- Email: [email protected]
Evaluation of micro-defects by DRAM data retention characteristics measurement
Nuclear Instrumentsand Methods in Physics Research B 127/ 128 ( 1997) 78-8
1 Beam Interactions with Materials 8 Atoms
EISEVIER
Evaluation of micro-defects by DRAM data retention characteristics measurement K.Miyoshi
*,
a, K. Terashima b, Y. Muramatsu a, N. Nishio a, T. Murotani a, S. Saito
a
a ULSI Device Development Laboratories, NEC Corporation, I120, Shimokuzawa, Sagamihara, Kanagawa 229, Japan b Microelectronics Research Laboratories, NEC Corporation, 34, Miyukigaoka. Tsukuba, Ibaraki 305, Japan
Abstract The influence of micro-defects induced intentionally by Sic ion implantation was investigated using data retention characteristics of dynamic random access memory (DRAM). The defect formation was controlled by Si+ implantation and subsequent annealing conditions. Micro-defects such as (311) defects having a size below 50 nm degraded the junction leakage current and data retention characteristics. Data retention characteristics was also affected by the existence of micro-defects such as point defect or its clusters, although the junction leakage current was low enough compared with unimplanted samples.
1. Introduction Doping by ion implantation is widely used in MOS device fabrication process. It is well known that residual defects induced by ion implantation degrade the electrical properties of MOS devices. The degradation of data retention characteristics is one of the major problems in dynamic random access memory (DRAM) devices [1,21 and it was mainly caused by high junction leakage current. The detailed analysis indicates the defects with sizes of pm range, such as dislocation loop, stacking fault, etc. Recently, submicron ULSI technologies require low temperature processing in order to fabricate shallow junctions. However, in lower temperature processing, the influence of residual defects on electrical properties of MOS devices would become more remarkable [3]. Therefore, it is important to evaluate the influence of micro-defects induced by low temperature process. In this work, the influence of the micro-defects caused by ion implantation was examined using DRAM data retention characteristics. Micro-defects induced intentionally by Si+ implantation was evaluated by transmission electron microscopy (TEM), deep level transient spectroscopy (DLTS) and photoluminescence (RL).
2. Experiments DRAM devices having conventional planar capacitors were fabricated on p-type (100) CZ silicon wafers. After
n+/p junction formation, Si ions were implanted at room temperature. The implant energy was 180 keV, and the projected range (R,) of Si existed within depletion region. The implant doses were 1 X 1013 and 1 X lOL4 cm-*. After implantation, annealing was carried out in N, atmosphere at 800°C for 30 min by furnace annealing (FA) to recover lattice damages and for 10 s by rapid thermal annealing (RTA) to introduce micro-defects intentionally, respectively. Electrical properties were examined by junction leakage current measurement on n+/p diodes and data retention characteristics measurement on DRAM devices. Thediffusedareaofn+/pdiodewas0.018mm2 (1.8 X lo4 p,m2). The morphology of defects in these samples was investigated by TEM operated at an accelerating voltage of 140 kV to avoid electron irradiation damage during observation. Electrically active damage was also studied by DLTS and PL measurement. DLTS measurements were performed using n+/p diodes at temperature range from 30 K to 300 K. For the determination of the energy level, isothermal frequency scan measurement was applied in the rate window range from 1 s- ’ to 2500 s- ’ . PL spectra were obtained at 4.2 k from bare Si wafers implanted with Si+ in the wavelength region of 1600-2500 nm. The samples were excited by the 488 nm line of an Ar ion laser. The emission from the samples was detected by a PbS detector.
3. Results and discussion 3.1. Junction
* Corresponding author. Fax: + 81-427-71-0938; email: [email protected].
leakage current measurement
Fig. 1 shows the junction leakage current characteristics of samples annealed at 800°C for 30 min by FA and for 10
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO168-583X(96)00855-5
K. Miyoshi et al./Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 78-81
0
2
4
Reverse
6 8 Bias (V)
10
Fig. 1.Junction leakage current characteristics of n+/p diodes for Si+ unimplanted samples. Annealing was carried out by FA at 800°C for 30 min and RTA at 800°C for 10 s.
s by RTA. In the FA samples, small leakage current below 10 pA was observed, which was almost the same as that for the samples without Si+ implantation. The I-V curve for the RTA samples at a dose of 1 X 10 I3 cm-* is almost the same as the curve for the samples without Si+ implantation. On the other hand, at a dose of 1 x lOI cm-2, leakage current increased by two orders of magnitude. 3.2. Data retention characteristics
measurement
Fig. 2 shows the DRAM dam retention characteristics of samples annealed by FA and RTA. The data retention characteristics curves for the FA samples are almost the same as the curve for the unimplanted samples. In the case of the RTA samples at a dose of 1 X 1Ol4 cm-*, a11bits failed even at.a short retention time since junction leakage current was too high, as shown in Fig. 1. However, in spite of no difference in junction leakage measurement, retention time for the samples at a dose of 1 X lOI cm-* was shorter than that for the Si+ unimplanted samples. These
Fig. 3. Cross-sectional TEM micrograph of RTA samples implanted at a dose of I X 1014 cm-*.
results suggest the existence of residual defects in the RTA samples at a dose of 1 X lOI cm-*. 3.3. Defect evaluation 3.3.1. TEM observation In order to identify the cause of degradation of electrical properties, the residual defects induced by Si + implantation was investigated by TEM. In the RTA samples at a dose of 1 X lOI cm-*, a high defect density (0.7 X 10” cm-*) was observed. The size of defects were from 3 nm to 40 nm. These results correspond to the remarkable degradation of junction leakage and data retention characteristics. The XTEM bright field image and HRTEM image of defects are shown in Figs. 3 and 4, respectively. The high resolution micrograph, shown in Fig. 4, clearly shows (3 11) defect image [4]. 1311) defects are known to occur under condition of electron irradiation [5] and ion implantation [6]. These (3 11) defects are not formed due to electron irradiation during TEM observation but due to ion implantation, because operation voltage of TEM was 140 kV in order to suppress knock-on damage [7]. These results indicate that the remarkable degradation of junction leakage and data retention characteristics in RTA at a dose of I X lOI cm-’ occurred due to (311) defects induced . by Si+ Implantation. On the other hand, in the RTA
Retention Time (au) Fig. 2. DRAM data retention characteristics of same samples, as shown in Fig. 1.
Fig. 4. HRTEM image of Fig. 3. Defect exists on (311) habit plane.
1. FUNDAMENTALS/BASICS
80
k Miyoshi et al./Nucl. _
Instr. and Meth. in Phys. Res. B 127/128
(1997) 78-81
3x10’”
2 s
2x10”
s ‘F; !! E
1 x10’3
8 50 0 0
50
100 150 200 250 300
Temperature (K) Fig. 5. DLTS spectra from the RTA samples implanted with a dose of 1 X 10” and IX lOI cm-*.
samples at a dose of 1 X lOI cm-*, apparent defects can not be detected by TEM. 3.3.2. DLTS measurement Fig. 5 shows the DLTS spectra from the RTA samples implanted with a dose of 1 X 1013 and 1 X 1OL4cm-*, respectively. No DLTS peak can be observed from the samples without Si+ implantation and samples at a dose of 1 X lOI cm-*. However, at a dose of 1 X 1014 cm-‘, the spectrum is dominated by the H (0.48) level, which is determined from the temperature dependence of hole emission rate from the defects, as shown in Fig. 6. Though the relation between H (0.48) level and (311) defects are not dear at the present time, its energy level near mid-gap suggests that it would be the dominant recombination center in p-type Si. 3.3.3. PL measurement Fig. 7 shows the PL spectra of the unimplanted sample, as-implanted sample, FA sample and RTA sample at a dose of 1 X lOI cm-‘, respectively. The PL intensity itself is weak and there is no significant peak in the PL spectra of the unimplanted and FA samples. On the other hand, a broad emission band with some structures appears
0.5
0.6
0.7
Photon
Energy
0.6
0.9
( eV )
Fig. 7. PL spectra of the unimplanted sample, as-implanted, and RTA samples at a dose of 1 X lOI cm-‘, respectively.
FA
in the PL spectra of the as-implanted and RTA samples. Comparing the spectral changes of the different samples in Fig. 7, the broad emission band is caused by the defects induced by ion implantation and RTA at 800°C for 10 s is not enough to remove them. From the result which apparent defects cannot be detected by TEM in this sample, micro-defects such as point defect or its clusters might be the origin of the broad emission band. The broad emission band also indicates the existence of some deep-level defects, which act as radiative recombination centers. Considering these results, we suggest that DRAM data retention characteristics is affected by the micro-defects induced by ion implantation.
4. Conclusions 3’.
18OkeV. lE14
8CCC”-10sec
fRTA)
-9 3.6
4 1000/T
4.4 (1
IK)
Fig. 6. Arrhenius analysis of the hole emission rate for the peak observed in the RTA samples implanted at a dose of 1 X IO” Here, e is the hole emission rate and T is the sample cm-‘.
temperature.
The relationship between micro-defects and the electrical characteristics of MOS devices was investigated. In the RTA samples at a dose of 1 X lOI cm-*, both junction leakage characteristics and DATA retention characteristics degraded remarkably by the existence of {311} defects. On the other hand, in the RTA samples at a dose of 1 X lOI cm -*, degradation of data retention characteristics was observed although there was no difference in junction leakage current. In this case, the broad emission band which might be due to micro-defects such as point defect or its clusters was observed in the PL spectrum. These results indicate that DRAM data retention characteristics
K. Miyoshi et al./Nucl.
Instr. and Meth. in Phys. Res. B 127/128
can be affected by not only micro-defects such as (311) defects having a size below 50 run but also micro-defects such as point defect or its clusters. Therefore, it is important to suppress the micro-defects formation in future device fabrication.
Acknowledgements
The authors are grateful to Dr. M. Tajima (Institute of Space and Astronautical Science) for his help in the PL measurement and K. Matsui in the evaluation of DRAM data retention characteristics. The authors would like to thank Drs. 0. Mizuno, K. Okada and K. Ikeda for their encouragement.
(1997) 78-81
81
References [l] H. Mikoshiba, N. Nishio, T. Matsumoto, H. Kikuchi, T. Kitano and H. Kaneko, Defect Engineering in Semiconductor Growth and Device Technology (Material Research Society, 1992) p. 629. [2] Sun S. Kim and W. Wijaranakula, J. Electrochem. Sot. 141 (1994) 1872. 131T. Nina, T. Ohmi, Y. Ishihara, A. Okita, T. Shibata, J. Sugiura and N. Ohwada, J. Appl. Phys. 67 (1990) 7404. 141 S. Takeda, Japan. J. App. Phys. 30 (1991) L639. [5] I.G. Salisbury and M.H. Loretto, Philos. Mag. A 39 (1979) 317. [6] D.J. Eaglesham, P.A. Stalk, H.-J. Gossmann, T.E. Haynes and J.M. Poate, Nucl. Instr. and Meth. B 106 (1995) 191. [7] W.A. Mckinley and H. Feshbach, Phys. Rev. 74 (19481 1759.
1 Beam Interactions with Materials 8 Atoms
EISEVIER
Evaluation of micro-defects by DRAM data retention characteristics measurement K.Miyoshi
*,
a, K. Terashima b, Y. Muramatsu a, N. Nishio a, T. Murotani a, S. Saito
a
a ULSI Device Development Laboratories, NEC Corporation, I120, Shimokuzawa, Sagamihara, Kanagawa 229, Japan b Microelectronics Research Laboratories, NEC Corporation, 34, Miyukigaoka. Tsukuba, Ibaraki 305, Japan
Abstract The influence of micro-defects induced intentionally by Sic ion implantation was investigated using data retention characteristics of dynamic random access memory (DRAM). The defect formation was controlled by Si+ implantation and subsequent annealing conditions. Micro-defects such as (311) defects having a size below 50 nm degraded the junction leakage current and data retention characteristics. Data retention characteristics was also affected by the existence of micro-defects such as point defect or its clusters, although the junction leakage current was low enough compared with unimplanted samples.
1. Introduction Doping by ion implantation is widely used in MOS device fabrication process. It is well known that residual defects induced by ion implantation degrade the electrical properties of MOS devices. The degradation of data retention characteristics is one of the major problems in dynamic random access memory (DRAM) devices [1,21 and it was mainly caused by high junction leakage current. The detailed analysis indicates the defects with sizes of pm range, such as dislocation loop, stacking fault, etc. Recently, submicron ULSI technologies require low temperature processing in order to fabricate shallow junctions. However, in lower temperature processing, the influence of residual defects on electrical properties of MOS devices would become more remarkable [3]. Therefore, it is important to evaluate the influence of micro-defects induced by low temperature process. In this work, the influence of the micro-defects caused by ion implantation was examined using DRAM data retention characteristics. Micro-defects induced intentionally by Si+ implantation was evaluated by transmission electron microscopy (TEM), deep level transient spectroscopy (DLTS) and photoluminescence (RL).
2. Experiments DRAM devices having conventional planar capacitors were fabricated on p-type (100) CZ silicon wafers. After
n+/p junction formation, Si ions were implanted at room temperature. The implant energy was 180 keV, and the projected range (R,) of Si existed within depletion region. The implant doses were 1 X 1013 and 1 X lOL4 cm-*. After implantation, annealing was carried out in N, atmosphere at 800°C for 30 min by furnace annealing (FA) to recover lattice damages and for 10 s by rapid thermal annealing (RTA) to introduce micro-defects intentionally, respectively. Electrical properties were examined by junction leakage current measurement on n+/p diodes and data retention characteristics measurement on DRAM devices. Thediffusedareaofn+/pdiodewas0.018mm2 (1.8 X lo4 p,m2). The morphology of defects in these samples was investigated by TEM operated at an accelerating voltage of 140 kV to avoid electron irradiation damage during observation. Electrically active damage was also studied by DLTS and PL measurement. DLTS measurements were performed using n+/p diodes at temperature range from 30 K to 300 K. For the determination of the energy level, isothermal frequency scan measurement was applied in the rate window range from 1 s- ’ to 2500 s- ’ . PL spectra were obtained at 4.2 k from bare Si wafers implanted with Si+ in the wavelength region of 1600-2500 nm. The samples were excited by the 488 nm line of an Ar ion laser. The emission from the samples was detected by a PbS detector.
3. Results and discussion 3.1. Junction
* Corresponding author. Fax: + 81-427-71-0938; email: [email protected].
leakage current measurement
Fig. 1 shows the junction leakage current characteristics of samples annealed at 800°C for 30 min by FA and for 10
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO168-583X(96)00855-5
K. Miyoshi et al./Nucl. Instr. and Meth. in Phys. Res. B 127/ 128 (1997) 78-81
0
2
4
Reverse
6 8 Bias (V)
10
Fig. 1.Junction leakage current characteristics of n+/p diodes for Si+ unimplanted samples. Annealing was carried out by FA at 800°C for 30 min and RTA at 800°C for 10 s.
s by RTA. In the FA samples, small leakage current below 10 pA was observed, which was almost the same as that for the samples without Si+ implantation. The I-V curve for the RTA samples at a dose of 1 X 10 I3 cm-* is almost the same as the curve for the samples without Si+ implantation. On the other hand, at a dose of 1 x lOI cm-2, leakage current increased by two orders of magnitude. 3.2. Data retention characteristics
measurement
Fig. 2 shows the DRAM dam retention characteristics of samples annealed by FA and RTA. The data retention characteristics curves for the FA samples are almost the same as the curve for the unimplanted samples. In the case of the RTA samples at a dose of 1 X 1Ol4 cm-*, a11bits failed even at.a short retention time since junction leakage current was too high, as shown in Fig. 1. However, in spite of no difference in junction leakage measurement, retention time for the samples at a dose of 1 X lOI cm-* was shorter than that for the Si+ unimplanted samples. These
Fig. 3. Cross-sectional TEM micrograph of RTA samples implanted at a dose of I X 1014 cm-*.
results suggest the existence of residual defects in the RTA samples at a dose of 1 X lOI cm-*. 3.3. Defect evaluation 3.3.1. TEM observation In order to identify the cause of degradation of electrical properties, the residual defects induced by Si + implantation was investigated by TEM. In the RTA samples at a dose of 1 X lOI cm-*, a high defect density (0.7 X 10” cm-*) was observed. The size of defects were from 3 nm to 40 nm. These results correspond to the remarkable degradation of junction leakage and data retention characteristics. The XTEM bright field image and HRTEM image of defects are shown in Figs. 3 and 4, respectively. The high resolution micrograph, shown in Fig. 4, clearly shows (3 11) defect image [4]. 1311) defects are known to occur under condition of electron irradiation [5] and ion implantation [6]. These (3 11) defects are not formed due to electron irradiation during TEM observation but due to ion implantation, because operation voltage of TEM was 140 kV in order to suppress knock-on damage [7]. These results indicate that the remarkable degradation of junction leakage and data retention characteristics in RTA at a dose of I X lOI cm-’ occurred due to (311) defects induced . by Si+ Implantation. On the other hand, in the RTA
Retention Time (au) Fig. 2. DRAM data retention characteristics of same samples, as shown in Fig. 1.
Fig. 4. HRTEM image of Fig. 3. Defect exists on (311) habit plane.
1. FUNDAMENTALS/BASICS
80
k Miyoshi et al./Nucl. _
Instr. and Meth. in Phys. Res. B 127/128
(1997) 78-81
3x10’”
2 s
2x10”
s ‘F; !! E
1 x10’3
8 50 0 0
50
100 150 200 250 300
Temperature (K) Fig. 5. DLTS spectra from the RTA samples implanted with a dose of 1 X 10” and IX lOI cm-*.
samples at a dose of 1 X lOI cm-*, apparent defects can not be detected by TEM. 3.3.2. DLTS measurement Fig. 5 shows the DLTS spectra from the RTA samples implanted with a dose of 1 X 1013 and 1 X 1OL4cm-*, respectively. No DLTS peak can be observed from the samples without Si+ implantation and samples at a dose of 1 X lOI cm-*. However, at a dose of 1 X 1014 cm-‘, the spectrum is dominated by the H (0.48) level, which is determined from the temperature dependence of hole emission rate from the defects, as shown in Fig. 6. Though the relation between H (0.48) level and (311) defects are not dear at the present time, its energy level near mid-gap suggests that it would be the dominant recombination center in p-type Si. 3.3.3. PL measurement Fig. 7 shows the PL spectra of the unimplanted sample, as-implanted sample, FA sample and RTA sample at a dose of 1 X lOI cm-‘, respectively. The PL intensity itself is weak and there is no significant peak in the PL spectra of the unimplanted and FA samples. On the other hand, a broad emission band with some structures appears
0.5
0.6
0.7
Photon
Energy
0.6
0.9
( eV )
Fig. 7. PL spectra of the unimplanted sample, as-implanted, and RTA samples at a dose of 1 X lOI cm-‘, respectively.
FA
in the PL spectra of the as-implanted and RTA samples. Comparing the spectral changes of the different samples in Fig. 7, the broad emission band is caused by the defects induced by ion implantation and RTA at 800°C for 10 s is not enough to remove them. From the result which apparent defects cannot be detected by TEM in this sample, micro-defects such as point defect or its clusters might be the origin of the broad emission band. The broad emission band also indicates the existence of some deep-level defects, which act as radiative recombination centers. Considering these results, we suggest that DRAM data retention characteristics is affected by the micro-defects induced by ion implantation.
4. Conclusions 3’.
18OkeV. lE14
8CCC”-10sec
fRTA)
-9 3.6
4 1000/T
4.4 (1
IK)
Fig. 6. Arrhenius analysis of the hole emission rate for the peak observed in the RTA samples implanted at a dose of 1 X IO” Here, e is the hole emission rate and T is the sample cm-‘.
temperature.
The relationship between micro-defects and the electrical characteristics of MOS devices was investigated. In the RTA samples at a dose of 1 X lOI cm-*, both junction leakage characteristics and DATA retention characteristics degraded remarkably by the existence of {311} defects. On the other hand, in the RTA samples at a dose of 1 X lOI cm -*, degradation of data retention characteristics was observed although there was no difference in junction leakage current. In this case, the broad emission band which might be due to micro-defects such as point defect or its clusters was observed in the PL spectrum. These results indicate that DRAM data retention characteristics
K. Miyoshi et al./Nucl.
Instr. and Meth. in Phys. Res. B 127/128
can be affected by not only micro-defects such as (311) defects having a size below 50 run but also micro-defects such as point defect or its clusters. Therefore, it is important to suppress the micro-defects formation in future device fabrication.
Acknowledgements
The authors are grateful to Dr. M. Tajima (Institute of Space and Astronautical Science) for his help in the PL measurement and K. Matsui in the evaluation of DRAM data retention characteristics. The authors would like to thank Drs. 0. Mizuno, K. Okada and K. Ikeda for their encouragement.
(1997) 78-81
81
References [l] H. Mikoshiba, N. Nishio, T. Matsumoto, H. Kikuchi, T. Kitano and H. Kaneko, Defect Engineering in Semiconductor Growth and Device Technology (Material Research Society, 1992) p. 629. [2] Sun S. Kim and W. Wijaranakula, J. Electrochem. Sot. 141 (1994) 1872. 131T. Nina, T. Ohmi, Y. Ishihara, A. Okita, T. Shibata, J. Sugiura and N. Ohwada, J. Appl. Phys. 67 (1990) 7404. 141 S. Takeda, Japan. J. App. Phys. 30 (1991) L639. [5] I.G. Salisbury and M.H. Loretto, Philos. Mag. A 39 (1979) 317. [6] D.J. Eaglesham, P.A. Stalk, H.-J. Gossmann, T.E. Haynes and J.M. Poate, Nucl. Instr. and Meth. B 106 (1995) 191. [7] W.A. Mckinley and H. Feshbach, Phys. Rev. 74 (19481 1759.