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EXCIMER LASERS
J.-P. Girardeau-Montaut
To cite this version:
J.-P. Girardeau-Montaut. EXCIMER LASERS. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-
225-C7-228. �10.1051/jphyscol:1987750�. �jpa-00227054�
EXCIMER LASERS
J.-P. GIRARDEAU-MONTAUT
Universitk Claude Bernard, Lyon I , Laboratoire des Interactions
~ a s e r - ~ a t g r i a u , 41, Bd du 11 novembre 1918, F - 6 9 6 2 2 V i l l e u r b a m e Cedex, France
A review of excimer l a s e r systems i s p r e s e n t e d , i n c l u d i n g b a s i c p h y s i c a l d a t a , technology, performances and a p p l i c a t i o n s . The l a s t few y e a r s have seen a r a p i d development of t h i s new type of gas l a s e r s and of many commercial systems pro- ducing e f f i c i e n t high power p u l s e s of u l t r a - v i o l e t r a d i a t i o n . A f t e r a s h o r t r e c a l l of p h y s i c a l p r i n c i p l e s , we g i v e a d e s c r i p t i o n of t h e i n t e r n a l s t r u c t u r e of nanose- cond and picosecond excimer l a s e r s , followed by t h e a n a l y s i s of p r i n c i p a l beam cha- r a c t e r i s t i c s : output power, e f f i c i e n c y , p u l s e s t a b i l i t y , beam p r o f i l e and l i f e t i m e . Some c o s t c o n s i d e r a t i o n s a r e a l s o examined. The p r e s e n t a t i o n of p r i n c i p a l applica- t i o n s i n v a r i o u s domains a s photochemistry, m a t e r i a l processing, non l i n e a r proces- s e s and medecine, shows how t h e s e l a s e r s a r e d e f i n i t e l y an u s e f u l t o o l .
The term "excimer l a s e r " does n o t d e s c r i b e a s i n g l e d e v i c e , b u t a group of p u l s e d gas l a s e r s t h a t d e l i v e r o p t i c a l r a d i a t i o n a t very h i g h peak and average po- wers. A l l emit p u l s e s l a s t i n g nanoseconds o r t e n s of nanoseconds, o r picoseconds f o r some ones a t wavelengths i n o r n e a r t h e u l t r a - v i o l e t . T h e i r comon f e a t u r e i s t h e l a s i n g s p e c i e s : a diatomic molecule t h a t i s bound only i n e l e c t r o n i c a l l y e x c i t e d s t a t e s , w h i l e i t s e l e c t r o n i c ground s t a t e i s r e p u l s i v e o r weakly bound (Figure 1 ) . Examples i n c l u d e homonuclear diatomics, e . g , He2 and Xe2 / I / . The "excimer" word o r i g i n a t e d a s a c o n t r a c t i o n of " e x c i t e d dimer". I t i s now u s e d i n a broader s e n s e f o r h e t e r o n u c l e a r and polyatomic molecules as w e l l , i n which t h e component atoms a r e o n l y bound i n t h e e x c i t e d s t a t e . A l l c o m e r c i a l l y a v a i l a b l e excimer l a s e r s o p e r a t e with t h e r a r e gas-halides compounds such a s : A ~ F * (193 nm), ~ r ~ l * (222 nm), K ~ F * (244 nm), ~ e ~ l " (308 nm) and XeF* (351 nm), which do n o t occur i n n a t u r e . Excimer l a s e r s can a l s o o p e r a t e with non-excimer media l i k e N 2 , F2 and C02 as w e l l .
The upper s t a t e of t h e excimer i s formed by chemical r e a c t i o n from i t s c o n s t i t u e n t s a f t e r one o r b o t h of them have been e l e c t r o n i c a l l y e x c i t e d o r i o n i z e d i n a very f a s t high v o l t a g e d i s c h a r g e . When an excimer drops from t h e e x c i t e d s t a t e t o t h e ground s t a t e , t h e f o r c e between t h e two atoms change from a t t r a c t i o n t o repul- s i o n and t h e molecule b r e a k s up, f r e e c o n s t i t u e n t s e n t e r i n g i n t h e pumping c y c l e a g a i n . This energy l e v e l s t r u c t u r e makes excimers v e r y good l a s e r m a t e r i a l s , with h i g h g a i n , t h e p o p u l a t i o n i n v e r s i o n being as long as t h e r e a r e molecules i n t h e ex-
c i t e d s t a t e s
1 I I
Laser transition 1
-
249 nrnF i g u r e 1 - P o t e n t i a l curves and l a s e r t r a n s i t i o n s f o r t h e K ~ F * excimer molecule.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987750
JOURNAL DE PHYSIQUE
Active medium
I n d i s c h a r g e e x c i t a t i o n , excimer l a s e r s c o n t a i n a gas-mixture a t t o t a l p r e s s u r e u s u a l l y below f i v e atmospheres. The b u l k of t h e mixture, h i g h e r than 88 % i s a b u f f e r g a s , which mediates energy t r a n s f e r : normally He o r Ne, although A r i s used i n some c a s e s . The r a r e gas t h a t does combine t o form excimer molecules i s pre- s e n t i n much s m a l l e r c o n c e n t r a t i o n s 0.5 % t o 12 % of t h e t o t a l p r e s s u r e . The halogen donor i s p r e s e n t i n c o n c e n t r a t i o n s of 0.5 % o r l e s s . I t may be e i t h e r a diatomic halogen such as F2, o r a halogen c o n t a i n i n g molecule such a s H C l o r NF3. A t y p i c a l gas m i x t u r e f o r K ~ F * w i l l c o n t a i n 4 t o r r F2, 120 t o r r K r and 2400 t o r r He. The o p t i - mum gas mixture is a complex f u n c t i o n of g a s k i n e t i c s and o p e r a t i n g c o n d i t i o n s . I t
i s d i f f e r e n t f o r d i f f e r e n t models of l a s e r s and t h e r e a r e a l s o l a r g e d i f f e r e n c e b e t - ween d i f f e r e n t excimer molecules.
Pump mechanism
The d e t a i l e d dynamic p r o c e s s e s t h a t l e a d t o emission from excimer l a s e r s a r e complicated 12-51. However, one can g e t an understanding of some key f e a t u r e s of l a s e r d e s i g n from t h e s i m p l i f i e d scheme of KrF a s given i n F i g u r e 1.
The upper l a s e r l e v e l i s an i o n i c a l l y bound s t a t e formed by three-body r e - combination of t h e K r + and F- i o n s i n t h e presence of a c o l l i s i o n p a r t n e r . The l i f e - time of spontaneous d e a c t i v a t i o n depends on p r e s s u r e and i s approximatively 2.5 n s e c . In o r d e r t o acheive a s i z a b l e p o p u l a t i o n (* 1015 cm-3), i t s i o n i c p r e c u r s o r s have t o be prepared on a time s c a l e and w i t h c o n c e n t r a t i o n s such t h a t t h e formation r e a c t i o n can produce s e v e r a l 1023 excimers x cm-3 x s-1. The n e c e s s a r y ions can b e prepared from t h e atomic s p e c i e s by f a s t avalanche d i s c h a r g e , e-beam o r microwave d i s c h a r g e . I t is t h e avalanche d i s c h a r g e e x c i t a t i o n which i s used i n a l l commercial systems.
To make e f f i c i e n t pumping, t h e r e a r e f u r t h e r requirements 121 : e l e c t r o n d e n s i t i e s of a 1015 cm-3, c u r r e n t d e n s i t i e s of lo3 ~ / c r n ~ , s o t h a t enough e l e c t t a n can 'provide the14 e v i o n i z a t i o n energy of K r . To meet t h e s e c o n d i t i o n s , a breakdown v o l t a - ge of 10-15 kV/cm i s need and t h e r e f o r e most l a s e r s o p e r a t e w i t h e l e c t r o d e s p a c i n g of 2 t o 3 cm and breakdown v o l t a g e s of 25-35 kV. To improve e n e r g y - t r a n s f e r dynamics and avoid a r c i n g , t h e l a s e r gas i s normally "pre-ionized" b e f o r e t h e main d i s c h a r g e i s f i r e d . P r e - i o n i z a t i o n i s normally accomplished with a p u l s e of W l i g h t from s p a r k s , o r by coronna e f f e c t 161. I n o r d e r t o t h e p r i n c i p a l e x c i t a t i o n p u l s e break:
down homogeneously t h e gas mixture, t h e s t a r t - u p e l e c t r o n d e n s i t y must b e about lo7- 108 a - 3 .
Technology
When broken down, t h e excimer gas mixtures have v e r y low impedance
*
10-1 fi s o t h a t e f f i c i e n t e x c i t a t i o n r e q u i r e s very f a s t low inductance, h i g h v o l t a g e c i r c u i t r y ( F i g u r e 2 ) . Lasers a r e pumped by d i s c h a r g i n g a high v o l t a g e c a p a c i t o r through a s u i t a b l e switch over t h e two l a s e r e l e c t r o d e s i n s i d e t h e l a s e r chamber.
Switching i s u s u a l l y w i t h t h y r a t r o n s which have j i t t e r of 1 t o 2 n s e c only. The upper l i m i t on p u l s e l e n g t h i s f u n c t i o n a l l y l i m i t e d t o t h e range of t e n s of nanose- conds by d i s c h a r g e i n s t a b i l i t i e s . P u l s e l e n g t h s of hundred of nanoseconds can b e reached only with e l e c t r o n beam o r microwave e x c i t a t i o n , b u t t h a t ' s about t h e upper l i m i t . Very r e c e n t l y 171, s h o r t e n i n g of p u l s e l e n g t h h a s been o b t a i n e d by p u l s e com- p r e s s i o n i n dye u n t i l %10 p s e c .
An excimer l a s e r c o n t a i n s a tube f i l l e d w i t h l a s e r - g a s mixture through which an e x c i t a t i o n p u l s e p a s s e s . I n discharge-driven l a s e r s , t h e e l e c t r i c a l p u l s e i s p e r p e n d i c u l a r t o t h e l a s e r beam a x i s . The l a s e r gas l i e s i n a r e s e r v o i r o u t s i d e of t h e e x c i t a t i o n r e g i o n , and t h e gas may b e a c t i v e l y flowed through t h e d i s c h a r g e zone. The l a s e r c a v i t y can be r e p e a t e d l y r e f i l l e d with d i f f e r e n t gas m i x t u r e s . This i s n e c e s s a r y because t h e l a s e r gas degradcsduring use. A f t e r t h e gas i s s p e n t , t h e c a v i t y must be emptied and r e f i l l e d . The l a s e r c a v i t y , o p t i c s and e l e c t r o d e s a r e designed t o r e s i s t c o r r o s i o n by t h e halogens.
2:
l a s e r chamber, PC : p r e ~ o n i r a t i o n c i r c u i t r y .O p t i c s
Excimer l a s e r s have such h i g h i n t e r n a l g a i n - t y p i c a l l y g % 5 t o 15 per centlcm- tha? t h e y a r e v i r t u a l l y s u p e r r a d i a n t and r e q u i r e l i t t l e o p t i c a l feedback.
The s t a n d a r d s t a b l e r e s o n a t o r c o n s i s t s of a p l a n e A1/MgF2 o r d i e l e c t r i c coated f u l l r e f l e c t o r and an uncoated CaF2 o r MgF2 o u t p u t c o u p l e r whose 8 % r e f l e c t i v i t y provi- des s u f f i c i e n t feedback. The windows a r e a f f i x e d d i r e c t l y t o t h e l a s e r c a v i t y and hence a r e exposed d i r e c t l y t o t h e l a s e r g a s . To p r e v e n t damage, r e f l e c t i v e c o a t i n g s a r e d e p o s i t e d on t h e o u t s i d e of t h e r e a r c a v i t y window. Laser emission i n d i s c h a r g e pumped excimer l a s e r s o c c u r i n g f o r only 10 t o 25 nsec, o p t i c a l feedback from t h e o p t i c a l r e s o n a t o r can only b e of use i f a t l e a s t two t o t h r e e o p t i c a l round t r i p s a r e p o s s i b l e i n t h e r e s o n a t o r d u r i n g l a s e r emission. T h i s l i m i t s r e s o n a t o r l e n g t h s t o
'L 100 cm.
Beam c h a r a c t e r i s t i c s
Wavelength o u t p u t power and temporal c h a r a c t e r i s t i c s of t h e major excimer l a s e r g a s e s a r e shown i n Table 1. As i n d i c a t e s , t h e r e a r e major d i f f e r e n c e s i n out- p u t power a v a i l a b l e from d i f f e r e n t excimers. With e x c e l l e n t f o c u s a b i l i t y of n e a r l y d i f f r a c t i o n - l i m i t e d W r a d i a t i o n , a l l o w s t h e g e n e r a t i o n of very h i g h i n t e n s i t i e s - up t o 1017 ~ / c m Z .
Due t o d i s c h a r g e parameters, p u l s e s l a s t from a few nanoseconds t o a few t e n s of nanoseconds. P u l s e l e n g t h s vary s i g n i f i c a n t l y among d i f f e r e n t g a s e s used i n t h e same l a s e r and i n d i v i d u a l p u l s e s of t e n have s t r u c t u r e w i t h i n them. P u l s e s a s long as 250 ns have been o b t a i n e d i n t h e l a b o r a t o r y by b a l l a s t i n g t h e d i s c h a r g e . Using s a t u r a b l e a b s o r b e r s i t was p o s s i b l e t o produce p u l s e s a s s h o r t a s 10 ps.
Table 1
-
Wavelength, maximum o u t p u t power and p u l s e l e n g t h of excimer l a s e r sK r C l 222
25 2.5 200
4-9 ArF
193 1000 100 250 8-14 Laser g a s
Wavelength (nm) P u l s e energy (mJ) Ang. power (W) Rep. r a t e (Hz) P u l s e l e n g t h (ns)
F2 157
12
% 1 100 6
KrF 249 1500 200 500 12-16 (0.01
XeCl 308 1000 100 250 8-40
XeF 351 400 50 200 8-18
JOURNAL
DE
PHYSIQUET y p i c a l w a l l p l u g e f f i c i e n c y f o r a d i s c h a r g e d r i v e n KrF l a s e r i s around 1.5 % t o 2 %. O t h e r excimer g a s e s a r e l e s s t h a n KrF.
The l a s e r t r a n s i t i o n s i n excimer l a s e r a r e broad due t o t h e l a c k of well- d e f i n e d energy l e v e l s i n t h e ground s t a t e . T y p i c a l l i n e w i d t h is a b o u t 0 . 3 nm w i t h o u t p u t wavelength t u n a b l e a c c r o s s about 3 nm. The broad f l u o r e s c e n c e bandwith of ArF* and K ~ F * makes them s u i t a b l e f o r g e n e r a t i o n o r a m p l i f i c a t i o n of subpicosecond p u l s e s .
Due t o t h e h i g h g a i n , low Q c a v i t y and low number of round t r i p s i n e x c i - mer o s c i l l a t o r s , mode c o m p e t i t i o n i s low and t h e o u t p u t c o n s i s t s of v e r y l a r g e num- b e r (% 107) of s p a t i a l modes. T h e r e f o r e , t h e s p a t i a l and temporal coherences of t h e r a d i a t i o n a r e v e r y s m a l l . Normally o u t p u t i s u n p o l a r i z e d , b u t p o l a r i z i n g o p t i c s can always be added.
With s t a n d a r d s t a b l e r e s o n a t o r o p t i c s , excimer l a s e r s t y p i c a l l y have r e c - t a n g u l a r beams roughly 1 x 2 cm. Beam d i v e r g e n c e i s 2 x 3 m i l l i r a d i a n s . U n s t a b l e r e s o n a t o r g e n e r a t e beams of s i m i l a r s i z e b u t much s m a l l e r d i v e r g e n c e (% 0 . 5 mrad)
.
A t l a s t , pulse-to-pulse v a r i a t i o n s c a n b e s i g n i f i c a n t i n p u l s e d excimer l a s e r s , w i t h s p e c i f i e d v a l u e s a v e r a g i n g i 5 % t y p i c a l l y . With a p u l s e s t a b i l i z a t i o n o p t i o n i t i s p o s s i b l e t o reduce v a r i a t i o n s t o around 1 % on s t r o n g l i n e s . I n t h e long term, o u t p u t energy d e c l i n e s a s t h e g a s a g e s .
Cost comparison
From a r e c e n t s t u d y / 8 / , i t a p p e a r s t h a t r e a l c o s t p e r k i l o w a t t - h o u r , i n - c l u d i n g f u e l g a s , maintenance and c a p i t a l i s minimum f o r l a s e r s w i t h a v e r a g e power between 20 W t o 50 W , v a l u e s which correspond t o n m e r o u s a p p l i c a t i o n s .
A p p l i c a t i o n s
Excimer l a s e r a p p l i c a t i o n s i n s c i e n c e , medecine and i n d u s t r y have become f a r t o o numerous t o d i s c u s s t h o r o u g h l y h e r e 191. The m a j o r i t y of t h e s e a p p l i c a t i o n s a r e i n r e s e a r c h , b u t a number of t h e s e l a s e r s a r e now b e i n g used i n i n d u s t r i a l pro- d u c t i o n o r c l i n i c a l a c t i o n a s w e l l . The t y p i c a l a p p l i c a t i o n s o f excimer l a s e r s i n - c l u d e : photochemistry / l o / , medecine I l l / , dye l a s e r pumping, m a t e r i a l p r o c e s s i n g /12/, non l i n e a r p r o c e s s e s , remote s e n s i n g and Raman s h i f t i n g , showing how t h e s e l a s e r s a r e an u s e f u l t o o l by now.
R e f e r e n c e s
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(1970) 329.
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(1986) 2124./11/ MULLER, D., L a s e r s and A p p l i c a t i o n s , may (1986) 85.
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