Charge trapping in irradiated SOI wafers measured by second harmonic generation

Total dose effects on silicon on insulator (SOI) UNIBOND wafers are studied via optical second harmonic generation (SHG). This technique is qualitatively compared with the pseudo-MOSFET technique for monitoring charges at the interfaces. Optical and electrical methods are combined to separate the contribution of the signal from each interface to the total SHG intensity. Radiation-induced oxide and interface traps increase the interface fields as determined from the SHG signals and the results are compared with electrical measurements.

I. Introduction

MOS TRANSISTORS fabricated on silicon on insulator (SOI) wafers have received great attention because of advantages in device isolation, speed, density, and scalability over bulk silicon devices [1]. Although SOI devices are naturally resistant to transient photocurrents and single event upset, total-dose irradiation may induce a parasitic conduction path at the buried oxide (BOX) interface due to radiation-induced oxide and interface traps [2]. The Si/BOX interface quality has been studied electrically through Fig. 1. Schematic diagrams of UNIBOND wafer structures used for optical measurements. Incident fundamental beam and selected SHG signals from (a) Si island and (b) BOX region. In (c) we show a schematic diagram of the incident and reflected beams. Doping is p-type for the Si film and substrate ; the thickness of the Si film is 72–160 nm; that of the BOX is 145 nm or 230 nm; and the area of the Si islands is 25 . various techniques such as I-V and C-V measurements. Wafer-level measurements via the pseudo-MOS technique are frequently used for evaluation of partially-processed wafers [3]– [5]. The limitation of this technique is that it damages the active device regions by directly probing the Si-film, and it is limited to characterization of the top interface. Recently, the second harmonic generation (SHG) technique has been proposed as a nondestructive and noninvasive probing technique since the SH signal detects the electric field at the interface without directly contacting the surface [6], [7]. Furthermore, the large penetration depth of the optical radiation allows us to use SHG as a sensitive probe of electric field at deeply buried SOI interfaces. Therefore, SHG can be an attractive alternative to investigate carrier dynamics in SOI wafers. In this paper, we apply SHG to characterization of multi-interfacial UNIBOND wafers. Significant effects on charge generation, trapping, and recombination at interfaces are measured for irradiated samples.