Ultrasonic cleaning achieves high particle removal efficiency without applying macroscopic mechanical pressure to the silicon substrate, thus safeguarding the structural integrity of the wafer .
The ultrasonic clean step is strategically positioned immediately following the CIS Backside Wafer Edg
e-Grind and prior to the Backside Si CMP steps . The edge-grinding process relies on mechanical abrasion to contour the wafer perimeter, which inherently generates significant quantities of silicon dust and abrasive particulate residue . If these submicron particles are not rigorously removed, they will act as parasitic abrasives during the subsequent CMP operations, leading to catastrophic micro-scratching and defect generation on the thinned silicon surface . Therefore, this cleaning step serves as a critical particulate decontamination barrier, ensuring a pristine backside surface to maintain the stability and defect-free requirements of the downstream planarization processes . The core physical mechanism of this step relies on the transmission of high-frequency acoustic waves through a liquid medium to remove adhered contaminants . Particles adhere to the wafer surface primarily through short-range van der Waals attraction and initial hydrogen bonding, which determine the baseline adhesion strength . The ultrasonic or megasonic waves generate alternating high and low pressure cycles in the fluid, resulting in the formation and violent collapse of cavitation bubbles . The implosion of these bubbles creates localized high-velocity micro-jets and intense acoustic streaming that exert extreme hydrodynamic shear forces on the wafer surface . When this applied hydrodynamic force exceeds the combined van der Waals and electrostatic adhesion forces, the particles are successfully dislodged from the substrate . To prevent re-deposition, the pH of the cleaning fluid is often tuned to modify the zeta potential of both the wafer surface and the dislodged particles, enforcing an electrostatic repulsion between them . A non-contact acoustic cleaning method is explicitly selected over contact-based scrubbing due to the fragile nature of the extensively thinned, bonded wafer pair . While full-contact brush cleaning can overcome high adhesion forces via direct normal and tangential mechanical stress, it poses a severe risk of mechanical damage to the delicately ground wafer edge . Ultrasonic cleaning achieves high particle removal efficiency without applying macroscopic mechanical pressure to the silicon substrate, thus safeguarding the structural integrity of the wafer . Furthermore, specialized equipment designs can utilize localized acoustic cleaning, where only the wafer backside contacts a surface-tension-elevated liquid meniscus, thereby avoiding full wafer immersion and further reducing mechanical stress on the bonded interface . For a 40nm BSI CMOS Image Sensor, the requirement for absolute surface integrity is extremely stringent because the backside silicon directly interfaces with incoming photons (Engineering Practice). Any residual submicron particles that cause nanoscale scratching during the subsequent CMP steps can introduce localized energy states within the silicon bandgap . These defect states act as carrier generation-recombination centers, which fundamentally degrade device performance by increasing dark current and white pixel defects . Consequently, optimizing the ultrasonic cleaning efficiency is essential for preserving the optoelectronic fidelity of the 40nm CIS architecture .
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