IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 36, NO. 5, zyxwvutsrqp MAY 1989 969 Infrared Microscopy Study of Anomdous Latchup Characteristics Due to Current Redistribution in Different Parasitic Paths CLAUD10 CANALI, FRANCESCO CORSI, MICHELE MUSCHITIELLO, AND ENRICO ZANONI, MEMBER, IEEE Abstract-Anomalous effects may arise during pulsed latchup tests, such as abrupt variations of the latchup current in steady-state con- ditions and “window” effects, i.e., the existence of a well-defined in- terval of IIO injected currents for latchup to occur, with lower and upper limits. Infrared microscopy allows the correlation of electrical characteristics with latchup current distribution and reveals that anomalous effects are due to the dynamic competition between different latchup paths, as confirmed zyxwvutsrqpo by the SPICE simulation of the lumped equivalent circuit of a CMOS output comprising two coupled p-n-p-n parasitic structures. I. INTRODUCTION 0 characterize electrical sensitivity to the latchup of T CMOS integrated circuits, different tests have been proposed, comprising 1) dc current tests, 2) injection of a positive/negative current by a curve tracer, and 3) pulsed overvoltages at I/O. Pulsed tests reduce device heating and consequent damage, provide a more realistic simu- lation of transients, and allow the study of the dynamic characteristics of latchup. However, due to the three-di- mensional current distribution within the same or different p-n-p-n structures on the same chip, anomalous effects may arise, such as the decrease of the latchup steady-state current, with an increase of the current injected into I/O, and “window” effects, i.e., the existence of a well-de- fined interval of injected currents with lower and upper limits for latchup to occur. Electrical characterization must be coupled with suita- ble analytical techniques, which allow localization of the active parasitic paths, identification of the triggering mechanism, and correction of the circuit layout. The ef- fectiveness of SEM techniques based on a capacitively coupled voltage contrast, electron-beam-induced current Manuscript received June zyxwvutsrqponm 13, 1988; revised December 16, 1988. This work was partially supported by the CNR, Progetto Finalizzato “Materiali e Dispositivi per 1’Elettronica a Stato Solido.” The review of this paper was arranged by Associate Editor R. R. Troutman. C. Canali and E. Zanoni are with the Dipartimento di Elettronica ed Informatica, Universita’ di Padova, Via Gradenigo 6a, 35131 Padova, It- aly. F. Corsi is with the Dipartimento di Elettrotecnica ed Elettronica, Univ- ersita’ di Bari, Via Re David 200, 70125 Bari, Italy. M. Muschitiello is with CSATA, Centro Studi e Applicazioni Tecnol- ogie Avanzate, Tecnopolis, Str. Prov. per Casamassima km. 3, 70100 Val- enzano, Bari, Italy. IEEE Log Number 8926431. (EBIC), and a stroboscopic voltage contrast in solving these problems has been previously demonstrated [4]-[6]. The task of solving latchup problems is further compli- cated by the presence of anomalous effects which may lead to neglecting potentially dangerous latchup paths and can induce serious errors in the evaluation and screening of CMOS integrated circuits [7]-[ 101. The aim of this paper is to demonstrate that infrared (IR) microscopy allows one to easily identify latchup sites and to correlate latchup current distribution with electrical characteristics, thus explaining anomalous effects. Re- sults of IR observations are further confirmed by the SPICE simulation of the lumped-equivalent circuit model of a CMOS output comprising two coupled parasitic paths, which reproduces electrical anomalies observed during pulsed tests on commercial devices. Potentials and limitations of IR microscopy for the study of latchup in CMOS are briefly reviewed in Section 11. Section I11 presents two examples of latchup analysis, which refer to a specific output of a) a 4-bit full adder, and b) a quad ex-or gate. Section IV reports results of the SPICE simulation on the lumped-equivalent circuit of a CMOS output, comprising two coupled latchup paths. Results are discussed in Section V, followed by the Con- clusions, Section VI. 11. THE INFRARED MICROSCOPY TECHNIQUE FOR LATCHUP ANALYSIS The latchup state induces a large current flow through the parasitic SCR structure. Latched areas can therefore be identified by detecting the near-IR radiation ( X = 1.1 pm ) emitted because of electron-hole recombination in Si junctions crossed by a large forward current [ 111 (see Fig. 1). To convert IR radiation into visible light, special IR- sensitive TV systems have to be used in conjunction with its objectives being modified and treated in order to min- imize aberrations and glare, and to optimize the contrast and image quality in the IR spectrum. To this purpose, PbS vidicon cameras are usually adopted as detectors; Fig. 2 shows the spectral response of the PbS vidicon of the Reichert Infrapol IR microscope adopted for this work. The response covers the range between 450 and 1800 nm, 0018-9383/89/0500-0969$01 .OO zyxwvut 0 1989 IEEE