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Soft error susceptibility mapping of DRAMs using a high-energy nuclear microprobe
Microelectronic Elsevier
Engineering
329
20 (1993) 329-338
Soft error susceptibility mapping of DRAMS using a high-energy nuclear microprobe H. Sayama”, H. Kimura’, Y. Ohno’, S. Satohb and M. Takai” “Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan bLSI Research and Development Laboratory, Mitsubishi Electric Corporation, Itami, Hyogo 664, Japan
Abstract.
Soft errors in 16Mbit dynamic random access memories (DRAMS) have been investigated using proton microprobes at 400 keV with a spot size of 1 x 1 urn’. The newly developed susceptibility mapping can reveal the correlation between the particle hitposition and the susceptibility to soft errors in a DRAM. The cell-mode soft-errors were found to take place by the incidence of ions within 6 pm around a monitored cell. These errors would be induced by minority carrier diffusion in a lateral direction. This result manifests the possibility of multiple-bit errors by the incidence of an energetic particle.
Keywords. Soft error; Dynamic random access memory; Proton microprobe; nuclear microprobe; Secondary electron image; Susceptibility mapping
High-energy
1. Introduction
The data stored in VLSI (very large scale integration) memories are lost by temporary upsets of bit-states, so called soft errors [l]. When a high-energy particle hits the substrate, soft errors are induced by a high number of electron-hole pairs created along the track of the particle. In particular, DRAMS with a small critical charge suffer from soft errors by a-particles emitted from the device package. Such soft errors are considered to be induced by three kinds of minority-carrier collections into a storage node: diffusion, drift and funneling [2]. A minority-carrier collection by funneling is induced only when an energetic particle hits p-n junctions. The created electron-hole pairs play the role of dipoles with the inverse field by the applied reverse bias, and hence the depletion layer cylindrically spreads along the particle track to maintain the applied potential. Moreover, the soft errors are considered to be classified into two error modes by the incident position of the energetic particle. Cell-mode soft errors are induced by the incidence of the energetic particle onto a memory cell capacitance, and a bit-line mode soft-error [3] by that onto a bit-line contact. 0167-9317/93/$06.00
(Q 1993 Elsevier Science Publishers
B.V. All rights reserved
330
H. Sayama et al. I Soft error susceptibility mapping
Hiokazu Sayama was born in Hyogo, Japan, on November 27, 1965. He received the BE and ME degrees in electrical engineering from Osaka University, Osaka, Japan, in 1988 and 1990, respectively. He is currently working toward the DrE at the same university, where he specializes in the study of evaluation of soft errors in a DRAM using nuclear microprobes and high-energy ion implantation into silicon. Mr. Sayama is a member of the Japan Society of Applied Physics.
Hiroshi Kimura was born in Hokkaido, Japan, in 1963. He received the BS degree in nuclear engineering from Hokkaido University, Sapporo, Japan, in 1987. In 1987 he joined the LSI Laboratory, Mitsubishi Electric Corporation, Hyogo, Japan. Since then he has been engaged in the research and development of DRAM process technology.
Yoshlkaxu Ohno was born in Osaka, Japan. He received the BS and MS degrees in material science from Osaka University, Osaka, Japan, in 1980 and 1982, respectively. He joined the LSI R&D Laboratory, Mitsubishi Electric Corporation, Hyogo, Japan, in 1982 and has been engaged in research on oxidation and diffusion process technology for MOS LSI. His current interest is in the physical mechanism of SiO, dielectric breakdown. Mr. Ohno is a member of the Japan Society of Applied Physics.
Shin-ichi Satoh was born in Gifu, Japan, on November 12, 1948. He received the BS degree in chemistry from Kobe University, Kobe, Japan, in 1972, and the PhD degree from Osaka University, Osaka, Japan, in 1985. He joined the Mitsubishi Electric Corporation, Tokyo, Japan, in 1972, and has been engaged in the development of process technologies. In 1977 he joined the Mitsubishi LSI Research and Development Laboratory, Hyogo, Japan, and is currently working on the development of device technologies of VLSI memories. Dr. Satoh is a member of the Japan Society of Applied Physics. Mikio Takai was born in Gifu, Japan, on February 26, 1949. He received the BE, ME, and DrE degrees in electrical engineering from Osaka University, Osaka, Japan, in 1971, 1973 and 1976, respectively. From 1977 to 1980, he was with the Fraunhofer Institute for Solid State Technology in Munich as a fellowship holder of the Alexander von Humboldt fo,undation (1977-1978) and a research fellow (1979-1980), where he worked on ion implantation in Si and compound semiconductors, minority-carrier lifetime, laser annealing and ion-beam analysis. In 1980 he joined the faculty of engineering science, Osaka University in Osaka, Japan, where he is engaged in research on beam-solid interactions, beam-induced chemical processing of semiconductors, crystal growth with semiconductors-oninsulator structures and ion-beam microanalysis, In 1987 and 1988, he stayed at University Erlangen-Niirnberg as a visiting professor. Since 1989 he is an associate professor of Osaka University. His current interest is on beam processing of materials, nanofabrication, nuclear microprobe analysis of semiconductor and its application to soft-error evaluation. He has published more than 150 papers. Dr. Takai is a member of the Japan Society of Applied Physics and the Material Research Society.
H. Sayama et al. I Soft error susceptibility mapping
331
Susceptibility to soft errors in DRAMS has been conventionally estimated by an a-particle source such as 241Am located over the DRAM. Two kinds of soft-error modes are distinguished by changes of cycle times of DRAM operations in this method. In some cases, DRAMS are irradiated with accelerated ion beams with a big spot-size [4]. Neither method can reveal locally susceptible positions in DRAMS against incident particles, since the incident position and angle of the particles on the DRAM cannot be identified. On the other hand, a local position of the elements in DRAMS can be irradiated with a high-energy nuclear microprobe, i.e., ion microprobe, which has been applied to three-dimensional structural analysis by Rutherford backscattering [5]. The ion microprobe technique was once adopted as an analytical method for the local susceptibility to soft errors in 16 kbit DRAMS by Campbell et al. [6]. They investigated the upset in the DRAMS using He+ microprobes collimated by a pinhole slit with a 2.5 km diameter. Geppert et al. also investigated 512 kbit DRAMS using microprobes [7]. However, both old-fashioned microprobe systems could not provide the exact incident position of ion beams, and hence the exact correlation between the incident position and upset could not be obtained. Recently, ion microprobes with a spot size of about 1 x 1 km2, almost as small as memory cell sizes of Mbit DRAMS, have been developed with precise quadrupole lenses. Such small ion microprobes, which can provide mapping images by a raster scan such as a secondary electron image, have been applied to soft-error evaluation in 16 kbit static-RAMS (SRAMs) by Horn et al. [8]. The operations of SRAMs are simpler than those of DRAMS, since the bit-states in a SRAM are kept without refresh operations and are slowly read out after irradiation by microprobes. On the contrary, DRAMS require refresh operations to keep the bit-state, and rapid reading-out is necessary. Our previous paper reported that a bit-state mapping could successfully reveal the change of the bit-state of the memory cell in a DRAM induced by nuclear microprobes [9]. It is experimentally proved that two soft-error modes exist according to the incident position of an energetic particle. In this study, a susceptibility mapping method, an improved bit-state mapping one, has newly been developed for Mbit DRAMS.
2. Experimental procedures Soft errors in a DRAM are investigated with the analytical system shown in Fig. 1. A proton microprobe at 400 keV from a 500 keV Disktron Accelerator [5] is selected as a probe, since the penetration depth of protons is longer than that of helium ions. The 400 keV protons have a projected range of 3.7 pm and the standard deviation in silicon is 0.17 pm [lo]. On the assumption that the lateral spread of the proton microprobe is equal to the standard deviation, it can be ignored since it is much smaller than the probe spot size of a few microns. The microprobe energy, flux, angle and position can be easily changed. Only one particular memory cell in the DRAM is operated by the word generator, since a huge apparatus is required if all the memory cells are
332
H. Sayama et al. I Soft error susceptibility mapping
v
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I
Fig. 1. Schematic view of the analytical system for soft error immunity in a DRAM using a nuclear microprobe. Microprobes are raster-scanned over the DRAM mounted on a 5-axis goniometer.
operated. The beam-scanning and acquisition of the output data are controlled by a microcomputer. Two mapping images are necessary for the investigation of soft errors using microprobes. One is a secondary electron image, consisting of probe-induced secondary-electron yields and probe-coordinates, to show the exact probe position in the irradiated area. Another is a bit-state mapping consisting of the bit-states of the monitored cell and probe-coordinates [9]. The “1” bit-state is repeatedly written and read in the monitored cell in every 1 ps (cycle time). Figure 2 shows the photograph of the outputs of the bit-state in the memory cell. Both bit-states can be observed in the photograph although the “1” bit-state is written in the monitored cell. The “0” bit-state is induced by soft errors. The pulses of the raw address strobe are also shown. The distortion of the output pulse is caused by the connection to a counter unit with a heavy load. The bit-state mapping, developed in our previous study [9], indicated at
Fig. 2. Output pulses (D,,,) of the monitored memory cell in the DRAM. Both bit states are observed due to the occurrence of soft errors. Row address strobe (RAS) signals are also indicated.
H. Sayama et al. / Soft error susceptibility mapping
333
least one change of the bit-state of the monitored cell, i.e., from “1” to “O”, during beam dwelling (10 ms). A further improvement in soft-error information, i.e., susceptibility mapping, has been newly introduced in this study. Only the pulses with “1” bit-state of outputs are counted during beam dwelling in each irradiated pixel of 64 x 64. The output counts of the monitored cell in each pixel become 10000 if no soft error takes place. In short, the reduction of the counts in each pixel indicates susceptibility to soft errors of the monitored cell in which the microprobes are irradiated. The location of positions in the DRAM that are susceptible to soft errors can be obtained by an overlap of the susceptibility mapping, the secondary electron image, and the pattern of Al wires around the monitored cell. A test element group (TEG) chip for 16 Mbit DRAM with a minimum feature size of 0.5 pm was used as a sample. The measured chip consists of stacked capacitor cells and gate regions with a length of 0.7 pm in a p-type substrate. The following biases were applied: 5 V to the supply (V,,), -3 V to the substrate, and 2.5 V to the capacitance. The passivation layer of the chip was removed so that clear probe-induced secondary electron images could be obtained. In this study, proton microprobes at an energy of 400 keV with a spot size of 1 X 1 pm2 were used for mapping of susceptibility to soft errors in the DRAM. The probe current is 25 pA. The proton microprobes are irradiated normal to the DRAM, and stop at a depth of about 1.2 km beneath the surface of the substrate after passing through the overlaid layers of 2.8 km on the substrate.
3. Results and discussion The energy of 400 keV protons decreases to about 140 keV when the protons reach the substrate. The microprobe with a current of 25 pA supplies about eight protons in the interval of 350 ns between the end of a writing process and the beginning of a reading one. The electron-hole pairs, created by the eight protons in this interval, are almost equivalent to the critical charge of the memory cell of 57 fC, corresponding to about 360,000 electrons, assuming that an electron-hole pair is created by 3.6 eV. Thus the proton microprobe can induce a soft error in the DRAM. Soft errors in DRAMS are induced by an a-particle (doubly-charged helium ion) at about 5 MeV. Energy transport of ions should be discussed when soft errors in DRAMS induced by a-particles are simulated using 400 keV protons. The energy transfer of incident ions to a target is referred to as stopping power or linear energy transfer (LET). Figure 3 shows the LETS of He+ and protons in silicon derived from the ZBL (Ziegler-Biersack-Littmark) equation based on the Bethe-Bloch stopping power formula [ll]. The LET of a probe proton comes close to the maximum value near the Bragg peak when the proton reaches the substrate, and is as high as that of He+ at 5 MeV. However, He+ transfers more energy to the target silicon before it stops at a depth of about
334
H. Sayama et al. I Soft error susceptibility mapping 40 -
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Fig. 3. Linear energy transfers as a function of energy for proton and helium ions in Si.
20 pm beneath the surface. The minority carriers, created by He’ at 5 MeV in a region deeper than about 1.2 km, cannot be simulated by a proton at 400 keV. Consequently, eight protons, supplied by a microprobe current of 25 pA in an interval of 350 ns, create about eight times as many electron-hole pairs as an a-particle emitted from packages in the region to a depth of 1.2 km beneath the surface, but they create no pairs in deeper regions. Figure 4(a) shows the susceptibility mapping around the monitored cell in the DRAM. The density of a pixel indicates the susceptibility to soft errors of the monitored cell when the microprobe irradiates a corresponding pixel. A dark mark reveals a high susceptibility of the monitored cell, and a white one means no soft error. Figure 4(b) shows the secondary electron image obtained in the same irradiated area. Al wiring patterns at the top layer can be observed to show the location of the monitored cell. Figure 5(a) shows the overlapped images of the susceptibility mapping, the secondary electron image, and Al wiring patterns. Rectangles correspond to each of the memory cells, and the solid rectangle at the corner of the memory cell array indicates the monitored cell. Figure 5(b) shows the enlarged susceptibility mapping around the monitored cell in Fig. 5(a). The incidence of ions within 6 km around the monitored cell contributes to cell-mode soft errors though the probe beam has a spot size of 1 x 1 km*, whereas the susceptible area is anisotropically located around the center of the monitored cell. No soft errors occur at the upper side of the monitored cell in Fig. 4(a). This is presumably due to additional Al wiring with a thickness of about 1.0 km, which prevents protons from reaching the substrate. Moreover, the pixels that are most susceptible to soft errors by 400 keV protons are not located at the monitored cell itself, but at positions nearby the monitored cell. Further investigation is necessary to reveal the inhomogeneous distribution of the susceptibility around the monitored cell. In any case, the area susceptible to soft errors is much larger than the memory cell area. This fact suggests the existence of a predominant chargecollection mechanism among drift, funneling and diffusion. The depletion layer
H. Sayama et al. I Soft error susceptibility mapping
335
I
Fig. 4. (a) Susceptibility mapping image around the monitored cell. (b) Probe-induced secondary electron image at the same irradiated area. The DRAM was irradiated by proton microprobes at 400 keV with a current of 25 pA. Counts in (a) indicate the frequency of normal output “ I” showing no soft error during beam dwelling of 10 ms.
336
H. Sayama et al. I Soft error susceptibility mapping
0
O(
Monitored
ceil
/
i .76 pm
LOW HIGH -
Fig.
SUSCEPTIBILITY COUNTS
+ HIGH LOW
5. (a) Overlapped image of susceptibility mapping, secondary pattern of Al wires of the DRAM. (b) Enlarged susceptibility
electron mapping
image and image.
the
H. Sayama et al. / Soft error susceptibility mapping
337
thickness of the monitored cell can be ignored, since it is not wider than 0.2 pm in lateral direction. In short, the mechanisms of funneling and drift contribute to minority-carrier collection only within the monitored cell. The minority carriers are collected by diffusion within about 6 brn outside the monitored cell. This result predicts the possibility of simultaneous soft errors in other cells, i.e., multiple-bit errors around the place where an energetic particle hits a device in a normal full cell operation.
4. Conclusions The susceptibility to soft errors in a TEG chip for 16Mbit DRAMS was experimentally measured using a 400 keV proton microprobe with a spot size of 1 X 1 pm”. The susceptibility mapping in addition to bit-state mapping has newly been developed. The susceptibility mapping can show the correlation between particle hit-position and susceptibility to upset. Cell-mode soft errors are induced by the incidence of ions within 6 pm around a cell. This fact indicates that minority carriers, created by a proton microprobe at 400 keV within about 6 p,rn around the monitored cell, are collected by carrier diffusion. Minority-carrier collection within a wide area predicts that soft errors in some memory cells, i.e., multiple-bits errors, would be induced if an energetic particle hits the place where memory cells are crowded.
Acknowledgments
This work was partly supported by’ the System of Joint Research with Industry in 1992 (the Ministry Education, Science and Culture and LSI R&D Laboratory, Mitsubishi Electric Corporation). The authors would like to thank H. Fujikawa, S. Hara, H. Andoh, K. Mino and K. Kawasaki for their help during experimental works.
References [l] T.C. May and M.H. Woods, IEEE Trans. Electron Devices ED-26 (1979) 2. [2] C.M. Hsieh, P.C. Murley and R.R. O’Brien, IEEE Electron Device Lett. EDL-2 (1981) 103. [3] D.S. Yaney, J.T. Nelson and L.L. Vanskike, IEEE Trans. Electron Devices ED-26 (1979) [4] ?A. Zoutendyk, L.D. Edmonds and L.S. Smith, IEEE Trans. Nucl. Sci. NS-36 (1989) 2267. [S] M. Takai, Znt. J. PZXE 2 (1992) 107. [6] A.B. Campbell, A.R. Knudson and E.A. Wolicki, Nucl. Znstrum. Methods 191 (1981) 437. [7] L.M. Geppert, U. Bapst, D.F. Heidl and K.A. Jenkins, IEEE Solid State Circuits 26 (1991) 132. [8] K.M. Horn, B.L. Doyle and F.W. Sexton, IEEE Trans. Nucl. Sci. NS-39 (1992) 7.
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[9] M. Takai, H. Sayama, (1993) 344.
H. Kimura,
W.S. Johnson and S.W. Mylorie, Projected Range Statistics - Semiconductors Hutchingson & Ross, Stroudsburg, 1975. J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985.
[lo] J.F. Gibbons,
and Related Materials, Dowden,
[ll]
Y. Ohno and S. Satoh, Nucl. Znstrum. Methods B 77
Engineering
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20 (1993) 329-338
Soft error susceptibility mapping of DRAMS using a high-energy nuclear microprobe H. Sayama”, H. Kimura’, Y. Ohno’, S. Satohb and M. Takai” “Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan bLSI Research and Development Laboratory, Mitsubishi Electric Corporation, Itami, Hyogo 664, Japan
Abstract.
Soft errors in 16Mbit dynamic random access memories (DRAMS) have been investigated using proton microprobes at 400 keV with a spot size of 1 x 1 urn’. The newly developed susceptibility mapping can reveal the correlation between the particle hitposition and the susceptibility to soft errors in a DRAM. The cell-mode soft-errors were found to take place by the incidence of ions within 6 pm around a monitored cell. These errors would be induced by minority carrier diffusion in a lateral direction. This result manifests the possibility of multiple-bit errors by the incidence of an energetic particle.
Keywords. Soft error; Dynamic random access memory; Proton microprobe; nuclear microprobe; Secondary electron image; Susceptibility mapping
High-energy
1. Introduction
The data stored in VLSI (very large scale integration) memories are lost by temporary upsets of bit-states, so called soft errors [l]. When a high-energy particle hits the substrate, soft errors are induced by a high number of electron-hole pairs created along the track of the particle. In particular, DRAMS with a small critical charge suffer from soft errors by a-particles emitted from the device package. Such soft errors are considered to be induced by three kinds of minority-carrier collections into a storage node: diffusion, drift and funneling [2]. A minority-carrier collection by funneling is induced only when an energetic particle hits p-n junctions. The created electron-hole pairs play the role of dipoles with the inverse field by the applied reverse bias, and hence the depletion layer cylindrically spreads along the particle track to maintain the applied potential. Moreover, the soft errors are considered to be classified into two error modes by the incident position of the energetic particle. Cell-mode soft errors are induced by the incidence of the energetic particle onto a memory cell capacitance, and a bit-line mode soft-error [3] by that onto a bit-line contact. 0167-9317/93/$06.00
(Q 1993 Elsevier Science Publishers
B.V. All rights reserved
330
H. Sayama et al. I Soft error susceptibility mapping
Hiokazu Sayama was born in Hyogo, Japan, on November 27, 1965. He received the BE and ME degrees in electrical engineering from Osaka University, Osaka, Japan, in 1988 and 1990, respectively. He is currently working toward the DrE at the same university, where he specializes in the study of evaluation of soft errors in a DRAM using nuclear microprobes and high-energy ion implantation into silicon. Mr. Sayama is a member of the Japan Society of Applied Physics.
Hiroshi Kimura was born in Hokkaido, Japan, in 1963. He received the BS degree in nuclear engineering from Hokkaido University, Sapporo, Japan, in 1987. In 1987 he joined the LSI Laboratory, Mitsubishi Electric Corporation, Hyogo, Japan. Since then he has been engaged in the research and development of DRAM process technology.
Yoshlkaxu Ohno was born in Osaka, Japan. He received the BS and MS degrees in material science from Osaka University, Osaka, Japan, in 1980 and 1982, respectively. He joined the LSI R&D Laboratory, Mitsubishi Electric Corporation, Hyogo, Japan, in 1982 and has been engaged in research on oxidation and diffusion process technology for MOS LSI. His current interest is in the physical mechanism of SiO, dielectric breakdown. Mr. Ohno is a member of the Japan Society of Applied Physics.
Shin-ichi Satoh was born in Gifu, Japan, on November 12, 1948. He received the BS degree in chemistry from Kobe University, Kobe, Japan, in 1972, and the PhD degree from Osaka University, Osaka, Japan, in 1985. He joined the Mitsubishi Electric Corporation, Tokyo, Japan, in 1972, and has been engaged in the development of process technologies. In 1977 he joined the Mitsubishi LSI Research and Development Laboratory, Hyogo, Japan, and is currently working on the development of device technologies of VLSI memories. Dr. Satoh is a member of the Japan Society of Applied Physics. Mikio Takai was born in Gifu, Japan, on February 26, 1949. He received the BE, ME, and DrE degrees in electrical engineering from Osaka University, Osaka, Japan, in 1971, 1973 and 1976, respectively. From 1977 to 1980, he was with the Fraunhofer Institute for Solid State Technology in Munich as a fellowship holder of the Alexander von Humboldt fo,undation (1977-1978) and a research fellow (1979-1980), where he worked on ion implantation in Si and compound semiconductors, minority-carrier lifetime, laser annealing and ion-beam analysis. In 1980 he joined the faculty of engineering science, Osaka University in Osaka, Japan, where he is engaged in research on beam-solid interactions, beam-induced chemical processing of semiconductors, crystal growth with semiconductors-oninsulator structures and ion-beam microanalysis, In 1987 and 1988, he stayed at University Erlangen-Niirnberg as a visiting professor. Since 1989 he is an associate professor of Osaka University. His current interest is on beam processing of materials, nanofabrication, nuclear microprobe analysis of semiconductor and its application to soft-error evaluation. He has published more than 150 papers. Dr. Takai is a member of the Japan Society of Applied Physics and the Material Research Society.
H. Sayama et al. I Soft error susceptibility mapping
331
Susceptibility to soft errors in DRAMS has been conventionally estimated by an a-particle source such as 241Am located over the DRAM. Two kinds of soft-error modes are distinguished by changes of cycle times of DRAM operations in this method. In some cases, DRAMS are irradiated with accelerated ion beams with a big spot-size [4]. Neither method can reveal locally susceptible positions in DRAMS against incident particles, since the incident position and angle of the particles on the DRAM cannot be identified. On the other hand, a local position of the elements in DRAMS can be irradiated with a high-energy nuclear microprobe, i.e., ion microprobe, which has been applied to three-dimensional structural analysis by Rutherford backscattering [5]. The ion microprobe technique was once adopted as an analytical method for the local susceptibility to soft errors in 16 kbit DRAMS by Campbell et al. [6]. They investigated the upset in the DRAMS using He+ microprobes collimated by a pinhole slit with a 2.5 km diameter. Geppert et al. also investigated 512 kbit DRAMS using microprobes [7]. However, both old-fashioned microprobe systems could not provide the exact incident position of ion beams, and hence the exact correlation between the incident position and upset could not be obtained. Recently, ion microprobes with a spot size of about 1 x 1 km2, almost as small as memory cell sizes of Mbit DRAMS, have been developed with precise quadrupole lenses. Such small ion microprobes, which can provide mapping images by a raster scan such as a secondary electron image, have been applied to soft-error evaluation in 16 kbit static-RAMS (SRAMs) by Horn et al. [8]. The operations of SRAMs are simpler than those of DRAMS, since the bit-states in a SRAM are kept without refresh operations and are slowly read out after irradiation by microprobes. On the contrary, DRAMS require refresh operations to keep the bit-state, and rapid reading-out is necessary. Our previous paper reported that a bit-state mapping could successfully reveal the change of the bit-state of the memory cell in a DRAM induced by nuclear microprobes [9]. It is experimentally proved that two soft-error modes exist according to the incident position of an energetic particle. In this study, a susceptibility mapping method, an improved bit-state mapping one, has newly been developed for Mbit DRAMS.
2. Experimental procedures Soft errors in a DRAM are investigated with the analytical system shown in Fig. 1. A proton microprobe at 400 keV from a 500 keV Disktron Accelerator [5] is selected as a probe, since the penetration depth of protons is longer than that of helium ions. The 400 keV protons have a projected range of 3.7 pm and the standard deviation in silicon is 0.17 pm [lo]. On the assumption that the lateral spread of the proton microprobe is equal to the standard deviation, it can be ignored since it is much smaller than the probe spot size of a few microns. The microprobe energy, flux, angle and position can be easily changed. Only one particular memory cell in the DRAM is operated by the word generator, since a huge apparatus is required if all the memory cells are
332
H. Sayama et al. I Soft error susceptibility mapping
v
Dynamic-RAM
I
I
Fig. 1. Schematic view of the analytical system for soft error immunity in a DRAM using a nuclear microprobe. Microprobes are raster-scanned over the DRAM mounted on a 5-axis goniometer.
operated. The beam-scanning and acquisition of the output data are controlled by a microcomputer. Two mapping images are necessary for the investigation of soft errors using microprobes. One is a secondary electron image, consisting of probe-induced secondary-electron yields and probe-coordinates, to show the exact probe position in the irradiated area. Another is a bit-state mapping consisting of the bit-states of the monitored cell and probe-coordinates [9]. The “1” bit-state is repeatedly written and read in the monitored cell in every 1 ps (cycle time). Figure 2 shows the photograph of the outputs of the bit-state in the memory cell. Both bit-states can be observed in the photograph although the “1” bit-state is written in the monitored cell. The “0” bit-state is induced by soft errors. The pulses of the raw address strobe are also shown. The distortion of the output pulse is caused by the connection to a counter unit with a heavy load. The bit-state mapping, developed in our previous study [9], indicated at
Fig. 2. Output pulses (D,,,) of the monitored memory cell in the DRAM. Both bit states are observed due to the occurrence of soft errors. Row address strobe (RAS) signals are also indicated.
H. Sayama et al. / Soft error susceptibility mapping
333
least one change of the bit-state of the monitored cell, i.e., from “1” to “O”, during beam dwelling (10 ms). A further improvement in soft-error information, i.e., susceptibility mapping, has been newly introduced in this study. Only the pulses with “1” bit-state of outputs are counted during beam dwelling in each irradiated pixel of 64 x 64. The output counts of the monitored cell in each pixel become 10000 if no soft error takes place. In short, the reduction of the counts in each pixel indicates susceptibility to soft errors of the monitored cell in which the microprobes are irradiated. The location of positions in the DRAM that are susceptible to soft errors can be obtained by an overlap of the susceptibility mapping, the secondary electron image, and the pattern of Al wires around the monitored cell. A test element group (TEG) chip for 16 Mbit DRAM with a minimum feature size of 0.5 pm was used as a sample. The measured chip consists of stacked capacitor cells and gate regions with a length of 0.7 pm in a p-type substrate. The following biases were applied: 5 V to the supply (V,,), -3 V to the substrate, and 2.5 V to the capacitance. The passivation layer of the chip was removed so that clear probe-induced secondary electron images could be obtained. In this study, proton microprobes at an energy of 400 keV with a spot size of 1 X 1 pm2 were used for mapping of susceptibility to soft errors in the DRAM. The probe current is 25 pA. The proton microprobes are irradiated normal to the DRAM, and stop at a depth of about 1.2 km beneath the surface of the substrate after passing through the overlaid layers of 2.8 km on the substrate.
3. Results and discussion The energy of 400 keV protons decreases to about 140 keV when the protons reach the substrate. The microprobe with a current of 25 pA supplies about eight protons in the interval of 350 ns between the end of a writing process and the beginning of a reading one. The electron-hole pairs, created by the eight protons in this interval, are almost equivalent to the critical charge of the memory cell of 57 fC, corresponding to about 360,000 electrons, assuming that an electron-hole pair is created by 3.6 eV. Thus the proton microprobe can induce a soft error in the DRAM. Soft errors in DRAMS are induced by an a-particle (doubly-charged helium ion) at about 5 MeV. Energy transport of ions should be discussed when soft errors in DRAMS induced by a-particles are simulated using 400 keV protons. The energy transfer of incident ions to a target is referred to as stopping power or linear energy transfer (LET). Figure 3 shows the LETS of He+ and protons in silicon derived from the ZBL (Ziegler-Biersack-Littmark) equation based on the Bethe-Bloch stopping power formula [ll]. The LET of a probe proton comes close to the maximum value near the Bragg peak when the proton reaches the substrate, and is as high as that of He+ at 5 MeV. However, He+ transfers more energy to the target silicon before it stops at a depth of about
334
H. Sayama et al. I Soft error susceptibility mapping 40 -
r’\
/
30;
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‘.
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, :
‘\
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‘\
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I
201
I
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H In SI HeIn 51
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--__ --__--__ --_.
zi 2
3
4
5
ENERGY (WV)
Fig. 3. Linear energy transfers as a function of energy for proton and helium ions in Si.
20 pm beneath the surface. The minority carriers, created by He’ at 5 MeV in a region deeper than about 1.2 km, cannot be simulated by a proton at 400 keV. Consequently, eight protons, supplied by a microprobe current of 25 pA in an interval of 350 ns, create about eight times as many electron-hole pairs as an a-particle emitted from packages in the region to a depth of 1.2 km beneath the surface, but they create no pairs in deeper regions. Figure 4(a) shows the susceptibility mapping around the monitored cell in the DRAM. The density of a pixel indicates the susceptibility to soft errors of the monitored cell when the microprobe irradiates a corresponding pixel. A dark mark reveals a high susceptibility of the monitored cell, and a white one means no soft error. Figure 4(b) shows the secondary electron image obtained in the same irradiated area. Al wiring patterns at the top layer can be observed to show the location of the monitored cell. Figure 5(a) shows the overlapped images of the susceptibility mapping, the secondary electron image, and Al wiring patterns. Rectangles correspond to each of the memory cells, and the solid rectangle at the corner of the memory cell array indicates the monitored cell. Figure 5(b) shows the enlarged susceptibility mapping around the monitored cell in Fig. 5(a). The incidence of ions within 6 km around the monitored cell contributes to cell-mode soft errors though the probe beam has a spot size of 1 x 1 km*, whereas the susceptible area is anisotropically located around the center of the monitored cell. No soft errors occur at the upper side of the monitored cell in Fig. 4(a). This is presumably due to additional Al wiring with a thickness of about 1.0 km, which prevents protons from reaching the substrate. Moreover, the pixels that are most susceptible to soft errors by 400 keV protons are not located at the monitored cell itself, but at positions nearby the monitored cell. Further investigation is necessary to reveal the inhomogeneous distribution of the susceptibility around the monitored cell. In any case, the area susceptible to soft errors is much larger than the memory cell area. This fact suggests the existence of a predominant chargecollection mechanism among drift, funneling and diffusion. The depletion layer
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I
Fig. 4. (a) Susceptibility mapping image around the monitored cell. (b) Probe-induced secondary electron image at the same irradiated area. The DRAM was irradiated by proton microprobes at 400 keV with a current of 25 pA. Counts in (a) indicate the frequency of normal output “ I” showing no soft error during beam dwelling of 10 ms.
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H. Sayama et al. I Soft error susceptibility mapping
0
O(
Monitored
ceil
/
i .76 pm
LOW HIGH -
Fig.
SUSCEPTIBILITY COUNTS
+ HIGH LOW
5. (a) Overlapped image of susceptibility mapping, secondary pattern of Al wires of the DRAM. (b) Enlarged susceptibility
electron mapping
image and image.
the
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thickness of the monitored cell can be ignored, since it is not wider than 0.2 pm in lateral direction. In short, the mechanisms of funneling and drift contribute to minority-carrier collection only within the monitored cell. The minority carriers are collected by diffusion within about 6 brn outside the monitored cell. This result predicts the possibility of simultaneous soft errors in other cells, i.e., multiple-bit errors around the place where an energetic particle hits a device in a normal full cell operation.
4. Conclusions The susceptibility to soft errors in a TEG chip for 16Mbit DRAMS was experimentally measured using a 400 keV proton microprobe with a spot size of 1 X 1 pm”. The susceptibility mapping in addition to bit-state mapping has newly been developed. The susceptibility mapping can show the correlation between particle hit-position and susceptibility to upset. Cell-mode soft errors are induced by the incidence of ions within 6 pm around a cell. This fact indicates that minority carriers, created by a proton microprobe at 400 keV within about 6 p,rn around the monitored cell, are collected by carrier diffusion. Minority-carrier collection within a wide area predicts that soft errors in some memory cells, i.e., multiple-bits errors, would be induced if an energetic particle hits the place where memory cells are crowded.
Acknowledgments
This work was partly supported by’ the System of Joint Research with Industry in 1992 (the Ministry Education, Science and Culture and LSI R&D Laboratory, Mitsubishi Electric Corporation). The authors would like to thank H. Fujikawa, S. Hara, H. Andoh, K. Mino and K. Kawasaki for their help during experimental works.
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