Fault Testing of CMOS Integrated Circuits Using Signature Analysis Method

Transcription

1 Fault Testing of CMOS Integrated Circuits Using Signature Analysis Method Prof. R.H. Khade 1 and Mr. Swapnil Gourkar 2 1 Associate Professor, Department of Electronics Engineering, Pillai Institute of Information Technology, New Panvel, Mumbai University 2 Lecturer, Department of Electronics Engineering, Pillai Institute of Information Technology, New Panvel, Mumbai University ABSTRACT Moore s law states that the number of transistors in integrated circuits doubles every 18 months. Increasing complexity of digital system over the past decade has made it essential to increase the awareness of need of fault testing and diagnosis. With the increase in complexity of the digital system, a test simulation along with diagnosis has become an important issue in VLSI testing. In this paper, a signature analysis method is presented for testing of CMOS integrated circuits. Keywords: VLSI, CMOS, Testing, Signature 1. INTRODUCTION There has been a continuous pressure on VLSI chip manufacturing industry to increase the manufacturing yield. Integrated circuit manufacturers are constantly trying to decrease the number of faulty parts they produce. The reliability of System-on-Chips must be ensured to a certain extent since a single fault is likely to make the whole chip useless. Therefore, fault diagnosis and fault repairing techniques are gaining importance these days. A manufacturer may be able to improve the circuit design or the manufacturing process by analyzing the parts that fail production tests and determining the cause of failure for each part. Detection of fault and the type of fault present in a circuit is known as fault diagnosis. With the growth of technology and advent of reconfigurable circuits like FPGAs, PLAs, PLDs etc. testing only for faulty chip is not adequate [1]. Fault location may be required to identify and then replace or discard the faulty sub-circuit. It can also be used to analyze the defect causing the faulty behavior. Fault diagnosis is executed upon manufactured chips, which are found to be faulty in order to identify the position and types of the faults present in them [2]. 2. HISTORICAL BACKGROUND Current measurement based testing of electronics components has always been an integral part of the testing since the birth of semiconductor industry. It is used to detect gross shorts and is generally referred to as static I DD test. The present form of quiescent current (I DDQ ) measurement based testing for CMOS VLSI, known as I DDQ testing, was first publicly proposed in 1981 [1] and then formulated in [2] and [4] for the detection of bridging faults. Around the same time, researchers at IBM also proposed the monitoring of switching current to detect transient failures (noise related failures) in memory devices [5]. 3. TESTING OF THE CHIPS Fault simulation is the process of simulating a circuit with a given set of test patterns and a set of faults, and comparing the response of the circuit with each fault to that of the fault-free circuit. INPUT PATTERN CIRCUIT UNDER TEST OUTPUT RESPONSE CORRECT RESPONSE COMPARATOR TEST RESULT Figure 1 Test set up for testing a circuit If the response does not match, the fault is considered detected by the given set of test patterns. Figure 1 illustrates the basic principle of digital testing. Input patterns called test vectors are applied to the inputs of the circuit. The response Volume 2, Issue 5, May 2013 Page 73

2 of the circuit is compared with the expected response. The circuit is considered good if the responses match, else the circuit is faulty. The quality of the tested circuit depends upon the thoroughness of the test vectors. 4. IDDQ TESTING I DDQ testing refers to the integrated circuit (IC) testing method based upon measurement of steady state powersupply current. I DDQ stands for quiescent power-supply current. Today, the majority of ICs are manufactured using complementary metal oxide semiconductor (CMOS) technology. In steady state, when all switching transients are settled-down, a CMOS circuit dissipates almost zero static current. The leakage current in a defect-free CMOS circuit is negligible (on the order of few nano amperes). However, in case of a defect such as gate-oxide short or short between two metal lines, a conduction path from power-supply (V DD ) to ground (GND) is formed and subsequently the circuit dissipates significantly high current. This faulty current is a few orders of magnitude higher than the fault-free leakage current. Thus, by monitoring the power-supply current, one may distinguish between faulty and fault-free circuit [3]. The steady state or quiescent current (I DDQ ) testing of CMOS integrated circuits is known to be very efficient for improving test quality [8]. The test methodology based on the observation of the quiescent current on power supply lines allows a good coverage of physical defects such as gate-oxide shorts, floating gates and bridging faults. These defects are neither well modelled by the classical fault models, nor detectable by conventional logic tests. In addition, I DDQ testing can be used as a reliability predictor due to its ability to detect defects that do not yet involve faulty circuit behaviour, but could be transformed into functional failures at an early stage of circuit life. Thus, I DDQ testing became a powerful complement to the conventional logic testing. Under the fault conditions, the normal values of I DDQ may be increased, decreased or generally distorted. Thus, fault detection can be accomplished by monitoring the Iddq current fluctuations using a current sensing circuit. In report, a simple built-in current sensor (BICS) is presented, which provides a digital output for supply current monitoring and testing in circuits. BICS is inserted in series with the power supply or the ground of the Circuit under test (CUT) to detect abnormal I DDQ current in the integrated circuit [6] as shown in Figure 2. Figure 2 Block diagram of I DDQ testing. Irrespective of all the advantages of I DDQ testing, there are some problems in using this method. Since the normal I DDQ is very low, measurements must be precise. Also, setting I DDQ threshold on bad devices can be hard. In case of open faults, the I DDQ becomes very small making it difficult to measure and compare with the threshold. To overcome all the problems occurring in the I DDQ testing a signature analysis method is used. 4. SIGNATURE ANALYSIS METHOD Signature analysis uses statistical inference techniques to deduce possible failure mechanisms with limited failure analysis. The goal of signature analysis (SA) is to significantly reduce the amount of failure analysis (FA) that a laboratory must perform. The knowledge obtained from prior comprehensive FA is used to assign a confidence level to Volume 2, Issue 5, May 2013 Page 74

3 the failure mechanism of an IC with a similar failure mode. In addition, SA can be used to determine the number of ICs that must be comprehensively analyzed to obtain a certain degree of confidence for a specific failure mechanism. The most powerful aspect of this method is the ability to implement it for single failures, making it useful for field returns, qualification failures, and other one of a kind failures. Another important benefit of this method is the ability to use a low degree of belief with any number of signatures that are not well known or rely entirely on expert opinion. This method obviously requires a standard set of failure mechanisms as well as a standard set of terms and definitions for failure analysis. This SA method has the ability to be incorporated into an automated IC diagnosis process that uses defect models and classes for site localization in addition to failure mechanism determination. Improved models and diagnosis procedures will help reduce the amount of testing and physical failure analysis necessary to determine a signature for the failure mechanism and its location. Signature analysis is potentially a powerful method for reducing the level of work and improving the efficiency of an FA laboratory. While this method may reduce the level of work in a different manner than other signature analysis methodologies, it is more flexible and provides structure to the FA process. In this paper, signature analysis method is used to detect the faults in integrated circuits. For this, we first find the signature of the circuit and compare it with the actual output of the circuit using software simulation. Then by enabling different faults in the circuit we are going to obtain different simulation results. The simulation results corresponding to different faults are stored and these results are then compared with the signature of the circuit. Figure 3 Circuit to obtain the signature of integrated circuits Figure 3 shows the circuit diagram used to test any NMOS or PMOS combinational circuit. Initially no input is applied to the combinational circuit to be tested. NMOS is initially off & capacitor has no charge stored in it. Now input is given to the NMOS so it turns ON. Then the current direction is V DD -NMOS-C-V DD. Now capacitor is charged to value V DD. Then input to NMOS is made low & it turns OFF. Now capacitor is charged but it does not have any path to get discharged. When input is given to the combinational circuit, it turns ON & capacitor gets a path to discharge. The capacitor then discharges through the circuit. The time required for the capacitor depends on various parameters of the NMOS & PMOS in the circuit. This time period is then compared with the time period obtained in the simulation results. Using the circuit shown in fig.(4), we can find the signature of an inverter IC which is then compared with the simulated results of the fault free IC. Figure 4 Circuit for finding the signature of the inverter IC Volume 2, Issue 5, May 2013 Page 75

4 Figure 5 Signature of a fault-free inverter IC Figure 6 Signature of the faulty inverter IC (Length of channel is less than the normal) Volume 2, Issue 5, May 2013 Page 76

5 Figure 7 Signature of the faulty inverter IC (Length of channel is larger than the normal) Figure 8 Signature of the faulty inverter IC (Source of NMOS transistor is open) Volume 2, Issue 5, May 2013 Page 77

6 Figure 9 Signature of the faulty inverter IC (Drain of PMOS transistor is open) Figure 10 Signature of the faulty inverter IC (Stuck-at-0 fault at the gate of PMOS) Volume 2, Issue 5, May 2013 Page 78

7 Figure 11 Signature of the faulty inverter IC (Stuck-at-0 fault at the gate of NMOS) Figure 12 Signature of the faulty inverter IC (Stuck-at-1 fault at the gate of PMOS) Volume 2, Issue 5, May 2013 Page 79

8 Figure 13 Signature of the faulty inverter IC (Stuck-at-1 fault at the gate of NMOS) Figure 14 Circuit for comparing the signature of the fault free IC and IC under test Figure 5 shows the signature of the fault free inverter IC. In this signature, it can be seen that when the input is given to the inverter, the capacitor C starts discharging through it. In this figure, for fault free circuit voltage across capacitor and output of inverter is shown. Figure 6 to 13 show the responses of the inverter IC with different faults. These faults occur when the length or width of the channel of the transistors is varied or in the presence of stuck-at faults at various Volume 2, Issue 5, May 2013 Page 80

9 nodes. Figure 14 shows the circuit which compares the signature of the faulty IC with the fault free IC. This circuit contains two counters which counts the number of pulses at the output of the inverter. The outputs of the two counters are 4-bit binary numbers which are then given to a comparator IC. If the IC under test is fault free then the outputs of both the counters will be same and the comparator will show no fault. But if there is any fault in the IC, the output of both the counters will be different and the comparator will show that the IC is faulty. 5. CONCLUSION A new method for signature analysis has been described. This method provides a robust and flexible way to implement signature analysis. The most powerful aspect of this method is the ability to implement it for single failures, making it useful for field returns, qualification failures. Another important benefit of this method is the ability to use a low degree of belief with any number of signatures that are not well known or rely entirely on expert opinion. This method obviously requires a standard set of failure mechanisms as well as a standard set of terms and definitions for failure analysis. This SA method has the ability to be incorporated into an automated IC diagnosis process that uses defect models and classes for site localization in addition to failure mechanism determination. Improved models and diagnosis procedures will help reduce the amount of testing and physical failure analysis necessary to determine a signature for the failure mechanism and its location. References [1] Groza, V.; Abielmona, R.; Assaf, M.H.; Elbadri, M.; El-Kadri, M.; Khalaf, A.; A Self-Reconfigurable Platform for Built-In Self-Test Applications, Instrumentation and Measurement, IEEE Transactions on, vol.56, no.4, pp , Aug [2] M. Abramovici, M. A. Breuer and A. D. Friedman, Digital Systems Testing and Testable Design, IEEE Press, [3] Rochit Rajsuman, Iddq Testing for CMOS VLSI, PROCEEDINGS OF THE IEEE, VOL. 88, NO. 4, APRIL [4] J. M. Acken, Testing for bridging faults (shorts) in CMOS circuits, in Design Auto. Conf., 1983, pp [5] R. Y. Li, S. C. Diehl, and S. Harrison, Power supply noise testing of VLSI chips, in Int. Test Conf., 1983, pp [6] D. Baschiera and B. Courtois, Testing CMOS: A challenge, in VLSI Design, Oct. 1984, pp [7] C. F. Hawkins and J. Soden, Electrical characteristics and testing for gate oxide shorts in CMOS ICs, in Int. Test Conf., 1985, pp [8] E. I. Muehldorf, A quality measure for LSI components, IEEE J. Solid State Circuits, pp , Oct [9] R. L. Wadsack, Fault modeling and logic simulation of CMOS and MOS integrated circuits, Bell Syst. Tech. J., pp , May June [10] J. Shen, W. Maly, and F. Ferguson, Systematic characterization of physical defects for fault analysis of MOS IC cells, in Int. Test Conf., 1984, pp [11] H.Walker and S. Director, VLASIC: A catastrophic fault yield simulator for integrated circuits, IEEE Trans. Computer-Aided Design, pp , Jan [12] M. Abramovici, M. A. Breuer and A. D. Friedman, Digital Systems Testing and Testable Design, IEEE Press, [13] Bob Duell, Iddq made easy CMOS Iddq test methodology Fundamental concepts, System Science, Inc., Revised edition,1997. [14] J.M. Soden, C.F. Hawkins, R.K. Gulati and W. Mao IDDQ testing: a review, Journal of Electronic Testing: Theory And Applications, vol.3, 1992, pp [15] S.D McEuen, Reliability benefits of IDDQ, J. of Electronic Testing: Theory and Application, Vol.3, 1992, pp [16] J.A. Segura, V.H. Champac, R.R. Montanes, J. Figueras and J.A. Rubio, Quiescent current analysis and experimentation of defective CMOS circuits, J. of Electronic Testing: Theory and Applications, Vol.3, 1992, pp [17] P. Nigh, W. Maly, Test generation for current testing, IEEE Design and Test of Computers, Feb.1990, pp [18] S.D McEuen, Reliability benefits of IDDQ, J. of Electronic Testing: Theory and Application, Vol.3, 1992, pp [19] K.J. Lee and J.J. Tang, A built-in current sensor based on current-mode design, IEEE Transactions on Circuits and Systems-II Analog and Digital Signal Processing, Vol. 45, No. 1, Jan.1998, pp [20] T.L. Shen, J. C. Daly, and J. C. Lo, On Chip current sensing circuit for CMOS VLSI, Proc. IEEE VLSI Test Symposium paper 16.2, 1992, pp [21] Christopher L. Henderson and Jerry M. Soden, A Signature Analysis Method for IC Failure Analysis, International Symposium for Testing and Failure Analysis, Nov I996. Volume 2, Issue 5, May 2013 Page 81

10 AUTHOR Rajendrakumar H. Khade is born in Akola district of Maharashtra state (India) on August 10, He has completed B.E. Electronics from S.G.G.S. Institute of Engineering and Technology, Vishnupuri Nanded (MS) India in 1987 and M.E. Electronics from V.J.T.I. Mumbai in 1999.He worked as faculty of Electronics department of R.A.I.T. Nerul, Navi Mumbai for 23 years. From July 2011 he joined Pillai Institute of Information Technology, New Panvel. His special fields of interest include VLSI, Digital System Design, Image Processing. Swapnil S. Gourkar received the B.E. degree in Electronics and Telecommunication Engineering in 2010 from Jawaharlal Darda Institute of Engineering and Technology, Yavatmal. Currently he is pursuing Masters Degree in Electronics Engineering from Pillai Institute of Information Technology, New Panvel and also working as a lecturer for under graduate programme at the same institute since His fields of interest are VLSI and Digital Electronics. Volume 2, Issue 5, May 2013 Page 82