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MELTING AND FREEZING KINETICS INDUCED
BY PULSED ELECTRON BEAM ANNEALING IN
ION-IMPLANTED SILICON
G. Chemisky, Damien Barbier, A. Laugier
To cite this version:
Colloque C5, suppl6ment au nO1O, Tome 44, octobre 1983 page C5-91
MELTING AND FREEZING KINETICS INDUCED BY PULSED ELECTRON BEAM
ANNEALING
I N ION-IMPLANTED SILICON
G . Chemisky, D . B a r b i e r a n d A. L a u g i e r
Laboratoire de Physique de Za MatiDre (LA 3581, I n s t i t u t National des Sciences Applique'es de Lyon, 20, avenue Albert Einstein, 69621 Villeurbanne Cedez, France
RESUME
Les mecanismes d'interaction faisceau d'energie-materiau determinent les phCnomenes d e recristallisation par 6pitaxie liquide. Dans c e travail, nous avons simule, pour un faisceau d'electrons polycinetique pulse, les cinetiques d e recristallisation d e silicium implante. Nous avons etudid I'influence du s p e c t r e d'6lectrons (12 et 15 keV d'energie moyenne), d e
2
l'energie superficielle (0,7
i
1,4 J / c m ) sur ces cinetiques pour deux temperatures initiales (20 e t 450°C).Apres irradiation, I'enthalpie dans la zone portee
i
la t e m p e r a t u r e d e fusion n'est pas uniformement distribuee. La vitesse d e recristallisation depend principalement du gradient thermique post-zone fondue.L'effet du prechauffage
i
450°C diminue l a vitesse d e recristallisation et allonge considerablement le temps d e sejouri
haute temperature d e l a couche superficielle.ABSTRACT
Annealing e f f e c t s of a pulsed energy beam in t h e liquid phase epitaxy regime mostly depend on beam material interaction mechanism.
In
this work t h e melting and freezing kinetics induced by a polykinetic electron beam pulse in ion-implanted silicon have been simulated for different electron beam parameters (energy spectrum and fluence) and t w o starting t e m p e r a t u r e 20 and 450°C.The enthalpy absorbed in t h e melting layer is not uniformly distributed s o t h a t t h e f r o n t s u r f a c e is turned into melt within t h e pulse duration while t h e melting layer back e n d
remains partially molten.
The melting-solid interface mostly depends on t h e thermal gradients in t h e solid beneath t h e melting layer and t h e l a t e n t h e a t fraction t o be evacuated in t h e bulk of t h e material.
The e f f e c t of amorphous layer thickness was also invistigated.
~ulse3(electron beam annealing on 450°C heated silicon produces a n increase of t h e fully molten layer thickness and a noticable decrease of t h e melting-solid i n t e r f a c e velocity by t h e same f a c t o r a s t h e thermal gradient inside t h e solid region. Consequently, h e a t release from t h e surface is slowed down s o t h a t a f t e r resolidification t h e silicon s u r f a c e temperature stays higher than 800°C a longer time.
INTRODUCTION
Ion implantation followed by a submicrosecond energy pulse (laser or electron) is now considered a s a very a t t r a c t i v e doping process either for t h e microelectronics o r for t h e solar cells industry /1,2!.
I t has been demonstrated t h a t short duration pulsed electron beam c a n be used t o remove implantation damage in silicon by liquid phase epitaxial regrowth of t h e ion implanted s u r f a c e layer starting \from t h e d e f e c t f r e e underlying crystal./3/.
JOURNAL DE PHYSIQUE
Thermal e f f e c t s such a s enthalpy deposition profile, maximum layer thickness, liquid phase duration and melt front velocity a r e determined by both t h e beam energy deposition profile and surface crystal structure.
In this paper, t h e computer simulation of t h e Pulsed Electron Beam Annealing (PEBA) induced thermal e f f e c t s have been performed. The particular f e a t u r e s of PEBA induced melting and freezing kinetics
are
investigated a s a function of t h e substrate starting t e m p e r a t u r e (20°C o r 450°C) and t h e amorphous layer thickness.Experimental d a t a a r e e x t r a c t e d from t h e monitoring system of a broad beam SPIRE 300 pulsed electron beam processor. This machine mostly designed f o r research applications produces polykinetic electron pulses of 50 ns in duration over a few square centimeters area. The pulse is obtained by discharge of a capacitor in a vacuum plasma field emission diode. A full description of t h e apparatus i s given in ref.141. The time-dependent electron energy deposition profile is computed starting from d i r e c t measurement of t h e diode current and voltage-wave- f o r m s during e a c h shot.
In this work, t h e mean electron energy have been varied from 10 t o 20 keV with fluences independently ranging from 0.7 t o 1.4 ~ / c m ~ .
I. COMPUTER STIMULATION O F THE TEMPERATURE PROFILES
Valuable simulation results a r e obtained with any thermal model only if t h e beam interaction mechanisms with t h e material a r e well known.
In t h e c a s e of electrons, t h e interaction mechanisms a r e nearly t e m p e r a t u r e and c r y s t a l structure independent. Moreover for incident electron energies less than 150 keV only t h e r m a l relaxation processes occur. In a previous work /5/, Monte-Carlo simulation has been used t o determine t h e normalized electron energy loss functions in silicon f o r various values of t h e incidence angle. The heat generation function @(x,t) of a polykinetic electron beam pulse c a n thus be precisely established. At any t i m e a f t e r t h e beginning of t h e pulse t h e depth-temperature profiles a r e obtained by solving t h e one-dimensional h e a t flow equation. L a t e r a l thermal gradients a r e neglected because t h e processed diameter i s much g r e a t e r than t h e heating depth. Then, t h e heat flow equation t o be solved i s :
where C(T) represents t h e specific h e a t per unit of volume and X(T) t h e thermal conductivity. The t e m p e r a t u r e dependence of these parameters is taken into account in t h e calculation and variation laws w e r e established starting from literature data. Numerical solution of equation (1) requires t h e use of boundary conditions which a r e imposed by t h e following physical considerations. Because of t h e short process t i m e ( < I'psec) radiation losses from t h e molten front surface a r e very weak and convection phenomena a r e negligible in t h e
-
6 vacuum process chamber (5x10 torr).In addition, t h e bulk of t h e wafer is assumed t o b e a thermostat a t fixed t e m p e r a t u r e To'
The phase change a t t h e melting point is taken into account by condisering t h e equivalence between specific h e a t and l a t e n t heat. This model implies t h a t t h e specific h e a t behaves like a Dirac function a t t h e melting point. For t h e simulation a narrow gaussian singularity is used s o t h a t t h e enthalpy at any depth in t h e material c a n be simply memorized through t h e temperature. The s a m e values have been used for t h e melt t e m p e r a t u r e of crystal or amorphous silicon. Equation (1) is digitized using t h r e e t i m e and depth levels. This method d e s ~ r i b e d by Bonaccina et a1./6/ is inconditionaly convergent. Moreover, i t does not require iterations on t h e thermophysical d a t a w h ~ c h a r e calculated on t h e intermediate time-level in order t o obtain a differential equation with constant coefficients. The thermal d a t a a r e taken in, theliterature /7,8,9/. The problem finally consists in solving t h e following matrix system a t e a c h time-step :
h h-1 h,h- 1
Bi depend only on t i m e s t e p s h and h-1. By solving t h e system we obtain t h e t e m p e r a t u r e distribution
(
TJ in t h e material a t any t i m e s t e p h.11. PEBA INDUCED MELTING
T h e PEBA t h e r m a l cycles include melting and freezing s t a g e which c a n b e described a s follows : within t h e pulse duration a silicon s u r f a c e layer is brought t o t h e melt point. The melting layer thickness depends on t h e e l e c t r o n energy deposition profile which is rather penetrating unlike t h e energy deposition profiles of a laser.
Thus a particular f e a t u r e of PEBA i s t o produce a deeper melting layer (1 m) than in t h e c a s e of laser annealing (0.3 y m) a t t h e s a m e fluence. However, t h e transient physical s t a t e of this melting layer depends on t h e absorbed l a t e n t h e a t fraction which is varying with depth a s
a function of both t h e c r y s t a l s t r u c t u r e and t h e energy deposition profile.
Fig.1 shows t h e enthalpy distribution a f t e r irradiation on silicon at 20°C a s
a
function -Z
of t h e pulse energy deposition profile f o r t h e s a m e superficial energy density (1 J / c m ). The presence of a n amorphous layer of thickness less t h a n 0.4 urn has no significant e f f e c t on t h e enthalpy distribution a f t e r irradiation because h e a t diffusion difference within t h e pulse dura- tion i s negligible. The energy g a p between t h e t o p and t h e bottom horizontal doted lines corresponds t o t h e l a t e n t h e a t LC f o r complete crystal silicon melting.
- MELTING THRESHOLD
0
1
2
3
4
5
DEPTH
Gq
1crone)
F I G 1; Enthalpy d ~ s t r i b u t l o n on S1 at 2 0 - C after ~ r r a d l a C l o n
a) 12 K e V mean energy electron beam
b) 15 K e V mean energy electron beam Pulse : 1 ( J / c r n Z >
O) Laser pulse
I
In t h e c a s e of a 12 keV m e a n energy electron beam pulse (type a ) on fig.1) crystal silicon is fully molten over 0.35 urn. In t h e c a s e of a 15 keV mean energy electron beam pulse (type b) o n fig.1) o n e c a n s e e t h a t t h e enthalpy required f o r complete melting of crystal silicon is n o t reached. Considering a 0.2 y m thick fully amorphous layer with l a t e n t h e a t La 40 % lower t h a n t h e crystal value LC /7/, t h e fully melting enthalpy limit is lowered (medium horizontal line on fig.1). So t h a t t h e t y p e b) e l e c t r o n beam pulse induces a n inhomogeneous physical s t a t e in t h e PEBA melting layer : t h e amorphous f r o n t layer is fully molten while a "melt like" s t a t e i s achiebed in t h e underlying crystal silicon over 0.4 LI m. Whith t h e t y p e a ) e l e c t r o n beam pulse t h e non fully molten layer thickness is less than of t h e computer simulation depht-step. Moreover t h e fully molten layer extends beyond t h e amorphous layer. In t h a t case, t h e problem of m e l t t e m p e r a t u r e difference between crystal and amorphous silicon i s eliminated /lo/, while i t should b e t a k e n into account f o r
a
type b) electron beam pulse. I t must b e noted t h a t t h e3
JOURNAL DE PHYSIQUE
The enthalpy profile induced by a pulsed laser is also plotted o n t y p e c ) fig.1. This profile is very similar t o a type a ) electron beam pulse but t h e fully molten layer thickness i s deeper for t h e electron beam pulse t h a n for t h e laser pulse.
where
i
( X d L is t h e l a t e n t heat fraction (between 0 and L) t o be evacuated a t t h e melting-solid i n t e r f a c e of abscissa XM,X
t h e thermal conductivity and ( d T / d ~ ) ~ t h e thermal gradient at depth XM inside t h e solid region.M
111. EFFECT OF SAMPLE STARTING TEMPERATURE ON THE FREEZING KINETICS
Computer simulations of t h e PEBA induced temperature profiles have been performed e i t h e r with a 20°C o r a 450°C s t a r t i n g temperature. Fig.2 shows t h e melting layer thickness
evolution versus fluence for t h e type a ) electron beam pulse.
On fig.3 a r e plotted t h e mean melting-solid i n t e r f a c e velocity V a s a function of fluence for a type a ) electron beam pulse a s deduced from computer s i m u l ~ t i o n using a 20°C and 450°C s t a r t i n g temperature. A
. 7 5
t
0 L 0;
.sa
VE
a
z . 2 5 -
2At 1 J / c m
,
t h e i n t e r f a c e velocity is nearly constant for a 20°C starting..
temperature. The strong variation below 1 ~ / c m ~ is related t o non-complete silicon melting (r) i 1). E f f e c t of heating silicon t o 450°C consists in a division by two of t h e melting-solidi n t e r f a c e velocity (2 m/s t o 1 m/s). According t o equation 3, this e f f e c t is consistent with t h e decrease of t h e thermal gradient ( d T / d ~ ) ~ from about 500°/ u m t o 350°/ urn.
M
FLUENCE
CJ/cm2>FIG 2: Variation of the?elt~ng layer thlckneap
-
v s fluence with ED 12 KeV1) Startlng tempereture 20'C
2) Starting temperacure 450'C
The e f f e c t s of heating silicon before pulsing only consists in a n constant increase of t h e induced melting layer thickness whatever t h e fluence value. In t h e c a s e of a type b) electron beam pulse t h e region where a "melt likett s t a t e is achieved undergoes t h e s a m e s h i f t a s t h e melting layer a s compared t o t h e e f f e c t s obtained with a 20°C starting temperature. During t h e freezing process t h e energy absorbed in t h e melting layer i s released by thermal conduction through t h e bulk of t h e material. The freezing kinetic is then controlled by t h e dissipation r a t e of t h e l a t e n t heat which mostly depends on t h e depth distribution of thermal gradients in t h e solid beneath t h e melting layer. The melting solid interface velocity V is given by :
FIG
flueno- for 2 values of the starting temperature with
r=
12 KoV1V. REMARKS AND CONCLUSION
Because of a rather penetrating energy deposition profile PEBA induces a deeper melting layer in silicon than pulsed lasers. However, t h e physical s t a t e of this melting layer is controlled by t h e energy deposition profile and also depends on t h e crystal structure of t h e surface layer. The e f f e c t of heating silicon up t o 450°C is t o increase t h e melting layer thickness and t o reduce t h e melting-solid interface velocity by t h e same factor a s t h e thermal gradient inside t h e solid region. Consequently heat release from t h e surface is slowed down so t h a t a f t e r resolidification t h e silicon surface temperature stays higher than 800°C a longer time than when using a 20°C starting temperature. We believe t h a t this cooling r a t e reduction should be equivalent t o a post-PEBA thermal annealing at high temperature as suggested by the decrease of t h e defect concentration in t h e regrowth layer when PEBA is performed on silicon heated above 400°C /11/.
REFERENCES
/ I / WHITE C.W., NARAYAN J., YOUNG R.T., M.R.S. Proceed.AIP 50 New York (1979) 175 /2/ GREENWALD A.C., KIRKPATRICK A.R., LITTLE R.G., MINNUCCI J.A., J.Appl.Phys.
3,
2 (1979) 783
/3/ WHITE C.W., NARAYAN J., YOUNG R.T., M.R.S. Proceed. AIP 50 New York (1979) 275 /4/ LAUGIER A., BARBIER D., CHEMISKY G., Proceed. of 4th E.C. Photovoltaic Conf. Reidel
edit. (1982) 1007
/ 5 / BARBIER D., BAGHDAD1 M., LAUGIER A., VICAR10 E., J.Microsc.Spectrosc.Electr.
6
(1981) 513
161 BONACCINA C.. COMINI G., FASANO A., PRIMICERO M., Intern. Journal of H e a t Mass Transfer,
&
(1973) 1825171. BAERI P., FOTI G., POATE J.M., CULLIS A.G., Phys.Rev.Lett. 45, 25 (1980) 2036
/8/ MAYCOCK P.D., Sold.Stat.Electr. @(I9671 161
/9/ SHASHKOV Y.M., GRISHIN V.P., Sov.Phys.Sol.State
4,
2 (1966) 447/ l o / RIMINI E., BAERI P., CAMPISANO S.U., FOTI G., M.R.S.Proceed. AIP 50, New York (1979) 259
