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Local Susceptibility Against Soft Errors in Dynamic Random Access Memories (Drams) Analyzed by Nuclear Microprobes

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Local Susceptibility Against Soft Errors in Dynamic Random Access Memories (Drams) Analyzed by Nuclear Microprobes Scanning Microscopy Volume 7 Number 3 Article 7 9-20-1993 Local Susceptibility Against Soft Errors in Dynamic Random Access Memories (DRAMs) Analyzed by Nuclear Microprobes H. Sayama Osaka University M. Takai Osaka University H. Kimura Mitsubishi Electric Corporation Y. Ohno Mitsubishi Electric Corporation S. Satoh Mitsubishi Electric Corporation Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Biology Commons Recommended Citation Sayama, H.; Takai, M.; Kimura, H.; Ohno, Y.; and Satoh, S. (1993) "Local Susceptibility Against Soft Errors in Dynamic Random Access Memories (DRAMs) Analyzed by Nuclear Microprobes," Scanning Microscopy: Vol. 7 : No. 3 , Article 7. Available at: https://digitalcommons.usu.edu/microscopy/vol7/iss3/7 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Scanning Microscopy, Vol. 7, No. 3, 1993 (Pages 825-835) 0891- 7035/93$5 .00 + .00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA LOCAL SUSCEPTIBILITY AGAINST SOFT ERRORS IN DYNAMIC RANDOM ACCESS MEMORIES (DRAMs) ANALYZED BY NUCLEAR MICROPROBES H. Sayama1 M. Takai1*, H. Kimura2, Y. Ohno2 and S. Satoh2 1 Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan 2 LSI Research and Development Laboratory, Mitsubishi Electric Corporation, Itami, Hyogo 664, Japan (Received for publication May 8, 1993, and in revised form September 20, 1993) Abstract Introduction A novel evaluation technique for soft errors in Trends in DRAM Mbit DRAMs (dynamic random access memories) Soft errors are recognized as a serious problem of has been developed using a 400 ke V proton micro­ future memory devices. For instance, memory cells probe system. This technique, which is called soft er­ in a dynamic-RAM (DRAM) are further shrunk and ror mapping, consists of a bit-state mapping image integrated with scaling laws [4]. As the metal-oxide­ and a secondary electron mapping image, and can re­ silicon (MOS) capacitor, a significant part of a veal the correlation between the incident position of memory cell in DRAMs, is also shrunk, the stored protons and susceptibility against soft errors in charge in the capacitor representing a bit-state is dras­ DRAMs. Soft errors are found to be induced by pro­ tically decreased. The memory capacitor with de­ ton incidence at 400 ke V within about 6 µm around creased stored charges becomes much more sensitive the memory cell in the case of DRAMs with a conven­ to external electronic noise, such as hot carriers [1], tional well. The susceptible area against proton inci­ alpha-particle irradiation emitted from device pack­ dence is much larger than the memory cell size. It is ages and wiring [12], and cosmic rays [14]. In the found that the area within 4 µm around the memory conventional environment, soft errors in DRAMs are cell is, in particular, highly sensitive to 400 keV pro­ caused by alpha-particles at an energy of less than 5 tons. A threshold dose to radiation hardness is esti­ Me V that create 1.4 x 106 electrons more than the mated by deterioration of the DRAMs during soft er­ stored charges in the dynamic memory cell. Some 3- ror mapping. A buried barrier layer, formed by high­ dimensional capacitor structures were invented to energy ion-implantation, was found to control the keep constant stored charges against the scaling law, charge collection of induced carriers and to suppress for examples, the trench capacitor [22] and the soft errors by 400 keV proton microprobes. stacked one [11]. Moreover, charge collection can be controlled by modifying the substrate, as in the case of silicon-on-insulator (SOI) structw-es [21], silicon­ implanted-oxygen (SIMOX) wafers [10], epitaxial wafers [9] and buried banier-layers formed by high­ Key Words: Dynamic random access memory, soft energy ion-implantation [24]. However, the adoption error, single event upset, nuclear microprobe, soft er­ of lower bias voltages makes memory cells more sen­ ror mapping, secondary electron mapping, radiation sitive in spite of such new structures. hardness, buried barrier layer, retrograde well, high­ energy ion-implantation. Soft error mechanism in DRAM Two modes of soft errors in DRAMs are consid­ ered to be distinguished by changing cycle times for * Address for correspondence: the operation; a cell mode [12] and a bit-line mode Mikio Takai [25]. Figure 1 shows the cell mode soft error which Faculty of Engineering Science, Osaka University, means the loss of the bit-state in a memory cell. The Toyonaka, Osaka 560, Japan electrons created by an alpha-pa11iclealong its track Phone: +81-6-844-1151 are collected in a storage capacitor. The bit-state is Fax: +81-6-857-7664 preserved in the case of the initial bit-state 'O' corre- 825 H. Sayama et al. Val {OV) Val (5V) Vcp (5V) -sub Depletion Layer V SUB (-3V) ! ! Energetic Energetic Particle Particle eeeee EB EB eeeee eeeee (a) (b) Figure 1 Dynamics of cell-mode soft errors in p-type Si induced by an energetic particle. The initial bit-state is 'O' in (a) and '1' in (b). Cell mode soft errors take place only in (b). Vslffi, VwL, VnL and Yer mean biases applied to substrate, word-line, bit-line and capacitor, respectively. 826 Local Susceptibility against Soft Errors in DRAMs sponding to the capacitor filled with electrons (Fig. la). On the contrary, the upset state '0' is produced by Radiation the collection of many electrons in the initial bit-state Source '1' corresponding to the capacitor without electrons (Fig. lb). ' .I I On the other hand, bit-line mode soft errors are ' Vacuum ' considered to be induced by the collection of elec­ 'I 'o Chamber ' ' trons to bit-line contacts in a floating time during a .'' ' ,': ~ read operation because bit-line mode errors are in­ ' ' : ' versely proportional to cycle times. All the two types / i of upsets, 'l' to '0' and '0' to 'l ', can be induced in the ' bit-line mode because an alpha-particle hits the con­ r' DRAM tacts of a bit-line and of the dummy bit-line. s1 Thus, these two modes of soft errors would de­ pend on the device element positions to which created ~1 IU7."; I .II S7 electrons are collected but share the same charge col­ lection mechanisms. The electrons created by an al­ pha-particle diffuse and drift within a depletion layer. Figure 2 Schematic view of a soft error evaluation Some electrons are collected by funneling [8], in method using a radioactive source. which the depletion layer spreads along the track of an energetic particle in the shape of a funnel because the created electron-hole pairs force to cancel the ap­ applied for the evaluation of soft errors in the early plied substrate potential. stage of DRAM generations. Campbell et al. investi­ gated 16 Kbit DRAMs using He+ microprobes colli­ Soft error measurement mated by a pin-hole slit with 2.5 µm diameter [3], and Soft error immunity has conventionally been Geppert et al. also investigated 2 x 256 Kbit DRAMs evaluated using energetic particles from a radioactive using He+ microprobes [6]. However, the exact inci­ source [2] or an accelerator [27]. Figure 2 shows the dent position of the microprobes was not unambigu­ schematic view of a soft error evaluation method us­ ously identified because the irradiated position was ing a radioactive source. The radiation source such as determined by movements of the mechanical stage an americium or a thorium source is located over with a low accuracy in both experiments. Hence the memory devices. This method can statistically mani­ exact correlation between the incident position of en­ fest soft error immunity of the whole devices. Soft ergetic particles and soft errors could not be obtained. errors only in a single device element, however, can­ Recently, nuclear microprobes with a spot size of not be evaluated because the incident position of the about 1 µm, almost as small as cell sizes of Mbit particles in a device chip is random. The incident DRAMs, have been developed using precise quadru­ angle of the particles is also random. Therefore, a pole lenses. Moreover, nuclear microprobes can pro­ new technique has been developed to clarify a hitting vide mapping images such as a secondary electron position and an angle to the device surface of a radia­ image by raster-scanning the focused beam. Such tion particle. small scanning microprobes have been applied to a soft error evaluation in 16 Kbit static-RAMs Microprobe application to soft error measure­ (SRAMs) by Horn et al. [7]. The application of the ment nuclear microprobe techniques to DRAMs is more Nuclear microprobes, high energy focused ion difficult than that to SRAMs because of the dynamic beams, have been developed for non-destructive operations. The bit states of SRAMs are kept and are analyses technology in archaeology, biology and slowly read out after probe irradiation, while DRAMs semiconductor, using particle-induced X-ray-emis­ require both refresh operation and rapid reading out. sion (PIXE) and/or Rutherford-backscattering (RBS) Moreover, a huge apparatus is required for operation [23]. Nuclear microprobes will be a powerful analyti­ of Mbit DRAMs if all the bit-states are monitored. cal tool for local susceptibility of soft errors because In this study, a one-bit memory cell was operated the incident position and angle to the devices can be because of the ease in dynamic operation, and proton easily controlled. Microprobe techniques were once microprobes at 400 ke V were applied to soft error 827 H.
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