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IR and electrical properties of thin silicon oxynitride films synthesized by ion implantation
Thin Solid Films, 91 (1982)45-53 ELECTRONICSAND OPTICS
45
IR AND E L E C T R I C A L P R O P E R T I E S O F T H I N SILICON O X Y N I T R I D E F I L M S S Y N T H E S I Z E D BY I O N I M P L A N T A T I O N * A. D. YADAV AND M. C. JOSHI Department of Physics, University of Bombay, Vidyanagari Campus, Bombay 400098 (India)
(ReceivedOctober 15, 1981;acceptedOctober 21, 1981)
Silicon oxynitride (SixOyNz) layers were synthesized by implanting singlecrystal silicon with ~4N 2 ÷ and 160 2 ÷ 30 keV ions in different proportions to doses varying from 1 x 10 ~7 to 1 x 10 ~8 ions cm -2. IR transmission techniques were used to investigate the structural dependence on the total ion dose and on the annealing temperature. Electrical properties, namely the dielectric strength and the currentvoltage and capacitance-voltage characteristics, of the ion-beam-synthesized SixOrN ~ layers and changes in them after annealing were measured.
1. INTRODUCTION Over the past decade the use of energetic ion beams for the doping of semiconductors has become an active area for research and new device development ~. In recent years, substantial efforts have also been made in the area of high dose ion implantation for producing materials with compositions and structures unattainable by conventional techniques 2. In semiconductor device technology, silicon-based oxide, nitride and oxynitride layers are widely used as passivating and dielectric thin films. Ion beam synthesis of such layers by these low temperature processing techniques has considerable technological importance and scope for potential applications in microelectronics. SiO2 and Si3N4 layers formed by ion implantation have been extensively studied a-~ x. In the present paper we describe the synthesis of silicon oxynitride (SixOyNz) layers by ion implantation and the identification of their structures by IR techniques. The structural dependence on the concentration of implanted N 2 ÷ and 0 2 ÷ ions and changes in the structures after annealing at different temperatures are also reported. The dielectric strength, current-voltage (/-1I) and capacitance-voltage (C-V) characteristics of the ionbeam-synthesized SixOrN ~ layers and their behaviour after annealing are presented.
* Paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea,Israel, September 21-25, 1981. 0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printedin The Netherlands
46 2.
A . D . YADAV, M. C. JOSHI
EXPERIMENTAL DETAILS
P-type single-crystal silicon wafers of resistivity 50 f~ cm, 250 lam thick, 19 mm in diameter, cut in the (111 ) direction, lapped and etch-polished were used as the substrate material. The silicon samples were bombarded at room temperature with 30 keV 14N2+ and 1602+ ion beams alternately at dose intervals of 2 x 1016 ions cm-2 to synthesize SixOrN z layers. Integrated beam current densities of about 10-15 taAcm -2 were used. The total ion fluences varied from 1 x 1017 to 1 × 1018 ions cm -2. The ion doses were measured with a current integrator using a secondary-electron suppressor. Ion beam scanning was utilized to ensure uniform dose distribution over the entire implanted area and to avoid ion beam heating effects. Dry N 2 and 0 2 gas mixtures were used as ion source feed-in material. The target region was surrounded by a liquid nitrogen jacket to produce good vacuum and a clean environment near the target and to prevent deposition of surface films due to cracking of hydrocarbons etc. The samples used for IR transmission studies were implanted on both optical faces to increase the effective path length for IR radiation. The samples selected for electrical measurements were implanted on only one side to a total dose of 1 x 1018 ions cm-2. Annealing of the implanted samples was performed in a vacuum system at 550 and 900 °C for 2.5 h at a vacuum of about 10-5 Torr. The IR transmission measurements were performed on a Perkin-Elmer model 457 double-beam spectrophotometer using an unimplanted matched silicon wafer in the reference beam. In each case, the reference sample had undergone the same cleaning, polishing and annealing steps as the implanted sample. For the electrical measurements, aluminium contacts to the back side of the silicon samples were evaporated in a vacuum of 10-5 Torr and were alloyed by maintaining the substrate temperature at 200 °C for about 40 min. Gold contact dots of different diameters (usually 1 mm and 0.4 mm) were evaporated through mechanical masks onto the top of the insulating layers synthesized by ion implantation to complete the MIS structures. The electrical contacts to the annealed samples were made after the required annealing treatments. All electrical measurements were performed at room temperature in darkness using an electrically screened and hermetically sealed sample holder box containing a dehumidifying agent (P205). The dielectric breakdown measurements were carried out on a Tektronix-575 curve tracer. An electrometer amplifier (Electronics Corporation of India Ltd., model EA-812) was used for the I - V characteristics. For each voltage setting the current readings were taken after 5-6 min to obtain reliable readings and to avoid transient effects 5' 12. The C - V measurements were performed at 1 MHz using an automatic C - V plotter (Princeton Applied Research, U.S.A., model 410) at various sweep speeds. 3.
RESULTS AND DISCUSSION
3.1. I R transmission studies
The IR transmission studies of the silicon implanted with 14N2+ and 1602 + ions indicated a strong dependence of the IR spectra on the total dose of implanted ions. The IR spectra for the silicon samples implanted to various dose levels are represented in Figs. 1 and 2. It can be seen that a considerably broad absorption
IR AND ELECTRICAL PROPERTIES OF SixOyN z FILMS
47
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Fig. 1. IR transmission spectra of Si=OyN, layers formed by ion implantation to various doses (unannealed; N2 + : 0 2 + implantation ratio, 1:1 ; ion energy, 30 keV): curve A, 3 x 101 ~ ions crn - 2; curve B, 5 x 101 ~ ions cm-2; curve C, 1 x 10 ~8 ions c m - 2 (implanted on only one side).
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Fig. 2. IR transmission spectra of SixOyN z layers formed by ion implantation to various doses (unannealed; N 2 ÷ :O 2 + implantation ratio, 3:1 ; ion energy, 30 keV): curve A, 1 x 10 t 7 ions c m - 2 ; curve B, 1 x 10~s ions cm -2.
band observed for low implantation doses becomes more intense and sharper when the total implantation dose is increased, and the peak maximum shifts towards higher frequencies. These changes in the IR spectra are more pronounced for low implantation doses than for high implantation doses, indicating that a state of saturation is reached for high dose levels. The peaks with maxima at 1050 and 800 cm- 1 observed for a dose of 1 × 1018
48
A . D . YADAV, M. C. JOSHI
ions cm-2 in Fig. 1 can be correlated with the peaks at 1036 cm-1 and 800 cm-1 reported for the ion-beam-synthesized SiO2 and Si3N 4 layers, corresponding to the stretching of bonds S i - - O and S i - - N respectively 3'7'8'1~. The observed shift towards higher frequencies and sharpening of the transmission band with increasing ion dose indicate a gradual transformation of the silicon surface into a complex structure composed of silicon, oxygen and nitrogen. Such a structure in the present work has been named silicon oxynitride (Si~OyNz). The saturation effect in the IR spectra can be explained on the basis of the fact that at any given ion energy there is a certain maximum number of ions that can be introduced into a substrate 2' 13. This occurs because of a state of equilibrium between the number of ions being implanted and the number being continuously sputtered away by the bombarding ions at 30 keV. From Figs. 1 and 2 it is seen that the IR spectra are very sensitive to the proportion of implanted oxygen to implanted nitrogen. The intensities of the absorption bands corresponding to the S i - - O and S i - - N stretching modes are observed to change with variations in the concentrations of implanted 0 2 ÷ and N 2 + ions. Thus the composition of the ion-beam-synthesized structure is essentially determined by the numbers of implanted 0 2 ÷ and N 2 ÷ ions. It should be pointed out here that Si~OrN z layers prepared by pyrolysis and the films of SiO2 and Si3N 4 mixtures have been reported to give only a single broad IR absorption band over the 1250-700 c m - ~frequency range14' 15. The presence of two distinct absorption peaks in the IR spectra of the ion-beam-synthesized Si~OyN~ layer can be attributed to the existence of SiO2 and Si3N 4. Thus the complex layer structure can be assumed to be composed of mixtures of SiO2 and SiaN 4, each present in a separate phase. However, in view of the broad unresolved bands, other forms of the complex structure consisting of silicon, oxygen and nitrogen cannot be ruled out. The silicon samples implanted to different dose levels were subsequently annealed in vacuum for 2.5 h at 550 and 900 °C. The resulting IR spectra given in Figs. 3 and 4 show significant changes after annealing. The absorption peaks shift towards higher frequencies and the bands become sharper when the annealing temperature is increased. A saturation of the annealing effects is observed for higher annealing temperatures. The above changes observed in the IR spectra after annealing can occur owing to release of bond strain and to disorder introduced into the layer due to the highly energetic implantation process. After annealing at 900 °C for 2.5 h, a sample implanted to a total dose of 1 x 1018 ions cm -2 with N2 ÷ and 0 2 ÷ ions in a concentration ratio of 3:1 showed two absorption peaks centred at 1060 c m - 1 (Si--O) and 830 c m - 1 (Si--N) (Fig. 4). The positions of the absorption peak maxima observed for annealed SixOrN Zfilms are very close to those reported for SiO2 and Si3N 4 films synthesized by ion implantation 3'%8'~1. This further supports our earlier statement that the ion-beam-synthesized oxynitride layers in the present work consist of SiO2, Si3N4 and some other intermediate complex structures or phases composed of silicon, oxygen and nitrogen.
3.2. Electrical properties of the ion-beam-synthesized SixOrNZlayers 3.2.1. Dielectric strength The dielectric strength of the SixOyNz layers prepared by ion implantation was found to depend on the polarity of the applied voltage with respect to the gold
IR AND ELECTRICAL PROPERTIES OF SixOrN = FILMS
49
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Fig. 3. IR transmission spectra of SixOyN= layers formed by ion implantation to various doses and subsequently annealed at 550 °C for 2.5 h (N2 ÷ :02 ÷ implantation ratio, 3:1; ion energy, 30 keV): curve A, 1 x 1017 ions cm-2; curve B, 1 x 1018 ions cm -2. 100-
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Fig. 4. IR transmission spectra of SixOyN= layers formed by ion implantation to various doses and annealed at 900 °C for 2.5 h (N2 +:02 ÷ implantation ratio, 3:1; ion energy, 30 keV): curve A, 1 x 101~ ions cm- 2; curve B, 1 x 1018 ions cm- 2. electrode on the insulating layer. A decreased value of the dielectric strength was observed after subsequent v a c u u m annealing treatments for both polarities of the applied voltage with respect to the gate electrode. The dielectric strength was independent of the polarity of the applied voltage after the samples had been annealed in v a c u u m at 900 °C for 2.5 h. An anneal at 900 °C for 2.5 h gave an average value of (1-2) x 107 V c m - 1 for the dielectric strength of the i o n - b e a m - s y n t h e s i z e d layer. The decrease in the dielectric strength upon annealing can be explained as follows: first, a decrease in the film thickness (which has been assumed in calculations to remain constant) occurs due to c o m p r e s s i o n of the layer during annealing and has not been taken into account in calculations; secondly, the highly disordered layer structure recovers towards a higher degree of ordering after annealing.
50
A . D . YADAV, M. C. JOSHI
3.2.2. Current-voltage characteristics
Figures 5 and 6 show the 1-1/characteristics of the ion-beam-synthesized SixOyNz layers for unannealed samples and for samples annealed in vacuum at 550 °C for 2.5 h. The l-l/curves for the application of a positive potential to the gold electrode on the oxynitride layer are shown in Fig. 5. Both curves show two distinct regions: an ohmic region, I ~ U, for lower applied voltages and a power law region, I oc U", for higher applied voltages where the exponent n takes the value 2.5 for the unannealed sample and 1.5 for the annealed sample. Figure 6 shows the I-1/curves when a negative potential is applied to the metal gate electrode on the oxynitride layer. These also show two regions: an ohmic lower voltage region and the other (power law) region for higher voltages with the exponent n changing from 5.3 for the unannealed sample to 1.6 for the annealed sample. The resistivities of the SixOyNz layers measured from the ohmic region of the/-l/characteristics (Figs. 5 and 6) and their changes after annealing are given in Table I. The increase in resistivity upon annealing can be associated with the annealing out of defects and traps present in the oxynitride layer formed by ion implantation s.
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Fig. 5. I - V characteristics for positive voltages applied to the top electrode: curve A, before annealing; curve B, after vacuum annealing at 550 °C for 2.5 h.
It is important to mention that for every step increase in the applied voltage a sudden increase in the current, which then decreased slowly to a certain value, was always observed during I- Vmeasurements. Plots of log J versus E 1/z (where E is the electric field and J is the current density) did not show a straight line over a wide
IR
AND
ELECTRICAL
PROPERTIES
SixOyN = F I L M S
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Fig. 6. I-V characteristics for negative voltages applied to the top electrode: curve A, before annealing; curve B, after vacuum annealing at 550°C for 2.5 h. voltage range. M o r e o v e r , the theoretical values of the coefficients expected for S c h o t t k y o r F r e n k e l - P o o l e m e c h a n i s m s at higher a p p l i e d fields were found to be m a n y times greater t h a n the values e s t i m a t e d from the e x p e r i m e n t a l plots. T h e transient c h a r a c t e r of the currents, the p o w e r law n a t u r e of the I - V characteristics a n d the d i s a g r e e m e n t between the e x p e r i m e n t a l a n d theoretical values of the coefficients for S c h o t t k y o r F r e n k e l - P o o l e m e c h a n i s m s suggest t h a t the currents flowing t h r o u g h i o n - b e a m - s y n t h e s i z e d o x y n i t r i d e layers are space charge limited. TABLE I THE
CHANGES
IN
RESISTIVITY
OF
THE
SixOyN = L A Y E R S
ON
ANNEALING
Sample conditions
Resistivity for positive applied voltage (~ cm)
Resistivity for negative applied voltage ([2 cm)
Unannealed Vacuum annealed at 550 °C for 2.5h
2.6 x 1011 7.0 x 10~~
2.8 x 1011 6.6 x 101~
3.2.3. Capacitance-voltage characteristics The C - V characteristics of the M I S structures with i o n - b e a m - s y n t h e s i z e d SixOyNz layers ( u n a n n e a l e d or a n n e a l e d at 550 a n d 900 °C for 2.5 h) as a dielectric are represented in Fig. 7. It can be seen that the c a p a c i t a n c e does n o t change with
52
A . D . YADAV, M. C. JOSHI
applied voltage for unannealed samples and for samples annealed at 550 °C for 2.5 h. This shows that the silicon below the ion-beam-synthesized SixOyNz layer is highly damaged and has not recovered to monocrystalline form even after 550 °C vacuum annealing treatments. From Fig. 7 it is observed that the C - V characteristics of samples annealed at 900 °C for 2.5 h exhibit a strong field dependence of the capacitance, which is typical of MIS structures. These results indicate that, after vacuum annealing at 900 °C for 2.5 h, the electrically dead layer of the damaged silicon has recovered the monocrystalline structure, thus giving a good siliconinsulator interface. The thickness of the ion-beam-synthesized oxynitride layer, calculated from C - V measurements for a sample annealed at 900 °C for 2.5 h and assuming a dielectric constant eSixOyNz ~---5, was found to be about 840 ~.
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+6
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Fig. 7. C - V characteristics for Au/SixOyNJSi structures: curve A, unannealed; curve B, after vacuum
annealing at 550 °C for 2.5 h; curveC, after vacuumannealingat 900 °C for 2.5 h. 4. CONCLUSIONS SixOyN= layers with various structural compositions can be synthesized by high dose ion implantation. The structure of these layers depends mainly on the relative concentrations of implanted 160 2 ÷ and 14N 2 ÷ ions. Annealing at 900 °C for 2.5 h is necessary to improve the structure and to remove the bond strain and disorder in the ion-beam-synthesized SixOyNz layer. The dielectric strength decreases with increasing annealing temperature and a value (1-2)x 107 V c m -1 was obtained after a 900 °C vacuum anneal for 2.5 h. Annealing of the samples increases the resistivity of the oxynitride layer. The I - V characteristics show that the currents flowing in these layers are ohmic in the low voltage region and space charge limited in the higher voltage range. C - V measurements show that the damaged silicon layer beneath the SixOyN=layer recrystallizes completely after a 900 °C anneal for 2.5 h. The thickness of the ion-beam-synthesized SixOyN, layer was found to be about 840/~. ACKNOWLEDGMENTS
We should like to thank Dr. N. B. Patii and his colleagues at Cotton
IR A N D ELECTRICAL PROPERTIES OF
SixOrNz FILMS
53
Technological Research Laboratory, Bombay, for allowing us to use the facilities for IR measurements and to Dr. B. M. Arora, Tata Institute of Fundamental Research, Bombay, for providing the facilities for dielectric strength and C - V measurements. One of us (A. D. Yadav) is grateful to the Department of Atomic Energy, Government of India, for the award of a Research Fellowship. REFERENCES 1 R.G. Wilson and G. R. Brewer, Ion Beams with Applications to Ion Implantation, Wiley, New York, 1973. 2 J.K. Hirvonen (ed.), Ion Implantation, Treatise Mater. Sci. Technol., 18 (1980). 3 J. Dylewski and M. C. Joshi, Thin Solid Films, 35 (1976) 327. 4 J. Dylewski and M. C. Joshi, Thin Solid Films, 37 (1976) 241. 5 J. Dylewski and M. C. Joshi, Thin Solid Films, 42 (1977) 227. 6 K.I. Kirov, E. D. Atanasova, S. P. Alexandrova, B. G. Amov and A. E. Djakov, Thin Solid Films, 48 (1978) 187. 7 A.D. Yadav and M. C. Joshi, Proc. 4th Int. Thin Film Congr., Loughborough, September 11-15, 1978, in Thin Solid Films, 58 (1979) 300. 8 A.D. Y a d a v a n d M . C. Joshi, ThinSolidFilms, 59(1979)313. 9 A. D. Yadav and M. C. Joshi, Proc. Annu. Syrup. on Nuclear Physics and Solid State Physics, Department of Atomic Energy, India, Vol. 20C, 1977, p. 200; Vol. 22C, 1979, p. 199. 10 A . D . Yadav and M. C. Joshi, Proc. 5th Int. Conf. on Ion Beam Analysis o f Materials, Sydney, February 16-20, 1981, in Nucl. Instrum. Methods, 191 (1981) 293-296; Proc. Annu. Symp. on Nuclear Physics and Solid State Physics, Vol. 23C, Department of Atomic Energy, India, 1980. 11 F.F. Komrov, I. A. Rogalevich and V. S. Tishkov, Radiat. Eft., 39 (1978) 163. 12 R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. 13 G. Dearnaley, J. H. Freeman, R. S. Nelson and J. Stephen, Ion Implantation, North-Holland, Amsterdam, 1973. 14 D.M. Brown, P. V. Gray, F. K. Heumann, H. R. Philipp and E. A. Taft, J. Electrochem. Soc., 115 (1968) 311. 15 T.L. Chu, J. R. Szedon and C. H. Lee, J. Electrochem. Soc., 115 (1968) 318.
45
IR AND E L E C T R I C A L P R O P E R T I E S O F T H I N SILICON O X Y N I T R I D E F I L M S S Y N T H E S I Z E D BY I O N I M P L A N T A T I O N * A. D. YADAV AND M. C. JOSHI Department of Physics, University of Bombay, Vidyanagari Campus, Bombay 400098 (India)
(ReceivedOctober 15, 1981;acceptedOctober 21, 1981)
Silicon oxynitride (SixOyNz) layers were synthesized by implanting singlecrystal silicon with ~4N 2 ÷ and 160 2 ÷ 30 keV ions in different proportions to doses varying from 1 x 10 ~7 to 1 x 10 ~8 ions cm -2. IR transmission techniques were used to investigate the structural dependence on the total ion dose and on the annealing temperature. Electrical properties, namely the dielectric strength and the currentvoltage and capacitance-voltage characteristics, of the ion-beam-synthesized SixOrN ~ layers and changes in them after annealing were measured.
1. INTRODUCTION Over the past decade the use of energetic ion beams for the doping of semiconductors has become an active area for research and new device development ~. In recent years, substantial efforts have also been made in the area of high dose ion implantation for producing materials with compositions and structures unattainable by conventional techniques 2. In semiconductor device technology, silicon-based oxide, nitride and oxynitride layers are widely used as passivating and dielectric thin films. Ion beam synthesis of such layers by these low temperature processing techniques has considerable technological importance and scope for potential applications in microelectronics. SiO2 and Si3N4 layers formed by ion implantation have been extensively studied a-~ x. In the present paper we describe the synthesis of silicon oxynitride (SixOyNz) layers by ion implantation and the identification of their structures by IR techniques. The structural dependence on the concentration of implanted N 2 ÷ and 0 2 ÷ ions and changes in the structures after annealing at different temperatures are also reported. The dielectric strength, current-voltage (/-1I) and capacitance-voltage (C-V) characteristics of the ionbeam-synthesized SixOrN ~ layers and their behaviour after annealing are presented.
* Paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea,Israel, September 21-25, 1981. 0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printedin The Netherlands
46 2.
A . D . YADAV, M. C. JOSHI
EXPERIMENTAL DETAILS
P-type single-crystal silicon wafers of resistivity 50 f~ cm, 250 lam thick, 19 mm in diameter, cut in the (111 ) direction, lapped and etch-polished were used as the substrate material. The silicon samples were bombarded at room temperature with 30 keV 14N2+ and 1602+ ion beams alternately at dose intervals of 2 x 1016 ions cm-2 to synthesize SixOrN z layers. Integrated beam current densities of about 10-15 taAcm -2 were used. The total ion fluences varied from 1 x 1017 to 1 × 1018 ions cm -2. The ion doses were measured with a current integrator using a secondary-electron suppressor. Ion beam scanning was utilized to ensure uniform dose distribution over the entire implanted area and to avoid ion beam heating effects. Dry N 2 and 0 2 gas mixtures were used as ion source feed-in material. The target region was surrounded by a liquid nitrogen jacket to produce good vacuum and a clean environment near the target and to prevent deposition of surface films due to cracking of hydrocarbons etc. The samples used for IR transmission studies were implanted on both optical faces to increase the effective path length for IR radiation. The samples selected for electrical measurements were implanted on only one side to a total dose of 1 x 1018 ions cm-2. Annealing of the implanted samples was performed in a vacuum system at 550 and 900 °C for 2.5 h at a vacuum of about 10-5 Torr. The IR transmission measurements were performed on a Perkin-Elmer model 457 double-beam spectrophotometer using an unimplanted matched silicon wafer in the reference beam. In each case, the reference sample had undergone the same cleaning, polishing and annealing steps as the implanted sample. For the electrical measurements, aluminium contacts to the back side of the silicon samples were evaporated in a vacuum of 10-5 Torr and were alloyed by maintaining the substrate temperature at 200 °C for about 40 min. Gold contact dots of different diameters (usually 1 mm and 0.4 mm) were evaporated through mechanical masks onto the top of the insulating layers synthesized by ion implantation to complete the MIS structures. The electrical contacts to the annealed samples were made after the required annealing treatments. All electrical measurements were performed at room temperature in darkness using an electrically screened and hermetically sealed sample holder box containing a dehumidifying agent (P205). The dielectric breakdown measurements were carried out on a Tektronix-575 curve tracer. An electrometer amplifier (Electronics Corporation of India Ltd., model EA-812) was used for the I - V characteristics. For each voltage setting the current readings were taken after 5-6 min to obtain reliable readings and to avoid transient effects 5' 12. The C - V measurements were performed at 1 MHz using an automatic C - V plotter (Princeton Applied Research, U.S.A., model 410) at various sweep speeds. 3.
RESULTS AND DISCUSSION
3.1. I R transmission studies
The IR transmission studies of the silicon implanted with 14N2+ and 1602 + ions indicated a strong dependence of the IR spectra on the total dose of implanted ions. The IR spectra for the silicon samples implanted to various dose levels are represented in Figs. 1 and 2. It can be seen that a considerably broad absorption
IR AND ELECTRICAL PROPERTIES OF SixOyN z FILMS
47
~00
90 C
i
?0-
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.......
c~ ~ 60-
50" ;,, . j '
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ffoo
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Fig. 1. IR transmission spectra of Si=OyN, layers formed by ion implantation to various doses (unannealed; N2 + : 0 2 + implantation ratio, 1:1 ; ion energy, 30 keV): curve A, 3 x 101 ~ ions crn - 2; curve B, 5 x 101 ~ ions cm-2; curve C, 1 x 10 ~8 ions c m - 2 (implanted on only one side).
100" 90"
80+ A,~
T70. B
~60,
50' 402000
;~oo'
idoo '
I}.oo WAVE NUMBER (CM-1)
'q'~oo '
,600
"
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Fig. 2. IR transmission spectra of SixOyN z layers formed by ion implantation to various doses (unannealed; N 2 ÷ :O 2 + implantation ratio, 3:1 ; ion energy, 30 keV): curve A, 1 x 10 t 7 ions c m - 2 ; curve B, 1 x 10~s ions cm -2.
band observed for low implantation doses becomes more intense and sharper when the total implantation dose is increased, and the peak maximum shifts towards higher frequencies. These changes in the IR spectra are more pronounced for low implantation doses than for high implantation doses, indicating that a state of saturation is reached for high dose levels. The peaks with maxima at 1050 and 800 cm- 1 observed for a dose of 1 × 1018
48
A . D . YADAV, M. C. JOSHI
ions cm-2 in Fig. 1 can be correlated with the peaks at 1036 cm-1 and 800 cm-1 reported for the ion-beam-synthesized SiO2 and Si3N 4 layers, corresponding to the stretching of bonds S i - - O and S i - - N respectively 3'7'8'1~. The observed shift towards higher frequencies and sharpening of the transmission band with increasing ion dose indicate a gradual transformation of the silicon surface into a complex structure composed of silicon, oxygen and nitrogen. Such a structure in the present work has been named silicon oxynitride (Si~OyNz). The saturation effect in the IR spectra can be explained on the basis of the fact that at any given ion energy there is a certain maximum number of ions that can be introduced into a substrate 2' 13. This occurs because of a state of equilibrium between the number of ions being implanted and the number being continuously sputtered away by the bombarding ions at 30 keV. From Figs. 1 and 2 it is seen that the IR spectra are very sensitive to the proportion of implanted oxygen to implanted nitrogen. The intensities of the absorption bands corresponding to the S i - - O and S i - - N stretching modes are observed to change with variations in the concentrations of implanted 0 2 ÷ and N 2 + ions. Thus the composition of the ion-beam-synthesized structure is essentially determined by the numbers of implanted 0 2 ÷ and N 2 ÷ ions. It should be pointed out here that Si~OrN z layers prepared by pyrolysis and the films of SiO2 and Si3N 4 mixtures have been reported to give only a single broad IR absorption band over the 1250-700 c m - ~frequency range14' 15. The presence of two distinct absorption peaks in the IR spectra of the ion-beam-synthesized Si~OyN~ layer can be attributed to the existence of SiO2 and Si3N 4. Thus the complex layer structure can be assumed to be composed of mixtures of SiO2 and SiaN 4, each present in a separate phase. However, in view of the broad unresolved bands, other forms of the complex structure consisting of silicon, oxygen and nitrogen cannot be ruled out. The silicon samples implanted to different dose levels were subsequently annealed in vacuum for 2.5 h at 550 and 900 °C. The resulting IR spectra given in Figs. 3 and 4 show significant changes after annealing. The absorption peaks shift towards higher frequencies and the bands become sharper when the annealing temperature is increased. A saturation of the annealing effects is observed for higher annealing temperatures. The above changes observed in the IR spectra after annealing can occur owing to release of bond strain and to disorder introduced into the layer due to the highly energetic implantation process. After annealing at 900 °C for 2.5 h, a sample implanted to a total dose of 1 x 1018 ions cm -2 with N2 ÷ and 0 2 ÷ ions in a concentration ratio of 3:1 showed two absorption peaks centred at 1060 c m - 1 (Si--O) and 830 c m - 1 (Si--N) (Fig. 4). The positions of the absorption peak maxima observed for annealed SixOrN Zfilms are very close to those reported for SiO2 and Si3N 4 films synthesized by ion implantation 3'%8'~1. This further supports our earlier statement that the ion-beam-synthesized oxynitride layers in the present work consist of SiO2, Si3N4 and some other intermediate complex structures or phases composed of silicon, oxygen and nitrogen.
3.2. Electrical properties of the ion-beam-synthesized SixOrNZlayers 3.2.1. Dielectric strength The dielectric strength of the SixOyNz layers prepared by ion implantation was found to depend on the polarity of the applied voltage with respect to the gold
IR AND ELECTRICAL PROPERTIES OF SixOrN = FILMS
49
100
l
................. ...................................
.L~_.B. . . . .
~gO-
""
70.
",,.j; 6O
5°2ooo
1~oo
~oo
1;.oo
1}oo
lOOO
8bo
60
WAVE NUMBER (CM-11
Fig. 3. IR transmission spectra of SixOyN= layers formed by ion implantation to various doses and subsequently annealed at 550 °C for 2.5 h (N2 ÷ :02 ÷ implantation ratio, 3:1; ion energy, 30 keV): curve A, 1 x 1017 ions cm-2; curve B, 1 x 1018 ions cm -2. 100-
B
9 0 . - -
1
, ...--"'
1
.~ 80' v <{
~ 70 60¸
5O
20oo
18'00
1~00
12.00 WAVE NUMBER (CM-1)
1~00
1~0
8bo
60~
I,,
Fig. 4. IR transmission spectra of SixOyN= layers formed by ion implantation to various doses and annealed at 900 °C for 2.5 h (N2 +:02 ÷ implantation ratio, 3:1; ion energy, 30 keV): curve A, 1 x 101~ ions cm- 2; curve B, 1 x 1018 ions cm- 2. electrode on the insulating layer. A decreased value of the dielectric strength was observed after subsequent v a c u u m annealing treatments for both polarities of the applied voltage with respect to the gate electrode. The dielectric strength was independent of the polarity of the applied voltage after the samples had been annealed in v a c u u m at 900 °C for 2.5 h. An anneal at 900 °C for 2.5 h gave an average value of (1-2) x 107 V c m - 1 for the dielectric strength of the i o n - b e a m - s y n t h e s i z e d layer. The decrease in the dielectric strength upon annealing can be explained as follows: first, a decrease in the film thickness (which has been assumed in calculations to remain constant) occurs due to c o m p r e s s i o n of the layer during annealing and has not been taken into account in calculations; secondly, the highly disordered layer structure recovers towards a higher degree of ordering after annealing.
50
A . D . YADAV, M. C. JOSHI
3.2.2. Current-voltage characteristics
Figures 5 and 6 show the 1-1/characteristics of the ion-beam-synthesized SixOyNz layers for unannealed samples and for samples annealed in vacuum at 550 °C for 2.5 h. The l-l/curves for the application of a positive potential to the gold electrode on the oxynitride layer are shown in Fig. 5. Both curves show two distinct regions: an ohmic region, I ~ U, for lower applied voltages and a power law region, I oc U", for higher applied voltages where the exponent n takes the value 2.5 for the unannealed sample and 1.5 for the annealed sample. Figure 6 shows the I-1/curves when a negative potential is applied to the metal gate electrode on the oxynitride layer. These also show two regions: an ohmic lower voltage region and the other (power law) region for higher voltages with the exponent n changing from 5.3 for the unannealed sample to 1.6 for the annealed sample. The resistivities of the SixOyNz layers measured from the ohmic region of the/-l/characteristics (Figs. 5 and 6) and their changes after annealing are given in Table I. The increase in resistivity upon annealing can be associated with the annealing out of defects and traps present in the oxynitride layer formed by ion implantation s.
©
-9[ 10
,
10-2
,
......
r
10"I
i
. . . . . . .
I
i
,
10 0
Jlllll
I0
I I
i
,
,,
....
I
10 2
u (vl
Fig. 5. I - V characteristics for positive voltages applied to the top electrode: curve A, before annealing; curve B, after vacuum annealing at 550 °C for 2.5 h.
It is important to mention that for every step increase in the applied voltage a sudden increase in the current, which then decreased slowly to a certain value, was always observed during I- Vmeasurements. Plots of log J versus E 1/z (where E is the electric field and J is the current density) did not show a straight line over a wide
IR
AND
ELECTRICAL
PROPERTIES
SixOyN = F I L M S
OF
51
-3 10
e~
~6E E u
@ 16 /
~,
10~
i
, , ....
10-2
,i
,
, ......
ig I
,
........
10°
i
,
, 'ILILII
101
10 2
u(v)
Fig. 6. I-V characteristics for negative voltages applied to the top electrode: curve A, before annealing; curve B, after vacuum annealing at 550°C for 2.5 h. voltage range. M o r e o v e r , the theoretical values of the coefficients expected for S c h o t t k y o r F r e n k e l - P o o l e m e c h a n i s m s at higher a p p l i e d fields were found to be m a n y times greater t h a n the values e s t i m a t e d from the e x p e r i m e n t a l plots. T h e transient c h a r a c t e r of the currents, the p o w e r law n a t u r e of the I - V characteristics a n d the d i s a g r e e m e n t between the e x p e r i m e n t a l a n d theoretical values of the coefficients for S c h o t t k y o r F r e n k e l - P o o l e m e c h a n i s m s suggest t h a t the currents flowing t h r o u g h i o n - b e a m - s y n t h e s i z e d o x y n i t r i d e layers are space charge limited. TABLE I THE
CHANGES
IN
RESISTIVITY
OF
THE
SixOyN = L A Y E R S
ON
ANNEALING
Sample conditions
Resistivity for positive applied voltage (~ cm)
Resistivity for negative applied voltage ([2 cm)
Unannealed Vacuum annealed at 550 °C for 2.5h
2.6 x 1011 7.0 x 10~~
2.8 x 1011 6.6 x 101~
3.2.3. Capacitance-voltage characteristics The C - V characteristics of the M I S structures with i o n - b e a m - s y n t h e s i z e d SixOyNz layers ( u n a n n e a l e d or a n n e a l e d at 550 a n d 900 °C for 2.5 h) as a dielectric are represented in Fig. 7. It can be seen that the c a p a c i t a n c e does n o t change with
52
A . D . YADAV, M. C. JOSHI
applied voltage for unannealed samples and for samples annealed at 550 °C for 2.5 h. This shows that the silicon below the ion-beam-synthesized SixOyNz layer is highly damaged and has not recovered to monocrystalline form even after 550 °C vacuum annealing treatments. From Fig. 7 it is observed that the C - V characteristics of samples annealed at 900 °C for 2.5 h exhibit a strong field dependence of the capacitance, which is typical of MIS structures. These results indicate that, after vacuum annealing at 900 °C for 2.5 h, the electrically dead layer of the damaged silicon has recovered the monocrystalline structure, thus giving a good siliconinsulator interface. The thickness of the ion-beam-synthesized oxynitride layer, calculated from C - V measurements for a sample annealed at 900 °C for 2.5 h and assuming a dielectric constant eSixOyNz ~---5, was found to be about 840 ~.
6°l ~_ B,,,
-10
-g
-6
50
-4
-2
0
÷2 +4 Bias Voltages (Volts)
+6
+8
+10
Fig. 7. C - V characteristics for Au/SixOyNJSi structures: curve A, unannealed; curve B, after vacuum
annealing at 550 °C for 2.5 h; curveC, after vacuumannealingat 900 °C for 2.5 h. 4. CONCLUSIONS SixOyN= layers with various structural compositions can be synthesized by high dose ion implantation. The structure of these layers depends mainly on the relative concentrations of implanted 160 2 ÷ and 14N 2 ÷ ions. Annealing at 900 °C for 2.5 h is necessary to improve the structure and to remove the bond strain and disorder in the ion-beam-synthesized SixOyNz layer. The dielectric strength decreases with increasing annealing temperature and a value (1-2)x 107 V c m -1 was obtained after a 900 °C vacuum anneal for 2.5 h. Annealing of the samples increases the resistivity of the oxynitride layer. The I - V characteristics show that the currents flowing in these layers are ohmic in the low voltage region and space charge limited in the higher voltage range. C - V measurements show that the damaged silicon layer beneath the SixOyN=layer recrystallizes completely after a 900 °C anneal for 2.5 h. The thickness of the ion-beam-synthesized SixOyN, layer was found to be about 840/~. ACKNOWLEDGMENTS
We should like to thank Dr. N. B. Patii and his colleagues at Cotton
IR A N D ELECTRICAL PROPERTIES OF
SixOrNz FILMS
53
Technological Research Laboratory, Bombay, for allowing us to use the facilities for IR measurements and to Dr. B. M. Arora, Tata Institute of Fundamental Research, Bombay, for providing the facilities for dielectric strength and C - V measurements. One of us (A. D. Yadav) is grateful to the Department of Atomic Energy, Government of India, for the award of a Research Fellowship. REFERENCES 1 R.G. Wilson and G. R. Brewer, Ion Beams with Applications to Ion Implantation, Wiley, New York, 1973. 2 J.K. Hirvonen (ed.), Ion Implantation, Treatise Mater. Sci. Technol., 18 (1980). 3 J. Dylewski and M. C. Joshi, Thin Solid Films, 35 (1976) 327. 4 J. Dylewski and M. C. Joshi, Thin Solid Films, 37 (1976) 241. 5 J. Dylewski and M. C. Joshi, Thin Solid Films, 42 (1977) 227. 6 K.I. Kirov, E. D. Atanasova, S. P. Alexandrova, B. G. Amov and A. E. Djakov, Thin Solid Films, 48 (1978) 187. 7 A.D. Yadav and M. C. Joshi, Proc. 4th Int. Thin Film Congr., Loughborough, September 11-15, 1978, in Thin Solid Films, 58 (1979) 300. 8 A.D. Y a d a v a n d M . C. Joshi, ThinSolidFilms, 59(1979)313. 9 A. D. Yadav and M. C. Joshi, Proc. Annu. Syrup. on Nuclear Physics and Solid State Physics, Department of Atomic Energy, India, Vol. 20C, 1977, p. 200; Vol. 22C, 1979, p. 199. 10 A . D . Yadav and M. C. Joshi, Proc. 5th Int. Conf. on Ion Beam Analysis o f Materials, Sydney, February 16-20, 1981, in Nucl. Instrum. Methods, 191 (1981) 293-296; Proc. Annu. Symp. on Nuclear Physics and Solid State Physics, Vol. 23C, Department of Atomic Energy, India, 1980. 11 F.F. Komrov, I. A. Rogalevich and V. S. Tishkov, Radiat. Eft., 39 (1978) 163. 12 R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. 13 G. Dearnaley, J. H. Freeman, R. S. Nelson and J. Stephen, Ion Implantation, North-Holland, Amsterdam, 1973. 14 D.M. Brown, P. V. Gray, F. K. Heumann, H. R. Philipp and E. A. Taft, J. Electrochem. Soc., 115 (1968) 311. 15 T.L. Chu, J. R. Szedon and C. H. Lee, J. Electrochem. Soc., 115 (1968) 318.