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Voltage Comparator System for Contactless Microcircuit Testing

IP.com Disclosure Number: IPCOM000082950D
Original Publication Date: 1975-Mar-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 48K

Publishing Venue

IBM

Related People

DeStafeno, JJ: AUTHOR [+3]

Abstract

In present contactless testing of integrated circuits (IC's) using an electron beam and a voltage-sensitive detector to measure internal circuit voltages, the accuracy of the voltage measurements is limited by noise and instability in the video signal. Various sources contribute to the noise and instability, e.g., fluctuations in electron-gun emission, charging of insulators in or near the IC chip, RF interference from other equipment, etc.

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Voltage Comparator System for Contactless Microcircuit Testing

In present contactless testing of integrated circuits (IC's) using an electron beam and a voltage-sensitive detector to measure internal circuit voltages, the accuracy of the voltage measurements is limited by noise and instability in the video signal. Various sources contribute to the noise and instability, e.g., fluctuations in electron-gun emission, charging of insulators in or near the IC chip, RF interference from other equipment, etc.

The voltage comparator system in Fig. 1 accurately measures a DC voltage at a particular internal node of an integrated circuit, in the presence of such noise and instability.

Let the physical location of the internal node to be measured be denoted by point P(M). There is another physical location on the IC chip at which the voltage is constant and known, e.g., a ground or power supply pad. This second point, P(R), is used as a reference for the voltage measurement. The system moves the electron beam back and forth repeatedly between points P(M) and P(R) at some predetermined fixed frequency f, and maintains the electron beam position at each of the two points for a predetermined fixed time internal (dwell time).

The beam position is controlled by a digital scan generator which has as alternate inputs the X and Y addresses of the two points P(M) and P(R). The digital scan generator also controls a beam blanking unit to blank the electron beam during the time that the beam addresses are changing between P(M) and P(R). This transit time, during which the beam is blanked, can be made very short compared with the dwell times at points P(M) and P(R), during which the beam is unblanked.

As the beam is switched back and forth between P(M) and P(R), the video signal output varies depending on the voltages at the point to be measured and the reference point. A comparator (discussed below) measures the difference or the ratio of the two signal levels V(M) and V(R)corresponding to points P(M) and P(R).In particular, P(R) can be chosen such that V(R) corresponds to ground potential.

The comparator suppresses noise and instability by suppressing signal frequency components above or below the predetermined switching frequency f. Thus, for example, if the DC level of the video signal shifts due to a change in electron-gun emission, the difference between V(M) and V(R) will remain constant, and the AC component of the video signal at frequency f will remain constant. Similarly, RF interference at a frequency higher than f can be filtered out by a low-pass filter incorporated into the comparator.

The comparator may be either an analog or a digital circuit. In the analog case, it is a standard lock-in amplifier synchronized to the switching frequency f. The frequency reference signal from the digital scan generator is phase-shifted in the lock-in amplifier to maximize the voltage signal outp...