Real Time Nanoscale Cleaning Phenomenon Observation during PVA Brush Scrubbing by Evanescent Field
Authors: Terayama Yutaka , Khajornrungruang Panart , Suzuki Keisuke , Kusatsu Kohei , Hamada Satomi , Wada Yutaka , Hiyama Hirokuni 
Published: February 2020

Cleaning process is indispensable in Post chemical mechanical polishing (Post-CMP) to remove the abrasive nano-particles remaining on the polished wafer surface. There are two main types of the cleaning process. One is contact cleaning with polyvinyl alcohol (PVA) sponge brush scrubbing, and the other is non-contact cleaning enforced with mega sonic by piezoelectric unit or dual (gas-liquid phase spray) jet cleaning. We have been focusing on the phenomenon of PVA brush contact cleaning process.

Generally, cleaning phenomenon is considered as abrasive nano-particle behaviors, which are removed from a wafer surface and occasionally reattach to the surface or still stay on the surface to be a residual contamination. However, these contaminations on a wafer surface are usually inspected before and after cleaning process, not during the cleaning process. Hence, we have been duplicated experimental cleaning process of thin film material on a silica glass surface, and have also been proposing a real time observing method to inspect the removal of abrasive nano-particle behaviors during the cleaning process.

Because these behaviors occur in a few hundred nm range from the wafer surface. Therefore, an evanescent light that localizes on the surface being cleaned in range of a few hundred nm can be applied to limitedly optically illuminate to observe either abrasive nano-particle behaviors or PVA brush movement. By this optical illumination, the contaminated nano-particles and/or PVA brush existing in an evanescent field will scatter their evanescent field to be the propagating light, which can reach to a camera in a microscopy to visualize the brush scrubbing cleaning phenomenon (figure(a)).

However, both scattering lights from PVA brush and nano-particles are same wavelength of illuminated laser. In this paper, 100±20 nm fluorescence silica-nano-particles were employed to identify the contaminated silica particles by fluorescent wavelength (λ=600 nm) that different from the illuminated laser wavelength (λ=450 nm).

A typical result of optical method is shown in figure(b). A silica particle that was already adhered on a glass surface was removed by sliding of PVA brush in pure water. Evidently, fluorescence scattering light (red light intensity : λ=600 nm) and PVA brush scattering light (blue light intensity : λ=450 nm) could be distinguished in the recorded phenomenon in rate of 120 frame per second by a RGB color camera.

Before coming of PVA brush, 100 nm silica particle already adhered on a glass surface. Next, PVA brush moved with a sliding speed of 2.5 mm per second by a rotation motor. Where the sliding speed was calculated by distance of PVA brush movement during 1/120 s (≈8.3 ms). Then, 100 nm silica particle was removed from surface after 33.3 ms as quantitatively shown in figure(c). These results imply that nano-particle was removing away at the same time during the attachment of PVA brush to the nano-particle or the approach near to the nano-particle. By using evanescent field and fluorescent nano-particle, we can observe nanoscale cleaning phenomenon during PVA brush scrubbing in real time.

Figure 1