THE APPLICATION OF LASER ANNEALING TO DOPANT PROFILING FOR SEMICONDUCTOR DEVICES

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THE APPLICATION OF LASER ANNEALING TO DOPANT PROFILING FOR SEMICONDUCTOR

DEVICES

A. Adams, S. Morgan

To cite this version:

A. Adams, S. Morgan. THE APPLICATION OF LASER ANNEALING TO DOPANT PROFILING

FOR SEMICONDUCTOR DEVICES. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-433-C5-

437. �10.1051/jphyscol:1983563�. �jpa-00223148�

THE APPLICATION OF LASER ANNEALING TO DOPANT PROFILING FOR SEMICONDUCTOR DEVICES

A . E . Adams and S.L. Morgan

GEC Research Laboratories, Hirst Research Centre, WembZey HA9 7PP, U.K.

Abstract For many device s t r u c t u r e s t h e r e i s a requirement f o r an annealing technique which has t h e f a c i l i t y t o t a i l o r j u n c t i o n dopant p r o f i l e s ; b i p o l a r t r a n s i s t o r s and IMPATT diodes a r e example, where t h e need i s f o r u n i f o r m l y doped p l a n a r j u n c t i o n s . Convent i o n a l l y , techniques such as vapour phase e p i t a x i a1 growth o f doped s i 1 ic o n l a y e r s , o r t h e concent r a t io n dependent d i f f u s i o n c o e f f i c i e n t s o f c e r t a i n dopants have been used t o approximate t o t h i s t y p e o f p r o f i l e . However, as ever f i n e r device geometries a r e sought, t h e j u n c t i o n g r a d i e n t s produced u s i n g these techniques become s i g n i f i c a n t

,

and t h e concornit ant degradation i n operating performance unacceptable. As an a l t e r n a t i v e lasers, e i t h e r pulsed o r scanned cw, i n c o n j u n c t i o n w i t h i o n - i m p l a n t a t i o n may be used. Rectangular doping p r o f i l e s are r e a d i l y a t t a i n a b l e by pulsed l a s e r annealing. This has immediate a p p l i c a t i o n t o h i g h frequency microwave devices and it has been shown t h a t t h i s may be r e a d i l y used. The a p p l i c a t i o n t o h i g h e f f i c i e n c y b i p o l a r t r a n s i s t o r s however depends on t h e a b i l i t y t o i n c o r p o r a t e t h e technique i n t o a b i p o l a r f a b r i c a t i o n process

.

The dopant p r o f i 1 es which are a t t a i nabl e by t h e usual met hods o f semi conductors f a b r i c a t i o n are not t h e optimum but r a t h e r a best compromise between t h e ion-imp1 anted Gaussian and t h e r e d i s t r i b u t i o n t h a t occurs v i a s o l i d s t a t e d i f f u s i o n , d u r i n g t h e subsequent furnace anneals. The a t t a i nabl e range o f device dimensions has been extended by e x p l o i t i n g anomal ous d i f f u s i o n e f f e c t s such as t h e concent r a t i o n dependent d i f f u s i o n c o e f f i c i e n t o f a r s e n i c i n s i l i c o n . The h i g h e r d i f f u s i o n c o e f f i c i e n t a t increased concent r a t ions permits t h e formation o f semi -rectangul a r p r o f i l e s of t h e t y p e r e q u i r e d f o r e m i t t e r s i n h i g h performance b i p o l a r t r a n s i s t o r s . Other methods e x i s t f o r t h e production o f r e c t a n g u l a r p r o f i l e s i n c l u d i n g t h e chemical d e p o s i t i o n from t h e vapour phase o f s i l i c o n and a dopant. This technique s t i l l r e q u i r e s t h e subst r a t e t o be heated t o temperatures o f 800 t o 1000°C f o r several minutes i f good c r y s t a l growth i s t o be achieved. This allows dopants t o d i f f u s e and causes j u n c t i o n s t o become graded over s i g n i f i c a n t distances. Figure 1 i s an example where n and p t y p e l a y e r s have been grown onto an n+ s u b s t r a t e t o form t h e t w o d r i f t regions o f a double d r i f t IMPATT diode. The temperature t i m e c y c l e has allowed arsenic t o d i f f u s e up from t h e s u b s t r a t e w i t h t h e r e s u l t t h a t t h e n/n+ i n t e r f a c e has become graded over a d i s t a n c e of 0.25 pm. The spreading o f j u n c t i o n s i n t h i s manner can have d e l e t e r i o u s e f f e c t s on device performance w i t h l o s s o f e f f i c i e n c y , lower output power, and reduced o p e r a t i n g frequency. Further, these problems become more acute as f i n e r dimensions are required.

Over t h e past few years t h e r e has been considerable i n t e r e s t i n a l t e r n a t i v e techniques 11-41 f o r a c t i v a t i n g dopants and r e c r y s t a l l i s a t i o n o f s i l i c o n layers. The e f f e c t s of l a s e r s , e i t h e r pulsed o r scanned cw, have been widely s t u d i e d [5-71. To achieve dopant a c t i v a t i o n w i t h a cw l a s e r , an argon-ion l a s e r beam i s u s u a l l y focussed t o a spot s i z e o f 20 pm and scanned across t h e surface w i t h a v e l o c i t y o f a few centimetres p e r second

.

During t h e dwell t i m e o f t h e beam, t y p i c a l l y 0.4 ms, t h e surface of t h e w a f e r i s heated t o a temperature which may be i n excess o f 1000°C, w e l l below t h e m e l t i n g p o i n t . These s h o r t temperature t i m e cycles r e s t r i c t dopant d i f f u s i o n t o a few 1 a t t ic e parameters, and consequently t h e annealed dopant p r o f i l e maintains t h e o r i g i n a l as-implanted semi -Gaussian shape.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983563

JOURNAL DE PHYSIQUE

n+ IjUBSTRAlE

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O f wider i n t e r e s t are techniques which can be used t o produce r e c t a n g u l a r p r o f i l e s . It i s w e l l known t h a t t h e d i f f u s i v i t y o f dopants i n molten s i l i c o n i s -108 t i m e s h i g h e r than i n t h e s o l i d phase. C l e a r l y i f a melt zone i s created on t h e surface o f a s i l i c o n wafer t o t h e r e q u i r e d j u n c t i o n depth f o r a t i m e s u f f i c i e n t t o a l l o w t h e dopant t o d i f f u s e over t h i s distance, t h e l i q u i d - s o l i d i n t e r f a c e w i l l act as a d i f f u s i o n b a r r i e r and a r e c t a n g u l a r p r o f i l e w i l l be formed.

One method by which t h i s may be attempted i s t o increase t h e power d e n s i t y a t which cw annealing i s performed. However, because o f t h e l o c a l i s e d nature o f t h e focussed l a s e r beam and t h e extreme nature

of

t h e surrounding thermal gradients, c o o l i n g i s n e c e s s a r i l y non-uniform and t h e process i s d i f f i c u l t t o c o n t r o l . The r e s u l t i s non p l a n a r j u n c t i o n s which would have poor e l e c t r i c a l performance. Pulsed l a s e r annealing i s a s u p e r i o r technique f o r t h i s purpose.

The l a s e r employed f o r t h e pulsed annealing s t u d i e s i s a 1.5 J Q-switched ruby w i t h a p u l s e width o f 35 ns. A beam homogeniser [8] i s used t o ensure t h a t an annealing u n i f o r m i t y o f b e t t e r t h a n 10% i s maintained. A study o f a t y p i c a l b i p o l a r e m i t t e r implant, 40 keV, 1016 ~ s / c m ~ , shows t h a t t h e s i l i c o n surface be i n s t o melt a t "0.2

J / C ~ and t h a t t h i s penetrates t h e amorphous region a t -0.3 J/cmj a l l o w i n g e p i t a x i a l regrowth t o occur. I f s i n g l e shot annealing i s used t h e melt time, 100-200 ns, i s i n s u f f i c i e n t t o a l l o w t h e dopant t o d i f f u s e t o t h e maximum extent o f t h e l i q u i d s o l i d i n t e r f a c e and so t h e j u n c t i o n evolves by 0.182 pm/J cm-2. However, i f m u l t i p l e shot annealing [9] i s employed t h e i n t e g r a t e d melt t i m e a l l o w s t h e dopant t o become u n i f o r m l y d i s t r i b u t e d and t h e j u n c t i o n increases by 0.285 pm J / c N 2 . For energy d e n s i t i e s g r e a t e r t h a n 0.5 J/cm2, t h e s i n g l e shot j u n c t i o n has t h e form of a complementary e r r o r f u n c t i o n but f o r t e n o r more shots, t h e j u n c t i o n i s doped

a l s o received a t e n shot anneal but t h e average energy d e n s i t y was v a r i e d from 1 t o 2

~ / c m 2 . This sequence r e s u l t e d i n npn s t r u c t u r e s w i t h t h e base d r i v e n t o a depth o f 0.6 pm, Figure 2. The second implant and anneal was used t o a d j u s t t h e e m i t t e r depth and hence t h e base width from 0.175 pm t o zero. It i s expected t h a t w i t h good c o n t r o l o f t h e l a s e r energy, base widths at l e a s t as narrow as 250 8, should be a t t a i n a b l e . A major d i f f i c u l t y which e x i s t s when attempting t o apply pulsed l a s e r annealing t o complex i n t e g r a t e d c i r c u i t s t r u c t u r e s i s c r a c k i n g o f t h e f i e l d oxides [lo] when t h e

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u n d e r l y i n g s i l i c o n melts. Although ways o f overcoming t h i s problem have been considered [lo, 111 it has prevented any e l e c t r i c a l assessment o f these s t r u c t u r e s

.

One family of devices where t h i s d i f f i c u l t y does not e x i s t i s IMPATT diodes.

I o n - i m p l a n t a t i o n and pulsed l a s e r annealing has been used as an a l t e r n a t i v e t o boron d r i f t i n g t o form t h e p+ r e g i o n o f 34 GHz s i n g l e d r i f t IMPATT diodes, Figure 3. The prepared s i n g l e d r i f t s t r u c t u r e s were i o n im l a n t e d w i t h

B

40 keV, 1016 B F ~ / C ~ ~ and p u l s e l a s e r annealed w i t h t e n shots a t 1.6 J/cm ,and t h e diodes were then completed as p a r t o f a t y p i c a l batch. The diodes were operated t o give a pulsed output and t h e i r e f f i c i e n c i e s measured, Figure 4. The e f f i c i e n c i e s compared we1 1 t o diodes f a b r i c a t e d c o n v e n t i o n a l l y and good output powers were a t t a i ned.

Forming t h e p+ region by t h i s technique has t h r e e advantages. The f i r s t i s t h a t h i g h e r surface concentrations are a t t a i n a b l e which decreases t h e avalanche breakdown

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JOURNAL DE PHYSIQUE

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voltage. 111e second, t h e j u n c t i o n gradient can be made more abrupt and hence reduce j u n c t i o n s e r i e s impedance, and f i n a l l y , by v a r y i n g t h e energy d e n s i t y o f t h e annealing pulses t h e depth o f t h e p+ r e g i o n and hence t h e ' d r i f t region, can be adjusted f o r maximum e f f i c i e n c y . T h i s leads t o t h e p o s s i b i l i t y o f h i g h e r o p e r a t i n g e f f i c i e n c i e s and g r e a t e r output powers when these techniques a r e a p p l i e d t o devices f o r operation a t h i g h e r frequencies, i.e. 94 GHz and above.

I n conclusion, l a s e r s e i t h e r scanned argon cw o r Q-switched ruby may be used t o anneal ion-implants. With t h e former, it i s necessary t o keep t h e surface o f t h e s i l i c o n wafer i n t h e s o l i d phase, t h i s r e s u l t s i n t h e f i n a l annealed p r o f i l e m a i n t a i n i n g t h e as I m p l a n t e d p r o f i l e . This technique i s t h e r e f o r e l i m i t e d 40 t h e annealing of shallow implants o f t h e t y p e required f o r f i n e dimension MOS where it i s l i k e l y t o be a s t r o n g candidate f o r e l i m i n a t i n g sideways dopant d i f f u s i o n i n t o t h e gate region.

A l t e r n a t i v e l y , r e c t a n g u l a r doping p r o f i l e s are r e a d i l y a t t a i n a b l e by pulsed l a s e r anneal ing. This has immediate a p p l i c a t i o n t o h i g h frequency microwave devices and it has been shown t h a t t h i s may be r e a d i l y achieved. The a p p l i c a t i o n t o h i g h e f f i c i e n c y b i p o l a r t r a n s i s t o r however depends on t h e a b i l i t y t o i n c o r p o r a t e t h e technique i n t o a b i p o l a r f a b r i c a t i o n process.

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