The dielectric breakdown properties and I–V characteristics of thin SiO2 films formed by high dose oxygen ion implantation into silicon

Thin Solid Films, 42 (1977) 227 235 ~') Elsevier Sequoia S.A., Lausanne

Printed in the Netherlands

227

THE DIELECTRIC BREAKDOWN PROPERTIES AND I-V C H A R A C T E R I S T I C S OF T H I N SiO 2 FILMS F O R M E D BY H I G H DOSE O X Y G E N ION I M P L A N T A T I O N INTO SILICON J. DYLEWSKI AND M. C. JOSHI

Department of Physics, University of Bombay, Bombay (India) Received August 31, 1975; accepted November 11, 1976)

The dielectric strength of SiO 2 layers formed by high dose oxygen ion implantation into silicon and its changes upon annealing at different temperatures were measured. The observed effects are explained taking into account various technological factors. The I - V characteristics of these layers are also presented for both negative and positive polarities of the applied voltage and for various annealing temperatures. The dielectric strength of the SiO 2 films is comparable with that of SiO 2 layers prepared by other conventional methods, and it decreases upon annealing. The I - V characteristics together with other observations indicate that the currents flowing through the oxide layers are ionic in the ohmic region and space charge limited in the non-ohmic region.

1. INTRODUCTION The application of high dose ion implantation into semiconductors is potentially very important, especially for microelectronics 1 because thin surface films with physical and chemical properties completely different from those of the substrate material can be obtained by this method. Watanabe and Tooi 2 first reported the possibility of forming thin SiO 2 layers by oxygen ion implantation into silicon; the dielectric strength of their layers was found to be 7 x 10 6 V c m - 1 and the resistivity w a s l017 ~ cm. The voltage range for which these results were obtained was not given. Further investigations of these thin SiO 2 layers have been carried out by relatively few workers a-5. Freeman et al. 3 reported a dielectric strength of 6 x l 0 6 V c m - 1 . To our knowledge neither the annealing behaviour of the dielectric strength nor full I - V characteristics of SiO 2 films formed by oxygen ion implantation have been reported in the literature; however, I - V characteristics for Si3N 4 layers formed by nitrogen ion implantation into silicon have been published by Astakhov et al. 6 The present work describes the detailed results of the dielectric strength of SiO 2 films formed by high dose oxygen ion implantations into silicon together with the I - V characteristics. The changes of these characteristics upon annealing at different temperatures are also reported.

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2. EXPERIMENTAL

For both the dielectric strength and the I - V measurements, p-type silicon wafers 300 gm thick and 32 mm in diameter with a resistivity of 60 f~ cm and cut in the ( 111 ) direction were used as the substrate material. The wafers were mirror polished on the optical face which would undergo implantation, but the other side of the wafers remained rough lapped to facilitate good electrical contact. All samples were implanted at room temperature with 1 6 0 2 + ions up to a dose of I x 1018 ions c m - 2 at 30 kV. Previous studies have shown that this ion dose is necessary and sufficient to ensure the formation of a stoichiometric SiO 2 layer v. A suitably adjusted isotope separator was used to perform the implantations. Ion beam current densities were about 15 I-tA cm 2 The mask used for implantations was circular and 2 cm in diameter. The ion doses were measured with a current integrator which had secondary electron suppression. Beam sweeping was introduced to ensure a uniform dose distribution over the whole implanted area and to reduce ion beam heating effects. Annealing of the implanted samples was performed in a vacuum system at a vacuum of about 10 -s Torr at either 550 °C or 800°C for 2 h. Aluminium was evaporated over the reverse side of the samples, which were heated during metallization to 200 °C to ensure better adherence of the metal film. For the I - V measurements aluminium was evaporated through a mechanical mask onto the SiO 2 layer in the form of a dot array, each dot being 1 mm in diameter. This size was chosen as a compromise between the sensitivity of the electrometer used for V measurements and the dielectric breakdown voltage. The value of this voltage is known to depend on the electrode area 8, and this limits the voltage range in which the ~ V measurements could be carried out. The annealed samples were metallized after annealing. For the dielectric strength measurements electrical contact to the oxide layer was made by means of a small rounded A1 probe tip rather than through evaporated aluminium contacts; this was in order to avoid as far as possible the influence of gross structural defects such as pinholes, weak spots etc.

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The dielectric breakdown measurements were carried out using a Tektronix 575 curve tracer, the samples being mounted on a commercial semiconductor wafer probing instrument. The applied voltage was raised continuously from 0 V to

DIELECTRIC BREAKDOWN IN SiO 2

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breakdown. Breakdown was assumed to have occurred when the current increased by about 100 laA and was sustained at any voltage. For the I - V measurements a Keithley 602 electrometer amplifier in its " n o r m a l " measuring mode was used. The measurements were performed in darkness in an electrically screened, hermetically sealed sample-holder box containing a dehumidifying agent (P205). Tefloninsulated connectors were used throughout the circuit, which is shown in Fig. 1. Each current reading was taken 5 min after application of the voltage in order to obtain reliable readings and to avoid the influence of transient effects 9. The thickness of the oxide layers was measured by multiple beam interferometry and was found to be about 800 A. 3. RESULTS AND DISCUSSION 3.1. Dielectric breakdown The breakdown processes depend significantly on the polarity of the applied voltage. Substantial changes in the values of the dielectric strength on annealing the samples were also recorded. When non-annealed samples were used and when a negative voltage was applied to the metal electrode on the oxide layer, an average value of 13.5 × 106 V cm-1 was obtained for the dielectric strength. The dielectric breakdown process for these voltages was not time dependent; in fact it was possible to maintain the voltage at 2 3 V (the resolution of the curve tracer) below the final breakdown value for extended periods of time (30-60 min) without any sign of increased conduction or breakdown. After this stressing, when the voltage was increased further, breakdown occurred at the same voltage as when the voltage was continuously increased from zero to the breakdown value. In the positive polarity biasing mode (positive potential applied to the metal electrode on the SiO 2 layer), a very pronounced time dependence of the breakdown process was observed. Increasing the voltage level at the same rate as in the negative voltage case gave an average value of 10.0 x 106 V c m - 1 . However, keeping the voltage at a value corresponding to a field intensity of about 4.5 x 106 V c m -1 caused dielectric breakdown within 30 s. This time decreased with increasing voltage; thus the value of the measured dielectric strength depends greatly on the rate of voltage increase. After annealing the implanted samples at 550°C for 2 h, a decrease in the dielectric strength of the oxide film was observed. When a negative voltage was applied to the metal electrode on the SiO 2 layer, this dielectric field strength was 10 × 106 V c m - 1 . At these negative voltages the breakdown value could again be approached to within a few volts without causing breakdown. However, a slightly time-dependent process was involved in the breakdown phenomena for this case (unlike the non-annealed samples). On increasing the voltage to a value close to the final breakdown voltage, breakdown did not occur immediately but quite often did occur after a delay of 1-5 s. When positive potentials were applied to the metal electrode on the SiO 2 layer the measured average dielectric strength was 9.4 x 106 V c m - 1. For this case a pronounced time dependence of the breakdown characteristic was again observed, e.g. a field of 6.25 x 106 V cm -1 applied for 30 s caused breakdown of the oxide film. Compared with the non-annealed samples, the time dependence of the breakdown process for positive voltages became less pronounced

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J. D Y L E W S K I ,

M. C. J O S H I

after annealing--the breakdown characteristic was less steep. A higher electric field was required to cause breakdown for voltages that were applied for the same length of time. An anneal of the samples at 800 °C for 2 h introduced further changes in the dielectric strength properties of the oxide films. After this annealing step the dielectric strength was the same for both polarities of the applied voltage and was equal to about 9.0 x l 0 6 V c m 1. Breakdown effects were again time dependent (Fig. 2) and equally so for both polarities. The measured dielectric properties for these annealed samples were totally independent of the polarity of the applied voltage to within the experimental accuracy. The time dependence of the breakdown process is shown in Fig. 2 for a sample annealed at 800 ' C for 2 h. The applied voltage is plotted against the time for which it had to be maintained to cause breakdown. u(v)

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Fig. 2. A plot of the valueof the voltageat whichbreakdown occurred w~.the time for which this voltage had to be applied to cause breakdown for a sample annealed at 800 °C for 2 h. The fact that dielectric breakdown is time dependent has been reported for the case of thermally grown oxide films also, although only for a positive voltage applied to the metal electrode on the oxide t°. This effect can be explained as caused by the contamination of the oxide with positive ions, most probably sodium ions 11 However, the effects we observed correspond closely to phenomena which are characteristic of a field intensification due to charged ions 12. The charged ions within the oxide film enhance the internal electric field. As the ions move through the oxide under the applied voltage it is possible for the internal field at the cathode to equal the dielectric strength of the film and thereby to cause breakdown. The time dependence of this field enhancement effect results from the ion drift in the oxide layer under an applied voltage, which is also a time-dependent process. An applied electric field can shift the mobile charge even at room temperature. As the positive ions drift under the applied field towards the silicom an increase in the electric field in the oxide near the silicon takes place. If the maximum breakdown strength of the oxide is exceeded, breakdown occurs. This ionic contamination results from the fact that, even though great care was taken during all steps of sample preparation, it cannot be claimed that the laboratory conditions conformed to the standard of a true "'clean MOS tech-

DIELECTRIC BREAKDOWN IN S i O 2

23 !

nology ''13, for technological reasons. It should be noted that in MOS technology the existence of 0.01 of a monolayer of sodium ions in an oxide can drastically change the properties of a semiconductor device. The presence of sodium ion contamination in the present work has been confirmed by the results of C- V studies reported elsewhere 14. It has generally been reported 1°' ~5 that, when a negative voltage is applied to a metal on an oxide layer, no time-dependent breakdown effects are observed. However, since sodium contamination is usually introduced just before metallization, it can be assumed that if a negative voltage is applied to the metal electrode on the oxide layer it will keep the positive sodium ions in place near the metal electrode; they will not then enhance the applied field. However, if the contamination is uniformly distributed through the oxide, a time dependence of the breakdown effect would be expected also when negative voltages are applied to the SiO2 layer. Since our contamination was not introduced on purpose, it is difficult to determine at which stage it took place. One possible explanation of the present results is that annealing the samples redistributes the sodium ions by thermal diffusion through the oxide layer from a position which is initially close to the metal-oxide interface. This would explain why the initially time-independent breakdown characteristics of the non-annealed samples for negative voltages change on annealing to time-dependent characteristics which are similar for both negative and positive voltages. We observed a general tendency for the dielectric strength of the oxide films to decrease on annealing, irrespective of the polarity of the applied voltage. Although it is difficult to pinpoint exactly all the possible processes responsible for this, clearly two at least have to be taken into consideration. The first is a densification process related to a shrinkage of the film thickness by about 10 ~ on annealing at 800 °C; this has been observed in SiO 2 layers prepared by other methods 16. Although we could not measure it directly in the present study, the decrease in porosity, i.e. the densification of SiO2 films formed by ion implantation on annealing, has been observed using IR transmission microscopy and has been reported elsewhere 7. We therefore expect a certain film thickness shrinkage to take place, although the experimental set-up used for the oxide thickness measurements gave similar results, to within its accuracy, for samples both before and after annealing. This effect has not been included in the calculations of the dielectric strength, and the film thickness was assumed to remain constant. A decrease in thickness would lower the value of the dielectric strength. It is well known that to obtain high values of the dielectric strength in silicon dioxide films a highly disordered amorphous film structure is essential ; the higher the degree of ordering and structure present in the film, the lower is the dielectric strength. A correlation between crystallization and dielectric breakdown has been reported by Meek and Braun 17 ; however, the crystallization of SiO 2 films prepared by other methods has been reported to take place both at room temperature ~8 and after high temperature processing ~9. This crystallization process, usually sluggish in SiO 2, is greatly accelerated by the presence of small quantities of network modifiers such as sodium 2°. In addition the annealing temperature of 800 °C lies in the region where the promotion of crystallization by sodium is at a maximum 2~. Also lattice

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defects created by ion implantation have been proposect as being responsible for local crystallization effects on annealing 8. These small randomly distributed crystallized regions are thought to be structurally similar to quartz, too minute to be detected by conventional techniques but large enough to cause breakdown by serving as flaws in the otherwise amorphous dielectric. The high values of dielectric strength that we obtained are consistent with values reported by other w o r k e r s / / f o r oxides prepared by other methods, ranging from 11 × 106 to 2 x 106 V c m - 1. By using electrodes of very small area (metal probe tips) we could largely exclude the influence of gross structural defects. The measured values are therefore close to the intrinsic (or electronic) dielectric strength and include the effects of the small crystallized regions. These breakdown modes are usually termed " p r i m a r y " and "secondary" respectively, as opposed to the "tertiary" breakdown mode associated with gross flaws and defects. 3.2. I - V characteristics

For all the measured samples it was observed that, when a voltage was applied to the sample or when a voltage was increased from one value to another, the current reading rose steeply by more than two to three times the final current value and then decayed very quickly to a much lower value; it decreased from this value at a much slower rate; finally after 3 - 4 min we could still observe some decay although it was much slower than initially. This slow decrease in current with time was more pronounced at lower voltages, whereas the steep rise was always found. We also observed polarization effects in the oxide. After these measurements a standing open-circuit voltage on the MOS structure could be detected, the value of which depended on the applied voltage during the previous measurements. Figures 3 and 4 show the l - V c u r v e s for non-annealed samples and for samples annealed at 550 °C and 800 °C for 2 h. Figure 3 shows the I - V curves for the application of a positive potential to the metal electrode on the SiO z layer. For all three curves two regions can be clearly distinguished: an ohmic region for lower applied voltages, and a region at higher voltages where a power law of the type I ~ U" (n = 2.5 3) applies. On annealing, the ohmic region of the I - V curves shifts towards lower currents (higher resistivities) and the "knee" of the curve, i.e. the transition from ohmic to nonohmic behaviour, shifts towards smaller voltages. After annealing the non-ohmic region becomes steeper. Figure 4 shows the I - V curves for application of a negative potential to the metal electrode on top of the SiO z. These also show two distinct regions ; the lower voltage region is ohmic; the other region is non-ohmic following a power law I ~ U", where n changes from 2.5 for non-annealed samples to 3 after an anneal at 550 °C and to 3.4 after annealing at 800 c'C. For the last annealing step the low voltage region is not ohmic but follows the power law with n = 1.3. An increase in the resistivity of the layers and a shift of the "knee" in the curve towards lower voltages was also observed. At higher voltages yet another region in the I - V curves was observed; this followed the power law with n = 7 11. However, as the measurements at these voltages became very noisy and unstable because of partial breakdown effects, reliable results were difficult to obtain. This higher voltage region could be due to breakdown effects rather than to intrinsic conduction mechanisms. The observed increase in the resistivity of the films on annealing can be

DIELECTRIC BREAKDOWN IN S i O 2

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Fig. 3. l-Vcharacteristics for positive voltages applied to the top electrode: curve A, before annealing; curve B, after annealing at 550 °C for 2 h; curve C, after annealing at 800 °C for 2 h. Fig. 4. 1- Vcharacteristics for negative voltages applied to the top electrode: curve A, before annealing; curve B, after annealing at 550 °C for 2 h; curve C, after annealing at 800 °C for 2 h.

TABLE I THECHANGESIN RESISTIVITYOFTHESiO2 LAYERSONANNEALING

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associated with an a n n e a li n g o u t o f defects a n d trap levels in the SiO 2 layer; e.g. a trap level 2.0 eV b e lo w the c o n d u c t i o n b a n d o f S i O 2 has been r e p o r t e d to exist 23. In the n o n - a n n e a l e d samples electrons can be injected into these levels an d m a y have sufficient m o b i l i t y to travel a l o n g t h e m t h r o u g h the oxide, e.g. by a h o p p i n g m e c h a n i s m . A n n e a l i n g the samples m a y r e m o v e the trap levels a n d m a y therefore i m p e d e the cu rr e n t flow t h r o u g h the oxide. T h e resistivities o f the SiO 2 layer m e a s u r e d in the o h m i c regions o f the I - V characteristics (Figs. 3 an d 4) a n d their changes on an n eal in g are given in T a b l e I. T h e y c o m p a r e well with values m e a s u r e d for SiO 2 films p r e p a r e d by o t h e r techniques 24. T h e shift t o w a r d s l o w er values o f the

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onset voltages of the non-ohmic regions in the ~ V curves on annealing indicates a decrease in the equilibrium density of the carriers in the dielectric layer. This follows because (if we assume that the currents flowing through the oxide are space charge limited) lower volume-generated carrier densities require lower voltages for the injected space charge carrier densities to predominate and to determine the character of the I - V curve 9. The measured I - V curves have also been plotted in the so-called Schottky coordinates l n J and E a'2 (where E is the electric field). If a Schottky or Poole Frenkel mechanism is responsible for the current flow, straight lines should be obtained. This was not so for our experiments, especially at lower field intensities. Even if the Schottky coefficient fls (as calculated from the slope of the curve in Schottky coordinates) is treated as linear at higher fields it is about 2.5-3 times lower than the theoretical value expected for a Schottky effect current flow mechanism and about 5 - 6 times smaller than that expected for the Poole Frenkel effect 25. This, together with the power law nature of the ~ V characteristics and the transient effects described previously and correlated with those observed for the case of space charge limited currents 9, all indicate that the currents flowing through the S i O 2 layer formed by oxygen ion implantation are space charge limited. It is interesting to note that the transient effect (the steep rise in current reading on increase in voltage) which is typical of space charge limited currents is also observed in the voltage range in which ohmic currents predominate in the steady state. This occurs because, when the voltage is increased, there is a transient high density of space charge carriers in the conduction b a n d - - a density that may exceed that of the volume-generated carriers, so that they determine the character of the ~ Vcurve. As the space charge carriers become trapped, their density falls below that of the volume-generated carriers and the latter lead to the steady state ohmic currents. At low fields the observed polarization effects (the standing open-circuit voltage) and the time-dependent conduction (the current slowly decreasing with time) indicate ionic rather than electronic conduction. Similar effects have been observed in various other studies of thin oxide films and have been associated with ionic conduction 2s. This is again substantiated by the fact that a certain amount of ionic contamination is present in the films we investigated. 4. CONCLUSIONS SiO 2 films formed by high dose oxygen ion implantations in silicon have good dielectric strength properties which are comparable with those of films prepared by thermal oxidation or by other methods. The observed effects include a time dependence of the breakdown processes. The time v e r s u s breakdown voltage characteristic was found to depend on the polarity of the voltage applied to the metal electrode on the SiO z layer and was sensitive to various annealing steps performed on the samples. These effects can easily be explained if we take into account the influence of some sodium ion contamination in the SiO2 films and if we consider the effects of annealing on the sodium ion distribution and the S i O 2 film structure. The SiO 2 films also have resistivities comparable with those of films prepared by conventional methods. At lower electric fields the I - Vcurves exhibit an ohmic behaviour and certain characteristics indicative of ionic conduction. At

DIELECTRIC BREAKDOWN IN SiO 2

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higher electric fields they show conduction that is governed by a power law. Certain observed transient effects are typical of space charge limited currents. Annealing of the samples increases the resistivity of the oxide films. The results of I R transmission studies, together with electron microscope and C- Vinvestigations of the SiO 2 films, are reported elsewhere 7, 14. ACKNOWLEDGMENTS

We are grateful to the Bhabha Atomic Research Centre, Trombay, for loaning the isotope separator to the University Department of Physics, University of Bombay. We would also like to thank the Tata Institute of Fundamental Research, Solid State Electronics Group, Bombay, for allowing the use of their facilities for the dielectric breakdown measurements and annealing of the samples, and the Nuclear Reaction Group for making their instruments available for the I - V measurements. REFERENCES 1 R.G. Wilson and G. R. Brewer, Ion Beams with Applications to Ion Implantation, Wiley, New York, 1973. 2 M. Watanabe and A. Tooi, Jpn. J. Appl. Phys., 5 (1966) 737. 3 J.H. Freeman, G. A. Gard, D. J. Mazey, J. H. Stephen and F. B. Whiting, European Conf. on Ion Implantation, Reading, 1970, Peregrinus, Stevenage, 1970, p. 74. 4 N . G . Blamires, D. N. Osborne, R. B. Owen and J. Stephen, in P. Glotin (ed.), Int. Conf. on Applications of lon Beams to Semiconductor Technology, Grenoble, 1967, Centre of Nuclear Studies, Electronic Services, Grenoble, p. 678. 5 J.A. Borders and W. Beezhold, in I. Ruge and J. Graul (eds.), Ion Implantation in Semiconductors, Springer Verlag, New York, 1971, p. 241. 6 V.P. Astakhov, T. B. Karashev and R. M. Aranovich, Soy. Phys. Semicond., 4 (1971) 1826. 7 J. Dylewski and M. C. Joshi, Thin Solid Films, 35 (1976) 327. 8 N.J. Chou and J. M. Eldridge, J. Electrochem. Soc., 117 (10) (1970) 1287. 9 R.H. Bube, Photoconductivity of Solids, Wiley, New York, 1960. 10 F.L. Worthing, J. Electrochem. Soc., 115 (1968) 88. 11 T.H. DiStefano, J. Appl. Phys.,44(1973)527. 12 C.M. Osburn and S. I. Raider, J. Electrochem Soc., 120 (1973) 1369. 13 S.R. Hofstein, Solid-State Electron., 10 (1967) 657. 14 J. Dylewski and M. C. Joshi, Thin Solid Films, 37 (1976) 241. 15 I. Raider, Appl. Phys. Lett., 23 (1973) 34. 16 W.A. Pliskin and H. S. Lehmann, J. Electrochem. Soc., 112 (1965) 1013. 17 R.L. MeekandR. H. Braun, J. Electrochem. Soc.,119(1972) 1538. 18 J. Drobek, J. Electrochem. Soc., 118 (1971) 325. 19 N. Nagasima and H. Enari, Jpn. J. Appl. Phys., 10 (1971) 441. 20 R. Schmidt, Silicon Interface Specialists Conf., Las Vegas, Nevada, 1968. 21 H. Rawson, Inorganic Glass Forming Systems, Academic Press, New York, 1967, p. 51. 22 N. Klein, IEEE Trans. Electron. Devices, 13 (1) (1966) 788. 23 R. Williams, Phys. Rev., Sect. A, 140 (1965) 569. 24 I.H. Pratt, SolidState Technol., 12 (1969) 55. 25 A.K. Jonscher, Thin Solid Films, 1 (1967) 213.