Consequently, the wet clean chemistry must be carefully optimized—often utilizing highly selective organic solvents or modified dilute cleans—to ensure complete residue removal without etching the highly doped junction .
This specific Ashing & Strip/Clean step immediately follows the intermediate
N-type Floating Diffusion (NFD) ion implantation in the shared-pixel architecture . During the preceding implant, the photoresist (PR) layer blocking the transfer gates and photodiode regions absorbs high-energy n-type dopants, transforming its upper layer into a highly crosslinked, carbonized crust known as high-dose ion-implanted photoresist (HDI-PR) . This hardened mask must be entirely eradicated before the subsequent P-Pinning photolithography step to ensure precise mask alignment and avoid introducing blocking defects . What distinguishes this step from other routine PR strips in the flow is the critical need to remove the physically hardened HDI-PR without altering the exceptionally sensitive, unannealed dopant profile of the newly formed floating diffusion node . The removal mechanism relies on a synergistic combination of plasma ashing and wet chemical stripping to overcome the physical barriers of the HDI-PR . The heavy ion bombardment from the NFD implant severely alters the surface mechanical properties of the resist, drastically increasing its hardness and elastic modulus . Standard liquid strippers cannot easily penetrate this dense carbonized shell, necessitating a preliminary plasma treatment to induce radical-driven polymer backbone scission and oxidation . High-energy reactive species, such as atomic oxygen and nitrogen radicals, lower the apparent activation energy of the stripping reactions, breaking down the crust so the underlying bulk resist can be dissolved . Following this mechanism, an activated wet stripper solution or a tailored solvent blend is utilized to lift off the residual polymer fragments and inorganic residues from the wafer surface . Material and chemistry selection during the wet clean phase is tightly constrained by the physical state of the underlying NFD silicon . Because the n-type dopants reside in a highly concentrated, unannealed, shallow surface layer, the silicon substrate is extremely vulnerable to chemical attack . Standard RCA SC1 (NH4OH/H2O2/H2O) cleaning sequences induce simultaneous oxidation and dissolution of the silicon surface, which can etch away critical nanometers of the substrate . For shallow, unannealed n-type implants, this chemical etching directly shaves off the peak dopant concentration, leading to massive dopant loss and an unacceptable reduction in active carrier concentration . Consequently, the wet clean chemistry must be carefully optimized—often utilizing highly selective organic solvents or modified dilute cleans—to ensure complete residue removal without etching the highly doped junction . At the 40nm CMOS Image Sensor node, controlling the exact dimensions and dopant volume of the floating diffusion is paramount because it dictates the node's capacitance and the pixel's conversion gain . Any plasma-induced charging or defect generation during the ash process could trap charges in the adjacent T1 and T2 transfer gate oxides, shifting the threshold voltage and inducing image lag . Furthermore, complete suppression of chemical dopant etching at this stage ensures that the resulting energy-band profile and barrier heights match the theoretical models required for optimal charge transfer .
Sign in to continue through all 417 steps