Quantitative Examination of Reverse-Biased Semiconductor Devices using Off- Axis Electron Holography

Abstract

The characterisation of device properties relies on quantitative determination of dopant distribution within the device structure. With the size of device features approaching the nanometre scale, the microelectronics industry requires characterisation techniques to image dopants with a spatial resolution of better than 10 nm. Electron holography satisfies this need, generating two-dimensional maps of electrostatic potential with sub 10 nm resolution using a field emission gun transmission electron microscope (FEGTEM) [1]. The technique uses an electrostatic biprism (typically a quartz wire coated with gold) to overlap an electron wave that has passed through an electron-transparent sample with a wave that has passed through vacuum. The interference pattern (or hologram) that forms in the overlap region contains information about the phase change of the electron wave caused by the sample. The electrostatic potential in the sample can be determined through examination of the reconstructed holographic phase image [2]. A systematic series of experiments have been conducted using a Philips CM300 FEGTEM, illustrating the application of electron holography to a Si p-n junction. The electrostatic potential distribution was examined as a function of applied reverse bias and membrane thickness. A single-tilt biasing holder (Fig. 1a) was used to examine cleaved wedge specimens, which have spring contacts made to their front and back faces. A membrane of uniform thickness was micro-machined in the cleaved samples using focused ion beam (FIB) milling (Fig. 1b).