Introduction to Advanced Semiconductor Memories

CHAPTER 1

INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES

1.1. SEMICONDUCTOR MEMORIES OVERVIEW

The goal of Advanced Semiconductor Memories is to complement the material already covered in Semiconductor Memories. The earlier book covered the following topics: random access memory technologies (SRAMs and DRAMs) and their application to speciﬁc architectures; nonvolatile technologies such as the read-only memories (ROMs), programmable read-only memories (PROMs), and erasable PROMs in both ultraviolet erasable (UVPROM) and electrically erasable (EEPROM) versions; memory fault modeling and testing; memory design for testability and fault tolerance; semiconductor memory reliability; semiconductor memories radiation effects; advanced memory technologies; and high-density memory packaging technologies [1]. This section provides a general overview of the semiconductor memories topics that are covered in Semiconductor Memories. In the last three decades of semiconductor memories’ phenomenal growth, the DRAMs have been the largest volume volatile memory produced for use as main computer memories because of their high density and low cost per bit advantage. SRAM densities have generally lagged a generation behind the DRAM. However, the SRAMs offer low-power consumption and high-performance features, which makes them practical alternatives to the DRAMs. Nowadays, a vast majority of SRAMs are being fabricated in the NMOS and CMOS technologies (and a combination of two technologies, also referred to as the mixed-MOS) for commodity SRAMs.

1 2 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES

Figure 1.1 Semiconductor memory market as a percentage of the total IC market [2].

In 1995, semiconductor memories accounted for 42% of the total IC market, but following 1995’s strong growth, memory prices collapsed for the next three years. In 1998, memory devices represented only 21% of the total IC market. During the 1990s, semiconductor memory sales averaged approximately 30% of total IC sales. It is forecasted that the memory portion of total IC sales will gradually increase through year 2005. Figure 1.1 shows the semiconductor memory market as a percentage of the total IC market [2]. In high-density and high-speed applications, various combinations of bipolar and MOS technologies are being used. In addition to MOS and bipolar memories, referred to as the ‘‘bulk silicon’’ technologies, silicon-on-insulator (SOI) isolation technologies have been developed for improved radiation hardness. SRAM density and performance are usually enhanced by scaling down the device geometries. Advanced SRAM designs and architectures for 4 to 16-Mb chips with submicron feature sizes have been developed and currently available as commodity chips. Application-specific memory designs include first-in-first- out (FIFO) buffer memory, in which the data are transferred in and out serially. The dual-port RAMs allow two independent devices to have simultaneous read and write access to the same memory. The content addressable memories (CAMs) are designed and used both as the embedded modules on SEMICONDUCTOR MEMORIES OVERVIEW 3 larger VLSI chips, and as stand-alone memory for specific system applications. A major improvement in DRAM evolution has been the switch from three-transistor (3T) designs to one-transistor (1T) cell design, enabling production of 4- to 16-Mb density chips that use advanced 3-D trench capacitor and stacked capacitor cell structure. Currently, 64-Mb to 1-Gb DRAM chips are in production, and multigigabit density chips are being developed. The technical advances in multimegabit DRAMs have resulted in greater demand for application-specific products such as the pseudostatic DRAM (PSRAM), which uses dynamic storage cells but contains all refresh logic on-chip that enables it to function similarly to an SRAM. Video DRAMs (VDRAMs) have been produced for use as the multiport graphic buffers. Some other examples of high-speed DRAM innovative architectures are synchronous DRAMs (SDRAMs), cache DRAMs (CDRAMs), and Rambus2+ DRAMs (RDRAMs). Nonvolatile memories (NVMs) have also experienced tremendous growth since the introduction in 1970 of a floating polysilicon gate-based erasable program read-only memory (EPROM), in which hot electrons are injected into the floating gate and removed either by ultraviolet internal photoemission or by Fowler—Nordheim tunneling. The EPROMs (also referred to as the UVEPROMs) are erased by removing them from the target system and exposing them to ultraviolet light. An alternative to EPROM (or UVEPROM) has been the development of electrically erasable PROMs (EEPROMs), which offer in-circuit programming flexibility. Several variations of this technology include metal—nitride—oxide—semiconductor (MNOS), silicon—oxide—nitride— oxide—semiconductor (SONOS), floating gate tunneling oxide (FLOTOX), and textured polysilicon. The FLOTOX is most commonly used EEPROM technology. An interesting NVM architecture is the nonvolatile SRAM, a combination of EEPROM and SRAM in which each SRAM has a correspond- ing ‘‘shadow’’ EEPROM cell. Flash memories based on EPROM or EEPROM technologies are devices for which contents of all memory array cells can be erased simultaneously, unlike the EEPROMs that have select transistors incorporated in each cell to allow for the individual byte erasure. Therefore, the flash memories can be made roughly two or three times smaller than the floating gate EEPROM cells. Flash memories are available in 8- to 512-Mb densities as production devices, and even higher densities in development. DRAMs are currently (and predicted to be in the future) the largest memory segment in terms of dollar sales. After DRAMs the SRAMs and flash markets represent the next two largest memory segments. In year 2000, the flash memory market surpassed the SRAM market and became the second-largest memory market segment. Both DRAM and flash market shares are expected to continue growing through 2005, although flash memory at a much faster pace. The remaining memory segments are predicted to remain stable. 4 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES

Figure 1.2a shows a comparison of different MOS technologies market share projected to year 2005 [2]. It is predicted that in year 2005, the DRAMs will account for just 60% of the memory market, whereas flash memory sales is forecast to account for 29% of the total memory market. Figure 2.2b shows percentages for each MOS memory technology market for the year 2000 and predicted values for the year 2005. Semiconductor Memories reviewed various memory failure modes and mechanisms, fault modeling, and electrical testing [1]. A most commonly used fault model is the single-stuck-at fault (SSF), which is also referred to as the classical standard fault model. However, many other fault models have also been developed for transition faults (TFs), address faults (AFs), bridging faults (BFs), coupling faults (CFs), pattern-sensitive faults (PSFs), and the dynamic (or delay) faults. A large percentage of physical faults occurring in the ICs can be considered as the bridging faults (BFs), consisting of shorts between the two or more cells or lines. Another important category of faults that can cause the RAM cell to function erroneously is the coupling or PSFs. In general, the memory electrical testing consists of the dc and ac parametric tests and functional tests. For RAMs, various functional test algorithms have been developed for which the test time is a function of the number of memory bits (n) and range in complexity from O(n) to O(n). The selection of a particular set of test patterns for a given RAM is influenced by the type of failure modes to be detected, memory bit density that influences the test time, and the memory automated test equipment (ATE) availability. Advanced megabit memory architectures are being designed with special test features to reduce the test time by the use of multibit test (MBT), line mode test (LMT), and built-in self-test (BIST). Application-specific memories such as the FIFOs, video RAMs, synchronous static and dynamic RAMs, and double- buffered memories (DBMs) have complex timing requirements and multiple setup modes that require a suitable mix of sophisticated test hardware, design for testability (DFT), and BIST approach. In general, the memory testability is a function of variables such as circuit complexity and design methodology. Therefore, the DFT techniques, RAM and ROM BIST architectures, memory error detection and correction (EDAC), and the memory fault tolerance are important design considerations. Structured design techniques are based upon the concept of providing uniform design to increase controllability and observability. The commonly used methodologies include the level-sensitive scan design (LSSD), scan path, scan/set logic, random access scan, and the boundary scan testing (BST). The RAM BIST implementation strategies include the use of algorithmic test sequence (ATS), the 13-N March algorithms with a data-retention test, a fault-syndrome-based strategy for detecting the PSFs, and built-in logic block observation (BILBO) technique. For the embedded memories, various DFT SEMICONDUCTOR MEMORIES OVERVIEW 5

Figure 1.2 (a) Comparison of different MOS memory technologies market share. (b) Percentages for each MOS memory technology market for year 2000 and predicted values for year 2005 [2].

and BIST techniques have been developed such as the scan-path-based flag- scan register (FLSR) and the random-pattern-based circular self-test path (CSTP). Advanced BIST architectures have been implemented to allow parallel testing with on-chip test circuits. The current generation megabit memory chips include spare row and columns (redundancies) in the memory array to 6 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES compensate for the fault cells. In addtion, to improve the memory chip yield, techniques such as built-in self-diagnosis (BISD) and built-in self-repair (BISR) are used. The errors in semiconductor memories can be broadly categorized as the hard failures caused permanent physical damage to the device and soft errors caused by alpha particles or the ionizing dose radiation environments. The most commonly used error-correcting codes (ECC) that are used to correct hard and soft errors are the single-error correction and double-error detection (SEC-DED) codes, also referred to as the Hamming Codes. Multimegabit DRAM chips have been developed that use redundant word and bit lines in conjunction with ECC to produce optimized fault tolerance effect. To recover from the soft errors (transient effects), memory scrubbing techniques are often used, which are based upon the probabilistic or deterministic models. These techniques can be used to calculate the reliability rate R(t) and mean time to failure (MTTF) of the memory system. Semiconductor Memories reviewed general reliability issues for semiconductor devices such as the memories, RAM failure modes and mechanisms, nonvolatile memory reliability, reliability modeling and failure rate prediction, design for reliability, and reliability test structures [1]. The general reliability issues pertaining to semiconductor devices in bipolar and MOS technologies are applicable to memories also. In addition, there are special reliability issues and failure modes, which are of special concern for the RAMs. These issues include gate oxide reliability defects, hot-carrier degradation, the DRAM capacitor charge-storage and data-retention properties, and DRAM soft-error failures. The memory gate dielectric integrity and reliability are affected by all processes involved in the gate oxide growth. The reduced MOS transistor geometries from scaling of the memory devices has made them more susceptible to hot-carrier degradation effects. Nonvolatile memories, just like volatile memories, are also susceptible to some specific failure mechanisms. In the floating gate technologies such as the EPROM and EEPROMs, data retention characteristics and number of write/erase cycles without degradation (endurance) are the most critical reliability concerns. Reliability failure modeling is the key to the failure rate prediction, and there are many statistical distributions that are used to model various reliability parameters. The method of accelerated stress aging for semiconductor devices such as memories is commonly used to ensure long-term reliability. An approach commonly used by the memory manufacturers in conjunction with the end-of-line product testing has been the use of reliability test structures and process (or yield) monitors incorporated at the wafer level and ‘‘drop-in’’ test sites on the chip. The purpose of reliability testing is to quantify the expected failure of a device at various points in its life cycle. SEMICONDUCTOR MEMORIES OVERVIEW 7

The space radiation environment poses a certain radiation risk to all electronic components on earth-orbiting satellites and the planetary mission spacecrafts. The cumulative effect of ionization damage from charged particles present in the natural space environment, such as the electrons and protons on semiconductor memories, can be significant. Ionizing radiation damage causes changes in memory circuit parameters such as standby power supply currents, I/O voltage threshold levels and leakage currents, critical path delays, and timing specification degradations. The single-event phenomenon (SEP) in the memories is caused by high-energy particles such as those present in the cosmic rays passing through the device to cause (a) single-event upsets (SEUs) or soft errors and (b) single-event latchup (SEL), which may result in hard errors. The impact of SEU on the memories, because of their shrinking dimensions and increasing densities, has become a significant reliability concern. The nonvolatile MOS memories are also subject to radiation degradation effects. The memory circuits can be designed for total dose radiation hardness by using optimized processes (e.g., hardened gate oxides and field oxides) and good design practices. The bulk CMOS memories have been hardened to SEU by using an appropriate combination of processes and design techniques. Radiation sensitivity of unhardened memory devices can vary from lot to lot; and for space applications, radiation testing is required to characterize the lot radiation tolerance. Semiconductor Memories discussed the following topics in detail: radiation-hardening techniques, radiation-hardening design issues, radiation testing, radiation dosimetry, wafer level testing, and test structures [1]. Advanced semiconductor memories technologies include ferroelectric RAMs (FRAMs or FeRAMs), magnetoresistive RAMs (MRAMs), analog memories, and quantum-mechanical switch memories. These technologies were briefly reviewed in Semiconductor Memories. The increasing requirements for denser memories have led to further compaction of standard packaging approach to hybrid manufacturing techniques and multichip modules (MCMs). For the assembly of MCMs, various interconnect technologies have been developed such as the wire-bonding, tape automated bonding (TAB), flip-chip bonding, and high-density interconnect (HDI). An extension of 2-D planar technology has been the 3-D concept, in which the memory chips are mounted vertically prior to the attachment of a suitable interconnect. The 3-D approach can provide higher packaging densities because of (a) reduction in the substrate size, module weight, and volume, (b) lower line capacitance and drive requirements, and (c) reduced signal propagation delay times. Semiconductor Memories reviewed commonly used memory packages, memory hybrids and 2-D MCMs, memory stacks and 3-D MCMs, memory MCM testing and reliability issues, memory cards, and high-density memory packaging future directions [1]. 8 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES

1.2. ADVANCED SEMICONDUCTOR MEMORY DEVELOPMENTS

This book, Advanced Semiconductor Memories, reviews in detail future advances in SRAMs, high-performance DRAMs, application-specific DRAM designs and architectures, nonvolatile memory technologies, embedded memory designs and applications, and future gigabit-to-terabit memory directions. These advanced developments are briefly summarized in this section. Advanced SRAM technology developments are reviewed in Chapter 2. SRAMs are currently available for both asynchronous and synchronous designs in a wide variety of speeds and architectures. However, synchronous designs are preferred and use one or more external clock signals to control the SRAM operations, and they result in improved timing controls. This allows the reduction of the device access times and cycle times to match the clock cycles of the fastest PC and RISC processors available. The synchronous SRAM (SSRAM) data buses are usually flow-through or pipelined. In the communication networks, SRAMs are being used as data buffers between the input and output ports, and they are also being used as high-speed lookup tables containing addresses and other information to route data stream from the data source to destination. SRAM speed has been enhanced by scaling down the device geometries, as well as by improvement in processes and circuit design techniques for the optimization of chip architecture. For low-voltage SRAMs, the designers are using various techniques to minimize the power consumption. Fast SRAMs have applications in cache memory system designs. Some examples of high- performance SRAM architectures are described, such as the flow-through SSRAMs, zero bus turnaround (ZBT2+) SRAMs, No Turnaround Random Access Memory (NtRAM2+) pipelined, quad-data-rate (QDR) SRAMs and double-data-rate (DDR) SRAMs. BiCMOS technology is more important in applications that require com- patibility with high-performance microprocessor clock speeds. However, Bi- CMOS process is more complex because of the additional steps required, compared to a standard CMOS process. The silicon-on-insulator (SOI) technology in SRAM applications offers significant performance advantages due to the reduction in device junction capacitance. The major advantages of the SOI technology include latchup free operation and improved soft-error rate (SER) performance. SOI technology SRAMs are finding applications in critical military and space applications with high total dose radiation and transient survivability requirements (see Semiconductor Memories, Chapter 7). The basic SRAM performance can often be improved by the addition of some extra logic control circuitry to provide specialty SRAMs. For example, in systems with multiple processors or devices requiring simultaneous access to the SRAM, multiport memories can be used. An extension of this concept is ADVANCED SEMICONDUCTOR MEMORY DEVELOPMENTS 9

first-in-first-out (FIFO) memories available in various configurations for the use as buffers for multiprocessors and serial communication networks. A specialty device is content addressable memory (CAM) that can output an address (or addresses) when data are presented to certain inputs. Design and architecture examples of three types of specialty memories are presented: multiport RAMs, FIFOs, and CAMs. Semiconductor Memories provided an introduction to the DRAM technology evolution, as well as technology developments in advanced architectures [1]. Chapter 3 (High-Performance Dynamic Random Access Memories) in the present book provides a detailed overview of further DRAM technology advances, scaling issues, and future trends. The new-generation DRAMs use various refresh schemes, as well as provide several modes for accessing data in the storage cells. For scaling to 64-Mb and higher densities, it is necessary to increase the cell’s storage capacity by using 3-D cell structures, along with trench or stacked capacitors. Enhanced DRAM (EDRAM) can be used to replace standard, slow full-page mode (FPM) or EDO DRAM for some higher performance applications that require large amounts of very fast memory. Extended-Data-Out (EDO) DRAMs have the advantage over conventional FPM memory is that it allows for a shorter page mode cycle time (or faster data rate) while accessing data within a single page in memory. An example of 64-Mb EDO DRAM available from Infineon Technologies is discussed. Single-data-rate synchronous DRAMs (SDRAMs) and synchronous graphic RAMs (SGRAMs) use the same basic memory cell and the same word-line drivers as the EDO DRAMs. However, their performance is limited by the interface requirements. Therefore, the double-data-rate (DDR) SDRAM/ SGRAM were introduced as an architectural enhancement by incorporating several major features. An example of architecture and functional operations of a commercially available 256-Mb SDRAM from Micron Technology is presented. The enhanced SDRAM (ESDRAM) is an evolutionary modification to the JEDEC standard for 16-Mb SDRAM, which incorporates changes to a standard DRAM to reduce latency, increase the bandwidth, and allow for concurrent operations to the same bank. An example of 16-Mb ESDRAM from Enhanced Memory Systems, Inc., is discussed. Cache DRAM (CDRAM) introduced by Mitsubishi Corporation is an SDRAM with an on-chip cache in the form of a separate SRAM array integrated with the DRAM array. However, in CDRAM (unlike the EDRAM), the SRAM and DRAM operations can be separately controlled. An example of 16-Mb CDRAM from Mitsubishi Corporation is provided. The virtual channel memory (VCM) memory core technology was developed by NEC Incorporated, to improve the memory data throughput efficiency and targeted for multiple users, multitasking, and interleaved access environ- 10 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES ment. Another example of advanced memory architecture is a 64-Mb fast-cycle RAM (FCRAM) that involves basic changes in the DRAM core concept by operating the memory like a synchronous SRAM using the dynamic core. Another example of 256-Mb DRAM architecture is given that provides up to ;32-bit-wide organization and uses an exchangeable, hierarchical data line scheme to minimize the number of sense amplifier circuits. Major gigabit DRAM scaling issues are discussed, using several examples of 1-Gb SDRAM cells and architectures that have been recently developed. In gigabit DRAM scaling, one of the major issues is the reduction of array power consumption without degrading the operating margin of the memory device and other characteristics. Multivalued and multilevel RAMs (MLDRAMs) schemes have been proposed, in which the amount of voltage placed across the capacitor is varied to represent the multiple states. If the number of states in a single memory cell is doubled, then the storage capacity of the memory cell can be doubled. Some of the proposed MLDRAM designs are reviewed, including a 4-Gb DRAM with multilevel storage memory cells that utilize data storage at four levels, where each level corresponds to a 2-bit data storage in a single memory cell. This approach can reduce the effective cell size by 50%. SOI DRAMs are under development for which the major advantages include a superior SER, better static data retention time characteristics, and potential for higher integration density than the bulk-Si-based DRAMs. An overview of various isolation processes used for SOI and feature comparison of the bulk-Si, partially depleted (PD) and fully depleted (FD) SOI transistors is provided. The technical advances in multimegabit DRAMs have resulted in greater demand for memory designs incorporating specialized performance requirements for applications, such as the high-end desktops/workstations, PC ser- vers/mainframes, 3-D graphics, network routers and switches. Some earlier examples of these application specific DRAMs, such as the pseudostatic DRAMs (PSRAMs), or virtual static DRAMs (VSRAMs) were discussed in Semiconductor Memories [1]. Chapter 4 in the present book describes latest developments in application specific memory architectures and designs in more details, such as the Video RAMs, SGRAMs, DDR SGRAMs, Rambus2+ Technology, Synchronous Link DRAMs (SLDRAMs), and 3-D RAMs. The video RAM (VRAM) was developed to increase the bandwidth of raster graphics display frame buffers. An example of the architecture of a 4-Mb VRAM is provided. A further improvement was SGRAMs that are very similar to the SDRAMs, except that they have several additional functions to improve their effectiveness in graphic systems design. Examples of a 64-Mb DDR SGRAM from Fujitsu Semiconductor, and a 256-Mb fast-cycle RAM (FCRAM2+) are provided. ADVANCED SEMICONDUCTOR MEMORY DEVELOPMENTS 11

Direct RDRAM is a high-speed memory for graphic applications and offers double the word width of the original RDRAM, offering storage capabilities of 64/72 Mb, 128/144 Mb, and 256/288 Mb. Concurrent RDRAMs perform two bank operations simultaneously, to allow high transfer rates using interleaved transactions. These memories can operate at speeds of 600 MHz, achieving data transfer speeds of 1.2 Gbytes/s. Various Rambus technologies and architectures including command sets, protocol formats, and functional blocks are discussed. SLDRAM is a new memory interface specification developed through the cooperative efforts of leading semiconductor memory manufacturers with a goal to meet the high data bandwidth requirements of emerging processor architectures, as described in IEEE Standard P1596.4. An example of a very-high-speed, packet-oriented, pipelined, 4-Mb;18 synchronous SLDRAM available from Micron Technology, Inc., is provided. Mitsubishi Corporation pioneered the introduction of a family of 3-D RAM for high-performance 3-D graphics hardware. The architecture and various functional blocks of this 3-D RAM family are discussed. Currently, in application-specific, high-performance memory designs, the competing technologies are SDRAM, DDRSDRAM, Rambus DRAM, and SLDRAMs. An overview of various memory system design considerations, such as the peak bandwidth performance comparison, granularity, and latency, is provided. Figure 1.3 provides a flow chart of various types of some commonly used RAMs.

Figure 1.3 A ﬂow chart of various types of some commonly used high speed RAMs. 12 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES

Nonvolatile memories (NVMs) are characterized by their ability to retain the stored data even with the device power off for indefinite periods of time, as compared to the volatile memories (such as the SRAMs and DRAMs) that lose the stored information under these conditions. Some examples of the NVMs are ROMs, PROMs, EPROMs, and EEPROMs designs and technologies. Special memories are also available such as nonvolatile random access memory (NOVRAM) or shadow RAM configurations that combine on the same chip, a SRAM array, and a backup EEPROM array of equal bits. In recent years, an area of interest in advanced nonvolatile memories has been the development of thin-film ferroelectric (FE) technology to build ferroelectric RAMs (FRAMs) as substitutes for the NOVRAMs, These NVM technologies were briefly discussed in Semiconductor Memories, Chapter 3 [1]. Serial EEPROMs are considered low-cost solution for the applications that do not require the high capability or short access times of traditional NOR and NAND type of architectures. The flash memory architecture has split along two main paths: traditional random access devices based on the NOR designs and byte-serial devices based on the NAND/AND architectures that have closer resemblance to the solid-state disk drives. Each of these approaches is suited for different applications, and finding wider usage throughout the industry. Chapter 4 of the present book provides a detailed overview of the EPROM/EEPROM and FRAM (or FeRAM) technology developments and architectures. In the growth of multipurpose flash memory market, the demands for all densities of flash are rising simultaneously, including that of low-density flash devices. In general, the flash memory technology can be divided into two broad categories: (a) NOR-based flash targeted toward the program code/data storage applications and (b) NAND-based flash ideal for mass storage applications. The NAND-based flash memories of 256-Mb and higher densities are already in mass production, with 512-Mb and higher densities also being targeted by the suppliers. The NOR-type flash memories are available with typical access times of 35 ns for 1-Mb memory devices and 45 ns for 128-Mb capacities. The latest approach to provide high storage densities in flash memories is the use of multilevel cell (MLC) charge storage per cell techniques. The EPROM/EEPROM technology is based on the charge storage in discrete trapping centers of an appropriate dielectric layer, or on a completely electrically isolated gate referred to as the ‘‘floating-gate’’ device. The floating- gate cell theory and operations, along with charge transport mechanisms such as the channel hot electron (CHE) injection and Fowler—Nordheim tunneling, are presented. The EPROM cell developments over last two decades include T-cell, X-cell, staggered virtual ground array (SVG) array cell, alternate metal ground (AMG) array cell, and so on. The EEPROM/flash memory cell arrays include ADVANCED SEMICONDUCTOR MEMORY DEVELOPMENTS 13 various NOR cell structures and NAND flash cells. An EEPROM technology that was developed earlier is the metal—nitride—oxide—silicon (MNOS) and (poly)silicon—oxide—nitride—oxide—semiconductor (SONOS), and floating- gate tunnel oxide (FLOTOX) cells that were briefly discussed in Semiconductor Memories [1]. Chapter 5 in the present book reviews the latest developments in EEP- ROM/flash cells and array structures. Four major flash architectures are reviewed: NOR, NAND, DINOR, and AND along with representative memory devices currently available from vendors, such as a 32-Mb simultaneous read/write NOR-based flash memory from Advanced Microdevices, Inc. (AMD), 32-Mb dual-plane flash memory from Intel Corp., 256-Mb Ultra- NAND flash memory from Samsung, Inc., and 16-Mb DINOR flash memory from Mitsubishi, Inc. The new developments include a proposed 3.3-V, 16-Mb nonvolatile memory using NAND flash architecture, which has operation virtually identical to that of a DRAM. The latest development in flash memory is the concept of multilevel (ML) that refers to the storage of more than one bit per cell, in order to increase the device density and reduce the cost per bit. In principle, the ML concept can be coupled with various types of memory architectures, such as the NOR and NAND to implement a 2-bit/cell scheme. However, there are several ML programming, sensing, and reliability issues that need to be addressed in each of these architectures. An example of this approach is Intel’s multilevel NOR-based architecture, which is currently capable of storing two bits per memory cell but may be scalable to three bits per cell. An overview of Intel’s 3-V StrataFlash NOR-based memory, a proposed multilevel 64-Mb NAND flash memory design, a 512-Mb NAND flash memory from Toshiba Corp., and a 256-Mb multilevel cell AND flash memory from Hitachi Corp. is provided. Semiconductor Memories reviewed general reliability issues such as the gate oxide breakdown, electromigration, hot-carrier degradation, metallization cor- rosion, and so on, which are generic among various semiconductor technologies [1]. However, there are a number of failure modes and mechanisms that are specific to the EPROM/EEPROMs such as data-retention characteristics and the number of write/erase cycles without degradation (endurance), which are critical reliability concerns. The major issues concerning yield and reliability of flash memories are flash overerase, program/read disturbs, program/read endurance, flash data retention failures, and flash hot carrier reliability effects. In general, the reliability of MLMs is more critical than that of the conventional two-level logic (1 bit/cell) because of the requirements for a larger threshold (VR) window (to keep adequate spacing among the stored levels) and/or reduced spacing between the adjacent levels (to limit the increase in VR window). Chapter 5 in the present book reviews the major reliability and yield issues for flash memories. 14 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES

FRAM is a RAM-based device that uses the ferroelectric (FE) effect as the charge storage mechanism, based on the ability of material to store an electrical polarization in the absence of an applied electric field; that is, a ferroelectric memory stores data within a crystalline structure. In FRAM, the memory readout is a destructive operation, and therefore each read access is accompanied by a precharge operation that restores the memory state. A write operation is very similar to a read operation and requires no system overhead. Some of the most widely used FE materials are PZT (PbZrVTi\VO) and SBT (SrBiTaO). An example of the FRAM is 256-Kb device that uses a two-transistor, two-capacitor (2T2C) memory cell design from Ramtron Corp. The new developments include one-transistor, one-capacitor (1T1C) memory cell design suitable for 1-Mb and higher-density designs. A proposed DRAM-like FeRAM cell array, referred to as the depletion FeRAM (DeFeRAM), and a 4-Mb FRAM with 1T1C cell design are discussed. A new chain FRAM (CFRAM) architecture has been proposed, which can realize 4F size memory cell and random access; and when 16 cells are connected in series, the chip size can be reduced to 63% to that of a conventional FRAM. Metal—ferroelectric semiconductor (MFS) devices that are considered candidates for high-density NVM applications are based on the principle that information can be stored as a polarization direction rather than as a charge on a capacitor. Ferroelectric films used as memory storage elements have significant reliability concerns, such as the aging/fatigue effects, thermal stabil- ity, effects of electric fields, and so on. These are briefly reviewed in Chapter 5. As the processor performance has increased from several hundred mega- hertz to 1 GHz and beyond, idle wait time in relatively slower DRAMs has increased, leading to a memory-processor performance gap. The fastest growing trend in advanced semiconductor memory is the embedded memories designs and applications. Memory technology for embedded memory has a wide variation, ranging from small blocks of ROMs, hundreds of kilobytes for the cache RAMs, high density (several megabits) of DRAMs, and small to medium density nonvolatile memory blocks of EEPROMs and flash memories. Embedded SRAM is one of the most frequently used memory embedded in logic chips. Chapter 6 discusses embedded memory designs and applications. Currently, the two major approaches for embedded memories development are fabricating memory in a logic-based process versus fabricating logic in a DRAM-based process technology. A recent trend driving the integration of DRAM into logic chips is the need to reduce power by eliminating the need for off-chip drivers and improving performance. Another advantage is noise reduction. A key advantage of the embedded memory approach is the higher packaging density and board space savings, which is a desirable feature for ADVANCED SEMICONDUCTOR MEMORY DEVELOPMENTS 15 applications such as the notebook computers and portable communication devices. The relative tradeoffs between the two approaches have spawned arguments regarding which technology should be preferred over the other. In a PC system, the major goal of using a cache memory is to increase the DRAM subsystem performance by reducing the latency and increasing bandwidth. Cache memory design tradeoffs are reviewed, and an example of cache architecture implementation for a popular TI DSP is provided. The demand for embedded memories is on the rise in current generation of ultra-large-scale integration (ULSI) and system-on-chip (SOC) level designs that require large amounts of SRAM, multiport RAM, DRAM, ROM, and EEPROM flash memories. Examples of some of the advanced SRAM macros are provided, such as one-transistor (1T) and four-transistor (4T) cell designs. In general, an ASIC with embedded memory will provide better system performance and a smaller part count as compared to a design that uses external memory. An example of early embedded DRAM development is Toshiba’s dRAMASIC process, based on two approaches, one of which utilizes one- transistor, one-capacitor (1T1C) architecture for providing high-density embedded DRAMs, and the other based on three-transistor cell that provides capability for implementation of low-density embedded DRAMs. Compiled DRAM macros of various densities, speeds, databus widths, dual-port configurations, and other features implemented in a merged DRAM/logic process are available from various companies. The latest developments include a 1-GHz eDRAM macro cell to serve large-capacity, on-chip L2 cache memory for gigahertz-level SOC designs. The embedding of DRAMs in a logic technology requires some additional steps to the standard logic process flow. Two examples of merged logic processor—DRAM architectures are discussed in detail: (1) Mitsubishi MR32/ D, a 32-bit RISC processor with DSP functions that uses 2 MB of DRAM plus 4 KB of cache SRAM on the same die and (2) a multimedia-oriented RISC processor that has a high-data-rate system and uses a concurrent RDRAM controller in a superscalar architecture. The alternative approach of DRAM technology with embedded logic architecture is mainly utilized by companies such as Mitsubishi, Samsung, Toshiba, and Infineon Technologies, which have DRAM manufacturing heri- tage and add some masking steps to include logic in their DRAM processes. Multimedia accelerators that require high data transfer rates between the frame buffers and the data processing units utilize embedded DRAM-logic approach. An example of Oki Electric Company’s multimedia accelerator, which inte- grates the MPEG-1 video/audio decoder, the 2-D graphic user interface (GUI) engine, and a RAMDAC (135-MHz, true color digital/analog converter),is provided. 16 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES

The University of California, Berkeley, approach for intelligent RAM (IRAM) supports designing the processor in a memory process, which has several advantages as well as disadvantages that are discussed. Another approach that has been proposed to implement processor-in-memory architecture is computational RAM (C·RAM) with a goal to make an effective use of internal memory bandwidth by pitch-matching simple process elements to memory columns. C·RAM can function either as a conventional memory chip or as a single-instruction stream, multiple-instruction data stream (SIMD) computer. The most popular examples of embedded flash memory devices are PLDs, FPGAs, DSPs, and microcontrollers. The embedded system designers prefer to use flash-based processors, which can be quickly programmed before transfer- ring their code to a more cost-effective ROM-based chip for high volume production. The use of embedded flash and EEPROM technologies in microcontrollers available from various suppliers is discussed. There is a growing need worldwide for small, inexpensive, rugged, and easily transportable forms of nonvolatile data storage. Flash card technology meets those requirements, and the use of flash cards is expected to grow exponentially over the next decade. Various flash card technologies are reviewed, such as the Advanced Technology Attachment (ATA), CompactFlash2+ Cards, Multi- Media Cards, and single-chip flash disk.

1.3. FUTURE MEMORY DIRECTIONS

Chapter 7 discusses mostly volatile and nonvolatile memory technologies that are in research and development, along with their future directions and potential for gigabit-to-terabit scaling. An example is magnetoresistive RAMs (MRAMs) that are nonvolatile magnetic storage devices based on the principle that a material’s magnetoresistance will change due to the presence of magnetic field. The MRAM technology has some attractive features such as the nondestructive readout (NDRO), high radiation tolerance, higher write/erase endurance compared to the FRAMs, and virtually unlimited power-off capabilities. Earlier MRAMs were based on the anisotropic magnetoresistance (AMR) effect, which has allowed realization of smaller elements with larger M-R effect and, therefore, a higher output signal. Some companies such as Honeywell, Nonvolatile Electronics, Inc. (NVE), have demonstrated working MRAM chips ranging from 64-Kb to 256-Kb densities. Many other companies such as Motorola, IBM, Infineon Technologies, Toshiba, and so on, are also actively developing MRAMs. The resonant tunneling diode (RTD) consists of an emitter region and collector region, and a double tunnel barrier structure, which contains a quantum well. Both tunnel diodes and RTDs exploit negative differential FUTURE MEMORY DIRECTIONS 17 resistance (NDR) characteristics of quantum-mechanical resonant tunneling currents. RTDs are of interest for use in multistate and compact memory, as well as in tunneling-based SRAM (TSRAM) cell designs and applications. A fully decoded 1-Kbit TSRAM prototype with DRAM-type high-speed signal- to-noise ratio sensing circuitry has been designed. An RTD-based sense amplifier circuit called a quantum MOS (QMOS) has been proposed that shows a good noise immunity and 20% faster sensing time as compared to the conventional CMOS design sense amplifier. RTDs based on III—V compound materials can achieve high peak current densities and appear to be the most likely candidates for advanced TSRAM designs and applications. In single-electron devices, the operating principle relies on the Coulomb repulsive force between electrons. These devices are expected to operate even at very small physical dimensions (atomic scale), making ultra-large-scale integration possible. Another potential advantage is ultra-low-power operation, because the device uses a very small number of electrons to perform basic operation. The majority of research in single-electron devices has been done at very low temperature, because room temperature operation requires very large Coulomb energy, accomplished only with sub-10-nm structures, which imposes lithographic limitations. Single-electron phenomena have also been observed at room temperature, and demonstration has included the concept of the floating-dot memory cell, in which nano-Si particles can replace the floating gate of the memory device. However, many major challenges need to be overcome before the commercial production of single-electron memory becomes feasible. Various single-electron memory configurations are under development, such as the SET flip-flop, electron-trap memory, SET ring memory, random background charge-independent memory, and single/multiple island memories. In a nanocrystal memory, the charge storage in a distributed floating gate offers several attractive characteristics such as the faster write times, operation at lower power than those for the EEPROMs, and better endurance characteristics than that for the flash EEPROMs. A 128-Mb single-electron memory prototype chip has been developed by Hitachi Central Laboratory, Japan, using the Coulomb blockade effect based on the electron repulsion within an ultrathin layer of polycrystalline material. The phase change memory technology stores information using structural phase changes in certain thin film alloys that typically utilize one or more elements from column VI of the periodic table (e.g., germanium and anti- monium). These phase change alloys are referred to as the chalcogenide materials. The phase change technology uses a thermally activated, rapid, reversible change in the structure of an alloy to store the data. Semiconductor memory elements using chalcogenide materials have been fabricated as technology demonstrators. Air Force Research Laboratories (AFRL) is funding 18 INTRODUCTION TO ADVANCED SEMICONDUCTOR MEMORIES development for a 64-Kb memory cell array as a characterization test chip, with plans for future migration to higher densities. A new area of research is protonic nonvolatile memories that are based on the observation that hydrogen ions (protons) can be used as the primary carriers of information in a silicon—silicon oxide—silicon (Si—SiO-Si) device, creating a memory function. Several patents have been granted based on this memory function concept. Chapter 7, Section 7.7 provides a few examples of the following new memory technology developments: · A proposed novel thyristor-based SRAM cell, called T-RAM, which has a cell area less than one-tenth the area of a conventional SRAM cell. It can provide DRAM densities, while the potential for speed is comparable to current generation of SRAMs. · An integrated content addressable read-only memory (CAROM) data storage system that uses data compression algorithm, which promises CD-ROM density, in an arbitrarily shaped data package without moving parts. A company, Autosophy, Inc., is planning to etch these arrays on foil, similar to active LCD production, which could result in foldable devices with hundreds of megabyte capacity that can fit in PC cards or other small cartridges. · Work (by IBM Research Group in Zurich, Switzerland) on development of a prototype called Millipede, which can store an amazing amount of data (e.g., 500 Gb/in.) as microscopic indentations on a flat polymer surface. This technology is similar to the operation of a phonographic stylus and is derived from the atomic force microscopy. · Holographic data storage that is considered a promising technology for achieving random access volumetric storage, offering orders of magnitude greater density than the surface storage. A unique advantage of holographic memories from the space applications perspective is their inherent radiation hardness. A holographic random access memory (HRAM) design has been proposed that can lead to the implementation of compact and inexpensive modules that can be used to construct large read—write memories. The greatest challenge for HRAM development is to improve its slow recording rate by several orders of magnitude.

REFERENCES

1. Ashok K. Sharma, Semiconductor Memories: Technology, Testing and Reliability, IEEE Press, New York, 1997. 2. B. McClean, B. Matas, and T. Yancey, The McClean Report: 2001 Edition, IC Insights, 2001.