Friedrich Hapke

X-masking during logic BIST and its impact on defect coverage

We present a technique for making a circuit ready for logic built-in self test by masking unknown values at its outputs. In order to keep the silicon area cost low, some known bits in output responses are also allowed to be masked. These bits are selected based on a stuck-at n-detection based metric, such that the impact of masking on the defect coverage is minimal. An analysis based on a probabilistic model for resistive short defects indicates that the coverage loss for unmodeled defects is negligible for relatively low values of n.

On Acceleration of SAT-Based ATPG for Industrial Designs

Due to the rapidly growing size of integrated circuits, there is a need for new algorithms for automatic test pattern generation (ATPG). While classical algorithms reach their limit, there have been recent advances in algorithms to solve Boolean Satisfiability (SAT). Because Boolean SAT solvers are working on conjunctive normal forms (CNFs), the problem has to be transformed. During transformation, relevant information about the problem might get lost and, therefore, is not available in the solving process. In this paper, we present a technique that applies structural knowledge about the circuit during the transformation. As a result, the size of the problem instances decreases, as well as the run time of the ATPG process. The technique was implemented, and experimental results are presented. The approach was combined with the ATPG framework of NXP Semiconductors. It is shown that the overall performance of an industrial framework can significantly be improved. Further experiments show the benefits with regard to the efficiency and robustness of the combined approach.

Embedded multi-detect ATPG and Its Effect on the Detection of Unmodeled Defects

The demand for higher quality requires more effective testing to filter out the bad devices. It is already known that multi-detection of single stuck-at faults results in more fortuitous detections of defects not behaving as stuck-at faults, which increases the test quality. Existing multi-detect tests, i.e., the well-known n-detect tests, suffer from significant test size increases. This paper shows that embedding multi-detection of faults within regular ATPG patterns results in a higher quality without a significant increase in test set size. High-volume silicon measurement results demonstrate that embedded multi-detect tests detect 2.3% to 4.7% more defective devices than conventional single-detect stuck-at tests.

Programmable deterministic Built-In Self-Test

In this paper, we propose a new programmable deterministic built-in self-Test (BIST) method that requires significantly lower storage for deterministic patterns than existing programmable methods and provides high flexibility for test engineering in both internal and external test. Theoretical analysis suggests that significantly more care bits can be encoded in the seed of a linear feedback shift register (LFSR), if a limited number of conflicting equations is ignored in the employed linear equation system. The ignored care bits are separately embedded into the LFSR pattern. In contrast to known deterministic BIST schemes based on test set embedding, the embedding logic function is not hardwired. Instead, this information is stored in memory using a special compression and decompression method. Experiments for benchmark circuits and industrial designs demonstrate that the approach has considerably higher overall coding efficiency than the existing methods.

Cell-Aware Test

This paper describes the new cell-aware test (CAT) approach, which enables a transistor-level and defect-based ATPG on full CMOS-based designs to significantly reduce the defect rate of manufactured ICs, including FinFET technologies. We present results from a defect-oriented CAT fault model generation for 1,940 standard library cells, as well as the application of CAT to several industrial designs. We present high volume production test results from a 32 nm notebook processor and from a 350 nm automotive design, including the achieved defect rate reduction in defective-parts-per-million. We also present CAT diagnosis and physical failure analysis results from one failing part and give an outlook for using the functionality for quickly ramping up the yield in advanced technology nodes.

Defect-oriented cell-aware ATPG and fault simulation for industrial cell libraries and designs

Industry is facing increasingly tougher quality requirements for more complex ICs. To meet these quality requirements we need to improve the defect coverage. This paper presents a new methodology to significantly increase the defect coverage of the test patterns generated by ATPG tools. The fault model used during the ATPG is enhanced to directly target layout-based intra-cell faults. In contrast to previous techniques, such as Gate-Exhaustive, N-Detect, or Embedded-Multi-Detect, which either are too complex for real-world designs or merely improve the probability of detecting intra-cell defects, the new approach targets the actual root causes of intra-cell defects. The newly proposed Cell-Aware-methodology has been evaluated for 90nm and 65nm technologies on 1671 library cells and on 10 real industrial designs with up to 50 million faults. The experimental results show an average increase of 1.2% in defect coverage and a reduction of 420ppm in escape rate for a 50mm2 design.

Efficient pattern mapping for deterministic logic BIST

Deterministic logic BIST (DLBIST) is an attractive test strategy, since it combines advantages of deterministic external testing and pseudo-random LBIST. Unfortunately, previously published DLBIST methods are unsuited for large ICs, since computing time and memory consumption of the DLBIST synthesis algorithms increase exponentially, or at least cubically, with the circuit size. In this paper, we propose a novel DLBIST synthesis procedure that has nearly linear complexity in terms of both computing time and memory consumption. The new algorithms are based on binary decision diagrams (BDDs). We demonstrate the efficiency of the new algorithms for industrial designs up to 2M gates.

Cell-aware Production test results from a 32-nm notebook processor

This paper describes a new approach for significantly improving overall defect coverage for CMOS-based designs. We present results from a defect-oriented cell-aware (CA) library characterization and pattern-generation flow and its application to 1,900 cells of a 32-nm technology. The CA flow enabled us to detect cell-internal bridges and opens that caused static, gross-delay, and small-delay defects. We present highvolume production test results from a 32-nm notebook processor to which CA test patterns were applied, including the defect rate reduction in PPM that was achieved after testing 800,000 parts. We also present cell-internal diagnosis and physical failure analysis results from one failing part.

Restrict Encoding for Mixed-Mode BIST

Programmable mixed-mode BIST schemes combine pseudo-random pattern testing and deterministic test. This paper presents a synthesis technique for a mixed-mode BIST scheme which is able to exploit the regularities of a deterministic test pattern set for minimizing the hardware overhead and memory requirements. The scheme saves more than 50% hardware costs compared with the best schemes known so far while complete programmability is still preserved.

PASSAT: efficient SAT-based test pattern generation for industrial circuits

Automatic test pattern generation (ATPG) based on Boolean satisfiability (SAT) has been proposed as an alternative to classical search algorithms. SAT-based ATPG turned out to be more robust and more effective by formulating the problem as a set of equations. In this paper, we present an efficient ATPG algorithm that makes use of powerful SAT-solving techniques. Problem specific heuristics are applied to guide the search. In contrast to previous SAT-based algorithms, the new approach can also cope with tri-states. The algorithm has been implemented as the tool PASSAT. Experimental results on large industrial circuits are given to demonstrate the quality and efficiency of the algorithm.