HAL Id: jpa-00223120
https://hal.archives-ouvertes.fr/jpa-00223120
Submitted on 1 Jan 1983
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
THE ANNEALING BEHAVIOUR OF HIGH DOSE As+ IMPLANTS
D. Hasko, R. Mcmahon, H. Ahmed
To cite this version:
D. Hasko, R. Mcmahon, H. Ahmed. THE ANNEALING BEHAVIOUR OF HIGH DOSE As+ IMPLANTS. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-223-C5-227.
�10.1051/jphyscol:1983535�. �jpa-00223120�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au nOIO, Tome 44, octobre 1983 page C5-223
THE A N N E A L I N G B E H A V I O U R OF H I G H DOSE AS+ IMPLANTS
D.G. Hasko, R.A. McMahon and H. Ahmed
Engineering Department, Cambridge University, Trwnpington Street, Cambridge CB2 IPZ, U. K.
~dsum6 - Le recuit de doses aussi importantes que 4.10'~ ions/cm2 , implantkes 5 l o a i d e de courants a l l a n t jusqu'8 lGmA, e s t d6crit. Les diffgrences entre l e recuit isothermique rapide e t l e recuit au four, introduites sur l a
mesure de l a re'sistance des f e u i l l e s , sont dues aux quantitgs diffgrentes de diffusion e t 2 l a diminution d'arsenic du f a i t de l'kvaporation. La qualit& du mate'riau n ' e s t pas affect6e par 1 1 i n t e n s i t 6 des f o r t s cornants
( 1 W ) u t i l i s k s t a n t que l'kl6vation de l a tempkrature, pendant llimplan- tation, r e s t e f a i b l e .
Abstract - The annealing behaviour of doses up t o 4. 1016 ions/cm2 implanted a t ion currents up t o lGmA is described. Differences between rapid isothermal and furnace annealing i n the measured sheet resistances a r e due t o different amounts of diffusion and t o l o s s of dopant by evaporation. Implantation a t high currents (1OnA) does not appear t o affect the quality of the regrown material provided the temperature r i s e during implantation is small.
1. INTRODUCTION
High resolution VLSI technology f o r mass production increasingly requires very high dose ion implants with e i t h e r insignificant o r closely controlled diffusion assoc- iated with the annealing of the implantation damage. Rapid isothermal annealing is capable of restoring a heavily damaged crystal by s o l i d phase epitaxy i n a time that is short compared t o t h a t needed f o r significant diffusion t o occur. multiplescan electron beam method i s a v e r s a t i l e isothermal technique f o r the rapid annealing of semiconductors ( 1 , 2 ) , and gives a thermal p r o f i l e which is a characteristic of all constant power density rapid isothermal annealing systems. The wafer temperature r i s e s i n i t i a l l y a t a r a t e determined by the power density, and eventually reaches an equilibrium temperature where radiated power balances input power. By employing a range of power densities various peak annealing t e m p e r a m s may be achieved.
This paper describes the annealing behaviour of s i l i c o n processed by rapid isothermal heating, using the multiple-scan electron beam method and compares it with furnace annealing. 80 keV As+ ions were implanted i n t o s i l i c o n , with doses i n the range 2.1015 t o 4.1016 ions/cm2 , using a comercial implanter (Nova Implantation Systems) capable of ion currents up t o 10mA. The implantation was carried out a s a batch pm- cess with the wafers clamped t o a spinning disc, with the ion beam scanning slowly along a radius. Normally the disc i s cooled during implantation, but some experiments were also done with the disc uncooled. However, even when the disc is cooled, the high intermittent dose r a t e causes significant fluctuations i n the temperature of the wafers and i n the pressure i n the implanter ( 3 ) . Particular problems i n annealing high dose implants implanted a t high r a t e s , might be expected t o a r i s e from local heating under the ion beam ( 4 ) , solid s o l u b i l i t y limits (5,6) and other dose r a t e related e f f e c t s .
2. RBS MEASUREMENTS
RBS measurements were performed on the implanted wafers using 1.5MeV. ~ e + + - ions backscattered a t 160; I n the case of an implant of low currents into a (cooled)
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983535
C5-224 JOURNAL DE PHYSIQUE
wafer so that the temperature r i s e is small there is a completely amorphous region, see Fig. 1. However, i f the temperature of the wafer r i s e s during exposure t o a high current ion bean it is possible f o r the local damage along the track of the ion t o anneal p a r t l y ( 3 ) . The r e s u l t i s a f u l l y amorphous region close t o the surface and a partly damaged layer between t h i s and the undamaged substrate, see Fig. 2,3.
The width of the f u l l y amorphous region ( .\. 900g) corresponds t o the range and devi- ation of 80keV As+ ions implanted i n t o silicon. The s i l i c o n backscatter yield i n t h a t region is reduced, see Fig. 4; indicating a displacement of s i l i c o n atoms, which is restored upon annealing.
Channel numDel Channel number
Fig. 1. 1.5 MeV He backscattering, Fig. 2. 1.5 MeV He backscattering random and channeled, as-implanted random and channeled, 80 keV As
100 keV As 4 1016. 4 1016 cooled.
An 80keV As+ ion entering the surface of the crystal can transfer a m a x i m of M(&)/ EM(&)
+
M(Si)] x 80keV t o a target s i l i c o n atom. Such a 'knock-ont target nucleus woula have a range of % 9008. A collison occuring deeper within the crystal would r e s u l t i n a smaller energy being transferred t o the target s i l i c o n atom, but the range is now the t o t a l of the distance travelled by the As ion and the range of the S i ion. Since these give m a x i m values f o r the energy and range it must be noted t h a t an average range would be somewhat l e s s and t h a t the standard deviations would include contributions from both the As and the S i paths. 'Fne e f f e c t i s t o deplete t h i s surface region of Si atoms and t o cause a region between the implanted layer and the substrate t o be damaged by many low energy S i ion collisons. Such damage would be much more susceptible t o s e l f annealing than regions d i r e c t l y dam- aged by the As+ implant.The rapid isothermal annealing behaviour of a wafer implanted a t lOmA on the cooled disc shown i n Fig. 2. Regrowth is nearly complete a f t e r annealing a t 950°C
(14W/cm2 ) leaving a surface damage peak with a buried damage peak a t % 12002 and a X min f o r the s i l i c o n of 12%. Annealing a t 12W°C ( 28w/cm2 ) and 13K1°C (40W/cnl! ) removes the buried damage peak and reduces the s i l i c o n r min t o 8% and 5.5% resp- ectively. For comparison wafers implanted on uncooled discs were a l s o annealed. In t h i s case the as-implanted spectrum indicates self-annealing of the buried layer.
Annealing of such a damaged layer, proceeds i n two stages, see Fig. 3. Annealing a t 800OC causes the f u l l y amorphous region t o regrow but does not greatly affect the buried damage peak. Annealing a t 12K1°C removes the buried damage peak, with a
x
min of 7.5% and annealing a t 13W°C reduces the small surface damage peak and x min f o r the c r y s t a l f a l l s t o 5.5%.counts counts
Channel number
1],7,
implanted2 0 0 3 0 0 2 0 0 3 0 0 ~ h a n n e l number
Fig. 3. 1.5 MeV He backscatkering, Fig. 4. 1.5 MeV He backscattering com- random and channeled, 80 keV A s parison of random spectra f o r 4
loi6
A s4 1016 uncooled. 80 keV.
Since annealing occurs i n a vacuum environment there i s a l o s s of dopant by evapor- ation which is shown by a reduction i n the s i z e of the As+ backscatter peak (ran- dom). The magnitude of t h i s l o s s is 10% a t 14W/cm2
,
30% a t 28w/crn2 and 5% a t 40W/cm2.
Channelling measurements indicate the amount of dopant on o r near l a t t i c e s i t e s and therefore expected t o be substitutional. Between 6 and 14 w/cm2 some 70%of the A s atoms appear t o become substitutional and t h i s fraction remains constant up t o the highest annealing powers, where it begins t o fall.
3. SHEET RESISTANCE
The sheet resistance was measured a f t e r rapid isothermal annealing by the four point probe method and the r e s u l t s plotted a s a function of the power density of the heat- ing electron beam, (Fig. 5 ) . The behaviour of the lowest dose material, 2.1015 ions/cm2, is typical of a wide range of sub-solid s o l u b i l i t y l i m i t implants i n giving a plateau i n sheet resistance over a wide range of annealing conditions and bounded on the low side by the minimum temperature required t o give regrowth of the implanted layer and on the upper side by diffusion of the i m p l a n t o r its l o s s by evaporation.
Fig. 5. Sheet resistance vs power Fig. 6. Sheet resistance vs power density f o r 80 keV As. density, comparison of cooled and
uncooled.
(I2 10) Rs
loo;
10 b
1 0 2 0 M 40 10 * 10 20 3 0 40
o ( w f c m Z ) o ( w ~ c d )
+./-./ (<ti:;
\
CooledJOURNAL DE PHYSIQUE
However, the behaviour of higher doses is more complex. The sheet resistances at the higher doses fall initially to approximately the same value, and they fall fur- ther at peak temperatures exceeding 1050°C, at rates which increase with dose.
However, above 1300°C the rate of fall of sheet resistance decreases. The annealing behaviour of implants on uncooled discs, Fig. 6, is characterised by the absence of any plateau region. There is a continuous fall in the sheet resistance, but the values are consistently higher than those for the implants using cooled disc5 except at the very highest annealing temperatures.
TABLE 1. Comparison of e-beam and furnace annealing for different implantation rates
Ion beam current(mA) lOmA 6rnA ZmA
Dose Expecte E-beam Furnace E-beam Furnace E-beam Furnace ions/cm2 Rs(n/o)f8) Rs(*/o) ~s(n/q Rs(n/o) Rs(fi/a) Rs(n/D) ~s(fi/o)
E
-
beam = w / c m 2,
5.7s (except * 24/cm2 Furnace = lOOO0C 30 mins N2+
1Wh 02Furnace annealing at 1000°C for 30 mins in a N2
+
10% O2 atmosphere was used to provide the comparative annealing behaviour, see table 1. Some diffusion is expec- ted, with the standard deviation of the dopant distribution increasing by 20 nm.The sheet resistance of the e-beam annealed samples is determined by the competing processes of diffusion, the increasing solubility of Asf at higher temperatures and evaporation of the dopant. Close agreement is seen for both the highest and lowest doses, but e-beam anneals gives higher R s values for the two intermediate doses.
4. CARRIER CONCENTRATIONS
The high substitutiondlity implied by the channelling measurements of the RBS, clearly do not result in the expected sheet resistance values (8). One explanation for this is that the As exists as a complex when present in high concentrations in silicon. This will limit the carrier concentration in the region of highest dopant concentration. Carrier concentrations have been measured by the anodic stripping techniques on Van derPauw samples and show saturated concentrations with low mobil- ities for the lower power anneals, see Fig. 7, but show regular distributions for the higher power anneals. Diffusion and evaporation reduce the peak concentration below the solid solubility limit as the peak annealing temperature increases, resulting in full activation of all of the dopant.
Fig. 7. Carrier concentration
and mobility depth p r o f i l e s . Carrier concentration Mobility
I I J W l C r n 2 10
BOO 'Oa" Depth A
Ion implants performed i n t o wafers clamped t o cooled d i s c s with currents of 2mA t o lOmA of As+ show no observable differences i n t h e i r annealing behaviour and are generalLy similar t o t h a t of low dose r a t e implants ( 2 ) . Differences between the sheet resistances r e s u l t i n g from e-beam and furnace annealing a r e due t o diffusion and, i n the former case, p a r t l y t o dopant evaporation. The activation of implant doses exceeding 2 1015 ions/cm2 i s s o l i d s o l u b i l i t y limited, which reduces the number of c a r r i e r s available.
ACKNOWLEDGEMENTS
The collaboration of Nova Implantation Systems, K. Steeples and G. Ryding is acknowledged.
REFERENCES
1. McMahon R.A., Ahmed H., Electron Lett. 1979, 15(2) p. 45-7.
2. McMahon R.A., Ahmed H., IEE Proc. 129(1) No. 3 1982 p. 105-10.
3. Ryding G. Dosimetry and Beam Quality. Fourth International Conference on Ion Implementation. Berchtesgaden. S e p t e d e r 1982.
4. Csepregi L., Kennedy E.G., L a S.S., Mayer J.W., Sigmon T.W., Applied F'hysics Lett. 29 (1976), No. 10 p. 645-8.
5. Tnvnbore F.A., Bell Sys. Tech. Jrnal. 1960, p. 295-32.
6. Williams J . S . , Elliman R.G., Nucl. I n s t . and Meths., 1982/183 (1981) p . 389-395.
7. Wilson S.R., Gregory R.B., Paulson W.M., Hamdi A.H., %Daniel F.D., Appl. Phys. L e t t . 41(10), 1982 p. 978-80.
8. Smith B.J., Stephen J. Handbook of Ion Implantation Data f o r Silicon Device Fabrication. AERE R9369.
