Detection of Hidden Gratings through Multilayer Nanostructures Using Light and Sound

Stephen Edward,Alessandro Antoncecchi,Hao Zhang,Vanessa Verrina,Stefan Witte,Irwan Setija,Paul Planken

doi:10.1103/physrevapplied.14.014015

Abstract

We report on the detection of diffraction gratings buried below a stack of tens of 18 nm thick $\mathrm{SiO_2}$ and $\mathrm{Si_3N_4}$ layers and an optically opaque metal layer, using laser-induced, extremely-high frequency ultrasound. In our experiments, the shape and amplitude of a buried metal grating is encoded on the spatial phase of the reflected acoustic wave. This grating-shaped acoustic echo from the buried grating is detected by diffraction of a delayed probe pulse. The shape and strength of the time-dependent diffraction signal can be accurately predicted using a 2D numerical model. Surprisingly, our numerical calculations show that the diffracted signal strength is not strongly influenced by the number of dielectric layers through which the acoustic wave has to propagate. Replacing the $\mathrm{SiO_2}$/$\mathrm{Si_3N_4}$ layer stack with a single layer having an equivalent time-averaged sound velocity and average density, has only a small effect on the shape and amplitude of the diffracted signal as a function of time. Our results show that laser-induced ultrasound is a promising technique for sub-surface nano-metrology applications.

Highlights

In semiconductor device manufacturing, techniques to detect micro and nanostructures buried below the surface of deposited layers are extremely important [1,2,3,4]
We show that the complex shape of the diffracted signal as a function of time can be reproduced using a comprehensive numerical model that includes the generation, propagation, and optical detection of the acoustic waves
Prior to experiments on samples with metallic and multiple dielectric layers, we first perform pump-probe experiments on relatively simple samples consisting of (i) a 10-nm amplitude 50% duty-cycle Au grating on a 522-nm flat Au layer deposited on glass [Fig. 3(a)] and, (ii) a 10-nm amplitude, 50% duty-cycle Ni grating on a 315-nm flat Ni layer deposited on glass [Fig. 3(b)]

Summary

INTRODUCTION

Techniques to detect micro and nanostructures buried below the surface of deposited layers are extremely important [1,2,3,4]. We show how we can detect buried gratings underneath optically opaque layers, by measuring transient optical diffraction from ultrafast, laser-induced, extremely high-frequency acoustic reflections from the grating. For the sample with ten bilayers of SiO2 and Si3N4 layers, after being generated, the acoustic wave has to travel through 42 layers in total before the acoustic echo reaches the glass-metal interface again where it is detected by diffraction of the optical probe pulse. Such phase shifts lead to destructive or constructive interference with the optical fields diffracted off acousticwave-induced gratings in the glass, which can strongly decrease or increase the amplitude of the diffracted signal Understanding these effects is crucial for a waferalignment application in an industry environment where a quantitative understanding of the signal is required to obtain a high measurement accuracy. Our results show that buried gratings can be detected through optically opaque layers on complex, multilayered samples, using laser-induced, extremely high-frequency ultrasound. The weaker part of the 800-nm beam is used as a probe

Detection of grating under metal layers

Detection of grating under metallic and multiple dielectric layers

Optical excitation of Au and Ni layers

Effective acoustic properties of the bilayer dielectric stack

CONCLUSION