Manufacturing Methods for a Flexible Optical Disk (FOD)
ONAGI Nobuaki, AMAN Yasutomo, MURATA Shozo, SUGIMOTO Yasunori, KAJIYAMA Takeshi*, KOIDE Daiichi*,
TAKANO Yoshimich**, TOKUMARU Haruki*
RICOH Co.,LTD.

*Japan Broadcast Corporation (NHK)**NHK Engineering service Inc.

1. AimThin optical disks (such as a flexible optical disk, FOD, hereafter) are drawing much attention because they canprovide high volumetric recording density on a thin disk with high-speed data transfer thanks to air stabilization.
Since the thickness of an FOD is in the range of 0.1 to 0.2 mm, however, conventional fabrication processes forDVDs cannot simple be applied as is to fabricate an FOD. This report describes some FOD fabrication methods
tested by the authors.

2. Trial production
The process flow for fabricating the FOD is shown in Figure 1. Owing to the thinness of the FOD, many steps ofthe FOD fabrication process are different from those of conventional processes; those steps—namely, pre-format
formation, recording-film formation, and overcoating—are described in the following sub-sections.

2-1. Pre-format formation for film
Ultraviolet (UV)-curing resin is dropped onto a stamper, and the resin is spread out by centrifugal force producedby coaxial spinning. After that, the resin is cured, and the cured film is peeled off the stamper. At that time,
variation in transfer-layer thickness, which is dependent on the characteristics of the UV-curing resin used and is
manifested in a significant difference in the focus-servo characteristic, can be seen. The appearance of the coating
by UV-curing resin is shown in Figures 2 and 3. As shown in Figure 2, the stamper is attached to the spinner, andthe UV-curing resin is spread out in a circular coating. After that, the circular film (which is cut bigger than thestamper) is placed on the stamper, and the spinner is rotated. During the spinning, the UV-cure resin is spread
across the whole stamper. After the resin is spun, and then cured by UV irradiation, the cured-resin film is peeled
off the stamper. The film thickness at that point varies in accordance with the type of UV resin used (see Figure 4).
According to Figure 4, the resin composed of monofunctional monomer (namely, “UV resin D”) represents the best
one from the viewpoint of low thickness variation. .

2-2. Formation of recording layer on film
When a normal retention jig for optical disks is used for sputtering, the coated film contains large undulations forthe following reason. Since the film is loosely fixed, temperature rise during the film-formation process causes thefilm to undulate, and the recording film is sputtered in the form of those undulations. The following two methods
for preventing these undulations were tried.
Method 1: The transfer film of the FOD is attached to a 0.6t hard-plastic substrate and sputtered by using astandard substrate-fixation jig. After processing such as transcription and film formation are completed, the film
and the 0.6t substrate are separated.
Method 2: A jig that applies tension to a circular film is used to form the recording layer in the form of a film intaut state.
The state of the film produced by method 1 is shown in Figure 5. It is thought that if the film can be bonded to thesubstrate without bubbling between the two, the recording layer can be formed. Bubbles are, however, oftengenerated between the film and the substrate, and these bubbles expand when the substrate is inducted into thevacuum chamber. Consequently, it has not been possible to attain a flat FOD.
The sputtering jig used in method 2 is shown in Figure 6. The recording layer is formed while radial tension isapplied to the circular film so that the film does not undulate. Using this method results in few undulations in thefilm after sputtering. In light of these conclusions regarding methods 1 and 2, the sputtering of the recording layeron the resin film was performed by method 2 (i.e., the tension-application jig).
The results—namely, measured undulation when the disk is revolving (“axial runout of FOD” in thefigure)—obtained by using methods 1 and 2 are, respectively, plotted in Figures 7 and 8. It is clear from theseresults that using the jig (method 2) makes it possible to perform sputtering of the recording layer with a standard
sputtering machine for optical disks.

2-3. Overcoat for recording layer
The protective sheet for the film disk after the recording layer has been formed on it is formed by applying andcuring a dry film resin. The dry film resin is processed in the form of a gel composed of a mixture of a UV-curingresin and a thermosetting resin. A key feature of forming the protective sheet by using a resin gel in this manner isthat it is easier in comparison with spinning a coating film on a film-like disk and curing it.

3. Characteristics fabricated FOD
The fabricated FOD can record and reproduce signals at the same recording density as that of a Blu-ray disk. A
concern, however, is variation with time of consequent warpage of the disk. To address that concern, aconstant-humidity-and-temperature test (80°C/85% RH for 200 hours) was carried out on the fabricated FOB toinvestigate time-varying warpage of the FOB surface. According to the test result, although deterioration of thesurface due to a certain degree of warpage was observed, the low value of the warpage (namely, less than 10microns) means that the warpage is not a concern.

4. Concluding remarks
Two trail fabrication methods for a flexible optical disk (FOD) were described. The FOD is fabricated by means ofsteps such as pre-format formation with UV-curing resin, sputtering under tension application, andprotective-sheet formation by using a dry-film resin. It can not only record and reproduce signals at the same
recording density as that of a Blu-ray disk but also provides satisfactory storage stability.

Film cutting (φ150 mm)

Pre format forming using UV-resin

Film cutting (φ130 mm)

Sputtering recording layer
Fig.2

Applying UV resin on a stamper

Fig.3

Spinning stamper with film

Bonding protective sheet (dry film resist)

Film cutting (φ120 mm)

Bonding center hubCompletion

UV resin thickness / μm

Fig.1 Manufacturing process for FOD

15141312111098765

UV resin A
UV resin B
UV resin C
UV resin D04590135180225270315

360

円周方向/°
Circumferential
direction / degree

Fig.4 Thickness variations of UV resin layer on various materials

Film

ボイド
Double ring

Airbubbles

Fig.5 Film bonding on 0.6t PC substrate

M3-103
2P_Sheet_3 103um
with the layer
like DVR
Stabilizer
pushvalue
2001/4/4
at 2000rpm
Guide position r:40mm

Fig.6 Sputtering jig

Guide push value
1000um1500um
2000um2500umProfile of Axial runout[um]1050-5-10-15-20-25-30-35
0.0

0.2

0.4

0.6

0.8
1.0
1.2
1.4
Angle position [rotation]

1.6

1.8

Fig.7 Profile of FOD axial runout made by bonding method

2.0

MA-Ref
PC100umwith the layer like DVR
2001/4/24
Stabilizer
push value
at 2000rpm
Guide position r:40mm

Guide push value
500um
1000um
1500um2000um50

Profile of Axial runout[um]0-50-100-150-200

-250
0.0

0.2

0.4

0.6

0.8
1.0
1.2
Angle position [rotation]

1.41.61.8

2.0

Fig.8 Profile of FOD axial runout made by double ring method