Organic Direct Optical Recording Media - American Chemical Society

22 Organic Direct Optical Recording Media L. A L E X A N D R U , M . A . H O P P E R , R. O. L O U T F Y , J. H . S H A R P , and P. S. V I N C E T T

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Xerox Research Centre of Canada 2600 Speakman Drive Mississauga, Ontario G . E. J O H N S O N and Κ. Y. LAW Xerox Corporation Webster Research Center Webster, NY 14580 The interaction of laser light with a wide variety of imaging materials has been actively explored over the past few years because of its potential in high density information storage and retrieval systems. These systems are applicable to the storage of x-ray and video pictures as well as to the storage of financial, satellite, computer and government statistical data. A n optical recording material, for example, can store up to 1011 bits of information on a 12" disk. This represents an increase in packing density of two orders of magnitude over a magnetic disk. The key component of the optical recording system is the recording medium, which is marked in one step and read in another step with a laser beam. Many materials have been considered for the purpose, including metal -films, organic dyes, dye-loaded polymers, metal-loaded polymers, discontinuous metal films, thermal coloration systems and bilayers (1-19). Tellurium- and gold-based optical disks have been the most widely studied materials to date. A major requirement of a suitable optical recording material is resolution since a spot size of 1 μm or less is required. This corresponds to a resolution of 500 lp/mm (lp = line pairs). A second requirement is marking speed. Information must be recorded at ~10 M H z , so that the recording process must be essentially instantaneous. Based on current laser sources a power density of 1 m W / c m can be delivered to the surface of the recording material. Hence for the generation of a 1 μm spot, recorded at 10 M H z , the imaging material must have an exposure sensitivity of at least 10- J / c m . With the advent of the development of high power solid-state diode lasers, which offer low power consumption, compactness, increased reliability and direct modulation capability and economy over gas lasers, a further requirement of the optical recording material will be infrared sensitivity. A summary of typical requirements of the optical recording material is given in Table I. The 2

2

0097-6156/84/0266-0435S06.00/0 © 1984 American Chemical Society

In Materials for Microlithography; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

436

MATERIALS FOR MICROLITHOGRAPHY

T A B L E I.

Typical Requirements of the Optical Recording Material VALUE


REQUIREMENT Resolution

1000 L P / M M

Writing Speed

10 M H z

Exposure Sensitivity

10

Storage time

1-10 Years

Spectral Sensitivity

Visible, I.R.

Defect Density

1 : 10

Readout Contrast

> 0.5

-2

J/cm

2

6

most intensively investigated materials to date (4-7) are those that involve a photoinduced thermal process. In this case the recording laser beam is focussed onto a thin film of the recording material that has been deposited onto a substrate. The absorbed radiation raises the temperature above a threshold value for pit formation. The marking process usually involves melting or vaporization. The recorded marks have a different reflectivity or transmissivity than the unmarked surface. The simplest recording medium is a bilayer structure. It is constructed by first evaporating a highly reflective aluminum layer onto a suitable disk substrate. Next, a thin film (15-50 nm thick) of a metal, such as tellurium, is vacuum deposited on top of the aluminum layer. The laser power required to form the mark is dependent on the thermal characteristics of the metal film. Tellurium, for example, has a low thermal diffusivity and a melting point of 452 °C which make it an attractive recording material. The thermal diffusivity of the substrate material should also be as low as possible, since a significant fraction of the heat generated in the metal layer can be conducted to the substrate. For this reason, low cost polymer substrates such as poly(methylmethacrylate) or polyvinyl chloride) are ideal. Relatively less work has been reported on ablative organic materials. These materials, which generally combine low thermal diffusivity with low melting points suggest that they may be more sensitive for ablative optical recording applications. Bell and Spong described bilayer structures consisting of dye films evaporated onto a reflective substrate (5). Subsequent activities based on Bell and Spong's bilayer structure using various dyes such as 6,6'diethoxythioindigo (7), di-indeno[l,2,3-cd:r,2',3'-lm]perylene (20), metallophthalocyanines (21-23), and platinum complexes of bis-(dithio-a-diketones) (24) have appeared in the patent literature. Trilayer structures using


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ALEXANDRU ET AL.

Organic Direct Optical Recording Media

437


squarylium dyes have also been reported very recently (25). Apparently Eastman Kodak Company has been or is working on a dye/polymer optical disk approach (26). During the past few years, two novel concepts, both involving the use of organic dye/polymer composites as the marking media, have been investigated at the Xerox Research Centers. The first, described as Laser Induced Dye Amplification ( L I D A ) , is based on the principle of dye diffusion. The second, described as Dye in Polymer (DIP), involves ablative optical marking using dissolved dyes in polymer films. Each of these concepts will be discussed in detail in the following sections. Laser Induced Dye Amplification (LIDA) Several investigators have explored the use of organic dyes for use in high sensitivity light marking processes. For example, Braudy (4) and Bruce and Jacobs (27) have studied the light induced transfer of volatile organic dyes from one polymer substrate to another. The concept of Light Induced Dye Amplification was first proposed and reduced to practice in 1976 by Alexandru and Novotny (9). The imaging process involves the simultaneous exposure and fixing of an image by photoinducing dye diffusion from a solid film of dye into a compatible polymer substrate. The simple structure of the recording film is shown in Figure 1. The laser radiation is absorbed by the solid dye and converted via non-radiative processes into thermal energy. The dye subsequently melts, flows in a lateral direction and also diffuses into the polymer substrate. A depression or pit is created in the solid dye film due to the diffusion of the dye into the polymer as well as the lateral flow. The imaging contrast results from the difference in the spectral characteristics of the diffused dye, which is similar to that for the dye in solution, and that of the solid dye film. In order to demonstrate the feasibility of the L I D A concept in high density optical recording systems, a number of dyes have been investigated and the results reported (9,10). In this paper, the results for two of these dyes, Disperse Red II, and Disperse Blue 60 are described in detail. Disperse Red II is manufactured by ICI and Disperse Blue 60 is manufactured by Dupont. Both are derivatives of 1,4-diaminoanthraquinone. Their molecular structures are shown in Figure 2.

Figure 1. LIDA optical disk structure.


438

MATERIALS FOR

0

NH

2

0CH

0

NH

MICROLITHOGRAPHY

3

2

DISPERSE RED II (ICI) Ο

Ν Ho Ο

li

ι

II

£

N-CH -CH -CH -OCH Downloaded by NORTH CAROLINA STATE UNIV on July 29, 2013 | http://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch022

2

2

2

3

DISPERSE B L U E 6 0 (DUPONT)

Figure 2. Molecular structure of Disperse Red and Disperse Blue dyes. LIDA Dye Characteristics. There are several important characteristics which a dye must posses in order to be useful in the L I D A concept. First, it must match the lasing frequency of the recording laser. The laser utilized in this work was an argon ion laser which could deliver up to 32 mW of power to the film surface at a wavelength of 5145A. The absorption spectrum of a thin evaporated solid film of Disperse Red 11 on a substrate of polyethyleneterephthalate is shown in Figure 3a and indicates that it is an excellent match to the 5145A emission of the argon ion laser. As stated earlier, the imaging contrast in the L I D A concept results from the difference in the spectral characteristics of the diffused dye and those of the solid film. Figure 3 also shows the absorption spectra of the polyethyleneterephthalate after dye diffusion has occurred. Figure 3b is the result of thermally induced diffusion while Figure 3c is the result of laser-induced dye diffusion. A blue shift is observed in each case. Both are similar to the spectrum of Disperse Red 11 in solution and indicate that the dye has diffused into the polymer substrate on the molecular level and has formed a solid solution with the polymer. Figure 4 shows the same spectra for the Disperse Blue 60 dye. The difference between the spectral characteristics of the diffused dye and that of the solid film is obvious in this case as well. The solid film exhibits a diffused absorption probably due to light scattering. Next, the thermal properties of the dye must be such that absorption of the laser energy will result in dye diffusion but not in decomposition. The melting temperature T , the latent heat of fusion, AH, and the specific heat for these dyes were determined by differential scanning calorimetry using a DuPont 990 Thermal Analyzer. The data are given in Table II. No thermal decomposition products for these dyes were detected upon heating to 600 °C for 20 msec. The energy density of the laser irradiation required to melt the dye and to induce diffusion will be given by: m

E

D

= mC AT p

+ m AH


(1)

22.

A L E X A N D R U ET AL.


ι

I

I 4000

Organic Direct Optical Recording Media 1

439

Γ

ι

I ^ 6000

ι

I I 8000

WAVELENGTH (A)

LIDA DISK ABSORPTION SPECTRA Figure 3. Absorption Spectra of the Disperse redlpolyethyleneterephthalate system, (a) Evaporated film, (b) Thermally diffused, (c) Laser diffused.

DISPERSE BLUE 6 0 ON POLYETHYLENETEREPHTHALATE α b C

SOLID FILM THERMALLY DIFFUSED LASER DIFFUSED

4000

y/\ \ S /

f *

v.

V i \

6000

8000

WAVELENGTH (A) Figure 4. Absorption Spectra of the Disperse Blue 60Ipoly ethyleneterephthalate system. (a) Evaporated film, (b) Thermally diffused, (c) Laer diffused.


440


Table II.

Thermal Properties of the Disperse Dyes DYE TYPE

Thermal Property

Disperse Blue 60

T

C O

250

200

AH

(J/g)

120

36

m


Disperse Red 11

C

p

@ 160°C (J/g-°C)

1.74

1.80

2

Using the data in Table II and a value of —0.1 mg/cm for the mass of dye/cm , m, in the solid film, E is computed to be —50 m J / c m for the Disperse Red 11 and —30 m J / c m for the Disperse Blue 60. The photophysical properties of the chosen dye must also be considered. Any absorbed energy which is re-emitted as fluorescence or phosphorescence can not be converted to thermal energy. Hence φρ and φ must be low. Therefore, the amount of absorbed energy that can be converted to thermal energy will be governed by the quantum yields of internal conversion, / and intersystem crossing, φ^. Since the optical recording medium must have the capability of read after write verification, the imaging process must be completed within 10~ sec. However, the conversion of electronic energy to thermal energy by intersystem crossing from Τ to So will be slow due to the long triplet lifetime (—10~ sec). Hence, the ideal dye for our application is one which has a quantum yield of internal conversion, /, approaching unity. 2

2

D

2

Ρ

c

7

3

c

Marking Results. Figure 5 shows a scanning electron micrograph of the marked bits on the Disperse Red 11 polyethyleneterephthalate system. The

Figure 5. SEM

of Marked Bits (x4800) in the LIDA disk. Laser Power 32 mW; Exposure time = I μ-s.


22.

Organic Direct Optical Recording Media

ALEXANDRU ET AL.

441


spots were marked using a power of 32 mW and an exposure time of 1 μδ. For a high density optical recording medium, two important parameters which characterize its capability are the size of mark and the threshold energy density required for mark creation. The size of the mark was found to be dependent on both the incident power and the exposure time. The size of the marked areas is shown in Figure 6 for different levels of power and exposure

Figure 6. Bit sizes as a function of laser power and exposure time in a 0.2 μm LIDA disk. times. Although the diameter of the laser beam spot size was kept constant at 1 μπι, the marks vary in size from less than a micron to several microns in diameter. As might be expected, increasing the laser beam power or the exposure time, increased the mark size. The power threshold, P , defined as the minimum power to induce dye diffusion, is obtained by extrapolating the curves in Figure 5 to zero bit area. When P is plotted as a function of exposure times less than 1 MS, reciprocity is maintained, demonstrating that the marking process is independent of exposure time and depends only on the marking energy. The threshold energy density required for marking, E , is defined by t

t

D

E

D

-

(2)

-L.

where P is the laser power, t is the exposure time and A is the area of the laser beam. The experimentally determined value for E is —100 mJ/cm . This is in good agreement with the value of 50 m J / c m calculated from the thermal properties of Disperse Red 11. t

2

D

2


442


It is worthwhile to discuss another interesting aspect of dye diffusion into polymer which is governed by polymer morphology. When the bit sizes marked with Disperse Blue 60 film are compared, relatively large systematic differences are observed between bits marked on Mylar (from Dupont) and on Kodar (from Kodak) substrates. The sizes of the bits obtained on Kodar substrate are substantially smaller (by a factor of up to 2) than the ones obtained on Mylar under the same conditions. Figure 7 shows the dependence of bit size marked


! BIT SIZE ON \μ. DISPERSE BLUE 6 0 SUBSTRATE: M-MYLAR K-KODAR POWER DENSITY: · 4.1 X l 0 W / c m 6

• 1.1 x l 0 w / c m 6

2

2

i 4

Organic Direct Optical Recording Media - American Chemical Society

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