Hyunyul Lim

A 3 Dimensional Built-In Self-Repair Scheme for Yield Improvement of 3 Dimensional Memories

A 3-dimensional Built-In Self-Repair (3D BISR) scheme is proposed for 3-dimensional (3D) memories. The proposed 3D BISR scheme consists of two phases: a parallel test-repair phase, and a serial test-repair phase. After all memory dice are simultaneously tested, only the faulty memory dice are serially tested and repaired using one Built-In Redundancy Analysis (BIRA) module. Thus, it is a faster test-repair with low area overhead. The proposed BIRA algorithm with a post-share redundancy scheme performs exhaustive searches for all combinations of spare rows and columns. Experimental results show that the proposed 3D BISR is up to two times faster than the 3D serial test-serial repair BISR when seven 2048 × 2048 bit memory dice are stacked. The proposed 3D BISR requires 44.55% of the area in comparison to a 3D parallel test-parallel repair BISR for four stacked memory dice (one 128 K RAM, two 256 K RAMs, and 512 K RAM). The yield of 3D memories is the highest due to the exhaustive search BIRA algorithm with the post-share redundancy scheme as shown in various experimental results.

Lifetime Reliability Enhancement of Microprocessors: Mitigating the Impact of Negative Bias Temperature Instability

Ensuring lifetime reliability of microprocessors has become more critical. Continuous scaling and increasing temperatures due to growing power density are threatening lifetime reliability. Negative bias temperature instability (NBTI) has been known for decades, but its impact has been insignificant compared to other factors. Aggressive scaling, however, makes NBTI the most serious threat to chip lifetime reliability in todays and future process technologies. The delay of microprocessors gradually increases as time goes by, due to stress and recovery phases. The delay eventually becomes higher than the value required to meet design constraints, which results in failed systems. In this article, the mechanism of NBTI and its effects on lifetime reliability are presented, then various techniques to mitigate NBTI degradation on microprocessors are introduced. The mitigation can be addressed at either the circuit level or architectural level. Circuit-level techniques include design-time techniques such as transistor sizing and NBTI-aware synthesis. Forward body biasing, and adaptive voltage scaling are adaptive techniques that can mitigate NBTI degradation at the circuit level by controlling the threshold voltage or supply voltage to hide the lengthened delay caused by NBTI degradation. Reliability has been regarded as something to be addressed by chip manufacturers. However, there are recent attempts to bring lifetime reliability problems to the architectural level. Architectural techniques can reduce the cost added by circuit-level techniques, which are based on the worst-case degradation estimation. Traditional low-power and thermal management techniques can be successfully extended to deal with reliability problems since aging is dependent on power consumption and temperature. Self-repair is another option to enhance the lifetime of microprocessors using either core-level or lower-level redundancy. With a growing thermal crisis and constant scaling, lifetime reliability requires more intensive research in conjunction with other design issues.

Optimized Built-In Self-Repair for Multiple Memories

A new built-in self-repair (BISR) scheme is proposed for multiple embedded memories to find optimum point of the performance of BISR for multiple embedded memories. All memories are concurrently tested by the small dedicated built-in self-test to figure out the faulty memories, the number of faults, and irreparability. After all memories are tested, only faulty memories are serially tested and repaired by the shared built-in redundancy analysis according to the sizes of memories in descending order. Thus, the fast test and repair are performed with low area overhead. To accomplish an optimal repair rate and a fast analysis speed, an exhaustive search for all combinations of spare rows and columns is proposed based on the optimized fault collection. Experimental results show that the proposed BISR has the optimal repair rate because of the exhaustive search. The performance of the proposed BISR is located in the optimum point between the test and repair time, and the area overhead. For example, the proposed BISR requires 49.6% of the area and 1.3 times of the test and repair time in comparison with parallel BISR scheme for four memories (one 128 K, two 256 K, and one 512 K memories). Furthermore, the more there are memories, the more superior performance in terms of the test and repair time, and the area overhead is shown.

Thermal-aware dynamic voltage frequency scaling for many-core processors under process variations

Process variations yield the asymmetry on core performance in many-core processors. Adaptive voltage scaling can hide the variations, but that results in the different thermal characteristics of cores. By using the thermal characteristics, the efficiency of energy optimization and temperature management can be improved. Experiments showed that the proposed dynamic voltage frequency scaling consumes up to 25.2% less energy than the existing thermal management technique while remaining the ratio of peak temperature violations under 1%.

FRESH: A New Test Result Extraction Scheme for Fast TSV Tests

Three-dimensional integrated circuits (3-D ICs) are considered to meet the performance needs of future ICs. The core components of 3-D ICs are through-silicon vias (TSVs), which should pass appropriate prebond and post-bond tests in 3-D IC fabrication processes. The test inputs must be injected into the TSVs, and the test results must be extracted. This paper proposes a new test result extraction scheme [fast result extraction by selective shift-out (FRESH)] for prebond and post-bond TSV testing. With additional hardware, the proposed scheme remarkably reduces the TSV test time. FRESH avoids unnecessary test result extraction when the number of faulty TSVs in the TSV set is 0 or exceeds the number of TSV redundancies in the set. These early fault analyses are executed in the checkers of TSV groups. The experimental results show that the proposed scheme can reduce the result extraction time in practical environments.

3-D Stacked DRAM Refresh Management With Guaranteed Data Reliability

The 3-D integrated dynamic random-access memory (DRAM) structure with a processor is being widely studied due to advantages, such as a large band-width and data communication power reduction. In these structures, the massive heat generation of the processor results in a high operating temperature and a high refresh rate of the DRAM. Thus, in the 3-D DRAM over processor architecture, temperature-aware refresh management is necessary. However, temperature determination is difficult, because in the 3-D DRAM, the temperature changes dynamically and temperature variation in a DRAM die is complicated. In this paper, a thermal guard-band set-up method for 3-D stacked DRAM is proposed. It considers the latency of the temperature data and the position difference between the temperature sensor and the DRAM cell. With this method, the data reliability of the on-chip temperature sensor-dependent adaptive refresh control is guaranteed. In addition, an efficient temperature sensor built-in and refresh control method is analyzed. The expected refresh power reduction is examined through a simulation.

A New 3-D Fuse Architecture to Improve Yield of 3-D Memories

A new 3-D fuse architecture is proposed to improve the yield of 3-D memories. Because the 2-D memories are stacked to form a 3-D memory, the repair status of the prebond is kept as the good status. However, if faults occur in the postbond on the same cells which were repaired in the prebond, they must be identified and repaired because they cannot be repaired by the previous methods. There is no research on the repair the same faulty cells which occur in the prebond and postbond yet. Therefore, the new 3-D fuse architecture is proposed to repair the faulty cells which occur in the prebond and postbond. The redundancies which repair the faulty cells in the prebond are invalidated. The faulty cells are repaired by other redundancies in the postbond by the proposed 3-D fuse architecture. Thus, the proposed technique can improve the yield of 3-D memories. The experimental results show that the proposed technique can achieve higher yields of 3-D memories because only the proposed technique can repair the same faulty cells occurring in the prebond and postbond, and verify the good repair status of the 3-D memories.

Scan cell reordering algorithm for low power consumption during scan-based testing

Power consumption during scan-based testing can be higher than that of normal mode operations, which can cause yield loss and degradation of reliability. This paper proposes a scan cell reordering algorithm to reduce the test power consumption during scan-based testing. The proposed algorithm considers both shift-out operations and shift-in operations. A cumulative weighted transition (CWT) is proposed and compared to minimize the test power consumption. Experimental results show that the proposed method greatly reduces the average power during scan testing.

Dynamic thermal management for 3D multicore processors under process variations

Stacking core layers is emerging as an alternative for future high performance computing, but thermal problems have to be tackled first. When adaptive voltage scaling is adopted to hide the growing variation in the performance of cores, as a result, heat generation of each core varies. By exploiting the static thermal characteristics, the efficiency of dynamic thermal management can be improved. The proposed thermal management reduces the energy consumption by up to 30.02% compared with existing techniques, while keeping the ratio of temperature violations around 1%.