40nm BSI CMOS Image SensorPreview

omplex fluorocarbon post-etch residues (PER) remain on the wafer [P1]. This specific Ashing & Strip/Clean step is critical because it directly precedes Ti/TiN barrier layer deposition, which requires an ultra-clean interface to form a low-resistance ohmic contact [T1]. Unlike earlier strip steps that may only deal with bulk dielectric etches, this step must completely clear the bottom of high-aspect-ratio contact holes without oxidizing the exposed silicon or eroding surrounding spacer and CESL structures [P2]. Any remaining polymeric sidewall residue or incomplete cleaning at the contact bottom will drastically increase the specific contact resistance or prevent proper adhesion of the subsequent metal layers [A1]. The removal process typically begins with an oxygen-based plasma ashing step to volatilize the bulk photoresist, but conventional high-temperature (e.g. [P3], 275 °C) ashing can cause the resist to crosslink and harden, leaving stubborn carbon-rich polymeric residues [P2]. To mitigate this, in-situ low-temperature plasma ashing is often utilized to balance reactive etching and surface passivation, preventing the thermal hardening of the resist crust [P2]. Following the dry ash, a wet strip/clean is required to dissolve the remaining highly crosslinked fluorinated and carbon-containing polymers that resist pure plasma removal [P1]. The wet chemistry functions by lowering the apparent activation energy of the stripping reactions, promoting polymer backbone scission and oxidation of the photoresist chains [P3]. Alternatively, ultraviolet (UV) irradiation can be used prior to wet cleaning to induce photochemical chain scission in C-C and C-F bonds, reducing the crosslink density of fluorocarbon residues and enabling their thorough dissolution by the solvent [P4]. The choice of wet cleaning chemistry is heavily dictated by the need to protect exposed sensitive materials while removing deeply embedded residues [A2]. Formulations often incorporate specific combinations of oxidizers, such as ammonium salts, and corrosion inhibitors like primary alkylamines, which selectively passivate delicate surfaces while oxidizing the target residues [A2]. Because fluorocarbon plasmas are used extensively in the preceding dielectric and CESL etches to ensure anisotropy, the resulting sidewall polymers are highly fluorinated and resistant to standard aqueous cleans [P4]. Therefore, advanced solvents mixed with specific additives or mechanical energy (such as megasonics) are selected to weaken the polymer structure and lift off the residues without causing dielectric under-etching or critical dimension loss [P1]. At the 40nm node, contact holes possess a very high aspect ratio, and the proximity of strained CESL layers makes the structures highly sensitive to both plasma damage and aggressive chemical attack [P1]. Traditional plasma descum or purely aqueous HF cleans can induce unacceptable dimension loss or damage the strain-inducing layers [P4]. Consequently, this step often employs carefully optimized low-damage schemes—such as UV modification combined with advanced organic solvents—to ensure the contact bottom is perfectly prepared [P4]. This pristine surface preparation fundamentally ensures the specific contact resistance remains sufficiently low for high-performance device operation [T1].