Ashing & Strip/Clean (F_DTI) - 40nm

Solvents with oxidative or reductive agents remove bromine passivation layers from DTI structures without damaging silicon .

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 . 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) . 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 . 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 . 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 . The physical and chemical mechanism of this cleaning step relies on a synergistic sequence of structural polymer modification followed by targeted wet dissolution . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 .

[High] Incomplete Residue Removal in Deep Trenches: Due to the extreme aspect ratio of the DTI structure, mass transport and fluid exchange of the wet stripping chemistry are severely diffusion-limited at the trench bottom . If the preceding fluorocarbon and brominated passivation polymers are not adequately modified via UV or plasma pre-treatment, the heavily crosslinked network will resist solvent penetration [P1, P3]. This leaves residual polymer that blocks the subsequent SiN fill, causing structural voids and compromising the electrical and optical isolation of the pixel .
[Medium] UV/Plasma-Induced Substrate Damage: High-dose UV irradiation or plasma activation is utilized to accelerate the scission of C–C and C–F bonds in the polymer residue . However, an excessive application of these high-energy treatments can damage delicate surrounding structures or induce unwanted secondary crosslinking of the residues, paradoxically reducing their solubility . In BSI CMOS Image Sensors, such overexposure can also generate interface trap states in the silicon substrate, which directly elevates dark current generation (Engineering Practice).
[Medium] Sidewall Attack and Profile Deformation: Overly aggressive ashing or the use of excessively active radical species in the stripping chemistry can inadvertently attack the exposed silicon sidewalls or the remaining oxide hard mask [P2, P4]. If the protective halogenated passivation layer is removed too rapidly in an uncontrolled chemical environment, the underlying structural dimensions may be etched laterally, leading to critical dimension (CD) bias and degraded electrical isolation between adjacent active regions .
[Low] Redeposition of Insoluble Species: During the wet stripping process, dissolved complex organohalogens or metallic trace residues can precipitate out of the solvent if local chemical concentration gradients or pH levels shift unfavorably within the confined space of the deep trench . These redeposited clusters adhere to the trench walls and act as localized stress concentration points or nucleation sites for defects during the subsequent SiN fill step .

Solvents with oxidative or reductive agents remove bromine passivation layers from DTI structures without damaging silicon .

[High] Incomplete Residue Removal in Deep Trenches: Due to the extreme aspect ratio of the DTI structure, mass transport and fluid exchange of the wet stripping chemistry are severely diffusion-limited at the trench bottom . If the preceding fluorocarbon and brominated passivation polymers are not adequately modified via UV or plasma pre-treatment, the heavily crosslinked network will resist solvent penetration [P1, P3]. This leaves residual polymer that blocks the subsequent SiN fill, causing structural voids and compromising the electrical and optical isolation of the pixel .
[Medium] UV/Plasma-Induced Substrate Damage: High-dose UV irradiation or plasma activation is utilized to accelerate the scission of C–C and C–F bonds in the polymer residue . However, an excessive application of these high-energy treatments can damage delicate surrounding structures or induce unwanted secondary crosslinking of the residues, paradoxically reducing their solubility . In BSI CMOS Image Sensors, such overexposure can also generate interface trap states in the silicon substrate, which directly elevates dark current generation (Engineering Practice).
[Medium] Sidewall Attack and Profile Deformation: Overly aggressive ashing or the use of excessively active radical species in the stripping chemistry can inadvertently attack the exposed silicon sidewalls or the remaining oxide hard mask [P2, P4]. If the protective halogenated passivation layer is removed too rapidly in an uncontrolled chemical environment, the underlying structural dimensions may be etched laterally, leading to critical dimension (CD) bias and degraded electrical isolation between adjacent active regions .
[Low] Redeposition of Insoluble Species: During the wet stripping process, dissolved complex organohalogens or metallic trace residues can precipitate out of the solvent if local chemical concentration gradients or pH levels shift unfavorably within the confined space of the deep trench . These redeposited clusters adhere to the trench walls and act as localized stress concentration points or nucleation sites for defects during the subsequent SiN fill step .

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