During the subsequent >1000 °C corner-rounding oxidation, the SiN layer effectively blocks oxygen diffusion, protecting the planar active area while allowing localized oxide growth strictly at the exposed trench corners .
In depth
In the Shallow Trench Isolation (STI) module, the SiN hard mask deposition
follows pre-cleaning and serves as the primary structural boundary for the active area . This silicon nitride layer is typically deposited over a thin thermal pad oxide to buffer interfacial stress between the nitride and the underlying silicon lattice . Unlike the subsequent SiO hard mask—which acts as a disposable pattern-transfer or anti-reflective layer—this SiN layer persists through the deep trench etch and acts as the definitive stopping layer during Chemical Mechanical Polishing (CMP) . Furthermore, it is distinctly different from later nitride hard masks used in spacer formation, because this specific film must withstand the high-temperature corner-rounding oxidation without oxidizing itself, thereby protecting the active area surface . The deposition of the SiN hard mask relies on Chemical Vapor Deposition (CVD) mechanisms where precursor gases, such as silane and ammonia, dissociate and react to form a solid continuous film . In plasma-enhanced (PECVD) systems, RF fields excite these reactant gases into highly reactive radicals, allowing deposition at significantly lower substrate temperatures . Conversely, Low-Pressure CVD (LPCVD) relies purely on thermal energy to drive the dehydrogenation and densification processes necessary for forming a near-stoichiometric Si3N4 network . Silane partial pressure directly determines the Si/N ratio in the resulting film, with lower silane flows favoring a denser atomic arrangement . Managing the intrinsic mechanical stress of this thick dielectric layer is critical; in plasma-assisted systems, this can be achieved by modulating ion bombardment energy through alternating high-frequency and low-frequency RF power . Silicon nitride is explicitly chosen for this step due to its high density, oxidation resistance, and excellent mechanical hardness, providing high etch selectivity relative to silicon dioxide . During the subsequent >1000 °C corner-rounding oxidation, the SiN layer effectively blocks oxygen diffusion, protecting the planar active area while allowing localized oxide growth strictly at the exposed trench corners . Minimizing the hydrogen content of the deposited SiN is essential, as excess hydrogen bonds (Si-H and N-H) act as precursors for bulk and interface trap generation . If hydrogen diffuses into the adjacent field oxide, it exacerbates radiation-induced positive charge trapping and causes parasitic field-oxide channel turn-on . Furthermore, the macroscopic stress imparted by the SiN film alters the silicon crystal's periodic atomic arrangement, which fundamentally modifies the local band structure . This mechanically induced strain changes carrier effective mass and mobility, directly impacting the drive current and subthreshold performance of the resulting devices . At the 40nm node, particularly for Backside Illuminated (BSI) CMOS Image Sensors, exact geometric control of the STI trench is required to minimize dark current and isolate highly sensitive adjacent pixels . As trench sidewalls become increasingly vertical at scaled dimensions, high-aspect-ratio gap-fill becomes highly susceptible to voiding . The SiN hard mask dictates the structural fidelity of the trench opening, meaning any critical dimension (CD) variation here propagates directly into the final isolation profile . Additionally, robust SiN coverage is necessary to mitigate corner loss during subsequent selective etching or cleaning steps, which would otherwise lead to localized electric field enhancement and catastrophic subthreshold leakage .
Risks & Challenges
[High] Intrinsic Film Stress Induced Defect Generation: Excessive compressive or tensile stress in the deposited SiN film can cause wafer warpage or induce localized dislocation defects in the underlying silicon lattice . This mechanical strain alters the local band structure and carrier mobility, leading to unpredictable variations in device drive current .
[High] Hydrogen-Induced Parasitic Leakage: If the deposition chemistry results in a highly hydrogenated SiN network, subsequent thermal steps will drive hydrogen diffusion toward the Si/SiO2 interfaces . This migration generates interface states and positive charge traps that lower the field-oxide threshold voltage, inducing parasitic subthreshold leakage .
[Medium] Low Density and Poor CMP Stop Integrity: Variations in silane partial pressure or plasma energy can produce a porous, non-stoichiometric film with diminished mechanical hardness . Such low-density regions exhibit a higher removal rate during the subsequent CMP process, leading to premature mask failure and active area erosion .
[Medium] Corner Loss Vulnerability: If the deposited SiN film exhibits poor edge conformity or insufficient chemical resistance, subsequent selective etching steps will cause the hard mask to recede at the pattern boundaries . Loss of this protective corner coverage exposes the silicon to localized electric field enhancement, significantly degrading the isolation reliability .