Investigation of some aspects of the liquid crystal optical voltage contrast technique for integrated circuit physical analysis

Abstract

This paper presents results of the optical voltage contrast mechanism using nematic liquid crystal on an operational amplifier integrated circuit (op-amp IC) connected up as a unity-gain inverting amplifier. The dependence of the intensity of the contrast mechanism on the peak-to-peak voltage (Vpk-pk), duty cycle (d) and frequency (f) was investigated for an input symmetrical, bipolar square waveform. It was found that the optical voltage contrast intensity varies as the square of Vpk-pk for a constant duty cycle square wave. For a constant Vpk-pk, the intensity dependence on duty cycle d is proportional to d for d < 50% and (1-d) for d > 50%. The low-frequency cutoff of the intensity response occurs at tens of hertz. This correlates well to the response time of the liquid crystal molecules which is in the region of tenths of seconds. The high-frequency falloff of the intensity response, at tens of kilohertz, to a 9-11 V peak-to-peak voltage is due to the finite slew rate of the op-amp test specimen which is typically around 0.5 V/μs. The slewing of the output waveform is equivalent to a deviation of the duty cycle from that of 50% which gives rise to the maximum intensity signal. This was confirmed by investigating the amount of slew of the output waveform for different input frequencies, fitting an increased duty cycle waveform to the slewed waveform and calculating the theoretical intensity response. The theoretical falloff in intensity correlates well with the experimental high-frequency rolloff. © 1994.