Ashing - 40nm BSI CMOS Image Sensor 40nm

Hydrogen radicals cleave sacrificial organic-layer bonds into volatile products, removing residue without oxidizing underlying inorganic films .

The Ashing step in the 40nm BSI CMOS Image Sensor process flow immediately follows the Sacrificial Lower OCL Etch (Engineering Practice). In advanced

image sensor manufacturing, sacrificial organic layers are utilized to planarize deep topography and facilitate controlled etch-back processes . Once the structural transfer is complete, it is critical to completely remove any remaining sacrificial polymer, photoresist, and etch-induced fluorocarbon residues . Ashing provides the necessary highly selective chemical removal to strip these hydrocarbon-based materials . This pristine surface preparation is strictly required before the subsequent Lower OCL Coating Deposition, ensuring optical uniformity and preventing adhesion failures at the dielectric interface . Plasma ashing operates primarily through a radical-dominated chemical reaction rather than physical ion sputtering . A plasma source generates monatomic reactive species, such as oxygen or hydrogen radicals, which diffuse to the wafer surface . In an O2-based chemistry, these oxygen radicals violently react with the hydrocarbon backbone of the sacrificial organic layer, acting essentially like a combustion process to produce volatile CO2 and H2O byproducts . To prevent physical damage to the delicate device surface, downstream or "afterglow" plasma etchers are typically employed . In this configuration, the plasma is generated in an upstream chamber, allowing neutral reactive species to reach the wafer while filtering out damaging high-energy ions, ensuring a purely chemical, isotropic etch . Furthermore, the chemical reaction rate is strongly temperature-dependent, governed by an Arrhenius relationship where elevated substrate temperatures provide the activation energy required for rapid polymer bond scission . The selection of the ashing chemistry involves a careful trade-off between removal rate and substrate preservation . While O2-based plasmas offer extremely high ashing rates, they carry a significant risk of oxidizing exposed silicon or silicon nitride surfaces . To mitigate this oxidation damage in highly sensitive devices, alternative reducing chemistries utilizing H2-based plasmas are frequently implemented . In H2 plasmas, reactive hydrogen radicals cleave C-C and C-H bonds to generate volatile molecules without oxidizing the underlying inorganic films . To compensate for the inherently lower ashing rates of pure H2, nitrogen gas (N2) can be introduced into the plasma mixture . The addition of N2 alters the plasma reaction pathways, suppressing radical recombination and enhancing the flux of active species to the surface, thereby optimizing the organic removal efficiency . At the 40nm node for BSI CIS devices, the tolerances for surface roughness and material loss are exceptionally tight due to their direct impact on optical signal integrity . Downstream plasma processing is particularly advantageous here, as it minimizes charging damage and ion-impact defects that could otherwise degrade the dark current performance of the photodiodes . Additionally, complete removal of the sacrificial LOCL is essential because any residual organic material would alter the localized refractive index, degrading the optical transmission into the active pixel array .

[High] Incomplete Organic Removal (Residues): If the substrate temperature drops or radical flux fluctuates, the Arrhenius-dependent chemical reaction rate will decrease exponentially . This leaves unreacted hydrocarbon polymers on the surface, which subsequently causes severe adhesion and optical uniformity issues during the Lower OCL Coating Deposition .
[High] Unintended Substrate Oxidation: Utilizing an excessively aggressive O2-based plasma chemistry can result in the rapid oxidation of exposed silicon or nitride layers . This alters the critical dimensions and optical properties of the underlying structures, a major failure mode that often necessitates a shift to H2-based chemistries .
[Medium] Plasma-Induced Surface Damage: If the downstream plasma configuration fails to sufficiently isolate the wafer from the primary discharge, energetic ions can strike the substrate . This direct ion bombardment introduces physical crystal damage and charging defects, which degrade device electrical performance .
[Low] Inorganic Ash Formation: Plasma ashing effectively volatilizes organic compounds but cannot remove metallic or silicon-based contaminants . If the sacrificial LOCL contained inorganic trace elements, these will remain on the wafer as solid ash, requiring subsequent wet chemical cleans to prevent micro-masking in later steps .

Hydrogen radicals cleave sacrificial organic-layer bonds into volatile products, removing residue without oxidizing underlying inorganic films .

The Ashing step in the 40nm BSI CMOS Image Sensor process flow immediately follows the Sacrificial Lower OCL Etch (Engineering Practice). In advanced

[High] Incomplete Organic Removal (Residues): If the substrate temperature drops or radical flux fluctuates, the Arrhenius-dependent chemical reaction rate will decrease exponentially . This leaves unreacted hydrocarbon polymers on the surface, which subsequently causes severe adhesion and optical uniformity issues during the Lower OCL Coating Deposition .
[High] Unintended Substrate Oxidation: Utilizing an excessively aggressive O2-based plasma chemistry can result in the rapid oxidation of exposed silicon or nitride layers . This alters the critical dimensions and optical properties of the underlying structures, a major failure mode that often necessitates a shift to H2-based chemistries .
[Medium] Plasma-Induced Surface Damage: If the downstream plasma configuration fails to sufficiently isolate the wafer from the primary discharge, energetic ions can strike the substrate . This direct ion bombardment introduces physical crystal damage and charging defects, which degrade device electrical performance .
[Low] Inorganic Ash Formation: Plasma ashing effectively volatilizes organic compounds but cannot remove metallic or silicon-based contaminants . If the sacrificial LOCL contained inorganic trace elements, these will remain on the wafer as solid ash, requiring subsequent wet chemical cleans to prevent micro-masking in later steps .

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges