40nm BSI CMOS Image SensorPreview

This Ashing & Strip/Clean step is critically positioned immediately following the highly aggressive Silicon Full Trench Etch (Anisotropic) [F_DTI] and before the Trench Vacuum dry and SiN Fill steps [P3]. The preceding Deep Reactive-Ion Etch (DRIE) process relies on fluorine and bromine chemistries to achieve the extreme aspect ratios required for deep trench isolation (DTI) [P3]. Consequently, the trench sidewalls and bottoms are left heavily coated with protective, highly crosslinked fluorocarbon and brominated passivation polymers, alongside the crusted remnants of the photoresist and oxide hard mask [P1, P3]. This Ashing & Strip/Clean step must completely eradicate these complex organic and organohalogen residues to expose pristine silicon and oxide surfaces [A3]. If left uncleaned, these hydrophobic polymer networks will severely degrade the adhesion and structural integrity of the subsequent SiN Fill step, leading to interface trap states that drastically increase dark current and white pixel defects in the BSI CMOS Image Sensor [P3]. What distinguishes this specific step from generic planar ashing operations is the profound mass transport restriction imposed by the extreme depth and narrow width of the DTI structures, necessitating highly specialized residue extraction mechanisms [P3]. The physical and chemical mechanism of this cleaning step relies on a synergistic sequence of structural polymer modification followed by targeted wet dissolution [P1]. Conventional oxygen plasma alone is often insufficient or overly damaging due to the C–F rich dense network of the fluorocarbon polymers, which strongly resists standard organic solvents [P1]. To overcome this, advanced hybrid approaches introduce ultraviolet (UV) irradiation or plasma liquid-vapor activation to structurally weaken the residues [P1, P4]. Under appropriate excitation, the high-energy photons or plasma-generated radicals induce photochemical scission of the C–C and C–F bonds within the polymer backbone [P1]. This chain scission reduces the molecular weight and crosslink density of the crust, whilst simultaneously introducing polar functional groups that lower the surface energy and improve solvent wettability [P1]. Once the structural integrity of the heavily crosslinked photoresist and passivation polymer is compromised, specially formulated wet stripping solvents can effectively penetrate, swell, and dissolve the modified residues [P2]. During this dissolution phase, mechanical energy such as megasonics may be coupled to the fluid to physically assist delamination and overcome the severe capillary forces governing the deep trench geometry [P2]. The selection of specific stripping chemistries and modification parameters requires a strict balance between residue removal efficiency and the preservation of the exposed substrate and hard mask [P2]. Solvents are engineered with specific oxidative or reductive agents capable of complexing with the halogenated etch by-products—such as the bromine passivation layers generated during the DTI etch—without causing aggressive pitting of the underlying silicon crystal [P3]. Furthermore, process parameters like UV dose or plasma activation voltage must be meticulously optimized within a narrow process window [P1, P4]. Insufficient activation energy yields incomplete polymer chain scission, preventing the solvent from dissolving the dense fluorocarbon network [P1]. Conversely, an excessive energy dose can drive unwanted secondary crosslinking of the residues or inflict structural damage upon the surrounding dielectric films, permanently altering their mechanical and electrical properties [P1, P4]. The subsequent Trench Vacuum dry step also dictates that this clean must conclude with a perfectly uniform, hydrophilic surface state free of any micro-watermarks or precipitated contaminants, ensuring flawless conformity for the proceeding SiN deposition [P3].