Thermal imaging of nanometer features

Abstract

To achieve the required performance with high speed switching transistors, the gate feature length in communication devices is as small as a few tens of nanometers in multi finger configurations and transistors are arrayed in a Monolithic Microwave Integrated Circuit (MMIC). The technology therefore, makes thermal characterization more and more difficult. We employ a transient thermal imaging technique to characterize the surface temperature of such nano-featured circuits. The setup is for a non-invasive and indirect thermoreflectance method with external light illumination and CCD imaging. Due to the diffraction limit, that is set by the optical properties of the objective lens in the microscope, optical and thermal images of features smaller than 300 nm blur. We propose an algorithm to resolve this problem by using a Gaussian approximation for the diffraction function in order to blur the thermoreflectance map obtained from modeling, and further use it to reconstruct the true thermal map of sub-diffraction sized devices. Thermal expansion of the device under test is another challenge for such high magnification microscope imaging. We employ a three dimensional Piezo stage controller to take the pixel-by-pixel thermoreflectance coefficients. With this combination, thermal imaging for wires with one-pixel width ~100 nm is achieved. Transient thermal imaging of multi hotspots provides the information of thermal invasion to the neighboring circuit by the thermal diffusion from the hotspots in the MMIC. We will demonstrate the technology component, which combined, could gain the required information for a potential 3-D thermal structure analysis for practical multiple nano-featured hotspots on a chip.