7 Polymers in Optical Recording
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James M. Pearson Corporate Research Laboratories, Eastman Kodak Company, Rochester, N Y 14650 Optical information storage, which has been a dream since the discovery of the laser, is now becoming a commercial reality. Read-only consumer products (video and digital audio disks) have provided a solid technological base for the development and introduction of the more sophisticated write-once and erasable recording systems. This chapter will review the current status of polymeric materials as substrates, protective layers, and active recording media in laser recording.
O P T I C A L I N F O R M A T I O N R E C O R D I N G A N D R E T R I E V A L have b e e n a d r e a m since the i n v e n t i o n of the laser 20 years ago. It is o n l y n o w b e c o m i n g a c o m m e r c i a l reality. R e a d - o n l y c o n s u m e r products such as the optical v i d e o disk a n d the d i g i t a l audio disk have established a disk m a n u f a c t u r i n g a n d p e r i p h e r a l s technological base that has accelerated the d e v e l o p m e n t of m o r e sophisticated w r i t e - o n c e a n d reusable r e c o r d i n g systems. T h e availability of small G a A l A s s e m i c o n d u c t o r diode lasers h a v i n g adequate p o w e r a n d stab i l i t y has e n a b l e d the design a n d construction of the compact l i g h t w e i g h t optical heads r e q u i r e d b y the technology. I n a d d i t i o n , these lasers have also d e f i n e d the w a v e l e n g t h region for r e c o r d i n g , r e a d i n g , a n d erasing a n d the sensitivity parameters r e q u i r e d of the m e d i a . T h e challenge to find appropriate r e c o r d i n g m e d i a has p r o v e n to be a significant one. A p p r o p r i a t e r e c o r d i n g m e d i a must have h i g h m a r k i n g s e n sitivity i n the n e a r - I R region of the s p e c t r u m c o u p l e d w i t h l o n g - t e r m stability for w r i t e - o n c e m e d i a a n d facile, r a p i d , a n d m u l t i c y c l e reversal for reusable materials. A t present, the o p p o r t u n i t y for p o l y m e r - b a s e d r e c o r d i n g materials appears to be i n the w r i t e - o n c e arena. I n " a b l a t i v e " (pit forming) w r i t e - o n c e systems, m a r k i n g occurs b y a t h e r m a l process, a n d l o w - m e l t i n g , l o w - v a p o r ization, l o w - f l o w - t e m p e r a t u r e materials w i t h l o w t h e r m a l c o n d u c t i v i t y are
0065-2393/88/0218-0331$06.75/0 © 1988 American Chemical Society
Bowden and Turner; Electronic and Photonic Applications of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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ELECTRONIC & PHOTONIC APPLICATIONS O F POLYMERS
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desirable. O r g a n i c a n d p o l y m e r i c materials are, therefore, i d e a l l y s u i t e d for this m o d e o f r e c o r d i n g (I). P o l y m e r materials are also the l e a d i n g candidates for disk substrates a n d for a n u m b e r o f disk packaging a n d protective configurations. I n j e c t i o n m o l d i n g o f v i d e o a n d compact disks has d e m o n s t r a t e d that this technology has the p o t e n t i a l for p r o d u c i n g the m o r e c o m p l e x a n d d e m a n d i n g substrates r e q u i r e d for r e c o r d i n g systems. A d v a n c e s i n materials a n d processing w i l l be necessary, b u t the appropriate research a n d d e v e l o p m e n t efforts appear to be i n place, a n d recent disclosures suggest that success is i m m i n e n t . T h i s chapter w i l l r e v i e w the present position a n d o p p o r t u n i t i e s for p o l y m e r i c materials i n w r i t e - o n c e a n d r e v e r s i b l e o p t i c a l r e c o r d i n g . A t p r e s ent, n o consensus has b e e n reached o n the o p t i m a l r e c o r d i n g or d i s k s u b strate materials. S e v e r a l alternatives are still b e i n g actively investigated.
7.1 Functional Performance Criteria A s u m m a r y o f some t y p i c a l performance r e q u i r e m e n t s for a w r i t e - o n c e m e d i u m is p r e s e n t e d i n T a b l e 7.1. T h e m a t e r i a l must b e capable o f p r o d u c i n g w e l l - d e f i n e d m i c r o m e t e r - s i z e d marks v i r t u a l l y instantaneously w i t h a foc u s e d solid-state d i o d e laser. F o r an optical system o p e r a t i n g at a w a v e l e n g t h of —800 n m w i t h a n u m e r i c a l aperture o f —0.5, this w i l l enable a storage d e n s i t y of about 1 0 b i t s / c m . T h e m e d i u m s h o u l d e x h i b i t a m a r k i n g s e n sitivity at 820 n m of b e t t e r than 2 0 - m W i n c i d e n t pulse p o w e r for 50-ns pulses, c o r r e s p o n d i n g to a 1 0 - M b i t s / s r e c o r d i n g rate. T h r e s h o l d sensitivity s h o u l d b e i n the 1 - 5 - m W range. 8
2
T h e actual m a r k i n g event s h o u l d be r a p i d e n o u g h to a l l o w for i m m e d i a t e r e a d after w r i t i n g , o r d i r e c t r e a d after w r i t e ( D R A W ) . T h i s c a p a b i l i t y w i l l e l i m i n a t e any t y p e o f postprocessing to create a detectable image a n d any slow i m a g e - f o r m i n g c h e m i s t r i e s . T h e D R A W feature e m p l o y s a r e a d - o n l y laser that is m o u n t e d o n the o p t i c a l h e a d u n i t along w i t h the r e c o r d i n g laser. T h e r e a d spot trails the w r i t e spot b y several m i c r o m e t e r s a n d verifies the Table 7.1. Performance C r i t e r i a for Write-Once M e d i a Characteristic Resolution Recording rate Sensitivity Threshold sensitivity CNR Packing density Real-time recording Areal defect density Stability fl
0 b
Criterion 10 Mbits/s
