RAPID ISOTHERMAL ANNEALING OF ION IMPLANTED SILICON DEVICES BY UNIFORM LARGE AREA IRRADIATION WITH A NEW ELECTRON BEAM SYSTEM

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RAPID ISOTHERMAL ANNEALING OF ION IMPLANTED SILICON DEVICES BY UNIFORM

LARGE AREA IRRADIATION WITH A NEW ELECTRON BEAM SYSTEM

L. Dori, M. Impronta, G. Lulli, P. Merli, M. Severi

To cite this version:

L. Dori, M. Impronta, G. Lulli, P. Merli, M. Severi. RAPID ISOTHERMAL ANNEALING OF ION IMPLANTED SILICON DEVICES BY UNIFORM LARGE AREA IRRADIATION WITH A NEW ELECTRON BEAM SYSTEM. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-415-C5-419.

�10.1051/jphyscol:1983560�. �jpa-00223145�

RAPID ISOTHERMAL ANNEALING OF I O N IMPLANTED S I L I C O N DEVICES BY UNIFORM LARGE AREA I R R A D I A T I O N WITH A NEW ELECTRON BEAM SYSTEM

L. Dori, M. Impronta, G. Lulli, P.G. Merli and M. Severi CIVR, I s t i t u t o LAMEL, Via Castagnoli 1, 40226 BoZogna, I t a l y

R6sums - On a pr6parQ des diodes ayant de bonnes caract6ristiques .Gleccriques par recuit isothermique rapide de silicium 100 lourdement dop6 par implan- tation de P + et AS'. Les traitements thermiques ont st6 effectuss en irradiant d'une manigre uniforme la surface post6rieure des Qchantillons avec un canon glectronique de nouvelle conception. La redistribution des impuretss est gran- dement rgduite, en comparaison avec un traitement thermique traditionnel Z 1000°C pour 30 min. Enfin, les mesures de capacit6-tension effectu6es sur des structures MOS ont montrQ que ce traitement ne provoque pas de d6fauts dans l'oxyde quand on irradie la surface post6rieure.

Abstract - Rapid isothermal annealing of P or As layers heavily implanted in

<loo> Si wafers has been carried out by using a new electron beam system.

Diodes with good electrical characteristics have been fabricated by irradia- ting the wafers on the back-side to a suitable thermal cycle. The impurity redistribution is greatly reduced as compared to conventional furnace annea- ling (1000°C. 30 min). Finally C-V measurements on MOS structures show that this technique does not cause significant oxide damage when the irradiation is performed on the back-side of the wafer.

1. INTRODUCTION

Successful annealing of shallow junctions for VLSI devices requires dopant activation, silicon epitaxial regrowth, point and extended defect removal without a significant dopant diffusion ( < 50 nm). Among the different attempts to find an adequate annealing process the newest and the most promising approach appears to be the rapid isothermal annealing /I/. In this work we will describe preliminary results concerning the electron beam annealing of As and P implanted Si. The electron beam annealing has been performed operating in an isothermal regime with a new electron gun, developed iy our laboratory, which is able to provide irraqation of a large area (tens of cm ) with a power density of up to tens of W/cm . ^The

following experiment3Pave b en carried out: a) measur ents of eJgctrica& profiles Fi

for P' (70 keV, 5x10 at/cm ) and AS' (100 keV, ZxlOfy and 5x10 atlcm 1 ^implan-

ted layers to determine activation and redistribution; b) fabrication of n p diodes to assess the effectiveness of the electron beam annealing in reducing the implan- ted damage; c) measurements of high and low-frequency C-V characterisitcs of Al-gate MOS capacitors to verify potential. gate oxide damage due to irradiation.

2. EXPERIMENTAL

Single-crystal 1 ohm.cm p-type (100, Si yafers were implanted atlyom temper ture 3n a random directiyn, with 100 keV As ioy? to dofjes +of 2x10 and 5x10 85

at/cm , and with 70 keV P ions to a dose of 5x10 at/cm . n p diodes were fabri- cated by performing these implants in the bare silicon through 1 mm-diameter windows opened in a 230 nm thick Si02. The e-beam annealing, using the experi- mental conditions described below, was performed irradiating either the back-side or the frontside of the wafers. To provide a standard of comparison for the e-beam

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983560

C5-416 JOURNAL DE PHYSIQUE

a n n e a l i n g (EBA) s e v e r a l samples were s u b j e c t e d t o 1000°C, 30 min f u r n a c e a n n e a l i n g (FA) i n flowing N2. A l l t h e w a f e r s were t h e n m e t a l l i z e d w i t h Ti/Ag and s u b j e c t e d t o a f i n a l anneal a t 400°C f o r 20 min i n a flowing N

MOS c a p a c i t o r s were f a b r i c a t e d by growing a 5; nrn t h i c k oxide i n an H and 0 combustion ambient a t 900°C on p-type S i s u b s t r a t e s , 7-10 ohm cm, C100> o s i e n t e d . 2 The oxide wai; i r r a d i a t e d i n t h e e-beam system a t a power d e n s i t y v a r y i n g between 15 and 30 W/cm

.

The electron-beam a n n e a l i n g was performed u s i n g t h e e l e c t r o n gun system r e p o r t e d i n F i g . 1 / 2 / . It p r o v i d e s a maximum power of -10 kW.

Fig. 1 - E l e c t r o n beam a n n e a l e r developed at LAMEL. I n t h e I n s e t a diagram of t h e e l e c t r o n o p t i c a l column i s r e p o r t e d .

A diagram of t h e e l e c t r o n o p t i c a l column r e p o r t e d i n t h e i n s e t shows: 1) Thermionic t r i o d e gun w i t h a d j u s t a b l e i n t e r e l e c t r o d e d i s t a n c e ; t h e massive t u n g s t e n cathode is i n d i r e c t l y heated by t h r e e f i l a m e n t s ( t h i s system p r o v i d e s good thermal s t a b i l i t y and r e g u l a r i t y o f e m i s s i o n ) ; 2 ) Gate v a l v e allowing t h e vacuum t o be p r e s e r v e d i n t h e gun chamber when t h e specimen chamber i s opened; 3 ) Beam d e f i n i n g a p e r t u r e ; 4) V a r i a b l e f o c a l l e n g t h magnetic l e n s ; it p r o v i d e s a v a r i a b l e beam s p o t d i a m e t e r w i t h a minimum v a l u e o f l m m ; 5 ) D e f l e c t i o q system p r o v i d i n g a scanning frequency o f up t o 1 KHz o v e r a s q u a r e a r e a o f 100 cm t o p r o c e s s 4-inch wafers; t h e minimum dwell time i s -. 10 s e c w i t h a beam having a diameter of lmm; 6 ) Wide specimen chamber w i t h d J f e r e n t wafer h o l d e r s t o a l l o w i r r a d i a t i o n under two d i f f e r e n t boundary c o n d i t i o n s : i ) thermal i n s u l a t i o n ; ii) good thermal c o n t a c t t o a s u b s t r a t e k e p t a t room temperature.

With t h i s e l e c t r o n o p t i c a l column t h e e l e c t r o n gun c a n work i n t o two d i f f e r e n t modes: a ) The f i r s t one, scanning mode, i s r e a l i z e d by f o c u s i n g t h e beam onto t h e specimen (dashed l i n e i n t h e i n s e t of Fig.1) and scanning it w i t h a frequency of up t o 500-1000 Hz. b) The second one, defocusing mode, s u i t a b l e f o r r a p i d i s o t h e r m a l t r e a t m e n t s , i s o b t a i n e d by f o c u s i n g t h e beam h i g h above t h e t h e r m a l l y i n s u l a t e d specimen ( c o n t i n o u s l i n e i n t h e i n s e t of F i g . 1 ) . T h i s mode, used i? o u r e x p e r i - ments, a l l o w s a n e a r l y uniform i r r a d i a t i o n of a l a r g e a r e a ( t e n s of cm ) .

The thermal c y c l e used f o r t h e a n n e a l i n g of t h e implanted specimens i s r e p o r t - ed i n Fig.2. The 2.8 cm y u a r e c h i p s , t h e r m a l l y i n s u l a t e d , were i r r a d i a t e d w i t h a power d e n s i t y of 7 . 1 W/cm f o r a time of 1 5 s. The temperature i n c r e a s e d a t a r a t e c o n t r o l l e d by t h e power d e n s i t y of t h e beam and by d e n s i t y , t h i c k n e s s and s p e c i f i c h e a t of t h e sample up t o a s t a t i o n a r y v a l u e dependent only on i n c i d e n t power d e n s i t y and e m i s s i v i t y of t h e specimen. A f t e r a time of 15 s., i n o r d e r t o reduce t h e d e n s i t y of r e s i d u a l 2 extended d e f e c t s / I / , t h e i n c i d e n t power d e n s i t y was i n c r e a s e d up t o 23.7 W/cm f o r an a d d i t i o n a l i r r a d i a t i o n time o f 2.5 s. The temper- a t u r e of t h e specimen reached a v a l u e of a b o u t llOO°C t h e n it decreased t o ambient temperature i n about 10 s.

C a l c u l a t e d thermal cy- c l e f o r t h e power den- s i t i e s ( D ) used i n t h e experiments.

-

2 C

2

500

Y I-

3. RESULTS AND DISCUSSION -D,,=23.7w/cm2

r C - * - - - .

- ,',"

I;,:

o ! ' l ' l ' l ' l '

Table I r e p o r t s t h e experimental s h e e t r e s i s t a n c e v a l u e s f o r e-beam annealed and f u r n a c e annealed P and A s doped l a y e r s a l o n g w i t h t h e v a l u e s p r e d i c t e d by a s l i g h t l y modified v e r s i o n of t h e computer program SUPREM I1 / 3 / .

0 5 10 15 20 25

TIME [sec]

TABLE I

S h e e t r e s i s t a n c e (ohm/n )

Implanted s p e c i e s Dose e-beam f u r n a c e

and Energy (at/cm2) (lOOO°C, 30 min)

measured measured s i m u l a t e d

The d a t a a r e t h e same f o r b o t h back-side and f r o n t s i d e i r r a d i a t i o n . The u n i f o r m i t y i s comparable t o t h e a c c e l e r a t o r dose s p e c i f i c a t i o n (+ 2 % ) . The d i f f e - rence i n s h e e t r e s i t a n c e between t h e e-beam annealed and f u r n a c e annealed samples i s due t o d i f f e r e n t i m p u r i t y p r o f i l e s . C a r r i e r c o n c e n t r a t i o n p r o f i l e s , determined

0.3 04 DEPTH [urn]

-as implanted

....

DEPTH [urn]

Fig. 3 - ~ l e c t r i c a l a ) 100 KeV, 5x10 A s p r p g i l e s a t e r EBA and a f t e r at/cm -5 ; b) 100 KeV, 2x10 E$ a t 109O0C f o r 30 min: at/cm . Continuous and dashed l i n e s r e p r e s e n t s i m u l a t e d p r o f i l e s .

C5-418 JOURNAL DE PHYSIQUE

by a n o d i c o x i d a t i o n and oxide s t r i p p i n g followed by r e s i s t i v i t y and H a l l e f f e c t measurements, a r e r e p o r t e d i n Fig.3 f o r each A s dose. A s can b e s e e n , t h e r e i s n e g l i g i b l e r e d i s t r i b u t i o n f o l l o w i n g EBA compared t o an i n depth p i f f u s i o q of about 150 nm f o r t h e FA. T h i s e f f e c t is more e v i d e n t f o r t h e 2x10 at/cm A s dose

( f i g . 3 b ) .

The as-implanted p r o f i l e s ( c a l c u l a t e d w i t h SUPREM 11) a r 5 d o c a l l y i n e x c e s s of t h e s o l i d s o l u b i l i t y l i m i t which f o r A s i s lower t h a n 3x10 at/cm3 /4/ i n t h e temperature range used i n o u r experiments. However m e t a s t a b l e e l e c t r i c a l l y a c t i v e c a r r i e r c o n c e n t r a t i o n s should n o t occur f o r t h e a n n e a l i n g time and temperature used i n our EBA experiments. Due t o t h e reduced d i f f u s i o n , t h e A s c o n c e n t r a t i o n remains h i g h e r t h a n t h e s o l u b i l i t y v a l u e and p r e c i p i t a t i o n occurs. I n f a c t t h e i n t e g r a t e d e l e c t r i c a l p r o f i l e i n d i c a t e s an a c t i v a t i o n of -80% f o r t h e EBA of t h e h i g h e s t A s dose. On t h e c o n t r a r y , i n t h e f u r n a c e annealed samples t h e f r a c t i o n of A s atoms p r e s e n t i n c o n c e n t r a t i o n s exceeding t h e s o l i d s o l u b i l i t y i s p r a c t i c a l l y e l i m i n a t e d due t o t h e d i f f u s i o n t h a t t a k e s p l a c e d u r i n g t h e h e a t t r e a t m e n t . Furthermore, t h e maximum c o n c e n t r a t i o n i s lower, t h u s r e s u l t i n g i n a s l i g h t l y h i g h e r e l e c t r o n m o b i l i t y .

Fig.4 shows histograms of t h e leakage c u r r e n t d e n s i t y a t 1 V r e v e r s e b i a s f o r A s doped d i o d e s e i t h e r e-beam annealed ( a ) o r f u r n a c e annealed ( b ) . S i m i l a r r e s u l t s were o b t a i n e d f o r P doped d i o d e s . A s can b e s e e n , t h e two histograms a r e very s i m i l a r , with t h e peak f y t h e e-beam annealed samples corresponding t o a c u r r e n t d e n s i t y of about 1 nA/cm , s l i g h t l y b e t t e r t h a n t h e one of t h e f u r n a c e annealed d i o d e s . The forward I - V c h a r a c t e r i s t i c s of e-beam annealed and f u r n a c e annealed d i o d e s a r e shown i n Fig.5. For o v e r f o u r o r d e r s of magnitude t h e c h a r a c t e r i s t i c s o f e-beam annealed d i o d e s can be f i t t e d t o t h e e x p r e s s i o n I = I ( ( e x p qV/mKT) - 1) w i t h m = 1.07, i n d i c a t i v e of a c u r r e n t flow dominated by d i f f h o n and p r a c t i c a l l y e q u a l t o t h a t o f f u r n a c e annealed diodes. A t low forwards b i a s , t h e c h a r a c t e r i s t i c s can be f i t t e d t o t h e same e x p r e s s i o n w i t h m = 1.33, i n d i c a t i v e of some recombina- t i o n c u r r e n t i n t h e space charge region. I t should be n o t e d t h a t t h e s e r e s u l t s a r e independent of t h e i r r a d i a t i o n mode (back o r f r o n t s i d e ) .

ARSENIC e-beam

6 E

furnace

LLJ

LEAKAGE CURRENT D E N S I T Y [ A ~ ~ ~ ]

Fig. 4 - Histogram of t h e leakage c u r r e n t F i g . 5 - Forward I V c h a r a c t e r - d e n s i t y a t l l Y r e v e r 1 e b i a s f o r A s implanted i s t i c ~ ~ of 9 implanted d i o d e s d i o d e s (2x10 at/cm ) ; a ) EBA, b) FA. (2x10 at/cm ) , a f t e r EBA and FA,

F i n a l l y , a s f a r a s p o t e n t i a l g a t e o x i d e damage due t o e-beam i r r a d i a t i o n i s concerned, t y p i c a l C-V c h a r a c t e r i s t i c s t a k e n on MOS c a p a c i t o r s f o l l o w i n g EBA on t h e

to, = 850A

-

backside e-beam exposure _-

-

-6 -4 -2 0 2 4 6 GATE VOLTAGE [V]

-backside -

exposure ---- frontside

exposure -

-

GATE VOLTAGE [V]

Fig. 6 - High- and low-frequency C-V MOS characteristics after EBA: a) irradia- tion from the back-side, b) irradiation from the back-side (-1 and from the frontside (--) .

back-side are shown iqFig.6a. These curves are independent of the power density in the range 15-30 W/cm . Practically no difference with respect to the unexposed control capacitors was observed. The average flat-band voltage is 1.14 V instead of 1.12 V for control wafers. The low-frequency curve is indicative of a low interfa- ce-state density. On the contrary, the C-V characteristics of the MOS capacitors irradiated on the frontside indicate an increase of both fixed-oxide charges and surface states (see Fig.6b). Furthermore these capacitors reveal an high percentage of shorted oxides (50-80%).

A potential concern with the use of EBA in VLSI technology is the introduction of neutral traps in the gate oxide which are not detected with C-V measurements.

However, avalanche carrier injection measurements /5/ indicate that e-beam irradia- tion on the back-side does not produce any significant oxide neutral trap forma- tion.

4. REFERENCES

1) Sedgwick T.O., in "VLSI Science and Technology/l982", The Electrochemical Society, Inc. 82-7, 130 and references therein.

2) Lulli G., Merli P.G., Mat. Chem., in press.

3) Masetti G., Severi M., Solmi S., IEEE Trans. Electron Devices ED-30 (1983) in press.

4) Nobili D. , Carabelas A., Celotti G. , Solmi S., J. Electrochem. Soc. 130 (1983) 922.

5) Razouk R.R., Delfino M., Fulks R.T., Powell R.A., Yep T.O., J. Appl. PhYs. 53

(1982) 800.