High-resolution microscopy through optically opaque media using ultrafast photoacoustics.

Alessandro Antoncecchi,Stefan Witte,Paul C M Planken,Hao Zhang,Vanessa Verrina,Stephen Edward

doi:10.1364/oe.405875

Alessandro Antoncecchi, Stefan Witte + Show 4 more

Open Access

https://doi.org/10.1364/oe.405875

Copy DOI

Abstract

We present a high-resolution microscope capable of imaging buried structures through optically opaque materials with micrometer transverse resolution and a nanometer-scale depth sensitivity. The ability to image through such materials is made possible by the use of laser ultrasonic techniques, where an ultrafast laser pulse launches acoustic waves inside an opaque layer and subsequent acoustic echoes from buried interfaces are detected optically by a time-delayed probe pulse. We show that the high frequency of the generated ultrasound waves enables imaging with a transverse resolution only limited by the optical detection system. We present the imaging system and signal analysis and demonstrate its imaging capability on complex microstructured objects through 200 nm thick metal layers and gratings through 500 nm thickness. Furthermore, we characterize the obtained imaging performance, achieving a diffraction-limited transverse resolution of 1.2 μm and a depth sensitivity better than 10 nm.

Highlights

IntroductionOptical metrology tools are employed for rapid positioning and accurate quantitative inspection of layer-to-layer placement in lithographic devices [1,2]
Optical imaging methods are essential in many areas of science and technology
Photoacoustic methods can provide contrast on micron-sized features commonly used in optical metrology, combined with the ability to detect such features through materials that are fully opaque to light

Summary

Introduction

Optical metrology tools are employed for rapid positioning and accurate quantitative inspection of layer-to-layer placement in lithographic devices [1,2] This type of metrology requires high-numerical-aperture imaging systems to be sensitive to sub-micrometersized metrology markers. Photoacoustic methods can provide contrast on micron-sized features commonly used in optical metrology, combined with the ability to detect such features through materials that are fully opaque to light. This combination of properties has already motivated the development of high-resolution photoacoustic imaging systems [17,18,19] using ultrafast-laser-induced ultrasound pulses. Et al [17] demonstrated the possibility to detect and image sub-optical-wavelength features through acoustic diffraction

Methods

Results

Conclusion