Dual-Lens Electron Holography for Junction Profiling and Strain Mapping of Semiconductor Devices

Abstract

Electron microscopes have been used extensively to look at structure at the nanometer scale. Most of the information obtained from electron microscopes is amplitude information. Yet, the phase information of electron microscopy, which can be obtained from off-axis electron holography, provides unique information on electronic structure and structural changes in a wide variety of materials.For the semiconductor industry, junction profiling and strain mapping in Si at high spatial resolution provide information that is critical for further scaling of semiconductor devices. Because of the complex process conditions involved to control the junction position relative to the gate position, determination of junction position at high spatial resolution can help to reduce the cost and cycle time of development. Bright-field holography can measure the phase change of electrons traversing the materials, which is directly related to the mean inner potential of Si, indicating the junction position at the nanometer scale. Furthermore, in recent years stressors have been incorporated into devices to change the semiconductor lattice constant in the channel region and thereby enhance hole and electron mobility. Like the junction definition, the extra processing steps involved to add strain in a device have increased development and manufacturing costs. One way to minimize development cycle time is to monitor, at a nanometer scale, changes in channel deformation resulting from process changes. In 2008, Hytch et al. reported that dark-field holography can provide a promising path to nanometer scale strain mapping [1]. Cooper et al. reported using dark-field holography to measure strain related to different process conditions [2, 3].The requirements of electron holography to inspect the current generation of semiconductor devices are: (1) a fringe width (fringe overlap) in the range of about 100 to 800 nm for an adequate field of view (FOV), (2) fringe spacing between 0.5 and 10 nm for meaningful spatial resolution, (3) visibility of the fringe contrast (10–30%) for useful signal-to-noise ratio, and (4) adjust-ability of both the FOV and the fringe spacing relative to the sample. In previous papers and patent disclosures, we reported that we had developed a dual-lens electron holography method on a JEOL instrument to meet the above requirements, and later we implemented the same method on FEI instruments to provide a similar operational range for electron holography [4–6]. This dual-lens operation allows electron holography to be performed from low to high magnification and provides the FOV and fringe spacing necessary for two-dimensional (2D) junction profiling and strain measurements for devices with various sizes.In this article, we describe the electron optics for this method. We also describe several examples of junction profiling and strain mapping to show how to use dual-lens electron holography to resolve semiconductor device issues at high spatial resolution.