Well-designed circuits are one key ldquoinsulatingrdquo layer between the increasingly unruly behavior of scaled complementary metal-oxide-semiconductor devices and the systems we seek to construct ...from them. As we move forward into the nanoscale regime, circuit design is burdened to ldquohiderdquo more of the problems intrinsic to deeply scaled devices. How this is being accomplished is the subject of this paper. We discuss new techniques for logic circuits and interconnect, for memory, and for clock and power distribution. We survey work to build accurate simulation models for nanoscale devices. We discuss the unique problems posed by nanoscale lithography and the role of geometrically regular circuits as one promising solution. Finally, we look at recent computer-aided design efforts in modeling, analysis, and optimization for nanoscale designs with ever increasing amounts of statistical variation.
Retention errors, caused by charge leakage over time, are the dominant source of flash memory errors. Understanding, characterizing, and reducing retention errors can significantly improve NAND flash ...memory reliability and endurance. In this paper, we first characterize, with real 2y-nm MLC NAND flash chips, how the threshold voltage distribution of flash memory changes with different retention age - the length of time since a flash cell was programmed. We observe from our characterization results that 1) the optimal read reference voltage of a flash cell, using which the data can be read with the lowest raw bit error rate (RBER), systematically changes with its retention age, and 2) different regions of flash memory can have different retention ages, and hence different optimal read reference voltages. Based on our findings, we propose two new techniques. First, Retention Optimized Reading (ROR) adaptively learns and applies the optimal read reference voltage for each flash memory block online. The key idea of ROR is to periodically learn a tight upper bound, and from there approach the optimal read reference voltage. Our evaluations show that ROR can extend flash memory lifetime by 64% and reduce average error correction latency by 10.1%, with only 768 KB storage overhead in flash memory for a 512 GB flash-based SSD. Second, Retention Failure Recovery (RFR) recovers data with uncorrectable errors offline by identifying and probabilistically correcting flash cells with retention errors. Our evaluation shows that RFR reduces RBER by 50%, which essentially doubles the error correction capability, and thus can effectively recover data from otherwise uncorrectable flash errors.
With continued scaling of NAND flash memory process technology and multiple bits programmed per cell, NAND flash reliability and endurance are degrading. Understanding, characterizing, and modeling ...the distribution of the threshold voltages across different cells in a modern multi-level cell (MLC) flash memory can enable the design of more effective and efficient error correction mechanisms to combat this degradation. We show the first published experimental measurement-based characterization of the threshold voltage distribution of flash memory. To accomplish this, we develop a testing infrastructure that uses the read retry feature present in some 2Y-nm (i.e., 20-24nm) flash chips. We devise a model of the threshold voltage distributions taking into account program/erase (P/E) cycle effects, analyze the noise in the distributions, and evaluate the accuracy of our model. A key result is that the threshold voltage distribution can be modeled, with more than 95% accuracy, as a Gaussian distribution with additive white noise, which shifts to the right and widens as P/E cycles increase. The novel characterization and models provided in this paper can enable the design of more effective error tolerance mechanisms for future flash memories.
Modern NAND flash memory chips provide high density by storing two bits ofdata in each flash cell, called a multi-level cell (MLC). An MLC partitions the threshold voltage range of a flash cell into ...four voltage states. When aflash cell is programmed, a high voltage is applied to the cell. Due to parasitic capacitance coupling between flash cells that are physically close to each other, flash cell programming can lead to cell-to-cell program interference, which introduces errors into neighboring flash cells. In order to reduce the impact of cell-to-cell interference on the reliability ofMLC NAND flash memory, flash manufacturers adopt a two-step programming method, which programs the MLC in two separate steps. First, the flash memory partially programs the least significant bit of the MLC to some intermediate threshold voltage. Second, it programs the most significant bit to bring the MLC up to its full voltage state. In this paper, we demonstrate that two-step programming exposes new reliability and security vulnerabilities. We experimentally characterize the effects of two-step programming using contemporary 1X-nm (i.e., 15-19nm)flash memory chips. Wefind that a partially-programmed flash cell (i.e., a cell where the second programming step has not yet been performed) is much more vulnerable to cell-to-cell interference and read disturb than a fully-programmed cell. We show that it is possible to exploit these vulnerabilities on solid-state drives (SSDs) to alter the partially-programmed data, causing (potentially malicious) data corruption. Building on our experimental observations, we propose several new mechanisms for MLC NAND flash memory that eliminate or mitigate data corruption in partially-programmed cells, thereby removing or reducing the extent of the vulnerabilities, and at the same time increasing flash memory lifetime by 16%.
As NAND flash memory continues to scale down to smaller process technology nodes, its reliability and endurance are degrading. One important source of reduced reliability is the phenomenon of program ...interference: when a flash cell is programmed to a value, the programming operation affects the threshold voltage of not only that cell, but also the other cells surrounding it. This interference potentially causes a surrounding cell to move to a logical state (i.e., a threshold voltage range) that is different from its original state, leading to an error when the cell is read. Understanding, characterizing, and modeling of program interference, i.e., how much the threshold voltage of a cell shifts when another cell is programmed, can enable the design of mechanisms that can effectively and efficiently predict and/or tolerate such errors. In this paper, we provide the first experimental characterization of and a realistic model for program interference in modern MLC NAND flash memory. To this end, we utilize the read-retry mechanism present in some state-of-the-art 2Y-nm (i.e., 20-24nm) flash chips to measure the changes in threshold voltage distributions of cells when a particular cell is programmed. Our results show that the amount of program interference received by a cell depends on 1) the location of the programmed cells, 2) the order in which cells are programmed, and 3) the data values of the cell that is being programmed as well as the cells surrounding it. Based on our experimental characterization, we develop a new model that predicts the amount of program interference as a function of threshold voltage values and changes in neighboring cells. We devise and evaluate one application of this model that adjusts the read reference voltage to the predicted threshold voltage distribution with the goal of minimizing erroneous reads. Our analysis shows that this new technique can reduce the raw flash bit error rate by 64% and thereby improve flash lifetime by 30%. We hope that the understanding and models developed in this paper lead to other error tolerance mechanisms for future flash memories.
With the continued globalization of the IC manufacturing supply chain, securing that supply chain is becoming increasingly difficult and this opens the door to a myriad of security threats such as ...unauthorized production, counterfeiting, IP theft, and hardware Trojan Horses. A parallel and related threat is posed by advanced reverse engineering capabilities, such that even chips manufactured at the most advanced technology nodes can be de-layered, imaged, and analyzed 1. While various manufacturing methodologies and camouflaged gates have been proposed, none fully address these threats, especially in combination. To address these concerns, we use post-manufacturing programmable camouflaged logic topology to simultaneously obscure the design IP from the manufacturer as well as combat reverse engineering. The basis of the design is a threshold-voltage-defined (TVD) logic gate topology that solely uses different threshold voltage implants to determine the logic gate function 2. Every gate has an identical physical layout and is post-manufacturing programmed with different threshold voltages for different Boolean functions using intentional directed hot-carrier injection (HCI). Similar intentional HCI techniques have previously been used to enhance SRAM margins, boost PUF reliability, and build TRNGs 34. The design is fully compatible with standard CMOS logic processes, requiring no special layers, structures, or process steps.
With the continued scaling of NAND flash and multi-level cell technology, flash-based storage has gained widespread use in systems ranging from mobile platforms to enterprise servers. However, the ...robustness of NAND flash cells is an increasing concern, especially at nanometer-regime process geometries. NAND flash memory bit error rate increases exponentially with the number of program/erase cycles. Stronger error correcting codes (ECC) can be used to tolerate higher error rates, but these have diminishing returns with increasing P/E cycles and can have prohibitively high power, area, and latency overheads. The goal of this paper is to develop new techniques that can tolerate high bit error rates without requiring prohibitively strong ECC. Our techniques, called Flash Correct-and-Refresh (FCR) exploit the observation that the dominant error source in NAND flash memory is retention errors, caused by flash cells losing charge over time. The key idea is to periodically read, correct, and reprogram (in-place) or remap the stored data before it accumulates more retention errors than can be corrected by simple ECC. Detailed simulations of a solid-state drive (SSD) storage system driven by measured experimental data from error characterization on real flash memory chips show that our techniques provide 46× average lifetime improvement on a variety of workloads at no additional hardware cost. We also find that our techniques achieve lifetime improvements that cannot feasibly be achieved with stronger ECC.
Single-Chip Heterogeneous Computing Chung, Eric S.; Milder, Peter A.; Hoe, James C. ...
2010 43rd Annual IEEE/ACM International Symposium on Microarchitecture,
12/2010
Conference Proceeding
To extend the exponential performance scaling of future chip multiprocessors, improving energy efficiency has become a first-class priority. Single-chip heterogeneous computing has the potential to ...achieve greater energy efficiency by combining traditional processors with unconventional cores (U-cores) such as custom logic, FPGAs, or GPGPUs. Although U-cores are effective at increasing performance, their benefits can also diminish given the scarcity of projected bandwidth in the future. To understand the relative merits between different approaches in the face of technology constraints, this work builds on prior modeling of heterogeneous multicores to support U-cores. Unlike prior models that trade performance, power, and area using well-known relationships between simple and complex processors, our model must consider the less-obvious relationships between conventional processors and a diverse set of U-cores. Further, our model supports speculation of future designs from scaling trends predicted by the ITRS road map. The predictive power of our model depends upon U-core-specific parameters derived by measuring performance and power of tuned applications on today's state-of-the-art multicores, GPUs, FPGAs, and ASICs. Our results reinforce some current-day understandings of the potential and limitations of U-cores and also provides new insights on their relative merits.
In deep sub-micron ICs, growing amounts of on-die memory and scaling effects make embedded memories increasingly vulnerable to reliability and yield problems. As scaling progresses, soft and hard ...errors in the memory system will increase and single error events are more likely to cause large-scale multi- bit errors. However, conventional memory protection techniques can neither detect nor correct large-scale multi-bit errors without incurring large performance, area, and power overheads. We propose two-dimensional (2D) error coding in embedded memories, a scalable multi-bit error protection technique to improve memory reliability and yield. The key innovation is the use of vertical error coding across words that is used only for error correction in combination with conventional per-word horizontal error coding. We evaluate this scheme in the cache hierarchies of two representative chip multiprocessor designs and show that 2D error coding can correct clustered errors up to 32times32 bits with significantly smaller performance, area, and power overheads than conventional techniques.
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