Real Time Nanoscale Cleaning Phenomenon Observation During Enforcing MHz Wave By Evanescent Field
Authors: Terayama Yutaka , Khajornrungruang Panart , Suzuki Keisuke , Mori Ryotaro , Hamada Satomi , Wada Yutaka , Hiyama Hirokuni 
Published: July 2022

In recent years, abrasive nanoparticles (sub Φ50 nm) have been applied in Chemical Mechanical Polishing (CMP) process to realize flattened surface of substrate for multilayer wiring structure and fine wiring width to achieve high performance Large Scale Integration Circuit. Definitely, the highly efficient removal of nanoparticles to be cause of the serious defect which remained on/in substrate in Post-CMP cleaning process is required. Therefore, it is indispensable to investigate the cleaning mechanisms by visualizing the cleaning phenomena that considered as nanoparticles’ behaviors during the process to realize optimal and efficient cleaning technology. However, the cleaning phenomena have not already clearly known because the residual contamination on the wafer surface has been usually inspected only before and after the cleaning process in dry condition, not in wet condition due to the surface scanning methods (e.g., scanning electron microscopy).

Generally, cleaning phenomenon is considered as abrasive nanoparticles’ behaviors, which are detached from a wafer surface and occasionally reattach to the surface or remain on the surface to be a residual contamination. These behaviors occurred in a few hundred nm range from the substrate surface. Hence, we have established the real-time visualization method to observe the nanoparticles’ behaviors in wet cleaning phenomena by applying an optical evanescent light that localized less than approx. 200 nm from the surface. Experimentally contact and non-contact cleaning phenomena also have been duplicated on an optical microscope that developed ourself.

In this paper, we visualized ultrasonic cleaning phenomena considered as Φ30, 50, 70 nm silica particles’ behaviors and/or cavitation effects movement during enforcing MHz wave by applying an evanescent light. Either Silica nanoparticles or cavitation effects that existing in an evanescent field were limitedly scatter to be the propagating light, which can reach to the optically CMOS area sensor of camera in a microscopy (Figure.1).

The typically observation results of the Φ30, 50, 70 nm silica particles’ detachment from the silica glass substrate surface during enforcing 1.1 MHz wave by oscillating piezo actuator with a power density of approx. 0.2 watt per square centimeter with the exposure time of 16 milliseconds in 60 frames per second as shown in Figure.2. The time-varying residual rate of silica particles on the surface was also evaluated by counting the Φ30, 50, 70 nm silica particles that attached on the surface in the observation area of 120 × 40 squared micrometers. Some of each silica nanoparticles were randomly intermittently detached from the surface or remained on the surface during the acoustic cavitation repeatedly irregularly occurs and decays near the surface by enforcing the MHz wave. The nanoparticle residual change rate showed trend that silica nanoparticles detached from the surface when the acoustic cavitation irregularly occurred near the surface.

The typically phenomenon was found that the residual silica nanoparticles were detached from the surface within 16 milliseconds (≒1/60 seconds) when the acoustic cavitation effects approached near to the surface that shown by background of high scattering light intensity as shown in Figure.3. Repeated occurrence of acoustic cavitation phenomenon seemed to accelerate the detachment of silica nanoparticles. By using the evanescent field, cleaning phenomena near the surface during enforcing MHz wave were visualized in real time.

Figure 1