A Soft Error Tolerant SRAM Design in 130NM CMOS Technology

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Soft error is a great concern for microelectronics circuits today. With the advanced development i n CMOS technologies, VLSI circuits are becoming more sensitive to external noise sources, especially radiation particle strikes, which are the cause of soft error. Soft errors are random and do not cause the permanent failure. However, it causes the corruption of stored information, which could turn to the failure in functionality of the circuits.

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Nội dung Text: A Soft Error Tolerant SRAM Design in 130NM CMOS Technology

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY UNIVERSITY OF SCIENCE LÊ THỊ LINH AN A SOFT ERROR TOLERANT SRAM DESIGN IN 130NM CMOS TECHNOLOGY Specialization: Electronic Engineering – Microelectronics Major Code: 60 52 70 MASTER DEGREE THESIS ELECTRONICS ENGINEERING – MICROELECTRONICS SUPERVISOR Dr. BÙI TRỌNG TÚ Ho Chi Minh City, 2010
ACKNOWLEDGEMENTS It is my pleasure to thank all the people who made this thesis possible. First of all, I would like to sincerely express my appreciation to my advisor, Dr. Bui Trong Tu, for his tremendous support, valuable guidance and constant encouragement during my studies. His technical advice made my master’s studies a meaningful learning experience. I am also grateful to Prof. Dang Luong Mo, Prof. Nguyen Huu Phuong, and Dr. Huynh Huu Thuan, who are the managers of this Microelectronics Master program. This is really an interesting course with enthusiastic and devoted professors, who are the experts in the IC industry. I also wish to thank my colleagues in TCAM team for all helpful discussion and valuable advice during my study. Appreciation is expressed for Silicon Design Solutions Company who have supported me about financial and let me join in this Master course during my work. Finally, my special thanks to my family who have always been with me throughout the difficulties and challenges of my master study. Ho Chi Minh City, November 2010 Le Thi LinhAn
ABSTRACT Soft error is a great concern for microelectronics circuits today. With the advanced development in CMOS technologies, VLSI circuits are becoming more sensitive to external noise sources, especially radiation particle strikes, which are the cause of soft error. Soft errors are random and do not cause the permanent failure. However, it causes the corruption of stored information, which could turn to the failure in functionality of the circuits. Meanwhile, the demand for a higher reliability of electronics applications is always a non-stop requirement. There are a lot of critical applications that need the extreme exactly in circuit functionality, such as the circuits used in space or biomedical equipment, as well as the military electronics and so on. Generally, soft errors in memories attracted more attention than soft errors in logic circuit. In addition, memories play an important part in modern system. Because of the high integration of storage cells, a large memory is more sensitive to particle strikes than logic. Due to that motivation, this thesis focuses to study about soft errors in memories. The thesis goes through the background knowledge of soft errors and its mitigation techniques. Then, a SRAM design with additional soft error tolerant feature will be presented. The SRAM is designed in 130nm CMOS technology, using circuit hardening and error correcting code techniques to mitigate the soft error effect. The soft error tolerant level is verified by some simulations. Not only focus on the soft error tolerant circuits, a whole SRAM architecture will be shown in detail, from circuit to physical implementation. The verification and simulation results are also included.
TABLE OF CONTENTS Acknowledgement Abstract Table of contents Abbreviations List of tables List of figures CHAPTER 1 - INTRODUCTION .................................................................................... 1 1.1. Problem and motivation ............................................................................................. 1 1.2. Contribution of the thesis ........................................................................................... 2 1.3. Thesis organization .................................................................................................... 2 CHAPTER 2 - BACKGROUND ....................................................................................... 4 2.1. Soft errors in semiconductor device........................................................................... 4 2.1.1. Radiation sources ................................................................................................. 4 2.2. Soft errors occurrence mechanism ............................................................................. 5 2.3. Soft errors mitigation techniques ............................................................................... 6 2.3.1. Device level techniques ....................................................................................... 6 2.3.2. Circuit level techniques ....................................................................................... 7 2.3.3. Block level techniques ......................................................................................... 7 CHAPTER 3 – SOFT ERROR TOLERANT SRAM DESIGN ................................... 10
3.1. SRAM specification ................................................................................................. 10 3.1.1. General information ........................................................................................... 10 3.1.2. Floorplan ............................................................................................................ 11 3.1.4. Operation brief description ................................................................................ 12 3.2. SRAM detail design ................................................................................................. 14 3.2.1. SRAM cell architecture ..................................................................................... 14 3.2.2. Replica path for Read operation ........................................................................ 15 3.2.3. Internal clock generator ..................................................................................... 17 3.2.4. Write circuit ....................................................................................................... 19 3.2.5. Decoder .............................................................................................................. 19 3.2.6. Input/output latches .......................................................................................... 21 3.3. Error detecting and correcting (EDC) block ............................................................ 22 3.3.1. Hamming code algorithm .................................................................................. 23 3.3.2. EDC block implementation ............................................................................... 24 3.3.3. EDC detail architecture...................................................................................... 26 CHAPTER 4 – DESIGN SIMULATION AND VERIFICATION .............................. 37 4.1. SRAM cell simulation .............................................................................................. 37 4.1.1. SRAM cell simulation to find device size ......................................................... 37 4.1.2. SRAM cell characteristic summary ................................................................... 42 4.1.3. Static noise margin comparison ......................................................................... 43 4.1.4. SRAM cell capacitance...................................................................................... 43 4.2. Soft error tolerant simulation ................................................................................... 44 4.2.1. Verification methodology .................................................................................. 44 4.2.2. Critical charge simulation .................................................................................. 45 4.2.3. Simulation results .............................................................................................. 46 4.2.4. Conclusion ......................................................................................................... 49 4.3. Post-layout simulation.............................................................................................. 50 4.3.1. Simulation setup ................................................................................................ 50
4.3.2. Cycle time definition and simulation result ....................................................... 52 4.3.3. Access time ........................................................................................................ 55 4.3.4. Setup time .......................................................................................................... 56 4.3.5. Timing delay of some critical paths................................................................... 57 4.3.6. Simulation results summary .............................................................................. 61 4.4. SRAM and EDC functional verification .................................................................. 61 4.4.3. Simulation setup ................................................................................................ 65 4.4.4. Functional verification result ............................................................................. 67 4.5. Physical verification ................................................................................................. 70 CHAPTER 5 – CONCLUSION AND FUTURE WORK ............................................. 75
ABBREVIATIONS VLSI Very large scale integration CMOS Complementary Metal-Oxide Semiconductor SEU Single Event Upset DRC Design Rule Check LVS Layout versus Schematic SRAM Static Random Access Memory ECC Error Correcting Code EDC Error Detecting and Correcting SNM Static noise margin LPE Layout Parasitic Extraction
LIST OF TABLES Table 3.1: Pin description ................................................................................................... 12 Table 3.2: Hamming code for 22 bits ................................................................................. 24 Table 4.1: Read current ...................................................................................................... 38 Table 4.2: Read leakage current ......................................................................................... 38 Table 4.3: Effect of leakage on read current ...................................................................... 38 Table 4.4: Write current ..................................................................................................... 40 Table 4.5: Static noise margin ............................................................................................ 41 Table 4.6: SRAM cell characteristic summary .................................................................. 43 Table 4.7: SNM comparison............................................................................................... 43 Table 4. 8: SRAM cell capacitance .................................................................................... 44 Table 4.9: Critical charge result of hardened SRAM cell .................................................. 46 Table 4.10: Critical charge result for normal SRAM cell .................................................. 48 Table 4.11: Performance result (SS_125_1.35) ................................................................. 61 Table 4.12: Timing delay between nodes ........................................................................... 61 Table 4.13: Design fault model .......................................................................................... 62
LIST OF FIGURES Figure 2.1: Redundancy........................................................................................................ 8 Figure 2.2: Concurrent error detection ................................................................................. 8 Figure 3.1: SRAM floorplan............................................................................................... 11 Figure 3.2: Write operation ................................................................................................ 13 Figure 3.3: Read operation ................................................................................................. 13 Figure 3.4: SRAM cell architecture.................................................................................... 15 Figure 3.5: Timing scheme for read operation ................................................................... 16 Figure 3.6: Reference IO cell and read circuit ................................................................... 17 Figure 3.7: Read clock generator circuit ............................................................................ 18 Figure 3.8: Write clock generator ....................................................................................... 19 Figure 3.9: Write circuit and sequential waveform ............................................................ 19 Figure 3.10: Row decoder block diagram .......................................................................... 20 Figure 3.11: Xdec circuit .................................................................................................... 21 Figure 3.12: Hardened latch architecture ........................................................................... 22 Figure 3.13: EDC block diagram........................................................................................ 25 Figure 3.14: Write encoder schematic ................................................................................ 27 Figure 3.15: Parity comparison schematic ......................................................................... 28 Figure 3.16: Syndrome decoder schematic ........................................................................ 29 Figure 3.17: Bit flipper block ............................................................................................. 30 Figure 3.18: Input select ..................................................................................................... 31 Figure 3.19: Output select and output latch........................................................................ 32
Figure 3.20: Top level layout view..................................................................................... 33 Figure 3.21: SRAM cell layout with only device layers shown ......................................... 34 Figure 3.23: Xdec cell layout ............................................................................................. 34 Figure 3.22: SRAM cell layout .......................................................................................... 34 Figure 3.24: Xdec array 1x256 ........................................................................................... 35 Figure 3.25: Control block .............................................................................................. 35 Figure 3.26: IO array 1x22 ................................................................................................. 36 Figure 4.1: Read current ..................................................................................................... 37 Figure 4.2: Write current .................................................................................................... 39 Figure 4.3: Inject a current source to an off NMOS drain ................................................. 45 Figure 4.4: The injected SEU current for hardened SRAM cell ........................................ 47 Figure 4.5: IBL waveform of hardened SRAM cell ........................................................... 47 Figure 4.6: The exchange state between IBL and IBLX .................................................... 47 Figure 4.7: The injected SEU current for normal SRAM cell............................................ 48 Figure 4.8: IBL waveform of normal SRAM cell .............................................................. 48 Figure 4.9: The exchange state between IBL and IBLX .................................................... 49 Figure 4.10: A part of LPE netlist containing capacitance value ....................................... 50 Figure 4.11: A part of LPE netlist containing resistor value .............................................. 51 Figure 4.12: A part of input waveform for performance simulation .................................. 51 Figure 4.13: Hspice option ................................................................................................. 52 Figure 4.15: Delay from clk rise to resetx rise ................................................................... 53 Figure 4.14: Cycle time must cover the internal clock....................................................... 53 Figure 4.17: Delay from clk rise to dmrbl rise ................................................................... 54
Figure 4.16: Cycle time must make sure all RBL be precharged fully .............................. 54 Figure 4.18: Cycle time must cover PWH of input latch plus for max setup time ............ 55 Figure 4.19: PWH of input latch ........................................................................................ 55 Figure 4.20: Access time definition.................................................................................... 56 Figure 4.21:Access time ..................................................................................................... 56 Figure 4.22: Address input path delay................................................................................ 57 Figure 4.23: Clock path delay ............................................................................................ 57 Figure 4.24: Delay from CLKA to intckx fall .................................................................... 58 Figure 4.25: Delay from intclk fall to rhcpx fall ................................................................ 58 Figure 4.26: Delay from rhcpx fall to latch rise ................................................................. 58 Figure 4. 27: Delay from rhcpx fall to echo rise ................................................................ 59 Figure 4.28: Delay from echo rise to resetx fall ................................................................. 59 Figure 4.29: Delay from resetx fall to intclk rise ............................................................... 59 Figure 4.30: delay from intclk rise to rhcpx rise ................................................................ 60 Figure 4.31: Delay from rhcpx rise to latch fall ................................................................. 60 Figure 4.32: Delay from intclk rise to resetx rise ............................................................... 60 Figure 4.33: Netlist of top level .......................................................................................... 66 Figure 4.34: A part of full test vector ................................................................................. 66 Figure 4.35: Hsim option .................................................................................................... 67 Figure 4.37: Waveform of SRAM functional simulation................................................... 68 Figure 4.36: Hsim log file .................................................................................................. 68 Figure 4.38: Waveform of EDC functional simulation ...................................................... 69 Figure 4.39: LVS Calibre report for hierachical checking ................................................. 71 Figure 4.40: Detail LVS report for top level ...................................................................... 72
Figure 4.41: DRC report ..................................................................................................... 74
CHAPTER INTRODUCTION 1 Pag e |1 CHAPTER 1 INTRODUCTION 1.1. Problem and motivation Reliability is the key challenge facing the modern VLSI system. The advanced development of CMOS technologies has resulted in the lower supply voltages, higher clock frequencies, and the increasing of transistor integration densities. Consequently, VLSI circuits are becoming more vulnerable to various noise sources. It can be listed here some well-known noise effects such as: the power and ground noise, capacitive coupling noise, radiation particle strikes … With the rapid scaling of technology, integrated circuits (ICs) are turned to be very sensitive to the radiation particles strikes. When a radiation particle strike at a sensitive region in a semiconductor device, the charges generated could corrupt the stored information in the memory element, resulting in an erroneous data at the output, or so called soft error. Soft errors are incidental and do not destroy the device. They just cause the temporary functional failure and the system still works well after that. The radiation particle striking is a random natural phenomenon; therefore they cannot be predicted or controlled by the designers. The charge particles could be the alpha particles, neutron induced 10B fission and high energy cosmic ray neutrons. The source of charge particles can be from the radioactive material or cosmic rays. In addition, it could also be the result of high energy particle interaction with semiconductor itself. Electronics applications nowadays always require a higher reliability level. Many critical applications such as biomedical circuits, as well as space and military electronics devices demand extreme high reliable circuit functionality. That means soft errors are becoming more and more unacceptable, even in the commercial
CHAPTER INTRODUCTION 1 Pag e |2 applications [1]. Therefore, soft error elimination is a major consideration of all VLSI circuits today. Memories always have a high density integration of storage elements. Hence, they are more sensitive to soft errors than in logic circuit. The soft errors in memories (SRAM and DRAM) were widespread studied from the end of the twentieth century [2]. However, it is still problematic up to now. Due to that motivation, this thesis focuses to study the soft errors on memories (specific in SRAM) and applies some mitigation techniques to design a SRAM with soft error tolerant feature. 1.2. Contribution of the thesis The thesis presents the detail design of a synchronous two-port SRAM in 130nm CMOS technology, with additional soft error tolerant feature. The design was applied two soft error mitigation techniques; those are circuit hardening and error correcting code (ECC) techniques. For the first technique, only some special parts of the design are selected to be hardened. They are the memory cell, the address input latches, data input and output latches, keeper circuits… These are parts that most easily suffer from soft error of a memory because they are storage elements. The second technique helps to recover the design if unfortunately the soft error occurred. It is a built-in error detecting and correcting (EDC) block for the SRAM. This block was applied the ECC techniques, used the Hamming code to detect if there is a single bit or double bit error in a memory array. And it will also function as a correcting circuit if there is a single bit upset. 1.3. Thesis organization The rest of thesis is organized as follows: Chapter 2 introduces the background knowledge of soft error, its mechanism as well as the mitigation techniques.
CHAPTER INTRODUCTION 1 Pag e |3 Chapter 3 describes detail about the SRAM design, including the SRAM specification, SRAM architecture, specific design for soft error tolerant feature and the physical implementation. Chapter 4 focuses on the verification methodologies and the simulations result. These simulations include the soft error tolerant level simulation, memory cell characteristic, post layout simulation, functional verification and physical verification. Finally, chapter 5 shows the conclusion and some discussions to improve this SRAM design.
CHAPTER BACKGROUND 2 Pag e |4 CHAPTER 2 BACKGROUND 2.1. Soft errors in semiconductor device Soft errors, also called Single Event Upset (SEU), are the errors in microelectronics circuit caused when the radiation particles strike at sensitive regions of the silicon devices. Soft errors are incident and no breakage of the device occurs [3]. They only flip the stored state of a memory element and will generate an erroneous value at output. Soft errors cause no permanent faults; the system still work well after suffering from an SEU. Therefore, they are named as soft error. This background section will help to get an overview of radiation sources which are the cause of soft errors and the mechanism of soft errors occurrence in semiconductor devices. 2.1.1. Radiation sources Radiation is kinetic energy in the form of high speed particles and electromagnetic waves [4]. Typically, three main sources of radiation causing soft errors in semiconductor device could be summarized as following: · Alpha particles: are the nuclei of helium atoms consisting of 2 protons and 2 neutrons. Alpha particles are generated from the radioactive decay process and when they collide with other atoms. Because of alpha particles cannot travel a long path in material, atmosphere therefore is not the main source of alpha particles, but an integrated circuit itself. Packaging and soldering contain traces of radioactive isotopes, which lead to release the alpha particles as well as other particles such as gamma and beta particles, as they decay to lower state. Alpha particles contain the kinetic energy in the range of 4 to 9 MeV [5]
CHAPTER BACKGROUND 2 Pag e |5 · High energy neutrons: when the cosmic radiation reacts with the atmospheres, it will cause the generation of secondary particles, includes protons, electrons, neutron … All of them can cause soft errors; however, charge generation property of neutron for the same energy is more than proton or electron. Neutron does not contain charge; therefore, the ionization in material cannot be caused by itself. However, when a neutron with energy above 1 MeV interacts with the silicon atoms, it will cause a nuclear reaction which generates charged particles. These charged particles cause ionization, lead to the soft error in the device. · Thermal neutrons: thermal neutrons are low energy neutrons with a kinetic energy of about 0.025 eV. The interaction of low energy cosmic neutrons and doping boron (isotope 10B and 11B) in semiconductor material generates the secondary radiation particles (the lithium atom and alpha particle). Both these particles can cause soft errors in the device 2.2. Soft errors occurrence mechanism In semiconductor device, there are some sensitive nodes which are easily to suffer from SEU. Those are the drain of the off NMOS and off PMOS transistors. Consider an off NMOS transistor, its source, gate and substrate terminals are connected to VSS. The drain is connected to VDD. The drain and substrate of this OFF transistor form a reverse-biased junction. Therefore, a strong electric field from drain to substrate exists in the depletion region of this junction. Because radiation particles generate the free electron hole pairs, this electric field will cause the collection of electron at drain and of hole at the substrate. That’s why these reverse-biased junctions are the most sensitive nodes to the particle strikes. When the particles strike at these sensitive nodes, due to the electric field of the reversed-biased junction, the generated charges are collected at the opposite voltage terminals (drain and substrate) of the reverse-biased junction. Electrons move towards the positive voltage while holes move toward the negative voltage.
CHAPTER BACKGROUND 2 Pag e |6 This event will cause a current pulse, flow from the n type diffusion to the p type diffusion in a very short duration. When the charge collection exceeds the critical charge, the storage value will be changed. Critical charge (Qcrit) is the minimum charge required to flip the cell. The Qcrit depends on the characteristic of the circuit, especially the supply voltage and the nodal capacitance of the drain [6]. When a particle strike discharges the charge stored at the drain of the OFF-NMOS transistor, it will flip from 1 to 0. Similarly for a 0 to 1 flip when it strikes the drain of the OFF-PMOS transistor. As technology scales down, to adapt the higher requirement for constraining the power and making the circuit transient faster, the supply voltage and nodal capacitance is decreasing swiftly. That makes the charge stored at the sensitive nodes of the device is reduced because Qnode = Cnode×Vdd, resulted in the more and more vulnerable to soft errors of SRAM. 2.3. Soft errors mitigation techniques In general, the soft error mitigation techniques could be classified into three categories: device level techniques, circuit level techniques and block level techniques 2.3.1. Device level techniques At this level, some methods were given out to edit the traditional fabrication process to make the device resistant to soft error. The manufacturers and designer could choose the appropriate material, package, as well as the better device geometries. For example, soft errors can be caused by alpha particles which are emitted by the materials or compounds used in packaging. Therefore, choosing the appropriate material which has the less probability of alpha particles could minimize the soft error rate. Or to reduce the soft error induced by the interaction of low energy cosmic neutrons and doping boron
CHAPTER BACKGROUND 2 Pag e |7 10 B, BPSG (boron phosphor silicate glass) is replaced by other insulators that do not contain boron. 2.3.2. Circuit level techniques The technique that is mostly used at this level to make the circuit resistant to SEU is radiation hardening technique [7]. With this technique, some special parts of the design are chosen to be hardened. Basic circuits element such as Inverter, Nand, Nor, flip-flop or latches are made SEU resistant by adding extra transistor than normal. Normally, this technique is often applied to the memory cell, keeper circuits, latches or flip-flops which are data storage element, thus are easily suffer from soft errors. It will help to increase the critical charge at sensitive nodes, making those nodes less susceptible to the SEU. This technique is widely used because the designer can predict which nodes are sensitive to protect them from the SEU. However, this technique causes the overhead in area and power consumption [8]. 2.3.3. Block level techniques Different with two approaches above, the block level techniques are used to detect the error and recover the design after being suffered from SEU, while the two approaches above mainly protect and enhance the design. There are two main mitigation techniques at this level: 2.3.3.1. Redundancy The redundancy techniques often clone to create the redundant circuit. However, this result the area and performance overhead, also higher power consumption. Triple modular redundancy is a classical method which has the high soft error reliability. Three identical copies of a circuit compute on the same data in parallel. The three outputs are then evaluated
CHAPTER BACKGROUND 2 Pag e |8 by the majority voting logic. It will return the value that occurred in at least two of three cases. By using this technique, the soft error will be detected if it occurs at one of the three circuits, assuming the other two circuits operated correctly. Figure 2.1: Redundancy Another example of redundancy is using concurrent error detection, from which, only a selected parts of the design are chosen to be duplicated. Figure 2.2: Concurrent error detection In these techniques, selecting the parts to be duplicated is very important. If the particle strike happens in the non-duplicated region, it cannot be detected. In contrast, if it occurs at the duplicated portion of the circuit, the checker could detect it. Therefore, must be careful to select the cutset logic in which the nodes have highest soft error susceptibility [9]. 2.3.3.2. Error correcting code and parity