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New techniques of implantation for near-term applications
Nuclear Instruments and/vlethods 189 (1981) 135-140 North-Holland Publishing Company
135
NEW TECHNIQUES OF IMPLANTATION FOR NEAR-TERM APPLICATIONS M. BRUEL, M. FLOCCARI, J. LABARTINO, J.F. MICHAUD and A. SOUBIE LETI, Commissariat d L 'Energie Atomique, 85X-38041 Grenoble, ~)'ance
We have studied new techniques associated with ion implantation. This includes recoil implantation and transient processes. The main results and some perspectives for industriai development of the R1TA process (Recoil Implantation plus Transient Anneaiing) arc presented. Theoretical and technical problems of flash-doping by ion implantation are discussed. The purpose of this process is to implant the total dose of required dopant in a time short enough to obtain a transient melting of the implanted layer. Our investigation shows that the present technology makes such studies possible, l'mphasis is put on the know-how and equipment development by controlled thermonuclear fusion research which will give more suitable equipment in the near future.
1. Introduction New technological processes are clearly needed for low cost production of devices like solar cells. The energy efficiency of the equipment involved in such processes must be as high as possible and will be one of the main parameters for the choice between technical solutions. Our research group has contributed to the study of such processes for doping semiconductors. The advantages of the RITA process (Recoild Implantation plus Transient Annealing) are presented in sect. 2. In sec. 3 the theoretical and technical feasibility of flashdoping with ion implanatation are discussed. The purpose of these experiments is to achieve doping of semiconductors (predeposition plus annealing) in only one step of a short duration (typically 1 gs).
o) RECOIL IMPLANTATION PRINCIPLE.
;+
b) CONCENTRATION PROFILES.
i
0
A . . . .
i
eA
CONCENTRATION
B
......... X
t
BEFORE IMPLANTATION.
2. The RITA process The principle of recoil implantation is illustrated by fig. 1. In the experiments presented here, an inert gas implantation was performed through an antimony layer vacuum evaporated onto silicon. The recoil efficiency ~ (number of antimony atoms trapped in silicon per incoming ion) has been measured for various energies and layer thicknesses (figs. 2 and 3). Obviously there exist ranges of energy and ranges of thickness where r~ does not vary strongly. It is possible to determine regions of relatively constant ~ in the 0029-554X/81/0000-0000/$02.50 © 1981 North-Itolland
* AFTER IMPLANTATION.
e
*
AFTER LAYER REMOVAL. I
I
e, Fig. 1. (a) Recoil implantation principle. (b) Concentration profiles at different stages. III. TEMPERATURE CONTROLLED IMPLANTS SPECIAL TECHNIQUES
M. Bmel et al. / New techniques of implantation
136 EFFICIENCY
]
'.
2 ~ ,_. iso~
400
. _ _ _ - - . _ 3oo~
::I]tl// / / / "/'~ / ,sLT/ I
EAr (keV)
1zoo ~ I
-
200
100 80 60
40
i •
t 20
.02 .A-; ENERGY(keV)
CE~..LEI't
46o
26o
4oo
Fig. 2. Efficiency versus Ar + energy. Recoil implantation: Ar +, 3 X 1014 cm -2, in Sb/Si. Parameter: thickness of the Sb layer (A).
EFFICIENCY
.8
.6 .4
.06
60
.04
0
I
.02 ~)q91lO
THICKNESS (k ,~)
CtA-LEr,
.2.
.
.
.4.
.6
i
'
Fig. 3. Efficiency versus Sb thickness. Recoil implantation: Ar +, 3 × 1014 cm -2, in Sb/Si. Parameter: Ar + energy (keV).
i
©eso CEA-LETI
THICKNESS (~.)
--3
o
8'00
Fig. 4. Some domains of quasi-const~t recoil efficiency.
(energy-thickness) plane (fig. 4). Further studies [1,2] show that this property is not exclusive to the combination of argon on antimony; we assume this result to be va/id for other combinations as long as neither the incident ion nor the target atom is of low mass. This is a very attractive aspect of recoil implantation, because precise controls of ion energy and layer thickness are not needed. These advantages could give rise to a new concept of simplified ion implanters. Furthermore the throughput of the ion implanter is enhanced by the recoil efficiency. We have measured = 5 for krypton on antimony. Our theoretical work predicts 77= 1 1 for antimony ions on antimony. Thus the very high current machines with 10 mA current on the target would be challenged by recoil systems with only one mA of current. The atom predeposition must be associated with an annealing process for achieving the doping of a semiconductor. Furnace annealing has been tested for R.I. layers (fig. 5). Unfortunately the layer resistivity obtained does not correspond to the predeposited dopant dose. This is explained by a strong out-diffusion of antimony during the annealing due to the very high surface concentration generated by recoil implantation. The out-diffusion problem does not exist with
M. Bruel et al. /New techniques of implantation Rn (~)
I
\ RECOIL IMPLANTATION \
tsb=620 /~ Ar = 85 keV
30(
~
Annealing : 600", 30 mn and 200"C, 5 h,
200
100
CLASSICAL ~ANTATION ~E1Ag.SLOET i 2x10 '~
v
~
,,
Dsb(cm-2 )
~ . - - ~ _ _ _ _ ~ 10's
10m
transient annealing. Experiments [3] conducted with a YAG laser and with the Spire Electronbeam machine show that the remaining dose after the annealing step is the same as before, within a precision o f 15%; neutron activation has been used for these measurements. As can be seen in fig. 6 the depth concentration profde after electron beam annealing shows a surface peak (due to segregation during solidification) and a bulk profile with a quasi-perfect Gaussian shape. These results are consistent with the surface melting model and antimony diffusion in liquid silicon. For comparison a double implantation (argon plus antimony) has been performed (the doses correspond to those o f recoil experiments). The corresponding profile after the same annealing is presented in fig. 7. This profile is similar to that of recoil. The sheet resistivity of these layers are 50 82 for the doubl~ implantation and 90 I2 for the recoiled layer. RBS experiments show that in both experiments 90% of antimony atoms are in substitutional sites (even above the solubility limit). However more detailed
Fig. 5. Annealing behaviour of classically- and recoil-implanted antimony layers.
•
CsbO= 0 '
~k(cm'=)
I
| Csbl21 10 z° ~
POINT/
POINT 2
1o"
RECOIL IMPLANTATION:
to" t 0 te
i0 ~e Si<1tl)
10~r
137
POINT 2
Si (11t) IMPLANTATIONS :
%\
Sb THICKNESS: 620 ~,
A r +, 1:50 keY, 10~Sern-2
Ar +, 1 5 0 keV, tOqScm"z
Sb+,130keV,2x1015cm "z
RECOILED Sb: 2x1015cm-2 ELECTRON BEAM ANNEALING : 1.13 J. cm"2 (SPI 6 0 0 0 , SPIRE CORP.,USA)
\
ELECTRON BEAM ANNEALING:
10~r
1.13 d x cm "z (SPI 6 0 0 0 ,
\
SPIRE CORP., USA)
\ ~) t98o
~1980 CeA-LET,
I0'00
2000
DEPTH (A) J 3000~ 4000
Fig. 6. 121Sb profile post transient annealing. Dashed line: xo = 0. w/-~-}-= 840 A C(x) = Cmax exp - [(x - xo)2/4Dt] and 1020 A.
etA- LEt,
DEPTH (.,~1 t000
2000
3000
4000
Fig. 7. 121Sb prof'fle post transient annealing. Dashed line: C(x) = Cmax exp - [(x - xo)2/4Dt] x 0 = 600 A. x / ~ = 980 A and 1200 A. III. TEMP. CONTROLLED IMPLANTS/SPECIAL TECHNIQUES
M. Bruel et al. / New techniques of implantation
13 8
PRECISE CONTROLS NOT NECESSARY NECESSARY
STEP . LAYER DEPOSITION
LAYER THICKNESS
YES tM6,CLIUMEVAPORATION)
![
........
j
~ -
GAS INERT IMPLANTATION YES
NO
.. , , J ~ . . . ~ . . .
[
l
DOSEpuRITY
BEAM
YES
[
]
E N E R G Y
TIME OF CHEMICAL ATTACK
CHEMICAL REMOVAL OF' THE LAYER
TRANSIENT ANNEALING BY ELECTRON BEAM
-
HOMOGENEITY ENERGY FLUX
Fig. 8. Actual principle of doping with recoil implantation associated with transient annealing.
experiments [2] indicate that the number of atoms above a concentration of approximately 7 × 10 ~9 c m - ~ are not electrically active even if they are substitutional. These annealing results are very attractive. Also, we think the RITA process could be very attractive for further developments. The steps of the RITA process are represented in fig. 8 in the present concept. Hopefully, steps like layer deposition and removal will be improved by means of E.B. flash-evaporation for deposition of the layer and by means of the same teclmique or sputtering for layer removal.
3. Flash-doping by ion implantation
A simple calculation gives the density of energy received by the wafer during ion implantation. For a 10 ~4 cm -2 implanted dose with ion energy 60 keV, 1 J cm -2 is deposited within the first layers of the wafer. It is notworthy that two main characteristics of the transient annealing processes are met here: energy density in the range 1 J cm - 2 , and energy deposition within the first layers. Therefore the questions is: is it possible to realize ion implantation in a
very short thne (1 /as typically) in order to use the ion energy deposition for heating and melting of the layer, the expected result being a perfect annealing comparable with that produced by laser or E.B. pulses? The main advantages of that process would be: - only one step for predeposition and annealing; - g o o d energy efficiency; only one J cm -2 used for deposition and annealing; - very fast process (typically 1/as); - energy deposition for annealing exactly adjusted to the profile of damages;
3.1. Theoretical consMerations The implanted ions lose their energy within a depth close to Rp + AR v. For ions between phosphorus and antimony Rp + ARp is nearly proportional to the energy Re Rp + z2xRp "~/-~o E , with Ro/Eo = 1.5 × 10-3 ktm/keV for P+, 0.75 × 10 -3 /am/keV for As +, 0.5 × 10-3 tan/keV for Sb +. Thus, the volumetric energy deposition is expressed by dW E° (keV//am)" D (cm -2) d ~ (Jcm_a)= 1.6 X 10 -12 Roe If the implantation is very short, the thermal diffusion length will be less than (Rp + ARp); this is the adiabatic case. For other conditions the themlal parameters will result from an energy deposition within a depth equal to the thermal diffusion length.
3.1.1. Adiabatic case The first condition of melting is dW/dV= 7000 J cm-3; 7000 J is the energy needed for heating 1 cm 3 of silicon to the melting point and the latent heat for the phase change. Also s -Re O(cm -2) = 4.4 × 101 ~ o (/am/keV). The second condition is adiabaticity; that means for example x/Dtht ~<0.5 (Rp + zXRp); at high temperature Dth = 0.l cm 2 s -1. This condition can then be written: t ( n s ) < 2 5 R[F--~o(/am/keV). E(keV)] 2 .
M. Bruel et al. / New techniques of implantation ~ION ENERGY ( k e Y )
L ['~I-A-LET'
I
[
CURRENT
102
D~NSITY ( • 10 3
" ), 104
Fig. 9. Relation between energy and current density o f flashdoping by ion implantation (adiabatic case),
From D and t one gets the expression of current density
~---dS(A cm -2) = 2.8 X 104 ~ o (keV//am)- E-2(keV).
139
(P+, As+, Sb+). The corresponding doses are indicated. The conclusion for the adiabatic case is that flashdoping is theoretically possible at very low doses (1.7 X 1012 cm -2 for Sb), whatever the energy is. This process would give controllable concentration profiles because only the penetration-depth is melted; that implies that the depth of the prof'fle would be controlled by the implantation energy and that the redistribution during melting is effective only within the penetration-depth. 3.1.2. General case
Fig. 10 represents the corresponding relations between dose and energy with current-density and duration as parameters. In this case the prof'de is rather controlled by the implantation duration and is not totally independent of the chosen dose and energy.
This relation is represented in fig. 9 tot three ions
DOSE ( cm -2 )
~4~Js I 101(
1015
1014
i
10
1o
2
1o
>
E N E R G Y ~t k e V )
Fig. 10. Relation between energy and dose for flash-doping by ion imphmtation (general case). Time and current density as parameters. III. TEMP. CONTROLLEI) IMPLANTS/SPECIAL TECHNIQUES
140
M. Bruel et aL / New techniques of implantation
3.2. Technical problems
Flash-doping requires current-densities in the range ( 1 - 1 0 0 0 ) A cm-2. Two means of realization are to be investigated. Steady-state systems with scanned focused beams and large area transient highdensity beams without focusing. 3.2.1. Steady state process The forming of a very-high-density beam of heavy particles requires: a very-high-brightness source; quiescent plasma conditions; a quasi-perfect space-charge neutralization. Ion-beams with a current-density of 1 A cnn-2 have been recently achieved [4]; a facility according to new concepts is now under construction in Orsay [5] for focusing ion currents as high as 10/~A within a diameter of 1 /an (which corresponds to I000 A cm-2). Consequently experimental facilities for flash doping with scanned ion beams are becoming available. A solution for avoiding problems related to spacecharge is the ionized cluster beam. The clusters consisting of N ions (N is in the range 103 -104), can be formed by adiabatic expansion of the vapor through a nozzle in vacuum. They are then single-ionized by electron bombardment and accelerated towards the target. The space-charge effects are strongly reduced in a cluster-beam compared with the corresponding effect in an ion-beam with the same atomic flux and same energy per atom. For example in a drift space the reducing factor would be N 2. Equivalent current-density between 0.1 and 1 A c m -2, with cluster energy in the range 500 k e V - 1 MeV, have already been obtained with hydrogen, by controlled fusion research groups in Karlsruhe and Fontenay-aux-Roses. The need for high-energy clusters (in the range 1 - 1 0 MeV or more) will be satisfied initially by conventional electrostatic or linear accelerators. In the future techniques of collective acceleration will perhaps solve this problem. Whatever the future will be, the ionic cluster implantation and the related physical effects are interesting to study; especially the interaction between the collision cascades induced by each atom of the cluster (as seen for clusters with two or three atoms) [6]. -
-
-
For large clusters one may expect the cluster penetration to give a microliquid phase, due to the large volumetric energy deposition, the result being a microtransient doping and annealing. 3.2.2. Transient process The recent development of intense pulsed ionsources (magnetically insulated source, reflex-triode, for exanaple) has been chiefly aimed at applications for controlled fusion. The extension of these new techniques to ions other than protons will give the opportunity to make machines achieving flash-dopin~ Other systems developed by CTF research could be suitable for such experiments, especially plasma accelerators (plasma focus, Hall accelerator); in these systems ion acceleration is provided by the Ampere force within the plasma. With such devices very high ionic-fluxes may be achieved.
4. Conclusion Most of the systems and know-how mentioned above are in an early state of development, but their interest for controlled fusion warrants further developments and thus these will be available soon for semiconductor doping. We are indebted to the Spire Corporation for the EB transient annealing.
References [1] M. Bruel and M. Floccari, Rapport de f'm d'dtude DGRST/CCM no. 78-7-2096. [2] M. Bruel, M. Floccari and J.P. Gailliard, Proc. 2nd Int. Conf. on Ion beam modification of materials, eds., R.E. Benenson, E.N. Kaufman, G.L. Miller and W.W. Scholz (North-Holland, Amsterdam, 1981) p. 93. [3] M. Floccari and M. Bruel: "Emploi des reciuts pulses darts la technologie des semi-conducteurs", Communication presented at Round Table discussion, Strasbourg, France, 29 May 1980. [4] R.L. Kubena et al., Abstracts of the Spriug Meeting of Electrochem. Soc., St. Louis, Missouri, May 11-16 1980. [5 ] J. Camplan, private communication. [6] G.L. Destetanis, J.P. Belle, J.M. Ogier-Collin and J.P. Gailliard, Nucl. Instr. and Meth. 182/183 (1981) 637.
135
NEW TECHNIQUES OF IMPLANTATION FOR NEAR-TERM APPLICATIONS M. BRUEL, M. FLOCCARI, J. LABARTINO, J.F. MICHAUD and A. SOUBIE LETI, Commissariat d L 'Energie Atomique, 85X-38041 Grenoble, ~)'ance
We have studied new techniques associated with ion implantation. This includes recoil implantation and transient processes. The main results and some perspectives for industriai development of the R1TA process (Recoil Implantation plus Transient Anneaiing) arc presented. Theoretical and technical problems of flash-doping by ion implantation are discussed. The purpose of this process is to implant the total dose of required dopant in a time short enough to obtain a transient melting of the implanted layer. Our investigation shows that the present technology makes such studies possible, l'mphasis is put on the know-how and equipment development by controlled thermonuclear fusion research which will give more suitable equipment in the near future.
1. Introduction New technological processes are clearly needed for low cost production of devices like solar cells. The energy efficiency of the equipment involved in such processes must be as high as possible and will be one of the main parameters for the choice between technical solutions. Our research group has contributed to the study of such processes for doping semiconductors. The advantages of the RITA process (Recoild Implantation plus Transient Annealing) are presented in sect. 2. In sec. 3 the theoretical and technical feasibility of flashdoping with ion implanatation are discussed. The purpose of these experiments is to achieve doping of semiconductors (predeposition plus annealing) in only one step of a short duration (typically 1 gs).
o) RECOIL IMPLANTATION PRINCIPLE.
;+
b) CONCENTRATION PROFILES.
i
0
A . . . .
i
eA
CONCENTRATION
B
......... X
t
BEFORE IMPLANTATION.
2. The RITA process The principle of recoil implantation is illustrated by fig. 1. In the experiments presented here, an inert gas implantation was performed through an antimony layer vacuum evaporated onto silicon. The recoil efficiency ~ (number of antimony atoms trapped in silicon per incoming ion) has been measured for various energies and layer thicknesses (figs. 2 and 3). Obviously there exist ranges of energy and ranges of thickness where r~ does not vary strongly. It is possible to determine regions of relatively constant ~ in the 0029-554X/81/0000-0000/$02.50 © 1981 North-Itolland
* AFTER IMPLANTATION.
e
*
AFTER LAYER REMOVAL. I
I
e, Fig. 1. (a) Recoil implantation principle. (b) Concentration profiles at different stages. III. TEMPERATURE CONTROLLED IMPLANTS SPECIAL TECHNIQUES
M. Bmel et al. / New techniques of implantation
136 EFFICIENCY
]
'.
2 ~ ,_. iso~
400
. _ _ _ - - . _ 3oo~
::I]tl// / / / "/'~ / ,sLT/ I
EAr (keV)
1zoo ~ I
-
200
100 80 60
40
i •
t 20
.02 .A-; ENERGY(keV)
CE~..LEI't
46o
26o
4oo
Fig. 2. Efficiency versus Ar + energy. Recoil implantation: Ar +, 3 X 1014 cm -2, in Sb/Si. Parameter: thickness of the Sb layer (A).
EFFICIENCY
.8
.6 .4
.06
60
.04
0
I
.02 ~)q91lO
THICKNESS (k ,~)
CtA-LEr,
.2.
.
.
.4.
.6
i
'
Fig. 3. Efficiency versus Sb thickness. Recoil implantation: Ar +, 3 × 1014 cm -2, in Sb/Si. Parameter: Ar + energy (keV).
i
©eso CEA-LETI
THICKNESS (~.)
--3
o
8'00
Fig. 4. Some domains of quasi-const~t recoil efficiency.
(energy-thickness) plane (fig. 4). Further studies [1,2] show that this property is not exclusive to the combination of argon on antimony; we assume this result to be va/id for other combinations as long as neither the incident ion nor the target atom is of low mass. This is a very attractive aspect of recoil implantation, because precise controls of ion energy and layer thickness are not needed. These advantages could give rise to a new concept of simplified ion implanters. Furthermore the throughput of the ion implanter is enhanced by the recoil efficiency. We have measured = 5 for krypton on antimony. Our theoretical work predicts 77= 1 1 for antimony ions on antimony. Thus the very high current machines with 10 mA current on the target would be challenged by recoil systems with only one mA of current. The atom predeposition must be associated with an annealing process for achieving the doping of a semiconductor. Furnace annealing has been tested for R.I. layers (fig. 5). Unfortunately the layer resistivity obtained does not correspond to the predeposited dopant dose. This is explained by a strong out-diffusion of antimony during the annealing due to the very high surface concentration generated by recoil implantation. The out-diffusion problem does not exist with
M. Bruel et al. /New techniques of implantation Rn (~)
I
\ RECOIL IMPLANTATION \
tsb=620 /~ Ar = 85 keV
30(
~
Annealing : 600", 30 mn and 200"C, 5 h,
200
100
CLASSICAL ~ANTATION ~E1Ag.SLOET i 2x10 '~
v
~
,,
Dsb(cm-2 )
~ . - - ~ _ _ _ _ ~ 10's
10m
transient annealing. Experiments [3] conducted with a YAG laser and with the Spire Electronbeam machine show that the remaining dose after the annealing step is the same as before, within a precision o f 15%; neutron activation has been used for these measurements. As can be seen in fig. 6 the depth concentration profde after electron beam annealing shows a surface peak (due to segregation during solidification) and a bulk profile with a quasi-perfect Gaussian shape. These results are consistent with the surface melting model and antimony diffusion in liquid silicon. For comparison a double implantation (argon plus antimony) has been performed (the doses correspond to those o f recoil experiments). The corresponding profile after the same annealing is presented in fig. 7. This profile is similar to that of recoil. The sheet resistivity of these layers are 50 82 for the doubl~ implantation and 90 I2 for the recoiled layer. RBS experiments show that in both experiments 90% of antimony atoms are in substitutional sites (even above the solubility limit). However more detailed
Fig. 5. Annealing behaviour of classically- and recoil-implanted antimony layers.
•
CsbO= 0 '
~k(cm'=)
I
| Csbl21 10 z° ~
POINT/
POINT 2
1o"
RECOIL IMPLANTATION:
to" t 0 te
i0 ~e Si<1tl)
10~r
137
POINT 2
Si (11t) IMPLANTATIONS :
%\
Sb THICKNESS: 620 ~,
A r +, 1:50 keY, 10~Sern-2
Ar +, 1 5 0 keV, tOqScm"z
Sb+,130keV,2x1015cm "z
RECOILED Sb: 2x1015cm-2 ELECTRON BEAM ANNEALING : 1.13 J. cm"2 (SPI 6 0 0 0 , SPIRE CORP.,USA)
\
ELECTRON BEAM ANNEALING:
10~r
1.13 d x cm "z (SPI 6 0 0 0 ,
\
SPIRE CORP., USA)
\ ~) t98o
~1980 CeA-LET,
I0'00
2000
DEPTH (A) J 3000~ 4000
Fig. 6. 121Sb profile post transient annealing. Dashed line: xo = 0. w/-~-}-= 840 A C(x) = Cmax exp - [(x - xo)2/4Dt] and 1020 A.
etA- LEt,
DEPTH (.,~1 t000
2000
3000
4000
Fig. 7. 121Sb prof'fle post transient annealing. Dashed line: C(x) = Cmax exp - [(x - xo)2/4Dt] x 0 = 600 A. x / ~ = 980 A and 1200 A. III. TEMP. CONTROLLED IMPLANTS/SPECIAL TECHNIQUES
M. Bruel et al. / New techniques of implantation
13 8
PRECISE CONTROLS NOT NECESSARY NECESSARY
STEP . LAYER DEPOSITION
LAYER THICKNESS
YES tM6,CLIUMEVAPORATION)
![
........
j
~ -
GAS INERT IMPLANTATION YES
NO
.. , , J ~ . . . ~ . . .
[
l
DOSEpuRITY
BEAM
YES
[
]
E N E R G Y
TIME OF CHEMICAL ATTACK
CHEMICAL REMOVAL OF' THE LAYER
TRANSIENT ANNEALING BY ELECTRON BEAM
-
HOMOGENEITY ENERGY FLUX
Fig. 8. Actual principle of doping with recoil implantation associated with transient annealing.
experiments [2] indicate that the number of atoms above a concentration of approximately 7 × 10 ~9 c m - ~ are not electrically active even if they are substitutional. These annealing results are very attractive. Also, we think the RITA process could be very attractive for further developments. The steps of the RITA process are represented in fig. 8 in the present concept. Hopefully, steps like layer deposition and removal will be improved by means of E.B. flash-evaporation for deposition of the layer and by means of the same teclmique or sputtering for layer removal.
3. Flash-doping by ion implantation
A simple calculation gives the density of energy received by the wafer during ion implantation. For a 10 ~4 cm -2 implanted dose with ion energy 60 keV, 1 J cm -2 is deposited within the first layers of the wafer. It is notworthy that two main characteristics of the transient annealing processes are met here: energy density in the range 1 J cm - 2 , and energy deposition within the first layers. Therefore the questions is: is it possible to realize ion implantation in a
very short thne (1 /as typically) in order to use the ion energy deposition for heating and melting of the layer, the expected result being a perfect annealing comparable with that produced by laser or E.B. pulses? The main advantages of that process would be: - only one step for predeposition and annealing; - g o o d energy efficiency; only one J cm -2 used for deposition and annealing; - very fast process (typically 1/as); - energy deposition for annealing exactly adjusted to the profile of damages;
3.1. Theoretical consMerations The implanted ions lose their energy within a depth close to Rp + AR v. For ions between phosphorus and antimony Rp + ARp is nearly proportional to the energy Re Rp + z2xRp "~/-~o E , with Ro/Eo = 1.5 × 10-3 ktm/keV for P+, 0.75 × 10 -3 /am/keV for As +, 0.5 × 10-3 tan/keV for Sb +. Thus, the volumetric energy deposition is expressed by dW E° (keV//am)" D (cm -2) d ~ (Jcm_a)= 1.6 X 10 -12 Roe If the implantation is very short, the thermal diffusion length will be less than (Rp + ARp); this is the adiabatic case. For other conditions the themlal parameters will result from an energy deposition within a depth equal to the thermal diffusion length.
3.1.1. Adiabatic case The first condition of melting is dW/dV= 7000 J cm-3; 7000 J is the energy needed for heating 1 cm 3 of silicon to the melting point and the latent heat for the phase change. Also s -Re O(cm -2) = 4.4 × 101 ~ o (/am/keV). The second condition is adiabaticity; that means for example x/Dtht ~<0.5 (Rp + zXRp); at high temperature Dth = 0.l cm 2 s -1. This condition can then be written: t ( n s ) < 2 5 R[F--~o(/am/keV). E(keV)] 2 .
M. Bruel et al. / New techniques of implantation ~ION ENERGY ( k e Y )
L ['~I-A-LET'
I
[
CURRENT
102
D~NSITY ( • 10 3
" ), 104
Fig. 9. Relation between energy and current density o f flashdoping by ion implantation (adiabatic case),
From D and t one gets the expression of current density
~---dS(A cm -2) = 2.8 X 104 ~ o (keV//am)- E-2(keV).
139
(P+, As+, Sb+). The corresponding doses are indicated. The conclusion for the adiabatic case is that flashdoping is theoretically possible at very low doses (1.7 X 1012 cm -2 for Sb), whatever the energy is. This process would give controllable concentration profiles because only the penetration-depth is melted; that implies that the depth of the prof'fle would be controlled by the implantation energy and that the redistribution during melting is effective only within the penetration-depth. 3.1.2. General case
Fig. 10 represents the corresponding relations between dose and energy with current-density and duration as parameters. In this case the prof'de is rather controlled by the implantation duration and is not totally independent of the chosen dose and energy.
This relation is represented in fig. 9 tot three ions
DOSE ( cm -2 )
~4~Js I 101(
1015
1014
i
10
1o
2
1o
>
E N E R G Y ~t k e V )
Fig. 10. Relation between energy and dose for flash-doping by ion imphmtation (general case). Time and current density as parameters. III. TEMP. CONTROLLEI) IMPLANTS/SPECIAL TECHNIQUES
140
M. Bruel et aL / New techniques of implantation
3.2. Technical problems
Flash-doping requires current-densities in the range ( 1 - 1 0 0 0 ) A cm-2. Two means of realization are to be investigated. Steady-state systems with scanned focused beams and large area transient highdensity beams without focusing. 3.2.1. Steady state process The forming of a very-high-density beam of heavy particles requires: a very-high-brightness source; quiescent plasma conditions; a quasi-perfect space-charge neutralization. Ion-beams with a current-density of 1 A cnn-2 have been recently achieved [4]; a facility according to new concepts is now under construction in Orsay [5] for focusing ion currents as high as 10/~A within a diameter of 1 /an (which corresponds to I000 A cm-2). Consequently experimental facilities for flash doping with scanned ion beams are becoming available. A solution for avoiding problems related to spacecharge is the ionized cluster beam. The clusters consisting of N ions (N is in the range 103 -104), can be formed by adiabatic expansion of the vapor through a nozzle in vacuum. They are then single-ionized by electron bombardment and accelerated towards the target. The space-charge effects are strongly reduced in a cluster-beam compared with the corresponding effect in an ion-beam with the same atomic flux and same energy per atom. For example in a drift space the reducing factor would be N 2. Equivalent current-density between 0.1 and 1 A c m -2, with cluster energy in the range 500 k e V - 1 MeV, have already been obtained with hydrogen, by controlled fusion research groups in Karlsruhe and Fontenay-aux-Roses. The need for high-energy clusters (in the range 1 - 1 0 MeV or more) will be satisfied initially by conventional electrostatic or linear accelerators. In the future techniques of collective acceleration will perhaps solve this problem. Whatever the future will be, the ionic cluster implantation and the related physical effects are interesting to study; especially the interaction between the collision cascades induced by each atom of the cluster (as seen for clusters with two or three atoms) [6]. -
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For large clusters one may expect the cluster penetration to give a microliquid phase, due to the large volumetric energy deposition, the result being a microtransient doping and annealing. 3.2.2. Transient process The recent development of intense pulsed ionsources (magnetically insulated source, reflex-triode, for exanaple) has been chiefly aimed at applications for controlled fusion. The extension of these new techniques to ions other than protons will give the opportunity to make machines achieving flash-dopin~ Other systems developed by CTF research could be suitable for such experiments, especially plasma accelerators (plasma focus, Hall accelerator); in these systems ion acceleration is provided by the Ampere force within the plasma. With such devices very high ionic-fluxes may be achieved.
4. Conclusion Most of the systems and know-how mentioned above are in an early state of development, but their interest for controlled fusion warrants further developments and thus these will be available soon for semiconductor doping. We are indebted to the Spire Corporation for the EB transient annealing.
References [1] M. Bruel and M. Floccari, Rapport de f'm d'dtude DGRST/CCM no. 78-7-2096. [2] M. Bruel, M. Floccari and J.P. Gailliard, Proc. 2nd Int. Conf. on Ion beam modification of materials, eds., R.E. Benenson, E.N. Kaufman, G.L. Miller and W.W. Scholz (North-Holland, Amsterdam, 1981) p. 93. [3] M. Floccari and M. Bruel: "Emploi des reciuts pulses darts la technologie des semi-conducteurs", Communication presented at Round Table discussion, Strasbourg, France, 29 May 1980. [4] R.L. Kubena et al., Abstracts of the Spriug Meeting of Electrochem. Soc., St. Louis, Missouri, May 11-16 1980. [5 ] J. Camplan, private communication. [6] G.L. Destetanis, J.P. Belle, J.M. Ogier-Collin and J.P. Gailliard, Nucl. Instr. and Meth. 182/183 (1981) 637.