An Investigation of Microplasma Noise in Zener Diodes with the SEM

Abstract

Avalanche breakdown and microplasma effects in silicon PN junctions are the direct source of noise generating mechanisms in these devices. Some of the noise sources are fundamentally related to the breakdown mechanism involved. However, in practical Zener diode structures, the mechanisms which contribute to excess noise often result from process induced defects and material imperfections. Breakdown mechanisms have been studied extensively using light emission optical miocroscopy,(1-5) but the technique is limited to shallow structures containing junction depths less than 1 ?m, particularly when bulk breakdown effects are being studied. These structures are generally free from overlying metal films and are designed to minimize optical absorption effects. Practical Zener diode structures are often more deeply diffused, and contain passivation layers and metal surface films. More recently, utilizing the electron beam induced current mode, the scanning electron microscope has been used to investigate electrical effects such as microplasma phenomena an and avalanche breakdown in PN junctions,(6-8) as well as structural imperfect ions in semiconductor devices. (9-12) This technique, provides the necessary means for the non-destructive investigation of avalanche breakdown sites which give rise to excess electrical noise in Zener diode structures. In the present investigation, a comparison is drawn between Zener diodes which exhibit surface and bulk breakdown mechanisms. Examples are presented which illustrate both typical breakdown noise in these structures, and excess noise resulting from specific process induced defects.