Precise Identification of Memory Faults Using Electrical Simulation

Low power circuits have become a necessary part in modern designs. Retention flip-flop is one of the most important components in low power designs. Conventional test methodologies are not sufficient to test the retention flip-flop thoroughly. This paper presents four new fault models and the testing of retention flip-flop. The four fault models are awake-mode stuck-at fault, sleep-mode stuck-at fault, awake-mode transition fault, and sleep-mode transition fault. The four faults model the defects that affect the retained value, wakeup time, and sleep time of retention flip-flops. Based on the new fault models, test patterns for retention flip-flop can be easily generated by current automatic test pattern generation tools. The proposed test methodology is validated by performing experiments on ISCAS'89 benchmark circuits and some realistic industrial low power designs. The experimental results show that the faults of retention flip-flops can be easily detected by our method and the average fault coverage is higher than 98%. The fault coverage of conventional single stuck-at fault and transition fault test can be increased by more than 1%.

We define a robust fault model as a model where the existence of an undetectable fault implies the existence of logic redundancy. The stuck-at fault model is robust, but other fault models such as certain bridging and interconnect open fault models are not. A robust fault model provides a mechanism to synthesize circuits in which all the target faults are detectable and 100% fault coverage is achievable. This is important since it provides a direct link between test quality and the circuit synthesis. We discuss robust fault models for bridging faults and interconnect open faults, and their use as part of a test generation process for a non-robust fault model.

Fault analysis of memory devices using defect injection and simulation is becoming increasingly important as the complexity of memory faulty behavior increases. In this paper this approach is used to study the effects of opens and shorts on the faulty behavior of embedded DRAM (eDRAM) devices produced by Infineon Technologies. The analysis shows the existence of previously defined memory fault models, and establishes new ones. The paper also investigates the concept of dynamic faulty behavior and establishes its importance for memory devices. Conditions to test the newly established fault models are also given.

The objective of this paper is to propose a new fault model suitable for test pattern generation for an FPGA configured to implement a given application. The paper demonstrates that the faults affecting the bit cells of the look-up tables (LUTs) are not redundant, although they store constant values. We demonstrate that these faults cannot be neglected and that the fault model corresponding to modifying the content of each LUT memory cell must be considered in order to cover the full range of possible faults. In order to evaluate the fault coverage of the proposed fault model, a set of circuits mapped on a Xilinx Virtex 300 FPGA have been considered. Test sequences generated by a gate-level commercial ATPG and an academic RT-level one have been fault simulated on these benchmark circuits. The obtained figures show that a high percentage of faults affecting the LUT bit cells are undetected, thus suggesting that suitable ATPG algorithms adopting the new fault model are required.

Recent work in IC failure analysis strongly indicates the need for fault models that directly analyze the function of circuit primitives. The input pattern (IP) fault model is a functional fault model that allows for both complete and partial functional verification of every circuit module, independent of the design level. We describe the IP fault model and provide a method for analyzing IP faults using standard SSL-based fault simulators and test generation tools. The method is used to generate test sets that target the IP faults of the ISCAS85 benchmark circuits and a carry-lookahead adder. Improved IP fault coverage for the benchmarks and the adder is obtained by adding a small number of test patterns to tests that target only SSL faults. We also conducted fault simulation experiments that show IP test patterns are effective in detecting non-targeted faults such as bridging and transistor stuck-on faults. Finally, we discuss the notion of IP redundancy and show how large amounts of this redundancy exist in the benchmarks and in SSL-irredundant adder circuits.

Software-based self-test (SBST) has recently emerged as an effective methodology for the manufacturing test of processors and other components in systems-on-chip (SoCs). By moving test related functions from external resources to the SoC's interior, in the form of test programs that the on-chip processor executes, SBST significantly reduces the need for high-cost, big-iron testers, and enables high-quality at-speed testing and performance binning. Thus far, SBST approaches have focused almost exclusively on the functional (programmer visible) components of the processor. In this paper, we analyze the challenges involved in testing an important component of modern processors, namely, the pipelining logic, and propose a systematic SBST methodology to address them. We first demonstrate that SBST programs that only target the functional components of the processor are not sufficient to test the pipeline logic, resulting in a significant loss of overall processor fault coverage. We further identify the testability hotspots in the pipeline logic using two fully pipelined reduced instruction set computer (RISC) processor benchmarks. Finally, we develop a systematic SBST methodology that enhances existing SBST programs so that they comprehensively test the pipeline logic. The proposed methodology is complementary to previous SBST techniques that target functional components (their results can form the input to our methodology, and thus we can reuse the test development effort behind preexisting SBST programs). We automate our methodology and incorporate it in an integrated software environment (developed using Java, XML, and archC) for the automatic generation of SBST routines for microprocessors. We apply the methodology to the two complex benchmark RISC processors with respect to two fault models: stuck-at fault model and transition delay fault model. Simulation results show that our methodology provides significant improvements for the two fault models, both for the entire processor (12% fault coverage improvement on average) and for the pipeline logic itself (19% fault coverage improvement on average), compared to a conventional SBST approach.

Summary form only given: Embedded memories have become the fastest growing segment of Systems on Chip (SoC) in recent years. According to the International Technology Roadmap for Semiconductors, embedded memories will continue to dominate the increasing SoC chip area in the future, approaching 94% within one decade. Hence, these memories will severely impact all aspects of SoC manufacturing including yield, quality and reliability. Additionally, nanotechnology is causing higher levels of device-parameter variations and new failure mechanisms that are not yet well understood. Precise fault modeling to design efficient tests is therefore essential in order to keep the test cost and test time within economically acceptable limits, while keeping higher product quality. The objective of this embedded tutorial is to provide test engineers, memory and ASIC designers, researchers, students and managers with an overview of state-of-the art of fault models for SRAMs (including traditional fault modeling and recent development in fault models for current and future technologies), together with the latest test algorithms. Next, it describes test plans, which are shown to depend on the size of the SRAM, its speed, and the application domain. Thereafter, industrial results are used to show the effectiveness of the proposed algorithms and stresses. Last, future challenges in embedded memory testing (e.g., in fault modeling, test design) are highlighted.

Although electrical simulation has become a vital tool in the design process of memory devices, memory testing has not yet been able to employ electrical simulation as an integral part of the test generation and optimization process. This is due to the exponential complexity of the simulation-based fault analysis, a complexity that made such an analysis impractical. This paper describes new methods to reduce the complexity of the fault analysis from exponential to constant with respect to the number of analyzed operations, thereby making it possible: 1) to use electrical simulation to generate test patterns; and 2) to perform simulation-based stress optimization of tests. The paper also discusses ways to analyze the impact of idle time on the faulty behavior. In addition, results of a fault analysis study performed to verify the new analysis method are shown, where the new analysis reduces the analysis time by a factor of 30.

Spot defects in memory devices are caused by imperfections in the fabrication process of these devices. In order to analyze the faulty effect of spot defects on the memory behavior, simulations have been performed on an electrical model of the memory in which the defects are injected, causing opens, shorts, or bridges. In this paper, simulation is used to analyze the faulty behavior of embedded DRAM (eDRAM) devices produced by Infineon Technologies. The paper applies the new approach of fault primitives to perform this analysis. The analysis shows the existence of most traditional memory fault models and establishes new ones. The paper also investigates the concept of dynamic faulty behavior and establishes its importance for eDRAMs. Conditions to test the newly established fault models, together with a test, are also given.

In this paper, a method of low power analog testing is proposed. In spite of having Oscillation Based Built in Self-Test methodology (OBIST), a look up table based (LUT) low power testing approach has been proposed to find out the faulty circuit and also to sort out the particular fault location in the circuit. In this paper an operational amplifier, which is the basic building block in the analog circuit, is designed and is taken for testing purpose. Fault coverage is identified after fault modeling, fault injection and fault simulation. More than 93% fault coverage is achieved and there is a scope of increasing more fault coverage. Since analog testing prefaces the challenge of power dissipation during testing, some power minimization techniques like sleepy stack method and current correlation method have adhered during the testing process. Test power reduction up to 84 % is achieved in this work.

Conventional fault models define their faulty behavior at the IO ports of standard cells with simple rules of fault activation and fault propagation. However, there still exist some defects inside a cell (intra-cell) or between two cells (dual-cell) that cannot be effectively detected by the test patterns of conventional fault models and hence become a source of DPPM. In order to further increase the defect coverage, many research works have been conducted to study the fault models resulting from different types of intra-cell and dual-cell defects, by SPICE-simulating each targeted defect with its equivalent circuit-level defect model. However, it was considered computationally infeasible to simulate every possible defective scenario for a cell library and obtain a complete set of cell-level fault models. In this paper, we present a new dual-cell-aware (DCA) framework based on examining the layout of two adjacent cells (i.e., a dual cell) to identify potential defects, where time-consuming RC extraction can be avoided and the runtime for SPICE simulation can be reduced. Experimental results and silicon data on a SoC product show that the proposed DCA framework can not only save runtime significantly but also maintain the promising efficacy of DCA tests for the objective of lowering DPPM.

We propose a new technique for computing exact fault coverage for any fault model. It consists of appropriate formation and counting of colors. Each color represents a set of faults, and the definition of colors varies according to the fault model. The technique utilizes the two aspects on which the fault coverage for any model depends, the circuit lines and the patterns applied to the circuit. Depending upon the sample space of faults for a given model, the representation of faults as colors differs. Colors are generated in a greedy and on demand manner ensuring they are unique. Even though the technique is simple in nature, it has never been used to calculate fault coverage for any fault model before. In this paper we apply it to calculate the fault coverage for the path delay fault model. Our experimental results show improvement over the existent techniques for the above mentioned model.

The March test is a widely-applied test method for memory arrays. Nowadays, the advanced memory technologies keep introducing new types of defects, which bring diverse multi-operation fault models in turn. The automatic March sequence generation methods are preferred to avoid the inefficient manual design. Most previous methods generate the March sequences that cover limited types of faults and strongly depend on the specific characteristics of known faults. To address this issue, this work proposes MarchGen, a March sequence generation method for memory faults with arbitrary number of operations. Firstly, the fault hierarchies, reduced taxonomy and generic test conditions of 2-composite faults are analyzed. Then, the heuristic March sequence generation workflow is designed accordingly, which takes either fault models or test sequences as input. Three optimization strategies are utilized to improve the fault coverage of the generated March sequences and the generation time consumption. The evaluation results show that the proposed method has low dependence on fault types. The generated March sequences reach 100% fault coverage for any simple and 2-composite faults or test sequences, and the generation time is roughly linear with the size of input set.

DRAMs play an important role in the semiconductor industry, due to their highly dense layout and their low price per bit. This paper presents the first framework of fault models specifically designed to describe the faulty behavior of DRAMs. The fault models in this paper are the outcome of a close collaboration with the industry, and are validated using a detailed. Spice-based analysis of the faulty behavior of real DRAMs. The resulting fault space is then used to derive a couple of new DRAM-specific tests, needed to detect some of the faults in practice

In this paper, a functional fault model for analog circuit diagnosis is proposed. A faulty module is modeled as a fault-free module in serial or in parallel with a fault module. To extract such a fault module, we adopt an iterative deconvolution technique to deconvolute the impulse response of the fault module from the faulty response. The test results show that with such a fault model and fault extraction technique the diagnostic resolution is improved significantly due to the separation of the fault and the system function. Moreover, such a fault model allows single-module fault tables to be applied to the diagnosis of a multi-module system.