Detection of Hidden Gratings through Multilayer Nanostructures Using Light and Sound

Abstract

We report on the detection of diffraction gratings buried below a stack of tens of 18-nm-thick SiO 2 and Si 3 N 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 are encoded on the spatial phase of the reflected acoustic wave. This grating-shaped acoustic reflection of the buried grating is detected by the diffraction of a delayed probe pulse. A detailed understanding of our measurements is essential for nanometrology applications in the semiconductor manufacturing industry, such as wafer alignment. 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. This allows us to identify the salient features in our measurements such as the presence of acoustic-wave-induced gratings inverted with respect to buried grating, the interference between the optical fields diffracted off multiple grating-shaped acoustic waves, and multiple thin dielectric layers behaving as a single effective acoustic medium. Our results show that laser-induced ultrasound is a promising technique for subsurface nanometrology applications.

Plasmonic enhancement of photoacoustic-induced reflection changes

“…Due to these high-frequency acoustic waves, we measure oscillatory changes of the reflection on the order of 1%. Analysis of these oscillations and their respective frequencies suggest that we observe multiple types of acoustic waves, three of which we identify as longitudinal waves (LWs) [3,27,28], surface waves [3,[48][49][50][51], and grating line quasi-NM oscillations [50,52,53]. We find that the phase of the LWs and the grating line NMs experiences a π phase shift when the wavelength is tuned around the SPP resonance wavelength.…”

“…Furthermore, these photoacoustic waves have been used to study a variety of material properties and physical phenomena, such as thermal and electron diffusion [7][8][9][10], electron-hole plasmas in semiconductors [11], hot-electron-induced pressure [12,13], elastic constants of single layer [14][15][16][17] and multilayer structures [18,19], the electronic structure of materials [20,21], surface deformation dynamics [22][23][24][25], and the generation of terahertz radiation [26]. Furthermore, laser-induced acoustic echoes have been used for the detection of buried gratings [27,28], showing promise for applications in the semiconductor device manufacturing industry.…”

“…Generally speaking, laser-induced acoustic echoes can be detected by measuring changes in optical properties, such as reflection from and transmission through a material [4], or by diffracting light from an acoustic-wave-induced grating [23,27,29]. Changes in optical reflection caused by acoustic waves are usually small and thus require fairly complicated setups involving high-repetition-rate laser oscillators and signal modulation/demodulation techniques.…”

Role of scattering by surface roughness in the photoacoustic detection of hidden micro-structures

“…We recently showed that laser-induced ultrasound can be employed for the detection of micro-and nano-structures buried underneath metal layers [31]. With this technique, we were able to detect gratings underneath optically opaque layers and dielectric multilayers, even after propagation of the acoustic wave through as many as 2 × 20 dielectric layers.…”

Photoacoustic detection of low duty cycle gratings through optically opaque layers

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] can be used to detect gratings buried below optically opaque layers. 19,20 In this technique, diffraction off an acoustic replica of the buried grating is used to detect the presence of the optically hidden grating. In this way, we were able to detect buried gratings even after propagation of the acoustic wave through as many as 2 Â 20 dielectric layers.…”