A STUDY OF HOT-CARRIER EFFECTS USING PHOTON EMISSION SPECTROSCOPY

Abstract

The growing interest in the spectral characteristics of the hot-carrier-induced light emission phenomenon is based on the belief that the emitted photons carry physical information on the hot carriers and can be useful in understanding the high field effects in silicon (Si) MOSFETs. The motivation for such investigation is in understanding the underlying mechanisms responsible for the light generation. Another motivating factor is the potential use of the photon emission spectra as a signature for distinguishing between the different physical mechanisms responsible for the light emission. Photon emission spectroscopy is a technique that can provide such information. In this project, a new spectroscopic photon emission microscope system (SPEMS) with panchromatic imaging and continuous wavelength spectroscopic capabilities has been developed. With the developed system, very low levels of light emissions from biased devices can be detected and high-resolution spectral analysis can be performed. The spectral analysis system is used to investigate the hot-carrier-induced light emission phenomenon in n-channel MOSFETs under both normal operation and hot carrier stressing conditions.

The new spectroscopic photon emission microscope system consists of two parts, a conventional emission microscope and a retractable spectroscopic module of high spectral resolution. With the specially-designed spectroscopic system, which includes a high-ellipticity semiellipsoidal mirror as the light collector and wellcoupled high-transmission optics, both the spectral analysis capability and sensitivity have been improved significantly. Results shown include those of metal-oxide semiconductor transistors biased into saturation, forward and reverse biased PN junctions, and oxide leakage. The measured high-resolution spectra raises the possibility of applying the "defect finger-printing" technique to device failure analysis, whereby a unique spectral signature is assigned to each failure mechanism. Spectral characterization is one of the essential steps towards the establishment of such a one-to-one relationship between the spectral signature and failure mechanism. Two normalisation methods are introduced for spectral characterization. The wavelength parameters, A LO and Aso¾ obtained from the normalisation, reveal bias-dependent characteristics which are functions of the electric field within the device.

Spectral analysis is used to analyse the hot-carrier-induced light emission from MOSFETs under normal operation in order to understand the underlying physical emission mechanisms. Direct spectrally-resolved measurements of the photon emission from n-channel MOS transistors operating in saturation have been obtained. The theoretical calculation, based on the bremsstrahlung radiation mechanism, does not agree well with the experimental results when the variation of the channel electric field with bias is accounted for. Based on these observations and the Monte Carlo simulation of the electron energy distribution, we conclude that the bremsstrahlung radiation of hot electrons in the coulombic field is unlikely to be the dominant mechanism of photon emissions in n-channel MOSFETs.

Hot-carrier-induced device degradation distinguishes itself from the other reliability concerns as it occurs when the transistor is operated under normal conditions. This has made hot-carrier reliability one of the more interesting research topics for the past two decades. Device degradation under hot-carrier conditions can be monitored and predicted by the light emission. Spectral analyses of photon emissions from nMOSFETs stressed under maximum substrate current [lsub(rnax)] and maximum gate current [Ig(max)] conditions have been performed. The factors affecting photon emissions for stressed MOSFETs include changes in the channel electric field and substrate current lsub, the effect of surface scattering by the generated interface states and the distribution of the drain current. Interface state generation increases surface scattering that enhances the efficiency of photon emissions, while trapped electrons by driving the drain current away from the surface of the device, decreases the efficiency of photon emissions. It was found that the difference spectra, obtained from subtracting the device's pre-stress spectra from the post-stress spectra, are dependent on the stress conditions and thus could be used for distinguishing the effects of interface states and trapped charges.