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Experimental method for measuring both atom and carrier concentration profiles in the same sample of ion-implanted silicon layers by radioactive-ion implantation
NUCLEAR
INSTRUMENTS
AND
METHODS
127 (1975) 93-98; © N O R T H - H O L L A N D
PUBLISHING
CO.
E X P E R I M E N T A L M E T H O D FOR M E A S U R I N G B O T H A T O M A N D CARRIER C O N C E N T R A T I O N P R O F I L E S IN THE S A M E S A M P L E OF I O N - I M P L A N T E D S I L I C O N LAYERS BY R A D I O A C T I V E - I O N I M P L A N T A T I O N MASAYA IWAKI, KENJI GAMO, KOHZO M A S U D A and S U S U M U N A M B A
Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, Japan SHINJI I S H I H A R A and ITSURO K I M U R A
Research Reactor Institute, Kyoto University, Kumatori, Sennan-Gun, Osaka, Japan and KATSUHIRO YOKOTA
Faculty of Engineering, Kansai University, Suita, Osaka, Japan Received 13 December 1974 and in revised form April 1975 An experimental method by radioactive-ion implantation for measuring both atom and carrier concentration profiles in the same sample of ion-implanted silicon layers is presented. Radioactive-ion implantation apparatus and a sample holder are produced for profiling operations. Atom and carrier concentration profiles are obtained by using successive layer-removal tech-
niques which involve anodic oxidation and oxide-layer removal by hydrofluoric acid: atom concentration profiles by measuring gamma rays from radioactive impurities in the solution of hydrofluoric acid, and carrier concentration profiles from Hall-effect and sheet-resistivity measurements. This technique is applied to arsenic-ion implantation in silicon, and some results are discussed.
1. Introduction
measured by means of Hall-effect and sheet-resistivity measurements combined with layer-removal techniques which involve anodic oxidation and HF strippinga). Concentration profiles of implanted impurities such as gallium 4) and arsenic 5) in silicon have been measured by means of neutron-activation analysis combined with layer-removal techniques. Neutron-activation analysis is the simplest and most sensitive technique for measuring the atom concentration profile if induced activity can be easily measured
One of the most important factors to characterize ion implantation into semiconductors is the concentration profile of atoms and carriers. Investigation of
these profiles and their correlation would be expected to give further understanding of the ion implantation. Several investigations have been performed on the concentration profiles of atoms or carriers. Carrier concentration profiles in silicon implanted with several impurities such as boron 1) and arsenic 2) have been
IONSOURCE ,INSULATOR MAGNET [
~~_~
EINZELLENS
[
/POSATCCELERATOR~ ~
~UTTER
F TTSuPU7
OILDIFFUSIONPUMP 1 1 ROTARYPUMP |
Fig. 1. Radioactive-ion implantation apparatus. The ion-beam transport is in the straight line by using the E x B mass separator. The ion source and target chamber are at high voltage and the separator at earth potential. Two sorption vacuum pumps are utilized near the ion source.
93
94
M. I W A K I et al. TABLE 1
Type and characteristics of implantation-system components presented in fig. 1. Ion gun Mass analyser
Nielsen type E x B type (permanent magnetic field: 0.33 Wb/m e) Ion energy up to 45 kV (extractor voltage: ~ 1 0 kV) (post-accelerator voltage: ,-,35 kV) X - Y scanner: ~-~5 kHz (X), ~0.1 kHz (Y) (saw wave) Typical beam current 1 #A (N +, As +, Sb+) Pumping two sets of oil-diffusion pump with sorption pump one set of oil-diffusion pump with rotary pump Typical pressure 7 x 10-6 torr
but cannot be applied to electrical measurements in the same sample, because the effect of neutron irradiation may involve the implanted layer. Independent measurements, however, may introduce the error into the depth from the surface. Therefore, it has been requested to measure both atom and carrier concentration profiles in the same ion-implanted sample. The object of the present work is to propose one method for measuring atom and carrier concentration profiles in the same sample by radioactive-ion implantation and to demonstrate the application of this method to arsenic-ion implantation. Radioactive 76As ions together with 75As ions were implanted in silicon. Carrier concentration profiles were determined by means of Hall-effect and sheet-resistivity measurements combined with successive layer removal by anodic oxidation and H F stripping. Atom concentration profiles were obtained by measuring g a m m a activity of 76As in the solution of hydrofluoric acid. We designed and manufactured samples and a sample holder as suited for electrical measurements, anodic oxidation, and H F stripping. The following, described in detail, are the experimental techniques and some results.
For protection against g a m m a rays from the ion source, it was covered with a lead shield about 3 cm thick. In the present work, ionized source material was 20 mg arsenic solid which was exposed to a thermalneutron flux of about 8 × 1013 neutrons/(cm2s) at the hydraulic conveyer of the Kyoto University Reactor ( K U R , 5 MW) for 10 hours. Its initial radioactivity was about 0.3 Ci. The radiation dose rate at about 50 cm from the outside of the lead shield was less than 1 mR/h. Ions extracted at 10 kV were focussed by a cylindrical einzel lens, mass-separated by the E × B mass separator, and accelerated by the post accelerator. With this apparatus, heavy ions can be accelerated up to 45 kV. To assure uniformity of the ion-beam intensity across the substrate area, saw-wave voltages were applied to the vertical and horizontal deflection plates between the post accelerator and the target. The ion beam swept a pattern roughly 4 cm squared on the target. The resultant angular dispersion of the ion beam was about + 1° from the center line. For implantation of about 5 x 1014 As/era 2, a run was needed of about 15 min, with a beam current density of about 0.1 # A / c m 2.
3. Experimental method for obtaining both concentration profiles The experimental procedure for measuring both atom and carrier concentration profiles in the same sample is presented in fig. 2. A t o m concentrations are SAMPLEPREPA~TION AND SOURCEPREPA~TIONBY NEUT~N ACTI~TION RADIOACTIVE ION IMP~NT~ION ANNEALING HALL EFFECTMEASUREMENT
ANODICIOXIDATION
2. Radioactive-ion implantation apparatus A schematic drawing of the apparatus for radioactive-ion implantation is shown in fig. 1. The type of components and main characteristics are presented in table 1. To prevent radioactive dust and gas from releasing into the laboratory, two sorption vacuum pumps were utilized near the ion source and the exhaust of a rotary p u m p near the target was jointed to a ventilation duct.
HFSTRIPPING SAMPLE
HF SOLVENT
GAMMARAYMEASUREMENT Fig. 2. Experimental procedure for measuring both atom and carrier concentration profiles in the same sample by means of NaI(TI) scintillation counting and Hall-effect measurement, combined with successive layer-removal techniques.
MEASURING
BOTH ATOM AND CARRIER
obtained by measuring gamma activity in the solution of hydrofluoric acid. Carrier concentrations are determined by measuring sheet Hall coefficient Rs and sheet resistivity ps before and after layer removal. Carrier concentration Ni in the ith layer can be given as follows6): =
--
--
ed i
i
i+1_[ /
, (1)
L\Ps/i
i+1
where (R~)~ and ( P s ) i are the sheet Hall coefficient and the sheet resistivity that are measured before removal of the ith layer of thickness dv Both concentration profiles can be obtained by repeating the above measurements combined with layer removal. Layer-removal techniques involve anodic oxidation and H F stripping. The anodic oxidation was accomplished by applying some voltage between a Pt cathode in the electrolyte and the back side of silicon. The electrolyte used was a 0.04M solution of potassium nitrate in N-methylacetamide (NMA). An anodic voltage under the constant current-density (10 mA/ cm 2) condition was taken as a parameter to determine the thickness of the removed silicon. The relationship between removed silicon thickness and anodic voltage is illustrated in fig. 3. These thicknesses were determined by a multiple-interference microscope measurement or neutron-activation analysis. The multipleinterference microscope measurement was carried out with mercury illumination. In the case of neutron150
1oo
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16x(AV)0"52
L)
~ so
CONCENTRATION
0
,
I
,
I
20 40 ANODIC VOLTAGE (volt)
,
Fig. 3. T h e thickness (d) o f the removed Si-layer as a function o f the anodic voltage (AV). T h e solid line indicates d = 1 6 x x (A V)°'52. T h e open a n d closed circles represent data o f Sba n d A s - d o p e d silicon, respectively, obtained by neutron-activation analysis. T h e triangles represent the data o f B-doped silicon obtained by the multiple-interference microscope technique.
CONTACT REGION
-,H ~-'-"'~ V
60
95
activation analysis, uniformly arsenic- and antimonydoped silicon substrates were exposed to a thermalneutron flux of about 8 x 10 la neutrons/(cm2s) for 10 hours, and about 3xl01Sneutrons/(cm2s) for 1 hour, respectively. After neutron irradiation, samples were sectioned by anodic oxidation and H F stripping. The thickness of the removed silicon was determined by comparing the net photoelectric peak area of the gamma-ray spectrum from 7 6 A s (0.55MeV) or 122Sb (0.56 MeV) contained in the resultant solutions with that for the known-thickness sample. The solid line in the figure indicates the equation d = 16 x x (A V)°'52, which was determined by a least-squares method with the observed points. In this equation, d and A V are the removed silicon thickness (A) and anodic voltage (V), respectively. Samples and a sample holder were designed and manufactured as suited for electrical measurements, anodic oxidation, and H F stripping. The sample shape with the van der Pauw 7) pattern for electrical measurements is shown in fig. 4. Contact regions of the van der Pauw pattern are made by diffusing the same type of impurities as the implanted radioactive ions. The area of the radioactive-ion-implanted region is about 0.5 cm z, which is large enough to maintain the sensitivity of gamma activity. The sample holder shown in fig. 5 is ideally suited to the sample shape. Main parts of the sample holder were made of teflon which does not easily adsorb chemicals (an electrolyte, hydrofluoric acid, and so on). An acrylic resin plate was used to isolate four electrical connections from one another easily. The electrical contacts were made by fastening Au wires to the contact regions by means of conducting Ag paste. The Au wires were led to the electrical connections on the acrylic resin. A silicon rubber-ring seal was inserted between the electrical contacts and the radioactive-ion-implanted region as shown in fig. 4 by the dot line, in order to protect the electrical contacts from anodic oxidation and H F stripping. Therefore, the sample holder allows electrical measurements, [ vllll_-_-:
0
PROFILES
..-.-. SILICONE RUBBER RING '"----RADIOACTIVE ION IMPLANTED REGION
20ram
Fig. 4. Sample shape with v a n d e r P a u w pattern.
96
M. I W A K I et al.
anodic oxidation, and H F stripping of the sample to be performed in it.
4. Application of arsenic-ion implantation Silicon substrates used were 2-6 f2 cm, boron-doped, (111) wafers cut from Czochralski-grown crystals. Before radioactive-ion implantation, the contact regions of the van der Pauw pattern shown in fig. 4 were produced by diffusing 75As atoms at 1100°C for 20 min, which were pre-deposited by ion implantation. R a d i o a c t i v e 76As ions together with 75As ions were implanted in silicon at 45 keV with a dose of about 5 × 1014 atoms/cm 2 at room temperature, by using the radioactive-ion implantation apparatus shown in fig. 1. The ion beam was misoriented by 8 ° off the <111) crystal axis in order to reduce the channeling effectS). After ion implantation, annealing was performed at 500°C and 600°C for 20 min in vacuum. Atom concentration profiles were obtained by measuring the gamma-ray spectrum emitted by 76As, contained in the H F solvent of each section, with a 1¼"x 2" well-type NaI(T1) scintillation counter and a 100 channel pulse-height analyser. A typical gamma-ray spectrum is shown in fig. 6. Atom concentration was determined by calculating the net photo-electric-peak area between 0.55 and 0.65 MeV shown in the figure. In order to obtain the absolute value of 76As, the photo-peak efficiency of the NaI(T1) scintillation counter was calibrated by using a standard source of 76As which was made separately. Carrier concentration profiles were obtained by repeating the electrical measurements combined with layer removal. Currents used for measuring the sheet Hall coefficient and the sheet resistivity were typically 100/tA at the beginning of a stripping run, and were reduced to 1/~A at the end of a stripping run. Magnetic [~.~.S ~.~.S ~ ......
Pt CATHODE
V////I///%I~
TEFLON I
1
SILCONE
field strength used for Hall-effect measurement was 5100 Oe. The removed silicon thickness per one stripping was about 100 A which corresponds to 30 V of the anodic voltage. The total thickness of the removed layer was measured directly by the multiple-interference microscope after the profiling operation and had almost the same value as the sum of the thicknesses of each section. A content of radioactive 76As ions was about 10-3% of total As ions. Use of a very small amount of radioactive ions can avoid the influences of 765e by fldecay of 76As and damages produced by gamma rays f r o m 76As on the electrical properties of the arsenicimplanted layer. Atom concentration profiles were obtained by utilizing radioactive 76As atoms, which were a small fraction of the implanted arsenic ions. The 75As atoms, a large fraction of the implanted ions, were employed for measuring carrier concentration profiles. The influence of the mass difference between 76As and 75As can be neglected in the comparison of both concentration profiles, because the difference between the average projected range of 76As and 75As implanted at 45 keV in silicon is about 1 ]k according to the numerical calculation from the LSS theory 9'1°), and much less than the removed thickness per one stripping, 100 A. Figs. 7 and 8 show atom and carrier concentration profiles after annealing at 500°C and 600°C, respectively. No difference was observed between atom concentration profiles before (not shown in the figure) and after annealing at 500 °C or 600 °C for 20 min, except near the surface. A scatter of the atom concentration
0.55 MeV
c~ o x
v 1
,--S
z
3
%. %,
FIA W Y//A
AI RESIN Ti PLATE
1.2 M e V
17"/A
I U
I 0.65MeV
0 ELECTRICAL
50 CHANNEL NUMBER
100
CONNECTIONS
REAR CONTACT
Fig. 5. Sample holder which allows Hall-effect measurement, anodic oxidation, and HF stripping to be performed in it.
Fig. 6. Typical gamma-ray spectrum from HF solvent measured by a NaI(T1) well-type scintillation counter and a 100 channel pulse-height analyser. The arsenic concentration was determined by the net photoelectric-peak area S.
MEASURING BOTH ATOM AND CARRIER CONCENTRATION PROFILES at the surface may be due to a contamination with 76As atoms (probably the neutral part of the ion beam). The atom concentration profile is composed of two parts: in the high-concentration region the profile is nearly Gaussian, whereas in the low-concentration region a small tail is visible. The average projected range and projected standard deviation of the profile were found to be almost the same as those predicted by the LSS theory for amorphous targetsg'~°). For the formation of the tail, Iwaki et al. 5) and recently Schwettman 11) have proposed the rapid diffusion process. For annealing at 500°C, the carrier concentration was found to be about 4% of the arsenic-atom concentration in the peak region where the amorphous phase had been produced. In the tail region, however, the concentration of the implanted arsenic atoms almost corresponds to the carrier concentration. For annealing at 600 °C, the carrier concentrations correspond to the arsenic-atom concentrations in the whole implanted region. The annealing at 500°C caused the carrier concentration at the tail region to recover to the corresponding atom concentration, and the annealing at 600°C caused recovery in the whole region and resulted in a complete agreement between both concentration profiles. This results show that implanted arsenic ions become electrically active by occupying the substitutional site
during the epitaxial recrystallization ~2) of the amorphous layer.
5. Summary and conclusions We proposed one method for measuring atom and carrier concentration profiles in the same sample by radioactive-ion implantation and demonstrated the application of this method to arsenic-ion implantation in silicon. The ion-implantation apparatus capable of handling radioactive ions has been produced. By using this ion-implantation apparatus, radioactive 76As ions together with 76As ions were implanted in silicon at room temperature with a dose of about 5 x 1014 cm-2. Annealing was performed at 500°C and 600°C for 20 min in vacuum. Atom and carrier concentration profiles were obtained by using successive-layer-removal techniques which involve anodic oxidation and H F stripping. Atom concentrations were obtained by measuring the gamma-ray spectrum from 76As, contained in the solution of hydrofluoric acid, with NaI(T1) scintillation counter. Carrier concentrations were determined by Hall-effect and sheet resistivity measurements before and after layer removal. A sample holder which is convenient to apply layer-removal techniques and electrical measurements was used.
As 45keY 6 × 1olZ'/crn 2
(}
~1o19
z o
/, 5 keV 5x1014/cm 2 ROOM TEMR
As
ROOM TEMP
El019
97
Z
2
.I
~i
< rY z
U Z
o 108
107 0
81018
I
500 DEPTH (/~,)
I
iooo
Fig. 7. Atom (O) and carrier (Q) concentration profiles in arsenic-implanted silicon layers for annealing at 500°C during 20 rain with a dose of 6 x 1014As/cm~.
107 o
I
I
500
1000
DEPTH ( ~ )
Fig. 8. Atom (O) and carrier (0) concentration profiles in arsenic-implanted silicon layers for annealing at 600°C during 20 rain with a dose of 5 x 1014As/cm2.
98
M. I W A K I et al.
W e a p p l i e d this m e t h o d to investigate the annealingt e m p e r a t u r e effect on b o t h c o n c e n t r a t i o n profiles. F r o m c o m p a r i n g a t o m c o n c e n t r a t i o n profiles with carrier c o n c e n t r a t i o n profiles, we conclude the following: 1) The carrier c o n c e n t r a t i o n profile agrees with the a t o m c o n c e n t r a t i o n profile in the tail region for annealing at 500 °C, a n d in the whole i m p l a n t e d region for annealing at 600 °C. 2) The recovery o f carrier c o n c e n t r a t i o n in arsenici m p l a n t e d silicon layers takes place first in the tail region a n d then proceeds t o w a r d s the surface. The a u t h o r s wish to t h a n k M r K. K a w a s a k i for his help in p e r f o r m i n g the experiments. This study was p e r f o r m e d by the c o o p e r a t i v e utiliz a t i o n o f the Research R e a c t o r Institute, K y o t o University.
References 1) w. K. Hofker, H. W. Werner, D. P. Oosthoek and N. J.
Koeman, Appl. Phys. 4 (1974) 125. 2) H. Mfiller, H. Kranz, H. Ryssel and K. Schmid, Appl. Phys. 4 (1974) 115. 3) j. A. Davies, G. C. Ball, F. Brown and B. Domeij, Can. J. Phys. 42 (•964) 1070. 4) K. Gamo, M. Iwaki, K. Masuda, S. Namba, S. Ishihara, I. Kimura, I. V. Mitchell, G. Iric, J. L. Witton and J. A. Davies, Japan. J. Appl. Phys. 12 (1973) 735. 5) M. Iwaki, K. Gamo, K. Masuda, S. Namba, S. Ishihara and I. Kimura, Ion implantation in semiconductors and other materials (B. L. Crowder, ed., Plenum Press, New York, 1973) p. 111. 6) R. Baron, G.A. Shifrin, O. J. Marsh and J. W. Mayer, J. Appl. Phys. 40 (1969) 3702. 7) E.J. Van der Pauw, Philips Res. Rept. 13 (1958) 1. s) G. Dearnaley, J. H. Freeman, G. A. Gard and H. A. Wilkins, Can. J. Phys. 46 (1968) 587. 9) j. Lindhard, M. Scharff and H. F. Schiott, Kgl. Danske Videnskab. Selskab, Mat.-Fys. Medd. 33 (1963) 14. 10) D. K. Brice, Radiation Effects 6 (1970) 77; and private communication. 11) F. N. Schwettmann, Appl. Phys. Letters 22 (1973) 570. 12) j. W. M~yer, L. Eriksson, S. T. Picraux and J. A. Davies, Can, J. Phys. 46 (1968) 663.
INSTRUMENTS
AND
METHODS
127 (1975) 93-98; © N O R T H - H O L L A N D
PUBLISHING
CO.
E X P E R I M E N T A L M E T H O D FOR M E A S U R I N G B O T H A T O M A N D CARRIER C O N C E N T R A T I O N P R O F I L E S IN THE S A M E S A M P L E OF I O N - I M P L A N T E D S I L I C O N LAYERS BY R A D I O A C T I V E - I O N I M P L A N T A T I O N MASAYA IWAKI, KENJI GAMO, KOHZO M A S U D A and S U S U M U N A M B A
Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, Japan SHINJI I S H I H A R A and ITSURO K I M U R A
Research Reactor Institute, Kyoto University, Kumatori, Sennan-Gun, Osaka, Japan and KATSUHIRO YOKOTA
Faculty of Engineering, Kansai University, Suita, Osaka, Japan Received 13 December 1974 and in revised form April 1975 An experimental method by radioactive-ion implantation for measuring both atom and carrier concentration profiles in the same sample of ion-implanted silicon layers is presented. Radioactive-ion implantation apparatus and a sample holder are produced for profiling operations. Atom and carrier concentration profiles are obtained by using successive layer-removal tech-
niques which involve anodic oxidation and oxide-layer removal by hydrofluoric acid: atom concentration profiles by measuring gamma rays from radioactive impurities in the solution of hydrofluoric acid, and carrier concentration profiles from Hall-effect and sheet-resistivity measurements. This technique is applied to arsenic-ion implantation in silicon, and some results are discussed.
1. Introduction
measured by means of Hall-effect and sheet-resistivity measurements combined with layer-removal techniques which involve anodic oxidation and HF strippinga). Concentration profiles of implanted impurities such as gallium 4) and arsenic 5) in silicon have been measured by means of neutron-activation analysis combined with layer-removal techniques. Neutron-activation analysis is the simplest and most sensitive technique for measuring the atom concentration profile if induced activity can be easily measured
One of the most important factors to characterize ion implantation into semiconductors is the concentration profile of atoms and carriers. Investigation of
these profiles and their correlation would be expected to give further understanding of the ion implantation. Several investigations have been performed on the concentration profiles of atoms or carriers. Carrier concentration profiles in silicon implanted with several impurities such as boron 1) and arsenic 2) have been
IONSOURCE ,INSULATOR MAGNET [
~~_~
EINZELLENS
[
/POSATCCELERATOR~ ~
~UTTER
F TTSuPU7
OILDIFFUSIONPUMP 1 1 ROTARYPUMP |
Fig. 1. Radioactive-ion implantation apparatus. The ion-beam transport is in the straight line by using the E x B mass separator. The ion source and target chamber are at high voltage and the separator at earth potential. Two sorption vacuum pumps are utilized near the ion source.
93
94
M. I W A K I et al. TABLE 1
Type and characteristics of implantation-system components presented in fig. 1. Ion gun Mass analyser
Nielsen type E x B type (permanent magnetic field: 0.33 Wb/m e) Ion energy up to 45 kV (extractor voltage: ~ 1 0 kV) (post-accelerator voltage: ,-,35 kV) X - Y scanner: ~-~5 kHz (X), ~0.1 kHz (Y) (saw wave) Typical beam current 1 #A (N +, As +, Sb+) Pumping two sets of oil-diffusion pump with sorption pump one set of oil-diffusion pump with rotary pump Typical pressure 7 x 10-6 torr
but cannot be applied to electrical measurements in the same sample, because the effect of neutron irradiation may involve the implanted layer. Independent measurements, however, may introduce the error into the depth from the surface. Therefore, it has been requested to measure both atom and carrier concentration profiles in the same ion-implanted sample. The object of the present work is to propose one method for measuring atom and carrier concentration profiles in the same sample by radioactive-ion implantation and to demonstrate the application of this method to arsenic-ion implantation. Radioactive 76As ions together with 75As ions were implanted in silicon. Carrier concentration profiles were determined by means of Hall-effect and sheet-resistivity measurements combined with successive layer removal by anodic oxidation and H F stripping. Atom concentration profiles were obtained by measuring g a m m a activity of 76As in the solution of hydrofluoric acid. We designed and manufactured samples and a sample holder as suited for electrical measurements, anodic oxidation, and H F stripping. The following, described in detail, are the experimental techniques and some results.
For protection against g a m m a rays from the ion source, it was covered with a lead shield about 3 cm thick. In the present work, ionized source material was 20 mg arsenic solid which was exposed to a thermalneutron flux of about 8 × 1013 neutrons/(cm2s) at the hydraulic conveyer of the Kyoto University Reactor ( K U R , 5 MW) for 10 hours. Its initial radioactivity was about 0.3 Ci. The radiation dose rate at about 50 cm from the outside of the lead shield was less than 1 mR/h. Ions extracted at 10 kV were focussed by a cylindrical einzel lens, mass-separated by the E × B mass separator, and accelerated by the post accelerator. With this apparatus, heavy ions can be accelerated up to 45 kV. To assure uniformity of the ion-beam intensity across the substrate area, saw-wave voltages were applied to the vertical and horizontal deflection plates between the post accelerator and the target. The ion beam swept a pattern roughly 4 cm squared on the target. The resultant angular dispersion of the ion beam was about + 1° from the center line. For implantation of about 5 x 1014 As/era 2, a run was needed of about 15 min, with a beam current density of about 0.1 # A / c m 2.
3. Experimental method for obtaining both concentration profiles The experimental procedure for measuring both atom and carrier concentration profiles in the same sample is presented in fig. 2. A t o m concentrations are SAMPLEPREPA~TION AND SOURCEPREPA~TIONBY NEUT~N ACTI~TION RADIOACTIVE ION IMP~NT~ION ANNEALING HALL EFFECTMEASUREMENT
ANODICIOXIDATION
2. Radioactive-ion implantation apparatus A schematic drawing of the apparatus for radioactive-ion implantation is shown in fig. 1. The type of components and main characteristics are presented in table 1. To prevent radioactive dust and gas from releasing into the laboratory, two sorption vacuum pumps were utilized near the ion source and the exhaust of a rotary p u m p near the target was jointed to a ventilation duct.
HFSTRIPPING SAMPLE
HF SOLVENT
GAMMARAYMEASUREMENT Fig. 2. Experimental procedure for measuring both atom and carrier concentration profiles in the same sample by means of NaI(TI) scintillation counting and Hall-effect measurement, combined with successive layer-removal techniques.
MEASURING
BOTH ATOM AND CARRIER
obtained by measuring gamma activity in the solution of hydrofluoric acid. Carrier concentrations are determined by measuring sheet Hall coefficient Rs and sheet resistivity ps before and after layer removal. Carrier concentration Ni in the ith layer can be given as follows6): =
--
--
ed i
i
i+1_[ /
, (1)
L\Ps/i
i+1
where (R~)~ and ( P s ) i are the sheet Hall coefficient and the sheet resistivity that are measured before removal of the ith layer of thickness dv Both concentration profiles can be obtained by repeating the above measurements combined with layer removal. Layer-removal techniques involve anodic oxidation and H F stripping. The anodic oxidation was accomplished by applying some voltage between a Pt cathode in the electrolyte and the back side of silicon. The electrolyte used was a 0.04M solution of potassium nitrate in N-methylacetamide (NMA). An anodic voltage under the constant current-density (10 mA/ cm 2) condition was taken as a parameter to determine the thickness of the removed silicon. The relationship between removed silicon thickness and anodic voltage is illustrated in fig. 3. These thicknesses were determined by a multiple-interference microscope measurement or neutron-activation analysis. The multipleinterference microscope measurement was carried out with mercury illumination. In the case of neutron150
1oo
w Z
16x(AV)0"52
L)
~ so
CONCENTRATION
0
,
I
,
I
20 40 ANODIC VOLTAGE (volt)
,
Fig. 3. T h e thickness (d) o f the removed Si-layer as a function o f the anodic voltage (AV). T h e solid line indicates d = 1 6 x x (A V)°'52. T h e open a n d closed circles represent data o f Sba n d A s - d o p e d silicon, respectively, obtained by neutron-activation analysis. T h e triangles represent the data o f B-doped silicon obtained by the multiple-interference microscope technique.
CONTACT REGION
-,H ~-'-"'~ V
60
95
activation analysis, uniformly arsenic- and antimonydoped silicon substrates were exposed to a thermalneutron flux of about 8 x 10 la neutrons/(cm2s) for 10 hours, and about 3xl01Sneutrons/(cm2s) for 1 hour, respectively. After neutron irradiation, samples were sectioned by anodic oxidation and H F stripping. The thickness of the removed silicon was determined by comparing the net photoelectric peak area of the gamma-ray spectrum from 7 6 A s (0.55MeV) or 122Sb (0.56 MeV) contained in the resultant solutions with that for the known-thickness sample. The solid line in the figure indicates the equation d = 16 x x (A V)°'52, which was determined by a least-squares method with the observed points. In this equation, d and A V are the removed silicon thickness (A) and anodic voltage (V), respectively. Samples and a sample holder were designed and manufactured as suited for electrical measurements, anodic oxidation, and H F stripping. The sample shape with the van der Pauw 7) pattern for electrical measurements is shown in fig. 4. Contact regions of the van der Pauw pattern are made by diffusing the same type of impurities as the implanted radioactive ions. The area of the radioactive-ion-implanted region is about 0.5 cm z, which is large enough to maintain the sensitivity of gamma activity. The sample holder shown in fig. 5 is ideally suited to the sample shape. Main parts of the sample holder were made of teflon which does not easily adsorb chemicals (an electrolyte, hydrofluoric acid, and so on). An acrylic resin plate was used to isolate four electrical connections from one another easily. The electrical contacts were made by fastening Au wires to the contact regions by means of conducting Ag paste. The Au wires were led to the electrical connections on the acrylic resin. A silicon rubber-ring seal was inserted between the electrical contacts and the radioactive-ion-implanted region as shown in fig. 4 by the dot line, in order to protect the electrical contacts from anodic oxidation and H F stripping. Therefore, the sample holder allows electrical measurements, [ vllll_-_-:
0
PROFILES
..-.-. SILICONE RUBBER RING '"----RADIOACTIVE ION IMPLANTED REGION
20ram
Fig. 4. Sample shape with v a n d e r P a u w pattern.
96
M. I W A K I et al.
anodic oxidation, and H F stripping of the sample to be performed in it.
4. Application of arsenic-ion implantation Silicon substrates used were 2-6 f2 cm, boron-doped, (111) wafers cut from Czochralski-grown crystals. Before radioactive-ion implantation, the contact regions of the van der Pauw pattern shown in fig. 4 were produced by diffusing 75As atoms at 1100°C for 20 min, which were pre-deposited by ion implantation. R a d i o a c t i v e 76As ions together with 75As ions were implanted in silicon at 45 keV with a dose of about 5 × 1014 atoms/cm 2 at room temperature, by using the radioactive-ion implantation apparatus shown in fig. 1. The ion beam was misoriented by 8 ° off the <111) crystal axis in order to reduce the channeling effectS). After ion implantation, annealing was performed at 500°C and 600°C for 20 min in vacuum. Atom concentration profiles were obtained by measuring the gamma-ray spectrum emitted by 76As, contained in the H F solvent of each section, with a 1¼"x 2" well-type NaI(T1) scintillation counter and a 100 channel pulse-height analyser. A typical gamma-ray spectrum is shown in fig. 6. Atom concentration was determined by calculating the net photo-electric-peak area between 0.55 and 0.65 MeV shown in the figure. In order to obtain the absolute value of 76As, the photo-peak efficiency of the NaI(T1) scintillation counter was calibrated by using a standard source of 76As which was made separately. Carrier concentration profiles were obtained by repeating the electrical measurements combined with layer removal. Currents used for measuring the sheet Hall coefficient and the sheet resistivity were typically 100/tA at the beginning of a stripping run, and were reduced to 1/~A at the end of a stripping run. Magnetic [~.~.S ~.~.S ~ ......
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field strength used for Hall-effect measurement was 5100 Oe. The removed silicon thickness per one stripping was about 100 A which corresponds to 30 V of the anodic voltage. The total thickness of the removed layer was measured directly by the multiple-interference microscope after the profiling operation and had almost the same value as the sum of the thicknesses of each section. A content of radioactive 76As ions was about 10-3% of total As ions. Use of a very small amount of radioactive ions can avoid the influences of 765e by fldecay of 76As and damages produced by gamma rays f r o m 76As on the electrical properties of the arsenicimplanted layer. Atom concentration profiles were obtained by utilizing radioactive 76As atoms, which were a small fraction of the implanted arsenic ions. The 75As atoms, a large fraction of the implanted ions, were employed for measuring carrier concentration profiles. The influence of the mass difference between 76As and 75As can be neglected in the comparison of both concentration profiles, because the difference between the average projected range of 76As and 75As implanted at 45 keV in silicon is about 1 ]k according to the numerical calculation from the LSS theory 9'1°), and much less than the removed thickness per one stripping, 100 A. Figs. 7 and 8 show atom and carrier concentration profiles after annealing at 500°C and 600°C, respectively. No difference was observed between atom concentration profiles before (not shown in the figure) and after annealing at 500 °C or 600 °C for 20 min, except near the surface. A scatter of the atom concentration
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Fig. 6. Typical gamma-ray spectrum from HF solvent measured by a NaI(T1) well-type scintillation counter and a 100 channel pulse-height analyser. The arsenic concentration was determined by the net photoelectric-peak area S.
MEASURING BOTH ATOM AND CARRIER CONCENTRATION PROFILES at the surface may be due to a contamination with 76As atoms (probably the neutral part of the ion beam). The atom concentration profile is composed of two parts: in the high-concentration region the profile is nearly Gaussian, whereas in the low-concentration region a small tail is visible. The average projected range and projected standard deviation of the profile were found to be almost the same as those predicted by the LSS theory for amorphous targetsg'~°). For the formation of the tail, Iwaki et al. 5) and recently Schwettman 11) have proposed the rapid diffusion process. For annealing at 500°C, the carrier concentration was found to be about 4% of the arsenic-atom concentration in the peak region where the amorphous phase had been produced. In the tail region, however, the concentration of the implanted arsenic atoms almost corresponds to the carrier concentration. For annealing at 600 °C, the carrier concentrations correspond to the arsenic-atom concentrations in the whole implanted region. The annealing at 500°C caused the carrier concentration at the tail region to recover to the corresponding atom concentration, and the annealing at 600°C caused recovery in the whole region and resulted in a complete agreement between both concentration profiles. This results show that implanted arsenic ions become electrically active by occupying the substitutional site
during the epitaxial recrystallization ~2) of the amorphous layer.
5. Summary and conclusions We proposed one method for measuring atom and carrier concentration profiles in the same sample by radioactive-ion implantation and demonstrated the application of this method to arsenic-ion implantation in silicon. The ion-implantation apparatus capable of handling radioactive ions has been produced. By using this ion-implantation apparatus, radioactive 76As ions together with 76As ions were implanted in silicon at room temperature with a dose of about 5 x 1014 cm-2. Annealing was performed at 500°C and 600°C for 20 min in vacuum. Atom and carrier concentration profiles were obtained by using successive-layer-removal techniques which involve anodic oxidation and H F stripping. Atom concentrations were obtained by measuring the gamma-ray spectrum from 76As, contained in the solution of hydrofluoric acid, with NaI(T1) scintillation counter. Carrier concentrations were determined by Hall-effect and sheet resistivity measurements before and after layer removal. A sample holder which is convenient to apply layer-removal techniques and electrical measurements was used.
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Fig. 8. Atom (O) and carrier (0) concentration profiles in arsenic-implanted silicon layers for annealing at 600°C during 20 rain with a dose of 5 x 1014As/cm2.
98
M. I W A K I et al.
W e a p p l i e d this m e t h o d to investigate the annealingt e m p e r a t u r e effect on b o t h c o n c e n t r a t i o n profiles. F r o m c o m p a r i n g a t o m c o n c e n t r a t i o n profiles with carrier c o n c e n t r a t i o n profiles, we conclude the following: 1) The carrier c o n c e n t r a t i o n profile agrees with the a t o m c o n c e n t r a t i o n profile in the tail region for annealing at 500 °C, a n d in the whole i m p l a n t e d region for annealing at 600 °C. 2) The recovery o f carrier c o n c e n t r a t i o n in arsenici m p l a n t e d silicon layers takes place first in the tail region a n d then proceeds t o w a r d s the surface. The a u t h o r s wish to t h a n k M r K. K a w a s a k i for his help in p e r f o r m i n g the experiments. This study was p e r f o r m e d by the c o o p e r a t i v e utiliz a t i o n o f the Research R e a c t o r Institute, K y o t o University.
References 1) w. K. Hofker, H. W. Werner, D. P. Oosthoek and N. J.
Koeman, Appl. Phys. 4 (1974) 125. 2) H. Mfiller, H. Kranz, H. Ryssel and K. Schmid, Appl. Phys. 4 (1974) 115. 3) j. A. Davies, G. C. Ball, F. Brown and B. Domeij, Can. J. Phys. 42 (•964) 1070. 4) K. Gamo, M. Iwaki, K. Masuda, S. Namba, S. Ishihara, I. Kimura, I. V. Mitchell, G. Iric, J. L. Witton and J. A. Davies, Japan. J. Appl. Phys. 12 (1973) 735. 5) M. Iwaki, K. Gamo, K. Masuda, S. Namba, S. Ishihara and I. Kimura, Ion implantation in semiconductors and other materials (B. L. Crowder, ed., Plenum Press, New York, 1973) p. 111. 6) R. Baron, G.A. Shifrin, O. J. Marsh and J. W. Mayer, J. Appl. Phys. 40 (1969) 3702. 7) E.J. Van der Pauw, Philips Res. Rept. 13 (1958) 1. s) G. Dearnaley, J. H. Freeman, G. A. Gard and H. A. Wilkins, Can. J. Phys. 46 (1968) 587. 9) j. Lindhard, M. Scharff and H. F. Schiott, Kgl. Danske Videnskab. Selskab, Mat.-Fys. Medd. 33 (1963) 14. 10) D. K. Brice, Radiation Effects 6 (1970) 77; and private communication. 11) F. N. Schwettmann, Appl. Phys. Letters 22 (1973) 570. 12) j. W. M~yer, L. Eriksson, S. T. Picraux and J. A. Davies, Can, J. Phys. 46 (1968) 663.