Direct observation of removal of SiO₂ nano-particles from silica surfaces: an evanescent field microscopy study and shear flow acting moment

In recent years, residual particles on the polished film surface or near the wiring of dozens of nanometer width/pitch can be a killer defect, which can lead to lower large-scale-integration circuit yield. ¹⁾ Definitely, removing smaller nano-particles (abrasive particles, cleaning waste, etc.) from the polished surfaces has been a significant challenge in the post-CMP (Chemical Mechanical Polishing) wet cleaning process. ²⁾

However, cleaning phenomena are not already clearly known because the residual contamination on the wafer surface has been usually inspected only after the cleaning process in dry conditions, not in wet conditions due to the surface scanning methods. Eventually, it is indispensable to investigate the cleaning mechanisms by observing the cleaning phenomena that are considered as the detachment of the nano-particle from the surface to be cleaned, and the occasional reattachment to the surface.

Thus, we have established a direct observation method of cleaning phenomena that occurred within the few hundred nanometers range from the surface by applying an evanescent light localized near the surface to be cleaned.

In this article, we have directly observed various-sized (Ø50, 70, and 300 nm) SiO₂ nano-particle behaviors near the SiO₂ surface by using developed evanescent wave microscopy in the inevitable shear flow field in a near parabolic velocity distribution with known velocity V_H to duplicate the rinse cleaning as shown in Fig. 1. After obtaining the evidence of each particle size that can be detached only by the rinse cleaning (shear flow), drag force and the moment (torque) acting on the nano-particles in the shear flow is also calculated to investigate quantitatively the size-dependent particle detachment from the surface.

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Evanescent light is generated in the range of a few hundred nanometers from the substrate surface when the total internal reflection occurs between the liquid that has a low refractive index (e.g., particle suspension, solution) and the substrate/thin film that has a higher refractive index in the incident angle θ_i condition above the critical angle θ_c (θ_i > θ_c = arcsin[n_L /n_H ]). ^3–5)

Particle detachment behaviors near the surface in wet conditions are observed in the shear flow with high contrast due to the limited scattering of the evanescent wave beam. ^6–9) Also, evanescent light can determine the nano-particle size which is attached to the surface by characterization of scattering light intensities. ^7,8) Thus, the cleaning phenomena can be observed in the shear flow after the particle size is determined.

Figure 2 shows the lab-made experimental setup, and Table SI (in the supplementary information) shows the experimental parameters. In this rinse cleaning experiment, a centrifugal force by practical rotation could be neglected, since the centrifugal force acting on the nano-particles is obviously less than 1% compared to the fluid drag force. A liquid flow cell was placed on the silica glass surface to observe nano-particle behaviors in the pulseless shear flow. We duplicated the flow velocity v_x (z) = z V_H /H, where z is the height from the surface, V_H = v_x (H) (≒ 70[μm/ms] at H = 1 μm) as shear flow during rinse cleaning. An evanescent field is generated by a laser to observe the nano-particle behavior near the surface as accumulated scattering light by an objective lens with a CMOS optical area sensor.

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Ø50, 70, and 300 nm standard SiO₂ particles attached by drying the suspension (Table SII shows that sample condition in detail), and the behaviors were observed near the silica glass surface as shown in Fig. 3 (sequentially observed images were shown in Fig. S1 in the supplementally information). It was recorded with an exposure time of 10 milliseconds in 100 frames per second. It showed the larger particle scatter higher light intensities, and detachment and remaining behaviors were quantitatively evaluated on a time axis. In practical rinse cleaning, it is well known that the residue particles were more detached from a SiO₂ surface, in high pH solution. However, it was difficult to detach residual sub-100 nm SiO₂ particles from the surface. The Ø300 nm SiO₂ particle was detached within 0.2 s.

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These results were also confirmed by surface contamination scanning inspection before and after the cleaning process by the imec group. ¹⁰⁾ It was implied that smaller particles did not receive enough detach energy. Hence, the magnitude of the drag force and moment acting on the residual particle on the surface were analyzed in the shear flow with a particle spherical curve. Previously, a shear flow as the Stokes flow ¹¹⁾ or a shear flow without particle spherical curve ¹²⁾ were widely calculated.

The drag force F_drag acted on the residual particle in the shear flow field with a linearly proportional velocity distribution with height z [μm] from the surface. The drag force acting on the differential sectional projected area dA was determined (Fig. S2 in the supplementally information),

$\,d{F}_{\mathrm{drag}}=\frac{1}{2}\rho {C}_{d}(z){[{v}_{x}(z)]}^{2}dA$

where ρ is the density of the solution, ν is the viscosity of the solution, C_d (z) is the drag coefficient, and v_x (z)[μm/ms] is the velocity distribution in the flow direction with height z [μm]. Integrating this over all the particle projection planes yields, F_drag is

$$\begin{eqnarray}&&{F}_{\mathrm{drag}}=6\rho v{V}_{{H}}{{\boldsymbol{D}}}^{{\bf{2}}}/H\end{eqnarray} \tag{ 1 }$$

Also, the moment action to particle was estimated as follows,

$$\begin{eqnarray}&&dM=d{F}_{{\rm{drag}}}\times z\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&M=4\rho \nu {V}_{{H}}{{\boldsymbol{D}}}^{{\bf{3}}}/H\end{eqnarray} \tag{ 3 }$$

For various constants given in the case of water, ρ = 1000 [kg m^–3], ν = 1.0 × 10⁻⁶ [m² s^–1], V_H = 70 [μm/ms], H = 1 [μm] is determined to calculate the absolute hydrodynamic drag force F_drag and moment action M as shown in Fig. 4,

$$\begin{eqnarray}&&{F}_{{\rm{drag}}}\left(D\right)=0.42{\left(D\left[{\bf{n}}{\bf{m}}\right]\right)}^{2}\left[{\rm{fN}}\right]\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&M\left(D\right)=0.28{\left(D\left[{\bf{n}}{\bf{m}}\right]\right)}^{3}[{\rm{fN}}\bullet {\rm{nm}}]\end{eqnarray} \tag{ 5 }$$

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The magnitude of the acting moment in the shear flow considerably increases compared with the increase in the magnitude of acting force, to be able to describe the detachability of the nano-particles on the surface. For this reason, the Ø300 nm particles were considered to be detached in the shear flow.

Acknowledgments