Radiation Effects on Polymers - American Chemical Society

Chapter 21

Thermal Marking of Amorphous Poly(ethylene terephthalate) 1

C. M. Roland, J. P. Armistead, and M. F. Sonnenschein

Downloaded by RUTGERS UNIV on November 30, 2016 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch021

Chemistry Division, Code 6120, Naval Research Laboratory, Washington, DC 20375-5000

It is shown that exposure of amorphous films of poly(ethylene terephthalate) (PET) to infrared light from a CO laser induces either crystallization or ablation, depending on the intensity of the radiation. The resolution of these thermal images is not only exempt from diffraction limitations, it is also found to be minimally affected by thermal diffusion. Crystallization and ablation can both serve as a basis for microlithography, providing a single step process producing high resolution images of good contrast and edge acuity. The extension of the technique to selective metallization of polymer films is demonstrated. 2

The use of polymeric materials as media for optical data storage has received much attention and a large body of relevant literature exists [1-7]· The desired attributes of an imaging process include the production of stable images which are high both i n contrast and in resolution. High resolution enables higher storage density on the polymer surface. When absorption of a single photon causes an incremental change i n the image intensity, the spatial resolution i s d i f f r a c t i o n limited. Hence "photon mode" processes rely on short wavelength radiation (ultraviolet, x-ray, and electron beam) to achieve high resolution. These approaches are employed in much photoresist technology [16]. A familiar commercial application of optical data storage i s the compact disk (CD) [7]. Information takes the form of a series of pits i n the surface of a polymer film, each depression having the same depth but being of different lengths and separations. The variation i n the Current address: Dow Chemical Research Center, Walnut Creek, C A 94598

This chapter not subject to U.S. copyright Published 1991 American Chemical Society

Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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reflection of a light beam as i t traverses alternately the smooth CD surface and the p i t s provides the data transcription. The pits i n a CD are created by a stamping process; a master engraves onto the polymer substrate a negative r e l i e f . Polycarbonate i s the current polymer of choice for CD's, due to i t s high glass transition temperature, toughness, r e l a t i v e l y low cost, and the low birefringence obtained i n injection moldings. An alternative to writing mechanisms based on photon or mechanically induced alterations i n the recording medium i s the use of heat to effect a physical or chemical change. Thermal processes selectively raise the temperature to some c r i t i c a l point at which image formation commences. Because a single photon provides i n s u f f i c i e n t energy to induce t h i s process, thermal methods are not d i f f r a c t i o n limited. There i s hence no a priori reason to employ short wavelength radiation; however, i t i s anticipated that thermal diffusion away from the d i r e c t l y heated regions w i l l smear the image and thus l i m i t resolution. Crystalline - amorphous phase transformations are a potential basis for thermal lithographic techniques. The conversion of an amorphous polymer to the c r y s t a l l i n e state alters the optical properties, introducing opacity and birefringence, and thus producing an image. A number of studies have employed the c r y s t a l l i z a t i o n of small molecule species residing on or i n a polymeric matrix [8-12]. The polymeric substrate i t s e l f can serve as the active medium. It must be initially amorphous, but yet highly c r y s t a l l i z a b l e , i n order to produce a high contrast image. Few polymers crystallize extensively above room temperature, while s t i l l c r y s t a l l i z i n g s u f f i c i e n t l y slowly to be obtainable i n the amorphous state. Poly(phenylene sulfide), poly(ether ether) ketone (PEEK), i s o t a c t i c polystyrene, and poly (ethylene terephthalate) are among the polymers which meet this c r i t e r i o n . An early demonstration of using the amorphous c r y s t a l l i n e phase change to impart an image involved poly(phenylene sulfide) [13]. Crystalline lines as narrow as 100 μπι were produced on a film of the f i l l e d polymer ("Ryton") by the application of heat from a C0 laser. I t i s unknown whether elimination of the f i l l e r would slow down c r y s t a l l i z a t i o n and thereby allow finer imaging. A C0 laser was also employed to selectively c r y s t a l l i z e PEEK films [14]. In these experiments image sizes were as large as a few mm, and the authors suggested the use of dyes to achieve high resolution. By placing the film on a substrate to slow the rate of cooling, the perfection of the c r y s t a l l i t e s was improved; the relationship of this to image quality was not addressed. It was reported that heating at a high rate for a short period rendered the PEEK amorphous, since ambient a i r quenching was sufficiently rapid to minimize recrystallization. An obvious advantage of using a physical change as the writing mechanism i s that such a 2

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Thermal Marking of Poly (ethylene terephthalate) 345

process i s reversible, potentially allowing for multiple read/write cycles. This r e v e r s i b i l i t y was demonstrated i n PET heated with an infrared laser, wherein c r y s t a l l i z a t i o n and melting were successively executed by adjustment i n the intensity or duration of the irradiation [15]. Directional c r y s t a l l i z a t i o n upon cooling of polyethylene oxide from the melt was carried out by translating films through an infrared laser beam [16]. Although lacking such r e v e r s i b i l i t y , a chemical change induced by heat can also serve as a lithographic method. Ablation at the surface of a polymer film, as well as other photochemical reactions, have been widely explored [17-28]. The use of an infrared laser to ablate oriented sheets of semi-crystalline PET ("Mylar") has been reported [29]. The texture arising from the presence of spherulites resulted in poor image quality. Photoetching and the photochemical degradation of poly (ethylene terephthalate) exposed to U.V. radiation has been extensively studied [30-34]. I t was demonstrated that, even i n the UV region, a thermal mechanism i s operative at high radiation intensities [29,33]. High resolution imaging v i a ablation using infrared radiation has recently been reported [35]. Reviewed herein are ongoing efforts [15,35,36] to accomplish thermal microlithography i n poly(ethylene terephthalate). Both physical and chemical changes can be induced by the application of heat. The u t i l i t y of these processes for optical data storage i s discussed. Experimental Experiments were carried out using extruded sheets (130 Mm thick) of amorphous PET ( i n t r i n s i c viscosity =0.75 dl/g) obtained from Eastman Chemicals, Kingsport, TN. Qualitatively similar results were obtained when a higher molecular weight PET (from the Goodyear Tire and Rubber Co. with I.V.= 1.04) was u t i l i z e d . Heating was accomplished by exposing the film to 10.6 Mm radiation from a Coherent Model 42 C0 laser. The intensity p r o f i l e of the laser beam was quite nonuniform; thus, average power measurements were of dubious value. To create an image, the laser beam flood illuminated a mask comprised of gold patterned on a GaAs wafer. The mask lay on top of the polymer, whereby radiation was selectively prevented from impinging on the polymer surface by r e f l e c t i o n . In an alternative arrangement, an image was produced by reflecting the laser light from an aluminum surface onto the PET film. Micrographs were obtained with an AMR Model 1000 scanning electronic microscope (SEM) and with a hot stage equipped Zeiss optical microscope. Differential scanning calorimetry was conducted on samples cut from the films using a Perkin-Elmer DSC-7 at 20 deg per minute. 2

Remits Laser Induced Crystallization.

In Figure 1 i s displayed


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Figure 1. Crystallization line produced i n amorphous PET by reflection of C0 laser radiation from an aluminum surface positioned near a film of PET. The irradiation time was 30 seconds. 2



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347 Thermal Marking of Poly (ethylene terephthalate)

amorphous PET film containing a c r y s t a l l i n e image, roughly 10 Mm wide, as seen through cross polarizers. This image was produced by heating the film with laser radiation (0.2 watts) reflected from a piece of aluminum. The localized c r y s t a l l i z a t i o n i s a consequence of heating the PET to above i t s c r y s t a l l i z a t i o n temperature. The high birefringence associated with the crystal phase produces a d i s t i n c t image through crossed polarizing f i l t e r s (as i l l u s t r a t e d i n the figure). The c r y s t a l l i t e s are s u f f i c i e n t l y large for significant light scattering, and consequently the images are also d i r e c t l y v i s i b l e . Although there i s a 10% reduction i n mass density upon c r y s t a l l i z a t i o n , image formation was not accompanied by any observable alteration of the surface topography of the films. From differential scanning calorimetry measurements i t was estimated that irradiation induced a level of c r y s t a l l i n i t y equal to about 34% (using 140 J/g as the perfect enthalpy of fusion [37]). The c r y s t a l l i n e phase melted at 245°C. This i s equivalent to the melting point obtained when the PET was c r y s t a l l i z e d at various temperatures over a range of from 140°C to 230°C [15]. From experiments on a hot stage and microscope, i t was determined that c r y s t a l l i z a t i o n commences at an observable rate at temperatures as low as 125°C. The laser induced crystallization presumably transpires within this temperature range. When a mask was used to define the image, the gold coating comprising the mask image was in contact with the polymer film. This probably reduces the temperature of the regions shielded from the radiation, since the gold functions as a heat sink. Representative images obtained by this method are seen i n Figures 2 and 3. With the mask i n contact with the film, higher power levels (circa 1 watt) were necessary to induce c r y s t a l l i z a t i o n , presumably to compensate for reflection losses at the interfaces (measured to be 30%) and heat conduction to the gold. The laser intensity p r o f i l e was non-uniform, and as a result the extent of c r y s t a l l i n e image formation was found to vary over the irradiated zones of the film, as can be seen i n Figure 2. Note that individual spherulites are v i s i b l e i n the irradiated films. A cross-section of a laser c r y s t a l l i z e d film i s displayed in Figure 4. I t i s observed that the crystalline phase extends completely through the film thickness. More opaque and more strongly biréfringent images were found to have a greater concentration of spherulites (Figure 4). In no case was there evidence of any gradient or accumulation of spherulites toward the irradiated side of the films. The absorptivity of the PET at 10.6 Mm was equal to about 90 cm". Assuming a heat capacity of 12 J/g-deg [37], and an amorphous density of 1.335 g/cm', a 0.25 watt laser could provide s u f f i c i e n t energy to heat a one square mm section of the film to beyond 125°C i n one second. This i s faster than the actual time scale for the laser writing. The efficiency of the marking process could not be American Chemical Society Library Clough and Shalaby; Effects 1155 Radiation 16th St., N.W.on Polymers 1

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DX»Society: 20036 ACS Symposium Series;ItestiiitetfMi. American Chemical Washington, DC, 1991.


Figure 2. Amorphous PET film irradiated through a light field gold on GaAs mask for 20 s at low laser power. The metallized side of the mask was in physical contact with the film. Images obtained on the P E T film were identical in size to those on the mask.


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Thermal Marking of Polyethylene terephthalate)


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ce

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Figure 4 . Cross-section of PET film (a) prior to exposure to the laser, wherein microtome marks can be seen; (b) after irradiation s u f f i c i e n t to give a weak c r y s t a l l i n e image; (c) after prolonged i r r a d i a t i o n that yielded a very opaque image.



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quantified with the present experimental apparatus and no effort was expended toward optimizing this efficiency. A representative dose-response curve i s shown i n Figure 5. The non-linearity of the response i s an attractive feature of thermal imaging processes. Crystallization as a mechanism for microlithography i s attractive only to the extent that high resolution marking can be achieved. As noted, the resolution of thermal imaging i s not subject to wavelength constraints due to d i f f r a c t i o n , and indeed features i n Figures 2 and 3 are s i g n i f i c a n t l y smaller than 10 μπι. As indicated by the data in Figure 5, there i s a threshold level of radiation absorption below which the consequent temperature r i s e i s insufficient to effect marking of the PET film. This threshold i s beneficial for obtaining both good contrast and high resolution. The resolution of a thermal process i s expected to be governed by the diffusion of heat away from the d i r e c t l y irradiated regions. When optical imaging by means of an amorphous to c r y s t a l l i n e phase transition was carried out with either polyphenylene sulfide [13] or poly(ether ether ketone) [14], the reported resolution was poor. The c r y s t a l l i z a t i o n extended beyond the d i r e c t l y irradiated regions. Modeling of the heat flow has suggested that thermal diffusion away from the irradiated areas w i l l smear the image unless extremely fast heating rates are employed [15]. Some representative results from these calculations are give i n Table I. Listed i s the temperature one micron away from the edge of a 10 micron wide irradiated area at the time a temperature of 120°C i s attained by the material one micron within the edge. Different times correspond to different heating rates. Note that rapid c r y s t a l l i z a t i o n commences i n PET at roughly 120°C.

Table I.

Calculated Temperatures i n Region Adjoining Directly Irradiated Areas* [15]

TIME (msec) TEMPERATURE (C)

1

10

100

1000

102

115

118

120

*At heating rates for which d i r e c t l y heated region reached 120°C.

A slow marking process i s predicted to be unable to y i e l d images as fine as 10 μπι. The reports of low resolution ( > ΙΟ μπι with polyphenylenesulf ide [13], and > ΙΟ μπι with PEEK [14]) are consistent with such expectations. Given these considerations, the image dimensions apparent i n 2

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ι

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1

30

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Figure 5. (a) The optical density (in arbitrary units) of a c r y s t a l l i n e mark formed by irradiation at 0.2 watt. (b) The depth ( i n Mm) of the ablation from exposure to 4 watts infrared laser radiation.


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Figures 1-3 are surprising. Although the c r y s t a l l i n e image quality i s not superb, i t should be noted that the extraneous c r y s t a l l i t e s are randomly situated. There i s no suggestion that this "noise" i s associated with the borders defining the images. I t i s certainly considered that more uniform films and a more uniform l i g h t source would improve the image quality. Figure 6a shows a thermally c r y s t a l l i z e d film of PET on which the c r y s t a l l i z a t i o n i n a small region has been erased by exposing the film to the C0 laser radiation. The amorphous spot i s transparent and non-birefringent, appearing through crossed polarizers as dark against the biréfringent c r y s t a l l i n e background. This erasure i s reversible, as demonstrated i n Figure 6b, which shows the "erased" spot following p a r t i a l overwriting by exposure to less intense laser l i g h t . With further irradiation the film could be made completely opaque and biréfringent, equivalent to i t s i n i t i a l condition. A disordering process such as melting can be accomplished significantly faster than the reverse operation of ordering the polymer segments into a c r y s t a l l i n e phase; therefore, i n principle, induced melting of c r y s t a l l i n e film i s potentially a faster marking process than c r y s t a l l i z a t i o n of amorphous polymer. However, whereas c r y s t a l l i z a t i o n commences at temperatures somewhat above the glass transition temperature, the reverse process requires heating to the higher temperature of melting, thus presumably requiring higher power input.


2

Ablation. As described above, at laser powers on the order of one watt or lower, c r y s t a l l i n e images can be produced i n amorphous PET, while very brief (< 1 sec) irradiation at s l i g h t l y higher intensities effects melting. When the PET i s exposed to s i g n i f i c a n t l y higher levels of the infrared radiation, however, a different process i s observed. As seen i n Figures 7-9, images produced by irradiation at several watts are three dimensional. Also, unlike the images produced by the amorphous-crystalline phase transformation, these images are not erasable. The more intense irradiation promotes significant chemical bond rupture i n the PET. The byproducts of this decomposition are then expelled as fragments or vaporized. This ablation at the PET surface creates an image. The dimension of the image i n Figure 9 i s less than 1 μπι, and as seen in both Figures 8 and 9, edge acuity i s quite good. Extraneous p i t s and debris are present on the original film. Although marks less than 1 μπι i n width can be produced, the masks have defects at this scale. The ultimate resolution achievable with this technique can not be determined presently. The depth of the laser etching i s proportional to the irradiation time (see Figure 5). A correspondence was also found between this depth and the weight loss measured for the films [35]. The steepness of the walls r e f l e c t s a minimal extent of thermal diffusion, which coupled with the Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


Figure 6. (a) Amorphous spot produced by laser melting in an i n i t i a l l y c r y s t a l l i n e PET f i l m (as viewed through crossed polarizing f i l t e r s ) . (b) The same spot after being p a r t i a l l y overwritten through r e c r y s t a l l i z a t i o n via laser heating. After complete overwriting, no evidence of the original spot remained.


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Thermal Marking of Polyfethylene terephthalate)


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Figure 7. Optical micrograph of ablated images formed in amorphous PET by exposure to infrared radiation through a mask. The scale mark is 300 μτη. Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Figure 8. Scanning electron micrograph of a portion of Figure 7. The imperfections are due to the condition of the original PET film and to mask errors.




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Figure 9. SEM of a portion of Figure 7. Notwithstanding the defect arising from an error i n the mask, the edge acuity of the line i s striking.



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expected existence of a threshold f o r thermal ablation f a c i l i t a t e s attainment of high resolution imaging. When the lower power level associated with induced c r y s t a l l i z a t i o n (< 1 watt typically) was continued well beyond the time required for c r y s t a l l i z a t i o n , only melting, but no ablation, resulted. Ablation of the PET could only be achieved using radiation levels of at least a few watts. The PET under the ablated areas remains amorphous, as inferred from the transparency and absence of birefringence of the images (Figure 10a and 10b). When the ablated film i s subsequently c r y s t a l l i z e d on a hot stage, both the unablated regions of the film and the imaged areas become opaque and biréfringent (Figure 10c and lOd). Since the laser light i s only attenuated roughly 20% upon passage through the film, i t i s expected that sufficient intensity i s available f o r significant heating of the underlying material during the course of the ablation. Evidently a temperature above the c r y s t a l l i n e melting point i s attained, whereby no c r y s t a l l i n i t y results. For PET films as thin as used herein (130 μια), a i r cooling i s s u f f i c i e n t l y fast to prevent c r y s t a l l i z a t i o n during cooling from the melting point down through the glass transition temperature. Ablative imaging was attempted on semi-crystalline PET, which was nonetheless transparent and optically clear due to the small size of the c r y s t a l l i t e s . Consistent with previous work [29], only very coarse, i l l - d e f i n e d images could be produced. This might seem surprising since prior to ablation, the material presumably i s heated above the crystalline melting temperature. Apparently a level of thermal energy s u f f i c i e n t for ablation i s attained i n a time period too short f o r thermal equilibration and consequent melting. High quality imaging requires i n i t i a l l y amorphous PET. The use of thermal ablation as an imaging method has been attempted with other polymers [35]. The efficiency and imaging quality varied widely, with the best overall results achieved with PET. Metallization. I t i s known that PET w i l l adhere to metals, and particularly to metal oxides, when raised above i t s melting point while i n contact with the metal [38]. Actually, for amorphous PET i t i s only necessary to heat the polymer above the glass transition temperature for bond formation to transpire. Localized metallization of PET can therefore be effected by i t s irradiation through a mask. The underside of the PET i s maintained i n contact with the metal, f o r example i n the form of a thin film, during exposure to the infrared l i g h t . After irradiation the metal i n the unexposed regions i s brushed or stripped away, leaving behind a metallized pattern. Representative results of this process are seen i n Figure 11. The bonding of PET i s superior to copper and aluminum than to gold. Attempts to remove the former result i n cohesive f a i l u r e of the polymer.


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359 Thermal Marking of Poly (ethylene terephthalate)


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Figure 10. Optical transmission micrograph of ablated images (a) observed with unpolarized l i g h t , where edge scattering enables the images to be seen and (b) seen through crossed polarizers. The material lying under the ablated area i s not biréfringent. After heating at 2 degrees per minute to 127°C., the entire film becomes translucent and biréfringent as respectively seen i n (c) unpolarized and (d) polarized l i g h t .

Continued on next page Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.



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Continuedfrompage 359. Figure 10. Optical transmission micrograph of ablated images (a) observed with unpolarized l i g h t , where edge scattering enables the images to be seen and (b) seen through crossed polarizers. The material lying under the ablated area i s not biréfringent. After heating at 2 degrees per minute to 127°C., the entire film becomes translucent and biréfringent as respectively seen i n (c) unpolarized and (d) polarized l i g h t .


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Thermal Marking of Poly (ethylene terephthalate)


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Summary In anticipation of smearing by thermal diffusion, i t i s generally believed that photon processes are necessary for high resolution lithography. The present results with infra-red laser marking of PET, however, indicate that thermal methods have potential for high resolution optical data storage applications. Both radiation induced c r y s t a l l i z a t i o n and ablation of PET film have been demonstrated. The two processes prevail at d i s t i n c t l y different levels of radiation intensity. While only the former process i s reversible, ablation may offer advantages with regard to contrast and resolution. These techniques can be readily extended to achieve select metallization of the polymer film.

Acknowledgments This work was sponsored by the Office of Naval Research. MFS acknowledges the National Research Council for an NRCNRL postdoctoral associateship.

Literature Cited

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Radiation Effects on Polymers - American Chemical Society

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