the polishing pad on material removal thickness (b), and surface roughness (c). The error bars in (c) denote standard errors (SE), obtained from five measured points for three different glass surfaces (15 measured points in total)

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Fig. 5. (a-c) Change in surface morphology of glass with polishing time obtained by WLI. The film material deposited on the polishing pads is Al (a), Cu (b), and Ce (c). (d, e) EDX spectra emitted from

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(d) Al and (e) Ce film before (as-deposited) and after polishing.

Fig. 6. Dependence of material removal characteristics on Ce film thickness. (a) Relationship between

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Ce film thickness and material removal rate (MRR). The inset shows the time dependence of material removal thickness (MR) for TCe=0.5, 0.8, 2.5 μm. (b) Change in surface roughness with polishing time

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continuous polishing samples, and in (b) denote SE.

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using different Ce film thickness. The error bars in (a) denote max. and min. values for the three

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Fig. 7. Dependence of material removal characteristics on polishing conditions. Effect of rotation rate of polishing pad (a) and polishing pressure (b) on MRR and surface roughness. The insets show the

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respectively.

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time-dependence of MRR. The error bars for MRR and Ra denote max. and min. values, and SE,

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Fig. 8. (a) Effect of polishing liquid pH on material removal characteristics, and comparison with conventional abrasive polishing using CeO2 particles. The error bars denote SE. (b-c) Surface morphology (upper) and section profile (lower) of glass surface polished for 10 min by Ce-film (b) or CeO2 particles (c). The inset in (a) shows SEM imagery of CeO2 particle (Scale bar: 2 μm)

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